Oscillating behavior of NO reduction reactions over rhodium studied on the atomic level

Abstract

Rate oscillations during the reduction of NO by H2 and NH3 over Pt(100) have been studied earlier in our laboratories. A vacancy model involving the surface coverages of NO, O, and the hydrogen source (H or NHx) was proposed. This paper describes the NO–H2 reactions over Rh studied by field-emission microscopy (FEM). Oscillations in both electron emission current and the images observed were recorded. The dynamic behavior of these reactions, such as traveling wave fronts moving over the surface in an oscillatory manner and the occurrence of surface explosions, was displayed on a fluorescent screen with a spatial resolution of 20 Å. All processes were recorded in real time with a charged coupled device video camera. The results obtained for the NO reduction over Pt(100) are compared with the FEM results obtained with Rh, where many crystal planes were studied at the same time. Several types of oscillatory behavior have been observed, with large differences in periodicity, frequency, and amplitude. The type of oscillation is determined by the NO–H2 ratio, the temperature, and the surface structure. Using probe-hole FEM local oscillations were observed on several surfaces. The nature of the wave fronts observed during oscillatory behavior of the NO–H2 reaction over Rh involves the removal of the adlayer by reaction of NO and its dissociation fragments with hydrogen.