Intricate behaviors of gas phase CO₂ photoreduction in high vacuum using Cu₂O-loaded TiO₂ nanotube arrays

Photoreduction of carbon dioxide (CO₂) in gas phase was investigated over composites of Cu₂O nanoparticles (CNPs) and anodized TiO₂ nanotube arrays (TNAs). CNPs with an average size of 90 nm were pulse-electrodeposited on vertically aligned hollow TiO₂ nanotubes having average pore size of 90 nm. Radical trapping experiments with electron spin resonance spectroscopy suggest that photoinduced charge separation occurs via direct Z-scheme mechanism in Cu₂O-loaded TNA (CNP/TNA) sample. Successively, CO₂ photoreduction for CNP/TNA from water/CO₂ mixture was monitored using a quadrupole mass analyzer in high vacuum, where the partial pressures of intermediate and final reaction species were measured in real time under ultraviolet-visible (UV–VIS) light irradiation (300–600 nm). The photoreduction of CO₂ on CNP/TNA happens through preferred generation of formaldehyde (HCHO) and methanol (CH₃OH), while platinum-loaded TNA (Pt/TNA) produced methane (CH₄) and hydrogen (H₂) instead of HCHO and CH₃OH. Carbon monoxide (CO) formation was commonly observed for both CNP/TNA and Pt/TNA specimens. These results reveal that CO₂ photoreduction occurs through hydrogenation in gas phase over CNP/TNA even in high vacuum, although CO₂ deoxygenation to CH₄ is conventionally dominant for gas phase reactions.