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

2018-02-23T14:37:11Z (GMT) by Michael Nolan
The conversion of CO<sub>2</sub> 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 CO<sub>2</sub>, the initial step of adsorption and activation by the catalyst is critical. Well-known metal oxides such as oxidised TiO<sub>2</sub> or CeO<sub>2</sub> 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 CO<sub>2</sub>, but the origin of this activity is not yet clear. Additionally, nanostructures can show enhanced activity towards CO<sub>2</sub>. In this paper we present density functional theory (DFT) simulations of CO<sub>2</sub> activation on heterostructured materials composed of extended rutile and anatase TiO<sub>2</sub> surfaces modified with nanoclusters with Bi<sub>2</sub>O<sub>3</sub> 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 Bi<sub>2</sub>O<sub>3</sub> nanoclusters are able to adsorb and activate CO<sub>2</sub>. Computed adsorption energies lie in the range of -0.54 eV to -1.01 eV. In these strong adsorption modes, CO<sub>2</sub> is activated, in which the molecule bends giving O-C-O angles of 126 - 130<sup>o</sup> and elongation of C-O distances up to 1.28 Å, with no carbonate formation. The electronic properties show a strong CO<sub>2</sub>-Bi-oxygen interaction that drives the interaction of CO<sub>2</sub> to induce the structural distortions. Bi<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub> heterostructures can be reduced to form Bi<sup>2+</sup> and Ti<sup>3+</sup> species. The interaction of CO<sub>2</sub> with this electron-rich, reduced system can produce CO directly, reoxidising the heterostructure or form an activated carboxyl species (CO<sub>2</sub><sup>-</sup>) through electron transfer from the heterostructure to CO<sub>2</sub>. These results highlight that a semiconducting metal oxide modified with suitable metal oxide nanoclusters can activate CO<sub>2</sub>, thus overcoming the difficulties associated with the difficult first step in CO<sub>2</sub> conversion.