Adsorption of CO on Heterostructures of BiO Nanocluster-Modified TiO and the Role of Reduction in Promoting CO Activation.

The capture and conversion of CO 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 CO, the initial step of adsorption and activation by the catalyst is critical. Well-known metal oxides such as oxidized TiO or CeO 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 CO, 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 CO adsorption on heterostructures composed of extended TiO rutile (110) and anatase (101) surfaces modified with BiO nanoclusters, highlighting in particular the role of heterostructure reduction in activating CO. 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 Bi lone pair. The reduction of BiO-TiO heterostructures can be facile and produces reduced Bi and Ti species. The interaction of CO with this electron-rich, reduced system can produce CO directly, reoxidizing the heterostructure, or form an activated carboxyl species (CO ) through electron transfer from the reduced heterostructure to CO. The oxidized BiO-TiO heterostructures can adsorb CO in carbonate-like adsorption modes, with moderately strong adsorption energies. The hydrogenation of the nanocluster and migration to adsorbed CO 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 CO, potentially overcoming the difficulties associated with the difficult first step in CO conversion.