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J. Phys. Chem. 1982, 86, 3468-3471
found evidence for particular solute/solvent associations in the waterldioxane system. (5) With the proposed equation, calculated values o f t for very dilute solutions as a function of concentration, with one exception, are in good agreement with experiment.
Acknowledgment. We thank Professor Manse1 Davies (Aberystwyth,Wales, U.K.)for comments and suggestions and particularly appreciate the assistance of Drs. R. Hill and L. Dissado (Chelsea College, University of London, U.K.) for their careful, detailed, and critical revision of the manuscript. The continuing support of the Conseio Na-
Strong Interaction of Metallotetraphenylporphyrlns with Supporting Metal Oxides Observed in the Catalytlc Decomposition of Hydrogen Peroxide Isao Mochlda, Aklnorl Yawtake, Hlroshl Fujltsu, and Kenjlro Takeshlta Reseerch Institute of Industrial Science, Kyushu Unhws& 86, Kasuga 816, Japan (Received: July 6, 1981; I n Finel Form: March 29, 7982)
The catalytic activities of metallotetraphenylporphyrins (M-TPP's) and their supported forms on some oxides for the decomposition of hydrogen peroxide were studied in buffers of pH ranging between 6.5 and 10.2. Unsupported M-TPP's including H,-TPP showed a small but definite constant activity at pH 10.2 regardless of their central metal ions and the first oxidation potentials. Supporting M-TPP on the oxides distinguished Co-TPP from other complexes and TPP ligand. The activity of the former complex was much enhanced by being supported on alumina and nickel oxide, whereas the others lost their activity, suggesting a strong and selective interaction of the catalyst material with the supports. The large conjugate ring of the ligand which has much the same oxidation potential regardless of the central metal ions may provide the active site for the catalysis common to the unsupported catalysts. In contrast, the active site of supported Co-TPP may be assumed to be the central metal ion, which can be modified to be active by the electronic transfer to the support through its axial coordination bond. Increased effective surface area and the hydrophilic nature of the supported catalyst may also contribute to the increase of the activity.
Introduction The catalytic decomposition of hydrogen peroxide has been extensively studied by using various metal ions and their complexes as a model for the catalysis promoted by catalase.' The reaction has often been explained in terms of the Haber-Weiss mechanism,2 including the redox of the central ion. We have reported that the activities of transition-metal ions exchanged onto zeolites vary in a bell-shaped pattern against their redox potential^.^ Such a dependency on their redox potentials suggests that the proper modification of the redox potential of an ion by any means may enhance its catalytic activity. Suitable coordination has been known to increase the catalytic activity! Some chelating ligands brought remarkable enhancement5 to give an activity comparable to that of catalase. A series of metallotetraphenylporphyrins (M-TF'P's) are known to show a sequence of redox potentials, and their catalytic activities have been examined in several sptems.6 Some of the oxide carriers are reported to be able to modify the properties of the metals and some complexes supported on them through their electronic interactions.' We found that Co-TPP showed a very high catalytic activity for the
reduction of nitric oxide after being supported on titanium dioxide. The enhancement can be assumed to be due to an electron transfer from the support to the complex.s Such a strong interaction of the catalytic material with the support may be a key factor for the development of a novel ~atalyst.~ In the present study, catalytic activities of some MTPP's for the decomposition of hydrogen peroxide were investigated to describe the roles of their metal ions and ligands in this catalysis and to reveal the effects of metal oxide supports on their activities since a sequence of the redox potentials and their strong interaction with the support may attract interest. The interaction can be evaluated not only in the catalytic reaction but also in the chemistry of the complex which may play a role as a probe to reflect the interaction. The supports were selected from fairly large varieties to define what properties were essential for the interaction. Although Okura et al.*O already reported the activities of Cu-TPP and Co-TPP supported on silica gel for the same reaction, their activities were rather limited to decomposing only 4 mol of the peroxide per mole of the catalyst even at 70 "C. Sigel et al." ex-
(1)Uri, N. Chem. Reu. 1952,50,375. Baxendale, J. H. Adu. Catal. 1953,4, 31. George, P.Ibid. 1952,4 , 367. (2)Haber, F.; Weiss, J. Naturwissenschaften 1932,20, 948. (3)Mochida, I.; Takeshita, K. J. Phys. Chem. 1974,78, 1653. (4)Sigel, H.; Wyss, K Fischer, B. E.; Prijs, B. Inorg. Chem. 1979,18, 1354. Sharma, V. S.;Schubert, J. Ibid. 1971,10,251. (5)Wang, J. H.J.Am. Chem. SOC.1955,77,822,4715. (6)Manassen, J. Catal. Reu. 1974,9,223. (7)Uchida, K.; Soma, M.; Onishi, T.; Tamaru, K. 2.Phys. Chem. (Frankfurt am Main) 1977,106,317.Jaeger, C.D.;Fan, F. F.; Bard, A. J. J. Am. Chem. SOC.1980,102,2592.
(8)Mochida, I.; Tsuji, K.; Suetsugu, K.; Fujitsu, H.; Takeshita, K. J . Phys. Chem. 1980,84,3159.Mochida, I.; Fujitau, H.; Takeshita,K.; Tsuji, K. Int. Congr. Catal., h o c . 7th 1980,1516. Mochida, I.; Suetsugu, K.; Fujitsu, H.; Takeshita, K.; Tsuji, K.; Sagara, Y.; Ohyoshi, A. J . Catal., submitted. (9)Vannice, M. A.; Garten, R. L. J. Catal. 1979,56,236. Kugler, E. L.Prep. Diu. Pet. Chem., Am. Chem. SOC.1980,25,564.Tauster, S.J.; Fung, S. C.; Garten, R. L. J . Am. Chem. SOC.1978,100,170. (10)Okura, I.; Kim-Thuan, N.; Keii, T. J . Mol. Catal. 1979,5, 293. (11) Erlenmeyer,H.; Waldmeier, P.; Sigel, H. Helu. Chim. Acta 1968, 51,1795. Waldmeier, P.;Sigel, H. Inorg. Chem. 1972,11, 2174.
0022-3654/82/2086-3468$01.25/0
0 1982 American Chemical Society
Catalytic Activities of Metallotetraphenylporphyrins
TABLE I: Oxide Supports and Their Properties BET surface oxide area, m*/g supplier silica gel 300 Wako Pure Chemical Industry, Ltd. silica-alumina L.A. 558 Shokubai Kasei Co. (14 wt % Al,O,)" silica-alumina H.A. 516 Shokubai Kasei Co. (29wt % Al,O,)b yalumina 130 Merck titanium dioxide 170 Titan Industry nickel oxide (green) 20 Wako Pure Chemical .Industry, Ltd nickel oxide (black) 170 calcined nickel carbonate a t 550 "C L.A. = low alumina.
H.A. = high alumina.
cluded the activity of tetradentated metal ion such as tetrasulfophthalocyanine. However, in the present study a relatively high pH of the reaction medium is found to enable the catalytic reaction to proceed at a high efficiency. The favorable effects of high pH for the reaction have been well recognized.12
Experimental Section Materials. Tetraphenylporphyrin (TPP)was synthesized according to Adler et al.13 Metal ions were introduced by refluxing TPP and corresponding metal acetates in N,N-dimethylformamide. Hydrogen peroxide was obtained from Mitsubishi Gas Chemical Co. Oxide supports and their suppliers are listed in Table I. Preparation of Catalysts. Metal oxide (2.45 g) was impregnated in 200 mL of a benzene solution of M-TPP (50 mg) overnight at room temperature, evacuated, and then dried at 80 "C. The M-TF'P content on the oxide was fixed at 2 w t %. Kinetic Procedure. After a glass flask containing 50 mL of buffer and the catalyst (170 mg) was immersed for 30 min in a constant-temperature bath (25 "C), 0.1 mL of H202was added. The mixture was magnetically stirred vigorously to avoid diffusion control, and the evolution of oxygen during the course of H202 decomposition was followed as a function of time by a mercury manometer connected to the flask. Catalytic activity was obtained as the initial rate of HzOzdecomposition, V (torr/min). For all of the reactions, V values were obtained within *5% error. Because H202dissociates to produce acidity, buffers were used as the reaction medium to maintain constant pH during the decomposition reaction. They were prepared from variable mixing ratios of Na2C03and NaHC03 to obtain pH values of 9.1 and 10.2. These carbonates may never coordinate to the metal ions of M-TPP, being different from phosphates, which were reported to influence the catalytic activities of metal ionsa4 Results and Discussion Catalytic Activities of M-TPP's. Catalytic activities of M-TPP's (unsupported fme powder) for the decomposition of hydrogen peroxide in aqueous solution at a pH 10.2 are plotted against their respective oxidation potentials in Figure 1, where those of aqueous metal ions are also included for comparison. The first oxidation potentials of M-TPP's referred to Manassen et al.14 are those of the central ions for Fe and Co complexes and those of the (12) Sigel, H.; MGller, U. Helu. Chim. Acta 1966, 49, 671. (13) Adler, A. D.; Longo, F. R.; Finarellii, J. D.; Goldmacher, J.;Assour, J.; Korsakoff, L. J.Org. Chem. 1967,32,476. Adler, A. D.; Longo,F. R.; Kampas, F.; Kim, J. J.Znorg. Nucl. Chem. 1970, 32, 2443. (14) Wolberg, A.; Manassen, J. J. Am. Chem. SOC.1970, 92, 2982.
The Journal of Physlcal Chemistry, Vol. 86, No. 17, 1982 3460
A8
I C
-1.5
-1.0
-05
0.0
Oxidation Potential
05
1.0
(V)
Flgwe 1. Catalytic actMty of MTPP correlated with oxidation potential. Reaction conditions: MTPP, 5 X 10" mol in 50 mL of buffer @H 10.2); H,Oz, 1 X mol; reaction temperature, 25 'C. M-TPP 0: (1) H,-TPP, (2) Cu-TPP, (3) Ni-TPP, (4) Zn-TPP, (5) Co-TPP, (6) FeCI-TPP. Metal ion A: (7) NiH, (8) As+, (9) Cow, (10) Mnw, (1 1) C p , (12) pb*, (13) C6'.
TABLE 11: Catalytic Activities of Some Metdoporphyrins Supported on Metal Oxidesa
lo2(ac tivi ty ), torr/min M-TPP H,-TPP FeCl-TPP CO-TPP CU-TPP
none 22 21 19 23
SiO,* 0.1 0.8 150 0.4
TiOZC 0 10 86 18
At 25 'C, pH 10.2. Activities of supports were n e g ligible. After subtracting the activity of the support.
macrocyclic ligands for Ni, Zn, and Cu complexes, respectively. The conversion of hydrogen peroxide finally reached 100% in an approximately first-order reaction profile with all complexes. The activities, which were not large but definite under the present conditions in spite of a previous report,1° were constant regardless of values and origins of their oxidation potentials. Even the acid form of the ligand, H2-TPP, showed the same activity. Such constant activities may suggest that the large conjugated *-electron system of the macrocyclic ligand may provide the active site for the decomposition. In contrast to M-TPP's, aqueous ions showed an activity order of a bell-shaped correlation against their oxidation potentials as observed with those supported in zeolites3 and on oxides.15 Catalytic Activities of M-TPP Supported on Metal Oxides. The catalytic activites of three M-TPP's and H2-TPPsupported on silica gel and titanium dioxide are summarized in Table 11. Supporting Co-TPP on silica gel increased its catalytic activity per 5 bmol of M-TPP by 8 times that of unsupported Co-TPP. The absolute rate and the catalytic turnover number in the present study were much larger than those reported by Okura et al.1° probably because of the high pH of the reaction medium. In contrast, the activity of other complexes and the ligand decreased several hundredfold by being supported on silica gel. The most marked decrease of the activity of H,-TPP should be noted. Titanium dioxide gave similar effects, although the extent of change was much smaller. Effects of some metal oxide supports on the catalytic activity of Co-TPP are summarized in Table 111. The activity increased by supporting Co-TPP on any oxides examined in the present study, but the extent of the in(15) 1505.
Kitajima, N.; Fukuzumi, s.; Ono, Y. J. Phys. Chem. 1978, 82,
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The Journal of Physicel Chemistry, Vol. 86, No. 17, 1982
Mochida et ai.
TABLE 111: Catalytic Activities of Co-TPP Supported on Metal Oxidesa
103v,c lOZV, carrier
L.A.~ H.A.e SiO, A1203
TiO, NiO f NiOB
CO-TPP
torr lmin
99
66 150 210 86b 41 520
torr gl (min m Z )
1.9 1.2 5.0
14 5.0
20 30.6 I
19
a At 25 'C, pH 10.2. V after subtracting the activity of carrier. Activity per surface area of the sup ort. L.A. = low alumina. e H.A. = high alumina. ?Green nickel oxide. Black nickel oxide.
crease depended very much on the kind of oxide. The activity of the support alone was negligible except for titanium dioxide and nickel oxide (black). Among the oxides in Table 111, nickel oxide (black) gave the greatest activity. The oxides of high surface area usually allowed large increases of activity, suggesting that better dispersion of Co-TPP is essential for the activity increase by increasing the number of Co-TPP's easily accessible to the substrate. Nevertheless, the marked difference far beyond the error in the activity due to the kind of oxide may indicate the surface area of the support is not the sole factor for the activity enhancement. The chemical environment around Co-TPP which the supporting oxide provides may ale0 play some essential roles in this catalysj~. Surprisingly high activities produced by y-alumina and nickel oxide (black) should be noted, because their surface areas are not so large. Titanium dioxide, which was the best support for the reduction of nitric oxide over Co-TPP,8showed an activity of only 1/8 that of nickel oxide (black), although their surface areas were similar. Although it is very difficult to count quanitatively the number of Co-TPPs really active in the catalysis increased by support, it may be reasonably assumed that the number is roughly proportional to the surface area of the support. Since a monolayer dispersion of the complex is never achieved on the present supports, a limited fraction of the complex which may interact with the support is responsible for the enhanced catalytic activity. In such a case, the activity per surface area of the supporting oxide (Table III) can be a measure of the extent of the activity increase induced by the support. It is meaningful to evaluate the extent since some strong interaction has been r e p ~ r t e d . ~ Both alumina and nickel oxide (black) of high activity exhibited also very high activities per surface area. It is of value to note a high value of green nickel oxide comparable to that of black nickel oxide, although the activity per weight of the former is very limited because of its limited surface area. Nickel oxide is, thus, suggested to interact favorably with Co-TPP for this catalysis. Although the strong interaction of titanium dioxide with the complex to produce an anion radical has been reported to enhance the reduction activity for nitric oxide: the oxide did not enhance so much the decomposition of hydrogen peroxide. The very limited activity increase by silica-alumina mixtures of strong acidity in spite of their alumina content and high surface area may suggest another factor influencing the catalytic activity. p H Dependence of Catalytic Activity. Catalytic activities of Co-TPP, H2-TPP, and their silica-supported counterparts in buffer media of variable pH values are
6
7
6
9 PH
1
0
Figure 2. Catalytic activlty of M-TPP or supported M-TPP at some values of pH. Reaction conditlons: MTPP, 5 X loa mol in 50 mL of buffer; H202, 1 X 10" mol; reaction temperature, 25 "C. (0) CO-TPP, ( 0 )CO-TPP/SIO~,(A)HZ-TPP, (0) H,-TPP/SIOp
illustrated in Figure 2. The catalytic activity increased sharply in the basic region beyond the pH value of 8.5 regardless of the catalysts. Above all, the increase of CoTPP/Si02 was largest. Since the strong dependence of this reaction on pH of the medium has been well-known,12 the low pH of the medium may be a definite reason that Okura et assumed no activity of unsupported M-TPP even at 70 "C. Similar dependence of the activities of unsupported Co-TPP and H2-TPP may suggest the same reaction mechanism on the same active site. A different profile of pH dependence appeared by supporting Co-TPP on Si02, indicating a different active site on the catalyst. Active Sites of M-TPP and Roles of Carrier in the Catalytic Decomposition of H2OP The catalytic decomposition of hydrogen peroxide has often been explained in terms of the Haber-Weiss mechanism2 (Scheme I), where the redox cycle of the catalyst is reflected at steps 2 and 4. Scheme I k
2H202 e2H+ + 2H00-
-- + -
(1) 2H00. 2(ML)(n-1)+ (2) 02 H202 2H00. (3) H202+ 2(ML)("-l)+ 20H- 2(ML)"+ (4)
2H00-
+ 2(ML)"+
+
+
The constant activities of unsupported M-TPP's including H2-TPP, regardless of their central metal ion and the first oxidation potentials and similar pH dependence, may suggest that the macrocyclic conjugated ?r-electron system of the ligand serves the active site for the catalysis. The oxidation potential of the ring is known to be almost indifferent to the central metal Cu and Zn ions in their rigidly tetracoordinated state such as in Cu- and Zn-TPP are generally accepted to be inactive for this catalysis.'l Cobalt ions chelated by N compounds generally lose their high a ~ t i v i t y .Waldmeier ~ and Sigel" have reported that cobalt(II1) hematoporphyrin has an activity, but other cobalt complexes including cobalt phthalocyanine have little activity. Such facts may support the above suggestion concerning the active site of unsupported M-TPP. M-TPP's except for Co-TPP, lost activities by being supported on silica gel in spite of their increased dispersion on the oxides, because the electronic configuration or the mobility of the ring system may be unfavorably modified by the carrier. The central metal ion of Co-TPP can be modified by the carriers to be active for this catalysis by reacting with
J. Phys. Chem. 1902, 86, 3471-3478
HOO-, although ita ring system may lose activity. The possibility of hexagonal coordination for the cobalt ion (only ion in the present study) may allow the effective modification by the carrier through its fifth coordination site to be active at ita sixth coordination site. Okura et al? assumed that oxidized C e T P P easily reads with HOO(step 2). The face-to-face cobalt porphyrin has been reported to form Gc-superoxo)dicobaltcomplex, allowing the electrochemical reduction of molecular oxygen to HzOz.16 (16)Collman, J. P.; Denisevich, P.; Konai,Y.; Marrocco, M.; Koval, C.; Anaon, F. C. J.Am. Chem. SOC.1980,102,6027.
3471
The support may encourage such redox properties of the central metal ion through a strong interaction. A large enhancement by nickel oxide and alumina may suggest that p-type semiconduction of the carrier (accepting an electron from Co-TPP) may be favorable for such a catalyst activation, in contrast to the case of N&H2 reaction.€ In addition, the enlarged effective surface area of MTPP and the local acid-base and hydrophilic natures of the carrier surface should strongly influence the activity. Limited activity increase by silica-alumina may be due tc its high acidity, which is not favorable for the dissociation of hydrogen peroxide (step 1) near the surface.
Rate of Demlcelllzatlon from the Dynamlc Surface Tensions of Micellar Solutions E. Rlllaerto and P. Jooo' DePament Cdbmb, Unlversitake Instelling Antwerpen, Universiteitsplein 1. 2610 Wllr!lk, Belgium (Received: October 27, 1981; In Final Form: April 6, 1982)
The dynamic surface tensions of micellar surfactant solutions (Triton X 100, cetyltrimethylammoniumbromide (CTAB), cetylpyridiniumchloride (CPCl),and myristyltrimethylammonium bromide (MTAB)) were measured by using the oscillating-jetand flowing-film methods. The experimental data are discussed by the assignmeni of a dilatation on a surface element. The diffusion equation considering demicellization as a chemical reactior was solved by using the concept of a diffusion penetration depth. From these experiments rate constants foi demicellization are obtained. The rate constants are of the same order of magnitudes as those obtained b5 other investigators using more classical techniques for fast reactions.
Introduction Both the oscillatingjet a d the flowing film are suitable methods for measuring the dynamic surface tension of surfactant solutions.1-3 The flowing film is a method recently developed in our laboratory and consists of flowing a surfactant solution as a thin layer (thickness of the order of 1 mm) over an inclined plate. In both methods, at a variable distance from the orifice of the jet or the inlet of the flowing film. the surface tension is measured. Following H&sen' a surface element a t a distance x has a surface age t given by t = x / u , (u, = the velocity a t the surface at x ) . The resulting surface tension as a function of time is currently discussed by diffusion of surfactant molecules from the bulk to the surface, and in many cases it is found that the adsorption kinetics are diffusion controlled. For some surfactants however (diols),P e t r P gave evidence that diffusion is not the rate-determining step. Considering surfactant solutions at concentrations exceeding the cmc, first monomers are diffusing from the bulk to the new surface. Secondly, the bulk liquid near the surface is depleted from monomers, the concentration becomes less than the cmc, and monomers are supplied by the micelles. Hence, besides diffusion, another relaxation process, micellar breakdown, becomes operative and affecta the dynamic surface tension. This dynamic surface tension depends on the rate of demicellization. This demicellization enters as a source term in the diffusion equation. Turning around the dynamic surface tension (1) R. S. Hansen, J. Phys. Chem., 68, 2012 (1964). (2)R.Defay and G. Petr6, "Surface and Colloid Science", Vol. 3, E. Matijevic, Ed., Wiley, New York, 1971,p 27. (3) R. Van den Bogaert and P. Jooe,J. Phys.Chem., 83,2244(1979); 84, 190 (1980). (4)G. Petr6, Ph.D. Thesis, Brussels, 1970. 0022-3654/82/2086-3471101.25/0
of micellar solutions yields information about the rate ai which the micelles are broken and seems to be, besides the more CO~~entional methods as T ,P j m P and Stopped flow a method to study demicellization kinetics.
Theory
As already said, to a given surface element at a distance x from the orifice or the inlet, a surface age t is attributed t = x/u. (1:
On the other hand, one can consider, following Cerro a n d Whitaker,5 the whole system as being stationary. The mathematical analysis is very complicated; a major diffi. culty is that the hydrodynamic and (convective) diffusior equations are coupled. The fluid velocity of the surface at the inlet (at x = 0) is zero and attains a maximum value after a transition regime (strictly for x a). Among othei things, this transition regime depends on the kind oj surfactant in solution and its concentration. Apparent13 a gradient in velocity along the surface, dvJdx, occurs. Tht dilatation 0 is defined as 0 = du,/dx. This stretching 01 the surface, expressed by the dilatation, is the highest ai the inlet and becomes zero a t x m. At last, adsorption to an empty surface, as treated bj Ward and Tordai? is characterized by a relaxation timt being the same as that coming from longitudinal waw experiments.' This was corroborated by Loglio et al.'
-
-
(5)R.L. Cerro and S.Whitaker, Chem. Eng. Sci., 26,785(1971a);J Colloid Interface Sei., 37,33 (1971). (6)Ward and L.Tordai, J. Chem. Phys. (7) J. Lucassen and D. Giles, J. Chem. Soc., Faraday Trans. 1,71,21' (1975). (8)G. Loglio, E.Rherta, and P. Joos, Colloid Polym. Sci., 259,1221 (1980);G.Loglio, V. Tesei, and R. Cini, J . Colloid Interface Sci., 71,311 (1979).
0 1982 Amerlcan Chemical Society
