A Study on Multi Doping Effect on Ceria Based Materials for Soot Oxidation Activity

Abstract

Ceria-based metal oxides are widely adopted as catalyst for various applications significantly due to its distinct features of cyclic redox properties, oxygen storage capacity, high metal solubility and thus, better solid solution formation. This work evaluates the impact of multi-dopants on ceria-based metal oxides for soot oxidation catalytic activity. An initial study on single metal oxides confirmed that the intrinsic nature of metal cation dictates the catalytic activity. Such that reducibility property controls the activity for redox metal oxides (CeO2, SnO2, Pr6O11, Mn3O4) and structural properties control for non-redox metal oxides (Gd2O3, La2O3, ZrO2, HfO2 and Sm2O3). Study on binary ceria-based metal oxide (CeO2-HfO2) confirms that the structural parameter (morphology and phase stability) controls the activity over whole composition range. With increase in dopant, phase separation occurs from fluorite phase, which in turn results in a morphological modification that reduces the catalytic activity. Ternary dopant (Mn) addition to the binary metal oxide (CeO2- HfO2 highest activity among binary metal oxide) at optimum composition enhances catalytic activity due to simultaneous reduction of Ce3+ and Mn4+ and improved oxygen vacancy. On further addition of dopant (Li, Ag, Ba and K) to CeO2-HfO2-Mn2O3 results in quaternary metal oxide that shows lowering of band gap, reactive planes and oxygen active sites that reduces active site and results in catalytic deactivation. Ternary doped sample along with binary metal oxides show the maximum activity due to its lower optical band gap, higher F2g content, higher reactive facet planes and higher reducibility ratio. Thus, it is not a single parameter that dictates the catalytic activity of ceria-based metal oxides. Intrinsic descriptors that control the activity are dopant nature, concentration and interference, apart from the extrinsic descriptor, that significantly modifies surface oxygen concentration. The thesis focuses the descriptors that actually controls the catalytic activity for multi doped ceria based metal oxides.