Defect Construction Of Cobalt Oxide Catalyst And Its Application In Catalytic Oxidation

With the rapid development of society and economy,resource depletion and environmental pollution have become major problems confronting human being.Compared to coal and oil energy,natural gas is a kind of comparably clean energy.However,its main component,methane(CH₄)is an important greenhouse gas with warming effect25 times that of CO₂,which contributing greatly to the global warming.Meantime,volatile organic compounds(VOCs)including toluene and formaldehyde on the one hand cause serious ecological environment destruction by destroying ozone layer and forming photochemical smog,on the other hand,the toxic VOCs pose a threat to human health.Therefore,the efficient removal of methane and VOCs by catalytic oxidation is the frontier research direction in the field of environment,which has attracted extensive research attention.Precious metal-based catalysts have demonstrated excellent performance,but their industrial application is greatly limited by their high price,resource scarcity and poor poison tolerance.Therefore,it is of great significance to design and construct highly efficient non noble metal catalysts.In this paper,we put our emphasis on the design of high-performance non-noble metal cobalt oxide catalyst.Taking defect engineering strategy(etching,doping,selective atom removal,etc.)as the breach,we tried to fully stimulate the intrinsic catalytic oxidation activity of Co₃O₄ thus promoting the substitution of noble metal catalysts.In the first part,we used a simple precipitation method to synthesize hexagonal Co₃O₄nano-sheet.Then hydrochloric acid etching method was adapted to engineer point defects on Co₃O₄ surface.This process could deliver higher specific surface area,more active oxygen species and better reducibility.When used as a catalyst for methane catalytic combustion reaction,the temperature corresponding to 50%methane conversion(T₅₀)is detected to be 311 ^oC with a mass hourly space velocity(WHSV)of 33,000 m L g^-1 h^-1,which is 31 ^oC lower than the original Co₃O₄.The conversion efficiency of methane is 4.1mmol g^-1s^-1 at 311 ^oC,which is 5.2 times that of the initial Co₃O₄.In the second part,we treated Co₃O₄ with high index(112)facets with ammonia to produce N-doped Co₃O₄(N-Co₃O₄-200).Compared with Co₃O₄,N-doping could bring a large number of oxygen vacancies,which greatly increases the amount of active oxygen species on its surface.In addition,N-doping can activate the Co-O bond,which improves the mobility of reactive oxygen species.Taking advantage of the above beneficial effect,N-Co₃O₄-200 showed excellent catalytic activity and stability in the catalytic oxidation of toluene.T₅₀ was found to be 208 ^oC at GHSV of 60000 h^-1,which was 32 ^oC lower than the pristine Co₃O₄.The activity was well maintained for 50 h without deactivation.In the last part,we developed a doping and de-doping strategy to construct the vacancy defect structure of Co₃O₄.Firstly,cationic Mg doped Co₃O₄(Mg-Co₃O₄)was synthesized by coprecipitation method.Based on this sample,we produced highly defective Co₃O₄(H-Mg-Co₃O₄-3h)by hydrochloric acid etching to partially remove the doped Mg atoms from the lattice.By systematic characterization such as HR-TEM,XPS etc.,we can detect abundant cation and anion vacancy defects in H-Mg-Co₃O₄-3h,which played significant role for the increase of active surface oxygen species.In the formaldehyde catalytic oxidation reactions the activity followed the sequence of H-Mg-Co₃O₄-3h>Mg-Co₃O₄>Co₃O₄.This indicated that the catalytic performance can be improved by cation doping,and the de-doping process can further tailor the defect structure and enhance its intrinsic activity.