(166d) Impact of Pore Size on Catalytic Behavior in Mesoporous Au@SiO2 Core-Shell Nanoparticles

Mesoporous silica-encapsulated gold core-shell nanoparticles (Au@SiO₂ CSNPs) were synthesized via a bottom up procedure to facilitate the aerobic, solvent-free oxidation of benzyl alcohol. The pore size and morphology of the catalyst allows for the unique opportunity to investigate the impacts of pore size on mass transport and surface activity discretely. The nanoparticles exhibit an average pore diameter of 25.5 Å, a size which enhances selectivity via physical constraint to inhibit the formation of the smaller, desired product (i.e. benzaldehyde) relative to larger, undesired products (i.e. benzoic acid/benzyl benzoate). In addition to improving selectivity, the CSNPs also demonstrate an activity maintenance effect. Despite the mesoporous silica structure obscuring much of the gold active surface, GC-FID analysis revealed the observed yield was far higher than on bare supported nanoparticles of equivalent nanoparticle size and gold mass.

The pore size of CSNPs was controlled via additional calcination and etching in a weakly alkaline environment. Under these conditions, investigation using TEM revealed the etching process to have minimal impact on gold core diameter. GC-FID analysis revealed both lower activity and benzaldehyde selectivity on CSNPs with higher pore sizes, suggesting the mesopores inhibit both the formation of large, undesired products and competitive adsorption of reactant phenyl groups, which block reactions on the surface.

The pore length of CSNPs was controlled via a series of sequential silica condensations around the gold core, increasing the thickness of the encapsulating silica shell. Investigation using TEM revealed higher shell growth using multiple additions of silica precursor compared to a single addition of equivalent mass. Benzyl alcohol oxidation using CSNPs with higher silica shell thickness demonstrated lower catalytic activity as shell thickness increases. An activation energy study revealed the Thiele modulus transitions from purely rate limited to the transition region over shell thicknesses studied.