Electrooxidation of Dimethoxymethane on a Platinum Electrode in

Direct methanol fuel cell (DMFC) is a promising power source for portable electronic devices and vehicles thanks to its high energy density, modest op...
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J. Phys. Chem. C 2008, 112, 19012–19017

Electrooxidation of Dimethoxymethane on a Platinum Electrode in Acidic Solutions Studied by in Situ FTIR Spectroscopy Zhi-You Zhou, De-Jun Chen, Hua Li, Qiang Wang, and Shi-Gang Sun* State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Department of Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: June 28, 2008; ReVised Manuscript ReceiVed: October 11, 2008

Electrooxidation of dimethoxymethane (DMM, H2C(OCH3)2), a promising alternative fuel to methanol in direct fuel cells, on a Pt electrode in acidic solutions was studied by in situ Fourier transform infrared (FTIR) reflection spectroscopy. The study revealed that the dissolved products of DMM oxidation are mainly methyl formate (MF, HCOOCH3), methanol and CO2, and the adsorbed species are linearly bonded CO (COL) that are derived from the further dissociative adsorption of methanol. The variation of products involved in DMM oxidation at different electrode potentials was analyzed quantitatively from in situ FTIR spectra. The results demonstrated that the MF and methanol are preferentially generated at low potentials or in the initial stage of DMM oxidation, while CO2 dominates at high potentials or in the final oxidation step. On the basis of experimental results, a reaction mechanism of DMM oxidation on Pt electrode was suggested at molecular level. 1. Introduction Direct methanol fuel cell (DMFC) is a promising power source for portable electronic devices and vehicles thanks to its high energy density, modest operating conditions, and safety.1-3 However, two critical issues should be addressed for its large-scale commercialization, i.e., the low activity of electrocatalysts toward methanol oxidation, and the crossover of methanol through proton exchange membrane.4-7 The high rate of methanol crossover leads to a mixed potential, and decreases significantly the cell voltage as well as the efficiency of fuel utilization. Therefore, to explore alternative organic fuels other than methanol with lower crossover, such as formic acid,8,9 ethanol,10,11 ethylene glycol,12,13 and dimethyl ether,14,15 has attracted extensive attention. Dimethoxymethane (DMM, H2C(OCH3)2) is also considered as a promising candidate due to its low crossover, high energy density (DMM: 5.62 Ah g-1, methanol: 5.02 Ah g-1), less toxicity, and availability as a derivative of natural gas or coal.16-18 The crossover rate of DMM is 1 order of magnitude lower than that of methanol.17 In addition, DMM may be expected to be more easily oxidized to CO2, since it has no strong C-C bond that is difficult to break.19 Several groups have investigated direct DMM fuel cell (DDMMFC) recently, and found that the performance of the DDMMFC is inferior or at best similar to that of the DMFC under typical operating conditions due to the low electro-activity of DMM on Pt-base catalysts.16,17,20 Therefore, it is important to understand the reaction mechanism of DMM electrooxidation for designing and preparing electrocatalysts with improved properties. Although a few fundamental studies of DMM oxidation on Pt electrodes were carried out using traditional electrochemical methods,21-23 the identification of products at molecular level and the elucidation of the reaction mechanism are still controversial. Electrochemical in situ FTIR spectroscopy is a powerful * Correspondence author. Telephone: +86-592-2180181. Fax: +86-5922183047. E-mail: [email protected].

tool for studying reaction mechanisms of electrooxidation of small organic molecules.24-27 Kerangueven et al. have studied DMM electrooxidation on several Pt single crystal electrodes and a Pt polycrystalline electrode by combining in situ reflection FTIR spectroscopy with cyclic voltammetry (CV), and detected adsorbed CO on Pt(100), adsorbed HCO species on Pt(111), and dissolved CO2 species in solutions.28,29 In another study, Miki et al. used surface-enhanced IR absorption spectroscopy (SEIRAS) to study the DMM electrooxidation on Pt film electrodes.30 Unfortunately, only adsorbed CO species were determined due to the attenuated total reflection mode used, which is merely sensitive to adsorbed species intrinsically.31 In the present study, we have employed in situ FTIR reflection spectroscopy working with a thin-layer IR cell to monitor the electrooxidation processes of DMM on a Pt polycrystalline electrode in 0.1 M H2SO4 solutions. By using this external IR reflection mode, we have successfully found several dissolved products as well as adsorbed species involved in DMM electrooxidation, and especially determined a large quantity of methyl formate (MF) for the first time, which indicates that the DMM oxidation is quite incomplete although it does not contain C-C bond. On the basis of IR spectroscopic results, a reaction mechanism was suggested at a molecular level. 2. Experimental Section 2.1. Electrochemical Experiments. All electrochemical experiments were carried out in a standard three-electrode cell. A 263A potentiostat/galvanostat (EG&G) was used to control electrode potentials. A platinized Pt foil was used as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. Electrode potentials reported in this paper were quoted versus the SCE scale. A polycrystalline Pt disk (φ)5 mm) embedded in a Teflon holder was used as working electrode. Prior to measurements, the Pt electrode was polished mechanically with alumina powder of sizes 1, 0.3, and 0.05 µm, and washed in an ultrasonic bath. The Pt electrode was then electrochemically cleaned by potential cycling between -0.26 and +1.20 V in 0.1 M H2SO4 at a scan rate of 50 mV s-1 until

10.1021/jp805695u CCC: $40.75  2008 American Chemical Society Published on Web 11/11/2008

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a reproducible and well-defined cyclic voltammogram was obtained. The electroactive surface area of the Pt electrode was evaluated to be 0.41 cm2 by measuring the electric charge of hydrogen adsorption (QH) and dividing the QH by the value of 210 µC cm-2, which is a value of electric charge density generally accepted for hydrogen adsorption/desorption on Pt electrode with an electro-active area of 1 cm2.32 The Pt electrode was then transferred to a freshly prepared 0.1 M DMM + 0.1 M H2SO4 solution, and cyclic voltammograms (CVs) were recorded. The current density was calculated by dividing the measured current by the electro-active surface area. 2.2. Electrochemical in Situ FTIR Reflection Spectroscopy. Electrochemical in situ FTIR reflection spectroscopic studies were carried out on a Nexus 870 spectrometer (Nicolet) equipped with a liquid-nitrogen-cooled MCT-A detector and an EverGlo IR source. The configuration of the thin-layer IR cell has been detailed previously.33 Prior to IR data collection, a clean procedure was applied to the Pt electrode; i.e., the electrode potential was first held at 1.0 V for 5.0 s to oxidize completely any adsorbates except for oxygen species; it was then stepped negatively to -0.25 V, at which surface oxygen species were reduced and the dissociative adsorption of DMM could be neglected; finally the Pt electrode was pushed against a CaF2 IR window to form a thin layer solution (∼1 µm), in order to reduce the intensive IR absorption by water. Infrared radiation sequentially passed through the CaF2 window and the thin-layer solution, and then reflected from the electrode surface. In such an external IR 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

∆R R(ES) - R(ER) ) R R(ER)

(1)

where R(ES) and R(ER) are the single-beam spectra collected at sample potential ES and reference potential ER, respectively. As a result, downward bands in the resulting spectra indicate the formation of products, while upward bands denote the consumption of reactants. In this paper, the ER was fixed at -0.25 V, and ES was varied from -0.20 to +0.70 V at an interval of 0.05 V. It is worthwhile noticing that, prior to IR data collection at each ES, the Pt electrode was first uplifted and cleaned by using the clean procedure stated above, then it was pushed down to the IR window to form a thin layer with renewed solution. In such a way, the influence of product accumulation and reactant consumption in the thin-layer solution on in situ FTIR spectrum collected at different ES has been avoided. 1000 singlebeam spectra were collected and coadded for each resulting spectrum to improve the signal-to-noise ratio, which took about 170 s. The spectral resolution was 8 cm-1. In case of transmission IR spectra collection, a transmission IR cell (Nicolet) was used, which consists of two CaF2 disks spaced by a Teflon gasket of 6 µm in thickness. 2.3. Other Conditions. All solutions were prepared with Millipore water (18 MΩ cm) purified in a Milli-Q Labo apparatus (Millipore Ltd., Japan), and were deaerated by bubbling high-purity N2 before measurements. DMM (98%) and sulfuric acid (98%) were purchased from Alfa Aesar; methyl formate, methanol, and formaldehyde (A.R. reagent) were obtained from China Medicine Shanghai Chemical Reagent Corp. All experiments were carried out at around 25 °C. 3. Results and Discussion 3.1. Cyclic Voltammetry. Figure 1a shows the cyclic voltammograms (CVs) recorded on Pt electrode in a fresh 0.1

Figure 1. (a) Cyclic voltammograms of Pt electrode recorded in fresh 0.1 M DMM + 0.1 M H2SO4 (solid line) and in 0.1 M H2SO4 (dotted line). (b) Effects of DMM hydrolysis on electrocatalytic activity. Key: solid line, fresh 0.1 M DMM; dashed line, partially hydrolyzed solution at ambient temperatures for 8 h; dotted line, 0.2 M CH3OH + 0.1 M HCHO. Inset is the magnification of the CVs in hydrogen region. Scan rate was 50 mV s-1.

M DMM + 0.1 M H2SO4 solution (solid line) and in 0.1 M H2SO4 solution (dotted line) at 50 mV s-1. In the positive-going potential scan, the oxidation of DMM starts at about 0.05 V. Along with the increasing electrode potential, an oxidation peak of 0.23 mA cm-2 appears at 0.33 V, followed by a small hump at 0.49 V. This hump may be attributed to the oxidation of adsorbed species (e.g., CO) generated from the dissociative adsorption of DMM. In the negative-going potential scan, the oxidation of DMM takes place right after the reduction of surface oxygen species, yielding a current peak of 0.20 mA cm-2 at 0.28 V. The hysteresis between positive- and negative-going scans is not significant as that observed in electrooxidation of methanol or formaldehyde,34 which indicates that the selfpoisoning effect in DMM oxidation is not pronounced. The existence of substantial hydrogen adsorption/desorption current in the hydrogen region (-0.25 to +0.10 V) confirms this point. It has been determined from the CV data that only about 23% of surface sites, mainly (100) sites of Pt at -0.02 V,35,36 were inhibited by adsorbed species derived from dissociative adsorption of DMM. DMM is an acetal molecule, and it can be hydrolyzed slowly to formaldehyde and methanol in acidic solutions.21 Thus, the hydrolyzed species of DMM may affect its electrochemical reactivity. Figure 1b is the comparison of the CVs recorded in a fresh 0.1 M DMM solution (solid line), in a partially hydrolyzed DMM solution at ambient temperatures for 8 h (dashed line), and in a mixed solution of formaldehyde and methanol solution (dotted line, i.e., the completely hydrolyzed

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Figure 2. In situ FTIR spectra of the electrooxidation of some organic molecules on Pt electrode at 0.30 V: 0.1 M DMM (A), 0.2 M CH3OH (C), 0.1 M HCHO (D), and 0.2 M CH3OH + 0.1 M HCHO (E). (B) Transmission spectrum of 0.1 M HCOOCH3 (MF).

TABLE 1: Assignments of the in Situ FTIR Spectrum of DMM Electrooxidation wavenumbers/cm-1 2343 ∼2060 1138 1106 1035 1717 1435 1385 1227 1172 1016

assignments νas(O)CdO) ν(C≡O) νas(C-O-C-O-C) νs(C-O-C-O-C) νas(C-O-C-O-C) ν(CdO) δas(CH3) δs(CH3) ν[(Od)C-O] r(CH3) ν(O-CH3) ν(C-O)

CO2 COL DMM (upward) HCOOCH3

CH3OH

DMM solution). In the partially hydrolyzed solution, it is clear that the oxidation current is nearly doubled, and the peak potential is shifted positively to 0.54 V. Moreover, the oxidation current recorded in the mixed solution of formaldehyde and methanol is about 1 order of magnitude larger than that measured in the fresh 0.1 M DMM solution between 0.20 and 0.75 V, and another big current peak of 7.5 mA cm-2 appears at about 1.1 V, whereas the DMM is completely inactive at such high potential. This result demonstrates that the intrinsic reactivity of DMM is very low. The inset to Figure 1b depicts the magnified CVs in the hydrogen region, in which we can see clearly that less surface sites were inhibited by the dissociative species in the fresh 0.1 M DMM solution. 3.2. FTIR Spectroscopic Studies of DMM Electrooxidation. Spectrum A in Figure 2 is an in situ FTIR spectrum of DMM oxidation on the Pt electrode at 0.30 V. The assignments of IR bands are listed in Table 1.37 Three upward IR bands at 1138, 1106, and 1035 cm-1 can be assigned to the characteristic IR absorption of DMM (νC-O-C-O-C), which was confirmed by transmission IR spectra of the DMM solution (see Supporting Information, Figure S1). Five downward bands at 1717, 1435, 1385, 1227, and 1172 cm-1 can be attributed to MF,38 as compared to its transmission spectrum (B). It is interesting that Kerangueven et al. have also detected a band at 1719 cm-1 on Pt(111) electrode for DMM electrooxidation, and they assigned

Zhou et al. the band to adsorbed HCO species.28 In our study, the band at 1717 cm-1 is undoubtedly attributed to νCdO of the dissolved MF, because the band center is independent of electrode potential (see below) and the IR features of fingerprint region in the in situ spectrum are in good agreement with those observed in the transmission spectrum of MF. The formation of MF indicates the oxidation of DMM on Pt electrode is quite incomplete, although it does not involve C-C bond cleavage. The downward band at 2343 cm-1 is ascribed to IR absorption of CO2, which is the complete oxidation product of DMM. However, its band intensity is considerably weaker than those measured in oxidation of methanol (C), formaldehyde (D), and their mixture (E) at the same electrode potential, which is consistent with the low activity of DMM. In addition, two downward bands near 1200 and 1055 cm-1 in the spectrum C, D, and E are attributed to the increase of HSO4- concentration in the thin layer solution,39 arising from the formation of proton when these organic molecules are oxidized. Some previous reports indicated that the DMM was electrochemically inactive and that the anodic current was mainly due to the oxidation of hydrolyzed products (i.e., formaldehyde and methanol).21,23 The current FTIR results clearly demonstrate that DMM can be directly oxidized to MF and CO2, although its reactivity is considerably lower than that of its hydrolyzed products. The downward band at 1016 cm-1 can be assigned to methanol (νC-O) that may be generated from the hydrolysis of methoxyl groups in DMM. It is worthwhile noticing that, although MF gives rise to a band at 1016 cm-1 (νs(C-O-C)), the intensity should not be so intensive. It was found that, in the transmission spectrum of MF (spectrum B), the ratio of the peak height of the band at 1714 over that at 1016 cm-1 is about 1: 0.095, while the corresponding ratio has been changed to 1: 0.37 in the in situ IR spectrum (spectrum A). Therefore, IR absorption of species other than MF may contribute to the enhancement of the band at 1016 cm-1. This feature becomes even more evident at a lower potentials (Supporting Information, Figure S2). Furthermore, Narayanan et al. have detected methanol in the anode side solution of a direct DMM fuel cell by gas chromatograph.18 As a consequence, this band could be reasonably assigned mainly to C-O stretching of methanol. Besides the dissolved products, DMM oxidation may also produce some adsorbed intermediates, as predicted by the suppression of hydrogen adsorption/desorption in CVs shown in Figure 1. In order to clarify this point, Figure 3 presents the magnified in situ FTIR spectra at around 2000 cm-1 of Figure 2. A band at about 2070 cm-1 can be seen clearly, and assigned to linearly bonded CO (COL) on Pt surface. The IR features of the COL, i.e. the band shape, position and intensity, generated from DMM are similar to those of the COL generated from methanol, but are considerably different from those of the COL produced by formaldehyde or the mixture. This fact suggests that methanol may be responsible for the production of COL on Pt electrode during DMM oxidation. The above FTIR studies demonstrate that MF, CO2, and methanol are the main dissolved products, and COL is an adsorbed intermediate for DMM oxidation on Pt electrode in H2SO4 solutions. 3.3. Potential Dependence of DMM Electrooxidation. Figure 4 illustrates in situ FTIR spectra of DMM oxidation on Pt electrode at potentials varying from -0.20 to 0.70 V. The IR bands of CO2, MF, and methanol appear nearly at the same potential of -0.050 V, which is 0.10 V lower than the initial oxidation potential of DMM measured from the CVs. Along with increasing electrode potential, the intensities of all IR bands

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Figure 5. Potential dependence of the change in relative concentration of DMM (X - 1), MF, CH3OH and CO2 (left y-axis), and integrated band intensity of COL (+ICOL, right y-axis).

Figure 3. In situ FTIR spectra of linearly bonded CO (COL) generated from the electrooxidation of DMM, CH3OH, HCHO, and CH3OH + HCHO.

Figure 4. In situ FTIR spectra of DMM electrooxidation on Pt electrode at different potentials. ES was varied from -0.20 to +0.70 V, ER ) -0.25 V. 1000 scans, 8 cm-1.

increase at first, and then decrease. Moreover, the frequencies of all IR bands do not shift with electrode potential except the νCO of COL. The dissolved product distributions of DMM oxidation at different potentials can be analyzed quantitatively from in situ FTIR spectra (detailed analytic method is illustrated in Figures S3-S6 of the Supporting Information). In brief, the contribution of each product to in situ FTIR spectra was evaluated by

subtracting its individual transmission spectrum collected in the same liquid-film thickness from the in situ FTIR spectra until the corresponding characteristic peaks disappear (1138 and 1035 cm-1 for DMM, 1717 and 1227 cm-1 for MF, 1016 cm-1 for methanol, and 2343 cm-1 for CO2). Thus, the change in relative concentrations of DMM, MF, methanol, and CO2 (∆CDMM, ∆CMF, ∆CMeOH, and ∆CCO2, respectively) in the thin-layer solution were obtained. The validity of the method was confirmed by analysis of transmission spectrum of a 1:1:1 mixed solution of 0.08 M DMM, 0.08 M MF, and 0.08 M methanol, and the relative error was within 6% (Supporting Information, Figure S5). It should be noted that, as for the analysis of in situ spectra, the effects of diffusion between the thin layer and bulk solution, as well as an uneven thickness of the solution film in the thin layer should be taken into account. 40,41 These effects can be nevertheless counteracted for the calculation of relative concentration in the same thin-layer solution. Figure 5 demonstrates the potential dependence of the ∆CDMM, ∆CMF, ∆CMeOH, and ∆CCO2 obtained from the in situ spectra shown in Figure 4, in which the ∆CDMM has been multiplied by a factor of -1 for facilitating the comparison (the signs of ∆CDMM and ∆CMF are opposite to ∆CDMM). Taking molecular structure into account, the oxidation of one DMM may produce one MF and one methanol. In Figure 5, the CMF is close to the CDMM, confirming that MF is difficult to be further oxidized under current conditions. The fraction of MF being oxidized is only 0.10-0.20 even in the potential region of 0.30-0.40 V, where MF exhibits the highest reactivity. On the other hand, the oxidation and dissociative adsorption of methanol are relatively easy, so ∆CMeOH is significantly smaller than ∆CDMM, especially at potentials above 0.30 V. The ∆CCO2 slowly increases below 0.25 V, and then it sharply rises and reaches a maximum at around 0.40 V. All these results indicate that DMM is preferentially oxidized to MF and methanol at potentials below 0.25 V, and to CO2 at higher potentials. The dependence of band intensity of COL on electrode potential is also illustrated in Figure 5 (right y-axis). At -0.20 V, no COL can be detected, indicating that dissociative adsorption is negligible at such a low potential. Along with potential increasing, the band intensity of COL increases quickly and reaches a plateau between 0.0 and 0.25 V, and then decreases to zero at 0.40 V due to its oxidation. 3.4. Time Dependence of DMM Electrooxidation. Several selected time-resolved FTIR spectra of DMM oxidation at 0.30 V collected at an interval of 0.17 s are presented in Figure 6a. The corresponding time dependence of ∆CMF and ∆CCO2 is shown in Figure 6b. The ∆CMF increases quickly in the initial 30 s, and then it reaches a stable value, while the ∆CCO2

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Zhou et al. SCHEME 1: Proposed Reaction Mechanism of DMM Electrooxidation on Pt Electrode in Acidic Solutions

In another experiment, we found that the oxidation of methanol to CO2 on Pt electrode occurs quite slowly at potentials below 0.30 V (Supporting Information, Figure S7). According to the mechanism illustrated in Scheme 1, DMM oxidation at a low potential (E < 0.25 V) should preferentially produce MF and methanol, which is consistent with the experimental results shown in Figure 5. 4. Conclusions Figure 6. (a) Several selected time-resolved FTIR spectra of DMM electrooxidation on the Pt electrode at 0.30 V. (b) Time-dependent relative concentration of MF and CO2. Time resolution used was 0.17 s.

continuously increases. In the early stage of DMM oxidation, ∆CMF is obviously much bigger than ∆CCO2. This result suggests that there is another product with one carbon atom, which is consistent with the assignment of methanol as a hydrolyzing product of DMM. 3.5. Proposed Mechanism of DMM Electrooxidation. Unlike methanol, DMM can not be oxidized in neutral or alkaline media,22 so H+ must play a key role in DMM electrooxidation. In addition, it has been reported that the hydrolysis of DMM in 0.1 M H2SO4 at room temperature is relatively slow (the half-life is about 11 days21) in comparison with the measurement time of in situ FTIR spectroscopy. As a result, the methanol detected by FTIR here (within 170 s) could not be generated from the direct hydrolysis of DMM itself, but rather from the hydrolysis of a reaction intermediate of DMM oxidation. Based on above results and analysis, a reaction mechanism of DMM electrooxidation on Pt electrode in acidic solutions is proposed and illustrated in Scheme 1. The reaction occurs as follows: (1) Similar to the initial steps of methanol dehydrogenation on Pt electrodes,42 DMM may be electrochemically adsorbed on Pt surface by the cleavage of one hydrogen atom. (2) One of the methoxyl groups in the adsorbed DMM will combine with H+ to form an oxonium ion, which is sequentially hydrolyzed to adsorbed methoxyl-hydroxymethane (a hemiacetal formation) and methanol. (3) The adsorbed methoxyl-hydroxymethane can be further oxidized to MF. (4) A small fraction of MF in the solution can be oxidized to CO2. (5) The methanol is oxidized further to CO2 either directly or via COL intermediate.

In this paper, the oxidation of DMM on a polycrystalline Pt electrode in 0.1 M H2SO4 solutions was investigated by means of cyclic voltammetry and in situ FTIR reflection spectroscopy together with transmission FTIR spectroscopy. On the basis of the FTIR results, a reaction mechanism was proposed at molecular level. The cyclic voltammetric studies indicate that the oxidation of DMM gives rise to a current peak of 0.20 mA cm-2 at about 0.30 V, and the hysteresis between the positive- and negativegoing potential scans is not pronounced, illustrating that the selfpoisoning effect is not significant in DMM electrooxidation. It has been determined that the reactivity of DMM is very low, which is only about one-tenth of that of its complete hydrolyzed products (i.e., methanol + formaldehyde). In situ FTIR results demonstrated that DMM is preferentially oxidized incompletely to methyl formate, which is difficult to be further oxidized. Moreover, one methoxyl group in DMM undergoes the hydrolysis to form methanol that can be oxidized to CO2, or generate linearly bonded CO through dissociative adsorption. Both the stationary and transient in situ FTIR spectra of DMM electrooxidation demonstrate that methyl formate and methanol are preferably produced at potentials below 0.25 V or in the initial stage of reaction, while CO2 dominates at higher potentials or in the final step of oxidation. The present study demonstrates that the oxidation of DMM on Pt electrode is quite incomplete, which will reduce significantly the utility efficiency of the fuel. Although the electro-activity and utility efficiency may be increased through the hydrolysis of DMM to methanol and formaldehyde, the self-poisoning effect will increase simultaneously. The key issue for the application of DMM in direct fuel cells consists in the exploration of better electrocatalysts that can either promote the oxidation of methyl formate or inhibit the methyl formate path. Acknowledgment. The study was supported by the NSFC (20503023, 20433060, and 20833005), the MOST (2007DFA40890), the NFFTBS (J0630429), and the Technological Innovation Program of Xiamen University (XDKJCX20041009).

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