Influencing the Electron Density of Nanosized Au Colloids via Immobilization on MgO to Stimulate Surface Reaction Activities.

Abstract

Heterogenization of colloidal gold on MgO is demonstrated to facilitate its catalytic surface reactivity. We show that the electron density on Au influenced by its immobilization on MgO along with the ensued metal-support interaction is one of the key parameters to obtain high activity. As elucidated by X-ray absorption spectroscopic (X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and extended X-ray absorption fine structure) studies, the presence of well-dispersed nanosized Au on MgO is observed to result in an enhancement in the electron density of Au. The consequence of this electron-rich gold on the catalytic activity is then investigated using the nitroarene reduction as a model reaction with a detailed kinetic study. The kinetic study is an attempt to use a true heterogeneous system rather than the usually studied quasi-homogeneous systems. The results obtained reveal that the Au/MgO catalyst has a surface rate constant of ∼1.39 × 10-3 mol m-2 s-1, which is significantly higher than those of the reported catalysts. While it validates the higher catalytic activity with a TOF of 9456 h-1 observed for Au/MgO, the increased adsorption constant for 4-nitrophenol on Au/MgO further reflects the efficacy of MgO as the support. This not only allows effective heterogenization of the Au nanoparticles keeping the catalyst stable under the reaction conditions and being reused several times but also renders a capability in reduction of other nitro group-containing substrates. Therefore, the results are believed to be of importance in designing heterogeneous catalysts utilizing the distinctive properties of the nanosized colloids and tuning their surface reactivity as well.