HYDROGEN CHEMISORPTION ON CERIA - INFLUENCE OF THE OXIDE SURFACE-AREA AND DEGREE OF REDUCTION

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 H-2 (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, H-2 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 H-2 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 H-2 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.