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

The dissociative chemisorption of N2 on Ru(0001) was studied by thermal adsorption. On the clean Ru(0001) surface the initial sticking coefficient was determined to be s0= 10−5.4 ± 0.7 × exp(− 36 ± 5kJ mol−1R−1T−1). With the deposition of less than 0.01 mono-layer of Au on the Ru(0001) surface the initial sticking coefficient of N2dramatically decreased to s0 = 10−3.5 ± 1.7 × exp (−126 ± 21 kJ mol−1R−1T−1), which corresponds to sevenorders of magnitude at 500 K. Since Au preferentially adsorbs at the stepson the Ru(0001) surface we conclude that N2 dissociation is exclusively taking place at the few step sites on the clean Ru(0001) surface. The desorption of N2 from the Ru(0001) surface was also strongly affected by the presence of small amounts of Au; the TPD peak was shifted up byroughly 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.