(7g) Understanding the Promotional Role of Pd in Dilute PdAu Alloys across Wide Compositional Ranges in Oxidative Alcohol Coupling Reactions

Oxidative coupling reactions enable biomass-derived oxygenates to serve as platform molecules for a much wider array of high-value chemicals. These reactions can be selectivity triggered over alloys wherein a highly active dopant metal such as Pd is diluted into a highly selective host metal such as Au. In alcohol oxidation, the promotional role of Pd is generally attributed to its ability to dissociate O₂, which is notably slow over typical Au catalysts. Whether subsequent aldehyde/alcohol coupling reactions to form esters then occur on Au-O species created by spillover or on Pd-Au interfacial sites is still debated. Additionally, the evolution of these active sites as a function of bulk composition and connections between Pd dilution and rates of elementary O₂ dissociation, oxidation, and coupling steps remain unclear. This work endeavors to assess these uncertainties by exploring the structures produced in a wide range of supported Pd₁Au_x catalysts and their impact on the oxidative coupling mechanism with simple oxygenates.

In this study, a range of supported Pd₁Au_x (x=5-200) alloy nanoparticles were synthesized using a colloid-mediated reductive deposition method. Characterization via TEM and ICP-OES reveal high degrees of particle sizes uniformity and composition control. Oxidative ethanol coupling to form ethyl acetate through an acetaldehyde intermediate was studied as a probe reaction. Reactivity trends indicate that both the rate of ethanol oxidation and the selectivity toward coupling increased with higher Pd loading, suggesting that the role of Pd goes beyond simple O₂ dissociation and spillover. Rate order and activation energy trends further support the notion that rapid coupling outcompetes acetaldehyde desorption at Pd-Au interfaces. FTIR spectroscopy and DFT calculations offer further insights into Pd microstructures in the presence of various key adsorbates. These findings lay groundwork to explain selectivity and activity control in a much wider range of oxidative functionalizations.