Nitric oxide interaction with Rh metal: kinetics of elemental steps and reaction with carbon monoxide

Nitric oxide adsorption, thermal desorption and decomposition on a Rh model catalyst was studied using Pulsed Field Desorption Mass Spectrometry (PFDM S). This relaxation-type technique was applied as a chemical probe allowing the composition analysis in selected areas of about 200 atomic surface sit es of the catalyst (with the latter being given in a nearly hemispherical m orphology of a small tip). The kinetics of thermal desorption were addresse d by varying the repetition frequency of the field pulses (usually between 100 kHz acid 1 Hz) during the ongoing adsorption process at different tempe ratures between 450 K and 548 K. Assuming first order kinetics, the mean li fetimes <(<tau>)over bar> before thermal desorption of NOad were evaluated from the (1 - 1/e) levels of the equilibrium coverages at long reaction tim es (i.e. low repetition frequencies). Evaluation in terms of an Arrhenius d iagram led to (E) over bar (d) = 102 kJ/mol for the activation energy and t au (o) = 4 x 10(-14) s for the preexponential factor. The NOad dissociation kinetics were followed by monitoring as a function of time and at a consta nt pulse repetition frequency the buildup of surface nitrogen acid oxygen. An activation energy E-dis = 27 kJ/mol was determined in this manner. A qua litative surface layer analysis was performed during the coadsorption and r eaction of NO and CO and provide information on the formation of an isocyan ate species, NCOad, at temperatures as high as 410K. (C) 2000 Elsevier Scie nce B.V. All rights reserved.