Mechanism Study On Low Temperature Oxidation Of Volatile Organic Compounds On Mn-based Catalysts For Catalytic Filtration

Volatile organic compounds（VOCs）are considered to be one of the main air pollutants,which display strong irritancy and toxicity and are extraordinarily harmful to human health and ecological environment.With the features of great emission intensity,high emission concentration,complex emission components and long emission duration,industrial sources are recognized as the largest emission sources of VOCs,which show significant impact on local air quality.In recent years,with the gradual development of key industries involving VOCs emissions,industrial VOCs emissions have shown a rapid growth trend in China.Therefore,it is imperative to carry out in-depth management on controlling VOCs pollutants.Catalytic oxidation technology is one of the most efficient technologies for VOCs abatement,in which the development of high-efficiency oxidizing catalysts is the core issue.At present,the operating temperature of commercial VOCs oxidizing catalysts is usually above 350°C,which cannot be applied to low-temperature emission occasions such as tobacco industry and spray paint industry.Therefore,the development of low-temperature oxidizing catalyst formulations is a major challenge in the treatment of VOCs emission.On the other hand,the components in industrial exhaust gas are complex.Industries such as petrochemical,tobacco,and spray coating not only emit a variety of VOCs,but also emit a large amount of dust.The coexistence situation of multiple pollutants puts tremendous pressure on air pollution control.In industrial applications,a single catalytic combustion device does not have the ability to remove dust,therefore it is still necessary to adopt a step-by-step method to control various pollutants in the flue gas,resulting in large equipment space,high controlling cost,and complex operation process.In this paper,the formulation of VOCs oxidizing catalysts suitable for low-temperature emission applications and the difficulty of stable operation of catalysts under complex conditions such as water-containing dust are studied.The structure-effect relationship of catalysts was systematically studied.The theoretical basis for low-temperature oxidizing catalyst design was proposed.A series of manganese-based low-temperature oxidizing catalyst formulations were developed.The influence mechanism of micro-nano scale adjustment on catalyst reaction performance was studied.The applicability of the catalyst under complicated operation conditions such as water-and dust-containing is expanded.The main results obtained in this paper are as follows:1.The structure-activity relationship of catalytic oxidation of VOCs was studied.Based on the VOCs oxidation reaction mechanism,manganese oxide with excellent redox properties and cerium oxide with excellent oxygen storage ability were combined,fabricating the manganese-cerium binary composite oxide catalyst through a citric acid method.When the molar ratio of Mn/Ce was 1:1,the obtained catalyst displayed the best activity in the low temperature range.We proposed the relationship between the structure,physicochemical properties and the catalytic performances.It was revealed that the bridging oxygen structure of Mn–O–Ce formed by the entry of manganese ions into the lattice of cerium oxide leaded to the lattice distortion and shrinkage of cerium oxide,where the oxygen ions escaped and formed abundant oxygen vacancies.Thereafter,oxygen in the environment was adsorbed and converted to surface active oxygen species,which significantly improved the activity of the catalyst at low temperature.In short,the bridging oxygen structure（Mn–O–Ce）in the bulk phase of the catalyst was proven to be the decisive factor of low temperature activity.2.A manganese based low temperature catalyst formulation was studied.Based on the structure-activity relationship above,we proposed a theory for the rational design of VOCs oxidizing catalyst at low temperature.A third substitution element was introduced on the basis of the manganese-cerium binary composite oxide to further broaden the lower temperature window.Four elements（Cu,Fe,Co,and La）,which have synergistic effects with Mn and Ce,were selected as substitution elements.The synergistic effect between Cu and Mn/Ce was found to be the strongest,since the low-temperature oxidation performance of VOCs on CuMnCeO_x catalyst was found to be the best.We concluded that the key factor determining this low-temperature activity is the special bridging oxygen structures such as Cu–O–Ce and Cu–O–Mn formed in the composite oxide.This theory has general applicability.Thereby,these results confirmed the universality of this theory.3.The influence mechanism of micro-nanoscale regulation on the catalytic performance was studied.The single crystal phase MnO₂ nanocatalysts with different crystal（α-,β-,γ-andδ-）forms were synthesized by hydrothermal method.It was found that the different stacking methods of octahedrons in the crystal structure ofα-,β-,γ-andδ-MnO₂ leaded to different tunnel structures and different lengths of Mn–O bonds,which were the key factors affecting the oxidation performance of VOCs.The morphologies of the catalysts were controlled through a two-step method by combing electrospinning technique and hydrothermal growth.The obtained MnO_x/TiO₂ series nanofiberous catalysts were composed of primary TiO₂ nanofibers and secondary MnO_x nanoparticles grown on the surface.Compared with MnO_x/TiO₂ nanoparticle catalyst with the same manganese loading prepared by impregnation method,it was revealed that the promotion on the mass transfer process in the heterogeneous catalytic reaction is the key factor affecting the catalytic activity.4.The applicability of manganese-based catalysts under water conditions was investigated.The above CuMnCeO_x catalyst was loaded on the commonly used oxide supports（TiO₂,Al₂O₃,and SiO₂）by the impregnation method.The influence mechanism of water vapor on the reaction performance of the catalyst was studied.It was found that H₂O can significantly inhibit the activity of the catalyst.The selection of a suitable support can reduce the inhibition on the catalyst activity by water vapor.Comparing the adsorption of VOCs on the catalyst surface under anhydrous and aqueous conditions,we concluded that the adsorption of water vapor on the surface of CuMnCeO_x/TiO₂ was the weakest,indicating that using TiO₂ as a support is beneficial to improve water resistance of manganese-based catalysts.There existed competitive adsorption between water vapor and VOCs on the active sites of catalyst surface,resulting in a decrease in catalyst activity.The stability test results showed that CuMnCeO_x/TiO₂ can maintain stable activity during a long-term test under water conditions,which proved that the catalyst possessed excellent hydrothermal stability.5.Development of catalytic filters for the synergistic removal of PM2.5 and VOCs.Aiming at the applicability of the catalyst under dusty conditions,a filterable catalytic composite material was designed,and the inert support was adjusted to the active support to realize the multi-functionality of the filter material.The above CuMnCeO_x catalyst was loaded on the PI filter by the emulsion impregnation method,and the obtained CuMnCeO_x/PI catalytic filter maintained the inherent excellent filtration performance,as well as 90%conversion of styrene oxidation was achieved at 193°C.Herein,synergistic removal of particulate matter and volatile organic compounds was realized.The stability test results showed that this catalytic filter displayed good thermal stability.In summary,this paper developed a novel CuMnCeO_x/PI catalytic filter with synergistic removal function of PM2.5 and VOCs,and provided a theoretical basis for the synergistic control of multi-pollutants in complex industrial flue gas.