Infrared Study of Surface Carbonates on Ceria-Niobia Catalyst Supports for Hydrogen Purification

S. HERNANDEZ, S. COLLINS, A. BONIVARDI

Sao Pedro, Brasil

Simposio; Fifth San Luis Symposium on Surfaces, Interfaces and Catalysis; 2010

Pan-American Advanced Studies Institute

Proton exchanged membrane fuel cells (PEMFCs) are currently being studied for energy production in mobile applications. The hydrogen produced by steam reforming of hydrocarbons or alcohols, and needed to feed those PEMFCs, must have less than 20 ppm CO  in order to avoid the poisoning of Pt electrodes. To achieve this low concentration of CO, the low temperature water gas shift reaction (LTWGS) and the preferential CO oxidation (PROX) appears as the most suitable processes, and gold-based catalysts have shown suitable performances in both reactions. For example, gold supported on ceria mixed oxides are among the materials with the highest activity to LTWGS. However, problems like deactivation by carbonate or hydroxy-carbonate should be addressed to improve the stability of these ceria-based catalysts and, at the same time, to preserve the redox properties of the support. The addition of an acidic cation to ceria, such as niobium(V), might decrease and/or weaken the carbonate formation on the oxide´s surface. In this work, ceria-niobia oxides CeO2/Nb2O5 molar ratio = 95/5, 90/10, 33/67) together with pure CeO2 and Nb2O5, were synthesizedand characterized in order to explore their surface acid-base properties. XRD showed only the fluorite-type structure of ceria for the whole set of ceria-niobia oxides. Carbonates formation after adsorbing CO2 was investigated by means of in-situ infrared spectroscopy. Mono-, bi- and poly-dentated carbonates (m-, b- and p-CO3=, respectively), and bicarbonates (HCO3-) species were identified through their internal vibrational modes. The presence of small amounts of Nb strongly decreased the amount of (bi)carbonate surface species, which is further reduced at higher concentrations of niobium. Particularly, the stabilities of HCO3-and m-CO3= on the ceria surface, are modified by niobium addition.