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

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

temperature programmed

[external_link_head]reaction spectroscopy,

and static secondary ion mass spectrometry.

For N-atom coverages below ≈0.15

[external_link offset=1]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

[external_link offset=2]desorption

at low NH3 coverage are 81 kJ/mol and 1013s−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.[external_footer]

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