Nitric oxide adsorption, decomposition, and desorption on Rh(100)

Abstract

Nitric oxide adsorption, decomposition, and desorption were studied on Rh(100) in the temperature range from 88 to 1100 K using electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). The EEL spectrometer was equipped with a multichannel detector for fast data acquisition. There are two adsorption states of NO on Rh(100), designated α1NO and α2NO, characterized by vibrational modes at 114 and 196 meV, respectively, and assigned to a lying down or highly inclined species and a vertically adsorbed species. The populations of the two states as functions of the total NO coverage were measured on the clean surface and with coadsorbed oxygen and CO. These coadsorbed species, whether adsorbed before or after the NO, increase the α2 population at the expense of α1. A model that includes an adsorbate–adsorbate interaction (range≈7 Å) which converts α1NO to α2NO and which permits adsorbing NO to diffuse so as to favor α1 adsorption fits the measured populations of the two species on the clean surface and produces a saturation coverage of 0.62 ML (1 ML=1.39×1015 molecule/cm2), in good agreement with the published result. Decomposition and desorption of NO at temperatures >90 K were studied by a series of temperature programmed EELS (TP-EELS) experiments at heating rates from 0.048 to 5.25 K/s and by TPD. At saturation, 62% of the NO decomposes as evidenced by the extent of N2 desorption in TPD peaks at 460 and 770 K. The remaining NO desorbs molecularly near 430 K with an activation energy Ea=28±3 kcal/mol and first order preexponential v=1014±1 s−1, as determined by TP-EELS. The decomposition of α1NO occurs near 170 K with Ea=10.5±0.7 kcal/mol and v=1011.8±0.7 s−1. The extent of the α2NO decomposition and its activation energy are strongly coverage dependent. The temperature at which its decomposition rate is a maximum approaches that of α1NO at low coverages, consistent with a decomposition mechanism involving an α1NO intermediate.