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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 6: 928 - 938

DOI: https://doi.org/10.7536/PC231005

Review

The Space Confinement Effect of Catalytic Materials and Its Application in Low Temperature Denitration

  • Wei Zhang , 1, 2, * ,
  • Qiao Wu 1, 2 ,
  • Yehao Fu 1, 2 ,
  • Yaocheng Liang 1, 2 ,
  • Min Ruan 1, 2 ,
  • Yanshan Yin 1, 2 ,
  • Shan Cheng 1, 2
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  • 1 School of Energy and Power Engineering, Changsha University of Science and Technology, Changsha 410114, China
  • 2 Renewable Energy Power Technology Key Laboratory of Hunan Province, Changsha 410114, China
* e-mail:

Received date: 2023-10-12

  Revised date: 2024-01-12

  Online published: 2024-03-15

Supported by

National Natural Science Foundation of China(52006016)

Hunan Natural Science Foundation(2023JJ30047)

Hunan Natural Science Foundation(2021JJ40573)

Hunan Natural Science Foundation(2020JJ4098)

Key Project of Scientific Research Project of Hunan Provincial Department of Education(21A0216)

Excellent Youth Project of Hunan Provincial Department of Education(202B041)

Excellent Youth Project of Hunan Provincial Department of Education(22B0291)

Changsha Natural Science Foundation(kq2014104)

Young Teachers' Growth Plan Project of Changsha University of Science and Technology(2019QJCZ044)

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Abstract

The spatial confinement effect of porous materials can change the surface electron distribution and electron transport performance,realize the local reaction in the micro-nano pore domain,effectively prevent the external environment from affecting the active substances in the confined space,and inhibit the agglomeration of the active center,which is an effective way to strengthen the denitrification performance of the catalyst.This paper focuses on the changes in surface energy,periodic boundary conditions and electronic energy levels of different catalytic materials,and discusses the formation mechanism of the spatial confinement effect.The effects of confinement effect on the dispersion of active species,redox ability and molecular adsorption strength in the reaction process and the regulation strategies of size effect,encapsulation effect and molecular sieve effect in confinement effect were described.The strengthening effects of confined catalysts on NH3adsorption performance,reaction selectivity,anti-toxicity and denitrification activity in the denitrification process were summarized.Finally,the development prospect of confined denitrification catalysts was prospected.

Contents

1 introduction

2 The influence of space confinement effect on catalytic reaction

2.1 Inhibiting the aggregation of active species

2.2 Promoting the migration of interface electrons

2.3 Enhancing the adsorption of reaction molecules

3 Spatial confinement effect regulation strategy

3.1 Control of confinement size effect

3.2 Control of confinement encapsulation effect

3.3 Control of confined molecular sieve effect

4 The application of spatial confinement effect in low-temperature denitrification

4.1 Strengthen the adsorption performance of NH3

4.2 Enhance the selectivity of the denitration reaction

4.3 Enhance the anti-toxicity of the catalyst

5 Conclusion and outlook

Key words： porous materials; confinement effect; catalytic mechanism; regulation strategy; low-temperature denitration

Cite this article

Wei Zhang , Qiao Wu , Yehao Fu , Yaocheng Liang , Min Ruan , Yanshan Yin , Shan Cheng . The Space Confinement Effect of Catalytic Materials and Its Application in Low Temperature Denitration[J]. Progress in Chemistry, 2024 , 36(6) : 928 -938 . DOI: 10.7536/PC231005

1 Introduction

in recent years,the high efficiency and stability of confined catalysts In the reaction process have attracted much attention.Nanoconfinement effects can be produced by partial or complete encapsulation of molecules or nanomaterials within nanoscale cavities,pores,two-dimensional surfaces,and channels[1]。 in the synthesis process of materials,nanoconfinement can not only serve as a physical reactor to determine the shape and size of nanomaterials,but also as a chemical microenvironment for the nucleation and growth of nanomaterials,resulting In the physical and chemical properties of confined materials different from those of their bulk analogues[2]。 Studies have shown that when the size of the material reaches the nanometer level,the surface of the catalyst has a very high surface energy,which makes the atom coordination insufficient and the surface atom has a high activity,resulting in a special nano-surface effect[3,4]; When the scale of the material is between the de Broglie wavelength and the light wavelength,the periodic boundary condition of the crystal is destroyed,and the atomic density of the surface layer is sharply reduced,resulting in small-scale effects[5]。 when the size of the particle reaches the nanometer level,especially When the size is less than or equal to the exciton Bohr radius of the material,the electron energy level near the Fermi level is split from the continuous state to the vertical state,and then the Kubo effect is generated[6]。
The structures of confined catalysts include core-shell,channel-encapsulated,two-dimensional induced and defect-derived structures[7]。 Nanocatalyst supports can be divided into zero-dimensional porous confinement materials,one-dimensional channel confinement materials and two-dimensional confinement materials according to the packaging dimension[8][9][10]。 Nanoporous materials are obviously different from conventional macroporous materials in charge distribution,surface free energy and delocalization encapsulation,and have excellent catalytic performance.Therefore,nanoconfined catalysts are widely used in many catalytic fields。
Among them,environmental protection and air quality have always been hot topics worldwide.Nitrogen oxide emissions are one of the main factors leading to air pollution and acid rain formation.Research and development of efficient and environmentally friendly denitrification catalysts is an important measure to reduce air pollution.Compared with traditional catalysts,the high selectivity,thermal stability and strong toxicity resistance of sterically confined catalysts provide new strategies and paths for denitrification technology.However,the self-assembly synthesis mechanism,performance regulation mechanism and special micro-effect excitation mechanism of confined SCR catalyst are not clear,and the interaction mechanism with guest molecules and elementary reaction mechanism need to be further studied。
in this paper,the influence and formation mechanism of spatial confinement effect on catalytic reaction are reviewed,and the control strategies of spatial confinement effect and its application In denitrification reaction are further summarized.Finally,the application of confined denitrification catalyst is summarized and prospected。

2 Steric confinement effect on catalytic reaction

Space-confined materials can increase the molecular orbital energy of the delocalized species,thereby changing the charge characteristics and reaction pathway of the catalyst,inhibiting the agglomeration of active species,thereby improving the selectivity of guest molecules and changing the reaction kinetics[11]。 the following mainly introduces some special effects of steric confinement on catalytic reactions and the mechanism of catalysis reported in the literature in recent years。

2.1 Inhibition of aggregation of active species

the confinement effect can restrict the migration of active species,promote the dispersion of active centers,anchor single atoms and inhibit the agglomeration and oxidation of active sites,which makes it easier for reactants and products to transfer in the confined space and provides a strong attractive catalytic environment for heterogeneous catalysis[12]。
the presence of electrostatic interactions,hydrogen bonds,van der Waals forces,and electromagnetic dipole-dipole interactions stabilize active particles in The confinement of particles on different dimensional spaces or interfaces[13]。 For example,the increased curvature of one-dimensional carbon nanotubes(Fig.1)will produce a larger reaction contact area and van der Waals interaction,and the in-tube encapsulation energy Eencis much larger than the external adsorption energy Eads.But in hollow graphitized carbon nanofibers(GNF),the encapsulated species exist at the inner step edge caused by the cup-stack structure,with maximized van der Waals contact,and their Eencmay still be higher than the Eadsdespite the reduced curvature[14]。 Compared with the zero-dimensional and one-dimensional confined systems,the vertical confined growth of the catalyst in the two-dimensional space has more optimal orientation and lattice strain[15]。 core-shell nanostructures are formed when metal nanoparticles are encapsulated by two-dimensional materials,in which the active Core structure is well protected by the shell,improving the stability of the catalyst[16]。
图1 CNT的三种结构及其对应的封装能和吸附能。（a）单臂纳米管，（b）多臂纳米管和（c）中空石墨化碳纳米纤维[14]

Fig. 1 Three structures of CNT and their corresponding encapsulation and adsorption energies (a) SCNT, (b) MCNT, and (c) GNF[14]

Recent studies have shown that the framework confinement structure can make the lattice distortion of ordered mesoporous TiO2to produce more defect sites,and fully contact with active nanoparticles to lead to stronger metal oxide interaction,which largely inhibits the agglomeration of active species and the redox ability of the catalyst[17]。 At the same time,the confinement effect of the aperture and the electron-rich environment of the confinement aperture can also effectively enhance the anchoring and capture of single atoms and increase the atom loading rate;the high surface energy of small pore size can prevent the aggregation of monoatomic active centers and limit the interaction between intermediates and active centers,thus improving the stability of the catalyst[18]。 Huo et al.Found that the good dispersion of Fe atoms on porous carbon was attributed to the rich pore structure and double constraints[19]。 the electrostatic interaction ensures the good pre-dispersion of the active precursor,while the spatial confinement effect further inhibits the aggregation of Fe during pyrolysis。

2.2 Facilitating the migration of interface electrons

When the material is reduced to nanometer scale,due to the surface effect,the highly active surface atoms not only cause the change of the transport and configuration of the surface atoms of the nanoparticle,but also cause the change of the surface electron spin conformation and electron energy spectrum.Therefore,the confinement effect can activate molecular bonds and promote the formation of oxygen vacancies by optimizing the charge layout of the catalytic system,thereby improving the redox ability of the catalyst。
In the denitration process,the adsorption of NOxon the catalyst is realized by activating the N—O bond of NO2,which only plays a supporting role for NO and NH3.In the molecular adsorption process,the electron-rich environment near the entrance of the confined space can transfer a large number of electrons to the adsorbed molecules,which makes the molecular bonds longer and weaker,thus facilitating the activation and dissociation of molecules[20][21]。 Yang et al.Found that the confinement effect in multi-walled nanotubes(MWCNTs)can change the electron density on the catalyst,making the step that determines the reaction rate easier to occur and improving the catalytic activity[22]。
Changing the charge distribution of the catalytic system can promote the mutual transformation between the active components with different valence States,and the difference of their electronic States can improve the high specific activity of the catalyst,so that the redox performance can be adjusted[23]。 The Ce3+species in the catalyst can lead to charge imbalance and enhance the formation of oxygen vacancies and unsaturated chemical bonds,which in turn produce more chemisorbed oxygen species[24]。 Mu et al.Found that the ordered mesoporous structure affected the charge density of the supported active particles,and guided the active components to produce a synergistic effect,so that more electrons were transferred to the support,thus promoting the generation of Ce3+species[25]。 Chen et al.Found that on the Cu/TNT（TiO2nanotube)catalyst,the Cu2+species accounted for 38.4%of the total surface copper,which was significantly higher than that of the Cu/P25(27.9%)catalyst[26]。 In addition,Yao et al.Found that the confinement effect of titanate nanotubes(TNT)could significantly enhance the electron transfer ability between encapsulated CeO2and supported CuO,and promote the formation and migration of active oxygen in the catalyst[27]。 When the single atom is used as the catalytic active center,the unsaturated coordination of the metal atom changes the electronic structure of the guest molecule,thus optimizing the adsorption of the reaction intermediate in the catalytic process.At the same time,the strong interaction between the metal single atom and the support promotes the electron transfer and changes the electronic state of the support,which is beneficial to the improvement of the catalyst activity[23]。

2.3 Enhanced adsorption of reactive molecules

the confinement effect can change the d-band state of the encapsulated particles,weaken the binding energy of the adsorbed molecules and expose more active centers,thus enhancing the adsorption performance of the catalyst and improving the mass transfer efficiency and selectivity of the catalytic reaction。
In the confined space,thed z 2 orbital of the encapsulated particle is perpendicular to the basal plane,which can maximize the overlap between the frontal orbital and the reactant orbital,and is beneficial to the adsorption/activation of the reactant[28]。 Zhan et al.Found that the Co d3of hollow CO nanoparticles has more overlap with C p2near the Fermi level,and the upshifted d-band center(εd)leads to the decrease of d-band orbital occupation,which is more favorable for its interaction with d-5σ(CO)and has stronger CO adsorption/activation ability[29]。 Active particles will produce stronger strain and deformation within the CNT,so that the d-band States move downward,and the lower d-band center will affect the occupation of the antibonding state of the adatom,resulting in the weakening of the adatom binding energy.And because that hybridization of the sp2deform more with the increase of the curvature,the nanotube has stronger confinement effect when the diameter of the nanotube is reduce[18]。
in a two-dimensional microenvironment,van der Waals interactions between opposing walls can impose strong geometric constraints on trapped interlayer molecules.in the case of an atom or molecule adsorbed to a two-dimensional space,the overlayer is lifted away from the surface,resulting in an increase In the potential energy and a decrease in the stability of the catalytic system[30]。 from the quantum effect,the overlayer acts as a constraint boundary for the molecular orbital of the adsorbate,so that the molecule cannot be sufficiently relaxed away From the surface,resulting in an increase in its orbital energy level[11]。 When the catalyst system with adsorbed molecules is close to the overlayer,the equilibrium distance between the molecules is shortened,and the overlayer and the adsorbed molecules are in a repulsive state,which strongly weakens the gas adsorption on the catalyst surface,and the adsorbed molecules are more affected by the confinement effect at higher positions above the surface[16]。 Zhou et al.Found that in the Gr/H/Ni system(Fig.2),the graphene coating changes the interaction between H and the metal surface,making H closer to Ni and generating steric hindrance,which increases the overlap of electron clouds between them,thus enhancing the repulsive part of the interaction[31]。
图2 Gr/H/Ni型二维催化剂模型图[31]

Fig. 2 Gr/H/Ni 2D catalyst model[31]

At the same time,when the molecular size is close to the pore size of the molecular sieve,the pore channel will guide the diffusion direction of the molecule,and the diffusion energy barrier will be greatly reduced,so that the diffusion is not affected by the concentration of the adsorbate,which greatly affects the molecular diffusion in the reaction process.Due to the shape selectivity of the limited area carrier,only reactant molecules with particle sizes smaller than the pore diameter are allowed to enter,so that macromolecular impurities in the flue gas are prevented from entering the nano reaction space to poison or occupy the reaction active center[32]。 In addition,the highly dispersed active material and the rich pore structure can expose more active centers and increase the contact opportunity between the reactant molecules and the active centers,thereby improving the mass transfer efficiency of the reactant and product molecules[33]。
to sum up,the confinement effect is beneficial to the charge redistribution of the catalyst and the adsorption of the reaction molecules,and can effectively improve the atom utilization rate of the active center and prevent the migration and agglomeration of the active center.However,due to the complexity of the restricted domain model and the limitation of computational methods,the strengthening mechanism of the low-dimensional restricted domain model needs to be further studied.In addition,how to control the confinement effect of the catalyst is also one of the problems to be solved。

3 Regulation strategy of spatial confinement effect

Nanostructures with different dimensions can be synthesized by chemical vapor deposition,etching,self-assembly and solvothermal methods to realize the construction of confined space[2]。 Regulating the confinement effect in the catalyst preparation process can effectively enhance the low-temperature NH3-SCR denitration performance,wherein regulating the size effect can promote the catalytic activity,regulating the encapsulation effect can enhance the stability of the catalyst,and optimizing the molecular sieve effect can effectively improve the selectivity of the catalyst to guest molecules[34]。

3.1 Control of confinement size effect

When the size of the constituent phase of the nanomaterial decreases to a certain critical size,the properties of the material will change significantly.From the geometric structure,the gradual exposure of the low-coordinated atoms and the increase of their proportion will significantly change the structure and proportion of the active centers of the catalytic material[35]; From the point of view of electronic structure,the significant change of electronic energy level due to quantum size effect will greatly affect the orbital hybridization and charge transfer between catalytic materials and reactants[36]。 Wang et al.Found that when the particle size of Pd nanoparticles is between 19.2 and 4.2 nm,the geometric effect dominates the right side of the volcano curve by controlling the size of HCs/LCs(the ratio of high and low coordination sites),while when the particle size is reduced to 4.1 nm,the electronic effect leads to an increase in binding energy,the Fermi level moves upward with the decrease of particle size,and the electron transfer of adsorbate is larger,which mainly controls the left side of the volcano curve[37]。 Therefore,changing the particle size and pore size of the support is the key to adjust the confinement size effect,thereby adjusting the selectivity and catalytic efficiency of the catalyst(Fig.3),which mainly includes the regulation of the pore size,shape and dimension of the porous or channel material。
图3 催化剂尺寸与催化效率的火山曲线图

Fig. 3 Volcanic curves of catalyst size and catalytic efficiency

In porous materials,the size of the zeolite product is affected by a large number of factors,such as impurities of the starting materials,slurry gel Si/Al ratio,pH,reaction temperature and time,formation of intermediate metastable phases,organic templates,nucleation and growth of more stable phases,and aging conditions[38][39]。 When Si/Al<5,SOD and other sodium aluminosilicate zeolites with low Si/Al ratios are usually formed;When Si/Al>5,high silica zeolite(BEA)and ZSM-11(MEL)are formed[40]。 Secondly,the aging treatment is beneficial to the formation of homogeneous molecular sieve Na-P1 with smaller particle size and larger specific surface area[41]。 different organic templates have Different interactions with zeolites.Compared with the mesoporous structure and slit-like large pore size of the products synthesized by tetrabutylammonium bromide(TBABr)template,the zeolites synthesized by tetrapropylammonium bromide(TPABr)template have better crystallinity and smaller particle size[42]。 At present,the development of methods that can improve the crystallization rate,simplify the treatment of residual solution after synthesis and improve the yield has become a research hotspot[43]。 Liang et al.Showed that the green synthesized nanosized small-pore zeolite SSZ-13 exhibited excellent catalytic performance in NH3-SCR due to a shorter mass transfer path,and its temperature window and resistance to SO2and H2O were superior to those of large-pore SSZ-13 prepared by traditional methods[44]。
For MOF,changing the reaction coordination,protonation/deprotonation,or adding surfactants can effectively control the dimension and size structure of the product[45]。 the coordination of the inorganic part determines the size and shape of the pore,the coordination number and the coordination orientation.A higher proportion of metal ions will increase the contact probability with organic molecules,and further make the metal ions and molecules better coordinated[46][47]。 At the same time,compared with monometallic nodes,the unique framework structure and mechanical stability of inorganic SBUs(secondary building units)also promote the development of high porosity MOF[48]; Organic linkers with different widths and functions determine the structure and morphological characteristics of MOF particles,and the use of mixed ligands to expand the metal carboxylic acid system is an effective way to obtain higher dimensional structures[46][49]。 the higher pH value in the synthesis system can make more deprotonated trimesic acid(BTC)ions coordinate with metal ions,which promotes the nucleation rate to increase and the crystal size to decrease[50]。 In addition,some cationic surfactants can be adsorbed on the negatively charged inorganic-organic building blocks as capping agents,thus inhibiting the conventional growth of MOF[51]; The electrostatic repulsion of anionic surfactants can provide sufficient growth and crystallization distance for MOF[52]。 In addition to the above three regulatory mechanisms,increasing the water/solvothermal reaction temperature can increase the coordination number of the central metal ion and the dimensionality of the MOF,while reducing the use of coordinated solvent molecules[53]。
In the channel structure,the confinement effect of titanate nanotubes(TNT)can shift the reduction peak to a lower temperature,which is beneficial to the enrichment of reduction potential and the adsorption of NH3molecules,and enhances the denitration activity of the catalyst at low temperature[54]。 In the hydrothermal synthesis of TNT,it is necessary to select the appropriate alkali treatment temperature and annealing temperature to maintain a highly stable tubular structure and control the size range[55,56]; When the pH value of the solution is close to neutral,TNT can show the largest specific surface area,the best species dispersion and the most active redox performance[57]。 In the synthesis of TNT by anodic oxidation,the increase of pH value can realize the synthesis of longer nanotubes[58]; the use of organic electrolyte will slow down The formation and chemical dissolution of oxides,and produce products with larger specific surface area[59]; Viscous electrolyte will inhibit the migration of fluoride ions and slow down the etching of TiO2,resulting in smaller nanotube diameter[60]; Grain boundary and dislocation are also the main factors that change the anodic oxidation reaction rate and ion diffusion coefficient,and the substrate with high roughness is beneficial to the formation and growth of nanotubes[61][60]; At the same time,the growth diameter of nanotubes increases with the increase of anodic oxidation potential[58]。 the introduction of an interlayer during the chemical template synthesis of TNT favors the formation of nanotubes,protects the fragile tubular structure,and prevents particle aggregation[62]。
The simple chemical composition and atomic bonding configuration of CNT make it show extremely rich diversity in structure.The main synthesis methods of CNT are arc discharge technology,laser ablation technology and chemical vapor deposition(CVD)[63][64]。 Sari et al.Studied the feasibility of synthesizing CNTs by arc discharge technique in liquid environment,and found that CNTs with length of 150μm or higher purity and quality could be prepared in deionized water and NaCl solution with different concentrations,and the length was proportional to the solution concentration[65]。 laser ablation technology involves the vaporization of solid target materials,and the product properties are affected by the material structure,chemical composition,synthesis environment,Laser characteristics,and the distance between the target and the substrate[66,67]。 with the increase of laser pulse power,the diameter of CNTs will decrease,while the diameter of CNTs will increase With the increase of furnace temperature[68,69]。 In CVD technology,the type and concentration of catalyst will affect the growth rate,morphology and growth space of CNTs.Reducing the concentration of catalyst can increase the growth space,yield and density of CNTs,otherwise,it will lead to the increase of product defects and the broadening of particle size distribution[70][71]。
It can be seen from the above that controlling the reaction conditions in the synthesis process of the confined carrier is an effective means to form the target confined dimension,regulate the pore size and overcome the shape selectivity,which lays a good structural foundation for the confined space encapsulation of active nanoparticles。

3.2 Regulation of confinement encapsulation effect

There are some differences in the way of encapsulation of nanoparticles with different confinement structures,and the feasibility of encapsulation will directly affect the stability and activity of catalysts[72]。
Among them,the in situ encapsulation method,also known as direct synthesis,mainly includes the intercalation and pyrolysis of active precursors.the specific method is to introduce metal precursors into the synthetic gel during the preparation of the carrier,so that they are confined in the pores of the carrier and react to form active centers[73]。 the results show that the in situ encapsulation method can absolutely capture the active sites in the CNT lumen,which ensures the reliability of the confinement effect results[72]。 Ning et al.Found that compared with direct pyrolysis,pyrolysis of Ni-MOF-74 after in-situ growth in the pores of mesoporous silica(SBA-15)could obtain Ni-PC composites confined in the pores,more highly dispersed,uniform size distribution and smaller particle size[74]。 in addition,the synthesis conditions of in situ encapsulation can affect the particle size and dispersion of the active species.When Wen et al.Anchored and encapsulated Co-MOFs by vacuum in-situ self-assembly method,they found that the ordered channels with larger pore size were beneficial to immobilize Co-MOFs and form smaller Co nanoparticles[75]。 At the same time,Wen et al.Also successfully converted the Co-MOFs@CeO2precursor into CeCoOxbimetallic oxides by precisely controlling the calcination temperature without destroying the ordered mesoporous structure,and found that with the increase of calcination temperature,the active Co species would migrate and aggregate,which would adversely affect the interaction between Ce and Co[76]。
Host-guest assembly refers to the physical or chemical combination of pre-synthesized catalyst active centers with supporting materials.Ring-shaped or cage-shaped carrier molecules are usually called"hosts",and metal ions or other small molecules are called"guests"[77]。 Typical metal precursors include metal salts and metal complexes,and the specific methods include wet impregnation,gas/liquid diffusion,and ion exchange[18]。 In the host-guest assembly process,catalysts with different active sites can be obtained by changing the ratio of support molecules to active sites.Sheng et al.Used the kinetically controlled coating method to cover the MnOxnanorods with different molar ratios of TiO2,and found that the obvious core-shell structure could be observed in the case of Mn/Ti=1:2,and the uniformly distributed TiO2shell protected the active center from the damage of toxic substances[78]。 At the same time,different synthesis temperatures will also affect the loading position of the active center.Low temperature can provide a mild environment for metal ions to be fixed in zeolite micropores or cages to form confined catalysts,while high synthesis temperature can easily cause nanoclusters to distribute on the outer surface of the carrier(Fig.4)[79]。 In addition to the reaction conditions,different preparation routes can also affect the particle size of the active substance and its dispersion on the confined support.Boutros et al.Found that compared with the ion exchange method,the Pd nanoparticles synthesized by the wet impregnation method had a smaller average diameter and were located inside the mesopores[80]。 It has been shown that when nanoparticles are introduced by wet impregnation,the weak interaction between the support and the active precursor leads to the local decomposition of active clusters during calcination,resulting in the formation of more dispersed and smaller nanoactive centers。
图4 合成温度对活性中心位置的影响[79]

Fig. 4 Influence of synthesis temperature on location of active center[79]

3.3 Regulation of restricted molecular sieve effect

In addition to controlled synthesis,modification of catalysts can increase the secondary pore size,change the pore volume,and increase the number of functional groups and defect sites,thus effectively controlling the molecular sieve effect of catalysts,enhancing the mass transfer efficiency and adsorption performance of catalysts,and inhibiting the reactant limitation of rigid molecular sieves[81]。
During the thermal aging process of the catalyst,the amorphous shell can be dehydrated and dehumidified to form a mesoporous structure with high specific surface area,thereby improving the activity of the catalyst.Wang et al.Found that the amorphous Al2O3shell could be converted intoγ-Al2O3with abundant mesopores during the high-temperature treatment at 800°C,which effectively reduced the loss of active particles and endowed the catalyst with relatively high oxidation reactivity[82]。
in addition to calcination and recrystallization,promoter modification is also one of the common means of catalyst modification.Among them,the structural promoter can improve the specific surface area,pore volume,pore size,mechanical strength and thermal stability of the catalyst.the results show that the use of template,dealumination in steam/acidic condition or desilication in high alkaline solution can delaminate the zeolite and increase the secondary porosity on the basis of the original zeolite,which is the main means to overcome the restriction of rigid zeolite on reaction molecules[83][84]。 Kwok et al.Corroded SiO2from the core by heat treatment in the presence of tetraphenylamine hydroxide(TPAOH)to form hollow layered zeolite crystals containing Fe nanoparticles,and additional mesopores were formed on the microporous structure due to the etching of TPAOH[85]。
Secondly,the functional promoter can change the surface properties of the catalyst.Sun et al.Used dopamine and 1,4-benzenedithiol(BDT)as polymerization monomers to functionally modify MOF,and found that the adsorption performance of MOF for metal sulfates was greatly improved,but the surface area and pore volume were decreased[86]。 Because the polymer coating has good stability and acid and alkali resistance,the catalyst material can obtain abundant functional groups and be protected by coating[87]。
The electronic promoter can adjust the outer electron distribution of the active component,which is beneficial to the adsorption and activation of the reactant.Tuan et al.Found that typical tannic acid(TAA)would release free proton（H+）to penetrate the original structure and convert the coordination framework of Co-MOF into a complex coordination network between it and Co2+,thus forming a hollow structure inside(Fig.5 )[88]。 The modified MOF has a higher degree of disordered carbon structure and more defect sites,which is beneficial to electron transfer and catalytic degradation.At the same time,the surface activity of CNTs catalyst can be fine-tuned by metal doping.In the catalytic conversion of N2O to N2,Pannopard et al.Found that the overlap of the d-orbital of the doped metal and the p-orbital of the N2O promoted the electron transfer between the nanotube and the N2O,which led to the redistribution of electrons between the catalyst and the reactant and reduced the reaction energy barrier[89]。
图5 （a）Co-MOF的化学蚀刻过程;（b~g）空心催化剂的SEM和TEM图[88]

Fig. 5 (a) chemical etching process of Co-MOF. (b~g) SEM and TEM images of hollow catalyst[88]

to sum up,adjusting the pore size of the support material,controlling the synthesis route and reaction conditions of the catalyst,and modifying the catalyst are effective strategies To control the confinement effect,which has a certain guiding role in the research and preparation of confined denitrification catalysts,and promotes the application of confined catalysts in low-temperature SCR reaction。

4 Application of Space Confinement Effect in Low Temperature Denitration

4.1 Enhanced adsorption properties of NH3

The chemisorption of NH3on the acidic sites on the catalyst surface is considered to be the first step of the NH3-SCR reaction,and the surface acidity of the catalyst is one of the determining factors for its SCR performance[90]。
The abundant functional group structure and pore hierarchy on the surface of the confined support increase the number and strength of the surface acid sites,thereby enhancing the adsorption of NH3.The results show that the abundant OH groups on the surface of TNT can be used as weak acid and medium-strong acid sites on the surface of the catalyst;The bent structure can cause more active atoms to be exposed on the lattice surface;The unique hollow tubular structure makes NH3molecules easy to enter and be trapped in the tubular framework,which accelerates the NH3-SCR reaction[91]。 Li et al.Used hydrothermal technique to introduce a MnOxcore into TNT,and then replaced part of the high-valent MnOxto generate a uniform Fe2O3nanosheet shell by an electrocouple replacement reaction,and the obtained core-shell catalyst maintained good catalytic performance in the temperature range of 150–360°C[92]。 Due to the interfacial effect between TNT and Fe2O3,the strong component interaction of MnOxis adjusted,and a large number of porous Fe2O3provides more acid sites for the reaction,and the acid sites of the catalyst increase gradually with the increase of Fe addition 。
In addition,the combination of surface modification and mesoporous confinement is one of the common means to manipulate the charge characteristics of the catalyst and enhance the adsorption of NH3molecules.Guo et al.Found that by coating a thin layer of titanium dioxide inside the silica mesopores,repeated coating improved the Brønsted acidity of the thin layer of titanium dioxide and expanded the coverage in the pore channels,so that the electronic state of Ti was regulated,and NH3molecules could be stably adsorbed onto the M-O-NH4+[93]。 A large number of structural defects on the layered TiO2make it obtain more adsorbed oxygen,and the low-temperature denitrification performance increases with the increase of coating thickness 。

4.2 Enhanced selectivity of denitration reaction

the confined environment regulates the selectivity of the reaction by imposing specific orientations and conformations on the reactant molecules to induce energetic and kinetic changes in the catalytic reaction[94]。 In the fast SCR reaction,the thermal decomposition of ammonium nitrate formed by the reaction of NO2adsorbed on the catalyst surface with NH3is the main way of N2O release at low temperature[95]。
The confined structure can prevent the formation of N2O by stabilizing the intermediate products in the denitration reaction process and blocking their decomposition.Chen et al.Showed that the selectivity of N2over small-pore Cu-CHA zeolite catalyst was higher than that over large-pore Cu-BEA zeolite catalyst,which was mainly due to the low activity of surface nitrate groups formed after Cu-CHA catalytic oxidation of NO[96]。 Meanwhile,the smaller pore opening of the Cu-CHA structure also increases the additional constraint of NH4NO3in the zeolite cage,thereby enhancing its thermal stability,and the high stability of NH4NO3contributes to the reduction of N2O generation over the Cu-CHA catalyst.In addition,microporous TiO2can also be stably decomposed into the reaction intermediate of N2O,so that its decomposition occurs at a higher temperature,effectively improving the N2selectivity of the reaction[97]。
the special closed space of the confined catalyst will affect the redox performance of the active center,and then affect the selectivity of the catalytic reaction products.When the inner diameter of CNT is reduced to 2 nm,the confinement effect will inhibit the oxidation reaction on the inner surface and promote the reduction reaction[98]。 From Kaptei's point of view,the formation of N2O can be explained by the oxidation of NH3to produce—NH or—N cannot react with NO to produce N2and can only leave the surface as N2O[99]。 Therefore,the confinement effect can inhibit the peroxidation of NH3to prevent the formation of N2O and improve the selectivity of the reaction.Chen et al.Found that compared with the traditional TiO2nanoparticle catalyst,in the TNT nanoconfined space,the XPS spectrum was shifted to the direction of higher binding energy,and the redox performance of the catalyst was inhibited,which led to the reduction of N2O production[26]。 Zhao et al.Confined the manganese oxide active species in Ho-modified titanium nanotubes,and the pore confinement effect of Ho-TNT on Mn increased the dispersion of Mn and promoted the interfacial effect-induced electron transfer between Mn and Ho to inhibit the generation of Mn4+,thereby promoting the redox ability and reducing the generation of by-products[100]。 Ho-TNTs@Mn has good catalytic activity and toxicity resistance,and can achieve more than 80%NO conversion and 100%N2selectivity in the range of 80~300℃ 。

4.3 Enhanced catalyst resistance to toxicity

In the denitration process,due to the existence of O2,NH3is easy to react with SO2to form NH4HSO4,which deposits on the surface of the catalyst to block the active sites,and the lower the reaction temperature is,the greater the negative impact of SO2is.Xu et al.Found that the main reason for the poisoning and deactivation of the Ce/TiO2catalyst was that SO2could react with the catalyst to produce Ce(SO4)2and Ce2(SO4)3with high thermal stability,which led to the destruction of the redox cycle between Ce(Ⅳ)and Ce(Ⅲ)and inhibited the formation and adsorption of nitrate[101]。 Therefore,inhibiting the formation of sulfate species and neutralizing the toxic species is the key to improve the catalyst lifetime。
The structural and chemical properties of the confined support itself can hinder the formation and deposition of sulfate species and prevent water attack on the interior of the catalyst.It was found that due to the synergistic effect of the tightness,hydrophobicity,acidity and redox properties of the zeolite ZSM-5 shell,the MnCeOx@Z5 catalyst showed strong water and sulfur resistance,hindered the formation of sulfate species,and had the highest N2activity and selectivity[102]。 At the same time,the abundant OH groups on the surface of the carrier also have a certain neutralization effect on toxic substances,which effectively reduces the competitive adsorption of toxic substances.Shi et al.Found that the unique hollow nanotube structure could effectively protect the active center on the inner surface from the poisoning of SO2or alkali metals[103]。 A large number of OH groups act as binding centers of SO2and alkali metals on the surface of the catalyst,resulting in the formation of sulfate species mainly in the form of surface sulfates,which delays the combination of alkali metals and Mn-Co active centers,thus delaying the poisoning process,so that the catalyst can also achieve excellent denitrification performance at 150℃in the presence of H2O,SO2and alkali metals 。
In addition,the restriction of the active material by the limited carrier will change the charge of the active center,which will affect the anti-poisoning performance of the catalyst.Xu et al.Showed that the confinement effect of TNT on Cu ensures the appearance of more active Cu+sites,which greatly weakens the adsorption and oxidation process of SO2,thus protecting the active component from SO2poisoning[104]。 The outstanding sulfur resistance of the catalyst is realized by the special structure-activity relationship that the confinement effect effectively adjusts the electron cloud density of the Cu surface.Meanwhile,Huang et al.Realized the presulfidation of NH3-SCR MnFe-TOS catalyst by simple hydrolysis of TiOSO4[105]。 The TiO2lattice distortion caused by the insertion of SO42−and the interface confinement effect disturb the local electron density,which strengthens the interaction between Mn-O-Fe,resulting in a large number of defects and surface acid sites,effectively inhibiting the adsorption and oxidation of SO2on the catalyst in the SCR process.The NO conversion of MnFe-TOS can reach more than 80%in the range of 180~380℃,and the selectivity of N2can be maintained at more than 90%in the whole temperature range 。

4.4 Denitration activity of enhanced catalyst

Catalyst activity is the core part in the reaction process.In the low temperature NH3-SCR reaction process,the catalytic activity is usually measured by NO conversion,selectivity and the active temperature zone of the reaction 。
As mentioned in Chapter 2,the enhancement of denitration catalyst activity is also inseparable from the promotion of interfacial electron transfer ability and the improvement of active species dispersion,which correspond to the improvement of redox ability and the effective utilization of active sites,respectively.Su et al.Found that due to the electronic interaction between the MnOxloaded in the channel and the inner surface of the nanotube,the catalyst had better oxygen supply ability and NO adsorption ability,showing higher NOxconversion[106]。 Li et al.Confined the active components such as Ce and W in the ordered mesoporous TiO2structure to form a framework-confined ordered mesoporous structure,which provided better specific surface area and abundant active sites for the catalyst.The confinement effect enhances the interaction between Ce,W and Ti,thereby increasing the frequency of electron exchange.The NO conversion rate exceeds 96%at 220℃,and the catalyst also shows excellent catalytic activity at higher temperatures[107]。
The adsorption performance of NH3is the decisive factor to promote the SCR reaction,the removal rate of NO and the selectivity of N2are the main criteria to measure the denitration activity of the catalyst,and the anti-poisoning performance is the key to ensure the stability and durability of the catalyst.In the process of strengthening low-temperature denitration,the strengthening of these three properties is indispensable,and the three together promote the improvement of denitration activity of the catalyst 。

5 Conclusion and prospect

To sum up,the formation of confinement effect of low-temperature NH3-SCR denitration catalyst can be attributed to the increase of molecular orbitals of confined molecules due to the influence of space quantum effect in confined space of different dimensions,thus changing the charge transfer and distribution ability of the catalytic system.The energy barrier in the reaction process is reduced,and the deactivation of active centers due to agglomeration is inhibited,thereby enhancing the product selectivity of denitration reaction,the adsorption performance of NH3and the toxicity resistance of the catalyst,thereby improving the low-temperature denitration activity of the catalyst(Fig.6).The confinement effect is highly dependent on the structural morphology,pore size,dispersion and stability of the active center of the confined support,which provides a certain guidance for the design and synthesis of low-temperature denitrification catalysts with space confinement.In this paper,the influence of spatial confinement effect on the catalytic reaction and its control strategy are discussed,and then its application in the denitrification process is summarized.Compared with the commonly used surface supported catalysts,the confined catalysts have broader prospects,but there are also some barriers,mainly in the following three aspects 。
图6 限域催化剂促进低温脱硝总结概念图

Fig. 6 Summary concept diagram of low-temperature denitration promoted by limited catalyst

(1)Due to the complexity and diversity of the spatial confinement structure,the mechanism of charge change and atomic migration is difficult to clarify,and the theoretical study of the confinement effect is limited;
(2)the high surface energy of single atom limits the loading of atomic-scale active centers,and the synthesis and denitrification application of confined single atom catalysts are still facing great challenges,and the service life of single atom catalysts is low due to the complexity of denitrification environment;
(3)the improvement of stability and toxicity resistance often requires the inhibition of oxidative adsorption of toxic substances,which also has a certain impact on the conversion and selectivity of the catalyst,making it difficult to balance the low-temperature activity and stability of the catalyst。
Therefore,in the development of confined catalysts,It is necessary to further develop new catalyst synthesis pathways and explore different functions and structural modifications of confined supports to ensure the dispersion of active sites and improve the utilization of active atoms;it is necessary to continue to improve the catalytic performance and reduce the competitive adsorption of toxic substances on the active sites by combining other effects with confinement effects,so that the stability and activity of the catalyst can be improved simultaneously during the reaction.At the same time,focusing on specific reactions and atoms from the micro level,exploring their electronic structure and energy changes is an effective way to clarify the reaction mechanism and provide a theoretical basis for experiments。

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