Adsorption, diffusion, and dissociation of NO, N and O on flat and stepped Ru(0001)

The chemisorption, diffusion and dissociation of nitric oxide, NO, on flat and stepped Ru(0001) surfaces are investigated using density functional theory. In the Perdew-Wang-91 GGA approximation (PW91) for the exchange-correlation energy, the NO chemisorption potential energy (calculated relative to the gas-phase NO) is -2.73 and -3.10 eV on fiat and stepped Ru(0001), respectively. The NO(a) experiences a diffusion energy barrier of 0.33 eV on the flat Ru(0001) terrace, while the barrier for diffusion across the steps is 0.9-1.1 eV. The barriers for attachment to and detachment from the steps are in the range of 0.4-0.8 eV. A number of strongly inclined, but metastable, NO configurations are found. The NO dissociation is calculated to be highly activated (E(a)similar or equal to 1.3 eV) on the flat Ru(0001) but is found to be only slightly activated (E(a)similar or equal to 0.1-0.5 eV) when monatomic Ru steps are present at the surface. The product N and O atoms prefer chemisorption in two- and threefold configurations right behind, or at, the step edges. The reason for this is the energetically higher Ru 4d-band positions at the step edges. The N and O atoms are subject to considerable energy barriers for diffusion over the Ru(0001) terraces (E-d(N)=0.79 eV and E-d(O)=0.54 eV), and they experience even larger barriers for interlayer diffusion. Combining the NO dissociation and the N plus O chemisorption results, two mechanisms are found to cause the higher reactivity of the step edges in terms of NO-bond activation. The main mechanism is the ability of the reaction site to provide rebonding of the non-interacting reaction products. The other mechanism is the minimization of the degree of repulsive interaction of the N and O atoms in the transition state. Chemisorption energies, and diffusion and dissociation energy barriers are further calculated in the PBE and RPBE exchange-correlation descriptions. The PBE results are, as expected, very close to the PW91 results. The RPBE results, however, show largely reduced bonding energies and energy barriers that vary by +/-0.2 eV depending on the reaction geometry. (C) 2000 Elsevier Science B.V. All rights reserved.