In situ FTIR spectroscopic studies of ethylene glycol electrooxidation on Pd electrode in alkaline solution: The effects of concentration

Journal of Electroanalytical Chemistry 688 (2013) 165–171

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In situ FTIR spectroscopic studies of ethylene glycol electrooxidation on Pd electrode in alkaline solution: The effects of concentration Jian-Long Lin, Jie Ren, Na Tian, Zhi-You Zhou ⇑, Shi-Gang Sun Department of Chemistry, College of Chemistry and Chemical Engineering, State Key Laboratory of Physical Chemistry of Solid Surfaces, 422 Siming South Road, Xiamen University, Xiamen 361005, China

a r t i c l e

i n f o

Article history: Available online 17 September 2012 Keywords: EG electrooxidation Electrocatalysis Concentration effects Direct alcohol fuel cells Reaction mechanism

a b s t r a c t We investigated the electrooxidation of ethylene glycol (EG) on polycrystalline Pd electrode in alkaline by in situ FTIR reflection spectroscopy, and focused on the effects of EG concentration (2 mM–1 M) on the reaction pathways. Voltammetric result shows the reaction order of EG is near 0.5. In situ FTIR results demonstrated that the reaction pathways and the product contribution strongly depend on the EG concentration. The oxidation degree of EG decreases with increasing EG concentration, and the main prod2 ucts gradually varies from CO2 3 , C2 O4 , glyoxylate, to glycolate. Bridge-bonded CO (COB), generated from dissociative adsorption of EG, can be easily oxidized at low EG concentration. These facts were attributed to the competitive adsorption of EG itself and reaction intermediates with water, which inhibits the formation of oxygen species from water dissociation, a key oxygen donor for deep oxidation of EG. Even though, EG is still much better than ethanol as candidate fuel for alkaline direct alcohol fuel cells. The present study is of importance for understanding reaction mechanism of EG electrooxidation. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In the last decades, the electrooxidation of ethylene glycol (EG) has attracted considerable attention due to its potential application in fuel cells and as a model C2 compound in electrocatalysis [1–7]. The electrooxidation of EG is, in principle, a complex process as up to 10 electrons per molecule is required for full oxidation to CO2. This reaction may proceed via several consecutive and/or parallel steps involving different reaction intermediate such as glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid, oxalic acid and formic acid [8–10]. Extensive efforts have been devoted to investigating the electrochemical adsorption and electrooxidation of EG on Pt electrodes [11–13]. There is little doubt that Pt and Pt-based electrocatalysts can exhibit an excellent performance for EG oxidation, yet they have some limitations for the applications in direct alcohol fuel cells (DAFCs), especially in terms of cost [14,15]. In this respect, Pd may be an alternative to Pt [16,17], yet few fundamental studies have been reported so far on the EG electrooxidation on Pd-based catalysts [18–20]. Recently, Shen group [21,22] has used in situ FTIR spectroscopy to study the electrocatalytic oxidation of EG (1 M) at Pd electrode in solutions with different pH values. Both C2 (glycolate, glyoxal, glyoxylate and oxalate) and C1 species (formate and carbonate) were detected. However, reaction selectivity

⇑ Corresponding author. Tel.: +86 592 2180181. E-mail address: [email protected] (Z.-Y. Zhou). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.08.027

was not quantitatively evaluated, and concentration effects have not been studied. It is well-known that the concentration of reactant plays a considerable effect on electrocatalysis [23–26]. For example, Camara and Iwasuta have investigated the effect of ethanol concentration on its electrooxidation on polycrystalline Pt electrode [24]. The results showed that the products requiring additional oxygen, namely CO2 and acetic acid, present a high yield at low ethanol concentration, whilst the acetaldehyde is favored at concentrated ethanol solutions. They suggested that at high ethanol concentration, Pt surfaces are blocked by weakly bonded intermediates, which impede OH formation and inhibit those pathways requiring additional oxygen atoms for oxidation. The pathway of ethanol dehydrogenation to acetaldehyde is less site demanding, similar to the electrooxidation of methanol to formaldehyde, and is governed by a reverse Eley–Rideal like type mechanism [23]. EG oxidation on Pt can proceed via two different pathways depended on whether CAC bond is broken [27]. Behm and co-workers used DEMS to study the effect of some external parameters such as electrode potential, reactant concentration, and catalyst loading, on the kinetics and mechanism of EG electrooxidation on Pt/C [28]. It was found that the current efficiencies of CO2 were below 25% at all EG concentration (1 mM–0.5 M). Despite the numerous contributions, the exact mechanism of EG electrooxidation, especially on Pd, has not been completely elucidated. As an attempt to get further insights about this matter, we carried out electrochemical in situ FTIR spectroscopic studies of the effect of EG concentration on its oxidation behavior on a polycrys-

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talline Pd electrode. Through the analysis of in situ FTIR spectra, the concentration-dependent reaction pathways of EG electrooxidation were illuminated.

3. Results and discussion

2. Experimental

Fig. 1a shows cyclic voltammograms of Pd electrode in 0.1 M NaOH containing different concentration of EG (CEG = 2 mM–1 M) at 50 mV s1. The open circuit potential of the Pd electrode in EG solutions was about 0.47 V. In the positive-going potential sweep, the peak current (ip) increases with EG concentration. The linear relationship between log(ip) and log(CEG) yields a reaction order of 0.45 for EG (Fig. 1b). Moreover, the peak potential (Ep) shifts positively as the EG concentration increases. For example, the Ep of 2 mM EG is located at 0.18 V, whereas it moves up to 0.02 V for 1 M EG. This phenomenon suggests that the adsorbed fragments of EG generating at low potentials are binding more strongly on the Pd surface when the EG concentration increases, which coincides with the previous study on the ethanol electrooxidation at polycrystalline Pt [24] and Pt (1 1 1) [26]. With the aim to interpret these behaviors in terms of the steps involved in the reaction and gain more knowledge about the concentration effect on the reaction pathways, we used in situ FTIR spectroscopy to monitor the reaction processes of EG oxidation.

2.1. Electrochemical experiments Electrochemical measurements were carried out in a standard three-electrode cell. A Pt foil was used as counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. All electrode potentials in this paper were reported versus the SCE scale. A polycrystalline Pd disk (/ = 5 mm) embedded into a Teflon holder was used as working electrode. Prior to electrochemical tests, the Pd electrode was polished mechanically with alumina powder of sizes 5, 1, and 0.3 lm, and washed in an ultrasonic bath. The Pd electrode was then electrochemically cleaned by potential cycling between 0.80 and 0.60 V in 0.1 M NaOH until a reproducible cyclic voltammogram (CV) was obtained. The Pd electrode was finally transferred to 0.1 M NaOH solution containing 2 mM, 20 mM, 0.1 M and 1 M EG, respectively. All the solutions were prepared with Millipore water (18 MX cm), and were deaerated by bubbling high-purity N2 before electrochemical measurements. The ohm drop by solution resistance has been compensated in the voltammetric tests.

3.2. In situ SPA-FTIR spectroscopic studies of EG electrooxidation at different concentration Fig. 2 displays in situ SPA-FTIR spectra obtained on the Pd electrode for EG electrooxidation at concentration varying from 2 mM

In situ FTIR measurements were carried out on a Nicolet-8700 FTIR spectrometer (Thermo Scientific, USA) equipped with a liquid-nitrogen-cooled MCT-A detector. The thin-layer IR cell has been detailed previously [29]. In this configuration, an un-polarized IR radiation sequentially passed through a CaF2 prism (13 mm in thickness) at an incident angle of 60° and a thin-layer (10 lm) solution, and then it was reflected by the electrode surface. In this external reflection mode, both dissolved substances in the thin-layer solution and adsorbed species on the electrode surface can be detected. The resulting spectra were reported as relative change in reflectivity, that is,

(a)

ip 1 M EG

3

0.1 M EG 20 mM EG 2 mM EG

i / mA

2.2. In situ FTIR experiments

DR RðES Þ  RðER Þ ¼ R RðER Þ

3.1. Voltammetric studies of EG oxidation on Pd at different concentration

2

1

0

ð1Þ -0.8

0.0

0.4

E / V(SCE) 10

-2

(b)

ip / mA cm

where R(ES) and R(ER) are the single-beam spectra collected at sample potential ES and reference potential ER, respectively. By this definition, the downward band in the resulting spectra indicates the formation of products, while upward bands denote the consumption of reactants. In this study, the ER was fixed at 0.80 V, where EG oxidation does not occur. To reduce the impact of reactant consumption and product accumulation in the thin-layer solution, we used single potential alternation FTIR (SPA-FTIR) method [30]. That is, after the collection of on set of single-beam spectra at the ER and ES, the thin-layer solution and electrode surface were renewed as following clean procedures: (1) The Pd electrode was lift up, and potential was held at 0.4 V for 15 s to oxidize completely any adsorbed species; then stepped negatively to 0.80 V (ER), where surface oxygen species were reduced, and the oxidation of EG did not occur; (2) The Pd electrode was then pushed against the CaF2 IR window to form a new thin-layer solution; (3) The single-beam spectra at the ER and the next ES were collected. Spectra were computed from the average of 200 interferograms (about 80 s). The spectral resolution was 8 cm1 without denotation. All experiments were carried out at room temperature (25 °C).

-0.4

1

0.1 0.01

0.1

1

CEG / M Fig. 1. (a) Cyclic voltammograms of polycrystalline Pd electrode recorded in 0.1 M NaOH containing EG with different concentrations. Scan rate: 50 mV s1. (b) Log– log plot between forward-scan peak current and EG concentrations.

J.-L. Lin et al. / Journal of Electroanalytical Chemistry 688 (2013) 165–171

(a)

(b)

(c)

(d)

167

Fig. 2. In situ SPA-FTIR spectra of EG oxidation on the Pd electrode in 0.1 M NaOH with different EG concentration. (a) 2 mM; (b) 20 mM; (c) 0.1 M; (d) 1 M. ES varied from 0.6 to 0.4 V, ER = 0.80 V, 8 cm1.

to 1 M. The reference spectrum was taken at 0.8 V, and sample potentials varied from 0.60 to 0.40 V. As noted in experimental section, each spectrum was collected with fresh thin-layer solution and cleaned Pd surfaces, i.e., the initial conditions are nearly the same. The peaks corresponding to the most important features are marked in the figure. For further details about the assignment of the bands can also be seen in Refs. [21,22,31]. In the 2 mM EG solution (Fig. 2a), a well-defined band at 1400 cm1, assigned to carbonate ions (CO2 3 ), appears at 0.5 V. The band intensity of CO2 increases with increasing potential and reaches a maximum 3 at 0.2 V, and then declines due to the low reactivity of Pd electrode at high potentials (Fig. 1a). In addition, a very weak band at 1309 cm1, assigned to oxalate (C 2 O2 4 ), emerges at potential of 0.3 V, and disappears at potential higher than 0 V. The maximal selectivity for C 2 O2 4 , gained at 0.2 V, is below 9%, as determined by a quantitative analytic method for in situ FTIR data we proposed previously [32,33]. No bands corresponding to other C2 species (such as glycolate, glyoxal, and glyoxylate) can be seen at this concentration. This result is unlike EG oxidation on Pt in acidic media, where the current efficiencies of CO2 were below 25% even in 1 mM EG solution [28]. Clearly, it is relatively easy for complete oxidation of EG on Pd in alkaline solution. When the EG concentration increases to 20 mM (Fig. 2b), there are characteristic bands for glycolate (1581, 1411, 1325, 1078 cm1) between 0.4 and 0.2 V, but the carbonate and oxalate ions dominate at higher potentials. Due to the OH depleted quickly by EG oxidation and its acidic

products (e.g., CH2 OH  CH2 OH þ 14OH ! 2CO2 3 þ 10H2 Oþ 10e), the thin-layer solution was gradually neutralized. As a result, a small fraction of carbonate was converted into CO2 when potential was higher than 0.20 V, as indicated by a small band at 2343 cm1. At higher concentration (Fig. 2c and d), the electrooxidation of EG mainly yields CO2, glycolate (1078 cm1) and glyoxylate (1099 cm1). Due to the grievous acidification of thin-layer solution, some carboxylate ions were even converted into ACOOH form (mC@O at 1730 cm1 and mCAOH at 1243 cm1) [22]. As for adsorbed species, bridge-bonded CO (COB) at about 1850 cm1, stemming from dissociative adsorption of EG on the Pd electrode, can be observed in all solutions. The band center of COB, generated at the same potential, shifts to high wavenumbers as the EG concentration increase. In order to complement the spectroscopic analysis, in situ FTIR spectra collected at the same potential but different EG concentration were re-plotted together, and shown in Fig. 3. It is interesting that the COB band (1890 cm1) is not clearly observed at 0.4 V for 2 mM EG, while it can be detected obviously in other three EG concentration. This result suggests that COB can be easily oxidized in EG solution with lower concentration. Fig. 4 depicts the potential dependence of band intensity of COB. The COB band intensity increases from 0.6 to 0.5 V due to the increase in dissociative adsorption of EG, and then declines due to the oxidation of COB at high potentials. For 20 mM EG solution, COB and glycolate are

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(a)

(b)

(c)

(d)

Fig. 3. Comparison of FTIR spectra collected with different EG concentration (indicated in the figure) at different potentials. (a) 0.4 V; (b) 0 V; (c) 0.1 V; (d) 0.2 V. Data were extracted from Fig. 2.

at higher concentration of EG can also be attributed to the inhibition the OHad formation due to the surface blockages by reaction intermediates. Additionally, for the solution containing 0.1 M EG, the band at 1099 cm1, assigned to glyoxylate, is well developed at 0.2 V (Fig. 3d), but such a band cannot be observed for 1 M EG. The result also suggests that at lower concentration, EG can be easily oxidized into further oxidation products even without the cleavage of CAC bond. The results presented in Figs. 2 and 3 suggest that the reaction pathways of EG electrooxidation highly depend on the EG concentration. The contributions of the pathways producing CO2 or CO2 3 are inhibited at high EG concentration, probably due to the limited availability of free sites for the adsorption of water to form adsorbed oxygen species.

0.08

2 mM

0.06

20 mM

ICO / a.u.

0.1 M 1M

0.04

0.02

0.00 -0.6

-0.4

-0.2

0.0

0.2

0.4

E / V (SCE)

3.3. Time-dependence spectroscopic studies of EG electrooxidation at different concentration

Fig. 4. Variation of COB band intensity with electrode potentials.

produced at potential 0.4 V; Oxalate and CO2 3 appear at higher potential. A large band at about 1580 cm1 appears for 0.1 M and 1 M EG, indicating that high EG concentration is prone to forming incomplete oxidation product [22], such as glycolate and glyoxylate. Comparing with the IR spectra between 0.1 M and 1 M EG in Fig. 3c and d, we can observe that more CO2 can be detected at 0.1 M EG than 1 M EG. This increasing resistance to generate CO2

Fig. 5 shows several set of time-resolved FTIR spectra of EG oxidation on the Pd electrode in 0.1 M NaOH with different EG concentration at 0 V. For 2 mM EG (Fig. 5a), only CO2 can be 3 observed, and its band intensity increases quickly in the initial 10 s, and then it reaches a maximum value at about 20 s, after which it decreases due to the diffusion of CO2 from thin-layer 3 solution to the bulk solution. For 20 mM EG (Fig. 5b), oxalate is produced beside CO2 3 . At the concentration of 0.1 M and 1 M, EG oxidation yields mainly CO2 and incomplete oxidized C2 products.

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(a)

(c)

(b)

(d)

Fig. 5. Time-resolved in situ FTIR spectra of EG electrooxidation on the Pd electrode at 0 V in 0.1 M NaOH with different EG concentration. (a) 2 mM; (b) 20 mM; (c) 0.1 M; (d) 1 M. Spectral resolution: 16 cm1; Time resolution: 6.67 s; ER = 0.8 V.

0.06

1M EG

0.4

0.04

0.3

0.03

0.2

0.02 0.01

0.1

0.00

0.0 0

40

80

CO2 band intensity

0.5

0.1M EG

0.05

COB band intensity

The IR band intensity of glycolate increases slowly in the initial time, but it up to a constant afterwards. As reaction time going, the OH ions in the thin-layer solution was depleted due to EG oxidation and the neutralization of acidic products. So CO2 can be seen after about 50 s. Fig. 6 presents the variation of IR band intensity of COB and CO2 with reaction time for 0.1 M EG and 1 M EG oxidation. Band intensity of COB diminishes with time increasing due to its oxidation while band intensity of CO2 increases. COB for 0.1 M EG can be completely oxidized in shorter time than 1 M EG (42 s vs 50 s). Due to its higher oxidation current of EG (Fig. 1a), depleting more OH, the oxidation of 1 M EG generates CO2 after 30 s, 10 s earlier than that of the oxidation of 0.1 M EG. What is more, the oxidation of 1 M EG produces less CO2. This fact is also in accordance with the above discussion.

120

t/s 3.4. Reaction pathways of EG electrooxidation at different concentration Based on above results and analysis, the EG concentration plays an important role in the reaction pathways of EG electrooxidation. Fig. 7 illustrates the main products of EG oxidation with different concentration at 0 V, close to the peak potential of EG oxidation in the CV (Fig. 1a). The main product in the solution containing 2 mM EG is CO2 (>98%), a product involving in the pathway of 3 the cleavage of CAC bond, whereas for 20 mM EG, its oxidation

Fig. 6. Time dependent band intensity of COB (circle) and CO2 (square) generated in 0.1 M (filled) and 1 M EG (open) solution.

2 yields both C 2 O2 4 (38%) and CO3 (62%). When the EG concentration increase to 0.1 M and 1 M, CO2 and incomplete oxidized C2 products (glycolate, glyoxylate) dominate. However, it is difficult to quantitatively determine the reaction selectivity for glycolate and glyoxylate, since the characteristic peaks to discriminate glycolate and glyoxylate at 1076 and 1099 cm1, respectively, are sig-

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EG concentration 2 mM 20 mM

Main products CO32- (>98%) C2O42- (38%), CO32- (62%)

0.1 M CO2, glycolate, glyoxylate 1M Fig. 7. Main products of EG electrooxidation on the Pd electrode at 0 V in 0.1 M NaOH with different EG concentration.

tion will not be exhausted completely. Fig. 8 shows the resulting in situ FTIR spectra. It is clear that the result is significantly different from that shown in Fig. 2c obtained in 0.1 M EG + 0.1 M NaOH. For example, at 0.30 V, the main products are CO2 3 (60%) and C 2 O2 4 (40%). Previously, we investigated ethanol electrooxidation on Pd electrode in alkaline solution, and found that the selectivity for CO2 species (CO2 + CO2 3 ) was as low as 2.5% in the potential region where Pd electrode exhibited considerable electrocatalytic activity (0.60 to 0.0 V), and main product was acetate [32]. If ethanol is only oxidized to acetate, it only provides four electrons per molecule, one third of the value for full oxidation to CO2. As for EG, the production of C 2 O2 has harvested eight electrons per mole4 cule, that is 80% of theoretical value. The present study indicates that as compared with ethanol, EG is easier to break CAC bond on Pd in alkaline solution, and more suitable as a fuel molecule in alkaline DAFCs. 4. Conclusions

Fig. 8. In situ SPA-FTIR spectra of EG oxidation on the Pd electrode in 0.1 M EG + 2 M NaOH. ES varied from 0.6 to 0.3 V, ER = 0.80 V, 8 cm1.

nificantly distorted by the thick CaF2 prism used in this study (CaF2 gradually becomes opaque for infrared radiation below 1100 cm1), and there are both carboxylic acids and carboxylates. It was reported that the production of acetaldehyde from ethanol oxidation on Pt is favored at concentrated ethanol solution [22]. Very similar with ethanol, EG oxidation on Pd is prone to producing glycolate and glyoxylate at high EG concentration. On one side, the cleavage of CAC generates C1 species that will eventually produce CO and CO2. On the other side, the formation of C2 intermediates need the dehydrogenation of EG and extra oxygen atoms from the dissociation of water. As EG concentration increases, the oxidation degree of EG gradually decreases: 2 CO2 3 ? C 2 O4 ? glyoxylate ? glycolate. This may be attributed to the limited availability of free sites for water adsorption to produce oxygen donor at high EG concentration. It is also worth noting that as for the FTIR test of EG oxidation in 0.1 and 1 M solution, the NaOH (0.1 M) in the thin-layer solution was exhaust easily, especially at potentials over 0.20 V as indicated by the appearance of CO2. Hence, the oxidation degree of EG (Fig. 7) obtained in the thin-layer cell may be lower than that in the normal cell. To evaluate the effect of NaOH concentration on the oxidation degree of EG electrooxidation, we further tested the FTIR spectra in 0.1 M EG + 2 M NaOH. As mentioned above, the complete oxidation of one EG molecule at most consumes 14 NaOH molecules. So, in this case, the NaOH in the thin-layer solu-

In this paper, the effects of EG concentration (2 mM–1 M) on its electrooxidation behaviors on Pd polycrystalline electrode in 0.1 M NaOH solution were studied by means of cyclic voltammetry and in situ FTIR reflection spectroscopy. The CV results indicate that the peak current of EG oxidation increases with increasing concentration, yielding a reaction order near 0.5. In addition, the peak potential in the forward scan also shifts positively from 0.18 to 0.02 V. FTIR spectroscopic studies demonstrated that the product contributions strongly depend on the EG concentration. For the concentration of 2 mM, EG electrooxidation only yields CO2 3 2 (>98%), whereas C 2 O2 4 (38%) and CO3 (62%) are the main products for 20 mM EG. At concentrated solution (0.1 M and 1 M), EG oxidation generates a majority of incomplete oxidized C2 products (glycolate and glyoxylate) and a small amount of CO2. Bridge-bonded CO (COB), generated from dissociative adsorption of EG can be observed in all solutions, and its band center shifts to high wavenumbers as the EG concentration increase. The detection of COB indicates that EG oxidation on Pd in alkaline media also follows the dual-path mechanism. The concentration dependence of reaction pathways may be a consequence of the competition for surface active sites among EG itself, all kinds of reaction intermediates and water. With the increase of EG concentration, the formation of adsorbed oxygen species from water can be inhibited, resulting in the fall of CAC bond cleavage and deep oxidation of EG that need extra oxygen atoms. Acknowledgments This study was supported by NSFC (21073152, 20933004, 20833005 and 21021002), Major State Basic Research Development Program of China (2012CB215500), Foundation for the Author of National Excellent Doctoral Dissertation of China (201126), Fundamental Research Funds for the Central Universities (2010121021) and Program for New Century Excellent Talents in University (NECT-11-0301 and NECT-10-0715). References [1] A. Dailey, J. Shin, C. Korzeniewski, Electrochim. Acta 44 (1998) 1147. [2] E. Peled, T. Duvdevani, A. Ahron, A. Melman, Electrochem. Solid State Lett. 4 (2001) A38. [3] E. Peled, V. Livshits, T. Duvdevani, J. Power Sources 106 (2002) 245. [4] V. Livshits, E. Peled, J. Power Sources 161 (2006) 1187. [5] H. Wang, Y. Zhao, Z. Jusys, R.J. Behm, J. Power Sources 155 (2006) 33. [6] V. Livshits, M. Philosoph, E. Peled, J. Power Sources 178 (2008) 687. [7] D. Kaplan, M. Alon, L. Burstein, Y. Rosenberg, E. Peled, J. Power Sources 196 (2011) 1078. [8] S.G. Sun, A.C. Chen, Electrochim. Acta 39 (1994) 969.

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