Water Chemistry on Model Ceria and Pt/Ceria Catalysts
Water Chemistry on Model Ceria and Pt/Ceria Catalysts
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DOI：
10.1021/jp302229x
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发表时间：
2012-06-07
影响因子：
3.7
通讯作者：
Libuda, Joerg
中科院分区：
文献类型：
--
作者：
Lykhach, Yaroslava;Johanek, Viktor;Libuda, Joerg
利用同步辐射光电子能谱（SRPES）、ce4d边缘共振光发射能谱（RPES）、红外反射吸收能谱（IRAS）和密度泛函（DF）计算，研究了水与化学计量CeO2(111)/Cu（111）、部分还原CeO2-x/Cu（111）和Pt/CeO2/Cu（111）模型催化剂的相互作用。在160 K的温度下，水在CeO2（111）薄膜上吸附形成的主要物质是化学吸附的分子水。在CeO2-x表面，水部分解离生成羟基。通过使用核心级PES，化学吸附水和羟基之间的区分由于相应光谱特征的重叠而变得复杂。然而，我们确定了氧化铈上OH基团的三个特征指标：(i)在价带（VB） PES中存在1 π和3 sigma态；（ii）晶格氧与OH/H2O的O 1s光谱组分之间的结合能（BE）分离增大；（iii） RPES中Ce3+共振幅度增大。化学吸附的水在400k以下解吸，羟基在500k时消失。用DF计算研究了化学吸附水和羟基的最有利构型。CeO2（111）和CeO2-x都涉及到强烈倾斜的H2O和OH，这使得它们的红外光谱检测变得复杂。在Pt/CeO2上，水在160 K时被分子吸附，但在退火过程中发生部分解离。在180 ~ 250k之间，水的解离伴随着氢向二氧化铈的溢出和羟基的形成。在250 K以上，羟基发生分解，氢从铈向铂反向溢出，随后发生分子水的解吸。
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.