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Our original proposal is to develop a multiscale approach to model desorption and adsorption of hydrogen in complex metal hydrides. The proposed research direction is now funded through a DOE hydrogen fuel initiative awards. We therefore explored new research directions with the support of the PRF grant. The grant was used to support students who worked on the supported transition metal as catalysts for CO₂ conversion and utilization. The major findings from the project were summarized in the following:

(1)   Effect of Pt Clusters on Methanol Adsorption and Dissociation over Perfect and Defective Anatase TiO₂(101) Surface: Methanol adsorption and dissociation on the perfect and defective anatase TiO₂(101) surfaces with and without Pt clusters have been studied using density functional theory periodic calculations. On the clean perfect anatase TiO₂(101) surface, the energies of methanol molecular and dissociative adsorption are almost degenerate. The dissociation via O—H scission with an activation barrier of 0.51 eV is the most favorable among three possibilities: O—H, C—O and C—H bond breaking. In contrast, the activation barrier for C—O bond cleavage is as high as 2.56 eV. In the presence of Pt clusters at perfect TiO₂ surface, both of methanol molecular adsorption and dissociation via C—O scission was enhanced. At the low temperatures, methanol prefers molecularly adsorbed at the interface of Pt/TiO₂. When temperature gets high enough, methanol begins dissociate and the product after its initial dissociation via C—O bond breaking is more stable than that via O—H or C—H scission. On the clean defective surface, methanol prefers dissociative adsorption at the oxygen vacancy site with its oxygen atom occupying the vacancy and the H atom on the surface two-coordinated oxygen site. When Pt particles occupied the oxygen vacancy site on the defective TiO₂(101) surface, the active sites for methanol dissociation became unavailable. In this case, the methanol molecules were forced to occupy less active site. The results were discussed in the context of photocatalytic mechanism over Pt/TiO₂ for hydrogen production from methanol.

(2)   CO₂ Adsorption and Activation over γ-Al₂O₃-supported Transition Metal Dimers: Catalytic conversion of CO₂ to liquid fuels has the benefit of reducing CO₂ emission. Adsorption and activation of CO₂ on the catalyst surface are key steps of the conversion. Herein, we used density functional theory (DFT) slab calculations to study CO₂ adsorption and activation over the γ-Al₂O₃-supported 3d transition metal dimers (M₂/γ-Al₂O₃, M = Sc ~ Cu). CO₂ was found to adsorb on M₂/γ-Al₂O₃ negatively charged and in a bent configuration, indicating partial activation of CO₂. Our results showed that both the metal dimer and the γ-Al₂O₃ support contribute to the activation of the adsorbed CO₂. The presence of a metal dimer enhances the interaction of CO₂ with the substrate. Consequently, the adsorption energy of CO₂ on M₂/γ-Al₂O₃ is significantly higher than that on the γ-Al₂O₃ surface without the metal dimer. The decreasing binding strength of CO₂ on M₂/γ-Al₂O₃ as M₂ changes from Sc₂ to Cu₂ was attributed to decreasing electron-donation by the supported metal dimers. Hydroxylation of the support surface reduces the amount of charge transferred to CO₂ for the same metal dimer and weakens the CO₂ chemisorption bonds. Highly dispersed metal particles maintained at a small size are expected to exhibit good activity toward CO₂ adsorption and activation.