Adsorption and reaction of methanol over CeO(X)(100) thin films.

Methanol was adsorbed on oxidized and reduced CeOX(100) thin films to probe the active sites and reaction selectivity of these surfaces compared to those of CeOX(111). Roughly twice as much methoxy was formed on oxidized CeO2(100) compared to that formed on CeO2(111). In addition to more methoxy, hydroxyl is also more stable on CeO2(100). Unlike on CeO2(111), however, methanol on CeO2(100) produced CO, CO2, and H2 in addition to water and formaldehyde. The behavior of CeO2(100) is related to its surface structure, which provides greater access to Ce cations and therefore more active adsorption sites and more highly undercoordinated Ce and O. The undercoordinated O may explain the enhanced dehydrogenation activity leading to CO and H2 formation. The reduction of ceria leads to increased methanol uptake on both CeO2 - X(100) and CeO2 - X(111). However, although the uptake doubled on reduced CeO2 - X(111) compared to the oxidized surface, it increased by only 10% on reduced CeO2 - X(100) compared to that on fully oxidized CeO2(100). Reduction of both surfaces leads to a greater production of CO and H2. Reaction on all surfaces progresses rapidly from methoxy to products. There is no spectroscopic evidence of formyl or formate intermediates. On CeOX(100), carbonate is detected that decomposes into CO2 at high temperature.