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REVIEW—NH3-SCR—Environmentally-benign catalysts for the selective catalytic reduction of NOx

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Environmentally-benign catalysts for the selective catalytic reduction of NO x from diesel engines:structure–activity relationship and reaction mechanism aspects

Fudong Liu,?Yunbo Yu ?and Hong He*

Selective catalytic reduction of NO x using NH 3or hydrocarbons (NH 3-SCR or HC-SCR)in oxygen-rich exhaust from diesel engines remains a major challenge in environmental catalysis.The development of highly e?cient,stable and environmentally-benign catalysts for SCR processes is very important for

practical use.In this feature article,the structure–activity relationship of vanadium-free catalysts in the NH 3-SCR reaction is discussed in detail,including Fe-,Ce-based oxide catalysts and Fe-,Cu-based zeolite catalysts,which is beneficial for catalyst redesign and activity improvement.Based on our research,a comprehensive mechanism contributing to the performance of Ag/Al 2O 3in HC-SCR is provided,giving a clue to the design of a catalytic system with high efficiency.

1.Introduction

Nitrogen oxides (NO x )are a major air pollutant that can lead to the formation of acid rain,photochemical smog and haze,thus

endangering the eco-environment and human health.1The NO x emissions resulting from human activity can be ascribed to stationary sources such as coal-fired power plants and mobile sources such as motor vehicles.2With the challenges arising from the energy crisis and global warming,the wide application of diesel engines in vehicles becomes more and more impor-tant because of their high fuel e?ciency.Two main pollutants from diesel engine exhaust are NO x and particulate matter (PM).Through the adjustment of heavy-duty diesel engines,

Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences,Beijing 100085,P.R.China.E-mail:honghe@https://www.sodocs.net/doc/df18176434.html,;Fax:+86-10-6284-9123;Tel:+

86-10-6284-9123

Fudong Liu

Fudong Liu was born in Tianjin,China,in 1982.He received his Bachelor degree in Environmental Engineering from Tsinghua University in 2005and PhD degree in Environmental Science from Research Center for Eco-Environmental Sciences (RCEES),Chinese Academy of Sciences (CAS)in 2010.He has been working as assistant professor in RCEES since 2010and was promoted to associate professor in 2013.He received Excellent

Doctoral Dissertation Award,CAS in 2011and Young Scientist Award,IACS in 2012.His research interests include environmental catalysis and air pollution control,especially the development of environmental-friendly catalysts for the selective catalytic reduction of NO x with NH 3

Yunbo Yu

Yunbo Yu received his PhD degree in Environmental Science from Research Center for Eco-Environmental Science (RCEES),Chinese Academy of Sciences in 2004.He has been an assistant professor of Environmental Science at RCEES since 2004,and was promoted to an associate professor in 2006.He then spent two years as a JSPS postdoctoral fellow in Prof.Haruta’s lab at Tokyo Metropolitan University,Japan.His research is concerned with environmental

catalysis and air pollution control,especially the development of catalysts for the selective catalytic reduction of NO x with hydrocarbons.

?Authors made equal contributions.

Received 11th February 2014,Accepted 4th April 2014DOI:10.1039/c4cc01098a

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ChemComm

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the emission of PM from diesel engines can be e?ectively reduced.However,due to the well-known‘‘trade-o?’’relation-ship between PM and NO x emitted from diesel engines,the increasing demand to eliminate NO x,especially under oxygen-rich conditions,has drawn great attention from both academia and industry.3,4Selective catalytic reduction(SCR)of NO x using reductants such as NH3,urea or hydrocarbons(HC)in the oxygen-rich exhausts is a highly e?cient way to reduce NO x emission,although it remains one of the major challenges in the field of environmental catalysis.

The SCR of NO x with NH3(NH3-SCR)was initially applied in the removal of NO x from stationary sources,and the commer-cially used catalyst systems are mainly WO3or MoO3doped V2O5/TiO2.5,6Vanadium-based NH3-SCR catalysts have also been successfully used for the deNO x process from diesel engines,since2005.7,8However,disadvantages remain for the vanadium-based catalysts,including the toxicity of V2O5,the narrow operation temperature window,the easy sublimation of V2O5and phase transformation of the TiO2support from anatase to rutile at high temperatures,which greatly restrict their further application,especially when stricter regulations are established for both PM and NO x for diesel engines,and the SCR converter should be installed downstream of the diesel particulate filter(DPF),with timed thermal shock(B8001C)in the regeneration processes of the DPF system.Therefore,many researchers are trying to develop vanadium-free NH3-SCR catalysts with high deNO x e?ciency,high N2selectivity,excellent hydrothermal stability and insensitivity to co-existing poisoning components in the SCR atmosphere such as H2O,SO2,HC or alkali metals etc.Currently,metal oxide catalysts and zeolitic catalysts are two well-studied types of vanadium-free catalysts for the NH3-SCR process in heavy-duty diesel engines,some of which show great potential in practical use.6,9,10However, despite the numerous studies on SCR performance under di?erent conditions,a comprehensive summarization of the structure–activity relationship of these catalysts in the NH3-SCR process is still lacking.In this article,taking Fe-,Ce-based oxide catalysts and Fe-,Cu-based zeolite catalysts as exam-ples,we will discuss the microstructure of active sites,the role of acidity and also the activation of reactants on these catalysts for the NH3-SCR reaction in detail,which is very important for the design of catalysts and study of reaction mechanisms.

On the other hand,since the pioneering work of Iwamoto et al.11and Held et al.,12many catalysts such as zeolitic oxide, base oxide/metal and noble metal catalysts have been found to be e?ective for the SCR of NO x with hydrocarbons(HC-SCR) in the presence of excess oxygen.3,4,7,13–19Among the catalysts proposed for HC-SCR technology,zeolitic oxide catalysts are effective for NO x reduction,while their water tolerance has been little improved.5Noble metal catalysts such as Pt/Al2O3 exhibit high deNO x activity in the low temperature range of 200–3001C while exhibiting a narrow operation temperature window.20,21Up to now,alumina-supported silver(Ag/Al2O3) has been known as one of the most effective catalysts for the HC-SCR even in the presence of water vapor and SO2.13,16,22–32 The distinctive advantage of HC-SCR is that the on-board fuel can be used as the reductant for NO x conversion,thus reducing the cost involved in infrastructure development for delivering the reductant to the automotive engine exhaust system.The past several years have witnessed a growth in research in NO x reduction by fuel-component hydrocarbons over Ag/Al2O3, while the light-off temperature for NO x reduction is still too high to be used for commercial application in diesel vehicles, even though it has been reported that low temperature activity can be promoted by the addition of H2.24,32–40More importantly, aromatic hydrocarbons,typically present in diesel fuel,exhibit low activity for NO x reduction even in the presence of H2.41 Indeed,when using diesel fuel(ultra-low sulfur diesel,US06)as the reductant,34Ag/Al2O3showed high initial activity for NO x reduction in the presence of3200ppm H2,while its activity gradually decayed with time,reaching a final conversion level similar

to that observed in the absence of H2.These results

indicate that the issue of catalyst deactivation by hydrocarbon

poisoning still needs to be resolved for the commercial applica-

tion of HC-SCR.To provide a guideline for developing an ideal

HC-SCR system,it is highly desirable to understand the mecha-

nism of NO x reduction by hydrocarbons at the molecular level.

2.The structure–activity relationship

of vanadium-free catalysts for

NH3-SCR of NO x

2.1.NH3-SCR of NO x over oxide catalysts

Many researchers have focused on the development of

vanadium-free oxide catalysts for the NH3-SCR process,which

can be divided into single metal oxide catalysts,supported-type

metal oxide catalysts and mixed metal oxide catalysts according

to the forms of the catalytic materials.Based on the active

components,the vanadium-free oxide catalysts can be further

classified as Fe-,Ce-,Mn-,or Cu-based materials,of which the Hong He

Hong He was born in1965,Hebei,

China.He received his PhD degree

in Chemistry at the University of

Tokyo in1994.After working as

researcher in Riken Corporation

(1994–1997),postdoctoral fellow

in University of Southern California

and University of Delaware

(1997–2000)and visiting scientist

in University of Toronto(2000–

2001),he became a full professor

in2001at Research Center for

Eco-Environmental Sciences

(RCEES),Chinese Academy of

Sciences(CAS).His research interests include environmental

catalysis and heterogeneous atmospheric chemistry,such as the

diesel emission control,indoor air pollution control and study on

the formation mechanism and control strategies of haze in China.

Feature Article ChemComm P

Fe-and Ce-based catalysts have been well studied in the deNO x process of diesel engines due to their compatibility with the typical exhaust temperature range.In the following sections,we will systematically summarize the recent research advances in understanding the structure–activity relationship of Fe-and Ce-based oxide catalysts in the NH 3-SCR reaction,which are important for their practical use.

2.1.1.Fe-based oxide catalysts.Early in the 1980s,Kato et al.42used Fe 2O 3as the active phase in a NH 3-SCR catalyst (i.e.Fe 2O 3–TiO 2mixed oxide catalyst)with high SCR activity and N 2selectivity at relatively high temperatures (350–4501C),and thereafter numerous types of Fe 2O 3-containing catalysts were developed by researchers,including Fe 2O 3-pillared layered clay (PILC),43Fe 2O 3–SiO 2,44Fe 2O 3supported on activated carbon (AC)45or activated carbon fiber (ACF)46etc.Yet at that time,no systematic studies of the microstructure of Fe species and its relationship with SCR performance were reported.

Several years ago,Apostolescu et al.47developed a new supported-type Fe 2O 3/WO 3/ZrO 2catalyst achieving total NO x conversion and high N 2selectivity in the NH 3-SCR reaction over the temperature range 280–4301C.Based on a series of characterization results including XRD and H 2-TPR,they pointed out that the Fe species in the Fe 2O 3/WO 3/ZrO 2catalyst were mainly present in the form of well-crystallized Fe 2O 3and also small Fe x O y particles.During the NH 3-SCR reaction,the Fe 3+species catalyzed the dehydrogenation of NH 3to NH 2while being reduced to Fe 2+species,and thereafter the NH 2species reacted with gaseous NO to produce N 2and H 2O,while O 2could re-oxidize Fe 2+species to Fe 3+species participating into the next redox cycle.The SCR reaction over this catalyst mainly followed an Eley–Rideal (E–R)mechanism.This is probably the first successful example elucidating the structure–activity rela-tionship of Fe species in the NH 3-SCR reaction for an Fe 2O 3-containing oxide catalyst.

Usually,it is also accepted that NO can be molecularly adsorbed on the transition metal ion or it can dissociatively chemisorb on oxide ion vacancies of the metal oxide catalysts,and react with adsorbed NH 3or NH 4+species to form SCR reaction intermediates and decompose into N 2and H 2O after-wards,following a Langmuir–Hinshelwood (L–H)mechanism.To effectively disperse the active species,anatase TiO 2is usually utilized as the catalyst support for the NH 3-SCR reaction.Although it is inert for NO x conversion,its acidity can inhibit the formation of sulfate species on the catalyst surface in SO 2-containing atmospheres above 2001C to avoid the deacti-vation of SCR catalysts.48Based on the above-mentioned ideas,Roy et al.49prepared an Fe ion substituted TiO 2,namely the Ti 0.9Fe 0.1O 2àd catalyst,by a novel single-step solution combus-tion method,showing total NO x conversion at ca.300–4251C with rather high N https://www.sodocs.net/doc/df18176434.html,pared with the well-crystallized ilmenite phase FeTiO 3over which the maximum NO x conversion did not exceed 20%,the catalytic activity of the Ti 0.9Fe 0.1O 2àd catalyst with ionic metal sites and oxide ion vacancies was much higher,indicating that such defect struc-tures in NH 3-SCR catalysts were important factors influencing their catalytic performance in the deNO x process.

Over the Ti 0.9Fe 0.1O 2àd catalyst,Roy et al.49considered that the Lewis acid sites were beneficial to the wide SCR operation temperature window,and for the Fe 2O 3/WO 3/ZrO 2catalyst pre-pared by Apostolescu et al.,47the importance of Lewis acid sites to the E–R mechanism of the NH 3-SCR reaction was also strongly emphasized.However,for the supported-type Fe 2(SO 4)3/TiO 2catalyst prepared by Ma et al.50with high NO x conversion at 350–4501C,they considered that the relatively strong Br?nsted acid sites induced by sulfate species contributed substantially to the deNO x efficiency.Thus,the roles of Lewis acid sites and Br?nsted acid sites in the NH 3-SCR reaction over Fe 2O 3-containing catalysts are still in debate,and in future studies,we strongly recommend that the elucidation of the involvement of these two types of acid sites in the NH 3-SCR reaction should be closely correlated to the reaction temperature.

In recent years,we have focused on the investigation of the structure–activity relationship of iron titanate (FeTiO x )catalysts in the NH 3-SCR reaction.This environmentally-friendly catalyst,when prepared by a co-precipitation method using Fe(NO 3)3and Ti(SO 4)2as precursors,showed high SCR activity,N 2selectivity and H 2O/SO 2durability in the medium temperature range (200–4001C),51and an operation tempera-ture window at least 50–1501C lower than those of the above-mentioned Fe 2O 3-containing https://www.sodocs.net/doc/df18176434.html,pared with the FeTiO x catalyst prepared using TiCl 4as a precursor and a Fe 2O 3/TiO 2catalyst prepared by an impregnation method,the existence of sulfate species in the preparation process could significantly inhibit the crystallization of mixed metal oxide phases,resulting in the formation of highly dispersed small iron titanate crystallites,which is totally different from the Fe 2O 3particles normally found in previously reported studies.52A similar promotion effect by sulfate species on the dispersion of Fe species was also observed by Ma et al.on their Fe 2(SO 4)3/TiO 2catalyst.50Thereafter,the microstructure of iron titanate crystallites in FeTiO x catalysts was studied in detail using various characterization methods,including N 2physisorption,Powder XRD,UV-vis DRS,Raman spectroscopy and XAFS etc.,and the surface chemical composition and redox behavior were studied using XPS and H 2-TPR.It was definitively concluded that the active iron titanate crystallites in the FeTiO x catalyst prepared at low calcination temperature were mainly in the form of a specific edge-shared Fe 3+–(O)2–Ti 4+structure (Scheme 1),and had large surface area,pore volume and abundant surface defects supplying rich catalytically active

sites

Scheme 1Proposed model of a homogeneous edge-shared Fe 3+–(O)2–Ti 4+structure in the FeTiO x catalyst.(Reproduced with permission from Elsevier Inc.)53

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for the NH 3-SCR reaction.In this specific edge-shared Fe 3+–(O)2–Ti 4+structure,the existence of an electronic inductive effect between Fe 3+species and Ti 4+species was confirmed,leading to higher NO adsorption and oxidation ability for Fe 3+species and thus higher SCR activity at low temperatures.52,53Although after high temperature calcination such as at 600or 7001C,the specific surface area and reactant adsorption ability of the FeTiO x catalyst were decreased to a certain extent due to the occurrence of sintering (i.e.well-crystallized Fe 2TiO 5was formed),the intrinsic SCR activity normalized by surface area was actually increased.54In future studies,the deposition of such iron titanate crystallites onto microporous or mesoporous materials with large surface area holds promise as a method to enhance its dispersion and thermal stability for practical utilization in industry.

Using in situ DRIFTS,transient response experiments and temperature programmed desorption/surface reaction meth-ods,the NH 3-SCR reaction mechanism over the FeTiO x catalyst was well studied (Scheme 2).55At temperatures below 2001C,on the Fe 3+–(O)2–Ti 4+structure,NO can be oxidized into nitrate species on Fe 3+sites and then reacts with adjacent adsorbed NH 3species on Ti 4+–OH Br?nsted acid sites to form intermedi-ate species (similar to ammonium nitrate species),followed by a subsequent reaction with gaseous NO to produce N 2and H 2O.An L–H mechanism is proposed accordingly for NH 3-SCR of NO x at low temperatures over the FeTiO x catalyst,in which the formation of reactive monodentate nitrate on Fe 3+sites is the rate-determining step.55An efficient method to improve the low temperature SCR activity of the FeTiO x catalyst is to enhance the NO x adsorption as monodentate nitrate,56as confirmed by subsequent experimental results in which the low temperature SCR activity was greatly enhanced through partial substitution of Fe species by Mn species in FeTiO x with considerably enhanced formation of monodentate nitrates.57If SO 2is

present in the NH 3-SCR atmosphere,the sulfate species will preferentially adsorb onto Fe sites to form Fe–O–S bonds (as evidenced by EXAFS results)competing with the adsorption of nitrate species,leading to the decline of low temperature SCR activity to a certain extent.58This inhibition effect is more prominent on Mn-substituted FeTiO x catalysts,mainly due to the easier blocking of the L–H reaction pathway at low tem-peratures by sulfate deposition on Mn sites.59In future studies,the simultaneous improvement of the low temperature SCR activity and SO 2durability of Fe-containing catalysts is still a great challenge,unless a completely new reaction pathway different from the L–H mechanism can be opened by catalyst modification or redesign.

Returning to the FeTiO x catalyst,at temperatures above 2001C,the Ti 4+–OH Br?nsted acid sites can be transformed into Lewis acid sites through dehydroxylation,and then adsorb NH 3to participate in the NO x reduction process.The adsorbed NH 3species can be activated into –NH 2species by neighboring Fe 3+sites through dehydrogenation,and thereafter react with gaseous NO to form a NH 2NO intermediate and decompose into N 2and H 2O.In this process,the Fe 3+species is first reduced to Fe 2+species and then reoxidized to Fe 3+species by gaseous O 2,completing a redox cycle.A typical E–R mechanism for NH 3-SCR of NO x at high temperatures over the FeTiO x catalyst is proposed accordingly,with the formation of NH 2NO being the rate-determining step.55Although sulfate species may also form on Fe 3+sites in this process,the activation of NH 3to –NH 2species is not influenced.Therefore,the E–R reaction pathway can still proceed over the sulfated FeTiO x catalyst,ensuring high deNO x efficiency at relatively high tempera-tures.58In brief,this was possibly the first time that different NH 3-SCR mechanisms over Fe-containing catalysts in the low and high temperature ranges with the involvement of both acid sites and redox sites were demonstrated.Since then,research-ers have started to pay attention to the influence of reaction temperature on the NH 3-SCR mechanism over different types of catalysts,such as the Fe–Ti spinel catalyst,60CeO 2/TiO 2,CeO 2–WO 3/TiO 2catalysts 61and CuO x /WO x –ZrO 2catalyst 62etc.In future studies,the hydrothermal stability and resistance to coexisting poisoning pollutants in the SCR atmosphere (i.e.H 2O,SO 2,HC,alkali metals etc.)of this FeTiO x catalyst still need to be investigated in more detail before its practical use in the deNO x process of diesel engines.

The above-mentioned Fe-containing catalysts are usually in the form of supported-type or mixed oxides,with the second or third component acting as a support or promoter.Actually,the well-designed Fe 2O 3oxide itself can also show good perfor-mance in the NH 3-SCR reaction,and this type of material has obvious advantages for the investigation of the structure–activity relationship.Very recently,the synthesis of Fe 2O 3nanomaterials with controlled crystal phase and morphology has achieved great success.63Over a -Fe 2O 3samples prepared by Yang et al.64by a hydrothermal route with nanocube and nanorod morphologies,the relationship between NH 3-SCR activity and the exposed crystal facet was established.They concluded that a -Fe 2O 3nanorods exposing more (110)

facets

Scheme 2Proposed SCR mechanisms over FeTiO x catalyst in di?erent temperature ranges.(Reproduced with permission from Elsevier Inc.)55

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with relatively high surface energy and higher density of Fe atoms showed much better NH 3-SCR activity than a -Fe 2O 3nanocubes mainly exposing (012)facets with low surface energy.Mou et al.65creatively synthesized novel a -Fe 2O 3nano-rods and g -Fe 2O 3nanorods by aqueous precipitation and calcination/refluxing methods,showing the same morphology but totally different exposed crystal facets.They clearly con-cluded that the g -Fe 2O 3nanorods enclosed by reactive (110)and (100)facets,simultaneously exposing Fe 3+and O 2àsites,were highly active for the activation of NH 3and NO,and thus highly active for the NH 3-SCR reaction.In contrast,the a -Fe 2O 3nanorods enclosed by (210)and (001)facets only exposed Fe 3+sites and showed relatively poor SCR activity,due to the lack of neighboring oxygen anions for the activation of NH 3and NO.These studies have significantly elevated the research on the structure–activity relationship of Fe 2O 3catalysts for the NH 3-SCR reaction to a new height,and in the near future studies on their practical utilization should be more focused.

2.1.2.Ce-based oxide catalysts.In previous studies,cerium oxide has usually been employed as the promoter or support for NH 3-SCR catalysts.For example,Chen et al.66added ca.10wt%Ce to a V 2O 5–WO 3/TiO 2catalyst with low vanadia loading (0.1wt%),which was able to greatly improve the NH 3-SCR activity in a broad operation temperature window.Long and Yang 67found that Ce doping could enhance the stability of Fe–ZSM-5catalysts,and Carja et al.68concluded that the synergistic effect between Fe and Ce in Fe–Ce–ZSM-5catalysts could greatly improve the deNO x efficiency.Wu et al.69,70con-cluded that a Ce-promoted Mn/TiO 2catalyst showed better SO 2resistance in the low temperature NH 3-SCR process.Li et al.71prepared a supported WO 3catalyst using CeO 2–ZrO 2mixed oxide as a support,exhibiting excellent NH 3-SCR activity and thermal stability,as a potential candidate for the deNO x process of diesel engines.

With the increase of the understanding on the e?ect of cerium oxide in the NH 3-SCR reaction,researchers have begun to realize that this material with excellent redox properties can also be used as an active component in SCR catalysts.For instance,Xu et al.72reported a novel Ce/TiO 2catalyst,prepared by a conventional precipitation method,showing high deNO x e?ciency and N 2selectivity from 250to 4001C.Afterwards,Gao et al.73,74investigated the influence of preparation methods (including sol–gel,impregnation and co-precipitation meth-ods)and concluded that the catalyst prepared by the sol–gel method showed the best catalytic performance.They consid-ered that the relatively strong interaction between Ce and Ti species and also the high dispersion of CeO 2crystallites on the TiO 2surface were important for the NH 3-SCR reaction.Although the SCR activity of the Ce/TiO 2catalyst has been proved to be outstanding,the resistance to SO 2poisoning and high space velocity (GHSV)still need to be enhanced for practical use.For example,the supported Ce/TiO 2catalyst can be deactivated by SO 2through the formation of highly ther-mally stable Ce(SO 4)2and Ce 2(SO 4)3,cutting off the redox cycle between Ce 3+and Ce 4+species.75In addition,with increasing GHSV,the insufficient SCR activity,especially at low temperatures,

cannot satisfy the requirements for practical use in diesel engines.72,73To overcome these shortcomings,Shan et al.76simply developed a homogeneous precipitation method characterized by a uniform increase of the pH value through decomposition of an organic base (urea in this study)to prepare a CeTiO x mixed oxide catalyst.During this process,the TiO x species was first precipitated at the initial stage and afterwards the CeO x species was precipi-tated uniformly onto the TiO x species,forming many catalytically active CeTiO x crystallites.77This CeTiO x catalyst showed much higher SCR activity than the supported Ce/TiO 2catalyst (Fig.1),with significantly improved resistance to high space velocity.

To better enhance the catalytic performance together with the thermal stability of this CeTiO x catalyst,doping with W species was carried out (Fig.1).78Over the optimal Ce 0.2W 0.2TiO x catalyst with Ce :W molar ratio of 1:1,NO x conversion could be maintained above 90%from 275to 4501C even at a rather high GHSV of 500000h à1.The W doping in the Ce 0.2W 0.2TiO x catalyst led to a much higher dispersion degree of Ce species,with a larger Ce 3+/Ce 4+ratio and more oxygen vacancies,resulting in higher NO oxidation ability and thus better SCR activity at low temperatures.In addition,due to the increased capacity for adsorption of NH 3species on both Br?nsted acid sites and Lewis acid sites induced by W doping,the NH 3unselective oxidation at high temperatures was also greatly inhibited,thus leading to higher SCR activity and N 2selectivity simultaneously.A similar promotion e?ect by W species on CeO 2/TiO 2supported-type catalysts was also observed by Chen et al.,79which they attrib-uted to the strong interaction between Ce and W species.

Actually,for TiO 2-containing NH 3-SCR catalysts,the activity loss at high temperatures above 6001C is usually associated with the occurrence of a phase transformation from anatase to rutile.Through analysis of the e?ect of W species in the Ce 0.2W 0.2TiO x catalyst,Shan et al.80had the idea to substitute all Ti species by W species.Under this inspiration,a novel and excellent CeWO x catalyst with high NH 3-SCR activity,outstand-ing N 2selectivity,improved thermal stability,good durability toward co-existing pollutants and especially much higher resis-tance to space velocity was developed (Fig.1).Even at a very high GHSV of 500000h à1,the NO x conversion over CeWO x could be maintained at 100%from 250to 4251C,which is

very

Fig.1NH 3-SCR activity over 20%Ce/TiO 2,Ce 0.2TiO x ,Ce 0.2W 0.2TiO x ,CeWO x catalysts in simulated diesel engine exhaust.

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beneficial for practical use in diesel vehicles with limited installa-tion space for SCR converters.Based on various characterization results,it was concluded that,compared with pristine oxides,the strong interaction between Ce and W species led to higher dispersion of CeO 2and WO 3crystallites and more abundant Ce 3+species with higher concentration of surface oxygen species,which were important for the oxidation of NO,thus promoting the ‘‘fast SCR’’reaction.The synergistic e?ect between Ce and W species produced more Br?nsted acid sites and Lewis acid sites for NH 3adsorption and also maintained enough capacity for NO x adsorption,achieving a broad operation temperature window in the NH 3-SCR reaction.This CeWO x catalyst is a fascinating candidate for the deNO x process from diesel engines,and Chen et al.81also found this advantageous combination of Ce and W species in a CeO 2–WO 3mixed oxide catalyst at nearly the same time.Peng et al.82,83found that this catalyst could resist greater amounts of alkali metals than V 2O 5–WO 3/TiO 2and that Mn doping could further improve its low temperature SCR activity.Over this CeO 2–WO 3mixed oxide catalyst,based on an in situ DRIFTS study,Chen et al.84proposed two reaction pathways of NH 3-SCR reaction which were similar to the E–R and L–H mechanisms.However,much more work needs to be done to better understand the intrinsic SCR reaction mechanism over this novel material,especially using other characterization methods.Besides the above-mentioned materials,CeO 2supported on Al 2O 3,85activated carbon fibers,86and carbon nanotubes 87together with a hydrothermally synthesized Ce–P–O catalyst,88were also developed by other researchers,all showing impressive deNO x efficiency,although the detailed structure–activity relationship of these CeO 2-containing catalysts in NH 3-SCR reaction still needs to be studied.In the aspect of catalyst structure design,Wang et al.89developed a novel titanium nanotube-confined CeO 2catalyst,showing higher NH 3-SCR activity than that of CeO 2supported on TiO 2nanoparticles,mainly due to the stronger redox ability and NH 3adsorption capacity,judging from comparison of the H 2-TPR and NH 3-TPD results,respectively.Owing to the presence of a ‘‘shell protection effect’’,Chen et al.90found that this titanium-nanotube-confined CeO 2catalyst showed a remarkable resistance to alkali metal poisoning in the deNO x process.This result provides a new route for synthesis of NH 3-SCR catalysts with higher poisoning resistance for practical use.Recently,the surface modification of CeO 2by WO x 91or sulfate species 92and the surface modifica-tion of CeO 2–ZrO 2solid solutions by nickel/sulfate species 93or phosphate species 94to promote the NH 3-SCR activity also achieved some progress,in which the roles of acid sites induced by surface modifiers and the inhibition of NH 3unselective oxidation at high temperatures were carefully addressed.In future studies,the surface modification of CeO 2or other metal oxides could be a promising way to prepare applicable catalysts that is also convenient for studying their structure–activity relationship in the NH 3-SCR reaction.2.2.

NH 3-SCR of NO x over zeolite catalysts

Zeolite catalysts have received much attention in recent years due to their excellent SCR activity and thermal stability for the deNO x process in diesel engines.3Owing to the complexity of

zeolite structures (e.g.ZSM-5,HBEA,MOR,USY,CHA etc.),the di?erence in preparation methods (e.g.aqueous ion-exchange,solid-state ion-exchange,incipient wetness impregnation,chemical vapor deposition,one-pot synthesis etc.)together with the di?erences in active metal sites (e.g.Fe,Cu etc.),a uni-versally accepted structure–activity relationship for zeolite catalysts in the NH 3-SCR reaction cannot be easily established.Fortunately,many researchers have devoted themselves to relatively fundamental research on zeolite NH 3-SCR catalysts besides work aimed toward their industrially practical use.The enlightening results obtained in this area will be introduced in the following sections classified by the Fe and Cu transition metals commonly used as active sites.

2.2.1.

Fe-based zeolite catalysts.Among the numerous Fe-based zeolite catalysts,the application of Fe-ZSM-5in the NH 3-SCR reaction has attracted much attention from research-ers since the end of the last century.In 1999,Ma and Gru

¨nert 95and Long and Yang 67both reported that Fe-ZSM-5catalysts prepared by over-exchange of Fe into H-ZSM-5through FeCl 3sublimation and ion-exchange of Fe into NH 4-ZSM-5using FeCl 2as a precursor,respectively,showed excellent NH 3-SCR activity even at high GHSV and in the presence of SO 2.Thereafter,more and more researchers focused on the study of Fe-ZSM-5catalysts,investigat-ing the influence of preparation methods,Fe exchange level and Si/Al ratio etc.on NH 3-SCR activity.10

Although di?erent research groups reported diverse optimal methods for preparation of Fe-ZSM-5catalysts with high apparent SCR activity,such as the improved aqueous ion-exchange method by Long and Yang,96the solid-state ion exchange method by Schwidder et al.,97and the chemical vapor deposition method by Iwasaki et al.,98the researchers eventually concluded that the preparation method was actually not a decisive factor in deter-mining the intrinsic catalytic activity of Fe-ZSM-5catalysts in the NH 3-SCR reaction (i.e.the turnover frequency),but the micro-structure of Fe species was.10,98Brandenberger et al.99recently systematically studied the di?erent Fe sites located in Fe–ZSM-5catalysts including monomeric,dimeric,clustered and oligomeric species,and correlated them with the measured NH 3-SCR activity (Scheme 3).They concluded that actually all Fe species in the Fe–ZSM-5catalyst were active,although they exhibited different

Scheme 3The Fe species probably present in Fe–zeolite NH 3-SCR

catalysts.

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temperature dependencies in the SCR reaction.Below 3001C,only monomeric Fe species contributed to the SCR reaction,and this type of Fe species did not catalyze the NH 3unselective oxidation below 5001C,which was beneficial to high N 2selectivity.The important role of monomeric Fe 3+sites was also emphasized by H?j et al.100in their Fe–BEA catalyst.Above 300,400and 5001C,respectively,the dimeric Fe species,the oligomeric species (e.g.trimeric and tetrameric Fe species)and partially uncoordinated Fe sites in the outmost layer of Fe x O y particles could also contribute to the NH 3-SCR reaction,and at high temperatures (45001C)the contribution of dimeric Fe species dominated.However,the unselective oxidation of NH 3would occur on Fe x O y particles above 3501C,resulting in low N 2selectivity,and the dimeric Fe species governed the NH 3oxida-tion up to 5001C.A similar effect by severely clustered Fe x O y species on the unselective consumption of the reducing agent NH 3in the SCR reaction was also proposed by Schwidder et al.101and Devadas et al.102Therefore,to obtain excellent NH 3-SCR activity at low temperatures together with good N 2selectivity at high temperatures,an Fe–zeolite catalyst with a maximum quantity of monomeric Fe 3+sites should be prepared.Although some other factors may also influence the apparent SCR activity of Fe–zeolite catalysts,103the conclusions drawn by Brandenberger et al.99in the above-mentioned study are generally useful for the design,synthesis and application of efficient Fe–zeolite catalysts.

Besides the nature and e?ect of Fe species in Fe–zeolite catalysts,researchers also have paid close attention to the influence of the acidic properties of zeolite materials on the NH 3-SCR reaction.On Fe-based zeolite catalysts with the same type of framework structure,the SCR activity usually increases with decreasing Si/Al ratio,104,105although the hydrothermal stability of zeolites deteriorates to a certain extent.Early work simply considered that the increase of SCR activity with low Si/Al ratio corresponded to the enhancement of Br?nsted acidity induced by aluminium sites (Al–OH),106and that the NH 4+species adsorbed on Br?nsted acid sites directly partici-pated in the SCR reaction to reduce NO x .107However,after extensive studies,the researchers finally realize that the Br?nsted acidic property is actually not a decisive factor for the high NH 3-SCR activity of Fe–zeolite catalysts,because over some zeolite materials without acidic sites,high NO x conver-sion rates could still be obtained.108Yet,in another aspect,a promotion effect by Br?nsted acidity on the NH 3-SCR reaction could still be observed if the catalysts contained the most favorable Fe species,and Schwidder et al.108concluded that this was probably due to the obvious promotion of an acid-catalyzed intermediate step in the NH 3-SCR reaction (e.g.the decomposition of NH 4NO 2)similar to the results reported by Li et al.109and Savara et al.,110which might be the rate-determining step at low temperatures.In addition,Brandenberger et al.111concluded that the Br?nsted acid sites might not be required directly in the SCR process for adsorbing or activating NH 3,but they were necessary to bind and disperse the reactive Fe 3+ions in the preparation process for Fe-based zeolite catalysts,similar to the results reported by Iwasaki et al.104Furthermore,the zeolite support with Br?nsted acidity could also play a role as an NH 3reservoir,regardless of the form in which the NH 3was stored,and in the NH 3-SCR reaction the stored NH 3could migrate to the active sites so as to undergo a reaction with NO.111A similar spill-over effect of NH 3on the zeolite support to the active Fe 3+sites in the NH 3-SCR reaction was also reported by Klukowski et al.112on their Fe–HBEA catalyst.Therefore,in brief summary,nowadays research-ers basically agree that in the standard NH 3-SCR reaction the oxidation function of Fe-based zeolite catalysts and the amount of active Fe species,but not the acidic properties of the zeolite supports,are the main factors controlling the deNO x efficiency.104,111

Hydrothermal stability is an important factor for the prac-tical use of Fe-based zeolite catalysts,to which the researchers also paid great attention.Hydrothermal treatment could result in the dealumination of zeolite supports,leading to the decrease of Br?nsted acid sites or breakdown of the framework structure,and more seriously the migration of active Fe 3+species to form clustered Fe x O y /Fe 2O 3species,leading to the decline of NH 3-SCR activity and N 2selectivity.113–116The remaining NH 3-SCR activity after hydrothermal aging under different conditions was mainly attributed to the residual monomeric Fe 3+species located at ion exchange sites.114The use of zeolite supports with high Si/Al ratio is expected to result in acceptable hydrothermal stability,but the SCR activity may be negatively affected owing to the presence of fewer Br?nsted acid sites to exchange,bind and disperse active Fe 3+sites.Recently,Iwasaki and Shinjoh 117suc-cessfully improved the hydrothermal stability of Fe–BEA catalysts by sequential ion-exchange of rare earth (RE)metals,and con-cluded that the improvement was dependent on the ionic radius of the exchanged metals.When using the RE metals with radii of 1.05–1.15?such as Ce,Nd,Sm,Gd and Tb,the dealumination of aged Fe–BEA catalyst was clearly reduced.This method could also be useful with other Fe-based zeolite catalysts for the effective improvement of hydrothermal stability in practical use.

Another important deactivation factor for Fe-based zeolite catalysts is hydrocarbon (HC)poisoning,which is closely related to the dimensional structure of zeolite supports.For example,on Fe–ZSM-5and Fe–BEA catalysts with three-dimensional structure,after HC poisoning the NH 3-SCR activity,especially at low temperatures,was markedly decreased mainly due to carbonaceous deposition.This resulted in the decline of surface area and pore volume together with suppressed NO oxidation ability for blocked Fe 3+sites,or the partial reduction of Fe 3+species to Fe 2+by HC,or the competitive adsorption of HC with NH 3/NO onto the catalyst surface.113,118,119The deacti-vation effect of HC on NH 3-SCR activity was less on the Fe–MOR catalyst with one-dimensional structure due to the difficulty in HC diffusion,and using this feature Ma et al.120effectively improved the HC resistance of a modified Fe–BEA monolith catalyst by coating another layer of MOR zeolite on the outer surface.This useful modification strategy is worthy of applica-tion in the preparation of other efficient NH 3-SCR monolith catalysts for the control of emissions from diesel engines.

In practical use,the low temperature NH 3-SCR activity of Fe-based zeolite catalysts still needs improvement to meet the diesel emission standards for cold-start and idle speed pro-cesses,121especially in the regions where Cu-based zeolite

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catalysts cannot be used,such as in Japan.122The relevant methods include the addition of catalyst promoters such as RE metal Ce 68and noble metal Pt,123together with the tuning of the reaction atmosphere,such as raising the NO 2ratio in NO x to facilitate the ‘‘fast SCR’’reaction 124,125and adding a small amount of NH 4NO 3solution as an e?ective oxidant for NO,creating similar ‘‘fast SCR’’reaction conditions.126In addition,to enable practical utilization of the catalysts,the deactivation e?ects of inorganic components contained in diesel exhaust on Fe-based zeolite catalysts are also worthy of investigation,including the combustion products of lubricant oil additives (i.e.Ca,Mg,Zn,P,B,Mo),the impurities of biodiesel fuel–urea solution (i.e.K,Ca)together with the aerosol particulates from intake air (i.e.Na,Cl);127these issues will not be discussed in detail due to the length limit of this article.

2.2.2.Cu-based zeolite catalysts.Since the use of zeolite catalysts in the NH 3-SCR reaction in the late 1970s,the Cu-based zeolite catalysts (mainly Cu-exchanged Y zeolites)have shown relatively good catalytic performance among the studied materials.5In the 1990s,the Cu-based zeolite catalysts attracted more attention from researchers in the field of catalytic deNO x from diesel engine exhaust.Cu–ZSM-5catalysts have been well studied in early studies,and showed excellent low temperature NH 3-SCR activity and a broad operation tempera-ture window even when urea was used as the reductant.128Sjo

¨vall et al.129

concluded that using a zeolite support with low Si/Al ratio and increasing the Cu loading could noticeably enhance the NH 3-SCR activity of Cu–ZSM-5catalysts.Qi et al.105reported that Cu–ZSM-5catalysts showed high deNO x performance at medium and low temperatures even at high GHSV,but the hydrothermal treatment could result in a decline in activity to a certain extent.Actually,comparative studies showed that the preparation method,the Cu loading and even the precursors used for ZSM-5synthesis could influence the hydrothermal stability of Cu–ZSM-5catalysts.105,128,130It is generally accepted that the active sites in the SCR reaction of Cu-based zeolites,including Cu–ZSM-5,Cu–beta and Cu–FAU,are mainly dimerized Cu 2+and Cu +species,and the facility of redox between Cu 2+and Cu +is beneficial to high SCR performance.10,128,131–135Through studies on the variation of SCR activity and the alteration of catalytically active sites,Park et al.131attributed the hydro-thermal deactivation of Cu–ZSM-5catalysts to the migration and re-distribution of Cu 2+species,leading to the decrease of active sites and the blockage of zeolite channels by sintered CuO,together with the dealumination and collapse of the zeolite support.After hydrothermal treatment of Cu–beta catalysts under different conditions (500–9001C,3%H 2O),Wilken et al.134found that the zeolite structure showed no obvious change below 8001C,yet the oxidation state of Cu species showed apparent change,with a decrease in Cu +species and increase in Cu 2+species as indicated by XPS results.Peden et al.135found that after severe hydrothermal treatment of Cu-beta catalysts at 9001C for 2h with 2%H 2O,the zeolite structure showed clear collapse.Therefore,in brief,the main reasons for the hydrothermal deactivation of Cu-based zeolite catalyst are the instability of active Cu species and of zeolite structures.Interestingly,Moden

et al.136reported that the active Cu species in Cu–FER catalysts with smaller pore size (0.42?0.54nm)showed higher hydro-thermal stability than that in Cu-beta catalysts with relatively larger pore size (0.66?0.67nm).Nanba et al.137–139also observed that in the presence of n -decane,NO x conversion in the NH 3-SCR reaction over the Cu–FER catalyst was less affected than that over Cu–ZSM-5.Thus,it is expected that using a zeolite support with smaller pore size may produce Cu-based zeolite catalysts with exceptional NH 3-SCR activity,outstanding hydrothermal stability and HC resistance simultaneously.

Recently,Kwak et al.140,141and Fickel et al.142reported a series of Cu–chabazite (Cu–CHA)catalysts including Cu–SSZ-13and Cu–SAPO-34,showing high NH 3-SCR activity,good N 2selectivity and excellent hydrothermal stability with great appli-cation potential in the deNO x process of diesel engines.Fickel et al.142considered that the excellent hydrothermal stability of Cu–SSZ-13and Cu–SAPO-34catalysts was mainly due to the unique structure of CHA zeolites containing eight-membered-ring pores with small pore size (0.38?0.38nm).For example,the 27Al-NMR results by Kwak et al.140showed that the hydro-thermally treated Cu–SSZ-13catalyst showed no noticeable change in the peak intensity of AlO 4species compared with the fresh catalyst;however,the peak intensity of AlO 4species in Cu–ZSM-5and Cu-beta catalysts decreased ca.57%and 31%,respectively,suggesting an obvious dealumination process.Based on the commercial Cu–CHA catalyst from BASF Corpora-tion,Schmieg et al.143found that even after hydrothermal treatment at 8001C for 16h,which was comparable to 135000mile vehicle-ageing,this catalyst still exhibited more than 70%NO x conversion from 200to 4501C.The small pore structure in Cu–CHA catalysts was also expected to be more resistant to HC poisoning due to the difficulty in diffusion of HC with larger size,such as isobutane,with a kinetic diameter of 5.5?.142

Using a variable-temperature XRD method,Fickel and Lobo 144proved that the ion-exchange of Cu species into NH 4–SSZ-13could greatly enhance its thermal stability.Through a comprehensive study using Rietveld refinement of the XRD data,in situ UV-Vis and XAFS techniques,Fickel and Lobo,144Korhonen et al.145and Deka et al.146finally concluded that only the isolated Cu 2+species located in the six-membered-rings of CHA structure coordinating with three oxygen atoms was the real active species in the NH 3-SCR reaction (Scheme 4).Using operando XAFS,

density

Scheme 4The structure of Cu–SSZ-13obtained from refinements of neutron scattering data,and magnification of the double six member (d6r)unit with Cu present as an isolated ion.(Reproduced with permission from American Chemical Society.)146

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functional theory (DFT)calculations,and first-principles thermo-dynamics models,Kispersky et al.147and McEwen et al.148proposed that under the standard NH 3-SCR conditions at steady state,the active Cu species was actually a mixture of Cu +species and Cu 2+species,indicating that the redox cycle between Cu +/Cu 2+is very important for high NH 3-SCR activity,similar to other Cu-based zeolite catalysts.Very recently,a further study using H 2-TPR and FTIR methods by Kwak et al.149revealed two distinct cationic positions in Cu–SSZ-13catalysts at different ion-exchange levels.They concluded that at low ion-exchange levels,the Cu 2+ions primarily occupied the sites in the six-membered-rings similar to those identified in early reports,while at high ion-exchange levels,the Cu 2+ions were present mostly in the large cages of the CHA structure.The latter Cu 2+species were much more easily reduced in the H 2-TPR process,indicating a high redox capability at low temperatures,which might be the main origin of the exceptional NH 3-SCR activity over Cu–SSZ-13catalysts.Therefore,although the NH 3-SCR performance of Cu–CHA catalysts has been well recog-nized,the understanding of the intrinsic active Cu sites is still in debate and ongoing.Besides,Kwak et al.150noticed that regardless of the Cu ion-exchange degree,the NO oxidation ability of Cu–SSZ-13catalysts was very low;even under ‘‘fast SCR’’conditions,the NO x conversion was only slightly improved,with little formation of N https://www.sodocs.net/doc/df18176434.html,pared with other Cu-based zeolite catalysts or even other NH 3-SCR catalysts,the above-mentioned ‘‘abnormal’’results indicate that a totally different NH 3-SCR reaction mechanism may exist over the Cu–SSZ-13catalyst,which needs to be investigated in detail using various methods in future research.In addition,new types of Cu-based zeolite catalysts are being developed constantly with high deNO x efficiency,such as the newly reported Cu–SSZ-39with AEI structure,which also shows extraordinary hydro-thermal stability.151Consequently,study of the structure–activity relationship of Cu-based zeolite catalysts in the NH 3-SCR reaction will always be a hotspot in the field of environmental catalysis.

Nowadays,most Cu–CHA zeolite catalysts are prepared by an ion-exchange method using Cu salt precursors and CHA zeolite supports,usually including ion-exchange,filtration,washing and calcination procedures.Due to the limits in channel size and exchange capacity of CHA zeolites,repeated ion-exchange proce-dures or longer ion-exchange time is necessary to increase the Cu loading.Furthermore,the synthesis of SSZ-13zeolites requires the structure-directing agent (SDA)N ,N ,N -trimethyl-1-adamant-ammonium hydroxide (TMAdaOH),which is very expensive,152limiting the wide application of this material in industry.Therefore,it is highly imperative to improve the synthesis method to reduce the cost of Cu–SSZ-13catalysts for the NH 3-SCR https://www.sodocs.net/doc/df18176434.html,ing the low-cost Cu-tetraethylenepentamine (Cu–TEPA)complex as a novel template,which is stable in a strong alkaline solution and matches the CHA cage structure very well,Ren et al.153,154successfully designed a one-pot synthesis method for Cu–SSZ-13catalysts,obtaining products with high Cu loading and high dispersion of Cu species simultaneously.Through adjusting the starting compo-sitions in the precursor gels,Cu–SSZ-13zeolites with different Si/Al ratios could be synthesized.The preliminary results showed that the

one-pot synthesized Cu–SSZ-13catalyst exhibited very good NH 3-SCR activity,especially in the low temperature range,yet much more work should be done to investigate its feasibility in practical use for the deNO x process of diesel engines.Picone et al.155used Cu cyclam (1,4,8,11-tetraazacyclotetradecane)and tetraethylammonium (TEA +)acting as co-templates to directly synthesize a Cu–SAPO STA-7catalyst with similar structure to that of Cu–SAPO-34,showing comparable performance in the NH 3-SCR reaction to that of Cu–ZSM-5prepared by an ion-exchange method and higher perfor-mance than that of Cu–SAPO STA-7with similar Cu content prepared by an aqueous ion-exchange method.The characterization results showed that a more homogeneous distribution of Cu species in Cu–SAPO STA-7was achieved via direct synthesis,which was probably the main reason for its high deNO x efficiency.The one-pot synthesis methods for Cu-based zeolite catalysts with small pore size using relatively cheap Cu-containing templates allow researchers to tune the Cu content,Cu distribution,and Si/Al ratio and thus the NH 3-SCR activity efficiently,although this approach is restricted to those zeolitic systems that can be prepared using coordination complexes as templates.156For practical use of this approach in industry,we believe,there is still a long but exciting way to go.

3.The mechanism responsible for the high performance of Ag/Al 2O 3for the SCR of NO x with oxygenated hydrocarbons

It has been widely accepted that the structure of hydrocarbons has a great influence on the activity of Ag/Al 2O 3for NO x reduction.13,16,157Oxygenated hydrocarbons such as ethanol,acetaldehyde,and propyl alcohol exhibit excellent NO x reduction activity on Ag/Al 2O 3,158,159which was also confirmed by our results.160–163More importantly,we attempted to deter-mine the intrinsic property responsible for the NO x reduction by oxygenated hydrocarbons over Ag/Al 2O 3.As shown in Fig.

Fig.2Activity of 4wt%Ag/Al 2O 3for NO x conversion by various hydro-carbons at di?erent temperatures.Conditions:NO 800ppm,O 210%,H 2O 10%,reductants (methanol 3030ppm,or ethanol 1565ppm,or acetaldehyde 1565ppm,or propene 1714ppm,or acetone 1043ppm,or IPA 1043ppm,or BA 783ppm,or TBA 783ppm),N 2balance,total flow =2000ml min à1,GHSV =50000h à1.

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methanol shows the poorest activity for NO x reduction,while ethanol,acetaldehyde,and butyl alcohol (BA)are most active,giving similar T 50(50%NO x conversion)at ca.2701C with a SV of 50000h à1.It be should noted that tert -butyl alcohol (TBA)exhibits a much lower NO x conversion in the temperature range 200–4501C,providing a T 50for NO x conversion at around 4201C,which is ca.1501C higher than that of the T 50with BA.The reductants with three carbon atoms (iso-propanol (IPA),and acetone)exhibit moderate activity for NO x reduction.Clearly,both the carbon chain length and configuration of oxygenated hydrocarbons have a pronounced effect on their properties for NO x reduction,therefore understanding such differences at the mechanism level may give clues for the design of a HC-SCR system with high efficiency.3.1.Partial oxidation of oxygenated hydrocarbons over Ag/Al 2O 3

As the initial step of HC-SCR,the partial oxidation of hydro-carbons has attracted much attention.In NO x reduction by ethanol over Ag/Al 2O 3,it has been proposed that acetate derived from the partial oxidation of ethanol plays a crucial role in the formation of isocyanate species (–NCO),as well as in the global NO x reduction process.13,30,31,167–169A possible mechanism for NO x reduction by ethanol over Ag/Al 2O 3was judged to be similar to that of C 3H 6:approximately,NO +O 2+C 2H 5OH -NO x (nitrate in particular)+C x H y O z (acetate in particular)-R–NO 2+R–ONO -–NCO +–CN +NO +O 2-N 2.23,24,30,31,167–171However,this mechanism does not sufficiently explain why ethanol has a much higher efficiency for the SCR of NO x over Ag/Al 2O 3than hydrocarbons such as propene (Fig.2).

In our earlier papers,162,172,173the formation and dynamic performance of partial oxidation products of ethanol over Ag/Al 2O 3were studied by an in situ DRIFTS method,and we found a novel enolic species originating from the partial oxidation of ethanol,with the structural feature of an oxygen atom adjacent to carbon–carbon double bonds (C Q C–O).As shown in Fig.3A,peaks at 1633,1416and 1336cm à1were assigned to the enolic species adsorbed on the surface of Ag/Al 2O 3.This assignment was confirmed by using 2,3-dihydrofuran as an enolic model compound,which has a structure containing C Q C bonded with an oxygen.From a comparison of the intensities of the respective peaks shown in Fig.3A,we deduced that the enolic species is predominant during the oxidation of ethanol on the Ag/Al 2O 3surface at low temperatures (within the range of 200–4501C).However,at high temperatures ranging from 500to 6001C,the surface acetate species exhibiting characteristic fre-quencies at 1570and 1466cm à1becomes dominant.

The partial oxidation intermediates of ethanol over Ag/Al 2O 3were further identified by synchrotron vacuum ultraviolet (VUV)photoionization mass spectrometry (PIMS),and

photoionization

Fig.3(A)In situ DRIFTS spectra of 4wt%Ag/Al 2O 3during partial oxidation of ethanol at di?erent temperatures;162(B)the photoionization e?ciency (PIE)spectra of m /z =44(C 2H 4O)measured in a flow of ethanol +O 2over Ag/Al 2O 3under low pressure at 3301C;164(C)in situ DRIFTS spectra of Ag/Al 2O 3with di?erent Ag loadings during partial oxidation of ethanol at 2501C;(D)molecular structure of the calculation model (inset figure)and its calculated FTIR spectrum for the surface C 4enolic species on Al 2O 3;(E)surface C 2enolic species on Ag/Al 2O 3;(F)C 4enolic species on Ag/Al 2O 3;165and (G)surface acetate on Ag/Al 2O 3.166(Reproduced with permission from Elsevier Inc.and American Chemical Society.)

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e?ciency(PIE)spectroscopy.164The PIE spectra of m/z=44 (C2H4O)(Fig.3B)at low pressure unambiguously illustrate the presence of ethenol(CH2Q CH–OH),with an ionization thresh-old of9.33eV.174At normal atmospheric pressure,however,this species was hardly observed in the gas phase,indicating a high level of activity.This result also indicates that Ag/Al2O3provides a suitable surface to stabilize the ethenol,because of its strong DRIFTS intensity on the catalyst surface under normal pressure (Fig.3A).Acetaldehyde,as the stable isomer of ethenol,was detected under low and atmospheric pressure with high ion intensity,giving the characteristic threshold of10.21eV.

Meanwhile,propenal(CH2Q CHCHO),acetone(CH3COCH3), and2-butenal(CH3CH Q CH–CHO)were also measured by VUV-PIMS during the partial oxidation of ethanol over Ag/Al2O3.This indicates that a condensation reaction occurs synchronously during the partial oxidation of ethanol over Ag/Al2O3,which also agrees with our TPD-MS experiment.173The condensation reactions of aldehydes,as well as ketones,are widely used in organic synthesis and are commonly catalyzed by zeolites,Al2O3,and TiO2.175–177 On a detailed mechanistic level,the role of silver species in the HC-SCR of NO x over Ag/Al2O3was mainly attributed to enhancement of the partial oxidation of reductants to form active intermediates,and their further reaction to produce N2.16,158,172,178In situ DRIFTS spectra showed that the surface species formed from the partial oxidation of ethanol over Ag/Al2O3(Fig.3C)were closely related to Ag loading.165Over pure Al2O3,the surface enolic species adsorbed on Al sites exhibited characteristic peaks at1655,1591,1392,and1336cmà1. After Ag loading,the peaks due to enolic species were observed at 1633,1416,and1336cmà1,the intensity of which gradually increased with increasing Ag content,indicating that this species would be bound on or close to Ag sites.

Density functional theory(DFT)calculations were also used to confirm the structure of adsorbed enolic species and acetate on Ag/Al2O3.162,165,166,173,179Two kinds of surface enolic species, adsorbed only on Al sites(CH2Q CH–CH Q CH–O–Al6O5(OH)8) and bound on or close to Ag(CH2Q CH–O–Ag,CH2Q CH–CH Q CH–O–Al–Ag)were suggested by DFT calculations over Ag/Al2O3and Al2O3,with the simulation molecular structure model and corresponding FTIR spectrum shown in Fig.3D–F, respectively.165As revealed by ICP measurements,165the molar ratio of Al/Ag on4wt%Ag/Al2O3was as high as60,while enolic species bound on Al sites were hardly observed(Fig.3C),strongly suggesting that the enolic species is preferably adsorbed on or close to silver sites.

On all Ag/Al2O3samples,UV-vis analysis shows that that Ag species are mainly present in the oxidized state(Ag+and Ag n d+).40,180–184Kinetic measurements confirm that such silver species,particularly strongly bound Ag+ions,are the active sites responsible for the reduction of NO x with ethanol.Thus, we propose that the enolic species on the Ag/Al2O3surface possibly adsorb on or close to isolated Ag+ions and/or Ag n d+ clusters,exhibiting an intimate contact with the active phase.This enolic species was clearly observed on the leached sample(Fig.3C), in which Ag+ions were predominant,further supporting our assumption.165By using precipitable silver compound supported catalysts such as Ag3PO4/Al2O3,Ag2SO4/Al2O3and AgCl/Al2O3,we also confirmed that the high dispersion Ag+cations were the active silver species for NO x reduction by ethanol,on which the reactive enolic species was formed having a strong infrared intensity in the typical temperature range of250–5001C.185

During the partial oxidation of ethanol on all samples, meanwhile,the peaks due to surface acetate were also clearly observed,with the characteristic vibrational frequencies (1574and1466cmà1),regardless of silver loading.These results indicate that the acetate species formed during the partial oxidation of ethanol may mainly adsorb on Al sites instead of Ag sites.The adsorption characteristics of acetate on the surface of Ag/Al2O3catalysts have also been investigated by DFT calcula-tions.166The calculated vibrational spectrum of acetate adsorbed on Al sites,present as CH3COO–Al2O(OH)2,is in good agreement with the experimental one(Fig.3G),while there is a large error between the calculated and experimental vibrational modes of the acetate species bound on silver sites,further confirming that acetate species over Ag/Al2O3are prone to interact with Al sites.

Based on the above results,the formation mechanism for surface enolic species during the partial oxidation of ethanol over Ag/Al2O3was proposed as shown in Scheme5.

Ethanol Scheme5Hypothesis for adsorbed enolic species formation and mechanism of ethanol-SCR of NO x over Ag/Al2O3.The intermediates in the gas phase such as acetaldehyde,ethenol,and2-butenal were confirmed by VUV-PIMS measurement.164Surface enolic species,acetate,and–NCO were identified by DRIFTS and DFT calculations.165,173(Reproduced with permission from Elsevier Inc.)165

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principally reacts with oxygen to form acetaldehyde,which is followed by isomerization to ethenol.Subsequently,a C2 enolic anion(CH2Q CH–Oà)–M+is formed by hydrogen extraction when ethenol is adsorbed on the surface of Ag/Al2O3.Meanwhile,the occurrence of aldol condensation of acetaldehyde,which has been confirmed by PIMS and TPD-MS measurement,may lead to the formation of C4 enolic species(CH2Q CH–CH Q CH–Oà)–M+as shown in Scheme5.164,173

Obviously,if the formation of surface enolic species follows the hypothesis described above,two prerequisites must be met.First,the selected reductant must contain at least one C–C bond.This has been clarified by our previous research,in which the enolic species was rarely observed when CH3OH(Fig.4A)and CH3OCH3were partially oxidized over Ag/Al2O3.161,186Secondly,it is widely accepted that the structural feature H–C–O–H is required for the partial oxida-tion of alcohols to aldehydes and/or ketones.That is,the OH group must be attached to a carbon atom with at least one hydrogen atom(denoted as a-H).Considering that enols are the tautomers of aledhyde/ketones,the presence of a-H is also a prerequisite for the formation of enolic species during partial oxidation of alcohols over Ag/Al2O3.To highlight this issue,butyl alcohol isomers with and without a-H were employed as reductants for NO x reduction over4wt% Ag/Al2O3.1631-Butanol(Fig.4B),sec-butyl alcohol and isobutyl alcohol,containing a-H,are favorable for the partial oxidation to form enolic species while the a-H participated reaction pathway is impossible for tert-butyl alcohol(Fig.4C)due to the absence of a-H.

Previous research further identified that surface enolic species were also produced during the partial oxidation of 1-propanol(and isopropyl alcohol)over Ag/Al2O3,160,161,187acet-aldehyde over both Ag/Al2O3172and Co/Al2O3,188and acetylene over ZSM-5.189Interestingly,substantial quantities of enols in the gas phase have been observed by VUV-PIMS during the combustion of hydrocarbons.190The above results strongly suggest that adsorbed enolic species and/or enols in the gas phase are common intermediates involved in the partial oxidation of hydrocarbons and oxygenated hydrocarbons.As a result, identification of their role is a key point in understanding of the pathway of HC-SCR.

3.2.Reactivity of intermediates originating from partial oxidation of oxygenated hydrocarbons over Ag/Al2O3

It has been widely accepted that the–NCO species is a vital intermediate for the SCR of NO x with ethanol and other hydrocarbons,thus much attention has been focused on its formation and reactivity.29,178,191–197With this in mind,the relationship between–NCO formation and the consumption of enolic species and acetate was investigated on Ag/Al2O3via the transient response of the DRIFTS method,and typical results are presented in Fig.5.165

After exposure of4wt%Ag/Al2O3to C2H5OH+O2at4001C for60min,the enolic species bound on or close to Ag sites exhibited strong peaks at1633,1412,and1338cmà1,while the characteristic vibration modes of the enolic species adsorbed on Al sites were hardly observed.Strong peaks assignable to acetate were also observed at1573and1466cmà1(Fig.5A). Switching the feed gas to NO resulted in a significant decrease in the intensity of peaks due to enolic species,while the decrease in the intensity of acetate peaks was much slower than that of enolic species on the same time scale.Meanwhile, this decrease was accompanied by the appearance of peaks due to–NCO,whose intensity increased with time on stream.These results confirm that the enolic species bound on or close to Ag sites are more active toward NO to form–NCO than acetate at this temperature(Fig.5B).

Under the same conditions as in the above experiments,we also analyzed the reactivity of enolic species and acetate when NO+O2was introduced over4wt%Ag/Al2O3(Fig.5C and D).In this case,a more significant decrease in the concentration of enolic species was observed compared with experiments carried out in the absence of O2.The trend of–NCO with time on stream follows the typical behavior of a reactive intermediate, increasing its intensity at the beginning,reaching a maximum at3min and then decreasing gradually.Meanwhile,the gas phase products such as N2and CO2were measured by

mass Fig.4In situ DRIFTS spectra of4wt%Ag/Al2O3during partial oxidation of methanol(A),1611-butanol(B),163and tert-butyl alcohol(C)163at di?erent temperatures.Conditions:the concentrations of reductants are the same as Fig.2,O210%,N2balance.(Reproduced with permission from Elsevier Inc.) Feature Article ChemComm P

spectrometry,and the results are shown in Fig.5E.Based on these data,the consumption of surface enolic species within 25min was estimated to be 283.1–566.2m mol g à1-cat,which is fairly close to that of the final product N 2,204.5m mol g à1-cat.These results quantitatively confirm that the enolic species in intimate contact with the active phase plays a crucial role in the reduction of NO x with ethanol.Over this catalyst,meanwhile,H 2–O 2titration showed that the accessible amount of silver sites was 85.34m mol g à1-cat,which is lower than that of surface enolic species.This result indicates that not only the Ag sites,but also Al sites closely linked to Ag participate in the formation of surface enolic species,further confirming our assignment for this species.

Further studies confirmed that the enolic species originating from the partial oxidation of butyl alcohol (BA),sec -butyl alcohol (SBA),and isobutyl alcohol (IBA),propyl alcohol,160isopropyl alcohol (IPA),161and acetaldehyde 162over Ag/Al 2O 3also exhib-ited high activity toward NO +O 2,and thus are responsible for NO x reduction by these oxygenated hydrocarbons.3.3.

The reaction pathway of HC-SCR over Ag/Al 2O 3

Based on the above results,a mechanism for NO x reduction by ethanol over Ag/Al 2O 3was proposed as shown in Scheme 5.In a typical reaction process,partial oxidation of ethanol results in

the formation of enolic species adsorbed on Al and Ag sites,as well as the formation of acetate on Al sites.Such di?erent features in the adsorption sites for the enolic and acetate species on Ag/Al 2O 3may contribute to their di?erent reactivity toward NO +O 2(and/or NO),and their role in the formation of –NCO during the reduction of NO x with ethanol.The enolic species intimately linked with Ag sites (RCH Q CH–O–Ag and RCH Q CH–O–Al–Ag),possessing high activity,further react with NO +O 2(and/or NO)to form Ag–NCO.The acetate bound on Al sites are less active when compared with the enolic species intimately linked with Ag sites,and thus play a minor role in –NCO formation.With an increase in the concentration of Ag–NCO,this species can transfer to Al sites,and as a result,the characteristic frequency of Al–NCO can be observed by the DRIFTS method.

The characteristic IR peaks of –NCO located around 2230and 2250–2260cm à1have been commonly observed on Ag/Al 2O 3during HC-SCR,while the assignments of their coordination sites are still controversial.Bion et al.191,192and Thibault-Starzyk et al.196observed the characteristic frequencies of –NCO on Ag/Al 2O 3located at 2255–2265and 2228–2240cm à1,which were assigned to –NCO species coordinated with octahedral Al 3+ions (Al VI –NCO)and tetrahedral Al 3+sites (Al IV –NCO),respectively.During the reduction of NO x with ethanol and propene

Fig.5Dynamic changes of in situ DRIFTS spectra over 4wt%Ag/Al 2O 3as a function of time in a flow of NO (A),and in a flow of NO +O 2at 4001C.(C)Before measurement,the catalyst was pre-exposed to a flow of C 2H 5OH +O 2for 60min at 4001C.Conditions:NO 800ppm,C 2H 5OH 1565ppm,O 210%(if used),N 2or Ar (for the case of (C))balance.(B)and (D)time dependence of the integrated areas of the peaks of enolic species (}),acetate (J ),and àNCO (n )for the cases of (A),and (C),respectively,and (E)time dependence of N 2and CO 2concentration calculated from the mass signals for the cases of (C).(Reproduced with permission from Elsevier Inc.)165

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Ag/Al2O3,the two peaks at2258–2262and2230–2235cmà1were detected by Ukisu et al.,198Kameoka et al.,23,199and Sumiya et al.29,168The high-frequency peak was assigned to Al–NCO, while the low-frequency one was attributed to adsorption on Ag sites(Ag–NCO).To identify the adsorption sites of–NCO on the Ag/Al2O3surface,DFT calculations were performed by Gao and He.194The calculated results indicate that the antisymmetric stretching mode of the Al–NCO group is located at2267cmà1 with strong intensity,which is in excellent agreement with the experimental value(around2260cmà1).The calculated mode of (OH)2Al–O–Ag–NCO shows the asymmetric stretching frequency of–NCO at2215cmà1,also close to the corresponding experi-mental one(around2230cmà1),indicating that the low-frequency peak may correspond to the–NCO species adsorbed on or close to Ag sites.Because of the weak intensity,the symmetric stretching mode for the surface structure containing Ag–NCO or Al–NCO groups has not been observed in the experimental IR spectra,which has also been proved by the DFT calculation.By using an elegant short time-on-stream in situ spectroscopic transient isotope experimental technique,more recently,Chansai et al.178,200proposed that there may be two types of–NCO species during the reduction of NO x with hydro-carbons over Ag/Al2O3.One is a slowly reacting spectator–NCO species,probably adsorbed on the oxide support of Al2O3,while another is related to reactive–NCO,possibly on or close to the active silver phase.

Organo-nitrite and-nitro species(R–ONO and R–NO2)have been regarded as potential intermediates in the SCR of NO x with hydrocarbons,which contributed to the formation of –NCO by reaction between the partial oxidation products of hydrocarbons and NO x(or ad-NO x).201As for ethanol-SCR over Ag/Al2O3,the two species may also participate in the formation of–NCO during exposure of the surface enolic species to NO and/NO+O2.In organic synthesis,enols and/or enolate anions are common intermediates in nitrosation reactions between aldehydes/ketones and substrates containing the NO group such as HONO,NO+,NOà,neutral NO,C(NO2)4,and [Fe(CN)5NO]2àto form the initial product of nitroso and nitro compounds.202–204This mechanism permitted us to hypo-thesize that similar reactions would occur during exposure of enolic species to NO on Ag/Al2O3,resulting in the formation of an organo-nitrite compound.Transformation of this organo-NO x compound to its enol tautomer,–CH Q N(OH),with subsequent dehydration to–CN and transformation to–NCO,has been proposed as a possible route for–NCO formation.181,191,205 The presence of O2promoted the reaction of enolic species toward NO,possibly resulting in the formation of an organo-nitro intermediate.This compound also contributed to produce –NCO via enol and–CNO formation.181,191,205

A general reaction pathway involving the formation of enolic species closely linked to active Ag sites was proposed to explain the NO x reduction by di?erent alcohols over Ag/Al2O3.206As shown in Scheme6,both the presence of a-H and at least C–C bonds in alcohols are necessary for the formation of enolic species.The employed alcohols with these two features thus exhibit high efficiency for NO x reduction,derived from the high concentration of reactive enolic species.As for NO x reduction by hydrocarbons such as alkanes and alkenes,however,the formation of acetate(or formate in the case of NO x reduction by CH4)and its further transformation to–NCO play a crucial role in NO x reduction.The low reactivity of acetate(or formate) results in a low efficiency for NO x reduction by hydrocarbons if compared with the NO x reduction by oxygenated hydrocarbons such as ethanol.

Water vapor is commonly present in diesel engine exhaust, the presence of which has a great influence on the reaction pathway of the HC-SCR.13,158,191Generally,the presence of water vapor usually suppressed the NO x reduction by hydro-carbons such as propene over Ag/Al2O3,while high NO x con-version was maintained when oxygenated hydrocarbons such as ethanol were employed as reductants.In our previous work,16in situ DRIFTS results showed that the presence of water vapor promoted the formation of enolic species inti-mately linked to Ag sites,while it suppressed the formation of acetate species.These opposite e?ects of water vapor on the two species may derive from their di?erent behaviors on the adsorbed sites,identification of which may provide important implications for understanding the e?ect of water vapor on the HC-SCR.

From a practical point of view,it is important to develop catalysts or catalyst-reductant systems that are active for NO x reduction in the presence of SO2.Numerous studies confirmed that the structure of hydrocarbons(oxygenated hydrocarbons) was not only responsible for the activity of Ag/Al2O3for NO x reduction,but also contributed to the sulfur tolerance of this silver catalyst.13,159,160In the presence SO2,the

deteriorated Scheme6Mechanism of HC-SCR of NO x over Ag/Al2O3.(Reproduced with permission from CAS/DICP.)206

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activity of Ag/Al 2O 3for HC-SCR was mainly due to the adsorp-tion and accumulation of sulfates on the Al 2O 3support,before which the oxidation of SO 2should have occurred on Ag sites.13,207,208As for NO x reduction by oxygenated hydrocarbons such as ethanol and acetaldehyde,large amounts of enolic species were formed,which adsorbed on or close to the active Ag sites.On the Ag sites covered with enolic species,the oxidation of SO 2would be suppressed,followed by the adsorp-tion and accumulation of sulfates.Therefore,it is reasonable that the catalyst–reductant system of Ag/Al 2O 3-oxygenated hydrocarbon exhibits high sulfur tolerance.The acetate,as the main product of partial oxidation of hydrocarbons,prefers to interact with Al sites.This situation is of benefit for SO 2oxidation on Ag sites,and thus a pronounced poisoning effect would be observed during NO x reduction by hydrocarbons such as C 3H 6over Ag/Al 2O 3.

Recently,it was found that the presence of H 2significantly enhanced the low temperature activity of Ag-based catalysts for NO x reduction by hydrocarbons.17,40,209–211Up to now,however,the interpretation of the H 2promotional effect on HC-SCR has been under debate.To increase the rate of NO x conversion,basically,the hydrogen must accelerate the rate(s)of the slow step(s)in the reaction of HC-SCR,while the slow step(s)are likely to vary depending on the reaction temperature and feed conditions.212We were the first to report the pronounced promotional effect of H 2on NO x reduction by ethanol over Ag/Al 2O 3even in the presence of water vapor.213Evidence provided by in situ DRIFTS revealed that the presence of H 2enhanced the formation of enolic species during the partial oxidation of ethanol over Ag/Al 2O 3at low temperatures.Such enhancement by H 2was also confirmed by GC-MS analysis,by promoting the formation of organic nitrogen-containing inter-mediates such as CH 3-NO 2and CH 3CN during the ethanol-SCR over Ag/Al 2O 3at low temperatures.By using in situ DRIFTS,we also found that the presence of H 2increased the activity of –NCO toward NO +O 2during the NO x reduction by ethanol over Ag/Al 2O 3.As revealed by VUV-PIMS analysis,meanwhile,the presence of H 2may trigger the hydrolysis of –NCO to NH 3,providing a new pathway for NO x reduction.206As a result,we proposed that the H 2effect on ethanol-SCR could be attributed to enhancement of the formation of enolic species and their further transformation to –NCO via the formation of organic nitrogen-containing species,which has been presented in Scheme 6.Such enhancement was also observed during the NO x reduction by C 3H 6over Ag/Al 2O 3in the presence of H 2.160,214

Typically,most of the studies dealing with the reaction mechanism of HC-SCR have been restricted to surface pheno-mena.However,Era

¨nen and co-workers 215,216proposed that the octane-SCR over Ag/Al 2O 3not only occurs on the surface of the catalyst but also continues in the gas phase,leading to the final products of N 2,H 2O,CO 2.Such a gas-phase reaction was proved by placing a commercial Pt-supported oxidation catalyst after the Ag/Al 2O 3catalyst in order to remove CO and unburnt hydrocarbons.When the oxidation catalyst was placed immedi-ately behind the Ag/Al 2O 3,NO x conversion to N 2was decreased dramatically.A similar gas-phase reaction was also observed to

occur during NO x reduction by ethanol over Ag/Al 2O 3.24,217,218The above results strongly suggest that very active N-containing intermediates in the gas phase were created over the Ag/Al 2O 3catalyst.Identification of these unstable intermediates is thus important in order to understand the mechanism of HC-SCR over Ag/Al 2O 3and to further optimize the catalysts.However,this may be a di?cult task because these gas phase intermediates

possess very high reactivity and a short lifetime.Era

¨nen et al.215proposed that gas phase reaction behind the silver catalyst may be involved in the reaction of activated NO x species with amines and https://www.sodocs.net/doc/df18176434.html,anic N-containing species such as R–NO 2,R–NCO,and R–CN are intermediates in the formation of amines and ammonia,which may also take part in this gas https://www.sodocs.net/doc/df18176434.html,ing a conventional electron-impact (EI)ionization mass spec-trometer and liquid nitrogen trap,unfortunately,only R–CN species were detected during the NO x reduction by octane over Ag/Al 2O 3,together with nitrogen-free species due to the partial oxidation of the reductant.215During the NO x reduction by ethanol or octane over Ag/Al 2O 3,large amounts of N-containing species such as NH 3and HCN were detected by a conventional MS technique.24Considering that the two gas phase species are not so active as to produce N 2in a catalyst-free system,this may not be the true story.Indeed,the real intermediates would have changed into the final product of N 2and/or more stable species owing to molecular collision and secondary reaction during the sampling and long retention times of a conventional mass spectrometer.Recently,synchrotron vacuum ultraviolet (VUV)photo-ionization combined with molecular-beam mass spectrometry (MBMS),also referred to as synchrotron VUV photoionization mass spectrometry (VUV-PIMS),was successfully applied in detecting reactive intermediates in hydrocarbon combustion chemistry.190,219In this case,molecular-beam sampling and high vacuum downstream can ensure free molecular flow of the sampled gas and reduce collision e?ects,making it possible to detect both stable and unstable intermediates;the employed vacuum ultraviolet single-photon photoionization can mini-mize fragmentation of target molecules because the total absorbed photon energy barely exceeds the ionization energy.Both of these advantages of VUV-PIMS may also provide an opportunity for identification of the reactive gas phase inter-mediates created by Ag/Al 2O 3during the HC-SCR and give deeper insight into its mechanism if this technique is employed properly.In fact,by using this powerful method,ethenol in the gas phase,an important intermediate during the catalytic oxidation of C 2–C 4alcohols over the Ag/Al 2O 3catalyst,was unambiguously identified,164,219further confirming the formation of surface enolic species during the NO x reduction by ethanol over Ag/Al 2O 3.

4.Conclusions and perspectives

The development of vanadium-free catalysts for the NH 3-SCR of NO x with high deNO x activity,N 2selectivity,hydrothermal stability and durability toward poisoning components is a con-tinuing research hotspot in the field of environmental catalysis.

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The study of the structure–activity relationship of these environmentally-benign catalysts in the NH 3-SCR reaction shows that the high dispersion of active species is a very important factor influencing the SCR performance,not only for metal oxide catalysts (such as the high dispersion of Fe 3+–(O)2–Ti 4+species in the FeTiO x catalyst and CeO x species in CeTiO x ,CeWTiO x and CeWO x catalysts),but also for zeolitic catalysts (such as the isolated Fe 3+and Cu 2+species located in ion-exchanged sites),which is a quite common view in heterogeneous catalysis.For metal oxide catalysts,the investigation of the effect of exposing different crystal facets in the NH 3-SCR reaction,especially for Fe 2O 3oxides,has achieved great success,which may be extended to other metal catalyst systems for better understanding of the intrinsic origin of excellent SCR performance on certain materials.The improvement of hydrothermal stability is highly needed for metal oxide NH 3-SCR catalysts,and is a main factor restricting their practical use in the deNO x process of diesel engines.Although the hydrothermal stability of Fe-and Cu-based zeolite catalysts is higher to a certain extent,due to the long-term operating conditions involving water vapor and timed thermal shock,the migration of isolated active sites can still occur,which requires researchers to develop novel zeolitic materials to meet the increasingly strict NO x emission standards.The Cu–CHA catalysts such as Cu–SSZ-13,Cu–SAPO-34and Cu–AEI catalyst such as Cu–SSZ-39are good examples in this field,and we believe in the near future,more and more novel,efficient zeolitic materials will be developed not only by conven-tional ion-exchange,chemical vapor deposition methods etc.but also by one-pot synthesis using cheaper templates.

The acidity of NH 3-SCR catalysts is always given close atten-tion by researchers,although many debates still remain.On metal oxide catalysts,both the Br?nsted acid sites and Lewis acid sites are usually considered to be involved in the NH 3-SCR process,while on zeolite catalysts,only the Br?nsted acid sites are proven to be useful for holding active metal sites and storing the reducing agent NH 3.The di?erences in under-standing of the e?ects of acid sites on metal oxide catalysts and zeolite catalysts may be due to the distinct dispersion,microstructure and neighboring local environments of active sites in these two types of materials.For the metal oxide catalysts,with their relatively large amount of active phase but small amount of acid sites,the proper increase in acidity may be helpful for the deNO x process;while for the zeolite catalysts,with relatively small quantities of isolated active sites but large amount of acid sites,maintaining the active sites in an isolated state is quite important.In further studies,with more advanced methods including ex situ,in situ ,and operando methods,we believe that a universally accepted structure–activity relationship for these vanadium-free catalysts for the NH 3-SCR reaction can be established.

The HC-SCR of NO x has attracted much attention as a possible alternative to the NH 3/urea-SCR.However,the activity and/or stability of catalysts developed for diesel-SCR are not su?cient to satisfy the demanding conditions of the deNO x application for diesel engines.On the other hand,the oxygenated hydrocarbons such as ethanol,acetaldehyde,propyl alcohols,and

butyl alcohols exhibit excellent NO x reduction activity on Ag/Al 2O 3,during which enolic species were formed and identified by in situ DRIFTS,VUV-PIMS measurements and DFT calculation.The enolic species originating from the partial oxidation of alcohols over Ag/Al 2O 3prefer to adsorb on or close to silver sites,in intimate contact with the active phase.This adsorption behavior of the enolic species contributes to their high activity for the formation of –NCO species and the final product N 2during the NO x reduction by oxygenated hydrocarbons over Ag/Al 2O 3.A general reaction pathway involving the formation of enolic species closely linked to active Ag sites was proposed to explain the NO x reduction by different alcohols over Ag/Al 2O 3.The presence of at least one a -H and one C–C bond is indispensable for the employed alcohols to produce enolic species during their partial oxidation over Ag/Al 2O 3.This intrinsic property responsible for NO x reduction by oxygenated hydrocarbons may provide a guideline for developing diesel-SCR of NO x systems with high efficiency for diesel engines.

Acknowledgements

The authors express their sincere thanks to the National Natural Science Foundation of China (51221892,51108446,21177142,51278486)and the Ministry of Science and Technology,China (2013AA065301,2010CB732304)for their support.

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