(158b) Targeted, Well-Defined, and Uniform Ag Nanostructures as Highly Selective Olefin Epoxidation Catalysts

Olefin epoxidation reactions involve partial oxidation of olefins to form epoxides. The catalyst used in this process is metallic Ag supported on inert supports. The critical issue in olefin epoxidation is the selectivity to epoxides. 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. [1]

Density functional calculations examining the competing isomerization kinetics of the oxametallacycle intermediate to EO and AC on Ag(111) show almost equivalent activation barriers. This has practical implications since large catalytic particles synthesized using impregnation techniques are dominated by the Ag(111) facet. Similar calculations performed on the Ag(100) surface have shown that the difference in activation barriers associated with forming AC and EO is by ~ 0.1 eV larger on Ag(100) than on Ag(111), indicating that the (100) surface should be intrinsically more selective to EO that the (111) surface. [2]

This result was confirmed experimentally by comparing steady state selectivity to EO on pentagonal Ag nanowire and nanocube catalysts, both dominated by (100) facets, to conventional Ag catalysts dominated in the (111) surface. By controlling the size and shape of catalytically active Ag particles the selectivity to EO was optimized for each shape resulting in 47% selectivity for conventional catalysts, while Ag nanowires were 65% selective and nanocubes were 75% selective. Furthermore we have used size-controlled synthesis of these nanoparticles to highlight the role of under-coordinated sites in dictating the selectivity for ethylene epoxidation. Lastly, the relative stability of the nanostructures under these harsh reaction conditions is examined. It was determined that the stability can be related to the size and internal crystal structure of the material.

The Ag nanowires and nanocubes exhibit characteristic plasmon resonances in the visible region, which makes these materials surface enhanced Raman spectroscopy (SERS)-active. We have utilized SERS to probe the geometry of the critical surface intermediates in epoxidation reactions.

1. Linic, S., Barteau, M.A., JACS, 2003, 125, 4034.

2. Christopher, P., Linic, S., JACS, 2008, 130, 11264.