B EtMlONMENTAL ELSEVIER Applied Catalysis B: Environmental 13 (1997) 3.543

FYI-IR and EPR spectroscopic analysis of La1 _,Ce,CoO3 perovskite-like catalysts

for NO reduction by CO

Lucia Fornia7*, Cesare Oliva, Tommy Barzettia, Elena Selli, Alexej M. Ezeretsb, Anatoli V. Vishniakovb

a Dipartimento di Chimica Fisica ed Elettrochimica, Universifb di Milano, Via Golgi 11.201339, Milano, Italy h D. I. Mendeleev University of Chemical Technology of Russia, Miusskaja Sq. 9, Moscow, Russia

Received 6 May 1996; received in revised form 2 September 1996; accepted 9 September 1996

Abstract

A IT-IR and EPR spectroscopic investigation has been carried out on a series of Lal_,Ce,CoO? (x=0-0.15) perovskite-type catalysts, which are quite active for the reduction of NO by CO, and a mechanism has been proposed for this reaction. The first step involves the oxidation of CO by the catalyst, followed by the dissociative adsorption of NO onto the catalyst surface. Finally, adsorbed nitrogen (Nads) yields N20, N2 and NCO,d, along three parallel paths. Thus, oxygen exchange between NO and CO seems to occur indirectly and involves a surface oxygen vacancy. The catalytic activity is decreased by the substitution of Ce4+ for La3+ in the perovskite-type structure, which reduces the mobility of bulk oxygen. Activity is partially restored for s>O.O5 due to the presence of a segregated cerium oxide phase.

Keywords: Perovskites; NO reduction by CO; IT-IR; EPR analysis

1. Introduction

LaM03 oxide mixtures with perovskite-type struc- ture (M=element of the first transition series) have been thoroughly investigated in recent years [l-7]. The main interest concerned their peculiar solid-state characteristics and their potential use as catalysts, particularly in exhaust gas depollution processes [8- 151. In fact, they can be regarded as valuable catalysts alternative to supported noble metals, which are easy to synthesize [16] with lower cost and having a great stability upto high temperatures. Moreover, their com-

*Corresponding author. Fax: (+39-2) 70638129.

position can be varied and their structure characterized easily. So, they are particularly suitable for activity- structure correlation studies.

The catalytic activity of a series of oxides with composition Lal_XCeXCoOj (x=04).20) has been investigated in a previous work [ 171. Two environ- mentally relevant redox reactions have been consid- ered: the oxidation of CO with air and the reaction,

NO + CO -+ CO2 +1/2N2 (1)

The latter leads to NO abatement with some advan- tages with respect to the more popular selective catalytic reduction (SCR) by NH3, as it is not affected by possible slipping of NH3, since it employs

0926-860X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO926-3373(96)00089-6

36 L. Fomi et al./Applied Catalysis B: Environmental 13 (1997) 3543

0 0.05 0.1 0.15 0.2 X

Fig. 1. Specific activity (accuracy of data = &lo%), defined as percent CO (0) and NO (0) conversion per unit catalyst surface area, and selectivity towards N20 (0) in reaction (1) over Lal_,Ce,Co03 catalysts at 573 K as a function of Ce stoichio- metric coefficient, x.

CO as reductant, a usual component of exhaust gases [181.

N20 was shown to form in parallel with reaction (1). Moreover, the specific activity of the catalysts employed in NO and CO conversion, referred to unit time and unit surface area of Lai_,Ce,CoOs oxide, was shown to depend on x value [17], i.e. on the percent substitution of cerium for lanthanum. A typi- cal trend is reported in Fig. 1. While lanthanum cobaltate (x=0) proved a good catalyst, specific activ- ity decreased after introducing Ce ions in the oxide, with a minimum for x=0.05. However, activity was partly restored for higher degrees of Ce substitution, levelling out for x>O.l.

In the present work, we report an IT-IR and EPR spectroscopic investigation on Lat-JZexCoOs oxides, aimed at elucidating the influence of cerium on the catalytic activity of the oxides and at getting informa- tion on the mechanism of the heterogeneous reduction of NO by CO. In particular, we carried out a FT-IR analysis of CO and NO separately or simultaneously adsorbed on series of oxides, aswellasanEPRanalysis, bothbeforeandaftertheiruseascatalystsinreaction(1).

2. Experimental

2.1. Materials

The oxide series Lai_JJe&oOs (x=0, 0.05, 0.10, 0.15) was prepared by the sol-gel technique in the

presence of acrylic acid, as already described [ 171. A limited amount of cerium could be inserted in the perovskitic lattice. A careful XRD analysis showed that less than 5% of La ions could be substituted by Ce ions. The rest of cerium was present as CeOz in a separate fee phase, so that a mixture of Lai_JZexCoOs (x < 0.05) and CeOz was finally obtained [ 171.

The gases employed as probes in the FT-IR analysis were CO (purity>99.90 vol %, O2

L.. Fomi et al. /Applied Catalysis B: Environmental 13 (1997) 3543 31

obtained in 2.2 min thereby excluding any significant heating of the samples. The background spectrum, recorded in the absence of the sample and under identical operating conditions, was always automati- cally subtracted. When investigating the change in composition of probe gases in contact with the catalyst at a given temperature, IR spectra were recorded in only 11 s, as an average of 10 records with a 8 cm- definition.

Most of the results are presented as difference- spectra, i.e. as the difference between the absorption spectra recorded after gas adsorption and the refer- ence spectrum (vide supra). This procedure is very useful for observing weak absorptions due to species formed during heat treatment, although it can some- times lead to artifacts [20].

Analysis by EPR was done ex-situ by means of a Bruker ESP 300 instrument, as already described [ 171. Both samples simply activated in air or in nitrogen and samples which had been employed as catalysts for reaction (1) were examined in the 135-293 K tem- perature range.

3. Results and discussion

3.1. Analysis by FT-IR

3.1.1. Pretreatments The effect of sample pretreatment at 923 K is shown

in Fig. 2. The transmittance IR-spectrum of a x=0.10

\ 2400 2200 2000 1600 1600 1400 1200 1000 BOO 600

Fig. 2. Transmittance spectrum of x=0.10 sample evacuated (a) for one night at room temperature and (b) for 2 h at 923 K (reference spectrum).

1 An

1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 navenumbers h-11

Fig. 3. Absorbance difference-spectra (a. u.) recorded after (a) NO adsorption on the x=0.10 sample at room temperature and successive 20 min evacuation at (b) 373 K, (c) 573 K and (d) 673 K.

sample evacuated at room temperature down to a residual pressure of 2.67x 10e3 Pa is reported in Fig. 2(a), whereas Fig. 2(b) shows the transmittance spectrum of the same sample after a 2 h evacuation at 923 K (reference spectrum). Heat treatment causes complete disappearance of the two absorption bands at 1050 and 852cm-, which do not reappear after regeneration of the sample with air. These bands can be attributed to carbonates formed after atmo- spheric CO2 adsorption [21]. In particular, the band at ca. 850 cm- 1s characteristic of the out-of-plane bending of the carbonate ion and it is always observed in the presence of this species. The untreated oxide has a rather low transmittance (Fig. 2a) and only the absorption bands due to vC@- appear in the spectrum in the relatively more transparent 1200-800 cm- region.

3.1.2. NO adsorption NO adsorption was carried out on catalysts at room

temperature (Fig. 3) and at 523 K. Almost the same adsorption trend was observed for all the perovskite oxides examined. The spectrum recorded after eva- cuation at 573 K following saturation at room tem- perature (Fig. 3c) still showed the absorption patterns characteristic of NO (vide infra), whereas almost complete desorption occurred after evacuation at 673 K (Fig. 3d). An identical behaviour was observed when adsorption was carried out at 523 K. This indi- cates that the same species form after NO adsorption at room temperature and at 523 K.

38 L. Forni et al. /Applied Catalysis B: Environmental 13 (1997) 35-43

The bands at 1898 and 1832 cm-, appearing in the room temperature spectrum (Fig. 3a), are character- istic of physically adsorbed, and thus slightly per- turbed, NO. The presence of a double band results from NO adsorption through either oxygen or nitrogen atom. A small frequency difference with respect to the gas phase vibration (1876 cm-) is observed in both cases, as polar interactions are responsible for adsorp- tion.

The spectrum at room temperature (Fig. 3a) is rather complex, especially at frequencies below 1600 cm-, due to the absorption of labile mono- and bi-dentate nitrate species [22]. On the other hand, there is no NO adsorption onto metal ions, as the corresponding bands would appear at higher frequen- cies [23].

The difference-spectra obtained after evacuation at 373-673 K (Fig. 3b-d) show only two neat absorption peaks at 1471 and 1381 cm- and no absorption in the 1000-1200 cm- region. Species such as NO,,,, NO,, or ONO& should generate at least one band in this region, besides a couple of bands in the 1300- 1500 cm- region [22,24-261. Hence, the band at 1381 cm- can be attributed to NO adsorbed in an anionic vacancy, as observed by Shin et al. [27] in analogous conditions, while the band at 1471 cm- could be due to a similar species adsorbed onto a different anionic vacancy.

The IR analysis of the gas phase during NO adsorp- tion gives no evidence of NO2 formation. Thus, the catalyst does not behave as an oxidant towards NO. Taking into account that NO adsorption also leads to the formation of N20 [17], we suggest, in agreement with other authors [ 16,281, that the following reactions occur after NO adsorption:

Fig. 4. (A) Absorbance difference-spectra (a. u.) relative to CO adsorption on the x=0.10 sample: spectrum recorded (a) after adsorption at 623 K and (b) successive 20 min evacuation at 673 K, (c) after adsorption at room temperature and successive 20 min evacuation at (d) 373 K and (e) 573 K; (B) Gas phase transmittance spectra recorded (a) at the beginning of CO contact with the x=0.10 sample at room temperature and (b) at equilibrium; (c) at the beginning of CO contact with the x=0.10 sample at 573 K and (d) at equilibrium.

Co- -Co+NO+Co-0-Co+Nads (2)

Nads + NO + N20 (3)

Nads + Nads + N2 (4)

0 representing a surface oxygen vacancy.

3.1.3. CO adsorption

however detectable due to physically adsorbed CO. Adsorption patterns were still observable after eva- cuation at 573 K (Fig. 4Ae), but disappeared after evacuation at 673 K. However, when the sample was saturated with CO at 623 K (Fig. 4Aa), evacua- tion at 773 K was needed, to desorb the species responsible for absorption. Thus, in contrast with NO, the higher the adsorption temperature, the more stable were the species generated by adsorption of CO.

The IR analysis was also carried out at various When samples saturated at room temperature were temperatures in the case of CO adsorption on different evacuated at 373 K (Fig. 4Ad), or when saturation was oxide samples. A large number of bands appeared, as carried out at 623 K (Fig. 4Aa), well resolved absorp- expected, after CO adsorption at room temperature tion bands appeared at 1470, 1399 cm- and 1471, (Fig. 4Ac). No absorption around 2150 cm- was 1389 cm-, respectively, together with a band around

L. Fomi et &./Applied Catalysis B: Environmental 13 (1997) 3543 39

855 cm-. As the latter is typical of the bending in plane of perturbed carbonate ions, we conclude, in line with Tejuca et al. [29,30], that CO adsorption leads exclusively to the formation of carbonate species. In fact, the higher frequency double band can be attrib- uted to a splitting of the 1415 cm- band due to the asymmetric stretching of carbonate ions. Moreover, a monodentate species should form [31]. In fact, the frequency difference Av between the two splitted bands is lower than 100 cm- (Fig. 4A). Av=71 cm- for species formed after CO adsorption at room temperature (Fig. 4Ad,e) and A~=82 cm- after adsorption at 623 K (Fig. 4Aa,b). This confirms the higher stability of carbonates obtained after CO adsorption at 623 K, as Av should increase with increasing the interaction strength [31]. No absorption was ever observed at frequencies higher than 2000 cm- indicating that CO does not adsorb onto metal ions.

Moreover, the IR analysis of the gas phase during CO adsorption evidenced that no CO2 was produced at room temperature (no bands at 2349 and 667 cm- appear in Fig. 4Ba,b), while CO, which is responsible for the 2143 cm- band, underwent oxidation when adsorption was carried out at 573 K (Fig. 4Bc,d). It is worth noting that no oxidant species was present in the gas phase and consequently oxygen must have been provided by the catalyst.

The following mechanism can thus be proposed for the reactions occurring onto the catalyst after CO adsorption:

C0-0-C0+C0-+C0~&+C0- 0 -co

(5)

(6)

(7)

Reaction (7) should have a high activation energy, as carbonates are formed at room temperature onto the catalyst surface, but no COz is detected in the gas phase. This mechanism is in line with the fact that we never observed the presence of steadily adsorbed CO by IR analysis. TDP-MS investigations also showed [17] that only COa desorption occurs independent of the CO adsorption temperature.

[b)

2M)o 2400 2200 2000 ied0 1600 1400 1200 1000 800 navenuabers km-11

Fig. 5. (A) Transmittance spectra relative to the x=0.10 sample: (a) reference spectrum, spectrum recorded (b) after CO adsorption at 623 K and (c) successive regeneration with air (200-300 Torr) at 773 K. (B) Transmittance spectrum recorded after CO adsorption (a) onto x=0 sample at 473 K, (b) onto x=0 sample at 523 K, (c) onto x=0.05 sample at 573 K, and (d) onto x=0.10 sample at 573 K.

The system under investigation presented a com- pletely different behaviour when CO was adsorbed onto the oxides at higher temperature. Fig. 5Ab, reporting the transmittance spectrum recorded after CO adsorption onto the x=0.10 sample at 623 K, evidences a change of the whole spectral shape with respect to the reference spectrum (Fig. 5Aa). How- ever, a spectrum identical to the latter could be obtained after the regeneration of sample in air (200-300 Tort-) at 773 K (Fig. 5Ac). A similar beha- viour was observed in analogous conditions for CO/ ZnO system [32].

A possible explanation is that at higher tem- perature also bulk oxygen becomes available for oxidation, in view of its rather high mobility in the perovskite-type structure, which presents many

40 L. Fomi et &/Applied Catalysis B: Environmental 13 (1997) 3543

defects. In fact, it is well known that oxygen mobility increases with increasing concentration of cationic vacancies [33-351. Consequently, the substi- tution of Ce4+ for La3+ would reduce bulk oxygen availability.

In order to verify this hypothesis, CO adsorption onto the x=0.10 sample was compared with CO adsorption onto the cerium-free x=0 sample and onto the x=0.05 sample. The x=0.05 and x=0.10 samples contain an equal amount of Ce4+ within the perovs- kite-type structure. Fig. 5B shows that the shape of the spectrum relative to the Ce-free sample (x=0) changes at relatively low temperatures (523 K, Fig. 5Bb). By contrast, the spectrum of the Ce-doped samples keeps the same shape upto higher temperatures (573 K, Fig. SBc,d). This means that in the x=0 sample, bulk oxygen becomes available at lower temperature. This is in line with an earlier observation [36] that kinetic parameters of CO oxidation with air onto LaCo03 change considerably between 473 and 523 K and confirms that the presence of Ce4+ reduces the avail- ability of bulk oxygen.

3.1.4. CO and NO co-adsorption The results relative to CO and NO co-adsorption

onto the x=0.10 sample at 573 K are shown in Fig. 6(a-c). Species were formed absorbing at fre- quencies both lower and higher than 1600 cm-. These species disappeared at temperatures higher than 723 K. When co-adsorption was carried out at 673 K (Fig. 6d), some species were still adsorbed at 973 K (Fig. 6e). No absorption appeared at frequencies higher than 2200 cm-, confirming that no adsorbed N20 species was formed.

The neat band at 2178 cm- can be attributed to a isocyanate species Co@+)+ - NCO- generated from the reaction between CO and Nads (vide infra, Eq. 8). Other authors observed the same species onto per- ovskites [37], metal oxides [38], and supported noble metals [39]. However, the isocyanate species should not be a reaction intermediate, as it is stable even after evacuation at 623 K (Fig. 6b). It rather appears to be a surface by-product, which confirms the formation of Nads.

The weak absorption band around 2000 cm- (Fig. 6) is attributable to NO adsorbed onto Co, which acts as an electron acceptor. This band exhibits an increase in vibration frequency with respect to the

Fig. 6. Absorbance difference-spectra (a. u.) relative to CO and NO co-adsorption onto the x=0.10 sample: spectrum recorded (a) after co-adsorption at 573 K and successive 20 min evacuation at (b) 623 K and (c) 723 K; (d) after co-adsorption at 673 K and (e) successive 20 min evacuation at 973 K.

corresponding gas phase absorption band (1876 cm-) [23,37]. It may be recalled that this band was not observed after adsorption of pure NO (vide supra). However, we noticed that all our oxide mixtures are much more transparent after co-adsorption of CO and NO and consequently, very weak absorption bands may become observable.

The patterns appearing at frequencies lower than 1600 cm- consist of only a couple of bands peaking at 1471 and 1399 cm- (A~=70 cm-) and of a band at 850 cm-, due to adsorbed monodentate carbonate species, for co-adsorption at 573 K (Fig. 6a). When co-adsorption was performed at 673 K (Fig. 6d), another couple of bands appeared, which were still present at 1518 and 1345 cm- after desorption at 973 K (Fig. 6e). Since in this case A~=170 cm-, we can assign this feature to a bidentate and therefore more resistant carbonate species.

The change in composition of the gas phase was always monitored during the co-adsorption process. An example is reported in Fig. 7. CO*, and not N20, formed as soon as the CO and NO mixture was put in contact with the catalyst. The absorption due to NzO (2224 and 1286 cm-) successively appeared (Fig. 7b-c) and constantly increased with time. This confirms that N20 should not be an intermediate species, but rather a product of a reaction parallel to N2 formation.

L Fomi et al. /Applied Catalysis B: Environmental 13 (1997) 3.543 41

!.. ,,, ___ _,, ,., ,,,I,,_,, ,,,,, 00 2Boa 2600 2400 2200 2ooa 1m 1EAJa 1400 12w iooo ml0

Navemhim b-1)

Fig. 7. Transmittance spectra of the gas phase at (a) the beginning, (b) half-time of CO and NO co-adsorption and (c) at equilibrium.

All these results lead us to propose the following mechanism for reaction (1) catalysed by Lat_,Ce,CoOs oxides:

Co-O-Co+CO-+CO~~s+CO-~-CO (5)

Co- ??-Co+NO-+Co-0-Co+Nads (2)

Nads + NO --+ N20 (3)

Nads + Nads ---f N2 (4)

Nads + Co + NC&j, (8)

The interaction of N20 with an oxygen surface vacancy, in a way similar to that of NO in Eq. 2, cannot be excluded on the basis of our experimental results. Moreover, CO should react directly from the gas phase, probably according to a Rideal-Eley mechanism, as also suggested by Tascon et al. [37]. Finally, the fact that the catalytic efficiency of per- ovskite-type oxides decreases with time-on-stream [17] could be ascribed to the formation of stable carbonate species. Furthermore, when the rate of reaction (5) slows down, as also suggested by our EPR analysis (vide infra), it can become rate-deter- mining.

3.2. Analysis by EPR

Some preliminary EPR results obtained with Lai_,Ce,CoOs samples have been already reported by us [17]. There we had observed that all fresh samples gave no EPR signal. Furthermore, the x=0.05 sample remained EPR silent even after ther-

mal activation for 1 h at 723 K, unlike the x=0.10 sample. In fact, the latter showed an EPR line after activation either in N2 or in air. That line was Lor- entzian-shaped and characterized by a width AH nearly independent of temperature. Therefore, the product A = I. AH2 was proportional to the concen- tration of the paramagnetic species, Z being the peak-to peak intensity of the first derivative. The following results were obtained: A(293 K)/A( 135 K)=0.28 (acti- vation in Nz), and A(293 K)/A(135 K)=O.33 (activa- tion in air).

A different situation was observed for the samples employed as catalysts in reaction (1) at 523 K. In fact, in all these cases the Lorentzian-shaped EPR line broadened linearly with temperature (Fig. 8), while the concentration of the paramagnetic species again decreased with temperature, being A(293 K)/ A(135 K)=0.29 (for x=0.10) and =0.77 (for x=0.15).

In our opinion, Corv (t&e:) and Con (t6,sei) low-spin paramagnetic states could be responsible [40] for all the above mentioned EPR patterns. In fact, the Lor- entzian shape of these lines indicates that they are due to spin-spin exchange overcoming the dipolar spin- spin interactions occurring between the two nearest neighbour Co low-spin paramagnetic ions. With acti- vated samples, less intense EPR lines were observed at higher temperature, but with nearly unchanged AH value. This indicates that in these cases the concen- tration of the EPR-active species should decrease at higher temperature. Therefore, such species should be C-o. The oxygen atom, in fact, would cause a superexchange effect between the two metal atoms, producing the observed exchange-narrowed Lorent- zian-shaped EPR line. At higher temperature the concentration decrease of Co-O-Co species would be a consequence of the increased oxygen mobility.

However, the EPR results obtained with samples after catalytic use (Fig. 8) cannot be interpreted only on this basis. In fact, in these cases the Lorentzian- shaped EPR band broadens at higher temperature.

The spin-spin exchange (or super exchange) inter- actions are strongly dependent on the spin-spin dis- tance. Therefore, they are modulated by lattice vibrations. However, it has been shown [41] that such a modulation does not provide any spin-lattice relaxa- tion pathway when the spin-spin exchange is isotro- pic. On the contrary, when the latter is anisotropic, the spin-phonon interaction can shorten the spin-lattice

L. Fomi et al/Applied Catalysis B: Environmental 13 (1997) 3543

I I I I I I I I I I I

0 1000 2000 3000 4000 5000 [Gl

Fig. 8. EPR spectra of x=0.10 sample, after use as catalyst in reaction (1) at 523 K, recorded at (a) 150 K, (b) 180 K, (c) 210 K, (d) 240 K, (e) 293 K.

relaxation time, causing a linearly temperature-depen- dent broadening of the Lorentzian-shaped (exchange- narrowed) EPR line.

Anisotropies can arise in the spin-spin interactions when the spin and orbit angular momenta are closely coupled, so that a given component of the spin may be associated with a given orbital wave-function having a lobe extending in the direction of a neighbouring ion [41]. This has been observed with layer-like crystals, which closely approximate two-dimensional antifer- romagnets, in particular with systems formed of layers of S = $Cuz+ sites [42-48] and other compounds 1491.

The fact that at higher detection temperature an EPR line-broadening is observed with the used sam- ples in the presence of cerium ions would suggest that in the present case also a layer-like structure forms, with oxygen-richer planes alternating with planes in which Co-a-Co structures prevail. This layer-like structure, in which electrostatic interactions favour the insertion of Co-Cl-Co planes, would be stabilized by the presence of the segregated CeOZ phase. In fact, in

the interface region between the perovskite-like and cerium oxide phases, the latter would behave as a sink of oxygen, favouring the conversion of Coo- Co sites into Co-O-Co sites. The stability of the Co- O-Co surface plane would reduce the catalytic activity of the sample, but at the same time it would also decrease the crystal strains, thus leading to an increase of catalyst duration.

4. Conclusions

The present IT-IR and EPR spectroscopic investi- gation allows us to propose the following mechanism for the reduction of NO by CO over these perovskite- type catalysts. The first step involves the oxidation of CO by the catalyst, followed by the dissociative adsorption of NO onto the catalyst surface. Finally, adsorbed nitrogen (Nads) yields NzO, N2 and NCO,d, along three parallel paths. Thus, oxygen exchange between NO and CO seems to occur indirectly and involves a surface oxygen vacancy. The catalytic

L. Fomi et al. /Applied Catalysis B: Environmental 13 (1997) 3543 43

activity is decreased by the substitution of Ce4+ for La3+ in the perovskite-type structure, which reduces the mobility of bulk oxygen. Activity is partially restored for x>O.O5, due to the presence of a segre- gated cerium oxide phase.

References

[ll

PI

[31

[41 [51

[61

[71 [81

[91

[lOI [ill

[121

Cl31 (141

U51

cl61

[171

[181

1191

[201

[211

[221

T. Nakamura, M. Misono and Y. Yoneda, Bull. Chem. Sot. Jpn., 55 (1982) 394. T. Nakamura, M. Misono and Y. Yoneda, Chem. Lett., (1981) 1589. T. Nakamura, M. Misono and Y. Yoneda, J. Catal., 83 (1983) 151. T. Nitadori and M. Misono, J. Catal., 93 (1985) 459. T. Nitadori, T. Ichiki and M. Misono, Bull. Chem. Sot. Jpn., 61 (1988) 621. K. Tabata, I. Matsumoto and S. Kohiki, J. Mater. Sci., 22 (1987) 1882. J.G. McCarty and H. Wise, Catalysis Today, 8 (1990) 231. J.M.D. Tascon, J.L.G. Fierro and L.G. Tejuca, Z. Phys. Chem. N. F., 124 (1981) 249. S.C. Sorensen, J.A. Wronkiewicz, L.B. Sis and G.P. Wirtz, Ceram. Bull., 53 (1974) 446. K. Tabata and M. Misono, Catalysis Today, 8 (1990) 249. N. Mizuno, M. Tanaka and M. Misono, J. Chem. Sot., Faraday Trans., 88 (1992) 91. Y. Teraoka, H. Fukuda and S. Kagawa, Chem. Lett., (1990) 1. D. Brynn Hibbert, Catal Rev. Sci. Eng., 34 (1990) 391. H. Yasuda, Y. Fujiwara, N. Mizuno and M. Misono, J. Chem. Sot., Faraday Trans., 90 (1994) 1183. B. Viswanathan, in L.G. Tejuca and J.L.G. Fierro (Editors), Properties and Applications of Perovskite-type Oxides, Marcel Dekker, New York, 1993, p. 271. L.G. Tejuca, J.L.G. Fierro and J.M.D. Tascon, , Adv. Catal., 36 (1989) 237. L. Forni, C. Oliva, F.P. Vatti, M.A. Kandala, A.M. Ezerets and A.V. Vishniakov, Appl. Catal. B: Environmental, 7 (1996) 269. W.F. Chu and F.J. Rohr, Solid State Ionics, 28-30 (1988) 1540. T. Barzetti, E. Selli, D. Moscotti and L. Fomi, J. Chem. Sot., Faraday Trans., 92 (1996) 1401. J.B. Peri, in J.R. Anderson and M. Boudart (Editors), Catalysis, Science and Technology, Vol. 5, Springer, Berlin, 1984, p. 171. J.M.D. Tascon, L.G. Tejuca, J. Chem. Sot., Faraday Trans. I, 77 (1981) 591. G. Busca and V. Lorenzelli, J. Catal., 72 (1981) 303.

~231

~241

~251 WI

~271

WI

P91

[301

1311 ~321

[331 t341

I351

[361

[371

[381 [391 [401

[411

[421 [431 WI

[451

C4f-2

[471

I481

[491

A. Terenin and L. Roev, Spectrochim. Acta, 11 (1959) 946. C.H. Rochester and S.A. Topham, J. Chem. Sot., Faraday Trans. I, 75 (1979) 872. M.J.D. Low and R.T. Yang, J. Catal., 34 (1974) 479. L. Cerruti, E. Modone, E. Guglielminotti and E. Borello, J. Chem. Sot., Faraday Trans. I, 70 (1974) 729. S. Shin, H. Arakawa, Y. Hatakeyama, K. Ogawa and K. Shimomura, Mater. Res. Bull., 14 (1979) 633. P.J. Gellings and H.J.M. Bouwmeester, Catalysis Today, 12 (1992) 1. J.M.D. Tascbn, L.G. Tejuca, Z. Phys. Chem. N. F., 121 (1980) 63. L.G. Tejuca, C.H. Rochester, J.L.G. Fierro and J.M.D. Tascon, , J. Chem. Sot., Faraday Trans. I, 80 (1984) 1089. G. Busca and V. Lorenzelli, Mater. Chem., 7 (1982) 89. G. Hussain, M.M. Rahman and N. Sheppard, Can. J. Spectrosc., 33 (1988) 138. N. Yamazoe and Y. Teraoka, Catalysis Today, 8 (1990) 175. D.M. Smyth, in L.G. Tejuca and J.L.G. Fierro (Editors), Properties and Applications of Perovskite-type Oxides, Marcel Dekker, New York, 1993, p. 47.. T. Seiyama, in L.G. Tejuca and J.L.G. Fierro (Editors), Properties and Applications of Perovskite-type Oxides, Marcel Dekker, New York, 1993, p. 215. S. George and B. Viswanathan, React. Kinet. Catal. Lett., 22 (1983) 411. J.M.D. Tascon, L.G. Tejuca and C.H. Rochester, J. Catal., 95 (1985) 558. J.W. London and A.T. Bell, J. Catal., 31 (1973) 96. J. Rasko, F. Solymosi, J. Catal., 71 (1981) 219. 0. Parkash, P. Ganguly, G. Rama Rao and C.N.R. Rao, Mat. Res. Bull., 9 (1974) 1173. A. Abragam and B. Bleaney, Electron Paramagnetic Reso- nance of Transition Ions, Dover, New York, 1986, Chaps. 9 and 10. T. Moriya, Phys. Rev. Lett., 4 (1960) 228. T. Moriya, Phys. Rev., 120 (1960) 91. T.G. Castner Jr. and M.S. Seehra, Physical Rev. B., 4 (1971) 38. L. Forni, C. Oliva, F.P. Vatti, N.A. Sinitsina, S.V. Sorochkin, A.V. Moev and A.V. Vishniakov, J. Chem. Sot., Faraday Trans., 88 (1992) 1041. C. Oliva, in S. Pizzini (Editor), Materials Science Forum, Trans Tech Publications, Aedermasnsdorf, Switzerland, 1993, Vol. 116, p. 121. D.L. Leslie-Pelecky, F. VanWijland, C.N. Hoff and J.A. Cowen, J. Appl. Phys., 75 (1994) 6489. B. Rameev, E. Kukovitskii, V. Kataev and G. Teitelbaum, Physica C, 246 (1995) 309. G. Mozurkevich, J.H. Elliott, M. Hardiman and R. Orbach, Phys. Rev. B, 29 (1984) 278.