Adsorption And Oxidation Of Formaldehyde On Ceria Catalyst: A Density Functional Theory Study

Formaldehyde is one of the most concerned pollutants among volatile organic compounds(VOCs) since it causes serious health problem.Therefore,the technology for formaldehyde elimination at low temperature is an urgent work.Recently,catalytic oxidation using metal/oxide catalysts has been widely applied to formaldehyde elimination. Due to its oxygen storage capacity(OSC),ceria(CeO₂) has been considered as one of the most important components in many catalytic systems.This research aims to elucidate the possible reaction mechanism of formaldehyde oxidation on ceria-based catalysts using density functional theory(DFT).First,six different periodic slab models with different terminations of CeO₂(111), (110) and(100) surface were systematically optimized.It shows that the CeO₂(111) with bridged oxygen termination is the most stable model for its lowest surface energy and highest band gap,followed by CeO₂(110) surface.O₂ adsorption on the clean CeO₂(111) and(110) surface is weak physisorption,while strong chemical adsorption occurs when the oxygen vacancy formed on the ceria surface with high adsorption energies,and the adsorbed O-O bond is activated.There are chemisorption and physisorption of formaldehyde on clean CeO₂(111) and (110) surface.For the strongly chemisorbed formaldehyde,the carbon and oxygen atoms of formaldehyde interact with the corresponding oxygen and cerium atoms of ceria surface with the formation of dioxymethylene intermediate.The adsorption energies decrease with increasing coverage of formaldehyde,and the interactions of the adsorbed formaldehydes could be negligible until the coverage is up to 0.25 monolayer(ML).The density of states (DOS) analysis indicates that the energy of the highest occupied molecular orbital (HOMO),no,shifts downward greatly,while the oxygen electrons of CeO₂ surface transfers to the lowest unoccupied molecular orbital(LUMO) of formaldehyde,Ï€^* co. Meanwhile,the new C-O_s and Ce-O_f bonds form and the corresponding C-O_f bonds of formaldehyde are elongated.However,the optimized structures of the physisorbed formaldehyde remain the planar structure as the free formaldehyde molecule with low adsorption energies,and the electron structures are slightly changed.To obtain a systematic understanding the reaction mechanism of formaldehyde oxidation,the stable configurations of the hydrogen dissociation as well as CO and CO₂ desorption are optimized.The energy barriers and corresponding transition states are calculated using climbing nudged elastic band(CNEB).On the clean CeO₂(111) surface, the hydrogen dissociation barriers of formaldehyde(1.71 eV) is much higher than that of formaldehyde desorption(0.81 eV).It is indicated that the hydrogen dissociation is very difficult in the absence of oxygen atmosphere,which is consistent with the temperature programmed desorption(TPD) experiment.It is unstable for the coadsorption of O₂ formaldehyde on clean(111) surface thermodynamically.Similar to the clean(111) surface, chemisorbed and physisorbed formaldehyde structures are also obtained on the oxygen vacancy surface and O₂/O_v species,accompanied by slight changes of the adsorption energies.The energy barrier of hydrogen dissociation is obviously decreased on the oxygen vacancy surface,1.30 eV.Furthermore,the O₂/O_v species greatly reduced the energy barriers for the first hydrogen dissociation(0.30 eV).All the above results indicated that formaldehyde could be oxidized on redox ceria surface at high temperature,and the oxygen vacancy on the surface played an important role in the formaldehyde oxidation.Then,the structures and energies of CO on CeO₂(111) and(110) surface are investigated.For the clean(111) surface,physisorption is found as weak interaction with the surface and the CO molecule.For the clean(110) surfaces,physisorption and strong chemisorption are found,in which the CO molecule bridged two oxygen atoms and pull these atoms out of their lattice sites with formation of a(CO₃) species.CO might strongly adsorb on the O₂/O_v surface to form carbonate intermediate,or directly lead to CO₂ without energy barrier,which is in good agreement with experimental infrared data.The carbonate intermediate might desorb as CO₂ with low energy barrier(0.28 eV of(111) surface).It is implied that the possible effects of ceria supporter on catalytic oxidation reaction are that O₂ adsorbs on the oxygen vacancy surface to form the reactive oxygen species,which in turn takes part into the CO oxidation reaction.At last,the adsorption behaviors of noble metal atom(Pt,Pd,Rh and Au) on CeO₂(111) surface are calculated.The corresponding M-O bonds formed since the strong interaction between the noble metal atom and the CeO₂(111) surface,indicating that the M-CeO₂ interface are energetically stable.A large support relaxation effect is observed on the M-CeO₂ interface and there may exist some active sites at the M-CeO₂ interfaces, suggesting that the noble metal atom may activate oxygen species and enhance the reactivity such as CO and HCHO oxidation.