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Studies on Indian Ocean Manganese Nodules PART 2. Physico-chemical Characteristics and Catalytic Activity of Heat-Treated Marine Manganese Nodules
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
179, 241–248 (1996)
0209
Studies on Indian Ocean Manganese Nodules PART 2. Physico-chemical Characteristics and Catalytic Activity of Heat-Treated Marine Manganese Nodules K. M. PARIDA, 1 P. K. SATAPATHY, N. N. DAS,
AND
S. B. RAO
Regional Research Laboratory, CSIR, Bhubaneswar 751 013, India Received June 9, 1995; accepted September 11, 1995
The effect of calcination temperature on the physico-chemical characteristics and catalytic activity of central Indian Basin manganese nodules have been investigated. TG, XRD, IR, and chemical analysis confirm the presence of amorphous oxyhydroxides of iron and d-MnO2 or todorokite, which convert to a-Fe2O3 and gMn2O3 or Mn3O4 phases at §4007C of calcination. The pore volume, average pore diameter, and pore size distribution practically remain unaffected up to 4007C of heat treatment. But in the temperature range 400 to 7007C, smaller pores coalesce to form larger ones and beyond 8007C the material becomes practically nonporous. The surface hydroxyl group and surface acidity are progressively decreased with a rise in calcination temperature. The surface area, surface oxygen, electrical conductivity, as well as catalytic activity for H2O2 decomposition and CO oxidation, however, follow a similar trend: a gradual increase up to 4007C of calcination and then a decrease with further increase in temperature of calcination. q 1996 Academic Press, Inc. Key Words: surface characteristics; catalytic activity; manganese nodules.
monoxide and hydrocarbon, methanation of CO, reduction of nitrogen oxides (5–9), demetallization, and desulphurization of topped petroleum crude in the presence of hydrogen (10, 11) are promising. As most of the heterogeneous catalytic reactions take place at elevated temperatures and pressures, it is important and necessary to know the possible changes in the physicochemical properties and hence the catalytic activity due to thermal treatment. Although it is clear that oceanic nodules have unique physico-chemical characteristics which have very significant effects on the catalytic activity, there has been only limited investigations (5, 12, 13) regarding the effect of heat treatment on the surface and textural properties. Also, no work has been carried out so far to correlate the physico-chemical properties and the catalytic activity of heat-treated samples. The present work investigates the effect of temperature on various physico-chemical characteristics, electrical conductivity, and catalytic activity of manganese nodules of the central Indian Ocean basin. EXPERIMENTAL
INTRODUCTION
Studies on deep sea manganese nodules have been subject of great interest to mineralogists, sedimentologists, and metallurgists due to their rich composition of metals such as manganese and iron and, especially Ni, Cu, Co, etc. (1–4). The effectiveness of these materials as catalysts for certain chemical reactions has attracted the attention of a number of workers because they exhibit quite remarkable features such as: (a) they occur in great abundance both in marine and fresh water sediments; (b) they possess high porosity ˚ ), and relatively (av. 62%) ultrafine grain size (av. 100 A 2 high surface area (100–300 m /g); and (c) they contain largely d-MnO2 and a-FeOOH along with a variety of transition metal ions in the interlayers and framework positions. Among many catalytic applications, oxidation of carbon 1
To whom correspondence should be addressed.
Sample Preparation Manganese nodules of the central Indian Ocean basin were obtained from the National Institute of Oceanography, Goa, India, and were collected during cruises of the Fernela F7. The samples were air dried, powdered, and sieved. The sieved fractions 075 / 45 mm were collected for further studies. About 10 g of sample was heated at each temperature (110–9007C) in a muffle furnace with a suitable controller to maintain offset temperatures with {57C. The samples were kept at the desired temperature for 3 h, cooled to about 507C, and then stored in a desiccator over fused CaCl2 . The samples thus prepared were labeled Mn–T, where T denotes the temperature of heat treatment in degrees Celsius. Characterization TG-DTA of the samples (about 15 mg) was carried out using a Shimadzu DT-40 model automatic thermal analyser in
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the temperature range 30–10007C at a heating rate of 107C min 01 . X-ray diffraction patterns of heat-treated samples were recorded on a Philips semiautomatic X-ray diffractometer with auto divergent slit and graphite monochromator using CuKa radiation operated at 40 kV and 20 mA. The FT-IR spectra (4000–400 cm01 ) of the samples in KBr phase were recorded using a JASCO Model 5300 spectrometer. The determination of surface acidity either by titration of solids (manganese nodules) suspended in benzene with nbutyl amine (14) or by spectrophotometric method using simple amines, (e.g., pyridine, cyclohexylamine) as adsorbate (15), could not be employed in this case due to partial solubility of some metal oxides in amines. As such, noninteracting amines (16), 2,6-dimethylpyridine (pka 6.9), 2,6-dimethylpiperidine (pka 11.11), and 2,6-dimethylmorpholine (pka 11.12) were used as adsorbates. In this method, freshly prepared amines (0.01 M) in cyclohexane were pipetted out into 100-ml stoppered conical flasks containing 0.2 g of nodules. The flasks were shaken for 1 h and then the contents were filtered. The absorbance of the filtrate was measured at preset wavelengths. The acid sites were calculated from the amount of substrate adsorbed on the surface. Other details have been described in literature (15). Surface excess oxygen and surface hydroxyl groups were determined by following reported methods (17, 18). Total Mn, Mn 2/ , Mn 3/ , and Mn 4/ were estimated as described earlier (19). Active oxygen (or available oxygen) was determined by the oxalate method (20) and the values were used for the calculation of x in MnOx . The percentage of weight losses at various calcination temperature were also verified from the TG curves of the samples. Nitrogen adsorption– desorption experiments were carried out at 78 K with a Quantasorb (Quantachrome, USA) and were analyzed for distribution of pores, cumulative pore volumes, average pore diameter, and specific surface area. Surface area was also measured using a high-speed surface area analyzer (Model 2200A, Micromeritics, USA). Electrical Conductivity The electrical conductivity ( s ) of the heat-treated samples in the temperature range 40–6007C was calculated by measuring the electrical resistance across the sintered pellet (21– 23) using a Kethley programmable electrometer (Model 617). As the nodules were calcined at various temperatures, the electrical conductivity in some cases were measured inbetween room temperature and calcined temperature. For this measurement the powdered samples were compressed into pellets by applying constant pressure (9 tons/in 2 ) and were electroded with silver paste. The pellets were mounted between two copper blocks of the sample holder. The sample holder contained in a tubular furnace was heated using a suitable temperature controller. A chromel–alumel thermocouple was used as temperature probe.
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FIG. 1. TG-DTA of air-dried manganese nodules.
Catalytic Activity The catalytic activity of manganese nodules under investigation were measured using H2O2 decomposition and CO oxidation reactions. The catalytic decomposition of aqueous H2O2 (AR) in the temperature range 15–357C and at atmospheric pressure was carried out by conventional gasometric method by measuring the volume of oxygen liberated (24). The apparatus and procedure have been discussed in an earlier communication (24). The catalytic activity for CO oxidation in the temperature range 100–6007C was carried out in a quartz reactor by feeding a gas mixture of CO (5%), O2 (5%), and N2 balance over the catalyst bed packed with 0.5 g catalyst at a flow rate of 6000 cm3 h 01 . The products were analyzed with a on-line gas chromatograph using a Poropak-Q column. RESULTS AND DISCUSSION
The TG curve (Fig. 1) of the sample indicates continuous loss in weight with rise in temperature up to 10007C. A considerable weight loss (14.5%) in the temperature range 30–1357C accompanied by a prominent endothermic peak at 827C corresponds to loss of physisorbed water. The second stage weight loss ( Ç8%) in the range 135–4257C may be due to decomposition of structural water from hydrous iron oxide and hydrous aluminosilicate phases. Above 4507C, the dehydrated sample further undergoes weight loss ( Ç8%), presumably due to decomposition of oxide phases. The total weight loss at 9007C recorded from the TG (about 30%) curve agreed well with that obtained from the mass balance of calcined product at 9007C (isothermal calcination in the furnace). The XRD patterns of the air-dried manganese nodules and calcined products are illustrated in Fig. 2. It is seen that the sample dried at room temperature possesses a few diffuse and broad peaks which are characteristic of todorokite (9.8 ˚ ) and d-MnO2 (2.41 and 1.41 A ˚ ). The todorokite phase, A however, disappeared at 1107C without development of any new materials. It is interesting to note that the position, as
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FIG. 2. Powder XRD patterns of heat-treated manganese nodules: (1) air dried and (2) 200, (3) 300, (4) 500, (5) 700, and (6) 9007C.
well, broadness and the intensity of XRD peaks of calcined samples (110–4007C) remains practically unchanged, indicating no change in the structure of the material. The peaks ˚ indicate the presence of quartz ( a-SiO2 ) at 3.32 and 3.18 A and zeolite minerals, respectively. Calcination at §4007C resulted in the appearance of strong peaks at 2.92 and 2.53 ˚ due to formation of a-Fe2O3 from amorphous goethite ( aA FeOOH). Around this temperature, d-MnO2 also decomposes to a-Mn2O3 , as indicated by the appearance of distinct ˚ . This also corroborates to the TG-DTA obserline at 2.49 A vation. The sharpness and intensity of the characteristic peaks of a-Fe2O3 and a-Mn2O3 increase with increase in calcination temperature beyond 5007C due to improved crystallinity and size of the particles. The presence of a distinct ˚ in the calcined product and sharp reflection line at 2.33 A beyond 8007C confirms the transformation of Mn2O3 and Fe2O3 to Mn3O4 and Fe3O4 , respectively. The XRD patterns and peak positions of preheated nodules are in good agreement with those reported by earlier workers (12, 13). Figure 3 displays the FT-IR spectra (4600–400 cm01 ) of the heat-treated nodules. The infrared spectra of the nodules are rather complicated due to their complex matrix. The various absorption peaks are interpreted with special reference to iron, silicon, and manganese oxides/oxyhydroxides and also to water molecules. The presence of OH groups as well as H2O (both adsorbed molecular water within the crystal structures) at lower calcination temperature ( £6007C) is revealed by the characteristic absorption peaks at around 3400 and 1640 cm01 , respectively, as OH stretching and
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bending modes of vibrations (25). These peaks either diminished or disappeared with progressive increase in calcination temperature. Surprisingly there is no peak at 3400 cm01 for the sample heated at 1107C. Nodules, generally, exhibit a very strong absorption band at around 1020–1040 cm01 , presumably due to Si–O or Si–O–Al vibrations. This band broadens by 50 cm01 with a rise in calcination temperature, indicating that the silicates become more crystalline (13). Further broadening at ú6007C indicates lattice disorder. Appearance of a peak at around 570 cm01 with the sample heated above 4007C indicates the formation of a-Fe2O3 (26). A strong peak invariably present at around 460–470 cm01 may be assigned to smectite-type clay minerals or Mn{O vibrations (13). Table 1 shows that the values of the O/Mn ratio gradually decrease with increase in calcination temperature in the range 110–4007C and then sharply beyond 5007C. These values (O/Mn Å 1.73–1.81) are comparable with those reported by Pattan et al. (27) for central Indian Ocean manganese nodules. The O/Mn ratio in the range 1.76–1.93 indicates that the deep sea manganese nodules contain largely Mn(IV) (73.81%). In the temperature range 500–6007C the MnO2 is completely converted to Mn2O3 , as indicated by the O/Mn ratio (1.42–1.59). These values are in good agreement with the appearance of characteristic XRD peaks of Mn2O3 around this temperature. Further decrease of the O/Mn ratio beyond 5007C is attributed to partial conversion of Mn2O3 to Mn3O4 . A similar behavior has also been reported for synthetic MnO2 by Kanungo et al. (28). The
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FIG. 4. Variation of surface acidity and surface hydroxyl group with respect to calcination temperature: 1, 2, and 3 are surface acidity values measured with 2,6-dimethylpyridine, 2,6-dimethylpiperidine, and 2,6-dimethylmorpholine, respectively.
FIG. 3. FT-IR spectra of heat-treated manganese nodules: (1) 110, (2) 200, (3) 300, (4) 400, (5) 500, (6) 600, (7) 700, (8) 800 and (9) 9007C.
various surface properties of heat-treated samples are summarized in Table 1. The surface oxygen increases with increase in calcination temperature, attains a maximum at around 4007C, and then decreases up to 9007C. The surface acidity and surface OH groups are, however, decreased progressively with a rise in calcination temperature (see Fig. 4). It is seen from the figure that the values of surface acidity
do not differ much although the amines with large pKa differences are used for its determination, indicating thereby that the manganese nodules largely contain the stronger acid sites (Brønsted type). It is well known that the Brønsted acidity is resulted due to the presence of surface OH groups. Thus, the decrease in surface OH groups due to dehydroxylation also causes a decrease in surface acidity values with a rise in calcination temperature (9). The surface area gradually increases with an increase in calcination temperature up to 4007C and then decreases very quickly as the temperature is raised further (cf. Table 1). An increase in surface area up to 4007C, presumably due to dehydration and dehydroxylation, resulted in higher N2 adsorption; the decrease in surface area beyond 4007C may result from the change in the crystal structure due to phase
TABLE 1 Effect of Calcination Temperature on Various Surface Properties and Electrical Conductivity of Manganese Nodules Sample number
Surface area (m2/g)
Surface oxygen (meq/g)
O/ Mn ratio
Weight loss (%)
N2 pore volume, 103 (cc/g)
Average pore ˚) diameter (A
Electrical conductivity, 107 1 s at 1007C, (ohm01 cm01)
Ea , eV
MN-110 MN-200 MN-300 MN-400 MN-500 MN-600 MN-700 MN-800 MN-900
103 101 113 125 60 11 5 2 1
0.175 0.150 0.200 0.255 0.100 0.042 0.031 0.010 0.006
1.94 1.93 1.81 1.76 1.59 1.42 1.44 1.43 1.38
15.2 21.5 22.8 25.3 26.4 27.9 28.7 29.0 29.7
111 94 99 109 53 14.5 6.6 3.8 1.2
42.9 37.2 35.2 34.9 42.1 52.8 57.5 — —
21.1 21.3 15.9 60.2 73.4 2.64 2.17 0.75 0.23
0.45 0.31 0.25 0.21 0.21 0.42 0.48 0.51 0.55
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FIG. 5. (a, b) Adsorption–desorption isotherms of heat-treated manganese nodules.
transformation and destruction of fine pores at higher temperatures. Weisz et al. (5) and Han et al. (12) have also observed a decrease in surface area with the heating ( §4007C) of the manganese nodules of other regions. In order to find the effect of temperature on pore diameter, pore volume, and pore size distribution we have studied the complete nitrogen adsorption–desorption isotherms of different heat-treated samples. Figures 5a and 5b represent N2 adsorption isotherms at liquid nitrogen temperature of a few samples. The hysteresis (either of type II or IV) occurs at a higher pressure region for samples heated at lower temperature ( £7007C) and indicates capillary condensation and hence mesopores. At higher temperature ( ú8007C) the straight line plots of Vads versus t observed up to P/P0 á 0.9, indicates the absence of porosity and seems to be intraparticle condensation (29, 30). Assuming the pores to be cylindrical and using the relationship r Å 2 (Vp /Sp ), where r is the average pore radius, Vp is the volume of the pores, and Sp is the specific internal surface area of the pores, the average pore diameter (Mn 0 110) is calculated to be 43 ˚ . It is worth noting that the predominant pore contributor A to the total pore volume shifts to larger pores with increase in calcination temperature ( £7007C, cf. Fig. 6). The pore size distribution curves show that predominant pores fall within the mesopore region. The cumulative pore volumes of heat-treated samples follow an exactly similar trend to that of surface area. Redon et al. (30) have also observed a similar trend with thermally treated goethite samples. A decrease in pore volume with a rise in temperature has been observed by Han et al. (12) due to closing of fine pores.
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FIG. 6. Effect of calcination temperature on pore size distribution of manganese nodules.
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TABLE 2 Kinetic Parameters for the Decomposition of Aqueous H2O2 over Manganese Nodules Samples, Catalyst, 20 mg, H2O2 , 0.02 M Rate constant (104 1 k1 s01) Sample number
15
25
357C
Ea (k cal/mol)
MN-110 MN-200 MN-300 MN-400 MN-500 MN-600 MN-700 MN-800 MN-900
9.0 8.56 12.9 15.5 9.9 1.04 0.29 — —
19.4 19.6 22.8 26.2 17.8 1.94 0.88 — —
33.9 34.8 39.2 43.2 31.9 3.47 2.27 1.91 0.75
13.0 13.3 10.1 9.1 10.8 14.7 18.7 — —
These values are slightly lower than those reported for pure MnO2 (0.8–1.4 eV) (22). Catalytic Activity
FIG. 7. Plots of log s versus inverse temperature.
Electrical Conductivity The electrical conductivity data for the heat-treated samples are shown in Fig. 7 as log s versus the reciprocal of temperature. It is interesting to note that these plots are linear with a break particularly at lower temperature ( õ1007C). The values of electrical conductivity at a particular temperature increase with calcination temperature, attain a maxima at around 5007C, and then decrease (Table 1). This increase in electrical conductivity is attributed to the dehydration and dehydroxylation of hydrous iron oxides and aluminosilicates, which in turn enhance the mobility of electrons. Above 5007C the conductivity decreases, most likely due to transformation of Mn2O3 and Fe2O3 to less conducting oxides, Mn3O4 and Fe3O4 , respectively. A semiconductor-type behavior is observed for all the samples: s continously increases with temperature in the 40–6007C range. The data can be represented by the general expression s Å A exp( 0 Ea /kT ),
[1]
where A is both a charge carrier concentration and material constant, T is the absolute temperature, Ea is the activation energy, and k is Boltzman’s constant. The activation energy (Ea ) of the heat-treated samples is presented in Table 1.
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The rate data and activation parameters for the decomposition of H2O2 on manganese nodules are presented in Table 2. The linear plots (Fig. 8) of the log of the initial rate of decomposition versus log[H2O2 ] with almost unit slopes indicate that the catalytic decomposition of H2O2 follows a first-order kinetics. Ahuja et al. (22) have also found a similar observation with pure MnO2 . The validity of first-order kinetics is further evident from the straight line plots of log (V` 0 Vt ) versus time, where Vt is the volume of oxygen liberated at time t and V` is the final volume of oxygen
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FIG. 8. log–log plot of initial rate of decomposition of H2O2 versus molar concentration of H2O2 .
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FIG. 9. Percentage of CO conversion over various heat-treated manganese nodules versus temperature at constant loading. The numbers 1, 2, 3, 4, 5, 6, 7, 8, and 9 represent samples heated at 110, 200, 300, 400, 500, 600, 700, 800, and 9007C, respectively.
liberated when all H2O2 has been decomposed. At lower temperature (157C), however, the first-order plot is found to be linear for about 75% of the total reaction period. Since all the samples follow a first-order kinetics, the specific rate constants are directly taken to correlate various surface properties with the catalytic activity for H2O2 decomposition. Further, the autodecomposition of H2O2 under the experimental conditions is negligibly small and, as such, the rate constant represents the rate of decomposition via a catalyzed path only. It is found that the specific rate constants for H2O2 decomposition and surface oxygen follow a similar trend: they increase with an increase in
FIG. 10. Effect of calcination temperature on catalytic activity of manganese nodules for H2O2 decomposition and CO oxidation.
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calcination temperature up to 4007C and then decrease on further increase in calcination temperature up to 9007C. The lowest activity of the sample calcined at 9007C is partially due to transformation of iron and manganese oxides to catalytically less active oxides, viz. Fe 3O4 and Mn3O4 . A similar behavior has been reported by others ( 20, 22 ) in the case of pure MnO2 . The values of apparent activation energy vary from 9 – 20 kcal /mol and compare well with the same reported for H 2O2 decomposition catalyzed by MnO2 ( 20, 22 ) . The high catalytic activity of the sample calcined at 4007C is presumably due to higher surface area, surface oxygen, and also to the existence of a suitable Mn 3/ – Mn 4/ couple. Mooi et al. ( 31 ) and Kanungo et al. ( 20 ) have shown that a favorable couple of Mn 3/ and Mn 4/ is necessary for the catalytic decomposition of H2O2 through electron exchange. It is observed that Ea for electrical conductivity follows a parallel relation with Ea for catalytic decomposition of H2O2 , indicating thereby that the species which are responsible for the conductivity are also the ones which partially contribute the active sites for decomposition reactions. The catalytic activities of heat-treated manganese nodules for CO oxidation at various temperature ( 100 – 6007C ) are presented in Fig. 9. The percentage of CO conversion as a function of calcination temperature is illustrated in Fig. 10. Similar to catalytic decomposition of H2O2 , the CO oxidation activity increases gradually with an increase in calcination temperature, but decreases sharply at higher calcination temperature, showing a maxima at 4007C. The drastic change in catalytic activity beyond 4007C might be related
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to the changes in surface area, surface oxygen, oxide phases, etc., which in turn reduced the number of active sites for the reaction. CONCLUSIONS
From the foregoing discussion it is clear that the nodules, due to the presence of ultrafine pores, contain a fair amount of water at normal conditions of temperature and pressure. The physically and chemically adsorbed water molecules evaporate continuously with a rise in temperature up to 400– 5007C. Heating nodules up to 4007C does not effect the phases of major components but transformed them into higher oxides at higher temperature. Distribution of pore size and pore volume practically remained unaffected at lower calcination temperatures but were influenced at higher temperatures. The smaller pores coalesce to form larger ones at higher temperatures. The surface acidity, which is mainly due to surface OH groups, decreased progressively with an increase in calcination temperature. On the other hand, the surface area, surface oxygen, electrical conductivity, and catalytic activity increased up to 4007C and then decreased sharply at higher calcination temperature. Thus, the porous texture and other surface properties of polymetallic oceanic nodules were retained up to relatively high temperatures, which allows one to consider their use as catalysts for several oxidation reactions as well as hydrodemetallization at õ4007C. ACKNOWLEDGMENTS The authors are thankful to Professor H. S. Ray, Director, Regional Research Laboratory, for his encouragement and permission to publish this paper. The financial assistance from the Department of Ocean Development, New Delhi, is gratefully acknowledged.
REFERENCES 1. Fuerstenau, D. W., and Han, K. N., Miner. Process. Technol. Rev. 1, 1 (1983).
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2. Ghosh, A. K., and Mukhopadhaya, R., Ind. J. Marine Sci. 20, 17 (1991). 3. Purnachandra Rao, V., Marine Geol. 74, 151 (1987), and references therein. 4. Momoi, H., Nakamoto, M., and Kamata, K., Miner. Geol. 42, 155 (1992). 5. Weisz, F., and Paul, B., J. Catal. 10, 407 (1968). 6. Farrel, E. E., Nicks, E. J., and Bauer, D. J., U.S. Bureau of Mines, Rep. Invest. R. 18330, 1979. 7. Caberra, A. L., Mapple, M. B., and Arrhenius, G., Appl. Catal. 64, 309 (1990). 8. Mastsuo, K., Nitta, M., and Aomura, K., Sekiya Gakkishi 22, 212 (1979). 9. Parida, K. M., Satapathy, P. K., Sahoo, A. K., and Das, N. N., J. Colloid Interface Sci. 173, 112 (1995). 10. Dodet, C., Noville, F., Crine, M., Marchot, P., and Pirard, J. P., Colloids Surf. 11, 187 (1984), and references therein. 11. Aizawa, R., Inoue, K., Koinuma, Y., Kushiyama, S., Kobayashi, S., Shimizu, Y., and Egi, K., Kogai 24, 169 (1989); CA 13, 194579r (1990). 12. Han, K. N., Hoover, M. P., and Fuerstenue, D. W., Marine Miner. 2, 139 (1979). 13. Mohapatra, B. K., Mishra, S. K., and Sahoo, R. K., Thermochim. Acta 145, 33 (1989). 14. Johnson, O., J. Phys. Chem. 59, 827 (1955). 15. Campelo, J. M., Garcia, A., Gutierrez, J. M., Luna, D., and Marinas, J. M., J. Colloid. Interface Sci. 95, 544 (1983). 16. Pritchard, J. G., and Long, F. A., J. Am. Chem. Soc. 79, 2365 (1957). 17. Uchijima, T., Takahashi, M., and Yoneda, Y., J. Catal. 9, 102 (1967). 18. Laragne, J. J., and Brenet, J., Bull. Soc. Chim. (France) 9, 3499 (1968). 19. Parida, K. M., Kanungo, S. B., and Sant, B. R., Electrochim. Acta 26, 435 (1981). 20. Kanungo, S. B., J. Catal. 58, 419 (1979). 21. Kramorz, W., and Zajecki, J., Pol. J. Chem. 54, 2075 (1980). 22. Ahuja, L. D., Rajeswar, D., and Nagpal, K. C., J. Colloid Interface Sci. 119, 481 (1987). 23. Taguchi, H., Naagao, M., and Shimadu, M., J. Solid State Chem. 92, 227 (1991). 24. Kanungo, S. B., Parida, K. M., and Sant, B. R., Electrochim. Acta 26, 1157 (1981). 25. Nakamato, K., ‘‘Infrared Spectra of Inorganic and Organic Coordination Compounds,’’ p. 107. Wiley, New York, 1963. 26. Kauffman, K., and Bazel, F., J. Inorg. Nucl. Chem. 37, 1139 (1975). 27. Pattan, J. N., and Mudholkar, A. V., Chem. Geol. 85, 171 (1990). 28. Parida, K. M., and Kanungo, S. B., Thermochim. Acta 64, 131 (1983). 29. Fierro, J. L. G., Mendioroz, S., and Olivan, A. M., J. Colloid Interface Sci. 100, 303 (1984). 30. Rendon, J. L., Cornejo, J., Arambarri, P. D., and Serna, C. J., J. Colloid Interface Sci. 92, 508 (1983). 31. Mooi, J., and Selowood, P. W., J. Am. Chem. Soc. 72, 4333 (1950).
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Studies on Indian Ocean Manganese Nodules PART 2. Physico-chemical Characteristics and Catalytic Activity of Heat-Treated Marine Manganese Nodules K. M. PARIDA, 1 P. K. SATAPATHY, N. N. DAS,
AND
S. B. RAO
Regional Research Laboratory, CSIR, Bhubaneswar 751 013, India Received June 9, 1995; accepted September 11, 1995
The effect of calcination temperature on the physico-chemical characteristics and catalytic activity of central Indian Basin manganese nodules have been investigated. TG, XRD, IR, and chemical analysis confirm the presence of amorphous oxyhydroxides of iron and d-MnO2 or todorokite, which convert to a-Fe2O3 and gMn2O3 or Mn3O4 phases at §4007C of calcination. The pore volume, average pore diameter, and pore size distribution practically remain unaffected up to 4007C of heat treatment. But in the temperature range 400 to 7007C, smaller pores coalesce to form larger ones and beyond 8007C the material becomes practically nonporous. The surface hydroxyl group and surface acidity are progressively decreased with a rise in calcination temperature. The surface area, surface oxygen, electrical conductivity, as well as catalytic activity for H2O2 decomposition and CO oxidation, however, follow a similar trend: a gradual increase up to 4007C of calcination and then a decrease with further increase in temperature of calcination. q 1996 Academic Press, Inc. Key Words: surface characteristics; catalytic activity; manganese nodules.
monoxide and hydrocarbon, methanation of CO, reduction of nitrogen oxides (5–9), demetallization, and desulphurization of topped petroleum crude in the presence of hydrogen (10, 11) are promising. As most of the heterogeneous catalytic reactions take place at elevated temperatures and pressures, it is important and necessary to know the possible changes in the physicochemical properties and hence the catalytic activity due to thermal treatment. Although it is clear that oceanic nodules have unique physico-chemical characteristics which have very significant effects on the catalytic activity, there has been only limited investigations (5, 12, 13) regarding the effect of heat treatment on the surface and textural properties. Also, no work has been carried out so far to correlate the physico-chemical properties and the catalytic activity of heat-treated samples. The present work investigates the effect of temperature on various physico-chemical characteristics, electrical conductivity, and catalytic activity of manganese nodules of the central Indian Ocean basin. EXPERIMENTAL
INTRODUCTION
Studies on deep sea manganese nodules have been subject of great interest to mineralogists, sedimentologists, and metallurgists due to their rich composition of metals such as manganese and iron and, especially Ni, Cu, Co, etc. (1–4). The effectiveness of these materials as catalysts for certain chemical reactions has attracted the attention of a number of workers because they exhibit quite remarkable features such as: (a) they occur in great abundance both in marine and fresh water sediments; (b) they possess high porosity ˚ ), and relatively (av. 62%) ultrafine grain size (av. 100 A 2 high surface area (100–300 m /g); and (c) they contain largely d-MnO2 and a-FeOOH along with a variety of transition metal ions in the interlayers and framework positions. Among many catalytic applications, oxidation of carbon 1
To whom correspondence should be addressed.
Sample Preparation Manganese nodules of the central Indian Ocean basin were obtained from the National Institute of Oceanography, Goa, India, and were collected during cruises of the Fernela F7. The samples were air dried, powdered, and sieved. The sieved fractions 075 / 45 mm were collected for further studies. About 10 g of sample was heated at each temperature (110–9007C) in a muffle furnace with a suitable controller to maintain offset temperatures with {57C. The samples were kept at the desired temperature for 3 h, cooled to about 507C, and then stored in a desiccator over fused CaCl2 . The samples thus prepared were labeled Mn–T, where T denotes the temperature of heat treatment in degrees Celsius. Characterization TG-DTA of the samples (about 15 mg) was carried out using a Shimadzu DT-40 model automatic thermal analyser in
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the temperature range 30–10007C at a heating rate of 107C min 01 . X-ray diffraction patterns of heat-treated samples were recorded on a Philips semiautomatic X-ray diffractometer with auto divergent slit and graphite monochromator using CuKa radiation operated at 40 kV and 20 mA. The FT-IR spectra (4000–400 cm01 ) of the samples in KBr phase were recorded using a JASCO Model 5300 spectrometer. The determination of surface acidity either by titration of solids (manganese nodules) suspended in benzene with nbutyl amine (14) or by spectrophotometric method using simple amines, (e.g., pyridine, cyclohexylamine) as adsorbate (15), could not be employed in this case due to partial solubility of some metal oxides in amines. As such, noninteracting amines (16), 2,6-dimethylpyridine (pka 6.9), 2,6-dimethylpiperidine (pka 11.11), and 2,6-dimethylmorpholine (pka 11.12) were used as adsorbates. In this method, freshly prepared amines (0.01 M) in cyclohexane were pipetted out into 100-ml stoppered conical flasks containing 0.2 g of nodules. The flasks were shaken for 1 h and then the contents were filtered. The absorbance of the filtrate was measured at preset wavelengths. The acid sites were calculated from the amount of substrate adsorbed on the surface. Other details have been described in literature (15). Surface excess oxygen and surface hydroxyl groups were determined by following reported methods (17, 18). Total Mn, Mn 2/ , Mn 3/ , and Mn 4/ were estimated as described earlier (19). Active oxygen (or available oxygen) was determined by the oxalate method (20) and the values were used for the calculation of x in MnOx . The percentage of weight losses at various calcination temperature were also verified from the TG curves of the samples. Nitrogen adsorption– desorption experiments were carried out at 78 K with a Quantasorb (Quantachrome, USA) and were analyzed for distribution of pores, cumulative pore volumes, average pore diameter, and specific surface area. Surface area was also measured using a high-speed surface area analyzer (Model 2200A, Micromeritics, USA). Electrical Conductivity The electrical conductivity ( s ) of the heat-treated samples in the temperature range 40–6007C was calculated by measuring the electrical resistance across the sintered pellet (21– 23) using a Kethley programmable electrometer (Model 617). As the nodules were calcined at various temperatures, the electrical conductivity in some cases were measured inbetween room temperature and calcined temperature. For this measurement the powdered samples were compressed into pellets by applying constant pressure (9 tons/in 2 ) and were electroded with silver paste. The pellets were mounted between two copper blocks of the sample holder. The sample holder contained in a tubular furnace was heated using a suitable temperature controller. A chromel–alumel thermocouple was used as temperature probe.
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FIG. 1. TG-DTA of air-dried manganese nodules.
Catalytic Activity The catalytic activity of manganese nodules under investigation were measured using H2O2 decomposition and CO oxidation reactions. The catalytic decomposition of aqueous H2O2 (AR) in the temperature range 15–357C and at atmospheric pressure was carried out by conventional gasometric method by measuring the volume of oxygen liberated (24). The apparatus and procedure have been discussed in an earlier communication (24). The catalytic activity for CO oxidation in the temperature range 100–6007C was carried out in a quartz reactor by feeding a gas mixture of CO (5%), O2 (5%), and N2 balance over the catalyst bed packed with 0.5 g catalyst at a flow rate of 6000 cm3 h 01 . The products were analyzed with a on-line gas chromatograph using a Poropak-Q column. RESULTS AND DISCUSSION
The TG curve (Fig. 1) of the sample indicates continuous loss in weight with rise in temperature up to 10007C. A considerable weight loss (14.5%) in the temperature range 30–1357C accompanied by a prominent endothermic peak at 827C corresponds to loss of physisorbed water. The second stage weight loss ( Ç8%) in the range 135–4257C may be due to decomposition of structural water from hydrous iron oxide and hydrous aluminosilicate phases. Above 4507C, the dehydrated sample further undergoes weight loss ( Ç8%), presumably due to decomposition of oxide phases. The total weight loss at 9007C recorded from the TG (about 30%) curve agreed well with that obtained from the mass balance of calcined product at 9007C (isothermal calcination in the furnace). The XRD patterns of the air-dried manganese nodules and calcined products are illustrated in Fig. 2. It is seen that the sample dried at room temperature possesses a few diffuse and broad peaks which are characteristic of todorokite (9.8 ˚ ) and d-MnO2 (2.41 and 1.41 A ˚ ). The todorokite phase, A however, disappeared at 1107C without development of any new materials. It is interesting to note that the position, as
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FIG. 2. Powder XRD patterns of heat-treated manganese nodules: (1) air dried and (2) 200, (3) 300, (4) 500, (5) 700, and (6) 9007C.
well, broadness and the intensity of XRD peaks of calcined samples (110–4007C) remains practically unchanged, indicating no change in the structure of the material. The peaks ˚ indicate the presence of quartz ( a-SiO2 ) at 3.32 and 3.18 A and zeolite minerals, respectively. Calcination at §4007C resulted in the appearance of strong peaks at 2.92 and 2.53 ˚ due to formation of a-Fe2O3 from amorphous goethite ( aA FeOOH). Around this temperature, d-MnO2 also decomposes to a-Mn2O3 , as indicated by the appearance of distinct ˚ . This also corroborates to the TG-DTA obserline at 2.49 A vation. The sharpness and intensity of the characteristic peaks of a-Fe2O3 and a-Mn2O3 increase with increase in calcination temperature beyond 5007C due to improved crystallinity and size of the particles. The presence of a distinct ˚ in the calcined product and sharp reflection line at 2.33 A beyond 8007C confirms the transformation of Mn2O3 and Fe2O3 to Mn3O4 and Fe3O4 , respectively. The XRD patterns and peak positions of preheated nodules are in good agreement with those reported by earlier workers (12, 13). Figure 3 displays the FT-IR spectra (4600–400 cm01 ) of the heat-treated nodules. The infrared spectra of the nodules are rather complicated due to their complex matrix. The various absorption peaks are interpreted with special reference to iron, silicon, and manganese oxides/oxyhydroxides and also to water molecules. The presence of OH groups as well as H2O (both adsorbed molecular water within the crystal structures) at lower calcination temperature ( £6007C) is revealed by the characteristic absorption peaks at around 3400 and 1640 cm01 , respectively, as OH stretching and
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bending modes of vibrations (25). These peaks either diminished or disappeared with progressive increase in calcination temperature. Surprisingly there is no peak at 3400 cm01 for the sample heated at 1107C. Nodules, generally, exhibit a very strong absorption band at around 1020–1040 cm01 , presumably due to Si–O or Si–O–Al vibrations. This band broadens by 50 cm01 with a rise in calcination temperature, indicating that the silicates become more crystalline (13). Further broadening at ú6007C indicates lattice disorder. Appearance of a peak at around 570 cm01 with the sample heated above 4007C indicates the formation of a-Fe2O3 (26). A strong peak invariably present at around 460–470 cm01 may be assigned to smectite-type clay minerals or Mn{O vibrations (13). Table 1 shows that the values of the O/Mn ratio gradually decrease with increase in calcination temperature in the range 110–4007C and then sharply beyond 5007C. These values (O/Mn Å 1.73–1.81) are comparable with those reported by Pattan et al. (27) for central Indian Ocean manganese nodules. The O/Mn ratio in the range 1.76–1.93 indicates that the deep sea manganese nodules contain largely Mn(IV) (73.81%). In the temperature range 500–6007C the MnO2 is completely converted to Mn2O3 , as indicated by the O/Mn ratio (1.42–1.59). These values are in good agreement with the appearance of characteristic XRD peaks of Mn2O3 around this temperature. Further decrease of the O/Mn ratio beyond 5007C is attributed to partial conversion of Mn2O3 to Mn3O4 . A similar behavior has also been reported for synthetic MnO2 by Kanungo et al. (28). The
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FIG. 4. Variation of surface acidity and surface hydroxyl group with respect to calcination temperature: 1, 2, and 3 are surface acidity values measured with 2,6-dimethylpyridine, 2,6-dimethylpiperidine, and 2,6-dimethylmorpholine, respectively.
FIG. 3. FT-IR spectra of heat-treated manganese nodules: (1) 110, (2) 200, (3) 300, (4) 400, (5) 500, (6) 600, (7) 700, (8) 800 and (9) 9007C.
various surface properties of heat-treated samples are summarized in Table 1. The surface oxygen increases with increase in calcination temperature, attains a maximum at around 4007C, and then decreases up to 9007C. The surface acidity and surface OH groups are, however, decreased progressively with a rise in calcination temperature (see Fig. 4). It is seen from the figure that the values of surface acidity
do not differ much although the amines with large pKa differences are used for its determination, indicating thereby that the manganese nodules largely contain the stronger acid sites (Brønsted type). It is well known that the Brønsted acidity is resulted due to the presence of surface OH groups. Thus, the decrease in surface OH groups due to dehydroxylation also causes a decrease in surface acidity values with a rise in calcination temperature (9). The surface area gradually increases with an increase in calcination temperature up to 4007C and then decreases very quickly as the temperature is raised further (cf. Table 1). An increase in surface area up to 4007C, presumably due to dehydration and dehydroxylation, resulted in higher N2 adsorption; the decrease in surface area beyond 4007C may result from the change in the crystal structure due to phase
TABLE 1 Effect of Calcination Temperature on Various Surface Properties and Electrical Conductivity of Manganese Nodules Sample number
Surface area (m2/g)
Surface oxygen (meq/g)
O/ Mn ratio
Weight loss (%)
N2 pore volume, 103 (cc/g)
Average pore ˚) diameter (A
Electrical conductivity, 107 1 s at 1007C, (ohm01 cm01)
Ea , eV
MN-110 MN-200 MN-300 MN-400 MN-500 MN-600 MN-700 MN-800 MN-900
103 101 113 125 60 11 5 2 1
0.175 0.150 0.200 0.255 0.100 0.042 0.031 0.010 0.006
1.94 1.93 1.81 1.76 1.59 1.42 1.44 1.43 1.38
15.2 21.5 22.8 25.3 26.4 27.9 28.7 29.0 29.7
111 94 99 109 53 14.5 6.6 3.8 1.2
42.9 37.2 35.2 34.9 42.1 52.8 57.5 — —
21.1 21.3 15.9 60.2 73.4 2.64 2.17 0.75 0.23
0.45 0.31 0.25 0.21 0.21 0.42 0.48 0.51 0.55
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FIG. 5. (a, b) Adsorption–desorption isotherms of heat-treated manganese nodules.
transformation and destruction of fine pores at higher temperatures. Weisz et al. (5) and Han et al. (12) have also observed a decrease in surface area with the heating ( §4007C) of the manganese nodules of other regions. In order to find the effect of temperature on pore diameter, pore volume, and pore size distribution we have studied the complete nitrogen adsorption–desorption isotherms of different heat-treated samples. Figures 5a and 5b represent N2 adsorption isotherms at liquid nitrogen temperature of a few samples. The hysteresis (either of type II or IV) occurs at a higher pressure region for samples heated at lower temperature ( £7007C) and indicates capillary condensation and hence mesopores. At higher temperature ( ú8007C) the straight line plots of Vads versus t observed up to P/P0 á 0.9, indicates the absence of porosity and seems to be intraparticle condensation (29, 30). Assuming the pores to be cylindrical and using the relationship r Å 2 (Vp /Sp ), where r is the average pore radius, Vp is the volume of the pores, and Sp is the specific internal surface area of the pores, the average pore diameter (Mn 0 110) is calculated to be 43 ˚ . It is worth noting that the predominant pore contributor A to the total pore volume shifts to larger pores with increase in calcination temperature ( £7007C, cf. Fig. 6). The pore size distribution curves show that predominant pores fall within the mesopore region. The cumulative pore volumes of heat-treated samples follow an exactly similar trend to that of surface area. Redon et al. (30) have also observed a similar trend with thermally treated goethite samples. A decrease in pore volume with a rise in temperature has been observed by Han et al. (12) due to closing of fine pores.
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FIG. 6. Effect of calcination temperature on pore size distribution of manganese nodules.
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TABLE 2 Kinetic Parameters for the Decomposition of Aqueous H2O2 over Manganese Nodules Samples, Catalyst, 20 mg, H2O2 , 0.02 M Rate constant (104 1 k1 s01) Sample number
15
25
357C
Ea (k cal/mol)
MN-110 MN-200 MN-300 MN-400 MN-500 MN-600 MN-700 MN-800 MN-900
9.0 8.56 12.9 15.5 9.9 1.04 0.29 — —
19.4 19.6 22.8 26.2 17.8 1.94 0.88 — —
33.9 34.8 39.2 43.2 31.9 3.47 2.27 1.91 0.75
13.0 13.3 10.1 9.1 10.8 14.7 18.7 — —
These values are slightly lower than those reported for pure MnO2 (0.8–1.4 eV) (22). Catalytic Activity
FIG. 7. Plots of log s versus inverse temperature.
Electrical Conductivity The electrical conductivity data for the heat-treated samples are shown in Fig. 7 as log s versus the reciprocal of temperature. It is interesting to note that these plots are linear with a break particularly at lower temperature ( õ1007C). The values of electrical conductivity at a particular temperature increase with calcination temperature, attain a maxima at around 5007C, and then decrease (Table 1). This increase in electrical conductivity is attributed to the dehydration and dehydroxylation of hydrous iron oxides and aluminosilicates, which in turn enhance the mobility of electrons. Above 5007C the conductivity decreases, most likely due to transformation of Mn2O3 and Fe2O3 to less conducting oxides, Mn3O4 and Fe3O4 , respectively. A semiconductor-type behavior is observed for all the samples: s continously increases with temperature in the 40–6007C range. The data can be represented by the general expression s Å A exp( 0 Ea /kT ),
[1]
where A is both a charge carrier concentration and material constant, T is the absolute temperature, Ea is the activation energy, and k is Boltzman’s constant. The activation energy (Ea ) of the heat-treated samples is presented in Table 1.
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The rate data and activation parameters for the decomposition of H2O2 on manganese nodules are presented in Table 2. The linear plots (Fig. 8) of the log of the initial rate of decomposition versus log[H2O2 ] with almost unit slopes indicate that the catalytic decomposition of H2O2 follows a first-order kinetics. Ahuja et al. (22) have also found a similar observation with pure MnO2 . The validity of first-order kinetics is further evident from the straight line plots of log (V` 0 Vt ) versus time, where Vt is the volume of oxygen liberated at time t and V` is the final volume of oxygen
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FIG. 8. log–log plot of initial rate of decomposition of H2O2 versus molar concentration of H2O2 .
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FIG. 9. Percentage of CO conversion over various heat-treated manganese nodules versus temperature at constant loading. The numbers 1, 2, 3, 4, 5, 6, 7, 8, and 9 represent samples heated at 110, 200, 300, 400, 500, 600, 700, 800, and 9007C, respectively.
liberated when all H2O2 has been decomposed. At lower temperature (157C), however, the first-order plot is found to be linear for about 75% of the total reaction period. Since all the samples follow a first-order kinetics, the specific rate constants are directly taken to correlate various surface properties with the catalytic activity for H2O2 decomposition. Further, the autodecomposition of H2O2 under the experimental conditions is negligibly small and, as such, the rate constant represents the rate of decomposition via a catalyzed path only. It is found that the specific rate constants for H2O2 decomposition and surface oxygen follow a similar trend: they increase with an increase in
FIG. 10. Effect of calcination temperature on catalytic activity of manganese nodules for H2O2 decomposition and CO oxidation.
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calcination temperature up to 4007C and then decrease on further increase in calcination temperature up to 9007C. The lowest activity of the sample calcined at 9007C is partially due to transformation of iron and manganese oxides to catalytically less active oxides, viz. Fe 3O4 and Mn3O4 . A similar behavior has been reported by others ( 20, 22 ) in the case of pure MnO2 . The values of apparent activation energy vary from 9 – 20 kcal /mol and compare well with the same reported for H 2O2 decomposition catalyzed by MnO2 ( 20, 22 ) . The high catalytic activity of the sample calcined at 4007C is presumably due to higher surface area, surface oxygen, and also to the existence of a suitable Mn 3/ – Mn 4/ couple. Mooi et al. ( 31 ) and Kanungo et al. ( 20 ) have shown that a favorable couple of Mn 3/ and Mn 4/ is necessary for the catalytic decomposition of H2O2 through electron exchange. It is observed that Ea for electrical conductivity follows a parallel relation with Ea for catalytic decomposition of H2O2 , indicating thereby that the species which are responsible for the conductivity are also the ones which partially contribute the active sites for decomposition reactions. The catalytic activities of heat-treated manganese nodules for CO oxidation at various temperature ( 100 – 6007C ) are presented in Fig. 9. The percentage of CO conversion as a function of calcination temperature is illustrated in Fig. 10. Similar to catalytic decomposition of H2O2 , the CO oxidation activity increases gradually with an increase in calcination temperature, but decreases sharply at higher calcination temperature, showing a maxima at 4007C. The drastic change in catalytic activity beyond 4007C might be related
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to the changes in surface area, surface oxygen, oxide phases, etc., which in turn reduced the number of active sites for the reaction. CONCLUSIONS
From the foregoing discussion it is clear that the nodules, due to the presence of ultrafine pores, contain a fair amount of water at normal conditions of temperature and pressure. The physically and chemically adsorbed water molecules evaporate continuously with a rise in temperature up to 400– 5007C. Heating nodules up to 4007C does not effect the phases of major components but transformed them into higher oxides at higher temperature. Distribution of pore size and pore volume practically remained unaffected at lower calcination temperatures but were influenced at higher temperatures. The smaller pores coalesce to form larger ones at higher temperatures. The surface acidity, which is mainly due to surface OH groups, decreased progressively with an increase in calcination temperature. On the other hand, the surface area, surface oxygen, electrical conductivity, and catalytic activity increased up to 4007C and then decreased sharply at higher calcination temperature. Thus, the porous texture and other surface properties of polymetallic oceanic nodules were retained up to relatively high temperatures, which allows one to consider their use as catalysts for several oxidation reactions as well as hydrodemetallization at õ4007C. ACKNOWLEDGMENTS The authors are thankful to Professor H. S. Ray, Director, Regional Research Laboratory, for his encouragement and permission to publish this paper. The financial assistance from the Department of Ocean Development, New Delhi, is gratefully acknowledged.
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