A catalytic and solid state study of lanthanum doped ceria
In this study of the catalytic oxidation of CO and CH4 over lanthanum doped ceria, it is demonstrated that dopant concentration is the dominant factor in the variation in catalytic properties. This composition dependency of catalytic properties is correlated with the variations in solid state and adsorption properties. The aim of the thesis is to determine precisely which of the dopant concentration dependent material properties are the major influencing factors in catalytic property variations. As heterogeneous catalysis is a surface phenomenon, full characterisation of the surface of the materials is necessary in a fundamental study. Surface composition is shown to vary significantly from that of the bulk, due to a surface segregation phenomenon which is studied in detail. A known model based on electrostatic interactions and atomistic simulations is successfully used to rationalise surface segregation. Catalytic properties and other surface characterisation data are subsequently interpreted with respect to surface composition. Catalyst surface composition studies are also used to estimate the variation in surface free oxide anion vacancy concentration with composition, based on the assumption that surface conductivity varies with composition similarly as does bulk conductivity. Surface conductivity is thus derived from literature bulk conductivity data. In the case of the catalytic oxidation of CO the addition of lanthanum dopant retards catalytic activity under the standard reaction conditions. The catalytic reaction rate equation activation energy and specific pre-exponential factor are demonstrated to vary similarly, peaking strongly at the surface composition coincident with maximum free oxide anion vacancy concentration. The catalytic properties are therefore proposed to be dependent on the free oxide anion vacancy population in the surface. In the case of the catalytic oxidation of CH4 the addition of lanthanum dopant is shown to promote activity under the standard reaction conditions. A synergistic promotion effect is demonstrated for surface base and redox functionalities which is shown to be maximised at the equicationic surface composition. The base function is shown to be enhanced with lanthanum surface concentration and is proposed to result in enhanced methane activation activity by acidlbase site deprotonation. The CeIv <-> CeIII redox function is attributed to surface cerium, resulting in enhanced oxidation of the methyl fragment following deprotonation on an adjacent acid/base site.