Facile synthesis of Mo-doped TiO2 for selective photocatalytic CO2 reduction to methane: Promoted H2O dissociation by Mo doping
Graphical abstract
Mo doping promotes H2O dissociation, thereby accelerating proton supply for CH4 generation, which is an important reason for the enhanced CH4 selectivity over Mo-doped TiO2.
Introduction
Photocatalytic CO2 reduction with H2O vapor on chemically stable and cheap TiO2 has gained extensive attention because it is a promising “green chemistry” strategy for direct conversion of CO2 to fuels or value-added chemicals driven by sunlight. [1,2] Both liquid-(HCOOH, CH3OH, etc.) and gas-phase (CH4) products have been monitored in liquid-solid systems where TiO2 is dispersed in CO2-saturated aqueous solutions, [[3], [4], [5]] while CO and CH4 are commonly produced at the solid–gas interface of TiO2 and reactants. [[6], [7], [8]] The solid-gas reaction mode obviously facilitates the separation of TiO2 and products and industrial amplification. However, it suffers from low selectivity to the CH4 product that has quite high heat of combustion, owing to the fact that CH4 generation requires eight photoinduced electrons and eight protons, involving multiple reaction steps. [9,10]
Several strategies were applied to enhance the CH4 selectivity over TiO2, such as surface modification, [11] heterojunction constructing [5], metal co-catalysis [12], and so on [[13], [14], [15]]. Among them, metal co-catalysis has been a popular technique since Kraeutler et al. first introduced Pt on TiO2 for photocatalytic CO2 reduction in 1978. So far, the employment of metal co-catalysts mainly focused on noble metals (such as Pt, Pd, Ru, Au, Ag and Ir) because of high chemical stability in oxidizing atmospheres. [[16], [17], [18], [19]] Nevertheless, the high cost and scarcity of noble metals are key obstacles for their extensive use [20]. As such, it is necessary to explore cheap alternatives to noble metals for selective photocatalytic CO2 reduction to CH4 [21,22], which requires understanding of the mechanism of promoting CH4 production by noble metals. Wang and co-workers research suggested that the efficient electron − hole separation by ultrafine Pt nanoparticles was the main reason attributable for the CH4-yield enhancement over Pt/TiO2; [23] it has been also reported by Dong and co-workers that the terrace sites of Pt nanoparticles are served as the active sites for methane generation, while the low-coordinated sites are more favorable in the competing hydrogen evolution reaction [24]. In our previous study, it was found that low H2O dissociation barrier on Pt play a key role in the formation of CH4. H2O dissociation on Pt nanoparticles supplies sufficient and readily available protons for CH4 formation in the photocatalytic CO2 reduction. [25] Thus, the substance capable of strongly dissociating adsorbed water molecules might be a good alternative to previous metals.
Recently, it was reported that ultra-small MoOx clusters as a cocatalyst of CdS nanowires can effectively activate the adsorbed water molecules, which is one of the key reasons for enhancing the photocatalytic hydrogen evolution activity, [26] so it was speculated that MoOx is a suitable modifier for selective photocatalytic CO2 reduction to CH4. Additionally, doping is a good strategy to obtain single MoOx or ultra-small MoOx clusters, being able to enhance the utilization efficiency of MoOx. Up to now, Mo-doped TiO2 samples, usually prepared at high temperatures and/or with multistep process, have been reported to apply to photocatalysis. [[27], [28], [29], [30], [31], [32], [33]] However, to the best of our knowledge, very few studies focused on photocatalytic CO2 reduction over Mo-doped TiO2. More importantly, it is unclear how Mo doping impacts the photocatalytic CO2 reduction over TiO2.
In this work, we report the successful fabrication of Mo-doped TiO2 photocatalysts via a one-pot hydrothermal method at a relatively low temperature (200 ℃). The nature of MoOx is determined via X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The surface structure of photocatalysts and the adsorption of reactants were probed by electron paramagnetic resonance and Fourier transform infrared spectroscopy. Reasons for the enhanced CH4 selectivity over Mo-doped TiO2 were elucidated, supported by optical characterization, surface-reactant interaction investigation and activity texts.
Section snippets
Synthesis of the photocatalysts
Bare TiO2 and Mo-doped TiO2 were prepared via a one-step hydrothermal method using dihydroxy bis (ammonium lactato) titanium (IV) (TALH; AR, Alfa Aesar) and (NH4)6Mo7O24·4H2O (AR; Sinopharm Chemical Reagent Co, Ltd) as Ti source and Mo source, respectively. In details, 0.5 mL of TALH and a desired amount of (NH4)6Mo7O24·4H2O were dissolved in 35 mL of deionized water under stirring, then the solutions were transferred into 50 ml Teflon-lined stainless steel autoclaves, which were sealed and
Structure and surface
N2 adsorption-desorption isotherms and pore size distribution curves of the TiO2, 0.1 %Mo/TiO2, 0.3 %Mo/TiO2 and 0.5 %Mo/TiO2 samples are shown in Fig. 1 A and B. All the isotherms are of the type-IV adsorption with a loop ring of type-H3 and there are no obvious saturation adsorption platforms, indicating the presence of irregular mesopores in the samples. [35] The pore size distributions of the photocatalysts are relatively concentrated with the most probable pore sizes at around 10 nm (Fig. 1
Conclusion
In summary, Mo-doped TiO2 photocatalysts were successfully prepared via a one-pot hydrothermal method at 473 K, characterized by XRD and Raman spectroscopy techniques revealing the lattice contraction of anatase TiO2. Element Mo exists in the form of Mo (Ⅵ) and Mo (Ⅴ), which suppresses the formation of Ti3+ at the surface of Mo-doped TiO2. The CH4 selectivity is increased with the Mo concentrations increasing and reaches 54.1 % at a Mo concentration of about 0.3 wt%, which can be attributed to
Author contribution statement
Shuaijun Feng & Jie Zhao conceived the idea and co-wrote the paper.
Shuaijun Feng, Yujie Bai & Ting Wang carried out the sample synthesis, characterization and CO2 reduction meansurement.
Xinxin Liang and Chuanyi Wang interpreted the data.
Declaration of Competing Interest
The authors declare no competing interests.
Acknowledgments
Financial support by the National Nature Science Foundation of China (Grant No. 21976116), and the Youth Talent Support Program of Shaanxi University of Science & Technology is gratefully appreciated.
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