Spatially Resolved Chemical Characterization with Scanning Photoemission Spectromicroscopy: Towards Near‐Ambient‐Pressure Experiments

Abstract The major experimental challenges in investigations of heterogeneous catalysis are the morphologically complex and dynamic micro‐ and nanosystems and the exploration of events that occur at the catalyst surface, which determine the catalyst activity and selectivity. Modern‐day investigations of catalytic reactions require a multitechnique experimental and computational approach, in which each tool provides specific and complementary information. The unique combination of surface and chemical sensitivity has ranked X‐ray photoelectron spectroscopy (XPS) as one of the most important experimental methods for the characterization of catalytic systems. After its invention, more than half a century ago, a revolutionary step in XPS development was the addition of sub‐micrometer lateral resolution intermediate between light microscopy and electron microscopy. XPS microscopes have responded to the increasing demands on nanotechnology to have access to the local chemical composition, electronic and magnetic structure, and reorganization processes at morphologically complex surfaces and interfaces. The high spatial resolution in XPS microscopes is achieved by two different approaches, magnification of the image of the irradiated surface area (X‐ray photoemission electron microscopy) or demagnification of the incident photon beam by using X‐ray focusing optics (scanning photoemission microscopy; SPEM). In the present article, using selected examples, we demonstrate the capabilities of SPEM for studies relevant to catalysis, and we will discuss the next steps in the ongoing development. In the first part, we present the successful characterization of the oxidation of (i) polycrystalline PtRh particles and (ii) Pd thin films that decorate carbon nanotubes. In the second part, we describe two new setups, developed at Elettra, to overcome the “pressure gap” for photoemission spectromicroscopy experiments, which is the major limitation in the exploration of “real world” catalytic reactions. The first measurements of core‐level photoemission spectroscopy and imaging obtained with spatial resolution of the order of 100 nm at near‐ambient pressure are presented.