The Effect of Ag Particle Shape and Surface Structure on Ethylene Epoxidation Selectivity

Introduction Approaching 100% selectivity in heterogeneous catalytic reactions is an ultimate objective of catalyst discovery and is critical for the design of efficient, environmentally friendly processes. This work focuses on an example where shape controlled synthesis of silver nano-particles has been utilized along with Density Functional Theory (DFT) calculations to design heterogeneous silver (Ag) catalysts that are more selective in partial oxidation of ethylene to form ethylene oxide (EO) than Ag catalysts prepared using conventional methods. These results indicate that advances in synthetic chemistry, which allow the synthesis of uniform metal particles of tunable shapes and sizes, might play a crucial role in the development of highly selective heterogeneous catalysts. Ethylene epoxidation involves the partial oxidation of ethylene to form ethylene epoxide and is an important industrial processes where catalytic performance is measured by selectivity to the epoxide. Silver is the only heterogeneous catalyst that can achieve reasonable selectivity to EO. The main by-products in the reaction are combustion products, CO2 and H2O. Ag catalysts synthesized using standard incipient wetness methods and supported on inert supports exhibit low selectivity, <20%, for small particle sizes, <20nm. It has been shown that pure Ag catalysts can achieve 40-50% selectivity when the particles are about 1 m diameter and dominated by the (111) facet. Industrial Ag catalysts are further promoted with various additives including Cs, Cl, and Re to achieve a selectivity of ~80-85%. An alternative approach to enhance the selectivity of Ag catalysts, discussed in this work, relies on the identification of Ag surfaces that are inherently more selective than Ag(111) and synthesis of catalysts which are dominated by these surfaces.