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

For investigating the mechanism of solid-state catalysed reactions, sine wave modulation of feed gas concentrations was used to induce dynamic variations in the concentrations of products, intermediates and reactants, as monitored in situ in a DRIFT cell. The phase shift Δϕ between the external perturbation of the feed gas and the signals of products, intermediates and reactants was examined in dependence on the modulation frequency ω. A micro-kinetic model consisting of several simple steps was used to describe the process. A relationship between the phase shifts and the constants characterising the single reaction step can be derived from the applied micro-kinetic model. Therefore, the evolution of each phase shift as function of the modulation frequencies is characteristic for the individual reaction step. Repeating the experiment at different temperatures made it possible to assess the temperature dependent constants characterising the model, like the Arrhenius energy EA.This new method was tested for the CO oxidation over a Pd25Zr75 based catalyst. Possible reaction orders for the reactants CO and O2 together with reaction constants were estimated.