Effect of the reaction conditions on the stability and structure of nitrogen layers on Rh(110) surfaces

Abstract

Parallel TPD and LEED studies are used to compare the behaviour of atomic nitrogen layers produced by NH 3 dissociation or by NO + H 2 reaction on Rh(110)1 × 1 and Rh(110)1 × 2 surfaces at reaction temperatures ranging from 310 to 480 K. Special attention is paid to the effect of “subsurface” oxygen and coadsorbed oxygen on the stability and desorption kinetics of the nitrogen layers. Depending on the reaction temperature, T r, nitrogen deposited by dissociative adsorption of NH 3 on a well reduced Rh(110)1 × 1 surface results in two N 2 TPD peaks, α and β 1, located at ≈470 and ≈530 K, respectively. The α-peak, which is the dominating feature for T r ≦ 330 K, is completely removed by the β 1-peak when the exposure is performed at T r ≧ 400 K. The nitrogen layers built by NO + H 2 reaction at T > 450 K are the most stable and desorb in a very sharp β 2-peak at 580 K. The same “explosive” desorption is shown by nitrogen layers deposited by dissociative adsorption of NH 3 at T r ≧ 400 K on Rh(110)1 × 2 or on Rh(110)1 × 1 with traces of “subsurface” oxygen. The desorption of the β 1- and β 2-states follows first-order and zeroth-order desorption kinetics, respectively. Ordered 3 × 1 and 2 × 1 LEED patterns characterize both higher temperature β-states. For the β-states the intensity of the fractional spots is comparable to that of the integer ones and almost does not change with increasing primary energy, but the Debye-Waller factor of the β 2-state is smaller. It is suggested that the accommodation of nitrogen atoms in these states involves a rearrangement of the substrate atoms. The observed differences in the behaviour of the atomic nitrogen layers indicates that a substantial restructuring of the substrate is required for optimization of nitrogen bonding. The degree of restructuring depends on the actual reaction conditions, the reaction temperature, the substrate surface symmetry and the presence of oxygen contaminants.