Oxidative steam reforming of ethanol over Ir/CeO2 catalysts: A structure sensitivity analysis

A series of Ir/CeO2 catalysts of different oxide and metallic phase dispersions was investigated by XRD, TPR, HRTEM, CO2 TPD/calorimetry, in situ DRIFT spectroscopy, and oxygen isotopic exchange techniques in order to elucidate the specific influence of catalyst morphology and structure on the oxidative steam reforming (OSR) of ethanol. Structure/texture sensitivity is demonstrated on the basis of the OSR mechanism, which involves, in order: (i) ethanol adsorption on the ceria surface, (ii) partial oxidation to acetate and surface migration of the C2 intermediates along the ceria support toward the Ir particles. (iii) cracking and further oxidation into Ir carbonyls and ceria carbonates, and (iv) hydrogen and carbon monoxide desorption from the Ir particles. Structure sensitivity is established by considering two types of sites: the ceria surface sites, which are essentially pairs of oxygen vacancies and basic OH groups, as well as interfacial sites between Ir and ceria phases, involving pairs of Ir coordinately unsaturated sites (CUSs) and the aforementioned Peripheral ceria sites. This structure sensitivity is revealed by TOF calculations based on these two types of sites, which correspond to the two main rate controlling steps of the ethanol OSR mechanism, namely steps (ii) and (iii). The materials considered display activity, selectivity, and resistance to aging (manifested by coke deposition and sintering) that are closely related to their initial structure and texture. Two domains in terms of iridium and ceria particle size are identified, leading either to stable and selective catalysts or to unstable and unselective catalysts. Such an original and quantified structure sensitivity analysis should prove useful for further process development. (C) 2011 Published by Elsevier Inc.