Supported catalysts for simultaneous removal of SO2,NOx, and Hg0 from industrial exhaust gases:A review

Ke Zho,Xin Sun,Chi Wng,Xin Song,Fei Wng,*,Ki Li,*,Ping Ning

a Faculty of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

b Faculty of Chemical Engineering,Kunming University of Science and Technology,Kunming 650500,China

Keywords:Supported catalysts Simultaneous removal SO2,NOx and Hg0 Industrial exhaust gases

ABSTRACT The simultaneous removal of SO2,NOx and Hg0 from industrial exhaust flue gas has drawn worldwide attention in recent years.A particularly attractive technique is selective catalytic reduction,which effectively removes SO2,NOx and Hg0 at low temperatures.This paper first reviews the simultaneous removal of SO2,NOx and Hg0 by unsupported and supported catalysts.It then describes and compares the research progress of various carriers,eg.,carbon-based materials,metal oxides,silica,molecular sieves,metal-organic frameworks,and pillared interlayered clays,in the simultaneous removal of SO2,NOx and Hg0.The effects of flue-gas components(such as O2,NH3,HCl,H2 O,SO2,NO and Hg0)on the removal of SO2,NOx,and Hg0 are discussed comprehensively and systematically.After summarizing the pollutantremoval mechanism,the review discusses future developments in the simultaneous removal of SO2,NOx and Hg0 by catalysts.

1.Introduction

Pollutants,such as sulfur dioxide (SO2),nitric oxides (NOx),trace heavy metals and particulate matters (PMs) are typical components in industrial exhaust flue gas,which are harmful to both the environment and human health.SO2and NOxare the main precursors of acid rain and toxic chemical smog [1,2].Elemental mercury (Hg0) is among the most toxic heavy metals owing to high toxicity,volatility and bio-accumulation[3].The flue gas contains three mercury species:Hg0,oxidized mercury(Hg2+),and particle bound mercury(Hgp)[4].Most of the Hgpand Hg2+can be effectively collected by conventional air-pollution control devices (APCDs) such as electrostatic precipitators,fabric filters and wet flue-gas desulfurization (WFGD) equipment,which are broadly installed in plants around the world [5].However,Hg0is relatively stable in the atmosphere,and (due to its chemical inertness,water insolubility,and volatility) is hardly removed by existing APCDs [6].

The current techniques for removing SO2,NOxand Hg0are selective catalytic reduction(SCR)[7,8],WFGD[9],and activatedcarbon injection [10,11].However,individual pollutant-treatment systems are expensive to construct and operate,complex,prone to instability,and require a high occupational area [12].The current technologies for simultaneous removal of SO2,NOxand Hg0include carbon-based material adsorption [13-15],photocatalytic oxidation [16,17],wet scrubbing [18-20],NH3-SCR [7,8]and nonthermal plasma(NTP)technologies[21,22].Carbon-based material adsorption and wet scrubbing have the disadvantages of secondary pollution and high investment costs.Photocatalytic oxidation and NTP have the disadvantage of limited industrial applications.Compared with these technologies,NH3-SCR catalytic technology has the advantages of high efficiency,low energy consumption and low operating cost.This makes the catalytic method to remove SO2,NOxand Hg0has become the main research direction for air pollution control.Initially,this research focused on SO2,NOxand Hg0removal by catalysts without carriers,but the removal effect was non-ideal[23].In later studies,the SO2,NOxand Hg0removal was improved by loading active components (transition metals,rare metals,precious metals)on various carriers(carbon materials,metal oxides,metal-oxide frameworks (MOFs),zeolites) [24-28].This review concludes the advantages and disadvantages of simultaneous removal methods,introduces the catalysts prepared by conventional and new carriers,and discusses the lowtemperature performances of the catalysts.The mechanism of pollutant removal by catalysts is also discussed,as well as the possible research directions and future prospects.

2.Removal technologies

Simultaneous removal technologies include carbon-based material adsorption [13-15],photocatalytic oxidation [16,17],wet scrubbing [18-20],NH3-SCR [7,8],and non-thermal plasma(NTP) technologies [21,22].The removal techniques are summarized in Fig.1.In carbon-based material adsorption,pollutants are often removed by modified activated carbon,active coke,or activated-carbon fibers,which have porous structures and strong adsorption capacities.However,carbon-based material adsorption is limited by high operating cost and quality degradation of fly ash[29].Under UV/visible radiation,pollutants are oxidized by positively charged holes on the photocatalyst surface.A photocatalytic oxidation system can be worked at relatively low temperature and pressure [30],but the UV light source raises the energy consumption.In wet scrubbing technology,the oxidant solution in the scrubbing system(such as NaClO2,KMnO4/NaOH or H2O2)can oxidize NO and Hg0to soluble NO2and Hg2+,which can then be removed by chemical solutions.However,this method produces large volumes of wastewater and high secondary pollution.NTP is a recently developed gas-phase oxidation method that removes multi-pollutants from flue gas [31,32].NTP technologies remove pollutants quickly and efficiently,operating at atmosphere pressure and room temperature with no secondary pollution [33,34].Their defects are high energy consumption and limited amount of flue gas;moreover,the high-voltage electricity poses a safety hazard that limits its industrial application [35].Synergistic removal of NO and Hg0by existing NH3-SCR systems promises to overcome the disadvantages of previous simultaneous multi-pollutant-removal technologies.In the NH3-SCR system,NO is reduced to N2and H2O by NH3,and Hg0is oxidized to Hg2+by O2.The generated Hg2+attaches to PM surfaces and is finally removed by electrostatic precipitators(ESP)or wet flue-gas desulfurization(WFGD) [36].The NH3-SCR system improves the purification efficiency and lowers the operating costs of industrial exhaust fluegas purification,without generating secondary pollution[12,37,38].The present article systematically reviews the catalyst types,the key factors controlling the reaction processes,and the proposed reaction mechanisms of NH3-SCR technologies.

Fig.1.Industrial exhaust gases source and simultaneous removal of SO2,NO and Hg0 technology.

3.Simultaneous catalytic removal

The existing catalysts for simultaneous removal of NO and Hg0have been studied in the absence [23,39,40]and presence of carriers.Carriers can be divided into two main categories:conventional and new.Carbon-based materials [15,41-44],metal oxides[26,27,45-47]and silica[48,49]are among the conventional carriers,whereas molecular sieves [50-53],MOFs [28,54]and pillared interlayered clays (PILCs) [24,55]are new carriers.This paper reviews the differences between the conventional and new supported catalysts in the simultaneous removal of SO2,NO and Hg.Most supported metal-oxide catalysts are prepared by the impregnation method.In the presence of different carriers,the same active component can exhibit very different activities.However,as most catalysts are studied only in the laboratory,more research on their practical applications is needed.

3.1.Catalysts

3.1.1.Unsupported catalyst

Many catalysts used to remove NOx,SO2and Hg0have been reported in the literature.Among the catalysts without carriers[23,39,40]are the CeO2-TiO2(CeTi)catalysts of Li et al.[39],which oxidize elemental mercury.They found that when the weight ratio of CeO2/TiO2is 1:2,the CeTi catalyst exhibits higher Hg0oxidation activity in 150-250°C,indicating that their superior performance was conferred by the high focuses of surface cerium and oxygen.Zhang et al.[23]researched TiO2-Al2O3-CeO2materials synthesized by the sol-gel method,which simultaneously remove NO and Hg0.They found that TiAl10Ce20nanoparticles supplemented by 10 wt%Al2O3removed NO and Hg0with superior efficiency(93.41%and 80.54%,respectively) in an SCR atmosphere at 300°C.Song et al.[56]studied the effect of Mn,Ti and Cu doping on simultaneous removal of NO and SO2of Fe2.5M0.5O4catalyst.The results show that the doping of the three metal oxides can significantly increase the removal rate of NO,but has no effect on the removal rate of SO2,because SO2can be completely removed under most conditions.Sun et al.[57]prepared a series of MFe2O4(M=Mn,Zn,Cu,Ni,Co) catalysts to remove NO and SO2by hydrothermal method.They found that the removal rate of SO2at room temperature can reach 100%,and the CuFe2O4catalyst has the highest activity compared to other catalysts.Shi et al.[58]studied the natural ferrous manganese ore to simultaneously remove NO,Hg0and toluene from flue gas.Activity evaluation experiments show that the maximum efficiencies of NO,Hg0oxidation are 95.29%,83.0%.The characterization results show that the oxidation activity of manganese ore is mainly provided by Mn4+and Fe3+in it.

3.1.2.Catalysts based on conventional supports

Carbon-based catalysts:Carbon-based materials are the most commonly used adsorbents,which are often used to remove multiair-pollutant from flue gas,including activated carbon,activated coke and activated carbon fiber[59-61].Using an activated-carbon carrier,Song et al.[62]prepared 1%M-6%Y/AC(where M is Ni,Al or Mo) by equal-volume impregnation.The catalytic activity of simultaneous SO2and NOxremoval was maximized on the 1%Ni-6%Y/AC catalyst.Sumathi et al.[63]prepared a palm shell activated carbon(PSAC)adsorbent and loaded several metal oxides(Ni,V,Fe and Ce)on the ride to simultaneously remove SO2and NOx.Among their sorbents,CeO2/PSAC yielded the maximum sorption capacity for the simultaneous removal of SO2and NOx.Sun[64]studied the catalytic activity of copper and iron-loaded microwave coconut shell activated carbon (MCSAC) with different loadings for simultaneous denitrification and mercury removal.The 15%Cu-5%Fe/MCSAC catalyst exhibited the best removal efficiency.The NOremoval rate was 86% and the mercury removal rate was 61% at 225°C.In addition,effect of NH3concentration on removal of NO and Hg0was investigated,as shown in Fig.2a.

Zhang et al.[65]studied the removal effect of FeCeOx/CNTs catalyst for NO and Hg and the SO2tolerance.They found that the NO and Hg removal rates of the catalyst were only 15.44% and 16.13%.Coating FeCeOx/CNTs catalyst with a layer of SiO2can significantly improve the activity and SO2tolerance of the catalyst.Li et al.[66]used activated coke as a medium to simultaneously remove SO2and NOxfrom flue gas.It is found that AC still has high desulfurization and denitration efficiency after regeneration cycle.Zhao et al.[67]employed a CuO-MnOx/AC-H catalyst for the simultaneous removal of Hg0and NO from flue gas.They pointed out the significant role of the reaction temperature in physisorption and chemical oxidation.The optimal temperature for simultaneous removal of Hg0and NOxwas 200°C.At this temperature,the 8%CuO-5%MnOx/AC-H catalyst achieved simultaneous removal efficiencies of 90%for mercury and 78% for NOx.Zhu et al.[68]synthesized a series of V2O5-xCeO2/AC catalysts to investigate the capabilities of the catalysts to simultaneously remove Hg0and NO in the temperature range from 120°C to 240°C.They discovered through experiments that compared to 1 V/AC and 1 V-8Ce/AC,8Ce/AC catalyst has the highest Hg0removal performance (EHg>98%) at 120-240°C.The 1 V-8Ce/AC catalyst showed the best NO conversion performance compared to 1 V/AC and 8Ce/AC,8Ce/AC,and the ENOincreased gradually from 55% to 86.9% with the increase of temperature (120-240°C),as shown in Fig.2b.Zhu et al.[44]used ammonium halide modified rice husk char to remove gaseous mercury.They found that the RHC adsorbent impregnated with ammonium bromide has excellent mercury removal performance,and can still reach an efficiency of about 80%for removing gaseous mercury.In addition,they found that this enhanced HgO removal adsorbent can synergistically remove SO2and NO.Gao et al.[69]studied a series of CoOx-CeO2loaded biomass activated carbon(CoCe/BAC)derived from maize straw.When removing both NO and Hg0at 230°C,the 15%Co0.4Ce0.6/BAC sample achieved 96.8% Hg0removal efficiency and 84.7% NO removal efficiency.Based on characterization analyses,the author attributed the best performance of 15%Co0.4Ce0.6/BAC to its largest specific surface area,lowest crystallinity and strongest redox ability than other samples in the series.The NO removal was unaffected by low SO2concentration in the gas stream,and the NO conversion efficiency reached 80%,as shown in Fig.2c.

Metal-oxide-based catalysts:Titania has stable physical and chemical properties,is non-toxic,and its surface area is below 100 m2/g[70].TiO2has a certain acidity which enables it to be the main site of NH3adsorption in the NH3-SCR reaction and thus beneficialtotheremovalofNOx.Therefore,TiO2isacommonsupport widely used in NH3-SCR reaction.The most prevalent commercial catalyst is V2O5-WO3/TiO2,which delivers high activity but is active only within a narrow temperature window(300-400°C)[71].Zhao etal.[46]found that adding CeO2tothe V2O5-WO3/TiO2catalyst can improve its catalytic activity for Hg0oxidation without using HCl.The co-catalyst CeO2shows satisfactory oxidation activity and improved water oxidation resistance.The promoted catalysts showed satisfactory oxidation activity and improved H2O resistance.Wang et al.[72]studied the oxidation catalytic performance of VWTi catalysts with different loadings of CuO.The results show that the 7%Cu/VWTi exhibited high Hg0oxidation and a desired NO removal efficiency at 280-360°C.Meng et al.[73]prepared a series of Mn-V-W/TiO2(Mn-VWT)catalysts by impregnation to simultaneously remove Hg0and NO.The results show that Mn-VWT-400 has the best simultaneous removal performance.Its removal efficiency of Hg0and NO reached 100%and 82%,respectively.Yang et al.[74]used CeO2to modify V2O5/TiO2for simultaneous removalof NO and Hg0.Experiments show that CeO2modification not only significantly improves the SCR performance,but also significantly enhances the oxidation of Hg0.Wang et al.[75]investigated the sulfated CuCl2/TiO2catalyst to simultaneously remove NO and Hg.By characterizing the catalyst before and after vulcanization,it was found that the CuSO4generated on the catalyst increased the number of acid sites on the catalyst surface,thereby improving the SCR activity.CuCl2(S18)/TiO2catalyst showed simultaneous catalytic oxidation of Hg0(73.9%) and NO (87%) at the temperature of 325°C.Wang et al.[76]synthesized CeO2supported on anatase TiO2with high-energy facets and used it in the NH3-SCR reaction.In the activity evaluation experiment,it was found that compared with Ce/P25 catalyst,it has high SCR activity.Through the characterization,it is concluded that the unique feature of the active energy(001)facet improves the thermal stability of CeO2,and the presence of Ti3+on the surface of TiO2effectively promotes the development of the SCR process,both of which lead to the remarkable catalytic performanceforthecatalyst.Chenetal.[77]studiedtheSCRactivity of MnCe oxide-supported titanate nanotubes(TNT).And compared with the conventional MnCe/TiO2and V2O5/TiO2nanoparticle catalysts,it is found that MnCe/TNTs catalysts have higher SCR activity,sulfur and water resistance.The characterization results show that the BET surface area of MnCe/TNTs is larger,and the adsorption capacity of NH3is greatly enhanced,which is beneficial tothe SCR activity.Liu et al.[78]designed the MnSmCo/Ti catalyst to simultaneously remove NO and Hg0at low temperature.The experimental results show that the catalyst exhibits NO conversion rate of 80% and Hg0removal rate of 100% at a gas hourly space velocity of 100,000 h-1in the temperature range of 150-250°C.Li et al.[79]prepared a series of molybdenum (Mo)-modified vanadium (V)-based selective catalytic reduction (SCR) catalyst samples using the impregnation method to remove elemental mercury(Hg0)and nitrogen oxides(NO)at the same time.Through experiments,it is found that the metal loading and the order of impregnation have no effect on the denitration activity of the catalyst,and have a particularly large effect on the mercury removal activity.The dipping sequence experiment found that MoO3(7) +WO3(3) →V2O5(0.5)/TiO2exhibited the highest mercury removal activity.The metal loading experiment found that V2O5(0.5) +MoO3(7)/TiO2catalyst exhibited the highest mercury removal activity.Liu et al.[80]studied the simultaneous removal of NO and Hg with Sb-modified Mn/TiO2catalyst under low-temperature SCR conditions.The results show that the MnSb/TiO2-0.25 catalyst can achieve NO removal (>90%) and Hg0oxidation (>80%) at 200-300°C.Li et al.[26]studied The effect of flue gas composition on the removal of Hg0and NO by a series of CeO2(ZrO2)/TiO2catalysts.At 250-400°C,the CeO2(ZrO2)/TiO2catalyst exhibited high Hg0catalytic oxidation efficiency(>95%)and high denitration efficiency(>95%).Liu et al.[45]prepared Co/TiO2catalysts by the sol-gel method,and determined the optimal Co loading for Hg0catalytic oxidation(7.5%).Yang et al.[81]prepared TiCe0.25Sn0.25Oxcatalystforsynergetic removalofNOandHg0fromfluegas.Through experiments they found TiCe0.25Sn0.25Ox has excellent NO and Hg0removal performance,and also has satisfactory SO2tolerance,and revealed the synergistic removal mechanism and the effects of flue gas components on synergetic removal of NO and Hg0,as shown in Fig.3a.Zhang et al.[82]loaded Co-Ce oxide on rod-like MnO2to simultaneously remove NO and Hg.They found that the Co-Ce/MnO2catalyst has excellent SCR activity and Hg capture efficiency.In addition,the Co-Ce/MnO2catalyst has a high tolerance to SO2.Finally,the excellent activity and high sulfur resistance of the catalyst are explained through characterization techniques,as shown in Fig.3b.Zhang et al.[83]researched Ce-Mo/TiO2catalyst to simultaneously remove NO and Hg.Ce-Mo/TiO2-0.3 catalyst exhibited an excellent Hg0oxidation and NO reduction efficiencies of over 90% at above 250°C.In addition,Ce-Mo/TiO2-0.3 catalyst also presented an excellent resistancetoSO2and H2O poisoningand agood stabilityandrecyclability.Theevaluationdata of thecatalytic activity of Ce-Mo/TiO2is shown in Fig.3c.Liu et al.[84]found that loading Nb on MnTiOxcan significantly improve the conversion ofNO and the oxidation of Hg by the catalyst.The removal efficiencyof different catalysts for NO and Hg are shown in Fig.3d.In addition to TiO2,MnO2is also used as a carrier for simultaneous removal of NO and Hg0from flue gas.

Fig.2.(a) Effect of NH3 concentration on removal of NO and Hg0 for 15%Cu-5%Fe/MCSAC catalyst.Reproduced with permission [64].Copyright 2019,Elsevier.(b)Simultaneous removal Hg0 and NO over 1 V/AC,8Ce/AC,and 1 V-8Ce/AC at different temperatures.Reproduced with permission [68].Copyright 2019,American Chemical Society.(c) Effect of active ingredient on removal of NO and Hg0 for 15%Co0.4Ce0.6/BAC.Reproduced with permission [69].Copyright 2018,Elsevier.

Fig.3.(a) Synergetic removal of elemental mercury and NO over TiCe0.25 Sn0.25Ox catalysts.Reproduced with permission [81].Copyright 2019,Elsevier.(b) Simultaneous removal of NO and Hg0 in flue gas over Co-Ce oxide modified rod-like MnO2 catalyst.Reproduced with permission[82].Copyright 2020,Elsevier.(c)The Mo-Ce/TiO2 catalyst

As a stable crystalline powder,Al2O3has good thermal stability,acid and alkali corrosion resistance,easy to form a fixed shape,and the surface area is less than 200 m2/g.These ideal properties have attracted extensive attention in the field of simultaneous removal of SO2,NOx,and Hg0.Wang et al.[47]prepared Cu and Fe-loaded Al2O3catalysts for simultaneous removal of NO and Hg0.The characterization results show that Al2O3as a support significantly increases the surface area and pore volume of the catalyst,improves the ability of the catalyst to adsorb gaseous reactants and therefore improves their catalytic efficiency.Moreover,Al2O3increased the absorbed oxygen amount,indicating an increased number of active oxidative sites and a stronger oxidation ability for Hg0.Yue et al.[85]prepared a series of CuaCebZrcO3/γ-Al2O3catalysts by impregnation method to simultaneously remove NOxand Hg0.Their research results show that 15% Cu1.4Ce0.55Zr0.25O3/γ-Al2O3has the highest removal efficiency of NO (93%) and Hg0(98%).

Silica based catalysts:Mesoporous silica has a large specific surface area,unique open-framework,a large pore volume and a high intrinsic surface activity,providing many adsorption sites for NO,SO2and Hg0.Chiu et al.[86]used the impregnation method to load V,Mn and Cu oxides onto SiO2to remove NO,SO2and Hg.The results show that the CuOx/SiO2catalyst exhibits the greatest Hg0and NO removal performance.In addition,he also studied the removal of Hg0and NO by MnOximpregnated V2O5-WO3/TiO2-SiO2catalyst[87].The catalysts loaded with 10%MnOxshowed the best activity for the removal of Hg0and NO.Liu et al.[49]loaded MnOx-CeOxon SiO2to remove NO and Hg.Through the activity evaluation of different loadings of MnOx(a%)-CeOx(b%)/SiO2,it is found that 20%MnOx-10%CeOx/SiO2has the best catalytic activity,and its mercury removal efficiency is 96%at 100-300 ℃.Secondly,they also found that the introduction of CeOxcan broaden the SCR activity temperature range of MnOx/SiO2catalysts,as shown in Figs.4a and b.Gu et al.[88]studied the catalytic NO removal efficiency of MnFeOx/SiO2catalysts under different Fe/Si conditions.They found that adding an appropriate amount of FeOxcan significantly widen the temperature window of the catalyst.Li et al.[89]prepared a TiO2-SiO2photocatalyst for the catalytic oxidation of Hg.Studies have shown that the catalyst has excellent catalytic oxidation activity for Hg,and the Hg removal mechanism is summarized,as shown in Fig.4c.Chen et al.[90]used the recycled Cu modified rice-husk-derived SiO2to remove NO and Hg.Studies have shown that 50% Cu/SiO2has the greatest Hg removal efficiency (88.2%),and 50%Cu-10%Ce/SiO2has 85% NO conversion at 100-300°C.Lin et al.[91]synthesized Cu-Mn-Ce oxideincorporated Mesoporous Silica by hydrothermal exfoliation of silicate to remove NO and Hg.The research results show that Cu2Mn8exhibits the highest Hg removal efficiency,and Ce doping can improve the NO removal efficiency.

3.1.3.Catalysts based on new supports

Molecular sieves-based catalysts:Zeolite-based materials have large specific surface area,good thermal stability,unique framework structure,excellent mechanical strength and adsorption,so they are considered as ideal carriers.Zeolite molecular sieves can chemically adsorb SO2,Hg and NO on the surface.The metal elements loaded on the surface of the zeolite molecular sieve and embedded in its pores can promote the oxidation or reduction of SO2,Hg and NO.Cao et al.[53]loaded Fe3O4,silver and V2O5on HZSM-5 by dipping.The experimental results show that at 150°C,the removal rate of Hg by the catalyst is 97%,the removal rate of Hg0and NO is 80%.Finally,the mechanism of simultaneous removal of NO and Hg by the catalyst is explained,as shown in Fig.5a.Yuan et al.[92]prepared nano-iron loaded ZSM-5 and combined vaporized H2O2and Na2S2O8,and Ca(OH)2solution to simultaneously remove SO2,NO and Hg0.Their research results show that when the mass of the catalyst is 0.3 g,the removal rate of SO2,NO and Hg0reaches the highest,respectively 99.9%,96.5%and 90.1%.Fan et al.[93]prepared Cu/HZSM-5 to simultaneously remove Hg0and NO.The experimental results show that the loading of copper significantly improves the activity of the catalyst,and the catalyst with a loading of 6%shows the best activity.Zhang et al.[67]used ZSM-5 modified with Mn-Fe to remove Hg at low temperatures and found that the introduction of Mn/Fe can effectively improve the mercury removal efficiency of ZSM-5.The catalyst with a Mn/Fe molar ratio of 1:2 has a 100%removal rate for Hg at 200-250°C.Applying an immersion method,Chui et al.[51]used the CuCl2/MCM prepared by the impregnation method is usedfor the simultaneous removal of SO2and NO.The experimental results show that the loading of CuCl2reduces the removal efficiency of SO2by 4%,but increases the removal efficiency of NO by 11%,as shown in Fig.5b.

Fig.4.(a) Removal of NO with MnOx-CeOx/SiO2.Reproduced with permission [49].Copyright 2017,Elsevier.(b) Removal of Hg0 with MnOx-CeOx/SiO2.Reproduced with permission [49].Copyright 2017,Elsevier.(c) Mechanisms of Hg capture and reemission on the surface of SiO2-TiO2 nanocomposite.Reproduced with permission [89].Copyright 2006,Elsevier.

Silico-aluminum-phosphate (SAPO) is a microporous crystal composed of three kinds of tetrahedral units,namely,PO4+,AlO4-and SiO2.Its pore structure can be controlled by changing different synthesis conditions and silicon content[94,95].SAPO are widely used as adsorbents,catalysts,and catalyst carriers.It is a new type of catalytic material with excellent shape selectivity,thermal stability and damp-heat stability.Because the framework is negatively charged,it has exchangeable cations and also exhibits proton acidity.SAPO can adjust the amount of acid by introducing various impurity atoms,so it can exhibit a catalytic performance ranging from medium strong acid to strong acid.It is well known that copper additives can improve the activity of SAPO-34,and copper-infused SAPO is very promising in industrial applications.Fickel et al.[94]showed that the NO conversion efficiency of Cuinfused SAPO approaches 100% at 200-400°C.Wang et al.[50]found that the content of Si and Al affects the content of Cu2+species on the Cu/SAPO-34 catalyst.In addition,the content of Si can affect the content of active sites (Si or Al) and the number of acid sites of the catalyst.To affect the SCR activity of the catalyst.Pang et al.[52]prepared Mn-exchanged SAPO-34 catalysts for lowtemperature SCR of NO with NH3.Among their samples,4 Mn/SAPO-34 achieved the highest SCR activity in the 120-210°C temperature range.The characteristic results suggested that species related to Mn3+and Mn4+were the active sites of the catalyst.

MOFs-based catalysts:MOFs are intriguing porous materials with the advantage of adjustable pore size that are self-assembled by organic ligands and metal ions [96].The adsorption performance of MOFs is boosted by sufficient porosity,large surface area,and versatile structure.In addition,MOFs promote the dispersion of active components,providing a transportation channel for catalytic reactants.The metal centers in MOFs are evenly distributed and highly dispersed,concentrating their activity in certain catalytic reactions.The advantages of MOF carriers have been exploited in the removal of SO2,NO and Hg0from flue gas.

Wang et al.[97]prepared a MOF called MIL-100(Fe)and used it in the SCR reaction.Experimental results show that the NOx conversion rate of MIL-100(Fe)exceeds that of conventional V2O5-WO3/TiO2catalysts below 300°C.Zhang et al.[54]investigated the MnOx-loaded MIL-100(Fe) was prepared by the impregnation method.The experimental results show that the catalyst supported by MIL-100(Fe)has higher Hg0adsorption and oxidation capacity than the catalyst supported by Fe2O3at all test temperatures.The Hg0removal efficiency was 77.4%at 250°C.In addition,the catalyst can be easily recovered from fly ash using magnets.The catalyst activity evaluation diagram and the removal mechanism diagram are shown in Fig.6a.Zhang et al.[28]prepared Mn-Ce supported MOF (MnCe@MOF) catalyst by impregnation method.When removing Hg0and NO from the flue gas,the loading of Mn and Ce significantly enhanced the activity of the catalyst.Characteristic analysis shows that the MOF scaffold has a developed porous structure and a high surface area.This makes the active phase highly dispersed and thus enhances the activity of the catalyst.The sulfur and water resistance experiments show that MnCe@MOF has high resistance to SO2and H2O.Fig.6b shows the removal efficiency of Hg0and NO on the MnCe@MOF catalyst and the morphology of the catalyst.The stronger adsorption of Hg0and NH3on MOF catalysts than on conventional materials might be explained by the special structure and Lewis acidity of MOF[98,99].

PILCs-based catalysts:PILCs is a new type of catalytic material with a two-dimensional pore-like molecular sieve structure.Schematic representation of the clay structure is shown in Fig.7a.It has the characteristics of adjustable pore structure,high porosity,high thermal stability,hydrothermal stability and exchangeable cations,so it is widely used to make catalysts and adsorbents [55,100-103].Chae et al.[55]synthesized a novel SCR catalyst with a Ti-PILC support,which demonstrated higher performance than conventional catalysts.Shen et al.[24]innovatively loaded MnOxin PILCs for low-temperature NH3-SCR denitration research.At the optimal loading of MnOx/Ti-PILC(10%) and 80°C,the NO conversion efficiency approached 100%,with good H2O resistance.Next,Shen et al.[104]systematically studied the low-temperature denitration activity of doped Mn-MOx/Ti-PILC.They found that Ce and La additives promoted the denitrification activity [104].He et al.[105]prepared a series of innovative Ce-Mn/Ti columnar clay (Ce-Mn/Ti-PILC) catalysts by the impregnation method for trapping elemental mercury(Hg0)at 100-350°C.The experimental results show that:in the temperature range of the study,the 6%Ce-6%MnOx/Ti-PILC catalyst performance is greater than 90% Hg0removal rate.The activity evaluation data of the catalyst and the Hg removal mechanism are shown in Fig.7b.It is found through experiments that in the first stage of the reaction,the main Hg0capture mechanism of the catalyst is adsorption.As the reaction progresses,the oxidizing ability of Hg0is greatly improved,which is contributed by the hydroxyl oxygen and lattice oxygen on the catalyst surface.Wang et al.[25]prepared a series of Mn-Ce/Ti-PILCs catalysts for simultaneous removal of NO and elemental mercury in simulated flue gas.The experimental results show that 6%Mn-6%Ce/Ti-PILCs exhibit excellent NO conversion rate (>95%) and Hg0removal efficiency(>90%)at 250°C.The characterization results show that the removal activity of NO and Hg0is strongly affected by the conversion between oxygen adsorbed on the catalyst surface and lattice oxygen.The ratio of Mn4+/Mn3+and Ce4+/Ce3+on the surface of the catalyst affects the conversion rate of NO and the removal of Hg0.The modified PILC behaves like a catalyst support.The NO and Hg removal efficiencies from 100°C to 300°C are shown in Fig.7c.

Fig.5.(a)magnetically responsive catalytic sorbent and its use for removal of Hg0 and NO.Reproduced with permission[53].Copyright 2017,Elsevier.(b)The synergistic effect of CuCl2 impregnated zeolites on simultaneous removal of Hg0, SO2 and NO.Reproduced with permission [51].Copyright 2014,Elsevier.

Catalyst with the same active component(s) shows very different activities in the simultaneous removal of SO2,NO and Hg0catalysts.Table 1 [24,25,28,47,49,54,64,65,69,75,80,82,106-111]lists the NO and Hg0removal efficiencies and SO2resistances of carriers with the same active components.Comparing the active metallic components among the carriers,we find that catalysts with Mn-Ce active sites are more resistant to SO2than catalysts with other metallic active sites.Therefore,catalysts with an Mn-Ce active component are potentially valuable in industrial applications.Furthermore,Mn-Ce@MOF is more SO2-resistant than Mn-Ce/SiO2,possibly because the MOF specific surface area is higher in the former than the latter.Finally,we find that new supported catalysts are more SO2-resistant than catalysts based on conventional supports and unsupported catalyst.

Table 1NO and Hg0 removal efficiencies and SO2 resistances of carriers with the same active components.

Fig.6.(a) The catalyst activity evaluation diagram and the removal mechanism diagram.Reproduced with permission [54].Copyright 2020,Elsevier.(b) The removal efficiency of Hg0 and NO on the MnCe@MOF catalyst and the morphology of the catalyst.Reproduced with permission [28].Copyright 2017,Elsevier.

Fig.7.(a)Schematic representation of the clay structure.Reproduced with permission[102].Copyright 1998,Elsevier.(b)The removal effect of Ce-Mn/Ti-PILC on Hg and the removal mechanism diagram.Reproduced with permission[105].Copyright 2014,American Chemical Society.(c)Simultaneous removal of NO and Hg by Ce-Mn/Ti-PILC and removal mechanism.Reproduced with permission [25].Copyright 2015,American Chemical Society.

3.2.Influence of gas composition on removal effect

3.2.1.Single component effect

Real industrial exhaust gas contains species such as O2,NO,H2O,NH3,HCl,and fly ash,whose concentrations depend on the industrial type and operational conditions.Because such variable gas concentrations remarkably affect the NO reduction and Hg0oxidation of catalysts,the impact of single flue-gas components on SO2,NO,and Hg0removal has attracted much interest.

Effect of O2:O2is an important component in the flue gas and plays a decisive role in most catalytic reactions [112,113].Fig.8a summarizes the effects of O2on SO2,NO,and Hg0.In the absence of O2,Hg can react with the chemisorbed and lattice oxygens on the catalyst surface.As the reaction proceeds,the limited oxygen onthe catalysts is gradually depleted,and the mercury removal efficiency is low.When O2is plentiful,the Hg0removal rate increases because the depleted lattice and chemisorbed oxygens are replaced with gas-phase O2[114,115].Increasing the O2concentration does not significantly affect the Hg0removal efficiency.However,However,a large increase in O2concentration will increase the rate of SO2oxidation to SO3.The excessive generation of SO3will not be conducive to the progress of the SCR reaction,because it reacts with NH3and H2O to produce ammonium sulfate and viscous ammonium bisulfate,gradually occupying the active sites of the catalyst and deactivating the catalyst.

Effect of NH3:NH3is one of the common reducing agents in industrial applications.The NH3content in the flue gas has a great influence on the reduction of NO and the oxidation of Hg0.Increasing the NH3/NO ratio can promote the reduction of NO in the NH3SCR reaction,but the increase in NH3content will cause it to compete with Hg0for adsorption on the catalyst,thereby inhibiting the oxidation of Hg0[26,113,116].Wang et al.testified that NH3inhibits Hg0oxidation over a CuO-MnO2-Fe2O3/γ-Al2O3catalyst[117].However,other studies have reported a significantly positive effect of NH3on Hg0oxidation [118].Li et al.[119]found that NH3consumes surface oxygen and limits the Hg0adsorption,thereby inhibiting Hg0oxidation over the catalyst.However,when the NH3supply is removed and O2is present,the inhibited Hg0oxidation activity is completely recovered.Therefore,adding O2at an appropriate concentration to a mixture of NO,SO2and Hg will preserve (to some extent) the Hg0oxidation by NH3.The mechanism by which NH3affects the SO2,NO and Hg0conversion is schematized in Fig.8a.

Effect of H2O:Water vapor is a main component in flue gases,and often leads to catalyst deactivation.Fig.8a shows the effects of H2O on SO2,NO,and Hg0.Water vapor decreases the Hg0oxidation and NO reduction over the supported catalyst [120],because it strongly competes with the reactive species for adsorption on the active sites of the catalyst [121].When H2O is removed from the simulated system,the efficiencies of NO reduction and Hg0oxidation are restored to some level slightly below the initial value.

Effect of HCl:HCl is regarded as the flue-gas component that most strongly affects the Hg0oxidation,because the main oxidized mercury species in coal-combustion flue gas is HgCl2[122].Fig.8a summarizes the effect of HCl on SO2,NO and Hg0.HCl is a critical oxidant for Hg0oxidation,especially by metal-based catalysts[123].Although the introduction of HCl can significantly promote the oxidation of Hg0,it cannot directly oxidize elemental mercury because it exists in a naturally reduced state [124].Hg0oxidation by HCl requires the assistance of oxygen.According to several studies[125,126].In the presence of HCl,HCl and O2will produce intermediate Cl2and active chlorine Cl*on the catalyst,making the catalyst oxidize Hg0through the Chlor-Deacon process[127-129].Wang et al.found that when present at low concentrations(10 ppm),HCl barely affects the NO conversion.However,at high concentrations,HCl prohibits the NO conversion because the chloride ions react with the metal on the catalyst surface,forming volatile metal-Cl compounds.These volatiles destroy the active sites and consequently deactivate the SCR reaction [130].

3.2.2.Interactive effect of SO2,NO and Hg0

The above-mentioned flue-gas components are mixed with various pollutants(SO2,NO and Hg0),which might influence each other during the catalytic reaction.Therefore,to simultaneously remove multiple pollutants,many researchers have investigated the interactive effects of SO2,NO,and Hg0during the pollutantremoval process.Their results are summarized in Fig.8b.

Interactive effect of SO2and NO:The effect of SO2on NO removal in the SCR process is contentious.Some researchers[131,132]believe that SO2inhibits NO removal throughout the reaction system.Proponents of this view consider that SO2reacts with metal oxides and NH3in the presence of O2to form metal sulfates and ammonium sulfates (such as NH4HSO4and(NH4)2SO4).Especially,ammonium sulfates formed at low temperature on the catalyst surfaces occupy the active sites,lowering the SCR catalytic activity [133].Other researchers[134,135]considered that the sulfate formed by SO2and metal oxides can enhance the acidity of the catalyst and help improve the low-temperature SCR activity of the catalyst,and some studies have found that introducing SO2into the SCR system can increase the removal rate of NO.Thus far,the influence of NO on SO2removal during the simultaneous removal of NO and SO2has not been investigated [14,136],possibly because the SO2reaction dominates over the NH3and O2reactions,meaning that NO does not affect the SO2removal.

Interactive effectofSO2and Hg0:The effectofSO2onHg0removal from the flue gas is complicated and inconclusive.Both promotional and inhibitory effects are reported in the literature[39,137-139].Li et al.[39]studied the effect of SO2on the oxidation of mercury on CeO2-TiO2catalyst.Through experiments,it is found that in the absence of oxygen in the simulated flue gas,SO2greatly inhibits theoxidation of Hg0,but once O2is introduced,SO2can promote the oxidation of Hg0.Tao et al.[140]explained the anoxic experimental phenomenon as competition by Hg0and SO2for similar active sites on the catalyst surface;therefore,when no O2exists in the flue gas,SO2inhibitstheHg0conversion.WhenO2ispresent,theoxidationof SO2to SO3is mediated by the chemisorbed oxygen generated by the Ce3+-related charge imbalance on the catalyst.However,the effect of Hg on SO2removal appears to be unreported in the literature.We speculate that Hg affects SO2removal by the same process as SO2affects Hg removal.

Fig.8.(a)A diagram showing possible effect mechanism of O2,HCl,H2 O and NH3 on SO2,NO and Hg0.(b)The interactive effects of SO2,NO and Hg0 on the catalyst during the pollutant-removal process.

Interactive effect of NO and Hg0:Wang et al.[15]studied the effect of NO on Hg0removal over an MnCe/TP catalyst.When the NO concentration increases from 200 ppm to 500 ppm,the removal efficiency of Hg0will slightly decrease.Wang believed that the Hg0oxidation was inhibited by other nonactive species such as nitrite,which might cover the active sites.Tao et al.[140]similarly found that in the absence of oxygen,increasing the NO concentration did not substantially change the Hg0removal efficiency.When O2was introduced to a flue gas containing NO,the Hg0removal efficiency slightly increased and a large amount of oxidized mercury was observed in the gas flow downstream of the sample.Li et al.[39]reported the same effect of NO on Hg oxidation over a CeTi catalyst.NO alone exerted a slightly propulsive effect on Hg0removal,especially at higher concentrations.The limited oxygen stored on the catalyst is consumed by both NO and Hg0oxidation [141].The propulsive phenomenon [69]might be explained by weak NO absorption to the sample surface.Some of these absorbed NO might react with the O2on the metal-oxide catalyst surface,generating species such as NO+and NO2that can oxidize Hg0[142].

Whether the Hg0removal also affects NO removal is a pertinent question.Several researchers [25,69,112,143]who studied the effect of Hg0on NO removal reached a consensual conclusion:Hg0exerts only a slight effect on the reduction of NO.The inhibitory effect has been attributed to two possible causes.First,Hg0might compete with NO and NH3for adsorption to catalytic sites [69].Second,the oxidation reaction of Hg0might accumulate HgO on the sample surface.If the accumulated Hg0covers some active sites,the NO removal efficiency will be slightly reduced[23,106,112,144].

3.3.Removal mechanism

Because the supported catalyst carrier generally has a rich specific surface area and a good pore structure,the supported active component is evenly dispersed on the surface and through the pores of the carrier.A supported catalyst ensures high catalytic activity for flue-gas removal.Thus far,no complete SO2,NO,and Hg0removal mechanism has been proposed.The mechanism of removal of SO2,NO and Hg0under the catalyst action is summarized in Fig.9.The pathways of SO2,NO and Hg0removal are demonstrated in Fig.9 with equations.

Fig.9.The mechanism of synergistic removal of SO2,NO and Hg0 by catalysts.The adsorbed surface oxygen is indicated as O*.

4.Conclusions and prospects

This paper presented the modern supported catalysts used in the removal of SO2,NO and Hg0from flue gas.The advantages and disadvantages of the formation technologies,the effects of crucial factors,and the removal mechanisms of the supported catalysts were carefully discussed.The technologies of carbon-based material adsorption technologies,photocatalytic oxidation technologies,wet scrubbing technologies,and NTP technologies face several problems:Large energy consumption,large equipment footprint,and secondary pollution.Solid-catalytic technology,which allows higher utilization of apparatus,incurs loweroperating costs,and requires less investment than multipollutant simultaneous removal technologies,has been widely investigated,and many supported catalysts have been researched in the past decade.MOF carriers for the low-temperature removal of NO and Hg0provide much higher SO2resistance than traditional supported catalysts (carbon-based materials,metal oxides,and silica).In addition,the composition of real flue gas is very complicated and remarkably affects the NO reduction and Hg0oxidation.The removal efficiencies of NO,SO2and Hg0are enhanced in the presence of O2.NH3inhibits Hg0oxidation and NO reduction in the absence of O2,but promotes these reactions in the presence of O2.H2O inhibits Hg0oxidation and NO reduction by competitively adsorbing to the active sites.After exploring the surface chemical processes,this review clarified the mechanisms of SO2,NO and Hg0removal,and proposed a theory of catalytic multi-pollutant purification by new supported catalysts.Reviewing the past solid catalytic technology and the current technological development direction,we finally proposed the future research direction.At present,catalysts based on new supports have advantages in sulfur resistance and denitrification and mercury removal compared with catalysts based on conventional supports.Among the catalysts based on the new carrier,the new MOFs material as the carrier has higher sulfur resistance at low temperature than other traditional materials such as carbon materials and metal oxides.This is because its regular pore structure and large surface area make the supported metal oxides highly dispersed.Compared with other materials,its sulfur resistance is poor at high temperature.This is because the thermal stability of MOFs at high temperatures is poor.The molecular sieve material has a synergistic removal effect of thimerosal compared to other materials at high temperature.In summary,the high-temperature activity of MOFs-based supported catalysts and the low-temperature activity of molecular sieve-based supported catalysts are worthy of in-depth study in the gas purification technology for removing SO2,NOxand Hg0from industrial exhaust gas.

Declaration of competing interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos.52000093,51968034,41807373 and 21667015),National Key R&D Program of China (No.2018YFC0213400),China Postdoctoral Science Foundation (Nos.2020T130271,2019M663911XB) and Open Fund of National Engineering Laboratory for Mobile Source Emission Control Technology (No.NELMS2019B03).