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Characterization of precious metal particles prepared using chitosan as a protective agent
15
Colloids and Surfaces, 55 (1991) 15-21 Elsevier Science Publishers B.V., Amsterdam
Characterization of precious metal particles prepared using chitosan as a protective agent Norio Ishizuki, Kanjiro Torigoe, Kunio Esumi’ and Kenjiro Meguro Department University
of Applied
Chemistry
of Tokyo, Kagurazaka,
and Institute
of Colloid and Interface
Shinjuku-ku,
Tokyo 162 (Japan)
Science,
Science
(Received 12 April 1990; accepted 10 July 1990)
Abstract Precious metal particles (Au, Pt, Pd, Rh) were prepared using chitosan as a protective agent. The particle sizes decreased with increasing amounts of chitosan, while the dispersion stability against electrolyte increased. Similarly, stable bimetallic particles of Au-Pd and Au-Rh were obtained and their particle sizes were almost independent of feed composition. Furthermore, these bimetallic particles exhibited characteristic properties for the decomposition of hydrogen peroxide and hydrogenation of methylvinylketone.
INTRODUCTION
Preparation of precious metal particles by reducing metal salts has been studied extensively [l-4]. In aqueous solution, precious metal particles have a tendency to flocculate and precipitate due to their hydrophobicity. To disperse the particles in aqueous solution, it is necessary to use protective agents [ 5-81, such as chelating agents, surfactants, and hydrophilic polymers. They adsorb on the surface of the particles and stabilize the particles in the dispersed state. Thus, protective agents have a very important role in obtaining a stable dispersion. Chitin, poly-/3- (1,4) -N-acetyl-D-glucosamine, is a cellulose-like biopolymer widely distributed in nature, especially in marine invertebrates, insects, fungi, and yeasts. Chitin also has unique properties, including toughness, bioactivity, and biodegradability. Recent application research [9,10] has largely been focused on chitosan, which is deacetylated chitin, because the free amino groups in this modified product contribute polycationic, chelating, and film-forming properties, along with ready solubility in dilute acetic acid. In particular, partially deacetylated chitin is water soluble, and is similar in behavior to hydrophilic polymers that adsorb on metal particle surfaces and form complexes. ‘To whom correspondence should be addressed.
0166-6622/91/$03.50
0 1991 -
Elsevier Science Publishers B.V.
16
In this work, various precious metal particles were prepared using chitosan as a protective agent and the particles were characterized by measuring particle size, dispersion stability, and catalytic activity. EXPERIMENTAL
Materials Aqueous chitosan was supplied by Ajinomoto Co., Ltd. The molecular weight of chitosan determined by a light scattering method was about 200 000. Chloroauric acid and chloroplatinic acid were obtained from Tanaka Kikinzoku Kogyo K.K. Methylvinylketone (MVK) was provided by Tokyo Kasei Co. Ltd. Palladium chloride, rhodium chloride, hydrazine and other chemicals were extra pure grade from Wako Pure Chemical Industries, Ltd, and used as received. Water used in this experiment was deionized by a Mini-Q water purification system. Preparation of precious metal particles All glassware was washed with aqua regia. Reducing agents, hydrazine monohydrate ( 1 cm3 ) dissolved in water (9 cm3 ) , formaldehyde ( 1 cm3 ) dissolved in 1 M potassium hydroxide (2 cm3), and pentaerythritol (1 g) dissolved in water (5 cm3) were made up just before use. A 1 cm3 sample of each metal salt 12.5 mmol dmp3 in aqueous solution and aqueous chitosan were mixed, and diluted to 49.7 cm3 with water, The solution was gently heated with a heating mantle and refluxed. After the solution boiled, 0.3 cm3 of reducing agent was added dropwise to the boiling solution. As soon as the color of the solution changed, it was cooled in an ice bath to terminate the reaction. Bimetallic particles were prepared using the same procedure as that described above. Here, the ratios of the respective salts were changed in a total volume of 1 cm3 in aqueous solution. Measurement Particle sizes were determined by a transmission electron microscope (Hitachi model H-800): the particle sizes were measured directly from the enlarged electron micrographs. The accuracy was ? 0.2 nm. Electron probe micro analysis (EPMA) [ 111 was performed with a Shimadzu Model EHAX-2001. UV-vis absorption spectra of suspensions were measured with a Hitachi Model 220 A spectrophotometer. The suspension containing metal particles (10 cm3) and 10 wt% sodium chloride ( 1 cm3) were mixed and they were centrifuged at 2000 rpm for 5 min.
17
The stability of the suspension was estimated as the absorbance ratio of supernatant and blank measured at 530 nm. A 1.5 wt% hydrogen peroxide solution (15 cm3) was diluted to 30 cm3 with 2 mol dmp3 H,SO,. The solution was placed in a thermostated water bath (40’ C ) . A bimetallic suspension of Au-Pt (0.1 cm3 ) was added to the solution to initiate decomposition of hydrogen peroxide. The reactant was titrated using 0.1 mol dmv3 KMnO,. Hydrogenation of MVK was carried out as follows. A 3.5 mg aliquot of MVK was diluted to 10 cm3 with water. The solution was placed in a thermostated heating mantle (60’ C ) and was saturated with hydrogen. Then, a Au-Pd suspension (1 cm3) was added to the solution to initiate the reaction. The reactant was measured by means of gas chromatography. RESULTS AND DISCUSSION
When formaldehyde, hydrazine or pentaerythritol was added to each metal salt dissolved in aqueous solution in the presence of chitosan, the reduction of metal salts was significantly affected by the kind of reducing agent used. Formaldehyde could reduce only HAuCl,, while both hydrazine and pentaerythritol reduced the four kinds of metal salts studied in this work. This result can be understood by considering the relative strengths of these reducing agents: as formaldehyde is a weak reducing agent compared to hydrazine and pentaerythritol, it cannot reduce H,PtCIG, PdC12, and RhCl,. Five systems provided well-dispersed metal particles: formaldehyde-HAuCl,, hydrazine-HAuCl,, hydrazine-PdCl,, hydrazine-RhCl,, and pentaerythritolRhCl,. In these systems, the particle sizes decreased with increasing amount of chitosan and their dispersion stability was dependent on their particle sizes. Figure 1 shows the particle sizes and dispersion stability of hydrazine-HAuCl, system as a function of feed concentration of chitosan. It is apparent that the particle size of Au particles decreases gradually with increasing amounts of chitosan and then becomes constant.On the other hand, the dispersion stability of Au particles was very low in solutions containing a smaller amount of chitosan, but above a critical amount of chitosan the dispersion stability increased sharply. These critical amounts of chitosan were almost the same for the five systems, around about 1*1O-3 wt%. However, among these five systems, there are important differences, so that, at the critical amount of each system, the particle size of Au is significantly larger than that of Pd and Rh. This suggests that chitosan as a protective agent is much more effective for Au particles than for Pd and Rh particles. Such protective action is expected to operate since chitosan is known [ 91 to interact strongly with metal ions or metals. Furthermore, in concentrations of 1. lo-’ wt%, all the particle sizes for the five systems ranged from 3 to 6 nm and the dispersion stability of these particles was scarcely affected by the electrolyte.
18 1.0
30
08
20 :
0.6
~ r -_ n 8
.-It; m IlJ T; .5 10 a
0.4
*
0.2
Cont. of Chitosan I wt%
Fig. 1. Change in particle size and dispersion stability of Au particles prepared by reduction of hydrazine as a function of feed concentration of chitosan.
Further, a preparation of bimetallic particles was carried out in the presence of 1*10-2 wt% chitosan. Here, hydrazine was used as a reducing agent for preparing bimetallic particles of Au-Pt, Au-Pd, and Au-Rh. When mixed salts in various proportions were reduced with hydrazine, a characteristic color change for each system was observed; the solution containing Au particles alone was a red color, and with increasing composition of Pt, Pd or Rh the solutions turned brown. To identify whether the particles obtained are bimetallic ones, the UV-vis spectra and EPMA measurements for the samples were made. As can be seen (Fig. Z), the spectra of 50Au50Pd were found to differ from those of their mixtures of respective samples, indicating that the samples prepared are not mixtures of monometallic particles but bimetallic ones. Similarly, the spectra of the other samples were different from those of the mixtures. Indeed, Michel and Schwartz [ll] also obtained a similar result for a Au-Pd system. The identification of the Au-Pt system could not be performed by means of UVvis spectra due to poor dispersion stability. Furthermore, the composition of the samples was determined by means of EPMA. The EPMA measurements show that the ratio of Au for the three systems is lower than that of the feed solution. In the Au-Pt system, the particle size increased gradually with increasing composition of platinum from 5 to 50 nm in diameter (Fig. 3). Similarly, as
19 0.6
I 480
Wave
I
,
560
640
I nm
length
Fig. 2. UV-vis spectra of Au-Pd system.
60.
08 E 40 c
0.6
w ‘iii 2 y :
0.4 20
2.= 2 zl
0.2 , I
0
_^
u.z
I
I
I
0.4
0.6
0.8
Fraction
of
1.0
Platinum
Fig. 3. Change in particle size and dispersion stability of bimetallic Au-Pt particles as a function of molar fraction of Pt.
the composition of platinum increased, the dispersion stability against sodium chloride decreased. On the other hand, in Au-Pd and Au-Rh systems, the particle sizes decreased gradually with increasing composition of Pd or Rh from 5 to 3 nm for the Au-Pd system and from 5 to 4 nm for the Au-Rh system, respectively. Both systems provided very stable dispersions against electrolyte, independent of the composition. The catalytic action of bimetallic particles prepared in this manner was investigated using two different reactions; decomposition of hydrogen peroxide
20
and hydrogenation of MVK. Hydrogen peroxide is decomposed to water and oxygen in the presence of a catalyst. As the reaction obeys first-order kinetics, the decomposition rate (RA) is expressed by RA = kA [ H,O, ] [C,] , where kA is the rate constant and [CA] is the concentration of catalyst. The decomposition rate constant kA of hydrogen peroxide was measured for the Au-Pt system, as shown in Fig. 4. The rate constant linearly decreased with increasing composition of Au and below Au=O& steeply decreased. This behavior was very similar to that obtained by Miner et al. [ 121. However, the activity expressed as kA/ [C,] in this work was lower than that by Miner et al. [ 121, because AuPt bimetallic particles probably agglomerated in water. MVK was hydrogenated to ethylmethylketone in the presence of Au-Pd bimetallic particles. At a constant hydrogen pressure (1 atm ), the reaction obeys first-order kinetics, so that the hydrogenation rate (RB) can also be expressed as RB = kB [ MVK] [ Cn] , where kB is the rate constant and [C, ] is the concentration of catalyst. Figure 4 also shows that the hydrogenation rate decreases with increasing composition of Au especially above a mole fraction of Au = 0.4. Thus, since the Au-Pt and Au-Pd systems exhibit a nonlinear change in these two reactions, the microstructure of bimetallic particles would seem to play an important role in their catalytic activity. This study shows that the particle sizes of Au, Pd, and Rh prepared using chitosan as a protective agent decrease with increasing amounts of chitosan, while the dispersion stability against electrolyte increases. Furthermore, in the preparation of bimetallic particles of Au-P& Au-Pd, and Au-Rh using chitosan, stable Au-Pd and Au-Rh particles were obtained, where their particle sizes were almost independent of the feed composition. A characteristic be-
2.0 0 1.0 _<
0
# 0.2 Molar
1 0.4 fraction
0.6 of
0.8 Au
Fig. 4. Rate constant versus molar fraction of Au: (. ) Au-Pt system; ( CJ) Au-Pd system.
21
havior of these bimetallic particles was observed in catalytic reactions the decomposition of hydrogen peroxide and hydrogenation of MVK.
such as
REFERENCES
8 9 10 11 12
J. Turkevich, R.C. Stevenson and J. Hiller, Discuss. Faraday Sot., 11 (1951) 55. K. Takiyama, Bull. Chem. Sot. Jpn., 31 (1958) 944. B.V. Enustun and J. Turkevich, J. Am. Chem. Sot., 85 (1963) 3317. L.D. Rampino and F.F. Nord, J. Am. Chem. Sot., 63 (1941) 2475. H. Hirai, J. Macromol. Sci. Chem., 13 (1979) 633. M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids Surfaces, 5 (1982) 209. K. Meguro, Y. Nakamura, Y. Hayashi, M. Torizuka and K. Esumi, Bull. Chem. Sot. Jpn., 61 (1988) 347. Y. Nakao and K. Kaeriyama, J. Colloid Interface Sci., 110 (1986) 82. J.P. Zikakis (Ed.), Chitin, Chitosan, and Related Enzymes, Academic Press, 1984. R.A.A. Muzzarelli, C. Jeuniaux and G.W. Gooday (Eds.), Chitin in Nature and Technology, Plenum Press, 1985. J.B. Michel and J.T. Schwartz, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987. p. 669. R.S. Miner Jr, S. Namba and J. Turkevich, in T. Seiyama and K. Tanabe (Eds.), Proc. 7th Int. Congr. Catalysis, Kodansha, 1981, p. 168.
Colloids and Surfaces, 55 (1991) 15-21 Elsevier Science Publishers B.V., Amsterdam
Characterization of precious metal particles prepared using chitosan as a protective agent Norio Ishizuki, Kanjiro Torigoe, Kunio Esumi’ and Kenjiro Meguro Department University
of Applied
Chemistry
of Tokyo, Kagurazaka,
and Institute
of Colloid and Interface
Shinjuku-ku,
Tokyo 162 (Japan)
Science,
Science
(Received 12 April 1990; accepted 10 July 1990)
Abstract Precious metal particles (Au, Pt, Pd, Rh) were prepared using chitosan as a protective agent. The particle sizes decreased with increasing amounts of chitosan, while the dispersion stability against electrolyte increased. Similarly, stable bimetallic particles of Au-Pd and Au-Rh were obtained and their particle sizes were almost independent of feed composition. Furthermore, these bimetallic particles exhibited characteristic properties for the decomposition of hydrogen peroxide and hydrogenation of methylvinylketone.
INTRODUCTION
Preparation of precious metal particles by reducing metal salts has been studied extensively [l-4]. In aqueous solution, precious metal particles have a tendency to flocculate and precipitate due to their hydrophobicity. To disperse the particles in aqueous solution, it is necessary to use protective agents [ 5-81, such as chelating agents, surfactants, and hydrophilic polymers. They adsorb on the surface of the particles and stabilize the particles in the dispersed state. Thus, protective agents have a very important role in obtaining a stable dispersion. Chitin, poly-/3- (1,4) -N-acetyl-D-glucosamine, is a cellulose-like biopolymer widely distributed in nature, especially in marine invertebrates, insects, fungi, and yeasts. Chitin also has unique properties, including toughness, bioactivity, and biodegradability. Recent application research [9,10] has largely been focused on chitosan, which is deacetylated chitin, because the free amino groups in this modified product contribute polycationic, chelating, and film-forming properties, along with ready solubility in dilute acetic acid. In particular, partially deacetylated chitin is water soluble, and is similar in behavior to hydrophilic polymers that adsorb on metal particle surfaces and form complexes. ‘To whom correspondence should be addressed.
0166-6622/91/$03.50
0 1991 -
Elsevier Science Publishers B.V.
16
In this work, various precious metal particles were prepared using chitosan as a protective agent and the particles were characterized by measuring particle size, dispersion stability, and catalytic activity. EXPERIMENTAL
Materials Aqueous chitosan was supplied by Ajinomoto Co., Ltd. The molecular weight of chitosan determined by a light scattering method was about 200 000. Chloroauric acid and chloroplatinic acid were obtained from Tanaka Kikinzoku Kogyo K.K. Methylvinylketone (MVK) was provided by Tokyo Kasei Co. Ltd. Palladium chloride, rhodium chloride, hydrazine and other chemicals were extra pure grade from Wako Pure Chemical Industries, Ltd, and used as received. Water used in this experiment was deionized by a Mini-Q water purification system. Preparation of precious metal particles All glassware was washed with aqua regia. Reducing agents, hydrazine monohydrate ( 1 cm3 ) dissolved in water (9 cm3 ) , formaldehyde ( 1 cm3 ) dissolved in 1 M potassium hydroxide (2 cm3), and pentaerythritol (1 g) dissolved in water (5 cm3) were made up just before use. A 1 cm3 sample of each metal salt 12.5 mmol dmp3 in aqueous solution and aqueous chitosan were mixed, and diluted to 49.7 cm3 with water, The solution was gently heated with a heating mantle and refluxed. After the solution boiled, 0.3 cm3 of reducing agent was added dropwise to the boiling solution. As soon as the color of the solution changed, it was cooled in an ice bath to terminate the reaction. Bimetallic particles were prepared using the same procedure as that described above. Here, the ratios of the respective salts were changed in a total volume of 1 cm3 in aqueous solution. Measurement Particle sizes were determined by a transmission electron microscope (Hitachi model H-800): the particle sizes were measured directly from the enlarged electron micrographs. The accuracy was ? 0.2 nm. Electron probe micro analysis (EPMA) [ 111 was performed with a Shimadzu Model EHAX-2001. UV-vis absorption spectra of suspensions were measured with a Hitachi Model 220 A spectrophotometer. The suspension containing metal particles (10 cm3) and 10 wt% sodium chloride ( 1 cm3) were mixed and they were centrifuged at 2000 rpm for 5 min.
17
The stability of the suspension was estimated as the absorbance ratio of supernatant and blank measured at 530 nm. A 1.5 wt% hydrogen peroxide solution (15 cm3) was diluted to 30 cm3 with 2 mol dmp3 H,SO,. The solution was placed in a thermostated water bath (40’ C ) . A bimetallic suspension of Au-Pt (0.1 cm3 ) was added to the solution to initiate decomposition of hydrogen peroxide. The reactant was titrated using 0.1 mol dmv3 KMnO,. Hydrogenation of MVK was carried out as follows. A 3.5 mg aliquot of MVK was diluted to 10 cm3 with water. The solution was placed in a thermostated heating mantle (60’ C ) and was saturated with hydrogen. Then, a Au-Pd suspension (1 cm3) was added to the solution to initiate the reaction. The reactant was measured by means of gas chromatography. RESULTS AND DISCUSSION
When formaldehyde, hydrazine or pentaerythritol was added to each metal salt dissolved in aqueous solution in the presence of chitosan, the reduction of metal salts was significantly affected by the kind of reducing agent used. Formaldehyde could reduce only HAuCl,, while both hydrazine and pentaerythritol reduced the four kinds of metal salts studied in this work. This result can be understood by considering the relative strengths of these reducing agents: as formaldehyde is a weak reducing agent compared to hydrazine and pentaerythritol, it cannot reduce H,PtCIG, PdC12, and RhCl,. Five systems provided well-dispersed metal particles: formaldehyde-HAuCl,, hydrazine-HAuCl,, hydrazine-PdCl,, hydrazine-RhCl,, and pentaerythritolRhCl,. In these systems, the particle sizes decreased with increasing amount of chitosan and their dispersion stability was dependent on their particle sizes. Figure 1 shows the particle sizes and dispersion stability of hydrazine-HAuCl, system as a function of feed concentration of chitosan. It is apparent that the particle size of Au particles decreases gradually with increasing amounts of chitosan and then becomes constant.On the other hand, the dispersion stability of Au particles was very low in solutions containing a smaller amount of chitosan, but above a critical amount of chitosan the dispersion stability increased sharply. These critical amounts of chitosan were almost the same for the five systems, around about 1*1O-3 wt%. However, among these five systems, there are important differences, so that, at the critical amount of each system, the particle size of Au is significantly larger than that of Pd and Rh. This suggests that chitosan as a protective agent is much more effective for Au particles than for Pd and Rh particles. Such protective action is expected to operate since chitosan is known [ 91 to interact strongly with metal ions or metals. Furthermore, in concentrations of 1. lo-’ wt%, all the particle sizes for the five systems ranged from 3 to 6 nm and the dispersion stability of these particles was scarcely affected by the electrolyte.
18 1.0
30
08
20 :
0.6
~ r -_ n 8
.-It; m IlJ T; .5 10 a
0.4
*
0.2
Cont. of Chitosan I wt%
Fig. 1. Change in particle size and dispersion stability of Au particles prepared by reduction of hydrazine as a function of feed concentration of chitosan.
Further, a preparation of bimetallic particles was carried out in the presence of 1*10-2 wt% chitosan. Here, hydrazine was used as a reducing agent for preparing bimetallic particles of Au-Pt, Au-Pd, and Au-Rh. When mixed salts in various proportions were reduced with hydrazine, a characteristic color change for each system was observed; the solution containing Au particles alone was a red color, and with increasing composition of Pt, Pd or Rh the solutions turned brown. To identify whether the particles obtained are bimetallic ones, the UV-vis spectra and EPMA measurements for the samples were made. As can be seen (Fig. Z), the spectra of 50Au50Pd were found to differ from those of their mixtures of respective samples, indicating that the samples prepared are not mixtures of monometallic particles but bimetallic ones. Similarly, the spectra of the other samples were different from those of the mixtures. Indeed, Michel and Schwartz [ll] also obtained a similar result for a Au-Pd system. The identification of the Au-Pt system could not be performed by means of UVvis spectra due to poor dispersion stability. Furthermore, the composition of the samples was determined by means of EPMA. The EPMA measurements show that the ratio of Au for the three systems is lower than that of the feed solution. In the Au-Pt system, the particle size increased gradually with increasing composition of platinum from 5 to 50 nm in diameter (Fig. 3). Similarly, as
19 0.6
I 480
Wave
I
,
560
640
I nm
length
Fig. 2. UV-vis spectra of Au-Pd system.
60.
08 E 40 c
0.6
w ‘iii 2 y :
0.4 20
2.= 2 zl
0.2 , I
0
_^
u.z
I
I
I
0.4
0.6
0.8
Fraction
of
1.0
Platinum
Fig. 3. Change in particle size and dispersion stability of bimetallic Au-Pt particles as a function of molar fraction of Pt.
the composition of platinum increased, the dispersion stability against sodium chloride decreased. On the other hand, in Au-Pd and Au-Rh systems, the particle sizes decreased gradually with increasing composition of Pd or Rh from 5 to 3 nm for the Au-Pd system and from 5 to 4 nm for the Au-Rh system, respectively. Both systems provided very stable dispersions against electrolyte, independent of the composition. The catalytic action of bimetallic particles prepared in this manner was investigated using two different reactions; decomposition of hydrogen peroxide
20
and hydrogenation of MVK. Hydrogen peroxide is decomposed to water and oxygen in the presence of a catalyst. As the reaction obeys first-order kinetics, the decomposition rate (RA) is expressed by RA = kA [ H,O, ] [C,] , where kA is the rate constant and [CA] is the concentration of catalyst. The decomposition rate constant kA of hydrogen peroxide was measured for the Au-Pt system, as shown in Fig. 4. The rate constant linearly decreased with increasing composition of Au and below Au=O& steeply decreased. This behavior was very similar to that obtained by Miner et al. [ 121. However, the activity expressed as kA/ [C,] in this work was lower than that by Miner et al. [ 121, because AuPt bimetallic particles probably agglomerated in water. MVK was hydrogenated to ethylmethylketone in the presence of Au-Pd bimetallic particles. At a constant hydrogen pressure (1 atm ), the reaction obeys first-order kinetics, so that the hydrogenation rate (RB) can also be expressed as RB = kB [ MVK] [ Cn] , where kB is the rate constant and [C, ] is the concentration of catalyst. Figure 4 also shows that the hydrogenation rate decreases with increasing composition of Au especially above a mole fraction of Au = 0.4. Thus, since the Au-Pt and Au-Pd systems exhibit a nonlinear change in these two reactions, the microstructure of bimetallic particles would seem to play an important role in their catalytic activity. This study shows that the particle sizes of Au, Pd, and Rh prepared using chitosan as a protective agent decrease with increasing amounts of chitosan, while the dispersion stability against electrolyte increases. Furthermore, in the preparation of bimetallic particles of Au-P& Au-Pd, and Au-Rh using chitosan, stable Au-Pd and Au-Rh particles were obtained, where their particle sizes were almost independent of the feed composition. A characteristic be-
2.0 0 1.0 _<
0
# 0.2 Molar
1 0.4 fraction
0.6 of
0.8 Au
Fig. 4. Rate constant versus molar fraction of Au: (. ) Au-Pt system; ( CJ) Au-Pd system.
21
havior of these bimetallic particles was observed in catalytic reactions the decomposition of hydrogen peroxide and hydrogenation of MVK.
such as
REFERENCES
8 9 10 11 12
J. Turkevich, R.C. Stevenson and J. Hiller, Discuss. Faraday Sot., 11 (1951) 55. K. Takiyama, Bull. Chem. Sot. Jpn., 31 (1958) 944. B.V. Enustun and J. Turkevich, J. Am. Chem. Sot., 85 (1963) 3317. L.D. Rampino and F.F. Nord, J. Am. Chem. Sot., 63 (1941) 2475. H. Hirai, J. Macromol. Sci. Chem., 13 (1979) 633. M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids Surfaces, 5 (1982) 209. K. Meguro, Y. Nakamura, Y. Hayashi, M. Torizuka and K. Esumi, Bull. Chem. Sot. Jpn., 61 (1988) 347. Y. Nakao and K. Kaeriyama, J. Colloid Interface Sci., 110 (1986) 82. J.P. Zikakis (Ed.), Chitin, Chitosan, and Related Enzymes, Academic Press, 1984. R.A.A. Muzzarelli, C. Jeuniaux and G.W. Gooday (Eds.), Chitin in Nature and Technology, Plenum Press, 1985. J.B. Michel and J.T. Schwartz, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987. p. 669. R.S. Miner Jr, S. Namba and J. Turkevich, in T. Seiyama and K. Tanabe (Eds.), Proc. 7th Int. Congr. Catalysis, Kodansha, 1981, p. 168.