Activation of atomically precise silver clusters on carbon supports for styrene oxidation reactions

Abstract

Metal clusters have distinct features such as large surface area, low-coordination-atom enriched surfaces, and discrete energy levels that influence their behavior during catalytic reactions. Atomically-precise Ag clusters, which are analogues of more well-studied Au clusters, are yet to be fully explored as catalysts for various chemical reactions. 2,4-Dimethylbenzenethiol-protected Ag₂₅ clusters were prepared and deposited onto carbon supports followed by calcination. Results from X-ray absorption fine structure (EXAFS) spectroscopy measurements and other characterization techniques indicated that thermal activation of carbon-supported Ag₂₅ clusters resulted in dethiolation of Ag clusters at 250 °C and beyond, and consequently mild growth in particle sizes of Ag clusters on carbon supports was seen with increasing activation temperatures. Both as-prepared and activated Ag₂₅ clusters were active for styrene oxidation reactions, with high selectivity towards styrene oxide, without using any promoter. Results show that mild activation at 250 °C yields the most active catalysts, and higher activation temperatures lead to decreased activities and slightly poorer selectivity to styrene oxidation as a result of cluster sintering. EXAFS data shows the resulting activated clusters are composed of Ag metal and that all thiols are removed from the Ag cluster surfaces, though XPS data shows that thiol oxidation products are still present in the sample.