Abstract
The capture and conversion of CO2 are 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 oxidized TiO2 or CeO2 are unable to promote this step. In addressing this difficult problem, a recent experimental work shows the potential for bismuth-containing materials to adsorb and convert CO2, the origin of which is attributed to the role of the bismuth lone pair. In this paper, we present density functional theory (DFT) simulations of enhanced CO2 adsorption on heterostructures composed of extended TiO2 rutile (110) and anatase (101) surfaces modified with Bi2O3 nanoclusters, highlighting in particular the role of heterostructure reduction in activating CO2. These heterostructures show low coordinated Bi sites in the nanoclusters and a valence band edge that is dominated by Bi–O states, typical of the Bi3+ lone pair. The reduction of Bi2O3–TiO2 heterostructures can be facile and produces reduced Bi2+ and Ti3+ species. The interaction of CO2 with this electron-rich, reduced system can produce CO directly, reoxidizing the heterostructure, or form an activated carboxyl species (CO2–) through electron transfer from the reduced heterostructure to CO2. The oxidized Bi2O3–TiO2 heterostructures can adsorb CO2 in carbonate-like adsorption modes, with moderately strong adsorption energies. The hydrogenation of the nanocluster and migration to adsorbed CO2 is feasible with H-migration barriers less than 0.7 eV, but this forms a stable COOH intermediate rather than breaking C–O bonds or producing formate. These results highlight that a reducible metal oxide heterostructure composed of a semiconducting metal oxide modified with suitable metal oxide nanoclusters can activate CO2, potentially overcoming the difficulties associated with the difficult first step in CO2 conversion.
Highlights
Increasing energy demand is leading to fossil fuel supply issues, and the use of fossil fuels has pushed the atmospheric CO2 concentration to 410 ppm
We have recently studied the interaction of molecules, such as CO2, with tin(II) oxide,[47] zirconia,[47] manganese oxide,[63] alkaline earth oxide,[64] and ceria[65] nanocluster-modified TiO2 heterostructures
The Bi2O3−TiO2 heterostructures can be reduced, with moderate energy costs, which should be possible through a thermochemical process or by the addition of hydrogen
Summary
Increasing energy demand is leading to fossil fuel supply issues, and the use of fossil fuels has pushed the atmospheric CO2 concentration to 410 ppm. Continuing CO2 emissions are projected to result in a higher than 2 °C rise in average global temperatures Given these severe societal problems, there is an urgent need to find materials that can convert the CO2 produced by the combustion of fossil fuels back to the fuels or to the precursors for the production of more useful chemicals. Irrespective of how electrons are provided to reduce CO2, the key step in any CO2 capture and use/conversion process is the initial adsorption of CO2, which should promote the “activation” of the molecule This activation can be a charge transfer to CO2, distortions to the molecular structure such as bending of O−C−O away from the linear elongations in the molecular C−O distances, or hydrogenation. The activation should avoid the production of overstabilized inactive surface-bound carbonates which would poison the catalyst
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