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Simultaneous kinetic and infrared studies of carbon monoxide oxidation on supported platinum catalyst

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  • Engineering
  • Chemical
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  • Physics

Abstract

Carbon monoxide oxidation on supported platinum catalysts has been studied with a well-mixed or gradientless reactor connected in parallel with an infrared cell. This arrangement allows simultaneous measurement of gas compositions within the reactor by mass spectrometry and observation of selected surface concentrations on the catalyst by infrared spectroscopy at conditions where the adverse effects of heat and mass transfer are negligible. Steady state and transient studies have been carried out to observe the nonlinear phenomena of steady state multiplicity, self-sustained oscillations, and rate enhancement during periodic operation.^ It was possible to characterize the entire charge in situ prior to detailed rate measurements by using a titration method developed in this work by using Ar tracer and balancing all species precisely. Isothermal steady state multiplicities for CO oxidation were observed with feed compositions containing excess oxygen. The multiplicity region shifted to higher temperatures and finally disappeared as the feed approached stoichiometric. As the reactor temperature was increased, the reaction rate increased slowly with high CO coverage and eventually jumped to high rate at the ignition temperature, and the coverage went down to a low value. When the temperature was decreased, the reaction rate stayed high; however, the surface CO started build up on the catalyst. The high rate was maintained up to about 90% coverage and was quenched to low rate at saturated surface coverage.^ Self-sustained oscillations were observed by adding either hydrogen or 1-butene into the CO and oxygen mixtures. A stoichiometric excess of hydrogen was necessary to develop oscillatory states. The oscillatory region disappeared and a multiplicity region was developed as the hydrogen concentration was decreased below stoichiometric. CO oxidation rate enhancement was observed by adding hydrogen in contrast to rate retardation which was observed by adding hydrocarbon. Significant rate enhancement was achieved by periodically introducing CO and oxygen to the reactor in order to maintain more desirable surface concentration on the catalyst.^ The extensive experiments involving titration, multiplicity, oscillation, and periodic operation can provide very severe tests of models proposed to describe the heterogeneous catalytic reaction. Models of the dynamic behavior have been proposed for future work. ^

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