Ind. Eng. Chem. Res. 1996, 35, 4767-4771
4767
Application of Statistical Experimental Strategies to H2O2 Production on Au/Graphite in Alkaline Solution Yat-June Li, Chia-Chin Chang, and Ten-Chin Wen* Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, Republic of China
By means of fractional factorial design and response surface methodology, the effect of such galvanostatic electrolysis conditions as pH, current density (CD), temperature, and MgSO4 concentration on H2O2 current efficiencies (CE) for Au/graphite electrodes has been systematically investigated. Fractional factorial analysis indicates that the effects of pH, current density, and temperature are key variables influencing the current efficiency on Au/graphite. Through response surface methodology, an empirical equation for H2O2 current efficiencies is fitted and plotted as contour diagrams in order to facilitate examination of experimental results. The contour plots show that the maximum H2O2 current efficiency can be obtained at a current density of 5 A m-2 and a temperature of 8 °C in 1 M KOH. Introduction Oxygen reduction is an industrially important electrochemical reaction, which is applied to fuel cells (Hayakawa et al. 1992), metal-air batteries (Wang et al., 1991), air-depolarized cathodes (Sudoh et al., 1988), and the production of hydrogen peroxide (Oloman and Watkinson, 1979). The applications of oxygen reduction are strongly dependent on the products involved, either OH- or HO2-. Oxygen reduction is considered to proceed primarily along two pathways (Yeager, 1984): one path is a four-electron reduction reaction without the formation of hydrogen peroxide; the other is an initial reduction reaction to produce HO2-, which is then possibly further electroreduced to OH-. The pathway of oxygen reduction is strongly determined by the electrode materials. Carbon is a good cathode for production of hydrogen peroxide and has been used to study H2O2 production via oxygen reduction by several researchers (Jiang et al., 1990; Baez and Pletcher, 1994; Tatapudi and Fenton, 1994). The technology of H2O2 production by electrochemical reduction of oxygen was recommended especially for on-site manufacture and was accepted by a U.S. pulp/paper plant through the Dow-Huron project (Baez and Pletcher, 1994). Although carbon cathodes are the most common choice for H2O2 production, they are not entirely satisfactory. Therefore, noble metal catalysts supported on carbons are widely adopted in industrial chemical processes, including liquid-phase hydrogenation and organic synthesis (Bird, 1987; Ramesh and Shukla, 1987) due to their specific properties such as high surface area and porosity or relatively low intrinsic chemical activity. Recently, Baez and Pletcher (1994) tried to develop alternative cathodes, e.g., a gold and mixed Ti-Au, to replace carbon. Unfortunately, coatings of mixed TiAu oxides show complicated behavior during oxygen reduction, exhibiting both 4e- and 2e- reactions, because oxygen reduction on gold is strongly influenced by foreign metal adatoms (Adzic et al., 1984). However, several researchers (Damjanovic et al., 1967; Wroblowa et al., 1976; Zurilla et al., 1978; Adzic et al., 1980; Fischer and Heitbaum, 1980) reported that gold cathodes showed two-step reduction of oxygen. RRDE * To whom correspondence should be addressed.
S0888-5885(96)00286-2 CCC: $12.00
(rotating ring disk electrode) experiments confirmed efficient H2O2 formation during the first stage of reduction. In addition, a number of papers (Ramesh and Shukla, 1987; Jiang et al., 1990; Suh et al., 1993; Yang et al., 1993; Baez and Pletcher, 1994; Tatapudi and Fenton, 1994) investigated metal catalysts supported on carbon for oxygen reduction. Therefore, Au/C catalysts were deemed by us worth studying for H2O2 production. In this study, fractional factorial design (Box et al., 1978a) was employed in planning the experiments for studying the effects of electrolysis variables on H2O2 current efficiency. In order to determine which experimental parameter settings affect current efficiency, the response surface procedure coupled with Central Composite Design (Box et al., 1978b) was employed in conjunction with additional experiments, the data of which were subjected to regression analysis to determine the conditions required for the optimum H2O2 production efficiency. Experimental Section Preparation of Electrodes. Graphite supports used in this study were supplied from Nippon Carbon Ltd. No. EGNPL (Japan). The plate electrodes were prepared by adsorption of the precursor, AuCl3‚xH2O (Johnson Matthey, 65% metal content). The precursor, in the appropriate molar ratios, was dissolved in an isopropyl alcohol solution containing 10% by volume concentrated HCl, giving a 0.5 mM total solution. Graphite supports were first degreased with soap and water. A particular Au coating was formed by baking the graphite at 85 °C for 12 h after it was dipped in its coating solution. The freshly prepared electrodes were coated with PTFE film and had exposed geometric areas of 50 cm2. Electrochemical Characterization. Electrochemical experiments were conducted using a cathodic linear scanning voltammetry technique. Voltammetry was performed by a BAS-100B (Bioanalytic System, Inc.) in a three-electrode cell, with an Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V vs NHE at 25 °C) acting as reference and a platinum wire serving as counter electrode. A Luggin capillary, whose tip was set at a distance of approximately 1 mm from the surface of the working electrode, was used to minimize errors due to iR drop. Note that all solutions used in this work were © 1996 American Chemical Society
4768 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 1. Factors and Levels for the 24-1 Fractional Factorial Design level factor
-1
+1
A, pH B, current density (A m-2) C, temperature (°C) D, MgSO4 concentration (ppm)
10.8 2 10 0
13.8 6 30 240
Table 2. Design Matrix and Experimental Data from the 24-1 Fractional Factorial Design with Defining Relation I ) ABCD factor
Figure 1. Cathodic linear sweep curves of graphite (curves 1 and 2) and Au/graphite (curve 3) electrode in 1 M KOH measured at sweep rate of 1 mV s-1 under purging nitrogen (curve 1) and oxygen (curves 2 and 3).
prepared with deionized water produced by a reagent water system (MILLI-Q SP, Japan) at 18 MΩ‚cm. In addition, cathodic linear scanning voltammetry investigations were performed under purging nitrogen or oxygen at 1 atm, and all potentials are quoted against Ag/AgCl. Voltammetry was carried out in a 1 M KOH solution prepared from special low-carbonate KOH pellets (Merck, GR). All measurements were conducted at 25 °C, maintained by means of a water thermostat (HAAKE D8 and G). Galvanostatic Electrolysis. Galvanostatic electrolysis was carried out in a divided cell with a Nafion 435, a platinum wire, and a Au/graphite electrode as separator, anode, and cathode, respectively. A 50 cm2 cathode surface area was used. The dc power was supplied by a HA-301 potentiostat/galvanostat system (Hokuto Denko Co., Japan). The charge was 54 C, and oxygen was saturated in the electrolyte (approximately 1.103 mM). After electrolysis, the hydrogen peroxide concentration in the cathodic electrolyte was determined by titrating with potassium permanganate solution. Results and Discussion Linear Sweep Voltammetry. The i-E behavior of graphite and Au/graphite with nitrogen/oxygen purging in a 1 M KOH solution at a scan rate of 1 mV s-1 is shown in Figure 1. By comparing the reduction currents of curves 1 and 2, it can be seen that the nitrogenpurged curve 1 currents are quite small before hydrogen evolution (approximately -1000 mV). The oxygenpurged curve 2 currents begin at approximately -200 mV and increase with increasing negative potential. These results show that the oxygen reduction reaction on graphite occurs between approximately -200 and -1000 mV. On curve 3, the reduction currents obviously represent the oxygen reduction reaction, which begins at approximately -200 mV and also increases with increasing negative potential. The reduction currents (negative to -300 mV) are larger on curve 3 than on curve 2. This difference is attributable to the better electrocatalytic activity of Au/graphite as opposed to graphite itself. This information, coupled with previous researches (Damjanovic et al., 1967; Wroblowa et al., 1976; Zurilla et al., 1978; Fischer and Heitbaum, 1980; Adzic et al., 1980; Jiang et al., 1990; Baez and Pletcher, 1994; Tatapudi and Fenton, 1994, who all proposed that
run
A
B
C
D
current efficiency (%)
1 2 3 4 5 6 7 8
-1 -1 -1 -1 +1 +1 +1 +1
-1 -1 +1 +1 -1 -1 +1 +1
-1 +1 -1 +1 -1 +1 -1 +1
-1 +1 +1 -1 +1 -1 -1 +1
51.42 48.44 60.36 55.89 68.56 63.34 73.77 71.54
Table 3. Estimates of Effects from the 24-1 Fractional Factorial Design with Defining Relation I ) ABCD effect
estimate
*A + BCD *B + ACD *C + ABD D + ABC AB + CD AC + BD BC + AD
15.275 7.450 -3.725 1.120 -0.745 0.000 0.375
carbon and gold are good cathodes for efficient H2O2 formation in the first stage of oxygen reduction), stimulated us to investigate H2O2 production on Au/graphite. Fractional Factorial Design. In order to investigate H2O2 formation on Au/graphite, the fractional factorial design method (Box et al., 1978a) and the response surface procedure (Box et al., 1978b) were used to identify the key variables influencing current efficiency from the following variables: (A) pH, (B) current density, (C) temperature, and (D) MgSO4 concentration. The purpose of the addition of MgSO4 is to inhibit the decomposition of hydrogen peroxide (Paleologou and Berry, 1991). The experimental design allows the influence of each process variable to be observed at a variety of other variable levels, as well as allowing observation of interactions among the variables. The design factors and levels for the 24-1 fractional factorial experiments are listed in Table 1, and the results of these experiments are given in Table 2. The level of each variable during a run is indicated in columns 2-5, with experimental results for H2O2 current efficiencies corresponding to each set of conditions being shown in column 6. The combination of observations used to estimate the effect of factor D (MgSO4 concentration) is identical to that used to estimate the three-factor interaction effect of factors A (pH), B (current density), and C (temperature); hence, the estimates of factor D and the interaction effect of factors A, B, and C are said to be “confounded” (Box et al., 1978a). Accordingly, the defining relation I ) ABCD, suggested by Box et al. (1978a), was established in order to identify the relationships between the effects. Estimates of the experimental variable effects are calculated following the procedure recommended by Box et al. (1978a) and are given in Table 3, which reveals that the effects of pH (A), current density (B), and temperature (C) are the
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4769
Figure 2. Effects of pH (A), current density (B), temperature (C), and MgSO4 concentration (D). The -1 and +1 terms represent low and high levels in Table 1.
key variables (indicated by asterisk symbols) influencing the current efficiency. All possible pathways of oxygen reduction can be put in terms of the following reactions (Babic and Metikoshukovic, 1993):
O2 + 2H2O + 4e- f 4OH-, E°H ) 0.401 V (NHE) (1) O2 + H2O + 2e- f HO2- + OH-, E°H ) -0.0649 V (NHE) (2) HO2- + H2O + 2e- f 3OH-, E°H ) 0.867 V (NHE) (3) Reaction (1) proceeds via a 4e- pathway in which oxygen reduction generates OH- directly. Reaction (2) shows a 2e- transfer and produces HO2-, which may be followed by reaction (3), another 2e- transfer. It is conceivable that, at high pH, reaction (2) is preferred among these three reactions. Now, an examination of Figure 2 reveals that H2O2 current efficiency at pH ) 13.8 is much higher than that at pH ) 10.8, but a simple positive correlation between pH and current efficiency does not provide a good variable for the type of minimum/ maximum studies used in this paper. Thus, in the following response surface methodology experiments, pH is fixed at 13.8 (1 M KOH). A further examination of Figure 2 also reveals that the effects of current density (B) and temperature (C) are significant and the addition of MgSO4 (D) is insignificant. Accordingly, in the following experiments, MgSO4 is not added. Path of Steepest Ascent. From either Table 3 or Figure 2, we have discussed the effects of A and D but not B and C. In fact, it is obvious that current density (B) and temperature (C) have positive and negative effects, respectively. In order to quantitatively elucidate the effects of all study variables, the H2O2 current efficiency data in Table 2 was subjected to regression analysis. The analysis generated the following equation:
CE ) 61.665 + 7.6375A + 3.725B - 1.8625C + 0.56D (4) where CE represents H2O2 current efficiency and A, B, C, and D are defined as in Table 1. The multiple correlation coefficient, R2, is equal to 0.998. A R2 value close to 1 means a perfect fit to the experimental data. Equation 4 represents the experimental data as a first-
Figure 3. Current efficiency diagrammed against current density or temperature on the path of steepest ascent. Table 4. Points on the Path of Steepest Ascent factor run
B
C
current efficiency (%)
1 2 3 4
-1 0 1 2
0.5 0 -0.5 -1
59.62 70.05 72.28 65.79
Table 5. Factors and Levels for Central Composite Design factor levels i, t -x2 -1 0 +1 +x2
CD (A
m-2)
1.76 3 6 9 10.24
temp (°C) 7.93 10 15 20 22.07
order model. As stated in the above discussion, we fixed A at pH ) 13.8 and D at zero addition of MgSO4. We then sought to evaluate B and C via the path of steepest ascent methodology and were very interested in these results since B and C possess the larger estimated absolute values and thus are the more important factors in eq 4. The path is followed by simultaneously moving C ) -1.8625 units in the temperature direction for every B ) +3.725 units moved in the current density direction or, equivalently, -1.8625/3.725 ) -0.5 units in the temperature direction for every 1 unit in current density. A series of points on the path of steepest ascent are listed in Table 4. The resulting points generated by a steepest ascent search are indicated by the circles shown in Figure 3, which suggest that maximum current efficiency is located in the neighborhood of run 3. Central Composite Design. On the basis of run 3, 11 Central Composite Design experiments (Box et al., 1978b) were performed. Design factors and levels are listed in Table 5, and the design matrix with the corresponding results are listed in Table 6. Again the data of H2O2 current efficiency were subjected to regression analysis and generated the following equation:
CE ) 72.41 - 8.95i + 1.89t - 10.78i2 - 0.53t2 + 4.27it (5) where the i and t terms respectively represent current
4770 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 6. Design Matrix and Experimental Data from the Central Composite Design for Quadratic Form Fit level run
i
t
current efficiency (%)
1 2 3 4 5 6 7 8 9 10 11
-1 -1 1 1 0 0 0 0 0 -x2 +x2
-1 1 -1 1 0 0 0 -x2 +x2 0 0
69.23 69.30 45.46 62.59 72.28 72.65 72.28 71.54 70.05 64.83 35.77
Table 7. Analysis of Variance for Fit of Current Efficiency from Central Composite Design source model error total R2 ) 0.9579
degree of freedom
sum of squares
mean square
5 5 10
1440.77 63.28 1504.05
288.15 12.66
F 22.76
density and temperature. The analysis of variance is presented in Table 7. The test statistics, F and R2, are defined:
F)
Figure 4. Constant current efficiency contour lines against current density and temperature. (Note: bottom i and left t coordinates are arbitrary level values as used in Table 5.)
MSR SSR , R2 ) MSE SST
resulting in the reduction of HO2- to OH- (reactions (2) and (3)). It might be worth noting that, in our experience, the diffusion of oxygen (untested in this study) might be another important cofactor in the oxygen reduction reaction. Regardless, the observed current efficiency decreases with decreasing current density for current < 3 A m-2 (i < -1). At low current density (6 A m-2), resulting in the decomposition of unstable HO2-. According to the above discussion, there must exist an optimum operating condition for current density (i) in the region between 3 and 6 A m-2 (-1 < i < 0). Contour Plot. In order to facilitate a straightforward examination of the dependence of H2O2 on current density and temperature, the contour plots (Figure 4) were constructed by using eq 5. The maximum current efficiency (approximately 74%) occurs in the region of 4-6 A m-2 and 15-20 °C. In addition, the oval shape in the maximum region with an inclined angle suggests that the effect of temperature on current efficiency is not obvious. In contrast to temperature, the current density has a great influence on current efficiency in the maximum region. Under these conditions, maximum current efficiency is thought to occur at approximately 5 A m-2 as discussed previously. Note that in the region of low current density (20 °C), low H2O2 current efficiency results from long electrolysis time and low dissolved oxygen, respectively. While in the region of high current density (>8 A m-2) and low temperature ( 6 A m-2 (i > 0). The high current density contributes to the high cathodic potential. At high cathodic potential the oxygen reduction reactions possibly favor a four-electron pathway,
Table 8. Current Efficiency of Hydrogen Peroxide at Optimum Conditions matrix
factor
run
sample
i
t
1, 2, 3 4, 5, 6
Au/graphite graphite
-0.33 -0.33
0.6 0.6
CD (A 5 5
m-2)
current efficiency (%)
temp (°C) 18 18
74.52 68.56
74.89 68.32
74.38 68.56
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4771
maximum current efficiency (approximately 74%) occurs at approximately 5 A m-2 and 18 °C in 1 M KOH, as confirmed in the following. Confirmation Experiments. In order to confirm the validity of the statistical experimental strategies, three additional experiments based on the above optimum condition were performed. The results for Au/ graphite together with those for graphite are listed in Table 8. These reproducibly high experimental current efficiency values for Au/graphite (74.52, 74.89, and 74.38%) validate that fractional factorial design and regression analysis are good and useful techniques in this field of study. In addition, a comparison between Au/graphite and graphite reveals that Au/graphite is a efficacious and, in fact, superior cathode for the production of hydrogen peroxide. Summary and Conclusion Statistical factorial experimental design has been shown to be a useful tool in reducing the number of trial runs. H2O2 current efficiency on Au/graphite is predominantly determined by the effects of pH, current density, and temperature. The experimental results from the steepest ascent path method and the response surface method, subjected to regression analysis and plotted as contour diagrams, were extremely useful in studying the effects of the key variables (temperature and current density) and determining the maximum value of H2O2 current efficiency on Au/graphite. The results show that maximum current efficiency for hydrogen peroxide production can be obtained at a current density of 5 A m-2 and a temperature of 18 °C in 1 M KOH. Acknowledgment The financial support of this work by the National Science Council of the Republic of China under Contract No. NSC 86-2214-E006-007 is highly appreciated. Literature Cited Adzic, R. R.; Tripkovic, A. V.; Markovic, N. M. Oxygen Reduction on Electrode Surfaces Modified by Foreign Ad-atoms: Lead Adatoms on Gold. J. Electroanal. Chem. 1980, 114, 37. Adzic, R. R.; Anastasijevic, N. A.; Dimitrijevic, Z. M. Oxygen Reduction on Ruthenium Electrode Modified by Foreign Metal Electrode. J. Electrochem. Soc. 1984, 131, 2730. Babic, R.; Metikoshukovic, M., Oxygen Reduction on StainlessSteel. J. Appl. Electrochem. 1993, 23, 352. Baez, V. B.; Pletcher, D. The Preparation and Characterization of Gold Coating on Titanium: the Reduction of Oxygen. J. Electroanal. Chem. 1994, 377, 231. Bird, A. J. In CatalysissScience and Supported Catalysts; Stiles, A. B., Ed.; Butterworth: Boston, 1987; pp 107-137.
Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for Experiments; Wiley: New York, 1978a; pp 374-433. Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for Experiments; Wiley: New York, 1978b; pp 510-539. Damjanovic, A.; Genshaw, M. A.; Bockris, J. O’M. Hydrogen Peroxide Formation in Oxygen Reduction at Gold. J. Electroanal. Chem. 1967, 15, 173. Fischer, P.; Heitbaum, J. Mechanistic Aspects of Cathodic Oxygen Reduction. J. Electroanal. Chem. 1980, 112, 231. Hayakawa, Y.; Kawashima, A.; Habazaki, H.; Asami, K.; Hashimoto, K. Amorphous Nickel-Valve Metal-Platinum Group Metal Alloy Electrodes for Hydrogen-Oxygen Sulphuric Acid Fuel Cells. J. Appl. Electrochem. 1992, 22, 1017. Jiang, S. P.; Lin, Z. G.; Tseung, A. C. C. Homogeneous and Heterogeneous Catalytic Reactions in Cobalt Oxide/Graphite Air Electrodes. I. Chemical Kinetics of Peroxide Decomposition by Co(II) Ions in Alkaline Solutions. J. Electrochem. Soc. 1990, 137, 759. Oloman, C.; Watkinson, A. P. Hydrogen Peroxide Production in Trickle-Bed Electrochemical Reactors. J. Appl. Electrochem. 1979, 9, 117. Paleologou, M.; Berry, R. M. (to PAPRICAN). U.S. Patent, 5,006,211, 1991. Ramesh, K. V.; Shukla, A. K. Carbon-Based Electrodes Carrying Platinum-Group Bimetal Catalysts for Oxygen Reduction in Fuel Cells with Acidic or Alkaline Electrolytes. J. Power Sources 1987, 19, 279. Sudoh, M.; Kodera, T.; Hino, H.; Shimamura, H. Electrochemical Production of Hydrogen Peroxide by Reduction of Oxygen. J. Chem. Eng. Jpn. 1988, 21, 198. Suh, D. J.; Park, T. J.; Ihm, S. K. Effect of Surface Oxygen Groups of Carbon Supports on the Characteristics of Pd/C Catalysts. Carbon 1993, 31, 427. Tatapudi, P.; Fenton, J. M. Simultaneous Synthesis of Ozone and Hydrogen Peroxide in a Proton-Exchange-Membrane Electrochemical Reactor. J. Electrochem. Soc. 1994, 141, 1174. Wang, C. C.; Goto, K. S.; Akbar, S. A. Demixing of (Ni, CO)O Under an Oxygen Potential Gradient Using a YSZ-Based Galvanic Cell. J. Electrochem. Soc. 1991, 138, 3673. Wroblowa, H. S.; Pan, Y.-C.; Razumney, G. Electroreduction of Oxygen. J. Electroanal. Chem. 1976, 69, 195. Yang, Y.; Zhou, Y.; Cha, C.; Carroll, W. M. A New Method for the Preparation of Highly Dispersed Metal-Carbon Catalyst Pd/C Catalyst and Its Properties. Electrochim. Acta 1993, 38, 2333. Yeager, E. Electrocatalysts for O2 Reduction. Electrochim. Acta 1984, 29, 1527. Zurilla, R. W.; Sen, R. K.; Yeager, E. The Kinetics of the Oxygen Reduction Reaction on Gold in Alkaline Solution. J. Electrochem. Soc. 1978, 125, 1103.
Received for review May 21, 1996 Revised manuscript received October 3, 1996 Accepted October 7, 1996X IE960286+
X Abstract published in Advance ACS Abstracts, November 15, 1996.
