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A highly efficient H2 evolution photocatalyst based on TiO2 supported subnanometer Au particles was developed on the basis of the reaction of a gold(I) molecular precursor [Au5Mes5] (Mes = 2,4,6-trimethylphenyl), with titanium dioxide partially dehydroxylated at 120 °C. IR, UV-vis, elemental analysis, XANES, and STEM-EDX show that the deposition of [Au5Mes5] onto TiO2 leads to the formation of both subnanometer Au particles and chemisorbed [Au5Mes5]. The remaining organic ligands are removed via a mild treatment under H2, yielding subnanometer gold(0) particles. A range of Au loadings (0.3, 0.9, 2.4 wt %) with similar particle sizes (<1 nm) on TiO2 are obtained and tested in methanol-assisted photocatalytic hydrogen production under UV light. These catalysts display significantly higher activity than a commercial reference Au-TiO2 catalyst. The presence of chemisorbed [Au5Mes5] in the as-synthesized catalyst further improved activity, albeit at the expense of stability. This work demonstrates a simple synthetic route to obtain subnanometer Au particles on TiO2 that display exceptional activity in photocatalysis. [ABSTRACT FROM AUTHOR]

Accomplishing acid-stable water oxidation is a critical matter for achieving both long-lasting water-splitting devices and other fuel-forming electro- and photocatalytic processes. Because water oxidation releases protons into the local electrolytic environment, it becomes increasingly acidic during device operation, which leads to corrosion of the photoactive component and hence loss in device performance and lifetime. In this work, we show that thin films of manganese-modified titania, (Ti,Mn)O x , topped with an iridium catalyst, can be used in a coating stabilization scheme for acid-stable water oxidation. We achieved a device lifetime of more than 100 h in pH = 0 acid. We successfully grew (Ti,Mn)O x coatings with uniform elemental distributions over a wide range of manganese compositions using atomic layer deposition (ALD), and using X-ray photoelectron spectroscopy, we show that (Ti,Mn)O x films grown in this manner give rise to closer-to-valence-band Fermi levels, which can be further tuned with annealing. In contrast to the normally n-type or intrinsic TiO 2 coatings, annealed (Ti,Mn)O x films can make direct charge transfer to a Fe(CN) 6 3-/4- redox couple dissolved in aqueous electrolytes. Using the Fe(CN) 6 3-/4- redox, we further demonstrated anodic charge transfer through the (Ti,Mn)O x films to high work function metals, such as iridium and gold, which is not previously possible with ALD-grown TiO 2 . We correlated changes in the crystallinity (amorphous to rutile TiO 2 ) and oxidation state (2+ to 3+) of the annealed (Ti,Mn)O x films to their hole conductivity and electrochemical stability in acid. Finally, by combining (Ti,Mn)O x coatings with iridium, an acid-stable water-oxidation anode, using acid-sensitive conductive fluorine-doped tin oxides, was achieved.

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C-H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heterolytic C-H or H-H activation via a 1,2 addition across a Co-O bond.