C-N COUPLING IN REACTIONS BETWEEN ATOMIC NITROGEN AND ETHYLENE ON RH(III)

Reactions between adsorbed atomic nitrogen and ethylene on Rh(lll)have been investigated by temperature-programmed reaction spectroscopy (TP RS), secondary ion mass spectrometry (SIMS), and work function measure ments. Coadsorption of a small amount of ethylene to 0.10 monolayer of atomic nitrogen results in the formation of a surface cyanide species , which is detected by SIMS through the Rh2CN+ cluster ion. Cyanide fo rmation has been followed by measuring the decrease of the atomic nitr ogen and carbon coverages and the accompanied increase in the CN cover age in temperature-programmed SIMS experiments. The CN formation kinet ics is described by a preexponential factor and an activation energy o f 10(11+/-1) s(-1) and 111 +/- 10 kJ/mol, respectively. In the absence of surface hydrogen, CN groups are stable up to similar to 700 K. CN decomposition results in reaction-limited desorption of Nz with a maxi mum at 800 K and is described by a preexponential factor and an activa tion energy of 10(13+/-1) s(-1) and 210 +/- 15 kJ/mol. Coadsorption of large amounts of ethylene to 0.10 monolayer of N-ads results in the d esorption of almost all nitrogen in the form of HCN between 500 and 70 0 K. Cyanogen was also observed as a reaction product although the sel ectivity was small, 3% at maximum. Work function measurements indicate that surface cyanide is present as a negatively charged species on th e Rh(lll) surface.