Dual Photocatalytic Roles of Light: Charge Separation at the Band Gap and Heat via Localized Surface Plasmon Resonance To Convert CO2 into CO over Silver-Zirconium Oxide.

Abstract

Confirmation of 13CO2 photoconversion into a 13C-product is crucial to produce solar fuel. However, the total reactant and charge flow during the reaction is complex; therefore, the role of light during this reaction needs clarification. Here, we chose Ag-ZrO2 photocatalysts because beginning from adventitious C, negligible products are formed using them. The reactants, products, and intermediates at the surface were monitored via gas chromatography-mass spectrometry and FTIR, whereas the temperature of Ag was monitored via Debye-Waller factor obtained by in situ extended X-ray absorption fine structure. With exposure to 13CO2, H2, and UV-visible light, 13CO selectively formed, while 8.6% of the 12CO mixed in the product due to the formation of 12C-bicarbonate species from air that exchanged with the 13CO2 gas-phase during a 2 h reaction. By choosing the light activation wavelength, the CO2 photoconversion contribution ratio was charge separated at the ZrO2 band gap (λ < 248 nm): 70%, localized at the Ag surface plasmon resonance (LSPR) (330 < λ < 580 nm): 28%, and characterized by a thermal energy of 295 K: 2%. LSPR at the Ag surface was converted to heat at temperatures of up to 392 K, which provided an efficient supply of activated H species to the bicarbonate species, combined with separated electrons and holes above the ZrO2, which generated CO at a rate of 0.66 μmol h-1 gcat-1 with approximately zero order kinetics. Photoconversion of 13CO2 using moisture was also possible. Water photo-oxidation step above ZrO2 was rate-limited, and the side reactions that formed H2 above the Ag were successfully suppressed instead to produce CO via the Mg2+ addition to trap CO2 at the surface.