(48c) Size and Shape Specific Chemistry of Uniform, Well-Defined Ag Nanoparticles of Different Shapes in Catalytic Ethylene Epoxidation

Ethylene epoxidation involves the partial oxidation of ethylene to form ethylene epoxide. This is an important industrial heterogeneous catalytic process, where selectivity is a crucial descriptor of the catalytic performance. The conventional catalyst used in this process is metallic Ag supported on inert supports and promoted with various additives. Recent studies have suggested that selectivity to epoxides is governed by a parallel reaction network in which surface oxametallacycle intermediates are isomerized on catalyst surface forming selective epoxide (EO) products and unselective aldehyde (AC) intermediates, which subsequently combust on the catalyst surface (Linic, S., Barteau, M.A., JACS, 2003, 125, 4034).

We employed density functional theory calculations (DFT) to study the underlying mechanisms governing the selectivity to EO on different Ag surface facets. These studies showed that the Ag(100) surface facets should be inherently more selective than the Ag(111) facet. The Ag(111) facet is the most abundant surface facet on spherical Ag particles, that are the standard form of Ag catalysts used in ethylene epoxidation. We employed synthesis techniques, developed recently in the field of nano-technology, to synthesize uniform catalytic particles of different shapes (mainly pentagonal nano-wires and nano-cubes) that are dominated by the desired Ag(100) surface facet. Steady-state reactor studies were used to investigate the proposed hypothesis. The studies showed that the shape of Ag particles play an important role in the selectivity of Ag catalysts as predicted by the DFT calculations. Furthermore, these studies have allowed us to develop a physically transparent model, which describes the effect of shape, size and external conditions on activity and selectivity. (Christopher, P., Linic, S., ChemCatChem, 2010, 2, 78. Christopher, P., Linic, S., JACS, 2008, 130, 11264.)