Theoretical Study On The Reaction Mechanism Of Small Molecule VOCs Catalytic Oxidation Over Cobalt-Based Catalysts

Air pollution is one of the largest environmental pollution problems in China.Among them,volatile organic compounds?VOCs?are numerous and have a wide range of hazards.The development of active catalysts is an effective way to eliminate VOC gases.The study of mechanism and nature of the catalytic reaction from the molecular scale has an important significance for the design of high activity catalysts.In this thesis,a series of Co₃O₄-based catalyst systems are studied by density functional theory calculation,including Co₃O₄?110?-A surface,Co₃O₄?110?-B surface,Co₃O₄ supported Pd catalyst and single-atom Pt₁/Co₃O₄.The properties of different surface active sites,the effect of surface oxygen vacancies and water,metal-oxide interfacial effects and single-atom doping effects are studied.And the processes of catalytic oxidation of several typical VOC gases are discussed in depth.The main research contents are as follows:?1?The existence form of the active site on Co₃O₄?110?-A and?110?-B surface was studied.The results show that on Co₃O₄?110?-A surface Co²⁺ ions have better catalytic activity than Co³⁺.On Co₃O₄?110?-B surface,Co²⁺ ions are indictive of the oxygen vacancies,which can activate oxygen and facilitate the oxidation of acetylene.In this paper,the reaction mechanism of acetylene catalytic oxidation on Co₃O₄ surface is also studied.The results show that the C₂H₂ molecules on both surfaces are first oxidized to be CHOCHO or CHOCO structure,which can easily break the C-C bond to form CO₂.?2?The catalytic oxidation of HCHO on Co₃O₄?110?-B surface is investigated.On perfect Co₃O₄ surface,HCHO molecules are decomposed by continuous C-H bonds cleavage and CO reacted with lattice oxygen to form CO₂.The role of oxygen is to fill surface oxygen vacancies and restore the surface.The reaction mechanism is Mars-Van Krevelen type,and the energy barrier of C-H cleavage is high.Suface oxygen vacancies can adsorb oxygen in the gas phase and further activate oxygen into active O₂-species,which can promote C-H bond cleavage and facilitate the oxidation of HCHO.?3?A little H₂ O can promote the oxidation of HCHO.The effect of H₂ O molecules is investigated.The results show that H₂ O can be adsorb on Co₃O₄ surface and decomposed into OH and H atoms.OH can adsorb on Co site and H atom can combine with lattice oxygen to form OH.The bonding of OH with Co leads to the decrease of electrons on Co and lattice oxygen,which is favorable to accept the electron of H atom generated by C-H bond cracking.Therefore,the presence of OH facilitates the cleavage of the C-H bond and the oxidation of HCHO.On the other hand,the presence of OH can change the adsorption mode of active oxygen and promotes the transfer of reactive oxygen species,which facilitates the catalytic oxidation of HCHO.?4?By comparing the mechanism of catalytic oxidation of HCHO over Co₃O₄?110?,Pd?111?and Pd/Co₃O₄,the nature of interfacial effect in HCHO oxidation reaction is described.Compared with Co₃O₄,the loading of Pd promotes C-H bonds cleavage and reduces the energy barrier of two C-H bonds cleavage during the reaction of HCHO oxidation.Compared with Pd?111?surface,the interface effect has less effect on C-H bonds and O-O bonds cleavage,but promotes CO oxidation.In the whole reaction process,Pd is the active center of C-H bonds cleavage and reduces the energy barrier of C-H bonds cleavage.More importantly,the interface effect between Pd and Co₃O₄ changes the adsoption mode of CO molecule.CO molecules can be adsorbed on hollow sites of Pd atoms on Pd?111?but the top site of Pd atoms at the interface.And CO molecules on top sites have better activity than that on hollow sites,which can promote oxidation of CO to CO₂.Pd/Co₃O₄ catalytic system has better HCHO catalytic activity than the pure Pd?111?surface and Co₃O₄ surface.?5?The role of single-atom Pt on CH₃OH oxidation over Pt₁/Co₃O₄ catalyst is studied.Pt atom can replace one Co²⁺ ion on Co₃O₄?111?surface to form a 3-coordinate Pt ion.The substitution of Pt is not favorable to the adsorption of O₂ and CH₃OH,but the surface interaction between Pt and Co₃O₄ can reduce the oxygen vacancy formation energy.In the presence of oxygen vacancies,CH₃OH and O₂ can be co-adsorbed on the catalyst surface.The reaction mechanism follows the Langmuir-Hinshelwood type.The complete CH₃OH oxidation reaction path: CH₃OH ? CH₃O + H,CH₃O ? CH₂ O + H,CH₂ O + OH ? CHO + H₂ O,CHO + OH ? CO + H₂ O,CO + O ? CO₂.Finally,we study the formation of new oxygen vacancies on Pt₁/Co₃O₄ to achieve catalyst surface recovery.Compared with the reaction on Co₃O₄?111?surface,the reaction energy barrier of CH₃OH oxidation over Pt₁/Co₃O₄ catalyst is significantly reduced,which indicates that the monoatomic can promote the catalytic oxidation activity of CH₃OH.