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

• SnO, ZrO2 and CeO2 modified TiO2 show reduced band gaps compared to unmodified TiO2.
• Formation of oxygen vacancies in all nanocluster modifiers is more favourable than in unmodified TiO2.
• CO2 adsorbs strongly at reduced ZrO2 to form carbonates.
• CO2 can interact with reduced CeO2 to break a CO bond and form free CO.

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 CO 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.

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