CO2 Activation on Heterostructures of Bi2O3-Nanocluster Modified TiO2: Promoting the Critical First Step in CO2 Conversion

The conversion of CO₂ to fuels is of significant importance in enabling the production of sustainable fuels, contributing to alleviating greenhouse gas emissions. While there are a number of key steps required to convert CO₂, the initial step of adsorption and activation by the catalyst is critical. Well-known metal oxides such as oxidised TiO₂ or CeO₂ are unable to promote this step. In addressing this difficult problem, recent experimental work shows the potential for bismuth-containing materials to activate and convert CO₂, but the origin of this activity is not yet clear. Additionally, nanostructures can show enhanced activity towards CO₂. In this paper we present density functional theory (DFT) simulations of CO₂ activation on heterostructured materials composed of extended rutile and anatase TiO₂ surfaces modified with nanoclusters with Bi₂O₃ stoichiometry. These heterostructures show low coordinated Bi sites in the nanoclusters and a valence band edge that is dominated by Bi-O states. These two factors mean that supported Bi₂O₃ nanoclusters are able to adsorb and activate CO₂. Computed adsorption energies lie in the range of -0.54 eV to -1.01 eV. In these strong adsorption modes, CO₂ is activated, in which the molecule bends giving O-C-O angles of 126 - 130^o and elongation of C-O distances up to 1.28 Å, with no carbonate formation. The electronic properties show a strong CO₂-Bi-oxygen interaction that drives the interaction of CO₂ to induce the structural distortions. Bi₂O₃-TiO₂ heterostructures can be reduced to form Bi²⁺ and Ti³⁺ species. The interaction of CO₂ with this electron-rich, reduced system can produce CO directly, reoxidising the heterostructure or form an activated carboxyl species (CO₂^-) through electron transfer from the heterostructure to CO₂. These results highlight that a semiconducting metal oxide modified with suitable metal oxide nanoclusters can activate CO₂, thus overcoming the difficulties associated with the difficult first step in CO₂ conversion.