Water Dissociation on CeO₂(100) and CeO₂(111) Thin Films

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Abstract

This study reports and compares the adsorption and dissociation of water on oxidized and reduced CeO2(100) and CeO2(111) thin films. Water adsorbs dissociatively on both surfaces. On fully oxidized CeO2(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 CeO2(111), recombining and desorbing between 200 and 300 K. Water produces 30% more hydroxyls on reduced CeO1.7(100) than on oxidized CeO2(100). The hydroxyl concentration increases by 160% on reduced CeO1.7(111) compared to oxidized CeO2(111). On reduced CeO1.7(100) most of the hydroxyls still recombine and desorb as water between 200 and 750 K. Most of the hydroxyls on reduced CeO1.7(111) react to produce H2 at 560 K, leaving O on the surface. A relatively small amount of H2 is produced from reduced CeO1.7(100) between 450 and 730 K. The differences in the adsorption and reaction of water on CeOX(100) and CeOX(111) are attributed to different adsorption sites on the two surfaces. The adsorption site on CeO2(100) is a bridging site between two Ce cations. This adsorption site does not change when the ceria is reduced. The adsorption site on CeO2(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 CeOX(111) is reduced, vacancy sites are produced which allows the water to adsorb and dissociate on the 3-fold Ce cation sites.

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