Transient Phenomena in CO and Hydrocarbon Oxidation over Supported Platinum Catalysts

Doktorsavhandling, 2004

The aim with this thesis work was to better understand transient phenomena involved in the low-temperature oxidation of CO and hydrocarbons by oxygen over typical oxidation catalysts. Supported platinum catalysts were prepared and carefully studied under transient conditions by means of flow-reactor experiments, different in situ spectroscopic (FTIR and XANES) methods in combination with mass spectrometry and mean-field kinetics simulations. The results show that deliberate manipulations of the reactant gas composition, so-called periodic operation, can enhance the low-temperature oxidation activity for both CO and hydrocarbons (propene and propane) over Pt/Al2O3 catalysts. In the case of CO and propene oxidation, the improved activity is due to a reactant adsorption induced change in the adsorbate coverage, i.e. reduced self-poisoning by CO and propene, respectively, favouring the reaction rate. For propane oxidation, however, the enhanced activity is instead connected with an optimum reaction rate for gas compositions close to the stoichiometric value, which can be approached by periodic operation. Moreover, this thesis provides new insight into the detailed bistable kinetics of CO oxidation over Pt/Al2O3 catalysts in oxygen excess at atmospheric pressure. It is shown that a platinum oxidation and reduction mechanism must complement the conventional three-step Langmuir-Hinshelwood reaction scheme in order to explain the extinction process and the observed reaction rate in the high active state. In this state the reaction rate was found to be several orders of magnitude lower than the CO impingement rate, which is at variance with the conventional mechanism. However, with the new mean-field kinetics model presented here, including CO interaction with a fully developed surface-oxide overlayer, such low reaction rates may be predicted.