Hydrogen chemisorption on ceria: influence of the oxide surface area and degree of reduction

Abstract

The chemisorption of hydrogen on two ceria samples (CeO₂-BS, 4 m² g^–1; CeO₂-SM, 56 m² g^–1) reduced at temperatures ranging from 623 to 1173 K has been studied by Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption followed by thermal conductivity (TPD-TC) and mass spectrometry (TPD-MS). The concentration of the oxygen vacancies created by the reduction treatments was determined by using a combination of O₂ pulses and temperature-programmed oxidation. According to our TPD-MS study, hydrogen can be desorbed from ceria as both H₂(reversible adsorption) and H₂O (irreversible adsorption), the relative contribution of these two forms depending on the reduction temperature. For samples reduced at 773 K or higher temperatures, H₂ was the only desorption product. From this observation, some earlier TPD-TC and TPR-TC results could be better understood. Upon reduction at 773 K, the amount of H₂ chemisorbed per mole of CeO₂ was ten times larger for CeO₂-SM than for CeO₂-BS. Likewise, the molar chemisorptive capability of CeO₂-SM strongly decreased (45 times) with the reduction temperature. No simple relationship could be observed between the amount of chemisorbed hydrogen and the total concentration of oxygen vacancies in the oxide. In contrast to earlier results on the contribution of a massive bronze-like phase when chemisorbing H₂ at 195–500 K, the results reported here show that the hydrogen chemisorption on reduced ceria is a surface-related process. Furthermore, the highest value for the hydrogen chemisorption we have obtained, 7.1 H atom nm^–2(BET), suggests a pure surface process.