Dissociative adsorption of N2 on Ru(0001):: A surface reaction totally dominated by steps

The dissociative chemisorption of N-2 on Ru(0001) was studied by thermal adsorption. On the clean Ru(0001) surface the initial sticking coefficient was determined to be s(0) = 10(-5.4+/-0.7) x exp(-36+/-5 kJ mol(-1) R-1 T-1). With the deposition of less than 0.01 monolayer of Au on the Ru(0001) surface the initial sticking coefficient of N-2 dramatically decreased to s(0)=10(-3.5+/-1.7) x exp(-126+/-21 kJ mol(-1) R-1 T-1), which corresponds to seven orders of magnitude at 500 K. Since Au preferentially adsorbs at the steps on the Ru(0001) surface we conclude that N-2 dissociation is exclusively taking place at the few step sites on the clean Ru(0001) surface. The desorption of N-2 from the Ru(0001) surface was also strongly affected by the presence of small amounts of Au; the TPD peak was shifted up by roughly 150 K. By simulating the TPD spectra we find that this shift corresponds to a difference in the desorption barrier of about 73 kJ/mol for desorption from the steps and from the terraces. The implications of the results for Ru as an ammonia synthesis catalyst are discussed. (C) 2000 academic Press.