Oxidation Catalysis by Novel Dilute Palladium-in-Gold Nanoparticles Supported on Porous Silica

Abstract

This thesis investigates the activity of dilute palladium-in-gold nanoparticles supported on porous silica for oxidative catalysis, exploring its capability to catalyze the oxidation of carbon monoxide and the selective oxidation of alcohols. The introduction summarizes the activity of model catalysts, emphasizing the activities of monometallic gold and monometallic palladium for oxidative reactions and the tunable nature of palladium-gold bimetallic alloys, that motivated the research interest in supported dilute palladium-in-gold nanoparticles for the oxidation of alcohols (Chapter 1). The methods used for the catalytic measurements of silica-supported nanoparticle catalysts in this thesis are then presented (Chapter 2). First, the ability of silica-supported dilute palladium-in-gold nanoparticles to dissociate molecular oxygen is probed with the oxidation of carbon monoxide and the distribution of palladium in the bimetallic nanoparticles is studied with spectroscopy (Chapter 3). The oxidation of carbon monoxide is further used to elucidate the gas-induced segregation of palladium to the surface and the kinetic evidence for the presence of both isolated palladium atoms and clustered palladium atoms under reaction conditions (Chapter 4). Next, it is shown that highly dilute palladium-in-gold nanoparticles on porous silica is more active, yet as highly selective as monometallic gold nanoparticles on porous silica (Chapter 5). The catalytic measurements align with a gold-based catalytic mechanism derived from model studies and suggest that clusters of palladium atoms promote molecular oxygen dissociation to initiate the catalytic process that is driven by gold. The 3 atomic % palladium-in-gold catalyst and the monometallic gold catalyst are further tested for the oxidation of higher molecular weight aliphatic alcohols. The gold nanoparticle-silica support interface contributes significantly to the activity of dilute palladium-in-gold nanoparticle catalysts towards the oxidation of higher molecular weight alcohols and the palladium at the surface of the bimetallic nanoparticles is blocked under these reaction conditions (Chapter 6). The activity of the 3 atomic % palladium-in-gold nanoparticles supported on silica for the oxidation of alcohols is proposed to be independent of the average concentration of palladium between 3 atomic % and 10 atomic %, based on the nearly identical specific activity of the two bimetallic catalyst for the oxidative self-coupling of methanol (Chapter 7). Finally, unpublished data of oxidative methanol self-coupling over a silica-supported monometallic palladium catalyst and a silica support with no metal nanoparticles (Appendix A), as well as the oxidation of high molecular weight aliphatic alcohols over the silica supported 3 atomic % palladium-in-gold nanoparticle catalyst (Appendix B), are included to describe findings that motivate further investigation.