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This study reports and compares the adsorption and dissociation of water on oxidized and reduced CeO 2 (100) and CeO 2 (111) thin films. Water adsorbs dissociatively on both surfaces. On fully oxidized CeO 2 (100) the resulting surface hydroxyls are relatively stable and recombine and desorb as water over a range from 200 to 600 K. The hydroxyls are much less stable on oxidized CeO 2 (111), recombining and desorbing between 200 and 300 K. Water produces 30% more hydroxyls on reduced CeO 1.7 (100) than on oxidized CeO 2 (100). The hydroxyl concentration increases by 160% on reduced CeO 1.7 (111) compared to oxidized CeO 2 (111). On reduced CeO 1.7 (100) most of the hydroxyls still recombine and desorb as water between 200 and 750 K. Most of the hydroxyls on reduced CeO 1.7 (111) react to produce H 2 at 560 K, leaving O on the surface. A relatively small amount of H 2 is produced from reduced CeO 1.7 (100) between 450 and 730 K. The differences in the adsorption and reaction of water on CeO X (100) and CeO X (111) are attributed to different adsorption sites on the two surfaces. The adsorption site on CeO 2 (100) is a bridging site between two Ce cations. This adsorption site does not change when the ceria is reduced. The adsorption site on CeO 2 (111) is atop a single Ce cation, and the proton is transferred to a surface O in a site between three Ce cations. When the CeO X (111) is reduced, vacancy sites are produced which allows the water to adsorb and dissociate on the 3-fold Ce cation sites.
Mullins et al. (Thu,) studied this question.