STEPS IN H-2 OXIDATION ON RH - OH DESORPTION AT HIGH-TEMPERATURES

The desorption of OH radicals over a polycrystalline Rh foil exposed to mixtures of H-2, 02, and H20 for surface temperatures between 1000 and 1800 K is measured by laser-induced fluorescence (LIF) and used to determine the elementary steps in H-2 oxidation on Rh. The results on Rh are compared to previous results obtained on Pt. OH desorption maxima and apparent OH desorption activation energies are slightly lower in H-2 oxidation on Rb compared to on Pt. However, hydroxyl desorption in H20 decomposition and its influence by 02 on Rh is markedly different than on Pt. Whereas 02 addition gives a monotonic increase in radical desorption asymptotically approaching a maximum on Pt, 02 addition enhances radical desorption only at low pressures on Rh. At high 02 pressures, the OH desorption rate decreases with additional 02. Modeling indicates that these differences are caused by the high activation energy for oxygen desorption on Rh and the resulting high coverage of O. Consequently, OH is a much less stable surface species on Rh than on Pt. The 12-step mechanism for reversible H-2 oxidation used to model the results on Pt is modified to allow for 2 types of 0 binding sites on Rh-competitive and noncompetitive sites, with the latter probably being a surface oxide. Simulation of the H2O + O2 experiments allows us to determine the equilibrium between the two types of 0 sites.