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Vibrational spectroscopy on platinum single-crystal electrodes
341
J. Eiectroanal. Chem., 239 (1988) 347-360 Elsevier Sequoia S.A., Lausanne - Printed
VIBRATIONAL ELECTRODES
in The Netherlands
SPECTROSCOPY
ON PLATINUM
SINGLE-CRYSTAL
PART I. IN-SITU INFRARED SPECTROSCOPIC STUDIES OF THE ADSORPTION AND OXIDATION OF CO ON Pt (111) IN SULPHURIC
N. FURUYA Department
and S. MOT00
of Apphed Chemrstry, Faculty of Engineering,
K. KUNIMATSU
Yamanashr
Vmverslty.
Kofu 400 (Japan)
l
Research Instrtute for Cata!vsrs. Hokkaido (Received
ACID
Vnrversrty, Sapporo 060 (Japan)
18th May 1987; in revised form 21st July 1987)
ABSTRACT
Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111). (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces. The adsorption and oxidation of CO on Pt (111) m 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.
INTRODUCTION
In situ infrared spectroscopic studies of electrocatalysis have increased rapidly since the early 1980s [l] but they have been limited mostly to polycrystalline electrodes of platinum-group metals, primarily due to the experimental difficulties in preparing clean and well-defined single-crystal electrode surfaces in contact with aqueous solutions. However, there has been much progress in the last several years in the preparation of clean and well-defined noble-metal single-crystal surfaces,
l
To whom correspondence
0022-0728/88/$03.50
should
be addressed.
0 1988 Elsevier Sequoia
S.A.
348
notably platinum [2-71, and experimental procedures to obtain clean and well-defined single-crystal surfaces in contact with electrolyte solutions without contamination of the surfaces by impurities from various sources are now established. The long dispute over the nature of the linear sweep voltammogram of hydrogen adsorption/desorption on Pt (111) in sulphuric and perchloric acid solutions first reported by Clavilier et al. [2,3] is now resolved as a result of the studies of various groups, notably Motoo and Furuya [4,5] who made a great effort to reproduce and develop further the method of Clavilier. Their results have finally been confirmed by Wagner and Ross [6] and more recently by Aberdam et al. [7], who reproduced the linear sweep voltammograms of hydrogen adsorption/desorption on Pt (111) in 0.1 M HClO,, reported by Clavilier [2,3] in 1980 and later by Motoo and Furuya [4,5] in 1984, starting with a platinum (111) crystal, cleaned and characterized by LEED and Auger in ultra high vacuum. The progress made in the last several years has made it quite feasible to conduct in situ infrared spectroscopic studies of electrochemical processes on platinum single-crystal electrodes, which can be prepared by melting platinum wires after the method of Clavilier [2,3]. Since the characterization of the single-crystal surfaces can be done by observing the linear sweep voltammograms of hydrogen adsorption/desorption in sulphuric or perchloric acid solutions without using ultra high vacuum apparatus, the practical problems in developing the vibrational spectroscopy on single-crystal electrodes are greatly reduced. However, one needs single-crystal surfaces with a substantial surface area to conduct infrared reflection measurements at high angles of incidence, which are necessary in order to improve the signal/noise ratio. Platinum single crystals of 2-3 mm diameter as employed in previous studies [2-51 are not suitable for infrared measurements without special focusing or other elaborate instrumental procedures. We report here the results of our attempts to make larger platinum single-crystal electrodes after the methods of Clavilier et al. [2,3] and Motoo and Furuya [4,5] for developing the in situ infrared spectroscopy on platinum single-crystal surfaces with (ill), (110) and (100) orientations. The adsorption and oxidation of CO on Pt (111) were studied as a model case and the effects of the adsorption potential and surface reconstruction on the nature and the oxidation mechanism of adsorbed CO are reported. EXPERIMENTAL
Platinum single-crystal beads of 5 mm diameter were prepared by melting platinum wires of 1 mm diameter with 4 N purity following the methods of Clavilier et al. [2,3] and Motoo and Furuya [4,5]. The crystals were oriented by the laser beam method and then cut to expose one of the planes of (ill), (110) and (loo), and the surfaces of the exposed planes were polished with a diamond paste of 3 pm and annealed at 1500 o C for 3 h in a gas + oxygen flame. For the electrochemical characterization of each surface, the single crystal was annealed at 1500°C for 5-10 s again in a hydrogen stream and quenched im-
349
-
?= Pt smgla
--q!+
crystal
/,
pt
Fig. 1. Schematic diagram of a platinum smgle-crystal electrode mounted in its all-glass holder utilizing syringes of different sizes.
mediately in water saturated with hydrogen. The structure of the electrode mounted in its all-glass holder is shown schematically in Fig. 1. The electrode was transferred to an electrochemical cell, while protecting the single-crystal surface with a drop of water. The structure of the cell is shown in Fig. 2. Measurements of the linear sweep voltammograms of hydrogen adsorption/desorption were conducted by the dipping method [2-51. After the electrochemical characterization of the surface, the electrode was transferred again, with similar precautions with a drop of the electrolyte solution, to an infrared cell previously filled with deaerated 0.5 M sulphuric acid. The infrared cell designed specifically for the single-crystal experiments was fitted with a CaF, prism window with 65 o bevelled edges and had a double syringe structure as shown in Fig. 3. The tip of the outer syringe plunger (B) can extend to the bottom of the single-crystal electrode (A), which is then pushed “gently” against the CaF, window by virtue of the presence of a flexible platinum spiral (C). The structure of the infrared cell enables one to conduct infrared spectroscopic measurements on the platinum-single crystal electrode but the electrochemical behaviour of the singlecrystal surface cannot be checked separately from that of the other platinum surfaces of the platinum single-crystal bead and wires, etc. exposed to the solution.
Fig. 2. Electrochemical cell by the dipping method.
for studying electrochemical processes
on platinum
single-crystal
electrodes
350
Fig. 3. Electrochemical infrared cell for in situ spectroscopic studies on platinum single-crystal surfaces. (A) Platinum single crystal; (B) tip of the syringe plunger used to push up the single crystal against the CaF, window: (C) platinum spiral; (D) platinum net counter-electrode; (E) Luggin capillary.
The adsorption of CO on Pt (111) was conducted at two initial adsorption potentials; 0.05 and 0.60 V (vs. RHE). The potential of the electrode had been set in 0.5 M sulphuric acid before CO gas was introduced into the cell. The infrared spectroscopic measurements were conducted using the polarization modulation method utilizing a rotating polarizer operating at 80 Hz. The angle of incidence was interface. The spectra were referred to the ca. 75” at the electrode/solution single-crystal surface free from the adsorbed CO layer, at 0.9 V (vs. RHE), in order to subtract the background signal coming mainly from the polarization characteristics of the grating. The electrolyte solution, 0.5 M sulphuric acid, was prepared from Merck Suprapur H,SO,. Water was purified by distilling Millipore water three times, the first time from alkaline permanganate solution. It was then pre-electrolysed overnight at 50 mA with platinized platinum electrodes. All glassware including the single crystal itself was cleaned by a hot mixture of freshly prepared H,SO, + HNO, (1 : 1) and rinsed thoroughly with Millipore water followed by the triply distilled Millipore water. The nitrogen gas (5 N purity) was used throughout the study, with a liquid nitrogen trap before the cell. Hydrogen gas was purified by a Pd/Ag hydrogen diffusion purifier. The reference electrode was Hg/Hg,SO,, but all potentials hereafter will be referred to the RHE scale. All the measurements were conducted at room temperature. RESULTS
Figure 4 shows the linear sweep voltammograms tion on the three low-index platinum single-crystal spectroscopic measurements. The general features single-crystal plane are in excellent agreement with smaller single-crystal planes [2-51. Since infrared spectroscopic measurements may cleanliness of the system by monitoring the stability
of hydrogen adsorption/desorpplanes prepared for the infrared of the voltammogram for each the previous results reported on take a few hours, we checked the of the voltammogram. Figure 5
351
12v (a)
40
t
(b)
+F+ E/V(RHE)
-2
Fig. 4. Linear sweep voltammograms of hydrogen adsorptlon/desorption observed on Pt (111). (110) and (100) m 0.5 M H,S04. The electrode diameter was ca. 5 mm and the sweep rate was 50 mV/s. Fig. 5. Change of the linear sweep between 0.05 and 0.75 V. (a) (-
voltammogmm on Pt (110) wth time under ) f = 0; (- - -) t =1 h. (b) f= 3 h.
continuous
cycling
shows the change of the voltammogram on Pt (110) with time when the potential was cycled repeatedly between 0.05 and 0.75 V for 3 h. It can be seen that the peak height decreases only slightly after 1 h and that it decreases by ca. 20% after 3 h. The essential features of the voltammogram are still preserved, however, and the total hydrogen charge between 0.05 and 0.40 V has decreased by only ca. 7%. The results demonstrated in Fig. 5 confirm that the clean single-crystal surfaces are essentially preserved during the infrared spectroscopic measurements. Figures 6a and 6b show a series of infrared spectra of CO adsorbed initially at 0.60 V, outside the hydrogen region on Pt (111) in 0.5 M H,SO,, as a function of the electrode potential. Figure 6a shows that only linear CO(a) is detected, giving rise to an intense C-O stretching band between 2050 and 2100 cm-‘. Fig. 6b demonstrates that the linear CO(a) layer is oxidized off the surface in a narrow potential range higher than 0.7 V and the surface becomes practically free from CO around 0.85 V. However, when CO adsorbs initially at 0.05 V starting from CO-free solution as described in the Experimental section, the behaviour of the adsorbed CO layer is somewhat different. Although only linear CO(a) is detected as shown by an EMIRS spectrum in Fig. 7, Fig. 8 shows the change of the infrared spectrum of the linear
352 (a)
(b) E/V(RHE) E/V(RHE)
I
2150 Wavenumber
/cm-’
2100
2050
Wavenumber
/cm-’
2000
Fig. 6. Infrared reflection absorption spectra of CO adsorbed on Pt (111) initially at 0.60 V m 0.5 M H,SO,. (a) Shift of the C-O stretching band wth potential before the onset of the electrooxidation of the adsorbed CO layer; (b) the change of the spectrum in the course of the electrooxidation at potentials higher than 0.7 V.
CO(a) as a function of the electrode potential. It can clearly be seen that oxidation of the adsorbed CO layer starts around 0.55 V, which is ca. 0.15 V lower compared to the oxidation of the CO(a) layer adsorbed initially at 0.60 V. From Figs. 6 and 8 we determined the potential dependence of the integrated band intensity and C-O stretching frequency, “c-O, as shown in Fig. 9. Figure 9a demonstrates the marked difference of the behaviour of the linear CO(a) layers adsorbed initially at different potentials, 0.05 and 0.60 V, respectively. It should be noted that a similar result was observed on a platinum polycrystalline electrode when CO was adsorbed initially at different potentials in the hydrogen region and double-layer region, 0.05 and 0.40 V, respectively [g-lo]. Figure 9b shows that v~_~ increases linearly with the potential at a rate of 28 cm-‘/V and that the sharp coverage decrease during the electrooxidation of the CO(a) layer does not affect the linearity for Eads = 0.60 V. However, for Eads = 0.05 thus deviating from V, vc-o decreases slightly at the onset of the electrooxidation, linearity but again it increases to regain linearity. This is quite a unique result found on this single-crystal electrode. When CO adsorbs at 0.05 V on a polycrystalline platinum electrode, vc_ o starts to deviate from linearity at the onset of electrooxidation and decreases continuously to lower wavenumbers with further decrease of the surface coverage of the adsorbed CO [8,9].
I
I
1 2100
I 2000
Waverwnber/cm-’
Fig. 7. EMIRS spectrum of CO adsorbed lmtially modulated between 0.05 and 0.30 V at 11 Hz. Fig. 8. Potential dependence Pt (111) electrode.
of the infrared
at 0.05 V on a Pt (111) electrode.
spectrum
of the linear CO adsorbed
The potential
initially
was
at 0.05 V on a
Finally, we investigated the influence of surface reconstruction on the nature and behaviour of the adsorbed CO layer. It has been shown that the Pt (111) surface structure is easily changed after potential excursion into the oxide formation region above 1.2 V and loses its characteristic voltammetric features for hydrogen adsorption/desorption [2,3,6,7]. Figure 10 shows the voltammograms observed in 0.5 M H,SO, before, (ill),, and after, (ill),, the restructuring of the (111) surface. Pt (111) was restructured by sweeping the potential between 0.05 and 1.45 V five times at 50 mV/s. This treatment of the (111) surface gives rise to the voltammogram designated by (ill),, which did not change appreciably with further cycling of the potential in the same potential range. One can see the marked difference between the voltammograms (ill), and (ill), but the total hydrogen charge obtained by integrating the current between 0.05 and 0.75 V is almost unchanged: the charge for (ill), is ca. 3% larger than that for (ill),. The true surface area calculated from the hydrogen charge remained practically unchanged despite the drastic change of the voltammogram.
354
0
IO
05 E /‘/(AHE)
Fig. 9. Potential dependence of (a) the integrated band intensity and (b) the C-O stretching the linear CO adsorbed on a Pt (111) electrode at 0.05 V (0) and 0.60 V (O), respectively.
frequency
of
Fig. 10. Linear sweep volt~mograms of a Pt (Ill) electrode in 0.5 M H,SO, before, (Ill),, and after, (ill),. potential cycling between 0.05 and 1.45 V five times at 50 mV/s. (------) First voltammogram of electrode oxidation starting from the (111 f N state.
Rg. Il. Companson of the EMIRS respectively, at 0.60 V.
spectra
of CO
adsorbed
on
Pt (1 II) N and
(1 II),
surfaces,
355
2150
2100 2050 Wavenumber /cm-’
2000
Comparison of the polarization modulated spectra of CO adsorbed on Pt (ill), and (ill), surfaces, respectively, at 0.60 V. The spectra were observed at the adsorptton potential (modulated: 0.1 - 0.6 V/RHE). Fig. 12.
However, we observed a significant change of the infrared spectrum of the linear CO(a) on the (ill), surface, as demonstrated by Figs. 11 and 12. Figure 11 compares the EMIRS spectra of CO adsorbed at 0.60 V on (ill), and (ill),, respectively, and we notice a large intensity increase and band shift to higher wavenumbers. The change is seen more clearly in the polarization modulated
I’
0
11
11
05’
1
’
1
IO
E/V(RHE)
Fig. 13. Potential dependence of (a) the Integrated infrared absorption Intensity and (b) the C-O stretching frequency of linear CO adsorbed on Pt (ill), at 0.05 V (0) and 0.60 V (0). respectwely.
356
spectra shown in Fig. 12. The peak height of the C-O stretching band almost doubles and the band position shifts to the higher wavenumber side by ca. 8 cm- ‘. This is a remarkable change when the surface area of the electrode has remained almost unchanged. However, despite the large increase of the spectral intensity and C-O stretching frequency, electrooxidation of the adsorbed CO layer takes place in almost the same potential range on both the (111) N and (111) n surfaces, as demonstrated by Figs. 13a and 9a. The intensity scale of Figs. 9a and 13a is the same, although its units are arbitrary. It can clearly be seen that the integrated intensity on (ill), is ca. 50% larger than that on (ill),. surface is shown in Fig. 13b. The potential dependence of r~,..~ on the (ill), The essential features are almost the same as those for the Pt (111) N surface, apart from the increase of “c-O by ca. 8 cm-’ and the increase of the slope of the pc_o vs. E linear line to 30 cm-‘/V compared with 28 cm-‘/V on the (ill), surface. DISCUSSION
Effect of the initial ahorption
potential
on the behaviour
of the adsorbed CO layer
The effect of the initial adsorption potential, where CO is adsorbed either in the hydrogen region or in the double-layer region, on Pt (111) in Figs. 9a and 13a has been observed previously on a platinum polycrystalline electrode by Kunimatsu et al. [X--10] using polarization modulated Fourier transformed infrared reflection adsorption spectroscopy (PM - FTIRRS). It was shown that significant amounts of bridged CO, ca. 10% of a monolayer, were produced when CO interacts with the platinum surface at 0.05 V and that the bridge CO plays an important role in determining the mechanism of the electrooxidation of the linear CO layer. On Pt (ill), however, bridged CO was not detected either by the polarization modulation method or by EMIRS for both of the initial adsorption potentials, which is in agreement with the electrochemical results of Motoo and Furuya [ll]. We checked the possibility of a change of the surface crystallographic orientation upon interaction of CO with the platinum surface at 0.05 V, as suggested in refs. 9 and 10. After establishing a saturated CO monolayer at 0.05 V, the solution was purged by nitrogen gas for 20 min and then the potential was changed linearly with time up to 1.15 V to oxidize the remaining CO layer on the (111) surface. The voltammogram of hydrogen adso~tion/deso~tion was checked immediately on the reverse scan from 1.15 to 0.05 V. Figure 14 compares the voltammogram thus observed with the one observed in 0.5 M sulphuric acid before the introduction of CO to the solution. We see no significant change in the voltammogram which is characteristic of the (111) surface. Now if we compare the integrated band intensities and C-O stretching frequencies of the CO layer established at 0.05 and 0.60 V, respectively, we notice a very small increase in the intensity as well as in vc._ o by ca. l-2 cm-’ for adsorption at 0.60 V. These data are seen in Figs. 9 and 13. This is usually an indication of a coverage increase of the linear CO, although it may be very small. The small
357
Fig. 14. Comparison of the linear sweep voltammograms of Pt (111) observed (1) and after adsorption of CO at 0.05 V and its electrooxidation (2).
initially
in 0.5 M H2S0,
coverage difference could be due to the difference in the number of either the vacant sites or the bridged CO molecules. Because vacant sites are needed for oxidation of the linear CO layer to adsorb oxygen-containing species, such as water or surface hydroxyl groups, the small difference of the surface coverage of the linear CO could be operating as the crucial factor in determining the rate of electrooxidation of the linear CO at a given potential. This point needs further investigation using other single-crystal surfaces with different crystallographic orientations, and the results will be reported in another paper. Oxidation
mechanism
of the adsorbed CO on Pt (I 11) and polyctystalline
Pt electrodes
As pointed out in the previous PM-FTIRRS studies of CO oxidation on a polycrystalline electrode by Kunimatsu et al. [8-lo], we can infer the mechanism of the electrooxidation of the adsorbed CO layer from the potential dependence of the C-O stretching band during the oxidation. For adsorption at a potential in the double-layer region, 0.60 V on Pt (111) in the present study, the potential dependence of v~_~ is not affected by the steep coverage decrease, as shown in Fig. 9. This suggests strongly that the oxidation proceeds from the edges of the CO islands keeping the vibrational interaction, dipole-dipole coupling, among the remaining CO molecules almost unchanged until very small coverages are attained. This was also the case on a polycrystalline platinum electrode in acids for CO adsorbed at 0.40 V [8-lo]. For CO adsorption at 0.05 V, however, the potential dependence of r+.o during the oxidation is markedly different on Pt (111) and polycrystalline Pt electrodes. The C-O stretching frequency decreases continuously once the oxidation starts on a polycrystalline Pt electrode, while on Pt (111) the frequency decreases slightly at the onset of oxidation, followed by an increase again to regain the linear shift of frequency with potential as shown in Figs. 9 and 13. The result on the polycrystalline electrode was interpreted in terms of a mechanism in which oxidation proceeds randomly in the adsorbed CO layer. The result demonstrated in Figs. 9 and 13 could be interpreted as follows: The oxidation is initiated at the vacant sites distributed randomly in the adsorbed layer, but as soon as the size of the domain of the vacant
358
sites increases, the CO molecules redistribute to form large islands. Further oxidation then proceeds at the edges of the islands. The reason why this is not the case for the polycrystalline Pt electrode may be that the surface is too heterogeneous, and the size of the domain of the vacant sites created by the initial oxidation of the bridged CO is too large for the redistribution to take place. Effect of surface reconstruction
on the nature of the adsorbed CO Iayer
The surface recons~uction of the Pt (111) surface demonstrated in the change of the volt~ogram of the hydrogen adso~tion/deso~tion in Fig. 10 gave rise to an intensity increase of ea. 50% and to an increase of the C-O stretching frequency of ea. 8 cm-‘. At the same time, it had no effect on the potential dependence of the electrooxidation. This is rather a surprising result in view of the almost constant surface area before and after the change of structure. The infrared absorption intensity at the band maximum was ca. 5%, as shown in Fig. 12, while the intensity was ca. 4% for the CO monolayer on the polycrystalline platinum electrode by PM-FTIRRS [8,9]. This is a great difference if we consider the band widths: ca. 19 studies at a in the present study and ca. 9 cm -’ in the PM-FTIRRS cm-’ polycrystalline platinum electrode. The integrated band intensity on the reconstructed (111) surface is therefore ca. 2.2 times larger than that on the polycrystalline Pt. If the higher infrared absorption intensity and higher C-O stretching frequency on (ill), could be attributed to a higher surface density of the linear CO than that on (ill),, it would shift the onset of electrooxidation to higher potentials. It is likely that there is a mechanism to give rise to the enhancement of the infrared absorption intensity on the (ill), surface but it is not clear at the moment whether the enhancement mechanism is correlated to the adsorbate overlayer structure or to the surface properties of the substrate Pt (ill),. Studies on other single-crystal planes in answering this question would certainly be interesting. On the other hand, it is not surprising that the catalytic property of the (ill), surface for CO oxidation is not appreciably different from a (Ill), surface, which has already been observed by Motoo and Furuya Ill]. The CO oxidation is not very st~cture-sensitive on platinum single-crystal electrodes as reported by Furuya and Motoo [12] and Lamy et al. 1131. Feasibility
of further
in situ vibrational
spectroscopic
studies on single-crystal
surfaces
In many cases, electrocatalytic processes are very often sensitive to the geometrical structure of the electrode surface. This was demonstrated clearly by Clavilier and co-workers [13,14] for the electrooxidation of formic acid on platinum single-crystal surfaces and also by Motoo and Furuya for the electrooxidation of formic acid [15] and methanol 1161 on platinum single-crystal surfaces. The importance of in situ vibrational spectroscopic methods to study the nature and behaviour of the adsorbed species involved in the electrocatalytic processes on single-crystal electrodes is well recognized.
359
Of the available in situ vibrational spectroscopic methods for studies at electrode/solution interfaces, however, Raman spectroscopy cannot cope with singlecrystal surfaces as it depends on a surface enhancement mechanism on roughened surfaces of silver, gold and copper. Recently, a similar surface enhancement phenomenon to SERS was discovered for infrared reflection absorption spectroscopy [17-191, although the enhancement factor is much smaller than SERS. The infrared enhancement also depends on the specific nature of the rough surfaces of silver and gold. In view of the present state of the in situ vibrational spectroscopies developed at electrode/solution interfaces, infrared reflection absorption spectroscopy at high angles of incidence utilizing various forms of modulation techniques, such as EMIRS and polarization modulation, is the only one that can cope with the single-crystal electrode surfaces. For infrared reflection measurements, however, one needs a single crystal of substantial surface area. With oriented commercial single crystals, difficulties will be encountered in the process of cleaning and characterization of the surface using ultra high vacuum apparatus equipped with surface analytical tools such as LEED and Auger. In addition, it is difficult to get the cleaned and characterized surface in contact with the electrolyte solution without contaminating the surface. It seems that this is the main reason why so many data apparently include effects of surface impu~ties. This is a common problem in many laboratories. For the development of surface vibrational spectroscopy on single-crystal surfaces, we always first confirm the cleanliness and surface crystallographic orientation by a suitable method such as hydrogen adsorption/desorption voltammetry, which is very sensitive to surface conta~nation as well as to the surface c~stallograp~~ orientation. On the basis of the results presented above, the success of the preparation and electrochemical characterization of large single crystals suitable for infrared reflection measurements is encouraging for further development of vibrational spectroscopy in studies of electrochemical processes on single-crystal electrode surfaces. Carbon monoxide is an easy reactant for study by infrared spectroscopy, but we believe that the study of hydrogen adsorption/desorption on platinum single-crystal surfaces will be feasible by EMIRs, which is the most suitable technique for that purpose. If this is accomplished, it will be clear that vibrational spectroscopy will contribute greatly to the understanding of the molecular processes at the electrode/solution interface.
REFERENCES
1 2 3 4 5 6
A. Bewick, K. Kummatsu, S. Pons and J.W. Russell, J. Electroanal. Chem., 160 (1984) 47. J. Clavilier. R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem.. 107 (1980) 205. J. Clavlher, J. Electroanal. Chem., 107 (1980) 211. S. Motoo and N. Furuya, J. Electroanal. Chem., 167 (1984) 309. S. Motoo and N. Furuya, J. Electroanal. Chem., 172 (1984) 339. F.T. Wagner and P.N. Ross, Jr., J. Electroanai. Chem., 150 (1983) 141.
360 7 8 9 10 11 12 13 14 15 16 17 18 19
D. Aberdam, R. Durand. R. Faure and F. El-Omar, Surf. Sci.. 171 (1986) 303. K. Kunimatsu, W.G. Golden and H. Seki, Langmuir, 1 (1985) 245. K. Kunimatsu, H. Seki, W.G. Golden, J.G. Gordon II and M.R. Philpott, Surf. Sci.. 158 K. Kunimatsu, H. Seki, W.G. Golden, M.R. Philpott and J.G. Gordon II, Langmmr, 2 S. Motoo and N. Furuya, Autumn Meeting of Electrochem. Sot. Jpn., Tokyo. 1982, AG205. N. Furuya and S. Motoo, Autumn Meeting of the Electrochem. Sot. Jpn., Tokyo, 1986, c113. C. Lamy, J.M. Leger. J. Clavdier and R. Parsons, J. Electroanal. Chem., 150 (1983) 71. J. Clavdier, R. Parsons, R. Durand, C. Lamy and J.M. Leger, J. Electroanal. Chem., 124 S. Motoo and N. Furuya, J. Electroanal. Chem., 184 (1985) 303. N. Furuya and S. Motoo in ref. 12. Abstr. No. C118. A. Hartstein, J.R. Kirtley and J.T. Tsang, Phys. Rev. Lett., 45 (1980) 201. A. Hatta, Y. Chiba and W. Suetaka, Surf. Sci., 158 (1985) 616. M. Ohsawa, M. Kuramusu, A. Hatta and W. Suetaka, Surf. Sci., 175 (1986) L787.
(1985) 596. (1986) 464. Abstr. No. Abstr.
No.
(1981) 321.
J. Eiectroanal. Chem., 239 (1988) 347-360 Elsevier Sequoia S.A., Lausanne - Printed
VIBRATIONAL ELECTRODES
in The Netherlands
SPECTROSCOPY
ON PLATINUM
SINGLE-CRYSTAL
PART I. IN-SITU INFRARED SPECTROSCOPIC STUDIES OF THE ADSORPTION AND OXIDATION OF CO ON Pt (111) IN SULPHURIC
N. FURUYA Department
and S. MOT00
of Apphed Chemrstry, Faculty of Engineering,
K. KUNIMATSU
Yamanashr
Vmverslty.
Kofu 400 (Japan)
l
Research Instrtute for Cata!vsrs. Hokkaido (Received
ACID
Vnrversrty, Sapporo 060 (Japan)
18th May 1987; in revised form 21st July 1987)
ABSTRACT
Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111). (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces. The adsorption and oxidation of CO on Pt (111) m 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.
INTRODUCTION
In situ infrared spectroscopic studies of electrocatalysis have increased rapidly since the early 1980s [l] but they have been limited mostly to polycrystalline electrodes of platinum-group metals, primarily due to the experimental difficulties in preparing clean and well-defined single-crystal electrode surfaces in contact with aqueous solutions. However, there has been much progress in the last several years in the preparation of clean and well-defined noble-metal single-crystal surfaces,
l
To whom correspondence
0022-0728/88/$03.50
should
be addressed.
0 1988 Elsevier Sequoia
S.A.
348
notably platinum [2-71, and experimental procedures to obtain clean and well-defined single-crystal surfaces in contact with electrolyte solutions without contamination of the surfaces by impurities from various sources are now established. The long dispute over the nature of the linear sweep voltammogram of hydrogen adsorption/desorption on Pt (111) in sulphuric and perchloric acid solutions first reported by Clavilier et al. [2,3] is now resolved as a result of the studies of various groups, notably Motoo and Furuya [4,5] who made a great effort to reproduce and develop further the method of Clavilier. Their results have finally been confirmed by Wagner and Ross [6] and more recently by Aberdam et al. [7], who reproduced the linear sweep voltammograms of hydrogen adsorption/desorption on Pt (111) in 0.1 M HClO,, reported by Clavilier [2,3] in 1980 and later by Motoo and Furuya [4,5] in 1984, starting with a platinum (111) crystal, cleaned and characterized by LEED and Auger in ultra high vacuum. The progress made in the last several years has made it quite feasible to conduct in situ infrared spectroscopic studies of electrochemical processes on platinum single-crystal electrodes, which can be prepared by melting platinum wires after the method of Clavilier [2,3]. Since the characterization of the single-crystal surfaces can be done by observing the linear sweep voltammograms of hydrogen adsorption/desorption in sulphuric or perchloric acid solutions without using ultra high vacuum apparatus, the practical problems in developing the vibrational spectroscopy on single-crystal electrodes are greatly reduced. However, one needs single-crystal surfaces with a substantial surface area to conduct infrared reflection measurements at high angles of incidence, which are necessary in order to improve the signal/noise ratio. Platinum single crystals of 2-3 mm diameter as employed in previous studies [2-51 are not suitable for infrared measurements without special focusing or other elaborate instrumental procedures. We report here the results of our attempts to make larger platinum single-crystal electrodes after the methods of Clavilier et al. [2,3] and Motoo and Furuya [4,5] for developing the in situ infrared spectroscopy on platinum single-crystal surfaces with (ill), (110) and (100) orientations. The adsorption and oxidation of CO on Pt (111) were studied as a model case and the effects of the adsorption potential and surface reconstruction on the nature and the oxidation mechanism of adsorbed CO are reported. EXPERIMENTAL
Platinum single-crystal beads of 5 mm diameter were prepared by melting platinum wires of 1 mm diameter with 4 N purity following the methods of Clavilier et al. [2,3] and Motoo and Furuya [4,5]. The crystals were oriented by the laser beam method and then cut to expose one of the planes of (ill), (110) and (loo), and the surfaces of the exposed planes were polished with a diamond paste of 3 pm and annealed at 1500 o C for 3 h in a gas + oxygen flame. For the electrochemical characterization of each surface, the single crystal was annealed at 1500°C for 5-10 s again in a hydrogen stream and quenched im-
349
-
?= Pt smgla
--q!+
crystal
/,
pt
Fig. 1. Schematic diagram of a platinum smgle-crystal electrode mounted in its all-glass holder utilizing syringes of different sizes.
mediately in water saturated with hydrogen. The structure of the electrode mounted in its all-glass holder is shown schematically in Fig. 1. The electrode was transferred to an electrochemical cell, while protecting the single-crystal surface with a drop of water. The structure of the cell is shown in Fig. 2. Measurements of the linear sweep voltammograms of hydrogen adsorption/desorption were conducted by the dipping method [2-51. After the electrochemical characterization of the surface, the electrode was transferred again, with similar precautions with a drop of the electrolyte solution, to an infrared cell previously filled with deaerated 0.5 M sulphuric acid. The infrared cell designed specifically for the single-crystal experiments was fitted with a CaF, prism window with 65 o bevelled edges and had a double syringe structure as shown in Fig. 3. The tip of the outer syringe plunger (B) can extend to the bottom of the single-crystal electrode (A), which is then pushed “gently” against the CaF, window by virtue of the presence of a flexible platinum spiral (C). The structure of the infrared cell enables one to conduct infrared spectroscopic measurements on the platinum-single crystal electrode but the electrochemical behaviour of the singlecrystal surface cannot be checked separately from that of the other platinum surfaces of the platinum single-crystal bead and wires, etc. exposed to the solution.
Fig. 2. Electrochemical cell by the dipping method.
for studying electrochemical processes
on platinum
single-crystal
electrodes
350
Fig. 3. Electrochemical infrared cell for in situ spectroscopic studies on platinum single-crystal surfaces. (A) Platinum single crystal; (B) tip of the syringe plunger used to push up the single crystal against the CaF, window: (C) platinum spiral; (D) platinum net counter-electrode; (E) Luggin capillary.
The adsorption of CO on Pt (111) was conducted at two initial adsorption potentials; 0.05 and 0.60 V (vs. RHE). The potential of the electrode had been set in 0.5 M sulphuric acid before CO gas was introduced into the cell. The infrared spectroscopic measurements were conducted using the polarization modulation method utilizing a rotating polarizer operating at 80 Hz. The angle of incidence was interface. The spectra were referred to the ca. 75” at the electrode/solution single-crystal surface free from the adsorbed CO layer, at 0.9 V (vs. RHE), in order to subtract the background signal coming mainly from the polarization characteristics of the grating. The electrolyte solution, 0.5 M sulphuric acid, was prepared from Merck Suprapur H,SO,. Water was purified by distilling Millipore water three times, the first time from alkaline permanganate solution. It was then pre-electrolysed overnight at 50 mA with platinized platinum electrodes. All glassware including the single crystal itself was cleaned by a hot mixture of freshly prepared H,SO, + HNO, (1 : 1) and rinsed thoroughly with Millipore water followed by the triply distilled Millipore water. The nitrogen gas (5 N purity) was used throughout the study, with a liquid nitrogen trap before the cell. Hydrogen gas was purified by a Pd/Ag hydrogen diffusion purifier. The reference electrode was Hg/Hg,SO,, but all potentials hereafter will be referred to the RHE scale. All the measurements were conducted at room temperature. RESULTS
Figure 4 shows the linear sweep voltammograms tion on the three low-index platinum single-crystal spectroscopic measurements. The general features single-crystal plane are in excellent agreement with smaller single-crystal planes [2-51. Since infrared spectroscopic measurements may cleanliness of the system by monitoring the stability
of hydrogen adsorption/desorpplanes prepared for the infrared of the voltammogram for each the previous results reported on take a few hours, we checked the of the voltammogram. Figure 5
351
12v (a)
40
t
(b)
+F+ E/V(RHE)
-2
Fig. 4. Linear sweep voltammograms of hydrogen adsorptlon/desorption observed on Pt (111). (110) and (100) m 0.5 M H,S04. The electrode diameter was ca. 5 mm and the sweep rate was 50 mV/s. Fig. 5. Change of the linear sweep between 0.05 and 0.75 V. (a) (-
voltammogmm on Pt (110) wth time under ) f = 0; (- - -) t =1 h. (b) f= 3 h.
continuous
cycling
shows the change of the voltammogram on Pt (110) with time when the potential was cycled repeatedly between 0.05 and 0.75 V for 3 h. It can be seen that the peak height decreases only slightly after 1 h and that it decreases by ca. 20% after 3 h. The essential features of the voltammogram are still preserved, however, and the total hydrogen charge between 0.05 and 0.40 V has decreased by only ca. 7%. The results demonstrated in Fig. 5 confirm that the clean single-crystal surfaces are essentially preserved during the infrared spectroscopic measurements. Figures 6a and 6b show a series of infrared spectra of CO adsorbed initially at 0.60 V, outside the hydrogen region on Pt (111) in 0.5 M H,SO,, as a function of the electrode potential. Figure 6a shows that only linear CO(a) is detected, giving rise to an intense C-O stretching band between 2050 and 2100 cm-‘. Fig. 6b demonstrates that the linear CO(a) layer is oxidized off the surface in a narrow potential range higher than 0.7 V and the surface becomes practically free from CO around 0.85 V. However, when CO adsorbs initially at 0.05 V starting from CO-free solution as described in the Experimental section, the behaviour of the adsorbed CO layer is somewhat different. Although only linear CO(a) is detected as shown by an EMIRS spectrum in Fig. 7, Fig. 8 shows the change of the infrared spectrum of the linear
352 (a)
(b) E/V(RHE) E/V(RHE)
I
2150 Wavenumber
/cm-’
2100
2050
Wavenumber
/cm-’
2000
Fig. 6. Infrared reflection absorption spectra of CO adsorbed on Pt (111) initially at 0.60 V m 0.5 M H,SO,. (a) Shift of the C-O stretching band wth potential before the onset of the electrooxidation of the adsorbed CO layer; (b) the change of the spectrum in the course of the electrooxidation at potentials higher than 0.7 V.
CO(a) as a function of the electrode potential. It can clearly be seen that oxidation of the adsorbed CO layer starts around 0.55 V, which is ca. 0.15 V lower compared to the oxidation of the CO(a) layer adsorbed initially at 0.60 V. From Figs. 6 and 8 we determined the potential dependence of the integrated band intensity and C-O stretching frequency, “c-O, as shown in Fig. 9. Figure 9a demonstrates the marked difference of the behaviour of the linear CO(a) layers adsorbed initially at different potentials, 0.05 and 0.60 V, respectively. It should be noted that a similar result was observed on a platinum polycrystalline electrode when CO was adsorbed initially at different potentials in the hydrogen region and double-layer region, 0.05 and 0.40 V, respectively [g-lo]. Figure 9b shows that v~_~ increases linearly with the potential at a rate of 28 cm-‘/V and that the sharp coverage decrease during the electrooxidation of the CO(a) layer does not affect the linearity for Eads = 0.60 V. However, for Eads = 0.05 thus deviating from V, vc-o decreases slightly at the onset of the electrooxidation, linearity but again it increases to regain linearity. This is quite a unique result found on this single-crystal electrode. When CO adsorbs at 0.05 V on a polycrystalline platinum electrode, vc_ o starts to deviate from linearity at the onset of electrooxidation and decreases continuously to lower wavenumbers with further decrease of the surface coverage of the adsorbed CO [8,9].
I
I
1 2100
I 2000
Waverwnber/cm-’
Fig. 7. EMIRS spectrum of CO adsorbed lmtially modulated between 0.05 and 0.30 V at 11 Hz. Fig. 8. Potential dependence Pt (111) electrode.
of the infrared
at 0.05 V on a Pt (111) electrode.
spectrum
of the linear CO adsorbed
The potential
initially
was
at 0.05 V on a
Finally, we investigated the influence of surface reconstruction on the nature and behaviour of the adsorbed CO layer. It has been shown that the Pt (111) surface structure is easily changed after potential excursion into the oxide formation region above 1.2 V and loses its characteristic voltammetric features for hydrogen adsorption/desorption [2,3,6,7]. Figure 10 shows the voltammograms observed in 0.5 M H,SO, before, (ill),, and after, (ill),, the restructuring of the (111) surface. Pt (111) was restructured by sweeping the potential between 0.05 and 1.45 V five times at 50 mV/s. This treatment of the (111) surface gives rise to the voltammogram designated by (ill),, which did not change appreciably with further cycling of the potential in the same potential range. One can see the marked difference between the voltammograms (ill), and (ill), but the total hydrogen charge obtained by integrating the current between 0.05 and 0.75 V is almost unchanged: the charge for (ill), is ca. 3% larger than that for (ill),. The true surface area calculated from the hydrogen charge remained practically unchanged despite the drastic change of the voltammogram.
354
0
IO
05 E /‘/(AHE)
Fig. 9. Potential dependence of (a) the integrated band intensity and (b) the C-O stretching the linear CO adsorbed on a Pt (111) electrode at 0.05 V (0) and 0.60 V (O), respectively.
frequency
of
Fig. 10. Linear sweep volt~mograms of a Pt (Ill) electrode in 0.5 M H,SO, before, (Ill),, and after, (ill),. potential cycling between 0.05 and 1.45 V five times at 50 mV/s. (------) First voltammogram of electrode oxidation starting from the (111 f N state.
Rg. Il. Companson of the EMIRS respectively, at 0.60 V.
spectra
of CO
adsorbed
on
Pt (1 II) N and
(1 II),
surfaces,
355
2150
2100 2050 Wavenumber /cm-’
2000
Comparison of the polarization modulated spectra of CO adsorbed on Pt (ill), and (ill), surfaces, respectively, at 0.60 V. The spectra were observed at the adsorptton potential (modulated: 0.1 - 0.6 V/RHE). Fig. 12.
However, we observed a significant change of the infrared spectrum of the linear CO(a) on the (ill), surface, as demonstrated by Figs. 11 and 12. Figure 11 compares the EMIRS spectra of CO adsorbed at 0.60 V on (ill), and (ill),, respectively, and we notice a large intensity increase and band shift to higher wavenumbers. The change is seen more clearly in the polarization modulated
I’
0
11
11
05’
1
’
1
IO
E/V(RHE)
Fig. 13. Potential dependence of (a) the Integrated infrared absorption Intensity and (b) the C-O stretching frequency of linear CO adsorbed on Pt (ill), at 0.05 V (0) and 0.60 V (0). respectwely.
356
spectra shown in Fig. 12. The peak height of the C-O stretching band almost doubles and the band position shifts to the higher wavenumber side by ca. 8 cm- ‘. This is a remarkable change when the surface area of the electrode has remained almost unchanged. However, despite the large increase of the spectral intensity and C-O stretching frequency, electrooxidation of the adsorbed CO layer takes place in almost the same potential range on both the (111) N and (111) n surfaces, as demonstrated by Figs. 13a and 9a. The intensity scale of Figs. 9a and 13a is the same, although its units are arbitrary. It can clearly be seen that the integrated intensity on (ill), is ca. 50% larger than that on (ill),. surface is shown in Fig. 13b. The potential dependence of r~,..~ on the (ill), The essential features are almost the same as those for the Pt (111) N surface, apart from the increase of “c-O by ca. 8 cm-’ and the increase of the slope of the pc_o vs. E linear line to 30 cm-‘/V compared with 28 cm-‘/V on the (ill), surface. DISCUSSION
Effect of the initial ahorption
potential
on the behaviour
of the adsorbed CO layer
The effect of the initial adsorption potential, where CO is adsorbed either in the hydrogen region or in the double-layer region, on Pt (111) in Figs. 9a and 13a has been observed previously on a platinum polycrystalline electrode by Kunimatsu et al. [X--10] using polarization modulated Fourier transformed infrared reflection adsorption spectroscopy (PM - FTIRRS). It was shown that significant amounts of bridged CO, ca. 10% of a monolayer, were produced when CO interacts with the platinum surface at 0.05 V and that the bridge CO plays an important role in determining the mechanism of the electrooxidation of the linear CO layer. On Pt (ill), however, bridged CO was not detected either by the polarization modulation method or by EMIRS for both of the initial adsorption potentials, which is in agreement with the electrochemical results of Motoo and Furuya [ll]. We checked the possibility of a change of the surface crystallographic orientation upon interaction of CO with the platinum surface at 0.05 V, as suggested in refs. 9 and 10. After establishing a saturated CO monolayer at 0.05 V, the solution was purged by nitrogen gas for 20 min and then the potential was changed linearly with time up to 1.15 V to oxidize the remaining CO layer on the (111) surface. The voltammogram of hydrogen adso~tion/deso~tion was checked immediately on the reverse scan from 1.15 to 0.05 V. Figure 14 compares the voltammogram thus observed with the one observed in 0.5 M sulphuric acid before the introduction of CO to the solution. We see no significant change in the voltammogram which is characteristic of the (111) surface. Now if we compare the integrated band intensities and C-O stretching frequencies of the CO layer established at 0.05 and 0.60 V, respectively, we notice a very small increase in the intensity as well as in vc._ o by ca. l-2 cm-’ for adsorption at 0.60 V. These data are seen in Figs. 9 and 13. This is usually an indication of a coverage increase of the linear CO, although it may be very small. The small
357
Fig. 14. Comparison of the linear sweep voltammograms of Pt (111) observed (1) and after adsorption of CO at 0.05 V and its electrooxidation (2).
initially
in 0.5 M H2S0,
coverage difference could be due to the difference in the number of either the vacant sites or the bridged CO molecules. Because vacant sites are needed for oxidation of the linear CO layer to adsorb oxygen-containing species, such as water or surface hydroxyl groups, the small difference of the surface coverage of the linear CO could be operating as the crucial factor in determining the rate of electrooxidation of the linear CO at a given potential. This point needs further investigation using other single-crystal surfaces with different crystallographic orientations, and the results will be reported in another paper. Oxidation
mechanism
of the adsorbed CO on Pt (I 11) and polyctystalline
Pt electrodes
As pointed out in the previous PM-FTIRRS studies of CO oxidation on a polycrystalline electrode by Kunimatsu et al. [8-lo], we can infer the mechanism of the electrooxidation of the adsorbed CO layer from the potential dependence of the C-O stretching band during the oxidation. For adsorption at a potential in the double-layer region, 0.60 V on Pt (111) in the present study, the potential dependence of v~_~ is not affected by the steep coverage decrease, as shown in Fig. 9. This suggests strongly that the oxidation proceeds from the edges of the CO islands keeping the vibrational interaction, dipole-dipole coupling, among the remaining CO molecules almost unchanged until very small coverages are attained. This was also the case on a polycrystalline platinum electrode in acids for CO adsorbed at 0.40 V [8-lo]. For CO adsorption at 0.05 V, however, the potential dependence of r+.o during the oxidation is markedly different on Pt (111) and polycrystalline Pt electrodes. The C-O stretching frequency decreases continuously once the oxidation starts on a polycrystalline Pt electrode, while on Pt (111) the frequency decreases slightly at the onset of oxidation, followed by an increase again to regain the linear shift of frequency with potential as shown in Figs. 9 and 13. The result on the polycrystalline electrode was interpreted in terms of a mechanism in which oxidation proceeds randomly in the adsorbed CO layer. The result demonstrated in Figs. 9 and 13 could be interpreted as follows: The oxidation is initiated at the vacant sites distributed randomly in the adsorbed layer, but as soon as the size of the domain of the vacant
358
sites increases, the CO molecules redistribute to form large islands. Further oxidation then proceeds at the edges of the islands. The reason why this is not the case for the polycrystalline Pt electrode may be that the surface is too heterogeneous, and the size of the domain of the vacant sites created by the initial oxidation of the bridged CO is too large for the redistribution to take place. Effect of surface reconstruction
on the nature of the adsorbed CO Iayer
The surface recons~uction of the Pt (111) surface demonstrated in the change of the volt~ogram of the hydrogen adso~tion/deso~tion in Fig. 10 gave rise to an intensity increase of ea. 50% and to an increase of the C-O stretching frequency of ea. 8 cm-‘. At the same time, it had no effect on the potential dependence of the electrooxidation. This is rather a surprising result in view of the almost constant surface area before and after the change of structure. The infrared absorption intensity at the band maximum was ca. 5%, as shown in Fig. 12, while the intensity was ca. 4% for the CO monolayer on the polycrystalline platinum electrode by PM-FTIRRS [8,9]. This is a great difference if we consider the band widths: ca. 19 studies at a in the present study and ca. 9 cm -’ in the PM-FTIRRS cm-’ polycrystalline platinum electrode. The integrated band intensity on the reconstructed (111) surface is therefore ca. 2.2 times larger than that on the polycrystalline Pt. If the higher infrared absorption intensity and higher C-O stretching frequency on (ill), could be attributed to a higher surface density of the linear CO than that on (ill),, it would shift the onset of electrooxidation to higher potentials. It is likely that there is a mechanism to give rise to the enhancement of the infrared absorption intensity on the (ill), surface but it is not clear at the moment whether the enhancement mechanism is correlated to the adsorbate overlayer structure or to the surface properties of the substrate Pt (ill),. Studies on other single-crystal planes in answering this question would certainly be interesting. On the other hand, it is not surprising that the catalytic property of the (ill), surface for CO oxidation is not appreciably different from a (Ill), surface, which has already been observed by Motoo and Furuya Ill]. The CO oxidation is not very st~cture-sensitive on platinum single-crystal electrodes as reported by Furuya and Motoo [12] and Lamy et al. 1131. Feasibility
of further
in situ vibrational
spectroscopic
studies on single-crystal
surfaces
In many cases, electrocatalytic processes are very often sensitive to the geometrical structure of the electrode surface. This was demonstrated clearly by Clavilier and co-workers [13,14] for the electrooxidation of formic acid on platinum single-crystal surfaces and also by Motoo and Furuya for the electrooxidation of formic acid [15] and methanol 1161 on platinum single-crystal surfaces. The importance of in situ vibrational spectroscopic methods to study the nature and behaviour of the adsorbed species involved in the electrocatalytic processes on single-crystal electrodes is well recognized.
359
Of the available in situ vibrational spectroscopic methods for studies at electrode/solution interfaces, however, Raman spectroscopy cannot cope with singlecrystal surfaces as it depends on a surface enhancement mechanism on roughened surfaces of silver, gold and copper. Recently, a similar surface enhancement phenomenon to SERS was discovered for infrared reflection absorption spectroscopy [17-191, although the enhancement factor is much smaller than SERS. The infrared enhancement also depends on the specific nature of the rough surfaces of silver and gold. In view of the present state of the in situ vibrational spectroscopies developed at electrode/solution interfaces, infrared reflection absorption spectroscopy at high angles of incidence utilizing various forms of modulation techniques, such as EMIRS and polarization modulation, is the only one that can cope with the single-crystal electrode surfaces. For infrared reflection measurements, however, one needs a single crystal of substantial surface area. With oriented commercial single crystals, difficulties will be encountered in the process of cleaning and characterization of the surface using ultra high vacuum apparatus equipped with surface analytical tools such as LEED and Auger. In addition, it is difficult to get the cleaned and characterized surface in contact with the electrolyte solution without contaminating the surface. It seems that this is the main reason why so many data apparently include effects of surface impu~ties. This is a common problem in many laboratories. For the development of surface vibrational spectroscopy on single-crystal surfaces, we always first confirm the cleanliness and surface crystallographic orientation by a suitable method such as hydrogen adsorption/desorption voltammetry, which is very sensitive to surface conta~nation as well as to the surface c~stallograp~~ orientation. On the basis of the results presented above, the success of the preparation and electrochemical characterization of large single crystals suitable for infrared reflection measurements is encouraging for further development of vibrational spectroscopy in studies of electrochemical processes on single-crystal electrode surfaces. Carbon monoxide is an easy reactant for study by infrared spectroscopy, but we believe that the study of hydrogen adsorption/desorption on platinum single-crystal surfaces will be feasible by EMIRs, which is the most suitable technique for that purpose. If this is accomplished, it will be clear that vibrational spectroscopy will contribute greatly to the understanding of the molecular processes at the electrode/solution interface.
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1 2 3 4 5 6
A. Bewick, K. Kummatsu, S. Pons and J.W. Russell, J. Electroanal. Chem., 160 (1984) 47. J. Clavilier. R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem.. 107 (1980) 205. J. Clavlher, J. Electroanal. Chem., 107 (1980) 211. S. Motoo and N. Furuya, J. Electroanal. Chem., 167 (1984) 309. S. Motoo and N. Furuya, J. Electroanal. Chem., 172 (1984) 339. F.T. Wagner and P.N. Ross, Jr., J. Electroanai. Chem., 150 (1983) 141.
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