Studies of Catalytic Low-Temperature CO Oxidation over Cobalt Oxide and Related Transition Metal Oxides

Doctoral thesis, 2002

The catalytic oxidation of carbon monoxide at low temperatures is important in many applications. The use of a low temperature active catalyst may lower the emissions during the cold-start of a car, and volatile organic compounds in exhaust gases from stationary sources might be treated without the need of preheating the gas. In this thesis the catalytic oxidation of carbon monoxide (CO) over metal oxides (MnO2, Fe2O3, Co3O4, NiO, CuO, ZnO and CeO2) has been studied. Of these metal oxides cobalt oxide is the most active for CO oxidation at low temperatures and can oxidise CO at room temperature. NiO, CuO and MnO2 are also active for CO oxidation at moderate temperatures. The main focus in this thesis has been on cobalt oxide which was studied with flow reactor experiments, in-situ FTIR, in-situ XRD, in-situ XANES and XPS. NiO, CuO and MnO2 were studied with flow reactor experiments and in-situ FTIR. The activity of all these catalysts when preoxidised decreased slowly with time during CO oxidation at constant temperature. The rate of deactivation could be decreased by increasing the stoichiometric ratio S = 2·[O2]/[CO], increasing the temperature or decreasing the CO or CO2 concentration. Different types of carbonate species were detected at the surfaces of the Co3O4 and NiO catalysts both upon CO or CO2 adsorption and during CO oxidation. Carbonyls and surface carbon were detected on Co3O4. Three main hypotheses for the deactivation of the catalysts during CO oxidation at constant temperature are discussed: Blocking of the active sites by carbonates, carbonyls or hydroxyl groups, a slow reduction of the oxidation state of the active metal ions, and a surface reconstruction leading to deactivation. For cobalt oxide the most probable explanation for the deactivation was a slow gradual surface reconstruction. Microkinetic modelling is an important tool in studying catalytic reactions. Some of the models proposed for the surface reactions involved in the CO oxidation over cobalt oxide were fitted with non-linear regression. The most common practical problems encountered in non-linear regression and different strategies for how these could be solved are discussed and a general strategy regarding non-linear regression is suggested.