Selective oxidation reactions, such as the conversion of propylene to propylene oxide, have been well studied on Au/TiO2 systems. Many studies have shown that these reactions rely on a co-feed of hydrogen and oxygen to create hydroperoxide intermediates responsible for the low temperature selective oxidation. Here the limiting step is the splitting of hydrogen which Au does poorly. Single atom alloys (SAA) of platinum group metals in coinage metals have been shown to have superior activity and selectivity for several hydrogenation reactions. In many cases such pairings are able to split hydrogen similar to platinum group metals without the loss of selectivity caused by large surfaces of Pt or Pd. In this work we describe the synthesis, characterization, and catalytic testing of a single atom alloy of Pd in Au nanoparticles for the selective oxidation of isopropanol to acetone as a probe reaction for the formation of hydroperoxide intermediates. SAA nanocrystals of Pd in Au were synthesized using an Au seed mediated colloidal technique. Pd/Au nanocrystals were loaded onto TiO2 and heat treated to remove ligands remaining from their synthesis. Nanocrystals were characterized using UV-Vis spectroscopy, transmission electron microscopy, and x-ray absorption techniques. Catalytic testing was conducted at atmospheric pressure in a reaction mixture containing isopropanol and oxygen in argon with and without hydrogen added. The activity and selectivity of two Pd/Au SAA were tested and compared to those of pure Au and Pd individually. Catalytic studies without hydrogen showed increasing conversion of isopropanol to acetone with increasing Pd. At high temperatures, the Pd catalysts became non-selective due to complete combustion. Tests with hydrogen added to the reaction mixture showed an increase in activity for all catalysts at temperatures below 150 oC, likely due to the creation of hydroperoxide intermediates. This increase was greatest for SAA of Pd/Au which also showed superior selectivity to both Pd and Au individually. This is due to the synergy between Pd atoms (which split hydrogen), Au nanocrystals (which create hydroperoxides with hydrogen and oxygen), and the TiO2 support (which stabilizes hydroperoxides). This work provides many opportunities for the further exploration of selective oxidation with Pd/Au SAA in the presence of hydrogen.

Synergistic effect in colloidal Pd/Au single atom alloy nanocrystals for selective oxidations

256th National Meeting (2018)

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