CuO/CeO2 catalysts for the preferential oxidation of CO in H2 rich mixture for fuel cell applications
Caputo, Tiziana (2006) CuO/CeO2 catalysts for the preferential oxidation of CO in H2 rich mixture for fuel cell applications. [Tesi di dottorato] (Inedito)
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This thesis is focused on CuO/CeO2 catalysts for the preferential oxidation of CO in H2 rich mixtures (CO PROX). After a brief introduction of the general problem (energy production with novel processes or raw materials, fuel cells) and a more detailed description of the CO PROX unit in a fuel processor in order to deliver to fuel cells a CO-free H2-containing stream. An overview of most representative and interesting catalytic systems proposed or utilised for the process has been also presented, before introducing the properties and the motivation of the choice of investigating catalysts based on copper oxide supported on ceria (or mixed ceria-zirconia). Results of catalytic tests carried out under experimental conditions in the range of interest for applications show that an increase of redox properties of catalyst obtained by supporting CuO on Ce-Zr mixed oxide is not directly linked to higher activity, the optimal catalyst formulation found corresponds to 4wt% CuO/CeO2, whose performances appear higher than commercially available Pt based catalysts. The catalyst is very selective (100% up to about 100°C, about 60-80% at higher temperatures), the selectivity is a unique function of temperature, slightly depending on the CO/H2 ratio, on O2 partial pressure and on the presence of CO2 and H2O in the gas. The kinetics has been modelled by considering reactions of both CO and H2 oxidation. The experimental data have been fitted by rate expressions based on LH (or Eley-Rydeal) reaction mechanism decribing the interaction between surface oxygen species and adsorbed CO or gas phase CO. For both reactions of CO and H2 oxidation a weak dependence on O2 partial pressure has been found, while the competitive adsorption with reactants of CO2 and H2O on the active sites limits th CO conversuion in the range of lower temperatures. The analysis of the redox properties evidenced that CO is a stronger reductant than H2 for catalyst sites. Indeed CO strongly interacts with completely oxidized sites already at room temperature, while hydrogen adsorbs exclusively on a pre-reduced sample. The re-oxidation rate of the active sites is faster than the hydrogen oxidation up to a temperature of 100°C, thus in this region the hydrogen oxidation does not take place and 100% selectivity is found for CO PROX process. The catalytic sites active in this temperature range (from room temperature up to 100°C) have been identified with TPR/TPD/TPO experiments with highly dispersed copper particles strongly interacting with ceria. Indeed it is strongly suggested that ceria also participates to redox processes already at very low temperatures and the sites at the borderline between ceria and copper have been identified as the main responsible for the activity and high selectivity in the process. At a temperature higher than 150°C CuO clusters are more active in the hydrogen oxidation, thus decreasing the overall efficiency of the CO PROX process.
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