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Oxygen storage materials (OSMs) have potential applications in various fields, including catalysis, wherein a high oxygen storage capacity is desired because lattice oxygen and oxygen vacancies can act as reactants in catalytic reactions. In this study, we demonstrated that orthorhombic FeNbO4has a high oxygen storage capacity, making it an effective catalyst for NO reduction and CO oxidation. FeNbO4could release and store lattice oxygen via reversible topotactic transformations while maintaining cation ordering in its crystal structure. The lattice oxygen release properties were significantly enhanced by doping FeNbO4with Ti at the Nb sites. The oxygen storage capacity of Ti-doped FeNbO4was further improved by reduction and reoxidation pretreatments at 773 K, in which sufficient lattice oxygen was released under H2atmosphere and reoxidized under O2atmosphere. In NO reduction by CO in the presence of O2, Ti-doped FeNbO4having high oxygen storage performance exhibited an excellent catalytic activity comparable to that of Pt/Al2O3, despite the absence of platinum group metals (PGMs). This could be explained by the participation of oxygen vacancies in the reaction via the Mars–van Krevelen mechanism, as revealed by temperature-programmed reactions with NO and CO. We propose that a catalyst design utilizing the highly reactive lattice oxygen and oxygen vacancies in OSMs can circumvent the need for expensive PGMs.

Towards the realisation of carbon neutrality by utilising renewable energy sources, the photocatalytic conversion of CO2with H2O—known as artificial photosynthesis—is important because H2O is non-toxic or non-hazardous, and an abundant source of protons for CO2reduction. Many studies on the photocatalytic conversion of CO2have revealed that Ag nanoparticles are an effective co-catalyst for the selective conversion of CO2to CO in water. To improve the activity of the photocatalytic conversion of CO2in water, modifying the surface of the photocatalyst is essential to load small Ag nanoparticles with high dispersity, which is difficult to achieve using conventional methods. In this study, ultrasonic reduction (USR) was used as an advanced modification method for photocatalysts with an Ag co-catalyst. Ag/ZnTa2O6prepared using the USR method exhibited good selectivity towards CO (>90%) evolution and a higher CO formation rate compared to those prepared using the conventional modification methods. High-resolution transmission electron microscopy images of the Ag co-catalyst revealed that Ag nanoparticles with a size of a single nanometre were loaded onto the surface of ZnTa2O6by the USR method, whereas much larger Ag particles loaded onto it were observed in the case of other methods. Accordingly, a small Ag co-catalyst with a single nanometre size exhibits superior activity towards the selective conversion of CO2to CO. Thus, we successfully achieved a high CO formation rate with high selectivity using a Ag/ZnTa2O6photocatalyst prepared viaUSR.

A SiO2-supported indium catalyst (In/SiO2) has shown high performance for the nonoxidative coupling of CH4(NOCM). Previous operandoX-ray absorption fine structure (XAFS) studies revealed that the In/SiO2catalyst undergoes dynamic structural changes during the activation process under flowing CH4, such as melting, new bond formation, and regeneration of the metallic form. To understand the effect of these dynamic changes, especially the formation of new bonds, on the catalytic properties, we examined the structures of an In/SiO2catalyst during the activation process under various gas flows (CH4, H2, and He) using operandoXAFS. Fresh In/SiO2catalysts are composed of metallic In particles covered with InOx. All of the samples melted at 430 K. In the XAFS spectrum of the sample under the He flow, peaks associated with the In–O bonds on the surface became visible at ∼800 K. In was partially evaporated at 1100 K because of the formation of volatile In2O. When the gas was switched from He to CH4, the In/SiO2was reduced to liquid-metal In. By contrast, in the In/SiO2under the H2flow, the In–O bonds were removed at ∼600 K. In the spectra of In/SiO2under the CH4flow, an In–Xpeak appeared at 800 K and then increased in intensity as the temperature was increased to 1100 K; this latter peak-increasing process was not observed under the He flow. The In–Xpeak was assigned to either In–CH3formed via the reaction with CH4or to In–O of surviving InOxspecies. The In–Xpeaks completely disappeared at 1100 K, and the catalyst became active toward the NOCM. Under all three gas-flow conditions, the liquid-metal In served as the active site during the reaction. The remarkable activity and long lifetime of the active site stem from the exceptional mobility and flexibility of the In atoms within the liquid metal.

Oxygen storage materials (OSMs) such as CeO2have attracted great attention as catalysts for the reverse water gas shift (RWGS) reaction, replacing metal-supported catalysts from the perspectives of thermal stability and CO selectivity. Because perovskite-type oxides are also known to be powerful OSM candidates, here we employed perovskite-type Mn-substituted SrTiO3as a catalyst for RWGS and investigated the effects of oxygen vacancies on the activity toward CO generation. Among the solid solutions with different Mn ratios, SrTiO3with 20% Mn substitution exhibited the highest activity. It is suggested that oxygen vacancies generated by the substitution of Mn ions for Ti ions in SrTiO3exhibit selective activity toward CO2splitting, whereas the redox of bulk Mn oxides does not have the capacity for CO generation in the control experiments. Quantitative H2-temperature programmed reduction and CO2-temperature programmed oxidation experiments revealed that half of the oxygen vacancies in Mn-substituted SrTiO3, which were generated by reduction with H2, were used as active sites for CO2splitting. We conclude that this generated active oxygen vacancy, which is attributed to the valence change of the Mn ions, contributes to efficient CO2splitting to form CO through the reverse Mars–van Krevelen mechanism.