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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.

Photocatalytic conversion of CO2by H2O is a promising method for solving energy and environmental problems. In this context, efficient photocatalysts that facilitate the selective conversion of CO2to the value-added chemical CO are essential. In this study, for the first time in the literature, we used an Mg-doped SrTiO3photocatalyst (Mg–SrTiO3) for the photocatalytic conversion of CO2to CO using H2O as the electron donor under monochromatic UV-light irradiation at 365 nm. Compared to pristine SrTiO3, Mg–SrTiO3, which was prepared viaa flux method, exhibited dramatically enhanced conversion of CO2to CO in the presence of an Ag–Co cocatalyst. Moreover, the selectivity toward CO evolution was >99%, which indicates suppression of the unnecessary and competitive H2evolution. Scanning electron microscopy of Mg–SrTiO3revealed edge-shaved cubic particles, which were correlated to the anisotropic distribution of photogenerated electrons and holes and the consequent enhancement of photocatalytic activity. Furthermore, the Mg-doping temperature and amount used to prepare Mg–SrTiO3influenced the substitution of Ti4+sites by Mg2+in the bulk of SrTiO3, thereby affecting the CO evolution. The apparent quantum efficiency of optimal Mg–SrTiO3in the photocatalytic conversion of CO2was determined to be 0.05%.

Hydrogen (H) species adsorbed on catalyst surfaces are key intermediates in catalytic hydrogenation reactions over supported metal catalysts. However, individual identification of H species on the metal catalysts has not been established to date. Here, we elucidated the H species on Pt/Al2O3by the combination of in situinelastic neutron scattering (INS) and density functional theory (DFT). Several H species in the presence of H2were successfully identified at different sites on the Pt surface and Al2O3support. The in situINS and FT-IR measurements revealed that the hydride/atop Pt–H, bridged perimeter/terrace Pt–H–Pt, and threefold Pt3–H are active intermediates in the C2H4hydrogenation reaction, whereas the edge Pt–H–Pt is a rather inert species. In addition, the in situmeasurements indicate that the Al2O3itself acts as hydrogen storage material by mediating AlO–H and Al–H–Al species, which serve H species in the C2H4hydrogenation reaction.

This study demonstrates that NO-selective reduction with C3H6and CO proceeds over platinum group metal (PGM; Pd, Rh, or Pt) catalysts supported on Sr3Fe2O7−δwith a layered perovskite-type oxide. Among the examined catalysts, Pd-loaded Sr3Fe2O7−δhaving high oxygen storage capacity shows the highest catalytic activity at stoichiometric oxygen concentration because the Pd catalyst can release the lattice oxygen of Sr3Fe2O7−δat lower temperature compared to the Rh- or Pt-loaded catalyst. When the NO-selective reduction at 773 K is carried out at various oxygen concentrations, PGM/Sr3Fe2O7−δshows superior catalytic activity over a wide range of oxygen concentrations to PGM/Al2O3, which does not have oxygen storage capacity. The oxygen vacancy sites in Sr3Fe2O7−δ, which are generated by the oxidation of C3H6and CO over PGM/Sr3Fe2O7−δ, are revealed to receive oxygen ions formed by NO reduction on the PGM species; as a result, the PGM species maintain their metal state and act as active sites for NO reduction. Namely, Sr3Fe2O7−δplays the role of an “oxygen buffer” in inhibiting the PGM-to-oxide transformation. Pt-loaded Sr3Fe2O7−δcan utilize the oxygen vacancy sites more effectively for the catalytic reaction than the Pd and Rh catalysts. To the best of our knowledge, this is the first report on the effective application of perovskite materials with oxygen storage capacity as catalyst supports for NO-selective reduction. The study results are expected to provide valuable information for designing a novel catalyst based on oxygen storage materials.

Because of the environmental problems and the resulting exigent demand for CO2recycling processes, great attention is being paid to the photocatalytic conversion of CO2into useful chemicals such as CO, HCOOH, HCHO, CH3OH, and CH4. We have previously reported that the Ag-loaded, Zn-modified Ga2O3photocatalyst exhibits excellent photocatalytic activity required for the conversion of CO2into CO by using H2O as a reductant and that the Ag particles that exist together with the Zn species act as good cocatalysts for the selective formation of CO. In this study, we demonstrated the photocatalytic activity of ZnTa2O6under UV light irradiation, which was prepared viasolid-state reaction, for the conversion of CO2in an aqueous NaHCO3solution. A Ag cocatalyst-loaded ZnTa2O6photocatalyst evolved CO as a reduction product of CO2with 46% selectivity toward CO evolution among the reduction products. In contrast, when Pt and Au were introduced as cocatalysts, the ZnTa2O6photocatalyst evolved H2with high selectivity (>99.9%).

Stoichiometric evolutions of CO, H2, and O2were achieved for the photocatalytic conversion of CO2with H2O as an electron donor using Ag-loaded Zn-modified Ga2O3. The selectivity toward the evolution of CO over H2can be controlled by varying the amount of Zn species added in the Ag-loaded Zn-modified Ga2O3photocatalyst. The production of H2gradually decreased with increasing amounts of Zn species from 0.1 to 10.0 mol%, whereas the evolution of CO was almost unchanged. The XRD, XAFS, and XPS measurements revealed that a ZnGa2O4layer was generated on the surface of Ga2O3by modification with Zn species. The formation of the ZnGa2O4layer eliminated the proton reduction sites on Ga2O3, although the crystallinity, surface area, and morphology of Ga2O3as well as the particle size and chemical state of Ag did not change. In conclusion, we designed a highly selective photocatalyst for the conversion of CO2with H2O as an electron donor using Ag (the cocatalyst for the CO evolution), ZnGa2O4(the inhibitor of the H2production), and Ga2O3(the photocatalyst).

Porphyrin-derivative-modified TiO2photocatalysts showed high photocatalytic activity for the selective catalytic reduction of NO with NH3in the presence of O2under visible-light irradiation. Tetra(p-carboxyphenyl)porphyrin (TCPP) was the most effective photosensitizer among the five porphyrin derivatives investigated. NO conversion and N2selectivity of 79.0% and 100%, respectively, were achieved at a gas hourly space velocity of 50 000 h−1. UV–Vis and photoluminescence spectroscopies revealed the presence of two species of TCPP on the TiO2surface; one was a TCPP monomer and the other was an H-aggregate of the TCPP molecules. It was concluded that the TCPP monomer is an active species for the photo-assisted selective catalytic reduction (photo-SCR). Moreover, an increase in the fraction of H-aggregates with increasing TCPP loading amount resulted in a decrease in the photocatalytic activity of the photo-SCR.

Effects of reaction temperature on the activity of photo-assisted selective catalytic reduction (photo-SCR) of NO with NH3were investigated under very high GHSV conditions (100 000 h−1). The reaction temperature had a significant effect on the photo-SCR activity over a TiO2photocatalyst. Maximum NO conversion was achieved at 433 K (NO conversion = 84%, N2selectivity = 100%). The apparent activation energies in the low and high temperature range were evaluated to be 9.0 kJ mol−1(353–433 K) and −2.7 kJ mol−1(493–593 K), respectively. Kinetic analysis revealed that the rate-determining step in the photo-SCR was decomposition of NH2NO intermediates in all the ranges of the reaction temperature (353–593 K). The reaction temperature affects not only the rate constant of decomposition of NH2NO intermediates but also the total number of active sites ([S]0) and the equilibrium constant of NH3adsorption, resulting in the dynamic change in the reaction rate.

This paper reviews the recent development of the photocatalytic emission control reaction and photooxidation with molecular oxygen, specifically focusing on efforts based on revealing the reaction mechanism by the authors' group. TiO2acts as an effective catalyst for the photo-assisted selective catalytic reduction of NO with NH3in the presence of O2(photo-SCR). Photooxidation of alcohols to carbonyl compounds proceeds selectively over Nb2O5without organic solvents. Usually, both TiO2and Nb2O5work only in the ultraviolet (UV) region because of the limit of their bandgap energies. However, both photo-SCR over TiO2and photooxidation of alcohols over Nb2O5proceed even under visible light irradiation up to ca. 450 nm. This indicates that these two reactions take place by the different photo-activation mechanism from the classical electron transfer mechanism in semiconductor photocatalysis, that is, the formation of an excited electron in the conduction band and the positive hole in the valence band. A mechanistic study using UV-Vis, ESR, FT/IR, kinetic study, and DFT calculations revealed the reaction mechanisms of photo-SCR and photooxidation of alcohols, and that the surface complex consisting of the adsorbed molecule and catalyst plays an important role in the photo-activation step. The surface complex is converted to the photo-activated species even under visible light irradiation, because the direct electron transition from a donor level derived from the adsorbed molecule to the conduction band of a photocatalyst takes place and a photo-generated hole is trapped on the adsorbed molecule to form the photo-activated radical species. The effective wavelength is shifted to a longer wavelength by the formation of the donor level derived from the adsorbed molecule during a chemical reaction (called here “in situ doping”). This unique photo-activation mechanism by “in situ doping” gives us attractive ways for removing the limit of bandgap energy, and the utilization of visible light.