J. Phys. Chem. 1903, 87, 5245-5248
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Kinetics of Adsorption-Desorption of Chromate on y-Al2O3Surfaces Using the Pressure-Jump Technique Naoki Mikami, Minoru Sasaki, Taizo Kikuchi, and Tatsuya Yasunaga Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima 730, Japan (Received: October 20, 1982; In Final Form: March 30, 1983)
In an aqueous -pAl,O, suspension containing chromate, double relaxations were observed by using the pressure-jump technique with electric conductivity detection. In the pH range 7-9, where HCr04- and Cr04" exist in the bulk phase, both fast and slow relaxation times decrease with increasing chromate concentration. The fast relaxation time is independent of pH, while the slow one increases with pH. From the static and kinetic results obtained, the fast and slow relaxations were attributed to the adsorption-desorption of di- and monovalent chromate ions on the protonated surface hydroxyl groups, respectively. The intrinsic values of the adsorption and desorption rate constants for monovalent chromate ion were determined to be klint = 5.3 X lo4 mol-' dm3 s-' and klint = 1.9 X 10 s-', and those for the divalent chromate ion k p t = 9.9 X lo4 mol-' dm3 s-l and kzint = 5.2 X 10 s-l, respectively, at I = 8 X and 25 O C . Introduction Very recently, kinetic studies of anion adsorption-desorption have been performed by using relaxation methods. In these studies, the mechanisms of the adsorption-desorption of OH- and IO3- on Ti02,1,2OH- on ~ e o l i t e ,and ~,~ C1- and C104- on a-FeOOH5have been elucidated. Such kinetic information is important to attain fundamental understanding of the anion adsorption. In a preceding paper,e we have reported the mechanism of the adsorption-desorption of mono- and divalent phosphate ions on r-Al,O, surfaces, which has been clarified from static and kinetic analyses on the basis of the model proposed by Davis, James, and Le~kie.~-lOFrom the above investigation, it has been revealed that the adsorption rate of divalent phosphate ions is much faster than that of monovalent ones. In order to discuss the difference between the adsorption-desorption rates of mono- and divalent anions, systematic investigations on the adsorption-desorption of anions such as chromate and arsenate, which are adsorbed strongly on oxide surfaces, are required. As a series of kinetic studies of the anion adsorption on -y-Al,03 surfaces, the present study was performed to clarify the dynamic aspects of the adsorption-desorption of chromate on r-AlzO, surfaces. In addition, the difference in the interfacial characteristics of chromate and phosphate was discussed. Experimental Section The y-A1203used was prepared according to the previous procedures.6J1 The total number of adsorption sites and the acidity constants were 3.7 X mol g-l, pKal = (1) Ashida, M.; Sasaki, M.; Hachiya, K.; Yasunaga, T. J . Colloid Interface Sci. 1980, 74, 572. (2) Hachiva. K.: Sasaki. M.: Karasuda. M.: Yasunaea. T. J . Phvs. Chem. 1980,-84, 2292. (3) Ikeda, T.; Sasaki, M.; Astumian, R. D.; Yasunaga, T. Bull. Chem. Soc. Jpn. 1981,54, 1885. (4) Ikeda, T.; Sasaki, M.; Yasunaga, T. J. Phys. Chem. 1982,86,1678. (5) Sasaki, M.; Moriya, M.; Yasunaga, T.; Astumian, R. D. J. Phys. Chem. 1983,87, 1449. (6) Mikami, N.; Sasaki, M.; Hachiya, K.; Astumian, R. D.; Ikeda, T.; Yasunaga, T. J. Phys. Chem. 1983, 87, 1454. (7) Davis, J. A.; James, R. 0.;Leckie, J. 0. J . Colloid Interface Sci. 1978, 63, 480. (8) Davis, J. A.; Leckie, J. 0. J . Colloid Interface Sci. 1978, 67, 90. (9) Davis, J. A.; Leckie, J. 0. J . Colloid Interface Sci. 1980, 74, 32. (10) James, R. 0.; Stinglich, P. J.; Healy, T. W. Adsorp. Aqueous Solutions (Proc. Symp.) 1981, 19. (11)Hachiya, K.; Ashida, M.; Sasaki, M.; Inoue, T.; Yasunaga, T. J . Phys. Chem. 1979,83, 1866. I
0022-3654/83/2087-5245$0 1.5010
6.8, and pKaz = 10.2, respectively.6 Sodium chromate (Wako reagent grade, Wako Chemical Co.) was used without further purification. The ionic strength of the y-Alz03-chromate suspension system was adjusted to 8 X with NaN03, and the pH of the suspension was varied with HN03. All measurements were performed on the suspensions equilibrated for 1 day, a t a particle concentration of C, = 30 g ~ m - ~ . The amount of adsorbed chromate was determined indirectly from the concentration change in the supernatant solution by means of colorimetric analysis with diphenylcarbazide a t a wavelength of 540 nm.12 Prior to the above measurement, samples of the y-Al,O, suspension containing chromate were centrifuged for 45 min at 20 OOOg in order to effect settling of the particles. The pressure-jump apparatus used was described previously in detail." The f-potentials were determined by microelectrophore~is.~~J~ Results a n d Discussion Double relaxations were observed in an aqueous 7-A1203 suspension containing chromate by using the pressurejump technique with electric conductivity detection, as shown in Figure 1. The directions of both of the relaxation signals indicate a decrease in conductivity of the suspension during the relaxation. Since these relaxations were not observed in y-A120, suspension, in NazCr04solution, nor in the supernatant solution of the y-Al2O3-chromate suspension system, the relaxations observed may be related to the adsorption-desorption of chromate on the y-A1203 surface. The concentration and pH dependences of both fast and slow reciprocal relaxation times, TF' and T;', are shown in Figures 2 and 3, respectively. As seen from Figure 2, the values of both 7 ~ 'and 7Q-l increase with the concentration of chromate. Meanwhile, the values of 7f1 are independent of pH, while those of 7;' show a decrease and then approach a constant value, as shown in Figure 3. According to the mechanism for phosphate adsorptiondesorption reported in the preceding paper: let us consider (12) Sandell, E. B. "Colorimetric Determination of Traces of Metals", 2nd ed.; Interscience: New York, 1950; pp 260-5. (13) Hachiya, K.; Sasaki, M.; Saruta, Y.; Mikami, N.; Yasunaga, T. J. Phys. Chem., in press. (14)Hachiya, K.; Sasaki, M.; Ikeda, T.; Mikami, N.; Yasunaga, T. J . Phys. Chem., in press.
0 1983 American Chemical Society
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Mikami et al
The Journal of Physical Chemistry, Vol. 87, No. 25, 1983 I
I
I
(
I
I
t
l
3
(a)
I
8 PH Figure 3. pH dependence of 7 ~ and ’ 7;’ in the y-Al,03-chromate system at C, = 30 g dm-3, added chromate concentration 3 X mol dm-3, and 25 O C .
Flgure 1. (a) Typical double relaxation curve in the y-Al,O,-chromate system observed by using the pressure-jump technique with electric conductivity detection at C , = 30 g dm-3 and 25 OC: sweep, 20 ms/division. (b) The semilogarithmic plot of the relaxation curve: sweep, 5 ms/division.
where e is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, the superscript int denotes intrinsic, and the subscript p refers to the plane of adsorbed chromate. For the case in which reaction I is faster than reaction 11, the expressions for the slow and fast reciprocal relaxation times, 7;l and 7f1, are given by1r6J5 7,-l
= hint( ex.(
?)(
[AIOHz+]+
(K1-l + [A10H2+l)[Cr042-]
K1-l + [A10H2+]+ [HCr04-]
I 0
I
I
2 L chromateconcn. , l o 3 mol dm-3
’0 6
Flgure 2. Dependence of the reciprocal fast and slow relaxation times, 7;’ and T;’, on the chromate concentration in the y-Al,O,-chromate system at C , = 30 g dm-3 and 25 O C .
the following reactions for adsorption-desorption of monoand divalent chromate ions on 7-A1203surfaces: k
AIOHz+ + HCr04- & A1OH2+-HCrO4k-i
(I)
W,)
AIOHz+ + C r 0 2 -
k
A10H2+-Cr042-
k,
(11)
(K,)
where A10H2+-HCr04- and A1OHz+-CrOt- denote the adsorbed states of HCr0; and CrOZ-, respectively; kl and kl are the adsorption and desorption rate constants of HCr04-, and kz and k2are those of C T O ~ ~respectively; -, and K is the equilibrium constant. K1 and Kz are defined
K1 = [A1OHZ+-HCrO4-] ex.( [AIOHz+][HCr04-],
5 )= KT
Kz = [A10H2+-Cr042-] exp( [A10H2+][Cr042-]B
G) =
where the square brackets indicate the equilibrium concentrations. In order to obtain the concentration terms in eq 3 and 4, measurements of the amount of chromate adsorbed and the {-potential were performed. The chromate concentration dependences of the amount of chromate adsorbed, the pH, and the {-potential are shown in Figure 4, and the pH dependences of the percent chromate adsorbed and the {-potential are shown in Figure 5 . As can be seen from Figures 4a and 5a, the amount of chromate adsorbed is independent of the chromate concentration in the region where the kinetic measurements were carried out, while it decreases with increasing pH. Meanwhile, the (-potential decreases slowly with increasing chromate concentration, and also decreases with increasing pH, as shown in Figures 4b and 5b, respectively. From these static results, the values of Klint,Kzint,\k, and the equilibrium concentrations were determined according to Davis, James, and Leckie’s theoretical treatment,sg where the value of C2 = 20 pF/cm2 (ref 7 ) and the value of the dissociation constant of HCr04-, pKHCro4-= 6.49,16 were used. Both values of Klintand Kzlntobtained are fairly smaller than those obtained in the y-A1203-phosphate system.6 This fact suggests that the interaction of chro(15) Ashida, M.; Sasaki, M.; Kan, H.; Yasunaga, T.; Hachiya, K.; Inoue, T. J. Colloid Interface Sci. 1978, 67, 219. (16) Weast, R. C. “CRC Handbook of Chemistry and Physics”, 60th ed.; CRC Press: Boca Raton, FL, 1980; D167.
,
The Journal of Physical Chemistry, Vol. 87, No. 25, 1983
Adsorption-Desorption of Chromate on y-AI,03 Surface I
O t
-20
I
I
\a
L
0
5247
6
4
2 4 6 c h r o m a t e concn., 10-3mol d r f 3
IO
8 PH
Figure 4. Chromate concentration dependences of (a) the amount of chromate adsorbed, (0),pH (O),and (b) the {-potential at C , = 30 g dm-3 and 25 OC.
r
Figure 5. pH dependences of the amount of chromate adsorbed (a) and {-potential (b) at C , = 30 g dm-3, added chromate concentration 3 X mol dm-3. and 25 'C.
TABLE I: Intrinsic Equilibrium Constants of Adsorption-Desorption of Chromate on y A l , O , Surface at C, = 180, I = 8 X and 25 "C
2.8
1.9
mate with surface sites may be weaker than that of phosphate with surface sites. The plots of 7;l and 7f1 vs. the concentration terms in eq 3 and 4 calculated by using these values did not fall on the straight lines passing through the origin. Consequently, the assumption that reaction I is faster than reaction I1 is not correct. Next, let us examine the opposite case, in which the reaction I1 is faster than reaction I. The expressions for ~ , - land ~ f are l expressed as1s6J5 T,-~
= k p t ( ex.(
")( 2kT
[A10H2+]
0
I
mol dm-3
Flgure 6. Plots of 7;' vs. F , in eq 5 at the various values of C , = 100 (0,A),180 (0, A),and 240 (0,A). Circles and triangles indicate the results obtained from the concentration and pH dependences of T o - ' , respectively.
l-
,
0
= kpt/exp( E)([A~OI-I~+] e*!3
,
+
(K2-l+ [AlOH,+])[HCrO,-] K,-l + [A10H2+]+ [CrO4'-1
7f1
1 .o
0.5
F,
+ [Cr042-])+
k2intFz
3
1 2 F2 , 10-3mol dm-'
Figure 7. Plot of 7;' vs. F , in eq 6 at C , = 180. Circles and triangles indicate the results obtained from the concentration and pH dependences of T , - ' , respectively.
(6)
The plot of vs. F1based on eq 5 is shown in Figure 6, where the value of the inner layer capacitance, C1,7-9was varied as a parameter.6 As can be seen from this figure, the plot gives the best linear relation passing through the
origin a t C1 = 180 (kF/cm2). Based on eq 6, meanwhile, the plot of 7f1 vs. F2 a t C1 = 180 is only shown in Figure 7 , since the concentration term, F2,is not sensitive to the value of C1. This plot also gives a linear relation passing through the origin. These facts confirm kinetically the validity of the mechanism proposed. The values of klint and kzint were obtained from the slopes of both straight lines, and the values of k-l'"t and k-2intwere calculated with
TABLE 11: Intrinsic Rate Constants of Adsorption-Desorption of Anions on r-Al,O, Surface a t 2 5 "C -
k_,i"t,
anion
kIht, mol-' d m 3 s - '
S-I
kZht mol-' dm; s-l
chromate phosphate
5.3 x i o 4 4.1 x 105
1 . 9 x 10 2.3
9.9 x 1 0 4 1.1x 107
I
k.,i"t, S-'
ref
5.2 X 10 2.7
present work 6
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J. Phys. Chem. 1983, 87, 5248-5251
those of Klintand Kzintin Table I and are listed in Table 11. As can be seen from this table, the values of klht and klint are of the same order of magnitude as those of kzint and kZint, respectively. This result indicates that there is no difference in the dynamic properties between monoand divalent chromate ions, in contrast with the result for phosphate adsorption that the adsorption rate of HP042is much faster than that of HzP0;.6 Comparing the values of the adsorption-desorption rate constants for chromate in Table I1 with those for phosphate reported previously,6 the values of the adsorption rate constants for H C r 0 4 and Cr042-are 1and 2 orders of magnitude smaller than those for HzP04-and HP042-,respectively, while the values of desorption rate constants for the former are 1 order of magnitude larger than those for the latter, respectively.
This fact also indicates that the interaction of chromate with A10H2+ is weaker than that of phosphate with A10H2+. The value of C1 = 180 determined above, which is equal to that reported in the y-Al,O3-phosphate system,6 is slightly larger than that reported by Davis et al.7-9 This fact may result from an increase of dielectric constant of the electrical double layer caused by the large amount of chromate adsorbed. Further systematic investigation will lead to a quantitative clarification of the dynamic behavior of anion adsorption-desorption on y-Alz03surfaces. Registry No. CrOd2-,13907-45-4;HCr04-,15596-54-0;A120,, 1344-28-1.
Dye Sensitization of a Zinc Oxide Electrode Studied by a Potential Modulation Technique Michlo Matsumura, * Kenro Mltsuda, and Hlroshl Tsubomura Department of Chemistry. Faculty of Englneering Science, Osaka Univefslty, Toyonaka, Osaka 560, Japan (Received: October 26, 1982; In Flnal Form: April 7. 1983)
A potential modulation technique has been developed and applied to the study of the dye sensitization phenomena. The lifetimes of oxidized sensitizing dyes, Rose Bengal and Rhodamine B, which are formed by electron transfer from the excited states to the conduction band of a ZnO electrode, are determined to be 60 h 15 and 100 f 25 ms, respectively. The rate constants for electron transfer from potassium iodide, L-tryptophan, and hydroquinone to oxidized Rose Bengal are also determined to be 1.2 (h0.5) X lo4, 4.0 (f1.7) X lo5, and 3.3 (f1.4) X lo6 M-I s-l, respectively.
Introduction The characterization of reaction intermediates formed on semiconductor electrodes is very important for establishing stable photoelectrochemical cells. For example, the photoanodic corrosion of an n-type semiconductor electrode can be suppressed if the electron supply from a reductant to the electrode is faster than the rate of photoanodic corrosion, which depends on the nature of the corrosion intermediates. The details of the mechanism and the kinetics of the reactions a t semiconductor electrodes have been studied by use of spin trapping,l rotating ring-disk electrode (RRDE),2electroluminescence,3 etc. For the dye sensitized semiconductor electrodes, the lifetime of an oxidized dye is a critical value for the stabilization of the photocurrent, since the dye-sensitized photocurrent decays by the irreversible reaction of the oxidized dye.4 The photocurrent can be stabilized when electrons are supplied to the oxidized dye from reductants in the solution. The fundamental properties of the dyesensitized photocurrent, e.g., quantum e f f i ~ i e n c y ad,~~~ sorptivity of the sensitizing dyes on the e1ectrode:J effects of reductants4” and surface s t a t e ~ , ~etc.,13 - l ~ have been (1) Jeager, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (2) Frese, K. W., Jr.; Madou, M. J.; Morrison, S.R. J. Electrochem. SOC.1981, 128, 1527. (3) Nakato, Y.; Tsumura, A.; Tsubomura, H. Chem. Lett. 1981, 383. (4) Matsumura, M.; Nomura, Y.; Tsubomura, H. Bull. Chem. SOC. Jon. 1977.50. 2533. ( 5 ) Tsubomura, H.; Matsumura, M.; Nakatani, N.; Yamamoto, K.; Maeda, K. Sol. Energy 1978, 21,93. ( 6 ) Matsumura, M.; Matsudaira, S.;Tsubomura, H.; Takata, M.; Yanagida, H Inod. Eng. Chem. Prod. Res. Deuelop. 1980, 19, 415. (7) Spitler, M. T.; Calvin, M. J . Chem. Phys. 1977, 66, 4294.
clarified. However, the details of the kinetics are not known. In this paper, we have determined the lifetimes of the oxidized dyes formed on a ZnO electrode by use of a potential modulation technique. The rate constants for electron transfer from the reductants to the oxidized dye will also be presented. The lifetimes of oxidized dyes have been reported by use of the RRDE technique14 or flash phot01ysis.l~ However, these methods give information about the dyes dissolved in solutions. In this paper we give results on the kinetics of the dye adsorbed on the semiconductor electrode, which is more important for the understanding of the dye sensitization phenomena a t a semiconductor electrode. Theory The energy models of an n-type semiconductor electrode, polarized anodically and a t the flat-band potential, and an excited dye molecule are shown in Figure 1. The electrons injected from the excited dye to the electrode surface move into the bulk when a depletion layer is (8) Watanabe, T.; Nakao, M.; Fujishima, A.; Honda, K. Ber. Bunsenges. Phys. Chem. 1980,84, 74. (9)Matsumura, M.; Nomura, Y.; Tsubomura, H. Bull. Chem. SOC. Jpn. 1979,52, 1599. (10) Arden, W.; Fromherz, P. J. Electrochem. SOC.1980, 127, 370. (11)Yamase. T.: Gerischer, H.: Lubke. M.: Pettinaer, B. Ber. Bunsenges. Phys. Chem. 1978,82, 1041. (12) Kojima, T.; Ban, T.; Kasatani, K.; Kawasaki, M.; Sato, H. Chem. Phys. Lett. 1982, 91, 319. (13) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31. (14) Watanabe, T.; Fujishima, A.; Honda, K. Chem. Lett. 1978, 735. (15) Usui, Y.; Iwanaga, C.; Koizumi, M. Bull. Chem. SOC.Jpn. 1969, 42, 1231.
0022-365418312087-5248$01.50/00 1983 American Chemical Society
