Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2025 Oct 8;8(20):15146–15156. doi: 10.1021/acsaem.5c02019

Efficient and Selective Photocatalytic Transformation of CO2 to CO with Mo6 Clusters Supported on Fe-Doped g‑C3N4

Jhon Sebastián Hernández 1, Marta Feliz 1,*
PMCID: PMC12570109  PMID: 41169692

Abstract

The carbon dioxide reduction reaction (CO2RR) driven by sunlight is expected to become a sustainable way of producing high-value compounds in the future. To achieve this goal, affordable and efficient photocatalysts still need to be discovered. In this work, we show that near-IR luminescent octahedral molybdenum iodido clusters decorated with isonicotinato ligands, (Bu4N)2[Mo6I8(O2CC5H4N)6] (Mo6), once combined with iron-doped carbon nitride (Fe-g-C3N4), provide the Mo6/Fe-g-C3N4 nanostructured materials, which are able to photocatalytically produce carbon monoxide (CO) from CO2 with high efficiency and selectivity. In a plausible mechanism, the Mo6 cluster acts as a photosensitizer, and its pyridine groups interact coordinatively with the embedded iron atoms, thus promoting the electronic conduction to the catalytic iron sites of the nanohybrid. The materials were characterized analytically, texturally, structurally, and spectroscopically using techniques such as inductively coupled plasma (ICP), specific surface area measurements, UV–vis–NIR diffuse reflectance spectra (DRS), powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDS), and photoluminescence. The CO2RR studies showed that this association induces a change in the selectivity and a significant increase in CO production compared to that produced separately by the Mo clusters and graphitic precursors. Considering the versatility of this building block strategy for preparing multicomponent hybrid nanomaterials, molybdenum metal clusters are regarded as promising catalysts for creating eco-friendly and cost-effective photocatalysts for the CO2RR.

Keywords: molybdenum clusters, iron, graphitic carbon nitride, CO2 photoreduction, nanohybrid material

graphic file with name ae5c02019_0009.jpg

graphic file with name ae5c02019_0007.jpg

1. Introduction

The increasing concentration of CO2 in the atmosphere represents one of the major global environmental concerns, significantly contributing to climate change and its devastating consequences. The pursuit of innovative and sustainable approaches to mitigate this issue has become a crucial priority in contemporary scientific research. In this context, new photocatalytic materials have emerged as a promising strategy for CO2 removal promoted by sunlight. −

The development of nanostructured photocatalysts with unique properties and enhanced catalytic performance is currently a great challenge. In fact, the modification of high-surface semiconductors as platforms to incorporate suitable photoactive and catalytically active molecules results in highly efficient hybrid materials with future prospects of sustainable transformations promoted by light in the framework of artificial photosynthesis and sustainable chemistry research. These materials leverage the synergetic effects of their individual components to offer a superior functionality compared to single-component systems. ,

Graphitic carbon nitride (g-C3N4), a material derived from the family of carbonitride polymers, has captured the scientific community’s attention due to its unique properties, including high stability, abundance, and the ability to absorb visible light. , These characteristics and the easy chemical modification of g-C3N4 with other catalysts and promoters make g-C3N4 and its composites ideal candidates for catalyzing the conversion of CO2, a greenhouse gas, into fuels such as CO as a valuable chemical compound. , Graphitic carbon nitride has also shown great potential in other applications such as photocatalytic hydrogen production, ,,, energy conversion and storage, as well as other photocatalysis processes, such as pollutant removal, ,,, organic synthesis, and biosensing. However, pure g-C3N4 exhibits some problems due to the low visible light utilization, small specific surface, and high rate of charge carrier recombination. The modification through metal doping is a methodology that allows overcoming these disadvantages and modifying the response of photocatalysts to improve the physicochemical properties. ,,,, This is because metals can act as electron acceptors, facilitating efficient charge carrier separation. Furthermore, this modification can broaden the absorption spectrum of the photocatalyst. This means that the modified catalyst can harness a wider range of light wavelengths, including visible light, which is abundant in sunlight. ,,,

As we face increasingly pressing environmental challenges, a thorough understanding and effective application of metal (M)-doped g-C3N4 (M-g-C3N4) catalysts in the CO2RR become critically important in the transition to more sustainable practices and in the search for solutions to mitigate the impact of climate change. However, M-g-C3N4 materials have been primarily used in photodegradation of organic contaminants and water splitting, and the reactivity is dependent on the nature of the dopant. ,, Thus, while Fe-g-C3N4 materials have proven to be active for oxidation processes, an effective functionalization enabling their use in reduction processes is less known. − To the best of our knowledge, only Hao and co-workers reported a mechanistic study of the CO2RR to CO catalyzed by an atomically dispersed Fe on the g-C3N4 material, with low surface area and catalytic performance (less than 1 μmol g–1 h–1).

The decoration of carbon nitride materials with other inorganic components by covalent or supramolecular interactions is an alternative strategy to obtain structured derivatives with enhanced properties. , Hexamolybdenum halide octahedral clusters emerge as suitable candidates for their incorporation onto carbon nitride surfaces. These nanosized compounds, denoted as [Mo6X8L6]n (n = charge), are robust metal units consisting of internal halide ligands (X) and apical (or terminal) ligands (L) of either organic or inorganic nature. These clusters show intrinsic electronic and optical properties, with an intense emission in the red and near-infrared (NIR) regions, featuring high quantum yields (Φem up to 88%) and relatively long lifetimes in the microsecond range (τem up to ∼300 μs) associated with phosphorescence. , The octahedral molybdenum clusters have been shown promising applications in photocatalytic hydrogen evolution reaction, biomedical applications, sensors, − and photonics. , In the field of the CO2RR, the incorporation of {Mo6Br8}4+ core clusters on the surface of graphene oxide enhanced the photocatalytic efficiency in the CO2RR to methanol in the presence of water, offering an intriguing strategy worth considering. As far as we know, the interaction of the molybdenum cluster with carbon supports modified with metal dopants, as well as its physicochemical and photocatalytic properties, has not been studied. In this work, Mo6/Fe-g-C3N4 nanostructured hybrids were prepared from Mo6 and Fe-g-C3N4 materials, and the catalytic activity and selectivity were assessed in the CO2RR.

2. Results and Discussion

2.1. Synthesis and Characterization

The Mo6/Fe-g-C3N4 nanocomposites were prepared by immobilization of the Mo6 cluster compound onto Fe-g-C3N4 supports, with different iron contents. The Mo6I8 cluster unit bearing isonicotinato ligands is considered a suitable building block for incorporation into Fe-g-C3N4 supports by coordinative interactions with iron atoms (Figure a), as demonstrated in similar molecular systems reported by Sokolov et al. , The iron-doped graphitic supports were obtained in a two-step synthesis: first, pristine g-C3N4 was obtained by thermal polycondensation of melamine, followed by a two-step thermal exfoliation to provide a 10-fold increase of the surface area (ca. 118 m2·g–1). − Second, iron­(III) chloride (1, 3, and 7% w/w) reacted with aqueous g-C3N4 at 90 °C to give Fe-g-C3N4-1, Fe-g-C3N4-3, and Fe-g-C3N4-7, respectively. The specific surface area of the Fe-g-C3N4 materials (114 m2·g–1 avg., Table S1) did not change substantially with respect to the graphitic precursor, whereas other authors observed an increase in area after metal doping. , The preparation of Mo6/Fe-g-C3N4 was achieved by mixing the molecular Mo6 cluster with the Fe-g-C3N4 support in THF with soft heating (40 °C) overnight. The incorporation of the molecular cluster induces a slight decrease in the surface area (9.5 m2·g–1 avg., Table S1), which is mainly attributed to pore plugging in the support. The metal content in the graphitic compounds was determined by ICP spectroscopy (Table S1). The presence of iron was confirmed in Fe-g-C3N4-1, Fe-g-C3N4-3, and Fe-g-C3N4-7 materials, indicating that the method used for metal doping of g-C3N4 was effective. The molybdenum content in the three corresponding Mo6/Fe-g-C3N4 nanomaterials is approximately 0.2% (w/w) in all cases (this value corresponds to roughly 0.8% of the cluster). Furthermore, after the cluster immobilization process, the Fe concentration in the materials remained unchanged. At least five attempts were made using different amounts of cluster (5, 10, 20, 30, and 50 mg) and various reaction times (2, 6, and 24 h) to increase the Mo6 content in the materials; however, none were successful. These results suggest that, regardless of the Fe concentration incorporated into the graphitic material, only a fraction of the Fe atoms would be accessible for the coordinative anchoring of Mo6 through pyridino groups, in agreement with the model depicted in Figure a.

1.

1

Open in a new tab

(a) Schematic representation of the interaction of Fe–N bonds between Mo6 and Fe-g-C3N4 (color codes for atoms: Fe: red, Mo, orange; I: yellow, O: malve, C: gray, N: dark blue; H atoms are omitted for clarity), (b) UV–vis DRS, (c) FTIR spectra, and (d) PXRD diffractograms of Mo6/Fe-g-C3N4-1, Fe-g-C3N4-1, and g-C3N4 materials.

All the nanocomposites and their precursors were characterized by additional spectroscopic, structural, analytical, and textural techniques. Considering that the graphitic materials provide similar characterization results independently of the iron content, for simplicity, the characterization of the material of greatest catalytic interest, Fe-g-C3N4-1 and Mo6/Fe-g-C3N4-1, is described next.

The UV–vis DRS of the Mo6/Fe-g-C3N4-1 material (Figure b) shows the characteristic fingerprint of the graphitic precursors, with no absorption of the Mo6 cluster detected (Figure S1). However, a red shift of the Mo6/Fe-g-C3N4-1 bands, and a slight increase in intensity, could be associated with the presence of the hexametallic cluster units, as was described in solution for similar systems with metal-pyridino linker–Mo6 coordinative interactions. , The band gap energy (E g) was calculated by the Tauc method and, taking into consideration the direct allowed transitions of carbon nitride, the values for g-C3N4, Fe-g-C3N4-1, and Mo6/Fe-g-C3N4-1 were 2.67, 2.62, and 2.61 eV, respectively (Figure b), indicating that the incorporation of Fe and Mo6 allows for improvement of visible light absorption ability.

The FTIR absorption spectra of the graphitic nanomaterials (Figure c) show a broad band between 3700 and 3000 cm–1, which corresponds to the vibrational mode of N–H bonds associated with amino groups and the O–H bonds of adsorbed water molecules. The peak at 1638 cm–1 is associated with the stretching vibrational modes of the C–N bond, while the signals at 1572, 1406, 1320, and 1240 cm–1 correspond to aromatic C–N stretching vibrations. The peak at 890 cm–1 indicates the out-of-plane bending mode of N–H, and the peak observed near 810 cm–1 represents the triazine ring breathing mode (Figure S2). The characteristic signals of the Mo6 cluster are not detected due to the low content and overlap with the graphitic IR signals (Figure S2). The similar vibrational fingerprints between the graphitic materials suggest that the g-C3N4 structure is retained after incorporation of Fe and Mo6. To verify the presence of clusters in Mo6/Fe-g-C3N4-1, Raman spectra of this hybrid and the Mo6 precursor were recorded in the characteristic Raman region of the octahedral molybdenum clusters. ,, Figure S3 shows intense bands with Raman shifts at 155, 198, 255, 310, and 420 cm–1 associated with the cluster-specific bonds Mo–Mo, Mo–I, and Mo–O. ,, Unfortunately, no signals were detected in the Mo6/Fe-g-C3N4-1 spectrum, probably due to the low concentration and/or the high luminescence of the Mo6 compound.

The PXRD patterns of the Mo6/Fe-g-C3N4-1 photocatalyst were measured and compared with the pristine materials. The diffractogram of g-C3N4 (Figure d) shows two typical signals at 12.4° (2θ), which is associated with the (100) plane and is due to the repetition of heptazine rings, and at 27.7°(2θ), which corresponds to the (002) plane and is associated with the stacking of the conjugated aromatic system, as observed in graphite. For the Fe-doped and the Mo6/Fe-g-C3N4-1 materials (Figure d), the peak of the (002) plane is little shifted to lower angles (0.2°) with respect to the support, which reveals a certain lattice disorder of g-C3N4 due to the presence of Fe, generating a less dense packing in the crystal lattice. This change is mainly associated with the interstitial positioning of Fe ions in carbon nitride by chemical coordination with the free electron pairs of the nitrogen atoms in its structure. Due to the low concentration of the Mo6 cluster deposited on the Fe-g-C3N4 surface, there are no diffraction peaks of crystalline Mo6 (Figure S4a) and no changes in the diffraction patterns of the graphitic support.

The morphology and microstructure of graphitic materials were investigated by FESEM, and the results are shown in Figure . The pristine g-C3N4 support exhibits a petal morphology and, in detail, looks like small sheets placed on top of each other. The same morphology is observed for Fe-g-C3N4-1 and Mo6/Fe-g-C3N4-1, as expected, considering the small changes in the surface area among the nanomaterials. A greater stacking of the sheets is detected for Mo6/Fe-g-C3N4-1, which agrees with a slight decrease in surface area after Mo6 immobilization. Mo and Fe are not detected due to their low concentration, whereas EDS analyses confirm the presence of Cl coordinated to iron atoms.

2.

2

Open in a new tab

FESEM images of (a) g-C3N4 (b) Fe-g-C3N4-1, and (c) Mo6/Fe-g-C3N4-1.

XPS analysis of Mo6/Fe-g-C3N4-1 was performed to verify the electronic nature of the metals. Figure a presents the survey scan XPS spectra of the Mo6/Fe-g-C3N4-1sample, which shows the characteristic peaks of C, O, N, Fe, I, and Mo at 285, 535, 398, 710, 619, and 230 eV, respectively. The resolution of the C 1s region (Figure b) shows two primary peaks with band energy values of 289.7 and 286.5 eV. The first one corresponds to the sp2 C–O and tri-s-triazine structures N–CN bonds of the support, and the second one can be associated with sp2 C–C of apical ligands in the Mo6 cluster and with the C absorbed on the sample surface. Figure c shows the Fe 2p region, and in this, two signals located at 711 and 726 eV can be observed, which correspond to the signals Fe 2p3/2 and Fe 2p1/2, respectively. These results confirm the presence of Fe in the material as Fe3+. The Mo 3d region (Figure d) displays the typical signals of Mo2+ at 230 and 233.5 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively. The I 3d region (Figure S5) shows the characteristic doublet of I– at 619.5 and 631.3 eV, corresponding to I 3d5/2 and I 3d3/2, respectively. Together, these results confirm the presence of the {Mo6I8}4+ cluster core on the surface of Fe-g-C3N4.

3.

3

Open in a new tab

XPS analysis of the Mo6/Fe-g-C3N4-1photocatalyst: (a) full survey spectrum, (b) C 1s, (c) Fe 2p, and (d) Mo 3d.

2.2. Photophysical Properties

Photoluminescence studies were carried out to elucidate possible mechanisms of energy and electron transfer between the Mo6 cluster and the Fe-g-C3N4 support under illumination conditions. For this purpose, steady-state emission spectra of Mo6/Fe-g-C3N4-1 and Fe-g-C3N4-1 were investigated and compared with those of their precursors. Upon light excitation (λexc = 365 nm), a broad emission band appears at 480 nm in the spectra of the graphitic materials (Figure a). This emission is characteristic of g-C3N4 and attributed to π* transitions from the valence band (VB) to the conduction band (CB), as well as to band edge transitions associated with lone pair states of nitrogen atoms present both within and between tri-s-triazine units. The emission band of the g-C3N4 increases upon incorporation of Fe into the graphitic support, contrary to what has been described in the literature, , suggesting a higher recombination of photogenerated charge carriers when this material is irradiated. The characteristic photoluminescence of Mo6 is detected in the spectra of the Mo6/Fe-g-C3N4-1 material, with an emission maximum close to that registered for the pristine cluster (705 nm, Figure S6), which is attributed to electronic transitions from triplet excited to the ground states of the hexametallic cluster unit. , Thus, the Mo6/Fe-g-C3N4-1 material displays two characteristic emission bands of the precursors, confirming that the Mo6 cluster is anchored to the support.

4.

4

Open in a new tab

(a) Steady-state emission spectra of Mo6, g-C3N4, and the hybrid nanomaterials measured in acetonitrile under a N2 atmosphere and λexc = 365 nm; temporal decay profiles of the emission of Mo6, g-C3N4, and Mo6/Fe-g-C3N4-1 nanomaterials recorded at (b) λexc = 365 nm and (c) λem = 705 nm. (d) Steady-state emission spectra of Mo6 in acetonitrile (0.25 mg·mL–1) with increasing volumes (in mL) of a suspension of Fe-g-C3N4-1 in acetonitrile, recorded in air with excitation at λexc = 365 nm.

The emission lifetimes were determined by fitting to a first-order exponential function, as shown in Figure b,c. The values calculated from the temporal decay profiles of the emission of the materials, recorded at λexc = 365 nm and λem = 480 nm, reveal that the emission lifetime of g-C3N4 is 3.2 ± 0.02 ns, whereas for Fe-g-C3N4-1 and Mo6/Fe-g-C3N4-1, the calculated values decrease to 2.4 ± 0.02 ns and 2.7 ± 0.02 ns, respectively, indicating that the recombination of photogenerated charge carriers in the hybrid materials is slightly faster. Additionally, the emission lifetime of the Mo6 cluster at λem = 705 nm was determined. After the data were fitted to a first-order exponential, the calculated value was 314 ± 0.002 ns. For the Mo6/Fe-g-C3N4-1 material, the calculated lifetime decreases to 210 ± 0.002 ns. This reduction in lifetime suggests the presence of a new (or additional) nonradiative deactivation pathway for the excited state of the cluster when it is in the carbon nitride environment. This implies that charge or energy transfer from Mo6 to Fe-g-C3N4-1 may occur due to strong interactions between the two components, facilitating additional nonradiative processes. Furthermore, it becomes evident that modification of the electronic environment of the cluster through interaction with the iron-doped carbon nitride alters its emissive efficiency. To support this hypothesis, photoluminescence measurements were recorded for a solution of the Mo6 cluster in acetonitrile (0.25 mg·mL–1), to which increasing amounts of a suspension of Fe-g-C3N4-1 in acetonitrile were added (Figure d). The progressive decrease in the intensity of the Mo6 cluster emission band (λem = 705 nm) and the increase in the characteristic emission band of the nitride (λem = 480 nm) upon adding increasing amounts of Fe-g-C3N4-1 suggest that an electron/energy transfer from the Mo6 cluster to the Fe-doped graphitic support occurs.

Based on all of the characterization data described above, the hybrid materials are potentially useful in photocatalytic processes. This is due to the effective immobilization of the molybdenum cluster on the graphitic support, probably associated with coordinative interactions, which leads to an enhancement of the physicochemical properties of the hybrid material. As a result, the material exhibits increased radiation absorption and more efficient charge carrier separation while maintaining its textural properties unaltered.

2.3. Photocatalytic CO2RR Studies

The performance of the Mo6/Fe-g-C3N4-n (n = 1, 3, 7) nanocomposites and their precursors, g-C3N4, Fe-g-C3N4-n, and Mo6, in the CO2 reduction, was assessed using a TEOA/acetonitrile (1% v/v) mixture and gentle heating (50 °C) for 24 h of irradiation, under standard reaction conditions. In the first study, the photocatalytic activity of the precursors was assessed. The g-C3N4 material provided H2 and CH4 (256 μmol and 22 μmol·g–1, respectively; Figure a), being the H2 produced 2-fold the H2 obtained by photocatalytic decomposition of TEOA (Figure S8). No additional C-based products were detected as products of the CO2RR.

5.

5

Open in a new tab

(a) Representation of products from photocatalytic CO2RR using g-C3N4-based photocatalysts; (b) influence of Fe content in the CO2RR using Mo6/Fe-g-C3N4-n (n = 1, 3, and 7) photocatalysts; (c) influence of temperature in the photocatalytic CO2RR promoted by Mo6/Fe-g-C3N4-1; and (d) reutilization tests of the Mo6/Fe-g-C3N4-1 photocatalyst.

Several studies have reported that the modification of g-C3N4 by incorporating an iron atom into the triazine rings enhances the photocatalytic performance of the support. Our results show that the Fe-g-C3N4-7 material produces H2, CH4, and CO in different amounts (1376, 43, and 184 μmol·g–1, respectively; Figure a), evidencing that the selectivity of the CO2RR toward CO changes with respect to g-C3N4. In addition, the efficiency in CO production is significantly higher than that reported for the same material exposed to less irradiation time (10 h) and dispersed in pure water. Studies conducted by DFT calculations showed that the VB of carbon nitride is a combination of the HOMO levels of the triazine monomer derived from Npz orbitals, while the CB consists mostly of LUMO levels of the Cpz orbitals. ,, In consequence, the transfer of the light-generated electrons in the triazine rings to the surface for their involvement in oxidation–reduction reactions proves challenging, leading to very rapid recombination processes of the photogenerated electron–hole pairs. The positive metallic species (Fe3+, as evidenced by XPS) in the interstitial positions of these rings act as attractive centers for photogenerated negative charge species (e–), improving charge separation and increasing their average lifetime, as confirmed by photophysical studies. This results in an enhancement of the photocatalytic efficiency of the materials in the presence of Fe. Thus, the trapping of electrons by Fe3+ species in Fe-g-C3N4 increases the quantity of available electron holes for oxidation processes, leading to a higher production of H2 as a product of TEOA oxidation.

Once Mo6 is supported onto Fe-g-C3N4-7, the resulting Mo6/Fe-g-C3N4-7 material achieves a maximum CO production of 3600 μmol·g–1 (Figure a), making it a highly efficient reaction (150 μmol·g–1·h–1). The high selectivity toward this product (87.8%) changed drastically with respect to the selectivity of the support. To give more insight into the role of the cluster compound, the photocatalytic activity of the molecular Mo6 cluster toward CO2 was tested under the same experimental conditions but in a homogeneous phase. After 24 h, the formation of CO, CH4, and H2 was observed, with CO being again the preferential product formed (Table S2). In contrast to the photophysical and photocatalytic characteristics of Mo6/Fe-g-C3N4, the isolated Mo6 clusters act as a photosensitizer and as a photocatalyst, confirming the dual role of the hexametallic cluster. ,

To enhance the performance of Mo6/Fe-g-C3N4-7 in the CO2RR, lower concentrations of Fe were tested on Mo6/Fe-g-C3N4-n (n = 1, 3) while keeping the reaction conditions unchanged. As shown in Figure b, the value of CO obtained (7752 μmol·g–1) with the Mo6/Fe-g-C3N4-1 catalyst shows a significant improvement with respect to the other two Mo6/Fe-g-C3N4 nanocatalysts tested and to most of the graphitic hybrid systems reported in the literature (Table S2), demonstrating that this three-component system is a benchmark in CO2-to-CO conversion among carbon nitride composites reported until date. The decrease in the amount of iron in the photocatalyst progressively enhances CO production with a slight decrease in selectivity (from 88 to 80%). This result supports the idea that by reducing the number of Fe3+ electron-trapping centers, more photogenerated electrons become available to promote reduction processes. However, the presence of Fe3+ in this material is crucial, as demonstrated by UV–vis DRS spectroscopy (Figure b) since the absorption spectrum of the material shifts slightly toward the visible region and, similarly, the energy band gap in the modified materials decreases compared to g-C3N4. Additionally, Fe3+ atoms act as anchoring points for the Mo6 cluster, which functions as a photosensitizer, allowing a greater number of photons to be captured by the system. Furthermore, considering the concentration of the Mo6 cluster in the material, it is assumed that the vast majority of Fe3+ atoms serve as anchoring points for the cluster. The catalytic results obtained can also be supported due to the large specific surface area calculated for the modified materials, which can promote the adsorption, desorption, and diffusion of products and reactants, favoring the photocatalytic performance.

Once demonstrated that Mo6/Fe-g-C3N4-1 shows the best efficiency for CO production, this nanomaterial was selected to study the effect of temperature (25, 50, and 75 °C) on the catalytic performance. The results obtained (Figure c) at room temperature show that the catalyst reduces its effectiveness in CO production to one-fourth of the efficiency achieved at 50 °C. However, when the temperature is increased to 75 °C, the efficiency and selectivity of the material remain unchanged. The increase in the photocatalytic activity of the materials with the temperature rise is mainly because of supplying more energy to the system, the molecules in the medium move faster, resulting in a higher collision frequency, and these collisions occur with greater force. This would mean that the reactants are more likely to overcome the activation energy barrier and transition into product formation. However, when the temperature in this system is increased to 75 °C, the CO production does not improve significantly. This is because the CO2 conversion is favored at lower temperatures due to the thermodynamic limit. The produced CO originating from CO2 was verified by GC-MS through the execution of an experiment with C-13 labeled carbon dioxide under the same reaction conditions at 50 °C (Figures S9 and S10). In the results, characteristic signals of 13-CO at 13, 16, and 29 m/z are observed. Additionally, it can be noted that the formed methane also originates from 13-CO2, as indicated by peaks at 14, 15, 16, and 17 m/z, corresponding to the molecular fractionation signals for mono-, bi-, tri-, and tetrasubstituted alkanes, respectively.

Reuse experiments were done (Figure d) and showed that Mo6/Fe-g-C3N4-1 loses activity after the first use, reducing the amount of CO produced by 19% for the first reuse and 25% for the second. This behavior may be due to aging of the material with prolonged exposure to light or surface contamination derived from the oxidation byproducts of TEOA present in the reaction system. However, it is important to highlight that leaching tests conducted by ICP measurements showed the absence of Fe or Mo in the solution, indicating the physical stability of the material after the reaction. Additionally, photoluminescence measurements carried out on the Mo6/Fe-g-C3N4-1 material after three cycles of use (Figure S11) showed that the emission band of the cluster decreased in intensity, suggesting partial decomposition of the Mo6 cluster, in agreement with the decrease in the material’s catalytic efficiency along the reuses.

2.4. Proposed Photocatalytic Reaction Mechanism

The selectivity of the CO2RR reaction could be due to the preferential CO2 to CO transformation, from a thermodynamic standpoint, which involves 2 electrons instead of 8 electrons required for CH4 production, as shown in eqs and : ,

CO2+2H++2e−→CO+H2O 1
CO2+8H++8e−→CH4+2H2O 2

From the perspective of the reaction mechanism, the high efficiency of Fe-g-C3N4-7 toward CO production is attributed to the coordinative interaction between the pyridyl group of the apical ligand of Mo6 and the iron atom, as illustrated in Figure b. The interaction of N–Fe bonds between the apical ligands of the Mo6 cluster and the Fe3+ atom incorporated into the g-C3N4 was assumed based on studies of similar systems that have demonstrated this behavior. , Two hypotheses are proposed to explain the high efficiency and selectivity of the hybrid in the reaction: (i) considering the molybdenum cluster as a photosensitizer; its anchoring would allow the transfer of photogenerated electrons to the Fe3+ atoms integrated into the graphitic structure, providing a greater number of electrons available to participate in reduction processes; (ii) considering the cluster as both a photosensitizer and a catalytic center, whereby the photogenerated electrons concentrate on the cluster unit, similar to what occurs in the molecular Mo6 cluster, while holes are transferred to the support. In the first hypothesis, the Fe3+ atoms are regarded as the catalytic centers, whereas in the second, the cluster unit itself is considered the active center, given that the selectivity of the transformation relative to the molecular Mo6 cluster is maintained.

Keeping these hypotheses in mind, and considering the mechanism proposed by Zhang and co-workers for Fe-g-C3N4 systems in the CO2 photocatalytic transformation, we present the possible mechanism reaction involved in this research (Figure ). Once the illumination of the material induces the electron excitation from the HOMO to the LUMO in the Mo6 cluster, the photogenerated electrons migrate to the CB of the iron-doped carbon nitride, improving the separation of the charge carriers (as a sensitizer mechanism). At the same time, the photogenerated holes in the hybrid material react with TEOA in solution to produce TEOA+, which could react with the CO2 adsorbed to form COOH· and TEOA, in a similar manner to what was proposed by Zhang and co-workers (in their case, the molecule oxidized is water to OH·). Then, the next step involves the cleavage of the C–O bond in COOH·, generating CO and another OH·. The OH· species are involved in the TEOA photooxidation, ultimately leading to the production of CO2 and H2 as main products. Besides, it is important to mention that all the photochemical process described before occurs at the active site of Fe, and this is due to the fact that the spin density is distributed mainly on the Fe atom in the interstitial positions of g-C3N4. On the basis of this scheme (Figure ), the Mo6 cluster would act as a photosensitizer and promote the electron injection into the iron reactive sites of the graphenic support.

6.

6

Open in a new tab

Schematic representation of the reaction mechanism for photoreduction of CO2 to CO using Mo6/Fe-g-C3N4 hybrid materials and TEOA as a sacrificial agent.

3. Conclusions

The immobilization of octahedral molybdenum clusters, bearing iodido and carboxylato ligands with terminal pyridyl groups, onto iron-doped carbon nitrides was effectively carried out using an impregnation method. The textural, structural, morphological, and spectroscopic characterization of the resulting nanostructured materials (Mo6/Fe-g-C3N4) shows good consistency with the physicochemical properties previously described for both the octahedral clusters and carbon nitride materials, individually.

The studies confirmed that the exfoliation processes applied to the carbon nitride led to an improvement in the specific surface area of the pristine carbon nitride. The incorporation of Fe3+ and the molybdenum cluster into the g-C3N4 structure causes a negligible decrease in the material’s surface area, which is not considered critical for catalytic applications and is attributed to pore blocking in the graphitic material. It was verified that the estimated Fe mass ratio is similar to that used during the doping reaction, while the maximum Mo6 content reached 0.8% w/w. The incorporation of iron and molybdenum into the g-C3N4 support leads to enhanced light absorption properties and a slight reduction in band gap energy (from 2.67 to 2.61 eV). The electronic properties of the molybdenum and iron precursors are preserved after their integration into the carbonaceous material. The photophysical properties of the Mo6/Fe-g-C3N4 hybrid materials show that both components retain their emission characteristics in the composite material, confirming the presence of the cluster anchored to the support. However, the decrease in the emission lifetime of the Mo6 cluster upon integration into the carbon nitride suggests an electronic interaction between the two components, which promotes nonradiative deactivation processes possibly attributable to charge or energy transfer toward the support.

The photocatalytic evaluation of the Mo6/Fe-g-C3N4 nanomaterials in the CO2 valorization demonstrated high efficiency and selectivity for CO in acetonitrile in the presence of TEOA as a sacrificial agent. While the incorporation of Fe (7% w/w) into the g-C3N4 structure leads to the formation of H2, CO, and CH4, the addition of the Mo6 cluster to the Fe-g-C3N4-7 material improves CO production, yielding 3600 μmol·g–1 catalyst after 24 h of reaction, thus indicating that the CO production efficiency of the tricomponent material increases by a factor of 20 once the Mo6 cluster is grafted on the iron-doped graphenic surface. Furthermore, the presence of the metal cluster enhances the selectivity of this transformation to 88%. Interestingly, CO production further increases to approximately 2-fold (7752 μmol·g–1) when the Fe content is reduced to 1% under the same reaction conditions, thereby verifying that the Mo6/Fe-g-C3N4-1 nanostructured material is the most active and efficient catalyst, with lower metal content. This phenomenon is attributed to the more homogeneous distribution of Fe in the material and the greater availability of photogenerated electrons to participate in the reduction processes. Neither decreasing nor increasing the temperature (to 25 or 75 °C) compared to the initial reaction conditions (50 °C) led to improved CO production. Produced CO originates exclusively from the photoreduction of CO2, as confirmed by 13C labeling experiments. The proposed catalytic reaction mechanism implies electron transfer from the Mo6 cluster to the Fe reactive sites of the graphenic support.

Supplementary Material

ae5c02019_si_001.pdf (1.4MB, pdf)

Acknowledgments

The authors thank funding resources from the Spanish Ministry of Science and Innovation (TED2021-130963B-C21, PID2021-123163OB-I00 and CEX2021-001230-S grants) funded by MCIN/AEI/10.13039/501100011033/ and FEDER, “A way of making Europe.” J.S.H. gratefully acknowledges to Programa Santiago Grisolía (Consejo Superior de Investigaciones Científicas and Generalitat Valenciana; grant number GRISOLIA/2021/054). The authors thank the technical team at the Instituto de Tecnología Química for providing us with all the facilities for all the characterizations, equipment, and instrumentation.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.5c02019.

  • Experimental details (synthesis, characterization, and catalytic processes); materials (specific surface area, ICP, UV–vis, FTIR, Raman, PXRD, XPS, NMR, photoluminescence); CO2RR products (GC and GC-MS); and comparison of the catalytic performance with the literature (PDF)

All authors contributed to the creation and preparation of the manuscript. J.S.H. synthesized the materials, performed the characterizations and photocatalytic experiments, and developed, analyzed, discussed, and validated the experimental data. M.F. contributed to the conceptualization, participated in discussions, contributed to the manuscript preparation, and supervised the research. All authors have read and agreed to the published version of the manuscript.

The authors declare no competing financial interest.

References

  1. Kumar S., Karthikeyan S., Lee A. F.. G-C3N4-Based Nanomaterials for Visible Light-Driven Photocatalysis. Catalysts. 2018;8(2):74. doi: 10.3390/catal8020074. [DOI] [Google Scholar]
  2. Xu Q., Xia Z., Zhang J., Wei Z., Guo Q., Jin H., Tang H., Li S., Pan X., Su Z., Wang S.. Recent Advances in Solar-Driven CO2 Reduction over g-C3N4-Based Photocatalysts. Carbon Energy. 2023;5(2):205. doi: 10.1002/cey2.205. [DOI] [Google Scholar]
  3. Fu J., Yu J., Jiang C., Cheng B.. G-C3N4-Based Heterostructured Photocatalysts. Adv. Energy Mater. 2018;8(3):1701503. doi: 10.1002/aenm.201701503. [DOI] [Google Scholar]
  4. Wudil Y. S., Ahmad U. F., Gondal M. A., Al-Osta M. A., Almohammedi A., Sa’id R. S., Hrahsheh F., Haruna K., Mohamed M. J. S.. Tuning of Graphitic Carbon Nitride (g-C3N4) for Photocatalysis: A Critical Review. Arab. J. Chem. 2023;16(3):104542. doi: 10.1016/j.arabjc.2023.104542. [DOI] [Google Scholar]
  5. Bhanderi D., Lakhani P., Modi C. K.. Graphitic Carbon Nitride (g-C3N4) as an Emerging Photocatalyst for Sustainable Environmental Applications: A Comprehensive Review. RSC Sustain. 2024;2(2):265. doi: 10.1039/D3SU00382E. [DOI] [Google Scholar]
  6. Liras M., Barawi M., De La Peña O’Shea V. A.. Hybrid Materials Based on Conjugated Polymers and Inorganic Semiconductors as Photocatalysts: From Environmental to Energy Applications. Chem. Soc. Rev. 2019;48(22):5454. doi: 10.1039/C9CS00377K. [DOI] [PubMed] [Google Scholar]
  7. Wang X., Chen X., Thomas A., Fu X., Antonietti M.. Metal-Containing Carbon Nitride Compounds: A New Functional Organic-Metal Hybrid Material. Adv. Mater. 2009;21(16):1609. doi: 10.1002/adma.200802627. [DOI] [Google Scholar]
  8. Liu Y., Wu X., Lv H., Cao Y., Ren H.. Boosting the Photocatalytic Hydrogen Evolution Activity of G-C3N4 Nanosheets by Cu2(OH)2CO3 - Modification and Dye-Sensitization. Dalton Trans. 2019;48(4):1217. doi: 10.1039/C8DT03579B. [DOI] [PubMed] [Google Scholar]
  9. Fajrina N., Tahir M.. A Critical Review in Strategies to Improve Photocatalytic Water Splitting towards Hydrogen Production. Int. J. Hydrogen Energy. 2019;44(2):540. doi: 10.1016/j.ijhydene.2018.10.200. [DOI] [Google Scholar]
  10. Ghosh U., Majumdar A., Pal A.. Photocatalytic CO2 Reduction over G-C3N4 Based Heterostructures: Recent Progress and Prospects. J. Environ. Chem. Eng. 2021;9(1):104631. doi: 10.1016/j.jece.2020.104631. [DOI] [Google Scholar]
  11. Nagella S. R., Vijitha R., Ramesh Naidu B., Krishna Rao K. S. V., Ha C. S., Venkateswarlu K.. Benchmarking Recent Advances in Hydrogen Production Using G-C3N4-Based Photocatalysts. Nano Energy. 2023;111:108402. doi: 10.1016/j.nanoen.2023.108402. [DOI] [Google Scholar]
  12. Todorova N., Papailias I., Giannakopoulou T., Ioannidis N., Boukos N., Dallas P., Edelmannová M., Reli M., Kočí K., Trapalis C.. Photocatalytic H2 Evolution, CO2 Reduction, and NOx Oxidation by Highly Exfoliated g-C3N4 . Catalysts. 2020;10(10):1147. doi: 10.3390/catal10101147. [DOI] [Google Scholar]
  13. Van M. N., Mai O. L. T., Do C. P., Thi H. L., Manh C. P., Manh H. N., Thi D. P., Danh B. D.. Fe-Doped g-C3N4: High-Performance Photocatalysts in Rhodamine b Decomposition. Polymers. 2020;12(9):1963. doi: 10.3390/polym12091963. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Ossman M. E., Abdelfattah M., Farouq R.. Photocatalytic Degradation of Red Dye: Optimisation Using RSM. J. Environ. Eng. Sci. 2016;11(4):96. doi: 10.1680/jenes.16.00016. [DOI] [Google Scholar]
  15. Chen L., Maigbay M. A., Li M., Qiu X.. Synthesis and Modification Strategies of G-C3N4 Nanosheets for Photocatalytic Applications. Adv. Powder Mater. 2024;3(1):100150. doi: 10.1016/j.apmate.2023.100150. [DOI] [Google Scholar]
  16. Zhou Z., Zhang Y., Shen Y., Liu S., Zhang Y.. Molecular Engineering of Polymeric Carbon Nitride: Advancing Applications from Photocatalysis to Biosensing and More. Chem. Soc. Rev. 2018;47(7):2298. doi: 10.1039/C7CS00840F. [DOI] [PubMed] [Google Scholar]
  17. Sakuna P., Ketwong P., Ohtani B., Trakulmututa J., Kobkeatthawin T., Luengnaruemitchai A., Smith S. M.. The Influence of Metal-Doped Graphitic Carbon Nitride on Photocatalytic Conversion of Acetic Acid to Carbon Dioxide. Front. Chem. 2022;10:825786. doi: 10.3389/fchem.2022.825786. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Wang X., Nan Z.. Highly Efficient Fenton-like Catalyst Fe-g-C3N4 Porous Nanosheets Formation and Catalytic Mechanism. Sep. Purif. Technol. 2020;233:116023. doi: 10.1016/j.seppur.2019.116023. [DOI] [Google Scholar]
  19. Luo W., Huang W., Feng X., Huang Y., Song X., Lin H., Wang S., Mailhot G.. The Utilization of Fe-Doped g-C3N4 in a Heterogeneous Photo-Fenton-like Catalytic System: The Effect of Different Parameters and a System Mechanism Investigation. RSC Adv. 2020;10(37):21876. doi: 10.1039/D0RA00993H. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Shi J., Bai X., Xu L., Jin X., Shi X., Jin P.. Facile Preparation of Fe-C3N4 Heterojunction for Enhanced Pollutant Degradation in Fenton-like Process. J. Water Process Eng. 2022;46:102628. doi: 10.1016/j.jwpe.2022.102628. [DOI] [Google Scholar]
  21. Zhao Z., Liu W., Shi Y., Zhang H., Song X., Shang W., Hao C.. An Insight into the Reaction Mechanism of CO2 Photoreduction Catalyzed by Atomically Dispersed Fe Atoms Supported on Graphitic Carbon Nitride. Phys. Chem. Chem. Phys. 2021;23(8):4690. doi: 10.1039/D0CP05570K. [DOI] [PubMed] [Google Scholar]
  22. Nguyen N. T. K., Lebastard C., Wilmet M., Dumait N., Renaud A., Cordier S., Ohashi N., Uchikoshi T., Grasset F.. A Review on Functional Nanoarchitectonics Nanocomposites Based on Octahedral Metal Atom Clusters (Nb6, Mo6, Ta6, W6, Re6): Inorganic 0D and 2D Powders and Films. Sci. Technol. Adv. Mater. 2022;23(1):547. doi: 10.1080/14686996.2022.2119101. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Mikhailov M. A., Brylev K. A., Abramov P. A., Sakuda E., Akagi S., Ito A., Kitamura N., Sokolov M. N.. Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}4+ Core Cluster Complexes by Terminal Carboxylate Ligands. Inorg. Chem. 2016;55(17):8437–8445. doi: 10.1021/acs.inorgchem.6b01042. [DOI] [PubMed] [Google Scholar]
  24. Akagi S., Fujii S., Kitamura N.. A Study on the Redox, Spectroscopic, and Photophysical Characteristics of a Series of Octahedral Hexamolybdenum­(II) Clusters: [{Mo6X8}­Y6]2‑ (X, y = Cl, Br, or I) Dalton Trans. 2018;47(4):1131. doi: 10.1039/C7DT04485B. [DOI] [PubMed] [Google Scholar]
  25. Feliz M., Puche M., Atienzar P., Concepción P., Cordier S., Molard Y.. In Situ Generation of Active Molybdenum Octahedral Clusters for Photocatalytic Hydrogen Production from Water. ChemSusChem. 2016;9(15):1963. doi: 10.1002/cssc.201600381. [DOI] [PubMed] [Google Scholar]
  26. Kirakci K., Shestopalov M. A., Lang K.. Recent Developments on Luminescent Octahedral Transition Metal Cluster Complexes towards Biological Applications. Coord. Chem. Rev. 2023;481:215048. doi: 10.1016/j.ccr.2023.215048. [DOI] [Google Scholar]
  27. Casanova-Chafer J., Feliz M., Garcia-Aboal R., Atienzar P., Llobet E.. Octahedral Molybdenum Iodide Clusters Supported on Graphene for Resistive and Optical Gas Sensing. ACS Appl. Mater. Interfaces. 2022;14(51):57122. doi: 10.1021/acsami.2c15716. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Ghosh R. N., Baker G. L., Ruud C., Nocera D. G.. Fiber-Optic Oxygen Sensor Using Molybdenum Chloride Cluster Luminescence. Appl. Phys. Lett. 1999;75:2885. doi: 10.1063/1.125180. [DOI] [Google Scholar]
  29. Casanova-Chafer J., Blanch X., Moreno A., Garcia-Aboal R., Atienzar P., Llobet E., Feliz M.. Graphene Based Mo6 Cluster Hybrid for Detecting Simulant Nerve Agent DMMP. Sens. Biosensing Res. 2024;43:100603. doi: 10.1016/j.sbsr.2023.100603. [DOI] [Google Scholar]
  30. Nerambourg N., Aubert T., Neaime C., Cordier S., Mortier M., Patriarche G., Grasset F.. Multifunctional Hybrid Silica Nanoparticles Based on [Mo6Br14]2‑ Phosphorescent Nanosized Clusters, Magnetic γ-Fe2O3 and Plasmonic Gold Nanoparticles. J. Colloid Interface Sci. 2014;424:132. doi: 10.1016/j.jcis.2014.03.008. [DOI] [PubMed] [Google Scholar]
  31. Segura-Sanchis E., Moreno A., Ramiro-Manzano F., Fenollosa R., Feliz M., Atienzar P.. Optoelectronic Properties of Octahedral Molybdenum Cluster-Based Materials at a Single Crystal Level. Dalton Trans. 2023;52(47):17818. doi: 10.1039/D3DT02501B. [DOI] [PubMed] [Google Scholar]
  32. Kumar P., Mungse H. P., Cordier S., Boukherroub R., Khatri O. P., Jain S. L.. Hexamolybdenum Clusters Supported on Graphene Oxide: Visible-Light Induced Photocatalytic Reduction of Carbon Dioxide into Methanol. Carbon. 2015;94:91–100. doi: 10.1016/j.carbon.2015.06.029. [DOI] [Google Scholar]
  33. Volostnykh M. V., Kirakosyan G. A., Sinelshchikova A. A., Loboda P. A., Dorovatovskii P. V., Mikhaylov M. A., Tsivadze A. Y., Sokolov M. N., Gorbunova Y. G.. Supramolecular Hybrids Based on Ru (II) Porphyrin and Octahedral Mo (II) Iodide Cluster. Dalton Trans. 2023;52(16):5354. doi: 10.1039/D3DT00251A. [DOI] [PubMed] [Google Scholar]
  34. Volostnykh M. V., Mikhaylov M. A., Sinelshchikova A. A., Kirakosyan G. A., Martynov A. G., Grigoriev M. S., Piryazev D. A., Tsivadze A. Y., Sokolov M. N., Gorbunova Y. G.. Hybrid Organic-Inorganic Supramolecular Systems Based on a Pyridine End-Decorated Molybdenum (II) Halide Cluster and Zinc (II) Porphyrinate. Dalton Trans. 2019;48(5):1835. doi: 10.1039/C8DT04452J. [DOI] [PubMed] [Google Scholar]
  35. Hu S., Ma L., You J., Li F., Fan Z., Lu G., Liu D., Gui J.. Enhanced Visible Light Photocatalytic Performance of G-C3N4 Photocatalysts Co-Doped with Iron and Phosphorus. Appl. Surf. Sci. 2014;311:164. doi: 10.1016/j.apsusc.2014.05.036. [DOI] [Google Scholar]
  36. Fao G. D., Jiang J. C.. Theoretical Investigation of CO2 Conversion on Corrugated G-C3N4 Surface Decorated by Single-Atom of Fe, Co, and Pd. Mol. Catal. 2022;526:112402. doi: 10.1016/j.mcat.2022.112402. [DOI] [Google Scholar]
  37. Sun Z., Wang H., Wu Z., Wang L.. G-C3N4 Based Composite Photocatalysts for Photocatalytic CO2 Reduction. Catal. Today. 2018;300:160. doi: 10.1016/j.cattod.2017.05.033. [DOI] [Google Scholar]
  38. Liu G., Dong G., Zeng Y., Wang C.. The Photocatalytic Performance and Active Sites of G-C3N4 Effected by the Coordination Doping of Fe (III) Chin. J. Catal. 2020;41:1564. doi: 10.1016/S1872-2067(19)63518-7. [DOI] [Google Scholar]
  39. Ma J., Yang Q., Wen Y., Liu W.. Fe-g-C3N4/Graphitized Mesoporous Carbon Composite as an Effective Fenton-like Catalyst in a Wide PH Range. Appl. Catal., B. 2017;201:232. doi: 10.1016/j.apcatb.2016.08.048. [DOI] [Google Scholar]
  40. Bicalho H. A., Lopez J. L., Binatti I., Batista P. F. R., Ardisson J. D., Resende R. R., Lorençon E.. Facile Synthesis of Highly Dispersed Fe (II)-Doped g-C3N4 and Its Application in Fenton-like Catalysis. Mol. Catal. 2017;435:156. doi: 10.1016/j.mcat.2017.04.003. [DOI] [Google Scholar]
  41. Puche M., García-Aboal R., Mikhaylov M. A., Sokolov M. N., Atienzar P., Feliz M.. Enhanced Photocatalytic Activity and Stability in Hydrogen Evolution of Mo6 Iodide Clusters Supported on Graphene Oxide. Nanomaterials. 2020;10(7):1259. doi: 10.3390/nano10071259. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Schoonover J. R., Zietlow T. C., Clark D. L., Heppert J. A., Chisholm M. H., Gray H. B., Sattelberger A. P., Woodruff W. H.. Resonance Raman Spectra of [M6X8Y6]2‑ Cluster Complexes (M) Mo, W; X, Y = Cl, Br, I. Inorg. Chem. 1996;35(22):6606. doi: 10.1021/ic960184b. [DOI] [PubMed] [Google Scholar]
  43. Wang X., Maeda K., Thomas A., Takanabe K., Xin G., Carlsson J. M., Domen K., Antonietti M.. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009;8(1):76. doi: 10.1038/nmat2317. [DOI] [PubMed] [Google Scholar]
  44. Xi X., Peng X., Xiong C., Shi D., Zhu J., Wen W., Zhang X., Wang S.. Iron Doped Graphitic Carbon Nitride with Peroxidase like Activity for Colorimetric Detection of Sarcosine and Hydrogen Peroxide. Microchim. Acta. 2020;187(7):383. doi: 10.1007/s00604-020-04373-w. [DOI] [PubMed] [Google Scholar]
  45. Št́astný M., Tolasz J., Bavol D., Kloda M., Sugahara Y., Lang K., Henych J., Kirakci K.. Concerted Reactive Adsorption and Photocatalytic Degradation of Bisphenol-S on Molybdenum Cluster-Modified Nanoceria. Inorg. Chem. 2025;64(36):18166. doi: 10.1021/acs.inorgchem.5c02157. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Zhang Y., Pan Q., Chai G., Liang M., Dong G., Zhang Q., Qiu J.. Synthesis and Luminescence Mechanism of Multicolor-Emitting g-C3N4 Nanopowders by Low Temperature Thermal Condensation of Melamine. Sci. Rep. 2013;3:1943. doi: 10.1038/srep01943. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Gan Z., Shan Y., Chen J., Gui Q., Zhang Q., Nie S., Wu X.. The Origins of the Broadband Photoluminescence from Carbon Nitrides and Applications to White Light Emitting. Nano Res. 2016;9(6):1801. doi: 10.1007/s12274-016-1073-2. [DOI] [Google Scholar]
  48. Gao J., Wang Y., Zhou S., Lin W., Kong Y.. A Facile One-Step Synthesis of Fe-Doped g-C3N4 Nanosheets and Their Improved Visible-Light Photocatalytic Performance. ChemCatChem. 2017;9(9):1708. doi: 10.1002/cctc.201700492. [DOI] [Google Scholar]
  49. Kitamura N., Kuwahara Y., Ueda Y., Ito Y., Ishizaka S., Sasaki Y., Tsuge K., Akagi S.. Excited Triplet States of [{Mo6Cl8}­Cl6]2‑, [{Re6S8}­Cl6]4‑, and [{W6Cl8}­Cl6]2‑ Clusters. Bull. Chem. Soc. Jpn. 2017;90(10):1164. doi: 10.1246/bcsj.20170168. [DOI] [Google Scholar]
  50. Marchuk M. V., Vorotnikov Y. A., Kuratieva N. V., Shestopalov M. A.. Cationic Cluster Complex [{Mo6I8}­(H2O)4(OH)2]2+ with Diphenyl Phosphate Anions: Structure and Optical Properties Behavior. Inorg. Chim. Acta. 2024;560:121819. doi: 10.1016/j.ica.2023.121819. [DOI] [Google Scholar]
  51. Li P., Wang F., Wei S., Li X., Zhou Y.. Mechanistic Insights into CO2 Reduction on Cu/Mo-Loaded Two-Dimensional g-C3N4 (001) Phys. Chem. Chem. Phys. 2017;19(6):4405. doi: 10.1039/C6CP08409E. [DOI] [PubMed] [Google Scholar]
  52. Bian S., Li X., Zhang L., Wang L., Wang J., Xu Q., Zang L., Zhang Y., Sun L.. Bimetal Cu and Fe Modified G-C3N4 Sheets Grown on Carbon Skeleton for Efficient and Selective Photocatalytic Reduction of CO2 to CO. J. Environ. Chem. Eng. 2023;11(2):109319. doi: 10.1016/j.jece.2023.109319. [DOI] [Google Scholar]
  53. Kumar P., Kumar S., Cordier S., Paofai S., Boukherroub R., Jain S. L.. Photoreduction of CO2 to Methanol with Hexanuclear Molybdenum [Mo6Br14]2‑ Cluster Units under Visible Light Irradiation. RSC Adv. 2014;4(20):10420. doi: 10.1039/c3ra47255h. [DOI] [Google Scholar]
  54. Qiu Y., Ma L., Zeng D., Li M., Cui D., Lv Y., Zhang S., Xiao R.. Efficient CO2 to CO Conversion at Moderate Temperatures Enabled by the Cobalt and Copper Co-Doped Ferrite Oxygen Carrier. J. Energy Chem. 2020;46:123. doi: 10.1016/j.jechem.2019.10.025. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ae5c02019_si_001.pdf (1.4MB, pdf)

Articles from ACS Applied Energy Materials are provided here courtesy of American Chemical Society

RESOURCES