Ammonia decomposition on (1 1 10) and (0 0 1) surfaces of ruthenium

Abstract

The decomposition of ammonia on stepped Ru(1 1 10) and flat Ru(0 0 1) surfaces has been investigated by Auger electron spectroscopy, low-energy electron diffraction, thermal-desorption studies and kinetic studies. The reaction takes place at ca. 400 K, and N₂ and H₂ are formed stoichiometrically. At lower temperatures (< 500 K) the reaction proceeds through the combination of N^*(ads), which is equilibrated with H₂ and NH₃ in the gas phase, and the rate of N₂ formation is expressed as follows: r_N2∝ exp (βθ_N*). The reaction takes place via a typical Temkin–Pyzhev mechanism in which an isotope effect for the hydrogen atoms in the ammonia molecule is observed, i.e. the rate of NH₃ decomposition is 1.5 times as fast as that of ND₃. The formation of N^*(ads), which is observed as a peak at 570 K in the thermal-desorption spectra for N₂, takes place preferentially on the stepped sites. Therefore, in the transient state the rate of N₂ formation on the stepped Ru(1 1 10) surface is an order of magnitude faster than that on the flat Ru(0 0 1) surface, although the steady-state reaction rate is twice that on the flat surface. This means that stepped sites on the Ru surface play an important role in breaking the N—H bond.

On the other hand, at higher temperatures (> 600 K) the rate of reaction is linearly dependent on ammonia pressure only and independent of hydrogen and nitrogen pressures; the amount of adsorbed hydrogen on the surface was negligible. Since no isotope effect was observed, the reaction is thought to proceed through the recombination of N(ads) on the surface.