Catalytic Oxidative Degradation Effect And Mechanism Of Ni-Cu/TiO₂ Catalyst For Toluene

In recent years,with the increasingly serious impact of volatile organic pollutants on the environment,the technology of degrading VOCs has been continuously improved.Thermal catalytic oxidation has the advantages of significant economic benefits,stability,low degradation temperature and no secondary pollution of degradation products,and is widely used in practical industries.Therefore,thermal catalytic oxidation deserves further study,the core of which is the choice of catalyst.In this paper,a supported binary composite transition metal catalyst was prepared by loading Ni-based and Cu-based composite oxides on TiO₂ by means of impregnation method using cheap and abundant transition metals Ni and Cu as parent catalysts.The effect of molar ratio of Ni/Cu and different mass ratios of Ni-Cu/TiO₂ on the catalytic oxidation of toluene.The relationship between the structure and catalytic performance of the catalyst was discussed by various characterization analyses,and the optimal catalytic conditions were selected to determine the stability of the catalyst.The properties of the catalyst were explored to degrade the toluene tail gas products and the mechanism was analyzed.The specific content and results are as follows:（1）A series of NiO_x,CuO_x and modified TiO₂ catalysts were synthesized by impregnation method,and Ni-Cu catalysts with different molar ratios and Ni-Cu/TiO₂catalysts with different mass ratios were further prepared to study their catalytic performance and performance of toluene oxidation.Various characterization results demonstrate the best efficient catalyst.Among them,the 5Ni-Cu/TiO₂ catalyst still showed good catalytic performance for toluene under the conditions of catalyst dosage of 0.3 g,toluene concentration of 2000 ppm,and space velocity of 40000 m L·g^-1·h^-1(T₅₀=220℃,T₉₀=350℃).When preparing the parent catalyst,it was found that the Cu group improved the effect of degrading toluene at low temperature,while the Ni group enhanced the effect of degrading toluene at medium temperature.（2）The characterization methods of TEM-mapping,XRD,XPS,FT-IR,H₂-TPR,O₂-TPD,Toluene-TPD and nitrogen adsorption/desorption were used to analyze the catalyst at the microscopic level.The results show that the addition of TiO₂ can change the morphology of the base material and the microstructure of the support through TEM-mapping,XRD and XPS,resulting in better dispersion of the original nickel-copper composite oxide,and the Ni-Cu₂ Cu²⁺in the catalyst is reduced to Cu⁺;more Ti ions are inserted into the original lattice by the characterization methods of XPS,FT-IR,O₂-TPD and nitrogen adsorption/desorption,resulting in more oxygen vacancies on the surface,increasing the Oxygen and lattice oxygen are adsorbed,and the pore size and specific surface area are improved;5Ni-Cu/TiO₂ has high redox properties from H₂-TPR and Toluene-TPD,and can catalyze the oxidation of toluene very well.（3）By changing the space velocity,toluene concentration,catalyst quality and temperature to explore the effect of changes in external conditions on the degradation of toluene by the catalyst.The results show that under the condition of empty tower,controlling a single variable,the degradation effect of toluene concentration of 500 ppm is better than that of 2500 ppm,and the degradation effect of space velocity of 20000m L·g^-1·h^-1 is better than 200000 m L·g^-1·h^-1;under the condition of 5Ni-Cu/TiO₂catalyst,control single variable,space velocity 40000 m L·g^-1·h^-1>20000 m L·g^-1·h^-1>100000 m L·g^-1·h^-1.Toluene concentration 2000 ppm>600 ppm>3000 ppm and catalyst mass 0.3 g>0.1 g>0.6 g.The catalytic performance of the 5Ni-Cu/TiO₂catalyst has good adaptability in the face of different reaction conditions.The optimal process conditions were finally confirmed by orthogonal experiments as follows:the reaction flow rate is 0.3 L/min,the toluene concentration is 2500 ppm,and the degradation temperature is 400°C,catalyst mass 0.4 g.And it still maintains a high activity stability in ten cycle stability tests and 24 h running stability tests.（4）The tail gas after degrading toluene at 300°C and 400°C was analyzed by GC-MS,and the used and unused 5Ni-Cu/TiO₂ catalysts were analyzed by XPS spectroscopy to explore the degradation of 5Ni-Cu/TiO₂ catalysts The reaction mechanism of toluene.The types and amounts of substances in the toluene tail gas products degraded at 300℃and the toluene tail gas products degraded at 400℃have changed;through the analysis of XPS images,the valence state changes of various substances in the catalyst are reflected,it is speculated that the catalyst may be The changes are:Cu⁺+Ni²⁺+Ni（?）Cu+n Ni²⁺.Therefore,this paper speculates the reaction mechanism of 5Ni-Cu/TiO₂ catalyst to degrade toluene,following the Mars-van Krevelen（MVK）mechanism.It is speculated that in the toluene environment,the degradation process on the catalyst surface is as follows:at a certain temperature,toluene is first adsorbed on the surface active sites of the 5Ni-Cu/TiO₂ catalyst.As the temperature increases,toluene can be gradually oxidized to form benzene and phenol,which are further decomposed into small molecular organic compound intermediates.After that,it is completely oxidized to H₂O and CO₂ by surface chemisorbed oxygen and lattice oxygen.The chemisorbed oxygen and lattice oxygen leave the surface of the5Ni-Cu/TiO₂ catalyst to form oxygen vacancies,and the internal Cu⁺is reduced to Cu,and Ni²⁺is reduced to Ni.Subsequently,the catalyst adsorbs oxygen from the air to supplement the oxygen vacancies,so that the surface Cu element is oxidized to Cu⁺species again,and the Ni element is oxidized to Ni²⁺species again.