18836
2007, 111, 18836-18838 Published on Web 12/06/2007
Surface Structure Dependent Electro-oxidation of Dimethyl Ether on Platinum Single-Crystal Electrodes Yujin Tong,† Leilei Lu,†,‡ Yi Zhang,† Yunzhi Gao,‡ Geping Yin,‡ Masatoshi Osawa,† and Shen Ye*,†,§ Catalysis Research Center, Hokkaido UniVersity, Sapporo, Japan, Department of Chemistry, Faculty of Science, Harbin Institute of Technology, Harbin, China, and PRESTO, Japan Science and Technology Agency (JST), Japan ReceiVed: October 3, 2007
The electro-oxidation of dimethyl ether (DME), which is regarded as a promising alternative fuel for fuel cells, on platinum single-crystal electrode surfaces in acidic solution has been characterized by electrochemical and in situ IR observations. It has been found that the electro-oxidation behavior of DME depends significantly on surface structures. The (100)-terrace shows a particularly high activity in DME electro-oxidation in which C-O bonding is supposed to be selectively cleaved on the (100) terraces.
Fuel cells are attracting a great deal of attention due to their important role in the fields of energy generation and environmental impact. Methanol (CH3OH) is regarded as one of the most promising fuels for fuel cells, but its high toxicity is a big barrier to its application. As an alternative fuel, dimethyl ether (DME, CH3-O-CH3) also has a high-energy density but very low toxicity. DME is expected to have a high electrochemical activity from its unique chemical structure.1-3 Although several studies on the electrochemical behavior of DME have been reported,4-7 there is little agreement in cyclic voltammgrams (CVs), reaction products, intermediates, etc. A possible reason is the differences in the surface structures of the electrodes used. It is well-known that electro-oxidation activities of C1 molecules, such as methanol and formic acid, largely depend on the surface structures of platinum electrodes. The use of platinum singlecrystal electrodes Pt(hkl) has contributed greatly to our understanding of these reactions.8-11 However, no systematic studies of DME on well-defined electrode surfaces have been carried out. In the present work, we examined the electro-oxidation of DME on Pt(hkl) and demonstrate its significant dependence on the surface structures. The (100)-terrace shows a particularly high electrocatalytic activity for DME. Based on the electrochemical and in situ IR observations, a reaction scheme for DME oxidation on Pt(hkl) is proposed. Pt(hkl) with various step-terrace structures was prepared by the Clavilier’s method.12-14 Before each experiment, the Pt(hkl) was annealed in an H2/air flame for a few seconds, then cooled in an H2/Ar atmosphere, and finally quenched in Milli-Q water. Experimental conditions for electrochemical and in situ IR spectroscopic measurements have been described elsewhere.13,15 Electrolyte solutions were deaerated with Ar for 30 min and then saturated with DME (≈1.65 M)4 by bubbling DME * To whom correspondence should be addressed. E-mail: ye@ cat.hokudai.ac.jp. † Hokkaido University. ‡ Harbin Institute of Technology. § PRESTO.
10.1021/jp7096907 CCC: $37.00
(99.99%, Sumitomo Chemicals Corp.) through the solution. All experiments were carried out at room temperature. Recently, Watanabe et al.16 reported that dimethoxymethane, which has a similar ether-like structure to DME, can be largely hydrolyzed to CH3OH and HCHO in acidic solution within ca. 10 days. We found that the hydrolysis of DME can be safely ignored under the present experimental conditions. Figure 1a shows CVs of Pt(111) in 0.5 M H2SO4 before (dotted) and after (red/blue) the introduction of DME at 0.05 V. In addition to the square shaped hydrogen wave (E < 0.3 V), the so-called “butterfly” region with a spike at 0.45 V is observed in the blank solution. The “butterfly” region has been attributed to the adsorption/desorption of bisulfate anions on the Pt(111) surface and the spike is ascribed to its orderdisorder phase transition.17,18 In the solution saturated by DME, an anodic current is superimposed in the “butterfly” region in both positive- and negative-going sweeps. Moreover, an anodic peak is observed at 0.8 V. This peak was not observed when the negative limit of the sweep was more positive than 0.45 V. These results suggest that some reaction intermediate(s) are formed by partial decomposition of DME in the “butterfly” region and subsequently oxidized at 0.8 V. That is, no direct oxidation of DME occurs. The partial decomposition of DME scarcely affects the adsorption of hydrogen on Pt(111). Rather, it competes heavily with bisulfate adsorption and is inhibited after an ordered ad-layer of bisulfate is formed as mentioned above. Similar electro-oxidation activity for DME was observed in 0.1 M HClO4, but the partial decomposition occurred in a wider potential region up to 0.70 V, ca. 0.25 V more positive than that in 0.5 M H2SO4, due to the weaker adsorption of perchlorate, confirming again that anion adsorption and desorption largely affect the decomposition of DME on Pt(111). Pt(110) showed similar activity for electro-oxidation of DME. In contrast, Pt(100) demonstrates a totally different behavior (Figure 1b). Three anodic peaks are observed at 0.32, 0.60, and 0.8 V in the first positive-going sweep (red), whereas a single © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18837
Figure 1. Cyclic voltammograms (CVs) of (a) Pt(111) and (b) Pt(100) electrodes in 0.5 M H2SO4 solution (dotted). Two successive CVs (red and blue) were recorded after the solution was saturated with DME and the potential held at 0.05 V. Scan rate, 0.05 V/s.
Figure 2. Chronoamperomertric profiles observed at +0.80 V for 600 s on Pt(111), Pt(110), Pt(100), Pt(910), Pt(310), and Pt(poly) electrode surfaces, in 0.5 M H2SO4 saturated by DME molecule.
peak is observed at 0.68 V in the reverse negative-going sweep. In the second cycle (blue) and further cycles, only two large anodic peaks at 0.80 and 0.68 V are observed in the positiveand negative-going sweeps, respectively. The hydrogen wave around 0.38 V was fully suppressed even after a potential cycle up to 0.5 V. As shown in Figure 1, the anodic peak at 0.8 V on Pt(100) in the positive-going sweep is approximately 30 times larger than on Pt(111). The peak at 0.68 V in the negativegoing sweep is only observed on Pt(100). In contrast to Pt(111), the two large anodic peaks on Pt(100) were always observed independent of the negative limit of the sweep. These results indicate that the electro-oxidation scheme of DME on Pt(100) is different from that on Pt(111). In further experiments using high index Pt(hkl) surfaces, the high oxidation current for DME was observed on the Pt(hkl) with (100) terrace, such as Pt(910) (9(100) × (110)) and Pt(310) (3(100) × (110)), suggesting that the (100) terraces have high oxidation activity for DME. A similar oxidation current profile has also been reported for the Pt(poly) electrode.6 This activity may be ascribed to the (100) facets on the Pt(poly) surface. The electro-oxidation of DME on Pt(hkl) was also evaluated by chronoamperometry at various potentials. Figure 2 shows the anodic current on Pt(hkl) recorded at 0.80 V (after a potential step from 1.0 V) as a function of time in 0.5 M H2SO4 saturated with DME. A large anodic current is observed on Pt(100) with only a small drop. A large current is also found on Pt(910) and Pt(310), whereas a much lower current is
observed on Pt(111) and Pt(110). The electro-oxidation activity for DME increases with an increase of (100) terrace length on the platinum surface, i.e., in the order Pt(100) > Pt(910) > Pt(310) . Pt(110) and Pt(111). It is noted that the electrooxidation activity for DME on Pt(poly) is higher than Pt(111) and Pt(110) but lower than Pt(100). To understand the structural dependence shown above, deeper insights into the electro-oxidation reaction mechanism of DME are necessary. It is helpful to discuss the case of DME by comparison with the reaction schemes proposed for CH3OH due to their structural similarity. Furthermore there are already many results published for the latter.8-11 Two possible initial steps are considered for electro-oxidation of DME on Pt(hkl) surfaces; these involve cleavage of C-H and C-O bonds. It has been proposed that CH3OH is first dehydrogenated on the Pt electrode surface through C-H cleavage in the potential region near the hydrogen adsorption region to yield CO.9-11 The CVs of DME on Pt(hkl) in the low potential region (0.05 ∼ 0.5 V) were found to be very similar to those of CH3OH. Our in situ IR observation showed that CO is formed on Pt(hkl) as a stable intermediate in DME electro-oxidation above ca. 0.3 V, and CO2 is generated at potentials higher than ca. 0.6 V. Recently, Cuesta19 demonstrated that cyanide adsorption can inhibit CO formation in the dehydrogenation step of CH3OH electro-oxidation on Pt(111). We also checked the behavior of DME on a cyanide modified Pt(111) in 0.5 M H2SO4 and found the anodic current in the “butterfly” region and the anodic peak at 0.8 V on the bare Pt(111) caused by DME completely disappear, whereas the hydrogen adsorption on the cyanide-modified electrode surface is not affected by the presence of DME (Figure 3). The cyanidemodified Pt(111) also gives a similar CV profile in the potential region between 0.05 and 0.6 V either in DME saturated solution or 0.5 M CH3OH solution. These results suggest that initial decomposition of DME on Pt(hkl) in the low potential region is analogous to dehydrogenation of CH3OH through C-H cleavage. DME needs at least three contiguous Pt atoms for the initial dehydrogenation step, which has been also proposed for CH3OH by Cuesta.19 However, it is different from those observed in CH3OH;19 CV observed on the cyanide-covered Pt(111) in a DME-saturated solution is almost identical to that in a DME-free solution (Figure 3). Furthermore, in situ IR observations show that neither CO nor CO2 was observed on the cyanide-covered Pt(111) between 0.05 and 1.1 V in 0.5 M H2SO4 saturated by DME, indicating that no direct oxidation of DME takes place on the Pt(111) surface. On the other hand, two different steps are considered for DME (C-O cleavage) and CH3OH (O-H cleavage) on Pt(hkl) at high potentials (ca. 0.6∼1.0 V). The C-O cleavage for DME to yield methoxy has been reported on many alumina-supported het-
18838 J. Phys. Chem. C, Vol. 111, No. 51, 2007
Letters selectively catalyzes a C-O cleavage process of DME, and thus plays an important role in its oxidation activity. Further experiments using the DEMS technique24 and theoretical works10 will be useful for exactly determining the products during the electro-oxidation of DME on the Pt(hkl) surfaces. Acknowledgment. This research was supported by a Grantin-Aid for Scientific Research (B) 19350099 from MEXT and PRESTO, Japan Science and Technology Agency (JST), and partially by the Natural Science Foundation of China (No. 20476020). L.L. acknowledges a fellowship from the Chinese government. We thank Prof. Paul Davies for a critical reading of the manuscript. Supporting Information Available: CVs of DME on other Pt(hkl). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 3. CVs of bare Pt(111) (black) and cyanide-modified Pt(111) electrodes (red) in 0.5 M H2SO4 solution (a) without and (b) with saturation of DME. Scan rate, 0.05 V/s.
erogeneous platinum metal surfaces.20 The methoxy is further decomposed to CO2. Electro-oxidation of CH3OH via a methoxy intermediate is also predicated by a recent DFT calculation.10 Interestingly, the calculation shows that the oxidation via the methoxy path is facilitated by introducing (100) sites on the (111) terrace.10 The different CV behaviors for the two molecules in the high potential region can be explained by the additional cleavage of a C-O bond in the case of DME comparing that of CH3OH. This C-O cleavage step of DME is expected to be facilitated by the existence of the (100) terrace and, therefore, contributes to the high electro-oxidation activity of DME on (100) terraces. It is noted here that formate, which has been reported as one oftheintermediatesforelectro-oxidationofmanyC1 molecules,21-23 has not been detected by in situ IR measurement in the present experiments on either Pt(hkl) or Pt(poly), indicating that the reaction scheme is definitely different from the C1 molecules. In conclusion, electro-oxidation of DME on Pt(hkl) surfaces depends significantly on the surface structure. The (100) terrace
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