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Electrochemical and Infrared Spectroscopic Quantitative Determination of the Platinum-Catalyzed Ethylene Glycol Oxidation Mechanism at CO Adsorption Potentials Bonnie Wieland, J. Patrick Lancaster, Cherokee S. Hoaglund, Paul Holota, and Wade J. Tornquist* Department of Chemistry, Eastern Michigan University, Ypsilanti, Michigan 48197 Received August 18, 1995X The electrochemical mechanism for ethylene glycol oxidation by polycrystalline platinum at 0.30 V/SCE in 0.10 M HClO4 is investigated by reflection infrared spectroscopic, coulometric, and voltammetric measurements of 0.10 M ethylene glycol, glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid, and oxalic acid. CO2, glycolic acid, and adsorbed CO are identified as reaction products for ethylene glycol and glycolaldehyde oxidation. A two-path mechanism is proposed for 0.30 V oxidation of ethylene glycol and glycolaldehyde: either the reacting molecule undergoes direct oxidation to desorbing glycolic acid or it undergoes direct dissociation of the carbon-carbon bond to form various amounts of aqueous CO2 and adsorbed CO. Calculations are performed, assuming the quantities of CO and CO2 depend statistically upon the identities of the two functional groups comprising the two-carbon reactant molecule and upon oxidation conditions. Calculation results for dissociation at 0.30 V show that nearly 100% of the carboxyl functional groups are oxidized to CO2, whereas 20% and 50-67%, respectively, of the alcohol and aldehyde groups are partially oxidized to adsorbed CO. About 20% of the ethylene glycol molecules undergo bond dissociation, whereas 25-40% of the glycolaldehyde molecules dissociate. In 70 s of electrochemical oxidation, about three times as many ethylene glycol molecules react as glycolaldehyde molecules.
Introduction
Scheme 1
The electrochemical oxidation of ethylene glycol has long attracted interest1 due to its possible application in fuel cell technology. In acidic environments, ethylene glycol can be completely oxidized to CO2 by Pt electrodes:
C2H6O2 + 2H2O f 2CO2 + 10H+ + 10eThe commonly accepted mechanism for this reaction is a complex series of parallel, two-electron oxidation steps depicted in Scheme 1. This mechanism was originally proposed to account for the results of product analysis after long-term electrolysis at various potentials. These experiments showed that increasing oxidation potentials generate increasing quantities of more heavily oxidized two-carbon compounds and CO2.2-4 Other investigators have also reported observing the production of glycolaldehyde,5 glycolic acid,5-7 glyoxal,6 glyoxylic acid,6 and oxalic acid,5,6 as well as formic acid7 and formaldehyde7 during electrolysis. Long-term electrolysis of ethylene glycol is not easily performed at double-layer potentials, due to the rapid formation of a strongly adsorbed poison on the electrode surface. A feature of reflection infrared spectroscopy is its ability to probe with great sensitivity electrocatalytic processes on a relatively short time scale. Reflection infrared studies in the double-layer potential region for Pt have identified adsorbed CO as the strongly adsorbed * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Weber, J.; Vasilev, Y. B.; Bagotskii, V. S. Electrokhimiya 1966, 2, 515. (2) Horanyi, G.; Inzelt, G. Acta Chim. Acad. Sci. Hung. 1979, 100, 229. (3) Inzelt, G.; Horanyi, G. Acta Chim. Acad. Sci. Hung. 1979, 101, 229. (4) Horanyi, G.; Kazarinov, V. E.; Vassiliev, Y. B.; Andreev, V. N. J. Electroanal. Chem. 1983, 147, 263. (5) Belgsir, E. M.; Bouhier, E.; Essiss Yie, H.; Kokoh, K. B.; Huser, H.; Leger, J.-M.; Lamy, C. Electrochim. Acta 1991, 36, 1157. (6) Vijh, A. K. Can. J. Chem. 1971, 49, 78. (7) Kadirgan, F. Ann. Chim. (Rome) 1989, 79, 517.
oxidation poison.8 Other IR studies have detected the formation of aqueous CO2 for potentials as low as 0.2 V/SCE9-11 and glycolic acid and oxalic acid for slightly higher double-layer potentials. In earlier studies, the determination of the mechanism of ethylene glycol oxidation at double-layer potentials was an important problem,1-4,13-17 in part because a well(8) Hahn, F.; Beden, B.; Kadirgan, F.; Lamy, C. J. Electroanal. Chem. 1987, 216, 169. (9) Christensen, P. A.; Hamnet, A. J. Electroanal. Chem. 1989, 260, 347. (10) Leung, L.-W.; Weaver, M. J. J. Phys. Chem. 1988, 92, 4019. (11) Leung, L.-W.; Weaver, M. J. Langmuir 1990, 6, 323. (12) Jiang, X.; Chang, S.-C.; Weaver, M. J. J. Chem. Soc., Faraday Trans. 1993, 89, 223. (13) Weber, J.; Vasilev, Y. B.; Bagotskii, V. S. Electrokhimiya 1966, 2, 522. (14) Vijh, A. K. Can. J. Chem. 1970, 48, 197. (15) Kokkinidis, G.; Jannakoudakis, D. J. Electroanal. Chem. 1982, 133, 307.
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proposed mechanism was expected to indirectly characterize the strongly adsorbed poison. After Hahn et al. spectroscopically identified the poison,8 investigators became interested in the sensitivity of ethylene glycol oxidation to surface structure18-21 or electrode material.5,22 To our knowledge, no detailed investigation of the mechanism in the acidic environment has been initiated since Hahn’s work;8 this is the general purpose of this report. Allowing for the possibility of CO formation at doublelayer potentials greatly complicates the mechanism of Scheme 1. Extending the analysis of Horanyi et al.,2-4 one would argue that CO can be produced directly by C-C bond dissociation in any molecule except, perhaps, oxalic acid, since the carbon atoms in oxalic acid are already in a higher oxidation state than the carbon atom in CO. Thus, each two-carbon compound represented in Scheme 1 has at least two paths available: Either the reactant undergoes functional group oxidation to the next molecule along the path, as shown, or it undergoes dissociation of the C-C bond to generate adsorbed CO and, perhaps, CO2. The specific purposes of this study are to determine the oxidation mechanism for ethylene glycol at the doublelayer potential 0.30 V/SCE and to perform a quantitative analysis of the products formed on a short time frame. To accomplish this task, oxidation products are analyzed both qualitatively and quantitatively at 0.30 V with infrared spectroscopy, cyclic voltammetry is used to determine quantities of adsorbed CO, and coulometry is used to determine the total quantity of charge produced by oxidation of ethylene glycol and its partial oxidation products at 0.30 V. Experimental Section Sample Preparation. Working electrodes were 9 mm diameter polycrystalline platinum (AESAR, 99.99%) disks sealed in glass and polished with successive grades of alumina. Prior to each experiment, the working electrode was polished with 0.05 µm alumina and immersed in water in an ultrasonic bath. The surface was then electrochemically treated by cycling at 1.0 V/s between the hydrogen evolution and oxygen evolution potential limits of the 0.10 M HClO4 (Aldrich, redistilled, 99.999%) supporting electrolyte, which was prepared with distilled, highpurity water (Barnstead Nanopure II). All reactant solutions were prepared from reagent organic compounds (Aldrich) in supporting electrolyte at 0.10 M. All experiments were performed at 23 °C. Coulometry and Cyclic Voltammetry. The electrochemical experiments were performed in a two-compartment glass cell, with a saturated calomel reference electrode and a platinum wire counter electrode. Potential programs were controlled and data were collected by an EG&G PAR 273A digital potentiostat. The following procedure was employed: The clean electrode was placed in supporting electrolyte in a sealed cell, and the electrolyte was purged with nitrogen for 10 min. To verify a clean system, the electrode potential was cycled twice at 100 mV/s between the limits of -0.27 and 1.28 V. The current was then measured at a sampling frequency of 21.4 Hz using the potential program of ref 1 for obtaining a clean surface and a reproducible adsorbed layer: With nitrogen gas bubbling during the first 60 s, the electrode potential was maintained for 10 s at 1.050 V and then stepped to 0.71 V for 70 s. The potential was then stepped down to 0.30 V for 70 s. The resultant data set was saved and later integrated over the final 70 s to obtain the coulometry data at (16) Kadirgan, F.; Beden, B.; Lamy, C. J. Electroanal. Chem. 1982, 136, 119. (17) Kazarinov, V. E.; Vassiliev, Y. B.; Andreev, V. N.; Horanyi, G. J. Electroanal. Chem. 1983, 147, 247. (18) Orts, J. M.; Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1990, 290, 119. (19) Sun, S.-G.; Chen, A.-C.; Huang, T.-S.; Li, J.-B.; Tian, Z.-W. J. Electroanal. Chem. 1992, 340, 213. (20) Sun, S.-G.; Chen, A.-C. Electrochim. Acta 1994, 39, 969. (21) Leung, L.-W.; Weaver, M. J. J. Phys. Chem. 1989, 93, 7218. (22) Chang, S.-C.; Ho, Y.; Weaver, M. J. J. Am. Chem. Soc. 1991, 113, 9506.
Langmuir, Vol. 12, No. 10, 1996 2595 0.30 V. Immediately following the program, a complete 1 V/s cyclic voltammogram was measured between 0.30 and -0.29 V at a sampling frequency of 1000 Hz. The second half of this data set was later integrated to determine the quantity of hydrogen desorbed from the surface. The cell was emptied of reference solution and refilled with 0.10 M organic sample solution, and the procedure was immediately repeated. Infrared Spectroscopy. Infrared spectra were collected with Digilab FTS-40 infrared spectrometers at 4 cm-1 resolution. Transmission spectra of organic compounds dissolved in 0.10 M HClO4 were obtained using a 14.9 µm path length cell (International Crystal Labs) fitted with CaF2 windows. Background transmission spectra were of 0.10 M HClO4. The standard reflection infrared electrochemistry cell and optics have been described previously.23 Potentials were controlled by an EG&G PAR 175 universal programmer and a 362 scanning potentiostat. Spectra were obtained using the following procedure: The cell was filled with 0.10 M reactant solution (0.20 M for glycolic acid), and the clean electrode was transferred to the cell with a protective drop of supporting electrolyte adhering to the surface. The contents of the cell were then purged with nitrogen for 10 min, and the electrode was brought under potential control at -0.25 V. The electrode potential was swept to 1.0 V at 1 V/s to guarantee a CO-free surface and stepped back to -0.25 V, and after 5 min, the electrode was pushed against the CaF2 window. Reference spectra were acquired at -0.25 V with 4 cm-1 resolution. The potential was then stepped to 0.20, 0.25, or 0.30 V, and sample spectra were acquired immediately and continuously in 32-scan packets. Spectra displayed in this report consist of five averaged packets, acquired over roughly 1 min, beginning after 6.5 min of accumulation of 0.30 V oxidation products (14 min for glyoxal). The infrared electrochemistry flow cell was a standard cell adapted for solution flow via side ports and lengths of Teflon tubing, which directed solution through the spectroscopic thin layer. Potentials were controlled by a Pine AFRDE4 potentiostat. Spectra were obtained with the following procedure: The cell was filled with 0.10 M supporting electrolyte, and the clean electrode was transferred to the cell. A reference spectrum was collected at 0.45 V with 4 cm-1 resolution. A 175 mL solution of 0.10 M ethylene glycol in 0.10 M HClO4 was then gravity fed through the cell for 10 min, and sample spectra were recorded. The spectrum displayed in this report is the average of 32 scans, which were acquired 70 min after the initiation of solution flow.
Results and Discussion Coulometry and Cyclic Voltammetry. Figure 1 displays a typical cyclic voltammogram of the aluminapolished platinum electrode in 0.10 M HClO4. Features near -0.2, 0.0, and 0.6 V are consistent with a clean polycrystalline Pt surface. Table 1 displays the average relative coulometry and cyclic voltammetry results for replicate analyses of each of the two-carbon compounds examined. The uncertainty ranges for 95% confidence are also tabulated. The relative Faradaic charge, QF,rel, is determined from the following formula
QF,rel ) (Qorg - Qbackg)/QHs
(1)
where Qorg and Qbackg are the 70 s coulometry charges measured in the presence and absence, respectively, of the organic compound tested. Whereas Qorg and Qbackg individually are sensitive to all electrochemical processes, their difference is primarily sensitive to the Faradaic reactions of the tested compound. The saturated hydrogen desorption charge, QHs, is determined by integrating the cyclic voltammogram hydrogen desorption region in organic-free electrolyte. When the coulometry charge difference is expressed relative to QHs, variations in results due to variations in electrode surface area are minimized. (23) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484.
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Figure 1. Cyclic voltammogram of clean polycrystalline platinum in 0.10 M HClO4, obtained prior to electrochemical analysis of organic oxidation products. Sweep rate is 100 mV/s. Table 1. Average Relative Charges for 70 s Oxidation at 0.30 V compound
QF,rela,c
θadb,c
ethylene glycol glycolaldehyde glyoxal glycolic acid glyoxylic acid oxalic acid
44 ( 3.9 10.8 ( 0.16 7.5 ( 1.3 2.5 ( 0.65 6.6 ( 1.5 2.0 ( 0.23
0.62 ( 0.024 0.76 ( 0.017 0.41( 0.031 0.09 ( 0.087 0.21 ( 0.020 -0.02 ( 0.031
a Coulometric charge difference between 0.10 M organic reactant and 0.10 M HClO4, reported relative to saturated hydrogen desorption charge. b Fraction of inactive hydrogen sites, due to adsorbate formation by 0.10 M organic reactant. c Uncertainties determined from 95% confidence limit calculations.
QF,rel increases with the quantity of material oxidized and with decreasing oxidation state of the compound tested. The hydrogen monolayer fractional adsorbate coverage, θad, is determined from the following formula
θad ) (QHs - QHorg)/QHs
(2)
where QHorg is the hydrogen desorption charge determined from cyclic voltammetry after 70 s of organic oxidation. Calculated this way, θad is equivalent to the fraction of the electrode surface which contains strongly bonded adsorbates. In the following discussion we will assume the only strongly adsorbed species present is CO,18 although there is evidence that other adsorbates are formed by ethylene glycol and oxalic acid at low potentials.11,12 The following trends are noted for the data in Table 1. First, compounds containing the alcohol functional group tend to be active, yielding relatively high Faradaic charges and high adsorbate coverages. Second, compounds containing the aldehyde functional group tend to yield moderate Faradaic charges and relatively high adsorbate coverages. Third, compounds containing the carboxylic (24) Lamy, C. Electrochim. Acta 1984, 29, 1581.
Figure 2. Infrared spectra of aqueous CO2 generated during oxidation of 0.10 M ethylene glycol in 0.10 M HClO4 by a polycrystalline Pt electrode held at (a) 0.30 V, (b) 0.25 V, and (c) 0.20 V. Reference spectra were obtained at -0.25 V. Sample spectra were collected for about 1 min following 6.5 min of oxidation.
acid functional group are relatively inactive, yielding low charges and low adsorbate coverages. These results are consistent with functional group electrocatalytic trends cited by other investigators.11,12,24 Infrared Spectroscopy. To characterize the products of ethylene glycol oxidation by platinum at double-layer potentials, reflection infrared spectra were obtained before and after the electrode underwent a step transition from a reference potential of -0.25 V to sample potentials of 0.20, 0.25, or 0.30 V. Figures 2 and 3 show the results obtained after 6.5 min of ethylene glycol oxidation at the sample potential. Besides adsorbed CO (not shown), aqueous CO2 is confirmed by a band at 2343 cm-1 at all three potentials (Figure 2). Peaks near 1240 cm-1, assigned to the C-O stretch of a compound or compounds
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Figure 3. Infrared spectra of carboxyl C-O stretching region. Products were generated during oxidation of 0.10 M ethylene glycol in 0.10 M HClO4 by a polycrystalline Pt electrode held at (a) 0.30 V, (b) 0.25 V, and (c) 0.20 V. The reference spectra were obtained at -0.25 V. Sample spectra were collected for about 1 min following 6.5 min of oxidation.
containing the carboxyl group, are also clearly detected at 0.25 and 0.30 V (Figure 3). These results are compared to those reported by Leung and Weaver,10 who also observed CO2 from ethylene glycol oxidation by polycrystalline Pt at 0.2 V; however, they reported first detecting a carboxylic acid peak near 0.4 V. The lower potential limit observed here is most likely due to the use of the potential step procedure, as opposed to a 2 mV/s potential sweep. To identify the carboxylic acid formed in Figure 3, we compared transmission cell spectra of candidates glycolic acid, oxalic acid, and glyoxylic acid to reflection spectra obtained during ethylene glycol oxidation. The spectra appear as c, d, and e in Figure 4; the absorbance scales are varied to simplify qualitative analysis. Spectrum a of Figure 4 was acquired during oxidation at 0.45 V in the flow cell; it consists of positive-going product absorbance bands. Spectrum b was obtained during oxidation at 0.30 V in the standard infrared electrochemical reflection cell; it includes both positive-going product and negative-going reactant absorbance bands. The spectrum obtained in the flow cell has four advantages over the spectrum obtained in the standard reflection cell. First, the higher oxidation potential used in the flow cell yields a decreased quantity of adsorbed CO and an increased quantity of acid products. Second, the large volume of reactant solution passing through the flow cell allows large quantities of products to form in the cell’s thin layer. Third, the use of organic-free electrolyte in the flow cell reference spectrum decreases reactant spectral interferences. Finally, the reference spectrum for the flow cell is acquired at the same potential as the sample spectrum, thereby minimizing interferences near 1110 cm-1, which have been attributed to potential dependent changes in the adsorption of perchloric acid.25 To determine the identity of the carboxyl oxidation product, three regions of each spectrum in Figure 4 are compared. First, each spectrum shows the presence of the carboxyl C-O stretching band near 1240 cm-1. The oxalic acid spectrum is unusual in that this band is observed below 1235 cm-1. A shoulder is observed to the blue of the C-O band in each spectrum, appearing unusually intense in the glyoxylic acid spectrum. This feature is also relatively intense at 1292 cm-1 in the standard reflection cell oxidation spectrum; however, its (25) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 239, 55.
Figure 4. Reflection infrared spectra of ethylene glycol oxidation products and transmission spectra of candidate carboxylic acids. (a) Ethylene glycol oxidation products in the flow reflection cell after 70 s of 0.45 V oxidation. (b) Ethylene glycol oxidation products in the standard reflection cell after 6.5 min of oxidation at 0.30 V. (c) 0.10 M glycolic acid in 0.10 M HClO4 solution. (d) 0.10 M oxalic acid in 0.10 M HClO4 solution. (e) 1 mL of glyoxylic acid in 50 mL of 0.10 M HClO4 solution.
appearance may be exaggerated by the loss of ethylene glycol reactant from the cell.9 The C-O region of the glycolic acid transmission spectrum provides the best match to the flow cell oxidation spectrum. Second, the spectra contain distinguishing features between 1200 and 1000 cm-1. The oxalic acid transmission spectrum is unique in its lack of detail, the glyoxylic acid transmission spectrum contains two peaks near 1103 and 1062 cm-1, and the glycolic acid spectrum contains a peak at 1093 cm-1. The 1093 cm-1 band may be the same peak obtained in the flow cell oxidation spectrum; however, it is noted that optical throughput approaches zero at 1050 cm-1 in spectrum a. Perchloric acid absorption near 1110 cm-1 severely limits the qualitative use of this region in spectrum b. Third, the glycolic acid transmission cell spectrum includes two weak defining peaks at 1357 and 1436 cm-1. Similar weak bands, partially masked by water vapor peaks, are observed in the ethylene glycol oxidation spectra of both the flow and standard reflection cells. The glyoxylic acid spectrum contains a weak band near 1400 cm-1, which is not apparent in the oxidation spectra. On the basis of the above observations, we conclude the glycolic acid transmission cell spectrum best matches the ethylene glycol oxidation spectrum of the flow cell at 0.45 V. Although we cannot rule out the possibility of more than one acid product, it is unlikely that glyoxylic acid and oxalic acid are major products at the low oxidation potentials investigated here. To determine possible reaction intermediates in the electrochemical oxidation of ethylene glycol by platinum, we compared infrared spectra acquired during 0.30 V oxidation of 0.10 M glycolaldehyde, 0.10 M glyoxal, and
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Figure 5. Infrared spectra of aqueous CO2 and adsorbed CO produced by 0.30 V oxidation of 0.10 M solutions of (a) ethylene glycol, (b) glycolaldehyde, (c) glyoxal, and (d) 0.20 M glycolic acid by a platinum electrode. Reference spectra were obtained at -0.25 V. Sample spectra were collected for about 1 min following 6.5 min of oxidation. The glyoxal spectrum was collected following 14 min of oxidation.
Figure 6. Infrared spectra of carboxyl C-O stretching region. Products were generated by 0.30 V oxidation of 0.10 M solutions of (a) ethylene glycol, (b) glycolaldehyde, and (c) glyoxal, and (d) 0.20 M glycolic acid by a platinum electrode. Reference spectra were obtained at -0.25 V. Sample spectra were collected for about 1 min following 6.5 min of oxidation. The glyoxal spectrum was collected following 14 min of oxidation.
0.20 M glycolic acid solutions in the standard reflection cell to similar spectra obtained for ethylene glycol (Figures 5 and 6). Trends are noted here for bands observed in the CO2, adsorbed CO, and carboxyl C-O stretching regions of the spectra. Figure 5 shows that each reactant produces aqueous CO2 during oxidation at 0.30 V, ethylene glycol and glycolaldehyde producing significantly more than glycolic acid and glyoxal. Ethylene glycol and glycolaldehyde show evidence for the production of large quantities of acid during 6 min of oxidation at 0.30 V (Figure 6, spectra a and b), ethylene glycol producing about twice as much as glycolaldehyde. The shape of the acid peak derived from glycolaldehyde is consistent with the same band in the ethylene glycol flow cell spectrum in Figure 4. Glyoxal produces only a small carboxyl C-O stretching band after about 15 min of oxidation (Figure 6, spectrum c). The true acid peak in this spectrum is probably masked somewhat by the presence of a negative-going absorption band near 1300 cm-1 due to the loss of glyoxal. Similarly, the glycolic acid spectrum, d, shows a negative absorbance band near 1240
cm-1. This result, which indicates a net loss of the acid functionality, suggests glycolic acid is not oxidized to oxalic acid during electrooxidation at this potential.11 Derivative bands near 2050 cm-1 in Figure 5 yield information regarding the relative amounts of adsorbed CO present before (downward partner) and after (upward partner) the system undergoes the potential step from -0.25 to 0.30 V. In general, frequency shifts to the blue occur as a result of increasing the electrode potential or increasing vibrational coupling through increased CO coverage.23 The positive-going band in the glycolic acid spectrum, d, indicates a small quantity of CO is present only at 0.30 V. Contrarily, the glycolaldehyde spectrum, b, shows large quantities of CO are present at both potentials, the steep slope at the zero crossing indicating the presence of similar CO coverages at both potentials. This band is compared to that from ethylene glycol, in which the partners are well separated in frequency; ethylene glycol produces only a moderate amount of adsorbed CO at -0.25 V and a greater amount at 0.30 V. The derivative band from glyoxal indicates that a CO coverage intermediate to those of ethylene glycol and glycolaldehyde is produced at -0.25 V; there is a net loss in adsorbed CO as a result of the potential step to 0.30 V. The observations regarding Figures 5 and 6 demonstrate that glycolaldehyde and ethylene glycol produce larger quantities of CO, CO2, and carboxylic acids at 0.30 V than glycolic acid or glyoxal; these results are consistent with the cyclic voltammetry and coulometry data for these molecules. The fact that acid production is greater and CO production is less for ethylene glycol than for glycolaldehyde at this potential suggests glycolaldehyde is not a required intermediate in the oxidation of ethylene glycol. Similarly, the comparatively smaller amounts of products formed by glyoxal and glycolic acid at 0.30 V, especially CO, suggest a glyoxal or glycolic acid intermediate is not necessary for CO and CO2 production in ethylene glycol and glycolaldehyde. Rather, we conclude that CO2 and adsorbed CO production by ethylene glycol and glycolaldehyde at double-layer potentials proceeds largely by direct C-C bond cleavage of the reactant molecule. Figures 5 and 6 together with Figure 4 also support a second path for oxidation of ethylene glycol and glycolaldehyde, wherein the reactant molecule is converted to unreactive glycolic acid, which then desorbs faster than it undergoes further oxidation. This mechanism has also been proposed by Christensen and Hamnet.9 In summary, we propose the oxidation mechanisms for both ethylene glycol and glycolaldehyde at 0.30 V include similar competing paths: One path involves functional group oxidation to glycolic acid, which, due to its unreactive nature, undergoes desorption from the electrode surface. The second path consists of direct C-C bond cleavage of the adsorbed reactant, followed by production of various amounts of CO2 and adsorbed CO. Scheme 2 shows these processes for ethylene glycol. In order to perform a quantitative mechanistic study, a spectroscopic determination of the relative quantities of glycolic acid and carbon dioxide produced during oxidation of ethylene glycol and glycolaldehyde is required. Table 2 summarizes average results of replicate analyses; the reported precision is limited by the standard deviation of several analyses. The surface coverages of adsorbed CO produced from the oxidation of reactant molecules are represented by ΓCO (1 monolayer CO ) 2 × 10-9 mol cm-2). These values were determined by comparing the inte-
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Langmuir, Vol. 12, No. 10, 1996 2599
Table 2. Quantities of Oxidation Products Detected at Given Potentials
a
compound
ΓCO (-0.25 V)a
ΓCO (0.30 V)a
QCO2 (0.30 V)a
corrected QCO2 (0.30 V)a
Qacid (0.30 V)a
Qacid/QCO2 (0.30 V)
ethylene glycol glycolaldehyde
0.6 1.98
1.4 1.88
1.6 1.57
1.6 1.48
4.2 1.8
2.55 1.19
Quantities are averages for several trials expressed in nmol cm-2.
grated CO band intensities with that obtained from a monolayer of CO formed from CO-saturated electrolyte. The quantities of CO2 and glycolic acid produced are represented by QCO2 and Qacid, respectively. QCO2 is obtained by comparing the area of the CO2 band from the reactant to that obtained from the oxidation of a monolayer of CO. Qacid is determined using the method of Leung and Weaver,10 which incorporates the integrated intensity of the 1240 cm-1 oxidation peak, the integrated molar absorptivity of the same glycolic acid band measured in a transmission cell (7100 M-1 cm-2), and the reflection enhancement factor for the reflection cell (2.5). Because the CO coverage for glycolaldehyde decreases as a result of stepping the potential from -0.25 to 0.30 V, the value for QCO2 reported in column 3 includes information from the oxidation of CO adsorbed at the lower potential. This value is corrected (column 4) by subtracting from it the calculated quantity of CO2 which is assumed to be generated as a result of the loss in adsorbed CO. The ratios Qacid/QCO2 for ethylene glycol and glycolaldehyde at 0.30 V appear in the last column of Table 2. Whereas glycolaldehyde produces acid and CO2 at similar rates, ethylene glycol favors formation of the acid over CO2 by a factor of about 2.5. The two molecules yield similar absolute amounts of CO2. Calculations The purpose of Table 2 is to provide an estimate for the relative amounts of acid and CO2 produced by glycolaldehyde and ethylene glycol at 0.30 V during the coulometry experiments. The Qacid/QCO2 values are required below in the quantitative determination of the respective oxidation mechanisms. Due to the inherent lower sensitivity of the infrared spectroscopy relative to the electrochemistry, and due to the diffusion limitations of the thin-layer arrangement of the standard reflection cell, it is impossible to replicate the coulometry experimental procedure in the infrared analysis. When using the spectroscopy data, one must consider the effects of the extended time frame of the infrared measurement as well as the role of CO adsorbed at -0.25 V and the role of the reactant in the standard reflection cell. Time Frame. Both the infrared and electrochemistry experiments require measurements taken over long time periods relative to the kinetics of adlayer formation. By 70 s of ethylene glycol coulometry analysis at 0.30 V, a CO overlayer is fully established1 which inhibits further oxidation and decreases Faradaic currents to values near zero. Between 1 and 7.5 min at 0.3 V in the reflection cell, the intensity of the adsorbed CO peak remains constant, and the amounts of acid and CO2 slowly increase. The average value of Qacid/QCO2 slowly decreases but does not change by more than 10%. Thus, the longer observation period required for the infrared analysis does not substantially change its estimation of Qacid/QCO2 for the coulometry experiment. Adsorbed CO. CO adsorbed at -0.25 V in the infrared experiment may interfere with the production of acid and CO2 after the potential is stepped to 0.30 V. For example, if CO adsorbed at -0.25 V is subsequently oxidized to CO2 at 0.30 V and then fresh CO is generated from organic reactant, Qacid/QCO2 is measured lower than the correct value for the process of interest. On the other hand, if
previously adsorbed CO is not oxidized at 0.30 V, it poisons oxidation sites which may otherwise preferentially produce CO2 or acid at that potential, and the direction of the error in the measured ratio is uncertain. However, there are a few clues regarding the impact of previously adsorbed CO on Qacid/QCO2. First, we have noted that CO adsorbed from ethylene glycol at 0.00 V prior to the initiation of the infrared experiment yields results more like those of glycolaldehyde; that is, less acid is produced, and a smaller Qacid/QCO2 value is realized at 0.30 V. Leung and Weaver10 have also observed essentially identical quantitative results for ethylene glycol and glycolaldehyde oxidation products during relatively slow 2 mV/s sweeps, where CO coverages are large. Second, we have monitored the ethylene glycol 0.30 V oxidation process over short intervals and observed Qacid/QCO2 to decrease as the CO coverage increases. We conclude the presence of adsorbed CO decreases Qacid/QCO2 for both ethylene glycol and glycolaldehyde. Chang and Weaver’s12 data for ethylene glycol oxidation at Pt(111) support this conclusion. The amount of CO adsorbed at -0.25 V for ethylene glycol is less than that for glycolaldehyde, suggesting a smaller error in the estimation of Qacid/QCO2 for ethylene glycol at 0.30 V. The large amount of previously adsorbed CO for glycolaldehyde suggests the possibility that our value for Qacid/QCO2 is a lower limit estimation for the true value for glycolaldehyde and that the true value may approach that of ethylene glycol.10 Reactants. The presence of larger quantities of unreacted molecules at -0.25 V relative to 0.30 V may create features in the spectral baseline which yield errors in the integrated absorbance of the 1240 cm-1 acid band. The shoulder near 1285 cm-1 in spectrum a of Figure 6 may be due to one edge of the 1258 cm-1 band in ethylene glycol. In such a case, the estimation for Qacid/QCO2 in Table 2 is no more than 25% larger than the actual value. Results. Table 3 summarizes the results obtained from substituting the information from Tables 1 and 2 into the equations developed in the Appendix. The values put in for the monolayer fractional coverage of CO, θCO, and the ratio of acid and CO2 product quantities, Qacid/QCO2, are included. Also listed are the hydrogen monolayer fractional coverages of reactants oxidized, θr,ox, and dissociated, θr,dis, and the fraction of molecules which undergo C-C bond dissociation, fr,dis; these values are averages calculated from θCO, θCO2, and θacid for each trial. Two other quantities are included in Table 3. These are the theoretical electrooxidative efficiencies specified in terms of relative yields of both CO2 and electrons. Because formation of CO2 is not a requirement for oxidation, the electron reaction efficiency is always greater than the CO2 efficiency. Two sets of results are listed for glycolaldehyde; one set is obtained when Qacid/QCO2 is the experimentally determined value 1.19 for glycolaldehyde, and one set is obtained when Qacid/QCO2 is the estimated upper limit for glycolaldehyde, 2.55, determined from the ethylene glycol analysis. The significant digits reported in Table 3 are limited by the results of 95% confidence limit calculations. The relative uncertainties are generally between 5 and 10%. The most important values determined by the calculations are the fractions of alcohol, aldehyde, and acid functional groups reacting to form adsorbed CO, fCO(alc),
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Table 3. Calculation Results for Specified Test Compounds at 0.30 V compound
Qacid/ QCO2
nCOa
nCO2b
θCOc
θCO2d
θacide
θr,oxf
θr,disg
fr,dish
glycolic acid glycolaldehyde glycolaldehyde ethylene glycol
0.00 1.19 2.55 2.55
2.84 1.54 1.43 3.00
2.81 4.24 4.42 5.00
0.09 0.76 0.76 0.62
0.8 1.45 1.02 2.8
1.7 2.6 7.1
0.44 2.8 3.5 8.8
0.44 1.11 0.89 1.7
1.0 0.39 0.25 0.19
fCO (acid)i
fCO (ald)j
fCO (alc)k
CO2 efficiencyl
electron efficiencym
0.50 0.67
0.18 0.18 0.18 0.18
0.90 0.257 0.1462 0.1581
0.9366 0.4759 0.3866 0.5020
0.008
a Average number of electrons required to form an adsorbed CO molecule from the given reactant molecule. b Average number of electrons required to form an aqueous CO2 molecule from the specified reactant molecule. c Hydrogen monolayer fractional quantity of adsorbed CO produced from oxidation of specified reactant molecule. d Hydrogen monolayer fractional quantity of CO2 produced from oxidation of specified molecule. e Hydrogen monolayer fractional quantity of glycolic acid produced from oxidation of specified molecule. f Hydrogen monolayer fractional quantity of specified reactant molecule undergoing oxidation. g Hydrogen monolayer fractional quantity of specified reactant molecule undergoing C-C bond dissociation during oxidation. h Fraction of specified reactant molecules undergoing C-C bond dissociation during reaction. i Fraction of acid functional groups reduced to CO during oxidation of specified reactant molecule. j Fraction of aldehyde functional groups oxidized to CO during oxidation of specified reactant molecule. k Fraction of alcohol functional groups oxidized to CO during oxidation of specified reactant molecule. l Relative yield of CO2 during oxidation of specified reactant molecule. m Relative yield of electrons during oxidation of specified reactant molecule.
fCO(ald), and fCO(acid), respectively. These results yield information regarding the relative kinetics for the competitive processes which produce adsorbed CO versus desorbed CO2 for each functional group. The value of less than 1% for fCO(acid) shows the carboxylic acid functional group nearly always forms CO2. This result is reasonable, given the carboxyl group must undergo reduction to form adsorbed CO. The calculations also predict that between 50 and 67% of the aldehyde functional groups form adsorbed CO, compared to about 20% for the alcohol group. These results agree with the observations of Weaver et al.11,12 and are in general agreement with the qualitative discussion regarding Table 1. The importance of adsorbed CO in poisoning heterogeneous catalysis at 0.30 V is highlighted when one compares the third and fourth rows of data in Table 3. Although roughly similar fractions of molecules undergo C-C bond dissociation in each case, about 20-25%, ethylene glycol, which contains no aldehyde groups (row 4), has a high probability of forming CO2, so nearly three times as many molecules are oxidized as for glycolaldehyde (row 3). The second most important set of results in Table 3 is the fractions of molecules undergoing C-C bond dissociation, fr,dis, listed in column 10. These results are an expression of the relative kinetics for the competitive processes which produce C-C bond rupture versus functional group oxidation in each reactant. As a test molecule, glycolic acid is assumed to be oxidized via bond rupture 100% of the time (fr,dis ) 1.0), whereas the test molecules glycolaldehyde and ethylene glycol favor functional group oxidation at this potential, undergoing bond cleavage 25-40% and 20% of the time, respectively. This trend suggests that probabilities for functional group oxidation are larger for compounds in low oxidation states. The fact that the carboxyl-bearing compounds are at once electrochemically inactive and poor producers of CO, whereas aldehyde-bearing molecules readily undergo C-C bond cleavage, even at -0.25 V, and are good producers of adsorbed CO, strongly suggests the most important step in C-C bond dissociation at low potentials involves a molecular precursor to adsorbed CO. Indeed, functional group oxidation of ethylene glycol and glycolaldehyde to carboxylic acids begins to become the favored pathway at potentials greater than 0.6 V,10 where carbon dioxide is the only product of C-C bond dissociation. Similarly, acetic acid, which contains an oxygen-free methyl group as well as an acid functionality, is almost electrochemically inert toward the production of adsorbed CO.26 Although the formation of CO has an important effect in limiting the oxidation process at this potential, the fraction of molecules dissociated, column 10, is, inversely, (26) Corrigan, D. S.; Krauskopf, E. K.; Rice, L. M.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 1596.
the best qualitative predictor for column 8, the number of monolayers of compound reacting. Indeed, glycolic acid, which can only be oxidized through bond cleavage, is relatively inert compared to glycolaldehyde and ethylene glycol, which oxidize about three and nine monolayers of molecules, respectively. Because there has been a long-time interest in fuel cell applications for these compounds, Table 3 includes CO2 and electron reaction efficiencies. The CO2 efficiency is heavily dependent upon the fraction of C-C bonds dissociated, fr,dis, because functional group oxidation to glycolic acid does not produce CO2 in the assumed mechanism. The electron efficiency is also influenced by the fraction of C-C bonds dissociated and the fraction of functional groups producing CO2, but it is more heavily influenced by the oxidation states of the carbon atoms in the reacting compound. Indeed, whereas ethylene glycol is not considered efficient in CO2 production, it is 50% efficient in electron production. Neither efficiency rating is important for predicting oxidation charges in Table 1. Two necessary quantities calculated for each compound are the average number of electrons required to make CO and CO2. For a bifunctionally symmetric molecule such as ethylene glycol, these values are determined a priori from oxidation number assignments. For functionally asymmetric molecules, such as glycolaldehyde and glycolic acid, the values are determined semiempirically. An interesting result of these calculations is the paradox that glycolic acid carbon atoms generate the same average charge, nCO and nCO2, whether they are oxidized to CO or CO2. Conclusions The qualitative and quantitative infrared spectroscopic experiments identify glycolic acid as a major product of ethylene glycol and glycolaldehyde oxidation at 0.30 V. The quantitative electrochemical experiments show that oxidation kinetics of bifunctional organic compounds is, to a certain extent, dependent upon the identity of the functional groups in the reactant molecule. A mechanism and mathematical model which assumes that CO and CO2 are formed when functional groups are oxidized after dissociation of the carbon-carbon bond in ethylene glycol, glycolaldehyde, and glycolic acid is consistent with the experimental observations. The results of the calculations show nearly 100% of carboxyl groups yield CO2, whereas 20% of alcohol functional groups and 50-67% of aldehyde functional groups yield adsorbed CO. Acknowledgment. This work was supported by the National Science Foundation Research Experiences for Undergraduates Program (J.P.L. and C.S.H.) and the Eastern Michigan University Graduate School. B.W.
+
+
Pt-Catalyzed Ethylene Glycol Oxidation Mechanism
Langmuir, Vol. 12, No. 10, 1996 2601
acknowledges a fellowship from the E. M. U. Honors Program. We thank Dr. Carol Korzeniewski for use of her laboratory equipment in performing the flow cell experiments. Appendix Mathematical Model. The mechanism described in the Results and Discussion section has ethylene glycol and glycolaldehyde either undergoing direct cleavage of the C-C bond, followed by production of CO or CO2 from the functional groups, or undergoing direct functional group oxidation to glycolic acid, followed by desorption from the metal surface. The primary purpose of the following calculations is to determine the fractions of ethylene glycol and glycolaldehyde molecules which use each of the two paths and the corresponding quantities of CO, CO2, and glycolic acid produced. A second purpose of these calculations is to determine the probabilities of producing CO and CO2 from the alcohol, aldehyde, and acid functional groups. The equations derived here assume that only CO, CO2, and glycolic acid are formed during electrooxidation, that background-subtracted coulometry measurements are a quantitative measure of Faradaic processes, that changes occurring in the cyclic voltammogram measurements as the result of the addition of organic reactant are directly related to the fractional coverage of CO, that any molecule undergoing C-C bond scission initially forms two fragments identical in form to the functional groups comprising the parent molecule, and that the fates of these fragments depend statistically upon the identities of the functional groups involved. For each of the three molecules examined, the total Faradaic charge measured by coulometry, QF,tot, is equal to the sum of QCO, QCO2, and Qacid, the charges required to make CO, CO2, and glycolic acid, respectively, from the reactant:
QF,tot ) QCO + QCO2 + Qacid
can make the substitution
θCO2(Qacid/QCO2) ) θacid
(A5)
and eq A3 can be expressed in terms of a single coverage unknown, θCO2, and three unknowns describing the number of electrons required to create each of the three product molecules. Solutions to eq A3 depend upon the reactant molecule of interest. The case for each molecule is outlined below. Ethylene Glycol. For the functionally symmetric molecule ethylene glycol, an analysis of carbon atom oxidation numbers determines nCO ) 3, nCO2 ) 5, and nacid ) 4. Using these constants, eq A3 can be solved for θCO2. θacid is then determined from eq A5, and the statistical probability that the alcohol functional group becomes adsorbed CO, fCO(alc), can be computed:
fCO(alc) ) θCO/(θCO + θCO2)
(A6)
1 ) fCO(alc) + fCO2(alc)
(A7)
The identity
can be employed to determine the probability of creating a CO2 molecule from the alcohol functional group. It is emphasized that our model assumes the values calculated in eqs A6 and A7 depend upon electrooxidation conditions and are independent of the parent molecule from which the alcohol functional group is derived. Glycolaldehyde. For glycolaldehyde nacid ) 2. The statistical factors nCO and nCO2 for the functionally asymmetric glycolaldehyde molecule cannot be determined directly from an analysis of oxidation numbers. Instead, these values are determined from a weighted average of the oxidation numbers for the alcohol and aldehyde functional groups:
(A1)
nCO ) 3fCO(alc) + 1fCO(ald)
(A8)
In the case of the reactant glycolic acid, Qacid is zero. If a monolayer of the general product p is defined to be the same quantity as a monolayer of hydrogen adsorption sites, then θp represents the hydrogen monolayer fractional coverage of p. θp is expressed by the following ratio:
where fCO(ald) is the probability that an aldehyde functional group is oxidized to CO. A similar weighted average exists for nCO2 derived from glycolaldehyde:
θp ) (Qp/np)/(QHs/nH)
Here we have employed identity A7 and an analogous identity for the aldehyde functional group. The fraction fCO(alc) is assumed to be the same as that determined in the ethylene glycol analysis; thus, eqs A8 and A9 can be substituted into eq A3 for glycolaldehyde, so that eq A3 is expressed in terms of two unknowns, fCO(ald) and θCO2. To solve the recast eq A3, we require an additional equation similar to eq A6:
(A2)
where np is the number of electrons required to generate a product molecule p from a reactant molecule, QHs is the hydrogen monolayer desorption charge measured by cyclic voltammetry, and nH is one electron per hydrogen atom. Equation A2 allows eq A1 to be recast in the following form:
QF,rel ) (QF,tot/QHs) ) nCOθCO + nCO2θCO2 + nacidθacid (A3) where nCO, nCO2, and nacid are the average numbers of electrons required to generate CO, CO2, and glycolic acid product molecules, respectively, from the reactant molecule of interest and QF,rel is given in Table 1. θCO in eq A3 is determined from the cyclic voltammetry measurements:
θCO ) (QHs - QHorg)/QHs
nCO2 ) 5(1 - fCO(alc)) + 3(1 - fCO(ald))
(fCO(alc) + fCO(ald))/2 ) θCO/(θCO + θCO2)
(A10)
Equation A10, which contains the same two unknowns fCO(ald) and θCO2, equates two expressions defining the fraction of carbon atoms forming adsorbed CO as a result of bond cleavage in the glycolaldehyde reactant. Glycolic Acid. For glycolic acid, nacid ) 0. Glycolic acid is asymmetric, so an analysis identical to that for glycolaldehyde is required, except eqs A8 and A9 depend upon the carboxyl functional group:
(A4)
where QHorg is the hydrogen desorption charge measured in the presence of the organic reactant. θCO2 and θacid in eq A3 are as yet to be determined, but their ratio is given by Qacid/QCO2 from the infrared experiments. Thus, we
(A9)
nCO ) 3fCO(alc) + (-1)fCO(acid)
(A8a)
nCO2 ) 5(1 - fCO(alc)) + 1(1 - fCO(acid)) (A9a) LA9506943
