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Abstract:
Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is becoming a popular characterization tool of surfaces. Developments in the last 20-30 years brought photoelectron spectroscopy into the realm of techniques that can be used under realistic use case scenarios in practical environments. Catalysis and surface science are still the dominant scientific fields that employ NAP-XPS most frequently. Heterogeneous catalysts are typically solid materials that convert reactants into products without being consumed in the reaction. This however does not mean that catalysts are rigid entities under reaction conditions. Just to the contrary: the as-synthesized catalyst can be best seen as only a precursor to the active-phase/ active-site ensemble, which is created under operational conditions. Phase transition, segregation, change of the nanoparticle morphology, surface reconstruction, surface melting, deposition of reactant fragments on and dissolution into the catalyst, and other phenomena may occur under the chemical potential of the reactants. NAP-XPS is a suitable experimental technique to study the electronic structure of a catalyst surface under reaction conditions. It provides a variety of potential information ranging from the actual surface phase/state of the catalyst under reaction conditions, the surface species present, and their coverage, up to more subtle information like work function changes, band bending, and more. Here, in this chapter, after some introductory paragraphs on technical aspects, sample contamination, and beam damage, we discuss various selected examples of NAP-XPS in heterogeneous gas-phase catalysis research. The examples are grouped together under the themes of (i) quantification of surface coverages, (ii) selected catalytic applications, and (iii) selected catalysts for more than one application. Due to the vast literature, it was not our intention to provide a comprehensive overview of all reactions pursued and scrutinized by NAP-XPS rather the examples were selected based on our own interests and with the goal to highlight many of the facets of catalyst dynamics. Finally, despite the broad adaptation of the technique, we also point out current shortcomings and limitations.
