Control of spillover processes in catalysis-porous and patterned electrodes

Abstract

Electrochemical promotion of catalysis (EPOC) is a result of backspillover phenomena which involves the migration of backspillover (oxygen ionic) species through the tpb onto the catalyst surface under the influence of electrical potential application. The present work studies the occurrence and the role of the backspillover species and also the mechanisms of the related processes including factors controlling this phenomenon in a Pt/YSZ system which utilises a model catalyst or sputtered patterned electrodes. In comparison to the electrodes prepared by painting the commercially available Pt catalysts on YSZ, the geometric characteristics and the type and amount of impurities on the Pt surface should not vary significantly among the samples except the tpb length (of the 3rd batch samples). Therefore, the electrochemical processes at the tpb can be studied systematically and the electrochemical data should be reproducible which are not the case when electrodes prepared by using the commercially available catalysts are involved. The variability in the electrochemical behaviour (charge transfer reaction) of the Pt/YSZ system, as depicted by CV results under a nonreactive condition at 400 oC, of the Pt electrodes prepared by using the commercially available catalysts is demonstrated in the first part of the study. The three commercially available catalysts provided by ESL, Heraeus and Metalor are shown to contain different type and amount of impurities and to have different Pt surface morphology. The impurities were analysed by ICP-OES while the surface morphology was investigated by SEM. In the second part of the study, the focus was on the Pt/YSZ system utilising the patterned electrodes where the correlations between the oxygen charge transfer (oxygen activity) and the tpb length were investigated by CV, EIS and current overpotential experiments under a nonreactive condition at temperatures between 300 oC and 450 oC .The term oxygen activity in this thesis refers to the kinetics of the oxygen charge transfer reaction. A proportional relationship between the oxygen charge transfer and the tpb length was not observed and this is explained by the changes in Pt surface morphology throughout the experimental works and most important the formation of surface oxides at the tpb which blocks the charge transfer processes. However, the changes in the oxygen activity at the tpb can be observed from the correlation between the current density and the peak current from the CV results. Another interesting observation is the appearance of additional peak in CV after exposure to hydrogen which can be associated with either the decomposition of impurity oxides or ii Pt oxides of different oxidation state at the binary interface. In the third part of the study, the catalytic and electrochemical behaviour of the system utilising the patterned electrodes was investigated during CO oxidation at temperatures between 300 oC and 450 oC. The Pt/YSZ system was found to exhibit (i) EPOC at 0.10 V, 0.20 V and 1.0 V, (ii) permanent EPOC and (iii) permanent poisoning effect. The backspillover species was found not only to enhance the catalytic rate but also to stop the open circuit rate oscillations and, at a high pO2, to restore the rate due to the permanent poisoning effect. Furthermore, five types of oxygen species are suggested to present at the tpb and on the Pt surface during the experimental works where the coverage of these species controls the amount of oxygen ionic species which can migrate through the tpb onto the Pt surface.