catalytic carbon monoxide oxidation over strontium, cerium and copper-substituted lanthanum...

Applied Catalysis A: General, 107 (1994) 201-227

Elsevier Science B.V., Amsterdam

201

APCAT A2648

Catalytic carbon monoxide oxidation over strontium, cerium and copper-substituted lanthanum manganates and cobaltates

K.S. Chan, J. Ma, !3. Jaenicke and G.K. Chuah

Department of Chemistry, National University of Singapore, Kent Ridge (Singapore 0511)

and

J.Y. Lee

Department of Chemical Engineering, National University of Singapore, Kent Ridge (Singapore 0511)

(Received 5 April 1993, revised manuscript received 17 August 1993)

Abstract

The influence of either A or B-site substitution in perovskite-type mixed oxides on the catalytic oxidation of carbon monoxide has been studied. The following systems were investigated: (La,Sr )MnO,, La(Mn,Cu)O,, (La,Sr)C!oO, and (La,Ce)CoO,. Cobaltates are generally more active than the manganates. Substitution in the A or B-site improved the catalytic activity with oxidation starting from 75°C. A volcano plot of activity versus composition was obtained for each series with up to a lo-fold increase in catalytic activity for the substituted compounds. Lattice oxygen participates in the reaction even under stoichiometric conmditions. The catalysts show a positive rate dependence on the carbon monoxide partial pressure so that under reducing conditions, the reaction is not inhibited. A bistability in the rate of catalytic oxidation at high carbon monoxide concentration was observed over Lai_,Sr,MnO, and LaMn, _$u,O, (0 Q r Q 0.2 ) . This bistability has been attributed to a carbon monoxide-driven reconstruction of the reduced s,urface, leading to pairs of Mn, ions with a Mn-Mn distance comparable to the spacing in the metal. These pairs provide reactive sites for carbon monoxide oxidation and oxygen chemisorption. Such metal-metal pairs are not found in the perovskite lattice but are a structural feature of the closely related hexagonal 4-layered packing which is the normal crystal structure of SrMnO,. The change back to the less active state is due to reoxidation of the surface. It was confirmed that a low mobility of lattice oxygen is a necessary condition for hysteresis in these oxides.

Key words: carbon monoxide oxidation; (La,Sr)MnOs; La(Mn / Cu)O,; (La / Sr)CoO, and (La

/ Ce)CoO,; oxygen mlobility; perovskites; steady-state multiplicity

Correspondence to: Dr. G.K. Chuah, Department of Chemistry, National University of Singapore,

Kent Ridge, Singapore 0511. Tel. (+65) 7722839, fax. (+65) 7791691, E-mail

CHMCGK@NUSVM.

0926-860X/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved.

SSZ)ZO926-860X(93)EOlS5-F

202 K.S. Chan et al. / Appl. Catal. A 107 (1994) 201-227

INTRODUCTION

Perovskite-type oxides have been extensively studied in various applications including catalysis [l-3], electrodes for fuel cells, superconductors, and pho- tocatalytic dissociation of water [ 4-71. Recently, renewed interest in the catalytic properties of these compounds stems from the suggestion to use them for low temperature catalytic combustion in power plants, thus avoiding the thermal NO, prolduction. Another novel application is the use as a medium for selective partial (oxidation in riser reactors.

The ideal perovskite structure, AB03, can be envisaged to consist of hexagonal closed packed sheets of A03, in which the big A cation is surrounded by six oxygen ions. In the cubic perovskite, these A03 sheets are stacked in the sequence abcabc and all octahedral holes formed by six oxygen ions are occupied by the small B cations. These B cations, which are usually transition metals, are the catalytically active centers. The A cations may be rare earth elements or base metals such as calcium, barium, strontium, bismuth or lead. They are believed to be catalytically inactive. Perovskites are frequently non- stoichiometric and should correctly be described by the formula ABO,,, where the nonstoichiometry 6 is usually between + 0.2 and - 0.5. By substitution of lower valency ions for part of the A or B ions, oxygen vacancies can be created and the oxidation state of the B ion can be changed. The stability of the perovskite structure makes these systems ideally suitable for studies into the correlation of catalytic activity with defect density.

Manganese and cobalt-containing perovskites are very active catalysts for the oxidation of carbon monoxide and hydrocarbons [8-141. Rare earth ions in the A-site increase the thermal stability of the material. In this work, we have investigated A- and B-site substitution in lanthanum manganates and cobaltates. In two series, (La,Sr )MnO, and (La,Sr ) COO,, the A-site ( La3+ ), was partly substituted with SF. This will result in a higher amount of Mn4+ or Co4+ due to charg e c ompensation. In another series, (La,Ce)CoO,, Ce4+ was substituted for part of the La3+. This should lead to the reduction of part of the Co3+ to Co’+ and the effect of this on the activity was also investigated. B-site substitution was examined in the system La (Mn,Cu)O, as copper manganates are known as active catalysts for a variety of reactions. The effects of these substitutions were investigated with respect to the oxidative activity, oxygen desorption capability and reducibility of the catalysts.

EXPERIMENTAL

Preparation

(La,Sr)Mn03 were prepared by coprecipitation of the metal nitrate solutions with an excess of aqueous ammonium carbonate at room temperature.

KS. Chan et al. / Appl. Catal. A 107 (1994) 201-227 203

The pink precipitates were filtered, washed with cold water, and dried over- night at 110°C. The resulting products were calcined in air at 860” C for 14 h. All other series, La(Mn,Cu)03, (La,Sr)CoO,, and (La,Ce)CoO,, were prepared by complexation of the metal nitrate solutions with citric acid. The resulting vitreous gels were evaporated to dryness at 60’ C in vacua using a rotary evaporator. The residues were heated at 200°C to decompose the citrate-nitrate complexes before calcining in air at 800’ C for 12 h. The structure was determined by X-ray powder diffraction (Philips PW 1729, Cr Kcu source). In agreement with earlier reports [E&11,15,16], it was found that single phase perovskite formed up to 60% substitution. Surface areas were determined by nitrogen adsorption (BET method) using a Micromeritics Flowsorb 2300.

Oxygen nonstoichiometry

(La,Sr)MnO, A wet chemical nnethod adapted from Yakel [ 171 was used for this analysis.

The oxygen content was determined in two steps. The first involved the deter- mination of the total oxidation power of the sample and the second, the total manganese in the catalysts.

The oxidation equivalent was determined as follows: a weighed amount of the catalyst was dissolved in a known amount of iron (II) sulfate solution, and the excess iron (II) was back titrated with standard potassium permanganate solution. To deternnine the total manganese, the catalyst was treated with dilute nitric acid. A few drops of hydrogen peroxide were added to facilitate dis- solution. The resulting clear solution was evaporated to near dryness to decompose the excess peroxide. The residue was dissolved in dilute sulfuric acid, followed by oxidation of the manganese to permanganate by sodium bismu- thate. A known volume of standard iron (II) sulfate was added to the permanganate and the excess iron (II) was titrated with standard potassium permanganate. From these two determinations, the amount of Mn3+ and Mn4+ can be calculated. The oxygen excess per formula unit is calculated assuming that the amount of strontium and lanthanum corresponds to the proportions used in the preparation and that all oxygen is present as 02-.

(La, Sr)CoO, The oxidation eiquivalent was determined by adding a weighed amount of

the sample into a ground glass-stoppered round-bottomed flask containing excess hydrochloric acid and potassium iodide [ 181. The flask was warmed to about 60°C until the sample dissolved. Both Co3+ and Co4+ are reduced by dilute hydrochloric acid to Co2+, liberating chlorine which in turn oxidizes the iodide to iodine. The liberated iodine was titrated against standard thiosulfate solution. The total amount of cobalt was determined as follows [ 191. A sample was dissolved in hot sulfuric acid and neutralized with excess sodium bicar-

204 KS. Chan et al. / Appl. Catal. A 107 (1994) 201-227

bonate. A few drops of hydrogen peroxide were added to form a green carbonato complex of Co3+. Excess hydrogen peroxide was decomposed by gentle heating. The solution was cooled, diluted with water and potassium iodide was added. The carbonato complex was reduced by the iodide to form Co2+. The iodine liberated was titrated with thiosulfate. Hence, from these measurements, the amount of Co3+ and Co4+ can be found.

Catalyst activity

Activity testing was carried out in a fixed-bed reactor operated under at- mospheric pressure. A Pyrex tube (10 mm I.D. x 200 mm) which is connected on both ends to 4 mm I.D. Pyrex tubing was used as the reactor. The catalyst was placed on a glass frit in the lo-mm I.D. part. The temperature of the catalyst bed was measured by a stainless steel-sheathed chromel-alumel thermocouple inserted coaxially in the reactor. Typically, 250 mg of catalyst were used, which occupied a volume of 0.5 ml. Before use, catalysts were pretreated in the reactor for 1 h at 400’ C in pure oxygen. A mixture of the reactant gases, carbon monoxide (purity>99.99%, typical flow-rate 8 ml/min) and oxygen (99.8%) in stoiNchiometric amounts (2 : 1 v/v), was made up in helium (99.9995%) to a total flow-rate of 50 ml/min and passed into the reactor. A constant flow-rate of the gases was maintained by mass flow controllers (MKS Inc. ) . The conce:ntration of oxygen, carbon monoxide and carbon dioxide in the effluent gases was analyzed by gas chromatography (molecular sieve 5A, 1 m, programmed 60-200’ C ) with a thermoconductivity detector (TCD ) . After determining the sensitivity factors for the different gases, the mass balance was generally within 3%. A blank experiment using quartz chips in place of the catalyst showed no carbon monoxide conversion under the experimental conditions.

Temperature-programmed desorption (TPD) of oxygen

Temperature-programmed desorption of oxygen was carried out at atmos- pheric pressure in a flow of helium. The sample was first conditioned by heating to 900°C in helium, followed by cooling to room temperature in flowing oxygen. Helium (flow-rate 50 ml/min) was then introduced in place of the oxygen and after flushing for 30 min, the temperature of the sample was ramped to 900°C at a rate of BO”C/min. A quadrupole mass spectrometer (Hiden HAL200 with tASYST software) was used to monitor the desorbed oxygen.

Amount of exchangeable oxygen

The following experiment was designed to determine qualitatively the lattice oxygen mobility and the ease of oxygen exchange. A catalyst is first reduced at

K.S. Chan et al. / Appl. Catal. A 107 (1994) 201-227 205

350’ C in a stream of carbon monoxide and then exposed to pure oxygen at a lower temperature (100 and 200’ C) for 30 min. It was found that treatment at 200°C leads to almost complete reoxidation of the samples but only partial oxidation is achieved at the lower temperature. Thus, the experiments at 200’ C probe the equi1ibriu.m whereas the experiments at 100’ C contain information on the kinetics of oxygen uptake at the lower temperature. The amount of oxygen incorporated into the lattice is equivalent to the carbon dioxide which is formed during the subsequent reduction at 350’ C. The total amount of carbon dioxide produced is therefore a measure for the ease of reoxidation at the low temperature, whereas the rate (production per unit time) is a measure of the lattice oxygen mobility at 350” C. To follow the evolution of carbon dioxide with time, the effluent was bubbled through a thermostated barium hydroxide solution whose conductivity was continuously recorded. The substituted cobaltates were irreversibly reduced under these conditions (see results section). The method had therefore to be modified in the following way. Freshly prepared cobaltates were reduced with carbon monoxide at 350 o C and the amount of carbon dioxide formed was determined as above. Reoxidation, even at 350’ C, does not reconstruct the original formula, but leads to an oxygen deficient compound.

Hysteresis study

The experiment was done in a temperature-programmed mode. A heating and cooling rate of 2’ C/min was chosen. The temperature rise in the catalyst bed during reaction has been shown to correlate well with the conversion, thus allowing the progress of reaction to be followed in real time. Since the details of this method have been described elsewhere [ 201, only a brief outline will be given here.

The oven temperature, T1 (measured to O.l”C by a chromel-alumel thermocouple placed outside the reactor at the same height as the catalyst bed) was ramped up and down at a speed of 2’ C/min. A second thermocouple placed in the center of the catalyst bed measures the temperature, T2, as the reactant gases were passed t:hrough the reactor. The total flow-rate was 100 ml/min and comprises of the carrier gas, helium, and the reactant gases, carbon monoxide and oxygen, in the ratio 46: 4 respectively. Blank runs were made using the same catalyst in a mixture of helium and nitrogen (the heat capacity of nitrogen being close to that of carbon monoxide and oxygen). The catalyst bed temperature in the absence of any reaction, T3, which corresponds to the temperature of the feed gases was thus determined. The conversion was calculated from these temperature measurements by a formula which equates the heat of reaction to the heat losses by conduction, flow and radiation in the steady state.

206 K.S. Ghan et al. / Appl. Catal. A 107 (1994) 201-227

RESULTS

Catalyst characterization

In the following discussion, “x” refers to the mole fraction of the substituent (strontium, copper or cerium). The surface areas of the different catalysts are summarized in Table 1. They are generally between 7 and 19 m2/g for the manganates and between 2 and 8 m2/g for the cobaltates. X-ray diffraction showed that samples up to x=0.6 crystallized in the perovskite structure, al- though LaMn,.,Cu,,G03 was accompanied by LaCu204 and CuO. For (La,Ce)CoO, with cerium content >20%, an additional phase of CeO, was detected.

TABLE 1

Properties of catalyst samples

Composition Surface area* M4+ x (m2/g) (%)

La, _,Srfln03 0 11.04 24 0.1 8.31 0.2 7.26 36 0.4 8.41 46 0.6 8.64 52 0.8 9.11 58

LaMn, _,Cu,03 0 11.04 24 0.2 13.14 0.4 17.32 0.6 19.46

La, _zSrzCoO, 0 0.2 0.4 0.6 0.8

3.99 2.4 8.50 3.0 2.23 3.1 3.23 6.8 2.19 42

La, _JZe,CoO, 0 3.!39 2.4 0.1 5.78 _

0.2 8.67 0.4 8.43

a Catalysts calcined at 800 o C except for (La,Sr )MnO, series which were calcined at 860 ’ C.

K.S. Ghan et al. / Appl. Catal. A 107 WW 201-227 207

Oxygen nonstoichiometry

The parent complounds of both series, LaCoO, (excess oxygen, S=O.O2) and LaMnO (6= 0.12 ), show oxidative nonstoichiometry. Oxidative nonstoichiometry has also been reported by other workers [ 17,211. By means of neu- tron diffraction, Tolfield and Scott [ 221 established that the oxidative nonstoichiometry in LaMnO, is due to cation vacancies on the lanthanum site rather than the presence of interstitial oxygen. 8% of the lanthanum sites in La- MnO,.,z are vacant, with only about 2% of the nonstoichiometry due to the presence of manganese vacancies. Fig. 1 shows that the oxygen nonstoichiometry, 8, changes almost linearly from +0.12 in LaMnO, to -0.11 in La,,,,Sr0,8Mn0,. For x>O.5, the structure is oxygen deficient. Hence, substitution of the trivalent lanthanum with bivalent strontium results not only in a higher amount of manganese(IV) but also leads to a loss of lattice oxygen (Table 1). Within the series (La,Sr ) Coo,, only LaCoO, shows slight oxidative nonstoichiometry I( 6= 0.02). Any strontium substitution resulted in oxygen deficiency. This agrees with the known instability of the Co4+ oxidation state. Charge compensation tends to be via oxygen deficiency rather than a promotion of Co3+ to Co4+ . Only at 80% strontium substitution does the amount of Co4+ increase sharply. This is probably effected by the constraints of the perovskite lattice which is stable only to 6= - 0.5. Measurements of the nonstoichiometry could not be carried out on La(Mn,Cu)03 and (La,Ce)CoO, because copper and cerium interfere with the redox titrations for manganese and cobalt, respectively. However, Vogel et al. [ 231 found by hydrogen thermogra- vimetry that all La(Mn,Cu)O, were oxygen deficient with manganese predom- inantly in the 4 + and copper in the 2 + state.

00 0.2 0.4 0.6 0.8

Strontium Substilution x

Fig. 1. Dependence of oxygen nonstoichiometry on strontium substitution for (0 ) La, _,Sr,MnO, and (0 ) La, _,SrzCoO,.


Carbon monoxide oxidation

The amount of carbon dioxide produced per unit surface area under stoichiometric conditions is shown in Fig. 2 for (La,Sr)MnO,. At 150°C the activity of the different catalysts in this series were rather similar and about 50% higher than LaMnO,. For temperatures between 150 and 200°C the rate of carbon monoxide oxidation was highest over La,,&,,MnO,. However, above this temperature, LaO,Sr,,zMnO, became the most active catalyst. The amount of carbon dioxide Iproduced per unit time and area is almost twice that over LaMnO,. Catalysts with strontium > 20% are not as active as La&+,,2Mn03, though their activities are still higher than that of unsubstituted LaMnO,.

Substitution of the B-site ion, manganese, with copper also resulted in an improved activity. At temperatures below 150” C, three regions of activity can be discerned as a result of increasing copper content (Fig. 3): (a) a sharp increase of activity for a small substitution up to X= 0.2 (b) fall in activity with further increase of copper for 0.2 <x < 0.6 and (c ) increase in activity for x > 0.6; at these compositions, considerable amounts of copper (II) oxide had been detected besides the perovskite phase. At 200°C and above, the activities of LaMn,_,Cu,O, (x= 0.2-0.6) were comparable and about thrice that of LaMn03.

Fig. 4 shows the conversion over the (La,Sr ) COO, catalysts versus temperature. Keeping tjhe catalyst loading constant (400 mg), the onset of reaction (conversion > l’% ) over La&Sr,,CoO, and La&+,,CoO, started at 75 ‘C, whereas over the unsubstituted compound, LaCoO,, conversion became meas-

200

Strontium Substitution x

Fig. 2. Carbon monoxide oxidation activity of Lal_,Sr,MnO, at different temperatures.


Fig. 3. Activity for carbon monoxide oxidation at 150°C of (a) LaI_ &,MnO,, (b) LaMn,_,Cu,O,, (c) La,_,Sr,CoO, and fd) Lal_,Ce,Co03.

60

temiperature (’ C)

Fig. 4. Percent carbon monoxide conversion over Lal_,Sr,CoO, vs. temperature at constant loading. (Catalyst weight 400 mg). (0 )LaCoO,, (0 ) La&3r,,.&oO,, (A ) La,,,SrO.,CoO,, (0 ) La0.&.&oOs, (m) Li4,.,Sr0.sCo03.

210 KS. Chan et al. 1 Appl. Catal. A 107 (1994) 201-227

urable only above 100’ C. Further increase in the strontium content shifted the conversion curve to higher temperatures relative to LaCoO,. The activity for carbon monoxide conversion at 150’ C reaches a maximum at the composition L~.$r,,,CoOs where about six times as much carbon dioxide is formed than over LaCoO, (Fig. 3). However, with the more highly substituted oxides, La,,,SrO&oOs and La,,,Sr,,&oO,, the activity dropped below that of LaCoO+

The (La,Ce) Coo, catalysts were all considerably more active than LaCoO,. Carbon monox:ide oxidation sets in at temperatures as low as 75°C. L~,,Ce,,,CoO, was found to be the most active catalyst of this series. At 150” C, the activity is about five times that of the unmodified LaCoO,. L~&e,,,CoO, and La,,&e,,,CoO, were about three times more active than the unsubstituted catalyst. Note that this activity is expressed per unit surface area. If the activity is expressed per unit weight rather than per unit area, then the increase observed for the cerium-substituted catalysts with their relatively high surface area (Table 1) would be even more impressive than that of the (La,Sr)CoO, series.

The apparent reaction order with respect to the partial pressures of carbon monoxide and oxygen was determined at 125-150” C (low temperature) and 200-225 o C (high temperature). The rates were measured by varying the partial pressure of one of the reactants while keeping the other reactant constant. The conversion was kept below 10%; in the experiments at high temperature, a smaller amount of catalyst was used and diluted with quartz chips so that the same space velocity as at 125-150” C was achieved.

Reaction orders for LaMnO, had to be measured at 160°C due to its very low activity at lf!5’ C. The reaction order with respect to oxygen, measured at low temperature, increases with strontium substitution from 0.36 for LaMnO to 1.0 for La&Sr,,sMnO, (Table 2). The values reported for low temperature apply only for low oxygen partial pressures. At oxygen partial pressures above about 150-160 m’bar, the rate becomes independent of oxygen. This is probably due to saturatio:n of adsorption sites according to the Langmuir isotherm f&,= bp/ (1 + bp). Reaction orders measured at 250” C show a significantly smaller dependence on oxygen for all catalysts in the (La,Sr)MnO, series. This indicates a reaction involving lattice oxygen. The data in Table 2 also show that the reaction order with respect to carbon monoxide does not vary much with temperature. At the lower temperature, there is a slight decrease in order on increasing strontium substitution. The order with respect to carbon monoxide was positive over the entire concentration range investigated. No inhibitory effect of carbon monoxide was observed on these oxides. At high carbon monoxide partial pressures, lattice oxygen reacts with carbon monoxide and thus prevents blocking of the surface sites. Moreover, the different adsorption isotherms for carbon monoxide and oxygen show that the two gases do not compete for the same adsorption sites.

For the other series of catalysts, the oxygen dependence was also positive

KS’. Ghan et al. / Appl. Catal. A 107 (1994) 201-227

TABLE 2

Reaction orders with respect to oxygen and carbon monoxide

211

Composition Order with respect to

Oxygen

125” 250°C

Carbon monoxide

125” 250°C

LaMnO, 0.36” 0.25 0.47” 0.77

La&&MnOs 0.38 0.19 0.64 0.52

Lao.GSrO.JJnOs 0.60 0.15 0.56 0.55 Lao.&.,MnO, 0.76 0.12 0.52 0.53

Lao.k&sMnOs 1.00 0.25 0.45 0.43

125°C 200°C 125°C 200°C

0.34 0.18 0.23 0 no rxn 0.08

150°C 200°C

0.19 0.19 0.70 0.34 0.43 0.38 0.44 0.31 0.39 0.40 0.37 0.53 0.38 0.38 0.20 0.32 0.28 0.48 0.33 0.35

125°C 200°C 125°C 200°C

0.58 0.57 0.64 0.57 no rxn 0.74

150°C 200°C

0.26 0.22 0.61 0.44 0.29 0.24 0.58 0.29 0.32 0.25 0.15 0.40

n Measured at 160 ’ C.

and less than one. However, the effect of substitution was not as pronounced as in (La,Sr )MnOB. Carbon monoxide dependence was again positive over the pressure range investigated, 80 <p < 400 mbar.

Apparent activation energies were calculated by fitting the rates of carbon dioxide formation at low carbon monoxide conversion to the Arrhenius equa- tion (Table 3 ). Substitution of strontium for lanthanum in (La,Sr )MnO, reduces the apparent activation energy to -48 kJ/mol from 66 kJ/mol in LaMnO,. The lower activation energy coincides with the observed increase in activity: the rate of carbon dioxide production at 150°C was higher over the substituted compounds than over LaMnO,. Similarly, in La (Mn,Cu) OS, the apparent activation energy was smaller for x=0.2 and 0.4 but increased in

212

TABLE 3

K.S. Chan et al. / Appl. Catal. A 107 (1994) 201-227

Apparent activation energy under stoichiometric conditions

Sample E,, (kJ/mol) Temperature range ( “C)

LaCoO, 100.8 125-175

L%.&eO.ZoG 42.7 75-125 La&e&oO, 46.2 75-125

La&eO.&oG 38.8 75-125

66.1 loo-250 46.6 loo-250 47.1 loo-250 47.8 loo-250 48.3 loo-250

66.1 loo-250 56.9 75-175 59.2 75-175

144 125-200

100.8 125-175 59.8 75-140 59.6 75-150 95.9 150-220 59.6 150-280

LaMn,,Cq,,O, where again the lowest activity was observed. The same correlation between a low apparent activation energy and high activity was noted for the cobaltates. La,,,Sr,,&oO, which has a relatively low apparent activation energy, 59.6 kJ/mol, was also found to be the most active catalyst in this series. However, the highly substituted compound L~.,Sr,,CoO, shows a similarly low apparent activation energy but is not as active. This may be related to structural factors which force the cobalt ion to assume the 4+ oxidation state; a lower oxidation state of 3+ would lead to the irreversible breakdown of the perovskite structure.

Amount of exchangeable oxygen

The amount of lattice oxygen which was removed by carbon monoxide is shown in Table 4 for two reoxidation temperatures, namely, 100 and 200°C. This is expressed as oxygen removed per formula unit. For a reoxidation temperature of 100 o C, La&SrO.~MnOs appears to be the most reducible oxide within its series, with the largest proportion of lattice oxygen which can be titrated by carbon monoxide. Strontium substitution in these catalysts introduces an-

KS. Chan et al. / Appl. Catal. A 107 (1994) 201-227

TABLE 4

Amount of oxygen taken up at various temperatures expressed as per formula unit

213

x 3+6 Oxygen uptake per formula unit (3+&sJ 3 + 6 calculated 1Oo”c 200°C (reduced) if Mn3+ or Co3+

La, _,Sr&fn03 0 3.12 0.2 3.08 0.4 3.03 0.6 2.96 0.8 2.89

O..ll 0.21 2.91 3.00 O..ll 0.18 2.90 2.90 0..15 0.19 2.84 2.80 0..20 0.21 2.15 2.70 0..13 0.27 2.62 2.60

LaMn, _$u,03 0.2 3.00” 0.4 3.00” 0.6 3.00”

0.13 0.22 2.78” 0.43 0.54 2.46” 0.27 0.36 2.64”

La, _$F,cOo, 0 3.01 0.2 2.92 0.4 2.82 0.6 2.73 0.8 2.81

0.48 0.52 2.49 3.00 0.70 0.79 2.12 2.90 0.92 1.04 1.78 2.80 0.73 1.05 1.68 2.70 0.59 0.73 2.08 2.60

La, _,Ce,CoO, 0.1 3.00” 0.86 0.89 2.11” 0.2 3.00” 0.88 0.95 2.05” 0.4 3.00” 0.80 1.48 1.52”

a Assuming oxygen stoichiometry of the oxidized form is 3.00.

ionic vacancies and1 therefore should facilitate the transport of oxygen into the lattice. However, this is counterbalanced by a decrease in the oxygen affinity which is related to the nonstoichiometry 6. Hence, at low temperatures, oxygen will accumulate under the surface. The resulting high chemical potential of the oxygen prevents more oxygen from being absorbed into the lattice. This effect is most pronounced in La,,.zSr0,sMn03. At the higher temperature, T = 200’ C, the amount of exchangeable oxygen is larger, but depends less on the strontium content of the oxides. This may be attributed to the higher mobility of lattice oxygen at 200’ C. The increased number of anionic vacancies at higher temperature will facilitate the movement of oxygen ions deep into the lattice.

In La(Mn,Cu)Ol,, the amount of lattice oxygen exchanged (based on the assumption of initial stoichiometric composition) at 100 and 200’ C increases with copper up to x = 0.4 before decreasing for higher substitution. This amount was higher in these oxides than in the (La,Sr ) MnO, series. For LaMn&&,,03, the most reducible oxide, a final composition after reduction ABO,,, was de-


termined. This is at the stability limit of the perovskite structure, which is given as AB02.5 [ 221.

The amount of lattice oxygen which could be removed with carbon monoxide at 350°C was also very high in LaCoO,. The final composition after reduction is LaCoO,,,. With strontium substitution, the amount of oxygen removed under the experimental conditions is even higher. It reaches 39% of the lattice oxygen for the composition L~,,Sq,,CoO,; in this case, reduction leads to the irreversible collapse of the perovskite structure. The cerium-substituted cobaltates could also be easily reduced, showing a loss of 26-50% of the lattice oxygen.

Fig. 5a shows the amount of carbon dioxide formed during the reduction of (La,Sr )MnO, as a function of time. The curves are normalized to the surface areas of the catalysts; the reoxidation temperature was 100’ C. This represen- tation has been chosen to emphasize the kinetic information gained from the measurements. The amount of oxygen removed from the lattice corresponds to more than two complete monolayers. The reduction is therefore obviously not confined to the surface, but extends considerably into the bulk of the compounds. To determine the oxygen mobility in the lattice, the amount of evolved carbon dioxide was plotted against 4. After correcting for the adsorbed oxygen, this resulted in curves with an initial linear part as shown in Fig. 5b. The slope of this linear part is taken as a measure of the lattice mobility (Fig. 5c ). From the figure, one sees that strontium substitution enhances the mobility of lattice oxygen up to x = 0.6.

The effect of copper substitution in the B-sites of La(Mn,Cu)03 is shown in Fig. 6a-c. The amount of carbon dioxide formed - and thus of lattice oxygen removed - has again been normalized to the surface area of the catalyst. Fol- lowing reoxidation at lOO”C, the amount of oxygen removed increases in the order: LaMnO, 4~ LaMn,,CuO.zOB < LaMn,,,,Cu,,,Os < LaMn,,&u,,,O~ < LaMn,,C~.,O,. The curves show a fast oxygen removal during the first 10 min; thereafter, very little additional oxygen was removed. The oxygen mobility at 350’ C is obviously far bigger in the Cu-substituted perovskites than in (La,Sr)MnO,. For LaMn,,,Cu,,O,, the reduction is so fast that the true slope cannot be resolved with our method.

OxygenTPD

All measurements are normalized to the surface area of the catalysts. LaMnO, shows two broad peaks (Fig. 7). A low-temperature a! peak appeared below 400°C while at higher temperatures, another desorption signal centered at 680°C was observed. The total amount of oxygen desorbed in the c~ peak cor-


(b) 60

0 20 40 60 60 100 0 1 2 3 4

Time (minutes)

@) ~~l---~

dtime

Strontium !Substitution x

Fig. 5. Kinetic reduction curves at 350°C for La1 _ .Sr,MnO, reoxidized at 100” C. (a) Carbon dioxide evolution as a function of time. (b) Data corrected for adsorbed oxygen versus square root of time (see text for details). (c) Mobility as function of strontium substitution 3~.

responds to 2.32.101’~ oxygen atoms/cm’ surface area (Table 5 ) . This value is comparable to the number of manganese atoms at the surface (ca. 3.3~10’~ atoms/cm’) [ 241, thus indicating a layer of oxygen adsorbed on top of the surface manganese ions. However, when strontium was substituted for part of lanthanum, the low-temperature peak disappeared. Only the high-temperature j? peak could be observed. The onset of this peak moved from 542°C in


(a) 120 La(Mn.Cu)0~

x=04

100

x=08

IYI

., ~.

80

%

$ 0

60. x=06

lb) . .

. . . x=04

.

-I

. . . . , x=0.8 / . . . . . / . . . x=06 . / . ‘. L .’

.’ .’

20 . , ,_,.C.'~O.2

.

0 <i'-~

/I

1 I

0 20 40 60 80 100 0 1 2 3 4

Time (minutes)

03 501-

40 1

f

h

-I

0.0 0.2 0.4 0.6 0.8 1.0

-\jtime

Copper Substitution x

Fig. 6. Kinetic reduction curves at 350°C for LaMn,_ Jk1,0, reoxidized at 100°C. (a) Carbon dioxide evolution as a. function of time. (b) Data corrected for adsorbed oxygen versus square root of time (see text for details). (c) Mobility as function of copper substitution x.

LaMnO, to 403 “C in La,,2Sr,,8Mn03 (Fig. 7) while the temperature of the peak maximum increases with strontium substitution. The integrated area of this peak is a function of the heating rate, thus suggesting that the P-oxygen originates from the lattice rather than from the surface.

Similarly with copper substitution in the B site, the cy peak was absent in LaMn,,&q,,O, and LaMn,.,C~,~O, (Fig. 8). Higher copper content, x 3 0.6, leads to the reappearance of this oxygen but with the peak maximum shifted

K.S. Chan et al. / Appl. Catal. A 107 (1994) 201-227

TABLE 5

Peak temperatures and amount of desorbed oxygen from TPD spectra

217

n a CY’ O/Sr P T!a atoms/cm* T, atoms/cm* T, atoms/cm2 (“C) (“Cl (“C)

La, _+SrJJnO, 0 241 2.32.10" - 0.2 - 0.4 - -

0.6 - 0.8 -

LaMn, _,Cu,03 0 241 2.32.10” 0.2 - 0.4 - 0.6 316 2.18.1O’5 1

La, _$r,CoO, 0 192 4.03.10’3 0.2 193 1.88.10’” 438 0.4 212 2.04.1015 440 0.6 226 1.02.10’5 467 0.8 233 1.83. 1015 498

La,_,Ce,CoO, 0 192 4.03*10’3 0.1 150 1.19*1013 0.2 178 2.35. 1Ol3 0.4 180 3.18. 1013 -

7.74.10’4 0.13 1.09.10’6 0.23 1.04*10’6 0.20 1.78.10’” 0.16

676 2.79*1015 714 2.77*1015 747 4.76~10’~ 799 7.63*1014 799 9.92*10’4

676 2.79.10’5 676 2.05.10’5 815 5.75.10’4 851 2.66.10=

793 3.07*10’4 807 4.58. 1Ol4 809 2.32. 1015 834 6.79. 1015 835 6.11*1015

793 3.07.10’4 815 1.45.10’5 823 1.40*10’5 835 5.79*1015

to higher temperatures, 315-366” C. The maximum for the /?-oxygen peak was likewise shifted, from 680 o C in LaMnO, to 851’ C in LaMn,,Cu,,,O,.

For (La,Sr)CoO,, the oxygen TPD shows a broad desorption plateau below 800°C and a sharp high temperature peak, /?, centered at N 820” C (Fig. 9). The broad plateau sieems to consist of two peaks, designated (Y and a’. The cy ’ - peak becomes noticeable for strontium content, x > 0.2. The intensity of all three peaks increases with strontium substitution. The peak maxima shift to higher temperatures with increasing X. With strontium substitution, more oxygen desorbs but the exchangeable oxygen is more strongly bound.

Since the (I! and CY’ peaks overlap, it is difficult to deconvolute them. How- ever, as a rough estimate of the amount of oxygen under each peak, a baseline was drawn from the minimum of the peaks. The area under each peak was then integrated. The amount of a oxygen is only 4.03*10’3 atoms/cm’ in LaCoO,

KS. Ghan et al. / Appl. Catal. A 107 (1994) 201-227

-3iiGM 0 600

Catalyst Temperature PC]

Fig. 7. TPD profile of oxygen from La,_,SrzMn03.

0 zoo 400 600 800 1000

Catalyst Temperature [“Cl

Fig. 8. TPD profile of oxygen from LaMn,_,Cu,O,.

but much higher in the strontium substituted compounds. As the strontium content increases’ from x = 0 to 0.8, the a-oxygen reaches a maximum for X= 0.4 (2.04. 1015 atoms/cm2) before decreasing for higher values of x. One monolayer is estimated to contain 9.6*1014 oxygen atoms/cm2. Hence, the area of the LY peak for La&Sr,,,,CoO, corresponds to about two layers of oxygen. For LaCoO, and La,&3r,,Co03, less than a monolayer of a-oxygen is desorbed while


200 400 600 800 1001


Fig. 9. TPD profile of ox:ygen from La,_,Sr,CoO,.

1 0 200 400 600 I300 lOO(


Fig. 10. TPD profile of oxygen from LaI_,Sr,CeOs.

La&3r,.,Co03 and L~~.$3r,,sCoO, desorb between 1 and 2 monolayers. In contrast, the amount of cr’-oxygen varies from 0.8 of a monolayer in L~.,Sr,.,CoO, to - 18 layers in La,,,Sr,,,,CoO 3. The latter value indicates that the reduction is not confined to the surface but is a bulk phenomenon.

In the cerium series, (La,Ce)CoO,, only the a! and /3 peaks are observed in


(a) 100 LaM%&wl.203

80

E ‘I t 60

z 6 40

8

‘_-., /

20 /

0 100 200 300

Catalyst Temperature (“C)

04 2 L~Mno.aC~o.zO~

1 EA q 26 t 1 kJ/mol

1000 K T

Fig. 11. (a) Hysteresis plot for carbon monoxide oxidation over LaMnO&u,,,O,. Total flow-rate (He:O,:CO=50:4:46)=100ml/ min. (b) Arrhenius plot of the apparent rate constant against reciprocal temperature. Catalyst: LaMn0.8Cu0,z03.

the TPD spectra. (Fig. 10). In La,&Je,,CoO,, the maximum for the a-oxygen is at the lowest temperature, 150°C. With increasing cerium substitution, the peak maximum shifts to higher temperatures though it is still lower than that observed for LaCoO,. The amount of a-oxygen desorbed is less than a monolayer in all cases, N l-3 - 1013 atoms/cm2. The high temperature p oxygen desorbs above 750’ C with a peak maximum at 815-835 ’ C. The amount of oxygen desorbed corresponds to l-5 layers.

Hysteresis plots

Fig. lla shows a plot of conversion versus catalyst temperature for a gas mixture of carbon monoxide : oxygen : helium of 46 : 4 : 50 (total flow = 100 ml/


min) over LaMn,,C~,,O,. At a catalyst temperature of 195”C, ignition of the catalyst suddenly occurred and the conversion rose from 20% to over 70%. Upon cooling, extinction was observed at a catalyst temperature of 170” C, about 25” C lower than the ignition temperature. Thus for the temperature range from 170” C to 195’ C, two different reaction rates can be observed. Both states are stable as a constant conversion was noted when the catalyst was kept at a fixed temperature in either the low or high activity range. The activation energy in both the high and low activity states were found to be rather similar, N 26 kJ/mol. (Fig. llb). In this series of oxides, LaMnO, and LaMn,,,CuO.zO, were the only other catalysts to show bistability under high carbon monoxide partial pressures. In the A-site substituted lanthanum manganates, (La,Sr)Mn03, it was found that La,,$3r0.,Mn0, and La&3r,.2Mn03 also showed the b&ability though the width of the hysteresis was narrower than in LaMnO,. Catalysts containing higher amounts of strontium (x> 0.2) did not exhibit any bistability.

DISCUSSION

An attractive feature of perovskites as oxidation catalysts is the ease with which oxygen is reversibly taken up into and removed from the structure. Since the transformations between the many related oxygen-defect structures within the perovskite lattice are largely topotactic, the catalyst particles are not broken down by the str’ess associated with repeated phase transitions. It has been established that oxidation reactions over perovskite-type metal oxides follow an intrafacial or Mars-van Krevelen mechanism where the reactant reacts with lattice oxygen and reduces the surface; the surface is reoxidized by uptake of oxygen from the ;gas phase or by mobile oxygen from the bulk. The oxygen mobility in the crystal lattice leads to an exchange between adsorbed oxygen, surface lattice oxygen, and bulk lattice oxygen. Here, surface adsorbed oxygen is a dissociated oxygen species which is bound to the surface and coordinated to only one or two metal ions. Surface lattice oxygen is embedded in the surface layer and multiply coordinated, while bulk lattice oxygen is the oxygen at the lattice sites below the surface. The different oxygen species are identified by the TPD spectra. The surface adsorbed species gives rise to the (Y peak. Whereas such a species is observed in all the cobaltates, it is only found in some of the manganates. Obviously, the presence of an a! peak is not related to the catalytic activity as measured. by carbon monoxide oxidation since some of the catalysts which displayed fairly high activity did not have any a peak. Both the cz’ and /3 peaks correspond to several monolayers of oxygen. The oxygen that desorbs in this temperature interval has therefore to originate from the bulk. a’-oxygen is only found in the substituted cobaltates. This species has been associated with oxygen frolm “void positions” [ 251, where void positions are defined


as sites which are empty in the (stable) reduced modification, but are occupied in the oxidized form. Such voids are introduced into the lattice by the partial substitution of Sr2+ for La 3+ Charge neutrality demands that for every two . Sr2+ ions, there must be one 02- vacancy. In the fully oxidized state, all vacancy sites may be occupied by excess oxygen; charge balance is maintained by oxidizing the B ions to a higher valency. The ratio of exchangeable oxygen to strontium should have values between 0 and 0.5. Table 5 shows that the experimental ratio is between 0.13-0.23, within the bounds of the model that associates cy ’ -oxygen with the oxide ion vacancies. It was thought that this fly’ - oxygen were readily available for oxidation reactions, so that the intensity of the desorption signal should be a measure for catalytic activity. Unfortunately, the activity within the cobaltate series does not correlate well with the mag- nitude of the ct? -desorption signal. The correlation with the a-oxygen is better. This implies that the reduction which gives rise to the (Y’ -desorption signal is already irreversible and therefore not directly related to the catalytic activity.

The catalytic iactivity for carbon monoxide oxidation using a stoichiometric mixture of carbon monoxide and oxygen (Fig. 3) was found to show a “volcano” plot for each series of A or B-site substituted perovskite catalysts. In the (La,Sr)MnO, series, La&&,,MnO, was the most active catalyst below 200°C. This catalyst was also found to be most easily reduced with carbon monoxide. Hence the catalytic activity at low temperatures appears to be influenced mainly by the mobility alf oxygen within the lattice framework. Above 200” C, the maximum activity was observed over La,,,Sr,,,Mn03 (x = 0.2 ) . At higher temperatures, the mobility of lattice oxygen is probably by a hopping mechanism and will therefore increase with strontium substitution due to the corresponding increase in the number of oxygen vacancies. If oxygen mobility were the pri- mary factor affecting activity, one expects the rate to reach a maximum at x= 0.8. However, the oxygen nonstoichiometry of the samples decreases with strontium content so that for x > 0.4, the compounds were deficient in oxygen. The opposite trend of these two factors could lead to the observed maximum in the catalytic activity at X= 0.2.

B-site substitution in La(Mn,Cu)03 leads to increased carbon monoxide oxidation activity. The observed maximum at low temperature for LaMq&q,,O, results from the interplay between increased oxygen mobility due to the copper substitution, reductive nonstoichiometry and a specific activity of copper itons for oxidation. Above 200” C, all copper-substituted oxides were more active than LaMnO,. This may be correlated with the greatly increased mobility of oxygen due to the copper substitution.

The higher activity of the cobaltates over the manganates may be attributed to the ease of oxygen loss. At room temperature, only LaCoOs had a very small oxygen excess while the strontium-substituted compounds were all oxygen deficient. Substitution of part of the La3+ sites with Sr2+ forces the promotion of co3+ to co4+ (as well as formation of oxygen vacancies in order to maintain


charge balance. As Co4+ is not stable, it tends to be easily reduced by release of oxygen. The ease at which the oxygen is lost is seen from the oxygen exchange experiments’ and the oxygen TPD. LaCoO, loses oxygen readily to the composition LaCoO,,, which is at the stability limit of the perovskite structure. This is much more than in LaMnO, where Mn4+ is a stable ion. Stron- tium substitution results in a high mobility of the lattice oxygen. The amount of oxygen that can be removed from the lattice by carbon monoxide titration at 350°C increases to 39% in L~,,Sr,,,CoO, so that the perovskite structure becomes unstable. From the amount of oxygen lost, the final composition con- sists of Laz03, SrO as well as some metallic cobalt. The ease of complete reduction to the cobalt metal was confirmed by temperature-programmed reduction in a mixture of hydrogen and helium.

Partial substitution of La3 + with Ce4+ was expected to lead to the formation of Co2+ ions from Co3+. However, the effect of Co2+ on the catalytic oxidation could not be studied1 over a wider concentration range because CeO, precipi- tated in addition to the perovskite phase for cerium substitution higher than 20%. Hence, the perovskite phase is expected to contain again A-site vacancies with promotion of Co3+ to Co4+ for charge balance. The reducibility of these catalysts is therefore similar to that of the strontium-substituted ones. L~.,Ce,,,CoO, had the highest carbon monoxide oxidation capability. For x 2 0.2, the activity was reduced though it was still higher than that of LaCoO,. The increased mobility of lattice oxygen may be responsible for the higher activity as compared with the unsubstituted oxide. The oxygen TPD for (La,Ce) COO, shows the presence of only two peaks, (x and /3. The temperature of the peak maximum is lowest in La0.&e,,lCo03 ( 150’ C ) and increases with cerium content. Hence, the position of the a-peak maximum parallels closely the catalytic activity.

The rate dependence on oxygen over all the oxides was ~1. For the (La,Sr)MnO, and L,a(Mn,Cu)O, series, the oxygen dependence was significantly larger at 125 ’ C than at the higher temperature of 200 o C (or 225 ’ C ) . At low temperatures,, adsorbed oxygen from the gas phase contributes more to the oxidation reaction than does lattice oxygen. For this case, the Langmuir- Hinshelwood model predicts a first order dependence on oxygen at low partial pressure, which goes towards zero order at high oxygen pressure. However, at high enough temperatures ( > 200 ’ C with these catalysts ) , the lattice oxygen becomes appreciably mobile and exchanges with the surface adsorbed oxygen. Hence the rate order becomes lower. For the cobalt-containing catalysts, the oxygen rate dependence was < 0.4 and independent of the temperature in the range studied. This indicates the participation of mobile lattice and/or surface lattice oxygen in the reaction.

The rate dependence in carbon monoxide was positive at all carbon monoxide concentrations. This is in contrast to the behavior of noble metal catalysts such as platinunn or ruthenium, where under high partial pressure of car-


bon monoxide, the activity is drastically decreased due to blocking of sites by adsorbed carbcm monoxide. Indeed, the oxidation of carbon monoxide to carbon dioxide over the perovskites was found to proceed even in the complete absence of oxygen in the feed stream, indicating clearly that the reaction is sustained by the lattice oxygen. At 350” C, the reduction of (La,Sr )MnO, pro- ceeds until nearly all the manganese is in the 3 + state. This amount of available oxygen is approximately equal to half an oxygen per strontium atom in (La,Sr)MnOs. However in (La,Sr)CoO,, the reduction occurs to metallic cobalt, leading to the destruction of the perovskite lattice. Reoxidation at 200°C does not restore the original structure.

The partial reduction of the surface can give rise to a kinetic bistability if the mobility of lattice oxygen is sufficiently low. Such a bistability has been observed over LaMnO, and Lal_,Sr,MnO, (0.1 <x< 0.2) when the gas mixture was very c,arbon monoxide-rich [ 241. The bistable regime opens up very suddenly at a carbon monoxide partial pressure of 200 mbar. In this regime, the catalyst has two states; one which is highly active and another with much lower conversion rates. The following mechanism has been proposed to explain the bistability: the B site cations, manganese or copper, are assumed to be the active centers. The surfaces of polycrystalline samples of most metal oxides are low index cleavage planes of the bulk structure. Fracturing takes place in such a way that the smallest number of cation-anion bonds is broken per unit area. For perovskites, the (100) and (111) surfaces are the dominant cleavage planes. Segregation effects are assumed to be minimal as the constituents are high melting oxides. In LaMnO,, the closest distance between two manganese ions is one lattice constant or 389 pm. Manganese ions at this spacing are found on (100) and (110) surfaces, whereas the distance on the (111) is 550 pm. With the active sites so far apart, the reaction steps of carbon monoxide adsorption and oxygen sorption and dissociation have to be spatially separated. Carbon dioxide can only be formed after the reaction partners diffuse towards one another. Calnsequently, the reaction rate is low over these surfaces.

Under reduci:ng conditions, a rearrangement of the manganese ions on the (111) surface becomes possible that leads to Mn-Mn pairs only 317 pm apart. This rearrangement is driven by the removal of rows of surface oxygen as out- lined in Fig. 12a. It is proposed that this reduced and reconstructed surface has a much higher catalytic activity because the spacing between the active centers makes them more suitable as a template for oxygen dissociation and carbon dioxide formation, A similar rearrangement is possible on the (100) surface. The manganese ions relax outward into four-fold coordinated positions. Half of the manganese ions are thought to move into (4 f 0) sites accompanied by the concerted removal of one third of the surface oxygen. Again, chains of closely spaced manganese will result (Fig. 12b). Whereas such close spacing of the B cations is not normally found in perovskites, it is a structural feature of many stacking variants of the ABO, structure. In the hexagonal CsNiCl,


00 (TJ La

00 M”

Fig. 12. (a) Schematic drawing of the (111) surface of LaMnOB. The filled and the open circles indicate the original pos:ition of the manganese ions. The rearrangement, indicated by arrows, leads to pairs with Mn-Mn interaction. The empty oxygen row may then be removed. (b) Sche- matic drawing of the (100) surface of LaMnO,. The surface manganese ions are initially 5-fold coordinated. It is proposed that the partially reduced ion becomes too big for the recessed position and moves higher into the surface layer where it assumes a d-fold coordinated position. As in the case of the (111) surface, a movement of some of the manganese ions leads to empty rows of oxygen which can be remtoved during reduction.

structure with a stacking sequence ababab, one-dimensional chains of the B cations form with a distance close to that found in the metal. Closely spaced pairs are also found in the structures intermediate between cubic perovskite and hexagonal packing like the hexagonal (6 layered) BaTiO, structure (AB- CACB stacking); the hexagonal (4 layered) BaMnO, structure (stacking se-


quence ABCB); and the hexagonal (9 layered) BaRuO, which contains linear Ru, groups. It is therefore likely that the perturbation at the surface is suffi- cient to allow for the formation of cation-cation bonds. Removal of part of the oxygen stabilizes the reconstruction. Movement of the cations back to the original position is then only possible after reoxidation of the surface. If this step is slow, a hysteresis in the reaction rate/temperature curve will appear.

The switching from the low to the high activity state is apparent in the Ar- rhenius plot of log (kz) vs. l/T (Fig. 1 lb ). Because of the strongly exothermic nature of the reaction, it is not possible to observe transitions from high+ low and low+ high and possible sustained oscillations under isothermal conditions. We assume that at temperatures > 200’ C and under reducing conditions, one-third to one-half of the surface oxygen is removed, followed or accompanied by the rearrangement of the manganese or copper ions in the surface. The reconstructed surface offers active sites of higher efficiency. Upon cooling, the surface is reoxidized and switches back to the low conversion state. Lattice oxygen rather than gas-phase oxygen is responsible for the re-oxidation. Therefore, a hysteresis can only be observed in oxides with low oxygen mobility. In our present study, only LaMn,,Cu,,,O, had a sufficiently low oxygen mobility, comparable to that of LaMnO,. Carbon monoxide oxidation over this catalyst indeed showed the expected hysteresis in agreement with our obser- vations on the (La,Sr)MnO, system.

CONCLUSIONS

Partial substitution of lanthanum manganates or cobaltates with strontium or copper lead to an increased activity for carbon monoxide oxidation. This is attributed to an increase in the oxygen mobility within the lattice of the perovskite. The oxides show a positive rate dependence on the carbon monoxide partial pressure so that their oxidative activity is not diminished in a reducing environment. In fact, some of the catalysts like La,_.Sr,MnO, and LaMn,_,Cu,O, (x< 0.2) can switch to a more active state under such conditions. Whereas the cobaltates are generally more active than the manganates, their reduction can easily go to the metallic state. This may lead to surface area loss and catalyst attrition under service conditions.

ACKNOWLEDGEMENT

Financial support by the National University of Singapore (RP880640) is gratefully acknowledged. We thank Mr S.T. Khor for help with some of the experiments. Th’e measurements on the cobaltates were generated by Mr F.H. Leom as an Honours Research Project.


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catalytic carbon monoxide oxidation over strontium, cerium and copper-substituted lanthanum...

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