Selective oxidation reactions are critically important industrially for creating a variety of oxygenated molecules. For some time, it has been known that Au nanocrystals immobilized on TiO2 are capable of selectively oxidizing propylene to propylene oxide in one step using a co-feed of hydrogen and oxygen. The reaction mixture employed is hypothesized to enable the formation of a hydro-peroxide intermediate capable of producing propylene oxide on the TiO2 support. In our past work we have shown that alloying small amounts Pd into Au increases the selective oxidation rate due to improved hydrogen dissociation for hydro-peroxide formation. This work aims to extend our understanding of the relationship between several active sites in supported Pd/Au catalysts (Pd atoms in Au, Au surfaces, and oxide sites) for selective oxidation reactions. Alloy nanocrystals were synthesized using a seed-mediated colloidal technique, immobilized on a variety of oxide and carbon supports, and were characterized using TEM, UV-Vis, and XAS techniques. Catalytic testing was conducted at atmospheric pressure using a reaction mixture that contained hydrogen, oxygen, and several alcohols. Different catalyst bed combinations allowed for separation of different active catalytic sites for comparison of their activity in the overall reaction. In-situ DRIFTS was conducted to monitor species on the catalytic surfaces during catalysis. Catalytic testing revealed that Pd/Au on metal oxide catalysts were capable of selectively oxidizing a variety of alcohols into their respective carbonyls using a mixture of hydrogen and oxygen in the feed stream. In all cases the alloyed particles outperformed monometallic Au, which showed low activity, and Pd, which showed poor selectivity and did not show activity improvement with hydrogen, suggesting that the Pd sites enhance hydrogen utilization in Au and that Au is necessary for the generation of selective oxidizing species. It was also found that a variety of metal oxides could be used as the site for selective oxidation of alcohols using hydro-peroxide species generated by Pd/Au but that carbon-based supports were inactive. These results suggest that the combination of three different active sites are necessary for the selective oxidation of alcohols in the presence of hydrogen where Pd atoms dissociate hydrogen, Au surfaces synthesize hydroperoxide species, and metal oxide surfaces utilize hydroperoxide species in selective oxidation reactions.

Synergistic selective oxidation of alcohols using dilute Pd/Au alloys on oxide supports

2019 Fall National Meeting

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