Ambient Pressure X-ray Photoelectron Spectroscopy for Probing Monometallic, Bimetallic and Oxide-Metal Catalysts Under Reactive Atmospheres and Catalytic Reaction Conditions

Abstract

Synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS) is an important in situ chemical probe in the toolbox of chemists and materials engineers. It uniquely aids in the investigation of the surfaces and interfaces of complex systems under dynamic environments, such as catalysts operating at the solid/gas interface. Nanoparticles (NPs) produced via colloidal chemistry offer the advantage of narrow particle distributions in APXPS studies of catalysts. They provide a narrow distribution in size, shape and composition of catalysts, which provide a closer correlation to actual catalysts than single crystal models for which APXPS is extensively employed. In this paper, some case studies of colloidaly-made uniform nanoparticles catalysts will be outlined. The examples will include monometallic, bimetallic and binary oxide-metal catalysts, where APXPS is used in different reactive atmospheres and during catalytic reactions. First, in situ CO oxidation studies of monometallic Rh NPs in the 2–7 nm range will be discussed. Next, APXPS studies of bimetallic NPs with size and composition control will be illustrated. NO-induced reversible core/shell restructuring of bimetallic PdRh NPs and gas-driven irreversible surface segregation of Cu in bimetallic CoCu NPs will be explained. To further illustrate the utility of the technique, APXPS and catalytic measurements carried out in parallel and under identical conditions will be described over bimetallic AuPd and CoPt NPs, during catalytic oxidation of CO. APXPS based structure–function correlations such as composition and ensemble dependence of catalytic activity will also be illustrated in this discussion. Finally, binary oxide-metal catalysts will be exemplified in APXPS studies of CeO2/Pt and TiO2/Co systems in hydrogen reducing atmospheres and/or during catalytic hydrogenation of CO2. Also, along with this idea, metal-support interactions in the forms of metal-induced reduction of oxide support, wetting and encapsulation of metal will be detailed in relation to catalytic properties.