Thermal Catalytic Oxidation Of Volatile Organic Compounds Over Porous-mineral-supported/transition-metal-mixed Manganese Oxides

Recently,the atmospheric pollution in China has severe impact on the human health and ecological balance.Volatile organic compounds?VOCs?are the major precursors of photochemical ozone and fine particle matters(PM_2.5),so that the elimination of VOCs is an urgent task for?Blue Sky Protection Campaign?.The thermal catalytic oxidation?TCO?is regarded as one of the effective,environmental-friendly,and economical way to reduce the emission of VOCs.The minerals with highly-developed porosity,large specific surface area,and good thermal stability provide sufficient room for loading active particles in catalysis.To overcome the agglomeration of particles and endow reactant molecules with more reactive sites,the natural porous minerals are utilized as catalyst support.The natural porous minerals are abundant,inexpensive,and diverse resources on the earthcrust.To increase the economic benefit from natural porous mineral,the exploitation of cheap porous-mineral-supported catalyst is a feasible way.The manganese-based catalysts are widely-used in thermal catalytic oxidation for VOCs due to many advantages,such as multiple oxidation state,fast electron transfer,and controllable morphology and structure et al.As the manganese combines with other transition metal elements,the physico-chemical properties of binary manganese-rich oxides are changed,including lattice defects,surface elemental composition and oxidation state,electron transfer rates,specific surface area,crystalline size,particles agglomeration,and magnetism et al.These physico-chemical properties of manganese-based catalyst influence the activity of TCO for VOCs in various degrees.The prepared catalysts in this study are classified into two categories.One kind is the mesoporous-halloysite-supported and macroporous-diatomite-supported manganese oxides.The aim is to increase the catalytic activity by the good dispersion of porous minerals with high thermal stability.The other kind is the copper or iron modified manganese oxides,which show much better reducibility through synergetic effect of two transition metals.The prepared porous-mineral-supported/transition-metal-mixed manganese oxides are utilized in the removal of formaldehyde,which is a typical VOC.The structural phase,dispersion,specific surface area,pore structure,oxidation state,and reducibility are investigated by different characterizations.The relationship between physico-chemical properties and catalytic activity of formaldehyde oxidation is also built.In addition,this study reveals the oxidation mechanism of formaldehyde over manganese oxides.More details and findings are the following.1)The nature of manganese source and the surface and morphology of halloysite determines the structural phase,oxidation state,and dispersion of manganese oxides on halloysite.These result in the difference on reducibility,catalytic activity of formaldehyde oxidation,and dispersion by using different manganese source in impregnation.This work is helpful for the halloysite-supported transition metal oxides by impregantation method.By using the nitrate,the 35-249 nm pyrolusite particles are scattered in the halloysite nanotubes.When the acetate is accepted as precursor,the 15-40 nm hausmannite particles are coated on the external surface of halloysite.However,as for samples prepared by permanganate,most amorphous K-containing manganese oxides aggregate on the external surface,the rest of particles enter into the halloysite lumen.The catalytic activity towards formaldehyde oxidation correlates with the reducibility of halloysite-supported manganese oxides?MnO_x/Hal?positively.Among three catalysts,the MnO_x/Hal by permanganate displays the best catalytic performance on account of the active component,amorphous K-containing manganese oxides.2)The occurrence of manganese species on diatomite is illustrated,and it is found that the amorphous MnO₂ is the activest component among manganese species.With the increase of manganese,the catalytic activity increases due to more active oxygen.The oxidation mechanism follows the MVK mode over diatomite-supported manganese oxides.This work is an interesting reference for the optimization of supporting amount and calcination temperature for supporting manganese oxides.From the material characterization,the manganese species on diatomite are amorphous MnO₂,low crystallized?-Mn₂O₃,and a little residual MnCO₃ precursor.The manganese particles are supported on the shells and the wall of pores.The total pore volume and specific surface area of MnO_x/Dt increase to 1.5 times of diatomite support.When the manganese amount increases to 14.1 wt%,the manganese particles aggregates together and the residual MnCO₃ becomes more on diatomite.By the analysis of structure-activity relationship,the increase of manganese retains the distribution and reducibility of manganese species.The catalytic performance depends on the reducibility of Mn⁴⁺in amorphous MnO₂.Although the distribution and reducibility are not altered as the increase of supporting amount,more active oxygen,which can oxidize VOCs to CO₂,results in higher catalytic activity.3)The local structure of amorphous Cu-Mn mixed oxides on palygorskite is similar to the spinel-like CuMn₂O₄.The role of Cu and Mn in catalytic oxidation of formaldehyde is identified by this study.Based on the MVK mechanism,the deactivation mechanism of palygorskite-supported Cu-Mn mixed oxides and one facile method for regeneration are proposed.These findings help to understand the structure-activity relationship and reaction mechanism in TCO of formaldehyde over Cu-Mn mixed oxides.The characterization results reveal that the copper-manganese oxides with the size of10-40 nm are distributed on the surface of palygorskite and in the intraparticle pores.The copper-manganese mixed oxides on palygorskite resemble the spinel-like CuMn₂O₄ in terms of structure,in which the Cu occupies at the tetrahedral site and the Mn occupies at the octahedral site.As the copper increases in Cu-Mn-O/Pal,the generation of formate species increases,and the catalytic activity as well as the resistance to water are also improved.All the prepared Cu-Mn-O/Pal achieves 99%CO₂ generation above 275 ^oC in thermal catalytic oxidation of formaldehyde.At a reaction temperature below 200 ^oC,the deactivation is ascribed to the accumulation of formate species on surface,which are important intermediate species during the complete oxidation of formaldehyde.In contrast,the deactivation at 200-250 ^oC is related to the gas-induced surface chemical transformation which leads to the loss of surface oxygen,the reduction of Cu and Mn,and the decrease of reducibility.Due to the decomposition of residual formate species,the increase of oxidation state,and the restoration of surface oxygen,the catalytic activity and reducibility of Cu-Mn-O/Pal recover partially.4)In the study of the effect of manganese amount and calcination temperature on the structure and surface properties of manganese spinel ferrite,it is observed that Mn⁴⁺is closely related to the catalytic activity in the TCO of formaldehyde.The prepared manganese spinel ferrite shows good stability in structure and catalytic performance,and the magnetism benefits for the separation and transfer.These make them a kind of potential candidate for the commercial catalyst.The manganese is successfully doped into the spinel structure in state of Mn³⁺and Mn⁴⁺.The substitution by manganese decreases the specific surface area and surface oxygen amount,obstructing the adsorption of formaldehyde and oxygen.However,the doping of manganese increases the reducibility of maghemite,facilitating the catalytic activity.When the precursor,manganese magnetite,is calcined at 200-600 ^oC,the spinel structure retains.The catalytic activity initially increases with the increment of calcination temperature until 400 ^oC,and then decreases.The manganese-substituted maghemite calcinated at 400 ^oC shows the best activity due to the highest reducibility of Mn⁴⁺.The prepared manganese spinel ferrite displays good reusablility and high-temperature water resistance.The preparation of porous-mineral supported manganese oxides broadens the way to develop high-value added product.It is easy for the magnetic manganese spinel ferrite to recycle and transfer in VOCs oxidation.The deactivation and regeneration mechanism of amorphous Cu-Mn mixed oxides helps to understand the VOCs oxidation mechanism over transition metal oxides.The structure-activity relationship analysis contributes to explore the significant aspects of manganese oxides on catalytic activity.