Oxygen desorption and oxidation-reduction kinetics with methane and carbon monoxide over perovskite type metal oxide catalysts

175

Applied Catalysis A: General, 194 (1993) 175-197 Elsevier Science Publishers B.V., Amsterdam APCAT A2618

Oxygen desorption and oxidation-reduction kinetics with methane and carbon monoxide over perovskite type metal oxide catalysts Per Salomonsson Chalmers Industriteknik,

S-412 88 Giiteborg (Sweden)

Timothy Griffin Asea Brown Boveri, Corporate Research Center Heidelberg, Eppelheimer Heidelberg (Germany)

Str. 82, D-6900

and Bengt Kasemo Department (Sweden)

of Applied Physics, Chalmers University

of Technology, S-412 96 Giiteborg

(Received 10 March 1993, revised manuscriptreceived 5 July 1993)

Abatract Selectedperovskite-based,metal oxide catalysts of composition La,_&&B:Ox (B: Mn,.Fe; B’: Fe or Cu) containing, in some cases, an excess of one or more of the B-position elements (i.e. y + z> 1) were investigatedfor their use in methane oxidation. A flow reactor/mass spectrometerarrangement was used to characterize the supported monolith catalyst samples. The catalysts possess significant activity for methane oxidation and also are able to exchangebulk oxygen in quantitiescorrespondingto severalhundredmonolayers. Much greaterquantities of oxygen could he removedunder reducingreaction conditions than by desorption in an inert argon atmosphere.With one catalyst, LaFe,,&.a,,~Oa, oxygen desorption occurred aheady at 206°C; otherwisetypical temperaturesfor deaorptiononset were 500600 aC. Lattice oxygen removal under reducing conditions and oxygen uptakeby reducedcatalysts were both found to exhibit a rather complex temperaturedependence. Investigatedcatalysts with an excess of B-position metals had a greatercapacity to exchange oxygen than the stoichiometric perovskites LaMnOs and LaFe~,,.,&q,~sO~Reoxidation of the reducedcatalystaoccmmd at lower temperatures than reduction. Oxidation of carbon monoxide consistently occurred at considerably lower temperaturesthan oxidation of methane. Complete oxidation of methane to carbon dioxide and water was observedunder most conditions with little carbon monoxide or higher hydmcarbon formation. Only for strongly reducedcatalystswas carbon monoxide production detected. Key words: combustion catalyst; methane oxidation; oxidation-reduction desorption; perovskite

kinetics; oxygen

Correspondence to: Dr. T. Griffin, Asea Brown Boveri, Corporate Research Center Heidelberg, Postfach 101332, D-69993 Heidelberg, Germany. Tel. (+49-6221)303978, fax. (+496221)304717,email [email protected]

0926-860X/93/$06.66

0 1993 Elsevier Science Publishers B.V. All rights reserved.

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INTRODUCTION

The introduction of catalysts into combustion processes has been proposed to both reduce thermal NO, production and to improve flame stability [l-3]. Due to the rather high operating temperatures of most combustion processes (T> 1250’ C), multicomponent metal oxides are likely to be suitable for such applications because they combine low volatility and reactivity with relatively high activity as oxidation catalysts [ 41. Noble metals (the normally most active materials for carbon monoxide and hydrocarbon oxidation) are susceptible to sintering and formation of volatile metal oxides in a high-temperature oxidative environment. Metal oxide catalysts have been the subject of many investigations focused on the methane oxidation reaction (for references see ref. 4). Much work has been attempted to determine a rational order in catalytic activity of different metal oxides for the oxidation of light hydrocarbons [5-71. Based on these studies it is apparent that oxides containing Co, Cr, Mn, Fe, Cu and Ni are very active for methane oxidation, and thus potentially interesting for natural gas combustion applications. In particular, metal oxide catalysts with the perovskite structure (ABO,) have been the subject of the greatest amount of recent research regarding use as high-temperature combustion catalysts. Several reviews concerning the relationship between catalytic activity and bulk structure and composition have appeared [8,9]. Arai et al. [lo] studied La-based LaB03 (B: Co, Mn, Fe, Cu, Ni and Cr) and partially substituted La1_&B03 perovskites. They found activities comparable to Pt/A1203 with the relative ranking, LaCoO,>LaMnOB, LaFeOs. Although the catalytic activity is generally determined by the B-position element [ 111, substitution of the A-position with an element of different valence, e.g. substitution of La3+ with Sr2+ can increase the catalytic activity [ 71. Such substitution at the A-position leads to either a change in the B-element valence (to higher values) or to the formation of oxygen ion defects. Mixing of B-site metals may invoke a strong synergistic effect, even when the host and the substitute cations have the same valence. This substitution may also involve a defect structure. Mixuno et al. [ 121 assumed a synergistic effect between copper and manganese for the catalysis of the carbon monoxide oxidation reaction; copper promotes the adsorption of carbon monoxide while manganese promotes the activity of lattice oxygen. Mixing manganese and copper in the B-position also allowed greater degrees of reduction without structural change; even in the grossly oxygen deficient phase, LaMn,-J&502.26, the perovskite structure was preserved. Catalysts based on oxides with the perovskite structure have, in parallel, been investigated for the reduction of nitric oxide with flue gas components:

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177

CO, Hz or C,H, [ 13,141. In one study [15], catalysts of composition La1_,Sr,B,,B: O3 (B: Mn, Fe; B’ : Fe or Cu ) containingan excessof one or more of the component oxides (i.e. y + z > 1) were investigated.These measurements demonstratedthat the catalytic activity of the pure or doped perovskite-type oxides could be increasedby this mixing with additionalcomponent metal oxides. It was suggestedthat the interfacesbetweenthe perovskiteoxide and the additionaloxides servedas the active centres for the reductionof nitric oxide. These catalyst compositions served as a startingpoint for the present work. This reportpresentsthe first resultsfrom the exploratorystageof this study. Specifically,we have investigated,for a number of catalysts, the activity for methaneoxidation in oxygen (in some cases also activityfor carbon monoxide oxidation for comparison), reduction and oxygen deaorptionkinetics, and oxygen uptake kinetics, respectively.The qualitativefeatures and differences/ similaritiesbetween catalyst compositions and how they compare with previously reported results are emphasized, rather than the quantitativeaspects. Part of the goal was to develop a suitable experimentalmethodology for the study of these catalysts. EXPERIMENTAL

Catalyst preparation and ch4zracterization

The compositions of the catalysts investigatedare given in Table 1. The nomenclatureused in the followingcorrespondsto the concentrationsof metal ions in the mixed oxides. Two of them are stoichiometricperovskites,LaMn and LaFeO,J&,, ( LaMnO and LaFe,-,.84C~.1603, respectively),and the other two, La,,.aSr0.2Mn0.J?eCu.,,.3 and LaFe,,Cu,,,, are prepared with excess of the B-position metal ions. Powder preparation

Catalyst powders were prepared using the citric acid sol-gel process [ 16181.With this method the starting materials are completely decomposed so TABLE 1 Catalysts compositions Catalyst

Relative metal content La

LaMn

0.5

LadhMrb.~eCw~ LaFe0.8&0.~~ Lfie&uo.d

0.25 0.5 0.266

Sr

0.066 -

Mn

Fe

0.5

_

0.25

0.333 0.42 0.633

cu

0.10 0.08 0.101

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that the composition of the product powder corresponds exactly to that of the starting materials. This was important in the case of catalysts containing greater than stoichiometric amounts of B-site component metals. The preparation of the catalysts is the subject of a concurrent study and thus only briefly described below. A more detailed description can be found elsewhere [ 191. An aqueous solution of metal nitrate, citric acid and glycol was prepared. The amounts of citric acid and glycol were calculated such that the molar ratio C (metal ions) : citric acid : glycol = 1: 1: 1. The mixture was then heated to 85 ’ C, resulting in partial decomposition, accompanied by nitrogen dioxide production. The product of this procedure was an amorphous solid, which was ground and slowly heated to 600” C. Subsequent calcining of the solid was performed at 800°C for 2 h. The formation of the perovskite phase was verified by performing X-ray diffraction (XRD ) analysis of the prepared powders. Coating of cordierite monoliths Samples were prepared supported on cordierite monoliths (Corning, Celcor ) of 200 cells/in .‘. Monolithic samples represent the likely form to be used in future applications, and offer advantages in sample handling and measurement. A powder slurry was “slipcoated” onto a pretreated (see below) monolith to give a 50-100 pm layer of homogeneous powder catalyst on the cordierite (as determined from scanning electron micrographs of representative catalyst cross sections). Pretreatment. The cordierite was initially coated five times with an aqueous nitrate solution of the catalyst composition ( Czmetal=1.25 mol/l) and dried at 180 ’ C. These honeycombs were then heated to 1100 ’ C for 2 h prior to the final coating. This initial procedure was performed to allow intimate interaction (and reaction) between catalyst and support, thereby enhancing the bond between catalyst and cordierite. It is also expected that this will reduce or eliminate catalyst/support interaction in the final powder coated samples. For the final preparation the powders were mixed with an organic binder to form a paste which was used for the coating of the pretreated cordierite honeycomb monoliths. The monoliths were coated with the highly viscous paste and dried in air at 200°C. Portions of the samples were then tested without additional heat treatment (“fresh” samples). Other honeycombs were sintered at 1000 OC for 30 min to investigate the effects of aging. The cordierite support itself was non-catalytic for the investigated reactions. This was tested by performing experiments with the uncoated cordierite. The samples used were cut to 22 mm length and 13.5 mm diameter. Physical characterization of the catalysts samples In Table 2 the phases of the prepared monolith samples detected by XRD are listed. For the samples with excess of perovskite B-site metals the presence

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179

TABLE 2 Phases of the powder coated monoliths detected by Catalyst

XRD

F%l8f3kOhNed

~GUlS LaMnO and/or LaFe03, MnaO, and/or FesO, LaFeOs, La&uO, (weak) LaFe03, FezOs

of additional oxide species was detected: for La&+o,,M~J?eCu,,~ an excess of MnsOl was observed and for LaFe2.4C~.4 an excess of Fe203 was found. In addition, for catalyst LaFeO.,,.,C~.lG,weak lines corresponding to La&uO, were found. It must be mentioned that the XRD spectra of LaFeOs and LaMnO, cannot be distinguished from each other; for this reason the extent of B-site mixing in La,,,,Sr,,M~J?eCu,,.~ could not be determined. Two samples were prepared for each of the two compositions, La,-&Sr,,Mn&FeC~.~ and LaFe2.4Cu,.4,of which one sample was sir&red at 1000°C for 30 min (referred to as “aged” in the following). All four samples were then reaction stabilized in 8% O2 and 2% CH, (remainder nitrogen) at 2’5 800” C for 1 h prior to measurements. Aging had an effect on the XRD phase composition measured. Sintering increased the Fez03 signal for LaFe,.4Cu,,4 and reduced weak lines corresponding to CuMnO, and other species in La,,.sSr0.2Mn0.$eCQ.3. BET surface areas were determined by adsorption of krypton on a Digisorb 2600 instrument from Micromeritics, assuming a cross-sectional area of 0.210 nm2 for the krypton atom. BET area values are listed below in Table 3 and ranged from 0.5 to 6 m2 total area for the samples used. Experimental system The system was composed of a quartz flow reactor with an on-lime mass spectrometer (MS), a temperature control unit and a gas handliig system. The system was similar to one described in detail earlier [ 20-221. The catalyst was mounted in the flow reactor, essentially a quartz tube (inner diameter 13.5 mm, length 350 mm) wrapped in a heating coil. The desired gas mixture (e.g. oxygen, methane in argon) was flown through the reactor at a pre-selected flow-rate, and the temperature of the reactor was varied as desired [e.g. constant temperature, or linear rise in temperature for temperature-programmed desorption/temperature-programmed reaction (TPD/TPR) 1. A quartz capillary, placed close to the catalyst, sampled a small amount of the gas (ca. low4 Torr l/s, 1 TOIT= 133.3 Pa) exiting the catalyst, which was fed to the MS. The

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chemical composition of the gas mixture can thus be continuously followed with the MS [ 221. The mass spectrometer (Balxers QMG 311/QMA 120 quadropole mass spectrometer) output was registered by a computer, in a multiplexing mode, so that up to 16 masses could be followed quasi-simultaneously (cycle time approx. 6 s for 7 masses ) . The recorded MS signal for each mass number was normalized to the argon carrier gas signal. This eliminated errors due to variations in sample gas mass flow, resulting from temperature or absolute pressure variations etc. [ 20,211. The directed, beam-like molecular flow of the sampled gas in vacuum, through the modified ion-source of the MS [ 221, helps to suppress signals from the background gas in the MS vacuum chamber. The temperature was measured at the catalyst inlet with a chromel-alumel thermocouple placed in one of the channels of the monolithic sample. Methods employed for kinetic studies The catalyst materials were characterized in a series of experiments with different gas mixtures, either at constant temperature, or by temperature programmed reaction (TPR) runs, in which the temperature is increased linearly with time in a given reactive gas stream. In the TPR runs the temperature was varied from 20°C to 800°C at a rate of 0.8”C/s. In all these flow experiments the same total flow-rate, 73.6 ml/min was used, except in the TPD experiments, when the flow-rate was 36.4 ml/min. RESULTS

Catalyst activity for methane oxidation; TPR runs TPR methane oxidation experiments were performed in a 3.2% methane, 12.8% oxygen mixture in argon. The catalysts were pretreated by performing one temperature ramp up to 800°C in the reactive mixture prior to measurement. Fig. 1 shows the activity versus temperature for the investigated catalysts. Their activity is compared to a commercially available supported platinum catalyst (1.5 g Pt/l monolith volume, 200 cell.s/in.2, AlsO,-waahcoat ). Among the metal oxide catalysts, LaMn is the most active (compare to platinum), which can, at least partly, be attributed to its relatively larger BET area. Also for the other catalysts the order of activity qualitatively follows the BET area. In Table 3 are also tabulated the temperature for 50% conversion, T5,,, and the apparent activation energies measured in the conversion range lo-70%. In order to correct for the depletion of methane in the monolith channels at high conversion, the &,‘s were extracted using a model assuming the reaction to be first order in CH, (CO ) concentration and zero order in oxygen concentration. In other words, the rate of change of methane concentration C(X) at a distance

P. Salomonsson et al. / Appl. Catal. A 104 (1993) 175-197

300

400

500

600

700

181

600

Temperature (“C) Fig. 1. Methane oxidation activity for catalysta in this repoti, (a) L&in, (b) F’t, (c) LsFeO&&,la, (d) La&&.~Mn&+C~.~~ (e) Lao.&.~Mno.PeCuo.~ (eged), (f) LsFe&uo.,, (g) LaFe&uo.~ (aged). Gas composition: 3.2% CH, and 12.8% O2 in argon. Heating rate: 0.8”C/s, flow-rate: 73.5 ml/min (space vel. = 1400 h-l). TABLE 3 Summary of activity measurements Catalyst

BETarea” (m’)

Z’& (“C)

E: (kJ/mol)

LaMn LaFeO.s.&uo.I~ Lao.8Ss.&rkPeCu0.3 Lao.&o.2Mno.sFeCk.9 (aged) La-Fe&uo.~ LaFel.&O.l (aged) Pt-A&O3 (1.5 g F’t/l)

5.6 4.0 2.0 1.2 1.0 0.5 61d

457 512 566 663 683 776 523

92 87 91 96 92 106 124

Total BET area for sample. bTemperature for 50% conversion. ‘Apparent activation energy. dMeasured with N, instead of Kr.

x into a monolith channel is assumed to be dC (x) /dx = - IZC(x ) , which when integrated over the catalyst length, 1, gives the outlet concentration C( 1) = C (0)exp ( - Kl). This was found to produce significantly better straight Arrhenius plots than obtained by only analyzing low conversions without considering reactant depletion effects. We emphasize that these activation energies are really “apparent” and should be used with caution for comparative purposes only. The reason is the present lack of a reliable kinetic model that can lend a physico-chemical meaning to the apparent activation energies, and

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that the validity of the model to extract the E,‘s has not been investigated in detail. With this model the measured apparent activation energy for methane oxidation over LaMnO, is 92 kJ/mol. This agrees well with the 91 kJ/mol reported by Arai et al. [lo], and is not too far from the 73 kJ/mol reported by McCarty and Wise [ 71. Aging causes a loss of surface area and an increase in Tso (cf. Fig. 1 and Table 3 for fresh and aged La,,.8Sro,M~.J?eC~,, and LaFe,.,Cuo.4 catalysts). Results below are presented for both fresh and aged catalysts. Differences between carbon monoxide and methane-oxidation over L.aFe.J& In the following, when we refer to conversion, it is generally only the carbon dioxide signal that is plotted. This is motivated by the observation that at all times carbon dioxide production is accompanied by a simultaneous consumption of methane in proportion, i.e. mass balance is observed with respect to carbon atoms. In addition no molecular hydrogen was observed. Only in experiments with a heavily reduced catalyst at high temperatures was carbon monoxide produced, accompanied by hydrogen production. Thus, we conclude that as long as no carbon monoxide or hydrogen is seen the oxidation is complete. In cases where the oxidation was incomplete this is indicated in the text. Carbon deposition could be a concern as the catalyst becomes oxygen delicient, i.e. after prolonged reduction in methane or carbon monoxide. An upper limit for carbon deposition was obtained by recording the carbon monoxide and carbon dioxide production during reoxidation of heavily reduced catalysts. It was concluded that less than 1% of the total amount of methane or carbon monoxide oxidized during reduction runs formed carbon on the surface. Since we did not investigate the kinetics on highly reduced catalysts in any detail, we must leave the possibility open that there is a significant carbon deposition rate on them. Such carbon deposition could even contribute to the decline in methane and carbon monoxide oxidation activity towards the end of runs like the ones in Fig. 3. TPR with CH, + 0, CO + 0, CH, and CO mixtures Fig. 2 illustrates for the catalyst LaFea:,Cu,,4, the difference in oxidation behaviour, for oxidation of methane and carbon monoxide to carbon dioxide with and without oxygen in the gas stream. The conditions of these experiments are given in the figure caption. Without gaseous oxygen, oxidation occurs exclusively with chemisorbed or lattice oxygen on the catalyst surface. From the order of decreasing activity we conclude that carbon monoxide reacts much more easily than methane and that molecular oxygen in the gas phase

P. Salmnonwon

et al. / Appl. Catal. A 104 (1993) 175-197

r

100

--CO+O.Jg);

183

8 _.,. .,

I

W+O,k+’ -:’

Co+O(s)

:::

0

0

100 200

300 400

Temperature

500

600

700

600

(“C)

Fig. 2. Comparison of methane and carbon monoxide-oxidation over catalyst LaFesJ&, in the presence and absence of gaseous oxygen, respectively. Pretreatment: For CO + Ox: TPR oxidation in 5% Ox in argon up to 899”C, followed by cooling to room temperature in the same atmosphere. The CO + 0 (s) experiment was performed directly after the CO + Ox experiment. For CH, + 0s: TPR with 3.2% CH,+ 12.8% 0, in argon, followed by cooling to room temperature in the same atmosphere. The CH, + 0 (8) experiment was performed directly after the CO + 0s experiment. Gas composition: CH,+Os: 3.2% CH,+12.8% 0,; CO+Os: 3.9% C0+4.7% 0,; CH,+O(s): 5% CH,; CO+O(s): 4% CO; balance argon in all cases. Heating rate: 0.8”C/s, flow-rate 73.5 ml/min (spacevel.=1499 h-l).

supplies oxygen to the surface that is either more accessible for reaction through a lower activation barrier, or merely supplies more oxygen than the oxide alone. An interesting feature in the curve for carbon monoxide oxidation, without oxygen in the gas phase, is the region with low conversion between ca. 100 and 25O”C, which after a numerical estimate appears to correspond to consumption of one monolayer” of oxygen. The arrow in the diagram indicates where the first monolayer of oxygen has been removed through reduction with carbon monoxide. With oxygen in the gas phase there is a continuous supply of surface oxygen and the reaction rate rises rapidly already at ca. 150’ C. For methane a large difference was observed between the apparent activation energies (calculated between 5 and 10% conversion) with and without gaseous oxygen (83 and 122 kJ/mol), which may indicate reaction with oxygen in different chemical binding states in the two cases. This is consistent with In order to relate the amount of oxygen removed in experiments in reducing atmosphere to the BET area, we will use the term “monolayer” of oxygen atoms, defined in the following way: based on a lattice parameter of 0.4 nm and the assumption that the stoichiometry of the bulk is preserved at the surface, the surface density of oxygen atoms is estimated to be 5-10’s atoms m-‘. By multiplying this number with the measured BET area of the sample we obtain the equivalent of one monolayer of oxygen atoms. When appropriate this oxygen monolayer equivalent is indicated with an arrow in the diagrams. This definition is also used for expressing the amounta of oxygen removed during temperature-programmed reduction and desorption (Tables 4 and 5).

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the study of McCarty and Wise [ 71, who performed detailed kinetic studies of methane oxidation with a LaFeOS catalyst. By varying the partial pressure of oxygen at different constant temperatures, and plotting the carbon dioxide production rate versus 6 they deduced activation energies for reaction with chemisorbed oxygen and with lattice-derived oxygen of 81 and 133 kJ/mol, respectively. In their study the lattice-derived oxygen began to contribute to the carbon dioxide production rate between 550 and 640’ C. The difference in conversion rate with and without gaseous oxygen is much less pronounced for methane compared to carbon monoxide in Fig. 2. This can be attributed to the higher temperatures necessary to initiate methane-oxidation. In that case, the oxygen supply from the lattice is obviously sufficient to maintain the reaction until the reduction has proceeded so far that the catalyst is depleted of oxygen. In the case of carbon monoxide, oxygen supply from the gas phase is, in contrast, vital at the lowest temperatures, which are too low for lattice oxygen diffusion within the catalyst.

Reduction in methane and carbon monoxide at constant temperature While TPR runs are powerful for obtaining overall activity as a function of temperature etc., especially when quasi steady state is established at each temperature, they suffer from the inherent problem that temperature and oxygen content vary simultaneously. Experiments at constant temperature are therefore necessary as a complement and are more accessible to interpretation and kinetic analysis. Fig. 3 shows three such runs with methane (at 600°C and 700°C) and carbon monoxide (700°C) as reducing agents. In these experiments the catalyst was brought to the desired temperature in 5% oxygen in argon, after which the reactor was flushed with argon. At t = 0 the methane or carbon monoxide gas flow was introduced and the carbon dioxide formation was monitored. The results are presented as rate of oxygen removal versus integrated amount of removed oxygen. For carbon monoxide there is for a long time 100% conversion to carbon dioxide (total mass transport control), until about 1500 pmol oxygen has been removed and the rate starts to be kinetically controlled. A total amount of about 4700 ~01 oxygen is removed before the rate becomes immeasurably small. Raising the flow-rate by a factor of 15 still results in mass transport control up to about the same amount of oxygen removed. Experiments at 600 ’ C produced almost identical results. Comparing the methane and carbon monoxide oxidation curves in Fig. 3, one should note that the oxygen removal capacity of methane is four times larger per molecule than for carbon monoxide. Therefore the maximum possible rate of carbon dioxide formation with methane was five times larger than with carbon monoxide (since the experiments were performed with 5% CH, and 4% CO in argon, respectively ) . With methane, the carbon dioxide production is not limited by the reactant

P. Sakvnonsson et al. /Appl. Catal. A 104 (1993) 175-197

0

2000 3000 4000 amount of oxygen removed @mole atomic oxygen)

1000 Total

185

5000

Fig. 3. Reduction at constant temperature for catalyst LaFez,,Cu,,.,. The figure displays the rate

of oxygen removal versus total amount of oxygen removed for reduction with methane at 600°C (a), with methane at 700°C (h), and with carbon monoxide at 700°C (c). Total conversion is assumed (see text), i.e. 4 oxygen-atoms are removed from the catalyst for every methane molecule that is converted to carbon dioxide. For reduction with carbon monoxide the rate is initially limited by the supply of carbon monoxide. Gas composition: 5% CH, and 4% CO, respectively (balance argon). Flow-rate: 73.5 ml/min.

supply in the initial stages as in the case of carbon monoxide. The maximum conversion is about 70%. The rate of oxygen-removalvaries considerablywith degree of reduction, as evidenced by the structurein the spectra. As seen in Fig. 3, the curvesfor 600“ C and 700oC are simii in shapeand the total amount of oxygen removed is the same for the two temperatures.However,the rate of removal is higher at the higher temperature,and therefore the reduction is then completed in a shorter time. Immediately after the reduction experiments, the reduced catalysts were reoxidized at the same temperatureas during reduction. The oxygen uptake was measuredby comparingthe systemresponseupon oxygenintroductionfor the reducedcatalyst with the correspondingresponse for an oxygen saturated catalyst (for details see ref. 24). The total amount of oxygen removed/taken up is largerwith carbon monoxide than with methane. During the reduction and subsequentoxidation at constant temperature4660+ 150 and 4700+ 100 ~01 O-atoms, respectively,were first removed and the reabsorbedin the carbon monoxide experiment. The corresponding numbers for methane were 14405 150 and 1410-t100 ~01, i.e. there is more than three times as much reductioncapacitywith carbon monoxide comparedto methaneat 600’ C (this difference is illustratedby the points where the rate approachesthe abscissa in Fig. 3). The oxygen amounts reduced away and reabsorbed correspond quantitatively,within error limits.

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TPR with methane (without gaseous oxygen) In order to obtain information about the total amount of oxygen that could be reversibly reduced/oxidized from different catalysts, TPR runs were performed with carbon monoxide and methane. TPR runs, rather than reduction runs at constant temperature, were chosen in this case, in order to obtain some information about the catalyst activity simultaneously in different temperature regimes. The experiments were performed with 5% CH4 (or 4% CO) in argon, directly following the activity measurements. The catalyst was cooled to room temperature in the CH1/02 mixture and the reactor was flushed with argon before the experiment was started. Before commenting on the results for individual catalysts some general features can be mentioned. The total amount of oxygen removed is always much larger than a monolayer, i.e. the predominant oxygen source is bulk oxygen. The TPR curves seem always to develop a double peak, indicating a change in activation energy (and maybe a change in the source or transport mechanism of oxygen) as the oxygen content is lowered. The TPR runs produce only carbon dioxide during the initial stages. Only at the highest degrees of reduction, at high temperature, was some carbon monoxide observed. The greatest amounts of carbon monoxide were measured with catalysts LaMn and with aged LaFe,,CQ, (carbon monoxide production for LaMn is shown in Fig. 6 1. The lack of carbon monoxide formation indicates either that carbon monoxide is not an intermediate species in the oxidation of methane or that the oxidation of carbon monoxide to carbon dioxide is fast compared to the initiating reaction step - which is likely to be the decomposition or reaction of methane to a lower hydrogen content. It is not possible to discriminate between these two alternatives with the present data; if carbon monoxide is an intermediate, it is not likely to survive as such to the catalyst exit, since carbon monoxide reacts at lower temperatures than methane (Fig. 2) and is less sensitive to oxygen depletion (Fig. 3 ) . Turning now to differences between different catalysts we first note that the initial activity, at low degree of reduction, is highest for LaFe,.&~.,, (Fig. 4)) followed by La&&.~Mn,-,.J?eC~.~ (Fig. 5) and LaMn (Fig. 7), which have comparable initial activities, while LaFez.J&, (Fig. 6) has the lowest activity. If instead peak conversion rates are inspected L~.eSro.2Mn0.J?eCu,,.~ has the highest activity (60% conversion at 660-730 ’ C ) , followed by LaFe2.&q,.4 (45% at 730 ’ C ) , LaFeO.&~.le (26% at 550 and 650 ’ C ) and LaMn ( 15% at 550 and 700” C ) (note the different scales used in the figures). The effect of aging is obvious from a comparison between the fresh and aged catalysts L~.aSr0.zMn&l?eCu,,.3 and LaFe,.,Cq,.4. Aging moves the TPR curves to higher temperature, while peak conversion rates are affected in opposite ways for these two catalysts. In the case of L~,,Sr,,Mq,.$eCq,.~, aging seems to alter the energetics, with higher peak intensity for the aged catalyst, but

P. Salomonsson et al. / Appl. Catal. A 104(1993) 175-197

25 I

0

I

I

187

I 0.5

100 200 300 400 500 600 700 600 Temperature (“C)

Fig. 4.TPR with 5% CH, in argon over oxidized LaFe,,,s.&~.le. The carbon dioxide and oxygen signals are plotted vs. temperature. Flow-rate: 73.5 ml/min, heating rate 0.8”C/s. The oxygen desorption in 100% Ar is also shown for comparison (flow-rate 36.4 ml/min, heating rate O.WC/ s).

0

100 200 300 400 500 600 700 600

Temperature (“C) Fig. 5. TPR with 5% CH, in argon over oxidized La,,,Sr0,2Mq,.sFeC~.s. Fresh (solid curve) and aged catalyst (dotted curve). Flow-rate: 73.5 ml/min, heating rate O.WC/s.

roughlythe same amount of oxygen removed (cf. Table 4). For LaFe2.,Cu,, on the other hand, the maximum rate occurs at the same temperaturewith the aged and fresh catalyst, but with less oxygen available for the aged catalyst. This decrease in removed oxygen for the aged catalyst scales with the loss of BET area (cf. Table 4). LaFeo,,J!u,.,6shows an interestingfeaturenot observedwith any other catalyst,namelya low temperatureoxygen desorptionpeak, appearingeuenin the presence of methane (dashed-dotted curve in Fig. 4). For comparison the oxygen temperature-programmeddesorption in argon is shown (dotted curve).

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189

low calcinationtemperatures5 650’ C, while highertemperaturesseemto produce a slightlyoxygen deficient LaFeOB_ 8 [ 261. Since our calcinationtemperaturewas 606°C we do not believethe observeddesorptionis due to LaFeOs+,+ In addition, experimentswith LaFeO, reportedin the literature [ 7,27,28] and separateexperimentsby us, produced very little low temperaturedesorption. The copper added to LaFeO, to form LaFeO.,&u,,.le createsonly a very slight change of the lines in the XRD spectrum. Therefore one can not conclude directly from XRD if a mixed La (Fe,Cu)O3 phase has formed. However, if a pure LaFeO, phase is formed, then there will be excess copper, which should appear as an additional phase (e.g. copper-oxides). Since no such lines were present in the XRD spectrum (the weak La&uO, was not strong enough to account for the 16% B site copper) we conclude that the copper was present in the lattice of the perovskite. Based on these observations, we attribute this observed low-temperature oxygen desorption to the partial substitutionof iron with copper. In analogy with the effect of replacingLa by Sr in La, _,SrJ?eOB[ 261, this is expected to produce more weaklybound oxygen due to the forced increasein iron valence. Temperature-programmed

oxidation after reduction

The reversiblereduction/oxidation behaviourof the catalystsis furtherelucidated by the oxygen uptake after reduction in methaneor carbon monoxide. After reduction in a TPR run, as described in the above section on TPR with methane, the catalyst was cooled to ambient temperaturein the reducingatmosphere.The reactor was flushedwith argon, a flow of 4.5% 0, in argon was then introduced,and the linear temperatureramp was started. Figs. 6-10 show oxygen uptake curvesfor the six differentcatalyst samples, LaFeo.,&,,lG and LaMn, fresh and aged L~,,Sro,Mn,,J?eC~.~, and fresh and aged LaFez,C~,, respectively. The kinetics of uptake are quite different for the different catalysts. The LaFe,,+&,ls catalyst absorbs most of the oxygen in a fairly narrow temperature interval 150-35O”C, while LaMn shows a very complex uptake pattern distributedover the whole temperaturerange 166-666oC. LaFe&&,, shows a similardouble minimumpattern as LaFe0.&u,,16and La,,.8Sr0.~Mn,,.J?eCu,,~ but over a wider temperaturerangethan for LaFeO.Jh.,,. Comparing the structuresin the temperature-programmedoxygen uptake curveswith the correspondingfeaturesof temperature-programmedreduction runs,we found double structuresor at least a tendency to double structuresin all cases except in the LaMn oxygen uptake curves. One may speculatethat this corresponds to oxygen in different binding states, perhaps related to the oxidation state of the metal ions, and/or to different activation barriers for dissociation or diffusion. Generally the uptake occurs at lower temperature

190

P.Submonssonetal./Appl. Cata1.A 104(1993)175-197

I

0

,

r

100 200 300 400 500 600 700 600 Temperature (“C)

Fig. 8. Oxygsn uptake during OS-TPR for LaFeO.&~.,s argon, flow-rate: 73.5 ml/m& heating rate O.g”C/s.

0

and L&In. Gas composition 4.5%O2 in

100 200 300 400 500 600 700 600 Temperature (“C)

Fig. 9.01ygen uptake during O,-TPR for J&&,.,M~.$eCq,~ catalyst (dotted curve). Conditions as in Fig. 8.

Fresh (solid curve) and aged

than reduction,which may be a signthat the reductionis not limitedby oxygen transport. In Table 4 the amountof oxygenremovedas carbon dioxide and waterduring the CHJ-TPR up to 800°C is listed for all catalysts. The second column contains the amount of oxygen absorbed during the subsequent oxygen uptake experiment, expressed as percent of the oxygen removed during CH,-TPR. There is throughouta good correlationbetween total oxygen uptake and preceding removal of oxygen by reduction (Table 4), but the oxygen uptake is systematicallya few percent lower. This can be due to measurementerrors, sincethe total uncertaintyin this measurementcould be quite large,up to lo%,

P. Salommwson et al. / Appl. Cati.

.z

80

Ii E Q,

80

i

40

0

.s

Fresh 20

LaFe2.4CuO.4

iii

2

191

A 104 (1993) 175-197

0

100

1

I

I 0

200

300

400

500

Temperature

800

700

800

(“C)

Fig. 10. Oxygen uptake during O,-TPR for LaFex,,C~.~ Fresh (solid curve) and aged catalyst (dotted curve). Conditions as in Fig. 8. TABLE 4 Amount of lattice oxygen available for reaction with oxidized cataly~ta Catalyst

LaMn LaFeO.&kI~ Lao.&~.xMno.eFeCuo.3 Lao.&.&k.JreC~.~ (aged) LaFe&uo.~ LaFe&&., (aged)

Oxygen removed

~Pmol)”

Monolayers*

281 397 670 624 575 307

12 24 80 128 143 145

Oxygen uptake (RI’

96 88 91 87 98 84

Total amount of oxygen reacting to form CO* and Hz0 during the CHI-TPR up to 8WC, assuming that for each CO2 molecule two Hz0 molecules are formed. *Same as “, but expressed in monolayers, as defined earlier. ‘Amount of oxygen absorbed during the oxygen uptake experiment with reduced catalysts, in percent of the oxygen removed during CHI-TPR.

but small irreversible catalyst changes or incomplete reoxidation cannot be excluded.

Temperature-programmed desorption of oxygen Information about oxygen binding and transport in the catalyst should in principle be obtainable from temperature-programmed desorption of oxygen. TPD of oxygen was performed after temperature-programmed oxidation runs. The oxidized catalyst was cooled to ambient temperature in the OJAr

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P. Salomonsson et al. / Appl. Catal. A 104 (1993) 175-197

mixture. The reactor was then flushed with argon before the TPD run was performed in flowing argon. Results are shown in Figs. 11-13. Except for LaFe0.8&~.16, as discussed earlier (Fig. 4), the desorption starts at quite high temperatures 2 450-500°C. The total amount of oxygen that desorbs is very small compared to the amount that can react with carbon monoxide or methane in the same temperature range (see Table 5 for a comparison). For LaMn and LaFe0.8&,16 about 20% and lo%, respectively, of the oxygen amount available for reaction, can be desorbed up to 800°C within the time of the experiment, while for the La,,8Sr,,,Mn,,,FeCu,3 and LaFe,,Cu,d catalysts only a few percent can be de0.30 ? ka 0.25 *a 5 E 0.20 s

I

,

:

...

LaMn ;

0

100 200

300 400

500

600

700

600

Temperature (“C) Fig. 11. TPD of oxygen in flowing argon for catalysts LaFe,,,&u,,,, ml/min,

36.4

,

0.20

‘; ** a~ 0.16 -z E 3

and LaMn. Flow-rate:

heating rate 0.8”C/s.

La0.8Sr0.2Mno.8FeCu0.3

0.12 --

5 .g 2

0.06 --

68

0.04 -0.00 -’ 200

I 300

400

500

600

700

800

Temperature (“C) Fig. 12. TPD of oxygen in flowing argon for Lao.sSro.zMno.sFeCuo,~. Fresh (solid curve) and aged catalyst (dotted curve 1. Conditions as in Fig. 11.

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P. Sakwnonsson et al. / Appl. CataL A 104 (1993) 175-197

7 $ j

E

.-5 z z 8 P

0.10

’

0.06

--

0.06

--

0.04

--

I

I

I

LaFe2.4CuO.4

0.02

0 I

0.00 200

300

400

500

Temperature

1

I

600

700

600

(“C)

Fig. 13. TPD of oxygen in flowing argon for J.&ez,,Cu,,.l. Fresh (solid curve) and aged catalyst (dotted curve). Conditions as in Fig. 11. TABLE 5 Amount of oxygen desorbed up to 800 ’ C during TPD of oxygen Catalyst

LaMn ~eO.wCuo.IG kkk~Mno.sFeCuo.3 LeO.&~.,Mrb.$eC&.~ ~ez.&uo.~ LaFe&uo.l (aged)

Oxygen desorbed

(aged)

(pa”

Monolayers*

52.2 46.3 20.0 7.9 14.1 8.8

2.2 2.8 2.4 1.6 3.5 4.2

“Total amount of oxygen desorbed up to 800°C. *Same as *, but expressed in monolayers, as defined earlier.

sorbed. This indicates a generally higher activation energy for the o”+O”+O$ reaction than for CO+OB+C02 or CH,+O”+CO, (where “s” stands for surface and “g” for gas). Table 5 also gives the amounts of oxygen desorbed in terms of monolayers. For LaMnO, a small desorption peak corresponding to about 1% of a monolayer is observed centered at T= 300°C. For the LaFez,Cq,.4 catalysts, a characteristic peak appears in the spectra at Tz 600°C. McCarty and Wise [ 71 reported a peak at 620°C (900K) for Fe203. This could explain the low-temperature behaviour of the LaFez.rCq,r catalyst, since it has an excess of iron which results in the formation of Fe,O,. The low-temperature desorption observed with the LaFeo.84Cq,ls catalyst is not seen for these two catalysts.

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P. Salomonsson et al. / Appl. Catal. A 104 (1993) 175-197

SUMMARY AND FINAL REMARKS

The observed activities for methane (and carbon monoxide) oxidation are quite high, evidenced by comparison of the LaMn catalyst to a supported platinum catalyst (Fig. 1) . The BET surface areas of the catalysts varied widely however (by a factor of lo), and for this reason, a meaningful ranking of activities baaed on turnover number is not yet possible. In addition, the surface areas and thus activities of the catalysts were reduced markedly upon aging. Apparently different rate limiting steps are operative for the methane and carbon monoxide oxidation reactions. The activities of the catalysts were in all cases higher for the carbon monoxide oxidation than for methane oxidation, which may be an indication that the first reaction step involving methane (e.g., CH,+O”-CHg +OH” or CH,-CH$ +H*) is rate limiting (We are not suggesting free methyl radical formation. The formulas just indicate two possible ways of initiating the methane decomposition). The methane oxidation reaction over the LaFez.lCG.l catalyst displayed a much greater activation energy in the absence of gaseous oxygen ( 122 vs. 83 kJ/mol). This marked difference in activation energies in oxidative and reducing environments indicates reaction with oxygen in different binding states in the two cases. This is consistent with the study of McCarty and Wise [ 71, who deduced activation energies for reaction with chemisorbed and lattice oxygen to be 81 and 133 kJ/mol, respectively. In their study, the lattice derived oxygen started to contribute to the carbon dioxide production rate between 560 and 640’ C [ 71. Thus, it appears that the reaction occurs by both “intrafacial” and “suprafacial” mechanisms [ 141, according to the availability of surface and lattice oxygen. Regardless of the availability of gaseous oxygen, all catalysts favored complete oxidation (formation of carbon dioxide and water) of methane. Only in cases of extreme oxygen depletion from the catalysts (corresponding to over 100 monolayers) or in the case of the poor oxygen-exchange catalyst LaMn, were carbon monoxide and hydrogen produced. The lattice oxygen in the catalysts is accessible for the reaction of methane in a reducing environment. The quantities of oxygen removed from catalysts were significantly greater than one monolayer. In one extreme case, with LaFe2.4Cu,.4, more than 500 monolayers were removed during reduction with carbon monoxide at 700°C. In regard to the characteristics of this lattice oxygen, the following points can be made: (i) Significantly more oxygen was removed in a reducing environment than in an inert argon environment (CH,TPR vs. 0,TPD ) , showing that the barrier for associative oxygen desorption generally is higher than for oxidation of methane and carbon monoxide. One exception to this is around the first one monolayer of the oxygen saturated LaFe O.&~.ls catalyst, which desorbs oxygen even in the presence of methane. (ii) Three times as much lattice oxygen was removed during reaction with

P. Salomnsson et al. / Appl. Catal. A 104 (1993) 175-197

195

carbon monoxide compared to methane at constant temperature for the LaFe2.4CQ.4;catalyst. (iii) These large amounts of oxygen, however, can be quantitatively reabsorbed into the lattice even after the extreme reduction of LaFez.,Cu,,, at 766” C in carbon monoxide. (iv) The reduction of the catalysts exhibits a complex temperature dependence. In general, the plots of carbon dioxide production versus temperature exhibited two peaks, with the higher temperature peak occurring at 2 650’ C. (v ) Beoxidation of the reduced catalysts (oxygen uptake) occurs at temperatures considerably lower than those where reduction takes place, also with a complex temperature dependence. The fact that uptake occurs at lower temperatures suggests that oxygen transportwithin the catalyst is not rate limiting for oxidation of methane over a partially reduced catalyst. The complex temperature dependence of both oxygen removal (due to reaction with methane) as well as oxygen uptake is most likely related to the sequential reduction/reoxidation of the metal ions in the oxides. It is known that manganese and iron can assume multiple valences within the perovskite structure. Reduction experiments performed at a constant temperature exhibit similar complexity. The BET surface area is not necessarily the effective area (active area) for the reduction of the catalysts. The aging may also alter the chemical composition of the surface. This is most clearly shown by the results for the fresh and aged catalysts L~,,Sr,,Mn,.~eCu,,.~. Although the aged catalyst had ca. one half the BET area of the fresh catalyst, roughly the same amount of oxygen was removed during reaction with methane under reducing conditions. The activation energy for 0 (a) diffusion in iron and cobalt perovskites has been measured to be 74 kJ/mol [ 231. For reduction of catalysts with methane (in the absence of gaseous oxygen) the extracted E, (for the LaFe,,Cu,,, catalyst) is greater than that corresponding to oxygen-diffusion. This result suggests that the methane decomposition is rate limiting in this case. The fact that oxygen uptake occurs at lower temperature than reduction in methane has implications for an intrafacial mechanism of oxidation. (i) Oxygen dissociating at the surface is obviously sufficiently mobile to fill oxygen vacancies in the bulk in the temperature range where methane oxidation occurs. (ii) With oxygen excess the intrafacial reaction is then expected to be zero order in oxygen partial pressure. Investigated catalysts with excess of B-position metals (based on perovskite stoichiometry) had a greater capacity to exchange oxygen than the stoichiometric perovskites LaMn and LaFeo.+&u,,ls. While this characteristic was apparently of importance in influencing the activity of these materials for nitric oxide reduction [ 151, it is not so important for the oxidation of methane in an oxidative environment. Future work will be emphasized on the investigation of the sequential re-

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duction of the catalysts by performing in situ XPS during reduction with carbon monoxide and methane. Oxygen transport within the catalysts will be studied through the use of “0 isotopes. ACKNOWLEDGEMENTS

We would like to acknowledge the contribution of Dr. Lutz Seffner of the Fraunhofer-Einrichtung fi.ir Keramische Technologien und Sintenverkstoffe (IKTS), Dresden, for the preparation of catalysts samples. In addition, we would like to thank Dr. Ingvar Andersson, of Chalmers Industriteknik (CIT) for his help in the organization of this project. This project was partially funded by The Swedish Board for Industrial and Technical Development (NUTEK ) , project number 90-01701. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

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