Mesoporous Silica Nanospheres Supported Co-based Bimetal Catalysts For Toluene Oxidation

Volatile organic compounds（VOCs）severly endanger the environment and human health,and reducing VOCs emissions has become an important topic.Catalytic oxidation is one of the methods to effectively control VOCs,and the key is the catalyst with high efficiency,high stability and low price.Cobalt oxides possess a variety of oxidation state and present excellent catalytic activity in the oxidation of VOCs.Recently,dendritic mesoporous silica nanospheres（DMSNs）with a central radial pore structure have been reported,which are expected to become a new type of catalyst support.Cobalt-based catalysts were synthesized by different methods using DMSNs as support.The precious metal palladium and transition metal cerium were further introduced to obtain bimetal supported nano-catalysts,the physicochemical properties of catalysts were characterized by a variety of techniques,and the catalytic activity for the toluene oxidation was investigated.Co-DMSNs-f and Co/DMSNs-i cobalt-based catalysts were synthesized by grafting and impregnation methods,respectively.It is found that the grafting method is more conducive to the formation of highly dispersed cobalt species.There are strong interaction between cobalt species and silica in the mesoporous structure,and it is mainly composed of Co（II）and cobalt silicate mixed oxide.Co²⁺species is the main center of oxygen activation.The abundant Co²⁺ions are beneficial to the activation of oxygen,thereby improving the catalytic oxidation activity of toluene.The dendritic mesoporous structure and central radial pores of DMSNs provide a higher contact area and diffusion effect for the catalytic reaction.The large specific surface area,unique structure,strong interaction between cobalt and the carrier and highly dispersed Co O_x speices promote the 3Co-DMSNs-f to exhibit excellent activity,stability and water resistance.In order to improve the low-temperature activity of the cobalt-based catalyst,metallic palladium and cerium were introduced by the impregnation method to prepare the bimetal catalyst.The loading of palladium promotes the reduction of active cobalt oxide species,and significantly improves the low-temperature catalytic activity.The strong interaction between Pd and Co O_x is helpful to the generation of oxygen vacancies and toluene adsorption sites.The 3%Pd1.7%Co/DMSNs-i shows the best catalytic performance,and T₉₀ can reach 151℃.Compared with cobalt oxide or cerium oxide catalysts,cobalt-cerium bimetallic catalysts with more Co²⁺and Ce³⁺species also have better low-temperature reducibility.Co²⁺and Ce³⁺species affect the charge and oxygen vacancies on the surface of catalyst,and generate more active oxygens.There is a synergistic effect between the cobalt-cerium bimetal,thereby increasing the oxygen mobility,and improving the redox ability and catalytic performance of the cobalt-cerium bimetallic catalysts.T₉₀ of Co₁Ce₁/DMSNs sample is 223℃.This research improves the catalytic performance of the catalyst in the toluene oxidation,and provides ideas for the design and preparation of other samples for the elimination of toluene-based VOCs.