Formation of $NH3$ and $N2$ from atomic nitrogen and hydrogen on rhodium (111)

Reactions of adsorbed N atoms on Rh(111) to $N2$ and $NH3$ were studied with temperature programmed desorption, temperature programmed reaction spectroscopy, and static secondary ion mass spectrometry. For N-atom coverages below $≈0.15$ monolayers, desorption of $N2$ follows simple second-order kinetics, but at higher coverages the desorption traces broaden to higher temperatures. Hydrogenation to $NH3$ can be described by a stepwise addition of H atoms to $Nads$ in which the reaction from $NH2,ads+Hads$ to $NH3,ads$ determines the rate. The activation energy for the rate determining step is 76 kJ/mol. The desorption of $NH3$ from Rh(111) was studied separately. The kinetic parameters for desorption at low $NH3$ coverage are 81 kJ/mol and $1013 s−1,$ but the rate of desorption increases rapidly with increasing $NH3$ coverage. It is argued that the remarkable coverage dependence of the desorption rate is unlikely to be caused by lateral repulsive interactions but may be due to a coverage dependence of the pre-exponential factor.