Use of periodic variations of reactant concentrations in time resolved DRIFT studies of heterogeneously catalysed reactions

For investigating the mechanism of solid-state catalysed reactions, sine wa ve modulation of feed gas concentrations was used to induce dynamic variati ons in the concentrations of products, intermediates and reactants, as moni tored in situ in a DRIFT cell. The phase shift Delta pi between the externa l perturbation of the feed gas and the signals of products, intermediates a nd reactants was examined in dependence on the modulation frequency omega. A micro-kinetic model consisting of several simple steps was used to descri be the process. A relationship between the phase shifts and the constants c haracterising the single reaction step can be derived from the applied micr o-kinetic model. Therefore, the evolution of each phase shift as function o f the modulation frequencies is characteristic for the individual reaction step. Repeating the experiment at different temperatures made it possible t o assess the temperature dependent constants characterising the model, like the Arrhenius energy E-A. This new method was tested for the CO oxidation over a Pd25Zr75 based catal yst. Possible reaction orders for the reactants CO and O-2 together with re action constants were estimated. (C) 2000 Elsevier Science B.V. All rights reserved.