Discussing the use of modified ceria as support for Pt catalysts on water-gas shift reaction

CeO2 was doped with 25 wt.% of MeOx (Me = Zr, Sm, or La) and Ce75Me25 mixed oxides were obtained. Pt (1 wt.%) was then deposited on these oxides and CeO2. The resulting catalysts were tested for the water-gas shift (WGS) reaction in a mixture whose composition was 5.49% CO, 4.10% CO2, 9.71% H-2, 30.75% H2O. Subsequently, CeO2 was loaded with various levels of ZrO2 (y wt.%), which gave another set of Ce100-yZry (y = 25, 40, 50, and 75) supports and the respective catalysts were also tested for the WGS. While XRD analysis showed that all the mixed oxides had retained the cubic structure of CeO2, Raman analysis suggested that the doped ceria supports contain more oxygen vacancies resulting in higher oxygen mobility than CeO2, which were corroborated by the TPR analyses of all modified catalysts. However, only catalysts of the Pt/CeZr family produced larger CO conversions than Pt/CeO2 (except for Pt/Ce25Zr75) indicating that reducibility is not a crucial factor to improve WGS activity. The chemical composition, on the other hand, revealed to be quite decisive. Long-term experiments indicated that deactivation occurs independently of the lanthanide introduced into the ceria lattice and therefore oxygen mobility/reducibility of the catalysts does not play any role in improving the catalyst stability either. Catalyst reactivation was also investigated by applying different thermal treatments aiming at removing superficial deposits of carbonate complexes. The results led to the conclusion that the carbonate species play only a minor role in catalysts deactivation, suggesting that another factor, acting simultaneously as soon as reactant flux is started, determines the catalytic stability of such systems based on mixed oxides of ceria. (C) 2009 Elsevier B.V. All rights reserved.