Alkaline earth metal oxide nanocluster modification of rutile TiO2 (110) promotes water activation and CO2 chemisorption

Abstract

Metal oxide photocatalysts are widely studied for applications in solar driven environmental remediation, antimicrobial activity, hydrogen production and CO₂ reduction to fuels. Common requirements for each technology include absorption of visible light, reduced charge carrier recombination and the ability to activate the initial molecule be it a pollutant, water or CO₂. The leading photocatalyst is some form of TiO₂. A significant amount of work has been undertaken to modifying TiO₂ to induce visible light absorption. The structure and composition of the catalyst should facilitate separation of electrons and holes and having active sites on the catalyst is important to promote the initial adsorption and activation of molecules of interest. In this paper we present a first principles density functional theory (DFT) study of the modification of rutile TiO₂ (110) with nanoclusters of the alkaline earth metal oxides (MgO, Ca, BaO) and we focus on the effect of surface modification on the key catalyst properties. The modification of rutile TiO₂ with CaO and BaO induces a predicted red shift in light absorption. In all cases, photoexcited electrons and holes localise on oxygen in the nanocluster and surface Ti sites, thus enhancing charge separation. The presence of these non-bulk alkaline earth oxide nanoclusters provides highly active sites for water and CO₂ adsorption. On MgO-rutile, water adsorbs molecularly and overcomes a barrier of only 0.36 eV for dissociation whereby hydroxyls are stabilised. On CaO- and BaO-modified rutile water adsorbs dissociatively. We attribute this to the high lying O 2p states in the alkaline earth oxide modifiers which are available to interact with water, as well as the non-bulk like geometry around the active site. Upon adsorption of CO₂ the preferred binding mode is as a tridentate carbonate-like species, as characterised by geometry and vibrational modes. The carbonate is bound by up to 4 eV. Thus these heterostructures can be interesting for CO₂ capture, helping alleviate the problem of CO₂ emissions.