Properties of LaCo1−tCrtO3 III. Catalytic activity for CO oxidation

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Applied Catalysis A: General 147 (1996) 189-205

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Properties of LaCOl_tCrtO 3 III. Catalytic activity for CO oxidation Bente Gilbu Tilset " Helmer Fjellv~tg a Arne Kjekshus a,, ,~se Slagtern by Ivar Dahl b a Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway b SINTEF, P.O. Box 124 Blindern, N-0314 Oslo, Norway

Received 17 May 1996; accepted 7 June 1996

Abstract The catalytic activity for CO oxidation has been studied for samples of the solid-solution series LaCo~_ tCrtO3, and NdCoO 3 for comparison. The catalytic activity over LaCo I - tCrtO 3 generally decreases with increasing chromium content. For samples with t = 0.70 and 0.80, distinct activity changes occur during the reaction, for t = 0.80 representing a permanent deactivation. Samples of LaCo ~_ tCrtO3 were also studied by XRD, TPR, TG and XPS. Variations in catalytic activity were compared with variations in structural, physical and chemical properties. A rough correlation was found between the starting temperature for reduction during TPR and the temperature required for 5% CO conversion. Furthermore, the activity pattern at 600 K follows the atom percentage of surface oxygen species as seen by XPS. Possible surface reactions on LaCo I tCrtO 3 during the catalytic oxidation of CO are discussed. Keywords: Perovskite-type oxides; CO oxidation; Activation and deactivation; Surface reactions

I. Introduction Oxidation of carbon monoxide in the gas phase proceeds via chain reactions, involving some 40 steps [1]. Many species other than CO and 02 are involved, and the rate depends to a large degree on contaminants like H 2 0 and C H 4. For catalyzed oxidations, the reaction scheme is considered to be much simpler. Oxides with the perovskite-type structure, denoted ABO 3, have attracted considerable attention as catalysts over the past few years, see, e.g., recent * Corresponding author. Tel.: (+ 47) 22855560; fax: (+ 47) 22855565. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 2 10-4

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reviews [2-5] and references therein. Such oxides catalyze various processes, e.g.: total oxidation of CO and CH4, oxidative coupling of methane, dehydrogenation of propane and reduction of NO x. CO oxidation over perovskite-type oxides has been considered to be a suprafacial process, where the electronic band structure near the Fermi level is believed to be of importance [6]. Thus, attempts have been made to correlate structural [7], magnetic [8,9] and electronic [10,11] features with the catalytic activity. Recently, the effects of oxygen availability and reactivity have been emphasized. Aspects like the binding energy of surface oxygen [12,13], concentration of oxygen vacancies [9], oxygen mobility [14], oxygen desorption [14] and redox properties of the oxide [ 14-16] have been examined. The process was suggested to be interfacial when the catalyst shows high oxygen mobility and suprafacial when the mobility is low [17]. Regular ABO 3 perovskite-type oxides and A-site substituted variants have received much attention in catalysis studies, while less work has been done to clarify the effect of B-site substitution [17]. For the present study of the latter category, the solid-solution series LaCo~ _~CrtO 3 was chosen. Both catalytic, physical and chemical properties of the end members differ appreciably. Within the L a B O 3 (B = V, Cr, Mn, Fe, Co, Ni) series, L a C o O 3 is the one most active, and LaCrO 3 among the least active for CO oxidation [18,19]. Furthermore, the structure [20-25], stability [26,27], and electric [22,28] and magnetic [25,29-32] properties vary widely when traversing from LaCoO 3 to LaCrO 3. We have previously studied the reduction and reoxidation of LaCo~_/CrtO 3 [27] and to reduce the extent of the present report, only absolutely necessary results are recapitulated. LaCol_tCrtO 3 easily undergoes topotactic bulk reduction to LaCo~_ tCrtO 3_ t / 2 and reoxidation in air occurs at moderate ( < 550 K) temperatures [27]. Moreover, the amount of surface oxygen species varies with the chromium content [27]. Hence, this system was regarded as an interesting candidate for an examination of relations between bulk and surface properties and catalytic activity.

2. Experimental 2.1. Sample preparation and characterization In order to minimize differences originating from the syntheses, all samples were prepared according to the same procedure. The L a C o l _ t C r t O 3 samples were prepared by decomposing citrate gels at 1073 K (1173 K for t = 0.40 and 1.00 to enhance crystallinity), followed by slow cooling in air, details are reported in Ref. [24]. NdCoO 3 was prepared similarly by decomposing citrate gels at 1073 K, followed by rapid cooling in air [33].

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All samples were characterized by powder X-ray diffraction (XRD) both in the as-prepared state and after the catalytic testing. Guinier-Hiigg cameras were used, with CrKa~ radiation and Si as internal standard. In-situ XRD studies of LaCoO 3 were performed at 770 K under conditions relevant for the catalytic testing in the reactors. Intensity data were collected with a Siemens diffractometer (D500) using Cu K a radiation and a secondary monochromator. The experiment was performed within an ATM 1000 XRD cell. Flowmeters were used to adjust gas concentrations to 2.0% CO and 1.0% 0 2 in N 2. The in- and outlet gases of the reaction cell were collected and analyzed on a Hewlett Packard 5880 gas chromatograph, and it was thereby confirmed that CO oxidation occurred during this experiment. Surface areas were measured using a Quantachrome Monosorb single-point BET apparatus, with nitrogen adsorption. Temperature programmed reduction (TPR) was performed at 5 K min-~ with 10% H 2 in Ar, using an Altamira AMI-1 Catalyst Characterization System. Details of sample handling, experimental set-up and instrument calibration are given in Ref. [27]. A Perkin Elmer 7 Series Thermal Analysis System was used for thermogravimetric (TG) analysis, both to determine whether carbonaceous species were present after catalytic testing, and to study the reoxidation of the samples after TPR. Carbon analysis was further undertaken with a Leco-CHN-600 instrument. Infrared spectroscopy (IR) was used to check for surface carbonates (Nicolet 5SX instrument; 360 to 3870 cm-~). X-ray photoelectron spectroscopy (XPS) of as-prepared and used catalysts was performed using a Vacuum Generators (VG) Microlab III X P S / S A M instrument with Mg Ko~ radiation. Samples were in general stored in air at room temperature for up to several weeks prior to examination. Details regarding analysis of the spectra are given in Ref. [27]. After heating the used t = 0.20 catalyst in air at 773 K, the XPS spectrum was found to be nearly identical to that for the as-prepared sample. Samples with t = 0.20 were reexamined after almost 2 years storage and the results showed that they had not changed significantly. 2.2. Catalytic measurements A pyrex plug-flow reactor with 9.6 mm inner diameter was used (Fig. 1). The catalysts (0.87 g; mesh 170-230) were diluted with quartz sand (1.75 g; mesh 35-70; preheated at 1273 K) to avoid large temperature gradients a n d / o r hot spots during the reaction. The resulting bed volume was 1.55 ml. To minimize the gas diffusion volume, the remaining volume of the reactor was filled with quartz sand of mesh < 35. The catalysts were heated in 99.99% N 2 before admitting the reactants; 2.00% CO and 1.00% 0 2 in N 2. The experiments were performed at atmospheric pressure, with a flow rate of 300 ml min- ~, giving GHSV = 11 600.

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The temperature in the bed was monitored with a type K thermocouple at the position with maximum temperature during catalysis (Fig. 1). The CO and 0 2 concentrations in the outlet gas were monitored by a Maihak Finor IR detector and a Rosemount Oxynos 100 detector, respectively. The catalytic activity was measured between 380 and 790 K. The measurements were normally conducted after constant temperature and conversion had been obtained, otherwise after 15 min. During intermittent heating and cooling, the reactor was shut off; hence the catalyst was subjected to the remaining reactant and product gases. The catalytic activity was normally measured for two complete cycles.

3. Results 3.1. Catalytic measurements

The results from the catalytic testing of L a C O l _ t C r t O 3 a r e given in Fig. 2. From parallel measurements on different samples (from the same batch) of t = 0.00, 0.20 and 0.80 (marked with solid squares and circles in Fig. 2), the reproducibility was judged to be satisfactory. Furthermore, the catalytic activities of the present LaCoO 3 and LaCrO 3 samples are in general agreement with those found in previous reports [7,13,18,19]. During most measurements, the catalytic activity was not stable. This is indicated in Fig. 2 by lines either above (deactivation) or below (activation) the individual data points. The lengths of the lines indicate the degree of activity change during the measurements (15 min). The deactivation for t = 0.40, 0.60, 0.80 and 1.00 was studied in more detail at 773 K. The samples were first heated in N 2, then the reactant gases were admitted and the catalytic activity was monitored until constant activity was obtained. The activities at the start (S) and end (E) points of these runs are marked in Fig. 2. The final values coincide with the lowest activities found during normal consecutive runs. Thus, the lower activity generally observed in the second catalytic run may represent a steady state for these samples. The activity for CO oxidation generally decreases with decreasing Co content (increasing t). This trend is further visualized in Fig. 3, in terms of the temperature required to obtain 50% conversion. The spread of the results is within the shaded region in Fig. 3. The sample with t = 0.90 showed a surprisingly high and stable activity, similar to that of the first runs for t = 0.20 and 0.40. This high activity is not in accord with the general trend of decreasing activity with increasing Cr content. The sample was carefully checked for phase segregations or impurities by XRD and XPS. Because no satisfactory explanation for the special behaviour of this sample was found, t = 0.90 is omitted from Figs. 2 and 3.

B. G. Tilset et al. / Applied Catalysis A: General 147 (1996) 189-205

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NdCoO 3 is slightly less active than LaCoO 3 (Fig. 4), but the activity profiles are quite similar and both compounds give rise to 100% conversion at high temperatures. The effects on the catalytic activity of the atmosphere and temperature during catalyst pretreatment was studied for LaCo0.20Cr0.800 3_ ~ (Fig. 5). The as-prepared sample (Fig. 5A, see also Fig. 2) deactivates considerably during the first run (filled symbols). This probably represents a permanent deactivation, as indicated by repeated measurements (open circles and squares). The initial activity of a fresh LaCoo.20Cro.8oO 3_ a catalyst brought to 770 K in N 2 (filled diamond, at S) is appreciably higher than for catalysts brought to 770 K during consecutive heating steps in the C O / O 2 - C O 2 atmosphere. The fresh catalyst deactivates rapidly and the steady-state activity level is reached within ca. 25 min (filled diamond, at E). An attempt to regenerate the catalyst in air at 770 K was unsuccessful; actually a further slight deactivation occurred (Fig. 5A, open triangles). How-

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ever, regeneration in air at 1070 K was possible (Fig. 5B). The reaction profiles for the as-prepared and regenerated samples are similar, but the activity for the latter sample is somewhat higher.

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measurement periods, activation occurs at low and deactivation at high temperatures. The catalytic activity during the second run is lower than in the first, although not as low as for the second runs in Fig. 5A and B, and the tendencies towards activation and deactivation during measurements are diminished.

3.2. Catalyst characterization by TPR, TG and XRD The temperatures where reduction of L a C o t _ t f r t O 3 starts during TPR and the region where reoxidation occurs during subsequent TG experiments are given in Fig, 6. Reduction of LaCoo.2oCro.8oO3 occurs at about the same temperature in 10% H 2 as in 10% CO (balance: N2). Thus, the reducing effect of H 2 and CO are about equal and it seems relevant to note the rough correlation between the temperature for 5% CO conversion and the temperature

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for onset of sample reduction during TPR (Fig. 7). These temperatures are below those where deactivation sets in and are believed to reflect the initial activity of the fresh samples. In order to assess whether bulk reduction of the catalyst occurs during catalysis, XRD data for the as-prepared (oxidized) samples and the used catalysts were compared. The resulting unit-cell dimensions were identical, strongly suggesting that no bulk reduction had occurred. The presence of a bulk reduced state should have been detected, despite the facts that the Co-rich samples reoxidize easily at 298 K (see Fig. 6) and that the samples were subjected to air prior to XRD. In order to further evaluate the state of the sample during the catalytic process, in-situ XRD experiments (2.0% CO and 1.0% 0 2 in N 2) were carried out for LaCoO 3, which is the composition most susceptible to reduction and reoxidation. At 770 K, the diffraction pattern corresponds to that obtained in air at the same temperature. Another indication of an overall oxidized state during catalysis is obtained from catalytic testing of a reduced t = 0 . 8 0 sample. The reduction to LaCo0.20Cro.8oO2.90 is accompanied by a phase transition and an increase in unit-cell volume per formula unit [27]. After two runs, the sample had reoxidized to LaCoo.2oCr0.s003.o0.

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At least for samples with t ~< 0.80, high reoxidation rates are expected at temperatures where the catalysis takes place (Fig. 6). This also leads one to expect an oxidized bulk state during the process. In spite of the mentioned XRD indications, one may envisage a reduction in the surface region of the samples during catalysis. This would not be detected if, e.g., the total amount of the reduced phase is below the detection limit for XRD, the surface reoxidation is rapid a n d / o r the reduced region is poorly ordered.

3.3. Catalyst characterization by XPS The binding energies for the LaCOl_tCrtO3_ ~ samples are equal to those given in Ref. [27], within the expected experimental uncertainty. As discussed in Ref. [27], the Oi (528.9 eV) peak is attributed to a normal O ls (0 2-) signal, Oii (530.9 eV) and OIn (532.9 eV) probably correspond to adsorbed oxygencontaining species, Cr I (575.9 eV) represents trivalent and Cr H (579.2 eV) higher-valent ( > 3 + ) surface species, Co I (780.1 eV) is assigned to Co 3+, whereas Coit (782.5 eV) is yet unassigned. The binding energies, and therefore also the valencies, were unchanged after the catalytic testing. No Co 2+ or metallic Co was detected in any of the samples. The total amounts of the different elements are, within possible errors, unchanged by the catalytic testing. Surface segregation of Co or Cr was not observed. However, the amount of higher valent chromium (Crii) decreased from 4.2 to 2.7% (averages), and was compensated by increased amounts of Cr 3+ (CrI). The decrease is within the possible experimental error, but since the

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decrease is consistent throughout the series, it is believed to reflect a general tendency. Such reduction of high valent chromium was also observed after TPR of the samples t = 0.80 and t = 1.00, where the first reduction step was assumed to involve higher valent Cr [27]. However, the amounts of Cr n (XPS) do not correlate with the catalytic activity, and it is probable that this reduction also involves Co 3÷ at the surface. The distribution of the different oxygen species is also changed after catalysis, as seen from Fig. 8, where least squares fitted lines reveal tendencies throughout the LaCol_tCrtO 3 series. The relative amounts of bulk (O 0 and surface (Oxi + O m) oxygen for the as-prepared samples vary with the composition (t). Upon the catalytic testing, the amount of surface oxygen decreases. A

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rough correlation between the catalytic activity (at 600 K) and the concentration of surface oxygen species (O I + O . ) seems to exist (Fig. 9, see especially the inset). For t = 0.70, the data point for the first run falls outside of the shaded region in the inset to Fig. 9; correlated with the severe deactivation below 600 K for this particular sample.

3.4. Activation / deactivation Generally, a much less severe deactivation occurs during each 15 min measurement period in the second run than in the first. In many cases, the second run seems to be close to a deactivated, steady state. Several deactivation processes were considered, among them coking, formation of carbonates and

B.G. Tilset et al./Applied Catalysis A: General 147 (1996) 189-205

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sample sintering, which would lead to loss of active surface area during catalysis. No carbon-containing species were deposited on the catalysts during testing, as evidenced by TG, carbon analysis, IR and XPS. Thus, coking a n d / o r carbonate formation are not involved in catalyst activation or deactivation. Significant sintering could be ruled out in view of the low temperatures used during the catalytic measurements. This was tested for the sample with t -- 0.80, and the surface area was found to be the same before and after catalytic testing and reheating in air at 1073 K. The activation and deactivation may reflect relative rates of sample reduction and reoxidation. With increasing t, the rate of reoxidation decreases, as seen from the increasing onset temperature found by TG, and the temperature regimes for reduction and reoxidation are the same for ca. 0.7 ~ t ~< ca, 0.9 (overlap of the shaded fields in Fig. 6). In this compositional region, unexpected and irregular features are observed during catalysis, For t = 0.70, the balance between reduction and reoxidation rates seems to be especially sensitive to details of experimental conditions, as both activation and deactivation occurs in the same temperature region, in consecutive runs (Fig. 2). For t = 0.80, the distinct first-run deactivation (Figs. 2 and 5) is probably due to an insufficient regeneration of active surface oxygen species (Fig. 9).

4. Discussion Among the many perovskite-type oxides which have been studied, the heterovalently substituted phases are the most active for total oxidation reactions, see, e.g., Refs. [17,34,35]. The high activity is probably due to their ability to take part in redox reactions where variations in valency a n d / o r oxygen stoichiometry are involved. The present study focuses on the effect of homovalent B-site substitution. In the case of the purely homovalent substitution, variations in stoichiometry or valency are not involved. For LaCo~ _tCr,O 3, however, Co can be reduced and reoxidized, thus influencing the vacancy distribution and the amount of mobile oxygen. Principal questions to be addressed are inter alia possible correlations between the activity and crystallographic or electronic parameters for LaCo~_tCr,O3, the state and role of surface species and importance of vacancies and topotactic redox reactions.

4.1. Catalytic activity versus bulk structure and physical properties The observed variations in the catalytic activity of LaCo~_~Cr,O 3 were compared with variations in physical properties, like unit-cell dimensions and structural arrangement [22-25] (including degree of structural distortion [25]),

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magnetic moment(s), temperature induced spin transitions or spin state equilibria [25] and electric conductivity [22,28]. The general decrease in catalytic activity occurs in parallel with increasing unit-cell dimensions, structural distortion and temperature for magnetic ordering. It is difficult to imagine that these variations alone would have impact on the catalytic activity, as corresponding correlations between electronic conductivity, magnetic moment and catalytic activity at relevant temperatures were not found. Contrary to previous reports [7,8,13], the activity of NdCoO 3 was found to be lower than that of LaCoO 3, rendering doubt about the hypothesis [8] that enhanced activity of NdCoO 3 is due to equal concentrations of low- and high-spin Co 3+. In summary, the activity differences do not seem to depend significantly on the mentioned bulk properties. The trend with a smooth decrease in activity with decreasing Co content (Figs. 2 and 3) is found only when the activity is evaluated per catalyst mass unit without considering the surface area. This indicates that the number of active sites is not principally governed by the surface area and that processes in an extended surface region must be taken into account. Relevant bulk parameters are oxygen activity and mobility, which are manifested inter alia in the topotactic reduction and reoxidation [27].

4.2. Surface reactions The reaction scheme for the heterogeneous catalytic oxidation of CO must involve reduction and reoxidation of the catalyst. Exposure to CO alone at elevated temperatures ( > 620 K) results in bulk reduction of the oxide and CO 2 formation, whereas reoxidation occurs in air already at 300-550 K. Both reactions are topotactic, and therefore relatively rapid. Our results suggest that the bulk is preserved in the oxidized state when stoichiometric amounts of CO and 02 react catalytically. However, oxygen vacancies generated in the surface region of the catalyst may participate in the process. Fig. 10 shows a schematic picture of different adsorbed carbonate and carbonyl species on a perovskite-type surface, following the notation in Ref. [36]. Earlier IR investigations have shown the presence of carbonates on LaCoO 3 [9] and LaCrO 3 [37] after CO adsorption at catalysis temperatures. For both compounds, carbonyl species have only been detected on samples where partial (surface) reduction has occurred [37]. Thus, carbonyl probably does not contribute significantly to the reaction on fully oxidized LaCo I _,CrtO 3. The surface of as-prepared samples contains chemisorbed oxygen (O ~- in Fig. 10). Let L a ( C o , f r ) O 3_ 8 ° nO denote such a surface, where the composition parameter 3 - 6 is used to allow for small variations in oxygen content; 6 is likely very close to 0. The surface reduction by CO to form 'surface carbonate' may then be written: La(Co,Cr)O3_ a • nO + nCO ~ La(Co,Cr)O3_ ~ •nCO 2

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g 3+

O y+ VACANCY

Fig. 10. Schematic representation of possible species on (100) of perovskite-type structure after 0 2 and CO adsorption. Dotted symbols show atoms in the layers above or below.

A corresponding reaction with H 2 takes place in the initial stage of TPR. The correlation between the temperatures for the onset of catalytic activity and the start of reduction indicates that reaction (1) is the first step in the catalytic oxidation of CO. A further indication of such an initial reaction step is the correlation between the initial surface oxygen (O n + O m) concentration and the catalytic conversion at 600 K during the first run (Fig. 9). The thus produced carbon dioxide is then desorbed: La(Co,Cr)O3_ a • nCO 2 ~ L a ( C o , C r ) Q _ ~ + nCO2(g )

(2)

The process is quicker for LaCoO 3 than for LaCrO 3 and this has been suggested as a reason for the higher catalytic activity of LaCoO 3 [38]. However, the present IR and XPS experiments did not show increased concentrations of adsorbed CO 2 on the catalysts after testing, thus CO 2 desorption is not considered to be the limiting factor under our conditions. Direct formation of CO 2 via the combined reactions (1) and (2) is considered unlikely. To sustain activity over a longer period of time, oxygen must be readsorbed: La(Co,Cr)O3_ ~ + n/ZO2(g ) ~ L a ( C o , C r ) Q _ a. nO

(3)

For the reduced t = 0.80 (6 = 0.10) sample, reaction (3) is probably the first step in the proceeding catalysis. Catalytic activity (Fig. 5C) starts in the temperature region where reoxidation starts during TG. This sample is reoxidized from LaCoo.aoCro.soOz9o to LaCo0.zoCr0.8oO 3 during the catalytic testing. In this process, reactive O 2 a n d / o r O - [39,40] may be formed. Not all of this 'intermediate' oxygen is incorporated into the bulk as O 2-, some of it reacts

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with CO and thus enhances the rate of CO oxidation. The gradual activation during the 15 min reaction periods in the lower temperature region may be due to a gradual increase in the surface concentrations of O~- and O - after exposure to the reactant gas mixture. For the oxidized samples ( 6 -- 0), readsorption of oxygen probably occurs at coordinatively unsaturated Co 2+ sites, thus regenerating Co 3÷. Therefore, C02+/C03+ is assumed to be the redox couple responsible for the major part of the catalytic activity. Finally, we can imagine the catalytic process over as-prepared (oxidized) samples to occur as follows: during the first step (1), adsorbed oxygen, perhaps together with some bulk oxygen in a dynamic surface region, is removed. Hereby, surface oxygen vacancies and Co 2+ are formed. Oxygen readsorption probably occurs at the thus generated vacant sites, giving active O~- or O species. This process is assumed to be rapid, owing to the presence of oxidizable Co 2÷ close to an oxygen vacancy and to the topotactic nature of the redox reaction in the event that an extended surface region is involved. Some of the active oxygen may even be built in at or near the surface as 0 2- . This could explain the increase in 01 as seen by XPS after catalysis (Fig. 8).

Acknowledgements The authors express their gratitude to cand. scient. Sissel JCrgensen for the XPS measurements and helpful discussions, to cand. scient. Knut Woxholt for preparing the NdCoO 3 sample and to The Research Council of Norway (NFR) for financial support.

References [1] K. v. Baczko (Editor), Chemisches Verhalten von CO und CO 2, Gmelins Handbuch der anorganischen Chemie, Kohlenstoff, Teil C2, Verlag Chemie, GmbH, Weinheim/Bergstrasse, 1972, Chap. 1.7. [2] M. Misono and E.A. Lombardo (Editors), Catal. Today, 8 (2) (1990). [3] L.G. Tejuca, J.L.G. Fierro and J.M.D. Tasc6n, Adv. Catal., 36 (1989) 237. [4] L.G. Tejuca and J.L.G. Fierro (Editors), Catal. Rev.-Sci. Eng., 34 (4) (1992). [5] L.G. Tejuca, J. Less-Common Metals, 146 (1989) 261. [6] RJ.H. Voorhoeve, J.P. Remeika and L.E. Trimble, Ann. N.Y. Acad. Sci., 272 (1976) 3. [7] D.K. Chakrabarty and D.Y. Rao, React. Kinet. Catal. Lett., 33 (1987) 131. [8] O. Parkash, P. Ganguly, G.R. Rao, C.N.R. Rao, D.S. Rajoria and V.G. Bhide, Mater. Res. Bull., 9 (1974) 1173. [9] B. Viswanathan and S. George, Indian J. Technol., 22 (1984) 348. [10] I. Kojima, H. Adachi and I. Yasumori, Surf. Sci., 130 (1983) 50. [11] R. Doshi, C.B. Alcock, N. Gunasekaran and J.J. Carberry, J. Catal., 140 (1993) 557. [12] T. Shimizu, Chem. Lett., (1980) 1. [13] M. Futai, C. Yonghua and Louhui, React. Kinet. Catal. Lett., 31 (1986) 47. [14] K.S. Chan, J. Ma, S. Jaenicke, G.K. Chuah and J.Y. Lee, Appl. Catal. A, 107 (1994) 201.

B.G. Tibet et al. /Applied Catalysis A: General 147 (1996) 189 205

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

205

T. Nakamura, M. Misono and Y. Toneda, Chem. Lett., (1981) 1589. T. Nakamura, M. Misono and Y. Yoneda, J. Catal., 83 (1983) 151. N. Yamazoe and Y. Teraoka, Catal. Today, 8 (1990) 175. L. Wachowski, Z. Phys. Chem. (Leipzig), 269 (1988) 743. J.M.D. Tasc6n and L.G. Tejuca, React. Kinet. Catal. Lett., 15 (1980) 185. G. Thornton, B.C. Tofield and A.W. Hewat, J. Solid State Chem., 61 (1986) 301. C.P. Khattak and D.E. Cox, Mater. Res. Bull., 12 (1977) 463. S.P. Tolochko, I.F. Kononyuk, Yu.G. Zonov and L.S. Ivashkevich, lzv. Akad. Nauk SSSR, Neorg. Mater., 23 (1987) 829 (Eng. transl.: Inorg. Mater., 23 (1988) 743). S.P. Tolochko, I.F. Kononyuk, V.A. Lyutsko and Yu. G. Zonov, lzv. Akad. Nauk SSSR, Neorg. Mater., 23 (1987) 1520 (Eng. transl.: Inorg. Mater., 23 (1988) 1342). B. Gilbu, H. Fjellv~g and A. Kjekshus, Acta Chem. Scand., 48 (1994) 37. B. Gilbu Tilset, H. Fjellv~g, A. Kjekshus and B.C. Hauback, in prep. T. Nakamura, G. Petzow and L.J. Gauckler, Mater. Res. Bull., 14 (1979) 649. B. Gilbu Tilset, H. Fjellv'~g and A. Kjekshus, J. Solid State Chem., 119 (1995) 271. R. Koc and H.U. Anderson, J. Mater. Sci., 27 (1992) 5477. N. Menyuk, K. Dwight and P.M. Raccah, J. Phys. Chem. Solids, 28 (1967) 549. P.M. Raccah and J.B. Goodenough, Phys. Rev., 155 (1967) 932. M.A. Sefiarls-Rodrlguez and J.B. Goodenough, J. Solid State Chem., 116 (1995) 224. E.F. Bertaut, J. Mareschal, G. de Vries, R. Al6onard, R. Pauthenet, J.P. Rebouillat and V. Zarubicka, IEEE Trans. Magn. 2 (1966) 453. K. Woxholt, Thesis, University of Oslo, Norway, 1994. T. Seiyama, Catal. Rev.-Sci. Eng., 34 (1992) 281. N. Mizuno, Y. Fujiwara and M. Misono, J. Chem. Soc., Chem. Commun., (1989) 316. G. Busca and V. Lorenzelli, Mater. Chem., 7 (1982) 89. L.G. Tejuca, C.H. Rochester, J.L.G. Fierro and J.M.D. Tasc6n, J. Chem. Soc., Faraday Trans., 80 (1984) 1089. J.L.G. Fierro and L.G. Tejuca, J. Chem. Tech. Biotechnol., 34A (1984) 29. A. Bielanski and J. Haber, Oxygen in Catalysis (Chem. Ind. Set., Vol. 43)~ Marcel Dekker, New York, 1991, p. 46. A. Bielanski and J. Haber, Oxygen in Catalysis (Chem. Ind. Ser., Vol. 43), Marcel Dekker, New York. 1991, p. 134.