N+N-]N-2 REACTION-RATES ON RH(111)

We have studied the reaction of N atoms adsorbed on Rh(111) to make gas phase N2, using a combination of temperature programmed desorption and kinetic modeling. Electron beam dissociation of adsorbed NO was used to prepare surfaces with the highest N atom coverages (≈0.5 monolayer) yet reported. Surfaces with coadsorbed N and O were also studied. Our experiments and modeling show that N atoms desorb as N2 from Rh(111) in at least three distinct desorption states with reaction rates that vary by about four orders of magnitude. Comparison of the N2 formation rates reported here with those measured during reaction of CO and NO at 16 Torr shows that under reaction conditions the surface most likely has an intermediate coverage of nitrogen atoms. In addition, our results show that at high N atom coverages N2 is formed at least 50 × faster than under reaction conditions, indicating that N atom recombination is not the rate limiting step in the reduction of NO by CO over Rh(111). © 1993 by Academic Press, Inc.