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Catalysis; Nonlinear Phenomena; Oscillations of Reaction Rate; Spectroscopy, Infrared; Surfaces
Abstract:
Based on previous work oscillations of the CO/O2 reaction on Pd catalysts were reinvestigated with an improved equipment. This allowed analysis of the Pd-L-CO band which was found to oscillate in phase with the absorbance in the B-CO band region. A model developed earlier could essentially be confirmed: The oscillations are driven by a CO induced transformation of the 2 × 2 oxygen adstructure to a (√3 × √3) R 30° structure. Oscillations were also observed on evaporated Pd films and analyzed by means of IR spectroscopy. The behaviour of these specimen resembles strongly that of supported materials. Oscillations of the CO/NO reaction on Pd were observed for the first time; they occurred at NO/CO ratios between 0.5 and 2 at temperatures around 600 K. The most prominent feature of this reaction is the low coverage of the Pd surface with molecular species under oscillatory conditions. By means of IR spectroscopy oscillations of NCO bands, counterphase to the reaction rate oscillations were observed, as well as small amplitude oscillations of the COad bands in phase with the reaction rate. These findings, together with results of kinetic investigations, led to a thermokinetic model involving blocking of the Pd surface with Nad and reactivation by desorption of N2. A simplified theoretical analysis revealed the generation of oscillations in such a model. By measurements on catalyst disks (wafers) spatial inhomogeneities of the reaction rate over the disk surface were detected which might be responsible for the occurrence of chaotic behaviour.
