C-F bond integration in polymeric carbon nitride enables oxygen-tolerant photocatalytic reduction of NO to N_2

Domain: physics
Published: 2025-12-10
Editorial outcome: major_revision

Abstract
The selective photocatalytic decomposition of NO under oxygen-rich conditions presents a significant challenge, largely due to the preferential reduction of O2 over NO. To overcome this limitation, we develop an F-modified polymeric carbon nitride catalyst (e-FCN). Under visible light irradiation in simulated ambient air (20% O2), the e-FCN catalyst achieves 85% NO conversion with 92% selectivity toward N2. It exhibits high apparent quantum yields for N2 formation (29.1% at 365 nm and ≥9.9% between 405 and 465 nm) and maintains its performance over 12 consecutive cycles without deactivation. Mechanistic investigations indicate that the incorporation of a C–F bond suppresses the detrimental C=O formation from O2 activation, thereby passivating the catalyst toward O2. Concurrently, e-FCN selectively adsorbs NO and promotes N–N coupling between adjacent NO molecules, ultimately converting NO to N2 via an N2O intermediate. This synergy of selective NO adsorption and controlled decomposition enables selective and oxygen-tolerant photocatalytic NO decomposition.

Introduction
The combustion of fossil fuels and vehicle emissions represent the primary sources of increasing nitrogen oxides (NOx) emissions. These NO compounds, with NO constituting approximately 95% of the total, exert significant detrimental effects on the environment and ecosystems1–3, which contribute to acid rain, greenhouse effect, photochemical smog, and secondary ultrafine particulate matter formation4,5. Currently, selective catalytic reduction (SCR) stands as the most established technology for NOx reduction. SCR employs a chemical reductant such as H2 or NH3 at temperatures exceeding 200°C6,7. Despite its effectiveness, post-SCR exhaust may contain several tens of ppmv NO. In indoor or urban environments, NO concentrations may be diluted to ppbv or several ppmv levels. However, these levels often still exceed the emission limits stipulated by the World Health Organization8,9. Additionally, SCR technology encounters challenges in removing low-concentration NO, highlighting the need for alternative methods to achieve effective elimination10,11. While the adsorption method can remove low-concentration NO, it carries the risk of generating N2O and NO2 through NO disproportionation, thereby contributing to secondary environmental pollution12,13. Another approach, photocatalytic oxidation of NO to NO3−, has been proposed as a promising technology for NO removal in indoor or urban settings14–16. Nonetheless, this method unavoidably generates more toxic NO2 as a byproduct, and the resulting nonvolatile NO3− species may accumulate and occupy surface active sites, leading to the deactivation of the photocatalyst17.

N2 could potentially be formed as another product of photocatalytic NO conversion. The inherent advantage of N2 as a non-toxic and non-adsorbing gas helps mitigate concerns about catalyst deactivation and secondary environmental impact18. Under ambient air conditions, the Gibbs free energy changes for the reduction of O2 to superoxide radical (•O2−) and hydrogen peroxide (H2O2) are 3 kJ·mol−1 and 117 kJ·mol−1, respectively19,20. These values are significantly lower than the energy required to cleave the N=O bond (607 kJ·mol−1) and form the N≡N bond (946 kJ·mol−1)21, both of which are necessary for the reduction of NO to N2. Consequently, the activation of O2 to form •O2− and H2O2 is thermodynamically favored. The resulting reactive oxygen species (•O2− and H2O2) should promote the oxidation of NO to NO3−, thereby inhibiting the reduction of NO to N2. Consequently, achieving selective photocatalytic reduction of NO in the presence of O2 remains a significant challenge.

Most works on photocatalytic NO decomposition have typically been carried out under anoxic conditions to minimize the influence of O2 22. While some studies have observed trace amounts of N2 in the presence of O2 23–25, NO decomposition to N2 in air conditioning is strongly hindered. Similarly, the photocatalytic reduction of CO2 in the presence of O2, akin to photocatalytic NO reduction, presents a formidable challenge. Recent efforts have therefore concentrated on incorporating CO2 adsorption sites into catalyst structures to lower local O2 concentrations and enable in situ CO2 capture and concentration26–31. Expanding on these methodologies, it is imperative to develop catalysts customized for the selective adsorption and decomposition of NO in the presence of O2.

Density functional theory (DFT) computations indicate that introducing a C–F bond into the polymeric carbon nitride (CN) model induces surface charge polarization around the F modification site. This polarization facilitates the selective adsorption of NO in the presence of O2 32. However, there are currently no experimental evidence supporting the selective adsorption of NO by F-modified CN in the presence of O2. In situ observations have revealed an unexpected accumulation of C=O groups on CN due to surface carbon oxidation during the oxygen evolution reaction (OER) in the water splitting process, leading to the deactivation of CN catalyst33. To address this issue, a surface fluorination strategy has been developed to mitigate the C=O accumulation by forming a C–F bond and to decrease OER barriers through the activation of adjacent N reactive sites. This approach significantly enhances the overall water-splitting efficiency of F-modified CN under visible light33. These findings propose the hypothesis that the introduction of the C–F bond in the CN structure should enable the selective NO adsorption capability while reducing interaction with O2.

Our prepared F-modified carbon nitride (e-FCN)34 enables oxygen-tolerant, selective photoreduction of NO to N2 under visible light. In contrast, O-modified carbon nitride (e-OCN)35 favors the activation of O2, shifting the reaction toward NO oxidation. This demonstrates that e-FCN uniquely possesses the capacity to preferentially adsorb and reduce NO even in O2, presenting a pioneering approach for selective NO reduction to N2 in oxygen-rich environments.

Results
Preparation and characterization of CN-based catalysts
The preparation of CN-based catalysts is illustrated in Fig. 1A. The bulk-OCN and bulk-FCN were synthesized by incorporating glycolic acid and NH4F, respectively, via the traditional condensation process34,35, followed by thermal exfoliation to produce e-OCN and e-FCN36. For comparison, bulk-CN and e-CN were prepared via melamine pyrolysis without additives. X-ray diffraction (XRD) analysis (Fig. S1A) revealed two characteristic peaks at 13.0° and 27.4°, corresponding to the (100) and (002) crystallographic planes, which are indicative of in-plane tri-s-triazine units and interlayer stacking, respectively37. A notable broadening of these peaks in the exfoliated catalysts suggests a reduction in crystallite size36. Atomic force microscopy (AFM) confirmed successful exfoliation, showing a decrease in average particle thickness from 40.1–48.2 nm for the bulk catalysts to 2.3–4.9 nm for the exfoliated catalysts (Fig. S2). Based on a theoretical monolayer thickness of 0.325 nm for CN38, this corresponds to a reduction from approximately 124–148 layers to merely 7–15 layers. This finding was corroborated by dynamic light scattering (DLS), which showed a substantial decrease in the average hydrodynamic diameter from approximately 900 nm to 120 nm post-exfoliation (Fig. S3). Collectively, these results confirm the successful exfoliation of the bulk CN (bulk CN)-based catalysts. Both AFM and DLS revealed a consistent exfoliation trend across the catalyst series. AFM measurements indicated that the particle thickness decreased in the order (Fig. S2): e-CN (4.3–4.9 nm) > e-OCN (2.8–3.4 nm) > e-FCN (2.3–2.7 nm). This trend was corroborated by DLS, which showed a corresponding decrease in hydrodynamic diameter (Fig. S3): e-CN (133.1±1.7 nm) > e-OCN (122.7±1.3 nm) > e-FCN (110.6±1.1 nm). Charge distribution calculations confirmed the strong electron-withdrawing nature of the O and F atoms, which generated a highly positive C center (Fig. S4). The greater electronegativity of F resulted in a more positive charge on this C center in the FCN model (+1.81) compared with that in the OCN model (+1.70). We therefore propose that this electron-withdrawing effect is the primary driver of enhanced exfoliation. The incorporation of O or F into the CN matrix attenuates the interlayer π-π interactions, thereby facilitating delamination. The pronounced efficacy of F stems directly from its higher electronegativity, which more effectively disrupts these cohesive forces and promotes exfoliation.

Solid-state nuclear magnetic resonance spectroscopy of 13C, 1H, and 19F (solid-state 13C, 1H, and 19F NMR) was employed to probe chemical environments (Fig. 1B–D). The 13C NMR spectra (Fig. 1B) showed consistent peaks at 156.4 ppm (C3N) and 164.8 ppm (C2N-NHx), confirming structural integrity39. For e-CN, a shoulder peak was observed at 162.9 ppm, which is assigned to the C2N-NH structure formed during the thermal exfoliation of bulk-CN40. This peak disappeared upon the introduction of electron-withdrawing O/F. We attribute this disappearance to a strong inductive effect from the O/F modifiers. The carbon atom in the C2N-NH structure is situated closer to the modification site than the carbon in the C2N-NHx structure (whose signal appears at 164.8 ppm). Consequently, the inductive effect induces a downfield shift of the C2N-NH signal, causing it to merge with the adjacent peak at 164.8 ppm and become unresolvable. 1H NMR (Fig. 1C) revealed peaks at 4.2 ppm (–NH2) and 9.4 ppm (–NH–), with an additional peak at 7.2 ppm (–OH) in e-OCN, verifying O modification41. The 19F NMR spectrum of e-FCN (Fig. 1D) exhibited a distinct peak at −145.5 ppm, confirming C–F bonding42.

X-ray photoelectron spectroscopy (XPS) analysis (Fig. S1B–D) corroborated these findings. The C 1s spectra displayed peaks at 284.8 eV (C–C), 286.4 eV (C–M, M=NH, OH, F), and 288.2 eV (N–C=N)39,41, while N 1s spectra showed contributions from N-C (398.6 eV), N-C3 (399.3 eV), and NHx (400.8 eV)39. The O 1s spectra (Fig. 1C) featured two typical peaks for all three catalysts: one at 533.1 eV (O–H from adsorbed H2O)43 and another at 532.0 eV (C–O–C). While the C–O–C signal in e-CN and e-FCN originates solely from adsorbed CO2 43, in e-OCN it has a dual origin from both structural O modifiers41 and adsorbed CO2 43. Furthermore, e-OCN is distinctive in exhibiting an additional –OH peak at 530.9 eV41. The F 1s spectrum of e-FCN (Fig. 1D) exhibited a peak at 687.3 eV, further verifying C–F bonding42,44. Electron paramagnetic resonance (EPR) spectroscopy (Fig. 1E) revealed a Lorentzian peak (g=2.005) in all catalysts except bulk-CN, attributed to unpaired electrons in graphitic rings45,46. Exfoliated catalysts showed enhanced signals (e-FCN > e-OCN > e-CN), indicating EPR increased defect density, whereas bulk catalysts exhibited weaker signals. In particular, bulk-CN exhibited no detectable EPR signal. These results demonstrate that O and F modifications introduce –OH and C–F moieties, respectively, which may influence photocatalytic activity in NO decomposition.

Photocatalytic NO conversion behaviors of modified CN catalysts
The comparative evaluation of photocatalytic NO decomposition in a 20% O2 atmosphere (simulating ambient air) was conducted using e-CN, e-OCN, and e-FCN in a continuous flow reactor under 425 nm light irradiation. To clearly present the catalytic reaction conditions, this study systematically summarized all relevant experimental parameters in Table S1. This consolidated table improved the logical organization of data presentation while enhancing the accessibility and clarity of the manuscript. As shown in Fig. 2A, significant differences are observed in NO removal efficiency among the catalysts: e-CN (20%), e-OCN (68%), and e-FCN (85%). Table S2 summarized the concentrations of N-containing products after 30 min of reaction over the three catalysts. In all cases, the total N concentration, which was calculated as the sum of all N-containing products, matched the initial NO concentration, meeting the N balance throughout the reaction. The data in Table S2 also revealed that N2 was the dominant product in NO decomposition over e-FCN, whereas N2O and NO3− were the primary products over e-CN and e-OCN. Based on these results, the product selectivity at 30 min was determined, demonstrating that e-FCN exhibited significantly higher N2 selectivity (92%) compared with e-CN (1%) and e-OCN (7%). Notably, e-OCN showed higher NO3− selectivity (55%) than e-CN (46%).

To identify the N source in N2 formation, isotopic labeling experiments were conducted using a 4% 15NO gas mixture in 20% O2/Ar within a sealed reactor. The system was rigorously prepared through multiple evacuation and purging cycles to ensure atmospheric integrity. Control pre-reaction analysis by GC-MS (Fig. S5A) confirmed the absence of N2 contamination, as only a characteristic O2 peak was observed in both the GC trace and the corresponding MS spectrum. Following a 2-h photocatalytic reaction, GC-MS analysis demonstrated that the N2 produced originated exclusively from the decomposition of 15NO, as indicated by the sole detection of 15N2 (Fig. 2B). A separate experiment was performed to ascertain the origin of the co-generated O2, utilizing a mixture of 4% NO in 20% 18O2/Ar. Control pre-reaction GC-MS analysis (Fig. S5B) confirmed the presence of exclusively 18O2, ruling out significant O2 contamination. After the photocatalytic reaction, the MS spectrum of O2 (Fig. 2C) revealed a composite signal for 16O2 and 18O2. The absence of a peak at m/z = 34 (16O18O) indicates that isotopic exchange between oxygen atoms did not occur. The experimental 16O2:18O2 ratio was approximately 1:11, which aligns accurately with the theoretically predicted ratio of 1:10.7 based on reaction selectivity. This result confirms that the newly generated O2 originates from the decomposition of NO.

To accurately quantify the N2 and O2 products, photocatalytic NO decomposition was conducted in an Ar atmosphere to eliminate interference from O2 (Fig. S6). As shown in Fig. S6C, e-FCN decomposed NO into nearly equimolar amounts of N2 and O2, consistent with previous reports17,25,47. Figures 2A and S6D revealed that the three catalysts exhibited similar N2 selectivity under both Ar and 20% O2/Ar atmospheres, despite following fundamentally different reaction pathways. Under the Ar atmosphere, e-CN, e-OCN, and e-FCN all underwent photocatalytic NO decomposition, producing N2O as a byproduct. In contrast, under the 20% O2/Ar atmosphere, the catalysts displayed distinct behaviors: e-FCN remained primarily governed by NO decomposition, with minimal NO oxidation, resulting in comparable N2 selectivity in both conditions. However, as shown in Fig. 2A, e-CN and e-OCN generated substantially more NO3− than N2O, indicating a predominant photocatalytic oxidation pathway (converting NO to NO3−) rather than decomposition. These differences underscore the selective catalytic preference for specific reaction pathways in the presence of O2.

The catalyst surface temperature was monitored using a thermocouple. As shown in Fig. S7A, the temperature increased from 25°C to 35°C during the photocatalytic reaction and stabilized thereafter, indicating that the LED light source did not induce significant heating of the catalyst. To exclude thermal contributions to NO decomposition, a control experiment was performed at 35°C in the absence of light. Figure S7B shows that all catalysts exhibited negligible activity under these conditions, confirming that NO decomposition was exclusively photocatalytic rather than thermally driven. To investigate the correlation between the initial NH4F amount and the F/C atomic ratio in the e-FCN catalyst, we first conducted quantitative elemental analysis of C, N, and F in e-FCN catalysts prepared with varying NH4F amounts. Based on the elemental composition data (Table S3), the corresponding F/C atomic ratios were calculated. As illustrated in Fig. S8A, a strong linear relationship (R2=0.9933) was observed between the initial NH4F amount and the F/C atomic ratio, demonstrating that the F content in e-FCN can be precisely tuned by adjusting the initial NH4F amount. Further analysis of the influence of F/C atomic ratios on catalytic performance (Fig. S8B) revealed that the e-FCN catalyst exhibited optimal activity at an F/C ratio of 0.10 at%. The process parameters, including catalyst mass and gas flow rate, were optimized to improve the photocatalytic NO decomposition efficiency (Figs. S9–10). Following this optimization, the apparent quantum yields (AQYs) for N2 formation were determined. As illustrated in Fig. 2D and Table S4, the AQYs of e-FCN are no lower than 9.9% within the 365–465 nm range, reaching a peak of 29.1% at 365 nm.

To evaluate the distinctiveness of the proposed C–F modification strategy, we systematically compared the performance of several catalysts under both Ar and 20% O2/Ar atmospheres. The tested catalysts included: e-FCN (containing C–F bonds), our previously developed mp-TiO2-x 25, surface-fluorinated F-mp-TiO2-x 48, and the BiOBr/BiOF system featuring O-Br/F bonds49. As shown in Fig. 3A, B, e-FCN exhibited pronounced catalytic performance under both conditions. Notably, while mp-TiO2-x demonstrated activity comparable to e-FCN in an Ar atmosphere, its NO removal efficiency and N2 selectivity decreased significantly in 20% O2/Ar, with substantial NO2 byproduct formation. Further analysis revealed that surface F doping (F-mp-TiO2-x) slightly improved NO removal efficiency but increased NO2 generation, suggesting that surface F doping primarily promotes the undesired oxidation of NO to NO2 rather than NO decomposition under oxygen-rich conditions. Similarly, the BiOF system (containing O–F bonds) showed marginally higher activity than BiOBr but remained ineffective for NO decomposition in an oxygen-rich environment. These comparative results clearly demonstrate the distinct advantage of the C–F modification strategy in developing highly efficient NO decomposition catalysts.

Although photocatalytic processes for NO removal have been extensively studied22, the majority of prior research has focused on the selective oxidation of NO to NO3−. We recently employed mp-TiO2-x to achieve selective NO reduction to N2 without a chemical reductant in anoxic conditions, but its performance was drastically reduced in the presence of O2 25. In contrast, e-FCN demonstrated pronounced efficiency in converting NO to N2 across a range of NO concentrations (0.5–2.0 ppmv; see Fig. 3C) and the concentration of O2 had a negligible effect on both NO removal efficiency and N2 selectivity (Fig. 3D). Conversely, oxygen-modified CN (e-OCN) promoted NO oxidation to NO3− in the presence of O2 (Fig. S11), shifting the primary byproduct from N2O to NO3− as O2 concentration increased from 0% to 20%. This behavior starkly contrasts with F-modified CN (e-FCN), which enables oxygen-tolerant NO decomposition to N2 regardless of O2 presence. As listed in Tables S5 and 6, e-FCN demonstrates competitive photocatalytic performance and AQYs for NO decomposition under 20% O2, even at an initial NO concentration as low as 0.5 ppmv, when compared with previously reported catalysts.

In durability tests, e-FCN demonstrated consistent NO removal efficiency and N2 selectivity over 12 consecutive cycles (Fig. 4A). In contrast, e-OCN exhibited a gradual decline in both NO removal efficiency and N2/NO3− selectivity (Fig. S12), with detectable NO2 formation emerging from the second cycle onward (Fig. S12B). Furthermore, measurements of accumulated NO3− on the catalyst surfaces revealed significantly higher deposition on e-OCN compared to e-FCN (Fig. 4B). These results suggest that e-OCN primarily oxidizes NO to NO3−, leading to catalyst deactivation via NO3− accumulation, whereas e-FCN predominantly decomposes NO into N2 and O2, minimizing NO3− generation and maintaining stable performance. Post-cycling characterization via XRD, solid-state NMR, and XPS (Fig. S13), along with elemental analysis (Table S3), confirmed the structural and compositional integrity of the e-FCN after 12 cycles.

As shown in Fig. 4C, e-FCN exhibited stable performance in both NO removal and N2 selectivity under continuous flow conditions over a 3-h period under RH=0% and constant light intensity. The catalyst surface temperature and the inlet gas flow rate were monitored in real time. Under irradiation from a 425 nm LED light source, the catalyst surface temperature showed no significant increase, remaining stable at approximately 35°C throughout the reaction (Fig. S14A). Concurrently, the gas flow rate at the reactor inlet remained constant without notable fluctuation (Fig. S14B). The NO removal efficiency and N2 selectivity fluctuated in response to changes in light intensity but remained generally stable under RH=45% (Fig. 4D). As previously demonstrated (Fig. S7B), thermal effects are negligible for NO decomposition at 35°C, eliminating temperature as a variable. Consequently, the observed performance fluctuations are primarily attributed to changes in light intensity, which directly modulate the yield of photogenerated charge carriers and thus the reaction kinetics within a certain range50.

In practical indoor environments, air contains a certain level of relative humidity (RH), and NO pollutants often coexist with other gases, such as CO2, NO2, and SO2 51. It is therefore essential to evaluate the effects of RH and coexisting gases on photocatalytic NO decomposition over e-FCN. Figures S15 and 16 systematically examined these influences by testing e-FCN under common indoor RH levels and typical concentration ratios of NO to CO2, NO2, and SO2. Figure S15 indicated that RH has a minor effect on photocatalytic NO decomposition, suggesting that H2O molecules do not significantly compete with NO for adsorption and activation sites on the catalyst surface. Similarly, coexisting CO2 exhibited a negligible impact (Fig. S16A, B). This observation aligns with prior theoretical calculations indicating that the adsorption energy of NO on F-CN is significantly more favorable than that of CO2, leading to the preferential adsorption of NO32. To evaluate the effect of NO2, the total initial NOx concentration was maintained at 2.0 ppmv while varying the NO2 proportion. The results indicate that neither the NOx removal efficiency nor the N2 selectivity was significantly affected (Fig. S16C, D). Furthermore, no NO2 was detected in the effluent. In this study, the initial NO concentration exceeded that of the coexisting NO2. Under these conditions, the two species primarily reacted via the fast SCR pathway in a 1:1 molar ratio, forming the unstable intermediate N2O3 52. This intermediate was then rapidly reduced by photogenerated electrons. This reaction pathway explains the absence of NO2 in the effluent. The presence of SO2 slightly inhibited the photocatalytic decomposition of NO (Fig. S16E, F). This inhibition can be attributed to SO2 acting as a scavenger for photogenerated holes, a mechanism previously documented in aqueous reaction systems53. The consumption of holes by SO2 disrupts the charge balance within the photocatalytic system, thereby altering the pathway for NO decomposition.

Charge carrier dynamics of modified CN-based catalysts
The prepared e-FCN photocatalyst successfully demonstrated an ability to reduce NO to N2 in the presence of 20% O2. This implies a capacity for the preferential adsorption and activation of NO over O2. To elucidate the key factors underlying this oxygen-tolerant NO reduction, we first investigated the charge carrier dynamics of e-CN, e-OCN, and e-FCN. UV-Vis diffuse reflectance spectra (UV-Vis DRS, Fig. S17A) revealed that: (1) e-FCN exhibits enhanced light absorption across both ultraviolet and visible regions compared with e-CN, a phenomenon attributed to F incorporation into the CN structure33; and (2) the absorption edge of e-OCN redshifts to the near-infrared region due to O modification35. To analyze charge carrier separation upon photoexcitation, we employed photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The PL spectra (Fig. S17B) showed a progressive redshift in the emission peaks from 455 nm (e-CN) to 461 nm (e-OCN) and 465 nm (e-FCN), with a concomitant decrease in peak intensity (e-CN > e-OCN > e-FCN). The TRPL results (Fig. S17B and Table S7) further indicated that the average emission lifetime of e-FCN (5.55 ns) is longer than those of e-CN (2.05 ns) and e-OCN (3.23 ns). The reduced PL intensity and prolonged emission lifetime collectively suggest that F incorporation enhances charge carrier separation. This conclusion is consistently supported by photocurrent and electrochemical impedance spectroscopy (EIS) measurements, which showed that the e-FCN electrode exhibits a higher photocurrent density (Fig. S17C) and lower electrical resistance (Fig. S17D) than e-CN and e-OCN.

Furthermore, determining the energy levels of these catalysts is essential for assessing how F modification influences the photocatalytic reaction from a thermodynamic perspective. The bandgap values, derived from Tauc plots, are 2.52 eV for e-CN, 2.05 eV for e-OCN, and 2.46 eV for e-FCN (Fig. S18A). The valence band (VB) edge positions, approximated from XPS VB spectra (Fig. S18B) according to the literature39, are +1.83 V RHE for e-CN, +1.52 V RHE for e-OCN, and +1.74 V RHE for e-FCN. Mott-Schottky plots measured at 1000 Hz (Fig. S18C) exhibit positive slopes, confirming n-type semiconductor behavior for all catalysts. The flat band potentials (Efb) are determined to be −0.69 V RHE for e-CN, −0.53 V RHE for e-OCN, and −0.72 V RHE for e-FCN; these values correspond to the conduction band (CB) edge levels54. An energy level diagram constructed from these findings is presented in Fig. S18D. The more negative CB edge of e-FCN (−0.72 V RHE), compared with e-CN (−0.69 V RHE) and e-OCN (−0.53 V RHE), indicates that e-FCN possesses a stronger driving force for the reduction of NO/NO2 to N2 [standard reduction potentials: E(NO/N2)=1.68 V RHE 55 and E(N2O/N2)=+1.77 V RHE 56]. However, thermodynamically, this more negative CB also provides a stronger driving force for the reduction of O2 to superoxide radical [•O2−, E(O2/•O2−)=-0.13 V RHE]57 or H2O2 [E(O2/H2O2)=0.68 V RHE]58. Thus, to resolve this thermodynamic conflict and identify the origin of the observed oxygen tolerance, subsequent investigations must focus on the charge carrier dynamics in NO/O2/Ar atmospheres.

To investigate the charge carrier dynamics of exfoliated CN-based catalysts under relevant conditions, microsecond transient absorption spectroscopy (μs-TAS) was employed, as its temporal resolution aligns with the timescales of the actual reaction kinetics. To distinguish the nature of the charge carriers, aqueous dispersions of e-CN were probed at 900 nm both with and without ethylenediaminetetraacetic acid dipotassium (EDTA). This well-established hole scavenger reveals the signature of long-lived electrons. As shown in Fig. S19A, the signal half-life (t50%) of e-CN was significantly prolonged from 35.5 μs to 63.6 μs upon the addition of EDTA. This observation confirms that the transient signal originates from photoinduced electrons, consistent with prior literature59.

Subsequently, the electron transfer kinetics of e-CN, e-OCN, and e-FCN were probed by introducing NO/O2/Ar atmospheres. Figures 5A, B and S19B demonstrated a decrease in the t50% values for all three catalysts under the NO or O2 atmosphere compared with the Ar atmosphere, confirming that electrons participate in activating both NO and O2 60. However, the catalysts exhibited distinct behavioral trends: e-CN showed similar reductions in t50% with NO or O2, indicating no significant specificity (Fig. S19B). On the other hand, e-OCN exhibited a larger reduction in t50% with O2 than NO (Fig. 5A), whereas e-FCN showed a substantial reduction with NO than O2 (Fig. 5B). These trends indicate that e-OCN preferentially facilitates the electron transfer to O2, whereas e-FCN favors the electron transfer to NO. To quantify this selectivity, the electron transfer efficiencies for O2 and NO, denoted as ETE(O2) and ETE(NO), were calculated from the t50% values according to an established literature method61. As listed in Table S8, e-FCN exhibited the highest ETE(NO) (30.2%) and the lowest ETE(O2) (6.2%), confirming its pronounced selectivity for activating NO over O2. This finding directly correlates with the observed catalytic performance, wherein e-FCN demonstrates an ability to reduce NO to N2 even in an oxygen-rich atmosphere.

In situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) was employed to probe light-induced binding energy shifts in exfoliated CN-based catalysts, thereby identifying functional groups active in photocatalysis62,63. Under illumination, the C 1s spectra exhibited a positive shift for C-M groups (where M=NHx, OH, F), indicating electron transfer from the C atoms to these functional groups (Figs. 5C and S20A, D). This interpretation was corroborated by a corresponding negative shift in the N 1s spectra of NHx groups, confirming their role as electron acceptors (Figs. 5D and S20B, E). In contrast, O 1s peaks associated with adsorbed H2O or CO2 exhibited no shift, indicating these species do not participate in the charge transfer process (Figs. 5E and S20C, F). Notably, in e-OCN, the C-OH peak in the O 1s spectrum exhibited a negative shift, identifying the OH group as an electron acceptor (Fig. S20F). An analogous negative shift was observed for the C–F group in the F 1s spectrum of e-FCN, indicating that F also acts as an electron acceptor (Fig. 5F). Collectively, these findings identify C-NHx, C-OH (in e-OCN), and C–F (in e-FCN) as direct participants in the photocatalytic reaction.

NO/O2 adsorption properties of modified CN-based catalysts
The above investigations into the charge carrier dynamics of CN-based catalysts have revealed significant variations in their electron transfer capabilities toward NO/O2. These findings warrant a further examination of the adsorption behavior and activation performance of NO/O2 on the CN-based catalyst surfaces. The nitrogen adsorption-desorption isotherms and pore-size distribution profiles of e-CN, e-OCN, and e-FCN are presented in Fig. S21. All three catalysts exhibit type II isotherms with an H3 hysteresis loop64. As summarized in Table S9, the Brunauer–Emmett–Teller (BET) surface area and pore volume increase in the order: e-CN < e-OCN < e-FCN. The preceding AFM and DLS analyses confirmed the successful exfoliation of the CN-based catalysts (Figs. S2 and 3). The introduction of electronegative O and F atoms effectively weakened the interlayer π-π interactions, thereby facilitating exfoliation. Owing to its higher electronegativity, F exerted a more pronounced effect, as evidenced by the decreasing order of both average particle thickness and average hydrodynamic particle size: e-CN > e-OCN > e-FCN. For layered materials, the specific surface area is primarily governed by the exposed surfaces of the exfoliated nanosheets. Theoretically, greater exfoliation reduces both the average number of layers and the overall thickness, resulting in a larger specific surface area36. The observed BET surface area and pore volume trend align well with this principle.

To evaluate the influence of exfoliation on photocatalytic NO decomposition activity, bulk CN-based photocatalysts were also prepared and tested as control samples under the identical reaction conditions. As illustrated in Fig. S22A–D, the photocatalytic NO removal efficiency and N2 selectivity of the bulk catalysts were significantly lower than those of the exfoliated catalysts in both Ar and 20% O2. The surface area and pore volume of the bulk catalysts are substantially smaller than those of the exfoliated catalysts (Fig. S21A, B and Table S9). This suggests that exfoliation increases the number of active sites exposed on the catalyst surface, thereby enhancing NO adsorption and subsequent activation36.

The NO and O2 adsorption capacities of e-CN, e-OCN, and e-FCN were evaluated using the NO/O2 temperature-programmed desorption (NO/O2-TPD) technique (Fig. 6A). To account for variations in specific surface area among the exfoliated catalysts, the TPD peak areas were normalized by the corresponding BET surface area, enabling a direct comparison of their intrinsic adsorption capacities. In the case of NO-TPD, e-FCN exhibited a higher desorption temperature and a larger normalized peak area than both e-CN and e-OCN, indicating a stronger and greater capacity for NO adsorption65. For O2-TPD, desorption events below 300°C were assigned to surface-adsorbed oxygen, while those between 300°C and 600°C were attributed to subsurface oxygen species66. In contrast to the NO adsorption results, e-FCN showed a significantly lower O2 desorption temperature and a smaller normalized peak area, suggesting a weaker and less extensive adsorption of O2 66. Conversely, e-OCN demonstrated the highest oxygen adsorption capacity of the three catalysts. The TPD peak areas were quantified by calibration with a standard gas mixture to establish the relationship between peak area and adsorbed gas concentration. As summarized in Table S10, this quantitative analysis confirms that e-FCN possesses the highest NO adsorption capacity (2.62 μmol·g−1) and the lowest O2 adsorption capacity (0.20 μmol·g−1), indicating a preferential adsorption of NO over O2. Collectively, the NO/O2-TPD results demonstrate that while e-CN and e-OCN exhibit comparable affinities for NO and O2, e-FCN remains largely inert towards O2.

To complement the NO/O2-TPD results, DFT calculations were conducted to analyze O2 and NO adsorption on the catalyst surfaces, despite the inherent challenge of modeling the exact catalyst under operational conditions. As shown in Fig. S23, the adsorption energies of O2 are more negative than those of NO for the optimized e-CN model [Eads(O2)=−0.18 eV, Eads(NO)=−0.09 eV] and the e-OCN model [Eads(O2)=−0.33 eV, Eads(NO)=-0.14 eV]. In contrast, for the e-FCN model [Eads(O2)=−0.08 eV, Eads(NO)=−0.21 eV], the adsorption energy of NO is significantly more negative than that of O2 (Fig. 6B, C). These theoretical results align with the experimental NO/O2-TPD data, confirming that e-FCN exhibits a strong preference for NO adsorption over O2, whereas e-CN and e-OCN show greater affinity for O2.

The above results have revealed that e-FCN preferentially adsorbs and activates NO, even in the presence of O2. However, the mechanism behind the inertness of e-FCN toward O2 adsorption, especially when compared with e-CN and e-OCN, requires further investigation. Previous studies have shown that the accumulation of the C=O bond, resulting from the oxidation of surface C atoms in CN catalysts, can be suppressed by surface fluorination that introduced the C–F bond, thereby mitigating C=O bond accumulation33. The formation of these C=O bonds on exfoliated CN-based catalysts was monitored using in situ Fourier-transform infrared (in situ FTIR) spectroscopy. Figure S24A–C showed that a newly generated band at 1725 cm−1 appeared upon exposure of e-CN, e-OCN, and e-FCN to 425 nm light irradiation, this band is consistent with the formation of a surface C=O bond. To verify this assignment, the in situ FTIR characterization was also carried out using 18O2 instead of O2. As shown in Fig. 7A–C, an analogous redshift to 1684 cm−1 occurred. The ratio of the wavenumbers, ν(C=18O)/ν(C=O), is close to the theoretical value of [μ(C=O)/μ(C=18O)]1/2=0.98, where μ(C=O)=6.86 and μ(C=18O)=7.20 represent the reduced masses of the respective C=O bonds67. Meanwhile, the FTIR bands associated with N-heterocyclic frameworks in the exfoliated catalysts and the C–F bond in e-FCN remained unchanged. These isotopic in situ FTIR results confirm the assignment of the 1725 cm−1 band to the C=O bond.

The exposure of both e-CN and e-OCN to O2/18O2 under 425 nm irradiation led to an increase in the band intensity corresponding to the C=18O/C=O bond (see Figs. 7A, B and S24A, B). On the other hand, the formation of the C=18O/C=O bond on e-FCN was significantly inhibited compared with that on e-CN and e-OCN. The spectral features of e-FCN remained consistent with those of N-heterocycles and the C–F bond (Figs. 7C and S24C). The time profiles of C=18O/C=O bond formation on e-CN, e-OCN, and e-FCN under irradiation in comparison with their dark control counterpart clearly show such spectral behavior (Figs. 7D and S24D). Collectively, these findings indicate that e-CN and e-OCN adsorb and activate O2 via light-driven C=O bond formation. The presence of C–F bonds in e-FCN inhibit this process. Consequently, e-FCN exhibits inert activity toward O2 activation but selectively adsorbs and activates NO for reduction under light irradiation, even in the presence of excess O2.

In situ monitoring of NO photoreaction over modified CN-based catalysts
The NO/O2 adsorption results demonstrate that the presence of the C–F bond in CN suppresses the formation of the C=O bond, rendering e-FCN inert towards O2. To investigate the NO photoconversion process in the presence of O2, in situ FTIR spectroscopy was employed to monitor the chemical transformation process. The FTIR spectra of initial NO adsorption on e-CN, e-OCN, and e-FCN reveal distinct bands (Fig. S25): one at 1690–1694 cm−1, corresponding to adsorbed (NO)2 species on the surface68, and others at 1273 and 1411–1413 cm−1, indicative of bridged (NO)2 species17. These bands (1273, 1411-1413, and 1694 cm−1) increase gradually over time (Fig. S25A, C, E), and e-FCN consistently exhibits higher band intensities compared with e-CN and e-OCN, thereby confirming stronger NO binding on e-FCN (Fig. S25B, D, F). The signals at 1273 and 1411–1413 cm−1 are typically attributed to the symmetric vibration of bridging bidentate NO3−69. The conventional assignment of the 1273 and 1411–1413 cm−1 bands to symmetric vibrations of bridging bidentate NO3− is inconsistent with the absence of asymmetric vibrational modes in the 1500–1700 cm−1 region. An alternative interpretation, which aligns with the observed spectral features, is that these bands originate from a bridged (NO)2 dimer adsorbed on the catalyst.

The assignment of the observed feature to an (NO)2 dimer was confirmed unequivocally using 15NO isotope-substituted in situ FTIR spectroscopy. As shown in Fig. S26, the spectrum exhibited a corresponding redshift to lower wavenumber. The experimental ν(15N)/ν(N) ratios showed close agreement with theoretical values predicted for an (NO)2 dimer (Table S11), thereby confirming this assignment70. While complementary DFT calculations were employed to investigate NO adsorption behavior and interactions between co-adsorbed NO molecules on the e-FCN surface, it is important to note the inherent difficulty in modeling the precise catalyst structure under realistic catalytic conditions. One or two NO molecules were introduced onto the e-FCN surface slab (Fig. 6C–F). Computed adsorption energies revealed a more stable configuration for the (NONO) dimer, with a total Eads(NONO) of -0.31 eV for the dimer system, compared with Eads(NO)=-0.21 eV for a single NO molecule (Fig. 6C, D). This enhanced stability arises from both π back-donation to the NO ligands and direct attractive interactions between the co-adsorbed NO molecules. Charge density analysis indicated significant electron accumulation at the adsorption sites, increasing from 0.36 |e| for a single NO molecule to 0.69 |e| for the dimer system (Fig. 6E, F), consistent with enhanced electron transfer from the e-FCN substrate. The total charge associated with the dimer was less than the sum of the two isolated NO adsorbates. This charge redistribution, combined with the adjacent geometry of the two N atoms, suggests a mechanism that mitigates inter-adsorbate repulsion and facilitates N–N coupling. The assignment of vibrational bands to an (NO)2 dimer, confirmed through in situ isotopic FTIR and theoretical calculations, indicates that N–N bond formation occurs via a partial overlap of electron density between adjacent N atoms25.

Upon irradiating NO adsorbed on the catalysts, a new band appears at 2314 cm−1, indicating the presence of adsorbed N2O on the catalyst surface, as shown in Fig. S27A, C, E71,72. Initially, this band increases with irradiation time for e-CN, e-OCN, and e-FCN, suggesting the formation of N2O as an intermediate in the decomposition of NO. However, after 18 min, the band begins to decrease on e-FCN (Fig. S27F), while it continues to rise on e-CN and e-OCN (Fig. S27B, D). This contrasting behavior between the catalysts suggests that the N2O intermediates undergo further transformation on e-FCN but accumulate on e-CN and e-OCN. This observation is consistent with the finding that N2O production during the photocatalytic NO conversion was minimal on e-FCN but significant on e-CN and e-OCN (see Fig. 2A). These results highlight the critical role of e-FCN in promoting the photocatalytic reduction of N2O intermediates to N2. Notably, a band at 1555 cm−1 was observed for all catalysts under light irradiation (Fig. S27A, C, E), indicating the accumulation of bidentate NO3− on the catalyst surface73. Among the catalysts, e-OCN exhibits the highest NO3− accumulation. This finding is consistent with the observation that NO3− selectivity for e-OCN is higher than that for e-CN and e-FCN (see above Fig. 2A). Definitive confirmation of the N2O and NO3− feature assignments was achieved using 15NO isotope-substituted in situ FTIR spectroscopy. The spectrum showed a corresponding redshift (Fig. 8), and the experimental ν(15N)/ν(N) ratios closely matched theoretical values for an N2O and NO3− (Table S11), validating the assignment70. The band area changes of 15N2O and 15NO3− were also consistent with those of N2O and NO3−. Consequently, it is revealed that NO photoconversion in O2 proceeds predominantly via an oxidative pathway on e-CN/e-OCN but shifts to a reductive pathway on e-FCN.

The role of direct electron transfer in photocatalytic NO decomposition can be evaluated by introducing potassium dichromate (K2Cr2O7) as an electron scavenger. Figure 9A shows that the addition of K2Cr2O7 significantly decreases NO removal and N2 selectivity for e-FCN under 425 nm light irradiation in 20% O2. This finding underscores the critical importance of electron transfer in NO conversion. Moreover, the production of NO2 during photocatalytic NO decomposition over e-FCN increases markedly in the presence of K2Cr2O7, suggesting that photoinduced holes, enhanced by electron scavenging, primarily drive the oxidation of NO to NO2 in the e-FCN/Cr system74. Conversely, the use of EDTA as a hole scavenger only moderately reduces NO removal and slightly decreases N2 selectivity under the identical reaction conditions (Fig. 9B). Similar trends are observed for e-OCN, as shown in Fig. S28, where the effects of K2Cr2O7 and EDTA are consistent with those observed for e-FCN. These results confirm that electron transfer, rather than hole transfer, plays the dominant role in the photocatalytic NO decomposition on both e-FCN and e-OCN.

The effect of hole scavenging on the reaction pathway was also investigated by carrying out the isotopic labeling experiments for the e-FCN/5EDTA catalyst. First, an experiment was performed with a 4% 15NO gas mixture in 20% O2/Ar. Pre-reaction GC-MS analysis (Fig. S29A) confirmed the absence of N2 contamination, with only a characteristic O2 peak detected. Following a 2-h photocatalytic reaction, GC-MS analysis revealed that the evolved N2 originated exclusively from the decomposition of 15NO, as indicated by the sole presence of 15N2 (Fig. S29B). This result demonstrates that scavenging photoinduced holes had no appreciable effect on N2 formation. Subsequently, an experiment was conducted with a 4% NO gas mixture in 20% 18O2/Ar. Pre-reaction analysis (Fig. S29C) confirmed the exclusive presence of 18O2, ruling out significant atmospheric O2 contamination. The post-reaction mass spectrum of O2 (Fig. S29D) displayed signals only for 18O2, indicating that no new O2 was generated through the hole-induced decomposition of EDTA. A parallel experiment conducted with EDTA in Ar corroborated these findings, showing a moderate reduction in NO removal and a slight decrease in N2 selectivity (Fig. S29E, F). Furthermore, hole scavenging inhibited O2 formation (Fig. 9C). Collectively, these results indicate that in the presence of EDTA, NO is converted to N2 through a photoreduction process without concomitant O2 generation.

To elucidate the pathway for the photocatalytic conversion of NO to O2, EPR spectra of e-FCN and e-FCN/5EDTA were compared before and after the reaction. As shown in Fig. S30A, the EPR signal intensity for the used e-FCN/5EDTA catalyst, measured at 273 K, was significantly higher than that of its pristine state, whereas the signal for e-FCN remained largely unchanged. For enhanced spectral resolution, EPR measurements were conducted at 77 K. At this temperature, the EPR spectrum of the used e-FCN/5EDTA exhibited an additional hyperfine structure with g-values of 2.002 and 1.995, along with an intensified signal at g=2.007 (Fig. 9D). These emergent signals are assigned to O3− species75. This observation effectively precludes the involvement of •O2−. This species is notoriously difficult to detect by direct EPR due to its rapid relaxation time and broad spectral features76, which contrast sharply with the well-defined hyperfine structure observed here. It is proposed that O3− forms via the reaction of O− with molecular O2 at 77 K, facilitated by weak covalent bonding between the π-electrons of O2 and the free electron on O−77. This adduct is unstable at elevated temperatures; heating above 150 K causes the dissociation of the weak bond, leading to the disappearance of O3− and the regeneration of O−78. This mechanism accounts for the intensified EPR signal at g=2.005 for the used e-FCN/5EDTA measured at 273 K: The signal for O− typically occurs at g=2.00579, which coincides with the intrinsic signal from unpaired electrons in CN-based catalysts. The accumulation of O− species thus enhances the overall signal intensity at this position. Conversely, the EPR spectrum of e-FCN at 77 K showed minimal change after the reaction (Fig. 9D). The EDTA scavenging tests and EPR results collectively reveal the NO conversion mechanism: NO is reduced by electrons to N2 and O−, which is then oxidized by holes to form O2. Scavenging holes with EDTA inhibit the final oxidation step, causing O− accumulation on e-FCN/5EDTA and preventing O2 formation, which was not observed on e-FCN.

The results of K2Cr2O7 scavenger tests indicate that the photoinduced holes in e-FCN and e-OCN oxidize NO to NO2 only in the presence of O2 (Figs. 9A and S28A, B). Notably, Fig. 2A demonstrates the formation of NO3− during the photocatalytic conversion of NO over e-FCN and e-OCN, suggesting that specific reactive oxygen species, such as •OH, 1O2, and •O2−, facilitate the oxidation of NO to NO3−22. Comparative analyses employing various scavengers, including tert-butanol (TBA) for •OH (Fig. S31), L-histidine (His) for 1O2 (Fig. S32), and L-ascorbic acid (AA) for •O2− (Fig. S33), were conducted. Figures S31 and 32 indicate that the introduction of TBA and His does not significantly affect NO removal efficiency or N2 selectivity, implying minimal contributions from •OH and 1O2 in the photocatalytic NO decomposition over e-FCN and e-OCN. In contrast, the use of AA as a •O2− scavenger moderately impairs NO removal, promotes NO2 formation, and suppresses NO3− production in the case of e-OCN (Fig. S33A, B). A similar trend is observed for e-FCN, where AA moderately reduces NO removal, increases N2 selectivity, and decreases NO3− formation (Fig. S33C, D). These findings indicate that •O2− selectively reacts with NO to produce NO3−14. Moreover, EPR spin-trapping experiments using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) to capture •O2− reveal that the amount of •O2− generated follows the order: e-OCN > e-CN > e-FCN (Fig. S30B). This trend aligns with catalytic performance (Fig. 2A) and in situ FTIR results (Figs. 8 and S27), which show a similar decreasing order for NO3− formation. Together, these results confirm that •O2− is the key species responsible for NO3− production during the photocatalytic conversion of NO over these catalysts.

The scavenger tests and EPR analysis results provide valuable insights into the mechanisms underlying photocatalytic NO decomposition. Initially, electron transfer plays a pivotal role in converting NO primarily into N2. Additionally, the following hole transfer facilitates the oxidation of O− species to O2, further contributing to NO decomposition. Moreover, O2 undergoes activation to form •O2−, which subsequently oxidizes NO to NO3−. Among the tested catalysts, e-FCN is almost inert towards O2, resulting in the lowest activity for O2 activation to •O2−. Consequently, e-FCN achieves the highest N2 selectivity and the lowest NO3− selectivity for NO decomposition.

Based on the above results, the photocatalytic reaction mechanism of NO conversion on e-FCN is proposed and illustrated in Fig. 10A. The ISI-XPS (Figs. 5C–F and S20) results identify C-NHx, C-OH (in e-OCN), and C–F (in e-FCN) as direct participants in the photocatalytic reaction. The NO/O2-TPD (Fig. 6A), theoretical calculations (Fig. 6B, C), and in situ FTIR for O2 photoactivation (Figs. 7 and S24) results indicate that e-FCN preferentially adsorbs NO in the presence of O2. The strongly electron-withdrawing F atom is bonded to the CN framework via C–F bonds, polarizing the adjacent π-ring and thereby enhancing its interaction with the NO molecule through the N atom12,80. The in situ FTIR (Figs. S25 and 26) and theoretical calculation (Fig. 6D–F) results from NO dark adsorption on e-FCN reveal a band corresponding to (NO)2, indicating that e-FCN forms (NO)2 on its surface after adsorbing two NO molecules, thereby promoting N-N coupling. The in situ FTIR results from NO photoreaction (Figs. 8 and S27) on e-FCN reveal that the (NO)2 band area is significantly reduced from that of NO dark adsorption, accompanied by the appearance of N2O intermediate. This suggests that (NO)2 adsorbed on the surface of e-FCN undergoes N–N coupling, forming N2O as an intermediate. Furthermore, these in situ FTIR results show that the N2O band area initially increases and then decreases over time, indicating that N2O intermediates on the e-FCN surface are further reduced to N2 by photo-induced electrons.

As shown in Figs. 9B, C and S29D, scavenging holes in e-FCN leads to a slight decrease in the NO removal rate and N2 selectivity, but completely inhibits O2 production. This confirms that the evolution of O2 during the photocatalytic decomposition of NO is driven by hole oxidation. Under irradiation, e-FCN should generate both electrons and holes, and the following interface charge transfer occurs simultaneously to maintain overall charge balance in the photocatalyst. However, the transfer of photoinduced electrons and holes occurs at different timescales due to differences in charge transfer kinetics25. After photoinduced electrons reduce (NO)2 to N2, N2 easily desorbs from the surface of e-FCN, leaving behind two O− species. These species are then oxidized by the photoinduced holes to produce O2, regenerating catalytic sites.

The photocatalytic results in Fig. 2A demonstrate that NO is oxidized to NO3− over e-OCN/e-CN, confirming that NO oxidation is the dominant reaction on the e-OCN/e-CN surface. EPR analysis of •O2− (Fig. S30B), along with •O2− scavenger experiments on e-OCN (Fig. S33A, B), revealed that •O2− is the primary active species responsible for oxidizing NO to NO3−. Notably, photoinduced electron scavenger experiments on e-OCN (Fig. S28A, B) led to a significant reduction in NO3− formation, accompanied by the generation of NO2 and an increase in its proportion. This suggests that photoinduced holes in e-OCN oxidize NO to NO2. The •O2− scavenger experiments yielded similar results, further supporting the role of photoinduced holes in oxidizing NO to NO2. Based on these findings, we propose a reaction mechanism for NO conversion on e-OCN and e-CN (Fig. 10B). NO primarily undergoes oxidation, where photoinduced holes facilitate its conversion to NO2, while O2 is reduced to •O2− by photoinduced electrons. Finally, •O2− oxidizes NO/NO2 to NO3−.

Methods
Chemicals
The chemicals used in this work include the following. Melamine, glycolic acid, ammonium bromide (NH4Br), ammonium fluoride (NH4F), titanium(III) chloride solution (10-15 wt% TiCl3), Pluronic F127 (C3H6O·C2H4O)x, bismuth(III) nitrate pentahydrate [Bi(NO3)3·5H2O], EDTA salt dihydrate, potassium dichromate (K2Cr2O7), L-ascorbic acid (AA), tert-butanol (TBA), L-histidine (His), DMPO, and ethanol were obtained from Merck Corporation. NO standard gas (500 ppmv NO, Ar balance), ultra-high purity Ar (99.999%), O2 standard gas (20 vol% O2, Ar balance), CO2 standard gas (5 vol% CO2, Ar balance), NO2 standard gas (500 ppmv NO2, Ar balance), and SO2 standard gas (500 ppmv SO2, Ar balance) were purchased from Deokyang Company. For isotopic labeling experiments, 15N-labeled sodium nitrite (Na15NO2) and 18O-labeled molecular oxygen (18O2) were obtained from Merck Corporation. 15NO was synthesized according to the reaction shown in Eq. 1, following a reported procedure81. The resulting 15NO gas was collected in a gas sample bag and subsequently introduced into the reactor using a mini gas pump. All chemicals used were of analytical grade and were used as received without further purification. Ultrapure deionized water (18 MΩ·cm) was produced using a Millipore system.

3Na15NO2 + H2SO4 → 215NO + Na15NO3 + Na2SO4 + H2O (1)

Catalyst preparation
Preparation of bulk-CN and thermally exfoliated CN (e-CN). The bulk-CN was prepared through the thermal decomposition of melamine (8.00 g, 63.43 mmol)37. Melamine was heated in a lidded crucible at 550°C for 3 h under static air, with a controlled heating rate of 5°C·min−1. The resulting product was collected and ground into a fine powder for subsequent applications.

The e-CN was prepared via a modified thermal exfoliation method36. In this process, 1.0 g of the prepared bulk-CN was heated in a lidded crucible at 500°C for 2 h under static air, with a heating rate maintained at 5°C·min−1. The final product was collected and ground into a fine powder for further use.

Preparation of bulk oxygen-modified CN (bulk-OCN) and thermally exfoliated oxygen-modified CN (e-OCN). The preparation of bulk-OCN was achieved by directly incorporating glycolic acid into the conventional condensation process35. Initially, melamine (8.00 g, 63.43 mmol) and glycolic acid (0.72 g, 9.52 mmol) were combined and homogenized through grinding. The mixture was then placed in a covered crucible and subjected to heating at 550°C for 3 h under static air conditions, with a controlled heating rate of 5°C·min−1. After the heating process, the crucible was allowed to cool naturally to room temperature. The resulting solid was then dispersed in water to remove any unreacted substances, stirred overnight, centrifuged, thoroughly washed with deionized water, and dried for further applications.

The e-OCN was prepared using the modified thermal exfoliation method based on the aforementioned procedure36. Specifically, 1.0 g of the prepared bulk-OCN was heated in a lidded crucible at 500°C for 2 h under static air, maintaining a heating rate of 5°C·min−1. The product obtained from this process was finely ground into powder for further use.

Preparation of bulk fluoride-modified CN (bulk-FCN) and thermally exfoliated fluoride-modified CN (e-FCN). The bulk-FCN was prepared through the direct addition of NH4F into the conventional condensation process34. Specifically, melamine (8.00 g, 63.43 mmol) and NH4F (1.06 g, 28.54 mmol) were combined and homogenized via grinding. The resulting mixture was transferred to a covered crucible and heated at 550°C for 3 h under static air conditions, with a controlled heating rate of 5°C·min−1. After cooling naturally to room temperature, the solid was dispersed in water to remove unreacted substances, stirred overnight, centrifuged, washed with deionized water, and then dried for further applications.

The e-FCN was prepared using a modified thermal exfoliation method based on the above procedure36. In this process, 1.0 g of the prepared bulk-FCN was heated in a covered crucible at 500°C for 2 h under static air conditions, maintaining a heating rate of 5°C·min−1. The resulting material was collected, finely powdered, and stored for further use.

The C and N content was determined using elemental analysis. A measured quantity of the catalyst was digested with concentrated HNO3 as an oxidant, and the F content in the resulting solution was quantified via ion chromatography. This measurement was then used to calculate the F content in the original catalyst. Based on this method, the atomic ratio of F/C was determined. Accordingly, the catalyst is denoted as e-F(X)CN, where X represents the atomic percentage (at%) of F relative to C.

For comparative analysis, four additional e-FCN catalysts were synthesized by varying the initial NH4F amount. These were labeled as e-F(0.05)CN, e-F(0.15)CN, e-F(0.20)CN, and e-F(0.25)CN, corresponding to initial NH4F amounts of 0.53 g (14.27 mmol), 1.59 g (42.81 mmol), 2.12 g (57.08 mmol), and 2.65 g (71.35 mmol), respectively. Unless otherwise stated, e-FCN in this study refers to e-F(0.10)CN.

Preparation of mesoporous TiO2-x (mp-TiO2-x). The mp-TiO2-x was prepared using a modified Ti3+ self-doping method25. A light-yellow, mesoporous TiO2-x powder was synthesized via a modified Ti3+ self-doping methodology. The process began with the dissolution of Pluronic F127 (0.25 g, 0.02 mmol) in ethanol (1.00 mL). An aqueous TiCl3 solution (2.60 mL, 4.00 mmol) was then combined with the ethanolic surfactant solution. Following solvent removal through heated stirring, the resultant gel was transferred to a crucible and subjected to thermal treatment. This calcination step involved a temperature ramp of 5°C·min−1 to a final temperature of 600°C, which was held for 3 h. The target mp-TiO2-x compound was obtained as a light-yellow solid after cooling to room temperature.

Preparation of F-doped mp-TiO2-x (F-mp-TiO2-x). The F-mp-TiO2-x was synthesized via an impregnation method48. First, a 0.1 M NH4F solution was prepared by dissolving 0.37 g (0.01 mol) of NH4F in 100 mL of deionized water. Next, 0.3 mL of this solution was diluted with 29.7 mL of deionized water to obtain 30 mL of a 1 mM NH4F solution. Subsequently, 1 g of mp-TiO2-x was dispersed in the diluted NH4F solution under continuous stirring. The mixture was then heated until complete solvent evaporation, yielding the final F-doped product, F-mp-TiO2-x. The F doping concentration in F-mp-TiO2-x matched that of the e-F(0.10)CN.

Preparation of BiOBr and BiOF. The BiOBr and BiOF were synthesized using a modified method based on previously reported literature49. For the preparation of BiOBr, NH4Br (0.98 g, 0.01 mol) was dissolved in 25 mL of deionized water to form an NH4Br solution. Simultaneously, Bi(NO3)3·5H2O (4.85 g, 0.01 mol) was dissolved in 25 mL of deionized water to prepare a Bi(NO3)3 solution. The NH4Br solution was then added dropwise into the Bi(NO3)3 solution. The resulting suspension was stirred for 18 h, followed by centrifugation. The precipitate was washed repeatedly with deionized water, dried, and collected as BiOBr. The same procedure was followed for the synthesis of BiOF, with NH4F (0.37 g, 0.01 mol) replacing NH4Br as the precursor.

Photocatalytic NO decomposition evaluation. As shown in Fig. S36, the NO decomposition photoreaction was performed using a 300 mL glass reactor with a closed circulation system25. For testing, a quartz plate containing a 2 cm × 2 cm groove was used to support a uniform dispersion of 0.15 g of catalyst. This plate was then placed inside the reactor, which allowed for light irradiation via a 3 cm-diameter quartz window. Various LED light sources (365, 385, 405, 425, 445, and 465 nm) were employed, and the light intensities reaching the catalyst were measured at 18, 23, 32, 50, 50, and 50 mW·cm−2, respectively, using a Newport 1918-R power meter. The catalyst surface temperature was monitored using a thermocouple. To examine the influence of temperature on catalytic performance, the reactor was externally heated using heating tape, enabling precise temperature control. A continuous gas flow containing 2.0 ppmv NO and varying concentrations (0, 5, 10, 15, and 20 vol%) of O2 was introduced into the glass reactor at different flow rates (10, 20, 30, and 40 mL·min−1). Ar was used as the balance gas in the reaction mixture. The RH of the gas stream was controlled by passing Ar gas through a stainless-steel water bottle. To evaluate the impact of coexisting gases (CO2, NO2, and SO2) on catalytic performance, varying concentrations of each gas were individually introduced into the reactor. The photocatalytic NO decomposition process was initiated by LED light irradiation after achieving the NO adsorption equilibrium.

The NOx levels were determined using a NOx analyzer (Thermo Scientific 17i), while gas chromatography-mass spectrometry (GC-MS, Agilent 7890A-5975B) was employed for the analysis of N2O, N2, and O2. After the reaction, the catalyst was suspended in water and agitated for 1 h to dissolve any absorbed nitrogen-containing compounds. The resulting mixture was filtered, and the filtrate was injected into an ion chromatograph (Thermo Scientific Dionex ICS-900) to quantify the NO3− concentration in the solution. To assess the accumulation of NO3− on the catalyst surface over 12 consecutive cycles, 12 parallel experiments were conducted, each subjected to 1-12 reaction cycles. Upon completion, the NO3− content adsorbed on the catalyst surface was quantified. NO removal efficiency and N2 selectivity were calculated based on inlet and outlet concentrations of reactants and products using Eqs. 2 and 3, respectively.

NO removal = ([NO]in - [NO]out) / [NO]in × 100% (2)

N2 selectivity = 2[N2]out / (2[N2]out + 2[N2O]out + [NO2]out) × 100% (3)

Isotopic labeling experiments. Isotopic labeling experiments were conducted in a sealed reactor. A 0.15 g catalyst sample was evenly dispersed into a 2 cm × 2 cm groove on a quartz glass plate. Prior to the photoreaction, the reactor was subjected to repeated evacuation and purging cycles. To identify the nitrogen source in N2 formation, the reactor was charged with a gas mixture of 4 vol% 15NO, 20% O2, and Ar (balance) at ambient pressure. Photocatalytic NO decomposition was then initiated under irradiation from a 425 nm LED light source. After a 2-h reaction, the gaseous products were analyzed by GC-MS using an HP-PLOT Molesieve 5A column (30 m × 0.32 mm × 15 μm) for the detection of 15N2 and O2. To determine the origin of the newly generated O2, a separate experiment was performed using a gas mixture of 4 vol% NO, 20% 18O2, and Ar (balance). Following an identical 2-h photoreaction, the product gases were analyzed by GC-MS to quantify N2, O2, and 18O2.

Characterizations. The study involves a comprehensive characterization of catalysts employing a variety of analytical techniques. XRD patterns were obtained using an XPERT-PRO X-ray diffractometer equipped with CuKα radiation. AFM imaging was performed using an NT-MDT Solver Next atomic force microscope. Particle size distribution was determined by DLS using an Omni zeta potential analyzer (Brookhaven Instruments Corporation, USA). Elemental analysis of C and N was conducted using an EA ICE-440 elemental analyzer.

XPS measurements were performed with a Thermo Scientific XPS K-alpha instrument, utilizing non-monochromatic Al-Kα radiation (with an electron work function Φ=3.90 eV). The VB edge positions were estimated from the XPS VB spectra using Eq. 4. ISI-XPS was performed using the ESCALAB 250Xi XPS system. During the characterization, a 425 nm LED light source was positioned approximately 20 cm from the prepared samples to examine light-induced changes in electron density on the catalyst surfaces. Deconvolution and peak fitting for C 1s, N 1s, O 1s, and F 1s were carried out using XPSPEAK software39.

E(RHE) = Φ + E(eV) - 4.44 (4)

[E(RHE) is the potential vs RHE at pH 7; E(eV) is the VB level from the XPS result; Φ=3.90 (the work function of the XPS equipment)].

EPR spectra were acquired using a JEOL JES-FA200 spectrometer under specific conditions: microwave power at 0.93 mW, frequency of 9.65 GHz, modulation amplitude of 10 G, and a time constant of 20.48 ms. The superoxide radical (•O2−) was detected using the radical spin-trapping reagent DMPO. The typical testing temperature for catalyst and •O2− analysis was 298 K; however, detecting the O3− signal required measurements at 77 K75,77,78. Solid-state nuclear magnetic resonance spectroscopy of 13C, 1H, and 19F (solid-state 13C, 1H, and 19F NMR) was performed on a Bruker Avance III 500WB spectrometer. Diffuse reflectance UV-visible absorption spectroscopy (UV-DRS) was conducted using a Shimadzu UV-2401PC spectrophotometer. The bandgap was determined from the Tauc plot using Eq. 5.

[F(R) × hν]2 = A × (hν - Eg) (5)

(h is Planck’s constant; ν is photon frequency; A is a constant; Eg is the bandgap).

Steady-state and time-resolved PL spectra were obtained using a Horiba FluoroMax-4 spectrophotometer with an excitation wavelength of 425 nm. Microsecond transient absorption spectroscopy (μs-TAS) was conducted in transmission mode using a nanosecond Nd:YAG laser (Spectra-Physics, LAB-130-10) as the pump source. The 355 nm laser output was focused onto the sample films with an excitation density of 0.5 mJ·cm−2 and a repetition rate of 10 Hz. A 100 W tungsten-halogen lamp (ASBN-W) generated the probe beam at 900 nm, with long-pass and short-pass filters placed between the lamp and the sample to minimize short-wavelength radiation and sample heating. The resulting absorption changes were directed into a monochromator (Spectral Products, CM110) for wavelength selection. A silicon photodiode detected the time-resolved signal, which was amplified by a broadband preamplifier and recorded using an oscilloscope (Tektronix DPO4012). Transient decay profiles averaged over 256 laser pulses to improve the signal-to-noise ratio. For in situ μs-TAS experiments, measurements were performed under simulated reaction conditions, with the sample films exposed to a controlled gas environment. Prior to testing, the cell was purged for 30 min with either Ar, O2, or a 500 ppmv NO/Ar mixture to ensure equilibrium adsorption of reactant molecules on the film surface. This approach enabled in situ μs-TAS studies under varying atmospheric conditions.

The N2 adsorption-desorption isotherms were obtained at 77 K, employing a Quantachrome NOVA 2200e instrument. Temperature-programmed desorption of NO or O2 (NO/O2-TPD) was performed using an AutoChem 2920 Chemisorption analyzer. The quantitative relationship between the TPD signal peak area and the adsorbed NO/O2 concentration was calibrated using an NO/O2 standard gas mixture, establishing a standard curve. This calibration was then applied to determine the NO/O2 adsorption capacity on the catalyst surface.

In situ Fourier-transform infrared spectroscopy (in situ FTIR) was employed to track the formation of reaction intermediates during the photocatalytic decomposition of NO. The measurements were conducted using a PerkinElmer IR spectrometer equipped with a diffuse-reflectance cell featuring a ZnSe window. O2 was adsorbed on the catalyst in the cell that was pretreated at 150°C under an Ar gas flow for 30 min. After cooling, a continuous flow of 500 ppmv NO and 20 vol% O2/Ar was introduced and maintained constant throughout the dark adsorption process and the following photoreaction under 425 nm light irradiation. According to the reported literature33, in situ FTIR spectroscopy was employed to monitor the formation of C=O bonds on CN-based catalysts. This was achieved by exposing the catalysts to O2 both in the dark and under 425 nm light irradiation.

To verify the FTIR peak assignments, isotopic labeling was performed using 15NO as the reaction gas. Upon substitution of N with 15N, all observed peaks shifted to lower wavenumbers, consistent with the expected isotopic effect67,70. Similarly, when monitoring C=O bond formation using 18O2 in place of O2, an analogous shift to lower wavenumbers was observed. The relationship between the vibrational frequency (ν), the reduced mass (μ), and the bond force constant (k) is given by Eq. 6. In most cases (e.g., C=O, NO dimer, and NO3−), the reduced mass (μ) can be calculated using Eq. 7, which is based on a diatomic vibrational model. However, for N2O, the calculation must follow Eq. 8 due to its triatomic vibrational model.

ν = 1 / (2π) * sqrt(k / μ) (6)

μ = (m1 × m2) / (m1 + m2) (7)

μ(N2O) = mN × (mN + 2mO) / (2 * (mN + mO)) (8)

Photoelectrochemical (PEC) measurements. PEC experiments were conducted using a GAMRY Interface 1010 electrochemical workstation with a three-electrode setup. The reference electrode was Ag/AgCl, and the counter electrode was a platinum sheet (1 cm × 1 cm). The working electrode was prepared by immobilizing the catalyst onto a plate of indium-doped tin oxide (ITO) conductive glass (1 cm × 1 cm), using a casting technique. To prepare the working electrode, 6 mg of catalyst was dispersed in a solution of 540 μL ethanol and 60 μL of a 5.0 wt% Nafion ethanolic solution. After sonication for 30 min, 100 μL of this suspension was applied to the pre-sonicated ITO electrode, ensuring a catalyst loading of 1 mg·cm−2. Once the solvent evaporated, the working electrode was ready for PEC testing.

The characterization of the photocurrent was conducted in a 0.1 M Na2SO4 solution at pH 7. Data were acquired at a bias of −0.5 V vs the reversible hydrogen electrode (VRHE) under chopped light irradiation at 425 nm, with alternating 30-s periods of light-on and light-off. EIS was performed by applying an AC voltage with a 5 mV amplitude across a frequency range from 105 Hz to 0.01 Hz, under dark open-circuit potential (OCP) conditions, in the same 0.1 M Na2SO4 solution at pH 7. The Mott-Schottky plots were measured in 0.1 M Na2SO4 solution at pH 7, using a fixed frequency of 1000 Hz in the dark. All recorded potentials were corrected for iR compensation and referenced to the RHE at pH 7, as specified by Eq. 9. It is important to note that a consistent set of photoelectrodes was used throughout all PEC measurements.

E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.197 (9)

AQYs for N2 formation. The quantum yields (AQYs) for N2 formation were determined at various wavelengths (365, 385, 405, 425, 445, and 465 nm) using Eq. 10.

AQY(%) = (niN2 × NA) / (EAλt / hc) (10)

In this equation, niN2 is the number of generated N2 molecules, i the number of electrons required for the reduction of NO to N2 (2NO+4e−→N2+2O2−, i=4), NA the Avogadro constant (6.02 × 1023), E the intensity of light (W·cm−2), A the area of light irradiation (A=4 cm2), λ the wavelength of the LED lamp, t the duration of illumination (t=1800 s), h the Planck constant (6.62 × 10−34 J·s), and c the speed of light (3.0 × 108 m·s−1).

Scavenging test for charge carriers. The scavenging agents employed for mechanistic investigation were EDTA, K2Cr2O7, L-ascorbic acid (AA), tert-butanol (TBA), and L-histidine (His), for scavenging hole (h+), electron (e−), •O2−, hydroxyl radical (•OH), and singlet oxygen (1O2), respectively82,83. To evaluate the hole scavenging effect, 0.15 g of e-FCN was dispersed in 10 mL aqueous solution of EDTA, followed by agitation and heating for water evaporation. Catalysts were denoted as e-FCN/1EDTA, e-FCN/3EDTA, and e-FCN/5EDTA, corresponding to EDTA concentrations of 1 mM, 3 mM, and 5 mM, with adsorbed EDTA acting as a hole scavenger. Similarly, employing analogous methods, catalysts were prepared using solutions of K2Cr2O7, AA, TBA, and His to scavenge e−, •O2−, •OH, and 1O2, resulting in e-FCN/1Cr, e-FCN/3Cr, e-FCN/5Cr (from 1 mM, 3 mM, and 5 mM K2Cr2O7); e-FCN/1AA, e-FCN/3AA, e-FCN/5AA (from 1 mM, 3 mM, and 5 mM AA); e-FCN/1TBA, e-FCN/3TBA, e-FCN/5TBA (from 1 mM, 3 mM, and 5 mM TBA); e-FCN/1His, e-FCN/3His, e-FCN/5His (from 1 mM, 3 mM, and 5 mM His). Subsequently, the prepared e-FCN/EDTA, e-FCN/Cr, e-FCN/AA, e-FCN/TBA, and e-FCN/His catalysts were assessed for photocatalytic NO decomposition. The e-OCN-based catalysts, including e-OCN/EDTA, e-OCN/Cr, e-OCN/AA, e-OCN/TBA, and e-OCN/His, were prepared using a similar procedure for testing photocatalytic NO decomposition.

Theoretical calculation methods. In this study, all calculations were conducted using a periodic slab model implemented in the Vienna Ab Initio Simulation Package (VASP)84–87 with the projected augmented wave (PAW) method88,89. Exchange-correlation effects were described within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional, and a plane-wave kinetic energy cutoff of 400 eV was applied. To minimize interactions between periodic images, a vacuum layer of 10 Å was introduced along the z-axis. Dispersion corrections were included via the DFT-D3 scheme90,91. The (F/O)CN catalyst models were constructed based on previously reported literature and subsequently optimized to eliminate any structurally unreasonable coordination geometries32,33,35,41. The atomic coordinates for all computational models used in this study are available in Supplementary Data 1. The adsorption energies (Eads) of NO and O2 were calculated as described in Eq. 11:

Eads = E*species - E* - Especies (11)

where E*species, E*, and Especies represent the total energies of the adsorbed system, the (F/O)CN substrate, and the isolated gas molecules, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript presents a study on a fluorine-doped carbon nitride (F-CN) photocatalyst for the selective reduction of NO to N2, demonstrating improved activity and selectivity. While the authors performed various characterizations and simulations to explore the structure–performance relationship, the actual role of the C–F bond in enhancing photocatalytic selectivity and activity remains insufficiently elucidated. Furthermore, the manuscript contains several conceptual inaccuracies and unsupported claims. I recommend a thorough revaluation of the experimental data and computational models to more accurately interpret the catalytic mechanism and correct the errors throughout the manuscript.\n1. The central issue lies in the unclear interpretation of how C–F coordination affects the selectivity for N2. The authors emphasize that the inert nature of O2 contributes to high N2 selectivity. However, experimental data show that the N2 selectivity across all three catalysts remains nearly unchanged regardless of the presence of O2, even though the non-F-doped samples (e-OCN and e-CN) exhibit strong O2 adsorption. This contradicts the argument that weak O2 adsorption due to F doping is responsible for enhanced N2 selectivity. If O2 adsorption truly limited N2 selectivity, one would expect significantly lower selectivity in e-OCN and e-CN under O2-rich conditions. The authors should reconsider the mechanistic interpretation and provide a more convincing explanation.\n2. The molecular models shown in Figure 1a appear to have incorrect bonding geometries. When F or O atoms are axially bonded to carbon, the coordination number of the carbon atom exceeds four, which violates basic valence rules. These structures need to be revised to reflect chemically plausible configurations, especially since they serve as the basis for DFT simulations.\n3. The authors should report the catalyst’s surface temperature under light irradiation to determine whether the reaction is purely photocatalytic or has a significant photothermal component, especially considering the gas–solid phase setup. Additionally, isotopic labelling experiments (e.g., using 15NO) are necessary to confirm that the nitrogen in the evolved N2 originates from NO, not from decomposition of the catalyst itself.\n4. To better understand the carrier dynamics and redox behaviour, the authors should conduct in situ XPS to trace the spatial distribution of photogenerated carriers, especially in e-FCN. Moreover, the band structures of all three catalysts should be experimentally determined (e.g., via UV-vis DRS, VB XPS, Mott–Schottky plots) to assess how F doping influences charge separation and redox potential—critical factors for selectivity.\n5. How to exclude the O disturbance as I can see from XPS results that C-O-C bond exists in these three samples.\n6. The data presented in Figure 5b must be normalized appropriately to enable valid comparisons among the three samples.\n7. The rationale for including relative humidity (RH) in the reaction system is unclear. Does RH significantly impact the photocatalytic performance or selectivity? If so, the mechanism should be discussed. If not, the necessity of its inclusion should be justified.\n8. The photocatalyst appears to exhibit deactivation after approximately three hours. The manuscript attributes this to NO3⁻ accumulation but provides no further evidence or mitigation strategy. The authors should explore the deactivation mechanism more thoroughly and suggest ways to minimize it, such as periodic regeneration.\n9. In the 13C NMR spectrum of the e-CN sample, a noticeable shoulder peak is present. The authors should assign this peak and discuss whether it represents a unique carbon environment or a defect structure.\n10. O2 adsorption capacity is only reported under dark conditions. Since light irradiation could influence surface charge and adsorption behaviour, the authors should also measure O2 adsorption under light to better understand its real-time interaction with the photocatalyst during operation.", "reviewer_2": "The manuscript addresses the significant challenge of photocatalytic selective NO decomposition to N2 under oxygen-rich conditions by introducing C-F bonds into the CN structure, developing an oxygen-tolerant e-FCN catalyst. The work achieves efficient photocatalytic NO-to-N2 conversion under oxygen-rich conditions, with experiments and theoretical calculations demonstrating that C-F bond engineering inhibits C=O bond formation to block O2 adsorption while selectively activating NO. This provides a novel strategy for designing oxygen-tolerant selective NO reduction catalysts. However, several concerns require clarification:\n1.Are the C-F bonds functioning as isolated sites or synergistically with other functional groups (e.g., -NH₂)? Further clarification of the active center is necessary.\n2.While in situ FTIR confirms NO consumption and N2 generation, the specific conversion pathway remains unclear. Time-resolved experiments or isotopic labeling should be incorporated to elucidate the dynamic NO transformation mechanism.\n3. The study focuses exclusively on the carbon nitride system and TiO2-x as a reference but lacks comparisons with other fluorinated materials. This omission weakens the claim of the uniqueness of C-F bond modification.\n4.High-temperature pyrolysis of NH4F may generate toxic byproducts (e.g., HF). A rigorous evaluation of the synthesis route’s environmental feasibility, including byproduct management and safety protocols, is strongly recommended.", "reviewer_3": "This manuscript reports the design and performance of a fluorine-modified polymeric carbon nitride photocatalyst for the selective reduction of NO to N₂ under oxygen-rich conditions. The authors claim a significant advance in achieving high N₂ selectivity and robust performance in the presence of O2. The work also proposes a mechanistic rationale for the observed selectivity. The manuscript is generally well written, the experimental procedures are detailed, and the supporting data are extensive. However, I have several comments regarding the novelty, evidence for the claims, and mechanistic interpretations.\n1. The manuscript should explain more clearly how this method is different from recent work using TiO₂, metal oxides, or doped carbon nitrides. Also, compare both performance and mechanism with these methods.\n2. The main claim is that C-F bonds help NO adsorption over O₂. The evidence comes mostly from TPD and DFT. Have the authors done direct surface tests with both gases together, like DRIFTS or co-adsorption?\n3. Main results are shown for single runs with little error analysis. Did the authors repeat key tests? Can they provide error bars or confidence intervals for main results?\n4. FTIR shows N₂O as a possible intermediate for N₂. Can the authors rule out other paths or surface nitrogen species? Is there isotopic data, like with ¹⁵NO, that tracks nitrogen atoms directly?\n5. The authors claim better charge separation in e-FCN by F doping, using PL, TRPL, and EIS. These changes are moderate. Can they show transient absorption or time-resolved EPR data to link this to NO reduction?\n6. The manuscript sometimes says \"20% O₂ and Ar atmosphere,\" but other conditions are not clear. What is the NO concentration? Are conditions the same for all tests? Please add a summary table of all test conditions.\n7. The method for tuning F content is not clear. How do the authors control F amount during synthesis? How do they confirm F content? Please describe the process and show data on F content and its reproducibility.\n8. The authors say e-FCN is stable for 3 hours of use. What steps kept reaction conditions steady? Were there changes in temperature, humidity, or flow rates? Please explain operational stability and note any changes during the test.\n9. The relation between electron density overlap and N–N bond comes from DFT and FTIR. More direct proof, like charge density plots or Bader analysis, would help. Can the authors explain how this overlap leads to N–N bonds?\n10. EPR changes after EDTA are said to show O⁻ anions. Little change without EDTA means O⁻ reacts to make O₂. Can the authors show O₂ release directly or use isotopic tests? Also, provide more EPR details to separate signals.\nOther minor issues:\nThe work needs a simple figure of electronic changes after F doping, like charge maps.\nTables S2 and S3: Make sure all cited values use the same NO levels and test conditions.\nMove key supporting data (like Table S1, Figures S6-7) into the main text for clarity.\nState if F content stays the same after long testing, or if any F is lost.", "reviewer_4": "The manuscript presents an intriguing strategy for achieving selective photocatalytic reduction of nitric oxide to dinitrogen under oxygen-rich conditions using a fluorine-modified polymeric carbon nitride. The authors claim high performance performance metrics in terms of NO conversion efficiency and N₂ selectivity. The study is well-structured and uses a broad array of experimental and theoretical tools. However, several major concerns need to be addressed to justify the significance and robustness of the claims.\n1. Comparative benchmarking is lacking for such a big claim as ‘first successful…). How does the performance quantitatively compare against other recent reports and what features distinguish the current work compared to literature (AQY, RH conditions, loading etc.)?\n2. The tests are performed at 20% O₂ under dry conditions. However, this does not fully simulate ambient air given that indoor environments possess varying humidity and gases. Authors must add additional data (or at least discuss) photocatalytic performance under variable RH (20–60%) and presence of NO₂, SO₂, or CO₂.\n3. The mass balance assumes equimolar N₂ and O₂ formation without direct quantification of oxygen evolution in all cases. Authors must provide full molar balances for N-containing species. Is there any undetected nitrogen retained on the catalyst surface?\n4. The proposed mechanism is plausible, but the assignment of FTIR peaks to this species is speculative which must be supported with isotope labeling experiments.\n5. The study shows stability over 6 cycles, but NO₃⁻ accumulation must be quantified per cycle and authors should aim for higher stability demonstration beyond 6 cycles (e.g., 10–15) to assess long-term operational efficacy.\n6. In this work, the reactor uses artificial gas mixtures. However, ambient conditions often include fluctuating light, temperature and humidity. Such experiments must be provided to pave the way towards real indoor applications.\n7. Data on exfoliation, particle size and morphology (change) on each modification step must be provided.\n8. Eventhough it is common in photocatalysis field to show results obtained from single experiment, average results with error bars obtained from minimum 3 experiments (especially on identifying selectivity) can improve reliability of data.\n9. Time-resolved PL data were collected under ambient conditions but can charge carrier dynamics be examined under photocatalytic reaction conditions (e.g., in NO/O₂/Ar atmospheres) to better reflect catalytic behavior under given experimental condition.\n10. In some places it is mentioned that increased NO adsorption on e-FCN based on TPD and FTIR results, but is it possible to quantify surface coverage of NO and O₂ (in µmol/m²) to assess catalyst efficiency per site?\n11. The bandgap of e-FCN is given, but band positions relative to NO reduction potential are not clearly shown. Flat band potential and/or valence/conduction band positions must be given and compared to NO/N₂O/N₂ redox potentials?\n12. Could water displace NO from active sites through adsorption?\n13. Exfoliation is said to weaken π–π interactions resulting in improved performance, but is there a method/data to prove the claim?\n14. Time-resolved PL data were collected under ambient conditions but can charge carrier dynamics be examined under photocatalytic reaction conditions (e.g., in NO/O₂/Ar atmospheres) to better reflect catalytic behavior under given experimental condition."}, "author_response": "Response Letter\nReviewer 1's Comments:\nThe manuscript presents a study on a fluorine-doped carbon nitride (F-CN) photocatalyst for the selective reduction of NO to N , demonstrating improved activity and selectivity. While the authors performed various characterizations and simulations to explore the structure-performance relationship, the actual role of the C-F bond in enhancing photocatalytic selectivity and activity remains insufficiently elucidated. Furthermore, the manuscript contains several conceptual inaccuracies and unsupported claims. I recommend a thorough revaluation of the experimental data and computational models to more accurately interpret the catalytic mechanism and correct the errors throughout the manuscript.\nComment 1: The central issue lies in the unclear interpretation of how C-F coordination affects the selectivity for N . The authors emphasize that the inert nature of O contributes to high N selectivity. However, experimental data show that the N selectivity across all three catalysts remains nearly unchanged regardless of the presence of O , even though the non-F-doped samples (e-OCN and e-CN) exhibit strong O adsorption. This contradicts the argument that weak O adsorption due to F doping is responsible for enhanced N selectivity. If O adsorption truly limited N selectivity, one would expect significantly lower selectivity in e-OCN and e-CN under O -rich conditions. The authors should reconsider the mechanistic interpretation and provide a more convincing explanation.\nResponse 1: We have elucidated the similar N selectivity observed for the e-CN, e-OCN, and e-FCN catalysts under both Ar and 20% O /Ar atmospheres.\n(p8-9) “Figures 2A and S6D revealed that the three catalysts exhibited similar N selectivity under both Ar and 20% O /Ar atmospheres, despite following fundamentally different reaction pathways. Under the Ar atmosphere, e-CN, e-OCN, and e-FCN all underwent photocatalytic NO decomposition, producing N O as a byproduct. In contrast, under the 20% O /Ar atmosphere, the catalysts displayed distinct behaviors: e-FCN remained primarily governed by NO decomposition, with minimal NO oxidation, resulting in comparable N selectivity in both conditions. However, as shown in Figure 2A, e-CN and e-OCN generated substantially more NO - than N O, indicating a predominant photocatalytic oxidation pathway (converting NO to NO -) rather than decomposition. These differences underscore the selective catalytic preference for specific reaction pathways in the presence of O .”\nComment 2: The molecular models shown in Figure 1A appear to have incorrect bonding geometries. When F or O atoms are axially bonded to carbon, the coordination number of the carbon atom exceeds four, which violates basic valence rules. These structures need to be revised to reflect chemically plausible configurations, especially since they serve as the basis for DFT simulations.\nResponse 2: We have evaluated the catalyst models and verified that the coordination geometries in these structures are chemically reasonable.\n(p41) “The (F/O)CN catalyst models were constructed based on previously reported literature and subsequently optimized to eliminate any structurally unreasonable coordination geometries.32,33,35,41”\nComment 3: (1) The authors should report the catalyst’s surface temperature under light irradiation to determine whether the reaction is purely photocatalytic or has a significant photothermal component, especially considering the gas-solid phase setup. (2) Additionally, isotopic labelling experiments (e.g., using 15NO) are necessary to confirm that the nitrogen in the evolved N originates from NO, not from decomposition of the catalyst itself.\nResponse 3: We respond to the comments in the following two sections.\n(1) We have monitored the catalyst surface temperature during the photocatalytic reaction and investigated the thermal effects on NO decomposition.\n(p9) “The catalyst surface temperature was monitored using a thermocouple. As shown in Figure S7A, the temperature increased from 25°C to 35°C during the photocatalytic reaction and stabilized thereafter, indicating that the LED light source did not induce significant heating of the catalyst. To exclude thermal contributions to NO decomposition, a control experiment was performed at 35°C in the absence of light. Figure S7B shows that all catalysts exhibited negligible activity under these conditions, confirming that NO decomposition was exclusively photocatalytic rather than thermally driven.”\nFigure S7. (A) Monitored surface temperature of catalysts during photocatalytic NO decomposition in 20% O atmosphere. (B) NO decomposition under dark conditions at 35°C in 20% O atmosphere.\n(2) We have performed isotopic labeling experiments to confirm the origin of N .\n(p8) “To identify the N source in N formation, isotopic labeling experiments were conducted using a 4% 15NO gas mixture in 20% O /Ar within a sealed reactor. The system was rigorously prepared through multiple evacuation and purging cycles to ensure atmospheric integrity. Control pre-reaction analysis by GC-MS (Figure S5A) confirmed the absence of N contamination, as only a characteristic O peak was observed in both the GC trace and the corresponding MS spectrum. Following a 2-hour photocatalytic reaction, GC-MS analysis demonstrated that the N produced originated exclusively from the decomposition of 15NO, as indicated by the sole detection of 15N (Figure 2B).”\nFigure 2B. GC-MS spectra of O and N generated from 2-hour photocatalytic decomposition of 15NO over e-FCN in 20% O atmosphere.\nComment 4: (1) To better understand the carrier dynamics and redox behavior, the authors should conduct in-situ XPS to trace the spatial distribution of photogenerated carriers, especially in e-FCN. (2) Moreover, the band structures of all three catalysts should be experimentally determined (e.g., via UV-Vis DRS, VB XPS, Mott-Schottky plots) to assess how F doping influences charge separation and redox potential, critical factors for selectivity.\nResponse 4: We respond to the comments in the following two sections.\n(1) We employed in-situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) to identify the functional groups active in the photocatalytic process.\n(p17) “In-situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) was employed to probe light-induced binding energy shifts in exfoliated CN-based catalysts, thereby identifying functional groups active in photocatalysis.62,63 Under illumination, the C 1s spectra exhibited a positive shift for C-M groups (where M = NH , OH, F), indicating electron transfer from the C atoms to these functional groups (Figure 5C and Figure S22A, D). This interpretation was corroborated by a corresponding negative shift in the N 1s spectra of NH groups, confirming their role as electron acceptors (Figure 5D and Figure S22B, E). In contrast, O 1s peaks associated with adsorbed H O or CO eshibited not shift, indicating these species do not participate in the charge transfer process (Figure 5E and Figure S22C, F). Notably, in e-OCN, the C-OH peak in the O 1s spectrum exhibited a negative shift, identifying the OH group as an electron acceptor (Figure S22F). An analogous negative shift was observed for the C-F group in the F 1s spectrum of e-FCN, indicating that F also acts as an electron acceptor (Figure 5F). Collectively, these findings identify C-NH , C-OH (in e-OCN), and C-F (in e-FCN) as direct participants in the photocatalytic reaction.”\nFigure 5C-F. X-ray photoelectron spectral band of (C) C 1s, (D) N 1s, (E) O 1s, and (F) F 1s for e-FCN in the dark and under light irradiation.\n(2) The energy levels of these catalysts were determined to assess how F modulates the photocatalytic reaction from a thermodynamic perspective.\n(p15-16) “Furthermore, determining the energy levels of these catalysts is essential for assessing how F modification influences the photocatalytic reaction from a thermodynamic perspective. The bandgap values, derived from Tauc plots, are 2.52 eV for e-CN, 2.05 eV for e-OCN, and 2.46 eV for e-FCN (Figure S20A). The valence band (VB) edge positions, approximated from XPS valence band spectra (Figure S20B) according to the literature,39 are +1.83 V (V denotes the RHE RHE potential relative to the reversible hydrogen electrode) for e-CN, +1.52 V for e-OCN, and +1.74 RHE V for e-FCN. Mott-Schottky plots measured at 1000 Hz (Figure S20C) exhibit positive slopes, RHE confirming n-type semiconductor behavior for all catalysts. The flat band potentials (E ) are fb determined to be -0.69 V for e-CN, -0.53 V for e-OCN, and -0.72 V for e-FCN; these RHE RHE RHE values correspond to the conduction band (CB) edge levels.54 An energy level diagram constructed from these findings is presented in Figure S20D. The more negative CB edge of e-FCN (-0.72 V ), compared with e-CN (-0.69 V ) and e-OCN (-0.53 V ), indicates that e-FCN possesses RHE RHE RHE a stronger driving force for the reduction of NO/N O to N [standard reduction potentials: 2 2 E(NO/N ) = 1.68 V 55 and E(N O/N ) = +1.77 V 56]. However, thermodynamically, this more 2 RHE 2 2 RHE negative CB also provides a stronger driving force for the reduction of O to superoxide radical 2 [•O -, E(O₂/•O -) = -0.13 V ]57 or H O [E(O /H O ) = 0.68 V ]58. Thus, to resolve this 2 2 RHE 2 2 2 2 2 RHE thermodynamic conflict and identify the origin of the observed oxygen tolerance, subsequent investigations must focus on the charge carrier dynamics in NO/O /Ar atmospheres.”\nComment 5: How to exclude the O disturbance as I can see from XPS results that C-O-C bond exists in these three samples.\nResponse 5: The origin of the C-O-C and O-H peaks in the O 1s XPS spectra was assigned.\n(p6) “The O 1s spectra (Figure 1C) featured two typical peaks for all three catalysts: one at 533.1 eV (O-H from adsorbed H O)43 and another at 532.0 eV (C-O-C). While the C-O-C signal in e- 2 CN and e-FCN originates solely from adsorbed CO ,43 in e-OCN it has a dual origin from both 2 structural O modifiers41 and adsorbed CO 43.” 2\nFigure 1C. 1H NMR, and O 1s X-ray photoelectron spectra of e-CN, e-OCN, and e-FCN.\nSubsequent analysis using ISI-XPS revealed that these peaks revealed no observable peak shift under light irradiation, confirming that the corresponding species are not involved in the photocatalytic reaction.\n(p17) “In contrast, O 1s peaks associated with adsorbed H O or CO eshibited not shift, indicating 2 2 these species do not participate in the charge transfer process (Figure 5E and Figure S22C, F). Notably, in e-OCN, the C-OH peak in the O 1s spectrum exhibited a negative shift, identifying the OH group as an electron acceptor (Figure S22F).”\nFigure 5E. X-ray photoelectron spectral band of O 1s for e-FCN in the dark and under light irradiation.\nComment 6: The data presented in Figure 5B must be normalized appropriately to enable valid comparisons among the three samples.\nResponse 6: The TPD peak areas were normalized to the corresponding BET surface area to facilitate a direct comparison of the intrinsic adsorption capacities.\n(p19) “To account for variations in specific surface area among the exfoliated catalysts, the TPD peak areas were normalized by the corresponding BET surface area, enabling a direct comparison of their intrinsic adsorption capacities.”\nFigure 6A.NO/O -TPD curves of e-CN, e-OCN, and e-FCN.\n2\n(p22) “The TPD peak areas were quantified by calibration with a standard gas mixture to establish the relationship between peak area and adsorbed gas concentration. As summarized in Table S11, this quantitative analysis confirms that e-FCN possesses the highest NO adsorption capacity (2.62 μmol·g-1) and the lowest O adsorption capacity (0.20 μmol·g-1), indicating a preferential 2 adsorption of NO over O .”\nResponse 7: We highlighted the necessity of investigating humidity and demonstrated that relative humidity (RH) exerted only a minor influence on photocatalytic NO decomposition.\n(p13) “In practical indoor environments, air contains a certain level of relative humidity (RH), and NO pollutants often coexist with other gases, such as CO , NO , and SO .51 It is therefore essential 2 2 2 to evaluate the effects of RH and coexisting gases on photocatalytic NO decomposition over e-FCN. Figures S15-16 systematically examined these influences by testing e-FCN under common indoor RH levels and typical concentration ratios of NO to CO , NO , and SO . Figure S15 2 2 2 indicated that RH has a minor effect on photocatalytic NO decomposition, suggesting that H O 2 molecules do not significantly compete with NO for adsorption and activation sites on the catalyst surface.”\nComment 8: The photocatalyst appears to exhibit deactivation after approximately three hours. The manuscript attributes this to NO - accumulation but provides no further evidence or mitigation strategy. The authors should explore the deactivation mechanism more thoroughly and suggest ways to minimize it, such as periodic regeneration.\nResponse 8: We have identified the causes of the marginal decline in catalyst performance observed during long-term experiments and have developed corresponding strategy to restore its catalytic performance.\n(p16-17 in Supp Info, Figures S14C-D) “As shown in Figure S14C, the e-FCN catalyst, when used directly in a new reaction cycle immediately after a 3-hour long-term performance, exhibited a slight decrease in both NO removal efficiency and N selectivity compared with the pristine 2 catalyst. However, quantitative analysis of NO - accumulation on the catalyst surface (Figure 4B) 3 revealed only a limited amount of deposition, suggesting that this is unlikely to be the primary cause of the decline in performance. In-situ FTIR results (see Figure 8 and Figure S29) indicated that both adsorbed (NO) and bridged (NO) dimer accumulated on the catalyst surface. During 2 2 the long-term reaction, the concentration of these NO species reaches a critical threshold, which subsequently impedes the further adsorption and activation of NO molecules, thereby diminishing both NO removal efficiency and N selectivity. To purge the accumulated NO species, the used e- 2 FCN catalyst was irradiated under 425 nm LED light in an Ar atmosphere for 30 minutes. Figures S14C-D demonstrate that this regeneration process successfully restored the photocatalytic activity of e-FCN to a level identical to that of the pristine catalyst.”\nFigure S14C-D. Regeneration of NO removal efficiency and product selectivity in used e-FCN after 3-hour durability.\nComment 9: In the 13C NMR spectrum of the e-CN sample, a noticeable shoulder peak is present. The authors should assign this peak and discuss whether it represents a unique carbon environment or a defect structure.\nResponse 9: The shoulder peak observed in the 13C NMR spectrum of e-CN was assigned. Furthermore, the absence of a corresponding peak in the spectra of e-OCN and e-FCN was explained.\n(p5-6) “The 13C NMR spectra (Figure 1B) showed consistent peaks at 156.4 ppm (C ) and 164.8 3N ppm (C ), confirming structural integrity.39 For e-CN, a shoulder peak was observed at 162.9 2N-NHx ppm, which is assigned to the C structure formed during the thermal exfoliation of bulk-CN.40 2N-NH This peak disappeared upon the introduction of electron-withdrawing O/F. We attribute this disappearance to a strong inductive effect from the O/F modifiers. The carbon atom in the C 2N-NH structure is situated closer to the modification site than the carbon in the C structure (whose 2N-NHx signal appears at 164.8 ppm). Consequently, the inductive effect induces a downfield shift of the C signal, causing it to merge with the adjacent peak at 164.8 ppm and become unresolvable.” 2N-NH\nFigure 1B. 13C NMR spectra of e-CN, e-OCN, and e-FCN.\nComment 10: O adsorption capacity is only reported under dark conditions. Since light irradiation 2 could influence surface charge and adsorption behavior, the authors should also measure O adsorption under light to better understand its real-time interaction with the photocatalyst during 2 operation.\nResponse 10: We investigated the O adsorption capabilities of e-CN, e-OCN, and e-FCN under 2 both dark conditions and light irradiation.\n(p21-22)\n“The above results have revealed that e-FCN preferentially adsorbs and activates NO, even in the presence of O . However, the mechanism behind the inertness of e-FCN toward O adsorption, 2 2 especially when compared with e-CN and e-OCN, requires further investigation. Previous studies have shown that the accumulation of the C=O bond, resulting from the oxidation of surface C atoms in CN catalysts, can be suppressed by surface fluorination that introduced the C-F bond, thereby mitigating C=O bond accumulation.33 The formation of these C=O bond on exfoliated CN- based catalysts was monitored using in-situ Fourier-transform infrared (in-situ FTIR) spectroscopy. Figures S26A-C showed that a newly generated band at 1725 cm-1 appeared upon exposure of e- CN, e-OCN, and e-FCN to 425 nm light irradiation, this band is consistent with the formation of a surface C=O bond. To verify this assignment, the in-situ FTIR characterization was also carried out using 18O instead of O . As shown in Figures 7A-C, an analogous redshift to 1684 cm-1 2 2 occurred. The ratio of the wavenumbers, ν(C=18O)/ν(C=O), is close to the theoretical value of [μ(C=O)/μ(C=18O)]1/2 = 0.98, where μ(C=O) = 6.86 and μ(C=18O) = 7.20 represent the reduced masses of the respective C=O bonds.67 Meanwhile, the FTIR bands associated with N-heterocyclic frameworks in the exfoliated catalysts and the C-F bond in e-FCN remained unchanged. These isotopic in-situ FTIR results confirm the assignment of the 1725 cm-1 band to the C=O bond.\nThe exposure of both e-CN and e-OCN to O /18O under 425 nm irradiation led to increasing 2 2 the band intensity corresponding to the C=18O/C=O bond (see Figures 7A-B and S26A-B). On the other hand, the formation of the C=18O/C=O bond on e-FCN was significantly inhibited compared with that on e-CN and e-OCN. The spectral features of e-FCN remained consistent with those of N-heterocycles and the C-F bond (Figures 7C, S26C). The time profiles of C=18O/C=O bond formation on e-CN, e-OCN, and e-FCN under irradiation in comparison with their dark control counterpart clearly show such spectral behavior (Figures 7D, S26D). Collectively, these findings indicate that e-CN and e-OCN adsorb and activate O via light-driven C=O bond formation. The 2 presence of C-F bonds in e-FCN inhibits this process. Consequently, e-FCN exhibits inert activity toward O activation but selectively adsorbs and activates NO for reduction under light irradiation, 2 even in the presence of excess O .” 2\nFigure 7. The in-situ FTIR spectral changes upon exposing 18O to e-CN, e-OCN, and e-FCN 2 surfaces in the dark and under light irradiation.\nReviewer 2's Comments:\nThe manuscript addresses the significant challenge of photocatalytic selective NO decomposition to N under oxygen-rich conditions by introducing C-F bonds into the CN structure, 2 developing an oxygen-tolerant e-FCN catalyst. The work achieves efficient photocatalytic NO-to- N conversion under oxygen-rich conditions, with experiments and theoretical calculations 2 demonstrating that C-F bond engineering inhibits C=O bond formation to block O adsorption 2 while selectively activating NO. This provides a novel strategy for designing oxygen-tolerant selective NO reduction catalysts. However, several concerns require clarification.\nComment 1: Are the C-F bonds functioning as isolated sites or synergistically with other functional groups (e.g., -NH )? Further clarification of the active center is necessary. 2\nResponse 1: We employed in-situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) to identify the functional groups active in the photocatalytic process.\n(p17) “In-situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) was employed to probe light-induced binding energy shifts in exfoliated CN-based catalysts, thereby identifying functional groups active in photocatalysis.62,63 Under illumination, the C 1s spectra exhibited a positive shift for C-M groups (where M = NH , OH, F), indicating electron transfer from the C x atoms to these functional groups (Figure 5C and Figure S22A, D). This interpretation was corroborated by a corresponding negative shift in the N 1s spectra of NH groups, confirming their x role as electron acceptors (Figure 5D and Figure S22B, E). In contrast, O 1s peaks associated with adsorbed H O or CO eshibited not shift, indicating these species do not participate in the charge 2 2 transfer process (Figure 5E and Figure S22C, F). Notably, in e-OCN, the C-OH peak in the O 1s spectrum exhibited a negative shift, identifying the OH group as an electron acceptor (Figure S22F). An analogous negative shift was observed for the C-F group in the F 1s spectrum of e-FCN, indicating that F also acts as an electron acceptor (Figure 5F). Collectively, these findings identify C-NH , C-OH (in e-OCN), and C-F (in e-FCN) as direct participants in the photocatalytic reaction.” x\nFigure 5C-F. X-ray photoelectron spectral band of (C) C 1s, (D) N 1s, (E) O 1s, and (F) F 1s for e-FCN in the dark and under light irradiation.\nComment 2: While in-situ FTIR confirms NO consumption and N generation, the specific 2 conversion pathway remains unclear. Time-resolved experiments or isotopic labeling should be incorporated to elucidate the dynamic NO transformation mechanism.\nResponse 2: The origin of the evolved N and O gases was initially elucidated using isotopic 2 2 labeling experiments.\n(p8) “To identify the N source in N formation, isotopic labeling experiments were conducted using 2 a 4% 15NO gas mixture in 20% O /Ar within a sealed reactor. The system was rigorously prepared 2 through multiple evacuation and purging cycles to ensure atmospheric integrity. Control pre- reaction analysis by GC-MS (Figure S5A) confirmed the absence of N contamination, as only a 2 characteristic O peak was observed in both the GC trace and the corresponding MS spectrum. 2 Following a 2-hour photocatalytic reaction, GC-MS analysis demonstrated that the N produced 2 originated exclusively from the decomposition of 15NO, as indicated by the sole detection of 15N 2 (Figure 2B). A separate experiment was performed to ascertain the origin of the co-generated O , 2 utilizing a mixture of 4% NO in 20% 18O /Ar. Control pre-reaction GC-MS analysis (Figure S5B) 2 confirmed the presence of exclusively 18O , ruling out significant O contamination. After the 2 2 photocatalytic reaction, the MS spectrum of O (Figure 2C) revealed a composite signal for O and 2 2 18O . The absence of a peak at m/z = 34 (16O18O) indicates that isotopic exchange between oxygen 2 atoms did not occur. The experimental 16O :18O ratio was approximately 1:11, which aligns 2 2 excellently with the theoretically predicted ratio of 1:10.7 based on reaction selectivity. This result confirms that the newly generated O originates from the decomposition of NO.” 2\nFigure 2B-C. (B) GC-MS spectra of O and N generated from 2-hour photocatalytic 2 2 decomposition of 15NO over e-FCN in 20% O atmosphere. (C) GC-MS spectra of O and N 2 2 2 generated from 2-hour photocatalytic decomposition of NO over e-FCN in 20% 18O atmosphere. 2\nSubsequently, peak assignments in the FTIR spectra were verified using 15NO as the reaction gas. The pathway for the NO photoreaction was then elucidated based on the in-situ FTIR results.\n(p23-25)\n“The NO/O adsorption results demonstrate that the presence of the C-F bond in CN suppresses 2 the formation of the C=O bond, rendering e-FCN inert towards O . To investigate the NO 2 photoconversion process in the presence of O , in-situ FTIR spectroscopy was employed to 2 monitor the chemical transformation process. The FTIR spectra of initial NO adsorption on e-CN, e-OCN, and e-FCN reveal distinct bands (Figure S27): one at 1690-1694 cm-1, corresponding to adsorbed (NO) species on the surface,68 and others at 1273 and 1411-1413 cm-1, indicative of 2 bridged (NO) species.17 These bands (1273, 1411-1413, and 1694 cm-1) increase gradually over 2 time (Figures S27A, C, E), and e-FCN consistently exhibits higher band intensities compared with e-CN and e-OCN, thereby confirming stronger NO binding on e-FCN (Figures S27B, D, F). The signals at 1273 and 1411-1413 cm-1 are typically attributed to the symmetric vibration of bridging bidentate NO -.69 However, the absence of corresponding signals for asymmetric vibrations in the 3 higher wavenumber range (1500-1700 cm-1) challenges this interpretation. Based on these spectral features, we attribute these two bands to the presence of a bridged (NO) dimer adsorbed onto the 2 catalysts.\nThe assignment of the observed feature to an (NO) dimer was confirmed unequivocally using 2 15NO isotope-substituted in-situ FTIR spectroscopy. As shown in Figure S28, the spectrum exhibited a corresponding redshift to lower wavenumber. The experimental ν(15N)/ν(N) ratios showed excellent agreement with theoretical values predicted for an (NO) dimer (Table S12), 2 thereby confirming this assignment.70 Complementary DFT calculations were employed to investigate NO adsorption behavior and co-adsorbed NO interactions on the e-FCN surface. One or two NO molecules were introduced onto the e-FCN surface slab (Figures 6C-F). Computed adsorption energies revealed a more stable configuration for the (NONO) dimer, with a total E (NONO) of -0.31 eV for the dimer system, compared with E (NO) = -0.21 eV for a single ads ads NO molecule (Figures 6C-D). This enhanced stability arises from both π back-donation to the NO ligands and direct attractive interactions between the co-adsorbed NO molecules. Charge density analysis indicated significant electron accumulation at the adsorption sites, increasing from 0.36 |e| for a single NO molecule to 0.69 |e| for the dimer system (Figures 6E-F), consistent with enhanced electron transfer from the e-FCN substrate. The total charge associated with the dimer was less than the sum of the two isolated NO adsorbates. This charge redistribution, combined with the adjacent geometry of the two N atoms, suggests a mechanism that mitigates inter- adsorbate repulsion and facilitates N-N coupling. The assignment of vibrational bands to an (NO) 2 dimer, confirmed through in-situ isotopic FTIR and theoretical calculations, indicates that N-N bond formation occurs via a partial overlap of electron density between adjacent N atoms.25\nUpon irradiating NO adsorbed on the catalysts, a new band appears at 2314 cm-1, indicating the presence of adsorbed N O on the catalyst surface, as shown in Figures S29A, C, E.71,72 Initially, 2 this band increases with irradiation time for e-CN, e-OCN, and e-FCN, suggesting the formation of N O as an intermediate in the decomposition of NO. However, after 18 minutes, the band begins 2 to decrease on e-FCN (Figure S29F), while it continues to rise on e-CN and e-OCN (Figures S29B, D). This contrasting behavior between the catalysts suggests that the N O intermediates undergo 2 further transformation on e-FCN but accumulate on e-CN and e-OCN. This observation is consistent with the finding that N O production during the photocatalytic NO conversion was 2 minimal on e-FCN but significant on e-CN and e-OCN (see Figure 2A). These results highlight the critical role of e-FCN in promoting the photocatalytic reduction of N O intermediates to N . 2 2 Notably, a band at 1555 cm-1 was observed for all catalysts under light irradiation (Figures S29A, C, E), indicating the accumulation of bidentate NO - on the catalyst surface.73 Among the catalysts, 3 e-OCN exhibits the highest NO - accumulation. This finding is consistent with the observation that 3 NO - selectivity for e-OCN is higher than that for e-CN and e-FCN (see above Figure 2A). 3\nDefinitive confirmation of the N O and NO - feature assignments was achieved using 15NO 2 3 isotope-substituted in-situ FTIR spectroscopy. The spectrum showed a corresponding redshift (Figure 8), and the experimental ν(15N)/ν(N) ratios excellently matched theoretical values for an N O and NO - (Table S12), validating the assignment.70 The band area changes of 15N O and 2 3 2 15NO - was also consistent with that of N O and NO -. Consequently, it is revealed that NO 3 2 3 photoconversion in O proceeds predominantly via an oxidative pathway on e-CN/e-OCN but 2 shifts to a reductive pathway on e-FCN.”\nFigure 8. The time evolution of in-situ FTIR spectra for 15NO photoreduction over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN under light irradiation in 20% O atmosphere. 2\nFigure S28. The time evolution of in-situ FTIR spectra for 15NO adsorption over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN in 20% O atmosphere. 2\nComment 3: The study focuses exclusively on the CN system and mp-TiO as a reference but 2-x lacks comparisons with other fluorinated materials. This omission weakens the claim of the uniqueness of C-F bond modification.\nResponse 3: The catalytic performance of e-FCN was compared against other fluorinated catalysts to demonstrate the superior efficacy of the C-F modification strategy.\n(p10-11) “To evaluate the uniqueness of the proposed C-F modification strategy, we systematically compared the performance of several catalysts under both Ar and 20% O /Ar atmospheres. The 2 tested catalysts included: e-FCN (containing C-F bonds), our previously developed mp-TiO ,25 2-x surface-fluorinated F-mp-TiO 48 and the BiOBr/BiOF system featuring O-Br/F bonds49. As 2-x shown in Figures 3A-B, e-FCN exhibited superior catalytic performance under both conditions. Notably, while mp-TiO demonstrated activity comparable to e-FCN in an Ar atmosphere, its NO 2-x removal efficiency and N selectivity decreased significantly in 20% O /Ar, with substantial NO 2 2 2 byproduct formation. Further analysis revealed that surface F doping (F-mp-TiO ) slightly 2-x improved NO removal efficiency but increased NO generation, suggesting that surface F doping 2 primarily promotes the undesired oxidation of NO to NO rather than NO decomposition under 2 oxygen-rich conditions. Similarly, the BiOF system (containing O-F bonds) showed marginally higher activity than BiOBr but remained ineffective for NO decomposition in an oxygen-rich environment. These comparative results clearly demonstrate the distinct advantage of the C-F modification strategy in developing highly efficient NO decomposition catalysts.”\nFigure 3A-B. Comparison of photocatalytic NO removal efficiency and product selectivity at 30 minutes between e-FCN, mp-TiO , F-mp-TiO , BiOBr, and BiOF in Ar and 20% O 2-x 2-x 2 atmospheres.\nComment 4: High-temperature pyrolysis of NH F may generate toxic byproducts (e.g., HF). A 4 rigorous evaluation of the environmental feasibility for synthesis route, including byproduct management and safety protocols, is strongly recommended.\nResponse 4: The environmental feasibility of the synthesis route was evaluated using a comprehensive life cycle assessment (LCA). The relevant data and descriptions are presented in Figures S17-S18, with a summary of these results provided in the Conclusion.\n(p32) “Finally, the synthesis of e-FCN, which involves high-temperature pyrolysis of NH F, may 4 release toxic byproducts such as HF, posing a considerable environmental burden. To evaluate these impacts, we conducted a comprehensive life cycle assessment (LCA) for e-FCN production and application, as summarized in Figures S17-18. The results reveal substantial net environmental benefits across the entire life cycle of e-FCN, effectively offsetting multiple environmental burdens.”\nFigure S18. (A) LCA characterization result for 1.0 kg e-FCN synthesis and usage. (B) Percentage contribution of different materials and energy in the lifecycle stage of 1.0 kg e-FCN. (C) Radar chart (GWP and OD) for the percentage of chemicals and energy consumption during the synthesis and use of 1.0 kg e-FCN. (D) LCA normalized result for 1.0 kg e-FCN synthesis and usage.\nReviewer 3's Comments:\nThis manuscript reports the design and performance of a fluorine-modified polymeric carbon nitride photocatalyst for the selective reduction of NO to N under oxygen-rich conditions. The 2 authors claim a significant advance in achieving high N selectivity and robust performance in the 2 presence of O . The work also proposes a mechanistic rationale for the observed selectivity. The 2 manuscript is generally well written, the experimental procedures are detailed, and the supporting data are extensive. However, I have several comments regarding the novelty, evidence for the claims, and mechanistic interpretations.\nComment 1: The manuscript should explain more clearly how this method is different from recent work using mp-TiO , metal oxides, or doped carbon nitrides. Also, compare both performance 2-x and mechanism with these methods.\nResponse 1: We delineated the distinctions between e-FCN and previously reported systems for photocatalytic NO removal.\n(p3-4) “While the photocatalytic oxidation of NO to NO - is well-established,22 achieving its 3 selective reduction to N without a chemical reductant in oxygen-rich environments remains 2 challenging.25 We demonstrated a breakthrough: the oxygen-tolerant, selective photoreduction of NO to N under visible light irradiation.” 2\nAdditionally, a comparative analysis of its catalytic performance against relevant benchmarks for photocatalytic NO decomposition was conducted.\n(p10-11)\n“To evaluate the uniqueness of the proposed C-F modification strategy, we systematically compared the performance of several catalysts under both Ar and 20% O /Ar atmospheres. The 2 tested catalysts included: e-FCN (containing C-F bonds), our previously developed mp-TiO ,25 2-x surface-fluorinated F-mp-TiO 48 and the BiOBr/BiOF system featuring O-Br/F bonds49. As 2-x shown in Figures 3A-B, e-FCN exhibited superior catalytic performance under both conditions. Notably, while mp-TiO demonstrated activity comparable to e-FCN in an Ar atmosphere, its NO 2-x removal efficiency and N selectivity decreased significantly in 20% O /Ar, with substantial NO 2 2 2 byproduct formation. Further analysis revealed that surface F doping (F-mp-TiO ) slightly 2-x improved NO removal efficiency but increased NO generation, suggesting that surface F doping 2 primarily promotes the undesired oxidation of NO to NO rather than NO decomposition under 2 oxygen-rich conditions. Similarly, the BiOF system (containing O-F bonds) showed marginally higher activity than BiOBr but remained ineffective for NO decomposition in an oxygen-rich environment. These comparative results clearly demonstrate the distinct advantage of the C-F modification strategy in developing highly efficient NO decomposition catalysts.\nAlthough photocatalytic processes for NO removal have been extensively studied,22 the majority of prior research has focused on the selective oxidation of NO to NO -. We recently employed mp- 3 TiO to achieve selective NO reduction to N without chemical reductant in anoxic condition but 2-x 2 its performance was drastically reduced in the presence of O .25 In contrast, e-FCN demonstrated 2 superior efficiency in converting NO to N across a range of NO concentrations (0.5-2.0 ppmv; 2 see Figure 3C) and the concentration of O had a negligible effect on both NO removal efficiency 2 and N selectivity (Figure 3D). Conversely, oxygen-modified CN (e-OCN) promoted NO 2 oxidation to NO - in the presence of O (Figure S11), shifting the primary byproduct from N O to 3 2 2 NO - as O concentration increased from 0% to 20%. This behavior starkly contrasts with F- 3 2 modified CN (e-FCN), which enables oxygen-tolerant NO decomposition to N regardless of O 2 2 presence. As listed in Tables S3-4, the photocatalytic performance and AQYs of e-FCN in NO decomposition under 20% O exceed those of previously reported catalysts, even at an initial NO 2 concentration as low as 0.5 ppmv.”\nFigure 3. (A, B) Comparison of photocatalytic NO removal efficiency and product selectivity at 30 minutes between e-FCN, mp-TiO , F-mp-TiO , BiOBr, and BiOF in Ar and 20% O 2-x 2-x 2 atmospheres. Effects of (C) initial NO concentration and (D) O concentration on photocatalytic 2 NO decomposition over e-FCN.\nComment 2: The main claim is that C-F bonds help NO adsorption over O . The evidence comes 2 mostly from TPD and DFT. Have the authors done direct surface tests with both gases together, like in-situ FTIR or co-adsorption?\nResponse 2: We initially investigated the O adsorption capabilities of e-CN, e-OCN, and e-FCN 2 under both dark conditions and light irradiation.\n(p21-22)\n“The above results have revealed that e-FCN preferentially adsorbs and activates NO, even in the presence of O . However, the mechanism behind the inertness of e-FCN toward O adsorption, 2 2 especially when compared with e-CN and e-OCN, requires further investigation. Previous studies have shown that the accumulation of the C=O bond, resulting from the oxidation of surface C atoms in CN catalysts, can be suppressed by surface fluorination that introduced the C-F bond, thereby mitigating C=O bond accumulation.33 The formation of these C=O bond on exfoliated CN- based catalysts was monitored using in-situ Fourier-transform infrared (in-situ FTIR) spectroscopy. Figures S26A-C showed that a newly generated band at 1725 cm-1 appeared upon exposure of e- CN, e-OCN, and e-FCN to 425 nm light irradiation, this band is consistent with the formation of a surface C=O bond. To verify this assignment, the in-situ FTIR characterization was also carried out using 18O instead of O . As shown in Figures 7A-C, an analogous redshift to 1684 cm-1 2 2 occurred. The ratio of the wavenumbers, ν(C=18O)/ν(C=O), is close to the theoretical value of [μ(C=O)/μ(C=18O)]1/2 = 0.98, where μ(C=O) = 6.86 and μ(C=18O) = 7.20 represent the reduced masses of the respective C=O bonds.67 Meanwhile, the FTIR bands associated with N-heterocyclic frameworks in the exfoliated catalysts and the C-F bond in e-FCN remained unchanged. These isotopic in-situ FTIR results confirm the assignment of the 1725 cm-1 band to the C=O bond.\nThe exposure of both e-CN and e-OCN to O /18O under 425 nm irradiation led to increasing 2 2 the band intensity corresponding to the C=18O/C=O bond (see Figures 7A-B and S26A-B). On the other hand, the formation of the C=18O/C=O bond on e-FCN was significantly inhibited compared with that on e-CN and e-OCN. The spectral features of e-FCN remained consistent with those of N-heterocycles and the C-F bond (Figures 7C, S26C). The time profiles of C=18O/C=O bond formation on e-CN, e-OCN, and e-FCN under irradiation in comparison with their dark control counterpart clearly show such spectral behavior (Figures 7D, S26D). Collectively, these findings indicate that e-CN and e-OCN adsorb and activate O via light-driven C=O bond formation. The 2 presence of C-F bonds in e-FCN inhibits this process. Consequently, e-FCN exhibits inert activity toward O activation but selectively adsorbs and activates NO for reduction under light irradiation, 2 even in the presence of excess O .” 2\nFigure 7. The in-situ FTIR spectral changes upon exposing 18O to e-CN, e-OCN, and e-FCN 2 surfaces in the dark and under light irradiation.\nSubsequently, peak assignments in the FTIR spectra were verified using 15NO as the reaction gas. The pathway for the NO photoreaction was then elucidated based on the in-situ FTIR results.\n(p23-25)\n“The NO/O adsorption results demonstrate that the presence of the C-F bond in CN suppresses 2 the formation of the C=O bond, rendering e-FCN inert towards O . To investigate the NO 2 photoconversion process in the presence of O , in-situ FTIR spectroscopy was employed to 2 monitor the chemical transformation process. The FTIR spectra of initial NO adsorption on e-CN, e-OCN, and e-FCN reveal distinct bands (Figure S27): one at 1690-1694 cm-1, corresponding to adsorbed (NO) species on the surface,68 and others at 1273 and 1411-1413 cm-1, indicative of 2 bridged (NO) species.17 These bands (1273, 1411-1413, and 1694 cm-1) increase gradually over 2 time (Figures S27A, C, E), and e-FCN consistently exhibits higher band intensities compared with e-CN and e-OCN, thereby confirming stronger NO binding on e-FCN (Figures S27B, D, F). The signals at 1273 and 1411-1413 cm-1 are typically attributed to the symmetric vibration of bridging bidentate NO -.69 However, the absence of corresponding signals for asymmetric vibrations in the 3 higher wavenumber range (1500-1700 cm-1) challenges this interpretation. Based on these spectral features, we attribute these two bands to the presence of a bridged (NO) dimer adsorbed onto the 2 catalysts.\nThe assignment of the observed feature to an (NO) dimer was confirmed unequivocally using 2 15NO isotope-substituted in-situ FTIR spectroscopy. As shown in Figure S28, the spectrum exhibited a corresponding redshift to lower wavenumber. The experimental ν(15N)/ν(N) ratios showed excellent agreement with theoretical values predicted for an (NO) dimer (Table S12), 2 thereby confirming this assignment.70 Complementary DFT calculations were employed to investigate NO adsorption behavior and co-adsorbed NO interactions on the e-FCN surface. One or two NO molecules were introduced onto the e-FCN surface slab (Figures 6C-F). Computed adsorption energies revealed a more stable configuration for the (NONO) dimer, with a total E (NONO) of -0.31 eV for the dimer system, compared with E (NO) = -0.21 eV for a single ads ads NO molecule (Figures 6C-D). This enhanced stability arises from both π back-donation to the NO ligands and direct attractive interactions between the co-adsorbed NO molecules. Charge density analysis indicated significant electron accumulation at the adsorption sites, increasing from 0.36 |e| for a single NO molecule to 0.69 |e| for the dimer system (Figures 6E-F), consistent with enhanced electron transfer from the e-FCN substrate. The total charge associated with the dimer was less than the sum of the two isolated NO adsorbates. This charge redistribution, combined with the adjacent geometry of the two N atoms, suggests a mechanism that mitigates inter- adsorbate repulsion and facilitates N-N coupling. The assignment of vibrational bands to an (NO) 2 dimer, confirmed through in-situ isotopic FTIR and theoretical calculations, indicates that N-N bond formation occurs via a partial overlap of electron density between adjacent N atoms.25\nUpon irradiating NO adsorbed on the catalysts, a new band appears at 2314 cm-1, indicating the presence of adsorbed N O on the catalyst surface, as shown in Figures S29A, C, E.71,72 Initially, 2 this band increases with irradiation time for e-CN, e-OCN, and e-FCN, suggesting the formation of N O as an intermediate in the decomposition of NO. However, after 18 minutes, the band begins 2 to decrease on e-FCN (Figure S29F), while it continues to rise on e-CN and e-OCN (Figures S29B, D). This contrasting behavior between the catalysts suggests that the N O intermediates undergo 2 further transformation on e-FCN but accumulate on e-CN and e-OCN. This observation is consistent with the finding that N O production during the photocatalytic NO conversion was 2 minimal on e-FCN but significant on e-CN and e-OCN (see Figure 2A). These results highlight the critical role of e-FCN in promoting the photocatalytic reduction of N O intermediates to N . 2 2 Notably, a band at 1555 cm-1 was observed for all catalysts under light irradiation (Figures S29A, C, E), indicating the accumulation of bidentate NO - on the catalyst surface.73 Among the catalysts, 3 e-OCN exhibits the highest NO - accumulation. This finding is consistent with the observation that 3 NO - selectivity for e-OCN is higher than that for e-CN and e-FCN (see above Figure 2A). 3\nDefinitive confirmation of the N O and NO - feature assignments was achieved using 15NO 2 3 isotope-substituted in-situ FTIR spectroscopy. The spectrum showed a corresponding redshift (Figure 8), and the experimental ν(15N)/ν(N) ratios excellently matched theoretical values for an N O and NO - (Table S12), validating the assignment.70 The band area changes of 15N O and 2 3 2 15NO - was also consistent with that of N O and NO -. Consequently, it is revealed that NO 3 2 3 photoconversion in O proceeds predominantly via an oxidative pathway on e-CN/e-OCN but 2 shifts to a reductive pathway on e-FCN.”\nFigure 8. The time evolution of in-situ FTIR spectra for 15NO photoreduction over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN under light irradiation in 20% O atmosphere. 2\nFigure S28. The time evolution of in-situ FTIR spectra for 15NO adsorption over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN in 20% O atmosphere. 2\nComment 3: Main results are shown for single runs with little error analysis. Did the authors repeat key tests? Can they provide error bars or confidence intervals for the main results?\nResponse 3: The catalytic experiments were repeated in triplicate, and the results are presented with error bars in the corresponding Figures and Tables. In some cases, error bars are shown but are smaller than the symbol size.\nComment 4: FTIR shows N O as a possible intermediate for N . Can the authors rule out other 2 2 paths or surface nitrogen species? Is there isotopic data, like with 15NO, that tracks nitrogen atoms directly?\nResponse 4: Initially, the concentrations of N-containing products after 30 minutes of reaction over the three catalysts were summarized.\n(p7-8) “Table S2 summarized the concentrations of N-containing products after 30 minutes of reaction over the three catalysts. In all cases, the total N concentration, which was calculated as the sum of all N-containing products, matched the initial NO concentration, meeting the N balance throughout the reaction. The data in Table S2 also revealed that N was the dominant product in 2 NO decomposition over e-FCN, whereas N O and NO - were the primary products over e-CN and 2 3 e-OCN. Based on these results, the product selectivity at 30 minutes was determined, demonstrating that e-FCN exhibited significantly higher N selectivity (92%) compared with e- 2 CN (1%) and e-OCN (7%). Notably, e-OCN showed higher NO - selectivity (55%) than e-CN 3 (46%).”\nTable S2. Summary of N species and calculated total N concentrations over e-CN, e-OCN, and e- FCN following a 30-minute photocatalytic reaction.a\nCatalyst [NO] [N2O] [N2] [NO2] [NO3 -] [Total N]\n(ppmv) (ppmv) (ppmv) (ppmv) (ppm) (ppm)b\ne-CN 1.600 ± 0.039 0.059 ± 0.005 0.049 ± 0.004 0 0.185 ± 0.019 2.001 ± 0.044\ne-OCN 0.638 ± 0.040 0.072 ± 0.006 0.231 ± 0.012 0 0.755 ± 0.025 1.999 ± 0.049\ne-FCN 0.298 ± 0.040 0.010 ± 0.003 0.781 ± 0.023 0 0.119 ± 0.011 1.999 ± 0.048\na Reaction condition: 2.0 ppmv NO in 20% O2/Ar, LED lamp (λ = 425 nm), inlet flow rate = 10 mL∙min-1, RH = 0%; b [Total N] = [NO] + ([N2O] + [N2]) × 2 + [NO2] + [NO3 -].\nSubsequently, the origin of the evolved N and O gases was initially elucidated using isotopic 2 2 labeling experiments.\n(p8) “To identify the N source in N formation, isotopic labeling experiments were conducted using 2 a 4% 15NO gas mixture in 20% O /Ar within a sealed reactor. The system was rigorously prepared 2 through multiple evacuation and purging cycles to ensure atmospheric integrity. Control pre-reaction analysis by GC-MS (Figure S5A) confirmed the absence of N contamination, as only a 2 characteristic O peak was observed in both the GC trace and the corresponding MS spectrum. 2 Following a 2-hour photocatalytic reaction, GC-MS analysis demonstrated that the N produced 2 originated exclusively from the decomposition of 15NO, as indicated by the sole detection of 15N 2 (Figure 2B). A separate experiment was performed to ascertain the origin of the co-generated O , 2 utilizing a mixture of 4% NO in 20% 18O /Ar. Control pre-reaction GC-MS analysis (Figure S5B) 2 confirmed the presence of exclusively 18O , ruling out significant O contamination. After the 2 2 photocatalytic reaction, the MS spectrum of O (Figure 2C) revealed a composite signal for O and 2 2 18O . The absence of a peak at m/z = 34 (16O18O) indicates that isotopic exchange between oxygen 2 atoms did not occur. The experimental 16O :18O ratio was approximately 1:11, which aligns 2 2 excellently with the theoretically predicted ratio of 1:10.7 based on reaction selectivity. This result confirms that the newly generated O originates from the decomposition of NO.” 2\nFigure 2B-C. (B) GC-MS spectra of O and N generated from 2-hour photocatalytic 2 2 decomposition of 15NO over e-FCN in 20% O atmosphere. (C) GC-MS spectra of O and N 2 2 2 generated from 2-hour photocatalytic decomposition of NO over e-FCN in 20% 18O atmosphere. 2\nFinally, peak assignments in the FTIR spectra were verified using 15NO as the reaction gas. The pathway for the NO photoreaction was then elucidated based on the in-situ FTIR results.\n(p23-25)\n“The NO/O adsorption results demonstrate that the presence of the C-F bond in CN suppresses 2 the formation of the C=O bond, rendering e-FCN inert towards O . To investigate the NO 2 photoconversion process in the presence of O , in-situ FTIR spectroscopy was employed to 2 monitor the chemical transformation process. The FTIR spectra of initial NO adsorption on e-CN, e-OCN, and e-FCN reveal distinct bands (Figure S27): one at 1690-1694 cm-1, corresponding to adsorbed (NO) species on the surface,68 and others at 1273 and 1411-1413 cm-1, indicative of 2 bridged (NO) species.17 These bands (1273, 1411-1413, and 1694 cm-1) increase gradually over 2 time (Figures S27A, C, E), and e-FCN consistently exhibits higher band intensities compared with e-CN and e-OCN, thereby confirming stronger NO binding on e-FCN (Figures S27B, D, F). The signals at 1273 and 1411-1413 cm-1 are typically attributed to the symmetric vibration of bridging bidentate NO -.69 However, the absence of corresponding signals for asymmetric vibrations in the 3 higher wavenumber range (1500-1700 cm-1) challenges this interpretation. Based on these spectral features, we attribute these two bands to the presence of a bridged (NO) dimer adsorbed onto the 2 catalysts.\nThe assignment of the observed feature to an (NO) dimer was confirmed unequivocally using 2 15NO isotope-substituted in-situ FTIR spectroscopy. As shown in Figure S28, the spectrum exhibited a corresponding redshift to lower wavenumber. The experimental ν(15N)/ν(N) ratios showed excellent agreement with theoretical values predicted for an (NO) dimer (Table S12), 2 thereby confirming this assignment.70 Complementary DFT calculations were employed to investigate NO adsorption behavior and co-adsorbed NO interactions on the e-FCN surface. One or two NO molecules were introduced onto the e-FCN surface slab (Figures 6C-F). Computed adsorption energies revealed a more stable configuration for the (NONO) dimer, with a total E (NONO) of -0.31 eV for the dimer system, compared with E (NO) = -0.21 eV for a single ads ads NO molecule (Figures 6C-D). This enhanced stability arises from both π back-donation to the NO ligands and direct attractive interactions between the co-adsorbed NO molecules. Charge density analysis indicated significant electron accumulation at the adsorption sites, increasing from 0.36 |e| for a single NO molecule to 0.69 |e| for the dimer system (Figures 6E-F), consistent with enhanced electron transfer from the e-FCN substrate. The total charge associated with the dimer was less than the sum of the two isolated NO adsorbates. This charge redistribution, combined with the adjacent geometry of the two N atoms, suggests a mechanism that mitigates inter- adsorbate repulsion and facilitates N-N coupling. The assignment of vibrational bands to an (NO) 2 dimer, confirmed through in-situ isotopic FTIR and theoretical calculations, indicates that N-N bond formation occurs via a partial overlap of electron density between adjacent N atoms.25\nUpon irradiating NO adsorbed on the catalysts, a new band appears at 2314 cm-1, indicating the presence of adsorbed N O on the catalyst surface, as shown in Figures S29A, C, E.71,72 Initially, 2 this band increases with irradiation time for e-CN, e-OCN, and e-FCN, suggesting the formation of N O as an intermediate in the decomposition of NO. However, after 18 minutes, the band begins 2 to decrease on e-FCN (Figure S29F), while it continues to rise on e-CN and e-OCN (Figures S29B, D). This contrasting behavior between the catalysts suggests that the N O intermediates undergo 2 further transformation on e-FCN but accumulate on e-CN and e-OCN. This observation is consistent with the finding that N O production during the photocatalytic NO conversion was 2 minimal on e-FCN but significant on e-CN and e-OCN (see Figure 2A). These results highlight the critical role of e-FCN in promoting the photocatalytic reduction of N O intermediates to N . 2 2 Notably, a band at 1555 cm-1 was observed for all catalysts under light irradiation (Figures S29A, C, E), indicating the accumulation of bidentate NO - on the catalyst surface.73 Among the catalysts, 3 e-OCN exhibits the highest NO - accumulation. This finding is consistent with the observation that 3 NO - selectivity for e-OCN is higher than that for e-CN and e-FCN (see above Figure 2A). 3\nDefinitive confirmation of the N O and NO - feature assignments was achieved using 15NO 2 3 isotope-substituted in-situ FTIR spectroscopy. The spectrum showed a corresponding redshift (Figure 8), and the experimental ν(15N)/ν(N) ratios excellently matched theoretical values for an N O and NO - (Table S12), validating the assignment.70 The band area changes of 15N O and 2 3 2 15NO - was also consistent with that of N O and NO -. Consequently, it is revealed that NO 3 2 3 photoconversion in O proceeds predominantly via an oxidative pathway on e-CN/e-OCN but 2 shifts to a reductive pathway on e-FCN.”\nFigure 8. The time evolution of in-situ FTIR spectra for 15NO photoreduction over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN under light irradiation in 20% O atmosphere. 2\nFigure S28. The time evolution of in-situ FTIR spectra for 15NO adsorption over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN in 20% O atmosphere. 2\nComment 5: The authors claim better charge separation in e-FCN by F doping, using PL, TRPL, and EIS. These changes are moderate. Can they show transient absorption or time-resolved EPR data to link this to NO reduction?\nResponse 5: Microsecond transient absorption spectroscopy (μs-TAS) was employed to probe the charge carrier dynamics of exfoliated CN-based catalysts in NO/O /Ar atmospheres. 2\n(p16-17)\n“To investigate the charge carrier dynamics of exfoliated CN-based catalysts under relevant conditions, microsecond transient absorption spectroscopy (μs-TAS) was employed, as its temporal resolution aligns with the timescales of the actual reaction kinetics. To distinguish the nature of the charge carriers, aqueous dispersions of e-CN were probed at 900 nm both with and without ethylenediaminetetraacetic acid dipotassium (EDTA). This well-established hole scavenger reveals the signature of long-lived electrons. As shown in Figure S21A, the signal half- life (t ) of e-CN was significantly prolonged from 35.5 μs to 63.6 μs upon the addition of EDTA. 50% This observation confirms that the transient signal originates from photoinduced electrons, consistent with prior literature.59\nSubsequently, the electron transfer kinetics of e-CN, e-OCN, and e-FCN were probed by introducing NO/O /Ar atmospheres. Figures 5A-B and S21B demonstrated a decrease in the t 2 50% values for all three catalysts under the NO or O atmosphere compared with the Ar atmosphere, 2 confirming that electrons participate in activating both NO and O .60 However, the catalysts 2 exhibited distinct behavioral trends: e-CN showed similar reductions in t with NO or O , 50% 2 indicating no significant specificity (Figure S21B). On the other hand, e-OCN exhibited a larger reduction in t with O than NO (Figure 5A), whereas e-FCN showed a substantial reduction 50% 2 with NO than O (Figure 5B). These trends indicate that e-OCN preferentially facilitates the 2 electron transfer to O , whereas e-FCN favors the electron transfer to NO. To quantify this 2 selectivity, the electron transfer efficiencies for O and NO, denoted as ETE(O ) and ETE(NO), 2 2 were calculated from the t values according to an established literature method.61 As listed in 50% Table 2, e-FCN exhibited the highest ETE(NO) (30.2%) and the lowest ETE(O ) (6.2%), 2 confirming its superior selectivity for activating NO over O . This finding directly correlates with 2 the observed catalytic performance, wherein e-FCN demonstrates an ability to reduce NO to N 2 even in an oxygen-rich atmosphere.”\nFigure 5A-B. μs-TAS decay kinetics of (A) e-OCN and (B) e-FCN in Ar, O , and NO atmospheres. 2\nTable 2. Data of μs-TAS decay kinetics for e-CN, e-OCN, and e-FCN under different atmospheres.\nCatalyst t (Ar) (μs) t (O ) (μs) t (NO) (μs) ETE(O ) (%) ETE(NO) (%) 50% 50% 2 50% 2\ne-CN 32.5 29.0 30.3 10.8 6.8\ne-OCN 27.3 20.2 24.3 26.0 11.0\ne-FCN 24.2 22.7 16.9 6.2 30.2\na ETE(O ) = [1 - (t (O )/t (Ar))] × 100%; b ETE(NO) = [1-(t (NO)/t (Ar))] × 100%. 2 50% 2 50% 50% 50%\nComment 6: The manuscript sometimes says \"20% O and Ar atmosphere\", but other conditions 2 are not clear. What is NO concentration? Are conditions the same for all tests? Please add a summary table of all test conditions.\nResponse 6: We have summarized the relevant reaction conditions in the manuscript.\n(p7) “To clearly present the catalytic reaction conditions, this study systematically summarized all relevant experimental parameters in Table S1. This consolidated table improved the logical organization of data presentation while enhancing the accessibility and clarity of the manuscript.”\nComment 7: The method for tuning F content is not clear. How do the authors control F amount during synthesis? How do they confirm F content? Please describe the process and show data on F content and its reproducibility.\nResponse 7: We initially established methods to quantify C, N, and F in e-FCN catalysts.\n(p34) “The C and N content was determined using elemental analysis. A measured quantity of the catalyst was digested with concentrated HNO as an oxidant, and the F content in the resulting 3 solution was quantified via ion chromatography. This measurement was then used to calculate the F content in the original catalyst. Based on this method, the atomic ratio of F/C was determined. Accordingly, the catalyst is denoted as e-F(X)CN, where X represents the atomic percentage (at.%) of F relative to C.”\nSubsequently, we investigated the correlation between the initial NH F amount and the resulting 4 F/C atomic ratio in the e-FCN catalysts.\n(p9-10) “To investigate the correlation between the initial NH F amount and the F/C atomic ratio 4 in the e-FCN catalyst, we first conducted quantitative elemental analysis of C, N, and F in e-FCN catalysts prepared with varying NH F amounts. Based on the elemental composition data (Table 4 1), the corresponding F/C atomic ratios were calculated. As illustrated in Figure S8A, a strong linear relationship (R2 = 0.9933) was observed between the initial NH F amount and the F/C 4 atomic ratio, demonstrating that the F content in e-FCN can be precisely tuned by adjusting the initial NH F amount. Further analysis of the influence of F/C atomic ratios on catalytic 4 performance (Figure S8B) revealed that the e-FCN catalyst exhibited optimal activity at an F/C ratio of 0.10 at.%.”\nFigure S8. (A) A linear relationship between the initial NH F amount and the F/C atomic ratio in 4 e-FCN (B) Effect of F modification content on photocatalytic NO decomposition over e-FCN in 20% O atmosphere. 2\nTable 1. Elemental analysis of e-CN, e-OCN, and e-FCN.\nCatalyst C (wt.%)a N (wt.%)a F (μg·g-1)b C/N (at.%) F/C (at.%)\ne-CN 37.10 61.17 - 70.73 -\ne-OCN 37.35 60.90 - 71.53 -\ne-F(0.05)CN 37.27 61.02 235.92 71.23 0.04\ne-F(0.10)CNc 37.34 60.92 590.81 71.48 0.10\ne-F(0.15)CN 37.37 60.87 886.86 71.60 0.15\ne-F(0.20)CN 37.41 60.81 1124.52 71.75 0.19\ne-F(0.25)CN 37.43 60.75 1535.69 71.85 0.26\nUsed e-F(0.10)CNc 37.34 60.92 590.73 71.48 0.10\na The quantitative analysis of C and N was conducted using elemental analysis. b After digesting a measured quantity of the catalyst with concentrated HNO3 as an oxidant, the F content in the resulting solution was quantitatively analyzed via ion chromatography. This measurement was subsequently used to determine the F content in the original catalyst. c Unless otherwise specified, e-FCN refers to e-F(0.10)CN.\nComment 8: The authors say e-FCN is stable for 3 hours of use. What steps kept reaction conditions steady? Were there changes in temperature, humidity, or flow rates? Please explain operational stability and note any changes during the test.\nResponse 8: Initially, we monitored the catalyst surface temperature during the photocatalytic reaction and investigated the thermal effects on NO decomposition.\n(p9) “The catalyst surface temperature was monitored using a thermocouple. As shown in Figure S7A, the temperature increased from 25°C to 35°C during the photocatalytic reaction and stabilized thereafter, indicating that the LED light source did not induce significant heating of the catalyst. To exclude thermal contributions to NO decomposition, a control experiment was performed at 35°C in the absence of light. Figure S7B shows that all catalysts exhibited negligible activity under these conditions, confirming that NO decomposition was exclusively photocatalytic rather than thermally driven.”\nFigure S7. (A) Monitored surface temperature of catalysts during photocatalytic NO decomposition in 20% O atmosphere. (B) NO decomposition under dark conditions at 35°C in 2 20% O atmosphere. 2\nWe subsequently monitored the catalyst surface temperature and the inlet gas flow rate in real time over a 3-hour period.\n(p13) “The catalyst surface temperature and the inlet gas flow rate were monitored in real-time. Under irradiation from a 425 nm LED light source, the catalyst surface temperature showed no significant increase, remaining stable at approximately 35°C throughout the reaction (Figure S14A). Concurrently, the gas flow rate at the reactor inlet remained constant without notable fluctuation (Figure S14B).”\nComment 9: The relation between electron density overlaps and N-N bond comes from DFT and FTIR. More direct proof, like charge density plots or Bader analysis, would help. Can the authors explain how this overlap leads to N-N bonds?\nResponse 9: A detailed elucidation of the charge density analysis results was presented.\n(p24) “Complementary DFT calculations were employed to investigate NO adsorption behavior and co-adsorbed NO interactions on the e-FCN surface. One or two NO molecules were introduced onto the e-FCN surface slab (Figures 6C-F). Computed adsorption energies revealed a more stable configuration for the (NONO) dimer, with a total E (NONO) of -0.31 eV for the dimer system, ads compared with E (NO) = -0.21 eV for a single ads NO molecule (Figures 6C-D). This enhanced stability arises from both π back-donation to the NO ligands and direct attractive interactions between the co-adsorbed NO molecules. Charge density analysis indicated significant electron accumulation at the adsorption sites, increasing from 0.36 |e| for a single NO molecule to 0.69 |e| for the dimer system (Figures 6E-F), consistent with enhanced electron transfer from the e-FCN substrate. The total charge associated with the dimer was less than the sum of the two isolated NO adsorbates. This charge redistribution, combined with the adjacent geometry of the two N atoms, suggests a mechanism that mitigates inter- adsorbate repulsion and facilitates N-N coupling. The assignment of vibrational bands to an (NO) 2 dimer, confirmed through in-situ isotopic FTIR and theoretical calculations, indicates that N-N bond formation occurs via a partial overlap of electron density between adjacent N atoms.25”\nFigure 6E-F. The spin-polarized charge densities of (E) one NO molecule and (F) two NO molecules adsorbed on e-FCN. The C, N, O, and F atoms are represented by dark grey, white, red, and cyan spheres.\nComment 10: EPR changes after EDTA are said to show O- anions. Little change without EDTA means O- reacts to make O . (1) Can the authors show O release directly or use isotopic tests? (2) 2 2 Also, provide more EPR details to separate signals.\nResponse 10: We respond to the comments in the following two sections.\n(1) The origin of the newly generated N and O was initially confirmed through isotopic labeling 2 2 experiments performed over e-FCN.\n(p8) “To identify the N source in N formation, isotopic labeling experiments were conducted using 2 a 4% 15NO gas mixture in 20% O /Ar within a sealed reactor. The system was rigorously prepared 2 through multiple evacuation and purging cycles to ensure atmospheric integrity. Control pre- reaction analysis by GC-MS (Figure S5A) confirmed the absence of N contamination, as only a 2 characteristic O peak was observed in both the GC trace and the corresponding MS spectrum. 2 Following a 2-hour photocatalytic reaction, GC-MS analysis demonstrated that the N produced 2 originated exclusively from the decomposition of 15NO, as indicated by the sole detection of 15N 2 (Figure 2B). A separate experiment was performed to ascertain the origin of the co-generated O , 2 utilizing a mixture of 4% NO in 20% 18O /Ar. Control pre-reaction GC-MS analysis (Figure S5B) 2 confirmed the presence of exclusively 18O , ruling out significant O contamination. After the 2 2 photocatalytic reaction, the MS spectrum of O (Figure 2C) revealed a composite signal for O and 2 2 18O . The absence of a peak at m/z = 34 (16O18O) indicates that isotopic exchange between oxygen 2 atoms did not occur. The experimental 16O :18O ratio was approximately 1:11, which aligns 2 2 excellently with the theoretically predicted ratio of 1:10.7 based on reaction selectivity. This result confirms that the newly generated O originates from the decomposition of NO.” 2\nFigure 2B-C. (B) GC-MS spectra of O and N generated from 2-hour photocatalytic 2 2 decomposition of 15NO over e-FCN in 20% O atmosphere. (C) GC-MS spectra of O and N 2 2 2 generated from 2-hour photocatalytic decomposition of NO over e-FCN in 20% 18O atmosphere. 2\nSubsequently, isotopic labeling experiments with the e-FCN/5EDTA demonstrated that the addition of EDTA inhibits O formation. 2\n(p27) “The effect of hole scavenging on the reaction pathway was also investigated by carrying out the isotopic labeling experiments for the e-FCN/5EDTA catalyst. First, an experiment was performed with a 4% 15NO gas mixture in 20% O /Ar. Pre-reaction GC-MS analysis (Figure S31A) 2 confirmed the absence of N contamination, with only a characteristic O peak detected. Following 2 2 a 2-hour photocatalytic reaction, GC-MS analysis revealed that the evolved N originated 2 exclusively from the decomposition of 15NO, as indicated by the sole presence of 15N (Figure 2 S31B). This result demonstrates that scavenging photoinduced holes had no appreciable effect on N formation. Subsequently, an experiment was conducted with a 4% NO gas mixture in 20% 2 18O /Ar. Pre-reaction analysis (Figure S31C) confirmed the exclusive presence of 18O , ruling out 2 2 significant atmospheric O contamination. The post-reaction mass spectrum of O (Figure S31D) 2 2 displayed signals only for 18O , indicating that no new O was generated through the hole-induced 2 2 decomposition of EDTA. A parallel experiment conducted with EDTA in Ar corroborated these findings, showing a moderate reduction in NO removal and a slight decrease in N selectivity 2 (Figure S31E-F). Furthermore, hole scavenging inhibited O formation (Figure 9C). Collectively, 2 these results indicate that in the presence of EDTA, NO is converted to N through a photoreduction 2 process without concomitant O generation.” 2\nFigure 9C. O evolution from photocatalytic NO decomposition over e-FCN in Ar. [NO] = 2.0 2 0 ppmv, RH = 0%, Ar.\n(2) We elucidated the signal changes in the EPR spectra by conducting measurements at 298 K and 77 K.\n(p27-28) “To elucidate the pathway for the photocatalytic conversion of NO to O , EPR spectra of 2 e-FCN and e-FCN/5EDTA were compared before and after the reaction. As shown in Figure S32A, the EPR signal intensity for the used e-FCN/5EDTA catalyst, measured at 273 K, was significantly higher than that of its pristine state whereas the signal for e-FCN remained largely unchanged. For enhanced spectral resolution, EPR measurements were conducted at 77 K. At this temperature, the EPR spectrum of the used e-FCN/5EDTA exhibited an additional hyperfine structure with g-values of 2.002 and 1.995, along with an intensified signal at g = 2.007 (Figure 9D). These emergent signals are assigned to O - species.75 This observation effectively precludes the involvement of 3 •O -. This species is notoriously difficult to detect by direct EPR due to its rapid relaxation time 2 and broad spectral features,76 which contrast sharply with the well-defined hyperfine structure observed here. It is proposed that O - forms via the reaction of O- with molecular O at 77 K, 3 2 facilitated by weak covalent bonding between the π-electrons of O and the free electron on O-.77 2 This adduct is unstable at elevated temperatures; heating above 150 K causes the dissociation of the weak bond, leading to the disappearance of O - and the regeneration of O-.78 This mechanism 3 accounts for the intensified EPR signal at g = 2.005 for used e-FCN/5EDTA measured at 273 K: The signal for O- typically occurs at g = 2.005,79 which coincides with the intrinsic signal from unpaired electrons in CN-based catalysts. The accumulation of O- species thus enhances the overall signal intensity at this position. Conversely, the EPR spectrum of e-FCN at 77 K showed minimal change after the reaction (Figure 9D). The EDTA scavenging tests and EPR results collectively reveal the NO conversion mechanism: NO is reduced by electrons to N and O-, which is then 2 oxidized by holes to form O . Scavenging holes with EDTA inhibits the final oxidation step, 2 causing O- accumulation on e-FCN/5EDTA and preventing O formation which was not observed 2 on e-FCN.”\nFigure 9D. EPR spectra of the pristine and used e-FCN with and without EDTA at 77 K.\nComment 11: The work needs a simple figure of electronic changes after F doping, like charge maps.\nResponse 11: The calculated charge distribution demonstrated the electron-withdrawing nature of the incorporated O and F atoms.\n(p5) “Charge distribution calculations confirmed the strong electron-withdrawing nature of the O and F atoms, which generated a highly positive C center (Figure S4). The greater electronegativity of F resulted in a more positive charge on this C center in the FCN model (+1.81) compared with that in the OCN model (+1.70).”\nFigure S4D, F. Charge distribution of the (D) e-OCN and (F) e-FCN. The C, N, H, O, and F atoms are represented by dark grey, white, pink, red, and cyan spheres.\nComment 12: Tables S3 and S4: Make sure all cited values use the same NO levels and test conditions.\nResponse 12: To enable direct comparison, Table S3-4 summarizes the catalytic performance and AQYs measured at an initial NO concentration of 0.5 ppmv.\n(p11) “As listed in Tables S3-4, the photocatalytic performance and AQYs of e-FCN in NO decomposition under 20% O exceed those of previously reported catalysts, even at an initial NO 2 concentration as low as 0.5 ppmv.”\nComment 13: Move key supporting data (like Table S1, Figures S6-7) into the main text for clarity.\nResponse 13: The results of the elemental analysis (Table 1) and μs-TAS (Table 2) are presented in the main text for clarity.\nComment 14: State if F content stays the same after long testing, or if any F is lost.\nResponse 14: The results of the recycling tests indicate that e-FCN maintains its compositional integrity, suggesting high stability and reusability.\n(p12) “Post-cycling characterization via XRD, solid-state NMR, and XPS (Figure S13), along with elemental analysis (Table 1), confirmed the structural and compositional integrity of the e-FCN after 12 cycles.”\nTable 1. Elemental analysis of e-CN, e-OCN, and e-FCN.\nCatalyst C (wt.%)a N (wt.%)a F (μg·g-1)b C/N (at.%) F/C (at.%)\ne-CN 37.10 61.17 - 70.73 -\ne-OCN 37.35 60.90 - 71.53 -\ne-F(0.05)CN 37.27 61.02 235.92 71.23 0.04\ne-F(0.10)CNc 37.34 60.92 590.81 71.48 0.10\ne-F(0.15)CN 37.37 60.87 886.86 71.60 0.15\ne-F(0.20)CN 37.41 60.81 1124.52 71.75 0.19\ne-F(0.25)CN 37.43 60.75 1535.69 71.85 0.26\nUsed e-F(0.10)CNc 37.34 60.92 590.73 71.48 0.10\na The quantitative analysis of C and N was conducted using elemental analysis. b After digesting a measured quantity of the catalyst with concentrated HNO3 as an oxidant, the F content in the resulting solution was quantitatively analyzed via ion chromatography. This measurement was subsequently used to determine the F content in the original catalyst. c Unless otherwise specified, e-FCN refers to e-F(0.10)CN.\nReviewer 4's Comments:\nThe manuscript presents an intriguing strategy for achieving selective photocatalytic reduction of nitric oxide to dinitrogen under oxygen-rich conditions using a fluorine-modified polymeric carbon nitride. The authors claim high performance metrics in terms of NO conversion efficiency and N selectivity. The study is well-structured and uses a broad array of experimental and 2 theoretical tools. However, several major concerns need to be addressed to justify the significance and robustness of the claims.\nComment 1: Comparative benchmarking is lacking for such a big claim as ‘first successful…). How does the performance quantitatively compare against other recent reports and what features distinguish the current work compared to literature (AQY, RH conditions, loading etc.)?\nResponse 1: We delineated the distinctions between e-FCN and previously reported systems for photocatalytic NO removal.\n(p3-4) “While the photocatalytic oxidation of NO to NO - is well-established,22 achieving its 3 selective reduction to N without a chemical reductant in oxygen-rich environments remains 2 challenging.25 We demonstrated a breakthrough: the oxygen-tolerant, selective photoreduction of NO to N under visible light irradiation.” 2\nComment 2: The tests are performed at 20% O under dry conditions. However, this does not fully 2 simulate ambient air given that indoor environments possess varying humidity and gases. Authors must add additional data (or at least discuss) photocatalytic performance under variable RH (20- 60%) and presence of NO , SO , or CO .\nResponse 2: The effects of CO , NO , and SO on the photocatalytic NO decomposition were 2 2 2 systematically examined to evaluate the performance of an e-FCN catalyst under practical indoor conditions.\n(p13-14) “In practical indoor environments, air contains a certain level of relative humidity (RH), and NO pollutants often coexist with other gases, such as CO , NO , and SO .51 It is therefore 2 2 2 essential to evaluate the effects of RH and coexisting gases on photocatalytic NO decomposition over e-FCN. Figures S15-16 systematically examined these influences by testing e-FCN under common indoor RH levels and typical concentration ratios of NO to CO , NO , and SO . Figure 2 2 2 S15 indicated that RH has a minor effect on photocatalytic NO decomposition, suggesting that H O molecules do not significantly compete with NO for adsorption and activation sites on the 2 catalyst surface. Similarly, coexisting CO exhibited a negligible impact (Figure S16A-B). This 2 observation aligns with prior theoretical calculations indicating that the adsorption energy of NO on F-CN is significantly more favorable than that of CO , leading to the preferential adsorption of 2 NO.32 To evaluate the effect of NO , the total initial NO concentration was maintained at 2.0 ppmv 2 x while varying the NO proportion. The results indicate that neither the NO removal efficiency nor 2 x the N selectivity was significantly affected (Figures S16C-D). Furthermore, no NO was detected 2 2 in the effluent. In this study, the initial NO concentration exceeded that of the coexisting NO . 2 x Under these conditions, the two species primarily reacted via the fast SCR pathway in a 1:1 molar ratio, forming the unstable intermediate N O .52 This intermediate was then rapidly reduced by 2 3 photogenerated electrons. This reaction pathway explains the absence of NO in the effluent. The 2 presence of SO slightly inhibited the photocatalytic decomposition of NO (Figures S16E-F). This 2 inhibition can be attributed to SO acting as a scavenger for photogenerated holes, a mechanism 2 previously documented in aqueous reaction systems.53 The consumption of holes by SO disrupts 2 the charge balance within the photocatalytic system, thereby altering the pathway for NO decomposition.”\nFigure S16. Effect of (A, B) CO , (C, D) NO , and (E, F) SO on photocatalytic NO decomposition 2 2 2 over e-FCN in 20% O atmosphere. 2\nComment 3: The mass balance assumes equimolar N and O formation without direct 2 2 quantification of oxygen evolution in all cases. Authors must provide full molar balances for N- containing species. Is there any undetected nitrogen retained on the catalyst surface?\nResponse 3: The concentrations of N-containing products after 30 minutes of reaction over the three catalysts were summarized.\n(p7-8) “Table S2 summarized the concentrations of N-containing products after 30 minutes of reaction over the three catalysts. In all cases, the total N concentration, which was calculated as the sum of all N-containing products, matched the initial NO concentration, meeting the N balance throughout the reaction. The data in Table S2 also revealed that N was the dominant product in 2 NO decomposition over e-FCN, whereas N O and NO - were the primary products over e-CN and 2 3 e-OCN. Based on these results, the product selectivity at 30 minutes was determined, demonstrating that e-FCN exhibited significantly higher N selectivity (92%) compared with e- 2 CN (1%) and e-OCN (7%). Notably, e-OCN showed higher NO - selectivity (55%) than e-CN 3 (46%).”\nTable S2. Summary of N species and calculated total N concentrations over e-CN, e-OCN, and e- FCN following a 30-minute photocatalytic reaction.a\nCatalyst [NO] [N2O] [N2] [NO2] [NO3 -] [Total N]\n(ppmv) (ppmv) (ppmv) (ppmv) (ppm) (ppm)b\ne-CN 1.600 ± 0.039 0.059 ± 0.005 0.049 ± 0.004 0 0.185 ± 0.019 2.001 ± 0.044\ne-OCN 0.638 ± 0.040 0.072 ± 0.006 0.231 ± 0.012 0 0.755 ± 0.025 1.999 ± 0.049\ne-FCN 0.298 ± 0.040 0.010 ± 0.003 0.781 ± 0.023 0 0.119 ± 0.011 1.999 ± 0.048\na Reaction condition: 2.0 ppmv NO in 20% O2/Ar, LED lamp (λ = 425 nm), inlet flow rate = 10 mL∙min-1, RH = 0%; b [Total N] = [NO] + ([N2O] + [N2]) × 2 + [NO2] + [NO3 -].\nThe results presented in Table S2 confirm that N was balanced throughout the reaction, thereby excluding the possibility of significant undetected N-containing products.\nComment 4: The proposed mechanism is plausible, but the assignment of FTIR peaks to this species is speculative which must be supported with isotope labeling experiments.\nResponse 4: Peak assignments in the FTIR spectra were verified using 15NO as the reaction gas. The pathway for the NO photoreaction was then elucidated based on the in-situ FTIR results.\n(p23-25)\n“The NO/O adsorption results demonstrate that the presence of the C-F bond in CN suppresses 2 the formation of the C=O bond, rendering e-FCN inert towards O . To investigate the NO 2 photoconversion process in the presence of O , in-situ FTIR spectroscopy was employed to 2 monitor the chemical transformation process. The FTIR spectra of initial NO adsorption on e-CN, e-OCN, and e-FCN reveal distinct bands (Figure S27): one at 1690-1694 cm-1, corresponding to adsorbed (NO) species on the surface,68 and others at 1273 and 1411-1413 cm-1, indicative of 2 bridged (NO) species.17 These bands (1273, 1411-1413, and 1694 cm-1) increase gradually over 2 time (Figures S27A, C, E), and e-FCN consistently exhibits higher band intensities compared with e-CN and e-OCN, thereby confirming stronger NO binding on e-FCN (Figures S27B, D, F). The signals at 1273 and 1411-1413 cm-1 are typically attributed to the symmetric vibration of bridging bidentate NO -.69 However, the absence of corresponding signals for asymmetric vibrations in the 3 higher wavenumber range (1500-1700 cm-1) challenges this interpretation. Based on these spectral features, we attribute these two bands to the presence of a bridged (NO) dimer adsorbed onto the 2 catalysts.\nThe assignment of the observed feature to an (NO) dimer was confirmed unequivocally using 2 15NO isotope-substituted in-situ FTIR spectroscopy. As shown in Figure S28, the spectrum exhibited a corresponding redshift to lower wavenumber. The experimental ν(15N)/ν(N) ratios showed excellent agreement with theoretical values predicted for an (NO) dimer (Table S12), 2 thereby confirming this assignment.70 Complementary DFT calculations were employed to investigate NO adsorption behavior and co-adsorbed NO interactions on the e-FCN surface. One or two NO molecules were introduced onto the e-FCN surface slab (Figures 6C-F). Computed adsorption energies revealed a more stable configuration for the (NONO) dimer, with a total E (NONO) of -0.31 eV for the dimer system, compared with E (NO) = -0.21 eV for a single ads ads NO molecule (Figures 6C-D). This enhanced stability arises from both π back-donation to the NO ligands and direct attractive interactions between the co-adsorbed NO molecules. Charge density analysis indicated significant electron accumulation at the adsorption sites, increasing from 0.36 |e| for a single NO molecule to 0.69 |e| for the dimer system (Figures 6E-F), consistent with enhanced electron transfer from the e-FCN substrate. The total charge associated with the dimer was less than the sum of the two isolated NO adsorbates. This charge redistribution, combined with the adjacent geometry of the two N atoms, suggests a mechanism that mitigates inter- adsorbate repulsion and facilitates N-N coupling. The assignment of vibrational bands to an (NO) 2 dimer, confirmed through in-situ isotopic FTIR and theoretical calculations, indicates that N-N bond formation occurs via a partial overlap of electron density between adjacent N atoms.25\nUpon irradiating NO adsorbed on the catalysts, a new band appears at 2314 cm-1, indicating the presence of adsorbed N O on the catalyst surface, as shown in Figures S29A, C, E.71,72 Initially, 2 this band increases with irradiation time for e-CN, e-OCN, and e-FCN, suggesting the formation of N O as an intermediate in the decomposition of NO. However, after 18 minutes, the band begins 2 to decrease on e-FCN (Figure S29F), while it continues to rise on e-CN and e-OCN (Figures S29B, D). This contrasting behavior between the catalysts suggests that the N O intermediates undergo 2 further transformation on e-FCN but accumulate on e-CN and e-OCN. This observation is consistent with the finding that N O production during the photocatalytic NO conversion was 2 minimal on e-FCN but significant on e-CN and e-OCN (see Figure 2A). These results highlight the critical role of e-FCN in promoting the photocatalytic reduction of N O intermediates to N . 2 2 Notably, a band at 1555 cm-1 was observed for all catalysts under light irradiation (Figures S29A, C, E), indicating the accumulation of bidentate NO - on the catalyst surface.73 Among the catalysts, 3 e-OCN exhibits the highest NO - accumulation. This finding is consistent with the observation that 3 NO - selectivity for e-OCN is higher than that for e-CN and e-FCN (see above Figure 2A). 3\nDefinitive confirmation of the N O and NO - feature assignments was achieved using 15NO 2 3 isotope-substituted in-situ FTIR spectroscopy. The spectrum showed a corresponding redshift (Figure 8), and the experimental ν(15N)/ν(N) ratios excellently matched theoretical values for an N O and NO - (Table S12), validating the assignment.70 The band area changes of 15N O and 2 3 2 15NO - was also consistent with that of N O and NO -. Consequently, it is revealed that NO 3 2 3 photoconversion in O proceeds predominantly via an oxidative pathway on e-CN/e-OCN but 2 shifts to a reductive pathway on e-FCN.”\nFigure 8. The time evolution of in-situ FTIR spectra for 15NO photoreduction over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN under light irradiation in 20% O atmosphere. 2\nFigure S28. The time evolution of in-situ FTIR spectra for 15NO adsorption over (A, B) e-CN, (C, D) e-OCN, and (E, F) e-FCN in 20% O atmosphere. 2\nComment 5: The study shows stability over 6 cycles, but NO - accumulation must be quantified 3 per cycle and authors should aim for higher stability demonstration beyond 6 cycles (e.g., 10-15) to assess long-term operational efficacy.\nResponse 5: The cycle times for e-OCN and e-FCN were extended from 6 to 12, and the accumulated NO - amount per cycle was quantified for each catalyst. 3\n(p12) “In durability tests, e-FCN demonstrated consistent NO removal efficiency and N 2 selectivity over 12 consecutive cycles (Figure 4A). In contrast, e-OCN exhibited a gradual decline in both NO removal efficiency and N /NO - selectivity (Figure S12), with detectable NO 2 3 2 formation emerging from the second cycle onward (Figure S12B). Furthermore, measurements of accumulated NO - on the catalyst surfaces revealed significantly higher deposition on e-OCN 3 compared to e-FCN (Figure 4B). These results suggest that e-OCN primarily oxidizes NO to NO -, 3 leading to catalyst deactivation via NO - accumulation, whereas e-FCN predominantly 3 decomposes NO into N and O , minimizing NO - generation and maintaining stable performance.” 2 2 3\nFigure 4A-B. (A) Durability of NO decomposition on e-FCN during the 12 repeated cycles. (B) Comparison of NO - accumulation on e-OCN and e-FCN over 12 repeated cycles. 3\nNotably, a method was established to quantify the accumulated NO - amount on a per-cycle 3 basis.\n(p36) “To assess the accumulation of NO - on the catalyst surface over 12 consecutive cycles, 12 3 parallel experiments were conducted, each subjected to 1-12 reaction cycles. Upon completion, the NO - content adsorbed on the catalyst surface was quantified.” 3\nComment 6: In this work, the reactor uses artificial gas mixtures. However, ambient conditions often include fluctuating light, temperature and humidity. Such experiments must be provided to pave the way towards real indoor applications.\nResponse 6: Initially, we monitored the catalyst surface temperature during the photocatalytic reaction and investigated the thermal effects on NO decomposition.\n(p9) “The catalyst surface temperature was monitored using a thermocouple. As shown in Figure S7A, the temperature increased from 25°C to 35°C during the photocatalytic reaction and stabilized thereafter, indicating that the LED light source did not induce significant heating of the catalyst. To exclude thermal contributions to NO decomposition, a control experiment was performed at 35°C in the absence of light. Figure S7B shows that all catalysts exhibited negligible activity under these conditions, confirming that NO decomposition was exclusively photocatalytic rather than thermally driven.”\nFigure S7. (A) Monitored surface temperature of catalysts during photocatalytic NO decomposition in 20% O atmosphere. (B) NO decomposition under dark conditions at 35°C in 2 20% O atmosphere. 2\nWe subsequently conducted a separate 3-hour stability test at RH = 45% under variable light intensity.\n(p13) “The NO removal efficiency and N selectivity fluctuated in response to changes in light 2 intensity but remained generally stable under RH = 45% (Figure 4D). As previously demonstrated (Figure S7B), thermal effects are negligible for NO decomposition at 35°C, eliminating temperature as a variable. Consequently, the observed performance fluctuations are primarily attributed to changes in light intensity, which directly modulate the yield of photogenerated charge carriers and thus the reaction kinetics within a certain range.50”\nFigure 4D. The 3-hour durability assessment of e-FCN for photocatalytic NO decomposition under RH = 45% and variable light intensities.\nComment 7: Data on exfoliation, particle size and morphology (change) on each modification step must be provided.\nResponse 7: Successful exfoliation was confirmed by atomic force microscopy (AFM) and dynamic light scattering (DLS).\n(p5) “X-ray diffraction (XRD) analysis (Figure S1A) revealed two characteristic peaks at 13.0° and 27.4°, corresponding to the (100) and (002) crystallographic planes, which are indicative of in-plane tri-s-triazine units and interlayer stacking, respectively.37 A notable broadening of these peaks in the exfoliated catalysts suggests a reduction in crystallite size.36 Atomic force microscopy (AFM) confirmed successful exfoliation, showing a decrease in average particle thickness from 40.1-48.2 nm for the bulk catalysts to 2.3-4.9 nm for the exfoliated catalysts (Figure S2). Based on a theoretical monolayer thickness of 0.325 nm for CN,38 this corresponds to a reduction from approximately 124-148 layers to merely 7-15 layers. This finding was corroborated by dynamic light scattering (DLS), which showed a substantial decrease in the average hydrodynamic diameter from approximately 900 nm to 120 nm post-exfoliation (Figure S3). Collectively, these results confirm the successful exfoliation of the bulk CN-based catalysts.”\nFigure S2. AFM images and height profiles along the lines of both bulk and exfoliated catalysts.\nFigure S3. DLS size distribution for aqueous dispersions of both bulk and exfoliated catalysts.\nComment 8: Even though it is common in photocatalysis field to show results obtained from single experiment, average results with error bars obtained from minimum 3 experiments (especially on identifying selectivity) can improve reliability of data.\nResponse 8: The catalytic experiments were repeated in triplicate, and the results are presented with error bars in the corresponding Figures and Tables. In some cases, error bars are shown but are smaller than the symbol size.\nComments 9/14: Time-resolved PL data were collected under ambient conditions but can charge carrier dynamics be examined under photocatalytic reaction conditions (e.g., in NO/O /Ar 2 atmospheres) to better reflect catalytic behavior under given experimental conditions.\nResponse 9/14: Given the overlap between Comment 9 and Comment 14, we provide a single consolidated response below. Microsecond transient absorption spectroscopy (μs-TAS) was employed to probe the charge carrier dynamics of exfoliated CN-based catalysts in NO/O /Ar 2 atmospheres.\n(p16-17)\n“To investigate the charge carrier dynamics of exfoliated CN-based catalysts under relevant conditions, microsecond transient absorption spectroscopy (μs-TAS) was employed, as its temporal resolution aligns with the timescales of the actual reaction kinetics. To distinguish the nature of the charge carriers, aqueous dispersions of e-CN were probed at 900 nm both with and without ethylenediaminetetraacetic acid dipotassium (EDTA). This well-established hole scavenger reveals the signature of long-lived electrons. As shown in Figure S21A, the signal half- life (t ) of e-CN was significantly prolonged from 35.5 μs to 63.6 μs upon the addition of EDTA. 50% This observation confirms that the transient signal originates from photoinduced electrons, consistent with prior literature.59\nSubsequently, the electron transfer kinetics of e-CN, e-OCN, and e-FCN were probed by introducing NO/O /Ar atmospheres. Figures 5A-B and S21B demonstrated a decrease in the t 2 50% values for all three catalysts under the NO or O atmosphere compared with the Ar atmosphere, 2 confirming that electrons participate in activating both NO and O .60 However, the catalysts 2 exhibited distinct behavioral trends: e-CN showed similar reductions in t with NO or O , 50% 2 indicating no significant specificity (Figure S21B). On the other hand, e-OCN exhibited a larger reduction in t with O than NO (Figure 5A), whereas e-FCN showed a substantial reduction 50% 2 with NO than O (Figure 5B). These trends indicate that e-OCN preferentially facilitates the 2 electron transfer to O , whereas e-FCN favors the electron transfer to NO. To quantify this 2 selectivity, the electron transfer efficiencies for O and NO, denoted as ETE(O ) and ETE(NO), 2 2 were calculated from the t values according to an established literature method.61 As listed in 50% Table 2, e-FCN exhibited the highest ETE(NO) (30.2%) and the lowest ETE(O ) (6.2%), 2 confirming its superior selectivity for activating NO over O . This finding directly correlates with 2 the observed catalytic performance, wherein e-FCN demonstrates an ability to reduce NO to N 2 even in an oxygen-rich atmosphere.”\nFigure 5A-B. μs-TAS decay kinetics of (A) e-OCN and (B) e-FCN in Ar, O , and NO atmospheres. 2\nTable 2. Data of μs-TAS decay kinetics for e-CN, e-OCN, and e-FCN under different atmospheres.\nCatalyst t (Ar) (μs) t (O ) (μs) t (NO) (μs) ETE(O ) (%) ETE(NO) (%) 50% 50% 2 50% 2\ne-CN 32.5 29.0 30.3 10.8 6.8\ne-OCN 27.3 20.2 24.3 26.0 11.0\ne-FCN 24.2 22.7 16.9 6.2 30.2\na ETE(O ) = [1 - (t (O )/t (Ar))] × 100%; b ETE(NO) = [1-(t (NO)/t (Ar))] × 100%. 2 50% 2 50% 50% 50%\nComment 10: In some places it is mentioned that increased NO adsorption on e-FCN based on TPD and FTIR results, but is it possible to quantify surface coverage of NO and O (in µmol·m-2) 2 to assess catalyst efficiency per site?\nResponse 10: For TPD, the NO/O adsorption capacities of the catalysts were quantified. 2\n(p20) “The TPD peak areas were quantified by calibration with a standard gas mixture to establish the relationship between peak area and adsorbed gas concentration. As summarized in Table S11, this quantitative analysis confirms that e-FCN possesses the highest NO adsorption capacity (2.62 μmol·g-1) and the lowest O adsorption capacity (0.20 μmol·g-1), indicating a preferential 2 adsorption of NO over O .” 2\nQuantifying absorption peaks in in-situ FTIR spectroscopy with high accuracy is inherently challenging. This study analyzes the dynamic behavior of surface species by tracking the temporal evolution of their characteristic FTIR peak areas. Consequently, the analysis is restricted to relative comparisons of concentration changes on a single catalyst and precludes quantitative comparisons of peak intensities between different catalysts. This approach therefore does not necessitate strictly quantitative intensity measurements.\nComment 11: The bandgap of e-FCN is given, but band positions relative to NO reduction potential are not clearly shown. Flat band potential and/or valence/conduction band positions must be given and compared to NO/N O/N redox potentials?\nResponse 11: The influence of F modification on the photocatalytic reaction was 2 2 thermodynamically assessed by determining catalyst energy levels and standard NO/N O/N redox 2 2 potentials.\n(p15-16) “Furthermore, determining the energy levels of these catalysts is essential for assessing how F modification influences the photocatalytic reaction from a thermodynamic perspective. The bandgap values, derived from Tauc plots, are 2.52 eV for e-CN, 2.05 eV for e-OCN, and 2.46 eV for e-FCN (Figure S20A). The valence band (VB) edge positions, approximated from XPS valence band spectra (Figure S20B) according to the literature,39 are +1.83 V (V denotes the RHE RHE potential relative to the reversible hydrogen electrode) for e-CN, +1.52 V for e-OCN, and +1.74 RHE V for e-FCN. Mott-Schottky plots measured at 1000 Hz (Figure S20C) exhibit positive slopes, RHE confirming n-type semiconductor behavior for all catalysts. The flat band potentials (E ) are fb determined to be -0.69 V for e-CN, -0.53 V for e-OCN, and -0.72 V for e-FCN; these RHE RHE RHE values correspond to the conduction band (CB) edge levels.54 An energy level diagram constructed from these findings is presented in Figure S20D. The more negative CB edge of e-FCN (-0.72 V ), compared with e-CN (-0.69 V ) and e-OCN (-0.53 V ), indicates that e-FCN possesses RHE RHE RHE a stronger driving force for the reduction of NO/N O to N [standard reduction potentials: 2 2 E(NO/N ) = 1.68 V 55 and E(N O/N ) = +1.77 V 56]. However, thermodynamically, this more 2 RHE 2 2 RHE negative CB also provides a stronger driving force for the reduction of O to superoxide radical 2 [•O -, E(O₂/•O -) = -0.13 V ]57 or H O [E(O /H O ) = 0.68 V ]58. Thus, to resolve this 2 2 RHE 2 2 2 2 2 RHE thermodynamic conflict and identify the origin of the observed oxygen tolerance, subsequent investigations must focus on the charge carrier dynamics in NO/O /Ar atmospheres.” 2\nComment 12: Could water displace NO from active sites through adsorption?\nResponse 12: We highlighted the necessity of investigating humidity and demonstrated that relative humidity (RH) exerted only a minor influence on photocatalytic NO decomposition.\n(p13) “In practical indoor environments, air contains a certain level of relative humidity (RH), and NO pollutants often coexist with other gases, such as CO , NO , and SO .51 It is therefore essential 2 2 2 to evaluate the effects of RH and coexisting gases on photocatalytic NO decomposition over e- FCN. Figures S15-16 systematically examined these influences by testing e-FCN under common indoor RH levels and typical concentration ratios of NO to CO , NO , and SO . Figure S15 2 2 2 indicated that RH has a minor effect on photocatalytic NO decomposition, suggesting that H O 2 molecules do not significantly compete with NO for adsorption and activation sites on the catalyst surface.”\nFigure S15. Effect of RH on photocatalytic NO decomposition over e-FCN in 20% O atmosphere. 2\nThe aforementioned findings demonstrate that the adsorption of an H O molecule is insufficient 2 to displace an adsorbed NO molecule from the active sites.\nComment 13: Exfoliation is said to weaken π-π interactions resulting in improved performance, but is there a method/data to prove the claim?\nResponse 13: The enhancement of exfoliation facilitated by electron-withdrawing O and F was elucidated initially.\n(p5) “Both AFM and DLS revealed a consistent exfoliation trend across the catalyst series. AFM measurements indicated that the particle thickness decreased in the order (Figure S2): e-CN (4.3- 4.9 nm) > e-OCN (2.8-3.4 nm) > e-FCN (2.3-2.7 nm). This trend was corroborated by DLS, which showed a corresponding decrease in hydrodynamic diameter (Figure S3): e-CN (133.1 ± 1.7 nm) > e-OCN (122.7 ± 1.3 nm) > e-FCN (110.6 ± 1.1 nm). Charge distribution calculations confirmed the strong electron-withdrawing nature of the O and F atoms, which generated a highly positive C center (Figure S4). The greater electronegativity of F resulted in a more positive charge on this C center in the FCN model (+1.81) compared with that in the OCN model (+1.70). We therefore propose that this electron-withdrawing effect is the primary driver of enhanced exfoliation. The incorporation of O or F into the CN matrix attenuates the interlayer π-π interactions, thereby facilitating delamination. The superior efficacy of F stems directly from its higher electronegativity, which more effectively disrupts these cohesive forces and promotes exfoliation.”\nFigure S4D, F. Charge distribution of the (D) e-OCN and (F) e-FCN. The C, N, H, O, and F atoms are represented by dark grey, white, pink, red, and cyan spheres.\nThe BET surface areas and pore volumes of the catalysts were subsequently compared.\n(p19) “As summarized in Table S10, the Brunauer-Emmett-Teller (BET) surface area and pore volume increase in the order: e-CN < e-OCN < e-FCN. The preceding AFM and DLS analyses confirmed the successful exfoliation of the CN-based catalysts (Figures S2-3). The introduction of electronegative O and F atoms effectively weakened the interlayer π-π interactions, thereby facilitating exfoliation. Owing to its higher electronegativity, F exerted a more pronounced effect, as evidenced by the decreasing order of both average particle thickness and average hydrodynamic particle size: e-CN > e-OCN > e-FCN. For layered materials, the specific surface area is primarily governed by the exposed surfaces of the exfoliated nanosheets. Theoretically, greater exfoliation reduces both the average number of layers and the overall thickness, resulting in a larger specific surface area.36 The observed BET surface area and pore volume trend aligns well with this principle.”\nFinally, the catalytic performance of the bulk and exfoliated catalysts was compared, demonstrating that exfoliation enhances catalytic performance.\n(p19) “To evaluate the influence of exfoliation on photocatalytic NO decomposition activity, bulk CN-based photocatalysts were also prepared and tested as control samples under the identical reaction conditions. As illustrated in Figures S24A-D, the photocatalytic NO removal efficiency and N selectivity of the bulk catalysts were significantly lower than those of the exfoliated 2 catalysts in both Ar and 20% O . The surface area and pore volume of the bulk catalysts are 2 substantially smaller than those of the exfoliated catalysts (Figures S23A-B and Table S10). This suggests that exfoliation increases the number of active sites exposed on the catalyst surface, thereby enhancing NO adsorption and subsequent activation.36”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The author has addressed my comments; thus, it can be accepted in the current form.", "reviewer_2": "The author has provided substantial evidence to address the reviewers' comments, and I have no further questions.", "reviewer_3": "The authors have satisfactorily addressed the reviewers’ concerns, and the manuscript is now suitable for publication in its current form.", "reviewer_4": "I am convinced with the additional tests and explanations provided by authors and can recommend for publication in this stage."}, "author_response": ""}]

Decision letter