(581h) Development of Methods for Precise, Multifactor Tuning of Shell Thickness and Pore Diameter of Silica-Encapsulated Gold Core-Shell Nanoparticles

A seeded encapsulation synthesis method was developed to facilitate precise, multifactor tuning of mesoporous silica-encapsulated gold core-shell nanoparticles (Au@SiO₂ CSNPs). The new seeded method prevents the Stöber synthesis portion of the reaction from impacting the gold core diameter. Altering the Stöber synthesis conditions thusly results in core-shell nanoparticles with different shell morphologies, without affecting the gold active phase. Because of this, remarkably simple catalytic studies can be performed to investigate the specific impact of pore morphology on catalytic performance. Both silica pore diameter and shell thickness (corresponding to pore length) were varied. The Au@SiO₂ CSNPs were used to catalyze the selective oxidation of benzyl alcohol to benzaldehyde. This is a common catalyst benchmark reaction, with the objective of maximizing selectivity towards benzaldehyde and minimizing the formation of further oxidized products (i.e., benzyl benzoate). The further oxidized products are more thermodynamically favored, resulting in an inverse relationship between conversion and selectivity towards the aldehyde product.

Alteration of pore length, via a control of the silica condensation kinetics, demonstrated that increasing pore length led to increased activities and selectivities for the oxidation of benzyl alcohol. Increasing pore width, via the use of swelling agents, increased activity at the cost of selectivity. This approach permitted, for the first time, a careful control of several morphological parameters without impacting each other. Molecular spectroscopy (FTIR and NMR) indicate that this is likely due to a microphase separation, via weak adsorption to the pore wall, providing for beneficial in-pore orientation and preferential mass transport. Thus, this platform enables fundamental catalytic investigations into pore geometry and catalyst design.