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

Abstract

Nitric oxide adsorption, thermal desorption and decomposition on a Rh model catalyst was studied using Pulsed Field Desorption Mass Spectrometry (PFDMS). This relaxation-type technique was applied as a chemical probe allowing the composition analysis in selected areas of about 200 atomic surface sites of the catalyst (with the latter being given in a nearly hemispherical morphology of a small tip). The kinetics of thermal desorption were addressed by varying the repetition frequency of the field pulses (usually between 100 kHz and 1 Hz) during the ongoing adsorption process at different temperatures between 450 K and 548 K. Assuming first order kinetics, the mean lifetimes τ before thermal desorption of NO ad were evaluated from the (1−1/e) levels of the equilibrium coverages at long reaction times (i.e. low repetition frequencies). Evaluation in terms of an Arrhenius diagram led to E ̄ d =102 kJ/mol for the activation energy and τ o =4×10 −14 s for the preexponential factor. The NO ad dissociation kinetics were followed by monitoring as a function of time and at a constant pulse repetition frequency the buildup of surface nitrogen and oxygen. An activation energy E dis =27 kJ/ mol was determined in this manner. A qualitative surface layer analysis was performed during the coadsorption and reaction of NO and CO and provide information on the formation of an isocyanate species, NCO ad, at temperatures as high as 410 K.