Preparation Of Hierarchical Cobalt-based Catalyst And Its Mechanism Of VOCs Purification

Nowadays,more attention has been paid to the curb of volatile organiccompounds（VOCs）,which mainly result in the atmospheric pollution,as environmental problems getting austerer.VOCs as a major group of significant air pollutants are pointed to two dominating reasons for the severe damages to environmental quality and public health.on the one hand,there is a part of VOCs themselves owning toxic nature,directly endanger human body health,on the other hand,some VOCs can react with SO₂ and NO_x emissions to the atmosphere under certain conditions and produce nitrate,sulfate and so on,as well as ozone which causes the ozone pollution in ground layer.In order to alleviate this situation,a large number of researchers are committed to the elimination of VOCs and put forward many methods,such as adsorption,condensation,thermal combustion,biotechnology,photocatalysis and catalytic oxidation,among which catalytic oxidation is generally believed to be one of the most efficient and cost-effective pathways for VOCs elimination owing to its high efficiency,low-operating temperature and no secondary pollutants.However,the key technical bottleneck of catalytic oxidation for VOCs elimination is to develop highly active catalysts.Compared with precious metal catalysts,transition metals have attracted more attention because of their low price,rich resources and not easily inactivated.The material of Co₃O₄ has been proved to be one of the most effective non-noble metal oxides to eliminate VOCs.Many scholars have put forward a lot of methods and ideas to further improve the catalytic oxidation performance of VOCs by Co₃O₄.In this paper,inspired by the cation-substituting effect on spinel Co₃O₄,MCo₂O₄（M=Co,Ni,Cu）hollow mesoporous spheres（HMS）and a series of mesoporous hollow nanospherical Mn_xCo_3-xO₄ solid solution catalysts were prepared,and their activity and physicochemical properties in the catalytic oxidation reaction of acetone were studied.The main contents are as follows:1.A series of spinel MCo₂O₄（M=Co,Ni,Cu）hollow mesoporous spheres （HMS）have been synthesized by solvothermal alcoholysis with goals to elaborate the cation-substituting effect on spinel Co₃O₄ for total oxidation of VOCs.The physicochemical properties of the catalysts were characterized by XRD、Raman spectra、FESEM、HRTEM、BET、XPS、H₂-TPR and O₂-TPD,and the active performances of the catalysts were evaluated by temperature-programmed oxidation reaction on the continuously flow fixed-bed reactor.The characterization results show that Co HMS,Ni HMS and Cu HMS are successfully prepared.In the activity test,the order of activity of three catalysts were:Cu HMS>Ni HMS>Co HMS.The outstanding performance can be attributed to the more oxygen vacancies and trivalent cobalt ions exposed on the surface of Cu HMS.The surface oxygen vacancies benefits the adsorption and activation of oxygen to produce O₂^-、O^-,which were considered to have strong oxidizing properties.The conducted kinetics study results excluded the influence of specific surface area on the catalytic activity.This excellent long-term stability and good water tolerance of Cu HMS hint its potential application value.2.The anticipative hollow mesoporous spheres Mn_xCo_3-xO₄ solid solutions with tunable Mn/Co molar ratios in spinel-type cubic crystal phase,successfully prepared by a facile solvothermal alcoholysis approach.The physicochemical characteristic of the catalysts were obtained by XRD、Raman spectra、EPR、FESEM、HRTEM、BET、XPS、H₂-TPR and O₂-TPD,and the active performances of the catalysts were evaluated by temperature-programmed oxidation reaction on the continuously flow fixed-bed reactor.Herein,Mn_1.20Co_1.80O₄ was undeniably considered as the optimal catalyst for acetone oxidation and without any carbon containing by-products other than CO₂ were detected.The excellent performance of Mn_1.20Co_1.80O₄ account for the strong synergistic effects between Mn and Co.And the effects embody on the following points:increasing the surface oxygen vacancies and the content of reactive oxygen species,enhancing the ability of the catalyst to adsorb and activate O₂ in the reaction atmosphere,increasing the redox capacity of the catalyst and the exposure of more trivalent cobalt ions.This indicates that the structure and properties of the surface of Mn_1.20Co_1.80O₄ are altered with the introduction of Mn.Finally,the Mn_1.20Co_1.80O₄ was found to have good stability and excellent water tolerance.