Activation of CO2 at Chromia-Nanocluster-Modified Rutile and Anatase TiO2

Converting CO₂ to fuels is required to enable the production of sustainable fuels and to contribute to alleviating CO₂ emissions. In considering conversion of CO₂, the initial step of adsorption and activation by the catalyst is crucial. In addressing this difficult problem, we have examined how nanoclusters of reducible metal oxides supported on TiO₂ can promote CO₂ activation. In this paper we present density functional theory (DFT) simulations of CO₂ activation on heterostructures composed of extended rutile and anatase TiO₂ surfaces modified with chromia nanoclusters. The heterostructures show non-bulk Cr and O sites in the nanoclusters and an upshifted valence band edge that is dominated by Cr 3d- O 2p interactions. We show that the supported chromia nanoclusters can adsorb and activate CO₂and that activation of CO₂ is promoted whether the TiO₂ support is oxidised or hydroxylated. Reduced heterostructures, formed by removal of oxygen from the chromia nanocluster, also promote CO₂ activation. In the strong CO₂ adsorption modes, the molecule bends giving O-C-O angles of 127 - 132^o and elongation of C-O distances up to 1.30 Å; no carbonates are formed. The electronic properties show a strong CO₂-Cr-O interaction that drives the interaction of CO₂ with the nanocluster and induces the structural distortions. These results highlight that a metal oxide support modified with reducible metal oxide nanoclusters can activate CO₂, thus helping to overcome difficulties associated with the difficult first step in CO₂ conversion.