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Electrochemical and In Situ FTIR Studies of Adsorption and Oxidation of Dimethyl Ether on Platinum Electrode
ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 10, October 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(10): 1739−1744.
ARTICLE
Electrochemical and In Situ FTIR Studies of Adsorption and Oxidation of Dimethyl Ether on Platinum Electrode Leiming Pan,
Zhiyou Zhou,
Dejun Chen,
Shigang Sun*
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, P. R. China
Abstract:
Dissociative adsorption and electrooxidation of dimethyl ether (DME) on a platinum electrode in different pH solutions
were studied using cyclic voltammetry (CV) and in situ FTIR reflection spectroscopy. The coverage of the dissociative adsorbed species was measured about 70% from hydrogen adsorption-desorption region (0.05−0.35 V (vs RHE)) of steady-state voltammogram recorded in 0.1 mol·L−1 H2SO4 solution. It was found that the electrochemical reactivity of DME was pH dependent, i.e., the larger the pH value was, the less the reactivity of DME would be. No perceptible reactivity of DME in 0.1 mol·L−1 NaOH solution could be detected. It was revealed that the protonation of the oxygen atom in the C−O−C bond played a key role in the electrooxidation of DME. In situ FTIR spectroscopic results illustrated that linearly bonded CO (COL) species determined at low potential region were derived from the dissociative adsorption of DME and behaved as ‘poisoning’ intermediate. The COL species could be oxidized to CO2 at potential higher than 0.55 V (vs RHE) and in the potential range from 0.75 to 1.00 V (vs RHE) DME was oxidized simultaneously via HCOOH species that were identified as the reactive intermediates. Key Words:
Dimethyl ether; Electrooxidation; pH effect; In situ FTIR spectroscopy; Platinum electrode
Direct alcohol fuel cells (DAFCs) own significant advantages over hydrogen-oxygen polymer electrolyte membrane fuel cell (PEMFC) as power sources for portable applications, consisting in the compact system, easy transport, and storage of fuels. However, the commercialization of DAFCs is still hindered by several limitations, e.g., the toxicity of methanol, the loss of efficiency due to fuel crossover[1−4], the low reactivity of anode catalysts, and the incomplete oxidation in case of ethanol caused by the difficulty of breaking C−C bond[5−9]. As the simplest ether, dimethyl ether (DME) does not contain C−C bond and has high energy density[10] with negligible crossover effect[11,12]. It is less toxic than methanol and is environmentally friendly. Moreover, DME can be stored in liquid state under a modest pressure (ca 5.06×105 Pa) and compatible with the existing liquefied petroleum gas (LPG) facilities and services. Müller et al.[12] reported that direct DME fuel cell (DDMEFC) may yield comparable power density and higher total efficiency than DMFC at 130
°C. Based on these facts, DME has been considered as one of the promising candidates of alternative fuels to alcohols for direct oxidation fuel cells. It is very important to understand the electrooxidation mechanism of DME for developing more active catalysts for DDMEFC. Since DME has an additional methyl group compared with methanol, its electrochemical behavior appears quite different and much more complicated. Müller et al.[12] studied the electrooxidation of DME on Pt electrode both in half-cell and DDMEFC by using cyclic voltammetry (CV) and gas chromatography (GC), respectively. They found that methanol is an intermediate in the electrooxidation of DME. Adzic et al.[13] investigated the electrooxidation process of DME at different concentrations on a Pt film electrode in 0.1 mol·L−1 HClO4 solutions by in situ ATR-SEIRAS. Besides adsorbed CO, (H2CO)ad was determined as a dissociative adsorbate when the DME concentration was high, whereas IR bands associated with adsorbed (CH3COO)ad or (CH3OCOO)ad were observed when the DME concentration was much
Received: April 12, 2008; Revised: June 17, 2008. *Corresponding author. Email: [email protected]; Tel: +86592-2180181. The project was supported by the National Natural Science Foundation of China (20433060, 20673091). Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
lower[13]. Coutanceau et al.[10] proposed that DME hydrolyzed first to (CH2OH)ad or methanol and then oxidized via at least three different paths to HCOOH or CO2. Tsutsumi et al.[14] have detected HCOOH in the anode compartment of a DDMEFC, whereas Yin et al.[15] reported that small amount of HCHO was generated during DME electrooxidation. Mitsushim et al.[16] found that methyl formate (HCOOCH3) and methanol were the main by-products in the anode reaction of a DDMEFC. Unlike the formation of HCOOCH3, the rate of methanol production did not depend on the current density but increased with an increase in temperature. These results indicated that methanol may be produced by DME hydrolysis. In this article, we report the results of electrooxidation of DME on platinum electrode in both acidic and alkaline solutions. In situ FTIR spectroscopy was used to investigate the intermediates and products and their variations with potential. Based on the results, a mechanism for the electrooxidation of DME on platinum electrode was proposed.
1
Experimental
DME (≥99.9%) was purchased from Sigma-Aldrich Inc., H2SO4 (GR), NaOH (GR), NaF/HF (AR), and Na2CO3/ NaHCO3 (AR) solutions were prepared using Millipore water (electric resistivity: 18 MΩ·cm) supplied from a Milli-Q lab equipment (Nihon Millipore Ltd.). The solutions were deaerated by bubbling ultra pure N2 prior to measurements. CV experiments were conducted on a CHI 631C electrochemical analyzer (CH Instruments) using a conventional three-electrode cell consisting of a polished Pt disk (φ=0.5 cm) as working electrode, a platinized Pt foil as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. In order to facilitate the comparison of results obtained in solutions of different pH values, all potentials measured with referring to the SCE were converted to a reversible hydrogen electrode (RHE) scale. Pt electrode was polished mechanically using alumina powder of size 5, 1, and 0.3 μm, then cleaned in an ultrasonic bath and then by potential cycling (0.05−1.50 V, 100 mV·s−1) in 0.1 mol·L−1 H2SO4 solution to remove any possible impurity on the surface. For calculating the current density (j), the electroactive surface area of the Pt electrode was determined by the charge of hydrogen adsorption (QH) integrated from CV curves in the potential range between 0.05 and 0.35 V. Cyclic voltammograms were recorded before and after bubbling DME gas into electrolytes of different pH values. The concentration of DME saturated in the solution under atmospheric pressure is ca 0.5 mol·L−1 at 30 °C[17]. In situ FTIR experiments were carried out on a Nexus 870 spectrometer equipped with a liquid-N2-cooled MCT detector. A CaF2 disk was used as IR window. A thin layer configuration of the IR cell was approached by pushing electrode against the window during FTIR measurements. The electrode potentials were controlled by a 263A potentiostat
(EG&G). The operation procedure was as follows: (i) cleaning adsorbed species on the electrode surface by polarizing the electrode at 1.10 V for 5 s; (ii) refreshing the electrode surface by shifting the potential to 0.10 V and polarizing for several seconds; (iii) pushing the electrode to the IR window to form a thin layer; (iv) collecting 200 interferograms at reference potential ER and sample potential ES, respectively, then co-add and average them to obtain single beam spectra R(ER) and R(ES) by Fourier transform at a spectral resolution of 8 cm−1. Finally, the spectra were calculated as the relative change in reflectivity by using following formula:
ΔR R( ES ) − R( ER ) = R R( ER )
(1)
All electrochemical experiments were carried out in a thermo-water bath of constant temperature ((30.0±0.1) °C), whereas in situ FTIR spectra were collected at room temperature.
2
Results and discussion
2.1 Electrooxidation of DME on Pt electrode in solutions of different pH values Fig.1a shows the stable CVs of Pt electrode recorded in 0.1 mol·L−1 H2SO4 solution saturated with DME (solid line) and without DME (dashed line). The peaks in the hydrogen adsorption-desorption region are significantly suppressed due to the dissociative adsorption of DME occurred on Pt electrode surface. It is known that about 70% of Pt active sites are blocked by the dissociative adsorbates when comparing the charges of hydrogen adsorption (QH) measured from the
Fig.1
CV curves of Pt electrode in 0.1 mol·L−1 H2SO4 (a) and 0.1 mol·L−1 NaOH (b) solutions
(——) saturated with DME, (- - -) without DME in the solution; sweep rate: 10 mV·s−1
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
two CVs. In the positive potential scan, no obvious faradaic current is observed in the double layer region (0.35−0.50 V). An oxidation current appears at approximately 0.53 V, indicating that adsorbed OH species from water dissociation[18−21] has involved in the electrooxidation of DME. The oxidation current arises quickly as potential increases, and a sharp peak I is observed at approximately 0.68 V corresponding to oxidation of the dissociative adsorbates; a large well-defined peak II occurs at 0.76 V, which can be ascribed to the ‘direct’ oxidation of DME on Pt surface sites released after the oxidation of adsorbed species. At higher potentials (above 0.80 V), the oxidation current drops down because of the deactivation of the Pt electrode surface covered with oxide species[22], implying that DME cannot be oxidized by these species. In the negative potential scan, the catalytic activity of Pt electrode recovers during the reduction of surface Pt oxide species, and two current peaks are observed at 0.73 V (peak III) and 0.50 V (peak IV). Being analogous to the oxidation behavior of other small organic molecules, such as methanol or formic acid[22−25], peaks III and IV may be both ascribed to the ‘direct’ oxidation of DME. However, the formation of the peak IV may be due to progressive poisoning of Pt electrode surface caused by the accumulation of dissociative adsorbed species. When the potential is swept into the hydrogen adsorption region, most of Pt surface sites (about 70%) are inhibited by dissociative adsorbed species, i.e., the adsorption of hydrogen is blocked again. Stable CVs of Pt electrode recorded in 0.1 mol·L−1 NaOH solutions saturated with DME (solid line) and without DME (dashed line) are illustrated in Fig.1b. It can be seen that the inhibition to the adsorption and desorption of hydrogen by dissociative adsorbates of DME is negligible. In the positive potential scan, a weak current peak is observed at 0.61 V and disappears at above 0.70 V. During the negative potential scan, no oxidation peak of DME can be observed except the peaks of the reduction of Pt oxide species and the adsorption of hydrogen. Therefore, these results indicate that DME is electrochemically inactive in strong alkaline solutions. As compared with the results shown in Fig.1a, this phenomenon comes mainly from different natures of these solutions, i.e., the difference in pH values. In order to investigate the pH effects on the electrooxidation of DME, a series of pH buffer solutions were prepared and investigated. In Fig.2, stable CVs of DME oxidation on Pt electrode in buffer solutions of different pH values are shown. During positive potential scan in acidic buffer solutions, two oxidation peaks at 0.67, 0.70 V and 0.65, 0.71 V are observed in both Fig.2a and Fig.2b. But in the case of alkaline solutions as shown in Fig.2(c, d), the first current peak drops down to an unnoticeable shoulder and the current of the second peak at 0.64 V is also very weak. In the negative potential scan, obvious oxidation peaks can be observed in Fig.2(a, b), which al-
Fig.2
CV curves of DME oxidation on Pt electrode in buffer solutions of different pH values
pH: (a) 3.92, (b) 5.82, (c) 9.45, (d) 11.35; sweep rate: 10 mV·s−1
most disappear in Fig.2(c, d). Some interesting facts can be observed: (1) The electrooxidation behaviors of DME in acidic buffer solutions are similar to those in 0.1 mol·L−1 H2SO4 solution, whereas the behaviors in alkaline solutions are comparable to those in 0.1 mol·L−1 NaOH solution; (2) Comparing the potentials of oxidation peaks with those measured in Fig.1, the first current peak presented in Fig.2 in the positive potential scan is associated with the oxidation of dissociative adsorbed species, and the second one is due to the ‘direct’ oxidation of DME on Pt electrode. Fig.3 illustrates the current density of oxidation peaks gradually decreases as the pH value of solution increases. The peak III disappears when the pH value is above 7, followed by peak IV when the pH value reaches 10. These results confirm that increasing the pH value of solutions will lead to a strengthened inhibition to the dissociative adsorption and ‘direct’ oxidation of DME on Pt electrode. The difference between the reactivity of DME in acidic and alkaline solutions is evident and contrasts sharply against other organic molecules. For example, methanol has considerable reactivity in both kinds of solutions[26]. The electrochemical reactivity of DME depends on the concentration of
Fig.3
Plots of lgj−pH for DME oxidation
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
H+ ions, namely, pH effect, implying that H+ ions have taken part in the oxidation. As a Louis base, DME can be protonated by free H+ ions to form CH3−O+H−CH3. Moreover, it is difficult to break the C−O−C bond at a neutral condition. Only at high temperature or in strong acid, such as HI acid, DME can be hydrolyzed into CH3OH or CH3I. Based on the results and analysis, it can be concluded that two facts should be considered for the cleavage of the C−O−C bond in electrocatalytic oxidization of DME: (1) the protonation of DME by H+ ions; (2) the adsorption of DME on Pt electrode surface. 2.2
In situ FTIR study
The comparison of FTIR spectral features of DME saturated in 0.1 mol·L−1 H2SO4 and 0.1 mol·L−1 NaOH solutions are illustrated in Fig.4. A series of IR bands of DME are observed in both solutions. The assignments of these bands were made referring to literature[27,28]. The bands at 1082 and 1161 cm−1 can be ascribed to vibration modes of v(COC) and r(CH3). The bands located at 1247 and 1459 cm−1 are assigned to r(CH3) and δ(CH3) of DME, respectively. The absorptions observed at 2831 and 3000 cm−1 are due to the symmetric stretching vibrations (vs) of CH3 group, whereas the bands close to 2941 and 2969 cm−1 can be assigned to the absorption of asymmetric stretching vibrations (vas) of CH3 group of DME. As clearly demonstrated in Fig.4, the intensity of corresponding IR bands of DME are comparable in the two solutions, which implies that DME has similar solubility in both alkaline and acidic media. It is thus confirmed that the remarkable difference between the electrochemical reactivity of DME in H2SO4 and NaOH solutions is originated from different concentrations of H+ ions in solutions. The in situ FTIR spectra of Pt electrode in 0.1 mol·L−1 H2SO4 solution saturated with DME are shown in Fig.5. The reference potential (ER) is 0.10 V, and the sample potential (ES) is gradually stepped from 0.15 to 1.00 V with an interval of 0.05 V. The IR features can be summarized as follows: (1) When ES=0.15 V, four positive-going bands and one negative-going band are observed. The positive-going bands
located at 1082 and 1161 cm−1 are ascribed to IR absorption of the v(COC) and r(CH3) of DME, and the bands near 1459 and 2833 cm−1 are assigned to the δ(CH3) and vs(CH3), respectively. As ES increases, the intensities of the four bands are augmented, indicating an increase in DME consumption. A negative-going band is observed at 1200 cm−1 when ES increases up to 0.60 V, which is due to the increase of concentration of sulfuric acid anions in the thin layer (between the electrode and IR window) and the adsorption of sulfuric acid anions on electrode surface at high potentials[29−31], and the intensity of this band increases as the potential increases, resulting in the decrease of the band close to 1161 cm−1 (positive-going). When ES reaches 0.95 V, the bands close to 1459 and 2833 cm−1 disappear completely, and the IR absorption of v(COC) also decreases, corresponding to the oxidation of Pt electrode surface at this potential, which blocks the adsorption and oxidation of DME. The negativegoing band around 2050 cm−1 is assigned to the linearly bonded CO (COL)[32−34]. This band blue-shifts linearly with increasing potentials below 0.55 V, then turns to red-shift at above 0.55 V with further increase in ES, indicating that 0.55 V is the initial potential of COL oxidation. Once ES exceeds 0.75 V, this band disappears, suggesting that COL has been totally oxidized. (2) When ES>0.50 V, a downward band at 1718 cm−1 is observed and its intensity changes slightly till 0.70 V, which can be assigned to the C=O stretching of formic acid in
Fig.5
In situ FTIR spectra of Pt electrode in 0.1 mol·L−1 H2SO4 solution saturated with DME
Fig.4
Comparison of FTIR spectra of saturated DME in
0.1 mol·L−1 H2SO4 (a) and 0.1 mol·L−1 NaOH (b) solutions
ER=0.1 V, ES values are indicated in the figure and vary with an interval of 0.05 V.
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
solution[35−37]. Since COL is totally desorbed by oxidation from Pt surface at above 0.70 V, the intensity of the band increases rapidly, then decreases at 0.90 V, corresponding to the oxidation of formic acid. This band finally disappears at 1.00 V. (3) Another downward band around 2345 cm−1 appears at 0.55 V, ascribing to IR absorption of CO2. It is interesting to note that the intensity of the CO2 band does not decrease at above 0.70 V, even if COL is totally oxidized, implying that CO2 is also yielded from the oxidation of other species. Because of the low reactivity of DME in NaOH solution, only a weak positive-going band at 1245 cm−1 can be observed in the in situ FTIR spectra (not shown), belonging to the r(CH3) vibration. No IR features of products by the electrooxidation of DME are detected. The potential dependence of the integrated intensities of main IR bands measured in Fig.5 is shown in Fig.6 and is described as below. (1) COL appears at about 0.15 V and reaches its maximum intensity at 0.55 V, indicating that only dissociative adsorption of DME occurs within this potential region, and CO is produced gradually as the ‘poisoning’ intermediate. CH3OCH3+H+→CH3O+HCH3 (2) CH3O+HCH3+2Pt→Pt2CH3O+HCH3 (3) (4) Pt2CH3O+HCH3+H2O→2PtCH2OH+3H++2e− PtCH2OH→PtCHO+2H++2e− (5) + − PtCHO→PtCO+H +e (6) (2) In the potential region between 0.55 and 0.70 V, the intensity of the COL band decreases obviously with increasing potential, implying that COL is oxidized to CO2 by the adsorbed OH species on Pt electrode surface and corresponding to the formation of current peak I of DME oxidation in Fig.1a. Meanwhile, the intensity of the band of HCOOH increases slightly. (7) PtCHO+H2O→HCOOH+H++Pt+e− PtCO+PtOH→CO2+H++2Pt+e− (8) (3) At potentials from 0.70 to 0.85 V, large amount of CO2 is still produced while the formation rate of HCOOH increases quickly, which indicates that DME begins to oxidize to CO2
via the reactive intermediate (HCOOH) path on Pt surface, corresponding to the second current peak in positive potential scan in Fig.1a. (9) HCOOH+2PtOH→CO2+2H2O+2Pt (4) In the potential region from 0.85 to 1.00 V, the intensities of both CO2 and HCOOH decrease quickly due to the formation of inert oxide species on the surface of Pt electrode. The overall reaction could be written as, H
+
CH3OCH3 + 3H 2O ⎯⎯⎯→ 2CO2 + 12H + + 12e − Pt
3
(10)
Conclusions
The electrooxidation of DME on Pt electrode in solutions of different pH values was investigated by CV. It is shown that the electrochemical reactivity of DME depends on the concentration of H+ ions in solution. No perceptible reactivity of DME in 0.1 mol·L−1 NaOH solution could be detected. Combined with the FTIR spectroscopic results, it was confirmed that H+ ions take part in the electrooxidation of DME, i.e., the protonation of the oxygen atom in DME molecule and the interaction between DME and Pt surface are key steps in the dissociative adsorption and oxidation of DME. From in situ FTIR spectroscopy, the IR features of COL, HCOOH, and CO2, as well as several positive bands due to the consumption of DME, have been detected. A reaction mechanism for the electrooxidation of DME on Pt electrode was proposed at molecule level. It has discerned CO as the ‘poisoning’ intermediate that is derived from the dissociative adsorption of DME at low potentials; when the COL is oxidized at high potentials (corresponding to the first oxidation peak), the oxidation of DME takes place via the reactive intermediate path; HCOOH acts as both reactive intermediate and product; CO2 is the final product (corresponding to the second oxidation peak).
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Fig.6
Variation of the integrated intensity of IR bands with potential ES
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ARTICLE
Electrochemical and In Situ FTIR Studies of Adsorption and Oxidation of Dimethyl Ether on Platinum Electrode Leiming Pan,
Zhiyou Zhou,
Dejun Chen,
Shigang Sun*
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, P. R. China
Abstract:
Dissociative adsorption and electrooxidation of dimethyl ether (DME) on a platinum electrode in different pH solutions
were studied using cyclic voltammetry (CV) and in situ FTIR reflection spectroscopy. The coverage of the dissociative adsorbed species was measured about 70% from hydrogen adsorption-desorption region (0.05−0.35 V (vs RHE)) of steady-state voltammogram recorded in 0.1 mol·L−1 H2SO4 solution. It was found that the electrochemical reactivity of DME was pH dependent, i.e., the larger the pH value was, the less the reactivity of DME would be. No perceptible reactivity of DME in 0.1 mol·L−1 NaOH solution could be detected. It was revealed that the protonation of the oxygen atom in the C−O−C bond played a key role in the electrooxidation of DME. In situ FTIR spectroscopic results illustrated that linearly bonded CO (COL) species determined at low potential region were derived from the dissociative adsorption of DME and behaved as ‘poisoning’ intermediate. The COL species could be oxidized to CO2 at potential higher than 0.55 V (vs RHE) and in the potential range from 0.75 to 1.00 V (vs RHE) DME was oxidized simultaneously via HCOOH species that were identified as the reactive intermediates. Key Words:
Dimethyl ether; Electrooxidation; pH effect; In situ FTIR spectroscopy; Platinum electrode
Direct alcohol fuel cells (DAFCs) own significant advantages over hydrogen-oxygen polymer electrolyte membrane fuel cell (PEMFC) as power sources for portable applications, consisting in the compact system, easy transport, and storage of fuels. However, the commercialization of DAFCs is still hindered by several limitations, e.g., the toxicity of methanol, the loss of efficiency due to fuel crossover[1−4], the low reactivity of anode catalysts, and the incomplete oxidation in case of ethanol caused by the difficulty of breaking C−C bond[5−9]. As the simplest ether, dimethyl ether (DME) does not contain C−C bond and has high energy density[10] with negligible crossover effect[11,12]. It is less toxic than methanol and is environmentally friendly. Moreover, DME can be stored in liquid state under a modest pressure (ca 5.06×105 Pa) and compatible with the existing liquefied petroleum gas (LPG) facilities and services. Müller et al.[12] reported that direct DME fuel cell (DDMEFC) may yield comparable power density and higher total efficiency than DMFC at 130
°C. Based on these facts, DME has been considered as one of the promising candidates of alternative fuels to alcohols for direct oxidation fuel cells. It is very important to understand the electrooxidation mechanism of DME for developing more active catalysts for DDMEFC. Since DME has an additional methyl group compared with methanol, its electrochemical behavior appears quite different and much more complicated. Müller et al.[12] studied the electrooxidation of DME on Pt electrode both in half-cell and DDMEFC by using cyclic voltammetry (CV) and gas chromatography (GC), respectively. They found that methanol is an intermediate in the electrooxidation of DME. Adzic et al.[13] investigated the electrooxidation process of DME at different concentrations on a Pt film electrode in 0.1 mol·L−1 HClO4 solutions by in situ ATR-SEIRAS. Besides adsorbed CO, (H2CO)ad was determined as a dissociative adsorbate when the DME concentration was high, whereas IR bands associated with adsorbed (CH3COO)ad or (CH3OCOO)ad were observed when the DME concentration was much
Received: April 12, 2008; Revised: June 17, 2008. *Corresponding author. Email: [email protected]; Tel: +86592-2180181. The project was supported by the National Natural Science Foundation of China (20433060, 20673091). Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
lower[13]. Coutanceau et al.[10] proposed that DME hydrolyzed first to (CH2OH)ad or methanol and then oxidized via at least three different paths to HCOOH or CO2. Tsutsumi et al.[14] have detected HCOOH in the anode compartment of a DDMEFC, whereas Yin et al.[15] reported that small amount of HCHO was generated during DME electrooxidation. Mitsushim et al.[16] found that methyl formate (HCOOCH3) and methanol were the main by-products in the anode reaction of a DDMEFC. Unlike the formation of HCOOCH3, the rate of methanol production did not depend on the current density but increased with an increase in temperature. These results indicated that methanol may be produced by DME hydrolysis. In this article, we report the results of electrooxidation of DME on platinum electrode in both acidic and alkaline solutions. In situ FTIR spectroscopy was used to investigate the intermediates and products and their variations with potential. Based on the results, a mechanism for the electrooxidation of DME on platinum electrode was proposed.
1
Experimental
DME (≥99.9%) was purchased from Sigma-Aldrich Inc., H2SO4 (GR), NaOH (GR), NaF/HF (AR), and Na2CO3/ NaHCO3 (AR) solutions were prepared using Millipore water (electric resistivity: 18 MΩ·cm) supplied from a Milli-Q lab equipment (Nihon Millipore Ltd.). The solutions were deaerated by bubbling ultra pure N2 prior to measurements. CV experiments were conducted on a CHI 631C electrochemical analyzer (CH Instruments) using a conventional three-electrode cell consisting of a polished Pt disk (φ=0.5 cm) as working electrode, a platinized Pt foil as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. In order to facilitate the comparison of results obtained in solutions of different pH values, all potentials measured with referring to the SCE were converted to a reversible hydrogen electrode (RHE) scale. Pt electrode was polished mechanically using alumina powder of size 5, 1, and 0.3 μm, then cleaned in an ultrasonic bath and then by potential cycling (0.05−1.50 V, 100 mV·s−1) in 0.1 mol·L−1 H2SO4 solution to remove any possible impurity on the surface. For calculating the current density (j), the electroactive surface area of the Pt electrode was determined by the charge of hydrogen adsorption (QH) integrated from CV curves in the potential range between 0.05 and 0.35 V. Cyclic voltammograms were recorded before and after bubbling DME gas into electrolytes of different pH values. The concentration of DME saturated in the solution under atmospheric pressure is ca 0.5 mol·L−1 at 30 °C[17]. In situ FTIR experiments were carried out on a Nexus 870 spectrometer equipped with a liquid-N2-cooled MCT detector. A CaF2 disk was used as IR window. A thin layer configuration of the IR cell was approached by pushing electrode against the window during FTIR measurements. The electrode potentials were controlled by a 263A potentiostat
(EG&G). The operation procedure was as follows: (i) cleaning adsorbed species on the electrode surface by polarizing the electrode at 1.10 V for 5 s; (ii) refreshing the electrode surface by shifting the potential to 0.10 V and polarizing for several seconds; (iii) pushing the electrode to the IR window to form a thin layer; (iv) collecting 200 interferograms at reference potential ER and sample potential ES, respectively, then co-add and average them to obtain single beam spectra R(ER) and R(ES) by Fourier transform at a spectral resolution of 8 cm−1. Finally, the spectra were calculated as the relative change in reflectivity by using following formula:
ΔR R( ES ) − R( ER ) = R R( ER )
(1)
All electrochemical experiments were carried out in a thermo-water bath of constant temperature ((30.0±0.1) °C), whereas in situ FTIR spectra were collected at room temperature.
2
Results and discussion
2.1 Electrooxidation of DME on Pt electrode in solutions of different pH values Fig.1a shows the stable CVs of Pt electrode recorded in 0.1 mol·L−1 H2SO4 solution saturated with DME (solid line) and without DME (dashed line). The peaks in the hydrogen adsorption-desorption region are significantly suppressed due to the dissociative adsorption of DME occurred on Pt electrode surface. It is known that about 70% of Pt active sites are blocked by the dissociative adsorbates when comparing the charges of hydrogen adsorption (QH) measured from the
Fig.1
CV curves of Pt electrode in 0.1 mol·L−1 H2SO4 (a) and 0.1 mol·L−1 NaOH (b) solutions
(——) saturated with DME, (- - -) without DME in the solution; sweep rate: 10 mV·s−1
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
two CVs. In the positive potential scan, no obvious faradaic current is observed in the double layer region (0.35−0.50 V). An oxidation current appears at approximately 0.53 V, indicating that adsorbed OH species from water dissociation[18−21] has involved in the electrooxidation of DME. The oxidation current arises quickly as potential increases, and a sharp peak I is observed at approximately 0.68 V corresponding to oxidation of the dissociative adsorbates; a large well-defined peak II occurs at 0.76 V, which can be ascribed to the ‘direct’ oxidation of DME on Pt surface sites released after the oxidation of adsorbed species. At higher potentials (above 0.80 V), the oxidation current drops down because of the deactivation of the Pt electrode surface covered with oxide species[22], implying that DME cannot be oxidized by these species. In the negative potential scan, the catalytic activity of Pt electrode recovers during the reduction of surface Pt oxide species, and two current peaks are observed at 0.73 V (peak III) and 0.50 V (peak IV). Being analogous to the oxidation behavior of other small organic molecules, such as methanol or formic acid[22−25], peaks III and IV may be both ascribed to the ‘direct’ oxidation of DME. However, the formation of the peak IV may be due to progressive poisoning of Pt electrode surface caused by the accumulation of dissociative adsorbed species. When the potential is swept into the hydrogen adsorption region, most of Pt surface sites (about 70%) are inhibited by dissociative adsorbed species, i.e., the adsorption of hydrogen is blocked again. Stable CVs of Pt electrode recorded in 0.1 mol·L−1 NaOH solutions saturated with DME (solid line) and without DME (dashed line) are illustrated in Fig.1b. It can be seen that the inhibition to the adsorption and desorption of hydrogen by dissociative adsorbates of DME is negligible. In the positive potential scan, a weak current peak is observed at 0.61 V and disappears at above 0.70 V. During the negative potential scan, no oxidation peak of DME can be observed except the peaks of the reduction of Pt oxide species and the adsorption of hydrogen. Therefore, these results indicate that DME is electrochemically inactive in strong alkaline solutions. As compared with the results shown in Fig.1a, this phenomenon comes mainly from different natures of these solutions, i.e., the difference in pH values. In order to investigate the pH effects on the electrooxidation of DME, a series of pH buffer solutions were prepared and investigated. In Fig.2, stable CVs of DME oxidation on Pt electrode in buffer solutions of different pH values are shown. During positive potential scan in acidic buffer solutions, two oxidation peaks at 0.67, 0.70 V and 0.65, 0.71 V are observed in both Fig.2a and Fig.2b. But in the case of alkaline solutions as shown in Fig.2(c, d), the first current peak drops down to an unnoticeable shoulder and the current of the second peak at 0.64 V is also very weak. In the negative potential scan, obvious oxidation peaks can be observed in Fig.2(a, b), which al-
Fig.2
CV curves of DME oxidation on Pt electrode in buffer solutions of different pH values
pH: (a) 3.92, (b) 5.82, (c) 9.45, (d) 11.35; sweep rate: 10 mV·s−1
most disappear in Fig.2(c, d). Some interesting facts can be observed: (1) The electrooxidation behaviors of DME in acidic buffer solutions are similar to those in 0.1 mol·L−1 H2SO4 solution, whereas the behaviors in alkaline solutions are comparable to those in 0.1 mol·L−1 NaOH solution; (2) Comparing the potentials of oxidation peaks with those measured in Fig.1, the first current peak presented in Fig.2 in the positive potential scan is associated with the oxidation of dissociative adsorbed species, and the second one is due to the ‘direct’ oxidation of DME on Pt electrode. Fig.3 illustrates the current density of oxidation peaks gradually decreases as the pH value of solution increases. The peak III disappears when the pH value is above 7, followed by peak IV when the pH value reaches 10. These results confirm that increasing the pH value of solutions will lead to a strengthened inhibition to the dissociative adsorption and ‘direct’ oxidation of DME on Pt electrode. The difference between the reactivity of DME in acidic and alkaline solutions is evident and contrasts sharply against other organic molecules. For example, methanol has considerable reactivity in both kinds of solutions[26]. The electrochemical reactivity of DME depends on the concentration of
Fig.3
Plots of lgj−pH for DME oxidation
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
H+ ions, namely, pH effect, implying that H+ ions have taken part in the oxidation. As a Louis base, DME can be protonated by free H+ ions to form CH3−O+H−CH3. Moreover, it is difficult to break the C−O−C bond at a neutral condition. Only at high temperature or in strong acid, such as HI acid, DME can be hydrolyzed into CH3OH or CH3I. Based on the results and analysis, it can be concluded that two facts should be considered for the cleavage of the C−O−C bond in electrocatalytic oxidization of DME: (1) the protonation of DME by H+ ions; (2) the adsorption of DME on Pt electrode surface. 2.2
In situ FTIR study
The comparison of FTIR spectral features of DME saturated in 0.1 mol·L−1 H2SO4 and 0.1 mol·L−1 NaOH solutions are illustrated in Fig.4. A series of IR bands of DME are observed in both solutions. The assignments of these bands were made referring to literature[27,28]. The bands at 1082 and 1161 cm−1 can be ascribed to vibration modes of v(COC) and r(CH3). The bands located at 1247 and 1459 cm−1 are assigned to r(CH3) and δ(CH3) of DME, respectively. The absorptions observed at 2831 and 3000 cm−1 are due to the symmetric stretching vibrations (vs) of CH3 group, whereas the bands close to 2941 and 2969 cm−1 can be assigned to the absorption of asymmetric stretching vibrations (vas) of CH3 group of DME. As clearly demonstrated in Fig.4, the intensity of corresponding IR bands of DME are comparable in the two solutions, which implies that DME has similar solubility in both alkaline and acidic media. It is thus confirmed that the remarkable difference between the electrochemical reactivity of DME in H2SO4 and NaOH solutions is originated from different concentrations of H+ ions in solutions. The in situ FTIR spectra of Pt electrode in 0.1 mol·L−1 H2SO4 solution saturated with DME are shown in Fig.5. The reference potential (ER) is 0.10 V, and the sample potential (ES) is gradually stepped from 0.15 to 1.00 V with an interval of 0.05 V. The IR features can be summarized as follows: (1) When ES=0.15 V, four positive-going bands and one negative-going band are observed. The positive-going bands
located at 1082 and 1161 cm−1 are ascribed to IR absorption of the v(COC) and r(CH3) of DME, and the bands near 1459 and 2833 cm−1 are assigned to the δ(CH3) and vs(CH3), respectively. As ES increases, the intensities of the four bands are augmented, indicating an increase in DME consumption. A negative-going band is observed at 1200 cm−1 when ES increases up to 0.60 V, which is due to the increase of concentration of sulfuric acid anions in the thin layer (between the electrode and IR window) and the adsorption of sulfuric acid anions on electrode surface at high potentials[29−31], and the intensity of this band increases as the potential increases, resulting in the decrease of the band close to 1161 cm−1 (positive-going). When ES reaches 0.95 V, the bands close to 1459 and 2833 cm−1 disappear completely, and the IR absorption of v(COC) also decreases, corresponding to the oxidation of Pt electrode surface at this potential, which blocks the adsorption and oxidation of DME. The negativegoing band around 2050 cm−1 is assigned to the linearly bonded CO (COL)[32−34]. This band blue-shifts linearly with increasing potentials below 0.55 V, then turns to red-shift at above 0.55 V with further increase in ES, indicating that 0.55 V is the initial potential of COL oxidation. Once ES exceeds 0.75 V, this band disappears, suggesting that COL has been totally oxidized. (2) When ES>0.50 V, a downward band at 1718 cm−1 is observed and its intensity changes slightly till 0.70 V, which can be assigned to the C=O stretching of formic acid in
Fig.5
In situ FTIR spectra of Pt electrode in 0.1 mol·L−1 H2SO4 solution saturated with DME
Fig.4
Comparison of FTIR spectra of saturated DME in
0.1 mol·L−1 H2SO4 (a) and 0.1 mol·L−1 NaOH (b) solutions
ER=0.1 V, ES values are indicated in the figure and vary with an interval of 0.05 V.
Leiming Pan et al. / Acta Physico-Chimica Sinica, 2008, 24(10): 1739−1744
solution[35−37]. Since COL is totally desorbed by oxidation from Pt surface at above 0.70 V, the intensity of the band increases rapidly, then decreases at 0.90 V, corresponding to the oxidation of formic acid. This band finally disappears at 1.00 V. (3) Another downward band around 2345 cm−1 appears at 0.55 V, ascribing to IR absorption of CO2. It is interesting to note that the intensity of the CO2 band does not decrease at above 0.70 V, even if COL is totally oxidized, implying that CO2 is also yielded from the oxidation of other species. Because of the low reactivity of DME in NaOH solution, only a weak positive-going band at 1245 cm−1 can be observed in the in situ FTIR spectra (not shown), belonging to the r(CH3) vibration. No IR features of products by the electrooxidation of DME are detected. The potential dependence of the integrated intensities of main IR bands measured in Fig.5 is shown in Fig.6 and is described as below. (1) COL appears at about 0.15 V and reaches its maximum intensity at 0.55 V, indicating that only dissociative adsorption of DME occurs within this potential region, and CO is produced gradually as the ‘poisoning’ intermediate. CH3OCH3+H+→CH3O+HCH3 (2) CH3O+HCH3+2Pt→Pt2CH3O+HCH3 (3) (4) Pt2CH3O+HCH3+H2O→2PtCH2OH+3H++2e− PtCH2OH→PtCHO+2H++2e− (5) + − PtCHO→PtCO+H +e (6) (2) In the potential region between 0.55 and 0.70 V, the intensity of the COL band decreases obviously with increasing potential, implying that COL is oxidized to CO2 by the adsorbed OH species on Pt electrode surface and corresponding to the formation of current peak I of DME oxidation in Fig.1a. Meanwhile, the intensity of the band of HCOOH increases slightly. (7) PtCHO+H2O→HCOOH+H++Pt+e− PtCO+PtOH→CO2+H++2Pt+e− (8) (3) At potentials from 0.70 to 0.85 V, large amount of CO2 is still produced while the formation rate of HCOOH increases quickly, which indicates that DME begins to oxidize to CO2
via the reactive intermediate (HCOOH) path on Pt surface, corresponding to the second current peak in positive potential scan in Fig.1a. (9) HCOOH+2PtOH→CO2+2H2O+2Pt (4) In the potential region from 0.85 to 1.00 V, the intensities of both CO2 and HCOOH decrease quickly due to the formation of inert oxide species on the surface of Pt electrode. The overall reaction could be written as, H
+
CH3OCH3 + 3H 2O ⎯⎯⎯→ 2CO2 + 12H + + 12e − Pt
3
(10)
Conclusions
The electrooxidation of DME on Pt electrode in solutions of different pH values was investigated by CV. It is shown that the electrochemical reactivity of DME depends on the concentration of H+ ions in solution. No perceptible reactivity of DME in 0.1 mol·L−1 NaOH solution could be detected. Combined with the FTIR spectroscopic results, it was confirmed that H+ ions take part in the electrooxidation of DME, i.e., the protonation of the oxygen atom in DME molecule and the interaction between DME and Pt surface are key steps in the dissociative adsorption and oxidation of DME. From in situ FTIR spectroscopy, the IR features of COL, HCOOH, and CO2, as well as several positive bands due to the consumption of DME, have been detected. A reaction mechanism for the electrooxidation of DME on Pt electrode was proposed at molecule level. It has discerned CO as the ‘poisoning’ intermediate that is derived from the dissociative adsorption of DME at low potentials; when the COL is oxidized at high potentials (corresponding to the first oxidation peak), the oxidation of DME takes place via the reactive intermediate path; HCOOH acts as both reactive intermediate and product; CO2 is the final product (corresponding to the second oxidation peak).
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