Effect Of Morphology Of Cobalt Tetroxide And Heteroatom Doping On Combustion Properties Of Toluene

Volatile organic compounds（VOCs）are one of the major pollutants in atmosphere,which induces the production of ground-level ozone and urban smog,and leads to environmental problems such as global warming,acid rain and air toxicity.At present,there are many ways to remove VOCs,such as thermal combustion,catalytic oxidation,adsorption,condensation,absorption,membrane infiltration and biological treatment.Among the above technologies,catalytic oxidation technology has been widely studied due to low energy consumption,high efficiency and no secondary pollution.The development of highly active and stable catalysts is the most crucial point in this process.Transition metal oxides,due to their low cost and high activity,have become potential alternative catalysts for noble metal.Co₃O₄ is one of the most potential catalysts in many catalytic reactions,and is widely used in the catalytic oxidation of CO,CH₄ and VOCs due to its low cost,environmental friendly and high catalytic activity.Morphology effect plays an important role for catalysts.Co₃O₄ catalysts with different shapes,such as sheet,strip,cube,plate and rod,mainly expose different crystal faces,resulting in different catalytic performance.Octahedral sites(Co³⁺)on the surface of Co₃O₄ are considered as the main active sites.Therefore,the cationic substitution effect is also a decisive factor affecting the catalytic activity of transition metal oxides.Increasing the amount of Co³⁺cations could improve the catalytic activity of spinel Co₃O₄.In addition,spinel catalysts AB₂O₄,especially cobalt spinel,have been widely studied in catalytic combustion due to their characteristics of high stability,abundant defects and adjustable denaturation.In this research,Co₃O₄ catalysts with different morphologies were synthesized via different methods.In addition,a series of MCo₂O₄ spinel（M=Cu,Ni and Zn）were prepared by co-precipitation method.The physical and chemical properties of catalysts and the catalytic oxidation performance for toluene oxidation were systematically analyzed by XRD,N₂ physical desorption,SEM,TEM,XPS,etc.The effects of morphologies and the second metal（M=Cu,Ni,Zn）on the catalytic performance for toluene oxidation were discussed.The details are as follows:（1）Co₃O₄ catalysts with different morphologies were prepared by hydrothermal synthesis method,co-precipitation method and agar gel method.The effects of different morphologies on the structure and properties of the catalysts were investigated.The catalytic oxidation of toluene obeys the order of Co₃O₄-flower>Co₃O₄-rod>Co₃O₄-sheet>Co₃O₄-particle.The T₉₀ of toluene oxidation over Co₃O₄-flower is 249°C.The durability test showed the activity remains unchanged over Co₃O₄-flower at 260°C for 50 h.（2）The results showed that the Co₃O₄-flower catalyst had the smallest particle size（10 nm）and the largest specific surface area(139.4 m² g^-1).Compared with Co₃O₄-rod,Co₃O₄-sheets and Co₃O₄-particle,Co₃O₄-flower has better reducibility at low temperature,more surface reactive oxygen species,and higher Co³⁺content,which is the fundamental reason for the excellent toluene catalytic oxidation performance of Co₃O₄-flower.（3）MCo₂O₄（M=Cu,Ni,Zn）spinel was prepared by co-precipitation method,and the effects of different transition metal cations on cobalt spinel were investigated.The catalytic activity obeys the sequence of Cu Co₂O₄>Zn Co₂O₄>Co₃O₄>Zn Co₂O₄.The Cu Co₂O₄exhibited the T₉₀ of 230°C.It also showed good stability under 250°C.（4）The ratio of Co²⁺/Co³⁺and oxygen mobility in the spinel structure are strongly affected by the substitution of the second metal.Compared with Ni Co₂O₄and Zn Co₂O₄,the interaction between Cu and Co made the catalyst have higher relative content of surface active Co³⁺and more surface reactive oxygen species,which greatly contribute to the best performance in the catalytic oxidation of toluene.