Cr-free Fe based catalysts for high-temperature water-gas shift reactions Xúc tác Cr-Fe nhiệt độ cao cho phản ứng water-gas shift Các phản ứng thay đổi nước và khí (WGSR) mô tả phản ứng của carbon monoxide và hơi nước để tạo thành carbon dioxide và hydrogen (hỗn hợp của carbon monoxide và hydrogen được gọi là khí nước)Các phản ứng thay đổi khí nước được phát hiện bởi nhà vật lý người Ý Felice Fontana trong năm 1780. Mãi đến sau này nhiều mà giá trị công nghiệp của phản ứng này đã được thực hiện. Trước những năm đầu thế kỷ 20, hydro thu được từ phản ứng của hơi nước dưới áp lực cao với sắt để sản xuất sắt, oxit sắt và hydrogen. Với sự phát triển của quá trình công nghiệp mà yêu cầu hydro, ví dụ như các HaberBosch tổng hợp amoniac, nhu cầu về một phương pháp ít tốn kém và hiệu quả hơn trong sản xuất hydro là cần thiết. 1 Như một giải pháp cho vấn đề này, các WGSR được kết hợp với các quá trình khí hóa than để sản xuất một sản phẩm hydro tinh khiết. Vì lý tưởng của nền kinh tế hydrogen tăng phổ biến, tập trung vào hydro như một nguồn nhiên liệu thay thế cho các hydrocacbon ngày càng tăng.
Trang 1jo u rn a l h o m e pa ge :w w w e l s e v i e r c o m / l o c a t e / c a t t o d
a Department of Chemical and Biological Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea
b Green School, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea
c Clean Energy Research Center, Korea Institute of Science and Technology (KIST) 39-1, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
a r t i c l e i n f o
Article history:
Received 31 August 2012
Received in revised form
20 December 2012
Accepted 21 December 2012
Available online 20 February 2013
Keywords:
Water-gas shift reaction
High-temperature shift
Fe/Cr catalysts: Cr-free catalysts
a b s t r a c t SinceitwaspatentedbyBoschandWildat1914,theFe/Cr-basedmixedoxidecatalysthasbeenusedfor water-gasshiftreactions(WGSRs).Untilthepresent,thiscatalysthasbeenusedastheprimarycatalyst forindustrialhigh-temperatureshift(HTS)reactions.However,becauseenvironmentalconcernsabout chromiumelementswereraisedintheearly1980s,thereplacementofchromiuminHTScatalystshas beenintenselystudiedbymanygroups.ThesestudieshavecontributednotableinsightsintoHTScatalysis usingFe-basedoxides,especiallyaboutthereactionmechanismandfunctionsofpromoterelements.In somecases,thepotentialofusingasubstituentmetalpreviouslyneglectedbecauseofpropertiesinferior
tothoseofchromiumwasrediscoveredafternoteworthyimprovementswereproducedbycombining
itwithothermetalsinpromotingtheFe-oxidecatalyst.ThispaperreviewstherecentstudiesofCr-free Fe-basedHTScatalysts,especiallyfocusingontherolesandfunctionsofthenon-chromiumpromoters
inthecatalysts
© 2013 Elsevier B.V All rights reserved
Water-gasshiftreaction(WGSR)isaredox-typereactionto
con-vertcarbonmonoxideand watervaporintocarbondioxideand
hydrogen(Eq.(1)),whichwasfirstdiscoveredbyanItalianphysicist
FeliceFontanain1780[1]
CO(g)+H2O(v)↔CO2(g)+H2(g) [H0=−41.1kJ/mol] (1)
WGSRisnowmostlyassociatedwiththesteamreformingof
hydrocarbons(naturalgas,petroleumgas,naphtha,gasoline,coals
andvarioustypesofbiomass)toproducehydrogenforuseinthe
synthesisofammoniaandmethanolandfortheFischer–Tropsch
process[2–4].WGSRgeneratesadditionalhydrogenusinggases
remaining after steam reforming, which is generally used to
optimize the H2/CO molar ratio optimal for the production of
(liquid)hydrocarbonsintheFischer–Tropschprocess.In
polymer-electrolyte membrane fuel cells (PEMFC) systems,it is usedto
removecarbonmonoxide,whichpoisonstheelectrodecatalysts
[5,6].ThroughWGSR,thecarbonmonoxidecontentisreducedfrom
10–15%to0.5–1%,whichis thenfurtherreducedtotracelevels
∗ Corresponding author at: Department of Chemical & Biological Engineering,
Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea.
Tel.: +82 2 3290 3299; fax: +82 2 926 6102.
E-mail address: kylee@korea.ac.kr (K.-Y Lee).
(<10–100ppm)bypreferentialoxidation(PROX)ormethanation catalysis[7]
WGSR is reversible and moderately exothermic, yielding 41.1kJ/mol of carbon monoxide The equilibrium constant for WGSR depends on the temperature, as expressed by Eq (2), whichimpliesthattheWGSRproducts(orH2/COratio)decrease withincreasingtemperature[1,8].AnequilibriumCOconversion (XCO,eq.)isdeterminedfromtheequilibriumconstantandgas com-positionusingEq.(3)[1]
Kp= yCO2 ,eq·yH2 ,eq yCO,eq·yH2 O ,eq =exp
4577.8
T −4.33
(2)
Kp=(yCO2 ,in+yCO,in·XCO,eq)(yH2 ,in+yCO,in·XCO,eq) [yCO,in(1−XCO,eq)](yH2O ,in−yCO,in·XCO,eq) (3)
yA,eqistheequilibriummolefractionofcomponentAatT;yA,inis themolefractionofcomponentAinthereactantmixture;XCO,eqis theequilibriumCOconversionatT;Tisthereactiontemperature (wheretheequilibriumstateisdefined)
Becauseofthisexothermicreversibility,WGSRistypically per-formed in two stages: a high-temperatureshift (HTS, typically 370–400◦C,10–60atm)andalow-temperatureshift(LTS,∼200◦C, 10–40atm)[1–4].Theunitsareconnectedviaaninter-stagecooler HTSischaracterizedbyfastkinetics,butthefinalCOconversion
islimitedbyequilibrium.Incontrast,LTSundergoesslow kinet-ics,but thethermodynamiclimitationismuch lessseverethan
0920-5861/$ – see front matter © 2013 Elsevier B.V All rights reserved.
Trang 2thatof HTS.Bycombining HTSandLTSin seriesand matching
both properly, adjustmentof the final gas composition (H2/CO
ratio)becomesfeasibleandfreefromthermodynamiclimitations
Becauseof the differentcorresponding reaction conditions and
environments,different catalysts areused in HTS compared to
anLTSprocess[1–3,9].InmostindustrialWGSRprocesses,
Fe/Cr-basedmixedoxides(Fe/Cr/Cu)andCu/Zn/Almixedoxidesareused
astheHTSandLTScatalysts,respectively.Inresearch,Co-based
cat-alysts(Co/Mn,Co/Cu,Co/Mo)haveattractedinterestduetotheir
sulfurtoleranceandhighHTSactivity[9–11].Au-supported
cat-alystshave beenextensively studiedfor theirhighLTSactivity
andpotentialstability inoxidativeatmospheres[1,12–14].PGM
(platinum-groupmetal) catalystshaverecentlygainedattention
duetotheirbroadapplicability,coveringbothHTSandLTS[1,15]
However,mostcommercialapplicationsstilladoptFe/Cr/Cuand
Cu/Zn/Alasworkingcatalystsbecauseoftheirhighactivity,
dura-bilityandreasonablemanufacturingcost
Steamtocarbonmonoxide(S/C)ratioisoneoftheimportant
controlledfactorstodeterminetheperformanceoftheWGSR.First,
theequilibriumCOconversionincreasesastheS/Cratioincreases,
increasingthefinalH2/COratiowithoutincreasingthecatalystload
ortemperature.Steamisamildoxidantthatslowsthereduction
ofthecomponent(metal)oxidesinaWGSRcatalyst,whichlargely
preventstheexcessivereductionandactivitylossofthecatalyst
duringthereaction.Steamalsoslowsmethanation(reaction
for-mula(4)and(5)),whichisanundesirablesidereactionofWGSR
becauseitdecreasesthehydrogenconcentrationandcausesaloss
ofsurfaceareabecauseofitsexothermicity
CO(g)+3H2(g)↔CH4(g)+H2O(v) [H0=−206.3kJ/mol]
(4)
CO(g)+H2(g)↔ 12CH4(g)+12CO2(g) [H0=−247.3kJ/mol]
(5)
Forthereasonspresentedabove,anappropriateamountofextra
steamisusuallyaddedtothegasstreamfortheWGS(water-gas
shift)reactor
InadditiontotheS/Cratio,thereductionfactor(orRfactor),
definedasthecontentsofreductivegases(COandH2)relativeto
thecontentsofnon-reductivegases(CO2andH2O)(Eq.(6)),isused
byseveralresearchersasthemeasureofthe‘reductive’natureof
reactantgasesinWGSR[16–22]
R= PCO+PH2
PCO2+PH2 O
(6)
Inpractice,theRfactorisusedtopredictwhetherthereactant
gascausesover-reductionofFe3O4(toFeOorFe)inaFe-basedHTS
catalyst.Itisgenerallyknownthatthegasdoesnotcause
over-reductionwhenRisbelow1.2,butover-reductionoccurswhenR
isabove1.6[18,19,22]
TheWGSreactantgascompositionvarieswidelybasedonthe
choiceoffeedstockandreformingconditions(S/Cratio,
temper-ature,pressure,w/f).Forexample,thetheoreticalcompositionof
biomass-reformedgas(40%H2,44%COand16%CO2)[23]isquite
differentfromthatoftypicalsteammethanereformate(56.7%H2,
10%CO,6.7%CO2and26.7%H2O)[24].ThevariationintheR
fac-torofHTSreactantgasescanbeobservedinTable1[18,23,25–33],
2and3.Ingeneral,theRfactorincreasesfrom0.3–1.0to1.0–1.6
asthesyngasH2contentincreaseswhenusingnaturalgasasthe
feedstockforsteamreforming(CH4hasthehighestH/Cratioamong
hydrocarbons),andtheadditionofextrasteamisrequiredto
pre-ventover-reductionofFe3O4bydecreasingRfactorofthereformed
Table 1
R factors adopted in various studies.
Cu/Mo/Fe 0.81 (2.00) [23]
Cu/Th/Fe 0.86 (6.00) [26]
Cu/Al/Fe 1.00 (1.00) [28]
Cu/Al/Fe 1.17 (1.00) [29]
Cu/Al/Fe 1.17 (1.00) [30]
Cu/Al/Fe 0.70 (2.00) 1.17 (1.00) [31]
Ni/Al/Fe 1.58 (2.50) [32]
V/Fe 1.00 (6.00) 2.33 (2.00) [19]
Cu/Al/Fe 1.00 (6.00) 1.40 (4.00) 2.33 (2.00) [33]
a Steam/CO ratio.
gas.However,lowerextra-steamadditionisdesirabletoreduce operationalcosts[20].Theamountofextra-steamshouldbe care-fullydetermined,becausetheperformanceofaWGScatalystwould changeifappliedunderdifferentRfactors
2 Fe/Cr-based HTS catalysts
2.1 GeneralbackgroundforFe/Cr-basedHTScatalysts Iron–chromium(ferrochrome,Fe/Cr)oxidewasfirstpatentedas
aWGScatalystbyBoschandWildin1914[34].Althoughnearly
acenturyhaspassed,Fe/Crisstillutilizedasaprimarycatalyst forHTSbecauseofitsreasonableactivityanddurabilityinmost applications
The pre-activatedcatalyst is generally composedof 87–95%
Fe2O3 (ferric oxide, mineral name: hematite), 5–10% Cr2O3 (chromium(III)oxide)andmiscellaneousothercomponents,such
asCuO,Co2O3 and/orMgO[3,9,35,36].TheFe/Crcatalystis usu-allysynthesized through the base-catalyzed co-precipitation of
Fe2(SO4)3and Cr2(SO4)3 usingNa2CO3[35].Theresidualsulfate ionsshouldbecarefullywashedtoavoidproducinghydrogen sul-fideduringactivation(pre-reduction)andreaction,whichpoisons theLTScatalystdownstream oftheHTScatalystbed[35].After calcination,themajorcatalystphaseisaFe2O3–Cr2O3mixture,in whichCr2O3isoccasionallyincorporatedintothe␣-Fe2O3lattice [9,35].Thecalcinedcatalystshouldbepre-reducedbeforeitsuse
inthereaction,throughwhichFe2O3 isturnedintoits catalyti-callyactiveFe3O4 (ferricferrousoxide,mineralname:magnetite) phase.Thepre-reductionisusuallyperformedat315–460◦Cusing reactantgas(syngas)[3];however,topreventover-reductioninto FeOormetallicFe,theRfactorofthereactantgasisadjustedto approximately1.0byaddingextrasteam[18,20]
In plant operation, thecommercial Fe/Crcatalysts generally decreasethecarbonmonoxideoutputfrom10%to15%infeedflow
to2–3%[2,35].TheactivityoftheFe/Crcatalystisimprovedbythe additionofapromoterelement,suchasCu,decreasingthe acti-vationenergy [37]and increasingselectivity(i.e.,inhibitingthe methanation[35])ofthecatalysis.SelectedtestresultsforFe/Cr catalystsarelistedinTable2[38–42]
TheWGSRmechanismofFe/Crcatalystsistypicallyunderstood
asaredox-typemechanism,whichhasbeeninterpreted intwo ways(Fig.1):(i)theregenerative(Rideal–Eleytype)and(ii)the associative(Langmuir–Hinshelwoodtype)mechanisms[3,9].The formerisoftenperceivedasmoresuitableforFe/Crcatalysts[3] Theregenerativemechanismisusuallyfacilitatedbytheexchange
ofelectronsbetweenFe2+andFe3+intheoctahedralsiteof mag-netiteduringWGScatalysis[29,43]
TheFe/Crcatalystisusedina processforanaverageof 3–5 yearswithoutexchangingwithfreshcatalyst[3,44].Theactivity decreaseismostlyduetothethermalsinteringofthemagnetite
Trang 3Table 2
HTS performances of Fe/Cr and Fe/Cr/Cu catalysts.
Fe/Cr S/C = 3.5 0.3d 60,000 1 400 ◦ C: 65% [97%] [39]
Fe/Cr S/C = 4.8 0.3 d 3000 20 360 ◦ C: 89% [89%] [40]
(93/7) S/G = 0.7
Fe/Cr/Cu S/C = 3.5 0.3 d 60,000 1 400◦C: 79% [97%] [38]
Fe/Cr/Cu S/C = 2.0 0.9d 10,000 10 380 ◦ C: 79% [87%] [41]
(90/8/2) S/G = 1.0 d
Fe/Cr/Cu S/C = 3.5 0.3d 60,000 1 400 ◦ C: 79% [97%] [42]
a Steam/CO ratio.
b Steam/gas ratio.
c “Wet gas” base.
d Estimated from the data given in the paper.
phase,butinplantoperation,increasingthereactiontemperature
compensatesforthisdecrease[45].Additionally,theFe/Crcatalyst
isseldomdeactivatedbysulfurpoisoning,unlikeLTScatalysts(e.g.,
Cu/Zn/Al)[45]
2.2 CatalyststructureofFe/Crcatalyst:roleofCr
Formanyyears,thechromiumintheFe/CrHTScatalystshas
beenpredominantlyrecognizedasastabilizertopreventthe
ther-malsintering of Fe3O4 and loss of surface areaof thecatalyst
Inafreshstate,Fe3O4/Cr2O3 hasamuchhigherspecificsurface
area(40m2/g for Fe3O4/8wt.% Cr2O3)than un-promotedFe3O4
(8m2/g)[46].Fe3O4/Cr2O3 alsoexhibitsmuch slowermagnetite
sintering than un-promoted Fe3O4 [9] Chinchen et al insisted
thatas thereactionprogresses,discrete Cr2O3 grainsgrowand
becomedispersedoverFe3O4domains,therebyblockingthe
ther-malagglomerationofFe3O4particles[47,48]
AdifferentopinionisthatCr3+ionsenterintotheinverse-spinel
latticeofFe3O4andformasolidsolution.Robbinsetal.foundthat
Cr3+ionsdissolveintotheFe3O4 latticeandoccupythe
octahe-dralsiteandthedisplacedFe2+andFe3+ions(fromtheoctahedral
sites)aretransferredtothetetrahedralsites[49].Edwardsetal
claimedthatthedissolvedCr3+isenrichedatthesurfaceregionof thecatalystandthattheCr-enrichedsurfaceshell,beingmore ther-modynamicallystablethantheFe-richcore,reducesiondiffusion andsinteringeffects [50].Natesakhawatetal.reportedthatthe
Cr3+inFe/CrwasoxidizedtoCr6+duringWGScatalysis[28].The
Cr3+↔Cr6+ oxidation–reductioncycle wasexpectedtoenhance theredoxrateofmagnetiteandpromotetheWGSactivityofthe catalyst
Today,itisgenerallyunderstoodthatchromiumactsasbotha textural(preventingthermalsintering)andfunctional(enhancing redoxefficiency)promoterinFe/CrHTScatalysts
2.3 Thenecessityofsubstitutingchromiumwithotherelements Hexavalentchromium(foundinchemicalcompounds contain-ingCr+6)isastrongcarcinogen,threateninghumanhealthandthe environment[51].Exposurethroughinhalationanddrinkingwater causescancerandseriousdamagetohumanorgansandskin.In contrast,trivalentchromium(Cr3+)hasverylowtoxicityandisa nutrientforthehumanbody[51]
Concernsabouttheenvironmentalhazardandtoxicityof hexa-valentchromiumhave beenraisedsince theearly1920s.Since
Trang 4Table 3
HTS performances of Cr-free Fe-based catalysts.
(mol ratio) S/G ratio b [WHSV] [Equil CO Conv.]
Fe/Al/Cu/Ce S/C = 10.0 1.0d 7000 h−1 20 350◦ C: 92% d [95%] [61]
(89/8/2/1) d S/G = 1.0
(96.3/3.4/0.3) S/G = 1.0 (3000 h −1 , dry gas base) 53% (after aging) [95%]
Fe/Al S/C = 1.0 1.0 d [0.060 m 3 /g cat /h] 1 400 ◦ C: 25% [77%] [28]
(91/9) S/G = 0.1
Fe/Al/Cu e S/C = 1.0 1.0 d [0.060 m 3 /g cat /h] 1 400 ◦ C: 46% [77%] [28]
(87/9/4) S/G = 0.1
Fe/Al/Cu f S/C = 1.0 1.2d [0.060 m 3 /g cat /h] 1 400 ◦ C: 57% [63%] [30]
(77/8/15) S/G = 0.1
Fe/Ni S/C = 3.7 1.2 [0.060m3 /g cat /h] 1 400◦C: 70% [78%] [20]
(80/20) S/G = 0.6
Fe/Ni S/C = 2.6 2.0 [0.025m3 /g cat /h] 1 400 ◦ C: 50% [65%] [21]
Fe/Ni S/C = 2.6 2.0 [0.025 m 3 /g cat /h] 1 400 ◦ C: 64% [65%] [21]
Fe/Ni/Cs S/C = 2.6 2.0 [0.075 m 3 /g cat /h] 1 400◦C: 61% [65%] [66]
Fe/Ni/Zn S/C = 2.6 2.0 [0.030 m 3 /g cat /h] 1 400 ◦ C: 65% [65%] [67]
Fe/Ni/Al g S/C = 3.0 1.4d 14,500 h −1 d 1 400◦C: 54% [57%] [32]
(37/22/41) S/G = 0.4 (10,000 h −1 , dry gas base)
Fe–Ni/Ce–Zr S/C = 3.7 1.1d 15,600 h −1 d 1 400 ◦ C: 75% [70%] [68]
(11–10/53–26) d S/G = 0.6 (10,000 h −1 , dry gas base) Slight methanation
a Steam/CO ratio.
b Steam/gas ratio.
c “Wet gas” base.
d Estimated from the data given in the paper.
e Prepared with co-precipitation.
f Prepared with sol–gel method.
g Prepared by solution-spray plasma method.
theU.S.NationalResearchCouncilpublishedthegeneral
guide-linesforchromiumcompoundriskassessmentsin1983,theEPA
haspublishedmanypracticalguidelinesfortheidentificationand
assessmentofhexavalentchromium[52].TheOccupationalHealth
andSafetyAdministration(OSHA)undertheU.S.Departmentof
Laborenforced strict regulationsregarding worker exposureto
hexavalentchromium in severalindustries [53] In Europe,the
recentlypublishedEuropeanRestrictionofHazardousSubstances
(RoHS)bannedtheuseofsixhazardousmaterials,including
hexa-valentchromium,inallelectronic–electricalequipment[54].Itis
onlyamatteroftimebeforetheseregulationsareexpandedtocover
entireindustries
ReturningtotheFe/CrHTScatalyst,thechromiumspeciesin
afreshFe/CrcatalystisusuallyCr+3(Cr2O3),whichismuchless
toxicthanCr+6.TheCr+6contentislow,but workersmusttake
precautionswhen handlingthecatalystthroughout thespanof
theoperation.Moreover,Cr+6iswater-solubleandisleachedfrom
thecatalystbycondensedsteamorcoldwater,whichcouldbea
threattotheenvironment,evenwithminimaldisposal[51].There
areseveralpossibilitiesforproducinghexavalentchromium
dur-ingthemanufacturingofthecatalyst.For instance,someofthe
Cr+3ionsthatwerenotprecipitatedcanbeoxidizedintoCr+6when
thecatalystiscalcinedathightemperaturewiththemineralbase
(Na+)presentintheprecipitates[51].Themotivationforreplacing
chromiumwithotherelementsmostlyliesintheseenvironmental
andhealthconcerns[55].InadditiontoWGSR,theissueof
replac-ingchromiumisbeingsimilarlydiscussedinthedevelopmentof
catalystsforfattyalcoholproduction(thehydrogenationoffatty
esters)[56,57]
Note:TheHTSactivitiesofCr-freeandCr-containingcatalysts arelistedinTables2and3.Itwasnotstraightforwardtocompare
acatalystwithanotherinactivity,becauseeachcatalysthadbeen testedinadifferentreactioncondition(S/Cratio,Rfactor,w/f, tem-perature,etc.).Sotheactivitieswerelistedinatableformatwith fullreactionconditionsprovided
3.1 Earlystudies:1980–1990 Chinchenfirst tried usinga Cr-free Fe-basedHTS catalystto replacechromiumwithanelementcapableofforminga spinel structureinironoxidewithoutundulydilutingthecatalytic activ-ity[58].Thechromereplacementswerechosenaccordingtoionic sizeandoxidationnumber[28,58].Amongthecandidates,Ca,Ce andZrwerecapableofformingspinelstructureswithFe.Fe/Ceand Fe/ZrshowedhighersurfaceareasthanthecommercialFe/Cr cata-lysts,buttheirspecificactivities(activitypertotalcatalystweight) werelowerthanthoseofcommercialcatalysts.Asimilarattempt
byRethwischandDumesicwasalsounsuccessful.Theytriedusing Zn(II)andMgtoreplaceCr,buttheactivitiesofFe/ZnandFe/Mg were30timeslower thanthatofmagnetite [59].It wasargued thattheelement(ZnorMg)displacedallof theFe2+ionsfrom theoctahedralsiteofinversespinellattice,preventingFe2+/Fe3+ redoxtransfers, whichare thedrivingforceoftheregenerative mechanismformagnetitecatalysis
Rethwisch et al dispersed un-promoted magnetite over a graphitesupport(Fe3O4/C)toenhancethecatalyticturnoverrate
Trang 5catalystinitiallyshowedhighactivity,whichthendecreasedafter
afewhours.Themagnetiteagglomeratedduringthereaction,but
theauthorsarguedthatthedeclineinactivitywasduetothe
con-strictionofporeswithinmagnetiteclusters,whichwasdrivenby
thesurfacehydroxylation ofmagnetite underwetatmospheres
Theactivityrecoveredtosomedegreeafterthecatalystwas
de-hydroxylatedunderadrycarbonmonoxideatmosphere
3.2 Fe/Al-basedcatalysts:Al/CeorAl/CuasreplacementsforCr:
1995–2010
The first promising results for developing Cr-free Fe-based
catalysts were arguably those of Ladebeck and Kochloefl [61],
whoreplacedCrinaFe/Cu/CrcatalystwithAl/Ce.Theresulting
Fe/Cu/Al/Cecatalystshowedactivitysuperiortothatofa
commer-cialcatalyst.Sincethen,Alhasbeenstudiedmoreintensivelythan
anyotherelementasareplacementforchromiuminFe-basedHTS
catalysts.Fromnow,itwillbereviewedinitsproperchronological
order
Araújo and Rangel proved that the activity promotedby Al
becomesmoreprominentwhenCuisincludedinthemagnetite
texture[33].Underlowsteam-to-gasratios(S/G=0.4;estimated
R factor=0.9),the Fe/Al/Cu catalyst wassimilarin HTSactivity
butbetterinselectivity(i.e.,methanationsuppression)compared
toa commercialCr-containingcatalyst.Theauthorsarguedthat
Al/Cupromotestheformationofthemagnetitephaseduring
pre-reductionandstabilizesthephaseagainstfurtherreduction.They
deemedthatCuactedasatexturalpromoterratherthana
func-tionalpromoter(creating orpromotingactivitybyaffectingthe
electronicpropertiesofthemajoractivespecies[23]).However,
thethermalstabilityoftheFe/Al/Cucatalystwasnotwithinthe
scopeofthestudy
Liuetal.studiedanAl/Ce-promotedFecatalyst(Fe/Al/Ce),which
wasgivenaproprietaryname,NBC-1[62,63].Thebasicideawas
toadopt ␥-Fe2O3 (maghemite) asthe backbone of thecatalyst,
whichwasthoughttobemoreeffectivethan␣-Fe2O3 in
incor-poratingpromoter elements, byutilizing the vacantsitesof an
imperfectspinelstructure [64] Fromthe context,it is inferred
thattheauthorsregarded Aland Cebothastexturalpromoters
forthemagnetitephase.Theauthorsclaimedthatthecatalystwas
activeandthermo-resistant,byshowingthatitwascomparableto
acommercialFe/CrcatalystinbothHTSactivityandspecific
sur-faceareameasuredafterhigh-temperatureaging(530◦C,15h).The
achievementwouldhavebeenmorepromisingifthecatalysthad
beentestedundermorereducibleconditions.(TheRfactorofthe
reactiongasisestimatedtobe0.6,whichismuchlowerthanthe
conventionalvalue,1.0.)
Regarding the Fe/Al/Cu HTS catalyst, the Ozkan group have
publishedseveralnoteworthystudiesinthelastdecade[28–31],
providingasystematicunderstandingofthecatalyst
Usinginsitu XRDandTPRstudies,thegroupproved thatAl
playedarolesimilartothatofCr,inhibitingthethermalgrowthof
themagnetitephaseandstabilizingthephaseagainstfurther
reduc-tiontoFeOandFe[28].However,theXPSstudiesindicatedthatAl
didnotpromotetheredoxrateoftheironoxidebecausetherewas
nochangeinitsoxidationstate(Al+3)duringWGScatalysis.Unlike
Al,Crchangeditsoxidationstate(+3↔+6)duringcatalysis,which
wasthoughttoallowittoactasafunctionalpromoterinpromoting
theWGSactivityoftheFecatalyst.SimilarlytotheresultsofAraújo
and Rangel’sstudy[33],theHTSactivitywasgreatly enhanced
whenAlandCuwereincludedinthecatalysttogetherasFe
promo-ters(Table3).CuwasconsideredafunctionalpromoterfortheFe/Al
catalystbecauseTPRanalysisindicatedthatCugreatlyenhanced
thereducibilityoftheironoxide[28].Theauthorsclaimedthat
therearetwowaysforCutoparticipateinthecatalysis:(1)theCu
Fig 2. BET surface areas of Fe/Ni catalysts FNxxyy denotes a Fe/Ni catalyst with Fe and Ni in xx and yy wt.%, respectively; “Aged” implies that the catalyst was aged with reaction for 3 h under 400 ◦ C, H 2 (56.7%), CO (10%), CO 2 (6.7%), H 2 O (26.7%) and WGSV = 0.025 m 3 /g cat /h.
speciesservesasanelectronic(functional)promoterandpromotes theredoxrateofthecatalysisand(2)theexcludedCuspeciesis presentonthecatalystsurface,whichisreducedtometallicCu dur-ingthereactionandprovidesadditionalactivesites,similartoCu
intheCu/Zn/AlcatalystinLTSreactions.However,becausemetallic
Cuisverypronetothermalsintering,itisdesirabletoincorporate allCuspeciesintoanironoxidestructureandformaperfectsolid solution[29].Thesol–gelpreparationofthecatalystfulfilledthis objective.Intheauthor’sfollowingpapers[29,30],itwasproven thatCuwasuniformlydistributedovertheironoxidematrixwhen thecatalystwaspreparedusingthesol–gelmethodatpH9with ironacetylacetonateastheFeprecursor(othersinnitrates),and
C2H5OH/NaOHwasusedasthesolvent/precipitant[29].Such uni-formitywasnotobtainedusingtheconventionalco-precipitation method.Thesol–gelmethodismoreadvantageousinthatitallows theformationof␥-Fe2O3tobeinducedbyadjustingtheFe2+/Fe3+ ratiointheprecursorsolutionandtheagingtime.TheTPRandXPS measurementsconfirmedthat␥-Fe2O3helpsincorporatethe pro-moterelements(AlandCu)intotheironoxidestructuretoform
auniformsolidsolution[29].Theauthorsfurtherimprovedthe preparationmethodusingpropyleneoxideasthegelationagent, whichimprovestheHTSactivityandstabilityfortheFe/Al/Cu cat-alyst[31]
3.3 Fe/Ni-basedcatalysts:HTScatalysisofFe/Ni/ZnandFe/Ni/Cs underhigh-R-factorconditions:2009–2011
Nihasbeenperceivedasunsuitableforuseasacomponentof WGScatalystsbecauseitiseasilyreducedunderWGSconditions andmanifestshighmethanationactivity[65]
However,ourresearchgrouphasfoundthat Niis capableof formingasolidsolutionwithironoxide,producingaFe/Nicatalyst thatexhibitsreasonableHTSactivityevenunderhighlyreducible conditions(Rfactor=2)ifpromotedbyanotherappropriate ele-ment[20,21,66,67].Ni/Fecatalystswerepreparedbyconventional co-precipitation,whichproducedinversespinelNiFeO4after cal-cinationinairat500◦C.TheinclusionofNiincreasedthesurface areaofthefreshcatalyst,buttheeffectwasdrasticallydiminished whenthecatalystisagedinHTSreaction(Fig.2 soitis techni-callyimpropertorefertoNiasatexturalpromoter.Nicaninstead
bereferredtoasafunctionalpromoter:underahighRfactorof2, theNi/Fe(67/33inmol%(Table3)or66/34inwt.%[21])catalyst showedhighinitialCOconversion(64%),closetotheequilibrium value (65%), whereas thecommercialFe/Cu/Cr catalyst showed only 50% conversion [21] TPR measurements confirmed the
Trang 6Fig 3.HTS activities of Cs-promoted Fe/Ni catalysts; FN: Fe/Ni (66/34 in wt.%),
xCsFN: x wt.%-Cs impregnated Fe/Ni (66/34 in wt.%); H 2 (56.7%), CO (10%), CO 2 (6.7%)
and H 2 O (26.6%), R factor = 2; 400◦C; WHSV = 0.075 m 3 /g cat /h [66]
Ni-enhancedredoxrateofironoxides[67].EvenwithahighR
fac-tor(R=2)andathightemperature(400◦C),thecatalystmanaged
tomaintainitsinitialactivityforover11h.However,partofthe
cat-alystwasreducedtoFeNi3(awaruite)duringthereaction,which
isattributedatleastinparttothemethanationsidereaction[21]
MethanewasproducedfrombothCOandCO2[21],fromwhichthe
selectivityoftheHTSreactionovertheFe/Nicatalystisestimated
as85–90%(Table3)
Theproblemoflowselectivity,thatis,theoccurrenceof
metha-nation,wasovercomebypromotingtheFe/Nicatalystwithcesium
[66]orzinc[67]
ByimpregnatingCsontheFe/Nicatalyst,theHTSactivitywas
greatlyenhancedandthemethanationwaseffectivelyrestrained
(Fig.3)[66].Thecatalystsweretestedunderaweighthourspace
velocity (WHSV=0.075m3/gcat/h) three times higher than that
usedinthepreviousstudy(Table3).Becauseoftheincrease in
WHSV,theCOconversionofun-promotedFe/Ni(NF,inFig.3)was
almosthalvedto32%.Undersuchadverseconditions,theFe/Ni/Cs
catalysts(3.9CsNFand6.0CsNF,inFig.3)showednear-equilibrium
COconversion(63%and61%)withalmost100%selectivity.Based
onCO2-TPDanalysis,theimprovementofthecatalyticperformance
wasattributedtotheincreaseinthenumberofweaklybasicsites
byCspromotion,onwhichtheformate-intermediatedassociative
mechanismwasthoughttoprogress
ZnpromotionalsoenhancedtheHTSactivityoftheFe/Ni
cat-alyst[67].Znwasco-precipitatedwithFeandNitoformasolid
solutionof(Zn,Ni)Fe2O4inversespinelspecies.TheZn-promoted
Fe/Ni (Fe/Ni/Zn) showed near-equilibrium CO conversion with
excellent methanation restraint (selectivity over 98%, Table 3
whichwassimilartothepreviousCs-promotedNi/Fecatalyst.The
catalystshowedverystableperformance,maintainingitsactivity
over15h.However,ZnpromotionisthoughttobeinferiortoCs
promotionintermsofactivityenhancementbecausesuchalevel
ofactivitywasobtainedunderaWHSVof0.035m3/gcat/h,whereas
Fe/Ni/Csachievedasimilarlevelofactivityunderanearlydoubled
WHSVof0.075m3/gcat/h(Table3).Znperformstheroleof
func-tionalpromoterfortheFe/Nicatalystverywell:first,Znprevents
thereductionordisintegrationoftheinversespinelphaseduring
reaction.Throughtime-dependentXRDanalysis,itwasfoundthat
FeNi3 wasformedfromthedisintegrationofanunstable,
incom-pletelayerofzinc–nickelferritenearthecatalystsurface.When
theunstablelayerwasusedup,thecorecrystalof(Zn,Ni)Fe2O4was
intact,andthereducedphase(FeNi3)didnotgrowfurther
through-outtherestofreaction[67].Second,Znenhancedthereducibility
ofthecatalyst,promotingCOoxidationwithlatticeoxygen,which
leadstoanincreaseintheWGSrateandselectivity.Theimproved reducibilityofFe/Ni/ZnwasconfirmedbyH2-TPRandCO-TGA mea-surements[67]
Watanabeetal.showedthatcombiningFe/NiwithAlresulted
in excellent HTS activity without significant methanation [32] (Table 3).The authors prepared the Fe/Ni/Al catalyst withthe solution-sprayplasmatechnique toproduceFe/Nispecies well-dispersedonthehollowAl2O3 sphere.Duringthereaction,the Fe/NispecieswerepartiallyreducedtoNi–Fealloy(FeNi3),which theauthorsnotedwasacrucialspeciesinsuppressinghydrogen adsorptionandCOmethanation.Thecatalystshowedthebest per-formanceintermsofHTSactivityandmethanationsuppression whentheFe/(Fe+Ni)atomicratiowasbetween0.5and0.8 TheauthorsdevelopedthisideaintodispersingFe/Nispecies
onthemesoporous CeO2–ZrO2 supportprepared bythe “hard-template method”usingKIT-6 asa template material [68].The purpose of this study wasalso to minimize methanation over theFe/Nispecies Thebasicideaswere,first,toimproveNi(or Fe–Ni)dispersionusingasupportwithalargespecificsurfacearea, andsecond,tousea reducibleoxidesupportthatimprovesthe transferrateoflatticeoxygen(inordertosuppressmethanation and increase the selectivity) Bothrequirements were simulta-neouslysatisfiedbyimpregnatingFe/Nispeciesonthemesoporous CeO2–ZrO2 preparedbythehardtemplate method.Thecatalyst showedimprovedthermalstability,HTSactivityandmethanation suppressioncomparedtothecatalystpreparedusingconventional, co-precipitatedCeO2–ZrO2 support.In particular,theformation
ofFeNi3 inahighlydispersedstateoverFe–Ni/CeO2–ZrO2(hard template)ledtoamoreeffectivesuppressionofmethanation 3.4 Othernoteworthystudies:1998–2011
Costaetal studiedtheuseofTh asa replacementforCrin Fe/Cr/Cucatalysts[26].BecausetheionicradiusofTh4+(0.94 ˚A)
isconsiderablylargerthanthatofFe3+(0.69 ˚A),Th4+wasnot incor-poratedintotheironoxidematrix;instead,itwenttothesurface, formingasegregatedphase.However,thepresenceofThresulted
intheformationofsmallerironoxideparticlesandhinderedthe thermal sinteringof theparticles.In addition,although Th was present atthesurface,it stabilized themagnetitephase against deeperreduction.Hence,Thcanbecategorizedasatextural pro-moterforFe-basedHTScatalysts.Exceptforitspresenceonthe surface,ThisalmostidenticaltoAlinitscharacteristicsandroleas
apromoter.LikeAl,itsactivityisalsolargelyenhancedbytheuse
ofCuasaco-promoter.TheauthorclaimedthattheFe/Th/Cu cat-alystismoreactivethanacommercialFe/Cr/Cucatalystat370◦C, S/G=0.6(S/C=6)andRfactor=0.8
Júnioretal.triedusingV(IV)(vanadium)asachrome replace-ment[19].Inthisstudy,vanadium-dopedmagnetitewasprepared
byheatingsol–gel-prepared,iron(III)–vanadium(IV) hydroxoac-etateunder nitrogen Because magnetite wasproduced directly withthismethod,pre-reductionwasnotneededwhenusingthis catalystintheHTSreaction.Vanadiumwaslocatedmainlyonthe surfaceasV2+andV5+species.Thereissomedoubtabouttheclaim thatvanadiumactedasatexturalpromoterforthemagnetitephase becausethespecificsurfaceareaofV-dopedmagnetitewasalready small(25–28m2/g)inthefreshstateandthedifferencefromthatof un-dopedmagnetitewasmarginal.However,thevanadium stabi-lizedFe3+andincreasedtheactivityandselectivityofthemagnetite phase,implyingthatitactsasafunctionalpromoter
Martosetal.studiedFe/Mo(VI)/CuasaCr-freeHTScatalyst,using theoxidation-reductionmethodtopreparethecatalyst[23].Mo6+ wasincorporatedperfectlyintothemagnetitestructureduetoits smallerionicradius(0.62 ˚A)comparedtoFe3+(0.69 ˚A).Although thecatalystwaspreparedwithoutthermalcalcination,the spe-cificsurfaceareaofFe/Mowasquitesmall(32m2/g).However,a
Trang 7linearrelationshipwasfoundbetweentheMocontentand BET
area,whichimpliesthatMoisatexturalpromoterforthe
mag-netitephase.However,theactivityenhancementandstabilization
ofthemagnetitephasewereobtainedwhenMowaspairedwith
Cu,whichwasverysimilartothecasesofFe/Al/CuandFe/Th/Cu
describedpreviously
Boudjemaa et al [69] examined the influence of acid-base
properties on Cr-free Fe-based catalysts in HTS reaction using
insituDRIFTmeasurementsasamajoranalyticaltool
Hematite-(later in reaction, magnetite-) supported SiO2, TiO2 and MgO
wereusedasthecatalysts,andtheorderofactivitywasrelated
to the basicity of the materials (measured by the activity in
isopropanol dehydrogenation): Fe2O3/MgOFe2O3/TiO2>Fe2O3
(notsupported)Fe2O3/SiO2.ThehighactivityofFe2O3/MgOwas
explainedintermsofa formate-intermediatedassociate
mecha-nism,inwhichthecarbonylspeciesadsorbedonFereactswith
hydroxylgroupsontheFe–MgOinterfacetoproduceformate
inter-mediates.Itwasproposedthatthedecompositionrateofformate
speciesgovernstheoverallreactionrate,whichisfacilitatedbythe
weakermetal-oxygenbondinthebasicoxides
Mahadevaiahetal.incorporatedFeintotheCeO2 crystalline
network,andtheresultantcatalystexhibitedimpressive
perform-ancesunderLTSandHTSconditions[70].Itisgenerallyaccepted
thatCeO2 isa goodredoxmaterialfortheregenerative
mecha-nisminWGScatalysis.InCeO2,thelatticeoxygeneasilyinteracts
withadsorbedCO(turningintoCO2),andthedepletedoxygensite
isrestoredwiththereleaseofH2fromH2O.TheWGSactivityis
furtheraugmentedifCOadsorptionispromotedbyanother
ele-ment.Thenoblemetals(Pt,Pd, Rh)areusuallychosenfor such
purpose[71],buttheauthorsusedFeinstead,partially
substitut-ingthe CeO2 withFe toform a solid solutionof Ce1−xFexO2−ı
Asaresult,FenotonlypromotedCOadsorptioninWGS
cataly-sisbutalsoenhancedtheoxygenstoragecapacitybysynergetic
redox interaction between Ce4+/Ce3+ and Fe3+/Fe2+ [70]
Near-equilibriumCOconversionwasachievedusingCe0.67Fe0.33O1.835
attemperaturesabove450◦C,andtheactivitywasexpandedto
theLTSregion(∼285◦C)whenPtwasco-dopedinsidethecatalyst
(Ce0.67Fe0.33Pt0.02O1.785)
AnotherexampleoffixingFeontoanon-magnetitestructurewas
suggestedbytheworkbySunetal.,inwhichtheperovskite
struc-turewasutilizedasacatalystmatrixforFe[72].Thebasicidea
wasasfollows:intheLaFeO3perovskitestructure,La3+lowersthe
bindingenergyofoxygeninFeO6octahedra,promotingthe
trans-ferrateoflatticeoxygen(␣-oxygen)totheadsorbedCO(possibly
onFe)toenhancetheWGSrateinaregenerative(redox)
mecha-nism.Inaddition,theauthorsadoptedageneralmethodtoincrease
theredoxpropertyoftheperovskitecatalyst,substitutinganother
cation(Ce4+)forthecationsinLaFeO3.Becauseoftherestriction
inionicradius,Ce4+isincorporatedexclusivelyintotheA-site(i.e.,
La3+),makingtheperovskitestructurenon-stoichiometric,which
is higherin oxygenstoragecapacity and thermal stability than
stoichiometricLaFeO3.Hence,thenon-stoichiometricLa0.9−xCexFeO3
catalystismoreactiveinHTSreactionsthantheLaFeO3catalyst
Inaddition,itsactivityattemperaturesabove550◦Cwashigher
thanthoseofcommercialFe/Cr/Cucatalystsoperatingat450◦C
TheaccommodationofCe4+inthiscatalystwaslimitedtolow
val-ues(x=0.2;above0.2,theperovskitebecameunstable);however,
asmallamountofCeO2wasalwaysfoundinthecatalysts.Itwas
claimedthatthesegregatedCeO2 phasealsoenhancestheWGS
activityviaitsintrinsicredoxproperty
WehavesummarizedtheresultsofpreviousstudiesofCr-free
Fe-basedHTScatalysts,especiallythoseaddressingthepromoters
usedtoreplaceCrinFe-basedHTScatalysts
(1)First, apromoter elementshouldformasolid solutionwith iron oxides or at least be located in the surface layer in a well-dispersedstate.IntheFe-basedWGScatalysts,therole
of promoter is divided into textural and functional roles.A texturalpromoter,whetheritexistsasindividualcrystalliteor fusesintoironoxidelattices,enhancesthecatalyst microstruc-ture(surfacearea,porosity,grainsize)andbehavesasabarrier forthermalgrowthofironoxidecrystallites(i.e.,thermal sin-tering).Afunctionalpromoterenhancestheredoxrateofthe catalystwithitsownredoxactivityorbyfacilitatingtheredox cycleofironoxide.Regardlessofrole,itismoredesirablefor
a promotertoformahomogeneoussolid solutionwithiron oxides
(2)Chromium in a commercial Fe/Cr or Fe/Cr/Cu HTS catalyst actsasatexturalandfunctionalpromoter.Amongthechrome replacementpromoters,AlandThhavefunctionalitiesas textu-ralpromoters,preventingthermalagglomerationandexcessive reductionofthemagnetitephase.CeandCufunctionas func-tionalpromotersforthemagnetiteor‘promoted’magnetite(e.g., Fe/Cr,Fe/Al,Fe/Th)phases,improvingtheredoxpropertiesof activespecies,whichinturnincreasestheintrinsicWGSactivity
ofthecatalyst
(3)To date, there is noknown singleelemental promoter that playsa dualrole(texturalandfunctional)witha promoting functionalitycomparabletothatofchromium.Indeveloping chrome replacement promoters,the mosteffective strategy
istocombinemorethantwonon-chromiumelements (usu-allyonetexturalandonefunctional),inwhichthematching betweenelementsisverycrucial.Forinstance,Znisnotaproper promoterforironoxideforitself;itincreasesthespecific sur-faceareabutdoesnotincreasetheWGSactivity[23].However,
Znplaysaprominentroleasafunctionalpromoterifpaired withNiinpromotingironoxide,whichnotonlyimprovesthe WGSactivitybutalsohindersmethanation[67]
(4)Ingeneral,magnetite-basedWGScatalysisfollowsa regener-ativemechanism Hence,itis usedtoproduceagoodresult whendispersingmagnetite(orpromotedmagnetitespecies)over reducibleoxidesupportwithsuperior latticeoxygen mobil-ity.Ceriumoxidesandtheirderivativescanbeusedforsuch
apurpose
(5)Theuseofbasepromoters,suchasalkalineoralkaline-earth metals,promotestheformate-intermediatedassociative mech-anisminWGScatalysis.Thisapproachcouldexhibitsynergy whencombinedwithotherfunctional/texturalpromotersfor promotingironoxidecatalysts
(6)TheimmobilizationofFeintoacrystallinematrixissometimes effectiveinstabilizingtheactivespecies(FeOx)against ther-malsinteringorexcessivereduction.Theuseofcerium-based perovskiteisagoodexample
Presently,thedevelopmentofCr-freeHTScatalystsisan impor-tant,ongoingtopicinthecatalystindustry.Wehaveintroduced somestudiesaboutCr-free“Fe-based”HTScatalysts,butthemajor developmentaltrendinvolvesnoblemetalcatalysts,whichexhibit highactivityin acompactcatalystbedand maintainthis activ-ityeveninoxidativeatmospheres[55].Thesecatalystsarequite promisinginspecificfields,suchasautomobileapplications How-ever,Fe-basedcatalystsarestilladvantageousintermsofmaterial cost, whichis highly desirablefor reducing theoperationcosts
of hydrogen plants [73] Thus, the need for reasonably priced HTS catalysts is providing an impetus for continuous studies
of Cr-free Fe-basedcatalysts Based onthis review, these stud-ies should focus on developing well-matched (non-chromium) promoter groups and crystalline matrixes for the active iron species
Trang 8ThisworkwassupportedbytheHumanResourcesDevelopment
oftheKoreaInstituteofEnergyTechnologyEvaluationand
Plan-ning(20114010203050)grant fundedbytheKoreagovernment
MinistryofKnowledgeEconomy
This work was supported from GTL-FPSO project (KIST,
2M29760)fundedbytheKoreangovernment,Ministryof
Knowl-edge Economy (Project number: MKE2011T100200023) and
Industries(DSME,KOGAS,JNKHeaters,Hy-lokKorea)
Dr.Dae-WonLeeissupportedbyaKoreaUniversitygrant
References
[1] D Mendes, A Mendes, L.M Madeira, A Iulianelli, J.M Sousa, A Basile,
Asia-Pacific Journal of Chemical Engineering 5 (2010) 111.
[2] L Lloyd, D.E Ridler, M.V Twigg, in: M.V Twigg (Ed.), Catalyst Handbook, 2nd
ed., Wolfe Publishing Ltd., London, 1989, pp 283–292.
[3] K Kochloefl, in: G Etrl, H Knozinger, J Weitkamp (Eds.), Handbook of
Hetero-geneous Catalysis, vol 4, Wiley-VCH, Weinheim, 1997, pp 1831–1843.
[4] C.N Satterfield, Heterogeneous Catalysis in Industrial Practice, 2nd ed.,
McGraw-Hill, New York, 1991, pp 442–446.
[5] J.J Baschuk, X.G Li, International Journal of Energy Research 25 (2001) 695.
[6] X Cheng, Z Shi, N Glass, L Zhang, J Zhang, D Song, Z-S Liu, H Wang, J Shen,
Journal of Power Source 165 (2) (2007) 739.
[7] E.D Park, D Lee, H.C Lee, Catalysis Today 139 (4) (2009) 280.
[8] J.M Moe, Chemical Engineering Progress 58 (2) (1962) 33.
[9] D.S Newsome, Catalysis Reviews: Science and Engineering 21 (2) (1980) 275.
[10] G.J Hutchings, R.G Copperthwaitet, F.M Gottschalk, R Hunter, J Mellor, S.W.
Orchard, T Sangiorgio, Journal of Catalysis 137 (2) (1992) 408.
[11] J.R Mellor, R.G Copperthwaite, N.J Coville, Applied Catalysis 164 (1–2) (1997)
69.
[12] D Andreev, V Idakiev, T Tabakova, A Andreev, R Giovanoli, Applied Catalysis
A: General 134 (2) (1996) 275.
[13] V Idakiev, Z.-Y Yuan, T Tabakova, B.-L Su, Applied Catalysis A: General 281
(1–2) (2005) 149.
[14] D Tibiletti, A Amieiro-Fonseca, R Burch, Y Chen, J.M Fisher, A Goguet, C.
Hardacre, P Hu, D Thompsett, Journal of Physical Chemistry B 109 (47) (2005)
22553.
[15] G Jacobs, L Williams, U Graham, G.A Thomas, D.E Sparksand, B.H Davis,
Applied Catalysis 252 (1) (2005) 66–74.
[16] J.W Lywood, M.V Twigg, Eur Patent Appl., 0361648, 1990.
[17] C Rhodes, G.J Hutchings, A.M Ward, Catalysis Today (1995) 43.
[18] C Rhodes, B.P Williams, F King, G.J Hutchings, Catalysis Communications 3
(2002) 381.
[19] I.L Júnior, J.M.M Millet, M Aouine, M.C Rangel, Applied Catalysis A: General
283 (2005) 91.
[20] J.Y Lee, D.-W Lee, K.-Y Lee, Y Wang, Catalysis Today 146 (2009) 260.
[21] J.Y Lee, D.-W Lee, Y.-K Hong, K.-Y Lee, International Journal of Hydrogen
Energy 36 (2011) 8173.
[22] G.K Reddy, P Boolch, P.G Smirniots, Journal of Catalysis 282 (2011) 258.
[23] C Martos, J Dufour, A Ruiz, International Journal of Hydrogen Energy 34 (10)
(2009) 4475.
[24] Y.S Oh, H.S Roh, K.W Jun, Y.S Baek, International Journal of Hydrogen Energy
28 (2003) 1387.
[25] S.H Kim, S.W Nam, T.H Lim, H.I Lee, Applied Catalysis B: Environmental 81
(2008) 97.
[26] J.L.R Costa, G.S Marchetti, M.C Rangel, Catalysis Today 77 (2002) 205.
[27] A.L.C Pereira, G.J.P Berrocal, S.G Marchetti, A Albornoz, A.O de Souza, M.C.
Rangel, Journal of Molecular Catalysis A: Chemical 281 (2008) 66.
[28] S Natesakhawat, X Wang, L Zhang, U.S Ozkan, Journal of Molecular Catalysis
A: Chemical 260 (2006) 82.
[29] L Zhang, X Wang, J.M.M Millet, P.H Matter, U.S Ozkan, Applied Catalysis A:
General 351 (2008) 1.
[30] L Zhang, J.M.M Millet, U.S Ozkan, Applied Catalysis A: General 357 (2009)
66.
[31] P Gawade, B Mirkelamoglu, B Tan, U.S Ozkan, Journal of Molecular Catalysis A: Chemical 321 (2010) 61.
[32] K Watanabe, T Miyao, K Higashiyama, H Yamashita, M Watanabe, Catalysis Communications 10 (2009) 1952.
[33] G.C Araújo, M.C Rangel, Catalysis Today 62 (2000) 201.
[34] C Bosch, W Wild, Canada Patent 153,379, 1914.
[35] W Ruettinger, O Ilinich, in: S Lee (Ed.), in: Encyclopedia of Chemical Processing, Taylor & Francis, 2006.
[36] L Lloyd, D.E Ridler, M.V Twigg, in: M.V Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp 293–295.
[37] C Rhodes, G Hutchings, Physical Chemistry Chemical Physics 5 (2003) 2719 [38] G.K Reddy, P.G Smirniotis, Catalysis Letters 141 (1) (2011) 27.
[39] A Khan, P Chen, P Boolchand, P.G Smirniotis, Journal of Catalysis 253 (2008) 91.
[40] V Idakiev, D Mihajlova, B Kunev, A Andreev, Reaction Kinetics and Catalysis Letters 33 (1) (1987) 119.
[41] C Martos, J Dufour, A Ruiz, International Journal of Hydrogen Energy 34 (2009) 4475.
[42] G.K Reddy, K Gunasekera, P Boolchand, J Dong, P.G Smirniotis, Journal of Physical Chemistry C 115 (2011) 7586.
[43] T Popa, G Xu, T.F Barton, M.D Argyle, Applied Catalysis A: General 379 (2010) 15.
[44] L Lloyd, D.E Ridler, M.V Twigg, in: M.V Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp 302–304.
[45] L Lloyd, D.E Ridler, M.V Twigg, in: M.V Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp 304–306.
[46] F Domka, A Basinska, R Fieldcrow, Surface Technology 18 (1983) 275 [47] G.C Chinchen, R.H Logan, M.S Spencer, Applied Catalysis 12 (1984) 89 [48] G.C Chinchen, R.H Logan, M.S Spencer, Applied Catalysis 12 (1984) 93 [49] M Robbins, G.K Wertheim, R.C Sherwood, D.N.E Buchanan, Journal of Physics and Chemistry of Solids 32 (1971) 717.
[50] M.A Edwards, D.M Whittle, C Rhodes, A.M Ward, D Rohan, M.D Shannon, G.J Hutchings, C.J Kiely, Physical Chemistry Chemical Physics 4 (2002) 3902 [51] C Pellerin, S.M Booker, Environmental Health Perspectives 108 (2000) a402 [52] P.C Grevatt, Toxicological Review of Hexavalent Chromium, U.S Environmen-tal Protection Agency, 1998.
[53] U.S Department of Labor, Hexavalent Chromium, OSHA 3373-10, 2009 [54] Official Journal of the European Union 46 (L37) (2003) 19–37.
[55] C Ratnasamy, J.P Wagner, Catalysis Reviews 51 (2009) 325.
[56] Y Hattori, K Yamamoto, J Kaita, M Matsuda, S Yamada, Journal of the Amer-ican Oil Chemists Society 77 (12) (2000) 1283.
[57] J Ladebeck, T Regula, Studies in Surface Science and Catalysis 121 (1999) 215 [58] G.C Chinchen, Eur Patent Appl 0062410, 1982.
[59] D.G Rethwisch, J.A Dumesic, Applied Catalysis 21 (1986) 97.
[60] D.G Rethwisch, J Phillips, Y Chen, T.F Hayden, J.A Dumesic, Journal of Catalysis
91 (1985) 167.
[61] J Ladebeck, K Kochloefl, Studies in Surface Science and Catalysis 91 (1995) 1079.
[62] Q Liu, W Ma, R He, Z Mu, Catalysis Today 106 (2005) 52.
[63] H Jin, Q Liu, Z Mu, L Diao, X Zhang, W Ma, Chin Patent ZL95 121834.4, 1999 [64] M.L Kundu, A.C Sengupta, G.C Maiti, B Sen, S.K Ghoshi, V.I Kuznetsov, G.N Kustova, E.N Yurchenko, Journal of Catalysis 112 (1988) 375.
[65] A Andreev, V Idakiev, D Mihajlova, D Shopov, Applied Catalysis 22 (1986) 385.
[66] J.Y Lee, D.-W Lee, M.S Lee, K.-W Lee, Catalysis Communications 14 (2011) 37 [67] M.S Lee, J.Y Lee, D.-W Lee, D.J Moon, K.-W Lee, International Journal of Hydro-gen Energy 37 (2011) 11218.
[68] K Watanabe, T Miyao, K Higashiyama, H Yamashita, M Watanabe, Catalysis Communications 12 (2011) 976.
[69] A Boudjemaa, C Daniel, C Mirodatos, M Trari, A Auroux, R Bouarab, Comptes Rendus Chimie 14 (2011) 532.
[70] N Mahadevaiah, P Singh, B.D Mukri, S.K Parida, M.S Hedge, Applied Catalysis B: Environmental 108–109 (2011) 117.
[71] T Blunesin, R.J Gorte, G.W Graham, Applied Catalysis B: Environmental 15 (1998) 107.
[72] Y Sun, S.S Hla, G.J Duffy, A.J Cousins, D French, L.D Morpeth, J.H Edwards, D.G Roberts, International Journal of Hydrogen Energy 36 (2011) 79 [73] FreedomCAR and Fuel Partnership, Hydrogen Production Roadmap: Technology Pathways to the Future (2009): www1.eere.energy.gov/ hydrogenandfuelcells/pdfs/h2 production roadmap.pdf