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Nakabayashi, I.; Masuda. S.;Yasumura, J. Nippon Kagaku Zasshi 1969, 9 0 , 344. 1073. Nakabayashi, I. Kogyo Kagaku Zasshi 1971, 74, 1527. Nakabayashi, I.; Hisano, T.: Terazawa, T. J . Catal. 1979, 58, 74 Rappoport, I. 6.; Silchenko. E. I. J . Appl. Chem. (USSR) 1937, 70, 1427 Wilson, C. L. J . Chem. SOC. 1945, 48. Yasumura, J.; Yoshino, T. Kogyo Kagaku Zasshi 1966, 69, 601. Yasumura, J.; Yoshino, T. Kogyo Kagaku Zasshi 1967, 7 0 , 141.
Yasumura, J.; Yoshino, T.; Abe, S. Ind. Eng. Chem. Prod. Res. De". 1968, .7 , 353 Yasumura, J . ; Nakabayashi, I. Chem. Left. (Jpn.) 1972, 511
Received for review F e b r u a r y 1, 1983 Revised manuscript received June 27, 1983 Accepted July 8, 1983
Synthesis Gas Reactions over Oxidized Intermetallic Compounds A. Shams1 and W. E. Wallace' Department of Chemistty, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Intermetallic compounds containing Ni or Co in chemical union with La, Ce, Th, or Zr were oxidized to form supported Ni or Co synthesis gas conversion catalysts. Oxidants used were O,, NO, synthesis gas, and in one case, (CeNi,), H,O. I n addition, catalysts consisting of Ni supported on CeO,, Tho,, and La,03 were prepared by a freeze-drying technique. All catalysts were active in the conversion of syngas into hydrocarbons, predominantly CH,. s-' at 205 'C-for the oxidized Large turnover frequencies were observed-as high as 19.6 X intermetallics. Even when the metal loading was comparable, significant differences were noted between the freezedried catalysts and those formed by intermetallic compound oxidation.
Introduction Intermetallic compounds containing rare earths or thorium in chemical union with Fe, Co, or Ni when exposed to synthesis gas at elevated temperatures are transformed (Coon et al., 1976; Luengo et al., 1977; Elattar et al., 1977, 1979; Coon, 1977; Moldovan et al., 1978; Chin et al., 1980; Atkinson and Nicks, 1977) into mixtures of rare earth orTh oxides and elemental Fe, Co, or Ni. These mixtures have been observed to be exceptionally active as syngas conversion catalysts. Examination of these materials by a varieity of techniques-Auger spectroscopy (Moldovan et al., 1978), X-ray diffraction (Coon, 1977), ESCA (Chin et al., 1980), and electron microscopy (Coon, 1977)-showed surfaces consisting of nodules of 3d transition metal of the order of 1 pm in size, growing out of or dispersed on the oxide. The converted material rather than the original alloy appears to be the active catalyst for the reaction. An interesting aspect of the reaction between the intermetallic compound and synthesis gas is that in this instance the latter is acting as an oxidizing agent, whereas it is normally a powerful reducing agent. Imamura and Wallace (1979) examined a number of Ni-Si and Co-Si intermetallics in connection with syngas conversion and observed behavior similar to that noted for the rare earth and thorium intermetallics; finely dispersed Ni or Co on Si02was produced. It was recognized in this and the earlier studies that oxidation of the intermetallic compound with synthesis gas constituted a new way of forming a supported catalyst. The work was immediately extended (Imamura and Wallace, 1979) to generate the new supported catalysts by controlled oxidation with O2 instead of syngas. The 02-produced catalysts were generally similar to the syngas-produced catalysts, but they differed in some important details. For example, CHI formation turnover frequencies were often higher for the catalysts formed by oxidation with 02. Since the characteristics of the catalysts depend on the nature of the oxidant involved, it was of interest to extend
the work to other oxidants. In the present work, selected intermetallic compounds containing Ni and Co were oxidized with NO and the resulting mixtures were examined as syngas conversion catalysts. This forms the main body of the present study. In addition, efforts were made to effect conversion of the intermetallic compounds by HzO, a point of interest in regard to establishing how syngas acts as an oxidant, but these were soon abandoned. Conversion effected was minimal and the mixtures formed showed insignificant activity. Intermetallics converted by oxidation with NO were for comparison purposes also oxidized by the conventional oxidants, syngas and O2 The present work constitutes an extension of the earlier work in that a number of new intermetallics have been examined as catalyst precursors. Also, and again for purposes of comparison, several supported catalysts formed by a special freeze-drying technique were examined. Results obtained using these latter catalysts as well as those obtained using the intermetallic compound-derived catalysts are presented. Experimental Methods
The intermetallic compounds were prepared by induction melting the metal constituents (>99.9% purity) on a water-cooled copper boat in a Ti-gettered Ar atmosphere. Most of the ingot samples were brittle. They were crushed with a stainless steel sample-crusher and then powdered in air using a mortar and pestle. The sample, after being ground into a fine powder (-120 pm), was weighed ( N 1 g) and supported on glass wool in the reactor tube. The oxidation treatment of the powdered sample was carried out by flowing the oxidizers oxygen, nitric oxide, and synthesis gas over the sample at experimental temperature and 1atm pressure. The powder X-ray diffraction method was used to identify the different phases which were formed during the oxidation treatment. Samples of composition 64% Ni/La20s, 63% Ni-Ce02, 8.390Ni/Ce02, and 8.9% Ni/Th02 were prepared by a
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freeze-drying technique (Tseung and Bevan, 1970). The starting materials were water-soluble salts, such as nitrates of the materials to be prepared. A mixture in the desired stoichiometric proportion of materials was dissolved into 200 mL of distilled water and sprayed as a fine jet into liquid nitrogen; then the frozen droplets were transferred into a 500-mL glass flask, which formed a part of the freeze-drying system (Devor et al., 1956), and evacuated by a mechanical vacuum pump at about torr for a few days until all traces of water had been removed. The dried powder was then decomposed at 300 "C, followed by quenching in oxygen and heating to the final experimental temperature for about 6 h. The total surface area was determined before and after synthesis gas reaction with the flow method developed by Nelsen and Eggertsen (1958). Carbon monoxide chemisorption was measured to estimate the number of active sites on the catalyst surface. Carbon monoxide chemisorption is made at room temperature by means of the adsorption flow method of Gruber (1962). It was assumed that there was one active site per molecule of CO adsorbed. The reactions were carried out in the microreactor used in the earlier studies (Imamura and Wallace, 1979). Prior to the catalytic activity measurement the sample (-0.5 g) was evacuated at 350 "C for 1 h and then reduced at 400 "C under a flow of hydrogen (30 mL/min) which had been purified by passing through a molecular sieve and a liquid nitrogen trap. This was carried out at 1 atm, for 2 h. The sample was cooled to experimental temperatures, and the hydrogen was then replaced by synthesis gas (H2/C0 = 3) which had passed through a molecular sieve at 1 atm total pressure. At steady-state conditions (when the reaction temperature had remained constant for approximately 30 min) the effluent gas was analyzed with a Gow-Mac gas chromatograph (GC) with a thermal conductivity detector and a '/* in. X 10 f t Porapax Q column. The output of the GC was recorded on a Sargent strip chart recorder with a disk integrator. A series of known gases were injected into the column of the GC. The direct comparison of the retention time of the product's components with retention time of the known samples permitted identification of the individual product. The product gas was also analyzed by a mass spectrometer equipped with a gas chromatograph. Gas flow rate was controlled by a needle valve and measured at the outlet by a soap bubble flow meter. The flow rates were kept in the range of 20 to 50 mL/min to maintain the CO conversion below 10% and minimize heat and mass transfer effects.
Results and Discussion A. Amounts of Conversion and Selectivity. The results obtained (Shamsi, 1981) are largely summarized in Table I-V. The freshly oxidized intermetallics generally contained a rare earth, thorium or zirconium oxide, and Ni or Co. Sometimes NiO or an oxide of Co was also present. Details of the phases present in the fresh and used catalyst are given in Table I. ZrNi, was included in the intermetallic compounds studied because it has been observed that CeNi, and ThNi, upon oxidation lead to unusually active catalysts. It appeared that the presence of a quadripositive cation in the support might be responsible for the exceptional activity. Zr is also quadripositive in its oxide. Hence ZrNi, was included in the materials being studied. Results in Table 11, however, do not indicate exceptional activity for catalysts derived from ZrNi,. Turnover frequencies (TN) were determined over a range of temperatures and were fitted to an Arrhenius type
Table I. Compounds Detected by X-ray Diffraction in Fresh and Used Catalysts precursor intermetallic
oxidized intermetallic
CeNi,, CeNi,, Ce,Ni,, ZrNi LaNi ,
Ni, NiO, CeO,
Ni, CeO,
Ni, ZrNi,, ZrO, Ni, NiO, La,O, CU-Co, CeO,, Co oxides
Ni, ZrNi, , ZrO, Ni, La,O, or-&, CeO,
ceco,
after syngas reactiona
Before exposure to synthesis gas t h e catalyst was reduced a t 400 "C under a flowof hydrogen (30 mL/min) for 2 h. 1000 I
1
t
\4
iL "15
16
1.8
17
I9
1000/T H / K )
Figure 1. Arrhenius plots for LaNi, oxized with NO: (A)CO conmethanation; oxidized with SG (X) CO consumption; sumption; (0) (0) methanation; oxidized with 02:(0) CO consumption; ( 0 )methanation. I00
I
/ 1 17
19
21
23
IOOO/T ( I / K )
Figure 2. Arrhenius plots for CeCo, oxidized with NO: (0) CO consumption; (X) methanation; LaCo, oxidized wiih O2 (second sample): (A) co consumption; (0)methanation.
equation [TN = A exp(-E,/RT)]. Turnover frequency (TN) is the number of molecules formed (or reacted) per metal site per second (Vannice, 1975). Representative plots are shown in Figure 1-4. Activation energies (FA) and frequency factors ( A ) derived from these plots are listed in Table 11. A pronounced compensation behavior is observed (see Figure 5). In this respect the catalytic behavior of the group of materials included in the present study resembles that exhibited by the systems studied by Coon (1977) and the RCo6-derived catalyst studied by Wallace et al. (1980). If TN is taken as a measure of catalytic activity, it is to be noted that no systematic behavior is noted in regard to the three oxidizers used. However, is is evident that the new catalysts formed by oxidation of intermetallic compounds are quite active when compared to conventional supported catalysts (Imamura and Wallace, 1980).
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Table IV. Syngas Conversion over Oxidized Intermetallicsa Product Distribution (mol %) precursor co intermetallic oxidizer conv, % c, C2(=) LaNi NO 3.8 85.2 1.6 SG 2.7 100 0 3.3 91.7 1.3 0 2 LaCo, NO 5.2 89.3 6.6 4.4 94.8 1.9 0 2 SG 4.8 95.4 3.1 CeNi NO 3.8 80.9 0 SG 2.1 84.3 1.7 5.2 75.1 0.3 0 2 5.8 89.8 2.0 H,O CeNi NO 4.5 76.8 0 89.7 0 SG 4.8 2.8 82.3 3.5 83.9 0 Ce ,Ni,, :b 5.5 84.8 0 SG 5.6 0.4 77.6 6.9 0 2 CeCo, NO 4.4 91.2 0.5 SG 2.2 91.5 3.4 2.6 89.2 7.5 0 2 ThCo, NO 3.0 93.8 1.3 SG 5.3 95.1 0 3.4 88.7 4 .O 0 2 TbNi NO 5.0 93.3 2.9 SG 5.3 89.1 0 0 2 5.8 81.1 0 ZrNi, NO 5.7 86.1 1.9 SG 6.0 88.9 0 0 2 6.6 85.6 2.4
c2
13.2 0 1.0 4.0 3.3 1.4 19.1 14 24.6 8.2 23.2 10.3 14.9 16.1 15.2 22.0 8.3 5.1 3.3 4.8 4.9 7.2 3.8 3.1 18.8 12 11.1 11.9
1983 585
Cz(=)lCz 0.12 0 1.3 1.6 0.58 2.2 0 0.12 0.01 0.24 0 0 0.19 0 0 0.02 0.06 0.67 2.3 0.27 0 0.56 0.76 0 0 0.16 0 0.20
The percent CO conversions listed apply t o a variety of temperatures ranging from 250 t o 350 "C. These data are presented primarily t o indicate product distributions. Because of the temperature variation some materials, e.g., H20-oxidized CeNi,, appear more active than they really are because the results obtained are for a higher temperature. Table V. Syngas Conversion over Catalysts Prepared by Freeze-Drying Product Distribution (mol%) conv, 64% Ni/La,O, 63% Ni/CeO, 8.3% Ni/CeO,
8.9%Ni/ThOZa
%
c,
C,(=)
c,
5.4 4.6 4.6 6.1
74.0 78.5 86.1 38.1
0.9 0.3
25.1 21.2 13 29.1
0.9 6.7
C*(=)/
-
4
c,
0.04 0.01 0.07 0.23
Product distributions (mol %) from C, to C, are: C, = 17.2; C, = 6.8; C, = 2.1. a
Nearly half of the new catalysts have TN's (at 205 "C) exceeding 5 X s-l, whereas supported Ni catalysts prepared by conventional wet chemical techniques have TN's at this temperature in the range 0.5 to 1 X s-l. B. Characteristics of Catalysts. Several features of the used catalysts merit comment. Generally among the catalysts formed from rare earth intermetallics the Cecontaining compounds were the most interesting from a catalytic viewpoint. They were transformed into systems with high surface area (metallic and total) and activity. Oxidation at different temperatures indicated that the catalytic activity is dependent on the degree of oxidation. Carbon deposition on many of the catalysts formed by syngas oxidation was evident by visual examination, Auger spectroscopy (AES), or photoemission studies (ESCA). LaNi5-, CeNi2-and CeNi6-derivedcatalysts showed strong carbon AES signals. The effect was even more pronounced with LaCo,. In fact, this material behaved in an unusual fashion in several ways. The CO chemisorption of 02oxidized LaCo5 (designated no. 1 in Table 11) increased from 4.1 to 22.6 pmol/g during the reaction, and the total surface area also increased considerably. The increase of total surface area is probably caused by carbon deposition during the synthesis gas reaction, and the reason for the increase in the CO uptake may be further oxidation and the drawing of cobalt to the surface, which is similar to
the behavior observed in ESCA measurements of ThNi5 after exposure to syngas (Chin et al., 1980). The Arrhenius plots for reactions using LaCo5-derivedcatalysts deviated from a straight line, and at reaction temperature the catalysts became deactivated. LaCo, (0,)(2) in Table I1 signifies that this sample was reduced for a second time and then used for synthesis gas conversion. The results obtained the second time are much higher than the first time. They give a straight line Arrhenius plot at low temperatures (Figure 2). The experimental TN values for LaCo5 oxidized with NO are higher than the others. In regard to the ThCo5-derived catalysts, these are of considerable interest because of extensive attention which has been paid to catalysts derived from the closely related ThNi5 compound (Elattar et al., 1979; Moldovan et al., 1978; Chin et al., 1980). Surface areas and CO chemisorptions before and after catalytic reaction are relatively high for the sample which has been oxidized with SG. The
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Figure 4. Arrhenius plots for freeze-dried 8.9% Ni/Th02: consumption; (0) methanation; freeze-dried 8.3% Ni/CeO,: consumption; (0) methanation.
(A) (X)
CO CO
X-ray diffraction pattern shows that (1) this system transformed into Tho,, Co, and Co oxide during reaction with O2and NO, and (2) there were no peaks which correspond to Co oxide in the reaction with synthesis gas. The Arrhenius plots are straight lines at low temperature and for a clean surface. The high total surface areas (Table 11) are possibly caused by carbon deposition during the oxidation reaction with SG. ESCA study of ThCo, showed a carbon peak which corresponds to the binding energy of graphite (Chin, 1980). A pronounced carbon peak was also observed in the Auger spectra of ThCo, after reaction and before catalytic reaction when it was oxidized with a mixture of H, and CO. Despite the high surface areas and CO chemisorptions of ThCo, oxidized with syngas, the experimental turnover frequencies are not significantly different from those obtained using catalysts formed with other oxidants. The apparent activation energies and preexponential factors are given in Table 11. Arrhenius plots for ThCo, oxidized with NO and 0, are not straight lines, and these samples become deactivated at higher reaction temperature. This deactivation may be caused by fast growth of carbidic and graphitic carbon on the catalyst surface during the catalytic reactions. The ZrNi, system is neither a rare earth nor an actinide intermetallic compound, but it is relevant to the present work because, as noted above, of the quadripositive valence state of Zr. In the present work the ZrNi, system was used to form catalysts by using as oxidizing agents NO and 02, thus supplementing the earlier studies (Ellatar et al., 1977; Moldovan et al., 1978) in which it was oxidized only with synthesis gas. Arrhenius plots deviated from a straight line at higher reaction temperature. This is probably caused by increasing carbon deposition on the active nickel sites. AES showed, in agreement with the earlier study (Moldovan e t al., 1978) that the nickel particles are heavily overlaid with graphite. Despite the carbon deposition on the nickel surface, the oxidized sample with synthesis gas showed, compared to those oxidized with 0, or NO, higher CO uptake and higher total surface areas before and after catalytic reactions. This indicates that a mixture of CO and Hz draws nickel to the surface of ZrNib, analogous to the effect observed for ThNi, (Chin et al., 1980). The higher total surface areas may be caused by carbon deposition. It was observed that cleaning the surface by flowing pure hydrogen over the sample for 23 min increases the formation of CH, by an order of magnitude. For example, the percentage of CH, formation at 570 K was 4.0, but as
K calzmclet
Figure 5. Compensation plot for intermetallic compounds oxidized with NO. The A and E A apply to the consumption of CO.
the temperature increased, this value dropped from 4.0 to 2.8 at 583 K. However, flowing hydrogen for 25 min increased the value from 2.8 to 4.2 at 583 K. This catalyst was not completely reactivated by flowing hydrogen over the sample because it was observed that the percentage of CH, formation at 583 K after cleaning was still only equal to that at 570 K. This means carbon deposition is one of the causes for the deactivation, and it cannot be removed entirely by hydrogen a t that temperature. Goodman et al. (1980) reported that the “carbidic” carbon on the nickel surface can easily be removed by heating the sample in H,. It is possible that during the reaction both carbidic and graphitic carbon were formed on the nickel surface and the carbidic carbon was removed by flowing hydrogen over the sample at 583 K. This is also true for the samples that were oxidized with O2 and NO. A t low temperature and for clean surface the Arrhenius plots were straight lines, whereas at high temperature the oxidized samples (with 02,NO and SG) became deactivated and the plots deviated from a straight line. This and previous studies of catalysts derived from oxidation of intermetallic compounds show that while these materials have impressive turnover frequencies, they have other features as catalysts which are disadvantageous. Almost all have small metallic and total surface areas, low dispersion, and highly crystallized metal particles on the surface. A few systems formed by oxidizing Ce and Th compounds are exceptions to this generalization. In regard to the large crystallites on the surface, thermomagnetic analysis (TMA) of the oxidized intermetallics showed that a large part of the total nickel present in the sample exists as ferromagnetic nickel. As examples, in the cases of CeNi, and LaNi, better than 75% of the Ni behaves ferromagnetically (see Figure 6). This therefore indicates that most of the nickel particles on the surface are large (similar magnetically to massive nickel) and hence they are not catalytically active. This is probably a consequence of the high Ni content in the precursor intermetallic (vide infra). C. Product Distribution. The percentage of CO conversion, the mole percent of products and the ratio of ethylene to ethane, which frequently reflects the olefinto-paraffin ratio in products, are shown in Tables IV and V. In almost all cases the CH, component of the products exceeded 80%. A few of them, such as CeNiz (O,), CeNi, (NO), CezNiI7(0,)and 64% Ni/Laz03(vide infra), formed about 75% methane. Total product distribution excluding H,O is given in Table VI. The amount of C02in product increases with increasing reaction temperature. The freeze-dried sample (vide infra) with 8.9% Ni supported on T h o z is a striking exception. This sample gave about 38% methane and 62% other hydrocarbons. Results from
Ind. Eng. Chem. Prod. Res. Dev., Vol. 2?, No. 4, 1983 587
Table VI. Total Product Distribution Excluding H,O (mol %) precursor CO conv intermetallic oxidizer a t 480 "C CH4 CeNi, 1.2 42.3 CeNi, :b 2.8 51.7 Ce,Ni,, 0 2 2.0 61.3 CeNi,, NO 2.2 63.4 CeCo NO 1.5 74.5 TbNi, 0, 1.6 64.7 ZrNi, SG 1.3 74.1 Thco, SG 2.7 82.9 Lac0 , 0 2 1.1 76.8 LaNi a NO 1.9 57.8 a
41.1 28.0 19.9 17.1 16.8 21.2 14.6 13.7 23.2 32.6
4.6 0 3.5 0 0.4 0 0 0 0 0.5
12.0 20.3 15.3 19.5 8.3 14.1 11.3 3.4 0 9.1
CO conversion a t 543 "C.
0
4 00
200 T
600
("C)
Figure 6. Magnetization versus temperature for Ni/Laz03(0) and Ni/CeOz (0) formed by oxidizing LaNi, and CeNi,, respectively, with NO.
CO chemisorption (Table 111)indicate that the freeze-dried sample (3.9% Ni/Th02) has higher metal dispersion than the other samples, and studies showed that the CO/H adsorption and selectivity to C2+hydrocarbons increases with increasing metal dispersion (Vannice, 1976; Bartholemew, 1980). Furthermore, catalysts with lower dispersion need to be run at higher temperature to get the same conversion. Therefore, selectivity to methane increases with increasing temperature. D. Catalysts Prepared by Freeze-Drying Technique. These catalysts were prepared to give a higher degree of dispersion and to intercompare with those obtained through the oxidation process. Results are summarized in Tables I11 and V and in Figures 3 and 4. Experiment showed that there is a significant difference between 64% Ni/La203and LaNi, oxidized with 02,which have almost the same loading of nickel. At 507 K the CO turnover number for the freeze-dried sample is about 17 times larger than that which was prepared by intermetallic compound oxidation, and the CHI turnover number is about 20 times higher than that for LaNi, oxidized with 02. Intermetallic compounds that oxidize readily and which, during the oxidation treatment, break into smaller particles (e.g., CeNiJ did not show a significant difference between these two preparation methods. Two freeze-dried Ni catalysts supported on CeOz with different loadings (63 and 8.3% Ni) were prepared and examined. This showed that despite a large change in Ni concentration, the changes in TN's are not significant. The highest values for the CO turnover number, the CH4 turnover number, and activity were obtained for 8.9%
Ni/ThOz at 507 K. This sample also has the highest CO chemisorptions and total surface areas among all the samples which have been studied in this laboratory so far. Imamura and Wallace (1980) reported on the activity (as syngas conversion catalysts) of 25% Ni/Th02 and 3.9% Ni/Th02 which had been prepared by the conventional impregnation technique and compared these with Ni/ Tho2 formed by the reaction of ThNi5 and 02.The 8.9% Ni/Th02 catalyst prepared by freeze-drying was superior to all of these samples. The total surface area of this sample is as high as 10 times larger and the CO chemisorption is almost 3 times larger than that of oxidized ThNi,. The percentage of CO conversion for 25% Ni/ Tho2 prepared by conventional wet chemical techniques (Imamura and Wallace, 1980) is 1.7% at 510 OC, whereas the value for the 8.9% freeze-dried sample is 11.0% at 205 "C. This shows a very significant improvement of freeze-dried sample over the impregnated samples. Acknowledgment This work was assisted by a contract with the Union Oil Company of California through its Science and Technology Division in Brea. CA.
Literature Cited Atkinson, G. 6.; Nicks, L. J. J. Catal. 1977, 46, 417. Bartholemew, C. H.; Pannell, R. B.; Butler, J. L. J. J. Catal. 1980, 65, 335. Chin, R. L. Ph.D. Dissertatlon, University of Pittsburgh, Pittsburgh, PA, 1980. Chin, R. L.; Elattar, E.; Wallace, W. E.; Hercules, D. M. J. phys. Chem. 1980, 84. 2895. Coon, V. T. Ph.D. Dissertation, University of Plttsburgh, Pittsburgh, PA, 1977. Coon, V. T.; Takeshita, T.; Wallace, W. E.; Craig, R. S. J. Phys. Chem. 1978, 8 0 , 1878. Devor, A. W.; Tiehener, E. 6.; Andre, C. E.; Frajola, W. J. J. Chem. Educ. 1956, 3 3 , 343. Elattar, A.; Takeshlta. T.; Wallace, W. E.; Craig, R. S. Science 1977, 196, 1093. Elattar, A.; Wallace, W. E.; Craig, R. S. Adv. Chem. S e r . 1979, No. 178, 7. Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates, J. T., Jr. J. Catal. 1980, 63,226. Gruber, H. L. Anal. Chem. 1962, 3 4 , 1820. Imamura, H.; Wallace, W. E. J. Phys. Chem. 1979, 8 3 , 2009. Imamura, H.; Wallace, W. E. J. Phys. Chem. 1979, 8 3 , 3261. Imamura. H.; Wallace, W. E. J. Catal. 1980, 65, 127. Luengo, C. A.; Cabrera, A. L.; McKay, H. B.; Maple, M. 8. J. Catal. 1977, 4 7 , 1. Moldovan, A. G.; Elattar, A.; Wallace, W. E. J. SolM State Chem. 1978 25, 23. Nelsen, F. M.; Eggertsen, F. T. Anal. Chem. 1958, 3 0 , 1387. Shamsi, A. Ph.D. Dissertation, Universlty of Pittsburgh, Pittsburgh, PA, 1981. Tseung, A. C. C.; Bevan, H. J. J. Met. Sci. 1970, 5 , 604. Vannice, M. A. J. Catal. 1875, 3 7 , 449: 1978, 4 4 , 152. Wallace, W. E.; Elattar, A.; Imamura, H.; Craig, R. S.;Moldovan, A. G. I n "The Science and Technology of Rare Earth Materials"; Subbarao, E. C.; Wallace, W. E., Ed.; Academic Press Inc.: New York, 1980; p 329.
Received for review December 14,1982 Accepted May 18,1983
