par Liénard, Georges 
;Degols, L.
Référence Bulletin des Sociétés chimiques belges, 99, 10, page (803-815)
Publication Publié, 1990
;Degols, L.Référence Bulletin des Sociétés chimiques belges, 99, 10, page (803-815)
Publication Publié, 1990
Article révisé par les pairs
| Résumé : | A method is described to acquire, in addition to catalytic reaction rates, the reactant adsorption rate, the surface coverage and its rate of formation. It is based on a non steady‐state transient technique, initiated by a pressure step of one reactant corresponding to adsorption (build up of the catalytic active surface) or to desorption (titration of adsorbed species). Deuterium is used instead of light hydrogen to differentiate by labelling, in the adsorption step, the reactant molecules (deuterated isomers) being desorbed without further reaction. The relaxation of the catalytic system is followed and time constants are defined (exponential variations with time), characterizing the different rates. A novel UHV apparatus, working in the pressure range 10−5 to 1 Torr, is described. Data are acquired by a computer that controls the electromagnet of a magnetic deflection mass spectrometer. A complete spectrum is scanned in less than 12s by a peak jumping method. The technique is applied to a typical experiment of hydrogenolysis of ethane, on a Rhodium evaporated film. Ethane rate of adsorption remains constant during the build up transient effect while the rate of hydrogenolysis and of perdeuterated isomer increase with time and are characterized by the same time constant as the rate of formation of the carbonaceous layer (Rcs). This indicates that the observed activity variations are not directly related to the adsorption rate. A quantitative relationship between Rcs, the rate of hydrogenolysis and the rate of formation of perdeutered isomer is established. The role of deuterium for the determination of the adsorption rate is analysed. Copyright © 1990 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim |
