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 ${\mathrm{N}}_{\mathrm{2}}$ and ${\mathrm{NH}}_{\mathrm{3}}$ were studied with temperature programmed desorption,

temperature programmed

[external_link_head]reaction spectroscopy,

and static secondary ion mass spectrometry.

For N-atom coverages below $\text{≈0.15}$

[external_link offset=1]monolayers, desorption

of ${\mathrm{N}}_{\mathrm{2}}$ follows simple second-order kinetics, but at higher coverages the desorption

traces broaden to higher temperatures. Hydrogenation to ${\mathrm{NH}}_{\mathrm{3}}$ can be described by a stepwise addition of H atoms to ${\mathrm{N}}_{\mathrm{ads}}$ in which the reaction

from ${\mathrm{NH}}_{\mathrm{2,ads}}{\mathrm{+H}}_{\mathrm{ads}}$ to ${\mathrm{NH}}_{\mathrm{3}}\mathrm{,ads}$ determines the rate. The activation energy

for the rate determining step is 76 kJ/mol. The desorption

of ${\mathrm{NH}}_{\mathrm{3}}$ from Rh(111) was studied separately. The kinetic parameters for

[external_link offset=2]desorption

at low ${\mathrm{NH}}_{\mathrm{3}}$ coverage are 81 kJ/mol and ${10}^{13}\hspace{0.17em}{\mathrm{s}}^{\mathrm{-1}},$ but the rate of desorption

increases rapidly with increasing ${\mathrm{NH}}_{\mathrm{3}}$ 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.[external_footer]