(53d) Insights into Partial Oxidation Catalysis over Supported Metal Nanoparticles

Partial oxidation reactions of either hydrocarbons or oxygenates over metal particles have inherent selectivity challenges in avoiding thermodynamically favored over-oxidation cascades. Significant changes in binding properties between reactants and products are ubiquitous in these chemistries, as oxidation products have much different functionality and/or reactivity relative to reactants. This increases the likelihood of kinetically relevant roles of products on rates and selectivities, as products can have different affinities for adsorption and differing propensity to form non-innocent coadsorbates. These impacts can be dependent on particle size and composition, as differences in reactant and product coverages are impacted by the identities and relative abundances of available adsorption site ensembles. In this presentation, I will survey the varying impacts of products observed during kinetic analyses of olefin epoxidations and of oxidative coupling of alcohols and amines. For example, in propylene epoxidation with H₂ and O₂ over Au/TS-1, rates were inhibited by propylene oxide but not significantly impacted by water and carbon dioxide; these effects were comparable over Au particles of varied size. In ethylene epoxidation with O₂ over promoted Ag particles, rates were inhibited by CO₂; the presence of co-fed CO₂ increases epoxidation selectivity. Finally, in oxidative coupling of methanol and dimethylamine to form dimethylformamide over bimetallic PdAu particles, co-fed water increased coupling rates (by ~10×) and selectivities over ~15 nm particles, but co-fed products were not impactful over 3 nm particles. While the impactful co-processed product in each case is distinct, these examples serve as reminders of the necessity to i) consider products (e.g., partial oxidation products, water, and CO₂) early in kinetic studies of oxidation reactions to ensure catalyst beds are differential throughout subsequent investigations, and ii) re-evaluate these effects if catalyst properties (e.g., metal particle sizes) are systematically varied.