Increased Affinity of Small Gold Particles for Glycerol Oxidation over Au/TiO2 Probed by NMR Relaxation Methods

The aerobic oxidation of glycerol in aqueous solution over Au/TiO2 catalysts has been studied, and the effect of Au loading by wet impregnation, in the range 0.5-5% Au, has been assessed. Low metal loading favors the deposition of smaller particles, whereas higher loadings lead to the formation of much larger gold particles, as revealed by scanning transmission electron microscopy (STEM) analysis. Reaction studies show that a higher metal loading has a detrimental effect on the catalyst activity, which decreases significantly as the Au load increases. In addition to reaction studies, H-1 NMRT1/T-2 relaxation time measurements have been used to assess the effect of metal loading and particle size on the adsorption properties of glycerol (reactant) and water (solvent) within the catalyst. The NMR results show that the adsorption properties of glycerol relative to water as a function of the Au loading have a similar trend to that observed for the reactivity, with glycerol exhibiting a higher surface affinity relative to water for the catalyst with low Au loading. The overall results indicate that metal loading significantly affects the typical Au particle size, which, in turn, affects both the reaction and adsorption properties of glycerol over the catalyst surface. In particular, the trend in T-1/T-2 ratio clearly indicates that glycerol has a much stronger affinity with smaller gold particles, which is an important factor in promoting glycerol oxidation. This result is of great significance in understanding the reactivity of polyols over supported gold catalysts and gives the first experimental evidence that smaller gold particles tend to be stronger adsorption sites for glycerol, in agreement with computational and theoretical studies.