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CO2 Activation on Heterostructures of Bi2O3-Nanocluster Modified TiO2: Promoting the Critical First Step in CO2 Conversion
preprintsubmitted on 22.02.2018, 23:59 and posted on 23.02.2018, 14:37 by Michael Nolan
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 - 130o 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.