N2 dissociation on Fe(110) and Fe/Ru(0001):: what is the role of steps?

In this paper, results from an experimental study of the growth and reactivity of Fe overlayers on Ru(0 0 0 1) in combination with density functional theory (DFT) calculations of nitrogen dissociation on closed-packed Fe surfaces are presented. Based on these, it is suggested that the N-2 dissociation on Fe(1 1 0) and Fe/Ru(0 0 0 1) surfaces is dominated by atomic steps/defects. By DFT we calculate that the activation barrier for N-2 dissociation on Fe/ Ru(0 0 0 1) is 36 kJ/mol lower than for Fe(1 1 0). Neither in the thermal nor the molecular beam experiments do we observe any sign of this huge activity difference between Fe overlayers on Ru(0 0 0 1) and Fe(1 1 0). From thermal data we extract an apparent activation barrier for N-2 dissociation on Fe/Ru(0 0 0 1) of 28 +/- 3 kJ/mol which is significantly lower than that calculated by DFT (71 kJ/mol) on the terrace, but in good agreement with that calculated for a step site on the same surface (39 kJ/mol). The low thermal barrier and the similarity between N-2 activation on Fe/Ru(0 0 0 1) and Fe(1 1 0) strongly indicates that steps and/or defects dominate the reaction on both Fe(1 1 0) and Fe/Ru(0 0 0 1). Temperature programmed desorption curves indicated a nitrogen induced reconstruction of the surface. (C) 2001 Elsevier Science B.V. All rights reserved.