Catalytic hydrogen oxidation on rhodium: : Bridging the materials gap

Abstract

The catalytic hydrogen is gaining importance as efforts increase to shift the energy production away from environmentally problematic fossil fuels. In theory, hydrogen could be won in a renewable way, e.g. by photocalytic water splitting with sunlight, and then converted to energy in a so-called fuel cell. For effcient fuel cells, good catalysts for the catalytic hydrogen oxidation need to be found. Studying potential catalyst materials has some issues, among others the so-called pressure and materials gap. Catalytic studies are often performed under ultra high vacuum conditions on simplified model systems and the results of such studies are not necessarily transferable to realistic conditions. In this work, it is attempted to partially bridge the materials gap by studying different Rh containing samples with increasing materials complexity: a polycrystalline Rh foil, an oxidized polycrystalline Rh foil and oxide supported Rh particles, which are similar to realistic industrial catalysts. To compare the catalytic behavior of the different model systems, kinetic phase diagrams were constructed using the kinetics by imaging approach. In such a kinetic phase diagram, the areas of low catalytic activity, high catalytic activity, and bistability are marked. The bistability area is a particularly interesting feature in the kinetic phase diagrams, it is a parameter space where the reaction can be in a steady state of low catalytic activity or high catalytic activity at the exact same external parameters, depending only on the prehistory of the system. Additionally, the structure-dependent oxidation of individual Rh(hkl) domains of a polycrystalline foil, was studied by photoemission electron microscopy (PEEM). In connection, the structure-dependence of multifrequential oscillations in the hydrogen oxidation was evaluated, where the formation of a subsurface oxide is assumed to be the feedback mechanism. The dependence of the oscillation frequency on the hydrogen pressure and oxygen pressure was also studied.