(521bl) Enhanced Catalytic Activity of Gold Nanoparticles in the Small Pore Environment of Mesoporous Silica

Previously, we have demonstrated a facile technique for depositing bare-surface and uniform-sized gold nanoparticles (3-4 nm dia.) inside the small pores of mesoporous silica (6.6 nm dia.)—porous gold catalysts. These catalysts are extremely active in the selective oxidation of benzyl alcohol without any thermal treatments. Their surface turnover frequency (26.4 s^-1), a normalized measurement for intrinsic catalytic activity, is higher than the control catalysts (20.7 s^-1), prepared by depositing the same gold nanoparticles onto the amorphous silica—nonporous gold catalysts. This indicates that the pore structure enhances catalysis. We have hypothesized that the porous gold catalysts do not induce mass-transfer (i.e., translation) limitations and restrain the random rotation of the reagent (i.e., benzyl alcohol). We have developed a nuclear magnetic resonance (NMR) based characterization (e.g., diffusion-ordered spectroscopy and T₁ relaxation) with the porous/nonporous gold catalysts to probe the translational and rotational motions of the reagent near the gold surface via the measurements of translational and rotational diffusivities. They are measured at different temperatures and correlated to the Arrhenius equation to determine the activation energy that quantifies the thermal energy barriers, where a higher activation energy indicates the increased restraint of that type of motion. A nearly 200 % reduction in the activation energy of translation and a 200 % increase in the activation energy of rotation were observed for the reagent in the porous gold catalysts compared to the nonporous gold catalysts. These results indicate that, compared to the control catalysts, the porous gold catalysts can better facilitate the transportation of the reagent to/from the gold surface and effectively reduce the random rotation of the reagent, potentially optimizing the adsorption of the reagent on the gold surface, and ultimately, improving the catalysts’ activity and selectivity.