Catalytic Removal Mechanism Of VOCs And NO_x Enhanced By Active Center Regulation Of Transition Metal Oxide

Fossil energy is the most important primary energy source in China.A large number of environmental air pollutants,such as nitrogen oxides(NOx)and volatile organic pollutants(VOCs),have been produced and emitted during the production and use processes of fossil energy,seriously affecting the social development and health of humans.Nano-thermal catalysis is one of the most effective methods to control air pollutants such as VOCs and NOx.Due to their flexible atomic coordination and abundant redox ability,the transition metals manganese oxides and copper oxides have broad application prospects in the catalytic elimination of VOCs and NOx.However,the activity of transition metal oxides at low temperatures is held back by the competitive adsorption of reactants,lower molecular oxygen and lattice oxygen activation abilities,and difficult desorption of intermediates.The fundamental solution to these problems requires a deeper understanding of the reactions occurring on the catalyst surface.Catalysis occurs at the surface active center,and the active center generally exists at the surface edges,unsaturated sites,or interlayers,and its quantity and quality directly determine the activity of the nanocatalyst.Efficient catalytic systems require a directional design to fully develop more complete active centers.Therefore,according to the characteristics and difficulties of VOCs and NOx catalytic elimination reactions,it is of great significance to regulate the active center of the nanocatalyst directionally,strengthen the low-temperature catalytic activity of manganeseand copper-based oxides,and realize the efficient control of VOCs and NOx.Toluene is one of the typical harmful VOCs in the atmosphere,with high ozone formation potential,and its treatment is typical and representative.Based on this,this thesis used toluene and NOx as model pollutants,and regulated the active centers of manganese-and copper-based oxides employing oxygen vacancy,cation vacancy,noble metal loading,and the construction of highly dispersed double sites.The relationships between the physical and chemical properties and catalytic efficiency of the catalysts were studied,and the key factors affecting the lowtemperature activity of the catalysts were clarified.Moreover,the catalytic reaction mechanism of toluene or NOx elimination over manganese-and copper-based oxides was investigated.The main research results are as follows:1.Manganese oxide(MnOx)nanocatalysts with oxygen defects were prepared by manganese organometallic framework(Mn-MOF)pyrolysis.It was found that the pyrolysis atmosphere had a significant effect on the structure and toluene degradation efficiency of MnOx catalysts.Compared to the MnOx-A nanocatalyst obtained by direct pyrolysis in an air atmosphere(T90=293℃;T90 represents the temperature at which toluene conversion reaches 90%)or the MnOx-AN nanocatalyst obtained by sequential pyrolysis in an air and nitrogen atmosphere(T90=259℃),the MnOx-NA nanocatalysts derived from Mn-MOF pyrolysis in an air atmosphere after pretreatment with nitrogen exhibited the best toluene degradation activity(T90=241℃).Moreover,MnOx-NA showed good catalytic stability(64 h)and water resistance.Results showed that the modified pyrolysis atmosphere could introduce oxygen vacancy into the MnOx catalyst,increase the Mn3+/Mn4+ratios,surface oxygen activity and content,and enhance the low-temperature reducibility of the catalyst.The MnOx-NA catalyst had the highest concentration of oxygen vacancies,resulting in the best catalytic activity.Further study found that the reducibility of Mn3+ at low temperatures was the key factor enhancing the catalytic activity of MnOx.In situ diffuse Fourier transform infrared spectroscopy(DRIFTS)results showed that oxygen vacancies on the MnOx surface played an important role in toluene adsorption and intermediate formation.2.The nano-structured cobalt manganese oxides CoMnOx with cationic defects(CMO-Ex,where x represents the acid concentration)were constructed by acid treatment to optimize the toluene oxidation activation of pristine CoMnOx(CMO-EO).Characterizations by positron annihilation spectrometry and HAADF-STEM measurements elucidated that cationic defect(manganese and cobalt defect)content was effectively regulated through acid concentration.Compared with CMO-EO(T90=257℃),the CMO-E0.05 sample modified by optimum manganese and cobalt defect content delivered the highest toluene catalytic degradation activity(T90=238℃).Moreover,the CMO-E0.05 sample possessed superior catalytic stability(60 h)and water resistance(5 vol%H2O).Our results showed that manganese and cobalt defects in CoMnOχ could boost the lattice oxygen activation and activity because of the increased cation valence state and shortened metal-oxygen bond,facilitating toluene oxidation.In situ DRIFTS study revealed that the cationic defects accelerated the toluene degradation rate by promoting the conversion of key intermediates(maleic anhydride)with highly active lattice oxygen.3.Manganese oxides with different platinum-loading amounts(Pt-MnOx)catalysts were prepared by the ball milling-molten salt method.The effect of Pt loading on toluene oxidation performance was studied.Results showed that 0.32 wt%Pt-MnOx catalyst had the highest catalytic activity(T90=175℃)with a WHSV of 60,000 mL·g1·h-1.Structure analysis revealed that MnOx existed in the form of Mn2O3 and Mn3O4 mixed oxides,and Pt nanoparticles with a size of 3-4 nm and Pt single atoms were highly dispersed on the surface of MnOx.Further study showed that Pt loading significantly affects the electronic structure of the catalyst.<0.32 wt%Pt loading could weaken the Mn-O bond strength,resulting in higher lattice oxygen activity and surface reactive oxygen species concentration,enhanced low-temperature reducibility of PtMnOx,and finally improved the catalytic efficiency of toluene degradation.When the Pt loading was further increased to 0.40 wt%,the dispersion of Pt particles decreased,which enhanced the Mn-O bond strength,impaired the lattice oxygen activity and surface reactive oxygen species,and therefore decreased the toluene degradation activity(T90=194℃).Compared with the aforementioned Mn-based catalysts CMOE0.05(T90=241℃)and MnOx-NA(T90=238℃),the 0.32 wt%Pt-MnOx catalyst showed the highest catalytic activity and water resistance(10 vol%H2O)and had a good application prospect.4.The copper-nickel-aluminum mixed oxides CuyNi3-yAlOx(y represents the Cu/Al ratio)with highly dispersed Ni—Cu dual active sites were constructed from the layered double hydroxide precursors to optimize the NH3-SCR activity.Compared with other CuyNi3-yAlx(<80%),Cu1.5Ni1.5AlOx delivered a NOx conversion as high as 90%at 200℃.Moreover,Cu1.5Ni1.5AlOx possessed superior N2 selectivity(92%above 200℃),catalytic stability(64 h),and H2O and SO2 resistance.Characterization demonstrated that Cu and Ni species in the CuyNi3-yAlOx mixed oxides had interactions that were derived from the electron transfer and influenced by the Ni/Cu ratio,inducing the formation of highly dispersed Ni-Cu sites.Cu1.5Ni1.5AlOx with a Ni/Cu ratio of 1 achieved the strongest interaction.That interaction could inhibit nanosheet agglomeration,increase surface acidity,and improve low-temperature reducibility,in favor of NH3 and NO adsorption-activation.It was discovered that the Ni-Cu sites participated in the NO and O2 activations,respectively,and accounted for the NH3 adsorption-activation synergistically.This study provides theoretical guidance and technical support for the directional design and preparation of high-efficiency VOCs and NOx catalytic elimination nanomaterials at low temperatures and affords theoretical support for achieving efficient and economic emission reduction of VOCs and NOx.