Dual effects of water vapor on ceria-supported gold clusters

Abstract

Atomically precise nanocatalysts are currently being intensely pursued in catalysis research. Such nanocatalysts can serve as model catalysts for gaining fundamental insights into catalytic processes. In this work we report a discovery that water vapor provokes the mild removal of surface long-chain ligands on 25-atom Au₂₅(SC₁₂H₂₅)₁₈ nanoclusters in a controlled manner. Using the resultant Au₂₅(SC₁₂H₂₅)_18−x/CeO₂ catalyst and CO oxidation as a probe reaction, we found that the catalytic activity of cluster/CeO₂ is enhanced from nearly zero conversion of CO (in the absence of water) to 96.2% (in the presence of 2.3 vol% H₂O) at the same temperature (100 °C). The cluster catalysts exhibit high stability during the CO oxidation process under moisture conditions (up to 20 vol% water vapor). Water vapor plays a dual role in gold cluster-catalyzed CO oxidation. FT-IR and XPS analyses in combination with density functional theory (DFT) simulations suggest that the “–SC₁₂H₂₅” ligands are easier to be removed under a water vapor atmosphere, thus generating highly active sites. Moreover, the O₂²⁻ peroxide species constitutes the active oxygen species in CO oxidation, evidenced by Raman spectroscopy analysis and isotope experiments on the CeO₂ and cluster/CeO₂. The results also indicate the perimeter sites of the interface of Au₂₅(SC₁₂H₂₅)_18−x/CeO₂ to be active sites for catalytic CO oxidation. The controlled exposure of active sites under mild conditions is of critical importance for the utilization of clusters in catalysis.