Surface reaction kinetics of NO on Rhlcub;110rcub;

摘要

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 xhyphen;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., THgr;NOle;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 energyEd=130plusmn;6 kJ/mol and a preexponential ngr;d=10(15.0plusmn;0.8)sminus;1for the limit of zero NO coverage in a first order process. The dissociation of molecular NOadcaused a shift of the N1score 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 energyEdisand the preexponentialk0disfor dissociation were determined from the TPSSIMS data by assuming first order kinetics in a Polanyindash;Wigner ansatz. Values ofEdis=15plusmn;2 kJ/mol andk0dis=10(1.9plusmn;0.5)sminus;1were found. These data are compatible with the rate constantkdis=0.18 sminus;1obtained in realhyphen;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 Kisliukrsquo;s precursorhyphen;mediated adsorption. ISS measurements indicated that oxygen atoms can also partly diffuse into the interior of the Rh(110) crystal. thinsp;