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Evidence for coh as an adsorbed intermediate in the anodic oxidation of methanol by ecdtms and sniftirs
327
J. Elecrroanal. Chem., 242 (1988) 327-333 Ekevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliiinary
note
EVIDENCE FOR COH AS AN ADSORBED INTERMEDIATE IN THE ANODIC OXIDATION OF METHANOL BY ECDTMS AND SNIFl-IRS
W. VIELSTICH Institute for Physical Chemistry, P.A. CHRISTENSEN,
University of Bonn, Bonn (F. RG.)
S.A. WEEKS and A. HAMNE’M
Inorganic Chemistry Laboratory,
University of Oxford
Oxford (Great Britain)
(Received 21st December 1987)
INTRODUCTION
Extensive electrochemical studies [l-7] of the oxidation of methanol on platinum electrodes have suggested that the reaction proceeds via a strongly adsorbed intermediate present at high coverage. Attempts to clarify the nature of this intermediate by a large number of electrochemical and spectroelectrochemical techniques have not fully characterised the main species involved, though the importance of its identification in the development of fuel cell technology cannot be overstated. Two parallel reaction steps have been postulated for the formation of the strongly adsorbed intermediate: CH,OH --) HCO,,, + 3 H+ + 3 e-
0)
or CH,OH + COH,,
+ 3 H+ + 3 e-
(2)
and CH,OH + CO,,, + 4 H+ + 4 e-
(3)
According to eqns. (l)-(3), three different adsorbed species are possible. In-situ infrared experiments by Bewick and co-workers and Foley et al. [&lo] resulted in the observation of linearly-bound CO, and this was identified as being one of the principal poisons of the electro-oxidation of small organic molecules on Pt. In addition, bridge-bonded C=O [9] was identified as another strongly bound adsorbate (these species were implicated as possible intermediates in the formation of CO2 [9]). However, a comparison of the intensity of the band due to linearly adsorbed CO obtained via the oxidation of HCOOH, CO and CH,OH clearly 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
328
indicates that another species other than CO or C==Oexists on the surface [ll] in the latter case, but conclusive identification of this additional species has not yet proved possible. Coulombic experiments [12-141 show that at low coverage the ratio Q&Q, (where Q,~s is the charge passed during formation of the adsorbate and Q, that passed during its oxidation), is near to one, indicating that the intermediate is either COH adsor HCO,,. The latter was excluded by an experiment with labelled isotopes ]I31. If, indeed, (Pt),COH does form, then it ought to be possible to identify it with in-situ IR provided a relatively strong IR absorption can be located. Bowman et al. [15] have calculated that, in the gas phase, (Pt),CO-H should absorb near 1380 cm-l. In solution, it is to be expected that this value will shift to the low v”side by about 100 cm-’ [16]. Recently, Sun et al. [17] have reported a feature near 1200 cm-’ in the SNIFTIRS spectrum of a single crystal of Pt (100) immersed in 0.25 M CH,OH + 0.5 A4 H,SO,. This was attributed to (Pt),CO-H; however, the high intensity of this band would make it unlikely that it originates from a monolayer, or submonolayer, coverage of intermediate, unless a strong enhancement mechanism could be plausibly advanced. Using ECTDMS we have shown the existence of H as well as CO in the adsorbate formed during the electrochemical oxidation of CH,OH on Pt. It was concluded that the intermediate was C-OH, and this conclusion has been supported by in-situ IR studies of the oxidation process. Using small platinum particles supported on glassy carbon, an absorption at 1200 cm-’ could be most plausibly assigned to the (Pt),C-OH species. Both the frequency and intensity were as expected for a monolayer of such a species. EXPERIMENTAL
Details of the methods employed in both the ECDTMS and SNIFTIRS techniques have been given elsewhere [18,19]. The sulphuric acid and methanol were of AristaR (BDH) quality. Water was first de-ion&d and double-distilled prior to pyrolysis at 1100 o C. The SNIFTIRS experiments employed a standard thin-layer cell configuration and a 7 mm dia. Pt disc 3 mm thick sheathed in epoxy resin. Potentials are quoted vs. the Hg/Hg,SO,/l M H,SO, electrode (MMSE) or vs. RHE. RESULTS AND DISCUSSION
ECTDMS experiments
Figure l(a) shows the thermal desorption mass spectrum obtained with a platinum electrode immersed in N,-purged 0.1 M H,S04 + 0.5 M CH,OH at 0.45 V vs. RHE for 120 s. For comparison, Fig. l(b) shows that obtained with Pt immersed in 0.1 M
329
m/e 20 2 -Id
y2x1 1' 6L
Xl
3bo ’
sbo
700
M
900
T/K
T/K
Fig. 1. (a) Thermal desorption diagrams of the methanol adsorbate on platinum. Methanol adsorption at + 0.45 V vs. RHE for 120 s from 0.1 M H2S04 + 0.5 M CH30H. (b) Thermal desorption diagrams of the adsorbate formed on a platinum electrode after immersion in 0.1 M H,SO, purged with 99% N, + 1% CO at 0.45 V vs. RHE for 300 s. In both cases the temperature scan was 5 K/s. Desorption diagrams are for: m/e= 28 (CO), m/e=44 (CO,), m/e=17 and 1 (H,O) and m/e=2 (Hz). m/e = 64 is due to SO4 from electrolyte.
H,S04 purged with a 99% N, + 1% CO mixture, after the platinum was held at 0.45 V vs. RHE for 300 s. Figure l(a) shows clearly the loss of two forms of CO (m/e = 28); the form lost at the higher temperature having hydrogen associated with it. Figure l(b) shows that the thermal desorption products from the Pt in CO-purged H,SO, have no hydrogen associated with them. Figure 2 shows the relative coverages of the Pt surface by the COH and CO species as a function of methanol concentration, obtained from ECDTMS data [18]. Clearly, the COH species is favoured at low coverage.
0.5
1
Covemge
Fig. 2. Mole fraction, M, of adsorbates on polished platinum afte-r methanol adsorption as a function of coverage, calculated from ECDTMS data [18]. Adsorption solutions: (0) 0.1 M H2S04 +0.005 M CH,OH; (+) 0.1 M H,SO, +0.5 M CH,OH. Adsorption potential 0.4 V vs. RHE.
330
SNIFTIRS experiments
We have reported [20] that single potential step in-situ IR experiments show that methanol is adsorbed even at relatively low potentials, giving rise to linearly-bound CO. The F of the C=O stretch increases by 30 cm-‘/V, in agreement with the results of Bewick and Pons [9]. However, unlike the results of Bewick and co-workers, the C=O did not appear to be an intermediate in the oxidation of CH,OH to CO,, since there was no significant change in the coverage of -CO even at potentials at which CO, is clearly being formed. In addition, it was observed that the equilibrium: SO;- +H++HSO,-
(4
was forced over to the right by the formation of Hf arising from the oxidation of the CH,OH to COz. A feature due to the HSO; appears in the region in which we might expect any absorption due to (Pt),C-O-H to occur. However, a weak absorption could be discerned near 1230 cm-’ which might be assigned to the C-O stretch of the (Pt),COH species. This feature was also observed when the sulphuric
IO2 A R/R
o-
-0.5 -
-I -
-15
-2.
MO
Mu
1100 13w
m
1100 lwo
wo
,rarenumkr/cm-’
Fig. 3. SNIFTIRS spectra from a glassy carbon electrode covered with small platinum particlea, immersed in 0.1 M H$O, +0.5 M CH30H electrolyte (CM, window). All spectra (69 scans, 8 cm-’ resolution) were normal&d to the base spectrum collected at - 0.55 V vs. MMSE, + 0.09 V vs. RHE. (a)-0.25; (b)-0.2; (c)-0.15; (d)-0.1; (e) 0; (f) 0.1; (g) 0.2; (h) 0.3; (i) 0.4 V.
331
lo2 A R/R
1
-0
-3. -4-
-5-8. -7 -8
Fig. 4. Conditions as for Fig. 3, except electrode was bulk platinum. 800-2000 cm-‘. (a)- 0.25; (b)-0.2; (c) -0.15; (d)-0.1; (e) 0; (f) 0.2; (g) 0.4 V.
acid electrolyte was replaced by 1 M HClO,, and its intensity was seen to change with potential in a systematic way, as discussed in detail below. Attempts to clarify the nature of this band at 1230 cm-’ by reducing the concentration of the H,SO, from 1 M to 0.1 M failed, because reaction (4) shows a larger swing the more dilute is the acid. It is known that small platinum particles adsorbed on carbon are capable of electro-oxidising methanol, albeit with lower activity. As the carbon surface is basic [21], it was hoped that the combination of the smaller amount of H+ produced, and buffering by the carbon surface would prevent the shift in the equilibrium in eqn. (4), and thus create a window in the spectral region where we expect the (Pt),COH C-O absorption to occur. Figure 3 shows the spectrum of a glassy carbon electrode covered with Pt particles immersed in 0.1 M H,SO, + 0.5 M MeOH in the spectral region of interest. For comparison, Fig. 4 shows the spectrum of a Pt electrode under the same conditions, showing the loss of the SO,‘- broad absorption at 1080 cm-‘, and a gain of the HSO; features at 1200 cm-’ and 1050 cm-‘. Figure 3 clearly shows the gain of a single feature at 1200 cm-‘, which cannot be assigned to HSO; as there is no accompanying feature at 1050 cm-‘. As was the case with the 1230 cm-’
332
feature observed in 1 M H,SO,, the intensity of the 1200 cm-’ band was found to follow that of the CO, gain feature at 2340 cm-‘, reaching a maximum at + 0.2 V vs. MMSE, + 0.84 V vs. RHE (in agreement with previous studies with bulk Pt). Above this potential, the feature decreased in size as the Pt surface became covered with oxide. The most plausible assignment of this band is thus the C-O stretch of the (Pt),C-O-H species on the surface. Interestingly, no CO,, species were observed in these experiments, in agreement with X-PES data on porous carbon electrodes impregnated with similar Pt particles, where the mode of poisoning appears to be quite different from that of bulk Pt.
CONCLUSIONS
Adsorption layers on platinum can be transferred from solution to UHV without any loss of material [18]. The direct proof of hydrogen in the adsorbate shows that, in addition to CO,,, large amounts of (Pt),COH are found during the oxidiation of methanol. The ratio of CO to COH depends upon the adsorption conditions, the COH intermediate being favoured at low coverage. In the SNIFIIRS experiments employing bulk Pt, the wavenumber region 1000-1300 cm-l is somewhat masked by HSO;/SOifeatures. However, Pt deposited on a glassy carbon substrate allows a “window” to be opened in the spectral region of interest and a feature is observed near 1200 cm-’ which appears most probably to be due to the C-O stretch of the (Pt),C-O-H surface species. The ECTDMS experiments showed that the (Pt),COH species is favoured at low coverage by intermediates. However, the SNIFTIRS experiments were all run by stepping the potential from ca.- 0.04 V vs. RHE, at which reactions (2) and (3) above are very slow, to potentials at which appreciable faradaic currents are passed. Since SNIFTIRS must be performed using very thin electrolyte layer configurations, oxidation of a significant proportion of the total methanol in the thin layer takes place on stepping the potential, and the IR data perforce refer to relatively low concentrations of methanol as diffusion into and out of the thin layer is inhibited severely. Thus the IR data are directly comparable to the ECTDMS data obtained at relatively low coverage of intermediate. The fact that, as the rate of oxidation of methanol increases, the coverage by the 1200 cm-’ feature also rises strongly suggests that it is a true intermediate, and the drop in coverage as we enter the oxide region is in accord with electrochemical expectations. A more detailed treatment of the FTIR data will be presented in a future paper ]201.
ACKNOWLEDGEMENTS
We should like to thank the EEC, Genetics financial support.
International
and the SERC
for
333 REFERENCES 1 2 3 4 S 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
W. Vielstich, Fuel Cells, Wiley, New York, 1970. W. Vielstich and U. Vogel, Ber. Bunsenges. Phys. Chem., 68 (1964) 688. O.A. Petry, B.I. Podlovchenko, A.N. Frumkin and H. Lal, J. EIectroanal. Chem., 10 (1965) 253. V.S. Bagotsky and Yu.B. Vassiliev, Electrochim. Acta, 11 (1966) 1439. V.E. Kazarinov, Yu.B. Vassalie.v, V.N. Andrew and S.A. Kuliev, J. Electroanal. Chem., 123 (1981) 34s. M.W. Breiter, Z. Phys. Chem. N.F., 98 (1975) 23. A. Capon and R. Parsons, J. EIectroanal. Chem., 45 (1973) 205. B. Beden, C. Lamy, A. Bewick and K. Kunimatsu, J. EIectroanaI. Chem., 121 (1981) 343. A. Bewick and S. Pons in RJ.H. Clark and RE. Hester @Is.), Advances in Infrared and Raman Spectroscopy, Vol. 12, Wiley Heyden, London, 1985, p. 1. J.K. Foley, C. Korzeniewski, J.L. Daschbach and S. Pons in A.J. Bard (Ed.), Electroanalytical Chemistry, A Series of Advances, Vol. 14, Marcel Dekker, New York, 1986, p. 309. K. Kunimatsu and M. Kita, J. EIectroanal. Chem., 218 (1987) 155. B.I. Podlovchenko and E.P. Gorgonova, Dokl. Akad. Nauk SSSR, 156 (1964) 673. T. Iwasita, W. Vielstich and E. Santos, J. Electroanal. Chem., 229 (1987) 367. S. Wiielm, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 238 (1987) 383. J.M. Bowman, J.S. Bittmann and L.B. Harding, J. Chem. Phys., 85 (1986) 911. S. Beyerimhoff, private communication, 1987. S.G. Sun, J. ClaviIier and A. Bewick, in Ext. Abstr. 38th ISE Meeting, Maastricht, 1 (1987) 35. S. Wilhelm, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 229 (1987) 377. P.A. Christensen, A. Hamnett and P.R TrevelIick, J. Electroanal. Chem., 242 (1988) 23. P.A. Christensen, A. Hamuett and S.A. Weeks, J. EIectroanal. Chem., to be submitted. J.P. Bandin in A.J. Bard (Ed.), Encyclopedia of the Electrochemistry of the Elements, Vol. 7, Marcel Dekker, New York, 1976, Ch. 1; H.P. Boehms in D.D. Eley, H. Pines and P.B. Weisz (Eds.), Advances in Catalysis, Vol. 16, Academic Press, New York, 1966, p. 179.
J. Elecrroanal. Chem., 242 (1988) 327-333 Ekevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliiinary
note
EVIDENCE FOR COH AS AN ADSORBED INTERMEDIATE IN THE ANODIC OXIDATION OF METHANOL BY ECDTMS AND SNIFl-IRS
W. VIELSTICH Institute for Physical Chemistry, P.A. CHRISTENSEN,
University of Bonn, Bonn (F. RG.)
S.A. WEEKS and A. HAMNE’M
Inorganic Chemistry Laboratory,
University of Oxford
Oxford (Great Britain)
(Received 21st December 1987)
INTRODUCTION
Extensive electrochemical studies [l-7] of the oxidation of methanol on platinum electrodes have suggested that the reaction proceeds via a strongly adsorbed intermediate present at high coverage. Attempts to clarify the nature of this intermediate by a large number of electrochemical and spectroelectrochemical techniques have not fully characterised the main species involved, though the importance of its identification in the development of fuel cell technology cannot be overstated. Two parallel reaction steps have been postulated for the formation of the strongly adsorbed intermediate: CH,OH --) HCO,,, + 3 H+ + 3 e-
0)
or CH,OH + COH,,
+ 3 H+ + 3 e-
(2)
and CH,OH + CO,,, + 4 H+ + 4 e-
(3)
According to eqns. (l)-(3), three different adsorbed species are possible. In-situ infrared experiments by Bewick and co-workers and Foley et al. [&lo] resulted in the observation of linearly-bound CO, and this was identified as being one of the principal poisons of the electro-oxidation of small organic molecules on Pt. In addition, bridge-bonded C=O [9] was identified as another strongly bound adsorbate (these species were implicated as possible intermediates in the formation of CO2 [9]). However, a comparison of the intensity of the band due to linearly adsorbed CO obtained via the oxidation of HCOOH, CO and CH,OH clearly 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
328
indicates that another species other than CO or C==Oexists on the surface [ll] in the latter case, but conclusive identification of this additional species has not yet proved possible. Coulombic experiments [12-141 show that at low coverage the ratio Q&Q, (where Q,~s is the charge passed during formation of the adsorbate and Q, that passed during its oxidation), is near to one, indicating that the intermediate is either COH adsor HCO,,. The latter was excluded by an experiment with labelled isotopes ]I31. If, indeed, (Pt),COH does form, then it ought to be possible to identify it with in-situ IR provided a relatively strong IR absorption can be located. Bowman et al. [15] have calculated that, in the gas phase, (Pt),CO-H should absorb near 1380 cm-l. In solution, it is to be expected that this value will shift to the low v”side by about 100 cm-’ [16]. Recently, Sun et al. [17] have reported a feature near 1200 cm-’ in the SNIFTIRS spectrum of a single crystal of Pt (100) immersed in 0.25 M CH,OH + 0.5 A4 H,SO,. This was attributed to (Pt),CO-H; however, the high intensity of this band would make it unlikely that it originates from a monolayer, or submonolayer, coverage of intermediate, unless a strong enhancement mechanism could be plausibly advanced. Using ECTDMS we have shown the existence of H as well as CO in the adsorbate formed during the electrochemical oxidation of CH,OH on Pt. It was concluded that the intermediate was C-OH, and this conclusion has been supported by in-situ IR studies of the oxidation process. Using small platinum particles supported on glassy carbon, an absorption at 1200 cm-’ could be most plausibly assigned to the (Pt),C-OH species. Both the frequency and intensity were as expected for a monolayer of such a species. EXPERIMENTAL
Details of the methods employed in both the ECDTMS and SNIFTIRS techniques have been given elsewhere [18,19]. The sulphuric acid and methanol were of AristaR (BDH) quality. Water was first de-ion&d and double-distilled prior to pyrolysis at 1100 o C. The SNIFTIRS experiments employed a standard thin-layer cell configuration and a 7 mm dia. Pt disc 3 mm thick sheathed in epoxy resin. Potentials are quoted vs. the Hg/Hg,SO,/l M H,SO, electrode (MMSE) or vs. RHE. RESULTS AND DISCUSSION
ECTDMS experiments
Figure l(a) shows the thermal desorption mass spectrum obtained with a platinum electrode immersed in N,-purged 0.1 M H,S04 + 0.5 M CH,OH at 0.45 V vs. RHE for 120 s. For comparison, Fig. l(b) shows that obtained with Pt immersed in 0.1 M
329
m/e 20 2 -Id
y2x1 1' 6L
Xl
3bo ’
sbo
700
M
900
T/K
T/K
Fig. 1. (a) Thermal desorption diagrams of the methanol adsorbate on platinum. Methanol adsorption at + 0.45 V vs. RHE for 120 s from 0.1 M H2S04 + 0.5 M CH30H. (b) Thermal desorption diagrams of the adsorbate formed on a platinum electrode after immersion in 0.1 M H,SO, purged with 99% N, + 1% CO at 0.45 V vs. RHE for 300 s. In both cases the temperature scan was 5 K/s. Desorption diagrams are for: m/e= 28 (CO), m/e=44 (CO,), m/e=17 and 1 (H,O) and m/e=2 (Hz). m/e = 64 is due to SO4 from electrolyte.
H,S04 purged with a 99% N, + 1% CO mixture, after the platinum was held at 0.45 V vs. RHE for 300 s. Figure l(a) shows clearly the loss of two forms of CO (m/e = 28); the form lost at the higher temperature having hydrogen associated with it. Figure l(b) shows that the thermal desorption products from the Pt in CO-purged H,SO, have no hydrogen associated with them. Figure 2 shows the relative coverages of the Pt surface by the COH and CO species as a function of methanol concentration, obtained from ECDTMS data [18]. Clearly, the COH species is favoured at low coverage.
0.5
1
Covemge
Fig. 2. Mole fraction, M, of adsorbates on polished platinum afte-r methanol adsorption as a function of coverage, calculated from ECDTMS data [18]. Adsorption solutions: (0) 0.1 M H2S04 +0.005 M CH,OH; (+) 0.1 M H,SO, +0.5 M CH,OH. Adsorption potential 0.4 V vs. RHE.
330
SNIFTIRS experiments
We have reported [20] that single potential step in-situ IR experiments show that methanol is adsorbed even at relatively low potentials, giving rise to linearly-bound CO. The F of the C=O stretch increases by 30 cm-‘/V, in agreement with the results of Bewick and Pons [9]. However, unlike the results of Bewick and co-workers, the C=O did not appear to be an intermediate in the oxidation of CH,OH to CO,, since there was no significant change in the coverage of -CO even at potentials at which CO, is clearly being formed. In addition, it was observed that the equilibrium: SO;- +H++HSO,-
(4
was forced over to the right by the formation of Hf arising from the oxidation of the CH,OH to COz. A feature due to the HSO; appears in the region in which we might expect any absorption due to (Pt),C-O-H to occur. However, a weak absorption could be discerned near 1230 cm-’ which might be assigned to the C-O stretch of the (Pt),COH species. This feature was also observed when the sulphuric
IO2 A R/R
o-
-0.5 -
-I -
-15
-2.
MO
Mu
1100 13w
m
1100 lwo
wo
,rarenumkr/cm-’
Fig. 3. SNIFTIRS spectra from a glassy carbon electrode covered with small platinum particlea, immersed in 0.1 M H$O, +0.5 M CH30H electrolyte (CM, window). All spectra (69 scans, 8 cm-’ resolution) were normal&d to the base spectrum collected at - 0.55 V vs. MMSE, + 0.09 V vs. RHE. (a)-0.25; (b)-0.2; (c)-0.15; (d)-0.1; (e) 0; (f) 0.1; (g) 0.2; (h) 0.3; (i) 0.4 V.
331
lo2 A R/R
1
-0
-3. -4-
-5-8. -7 -8
Fig. 4. Conditions as for Fig. 3, except electrode was bulk platinum. 800-2000 cm-‘. (a)- 0.25; (b)-0.2; (c) -0.15; (d)-0.1; (e) 0; (f) 0.2; (g) 0.4 V.
acid electrolyte was replaced by 1 M HClO,, and its intensity was seen to change with potential in a systematic way, as discussed in detail below. Attempts to clarify the nature of this band at 1230 cm-’ by reducing the concentration of the H,SO, from 1 M to 0.1 M failed, because reaction (4) shows a larger swing the more dilute is the acid. It is known that small platinum particles adsorbed on carbon are capable of electro-oxidising methanol, albeit with lower activity. As the carbon surface is basic [21], it was hoped that the combination of the smaller amount of H+ produced, and buffering by the carbon surface would prevent the shift in the equilibrium in eqn. (4), and thus create a window in the spectral region where we expect the (Pt),COH C-O absorption to occur. Figure 3 shows the spectrum of a glassy carbon electrode covered with Pt particles immersed in 0.1 M H,SO, + 0.5 M MeOH in the spectral region of interest. For comparison, Fig. 4 shows the spectrum of a Pt electrode under the same conditions, showing the loss of the SO,‘- broad absorption at 1080 cm-‘, and a gain of the HSO; features at 1200 cm-’ and 1050 cm-‘. Figure 3 clearly shows the gain of a single feature at 1200 cm-‘, which cannot be assigned to HSO; as there is no accompanying feature at 1050 cm-‘. As was the case with the 1230 cm-’
332
feature observed in 1 M H,SO,, the intensity of the 1200 cm-’ band was found to follow that of the CO, gain feature at 2340 cm-‘, reaching a maximum at + 0.2 V vs. MMSE, + 0.84 V vs. RHE (in agreement with previous studies with bulk Pt). Above this potential, the feature decreased in size as the Pt surface became covered with oxide. The most plausible assignment of this band is thus the C-O stretch of the (Pt),C-O-H species on the surface. Interestingly, no CO,, species were observed in these experiments, in agreement with X-PES data on porous carbon electrodes impregnated with similar Pt particles, where the mode of poisoning appears to be quite different from that of bulk Pt.
CONCLUSIONS
Adsorption layers on platinum can be transferred from solution to UHV without any loss of material [18]. The direct proof of hydrogen in the adsorbate shows that, in addition to CO,,, large amounts of (Pt),COH are found during the oxidiation of methanol. The ratio of CO to COH depends upon the adsorption conditions, the COH intermediate being favoured at low coverage. In the SNIFIIRS experiments employing bulk Pt, the wavenumber region 1000-1300 cm-l is somewhat masked by HSO;/SOifeatures. However, Pt deposited on a glassy carbon substrate allows a “window” to be opened in the spectral region of interest and a feature is observed near 1200 cm-’ which appears most probably to be due to the C-O stretch of the (Pt),C-O-H surface species. The ECTDMS experiments showed that the (Pt),COH species is favoured at low coverage by intermediates. However, the SNIFTIRS experiments were all run by stepping the potential from ca.- 0.04 V vs. RHE, at which reactions (2) and (3) above are very slow, to potentials at which appreciable faradaic currents are passed. Since SNIFTIRS must be performed using very thin electrolyte layer configurations, oxidation of a significant proportion of the total methanol in the thin layer takes place on stepping the potential, and the IR data perforce refer to relatively low concentrations of methanol as diffusion into and out of the thin layer is inhibited severely. Thus the IR data are directly comparable to the ECTDMS data obtained at relatively low coverage of intermediate. The fact that, as the rate of oxidation of methanol increases, the coverage by the 1200 cm-’ feature also rises strongly suggests that it is a true intermediate, and the drop in coverage as we enter the oxide region is in accord with electrochemical expectations. A more detailed treatment of the FTIR data will be presented in a future paper ]201.
ACKNOWLEDGEMENTS
We should like to thank the EEC, Genetics financial support.
International
and the SERC
for
333 REFERENCES 1 2 3 4 S 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
W. Vielstich, Fuel Cells, Wiley, New York, 1970. W. Vielstich and U. Vogel, Ber. Bunsenges. Phys. Chem., 68 (1964) 688. O.A. Petry, B.I. Podlovchenko, A.N. Frumkin and H. Lal, J. EIectroanal. Chem., 10 (1965) 253. V.S. Bagotsky and Yu.B. Vassiliev, Electrochim. Acta, 11 (1966) 1439. V.E. Kazarinov, Yu.B. Vassalie.v, V.N. Andrew and S.A. Kuliev, J. Electroanal. Chem., 123 (1981) 34s. M.W. Breiter, Z. Phys. Chem. N.F., 98 (1975) 23. A. Capon and R. Parsons, J. EIectroanal. Chem., 45 (1973) 205. B. Beden, C. Lamy, A. Bewick and K. Kunimatsu, J. EIectroanaI. Chem., 121 (1981) 343. A. Bewick and S. Pons in RJ.H. Clark and RE. Hester @Is.), Advances in Infrared and Raman Spectroscopy, Vol. 12, Wiley Heyden, London, 1985, p. 1. J.K. Foley, C. Korzeniewski, J.L. Daschbach and S. Pons in A.J. Bard (Ed.), Electroanalytical Chemistry, A Series of Advances, Vol. 14, Marcel Dekker, New York, 1986, p. 309. K. Kunimatsu and M. Kita, J. EIectroanal. Chem., 218 (1987) 155. B.I. Podlovchenko and E.P. Gorgonova, Dokl. Akad. Nauk SSSR, 156 (1964) 673. T. Iwasita, W. Vielstich and E. Santos, J. Electroanal. Chem., 229 (1987) 367. S. Wiielm, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 238 (1987) 383. J.M. Bowman, J.S. Bittmann and L.B. Harding, J. Chem. Phys., 85 (1986) 911. S. Beyerimhoff, private communication, 1987. S.G. Sun, J. ClaviIier and A. Bewick, in Ext. Abstr. 38th ISE Meeting, Maastricht, 1 (1987) 35. S. Wilhelm, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 229 (1987) 377. P.A. Christensen, A. Hamnett and P.R TrevelIick, J. Electroanal. Chem., 242 (1988) 23. P.A. Christensen, A. Hamuett and S.A. Weeks, J. EIectroanal. Chem., to be submitted. J.P. Bandin in A.J. Bard (Ed.), Encyclopedia of the Electrochemistry of the Elements, Vol. 7, Marcel Dekker, New York, 1976, Ch. 1; H.P. Boehms in D.D. Eley, H. Pines and P.B. Weisz (Eds.), Advances in Catalysis, Vol. 16, Academic Press, New York, 1966, p. 179.