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

Formaldehyde adsorption on CeO2(111) surface terminated by bridge O atom was systematically investigated by periodic density functional theory (DFT) 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(111) surface. For the chemisorbed formaldehyde, the carbon and oxygen atoms of formaldehyde interact with the corresponding oxygen and cerium atoms of CeO2(111) surface, and form chemical bonds. The adsorption energies decrease with the increase of formaldehyde coverage on CeO2(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-1. 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, π*CO. 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.