Surface reaction kinetics of NO on Rh{110}

Abstract

The reaction of NO with Rh(110) single crystal surfaces was studied by means of various surface analytical tools including static secondary ion mass spectrometry (SSIMS), temperature programmed desorption (TPD), monochromatized x-ray photoelectron spectroscopy (MXPS), and ion scattering spectroscopy (ISS). Molecular NO adsorption was found to occur below 200 K. Higher temperatures caused dissociation of adsorbed NO molecules. Complete dissociation occurred only below a critical coverage, i.e., ΘNO≤0.12. Nitrogen and oxygen molecules were the only desorption products in TPD under these conditions. For higher coverages, thermal desorption of NO occurred and was found to be associated with an activation energy Ed=130±6 kJ/mol and a preexponential νd=10(15.0±0.8) s−1 for the limit of zero NO coverage in a first order process. The dissociation of molecular NOad caused a shift of the N1s core level binding energies from 400.3 to 397.6 eV. SSIMS studies were performed in either real time or in a temperature programmed manner (TPSSIMS) and the calibrated intensities of ionic species were taken to evaluate the dissociation kinetics. Both the activation energy Edis and the preexponential k0dis for dissociation were determined from the TPSSIMS data by assuming first order kinetics in a Polanyi–Wigner ansatz. Values of Edis=15±2 kJ/mol and k0dis=10(1.9±0.5) s−1 were found. These data are compatible with the rate constant kdis=0.18 s−1 obtained in real-time measurements during the ongoing NO adsorption and dissociation at 300 K. In this case, the SSIMS data were evaluated on the basis of consecutive reaction kinetics, including site inhibition by oxygen and Kisliuk’s precursor-mediated adsorption. ISS measurements indicated that oxygen atoms can also partly diffuse into the interior of the Rh(110) crystal.