100 anos de sintese da amonia

ChineseJournalofCatalysis35(2014)16191640

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Review Ammoniasynthesiscatalyst100years:Practice,enlightenmentandchallengeHuazhangLiu*InstituteofIndustrialCatalysisofZhejiangUniversityofTechnology,Hangzhou310014,Zhejiang,China

A R T I C L E I N F O

A B S T R A C T Articlehistory:Received19March2014Accepted23April2014Published20October2014

Ammonia synthesis catalyst found byHaberBosch achieves its history of 100 years. The currentunderstandingandenlightenmentfromfoundationanddevelopmentofammoniasynthesiscatalystarereviewed,anditsfutureandfacingnewchallengeremainedtodayareexpected.Catalyticammoniasynthesistechnologyhasplayedacentralroleinthedevelopmentofthechemicalindustryduring the20thcentury.During100years, ammonia synthesis catalysthas come throughdiversifiedseedtimesuchasFe3O4basedironcatalysts,Fe1xObasedironcatalysts,rutheniumbasedcatalysts,anddiscoveryofaCoMoNsystem.Oftennewtechniques,methods,andtheoriesofcatalysishaveinitiallybeendevelopedandappliedinconnectionwithstudiesofthissystem.Similarly,newdiscoveriesinthefieldofammoniasynthesishavebeenextendedtoother fieldsofcatalysis.Thereisnootherpracticallyrelevantreactionthatleadstosuchacloseinterconnectionbetweentheory,modelcatalysis, andexperimentasthehighpressuresynthesisofammonia.Catalyticsynthesisammoniareactionisyetaperfectmodelsystemforacademicresearchinthefieldofheterogeneouscatalysis.Understanding themechanismand the translation of the knowledge into technical perfection hasbecomea fundamentalcriterionforscientificdevelopment incatalysisresearch.Theneverendingstoryhasnotendedyet.Inadditiontoquestionsabouttheelementarystepsofthereactionandtheimportanceoftherealstructureandsubnitridesforthecatalystefficiency,aswellasthewideopenquestionaboutnewcatalystmaterials,therearealsodifferentchallengesthrowndownbytheoryfortheexperimentalistinthepredictionofabiomimeticammoniasynthesispathatroomtemperatureand atmospheric pressure including electrocatalysis, photocatalysis and biomimetic nitrogen fixation.

2014,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:AmmoniasynthesiscatalystDiscoveryDevelopmentChallengePracticeEnlightenment

1. Theinventionandenlightenmentofammonia synthesiscatalyst

The ammonia synthesis industry has developed rapidlysincethefirstammoniasynthesisdeviceovertheworldstartedtoproduceammonia inSeptember9th,1913.Toearly2000s,theammoniasynthesisdeviceswithdailyproductioncapabilityof1000or2200tareworldwide.Ammoniasynthesishasbeenapillarof chemical industryandamilestone in thehistoryofconquestofnaturemadebyhumanbeings.

In theprocessof thisgreat invention,unprecedenteddifficulties have been encountered [1]. In 1787, C. L. Bertholletproposed that ammonia consisted of elemental nitrogen andhydrogen.Manydistinguishedchemistsatthattime, includingW.H.Nernst,W.Ostward,F.Haber,etc., immediatelycontributed great efforts into research about ammonia synthesis byelemental nitrogen and hydrogen.However, the first obstaclethey facedwas chemical equilibrium. The law ofmass actionand the lawof chemical equilibriumdidnot be found at thattime,sothatconcentrationofammoniaintheequilibriumwas

*Correspondingauthor.Tel:+8657188320063;Fax:+8657188320259;Email:[email protected]:10.1016/S18722067(14)601182|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.35,No.10,October2014

1620 HuazhangLiu/ChineseJournalofCatalysis35(2014)16191640

unclear.Atatmosphericpressure,ammoniawasonlygeneratedatvery low temperature,but itdecomposedathigh temperature.Therefore,manyscientistsevenbelievedthatthegenerationofammoniabytheelementalhydrogenandnitrogenwasaninsurmountableobstacle.

At that critical moment, Haber first proposed to use highpressurereactiontechnique.However,itwasstillhardtorealizeindustrialscaleproductionduetolowconversionperpassofammonia.SoHaberabandonedthepopularstaticviewandadoptsadynamicmethodbyintroducinganimportantconcept,thereactionrate,whichusingspacetimeyieldtoreplacereaction yield. Based on this important principle, he developedclosedprocessflowandloopoperationtechnology.Thesethreetechnologiesandconceptof reactionratewereagreat invention that provided the basis for the construction of experimental apparatus to produce ammonia and achieved the firstpressurized catalytic process in industrial history. Thiswas amilestone in the development history of the catalytic processthatrepresentedthebeginningofaneweraofindustrialcatalysis.Onlya fewyears later,methanol synthesis,FischerTropsch synthesis and highpressure reaction technology in thepresence of heterogeneous catalysts that appeared subsequentlyhavebecomeessentialpracticesinthefieldoforganicchemistry,andpromoted theentirechemicalandmaterial industries.Habersunprecedentedcreationsestablishedthebasisfortheentirechemicalengineeringscience.

InFebruary1908,Haber signedanagreementwithBadenAnilineandSodaCompany(BASF).BASFassigned the taskofindustrialdevelopment toCarlBosch.Bosch immediatelywasaware enough of the fact that he had to address threemajorchallenges: designingmethods to produce lowcost hydrogenandnitrogen;exploringanefficientandstablecatalyst;developing equipment and materials for highpressure ammoniasynthesis.

Haberandotherscientistsenergeticallyexploredcatalysts.Haberdiscovered thatosmiumanduraniumuraniumcarbidecatalystsdisplayedexcellentperformanceonammoniasynthesis.BASFCorporationacquiredpurchaserightsforosmiuminstock all over theworld, a total of about 100 kg. Although itsoundsincredibletoday,itdidfullyreflectthepassionofscientistsandentrepreneursat that time.However,HaberwasappointedthedirectoroftheInstituteofPhysicalChemistryand

ElectrochemistryKaiserWilhelmInstitutein1912,whichalsomarked the end of Habers research activities in the field ofammoniasynthesis.

Boschassignedthetaskonfindingefficientandstablecatalysts to his assistant AlwinMittasch.Mittasch first conductedextensivestudiesonmetalnitridesinanattempttofixthenitrogeninairbytheindirectroute.Althoughthattechniquewasunsuccessful for the ammonia synthesis, it provided valuableinformationonthecatalyticpropertiesofalmostall themetalelementsinperiodictable.Herecognizedthatmanyofmetalsitselfpresentedonlylittleornocatalyticeffect,butanadditivecouldimprovetheircatalyticactivity.Basedonthesefindings,inFebruary1909hemadeanunprovenhypothesis:"thewinning catalyst should be a multicomponents system" and itneededaverylargenumberofteststodetermine.Forthisreason, BASF produced a variety of model reactors for catalysttests.From1909to1911, inaboutayearandahalf,2500ofdifferent catalysts were tested at 6500 times. That amazingcatalystselectiontrial,continueduntil1922beforeitwasover,with a totalof 20000 timesof testing forover5000differentcatalystsystems.

Ironhas been knownas an effective catalyst for ammoniasynthesissincetheyearof1905.However,itwasprovedtobedisappointing in BASFs initial experiments. Someday Mittaschs assistant Wolf inadvertently used SwedishproducedGallivareironoresampleswhichhadbeenplacedontheshelfofthelaboratoryafewyearstotestthesynthesisofammonia,andreceivedunexpectedresults.Hefoundthatifafewpercentof alumina, a small amount of calcium oxide and potassiumalkaliwerefusedintopureiron,asuitablecatalystforthesynthesisofammoniawasobtained.Thebestcatalystwasprovedtobeamulticomponentmixture,whichcomprisedthesimilarcomposition of Gallivare magnetite. That is the magnetitebasedfusedironcatalystwithasmallamountofpromoterwhichisstillusedtoday.Themixedcatalystisprovedtobesoeffectivethatevennowallammoniacatalystsintheworldarestillmanufacturedbasedonthisprinciple.

Haber,Bosch,Mittasch,andErtl these fourgreat scientistshavemade a great contributionon the creation and development of ammonia synthetic industry, among whom Haber,Bosch,andErtlwereawardedtheNobelPrizeinChemistry.

Thesuccessfuldevelopmentofsyntheticammoniaindustry

FritzHaber(18681934)Laidthetheoreticalbasisonsynthesisofammonia,awardedthe1919NobelPrizeinChemistry.

CarlBosch(18741940)Realizedtheindustrialsynthesisofammonia,awardedthe1931NobelPrizeinChemistry.

AlwinMittasch(18691953)Themajordeveloperforfusedironcatalyst,whoproposedtheconceptofmixedcatalyst.

GerhardErtl (1936)Greatcontributiononironcatalystsurfacechemistryresearch,awardedthe2007NobelPrizeinChemistry.

HuazhangLiu/ChineseJournalofCatalysis35(2014)16191640 1621

isnotonlyagreattechnicalachievementbutalsoamasterpieceof the organizationwork,whichbecome a precedent in earlystage for today's prevalent collaborative innovation (teamwork).Inthecreationprocess,Haber,BoschandMittascsthegreat creation, brilliant scientific ideas and innovative spirit,thepassionandspiritofcooperationfromscientistsandentrepreneurs,aswellastheteamspiritofcooperationandcollaborativeinnovationamongchemists,engineers,physicists,materials scientists and a variety of artisans group, areworthy ofouradmirationandlearning.

Greatsuccessonammoniaindustryhaschangedthehistoryofworld foodproduction.According to the statistics fromUNFoodandAgricultureOrganization(FAO),fertilizercontributesmorethan40%tofoodproduction.Thus,thecatalyticammoniasynthesistechnologyinventedbyHaberandBoschisconsidered to be one of the greatest contributions to human beings. From the technological invention to the present, theEarthspopulationhas grownby4.2 times from1.7billionatthebeginning of the 20th century,while foodproductionhasincreasedby7.8 times.Humans can still produce ample foodandclothingunder the limited landresources,mainly relyonsuch technology created by Haber and Bosch. Now, 50% ofnitrogen in our body is from ammonia synthesis [2], whichmean, if without such invention, 50% of people in the Earthcannotsurvive.Chinaisalsounlikelytofeed20percentoftheworldspopulationbyonly7%ofarablelandallovertheworld.

After a century of development, catalytic synthesis of ammoniahasmadetremendousprogress.Theproductioncapacityofsinglesetequipmenthasbeenimprovedfromtheoriginal5tofdailyammoniaproductiontothecurrent2200t.Thereaction pressure has dropped to 1015MPa from the original100 MPa. The energy consumption has decreased to 27.2 GJfromtheoriginal78GJ,whichisclosetothetheoreticalenergyconsumptionof20.1GJ.Butasthesecondlargestchemicals,theammoniaproductionstillconsumes2%oftotalenergysupplyintheworldandreleasesmorethan400MtofCO2,whichaccountsfor1.6%oftotalglobalCO2emissions.

2. Thedevelopmentandenlightenmentofammonia synthesiscatalysts

The inventionof fused ironcatalystcreatesacatalyticammoniasynthesisindustry.Ironcatalystsforammoniasynthesisbecomeoneofthemostsuccessfulandstudiedthoroughcatalystsintheworld.Withthedevelopmentofpetrochemical,coalchemical, biochemical, polymer, materials, energy and environment,therelativepositionofresearchonammoniasynthesiscatalystinthecatalyticdomaingraduallydeclines,anditisno longerthemainaspectsofcatalysisresearch,but therigiddemandforfooddecidestheirreplaceabletraditionalammoniaindustrycanonlyrelyontechnologicalprogresstoconstantlyevolve. The catalyst of any progress can improve thermodynamicefficiencyandlowerproductprices.Therefore,advancesofammonia industryanditscatalysttechnologywillnotstop.Initially, the suitable Fe catalyst was only found by F. Haberfromabout5000triedcatalysts;currently, inorderto furtherimprove theprocess and reduceenergy consumption, further

improvingthecatalystisstilltheonlyhope.

2.1. DevelopmentofammoniasynthesiscatalystCurrentlyfusedironcatalystsstilloccupytheabsoluteposi

tion in industrywith tens of catalyst product types, ofwhichmorethantenkindsaredevelopedbyChinese.NanjingChemicalIndustryCompanydevelopedtheA102ammoniasynthesiscatalyst in 1951, which was the first Chinese selfdevelopedammoniasynthesiscatalyst,andfollowedbysuccessfuldevelopmentoftypeA106andA109ammoniasynthesiscatalysts.In1979, Zhejiang University of Technology successfully developedtypeA1102lowtemperatureammoniasynthesiscatalyst[3].AfterthattheNanjingChemicalIndustryResearchInstitute,FuzhouUniversity,LinQuCatalystPlant,ZhengzhouUniversity,Hubei Institute of Chemistry, etc. successfully developed typeA1101, A110 3, A1104, A1105Q (spherical) and A1106catalysts,whichformedawidelyappliedA110catalystsfamilysince1980s[4].

CocontainingcatalystisanimportantdevelopmentforthetraditionalFe3O4basedfusedironcatalyst.TheBritishcompany ICI applied patents on cobaltcontaining catalyst in 1978,andsuccessfullydeveloped741typecobaltcontainingcatalystin 1979. In 1985, Fuzhou University successfully developedA201typecobaltcatalyst[5],thentheamountofcobaltinA201wasfurtherreducedandCeO2wasadded,whichwascalledasthe typeA202 cobaltcontaining catalyst in1995 [6]. In addition, South ChinaUniversity of Technology,Nanjing ChemicalCompany, Zhengzhou University, also developed their cobaltcontainingcatalysts[4,7].

Since rutheniumbased catalyst for ammonia synthesis isinvented in the 1990s, most of scientists have shifted theirmain research directions and attention to the studyof rutheniumbasedcatalyst,andthusfusedironcatalystresearchhasbecome less popular. Only a few of universities and researchinstitutionsaroundtheworldarestillstudyingfusedironcatalysts, such as Szczecin University of Technology in Poland[810], Fuzhou University [11] and Zhejiang University ofTechnology [12]. Others such as the Fritz Haber InstituteMaxPlanckSociety in Germany [13] occasionally publishedresearchpapersonironcatalystsforammoniasynthesis.

To 1970s, the fused iron catalyst was considered wellconsolidatedandnospecial improvementwasstillexpected.The industrial iron catalyst presently used is not basicallydifferent from that developed 100 years ago [14]. It has becomemore difficult to achieve significant progress. This willencourage people to seek a major technological breakthroughonekindof jumpingordiscontinuous technologicalprogress. Nearly 30 years, the discoveries of Fe1xObasedcatalyst system,rutheniumbasedcatalystandcobaltandmolybdenumbimetallicnitridecatalystareexpressingtheideaofseekingtechnicalbreakthroughs(Table1).

2.1.1. ThediscoveryofFe1xObasedammoniasynthesiscatalyst Inthepastcentury,scholarsalwaysbelievedthatwhenthe

precursorof fused iron catalystswasFe3O4, catalysts showedthehighest activity. Therefore, people confined their thinking


onFe3O4 catalyst in thepast fused iron catalyst research anddevelopment,andimprovedthecatalystactivityandlifejustbychangingthetypeandnumberofpromoter,whileignoringtheimpactofcatalystprecursorphase.Althoughresearchandimprovementhadbeendoingbyscientistsincountries,magnetitewasstillofthedominance[15].In1986ZhejiangUniversityofTechnology[1621] inventedFe1xObasedcatalystwithWustitestructure,whichbroketheshackleofthetraditionalconclusion"thefusedironcatalystwithFe3O4asprecursorshowsthehighest activity", and found a breakthrough in improving theperformanceof fused iron catalystWustite catalyst system. Itmarked80yearsofresearchonfusedironcatalystshadbeenmade a substantial progress which kept the development offusedironcatalystalive.Fe1xObasedammoniasynthesiscatalystismostactivefusedironcatalystintheworld.Thisdiscovery causeswidespread concern and interest in domestic andforeign scholars [13,14,22,23], and has been widely used inindustry.

The author [24] has the opportunity to witness the constructionanddevelopmentofChinasammonia industry fromthe1960s,anddevoteshislifetothedevelopmentofcatalyticammonia technology.He contributeshis efforton thevariousresearch stages of the catalystwhich start from Fe3O4based,cobaltcontaining Fe3O4based, Fe1xObased to rutheniumbasedcatalysts.HecreatedtheFe1xObasedcatalystanditstheory system based on themonophase principle of iron oxides, and collaboratedwith his colleagues to successfully developnewindustrialcatalystsoftypeA1102,A301,ZA5,etc.,whichhavebecomeoneofmajorindustrialammoniasynthesiscatalystsinnearly30years.

2.1.2. Thediscoveryofrutheniumbasedcatalystsforammoniasynthesis

The Fe3O4based iron catalyst was considered well consolidated and no special improvement was still expected.Scientists abroad started to look for nonferrous noblemetalcatalysts.40yearsago,Ozakietal.[25]inareviewarticleproposed that chemical adsorption of nitrogen and catalytic efficiency of elements in ammonia synthesis and decompositioncould be associated, and thus could obtain a volcanoshapedcurvetoquantitativelydescriptthecatalyticefficiencyofmetalelements in ammonia synthesis. In this graph, ruthenium, osmium,andironareatthetopofthevolcanoshapedcurve.Under industrial conditions, the use of Ru and Os catalysts hasbeenclosetotheoptimumpoint.BoththeoreticalandpracticalstudiesinnearlyahundredyearshaveshownthatRu,OsandFearethebestpuremetalcatalysts.

The development of ruthenium catalyst has a long history[26].Thefirstreportabouttheapplicationofrutheniumcatalyst insynthesisofammoniawaspublishedin1917, inwhich

Mittasch,etc.believedthatcatalyticactivityofrutheniumcatalystsintheammoniaprocessisnotasgoodasthatofironcatalyst.Thenrutheniumcatalysthadnotbeenreportedina longtimeperiod.In1969,Tamaru[27]proposedatransitionmetalelectrondonoracceptortype(EDA)ofammoniasynthesiscatalystsystem.Inthiscatalystsystem,theychosethealkalimetalpotassiumorsodiumaselectrondonors,transitionmetalssuchas iron, ruthenium, osmium, cobalt, etc. as electron acceptorsand staffwith electrons transport capability such as phthalocyanine, polyphenylene quinone, graphite or graphitized carbonascarriers,anditshowedhighcatalyticactivityinammonia synthesisundermild conditions. In1972,Ozaki et al [28]foundthatwhenrutheniumasanactivecomponent,potassiumasametalpromoter, carbonas a catalyst carrier, the catalystsystemshowedhighactivity forammoniasynthesis.Thatdiscovery once again sparked scientists interest in studying rutheniumcatalysts.Afterthat,researchersinJapan,Russia,UK,USA,Italyandothercountries,aswellasZhejiangUniversityofTechnology, Fuzhou University, Xiamen University, Dalian InstituteofChemicalPhysics(DICP)andotherunitsinourcountry[2932]putalargeamountofenergyintothedevelopmentof ruthenium catalysts in order to replace traditionalironbased catalysts. British Petroleum (BP) was responsibleforloadingrutheniumcarbonylcompoundsongraphitecarboncarrierstobeanewRu/Ccatalyst.KelloggwasresponsiblefordevelopingtheammoniasynthesisprocessbyusingthatRu/Ccatalyst.With10yearsofjointefforts,theysuccessfullydevelopedin1992anewammoniasynthesisprocessKAAP(KelloggAdvanced Ammonia Process) which was applicable to Ru/Ccatalyst,andachieveditsindustrialapplications[3335].

Althoughrutheniumcatalystsarehighlyactive,theirstronginhibitionofH2andthemethanationofcarbonmaterialofthecarriersinRucatalystunderconditionsofammoniasynthesiswhichresultsinlossofactivecarboncarrierandshorteningthelife of catalyst, are weaknesses of the ruthenium catalysts.Meanwhile,theRuandOsareveryexpensive,whichislackofcommercial appealing compared to the thirdbest Fe catalyst[36].OsandUareabandonedbyHaberintheearly20thcentury.Ru/Ccatalystisnotmuchadvantageinenergyefficiency(Table 2). From1992 to 2010, only 16 ammonia plants usedruthenium catalysts. Therefore, it canbe said that theoreticalmeaning of ruthenium catalysts is larger than its practicalmeaning.The industry is still necessary to findmoreefficientandcheapercatalyststhanrutheniumcatalysts.

2.1.3. ThediscoveryofCoMonitridecatalystforammonia synthesis

Nrskovetal[36]proposedanalloycatalystisdesignedbyinterpolation in the periodic table. This catalyst developmentstrategywasobtainedbysimplephysicalprinciples,soitsbasic

Table1Developmentofammoniasynthesiscatalysts.Developmentstage Year Inventor Catalysttype Chemicalcomposition(1)Fe3O4basedcatalyst 1913 BASF,Germany S610,KM Fe3O4+Al2O3+K2O+CaO+(2)Fe1xObasedcatalyst 1986 ZhejiangUniv.ofTechnol.,China A301,ZA5 Fe1xO+Al2O3+K2O+CaO+(3)Rubasedcatalyst 1992 UKBP,Japan KAAP RuBaK/AC


principlecouldbewidelyapplied.Accordingtothisprinciple,areasonable assumption was that the elements which reactedwithnitrogenveryactivelyandveryinactivelyinA.Ozakivolcanoshapedcurve together formalloy toconstructanewactivesurfaceinordertoachievethemostoptimalperformance.The result showed that theactivityof cobaltmolybdenumnitridecatalystwashigherthanthatofRuandOscatalysts;wasalso better than the activity of either single component forammoniasynthesis;wasevenbetterthanFeandRuatlowNH3concentration [3741]. The discovery of cobaltmolybdenumnitridecatalystisconsideredtobethelatestvertexsofarinthestudy of ammonia synthesis catalyst according to theoreticalpredictions[42,43].

TheexperimentsofErtl[44]andSomorjai[45]canimproveunderstanding on ammonia synthesis and allow quantitativetheoreticaldescriptionandpredictiononthereaction.First,onthebasisofbasicknowledgeonreactionpathwaysandtransition state theory, thequantitativedescriptionof catalytic efficiency of the elements in the ammonia synthesis can be obtained. It thus canpredict the catalytic efficiencyof alloy systems[36,46].ResearchresultsontheCoMoNsystem[36,47]confirm that the both theory and experience in the choice ofcatalyst are equally useful. This impressive success storiesshowthataccordingtothetargetreactionprocess,anewcatalystsystemcanbedesignedbaseonpuretheory[36,48].Thus,thediscoveryofnonferrousandalloycatalystswillonceagainpromotethedevelopmentofheterogeneouscatalysisscience.

Herein,thatweneedtoconcerniswhattheresearchmethods of discovery and development of catalyst for ammoniasynthesis can give inspiration to us. During the invention ofammonia synthesis catalyst,Mittasch, etc. used test screeningmethod which relied on a large number of experiments andwasacompletelynovelapproachatthattime.Thatmethodissoeffective thatpeople are still following it. Thediscoveryofcobalt molybdenum nitride catalyst provide us with anothernew research method, which means that catalyst can be designedbypure theory, includingthe interpolation intheperiodictabletodesigncatalysts.Withunderstandingthetheoreticalknowledgeandregularitiesofcatalyticscienceindepth,aswell as accumulation of a lot of information and experience,especiallywith the development of computer technology, designofcatalystbasedonthetheoryandinthe"molecular"levelchangestobepossible.Inrecentyears,avarietyofexpertsystemstoassistthedesignofcatalystshavedeveloped[4952].

2.2. PeculiarityoffusedFecatalystandinspirationsfromitstheoryandpractice

The catalytic chemistry in ammonia synthesis has specialcharm. so that attracts the attention and interest frommany

chemists., Many famous physical chemist and catalytic scientists at the present age, such asW.H. Nernst,W. Ostward, F.Haber,C.Bosch,M.I.Temkin,G.Ertl,P.Emmett,A.Nielsen,H.Topse,G.A.Samorjai, J.A.Dumesic, J.K.Norskov,M.Boudart ,etc. have been attended or involved in research of catalyticammoniasynthesis[5358],andpublishedanumberofmonographs[24,5962].Thisisbecausethefusedironcatalystshavesomespecialproperties.

(1)Inthedevelopmentofthechemicalindustryinthe20thcentury,thecatalyticammoniatechnologyplayedacriticalrole[63].Theimportanceofthisindustryassociatedwiththatpeoples strong interest on understanding of important scientificvalue and technological progress on the ammonia synthesiscatalyst.Typically, thedevelopmentofnew technologies,newmethodsandnewtheorywhichwererelativetocatalysisusuallystartedfromthisreactionsystem,orwerefirstappliedtothis reaction system. Similarly,newdiscoveries in the fieldofcatalytic synthesis of ammoniawere often extended to othercatalyticfields.Thedevelopmentoffinecharacterizationtechniques, dynamic analysis and new theoretical models havegreatlypromotedtotheindepthunderstandingofthefoundationofammoniasynthesiscatalysts.

Evenbeingconstantlyimprovedforcenturies,thenatureoffused iron catalyst stillunchanged.Todate, all studieson thesynthesis of ammonia have been based on this catalyst. Forexample,thecompletionofwellknownBETadsorptiontheoryon the iron catalyst;method for determining the active componentofthecatalystsurfacebyselectivechemicaladsorptioninventedbyP. Emmett; theworkbasedonnitrogen selectiveadsorptiononFe(111)crystalfacelaidthefoundationofmetalcluster catalysis theorywhichgradually formed in the1980s;the important assumption of "crystal surfacewith the largestligand number shows the greatest catalytic activity" and theconceptof structuresensitive reactionsproposedbyG.A. Somorjai;theconceptofstoichiometricnumberwasproposedbyJ.Horiutitoverifythekineticsmechanismofammoniasynthesisreaction;M.I.Temkintheoryandhisfamousammoniasynthesiskineticequation,wasthefirstsuccessfullyemployedandisnowstillbeingusedindesignofindustrialreactors,andalsolaidthefoundationforheterogeneouscatalyticreactionkinetics.Thesetheoriesandconcepts, ledthedevelopmentofaseriesofbasic theory, laidthe foundation forheterogeneouscatalysisscience.Historyofammoniasynthesisreactionandthecatalystisamicrocosmofthehistoryofheterogeneouscatalysis.

Temkins theory of catalytic reaction kinetics on nonuniformsurfacenotonlyhasbeenprovenbydataofoverallreactionkineticsonammoniasynthesisusingironcatalyst,butalsomore importantly, perhaps, can induce some very useful anduniversal results. For example, firstly, Temkin equation is re

Table2Comparisonofironcatalystsandrutheniumcatalysts.Catalysttype Resource Manufacturingcost(103*Yuan/m3)

Conditions Energyconsumption(GJ/t)T/C P/MPa H2/N2

Fe abundant 30 350525(wide) 1030 23 ~27Ru/AC scarce 1600 325450(narrow) 10 2 ~27


ducedbasedonatwostepmechanismorthosecanbesimplifiedasatwostepmechanism,andnonuniformityofthecatalystsurfacecanusuallybetreatasauniformsurfaceinthekinetics of catalysis. Therefore it can be applied to any type ofcatalytic reactions. Secondly, as formultiple sites adsorption,thelargedifferenceoncatalyticactivityfromdifferentcatalystsis driven from themultiple reasoning to active site, which isextremely important to thediscussionof the structural sensitivityofheterogeneouscatalyticreactions.Thirdly,theconceptofdistributionfunctiononactivesiteisintroducedinthederivation of the Temkin theory, which can induce manywellknown adsorption isotherms such as Freundlich, FrumkinTemkinexpressionsandtheir formulas,andother lawsofadsorption rate such as Elovich equation. It can deduce thatoptimumactivesiteorbestactivecatalystshouldhavemoderate affinity value, which means energy distribution is in thecenterofactivesitesof thesurface.Theseresults canbeconsidered as the Sabatier principle which best catalyst can beeasily formed sufficiently stablebutnot too stable intermediates[64].

These theories provide valuable information for catalysisstudy that, the affinity valuemustbe changed inorder to getthebestcatalyst.Forexample,thefollowingthreemethodscanbe used for the metal catalysts: First, changing the exposedcrystalsurfaceortheparticlesizetoalterthesurfacestructure,whichincludesthechangesoftherelativedistributionratioofatoms on the surface with different coordination numbers;Second,forminganalloy(e.g.,copperisaddedintonickel)oraddingsurfaceimpurities(e.g.,sulfur,carbon,oxygenornitrogen)tothemodifiedmetalcatalysts[9];Third,accordingperiodictabletochangesmetalcomponentsinthecatalystinorderto select the best catalyst, such as the discovery of cobaltmolybdenum nitride catalyst. To make this approachmeaningful,itmustassumethatreactionmechanismdoesnotchange.However,whentheactivityofacatalystincreasestoacertain level, the further increaseof thembecomesverydifficult.Tobreakthroughthislevel,ithastofindadifferentreactionmechanism.

Practiceshowsthatthereisnootherreactionlikeammoniasynthesis reaction, which can link the theories, models andexperimentstogether.Theresultsobtainedinthelowpressureexperiments canbe confirmedbyhighpressure experiments;dynamicsresultingobtainedundertheultrahighvacuumconditioncanbeextrapolatedtotheindustrialconditions;studiesonasinglecrystalcanbedescribedbythetheory[44,65].Thissituationcannotonlybeenappliedtotheironcatalyst,butalsototherutheniumcatalystandCo3Mo3Ncatalyst.Moreover,therequiredtimetounderstandthesecatalysts isgettingshorter,even though the structure and chemical composition of catalystsbecomemore complex [13].Therefore, the catalytic ammonia synthesis reaction is still an ideal model system forstudyingtheoriesofheterogeneouscatalysis.

(2)Theammonia synthesis reaction isoneof the simplestchemicalreactionswhichdoesntgenerateabyproductandisagreenchemicalreactionwith100%ofatomicutilization.Thecogeneration of ammonia synthesiswith CO2 for urea or ammoniumbicarbonateisacleanproductionprocesswithoutany

emission,whichisarareandmaturedindustrialtechnologyofcombining theCO2 capture, storageanduse [66]; In industry,achieving the ammonia synthesis reaction is one of themostcomplexandtypicalchemicalprocess;Intheory,thereactionisable to be completed at room temperature and atmosphericpressurebutitpracticallyisverydifficulttobeachievedunlessat high temperature and high pressure conditions. Therefore,understanding the mechanism of the catalytic ammonia synthesisreactionandconvertingit intoaperfecttechnologyhasbeentheprimarystandardondevelopmentofcatalyticdomain.

(3) Themodern industrial iron catalysts are a nanostructuredmetastablesubstance,which is formedduringthesurprisinglycomplexsynthesisoftheoxideprecursor[67,68].Itsmetastabilityisalsothereasononsensitivityofoverheatstressgenerated during the activation and oxidative activation ofmaterials. The pathway to prepare nanostructures can be selected, suchas,Fe3O4Fe1xO [6972], anda seeminglyverysimplestructureoftheironcatalystisactuallyverycomplicated.Astartlingexampleisthatjustusingdifferentcatalystprecursors causes a tremendous change in thenanostructuresofmetal surface.Wustitebased catalyst has been demonstratedtobemoreactivethanthemagnetitebasedcatalyst[23].Quantitativeanalysis[73]revealsthat,onlylessthan1%oftheironsurfaceof the iron catalyst is involved in the activationofnitrogen,andtheremaining99%oftheirononlyplaystheroleofa support. If there can bemore exposed surface on iron, theactivityof the catalystwill be greatly enhanced. For instance,scientists used the iron catalyst as carrier, coated its surfacewithnanoirontopreparecoatednanoironcatalyst.

(4)Ammoniasynthesiscatalystisthemoststablecatalystinallindustrialcatalysts.Thestructureofsuchmetastablematerialswithnanostructureisalmostunchangedevenbeingusedundertheharshreactionconditionsformorethan15years.Alotofresearchesandcharacterizationsregardingthispropertyhavebeenconducted,andmanymodelshavebeenproposedtoexplainthestabilityoftheactivesurfaceandmechanismofitsformation[10,55,7479].

(5)Thecatalyticsynthesisofammoniawhich tightlyassociatedwithindustryisstillakeyreactionforcreatingnewlifeandaprototypemodelreactionthathelps ingaininga fundamental understanding of catalysis in general and therefore ofconsiderablescientificandculturalimportance.Itismainlythisreasonthatdrivestheresearchinammoniasynthesisforward,especiallysinceevidenceforaknowledgebasedimprovementofacatalystwouldhaveastrongsignalingeffectonotherfieldsofcatalysisresearch[80].

For example, during ammonia synthesis process, from thegasification, purification to the synthesis, the major chemicalreactions are heterogeneous catalytic process, so the catalystplays a very important role. Nine catalysts are used in thesteam conversion ammonia synthesis byusingnatural gas ornaphtha as rawmaterials,which includehydrocarbonhydrogenation catalyst, steam reforming catalyst in first stage andsecond stage, high and low temperature shift catalyst,methanationcatalyst,ammoniasynthesiscatalyst,COselectiveoxidationcatalyst,etc.;thepartialoxidationprocessusingresidue as rawmaterial and coal pressured gasification process


also use sulphurresistant CO shift catalysts, Claus sulfur recoverycatalyst,CO2dehydrogenationcatalysts,variousdetoxification catalyst and molecular sieve drying agents [81].Whichever the ammonia synthesis process, shift catalyst andammoniasynthesiscatalystareindispensableandarethecorecatalystinammoniaindustry.

This dozen of catalyst,most of them are basic catalysts inotherchemicalprocesses,suchascoalchemical,petrochemical,naturalgaschemical,biochemical,energychemical,oilrefiningindustryaswellasenvironmentalprotectionandotherchemicalprocess.Inaddition,ammoniasynthesisindustryalsocontainsaseriesofhightechandcommonkeytechnologiesneedtobesolvedinemergingindustriesofstrategicimportance.Thecatalyticammoniasynthesisprocessitselfalsocontainsahugepotential for energy saving. People will continue to improvethese catalysts. Therefore, the development of synthetic ammonia catalysts will promote the development of other catalysts.Understandingammoniasynthesiscatalystsandprocesshasastronginspirationandreferenceonaseriesofcommon,keytechnologiesinthemodernchemicalindustry,energy,materials and environmental protection fields, especially for energy saving of traditional industries, modern coal chemicalindustry, hydrogen production and clean energy and otheremergingindustriesofstrategicimportance[82].

3. ThechallengesoftheammoniasynthesiscatalystInthe21stcentury,ammoniasynthesiswascalledas"sun

setindustry".Somescientistsalsolamentallquietatthenitrogenfrontandthelowvisibilityofresearchinnitrogenfixationingeneral [83]. In thisregard, theGermanscientistR.Schlgl[13]publishedareportentitled"CatalyticSynthesisofAmmoniaANeverEndingStory?",pointedoutthestoryofcatalyticammoniasynthesisisneverover.

(1)Thenitrogencyclingisoneofmostimportantcyclinginnature to sustain life on Earth. Ammonia is also an essentialrawmaterialfortheoperationofmodernsociety,whichgivestheammoniaindustryexuberantvitality.Thesynthesisofthesematerials requires ammonia as the activated state nitrogen.Catalyticammoniasynthesisisanimportantpartofthenitrogencyclinginnature,alsoanimportantcomplementneededbyorganisms (includinghumans), and currently theonlyway toobtain activated state nitrogen in industrialscale. Currentlygenerating activated state nitrogen through othermethods isstill only the subject of scientific research. The production ofammonia requires the use of a variety of carboncontainingfuels toobtainH2gas,however,nomatterhowscarceenergysupplies and how strict the environmental controls will be,the rigid demand for food determines the ammonia industrymust relyon scientific and technologicalprogress to face thisgrimsituationandcontinueitsdevelopmentandpromotethecontinuousimprovementandinnovationonammoniasynthesiscatalyst tomeet theneedsofhumansexistenceandsocialdevelopment.

Therefore,ammoniaindustryisanirreplaceabletraditionalindustrywithvitality.

(2)Ammoniaproductionofrawmaterialsandfuelsbothare

energy. Current global focuses on energy issues are closelyrelativetotheammoniaindustry.TheemissionofCO2willalsobeseverelyrestrict,savingenergyandreducingemissionhavealways been the major issues to the ammonia industry. Thecomprehensive energy consumption of advanced ammoniaplant by using natural gas as a raw material has reachedaround27.5GJ/t,with the process total thermal efficiency ofmorethan70%[84].Commercialironcatalystandrutheniumcatalyst both can achieve the above benefits [85,86]. Any advancement in the catalyst can improve the thermodynamicefficiencyandreducethepriceoftheproduct[87].Itshouldbeemphasized that, superficially, the energy loss mainly comesfrom the transformation process and essentially should befrom synthesis of ammonia. The power consumption whichaccountsthetotalenergyconsumptionforabout30%ismainlyfortheserviceofsynthesis[88].Thehighpressureinammoniasynthesisisusedtoovercometheactivationbarrierofreaction,whichdependsonthecatalystactivity.Toovercomethisreaction energy barrier we pay a how high price! Therefore, thedevelopment of new catalyst for lowpressure ammonia synthesisismeaningful.

(3)HaberBoschnitrogenfixationprocessdoesnotinvolvetheuseofotherformsofenergyincatalyticreactions,suchaselectricenergy,lightenergy,etc.,neithertheroleofcatalystindifferentenergytransformation.Inrealityproductionpractice,thetransformationofotherformsofenergy,suchasthechemicalenergy,solar,wind,hydroandnuclearenergytransferringintoelectricenergy;orelectricenergy,lightenergytransferringintochemicalenergy;etc.issoextraordinarilyinteresting.

In the HaberBosch nitrogen fixation process which usesheatenergyfromfossilfuelasthesoledrivingforce,evenintheammonia plantwith themost advancedwaste heat recoveryandcascadeutilizationofenergy(totalenergyefficiencyisupto74%),thereisnotonlymorethan20%oftheenergysavingpotential,butalsoconsumptionof fossil fuels forat least27.5GJ/tofenergy.Eveninthelimitstate(thetotalenergyefficiencyis100%),itstillhastoconsumefossilfuelsfor20.13GJ/tofenergy.

Therefore,theintroductionofelectricenergy,solarenergy,andradiationenergyintoammoniasynthesistoassisttheactivationofnitrogenmoleculeor change the reactionpathways,and the study of the role of a catalyst in the transformationbetweendifferentformsofenergy,areinthepracticalandtheoreticalsignificance.

(4) As everyone knows, focus and difficulties of catalyticchemistryresearchistheactivationofthemoststableofseveralsmallmoleculesinnature(CO2,H2O,CO,CH4,H2,N2,O2).Nitrogen molecule is one of hardest activated elemental substancesandchemicalbond.ThedissociationenergyofNNtriplebondisthe942kJ/mol,anditsbreakrequireshighenergy.Howtoactivatenitrogenmoleculeisakeytheoreticalissuestonitrogenfixation.Nitrogenmoleculehasalsobecomeoneofthe prototype molecules in chemistry and catalysis researchwithatypicalrepresentativesignificance.

(5)Thestandardequilibriumconstantofammoniasynthesisreactionat25Cisashighas6.8105.Therefore,theammoniasynthesisatroomtemperatureandunderatmospheric


pressure is theoretically possible, but the reaction rate is almostimpossibletobedetected.Thisisanewchallenge.Sincenitrogen fixation is important for human survival and development, achieving ammonia synthesis atnormal temperatureandpressurehasbeenthegoalwithrelentlesspursuitbyhumanbeings.

In summary, reducing energy consumption of the existingammoniatechnology,lookingfornewwaysandmeansofammoniasynthesis,exploringthepossibilityofammoniasynthesisatnormaltemperatureandpressure,etc.,arethenewchallengesfacedbythecatalyticammoniatechnology.

Thekeytoachieveammoniasynthesisatnormaltemperature and pressure is the activation of nitrogenmolecule andformsandwaystoprovideenergy.

3.1. ActivationofnitrogenmoleculeThe process of converting the free state of nitrogen in air

intonitrogencompoundsisknownasnitrogenfixation,includingchemicalandbiologicalnitrogenfixation.Therearemainlythreewaystochemicallyactivatenitrogenmolecule:

(1)Reductionmethod,byusingareducingagenttogiveN2electrons. Catalytic ammonia synthesis belongs to reductionmethod.

(2) Oxidationmethod, by using an oxidizing agent to taketheelectronsawayfromnitrogenmolecule.SincethefirstionizationenergyofN2ishigh,suchastrongoxidanthasnotbeenfoundtoformasuitablecatalyticcycle.

(3)physicalchemicalmethod(activationmethod),byusingstrongconditionssuchashighVoltwithdischarge,plasmaandotherphysicalmeanstoexcitetheN2moleculefromthegroundstatetothehighenergystate,oreventakeitaparttomakeitbecomeanitrogenatomoranitrogenioninordertoreactwithothersubstances.Forexample:arcmethodandcalciumcyanamidemethod at early stage. Enormous energy consumptiongreatly limits the industrialapplicationof these twomethods.Inrecentyears,ammoniasynthesisbytheplasma[8992],themagnetic induction method [93] and other studies are alsoactive,buttheyarestillintheexploratorystage.

Thus, the catalytic reduction method occupies undisputeddominance,whichisalsocurrentlytheonlyindustrialscaleofchemicalnitrogen fixationmethod.Aftera long timeresearchand exploration, under the catalysis of ruthenium andFe1xObasedcatalysts,theinitialactivetemperatureofthecatalystscanbereducedtoabout200C[94].Forexample,intheindustrialprocessofhighpurityammonia,byusingZA5catalyst,undertheconditionsof8MPaofpressureandthereactiontemperatureattheinletandexitofreactoris215Cand363Crespectively, thenetvalueofammonia is themorethan10%,whichhasmettheeconomicrequirementfornetvalueofammonia in industry, the key is the development of the correspondinglowpressuresprocess.Butitisexpectedlydifficulttofurtherdevelophigheractivity catalystunder lower temperatures.

Theresearchoncatalyticammoniasynthesis,whichlastsfora century, is the study of the activation of N2 and its nature.Many kinds of modern physicalchemical instruments have

beenusedtostudythemechanismofactivationofN2.However,sofar,itstilliscontroversialabouttheactivationofnitrogenonironcatalysts,which is everbelonging todissociativeadsorptionormolecularadsorption.Themostofexperimentssupportthedissociativeadsorption[70,9597].Forexample,toFecatalysts,G.Ertl[98]proposedamechanismofcatalyticammoniasynthesisreactionandpotentialenergydiagramofitsthermodynamicsandkineticsbasedonN2dissociativeadsorption(Fig.1).ThisisoneofrepresentativeachievementswhenG.ErtlwontheNobelPrizeinChemistryin2007[99].

M.Boudart[80]consideredthatFigure1gavealotofguidance.Toreallyunderstandthemechanismofcatalyticreaction,itshouldbeabletoprovideakindofthermalchemicalkineticprofiles as clear as that of ammonia catalytic reaction shownFig.1.Theoristsaretryingtocalculatealotofmissingenergyvalueoftheelementarystepsinthecatalyticcycle[100].

TherearealsomanyexperimentstosupporttheN2molecularadsorption[65,101106].Forexample,Liaoetal.[107,108]studied bothmechanisms of ammonia synthesis, the associativeandthedissociative,onironcatalystssurfacebyusingthemolecule design system for heterogeneous catalysis basedonreactive energetics, the Bond Order ConservationMorse Potential (BOCMP)approachandantideuterium isotopeeffect[109].Thecalculationsshowedthattheactivationenergybarrier of ratedetermining step (rds) on associativemechanismwas below to that of rds on dissociativemechanism, but theactivation energy barrier of reaction was significantly lowerthanthatofrdsondissociativemechanism.Itcouldbeinferredthattherearetwocompetingreactionpathwaysonthesurfaceoftheironcatalyst.

In the ammonia synthesis by using iron catalyst, the stoichiometricnumberrdsofoverall reactionandratedeterminingstepwhicharedetectedfromtransferofthechemicaltracers canbeequal to1or2 (Fig.2).Bothvalueshavebeen reportedintheexperimentalwork.Horiutietal.[110,111]foundthatrds=2nearequilibrium.Tanaka[112] foundthatrdsofsynthesisreactionequals to2away fromtheequilibrium,buttherdsofdecompositionreactionequalsto1.rds=2isgoodfor the ratedetermining step in step2of Fig. 2(a), butmanyevidences indicates that thenitrogenadsorption isratedetermining step,whererds equals to1.However it cannotdeterminewhethertheadsorptionisdissociationornotwhenrds=

Fig.1.Mechanismandpotentialenergydiagramofammoniasynthesisoniron[98].TheenergyisinkJ/mol.


1. It is conceivable that the reaction sequence shown in Fig.2(b)exists,whereinthefirsttwostepsareconsideredtobethepathwaythroughwhichthenitrogenaserealizesnitrogenfixation. If so,rds still equals to1.However, the evidenceon thedissociative adsorption of nitrogen on iron catalyst is nowoverwhelming. The dissociation on iron catalysts andnondissociation on the nitrogenase respectively just characterizeindustrialcatalysisathightemperatureandenzymecatalysisatlowtemperature.Therefore,inordertoimprovetheactivityoftheironcatalystonammoniasynthesis,itmayneedto essentially change thenatureof each step in the ironcatalyzedreactionsequence[10].

Thus,theproblems,suchastheactivatedformsofN2showninFigs.1and2,thebasicstepsofammoniasynthesisreactionand the real structure etc., still containmany science implies.Thenewandefficientmethodonactivationofnitrogenmoleculeisstillbeingexplored[57].Theactivationofnitrogenmoleculeisstillachallengeinchemistryandcatalysisscience,andstillhastheoreticalandpracticalsignificance.

3.2. Newchallengesintheoryofcatalyticammonia Althoughtheheatvalueandtheeffectiveenergy(exergy)of

ammoniais21.29and20.13GJ/t,respectively,theactualenergy consumption ismuchhigher. Sonomatterwhatmaterialsand process are used to synthesize ammonia, the providedeffective energy cannot be less than 20.13 GJ/t. In the HaberBosch process of nitrogen fixation, because the effectiveenergyvalueoftherawmaterialismorethaneffectiveenergyvalueoftheproduct,idealworkoftheprocessispositive.Eachproductionof1tonofsaturated liquidammonia, theoretically,isexternalworkprocess(Table3).Forexample,theammoniasynthesisprocessbyusingpureH2andN2asrawmaterialscanprovide0.63GJ/tofexternalwork,butdirectlyusingthewaterandairasrawmaterialsforthenitrogenfixationprocessmust

consumeexternalworkof20.31GJ/tatleast.Thecomparisonshows that themain energy loss of the process derives fromextractions of nitrogen in air and the hydrogen in water.Therefore, if other forms of energy, such as electric,lightenergy,etc.,canbeintroducedintonitrogenfixationprocesstotakethehydrogenoutofwater,thenthereactionpathwaycanbechanged.Althoughatleast20.31GJofelectricworkneeds to be consumed, electric energy can be derived fromrenewableenergysources,suchassolar,wind,hydroornuclearenergy.Ifnitrogenfixationprocessdonothavetousefossilfuels,thatwillbeacompletelyrevolutiontoammoniaindustry!

Therefore,introducingtheelectricenergy,solarenergy,etc.intonitrogen fixationprocess, changing the reaction pathwayorbiomimeticsynthesisisoneofthemajorchallengesthrowndowntocatalyticscientistsandhasgreattheoreticalandpracticalsignificance.

3.2.1. ThestudiesofelectrocatalysiscatalystsElectrocatalysis can promote the thermodynamic non

spontaneousreactionN2+3H2O=2NH3+1.5O2(K298=10120)tooccurbyelectricenergy,thusexpandstheammoniasynthesisresearchfield;Italsoallowstheammoniasynthesisreactionwhichislimitedbytheequilibriumisnotorlessaffectedbythethermodynamicequilibrium.Therefore,theintroductionoftheelectricenergyintoammoniasynthesisprocesstoactivateactivationofnitrogenmoleculeor change the reactionpathwayhasbeenoneoftheconcernedresearchareas.Electrochemicalsynthesis method has similar efficiency with that of existingmethods and is a desirable method to synthesize ammoniaunder normal temperature and pressure [114]. For example,fortheelectrochemicalprocessofammoniasynthesisatahightemperature (570 C) and atmospheric pressure, the conversionofhydrogeniscloseto100%.Thus,inrecentyears,studiesof electrochemical method for ammonia synthesis at normaltemperature and pressure ammonia are also very active

Step

1 N2 + 2 * N2 *

2 N2 * + H2 *

3 N2H2 * + H2 N2H4*

2NH3 + *

N2 + 3H2 2NH3

4

i1

1

1

1

N2H2 *

N2H4 * + H2

Step

1 N2 + 2 * 2N *

2 N * + H * NH * + *

3 NH * + H * NH2 * + *

NH2 * + H * NH3 + 2 *

H2 + 2 * 2H *

N2 + 3H2 2NH3

4

5

i1

2

2

2

3

(a) (b)

Fig.2.ThemechanismsofN2dissociativeadsorption(a)andmolecularadsorption(b)andtheirstoichiometricnumber.

Table3Theoreticalenergyconsumptionofammoniasynthesis[113].Rawmaterial Totalreactionequationofprocess H=Hv Theoreticalenergyconsumptionofprocess(GJ/t)

Theoreticalenergyconsumptioofproduct(GJ/t)

Water,air H2O+N2NH3+O2 21.26 20.31 20.13Water,air,coal C+H2O+(N2+O2)NH3+CO2 0.80 0.19 20.13Water,air,naturegas CH4+H2O+(N2+O2)NH3+CO2 1.85 0.94 20.13Water,air,lightoil C9H2O+H2O+(N2+O2)NH3+CO2 1.41 1.65 20.13PureH2andN2 H2+N2NH3 3.95 0.63 20.13


[115118].Themainelectrocatalystswhichhavebeenstudiedinclude

iron phthalocyanine catalystloaded gas diffusion electrodes,the ceramic solid electrolyte and the molten salts(LiCl/KCl/CsCl) and so on. Using solid electrolytes with highprotonconductivityat roomtemperature to improve the current efficiency and the stability of electrodes is an importantdirectionforfutureresearchonelectrochemicalammoniasynthesis[87,119,120].

Lowcurrentefficiencyisthekeytoinfluencetheefficiencyandproductcostsofelectrochemicalammoniasynthesis.Withtheindepthstudyoftheelectrochemicalammoniasynthesis,ifcurrent efficiency and conversion rates can be significantlyimprovedsothatthecostoftheelectrochemicalammoniasynthesis can be focused on consumption of electric energy, theelectrochemical ammonia synthesis in the remote districts ofsufficient inelectricenergy,oreffectivelyconvertingsolarenergy,windandwaterenergyintoelectricityisexpectedtohaveitsplace.Especiallywhentheenergycrisisinthefutureleadstopricesofoil,naturegasandcoaletc.raisesharplywhichresultsincostsofHaberBoschammoniasynthesisgrowingexponentially,theelectrochemicalammoniasynthesiswillberegardedasausefulalternative.Therefore,thestudyofelectrochemicalammoniasynthesisstillhaspotentialapplication[115].

3.2.2. ResearchonphotocatalyticammoniasynthesiscatalystsThemost familiarphotocatalyticreaction isnaturalphoto

synthesis:CO2+H2OCH2O+O2.Greenplantsabsorbsunlightbychlorophyll(photosensitizer),convertCO2andH2ObyplantenzymestocarbohydrateandreleaseO2.Photosynthesisisthemostimportantwaytoconvertsolarenergyintochemicalenergy.Themostcriticalstepincomplexprocessofphotosynthesis is thesubstances inphotosyntheticreactioncenterabsorbphoto energy to release electrons which are transferred intocellstocausechemicalsynthesisreactionsothatsolarenergyisstoredup[121].

Atroomtemperatureandatmosphericpressureusingwaterasahydrogensourceandsolarenergyasenergy,aphotocatalyticwaytodirectly transformthenitrogen inair intoammonia:N2+3H2O2NH3+1.5O2,needtoresolvethesolarenergyinputandphotocatalystsofproblems.

These two reactions are both thermodynamically nonspontaneousreactions,andN2isactivatedtheharderthanCO2,but their photocatalysis are both theoretically achievable.WhicheverthenaturalCO2reductionreactionorartificialwater reduction (producing H2) and oxidation (producing O2)reaction,isaverycomplexcatalyticprocessthatusuallyoccursthroughmultipleelectronspathway, andcombinationsofelementaryreactions.MichelandDeisenhoferwhocowinnersoftheNobelPrizeinChemistryin1988[122,123]usedtheoretical calculations to conclude that the common effect of asymmetric nuclear Frankcondon factors and the electronic couplingis likelythemainreasonforunidirectionelectrontransfer.Thestudyresultsofmechanismofphotosynthesisand itscenter structure will provide inspiration for photocatalyticammoniasynthesis.

The researchonphotocatalysishasmore than50yearsof

history [124].Most of the photocatalyst used as thematerialhavingsemiconductorcharacteristics ,suchasdifferentseriesof metaldoped TiO2 and WO3 series [125], and CdS/GaPPt,Fe2O3Nd2O3catalystandsoon[126132].Currentlypeopleareconstantlydevelopingmoreeffectivecatalystandthenewmethods for ammonia synthesis at normal temperature andpressure [133135]. This shows that people are exploring tothislongtermgoal.

3.2.3. Studiesonchemicalsimulationofnitrogenaseammoniasynthesis

Innature,thereisamicroorganism,whichcomprisesacatalystwith a special abilitynitrogenase, that can directly reducethenitrogeninairtoammoniaatnormaltemperatureandpressure conditions. Its nitrogen fixation capacity is thousandfoldofHaberBoschchemicalnitrogenfixationprocess.Itis estimated that today the biological fixation of nitrogenreached200milliontons,coveringabout48%ofthecombinednitrogen in earth surface (the remaining 52% is provided bycatalyticammoniasynthesis).Biologicalnitrogenfixation,bothitsrequiredconditionsandnitrogenfixationcapability,ismuchhigherthanchemicalnitrogenfixation.Biologicalnitrogenfixation can be divided into biological and biomimetic chemistrynitrogenfixation.

Biomimetic chemistry nitrogen fixation uses chemicalmethodstosimulatethefunctionofnitrogenaseinvivotoprepare fine chemical catalyst in order to achieve ammonia synthesisatnormaltemperatureandpressure.Thisisachallengefacingcatalysisscientists.Itisboththeoreticallyandpracticallysignificanceonstudiesaboutmechanismofbiologicalnitrogenfixation. It can provide an important basis for the chemicalsimulationof biologicalnitrogen fixation.To achievenitrogenfixation by nitrogen fixation microbes, there are three basicconditions [136]: (l) nitrogenase; (2) MgATP2; (3) electrondonors, such as reduced ferredoxin, reduced flavodoxin, orartificial Na2S2O4 to provide electrons for N2 reduction. The1970s1980s,agroupofChinesescientists ledbyfamousscientistsAoqingTang, Jiaxi Lu andQirui Cai indepthly studiedthe nitrogenase and its chemical simulation [137], and proposed amodel of the active center of the nitrogenase [138].After continuous efforts of scientists around the world, thechemical simulation of nitrogenase has been developed, andnitrogen fixationmolecular genetics has been created, whichhave made biological nitrogen fixation research significantprogress.SincetheAmericanscholarReesetal[139143]clarifiedthethreedimensionalstructureofnitrogenaseactivecentralatomclustersandpolypeptidesaround, thestudiesaboutchemical simulation of biological nitrogen fixation are onceagain on the rise [144151]. Meanwhile, the development ofselectiveenzymeswillbearichsourceofcatalystsfororganicchemistry and biotechnology [152]. Although no satisfactorypracticalresulthasbeenachievedsofar, theresearchandexploration for biological nitrogen fixation and biomimetic ammoniasynthesiswillnotbestopped.

In addition, in the biological azotobacter nitrogen fixationmethod,mainlynonleguminouscropsareinoculatedbyrhizobiumusingbioengineeringtechnologyto introducethenitro


genase genes and other related genes so that it can achieveselfprovidednitrogen [153].With thedevelopmentofbreeding transgenic technology to achieve nonleguminous cropswithinoculationofrhizobiumforselfprovidednitrogenandalarge number of planting leguminous and other oil plants(nitrogencontaining 7%8%), and reducing the amount ofchemicalnitrogenfertilizerisonewaytosolvenegativeeffectsof a lot of fertilizer application in the agricultural production[154].

4. ConclusionsAmmonia synthesis in catalytic chemistry is charismatic.

Fused iron catalysts have some special properties which arebeyondcomparetoanumberofothercatalysts.Theammoniasynthesis reaction of high industrial relevance is also a keyreactionforcreatingnewlifeandaprototypicalmodelreactionthathelpsingainingafundamentalunderstandingofcatalysisin general and therefore of considerable science and cultureimportance.Understandingof themechanismof catalytic ammonia synthesisandconverting it into theperfect technologyhas been a basic standard in the catalytic domain, especiallysinceevidenceforaknowledgebasedimprovementofacatalyst would have a strong signaling effect on other fields ofcatalysis research.Therefore, it is still an idealmodel systemforheterogeneouscatalysisresearch.

Catalytic ammonia technology plays a central role in thedevelopmentofthechemicalindustryinthe20thcentury.Humansneedfood,andfoodneedsnitrogen,sothestoryofcatalyticammonia synthesis isneverend.Ammonia isalsoanessentialrawmaterialfortheoperationofmodernsociety,whichgivesexuberantvitalitytoammoniaindustry,andwillcontinueto promote the improvement and innovation on catalyst for

ammonia synthesis. In the 21st century, catalytic ammoniatechnologywillfacenewchallengesintheoryandpracticeandinnewapplicationofammonia.Reducingtheenergyconsumptionofexistingcatalyticammoniasynthesistechnology, introducing electric energy, light energy into ammonia synthesisprocess,lookingfornewwaysofammoniasynthesis,exploringelectrocatalysis, photocatalysis and chemical simulation biologicalnitrogenfixationonammoniasynthesisatnormaltemperatureandpressureareconcernedresearchfield.

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Pagenumbersrefertothecontentsintheprintversion,whichincludeboth theEnglishandChinese versions of thepaper.The online versiononlyhastheEnglishversion.ThepageswiththeChineseversionareonlyavailableintheprintversion.

Ammonia synthesis catalyst 100 years: Practice, enlightenment and challenge1. The invention and enlightenment of ammonia synthesis catalyst2. The development and enlightenment of ammonia synthesis catalysts2.1. Development of ammonia synthesis catalyst2.1.1. The discovery of Fe1xObased ammonia synthesis catalyst2.1.2. The discovery of rutheniumbased catalysts for ammonia synthesis2.1.3. The discovery of CoMo nitride catalyst for ammonia synthesis

2.2. Peculiarity of fused Fe catalyst and inspirations from its theory and practice

3. The challenges of the ammonia synthesis catalyst3.1. Activation of nitrogen molecule3.2. New challenges in theory of catalytic ammonia3.2.1. The studies of electrocatalysis catalysts3.2.2. Research on photocatalytic ammonia synthesis catalysts3.2.3. Studies on chemical simulation of nitrogenase ammonia synthesis

4. ConclusionsReferences

100 anos de sintese da amonia

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