Urea photosynthesis over a MOF-on-MOF S-scheme heterojunction

Domain: neuroscience
Published: 2026-02-06
Editorial outcome: major_revision

Abstract
Photocatalytic nitrate reduction reaction coupled carbon dioxide reduction reaction holds great promise for sustainable urea synthesis, the design of photocatalysts with efficient C-N coupling and charge separation is crucial yet challenging. Herein, we report the synthesis of a novel MOF-on-MOF S-scheme heterojunction photocatalyst with interfacial dual active sites for efficient urea production via nitrate reduction reaction coupled carbon dioxide reduction reaction. The integration of semiconducting Zr-MOF and Co-MOF produces interfacial Co and Zr dual sites for NO3- activation and CO2 adsorption, synergistically promoting C-N coupling with reduced energy barriers. Besides, the S-scheme heterojunction enables the strong redox capability and facilitates the charge transfer. Consequently, the rationally designed MOF-on-MOF heterojunction achieves a high urea yield of 3523.4 μg g⁻¹ h⁻¹ and an apparent quantum yield of 1.16% at 365 nm in the absence of sacrificial agents. This work paves the way for the development of high-performance photocatalysts for urea production.

Introduction
Urea (CO(NH2)2) serves as the most widely used nitrogen fertilizer in agriculture and a vital feedstock for chemical production1–3. The industrial urea production is primarily based on the Bosch-Meiser process by the reaction of ammonia (NH3) and carbon dioxide (CO2) under harsh conditions (150°C–200°C, 150–250 bar), which consumes large amounts of fossil fuels and emits substantial CO2 4–7.

Photocatalytic N2 and CO2 co-reduction (PNRR&CRR) offers an ambient and sustainable alternative approach for urea production by using sunlight as the energy input, N2, CO2 and H2O as the raw materials8,9. However, the efficiency of PNRR&CRR remains unsatisfactory with urea yields lower than 1500 µg h⁻¹ g⁻¹cat in pure water, possibly ascribed to the low solubility of N2 and high dissociation energy of N≡N bond (941 kJ mol⁻¹)10,11.

Compared with inert N2, nitrate (NO3-) is more reactive with lower N=O cleavage energy (204 kJ mol⁻¹) and higher aqueous solubility11,12. Substituting N2 with NO3- to couple with CO2 is energetically more favorable for urea photosynthesis13. Besides, CO2 is a common greenhouse gas and NO3- is a widespread environmental pollutant in wastewater14,15. Photocatalytic co-reduction of NO3- and CO2 (P-NitRR&CRR) thus represents a promising strategy for simultaneous environmental remediation and value-added chemical production. Even the NitRR&CRR route has been reported in the field of electrocatalysis16,17, the alternative P-NitRR&CRR route is hardly achieved. There is only report that delivered a urea production yield of just 0.22 μg h⁻¹ g⁻¹cat using Cs2CuBr4/TiO2-x as the photocatalyst13. The development of high-performance photocatalysts to drive P-NitRR&CRR for urea synthesis remains a critical challenge.

As an important class of porous crystalline materials, metal-organic frameworks (MOFs) hold great promise as efficient photocatalysts owing to their large specific surface areas, high porosities and organic light-harvesting units, ligand-metal charge transfer pathway, as well as high tunability in compositions and structures13,18,19. Even MOFs photocatalysts have been extensively investigated, their application in photocatalytic urea synthesis remains unexplored.

Compared with traditional MOFs, construction of MOF-on-MOF heterostructures as heterojunction photocatalysts represents an emerging strategy for further improving the photocatalytic properties by enhancing light absorption, promoting charge separation and offering additional interfacial active sites19. To date, MOF-on-MOF photocatalysts have demonstrated promising potentials in the fields of N2 fixation19, water splitting20, CO2 reduction21 and pollutant degradation22, however the design of such photocatalysts for urea synthesis via the P-NitRR&CRR strategy has not been reported.

Herein, we report a MOF-on-MOF S-scheme heterojunction photocatalyst with interfacial dual active sites for efficient P-NitRR&CRR to urea production (Fig. 1). By growing Co-HHTP (HHTP=2,3,6,7,10,11-hexahydroxytripehenylene) on NU-1000 (NU = Northwestern University), NU-1000@Co-HHTP is formed (Fig. 1a) with a S-scheme heterojunction (Fig. 1b) and interfacial Zr-Co dual active sites (Fig. 1c). The Co sites facilitate the reduction of NO3- to *NO intermediates, while the Zr sites adsorb CO2 for subsequent C-N coupling with *NO on neighboring Co sites, lowering the energy barrier for the generation of key C-N coupling intermediates. Further combined with the facilitated charge separation and strong redox capability of S-scheme heterojunction (Fig. 1b), the elaborately designed NU-1000@Co-HHTP photocatalyst exhibits a urea yield rate of 3523.4 μg g⁻¹ h⁻¹ and an apparent quantum yield (AQY) of 1.16% at 365 nm, competitive with the reported photocatalysts.

Results
The synthesis procedure of NU-1000@Co-HHTP heterostructure is schematically presented in Fig. 1a. Firstly, NU-1000 was prepared through a solvothermal reaction employing ZrOCl2·8H2O as metal sources and 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl)tetrabenzoate (H4TBAPy) as ligands20,23. The field-emission scanning electron microscope (SEM) and transmission electron microscopy (TEM) images of the resultant NU-1000 show a well-defined hexagonal rod-like morphology with smooth surface and solid nature (Supplementary Fig. 1a–c). The length and diameter are measure to be approximately 890 and 370 nm, respectively. The powder X-ray diffraction (XRD) characteristic diffraction peaks of NU-1000 (Supplementary Fig. 1d) matches well with the simulated crystal structure (CCDC 1580411)24.

By subsequent reaction of NU-1000 with Co2+ and HHTP in 1-propanol solution, the NU-1000@Co-HHTP heterostructure was obtained25. The SEM image (Fig. 2a) of NU-1000@Co-HHTP exhibits uniform microrods with rough surface. At higher magnification (Fig. 2b), Co-HHTP nanorods with an average length of 71 nm and diameter of 39 nm are found densely adhered on the surface of NU-1000. From the SEM image of one broken particle (Supplementary Fig. 2) and the TEM image (Fig. 2c), the formation of rod-on-rod core-shell heterostructure is clearly demonstrated. Figure 2d displays the HRTEM image collected from a single nanorod on the shell. The lattice fringes with a d-spacing of 1.91 nm (indicated by the yellow circle) correspond to the (100) plane of crystalline Co-HHTP26. The spatial elemental distribution was then examined by energy dispersive X-ray (EDX) spectroscopy. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images as well as the line scanning spectra of NU-1000@Co-HHTP are shown in Fig. 2e, f. The signals of C and O elements are evenly detected throughout the whole particle, while those of Zr and Co elements are predominately distributed in the internal core and external shell regions, respectively, further verifying the core-shell heterostructure of NU-1000@Co-HHTP. Moreover, the Co and Zr mass ratio of NU-1000@Co-HHTP was measured to be 13.02% and 8.29%, respectively, via inductively coupled plasma-optical emission spectrometry (ICP-OES, Supplementary Table 1). The Zr/Co molar ratio was thus calculated to be 10.11:10.00, corresponding to a NU-1000/Co-HHTP mass ratio of 1.25/1.00.

To investigate the crystalline and chemical structures of NU-1000@Co-HHTP, XRD and Fourier-transform infrared (FTIR) spectroscopy analyses were performed. In addition to NU-1000, Co-HHTP nanorods (Supplementary Fig. 3) were also prepared as a control sample by using the similar synthesis protocol of NU-1000@Co-HHTP without the addition of NU-1000. The mass ratios of Co and Zr of Co-HHTP and NU-1000 were determined as 18.66% and 23.44%, respectively, close to the theoretical values. As shown in Fig. 2g, h, the characteristic diffraction peaks of both NU-1000 and Co-HHTP are detected in the XRD pattern of NU-1000@Co-HHTP. In the FTIR spectrum of NU-1000@Co-HHTP (Supplementary Fig. 4), the bands at 1454, 1365, and 1215 cm⁻¹ are attributed to the benzene skeleton vibrations of Co-HHTP27. In addition, the peak of adsorbed H2O is also found at 1655 cm⁻¹.

N2 sorption analysis (Fig. 2i) was used to characterize the textural properties of NU-1000, Co-HHTP and NU-1000@Co-HHTP. The N2 sorption isotherm of Co-HHTP shows a type I hysteresis loop with microporous nature. Different from Co-HHTP, NU-1000 and NU-1000@Co-HHTP exhibit a micro-mesopore coexisting structure28. The Brunauer-Emmett-Teller (BET) specific surface areas of NU-1000, Co-HHTP and NU-1000@Co-HHTP are calculated to be 1150.1, 322.78, and 933.42 m² g⁻¹, with corresponding pore volumes of 0.86, 0.43, and 0.75 cm³ g⁻¹, respectively (Supplementary Table 2). From the pore size distribution curve of NU-1000@Co-HHTP, both micropores of 1.12 nm from Co-HHTP and mesopores of 2.30 nm originated from NU-1000 are observed (Supplementary Fig. 5). The above results demonstrate the successful fabrication of NU-1000@Co-HHTP heterostructures.

The electronic structure and coordination environment of the samples were investigated using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis. As shown in Fig. 3a, the Co K-edge absorption for NU-1000@Co-HHTP exhibits an increased edge energy than that for Co-HHTP. In contrast, the Zr K-edge position for NU-1000@Co-HHTP negatively shifts compared to that for NU-1000. The observations suggest the electron transfer from Co-HHTP to NU-1000 within NU-1000@Co-HHTP, resulting in the generation of electron deficient Co and electron-sufficient Zr sites. In the Co K-edge Fourier transform (FT) EXAFS spectra (Fig. 3b, Supplementary Table 3), the characteristic peak at 1.71 Å is assigned to Co-O coordination between Co and ligands in Co-HHTP. For NU-1000@Co-HHTP, apart from the Co-O bond, a distinct peak attributed to the Co-O-Zr bond emerges at 2.75 Å (Fig. 3b). Figure 3e displays the Zr K-edge spectrum of NU-1000, where the peaks located at around 1.51 and 3.29 Å correspond to the Zr-O and Zr-O-Zr bond, respectively. For NU-1000@Co-HHTP, an additional peak of Zr-O-Co bond is found at ≈2.77 Å, in accordance with the results of Co K-edge spectra. Compared to the Zr-O-Zr bond, the shorter bond length of Zr-O-Co is attributed to the smaller radius of Co than Zr. Moreover, the derived wavelet transform diagrams (Fig. 3c, f) offer visual evidence for the formation of Zr-O-Co chemical bonds at the NU-1000@Co-HHTP heterointerface.

The optical properties and electronic band structures of the samples were explored by ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) and Mott-Schottky analysis. From the UV-vis DRS spectra and corresponding Tauc plots (Figs. 4a and 4b), NU-1000 exhibits an absorption edge at 460 nm with a band gap (Eg) of 2.65 eV, close to the reported results29. After combination with Co-HHTP with a narrower band gap of 1.40 eV, NU-1000@Co-HHTP shows significantly enhanced light absorption intensity than NU-1000 from UV to near-infrared region. Figure 4c shows the Mott-Schottky curves of NU-1000 and Co-HHTP at the frequencies of 500, 1000, and 1500 Hz. The positive slopes of two samples reflect their n-type semiconductor features29. Based on the transverse intercept values, the flat-band potentials of NU-1000 and Co-HHTP were determined to be 0.01 and -0.09 V, respectively. For n-type semiconductors, the conduction band (CB) position is typically ~0.1 eV below the flat-band potential30,31. Therefore, the CB potentials of NU-1000 and Co-HHTP are calculated to be -0.09 and -0.19 V (vs RHE), respectively.

X-ray photoelectron spectroscopy (XPS) was applied for unveiling the charge transfer between NU-1000 and Co-HHTP within NU-1000@Co-HHTP heterostructure. In addition to NU-1000 and Co-HHTP, a physically mixed NU-1000/Co-HHTP sample was also prepared as comparison. The full survey spectrum of NU-1000@Co-HHTP (Supplementary Fig. 6) reveals the presence of Zr, Co, O and C elements, consistent with the element mapping observations. The high-resolution XPS spectra of Co 2p, Zr 3d and O 1s collected without irradiation are shown in Fig. 4d–f. The Co 2p spectrum of Co-HHTP (Fig. 4d) could be deconvoluted into Co 2p3/2 and 2p1/2 peaks of Co2+ at 781.53 and 797.13 eV, and two satellite peaks at 786.09 and 802.79 eV, respectively. By integrating with NU-1000, the binding energies of Co 2p peaks for NU-1000@Co-HHTP increase by 0.25 eV, corresponding to the decreased electron density of Co. The Zr 3d spectrum of NU-1000 presents two distinct peaks at 182.89 and 185.24 eV, attributed to Zr 3d5/2 and Zr 3d3/2, respectively. In opposite to Co, the Zr 3d peaks of NU-1000@Co-HHTP exhibit a negative shift of ≈0.29 eV, manifesting the electron transfer from Co in Co-HHTP to Zr in NU-1000. The O 1s spectrum of NU-1000 is displayed in Fig. 4f, which can be fitted into two peaks of O-C=O group and Zr-O bond at 532.67 and 531.14 eV, respectively. For Co-HHTP, the two peaks at 531.85 and 530.24 eV belong to the C-O group and Co-O bond, respectively. The combination of the two components generates a distinct peak at 530.73 eV in the spectrum of NU-1000@Co-HHTP, positioned between Co-O peak of Co-HHTP and Zr-O peak of NU-1000, suggesting the formation of Zr-O-Co chemical bonds at the NU-1000/Co-HHTP interface22,32. In contrast to the significant variations in binding energies of the elements observed in NU-1000@Co-HHTP, the electron states of Co and Zr in NU-1000/Co-HHTP exhibit negligible changes compared to NU-1000 and Co-HHTP, suggesting the more intimate interfacial contact between the two components in NU-1000@Co-HHTP.

To elucidate the electron transfer of NU-1000@Co-HHTP under visible-light illumination, in situ irradiated XPS (ISIXPS) measurement was further carried out. Compared to the spectra in darkness, the binding energies of Co 2p peaks in the ISIXPS spectrum decrease by 0.31 eV (Fig. 4d), while those of Zr 3d peaks present a positive shift of 0.3 eV. The observations indicate the transportation of photo-generated electrons from NU-1000 to Co-HHTP under light irradiation. Subsequently, ultraviolet photoelectron spectroscopy (UPS) was employed to measure the work function (Φ) and Fermi levels (Ef) of NU-1000 and Co-HHTP. Supplementary Fig. 8 shows the UPS spectra measured at -10 V bias, where the Φ values of NU-1000 and Co-HHTP were determined to be 4.38, and 3.99 eV, respectively, using the formula of Φ=hv-(Ecutoff-Efermi), where Ecutoff and Efermi represent the secondary electron cutoff and Fermi edge, respectively. Subsequently, the Ef values of NU-1000 and Co-HHTP were calculated to be -4.38 and -3.99 eV (vs vacuum level), respectively, according to the formula of Φ=Evac-Ef (Evac=0)33. Further based on UPS measurements performed without external bias (Supplementary Fig. 9), the valence band maximum (VBM) positions were determined to be 6.60 eV for NU-1000 and 5.23 eV for Co-HHTP using the equation of EVB=21.22-(Ecutoff-Efermi)20. Then, the VB potentials relative to NHE are calculated as 2.57 V for NU-1000 and 1.20 V for Co-HHTP according to the formula of EVB (vs. RHE)=EVB-4.03, well matching with the values measured by Mott–Schottky diagrams and Tauc plots. Besides, valence band XPS (VB-XPS) measurement was performed to further analyze the valance band of the samples. Based on the VB-XPS spectra (Supplementary Fig. 10), the VB values of NU-1000 and Co-HHTP were determined as 2.58 and 1.22 V (vs. RHE), respectively. In combination with the results of Mott–Schottky diagrams and Tauc plots, the band structures of NU-1000 and Co-HHTP are depicted in Fig. 4g.

Based on the UV-vis DRS, Mott-Schottky, XPS and UPS results, the charge transfer mechanism within the NU-1000@Co-HHTP heterojunction is postulated. As the energy band structures (Fig. 4g) reveals higher band positions and Ef of Co-HHTP than NU-1000. Upon contact to form a heterojunction, the free electrons in Co-HHTP are transferred to NU-1000 until the Ef equilibrium is reached, leading to an internal electric field (IEF) pointing from Co-HHTP to NU-1000 at the interface (Fig. 4h). Driven by the IEF, the bands of Co-HHTP and NU-1000 bend upward and downward, respectively. Under illumination, electrons are excited from the VB of Co-HHTP and NU-1000 to their CB, leaving photogenerated holes in the VB. Subsequently, the IEF and band bending facilitates the migration of photogenerated electrons in CB of NU-1000 to VB of Co-HHTP with the photogenerated holes consumed (Fig. 4i). Consequently, the photogenerated electrons and holes are accumulated in CB of Co-HHTP and VB of NU-1000 with maximumly preserved redox capability. The charge transfer manner of NU-1000@Co-HHTP heterojunction well aligns with the S-scheme mechanism34.

The photocatalytic performance of urea synthesis via co-reduction of NO3- and CO2 was evaluated in a sealed quartz photoreactor system containing CO2-saturated NO3- aqueous solution (Supplementary Fig. 11). The yield of generated urea was quantified by colorimetric method and 1H nuclear magnetic resonance (NMR) spectroscopy (Supplementary Figs. 12 and 13). As shown in Fig. 5a and Supplementary Fig. 14, NU-1000@Co-HHTP with an optimized dosage of 0.5 mg mL⁻¹ delivers a urea production amount of 4.70 µmol within 4 h, exceeding NU-1000 (0.26 µmol), Co-HHTP (0.89 µmol) and NU-1000/Co-HHTP (0.59 µmol). Moreover, the urea production rate of NU-1000@Co-HHTP is calculated to be 3523.4 µg h⁻¹ g⁻¹, (Fig. 5b), ~18.4, 5.3 and 8-folds higher than that of NU-1000 (191.8 µg h⁻¹ g⁻¹), Co-HHTP (664.7 µg h⁻¹ g⁻¹) and NU-1000/Co-HHTP (440.9 µg h⁻¹ g⁻¹), respectively, indicating the enhanced photocatalytic activity of NU-1000@Co-HHTP. Furthermore, quantitative 1H NMR analysis indicates that the urea yield is consistent with the result obtained from the colorimetric method (Supplementary Fig. S15).

Except for urea, other possible byproducts including NH3, NO2-, CO and H2 were also detected by UV-Vis spectroscopy and gas chromatography (Fig. 5b, Supplementary Figs. S16 and S17). When catalyzed by NU-1000, CO is generated as the main byproduct with the highest yield of 853.2 µg h⁻¹ g⁻¹ among all samples. In contrast, Co-HHTP predominately produces NH3 with a yield 727.6 µg h⁻¹ g⁻¹, higher than that of NU-1000 (195.2 µg h⁻¹ g⁻¹), NU-1000/Co-HHTP (242.4 µg h⁻¹ g⁻¹) and NU-1000@Co-HHTP (166.4 µg h⁻¹ g⁻¹). Besides, NO2- is almost undetectable with only trace amounts of H2 for all samples. Based on these results, it is speculated that NU-1000 and Co-HHTP mainly favor CO2RR and NO3-RR, respectively. By integrating NU-1000 and Co-HHTP, the C-N coupling toward urea production over NU-1000@Co-HHTP is greatly promoted.

Except for the P-NitRR&CRR process, the coupled water oxidation reaction was also explored. As shown in Supplementary Fig. 18, the yield of O2 as the oxidation product follows the order of NU-1000@Co-HHTP (962.3 µmol g⁻¹ h⁻¹) > Co-HHTP (213.2 µmol g⁻¹ h⁻¹) > NU-1000/Co-HHTP (184.9 µmol g⁻¹ h⁻¹) > NU-1000 (101.2 µmol g⁻¹ h⁻¹).

Afterwards, the apparent quantum yield (AQY) was examined to assess the light utilization efficiency under monochromatic light irradiation from 365 to 650 nm. As presented in Fig. 5c, the AQY of NU-1000@Co-HHTP exhibits a wavelength-dependent trend, in line with the absorption spectrum. A maximum AQY value of 1.16% is achieved at 365 nm (Fig. 5c, Supplementray Table 4). Both the urea yield and AQY of NU-1000@Co-HHTP are competitive with the reported photocatalysts used in both NRR&CRR and NitRR&CRR systems (Supplementary Table 5)8,10,13,31,35–38. Nevertheless, the performance of NU-1000@Co-HHTP-based P-NitRR&CRR system is inferior to the state-of-the-art electrocatalytic NitRR&CRR (E-NitRR&CRR) systems (Supplementary Table 6), inspiring further advances in the design of efficient photocatalysts and photocatalytic systems.

To study the urea formation pathway through CO2 and NO3- co-reduction, an isotopic labeling experiment was conducted. In the 1H NMR spectrum (Fig. 5d), the characteristic signals corresponding to (15NH2)2CO and (14NH2)2CO are detected when using 15NO3- and 14NO3- as the nitrogen source, respectively38. Upon changing the feeding gas to 13CO2, a typical singlet peak of 13CO(NH2)2 is observed in the CNMR spectrum (Supplementary Fig. 19), indicating that the carbon in urea is derived from CO2. In addition, almost no urea could be formed in the reaction systems with only CO2 or NO3-, and without catalysts or light irradiation, verifying that urea is produced from P-NitRR&CRR (Fig. 5e). Except for activity, the catalytic stability of NU-1000@Co-HHTP was also explored. As shown in Fig. 5f, ≈97.3% of the original urea yield is preserved after ten consecutive reaction cycles with each cycle lasting for 4 h, suggesting the stability of NU-1000@Co-HHTP. Additionally, comparative characterizations of NU-1000@Co-HHTP before and after photocatalysis using SEM, TEM, XRD, FTIR and ICP (Supplementary Figs. 20, 21 and Table 7) reveal almost unchanged morphology, crystal structure and chemical composition. In the supernatant after reaction, only trace amounts of Co and Zr were detected (0.030 and 0.024 mg/L, respectively). The mass ratios of leached Co and Zr from NU-1000@Co-HHTP are thus estimated as 0.07% and 0.03%, respectively, further demonstrating the structural stability of NU-1000@Co-HHTP. Besides, the decreased intensity of satellite peak of Co2+ in the XPS spectrum may be caused by the increased oxidation state of Co sites by adsorbing NO3-39.

To unravel the mechanism of photocatalytic C-N coupling, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to monitor the reaction intermediates in real-time. The DRIFTS spectra and corresponding contour projection diagrams of NU-1000, Co-HHTP and NU-1000@Co-HHTP are shown in Supplementary Figs. 22, 24 and Fig. 5g, h. In the spectra of NU-1000@Co-HHTP collected in darkness, the peaks assigned to the adsorbed CO2, H2O and NO3- are observed at 3555, 1642 and 978 cm⁻¹, respectively38,40–43. Under light illumination for 20 min, the peak intensities of NO3- and H2O are reduced with an additional peak of *NO intermediate generated at 1886 cm⁻¹44. At 40 min, the peaks of *NO and CO2 are synchronously weakened, accompanied with the formation of *O2CNO at 2056 cm⁻¹, a key C-N coupling intermediate via the interaction between CO2 and *NO45. Compared to the widely recognized *OCNO intermediate (∼2075 cm⁻¹)45, the peak of *O2CNO exhibits a redshift of ~19 cm⁻¹, which maybe attributed to the lower C-N bond order of *O2CNO than that of *OCNO. In addition, other peaks attributed to the bending and wagging modes of -NH2 (1206 and 3630 cm⁻¹) and stretching mode of C-N bond (1425 cm⁻¹) of urea also emerge5,46,47. By prolonging the reaction time to 60 min, the peak intensities of CO2, H2O, NO3- and *NO further decrease while those of *CO2NO and urea are gradually intensified, indicating the continuous occurrence of P-NitRR&CRR toward urea production.

For NU-1000 (Supplementary Fig. 22a, b), the spectra collected under dark conditions also exhibit strong peaks of the three reactants. After irradiation of 60 min, the adsorbed CO2 is largely consumed with only marginally reduced NO3- peak. The peaks of *O2CNO intermediate and urea are much weaker than those of NU-1000@Co-HHTP. Moreover, the peak at 1460 cm⁻¹ belongs to the *CO intermediate, suggesting CO as the main byproduct from CO2RR over NU-100048. In addition, the absence of *CO signals in the in-situ DRIFTS spectra for NU-1000@Co-HHTP and Co-HHTP maybe ascribed to the low CO2 reduction activity and weak *CO adsorption at the Co sites, which result in undetectably low surface *CO concentrations49–51. The speculation is further supported by the CO temperature-programmed desorption (CO-TPD) measurement. As shown in Supplementary Fig. 23, the CO-TPD profile of NU-1000 exhibits two distinct peaks at 133 and 390 °C, assigned to the physisorption and chemisorption of CO, respectively. For Co-HHTP, both the physisorption and chemisorption temperature is significantly lower than that of NU-1000, along with reduced peak intensities, indicating weaker CO adsorption at Co sites of Co-HHTP than NU-1000. After forming heterojunction, the CO adsorption strength of NU-1000@Co-HHTP lies between that of the two individual components. In contrary to NU-1000 (Supplementary Fig. 24a, b), the photocatalytic reaction over Co-HHTP is dominated by NitRR to NH3 (1301 cm⁻¹)52,53. The limited CO2 adsorption and activation abilities result in the low C-N coupling efficiency. The evolution trend of byproducts and products in in-situ DRIFTS spectra well match with the photocatalytic results.

Linear sweep voltammetry (LSV) method was employed to assess the CO2 and NO3- reduction activity of the samples. As shown in Supplementary Figs. 25 and 26a–c, the current densities of NU-1000@Co-HHTP in the presence of NO3-, CO2 or both NO3- and CO2 are larger than that in 0.5 M Na2SO4 without reactants. Moreover, NU-1000 exhibits higher current densities than Co-HHTP in the CO2-saturated Na2SO4 electrolyte (Supplementary Fig. 26a), whereas a reversed trend is observed in Ar-saturated KNO3 electrolyte, indicating the predominated CO2RR and NitRR process over NU-1000 and Co-HHTP, respectively. Upon forming heterojunction, both the CO2 and NO3- activation capabilities of NU-1000@Co-HHTP are enhanced compared to the individual materials. Moreover, in CO2-saturated KNO3 electrolyte, NU-1000@Co-HHTP exhibits a significantly lower onset potential and higher current density than NU-1000 and Co-HHTP, demonstrating facilitated co-reduction reaction of CO2 and NO3- for C-N coupling (Supplementary Fig. 26c).

Owing to the much lower solubility and diffusion efficiency of CO2 compared to NO3- in aqueous media, CO2 adsorption capacity of catalysts has been recognized as a critical rate-limiting factor that governs the C-N coupling efficiency14,54–56. Therefore, CO2 sorption and CO2-TPD were further carried out to investigate the CO2 adsorption behaviors on different samples. As shown in the CO2 sorption isotherms (Supplementary Fig. 26d), NU-1000@Co-HHTP exhibit a CO2 adsorption capacity of 44.76 cm³ g⁻¹ at 273 K (Supplementary Table 8), locating between NU-1000 (68.43 cm³ g⁻¹) and Co-HHTP (18.18 cm³ g⁻¹). The CO2-TPD profiles of NU-1000, Co-HHTP and NU-1000@Co-HHTP are presented in Supplementary Fig. 26e, where the peaks in the low-temperature region (<200 °C) and high-temperature (200-500 °C) refer to the physical and chemical adsorption of CO2, respectively. The temperature and area of physisorption peak follow the order of NU-1000 > NU-1000@Co-HHTP > Co-HHTP, possibly ascribed to the largest specific surface area of NU-1000. The chemisorption temperature of NU-1000 is also higher than Co-HHTP but lower than NU-1000@Co-HHTP, suggesting the promoted CO2 chemisorption by forming heterojunction. The results of photocatalysis tests, in-situ DRIFTS spectra, LSV curves, CO2 sorption isotherms and CO2-TPD spectra collaboratively reveal that NU-1000@Co-HHTP simultaneously integrates the strong CO2 adsorption capability and NitRR activity, resultantly improving the C-N coupling efficiency for urea production.

Subsequently, the charge separation and transfer kinetics of the samples were examined by using photoelectrochemical (PEC) and photoluminescence (PL) measurements. As shown in Supplementary Figs. 26f and Fig. 27a, NU-1000@Co-HHTP has the highest transient photocurrent response and smallest arc radius among all samples, indicating its boosted charge separation efficiency. The fitted EIS spectra and corresponding equivalent circuit are displayed in Supplementary Fig. 27a and Table 9. When integrated with the highly conductive Co-HHTP (80.12±2.45 Ω cm⁻²), NU-1000@Co-HHTP shows a lower charge transfer resistance (Rct) of 65.23±2.17 Ω cm⁻² than that of NU-1000 (179.80±4.23 Ω cm⁻²) and NU-1000/Co-HHTP (84.95 Ω±4.56 cm⁻²), implying the stronger transport capability. In addition, the charge recombination behavior was then explored by open-circuit voltage decay (OCVD) measurements (Supplementary Figs. 26g and 27b). Compared to NU-1000, Co-HHTP and NU-1000/Co-HHTP, NU-1000@Co-HHTP affords higher open-circuit voltage under irradiation and slower photovoltage decay after the light is turned off, suggesting the slowest carrier recombination kinetics and longest photogenerated charge lifetime56,57. In the steady PL spectra (Supplementary Fig. 26h), NU-1000@Co-HHTP possesses the lowest fluorescence intensity with inhibited photogenerated electron-hole recombination. The charge carrier dynamics were further probed by time-resolved PL (TRPL) spectroscopy fitted with a biexponential function (Supplementary Fig. 26i). As seen, NU-1000@Co-HHTP exhibits an obviously longer average fluorescence lifetime of charge carriers (28.69 ns) than NU-1000 (6.31 ns), Co-HHTP (18.20 ns) and NU-1000/Co-HHTP (14.32 ns), demonstrating the longer-lived charge pairs in NU-1000@Co-HHTP. The improvements in charge transfer and separation also contributes to the enhanced photocatalytic efficiency of NU-1000@Co-HHTP.

To further understand the enhanced performance of NU-1000@Co-HHTP, density functional theory (DFT) calculations were performed. Density functional theory (DFT) calculations (Supplementary Data 1 and 2) were performed to support the experimental findings. The charge difference distribution diagram (Fig. 6a) shows the electron accumulation around the Zr atoms and electron depletion from the Co atoms, suggesting the electron transfer from NU-1000 to Co-HHTP, in accordance with the XPS and UPS results. Besides, the adsorption energies of NO3- and CO2 reactants on different sites for NU-1000, Co-HHTP and NU-1000@Co-HHTP were calculated (Fig. 6b and Supplementary Fig. 28). For Co-HHTP, the adsorption energy of *NO3- on Co is -0.96 eV, significantly lower than that of CO2 (-0.12 eV, Fig. 5b), suggesting the preferential *NO3- adsorption on Co sites. In contrast for NU-1000, the CO2 adsorption is more favorable on Zr sites, as supported by the lower adsorption energy of CO2 (-0.37 eV) than NO3- (-0.16 eV). Compared to Co-HHTP and NU-1000, the adsorption energies of NO3- (-1.33 eV) on Co sites and CO2 (-0.52 eV) on Zr sites are more negative, revealing the enhanced reactant adsorption within the heterostructure.

Furthermore, the Gibbs free energy change (ΔG) of each elementary step during the NitRR@CRR process are presented in Fig. 6c with the corresponding adsorption configurations illustrated in Supplementary Figs. 29 and 30. According to the S-scheme charge transfer mechanism, the Co centers serve as reduction sites within NU-1000@Co-HHTP. Therefore, the reaction pathway for urea synthesis was initiated from NO3- reduction. As seen, the potential determination step (PDS) for NU-1000@Co-HHTP was recognized as the first C-N coupling step (*NO + CO2 → *CO2NO) with ΔG of 0.2 eV, lower than that for Co-HHTP (0.58 eV). A similar trend is also found in the second C-N coupling step (*CONH2 + *NO → *CONONH2) with ΔG of 0.15 and 0.44 eV for NU-1000@Co-HHTP and Co-HHTP, respectively. The observations suggest the promoted C-N coupling over NU-1000@Co-HHTP. To gain further insight into the C-N coupling step, the transition state (TS) profiles and optimized configurations of the PDS for the two samples are displayed in Figs. 6d, e and Supplementary Fig. 31. The energy barrier from *NO on Co site to TS (ΔGTS) via the coupling with *CO2 on neighboring Zr site is calculated to be 0.3 eV for NU-1000@Co-HHTP, significantly lower than that for Co-HHTP (0.79 eV) that experiences the C-N coupling between *NO and free CO2. Therefore, the coupling between *NO and CO2 is kinetically more favorable over NU-1000@Co-HHTP with interfacial Zr-Co dual sites, facilitating the formation of C-N bonds toward boosted urea synthesis. Given the difficulties in modelling the exact structures under catalytic conditions using DFT calculations, the theoretical results primarily serve to support the experimental findings.

The experimental results with the aid of DFT calculations reveal the crucial role of MOF-on-MOF S-scheme heterojunction with interfacial dual sites in enhancing photocatalytic C-N coupling efficiency for urea synthesis. During the photocatalysis process, the Co sites within the NU-1000@Co-HHTP heterojunction are responsible for the adsorption and reduction of NO3- to *NO intermediate. The adjacent Zr sites at the interface supply CO2 for the C-N coupling with *NO on Co sites, reducing the energy barrier for the formation C-N intermediates (e.g., *NOCO2 and *CONONH2). For NU-1000 with single Zr sites and Co-HHTP with single Co sites, it is not noted that the adsorption energy of either Zr or Co sites for both CO2 and NO3- adsorption is negative. Thus, the chance of urea formation via P-NitRR&CRR cannot be avoided, as evidenced by the formation of urea with a low yield in the two individual MOF catalysts. However, the adsorption energy of Co sites in Co-HHTP for CO2 is the highest among all test groups, the same is the NO3- adsorption energy of Zr sites in NU-1000, in contrast to NU-1000@Co-HHTP that has the lowest adsorption energy for both CO2 and NO3- adsorption. Therefore, the overall performance of NU-1000 and Co-HHTP is significantly inferior than NU-1000@Co-HHTP. In addition, the S-scheme charge transfer pathway facilitates the electron-hole separation and offers strong redox capability. Collectively, the enhanced performance of urea photosynthesis is achieved.

Methods
Chemicals
Zirconyl chloride octahydrate (ZrOCl2·8H2O, 98%), cobalt(II) acetate tetrahydrate (Co(CH3COO)2·4H2O, 99%), 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl)tetrabenzoate (H4TBAPy, 98%), 4-phenylbenzoic acid (98%), 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP, 99%), ferric chloride anhydrous (FeCl3, 99.99%), diacetyl monoxime (99%), thiosemicarbazide (99%), p-aminobenzenesulfonamide (99%), N-(1-naphthyl) ethylenediamine dihydrochloride (99%), N,N-dimethylformamide (DMF, 99.8%), 1-propanol (C3H8O, ≥95.0%), 1-propanol (C3H8O, ≥95.0%) and ethanol (C2H5OH, 99%) were purchased from Adamas-beta. Sodium hypochlorite solution (NaClO, 6-14%), potassium hydroxide (KOH, 96%), sodium hydroxide (NaOH, 96%), sodium salicylate (C7H5NaO3, 99.5%) and sodium nitroferricyanide dehydrate (C5FeN6Na2O, 99%) were purchased from Aladdin. Sodium citrate (98%), sulfuric acid, salicylic acid (99%), hydrochloric acid (36-38%), sulfuric acid (98%) and phosphoric acid (85%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the solutions used in the experiments were prepared by ultrapure water. All the materials were used as received without further purification.

Synthesis of NU-1000
In a typical synthesis, 40 mg of ZrOCl2·8H2O and 1.8 g of 4-phenylbenzoic acid were dissolved in 5 mL of DMF in a vial. 10 mg of TBAPy was dissolved in 3 mL of DMF in another vial. The two vials were heated at 90°C for 1 h. After cooling down to room temperature, the two solutions were mixed by sonication treatment for 10 min. The resultant suspension was then heated at 120°C for 1 h. Finally, the products were collected by centrifugation, washed with DMF and dried at 55°C for further use.

Synthesis of NU-1000@Co-HHTP
5 mg of NU-1000 was dispersed into a mixture solution containing 4 mL of 1-propanol and 3.5 mg of HHTP by sonication for 20 min. Subsequently, 4 mL of Co(CH3COO)2·4H2O aqueous solution (0.8 mg mL⁻¹) was poured into the above suspension. By further heating at 55°C for 2 h, the final products were collected by centrifugation, washed with H2O and ethanol for three times and dried overnight.

Synthesis of Co-HHTP
Co-HHTP nanorods were prepared by using the similar synthesis process of NU-1000@Co-HHTP except without the addition of NU-1000.

Material characterization
Scanning electron microscopy (SEM) images were collected by scanning electron microscope (HITACHI-S4800). Transmission electron microscopy (TEM) were performed on a JEOL 2100F instrument at 200 KV. A field-emission scanning electron microscope (Helios G4 UX, FEI Inc. USA) equipped with a focused ion beam (FIB) system and an energy-dispersive X-ray spectroscopy (EDS) detector (X-Max 150T, Oxford Instruments, UK) was used to probe the elemental distribution. X-ray diffraction (XRD) patterns were acquired on a diffractometer (Bruker D8 Advanced) equipped with Cu Kα X-ray source (λ=0.154 nm). X-ray photoelectron spectroscopy (XPS) tests were carried out on a PHI Quantera II ESCA instrument by using Al Kα radiation (1486.8 eV). The concentrations of metal ions were measured by Agilent 730 inductively coupled plasma-optical emission spectrometry (ICP-OES). Diffuse reflectance UV-vis absorption spectroscopy was recorded on a UV-vis spectrophotometer (Lambda 950). N2 sorption measurements and CO2 adsorption were performed on a Micromeritics ASAP-2460 at 77 K and 273 K, respectively. The samples were degassed in vacuum at 423 K for 24 h before analysis. The temperature-programmed desorption of CO2 (CO2-TPD) was carried out on a Micromeritics Chemisorb 2750. Ultraviolet photoelectron spectroscopy (UPS) measurements were carried out on an ESCALAB 250XI setup by using a monochromatic He (21.22 eV) radiation source. PL spectra were recorded on an Edinburgh FS5 spectrofluorometer. Time-resolved photoluminescence (TRPL) spectra were collected on a FSL980 transient fluorescence spectrometer. FTIR spectra were recorded on a Nicolet Fourier spectrophotometer using KBr pellets. A nuclear magnetic resonance (NMR) spectrometer (600 MHz 1H NMR, Bruker AVANCE III HD) was used to quantify urea and ammonia concentration.

Evaluation of photocatalytic activity
Typically, 20 mg of photocatalyst was dispersed in 40 mL of KNO3 (1 M) by ultrasonication for 10 min. After continuously bubbled with CO2 for 30 min to remove air residual, a 300 W xenon lamp (PLS-SXE300D/300DUV, Perfect Light) was used as the visible light source (λ>400 nm) with a light intensity of 100 mW cm⁻² to initiate the reaction. The photocatalytic reaction was conducted for 4 h under stirring at 600 rpm and the temperature of the reaction solution was kept at 25°C by an external circulating cooling water system. Throughout the reaction, 1 mL of reaction solution was collected at regular time intervals (1 h) and filtrated with 0.22 µm filters to remove the photocatalysts for quantitative analysis. The produced urea in the reaction solution was quantified by diacetylmonoxime-thiosemicarbazide (DAMO-TSC) and NMR methods, respectively. All error bars of the results were derived from three independent replicate experiments.

Calculation methods for apparent quantum yield (AQY)
To calculate the AQY, the incident light was passed through a monochromator with the band pass filter at wavelengths of 365 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, and 650 nm as the light source, respectively. In addition, an optical power meter (PL-MW2000, Beijing Perfectlight Technology) was utilized to detect the light intensity. The errors of urea yield represent the standard deviations of three independent measurements. The AQY was calculated by the following equation:
AQY(%) = (number of reacted electrons / number of incident photons) × 100%
= (number of evolved urea molecules × 16 / number of incident photons) × 100%

Determination of product concentration
Urea quantification by DAMO-TSC method. Firstly, 500 mg of diacetylmonoxime and 10 mg of thiosemicarbazide were dissolved in 100 mL of H2O to prepare reagent A, and 10 mL of H3PO4, 30 mL of H2SO4, and 10 mg of FeCl3 were dissolved in 60 mL of H2O to prepared reagent B. Afterwards, 1 mL of the reaction solution was collected by mixing 1 mL of reagent A and 2 mL of reagent B. After reacting in a boiling water bath at 100°C for 20 min, the absorbance of the mixture solution was determined by UV-vis spectrophotometer at 655 nm for quantifying the urea concentration according to the standard calibration curve.
1H NMR measurement was also conducted to quantitatively detect urea. 5 mL of the reaction solution was collected, concentrated via rotary evaporation, and subsequently redissolved in a mixture containing 500 µL of DMOS solution, 50 µL of DMSO-d6 and 20 µL of maleic acid (1 ppm). The resulting solution was transferred to an NMR tube for analysis.
Determination of NO2-. The mixed solution of p-aminobenzenesulfonamide (4 g), N-(1-naphthyl) ethylenediamine dihydrochloride (0.2 g), DI water (50 mL) and phosphoric acid (10 mL) was used as the color reagent. 200 µL of reaction solution was mixed with 0.1 mL of color reagent. After sitting for 20 min, the absorption intensity of the solution was measured by a UV-vis spectrophotometer at a wavelength of 540 nm. The concentration of NO2- was then quantified according to the standard curve (Supplementary Fig. 16)40.
Determination of NH3. The concentration of ammonia product was measured by NMR. 5 mL of 0.1 M HCl, 50 µL of D2O and 5 mL of maleic acid (50 ppm, an internal standard solution) were added into 5 mL of the diluted reaction solution. Afterwards, the concentration of generated NH3 was determined by NMR according to the standard curve (Supplementary Fig. 17)40.

Photoelectrochemical test
Photoelectrochemical measurements were performed on a Chenhua CHI760E electrochemical workstation in 0.1 M Na2SO4 electrolyte (pH = 7 ± 0.2). A typical three-electrode setup was employed, consisting of a Pt wire counter electrode, an Ag/AgCl reference electrode, and a working electrode made from photocatalyst-modified indium tin oxide (ITO). The detailed procedure for preparing working electrode was as follows: 20 mg of the catalysts was first dispersed in 60 µL of a 10% Nafion solution. The resultant slurry was then drop-casted onto a 1 cm² area of ITO glass and dried at 55°C overnight. To record photocurrent signals, a 300 W xenon lamp (PLSLight, model SXE300D/300DUV) was used as the light source. EIS measurements were performed using an AC perturbation signal of 10 mV over a frequency range of 0.1-105 Hz. Mott-Schottky plots were examined with a scan rate of 5 mV s⁻¹ at 500 Hz and 1500 Hz. During the transient open-circuit voltage decay (OCVD) tests for 800 s, the visible-light source was switched on and off with a time interval of 300 s. The average carrier lifetimes (τn) were determined from the open-circuit voltage (Voc) decay using the following formula:
τn = (kBT / q) × (dt / dVoc)
where τn represents the average lifetime, Voc is open-circuit voltage, kB is the Boltzmann constant, T is the temperature (in Kelvin), and q is the unsigned charge of an electron.
The flat-band potential values are determined using the Mott-Schottky equation:
1/C2 = 2 / (ε0εND) × (E - Efb - kBT/q)
where C is the space charge capacitance, ND is the donor density, ε0 and ε are the dielectric constants of free space and the film electrode, respectively, E is the applied potential, Efb is the flat-band potential, kB is Boltzmann constant, T is the temperature, and q is the electronic charge. The Efb value can be determined from the extrapolation to 1/C2=0. For n-type semiconductors, the conduction bands are 0.1 eV higher than the flat potentials. All the potentials were calibrated with a reversible hydrogen electrode (RHE)
ERHE = EAg/AgCl + 0.0591 × pH + 0.197
All the electrochemical tests were performed without IR compensation.

In situ diffuse reflectance infrared Fourier transform spectroscopy measurements
40 mg of photocatalyst was vigorously stirred in 25 mL of 1.0 M KNO3 solution for 6 h, followed by centrifugation and drying at 55°C for 12 h. Afterwards, 25 mg of the NO3- adsorbed catalyst was placed in a diffuse reflectance cell at room temperature with continuous CO2 (99.999%) purging. The time-dependent FTIR profiles were then recorded under 300 W Xeon lamp irradiation.

Computational details
Vienna Ab initio Simulation Package (VASP) was employed to perform the Spin-polarized density functional theory (DFT) calculations58,59. The generalized gradient approximation (GGA) in combination with the Perdew–Burke–Ernzerhof (PBE) functional was used to describe the exchange–correlation interactions60. In addition, the relation between ionic cores and valence electrons was modeled using the projected augmented wave (PAW) method61,62, and the electronic wave functions were expanded in a plane-wave basis set with a kinetic energy cutoff of 500 eV. Gaussian smearing approach with a width of 0.05 eV was applied to simulate the partial occupancies of the Kohn-Sham orbitals. Structural optimizations were conducted until the total energy and force converged to within 10⁻⁵ eV and 0.02 eV/Å, respectively. The initial models were built according to the standard crystal structures of NU-1000 and Co-HHTP. To minimize undesired periodic interactions, a vacuum layer of 18 Å was introduced along the direction normal to the material plane. For Brillouin-zone sampling during relaxation and self-consistent calculations, a 2×1×1 Monkhorst–Pack k-point mesh was employed63. The adsorption energies (Eads) of reactants and intermediates were determined according to the formula of Eads = Ead/sub - Ead - Esub, where Ead/sub, Ead, and Esub refer to the total energies of optimized adsorption configuration, individual adsorbate and substrate, respectively. Besides, the Gibbs free energy (G) was determined based on the equation:
G = E + ZPE - TS
where E is the total energy, ZPE refers to the zero-point energy, and TS represents the entropic contributions.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "“Urea Photosynthesis with Carbon Dioxide and Nitrate over A MOF-on-MOF S-Scheme Heterojunction” : the work established the effectiveness of NitRR&CRR photocatalysts for urea production. Some important remarks about the manuscript are:\n1. In page 1 line no 24, the authors mentioned : “ Photocatalytic co-reduction of NO3- and CO2 (P-NitRR&CRR) holds great promise for sustainable urea synthesis”. Meaning of short form P-NitRR&CRR should be mentioned for the first time somewhere in the manuscript here in abstract. Authors should check thoroughly whole manuscript.\n2. In line 85 “ an apparent quantum yield (AQY) of 1.16% at 365 nm, superior to the reported photocatalysts”. As per authors 1.16% QY is superior. Does it mean no other photocatalyst exhibits better QY than this? Authors should justify their claim.\n3. In line 107 it is mentioned “ Fig. 1c further demonstrates the formation of rod-on-rod core-shell heterostructure”. It is not clear how authors clarify the image as core-shell structure. It is not clear from given image.\n4. FTIR analysis is not very clear. There are many strong peaks which have not demonstrated. For example the peak around 1600 in Co HHTP.\n5. In line 153 it is mentioned “ significantly enhanced light absorption intensity than NU-1000 from UV to near-infrared region” . What is the outcome of that? Authors should explain why the enhancement of light absorption intensity?\n6. It seems that authors have performed too many experiments to justify the goal. Some experiments are not required. For example without FTIR analysis also the same goal can be achieved.\nFor Nature communication publication the work should be very much effective as per society need. The heading was interesting but the overall manuscript is very much crowed with too many experiments. Manuscript is not suitable for publication in the present form.", "reviewer_2": "General Comments: In this study, Xi et al. reported the design of a NU-1000@Co-HHTP S-scheme heterojunction photocatalyst with interfacial dual sites for P-NitRR&CRR, achieving a high urea yield of 3523.4 μg g-1 h-1. The MOF-on-MOF photocatalyst design is interesting and the dual-site catalytic mechanism was elucidated via a series of in-situ characterizations and theoretical calculations. Overall, the paper is well organized and will make a valuable contribution to the field of photocatalysis and green urea synthesis. I am pleased to recommend its publication after following issues are addressed.\nComment 1. Test results of the oxidation reaction over NU-1000@Co-HHTP in water without sacrificial agents should be provided.\nComment 2. The EIS spectra should be fitted with corresponding equivalent circuit included.\nComment 3. The authors report urea production in terms of μg g-1 h-1. However, the time course of urea production was not shown.\nComment 4. The authors examined the conduction band potentials of via Mott-Schottky plots. In order to further confirm the band structure shown in Figure 2g, the results of valence-band XPS should also be proved.\nComment 5. Isotopic labeling measurement (e.g., ¹³C) should be performed to verify that the carbon in urea is derived from CO₂, rather than the decomposition of MOF catalysts.\nComment 6. The structural stability is highly important for the evaluation of catalyst stability. The authors should confirm whether there is metal leaching during photocatalytic test.\nComment 7. Scheme S1 should be removed as it duplicates the content presented in Scheme 1a.\nComment 8. There are some minor mistakes in the text and graphs, the authors should carefully check the manuscript. For example, \"References\"-not \"Reference\"; \"exhibits\"-not \" exbibits \"; the legends of x-axis in Figures 5c are missing; the names of NU-1000@CoHHTP, NU-1000@Co HHTP, NU-1000@Co-HHTP should be unified.", "reviewer_3": "In this work, Xi and co-workers reported a MOF-on-MOF S-scheme heterojunction for efficient photocatalytic urea production. By integrating NU-1000 and Co-HHTP, dual Co and Zr active sites were constructed, which synergetically promoted C–N coupling. However, this work lacks novelty because the MOF-on-MOF S-scheme heterojunction has already been reported (Angew. Chem. Int. Ed. 2025, 64, e202502009.). Meanwhile, the data presented in this article do not substantiate the author's standpoint. Thus, the research work is still insufficient to meet the requirements of Nature Communications. The reasons are as follows:\nMajor issues:\n1. The analysis of the S-scheme heterojunction structure was paradoxical. Though the authors employed various measurements to elucidate the charge transfer mechanism within the NU-1000@Co-HHTP heterojunction, the results do not match each other, leading to potentially erroneous conclusions:\n(1) As semiconductors, the Fermi level is typically located between the conduction and valence bands. Thus, the energy of the valence band, relative to the Fermi level, should be smaller than the band gap. However, the UPS spectrum (valence band zone on the right side of Fig. S5) suggested that the energy of the valence band for both Co-HHTP and NU-1000 was significantly larger than their band gaps. This discrepancy in the UPS data led to a misinterpretation of the work functions, thus, the conclusion of the S-scheme heterojunction was not convincing.\n(2) The band structure obtained by Mott-Schottky tests was contradictory to the UPS findings.\n2. Though the authors performed systematic investigations to study the formation of interfacial Co-Zr dual sites, the presented evidence remains inadequate. XPS technique does not have the fine structure resolution characteristics. Thus, the peak at 530.69 eV in the O 1s XPS spectrum of NU-1000@Co-HHTP, positioned between the Co–O peak of Co-HHTP and Zr–O peak of NU-1000, was more likely indicative of a signal overlap rather than the formation of Zr–O–Co chemical bonds.\n3. The identification of in situ DRIFTS spectra was not rigorous.\n(1) During in situ DRIFTS tests, both H2O and CO2 existed. The adsorbed CO2 and H2O were commonly reported at 2300–2400 cm‒1 (J. Am. Chem. Soc. 2025, 147, 24370.) and 3300–3700 cm‒1 (Angew. Chem. Int. Ed. 2023, 62, e202307808.), respectively. However, the peaks at 3555 cm‒1 were assigned to the adsorbed CO2 in this manuscript.\n(2) The peaks observed at 1425 cm‒1 in the in situ DRIFTS spectra of both NU-1000@Co-HHTP (Fig. 3g and 3h) and NU-1000 (Fig. 3i and 3j) were assigned to C–N and *CO intermediate, respectively.\n(3) No distinct signal corresponding to the *CO intermediate was detected in the in situ DRIFTS spectra of either NU-1000@Co-HHTP or Co-HHTP. How does the CO product form during photocatalytic reactions?\n4. The distinction of reaction pathways is ambiguous. Reference 41, cited by the authors, identified the peak at 2075 cm‒1 as corresponding to the *OCNO species. However, in the in situ DRIFTS spectra of NU-1000@Co-HHTP, the peak at 2056 cm‒1 was assigned to the *CO2NO. How do the authors distinguish the *OCNO and *CO2NO species? Furthermore, why does this work propose *CO2NO as the key intermediate during the C–N coupling process, instead of following the *OCNO pathway?\n5. The authors should compare the consistency of urea production detected by the different assays.\n6. The authors claimed that NU-1000@Co-HHTP exhibited superior carrier kinetics compared to other samples. However, based on the PEC and OCVD results (Fig. 4f, g), the integration of NU-1000 and Co-HHTP actually decreased charge transfer and separation efficiency of Co-HHTP. This result contradicted the findings from the PL and TRPL tests (Fig. 4h, i), which suggested enhanced carrier dynamics.\nMinor issues:\n1. The noise-signal ratio in the 1H NMR spectra decreased as the concentration of the urea standard solution increased. To avoid any misunderstandings, the internal standard should also be included in the 1H NMR spectra.\n2. The standard curves for NO2− and NH3 should also be provided.\n3. HHTP contains abundant chemical bonds between C and O atoms. I wonder why there was no signal corresponding to O–C=O or O–C observed in O 1s XPS spectra of Co-HHTP.\n4. The relative Co2+ peak intensity to satellite (Figure S10b) significantly changed after cycling tests. Is there any other oxidation state of Co formed?\n5. “Fig. 4k, l” in page 12 should be corrected to “Fig. 3k, l”. Besides, there are many spelling and grammar errors in this manuscript (e.g., “donated as NU-1000/Co-HHTP”). The author should check the manuscript carefully.", "reviewer_4": "General comments: In this manuscript, the authors reported a MOF-on-MOF (NU-1000@Co-HHTP) S-scheme heterojunction with interfacial dual sites for urea synthesis via photocatalytic co-reduction of NO₃⁻ and CO₂ (P-NitRR&CRR). The photocatalyst delivered an impressive urea yield of 3523.4 μg g-1 h-1 and a high apparent quantum yield of 1.16% at 365 nm in pure water, outperforming the reported materials. The photocatalytic mechanism was also in depth elucidated by both experimental results and theoretical simulations. The overall experimental design is reasonable, the research content is substantial, and the article logic is clear. Therefore, the manuscript could be accepted after addressing the following concerns.\nComment 1. NU-1000 forms a S-scheme heterojunction with Co-HHTP. What is the difference between S-scheme heterojunction and Z-scheme heterojunction, and how to determine whether the material forms an S-scheme heterojunction instead of a Z-scheme heterojunction?\nComment 2. To further support the crucial role of forming heterojunction, XPS results of physically mixed samples should be provided.\nComment 3. The authors should provide more detail process about the conductive band potential calculation from Mott-Schottky curves and flat band potential.\nComment 4. To conclusively trace the carbon source in urea synthesis, isotopic labeling experiments (e.g., using ¹³C-labeled CO₂) should be performed.\nComment 5. The flat-band potential values of NU-1000 and Co-HHTP presented in Figure 2c are inconsistent with those in the main text. The flat-band potentials of NU-1000 and Co-HHTP should be -0.60 and -0.7 eV, respectively.\nComment 6. The manuscript only describes the co-reduction of CO2 and NO3- to synthesize urea, but does not identify the oxidation process.\nComment 7. The proposed \"*CO2+*NO\" coupling pathway is interesting. However, given that \"*CO (from CRR) + *NO\" coupling has been extensively documented in electrocatalysis literatures, could the authors further clarify how they excluded this alternative pathway in their mechanism?\nComment 8. There are some errors in the manuscript, for instance, the units of conduction band and valance band potential are \"V\" rather than \"eV\"; the position of \"on/off\" labels in Figure 4f is incorrect; the sample name should be unified throughout the whole manuscript.", "reviewer_5": "The work by Chao Liu reports a MOF S-scheme heterojunction catalyst with dual Zr and Co sites that promotes efficient C-N coupling for the photochemical synthesis of urea from NO3 and CO2. The authors perform synthesis and characterisation of the material using TEM, elemental mapping and XRD. Optical properties of the materials are analysed with XPS and UV-vis methods. The area of the catalyst was measured with N2 adsorption/desorption or BET measurements, and the adsorption properties towards NO3 and CO2 were also evaluated. Phochemical and photoelectrochemical measurements were run to assess performance. FTIR and NMR were used to address the reaction mechanism and product formation. This paper shows a photocatalytic material with interesting properties toward urea production via the photosynthetic process, which I think is valuable knowledge for the scope and the readership of this journal. However, I think different parts of the paper could be discussed more. Below, I add my comments:\n1. I understand that the presence of dual sites or vicinal Zr and Co is key to catalysing the carbon-nitrogen coupling step that is limiting for the production of urea. The need for dual sites for C-N coupling is also observed in electrocatalysis. However, the other photocatalysts reported in this work, with single sites of Co or Zr, or a mixture of them (1000-Nu/CoHHTP), also produce a significant amount of urea, despite not having dual sites. What explains the production of urea in the other catalysts?\n2. Could urea be produced if there were either Zr dual sites or Co dual sites rather than Co-Zr?\n3. Figure 4 shows the photoelectrochemical current of the different materials toward different environments containing KNO3, CO2, or both. How is the current in the blank electrolyte, I mean, without reactant (NO3 or CO2), only in Na2SO4?\n4. The authors determine the area of the catalysts with N2 adsorption, but is the electroactive area the same? Is it possible to provide capacitance measurements of the catalysts to get information on the amount of Co and Zr sites available for catalysis?\n5. What is the loading or amount of Co and Zr in each catalyst?\n6. The authors explain the presence of dual or vicinal Co and Zr sites that enhance the production of urea, but which characterisation method confirms or supports that they form dual sites in the prepared material?\n7. The authors evaluate the catalyst structure after photocatalysis, but how long does it take to carry out each cycle, or for how long is the urea synthesis carried out in hours?\n8. I suggest incorporating a table that compares the state of the art of photo and electrocatalysts for the production of urea. How is the performance of this catalyst compared to other ones reported in the literature?"}, "author_response": "Point-to-point response and revisions\n(Original Manuscript ID: NCOMMS-25-53753)\nReviewer 1:\nGeneral comments:\n“Urea Photosynthesis with Carbon Dioxide and Nitrate over A MOF-on-MOF S-\nScheme Heterojunction”: the work established the effectiveness of NitRR&CRR\nphotocatalysts for urea production. Some important remarks about the manuscript are:\nResponse: We thank Reviewer 1 for the comments. Your suggestions have encouraged\nus to further improve the quality of our work.\nComment 1. In page 1 line no 24, the authors mentioned: “Photocatalytic co-reduction\nof NO - and CO (P-NitRR&CRR) holds great promise for sustainable urea synthesis”.\n3 2\nMeaning of short form P-NitRR&CRR should be mentioned for the first time\nsomewhere in the manuscript here in abstract. Authors should check thoroughly whole\nmanuscript.\nResponse: We thank Reviewer 1 for the helpful comments. We have thoroughly\nreviewed whole manuscript and ensured that all abbreviations are defined when they\nare first mentioned.\nThe corresponding descriptions have been added in Page 1 in the revised manuscript as\nfollows:\n\"Photocatalytic nitrate reduction reaction coupled carbon dioxide reduction reaction (P-\nNitRR&CRR) holds great promise for sustainable urea synthesis, the design of\nphotocatalysts with efficient C-N coupling and charge separation is crucial yet\nchallenging.\"\nComment 2. In line 85 “an apparent quantum yield (AQY) of 1.16% at 365 nm,\nsuperior to the reported photocatalysts”. As per authors 1.16% QY is superior. Does it\nmean no other photocatalyst exhibits better QY than this? Authors should justify their\nclaim.\nResponse: We appreciate Reviewer 1’s comment. There is only one report on the P-\nNitRR&CRR for urea production with an AQY value of only 0.02%, largely inferior\nthan our results. Further clarifications have been added in the revised manuscript as\nfollows:\n\"A maximum AQY value of 1.16% is achieved at 365 nm (Fig. 4c), which largely\nsurpasses the state-of-the-art P-NitRR&CRR catalysts.13\"\nComment 3. In line 107 it is mentioned “Fig. 1c further demonstrates the formation of\nrod-on-rod core-shell heterostructure”. It is not clear how authors clarify the image as\ncore-shell structure. It is not clear from given image.\nResponse: We appreciate Reviewer 1’s thoughtful comment. To clearly demonstrate\nthe core-shell structure of the NU-1000@Co-HHTP, the SEM image of one broken\nparticle is added in the revised manuscript as Fig. S2 in the revised manuscript as below.\nFig. S2 SEM image of one broken NU-1000@Co-HHTP particle.\n\"From the SEM image of one broken particle (Fig. S2) and the TEM image (Fig. 1c),\nthe formation of rod-on-rod core-shell heterostructure is demonstrated.\"\nComment 4. FTIR analysis is not very clear. There are many strong peaks which have\nnot demonstrated. For example the peak around 1600 in Co-HHTP.\nResponse: We thank Reviewer 1 for the insightful comment. The peak at ≈1655 cm⁻¹\nin the spectrum of Co-HHTP can be attributed to O-H bending vibrations from adsorbed\nwater molecules (Ref. J. Am. Chem. Soc. 2024, 146, 18061-18073; Angew. Chem. Int.\nEd. 2025, e202505046).\nCorresponding descriptions have been added in Page 7 in the revised manuscript as\nfollows:\n\"In addition, the peak of adsorbed H O is also found at 1655 cm-¹.\"\nintensity than NU-1000 from UV to near-infrared region”. What is the outcome of that?\nAuthors should explain why the enhancement of light absorption intensity?\nResponse: We are grateful for Reviewer 1’s helpful comment. The significantly\nenhanced light absorption of NU-1000@Co-HHTP is originated from the hybridization\nwith Co-HHTP, which exhibits a narrower bandgap (∼1.40 eV) than NU-1000 (2.65\neV).\nThe corresponding descriptions have been added in Page 8 in the revised manuscript.\n\"After combination with Co-HHTP with a narrower bandgap of 1.40 eV, NU-\n1000@Co-HHTP shows significantly enhanced light absorption intensity than NU-\n1000 from UV to near-infrared region.\"\nComment 6. It seems that authors have performed too many experiments to justify the\ngoal. Some experiments are not required. For example without FTIR analysis also the\nsame goal can be achieved.\nResponse: We thank Reviewer 1 for the thoughtful comments. The FTIR spectra have\nbeen moved to the supporting information.\nFor Nature communication publication the work should be very much effective as per\nsociety need. The heading was interesting but the overall manuscript is very much\ncrowed with too many experiments. Manuscript is not suitable for publication in the\npresent form.\nResponse: We thank Reviewer 1 for the constructive comments. In the revised\nmanuscript, we have optimized the data presentation in the main text to better highlight\nthe main conclusions. Specifically, the FTIR spectra, the in-situ DRIFTS spectra of NU-\n1000 and Co-HHTP, as well as the original Figure 4 have been moved to the supporting\ninformation. Furthermore, N sorption isotherms have been moved from the supporting\n2\ninformation to the main text.\nReviewer 2:\nComments:\nIn this study, Xi et al. reported the design of a NU-1000@Co-HHTP S-scheme\nheterojunction photocatalyst with interfacial dual sites for P-NitRR&CRR, achieving a\nhigh urea yield of 3523.4 μg g-1 h-1. The MOF-on-MOF photocatalyst design is\ninteresting and the dual-site catalytic mechanism was elucidated via a series of in-situ\ncharacterizations and theoretical calculations. Overall, the paper is well organized and\nwill make a valuable contribution to the field of photocatalysis and green urea synthesis.\nI am pleased to recommend its publication after following issues are addressed.\nResponse: We thank Reviewer 2 for the positive comments. Your suggestions have\nencouraged us to further improve the quality of our work. In the revised manuscript, we\nhave conducted additional experiments with further revisions to address your concerns\n(see details below).\nComment 1. Test results of the oxidation reaction over NU-1000@Co-HHTP in water\nwithout sacrificial agents should be provided.\nResponse: We are grateful for Reviewer 2’s helpful comment. Generally, the\nphotogenerated holes are consumed for water oxidation into O in sacrificial agent-free\n2\nsystems. Therefore, the generated amount of O has been quantified, following the order\n2\nof NU-1000@Co-HHTP (962.3 μmol g-1 h-1) > Co-HHTP (213.2 μmol g-1 h-1) > NU-\n1000/Co-HHTP (184.9 μmol g-1 h-1) > NU-1000 (101.2 μmol g-1 h-1). The results have\nbeen added as Fig. S18 in the revised manuscript as below.\nFig. S18 O production rates of NU-1000, Co-HHTP, NU-1000/Co-HHTP and NU-\n2\n1000@Co-HHTP.\nThe corresponding descriptions have been added in Page 12 in the revised manuscript\nas follows:\n\"Except for the P-NitRR&CRR process, the coupled water oxidation reaction was also\nexplored. As shown in Fig. S18, the yield of O as the oxidation product follows the\n2\norder of NU-1000@Co-HHTP (962.3 μmol g-1 h-1) > Co-HHTP (213.2 μmol g-1 h-1) >\nNU-1000/Co-HHTP (184.9 μmol g-1 h-1) > NU-1000 (101.2 μmol g-1 h-1).\"\nComment 2. The EIS spectra should be fitted with corresponding equivalent circuit\nincluded.\nResponse: We thank Reviewer 2 for the helpful comments. Equivalent circuit fitting\nhas been performed to analyze the EIS spectra (Fig. S26 and Table S6, see below).\nFig. S26 (a) EIS spectra of NU-1000, Co-HHTP, NU-1000/Co-HHTP and NU-\n1000@Co-HHTP.\nTable S6. Equivalent circuit of EIS plots and calculated R values.\nct\nSample R (Ω cm-2)\nct\nNU-1000 179.80\nCo-HHTP 80.12\nNU-1000/Co-HHTP 84.95\nNU-1000@Co-HHTP 65.23\nThe corresponding descriptions have been added in Page 16 in the revised manuscript\nas follows:\n\"The fitted EIS spectra and corresponding equivalent circuit are displayed in Fig. S26a\nand Table S6. When integrated with the highly conductive Co-HHTP (80.12 Ω cm-2),\nNU-1000@Co-HHTP shows a lower charge transfer resistance (R ) of 65.23 Ω cm-2\nct\nthan that of NU-1000 (179.80 Ω cm-2) and NU-1000/Co-HHTP (84.95 Ω cm-2),\nimplying the stronger transport capability.\"\nComment 3. The authors report urea production in terms of μg g-1 h-1. However, the\ntime course of urea production was not shown.\nResponse: We are grateful for Reviewer 2’s valuable comment. The time course of urea\nproduction has been added as Fig. S14 in the revised manuscript.\nFig. S14 Time courses of urea production for NU-1000, Co-HHTP, NU-1000/Co-HHTP\nand NU-1000@Co-HHTP.\nCorresponding descriptions have been added in the revised manuscript as follows:\n\"As shown in Fig. S13 and S14, NU-1000@Co-HHTP with an optimized dosage of 0.5\nmg mL-1 delivers a urea production amount of 4.70 µmol within 4 h, exceeding NU-\n1000 (0.26 µmol), Co-HHTP (0.89 µmol) and NU-1000/Co-HHTP (0.59 µmol).\nMoreover, the urea production rate of NU-1000@Co-HHTP is calculated to be 3523.4\nµg h⁻¹ g⁻1 (Fig. 3a), ~18.4, 5.3 and 8-folds higher than that of NU-1000 (191.8 µg h-1 g-\n1), Co-HHTP (664.7 µg h-1 g-1) and NU-1000/Co-HHTP (440.9 µg h-1 g-1), respectively,\nindicating the enhanced photocatalytic activity of NU-1000@Co-HHTP.\"\nComment 4. The authors examined the conduction band potentials of via Mott-\nSchottky plots. In order to further confirm the band structure shown in Figure 2g, the\nresults of valence-band XPS should also be proved.\nResponse: According to Reviewer 2’s useful suggestion, valence band XPS (VB-XPS)\nmeasurement has been performed to determine the valance band of photocatalysts. The\nresults have been added as Fig. S10 in the revised manuscript as below.\nCo-HHTP\n)\nu.\na. 0.60 eV\n(\ny\nt\nsi\nn\ne NU-1000\nt\nI\nn\n1.96 eV\n0 2 4 6 8 10\nBinding energy (eV)\nFig. S10 VB-XPS spectra of NU-1000 and Co-HHTP.\nCorresponding revision has been made in Page 11 in the revised manuscript as follows:\n\"Besides, valence band XPS (VB-XPS) measurement was performed to further analyze\nthe valance band of the samples. Based on the VB-XPS spectra (Fig. S10), the valence\nband values of NU-1000 and Co-HHTP were determined as 2.17 and 0.81 eV,\nrespectively. In combination with the results of Mott–Schottky diagrams and Tauc plots,\nthe band structures of NU-1000 and Co-HHTP are depicted in Fig. 3g.\"\nComment 5. Isotopic labeling measurement (e.g., ¹³C) should be performed to verify\nthat the carbon in urea is derived from CO₂, rather than the decomposition of MOF\ncatalysts.\nResponse: We thank Reviewer 2 for the valuable comments. To ensure that the carbon\nin urea is derived from CO , an isotopic labeling experiment has been carried out by\n2\nusing 13CO as the feeding gas. The results have been added as Fig. S19 in the revised\n2\nmanuscript as below.\nFig. S19 13C NMR spectra of the product obtained by using 13CO as feeding gas and\n2\nstandard 13CO(NH ) .\n2 2\nThe corresponding descriptions have been added in Page 13 in the revised manuscript\nas follows:\n\"Upon changing the feeding gas to 13CO , a typical singlet peak of 13CO(NH ) is\n2 2 2\nobserved in the C NMR spectrum (Fig. S19), indicating that the carbon in urea is\nderived from CO₂.\"\nComment 6. The structural stability is highly important for the evaluation of catalyst\nstability. The authors should confirm whether there is metal leaching during\nphotocatalytic test.\nResponse: We appreciate Reviewer 2’s insightful comments. In order to confirm\nwhether there is metal dissolution during the process of photochemical synthesis of urea,\nthe metal content in the reaction solution after continuous test for 4 h was measured by\nICP-OES. Only trace amounts of Co and Zr could be detected (Table S4), implying the\nstructural stability of NU-1000@Co-HHTP.\nTable S4. ICP-OES results of the reaction solution after photocatalytic test.\nSample Zr (mg/L) Co (mg/L)\nReaction solution\n0.024 0.030\nafter test\nThe corresponding descriptions have been added in Page 14 in the revised manuscript\nas follows:\n\"In the supernatant after reaction, only trace amounts of Co and Zr were detected (0.030\nand 0.024 mg/L, respectively). The mass ratios of leached Co and Zr from NU-\n1000@Co-HHTP are thus estimated as 0.07% and 0.03%, respectively, further\ndemonstrating the structural stability of NU-1000@Co-HHTP.\"\nComment 7. Scheme S1 should be removed as it duplicates the content presented in\nScheme 1a.\nResponse: We thank Reviewer 2 for the helpful comment. The Scheme S1 has been\nremoved in the revised supporting information.\nComment 8. There are some minor mistakes in the text and graphs, the authors should\ncarefully check the manuscript. For example, \"References\"-not \"Reference\";\n\"exhibits\"-not \" exbibits \"; the legends of x-axis in Figures 5c are missing; the names\nof NU-1000@CoHHTP, NU-1000@Co HHTP, NU-1000@Co-HHTP should be unified.\nResponse: We thank Reviewer 2 for pointing out our mistakes, which have been\ncarefully revised in the manuscript as below.\n\"As seen, NU-1000@Co-HHTP exhibits an obviously longer average fluorescence\nlifetime of charge carriers (28.69 ns) than NU-1000 (6.31 ns), Co-HHTP (18.20 ns) and\nNU-1000/Co-HHTP (14.32 ns), demonstrating the longer-lived charge pairs in NU-\n1000@Co-HHTP.\"\n\"Fig. 1 (a, b) SEM, (c, d) TEM, (e) HADDF-STEM and element mapping images, and\n(f) line scanning elemental distribution profile collected along the while line in (e) of\nNU-1000@Co-HHTP. (g) XRD patterns, (h) locally enlarged pattern of (g), and (i) N\n2\nsorption isotherms of NU-1000, Co-HHTP and NU-1000@Co-HHTP.\"\nFig. 5 (a) Calculated charge difference distribution of NU-1000@Co-HHTP (the yellow\nand blue regions refer to electron accumulation and depletion regions, respectively), (b)\nadsorption energy of NO - and CO on the Zr and Co sites in Co-HHTP, NU-1000 and\n3 2\nNU-1000@Co-HHTP, (c) Gibbs free energy diagram for NitRR&CRR on Co-HHTP\nand NU-1000@Co-HHTP, (d) energy barrier of coupling between *NO and CO to\n2\n*CO NO for Co-HHTP and NU-1000@Co-HHTP, (e) optimized configurations of TS\n2\nfor Co-HHTP and NU-1000@Co-HHTP. Color code: Co, cyan; Zr, green; C, gray; N,\nblue; O, red; H, white.\nBesides, the names of NU-1000@CoHHTP, NU-1000@Co HHTP, NU-1000@Co-\nHHTP have been unified throughout the whole manuscript.\nReviewer 3:\nGeneral comments:\nIn this work, Xi and co-workers reported a MOF-on-MOF S-scheme heterojunction for\nefficient photocatalytic urea production. By integrating NU-1000 and Co-HHTP, dual\nCo and Zr active sites were constructed, which synergetically promoted C–N coupling.\nHowever, this work lacks novelty because the MOF-on-MOF S-scheme heterojunction\nhas already been reported (Angew. Chem. Int. Ed. 2025, 64, e202502009.). Meanwhile,\nthe data presented in this article do not substantiate the author's standpoint. Thus, the\nresearch work is still insufficient to meet the requirements of Nature Communications.\nThe reasons are as follows:\nResponse: We thank Reviewer 3 for the critical comments. In the literature mentioned\nby you, a Cd-TBAPy/Cu-TCPP S-scheme heterojunction was synthesized for\nphotocatalytic water splitting. MOF-on-MOF hybridization is an emerging but\nimportant field in the research of MOFs, where the design of new structures and new\nfunctions of MOF-on-MOF heterostructures is great significance. In our work, both the\ncombination of NU-1000 and Co-HHTP and the application of urea production are\nreported for the first time in the MOF-on-MOF field. Besides, your concerns on the\ndata presentation are also highly valuable. We have thoroughly revised the manuscript\nto ensure that the claims are well supported by the experimental data (see details below).\nMajor issues:\nComment 1. The analysis of the S-scheme heterojunction structure was paradoxical.\nThough the authors employed various measurements to elucidate the charge transfer\nmechanism within the NU-1000@Co-HHTP heterojunction, the results do not match\neach other, leading to potentially erroneous conclusions:\n(1) As semiconductors, the Fermi level is typically located between the conduction and\nvalence bands. Thus, the energy of the valence band, relative to the Fermi level, should\nbe smaller than the band gap. However, the UPS spectrum (valence band zone on the\nright side of Fig. S5) suggested that the energy of the valence band for both Co-HHTP\nand NU-1000 was significantly larger than their band gaps. This discrepancy in the UPS\ndata led to a misinterpretation of the work functions, thus, the conclusion of the S-\nscheme heterojunction was not convincing.\nResponse: We appreciate Reviewer 3’s thoughtful comments. We agree with you that\nthe valence band maximum (VBM) energy should be smaller than the band gap. We\nhave carefully re-examined the test method and found that the UPS spectra presented\nin the original manuscript (Fig. S8) were acquired at a bias of -10 V, which is generally\nused for measuring the secondary electron cutoff (E ) and the Fermi edge (E )\ncutoff Fermi\nrather than the VBM. To determine the VBM, the UPS measurement has been\nperformed at 0 V bias (Angew. Chem. 2025, e202502009) in this revision. The\ncorresponding spectra of NU-1000 and Co-HHTP have been added as Fig. S9 in the\nrevised manuscript (see below).\nAs depicted in the Fig. S9, the distances from the Fermi level to the VBM (E ) for\ncut on\nNU-1000 and Co-HHTP are 2.22 and 1.24 eV, respectively. Based on the UPS spectra\nat -10 and 0 V, the VBM values (E ) of NU-1000 and Co-HHTP are determined to be\nVB\n6.60 and 5.23 eV, respectively, according to the equation: E = 21.22 − (E - E\nVB cut off Fermi\n- E ). Further using the formula of E (vs. NHE) = E − 4.44, the VB potentials\ncut on VB VB\nrelative to NHE are calculated as 2.16 V for NU-1000 and 0.79 V for Co-HHTP, in\naccordance with the values measured by Mott–Schottky diagrams and Tauc plots.\nFig. S8 UPS spectra of NU-1000 and Co-HHTP at -10 V bias.\nFig. S9 UPS spectra of NU-1000 and Co-HHTP at 0 V bias.\nThe corresponding descriptions have been added in Page 10 in the revised manuscript\nas follows:\n\"Fig. S8 shows the UPS spectra measured at -10 V bias, where the Φ values of NU-\n1000 and Co-HHTP were determined to be 4.38, and 3.99 eV, respectively, using the\nformula of Φ= hv- (E - E ), where E and E represent the secondary\ncutoff fermi cutoff fermi\nelectron cutoff and Fermi edge, respectively. Subsequently, the E values of NU-1000\nf\nand Co-HHTP were calculated to be -4.38 and -3.99 eV (vs vacuum level), respectively,\naccording to the formula of Φ = E – E (E = 0).33 Further based on UPS measurements\nv f v\nperformed without external bias (Fig. S9), the valence band maximum (VBM) positions\nwere determined to be 6.60 eV for NU-1000 and 5.23 eV for Co-HHTP using the\nequation of E = 21.22 − (E - E - E ).20 Then, the VB potentials relative to\nVB cut off Fermi cut on\nNHE were calculated as 2.16 V for NU-1000 and 0.79 V for Co-HHTP according to the\nformula of E (vs. NHE) = E − 4.44, well matching with the values measured by\nVB VB\nMott–Schottky diagrams and Tauc plots.\"\n(2) The band structure obtained by Mott-Schottky tests was contradictory to the UPS\nfindings.\nResponse: We acknowledge Reviewer 3’s constructive comments. We have answered\nto your question in our response to your comment 1-(1).\nComment 2. Though the authors performed systematic investigations to study the\nformation of interfacial Co-Zr dual sites, the presented evidence remains inadequate.\nXPS technique does not have the fine structure resolution characteristics. Thus, the peak\nat 530.69 eV in the O 1s XPS spectrum of NU-1000@Co-HHTP, positioned between\nthe Co–O peak of Co-HHTP and Zr–O peak of NU-1000, was more likely indicative of\na signal overlap rather than the formation of Zr–O–Co chemical bonds.\nResponse: We acknowledge Reviewer 3’s insightful comment. To further probe the\nformation of interface Zr-O-Co bonds, X-ray near-edge absorption spectroscopy\n(XANES) and extended X-ray absorption fine structure (EXAFS) analysis were further\nperformed. The new results have been added as Fig. 2 in the revised manuscript as\nbelow.\nFig. 2 (a) Co K-edge and (d) Zr K-edge XANES spectra of different samples. Insets in\n(a) and (d) are magnified images. (b) Co K-edge and (e) Zr K-edge FT-EXAFS spectra\nof different samples. Wavelet transforms for (c) Co and (f) Zr K-edge EXAFS.\nThe corresponding descriptions have been added in Page 7 in the revised manuscript as\nfollows:\n\"The electronic structure and coordination environment of the samples were\ninvestigated using X-ray absorption near-edge structure (XANES) and extended X-ray\nabsorption fine structure (EXAFS) analysis. As shown in Fig. 2a, the Co K-edge\nabsorption for NU-1000@Co-HHTP exhibits an increased edge energy than that for\nCo-HHTP. In contrast, the Zr K-edge position for NU-1000@Co-HHTP negatively\nshifts compared to that for NU-1000. The observations suggest the electron transfer\nfrom Co-HHTP to NU-1000 within NU-1000@Co-HHTP, resulting in the generation\nof electron deficient Co and electron-sufficient Zr sites. In the Co K-edge Fourier\ntransform (FT) EXAFS spectra (Fig. 2b), the characteristic peak at 1.71 Å is assigned\nto Co-O coordination between Co and HHTP ligands in Co-HHTP. For NU-1000@Co-\nHHTP, apart from the Co-O bond, a distinct peak attributed to the Co-O-Zr bond\nemerges at 2.75 Å (Fig. 2b). Fig. 2e displays the Zr K-edge spectrum of NU-1000,\nwhere the peaks located at around 1.51 and 3.29 Å correspond to the Zr-O and Zr-O-Zr\nbond, respectively. For NU-1000@Co-HHTP, an additional peak of Zr-O-Co bond is\nfound at ≈2.77 Å, in accordance with the results of Co K-edge spectra. Compared to\nthe Zr-O-Zr bond, the shorter bond length of Zr-O-Co is attributed to the smaller radius\nof Co than Zr. Moreover, the derived wavelet transform diagrams (Fig. 2c, f) offer\nvisual evidence for the formation of Zr-O-Co chemical bonds at the NU-1000@Co-\nHHTP heterointerface.\"\nComment 3. The identification of in situ DRIFTS spectra was not rigorous.\n(1) During in situ DRIFTS tests, both H O and CO existed. The adsorbed CO and\n2 2 2\nH O were commonly reported at 2300-2400 cm-1 (J. Am. Chem. Soc. 2025, 147, 24370.)\n2\nand 3300-3700 cm-1 (Angew. Chem. Int. Ed. 2023, 62, e202307808.), respectively.\nHowever, the peaks at 3555 cm-1 were assigned to the adsorbed CO in this manuscript.\nstretching vibrations arising from adsorbed H O or surface hydroxyl groups exhibit a\n2\nbroad spectral band at 3640 cm-1 (e. g., Nat. Commun. 2021, 12, 6141; Nat. Commun.\n2022, 13, 5337; Nat. Commun. 2025, 16,7288; Top Catal. 2017, 60, 1709-1721).\nHowever, in the in-situ DRIFTS spectra in our work, a sharp peak is observed at 3555\ncm⁻¹, which could be attributed to the adsorbed CO according to the literature (e.g.,\nChem. Soc. 2024, 146, 6072; Angew. Chem. Int. Ed. 2025, e202512615).\nCorresponding descriptions have been corrected in Page 14 in the revised manuscript\nas follows:\n\"In the spectra of NU-1000@Co-HHTP collected in darkness, the peaks assigned to the\nadsorbed CO , H O and NO - are observed at 3555, 1642 and 978 cm-1,\n2 2 3\nrespectively.39,40-43\".\n(2). The peaks observed at 1425 cm-1 in the in situ DRIFTS spectra of both NU-\n1000@Co-HHTP (Fig. 3g and 3h) and NU-1000 (Fig. 3i and 3j) were assigned to C–N\nand *CO intermediate, respectively.\nResponse: We thank Reviewer 3 for the helpful comment. We are sorry for the incorrect\ndescriptions. There are two peaks in the region of 1400-1500 cm-1 including one peak\nof C-N bond at 1425 cm-1 and another of *CO at 1460 cm-1. In the revised manuscript,\nFig. 3i has been redrawn to indicate the two peaks more clearly (see below).\nCorresponding descriptions have been corrected in Page 15 in the revised manuscript\nas follows:\n\"Moreover, the peak at 1460 cm-1 belongs to the *CO intermediate, suggesting CO as\nthe main byproduct from CO RR over NU-1000.48\"\n(3). No distinct signal corresponding to the *CO intermediate was detected in the in situ\nDRIFTS spectra of either NU-1000@Co-HHTP or Co-HHTP. How does the CO\nproduct form during photocatalytic reactions?\nResponse: We thank Reviewer 3 for the insightful comments. For NU-1000@Co-\nHHTP and Co-HHTP, the reduction of CO to Co occurs at the Co sites in Co-HHTP.\n160812; Angew. Chem. Int. Ed. 2022, 61, e202210576; Angew. Chem. Int. Ed. 2025, 64,\ne202504423), resulting in the rapid desorption of *CO. Besides, the CRR activities of\nNU-1000@Co-HHTP and Co-HHTP are much lower than that of NU-1000. Possibly\nascribed to the above two reasons, the *CO concentrations on the surface of NU-\n1000@Co-HHTP and Co-HHTP are extremely low, resulting in the absence of *CO\nsignal in the in-situ DRIFTS spectra. The similar phenomenon has also been observed\nin literature (eg., Angew. Chem. Int. Ed.2023, 62, e202307123; Angew. Chem. Int. Ed.\n2024, 63, e202402215; Appl. Catal. B: Environ. 2023, 338, 122962; Appl. Catal. B:\nEnviron. 2023, 336, 122917; Chem. Eng. J. 2025, 507, 160812).\nCorresponding descriptions have been corrected in Page 14 in the revised manuscript\nas follows:\n\"In addition, the absence of *CO signals in the in-situ DRIFTS spectra for NU-\n1000@Co-HHTP and Co-HHTP may be ascribed to the low CO₂ reduction activity and\nweak *CO adsorption at the Co sites, which result in undetectably low surface *CO\nconcentrations.49-51\"\nComment 4. The distinction of reaction pathways is ambiguous. Reference 41, cited\nby the authors, identified the peak at 2075 cm-1 as corresponding to the *OCNO species.\nHowever, in the in situ DRIFTS spectra of NU-1000@Co-HHTP, the peak at 2056 cm-\n1 was assigned to the *CO NO. How do the authors distinguish the *OCNO and\n2\n*CO NO species? Furthermore, why does this work propose *CO NO as the key\n2 2\nintermediate during the C–N coupling process, instead of following the *OCNO\npathway?\nResponse: We thank Reviewer 3 for the insightful questions. The peak of *O₂CNO\nintermediate identified in our work at 2056 cm⁻¹ exhibits a red shift of ~19 cm⁻¹\ncompared to that of the *OCNO species (∼2075 cm⁻¹) reported in literature (e.g.,\nRef.41, Adv. Mater. 2024, 36, 2408510), which may be caused by the lower C-N bond\norder of *O₂CNO than that of *OCNO. Besides, according to the DFT calculation\nresults (Fig. 5b), the Zr centers of NU-1000 in NU-1000@Co-HHTP serve as the\nbinding sites for CO adsorption. Further according to the S-scheme charge transfer\n2\npathway (Fig. 3g), the photogenerated holes are mainly accumulated on NU-1000 in\nNU-1000@Co-HHTP, precluding the occurrence of CO reduction at NU-1000. In\n2\nother words, the CO molecules are adsorbed on the Zr sites and directly participate in\n2\nthe C-N coupling with the *NO intermediate on adjacent Co sites. In combination with\nthe in-situ DRIFTS spectra, DFT calculations and S-scheme mechanism, we propose\n*O CNO as the key intermediate, which has also been identified in the studies on\n2\nelectrocatalytic C-N coupling (Ref. Natl. Sci. Rev. 2023, 10, nwac209, ACS Catal. 2023,\n13, 14540, Nat. Sustain. 2021, 4, 868). Further clarifications have been added in Page\n14 in the revised manuscript as follows:\n\"Compared to the widely recognized *OCNO intermediate (∼2075 cm⁻¹),45 the peak of\n*O₂CNO exhibits a red shift of ~19 cm⁻¹, which may be attributed to the lower C-N\nbond order of *O₂CNO than that of *OCNO.\"\nComment 5. The authors should compare the consistency of urea production detected\nby the different assays.\nResponse: We are grateful for Reviewer 3’s constructive comments. We have\ncompared the urea production rates measured by 1H NMR and diacetylmonoxime-\nthiosemicarbazide (DAMO-TSC) colorimetric method. As shown in Fig. S15, the\nrelative error of the production rates determined by the two methods is lower than 2.3%,\nverifying the reliability of the data.\nFig. S15 Urea yield rates of the sample detected by DAMO-TSC and 1H NMR.\nThe corresponding descriptions have been added in Page 11 in the revised manuscript\nas follows:\n\"Furthermore, quantitative 1H NMR analysis indicates that the urea yield is consistent\nwith the result obtained from the colorimetric method (Fig. S15).\"\nComment 6. The authors claimed that NU-1000@Co-HHTP exhibited superior carrier\nkinetics compared to other samples. However, based on the PEC and OCVD results\n(Fig. 4f, g), the integration of NU-1000 and Co-HHTP actually decreased charge\ntransfer and separation efficiency of Co-HHTP. This result contradicted the findings\nfrom the PL and TRPL tests (Fig. 4h, i), which suggested enhanced carrier dynamics.\nResponse: We thank Reviewer 3 for the helpful comments. We are sorry for the\nmislabeled samples in the origin Fig. 4f, g and S11. The mistakes have been corrected\nin the new version as below.\nFig. 25 (f) transient photocurrent response profiles, (g) average electron lifetimes (τ )\nn\nof NU-1000, Co-HHTP, NU-1000/Co-HHTP and NU-1000@Co-HHTP.\nFig. S26 (a) EIS spectra and (b) transient OCVD curves of NU-1000, Co-HHTP, NU-\n1000/Co-HHTP and NU-1000@Co-HHTP.\nMinor issues:\nComment 1. The noise-signal ratio in the 1H NMR spectra decreased as the\nconcentration of the urea standard solution increased. To avoid any misunderstandings,\nthe internal standard should also be included in the 1H NMR spectra.\nResponse: We are grateful for Reviewer 3's insightful comments. According to your\nsuggestion, we have included maleic acid as the internal standard in the 1H NMR\nspectra (Fig. S12, see below).\nFig. S12 (a) 1H NMR spectra of a series of 14NH CO14NH standard solutions (0.1 ppm,\n2 2\n2 ppm, 5 ppm, 10 ppm, and 15 ppm).\nComment 2. The standard curves for NO - and NH should also be provided.\n2 3\nResponse: We appreciate Reviewer 3’s helpful comments. The standard curves for\nNO - and NH have been added in the revised supporting information as follows:\n2 3\nFig. S16 The concentration-absorbance calibration curves of NO -.\nrevised supporting information as follows:\n\"The concentration of NO - was then quantified according to the standard curve (Fig.\n\"The concentration of NH was then quantified according to the standard curve (Fig.\nComment 3. HHTP contains abundant chemical bonds between C and O atoms. I\nwonder why there was no signal corresponding to O–C=O or O–C observed in O 1s\nXPS spectra of Co-HHTP.\nResponse: We thank Reviewer 3 for the professional comments. We have reanalyzed\nthe O 1s XPS spectra of Co-HHTP. In the updated O 1s spectra (see below), two peaks\nassigned to C-O and Co-O are fitted at 531.85 and 530.24 eV, respectively.\nFig. 3f XPS spectra of (f) O 2p of NU-1000, Co-HHTP and NU-1000@Co-HHTP.\nCorresponding descriptions have been added in Page 10 in the revised manuscript as\nfollows:\n\"For Co-HHTP, the two peaks at 531.85 and 530.24 eV belong to the C-O group and\nCo-O bond, respectively.\"\nComment 4. The relative Co2+ peak intensity to satellite (Fig. S10b) significantly\nchanged after cycling tests. Is there any other oxidation state of Co formed?\nResponse: We are grateful for Reviewer 4’s insightful comments. During the\nphotocatalytic process, the adsorption of oxidative NO - on the Co sites may increase\n3\nthe oxidation state of Co, thereby resulting in the decreased intensity of satellite peak\nof Co2+ after cycling test. The similar phenomenon has also been observed in literature\n(eg., Nat. Commun. 2020, 11, 3043.)\nCorresponding descriptions have been added in Page 14 in the revised manuscript as\nfollows:\n\"Besides, the decreased intensity of satellite peak of Co2+ in the XPS spectrum may be\ncaused by the increased oxidation state of Co sites by adsorbing NO -.39\"\nare many spelling and grammar errors in this manuscript (e.g., “donated as NU-\n1000/Co-HHTP”). The author should check the manuscript carefully.\nResponse: We thank Reviewer 3 for pointing out our mistakes. We have carefully\nchecked and revised the spelling and grammar throughout the whole manuscript.\nReviewer 4:\nGeneral comments:\nIn this manuscript, the authors reported a MOF-on-MOF (NU-1000@Co-HHTP) S-\nscheme heterojunction with interfacial dual sites for urea synthesis via photocatalytic\nco-reduction of NO₃⁻ and CO₂ (P-NitRR&CRR). The photocatalyst delivered an\nimpressive urea yield of 3523.4 μg g-1 h-1 and a high apparent quantum yield of 1.16%\nat 365 nm in pure water, outperforming the reported materials. The photocatalytic\nmechanism was also in depth elucidated by both experimental results and theoretical\nsimulations. The overall experimental design is reasonable, the research content is\nsubstantial, and the article logic is clear. Therefore, the manuscript could be accepted\nafter addressing the following concerns.\nResponse: We thank Reviewer 4 for the positive comments. We have made revisions\nin the revised manuscript to fully address your comments by adding new experimental\nresults and clarifications (see details below).\nComment 1. NU-1000 forms a S-scheme heterojunction with Co-HHTP. What is the\ndifference between S-scheme heterojunction and Z-scheme heterojunction, and how to\ndetermine whether the material forms an S-scheme heterojunction instead of a Z-\nscheme heterojunction?\nResponse: We thank Reviewer 4 for the thoughtful comments. Z-scheme\nheterojunction includes traditional Z-scheme, all-solid-state Z-scheme, and direct Z-\nscheme heterojunction. Specifically, the traditional Z-scheme heterojunction is\ncomposed of two semiconductors with staggered band structure configurations and\nsuitable intermediate couples (e.g., Fe3+/Fe2+, IO -/I-, and I -/I-) in solution. The all-\n3 3\nsolid-state Z-scheme heterojunction is consisted of two different semiconductors with\na solid electron mediator (e.g. Au, Ag, Pt) at the interface between them. Direct Z-\nscheme heterojunction photocatalyst is directly formed by combining two different\nsemiconductors in the absence of redox couples and electron mediators. The S-scheme\nheterojunction can be directly distinguished from the traditional Z-scheme and indirect\nZ-scheme heterojunction based on the different constitutions. The direct Z-scheme\nheterojunction follows the same charge transfer mechanism with S-scheme\nheterojunction. The concept of \"S-scheme heterojunction\" was proposed based on\ndirect Z-scheme heterojunction by Prof. Jiaguo Yu, aiming to avoid the consequences\nof the failure of traditional Z-scheme photocatalysts and all-solid-state Z-scheme\nphotocatalysts, and describe the charge transfer path more clearly (e.g. Chem 2020, 6,\n1543; Adv. Mater. 2022, 34, 2107668). To validate the S-scheme or direct Z-scheme\nmechanism, the techniques such as in situ irradiated XPS and Kelvin Probe Force\nMicroscopy that are used in our study are recognized as the most efficient methods.\nComment 2. To further support the crucial role of forming heterojunction, XPS results\nof physically mixed samples should be provided.\nResponse: We thank Reviewer 4 for the constructive comments. According to your\nsuggestion, the XPS spectra of physically mixed NU-1000/Co-HHTP have been\nprovided and compared with those of individual NU-1000 and Co-HHTP in the revised\nmanuscript (Fig. S7, see below).\nFig. S7 XPS spectra of (a) Zr 2p, (b) Co 2p and (c) O 2p of NU-1000/Co-HHTP.\nThe corresponding descriptions have been added in Page 9 and 10 in the revised\nmanuscript:\n\"In addition to NU-1000 and Co-HHTP, a physically mixed NU-1000/Co-HHTP\nsample was also prepared as comparison.\"\n\"In contrast to the significant variations in binding energies of the elements observed in\nNU-1000@Co-HHTP, the electron states of Co and Zr in NU-1000/Co-HHTP exhibit\nnegligible changes compared to NU-1000 and Co-HHTP, suggesting the more intimate\ninterfacial contact between the two components in NU-1000@Co-HHTP.\"\nComment 3. The authors should provide more detail process about the conductive band\npotential calculation from Mott-Schottky curves and flat band potential.\nResponse: We thank Reviewer 4 for the useful comment. The detailed calculation\nprocess has been added in the experimental section in the revised supporting\ninformation as follows:\n\"The flat-band potential values are determined using the Mott-Schottky equation:\n1 2 𝑇\n= (𝐸 −𝐸 −\n𝐵）\n𝐶2 𝑁 𝑓𝑏 𝑞\n0 𝐷\nwhere C is the space charge capacitance, N is the donor density, ε and ε are the\nD 0\ndielectric constants of free space and the film electrode, respectively, E is the applied\npotential, E is the flat-band potential,  is Boltzmann constant, T is the temperature,\nfb B\nand q is the electronic charge. The E value can be determined from the extrapolation\nfb\nto 1/C2 = 0. For n-type semiconductors, the conduction bands are 0.1 eV higher than\nthe flat potentials.\"\nComment 4. To conclusively trace the carbon source in urea synthesis, isotopic labeling\nexperiments (e.g., using ¹³C-labeled CO₂) should be performed.\nResponse: We thank Reviewer 4 for the constructive comments. To ensure that the\ncarbon in urea is derived from CO , an isotopic labeling experiment has been carried\n2\nout by using 13CO as the feeding gas. The results have been added as Fig. S19 in the\n2\nrevised manuscript as below.\nFig. S19 13C NMR spectra of the product obtained by using 13CO as feeding gas and\n2\nstandard 13CO(NH ) .\n2 2\nThe corresponding descriptions have been added in Page 13 in the revised manuscript\nas follows:\n\"Upon changing the feeding gas to 13CO , a typical singlet peak of 13CO(NH ) is\n2 2 2\nobserved in the C NMR spectrum (Fig. S19), indicating that the carbon in urea is\nderived from CO₂.\"\nComment 5. The flat-band potential values of NU-1000 and Co-HHTP presented in\nFig. 2c are inconsistent with those in the main text. The flat-band potentials of NU-\n1000 and Co-HHTP should be -0.60 and -0.7 eV, respectively.\nResponse: We thank Reviewer 4 for pointing out our mistakes. The flat-band potential\nvalues of NU-1000 and Co-HHTP in the main text have been revised as follows:\n\"Based on the transverse intercept values, the flat-band potentials of NU-1000 and Co-\nHHTP were determined to be -0.60 and -0.7 V, respectively.\"\nComment 6. The manuscript only describes the co-reduction of CO and NO - to\n2 3\nsynthesize urea, but does not identify the oxidation process.\nResponse: We thank Reviewer 4 for the valuable suggestions. Generally, the\nphotogenerated holes are consumed for water oxidation into O in sacrificial agent-free\n2\nsystems. Therefore, the generated amount of O was further quantified (Fig. S18 as\n2\nbelow), following the order of NU-1000@Co-HHTP (962.3 μmol g-1 h-1) > Co-HHTP\n(213.2 μmol g-1 h-1) > NU-1000/Co-HHTP (184.9 μmol g-1 h-1) > NU-1000 (101.2 μmol\ng-1 h-1). The results have been added as Fig. S18 in the revised manuscript.\nFig. S18 O production rates of NU-1000, Co-HHTP, NU-1000/Co-HHTP and NU-\n2\n1000@Co-HHTP.\nThe corresponding descriptions have been added in Page 12 in the revised manuscript\nas follows:\n\"Except for the P-NitRR&CRR process, the coupled water oxidation reaction was also\nexplored. As shown in Fig. S18, the yield of O as the oxidation product follows the\n2\norder of NU-1000@Co-HHTP (962.3 μmol g-1 h-1) > Co-HHTP (213.2 μmol g-1 h-1) >\nNU-1000/Co-HHTP (184.9 μmol g-1 h-1) > NU-1000 (101.2 μmol g-1 h-1).\"\nComment 7. The proposed \"*CO +*NO\" coupling pathway is interesting. However,\n2\ngiven that \"*CO (from CRR) + *NO\" coupling has been extensively documented in\nelectrocatalysis literatures, could the authors further clarify how they excluded this\nalternative pathway in their mechanism?\nResponse: We appreciate Reviewer 4's insightful comment. According to the DFT\ncalculation results (Fig. 5b), the Zr centers of NU-1000 in NU-1000@Co-HHTP serve\nas the binding sites for CO adsorption. Further according to the S-scheme charge\n2\ntransfer pathway (Fig. 3g), the photogenerated holes are mainly accumulated on NU-\n1000 in NU-1000@Co-HHTP, precluding the occurrence of CO reduction. In\n2\nother words, the CO molecules are adsorbed on the Zr sites and directly participate in the C-\nin-situ DRIFTS spectra, DFT calculations and S-scheme mechanism, we propose\n*O CNO as the key intermediate, which has also been identified in the studied on\n2\nelectrocatalytic C-N coupling (Ref. Natl. Sci. Rev 2023, 10, nwac209, ACS Catal. 2023,\n13, 14540, Nat. Sustain. 2021, 4, 868). Further clarifications have been added in Page\n14 in the revised manuscript as follows:\n\"Compared to the widely recognized *OCNO intermediate (∼2075 cm⁻¹),45 the peak of\n*O₂CNO exhibits a red shift of ~19 cm⁻¹, which may be attributed to the lower C-N\nbond order of *O₂CNO than that of *OCNO.\"\nComment 8. There are some errors in the manuscript, for instance, the units of\nconduction band and valance band potential are \"V\" rather than \"eV\"; the position of\n\"on/off\" labels in Fig. 4f is incorrect; the sample name should be unified throughout the\nwhole manuscript.\nResponse: We thank Reviewer 4 for pointing out our mistakes, which have been\ncarefully revised in the manuscript as below.\nFig. 3 (a) UV-vis DRS spectra of NU-1000, Co-HHTP, NU-1000/Co-HHTP, NU-\n1000@Co-HHTP, (b) Tauc plots and (c) Mott-Schottky plots of NU-1000 and Co-\nHHTP, XPS spectra of (d) Co 2p, (e) Zr 2p and (f) O 2p of NU-1000, Co-HHTP and\nNU-1000@Co-HHTP, (g-i) schematic illustration of the S-scheme charge transfer\nmechanism of NU-1000@Co-HHTP.\nFig. S25 (a-c) LSV curves of NU-1000, Co-HHTP and NU-1000@Co-HHTP under\ndifferent conditions, (d) CO adsorption curves and (e) CO -TPD spectra of NU-1000,\n2 2\nCo-HHTP and NU-1000@Co-HHTP, (f) transient photocurrent response profiles, (g)\naverage electron lifetimes (τ ), (h) PL spectra and (i) TRPL spectra of NU-1000, Co-\nn\nHHTP, NU-1000/Co-HHTP and NU-1000@Co-HHTP.\nReviewer 5:\nGeneral comments:\nThe work by Chao Liu reports a MOF S-scheme heterojunction catalyst with dual Zr\nand Co sites that promotes efficient C-N coupling for the photochemical synthesis of\nurea from NO - and CO . The authors perform synthesis and characterization of the\n3 2\nmaterial using TEM, elemental mapping and XRD. Optical properties of the materials\nare analysed with XPS and UV-vis methods. The area of the catalyst was measured with\nN adsorption/desorption or BET measurements, and the adsorption properties towards\n3 2\nmeasurements were run to assess performance. FTIR and NMR were used to address\nthe reaction mechanism and product formation. This paper shows a photocatalytic\nmaterial with interesting properties toward urea production via the photosynthetic\nprocess, which I think is valuable knowledge for the scope and the readership of this\njournal. However, I think different parts of the paper could be discussed more. Below,\nI add my comments:\nResponse: We thank Reviewer 5 for the positive comment stating that \"This paper\nshows a photocatalytic material with interesting properties toward urea production via\nthe photosynthetic process, which I think is valuable knowledge for the scope and the\nreadership of this journal.\" Your insightful comments are highly valuable and important\nfor us to improve the quality of our work. In the revised manuscript, we have conducted\nadditional experiments with further revisions to address your comments (see details\nbelow).\nComment 1. I understand that the presence of dual sites or vicinal Zr and Co is key to\ncatalysing the carbon-nitrogen coupling step that is limiting for the production of urea.\nThe need for dual sites for C-N coupling is also observed in electrocatalysis. However,\nthe other photocatalysts reported in this work, with single sites of Co or Zr, or a mixture\nof them (1000-Nu/CoHHTP), also produce a significant amount of urea, despite not\nhaving dual sites. What explains the production of urea in the other catalysts?\nResponse: We thank Reviewer 5 for the thoughtful comments. Based on the DFT\ncalculation results in Fig. 5b, the Zr and Co sites prefer to adsorb CO₂ and NO₃⁻,\nrespectively, synergistically promoting the C-N coupling over NU-1000@Co-HHTP.\nFor NU-1000, the adsorption energies of CO₂ (-0.37 eV) and NO₃⁻ (-0.16 eV) on Zr\nsites are both negative, suggesting that the co-reduction of NO - and CO can proceed\n3 2\nover NU-1000. Similarly, Co-HHTP with adsorption energies of -0.96 eV for NO₃⁻ and\n-0.12 eV for CO₂ is also capable of simultaneously adsorbing and activating these two\nmolecules. This explains the occurrence of C-N coupling and urea production over NU-\n1000, Co-HHTP and NU-1000/Co-HHTP. However, owing to the lack of interfacial\nCo/Zr dual sites, the side reactions of CRR and NitRR are predominated over NU-1000\nand Co-HHTP, respectively, with significantly higher energy barrier of the C-N\ncoupling step than NU-1000@Co-HHTP. Together with the lower charge separation\nefficiencies, NU-1000, Co-HHTP and NU-1000/Co-HHTP exhibit inferior urea\nproduction performances than NU-1000@Co-HHTP. To avoid confusion, further\nclarifications have been added in Page 19 in the revised manuscript as follows:\n\"For NU-1000 with single Zr sites and Co-HHTP with single Co sites, it is noted that\nthe adsorption energy of either Zr or Co sites for both CO and NO - adsorption is\n2 3\nnegative. Thus, the chance of urea formation via P-NitRR&CRR cannot be avoided, as\nevidenced by the formation of urea with a low yield in the two individual MOF catalysts.\nHowever, the adsorption energy of Co sites in Co-HHTP for CO is the highest among\n2\nall test groups, the same is the NO - adsorption energy of Zr sites in NU-1000, in\n3\ncontrast to NU-1000@Co-HHTP that has the lowest adsorption energy for both CO\n2\nand NO - adsorption. Therefore, the overall performance of NU-1000 and Co-HHTP is\n3\nsignificantly inferior than NU-1000@Co-HHTP.\"\nComment 2. Could urea be produced if there were either Zr dual sites or Co dual sites\nrather than Co-Zr?\nResponse: We are grateful for Reviewer 5’s thoughtful comment. To synthesize the\ncatalyst with dual Co sites, Co-TBAPy was synthesized by changing the Zr center in\nNU-1000 into Co for growing Co-HHTP, resulting in Co-TBAPy@Co-HHTP. The\nphotocatalytic results show that Co-TBAPy@Co-HHTP could also produce urea with\na production rate of 750.9 µg g-1 h-1, significantly inferior than NU-1000@Co-HHTP,\nfurther suggesting the advantages of Co-Zr dual sites in promoting P-NitRR&CRR.\nWe have also tried to synthesize the catalyst with dual Zr sites by substituting the Co\ncenter in Co-HHTP with Zr. However, the Zr-HHTP was hardly obtained using\nprevailing method. We have also carefully surveyed the literatures and find that the\ncoordination of Zr with HHTP into Zr-HHTP has not been reported.\nFig. R1 (a, b) SEM images of Co-TBAPy and Co-TBAPy@Co-HHTP, (c) XRD\npatterns of Co-TBAPy, Co-HHTP and Co-TBAPy@Co-HHTP.\nFig. R2 Urea yield of Co-TBAPy@Co-HHTP.\nComment 3. Figure 4 shows the photoelectrochemical current of the different materials\ntoward different environments containing KNO , CO , or both. How is the current in\n3 2\nthe blank electrolyte, I mean, without reactant (NO - or CO ), only in Na SO ?\n3 2 2 4\nResponse: We are grateful for Reviewer 5’s insightful comments. The LSV curve in\nblank electrolyte without NO - and CO reactant has been recorded (Fig. S24, see\n3 2\nbelow).\nFig. S24 LSV curve of NU-1000@Co-HHTP in Na SO solution.\n2 4\nThe following descriptions have been added in the revised manuscript:\n\"As shown in Fig. S24 and S25a-c, the current densities of NU-1000@Co-HHTP in the\npresence of NO -, CO or both NO - and CO are larger than that in 0.5 M Na SO\n3 2 3 2 2 4\nwithout reactants.\"\nComment 4. The authors determine the area of the catalysts with N adsorption, but is\n2\nthe electroactive area the same? Is it possible to provide capacitance measurements of\nthe catalysts to get information on the amount of Co and Zr sites available for catalysis?\nResponse: We thank Reviewer 5 for the constructive comments. The electrochemical\nactive surface areas (ECSA) of the catalysts were evaluated based on the double-layer\ncapacitances (C ) by recording the cyclic voltammograms (CV) curves at different\ndl\nscanning rates of 10-100 mV s-1 in the non-Faradaic voltage region. As shown in Fig.\nR3 and Table R1 (see below), NU-1000@Co-HHTP possesses an ECSA value of 0.180\ncm2 mg-1, slightly higher than NU-1000 (0.135 cm2 mg-1) and Co-HHTP (0.152 mF cm2\nmg-1, Table R1). This is possibly ascribed to the enhanced charge carrier transfer\ncapability of NU-1000@Co-HHTP by the formation of internal electric field (as\nevidenced by the UPS and XPS results), which facilitates the electric double-layer\ncharging across the materials on electrode.\nFig. R3 (a-c) CV curves of Co-HHTP, NU-1000 and NU-1000@Co-HHTP at different\nscan rates, (d) CV current density versus scan rate. The linear slope is equivalent to the\nC of Co-HHTP, NU-1000 and NU-1000@Co-HHTP.\ndl\nTable R1. ECSA of NU-1000, Co-HHTP and NU-1000@Co-HHTP.\nSample ECSA (cm2 mg-1)\nNU-1000 0.135\nCo-HHTP 0.152\nNU-1000@Co-HHTP 0.180\nComment 5. What is the loading or amount of Co and Zr in each catalyst?\nResponse: We appreciate Reviewer 5’s thoughtful comment. The mass ratios of metals\nin NU-100, Co-HHTP and NU-1000@Co-HHTP have been measured by inductively\ncoupled plasma-optical emission spectrometry (ICP-OES) and summarized in Table S1\n(see below).\nTable S1. Mass ratios of metals in NU-1000, Co-HHTP and NU-1000@Co-HHTP.\nSample Zr (%) Co (%)\nNU-1000 23.44 -\nCo-HHTP - 18.66\nNU-1000@Co-HHTP 13.02 8.29\nThe following descriptions have been added in the revised manuscript:\n\"Moreover, the Co and Zr mass ratio of NU-1000@Co-HHTP was measured to be 13.02%\nand 8.29%, respectively, via inductively coupled plasma-optical emission spectrometry\n(ICP-OES, Table S1). The Zr/Co molar ratio was thus calculated to be 10.11:10.00,\ncorresponding to a NU-1000/Co-HHTP mass ratio of 1.25/1.00.\"\n\"The mass ratios of Co and Zr of Co-HHTP and NU-1000 were determined as 18.66%\nand 23.44%, respectively, close to the theoretical values.\"\nComment 6. The authors explain the presence of dual or vicinal Co and Zr sites that\nenhance the production of urea, but which characterisation method confirms or supports\nthat they form dual sites in the prepared material?\nResponse: We thank Reviewer 5 for the constructive comments. To further probe the\nformation of interface Zr-O-Co bonds, X-ray near-edge absorption spectroscopy\n(XANES) and extended X-ray absorption fine structure (EXAFS) analysis were further\nperformed. The new results have been added as Fig. 2 in the revised manuscript as\nbelow.\nFig. 2 (a) Co K-edge and (d) Zr K-edge XANES spectra of different samples. Insets in\n(a) and (d) are magnified images. (b) Co K-edge and (e) Zr K-edge FT-EXAFS spectra\nof different samples. Wavelet transforms for (c) Co and (f) Zr K-edge EXAFS.\nThe corresponding descriptions have been added in Page 7 in the revised manuscript as\nfollows:\n\"The electronic structure and coordination environment of the samples were\ninvestigated using X-ray absorption near-edge structure (XANES) and extended X-ray\nabsorption fine structure (EXAFS) analysis. As shown in Fig. 2a, the Co K-edge\nabsorption for NU-1000@Co-HHTP exhibits an increased edge energy than that for\nCo-HHTP. In contrast, the Zr K-edge position for NU-1000@Co-HHTP negatively\nshifts compared to that for NU-1000. The observations suggest the electron transfer\nfrom Co-HHTP to NU-1000 within NU-1000@Co-HHTP, resulting in the generation\nof electron deficient Co and electron-sufficient Zr sites. In the Co K-edge Fourier\ntransform (FT) EXAFS spectra (Fig. 2b), the characteristic peaks at 1.71 Å is assigned\nto Co-O coordination between Co and HHTP ligands in Co-HHTP. For NU-1000@Co-\nHHTP, apart from the Co-O bond, a distinct peak attributed to the Co-O-Zr bond\nemerges at 2.75 Å (Fig. 2b). Fig. 2e displays the Zr K-edge spectrum of NU-1000,\nwhere the peaks located at around 1.51 and 3.29 Å correspond to the Zr-O and Zr-O-Zr\nbond, respectively. For NU-1000@Co-HHTP, an additional peak of Zr-O-Co bond is\nfound at ≈2.77 Å, in accordance with the results of Co K-edge spectra. Compared to\nthe Zr-O-Zr bond, the shorter bond length of Zr-O-Co is attributed to the smaller radius\nof Co than Zr. Moreover, the derived wavelet transform diagrams (Fig. 2c, f) offer\nvisual evidence for the formation of Zr-O-Co chemical bonds at the NU-1000@Co-\nHHTP heterointerface.\"\nComment 7. The authors evaluate the catalyst structure after photocatalysis, but how\nlong does it take to carry out each cycle, or for how long is the urea synthesis carried\nout in hours?\nResponse: We thank Reviewer 5 for the helpful comments. The detailed descriptions\nhave been added in the experimental section in the revised manuscript and supporting\ninformation as follows:\n\"The photocatalytic reaction was conducted for 4 h under stirring at 600 rpm and the\ntemperature of the reaction solution was kept at 25 °C by an external circulating cooling\nwater system. Throughout the reaction, 1 mL of reaction solution was collected at\nregular time intervals (1 h) and filtrated with 0.22 µm filters to remove the\nphotocatalysts for quantitative analysis.\"\n\"As shown in Fig. 4f, ≈97.3% of the original urea yield is preserved after ten\nconsecutive reaction cycles with each cycle lasting for 4 h, suggesting the excellent\nstability of NU-1000@Co-HHTP.\"\nComment 8. I suggest incorporating a table that compares the state of the art of photo\nand electrocatalysts for the production of urea. How is the performance of this catalyst\ncompared to other ones reported in the literature?\nResponse: According to Reviewer 5’s valuable suggestion, a Table that compares the\nperformance of NU-1000@Co-HHTP with the state of the art of photo and\nelectrocatalysts for the production of urea has been added as Table S3 in the revised\nmanuscript. Clearly, the performance of NU-1000@Co-HHTP photocatalyst is the best\namong the reported photocatalysts. However, compared with the electrocatalysts, the\nurea production rate of NU-1000@Co-HHTP is several orders of magnitude lower. The\nfar inferior performance of photocatalysis than electrocatalysis is a common\nphenomenon in various reactions such as hydrogen evolution, water oxidation, CO\n2\nreduction, N fixation, nitrate reduction and two-electron oxygen reduction. This\n2\ninspires us and the researchers in the field of photocatalysis to develop more efficient\nphotocatalysts and photocatalytic systems in future works.\nTable S3. Performance comparison of NU-1000@Co-HHTP with reported the state of\nthe art of photocatalysts and electrocatalysts.\nCatalyst System Urea yield References\n(μg h-1 g -1)\ncat\nCdS@BiOBr P-NRR&CRR 900 ACS Sustainable Chem.\nEng. 2023, 11, 1962.\nNi -CdS/WO P-NRR&CRR 624 Angew. Chem. Int. Ed.\n1 3\n2024, 63, e202405637.\nRu /CeO -V P-NRR&CRR 823.8 Adv. Funct. Mater. 2025,\n1 2 O\n2506296.\nNi /TiO P-NRR&CRR 943.8 Angew. Chem. Int. Ed.\n1 2-x\n2025, e202419628.\nRu-TiO P-NRR&CRR 1497 Adv. Energy Mater. 2024,\n2\n14, 2400201.\nCu SA-TiO P-NRR&CRR 432.1 Adv. Mater. 2022, 34,\n2\n2207793.\nCeO -500 P-NRR&CRR 387.5 Angew. Chem. Int. Ed.\n2\n2023, 62, e202312076.\nRu-Cu/CeO P-NRR&CRR 1002 Sci. Bull. 2025, 70, 1118.\n2\n13, 1855.\nCdS@Bi WO P-NRR&CRR 600 ACS Sustainable Chem.\n2 6\nEng. 2024, 12, 14505.\nCCBT-Ar P-NitRR&CRR 0.2 Appl. Catal. B: Environ.\n2025, 360, 124511.\nCu-TCPP(Co)-NF P-NitRR&CRR 2459.8 Angew. Chem. Int. Ed.\n2025, e202512615.\nCu/Cu O E- 6.3×105 Nat. Commun. (2025)\nCuPc-Amino E- 6.2×106 Nat. Commun. (2024)\nNitRR&CRR 15:8858\nCuWO E- 98500 Nat. Commun. (2023)\nPt/C E- 6.0×106 Nat. Cata. 2024, 7, 785.\nNitRR&CRR\nRP-AuCu E- 1.4×106 J. Am. Chem. Soc. 2025,\nNitRR&CRR 147, 8871.\nMo-PCN-222(Co) E- 8.4×105 Angew. Chem. Int. Ed.\nNitRR&CRR 2024, 63, e202402215.\nCu1/NC E- 5.9×105 Nat. Commun. 2024, 15,\nNitRR&CRR 176.\nPd Au /RuO E- 4.4×106 Angew. Chem. Int. Ed.\n2 1 2\nNitRR&CRR 2025, 64, e202421266.\nSrCo Ru O E- 1.5×105 Angew. Chem. Int. Ed.\n0.39 0.61 3-δ\nNitRR&CRR 2024, 63, e202401943.\nCu-PMOF E- 1.5×106 Angew. Chem. Int. Ed.\nNitRR&CRR 2025, 64, e202513441.\nPcNi-Fe-O E- 2.1×106 Angew. Chem. Int. Ed.\nNitRR&CRR 2024, 63, e202410625.\nNi (BO ) -150 E-NRR&CRR 5.8×105 Energy Environ. Sci. 2021,\n3 3 2\n14, 6605.\nMOF Co-PMDA-2- E-NRR&CRR 8.7×105 Energy Environ. Sci. 2022,\nmbIM 15, 2084.\nGa Cu Mo @C E-NRR&CRR 1.7×106 Angew. Chem. Int. Ed.\n79 11 10\n2024, 63, e202402236.\nZn-Mn N,Cl E-NRR&CRR 2.4×105 Angew. Chem. Int. Ed.\n2023, 62, e202305447.\nNU-1000@Co- P- 3523.4 This work\nHHTP NitRR&CRR\nThe corresponding results have been added in Page 12 in the revised manuscript as\nbelow:\n\"Notably, such an exceptional urea yield is also superior than the reported\nphotocatalysts used in both NRR&CRR and NitRR&CRR systems (Fig. 3b, Table\nS3).8,10,13,31,35-38 Nevertheless, the performance of NU-1000@Co-HHTP-based P-\nNitRR&CRR system is inferior to the state-of-the-art electrocatalytic NitRR&CRR (E-\nNitRR&CRR) systems (Table S3), inspiring further advances in the design of efficient\nphotocatalysts and photocatalytic systems.\""}, {"round": 2, "reviewer_comments": {"reviewer_1": "“Urea Photosynthesis with Carbon Dioxide and Nitrate over A MOF-on-MOF S-Scheme Heterojunction” : the manuscript has now been revised as per comments by reviewers. All comments have been addressed. Though the overall manuscript is very much crowded with too many experiments. Authors may remove some unnecessary experimental data. After that the Manuscript is suitable for publication.", "reviewer_2": "The revised version can be accepted.", "reviewer_3": "I have read the author’s responses and the revised manuscript with interest. Owing to the improved novelty and more in-depth experimental data analysis in this version, I am more positive in my recommendation. However, I still have some concerns about this manuscript. Considering that reports in this research area remain limited, publication of this work will help enrich the field. Therefore, I recommend a minor revision and the following comments:\n1. The authors should provide quantitative EXAFS fitting parameters, including coordination numbers, bond distances, Debye-Waller factors, and the associated fitting residuals.\n2. I recommend that the authors provide additional evidence to support the claim of weak *CO adsorption, such as CO-TPD or controlled CO-DRIFTS experiments. These measurements would help directly verify the adsorption/desorption behavior of CO on NU-1000, Co-HHTP, and NU-1000@Co-HHTP and clarify why no CO signal is observed in the in situ DRIFTS spectra.", "reviewer_4": "The questions raised have been well addressed, and it is recommended to accept the article.", "reviewer_5": "The authors have fully addressed my comments, and it seems that the other reviewer´s comments have also been addressed.\nIn particular, they have expanded the experimental data and characterisation of dual sites, and added a discussion of the state-of-the-art photo- and electrocatalysts for urea synthesis, which I think improves the discussion and quality of the manuscript.\nGiven that the topic and materials presented here are of great interest, I think the article is ready for acceptance."}, "author_response": "Point-to-point response and revisions\n(Original Manuscript ID: NCOMMS-25-53753)\nReviewer 3:\nGeneral comments:\nI have read the author’s responses and the revised manuscript with interest. Owing to\nthe improved novelty and more in-depth experimental data analysis in this version, I\nam more positive in my recommendation. However, I still have some concerns about\nthis manuscript. Considering that reports in this research area remain limited,\npublication of this work will help enrich the field. Therefore, I recommend a minor\nrevision and the following comments:\nResponse: We thank Reviewer 3 for the positive comment.\nComment 1. The authors should provide quantitative EXAFS fitting parameters,\nincluding coordination numbers, bond distances, Debye-Waller factors, and the\nassociated fitting residuals.\nResponse: We thank Reviewer 3 for the helpful comment. The quantitative EXAFS\nfitting parameters have been presented in Table S3 in the revised supporting information\nas follows:\nTable S3. Fitting results of Zr/Co K-edge EXAFS spectrum.\nEdge Samples Shell CNa R(Å)b σ2(Å2)c Rd factor\nNU-1000 Zr-O 8.0±0.5 1.51 0.0062 0.0019\nZr K-edge\nZr-O 7.9±0.3 1.61 0.0016\nNU-1000@Co-\n0.0032\nHHTP\nZr-O-Co 3.5±0.4 2.77 0.0032\nCo-HHTP Co-O 6.0±0.5 1.71 0.0065 0.0041\nCo K-edge\nCo-O 5.9±0.4 1.62 0.0030\nNU-1000@Co-\nCo-O-Zr 3.4±0.3 2.75 0.0045 0.0110\nHHTP\naCN: coordination number; bR: bond distance; cσ2: Debye-Waller factor; dR factors:\ngoodness of fitting.\nThe corresponding descriptions have been added in Page 8 in the revised manuscript as\nfollows:\n\"In the Co K-edge Fourier transform (FT) EXAFS spectra (Fig. 2b, Table S3), the\ncharacteristic peak at 1.71 Å is assigned to Co-O coordination between Co and HHTP\nligands in Co-HHTP.\"\nComment 2. I recommend that the authors provide additional evidence to support the\nclaim of weak *CO adsorption, such as CO-TPD or controlled CO-DRIFTS\nexperiments. These measurements would help directly verify the adsorption/desorption\nbehavior of CO on NU-1000, Co-HHTP, and NU-1000@Co-HHTP and clarify why no\nCO signal is observed in the in situ DRIFTS spectra.\nResponse: We acknowledge Reviewer 3’s valuable comment. We have performed CO-\nTPD measurements to investigate the adsorption/desorption behavior of CO on NU-\n1000, Co-HHTP and NU-1000@Co-HHTP. The results have been added as Fig. S23 in\nthe revised manuscript as below.\nFig. S23 CO-TPD spectra of NU-1000, Co-HHTP and NU-1000@Co-HHTP.\nThe following descriptions have been added in Page 14 in the revised manuscript:\n\"The speculation is further supported by the CO temperature-programmed desorption\n(CO-TPD) measurement. As shown in Fig. S23, the CO-TPD profile of NU-1000\nexhibits two distinct peaks at 133 and 390 °C, assigned to the physisorption and\nchemisorption of CO, respectively. For Co-HHTP, both the physisorption and\nchemisorption temperature is significantly lower than that of NU-1000, along with\nreduced peak intensities, indicating weaker CO adsorption at Co sites of Co-HHTP than\nNU-1000. After forming heterojunction, the CO adsorption strength of NU-1000@Co-\nHHTP lies between that of the two individual components.\""}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have well addressed my concerns. Thus, I think this manuscript is suitable for the publication in Nature Communications in its current form."}, "author_response": ""}]

Decision letter