- Email: [email protected]
carbon black in acid solution
Journal of Electroanalytical Chemistry, 370 (1994) 159-164
159
Homogeneous and heterogeneous catalytic decomposition of hydrogen peroxide by metalloporphyrin/ carbon black in acid solution Y. Yang
l
and A.C.C. Tseung
Chemical Energy Research Centre, Department of Chembtry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ (UK)
Z.G. Lin State Key Laboratory for Physical Chemistry of the Solid Swface, Department of Chemistry, Xiamen University Xiamen, Fujian 361005 (China)
(Received 18 June 1993; in revised form 5 October 1993)
Abstract The kinetics of hydrogen peroxide decomposition by heat-treated Co-tetramethoxylphenyl-porphyrin(Co-TMPP)/carbon black and Fe-tetraphenyl-porphyrin(Fe-TPP)/carbon black has been studied by a gasometric method in sulphuric acid. In the former case, the reaction order is 1.26 f 0.11, indicating that a combined homogeneous and heterogeneous mechanism is involved. In the latter case, the reaction order is 1.02 f 0.11, indicating that a heterogeneous mechanism is involved. The nature of the dissolved species from the heat-treated sample has been studied using UV-visible spectroscopy.
1. Introduction The use of metalloporphyrin or metallophthalocyanines supported on carbon as oxygen reduction catalysts in alkaline and acid solutions has been widely reported [l-19]. When a metalloporphyrin/carbon or metallophthalocyanine/ carbon system is heated at high temperature (ca. 800°C) in an inert atmosphere, the activity for oxygen reduction is improved and the electrode possesses a higher stability than for non-heat treated samples. Although some researchers have suggested that the decomposition of hydrogen peroxide by these catalysts of the electroreduction of oxygen plays an important role in the improvement of the electrode activity and stability [7-91, the kinetics of catalytic decomposition have not been studied in detail [8,9]. In addition, some authors have suggested that the Me-N, species is still present after pyrolysis and works as a possible “active centre”. The nature of the pyrolysed
l
To whom correspondence should be addressed, at Department Chemistry, Xiamen University, Fujian 361005, China.
0022-0728/94/$7.00 SSDI 0022-0728(94)03201-Y
of
metalloporphyrin formed on the carbon surface is not yet thoroughly understood. A better insight into the nature of the “active species” will have considerable theoretical and practical significance [ 111. From a practical viewpoint, the formation of hydrogen peroxide on the carbon electrode not only decreases the oxygen electrode potential but also promotes oxidative degradation of the carbon support, leading to reduced activity and ultimate failure of the electrode. Therefore improving the hydrogen peroxide decomposition of the oxygen electrode is of considerable importance [20,21]. van Veen and Visser [8] concluded that the decomposition of H,O, by heat-treated Me-Pc/Norit BRX (where Me is Co, Fe, Mn, Pt, Ru etc.) in 8 N H,SO, was basically a first-order reaction. Unfortunately, not enough experimental results were presented in their paper to enable a more detailed kinetic analysis to be performed. Up to now, the decomposition of hydrogen peroxide on metallomacrocyclics/ carbon black has usually been explained in terms of a heterogeneous mechanism. Recently, Jiang et al. [22,231 have indicated that a combined homogeneous and heteroge0 1994 - Elsevier Sequoia. All rights reserved
160
Y. Yang et al. / Catalytic
decomposition
neous mechanism is involved in a Co,O,/graphite electrode in alkaline solution. This conclusion is based on the fact that Co,O, is slightly soluble in potassium hydroxide solution. The dissolved Co2+(HCoO;) in KOH is very active for peroxide decomposition and the reaction order is 1.2, indicating that a combined homogeneous and heterogeneous mechanism is involved. Moreover, recent cyclic voltammetric studies on glassy carbon electrodes and rotating ring-disc electrodes have provided further conclusive evidence of the role of HCoO; in the decomposition of hydrogen peroxide [24]. In addition, a combined homogeneous and heterogeneous mechanism for the decomposition of H,O, on Mn(OH), in alkaline solution has also been confirmed 1251. Mijssbauer studies by Scherson et al. [lo] on heated Co-TMPP/ carbon failed to reveal the presence of Co-N, centres; only cobaltous oxides were detected. Therefore it is likely that a similar homogeneous and heterogeneous mechanism is involved in the decomposition of hydrogen peroxide in alkaline solution. However, it is not certain whether a similar combined homogeneous and heterogeneous mechanism is operating in the decomposition of hydrogen peroxide by heated metallomacrocyclic/ carbon catalysts in acidic solution. In this paper we report kinetic studies of hydrogen peroxide decomposition in 1 M H,SO, using different non-heat-treated and heat-treated metalloporphyrin/ carbon catalysts. The metalloporphyrins were Co-tetramethoxyl + phenyl porphyrin (Co-TMPP) and Fe-tetraphenyl-porphyrin (Fe-TPP). The nature of dissolved species of these systems in 1 M H,SO, was investigated using UV-visible spectroscopy. 2. Experimental The metal macrocyclics were used as purchased. The following metalloporphyrins (Aldrich) were used: 5,10,15,20-tetraphenyl-21H,23H_porphine iron010 chloride (> 97%), 5,10,15,20-tetra(4-methoxyl)phenyl21H,23H-porphine cobalt and 5,10,15,20-tetra(4methoxyl)phenyl-21H,23&porphine (H,-TMPP) (> 97%). Lampblack was obtained from Fisons Ltd. The electrolyte (1 M H2S04) was prepared by diluting 98% H,SO, with the double-distilled water. A conventional gasometric technique [221 was used to measure the amount of oxygen evolved. In the experiments, the weight of catalyst (heated and unheated samples) was 100 mg and the volume of 1 M HiSO, was 50 ml. An appropriate volume of 1.78 M H,O, was added to metalloporphyrin/ carbon black + 1 M H,SO, system to study the effect of H,O, concentration. Unless otherwise stated, the temperature of the reaction vessel was controlled at 60°C and the stirring rate of the
of b20, by metalloporphyrin /carbon black
magnetic stirrer was fixed at mark 4 (SS3H hotplate stirrer, Chemlab, UK). UV-visible spectra were obtained using a Philips PU8700 series UV-visible spectrometer at a bandwidth of 2 nm and a scan rate of 500 or 1000 nm min-‘. The cell width was 1 cm. Both plastic and silica cells were used. The silica cell was used when it was necessary to reduce the wavelength to less than 250 nm. Co-TMPP, H,-TMPP and Fe-TPP were preadsorbed on carbon using a method similar to that described by Scherson et al. [lo]. After dissolving 100 mg of metalloporphyrin sample (Co-TMPP, 100 mg; H,TPP, 100 mg; Fe-TPP, 88.9 mg) in 200 ml of acetone, 2 g of carbon were added under ultrasonic agitation for 1.5 h. The solvent was removed with a water aspirator at room temperature. The amount of metalloporphyrin added corresponds to 4.8% (w/w) for Co-TMPP and H,-TMPP and 4.25% (w/w> for Fe-TPP. This should be more than sufficient to form a monolayer of the complex on the carbon surface since the surface area of the lampblack is only 20-22 m2 g-’ (ref. 26, p. 5). Some of the powder (ca. 500 mg) was placed in ceramic boats and heated at 800°C for 2 h under a continuous flow of nitrogen in a tube furnace. After switching off the furnace, the sample was cooled in a nitrogen stream to avoid oxidation. If no cobalt was lost during heat treatment, the total amount that could be dissolved in 55 ml of 1 M H,SO, as Co2+ was 0.119 mM, A separate solution containing 0.119 mM Co2+ was prepared by dissolving 8.8 mg of Co0 in 100 ml of 1 M H,SO,. 3. Results and discussion 3.1. Comparison of the catalytic activity towards hydrogen peroxide of heated carbon black, unheated and heated Co- TMPP / carbon black, Hz- TMPP / carbon black, FeTPP/carbon black and the blank solution The catalytic decomposition of hydrogen peroxide by unheated Co-TMPP/ carbon black, heated carbon black, heated H,-TMPP, Fe-TPP and Co-TMPP/ carbon black and in the blank 1 M H2S0, solution are shown in Fig. 1 (evolved gas volume vs. time). The heated Fe-TPP/carbon black has the highest activity followed by Co-TMPP/ carbon black. 3.2. Homogeneous and heterogeneous mechanism for the Co- TMPP /carbon
If we use calculate the carbon black (ln[ v,,/( v,, good enough tion.
black system
the first-order reaction rate equation to decomposition data of heated Co-TMPP/ in 1 M H,SO, (Fig. 21, the fit of the data - V)] vs. time) to a straight line is not to confirm that this is a first-order reac-
Y. Yang et al. / Catalytic decomposition of H202 by metalloporphyrin /carbon
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400 300 Time /I
500
600
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I
I
I
I
I
-0.95
-0.9
-0.85
-0.6
-0.75
-0.7
Log (CCCl/mol
Fig. 1. Time-dependence of the volume of gas evolved in the decomposition of H,O by untreated and heat-treated Co-TMPP/C and Fe-TPP/C in 1 M H,SO, at 60°C. Pretreatment conditions are shown on the figure.
In order to deduce the rate equation of the decomposition reaction and to calculate the reaction order, log u,, was plotted against log Ci (Fig. 3), where u0 is the initial rate of H,O, decomposition and c is the initial H,O, concentration. The slope of the line obtained before correcting for the self-decomposition of hydrogen peroxide is 1.16 +_0.11. After correcting for the self-decomposition of hydrogen peroxide under the same experimental conditions, the reaction order is 1.26 + 0.11 and the calculated rate constant is 3.2 X lo-5 ~-0.26 s‘. The initial concentration dependence of the rate constant k,,,, is shown in Fig. 4. It can be seen that ki.26 is virtually independent of the initial concentration of hydrogen peroxide. This indicates that this reaction order is appropriate for this system. Since the reaction order is not unity, more than one reaction mechanism is involved, possibly both homogeneous and heterogeneous mechanisms [22]. The reac-
0.25 ((
-0.65
F)
Fig. 3. Log-log plots of the initial rate of H,O, decomposition vs. initial concentration of H,O, catalyzed by heat-treated Co-TMPP/C: not corrected for self-decomposition of H,O,; n corrected for self-decomposition of H,O, under the same experimental conditions.
l
tion order is different from that reported by van Veen and coworkers [7,8] who concluded that the decomposition was a first-order reaction. Since they did not report their kinetic data [7], it is impossible to perform a more rigorous kinetic analysis of their results. The above results imply that a soluble Co-N, complex and/or cobalt ions may also play some role as a homogeneous catalyst in the decomposition of hydrogen peroxide by heat-treated Co-TMPP/ carbon black in acid solution. Many physical and chemical changes could occur after heat treating the sample at 800°C under a nitrogen atmosphere. The metalloporphyrin ring would been broken, and it has been suggested that the residual compounds are metal carbides [l-3,111 and oxides [7,10]. In addition, the carbon support undergoes char-
% w d h .
5E
Bx
A
A
w3 I 2
I/
I too
I
I
I
0.00
0.1
0.12 Initial
Time/s
Fig. 2. First-order reaction plot of H,O, decomposition by 100 mg of heat-treated Co-TMPP/C catalyst (0.173 M H,O,; 1 MH,SO,).
1
0.14 concentmtion
I
0.16
I
0.16
I
0.2
I 0.22
C,/M
Fig. 4. Relationship between initial concentration of H,O, and the rate constant k,.26 in the presence of 100 mg of heat-treated CoTMPP/C in 1 M H,SO, at WC.
162
Y. Yang et al. / Catalytic decomposition
of H202 by metaliopophyrin
/carbon
black
0.5 II1 -2.001 -1.15
’
’
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1
I
I
I -0.65
cog CL
01 0.06
I
I
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I
0.06
0.1
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0.14
Initial
Fig. 5. Log-log plots of the initial rate of H,O, decomposition vs. the initial concentration of H,O, catalysed by 100 mg of heat-treated Fe-TPP/C: n Before correcting for self-decomposition of H,O,; A after correcting for self-decomposition of H,Oz under same experimental conditions.
acteristic physical and chemical changes due to decomposition of the surface oxide species on carbon (ref. 26, p. 132). These include changes in the wetting properties and the surface area of the carbon support [15]. In the present study, it was observed that Co-TMPP/ carbon black which had not been pre-treated floated on water. However, after pyrolysis most of the carbon particles dropped to the bottom of a glass beaker containing water. Peroxide decomposition studies using a solution containing 100 mg of heat-treated carbon (lamp black) and dissolved cobalt ion ‘at a concentration equivalent to the maximum amount of cobalt that could be leached from pyrolysed Co-TMPP/carbon black catalyst (0.119 mM) showed that the activity is very much lower. This suggests that species other than the dissolved cobalt ion are also involved in the decomposition of hydrogen peroxide.
3.3. Measurement of the reaction order for the Fe-TPP/ carbon black system
The kinetics of the decomposition of hydrogen peroxide on an Fe-TPP/carbon black system were also evaluated. The log-log plot (Fig. 5) indicates that the reaction order is unity and that there is only one heterogeneous mechanism. The reaction order calculated from Fig. 5 is 0.98 f 0.07 (after correcting for self-decomposition of hydrogen peroxide under the same experimental conditions it is 1.02 f 0.11). The rate constant is 1.8 X lo-’ s-l. The initial concentration dependence of k, is shown in Fig. 6. This confirms that the reaction order is unity.
I
I
0.16
concentration
0.16
I
0.2
I 0.22
G/M
Fig. 6. Plot of the initial concentration of H,O, vs. the first-order rate constant in the presence of 100 mg of heat-treated Fe-TPP/C in 1 MH,SO, at 60°C.
3.4. W-visible spectroscopic characterization of dissolved species from the Co-TMPP /carbon black, CoTPP/ carbon black and Fe- TPP / carbon black systems
The cobalt species dissolved in 1 M H,SO, should be detectable using UV-visible spectroscopy. Figure 7(a) show the solution extracted from the catalyst (CoTMPP/carbon black) + 1 M H,SO, (500 mg of catalyst + 7 ml 1 M H,SO,) after immersion for 30 min at 60°C. As a comparison, the UV-visible spectrum of 4.8 X 10e3 M CoSO, is also shown in the figure. There are three peaks at ca. 518 nm, 630 nm and 712 nm (the last two peaks become more distinctive when the recorded spectra stabilize). The peak at 518 nm is attributed to [Co(H,O),l*’ [27]. The peaks at ca. 630 nm and 712 nm can be attributed to the corresponding cobalt complex. From the position of the peaks, it can be concluded that this species has a tetrahedral structure, similar to that of (CoCl,)*-. According to the usual spectrochemical series, the stronger the ligand,
r----i
(0)
r--l
Wovelangth/nm
(b)
Fig. 7. UV-visible spectra of the solution extracted from heat-treated Co-TMPP/C catalyst (500 mg) + 1 M HaSO, (7 ml): (a) immersed for 30 min at 60°C; --- 4.8~ 1O-3 M CoSO, + 1 M HaSO,; (b) after addition of HaO, (cH202 = 0.25 M) for 96 h.
Y. Yang et al. / Catalytic decomposition of H,O,
the larger is the splitting of the d orbital. The N group usually has a stronger complexing ability than the Cl[28]. However, our results suggest that the dissolved N, group has a weaker complexing ability than Cl-. Recently, ex-situ X-ray photoelectron spectroscopy results 141 showed that a new N 1s spectral peak of the N species on carbon black appeared after pyrolysis. This implies that the coordination ability of the nitrogen atom decreases in some of Me - N, species. This is in agreement with the above spectral results. If the combined energy of the electron of the nitrogen atom becomes higher, the negative charge of the nitrogen will decrease resulting in a decrease in the coordination ability of the nitrogen atom in the Me - N, species. On adding some hydrogen peroxide into this system (eg. adding 1 ml of 4N H,O, to solid catalyst + 1 M H,SO,) the peaks at higher wavelength become weaker and difficult to distinguish (Fig. 7(b)). Only the peak at 518 nm is still present in the spectrum. This suggests that the main state of Co’+ in the solution after the addition of hydrogen peroxide is [Co(H,0)J2+. The other cobalt species could be attacked and decomposed by hydrogen peroxide. When the same method was used to detect dissolved species extracted from the Fe-TPP/carbon black + 1 M H,SO, system, no distinguishable peaks were observed between 350 and 900 nm. It is very interesting to note the presence of two different mechanisms in the two heated metalloporphyrin/carbon black systems. This may imply that two different mechanisms exist during the pyrolysis processes [15]. In addition, in previous studies [10,29,30] evidence has been found that some cobalt and iron oxide remained on the electrode after heat treatment. Therefore our results suggest that there are not enough soluble Fe ions in the heated Fe-TPP carbon black system to affect its activity towards the decomposition of hydrogen peroxide. Hence a similar mechanism to that present in the heated Co-TMPP/carbon black system could not exist. Further work will be directed towards the study of the pyrolysis mechanism, the synthesis of possible “active centres” using other chemical or metallurgical methods, and evaluating their activity towards the decomposition of hydrogen peroxide and the electroreduction of oxygen in acid solution. 4. Conclusions (1) The catalytic ability of metalloporphyrin/ carbon systems towards the decomposition of hydrogen peroxide shows a significant increase after they have been pyrolysed. The activity of Fe-TPP/carbon black is better than those of Co-TMPP/carbon black and CoTPP/carbon black.
by metalloporphyrin /carbon
black
163
(2) We have investigated the chemical kinetics of the decomposition of hydrogen peroxide by heattreated Co-TMPP/carbon black catalyst in acid solution. The reaction order is 1.16 f 0.11 (corrected value, 1.26 * 0.11). This suggest that both a homogeneous and a heterogeneous mechanism are involved. The reaction order for heat-treated Fe-TPP/carbon black is 0.98 k 0.07 (corrected value, 1.02 + 0.111, which implies that a heterogeneous mechanism is involved. (3) Some dissolved Co2+ species (i.e. aqueous c02+ ions) possibly a Co-N complex, have been detected using UV-visible spectroscopy. However, no dissolved Fe2+ species were detected between 350 and 900 nm. These results support the hypothesis that two different peroxide decomposition mechanisms are involved. Acknowledgement YY would like to thank the British Council for financial support under the ALCS scheme. References 1 P. Vasudevan, Santosh, N. Man and S. Tyagi, Transition Met. Chem., 15 (1990181. 2 J.A.R. van Veen and J.F. Van Baar, Rev. Inorg. Chem., 4 (1982) 293. 3 M.R. Tarasevich and K.A. Radyushkina, Russ. Chem. Rev., 49 (1980) 718. 4 A. Widelov and R. Larsson, Electrochim. Acta, 37 (1992) 187. 5 S.L. Gupta, H. Huang and E.B. Yeager, Electrochim, Acta, 36 (1991) 2165. 6 Z.Y. Zeng, S.L. Gupta, H. Huang and E.B. Yeager; J. Appl. Electrochem., 21 (1991) 973. Phys. Chem., 85 (1981) 7 J.A.R. van Veen et al., Ber. Bunsenges. 693, 700. Acta, 24 (1979) 921. 8 J.A.R. van Veen and C. Visser, Electrochim. React. Kinet. Catal. Lett., 6 9 M.R. Tarasevich and G.I. Zakharkin, (1977) 77. 10 D.A. Scherson, S.L. Gupta, C. Fierro, E.B. Yeager, M.E. Kerdesky, J. Eldridge, R.W. Hoffman and J. Blue, Electrochim. Acta, 29 (1983) 1205. 11 A.J. Applepy and F.R. Foulkes (Eds.) Fuel Cell Handbook, Van Nostrand Reinhold, New York, 1989, Ch. 12. 12 R. Holze, Electrochim. Acta, 36 (1991) 999. 13 A. Van der Putten, A. Elzing, W. Visscher and E. Barendrecht, J. Electroanal. Chem., 205 (1986) 223. 14 J.A. van Veen, J. van Baar and K. Kroese, J. Chem. Sot. Faraday Trans. I, 77 (1981) 1021. 15 J.C. Wang in R.E. White and A.J. Appleby (Eds.), Symp. on Fuel Cells, San Francisco, CA, Electrochemical Society, Pennington, NJ, 1989, p. 268. 16 F. Solomon, Ext. Abstr. Electrochemical Society, Spring Meeting, Toronto, 1985. 17 E. Yeager, Presentation at DOE Contract Meeting (Technology Base Research Project), Cleveland, OH, 1986. 18 F. Beck, J. Appl. Electrochem., 7 (1977) 239. 19 A.A. Tanaka, C. Fierro, D. Scherson and E.B. Yeager, J. Phys. Chem., 91 (1987) 3799.
164 20 R. Manoharan,
21 22 23 24 25
Y. Yang et al. / Catalytic decomposition of H,O, by metalloporphyrin /carbon black
J.B. Goodenough, A. Hamnett, K.R. Jamieson, CA. Paliteiro and A.K. Shukla, IChemE Symp. Ser. (1986) 235. J.R. Goldstein and A.C.C. Tseung, J. Phys. Chem., 76 (1972) 3646. S.P. Jiang, Z.G. Lin and A.C.C. Tseung, J. Electrochem. Sot., 137 (1990) 759. S.P. Jiang, Z.G. Lin and A.C.C. Tseung, J. Electrochem. Sot., 137 (1990) 764. Y. Yang, A.C.C. Tseung and Z.G. Lin, unpublished results. S.P. Jiang, W.R. Ashton and A.C.C. Tseung, J. Catal., 131 (1991) 88.
26 K. Kinoshita, Carbon, Electrochemical and Physicochemical Properties, Wiley, New York, 1988. 27 F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (5th edn.), Wiley, New York, 1988, p. 730. 28 N.N. Greenwood and A. Earnshaw, The Chemistry of the Elements, Pergamon Press, 1986, p. 1096. 29 E. Yeager, Electrochimica Acta, 29 (1984) 1527. 30 S. Dong and R. Jiang, Ber. Bunsenges. Phys. Chem., 91 (1987) 479.
159
Homogeneous and heterogeneous catalytic decomposition of hydrogen peroxide by metalloporphyrin/ carbon black in acid solution Y. Yang
l
and A.C.C. Tseung
Chemical Energy Research Centre, Department of Chembtry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ (UK)
Z.G. Lin State Key Laboratory for Physical Chemistry of the Solid Swface, Department of Chemistry, Xiamen University Xiamen, Fujian 361005 (China)
(Received 18 June 1993; in revised form 5 October 1993)
Abstract The kinetics of hydrogen peroxide decomposition by heat-treated Co-tetramethoxylphenyl-porphyrin(Co-TMPP)/carbon black and Fe-tetraphenyl-porphyrin(Fe-TPP)/carbon black has been studied by a gasometric method in sulphuric acid. In the former case, the reaction order is 1.26 f 0.11, indicating that a combined homogeneous and heterogeneous mechanism is involved. In the latter case, the reaction order is 1.02 f 0.11, indicating that a heterogeneous mechanism is involved. The nature of the dissolved species from the heat-treated sample has been studied using UV-visible spectroscopy.
1. Introduction The use of metalloporphyrin or metallophthalocyanines supported on carbon as oxygen reduction catalysts in alkaline and acid solutions has been widely reported [l-19]. When a metalloporphyrin/carbon or metallophthalocyanine/ carbon system is heated at high temperature (ca. 800°C) in an inert atmosphere, the activity for oxygen reduction is improved and the electrode possesses a higher stability than for non-heat treated samples. Although some researchers have suggested that the decomposition of hydrogen peroxide by these catalysts of the electroreduction of oxygen plays an important role in the improvement of the electrode activity and stability [7-91, the kinetics of catalytic decomposition have not been studied in detail [8,9]. In addition, some authors have suggested that the Me-N, species is still present after pyrolysis and works as a possible “active centre”. The nature of the pyrolysed
l
To whom correspondence should be addressed, at Department Chemistry, Xiamen University, Fujian 361005, China.
0022-0728/94/$7.00 SSDI 0022-0728(94)03201-Y
of
metalloporphyrin formed on the carbon surface is not yet thoroughly understood. A better insight into the nature of the “active species” will have considerable theoretical and practical significance [ 111. From a practical viewpoint, the formation of hydrogen peroxide on the carbon electrode not only decreases the oxygen electrode potential but also promotes oxidative degradation of the carbon support, leading to reduced activity and ultimate failure of the electrode. Therefore improving the hydrogen peroxide decomposition of the oxygen electrode is of considerable importance [20,21]. van Veen and Visser [8] concluded that the decomposition of H,O, by heat-treated Me-Pc/Norit BRX (where Me is Co, Fe, Mn, Pt, Ru etc.) in 8 N H,SO, was basically a first-order reaction. Unfortunately, not enough experimental results were presented in their paper to enable a more detailed kinetic analysis to be performed. Up to now, the decomposition of hydrogen peroxide on metallomacrocyclics/ carbon black has usually been explained in terms of a heterogeneous mechanism. Recently, Jiang et al. [22,231 have indicated that a combined homogeneous and heteroge0 1994 - Elsevier Sequoia. All rights reserved
160
Y. Yang et al. / Catalytic
decomposition
neous mechanism is involved in a Co,O,/graphite electrode in alkaline solution. This conclusion is based on the fact that Co,O, is slightly soluble in potassium hydroxide solution. The dissolved Co2+(HCoO;) in KOH is very active for peroxide decomposition and the reaction order is 1.2, indicating that a combined homogeneous and heterogeneous mechanism is involved. Moreover, recent cyclic voltammetric studies on glassy carbon electrodes and rotating ring-disc electrodes have provided further conclusive evidence of the role of HCoO; in the decomposition of hydrogen peroxide [24]. In addition, a combined homogeneous and heterogeneous mechanism for the decomposition of H,O, on Mn(OH), in alkaline solution has also been confirmed 1251. Mijssbauer studies by Scherson et al. [lo] on heated Co-TMPP/ carbon failed to reveal the presence of Co-N, centres; only cobaltous oxides were detected. Therefore it is likely that a similar homogeneous and heterogeneous mechanism is involved in the decomposition of hydrogen peroxide in alkaline solution. However, it is not certain whether a similar combined homogeneous and heterogeneous mechanism is operating in the decomposition of hydrogen peroxide by heated metallomacrocyclic/ carbon catalysts in acidic solution. In this paper we report kinetic studies of hydrogen peroxide decomposition in 1 M H,SO, using different non-heat-treated and heat-treated metalloporphyrin/ carbon catalysts. The metalloporphyrins were Co-tetramethoxyl + phenyl porphyrin (Co-TMPP) and Fe-tetraphenyl-porphyrin (Fe-TPP). The nature of dissolved species of these systems in 1 M H,SO, was investigated using UV-visible spectroscopy. 2. Experimental The metal macrocyclics were used as purchased. The following metalloporphyrins (Aldrich) were used: 5,10,15,20-tetraphenyl-21H,23H_porphine iron010 chloride (> 97%), 5,10,15,20-tetra(4-methoxyl)phenyl21H,23H-porphine cobalt and 5,10,15,20-tetra(4methoxyl)phenyl-21H,23&porphine (H,-TMPP) (> 97%). Lampblack was obtained from Fisons Ltd. The electrolyte (1 M H2S04) was prepared by diluting 98% H,SO, with the double-distilled water. A conventional gasometric technique [221 was used to measure the amount of oxygen evolved. In the experiments, the weight of catalyst (heated and unheated samples) was 100 mg and the volume of 1 M HiSO, was 50 ml. An appropriate volume of 1.78 M H,O, was added to metalloporphyrin/ carbon black + 1 M H,SO, system to study the effect of H,O, concentration. Unless otherwise stated, the temperature of the reaction vessel was controlled at 60°C and the stirring rate of the
of b20, by metalloporphyrin /carbon black
magnetic stirrer was fixed at mark 4 (SS3H hotplate stirrer, Chemlab, UK). UV-visible spectra were obtained using a Philips PU8700 series UV-visible spectrometer at a bandwidth of 2 nm and a scan rate of 500 or 1000 nm min-‘. The cell width was 1 cm. Both plastic and silica cells were used. The silica cell was used when it was necessary to reduce the wavelength to less than 250 nm. Co-TMPP, H,-TMPP and Fe-TPP were preadsorbed on carbon using a method similar to that described by Scherson et al. [lo]. After dissolving 100 mg of metalloporphyrin sample (Co-TMPP, 100 mg; H,TPP, 100 mg; Fe-TPP, 88.9 mg) in 200 ml of acetone, 2 g of carbon were added under ultrasonic agitation for 1.5 h. The solvent was removed with a water aspirator at room temperature. The amount of metalloporphyrin added corresponds to 4.8% (w/w) for Co-TMPP and H,-TMPP and 4.25% (w/w> for Fe-TPP. This should be more than sufficient to form a monolayer of the complex on the carbon surface since the surface area of the lampblack is only 20-22 m2 g-’ (ref. 26, p. 5). Some of the powder (ca. 500 mg) was placed in ceramic boats and heated at 800°C for 2 h under a continuous flow of nitrogen in a tube furnace. After switching off the furnace, the sample was cooled in a nitrogen stream to avoid oxidation. If no cobalt was lost during heat treatment, the total amount that could be dissolved in 55 ml of 1 M H,SO, as Co2+ was 0.119 mM, A separate solution containing 0.119 mM Co2+ was prepared by dissolving 8.8 mg of Co0 in 100 ml of 1 M H,SO,. 3. Results and discussion 3.1. Comparison of the catalytic activity towards hydrogen peroxide of heated carbon black, unheated and heated Co- TMPP / carbon black, Hz- TMPP / carbon black, FeTPP/carbon black and the blank solution The catalytic decomposition of hydrogen peroxide by unheated Co-TMPP/ carbon black, heated carbon black, heated H,-TMPP, Fe-TPP and Co-TMPP/ carbon black and in the blank 1 M H2S0, solution are shown in Fig. 1 (evolved gas volume vs. time). The heated Fe-TPP/carbon black has the highest activity followed by Co-TMPP/ carbon black. 3.2. Homogeneous and heterogeneous mechanism for the Co- TMPP /carbon
If we use calculate the carbon black (ln[ v,,/( v,, good enough tion.
black system
the first-order reaction rate equation to decomposition data of heated Co-TMPP/ in 1 M H,SO, (Fig. 21, the fit of the data - V)] vs. time) to a straight line is not to confirm that this is a first-order reac-
Y. Yang et al. / Catalytic decomposition of H202 by metalloporphyrin /carbon
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too
200
400 300 Time /I
500
600
-2 -1.15
700
I -1.1
I
I
-1.05
-I
I
I
I
I
I
I
-0.95
-0.9
-0.85
-0.6
-0.75
-0.7
Log (CCCl/mol
Fig. 1. Time-dependence of the volume of gas evolved in the decomposition of H,O by untreated and heat-treated Co-TMPP/C and Fe-TPP/C in 1 M H,SO, at 60°C. Pretreatment conditions are shown on the figure.
In order to deduce the rate equation of the decomposition reaction and to calculate the reaction order, log u,, was plotted against log Ci (Fig. 3), where u0 is the initial rate of H,O, decomposition and c is the initial H,O, concentration. The slope of the line obtained before correcting for the self-decomposition of hydrogen peroxide is 1.16 +_0.11. After correcting for the self-decomposition of hydrogen peroxide under the same experimental conditions, the reaction order is 1.26 + 0.11 and the calculated rate constant is 3.2 X lo-5 ~-0.26 s‘. The initial concentration dependence of the rate constant k,,,, is shown in Fig. 4. It can be seen that ki.26 is virtually independent of the initial concentration of hydrogen peroxide. This indicates that this reaction order is appropriate for this system. Since the reaction order is not unity, more than one reaction mechanism is involved, possibly both homogeneous and heterogeneous mechanisms [22]. The reac-
0.25 ((
-0.65
F)
Fig. 3. Log-log plots of the initial rate of H,O, decomposition vs. initial concentration of H,O, catalyzed by heat-treated Co-TMPP/C: not corrected for self-decomposition of H,O,; n corrected for self-decomposition of H,O, under the same experimental conditions.
l
tion order is different from that reported by van Veen and coworkers [7,8] who concluded that the decomposition was a first-order reaction. Since they did not report their kinetic data [7], it is impossible to perform a more rigorous kinetic analysis of their results. The above results imply that a soluble Co-N, complex and/or cobalt ions may also play some role as a homogeneous catalyst in the decomposition of hydrogen peroxide by heat-treated Co-TMPP/ carbon black in acid solution. Many physical and chemical changes could occur after heat treating the sample at 800°C under a nitrogen atmosphere. The metalloporphyrin ring would been broken, and it has been suggested that the residual compounds are metal carbides [l-3,111 and oxides [7,10]. In addition, the carbon support undergoes char-
% w d h .
5E
Bx
A
A
w3 I 2
I/
I too
I
I
I
0.00
0.1
0.12 Initial
Time/s
Fig. 2. First-order reaction plot of H,O, decomposition by 100 mg of heat-treated Co-TMPP/C catalyst (0.173 M H,O,; 1 MH,SO,).
1
0.14 concentmtion
I
0.16
I
0.16
I
0.2
I 0.22
C,/M
Fig. 4. Relationship between initial concentration of H,O, and the rate constant k,.26 in the presence of 100 mg of heat-treated CoTMPP/C in 1 M H,SO, at WC.
162
Y. Yang et al. / Catalytic decomposition
of H202 by metaliopophyrin
/carbon
black
0.5 II1 -2.001 -1.15
’
’
1
1
1 -0.90
1
1
I
I
I -0.65
cog CL
01 0.06
I
I
I
I
0.06
0.1
0.12
0.14
Initial
Fig. 5. Log-log plots of the initial rate of H,O, decomposition vs. the initial concentration of H,O, catalysed by 100 mg of heat-treated Fe-TPP/C: n Before correcting for self-decomposition of H,O,; A after correcting for self-decomposition of H,Oz under same experimental conditions.
acteristic physical and chemical changes due to decomposition of the surface oxide species on carbon (ref. 26, p. 132). These include changes in the wetting properties and the surface area of the carbon support [15]. In the present study, it was observed that Co-TMPP/ carbon black which had not been pre-treated floated on water. However, after pyrolysis most of the carbon particles dropped to the bottom of a glass beaker containing water. Peroxide decomposition studies using a solution containing 100 mg of heat-treated carbon (lamp black) and dissolved cobalt ion ‘at a concentration equivalent to the maximum amount of cobalt that could be leached from pyrolysed Co-TMPP/carbon black catalyst (0.119 mM) showed that the activity is very much lower. This suggests that species other than the dissolved cobalt ion are also involved in the decomposition of hydrogen peroxide.
3.3. Measurement of the reaction order for the Fe-TPP/ carbon black system
The kinetics of the decomposition of hydrogen peroxide on an Fe-TPP/carbon black system were also evaluated. The log-log plot (Fig. 5) indicates that the reaction order is unity and that there is only one heterogeneous mechanism. The reaction order calculated from Fig. 5 is 0.98 f 0.07 (after correcting for self-decomposition of hydrogen peroxide under the same experimental conditions it is 1.02 f 0.11). The rate constant is 1.8 X lo-’ s-l. The initial concentration dependence of k, is shown in Fig. 6. This confirms that the reaction order is unity.
I
I
0.16
concentration
0.16
I
0.2
I 0.22
G/M
Fig. 6. Plot of the initial concentration of H,O, vs. the first-order rate constant in the presence of 100 mg of heat-treated Fe-TPP/C in 1 MH,SO, at 60°C.
3.4. W-visible spectroscopic characterization of dissolved species from the Co-TMPP /carbon black, CoTPP/ carbon black and Fe- TPP / carbon black systems
The cobalt species dissolved in 1 M H,SO, should be detectable using UV-visible spectroscopy. Figure 7(a) show the solution extracted from the catalyst (CoTMPP/carbon black) + 1 M H,SO, (500 mg of catalyst + 7 ml 1 M H,SO,) after immersion for 30 min at 60°C. As a comparison, the UV-visible spectrum of 4.8 X 10e3 M CoSO, is also shown in the figure. There are three peaks at ca. 518 nm, 630 nm and 712 nm (the last two peaks become more distinctive when the recorded spectra stabilize). The peak at 518 nm is attributed to [Co(H,O),l*’ [27]. The peaks at ca. 630 nm and 712 nm can be attributed to the corresponding cobalt complex. From the position of the peaks, it can be concluded that this species has a tetrahedral structure, similar to that of (CoCl,)*-. According to the usual spectrochemical series, the stronger the ligand,
r----i
(0)
r--l
Wovelangth/nm
(b)
Fig. 7. UV-visible spectra of the solution extracted from heat-treated Co-TMPP/C catalyst (500 mg) + 1 M HaSO, (7 ml): (a) immersed for 30 min at 60°C; --- 4.8~ 1O-3 M CoSO, + 1 M HaSO,; (b) after addition of HaO, (cH202 = 0.25 M) for 96 h.
Y. Yang et al. / Catalytic decomposition of H,O,
the larger is the splitting of the d orbital. The N group usually has a stronger complexing ability than the Cl[28]. However, our results suggest that the dissolved N, group has a weaker complexing ability than Cl-. Recently, ex-situ X-ray photoelectron spectroscopy results 141 showed that a new N 1s spectral peak of the N species on carbon black appeared after pyrolysis. This implies that the coordination ability of the nitrogen atom decreases in some of Me - N, species. This is in agreement with the above spectral results. If the combined energy of the electron of the nitrogen atom becomes higher, the negative charge of the nitrogen will decrease resulting in a decrease in the coordination ability of the nitrogen atom in the Me - N, species. On adding some hydrogen peroxide into this system (eg. adding 1 ml of 4N H,O, to solid catalyst + 1 M H,SO,) the peaks at higher wavelength become weaker and difficult to distinguish (Fig. 7(b)). Only the peak at 518 nm is still present in the spectrum. This suggests that the main state of Co’+ in the solution after the addition of hydrogen peroxide is [Co(H,0)J2+. The other cobalt species could be attacked and decomposed by hydrogen peroxide. When the same method was used to detect dissolved species extracted from the Fe-TPP/carbon black + 1 M H,SO, system, no distinguishable peaks were observed between 350 and 900 nm. It is very interesting to note the presence of two different mechanisms in the two heated metalloporphyrin/carbon black systems. This may imply that two different mechanisms exist during the pyrolysis processes [15]. In addition, in previous studies [10,29,30] evidence has been found that some cobalt and iron oxide remained on the electrode after heat treatment. Therefore our results suggest that there are not enough soluble Fe ions in the heated Fe-TPP carbon black system to affect its activity towards the decomposition of hydrogen peroxide. Hence a similar mechanism to that present in the heated Co-TMPP/carbon black system could not exist. Further work will be directed towards the study of the pyrolysis mechanism, the synthesis of possible “active centres” using other chemical or metallurgical methods, and evaluating their activity towards the decomposition of hydrogen peroxide and the electroreduction of oxygen in acid solution. 4. Conclusions (1) The catalytic ability of metalloporphyrin/ carbon systems towards the decomposition of hydrogen peroxide shows a significant increase after they have been pyrolysed. The activity of Fe-TPP/carbon black is better than those of Co-TMPP/carbon black and CoTPP/carbon black.
by metalloporphyrin /carbon
black
163
(2) We have investigated the chemical kinetics of the decomposition of hydrogen peroxide by heattreated Co-TMPP/carbon black catalyst in acid solution. The reaction order is 1.16 f 0.11 (corrected value, 1.26 * 0.11). This suggest that both a homogeneous and a heterogeneous mechanism are involved. The reaction order for heat-treated Fe-TPP/carbon black is 0.98 k 0.07 (corrected value, 1.02 + 0.111, which implies that a heterogeneous mechanism is involved. (3) Some dissolved Co2+ species (i.e. aqueous c02+ ions) possibly a Co-N complex, have been detected using UV-visible spectroscopy. However, no dissolved Fe2+ species were detected between 350 and 900 nm. These results support the hypothesis that two different peroxide decomposition mechanisms are involved. Acknowledgement YY would like to thank the British Council for financial support under the ALCS scheme. References 1 P. Vasudevan, Santosh, N. Man and S. Tyagi, Transition Met. Chem., 15 (1990181. 2 J.A.R. van Veen and J.F. Van Baar, Rev. Inorg. Chem., 4 (1982) 293. 3 M.R. Tarasevich and K.A. Radyushkina, Russ. Chem. Rev., 49 (1980) 718. 4 A. Widelov and R. Larsson, Electrochim. Acta, 37 (1992) 187. 5 S.L. Gupta, H. Huang and E.B. Yeager, Electrochim, Acta, 36 (1991) 2165. 6 Z.Y. Zeng, S.L. Gupta, H. Huang and E.B. Yeager; J. Appl. Electrochem., 21 (1991) 973. Phys. Chem., 85 (1981) 7 J.A.R. van Veen et al., Ber. Bunsenges. 693, 700. Acta, 24 (1979) 921. 8 J.A.R. van Veen and C. Visser, Electrochim. React. Kinet. Catal. Lett., 6 9 M.R. Tarasevich and G.I. Zakharkin, (1977) 77. 10 D.A. Scherson, S.L. Gupta, C. Fierro, E.B. Yeager, M.E. Kerdesky, J. Eldridge, R.W. Hoffman and J. Blue, Electrochim. Acta, 29 (1983) 1205. 11 A.J. Applepy and F.R. Foulkes (Eds.) Fuel Cell Handbook, Van Nostrand Reinhold, New York, 1989, Ch. 12. 12 R. Holze, Electrochim. Acta, 36 (1991) 999. 13 A. Van der Putten, A. Elzing, W. Visscher and E. Barendrecht, J. Electroanal. Chem., 205 (1986) 223. 14 J.A. van Veen, J. van Baar and K. Kroese, J. Chem. Sot. Faraday Trans. I, 77 (1981) 1021. 15 J.C. Wang in R.E. White and A.J. Appleby (Eds.), Symp. on Fuel Cells, San Francisco, CA, Electrochemical Society, Pennington, NJ, 1989, p. 268. 16 F. Solomon, Ext. Abstr. Electrochemical Society, Spring Meeting, Toronto, 1985. 17 E. Yeager, Presentation at DOE Contract Meeting (Technology Base Research Project), Cleveland, OH, 1986. 18 F. Beck, J. Appl. Electrochem., 7 (1977) 239. 19 A.A. Tanaka, C. Fierro, D. Scherson and E.B. Yeager, J. Phys. Chem., 91 (1987) 3799.
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26 K. Kinoshita, Carbon, Electrochemical and Physicochemical Properties, Wiley, New York, 1988. 27 F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (5th edn.), Wiley, New York, 1988, p. 730. 28 N.N. Greenwood and A. Earnshaw, The Chemistry of the Elements, Pergamon Press, 1986, p. 1096. 29 E. Yeager, Electrochimica Acta, 29 (1984) 1527. 30 S. Dong and R. Jiang, Ber. Bunsenges. Phys. Chem., 91 (1987) 479.