Reaction Mechanism Of Catalytic Oxidation Of Toluene Over Copper-manganese Bimetallic Oxides

Volatile organic compounds?VOCs?are a kind of air pollutants emitted from a wide range of sources,which can seriously endanger human health and the natural environment.It is urgent to control and reduce their emissions.Up to date,the most promising VOCs control technology is catalytic oxidation,which has low energy consumption,high efficiency and less secondary pollution.As the core of catalytic oxidation technology,the catalysts have attracted extensive attention in recent years.The large-scale application of noble metal catalysts is limited by scarce resources and high prices.Composite metal oxide catalysts have good stability and strong redox ability.They are a kind of catalytic oxidation catalysts with broad application prospects.In this study,the representative toluene in benzene series was chosen as the target pollutant,and CuMn bimetallic oxide was chosen as the catalyst to investigate the effect of molar ratio of Cu/Mn on the catalytic activity of toluene and the reaction mechanism of toluene oxidation over CuMn catalyst.Moreover,the effect of alkali metals on CuMn catalyst was studied,and the effect mechanism of alkali metals on the activity of CuMn catalyst was systematically investigated.Firstly,CuO,Mn₃O₄ and CuMn catalysts with different molar ratios of Cu/Mn were prepared by co-precipitation method.The toluene catalytic oxidation activity studies showed that when the molar ratio of Cu/Mn was 2,the prepared Cu₂Mn₁ catalyst had the best catalytic activity,excellent reusability and water resistance.The reason is that increasing the molar ratio of Cu/Mn properly can enhance the specific surface area and the toluene adsorption capacity of the catalyst.It was also found that lattice oxygen was the main oxygen species involved in toluene oxidation on CuMn catalyst,and the lattice oxygen content increased with the increase of the molar ratio of Cu/Mn in a certain range.In situ DRIFT showed that CuO was the adsorption sites and oxidation sites of toluene on CuMn catalyst.The increase of Cu/Mn molar ratio increased the content of Mn⁴⁺on the catalyst surface,promoted the circulation between Cu⁺and Cu²⁺,thus facilitated the catalytic activity of toluene oxidation.Steady-state kinetic studies showed that the oxidation of toluene on CuMn catalyst is not a first-order reaction related to toluene concentration.The reaction order increases with the increase of temperature.This is due to the influence of temperature on the toluene adsorption capacity and oxidation ability of CuMn catalyst.The oxidation ability of CuMn catalyst is the main limiting factor in the reaction at low temperature,while the toluene adsorption capacity of CuMn catalyst is the main limiting factor in the reaction at high temperature.Then,considering that alkali metal could promote the interaction between bimetallic metals,Cu₂Mn₁ catalysts with different K loading amounts were prepared by impregnation method.The toluene catalytic oxidation activity results showed that when K loading amount was low or high,K would inhibit the activity of the catalyst.When K loading amount was moderate,K would promote the activity of the catalyst.Among the K/Cu₂Mn₁ catalysts with different K loadings,0.5K/Cu₂Mn₁ showed the best activity.The characterization results showed that K could inhibit the redox ability and reduce the specific surface area of the catalyst and the toluene adsorption capacity.At the same time,the contents of Mn⁴⁺and surface lattice oxygen on the catalyst surface can also be increased to a certain extent by K loading.In situ DRIFT showed that the oxidation rate of benzene ring and methyl was the fastest on 0.5K/Cu₂Mn₁ catalyst with the highest Mn⁴⁺content,thus it has the best catalytic activity.