A revisitation of transient CO2 methanation over Rh/CeO2 and Rh/CeO2-ZrO2 based catalysts

The aim of this thesis was the study of CO2 activation over Rh/CeO2 and Rh/CeO2-ZrO2 catalysts. Rh/CeO2 catalysts are particularly interesting since a high activity peak during transient state have been obtained at conditions such that other Me/Redox catalysts do not (Me: noble metal, Redox: Redox support). Because of that, this work had two main specific objectives: The first was to study the effect of pretreatment conditions on the behavior of catalysts with higher reduction degrees than polycrystalline ceria, such as nanoshaped ceria and ceria-zirconia based catalysts. The second was to determinate if the high activity during transient state is related to the reducibility degree of ceria-based catalysts, i.e. the role of oxygen vacancies on CO2 activation. To reach the objectives, rhodium over several supports (namely ceria nanocubes, ceria nanorods, polycrystalline ceria, ceria-zirconia mixes and zirconia) were prepared, characterized and tested. The results indicated that samples with higher reduction degrees were not only more active during transient state, but also this effect was longer on the time, i.e. it tooks more time to reach the steady state. These findings agree with the proposal of the oxygen vacancies performing a key role during CO2 activation