Determining Structure-Reactivity Correlations of High Surface Index Gold Nanocrystals

One of the main challenges in catalysis research is the design of heterogeneous catalysts that can reach 100% selectivity in the formation of one desired product. The development of highly selective catalysts will have a worldwide benefit by minimizing purification processes and chemical waste production. In order to achieve this goal, and prepare highly selective catalysts it is essential to identify the ways by which different surface sites direct the reactivity and products selectivity of catalytic nanostructures. This fundamental understanding of structure-reactivity correlations within catalytic nanoparticles will enable the development of highly selective catalysts based on rational design.

In order to uncover structure-reactivity correlations in catalytically-active nanostructures we have synthesized low and high surface index Au nanocrystals and tested their reactivity. The high density of surface defects (e.g. steps) in high surface index nanocrystals makes them an ideal model system for identifying the ways by which surface defects direct the catalytic properties of metallic nanoparticles. Au crystals with facets having different miller indices were synthesized. Au nanostars with high surface index of {742} were synthesized and characterized. Their average size was 200±25 nm. In addition, low index nanocrystals were also synthesized: icosahedra with a surface index of {101} and a size distribution of 100±15 nm, and decahedra with a surface index of {110} and a size distribution of 115±20 nm. The surface index directly relates to the density of steps and kinks on the surface, with high index crystals having up to two orders of magnitude higher density of surface steps when compared to crystals with low surface index. The reactivity of the nanocrystals was tested in various hydrogenation and oxidation reactions: Hydrogenation of cinnamaldehyde and hydroxymethyl furfural, and oxidation of benzyl alcohol. The different yields and selectivities obtained with the nanocrystals relates to the density and type of surface defects.