Adsorption and decomposition of ammonia on an Fe(1 × 1) overlayer on an Ru(001) surface with or without co-adsorbed oxygen

Abstract

The adsorption and decomposition of ammonia on Fe(1 × 1) overlayers on an Ru(001) surface with or without co-adsorbed oxygen have been investigated by means of Auger electron spectroscopy, low-energy electron diffraction and thermal desorption, and also by kinetic studies. The adsorption of ammonia at 420 K on an epitaxial Fe overlayer deposited on an Ru(001) surface gave a newly-ordered (√7 ×√7)R19° structure over a wide range of Fe coverages from 0.14 to 2 monolayers. The desorption of N₂ from the ordered adsorbate layers showed a sharp peak at 870 K which was replaced by a peak at 970 K above 0.6 monolayer Fe coverage, showing similarity to the decomposition of surface nitride, Fe₄N, observed for Fe single-crystal planes. The decomposition of ammonia on Fe/Ru(001) surface proceeded via two consecutive reaction steps that were dynamically balanced. The activation energy of the formation of atomic nitrogen varied from 33 to 8 kJ mol^–1 with an increase in Fe coverage from 0 to 1 monolayer, while the rate of desorption of N₂ was reduced. Accordingly, the steady-state rate of decomposition of ammonia exhibited an optimum Fe coverage to reach the maximum rate. In contrast, a c(4 × 2) ordered overlayer appeared upon the adsorption of oxygen on an annealed Fe/Ru(001) surface. The steady-state rate of decomposition of ammonia was enhanced 3.5 times by co-adsorbed oxygen, because nitrogen was effectively activated by surrounding oxygen atoms in c(4 × 2) mixed overlayers.