Syngas production by photoreforming of formic acid with 2D V_xW_1−xN_1.5 solid solution as an efficient cocatalyst

Abstract

Formic acid (FA) is a potential biomass resource of syngas with contents of carbon monoxide (CO, 60 wt.%) and hydrogen (H₂, 4.4 wt.%). Among the technologies for FA conversion, the photoreforming of FA has received widespread attention due to its use of green solar energy conversion technology and mild reaction conditions. Herein, a V–W bimetallic solid solution, V_xW_1−xN_1.5 with efficient co-catalytic properties was first and facilely synthesized. When CdS was used as a photocatalyst, the activity performance of the V_0.1W_0.9N_1.5 system was over 60% higher than that of the W₂N₃ system. The computational simulations and experiments showed the V_0.1W_0.9N_1.5 had great metallic features and large work functions, contributing a faster photo-generated carrier transfer and less recombination, finally facilitating a great performance in cocatalyst for syngas production in photoreforming FA. This work provides an approach to synthesizing novel transition metal nitrides for photocatalysis.

Graphical abstract

Keywords

photocatalysis / syngas / formic acid / cocatalyst / solid solution

1 Introduction

As environmental issues become more prominent, the utilization and development of clean energy are still full of challenges. Formic acid (FA) could be produced from biomass conversion or reduction of carbon dioxide (CO₂) [1–10], which is a potential intermediate for low-carbon emission technologies. With contents of carbon monoxide (CO, 60 wt.%) and hydrogen (H₂, 4.4 wt.%) [11,12], reforming of FA (dehydrogenation or dehydration) is an attractive solution for producing H₂ or syngas (mixture of CO and H₂) [13]. Both H₂ and syngas are important raw chemical materials and could be further used for synthesizing high-value chemicals through the Fischer–Tropsch process [11,14–16].

When FA is used as an intermediate for green energy conversation, especially for solar energy, it could be a potential economic energy carrier as liquid sunshine [7]. Typically, solar energy will be stored in the form of chemical energy in FA first, and then FA is used to produce chemical raw materials such as reformed to H₂ and syngas. For lower carbon emissions, direct solar-driven photoreforming has received attention in FA reforming at room temperature [17,18]. The cocatalyst–photocatalyst system has been used as an efficient photocatalysis system for photoreforming of FA because the cocatalyst promotes the charge separating and provides excellent active sites. In the past, a large number of photoreforming of FA systems with noble metal cocatalyst [18], such as AuPd/TiO₂ [19], AgPd/g-CN [20], and Pt/CdS [21], were investigated, among which, the low cost CdS have a suitable band structure for driving the reforming of FA with ultilization of visible light, and is therefore considered as a potential photocatalyst for photoreforming of FA [22–24]. However, considering the cost and rarity of cocatalysts (Pt, Au, Ag, and Pd), earth-abundant elements based cocatalyst–photocatalysts are more suitable for potential large-scale photoreforming FA systems. In the past few years, some excellent non-noble metal photocatalyst systems based on the CdS photocatalyst have been reported. For example, due to the excellent hydrogen evolution reaction (HER) and H₂ desorption ability of ultrasmall CoP, the CdS/CoP@RGO hybrid can achieve a high apparent quantum yield (AQY) of 32% for reforming FA at 420 nm [23]. A FeP@CdS nanorods photocatalysts were reported with a high photoreforming FA activity due to improved carrier separation, and the AQY of the system at 420 nm reached 54% [22]. Nevertheless, the utilization rate of solar energy is still low. Recently, transition metal nitrides (TMNs) have been an emerging class of materials as cocatalysts in photocatalysis systems [25], especially for photoreforming of FA [26,27]. Most TMNs materials show an excellent electrical conductivity, unique electronic structure, and a good mechanical behavior [28]. In previous work [27], the 1D CdS/2D W₂N₃ photocatalysis system is reported to reach an AQY at 420 nm of 76.84% for syngas production in FA/SF (sodium formate) solution, but it is still far below the requirements of industrial production. The main reasons limiting photocatalytic activity are often attributed to high carrier recombination and lack of excellent reactive sites. Thus, a more rationally designed cocatalyst can effectively extract photogenerated carriers from photocatalysts and provide excellent reaction active sites. However, the performance of photoreforming of FA still do not meet the actual needs of application field, and the rational design strategies for cocatalysts–photocatalysts systems are still insufficient.

For further improving the photocatalytic activity of the W₂N₃-based photoreforming FA system, some novel design strategies for cocatalysts are needed. Among them, doping TMNs with metal heteroatom dopants is a common and effective strategy, which could disturb the original distribution of materials, further improving their chemical and electronic properties and affecting their catalytic activity [29,30]. Among the various doping metals, the V atoms as dopants were developed due to their lower d-band electron density and suitable atomic radius [30,31]. V-doped Co₄N (V-Co₄N, Co:3d74s2) has been reported to have a high electrocatalytic activity toward HER because V doping reconfigures the electronic structure of V-Co₄N and transfers electrons from V to Co, and doped V atoms also leads to a lower center of the d-band, a lower valence band, and a weakened adsorption of H* adsorption [30]. Notably when the content of dopants was increasing in the metal atom heteroatoms doping material, there may be alloying in places or even as a whole solid solution. The alloying is also used to optimize the chemical and physical properties of 2D TMNs [32]. For example, it has been reported that two-dimmensional (2D) V_0.2Mo_0.8N_1.2 bimetallic nitridene solid solution was synthesized, in which the surface Mo atoms become electron-rich active sites due to the donated electrons from alloyed V atoms, thereby promoting hydrogen coupling [32]. However, there is still no work reported on the modification of W₂N₃ using alloying strategy and used for photoreforming of FA. In addition, further studies on the cocatalytic properties of W₂N₃-based solid solutions and related mechanisms are lacking.

Herein, a 2D V_xW_1−xN_1.5 solid solution was first reported as an efficient cocatalyst for photoreforming FA to produce syngas. In the most efficient 2D V_xW_1−xN_1.5 solid solution in CdS based photocatalytic reaction system, V_0.1W_0.9N_1.5, the V atoms were uniformly distributed on the V_0.1W_0.9N_1.5. The density functional theory (DFT) calculations, ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS) confirmed that the excellent cocatalytic activity of V_0.1W_0.9N_1.5 was originated from the strong intereaction of heterojunction and further optimizing of the charge separation on the interface of CdS and 2D V_0.1W_0.9N_1.5 solid solution. This work offers an avenue for contributing efficient TMNs-based catalysts and optimizing the related heterojunction materials.

2 Experimental section

2.1 Chemical and materials

All of the used materials were of analytical grade and used as received without further purification. FA (HCOOH, ≥ 98%), ethanol absolute (C₂H₅OH, ≥ 99.7%), sodium sulfide nonahydrate (Na₂S·9H₂O), cadmium acetate dihydrate (Cd(Ac)₂·2H₂O), sodium tungstate dihydrate (Na₂WO₄·2H₂O), tungsten trioxide (WO₃), and vanadium pentoxide (V₂O₅) were purchased from Sinopharm Chemical Reagent Co., Ltd. or Sigma Andrich. Ultrapure water (resistivity: ~18.5 MΩ·cm) was used when needed.

2.2 Preparation of 2D V_xW_1−xN_1.5 solid solution

The 2D layered V_xW_1−xN_1.5 nanosheets were synthesized via a molten salt-directed synthesis method reported in Refs. [27,32]. Initially, V₂O₅ and WO₃ were combined in mole ratios of 2 : 98, 10 : 90, and 20 : 80, with a total molar quantity of 0.1 mol. This V₂O₅/WO₃ mixture was then mixed with 3.30 g of Na₂WO₄·2H₂O powder using a mill system (3 cycles, every cycle with 400 r/min for 20 min) to obtain the mixture as the precursor. Subsequently, 100 mg of this mixture was evenly placed into a porcelain boat. The 2D V_xW_1−xN_1.5 sample was formed by annealing the as-prepared precursor at 750 °C for 5 h with a heating rate of 1 °C/min at 5% NH₃/Ar atmosphere. The resulting product was cleaned in deionized water using ultrasonic treatment for 30 min, then filtered to remove the salt, and freeze-dried to obtain the V_xW_1−xN_1.5 nanosheets sample. The 2D W₂N₃ samples were synthesized using a similar method as the preparation of V_xW_1−xN_1.5 solid solution nanosheets, without addition of V₂O₅.

2.3 Preparation of CdS nanoparticle

CdS nanoparticles (CdS NPs) were prepared according to the methods in Cao et al. [23]. In a typical synthesis, 200 mL of aqueous Na₂S (0.2 mol/L) was added to 250 mL of 0.2 mol/L (CH₃COO)₂Cd aqueous solution under vigorous stirring to obtain a yellow precipitate. The mixture was stirred for 24 h and kept undisturbed at 25 °C for another 24 h. The solids were separated by filtration and dispersed into 160 mL of water, and then the mixture was added to a Teflonlined stainless steel autoclave (2 × 100 mL) and heated at 200 °C for 72 h. Finally, the CdS NPs obtained were separated by centrifugation and washed by water at least three times.

2.4 Preparation of CdS/V_xW_1−xN_1.5 hybrid photocatalysts

CdS/2D V_xW_1−xN_1.5, which are denoted as CdS/V_xW_1−xN_1.5 in this work, was synthesized by simple physical mixing. Generally, the CdS/V_xW_1−xN_1.5 hybrid was prepared by mixing different ratios of CdS NPs and 2D V_xW_1−xN_1.5 in 5 mL of ethanol absolute by sonicating for 1 h and then stirred for 12 h at room temperature. After that, the sample was vacuum-dried at 65°C and collected. CdS/VN_x was prepared from CdS and VN_x using the method described above.

2.5 Evaluation of photocatalytic performance

Photocatalytic reforming of FA was conducted using a custom-built reaction system. Typically, 80 mL of FA aqueous solution and 4 mg of photocatalyst were added to a Pyrex glass cell with a volume of 105 mL. The mixture was then subjected to ultrasonication for a minimum of 10 min. Subsequently, the reactor was purged with argon for 20 min. The reaction cell was then irradiated by simulated sunlight, which was provided by a Xenon lamp source (Microsolar 300, Beijing Perfectlight, AM 1.5G filter) with an intensity of approximately 0.56 W/cm². The magnetic stirring was kept during the photocatalytic reaction, and the reaction system was maintained at room temperature (25 °C) using cooled circulating water. During the process, 100 μL of gas was collected from the reactor at each sampling time using a gas syringe. The generated gas products were analyzed using a gas chromatograph (GC) equipped with a thermal-conductivity detector (Agilent 7890A with argon as the carrier gas) and a PLOT C-2000 chromatographic column. Specific amounts of CO, H₂, and CO₂ were determined using the external standard method. Additionally, a cycling stability test was conducted by refreshing the reaction system through bubbling with argon.

3 Results and discussion

3.1 Structural characterization of V_xW_1−xN_1.5

Several samples of V_xW_1−xN_1.5 were prepared by the modified melt salt method with different ratios of vanadium source and tungsten source. Generally, the mixture precursor of V₂O₅ (vanadium source), WO₃ (tungsten source), and Na₂WO₄·2H₂O (salt) was processed at 750 °C under 5% NH₃/Ar atmosphere, then the samples were collected after washing out the salt. First, the crystal structure of as-prepared V_xW_1−xN_1.5 samples was analyzed by XRD. Both V_0.02W_0.98N_1.5 and V_0.1W_0.9N_1.5 have similar crystal structures as hexagonal-phase W₂N₃ (PDF#01-081-9140) (Fig.1(a)) [33,34]. However, near 37.6, 43.7, and 63.7 degrees, the character peaks belonging to VN_x could be observed at the pattern of V_0.2W_0.8N_1.5, which confirmed that the V atoms would form VN_x other than a uniform solid solution (Fig.1(b)). This might be due to the limited solubility of V atoms in 2D layered TMNs solid solution [32], which further suggested the V/W molar ratio 1:9 could be the limit to form V_xW_1−xN_1.5 solid solution with no VN_x formation. Scanning electron microscopy (SEM) further showed the W₂N₃ and all V_xW_1−xN_1.5 samples were essentially 2D in shape (Fig. S1). However, the presence of some particles in the V_0.2W_0.8N_1.5 sample, which are believed to be VN_x, further suggests that a high proportion of V resource can lead to the formation of VN_x. Therefore, 2D V_0.1W_0.9N_1.5 is used as a typical example in this work to further demonstrate and discuss the characterizations of V_xW_1−xN_1.5 solid solution. The high-resolution transmission electron microscopy (HRTEM) mages showed the 2D morphology and atomic structure of V_0.1W_0.9N_1.5, which had an in-plane spacing of 0.226 nm similar to that in Ref. [103] of W₂N₃ (Fig.2(a) and Fig.2(b)). In the scanning transmission electron microscopy (STEM) mode, the energy-dispersive X-ray spectroscopy (EDS) (Fig. S2) and its mapping images show that the V atoms are uniformly dissolved into W–N lattice successfully (Fig.2(d)–Fig.2(g)). Then, the inductively coupled plasma-mass spectrometry (ICP-MS) results show that the V element content in the V_xW_1−xN_1.5 sample increases with increasing content of V precursor (Table S1). Furthermore, as shown in the XRD pattern (Fig.3(a)), the peak near 64.6 degrees shifts to the right as the amount of V increases, indicating that the lattice constants of V_xW_1−xN_1.5 are larger than those of W₂N₃, which might be due to the substitution of V (a smaller atomic radius) for W (a large atomic radius), resulting in lattice distortion. Then, the chemical state of elements in 2D V_0.1W_0.9N_1.5 sample was analyzed by XPS. By analyzing the fine spectrum of V 2p, peaks at 513.5, 515.3, and 517.2 eV were found, corresponding to V 2p_3/2, indicating the presence of V²⁺, V³⁺, and V⁵⁺ oxidation states (Fig. S3), suggesting that vanadium in the solid solution exists as a complex mixed valence state. Fig.3(b) further shows that the intensity of V characteristic peaks in V 2p spectra increases with the amount of V, whereas the shape and position of the peaks of V_0.1W_0.9N_1.5 were different from VN_x. It suggests the V is successfully alloyed in V_0.1W_0.9N_1.5 without phase transformation or separation similar to that occurring in V_0.2W_0.8N_1.5, which has a similar V 2p spectrum of VN_x. The XPS W 4f spectra of V_0.1W_0.9N_1.5 and W₂N₃ reveal two main peaks at approximately 32.7 and 34.9 eV, which are attributed to the W 4f_7/2 and W 4f_5/2 of W–N, respectively. However, a slightly positive peak shift is observed in V_0.1W_0.9N_1.5 compared to W₂N₃, indicating a higher W valence in V_0.1W_0.9N_1.5 (Figs. S4 and Fig.3(c)). This suggests that the presence of alloyed V atoms leads to charge redistribution within the solid solution materials. The calculation of average Bader charge further shows that the Bader charge of W on the V_0.1W_0.9N_1.5 is lower than that on the pristine W₂N₃ (Fig.3(e) and Fig.3(f)), which is consistent with XPS results and confirms the charge transfer between the W and V. The valence band spectra of XPS show that the intensity of both W₂N₃ and V_0.1W_0.9N_1.5 samples is not zero at the Fermi level (Fig.3(d)), suggesting the V_0.1W_0.9N_1.5 still maintains the metallic features which are similar to those of W₂N₃ [25,27] and a suitable number of the alloyed V atoms could not significantly change the energy band of material near the Fermi level. Moreover, as shown in Fig.3(g) and Fig.3(h), there is still intensity of density of state (DOS) at the Fermi energy level of V_xW_1−xN_1.5 like that of W₂N₃, and the contribution of V 3d in the total density of state is not significant, further indicating the alloyed V atoms in tungsten nitride would not significantly change the metallic features of samples and the V_xW_1−xN_1.5 still have good metallic features.

3.2 Photoreforming of FA with CdS/V_xW_1−xN_1.5 hybrid photocatalysts

First, the chopped-light linear sweep voltammograms (LSVs) and UV-visible (UV-vis) spectra were detected to discuss the role of V_xW_1−xN_1.5 in the photocatalytic system. As shown in Figs. S5 and S6, there were no photocurrent responses of V_xW_1−xN_1.5 solid solution under irradiation, and no obvious absorption edge existence in the wide spectrum (300–1200 nm), indicating that the 2D V_xW_1−xN_1.5 solid solution could not produce photo-generated carriers and it could act as a cocatalyst in the system like W₂N₃ [27]. The cocatalytic performance of 2D V_xW_1−xN_1.5 solid solution was tested under the CdS photocatalyst system in FA aqueous solution (Figs. S7 and S8). According to the controlled experiments (Table S2), the photoreforming of FA is considered as the primary reaction in the CdS/V_0.1W_0.9N_1.5 photocatalytic reaction system. As shown in Fig.4(a), both W₂N₃ and V_xW_1−xN_1.5 could greatly enhance the performance of photoreforming of FA. Among the different CdS/V_xW_1−xN_1.5 samples, the CdS/V_0.1W_0.9N_1.5 showed the highest improvement in photoreaction. Notably, no gas products could be detected in control experiments without CdS, light, or FA, suggesting that the reforming reaction of FA is based on CdS photocatalyst and the V_xW_1−xN_1.5 is as a cocatalyst (Table S2). In particular, after optimizing the ratio of CdS and V_0.1W_0.9N_1.5, the CdS/V_0.1W_0.9N_1.5 hybrid photocatalyst could reach 184.04 μmol/h of H₂ evolution (Fig.4(b)). The H₂ evolution activity enhancement of CdS/V_0.1W_0.9N_1.5 70% is higher than that of CdS/W₂N₃, which suggests that 2D V_0.1W_0.9N_1.5 could be a better photocatalytic cocatalyst. However, the CdS/V_0.2W_0.8N_1.5 hybrid photocatalysts showed a lower activity than that of CdS/V_0.1W_0.9N_1.5 due to the presence of VN_x in V_0.2W_0.8N_1.5, which could not be an ideal cocatalyst for photoreforming of FA (Table S2). It is worth noting that the excess content of V_0.1W_0.9N_1.5 would harm the activity due to the shielding effect of cocatalyst. Furthermore, V_0.1W_0.9N_1.5 also shows similar cocatalytic performance as the other typical photocatalyst like TiO₂ and g-C₃N₄ (Fig. S9), indicating V_xW_1−xN_1.5 solid solution has extensibility as cocatalysts. On the other hand, for the production of syngas by photoreforming of FA, selectivity is also a key indicator. The selectivity in photoreforming FA for CdS/W₂N₃ and CdS/V_0.1W_0.9N_1.5 is similar (Fig.4(a)), suggesting that the alloyed V atoms has not significantly affected the selectivity of reforming of FA. In addition, the H₂/CO₂ ratio larger than 1∶1 (the theoretical stoichiometric ratio) may be attributed to the higher solubility of CO₂ in FA aqueous solution than H₂ (Table S2). The cycling test results demonstrated the stability of the reaction system, showing that the photocatalytic activity did not significantly decrease after 3 cycles. This suggests that the photocatalytic system established was relatively stable (Fig.4(c)). Additionally, the lower level of photocorrosion in CdS/V_0.1W_0.9N_1.5 compared to CdS, as further confirmed by ICP-MS (Table S3), indicates a higher stability for the CdS/V_0.1W_0.9N_1.5 composite. These above results indicate V_xW_1−xN_1.5 has an excellent cocatalytic performance for photoreforming of FA in the CdS based system, and solid solution for bimetallic transition metal nitrides could be a potential strategy for novel cocatalysts.

3.3 Mechanism investigation of efficiency photocatalysis

To elucidate the mechanism by V_xW_1−xN_1.5 enhances the activity of photoreforming of FA. The light spectrum utilization of CdS and CdS/V_0.1W_0.9N_1.5 hybrid photocatalysts was first analyzed using UV-vis spectroscopy. The shape of the UV-vis spectra of CdS/V_0.1W_0.9N_1.5 hybrid is similar to that of CdS (Fig. S10(a)). Still, the CdS/V_0.1W_0.9N_1.5 composite has a higher absorbance intensity in the region greater than 550 nm, due to the extremely high absorbance intensity of all light spectrum of V_0.1W_0.9N_1.5. The Tauc plot further confirms that there is no significant change in the band gap, indicating that the ability of CdS to utilize the light to generate photogenerated carriers in CdS/V_0.1W_0.9N_1.5 is similar to that of bare CdS (Fig. S10(b)). Thus, the possible origins of the enhancement of photocatalytic activity by V_0.1W_0.9N_1.5 could be more efficient carrier transfer and less carrier recombination. Then, to confirm the above hypotheses, the interaction between CdS and the V_0.1W_0.9N_1.5 heterojunction, which affects the photogenerated carrier transfer, was studied. As shown in Fig. S11(a) and S11(b), after contacting between CdS and V_0.1W_0.9N_1.5, the W 4f peaks and the Cd 3d peaks in CdS/V_0.1W_0.9N_1.5 were negatively and positively shifted, respectively, indicating the increased surface electron density in V_0.1W_0.9N_1.5 and decreased surface electron density in CdS. These results imply that V_0.1W_0.9N_1.5 receives the electrons from CdS and confirm the strong interaction formatted between V_0.1W_0.9N_1.5 and CdS in CdS/V_0.1W_0.9N_1.5 hybrid, which promotes transfer and separation of carriers. Furthermore, as shown in Fig. S12, the E_cutoff of V_0.1W_0.9N_1.5 and CdS was 16.74 and 17.51 eV, respectively. Thus, the work function of V_0.1W_0.9N_1.5 (4.48 eV) is larger than that of CdS (3.71 eV), suggesting that a strong built-in electric field between CdS and V_0.1W_0.9N_1.5 develops when these two materials are in contact. The formation of the strong built-in electric field in heterojunction complex photocatalysts would reduce the recombination of photogenerated electron-hole and improve the separation of carriers. Steady-state photoluminescence/time-resolved photoluminescence (PL/TRPL) spectra was used to further study the charge carrier transfer process. As shown in the PL spectra of different samples (Fig.5(a)), the emission peak intensity of CdS/V_0.1W_0.9N_1.5 is much lower than that of CdS and even lower than that of CdS/W₂N₃, indicating that the CdS/V_0.1W_0.9N_1.5 system has an efficient charge transfer. Further, TRPL was introduced to investigate the lifetime of photogenerated carriers (Fig.5(c) and Fig.5(d)). Correspondingly, τ1 and τ2 are associated with the carriers recombination lifetime from the near-band-edge emission and hole traps in deeply located states, respectively [35,36]. In the comparison of CdS with CdS/V_0.1W_0.9N_1.5, the increase in τ1 from 31 to 36 ps indicates a more efficient charge transfer near the conduction band (CB) state for CdS/V_0.1W_0.9N_1.5. Correspondingly, the τ2 value of CdS (493 ps) is larger than that of CdS/V_0.1W_0.9N_1.5 (453 ps), suggesting that the surface state of CdS is changed by contacting between CdS and V_0.1W_0.9N_1.5 in which a charge transfer pathway is formed in the V_0.1W_0.9N_1.5-CdS interface [11]. Furthermore, the average exciton lifetime was calculated as 0.457 ns (CdS) and 0.408 ns (CdS/V_0.1W_0.9N_1.5), indicating that the formation heterojunction between CdS and V_0.1W_0.9N_1.5 was beneficial for the charge separation and extraction [37]. The electrochemistry impedance spectroscopy (EIS)-derived bode plots of different samples were also studied. As shown in Fig.5(b), the CdS/V_0.1W_0.9N_1.5 demonstrates a smaller semicircle radius than bare CdS and CdS/W₂N₃, representing a better carrier mobility and a lower charge-transfer resistance [38]. These above results confirm that the V_0.1W_0.9N_1.5 solid solution has an excellent carrier transfer ability, which is attributed to the stronger interaction between the interface of V_0.1W_0.9N_1.5 and CdS.

The mechanism of photoreforming of FA over CdS/V_0.1W_0.9N_1.5 was further explored. To detect oxidative active species by inhibition experiments, isopropanol (IPA) and ethylenediaminetetraacetic acid disodium (EDTA-2Na) were used in the system as the radical scavengers of OH and h⁺, respectively. According to the photocatalytic activity after the addition of IPA and EDTA-2Na in the reaction system (Fig.6(c)), the hole (h⁺) could be the main active species that participate in the photoreforming of FA, which is in agreement with the previous work [27]. The radical or intermediate of the photoreforming FA reaction was further detected by the in situ electron paramagnetic resonance (EPR) test. For the CdS/V_0.1W_0.9N_1.5 photocatalyst system, no signal could be seen under dark conditions, whereas only 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)–{\mathrm{C}\mathrm{O}}_2^{-} signal could be detected under irradiation, which is similar to that of CdS and CdS/W₂N₃ (Fig.6(a) and Fig.6(b)). The band structure further confirms that CdS could provide sufficient thermodynamic driving force to oxidation HCOO- to {\mathrm{C}\mathrm{O}}_2^{-} (Fig. S12). It suggests that the main intermediate of photoreforming reaction is {\mathrm{C}\mathrm{O}}_2^{-} other than others, and also indicates that the alloyed V atoms in the V_xW_1−xN_1.5 solid solution would not change the reaction steps of photoreforming FA. Therefore, the mechanism of FA reforming in the CdS/V_0.1W_0.9N_1.5 photocatalytic system can be proposed as follows (Fig.6(d)): CdS produces electron-hole pairs when it absorbs light with the right energy. Then, the electron-hole pair in the presence of a CdS/V_0.1W_0.9N_1.5 heterojunction will separate, where the electrons will be directed to V_0.1W_0.9N_1.5 and the holes will remain on the CdS. Afterwards, the H⁺ that comes from FA participates in HER on V_0.1W_0.9N_1.5, the HCOO-anion is oxidized by holes on VB of CdS to produce the intermediate {\mathrm{C}\mathrm{O}}_2^{-}, and the {\mathrm{C}\mathrm{O}}_2^{-} would further be converted to CO₂ or CO.

4 Conclusions

In summary, a 2D layered V_0.1W_0.9N_1.5 solid solution was synthesized for efficient photoreforming of FA as a novel cocatalyst. Compared with pristine W₂N₃, the V_0.1W_0.9N_1.5 solid solution further boosted the syngas production rate in the CdS-based system. Furthermore, the experimental results and DFT calculation confirmed that V_0.1W_0.9N_1.5 has a larger work function and a modified electronic structure, which causes a stronger interaction between the heterojunction of CdS and 2D V_0.1W_0.9N_1.5 and further facilitates the photogenerated carrier separation. This work offers a strategy for rational design of 2D TMNs materials as cocatalysts in the photoreforming of FA and other photocatalytic reaction systems.

References

[1] Qi L, Mui Y F, Lo S W. . Catalytic conversion of fructose, glucose, and sucrose to 5-(hydroxymethyl)furfural and levulinic and formic acids in γ-valerolactone as a green solvent. ACS Catalysis, 2014, 4(5): 1470–1477

[2] Voß D, Pickel H, Albert J. Improving the fractionated catalytic oxidation of lignocellulosic biomass to formic acid and cellulose by using design of experiments. ACS Sustainable Chemistry & Engineering, 2019, 7(11): 9754–9762

[3] Kato N, Mizuno S, Shiozawa M. . A large-sized cell for solar-driven CO₂ conversion with a solar-to-formate conversion efficiency of 7.2%. Joule, 2021, 5(3): 687–705

[4] Antón-García D, Edwardes Moore E, Bajada M A. . Photoelectrochemical hybrid cell for unbiased CO₂ reduction coupled to alcohol oxidation. Nature Synthesis, 2022, 1(1): 77–86

[5] Wei D, Sang R, Sponholz P. . Reversible hydrogenation of carbon dioxide to formic acid using a Mn-pincer complex in the presence of lysine. Nature Energy, 2022, 7(5): 438–447

[6] Schwarz F M, Moon J, Oswald F. . Biological hydrogen storage and release through multiple cycles of bi-directional hydrogenation of CO₂ to formic acid in a single process unit. Joule, 2022, 6(6): 1304–1319

[7] Zhai S, Jiang S, Liu C. . Liquid sunshine: Formic acid. Journal of Physical Chemistry Letters, 2022, 13(36): 8586–8600

[8] Pan H, Heagy M D. Photons to formate: A review on photocatalytic reduction of CO₂ to formic acid. Nanomaterials, 2020, 10(12): 2422

[9] Chen Z, Junge H, Beller M. Amino acid promoted hydrogen battery system using Mn-pincer complex for reversible CO₂ hydrogenation to formic acid. Frontiers in Energy, 2022, 16(5): 697–699

[10] Chen X, Jin F. Photocatalytic reduction of carbon dioxide by titanium oxide-based semiconductors to produce fuels. Frontiers in Energy, 2019, 13(2): 207–220

[11] Hu X, Jin J, Wang Y. . Au/MoS₂ tips as auxiliary rate aligners for the photocatalytic generation of syngas with a tunable composition. Applied Catalysis B: Environmental, 2022, 308: 121219

[12] Zhou H, Wang M, Wang F. Oxygen-controlled photo-reforming of biopolyols to CO over Z-scheme CdS@g-C₃N₄. Chem, 2022, 8(2): 465–479

[13] Gao Y, Hu E, Huang B. . Formic acid dehydrogenation reaction on high-performance Pd_xAu_1−xalloy nanoparticles prepared by the eco-friendly slow synthesis methodology. Frontiers in Energy, 2023, 17(6): 751–762

[14] Liang Z, Song L, Sun M. . Tunable CO/H₂ ratios of electrochemical reduction of CO₂ through the Zn-Ln dual atomic catalysts. Science Advances, 2021, 7(47): eabl4915

[15] Guo S, Zhao S, Wu X. . A Co₃O₄-CDots-C₃N₄ three component electrocatalyst design concept for efficient and tunable CO₂ reduction to syngas. Nature Communications, 2017, 8(1): 1828

[16] Andrei V, Reuillard B, Reisner E. Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite–BiVO₄ tandems. Nature Materials, 2020, 19(2): 189–194

[17] Toe C Y, Tsounis C, Zhang J. . Advancing photoreforming of organics: Highlights on photocatalyst and system designs for selective oxidation reactions. Energy & Environmental Science, 2021, 14(3): 1140–1175

[18] Puga A V. Photocatalytic production of hydrogen from biomass-derived feedstocks. Coordination Chemistry Reviews, 2016, 315: 1–66

[19] Zhang Z Y, Cao S W, Liao Y S. . Selective photocatalytic decomposition of formic acid over AuPd nanoparticle-decorated TiO₂ nanofibers toward high-yield hydrogen production. Applied Catalysis B: Environmental, 2015, 162: 204–209

[20] Xiao L, Jun Y S, Wu B. . Carbon nitride supported AgPd alloy nanocatalysts for dehydrogenation of formic acid under visible light. Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2017, 5(14): 6382–6387

[21] Li Y X, Hu Y F, Peng S Q. . Synthesis of CdS nanorods by an ethylenediamine assisted hydrothermal method for photocatalytic hydrogen evolution. Journal of Physical Chemistry C, 2009, 113(21): 9352–9358

[22] Wang T, Yang L, Jiang D. . CdS nanorods anchored with crystalline FeP nanoparticles for efficient photocatalytic formic acid dehydrogenation. ACS Applied Materials & Interfaces, 2021, 13(20): 23751–23759

[23] Cao S, Chen Y, Wang H. . Ultrasmall CoP nanoparticles as efficient cocatalysts for photocatalytic formic acid dehydrogenation. Joule, 2018, 2(3): 549–557

[24] Zhou P, Zhang Q, Xu Z. . Atomically dispersed Co-P₃ on CdS nanorods with electron-rich feature boosts photocatalysis. Advanced Materials, 2020, 32(7): 1904249

[25] Wang Y L, Nie T, Li Y H. . Black tungsten nitride as a metallic photocatalyst for overall water splitting operable at up to 765 nm. Angewandte Chemie, 2017, 129(26): 7538–7542

[26] Wang T, Chen M, Wu J. . Highly efficient photocatalytic formic acid decomposition to syngas under visible light using CdS nanorods integrated with crystalline W₂N₃ nanosheets. Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2023, 11(5): 2246–2251

[27] Ye X, Dong Y, Zhang Z. . Mechanism insights for efficient photocatalytic reforming of formic acid with tunable selectivity: Accelerated charges separation and spatially separated active sites. Applied Catalysis B: Environmental, 2023, 338: 123073

[28] Wang H, Li J, Li K. . Transition metal nitrides for electrochemical energy applications. Chemical Society Reviews, 2021, 50(2): 1354–1390

[29] Tang Y, Yang C, Xu X. . MXene nanoarchitectonics: Defect-engineered 2D MXenes towards enhanced electrochemical water splitting. Advanced Energy Materials, 2022, 12(12): 2103867

[30] Chen Z, Song Y, Cai J. . Tailoring the d-band centers enables Co₄N nanosheets to be highly active for hydrogen evolution catalysis. Angewandte Chemie International Edition, 2018, 57(18): 5076–5080

[31] Zhang H M, Wang J J, Meng Y, et al. Review on intrinsic electrocatalytic activity of transition metal nitrides on HER. Energy Material Advances. 2022, 2022: 0006

[32] Jin H, Yu H, Li H. . MXene analogue: A 2D nitridene solid solution for high-rate hydrogen production. Angewandte Chemie International Edition, 2022, 61(27): e202203850

[33] Jin H, Li L, Liu X. . Nitrogen vacancies on 2D layered W₂N₃: A stable and efficient active site for nitrogen reduction reaction. Advanced Materials, 2019, 31(32): 1902709

[34] Wang S, Yu X, Lin Z. . Synthesis, crystal structure, and elastic properties of novel tungsten nitrides. Chemistry of Materials, 2012, 24(15): 3023–3028

[35] Zhang K, Kim J K, Park B. . Defect-induced epitaxial growth for efficient solar hydrogen production. Nano Letters, 2017, 17(11): 6676–6683

[36] Tian B, Gao W, Zhang X. . Water splitting over core-shell structural nanorod CdS@Cr₂O₃ catalyst by inhibition of H₂–O₂ recombination via removing nascent formed oxygen using perfluorodecalin. Applied Catalysis B: Environmental, 2018, 221: 618–625

[37] Li R, Takata T, Zhang B. . Criteria for efficient photocatalytic water splitting revealed by studying carrier dynamics in a model Al-doped SrTiO₃ photocatalyst. Angewandte Chem International Edition, 2023, 62: e202313537

[38] Qi M Y, Li Y H, Zhang F. . Switching light for site-directed spatial loading of cocatalysts onto heterojunction photocatalysts with boosted redox catalysis. ACS Catalysis, 2020, 10(5): 3194–3202

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