Reactivity of metal oxide nanocluster modified rutile and anatase TiO₂: Oxygen vacancy formation and CO₂ interaction

The reduction of CO₂ to fuels is an active research topic with much interest in using solar radiation and photocatalysts to transform CO₂ into higher value chemicals. However, to date there are no photocatalysts known that can use solar radiation to efficiently reduce CO₂. One particularly difficult problem is activating CO₂ due to its high stability. In this paper we use density functional theory simulations to study novel surface modified TiO₂ composites, based on modifying rutile and anatase TiO₂ with molecular-sized metal oxide nanoclusters of SnO, ZrO₂ and CeO₂ and the interaction between CO₂ and nanocluster-modified TiO₂. We show that reduction of the supported nanocluster is favourable which then provides reduced cations and sites for CO₂ adsorption. The atomic structures and energies of different adsorption configurations of CO₂ on the reduced modified TiO₂ composites are studied. Generally on reduced SnO and CeO₂ nanoclusters, the interaction of CO₂ is weak producing adsorbed carbonates. On reduced ZrO₂, we find a stronger interaction with CO₂ and carbonate formation. The role of the energies of oxygen vacancy formation in CO₂ adsorption is important because if reduction is too favourable, the interaction with CO₂ is not so favourable. We do find an adsorption configuration of CO₂ at reduced CeO₂ where a C-O bond breaks, releasing CO and filling the oxygen vacancy site in the supported ceria nanocluster. These initial results for the interaction of CO₂ at surface modified TiO₂ provide important insights for future work on CO₂ reduction using novel materials.