Steering accessible oxygen vacancies for alcohol oxidation over defective Nb2O5 under visible light illumination
Graphical abstract
The functionality of oxygen vacancies is strongly dependent on the properties of dopants during photocatalytic alcohol oxidation reactions.
Introduction
Conventional aldehyde production from alcohol oxidation is generally subject to severe environmental and economic issues [[1], [2], [3]]. Greener and more cost-effective approaches are strongly desired. As a flagship example, photocatalytic alcohol oxidation represents a promising strategy for aldehyde production, in which all reactions can be performed under ambient conditions with water as the solvent [4,5]. To avoid the secondary pollution from the photo-corrosion of photocatalysts, stable photocatalysts are of significant importance to the development of eco-friendly photocatalytic system for selective alcohol oxidation [6,7]. Although many stable metal oxide semiconductors (MOS) such as TiO2, Nb2O5, Cu2O and SrTiO3 have been widely used as photocatalysts for water splitting, CO2 reduction, and nitrogen fixation [[8], [9], [10], [11]], their activity for selective oxidation of alcohols is generally less explored. In addition, these MOS all have wide band gap which have poor light absorption to solar photons. Modifying MOS by doping techniques serves as a widely used strategy for effective regulation of their band structure by creating local energy states in their forbidden band gap [[12], [13], [14]]. Various types of defects such as oxygen vacancies (OVs) can be formed during doping processes, particularly for aliovalent doping whereby charge neutrality has to be satisfied [15,16]. Previous studies showed that OVs exhibit great potential in photocatalytic alcohol oxidation, not only serving as electrons trap centers that prevent the recombination of charge carriers [17], but also acting as Lewis acid sites for effective adsorption of reagents [18,19]. In aerobic oxidation, O2 adsorption on OVs had been demonstrated to promote the production of superoxide radicals [20], while for a direct pathway, alcohol substrates adsorption on OVs is likely more available to improve the photocatalytic activity [21]. Although the strong correlation between OVs and photocatalytic activity has been recognized, some key issues about the functionality of oxygen vacancies during alcohol oxidation as well as how they improve alcohol photo-oxidation lacks comprehensive understanding.
As a typical type of MOS, Nb2O5 is highly suitable photocatalyst because of its good chemical stability and photocorrosion resistance [[22], [23], [24], [25]]. For instance, Zhang et al. utilized bulk Nb2O5 in converting 5-hydroxymethylfurfural (5-HMF) into 2,5-diformylfuran (DFF) under visible light (λ > 400 nm), and they obtained ∼20 % conversion and ∼90 % selectivity using benzotrifluoride as a solvent [23]. Su et al. fabricated nitrogen-modified Nb2O5 with different N sources, and they found that both NH4Cl and melamine were able to generate oxygen vacancies that were beneficial in photo-oxidation [22]. Tanaka and co-workers have carried out a series of studies on the photo-oxidation of alcohol/amine over Nb2O5, and they analyzed the mechanism of interaction between organic substrates and Nb2O5 surface during photo-oxidation. Nevertheless, the OVs were not considered in their studies which were crucial in interfacial photo-chemical process [[26], [27], [28], [29]]. Here, taking Nb2O5 as a model MOS, we explored the role of OVs during photocatalytic alcohol oxidation by doping Ti and Ce into the Nb2O5 structure. Ti and Ce are both tetravalent ions which satisfies the aliovalent doping principle to generate oxygen vacancies in Nb2O5, moreover, they possess suitable ionic radius to be induced readily into the Nb2O5 structural lattice. In this work, using benzyl alcohol (BA) and 5-HMF as the representative alcohol, we performed a comparative study over bare Nb2O5 (designated as B-NbO), Ti doped Nb2O5 (designated as Ti-NbO) and Ce doped Nb2O5 (designated as Ce-NbO) for photocatalytic selective alcohol oxidation. With an aid of Pt nanoparticles (NPs) as cocatalysts, both Ti-NbO and Ce-NbO exhibited superior activity for conversion of alcohol compared to bulk B-NbO with a high selectivity. More interestingly, Ce-NbO displayed a better performance than Ti-NbO, especially for oxidation of 5-HMF which was more inert to oxidation than BA. Further analysis suggested that compared with Ti dopants, Ce dopants induced sufficient available OVs at the surface without distinctly distorted the Nb2O5 structure (Scheme 1). These Ce induced OVs favored efficient dissociative chemisorption of the alcohol hydroxyl group at the surface of Nb2O5 with a lower binding energy. Dissociative chemisorption of alcohol substrates at the interface of defective Nb2O5 photocatalysts contributed to the efficient ligand-to-metal charge transfer (LMCT) under light irradiation and their subsequent photocatalytic oxidation of alcohol substrates.
Section snippets
Chemical materials
All reagents were used without further purification. Ethylene glycol (C2H6O2, GC, > 99.0 %), methanol (CH4O, AR, > 99.0 %), benzyl alcohol (C7H8O, AR, > 99.0 %), 5-Hydroxymethylfurfural (C6H6O3, AR, > 99.0 %) tetrabutyl titanate (IV) (C16H36O4Ti, AR, > 99.0 %), cerium (IV) ammonium nitrate (Ce(NH4)2(NO3)6, AR, > 99.0 %) and chloroplatinic acid hexahydrate (H2PtCl6∙6H2O, AR, > 98.0 %) were purchased from Aladdin Biochemical Technology Co. Ltd (Shanghai, China). Citric acid monohydrate (C6H8O7∙H2
Characterization of metal-doped Nb2O5
XRD analysis (Fig. 1a) showed that Ti-NbO-3 (where 3 indicated the initial doping amount of 3 mol%), Ce-NbO-3, and B-NbO own nearly the same crystal structure when calcined at 600 °C. All the characteristic reflections matched well to the standard patterns of Nb2O5 (JPCDS No. 28-0317), indicating that the pseudohexagonal (TT phase) structure of Nb2O5 was maintained after doping Ti and Ce [25,30]. The pseudohexagonal Nb2O5 could be transformed into orthorhombic Nb2O5 (T phase) when the
Conclusion
We successfully modified Nb2O5 photocatalysts with abundant oxygen vacancies by introducing the two dopants Ti and Ce. We systematically investigated their performance for photocatalytic selective alcohol oxidation under visible light illumination. Our results indicated that oxygen vacancies induced by Ti/Ce doping played a critical role in alcohol oxidation. The formation of adsorbed alkoxyl species at these defective sites was significantly important for the formation of the mediated complex
CRediT authorship contribution statement
Yingtong Zou: Conceptualization, Verification, Investigation, Formal analysis, Data curation, Original draft. Yezi Hu: Methodology, Data curation. Andrej Uhrich: Data curation, Software. Zewen Shen: Data curation, Formal analysis. Baoxiang Peng: Data curation, Supervision. Zhuoyu Ji: Formal analysis, Supervision. Martin Muhler: Formal analysis, Supervision. Guixia Zhao: Supervision, Project administration, Funding acquisition. Xiangke Wang: Supervision, Funding acquisition. Xiaoxiang Xu:
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
We sincerely acknowledge the financial supports by the National Natural Science Foundation of China (21976054, 51972233), the National Key Research and Development Program of China (2017YFA0207002), Science Challenge Project (TZ2016004), the Max Planck Society (Max Planck Fellowship), the Deutsche Forschungsgemeinschaft (DFG) (SFB/TRR 247) and Fundamental Research Funds for the Central Universities (2020MS036).
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