(621ag) Dynamic Surface Reconstructions of Nanoporous Gold Catalysts during Activation and Selective Oxidation Reactions
Understanding how to reproducibly synthesize and activate bimetallic catalysts is critical to their function and stability. Furthermore, developing a framework for tuning their activity and selectivity is important for the energy efficiency of the catalytic processes that utilize bimetallic materials. Nanoporous gold (npAu), an unsupported metallic Au alloy with dilute Ag content, requires activation with ozone in order to selectively catalyze the O-assisted coupling of methanol and other alcohols. We have shown that treatment of npAu by ozone at 150 °C followed by exposure to a methanol-oxygen mixture results in catalysts of high catalytic activity and improved stability. We have used a combination of atmospheric-pressure X-ray photoelectron spectroscopy (AP-XPS) and electron microscopy (including environmental-TEM) to show significant changes in surface structure and morphology of the npAu at different stages of the activation process. The ozone treatment causes npAu restructuring that drives the segregation of silver in the near surface region into quasi-crystalline regions of silver oxide and the formation of a gold surface oxide. When the oxidized catalyst is placed in a reducing (i.e. reaction) atmosphere, the oxygen is removed from the gold and the silver oxide crystals are reduced to small silver-rich particles that activate dioxygen. These results shed light on the mechanism by which a minor amount of silver drives selective oxidation reactions on npAu and provide a basis for the design of new bimetallic materials.