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

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

Surf. Sci., **459**, 323 (2000).

Abstract
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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 ex\\-change-correlation energy, the NO chemisorption potential
energy (calculated relative to the gas-phase NO) is -2.73 eV and -3.10
eV on flat 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 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
($E\_a\\simeq 1.3$ eV) on the flat Ru(0001) but is found to be only
slightly activated ($E\_a\\simeq 0.1$-$0.5$ eV) when monatomic Ru steps
are present at the surface. The product N and O atoms prefer
chemisorption in two- and three-fold configurations right behind or at
the step edges. The reason for this is ascribed to 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 $\\pm$0.2
eV depending on the reaction geometry.