(677e) Optimizing Adsorption on Gold Surface in the Small Pores of Mesoporous Silica

Previously, we have demonstrated that even with the normalization of the particle size, surface area, and gold loading (i.e., surface turnover frequency), the gold nanoparticles (with a bare surface) in the small pores of mesoporous silica (26.4 s^-1) are more active in the selective oxidation of benzyl alcohol molecules (C₆H₅CH₂OH) than those on the amorphous silica (20.7s^-1). This indicates that the pore structure enhances catalysis. Furthermore, we have developed a nuclear resonance magnetic (NMR) based characterization to probe the molecules’ motions (i.e., translation & rotation) near the gold surface. The small pores are shown to restrain the random rotation of the molecules and do not induce mass-transfer (i.e., translation) limitations. We have hypothesized that the restrained rotation optimizes the adsorption of the reagent (i.e., benzyl alcohol) on the gold surface, ultimately improving the catalysts’ activity. The ‘unfavorable’ adsorption of the reagent (i.e., C₆H₅-) is expected to block the active sites on the gold surface and hinder catalysis. Since the direct investigation of adsorption is difficult in liquids, the hypothesis is tested by measuring the desorption energy of C_6­H₅- (represented by benzene) in the porous/nonporous silica via temperature-programmed desorption. It is further tested by measuring the heat (i.e., enthalpy) evolved during the solid-liquid interactions while immersing the gold nanoparticles supported by the porous/nonporous silica in liquid benzene via immersion calorimetry. Compared to nonporous silica, a lower enthalpy of immersion and a reduction in the desorption energy are observed in the porous silica. These results indicate that within the porous silica, the restrained rotation of the phenyl rings can reduce the degrees of freedom of ‘unfavorable’ adsorption on the gold surface and weaken the binding strength required to separate the adsorbed phenyl rings from the gold surface, creating more available reaction sites and facilitating a successful reaction.