Surface Raman spectroscopy as a versatile technique to study methanol oxidation on rough Pt electrodes

Electrochimica Acta 46 (2000) 193– 205 www.elsevier.nl/locate/electacta

Surface Raman spectroscopy as a versatile technique to study methanol oxidation on rough Pt electrodes B. Ren a,1, X.Q. Li a,2, C.X. She a, D.Y. Wu a, Z.Q. Tian b,* b

a Department of Chemistry, Xiamen Uni6ersity, Xiamen 361005, People’s Republic of China State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen Uni6ersity, Xiamen 361005, People’s Republic of China

Received in revised form 24 March 2000

Abstract The emphasis in the present study was placed on developing Raman spectroscopy into a versatile technique, which offers intriguing opportunities for investigating electrocatalytic reaction. Through the in-situ Raman spectroscopic study, with a confocal Raman microscope, on the methanol electrooxidation on platinum electrodes with various surface roughness, it has shown the advantage in obtaining the informative spectra during the surface reaction with high faradaic current. This is hard to be performed by the other spectroscopic methods that have to use the thin-layer cell. The ability to obtain both the low frequency and the high frequency vibrations of Pt– C and CO bands allows the assignment of surface species unambiguously. The ease of studying the surface bonding, investigating highly roughened surfaces with dark color and using high concentration electrolyte may provide a way to bridge the gap between the systems of fundamental research and practical applications. The results reveal the surface roughness effect on the electrooxidation process and provide clear evidence for the parallel oxidation mechanism. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Raman microscopy; Methanol; Platinum; Electrooxidation; Carbon monoxide

1. Introduction The methanol electrooxidation on Pt-based electrodes is one of the most extensively studied systems in terms of both fundamental and applied research in surface (interfacial) electrochemistry. It involves a very complex reaction of an exchange of 6e− occurring in many steps and various kinds of surface (interfacial) species such as reactants, intermediates, poisons, supporting electrolytes and even solvent molecules. There* Corresponding author. Tel.: + 86-592-2181906; fax: +86592-2085349. E-mail address: [email protected] (Z.Q. Tian). 1 Also corresponding author. 2 Present address: Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA.

fore, the characterization of different surface species and surface structures during electrochemical processes is of central concern. In the past several decades, numerous electrochemical and non-electrochemical methods have been employed for the study of the methanol oxidation process, which led to huge amount of experimental information available in the literature and to many articles giving extensive review from various aspects [1 – 9]. Although great efforts have been made, an understanding of the mechanism of methanol electrooxidation at molecular level remains elusive [1 – 9]. Some important issues, such as the reaction path, the nature of the surface oxidant and intermediates, the influence of the surface morphology, the surface roughness and the foreign metals on the reaction process, are still in controversy. The techniques applied to this system have various individual disadvantages because of the very

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complicated reaction process. In-situ optical spectroscopies including infrared (IR) [2,10–12], second harmonic generation (SHG) [13], sum frequency generation (SFG) [14,15] and ellipsometry [16] have been used extensively. Among them, IR is the most intensively used technique and has made great contributions to reveal the nature of the adsorbed species and the strong influence of the surface crystallography (with single crystals) on the reaction [3,10–12,17–22]. In order to obtain good quality spectral signals, the electrode surfaces used in these optical spectroscopies are required in general to be of high surface reflectivity, such as single crystal electrodes, smooth polycrystalline electrodes or surfaces of mild roughness. However, the real catalysts and electrodes for practical application are those with high surface roughness. By contrast, many non-optical methods such as mass spectroscopy [23], NMR [24], and X-ray techniques [25,26] have been applied to study the rough surfaces or porous (powder) electrodes and provided much useful information. However, they also have their own limitation, especially in identifying of surface species at molecular level. For instance, although differential electrochemical mass spectroscopy (DEMS) could readily be used to study the technical electrode in situ, the limitation that only volatile species can be detected greatly restricts its wide application. So, it seems to be vitally important to further develop the methodology particularly in two aspects: (i) hyphenating different techniques for the complementary study; and (ii) establishing more versatile techniques to study electrocatalysts under the rigorous reaction condition. So far, Raman spectroscopy has not been considered as a technique that suitable for investigating the methanol oxidation system, this technique has not been included even in the latest two reviews [3,4]. As a powerful vibrational spectroscopy in surface (interfacial) electrochemistry [27–30], Raman spectroscopy is the only vibrational spectroscopy suited to study highly rough electrode surfaces. Furthermore, Raman can more readily obtain spectral information of surface adsorbate bands locating in the low frequency region, such as below 600 cm − 1, where it is difficult for other in-situ vibrational spectroscopy such as IR and SFG to investigate. This advantage in yielding important information on diverse surface bonding leads Raman spectroscopy to reveal with detail the nature of the surface interaction, coverage and coadsorption. Unfortunately, the advantages aforementioned are overwhelmed by a vital drawback of surface Raman spectroscopy, which is originated from the very weak quantum yield of Raman process. For the detection of surface Raman signals, this disadvantage becomes significant inevitably because the surface species are normally in the monolayer or sub-monolayer amount. Consequently, the signal is too weak to be detected, especially for those molecules, including CO, having

very small Raman cross section. Although surface-enhanced Raman spectroscopy (SERS) has extremely high surface sensitivity and has been applied in surface electrochemistry [27– 30], only Ag, Au and Cu surfaces can provide giant surface enhancement, but not Pt group metals. Of particular interest in further developing this technique is to extend the study to transition metals widely used as electrocatalysts. Great efforts have been made to extend SERS study to transition metals in the past decades [31– 43]. The most successful approach could be to deposit ultrathin transition metal films (one to five monolayers) onto SERS active electrodes, making use of the long-range enhancement (electromagnetic mechanism) created by the SERS active substrate underneath [32– 39]. For example, Weaver and co-workers obtained good quality SERS spectra of methanol dissociatively adsorbed on Pt-coated Au electrodes [36]. However, with so thin a film on the very rough surface, some residual substrate sites are usually exposed, so it is very difficult to eliminate entirely the possibility that the adsorbate is bound to the exposed substrate or to the boundary of the two metals rather than to the overlayer sites. Recently, they have developed an electrodeposition method to produce ‘pinhole free’ films on the SERS active Au substrate [37– 39]. It is certainly a significant progress compared with what have been reported before. The thin-film strategy can lead the transition metal films to generate the surface enhancement as much as of four order [39]. It shows great advantage especially when the surface species studied is of very low Raman cross section, and the high detection sensitivity is needed. However, in some special situation, that the electrochemical system has to be studied in a very wide potential range and under the rigorous reaction condition the stability and reversibility of this kind of ultrathin film electrode will be a problem. The most reliable but also difficult way is to obtain SERS spectra directly from bare transition metals. There have been only several reports about the observation of SERS from bare Pt electrodes [40– 43]. However, no further studies continued in the past five years mainly due to that the insufficiency of the surface Raman signals detected. It was extremely difficult to correlate the Raman data with electrochemical results and provide the meaningful information. For example, in 1977 Cooney et al. reported a SERS study of CO adsorption on a platinized Pt electrode, two extremely weak bands at ca. 2096 and 2081 cm − 1 were recorded and attributed to the stretching vibration of linearly adsorbed CO [40]. However, their result was unable to be repeated by other groups. To our best knowledge, there has been no SERS study on methanol dissociation and electrooxidation on bare Pt-based electrodes. In fact Raman spectroscopy has not yet been considered as a general surface tool as IR spectroscopy used widely in electrochemistry and surface science.

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Recently, with the latest advances in Raman instrumentation equipped with CCD detector, the confocal microscope, and holographic notch filter [44], combined with the use of special surface roughening procedures, we are able to extend SERS study to many bare transition metals including Pt, Rh, Ni, Co and Fe [45–51]. For example, we have obtained high quality surface Raman spectra of CO adsorbed on bare roughened Pt electrodes in base solution and studied the influence of OH− on the CO adsorption [45]. Except our own communication [49,50], there is still no other Raman study of methanol dissociative adsorption on roughened platinum surfaces. In this paper, the methodology of using surface Raman spectroscopy for the study of the methanol oxidation process is discussed through investigating the effects of surface roughness and methanol concentration on methanol electrooxidation on roughened Pt electrodes.

2. Experimental Raman measurements were performed using a confocal microprobe Raman system (LabRam I). The slit and pinhole of the system were set at 200 and 800 mm, respectively. The microscope attachment was based on an Olympus BX40 system and used a 50 × long working-length objective (8 mm) so that the objective will not be immersed in the electrolyte. The laser spot on an electrode surface has a size of ca. 3 mm in diameter. A holographic notch filter was equipped to filter the excitation line and two selective holographic gratings (1800 and 300 g/mm) were employed for different purposes. A CCD with 1024 ×256 pixels was used as the photon detector. The excitation line was 632.8 nm from an internal He–Ne laser with a power of 12 mW on the sample. The working electrode was a polycrystalline Pt disk with a geometric area of 0.1 cm2 embedded in a Teflon shroud. It was polished successively with 0.3 and 0.05 mm alumina slurry (Buehler Ltd.) to a mirror finish, and sonicated in triply distilled water. Careful polishing with finer alumina powder is necessary to obtain homogeneous and SERS-stable roughened Pt surfaces. The preparation and characterization of the Pt working electrode will be given in the next section. A large Pt ring served as the counter electrode. The reference electrode was a saturated calomel electrode (SCE), thus all the potential quoted here are with respect to SCE. The detailed description on the spectroelectrochemical measurement and the roughening procedure has been given elsewhere [45,46]. A Raman spectrum was normally acquired after keeping the Pt electrode at a fixed potential for 2 min, and the potential dependent Raman spectra were acquired by moving the potential stepwise in either a positive or negative direction. All experiments were performed at room

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temperature. All the chemicals used were analytical reagent and the solutions were prepared using Milli-Q water.

3. Results and discussion

3.1. Preparing roughened Pt electrodes As has been widely accepted that a proper surface roughening procedure is essential to obtain reasonably good-quality spectra in order to acquire intensity– potential and wavenumber– potential profiles of adsorbates at electrodes owing to the enlargement of the surface area and the possible contribution of surface enhancement effect [46,49]. In the present study the roughening procedure of Pt followed what has been established originally by Arvia and his coworkers [52,53]. They reported that the surface after this unique pretreatment shows very good stability and they could even prepare surface with different preferred orientation. In our case, slightly modification was made on the procedure to make the electrode more suitable to be used in the Raman study. In brief, a square wave of 1.5 kHz with upper and lower potentials of 2.4 and −0.2 V was applied to the electrode in 0.5 M H2SO4 for a few minutes, and then the potential was held at − 0.2 V until the electroreduction of the surface oxide layer was completed. The longer time of potential modulation favors the obtaining of larger surface area, e.g. 1 min for the roughness factor R : 50 and 5 min for R: 200 (In the present study, the former electrode is defined as the mildly roughened electrode and the latter is dubbed as the highly roughened electrode. This kind of definition is appropriate in fundamental research. However, in practical application, this highly roughened electrode can only be considered as mildly roughened surface). Afterwards, the electrode was further subject to potential cycling in a fresh 0.5 M H2SO4 solution until the reproducible cyclic voltammograms were obtained. This is called electrochemical cleaning process for removing the unstable surface sites and impurities generated during the roughening process. Then the electrode is ready for spectroscopic measurement. The cyclic voltammogram of a Pt electrode undergoing such kind of roughening in 1.0 M H2SO4 is given in Fig. 1. The curve is very similar to that of the typical smooth Pt electrode, indicating that the roughened platinum retains essentially the properties of a normal platinum electrode. Conventional electrochemical studies on this Pt system have revealed that the charge flowing through the hydrogen adsorption or desorption region is right about the amount of one monolayer of hydrogen discharge/charge on the Pt surface. The typical value for an ideally smooth polycrystalline Pt electrode is ca. 210 mC cm − 2. Taking this consideration in

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mind, the surface roughness factor, which is defined as the ratio of the real surface area to the geometric surface area, was calculated by the ratio of the charge in the hydrogen adsorption region for the roughened Pt electrodes and 210 mC cm − 2. It should be pointed out that the Pt electrode surface after the electrochemical roughening and cleaning pretreatment shows very good stability and reversibility in both electrochemical and Raman spectroscopic measurements. This is demonstrated convincingly in Fig. 2. It gives the normalized integrated SERS intensities of the ring breathing mode (at ca. 1008 cm − 1) of pyridine adsorbed on Pt, Au, and Ag surfaces at different potentials. It is evident from Fig. 2(a) that the signal from Pt electrodes only undergoes slightly change after a potential excursion at very negative potentials. While the typical SERS electrodes (Au and Ag) sustain severe change, especially for the Ag electrode, see Fig. 2(b) and (c). This change is due to the decomposition of the SERS active sites that are very unstable at the desorption potentials of the surface species. For every new SERS experiment, the noble metal electrodes have to be polished and roughened again to recreate the SERS activity. However, for the Pt electrode, it could be used again both for electrochemical and spectroscopic studies without reroughening. Before each experiment, it needs only to be subject to the electrochemical cleaning in a H2SO4 solution until the reproducible cyclic voltammogram presents. It excludes very effectively the possible influence of some SERS ‘hot’ spot or ‘anomalous’ sites commonly existing in the noble metal systems, since these sites with high SERS activity are very unstable and can be removed by potential cycling or excursion at very negative or positive potentials. This surface pretreatment approach for the Pt electrode is especially important in

Fig. 1. Cyclic voltammogram of a roughened Pt electrode in 1.0 M H2SO4 solution. Scan rate: 20 mV/s.

Fig. 2. The integrated intensity-potential profile for ring breathing mode ( 1008 cm − 1) of adsorbed pyridine (Py) on various metal surfaces (a) Pt, (b) Au, (c) Ag.

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the present potential dependent study covering a very wide potential range. Thus, the electrochemical and spectroscopic results obtained from the Pt surface are highly comparable. It is necessary to keep in mind that the surface uniformity may be a problem for highly rough surface. With the help of a microscope, the sample size in the dimension of several microns is enough for the measurement, thus it is very easily applied to investigate a very rough and dark color surface. In the present study the spectra from different area of the surface were acquired to check the uniformity before each series of study. The most typical points were selected as the surface to be studied. More than three spectra were recorded for each data in the intensity–potential profiles. In fact, we found the roughened Pt surface is quite uniform if the surface pretreatment is successful.

3.2. In-situ studying electrochemical reaction Although electrochemical reaction processes including methanol electrooxidation have significance to many applications of technological importance, the interfacial structure under reaction condition has been studied much less extensively than that in the nonfaradaic potential region either by conventional electrochemical methods or by spectroelectrochemical techniques. In the differential capacitance, impedance and chronoamperometric studies, the information about adsorption processes and interfacial structural changes are submerged inevitably by a huge faradaic current. Moreover, the large gas bubbles, such as CO2, H2 or O2, generated at the surface can change the composition of the electrolyte near the surface and alter

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the light intensity irregularly, which in general causes destructive interference to the in-situ spectroelectrochemical measurements or indeed makes the optical measurement impossible. Using the optical internal reflection technique, by which the light does not go through the bulk electrolyte, makes the measurement under bubbling possible. However, its special requirement on the electrode preparation greatly limits its application. An alternative way to overcome the problem is to use the thin-layer optical configuration. In this case, the electrode is pushed as close as possible to the cell window so that the surface gas is dispersed with very fine bubbles [54]. However, the thin-layer configuration leads to the large ohmic drop in the ultrathin solution layer, which becomes more severe under high faradaic current. Therefore, it is important to develop a versatile technique suitable for investigating the electrochemical reaction with high faradaic current, such as the severe hydrogen evolution [54] and methanol oxidation. Raman spectroscopy, especially SERS, is probably one of the best techniques feasible to in-situ investigate the electrochemical systems under reaction condition with high faradaic current. For the Raman study with visible light excitation, there is almost no or only very weak absorption of the optical window and the aqueous solution to the incident and scattered lights. There is no interference of the ambient air to the measurements, so that the thin-layer configuration and even optical windows are not necessary except under some very special conditions. Therefore, Raman spectroscopy has its special application in the study of electrochemical reaction of high faradaic current. The optical configuration of the Raman cell can be similar to that of the conventional electrochemical cells which have relatively thick solution layer. It makes the cell easily to match the conditions of high reaction current and of electrochemical transient measurements. Especially, in confocal Raman microscopic study, the objective collects scattering lights only from very small area and volume of the sample near the surface. So the open cell without window is easy to be used in the present study, see Fig. 3. As a consequence, the investigation under severe bubbling becomes much easy, in particular if SERS enhancement of two order for the Pt electrode in the present study is operated [46,49]. Accordingly, it is essential to take the advantages mentioned above to perform Raman study on the electrochemical reaction, in particular the methanol system.

3.3. Studying surface roughness effect

Fig. 3. The electrochemical Raman cell used in this study. The cell body is made of Teflon. WE, working electrode (Pt); CE, counter electrode (Pt ring); RE, reference electrode (SCE).

The obtaining of stable and potential reversible Pt electrode surfaces enables us to perform systematic study on the methanol oxidation process. Fig. 4 displays the cyclic voltammograms (CVs) of three bare Pt

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Fig. 4. Cyclic voltammograms of platinum electrodes in 1.0 M CH3OH+ 1.0 M H2SO4. The roughness factor of Pt is about (a) smooth (b) 50, (c) 200. Scan rate: 20 mV/s.

electrodes with different roughness in 1.0 M CH3OH+ 1.0 M H2SO4. Their CVs have several similar features in the potential region studied: the inhibition of the hydrogen adsorption/desorption peaks due to the dissociative adsorption of methanol in the hydrogen adsorption region ( − 0.3 to 0 V); the relatively low but steadily growing of the methanol oxidation current in the double layer region (0 to 0.4 V); a peak in the

oxidation current near 0.6 to  0.85 V on the positive potential scan, followed by a decline due to the surface blockage by the formation of the surface oxides; further oxidation of methanol on the oxide surface upon increasing the potential up to 1.25 V; auto-oxidation of methanol on the freshly revealed Pt surface after the stripping of the oxide layer on the negative potential scan [55]. However, it is necessary to note that the CVs show some slight difference for these electrodes. Although it is not easy to distinguish the difference in the onset potential of the anodic current (vide infra), one can obviously see the difference in the potential of the peak current, which is at  0.62, 0.66 and 0.71 V for the smooth, mildly rough and highly rough electrodes, respectively. These differences in the CVs indicate that the surface roughness may have some effect on the methanol dissociation and electrooxidation behavior. Obviously, it is difficult and somewhat ambiguous to explain the surface roughness effect on the oxidation behavior of methanol at the molecular level based only on the cyclic voltammetric results above. More persuasive evidence for interpreting the difference could be obtained after examining on the corresponding surface Raman spectra of the system investigated. Because we were not able to obtain any detectable surface Raman spectra from the smooth electrode, we present here only the Raman spectra from two roughened Pt electrodes with small and large roughness factors (R : 50 and R: 200), respectively. Fig. 5(a) shows the surface Raman spectra of the Pt electrode with R :200 in 1.0 M CH3OH+1.0 M H2SO4. These spectral bands can be divided into two categories based on the independence or dependence of the band frequency on the applied potential. The bands at around 450, 590, 980, 1018 and 1051 cm − 1 are classified to the first category as their frequencies remained unchanged in the whole potential region studied. It indicates that they are from the solution species near the Pt surface (vide infra). Emphasis should be placed on two bands belonging to the second category showing strong characteristic of potential dependence, that is most likely related to the molecular vibration of the intermediate of the methanol dissociative adsorption and oxidation. The band at  2050 cm − 1 could be assigned to the stretching vibration of the linearly bonded CO by analogy with the IR results [2,3], and a strong and sharp band at ca. 490 cm − 1 could be attributed to the Pt– C vibration of the linearly adsorbed CO. The latter located in the low frequency region will be discussed in more detail in the next section. It can be seen clearly that the band intensity of the surface CO is considerably stronger for the highly rough surface with larger surface area than that of the mildly rough surface in spite of the fact that the former has much darker color. This demonstrates that Raman microscopy, unlike many optical reflectance spectroscopies, is feasible to

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collect the scattering lights from the highly rough surface with very dark color. After carefully comparing Fig. 5(a) and (b), one can find out that the C–O vibration of linearly bonded CO disappear at 0.525 V on the Pt electrode with mild surface roughness, while they can still be observed even

Fig. 5. Potential-dependent surface Raman spectra of roughened Pt electrodes in 1.0 M CH3OH+ 1.0 M H2SO4. The roughness factor of Pt is about (a) 200, (b) 50. Excitation line: 632.8 nm.

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at 0.575 V on the Pt electrode with high surface roughness. The results indicate that the surface CO is more difficult to be oxidized upon the increase of the surface roughness. This trend is in consistence with IR results reported by Beden et al. about methanol dissociative adsorption at smooth and mildly roughened Pt electrodes (RB 30). They assumed that the main reason for the difference might be the particle size effects related to specific crystallographic sites, whose population varies with the particle size and morphology [56,57]. Indeed, the main difference between Pt electrodes with different roughness is that there are more surface defects such as edge, step and kink sites on Pt electrodes with higher surface roughness (e.g. the roughness factor of 200). These sites are more active and thus, play an important role in the electrocatalytic reaction [58]. For instance, McClellan et al. studied the binding energy of CO on the step sites and terrace sites on the Pt (321) surface, which is estimated to be 151 and 96 kJ/mol, respectively. They concluded that this extra 55 kJ/mol energy enhances the CO stability on stepped sites, thus the CO adsorbed on step sites is more difficult to be oxidized to CO2 [59]. Xu et al. in their study of the oxidation of CO on the Pt (335) surface, found that CO adsorbs on the step site preferentially and is harder to be oxidized compared with that on the terrace sites [60]. From these facts, it is reasonable to assume that once CO is formed during methanol electrooxidation on the roughened Pt surface, it tends to adsorb first on defect sites where it is harder to be oxidized. The facts that there are more surface defects on the highly rough Pt electrode and the CO– substrate interaction is stronger at defect sites, support that CO is more difficult to be oxidized on this surface. Accordingly, the C – O band could still be observed at more positive potentials on the Pt electrode with high surface roughness. It should be noted from Fig. 5 that, the stretching vibration of CO on both surfaces blue shifts first and then red shifts with the positive moving of the electrode potential from − 0.2 V. The frequency change of the adsorbate has been interpreted by three effects [58]: electrochemical Stark effect, dipole– dipole coupling effect and surface structural effect (morphology). The above frequency shift can only be satisfactorily explained by the combination of these three effects. It could be seen from Fig. 5 that on both surfaces, the frequency of the CO stretch blue shifts with the potential moving positively before CO oxidation, this could be well explained by the electrochemical Stark effect. However, the slight red shifts in frequency upon the initial oxidation of CO could be due to the lowering of surface coverage of CO as a result of surface oxidation as well as the surface structural effect. It is worthy of mentioning again that there are more defects on the rough surface than that on the smooth surface, and the

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CO adsorbed on terrace site are more easily to be oxidized [57]. Fig. 5(a) and (b) show that the shape of CO stretch band is broad and asymmetric. The long tail at the low frequency part is most likely from those CO adsorbed on the defects. The relatively sharp band at around 2057 cm − 1 at 0.2 V decreased substantially when the potential moved positively to the initial potential of CO oxidation. It is very possible from the CO on the terrace sites since this kind of CO is less stable than that on the defect site, thus could be oxidized first at the lower potentials. More CO on defects on the R :200 surface may result in the more positive oxidation potential for the CO, as could be seen in Fig. 5(a). Further positive moving of the electrode potential, the CO on defect was also oxidized, thus the frequency of the tail decreased due to the weakening of dipole– dipole coupling effect as the result of the lowering of surface coverage.

3.4. Probing interfacial species and the concentration effect As mentioned above, five intense bands in Fig. 5 are assigned to be from the solution species. The bands at around 450, 590 cm − 1, 980 cm − 1 and 1051 cm − 1 are related to the vibration of SO24 − and HSO− 4 , and the 1018 cm − 1 band is from C–O stretching vibration of CH3OH. Although the frequency of these bands from the solution species is not affected by the applied potential, the intensity of the same bands depends considerably on the potential in more positive potential region when the methanol is oxidized. It is therefore necessary to analyze their behaviors. Compared with the conventional Raman spectroscopy, confocal Raman microscopy has not only much higher sensitivity to detect surface species but also higher vertical resolution. Because only the scattered lights from the laser focus,  5 mm3 in volume, can eventually go through the spectrometer to be detected. The signal from other plane of the solution is discriminated by the con-focused pinhole. This confocal optical configuration ensures the eliminating of the signal from bulk solution efficiently and thus increases the sensitivity to the concentration change of the solution in the interface region. It could be seen in Fig. 5(a), with the positive going of the electrode potential to that of methanol oxidation, some Raman signals from the solution phase changed obviously. The 1018 cm − 1 band assigned to the C–O stretching of methanol decreased steadily with the oxidation of methanol. However, the two peaks assigned to the symmetric vibration of SO24 − (980 cm − 1) and HSO− (1051 cm − 1) respectively change 4 slightly in their relative intensity. This change became distinctive when the supporting electrolyte was changed to 0.10 M H2SO4. The peak related to HSO− 4 increased with the oxidation of methanol, while SO24 − decreased

considerably, as shown in Fig. 5(a). This could be well understood by the total reaction formula of the methanol electrooxidation in this solution, where CH3OH+H2O“ CO2 + 6H+ + 6e − Thus, it is expected that the oxidation of methanol will decrease the methanol concentration and increase the concentration of H+ near the surface substantially, hence influences the following balance: H2SO4 “ H+ + HSO4− HSO4− l H+ + SO24 − Thus, the concentration of SO24 − and HSO− 4 changes, leading to the change of the relative intensity. The experiment was done by focusing the laser on the electrode surface while changing the electrode potential. On the other hand, the laser could be focused at different layer of the solution by moving vertically the cell fixed in the X– Y–Z stage to perform layer analysis, i.e. to change the distance between the laser focus and the electrode surface, to detect the concentration gradients of this solution during reaction. If not with the help of confocal system, this kind of analysis is almost impossible because the signal from the interfacial region or the individual layers will be overwhelmed by the strong signal from the bulk, especially when the bulk concentration is very high (in our case, 1.0 M methanol and 1.0 M H2SO4). Moreover, the solution layer we used is relatively thick (250– 500 mm) in comparison with the thin solution layer of few tens micron meters for the other optical spectroscopies. We have found that it is important to choose appropriate thickness of solution layer for obtaining good-quality Raman spectra. The thinner solution layer can distort less the optical configuration of the light collecting system consisting of the microscope, solution and the surface. Thus it could provide signals of good signal-to-noise ratio [61]. A thicker solution layer up to 1 – 2 mm can be used if the surface signal is sufficiently strong. To avoid the strong interference by the solution signal, we tried to use 0.10 M H2SO4 as the supporting electrolyte. The Raman spectra, from the Pt electrode with surface roughness factor of ca. 200, does show less interference by the electrolyte signal, see Fig. 6(a). Furthermore, the pH change in the interface, which is reflected by the change of the relative intensity of SO24 − and HSO− 4 in the solution. The methanol consumption during the reaction can also be strongly evident. However, we found that the concentration of the supporting electrolyte affected significantly the electrochemical behavior of the methanol electrooxidation process. The potential dependent Raman signal of the CO bands and the relevant CV in Fig. 6(b) display totally different features in comparison with that in the higher electrolyte concentration. The oxidation of methanol is

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Fig. 6. (a) Raman spectra and (b) Cyclic voltammogram of roughened Pt in 1.0 M CH3OH+ 0.10 M H2SO4. The roughness factor of Pt is about 200. Scan rate: 20 mV/s.

severely retarded in 1.0 M CH3OH+ 0.10 M H2SO4, the potential for complete oxidation of CO is 200– 300 mV more positive than that in 1.0 M CH3OH+ 1.0 M H2SO4, see Fig. 6(a). The oxidation current of methanol increases and decreases linearly upon the potential scanning between ca. 0.4 and 1.2 V in the positive and negative scan respectively, current oscillation occurs between 0.7 and 0.9 V on the negative scan. It is very possible that these distinctive differences in both the spectra and CV are due to the ohmic resistances within the electrolytic and metallic structures of the highly roughened electrode. Though the actual form of roughness is unknown, it is obvious that such electrodes must behave more as 3D than as 2D electrodes. In other words, for the highly roughened surface, metal and electrolyte are intermixed which forms sponge-type,

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cavernous, porous structures. Because of the small intermixed structures, the transfer of charges (ions and electrons) is hindered, and the diffusion may also be hampered. Thus, the high current density may cause that the local potential drop across the interface varies from the outer region to the deeper, more buried regions of such 3D electrodes. This affects their electrochemical as well as their spectroscopic behaviors and this effect becomes more severe on the electrode with roughness factor of 200 than on the electrodes with smaller roughness factor. Therefore, such effects must also be taken into account, for instance, when studying the role of CO, the potential dependence of the CObands, or the blocking of Pt oxidation in the presence of 1.0 M methanol. Accordingly, one must be very careful in choosing appropriate concentration of the reactant and supporting electrolyte, and performing the measurement. This effect will not be a problem at the low current situation. However, it becomes significant at the highly roughened surface under high oxidation current in the concentrated methanol system. Increasing the concentration of the sulfuric acid to  1.0 M could almost surmount the problem due to the lowering of the resistance. It should be pointed out that this problem could be more severe for the other optical spectroscopies, such as the IR study, in which a thin layer cell ( 10–30 mm) is normally used in order to acquire the IR signal of surface species with reasonable sensitivity. With so thin a layer, the solution resistance will be very large, especially in vigorous reaction condition. Thus, the potential given by the potentiostat will not be completely applied for the reaction, some of which is consumed as the ohmic drop across the solution layer. While in our Raman study the layer  200– 500 mm is used, this effect could be neglected, at least to a great extent. The disadvantage of Raman spectroscopy for studying solution species is its low detection sensitivity. For the study of methanol system the reaction product, CO2, is a very strong IR absorbent, on the other hand, is a very weak Raman scatter. It is impossible to detect this species in the present system because the solution volume to be sampled is very small. Overall Raman spectroscopy has the advantage of vertically spatial resolution in investigating the interfacial system and obtaining the concentration profile normal to the electrode surface during the reaction. A paper with discussion on this aspect and more detailed description of monitoring the relative intensity change of the interfacial species is under preparation [61].

3.5. Detecting surface bonding and formation of oxides One interesting spectral feature in the low frequency region should be further examined in Figs. 5 and 6. To be specially pointed out, with the conventional IR

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spectroscopy, it is almost impossible to obtain the information of the Pt–C vibration in the low frequency region. Although the use of synchrotron IR radiation has overcome partially this problem [62], it is still very difficult to be a general tool widely used for such kind of study. This illustrates the advantage of Raman spectroscopy in detecting the signals of metal-adsorbate in the low frequency region. In Fig. 4 particular attention is paid to the strong and sharp band at ca. 490 cm − 1 emerging between two solution bands related to SO24 − . In comparison with the previous electron energy loss spectra (EELS) [63] and SERS [36] results, this band could be attributed to the Pt–C vibration of linearly adsorbed CO. The CCD detector equipped in the present Raman system, has a good quantum efficiency (38%) in the low frequency region (400  600 cm − 1) with 632.8 nm excitation. This is beneficial to detect the Pt–C stretching band of good quality in comparison with the C–O stretching band in the high frequency region. To analyze the formation of other surface species during the methanol oxidation is important. However, it is hard to clearly distinguish the other spectral bands in addition to surface CO and the solution species because the high electrolyte concentration of 1.0 M H2SO4 is used. To extract the weak surface signal that is interfered strongly by the solution signal, we manipulated the spectra in the low frequency region of Fig. 5 by subtracting the solution signal. After manipulation, a broad band at the positive potentials can be clearly observed in Fig. 7 and its potential dependence can be analyzed. In our previous paper [45] and that of Weaver’s on the thin Pt film electrodes [64,65], the bands with similar frequency have been assigned to be from the platinum oxide during the oxidation of the Pt

electrode. In order to analyze the origin of the broad band centered at 580 cm − 1 in this system, a Raman study of Pt in the 1.0 M H2SO4 was performed as the blank sample. The spectra were given in Fig. 7(b). It could be seen that, a weak band at ca. 510 cm − 1 is discernible at 0.7 V, which is very possibly due to the vibration of oxygen-containing species such as Pt– O or Pt– OH at the surface. With further positive moving of the electrode potential to 0.9 V, this band blue shifts to ca. 570 cm − 1 and develops into a very broad and strong band which may be overlapped by several bands contributed by various kinds of platinum oxides. It should be pointed out that, on the Pt electrode with R: 200, no band related to platinum oxide can be observed at 0.7 V in the methanol solution. It indicates that the oxidation of platinum itself is suppressed in the presence of methanol. There are several possible reasons that may jointly lead to the suppression of the platinum oxide formation: First, the adsorption of oxygen species is hindered by the adsorption of CO so that the oxidation of Pt electrode is retarded. Secondly, protons formed during methanol electrooxidation result in the decrease of pH near the electrode surface which even alters the HSO− 4 / SO24 − equilibrium, especially in the case of the Pt with high surface roughness. Consequently, the concentration of OH− in the vicinity of the surface decreases dramatically. Thirdly, there is sufficient methanol near the surface, indicated by the 1018 cm − 1 band that can still be observed at potentials higher than 0.7 V, thus the methanol electrooxidation, as a part of the whole oxidation reaction, competed with the platinum oxidation. The capability to detect various Raman bands is certainly very helpful to investigate the nature of sur-

Fig. 7. Potential-dependent surface Raman spectra of a roughened Pt in 1.0 M CH3OH+1.0 M H2SO4 (a) and 1.0 M H2SO4 without methanol (b). The surface roughness factor is ca. 200. Excitation line: 632.8 nm.

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Fig. 8. Dependence of static state current Istat and band intensity ICO for the stretching vibration of the linearly adsorbed CO on the electrode potential in 1.0 M CH3OH+ 1.0 M H2SO4. The roughness factor of the electrode used is about (a) 200, (b) 50.

face bonding, the coordination of surface species and the occurrence of surface oxidation. This advantage has been taken to study the methanol dissociate adsorption and pure CO adsorption. Some differences have been revealed in the nature of CO adsorption of these two systems, which will be presented elsewhere [61].

3.6. In6estigating the oxidation mechanism As has been introduced in the first part, the molecular level understanding of the mechanism of methanol electrooxidation remains elusive. In particular, whether the formation of CO is a necessary intermediate (serial way) or a by-product (parallel way) is still controver-

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sial [6 – 9]. Willsau et al. found in their DEMS studies that the entire oxidation of methanol took place only after the adsorbed CO was oxidized on smooth Pt electrodes, which was interpreted in favor of the serial mechanism involving adsorbed CO [6]. On the other hand, many researchers held different opinions. Sriramulu et al. observed non-zero CO2 yields on both Pt(100) and Pt (111) surfaces at potentials where adsorbed CO was not oxidized [66]. In one of such experiments, 13CO was allowed to adsorb on the Pt electrode; thereafter, the solution was exchanged with that containing 13C-labeled methanol [7]. The IR spectra showed no change in the frequency of the C – O stretch during the electrooxidation, suggesting that CO acts only as a surface stable product and not a necessary intermediate [7]. These reports provide strong evidence for the existence of the parallel mechanism. While most of these reports concentrated on the study of smooth or single crystal Pt electrodes, it is necessary to pay attention on this aspect for roughened Pt electrodes that are more close to practical catalysts. As the first effort, we simply compare the Raman intensity of CO and the oxidation current of methanol from rough electrodes, which could provide insights into the mechanism for methanol oxidation. Fig. 8 gives the dependence of the steady state current and the band intensity of CO stretching vibration of linearly adsorbed CO on the electrode potential in 1.0 M CH3OH+1.0 M H2SO4. On both roughened Pt electrodes, the oxidation current of methanol increased rapidly with the oxidation of CO at a similar potential around 0.3 V, and reached its maximum at around 0.50 V. At more positive potentials the oxidation current decreased rapidly. The onset oxidation potential is independent of the surface roughness, inferring that the oxidation of the adsorbed CO occurs at the terrace instead of kinks and edges. The current kept growing with the oxidation of surface CO that is deduced from the relevant Raman intensity. For the highly rough surface, the oxidation current almost reached the maximum when the band intensity of CO decreased to 30% of the maximum. While the Raman intensity dropped more sharply for the mild rough surface, suggesting that the adsorbed CO at the surface of less defect sites is relatively easy to be completely oxidized. Nevertheless, Fig. 8 concludes quite confidently that the severe methanol oxidation can still go on with the existence of CO on the rough surface. The oxidation of methanol can go through the direct oxidation of methanol through the reactive intermediate and the oxidation of the poisoning intermediate of CO. This is a convincing evidence for the parallel mechanism. According to the parallel mechanism, it is essential to get the information from not only the poisoning intermediates but also the reactive intermediates. Unfortunately, up to now, we are still not able to get the

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exact information of the reactive intermediates probably because of their instability or weak interaction with Pt electrodes. Therefore, it will be possible to obtain the Raman signal of these species after carefully designing the experimental procedure. More systematic work has been carried out, including the improvement of the experimental condition and theoretical calculation, in order to investigate the mechanism in more detail.

Acknowledgements

4. Concluding remarks

References

Through this preliminary study on methanol electrooxidation on rough platinum electrodes, we have shown the strategy of developing Raman spectroscopy, especially confocal Raman microscopy, into a versatile tool in electrocatalysis. We have also demonstrated that the availability of confocal Raman microscopy offers intriguing opportunities for investigating catalytic reaction in electrochemical environments. From methodological point of view, this technique has an important advantage in obtaining the informative spectra during the surface reaction in high faradaic current, which is hard to be obtained by the other spectroscopic methods using the thin-layer cell due to the severe ohmic drop and gas interference. With the use of confocal optical setup, it makes the spectroscopic investigation of the higher concentration electrolyte and the system with thick solution layer possible. The ease of studying the surface bonding and investigating highly roughened surfaces with dark color may provide a way to bridge the gap between the systems of fundamental research and the technical application of practical importance. In the present study the emphasis was placed on the methodology. It would be more essential if Raman spectroscopy could challenge itself to some key issues and make definite conclusions on the methanol oxidation. Several approaches have been made in our lab, including the detailed characterization of the reaction intermediates, quantitatively study of the proportions between different surface species, and identification of the surface oxygen source. Furthermore, the use of the optical fiber technique in Raman spectroscopy has been considered. It may develop into a flexible method in the remote detection and control. The fiber Raman technique can be used as the sensor during oxidation process of methanol on those practical electrocatalysis, and it will be possible to eventually investigate the vigorous reaction taking place on the catalysis in real fuel cell systems. Therefore, it is reasonable to be optimistic that Raman spectroscopy will be developed, into a complementary tool to many other techniques that have been extensively used, for studying electrocatalysis involving complex reaction.

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