甲醛在CeO_2(111)表面吸附的密度泛函理论研究

A Density Functional Theory Study of Formaldehyde Adsorption on CeO_2(111) Surface

采用基于第一性原理的密度泛函理论和周期平板模型,研究了甲醛在以桥氧为端面的CeO2(111)稳定表面上的吸附行为.通过对不同覆盖度,不同吸附位的甲醛吸附构型、吸附能及电子态密度的分析发现,甲醛在CeO2(111)表面存在化学吸附与物理吸附两种情况.化学吸附结构中甲醛的碳、氧原子分别与表面的氧、铈原子发生相互作用,形成CH2O2物种;吸附能随着覆盖度的增加而减小.与自由甲醛分子相比,物理吸附的甲醛构型变化不大,其吸附能较小.利用CNEB(climbing nudged elastic band)方法计算了甲醛在CeO2(111)表面的初步解离反应活化能(约1.71eV),远高于甲醛脱附能垒,这与甲醛在清洁CeO2(111)表面程序升温脱附实验中产物主要为甲醛的结果相一致.

Formaldehyde adsorption on CeO2（lll） surface terminated by bridge O atom was systematically investigated by periodic density functional theory （DFF） with the generalized gradient approximation （GGA） developed by projector augmented wave （PAW）. According to the analysis of the optimized structures of adsorbed formaldehyde, adsorption energies at different coverages and different adsorption sites, it is found that there are two types of adsorbed formaldehyde on CeO2（l 1 l） surface. For the chemisorbed formaldehyde, the carbon and oxygen atoms of formaldehyde interact with the corresponding oxygen and cerium atoms of CeO2（lll） surface, and form chemical bonds. The adsorption energies decrease with the increase of formaldehyde coverage on CeOa（111） surface. However, the optimized structures of the physically adsorbed formaldehyde are almost unchanged compared with the free formaldehyde molecule. The corresponding adsorption energies are generally lower than -27 kJ -mol-~. It is learned from the density of states of the chemically adsorbed formaldehyde that the energy of the highest occupied molecular orbital （HOMO）, no, shifts downward greatly, while the oxygen electrons of CeO2（111） surface transfer to the lowest unoccupied molecular orbital （LUMO） of formaldehyde, ~~o. Therefore, new Ce---O2 bonds form and the corresponding C--O1 bonds of formaldehyde are elongated. Based on the investigation above, the potential energy for the first H atom dissociation reaction of formaldehyde was calculated using climbing nudged elastic band （CNEB） to be ca 1.71 eV, which was much higher than that of the barrier for the desorption of formaldehyde, 0.80 eV. It is indicated that the main reaction for the temprature programmed desorption （TPD） of formaldehyde over clean CeO2（111） surface is the desorption of formaldehyde instead of its dissociation reaction, which is well consistent with experimental data.