In situ dynamic study of hydrogen oxidation on rhodium.
The reaction of hydrogen/oxygen gas mixtures with rhodium single crystals was studied using video-FIM (Field Ion Microscopy) at temperatures between 350 and 550 K and up to 2 x 10(-2) Pa total pressure. Imaging at 500 K in a hydrogen rich gas mixture (H2:O2 = 9) revealed considerable morphological changes of the (0 0 1)-oriented field emitter tip, i.e. the growth of low-index at the expense of high-index planes and strong crystal coarsening. Decreasing the hydrogen partial pressure led to chemical and structural changes of the Rh sample. Starting on the [1 1 0] planes a surface oxide formed, which spread anisotropically across the surface until it finally covered the whole visible surface area. The transformation was reversible upon increasing the hydrogen pressure back to its initial value. However, a hysteresis behavior was observed, i.e. a larger hydrogen partial pressure was found to be necessary to re-establish the initial patterns of a reactive Oad/Had layer. By varying the temperature from 400 to 500 K a phase diagram was established for the Oad/Had system. Increasing the electric field proved to shift the phase diagram towards higher H2 pressures. At 550K self-sustained kinetic oscillations with a cycle time of approximately 40s could be observed.
Oscillatory behaviour of the reduction of NO by H2 and NH3 over Rh studied by FEM
Abstract The NO reduction by H 2 and NH 3 on Rh has been studied by field emission microscopy (FEM). The NO-H 2 reaction shows oscillatory behaviour at 460 K and P NO = 1.5×10 -7 Torr and P H 2 = 1×10 -6 Torr. Unique features of FEM that can be used to study the oscillations in detail are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is displayed in real-time on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H 2 and NO-NH 3 reactions over Pt(100) are observed on Rh as well, including the surface explosion. The NO-NH 3 reaction does also show the surface explosion. Quite remarkable features are observed in the latter case. Single planes show oscillatory behaviour under conditions where others do not. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.
CO adsorption on unreconstructed and reconstructed Rh(110) surfaces: LEED and XPS studies
Abstract The adsorption of CO on the (1 × 1) unreconstructed and (1 × 2) reconstructed Rh(110) surfaces has been studied by means of XPS and LEED. The O 1s spectra of CO were used for measuring the coverage and determining the binding sites of the adsorbed molecules. It has been found that on both surfaces CO adsorption occurs in two bonding configurations characterized by O 1s binding energies at 530.8 and 531.9 eV. They are assigned to occupation of bridge and on-top sites, respectively. On the (1 × 1) surface initially the on-top sites are occupied yielding an ordered c(2 × 2) layer at a coverage of 0.5 ML. At higher coverage CO changes its bonding configuration and a (2 × 1)p2mg structure is formed near saturation when the coverage approaches 1 ML. It is suggested that the (2 × 1)p2mg structure consists of zig-zag chains of bridging tilted CO. When CO adsorbs on the (1 × 2) surface simultaneous occupation of on-top and bridge sites is observed. Molecules in both bonding configurations contribute to the observed (2 × 2), c(2 × 4) and (2 × 2)p2mg structures formed on the (1 × 2) surface. The intensities of the CO O 1s spectra indicate that the amount of CO adsorbed on the (1 × 1) and (1 × 2) surfaces is the same. Comparable amounts of CO adsorbed on the [110] rows and (111) microfacets of the (1 × 2) surface are suggested in order to explain the observed (2 × 2), c(2 × 4) and (2 × 2)p2mg structures.