Size-Specific Chemistry of Ag Nanostructures in Catalytic Ethylene Epoxidation

The low selectivity of heterogeneous catalysts has been one of the critical obstacles to the wider use of heterogeneous processes in the commercial production of high value chemicals. The limited selectivity is related to a number of issues, including the lack of predictive theories that guide the discovery of the optimal catalytic site, the dearth of strategies to synthesize the targeted sites at high concentrations, and the difficulties associated with preserving these sites under reaction conditions. 2] Advances in the fields of theoretical and synthetic chemistry are beginning to address some of these issues and provide a framework for the identification (based on molecular insight) and synthesis of highly selective uniform catalytic structures with high concentration of targeted surface sites. We recently showed that the shape of catalytic silver particles affects selectivity in the epoxidation of ethylene to form ethylene oxide (EO; C2H4+ =2 O2!C2H4O) on alumina-supported Ag catalysts. The selective product in the process is EO, whereas H2O and CO2 are undesired byproducts. The studies showed that the selectivity to EO on Ag nanowire catalysts was much higher than that on conventional spherical Ag particles with identical external conditions. The enhanced EO selectivity of the nanowire catalysts was attributed to a higher concentration of the Ag(100) surface facets on nanowires in comparison to spheres. Density functional theory (DFT) calculations showed that the Ag(100) surface facet is inherently more selective towards EO than the Ag(111) facet. Herein we show that the selectivity to EO in the ethylene epoxidation reaction is further enhanced on uniform supported Ag nanocube catalysts. By comparing the catalytic performance of Ag nanocubes, pentagonal nanowires, and spherical catalytic particles of varying size, we derive a simple model, which can account for the impact of chemical (for example, inherently different outcome of a chemical process on catalytic particles of different shapes), and physical (size of catalytic particles, and the impact of external operating conditions) factors on the reaction selectivity. Our studies show that catalytic particles of controlled size and shape not only represent promising heterogeneous catalysts for selective production of chemicals, but also act as a critical platform to study heterogeneous catalytic processes and to identify crucial factors that impact process selectivity.