Spontaneous and photoinduced conversion of CO2 on TiO2 anatase

Spontaneous isotopic exchange of oxygen atoms between gaseous C¹⁸O₂ and Ti¹⁶O₂ nanoparticles has been studied using high-resolution Fourier transform infrared absorption and first principles density functional theory calculations. The rate of formation of gaseous C¹⁶O₂ is found to be highly dependent on the nature of the titania sample, growing with increasing calcination temperature for both amorphous and crystalline nanoparticles. This is attributed to an increase in the population of oxygen atom defects and for uncoordinated sites at higher calcination temperatures. Additional support for the experimental observations is provided by calculations of the activation barriers for oxygen exchange on crystalline anatase and on a (TiO₂)¹⁰ cluster. Photoirradiated Ti¹⁶O₂ is known to reduce carbon dioxide to methane. This multistep reaction is commonly represented as a sequence of proton-coupled two-electron reactions leading from carbon dioxide to formate to formaldehyde to methanol and to methane. The actual reaction mechanism is more complex catalytic cycle together with the radical chemistry. The implications of these reactions for photocatalytic methanogenesis and photosynthetic fixation of atmospheric carbon in prebiotic nature are discussed.