| Volatile organic compounds?VOCs?can be not only hazardous to human health but also harmful to the environment,which are considered one of the main atmospheric pollutants.The greatest challenge of this technology is the development of catalysts with high performance.In this paper,cobalt-based catalysts with high performance were explored for catalytic oxidation of toluene as probe reactant.The relationship between physicochemical properties and catalytic activity of cobalt-based catalysts was studied.And the reaction mechanism of toluene catalytic oxidation over the surface of cobalt-based catalyst was investigated.The main conclusions are as follows:1).A template-free hydrothermal method was applied to successfully synthesize three kinds of bulk cobalt oxides with different morphologies including 1D-Co3O4 nanoneedle,2D-Co3O4 nanoplate,and 3D-Co3O4 nanoflower.The 3D-Co3O4 nanoflower performed the excellent activity and the temperature required for achieving a toluene conversion of 50%and 90%(T50%and T90%)at approximately 229°C and 238°C,respectively.It has been found that low temperature reducibility,highly defective structure with abundant surface active oxygen species and rich Co3+cationic species were responsible for the excellent catalytic performance of 3D-Co3O4 nanoflower.Meanwhile,it shows good long-term catalytic stability and reusability.The results of in situ DRIFT show that toluene is more easily oxidized on the surface of 3D-Co3O4 nanoflower catalyst than that on the surface of1D-Co3O4 nanoneedle catalyst,which is consistent with the tendency of their catalytic activity.Moreover,the reaction pathways of toluene oxidation over the surface of 3D-Co3O4nanoflower catalyst could be as follows:toluene?benzyl alcohol?benzaldehyde?benzoate,and finally completely oxidized into CO2 and H2O.2).The 3D hierarchical Co3O4 nanocatalysts with different morphologies exposed with various crystal planes have been synthesized via a hydrothermal process and subsequent direct thermal decomposition,including 3D hierarchical cubes-stacked Co3O4 microsphere?C?,3D hierarchical plates-stacked Co3O4 flower?P?,3D hierarchical needles-stacked Co3O4 two-spheres with an urchin-like structure?N?,and 3D hierarchical sheets-stacked fan-shaped Co3O4?S?,which are primarily exposed with?111?,?110?,?110?,and?112?crystal planes,respectively.C sample performed the better activity and the temperature required for achieving a toluene conversion of 50%and 90%(T50%and T90%)have been obtained at approximately 240°C and 248°C,respectively.It is concluded that large specific surface area,highly defective structure with more surface adsorbed oxygen species and abundant high valence Co ions were responsible for the excellent catalytic performance of C sample.Moreover,the specific toluene reaction rate for toluene oxidation of these crystal planes follows?110?>?111?>?112?.C sample shows good long-term catalytic stability and reusability.3).The Mn O2-x@Co3O4,ZIF-67-derived porous Co3O4 in situ grown on 1D Mn O2nanomaterials were successfully fabricated.The resultant Mn O2-x@Co3O4,especially for Mn O2-3@Co3O4,shows an excellent catalytic activity with the temperature required for achieving a toluene conversion of 90%(T90%)of approximately 229?,which is lower than the pure?-Mn O2 nanowire and Co3O4-b due to increasing surface oxygen species,and the constructed heterogeneous interface between 1D Mn O2 and porous Co3O4.The catalytic activities of Mn O2-x@Co3O4 were different dependence upon the various 1D Mn O2-x.The results of in situ DRIFT show Mn O2-3@Co3O4 is more capable of activating gaseous oxygen molecules into actived oxygen species than 1D Mn O2-3.Moreover,benzoate species was rapidly transformed from toluene over the surface of Mn O2-3@Co3O4 caralyst,and finally completely oxidized into CO2 and H2O.4).A series of carrier Co3O4 with different morphologies were fabricated,and Pt particles prepared by reduction method were loaded on Co3O4 with different morphologies by the method of electrostatic adsorption.Compared with the Co3O4,Pt/Co3O4 catalyst showed significantly improved catalytic activity of toluene.Pt/Co3O4-p performed the excellent activity and the temperature required for achieving a toluene conversion of 50%and 90%(T50%and T90%)have been obtained at approximately 154 and 167°C,respectively.The strong interaction between noble metal Pt and carrier Co3O4 improves the redox performance of Pt/Co3O4 catalyst,weakens the Co-O bond,and increases the mobility of lattice oxygen and the content of adsorbed oxygen.According to the results of in situ DRIFT,it can be speculated the reaction pathways of Co3O4-p and Pt/Co3O4-p catalysts in toluene/nitrogen atmosphere could be as follows:toluene?benzyl alcohol?benzoate,and only partial complete oxidation to CO2 and H2O.By contrast,the reaction pathways of Pt/Co3O4-p catalysts in toluene/air atmosphere could be as follows:toluene?benzyl alcohol?benzaldehyde?benzoate?maleic anhydride,and finally complete oxidation to CO2 and H2O.The possible mechanism of catalytic oxidation of toluene on the surface of Co3O4-p and Pt/Co3O4-p catalysts is mainly follows the Langmuir-Hinshelwood mechanism at lower reaction temperature,while it is mainly follows Mars-van Krevelen mechanism at higher reaction temperature. |
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