Langmuir 1995,11, 1009-1013
1009
Electrochemical Properties of Iron Phthalocyanine Immobilized on Titanium(n7) Oxide Coated 6n Silica Gel Surface Lauro T. Kubota,? Yoshitaka Gushikem,*vt Joelma Perez,$ and Auro A. TanakaS>s Instituto de Quimica, Unicamp, C.P. 6154, 13083-970 Campinas, Sdo Paulo, Brazil, Instituto de Fisica e Quimica de Sdo Carlos, USP, C.P. 369, 13560-970, Siio Carlos, Sdo Paulo, Brazil Received May 13, 1994. I n Final Form: November 29, 1994@ Electrochemicalstudies ofiron tetrasulfonatedphthalocyanine(FeTsPc)immobilized on silica gel modified with titanium(IV)oxide were carried out in aqueous solutions. Cyclic voltammogramsrecorded in different supporting electrolyte solutions presented a redox process that became better defined under acidification. The position of the peak potentials (0.35-0.45 V us SCE) and their invariance with solution pH indicated that these processes can be assigned to the FeI1TsPc/Fe1I1TsPcredox couple, with characteristics similar to those observed for FeTsPc adsorbed on graphite surfaces. The nature ofthe supporting cation in solution does not shift the midpoint potential (E,) of the redox process, when Li+, Na+, K+, and NH4+ are used. In contrast, the supporting anion has a strong effect, i.e., the E m shifts toward more positive values in the ck>so4=.Preliminary experiments indicated that the modified electrode presents order ClO4->NO3-> catalytic activity for hydrazine oxidation in neutral solution.
Introduction Metallophthalocyanines constitute a n important class of catalysts which have some advantages over metal oxides and provide interesting models for theoretical and experimental studies involving electron mediated processes, since most of the catalytic reactions involve the transfer of electrons. lV3 Electrochemical studies of these compounds have supplied important information about the mechanism of electron mediation in catalytic reaction^.^,^ Recently, papers on electrochemical studies of metallophthalocyanines and hexacyanometallates adsorbed on different matrixes have been reported.6-8 The possibilities of characterizing the structures of the surfaces modified with electroactive species have increased the interest in this fieid.9~0 In our laboratory, we have prepared metal oxides coated on a silica gel surface. These materials were able to adsorb hexacyanoferrate anion complexes.11,12 Studies of the electrochemical properties of modified silica surfaces are possible due to the development of carbon paste electrodes
* Author for correspondence. + Instituto de Quimica, Unicamp.
* Instituto de Fisica e Quimica de SLo Carlos.
Present address: Departamento de Quimica, Universidade Federal do Maranhlo, 65080-040 S l o Luis, MA,Brazil. @Abstractpublished i n Advance A C S Abstracts, February 15, 1995. (1)Machida, K.; Anson, F. C. J.Electroanal. Chem. 1988,256,463. (2)Kirschenmann., M.:, Wohrle. D. and Vielstich. W. Phrs. Chem. 1988,&, 1403. (3) Biloul, A.; Contamin, 0.; Scarbech, G.; Savy, M. J.Electroanal. Chem. 1992,335,162. (4)Fierro, C.: Anderson, A. B.; Scherson, D. A. J.Phys. Chem. 1988, 92,6902. ( 5 ) Ikeda, 0.; Itoh, S.;Yoneyama, H. Bull. Chem. Soc.Jpn. 1988,61, 1428. (6) Fisher, H.; Schulz-Ekloff,G.; Buck, T.; Wohrle, D.; Vassileva, M.; Andeev, A. Langmuir 1992,8, 2720. (7)Zagal, J. H.Coord. Chem. Rev. 1992,119,89. (8)Zaldivar, G.A. P.; Gushikem, Y.; Kubota, L. T. J . Electroanal. Chem. 1991,318,247. (9)Siperko, L.M.; Kuwana, T.; J.Electrochem. SOC.1986,133,2439. Villar, E.; Ureta-Zanartu, S. J.Electroanal. Chem. (10)Zagal, J.H.; 1982,135,343. (11)Kubota, L. T.; Gushikem, Y. Electrochim. Acta 1992,37,2437. (12)Andreotti, E.I. S.; Gushikem, Y.; Kubota, L. T. J. Braz. Chem. SOC.1992,3,21.
with these materials.13 Such electrodes are very important, particularly in connection with their catalytic properties14 due to their high porosity and high surface area. The modification of the silica surface with titanium(IV1 oxide and the adsorption of iron phthalocyanine on the modified surface are described in the present work. The electrode properties and the electrocatalytic activity of hydrazine oxidation using a n electrode made by this material were studied.
Experimental Section
Silica-Titanium Preparation. Silica gel 60 (Merck), with a specific surface area of 500 m2g-l and particle size between 0.025and 0.20mm, was previously dried a t 423 Kunder high vacuum. This activated material (40g) was immersed in 400 mL of C C 4 solution of titanium tetrachloride 10% (vh) and the mixture was refluxed for 10 h under nitrogen atmosphere. The resulting solid was filtered in a Schlenk apparatus under nitrogen and washed with carbon tetrachloride until all unreacted titanium tetrachloride was eliminated. The product was then heated a t about 423 K under high vacuum, in order to remove all adsorbed solvent and trapped HC1 gas. The solid was carefully hydrolyzed, washed with demineralized water, and, finally, dried in a n oven a t 373 K. The following equations can be written to describe the preparation reactions:
+ - (=SiO)nTiC14-n + nHCl (=SiO)nTiC14-n+ (4 - n)H,O - (=SiO)nTi(OH)4-n+ n ( G S i 0 H ) TiC1,
(4 - n)HC1
where ESiOH stands for silanol group. For the sake of brevity, the chemically modified silica, ( ~ s i O ) ~ T i ( o H ) 4 - ~ , hereafter will be denoted as SiOz/TiOz. (13)Kubota, L.T.;Gushikem, Y. J . Electroanal. Chem. 1993,362, 219. (14)Collman, J.P.;Marrocco, M.; Denisevich, P.; Koval, C.; Anson, F.C. J. Electroanal. Chem. 1979,101,717.
0743-746319512411-1009$09.00/00 1995 American Chemical Society
1010 Langmuir, Vol. 11, No. 3, 1995
Kubota et al.
The determination of titanium concentration on the substrate was carried out by leaching the metal from the surface with hot concentrated HC1 solution followed by a gravimetric analysis. The specific surface area was determined by BET multipoints method, using a Micromeritics FlowSorb I1 2300. Adsorption of Iron Tetrasulfonated Phthalocyanine. About 5 g of SiOJl’iOz was immersed in 100 mL of 0.01 M aqueous solution of iron tetrasulfonated phthalocyanine (FeTsPc). The mixture was shaken for 20 min and then filtered, washed, and dried a t 373 K under vacuum. The amount of adsorbed iron complex was determined by atomic absorption spectrometry. The quantity of FeTsPc- was 0.65 mmol g-l. W-vis Spectoscopy. W-vis spectrum of the material was obtained by suspending the material in carbon tetrachloride, as previously described,15and iron tetrasulfonated phthalocyanine in dichloromethane solution, using a Carry 2300 spectrophotometer. Cyclic Voltammetry (CV) Experiments. Carbon paste electrodes were prepared by mixing high purity graphite (Fluka) and the material in a 3:4 (w/w) ratio with a few drops of mineral oil. The electrochemical measurements were performed using a PAR-273 potentiostat. A system with three electrodes was used for all experiments. The carbon paste electrode of the material was used as the working electrode, platinum wire as the auxiliary electrode, and the saturated calomel electrode (SCE) as the reference. All measurements were carried out under high-purity nitrogen. The acidity of the supporting electrolyte solutions was adjusted by addition of acid solution of the same anion. Electrocatalytic Oxidation of Hydrazine. The electrocatalytic activity of [FeTsPcl adsorbed on SiOf102, hereafter denoted as SiOJl’iOfleTsPc, was studied by a cyclic voltammetry technique in neutral KC1 1.0 M solution, with different concentrations of hydrazine sulfate.
Results and Discussion Characteristics of the Material. The amount of titanium grafted on silica gel surface was about 1.0 mmol g-l, and the BET specific surface area was 464 m2g-l. The material is thermally stable and showed a good chemical stability in acid solutions. The exchange properties of the coated titanium(IV1 oxide were similar to those observed for pure hydrated titanium(W oxide in adsorbing anionic species from acid ~ o l u t i o n s . ~In~ this J ~ case the adsorption of FeTsPc by SiOJl’iO2 can be described by the following equation:16
3TiOH
+ H+ + [FeTsPcl-
-
=Ti+[FeTsPcl-
+ H,O
where TiOH stands for Ti(IV) oxide coating. FeTsPc- strongly adhered to the surface and could not be leached from the surface even with 3.0 M HN03 or 1.0 M HC1 solutions, but was completely leached out with a 0.5 M NaOH solution. This behavior is presumably due to the amphoteric character of the material as observed for most of the metal 0xides.l’ Figure 1B shows the absorption spectrum ofthe material between the 300 and 900 nm range. Two absorption bands with maxima a t 363 and 647 nm are observed. These (15)Gushikem, Y.;Moreira, J. C. J. ColloidInterfuce Sci. 1986,107, 70. (16) Kubota, L.T.;Gushikem, Y.: Castro, S. C.: Moreira. J. C. Colloids Surf. 1991,57,11. (17)Strelko, V. V.; Khainakov, S. A.; Kvashenko, A. P.; Belyakov, V. N.;Bortun, A. I. J. Appl. Chem. USSR, 1988,61, 1922.
c
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a a
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m
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a
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333
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WAVELENGTH/nm Figure 1. Absorption spectraof (A)FeTsPc in dichloromethane and (B) FeTsPc immobilized on S i O n O z .
pH 6.0
%:
-\
-
pH 2.0
-
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bands are similar to those obtained in dichloromethane solution (Figure 1A) and to those reported in the literature,ls which indicate that immobilization does not have a significant influence on the structure of the complex. Redox Properties of theMateria1. The carbon paste electrode with SiOfliOZ presented the voltammogram shown in Figure 2. The potential was scanned from 0.2 up to 0.8 V in both acid solutions, and in neutral solutions did not present any current peak. When SiOflOfleTsPc was used, no current peak was observed in neutral solution. However, in acid solution (pH I 2), two current peaks appeared as can be seen in Figure 3. The anodic and cathodic peaks of cyclic voltammograms recorded in different supporting electrolyte solutions presented a better defined redox process under acidification, as illustrated in Figure 4. The value of the midpoint potentials (0.40 V vs SCE) and its invariance for solution pH I 2.0, suggest that the redox process can be attributed to the FeI1TsPc/Fe1I1TsPcredox couple. This feature is similar to that observed for FeTsPc adsorbed on graphite surfaces.lg The increase of the peak current under (18)Knothe, G.; Wohrle, D. Mukromol. Chem. 1989,190,1573. (19) Contamin, 0.; Levart, E.; Savy, M. J. Electroanul. Chem. 1980, 115,267.
Studies of FeTsPc on Silica Gel
7.5
Langmuir, Vol. 11, No. 3, 1995 1011
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E/V Figure 3. Cyclic voltammograms of carbon paste electrode with SiOfliOfleTsPc obtained in NaN03 1.0 M at pH = 2.0 (-) and pH = 6.0 (..-) and scan rate of 10 mV s-l.
I
I
0.4
0.1
0.5 0.7 0.9 E/V Figure 4. Cyclic voltammograms of carbon paste electrode 0.3
with SiOmOfleTsPc obtained in different solutions pH in 1.0 M NaN03 and at 2 mV s-l. Table 1. Midpoint potential (E,) of Si02/TiOz/FeTsPcin Different Supporting Electrolytes (10 M Solution Concentration) supporting electrolyte E, /mV vs SCE LiCl 447 f 5 NaCl 448 f 5 KC1 446 f 5 NHiCl 443 f 5
acidification can be attributed to the protonation of the sulfonic groups and increase of Lewis acid sites on SiOd Ti02 surface that facilitates the approach of the anion from the supporting electrolyte solution to the electrode interface. However, in the present case the concentration of the electroactive species, involved in the redox process, can be increased as the ionic strength is changed (not controlled in the present experiment) a t lower pH. This fact can also contribute to the increase the current peak. The nature of the cation of the supporting electrolyte does not affect the midpoint potential (E,) of the redox process (Table 1). The invariance of the peak potential position with the nature of the cation in the electrolyte suggests that the cation does not interact with the metal center of the electroactive species. The cations do not interact with electroactive species presumably because they are unable to penetrate into the carbon paste. In contrast, the nature ofthe anion has a considerable effect, Le., the E , shifts toward more positive values in the order
01 1
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Figure 6. Scan rate dependence on the peak current.
C104- > Nos- > C1- > S 0 2 - , according to the obtained results presented in Figure 5 . Similar behavior was observed for nickel tetrazacomplexes by Taraszewska et. aZ.,2O who correlated the potential shift to the anion penetration capacity into the carbon paste and interaction with the metal center. The coordination power of the anions with iron atom could explain this behavior. The anion can drift the electronic density to the iron atom stabilizing the oxidized form and, consequently, the midpoint potential shifts toward more negative values. Thus, the more coordinating the anion, the more negative is the midpoint potential as reported by Lever,21 for complexes in solution. Thus, in this case the nature of the anion of the supporting electrolyte has a significant influence on the cyclic voltammetry responses. The behavior for adsorbed species, the anodic and cathodic charge ratio, was about 1.0. The peak to peak separations were not zero, and for scan rates ( u ) higher than 2 mV s-l, the plot of peak current against u1I2 is linear (Figure 6). This behavior is similar to that observed where the process is under mass-transport control, although the complex in this case is adsorbed on the matrix. It can be attributed to transport of the ion of the electrolyte to and from or through the electrode interface for charge compensation. Repulsive interactions between the ions on the material surface and the resistance of the electrode could also influence the shape of the current(20) Taraszewska, J.; Roslonek, G.; Darlewski, W. J. Electroanal. Chem. 1994, 371, 223. (21)Lever, A. B. P. Inorg. Chem. 1990,29,1271.
1012 Langmuir, Vol. 11, No. 3, 1995 potential wave since the material is not a conductor. Another factor that could affect the shape of the voltammograms is the mechanism ofthe electron transfer. This mechanism could be occurring by a jump process as observed in polymeric films.22 The relatively small capacitance observed in data presented in Figure 6 suggests that the resistance of the electrode is not very high, indicating that the insulator character of the silica is compensated by titanium oxide and graphite carbon paste, or the high conductivity of the proton, since this experiment was performed in acid media. The stability of the electrode response was checked and the voltammograms did not present any significant modification even after several scans in slightly acid media. A decrease of 20% in the current after 200 cycles, in solutions with pH 2, is observed. Similar behavior was observed by Andreotti12in a n electrode with immobilized hexacyanoferrate on silica gel surface. In solutions with lower pH an increase in the current is observed (Figure 4),but the stability is not so good. A decrease of 70%in the current is observed in solutions with acid concentrations of 1.0 M, after 200 cycles. Electrocatalytic Activity of the Material for Hydrazine Oxidation. The electro-oxidation of hydrazine catalyzed by sulfonated phthalocyanines adsorbed on a graphite electrode has been investigated by Zaga1.23-25 Although these studies show that the redox couples have negative potentials, it should be remembered that the midpoint potentials of these complexes are pH dependent, as shown by Zagal et aLZ6 Therefore, the catalytic mechanism can be different a t different pH. The catalytic mechanism has been studied in alkaline media because the phthalocyanine complexes, adsorbed on ordinary pyrolytic graphite, are not stable in acid media.' The studies of catalytic properties of the phthalocyanine complexes in acid media can be very important, since the FeTsPc adsorbed on SiOmOZ was stable in acid media. This stability may be attributed to the strong interaction between the complex and the oxide matrix in acid media. In acid solutions no catalytic current was observed in the presence of hydrazine for these electrodes. In neutral solution the electrodes constructed with SiOfliOz did not present any catalytic peak current even in the presence of hydrazine, as can be seen in Figure 7. Hydrazine can be oxidized via four electron to give nitrogen (E = 1.16 V us SHE) on a platinum disk electr~de.~'However, the electrodes constructed with SiOfliOfleTsPc presented an oxidation current with maximum a t about 0.8 V (Figure 81, similar to those of catalytic current. This current was proportional to the hydrazine concentration, indicating that iron phthalocyanine complex catalyzes the hydrazine oxidation in neutral media (Figure 9). These observations can be explained by the fact that in acid media, the interaction between the active iron complex site and the protonated hydrazine is weaker. A stronger interaction of the complex and the positively charged matrix surface is expected to occur. It must be remembered that the catalytic activity depends on the adsorption capacity of (22) Shigehara,K.; Oyama, N.;Anson, F. C. J . Am. Chem. SOC.1981, 103,2552. (23) Zagal, J. H.; Ureta-Zanartu, S.; J . Electrochem. SOC.1982,129, 2242. (24) Zagal, J. H.; Lira, S.; Ureta-Zanartu, S. J. Electroanal. Chem., 1986,210,95. (25) Zagal, J. H. J . Electroanal. Chem. 1980,109,389. (26) Zagal, J. H.; PBez, M.; Santos, J. R., Jr.;Tanaka, A. A.; Linkous, C . A. J . Electroanal. Chem. 1992,339,13. (27) Tamura, K.;Kahara, T. J . Electrochem. SOC.1976,123,776.
Kubota et al.
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with SiOdIYOfleTsPc obtained in neutral solution with different hydrazine concentration at 10 mV 8-l: (a) without M; (c) 1.0 x M, (d) 2.0 x M; hydrazine, (b) 0.5 x (e) 3.0 x M; (0 4.0 x M; (g) 5.0 x M; (h) 10.0 x 10-3 M. the hydrazine on the active site, i.e. interaction with metallic center28 Figure 10 illustrates the dependence of the currents on hydrazine concentration. From the slope of the plot in this figure the order can be obtained in hydrazine electrooxidation process on FeTsPc and is equal to VZ. This value
(28) Steinbach,F.; Zobel, M. J . Chem. Soc., Faraday Trans.I, 1979, 82,113.
Langmuir, Vol. 11, No. 3, 1995 1013
Studies of FeTsPc on Silica Gel
catalytic activity could be attributed to the protonation of the hydrazine molecule since this could hinder the interaction of protonated hydrazine with the active site.
-1.5
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Conclusions
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is different of those found by Z a g a P which indicates that the mechanism should be different. In the present case the experiment was carried out at pH about 6.0 while that of Zaga123was carried out a t pH =. 10.0. The poor
The most important feature is the stability presented by the adsorbed phthalocyanine in acid media. SiOJl’iOz can be used as a n alternative to other high area supporting matrices such as carbon blacks, active charcoals, and pyrolitic graphite. Although the electrocatalytic activity of the material in the hydrazine oxidation was observed to be poor, its use in electrocatalyzed process for other substrates is promising. The weak interaction between hydrazine and the iron center in neutral and acidic solutions may be responsible for the low electro-oxidation capacity. In addition, the diffusion of chemical species into the SiOJl’iOdFeTsPc carbon paste solid solution interface must be considered.
Acknowledgment. The authors acknowledge to FAPESP, FINEP, and CNPq for financial support. LA940400Y
