Size effects in the catalytic hydrogen oxidation on rhodium

Abstract

Polycrystalline noble metal foils and µm-sized "curved" crystals, consisting of many µm-sized domains or facets of different structures, may play a role of "surface structure libraries". The application of such "surface structure libraries" in combination with spatially resolved kinetic experiments allows simultaneous determination of inherent catalytic properties of different crystallographic orientations under identical reaction conditions. This approach allows new insights into catalytic ignition and reaction front propagation. In the present work the hydrogen oxidation reaction was studied on different Rh samples in the pressure range of 10-6 mbar. The probed Rh samples, that differed by their size and shape, have exposed surfaces of different crystallographic orientation: (i) The apex of a nanometer-sized Rh tip consisting of nm-sized Rh(hkl) facets; (ii) a curved µm-sized Rh crystal sample consisting of sub-µm Rh(hkl) facets. The hemispherical shape of these two samples generates complete fcc ''surface structure libraries'' and a shape which is comparable to the particles used in catalysis; (iii) a polycrystalline Rh sample exhibiting flat differently oriented Rh(hkl) domains of 20 to 100 µm in size. All these Rh surfaces represent model catalytic systems ranging from few nm to hundreds of µm along a size axis. Two types of microscopes were used to image in situ the hydrogen oxidation on the Rh samples of different sizes: the field emission microscope (FEM) and the photo emission electron microscope (PEEM). Both techniques are able to distinguish the local surface coverage of adsorbed species during an ongoing hydrogen oxidation reaction. Using the kinetics by imaging approach, a relationship between image brightness and catalytic activity (high or low activity state) could also be established. The catalytic behavior of hydrogen oxidation on Rh was studied and quantitatively summarized in kinetic phase diagrams. A size effect in hydrogen oxidation on Rh was studied by comparison of the nm-sized and µm-sized Rh specimens. Kinetic transitions between the high and low catalytic activity steady states were studied. Such transitions are accompanied by propagating reaction fronts which were monitored in the temperature range of 413 to 493 K. On the µm-sized Rh(hkl) domains of the polycrystalline Rh foil nucleation of reaction fronts was observed on microscopic defects. It could be shown, that stepped Rh domains affect the predominant direction of front propagation and grain boundaries do not prevent the hydrogen front from crossing over to adjacent domains. Using the FIM, water molecules appearing as a product of hydrogen oxidation were ionized and used for imaging of the Rh specimen surface. In this way, catalytically active surface sites were in situ imaged. During a kinetic transition to the active state the Rh{112} facets were identified as centers of reaction front nucleation. In addition, by FEM imaging, it could be shown that the adjacent facets to Rh{112} of different crystallographic orientations guide reaction fronts across the entire surface of a Rh curved sample in a temperature dependent way along certain ''front propagation paths''. This can be explained by temperature dependent oxygen-induced reconstructions during hydrogen oxidation.