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.
Oxygen-Induced Reconstruction and Surface Oxidation of Rhodium
The reaction of oxygen with Rh single-crystal tips was investigated using field ion microscopy (FIM). Emphasis was laid on revealing the atomic structure of individual tip surface planes along with their influence on the early stages of the reaction, i.e., chemisorption and nucleation. Under field-free conditions, the interaction of oxygen with a (001) oriented Rh tip was found to lead to major reconstructions at temperatures above 400 K. While the original shape of the tip was nearly hemispherical before the reaction, it was polyhedral thereafter. In particular, the {011} and {113} planes were seen to adopt a “missing row” type reconstruction in the presence of oxygen adsorbate. On the Rh{113} plane, a (1 × 3) reconstruction prevailed for oxygen exposures larger than 60 L (1 L = 1.3 × 10-4 Pa·s) and temperatures T ≥ 550 K. In this type of missing-row reconstruction two adjacent dense-packed chains of atoms are absent. Surface oxidation was found to be promoted by the presence of an external electric fiel...
Nitrogen layers on Rh(110)1 × 1 and Rh(110)1 × 2 surfaces produced by NO + H2 reaction: structure, stability and desorption kinetics
We present LEED, AES and thermal desorption studies on the behaviour of atomic nitrogen layers, deposited on Rh(110) 1 × 1 and Rh(110) 1 × 2 surfaces by NO + H2 reaction at 450 K. The desorption measurements are performed in two modes, with a linear temperature ramp (TPD) and at constant temperature. The N2 TPD spectra are used for evaluation of the desorption parameters. The LEED patterns change sequentially from a streaky 3 × 1 to 2 × 1 with growth of the nitrogen layer on Rh(110)1 × 1. The sharpness of the extra spots increases after heating to 480 K or when the layers are produced in a partial desorption mode. This indicates that the formation of well ordered structures is an activated process. The better ordering leads to higher thermal stability of the layers, as reflected by the increase of the desorption energies from 90 to 130 kJ/mol. The N2 TPD spectra are very sharp and are best fitted to a zero order rate process. The changes in the desorption kinetics at a constant temperature are attributed to competing desorption from 2 × 1, 3 × 1 and disordered phases, coexisting on the surface within certain coverage range. Nitrogen layers deposited on Rh(110) 1 × 2 at T ⩽ 400 K form a (2 × 2)pg structure. At T > 440 K the 1 × 2 reconstruction is lifted and the (2 × 2)pg structure converts sequentially into the same 2 × 1, 3 × 1 and 1 × 1 structures, observed for nitrogen adsorbed on Rh(110)1 × 1. We suggest that the 2 × 1, 3 × 1 and (2 × 2)pg structures involve N-induced restructuring of the substrate surface with some nitrogen penetrating underlayer.