Abstrakt
We have studied the interaction of water with stoichiometric CeO2(111)/Cu(111), partially reduced CeO2-x/Cu(111), and Pt/CeO2/Cu(111) model catalysts by means of synchrotron-radiation photoelectron spectroscopy (SRPES), resonant photoemission spectroscopy (RPES) at the Ce 4d edge, infrared reflection absorption spectroscopy (IRAS), and density functional (DF) calculations. The principal species formed during adsorption of water at 160 K on CeO2(111) films is chemisorbed molecular water.
On the surface of CeO2-x water partially dissociates yielding hydroxyl groups. By use of core-level PES, differentiation between chemisorbed water and hydroxyl groups is complicated by the overlap of the corresponding spectral features.
Nevertheless, we determined three characteristic indicators for OH groups on ceria: (i) the presence of 1 pi and 3 sigma states in valence band (VB) PES; (ii) an increase of the binding energy (BE) separation between the O 1s spectral components of lattice oxygen and OH/H2O; (iii) an increase of the amplitude of the Ce3+ resonance in RPES. Chemisorbed water desorbs below 400 K and hydroxyl groups vanish at 500 K.
The most favorable configurations of chemisorbed water and hydroxyl groups have been investigated by DF calculations. Both CeO2(111) and CeO2-x involve strongly tilted H2O and OH species which complicate their detection by IRAS.
On Pt/CeO2, water adsorbs molecularly at 160 K but undergoes partial dissociation during annealing. The dissociation of water is accompanied by spillover of hydrogen to ceria and formation of hydroxyl groups between 180 and 250 K.
Above 250 K, decomposition of hydroxyl groups and reverse spillover of hydrogen from ceria to Pt occurs, followed by desorption of molecular water.