HYDROGEN CHEMISORPTION ON CERIA - INFLUENCE OF THE OXIDE SURFACE-AREAAND 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 a nd temperature-programmed desorption followed by thermal conductivity (TPD-TC) and mass spectrometry (TPD-MS). The concentration of the oxyg en vacancies created by the reduction treatments was determined by usi ng a combination of O2 pulses and temperature-programmed oxidation. Ac cording to our TPD-MS study, hydrogen can be desorbed from ceria as bo th H-2 (reversible adsorption) and H2O (irreversible adsorption), the relative contribution of these two forms depending on the reduction te mperature. For samples reduced at 773 K or higher temperatures, H-2 wa s the only desorption product. From this observation, some earlier TPD -TC and TPR-TC results could be better understood. Upon reduction at 7 73 K, the amount of H-2 chemisorbed per mole of CeO2 was ten times lar ger for CeO2-SM than for CeO2-BS. Likewise, the molar chemisorptive ca pability of CeO2-SM strongly decreased (45 times) with the reduction t emperature. No simple relationship could be observed between the amoun t of chemisorbed hydrogen and the total concentration of oxygen vacanc ies in the oxide. In contrast to earlier results on the contribution o f 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), su ggests a pure surface process.