Krishnamurthy, K. R.

Sastri, M. V. C.

Viswanathan, B.

English

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-----~-- ----IJudie Journal of ChemistryVol. 15A, March 1977,pp. :105-:109--~---~-------~~~~,-------1III.CatalyticOxidationof CarbonMonoxideon Spinel Type Ferrites:Kinetic CharacteristicsK. R. KRISHNAMURTHY*, B. VISWANATHAN &; M. V. c. SASTRltDepartmentof Chemistry,IndianInstituteof Technology,Madras600036Received25 June 1976; accepted9 Au~usl 1976Theoxidationof CO onspineltypeferritesof theformulaeMFe20, or M.YN"FesO,is foundtoobeyafirstorderratelaw. Thereactionrateis foundtobeindependentofthepartialpressureofeitheroxygenor carbondioxidethusshowingthattheadsorptionofCOis theratecontrollinl1stepof theoveraUprocess.Linearcorrelationshavebeenobtainedbetweenphysi-cochemicalpropertieslikeunitcellparameter,oxygenbondenergy,activationenergyfor electricalconduc-tionandactivity.FERRITES of the general formula MFe!O"whereM is a divalentcationlike Mn2+,Fe2+,C02+,Ni2+,CU20f and Zn2of, crystallizein aspinel type structurel,in which cations,namelyM2+and Fes+,can occupyeither the tetrahedraloroctrahedralholescreatedbytheoxideions. Thisdistributionof cationsat differentcrystallographicenvironmentsincreasesthe surfaceheterogeneity.Hence, this class of compoundsexhibits a widespectrumof activity for reactionslike oxidation2,isotopic exchangeof 01 (ref. 3) and H2 (ref.4)dehydrogenationand dehydrationS,oxidativede-hydrogenation6andN20decomposition?Thespinellatticeimpartsextrastabilityto thecatalystsundervariousreactionconditions,suchthatthesesystemshavesustainedactivitiesandselectivitiesfor longerperiods8.For these reasons,ferrites have beenusedextemivelyas catalystsfor someindustriallyimportantprocesseslike oxidativedehydrogenationof butenes to butadieneD(MgFe20" ZnFe20",),hydrodesulphurizationf petroleumcrudes(CoFe20"and NiFe20,,)lOand the treatmentof automobileexhaustsll.The studiesso far reportedin literatureon thecatalyticpropertiesof ferriteshavebeenaimedatelucidatingthe influenceof (i) the natureof thedivalentmetalion, and (ii) the distributionof thecationsat the two crystallographicenvironments.For example,Schwabet al.12 have reportedthatnormal ferritesare more active for CO-oxidationthan the inversefecritesand that the activity isdependentupon the numberof octahedrallyco-ordinatedferricions. On theotherhand,Boreskovetal.2haveobservedthatinverseferriteslikeNiFellO"CoFe20, and CuFe20, are more active than thenormalferritesfor the oxidationof hydrogenandmethane. For the conversionof butenesto buta-diene,MgFe20"an inversespinel,wasfoundto bea bettercatalystthan ZnFe20" a normalferrite8.Gradationin the activity of the inverseferritesisalsoobservedby CaresandHightower6,whoreport-*Presentaddress:ResearchChemist,IndianPetrochemicalsCorpnLtd, P.O. ]awaharnagar,Baroda391320.tPresentaddress:Head,MaterialsScienceResearchCentre,IndianInstituteof Technology,Madras600036.ed CuFe20, to be moreactive than CoFe20,forthe conversionof butene to butadiene. It isclearfromtheseresultsthat the relativeactivitiesof Fe3+ions at octahedraland tetrahedralsitesandtheeffectof thenatureof divalentions in thespinel lattice have not been establishedunequi-vocally.A systematicstudy of the catalyticactivity ofthe ferritesand rationalizationof the activity onthebasisof theirstructural,electricalandmagneticpropertieswould, therefore,greatly help in theunderstandingof their catalyticaction.With this aim in mind, a numberof simpleferrites(MFe.O,whereM=Mn2+,Fe2+,C02+,Nilof,Cu2+'and Zn2+)and mixedferrites(Ni"Znl_",Fe.O,with x=0·2,0,5 and 0·8) havebeeninvestigatedas catalysts for the oxidation of CO. Variouskinetic parameters,like rate constants,activationenergyand frequencyfactor,havebeenevaluatedwith a view to correlatingthem with the solidstatepropertiesof the ferrites. In orderto havemeaningfulcorrelations,thesekinetic parametershave been evaluatedunder specificexperimentalconditionsfor thecatalystsso asto evolvea syste-matic study of the kinetics of CO-oxidationonferrites. The presentpaper deals only with theresultsobtainedon the kineticsof CO-oxidation.The mechanisticaspectsof this reactionwill bediscussedin a subsequentpaper.MaterialsandMethodsTheferritecatalysts(exceptMnFe20,andFesO,)werepreparedby co-precipitationmethodas de-scribedby Boreskovetat.lS. MnFe20,wasobtain-edby the solidstatereactionbetweenMnC03andFeiOs.FeaO,wasobtainedby the methoddescrib-ed by David and We1ch14.• The products wereanalysedto ascertaintheir stoichiometryand theirphasecharacterizationsweredoneby X-ray diffrac-tion. ElectricalconductivityandSeebeckcoefficientmeasurementswere carried out using speciallydesignedcells1".The cation distributions werecomputedfromthe intensitiesof theX-ray diffrac-tion lines. The surfaceareasof the samplesweredeterminedby the BET method from nitrogen205INDIAN J. CHEM., VOL. 15A, MARCH 1977TABLE 1- PHYSICO-CHEMICALPROPERTIES OF SPINEL TYPE FERRITE CATALYSTSFed·iteUnit cellSemicond.Ea for el ctricalSp. satnR te fSurfaceparameter(a)typconductionmagntznhomo learare (BET)-AeV(CGS units)O2 x hangesq. m/gMnFe. 48,500n (>300°)0,44 «300°)8 ·1-6X 1011·5P (<300°)O·56 (>300°)Fep438--87·0-30·1CoFep· 1p 0·57 782·4 07,NIFe20' 549 5 ·8CuFe20= ·2020 °) 626 4-x 9c=8·72P «2 °)'3 «3 0°)ZnFe2042n 1·02 -6,9x 096 6Zno'8Ni.,Fe,O,-p 10 74'56 '5Fe20,· « 8 °)1 3-2·3Zno.2N+8FesO,O·56 (>280°)- P 2 4769,'01·10 (>470°)Fig. 2 - First order kinetic plots for the oxidation of COon ZnFe20,(at 300°)and CoFe!O, (at 280°)ToMe LeodgaugeTo highvacuum system• Zn Fez 04 (JOO·C)GCoFe2~ (Z80·C)Fig. 1- Static reactor system0,rI"1110987N 5!~;;: &Q.0-'" 524adsortion measurementsat liqujd nitrogentem-peratre (-196.8°). Thephysicc-chemicalpropertiesof spi el typeferritecatalystsaregivenin Table 1.Car on monoxidewaspreparedby the reductionof Ca 03 with zinc dust16. Compressedoxygen,nitrogn, carbondioxideand heliumof betterthan99'9~tpurity, obtainedfrom commercialsources,were urifiedfurtherby standardprecedures.As atic ecirculatoryreactor,shownschemati allyin Fi . 1, was employedfor kinetic studies. Therate f reactionwasfollowedby admittinga stoi-chio tric mixturecf CO and 02+dih:.ent,if any,to th systemto give an initial total pressureof70±1 torr, noting the declineof pressureas afunct"n of time and applyingthe stoichiometricequaton. Where necessary,the increasein CO2concetration was detelminedby gas chromato-grap . The kineticswerestudiedat thetempera-tures specific for each catalyst. The weight ofcatal st placedin thereactorvariedfrem1'5to 2 g.Pri r to eachrun, thecatalystswereactivatedbyevac tion at 10-5 torr at 4000 for 6 hr and pre-treatd with oxygenat a pressureof about 100torr or 10 hr at the reactiontemperature.Thisproceure was found to give reproduciblekineticdata.Resuts andDiscussionKi etic studies- Detailedkinetic measurementshave eencarriedout ontworepresentativef rrites,namey CoFe204'a p-type ferrimagneticinversespine, and on ZnFe204,an n-type paramagneticnorm1 spinel. The rate of oxidation of CO onCaFe04 at 2800 and on ZnFe204 at 3000 hasbeenmeasred at an initial total pressure(PT) of 70±1torr ith a stoichiometricmixtureof CO and 02'The eactionobeysa first order rate law as seenfrom the linear plot shown in Fig. 2 betweenlog ( TiP) andtimet, whereP standsfor the totalpressre at any time t. The validity of the firstorde ratelaw hasbeenfurthercheckedby varyingthe i itial total pressureand the partial pressuresof C and 02'V iation of initial totalpressure- The rate ofoxid tion of CO was measuredin the initial totalpressre range of 40-210 torr. Typical ratecurv s obtainedon ZnFe204 at 3000 are given inFig. 3 and the valuesof the initial rates (Ri) atvari s initial total pressures4,07, 6·88and 14·52206II1 III 1',11KRISHNAMURTHY et al.: CATALYTIC OXIDATION OF CARBON MONOXIDEFig. 4- Kineticsof CO-oxidationon ZnFe.O,at differenttemperaturesfrom 10to 50 torr keepingthe partial pressureofoxygenconstantat 25 torr. From the ratecurves,thevaluesof the initial rates(RiCO) werecalculatedand are givenin Table 2. The plots of log (RiCO}versuslog Pco are linearwith slopeequalto one,thusestablishingthat thereactionis first orderwithrespectto CO.Effect of carbondioxide- On both the catalysts,it was found that 2CO+Os+I0% CO2and 2CO+0s+10% He gave coincidentrate curves. Thatis, the reactionrate is unaffectedby the presenceor build-upof COs in the system.Effect of varying temperatureand evaluation ofapparent activation energy- Typical kinetic plots(logP co vs t) for the oxidationof CO on ZnFe20<land Nio'5Zno'5FesO,at various temperaturesaregivenin Figs. 4 and5 respectively.All theferritesare found to be active in the temperaturerange200-450°,exceptFe304which getsoxidizedunderreactionconditionsabove 2000•The valuesof the rate constantper unit area,apparentactivationenergy(E) obtainedfrom thecorrespondinglinearArrheniusplots and frequencyfactor obtainedfor the variousferrites are givenin Table 3.Compensationeffect- The operationof a compen-sation effectis testifiedby the linear plot of Eaversusthe fre'quencyfactor. This effectseemstobe commonto catalysisby spinelferrites,as it has704ll 280·CA 300 ·C• 320·C6030 40 50Tim~ (min)20101.461.421.141.10o1.381.341.300t1.uCIl 1.2621.22.18xPT(em Hg)4ll 4.07A 6.88• 14.52X 21.301.0o 20.8- III: :e!:.6N0lJQ. 0.4Pco Po.H.RateemHgemHgem Hg/minCaFe.O,(TEMP. 280°)4·922-461·757x 10-13·70·1 221·28 x to-I2 46·g'63 x1 233 704 3 5X 10-3ZnFeIO,300°)4,592 181X '' 41 130 '15, 45 45Fig.3 - RatesofCOoxidationonZnFe.O,at300°at varioustotalpressuresTABLE 2-EFFECT OF VARIATION OF PARTIAL PRESSURE OFCO ON THE RATE OF OXIDATIONcm Hg are 1'222X 10-2,2·181X 10-2and 5'038xl0-2cm Hg min-1respectivelyat 300°and CO:02 ratioof 2:1. The increasein the rate of oxidationwasobservedonly up to PT=140 torr. Above thispressure,the rate was found to be independentof the initial total pressure. This can be attribut-ed to either (i) the saturationof the surfacesitesby the adsorptionof the reactantmoleculesor(ii) to the reactionbeing controlledby the bulkdiffusionof thegasmixtureto the catalystsurface.The plot of log Ri VS log PT is linearwith a slopeequalto one,whichsupportsthe earlierconclusionthat the reactionis first order.Variation of partial pressure of oxygen- Therate curvesobtainedfor the oxiaation6f CO onCoFe20, at 2800 and on ZnFe204at 300° showthat thevariationsof thepartialpressureof oxygendo not changethe rateof oxidation,indicatingzeroorderwith respectto oxygen.Variation of the partial pressure of co- Usingheliumas diluentso as to maintaina totalpressureof 70 torr, the partial pressureof CO was varied207'1.5&INDIAN J. CHEM., VOL. 15A,MARCH 19778.8cell-2.2unite Mn• Fea Co• HiA Cu• Znand•6- Correlationbetweenactivityparameter11·4(b'L aMnGICo11·0 •Nib. ~ J ~ loJ10'61- •ZGI0· GOI.10·2 .. 0> I "I -l0.55c: ~ 9·8uf ''[ I""1"~6.0.45 I.lJ- g·o f;.-,-10'408· -I ,'+/ r7·8II10··30-.3'6-3·2-2'B'4log k82-3.6 -3.4 -3.2 -3.0 -2.8 -2.6log k8.687....,8.5.«'-'ClFig.Fig. 7- Correlationbetweenactivity and (a) activationenergyfor electricalconductionand(b)rateof homonuclearisotopicoxygenexchangealso been observedin the case of oxidation ofmethane2,hydrogenand propylene1?on MFe204(M=Cdz+,Ni2+,Co2+,Mn2+,Mg2+and Znz+).In view of the observedcompensationeffectitwould not be correctto use activationenergyasan index of activity. The valuesof the rate COn-stant at 2800 normalizedto unit areaarethereforeused for comparisonof activity. The gradationobserved is CuFe204>Fe;104>MnFe204>ZnFe204>CoFeZ04>NiFe204·In order to explain this pattern of activity,correlationshavebeenformulatedbetweenactivityand the solid state propertieslisted in Table 1.In the caseof simple ferrites,linear correlationshave beenobservedbetweenactivity and (i) unitcell parameter,(ii) activationenergyfor electricalconduction (Eal and (iii) oxygen bond energy.Frequencyfactormin-'m-o0·2256,35X 10'0·180'8360'23839,727·212-1Eakcaljmolea 280'CA 290'C• 300'C3·51X 10-03,08x 10-02·49X 10-'2·02x 10-'MAGNETITEMANGANESE FERRITE4·50X 10-·3·03X 10-514'52·18X 10-·COBALT FERRITE3·17X 10-'2 12 -&7,31·02X 10-'NICKE I1 96 1 67 '8 8·19 &PPERFERRITE3·01X 10-0 6 0'22 33 -0ZINC FER ITE6'91X 10-&5·33X 10-&3·66X 10-&Rateconstantsmin-l m-O1.241.n61.31.201.42f52 02 511I,ooY'"2 ,.2102 00TABLE 3 - VALUES OF RATE CONSTANTS,ACTIVATION ENERGYAND FREQUENCY FACTOR,1,2o 10 20 30 40 50 60 70f T:me(mln)Fig. - Kineticsof CO-oxidationon Nio'5Zno'5Fep.atdifferentemperaturesill III WI',III II :II '.; 1'IIII"iI' I Pllld'lKRISHNAMURTHY et al.: CATALYTIC OXIDATION OF CARBON MONOXIDEThese correlationsare shown in Figs. 6 and 7.While linear relationshipsbetweenactivity andunit cellparameterandoxygenbondenergyindicatethe role of metal-oxygenbondenergy18,correlationbetweenEa and activity showsthe importanceofchargetransferstep in the overall oxidationre-action. The significanceof thesecorrelationswillbe discussedin detail in relationto the mechanismof oxidationin a subsequentpaper.AcknowledgementThis work forms part of a ResearchProjectfundedby the Departmentof ScienceandTechno-logy. Awardof a researchfellowshipby theCSIR,New Delhi, to oneof the authors(K.R.K.) is alsoacknowledged.References1. GORTER, E. W., PhiliPs Res. Rep., 9 (1954),295.2. BORESKOV, G. K., POPOVSKII, V. V., LEBEDEVA, N. E.,SAZONOV, V. A. & ANDRUSHKEVITCH, T. V., Kinet.Katal., 11 (1970),1253; 12 (1971),979.3. YONEDA, Y., MAKISHIMA, S. & HURASA, J., J. Am.chem.Soc.,80 (1958),4503.4. SQUIRES, R. G. & PARRAVANO, G., J. Catalysi~,2 (1963),324.5. LITCHNER, E. & SZALEK, G., Z. Chem.,8 (1968),314.6. CARES, W. R. & HIGHTOWER, J. W., J. Catalysis, 13(1971),193.7. GWAN, K. & SQUIRES, R. G., Chem.EngngProgr. Symp.Ser., 64 (1968),24.8. RENNARD, R. J. & KHEL, W. L.,]. Catalysis,21(1971),282. MASSOTH, F. E. & SCARPIELLOW, P. A., J. Cata-. lysis, 21 (1971),294.9. KHEL, W. L. & RENNARD, R. ]., US Pat. 34,507,886and 34,507,887,1969.10. RYLADER (Jr), P. N. & ZIMMERSCHIED, W. J., US Pat.2,805.187,1957.11.KEIZO, I., TOSHIO, T., MAso, K. & TOSHIKAZU, A.,Japan Kokai, 74: 102,590,74: 123,174,74: 120,886.1974.12. SCHWAB, G. M., ROTH, E., GRINTZOS, CH. & MAVRAKIS,N., Structureandpropertiesof solid surfaces,edited byR. Gomer & C. S. Smith (University of Chicago Press,.Chicago), 1953,464.13. BORESKOV, G. K., DZEVENSKI, Z., POPOVSKII, V. V.,MUZYKANTOV, V.I., ELIZAROVA, G.L., MATVIENKO, L.G.,PLyASOVA, L. M., KARAKCHIEV, L. G., OSTEN'KOVICH.A. A. & SHKRABINA, R. A., Kinet. Katal., 11 (1970),793.14. DAVID, I. & WELCH, A. J. E., Trans. Faraday Soc., 52(1956),1642. ,15. KRISHNAMURTHY, K. R., Ph.D. thesis, lIT, Madras, 1975.16. WEINHOUSE, S., J. Am. chem.Soc.,70 (1948),442.17. ZANDERIGHI, L., FAEDDA, M. P. & CARRA,S., J. Catalysis,35 (1974),427.18.BORESKOV, G. K., POPOVSKII, V. V. & SAZONOV, V. A.,Kinet. Katal., 9 (1968),302; 13 (1972),344.209, . ._,..t . ~_~ _

Catalytic Oxidation of Carbon Monoxide on Spinel Type Ferrites : Kinetic Characteristics

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Catalytic Oxidation of Carbon Monoxide on Spinel Type Ferrites : Kinetic Characteristics

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