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

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Reactions

of adsorbed N atoms on Rh(111) to $N2$ and $NH3$ were studied with temperature programmed desorption,

temperature programmed

and static secondary ion mass spectrometry.

For N-atom coverages below $≈0.15$

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

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