Carbon dioxide photo-reduction by a gold-ruthenium cluster deposited upon titanium dioxide

Solar photo-reduction of CO₂ is emerging as a highly attractive renewable energy source, as it provides both a renewable fuel and a non-atmospheric endpoint for greenhouse gas emissions. Photocatalytic conversion of CO₂ and H₂O to hydrocarbon fuels is well-established to occur using titanium dioxide decorated with metal nanoparticles, however the potential for atomically-precise metal clusters to act as co-catalysts upon TiO₂ for this reaction has been unexplored to date. In this work, a bimetallic Ru₃(μ-AuPPh₃)(μ-Cl)(CO)₁₀ cluster deposited upon TiO₂ (referred to as “AuRu₃-TiO₂”) is demonstrated to heterogeneously reduce CO₂ with water under ultraviolet irradiation. This is the first report of a cluster co-catalyst upon TiO₂ effectively photo-catalysing this reaction, and is found to perform more efficiently than a literature standard of platinum nanoparticle decorated TiO₂. A range of C₁-C₃ hydrocarbon products are detected from AuRu₃-TiO₂, with molecular hydrogen also evolved from concomitant reduction of water. The dependences of photo-activity upon factors such as reaction temperature and reagent partial pressures are explored. Changing the co-catalyst by a single atom strongly influences both photoactivity and product selectivities. Photo-generation rates of all products of interest decay significantly under repeated testing, with only methane production remaining non-zero after four consecutive tests. Isotopic substitution of D₂O for H₂O shows less than 30% deuterium incorporation into generated hydrogen, but kinetic isotope effects suggest substantial deuteration of hydrocarbon products. Diffuse reflectance spectroscopy reveals the emergence of a surface plasmon absorption band from AuRu₃-TiO₂ after photocatalysis, highly characteristic of cluster aggregation into larger gold nanoparticles. X-ray photoelectron spectroscopy indicates that the cluster remains intact on heating in vacuum, but forms a ruthenium oxide species on the TiO₂ surface after photo-reduction testing. N₂ adsorption, thermo-gravimetric analysis and temperature-programmed desorption experiments all provide evidence for partial de-ligation of the AuRu₃ cluster on heating in vacuum. Deposition of de-ligated AuRu₃ upon anatase TiO₂ is modelled by density functional theory, using a Ti₁₄O₄₅H₂₆ model of the anatase (101) surface. In a neutral charge state, the lowest energy structure of AuRu₃ on the surface has octet multiplicity and a pseudo-trigonal pyramid geometry. The cluster inserts filled electronic states into both the TiO₂ valence band and band-gap, but effective orbital overlap occurs only with the former. Binding of H₂O and CO₂ to the surface is found to be weakened by the cluster withdrawing electron density from the adsorbate-surface interactions. CO ligands are bound to both the cluster or the anatase surface in approximately equal strength, and binding of a triphenylphosphine ligand significantly distorts the cluster geometry on the surface. Modelling of Au₁ and Ru₃ clusters on the surface indicates that the gold atom of AuRu₃ is much more weakly bound than ruthenium to both the bare surface and Ru₃-TiO₂, suggesting AuRu₃ likely aggregates on the surface via initial fragmentation into elemental components.Thesis (M.Phil.) -- University of Adelaide, School of Biological Sciences, 2017