J. Phys. Chem. 1902, 86, 4413-4417
4413
Kinetic Studies on Intercalation-Deintercalation of OH- and CI- in a Hydrotalcite-Like Compound, Nio,75Alo~25(0H)2Clo~25, Using the Pressure-Jump Technique Mlnoru Sasakl, Naokl Mlkaml, Tetsuya Ikeda, Karuakl Hachlya, and Tatsuya Yasunaga" Department of Chemistty, Faculty of Science, Hiroshima University, Hiroshima 730, Japan (Received: April 5, 7982; In Final Form: June 14, 1982)
In basic aqueous suspensions of a synthetic hydrotalcite-likecompound, Nio.75Alo.25(OH)2Clo.25~mH20, double relaxation curves of the order of milliseconds were observed by using the pressure-jump technique with conductivity detection. The slow reciprocal relaxation time increases with OH- concentration while the fast one decreases. The amount of OH- adsorbed is slightly larger than that of C1- released. From kinetic and static results obtained, the fast and slow relaxations were attributed to intercalation-deintercalation processes of C1- and of OH-, respectively, in host sites of the layered intercalation compound, preceded by very fast adsorption-desorption of these ions on the surface of the compound. The intercalation and deintercalation rate constants were determined to be 1.4 X lo3and 1.8 x 10 s-l for C1-, and 9 X lo3 and 6 s-' for OH-, respectively, at 25 "C.
Introduction Layered intercalation compounds have accessible unoccupied lattice positions with mobile species (atoms, molecules, or ions) in contact with the solid host lattice and are characterized by an intercalation process leading to their formation and a reversible topotactic bulk reaction of a solid, in which the guest species diffuse from the outer surface into the bulk of the solid and an interlayer distance changes by the intercalation. The physical properties of the compounds can be modified significantly by the intercalation of various guest species. Recently, intercalation has become a topic of considerable interest among materials scientists, catalytic chemists, and physicochemists.'-lo Apart from academic interest one wishes to know how the guest species intercalate into the layers accompanying the change in the interlayer distance and what the elementary process is in the intercalation. However, the usual intercalation and guest species exchange processes in the compound are too fast to be observed by ordinary methods. Very recently, kinetic studies of ion exchange of alkali-metal ions and organic ions for Na+ in zeolites which have cage structures as host positions have been performed by using the pressure-jump technique and the detailed mechanisms for the ion exchange were elucidated."J2 Among the layered intercalation compounds, a synthetic hydrotalcite-like compound is utilized as an antiacid, antipeptin, catalyst support, ion exchanger, and adsorbent for acid and anionic substances.6J0 In spite of the importance and usefulness of these compounds, however, the (1) A. Clearfield and J. A. Stynes, J. Znorg. Nucl. Chem., 26, 117 (1964). (2) S. Yamanaka, Inorg. Chem., 15, 2811 (1976). (3) R. Schollhorn, R. Kuhlmann, and J. 0. Besenhard, Mater. Res. Bull., 11, 83 (1976). (4) S. Yamanaka, Y. Horibe, and M. Tanaka, J . Inorg. Nucl. Chem., 38, 323 (1976). (5) G . Alberti, Acc. Chem. Res., 11, 163 (1978). (6) S. Miyata and T. Hirose, Clays Clay Miner., 26, 441 (1978). (7) R. Schollhorn, Physica, SSB, 89 (1980). (8) A. H. Thompson, Physica, SSB, 100 (1980). (9) P. Collombet and M. Donot, Physica, SSB, 117 (1980). (10) S. Miyata, Clays Clay Miner., 28, 50 (1980). (11) T. Ikeda, M. Sasaki, N. Mikami, and T. Yasunaga, J. Phys. Chem., 85, 3896 (1981). (12) T. Ikeda, M. Sasaki, and T. Yasunaga, J. Phys. Chem., 86,1680 (1982).
mechanisms for the intercalation and ion-exchange reactions in the compound have not been well established. The purpose of the present investigation is to elucidate kinetically the intercalation and ion-exchangereactions of C1- and OH- in a basic aqueous suspension of a synthetic hydrotalcite-like compound (Ni-HT(C1)) using the pressure-jump technique with conductivity detection.
Experimental Section The pressure-jump apparatus used is the same as that reported previ~usly,'~ and the time constant of the pressure jump is 80 p. The hydrotalcite-like compound Ni-HT(C03) ( N i o . ~ 5 A ~ , 2 5 ( O H ) ~ ( C 0 3 ) o , was l ~ 5 ~supplied ~ H z O )from the Kyowa Chemical C0.14 Carbonate ion in this compound is a mobile guest species in contact with the solid host lattice. The chloride-type hydrotalcite-like compound NtHT(C1) was prepared by ion exchange of C1- for C032in the Ni-HT(C03) adding hydrochloric acid dropwise above pH 3.5. All reagents (Wako Chemical Co., reagent grade) were used without further purification. The amount of C1- released was determined by means of colorimetric analysis with mercuric thiocyanate at a wavelength of 460 nm.15 Prior to the measurement, the samples were centrifuged for 30 min at 1OOOOg in order to complete settling. The pressure-jump measurement as well as the static experiments were carried out on samples of 20 g cm-3 particle concentration C,. In the experimental determination of the saturated amount of OH- adsorbed, the C, was 1 g dm-3. The { potential of the suspension was measured by an ultramicroelectrophoresis method.16 Results and Discussion Kinetic measurements were carried out in basic aqueous suspensions of Ni-HT(C1) by using the pressure-jump technique with conductivity detection. As is apparent from the semilogarithmic plot of the relaxation curve shown in Figure 1,two relaxations were observed, where both slopes (13) K. Hachiya, M. Ashida, M. Sasaki, H. Kan, T. Inoue, and T. Yasunaga, J. Phys. Chem., 83, 1866 (1979). (14) S. Miyata, Clays Clay Miner., 23, 369 (1975). (15) S. Uchimi, Nippon Kagaku Zasshi, 73, 835 (1952). (16) K. Hachiya, J . Sci., Hiroshima Uniu., Ser A . , 45, 157 (1981).
0022-3654/82/2086-4413~0~.25/0 0 1982 American Chemical Society
4414
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982
1 ' '
(a)
Sasaki et al.
1
0
2
6
4 [OH-]
8
N
Figure 3. Dependence of the reciprocal slow relaxation time on the O K concentration in the basic aqueous suspension of Ni-HT(CI) at C, = 20 g d m 3 and 25 O C . c2t
s
i
b
I
Figure 1. (a) Typical double relaxation curves in the basic aqueous suspension of Ni-M(Ci) observed by the pressure-jump technique with conductivity detection at C, = 20 g d d , pH 11.7, and 25 OC; sweep 50 ms/division. (b) Semilogarithmic plot of the relaxation curve; sweep 10 ms/division. I
a 10.
a
0
,
,
1
2
added NaOH , 10-'N
Figure 4. pH dependence on the concentration of added NaOH in the basic aqueous suspension of Ni-M(CI) at C, = 20 g dm3 and 25 'C. 0
2
4 [OH'] ,
6
a
N
Figure 2. Dependence of the reciprocal fast relaxation time on the OH- concentration in the basic aqueous suspension of Ni-HT(CI) at C, = 20 g dm-3 and 25 OC.
of the relaxation signals indicate a decrease in the conductivity of the suspension during the relaxations. The reason will be discussed later in detail. No relaxation was observed in the suspension below pH 10.2, in the supernatant solution of the suspension, or in the Ni-HT(C0J suspension of the same pH. These facts suggest that the two relaxations may be due to the ion exchange of OHfor C1- or the intercalations of these ions into the layers of the Ni-HT(C1). The OH- concentration dependence of the fast and slow reciprocal relaxation times, 7r1and 7;l, are shown in Figures 2 and 3, respectively. As can be seen from these figures, 7~~ increases with the concentration, while 7c' decreases. To obtain the equilibrium parameters, an extensive study of the static properties of the suspensions was carried out. The base titration curve shown in Figure 4 essentially differs from that reported for zirconium phosphate, in which a constant pH region appeared in the titration ~ u r v e . This ~ * ~fact indicates that a phase transition during the exchange does not occur in the present system. Thus, the phase transition can be eliminated as the possible ion-exchange mechanism. In order to obtain static information on behavior of the ion exchange of OH- for C1in the Ni-HT(C1) suspension, the amounts of OH- adsorbed and C1- released, [OH-Iadand [Cl-],, were measured and the results are shown in Figure 5. As can be seen from this figure, C1- released exists even in the base-free sus-
d i
I
0
2
1
added NaOH ,
N
Figure 5. Adsorption isotherm of OH- (0) and the amounts of Clreleased (0)at C, = 20 g d m 3 .
pension, and above pH 10.2 the amount of OH- adsorbed is slightly larger than that of C1- released. These facts suggest that the results cannot be explained by the simple ion exchange of OH- for C1-: s c J
5
sot-
(1)
3r
where the symbols S(C1) and S(0H) denote the bound state of C1- on the host site of Ni-HT(C1) and the released state of C1- induced by the OH- adsorption, respectively. As can be predicted from the kinetic data, at least a two-step mechanism of ion exchange must be considered.
The Journal of Physical Chemistry, Vol. 86, No. 22, 7982 4415
Intercalation-Deintercalation of OH- and CI-
-40r > E
TABLE I: Thermodynamic Parameters of IntercalationDeintercalation and Adsorption-Desorption of OH- and C1- in the Aqueous Ni-HT(C1) System at 25 "C
P
Kzht, mol-' dm3
10:3 . K3mt, mol dm-3
9
2
10-3. 10-'K,, mol-' dm3 1.0 -f. 0.2
1o2Kb, mol dm-3
10ZK,
3.1-f.0.9 1.3 ( l . l ) a
10-3K, 1.5 (1.5)'
a These values were determined from eq 7 by using the values of K,, Kb, Kzint, and K3ht.
0
2
1
added NaOH ,
I
N
'
7
Flgwe 6. { potential as a function of the concentration of added NaOH.
I
I
0
0
2
4 [CL'I'', IO2 mol' dm3
2
6
4
8
1
0
exp(3)(LS(CI)l +[OH']) , 16'n-d dm-3
Figure 7. Plot of ([S(CbOH)] i- [S(OH)])/([S(CI)] [OH-]) vs. [CI-I-' in eq 1.
Flgure 8. Plot of [OH-]) in eq 2.
T , - ~exp[ePol(2kT)]
vs. exp[e\k,lkT]([S(CI)]
+
Two kinds of two-step mechanisms have already been reported by the authors as follows: 11~12~17
\*
SICI!
S+
\
\r
CI
SICI)
S(OH!
S(CI.OH)
OH-
(Ka)
(11)
OH S(OH)
CI
(111)
-
(Kb)
where the symbols S+ and S(C1.OH) denote the positive host site and the adsorbed state on the S(Cl), respectively, K is the equilibrium constant, and k is the rate constant. According to mechanism I1 both reciprocal relaxation times must increase with OH- concentration. However, the experimental results shown in Figure 2 contradict the prediction. Therefore, mechanism I1 is excluded. In the case that the first step in mechanism I11 is faster than the second one, qualitative tendencies of the reciprocal relaxation times are roughly in agreement with the experimental results. Since an electrostatic potential in the vicinity of the surface of a solid plays an important role in the kinetic behavior of an ion on the ~ u r f a c e , ~the * J ~{ potential was measured and the results are shown in Figure 6. This figure shows that the { potential is created by the positive vacant site S+ in the surface of Ni-HT(C1). As can be seen from Figures 5 and 6, the amount of this site is appreciably smaller than that of OH- adsorbed and C1-
(17) T. Ikeda, J. Nakahara, M. Sasaki, and T. Yasunaga, J. Phys. Chem., submitted. (18) M. Ashida, M. Sasaki, H. Kan, T. Yasunaga, K. Hachiya, and T. Inoue, J. Colloid Interface Sci., 67, 219 (1978). (19) M. Sasaki, M. Moriya, R. D. Astumian, T. Yasunaga, and Z. A. Schelly, J. Phys. Chem., submitted for publication.
0.5 exd%([S(OH) 2kT
Flgure 9. Plot of
T,-~
+
1 [Cl-l)fb .16'mol dm-3
f , vs. the concentration term in eq 3.
released and decreases drastically with the OH- concentration. Above pH 10.2, thus, the amount of S+ can be neglected in the kinetic analysis, while the contribution of the { potential to rate constants of ions must be considered. According to mechanism 111, the equilibrium constants, K , and Kb, are given by [S(Cl*OH)]+ [S(OH)] [S(C1)l[OH-l
= K , 4- KaKb-
1
LC1-1
(1)
The site concentration was calculated from the amount of OH- adsorbed at saturation, where the value was determined to be 9.5 X lo-* mol g-' from the adsorption isotherm measured at C, = 1g dm-3. The results calculated by using this value are shown in Figure 7. In the region that the value of the { potential becomes smaller, an excellent linearity of the plots was obtained, and the values of K, and Kb obtained from the slope and the intercept are listed in Table I. This fact suggest the possibility of mechanism 111. Taking into account the surface poten-
4416
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982 3 t ;.
Sasaki et at.
i
3
2
0
F
' 7
-
2
r 1
1
_L
0
1
2
1 f, , 10-3
f ,107
Figure 10. Plot of the reciprocal fast relaxation time vs. the concentration term, f ,, in eq 4.
Figure 11. Plot of the reciprocal slow relaxation time vs. the concentration term, f,, in eq 5.
tials,1"20 the fast and slow reciprocal relaxation times for mechanism I11 are given by the following equations:
TABLE 11: Kinetic Parameters of IntercalationDeintercalation of OH' and C1- in the Aqueous Ni-HT(C1) System at 25 "C
rrl
5)=
10-3k4,
exp( 2 k T
k?
ex.(
1 0 - ' k , , s-'
+ [OH-]) + k_Pt( 2 )
,)([S(Cl)] eQ0
1.8
10-3k.1,
1.4
s-l
s-'
9
k.4, s-' 6
equations of the reciprocal relaxation times for the first and fourth steps coupled to the other rapid steps are expressed as
with
K;l
fb
+ [OH-] + [S(Cl)]
3) 2kT [OH-] + [S(Cl)]
= ex.(
where \koand \k, denote the potentials created by the OHand C1- adsorbed, respectively, the superscript int denotes intrinsic, k is the Boltzmann constant, and T is the absolute temperature. The experimental results for 7r1and ~ , - lwere analyzed by using these equations, and the results are shown in Figures 8 and 9, where the values of \ko and \k, were estimated according to the theoretical treatment of Davis and Leckie.20As can be seen from these figures, the plots for 7f1 fell on the straight line, while the plots for 7;l did not show the linear relation and had a small negative slope, which is physically without meaning. Therefore, mechanism I11 is also eliminated as a possibility. On the basis of the fact that the static results could be explained by mechanism I11 as described above, mechanism I11 was further divided into the intercalation-deintercalation and adsorption-desorption reactions of C1- and OH- in the Ni-HT(Cl)-OH- systems as SiCIl,
+
S(CIi,
2_' 3-
S C O H ) , e S(OH),
S\CI*OHI,
( K ,1
(K,)
(fast)
(very fast)
21
(K,) (K,) (very fast) ( s l o w )
(IV)
where the subscripts 1 and s denote the layer and the surface, respectively. The relaxation times for the second and third steps, in the case that the other steps are very fast or very slow, are given by equations similar to eq 2 and 3. Therefore, the equations derived could not be applied to the experimental kinetic results. Meanwhile, the relaxation times for the first and fourth steps, where the other steps are very slow, are expected to be independent of the concentration of species, which also contradicts the experimental fact. Thus, we examine the sole plausible case that the second and third steps are very fast ones, which may be diffusion-controlled processes. The (20) J. A. Davis, R. 0. James, and J. 0. Leckie, J . Colloid Interface Sci., 63, 480 (1978).
By plotting 7r1and 7L1vs. the concentration terms in eq 4 and 5, as shown in Figures 10 and 11, respectively, we obtained fairly good linearities, where Kzhtand K3htwere determined by minimizing the standard deviation of the plots from the straight lines. The linearity of these plots suggests the possibility of mechanism IV. Values of rate constants for steps 1 and 4 were obtained from the slopes and the intercepts of the straight lines and are listed in Table 11. The equilibrium constants, K1 and K4, calculated kinetically from the ratio of the experimental rate constants are listed in Table I. On the other hand, these values were also determined statically from the following equations: Kb = K3int(l+ K4) K , = Kqint/(l + K1-l) (7) As shown in Table I, the values of K1 and K4 obtained kinetically are in good agreement with those determined statically. This fact lends further support for the validity of mechanism IV. According to this mechanism, the fact that the conductivity decreases following the sudden drop in pressure as described above results from the change in the concentration of the species in the intercalationdeintercalation processes of OH- and C1- accompanying the very fast adsorption-desorption equilibrium of the ions on the surface of the Ni-HT(C1), where the conductivity decreases in the same manner as the relaxations observed in the Ti02-H+, Ti02-OH-, y-A1203-Pb2+,and zeolitesOH- reported previously.1'~13~16~1g~21~z2 Among the rate constants listed in Table 11, the value of the intercalation (21) M. Ashida, M. Sasaki, K. Hachiya, and T. Yasunaga, J . Colloid Interface Sci., 74, 572 (1980). (22) T. Ikeda, M. Sasaki, R. D. Astumian, and T. Yasunaga, Bull. Chem. Soc. Jpn., 54, 1885 (1981).
4417
J. Phys. Chem. 1902, 86,4417-4422
rate constant for OH- is 5 times larger than that for C1-, while the value of the deintercalation rate constant for OHis 3 times smaller than that for C1-. This indicates that OH- may interact strongly with the solid host lattice in the layers of the Ni-HT(C1). Similar kinetic investigations in aqueous suspensions of
layered intercalation compounds are in progress and the results will be reported in the near future.
Acknowledgment. We thank Kyowa Chemical Co. for the supply of the synthetic hydrotalcite-likecompound and thank Mr. Shigeo Miyata for his useful suggestions.
Mechanism of Reduction of Bis( 2-hydroxyethyl) Trisulfide by eaq- and CO,-. and Scavenglng of RSS. Radicals
Spectrum
Zhennan Wu,' Thomas G. Back, Rizwan Ahmad, Raghav Yamdagnl, and David A. Armstrong' Department of Chemlstry, University of Calgary, Calgary, Alberta, Canada T2N 1N4 (Received: May IO, 1982; In Final Form: June 29, 1982)
Each of the reducing radicals e,; CO;, and (CH3)&OHcleaves bis(Bhydroxyethy1) trisulfide in aqueous solution to produce (2-hydroxyethy1)perthiyl(RSS-)radicals and 2-hydroxyethanethiol. The RSS. radical had h,, = 374 nm, cmru = 1630 f 50 M-' cm-l, and a second-order rate constant for dimerization 2kI2 = (1.4 f 0.3) x lo9 M-l s-l (2RSS- RSIR (12)). It is able to abstract H atoms from dihydroflavin adenine dinucleotide (FH,), but not from formate. Thus, in aqueous solution DRSS+ must be greater than 60 but less than 90 kcal mol-'. Although tetrasulfide and thiol are the major initial products of reduction, secondary thermal reactions occur and produce sulfanes (RS,H) and polysulfides (RS,R). In the early stages of reduction products of n = 2 dominate. However, on prolonged reaction with -COP-elemental sulfur is precipitated, and this is probably formed by elimination from sulfanes with n much larger and in the region of 8.
-
Introduction Thermal and free-radical-initiated decompositions of polysulfides (RS,R) and sulfanes (HS,H) have been subjects of interest for many year^.^-^ However, although the products and rates of decomposition of higher polysulfides (n L 3) have been studied, fast reaction techniques do not appear to have been utilized to characterize short-lived intermediates and their reactions. Surprisingly, this situation exists despite relatively intense activity in the application of pulse radiolysis and other methods to the study of neutral (e.g., RS*),697cationic (e.g., RS+R,RS.SR+),B-lo and anionic (RS-SR-)" radical species in solutions of thiols, sulfides, and disulfides. The major reasons are probably the relatively low solubility of higher polysulfides in water, which is the best solvent for the application of radiation (1)On leave of absence from the Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, China. (2)I. Kende, T. L. Pickering, and A. V. Tobolsky, J.Am. Chem. SOC., 87. 5582 (1965). - - - ~ (3)T. L.Pickering, K. J. Saunders, and A. V. Tobolsky, J . Am. Chem. SOC.,89,2364 (1967). (4) E. Miiller and J. B. Hyne, J. Am. Chem. SOC.,91,1907 (1969). ( 5 ) L.Field in 'Oreanic Chemistrv of Sulfur". S. Oae.. Ed.., Plenum Press, New York, 1977,Chapter 7. (6) G.G. Jayson, D. A. Stirling, and A. J. Swallow, Int. J.Radiat. Biol. Re&. Stud. Phys., Chem. Med., 19, 143 (1971). (7)J. W.Purdie, H. A. Gillis, and N. V. Klassen, Can. J . Chem., 51, 3132 (1973). (8)M.BonifaEi6 and K.-D. Asmus, J. Phys. Chem., 80,2426(1976),and earlier work cited therein. (9)K.-D. Asmus, D. Bahnemann, Ch.-H. Fischer, and D. Veltwisch, J. Am. Chem. SOC.101, 5322 (1979),and earlier work cited therein. (10)A. J. Elliot, R. J. McEachern, and D. A. Armstrong, J. Phys. Chem., 85,68 (1981). (11)M. Z.Hoffman and E. Havon. J. Am. Chem. SOC..94,7950(1972). and references cited therein.
.
, - - - - I
-
0022-3654/82/2086-4417$01.25/0
chemical techniques, and secondly the fact that these molecules are rapidly interconverted, e.g. 2RS3R + RSzR + RSdR (1) in the presence of base.5 The latter feature seriously limits the conditions of pH under which polysulfides can be investigated. However, with the proper choice of polysulfide, studies can be made in the pH region 4-7. In this investigation the standard techniques of radiation chemistryI2have been utilized to produce the radicals e, -, COS-, and (CH3)2COHvia reactions 2-5 and to study 4.2H20
--
2.7ea,-
+ 0.6H. + 2.8.OH + 2.7H,,+ + 0.4H2 + 0.7H202
-
(CH3),CHOH + .OH (or He) (CH,),COH eaq-+ NzO (+ H20) HC02- + -OH (or H.)
+ HzO (or H,) .OH + Nz + OHH 2 0 (or H2) + C0,-
-
-
short-lived products of their reactions with bis(2hydroxyethyl trisulfide by pulse radiolysis. This trisulfide was chosen for its low vapor pressure, water solubility, and stability in aqueous solution. The perthiyl RSS. radical, which has previously been postulated in thermal decomp o s i t i o n ~ and ~ , ~ reported in flash photolysis13 and oneelectron oxidations1° of certain disulfides, emerged as the major intermediate. (12)I. G. DraganiE and Z. D. DraganiE, "The Radiation Chemistry of Water", Academic Press, New York, 1971. (13)G. H. Morine and R. R. Kuntz, Photochem. Photobiol., 33, 1 (1981).
0 1982 American Chemical Society
