Comparison of Colloidal versus Supported Gold Nanoparticle Catalysis

Immobilization of colloidal gold nanoparticles (AuNPs) on metal oxide supports is often done to enhance stability and recoverability over multiple reaction cycles but with a reduction in available surface area. It has been theorized that colloidal AuNPs are catalytically more active and selective than the supported counterparts on a gold mass basis. Direct comparison of catalytic activity and available surface area of 5 nm diameter colloidal gold nanoparticles (AuNP) dispersed in solution and supported on titania has been conducted in this work. A versatile spectroscopic organothiol ligand adsorption method was used to measure available surface area of both colloidal and supported catalysts in aqueous media for direct comparison, circumventing phase limitations of other characterization techniques like chemisorption. Turnover frequency measurements on a gold mass basis showed a reduction in catalytic activity for supported AuNP compared to colloidal AuNP, whereas turnover frequency normalized to available surface area was increased for supported AuNP. This suggests enhancement of AuNP catalytic activity by the metal oxide support for this particular model reaction and led to quantification of the synergistic support effect. An induction time was observed, which increased with increasing reactant concentration with respect to available surface area. The effect of the reactant addition sequence and reactant concentration on the induction time was explored and suggests competitive adsorption of reactant species on the AuNP surface and also depicts a threshold reactant to available surface area ratio, above which emergence of induction time is observed