Near-unity CO_2-to-ethylene photoconversion over low coordination single-atom catalysts

Domain: neuroscience
Published: 2026-01-27
Editorial outcome: major_revision

Abstract
Photocatalytic conversion of carbon dioxide to value-added chemicals, particularly multi-carbon products, offers a promising route toward carbon-neutral cycles. However, achieving high activity and selectivity remains extremely challenging due to the instability of key reaction intermediates and limited C–C coupling efficiency. Herein, we report a low-coordination manganese single-atom catalyst embedded in zinc sulfide (Mn–ZnS) that enables efficient and selective CO -to-C conversion. In-situ spectroscopic analyses and density functional theory calculations reveal that sulfur vacancies are created at the Mn single-atom coordination sites and induce the formation of coordination-unsaturated Mn-S configuration. The asymmetric coordination environment of Mn modulates local charge distribution, strengthens *CO adsorption, and promotes *CO and *CHO coupling to form the *COCHO intermediate for efficient C–C coupling. As a result, the Mn–ZnS catalyst achieved 99.1% selectivity for ethylene with a formation rate of 76.6 μmol g-1 h-1. This study highlights the critical role of atomic-level coordination engineering in advancing photocatalytic CO -to-C conversion.

Introduction
Solar-driven conversion of carbon dioxide (CO ) into multi-carbon (C ) solar fuels has garnered significant attention as a promising green solution to address global energy demands and mitigate the impacts of climate change. Although recent advances have demonstrated the potential of photocatalytic systems for CO -to-C conversion, achieving high selectivity remains a critical challenge, because the formation of C products relies heavily on the stability of carbon-based intermediates at the catalytic active sites to enable efficient C–C coupling.

Among various developed photocatalysts for CO reduction reaction (CO RR), transition metal sulfides, such as CdS, Bi S , CulnSnS , have attracted particular interest due to their tunable d-orbitals. However, these materials often exhibit weak adsorption of C intermediates, e.g., *CO, leading to premature desorption and preferential formation of C products, such as CO, CH , and HCOOH, rather than the desired C compounds. To address this limitation, several effective strategies, including heterojunction construction, crystal facet engineering, doping, and defect engineering, have been explored to enhance C selectivity by stabilizing carbon-based intermediates. For instance, in MoS / Fe O system, d-porbital hybridization at Mo-Fe sites reduced electrostatic repulsion between *CO and *COH intermediates, promoting ethylene (CH ) formation. Co-O-Fe triatomic sites in partially oxidized FeCoS increased local charge density, enriching C intermediates and directing the reaction pathway toward C products. Incorporation of Co into NiS tailored the coordination number and oxidation state of Ni sites, facilitating asymmetric C–C coupling between *CO species in atop and bridge adsorption configuration, achieving 75% selectivity for C H . Despite these advances, achieving high activity and selectivity for photocatalytic CO -to-C conversion remains an unmet goal.

Single-atom catalysts (SACs), with atomically dispersed metal centers and well-defined coordination environments, provide a powerful platform for tuning intermediate adsorption and enhancing CO RR activity and selectivity. Particularly, low-coordination SACs can break geometric symmetry and modulate the local electronic structure, strengthening the adsorption of carbon-based intermediates and facilitating C–C coupling. Therefore, integrating low-coordination SACs into binary sulfides holds great potential for stabilizing *CO and enabling efficient photocatalytic CO -to-C conversion.

Herein, we develop a low-coordination Mn single-atom embedded zinc sulfide (Mn–ZnS) via a microwave irradiation-induced targeted defect engineering strategy for enhancing CO -to-CH photoreduction. Structural analyses reveal that Mn atoms are atomically dispersed and coordinated with a reduced number of sulfur atoms, forming Mn–Zn coupled sites with modulated electronic structures. Density functional theory (DFT) calculations and in-situ spectroscopic studies demonstrate that these low-coordination Mn sites effectively stabilize C H -related intermediates and promote asymmetric *CO–*CHO coupling into *COCHO, a key intermediate for C–C bond formation. As a result, Mn–ZnS exhibits attractive photocatalytic performance for CO reduction, achieving a C H production rate of 76.6 μmol g−1 h−1 with nearly 100% selectivity, significantly emphasizing the capability of regulating the coordination environment of sing-atom catalytic sites for photoconversion CO into C products.

Results
Enhanced C–C coupling via low-coordination Mn sites
To assess *CO formation during the CO RR, we calculated the free energy profiles for CO -to-*CO conversion on ZnS, Mn–ZnS, and Mn–ZnS. Among these, Mn–ZnS exhibits the lowest energy barrier (0.56 eV), significantly lower than that of ZnS and Mn–ZnS, indicating that low-coordination structure of Mn single-atom is favorable for CO activation and *CO generation.

To investigate the behavioral of *CO intermediates on different coordination structures, the *CO adsorption energy and protonation energy barriers of Mn–ZnS and Mn–ZnS were compared. The interaction between the Mn 3d orbital and the 2π* (LUMO) and 5σ (HOMO) orbitals of *CO underpins the modulation of adsorption behavior. The low-coordination Mn site in Mn–ZnS strengthens the Mn-CO π backbonding, resulting in a significantly enhanced *CO adsorption energy of −1.36 eV, much stronger than that of coordination-saturated Mn–ZnS. Furthermore, the incorporation of low-coordination Mn single atoms ingeniously balances the competitive adsorption of *CO intermediates and *H species. The appropriate binding inhibits premature *CO desorption and enables its participation in subsequent protonation steps. It is worth noting that the energy barrier for *CO hydrogenation on Mn–ZnS is reduced to 0.75 eV, compared to 1.25 eV on Mn–ZnS, confirming the regulatory role of the low-coordination structure in steering the *CO reaction pathway.

To probe the origin of the stronger *CO adsorption, we investigated the electronic structure differences between Mn–ZnS and Mn–ZnS. Differential charge density analysis reveals a significant electron transfer (1.08 |e|) from the Mn single atom to the adsorbed *CO in Mn–ZnS, compared to just 0.32 |e| in Mn–ZnS, highlighting the enhanced electronic interaction in the low-coordination structure. Furthermore, in Mn–ZnS (coordination number, CN=2), the Mn 3d orbital center shifts upward relative to Mn–ZnS, resulting in a higher d-band center. This upshift implies an increased population of anti-bonding states, favoring stronger adsorption of reaction intermediates and facilitating the key C–C coupling process.

To validate this mechanistic insight, we calculated the free energy barriers for C–C coupling through different C species (*CO, *CHO, *COH) on Mn–ZnS. The results suggested that direct dimerization of two *CO species into *COCO is energetically unfavorable (ΔG=1.91 eV). Instead, *CO preferentially undergoes hydrogenation to *CHO, which subsequently couples with another *CO to form the *COCHO intermediate via an asymmetric C–C bond formation pathway. This pathway is thermodynamically more favorable and constitutes the lowest-energy pathway for CO -to-CH conversion. Thus, low-coordination Mn single-atom sites in Mn–ZnS significantly enhance *CO adsorption and facilitates the asymmetric coupling of *CO and *CHO, ultimately leading to the selective generation of C H . The theoretical screening of coordination structures serves as a guide for establishing trends in *CO adsorption and the subsequent reaction pathways.

Catalyst synthesis and characterization
Guided by the DFT-screened Mn–ZnS structure, a Mn single-atom substituted ZnS catalyst with targeted sulfur vacancies (S ) was successfully synthesized via a microwave irradiation-induced targeted defect strategy, followed by H O etching under an Ar atmosphere. Inductively coupled plasma optical emission spectrometry (ICP-OES) confirmed a Mn content of 0.6 wt%. X-ray diffraction (XRD) patterns confirmed the formation of cubic-phase ZnS with an average crystallite size of 41.3 ± 0.2 nm, as estimated using the Scherrer equation.

High-resolution transmission electron microscopy (HRTEM) revealed distinct lattice fringes with an interplanar spacing of 0.312 nm, corresponding to the (111) plane of ZnS. Scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping images and the STEM-EDS line scanning profiles corroborated the even distribution of Zn, Mn, and S throughout the sample, with no observable aggregation of Mn into large clusters or particles. Aberration-corrected annular dark-field STEM (ADF-STEM) further confirmed the atomic dispersion of Mn, with isolated single atoms highlighted by yellow circles and localized intensity dips in the corresponding line-scan profile across a representative region.

To confirm the formation of S , electron paramagnetic resonance (EPR) spectroscopy was performed. A clear signal at g=2.005 indicated the presence of S in Mn–ZnS. Additionally, EDS point analysis showed an increased metal-to-sulfur atomic ratio, further supporting the generation of S , which increased with prolonged H O etching time. Quantitative analysis from ICP-OES and EPR revealed an isolated S concentration of 1.54% for Mn–ZnS, closely matching the theoretically identified optimal configuration by DFT.

To probe the local coordination environment of Zn and Mn atoms, X-ray absorption spectroscopy was conducted. X-ray absorption near-edge structure (XANES) spectra showed that Zn maintained an oxidation state of approximately +2.0. The Mn K-edge positions in both Mn–ZnS and Mn–ZnS were located between those of metallic Mn and MnO, indicating an average Mn valence between 0 and +2. Notably, Mn in Mn–ZnS exhibited a lower oxidation state than in Mn–ZnS, attributed to the low-coordination environment induced by S formation.

Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) analysis displayed distinct peaks corresponding to Zn–S and Zn–Zn coordination, while a prominent peak at 1.96 Å was assigned to Mn–S bonding. The absence of Mn–Mn and Mn–O coordination in both Mn–ZnS and Mn–ZnS indicates that Mn atoms are atomically dispersed and coordinated with neighboring S atoms, consistent with ADF-STEM results.

EXAFS fitting revealed that the first-shell Mn–S coordination number was 3.7 for Mn–ZnS and 1.9 for Mn–ZnS. These values are in excellent agreement with DFT-optimized structures, confirming that Mn atoms in Mn–ZnS are coordinated with only two sulfur atoms. No Mn–Mn path in second-shell coordination and wavelet transform (WT) analysis further confirmed the atomic dispersion and low-coordination environment of Mn in Mn–ZnS.

To investigate the effect of low-coordination Mn single-atoms on light absorption efficiency, UV–Vis diffuse reflectance spectra and X-ray spectroscopy (XPS) valence band spectroscopy were conducted. The optical absorption band edge of Mn–ZnS is observed at 428 nm, corresponding to a smaller band gap of 2.88 eV. Ultraviolet photoelectron spectroscopy measurements revealed work functions (Φ) of 6.13 and 5.60 eV for Mn–ZnS and Mn–ZnS, respectively. The valence band maxima for Mn–ZnS and Mn–ZnS were located at 1.00 and 0.55 eV below the Fermi level, and the electronic band structure versus Normal Hydrogen Electrode could be elucidated, which is thermodynamically favorable for the CO -to-CH conversion. These results are well-consistent with those obtained from Mott–Schottky plots, clearly suggesting the ability for CO -to-CH conversion and O evolution. Upon irradiation, photogenerated electrons are transferred from S (electron donor) to Mn (electron trap), and then rapidly injected into the adsorbed CO molecules through the electron transfer channel. Moreover, improved charge-transfer dynamics induced by low-coordination Mn sites were recorded.

To further elucidate intermediate adsorption and reaction pathways in photocatalytic CO reduction, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed. Under simulated solar irradiation, Mn–ZnS exhibited characteristic signals at 1618, 1655, and 1695 cm−1 (*COOH species), 2078 cm−1 (*CO), 1122 and 1415 cm−1 (*CHO), and a key band at 1315 cm−1 corresponding to *COCHO, a critical intermediate in C–C coupling toward C H . In contrast, Mn–ZnS exhibited a stronger *CO signal at 2078 cm−1, indicating enhanced stabilization and coverage of *CO intermediate. The progressive intensification of the 1315 cm−1 band over time confirmed efficient *CO–*CHO coupling and *COCHO formation, which was further verified by the 13CO labeling experiment. Additional bands at 1442, 1462, and 1364 cm−1 were attributed to *CH and *CH intermediates, confirming the high activity and selectivity of Mn–ZnS toward C H production.

To further probe CO adsorption behavior, CO-adsorbed DRIFTS was performed on both catalysts. Prominent peaks at ~2084 and 2036 cm−1 were assigned to linearly bonded CO (CO ) on Mn single atoms. Mn–ZnS displayed significantly higher peak intensities than Mn–ZnS, indicating a stronger CO binding capability, consistent with DFT predictions. Based on the in situ DRIFTS data and theoretical insights, a photocatalytic reaction pathway for CO conversion on Mn–ZnS was proposed, highlighting the critical role of low-coordination Mn single-atom sites in stabilizing reaction intermediates, enabling *CO–*CHO coupling, and ultimately promoting selective C H production.

Evaluating catalyst performance for CO photoreduction
The CO photoreduction performance of Mn–ZnS catalysts was assessed under visible light irradiation (λ≥380 nm) without the use of photosensitizers or sacrificial agents. As shown in Fig. 5a, pristine ZnS primarily produced CO at a rate of 64.6 μmol g−1 h−1, with a low C H selectivity of only 5.6%. Compared to pristine ZnS, ZnS with sulfur vacancies (ZnS) exhibits negligible enhancement in C H yield and selectivity (7.3%). Upon incorporation of saturated-coordination Mn single atoms (Mn–ZnS), both activity and selectivity improved significantly, achieving a C H formation rate of 47.5 μmol g−1 h−1 and a selectivity of 74.5%. Concurrently, CO evolution decreased to 28.4 μmol g−1 h−1 and the competing hydrogen evolution reaction was markedly suppressed, likely due to the *CO-stabilizing effect of Mn sites that promote its protonation and subsequent C–C coupling.

Notably, the low-coordination Mn–ZnS catalyst delivered a significantly enhanced C H formation rate of 76.6 μmol g−1 h−1, nearly 58.9 times higher than that of pristine ZnS, with a selectivity of 99.1% for C H production. Meanwhile, CO formation further decreased to 4.2 μmol g−1 h−1 and no liquid products were detected, underscoring the pivotal role of Mn coordination environment in modulating *CO adsorption and steering the reaction pathway toward selective C hydrocarbon formation.

To further validate the role of low-coordination Mn SAs in promoting C–C coupling, a CO substitution experiment was carried out. This resulted in a C H yield of 198.8 μmol g−1, confirming that *CO is a crucial intermediate for C–C coupling and effectively activating and stabilizing by the low-coordination Mn sites. The apparent quantum efficiency (AQE) for CO photoreduction over Mn–ZnS reached 8.1% at 420 nm. Control experiments demonstrated that C H formation occurred only under light irradiation in the presence of both CO and H O, excluding contributions from dark reactions or carbon-containing contaminants. Isotope-labeling experiments using 13CO and D O further confirmed CO and H O as the carbon and proton sources, respectively, for C H formation. Considering that the activation of H O into active hydrogen species may be an important factor that affecting the turnover-limiting step during the photocatalytic CO RR to C H , the KIE of H/D value over the Mn–ZnS catalyst is close to 1, further confirming the *CHO formation as the rate-determining step suggested by DFT results.

The photocatalytic stability of Mn–ZnS was evaluated over 50 consecutive cycles (200 h), during which no significant decline in activity or selectivity was observed. Post-reaction characterizations, including TEM, XRD, and XPS, confirmed the preservation of the structural and electronic integrity of the catalyst. No characterized signals of Mn–Mn bond over Mn–ZnS after long-term reaction further proved the great stability of the coordination environment of Mn atoms, ensuring effective photocatalytic performance. Furthermore, the well-balanced charge consumption between the oxidation and reduction half-reactions (R ≈1) minimizes charge accumulation and effectively suppresses photocorrosion. Notably, under sacrificial-agent-free conditions, a comparative analysis of photocatalytic performance between our study and previous reported catalysts is displayed in Supplementary Table 6. Both the competitive C H yield and selectivity of Mn–ZnS underscores its potential as a highly efficient and robust CO -to-CH conversion.

To evaluate the general applicability of low-coordination transition metal single atoms in ZnS matrices, analogous catalysts incorporating Fe, Co, Ni, and Cu were synthesized and tested. All M–ZnS (M=Fe, Co, Ni, Cu) variants exhibited substantially higher C H yields than both pristine ZnS and their saturated-coordination M–ZnS counterparts, indicating a certain enhancement in photocatalytic performance induced by low-coordination metal centers.

To elucidate the electronic origin of this enhancement, projected density of states analyses were conducted for the 5σ and 2π orbitals of adsorbed CO and the metal 3d orbitals (3d , 3d , 3d ) of the M–ZnS systems. Among all the systems, Mn–ZnS exhibits the highest d-band center at −1.85 eV, closer to the Fermi level. This upshift results in decreased population of antibonding d–5σ and bonding d–2π* states, thereby enhancing *CO adsorption.

Furthermore, integrated crystal orbital Hamilton population values for M–*CO bonds exhibited a clear linear decrease from Cu–ZnS to Mn–ZnS, further validating the stronger *CO binding affinity of Mn sites. In situ DRIFTS spectra and free energy of reaction pathway presented that Mn–ZnS exhibits significantly stronger *CO absorption peaks and comparatively favorable energetics for *CO protonation and coupling than that of Cu–ZnS, which is consistent with the results from the catalytic performance. These findings conclusively demonstrate that low-coordination transition metal single atoms, particularly Mn, enhance *CO stabilization and facilitate the critical C–C coupling step, thereby driving selective C product formation in CO photoreduction.

Methods
Chemical reagents
Zinc acetate (Zn(OAc)), Manganese acetate (Mn(OAc)), Copper acetate (Cu(OAc)), Iron acetate (Fe(OAc)), Nickel acetate (Ni(OAc)), Cobalt acetate (Co(OAc)), thiourea, ethylene glycol were purchased from Aladdin Reagent Co., Ltd., Shanghai. All chemicals are of analytical grade and used without further purification. The deionized water was supplied by a Millipore system (Outlet water resistivity >18 MΩ cm).

Sample preparation
To synthesize ZnS catalyst, typically 6 mmol of Zn(OAc) and 12 mmol of thiourea were added to 100 mL of ethylene glycol. After continuous agitation for 30 min, the mixture was moved to a 250 ml three-necked flask with round bottom, and placed in a microwave reactor with a reflux unit. It was heated from room temperature to 150°C with a heating rate of 25°C min−1 and maintained for 10 min. After the reaction cooled to room temperature naturally, the resulting powder was washed with deionized water and ethanol for three times and dried under vacuum at 80°C for 6 h. The obtained samples were named as ZnS.

To synthesize Mn–ZnS catalyst, 6 mmol of Zn(OAc) and 12 mmol of thiourea were added to 100 mL of ethylene glycol. After continuous agitation for 30 min, the different mass ratios of Mn(OAc) (0.2%, 0.6%, 1%) was added to the suspension and continued stirring for 10 min. The mixture was moved to a 250 ml three-necked flask with round bottom, and placed in a microwave reactor with a reflux unit. It was heated from room temperature to 150°C with a heating rate of 25°C min−1 and maintained in Ar flow for 10 min. After the reaction cooled to room temperature naturally, the resulting powder was washed with deionized water and ethanol for three times and dried under vacuum at 80°C for 6 h. The obtained samples were named as Mn–ZnS-0.2, Mn–ZnS-0.6 and Mn–ZnS-1.0, respectively.

To synthesize Mn–ZnS, 100 mg Mn–ZnS was immersed in H O solution (0.05 mol L−1) under the protection of Ar in microwave reaction treated for 5, 10, and 15 s, respectively, all the obtained samples were carefully washed and dried using the method described above. The resulting samples were labeled as Mn–ZnS with different S-vacancy concentrations.

The preparation process for ZnS is the same as that of Mn–ZnS, except that the precursor Mn salt was not added. The synthesis steps for Fe–ZnS, Co–ZnS, Ni–ZnS, and Cu–ZnS are the same as Mn–ZnS, except that the precursor salt was replaced with the corresponding acetate. Unless otherwise specified, the precursor transition metal salt loading for the ZnS sample was 0.6%.

Characterization
The phase structure of the photocatalysts was performed on a powder X-ray diffractometer (XRD, Shimadzu XRD 6000). The morphology was observed via field emission scanning electron microscopy (FEI Quanta 200F), transmission electron microscopy (TEM, JEOL JEM 2100). STEM-ADF images and EDS mapping were obtained by Hitachi HF5000, working at an accelerating voltage of 200 kV. To interpret the light absorption characteristics of the catalysts, UV–Vis DRS were detected on an UV–vis spectrophotometer (Shimadzu UV-2600) over a range of 200–800 nm. To study on the chemical bonds or functional information, in situ DRIFTS were examined using the IRAffinity-1 FTIR spectrometer. The Zn, Cu, and Mn K-edge X-ray absorption spectra were acquired from 4B9A beamline in Beijing Synchrotron Radiation Facility (BSRF). Isotopic labelling experiments were performed on an Agilent 5977B GC/MS system, using 13CO and D O as reactants under the photocatalytic CO reduction conditions. The GC-MS spectra reflect the relationship between the m/z of all ion fragments and their relative abundance. Photoluminescence spectra were analyzed on FluoroMax+ spectrophotometer (Hitachi, Japan) with excitation wavelength at 310 nm, and the receiving fluorescence range was from 400 to 700 nm. X-ray photoelectron spectroscopy (XPS) and valence band-XPS tests were conducted on the PerkinElmer PHI-1600 ESCA spectrometer with a Mg Kα X-ray source. All the calibration for the binding energies were based on the C 1s at 284.8 eV. Carbon dioxide-temperature programmed desorption (CO -TPD) was recorded on a BSD-C200 chemisorption system (BSD Instrument).

In-situ DRIFT spectra measurement
In-situ DRIFT spectra were obtained from a Thermo Scientific Nicolet iS50 FTIR spectrometer. The compressed catalysts were placed in a diffuse reflectance cell (Harrick) equipped with a mixed atmosphere of CO and H O vapor for in situ experiments. In the pretreatment stage, the system was progressively heated from room temperature to 200°C under continuous N purging to remove surface impurities. After cooling to room temperature, the data were collected at a flow rate of 50 mL min−1 in a mixed atmosphere. The infrared spectra obtained in the dark were used as background data. Subsequently, time-dependent IR spectra were collected under illumination for 30 min.

In situ CO-adsorbed DRIFT spectroscopy measurement
In situ CO-adsorbed DRIFT spectroscopy measurement was carried out on a Nicolet 6700 instrument equipped with a mercury cadmium telluride detector. 10 mg catalyst was placed in an infrared diffuse reflection high-temperature reaction cell with a ZnSe window (Pike Technologies), pretreated in 10% H /N flow at 300°C for 30 min, then cooled to room temperature. This was followed by the introduction of CO flow for 30 min and subsequent evacuation. The CO-adsorbed FT-IR spectra were recorded at an apart of 2 min.

Photoelectrochemical measurements
Photoelectrochemical (PEC) measurements were carried out in the three-electrode system by utilizing an electrochemical workstation (CHI660E). The prepared coated catalysts serve as the working electrode, with an effective surface area of 1 cm2. Pt net and Ag/AgCl electrode were employed as the counter electrode and reference electrode, respectively. The Na SO solution (0.1 mol L−1) was acted as electrolyte (pH=7.02±0.05). In order to prepare the working electrode, 25 mg catalyst and 10 μL Nafion were dispersed in 384 μL deionized water and 96 μL ethanol, then followed by ultrasonication for 25 min. The evenly mixed ink (5 μL) was dropped on the surface of the glass-carbon electrode, and dried naturally at room temperature. The photocurrent response was recorded under the light on/light off. The electrochemical impedance spectra was performed, traversing a frequency range from 0.1 to 100000 Hz. Resistance (R) was measured to be 0.60 ± 0.02 Ω at the open circuit potential.

Tests on photocatalytic CO reduction with H O
The photocatalytic CO reduction tests were performed in a sealed reaction system by employing the Labsolar 6A reactor (Beijing Perfectlight Technology Co., Ltd.). 0.2 g of photocatalysts was dispersed in 5 mL of deionized water within a glass dish (diameter is ~4.0 cm). The mixed sample was then placed inside a reactor (volume of ~370 mL) of the photoreaction system, at a distance of 15.0 cm from the light source. The reaction system was subjected to three cycles of vacuuming and refilling with high-purity CO , achieving a final CO pressure of 80.0 kPa in the reactor. And a cooling circulating water device was utilized to maintain the reaction system a constant temperature at 25±1.5°C. To simulate the natural photosynthesis, a 300 W xenon lamp (~100 mW cm−2) equipped with a 380 nm external filter served as light source. The light intensity of catalyst surface was measured by a ThorLabs PM100D Power with a photodiode sensor, with the average value determined from multiple regions of the watch glass. After starting the light source and the gas chromatograph automatic sampling device, the sample was analyzed every 30 min, and the reaction test was carried out for 4 h. The amount of C H , CH , CO, H , and O products evolved was analyzed online by a GC-9560 gas chromatograph (HuaAiSePu Company) using the external standard calibration method. For the liquid product, the amount of HCOOH was determined by a proton nuclear magnetic resonance (1H NMR) analysis, taking DMSO as an internal standard. No other products were detected above the detection limit of instrument.

The product selectivity for CO photoreduction to C H was calculated according to the following equations:
S (C2H4) = (12n C2H4) / (12n C2H4 + 2n(CO) + 8n CH4 + 2n(CHOOH)) × 100%
where n(product) is the molar amount of product generated.

To quantify the efficiency of photocatalytic light-energy conversion, the apparent quantum efficiency (AQE) was measured. The incident photon numbers were determined using monochromatic light sources at specified wavelengths (λ=400, 420, 450, 500, 520, and 600 nm). AQE was defined as the ratio of consumed photons to incident photons, and according to the following equations:
AQE = (12 × Number of reacted electrons) / (Number of incident photons) × 100% = (12 × n C2H4 × N A) / (S × I × t × λ / h × c) × 100%
where S is the irradiated area (cm2), I is the irradiation light intensity (W cm−2), t is the irradiation time (s), λ is the equivalent wavelength (m), h is Planck’s constant (6.626×10−34 Js−1), c is the speed of light (2.998×108 ms−1) and N A is Avogadro’s number (6.022×1023 J mol−1). n C2H4 is the amount of C H produced in 4 h.

The incident photon-to-current efficiency is defined as the ratio of the incident monochromatic photons converted to collected electrons and can be calculated by using Eq. (3):
IPCE = (j × λ × 1240) / (I × λ) = (j × 1240) / I
where j is the photocurrent density (A cm−2), λ is the equivalent wavelength (m), and I is the irradiation light intensity (W cm−2).

Density functional theory (DFT) calculations
DFT calculations were carried out in the framework of the Vienna ab initio Simulation Package (VASP). The exchange and correlation energies were established by the Perdew–Burke–Ernzerhof functional with spin-polarized generalized gradient approximation. The core electrons were described by the projected augmented wave pseudopotentials. For geometry optimization, a cutoff energy of 450 eV was used to expand the wave functions, and the Brillouin zone was sampled with 2×2×1 k-points. All the structures and energy were allowed to relax below 0.05 eV Å−1. The DFT-D3 method of Grimme et al. was employed to involve the van der Waals interaction. The calculations of charge density difference were employed to analyze the movement and distribution of the charge. For the elementary reaction barriers, the transition states were determined by the climbing image nudged elastic band method and were confirmed by further frequency calculations showing one and only one imaginary frequency.

According to the XRD results, a 5×5 slab model composed of three layers along the (111) direction was constructed with the space group F-43m model of ZnS carrier. A 20.0 Å vacuum region between the slabs was constructed to avoid the interlayer interaction. The doped ZnS with other transition metal elements was simulated by replacing one of the Zn atoms. The S-vacancy was simulated by removing S atoms off, and the theoretical value of S is 1.33%. Considering the difference in the performance of as-prepared catalysts, ICP quantified vacancy concentrations, specific configuration of sulfur vacancies was adopted to model the surface. The atomic coordinates of the DFT models are provided as a Supplementary Data 1 file. The formation energy of sulfur vacancy ΔE VS was calculated as follows:
ΔE VS = E slab-VS - E slab - E S
in which E slab-VS and E slab represent the energy of the slab after and before V s . The adsorption energy E ads was calculated by a standard formula:
E ads = E catalyst-* - (E catalyst + E*)
in which E catalyst-* and E* is the total energy of the slab with and without intermediates.

The Gibbs free energy change (ΔG) for the reaction was calculated by
ΔG = ΔE + ΔE ZPE - TΔS
in which ΔE is reaction energy obtained from DFT calculations, ΔE ZPE represent the change of the zero-point energy, TΔS is the entropic contribution (T was set to 300 K), which were obtained from the vibrational frequency calculations through the VASPKIT code. The free energy of the proton–electron pair is equal to half the free energy of the hydrogen molecule according to the computational hydrogen electrode method.

Ab initio molecular dynamic simulations are carried out by CP2K package and the QUICKSTEP module with fully explicit solvent water molecules. Godecker-Teter-Hutter (GTH) pseudopotentials were adopted to model the core electrons, and double-ζ valence single polarization (DZVP)-molecularly optimized (MOLOPT)-short-ranged-GTH basis set was employed to obtain optimized structure. Valence electrons were expanded in an orthonormal plane-wave basis using a cutoff energy of 400 Ry, along with Grimme D3 dispersion correction. The self-consistent field convergence criterion is set to 10−5 Ry for the total energies, and the simulation temperature (300 K) was controlled through a Nosé-Hoover thermostats with timestep of 1.0 fs. DFT offers valuable insights for screening coordination structures and exploring reaction pathways, yet it remains inherent limitations in simulating practical catalytic conditions (e.g., actual illumination, solvation effects). Thus, the computationally identified coordination structures should serve as qualitative guides for catalyst design and mechanistic investigation.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents an interesting study where atomic-level coordination control in a sulfide host translates directly to near-unity C₂ selectivity in sacrificial-agent-free CO₂ photoreduction. The key contribution of this work is the deliberate creation of sulfur vacancies around the Mn center, lowering its coordination from the usual tetrahedral (CN = 4) to CN ≈ 2. This unsymmetrical Mn–S₂ motif is argued to (i) strengthen *CO adsorption, (ii) lower the *CO → *CHO barrier, and (iii) enable *CO–*CHO asymmetric coupling to *COCHO, the gateway intermediate for C–C coupling. The DFT-guided synthesis, operando DRIFTS, XAFS, AC-STEM, and isotope (¹³CO₂/D₂O) labeling are combined to provide an unusually complete mechanistic picture for a photocatalytic CO₂RR study. As such, I would like to recommend to publish this work with major revisions.\n1. EPR (g = 2.005) and S-content from EDS indicate Sv formation, yet the absolute Sv concentration (1.54 %) is derived from a spin-counting model that assumes isolated S-vacancies. Clustering of vacancies could alter Mn coordination and electronic structure; spatially resolved EELS or positron annihilation spectroscopy would help to get S vacancy quantification and homogeneity.\n2. Only 0.6 wt % Mn is loaded; at this level, even a small fraction of MnOx or MnS₂ clusters (<1 nm) could be missed by AC-STEM. A Mn K-edge EXAFS fitting range beyond 3 Å should be inspected for Mn–Mn contributions.\n3. Photocurrent and EIS (Supplementary Figs. 18–19) show improved charge-transfer kinetics for Mn₁–ZnSᵥ, but the incident-photon-to-current efficiency (IPCE) at 420 nm is not reported; this would directly corroborate the 8.1 % AQE claim.\n4. Mott–Schottky plots give flat-band potentials, yet the absolute band positions (vs. NHE) are extracted assuming no Fermi-level pinning by Mn states; in situ ambient-pressure XPS under illumination could verify band alignment under operating conditions.\n5. D₂O experiments confirm water as the proton donor, but no KIE (k_H/k_D) for C₂H₄ formation is provided. A primary KIE would strengthen the claim that *CHO formation is turnover-limiting.\n6. The H₂ evolution rate is suppressed to ≈1 µmol g⁻¹ h⁻¹, but its potential role as a competing pathway is not quantitatively modeled.\n7. Ten 4-h cycles (40 h total) show no loss, yet photocorrosion of ZnS in aqueous media is well-known. Longer-term tests (>100 h) or continuous-flow operation should be addressed.\n8. Microwave/H₂O₂ etching is elegant for lab scale (~100 mg batches); scalability to gram or kilogram scale is not addressed.\n9. Only the ZnS(111) facet is modeled while ZnS also exposes (110) and (100) surfaces with different Sv formation energies. It will be necessary to model other two surfaces using DFT calculations for comparison.\n10. Solvation and entropic contributions to *COCHO stability are not explicitly included.\n11. ab-initio molecular dynamics simulations should be performed to capture dynamic vacancy reconstruction (e.g. 300 K).\n12. Band-gap determination from UV–Vis (Fig. S13) uses a Tauc plot without stating whether direct or indirect transition is assumed; please clarify.\n13. The reaction chamber volume, light intensity mapping, and temperature rise under focused Xe lamp are not documented; local heating can accelerate C–C coupling.\n14. DRIFTS spectra are normalized to the ZnS phonon band at 1122 cm⁻¹, but absolute absorbance calibration would allow estimation of surface *CO coverage.\n15. The error bars for AQE (currently single value at 420 nm) should be provided.", "reviewer_2": "The manuscript addresses a significant challenge for the energy community – how to efficiently convert CO2 into valuable C2 products using solar energy directly. The manuscript is well-written, clearly organized, and contains a wide spectrum of characterization methods. My review will focus primarily on the photochemical characterization experiments, which require some clarifications and the filling of missing data points to assess the paper's merit accurately.\n1. The paper’s main point is that using low-coordination Mn sites to stabilize CO2R intermediates results in a higher efficiency towards C2+ products, as otherwise these critical intermediates are not able to react further. To support this claim, the authors should also perform control experiments using CO as feed for the ZnS catalyst and add this to Fig 5c. This is a critical point to address.\n2. Secondly, the authors claim that they excelled in two metrics important for photochemical systems: ethylene production rate and ethylene selectivity. Given this is a primary claim, the authors should provide more details on how the experiments were conducted and how the metrics were calculated. Also, were the experiments repeated for reproducibility? The error bars should be added to the photochem experiments. The authors should also include their calculations of apparent quantum efficiency (specific values used, not only the formula).\n3. Important details are missing in the experimental description -e.g., what was the total amount of CO2 injected into the reactor, and how was the photocatalyst immobilized?\n4. Fig 5d: control condition 5, should it be “replacing CO2 with Ar” instead of “replacing Ar with CO2”?\n5. What do authors mean as “efficiency of tourism” (SI, section on Tests on photocatalytic CO2 reduction with H2O)? there is also a typo in Eq1. Looks like this was an auto-correct mistake, please review all your writing carefully.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "Tang et al. report a Mn single-atom catalyst embedded in ZnS with sulfur vacancies (Mn1–ZnSv) for photocatalytic CO2 conversion to C2H4. Photocatalysts were modified by inclusion of Mn, hypothesizing that “single-atom” sites with Mn would increase binding energy for *CO and steer selectivity to C2 products, Figure 1. Nanoparticle catalysts were made by solution-based method: ZnS and Mn-ZnS with 3 different mass ratios. Fe, Co, Ni, and Cu were also employed as dopants. S vacancies were induced by microwave treatment for 5-15 seconds: Zn-ZnSv series. While a large number of samples appear to have been made, study focuses on one of them (page 6): “Inductively coupled plasma optical emission spectrometry (ICP-OES) confirmed a Mn content of 0.6 wt% (Supplementary Table 1).” It does appear, EPR in Figure S8, that this sample has more unpaired electrons than control which did not undergo the microwave treatment. A very high selectivity for this preparations is claimed: 76.6 μmol·g-1·h-1 under sacrificial-agent-free conditions.\nNo sacrificial species were employed and 13C and 2D labeling was used to ensure that carbon and protons come from CO2 and water, respectively. O2 production is also reported, Figure 22. Study thus follows “best practices” for experimental reports of this type, see 10.1021/acsenergylett.8b00196 and 10.1002/anie.201207199. Also, interestingly, Mn-ZnSv is able to convert CO to C2H4 (this is not always the case for CO2R photocatalysts). They estimate an AQE of 8% at 420 nm. The authors attribute the performance to *CO stabilization and *CO–*CHO coupling, supported by in situ spectroscopy and DFT calculations.\nReview prompts are addressed as follows:\nWhat are the noteworthy results?\nThe performance of the photocatalysts, both in terms of activity/selectivity, is very good compared to other reports in the literature, Figure 5f.\nWill the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.\nWhile the performance metrics are high, the novelty is limited. Similar catalyst architectures and comparable activity/selectivity have been reported (e.g., Energy Environ. Sci. 17, 5060–5069 (2024); J. Am. Chem. Soc. 145, 422–435 (2023)), and the present study does not clearly distinguish itself conceptually or mechanistically.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nIn part, see detailed comments below.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nRevisions are needed in the data analysis, see detailed comments below.\nIs the methodology sound? Does the work meet the expected standards in your field?\nYes. Again, the use of isotope labeling, CO substitution, etc. are best practices in the field.\nIs there enough detail provided in the methods for the work to be reproduced?\nYes.\nSummary statement.\nThe manuscript reports strong catalytic performance but lacks sufficient novelty and mechanistic depth to meet the standards of Nature Communications. Addressing the above points, particularly clarifying the Mn–vacancy interplay, rigorously validating the universality claim, and strengthening stability and mechanistic evidence, would be essential to enhance the originality and impact of the work.\nDetailed comments.\nThe interplay between sulfur vacancies and Mn single atoms is insufficiently resolved. Figure S26 presents 4 h C2H4 yields for Fe, Co, Ni, and Cu-doped catalysts subjected to the microwave treatment. Yields are ~70-120 umol/g. What is not shown on this graph is the Mn-ZnS not subjected to the microwave treatment: 190 umol/g. This data does not support the statement: “indicating a universal enhancement in photocatalytic performance induced by low-coordination metal centers.” So it is not clear whether Mn substitution, which appears to be the original hypothesis, or the S vacancie is the main effect. At the very least, a ZnSᵥ control (without Mn) should be synthesized, characterized, tested, and included in DFT calculations. Figures 4a and 4b do not convincingly isolate the effect of sulfur vacancies. Full catalytic cycle calculations with corresponding energy profiles are needed for both cases, and any synergistic effect should be explicitly demonstrated in the mechanism.\nThe assertion that multiple metals (Fe, Co, Ni, Cu) can be incorporated as single sites requires direct evidence. At least one alternative metal (e.g., Cu) should be systematically compared with Mn via in situ DRIFTS, XAS, and full DFT energy profiles, etc., with a deeper discussion of performance differences beyond the d-band center argument.\nFigure 2. In the free energy diagram, conversion of CO2(g) to *CO is shown to have a negative free energy (in principle, spontaneous). What conditions were assumed to make this reaction downhill. I suspect that that the authors are using the computational hydrogen electrode that the reaction is proceeding via proton-coupled electron transfers. If this is the case, what potential was assumed and why was it though that this could be realized in photocatalytic experiments?\nMore generally, the influence of light on the catalytic process, particularly from a mechanistic perspective, is not discussed in sufficient detail. DFT calculations, especially those related to Supplementary Fig. 3, should incorporate the effect of photoexcitation to better reflect the actual reaction conditions.\nSupplementary Fig. 25 is ambiguous. Given the claimed 200-hour stability (Fig. 5e), post-reaction characterizations, especially XAS, should be shown for the spent catalyst. More frequent GC sampling (e.g., every 5–10 min) is recommended to capture potential fluctuations.\n13CO labeling, combined with in situ DRIFTS, would provide stronger evidence for the proposed asymmetric *CO–*CHO coupling pathway.\nFigure S24. This is said to be a GC-MS spectrum. But multiple analytes are present. Were chromatograms summed to produce this data?\nThe following is said about Figure 3b. “Elemental mapping via energy-dispersive X-ray spectroscopy (EDS, Fig. 3b) showed homogeneous distribution of Zn, Mn, and S throughout the sample, with Mn atoms clearly penetrating into the ZnS matrix.” On what basis can it be said that the Mn distribution is homogenous? What would an inhomogeneous distribution look like? Also “Mn atoms clearly penetrating into the ZnS matrix.” It is not possible to see if Mn atoms are inside the ZnS with TEM as it produces a 2D projection of a 3D object.\nFigure 3c caption. What do the yellow circles mean and what quantitative criterion was used to draw them?\nPage 6. “with directional sulfur vacancies (Sv).” What is a directional sulfur vacancy? ZnS has the zinc-blende structure, Figure 6. There is only one type of S in this structure and just one type of S vacancy. Also, how can a vacancy have a direction (vectorial?).\nPage 7. “…revealed a Sv concentration of 1.54% 156 for Mn1–ZnSv, closely matching the theoretical value predicted by DFT.” How can DFT predict the vacancy concentration produced by the microwave treatment?\nPage 9. “In contrast, Mn1–ZnSv exhibited a stronger *CO signal at 2078 cm⁻1 (Fig. 4b), indicating enhanced generation and stabilization of *CO.” Authors cannot conclude that CO2 -> CO is faster on the Mn-doped material. The steady-state concentration of *CO depends both on the generation and desorption rate.", "reviewer_5": "Response to Reviewers’ Comments\nManuscript number: NCOMMS-25-62587A\nTitle: Near-unity CO -to-ethylene photoconversion over low-coordination manganese single-\n2\natom sites on binary sulfide\nA Point-to-point Response to Reviewer’s comments\nDear Reviewers,\nThank you for checking our manuscript and proposing valuable comments! The comments are\nhighly appreciated and helpful to improve our work. We have made corresponding changes to\nthe manuscript (marked in RED) and Supplementary information to address the reviewers’\nconcerns; these changes are specified and discussed in a point-to-point response to the\nreviewers’ comments, as shown below:\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis manuscript presents an interesting study where atomic-level coordination control in a\nsulfide host translates directly to near-unity C₂ selectivity in sacrificial-agent-free CO₂\nphotoreduction. The key contribution of this work is the deliberate creation of sulfur vacancies\naround the Mn center, lowering its coordination from the usual tetrahedral (CN = 4) to CN ≈\n2. This unsymmetrical Mn–S₂ motif is argued to (i) strengthen *CO adsorption, (ii) lower the\n*CO → *CHO barrier, and (iii) enable *CO–*CHO asymmetric coupling to *COCHO, the\ngateway intermediate for C–C coupling. The DFT-guided synthesis, operando DRIFTS, XAFS,\nAC-STEM, and isotope (¹³CO₂/D₂O) labeling are combined to provide an unusually complete\nmechanistic picture for a photocatalytic CO₂RR study. As such, I would like to recommend to\npublish this work with major revisions."}, "author_response": "Response to Reviewers’ Comments\nManuscript number: NCOMMS-25-62587A\nTitle: Near-unity CO -to-ethylene photoconversion over low-coordination manganese single-\n2\natom sites on binary sulfide\nA Point-to-point Response to Reviewer’s comments\nDear Reviewers,\nThank you for checking our manuscript and proposing valuable comments! The comments are\nhighly appreciated and helpful to improve our work. We have made corresponding changes to\nthe manuscript (marked in RED) and Supplementary information to address the reviewers’\nconcerns; these changes are specified and discussed in a point-to-point response to the\nreviewers’ comments, as shown below:\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis manuscript presents an interesting study where atomic-level coordination control in a\nsulfide host translates directly to near-unity C₂ selectivity in sacrificial-agent-free CO₂\nphotoreduction. The key contribution of this work is the deliberate creation of sulfur vacancies\naround the Mn center, lowering its coordination from the usual tetrahedral (CN = 4) to CN ≈\n2. This unsymmetrical Mn–S₂ motif is argued to (i) strengthen *CO adsorption, (ii) lower the\n*CO → *CHO barrier, and (iii) enable *CO–*CHO asymmetric coupling to *COCHO, the\ngateway intermediate for C–C coupling. The DFT-guided synthesis, operando DRIFTS, XAFS,\nAC-STEM, and isotope (¹³CO₂/D₂O) labeling are combined to provide an unusually complete\nmechanistic picture for a photocatalytic CO₂RR study. As such, I would like to recommend to\npublish this work with major revisions.\nResponse: We appreciate the reviewer’s detailed summary of our work and are pleased that\nthe novelty and strengths of our coordination design and its photocatalytic performance were\nwell recognized. We have carefully addressed each of the reviewer's concerns below.\n1. EPR (g = 2.005) and S-content from EDS indicate S formation, yet the absolute S\nv v\nconcentration (1.54 %) is derived from a spin-counting model that assumes isolated S-\nvacancies. Clustering of vacancies could alter Mn coordination and electronic structure;\nspatially resolved EELS or positron annihilation spectroscopy would help to get S vacancy\nquantification and homogeneity.\nResponse: We sincerely thank the reviewer for this insightful and professional comment. We\nagree that clarifying the distribution of S-vacancies is crucial for confirming the precise\ncoordination environment of the Mn center. We have addressed this concern through additional\nexperiments and deeper analysis as detailed below.\nFirst, from a theoretical perspective, a facile and thermally activated hydrogen peroxide\n(H O ) chemical etching approach is proposed to introduce homogeneously distributed single\n2 2\nS-vacancies onto the Mn –ZnS surface. Then, the systematic adjustment of the etching duration\n1\nand etching temperature contributes to precise tuning of the single S-vacancy concentration.\nBased on the DFT calculations, Mn –ZnS means that two single S atoms in the pristine Mn –\n1 v 1\nZnS are removed, resulting in a theoretical S-vacancy concentration of 1.33% (a total of 75 S\natoms in the pristine Mn –ZnS model). The structure model and electron localization function\n1\n(ELF) of ZnS, Mn –ZnS and Mn –ZnS visually demonstrate the existence of S vacancy (Fig.\n1 1 v\nR1).\nSecond, to confirm the formation of S , electron paramagnetic resonance (EPR)\nv\nspectroscopy was performed (Fig. R2a). A clear signal at g = 2.005 indicated the presence of\nS in Mn –ZnS . The narrow and symmetric line shape of our EPR signal (g=2.005) suggests\nv 1 v\nnegligible dipole-dipole interaction between the unpaired electrons, consistent with isolated\ndefects (J. Am. Chem. Soc. 2020, 142, 9, 4298–4308). Additionally, EDS point analysis (Fig.\nR2b) showed an increased metal-to-sulfur atomic ratio, further supporting the generation of S ,\nv\nwhich increased with prolonged H O etching time. Quantitative analysis from ICP-OES and\n2 2\nEPR (Fig. R2c, d) revealed an experimental value of S concentration is 1.54% for Mn –ZnS ,\nv 1 v\nclosely matching the optimal configuration screened by DFT.\nThird, to further verify the consistency of the coordination environment of atomic Mn site,\nmore precise fitting of the Mn K-edge EXAFS spectra was conducted (Fig. R3). The EXAFS\nfitting results show that the coordination number of Mn-S in the Mn –ZnS sample was stable\n1 v\nat ~1.9, and no Mn-Mn or Mn-O bond were observed, suggesting that S exists in isolation\nv\nrather than in clusters.\nFinally, following the reviewer's excellent suggestion, we performed spatial-resolved EELS\nmapping around individual Mn atoms. The oxidation state of Mn was determined by\ncalculating the intensity ratio between the L3 (642.4 eV) and L2 (657.9 eV) peaks in the Mn\nL-edge EELS spectra (Sci Rep, 2023, 13, 14132). The results confirm that Mn exists\npredominantly in the +2 oxidation state in Mn –ZnS (Fig. R4), consistent with XAS result,\n1 v\nindirectly indicating that the existence state of S is isolated.\nv\nIn the revised manuscript, we have included these results and added related discussion in the\nrevised manuscript and the revised supplementary information (Fig. 3e, Supplementary Fig.\n16 and 19, line 159-162, 173-174).\nFig. R1 Top- , side-view and ELF of the slab model of ZnS, Mn –ZnS and Mn –ZnS (111)\n1 1 v\nsurface (blue: Zn, yellow: S, red: Mn).\nFig. R2 (a) EPR spectra of the pristine Mn –ZnS and Mn –ZnS with different S-vacancy\n1 1 v\nconcentrations. (b) HAADF-STEM images, the corresponding EDS elemental mapping and\natomic ratio (the number of M is the sum of Zn and Mn atoms) of Mn –ZnS with different\n1 v\nH O etching time. (c) The linear fitting between ICP calculated S-vacancy and EPR quantified\n2 2\nS-vacancy concentration over as-prepared catalysts. (d) ICP calculated M/S ratios as a function\nof the H O etching treatment time (the number of M is the sum of Zn and Mn atoms).\n2 2\nFig. R3 (a) Fourier-transform (FT) spectra of EXAFS oscillations for Mn foil, MnO, Mn O ,\n2 3\nMn –ZnS and Mn –ZnS . (b) Mn K-edge EXAFS fitting analyses for Mn –ZnS samples in R\n1 1 v 1 v\nSpace.\nMn -ZnS\n1 v\nL\n3\n)\n.\nu\n.\na\n(\n s\nt\nn L\nu 2\no\nC\nL /L = 3.52\n3 2\n640 645 650 655 660 665\nEnergy loss (eV)\nFig. R4 Mn-L electron energy loss spectrum (EELS) spectra for Mn –ZnS .\n2,3 1 v\n2. Only 0.6 wt % Mn is loaded; at this level, even a small fraction of MnOx or MnS₂ clusters\n(<1 nm) could be missed by AC-STEM. A Mn K-edge EXAFS fitting range beyond 3 Å should\nbe inspected for Mn–Mn contributions.\nResponse: Thank you for your constructive comments. In response to the reviewer’s\nsuggestion, the FT-EXAFS spectra of standard MnO and Mn O was supplemented in Fig. R3,\n2 3\nand the peak at 1.52 Å is attributed to Mn-O bond (Angew. Chem. Int. Ed. 2025, 64,\ne202509741). No characteristic signals of the Mn-Mn and Mn-O bonds were detected,\ndemonstrating that the Mn atoms are atomically dispersed in the Mn –ZnS , and no MnO\n1 v x\nforms are present.\nTo verify the contribution of the Mn-Mn bond, the second coordination shell of the Mn K-\nedge EXAFS over Mn –ZnS , beyond the range of 3 Å, was fitted in Fig. R3 and Table R1.\n1 v\nThe EXAFS fitting results show that Mn atom mainly exists in the Mn-S path (the first\ncoordination shell, 1.96 Å) and the relatively weak Zn-Mn path (the second coordination shell,\n3.72 Å), without Mn-Mn path coordination. In addition, the independent Mn-Mn path was\nintroduced for EXAFS fitting of Mn –ZnS , and the coordination number of Mn-Mn path was\n1 v\nstatistically less than 0.1, with no significant improvement in R-factor. It further confirms the\natomic dispersion of Mn atoms and the absence of clusters of Mn compounds in Mn –ZnS\n1 v\n(Nat Commun 2025, 16, 4622).\nIn the revised manuscript, we have included these results in Fig. 3e and added related\ndiscussion in the line 173-174, 179-181 of the revised manuscript and Supplementary Fig. 19\nof the revised supplementary information.\nTable R1 EAXFS fitting results of the Mn K-edge for the Mn –ZnS .\n1 v\nSample Path N R/ Å σ2/10-3 Å2 ΔE/ eV R-factor\nZn-S1 3.8±0.1 2.26±0.01 7.2±0.1 -1.9±0.1\nMn –ZnS Mn-S 1.9±0.2 1.96±0.02 7.3±0.2 -1.5±1.1 0.009\n1 v\nZn-Mn 11.8±0.2 3.72±0.01 6.8±0.2 -1.6±0.2\n3. Photocurrent and EIS (Supplementary Figs. 18–19) show improved charge-transfer kinetics\nfor Mn₁–ZnSᵥ, but the incident-photon-to-current efficiency (IPCE) at 420 nm is not reported;\nthis would directly corroborate the 8.1 % AQE claim.\nResponse: Thanks for your constructive comments. According to your suggestion, the incident\nphoton-to-current efficiency (IPCE) was measured based on the photocurrent density under\nmonochromatic light irradiation to directly evaluate the photoelectric conversion efficiency of\nMn –ZnS . The IPCE can be calculated by using equation (1):\n1 v\nℎ𝑐\n×𝑗\n𝐼𝑃𝐶𝐸 = λ (1)\n𝐼𝑒\nwhere j is the photocurrent density (A cm-2), I is the irradiation light intensity (W cm-2), λ is\nthe equivalent wavelength (m), h is Planck's constant (6.626×10-34 J s-1), c is the speed of light\n(2.998×108 m s-1).\nAs shown in Fig. R5a, the corresponding IPCEs of pristine ZnS, Mn –ZnSand Mn –ZnS\n1 1 v\nwere calculated and consistent with the AQE results (Fig. R5b). Compared to the ZnS and\nMn –ZnS, the Mn –ZnS showed an obvious enhancement of the IPCE in the range of 400–\n1 1 v\n600 nm. At 420 nm, the IPCE of the Mn –ZnS exhibited a maximum value of 3.4%, which is\n1 v\n1.6 times higher than that of Mn –ZnS, implying excellent carrier transfer and separation. Then,\n1\nthe AQE of 420 nm was calculated to be 8.12% in combination with the measurement\nparameters of IPCE. In conclusion, combining photocurrent, EIS, and IPCE, a solid and\nconsistent verification of AQE results is provided from the kinetic perspective of charge\nseparation and transport.\nWe have included the above results and added related discussions in the revised manuscript\n(the part of Tests on photocatalytic CO reduction with H O) and the revised supplementary\n2 2\ninformation (Supplementary Fig. 19).\nFig. R5 (a) IPEC plots of ZnS and Mn –ZnS, the Mn –ZnS . (b) The AQE of C H evolution\n1 1 v 2 4\ncatalyzed by Mn –ZnS catalyst and the correlation between the related absorption spectra and\n1 v\nabsorption wavelengths.\n4. Mott–Schottky plots give flat-band potentials, yet the absolute band positions (vs. NHE) are\nextracted assuming no Fermi-level pinning by Mn states; in situ ambient-pressure XPS under\nillumination could verify band alignment under operating conditions.\nResponse: We thank the reviewer for professionally reviewing our manuscript and giving\nconstructive comments to improve our manuscript. Following the reviewer's insightful\nsuggestion, to directly probe the band alignment in the working state, in situ ambient-pressure\nVB-XPS experiments were performed under visible light irradiation. Under light irradiation,\nthe valence band maxima for Mn –ZnS and Mn –ZnS were located at 1.00 and 0.55 eV below\n1 1 v\nthe Fermi level (Fig. R6), respectively, with a slight redshift compared to those in the dark,\nwhich is direct evidence for band bending caused by the accumulation of photogenerated\ncarriers. Combining the results of UPS and UV-Vis, the conduction band minimum (CBM) of\nMn –ZnS and Mn –ZnS were determined to be -1.16 and -1.23 eV, respectively. The CBM\n1 1 v\nvalues of Mn –ZnS (1.06 eV) and Mn –ZnS (1.15 eV) obtained by Mott-Schottky analysis are\n1 1 v\nvery close to those obtained by in situ XPS measurement (ΔE<0.1 eV), indicating that the\nintroduction of Mn did not cause severe Fermi-Level pinning.\nIn the revised manuscript, the electronic band structure versus Normal Hydrogen Electrode\n(NHE) was obtained by in situ XPS measurement (Fig. R6). The actual CBM under operating\nconditions is thermodynamically favorable for the CO -to-C H conversion. We have included\n2 2 4\nthe above results in Supplementary Fig.s 22, 23 and added related discussions in the revised\nmanuscript (line 198-202).\nFig. R6 In situ irradiated VB-XPS spectra of (a) Mn –ZnS and (b) Mn –ZnS in dark and\n1 1 v\nirradiation.\n-3\nZnS Mn -ZnS Mn -ZnS\n1 1 v\n-2 Ec = -0.98 eV -1.16 eV -1.23 eV\n)\nV\ne -1\nE ( CO /C H = -0.38 eV\n2 2 4\nH\nN 0\ns Eg = 3.48 eV 3.26 eV 2.88 eV\nv\nl 1\na H O/O = 0.82 eV\ni 2 2\nt\nn\ne\nt 2\no\nP\n1.65 eV\n3\n2.10 eV\nEv = 2.50 eV\n1 1 v\n5. D₂O experiments confirm water as the proton donor, but no KIE (k_H/k_D) for C₂H₄\nformation is provided. A primary KIE would strengthen the claim that *CHO formation is\nturnover-limiting.\nResponse: We sincerely appreciate the reviewer’s valuable comment and suggestion. In light\nof this suggestion, we have conducted the kinetic isotopic effect (KIE) of H/D over ZnS, Mn –\n1 v\ncatalyst dosage, volume of reaction gas, etc.). Our DFT calculations (Fig. R8a) suggest that\nthe *CO hydrogenation into *CHO may be the turnover-limiting step during the photocatalytic\nCO RR to C H . The activation of H O to active hydrogen species may thus be an important\n2 2 4 2\nstep that affects the *CHO formation. To understand the role of H O activation in\n2\nphotocatalytic CO RR to C H product, we have measured the KIE of H/D over as-prepared\n2 2 4\ncatalysts. As displayed in Fig. R8b, the KIEs of H/D, which are defined as the ratio of C H\n2 4\nformation rates in H O and D O, were calculated to be 1.67, 1.36 and 1.18 over ZnS, Mn –ZnS\n2 2 1\nand Mn –ZnS catalysts, respectively. It is generally believed that the KIE value of 2 is\n1 v\nmanifested as a distinct primary KIE, indicating that the H O activation is involved in the rate-\n2\ndetermining step (Nat. Commun. 2019, 10, 892). By contrast, the KIE over the Mn –ZnS\n1 v\ncatalyst is close to 1. Therefore, the H O dissociation is not regarded as the rate-determining\n2\nstep over the Mn –ZnS catalyst. That is to say, the KIE further strengthens the claim that\n1 v\n*CHO formation is turnover-limiting step. We have included the KIE of H/D in Supplementary\nFig. 41 and added related discussion in the revised manuscript (line 228-269).\nFig. R8 (a) The free minimum-energy diagram for *CO hydrogenation and subsequent C-C\ncoupling on Mn –ZnS (111) facet.\n1 v\n(b) The C H formation rate in H O and D O over pristine\n2 4 2 2\nZnS, Mn –ZnS and Mn –ZnS catalysts.\n1 1 v\n6. The H₂ evolution rate is suppressed to ≈1 µmol g⁻¹ h⁻¹, but its potential role as a competing\npathway is not quantitatively modeled.\nResponse: Thank you sincerely for your constructive comments on our studies. The hydrogen\nevolution reaction (HER) is usually regarded as a competing reaction for the photoreduction\nreaction of carbon dioxide (CO RR). However, the adsorbed hydrogen species (*H) generated\n2\nby H O dissociation, as a proton donor, is crucial for CO photoreduction to hydrocarbons. To\n2 2\nfurther explore the mechanism of the HER inhibition and the enhanced C H selectivity, the\n2 4\ncompetitive adsorption behavior of *H species and *CO intermediates on the catalyst was\ninvestigated by simulation calculations. In our study, the introduction of low-coordinated Mn\nsingle atoms can lead to the nonequivalence of surface sites, thus modulating the distribution\nof *CO intermediates (Fig. R9). Our computational results show that the incorporation of low-\ncoordinated Mn single atoms ingeniously balances the competitive adsorption of *CO\nintermediates and *H species (Fig. R10). Compared with *H species, *CO exhibits a stronger\nadsorption capacity (-1.36 eV) at the Mn active site in Mn –ZnS , which ensures that there are\n1 v\nsufficient protons participating in the subsequent hydrogenation process without affecting the\nadsorption of key carbon-based intermediates at the active sites. Furthermore, the Gibbs free\nenergy (|ΔG |) for the HER on Mn –ZnS (0.56 eV) is higher than that of Mn –ZnS (0.22 eV)\nH* 1 v 1\n(Fig. R11), which not only effectively suppress the HER but also facilitate *CO protonation\ninto *CHO. The kinetic barriers of *CO protonation (Fig. R12a) and the products yield (Fig.\nR12b) were conducted to validate that HER was not only suppressed but also ensured that the\n*H species was efficiently used for CO reduction and its subsequent C-C coupling process.\n(Supplementary Fig. 4, line 100-102, 245-248).\n-0.4\n-0.63\n-0.35 -0.71\n-0.48\n-0.8\nV ) -1.36\ne -1.2 -1.02\n(\nO\nC -1.6 = 5.084 Å\n*\ns = 7.107 Å\nd\na\nE -2.0 6 5 = 4.401 Å\n1 4\n3 = 3.330 Å\n-2.4 2\n= 3.816 Å\nZn Zn Zn Zn Zn Mn\n1 2 3 4 5 6\nFig. R9 The *CO adsorption energy at different sites of Mn –ZnS . The insets are top views\n1 v\nof the structure of different sites and the distances between different Zn sites and Mn sites. The\nMn, Zn, and S atoms are shown in pink, blue, and yellow colors, respectively.\nFig. R10 The comparison of *CO and *H adsorption energy of Mn –ZnS and Mn –ZnS with\n1 1 v\ndifferent sites (Mn: pink, Zn: blue, S: yellow).\nFig. R11 The Gibbs free energy diagrams for hydrogen adsorption of ZnS, Mn –ZnS and\n1 v\nFig. R12 (a) Reaction energy diagram from *CO to *CHO on ZnS, Mn –ZnS and Mn –ZnS\n1 1 v\nwith molecular structures of initial, transition and final states inserted. (b) The products yield\nover ZnS, Mn –ZnS and Mn –ZnS catalysts.\n1 1 v\n7. Ten 4-h cycles (40 h total) show no loss, yet photocorrosion of ZnS in aqueous media is\nwell-known. Longer-term tests (>100 h) or continuous-flow operation should be addressed.\nResponse: Thank you sincerely for pointing out our shortcomings. As shown in Fig. 5e, we\nhave provided the photocatalytic stability tests for CO photoreduction to C H over 50\n2 2 4\nconsecutive cycles (200 hours), during which no significant decline in activity or selectivity\nwas observed. We apologize for the writing errors in the main text and have now corrected\nthem (line 270).\n8. Microwave/H₂O₂ etching is elegant for lab scale (~100 mg batches); scalability to gram or\nkilogram scale is not addressed.\nResponse: Thank you for your careful consideration. Scalability is the core issue that must be\nfaced and solved from laboratory research to industrial application. In our study,\nmicrowave/H O etching, as a novel method, was carried out to complete the proof of concept\n2 2\nand fundamental mechanism research on a lab scale (~100 mg). Small-scale experiments allow\nus to conduct rapid screening and optimization of a large number of parameters at a lower cost\nand with higher efficiency, which is crucial for understanding the nature of reactions and\nidentifying key influencing factors.\nIn view of the scalability problem, a series of gram-level amplification experiments (1, 2\nand 5 g) over Mn –ZnS in the microwave/H O etching process were conduct under the\n1 2 2\ncondition of maintaining the same H O concentration, precursor adding mode and microwave\n2 2\npower. The products yield and C H selectivity of the amplification experiments is illustrated\n2 4\nin the Fig. R13. As the amount of Mn –ZnS was scaled up to gram level (1, 2 and 5 g), C H\n1 2 4\nyield and selectivity did not change significantly compared with 100 mg, and the C H\n2 4\nselectivity could still reach 99%. We will actively explore the reaction equipment and reaction\nparameters suitable for larger gram experiments, so as to provide a solid theoretical and\nexperimental basis for the scale-up application of the catalyst. The corresponding discussion\nhave been added in manuscript (Supplementary Fig. 34, line 237-239).\n)350\n1\n-g\n100) \nol 300 %\nm\ny\n(\nμ250 t\n(\nd ti\nvi\nel200 c\ns\nyi\n150\n50\ns\nel\ne\nc\nt\n4\nu H\nd100 2\no C\nr\nP 50\n0 0\n100 mg 1 g 2 g 5 g\nFig. R13 The products yield and C H selectivity over different amounts of Mn –ZnS (100 mg,\n2 4 1\n1 g, 2 g and 5 g) in microwave/H O etching amplification experiments.\n2 2\n9. Only the ZnS(111) facet is modeled while ZnS also exposes (110) and (100) surfaces with\ndifferent S formation energies. It will be necessary to model other two surfaces using DFT\nv\ncalculations for comparison.\nResponse: Thanks to the reviewer for highlighting this important issue and for your valuable\nsuggestion. Based on your critical observation, we have accordingly performed DFT\ncalculation of ZnS, Mn –ZnS and Mn –ZnS with exposed (110) and (100) facet, respectively.\n1 1 v\nThe structural models and vacancy formation energies of catalysts with different exposed facet\nwere presented in Fig. R14. In contrast to Mn –ZnS with exposed (111) facet, the S-vacancy\n1 v\n(S ) formation energies of Mn –ZnS with exposed (110) and (100) facet were higher,\nv 1 v\nindicating that S formed preferentially on the (111) facet (Fig. R15). In the revised manuscript,\nv\nwe have included these results in Supplementary Figs. 2, 3 and added the related discussions\n(line 90).\nFig. R14 Top-view and side-view of the slab model of ZnS, Mn –ZnS and Mn –ZnS with\n1 1 v\nexposed (100) and (110) surface (blue: Zn, yellow: S, red: Mn).\nFig. R15 The vacancy formation energy (ΔE ) of Mn –ZnS with (111), (100) and (110) facet,\nVs 1 v\nrespectively.\n10. Solvation and entropic contributions to *COCHO stability are not explicitly included.\nResponse: Thanks for your constructive suggestion. Ab-initio molecular dynamics (AIMD)\nsimulations with fully explicit solvation were performed to elucidate solvation and entropic\ncontributions to *COCHO stability. In our simulation model, we placed 202 water molecules,\nleading to the average density of ∼1 g·cm–3 to model the solvent water environment. The Gibbs\nfree energy (∆G) for the C–C coupling (*CO + *CHO → *COCHO) on the Mn\n1\n–ZnS\nv\nsurface\nreaction was calculated by\n∆G = ∆E + ∆E −T∆S−𝑒𝑈 (2)\nZPE\nin which ∆E represent the change of the zero-point energy (ZPE), T∆S is the entropic\nZPE\ncontributions (T was set to be 300 K), which were obtained from the vibrational frequency\ncalculations through the VASPKIT code. e is the elementary charge and U is the conduction\nband potential of semiconductor photocatalysts. The corresponding equilibrated structures\nobtained from the 5 ps AIMD simulation are presented in Fig. R16a.\nConsidering the solvation and entropy contributions, the simulation results further provide\nunderstanding on that a water environment close to the reaction condition can reduce the barrier\nof *CO–*CHO coupling into *COCHO from 0.62 eV to 0.54 eV (Fig. R16b) over Mn –ZnS .\n1 v\nThis highlights the importance of considering the solvation environment and entropy\ncontribution, especially for *COCHO stability, which forms strong hydrogen bonds with the\ninterfacial water. Specifically, the Mn–C bonds between *COCHO and the interface shorten,\nleading to increased charge transfer between Mn and C and stronger stability of *COCHO (Fig.\nR16c). Collectively, taking into account the solvation and entropic effects, the energy barrier\nof C-C coupling decreased, further stabilizing the *COCHO intermediate and making the\nreaction pathway more thermodynamically and kinetically favorable.\nThe corresponding revision and discussion have been added in the revised manuscript\n(Supplementary Fig. 6, line 130-132, section on Density functional theory (DFT)\ncalculations).\nFig. R16 (a) Side view of the simulation models of Mn –ZnS . Hydrogen bonds are depicted\n1 v\nby thin dashed lines. (b) Free energy diagram for C–C coupling (*CO + *CHO → *COCHO)\non the Mn –ZnS surface in vacuum and solvation environment, respectively (IS, TS, FS are\n1 v\ninitial state, transition state, and final state, respectively). (c) The charge transfer between Mn\nand C atoms and the Mn–C bond length of Mn –ZnS in vacuum and solvation environment.\n1 v\nThe zinc, manganese, sulfur, carbon, oxygen and hydrogen atoms are colored in blue, pink,\nyellow, gray, red and white, respectively.\n11. ab-initio molecular dynamics simulations should be performed to capture dynamic vacancy\nreconstruction (e.g. 300 K).\nResponse: Thank you for the constructive and valuable comments. According to your\nexcellent suggestion, ab-initio molecular dynamics (AIMD) simulations were performed for a\nduration of 10 ps at 300 K with time step of 1.0 fs (Fig. R17). As shown in Fig. R17a, time-\nresolved structural snapshots capture the vacancy structure remaining stable during the\ndynamic process without any collapse, diffusion or reorganization. The Mn-S bond length\nevolution (Fig. R17b) displays that in Mn –ZnS , the distance between the Mn active center\n1 v\nand the S atom coordinated with it thermally oscillates only around the equilibrium value. The\nsulfur vacancies persisted throughout the AIMD simulations and no structural reconstruction\nwas observed, which strongly proves that our proposed low-coordination Mn single atom\nstructure is stable. These results are supplement in the revised manuscript and supplementary\ninformation (Supplementary Fig. 17, line 159-162, section on Density functional theory (DFT)\ncalculations).\nFig. R17 (a) Representative snapshots from AIMD simulation for the vacancy reconstruction\nprocess. Blue, red and yellow balls represent Zn, Mn and S atoms, respectively; (b) Variations\nof Mn-S bond during the AIMD simulation.\n12. Band-gap determination from UV–Vis (Fig. S13) uses a Tauc plot without stating whether\ndirect or indirect transition is assumed; please clarify.\nResponse: Thank you very much for your rigorous consideration. The Tauc plot is derived\nbased on the direct transition model, as ZnS is a well-known direct bandgap semiconductor.\nThe bandgap energy (Eg) was determined by extrapolating the linear region of the plot of (αhν)2\nversus photon energy (hν) (Nano Energy 2022, 93, 106809). We have marked \"direct\ntransition\" in the captions of Supplementary Fig. 20 in the supplementary information to make\nthe analysis of UV–Vis more complete.\n13. The reaction chamber volume, light intensity mapping, and temperature rise under focused\nXe lamp are not documented; local heating can accelerate C–C coupling.\nResponse: Thank you for your careful check and suggestion. The photocatalytic reactions were\ncarried out in Labsolar 6A device with a reactor volume of 370 mL and a system circulation\npipeline volume of ~100 mL. In order to simulate the photosynthesis of natural plants, a 300\nW Xe lamp was chosen as the light source. The average light intensity irradiated on the surface\nof the catalyst is ~15 mW cm-2 measured by a ThorLabs PM100D Power with a photodiode\nsensor.\nIn the photocatalytic experiment process, the temperature of the reaction system was\ncontinuously maintained at 25 ± 1.5 oC through a cooling circulating water system. To address\nyour concerns about \"local heating\", we conducted an additional verification experiment: a fine\nmicro-thermometer was placed inside the reactor, adjacent to the catalyst area, to monitor the\ntor monitor the\ntemperature changes under light irradiation in real time. As shown in the Table R2, under the\nsame reaction light intensity, the measured local temperature change is extremely small (ΔT <\n3.0 oC). According to the Arrhenius equation, the influence of such a small temperature change\non the formation rate of products can be ignored. The cooling circulating water system\neffectively removes the heat generated by light irradiation, maintaining the stability of the bulk\nphase temperature. In conclusion, through strict temperature control and real-time monitoring,\nit has been confirmed that the influence of local thermal effects can be ignored, and the\nobserved activity originates from the photocatalytic process.\nAccording to your suggestions, we have supplemented the above information in detail in the\nexperimental section of the revised manuscript (section on Tests on photocatalytic CO\n2\nreduction with H O).\n0 24.8\n15 25.3\n30 25.1\n60 25.3\n120 25.9\n240 27.2\n14. DRIFTS spectra are normalized to the ZnS phonon band at 1122 cm⁻¹, but absolute\nabsorbance calibration would allow estimation of surface *CO coverage.\nResponse: Thank you for your meticulous suggestion. In our work, we normalized the in situ\nDRIFTS spectra to highlight the relative variation trends of the signal intensity of key\nintermediates (such as *CO, *COCHO, etc.) related to the reaction mechanism, thereby\nverifying the reaction path proposed. According to your suggestion, we have performed\ncontour plot processing on the original in situ DRIFTS data of Mn –ZnS and Mn –ZnS with\n1 1 v\nabsolute absorbance values (Fig. R18), which is a key step to turn the mechanism\ndemonstration from qualitative to quantitative. As shown in Fig. R18, the magnitude of the\nabsolute absorbance signal at 2078 cm-1, attributed to the *CO intermediate, can visually reflect\nthe relative change of surface *CO coverage. Compared with Mn –ZnS, the *CO peak signal\n1\nof Mn –ZnS is stronger during the photocatalytic CO RR. To further verify this result, CO-\n1 v 2\nadsorbed DRIFTS was performed (Fig. R19), indicating a higher surface *CO coverage on\nMn –ZnS . In addition, a correlation analysis was conducted between the changing trend of\n1 v\nthe absolute absorbance value of the *CO and the C H formation rate and selectivity over\n2 4\nMn –ZnS and Mn –ZnS (Fig. R20). The favorable positive correlation trend presented\n1 1 v\nindicates that a higher surface *CO coverage is conducive to improving the C-C coupling\nefficiency, which is also the key reason for the high selectivity of C H . In response to the\n2 4\nreviewer’s suggestion, we have added a corresponding description to better evaluate surface\n*CO coverage and corroborate the key role of surface *CO coverage in driving C H selectivity\n2 4\n(Supplementary Fig. 30, line 221-225, 256-259).\nFig. R18 In situ DRIFTS for CO photoreduction reaction with the absolute absorbance over\n1 1 v\nFig. R19 CO-adsorbed DRIFTS for Mn\n1\n–ZnS (a) and Mn\n1\n–ZnS\nv\n(b).\n0.027 C H selectivity 99.1% 100\n2 4 1)\n-h\n0.026 1\n-g\nu.) 0.025 80\nm\nol\na. 74.5% μ\ne\n( 0.024 (\ne\nb\na n\nc\n0.023 60\nn\nr\nat\nr o\ns o 0.022 ati\nb m\nA\n0.021 40 o r\nf\n0.020 2\nC\n0.019 20\nMn\n1\n-ZnS Mn\n1\n-ZnS\nv\nFig. R20 The correlation analysis between the changing trend of the absolute absorbance value\nof the *CO and the C H formation rate and selectivity over Mn –ZnS and Mn –ZnS .\n2 4 1 1 v\n15. The error bars for AQE (currently single value at 420 nm) should be provided.\nResponse: We thank the reviewer for the good suggestion. As suggested, we have provided\nthe error bars for AQE in Supplementary Fig. 39 of the revised supplementary information.\n1.0\n8\n0.8\n)\nu. 6\na.0.6 A\n( Q\ne E\nn\nc\n4 (\na0.4 %\nb )\nr\no\ns 2\nb0.2\nA\n0.0 0\n400 500 600\nWavelength (nm)\nFig. R21 The AQE of C H evolution catalyzed by Mn –ZnS catalyst and the correlation\n2 4 1 v\nbetween the related absorption spectra and absorption wavelengths.\nReviewer #2 (Remarks to the Author):\nThe manuscript addresses a significant challenge for the energy community – how to efficiently\nconvert CO into valuable C2 products using solar energy directly. The manuscript is well-\n2\nwritten, clearly organized, and contains a wide spectrum of characterization methods. My\nreview will focus primarily on the photochemical characterization experiments, which require\nsome clarifications and the filling of missing data points to assess the paper's merit accurately.\nResponse: We sincerely thank your positive feedback and for recognizing the significance of\nour work. We are also grateful for the constructive comments and valuable suggestions, which\nhave helped us enhance the quality of the manuscript. Below, we have provided detailed\nresponses to your comments. The point-to-point response is as follows and related changes are\nmarked in RED in the revised manuscript and the supplementary information.\n1. The paper’s main point is that using low-coordination Mn sites to stabilize CO RR\n2\nintermediates results in a higher efficiency towards C products, as otherwise these critical\n2+\nintermediates are not able to react further. To support this claim, the authors should also\nperform control experiments using CO as feed for the ZnS catalyst and add this to Fig 5c. This\nis a critical point to address.\nResponse: Thanks to the reviewer for highlighting this important issue and for your valuable\nsuggestion. Following the suggestion, we performed control experiments by supplying 100%\nCO as the reactant under otherwise identical reaction conditions over the ZnS catalyst.\nAs shown in Fig. R22, the ZnS catalyst exhibits markedly lower C H yield with CO as the\n2 4\nreactant compared to CO , while also underperforming significantly against the Mn -ZnS\n2 1\ncatalyst under the same conditions. This result underscores that the low-coordination Mn sites\nnot only stabilize the CO intermediate, but also facilitate CO activation, thereby forming a\n2\nhighly active adsorbed *CO species favorable for subsequent C-C coupling. The synergistic\n\"activation-stabilization\" mechanism is the essential reason for the high selectivity of the Mn -\nv 2 4 2\nIn response to the reviewer’s suggestion, we have added the related explanations in the\nrevised manuscript (Fig. 5c, line 256-259).\n1000\nreactant CO CO\n) 2\n- 1 g800 H 2 H 2\nol CH\n4\nm\nCO\nμ\n(\nd\n4\nel\nyi\ns\n400\nt\nc\nu\nd\no200\nr\nP\n1 1 v\nFig. R22 The products yield over ZnS, Mn –ZnS and Mn –ZnS catalysts using CO and CO\n1 1 v 2\nas reactant, respectively.\n2. Secondly, the authors claim that they excelled in two metrics important for photochemical\nsystems: ethylene production rate and ethylene selectivity. Given this is a primary claim, the\nauthors should provide more details on how the experiments were conducted and how the\nmetrics were calculated. Also, were the experiments repeated for reproducibility? The error\nbars should be added to the photochem experiments. The authors should also include their\ncalculations of apparent quantum efficiency (specific values used, not only the formula).\nResponse: We sincerely appreciate the valuable comments. The specific details on\nexperimental part and performance indicators have been provided point by point.\n(1) Based on your suggestions, we have supplemented and explained more details regarding\n(i): Photochemical testing: The photocatalysts (0.2 g) was dispersed at the bottom of watch\nglass (diameter is ~4.0 cm) with 5 mL deionized water. The mixed sample was uniformly\nultrasound and put into the photoreaction system with a reactor volume of 370 mL. Before\nintroduction of light irradiation, the photoreaction system was thoroughly vacuum-treated (3\ntimes), and then pure CO was injected into the internal circulation reactor for 30 min. And a\n2\ncooling circulating water device was utilized to maintain the reaction system a constant\ntemperature at 25 ± 0.5 oC. In order to simulate the photosynthesis of natural plants, one xenon\nlamp (300 W) was chosen as the light source, and added a 380 nm external filter to make the\nlight source only retain the visible light. The light intensity was measured by a ThorLabs\nPM100D Power with a photodiode sensor (~15 mW cm-2). After starting the light source and\nthe gas chromatograph automatic sampling device, the sample was analyzed every 15 min, and\nthe reaction test was carried out for 4 h. (ii) Yield and selectivity calculations: The real-time\npeak area of the products was recorded during the photocatalytic test, and the product yield\nwas obtained by the standard working curve. The selectivity of CO reduction to carbon\n2\nproducts (S ) is obtained by the following formula (3):\nCO2\n𝑆 (%) =\n12𝑛(𝐶2𝐻4)+2𝑛(𝐶𝑂)+8𝑛(𝐶𝐻4)+ 2𝑛(𝐶𝐻𝑂𝑂𝐻)\n×100% (3)\n𝐶𝑂2\n12𝑛(𝐶2𝐻4)+2𝑛(𝐶𝑂)+8𝑛(𝐶𝐻4)+ 2𝑛(𝐶𝐻𝑂𝑂𝐻)+2𝑛(𝐻2)\nThe selectivity of C H reduction is obtained by the following formula (4):\n2 4\n𝑆 (%) =\n12𝑛(𝐶2𝐻4)\n×100% (4)\n𝐶2𝐻4\n12𝑛(𝐶2𝐻4)+2𝑛(𝐶𝑂)+8𝑛(𝐶𝐻4)+ 2𝑛(𝐶𝐻𝑂𝑂𝐻)\n(2) The repeated photoactivity test was carried out and an error bar for C H yield was added\n2 4\nin Fig. R23.\nFig. R23 The evolution of C H product during photocatalytic CO reduction over time.\n2 4 2\n(3) The ratio of the number of photons converted by the reaction to the number of incident\nphotons is the value of AQE, and the specific formula is as follows:\n𝐴𝑄𝐸 =\n12×Number of reacted electrons\n×100 % =\n12×nC2H4 ×NA×100\n% =\n𝐶2𝐻4 Number of incident photons S×I×t× λ\nh×c\n12×76.6 ×0.2×4×10−6×6.022×1023\n= 0.0812 = 8.12% (5)\n𝜋×22×15×10−3×14400×\n420×10−9\n6.626×10−34×2.998×108\nwhere S is the irradiated area (cm2), I is the irradiation light intensity (W cm-2), t is the\nirradiation time (s), λ is the equivalent wavelength (m), h is Planck's constant (6.626×10-34 J s-\n1), c is the speed of light (2.998×108 m s-1) and N is Avogadro's number (6.022×1023 J mol-1).\nA\nn is the amount of C H produced in 4 h.\nC2H4 2 4\nIn the revised manuscript, we have included these results in Fig. 5b and added the related\ndiscussions in the method section (Supplementary Table 4 and section on Tests on\nphotocatalytic CO reduction with H O).\n2 2\n3. Important details are missing in the experimental description -e.g., what was the total amount\nof CO injected into the reactor, and how was the photocatalyst immobilized?\nwere carried out in a reactor volume of 370 mL and a system circulation pipeline volume of\n~100 mL. The pure CO was injected into the internal circulation reactor (50 mL min-1) until\n2\nthe system pressure reached 80 kPa. The photocatalyst power (0.2 g) was dispersed in\ndeionized water (5 mL) and sonicated to form a homogeneous slurry at the bottom of watch\nglass (diameter is ~4.0 cm), and place this glass in the reactor.\nWe have supplemented this information in the revised manuscript to enhance its rigor and\nrepeatability (section on Tests on photocatalytic CO reduction with H O).\n2 2\n4. Fig 5d: control condition 5, should it be “replacing CO with Ar” instead of “replacing Ar\n2\nwith CO ”?\n2\nthe revised manuscript (Fig. 5d).\n5. What do authors mean as “efficiency of tourism” (SI, section on Tests on photocatalytic CO\n2\nreduction with H O)? there is also a typo in Eq1. Looks like this was an auto-correct mistake,\n2\nplease review all your writing carefully.\nResponse: Thank the reviewer for checking our work carefully and providing valuable\ncomments. We sincerely apologize for the obvious clerical error in \"efficiency of tourism\" here,\nwhich should be \"turnover frequency\". For better expression, we have made corrections in the\ncorresponding section: “The ratio of the number of photons converted by the reaction to the\nnumber of incident photons is the value of AQE”. We have conducted a comprehensive review\nof all formulas, charts and main text to ensure that such errors have been completely corrected.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse：Thanks for your professional comments and constructive suggestions. Those\ncomments are all valuable and very helpful to us. We have tried our best to improve the\nmanuscript and make the corresponding modification in the revised manuscript. These\nrevisions do not affect the framework of this paper. We sincerely hope that these modifications\nand explanations will be acknowledged. Once again, thank you very much for your comments\nand suggestions.\nReviewer #4 (Remarks to the Author):\nTang et al. report a Mn single-atom catalyst embedded in ZnS with sulfur vacancies (Mn –\nv 2 2 4\nof Mn, hypothesizing that “single-atom” sites with Mn would increase binding energy for *CO\nand steer selectivity to C2 products, Figure 1. Nanoparticle catalysts were made by solution-\nbased method: ZnS and Mn-ZnS with 3 different mass ratios. Fe, Co, Ni, and Cu were also\nemployed as dopants. S vacancies were induced by microwave treatment for 5-15 seconds: Zn-\nZnSv series. While a large number of samples appear to have been made, study focuses on one\nof them (page 6): “Inductively coupled plasma optical emission spectrometry (ICP-OES)\nconfirmed a Mn content of 0.6 wt% (Supplementary Table 1).” It does appear, EPR in Figure\nS8, that this sample has more unpaired electrons than control which did not undergo the\nmicrowave treatment. A very high selectivity for this preparations is claimed: 76.6 μmol·g-1·h-\n1 under sacrificial-agent-free conditions.\nNo sacrificial species were employed and 13C and 2D labeling was used to ensure that carbon\nand protons come from CO and water, respectively. O production is also reported, Figure 22.\n2 2\nStudy thus follows “best practices” for experimental reports of this type, see\n10.1021/acsenergylett.8b00196 and 10.1002/anie.201207199. Also, interestingly, Mn -ZnS is\n1 v\nable to convert CO to C H (this is not always the case for CO RR photocatalysts). They\n2 4 2\nestimate an AQE of 8% at 420 nm. The authors attribute the performance to *CO stabilization\nand *CO–*CHO coupling, supported by in situ spectroscopy and DFT calculations.\nReview prompts are addressed as follows:\nWhat are the noteworthy results?\nThe performance of the photocatalysts, both in terms of activity/selectivity, is very good\ncompared to other reports in the literature, Figure 5f.\nWill the work be of significance to the field and related fields? How does it compare to the\nestablished literature? If the work is not original, please provide relevant references.\nWhile the performance metrics are high, the novelty is limited. Similar catalyst architectures\nand comparable activity/selectivity have been reported (e.g., Energy Environ. Sci. 17, 5060–\n5069 (2024); J. Am. Chem. Soc. 145, 422–435 (2023)), and the present study does not clearly\ndistinguish itself conceptually or mechanistically.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nIn part, see detailed comments below.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\nRevisions are needed in the data analysis, see detailed comments below.\nIs the methodology sound? Does the work meet the expected standards in your field?\nYes. Again, the use of isotope labeling, CO substitution, etc. are best practices in the field.\nIs there enough detail provided in the methods for the work to be reproduced?\nYes.\nSummary statement.\nThe manuscript reports strong catalytic performance but lacks sufficient novelty and\nmechanistic depth to meet the standards of Nature Communications. Addressing the above\npoints, particularly clarifying the Mn–vacancy interplay, rigorously validating the universality\nclaim, and strengthening stability and mechanistic evidence, would be essential to enhance the\noriginality and impact of the work.\nResponse: We would like to extend our deepest gratitude to the reviewer for your detailed and\nthorough feedback. We have taken all the concerns seriously and tried our best to address them\nby numerous additional experiments and/or substantial revisions. As you will see in the revised\nmanuscript and the detailed point-by-point responses, your concerns about the consistency of\nexperimental data with theoretical calculations should have been fully addressed. In the current\nversion, the scientificity of this work have been significantly improved compared to the\nprevious submission. Please see our point-by-point responses as follows.\nDetailed comments.\n1. The interplay between sulfur vacancies and Mn single atoms is insufficiently resolved.\nFigure S26 presents 4 h C H yields for Fe, Co, Ni, and Cu-doped catalysts subjected to the\n2 4\nmicrowave treatment. Yields are ~70-120 umol/g. What is not shown on this graph is the Mn-\nZnS not subjected to the microwave treatment: 190 umol/g. This data does not support the\nstatement: “indicating a universal enhancement in photocatalytic performance induced by low-\ncoordination metal centers.” So it is not clear whether Mn substitution, which appears to be the\noriginal hypothesis, or the S vacancies is the main effect. At the very least, a ZnSᵥ control\n(without Mn) should be synthesized, characterized, tested, and included in DFT calculations.\nFigures 4a and 4b do not convincingly isolate the effect of sulfur vacancies. Full catalytic cycle\ncalculations with corresponding energy profiles are needed for both cases, and any synergistic\neffect should be explicitly demonstrated in the mechanism.\nResponse: We thank the reviewer for the insightful suggestion. As suggested, we have\nconducted corresponding supplementary experiments and calculations to clarify the interaction\nbetween the Mn single atom and sulfur vacancies. The following are our point-by-point\nresponses:\n(1) ZnS sample was synthesized according to exactly the same microwave treatment method\nv\n(without the addition of Mn precursors) and characterized, tested and DFT calculated (Fig.\nR24). The clear signal at g=2.005 and the relative red shift of XPS indicate that sulfur\nvacancies have been successfully created in ZnS . The absorption edge of ZnS is redshifted\nv v\nrelative to pure ZnS, further confirming the successful construction of vacancies.\nPhotocatalytic CO reduction performance reveals that ZnS with sulfur vacancies (ZnS )\n2 v\nexhibits negligible enhancement in C H yield and selectivity (7.3%) compared to pristine ZnS\n2 4\n(5.6%) (Fig. R25). In stark contrast, Mn single atom-doped ZnS (Mn –ZnS) achieves a\n1\nsubstantial improvement in C H selectivity (74.5%), demonstrating that isolated sulfur\n2 4\nvacancies are inactive for C H generation. Instead, the introduction of doped metal atoms,\n2 4\nestablishing bimetallic sites, is essential for activating the C H pathway. Remarkably, Mn –\n2 4 1\nZnS synthesized via vacancy engineering from Mn –ZnS precursor delivers near-unity C H\nv 1 2 4\nselectivity (99.1%), which unambiguously identifies low-coordination Mn single atoms as the\npivotal active centers driving high C H selectivity.\n2 4\n(2) To further explain the general applicability of low-coordination transition metal single-\natom doping in ZnS materials, we supplemented the photocatalytic performance of saturated-\ncoordination M -ZnS (M = Fe, Co, Ni, Cu) and low-coordination M -ZnS in Fig. R26. All\n1 1 v\nM –ZnS variants exhibited substantially higher C H yields than their saturated-coordination\n1 v 2 4\nM –ZnS counterparts, indicating a certain enhancement in photocatalytic performance induced\n1\nby similar low-coordination metal centers. The results of general applicability test further\nemphasize that the introduction of another metal site is a necessary condition for generating\nC H , and a low-coordination environment is a key strategy driving the improvement of C H\n2 4 2 4\nselectivity.\n(3) Based on your suggestions, the free energy of the entire reaction path from CO to C H\n2 2 4\nwere carried out over ZnS, ZnS , Mn –ZnS and Mn –ZnS . As shown in the Fig. R24f, the\nv 1 1 v\nenergy barrier of the ZnS path is not significantly lower than that of ZnS and the main product\nv\nis still CO. Both Mn –ZnS and Mn –ZnS enhance the adsorption of *CO intermediates,\n1 1 v\nindicating that the Mn active sites facilitate the reduction of hydrogenation energy barriers.\nCrucially, the low-coordination Mn –ZnS exhibits the lowest energy barriers for *CO\n1 v\nprotonation (*CHO formation) and the asymmetric coupling of *CO-*CHO (*COCHO\nformation). It demonstrates that S acts as “electronic modulators”, synergizing with Mn\nv\ncenters to optimize the C-C coupling pathway, thereby enabling high C H yield and selectivity.\n2 4\nIn the revised manuscript, we have corrected and supplemented the relevant discusses to\nelaborate on the synergistic effect between the Mn active center and sulfur vacancies\n(Supplementary Figs. 35, 36 and 46, line 241-242, 245-248, 295-298).\nFig. R24 (a) Top-, (b) side- view of the slab model of ZnS (blue: Zn, yellow: S, red: Mn). (c)\nv\nEPR spectra of ZnS , Mn –ZnS and Mn –ZnS . XPS spectra of (d) Zn 2p and (e) UV-Vis DRS\nv 1 1 v\nover ZnSand ZnS . (f) Reaction pathways and C-C coupling step for CO photoreduction over\nv 2\nZnS, ZnS , Mn –ZnS and Mn –ZnS .\nv 1 1 v\n1 )400 100\n-g\nol 350 % )\nm\n300 y\n(\nμ t\n(\nd 250 ti\nvi\nel c\ns\nyi200 50\ns\nel e\nc\nt150\n4\nu H\no\nd100\nC\n2\nr\nP 50\n0 0\nZnS ZnS\nv\nMn\n1\n-ZnS Mn\n1\n-ZnS\nv\nFig. R25 The photocatalytic CO reduction performance over pristine ZnS, ZnS , Mn –ZnS\n2 v 1\nand Mn –ZnS catalysts.\n1 v\n400\ncatalyst M-ZnS M-ZnS\n1 v 1\n) CH4 CH4\n-1\nol\ng 300\nC\nCO\n2H4 C\nCO\n2H4\nm\nμ M= Mn, Fe, Co, Ni, Cu\n(\nd\nel 200\nyi\ns\nct\nu\nd\nr o 100\nP\nFig. R26 The products yield of CO reduction based photocatalytic reaction with different\n2\ntransition metal elements over M –ZnS and M –ZnS (M=Mn, Fe, Co, Ni, Cu) catalysts under\n1 1 v\n4 h irradiation.\n2. The assertion that multiple metals (Fe, Co, Ni, Cu) can be incorporated as single sites\nrequires direct evidence. At least one alternative metal (e.g., Cu) should be systematically\ncompared with Mn via in situ DRIFTS, XAS, and full DFT energy profiles, etc., with a deeper\ndiscussion of performance differences beyond the d-band center argument.\nResponse: Thank you to the reviewer for the valuable comment. As suggested, we have\nsystematically selected Cu as the representative contrast metal, and completed the following\nin-depth supplementary research.\nTo probe the local coordination environment of Cu atoms in Cu –ZnS , X-ray absorption\n1 v\nspectroscopy (XAS) was conducted. X-ray absorption near-edge structure (XANES) spectra\n(Fig. R27a) showed that the absorption edge position of Cu –ZnS lies between those of the\n1 v\nCu O and CuO reference standards, indicating that Cu exists in Cuδ+ (1 < δ < 2). The Cu K-\n2\nedge extended X-ray absorption fine structure (EXAFS) spectra in R-space (Fig. R27b) reveal\nan absence of the Cu-Cu bond peak at 2.2 Å, while displaying a prominent peak at 1.8 Å,\ncorresponding to the Cu-S bond in the first coordination shell, which strongly supports the\nformation of Cu SAs. Further EXAFS fitting analysis determined the Cu-S coordination\nnumber to be 2.0, and the Cu-S bond length to be 2.13 Å, which is longer than that of Mn-S\nbond (1.96 Å). The above characterization collectively indicates the successful synthesis of\nCu –ZnS structure.\n1 v\nTo elucidate the underlying mechanism of performance differences between the Cu –ZnS\n1 v\nand Mn –ZnS , in situ DRIFTS spectra, which are sensitive to the real-time reaction\n1 v\nintermediates, were measured. As illustrated in Fig. R28, a distinct vibrational signal was\ndetected at 2078 cm-1, corresponding to linearly adsorbed CO (*CO ). In contrast, the Cu –\nL 1\nZnS exhibited significantly weaker *CO absorption peaks under identical conditions,\nv L\nsuggesting that Mn –ZnS featured a superior ability to adsorb *CO intermediates for further\n1 v\nC-C coupling. This strengthened adsorption was attributed to the stronger d→2π backbonding\nbetween the d-orbital of low-coordination Mn atoms in the Mn –ZnS and the 2π* antibonding\n1 v\norbital of *CO, which is more pronounced than for low-coordination Cu atoms in the Cu –\nv 1 v\nand coupling into *COCHO, as manifested by the peak at 1315 cm-1, which served as a critical\nintermediate in the pathway to C H formation. The performance differences, supported by\n2 4\ntheoretical calculations (Fig. R29), could be attributed to the higher ΔG associated with the\n*CO-to-*CHO conversion for Cu –ZnS (0.82 eV)than for Mn –ZnS (0.62 eV), indicating\n1 v 1 v\ncomparatively less favorable energetics of Cu –ZnS . In addition, the free energy for *CO-\n1 v\n*CHO coupling on Mn –ZnS is significantly lower than that of on Cu –ZnS , which suggests\n1 v 1 v\nthat the stronger free energy driving force on Mn –ZnS likely contributes to its enhanced\n1 v\ncatalytic activity.\nIn the revised manuscript, we have included these results in Supplementary Figs. 45, 49, 50\nand added the related discussions (line 293-298 and supplementary information).\nFig. R27 (a) Normalized Cu K-edge XANES spectra and (b) the corresponding Fourier spectra\nin R-space of Cu –ZnS , Cu O, CuO and Cu foil. (c) FT-EXAFS fitting curves of Cu –ZnS .\n1 v 2 1 v\nFig. R28 In situ DRIFTS for CO photoreduction reaction over Cu –ZnS (a) and Mn –ZnS\n2 1 v 1 v\n(b) catalysts.\n1.2\n0.8 Cu 1 -ZnS v\nV) 0.4 Mn 1 -ZnS v\ny ( e 0.0 *COOH *CHO *CHOCHOH *CHOHCH\ng\nn\ner-0.4 CO\n2 *CO *COCHOH\ne\ne-0.8\n*COCHO\n*CHOCH\nFr\ne-1.2\n*CHCH\n2\n-1.6\n*CHCH\n2 2\nReaction coordinate\nFig. R29 Free energy diagrams of the CO reduction pathway to C H on Cu –ZnS and Mn –\n2 2 4 1 v 1\nZnS , respectively.\nv\n3. Figure 2. In the free energy diagram, conversion of CO (g) to *CO is shown to have a\n2\nnegative free energy (in principle, spontaneous). What conditions were assumed to make this\nreaction downhill. I suspect that that the authors are using the computational hydrogen\nelectrode that the reaction is proceeding via proton-coupled electron transfers. If this is the case,\nwhat potential was assumed and why was it though that this could be realized in photocatalytic\nexperiments?\nResponse: We appreciate the reviewer’s suggestion to clarify the CO (g)-to-*CO reaction\n2\nprocesses. Photocatalytic CO reduction reaction (CO RR) is a complex proton-coupled\n2 2\nelectron transfers (PCET) process, and its photocatalytic efficiency is influenced by various\nfactors such as thermodynamics and kinetics. The computational hydrogen electrode (CHE)\nmodel was employed to obtain the free energy of the elementary step in the theoretical research\non CO RR (J. Phys. Chem. B, 2004, 108, 17886–17892.). The whole elementary steps of\n2\nphotocatalytic CO -to-*CO conversion could be described as following:\n2\n* + CO + 2H+ + 2e- → *CO + 2H+ + 2e- (6)\n2 2\n*CO + 2H+ + 2e- → *COOH + H+ + e- (7)\n2\n*COOH + H+ + e- → *CO + H O (8)\n1 v\nunder light irradiation is a prerequisite for the occurrence of photocatalytic reactions. The\npotential at the conduction band minimum of the Mn –ZnS catalyst is sufficiently negative\n1 v\nthan the equilibrium potential of the reaction to drive the CO reduction reaction. In the actual\n2\nreaction, although the electrons come from the photogenerated electrons in the excited state of\nthe Mn –ZnS catalyst and the H+ originates from the cleavage of water molecules in\n1 v\nsolution, the energy of the proton–electron pair is equal to half the energy of the hydrogen\nmolecule according to the computational hydrogen electrode method (J. Electrochem.\nSoc. 2005, 152, J23). Thus, the Gibbs free energy of the reaction can be obtained by the\nfollowing equation:\n∆G = ∆E + ∆E −T∆S−𝑒𝑈 (9)\nZPE\nin which ∆E represent the change of the zero-point energy (ZPE), T∆S is the entropic\nZPE\ncontribution (T was set to be 300 K), which were obtained from the vibrational frequency\ncalculations through the VASPKIT code. e is the elementary charge and U is the conduction\nband potential of semiconductor photocatalysts. Specifically, the calculation formulas are as\nfollows:\nΔG[*CO ] = G(*CO ) – G(*) – G(CO ) (10)\n2 2 2\nΔG[*COOH] = G(*COOH) – G(*) – G(CO ) – 0.5 × G(H ) (11)\n2 2\nΔG[*CO] = G(*CO) + G(H O) – G(*) – G(CO ) – G(H ) (12)\n2 2 2\nBased on the above discussion, photocatalytic CO -to-*CO conversion on the Mn –ZnS\n2 1 v\nsurface with an appropriate *CO adsorption energy is thermodynamically favorable. The\ncorresponding supplementary discussion have been added in the revised manuscript (section\non Density functional theory (DFT) calculations).\n4. More generally, the influence of light on the catalytic process, particularly from a\nmechanistic perspective, is not discussed in sufficient detail. DFT calculations, especially those\nrelated to Supplementary Fig. 3, should incorporate the effect of photoexcitation to better\nreflect the actual reaction conditions.\nResponse: We sincerely thank the reviewer for the constructive comments to further elucidate\nthe photocatalytic mechanism.\nFirstly, to explore the light absorption capacity, the band structure of the prepared Mn –ZnS\n1 v\nphotocatalyst, as determined by UV-Vis DRS, VB-XPS, UPS and Mott-Schottky plots\n(Supplementary Figs. 20-25), possesses a suitable conduction band potential (-1.23 eV) for\nCO -to-C H and valence band potential (1.65 eV) for H O-to-O , enabling the generation of\n2 2 4 2 2\nelectron-hole pairs. This is the essential first step that provides the thermodynamic driving\nforce (-eU in our DFT model) for the CO reduction reaction.\ntransient photocurrent measurements (Supplementary Fig. 28) revealed that low-coordination\nMn single atom site exhibited significantly enhanced charge separation and migration\nefficiency compared to saturated-coordination Mn single atom site. What’s more, TRPL decay\ncurves and SPV (Supplementary Fig. 27) were carried out to confirm the long lifetime of\ncarriers and a high concentration of photogenerated electrons accumulated on the Mn –ZnS\n1 v\ncatalyst surface.\nTo gain insight into the electron transfer mechanism under light illumination, the Quasi-in\nsitu XPS measurement of Mn –ZnS was studied (Fig. R30). Under illumination, Quasi-in\n1 v\nsitu XPS analysis reveals a distinct positive shift for S 2p accompanied by a negative binding\nenergy shift for Mn 2p, providing direct spectroscopic evidence that photogenerated electrons\nare transferred from S (electron donor) to Mn (electron trap). Subsequently, photoelectrons are\nrapidly injected into the adsorbed CO molecules (Fig. R31) through the electron transport\n2\nchannel. Activated CO undergoes a series of proton-coupled electron transfer (PCET)\n2\nprocesses to obtain *CO intermediate (Fig. 2b). The localized electron-rich environment of\nlow-coordinated Mn single-atom site enhances the adsorption of *CO and drives the multi-\nelectron pathway toward selective C H formation (Supplementary Fig. 7).\n2 4\nFinally, in the photocatalytic process, the generation of electron-hole pairs by Mn –ZnS\n1 v\ncatalyst under light irradiation is a prerequisite for the occurrence of photocatalytic reactions.\nThe potential at the conduction band minimum of the Mn –ZnS catalyst is sufficiently\n1 v\nnegative than the equilibrium potential of the reaction to drive the CO reduction reaction.\nconditions, -eU represents the thermodynamic driving force for Mn –ZnS to utilize\n1 v\nphotogenerated electrons (Nat Catal, 2025, 8, 728–739), and the energy of the proton–electron\npair is equal to half the energy of the hydrogen molecule according to the computational\nhydrogen electrode method (J. Electrochem. Soc. 2005, 152, J23). Thus, the Gibbs free energy\nof the reaction can be obtained by the following equation:\n∆G = ∆E + ∆E −T∆S−𝑒𝑈 (13)\nZPE\nin which ∆E represent the change of the zero-point energy (ZPE), T∆S is the entropic\nZPE\ncontribution (T was set to be 300 K), which were obtained from the vibrational frequency\ncalculations through the VASPKIT code. e is the elementary charge and U is the conduction\nband potential of semiconductor photocatalysts.\nIn response to the reviewer’s suggestion, we have added the related explanations in the\nrevised manuscript (Supplementary Fig. 26, line 198-208, section on Density functional\ntheory (DFT) calculations).\nFig. R30 Quasi-in situ XPS measurement of (a) Mn 2p and (b) S 2p over Mn –ZnS in\n3/2 1 v\ndark and in light.\nCO -TPD\n2\n397\n)\n. u\n. Mn -ZnS\na 1 v\n(\ny\nt 308\ni\ns\nn Mn -ZnS\ne 1\nt\nn\nI\nZnS 219\n150 200 250 300 350 400\nTemperature (oC)\nFig. R31 CO -TPD profiles of ZnS, Mn –ZnS and Mn –ZnS .\n2 1 1 v\n5. Supplementary Fig. 25 is ambiguous. Given the claimed 200-hour stability (Fig. 5e), post-\nreaction characterizations, especially XAS, should be shown for the spent catalyst. More\nfrequent GC sampling (e.g., every 5–10 min) is recommended to capture potential fluctuations.\nResponse: We thank the reviewer for the valuable suggestion. The validation of catalyst\nstability and experimental details are essential to enhance the rigor and persuasive power of\nthis work. The following is a point-by-point response to your comment.\n(1) The structural integrity of Mn –ZnS over several cycles was evaluated and the results\n1 v\nwere shown in Supplementary Fig. 43. SEM images (Supplementary Fig. 43a) and XRD\n(Supplementary Fig. 43b) show that the crystallite morphology and crystal structure remain\nintact after extended operation, respectively. The consistent XPS spectra of fresh\n(Supplementary Fig. 434c-f) and reused (Supplementary Fig. 43g, h) samples further support\nthe chemical stability.\n(2) To confirm that local coordination environment of Mn atoms in spent sample, XAS was\nconducted. There are no characterized signals of Mn-Mn bond in Mn –ZnS after reactions and\n1 v\nthe coordination number of Mn atoms remains at 1.9±0.3 after long-term reaction (Fig. R32).\nThe stability of the coordination environment ensures the efficient photocatalytic performance\nof the Mn –ZnS catalyst.\n1 v\n(3) As you suggest, we have conducted more frequent GC sampling (every 15 min) to\nevaluate the photocatalytic activity and selectivity of the Mn –ZnS catalyst. As shown in Fig.\n1 v\nR33, the photocatalytic stability of Mn –ZnS was evaluated over 50 consecutive cycles (200\n1 v\nh), during which no significant decline in activity or selectivity was observed.\nFollowing your suggestion, the corresponding revision and discussion have been added in\nthe revised manuscript (Supplementary Figs. 42-44, line 274-279).\nFig. R32 (a) Normalized Mn K-edge XANES spectra and (b) Fourier-transform (FT) spectra\nof EXAFS oscillations of Mn –ZnS , the spent Mn –ZnS and the related reference samples.\n1 v 1 v\n360\n100\n)\n330 %\n-\n1\ng\n)\ny\n(\nol 300 vi\nt\nm ti\nc\nμ 270 e\nd\n( el\nS\nYi\nel\n210 90\nMn -ZnS\n1 v\n180\n0 8 16 24 32 40 184 192 200\nTime (h)\nFig. R33 The cycling tests for CO\n2\nphotoreduction to C\n4\nover Mn 1–ZnS\nv\nphotocatalyst under\nthe same as reaction conditions.\n6. 13CO labeling, combined with in situ DRIFTS, would provide stronger evidence for the\nproposed asymmetric *CO–*CHO coupling pathway.\nResponse: Thank you for the insightful comments. As suggested, we have conducted the in\nsitu DRIFTS tests with labeled 13CO as adsorbate to further verify the asymmetric *CO–*CHO\ncoupling over Mn –ZnS catalyst during photocatalytic reduction reaction. In situ DRIFTS of\n1 v\nMn –ZnS exhibit characteristic isotope effect peaks involving *CO and *CHO species (Fig.\n1 v\nR34). Furthermore, the increasing tendency of the characteristic isotope effect peaks related\nto *COCHO intermediates, *13CO13CHO, was observed over Mn –ZnS with the light\n1 v\nirradiation during the photocatalytic process, which strongly supports the asymmetric coupling\nof *CO and *CHO. Combined with the results of GC-MS of photocatalytic 13CO reduction\n2\nwith D O over Mn –ZnS catalyst, the formation of 13C D further provides strong evidence\n2 1 v 2 4\nfor the asymmetric coupling of *CO–*CHO.\nIn response to the reviewer’s suggestion, we have added the related explanations in the\nrevised manuscript (Supplementary Fig. 31, line 221-225).\n13CO\n)\nu . 2078 1317\na . *13CO 1684 *13CO13CHO\n( *13C H\ne 2 4\nc\nn\na 30 min\nb\nr\no\ns\nb\nA\n1364, 1462 1168\nLight on\n*13CH\n2\n*13CHO\n2200 2000 1800 1600 1400 1200\nWavenumber cm-1\n( )\nFig. R34 In situ DRIFTS of 13CO chemisorption over Mn –ZnS catalyst during photocatalytic\n1 v\nreduction reaction.\n7. Figure S24. This is said to be a GC-MS spectrum. But multiple analytes are present. Were\nchromatograms summed to produce this data?\nResponse: Thank you to the reviewers for raising the important question to give us the\nopportunity to clarify our data analysis methods more clearly. Supplementary Fig. S24 shows\nthe conversion of profile spectrum into centroid spectrum in the gas chromatography-mass\nspectrometry (GC-MS) coupled analysis. The GC-MS spectra reflect the relationship between\nthe m/z of all ion fragments and their relative abundance rather than later artificial summary\nof chromatograms, and is used to visually infer the structure of the target products of\nphotocatalytic 13CO reduction with D O over Mn –ZnS catalyst. The presence of multiple\n2 2 1 v\nanalytes in the mass spectrum corresponds to characteristic ion fragments with different m/z,\nand the product was identified as C H by comparison with the NIST standard mass\n2 4\nspectrometric library. We have supplemented relevant content in the method section of the\nrevised manuscript to eliminate misunderstandings (line 264-265, section on\nCharacterization).\n8. The following is said about Figure 3b. “Elemental mapping via energy-dispersive X-ray\nspectroscopy (EDS, Fig. 3b) showed homogeneous distribution of Zn, Mn, and S throughout\nthe sample, with Mn atoms clearly penetrating into the ZnS matrix.” On what basis can it be\nsaid that the Mn distribution is homogenous? What would an inhomogeneous distribution look\nlike? Also “Mn atoms clearly penetrating into the ZnS matrix.” It is not possible to see if Mn\natoms are inside the ZnS with TEM as it produces a 2D projection of a 3D object.\nResponse: We thank the reviewer for the critical observations regarding the interpretation of\nour EDS results, which help us present our findings with greater scientific rigor.\nWe fully agree that a qualitative assessment from a single EDS elemental mapping is\ninsufficient to definitively claim “homogeneous distribution”. In response to your concern,\nline-scan profiles were carried out to offer further insights into the spatial distribution of Zn,\nMn, and S throughout the sample (Fig. R35). EDS elemental mapping combined with line-\nscan profiles provides strong evidence for the uniform distribution of elements (Nat.\nMater. 2024, 23, 1355–1362; ACS Catal. 2024, 14, 17, 13400–13407). Meanwhile, we have\nmodified from \"homogeneous distribution\" to a more accurate description: “Elemental\nmapping via energy-dispersive X-ray spectroscopy (EDS, Fig. 3b) and the STEM-EDS line\nscanning profiles (Supplementary Fig. 12) corroborated the even distribution of Zn, Mn, and S\nthroughout the sample, with no observable aggregation of Mn into large clusters or particles.”\nThe conventional EDS provides a 2D projection of a 3D object and cannot directly resolve\nthe internal location of dopant atoms. To confirm the integration of Mn and ZnS lattices, the\nresults of XRD, XPS and EXAFS were further discussed. The absence of any diffraction peaks\ncorresponding to Mn oxide or sulfide phases, coupled with the slight red shift in ZnS peak\npositions (Fig. R36a), indicates lattice incorporation. Then, the binding energy of Mn 2p peaks\nconfirm that Mn exists in the oxidation state of +2 (Fig. R36b), which is consistent with the\nexpectation of replacing the Zn2+ site. In addition, EXAFS fitting revealed that the first-shell\nMn–S coordination number was 3.7 for Mn –ZnS (Fig. R36c), which is one of the direct pieces\n1\nof evidence proving that Mn atoms have replaced Zn atoms and exists within the ZnS lattice.\nIn response to the reviewer’s suggestion, we have corrected the original manuscript and\nadded the related explanations in the revised manuscript (Supplementary Fig. 12, line 148-151).\nFig. R35 STEM image of Mn –ZnS (a) and Mn –ZnS (b), and the corresponding STEM-EDS\n1 1 v\nelemental line scan (c, d), respectively.\nFig. R36 (a) XRD pattern of pristine ZnS, Mn –ZnS and Mn –ZnS . (b) XPS spectra of Mn 2p\n1 1 v\nover Mn\n1\n–ZnS and Mn\n1\n–ZnS\nv\n. (c) EXAFS spectrum fitting result of Mn\n1\n–ZnS in R space.\n9. Figure 3c caption. What do the yellow circles mean and what quantitative criterion was used\nto draw them?\nResponse: We thank the reviewer for the comment. Aberration-corrected annular dark-field\nscanning transmission electron microscope (ADF-STEM) image was employed to further\nconfirm the atomic dispersion of Mn in Mn –ZnS catalyst. As shown in Fig. R37, several\n1 v\nisolated dark spots (marked by yellow cycles) can be captured by utilizing the Z-contrast\ndifference between the lighter Mn and the heavier Zn atoms, which indicate successful atomic\nsubstitution of Zn by Mn atoms. A line-scan intensity profile across a representative region\n(Profile 1–2, Fig. R37) reveals localized intensity dips, confirming atomic-level dispersion of\nMn atoms (Angew. Chem. Int. Ed. 2025, e202516903).\nWe have included the above results in Fig. 3c and added the related discussions in the line\n150-154 in the revised manuscript.\nFig. R37 HAADF-STEM image of Mn –ZnS with Mn single atoms marked by yellow circles,\n1 v\nand atomic intensity corresponding to the marked area of the line profile 1 and profile 2.\n10. Page 6. “with directional sulfur vacancies (S ).” What is a directional sulfur vacancy? ZnS\nv\nhas the zinc-blende structure, Figure 6. There is only one type of S in this structure and just\none type of S vacancy. Also, how can a vacancy have a direction (vectorial?).\nResponse: Thank you to the reviewer for raising the important issue, which provides us with\nan opportunity to clarify a key innovation point in our work. From a crystallographic\nperspective, there is indeed only one type of S vacancy. The expression \"directional\" in our\noriginal manuscript might not be precise enough to cause a misunderstanding. We have\ncorrected \"directional sulfur vacancies\" to \"targeted sulfur vacancies\". Specifically, S\nvacancies are preferentially created in specific chemical environments (around Mn single-atom\nsites) via innovative selective-etching strategy.\nThe innovative selective etching strategy refers to the selective breaking of chemical bonds.\nMicrowave thermally activated hydrogen peroxide (H O ) generates hydroxyl radicals (·OH)\n2 2\nradicals that preferentially attack and break the Mn-S bonds, with weaker bond energy, to\ncreate targeted vacancies at single-atom coordination sites (J. Am. Chem. Soc. 2025, 147, 3,\n2689–2698). Therefore, the regulation of the spatial distribution of vacancies is achieved\nthrough thermodynamically and kinetically controlled synthesis, enriching vacancies around\nthe designed active sites, thereby constructing the expected low-coordination Mn active sites.\nWe have made corresponding revisions to the manuscript (Supplementary Fig. 9, line 138-\n141).\n11. Page 7. “…revealed a S concentration of 1.54% for Mn –ZnS , closely matching the\nv 1 v\ntheoretical value predicted by DFT.” How can DFT predict the vacancy concentration produced\nby the microwave treatment?\nResponse: Thank the reviewer for highlighting this valuable issue, allowing us to clearly\nclarify the innovative design concept of combining theoretical guidance and experimental\nresults in our work. We fully agree that traditional DFT calculations indeed cannot directly\npredict the precise defect concentration produced by microwave treatment. The word \"predict\"\nin the manuscript may be a misleading expression. We sincerely apologize for this and have\nmade corrections in the revised manuscript.\nIn fact, our research approach embodies a strategy of \" theory-guided directional design and\nexperimental verification \", which is specifically described as follows:\n(1) The role of theoretical calculation is to screen the optimal configuration rather than\npredict the synthesis concentration. We systematically constructed Mn single-atom doped\nZnS models with different coordination structures and evaluated their C-C coupling capabilities\nby key descriptors (such as *CO adsorption energy). The DFT results revealed that a specific\nlow-coordination (N=2) Mn active site, with a sulfur vacancy (S ) concentration of 1.33%,\nv\ntheoretically exhibits the optimal adsorption strength for *CO intermediates, which is the most\nfavorable for C-C coupling. The concentration value of 1.33% is a \"target optimal value\",\nwhich stems from the theoretical exploration of the structure-activity relationship.\n(2) The goal of experimental synthesis is directed toward the optimal configuration.\nBased on the above theoretical screening results, we developed and optimized the microwave-\ninduced H O etching, a targeted defect engineering strategy, aiming to prepare the Mn –ZnS\n2 2 1 v\ncatalyst with near-ideal vacancy concentrations.\n(3) The validation of experimental results demonstrated the exceptional alignment\nbetween theory, structure, and performance. The experimental S concentration in the Mn –\nv 1\nZnS catalyst prepared was determined to be 1.54% by multiple characterization methods (ICP,\nv\nEPR, EDS Mapping), which was highly consistent with the theoretical target value (1.33%).\nMoreover, the Mn –ZnS catalyst demonstrates outstanding C H yield and selectivity.\n1 v 2 4\nTherefore, the optimal structure model was screened out through theoretical calculation, and\nthen the catalyst with key parameters highly consistent with the ideal model was successfully\nprepared via precise synthesis experiments. The closed-loop from theoretical design to\nexperimental implementation is precisely one of the core innovative points of our work, which\nsignificantly surpasses the traditional catalyst development model.\nWe have corrected the relevant expressions and added related discussions in the revised\nmanuscript (line 138-141, 159-162, section on Density functional theory (DFT) calculations).\n12. Page 9. “In contrast, Mn –ZnS exhibited a stronger *CO signal at 2078 cm-1 (Fig. 4b),\n1 v\nindicating enhanced generation and stabilization of *CO.” Authors cannot conclude that CO\n2\n-> CO is faster on the Mn-doped material. The steady-state concentration of *CO depends both\non the generation and desorption rate.\nResponse: We sincerely thank the reviewer for this insightful comment, which correctly\nhighlights the distinction between the steady-state concentration of an intermediate and its rate\nof formation. We agree that a stronger *CO signal in in-situ DRIFTS could indeed result from\neither a faster formation rate or a slower desorption rate. The corresponding expression is\nmodified as below: “In contrast, Mn –ZnS exhibited a stronger *CO signal at 2078 cm-1 (Fig.\n1 v\n4b), indicating enhanced stabilization and coverage of *CO.”\nTo further provide strong evidence for the enhanced stabilization of *CO, performance\nexperiments and theoretical calculations were carried out. DFT calculations (Fig. R38a)\ndemonstrate that the *CO adsorption energy is stronger on the Mn –ZnS compared to other\n1 v\nstructures, which is consistent with the high steady-state of *CO. In addition, Compared with\nMn –ZnS, the significantly increased C H yield and decreased CO yield on Mn –ZnS (Fig.\n1 2 4 1 v\nR38b) indicate that more *CO intermediates are involved in the C-C coupling pathway, which\nfurther explains the enhanced stabilization of *CO observed in in-situ DRIFTS experiment. In\nresponse to the reviewer’s suggestion, we have revised the related explanations in line 221-225\nin revised manuscript.\nFig. R38 (a) Adsorption energies of *CO on Mn\n1\n–ZnS and Mn\n1\n–ZnS\nv\n. (b) Product yields of\nproducts (HCOOH, CO, C H and CH ) and the selectivity of C H product over ZnS, Mn –\n2 4 4 2 4 1\nZnS and Mn –ZnS catalysts.\n1 v\nReviewer #5 (Remarks to the Author):\nThe paper reports on the Mn –ZnS photocatalyst for CO reduction to C H . The study\n1 v 2 2 6\nemploys theoretical guidance in experimental design, and the overall structure is complete,\nwith clear logic and theoretical foundations. However, the following points require further\ndiscussion:\nResponse: We sincerely appreciate the reviewer’s detailed summary of our work, and are\npleased that the novelty and strengths of our designed Mn –ZnS photocatalyst and its near-\n1 v\nunity selectivity for CO -to-C H photoconversion were well recognized. We have carefully\n2 2 4\naddressed each of the reviewer's concerns below.\n1. On page 5, the statement “The low-coordination Mn site in Mn –ZnS strengthens the Mn–\n1 v\nCO π backbonding” – what does this mean? Please provide a detailed explanation. What is the\nrelationship between this and the differential charge density analysis?\nResponse: Thanks to the reviewer for highlighting this important issue and for your valuable\nsuggestion. We are sorry for our unclear or inadequate descriptions. Typically, π backbonding\nis a pivotal concept in metal–ligand bonding (Fig. R39). In the Mn–*CO interaction, it involves\ntwo synergistic electron processes: σ-donation and π-backbonding (π-backdonation). (1) The\nCO molecule donates its lone pair electrons from the carbon atom to fill an empty d-orbital of\nthe Mn atom, forming a σ-bond. (2) The Mn atom, in turn, backdonates electrons from its filled\nd-orbitals into the π-antibonding orbital of CO (ACS Catal. 2024, 14, 9, 7011–7019). In our\nmanuscript, low-coordination Mn sites, characterized by higher electron density and an\nelectron-rich state, significantly enhance the strength of the π-backbonding process.\nThe enhanced π backbonding was directly verified through Bader charge and charge density\ndifference analyses (Fig. R40). Compared to Mn –ZnS, more electron transfer of Mn –ZnS\n1 1 v\n55\nclearly demonstrates that low-coordination Mn sites strengthen π-backbonding. Furthermore,\nthe intensified π backbonding significantly enhances the adsorption strength of the *CO\nintermediate on the low-coordination Mn sites, while simultaneously activating the adsorbed\n*CO species to a higher degree for subsequent C–C coupling.\nWe have supplemented the corresponding descriptions and the changes are highlighted in\nred in the revised manuscript (line 97-100, 119-122).\nFig. R39 Illustration of the interaction between the 5σ and 2π* of *CO and metal surfaces.\nFig. R40 Differential charge density and Bader charge of *CO adsorbed on Mn –ZnS (a) and\n1 v\nMn –ZnS (b) (the isosurface is 0.005 e/Å3). Blue and purple represent charge gaining and\n1\nlosing, respectively.\n56\n2. To ensure greater rigor, in addition to calculating the adsorption of *CO on the Mn atom in\nboth Mn –ZnS and Mn –ZnS structures, the authors should also consider the adsorption on\n1 1 v\nZn atoms surrounding the Mn site. Although *CO does not readily adsorb on Zn atoms in pure\nZnS, the electronic structure modulation induced by Mn may alter the adsorption behavior on\nneighboring Zn atoms.\nResponse: Thanks to the reviewer for this important and meaningful suggestion. We agree that\nin single-atom catalyst systems, the electronic modulation effect of the central metal atom on\nits surrounding coordination environment is crucial and may significantly alter the adsorption\nbehavior of adjacent sites. The adsorption energies of the *CO intermediate at different\nadsorption sites on the Mn –ZnS model have been systematically compared. Fig. R41 shows\n1 v\nthat the adsorption energy of *CO at the Mn site is much lower than that at the surrounding Zn\nsites, indicating that the modulation of Mn is insufficient to cause strong adsorption of *CO at\nthe Zn sites. It was further verified that the low-coordination Mn site stabilizes the *CO\nintermediate through an enhanced Mn-*CO π-backbonding, creating the necessary conditions\nfor the subsequent C-C coupling. In the revised manuscript, we have included the above results\nin Supplementary Fig. 4 and added the related discussions in line 97-102 in the revised\nmanuscript.\n-0.4\n-0.63\n-0.35 -0.71\n-0.48\n-0.8\nV ) -1.36\ne -1.2 -1.02\n(\nO\nC -1.6 = 5.084 Å\n*\ns = 7.107 Å\nd\na\nE -2.0 6 5 = 4.401 Å\n1 4\n3 = 3.330 Å\n-2.4 2\n= 3.816 Å\nZn Zn Zn Zn Zn Mn\n1 2 3 4 5 6\n1 v\nof the structure of different sites and the distances between different Zn sites and Mn sites. The\nMn, Zn, and S atoms are shown in pink, blue, and yellow colors, respectively.\n3. On page 6, why can microwave radiation directionally induce defects? What advantages\ndoes microwave irradiation offer over other synthesis methods specifically for ZnS materials?\nHow does H O etch S? Is it possible that O atoms are introduced into ZnS during this process?\n2 2\nResponse: Thank you to the reviewer for raising these key questions related to the synthetic\nmechanism. We have supplemented these key points in detail, and the specific explanations\nare as follows:\n(1) Innovative selective-etching strategy were adopted to synthesize Mn –ZnS catalyst via\n1 v\na microwave irradiation–H O etching treatment under an Ar atmosphere. Microwave\n2 2\nthermally activated hydrogen peroxide (H O ) generates hydroxyl radicals (·OH) radicals that\n2 2\nselectively cleave metal-S bonds, with weaker bond energy, to create targeted vacancies at\nsingle-atom coordination sites (J. Am. Chem. Soc. 2025, 147, 3, 2689–2698). The microwave\nenergy is uniformly and transiently injected into the bulk phase under the Ar atmosphere,\nresulting in a uniform defect distribution during the H O etching process.\n2 2\n(2) In contrast to conventional external heating methods, microwave energy directly interacts\nwith the molecular dipoles of ethylene glycol to achieve rapid, bulk phase heating of the\nmaterial. Specifically, i) High Efficiency and Energy Saving: The reaction rate is significantly\naccelerated compared to conventional hydrothermal/solvothermal methods, substantially\nreducing synthesis time; ii) Uniform Size Distribution: The bulk phase heating characteristic\nfacilitates the formation of more homogeneous size distributions, avoiding the issue of particle\nsize heterogeneity caused by thermal gradients in traditional approaches. iii) Morphology and\nthereby exposing larger active surface areas.\n(3) The essence of H O etching is an oxidation process, that is, ZnS + H O → Zn2+ + S2- +\n2 2 2 2\n2OH-. Subsequently, S2- can be further oxidized to soluble species (such as SO 2-, SO 2-),\n3 4\nthereby removing S atoms from the lattice to obtain sulfur vacancies (S ). Theoretically, it is\nv\npossible for a small amount of O²⁻ or OH⁻ in solution to be incorporated into the lattice during\nthe etching process.\nTo address this concern regarding the possible introduction of O atoms into the ZnS lattice\nduring the H O etching process, we have performed the following additional characterizations\n2 2\nand provided a detailed discussion in the revised Supplementary Information: i) XPS analysis:\nAs shown in Fig. R42, the O 1s XPS spectra were deconvoluted into two primary peaks at\n531.5 and 532.4 eV, corresponding to the surface adsorbed oxygen (O ) and oxygen in\nads\nadsorbed water (O ) (Proc. Natl. Acad. Sci. 2024, 121 (11) e2319427121), respectively.\nw\nCrucially, the lattice oxygen in metal oxide at 530.0 eV wasn’t observed, indicating that the\ndoping of O atoms can be disregarded. ii) XRD analysis: We re-examined the XRD patterns of\nMn –ZnS before and after H O treatment (Fig. R43). No new diffraction peaks corresponding\n1 2 2\nto any zinc oxide (ZnO) or zinc oxysulfide (ZnOₓSᵧ) phases were detected. Based on the results,\nH O acts as an oxidant and etching agent for generating sulfur vacancies, rather than a source\n2 2\nfor O-doping.\nIn response to the reviewer’s suggestion, we have added a corresponding description in\nSupplementary Fig. 9 of the supplementary information.\nO\nads\nO\nu. ) W Mn\n1\n-ZnS\nv\na.\n(\ny\nt\nsi\nn\ne\nn t O\nI ads\nO\nW Mn -ZnS\n1\n536 534 532 530 528 526\nBinding energy (eV)\nFig. R42 XPS spectra of O 1s of Mn –ZnS and Mn –ZnS catalysts.\n1 1 v\n(111)\n(220)\n(311)\n. ) Mn -ZnS\nu 1 v\n.\na\n(\ny\nt\ni\ns Mn -ZnS\nn 1\ne\nt\nn\nI\nZnS\nZnS(PDF#05-0566)\n20 30 40 50 60 70 80\n2θ (°)\nFig. R43 XRD pattern of pristine ZnS, Mn –ZnS and Mn –ZnS .\n1 1 v\n4. In Figure 3e, how can it be confirmed that the bond is Mn–S rather than Mn–O?\nResponse: Thank you for your critical comment to confirm the precise chemical structure of\nthe Mn active center. We supplemented the Fourier transform spectra of EXAFS of standard\nMnO and Mn O , and compared them with Mn –ZnS and Mn –ZnS catalysts. As shown in\n2 3 1 1 v\nFig. R44, MnO and Mn O both exhibit the characteristic peak of the Mn-O bond at 1.52 Å.\n2 3\nIn contrast, a prominent peak at 1.96 Å was assigned to Mn–S bonding in Mn –ZnS and Mn –\n1 1\nv\nevidence for distinguishing Mn-O bonds from Mn-S bonds. The absence of Mn–Mn and Mn-\nO coordination in both Mn –ZnS and Mn –ZnS indicates that Mn atoms are atomically\n1 1 v\ndispersed and coordinated with neighboring S atoms.\nIn the revised manuscript, we have included these results in Fig. 3e and added related\ndiscussion in the line 173-174 of the revised manuscript.\nMn-O\nMnO\nMn-O\nMn O\n2 3\nu. ) Mn-Mn\na.\nMn foil\n(\ny\nsi t Mn-S\nn\ne Mn -ZnS\nt 1\nn\nI\nMn-S\nMn -ZnS\n1 v\n0 1 2 3 4 5 6\nRadial distance (Å)\nFig. R44 Fourier-transform (FT) spectra of EXAFS oscillations for Mn foil, MnO, Mn O ,\n2 3\nMn –ZnS and Mn –ZnS .\n1 1 v\n5. In Figure 3c, given that the atomic masses of Mn and Zn are very similar and the Mn –ZnS\n1 v\nis bulk rather than ultrathin sheet-like, it is generally challenging to distinguish them via ADF-\nSTEM. Can the authors provide a reasonable explanation for this observation?\nResponse: Thank the reviewer very much for checking our manuscript and making\nprofessional comments!\nSTEM) image was employed to further confirm the atomic dispersion of Mn in Mn –ZnS\n1 v\ncatalyst. As shown in Fig. R45, several isolated dark spots (marked by yellow cycles) can be\ncaptured by utilizing the Z-contrast difference between the lighter Mn and the heavier Zn atoms,\nwhich indicate successful atomic substitution of Zn by Mn atoms. A line-scan intensity profile\nacross a representative region (Profile 1–2, Fig. R45) reveals localized intensity dips,\nconfirming atomic-level dispersion of Mn atoms (Angew. Chem. Int. Ed. 2025, e202516903).\nIn addition, EDS elemental mapping images and STEM-EDS line scanning profiles show a\nhomogeneous dispersion of Zn, Mn, and S atoms throughout the Mn –ZnS sample (Fig. R46).\n1 v\nMore importantly, EXAFS fitting revealed that the existence of first-shell Mn–S path and\nthe absence of Mn-Mn path and Mn-O path in Mn –ZnS catalyst (Fig. R47), which provided\n1 v\nstrong evidence for the ADF-STEM observation.\nAccording to the reviewer’s comments, the detailed discusses have been added in the revised\nmanuscript. (Figs. 3c and 3e, Supplementary Fig. 12, line 148-154).\n1 v\nand atomic intensity corresponding to the marked area of the line profile 1 and profile 2.\nFig. R46 (a) EDS elemental mapping of Mn –ZnS . (b) STEM image of Mn –ZnS and (c) the\n1 v 1 v\ncorresponding STEM-EDS elemental line scan.\n1\n–ZnS and Mn\n1\n–ZnS\nv\n.\n6. Please include the energy band structures related to other possible CO reduction products\n2\nto further support the claim that CO is more readily reduced to C H .\n2 2 4\nResponse: We thank the reviewer for this suggestion regarding the energy band structures to\ndeepen the understanding of C H product selectivity. We have included the energy band\n2 4\nstructures related to possible CO reduction products in Table R3. The energy band position\n2\nof the photocatalyst is crucial for determining the thermodynamic feasibility of the CO RR. In\n2\nthis system, the relatively negative conduction band position of Mn –ZnS (-1.23 eV) indicates\n1 v\nits strong reducing ability, which is sufficient to trigger the reduction products such as C H\n2 4\ncurrently studied by the researchers. However, the separate energy band structure is not enough\nto explain the dynamics of specific C H selectivity. To provide a more rigorous and\n2 4\nmechanistic explanation for the high C H selectivity, we have performed and included\n2 4\nadditional theoretical calculations of the Gibbs free energy for the possible products from\nadsorbed *CO, including HCHO, CH OH, CH , and C H . The calculations reveal that the C–\n3 4 2 4\nC coupling step (*CO + *CHO → *CHOCO) on the low-coordination Mn site in the Mn –\nv\n64\nreaction pathways (Fig. R48). In conclusion, the energy band position determines the\nthermodynamic feasibility of CO -to-C H conversion over Mn –ZnS , while the unique local\n2 2 4 1 v\ncoordination environment of the low-coordination Mn site promotes C-C coupling kinetically.\nIn response to the reviewer’s suggestion, we have added the related explanations in line 198-\n202 and Supplementary Fig. 24 of the supplementary information.\nTable R3 Photocatalytic reduction of CO and its standard redox potential.\nCO + 2H+ + 2e- → CO + H O -0.53\n2 2\nCO + 8H+ + 8e- → CH + 2H O -0.24\n2 4 2\nCO + 12H+ + 12e- → C H + 4H O -0.38\n2 2 4 2\nCO + 6H+ + 6e- → CH OH + H O -0.39\n2 3 2\nCO + 4H+ + 4e- → HCHO + H O -0.48\n2 2\nCO + 2H+ + 2e- → HCOOH -0.61\ncoupling on Mn –ZnS (111) facet.\n1 v\n7. Since water is present in the photocatalytic reaction system, were any products detected in\nthe aqueous solution?\nnuclear magnetic resonance (NMR) analysis for the liquid product. For pristine ZnS, formic\nacid (HCOOH) (12.1 μmol g−1 h−1) was detected, and its selectivity was 8.9%. No liquid\nproducts were detected on the Mn –ZnS and Mn –ZnS catalysts above the instrument's\n1 1 v\ndetection limit. These experimental results provided further insight into the high C H\n2 4\nselectivity of the Mn –ZnS catalyst. We have added the related discusses in the revised\n1 v\nmanuscript (Fig. 5a, line 253).\n8. It would be beneficial if the authors could calculate the consumption and balance of\nphotogenerated carriers throughout the reaction to further elucidate the stability of the catalyst.\nResponse: We appreciate the reviewer’s suggestion to investigate the consumption and balance\nof photogenerated carriers throughout the reaction to gain a thorough understanding of the\nphotochemical stability of Mn –ZnS catalyst. In our system, the main half-reaction is as\n1 v\nfollows:\nReaction: 6CO + 12 H+ + 12 e- → C H + 6H O (14)\n2 2 4 2\nOxidation: 6H O → 3O + 12H+ + 12e- (15)\n2 2\nTo quantify the consumption and equilibrium of photogenerated carriers, that is, the total\nelectron consumption in the reduction reaction and the hole consumption in the H O oxidation\n2\nreaction, the number of electrons transferred in the formation of the two half-reaction products\nwere precisely measured. The ratio of the number of electrons transferred in the oxidation\nreaction to the number of electrons transferred in the reduction reaction is defined as R . The\nO/R\ncloser the R value is to 1, the better the photochemical stability of the catalysts. As shown\nO/R\nin Table R4, the R values of ZnS, Mn –ZnS and Mn –ZnS are all close to 1, which means\nO/R 1 1 v\nthat the consumption of photogenerated electrons and holes on the catalysts’ surface is nearly\n66\nbalanced, directly avoiding the oxidation corrosion or photocorrosion of the catalysts caused\nby charge accumulation. Thus, it explains the reason for the excellent photochemical stability\nof the catalysts from the perspective of mechanism. In addition, the cyclic stability test (Fig.\n5e) and the structural characterization before and after the reaction (Supplementary Figs. 43\nand 44) in the manuscript also confirmed the internal reasons for the excellent photostability\nof Mn –ZnS . In response to the reviewer’s suggestion, we have added the related discusses in\n1 v\nthe revised manuscript (Supplementary Table 4, line 276-279).\nTable R4 The formation rates and the value of R over ZnS, Mn –ZnS and Mn –ZnS\nO/R 1 1 v\ncatalysts.\nFormation rate [μmol g−1 h−1]\nSample Reduction O xidation R *\nO/R\nC H CO CH HCOOH H O\n2 4 4 2 2\nZnS 1.3 64.6 12.9 12.1 113.5 130.7 1.05\nMn –ZnS 47.5 28.4 17.3 N.d.d 39.2 238.3 1.13\n1 v\n* 𝑅 = \n4×𝑛𝑂2\n𝑂/𝑅\n12×𝑛𝐶2𝐻4 +2×𝑛𝐶𝑂+8×𝑛𝐶𝐻4 +2×𝑛𝐻𝐶𝑂𝑂𝐻+2×𝑛𝐻2\nManuscript number: NCOMMS-25-62587A\nTitle: Near-unity CO -to-ethylene photoconversion over low-coordination manganese single-\n2\natom sites on binary sulfide\nA Point-to-point Response to Reviewer’s comments\nDear Editor and Reviewers,\nThank you for checking our manuscript and proposing valuable comments! The comments are\nhighly appreciated and helpful to improve our work. We have made corresponding changes to\nthe manuscript and Supplementary information (highlighted in yellow in the revised versions)\nto address the reviewers’ concerns; these changes are specified and discussed in a point-to-\npoint response to the reviewers’ comments, as shown below:\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nFollowing a comprehensive re-evaluation of the manuscript, it has been concluded that the\nmethod for free energy calculation remains flawed. Specifically, the use of the electrochemical\npotential (U) in equation 5 is inappropriate, as it represents the conduction band potential of\nthe semiconductor photocatalyst, which is not applicable in this context. Furthermore, in\nequation 3, the energy should correspond to the entire catalyst surface slab rather than just the\nsurface. Additionally, the manuscript does not provide sufficient details regarding the\ncalculation of the free energy barrier. In light of these significant issues, I regret that I cannot\n1\nrecommend this work for publication in its current form.\nResponse: We sincerely appreciate the reviewer’s insightful and constructive comments.\nYour suggestions accurately identified the key issues in the free-energy calculation methodology\nand its expression in our original manuscript, and they have provided valuable guidance for\nimproving the scientific rigor of this work. In response, we have added relevant theoretical\nreferences, clarified the calculation framework, and corrected all free-energy results to ensure\nthat every concern is fully resolved. We respectfully hope that the revised version may be given\na new opportunity for consideration in Nature Communications.\n1. Addressing concerns about the “-eU” item in the equation 5 for free energy\n(i) We fully agree with the reviewer that the use of the electrochemical potential (U) in\nequation 5 was inappropriate under the computational context of this work. After careful\nexamination, we recognize that your judgment is scientifically sound and directly points to the\ndirection needed for methodological revision.\nIn our preliminary theoretical screening, we evaluated two parallel computational schemes:\n(1) the classical Gibbs free-energy formalism, ∆G = ∆E + ∆E −T∆S (Nat Commun\nZPE\n2024, 15, 10589, J. Am. Chem. Soc. 2023, 145, 51, 28276, Sci. Adv. 2020, 6, eaaz2060), and\n(2) a modified form incorporating –eU, ∆G = ∆E + ∆E −T∆S−𝑒𝑈 (Nat Catal 2025, 8,\nZPE\n728, Nat Catal 2023, 6, 574, J. Chem. Phys. 2013, 139, 044103), where U represents the\nconduction-band potential of the semiconductor and –eU approximates the thermodynamic\ndriving force from photogenerated electrons.\nBased on your valuable comments, we re-examined the applicability of the two calculation\nschemes. Considering the research system and computational scenarios, the introduction of “-\neU” in the equation 5 for free energy indeed cannot represent a complete photodynamic\ndescription process with inherent complexity. It only offers a balanced and computationally\ntractable approach for initial mechanistic insight and identifying activity trends. The generally\n2\nrecognized classical Gibbs free energy equation (∆G = ∆E + ∆E −T∆S) is more in line\nZPE\nwith the free energy calculation logic of this study. Therefore, we have corrected all the\ncalculation results involving Gibbs free energy in the original manuscript to those using the\nclassical Gibbs free energy equation (Fig. R1).\nTo further verify the reliability of the revised conclusion, we compared the Gibbs free energy\ndiagram calculated by the two equations, and found that the trends of the theoretical screening\nresults of the two were completely consistent. The core theoretical guiding conclusion was not\naffected, ensuring the robustness of the research conclusion.\nFig. R1 a, The free energy diagram for CO reduction reaction to display the conversion of\n2 1 1 v\n*CO hydrogenation and subsequent C-C coupling on Mn –ZnS (111) facet. c, Reaction energy\n1 v\ndiagram from *CO to *CHO on ZnS, Mn –ZnS and Mn –ZnS with molecular structures of\n1 1 v\ninitial, transition and final states inserted. d, Reaction pathways and C-C coupling step for CO\n2\nphotoreduction over ZnS, Mn –ZnS and Mn –ZnS .\n1 1 v\n3\n(ii) The considerations for using equation 5 in the original manuscript are as follows. In the\nphotocatalytic process, the generation of electron-hole pairs by Mn –ZnS catalyst under light\n1 v\nirradiation is a prerequisite for the occurrence of photocatalytic reactions. The potential at the\nconduction band minimum of the Mn –ZnS catalyst is sufficiently negative than the\n1 v\nequilibrium potential of the reaction to drive the CO reduction reaction. The methodology\n2\npresented in equation 5 is based on the Computational Hydrogen Electrode (CHE) model, a\nstandard method in electrocatalysis (J. Phys. Chem. B 2004, 108, 17886–17892). It explicitly\nintroduces the electrode potential (U) into the Gibbs free energy calculation to describe the\nreaction free energy at a given potential. To account for the influence of photoexcitation on\nreaction free energy and better approximate realistic conditions, the conventional\ncomputational hydrogen electrode (CHE) model was extended to photocatalytic system. In our\nwork, the \"electrode potential (U)\" is redefined as the effective conduction band potential of\nthe semiconductor photocatalysts, while -eU represents the thermodynamic driving force\nprovided by photogenerated electrons in Mn –ZnS (J. Chem. Phys. 2013, 139, 044103). This\n1 v\nadaptation allows for a self-consistent mapping of the thermodynamic landscape in\nphotocatalytic systems. Consequently, the -eU correction is introduced to the standard\nexpression, yielding a photocatalytic free energy equation: ΔG = ΔE + ΔE - TΔS - eU, with\nZPE\nthe core rationale supported by references, such as Nat Catal 2025, 8, 728–739, Nat Catal 2023,\n6, 574–584, J. Chem. Phys. 2013, 139, 044103.\n(iii) According to your valuable suggestions and a large number of literatures on\nphotocatalytic CO reduction (Nat Commun 2024, 15, 10589, J. Am. Chem. Soc. 2023, 145, 51,\n2\n28276–28283, Sci. Adv. 2020, 6, eaaz2060), the modified equation still has the limitation that\nit cannot accurately describe the complete photodynamic process, and it only provides a\nreference for the theoretical screening trend. Therefore, we have made comprehensive revisions\n4\nto the relevant contents of the original manuscript (Fig. 2, Supplementary Figs. 4-7,\nSupplementary Figs. 24 and 49, line 90-92, 127-129, 461).\n2. The expression of the energy calculation object in equation 3\nWe are very sorry for the misunderstanding of the energy calculation object in equation 3\ndue to our inaccurate expression. Thank you for pointing out this problem in time.\nIn the actual calculation process, the total energy of the complete slab model of the catalyst is\nalways used for calculation, rather than only the surface energy. The expression of this\ncalculation equation strictly follows the reference: Sci. Adv. 2020, 6, eaaz2060, Angew. Chem.\nInt. Ed. 2025, 64, e202506072. In the original manuscript, \"overall slab energy\" is expressed\nas \"surface energy\", which is a habitual and inappropriate expression to cause the possible\nambiguity. In the revised manuscript, we will formally correct the expressions in equation 3\nto ΔE = E −E −E , in which E and E represent the energy of the slab\nVS slab-VS slab S slab-VS slab\nafter and before V , and verify the relevant expressions throughout the manuscript to avoid\ns\nsimilar ambiguities (line 454-456).\n3. The detail regarding the calculation of the free energy barrier\nAll the supplementary calculation details mentioned above will be elaborated in detail in the\n\"Density functional theory (DFT) calculations\" section of the revised manuscript (line 436-446)\nand Supplementary Table 1 of the revised supplementary information, and the detailed contents\nare as follows:\nDFT calculations were carried out in the framework of the Vienna ab initio Simulation\nPacklage (VASP). The exchange and correlation energies were established by the Perdew-\nBurke-Ernzerhof (PBE) functional with spin-polarized generalized gradient approximation\n(GGA) (Phys. Rev. Lett. 1996, 77, 3865). The core electrons were described by the projected\naugmented wave (PAW) pseudopotentials. For geometry optimization, a cutoff energy of 450\n5\neV was used to expand the wave functions, and the Brillouin zone was sampled with 2 × 2 × 1\nk-points. All the structures and energy were allowed to relax below 0.05 eV Å−1. The DFT-D3\nmethod of Grimme et al. was employed to involve the van der Waals interaction (J. Comput.\nChem. 2006, 27, 1787–1799). The calculations of charge density difference were employed to\nanalyze the movement and distribution of the charge. For the elementary reaction barriers, the\ntransition states (TSs) were determined by the climbing image nudged elastic band (CINEB)\nmethod and were confirmed by further frequency calculations showing one and only one\nimaginary frequency.\nTable R1 Calculated free energy for reaction process and corrections for intermediates\nElementary steps Intermediate ZPE/eV TS/eV G/eV\n*+CO → *CO *CO 0.31 0.18 0.17\n2 2 2\n*CO + H+ + e- → *COOH *COOH 0.6 0.22 0.56\n2\n*COOH + H+ + e- → *CO + H O *CO 0.19 0.15 -0.14\n2\n*CO + H+ + e- → *CHO *CHO 0.48 0.15 0.51\n*CO + *CHO → *COCHO *COCHO 0.72 0.19 -0.51\n*COCHO + H+ + e- → *COCHOH *COCHOH 1.04 0.24 -0.15\n*COCHOH + H+ + e- → *CHOCHOH *CHOCHOH 1.06 0.2 0.09\n*CHOCHOH + H+ + e- → *CHOCH + H O *CHOCH 0.84 0.12 -0.27\n2\n*CHOCH + H+ + e- → *CHOHCH *CHOHCH 1.17 0.11 -0.19\n*CHOHCH + 2H+ + 2e- → *CH CH + H O *CH CH 1.26 0.08 -0.89\n2 2 2\n*CH CH + H+ + e- → *CH CH *CH CH 1.38 0.06 -1.24\n2 2 2 2 2\nWe sincerely appreciate the reviewer’s time and effort in evaluating our manuscript, as well\nas the insightful comments and constructive suggestions. We have carefully addressed all\nconcerns raised, and hope the revisions will meet with approval.\nThe authors addressed my comments.\nResponse: We appreciate the reviewer’s effort in reviewing our manuscript, positive\nrecommendation and insightful comments very much.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We appreciate the reviewer’s effort in reviewing our revised manuscript and\npositive recommendation very much.\nReviewer #4 (Remarks to the Author):\nRevised ms. which contains new experimental data and clarifies many points is recommended\nfor publication.\nResponse: We sincerely appreciate the reviewer’s time and thoughtful evaluation of our\nmanuscript, which have been invaluable in enhancing the quality of this work.\nReviewer #5 (Remarks to the Author):\nThe authors have already provided reasonable answers to all the questions that were raised.\nResponse: We appreciate the reviewer’s effort in reviewing our revised manuscript and\npositive recommendation very much.\nManuscript number: NCOMMS-25-62587B\nTitle: Near-unity CO -to-ethylene photoconversion over low coordination single-atom\n2\ncatalysts\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors have provided reasonable replies to all my queries and have appropriately revised\nthe manuscript accordingly. Now I do not have any further questions regarding the manuscript.\nResponse: We sincerely appreciate the reviewer’s time and thoughtful evaluation of our\nmanuscript, which have been invaluable in enhancing the quality of this work.\n1 v 2 2 6\ntheoretical guidance in experimental design, and the overall structure is complete, with clear logic\nand theoretical foundations. However, the following points require further discussion:\n1. On page 5, the statement “The low-coordination Mn site in Mn –ZnS strengthens the Mn–CO π\n1 v\nbackbonding” – what does this mean? Please provide a detailed explanation. What is the relationship\nbetween this and the differential charge density analysis?\n2. To ensure greater rigor, in addition to calculating the adsorption of *CO on the Mn atom in both\nMn –ZnS and Mn –ZnS structures, the authors should also consider the adsorption on Zn atoms\n1 1 v\nsurrounding the Mn site. Although *CO does not readily adsorb on Zn atoms in pure ZnS, the\nelectronic structure modulation induced by Mn may alter the adsorption behavior on neighboring\nZn atoms.\n3. On page 6, why can microwave radiation directionally induce defects? What advantages does\nmicrowave irradiation offer over other synthesis methods specifically for ZnS materials? How does\nH O etch S? Is it possible that O atoms are introduced into ZnS during this process?\n2 2\n4. In Figure 3e, how can it be confirmed that the bond is Mn–S rather than Mn–O?\n5. In Figure 3c, given that the atomic masses of Mn and Zn are very similar and the Mn –ZnS is\n1 v\nbulk rather than ultrathin sheet-like, it is generally challenging to distinguish them via ADF-STEM.\nCan the authors provide a reasonable explanation for this observation?\n6. Please include the energy band structures related to other possible CO reduction products to\n2\nfurther support the claim that CO is more readily reduced to C H .\n2 2 6\n7. Since water is present in the photocatalytic reaction system, were any products detected in the\naqueous solution?\n8. It would be beneficial if the authors could calculate the consumption and balance of\nphotogenerated carriers throughout the reaction to further elucidate the stability of the catalyst."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Following a comprehensive re-evaluation of the manuscript, it has been concluded that the method for free energy calculation remains flawed. Specifically, the use of the electrochemical potential (U) in equation 5 is inappropriate, as it represents the conduction band potential of the semiconductor photocatalyst, which is not applicable in this context. Furthermore, in equation 3, the energy should correspond to the entire catalyst surface slab rather than just the surface. Additionally, the manuscript does not provide sufficient details regarding the calculation of the free energy barrier. In light of these significant issues, I regret that I cannot recommend this work for publication in its current form.", "reviewer_2": "The authors addressed my comments.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "Revised ms. which contains new experimental data and clarifies many points is recommended for publication.", "reviewer_5": "The authors have already provided reasonable answers to all the questions that were raised."}, "author_response": "Response to Reviewers’ Comments\nManuscript number: NCOMMS-25-62587A\nTitle: Near-unity CO -to-ethylene photoconversion over low-coordination manganese single-\n2\natom sites on binary sulfide\nA Point-to-point Response to Reviewer’s comments\nDear Editor and Reviewers,\nThank you for checking our manuscript and proposing valuable comments! The comments are\nhighly appreciated and helpful to improve our work. We have made corresponding changes to\nthe manuscript and Supplementary information (highlighted in yellow in the revised versions)\nto address the reviewers’ concerns; these changes are specified and discussed in a point-to-\npoint response to the reviewers’ comments, as shown below:\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nFollowing a comprehensive re-evaluation of the manuscript, it has been concluded that the\nmethod for free energy calculation remains flawed. Specifically, the use of the electrochemical\npotential (U) in equation 5 is inappropriate, as it represents the conduction band potential of\nthe semiconductor photocatalyst, which is not applicable in this context. Furthermore, in\nequation 3, the energy should correspond to the entire catalyst surface slab rather than just the\nsurface. Additionally, the manuscript does not provide sufficient details regarding the\ncalculation of the free energy barrier. In light of these significant issues, I regret that I cannot\n1\nrecommend this work for publication in its current form.\nResponse: We sincerely appreciate the reviewer’s insightful and constructive comments.\nYour suggestions accurately identified the key issues in the free-energy calculation methodology\nand its expression in our original manuscript, and they have provided valuable guidance for\nimproving the scientific rigor of this work. In response, we have added relevant theoretical\nreferences, clarified the calculation framework, and corrected all free-energy results to ensure\nthat every concern is fully resolved. We respectfully hope that the revised version may be given\na new opportunity for consideration in Nature Communications.\n1. Addressing concerns about the “-eU” item in the equation 5 for free energy\n(i) We fully agree with the reviewer that the use of the electrochemical potential (U) in\nequation 5 was inappropriate under the computational context of this work. After careful\nexamination, we recognize that your judgment is scientifically sound and directly points to the\ndirection needed for methodological revision.\nIn our preliminary theoretical screening, we evaluated two parallel computational schemes:\n(1) the classical Gibbs free-energy formalism, ∆G = ∆E + ∆E −T∆S (Nat Commun\nZPE\n2024, 15, 10589, J. Am. Chem. Soc. 2023, 145, 51, 28276, Sci. Adv. 2020, 6, eaaz2060), and\n(2) a modified form incorporating –eU, ∆G = ∆E + ∆E −T∆S−𝑒𝑈 (Nat Catal 2025, 8,\nZPE\n728, Nat Catal 2023, 6, 574, J. Chem. Phys. 2013, 139, 044103), where U represents the\nconduction-band potential of the semiconductor and –eU approximates the thermodynamic\ndriving force from photogenerated electrons.\nBased on your valuable comments, we re-examined the applicability of the two calculation\nschemes. Considering the research system and computational scenarios, the introduction of “-\neU” in the equation 5 for free energy indeed cannot represent a complete photodynamic\ndescription process with inherent complexity. It only offers a balanced and computationally\ntractable approach for initial mechanistic insight and identifying activity trends. The generally\n2\nrecognized classical Gibbs free energy equation (∆G = ∆E + ∆E −T∆S) is more in line\nZPE\nwith the free energy calculation logic of this study. Therefore, we have corrected all the\ncalculation results involving Gibbs free energy in the original manuscript to those using the\nclassical Gibbs free energy equation (Fig. R1).\nTo further verify the reliability of the revised conclusion, we compared the Gibbs free energy\ndiagram calculated by the two equations, and found that the trends of the theoretical screening\nresults of the two were completely consistent. The core theoretical guiding conclusion was not\naffected, ensuring the robustness of the research conclusion.\nFig. R1 a, The free energy diagram for CO reduction reaction to display the conversion of\n2 1 1 v\n*CO hydrogenation and subsequent C-C coupling on Mn –ZnS (111) facet.\n1 v\n(ii) The considerations for using equation 5 in the original manuscript are as follows. In the\nphotocatalytic process, the generation of electron-hole pairs by Mn –ZnS catalyst under light\n1 v\nirradiation is a prerequisite for the occurrence of photocatalytic reactions. The potential at the\nconduction band minimum of the Mn –ZnS catalyst is sufficiently negative than the\n1 v\nequilibrium potential of the reaction to drive the CO reduction reaction. The methodology\n2\npresented in equation 5 is based on the Computational Hydrogen Electrode (CHE) model, a\nstandard method in electrocatalysis (J. Phys. Chem. B 2004, 108, 17886–17892). It explicitly\nintroduces the electrode potential (U) into the Gibbs free energy calculation to describe the\nreaction free energy at a given potential. To account for the influence of photoexcitation on\nreaction free energy and better approximate realistic conditions, the conventional\ncomputational hydrogen electrode (CHE) model was extended to photocatalytic system. In our\nwork, the \"electrode potential (U)\" is redefined as the effective conduction band potential of\nthe semiconductor photocatalysts, while -eU represents the thermodynamic driving force\nprovided by photogenerated electrons in Mn –ZnS (J. Chem. Phys. 2013, 139, 044103). This\n1 v\nadaptation allows for a self-consistent mapping of the thermodynamic landscape in\nphotocatalytic systems. Consequently, the -eU correction is introduced to the standard\nexpression, yielding a photocatalytic free energy equation: ΔG = ΔE + ΔE - TΔS - eU, with\nZPE\nthe core rationale supported by references, such as Nat Catal 2025, 8, 728–739, Nat Catal 2023,\n6, 574–584, J. Chem. Phys. 2013, 139, 044103.\n(iii) According to your valuable suggestions and a large number of literatures on\nphotocatalytic CO reduction (Nat Commun 2024, 15, 10589, J. Am. Chem. Soc. 2023, 145, 51,\n2\n28276–28283, Sci. Adv. 2020, 6, eaaz2060), the modified equation still has the limitation that\nit cannot accurately describe the complete photodynamic process, and it only provides a\nreference for the theoretical screening trend. Therefore, we have made comprehensive revisions\n4\nto the relevant contents of the original manuscript (Fig. 2, Supplementary Figs. 4-7,\nSupplementary Figs. 24 and 49, line 90-92, 127-129, 461).\n2. The expression of the energy calculation object in equation 3\nWe are very sorry for the misunderstanding of the energy calculation object in equation 3\ndue to our inaccurate expression. Thank you for pointing out this problem in time.\nIn the actual calculation process, the total energy of the complete slab model of the catalyst is\nalways used for calculation, rather than only the surface energy. The expression of this\ncalculation equation strictly follows the reference: Sci. Adv. 2020, 6, eaaz2060, Angew. Chem.\nInt. Ed. 2025, 64, e202506072. In the original manuscript, \"overall slab energy\" is expressed\nas \"surface energy\", which is a habitual and inappropriate expression to cause the possible\nambiguity. In the revised manuscript, we will formally correct the expressions in equation 3\nto ΔE = E −E −E , in which E and E represent the energy of the slab\nVS slab-VS slab S slab-VS slab\nafter and before V , and verify the relevant expressions throughout the manuscript to avoid\ns\nsimilar ambiguities (line 454-456).\n3. The detail regarding the calculation of the free energy barrier\nAll the supplementary calculation details mentioned above will be elaborated in detail in the\n\"Density functional theory (DFT) calculations\" section of the revised manuscript (line 436-446)\nand Supplementary Table 1 of the revised supplementary information, and the detailed contents\nare as follows:\nDFT calculations were carried out in the framework of the Vienna ab initio Simulation\nPacklage (VASP). The exchange and correlation energies were established by the Perdew-\nBurke-Ernzerhof (PBE) functional with spin-polarized generalized gradient approximation\n(GGA) (Phys. Rev. Lett. 1996, 77, 3865). The core electrons were described by the projected\naugmented wave (PAW) pseudopotentials. For geometry optimization, a cutoff energy of 450\n5\neV was used to expand the wave functions, and the Brillouin zone was sampled with 2 × 2 × 1\nk-points. All the structures and energy were allowed to relax below 0.05 eV Å−1. The DFT-D3\nmethod of Grimme et al. was employed to involve the van der Waals interaction (J. Comput.\nChem. 2006, 27, 1787–1799). The calculations of charge density difference were employed to\nanalyze the movement and distribution of the charge. For the elementary reaction barriers, the\ntransition states (TSs) were determined by the climbing image nudged elastic band (CINEB)\nmethod and were confirmed by further frequency calculations showing one and only one\nimaginary frequency.\nTable R1 Calculated free energy for reaction process and corrections for intermediates\nElementary steps Intermediate ZPE/eV TS/eV G/eV\n*+CO → *CO *CO 0.31 0.18 0.17\n2 2 2\n*CO + H+ + e- → *COOH *COOH 0.6 0.22 0.56\n2\n*COOH + H+ + e- → *CO + H O *CO 0.19 0.15 -0.14\n2\n*CO + H+ + e- → *CHO *CHO 0.48 0.15 0.51\n*CO + *CHO → *COCHO *COCHO 0.72 0.19 -0.51\n*COCHO + H+ + e- → *COCHOH *COCHOH 1.04 0.24 -0.15\n*COCHOH + H+ + e- → *CHOCHOH *CHOCHOH 1.06 0.2 0.09\n*CHOCHOH + H+ + e- → *CHOCH + H O *CHOCH 0.84 0.12 -0.27\n2\n*CHOCH + H+ + e- → *CHOHCH *CHOHCH 1.17 0.11 -0.19\n*CHOHCH + 2H+ + 2e- → *CH CH + H O *CH CH 1.26 0.08 -0.89\n2 2 2\n*CH CH + H+ + e- → *CH CH *CH CH 1.38 0.06 -1.24\n2 2 2 2 2\nWe sincerely appreciate the reviewer’s time and effort in evaluating our manuscript, as well\nas the insightful comments and constructive suggestions. We have carefully addressed all\nconcerns raised, and hope the revisions will meet with approval.\nThe authors addressed my comments.\nResponse: We appreciate the reviewer’s effort in reviewing our manuscript, positive\nrecommendation and insightful comments very much.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We appreciate the reviewer’s effort in reviewing our revised manuscript and\npositive recommendation very much.\nReviewer #4 (Remarks to the Author):\nRevised ms. which contains new experimental data and clarifies many points is recommended\nfor publication.\nResponse: We sincerely appreciate the reviewer’s time and thoughtful evaluation of our\nmanuscript, which have been invaluable in enhancing the quality of this work.\nReviewer #5 (Remarks to the Author):\nThe authors have already provided reasonable answers to all the questions that were raised.\nResponse: We appreciate the reviewer’s effort in reviewing our revised manuscript and\npositive recommendation very much.\n7"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have provided reasonable replies to all my queries and have appropriately revised the manuscript accordingly. Now I do not have any further questions regarding the manuscript."}, "author_response": ""}]

Decision letter