Peculiar nitrogen desorption on rhodium surfaces and the decomposition mechanism of nitrogen oxides

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Improvements of cooling performance and pumping rates ; A sample crystal in the angle-resolved thermal desorption apparatus was successfully improved to be cooled from 90K to 50K. The pumping rate was increased about 5 times and high enough to reduce the background signal level to one fifth.N_2O decomposition on clean Rh(110) ; N_2O is already decomposed from 60 K and yields five N_2 desorption peaks in the range of 70-170 K. β_1-N_2 peaking at around 165 K shows a cosine distribution, indicating desorption from N_2 trapped once on the surface after N_2O dissociation. Β_2-N_2 peaked at 140 K and β_ 3-N_2 at 120K. These peaks appeared at higher N_2O exposures and collimate at 68 degrees from the surface normal in the plane in the [001] direction. On the other hand, both β_4-N_2 and β_5-N_2 were observed at small N_2O exposures. These collimate 68 degrees off the surface normal into the [001] direction.N_2 desorption is sensitive to the amount of residual surface oxygen. The amount was controlled by selecting pre-heating temperature, N_2O is not decomposed when the pre-heating is below 1000 K. After the pre-heating at 1100 K, N_2O is decomposed at 100-170 K, emitting N_2 collimated at around 30 degrees along the [001] direction. A (1x2) missing-row structure was observed by LEED. The 30 degree component was proposed to be formed on declining terraces on the missing-row structure.The orientation of adsorbed N_2O on Rh(110) was examined at 70 K by near-edge X-ray absorption fine structure. N_2O is partly dissociated at 70 K at low N_2O exposures. At higher N_2O coverage, two adsorption forms, standing and lying, were proposed.Similar results were obtained on Pd(110) for N_2O decomposition and adsorption.