Single iron redox sites boost Methanol-SCR at low temperature

Domain: physics
Published: 2025-12-20
Editorial outcome: major_revision

Abstract
Selective catalytic reduction of NO to N by methanol as a reducing agent presents a prospective solution for the removal of NO from the exhaust of methanol engines and the coal-fired power plant, but it confronts the problem of insufficient deNO activity at low temperatures (<350 °C). Here, we discover a strategy for boosting the activity of low-temperature Methanol-SCR by the collaboration of zeolitic acid sites with single iron redox sites. The further pilot-scale bench test using the coated monolithic catalyst also exhibits a remarkable NO conversion and a high stability at low temperature. The location of different Fe sites in FER zeolite is identified by X-ray absorption spectroscopy, Mössbauer spectroscopy, as well as 2D 1H-1H DQ MAS NMR technology. While the dynamic evolution of [FeO]+ as the critical redox site for NO oxidation is successfully captured by in situ Mössbauer spectroscopy, which unravels the mechanism for the generation of key intermediate HONO on [FeO]+ sites contributing the low-temperature Methanol-SCR activity. These results indicate the great potential application of Fe-FER zeolite catalyst in industrial Methanol-SCR and provide an atomic-level comprehension of how the dual-active-sites contribute to low-temperature Methanol-SCR activity.

Introduction
The application of methanol as a leading alternative-fuel candidate is growing steadily in the transportation field owing to its economy and easy-to-store. Approximately 90% less CO emissions running on methanol engines using green methanol than conventional liquid fuels are very attractive under the objective of carbon neutrality. Despite the utilization of methanol fuel drastically reduces sulfur and particulate emissions, the emission of NO is still existing issue. Selective catalytic reduction technology by NH (NH -SCR) is currently one of the most widely used technologies for NO abatement. However, in the exhaust gas emitted from methanol as fuel, toxic HCN will be generated in NH -SCR process inevitably as a consequence of the side reaction of NH with HCHO. Moreover, NH -SCR process also confronts a difficulty for the deNO treatment in some applications scenarios containing high concentration of SO , such as coal-fired power plants and marine diesel engines using heavy fuel oil (HFO). Ammonium sulfates are known to be a deactivation species for SCR catalysts according to the inevitable side reaction between NH and SO , which usually leads to the deactivation of catalysts and pipe blockage. Methanol as a reducing agent instead of NH can avoid the formation of ammonium sulfates and the utilization of methanol from exhaust directly as a reducing agent for NO removal, not only enhancing the safety of the deNO process, but also saving space for the storage of deNO reductant. Accordingly, selective catalytic reduction NO to N by methanol (Methanol-SCR) is a prospective technology for the deNO treatment both in the combustion of conventional fuels and promising methanol fuel.

Methanol-SCR catalysts have been developed by supported metal or metal oxide, such as Al O -based catalysts and zeolite-based catalysts. However, the deNO efficiency for Methanol-SCR on the above-reported catalysts is nowhere close to that of NH -SCR on the commercialized V-based oxide deNO catalysts, especially at low temperature (<350 °C). The rational design of catalysts with high activity requires guidance based on an understanding of the structural features, which is vital for establishing the relationship between the structure of the catalyst and its performance during the reaction. Generally, the strong acidity and excellent redox property are two crucial factors for designing NH -SCR catalysts. Strong and abundant acidity is conducive to the adsorption of NH /NH + species, which react either with adsorbed nitrites/nitrates following the Langmuir-Hinshelwood (L-H) mechanism or directly with gaseous NO following the Eley-Rideal (E-R) mechanism. Prominent redox ability is favorable to the formation of active NH or gaseous NO intermediates, which can improve the deNO efficiency through the E-R mechanism or “Fast SCR”, respectively. Likewise, the acidity and redox properties are pivotal for the Methanol-SCR catalyst to adsorb/activate methanol and NO , respectively. The framework aluminum sites in zeolites, namely Brønsted acid sites (BASs), can effectively control the adsorption and activation of methanol molecules. The presence of extra-framework aluminum (EFAL) species as Lewis acid sites (LASs) in the pores of zeolite has also been reported to accelerate the activation of methanol by exerting a big influence on the acid strength of BAS. Abundant and strong acidity contributes to the adsorption/activation of methanol molecules is essential for Methanol-SCR catalysts, has been reported according to our previous research. However, the direct synthesis of H-FER zeolite with the controllable amounts of extra-framework aluminum (EFAL) species is very difficult and with poor repeatability, thus it is not conducive to industrial applications. Additionally, Bi-FER zeolite with the controllable Bi amounts was prepared for improving the low-temperature Methanol-SCR activity, while the yield of N O as the by-product was also highly enhanced. Therefore, a potential industrial Methanol-SCR catalyst with remarkably low-temperature activity and low cost is still highly required.

HONO has also been proposed as an intermediate participant in the Methanol-SCR reaction by adsorbing on EFAL species to form nitrites, which can further react with methyl groups produced on BASs to generate the key intermediate CH NO . The formation of HONO is closely related to NO, NO , and H O. Redox sites are favorable to the formation of active intermediates during the NH -SCR reaction. Therefore, excellent redox properties to promote the NO oxidation and the generation of HONO by the reaction of NO with H O are equally important in Methanol-SCR catalysts.

Herein, we developed a series of Fe-FER zeolites as Methanol-SCR catalysts, of which the structural features contain the acidity of the zeolite and the redox property of Fe sites. The introduction of Fe enhances the catalytic activity of Methanol-SCR on FER zeolite remarkably at the low temperature region (250~350 °C). Atomic-level exploration of Fe sites under SCR remain obscure due to the complexity of Fe active sites and a lack of direct observation of the catalytic performance on specific sites. But in this work, the geometric configurations and atomic compositions, as well as the location of different Fe sites in FER zeolite, were distinguished precisely by multi-characterization methods such as spherical aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy, as well as 2D 1H-1H double quantum Magic Angle Spinning Nuclear Magnetic Resonance (2D 1H-1H DQ MAS NMR) technology. The dynamic evolution of [FeO]+ to Fe2+ sites during the NO oxidation was also probed by performing the in situ Mössbauer spectroscopy. Combined with the in situ FTIR and theoretical calculations based on density functional theory (DFT), the reaction mechanism of [FeO]+ sites enhances the Methanol-SCR activity by promoting the NO oxidation and generation of key intermediate HONO was revealed, which broadens the insight for low temperature Methanol-SCR catalyst design.

Results
Synthesis and characterization of Fe-FER zeolite
Fe-FER zeolites have been successfully prepared using the hydrothermal synthesis method by means of ligand-stabilized metallic species precursors mixed with synthesis gel during the crystallization. Pyrrolidine is a preferred organic structure direct agent (OSDA) of FER zeolite to complex with iron ions, enabling to achieve atomically dispersion of Fe sites in crystalline zeolite. As shown in Fig. 1a, 0.6Fe-FER zeolite, as a representative sample, exhibits a typical plate-like shape of FER zeolite and no bright nanoparticles can be observed by HAADF-STEM image, despite highly dispersed Fe sites existing in the FER zeolite, as corroborated by the energy dispersive X-rays spectra (EDS) mapping images. Spherical aberration-corrected HAADF-STEM technology was exploited to further investigate the state of Fe sites. The microporous of zeolite, along with eight-membered ring channels, can be clearly visualized, and the bright spots observed at the higher accelerating current reveal the presence of atomically dispersed Fe sites in the 0.6Fe-FER zeolite. Nevertheless, highly dispersed tiny nanoparticles gradually appear as the iron content increases to 1.2 wt%. The UV–vis spectra of 1.2Fe-FER also reveal the presence of small oligomeric Fe O clusters at 346 nm, in addition to isolated Fe ion sites at 226 and 272 nm. X-ray absorption near edge structure (XANES) of Fe K-edge for the samples with Fe contents of 0.4, 0.6 and 1.2 wt% are closer to that of the reference Fe O , suggesting that Fe3+ is the major component in Fe-FER zeolites. Although linear combination analysis (LCA) of XANES spectra is a straightforward and widely used method in catalysis for estimating the Fe3+/Fe2+ ratio, the use of well defined reference compounds such as bulk oxides as standards often fails to accurately replicate the XANES features of actual catalyst samples due to differences in local coordination environments. The pre-edge peak in the spectra corresponds to the dipole-forbidden 1s→3d transition. The position of the pre-edge peak can also be used as a direct measure of the Fe oxidation state, making an unambiguous distinction between Fe2+ (~7112.5 eV) and Fe3+ (~7113.5 eV). Graphical analysis of the correlation between integrated intensities and positions of pre-edge features in the variogram was employed to further discriminate various oxidation states and coordination environments of Fe. Since the 1s→3d transition is a quadrupolar transition, it exhibits weak intensity in coordination geometries with an inversion center (centrosymmetric geometry), such as octahedral coordination. But in fivefold or fourfold geometry, 3d–4p orbital hybridization facilitates dipole-allowed 1s→4p transitions, resulting in significantly higher intensity. As shown in Fig. 1c and Fig. 1d, the higher integrated intensities of Fe-FER zeolites pre-edge analysis than Fe3+ Oh, indicating that Fe sites in a lower geometry in Fe-FER zeolites than the bulk α-Fe O . The Fe3+/Fe2+ ratio obtained by curve fitting analysis of pre-edge peak in Fe-FER zeolites exhibits an increasing trend with increasing iron loading. It can also be demonstrated in the phase-uncorrected Fourier transform X-ray absorption fine structure (FT-EXAFS) spectra that the first contribution of Fe-FER zeolites is closer to the Fe-O scattering path and the absence of the second contribution demonstrates that Fe sites are highly dispersed in FER-zeolites in the form of coordination with oxygen. According to curve-fitting results as shown in Table S3, the average distance and coordination number of Fe-O for 0.6Fe-FER zeolite are estimated to be 2.05 Å and 4.7, respectively, which are consistent with the results of pre-edge analysis. Although the Fe–O–Fe peaks are clearly observed in α-Fe O , they are difficult to detect in the second-shell contributions of zeolite catalysts when the metal species or oxide particles are very small and highly dispersed. As for the FT-EXAFS spectra of 1.2Fe-FER, similarly, the absence of the second contribution can also be perceived, demonstrating that although the presence of nanoparticles in 1.2Fe-FER, they are still highly dispersed as iron oxide nanoparticles without any apparent agglomeration. Moreover, the absence of higher R-space signals by the wavelet transform analysis also validate the high dispersion of Fe sites.

Mössbauer spectroscopy is an effective technique to provide reliable information on the forms of iron sites and the environment where iron ions exist in zeolite by measuring the hyperfine interaction between nuclear moments and electromagnetic fields. Consequently, Mössbauer spectroscopy was further employed to precisely characterize the Fe sites in 57Fe-FER zeolites. Mössbauer spectra are shown in Fig. 1f, and the corresponding Mössbauer parameters are listed in Table 1. The species and locations of iron can be pinpointed by the hyperfine parameters of isomer shift (IS) and quadrupole splitting (QS). Typically, 0.1 ≤ IS < 0.6 mm/s and 0.6 ≤ IS ≤ 1.4 mm/s are assignable to Fe3+ and Fe2+ component, respectively. The value of QS is correlated with symmetry. Larger QS values represent the worse symmetry around iron ions. As shown in Table 1, the spectral contributions of Fe-FER samples advocate that Fe3+ is the major component in Fe-FER zeolites. Concomitantly, based on the results of 27Si MAS NMR and 27Al MAS NMR, the introduction of Fe3+ has no effect on the framework of zeolite, indicating that Fe3+ is primarily in the extra-framework of zeolite. FER zeolite comprises ten-membered ring channels (4.2×5.4 Å) intersected by eight-membered ring channels (3.5×4.8 Å). As depicted in Fig. 1g, four characteristic positions denoted as B, C, F, and G can be occupied by isolated cations in FER zeolite as reported in previous literatures and the C-sites remain unoccupied at low iron concentration.

Three kinds of iron sites were found to be present in the 0.4Fe-FER and 0.6Fe-FER zeolites. The component I of 0.4Fe-FER zeolite with IS=0.28 mm/s and QS=1.53 mm/s and 0.6Fe-FER zeolite with IS=0.25 mm/s and QS=1.7 mm/s can be assigned to the [FeO]+ at the six-membered ring (G-site). The component II of 0.4Fe-FER zeolite with IS=0.52 mm/s and QS=2.83 mm/s and 0.6Fe-FER zeolite with IS=0.5 mm/s and QS=2.8 mm/s has a high value of QS, suggesting that the component is in a position that confers it less symmetry. Accordingly, the Fe3+ at the B-site can be assigned to the component II, as reported previously. The component III of the Fe-FER zeolites with IS>0.6 mm/s is assigned to Fe2+, while the lower value of QS (<0.5 mm/s) indicates that Fe2+ is most likely located at the G-site and stabilized with the negative charge provided by the two aluminum. Compared to 0.4Fe- and 0.6Fe-FER zeolite, the component IV with IS=0.30 mm/s and QS=1.10 mm/s was observed in the 1.2Fe-FER zeolite, which is assignable to the tiny Fe O nanoparticles in accordance with the results of TEM and the reference. Collectively, single Fe3+ sites consist mainly of [FeO]+ at G-sites and Fe3+ at B-sites in Fe-FER zeolites, whereas tiny Fe O nanoparticles appear as the Fe content increases to 1.2 wt%. The perturbed T-O-T vibrational modes (1000~800 cm−1 in the IR spectra) of the zeolite framework also provide an effective method to distinguish different Fe sites. The IR bands at 903, 895, 885, and 873 cm−1 are attributed to T-O-T oxygen bridges perturbed by Fe2+, Fe3+, [FeO]+, and small oligomeric Fe O clusters, respectively. The quantitative results are in good agreement with those obtained from the Mössbauer spectra.

The exploitation of 2D 1H-1H DQ MAS NMR spectroscopy has enriched the material characterization methods at the atomic scale, allowing for in-depth exploration of the proximity between active sites, which has often been interpreted as ‘the closer the better’ for locating metal and acid sites. Two off-diagonal cross-peak pairs at (4.0, 6.7) and (2.7, 6.7) ppm, as well as (2.9, 5.4) and (2.5, 5.4) ppm corresponding to the adjacent Si-O(H)-Al sites (BASs) and EFAL species (Al-OH) can be distinguished in H-FER zeolite, in addition to the diagonal autocorrelation peaks corresponding to close proximity of Si-O(H)-Al sites at (3.9, 7.8) ppm and silanols at (1.7, 3.4) ppm. The absence of autocorrelation peak at (3.9, 7.8) ppm in Fe-FER zeolites regardless of the amount of iron as shown in Fig. 2b–d suggests that Fe sites prefer to occupy one of the adjacent Si-O(H)-Al sites, resulting in the disappearance of autocorrelation peaks of BASs, as depicted in Fig. 2e. The reduced amount of BASs and enhanced amount of LASs in Fe-FER zeolites quantified by the FT-IR spectra of NH adsorption also confirm that Fe sites, substituting proton counter-cations, exhibit Lewis acid properties by acting as electron pair acceptors. The amounts of BASs determined by the Al sites framework are consistent with the results obtained from the FT-IR spectra of NH adsorption. Moreover, the content of Si-O(H)-Al sites at 3.9 ppm decreases 54% to 20% as the iron content increases in 1H MAS NMR results and the reduction of desorption band at 300~600 °C measured by temperature-programmed desorption of NH (NH -TPD) experiment also suggesting that Si-O(H)-Al sites are partially occupied by Fe sites. No significant difference is observed before 650 °C between the TCD and MS signals, indicating that ammonia decomposition does not occur in Fe-FER zeolites. The BASs formed by framework aluminum in zeolites are primarily located within the internal channels. The introduction of iron species mainly exists as ion sites that replace BASs and are incorporated into the zeolite channels. In contrast, EFAL species are also distributed on the external surface of the zeolite. Therefore, the introduction of iron has a more significant impact on the BASs compared to the EFAL species.

Catalytic performance of Fe-FER zeolites
Since the excellent redox properties are crucial for the SCR reaction, the NO oxidation performance of the Fe-FER zeolites was first investigated. The conversion of NO increases gradually can be observed with the increment of iron content and the activation energy for NO oxidation of the 0.6Fe-FER zeolite (E = 31.2 kJ/mol) is lower than that of the H-FER zeolite (E = 56.6 kJ/mol), suggesting that the introduction of Fe sites is conducive to the oxidation of NO. Concomitantly, investigations were steered towards the correlation between the content of different Fe sites and the reaction rate of NO oxidation. The reaction rates for NO oxidation, which excludes the limitations of internal and external diffusion, manifest an immensely good linear relationship with the amount of [FeO]+ present in Fe-FER zeolites. On the contrary, neither the amount of the Fe3+ nor the Fe2+ correlates with the reaction rate of NO. Reaction order with respect to NO was measured over the 0.6Fe-FER zeolite by analyzing the reaction rate dependency on reactant concentration. As shown in Fig. S14, the reaction rate was found to have linear dependence with respect to NO, with the slope of the linear fit being approximately 1, indicating that the reaction follows first-order kinetics.

In situ Mössbauer and FT-IR spectroscopy were employed to monitor the changes of Fe sites during the NO oxidation reaction. Mössbauer spectra are shown in Fig. 3c and the corresponding Mössbauer parameters are listed in Table 2. Firstly, the catalyst was oxidized under the reaction atmosphere of 14% O /N . With the introduction of 1000 ppm NO/N, the content of component I decreases from 53% to 10%, accompanied by the appearance of a Fe2+ site (component V), signifying that the [FeO]+ was reduced to Fe2+. Compared to component I, the insignificant change in QS values of component V points to the [Fe2+-NO −]+ structure maybe formed at the G-site. When 14% O /N was introduced instead of NO, the oxidation reaction from Fe2+ to [FeO]+ took place according to the disappearance of component V in conjunction with the component I increasing from 10% to 52%. Time-resolved in situ FT-IR spectra on 0.6Fe-FER zeolite were also employed as a complementary technique to monitor changes in Fe sites during the NO oxidation reaction. The introduction of 1000 ppm NO/N induced a distinct shift of the T–O–T vibrational peak to higher wavenumbers in IR spectra, suggesting increased formation of Fe2+ species with a significant decrease in the [FeO]+ site. This shift is accompanied by the emergence of IR bands at 1870, 1590, and 1478 cm−1, which are attributed to Fe2+–NO, nitrates, and Fe2+–NO − species, respectively. Accordingly, the IR band at 909 cm−1 can be assigned to T–O–T oxygen bridges perturbed by Fe2+–NO − species. When 14% O /N was introduced instead of NO, a time-dependent shift of the T–O–T vibrational peak to lower wavenumbers was observed, accompanied by the appearance of a band at 1625 cm−1 (attributed to NO adsorption) and a decrease in the Fe2+–NO − species at 1478 cm−1. These observations suggest that NO oxidation proceeds via Fe2+–NO − as an intermediate, as depicted in Fig. 3e, which is in good agreement with the results of the Mössbauer spectra. The significant variance observed in [FeO]+ demonstrates its pivotal role as an active site in the NO oxidation reaction. The [FeO]+ site can react with NO to form [Fe2+-NO −]+ intermediate. Subsequently, NO is generated via electron transfer from NO ⁻ to the oxygen atom (originating from O ), forming the [Fe2+-O−]+ intermediate. Further electron transfer from Fe2+ to O⁻ facilitates the formation of [FeO]+. Time-resolved in situ FT-IR experiment was also conducted to investigate the influence of methanol on Fe sites under reaction conditions. As shown in Fig. S15, the introduction of methanol causes no noticeable change in the perturbed T–O–T vibration of Fe sites, implying that methanol exerts a negligible influence on the iron sites under the reaction conditions.

Methanol-SCR activities were investigated on H-FER zeolite and a series of Fe-FER zeolites. As shown in Fig. 4a, Fe-FER zeolites significantly enhance the catalytic activity at low temperature region (250~350 °C) in comparison with H-FER zeolite. With the increasing Fe content up to 0.6 wt%, the conversion rate of NO at the low temperature region increases gradually with the optimal reaction temperature for the maximum NO conversion (T ) shifting down to a lower temperature (350~300 °C), as shown in Fig. S16. When the Fe content exceeded 0.6 wt%, the catalytic activities gradually decreased at all the entire temperature range. The decrease in the conversion rate of NO on Fe-FER zeolites at high temperature (>300 °C) can be ascribed to the oxidation of methanol to CO and CO , which leads to a shortage of the reductant for the Methanol-SCR reaction, as corroborated by the reaction of methanol oxidation in the absence of NO in Fig. S18. It is noteworthy that Fe-FER zeolite with high iron content (e.g. 1.2 wt%) is more likely to oxidize methanol to CO due to the presence of iron oxide nanoparticles. Meanwhile, under the optimum reaction temperature window (225~300 °C), 0.6Fe-FER zeolite exhibits the best yield of N , accompanied by the least N O formation, as illustrated in Figs. S19 and S20. A progressive increase in iron content leads to a corresponding decline in both deNO activity and N selectivity during the Methanol-SCR reaction, primarily owing to the loss of BASs in the catalysts, as BASs have been reported to be closely associated with the high temperature activity and N selectivity in Methanol-SCR reactions. Moreover, the Fe-FER zeolites exhibit a competitive advantage in low-temperature deNO efficiency compared to other Methanol-SCR catalysts, such as the reported Ag/Ag O and Cu-containing catalysts. The products derived from methanol were also analyzed, as shown in Fig. 4b. Dimethyl ether was readily formed at low temperature (<250 °C) and a small amount of formaldehyde was also detected as by-products in addition to CO and CO . Compared with the serious deactivation of NH -SCR catalysts such as commercial V/W/TiO and Cu-CHA, 0.6Fe-FER zeolite shows only <5% loss of activity at the low temperature region (<300 °C) after withstanding the same SO poisoning treatment as that reported for V/W/TiO catalyst, while the activity at high temperature region (>300 °C) is virtually unaffected. For potential industrial application, the bench test was also conducted under the reaction condition simulating methanol engine exhaust, using a 150×150×150 mm monolithic catalyst coated with 0.6Fe-FER zeolite. As shown in Fig. 4c, the coated monolithic catalyst still remains higher catalytic activity, achieving NO conversion above 80% within the temperature range of 300 to 400 °C, and a remarkable catalytic stability with more than 200 h could also be achieved at 300 °C.

The activation of methanol in zeolite to form methyl groups plays a strikingly important role in the Methanol-SCR reaction on the basis of our previous work. As displayed in Fig. S27, when CH OH and O were introduced, three identical methyl signals at 60.9, 59.2 and 50.5 ppm are observed on both H-FER and 0.6-Fe-FER zeolites corresponding to side-on adsorbed dimethyl ether (DME), methyl groups generated on BAS and adsorbed methanol, respectively. No new methyl signals appeared on 0.6Fe-FER zeolite, signifying that methanol is mainly activated on BASs rather than Fe sites. The adsorption capacity of NO, NO and CH OH molecules on BASs was also explored by DFT calculations. It reveals that the BASs in FER zeolite display a much stronger adsorption capacity for CH OH molecules (25.2 kcal/mol) than that for NO (5.2 kcal/mol) and NO (8.6 kcal/mol), as manifested in Figs. 4e and S28. Together with the results of 13C CP/MAS NMR measurements as illustrated in Fig. S27, it can be reasonably inferred that the BASs will be preferentially occupied by the methanol molecules and further activated to form methyl groups during the Methanol-SCR reaction.

The role of Fe sites in Methanol-SCR reaction
In practical SCR reactions, the abundant presence of water vapor inevitably facilitates the interaction of NO with H O, thereby leading to the formation of nitrites or nitrates that act as reactive intermediates in the overall process. In both NH -SCR and Methanol-SCR processes, HONO has been identified as a crucial nitrogen-containing reaction intermediate, and its formation is also influenced by the NO/NO ratio. Therefore, the formation of HONO on the catalysts was further investigated. Time-resolved in situ Fourier transform infrared (FT-IR) experiments were implemented to further reveal the effect of Fe sites on Methanol-SCR reaction. As shown in Fig. 5a, when NO and O were flowed in, the adsorbed NO (1630 cm−1) was first observed on H-FER zeolite; meanwhile, the adsorbed nitrates (1590 cm−1), N O (1700 cm−1), and NO+NO (2235 cm−1) species appeared gradually. Subsequently, CH OH was introduced in place of NO, and absorption peaks at 1370, 1460, 2855, and 2965 cm−1 corresponding to methyl groups appeared, accompanied by the consumption of absorption peaks at 3590 cm−1 and 3650 cm−1, owing to the absorption and activation of methanol on BAS and EFAL species (Al-OH), respectively. Compared to H-FER zeolite, 0.6Fe-FER zeolite exhibits different adsorption species when NO and O are flowed in. As shown in Fig. 5c, no adsorbed N O species were observed on the 0.6Fe-FER zeolite, while the formation of nitrite (1470 cm−1) was detected. HONO has been proposed as a key intermediate in the SCR reaction over both copper- and iron-containing zeolites, and is also regarded as a nitrite-precursor. However, it is difficult to detect directly as a highly reactive intermediate. Once formed, it is rapidly adsorbed by hydroxyl groups and subsequently converted into nitrite species via reactions such as Al–OH + HONO → Al–ONO + H O. Therefore, nitrite species are generally accepted as indirect evidence for the formation of HONO. In the Methanol-SCR reaction, HONO produced by the interaction of NO with H O corresponds to a general reaction occurring on zeolites: NO+NO +H O→2HONO. In comparison, under dry conditions, more NO+NO and adsorbed NO species were formed on the 0.6Fe-FER zeolite, whereas the adsorbed NO species were not further converted to HONO, as shown in Fig. S32. As the purging process commences, the hydroxyl vibration at ~3650 cm−1 rapidly disappears, whereas the IR absorption band at 1630 cm−1 remains, suggesting that the species responsible for the IR absorption band at 1630 cm−1 is primarily adsorbed NO rather than water. Accordingly, the absence of H O leads to a significant decrease in Methanol-SCR reactivity. Furthermore, when CH OH was introduced instead of NO, the generation and consumption of IR absorption band at 1690 cm−1 can be captured, as manifested in Fig. 5d. Notably, the species with the IR absorption band at 1690 cm−1 is assigned to formamide, another key intermediate in the Methanol-SCR reaction, which can be formed by the conversion of nitromethane resulting from the reaction of methyl groups and adsorbed HONO. These reaction intermediates are consistent with those previously reported for the Methanol-SCR process. With the aid of SVUV-PIMS spectroscopy, Chen et al. found the presence of nitromethane but with a low ion signal strength in the Ethanol-SCR and (NH +Ethanol)-SCR processes. The above elaboration points to the fact that the introduction of Fe sites not only facilitates the oxidation of NO, but also further promotes the generation of the key intermediate HONO, thus enhancing the reactivity of Methanol-SCR. A comprehensive reaction pathway for the Methanol-SCR process was proposed, as depicted in Fig. S34. BASs contribute to the adsorption and activation of methanol to generate active methyl groups, while [FeO]+ sites are conductive to produce HONO, thereby accelerating the generation of the key intermediate nitromethane, which can transform to formamide and subsequently react with NO to produce N .

Since the component III of Fe2+ at G-site is insensitive to oxidizing and reducing treatment and shows little tendency to participate in the reaction, DFT calculations were adopted to further explore the role of Fe3+ and [FeO]+ sites in the formation of HONO. According to the results of Mössbauer spectra, the models of [FeO]+ at the G-site and Fe3+ at the B-site are constructed in FER zeolite, and the reaction pathways for generating HONO are proposed, as depicted in Fig. S35. On the [FeO]+ at G-site, NO first reacts with [FeO]+ to form Fe2+-NO −, then NO − reacts with H O to produce Fe-OH and a HONO with an energy barrier of 26.3 kcal/mol. Afterwards, the Fe-OH should overcome a barrier of 30 kcal/mol, and react with NO to form [FeO]+ and another HONO. The reaction path follow a general reaction in zeolite: NO+NO +H O→2HONO. On the Fe3+ at the B-site, NO is more likely to adsorb on the Fe3+ site first, and a prohibitively high energy barrier (77.5 kcal/mol) need to be overcome to generate HONO (Fig. 5e). The above results suggest that [FeO]+ at the G-site can function as a promoter of HONO production, whereas Fe3+ at the B-site does not. The formed HONO will further react with the activated methyl group at the BAS to produce the key intermediates nitromethane and formamide, thereby enhancing the catalytic performance of Methanol-SCR.

Overall, [FeO]+ is a preferred active site for the NO oxidation and Methanol-SCR. The dynamic evolution of Fe sites provides an interpretation that Fe sites with reactive oxygen are susceptible to the oxidation and reduction treatment, accompanied by variation in the valence state of the Fe sites. Conversely, the respective Fe2+ at the G-site and Fe3+ at the B-site are difficult to activated during the reaction process. Notably, [FeO]+ has two fewer positive charges than Fe3+, so less negative charge is required to balance the [FeO]+ site in the framework of zeolite, which allow more Si-O−-Al sites to be released to produce BASs. BASs also play a critical role in Methanol-SCR. On one hand, methanol can be activated on BASs to generate methyl groups to participate in the reaction. On the other hand, BASs are directly related to N selectivity, as reported in the previous studies. CHA and FER zeolites with a Cu loading of 0.6 wt% were prepared and evaluated for the Methanol-SCR and NO oxidation reaction. The Cu-FER zeolite prepared by the ion exchange method (0.6Cu-FER-IE) exhibits high catalytic activity in the temperature range of 350~450 °C with the conversion of NO above 70%, while the Cu-CHA zeolite, commonly used as a commercial NH -SCR catalyst, showed less than 20% activity across the entire temperature range. The catalytic performance of the Cu-FER zeolite in the Methanol-SCR reaction appear to depend primarily on the BASs of the FER framework, as its activity is similar to that of the H-FER zeolite. Moreover, neither the Cu-FER nor the Cu-CHA zeolite exhibits significant NO oxidation activity. The pronounced difference in activities between the Cu-FER zeolites synthesized by different methods suggest the presence of distinct copper species in the two catalysts, a phenomenon that needs further investigation. A methanol oxidation experiment was also conducted for the Cu-CHA zeolite. As shown in Fig. S36, in contrast to the Fe-FER zeolite, the Cu-CHA zeolite predominantly oxidizes methanol to CO rather than CO, suggesting that the two catalysts exhibit markedly different behaviors in methanol oxidation. This implies that the carbon-containing intermediates formed during the Methanol-SCR process are likely different for these two catalysts. FER zeolite is a medium-pore zeolite consisting of 10-membered ring (4.2×5.4 Å) channels intersected by 8-membered ring (3.5×4.8 Å) channel, whereas the CHA zeolite features a three-dimensional microporous framework composed of cha cages interconnected by eight-membered ring (8-MR) windows (~3.8×3.8 Å). The low activity of the CHA zeolite in the Methanol-SCR reaction is likely attributed to its unique microporous structure, which easily causes carbon deposition during the methanol-related reaction. It indicates that the zeolite topology also plays a crucial role in the Methanol-SCR catalytic performance.

In this work, we have demonstrated that the collaboration of acid sites and iron redox sites in zeolite can promote the low-temperature Methanol-SCR activity on a series of Fe-FER zeolites, which boosts the catalytic activity of Methanol-SCR significantly at low temperature region (250~350 °C). Pilot-scale bench test using the coated monolithic catalyst also exhibited a remarkable NO conversion and a high stability at low temperature, clearly indicating the great potential application of Fe-FER zeolite catalyst in industrial Methanol-SCR. Further mechanistic studies indicated that BASs in FER zeolite could generate methyl groups by activating methanol, whereas an atomically dispersed [FeO]+ site would further improve the redox properties of the Methanol-SCR catalysts by facilitating the oxidation of NO and the generation of key intermediate HONO. This work discloses the similar design principles of SCR catalysts, and deepens the cognition and understanding of the redox sites for SCR reaction. It not only provides a powerful way to design promising Methanol-SCR catalysts, but also offers a guiding direction for the establishment of the structure-activity relationship of such SCR catalysts.

Methods
Catalyst preparation
FER zeolite catalysts were prepared by the hydrothermal method with pyrrolidine and ethylene glycol as structure-directing agents (SDAs). Firstly, 0.245 g of NaOH and 0.48 g of NaAlO were dissolved in 10 g of H O. Then, 3 g of fumed silica was added to the mixture under rigorous stirring, followed by 21 g of ethylene glycol being dropped in the above solution. The obtained gel was transferred into an autoclave with a PTEE liner for hydrothermal synthesis at 180 °C for 24 h. Subsequently, a mixture prepared by ferric sulfate with different masses and 0.392 g of pyrrolidine dissolved in 3.5 g of H O was added to the cooled gel and continued to hydrothermal synthesis at 180 °C for 72 h. The obtained product was separated, followed by washing with deionized water, drying at 100 °C overnight. Finally, the obtained solid was calcinated in air at 600 °C for 6 h to remove the SDAs and calcined at 550 °C for 2 h followed by ion exchanging with the solution of NH Cl for 3 times to get Fe-FER catalysts. The final product was denoted as xFe-FER, of which the suffix of x referred to the mass percentage of the loading of Fe. H-FER was prepared by a similar procedure except for the addition of ferric sulfate. 0.6Cu-FER-one pot zeolite was successfully synthesized using copper sulfate as the Cu precursor, following the same protocol as 0.6Fe-FER zeolite. Cu-FER-IE and Cu-CHA zeolites were prepared by ion exchanging with the solution of copper acetate. The detailed procedure is as follows: 0.031 g of copper acetate was dissolved in 10 g of H O, followed by the addition of 3 g of NH -FER or NH -CHA zeolite to the copper acetate solution. The mixture was stirred vigorously at 80 °C for 2 h. The resulting Cu-FER-IE or Cu-CHA sample was then obtained by filtration, washed with deionized water, dried at 100 °C overnight, and finally calcined at 550 °C for 2 h.

XRD measurement
The framework of the FER samples was confirmed by powder XRD patterns with Cu Kα radiation (λ=1.5418 Å) and with a scan rate of 2θ=8.0 °/min at 3~55° (Rigaku MiniFlex 600 Benchtop X-ray diffractometer).

Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
The content of Fe present in FER zeolite was determined by ICP-AES (Agilent Technology 4210).

UV-vis spectra
The UV-vis spectra were depicted with a UV-vis spectrophotometer (UV-2600, Shimadzu).

N 2 adsorption and desorption measurement
N adsorption/desorption experiments of samples were performed on 3H-2000PM2 (BEISHIDE), and the samples were degassed under vacuum at 200 °C for 5 h before analysis. Typically, the Brunauer-Emmett-Teller (BET) equation was used to calculate the specific area of the sample, and the micropore specific surface area was evaluated using t-plot method.

TEM
HR-TEM, HAADF, and EDS images were taken using a FEI Talos F200X S/TEM with a field-emission gun at 200 kV. A spherical aberration-corrected HAADF-STEM image was taken by JEM-ARM300F with a field-emission gun at 300 kV. The pores of zeolite and Fe single atom were operated at the accelerating current of 5 µA and 15 µA, respectively.

XAS measurement
Fe K-edge XAS were carried out using a synchrotron radiation ring at PF-12C beamline (Photon Factory, KEK) equipped with a Si(111) single-crystal monochromator in a fluorescence mode. The data analysis of the XAS spectra was conducted by the Demeter software package ATHENA and ARTEMIS. The Fourier transform of the k3-weighted EXAFS was carried out over a k range of 3–10 Å−1. Samples were pretreated to remove the adsorbed water at 200 °C for 2 h in flowing synthetic air before sealing in polyethylene cells for XAS measurements.

In situ Mössbauer spectroscopy measurements
For Mössbauer spectroscopy measurements, 57Fe was introduced into FER zeolites by the above-described methods. The transmission 57Fe Mössbauer spectra were collected at 300 °C using 57Co in Rh matrix as a source. The velocity scale was calibrated using sodium nitroprusside (SNP), and the isomer shift quoted in this work is relative to that of the α-Fe. The spectra were analyzed using the MossWinn software. Prior to the Mössbauer spectra test, all the samples were dehydrated at 500 °C in 14% O /N for 2 h. Thereafter, the reaction gas at a flow of 50 mL/min was introduced at 300 °C for 1 h in the following order: 14% O /N ; 1000 ppm NO/N ; 14% O /N).

NMR measurements
All NMR experiments were performed on a Bruker Avance III 400 MHz spectrometer with resonance frequencies of 400.1 and 100.6 MHz for 1H and 13C, respectively. 1H MAS NMR spectra were collected at a MAS rate of 10 kHz, using a single-pulse excitation of π/2, pulse duration of 2.6 µs, and a repetition time of 20 s. 2D 1H-1H DQ MAS NMR experiments were executed using a POST-C7 pulse sequence with a recycle delay of 20 s and a MAS rate of 10 kHz. 16 scans were acquired for each t1 increment. Note that prior to 1H NMR test, all the samples were dehydrated at 420 °C in vacuum (below 10−3 Pa) for 12 h. 13C CP/MAS NMR spectra were collected using a 4.0 mm HX double resonance MAS NMR probe at a MAS rate of 10 kHz. An number of scans of 1800 and a repetition time of 4 s were used. The samples pelletized in 80-100 mesh were firstly pretreated at 500 °C for 2 h in synthetic air. Thereafter, the reaction gas at a flow of 50 mL/min was introduced at 300 °C CH OH+O (1000 ppm 13CH OH and 7% O in N atmosphere). Finally, the sample was quickly cooled down for 13C NMR measurement.

NH -TPD measurements
NH -TPD experiments were performed on a chemisorption analyzer (PCA-1200 Builder Technology). The samples were pretreated at 400 °C for 1 h in synthetic air with a flow of 50 mL/min to remove the adsorbed water. The temperature was cooled down to 100 °C and saturated with 1% v/v NH /He mixture with a flow rate of 20 mL/min for 1 h. Physisorbed ammonia was removed at 100 °C for 1 h in helium flow until the TCD signal was stabilized. The sample was heated from 100 °C to 700 °C with a 5 °C/min ramping rate in a helium flow rate of 20 mL/min−1. Desorption of NH 3 was monitored by mass spectrometry (m/z=16).

In situ FT-IR spectroscopy
In situ FT-IR experiments were conducted on a Thermal Scientific Nicolet iS10 instrument with a DTGS detector. Infrared spectra were overlapped 16 times at a resolution of 4 cm−1. ~10 mg of the sample was pressed into a slice (0.8 MPa) with a diameter of 13 mm and then transferred to the transmission cell equipped with ZnSe windows. The sample was pretreated at 500 °C for 2 h with a flow of 50 mL/min in synthetic air before the adsorption of the reaction gas. The transient reaction of Methanol-SCR was monitored by FT-IR with gas composition as follows: NO+O (500 ppm NO and 7% O in N atmosphere); CH OH+O (1000 ppm CH OH and 7% O in N atmosphere), and N balanced. Simultaneously, H O was introduced by bubbling with N (1 mL/min) at 30 °C. The total flow rate of gas was 50 mL/min. The background spectrum was collected after the bubbling of H O with N for 30 min before the reaction gas was flowed into the cell to perform in situ measurements. The perturbed T-O-T vibrational modes (1000~800 cm−1 in the IR spectra) of the zeolite framework to distinguish different Fe sites were obtained at 175 °C, and the background spectrum was collected without the sample in the synthetic air. The reducing and oxidizing half reactions of NO oxidation were monitored by FT-IR at 300 °C with gas composition in the following order: 14% O /N ; 1000 ppm NO/N; 14% O /N ). Prior to NH adsorption experiments, the samples were evacuated (10−7 mbar) at 450 °C for 1 h. Then, FTIR spectra were acquired at 150 °C after adsorption of excess NH (10 mbar). Extinction coefficient of 0.11 cm2 µmol−1 for NH + deforming vibration bands (1460 cm−1) and 0.026 cm2 µmol−1 for NH deforming vibration bands (1620 cm−1) in IR spectra as described in previous publications, were employed to calculate the amount of Brønsted acid sites and Lewis acid sites, respectively.

Activity tests
Methanol-SCR reaction was executed in a fixed-bed reactor with the reactant gas mixture containing 2000 ppm CH OH, 1000 ppm NO, 14% O , and 5% H O in N balance. 1000 ppm NO and 14% O were used for the reaction of NO oxidation. The total flow rate of gas through the catalysts was 1000 mL/min with a gas hourly space velocity (GHSV) of 30,000 h−1. CH OH/H O solution was injected by a liquid plunger pump and flew through the evaporator. 2 mL of catalyst used for the catalytic activity test was prepared by pressing the catalyst powders under a pressure of 0.9 MPa, then pelletized and sieved to 18~30 mesh. The temperature was measured in the range of 200 °C to 500 °C. CH OH, CH OCH , CO, CO , NO, NO , and N O of the outlet gas were quantified by an FTIR spectrometer (Nicolet iS10). HCHO of outlet gas was analyzed by a KITAGAWA gas detector tube (Komyo Rikagaku Kogyo). SO poisoning of the Fe-FER catalyst was carried out under the conditions referred to in the previous work.

The bench test was conducted at the Shanghai Marine Diesel Engine Research Institute using a 150×150×150 mm monolithic catalyst coated with 0.6Fe-FER zeolite. The reaction condition refers to methanol engine exhaust parameters containing 2000 ppm CH OH, 1000 ppm NO, 14% O , 10% CO , and 5% H O in N balance with a GHSV of 15,000 h−1. The conversion of NO and N yield were calculated by following equation:

NOx conversion % = (NO xinlet - NO xoutlet) / NO xinlet × 100 %

N 2 yield % = (2 × N 2 outlet) / (NO xinlet + NO xoutlet) × 100 %

Thereaction rate was calculated using the following equation:

Reaction rate (mmol g−1 h−1) = (Feed mL h−1 × X NOx (%)) / (m catal (g) × 22.4 (L mol−1) × 10−3 (mol mmol−1) × 103 (mL L−1))

where Feed NOx represents the concentration of NO , X NOx represents the conversion of NO , m catal represents the mass of catalyst.

Kinetic measurements
The limitations of internal and external diffusion were evaluated by reactions under different gas velocity and grain mesh before the kinetic measurements of the NO oxidation reaction. As shown in Fig. S37a, the conversions of NO are almost unchanged with different particle size from 40~100 mesh under GHSV=30,000 h−1. Thus, internal diffusion limitation was excluded with the particle size of 40~60 mesh. Furthermore, as shown in Fig. S37b, the conversions of NO are almost unchanged with the GHSV≥800,000 h−1 indicating that the external diffusion limitation was also excluded with the GHSV=800,000 h−1. According to the above experiments, the GHSV and particle size were selected as 800, 000 h−1 and 40~60 mesh, respectively. Total gas flow through the catalysts was 1000 mL/min. The Arrhenius plots was drawn according to the reaction rate with the conversion of NO less than 20 %. The reaction order with respect to NO was also determined under the condition at 175 °C, a feed containing 14% O with N as balance gas was used and NO concentrations were varied in the range of 100~900 ppm.

Computation details
The FER zeolite model used for the adsorption of reaction gas was constructed by truncating a 60T cluster from the crystal structure. In order to reduce the computational cost, the computational model employed for the calculation of the reaction path was constructed by truncating an 18T cluster from the 60T cluster. One silicon atoms in the framework were replaced by one aluminum atom to construct H-FER zeolite model. Fe-FER zeolite was modeled by the construction of [FeO]+ and Fe3+ sites at the G-site and B-site, respectively. Atoms far away from the reactants were kept fixed to their equilibrium positions during the optimization process for the skeleton stabilization. All of the associated structures were optimized at the level of DFT with Becke’s three-parameter exchange and Lee-Yang-Parr correlation functional (B3LYP) implemented in the Gaussian 09 package. The pseudo-potential basis set SDD was utilized for Fe, and the polarized 6-31g (d, p) basis set was used for Si, Al, O, H, C, and N atoms. The combination of B3LYP/6-31g (d, p) has been previously reported to provide accurate results in zeolite-catalyzed reactions. Berny algorithms were adopted to search and optimize the transition states. The Hessian matrix was calculated to validate that only one imaginary frequency was observed for each transition state.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper describes an Fe-FER zeolite which is catalytic for NOx reduction with methanol (MeOH). Several characterization methods were used to characterize the active site of Fe in the FER zeolite. While the NOx conversions are interesting, there are a number of issues that need to be addressed before this paper can be accepted.\nComments:\n1. The figures for the characterizations are very small and important features can not be evaluated. In addition, five Fe sites are proposed from the characterizations, which appears to result from overfitting of the data. Justification for the validity of fitting several different sites needs to be justified. The tables report 3 significant figures for Mossbauer (T1 and T2), 4 significant figures for H NMR (TS3) and 3 significant figures for XAS. The small features and noise, which can’t be judged from the figures, would not appear to justify the fits. A more complete analysis of all samples and figures in the SI is required.\n2. The number of H+ sites was determined by H NMR (Fig S4 and TS3, for example) and NH3 TPD. The NH3 TPD still has a significant overlap of the physisorbed NH3 with the H+ sites, which makes quantification of the H+ sites inaccurate. Better methods to determine the number of H+ sites by NH3 TPD is necessary. The number of H+ sites should be compared to the NH3 TPD and the Al composition. Without consistency for these results, the data cannot be considered correct.\n3. The XAS data is incompletely and poorly fit. What is the pre-edge XANES energy of every sample and reaction condition? The pre-edge energy of Fe+2 and Fe+3 are different. Mossbauer shows a change to more Fe+2 for NO oxidation. Is there a change in the pre-edge peaks. There is also an increase in the formation of more FeO+. Is there a simultaneous increase in the Fe-O CN. What is this bond distance, separate from the Fe states that do not change? If this is the active Fe, then this Fe-O bond distance is important, for example for modeling the catalytic process.\n4. The reaction chemistry for NO oxidation does not make sense. Mossbauer show a change in the I peak decreasing from about 45% to 10% (T2). The other sites occupancies remain about the same, within about 5%. A new peak V is fit which increases with NO/O2 and decreases with O2 (T2). (There is no experimental procedure for NO oxidation given in the experimental section and the text and table are unclear of the exact reaction conditions. I assume that NO oxidation has NO + O2.) Peak V is described at Fe+2 with bond NO2 with -1 charge (?). Does NO react at the FeO+ site? If so, why not treat the oxidized catalyst with NO and show the formation of bond NO2. The Mossbauer might show completed loss of Peak 1, with NO at 300C. Then reaction with O2 of this state might regenerate Peak 1. The reaction of O2 often requires multiple active sites, i.e., 2 Fe sites in molecular proximity. ½ O2 is not a reactant as show in Fig. 3d. (If there are 3 significant figures in the Mossbauer fits, then all sites are active, since they change by more than 0.1%!)\n5. If FeO+ is the catalytic site, Mossbauer indicates that 0.4, 0.6 and 1.2% Fe have 47, 53 and 45%, respectively. If the fits are good to 5% (not 0.1%) then the number of active sites in all catalysts is similar. Why is the deNOx rate (Fig 4a) of the 1.2% Fe much lower than that of the former samples? Also, why does the rate of the 0.4% Fe remain higher at high T, compared to the other 2 samples?\nThe catalytic performance for NO oxidation and deNOx reaction needs to be more clearly determined. Kinetic rates, and TOR if FeO+ is the active site, activation energies and how these changes with gas concentrations need to be determined. While deNOx by methanol leads to partially oxidized hydrocarbons, there are readily oxidized to CO2 in a subsequent catalytic step. However, the formation of N2O, which is a much more intensive green house gas than NOx and CO2, will not be attractive for practical applications. While the type of characterizations appears to be sufficient to characterize the active site of catalyst, these methods need to be more thoroughly analyzed and with additional experiments are needed to confirm the active site. With a mixture of Fe states, both oxidation states and site locations, well designed experiments are needed to show which of these site change, and which are inactive. Then the structure of this site needs to be isolated from the spectra in each method. For oxidation reactions (NO oxidation or deNOx) catalyst characterizations for the two half reactions, i.e., reducing and oxidizing half reactions, allow for more accurate identification of the sites that are changing structure and oxidation state.\nAdditional experiments and analysis are required before this manuscript can be accepted.", "reviewer_2": "This work provides important mechanistic insights into the role of [FeO]⁺ in Methanol-SCR combining experimental and computational approaches to elucidate its function in NO oxidation and HONO formation. While the work is well-structured and presents compelling findings, additional information is required to enhance the study’s robustness. The detailed comments are as follows.\nSpecific Comments\n1. The results from Mössbauer spectroscopy and the CO₂ concentration data from methanol oxidation suggest that a notable proportion of small FexOy species are present in the 1.2Fe-FER catalyst. However, it is interesting to note that no related structural information was observed in other characterization techniques, such as XRD and EXAFS. To provide a more comprehensive understanding, the authors may consider supplementing their discussion with additional analyses, such as Wavelet transform spectra. Furthermore, UV-Vis spectroscopy could be a useful tool to determine the presence of oligomeric Fe species, which would help clarify this point.\n2. In Lines 157–163, it would be beneficial for the authors to explicitly clarify which catalyst the IS and QS values of species I, II, and III correspond to. Specifically, do these values represent those of 0.6Fe-FER, or are they averaged across different catalysts? Clarifying this aspect would enhance the readers’ understanding of the presented data.\n3. The 2D 1H-1H DQ MAS NMR spectroscopy data of H-FER reveal two distinct cross-peak pairs, which the authors attribute to interactions between adjacent BAS and EFAL sites. The authors further propose that Fe incorporation into BAS sites during hydrothermal synthesis led to the disappearance of autocorrelation peaks. However, it is observed that only one cross-peak pair was significantly affected and converged, while the other remained unchanged. A more detailed discussion on why certain BAS sites were preferentially affected by Fe incorporation would strengthen the interpretation of the NMR results.\n4. To further support the discussion on BAS acidity changes, the authors may consider providing FTIR analysis using NH₃ as a probe molecule. Such an analysis would allow for a clearer evaluation of how Fe substitution influences the acidic properties of the catalyst.\n5. Since Fe is known to exhibit ammonia decomposition activity at elevated temperatures, it would be valuable for the authors to discuss whether this effect could have influenced the NH₃-TPD results. Providing MS data alongside NH₃-TPD would help confirm whether ammonia decomposition contributed to the observed desorption profiles.\n6. To more effectively illustrate the suppression of N₂O formation, the inclusion of N₂ selectivity data, in addition to N₂ yield, would be highly beneficial. Selectivity provides a clearer perspective on how efficiently NOx is converted to N₂ relative to other byproducts, thereby offering a more comprehensive assessment of catalyst performance.\n7. In Fig. 4(b), the total carbon concentration at 200°C appears inconsistent with the trend observed in the dataset. Given that CH₃OCH₃ contains two carbon atoms, its contribution to the total carbon balance should be adjusted accordingly. A re-evaluation of this calculation may improve the accuracy of the presented data.\n8. The authors propose that the oxidation state of [FeO]⁺ changes CH₃OH-SCR. If this is the case, one might expect corresponding variations in TOT vibrations and M-O stretching vibrations under reaction conditions. Providing additional spectroscopic analyses, such as in situ FTIR or Raman spectroscopy, could further substantiate this claim and strengthen the overall discussion.\n9. While this study provides valuable insights into the role of [FeO]⁺ in NO oxidation and Methanol-SCR, the reliance on Mössbauer spectroscopy for Fe species assignment raises some concerns as the deconvolution process could introduce variability in some cases (Fig. 1e). Additionally, XANES data (Fig. 1b) suggest that most of Fe species are observed in the Fe³⁺ oxidation state, which calls for further discussion on the oxidation state assignments. Given that cation location in FER zeolite frameworks is known to influence perturbed vibrational modes (~900 cm⁻¹ in IR), employing IR spectroscopy could provide a complementary method to distinguish Fe site locations, as seen in studies on Co-FER and Pd-FER (There are numerous literatures on this). Furthermore, while the manuscript attributes the superior NO oxidation ability of [FeO]⁺ to its coordinated oxygen, it remains unclear whether this effect is primarily due to reactive oxygen species or intrinsic geometric differences arising from distinct cation positions. Addressing these aspects through additional spectroscopic validation would further strengthen the conclusions of this study.\n10. Although the authors claim HONO as a key intermediate in the Methanol-SCR mechanism, additional evidence is required. The presented IR data primarily shows the presence of methyl groups, nitrates, and nitrites, but does not explicitly confirm the formation or participation of HONO in the reaction pathway. I am not sure whether the authors' previous work may provide further justification or not, it would be beneficial to discuss whether HONO can be directly observed through spectroscopic techniques. If direct detection is challenging, exploring indirect validation methods such as isotope labeling experiments or kinetic studies could provide additional support for its role.\n11. Lastly, I think a broader comparison with other catalysts should be provided. Since Cu-based zeolites are generally known for their superior redox properties compared to Fe-based ones, they may exhibit even higher activity in this reaction. Including Cu-exchanged zeolite as a reference catalyst could offer useful context for evaluating the advantages and limitations of Fe-FER, particularly regarding NO oxidation and overall Methanol-SCR performance.", "reviewer_3": "This paper reports on a mechanistic study of the Selective Catalytic Reduction of NO over Fe-zeolite catalysts using methanol as the reducing agent. The authors claim that the collaboration between zeolitic acid sites and Fe redox sites provides a strategy for boosting the low-T MeOH SCR activity: however, I have some doubts about the novelty of this concept and the conclusive nature of the evidence included in the manuscript. Please see my points below.\n1. As a preliminary general comment, the English usage needs to be improved: there are several incomplete or incorrect sentences and typos, which make sometimes difficult to follow the text flow.\n2. A major concern is the identification of the oxidation of NO to NO2 as the rate limiting step of the SCR reactions (line 188). This is a controversial assumption, which has been shown in the past to contradict several experimental findings. For example, i) the rate of NO oxidation is typically much lower than the SCR rate, ii) H2O has opposite effects on the rates of NO oxidation and of the NH3-SCR reaction, iii) the NO oxidation activities of Fe-zeolite and Cu-zeolite catalysts do not correlate with the corresponding NH3-SCR activities ….\n3. Figure 3b shows a linear correlation between the NO oxidation rate at 300°C and the amount of [FeO]+. Notice that at 300°C the NO oxidation rate is already clearly limited by chemical equilibrium: see Figure 3a where the NO conversion at 300°C is in the descending branch of the lightoff curves for all the catalysts. Therefore, the rates plotted in Figure 3b are hardly representative of the intrinsic NO oxidation kinetics, being at least in part controlled by thermodynamics.\n4. How were the NO oxidation rates extracted from the integral NO conversion data of Figure 3a?\n5. Apparently, the authors propose a NO activation pathway wherein NO is first oxidized to NO2 over [FeO]+ and then HONO is formed. NO oxidation to NO2 is a two-electron process, so how can it proceed over single Fe sites, which cycle between Fe3+ and Fe2+? Can you rule out an alternative pathway where NO is directly oxidized to HONO over Fe3+OH, as proposed in the literature for both Fe- and Cu-zeolite catalysts?"}, "author_response": "Response to comments\nManuscript ID: NCOMMS-24-78027A-Z\nFirst of all, we are grateful for all reviewers’ in-depth comments and suggestions, which have\nhelped us greatly to improve the quality of the present manuscript. We have considered all the\ncomments and suggestions seriously, and carried out the related experiments following the\nsuggestions of the reviewers. We have made detailed and point to point response (written in\nBlue) and revised the manuscript accordingly with all changes being highlighted in Yellow in\nthe revised Manuscript and Supporting Information.\n------------------------------------\nReviewer #1:\nGeneral Comments: This paper describes an Fe-FER zeolite which is catalytic for NOx\nreduction with methanol (MeOH). Several characterization methods were used to characterize\nthe active site of Fe in the FER zeolite. While the NOx conversions are interesting, there are a\nnumber of issues that need to be addressed before this paper can be accepted.\nOur response: We thank you very much for endorsing the scientific contribution of our work\nand your positive and insightful comments.\nComment 1. The figures for the characterizations are very small and important features can\nnot be evaluated. In addition, five Fe sites are proposed from the characterizations, which\nappears to result from overfitting of the data. Justification for the validity of fitting several\ndifferent sites needs to be justified. The tables report 3 significant figures for Mossbauer (T1\nand T2), 4 significant figures for H NMR (TS3) and 3 significant figures for XAS. The small\nfeatures and noise, which can’t be judged from the figures, would not appear to justify the fits.\nA more complete analysis of all samples and figures in the SI is required.\nOur response: Thanks for your insightful suggestion. Now all figures in the manuscript have\nbeen revised to improve the visibility of key features and to better reveal subtle details and\nnoise, thereby supporting the quality of the fits. Additionally, the data in the tables have been\nrounded to two significant figures.\nComment 2. The number of H+ sites was determined by H NMR (Fig S4 and TS3, for example)\nand NH -TPD. The NH -TPD still has a significant overlap of the physisorbed NH with the\n3 3 3\nH+ sites, which makes quantification of the H+ sites inaccurate. Better methods to determine\nthe number of H+ sites by NH -TPD is necessary. The number of H+ sites should be compared\n3\nto the NH -TPD and the Al composition. Without consistency for these results, the data cannot\n3\nbe considered correct.\nOur response: Thanks for your valuable suggestion. As noted in the reviewer’s comment, the\nNH -TPD results still exhibit significant overlap with physisorbed NH , which compromises\n3 3\nthe accuracy of H+ site quantification. To more accurately quantify the acidic sites in the\ncatalysts, the FT-IR experiments of NH adsorption were supplemented, as they are widely\n3\nreported for characterizing acidic sites in zeolite materials (J. Phys. Chem. C 123, 2019,\n27436−27447; Chemical Engineering Journal 2014, 237, 372–379). As shown in Fig. R1\n(New Figure shown below and also in the supporting information as Fig. S8), the reduced\namount of BASs and enhanced amount of LASs in Fe-FER zeolites also confirm that Fe sites,\nsubstituting proton counter-cations, exhibit Lewis acid properties by acting as electron pair\nacceptors. The amounts of BASs determined by the Al sites is consistent with the results\nframework\nobtained from the FT-IR spectra of NH adsorption (Fig. R2, New Figure shown below and\n3\nalso in the supporting information as Fig. S9).\nFig. R1. (a) The FT-IR spectra of NH adsorption on the H-FER and Fe-FER zeolites. (b) The\n3\nacid amounts determined by the FT-IR spectra of NH adsorption. Extinction coefficient of\n3\n0.11 cm2 μmol-1 for NH + deforming vibration bands (1460 cm-1) and 0.026 cm2 μmol-1 for\n3\nwere employed to calculate the amounts of BASs and LASs, respectively.\nFig. R2. The amounts of BASs determined by the FT-IR spectra of NH adsorption and Al site\n3\nin the framework of the Fe-FER zeolite calculated by multiplying the spectral contribution of\n4-coordinated framework Al in 27Al MAS NMR spectra and the total Al content determined by\nICP.\nRevisions: For improving the understanding, the aforementioned discussions have been added\nin the revised manuscript in a following manner: “The reduced amount of BASs and enhanced\namount of LASs in Fe-FER zeolites quantified by the FT-IR spectra of NH adsorption also\n3\nconfirm that Fe sites, substituting proton counter-cations, exhibit Lewis acid properties by\nacting as electron pair acceptors.33 The amounts of BASs determined by the Al sites is\nframework\nconsistent with the results obtained from the FT-IR spectra of NH adsorption (Fig. S8 and Fig.\nComment 3. The XAS data is incompletely and poorly fit. What is the pre-edge XANES energy\nof every sample and reaction condition? The pre-edge energy of Fe+2 and Fe+3 are different.\nMossbauer shows a change to more Fe+2 for NO oxidation. Is there a change in the pre-edge\npeaks. There is also an increase in the formation of more FeO+. Is there a simultaneous increase\nin the Fe-O CN. What is this bond distance, separate from the Fe states that do not change?\nIf this is the active Fe, then this Fe-O bond distance is important, for example for modeling the\ncatalytic process.\nOur response: Thanks for your valuable comments. We would like to clarify that the XAS\nexperiments in this manuscript were not conducted under reaction conditions, but rather on\ndehydrated Fe-FER zeolites. Consequently, the changes of Fe sites in the NO oxidation reaction\ncould not be obtained by the XAS results. But the pre-edge peaks were further analysis to\nidentify the oxidation states and coordination environments of iron. The pre-edge peak in the\nspectra corresponds to the dipole-forbidden 1s→3d transition. The position of the pre-edge\npeak can be also used as a direct measure of the Fe oxidation state, making an unambiguous\ndistinction between Fe2+ (~7112.5 eV) and Fe3+ (~7113.5 eV). Graphical analysis of the\ncorrelation between integrated intensities and positions of pre-edge features in the variogram\nwas employed to further discriminate various oxidation states and coordination environments\nof Fe. Since the 1s→3d transition is a quadrupolar transition, it exhibits weak intensity in\ncoordination geometries with an inversion center (centrosymmetric geometry), such as\noctahedral coordination. But in fivefold or fourfold geometry, 3d–4p orbital hybridization\nfacilitates dipole-allowed 1s→4p transitions, resulting in significantly higher intensity (J.\nSynchrotron Rad. 2015, 22, 410–426). As shown in Fig. 1c and 1d (updated in Fig. 1, as shown\nbelow), the higher integrated intensities of Fe-FER zeolites pre-edge analysis than Fe3+ ,\nOh\nindicating that Fe sites in lower geometry in Fe-FER zeolites than the bulk Fe O . The Fe3+/Fe2+\n2 3\nratio obtained by curve fitting analysis of pre-edge peak in Fe-FER zeolites exhibits an\nincreasing trend with increasing iron loading (Table R1, new Table shown below and also in\nthe supporting information as Table S2).\nTable R1. The fraction of Fe3+ and Fe2+ in the Fe-FER zeolites obtained by curve fitting\nanalysis of pre-edge peak.\nSample Fe2+ Fe3+ Fe3+/Fe2+\n0.4Fe-FER 38% 62% 1.63\n0.6Fe-FER 31% 69% 2.23\n1.2Fe-FER 17% 83% (23% Fe O ) 4.88\n2 3\nRevisions: For improving the understanding, the above discussions have been added in the\nrevised manuscript. (See. Lines 1 to 15 of Page 6)\nFig. 1. (a) HAADF-STEM and spherical aberration-corrected HAADF-STEM images of\n0.6Fe-FER zeolite as well as HAADF-STEM image of 1.2Fe-FER zeolite. (b) XANES spectra\nof Fe-FER zeolites with Fe foil, FeO and Fe O as references. (c) Pre-edge spectra of Fe-FER\n2 3\nzeolites with FeO and Fe O as references. The fitted peaks in blue, purple and light grey,\n2 3\nrepresent Fe2+, Fe3+ and bulk Fe O , respectively. (d) Variogram of the integrated intensity and\n2 3\ncentroid position of the pre-edge peak of the Fe-FER zeolites. (e) Wavelet transform spectra of\nFe foil, FeO, Fe O and 0.6Fe-FER. (f) Mössbauer spectra collected at 300 °C of 0.4Fe-FER,\n2 3\n0.6Fe-FER and 1.2Fe-FER zeolites. The fitted peaks in red, green, blue and purple represent\ncomponent I, component II, component III, and component IV, respectively. (g) Position of B-,\nC-, F- and G-site in the extra-framework of FER zeolite looking along the ten-ring channels\n(left) and the eight-ring channels (right).\nComment 4. The reaction chemistry for NO oxidation does not make sense. Mossbauer show\na change in the I peak decreasing from about 45% to 10% (T2). The other sites occupancies\nremain about the same, within about 5%. A new peak V is fit which increases with NO/O and\n2\ndecreases with O (T2). (There is no experimental procedure for NO oxidation given in the\n2\nexperimental section and the text and table are unclear of the exact reaction conditions. I\nassume that NO oxidation has NO + O2.) Peak V is described at Fe+2 with bond NO2 with -1\ncharge (?). Does NO react at the FeO+ site? If so, why not treat the oxidized catalyst with NO\nand show the formation of bond NO2. The Mossbauer might show completed loss of Peak 1,\nwith NO at 300C. Then reaction with O2 of this state might regenerate Peak 1. The reaction of\nO2 often requires multiple active sites, i.e., 2 Fe sites in molecular proximity. ½ O2 is not a\nreactant as show in Fig. 3d. (If there are 3 significant figures in the Mossbauer fits, then all\nsites are active, since they change by more than 0.1%!)\nOur response: Thanks for your insightful question. The experimental procedure for the\nreaction of NO oxidation had given as the reviewer suggested in the original manuscript. In\nsitu Mössbauer spectra of 0.6Fe-FER zeolite were collected at 300 °C with 14% O /N flowed\n2 2\nin; with 1000 ppm NO/N flowed in instead of 14% O /N ; with 14% O /N flowed in instead\n2 2 2 2 2\nof 1000 ppm NO/N . The arrows in Fig. 3c represent the order in which the gases were\n2\nintroduced. Firstly, the 0.6Fe-FER zeolite was oxidized under the reaction atmosphere of 14%\nO /N . With the introduction of 1000 ppm NO/N , the content of component I decreases from\n2 2 2\n53% to 10%, accompanied by the appearance of a new Fe2+ site (component V), signifying that\nthe [FeO]+ was reduced to Fe2+. Compared to component I, the insignificant change in QS\nvalues of component V points to the [Fe2+-NO -]+ structure may be formed at the G-site. When\n2\n14% O /N was introduced in instead of NO, the oxidation reaction from Fe2+ to [FeO]+ was\n2 2\ntaken place according to the disappearance of component V in conjunction with the component\nI increases from 10% to 52%. In situ FT-IR spectroscopy was also employed as a\ncomplementary technique to monitor changes in Fe sites during the NO oxidation reaction (Fig.\n3d, updated in Fig. 3 as shown below). Correspondingly, the introduction of 1000 ppm NO/N\n2\ninduced a distinct shift to higher wavenumbers in the IR spectra (1000~800 cm-1), suggesting\nincreased formation of Fe2+ species with a significant decrease in the [FeO]+ site. This shift is\naccompanied by the emergence of IR bands at 1870, 1590, and 1478 cm-1, which are attributed\nto Fe2+–NO, nitrates, and Fe2+–NO - species, respectively. Accordingly, the IR band at 909 cm-\n2\n1 can be assigned to T-O-T oxygen bridges perturbed by Fe2+–NO - species. When 14% O /N\n2 2 2\nwas introduced instead of NO, a distinct shift to lower wavenumbers was observed,\nFig. 3. (a) Effect of the amount of Fe on the activity of NO oxidation on FER zeolites. (b) The\nrelationship of the amount of [FeO]+ with reaction rate for NO oxidation at 175 °C. (c) In situ\nMössbauer spectra of 0.6Fe-FER zeolite were collected at 300 °C with 14% O /N flowed in;\n2 2\nwith 1000 ppm NO/N flowed in instead of 14% O /N ; with 14% O /N flowed in instead of\n2 2 2 2 2\n1000 ppm NO/N . The arrows represent the order in which the gases were introduced. The\n2\nfitted peaks in orange represents the component V. (d) In situ FT-IR spectra of 0.6Fe-FER\nzeolite collect at 300 °C with 14% O /N flowed in; with 1000 ppm NO/N flowed in instead\n2 2 2\nof 14% O /N ; with 14% O /N flowed in instead of 1000 ppm NO/N . (e) Structural schematic\n2 2 2 2 2\nof the dynamic evolution of Fe sites during the NO oxidation reaction.\naccompanied by the appearance of a band at 1625 cm-1 (attributed to NO adsorption) and a\n2\ndecrease in the Fe2+–NO - species at 1478 cm-1. These observations suggest that NO oxidation\n2\nproceeds via Fe2+–NO - as an intermediate as depicted in Fig. 3e (updated in Fig. 3 as shown\n2\nabove), which is in good agreement with the results of the Mössbauer spectra. The significant\nvariance observed in [FeO]+ demonstrates its pivotal role as active sites in the NO oxidation\nreaction. The [FeO]+ site can react with NO to form [Fe2+-NO -]+ intermediate. Subsequently,\n2 2 2\nforming the [Fe2+-O-]+ intermediate. Further electron transfer from Fe2+ to O⁻ facilitates the\nformation of [FeO]+.\nExperimental uncertainties limit the precision in quantifying changes in Fe sites. Although\nit cannot be definitively concluded that the Fe3+ and Fe2+ sites are completely inactive in the\nreaction, the Mössbauer spectra fits with deviations within 5% are considered acceptable (J.\nCatal. 2014, 312, 123-138).\nRevisions: For improving the understanding, the aforementioned discussions have been\nincluded in the revised manuscript. (See. Lines 18 to 27 of Page 11, Lines 4 to 6 of Page 12,\nLines 1 to 12 of Page 13)\nComment 5. If FeO+ is the catalytic site, Mossbauer indicates that 0.4, 0.6 and 1.2% Fe have\n47, 53 and 45%, respectively. If the fits are good to 5% (not 0.1%) then the number of active\nsites in all catalysts is similar. Why is the deNOx rate (Fig 4a) of the 1.2% Fe much lower than\nthat of the former samples? Also, why does the rate of the 0.4% Fe remain higher at high T,\ncompared to the other 2 samples?\nOur response: Thanks for your valuable question. The spectral contribution does not directly\nrepresent the amount of [FeO]+ site in the catalysts. The quantities of the different Fe species\nwere calculated and presented in the tables by multiplying their spectral contributions and the\ntotal iron content determined via ICP analysis (updated in Table 1 & 2 and Table S4 (new\nTable as shown below)). The amounts of different Fe sites show an increase with increasing\ntotal iron content. Moreover, although [FeO]+ is considered an active site in the NO oxidation\nreaction, the deNO activity in the Methanol-SCR reaction is governed by the synergistic\nx\ninteraction between zeolitic Brønsted acid sites (BASs) and iron redox sites. Notably, BASs\nalso play a pivotal role in deNO activity at high temperatures (>350 °C). The incorporation\nx\nof iron sites inevitably leads to a decrease in the amounts of BASs, as shown in Fig. S8,\nresulting in a significant reduction in deNO activity at high temperature (>350 ℃).\nx\nTable S4. IR peak position, Fe sites, spectral contribution, and Fe sites amounts of Fe-FER\nzeolites.\nFe sites\nPeak position/ Spectral\nSample Fe sites amount* /\n(cm-1) contribution\n(wt%)\n885 [FeO]+ 44% 0.18\n0.4Fe-FER 895 Fe3+ 15% 0.06\n885 [FeO]+ 54% 0.32\n0.6Fe-FER 895 Fe3+ 13% 0.08\n885 [FeO]+ 46% 0.55\n1.2Fe-FER\nx y\n*The amounts of Fe sites were obtained by multiplying the spectral contribution and the total\niron content determined by ICP.\nRevisions: For improving the understanding, the aforementioned discussions have been\nincluded in the revised manuscript. (See. Lines 3 to 4 of Page 9, Lines 13 to 14 of Page 13,\nLines 3 to 4 of Page 12 in SI)\nComment 6. The catalytic performance for NO oxidation and deNOx reaction needs to be more\nclearly determined. Kinetic rates, and TOR if FeO+ is the active site, activation energies and\nhow these changes with gas concentrations need to be determined. While deNOx by methanol\nleads to partially oxidized hydrocarbons, there are readily oxidized to CO in a subsequent\n2\ncatalytic step. However, the formation of N O, which is a much more intensive green house gas\n2\nthan NOx and CO , will not be attractive for practical applications. While the type of\n2\ncharacterizations appears to be sufficient to characterize the active site of catalyst, these\nmethods need to be more thoroughly analyzed and with additional experiments are needed to\nconfirm the active site. With a mixture of Fe states, both oxidation states and site locations,\nwell designed experiments are needed to show which of these site change, and which are\ninactive. Then the structure of this site needs to be isolated from the spectra in each method.\nFor oxidation reactions (NO oxidation or deNOx) catalyst characterizations for the two half\nreactions, i.e., reducing and oxidizing half reactions, allow for more accurate identification of\nthe sites that are changing structure and oxidation state.\nOur response: Thanks for your insightful suggestion. Reaction order with respect to NO was\nmeasured over the 0.6Fe-FER zeolite by analyzing the reaction rate dependency on reactant\nconcentration. As shown in Fig. R3 (New Figure shown below and also in the supporting\ninformation as Fig. S14), the reaction rate was found to have linear dependence with respect to\nNO, with the slope of the linear fit being approximately 1, indicating that the reaction follows\nfirst-order kinetics.\nFig. R3. Dependence of NO oxidation rates on NO concentration at 175 ℃. Reaction condition:\n14% O , 100~900 ppm NO. Catalyst: 0.6Fe-FER zeolite. Balance gas: N . Total gas flow:1000\n2 2\nmL/min. GHSV=800,000 h-1.\nN O production in this work is negligible, particularly over the 0.6Fe-FER zeolite.\nshown in Fig. S19, the yield of N O remains below 3% at the optimal reaction temperature\n2\nrange (275~300 °C). Moreover, the formation of N O is associated with the BASs in the zeolite,\n2\nsuggesting that fine-tuning the acidity and redox properties, such as adjusting the SiO /Al O\n2 2 3\nratio (SAR), offers a promising strategy to minimize N O formation and enhance N yield.\n2 2\nThe pre-edge analysis of XAS spectra and the perturbed T-O-T vibrational analysis\n(1000~800 cm-1 in the IR spectra) of the zeolite framework were also employed as\ncomplementary techniques to discriminate various oxidation states and coordination environments\nof Fe sites (Fig. 1c, Fig. 1d, Table S2, Fig. S7 (New Figure shown below as Fig.\nR4) and Table S4). In situ Mössbauer spectroscopy and in situ FT-IR spectroscopy were\nconducted to identify the change of Fe sites by the reducing and oxidizing half reactions of NO\noxidation (Fig. 3c, Table 2 and Fig. 3d).\nFig. R4. IR spectra collected at 175 °C of (a) 0.4Fe-, (b) 0.6Fe- and (c) 1.2Fe-FER zeolites.\nThe peak position corresponding to component I, II, III, IV is 903, 895, 885, and 873 cm-1,\nrespectively.\nRevisions: For improving the understanding, the aforementioned discussions have been\nincluded in the revised manuscript. (See. Lines 27 to 29 of Page 8, Lines 1 to 2 of Page 9, Lines\n12 to 15 of Page 11, Lines 18 to 27 of Page 11, Lines 1 to 12 of Page 13)\nReviewer #2:\nGeneral Comment: This work provides important mechanistic insights into the role of [FeO]+\nin Methanol-SCR combining experimental and computational approaches to elucidate its\nfunction in NO oxidation and HONO formation. While the work is well-structured and presents\ncompelling findings, additional information is required to enhance the study’s robustness. The\ndetailed comments are as follows.\nOur response: We would like to thank you for the positive comments and appreciation of our\nwork. Below we elaborate on the shared concerns.\nComment 1. The results from Mössbauer spectroscopy and the CO2 concentration data from\nmethanol oxidation suggest that a notable proportion of small FexOy species are present in the\n1.2Fe-FER catalyst. However, it is interesting to note that no related structural information\nwas observed in other characterization techniques, such as XRD and EXAFS. To provide a\nmore comprehensive understanding, the authors may consider supplementing their discussion\nwith additional analyses, such as Wavelet transform spectra. Furthermore, UV-Vis\nspectroscopy could be a useful tool to determine the presence of oligomeric Fe species, which\nwould help clarify this point.\nOur response: Thanks for your suggestive advice. The wavelet transform analysis of the\n1.2Fe-FER zeolite has been added in Fig. R5 (New Figure shown below and also in the\nsupporting information as Fig. S5). No significant differences are observed between the 0.6Fe-\nFER and 1.2Fe-FER samples, which can be attributed to the high dispersion of tiny\nnanoparticles.\nFig. R5. Wavelet transform spectra of the 0.4Fe-FER and 1.2Fe-FER zeolites.\nAs you suggested, UV-vis spectroscopy was also utilized to determine the presence of\noligomeric Fe species. Clearly, the UV-vis spectra of 1.2Fe-FER zeolite reveal the presence of\nsmall oligomeric Fe O clusters at 346 nm, in addition to isolated Fe ion sites at 226 and 272 nm\nx y\n(Fig. R6, new Figure shown below and also in the supporting information as Fig. S3).\nFig. R6. UV-vis spectra of the 0.6Fe-FER and 1.2Fe-FER zeolites.\nRevisions: For improving the understanding, the aforementioned discussion has been included\nin the revised manuscript: “The UV–vis spectra of 1.2Fe-FER also reveal the presence of small\noligomeric Fe O clusters at 346 nm, in addition to isolated Fe ion sites at 226 and 272 nm\nx y\n(Fig. S3).” (See. Lines 24 to 25 of Page 5)\nComment 2. In Lines 157–163, it would be beneficial for the authors to explicitly clarify which\ncatalyst the IS and QS values of species I, II, and III correspond to. Specifically, do these values\nrepresent those of 0.6Fe-FER, or are they averaged across different catalysts? Clarifying this\naspect would enhance the readers’ understanding of the presented data.\nOur response: Thanks for your constructive suggestion. A more detailed classification of the\ncomponents has been revised in the manuscript. The component I of 0.4Fe-FER zeolite with\nIS=0.28 mm/s and QS=1.53 mm/s and 0.6Fe-FER zeolite with IS=0.25 mm/s and QS=1.7\nmm/s can be assigned to the [FeO]+ at six-membered ring (G-site). The component II of 0.4Fe-\nFER zeolite with IS=0.52 mm/s and QS=2.83 mm/s and 0.6Fe-FER zeolite with IS=0.5 mm/s\nand QS=2.8 mm/s has a high value of QS, suggesting that the component is in a position which\nconfers it a less symmetry. Accordingly, the Fe3+ at B-site can be assigned to the component II,\nas reported previously. The component III of the Fe-FER zeolites with IS>0.6 mm/s is assigned\nto Fe2+, while the lower value of QS (<0.5 mm/s) indicates that Fe2+ is most likely located at\nG-site and stabilized with the negative charge provided by the two aluminum.\nRevisions: The aforementioned discussion has been included in the revised manuscript. (See.\nLines 15 to 22 of Page 8)\nComment 3. The 2D 1H-1H DQ MAS NMR spectroscopy data of H-FER reveal two distinct\ncross-peak pairs, which the authors attribute to interactions between adjacent BAS and EFAL\nsites. The authors further propose that Fe incorporation into BAS sites during hydrothermal\nsynthesis led to the disappearance of autocorrelation peaks. However, it is observed that only\none cross-peak pair was significantly affected and converged, while the other remained\nunchanged. A more detailed discussion on why certain BAS sites were preferentially affected\nby Fe incorporation would strengthen the interpretation of the NMR results.\nOur response: Thanks for your helpful suggestion. The BASs formed by framework aluminum\nin zeolites (Si-O(H)-Al) are primarily located within the internal channels. The introduction of\niron species mainly exists as ion sites replace BASs (Si-O(Fe)-Al) and are incorporated into\nthe zeolite channels. In contrast, extra-framework aluminum (EFAL) species are also\ndistributed on the external surface of the zeolite. Therefore, the introduction of iron has a more\nsignificant impact on the BASs compared to the EFAL species.\nRevisions: The aforementioned discussion has been included in the revised manuscript. (See.\nLines 8 to 13 of Page 10)\nComment 4. To further support the discussion on BAS acidity changes, the authors may\nconsider providing FTIR analysis using NH as a probe molecule. Such an analysis would\n3\nallow for a clearer evaluation of how Fe substitution influences the acidic properties of the\ncatalyst.\nOur response: Thanks for your insightful suggestion. To more accurately quantify the acidic\nsites in the catalysts, the FT-IR experiments of NH adsorption were supplemented as shown\n3\nin Fig. R1 (New Figure shown below and also in the supporting information as Figure S8). The\nreduced amount of BASs and enhanced amount of LASs in Fe-FER zeolites quantified by the\nFT-IR spectra of NH adsorption also confirm that Fe sites, substituting proton counter-cations,\n3\nexhibit Lewis acid properties by acting as electron pair acceptors (Please also see our response\nto Reviewer 1, Comment 2).\nComment 5. Since Fe is known to exhibit ammonia decomposition activity at elevated\ntemperatures, it would be valuable for the authors to discuss whether this effect could have\ninfluenced the NH -TPD results. Providing MS data alongside NH -TPD would help confirm\n3 3\nwhether ammonia decomposition contributed to the observed desorption profiles.\nOur response: Thanks for your valuable suggestion. The TCD signals were provided alongside\nMS signals in NH -TPD-MS measurements, as shown in Fig. R7 (new Figure shown below\n3\nand also in the supporting information as Fig. S11). Before 650 °C, no significant difference is\nobserved between the TCD and MS signals, indicating that ammonia decomposition does not\noccur in Fe-FER zeolites. This could be well supported by the FT-IR experiments of NH\n3\nadsorption (Fig. R1, also see our response to Reviewer 1, Comment 2).\nFig. R7. NH -TPD-MS measurements on the H-FER and different Fe-FER zeolites. The peak\n3\nat the low temperature (<300 °C) is attributed to NH desorption at the weak acid site, while\n3\nthe peak in the temperature region of 300~600 °C is assignable to the desorption of NH at the\nRevisions: The aforementioned discussion has been included in the revised manuscript. (See.\nLines 8 to 9 of Page 10)\nComment 6. To more effectively illustrate the suppression of N2O formation, the inclusion of\nN2 selectivity data, in addition to N2 yield, would be highly beneficial. Selectivity provides a\nclearer perspective on how efficiently NOx is converted to N2 relative to other byproducts,\nthereby offering a more comprehensive assessment of catalyst performance.\nOur response: Thanks for your valuable suggestion. Selectivity of N for Methanol-SCR on\nshown below and also in the supporting information as Fig. S20). A progressive increase in\niron content leads to a corresponding decline in N selectivity during the Methanol-SCR\n2\nreaction, primarily owing to the loss of BASs in the catalysts, as BASs have been reported to\nbe closely associated with the high temperature activity and N selectivity in Methanol-SCR\n2\nreactions.\nFig. R8. Selectivity of N for Methanol-SCR on the FER zeolites.\nin the revised manuscript in a following manner: “A progressive increase in iron content leads\nto a corresponding decline in both deNO activity and N selectivity during the Methanol-SCR\nx 2\nreaction, primarily owing to the loss of BASs in the catalysts, as BASs have been reported to\nbe closely associated with the high temperature activity and N selectivity in Methanol-SCR\n2\nreactions (Fig. S20).” (See. Lines 14 to 17 of Page 14)\nComment 7. In Fig. 4(b), the total carbon concentration at 200°C appears inconsistent with\nthe trend observed in the dataset. Given that CH OCH contains two carbon atoms, its\n3 3\ncontribution to the total carbon balance should be adjusted accordingly. A re-evaluation of this\ncalculation may improve the accuracy of the presented data.\nOur response: Thanks for your insightful suggestion. We have checked the data and adjusted\nthe concentration of CH OCH in the revised manuscript, as shown in Fig. 4b.\n3 3\nComment 8. The authors propose that the oxidation state of [FeO]⁺ changes CH₃OH-SCR. If\nthis is the case, one might expect corresponding variations in TOT vibrations and M-O\nstretching vibrations under reaction conditions. Providing additional spectroscopic analyses,\nsuch as in situ FTIR or Raman spectroscopy, could further substantiate this claim and\nstrengthen the overall discussion.\nOur response: Thanks for your valuable suggestion. The perturbed T-O-T vibrational modes\n(1000~800 cm-1 in the IR spectra) of the zeolite framework also provide an effective method to\ndistinguish different Fe sites. The IR bands at 903, 895, 885 and 873 cm-1 are attributed to T-\nO-T oxygen bridges perturbed by Fe2+, Fe3+, [FeO]+ and small oligomeric Fe O clusters,\nx y\nrespectively (Fig. R4 as shown below, which was also provided in the supporting information\nas Fig. S7). The quantitative results are in good agreement with those obtained from the\nMössbauer spectra (Table S4). In situ FT-IR spectroscopy was also employed as a\ncomplementary technique to monitor changes in Fe sites during the NO oxidation reaction (Fig.\n3d, as a new Figure updated in Fig. 3). Correspondingly, the introduction of 1000 ppm NO/N\n2\ninduced a distinct shift to higher wavenumbers in the IR spectra, suggesting increased formation\nof Fe2+ species with a significant decrease in the [FeO]+ site. This shift is accompanied by the\nemergence of IR bands at 1870, 1590, and 1478 cm-1, which are attributed to Fe2+–NO, nitrates,\nand Fe2+–NO - species, respectively. Accordingly, the IR band at 909 cm-1 can be assigned to\n2 2 2\ninstead of NO, a distinct shift to lower wavenumbers was observed, accompanied by the\nappearance of a band at 1625 cm-1 (attributed to NO adsorption) and a decrease in the Fe2+–\n2\ngood agreement with the results of the Mössbauer spectra. The significant variance observed\nin [FeO]+ demonstrates its pivotal role as active sites in the NO oxidation reaction. The [FeO]+\nsite can react with NO to form [Fe2+-NO -]+ intermediate. Subsequently, NO is generated via\n2 2\nelectron transfer from NO ⁻ to the oxygen atom (originating from O ), forming the [Fe2+-O-]+\n2 2\nintermediate. Further electron transfer from Fe2+ to O⁻ facilitates the formation of [FeO]+.\nFig. R4. IR spectra collected at 175 °C of (a) 0.4Fe-, (b) 0.6Fe- and (c) 1.2Fe-FER zeolites.\nThe peak position corresponding to component I, II, III, IV is 903, 895, 885, and 873 cm-1,\nrespectively.\nRevisions: For improving the understanding, all the new Figures have been provided in the\nrevised manuscript and the corresponding discussions have also included in the main text. (See.\nLines 27 to 29 of Page 8, Lines 1 to 2 of Page 9, Lines 1 to 12 of Page 13)\nComment 9. While this study provides valuable insights into the role of [FeO]+ in NO oxidation\nand Methanol-SCR, the reliance on Mössbauer spectroscopy for Fe species assignment raises\nsome concerns as the deconvolution process could introduce variability in some cases (Fig.\n1e). Additionally, XANES data (Fig. 1b) suggest that most of Fe species are observed in the\nFe3+ oxidation state, which calls for further discussion on the oxidation state assignments.\nGiven that cation location in FER zeolite frameworks is known to influence perturbed\nvibrational modes (~900 cm-1 in IR), employing IR spectroscopy could provide a\ncomplementary method to distinguish Fe site locations, as seen in studies on Co-FER and Pd-\nFER (There are numerous literatures on this). Furthermore, while the manuscript attributes\nthe superior NO oxidation ability of [FeO]+ to its coordinated oxygen, it remains unclear\nwhether this effect is primarily due to reactive oxygen species or intrinsic geometric differences\narising from distinct cation positions. Addressing these aspects through additional\nspectroscopic validation would further strengthen the conclusions of this study.\nOur response: Thanks for your valuable suggestion. The pre-edge analysis of XAS spectra\nand the perturbed T-O-T vibrational analysis (1000~800 cm-1 in the IR spectra) of the zeolite\nframework were also employed as complementary techniques to discriminate various oxidation\nstates and coordination environments of Fe sites (Figs. 1c & 1d (new Figures updated in Fig .1),\nTable S2, Fig. S7 and Table S4). The pre-edge peak in the spectra corresponds to the dipole-\nforbidden 1s→3d transition. The position of the pre-edge peak can be also used as a direct\nmeasure of the Fe oxidation state, making an unambiguous distinction between Fe2+ (~7112.5\neV) and Fe3+ (~7113.5 eV). Graphical analysis of the correlation between integrated intensities\nand positions of pre-edge features in the variogram was employed to further discriminate\nvarious oxidation states and coordination environments of Fe. Since the 1s→3d transition is a\nquadrupolar transition, it exhibits weak intensity in coordination geometries with an inversion\ncenter (centrosymmetric geometry), such as octahedral coordination. But in fivefold or fourfold\ngeometry, 3d–4p orbital hybridization facilitates dipole-allowed 1s→4p transitions, resulting\nin significantly higher intensity. As shown in Fig. 1c and 1d, the higher integrated intensities\nof Fe-FER zeolites pre-edge analysis than Fe3+ , indicating that Fe sites in lower geometry in\nOh\nFe-FER zeolites than the bulk Fe O .The Fe3+/Fe2+ ratio obtained by curve fitting analysis of\n2 3\npre-edge peak in Fe-FER zeolites exhibits an increasing trend with increasing iron loading\n(Table S2). (Please also see our response to Reviewer 1, Comment 3)\nThe perturbed T-O-T vibrational modes (1000~800 cm-1 in the IR spectra) of the zeolite\nframework also provide an effective method to distinguish different Fe sites. The IR bands at\n903, 895, 885 and 873 cm-1 are attributed to T-O-T oxygen bridges perturbed by Fe2+, Fe3+,\n[FeO]+ and small oligomeric Fe O clusters, respectively (Fig. S7). The quantitative results are\nx y\nin good agreement with those obtained from the Mössbauer spectra (Table S4).\nIn situ Mössbauer and FT-IR spectroscopy were employed to monitor the changes of Fe\nsites during the NO oxidation reaction. Mössbauer spectra are shown in Fig. 3c and the\ncorresponding Mössbauer parameters are listed in Table 2. Firstly, the catalyst was oxidized\nunder the reaction atmosphere of 14% O /N for 1 h. With the introduction of 1000 ppm NO/N ,\n2 2 2\nthe content of component I decreases from 53% to 10%, accompanied by the appearance of a\nnew Fe2+ site (component V), signifying that the [FeO]+ was reduced to Fe2+. Compared to\ncomponent I, the insignificant change in QS values of component V points to the [Fe2+-NO -]+\n2\nstructure may be formed at the G-site. When 14% O /N was introduced in instead of NO, the\n2 2\noxidation reaction from Fe2+ to [FeO]+ was taken place according to the disappearance of\ncomponent V in conjunction with the component I increases from 10% to 52%. In situ FT-IR\nspectroscopy was also employed as a complementary technique to monitor changes in Fe sites\nduring the NO oxidation reaction (Fig. 3d). Correspondingly, the introduction of 1000 ppm\nNO/N induced a distinct shift to higher wavenumbers in IR spectra, suggesting increased\n2\nformation of Fe2+ species with a significant decrease in the FeO+ site. This shift is accompanied\nby the emergence of IR bands at 1870, 1590, and 1478 cm-1, which are attributed to Fe2+–NO,\nnitrates, and Fe2+–NO - species, respectively. Accordingly, the IR band at 909 cm-1 can be\n2\nassigned to T-O-T oxygen bridges perturbed by Fe2+–NO - species. When 14% O /N was\n2 2 2\nintroduced instead of NO, a distinct shift to lower wavenumbers was observed, accompanied\nby the appearance of a band at 1625 cm-1 (attributed to NO adsorption) and a decrease in the\n2\nof the Mössbauer spectra. The significant variance observed in [FeO]+ demonstrates its pivotal\nrole as active sites in the NO oxidation reaction. The [FeO]+ site can react with NO to form\n[Fe2+-NO -]+ intermediate. Subsequently, NO is generated via electron transfer from NO ⁻ to\n2 2 2\nthe oxygen atom (originating from O ), forming the [Fe2+-O-]+ intermediate. Further electron\n2\ntransfer from Fe2+ to O⁻ facilitates the formation of [FeO]+. (Please also see our response to\nReviewer 1, Comment 4)\nRevisions: For improving the understanding, the aforementioned discussion has been included\nincluded\nin the revised manuscript and highlighted in yellow. (See. Lines 1 to 15 of Page 6, Lines 27 to\n29 of Page 8, Lines 1 to 2 of Page 9, Lines 18 to 27 of Page 11, Lines 1 to 12 of Page 13)\nComment 10. Although the authors claim HONO as a key intermediate in the Methanol-SCR\nmechanism, additional evidence is required. The presented IR data primarily shows the\npresence of methyl groups, nitrates, and nitrites, but does not explicitly confirm the formation\nor participation of HONO in the reaction pathway. I am not sure whether the authors' previous\nwork may provide further justification or not, it would be beneficial to discuss whether HONO\ncan be directly observed through spectroscopic techniques. If direct detection is challenging,\nexploring indirect validation methods such as isotope labeling experiments or kinetic studies\ncould provide additional support for its role.\nOur response: Thanks for your valuable suggestion. HONO is difficult to detect directly as a\nhighly reactive intermediate. Once formed, it is rapidly adsorbed by hydroxyl groups and\nsubsequently converted into nitrite species via reactions such as Al–OH + HONO → Al–ONO\n+ H O. HONO has been proposed as a key intermediate in the SCR reaction over both copper-\n2\nand iron-containing zeolites, and is also regarded as a nitrite-precursor. (See former Refs:\nApplied, Catalysis B: Environmental, 2022, 316,121522; ACS Catal. 2022, 12, 9983−9993)\nTherefore, nitrite species are generally accepted as indirect evidence for the formation of\nHONO.\nComment 11. Lastly, I think a broader comparison with other catalysts should be provided.\nSince Cu-based zeolites are generally known for their superior redox properties compared to\nFe-based ones, they may exhibit even higher activity in this reaction. Including Cu-exchanged\nzeolite as a reference catalyst could offer useful context for evaluating the advantages and\nlimitations of Fe-FER, particularly regarding NO oxidation and overall Methanol-SCR\nperformance\nOur response: Thanks for your helpful suggestion. Fe-FER zeolites prepared in this work\nexhibit a competitive advantage in low-temperature deNO efficiency compared to other\nx\nMethanol-SCR catalysts, such as the reported Ag/Ag O and Cu-containing catalysts (See Fig.\n2 3\nR9, new Figure shown below and also in the supporting information as Fig. S22). CHA and\nFER zeolites with a Cu loading of 0.6 wt% were synthesized and evaluated for the Methanol-\nSCR reaction and NO oxidation reaction (See Fig. R10, new Figure shown below and also in\nthe supporting information as Fig. S21). The Cu-FER zeolite exhibits high catalytic activity in\nthe temperature range of 350~450 °C with the conversion of NO above 70%, while the Cu-\nx\nCHA zeolite, commonly used as a commercial NH -SCR catalyst, showed less than 20%\n3\nactivity across the entire temperature range.\nFig. R9. Comparison of deNO efficiency for Methanol-SCR on the Fe-FER catalysts with the\nx\nreported catalysts. The reaction rate was measured with the conversion of NO is ~ 20% of\nx\nwhich the limitations of internal and external diffusion were excluded.\nFig. R10. (a) NO conversion for Methanol-SCR and (b) NO conversion for NO oxidation on\nx\nthe Cu-CHA and Cu-FER zeolites. The reaction conditions are consistent with those of the Fe-\nFER zeolites.\nThe catalytic performance of the Cu-FER zeolite in the Methanol-SCR reaction appears\nto depend primarily on the BASs of the FER framework, as its activity is comparable to that of\nthe H-FER zeolite. Moreover, neither the Cu-FER nor the Cu-CHA zeolite exhibits significant\nNO oxidation activity. The very low activity of the Cu-CHA zeolite is likely attributed to its\nunique microporous structure of CHA-type, which can easily lead to carbon deposition during\nthe methanol-related reaction. (Fuel, 2025, 395, 135207) It indicates that the zeolite topology\nalso plays a crucial role in the Methanol-SCR catalytic performance.\nRevisions: For improving the understanding, the aforementioned Figures and the\ncorresponding discussion has been included in the revised manuscript and highlighted in yellow.\n(See. Lines 17 to 20 of Page 14, Lines 21 to 29 of Page 18, Lines 1 to 3 of Page 19, Lines 4 to\n5 of Page 20)\nReviewer #3:\nGeneral Comment: This paper reports on a mechanistic study of the Selective Catalytic\nReduction of NO over Fe-zeolite catalysts using methanol as the reducing agent. The authors\nclaim that the collaboration between zeolitic acid sites and Fe redox sites provides a strategy\nfor boosting the low-T MeOH SCR activity: however, I have some doubts about the novelty of\nthis concept and the conclusive nature of the evidence included in the manuscript. Please see\nmy points below.\nOur response: We would like to express our gratitude for your valuable comments and\nconstructive suggestions, which has contributed significantly to increasing the overall quality\nof our manuscript.\nComment 1. As a preliminary general comment, the English usage needs to be improved: there\nare several incomplete or incorrect sentences and typos, which make sometimes difficult to\nfollow the text flow.\nOur response: Thanks for your valuable suggestion. We have thoroughly polished the\nmanuscript for a better readability.\nComment 2. A major concern is the identification of the oxidation of NO to NO as the rate\n2\nlimiting step of the SCR reactions (line 188). This is a controversial assumption, which has\nbeen shown in the past to contradict several experimental findings. For example, i) the rate of\nNO oxidation is typically much lower than the SCR rate, ii) H O has opposite effects on the\n2\nrates of NO oxidation and of the NH -SCR reaction, iii) the NO oxidation activities of Fe-\n3\nzeolite and Cu-zeolite catalysts do not correlate with the corresponding NH3-SCR activities ….\nOur response: Thank you very much for this remark, and now the corresponding descriptions\nhave been corrected in a following manner:\n“Since the excellent redox properties are crucial for the SCR reaction, NO oxidation\nperformance of the Fe-FER zeolites was first investigated.”\nComment 3. Figure 3b shows a linear correlation between the NO oxidation rate at 300 °C\nand the amount of [FeO]+. Notice that at 300 °C the NO oxidation rate is already clearly\nlimited by chemical equilibrium: see Figure 3a where the NO conversion at 300 °C is in the\ndescending branch of the lightoff curves for all the catalysts. Therefore, the rates plotted in\nFigure 3b are hardly representative of the intrinsic NO oxidation kinetics, being at least in part\ncontrolled by thermodynamics.\nOur response: Thanks for your constructive suggestion. To completely exclude\nthermodynamic effects, we re-plotted the correlation between the reaction rate and the amount\nof [FeO]+ site at 175 °C (see updated Fig. 3b). The reaction rates for NO oxidation which\nexcluding the limitations of internal and external diffusion also manifest an immensely good\nlinear relationship with the amount of [FeO]+ present in Fe-FER zeolites.\nRevisions: For improving the understanding, the corresponding discussion has been modified\nand included in the revised manuscript and highlighted in yellow. (See. Lines 8 to 10 of Page\n11, Lines 2 to 3 of Page 12, Lines 12 to 14 of Page 22)\nComment 4. How were the NO oxidation rates extracted from the integral NO conversion data\nof Figure 3a?\nOur response: Thanks for your insightful question. The reaction rates of NO oxidation reaction\nwere calculated using the following equation:\nFeed (mL∙h-1)× X (%)\nReaction rate (mmol∙g-1∙h-1)= NOx NOx\nm (g)×22.4 (L∙mol-1)×10-3(mol∙mmol-1) ×103 (mL∙L-1)\ncatal\nwhere Feed represents the concentration of NO , X represents the conversion\nNOx x inlet NOx\nof NO , m represents the mass of catalyst.\nx catal\nRevisions: For improving the understanding, the aforementioned equation has been provided\nin the revised manuscript and highlighted in yellow. (See. Lines 9 to 12 of Page 23)\nComment 5. Apparently, the authors propose a NO activation pathway wherein NO is first\noxidized to NO over [FeO]+ and then HONO is formed. NO oxidation to NO is a two-electron\n2 2\nprocess, so how can it proceed over single Fe sites, which cycle between Fe3+ and Fe2+? Can\nyou rule out an alternative pathway where NO is directly oxidized to HONO over Fe3+OH,\nas proposed in the literature for both Fe- and Cu-zeolite catalysts?\nOur response: Thanks for this hint. The oxidation of NO (2NO+O →2NO ) involves a four-\n2 2\nelectron transfer process, as illustrated in Fig. 3e (updated Figure in Fig. 3). The [FeO]+ site\ncan react with NO to form [Fe2+-NO -]+ intermediate. Subsequently, NO is generated via\n2 2\nelectron transfer from NO ⁻ to the oxygen atom (originating from O ), forming the [Fe2+-O-]+\n2 2\nintermediate. Further electron transfer from Fe2+ to O⁻ facilitates the formation of [FeO]+.\nAs suggested by the reviewer, Fe3+–OH has been reported as an alternative active site\nwhere NO can be directly oxidized to HONO. However, the characteristic IR band of Fe3+–OH\n(~3680 cm-1) was not observed on the Fe-FER zeolites (Fig. R11, new Figure as shown below).\n(ACS Catal. 2022, 12, 9983−9993) Therefore, Fe3+–OH was not considered an active site in\nthis work.\nFig. R11. The FTIR spectra of Fe-FER zeolites.\nRevisions: For improving the understanding, the corresponding discussion has been included\nin the revised manuscript and highlighted in yellow. (See. Lines 9 to 12 of Page 13)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised manuscript has been improved; however, several questions remain to be addressed before the manuscript can be accepted.\nComments:\n1. From the in situ Mossbauer is it is clear that Peak I, which is assigned to FeO+, is redox active for NO oxidation. The fraction of Peak I decreases on reaction with NO with formation of a new Peak V and oxidizes back to Peak I with O2. The in situ reaction, however, is not MeOH SCR. It’s unfortunate, that the NO reduced Peak V species was not reacted with MeOH and shown to make N2 and the original oxidized Fe, i.e., Peak I. Why does the confirmation of the active site for NO oxidation lead to identification of the active species for MeOH SCR? The connection between the two reactions is not clearly discussed.\n2. The XAS analysis is incomplete with little discussion of the results. The XANES pre-edge peak was fit with Fe+2, Fe+3 and Fe2O3. It is not clear what was used for the Fe+2 and Fe+3 standards. Additionally, Fe2O3 has a higher shell Fe-O-Fe peak, see figure 1e, while samples have no higher shell peaks, also see figure 1e. Why is there a good fit of the small pre-edge peak using and incorrect standard? The fractional agreement of Peak III (Fe+2) by Mossbauer with the XANES pre-edge peak is a reasonable agreement for the 0.6% Fe sample. What about the other samples where the Mossbauer has been done, Table 1. Why is there any agreement since you don’t have pure samples? Have these small peaks been overfit?\n3. The EXAFS of one sample has been fit. The Fe-O CN was 4.7, Table S3. All the Fe references have 6 Fe-O bonds, so what does it mean when the CN is 4.7? Since there are multiple Fe species by Mossbauer, what information does the bond distance and CN provide? The small size of the FT of the EXAFS, figure S4 is difficult to see if any of the samples are different. From the Mossbauer, there should be significant differences between the samples.\n4. In previous work, it has been proposed that HONO species are important reaction intermediates. However, these were shown to be formed by NOx and H2O, see line 340. Why not conduct Mossbauer to show that FeO+ reduced to the reaction intermediate with NOx and H2O? H2O often has a large effect on deNOx chemistry and dry NO may not give the same results as NO + H2O. How has HONO been shown to be the active species for N2 formation (in this study) that is different from previous studies?\n5. IR seems to imply that additional reaction intermediates are formed prior to N2 formation. Mossbauer of the same reactions would provide confirmation of the proposed intermediates.\n6. If FeO+ is the deNOx active site, the fraction in each sample is approximately the same, Table 1. I don’t understand why having more sites, i.e., same fraction, but higher Fe loading leads to lower rates. The explanation that MeOH is oxidized with high loadings, but not low, doesn’t make sense. If MeOH oxidation is high and nearly completely oxidized with high loadings, but not low loadings, this indicates that high MeOH with higher loaded catalysts will have higher rates. It also suggests that significant oxidation of MeOH will occur on these catalysts. To get high NOx conversions, large amounts of MeOH will be required. This will become a practical problem. I would like to see the highly loaded Fe catalysts kinetics at the same MeOH outlet concentration as that of the low loaded catalysts. Are the TOR the same (based on FeO+)? This would also confirm the active site is FeO+.\n7. The oxidation of MeOH would also suggest that other C species may be the reactants, e.g. CO or H2CO. MeOH oxidation would also suggest that FeO+ is partially reduced. How does one know other Fe species are not formed and are the active site? In situ Mossbauer, which is the very interesting part of this paper, could be highly informative.\nThe conformation of the active site as FeO+, or Fe species formed by reaction with MeOH, can be confirmed by testing each condition at differential conversions and determining the TOR. The amount of MeOH in the outlet gas needs to be high and approximately the same.", "reviewer_2": "We sincerely thank the authors for thoroughly addressing the requested additional analyses. Many of the previous concerns have been sufficiently clarified. However, a few points still remain open to discussion, and we would like to offer some additional comments in this regard.\nSpecific Comments\n1. It is noted that the concentration of CH₃OCH₃ in Fig. 4b has been modified. However, my initial concern was related to a potential error in the calculation or reporting of the total carbon concentration, not in the raw data value of CH₃OCH₃ itself. Therefore, the revision of the CH₃OCH₃ concentration is rather unexpected. To clarify this point, I would appreciate it if the authors could provide the corresponding real-time FTIR data supporting this experiment.\n2. Regarding the revised Fig. 3 (the in-situ FT-IR spectra): While I agree with the overall experimental design, I would kindly ask the authors to present the in-situ FT-IR spectra as a time-resolved overlay (with at least 1-minute intervals). the in-situ FT-IR is a powerful technique for tracking the dynamic evolution of surface species in real time. For example, during the gas-switching step from NO to O₂, observing the time-dependent shift of the T–O–T vibrational peak would provide stronger evidence for the proposed consumption of the Fe–NO₂ intermediate. Presenting the data in a time-resolved manner would substantially strengthen the authors’ mechanistic interpretation.\n3. We appreciate the authors’ effort in preparing and evaluating two types of Cu ion-exchanged catalysts in response to our previous request. However, we believe that the superior performance of the Fe-FER catalyst is likely attributable to the in-situ incorporation of Fe ions during the hydrothermal synthesis, which enables atomic-level dispersion within the zeolite framework. Therefore, if feasible, we kindly encourage the authors to synthesize a Cu-based catalyst using the same synthetic protocol as that of Fe-FER—by simply replacing the Fe precursor with a Cu precursor. This would allow for a more meaningful and direct comparison between the two metal systems under identical synthetic and structural conditions.", "reviewer_3": "I am satisfied with the authors' response to my comments, but for Comment 3.\nIn this respect, please notice that the best fit straight line of the reaction rates in the updated Fig. 3b does not pass through the origin. It seems therefore that the rate of NO oxidation is at least in part controlled by something else than just the amount of [FeO]+ sites.\nAccordingly, I am even more convinced that the oxidation of NO to NO2 may not be the best reaction to probe the redox properties of the Fe-FER catalyst. Again, the correlation of the NO oxidation activity with the SCR DeNOx activity remains questionable.\nThese aspect deserve some more thorough discussion in my view."}, "author_response": "Response to comments\nManuscript ID: NCOMMS-24-78027B\nFirst of all, we are grateful for all reviewers’ in-depth comments and suggestions, which have\nhelped us greatly to improve the quality of the present manuscript. We have considered all the\ncomments and suggestions seriously, and carried out the related experiments following the\nsuggestions of the reviewers. We have made detailed and point to point response (written in\nBlue) and revised the manuscript accordingly with all changes being highlighted in Yellow in\nthe revised Manuscript and Supporting Information.\n------------------------------------\nReviewer #1:\nGeneral Comments: The revised manuscript has been improved; however, several questions\nremain to be addressed before the manuscript can be accepted.\nComment 1. From the in situ Mossbauer is it is clear that Peak I, which is assigned to FeO+,\nis redox active for NO oxidation. The fraction of Peak I decreases on reaction with NO with\nformation of a new Peak V and oxidizes back to Peak I with O . The in situ reaction, however,\n2\nis not MeOH SCR. It’s unfortunate, that the NO reduced Peak V species was not reacted with\nMeOH and shown to make N and the original oxidized Fe, i.e., Peak I. Why does the\n2\nconfirmation of the active site for NO oxidation lead to identification of the active species for\nMeOH SCR? The connection between the two reactions is not clearly discussed.\nOur response: Thank you for your insightful comments. Generally, strong acidity and\nexcellent redox properties are two crucial factors in the design of NH -SCR catalysts (Nat.\nfacilitates the adsorption of NH /NH + species, which can react with adsorbed nitrates/nitrites\n3 4\nvia the Langmuir-Hinshelwood (L-H) mechanism or directly with gaseous NO through the\nx\nEley-Rideal (E-R) mechanism. A prominent redox ability favors the formation of active NH\n2\nor gaseous NO intermediates, which can enhance deNO efficiency via the E-R mechanism or\n2 x\n\"Fast SCR,\" respectively. Based on this design rationale, we propose that the acidity and redox\nproperties are also pivotal for the Methanol-SCR catalyst. The abundant and strong acidity of\nzeolite is essential for the adsorption and activation of methanol molecules in Methanol-SCR\ncatalysts, as reported in previous research (ACS Catal. 2022, 12, 2403-2414; Catal. Today 2020,\n355, 443-449). However, the direct synthesis of H-FER zeolite with the controllable amounts\nof extra-framework aluminum (EFAL) species as Lewis acid sites (LASs) is very difficult and\nwith poor repeatability, thus is not conducive to industrial applications. Herein, we have\ndeveloped a series of Fe-FER zeolites as Methanol-SCR catalysts, which possess the structural\nfeatures of zeolite acidity and the redox properties of Fe sites. The primary purpose of the\nMössbauer spectroscopy experiments was to verify that Fe species can be incorporated into\nFER zeolites as redox-active sites through the NO oxidation reaction, and to further distinguish\nmultiple Fe species, among which [FeO]+ was identified as the key redox center. Although we\nintended to further validate the Methanol-SCR process using Mössbauer spectroscopy, the\ntesting facility prohibited the introduction of water or methanol into the cell to protect the\nwindow materials.\nIn practical SCR reactions, the abundant presence of water vapor inevitably facilitates the\ninteraction of NO with H O, thereby leading to the formation of nitrites or nitrates that act as\nx 2\nreactive intermediates in the overall process. In both NH -SCR and Methanol-SCR processes,\nformation is also influenced by the NO/NO ratio (Environ. Sci. Technol. 2023, 57, 16289−\n2\n16295; ACS Catal. 2022, 12, 9983−9993; J. Phys. Chem. C 2021, 125, 21975−21987; ACS\nCatal. 2022, 12, 2403-2414). Therefore, the formation of HONO on the catalysts was further\ninvestigated.\nTime-resolved in situ Fourier transform infrared (FT-IR) experiments were conducted to\nfurther elucidate the effect of Fe sites on the Methanol-SCR reaction. As depicted in Figure 5a,\nupon the co-introduction of NO and O in the presence of H O, adsorbed NO (at 1630 cm-1)\n2 2 2\nwas initially detected on H-FER zeolite, followed by the gradual appearance of adsorbed\nnitrates (at 1590 cm-1), N O (at 1700 cm-1), and NO+NO (at 2235 cm-1) species. Compared to\n2 3 2\nH-FER zeolite, 0.6Fe-FER zeolite exhibits different adsorption species when NO and O were\n2\nflowed in. As shown in Fig. 5c, no adsorbed N O species were observed on the 0.6Fe-FER\n2 3\nzeolite, while the formation of nitrite (1470 cm-1) was detected. HONO has been proposed as\na key intermediate in the SCR reaction over both copper- and iron-containing zeolites, and is\nalso regarded as a nitrite-precursor (Appl. Catal. B-Environ. 2022, 316,121522; ACS Catal.\n2022, 12, 9983−9993). However, it is challenging to detect directly because it is a highly\nreactive intermediate. Once formed, it is quickly adsorbed by hydroxyl groups and\nsubsequently converted into nitrite species through reactions such as Al–OH + HONO → Al–\nONO + H O. Therefore, nitrite species are generally accepted as indirect evidence for the\n2\nformation of HONO. In the Methanol-SCR reaction, HONO produced through the interaction\nof NO with H O corresponds to a general reaction occurring on zeolites: NO + NO + H O →\nx 2 2 2\n2HONO.\nFig. 5. Time-resolved in situ FT-IR spectra on H-FER zeolite with (a) 500 ppm NO and 7% O\n2\nflowed in at 300 °C for 30 min; (b) 1000 ppm CH OH and 7% O flowed in at 300 °C for 30\n3 2\nmin after 500 ppm NO flowed in at 300 °C for 30 min. Time-resolved in situ FT-IR spectra on\n0.6Fe-FER zeolite with (c) 500 ppm NO and 7% O flowed in at 300 °C for 30 min; (d) 1000\n2\nppm CH OH and 7% O flowed in at 300 °C for 30 min after 500 ppm NO flowed in at 300 °C\n3 2\nfor 30 min. H O was introduced by bubbling with N (1 mL/min) at 30 °C during all of the\n2 2\ntests for the simulation of Methanol-SCR reaction with the co-existence of H O. (e)\nat B-site in FER zeolite.\nFurthermore, when CH OH was introduced instead of NO, the generation and\n3\nconsumption of IR absorption band at 1690 cm-1 can be captured on the 0.6Fe-FER zeolite,\nas manifested in Fig. 5d. Notably, the species with the IR absorption band at 1690 cm-1 is assigned\nto formamide, another key intermediate in the Methanol-SCR reaction, which can be formed\nby the conversion of nitromethane resulting from the reaction of methyl groups and adsorbed\nHONO. These reaction intermediates are consistent with those previously reported for the\nMethanol-SCR process (ACS Catal. 2022, 12, 2403-2414). The above elaboration points to the\nfact that the introduction of Fe sites not only facilitates the oxidation of NO, but also further\npromotes the generation of the key intermediate HONO, thus enhancing the reactivity of\nMethanol-SCR. A comprehensive reaction pathway for the Methanol-SCR process was\nproposed, as depicted in Fig. R1 (New Figure shown below and also in the supporting\ninformation as Fig. S33). Brønsted acid sites (BASs) contribute to the adsorption and activation\nof methanol to generate active methyl groups, while [FeO]+ sites facilitate the production of\nHONO, thereby accelerating the generation of the key intermediate nitromethane.\nNitromethane can transform into formamide and subsequently react with NO to produce N .\nx 2\nFig. R1. The proposed reaction path of Fe-FER zeolite on the Methanol-SCR reaction.\nRevisions: For improving the understanding, the aforementioned Figures and discussions have\nbeen included in the revised manuscript and highlighted in yellow. (See. Lines 26 to 28 of Page\n17, Lines 1 to 29 of Page 18, Lines 1 to 8 of Page 19)\nComment 2. The XAS analysis is incomplete with little discussion of the results. The XANES\npre-edge peak was fit with Fe+2, Fe+3 and Fe O . It is not clear what was used for the Fe+2\n2 3\nand Fe+3 standards. Additionally, Fe O has a higher shell Fe-O-Fe peak, see figure 1e, while\n2 3\nsamples have no higher shell peaks, also see figure 1e. Why is there a good fit of the small pre-\nedge peak using and incorrect standard? The fractional agreement of Peak III (Fe+2) by\nMossbauer with the XANES pre-edge peak is a reasonable agreement for the 0.6% Fe sample.\nWhat about the other samples where the Mossbauer has been done, Table 1. Why is there any\nagreement since you don’t have pure samples? Have these small peaks been overfit?\nOur response: Thanks for your valuable question. It was previously shown that the pre-edge\npeak in Fe K-edge absorption spectra can directly reflects the oxidation state and coordination\ngeometry of iron in oxide environments. A pre-edge variogram derived from reference spectra\nof well-defined oxidic iron species can therefore serve as a framework for the qualitative and\nquantitative characterization of iron species in unknown compounds. FeO and α-Fe O are two\n2 3\ncommonly used standards in the pre-edge peak analysis of Fe-containing zeolites. (J. Am. Chem.\nSoc. 2025, 147, 1677-1693; J. Am. Chem. Soc. 2024, 146, 31969−31981; J. Synchrotron Rad.\n2015, 22, 410-426) In this study, the XANES pre-edge peaks were fitted using FeO and α-\nFe O as the Fe2+ and Fe3+ standards, respectively. The fitted peaks in blue and purple\n2 3\ncorrespond to Fe2+ and Fe3+, respectively. A distinctive feature of the pre-edge spectrum of bulk\nα-Fe O is the contribution at approximately 7115 eV (shown in light grey), which is attributed\n2 3\nto non-local transitions into the orbitals of nearest-neighbour Fe atoms. Although the Fe–O–Fe\npeaks are clearly observed in α-Fe O , they are difficult to detect in the second-shell\n2 3\ncontributions of zeolite catalysts when the metal species or oxide particles are very small and\nhighly dispersed ( Catal. Today 2025, 446, 115104; Science 2021, 373, 315-320; J. Am. Chem.\nSoc. 2023, 145, 5888-5898). The initial contribution of Fe-FER zeolites is attributed to the Fe-\nO scattering path, and the absence of a second contribution further indicates that Fe sites are\nhighly dispersed within FER zeolites, coordinated with oxygen (see Fig. S4). As for the FT-\nEXAFS spectra of 1.2Fe-FER, similarly, the absence of the second contribution can also be\nperceived, demonstrating that albeit the presence of nanoparticles in 1.2Fe-FER, they are still\nhighly dispersed as iron oxide nanoparticles without any apparent agglomeration.\nRevisions: For improving the understanding, the aforementioned discussions have been added\nin the revised manuscript and highlighted in yellow. (See. Lines 18 to 21 of Page 6, Lines 24\nto 29 of Page 6, Lines 5 to 9 of Page 7)\nComment 3. The EXAFS of one sample has been fit. The Fe-O CN was 4.7, Table S3. All the\nFe references have 6 Fe-O bonds, so what does it mean when the CN is 4.7? Since there are\nmultiple Fe species by Mossbauer, what information does the bond distance and CN provide?\nThe small size of the FT of the EXAFS, figure S4 is difficult to see if any of the samples are\ndifferent. From the Mossbauer, there should be significant differences between the samples.\nOur response: Thanks for your insightful comment. XAFS analysis inherently provides the\naverage structural information of metal species within zeolite materials, making it challenging\nto accurately resolve the distinct states of multiple metal species. For instance, the coordination\nnumbers and bond lengths obtained from the analysis can only be interpreted as average values\nrepresenting the overall metal species present in the zeolite framework. The pre-edge peak in\nthe spectra corresponds to the dipole-forbidden 1s→3d transition. The position of the pre-edge\npeak can be also used as a direct measure of the Fe oxidation state, making an unambiguous\ndistinction between Fe2+ (~7112.5 eV) and Fe3+ (~7113.5 eV). Graphical analysis of the\ncorrelation between integrated intensities and positions of pre-edge features in the variogram\nwas employed to further discriminate various oxidation states and coordination environments\nof Fe. Since the 1s→3d transition is a quadrupolar transition, it exhibits weak intensity in\ncoordination geometries with an inversion center (centrosymmetric geometry), such as\noctahedral coordination. But in fivefold or fourfold geometry, 3d–4p orbital hybridization\nfacilitates dipole-allowed 1s→4p transitions, resulting in significantly higher intensity. As\nshown in Fig. 1c and Fig. 1d, the higher integrated intensities of Fe-FER zeolites pre-edge\nanalysis than Fe3+ , indicating that Fe sites in lower geometry in Fe-FER zeolites than the bulk\nOh\nα-Fe O . According to curve-fitting results as shown in Table S3, the average distance and\n2 3\ncoordination number of Fe-O for 0.6Fe-FER zeolite are estimated to be 2.05 Å and 4.7,\nrespectively, which are consistent with the results of pre-edge analysis.\nFig. 1. (a) HAADF-STEM and spherical aberration-corrected HAADF-STEM images of\n0.6Fe-FER zeolite as well as HAADF-STEM image of 1.2Fe-FER zeolite. (b) XANES spectra\nof Fe-FER zeolites with Fe foil, FeO and α-Fe O as references. (c) Pre-edge spectra of Fe-\n2 3\nFER zeolites with FeO and α-Fe O as references. The fitted peaks in blue and purple represent\n2 3\nFe2+ and Fe3+, respectively. A distinctive feature of the pre-edge spectrum of bulk α-Fe O is\n2 3\nthe contribution at approximately 7115 eV (shown in light grey), which is attributed to non-\nlocal transitions into the orbitals of nearest-neighbour Fe atoms. (d) Variogram of the integrated\nintensity and centroid position of the pre-edge peak of the Fe-FER zeolites. (e) Wavelet\ntransform spectra of Fe foil, FeO, α-Fe O and 0.6Fe-FER. (f) Mössbauer spectra collected at\n2 3\n300 °C of 0.4Fe-FER, 0.6Fe-FER and 1.2Fe-FER zeolites. The fitted peaks in red, green, blue\nand purple represent component I, component II, component III, and component IV,\nrespectively. (g) Position of B-, C-, F- and G-site in the extra-framework of FER zeolite looking\nalong the ten-ring channels (left) and the eight-ring channels (right).\nRevisions: The aforementioned discussions have been included in the revised manuscript and\nhighlighted in yellow. (See. Lines 5 to 16 of Page 6, Lines 21 to 24 of Page 6)\nComment 4. In previous work, it has been proposed that HONO species are important reaction\nintermediates. However, these were shown to be formed by NO and H O, see line 340. Why\nx 2\nnot conduct Mossbauer to show that FeO+ reduced to the reaction intermediate with NO and\nx\nH O? H O often has a large effect on deNOx chemistry and dry NO may not give the same\n2 2 x\nresults as NO + H O. How has HONO been shown to be the active species for N formation\n2 2\n(in this study) that is different from previous studies?\nOur response: Thank you for your valuable question. The main objective of the Mössbauer\nspectroscopy experiments was to confirm that Fe species can be incorporated into FER zeolites\nas redox-active sites via the NO oxidation reaction, and to additionally differentiate between\nmultiple Fe species, with [FeO]+ being identified as the primary redox center. Since the testing\nfacility prohibited the introduction of water or methanol into the cell to protect the\nwindow materials during Mössbauer spectra testing, time-resolved in situ FT-IR experiments were\nconducted to investigate the influence of Fe sites on HONO formation, as depicted in Figure\n5.(Please see our response to Comment 1).\nIn comparison, under dry conditions, more NO+NO (2235 cm-1) and adsorbed NO (1630\n2 2\ncm-1) species were formed on the 0.6Fe-FER zeolite, whereas the adsorbed NO species were\n2\nnot further converted to HONO, as shown in Fig. R2 (New Figure shown below and also in\nthe supporting information as Fig. S31). Accordingly, the absence of H O leads to a significant\n2\ndecrease in Methanol-SCR reactivity (Fig. R3, New Figure shown below and also in the\nsupporting information as Fig. S32).\nFig. R2. Time-resolved in situ FT-IR spectra on 0.6Fe-FER zeolite with 500 ppm NO and 7%\nO flowed in at 300 °C for 30 min.\n2000 ppm CH OH, 1000 ppm NO, and 14% O in N balance.\n3 2 2\nRevisions: For improving the understanding, the aforementioned Figures and the\ncorresponding discussions have been included in the revised manuscript and highlighted in\nyellow. (See. Lines 20 to 24 of Page 18)\nComment 5. IR seems to imply that additional reaction intermediates are formed prior to N2\nformation. Mossbauer of the same reactions would provide confirmation of the proposed\nintermediates.\nOur response: Thanks for your insightful suggestion. Although we intended to further validate\nthe Methanol-SCR process via Mössbauer spectroscopy, the testing facility prohibited the\nintroduction of water or methanol into the cell to protect the window materials.\nAs manifested in Fig. 5d, when CH OH was introduced instead of NO, the generation and\n3\nconsumption of IR absorption band at 1690 cm-1 can be captured. Notably, the species with the\nIR absorption band at 1690 cm-1 can be assigned to formamide, another key intermediate in the\nMethanol-SCR reaction, which can be formed by the conversion of nitromethane resulting from\nthe reaction of methyl groups and adsorbed HONO. These reaction intermediates are consistent\nwith those previously reported for the Methanol-SCR process and follow a similar reaction\npathway as depicted in Fig. R1. (Please see our response to Comment 1).\nComment 6. If FeO+ is the deNOx active site, the fraction in each sample is approximately the\nsame, Table 1. I don’t understand why having more sites, i.e., same fraction, but higher Fe\nloading leads to lower rates. The explanation that MeOH is oxidized with high loadings, but\nnot low, doesn’t make sense. If MeOH oxidation is high and nearly completely oxidized with\nhigh loadings, but not low loadings, this indicates that high MeOH with higher loaded catalysts\nwill have higher rates. It also suggests that significant oxidation of MeOH will occur on these\ncatalysts. To get high NOx conversions, large amounts of MeOH will be required. This will\nbecome a practical problem. I would like to see the highly loaded Fe catalysts kinetics at the\nsame MeOH outlet concentration as that of the low loaded catalysts. Are the TOR the same\n(based on FeO+)? This would also confirm the active site is FeO+.\nOur response: Thanks for your helpful suggestion. [FeO]+ has been identified as an active\ncenter for NO oxidation, but it is not the sole active site responsible for deNO . Brønsted acid\nx\nsites (BASs) in FER zeolites also play a pivotal role in the Methanol-SCR reaction. As shown\nin Figure S26, upon the introduction of CH OH and O , three identical methyl signals at 60.9,\n3 2\n59.2, and 50.5 ppm are observed on both H-FER and 0.6Fe-FER zeolites, corresponding to\nside-on adsorbed dimethyl ether (DME), methyl groups generated on BASs, and adsorbed\nmethanol, respectively. No new methyl signals appeared on 0.6Fe-FER zeolite, signifying that\nmethanol is mainly activated on BASs rather than Fe sites. The adsorption capacity of NO,\nNO and CH OH molecules on BASs was also explored by DFT calculations. It reveals that\n2 3\nthe BASs in FER zeolite display a much stronger adsorption capacity for CH OH molecules\n3\n(25.2 kcal/mol) than that for NO (5.2 kcal/mol) and NO (8.6 kcal/mol), as manifested in Fig.\n2\n4e and Fig. S27. Together with the results of 13C CP/MAS NMR measurements as illustrated\nin Fig. S26, it can be reasonably inferred that BASs are preferentially occupied by methanol\nmolecules and subsequently activated to form methyl groups that participate in the Methanol-\nSCR reaction, which has been confirmed by previous studies (ACS Catal. 2022, 12, 2403-2414).\nConsequently, both [FeO]+ and BASs are kinetically important in the Methanol-SCR reaction.\nAs depicted in Fig. R1, BASs contribute to the adsorption and activation of methanol to\ngenerate active methyl groups, while [FeO]+ sites are conductive to produce HONO, thereby\naccelerating the generation of the key intermediate nitromethane, which can transform to\nformamide and subsequently react with NO to produce N . (Please see our response to\nx 2\nComment 1)\nNotably, there is a 'see-saw' effect where the introduction of [FeO]+ and Fe3+ requires more\nSi-O--Al sites to balance the charge, thereby inevitably leading to a decrease in BASs, as shown\nin Fig. S8. A progressive increase in iron content leads to a corresponding decline in deNO\nx\nactivity during the Methanol-SCR reaction, primarily owing to the loss of BASs, which are\nresponsible for methanol activation.\nThe methanol outlet concentrations during the Methanol-SCR reaction over 0.6Fe-FER\nand 1.2Fe-FER zeolites are shown in Fig. R4 (New Figure shown below and also in the\nsupporting information as Fig. S28). At temperatures below 300 °C, the methanol outlet\nconcentration of 1.2Fe-FER is higher than that of 0.6Fe-FER. Given that methanol oxidation\ndoes not occur below 300 °C (Fig. R5, Figure also in the supporting information as Fig. S17),\nthe lower activity of 1.2Fe-FER is attributed predominantly to the reduced BAS rather than\nmethanol consumption via oxidation.\nFig. R4. The CH OH outlet concentration in the Methanol-SCR reaction on the 0.6Fe-FER and\n3\n1.2Fe-FER zeolites.\nFig. R5. The concentration of CO and CO in the reaction of Methanol oxidation on the H-\n3 2\nH O with N as balance gas.\n2 2\nHowever, the decrease in NO conversion over Fe-FER zeolites at high temperature\nx\n(>300 °C) can be influenced by both the loss of BASs and the oxidation of methanol. Increasing\nthe CH OH inlet concentration enhances Methanol-SCR activity at high temperatures\n3\n(>300 °C), further supporting this speculation that the oxidation of methanol to CO and CO\n2\nmay leads to a shortage of the reductant for Methanol-SCR reaction, as shown in Fig. R6 (New\nFigure shown below and also in the supporting information as Fig. S29).\nFig. R6. (a) The conversion of NO on 0.6Fe-FER zeolite with 2000 ppm and 3500 ppm\nx\nCH OH inlet concentration. (b) The conversion of NO on 1.2Fe-FER zeolite with 2000 ppm\n3 x\nand 3000 ppm CH OH inlet concentration.\nturnover rate (TOR) based solely on [FeO]+ cannot serve as a definitive measure of catalytic\nactivity. Accordingly, the reaction rates were calculated based on the catalyst mass in this work.\nTo ensure similar CH OH outlet concentrations, the reaction rates were measured for 0.6Fe-\n3 3\nconcentration of 3000 ppm at temperatures below 300 °C. At temperatures above 300 °C, the\nreaction rates were determined for 0.6Fe-FER with a CH OH inlet concentration of 3500 ppm\n3\nand for 1.2Fe-FER with a CH OH inlet concentration of 4000 ppm. Although the CH OH outlet\n3 3\nconcentration remained similar, the 0.6Fe-FER zeolite still exhibits a higher reaction rate than\nthe 1.2Fe-FER zeolite, indicating that even under conditions of sufficient reductant availability,\nthe loss of BASs substantially impairs Methanol-SCR activity over the entire temperature range\n(Fig. R7, New Figure shown below and also in the supporting information as Fig. S30).\nFig. R7. The reaction rates calculated based on the catalyst mass for 0.6Fe-FER and 1.2Fe-\nFER zeolites under similar CH OH outlet concentrations.\nrevised manuscript and the corresponding discussions have also included in the main text. (See.\nLines 9 to 15 of Page 16, Lines 1 to 5 of Page 17, Lines 7 to 24 of Page 17)\nComment 7. The oxidation of MeOH would also suggest that other C species may be the\nreactants, e.g. CO or H CO. MeOH oxidation would also suggest that FeO+ is partially\n2\nreduced. How does one know other Fe species are not formed and are the active site? In situ\nMossbauer, which is the very interesting part of this paper, could be highly informative.\nOur response: Thanks for your valuable question. The synergistic interaction between iron\nsites and BASs can substantially enhance the activity of the Methanol-SCR reaction at low\ntemperatures (<300 °C), as shown in Fig. 4a. BASs contribute to the adsorption and activation\nof methanol to generate active methyl groups, while [FeO]+ sites are conductive to produce\nHONO, thereby accelerating the generation of the key intermediate nitromethane, which can\ntransform to formamide and subsequently react with NO to produce N . (Please see our\nx 2\nresponse to Comment 1 and Comment 6) As shown in Fig. S17, the oxidation of methanol to\nCO and CO in the absence of NO starts to take off above 300 °C and without the generation\n2 x\nof HCHO, indicating that Fe species remain unaffected by methanol at temperatures below\n300 °C. Furthermore, the catalytic performance of H-FER zeolite demonstrates that acid sites\nare capable of sustaining high Methanol-SCR activity at temperatures above 300 °C, whereas\nthe introduction of Fe caused a significant decrease in the activity at high temperatures.\nIt suggests that although Fe species are partially reduced at temperatures above 300 °C,\nthey do not contribute significantly to Methanol-SCR activity. Of course, the utilization of in situ\nMössbauer spectroscopy for identifying the dynamic structures of Fe species is very interesting.\nUnfortunately, the testing facility prohibited the introduction of water or methanol into the cell\nto protect the window materials.\nThe conformation of the active site as FeO+, or Fe species formed by reaction with MeOH, can\nbe confirmed by testing each condition at differential conversions and determining the TOR.\nThe amount of MeOH in the outlet gas needs to be high and approximately the same.\nOur response: Thanks for your valuable advice. We have supplemented the experiments and\nthe corresponding discussions have been included in the revised manuscript and highlighted in\nyellow. (Please see our response to Comment 6)\nReviewer #2:\nGeneral Comment: We sincerely thank the authors for thoroughly addressing the requested\nadditional analyses. Many of the previous concerns have been sufficiently clarified. However,\na few points still remain open to discussion, and we would like to offer some additional\ncomments in this regard.\nOur response: We would like to thank you for the positive comments of our work. Below we\nelaborate on the shared concerns.\nComment 1. It is noted that the concentration of CH OCH in Fig. 4b has been modified.\n3 3\nHowever, my initial concern was related to a potential error in the calculation or reporting of\nthe total carbon concentration, not in the raw data value of CH OCH itself. Therefore, the\n3 3\nrevision of the CH OCH concentration is rather unexpected. To clarify this point, I would\n3 3\nappreciate it if the authors could provide the corresponding real-time FTIR data supporting\nthis experiment.\nOur response: Thanks for your insightful comment. The corresponding experimental data are\nprovided in the Table R1.\nTable R1. The product concentration of methanol conversion in Methanol-SCR reaction on\n0.6Fe-FER zeolite.\nTotal\nTemperature CH OH CH OCH 2x CH OCH HCHO CO CO concentration\n3 3 3 3 3 2\n/°C /ppm /ppm /ppm /ppm /ppm /ppm of carbon\n/ppm\n200 1160 350 700 31 53 51 1995\n250 996 71 142 156 320 407 2021\n300 466 0 0 168 566 822 2022\n350 98 0 0 148 584 1188 2018\n400 33 0 0 36 627 1343 2039\nWe would like to clarify that in the first version of the manuscript, due to an oversight on\nour part, the values of 2x CH OCH concentration (Column 4) which used to calculate the total\n3 3\ncarbon concentration, were directly depicted into the Fig. 4b. After being alerted by the\nreviewer, we discovered this error and corrected the concentration value of CH OCH (Column\n3 3\n3)in the first revised version of the manuscript.\nComment 2. Regarding the revised Fig. 3 (the in-situ FT-IR spectra): While I agree with the\noverall experimental design, I would kindly ask the authors to present the in-situ FT-IR spectra\nas a time-resolved overlay (with at least 1-minute intervals). the in-situ FT-IR is a powerful\ntechnique for tracking the dynamic evolution of surface species in real time. For example,\nduring the gas-switching step from NO to O2, observing the time-dependent shift of the T-O-T\nvibrational peak would provide stronger evidence for the proposed consumption of the Fe-NO2\nintermediate. Presenting the data in a time-resolved manner would substantially strengthen the\nauthors’ mechanistic interpretation.\nOur response: Thanks for your valuable suggestion. Time-resolved in situ FT-IR spectra on\n0.6Fe-FER zeolite were supplemented, as shown in Fig. 3d. The introduction of 1000 ppm\nNO/N induced a distinct shift of the T–O–T vibrational peak to higher wavenumbers in IR\n2\nspectra, suggesting increased formation of Fe2+ species with a significant decrease in the [FeO]+\nsite. This shift is accompanied by the emergence of IR bands at 1870, 1590, and 1478 cm-1,\nwhich are attributed to Fe2+–NO, nitrates, and Fe2+–NO - species, respectively. Accordingly,\n2\nthe IR band at 909 cm-1 can be assigned to T–O–T oxygen bridges perturbed by Fe2+–NO -\n2\nspecies. When 14% O /N was introduced instead of NO, a time-dependent shift of the T–O–\n2 2\nT vibrational peak to lower wavenumbers was observed, accompanied by the appearance of a\nband at 1625 cm-1 (attributed to NO adsorption) and a decrease in the Fe2+–NO - species at\n2 2\n1478 cm-1. These observations suggest that NO oxidation proceeds via Fe2+–NO - as an\n2\nintermediate as depicted in Fig. 3e, which is in good agreement with the results of the\nMössbauer spectra. .\nFig. 3. (a) Effect of the amount of Fe on the activity of NO oxidation on FER zeolites. (b) The\nrelationship of the amount of [FeO]+ with reaction rate for NO oxidation at 175 °C. (c) In situ\nMössbauer spectra of 0.6Fe-FER zeolite were collected at 300 °C with 14% O /N flowed in;\n2 2\nwith 1000 ppm NO/N flowed in instead of 14% O /N ; with 14% O /N flowed in instead of\n2 2 2 2 2\n1000 ppm NO/N . The arrows represent the order in which the gases were introduced. The\n2\nfitted peaks in orange represents the component V. (d) Time-resolved in situ FT-IR spectra of\n0.6Fe-FER zeolite collect at 300 °C with 14% O /N flowed in; with 1000 ppm NO/N flowed\n2 2 2\nin instead of 14% O /N ; with 14% O /N flowed in instead of 1000 ppm NO/N . The spectra\n2 2 2 2 2\nwere collected with 1-minute intervals. (e) Structural schematic of the dynamic evolution of Fe\nsites during the NO oxidation reaction.\nRevisions: The new Figures have been provided in the revised manuscript and the\ncorresponding discussions have also included in the main text. (See. Lines 12 to 24 of Page 12,\nLines 1 to 4 of Page 14)\nComment 3. We appreciate the authors’ effort in preparing and evaluating two types of Cu ion-\nexchanged catalysts in response to our previous request. However, we believe that the superior\nperformance of the Fe-FER catalyst is likely attributable to the in-situ incorporation of Fe ions\nduring the hydrothermal synthesis, which enables atomic-level dispersion within the zeolite\nframework. Therefore, if feasible, we kindly encourage the authors to synthesize a Cu-based\ncatalyst using the same synthetic protocol as that of Fe-FER—by simply replacing the Fe\nprecursor with a Cu precursor. This would allow for a more meaningful and direct comparison\nbetween the two metal systems under identical synthetic and structural conditions.\nOur response: Thanks for your helpful suggestion. Cu-FER zeolite was successfully\nsynthesized using copper sulfate as the Cu precursor following the same protocol as 0.6Fe-FER\nzeolite. The final product, denoted as 0.6Cu-FER-one pot, was also evaluated for the Methanol-\nSCR reaction. As shown in Fig. R8 (New Figure shown below and also in the supporting\ninformation as Fig. S21), although the activities of both Cu-FER zeolites are significantly lower\nthan that of the 0.6Fe-FER zeolite, the one-pot synthesized 0.6Cu-FER zeolite exhibits lower\nMethanol-SCR activity at low temperatures (<400 °C) compared to 0.6Cu-FER prepared by\nion exchange method (0.6Cu-FER-IE). The pronounced difference in activities between the\nCu-FER zeolites synthesized by different methods suggests the presence of distinct copper\nspecies in the two catalysts, a phenomenon that needs further investigation.\nFig. R8. (a) NO conversion for Methanol-SCR and (b) NO conversion for NO oxidation on\nx\nthe Cu-containing zeolites. The reaction conditions are consistent with those of the Fe-FER\nzeolites.\nRevisions: The new Figure and the corresponding discussions have been provided in the\nrevised manuscript. (See. Lines 26 to 29 of Page 20, Lines 1 to 11 of Page 21, Lines 13 to 15\nof Page 22)\nReviewer #3:\nGeneral Comment: I am satisfied with the authors' response to my comments, but for Comment\n3. In this respect, please notice that the best fit straight line of the reaction rates in the updated\nFig. 3b does not pass through the origin. It seems therefore that the rate of NO oxidation is at\nleast in part controlled by something else than just the amount of [FeO]+ sites. Accordingly, I\nam even more convinced that the oxidation of NO to NO may not be the best reaction to probe\n2\nthe redox properties of the Fe-FER catalyst. Again, the correlation of the NO oxidation activity\nwith the SCR DeNOx activity remains questionable. These aspect deserve some more thorough\ndiscussion in my view.\nOur response: Thanks for your insightful suggestion. In the updated Fig. 3b, the fitted straight\nline of the reaction rates does not pass through the origin, which can be attributed to the intrinsic\nNO oxidation activity of the acidic sites in the H-FER zeolite, as shown in Fig. 3a. Although\n[FeO]+ sites are not the only redox centers in Fe-FER zeolites, their presence further enhances\nthe oxidation of NO to NO of the catalyst.\ninteraction of NO with H O, thereby leading to the formation of nitrites or nitrates, which\nx 2\nsubsequently serve as reactive intermediates in the overall process. In both NH -SCR and\nintermediate, and its formation is also influenced by the ratio of NO and NO (Environ. Sci.\n125, 21975−21987; ACS Catal. 2022, 12, 2403-2414). Therefore, the formation of HONO on\nthe catalysts was further investigated. A comprehensive reaction pathway for the Methanol-\nSCR process was proposed through the combined analysis of time-resolved in situ FT-IR\nspectra and DFT calculations, as shown in Fig. 5 and the reaction pathway was depicted in Fig.\nR1 (New Figure shown below and also in the supporting information as Fig. S33). BASs in\nFER zeolite contribute to the adsorption and activation of methanol to generate active methyl\ngroups, while [FeO]+ sites are conductive to produce HONO, thereby accelerating the\ngeneration of the key intermediate nitromethane, which can transform to formamide and\nsubsequently react with NO to produce N .\nx 2\nFig. R1. The proposed reaction path of Fe-FER zeolite on the Methanol-SCR reaction.\nRevisions: For improving the understanding, the aforementioned Figure and discussions have\nbeen included in the revised manuscript and highlighted in yellow. (See. Lines 26 to 28 of Page\n17, Lines 1 to 3 of Page 18, Lines 3 to 8 of Page 19)"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The additional data and explanations have significantly improved the paper making it more through and understandable for the reader.\nThe paper can be accepted without further revisions.", "reviewer_2": "I am generally satisfied with the authors’ previous responses and the additional analyses provided. Nevertheless, I have developed some concerns regarding the way in which the authors describe the key intermediates and active sites involved in the reaction. After carefully reviewing the comments raised by other reviewers, I am inclined to believe that there may be flaws in the proposed reaction pathway, which I would like to point out in this round of review.\nSpecific Comments\n1. According to the newly proposed reaction path (Fig. R1), the primary role of [FeO] is to generate HONO as the initiator of the reaction. However, it has been reported that Cu-CHA forms HONO more rapidly than Fe-CHA and exhibits superior low-temperature NH₃-SCR activity. In this context, the unexpectedly poor catalytic performance of Cu-CHA in the Methanol-SCR reaction remains puzzling. Could the authors provide additional experimental evidence or a new rationale to substantiate the claim that NO oxidation ability is the rate-determining factor for the overall reaction pathway? Furthermore, is methanol activation strictly confined to BAS sites? If possible, we strongly encourage the authors to evaluate the methanol oxidation ability of Cu-CHA and present the results, as this would greatly strengthen their mechanistic argument\n2. Further inquiries regarding Fig. R1: If nitromethane is proposed as a key intermediate, its inclusion should be supported by direct experimental evidence or a clearly cited reference. Upon reviewing reference 47, which the authors have indicated, I found no explicit mention of nitromethane, and thus it appears insufficient to substantiate such an important mechanistic claim.\n3. There may be some ambiguity in the current peak assignments in Fig 5. Specifically, the band observed at 2230 cm⁻¹ appears more consistent with NO⁺ adsorbed on BAS rather than NONO₂⁺. Likewise, although the authors attribute the 1630 cm⁻¹ band to adsorbed NO₂ species, a closer inspection of the enlarged spectrum in Fig. R2 suggests that it more closely resembles the well-known bending vibration of water. It is plausible that upon introduction of NO + O₂, the proton from BAS is exchanged with NO⁻, subsequently combining with a neighboring hydroxyl group to form water. To verify the validity of the current assignments, I kindly suggest that the authors conduct a simple dehydration experiment, which could provide a straightforward means of confirming these interpretations.\n4. The authors demonstrate, through in situ Mössbauer experiments with NO + O₂, that the relative proportions of [FeO]⁺ and other Fe sites vary during the reaction cycle. This is an interesting and valuable observation. However, it raises an important question: would the spectral contributions shown in Table 1 (ex situ results) also evolve dynamically if methanol were introduced under reaction conditions? In other words, are reversible changes between different Fe components possible depending on the reaction environment? Clarification on this point would greatly enhance the mechanistic interpretation.", "reviewer_3": "The revised manuscript is now acceptable for publication."}, "author_response": "Response to comments\nManuscript ID: NCOMMS-24-78027C\nFirst of all, we sincerely appreciate the recognition and acceptance of our manuscript by\nReviewer 1 and Reviewer 3, and are also grateful for the in-depth comments and suggestions\nfrom Reviewer 2, which have greatly helped us to improve the quality of the present manuscript.\nWe have considered all the comments and suggestions seriously, and carried out the related\nexperiments following the suggestions of the reviewer. We have made detailed and point to\npoint response (written in Blue) and revised the manuscript accordingly with all changes being\nhighlighted in Yellow in the revised Manuscript and Supporting Information.\n------------------------------------\nReviewer #2:\nGeneral Comment: I am generally satisfied with the authors’ previous responses and the\nadditional analyses provided. Nevertheless, I have developed some concerns regarding the way\nin which the authors describe the key intermediates and active sites involved in the reaction.\nAfter carefully reviewing the comments raised by other reviewers, I am inclined to believe that\nthere may be flaws in the proposed reaction pathway, which I would like to point out in this\nround of review.\nOur response: We would like to thank you for the positive comments on our work. Below,\nwe elaborate on your concerns and hope that our clarification addresses them all.\nComment 1. According to the newly proposed reaction path (Fig. R1), the primary role of [FeO]\nis to generate HONO as the initiator of the reaction. However, it has been reported that Cu-\nCHA forms HONO more rapidly than Fe-CHA and exhibits superior low-temperature NH -\nMethanol-SCR reaction remains puzzling. Could the authors provide additional experimental\nevidence or a new rationale to substantiate the claim that NO oxidation ability is the rate-\ndetermining factor for the overall reaction pathway? Furthermore, is methanol activation\nstrictly confined to BAS sites? If possible, we strongly encourage the authors to evaluate the\nmethanol oxidation ability of Cu-CHA and present the results, as this would greatly strengthen\ntheir mechanistic argument.\nOur response: Thanks for your insightful comment. The 0.6Fe-CHA zeolite was also prepared\nand evaluated for its Methanol-SCR activity under the same reaction conditions. As depicted\nin Fig. R1, both the 0.6Fe-CHA and 0.6Cu-CHA zeolites exhibit lower activity in the\nMethanol-SCR reaction compared to the 0.6Fe-FER and 0.6Cu-FER zeolites, respectively.\nThis indicates that different zeolite topologies have distinct effects on the Methanol-SCR\nreaction. FER zeolite is a medium-pore zeolite consisting of 10-membered ring (4.2 × 5.4 Å)\nchannels intersected by 8-membered ring (3.5 × 4.8 Å) channel, whereas the CHA zeolite\nfeatures a three-dimensional microporous framework composed of cha cages interconnected\nby eight-membered ring (8-MR) windows (~3.8 × 3.8 Å). The low activity of the CHA zeolite\nin the Methanol-SCR reaction is likely attributed to its unique microporous structure, which\neasily causes carbon deposition during the methanol-related reactions. (Fuel, 2025, 395,\n135207)\nFig. R1. NO conversion for the Methanol-SCR reaction on 0.6Fe-CHA and 0.6Cu-CHA\nx\nzeolites.\n[FeO]+ has been identified as an active center for NO oxidation on Fe-FER zeolite, but it\nis not the sole active site responsible for deNO . As Brønsted acid sites (BASs) in FER zeolites\nx\nalso play a pivotal role in the Methanol-SCR reaction. Therefore, it is not easy to ascertain\nwhether the NO oxidation is the rate-determining step of the reaction. As shown in Fig. S27,\nupon the introduction of CH OH and O , three identical methyl signals at 60.9, 59.2, and 50.5\n3 2\nppm are observed on both H-FER and 0.6Fe-FER zeolites, corresponding to side-on adsorbed\ndimethyl ether (DME), methyl groups generated on BASs, and adsorbed methanol, respectively.\nNo new methyl signals appeared on 0.6Fe-FER zeolite, signifying that methanol is mainly\nactivated on BASs rather than Fe sites. The adsorption capacity of NO, NO and CH OH\n2 3\nmolecules on BASs was also explored by DFT calculations. It reveals that the BASs in FER\nzeolite display a much stronger adsorption capacity for CH OH molecules (25.2 kcal/mol) than\n3\nthat for NO (5.2 kcal/mol) and NO (8.6 kcal/mol), as manifested in Fig. 4e and Fig. S28.\nbe reasonably inferred that BASs are preferentially occupied by methanol molecules and\nsubsequently activated to form methyl groups that participate in the Methanol-SCR reaction,\nwhich has been confirmed by previous studies (ACS Catal. 2022, 12, 2403-2414).\nAs you suggested, an additional methanol oxidation experiment was conducted for the\nCu-CHA zeolite. As shown in Fig. R1 (New Figure shown below and also in the supporting\ninformation as Fig. S36), in contrast to the Fe-FER zeolite, the Cu-CHA zeolite predominantly\noxidizes methanol to CO rather than CO, suggesting that the two catalysts exhibit markedly\n2\ndifferent behaviors in methanol oxidation. This implies that the carbon-containing\nintermediates formed during the Methanol-SCR process are likely different for these two\ncatalysts.\nFig. R2. The conversion of methanol in methanol oxidation reaction on 0.6Cu-CHA zeolite.\nRevisions: The new Figure and the corresponding discussions have been provided in the\nrevised manuscript. (See. Lines 9 to 17 of Page 16, Lines 1 to 3 of Page 17, Lines 9 to 20 of\nPage 21)\nComment 2. Further inquiries regarding Fig. R1: If nitromethane is proposed as a key\nintermediate, its inclusion should be supported by direct experimental evidence or a clearly\ncited reference. Upon reviewing reference 47, which the authors have indicated, I found no\nexplicit mention of nitromethane, and thus it appears insufficient to substantiate such an\nimportant mechanistic claim.\nOur response: Thanks for your valuable suggestion. As reported in our previous studies (ACS\nCatal. 2022, 12, 2403-2414; Appl. Catal. B-Environ. 2022, 318, 121883), nitromethane can be\nrapidly decomposed into formamide under reaction conditions, making it challenging to\ndirectly observe nitromethane species during the reaction. With the aid of SVUV-PIMS\nspectroscopy, Chen et al., found the presence of nitromethane but with a low ion signal strength\nin the Ethanol-SCR and (NH +Ethanol)-SCR processes (Energy & Fuels, 2022, 36, 9230). The\n1(owing to formamide), and which could indirectly reflect the nitromethane route.\nFor improving the understanding, as you suggested, the presence of nitromethane and our\nprevious studies concerning the conversion of nitromethane into formamide has been referred\nin the revised manuscript.\nRevisions: The references have been provided in the revised manuscript. (See. Lines 26-29 of\nPage 18, Lines 1 to 3 of Page 19)\nComment 3. There may be some ambiguity in the current peak assignments in Fig 5.\nSpecifically, the band observed at 2230 cm⁻¹ appears more consistent with NO⁺ adsorbed on\nBAS rather than NONO₂⁺. Likewise, although the authors attribute the 1630 cm⁻¹ band to\nadsorbed NO₂ species, a closer inspection of the enlarged spectrum in Fig. R2 suggests that it\nmore closely resembles the well-known bending vibration of water. It is plausible that upon\nintroduction of NO+O₂, the proton from BAS is exchanged with NO⁻, subsequently combining\nwith a neighboring hydroxyl group to form water. To verify the validity of the current\nassignments, I kindly suggest that the authors conduct a simple dehydration experiment, which\ncould provide a straightforward means of confirming these interpretations.\nOur response: Thanks for your helpful suggestion. We agree with you that water could be\nformed upon introduction of NO+O₂. While at the high reaction temperature of 300 oC, the\nformed water could be rapidly desorbed from the active sites. To differentiate the adsorption\nspecies on the catalyst, a nitrogen purge step was supplemented following the introduction of\nNO and O . As shown in Fig. R3 (New Figure shown below and also in the supporting\n2\ninformation as Fig. S32), as the purging process commences, the hydroxyl vibration at ~3650\ncm-1 rapidly disappears, whereas the IR absorption band at 1630 cm-1 remains, suggesting that\nthe species responsible for the IR absorption band at 1630 cm-1 is primarily adsorbed NO\n2\nrather than water. (J. Catal. 2010, 273, 29-38)\nFig. R3. Time-resolved in situ FT-IR spectra on 0.6Fe-FER zeolite with 500 ppm NO and 7%\nO flowed in at 300 °C for 30 min.\n1 gradually diminishes due to the desorption of NO , while the IR absorption band at 2235 cm-\n2\n1 simultaneously shifts toward lower wavenumbers. Considering that the vibration of NO+\nadsorbed on zeolitic BAS is commonly assigned to the band at ~2180 cm-1 in previous studies\n(Catal. Today 2017, 283, 54–65; J. Catal. 2015, 325, 68–78; Fuel 2025, 394, 135117), the\nadsorption band observed at 2235 cm-1 can be attributed to the interaction between NO and\nto attribute the absorption band at 2235 cm-1 to NO+NO species. This interpretation is further\n2\nsupported by previous literature reports. (J. Phys. Chem. C 2021, 125, 13889−13899; J. Phys.\nChem. C 2021, 125, 1913-1922)\nRevisions: The new Figure and the corresponding discussions have been provided in the\nrevised manuscript. (See. Lines 20 to 23 of Page 18)\nComment 4. The authors demonstrate, through in situ Mössbauer experiments with NO + O₂,\nthat the relative proportions of [FeO]⁺ and other Fe sites vary during the reaction cycle. This\nis an interesting and valuable observation. However, it raises an important question: would the\nspectral contributions shown in Table 1 (ex situ results) also evolve dynamically if methanol\nwere introduced under reaction conditions? In other words, are reversible changes between\ndifferent Fe components possible depending on the reaction environment? Clarification on this\npoint would greatly enhance the mechanistic interpretation.\nOur response: Thanks for your helpful suggestion. The utilization of in situ Mössbauer\nspectroscopy for identifying the dynamic structures of Fe species is very interesting.\nUnfortunately, the testing facility prohibited the introduction of water or methanol into the cell\nto protect the window materials. However, according to your suggestions, we also employed\ntime-resolved in situ FT-IR experiments to investigate the influence of methanol on the Fe\nspecies upon the introduction of methanol under reaction conditions. As shown in Fig. R4\n(New Figure shown below and also in the supporting information as Fig. S15), the introduction\nof methanol causes no noticeable change in the perturbed T–O–T vibration of Fe sites (~900\ncm-1 in the IR spectra), implying that methanol exerts negligible influence on the iron sites\nunder the reaction conditions. We hope that our clarification addresses your concerns, and\nthanks for you understanding.\nFig. R4. Time-resolved in situ FT-IR spectra of 0.6Fe-FER zeolite collect at 300 °C with 2000\nppm CH OH flowed in.\nrevised manuscript. (See. Lines 28 to 29 of Page 12, Lines 1 to 3 of Page 13)"}, {"round": 4, "reviewer_comments": {"reviewer_2": "I appreciate the authors’ diligent efforts in addressing the additional requests throughout the review process. These revisions have clearly improved the overall quality and completeness of the manuscript. To further enhance reproducibility for other researchers, I ask the authors to provide a more detailed synthetic recipe for the catalysts prepared via the ion-exchange method that was newly introduced during the peer-review process. Aside from this point, I have no further comments, and I agree with the publication of the manuscript."}, "author_response": "Response to comments\nManuscript ID: NCOMMS-24-78027D\nFirst of all, we sincerely appreciate the recognition and acceptance of our manuscript by\nReviewer 2, which have greatly helped us to improve the quality of the present manuscript.\nWe have made detailed and point to point response (written in Blue) and revised the manuscript\naccordingly with all changes being highlighted in Yellow in the revised Manuscript.\n------------------------------------\nReviewer #2:\nGeneral Comment: I appreciate the authors’ diligent efforts in addressing the additional\nrequests throughout the review process. These revisions have clearly improved the overall\nquality and completeness of the manuscript. To further enhance reproducibility for other\nresearchers, I ask the authors to provide a more detailed synthetic recipe for the catalysts\nprepared via the ion-exchange method that was newly introduced during the peer-review\nprocess. Aside from this point, I have no further comments, and I agree with the publication of\nthe manuscript.\nOur response: We would like to thank you for the positive comments on our work. The\ndetailed procedure of catalysts prepared by ion-exchange method has added in the revised\nmanuscript. (See. Lines 14 to 18 of Page 14)"}]

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