Heterogeneous catalysts are complex materials with various interfaces. The understanding of complex metal-oxide interfaces is identified as a transformative opportunity in catalysis due to their ability to enable highly desirable chemical transformations. Dilute Pd and Ag alloys are promising catalysts for complicated chemical transformations such as highly selective oxidation reactions. Ag and Pd are highly miscible in the bulk but can form kinetically trapped states such as PdAg surface alloys. These metastable structures have often been rendered the active phase in applied catalysis which makes them inherently relevant for study with advanced surface science techniques. Despite some studies on PdAg catalysts there is a lack of fundamental studies on oxidized PdAg surface alloys, relevant to selective oxidation reactions. Our work demonstrates the dynamic nature of the metal/metal oxide interface in dilute Pd/Ag(111) alloys and its effect on reactivity towards CO by comparing alloys to pure metals as well as comparing two different alloy structures. The enhanced rate of reduction of Pd oxide by CO as observed by ambient-pressure XPS (AP-XPS) shows the favorable synergistic interplay of Pd and Ag. Moreover, the alloy surface reorganizes after oxidation in O2 from single-layer Pd islands to multi-layer PdOx islands. Another reorganization occurs after subsequent reduction to form multi-layer intermixed PdAg alloy islands as indicated by STM. Furthermore, the preparation method has a major effect on the structure of the resulting oxidized Pd/Ag surface. Small and well-distributed Pd/PdOx clusters exhibit a faster rate of reduction towards CO than large, non-uniformly distributed Pd islands. This suggests how the structure and local composition of Pd/Ag alloys can drastically affect reactivity while having the same chemical composition on the macroscale. 

Dynamic metal/metal oxide interface affects PdAg reactivity

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.