TRANSIENT FTIR STUDIES AND ELEMENTARY STEP MODELING OF CARBON MONOXIDE AND ETHYLENE OXIDATION ON SILICA SUPPORTED PLATINUM AND PALLADIUM CATALYSTS (INFRARED).

摘要

Transient methods of Temperature-Programmed Reaction (TPR) and Concentration-Programmed Reaction (CPR) have been used along with Fourier Transform Infrared Spectroscopy (FTIR) to study the multiplicity features and oscillatory behavior of CO oxidation and ethylene oxidation over Pt and Pd supported catalysts. Selected area FTIR and surface temperature measurements have shown that self-sustained oscillations in the above reactions involve spatially propagating regions of non-uniform coverage and surface temperature, even under recycle reactor conditions. The results elucidate how the multipeak structure of the oscillations can arise from inhomogeneous patterns in surface reaction rate controlled by localized CO or ethylene inhibition. Each TPR and CPR experiment depicts the entire temperature or concentration dependence of the reaction (bifurcation diagram) including ignition, extinction and hysteresis behavior. Such experiments have been used to construct bifurcation cross-section diagrams in the T(,g) vs. C plane for CO and ethylene oxidation on Pt/SiO(,2). The results for CO oxidation have been used to fit an elementary step model including transport effects and kinetic constants taken from single crystal studies. The model reproduces qualitatively well the features of TPR and CPR experiments and bifurcation cross-section diagrams obtained at low and high oxygen concentrations. Both the experiment and the theory show a local maximum in the extinction branch as well as a local minimum near the stoichiometric point at low oxygen concentrations. The model thus correctly predicts the existence of isolated steady-state branches including bifurcation diagrams with one ignition and two extinction points that have been previously reported, as well as a new type of bifurcation diagram having one ignition and three extinction points that occurs only at low oxygen concentration. It is also shown that the model can provide insight into the rate limiting step that prevails under different conditions. Using a forcing function that simulates local quenching (pseudo-inhomogeneous approach), the model provides some understanding of the local temperature excursions observed experimentally during ignition, extinction and oscillatory states. The model is also used to evaluate an oxidation-reduction mechanism to describe the oscillations, and to reproduce the features of forced periodic operation reported in the literature for CO oxidation.