Soc. , 30, No. 6
Characte rization and Catalytic Properties of Surface La-rich LaFeO
?Perovskite
Young-Gil Cho, Kyong-Hoon Choi： Yong-Rok KimJJin-Seung Jung,* andSung-HanLee*
Department ofChemistry, Yonsei University, Wonju 220-710,Korea. * E-mail: [email protected]
‘Department of Chemistry, Yonsei University, Seoul 120-749, Korea
^Department ofChemistry,Kangnung National University, Gangnung210-320,Korea ReceivedDecember 8, 2008,Accepted January8, 2009
Key Words:LaFeO3 perovskite, Totaloxidation of methane, Oxidativecoupling of methane
Perovskite-type LaBO3 (B = transition metal) oxides are proved to behighlyactivecatalysts foroxidation reactionsin connection with airpollution control. Especially, they are considered aspotential catalysts in substitution of noblemetal catalysts because of their high thermostability.Many investi
gatorshavesuggested that oxygen vacancies in the perovskite oxides play a major role in catalyticoxidations although there is still debate on the origin of their catalyticactivity.1-7 In LaBO3 peovskite, oxygen vacancies canbe generatedby a partial substitution of the lanthanum ion or transition metal ion with an elementof different valence, orby altering the La/B ratio in thepreparation.8 To obtaina homogeneousand single phase LaBO3 perovskite, the sol-gel method or the co-precipitation method is usually used. However, a part of transitionmetal atoms in LaBO3 perovskitecan exist in higher or lower oxidation states, whichmayinducethe formation of anionor cation defects in theoxide. Accordingly,thesurface compositionmay be different from thebulkone,whichmay significantly modify the catalytic properties of pure LaBO3
perovskite.
LaFeO3 perovskitecatalyst is knownto behighlyactivein total oxidationof methane. Itsactivityisgenerallyexplained bythe fluctuation between twostable oxidation states ofFe, Fe2+and Fe3+,where surface oxygen species are partly con
sumed and regenerated during a continuouscycle.7 However, the iron atoms attheB site maybe present inhigher oxidation state as suggestedbyseveral investigators.2,9 If some Fe4+ions exist in LaFeO3, some lanthanumionscould be removedfrom the lattice,basedon theprinciple of controlled valency. The lanthanum oxideliberated from the LaFeO3 perovskite structure maychangethecatalytic properties of the perovskitecatalyst.
In thepreviouspaper,10 LaFeO3 and LaCoO3 prepared bythe citrate sol-gel method revealed some activities in the oxidative coupling of methaneabove 650 oCand theactivities weresuggestedtobe attributable tolanthanum oxide unincor porated into theperovskite lattice. La2O3 catalystisstrongly basic and efficientlygenerates methyl radicals in the oxidative coupling of methane.11,12 If a small amount of lanthanum oxide is isolatedfromthe LaFeOs perovskite latticedue to the presence of some Fe4+ ions, the perovskite catalyst would produce some C2 hydrocarbons inthe oxidativecoupling of methane. The aimof this workisto find thelanthanum oxide liberated from the LaFeO3 perovskite structureand to inves
tigate its catalyticbehaviors in CH4/O2 reaction above 600 oC.
To do this, LaFeO3 and La0.9FeO3 perovskite oxides were
prepared bythecitrate sol-gelmethod and wereexaminedas catalysts for methaneoxidationat a CH4/O2 ratio of5 in the temperature range of 600-800 oC at atmospheric pressure.
They were characterized byBET surface area measurement, XRD,DSC,EDX, FT-IR,and XPS.
Experiment시
LaFeO3 oxideswith La/Featomicratios of 0.9and 1 were preparedfromlanthanumnitrate (La(NO3)3・6H2。，Aldrich,
> 99.99%) and ferric nitrate(Fe(NO3)3・9H2。, Aldrich, > 99.99%).
Boththe lanthanum nitrate and iron nitrate were weighedto obtain the desired atomic ratioof La/Fe, and werethen dis
solvedindeionizedwater. The resultingsolution wasmixed with anaqueoussolution of citric acid to havethesame amount of equivalents.Waterwasevaporated from themixed solution at 70 oC under vacuum toproduceaviscousgel.Theviscous gel was incubated at 120 oC overnight to produce a solid amorphous mixture.The resulting mixture wasground,heated to 400 oCfor 8 h,furtherheatedat 800 oC for 3 h, and then cooled to room temperature at a cooling rateof 60oC/h. The La2O3 sample was prepared bydecomposition of lanthanum
Figure 1. X-ray diffraction patterns of LaFe。9O3, LaF eO3, and La2O3 samples. La2O3 (JCPDS no. 24-0554); LaFeO3 (JCPDS no. 15-0148).
nitrate at 600 oC andcalcination at 800 oC in air. TheBET surface areas of the sampleswere determined by N2 adsorp
tion at the temperature of liquid nitrogen using a Micro- meriticsAccusorb2100 systemand thevalueswere5.3 m2/g for the LaFeO3 and 4.2m2/gfor the La0.9FeO3 sample. X-ray powder diffraction (XRD) analysis was performed at room temperatureby using aPhilipsPW-1710diffractometer with Cu-Ka radiation. A differential scanning calorimetry (DSC) analysis was carried outby a Shimadzu DSC 60 equipment in flowing dried air (10cm3/min) at a heatingrateof10 oC/min up to 600 oC. TheDSC curves of the LaFeO3 and La0.9FeO3 samplesafterexposure to wet airfor 72 hat roomtemperature are presented in Figure 2, in which the La0.9FeO3 sample shows no endothermpeaks while the LaFeO3 sampleshows a weak endotherm peak at 320 oC. To measure the bulk com position, EDX (energy dispersive X-ray) analysis was per formedfor the catalystsby a Genesis EDX32. Theconcen
trations ofLaand Fe weredeterminedfrom their K-lines and the X-ray yields were adjustedby the ZAF correction. The La/Fe atomic ratios determinedby EDX were 1.05 for the LaFeO3 and0.94for the La0.9FeO3 sample, indicating that the values areclose to thenominal ones. The surfacecomposition and thevalence state of eachelement were investigatedby X-ray photoelectron spectroscopy (XPS). XPS ansalysis of the catalystswasperformed by a a KratosAxis-Nova photo
electron spectrometer unit with a monochromated Al Ka
source. The base pressure in the analyzer chamber was less than 5.0 x 10-9Torr.The resulting binding energyvalues were corrected using theC(1s)peak at 285.0 eV.The surfacecom position wascalculated from the XPS peak areaof eachelement by using theatomicsensitivityfactorsprovidedbythemanu facturer. The surface La/Fe ratiosdeterminedby XPS were 1.18 for theLaFeOs and1.06 for theLawFeOa FT-IR spectra were obtainedwith a Fourier transform infrared (FT-IR) spec trometer (Perkin- Elmer Co.)at a resolution of 4.0cm-1and32 scans. Thesample disk for theFT-IR spectroscopic analysis was made bycompressingthe mixture of sample (10%) and KBr. The FT-IR spectra of LaFeO3, La0.9FeO3, and La2O3
samples afterexposure to wet airfor 72 h at room temperature.
As presented in Figure 3, the La0.9FeO3 sample showed no
Temp (oC)
Figure 2. DSC curves of La°.9FeO3 and LaFeO3 samples after exposure to wet air.
significant bands, while the LaFeO3 sample showed main bands or shoulders in therange of 2000-500cm-1.The bandat
1640 cm-1 is ascribedto an O-H bending mode of hydroxide phase, thebandat640 cm-1ischaracteristic ofa bendingOH vibration of the lanthanum hydroxidephase, and the bands at
1080 and 850 cm-1 areduetothecarbonates.13-16
Catalytic reaction was carried out in a continuous flow tubularreactor which consistedofa 0.8-cm-i.d.and 3-cm-long quartztube sealed to 0.4-cm-i.d. quartz tubes ontwo ends.
The catalyst bed was placed in the0.8cm-i.d. portion which was placed in a electrical furnace. To control the reaction temperature, aK-type thermocouple was placed in contact withthe outside wallof the reactor next to the catalyst bed.
The amount of catalyst was typically 50 mg. The catalyst loaded in the reactor was pre-treatedin a flow of air (20 cm3/min)at 800 oCfor 3 h. Thefeed flow rate of thereaction mixture atambient conditionwasCH4/O2/He= 10/2/28cm3/
min and theflows of the gaseswere controlledby electronic mass flow controllers (Kofloc 3660).Thepurity of thegases was greater than 99.99% and the gases were dehydrated and purified with suitable filters.The gas products wereanalyzed by on-line gas chromatography (HP model 5890 plus) equipped with a thermal conductivity detector (TCD) and a flame ionizationdetector (FID). A cold trap wasplacedatthereactor exit to remove water vaporfrom the gaseous mixture. The conversion of methanewas calculated from the amounts of products and themethane introducedinto the feed stream.The selectivities werecalculated on the basis of theconversion of methaneto each product.Themethane conversion and product selectivities weretypicallycompared after 1 h time-on-stream.
Figure 3. FT-IR spectra of La2O3, La0.9FeO3, and LaFeO3 samples after exposure to wet air.
Soc. , 30, No. 6
The blank testsperformed inaflow of the CHq/Oz/He (= 10/ 2/28cc/min) reaction mixtureshowed methane conversion of 0-2.6% to C2 hydrocarbons in the temperature range of 600-800oC.The details ofcatalytic experimentwere described in theprevious paper.17,18
Results and Discussion
Figure1 showsthe X-ray powder diffractionpatterns of the LaFeO3 and LawFeOa samples, which indicates the formation of monophase perovskiteoxides havingorthorhombic structure.
In thiswork,thesurfaceLa/Featomic ratio, 1.18, determined byXPS for theLaFeOa samplewas higher than the nominal one,indicating that lanthanum is more rich onthesurface than in the bulk. Toverify the presence of lanthanum oxide liberated from theperovskite lattice, DSC analysiswasperformed for thesample after exposure towet air and theresultsareshown in Figure 2. Becauselanthanumoxide is easilyconvertedto the hydroxide form even at room temperature in air, the lanthanumoxide aged in air revealsanendothermpeakdue to thedehydration.13A weak endothermpeakobserved at 320 oC is attributable to the removal of hydroxyl groupsfrom lanthanum hydroxide formed byhydration of lanthanum oxide in wet air.
The result implies that lanthanum oxide ina small amount existsinthe oxide. Figure 3 comparesthe FT-IR spectraof LaFeOs, La°9FeO3,andLa2O3 samples after exposure to wet airatroom temperature. The FT-IRspectrumofLaFeO3 sample displays absorption bands at 1640 cm-1 due to anO-H bending mode of adsorbed H2O and 640 cm-1 due to a bending OH vibration of the lanthanum hydroxide phase.13-16 The result
supports that lanthanumoxide exists in the LaFeO3 sample.
The isolated lanthanum oxide which was not detected by XRDpresumably exists asnanoparticlesoramorphous in the perovskite. The liberation of lanthanum oxide from the LaFeO3 perovskitelattice is considered tobedueto the unusual oxida tion state of iron, Fe4+, intheoxide.When a part of Fe3+ ions is oxidized to Fe4+ions, a part of lanthanum ions would be removed from the peropvskite latticebased onthe principleof controlled valency.According to Ciambelli et al.19 andBarbero etal.,20 the Fe4+ content determined from the TPR data of LaFeO3
perovskite is 2.8% aftercalcinationat 600 oC and 1.3%after calcination at 800 oC, indicating thatthe Fe4+ contentdecreases withincreasing thecalcination temperature. Their results are agreeable to the present results showing the presence of isolatedlanthanum oxide inthe LaFeO3 sample calcinedat 800oC.
Figure 4 reveals the XPS spectra of O(1s), La(3d), and Fe(2p). The bindingenergiesof La 3ds/2, 833.5 eVfor the LaFeO3 sample and 833.8 eV for the La°9FeO3 sample, corre
spondtoLa3+ ions in oxide form and the binding energy of Fe 2p3/2, 710.2 eV for both the samples, corresponds to Fe3+ ions in oxide form.9,21 Wecould notdistinguish Fe3+andFe4+ from the Fe(2p) XPS signal. The O(1s) XPS signal ofLa0.9FeO3
sample was dividedinto two peaks at 529.9 and532.1 eV, while that of LaFeO3 samplewasdividedintothreepeaksat 529.4, 531.9, and 534.4 eV. The O(1s) binding energies at 529.9 eV and 529.4 eV are ascribed tolatticeoxygen species.
The binding energies at 532.1 and531.9eV can beassigned to chemisorbed oxygen species(O~)or OH 一species because the O(1s) binding energy of O" or OH" ionis generallyhigher by
Figuie 4. La(3d), Fe(2p), and O(1s) XPS spectra of La°.9FeO3 and LaFeO3 samples.
80
O co
♦ co2
O C2H4
70
Temperature ( C)
Figure 5. Selectivities and methane conversion of LaFeO3 catalyst for methane oxidation at a CH4/O2 ratio of 5 in the temperature range of 600-800 oC.
Temperature (oC)
Figure 6. Selectivities and methane conversion of La°.9FeO3 catalyst for methane oxidation at a CH4/O2 ratio of 5 in the temperature range of 600-800 oC.
a 2.1-2.5 eV than that of latticeoxygen.22 The binding energy at534.4 eV observed for the LaFeO3 sample is attributedto the adsorbed water species.22 Because lanthanum oxide is easilyhygroscopic when it is exposedto air, thehigherO(1s) binding energy (534.4 eV) isconsideredto be due to water moleculeassociatedwiththe surfacelanthanum oxide.
In theCH4/O2 reaction,the LaFeO3 catalyst produced CO2, C2H4, and C2H6 as major products in the temperaturerange of 600-800 oC. Figure 5 showsmethane conversion andproduct selectivities of the LaFeO3 catalyst, in whichthe C2 yieldat 800oCis 5.8% in aselectivity of 35.8%. Onthe other hand, the La0.9FeO3 catalyst showed total oxidationactivity for the reaction as shown in Figure 6. The LawFeOa catalystproduced C2 hydrocarbons in a selectivity below 9.5% in the temper aturerangeof725 - 800oC and the C2 yield at800 oC was merely
1.2%. In the previous paper, we reported that the CH4/O2
reaction in a quartz flowreactor produces C2 hydrocarbons below800oC in the absence ofcatalyst.17In this work, the blank testshowedaC2 yield of 2.6%at 800 oC and thevalue is higherthan 1.2%of theLa°9FeO3 catalyst, suggestingthe C2 production catalyzed by the La0.9FeO3 to be negligible.
Because the specific surface areas of the LaFeOs and LawFeOm
samples, 5.3 and 4.2 m2/g, are very low and do notdiffer significantly, thedifferences in catalytic propertiesof both the catalysts is inferred to be closely related to their surface structures.
Many investigationsfor theoxidativecoupling of methane over metal oxide catalyst have shownthat less electrophilic oxygen speciesadsorbed on oxygenvacancysiteseffectively generatemethylradicals whicharecoupled to form ethanein
the gas phase.23 ThepresentLa0.9FeO3 catalystwas found to be a single phase perovskitehaving orthorhombicstructure by XRD.The non-stoichiometricLa°9FeO3 oxidecanbe written as La0.9FeO2.85. Ifsome Fe4+ ionsare presentin theoxide,the oxidecould be written as La0.9(Fe3+； Fe4+)O2.85+。血.High activity of the La0.9FeO3 catalyst for totaloxidation of methane is believed to becloselyrelatedto oxygen vacancyassuggested by other investigators.3,23 Because the La0.9FeO3 catalyst negligiblyproducedC2 hydrocarbons in thisreaction,oxygen vacancy seems tobenot directlyrelated to the formationof C2
hydrocarbons. As mentioned above, the LaFeO3 sample is considered to be a mixedphase of isolatedlanthanum oxide and lanthanum-deficientLaFeO3 perovskite oxide.It can be represented as La2O3/La】_gFe：二 Fe^g. Theisolatedlan thanum oxide may be related to thenature and reactivity of the perovskitesurface.Lanthanumoxide isknown to bean active andselective catalyst for the oxidative coupling of methane,12,23 whichenables us to consider thatthe C2 production over the LaFeO3 catalyst is attributed to the surface lanthanumoxide.
In the oxidative coupling of methane overmetal oxide catalyst, COis generally produced as a major product. However, the present LaFeO3 catalyst scarcely produced COin the reaction as shown in Figure 6, whichimplies that CO can be further oxidized toCO2 onthe perovskite containingoxygen vacancies which act as active site for total oxidationof methane.
Consequently,whenthe LaFeO3 perovskite is prepared by the citrate sol-gelmethod and calcinedat 800 oC, lanthanum deficiencies are created in the LaFeO3 perovskite owing tothe presence ofFe4+ ion in theperovskite, resulting inthe libera
Soc. , 30, No. 6
tionof lanthanum oxide from the perovskite lattice. The LaFeO3
perovskitecatalystshowstotal oxidation activityin the CH4/ O2 reactionbelow700 oC, but above700 oC it shows some activity for the oxidative coupling of methane. The surface lanthanum oxide liberated from the perovskite structure is responsible for the modification ofcatalytic properties of LaFeO3.
Acknowledgments. This work was supported by research fUnds fromYonseiUniversity in 2008 and theKorea Science andFoundation throughthePioneer Converging Technology Program (No. M10711160001-08M1116-00110). Y.-G. Cho and K.-H. Choithank the fellowship of the BK21 program from the Ministry of Education and Human Resources Development.
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