Theoretical Study On The Catalytic Oxidation Of HCHO And CH₃OH Over Ceria-based Catalysts

With the rapid development of industrialization and urbanization,volatile organic compounds（VOCs）have become the main contributors to air pollution.VOCs have the characteristics of volatility,toxicity,teratogenicity,carcinogenicity,photochemical activity,etc.,which not only adversely affect human health,but also form secondary pollutants（such as tropospheric ozone,peroxyacetyl nitrate and secondary organic aerosols,photochemical smog）.At present,air pollution purification has received extensive research and becomes one of the public concerns.Catalytic oxidation technology is one of the most widely used and most efficient VOCs treatment method due to its high economic feasibility,low cost,and no secondary pollution.The choice of catalyst is a key factor in the implementation of catalytic oxidation technology.Based on the density functional theory,understanding the properties of reactive molecules and nanomaterials at the atomic scale and revealing the mechanism of catalytic reactions are essential for the rational design and preparation of high-activity and low-cost catalytic nanomaterials.In this article,density functional theory（DFT）was used to study the oxidation reaction of HCHO and CH₃OH on ceria-based catalysts.The Au-doped CeO₂（111）,（110）and（100）catalysts,Pt₁-CeO₂ catalysts with stabilizing platinum atom on oxygen vacancies and Pt₁-Pt_nco-supported CeO₂（111）catalysts were constructed,to study the surface properties of these catalysts and the catalytic oxidation reaction mechanism of HCHO and CH₃OH on the atomic scale.From a theoretical point of view,to make an in-depth explanation of the interaction between metal and support,the existence state of metal,the effect of crystal face of support,the influence of oxygen vacancy and concentration,the active site of reaction,the structure-activity relationship,etc.Combined with the method of micro-kinetics simulation,the kinetic problems in the reaction process are studied.The main research contents and conclusions are as follows:（1）The reaction mechanism of HCHO oxidation on Au-CeO₂（111）catalyst.On the pefective AuCe_1-xO₂（111）and defective AuCe_1-xO_2-y（111）surfaces,the reaction follows the Mars van Krevelen（MVK）type mechanism in which adsorbed HCHO reacts with surface oxygen atoms.On the surface of the defect AuCe_1-xO_2-2y（111）with two oxygen vacancies,due to the co-adsorption of HCHO and O₂ and the surface reduction by the removal of lattice oxygen,and the reaction follows the co-action of Langmuir-Hinshelwood（L-H）and Mars van Krevelen（MVK）mechanisms.At the same time,the role of Auspecies and the effect of multiple oxygen vacancies on the HCHO oxidation reaction were also studied.The Auatoms doped in AuCe_1-xO₂（111）assist the transfer of H and lower the energy barriers for the two-step C-H bond cleavages.The active oxygen species formed by Audoping contribute to the formation of surface oxygen vacancies,thereby affecting HCHO adsorption and oxidation reactions.Due to the different adsorption configurations of HCHO on the perfective and defective Au-doped CeO₂ surfaces,it is difficult for Auon the defective surface to accept and assist the transfer of H atom.On the AuCe_1-xO_2-2y（111）surface with two oxygen deficiencies,the adsorbed O₂^-species contributes to the breaking of the C-H bond and the reduction of the reaction energy barrier due to the co-adsorption of HCHO and O₂.The Au-doped CeO₂ surface with multiple oxygen vacancies is more conducive to the adsorption and oxidation of HCHO.（2）The reaction mechanisms of HCHO oxidation catalyzed by Au-CeO₂（110）,（100）were studied.Through DFT+U calculation and micro-kinetic simulation,the structure-activity relationship of Au-CeO₂ catalyst was studied.On the perfective AuCe_1-xO₂（110）,（100）surface,HCHO oxidation follows the MVK mechanism.With the spontaneous migration of Auatoms,AuCe_1-xO₂（110）forms a more stable structure,showing enhanced HCHO adsorption and oxidation performance.On the defective AuCe_1-xO_2-y（110）and（100）surfaces,HCHO oxidations undergo an co-action of L-H and MVK mechanisms.The adsorbed O₂ species contributes to the reduction of the energy barrier of the reaction step in HCHO oxidation.Whether stoichiometric surfaces or defective surfaces,（110）is most active for HCHO oxidation with the lowest activation energy for the rate-determining step,followed by（111）,and then（100）.The dopant Auactivates surface oxygen and promotes the formation of oxygen vacancies on each surface.Among them,AuCe_1-xO₂（110）has the easiest ability to form oxygen vacancies.Aualso affects HCHO oxidation.Compared to the pristine ceria surfaces,Aureduces the energy barriers of key reaction steps on AuCe_1-xO₂（111）（110）（100）surfaces.The Au-doped CeO₂（110）catalyst has the characteristics of easy formation of oxygen vacancies,strong HCHO adsorption and low reaction energy barrier,exhibits excellent HCHO oxidation performance.Combined with the micro-kinetic study of the HCHO catalytic oxidation reaction,it was confirmed that the HCHO oxidation reaction is very sensitive to the exposed crystal faces,the stability of Auand the oxygen vacancies.（3）CH₃OH oxidation reaction catalyzed by Pt₁-CeO₂ catalysts with stabilizing platinum atom on ceria oxygen vacancies of different exposed surfaces was studied.The lower oxygen vacancy formation energy on the surface of cerium oxide indicates that it is easy to form oxygen vacancies.On the（111）,（110）and（100）surfaces,the exposed oxygen vacancies can trap Pt atom,making it easily adsorbed on the defect sites.Due to the strong interaction between the metal and the support,the Pt-O-Ceinterface deforms spontaneously to obtain thermodynamic stability.Compared with Pt₁-CeO₂（110）and Pt₁-CeO₂（111）catalysts,Pt₁-CeO₂（100）has the strongest and most stable deformed structure.The deformation of the structure not only causes the changes in charge and valence,but also exposes oxygen vacancies,which significantly improves the ability to adsorb methanol and oxygen.The CH₃OH oxidation reaction mechanism on single-atom Pt₁-CeO₂（100）follows the co-action of L-H and MVK mechanisms.Compared with the surface of CeO₂（100）,the energy barrier is lower.This material shows excellent efficiency and thermal stability in removing methanol oxidation.（4）Studying the methanol oxidation reaction catalyzed by the Pt₁-Pt_n co-supported CeO₂catalyst.Compared with CeO₂（111）catalysts,the formation energies of oxygen vacancies on a series of Pt₁-CeO₂（111）,Pt_n-CeO₂（111）and Pt₁-Pt_n-CeO₂（111）catalysts are all reduced.Comparing the adsorption of CH₃OH and O₂ on different surfaces,Pt₁ sites help the adsorption of CH₃OH,and Pt_n clusters greatly promote the adsorption of O₂.The Pt₁-Pt_n co-supported CeO₂ catalyst has lower oxygen vacancy formation energy and stronger adsorption energy of CH₃OH and O₂.The defective Pt₁-Pt_n-CeO_2-y（111）surface can also promote the adsorption of CH₃OH and O₂.It reveals the methanol oxidation mechanism catalyzed by the defective Pt₁-Pt_n-CeO_2-y（111）and the recovery process of catalyst.The CH₃OH catalytic oxidation reaction follows the co-action of L-H and MVK mechanisms.Compared with the CeO₂（111）surface,the energy barriers of methanol oxidation reaction are lower,and Pt₁-Pt_n co-supported CeO₂（111）catalyst promoted the methanol oxidation reaction.