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Properties of perovskite-type oxides I: Bulk and surface studies
of the Less-Common
Jourml
Metals,
146 (1989)
PROPERTIES OF PEROVSKITE-TYPE I: BULK AND SURFACE STUDIES LUIS
251 - 259
251
OXIDES
G. TEJUCA
Znstituto
de Catalisia y Petroleoquimica,
(Received
July
C.S.Z.C. Serrano 119, 28006 Madrid (Spain)
19,1988)
Summary The methods of preparation and some bulk and surface properties of perovskite oxides, mainly lanthanide perovskites, are reviewed. These properties include oxidative and reductive non-stoichiometry, formation of cation and anion vacancies, behaviour in a reducing atmosphere and gas adsorption with reference to the characterization of perovskites and the role of adsorbed species in their catalytic activity. Emphasis has been laid on the role of the A and B cations in the reactivity of these compounds.
1. Introduction Perovskite-type oxides constitute a group of isomorphic compounds with a cubic structure and unit formula ABOs. The larger cation A, situated at the centre of the cube, is twelve-coordinated with the oxide anions. The B cations occupy the corners of the cube and are in six-fold (octahedral) coordination with the anions. The oxygen atoms situated at the midpoints of the edges are each surrounded by two cations in position B and four cations in position A. The lower limits required for the ionic radii are rA > 0.090 nm and FB > 0.051 nm. However, the ionic radii must follow the equation rA + r0 = t x 2”2 (ra + re) where t is the tolerance factor as defined by Goldschmidt [ 1 J. The perovskite structure is stable for t values within the limits 0.75 < t < 1. Nevertheless the ideal structure occurs only in a very few cases. Orthorhombic and rhombohedral distortions are frequently found. Other less common distortions, i.e. tetragonal, monoclinic and triclinic, are also known to occur. Practically all the natural metallic elements of the periodic table are stable in a perovskite oxide structure. This, together with the possibility of synthesising multicomponent perovskites by partial substitution of cations in positions A and B, accounts for the ample diversity of properties which these compounds exhibit. The position A in the structure is most frequently filled by alkaline, alkaline earth or rare earth (Ln) ions although other cations may 0022-5088/89/$3.50
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have the proper size for occupation of these sites. Lanthanide cations may also be placed in B sites. Thus, ~terl~th~ide perovskites where lanthanide cations occupy both A and B positions have been described [ 21. However, a transition metal is the most usual B component in perovskite oxides. The 4f orbit& of the rare earth ions are so small in spatial extent that they have very little interaction with each other or with the d-electrons of the transition metal cations in position B. Therefore the influence of the rare earth ions is derived mainly from the effect of ionic size on the unit cell parameters. These changes in the crystal dimensions may be expected to produce variations in the A-O and B-O interactions and in the physical and chemical properties of the compounds. Thus, Shin-ike et al. [3] showed by means of infrared and ultraviolet analyses that the V-O bond length became shorter and the V-O strength increased with increasing atomic number of the l~th~de element in the series LnVOs (Ln =Nd through to Yb). Greedan [4] found that the LnTiOs series exhibits a rich variety of physical properties, e.g. metallic, semimetallic and semiconducting behaviour and virtually every type of magnetic behaviour. These properties depend strongly on the lanthanide element although they are also associated with the Ti(II1) sublattice. The superconducting character of the series of LnBa&usO, (Ln, cerium through to luteti~) perovskites seems also to depend on the rare earth ion in position A of the structure [ 5].- The compounds containing cerium and praseodymium do not superconduct whereas those containing neodymium or heavier elements, with the exception of terbium, show transition temperatures T, higher than 90 K. Under the conditions employed for prep~ation, cerium and terbium are oxidized to their 4+ states, and phases with these ions in the perovskite B site are obtained. Praseodymium readily forms non-superconducting tetragonal PrBa,CusO,. In general, T, decreases with decreasing size of the rare earth ion. This paper will be concerned mainly with LnMOs oxides, M being a transition element, although perovskites containing other less common metals will be also studied. The general methods of prep~ation of these compounds are briefly discussed. Some bulk and surface properties such as their non-stoichiometric character, their behaviour in a reducing atmosphere and their adsorption of simple and organic molecules are also reviewed. Emphasis has been laid on the role of the A and B cations in the reactivity of these compounds.
2. Preparation Several methods have been described for the preparation of perovskites. Briefly, they can be classified according to the criterion of Courty and Marcilly [6] for mixed oxides in solid-solid reactions (ceramic method) and liquid-solid reactions. Within this last category the physical methods used were dry evaporation, spray-drying, freeze-drying and explosion. However, chemical methods such as crystallization, coprecipitation and complexation
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were the most frequently employed. These compounds when deposited on adequate support materials are more resistant to sintering and are more efficient as total oxidation catalysts. Lanthanide oxides appear to play an important role in the preparation of supported perovskites. For example, LaO.&-,&oOs cannot be supported directly on A1203 since cobalt atoms are incorporated into the bulk of the support yielding a spinel. This reaction can be avoided by depositing the perovskite on cordierite ( 2A1,03. 5Si02. 2MgO) precoated with Laz03 [7]. Two phases of lanthanum oxide, hexagonal and monoclinic, appear in La,O,-cordierite, the percentage of monoclinic Laz03 being a maximum for a loading of 18 wt.% La*Os. The catalytic activity of supported La,.sSr,.,CoOs in propane oxidation also shows a maximum which is coincident with that found for monoclinic LazOs. This suggest that the perovskite is preferentially formed on the surface of this variety of lanthanum oxide. Fujii et al. [8] found an additional important support effect when depositing the above perovskite as a thin layer or as fine particles on metal oxides such as Laz03, CeO,, Nd203, SmzOs, Gdz03 and Zr02. The catalytic activity in propane oxidation was found to be higher for samples supported on ZrOz and CeOz. It appears that La,,8Sr0.ZCo03 is deposited in a highly dispersed state on the surface of these two oxides.
3. Adsorbed and lattice oxygen Nakamura et al. [9] found that there was an increase in desorbed oxygen as measured by temperature-programmed desorption (TPD) for increasing values of 3c in La,_,Sr,CoO, By assuming the surface oxide density to be 9.6 X lOi atom m- 2 the desorbed oxygen amounted to or. 0.7 surface layers for x = 0.2 and several layers for x = 0.6. These results cannot be accounted for in terms of adsorbed oxygen alone, and therefore it can be inferred that more than one oxygen species was desorbed. These species could not be distinguished in the TPD spectra since a continuous change from oxygen desorbed at low temperatures (adsorbed oxygen or weakly bonded lattice oxygen) to oxygen desorbed at higher temperatures (lattice oxygen) was observed. Nitadori and Misono [lo] also detected two types of oxygen in La,_,Sr,FeO, and Lai_.&exMOJ (M = Fe, Co). However, in Lar_,Ce*FeO, the desorbed oxygen decreases with increasing cerium substitution x as opposed to what is observed in the strontium-substituted compounds. Yamazoe et al. [ll] found two clearly separated peaks which increased with strontium content in the TPD spectra of oxygen desorbed from Lai_,Sr,CoOJ. By studying the evolution of the X-ray photoelectron spectroscopic (XPS) photolines of oxygen 1s as a function of x and the outgassing temperature of the perovskite, these authors assigned the two peaks observed to absorbed oxygen (TPD peak below 800 “C; XPS signal with a binding energy (BE) of 530.2 - 531.4 eV) and lattice oxygen (TPD peak above 800 “C; XPS signal at BE of 528.2 eV). The desorbed oxygen corresponding
to each of the TPD peaks amounted to more than one monolayer. The better resolution of these TPD spectra as compared with those of Nakamura et al. [9] and Nitadori and Misono [lo] may be due to the rather different temperature ramps used, i.e. 10 “C min-’ [ll] us. 20 “C mm-’ [9, lo]. Also, the desorption peaks of oxygen in these two sets of experiments appeared at substantially different temperatures as could be expected from the different adsorption temperatures used, i.e. 300 “C [9, lo] and 800 “C [ll]. Other authors [12, 131 have also detected two types of oxygen in perovskite oxides. The change in these, brought about by partial substitution of the lanthanide cation in position A, is accompanied by changes in the reducibility of the compounds, in the sum of the A-O and B-O bond energies, in their non-stoichiometric character and in their catalytic activity in reactions where oxygen participates as a reactant. Some of these effects will be considered below.
4. Non-stoichiometry Perovskite oxides offer a wide field of non-stoichiometric compounds made possible by the presence of vacancies in A and/or oxygen sites. The stability of the structure is derived from the Madelung energy of the stacking of B06 octahedra. This means that vacancies in position B are rare and when found in these compounds their concentration is very low [14]. Since the B06 octahedra form a stable network in the perovskite structure the 12coordinated A cations can be missing without collapse of the structure. The limiting case for the A-deficient structure is that of ReOs where the tunnels in the M402s units are empty whereas in the A,W03 bronzes they are partially and randomly occupied and in the ABOs stoichiometric perovskites they are fully occupied [15]. However, the most common nonstoichiometry is that produced by vacancies in oxygen sites. In what follows we will refer to oxidative (excess of oxygen) and reductive (deficiency of oxygen) non-stoichiometry. Perhaps the best characterized perovskite exhibiting oxidation nonstoichiometry is LaMnO, + h where the excess oxygen decreases with increasing firing temperatures [ 16, 171. The oxygen content may also be changed by varying the oxygen partial pressure in contact with the oxide in a reducing atmosphere at high temperatures. Thus, Kamata et al. [ 181 reported that a sample of composition LaMnOs.es at 1200 “C and an oxygen pressure of 10’ Pa loses oxygen and becomes a stoichiometric perovskite at 10-3*79 Pa oxygen. Nakamura et al. [ 191 found a similar evolution for LaMnO,.,,. The non-stoichiometry in this perovskite can also be controlled by partial substitution of the lanthanum. For example, with increasing values of 3t in LaI_.$aXMnOs +h the charge compensation is achieved by oxidation of Mn3+ to Mn4+ and to a smaller extent by a decrease in oxygen content and eventual appearance of oxygen vacancies [ 201. When x = 1, X reaches a value of -0.06. Mn4+ appears to be very stable in these substituted perovskites.
255
Thus, Jonker and van Santen [IS] reported that in La,%-,MnOs, the Mn4+ content ranged from 17% (x = 0) to 85% (z = 1). This explain the narrow range of reductive non-stoi~~omet~ found for this perovskite. This behaviour is in contrast with that observed for cobalt perovskites where the oxygen vacancies play a more important role as charge compensators because of the lower stability of Co4+ as compared with Mn4+ [ 211. Tofield and Scott [22] showed by powder neutron diffraction that LaMnO,+&, rather than containing inte~titi~ oxide ions, has a kind of non-stoichiometry which involves cation vacancies mainly on the A sites and to a lesser extent on the B sites. Likewise, in other perovskites such as lanthanum-substituted BaTiOs and lanthanum-substituted PbTiOs it has been suggested that the extra oxygen is accommodated over a wide range of oxidative non-stoichiometry by the introduction of defects in both A and B metal lattices. For this to occur the hirer-oxidation-sag ion responsible for the non-stoichiometry must be smaller than the host cation. A-site vacancies as charge-compensators were also postulated by Nitadori and coworkers [lo, 231 on the basis of X-ray diffraction data from Lal_,Ce,MO, (M = Mn, Fe, Co) with low cerium content. These perovskites may then be formulated as LaI_,Ce,&_~MOs (Ip, A-cation vacancy). Other perovskites such as LaCrOs +A [ 171, LnMnO s+h (Ln = Y, Nd, Sm, Dy, Er) [24, 251 and LaFe03 +h [26] were reported to exhibit oxidative non-stoichiometry. More details of compounds with excess oxygen have been given by other authors [27, 281. As stated above, even typical perovskites with excess oxygen such as manganites can exhibit reductive non-stoichiomet~ following appropriate substitution of the lanthanide cation by a lower-valence cation [ 16, 201. However, the most representative examples of oxygen-deficient perovskites are those which are easily reducible like LnCoO,. Patil et al. [29] found by means of thermogravimetric and differential thermal analyses that the series of perovskites LnI_xBawCoOJ fLn = La, Nd, Sm, Dy) lose oxygen readily when the tem~rat~e is increased. This oxygen loss increases with the barium content and with decreasing oxygen partial pressure in the surrounding atmosphere (0, < air < N2). The lanthanide cation appears also to have a noticeable influence since the oxygen loss in NdI_,Ba,CoOs is substantially larger than that observed in LaI_,Ba,CoOs. This effect of the A cation can be explained in terms of the greater oxygen-binding energy of the metaloxygen bond (A-0 and B-O energies) of LaCoOs (286.3 kJ mol-‘) as compared with that of NdCoO, (276.5 kK mol-‘) [30]. The composition of the samples can then be written as Lnl_,Ba,CoO,_h where the oxygen deficiency is a function of the variables mentioned above. These results are consistent with those of Misono and Nitadori 1311 and Jonker and van Santen 1213. The later authors found that the charge compensation in Lar_,Sr,CoOs_~ is accomplished by oxidation of Co3+ to Co4+ where x < 0.4. With a higher strontium content the percentage of Co4+ decreases (because of its instability) and anion vacancies appear giving rise to oxygendeficient perovskites.
256
An interesting example of reductive non-stoichiometry has been provided by Gibb et al. [32]. These authors found by means of MSssbauer spectroscopy that substitution of iron for Ru4* in SrRuOs takes place exclusively as Fe3+. When x < 0.3 the charge compensation is reached through formation of anion vacancies, and an oxygen-deficient perovskite SrRul-,Fe,03-~ appears. The structure is unable to tolerate more than cu. 4% oxygen deficiency and, as a result, when x > 0.3 partial oxidation of Ru4+ to RI.?+ occurs. Other perovskite systems such as SrV03_k [33] SrFe03_h [34] and LaNi03_h [26] have been reported as oxygen-deficient compounds. Amongst these, it is worth noting the tendency to reductive non-stoichiometry of SrFe03,_h as opposed to the behaviour of LaFe03 +A [26]. This effect is caused by the charge-compensating factor, resulting in an oxygen loss, introduced by Sr*+ in the lattice of SrFe03_k. A remarkable example of reductive non-stoichiomet~ is that represented by LnBa,Cu, 0, _A oxide superconductors [ 351. The AB03*h oxides exhibiting oxidative or reductive non-stoichiometry have the remarkable property of being susceptible to synthesis with a considerable concentration of A-site or oxygen vacancies without substantial modification of the structure, by controlled substitution of the lanthanide cation in position A, Given the importance of these defects in catalysis, when studying the catalytic activity of multicomponent perovskites, due consideration should be given to their non-stoichiometric character which is highly dependent on the substituting cation, on the temperature and on the atmosphere inside the reactor.
5. Reduction studies Reduction studies in hydrogen were effected mainly on LnMOs oxides (M, first-row transition metal) by means of temperature-programmed reduction (TPR). LaCr03 was found to be highly stable [36(a)]. At 1000 “C it underwent a reduction smaller than 0.1 e- per molecule (3e- per molecule would correspond to the reduction of M3+ to MO). LaMnO, [36(b)] showed a single reduction step of 1 e- per molecule at 800 “C (temperature at which the weight loss corresponding to the reduction indicated is reached). LaFeO, [36(c)] was reduced to 3 e- per molecule at 1000 “C. The total weight loss observed was higher than that expected for reduction of Fe3+ to Fe* indicating oxidative non-stoichiometry in this perovskite. LaCoOs [37] and LaNiOs [38(a)] both exhibited two reduction steps of 1 e- and 3 e- per molecule at relatively low temperature (7’ < 625 “C). A similar behaviour has been reported for LnCoOa perovskites [ 301. The reducibility (ease of reduction) increases, therefore, from Cr3+ to Ni3+ in the series of LaM03 oxides. This order of stability has also been observed by Nakamura et al. 119) by means of thermogravimetric measurements in a reducing atmosphere. The mixed oxides LaM03 (M = Fe, Co, Ni) were found to be more stable in a hydrogen atmosphere than the simple oxides Fez03, Co304 and NiO [26].
257
This shows the increased stability of these cations in a perovskitestructure. to different rczdu&km mechbms;, These can be ~st~n~s~ed by redw&ion under X’suGwmafcrtnditicms, Thus, FWoQS [3&(b)] and LaNi03 [38(a)] are both reduced from M3* to M’+ according ta the contracting sphere model and from M2+ to MU according to the nucleation model. The distinction between these reduction mechanisms is somewhat arbitrary because the contracting sphere model starts with a very fast nu~~eat~o~ and the nucleation rne~~~rn ends according to a ~o~t~a~t~ngsphere model, The cation is position A plays an important role in the reducibility of perovskites. Thus Arakawa et al. [39] showed by thermogravimetric analysis that the extent of reduction in hydrogen of LnCoO, oxides increased from LaCoOJ to EuCoOs, Le. with decreasing ionic radius of the ~~tb~de element. The same reduction sequence was found by Futai et ab. [30f by means of TPR experiments. However, the perovskites of gadolinium, terbium aJnddysprosium were less reducible, so that B maximum in reducibility was found for EuCoQ. These authors [30] reported a direct relationship between the reducibility and the sum of the energies of the Ln0 aad Co-0 bonds, he, the ease of reduction incz~ases with decreasing bond energy, In addition, the scanty in hydrogen of LaFeO, and I&3&& (with a higher ionic radius of the cation in position A) wzu found to be higher than that of the corresponding yttrium perovskites [40]. Moreover, Katsura et al. [C&I] found a linear relationship between the Gibbs free energy of formation AG of a series of LnFeO, (Ln from La through to Lu) perovskites from Ln2Us and Fe&l3 (in the ~mpera~ ~~te~~ 877 = 1024 “C) and the toferzincefactor t [I], i.e. AC becomes more negative with increasing values of Eor increasing ionic radius of the lanthanidr? cation. A similar relationship ~89 found for the Gibbs free energy of formation of LnFeOJ oxides from metallic iron, Ln,O, and oxygen at 1200 ~1297 “C 1423. Tlrese rem&s kdica&e that the stability of these oxides increases with ~&~~~~~ size of the cathode ion in the tem~rat~e interval studies, and are cons%tent with those of Arakawa et at, 139f for rsZCo0, perovskites. Partial substitution of the A ion by an ion of different oxidation state as in Lal_xSr, COO, may also cause significant changes in reducibility. The increasing concentration of Co4+ {unstable) and/or of oxygen vacancies (which favours the diffusiun of fat&e oxygen from bulk to surface) as charge ~orn~~tors~ with ~n~r~~g xi accounts for the increase in redurribility of this oxide with increasing strontium content [9]_ In contrast, the reducibility of La0,sTh0,,Co03 is lower than that of LaCoO, sirrce part of the cobalt is present as Co2* [43]. In the X-ray diffraction pattern of LaCoOJ reduced to 3 e” per molecule fat 500 “r=)Crespin and Hz& f37] only fowJd &es for I&J& ~d~~at~g that the metallic cobalt was bigbIy dispersed in a matrix of ~~~~~ sesquioxide, A high de#ee of dispersion of the metal in position B has also been found by Crespin at al. [44] &er reduction of LaNiOJ, and by Reller et al. [45] following reduction-oxidation cycles in CaRu03 at law temper-
Different TPR steps may eorrqxmd
258
atures. Similar reduction treatments under controlled conditions may provide a promising pathway for the preparation of highly active metallic catalysts using perovskite oxides as starting materials. 6. Adsorption studies The adsorption studies which have been carried out were aimed at perovskite characterization or the determination of the role of the adsorbed species in the catalytic activity. These include simple molecules such as HZ, CO, CO? and NO, and hydrocarbons. Adsorption of oxygen has been discussed above. Ichimura et al. [46] reported TPD spectra showing superposed peaks of Hz after adsorption of this molecule on several perovskites. The species desorbing above 70 “C were considered reponsible for the hydrogenation and hydrogenolysis of CZ hydrocarbons. CO and CO* adsorption [47(a) and (b)] has been studied in connection with the mechanism of CO oxidation [47(c)]. NO has been used in the characterization of the active centres involved in adsorption and in catalytic processes on simple oxides. These studies have been extended recently to perovskite oxides. Thus, Ulla et al. [48] used the poisoning effect of NO adsorption in ethylene hydrogenation at low temperatures for the estimation of metallic centres in reduced LaCo03. Ichimura et al. [46] found TPD peaks of CH4 after adsorption of C2H, and C&H4on LaCoOs at 27 “C. However, the spectra obtained after adsorbing C&H, and C&H4on LaAlOs and LaFe03 contained only peaks of the undissociated molecules. These results showed the strong contribution from the cobalt ions in the perovskite structure to the carbon-carbon bond scission. References 1 V. M. Goldschmidt, Skr. Nor. Vidensk.Akad. Oslo, I, 8 (1926). 2 U. Berndt, D. Maier and C. Keller, J. Solid State Chem., 13 (1975) 131. 3 T. Shin-ike, T. Sakai, T. Sakai, G. Adachi and J. Shiokawa, Mater. Res. Bull., 12 (1977) 685. 4 J. E. Greedan, J. LessCommon Met., 111 (1985) 335. 5 L. F. Schneemeyer, J. V. Waszczak, S. M. Zahorak, R. B. van Dover and T. Siegrist, Mater. Res. Bull., 22 (1987) 1467. 6 P. Courty and C. Marcilly, in B. Deimon, P. A. Jacobs and G. Poncelet (eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 119. 7 N. Mizuno, H. Fujii and M. Misono, Chem. Lett., (1986) 1333. 8 H. Fujii, N. Mizuno and M. Misono, Chem. Lett., (1987) 2147. 9 T. Nakamura, M. Misono and Y. Yoneda, Bull. Chem. Sot. Jpn., 55 (1982) 394. 10 T. Nitadori and M. Misono, J. Catal., 93 (1985) 459. 11 N. Yamazoe, Y. Teraoka and T. Seiyama, Chem. Left. (1981) 1767. 12 K. Ichimura, Y. Inoue and I. Yasumori, Bull. Chem. Sot. Jpn., 53 (1980) 3044. 13 J. L. G. Fierro and L. G. Tejuca, Appl. Surf. Sci., 27 (1987) 453. 14 R. J. H. Voorhoeve, in J. J. Burton and R. L. Garten (eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977, p. 129. 15 B. Raveau, Prvc. Indian Nat. Sci. Acad., 52A (1986) 67.
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16 G. H. Jonker and J. H. van Santen, Physica, 16 (1950) 337. 17 R. J. H. Voorhoeve, Ann. N. Y. Acad. Sci., 272 (1976) 3. 18 K. Kamata, T. Nakajima, T. Hayashi and T. Nakamura, Mater. Res. Bull., 13 (1978) 49. 19 T. Nakamura, G. Petzow and L. J. Gauckler, Mater. Res. Bull., 14 (1979) 649. 20 W. Chengxian, D. Bosheng, F. Shurong, Y. Zuolong, X. Xiaofan and W. Yue, Sci. Sin., 27B (1984) 778. 21 G. H. Jonker and J. H. van Santen, Physica, 19 (1953) 120. 22 B. C. Tofield and W. R. Scott, J. Solid State Chem., 10 (1974) 183. 23 T. Nitadori, S. Kurihara and M. Misono, J. Catal., 98 (1986) 221. 24 K. Kamata, T. Nakajima and T. Nakamura, Mater. Res. Bull., I4 (1979) 1007. 25 N. Kamegashira and Y. Miyazaki, Mater. Res. Bull., 19 (1984) 1201. 26 L. Wachowski, S. Zieiinski and A. Burewicz, Acta Chim. Acad. Sci. Hung., 106 (1981) 217. 27 N. Kamegashira and Y. Miyazaki, Mater. Chem. Phys., I1 (1984) 187. 28 D. M. Smyth, Annu. Rev. Mater. Sci., 15 (1985) 329. 29 S. B. Patil, A. Bandyopadhyay, D. K. Chakrabarty and H. V. Keer, Thermochim. Acta, 61 (1983) 269. 30 M. Futai, C. Yonghua and L. Hui, React. Kinet. Catal. Lett., 31 (1986) 47. 31 M. Misono and T. Nitadori, in M. Che and G. C. Bond (eds.), Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 409. 32 T. C. Gibb, R. Greatrex, N. N. Greenwood and K. G. Snowdon, J. Solid State Chem., 14 (1975) 193. 33 P. Dougier, J. C. C. Fan and J. B. Goodenough, J. Solid State Chem., 14 (1975) 247. 34 J. B. MacChesney, R. C. Sherwood and J. F. Potter, J. Chem. Phys., 43 (1965) 1907. 35 E. M. Engler, Chemtech, 17 (1987) 542. 36 (a) J. L. G. Fierro and L. G. Tejuca, J. Catal., 87 (1984) 126; (b) J. L. G. Fierro, J. M. D. Tascon and L. G. Tejuca, J. Catal., 89 (1984) 209; (c) J. M. D. Tasc&r, J. L. G. Fierro and L. G. Tejuca, J. Chem. Sot., Faraday Trans. 1, 81 (1985) 2399. 37 M. Crespin and W. K. Hail, J. Catal., 69 (1981) 359. 38 (a) J. L. G. Fierro, J. M. D. Tascbn and L. G. Tejuca, J. Catal., 93 (1985) 83; (b) J. L. G. Fierro, M. A. Pena and L. G. Tejuca, J. Mater. Sci., 23 (1988) 1018. 39 T. Arakawa, N. Ohara and J. Shiokawa, J. Mater. Sci., 21 (1986) 1824. 40 L. Carreiro, Y.-T. Qian, R. Kershaw, K. Dwight and A. Wold, Mater. Res. Bull., 20 (1985) 619. 41 T. Katsura, T. Sekine, K. Kitayama, T. Sugihara and N. Kimizuka, J. Solid State Chem., 23 (1978) 43. 42 T. Katsura, K. Kitayama, T. Sugihsra and N. Kimizuka, Bull. Chem. Sot. Jpn., 48 (1975) 1809. 43 J. A. Marcos, R. H. Buitrago and E. A. Lombardo, J. Catal., 105 (1987) 95. 44 M. Crespin, P. Levitz and L. Gatineau, J. Chem. Sot., Faraday Trans. 2, 79 (1983) 1181. 45 A. ReiIer, G. Davoodabady, A. Portmann and H. R. Oswald, Proc. 8th Eur. Congr. on Electron Microscopy, Budapest, 1984, p. 1165. 46 K. Ichimura, Y. Inoue, I. Kojima, E. Miyazaki and I. Yasumori, in T. Seiyama and K. Tanabe (eds.), New Horizons in Catalysis, Proc. 7th Znt. Congr. on Catalysis, B, Elsevier-Kodansha, Tokyo, 1981, p. 1281. 47 (a) J. M. D. Tascon and L. G. Tejuca, 2. Phys. Chem., Neue Folge, 121 (1980) 63; (b) J. M. D. Tasc6n and L. G. Tejuca, J. Chem. Sot., Faraday Trans. 1, 77 (1981) 591; (c) J. M. D. Tascon, J. L. G. Fierro and L. G. Tejuca, 2. Phys. Chem., Neue Folge, I24 (1981) 249. 48 M. A. Ulla, E. E. Mir6 and E. A. Lombardo, Proc. 8th Iberoamer. Symp. on Catalysis, La Rabida, Spain, 1982, p. 475.
Jourml
Metals,
146 (1989)
PROPERTIES OF PEROVSKITE-TYPE I: BULK AND SURFACE STUDIES LUIS
251 - 259
251
OXIDES
G. TEJUCA
Znstituto
de Catalisia y Petroleoquimica,
(Received
July
C.S.Z.C. Serrano 119, 28006 Madrid (Spain)
19,1988)
Summary The methods of preparation and some bulk and surface properties of perovskite oxides, mainly lanthanide perovskites, are reviewed. These properties include oxidative and reductive non-stoichiometry, formation of cation and anion vacancies, behaviour in a reducing atmosphere and gas adsorption with reference to the characterization of perovskites and the role of adsorbed species in their catalytic activity. Emphasis has been laid on the role of the A and B cations in the reactivity of these compounds.
1. Introduction Perovskite-type oxides constitute a group of isomorphic compounds with a cubic structure and unit formula ABOs. The larger cation A, situated at the centre of the cube, is twelve-coordinated with the oxide anions. The B cations occupy the corners of the cube and are in six-fold (octahedral) coordination with the anions. The oxygen atoms situated at the midpoints of the edges are each surrounded by two cations in position B and four cations in position A. The lower limits required for the ionic radii are rA > 0.090 nm and FB > 0.051 nm. However, the ionic radii must follow the equation rA + r0 = t x 2”2 (ra + re) where t is the tolerance factor as defined by Goldschmidt [ 1 J. The perovskite structure is stable for t values within the limits 0.75 < t < 1. Nevertheless the ideal structure occurs only in a very few cases. Orthorhombic and rhombohedral distortions are frequently found. Other less common distortions, i.e. tetragonal, monoclinic and triclinic, are also known to occur. Practically all the natural metallic elements of the periodic table are stable in a perovskite oxide structure. This, together with the possibility of synthesising multicomponent perovskites by partial substitution of cations in positions A and B, accounts for the ample diversity of properties which these compounds exhibit. The position A in the structure is most frequently filled by alkaline, alkaline earth or rare earth (Ln) ions although other cations may 0022-5088/89/$3.50
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in The Netherlands
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have the proper size for occupation of these sites. Lanthanide cations may also be placed in B sites. Thus, ~terl~th~ide perovskites where lanthanide cations occupy both A and B positions have been described [ 21. However, a transition metal is the most usual B component in perovskite oxides. The 4f orbit& of the rare earth ions are so small in spatial extent that they have very little interaction with each other or with the d-electrons of the transition metal cations in position B. Therefore the influence of the rare earth ions is derived mainly from the effect of ionic size on the unit cell parameters. These changes in the crystal dimensions may be expected to produce variations in the A-O and B-O interactions and in the physical and chemical properties of the compounds. Thus, Shin-ike et al. [3] showed by means of infrared and ultraviolet analyses that the V-O bond length became shorter and the V-O strength increased with increasing atomic number of the l~th~de element in the series LnVOs (Ln =Nd through to Yb). Greedan [4] found that the LnTiOs series exhibits a rich variety of physical properties, e.g. metallic, semimetallic and semiconducting behaviour and virtually every type of magnetic behaviour. These properties depend strongly on the lanthanide element although they are also associated with the Ti(II1) sublattice. The superconducting character of the series of LnBa&usO, (Ln, cerium through to luteti~) perovskites seems also to depend on the rare earth ion in position A of the structure [ 5].- The compounds containing cerium and praseodymium do not superconduct whereas those containing neodymium or heavier elements, with the exception of terbium, show transition temperatures T, higher than 90 K. Under the conditions employed for prep~ation, cerium and terbium are oxidized to their 4+ states, and phases with these ions in the perovskite B site are obtained. Praseodymium readily forms non-superconducting tetragonal PrBa,CusO,. In general, T, decreases with decreasing size of the rare earth ion. This paper will be concerned mainly with LnMOs oxides, M being a transition element, although perovskites containing other less common metals will be also studied. The general methods of prep~ation of these compounds are briefly discussed. Some bulk and surface properties such as their non-stoichiometric character, their behaviour in a reducing atmosphere and their adsorption of simple and organic molecules are also reviewed. Emphasis has been laid on the role of the A and B cations in the reactivity of these compounds.
2. Preparation Several methods have been described for the preparation of perovskites. Briefly, they can be classified according to the criterion of Courty and Marcilly [6] for mixed oxides in solid-solid reactions (ceramic method) and liquid-solid reactions. Within this last category the physical methods used were dry evaporation, spray-drying, freeze-drying and explosion. However, chemical methods such as crystallization, coprecipitation and complexation
253
were the most frequently employed. These compounds when deposited on adequate support materials are more resistant to sintering and are more efficient as total oxidation catalysts. Lanthanide oxides appear to play an important role in the preparation of supported perovskites. For example, LaO.&-,&oOs cannot be supported directly on A1203 since cobalt atoms are incorporated into the bulk of the support yielding a spinel. This reaction can be avoided by depositing the perovskite on cordierite ( 2A1,03. 5Si02. 2MgO) precoated with Laz03 [7]. Two phases of lanthanum oxide, hexagonal and monoclinic, appear in La,O,-cordierite, the percentage of monoclinic Laz03 being a maximum for a loading of 18 wt.% La*Os. The catalytic activity of supported La,.sSr,.,CoOs in propane oxidation also shows a maximum which is coincident with that found for monoclinic LazOs. This suggest that the perovskite is preferentially formed on the surface of this variety of lanthanum oxide. Fujii et al. [8] found an additional important support effect when depositing the above perovskite as a thin layer or as fine particles on metal oxides such as Laz03, CeO,, Nd203, SmzOs, Gdz03 and Zr02. The catalytic activity in propane oxidation was found to be higher for samples supported on ZrOz and CeOz. It appears that La,,8Sr0.ZCo03 is deposited in a highly dispersed state on the surface of these two oxides.
3. Adsorbed and lattice oxygen Nakamura et al. [9] found that there was an increase in desorbed oxygen as measured by temperature-programmed desorption (TPD) for increasing values of 3c in La,_,Sr,CoO, By assuming the surface oxide density to be 9.6 X lOi atom m- 2 the desorbed oxygen amounted to or. 0.7 surface layers for x = 0.2 and several layers for x = 0.6. These results cannot be accounted for in terms of adsorbed oxygen alone, and therefore it can be inferred that more than one oxygen species was desorbed. These species could not be distinguished in the TPD spectra since a continuous change from oxygen desorbed at low temperatures (adsorbed oxygen or weakly bonded lattice oxygen) to oxygen desorbed at higher temperatures (lattice oxygen) was observed. Nitadori and Misono [lo] also detected two types of oxygen in La,_,Sr,FeO, and Lai_.&exMOJ (M = Fe, Co). However, in Lar_,Ce*FeO, the desorbed oxygen decreases with increasing cerium substitution x as opposed to what is observed in the strontium-substituted compounds. Yamazoe et al. [ll] found two clearly separated peaks which increased with strontium content in the TPD spectra of oxygen desorbed from Lai_,Sr,CoOJ. By studying the evolution of the X-ray photoelectron spectroscopic (XPS) photolines of oxygen 1s as a function of x and the outgassing temperature of the perovskite, these authors assigned the two peaks observed to absorbed oxygen (TPD peak below 800 “C; XPS signal with a binding energy (BE) of 530.2 - 531.4 eV) and lattice oxygen (TPD peak above 800 “C; XPS signal at BE of 528.2 eV). The desorbed oxygen corresponding
to each of the TPD peaks amounted to more than one monolayer. The better resolution of these TPD spectra as compared with those of Nakamura et al. [9] and Nitadori and Misono [lo] may be due to the rather different temperature ramps used, i.e. 10 “C min-’ [ll] us. 20 “C mm-’ [9, lo]. Also, the desorption peaks of oxygen in these two sets of experiments appeared at substantially different temperatures as could be expected from the different adsorption temperatures used, i.e. 300 “C [9, lo] and 800 “C [ll]. Other authors [12, 131 have also detected two types of oxygen in perovskite oxides. The change in these, brought about by partial substitution of the lanthanide cation in position A, is accompanied by changes in the reducibility of the compounds, in the sum of the A-O and B-O bond energies, in their non-stoichiometric character and in their catalytic activity in reactions where oxygen participates as a reactant. Some of these effects will be considered below.
4. Non-stoichiometry Perovskite oxides offer a wide field of non-stoichiometric compounds made possible by the presence of vacancies in A and/or oxygen sites. The stability of the structure is derived from the Madelung energy of the stacking of B06 octahedra. This means that vacancies in position B are rare and when found in these compounds their concentration is very low [14]. Since the B06 octahedra form a stable network in the perovskite structure the 12coordinated A cations can be missing without collapse of the structure. The limiting case for the A-deficient structure is that of ReOs where the tunnels in the M402s units are empty whereas in the A,W03 bronzes they are partially and randomly occupied and in the ABOs stoichiometric perovskites they are fully occupied [15]. However, the most common nonstoichiometry is that produced by vacancies in oxygen sites. In what follows we will refer to oxidative (excess of oxygen) and reductive (deficiency of oxygen) non-stoichiometry. Perhaps the best characterized perovskite exhibiting oxidation nonstoichiometry is LaMnO, + h where the excess oxygen decreases with increasing firing temperatures [ 16, 171. The oxygen content may also be changed by varying the oxygen partial pressure in contact with the oxide in a reducing atmosphere at high temperatures. Thus, Kamata et al. [ 181 reported that a sample of composition LaMnOs.es at 1200 “C and an oxygen pressure of 10’ Pa loses oxygen and becomes a stoichiometric perovskite at 10-3*79 Pa oxygen. Nakamura et al. [ 191 found a similar evolution for LaMnO,.,,. The non-stoichiometry in this perovskite can also be controlled by partial substitution of the lanthanum. For example, with increasing values of 3t in LaI_.$aXMnOs +h the charge compensation is achieved by oxidation of Mn3+ to Mn4+ and to a smaller extent by a decrease in oxygen content and eventual appearance of oxygen vacancies [ 201. When x = 1, X reaches a value of -0.06. Mn4+ appears to be very stable in these substituted perovskites.
255
Thus, Jonker and van Santen [IS] reported that in La,%-,MnOs, the Mn4+ content ranged from 17% (x = 0) to 85% (z = 1). This explain the narrow range of reductive non-stoi~~omet~ found for this perovskite. This behaviour is in contrast with that observed for cobalt perovskites where the oxygen vacancies play a more important role as charge compensators because of the lower stability of Co4+ as compared with Mn4+ [ 211. Tofield and Scott [22] showed by powder neutron diffraction that LaMnO,+&, rather than containing inte~titi~ oxide ions, has a kind of non-stoichiometry which involves cation vacancies mainly on the A sites and to a lesser extent on the B sites. Likewise, in other perovskites such as lanthanum-substituted BaTiOs and lanthanum-substituted PbTiOs it has been suggested that the extra oxygen is accommodated over a wide range of oxidative non-stoichiometry by the introduction of defects in both A and B metal lattices. For this to occur the hirer-oxidation-sag ion responsible for the non-stoichiometry must be smaller than the host cation. A-site vacancies as charge-compensators were also postulated by Nitadori and coworkers [lo, 231 on the basis of X-ray diffraction data from Lal_,Ce,MO, (M = Mn, Fe, Co) with low cerium content. These perovskites may then be formulated as LaI_,Ce,&_~MOs (Ip, A-cation vacancy). Other perovskites such as LaCrOs +A [ 171, LnMnO s+h (Ln = Y, Nd, Sm, Dy, Er) [24, 251 and LaFe03 +h [26] were reported to exhibit oxidative non-stoichiometry. More details of compounds with excess oxygen have been given by other authors [27, 281. As stated above, even typical perovskites with excess oxygen such as manganites can exhibit reductive non-stoichiomet~ following appropriate substitution of the lanthanide cation by a lower-valence cation [ 16, 201. However, the most representative examples of oxygen-deficient perovskites are those which are easily reducible like LnCoO,. Patil et al. [29] found by means of thermogravimetric and differential thermal analyses that the series of perovskites LnI_xBawCoOJ fLn = La, Nd, Sm, Dy) lose oxygen readily when the tem~rat~e is increased. This oxygen loss increases with the barium content and with decreasing oxygen partial pressure in the surrounding atmosphere (0, < air < N2). The lanthanide cation appears also to have a noticeable influence since the oxygen loss in NdI_,Ba,CoOs is substantially larger than that observed in LaI_,Ba,CoOs. This effect of the A cation can be explained in terms of the greater oxygen-binding energy of the metaloxygen bond (A-0 and B-O energies) of LaCoOs (286.3 kJ mol-‘) as compared with that of NdCoO, (276.5 kK mol-‘) [30]. The composition of the samples can then be written as Lnl_,Ba,CoO,_h where the oxygen deficiency is a function of the variables mentioned above. These results are consistent with those of Misono and Nitadori 1311 and Jonker and van Santen 1213. The later authors found that the charge compensation in Lar_,Sr,CoOs_~ is accomplished by oxidation of Co3+ to Co4+ where x < 0.4. With a higher strontium content the percentage of Co4+ decreases (because of its instability) and anion vacancies appear giving rise to oxygendeficient perovskites.
256
An interesting example of reductive non-stoichiometry has been provided by Gibb et al. [32]. These authors found by means of MSssbauer spectroscopy that substitution of iron for Ru4* in SrRuOs takes place exclusively as Fe3+. When x < 0.3 the charge compensation is reached through formation of anion vacancies, and an oxygen-deficient perovskite SrRul-,Fe,03-~ appears. The structure is unable to tolerate more than cu. 4% oxygen deficiency and, as a result, when x > 0.3 partial oxidation of Ru4+ to RI.?+ occurs. Other perovskite systems such as SrV03_k [33] SrFe03_h [34] and LaNi03_h [26] have been reported as oxygen-deficient compounds. Amongst these, it is worth noting the tendency to reductive non-stoichiometry of SrFe03,_h as opposed to the behaviour of LaFe03 +A [26]. This effect is caused by the charge-compensating factor, resulting in an oxygen loss, introduced by Sr*+ in the lattice of SrFe03_k. A remarkable example of reductive non-stoichiomet~ is that represented by LnBa,Cu, 0, _A oxide superconductors [ 351. The AB03*h oxides exhibiting oxidative or reductive non-stoichiometry have the remarkable property of being susceptible to synthesis with a considerable concentration of A-site or oxygen vacancies without substantial modification of the structure, by controlled substitution of the lanthanide cation in position A, Given the importance of these defects in catalysis, when studying the catalytic activity of multicomponent perovskites, due consideration should be given to their non-stoichiometric character which is highly dependent on the substituting cation, on the temperature and on the atmosphere inside the reactor.
5. Reduction studies Reduction studies in hydrogen were effected mainly on LnMOs oxides (M, first-row transition metal) by means of temperature-programmed reduction (TPR). LaCr03 was found to be highly stable [36(a)]. At 1000 “C it underwent a reduction smaller than 0.1 e- per molecule (3e- per molecule would correspond to the reduction of M3+ to MO). LaMnO, [36(b)] showed a single reduction step of 1 e- per molecule at 800 “C (temperature at which the weight loss corresponding to the reduction indicated is reached). LaFeO, [36(c)] was reduced to 3 e- per molecule at 1000 “C. The total weight loss observed was higher than that expected for reduction of Fe3+ to Fe* indicating oxidative non-stoichiometry in this perovskite. LaCoOs [37] and LaNiOs [38(a)] both exhibited two reduction steps of 1 e- and 3 e- per molecule at relatively low temperature (7’ < 625 “C). A similar behaviour has been reported for LnCoOa perovskites [ 301. The reducibility (ease of reduction) increases, therefore, from Cr3+ to Ni3+ in the series of LaM03 oxides. This order of stability has also been observed by Nakamura et al. 119) by means of thermogravimetric measurements in a reducing atmosphere. The mixed oxides LaM03 (M = Fe, Co, Ni) were found to be more stable in a hydrogen atmosphere than the simple oxides Fez03, Co304 and NiO [26].
257
This shows the increased stability of these cations in a perovskitestructure. to different rczdu&km mechbms;, These can be ~st~n~s~ed by redw&ion under X’suGwmafcrtnditicms, Thus, FWoQS [3&(b)] and LaNi03 [38(a)] are both reduced from M3* to M’+ according ta the contracting sphere model and from M2+ to MU according to the nucleation model. The distinction between these reduction mechanisms is somewhat arbitrary because the contracting sphere model starts with a very fast nu~~eat~o~ and the nucleation rne~~~rn ends according to a ~o~t~a~t~ngsphere model, The cation is position A plays an important role in the reducibility of perovskites. Thus Arakawa et al. [39] showed by thermogravimetric analysis that the extent of reduction in hydrogen of LnCoO, oxides increased from LaCoOJ to EuCoOs, Le. with decreasing ionic radius of the ~~tb~de element. The same reduction sequence was found by Futai et ab. [30f by means of TPR experiments. However, the perovskites of gadolinium, terbium aJnddysprosium were less reducible, so that B maximum in reducibility was found for EuCoQ. These authors [30] reported a direct relationship between the reducibility and the sum of the energies of the Ln0 aad Co-0 bonds, he, the ease of reduction incz~ases with decreasing bond energy, In addition, the scanty in hydrogen of LaFeO, and I&3&& (with a higher ionic radius of the cation in position A) wzu found to be higher than that of the corresponding yttrium perovskites [40]. Moreover, Katsura et al. [C&I] found a linear relationship between the Gibbs free energy of formation AG of a series of LnFeO, (Ln from La through to Lu) perovskites from Ln2Us and Fe&l3 (in the ~mpera~ ~~te~~ 877 = 1024 “C) and the toferzincefactor t [I], i.e. AC becomes more negative with increasing values of Eor increasing ionic radius of the lanthanidr? cation. A similar relationship ~89 found for the Gibbs free energy of formation of LnFeOJ oxides from metallic iron, Ln,O, and oxygen at 1200 ~1297 “C 1423. Tlrese rem&s kdica&e that the stability of these oxides increases with ~&~~~~~ size of the cathode ion in the tem~rat~e interval studies, and are cons%tent with those of Arakawa et at, 139f for rsZCo0, perovskites. Partial substitution of the A ion by an ion of different oxidation state as in Lal_xSr, COO, may also cause significant changes in reducibility. The increasing concentration of Co4+ {unstable) and/or of oxygen vacancies (which favours the diffusiun of fat&e oxygen from bulk to surface) as charge ~orn~~tors~ with ~n~r~~g xi accounts for the increase in redurribility of this oxide with increasing strontium content [9]_ In contrast, the reducibility of La0,sTh0,,Co03 is lower than that of LaCoO, sirrce part of the cobalt is present as Co2* [43]. In the X-ray diffraction pattern of LaCoOJ reduced to 3 e” per molecule fat 500 “r=)Crespin and Hz& f37] only fowJd &es for I&J& ~d~~at~g that the metallic cobalt was bigbIy dispersed in a matrix of ~~~~~ sesquioxide, A high de#ee of dispersion of the metal in position B has also been found by Crespin at al. [44] &er reduction of LaNiOJ, and by Reller et al. [45] following reduction-oxidation cycles in CaRu03 at law temper-
Different TPR steps may eorrqxmd
258
atures. Similar reduction treatments under controlled conditions may provide a promising pathway for the preparation of highly active metallic catalysts using perovskite oxides as starting materials. 6. Adsorption studies The adsorption studies which have been carried out were aimed at perovskite characterization or the determination of the role of the adsorbed species in the catalytic activity. These include simple molecules such as HZ, CO, CO? and NO, and hydrocarbons. Adsorption of oxygen has been discussed above. Ichimura et al. [46] reported TPD spectra showing superposed peaks of Hz after adsorption of this molecule on several perovskites. The species desorbing above 70 “C were considered reponsible for the hydrogenation and hydrogenolysis of CZ hydrocarbons. CO and CO* adsorption [47(a) and (b)] has been studied in connection with the mechanism of CO oxidation [47(c)]. NO has been used in the characterization of the active centres involved in adsorption and in catalytic processes on simple oxides. These studies have been extended recently to perovskite oxides. Thus, Ulla et al. [48] used the poisoning effect of NO adsorption in ethylene hydrogenation at low temperatures for the estimation of metallic centres in reduced LaCo03. Ichimura et al. [46] found TPD peaks of CH4 after adsorption of C2H, and C&H4on LaCoOs at 27 “C. However, the spectra obtained after adsorbing C&H, and C&H4on LaAlOs and LaFe03 contained only peaks of the undissociated molecules. These results showed the strong contribution from the cobalt ions in the perovskite structure to the carbon-carbon bond scission. References 1 V. M. Goldschmidt, Skr. Nor. Vidensk.Akad. Oslo, I, 8 (1926). 2 U. Berndt, D. Maier and C. Keller, J. Solid State Chem., 13 (1975) 131. 3 T. Shin-ike, T. Sakai, T. Sakai, G. Adachi and J. Shiokawa, Mater. Res. Bull., 12 (1977) 685. 4 J. E. Greedan, J. LessCommon Met., 111 (1985) 335. 5 L. F. Schneemeyer, J. V. Waszczak, S. M. Zahorak, R. B. van Dover and T. Siegrist, Mater. Res. Bull., 22 (1987) 1467. 6 P. Courty and C. Marcilly, in B. Deimon, P. A. Jacobs and G. Poncelet (eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 119. 7 N. Mizuno, H. Fujii and M. Misono, Chem. Lett., (1986) 1333. 8 H. Fujii, N. Mizuno and M. Misono, Chem. Lett., (1987) 2147. 9 T. Nakamura, M. Misono and Y. Yoneda, Bull. Chem. Sot. Jpn., 55 (1982) 394. 10 T. Nitadori and M. Misono, J. Catal., 93 (1985) 459. 11 N. Yamazoe, Y. Teraoka and T. Seiyama, Chem. Left. (1981) 1767. 12 K. Ichimura, Y. Inoue and I. Yasumori, Bull. Chem. Sot. Jpn., 53 (1980) 3044. 13 J. L. G. Fierro and L. G. Tejuca, Appl. Surf. Sci., 27 (1987) 453. 14 R. J. H. Voorhoeve, in J. J. Burton and R. L. Garten (eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977, p. 129. 15 B. Raveau, Prvc. Indian Nat. Sci. Acad., 52A (1986) 67.
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