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 CO<sub>2</sub> 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 CO<sub>2</sub>. The leading photocatalyst is some form of TiO<sub>2</sub>. A significant amount of work has been undertaken to modifying TiO<sub>2</sub> 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 TiO<sub>2</sub> (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 TiO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> capture, helping alleviate the problem of CO<sub>2</sub> emissions.

Adsorption of CO2 on Heterostructures of Bi2O3 Nanocluster-Modified TiO2 and the Role of Reduction in Promoting CO2 Activation

“…The other adsorption modes in Figure 6 are of more interest, and these show new C−O interactions with the supported nanoclusters, and the adsorption structure is consistent with a carbonate-like adsorption mode, albeit without the strong stabilization usually found for carbonate adsorption, 64,65 and consistent with the CO 2 adsorption on the Bi-oxide-terminated pyrochlore. 44 Table 3 which is consistent with its larger adsorption energy and distortions away from the gas-phase CO 2 geometry.…”

“…In our earlier work, the emphasis has been on new heterostructured materials with predicted visible light absorption 47−59 and reduced charge recombination. 48−52,60−62 We have recently studied the interaction of molecules, such as CO 2 , with tin(II) oxide, 47 zirconia, 47 manganese oxide, 63 alkaline earth oxide, 64 and ceria 65 nanocluster-modified TiO 2 heterostructures. We have found that such nanoclustermodified TiO 2 heterostructures can adsorb CO 2 , 47 but that some heterostructures, such as reduced MnO x −TiO 2 , do not show favorable CO 2 adsorption, even though reduced cations are present.…”