In Situ XAS Investigations of Chemical Oscillations in the Oxidation of Co on Supported Pd Catalysts

Self-sustained temperature as well as rate oscillations during the oxidation of CO at atmospheric pressure on supported Pd catalysts were studied in-situ at the Pd K edge by employing energy-dispersive XAS. From the absorption spectra a phase transition from palladium oxide to palladium clusters can be excluded as driving force of the observed oscillations. Nevertheless, a constant phase correlation between edge position and the observed temperature oscillations could be revealed. This points toward a periodical surface oxidation / reduction process in phase with deactivation / activation cycles of the catalyst. Furthemore, an oscillatory behaviour in height and position of the first Pd peak could be seen in the radial distribution function. From theoretical cluster calculations using FEFF6 it can be deduced, that the evaluated oscillations in Pd coordination number and Pd-Pd distance indicate an oscillatory change of surface coverage from CO to oxygen and vice-versa. Taking these surface-coverage oscillations and the periodical oxidation / reduction into account, a surface activation / deactivation process is proposed as kinetic mechanism for the observed chemical oscillations