J. Phys. Chem. 1996, 100, 8443-8447
8443
New Uniform Solid Catalyst for the Low-Temperature Oxidation of Carbon Monoxide: A Triple-Layered Rare Earth Perovskite Containing Co and Cu Ions† Sivarajan Ramesh and Manjanath S. Hegde* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ReceiVed: December 13, 1995; In Final Form: February 14, 1996X
Oxygen reactivity and catalytic activity of the cobalt-containing layered defect perovskites, YBa2Cu2CoO7+δ and LaBa2Cu2CoO7+δ, in comparison with LaBa2Cu3O7-δ have been investigated employing temperatureprogrammed desorption (TPD) and temperature-programmed surface reactions (TPSR) in the stoichiometric and catalytic mode using carbon monoxide as a probe molecule. TPD studies showed evidence for the presence of two distinct labile oxygen species, one at (0 0 1/2) sites and the other at (0 1/2 0) sites in LaBa2Cu2CoO7+δ against a single labile species at (0 1/2 0) in the case of two other oxides. The activation energies for the catalytic oxidation of carbon monoxide by oxygen over LaBa2Cu3O7-δ, YBa2Cu2CoO7+δ, and LaBa2Cu2CoO7+δ have been estimated to be 24.2, 15.9, and 13.6 kcal/mol, respectively. The reactivity and catalytic activity of the oxide systems have been interpreted in terms of the structural changes brought about by substituents, guided by a directing effect of the larger rare earth cation. TPSR profiles, structural analysis, and infrared spectroscopic investigations suggest that the oxygen present at (0 0 1/2) sites in the case of LaBa2Cu2CoO7+δ is accessible to catalytic oxidation of CO through a Mars-Van Krevelen pathway. Catalytic conversion of CO to CO2 over LaBa2Cu2CoO7+δ occurs at 200 °C. The enhanced reactivity is explained in terms of changes brought about in the coordination polyhedra around transition metals, enhanced basal plane oxygen diffusivity, and redox potentials of the different transition metal cations.
1. Introduction Mixed metal oxides of the perovskite (ABO3), sheelite (ABO4), and related oxide types serve as ideal model materials for the study of oxygen reactivity and catalytic activity.1-3 Discovery of superconductivity at 90 K in YBa2Cu3O7-δ, a triple-layered, defect perovskite oxide, led to a renewed interest in perovskite-related oxide systems. The effect of oxygen stoichiometry and cationic substituents on the structural and physical properties of the YBa2Cu3O7-δ system has been extensively studied.4,5 However, this oxide claimed attention as a potential oxide catalyst followed the work of Hansen et al.6 toward the ammoxidation of toluene to benzonitrile. Advantages of using YBa2Cu3O7-δ-related materials as an oxide catalyst stem from the following facts: (a) about a mole of atomic oxygen per mole of the YBa2CU3O7-δ compound can be reversibly intercalated in the a-b basal planes; (b) a wide variety of substitutions for Y, Ba, and Cu can be made to modify the structure and activation energy for oxygen desorption; and (c) this type of solid falls in the class of uniform heterogeneous catalysts7 wherein the solid as a whole participates in a catalytic reaction. Catalytic oxidation of carbon monoxide to carbon dioxide at temperatures near ambient is one of the challenging problems in environmental chemistry. Carbon monoxide oxidation has been carried out over the YBa2Cu3O7-δ system by a number of groups in stoichiometric and catalytic modes.8-11 Among the various substitutions made for copper in the YBa2Cu3O7-δ system, trivalent cations such as Fe3+, Co3+, and Al3+ are known to occupy the Cu(1) (chain copper) sites, resulting in an increased oxygen content. The excess oxygen was found to occupy the (1/2 0 0) sites, rendering the structure tetragonal. The influence of a central rare earth ion on the crystal chemistry † Contribution no. 1172 from the Solid State and Structural Chemistry Unit. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03701-4 CCC: $12.00
of the RBa2Cu3O7-δ series of oxides was recently analyzed by a method of bond valence sums (BVS) for R spanning over the complete rare earth series.12 This study gave an important indication that larger rare earth ions such as La and Pr in the central cage show a larger bond valence (than the normal value of 3), indicative of a compressional strain in the central cage of the layered system. A recent neutron Rietveld refinement study13 of the LaBa2Cu2CoO7.35 oxide has shown that (a) the oxide crystallizes in an orthorhombic structure similar to YBa2Cu3O7-δ; (b) about 12% of cobalt ions occupy the Cu(2) sites; and (c) about 0.25 out of 7.35 oxygens occupy the (0 0 1/2) sites in the rare earth plane. A similar substitution of Fe ions in Cu(2) sites has been made in LaBa2Cu3-xFexO7+δ.14 However, the effect of the rare earth ionic radius on the site preferences on the substituent transition metal cations, and hence on the reactivity and catalytic activity of the oxides, has not been studied so far. In the present study we report the stoichiometric and catalytic oxidation of carbon monoxide over YBa2Cu2CoO7+δ, LaBa2Cu3O7-δ, and LaBa2Cu2CoO7+δ. It has been shown that a combined substitution of La for Y and Co for Cu results in the inducement of reactive oxygen at (0 0 1/2) sites in addition to the (0 1/2 0) basal plane oxygen. 2. Experimental Section The catalyst oxides LaBa2Cu3O7-δ, YBa2Cu2CoO7+δ, and LaBa2Cu2CoO7+δ were synthesized by a ceramic method. Stoichiometric amounts of Y2O3, La2O3, BaO2, CuO, and Co(C2O4)‚2H2O were ground and calcined at 940 °C for 48 h, with several intermediate grindings, and cooled to room temperature at a rate of 1 °C/min. The resulting products were analyzed for phase purity and structure by a powder X-ray diffraction method employing a JEOL-JDX8 powder diffractometer. The particle morphology and the size of the samples were studied employing a Cambridge scanning electron microscope (SEM). The oxygen contents of the samples were determined by iodometric titration. The thermal desorption of © 1996 American Chemical Society
8444 J. Phys. Chem., Vol. 100, No. 20, 1996
Figure 1. X-ray diffractograms of (a) LaBa2Cu3O7-δ catalyst; (b) spent solid in the stoichiometric reaction; and (c) spent solid in the catalytic reaction.
Ramesh and Hegde
Figure 2. X-ray diffractograms of (a) YBa2Cu2COO7+δ catalyst; (b) spent solid in the stoichiometric reaction; and (c) spent solid in the catalytic reaction.
oxygen and stoichiometric and catalytic oxidations were carried out employing home-built equipment.15 In a typical desorption experiment, 250 mg of the solid was placed in an 8 mm diameter, continuous flow, cylindrical quartz reactor. The reactor was evacuated to 10-5 Torr and heated up to 600 °C. The temperature of the reaction was measured by a fine chromel-alumel thermocouple kept immersed in the sample bed. The amount of oxygen emanating from the solid oxide was measured employing a VG-QXK300 quadrupole mass spectrometer. In the case of stoichiometric (CO over the catalyst) and catalytic (CO + O2) reactions, the flow rates of CO and O2 were each maintained at 15-20 µmol/s. The thermograms of all the reactants and the products were constructed from the intensity data acquired over 30-600 °C. The infrared spectra of the catalysts and the spent solids were recorded employing a Perkin-Elmer infrared spectrometer by a KBr disc method. 3. Results 3.1. Structural Changes. Figure 1 (curve a) shows the X-ray diffractogram of LaBa2Cu3O7-δ. The oxide crystallized in an orthorhombic structure. There is a small amount of BaCuO2 impurity phase marked by an asterisk. The X-ray diffractograms of the spent solids obtained in the stoichiometric and catalytic reactions are shown in curves b and c, respectively. The LaBa2Cu3O7-δ phase, under stoichiometric reaction conditions up to 600 °C, was stable but underwent an orthorhombic to tetragonal structural transition, as could be verified from the reversal in intensities of the (0 2 0) and (2 0 0) peaks (compare curves a and b). However, the oxide showed signs of deterioration. In the catalytic reaction, the oxide was stable (curve c). Figure 2 shows the structural transformation of YBa2Cu2CoO7+δ. The compound as prepared was tetragonal (see curve a). This oxide decomposed under stoichiometric reaction (curve b). The
Figure 3. X-ray diffractograms of (a) LaBa2Cu2CuO7+δ catalyst; (b) spent solid in the stoichiometric reaction; and (c) spent solid in the catalytic reaction.
structure of the spent solids in the catalytic reaction remained tetragonal (compare curves a and c of Figure 2). Figure 3 shows the structural transformations that occurred in LaBa2Cu2CoO7+δ. Stoichiometric CO oxidation over this oxide resulted in the deterioration of the parent phase (curve b). Orthorhombic structure of LaBa2Cu2CoO7+δ (curve a) was retained in the catalytic oxidation of CO (curve c). Cell parameters of the parent and spent solids as well the oxygen contents are given
Solid Catalyst for the Low-Temperature Oxidation of CO
J. Phys. Chem., Vol. 100, No. 20, 1996 8445
TABLE 1: Crystal Symmetry and Cell Parametersa of the Oxide Catalysts and Spent Solids system
structure
a
b
c
oxygen content
YBa2Cu2CoO7+δ LaBa2Cu3O7-δ LaBa2Cu2CoO7+δ YBa2Cu2CoO7+δ (CO/anaerobic) LaBa2Cu3O7-δ (CO/anaerobic) LaBa2Cu2CoO7+δ (CO/anaerobic) YBa2Cu2CoO7+δ (CO + O2, 1:1) LaBa2Cu3O7-δ (CO + O2, 1:1) LaBa2Cu2CoO7+δ
T O O
3.884 3.888 3.923
3.884 3.923 3.949
11.661 11.745 11.820
7.21 6.98 7.34
T
3.938
3.938
11.821
T
3.892
3.892
11.680
T
3.936
3.936
11.837
O
3.926
3.947
11.852
a
Figure 5. TPR profiles for (a) stoichiometric and (b) catalytic oxidation of carbon monoxide over the LaBa2Cu3O7-δ system.
Accurate within (0.004.
Figure 6. TPR profiles for the (a) stoichiometric and (b) catalytic oxidation of carbon monoxide over the YBa2Cu2CoO7+δ system. Figure 4. Temperature-programmed desorption profiles of oxygen from (a) LaBa2Cu3O6.98; (b) LaBa2Cu2CoO7.35; and (c) YBa2Cu3O6.9. Inset shows the TPD of oxygen from YBa2Cu2CoO7.17.
in Table 1. Particle sizes of the oxides estimated from scanning electron microscopy were in the range 2-8 µm. 3.2. Temperature-Programmed Desorption. Figure 4 shows the temperature-programmed desorption (TPD) profiles from LaBa2Cu3O7-δ and LaBa2Cu2CoO7+δ. TPD of oxygen from YBa2Cu3O7-δ is also given in the figure for comparison. The onset temperature of oxygen desorption and the peak temperature from LaBa2Cu3O7-δ and YBa2Cu3O7-δ are about the same, indicating that the desorbed oxygen emanates from the same (0 1/2 0) sites in both the oxides. This is to be expected, since both these oxides have the same structure. The TPD of oxygen from LaBa2Cu2CoO7+δ showed two distinct peaks, one at 390 °C and the other at 500 °C. Another interesting observation in the case of LaBa2Cu2CoO7+δ is that the oxide heated to 450 °C had an oxygen content of 7.02 and the structure remained orthorhombic, suggesting that the basal plane oxygens at (0 1/2 0) may be intact. The structure changed to tetragonal on further desorption of oxygen up to 600 °C. Therefore, the 500 °C desorption peak falls in line with the LaBa2Cu3O7-δ, YBa2Cu3O7-δ system which can be assigned to the oxygen at the O(1) site, and the desorption peak at 390 °C can then be assigned to the rare earth plane oxygen. The TPD profile of YBa2Cu2CoO7+δ given in the inset of Figure 4 shows a single desorption peak at 430 °C. In this compound, oxygen over 6 is known to be occupied in the (1/2 0 0) and (0 1/2 0) sites, but being equivalent sites in the tetragonal structure, only a single desorption peak is expected. Two distinct oxygen desorption peaks from LaBa2Cu2CoO7+δ oxide therefore confirm two labile oxygen sites. Activation energies of oxygen desorption were estimated from an Arrhenius plot (ln(I) vs 1/T, where I is the intensity of the O2 mass peak at a temperature T)
in each of these cases, and the values are 27, 23, and 19 kcal/ mol, respectively, for LaBa2Cu3O7-δ, YBa2Cu2CoO7+δ, and LaBa2Cu2CoO7+δ. 3.3. Stoichiometric and Catalytic CO Oxidation. Temperature-programmed reaction (TPR) profiles of stoichiometric and catalytic oxidation of carbon monoxide to carbon dioxide over LaBa2Cu3O7-δ are shown in Figure 5a,b, respectively. The stoichiometric reaction shows an appreciable conversion at 250 °C, and the reaction maximum was observed at 460 °C. The onset temperature for the catalytic oxidation over the same oxide was found to be about 250 °C, and the maximum reaction rate was achieved at 375 °C. Figure 6a,b gives the TPR profiles representing the stoichiometric and catalytic reactions over YBa2Cu2CoO7+δ. The stoichiometric oxidation over YBa2Cu2CoO7+δ shows a broad nondistinct feature (R1) over the temperature range 275-475 °C and a sharp feature (β1) showing a maximum at 600 °C. The onset temperature for the catalytic reaction in this case was around 250 °C, and the maximum conversion was achieved at about 375 °C, as in the case of LaBa2Cu3O7-δ. TPR profiles of stoichiometric and catalytic oxidation over LaBa2Cu2CoO7+δ are shown in Figure 7a,b. In this case, TPR of stoichiometric oxidation of CO shows two distinct CO2 peaks, one at 300 °C (R2) and the other at 450 °C (β2). The onset temperature of about 200 °C for the catalytic oxidation over LaBa2Cu2CoO7+δ was the lowest among the three oxide systems. Assuming first-order desorption without readsorption of the product CO2, the expression for the concentration, C, of CO2 at any given time at the product mixture for a tubular reactor can be written as
C ) C0 exp[(kτ) exp(-Ea/RT)]
(1)
where k is the rate constant and τ is the space time. C0 is a
8446 J. Phys. Chem., Vol. 100, No. 20, 1996
Ramesh and Hegde
Figure 7. TPR profiles for the (a) stoichiometric and (b) catalytic oxidation of carbon monoxide over the LaBa2Cu2CoO7+δ system.
Figure 9. Infrared spectra of (a) spent solid in the catalytic oxidation of CO over (a) LaBa2Cu2CoO7+δ; (b) lanthanum carbonate; and (c) barium carbonate.
oxygen and insignificant readsorption of CO2 in a catalytic reaction. 4. Discussion Figure 8. TPR profiles of catalytic oxidation of CO over LaBa2Cu3O7-δ, YBa2Cu2CoO7+δ, and LaBa2Cu2CoO7+δ fitted to the expression C ) C0 exp[(kτ) exp(-Ea/RT)].
nonzero initial value for the CO2 in a catalytic reaction. The data from the experimental CO2 product profiles of all three catalysts were fitted to expression 1 to estimate the activation energies of formation of CO2 under catalytic oxidation conditions. The product kτ was varied as a single constant during the regression cycles. The activation energies of CO2 formation over LaBa2Cu3O7-δ, YBa2Cu2CoO7+δ, and LaBa2Cu2CoO7+δ thus estimated were 24.2, 15.9, and 13.6 kcal/mol, respectively. Experimental and calculated reaction profiles for all three oxides are shown in Figure 8. 3.4. IR Spectral Studies. Carbon monoxide oxidation over metal oxides is known to form surface carbonates. The IR spectrum of the spent solid in the case of LaBa2Cu3O7-δ showed intense absorptions at 1415, 860, and 690 cm-1, characteristic of BaCO3. A similar observation has been made by Lin et al.11 in the case of stoichiometric oxidation of CO by YBa2Cu3O7-δ. The intensity of the carbonate peaks in YBa2Cu2CoO7+δ was very weak. On the other hand, the IR spectra of the spent solid in the stoichiometric reaction of CO over LaBa2Cu2CoO7+δ showed absorption peaks at 1415, 1380, 1015, 855, and 815 cm-1, suggesting that the surface carbonates formed on this oxide may be different from those in the case of LaBa2Cu3O7-δ and YBa2Cu2CoO7-δ. Figure 9 shows the IR spectra of both standard BaCO3 and lanthanum carbonate recorded under identical conditions and the IR spectra of the spent solid. A comparison of the IR spectra of the spent solid with those of standard lanthanum and barium carbonates recorded under identical conditions and standard assignments for lanthanum carbonate16 suggests that the spent solid in the case of LaBa2Cu2CoO7+δ contains a mixture of lanthanum and barium carbonates. IR spectra of the spent solids in the case of catalytic CO oxidation over all three oxides did not show any absorption due to carbonates. This suggests nonconsumption of the lattice
The orthorhombic YBa2Cu3O7-δ structure supports two types of copper atoms, Cu(1) at (0 0 0) sites adopting a square planar geometry and Cu(2) at (0 0 z) in a square pyramidal coordination. The YBa2Cu3O6 member adopts a tetragonal structure, the Cu(1) sites are 2-fold coordinated, and the oxygen vacancies are ordered. La Graff et al.17 proposed that the diffusion of oxygen along the Cu(1)-O(1)-Cu(1) chain was favorable if the oxygen started at the end of the chain and the adjacent chain site was vacant. Pollert et al.,18 while investigating the substitutional effects of cobalt in YBa2Cu3O7-δ at Cu(1) sites, observed the inducement of additional oxygen at O(5) positions and the displacement of cobalt and oxygen ions from their regular sites. YBa2Cu3O7-δ, a triple-layered perovskite, has been viewed19 as an intergrowth of fixed oxygen composition layers and layers of variable composition [M(1)Ox]+ separated by [Y3+] and [BaO]0 layers with a sequence of [M(1)Ox]+[BaO]0[M(2)O2]2-[Y3+][M(2)O2]2[BaO]0[M(1)Ox]+ The cation anion electron transfer energy within these layers depends on the geometry as well as the activation energy Ea for transfer of an electron in a reaction of the type
Mn+ + O2- h M(n-1)+ + OEa in turn, depends on the electronegativity difference, X, between the metal ions placed at Cu(1)/Cu(2) sites and oxygen. On the basis of these arguments Pollert et al.18 concluded that the presence of cobalt increased the ionicity of the M(1)-O(1) bonds. This suggests that in the case of YBa2Cu2CoO7+δ reactive holes may be located both on Cu and Co and coupled in the form of
Cu3+ + O2- h Cu2+ + O1Co4+ + O2- h Co3+ + O1-
Solid Catalyst for the Low-Temperature Oxidation of CO
J. Phys. Chem., Vol. 100, No. 20, 1996 8447 Also, the absence of lanthanum carbonate in the case of LaBa2Cu3O7-δ verifies the utilization of only O(1) oxygen. These surface carbonates can be regarded as intermediate surface species formed during the CO oxidation.22,23 5. Conclusions
Figure 10. Projection diagrams along the a-c plane for the (a) YBa2Cu2CoO7+δ; (b) LaBa2Cu3O7-δ; and (c) LaBa2Cu2CoO7+δ system of oxides.
Such an influence of cobalt ion and excess oxygen in YBa2Cu2CoO7+δ thus can in combination account for a lower oxygen desorption energy than in the case of YBa2Cu3O7-δ and LaBa2Cu3O7-δ. The mechanism of the catalytic CO oxidation can be interpreted in terms of a Mars-Van Krevelen pathway,20 wherein the consumed basal plane oxygens in LaBa2Cu3O7-δ and YBa2Cu2CoO7+δ are replenished by the feed stream oxygen in a consequent step. Pickering and Thomas,10 in explaining the shallow minima in catalytic activity of YBa2Cu3O6+δ toward CO oxidation as a function of δ, observed that a Mars-Van Krevelen pathway involving the O(1) oxygens dominates at higher values of δ. The observed, highest reactivity in a stoichiometric reaction and catalytic activity of LaBa2Cu2CoO7+δ can then be explained only on the basis of a different structural model, envisaging the role played by the larger ionic size and basicity of the lanthanum ion influencing the oxygen coordination of substituent transition metal cations. Based on the structures of LaBa2Cu3O7-δ, YBa2Cu2CoO7+δ, and LaBa2Cu2CoO7+δ, projection diagrams along the a-c plane for all three oxides are shown in Figure 10. Figure 10c shows the idealized model of LaBa2Cu2CoO7+δ, in which the combined effect of a larger lanthanum ion and significant cobalt at Cu(2) sites results in the enrichment of the oxygen content at the rare earth plane. The distinct CO2 peaks (R2) in the thermogram of the stoichiometric CO oxidation over LaBa2Cu2CoO7+δ can then be assigned to the consumption of the intersheet oxygens, as these are the only sites with reversibly desorbable oxygen in the triple cell in addition to the reactive basal plane oxygens. A similar case of the utilization of rare earth plane oxygen in a stoichiometric oxidation of ammonia has been reported in the case of PrBa2Cu2CoO7+δ.21 Lin et al. explained the formation of BaCO3 by the reaction of O(1) oxygen sandwiched between two [Ba-O] layers with carbon monoxide proceeding through intermediate carbonate formation. Thermodynamically stable BaCO3 is formed instead of CuCO3. Along similar lines, the presence of lanthanum carbonate in the spent solid in the case of LaBa2Cu2CoO7+δ can only be explained by the reaction of CO with the rare earth plane oxygen to form an intermediate lanthanum carbonate.
YBa2Cu3O7-δ is a versatile, defect, triple-layered perovskite amenable to a variety of cationic substitutions. Simultaneous substitution of Y by La and one Cu by Co, forming a new oxide LaBa2Cu2Co7+δ, retaining the parent orthorhombic structure, shows two distinct labile oxygen sites, one at (0 1/2 0) and the other at (0 0 1/2). The activation energy of oxygen desorption is decreased from 27 to 19 kcal/mol, and the oxygen desorbing at lower temperature is from the (0 0 1/2) sites in the La plane. While oxygen from both sites in LaBa2Cu2CoO7+δ is utilized in the uniform heterogeneous catalytic oxidation of CO to CO2, the oxygen in the La plane reacts with CO at a temperature as low as 200 °C with a substantially low activation energy of 13.6 kcal/mol. Acknowledgment. The authors thank Professor K. S. Gandhi for useful discussions. Financial support from the Department of Science and Technology, Government of India, is gratefully acknowledged. References and Notes (1) Voorhoeve, R. J. H. In AdVanced Materials in Catalysis; Burton, J. J., Garten, R. L., Eds.; Academic Press: New York, 1977. (2) Tejuca, L. G.; Fierro, J. L. G.; Tascon, J. M. D. AdV. Catal. 1989, 36, 237. (3) Sleight, A. W. In AdVanced Materials in Catalysis; Burton, J. J., Garten, R. L., Eds.; Academic Press: New York, 1977. (4) Tarascon, J. M.; McKinnon, W. R.; Greene, L. H.; Hull, G. W.; Vogel, E. M. Phys. ReV. 1987, B36, 226. (5) Jorgenson, J. D.; Veal, B. W.; Paulickas, A. P.; Nowicki, L. J.; Crabtree, G. W.; Claus, H.; Kwok, W. K. Phys. ReV. 1990, B41, 1863. (6) Hansen, S.; Otamiri, J.; Bovin, J.; Anderson, A. Nature 1988, 334, 143. (7) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 1673. (8) Jiang, P.; Peng, Y.; Zhou, Q.; Gao, P.; Yuan, H.; Deng, J. Catal. Lett. 1989, 3, 235. (9) Halasz, I.; Brenner, A.; Shelef, M.; Simon Ng, K. Y. J. Catal. 1990, 126, 109. (10) Pickering, I. J.; Thomas, J. M. J. Chem. Soc., Faraday Trans. 1991, 87, 3067. (11) Lin, J.; Neoh, K. G.; Li, N.; Tan, T. C.; Wee, A. T. S.; Huan, A. C. H.; Tan, K. L. Inorg. Chem. 1993, 32, 3093. (12) Ramesh, S.; Hegde, M. S. Physica C 1994, 230, 135. (13) Ramesh, S.; Vasanthacharya, N. Y.; Hegde, M. S.; Subbanna, G. N.; Rajagopal, H.; Sequiera, A.; Paranjpe, S. K. Physica C 1995, 235, 43. (14) Slater, P. R.; Wright, A. J.; Greaves, C. Physica C 1991, 183, 111. (15) Hegde, M. S.; Ramesh, S.; Ramesh, G. S. Proc. Indian Acad Sci. (Chem. Sci.) 1992, 104, 591. (16) Ross, S. D. Inorganic Infrared and Raman Spectra; McGraw-Hill: London, 1972. (17) La Graf, J. R.; Payne, D. A. Physica C 1993, 212, 487. (18) Pollert, E.; Sedmidubsky, D.; Knizek, K.; Jirak, Z.; Vasek, P.; Janececk, I. Physica C 1992, 197, 371. (19) Goodenough, J. B. Supercond. Sci. Technol. 1990, 3, 26. (20) Mars, P.; Van Krevelen, D. W. Chem. Eng. Sci. Suppl. 1954, 3, 41. (21) Hegde, M. S.; Ramesh, S.; Panchapagesan, T. J. Solid State Chem. 1993, 102, 306. (22) Stone, F. S. AdV. Catal. 1962, 13, 1. (23) Salvador, P. J. Phys. Chem. 1989, 93, 8278.
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