Study of the methanol adsorbates on Pt(100) and Pt(111) single crystal surfaces

W134686,88 13.00+0.00 6 ,988. Pcrsmwn PKUpk.

STUDY

OF THE METHANOL ADSORBATES ON Pt(100) Pt( 111) SINGLE CRYSTAL SURFACES B. BITTINS-CATTANEO,

AND

E. SANTOS,* W. V~ELSTICH and U. LINKE+

Institut fiir Physikalische Chemie, Universitiit Bonn, Wegelerstr. 12, D-5300 Bonn, F.R.G. (Received 9 November 1987; in revised form 29 March 1988) Abstract-The formation of methanol adsorbate as a structure sensitive process was studied on platinum single crystal surfaces. The measurements were performed on reproducibly prepared, clean and either well ordered (“undisturbed”) or cycled (“disturbed”) Pt(100) and Pt(ll1) single crystal electrodes. Essential differences were found. The data are interpreted in terms of parallel pathways leading to both COH and CO as adsorbed intermediates in a ratio depending on the surface structure present on the single crystal electrode. during the adsorption and subsequent oxidation of methanol on the respective Pt single crystal electrodes. For the sake of reproducibility all measurements were performed on clean, either well ordered ie “undisturbed” or “disturbed” (by electrochemical cycling into the oxide region) Pt( 100) and Pt( 111) single crystal surfaces.

INTRODUCTION In the course of electrocatalysis research the nature of the strongly adsorbed intermediate originating from CH,OH oxidation on polycrystalline Pt in acidic solution has been studied extensively, as its presence on the electrode reduces the reaction rate considerably[l, 21. Different methods (in situ and ex situ) have been employed: electroanalytical[3-71; ir-spectroscopic[8%11]; mass-spectroscopic (DEMS[7, 12, 133 and ECTDMS[ 14, 151) and radiometric[ 16, 171. As adsorbed intermediates species with composition COH[S, 7, 14, 16, 171, HCO[3], CO[4,6,8-IO] and mixtures of CO/COH[l5] have been proposed. Recently, Wilhelm et a/.[ IS] presented charge measurements during methanol adsorbate formation and with Electrochemical oxidation in combination Thermal Desorption Mass Spectroscopy (ECTDMS) performed on various kinds of polycrystalline Pt electrodes. They found strong evidence for parallel pathways in the methanol adsorption process producing both COH and CO as adsorbed intermediates in a ratio varying with coverage degree and methanol bulk concentration. Moreover, due to the progress in single crystal preparation (see references in[lS, 193) attempts have been made to investigate the sensitivity of CH,OH oxidation (and simultaneously of CO, HCOOH and H,CO oxidation) to a controlled surface structure[19-291. Up to now, except for measurements presented by our group[l9], these studies were limited either to bulk fuel oxidation or adsorbate oxidation in the presence of bulk fuel or adsorbate formation under open circuit conditions. In this paper, in addition to measurements of bulk CH,OH oxidation, we present information on the strongly bound intermediate originating from CH,OH oxidation on Pt(lOO) and Pt(ll1) single crystal electrodes. This was achieved by measuring in flow cell technique under strict potential control[30], the respective desorption spectra and the charges released

EXPERIMENTAL Solutions

and electrode

preparation

The efectiolyte solutions were prepared with Millipore water and analytical grade chemicals: HCIO, (Merck p.a.) and CH,OH (Merck p.a.). The electrolyte composition (0.05 M HCIO,) was chosen to minimize anion adsorption. All solutions were carefully deaerated with Ar (99.996 I”,) further purified through Oxysorb (Messer Griesheim). The preparation of the high purity single crystal electrodes Pt(lO0) and Pt(1 II) (diameter 8 and 6 mm, thickness 2.5 and 1.5 mm respectively) is described elsewhere[19].

Flow

*present address: INFIQC, Faculdad de Ciencias Quimicas. Universidad National de Cbrdoba, Argentina. filr Institut Jilllch, address: KFA TPermanent Grenzfliichenforschung und Vakuumphysik, F.R.G.

cell technique

Measurements were performed in a flow cell made of glass containing approximately 18 ml of electrolyte and thermostated at 20°C. In order to avoid contamination with air the solution containers were connected to the cell by glass tubes only and the entire Ar flow system was made of steel. The mounting of the Pt single crystals is shown in Fig. 1. To eliminate edge effects the cyclindrical Pt(lOO) and Pt(ll1) single crystal electrodes were fixed on to a steel contact oia a PTFE screw mechanism, yielding an effective geometric contact surface to the electrolyte of 23.8 mm’ and 19.6 mm’, respectively. All currents measured are given in current densities with respect to the geometric area. The potentials were measured us a reversible hydrogen electrode (rhe) in the same solution. A smooth, large area Pt electrode served as counter electrode.

1499

B. BITTINS-CATTANEOet al.

1500

PTFE screw mechanism

wLPt

Fig. 1. Mounting Preparation surfaces

and

singlecrysta1

of the Pt single crystal electrodes.

characterization

of stab/e

electrode

Prior to each series of experiments the Pt single crystal electrodes were cleaned in a controlled way to obtain a reproducible surface structure. After annealing in a gas-oxygen flame the electrodes were quenched in deaerated H,O and immediately mounted in the PTFE sample holder under protection of a droplet of H,O to avoid contact with air. To remove any contamination from the surface the single crystals were cleaned electrochemically in situ by slow (10 mV s-‘) CO adsorption+iesorption cycles in saturated CO solution during several hours. The single crystals were kept at potentials lower than 0.85 V in the case of Pt(100) and 1.0 V in the case of Pt(ll1) to exclude a rearrangement of the surface atoms by oxide formation([l8] and references therein). In a second series of experiments these “undisturbed” ie well ordered single crystal surfaces were cycled repeatedly (100 times at 50 mV s-‘) up to 1.45 V, thus prodbcing “disturbed” single crystal surfaces with stable surface structures. The hydrogen adsorption-desorption profile was used as an indicator of the state of a given surface structure. The respective voltammograms in 0.05 M HCIO, are shown in Fig. 2. They compare well with those published by Clavilier[31] and Aberdam et

100

F

A

al.[32] for Pt( 111) and Scortichini ef a/.[333 and Love et a[.[341 for Pt(lO0) and Pt(ll1). For Pt(lO0) during cycling a redistribution between weakly and more strongly bonded hydrogen takes place favouring the more strongly bonded hydrogen in the “disturbed” state. From the hydrogen region in Fig. 2 one can calculate an enhancement of the real surface area by a factor of 2.5 for cycled Pt( 100). The uncycled Pt( 111) shows the reversibIe group of peaks between 0.6 V and 0.85 V interpreted as very strongly adsorbed hydrogen[3 t], characteristic for a clean and well-ordered Pt( I 11) surface. These hydrogen states disappear during cycling promoting the growth of a sharp hydrogen desorption peak at a very low potential. As in the case of Pt(lO0) the onset of oxide formation shifts to more cathodic potentials upOn cycling and the real surface area is increased (by a factor of 1.4). RESULTS Oxidation

qf bulk methanol

The oxidation of bulk methanol in a 0.05 M HCIO, +O.Ol M CH,OH solution on Pt(lO0) and Pt(tll) “undisturbed” and “disturbed” is shown in Figs 3 and 4, respectively. in all cases hydrogen adsorption is not completely suppressed. Under our conditions methanol oxidation starts not before 0.5 V for “undisturbed” Pt(tO0) and Pt(l11). Whereas the peak potentials (0.75 V for the positive going sweep) nearly agree, the maximum current density (relative to the real surface area) is higher on “undisturbed” Pt(ll1) than on “undisturbed” Pt(lOO) (1.51 and 0.54mAcm-‘, respect ively). The shape of the methanol voltammograms are quite similar to those reported in[25]. Upon cycling the Pt single crystal electrodes into the region of oxide formation, the activity for methanol oxidation increases as concerns the onset of methanol oxidation (0.45 V for “disturbed” Pt( 100) and Pt( 1 I 1)). For Pt(100) a pronounced effect on the current is observed. The maximum current density (relative to the real surface area) is increased by a factor of 2.0. For

Pt(1001

Fig. 2. Cyclic voltammograms in 0.05 M HClO, of single crystal surfaces Pt( 1OO)and Pt(ll1); “undisturbed” (- ~ -, cycled up to 0.85 V and 1.0 V, respectively) and “disturbed” (-, cycled up to 1.45 V): current density = 50 mVs_ ‘. referred to geometric area Aacorn= 0.238 cm’ for Pt(lOO)and Agcom = 0.196 cm* forPt(lll):u

1501

Methanol adsorbates on Pt single crystal surfaces Pt 11001 (b) “disturbed”

(a) “undisturbed”

‘._,

potential

potential

vs. RHE/V

vs. RHE/ V

for the oxidation of bulk methanol on h(100) single crystal surfaces (a) 0.05 h4 HCIO,, + 0.01 M CH,OH; “undisturbed” and (b) “disturbed”; (---) 0.05 M HCIO .,; (-) U= tOOmvs-‘;secoudsweep.

Fig. 3. Cyclic voltammograms

Pt(lll1 ,(a) “undisturbed”

potential Fig. 4. Cyclic voltammograms

“undisturbed” Pt(tll) the enhancement

Oxidation

potential

vs. RHE/ V

vs. RHE/V

for the oxidation of bulk methanol on Pt(lll)

and (b) “disturbed”;

single crystal surfaces (a) (- - -) 0.05 M HClO,; (---) 0.05 M HC104 +O.Ol M CHaOH; v = 100 mV s-‘: second sweep.

increase of current is caused of the real surface area only.

by

the

of methanol adsorbate

The flow cell procedure used here is described in detail elsewhere[7]. The methanol adsorbate was formed out of a 0.05 M HCIO, +O.Ol M CHaOH solution at 400 mV or 450mV, potentials at the beginning of the double layer region (Fig. 2) and just before the onset of bulk methanol oxidation (Figs 3 and 4). During methanol adsorption the charge Qaa was measured, the degree of coverage with adsorbate depending on the time of adsorption (tad = l&1380 s). The adsorption was interrupted by replacing the solution by pure supporting electrolyte under potential

control. A typical current-time transient taken during methanol adsorption is shown in Fig. 5. A potential sweep was started in cathodic direction to determine the adsorbate desorption spectra for the respective Pt single crystal surfaces and to evaluate the charge released during the oxidation of the adsorbate Q,, . The second sweep (coinciding well with the voltammogram in pure electrolyte) was taken as proof that no adsorbate remained at the electrode surface and that the electrolyte exchange was complete. From the suppression of hydrogen desorption the degree of coverage with adsorbate 8 was evaluated. Figure 6 shqws the desorption of methanol adsorbate on “undisturbed” and “disturbed” Pt(100) surfaces. In both cases the more strongly bonded

B. BITTINS-CATTANEO

1502 005M HCLOL

I

0.05t-i HCLOL+O.OIM

et al.

CH30H

I

0.05M

HC!O,

I I

< =A 1.5 \ %

I I I I

0 Fig. 5. Current-time

1

4

5 time / min

of methanol on “undisturbed” solution at E, = 450 mV.

transient during the adsorption HCIO., + 0.01 M CH,OH

Pt (100 (a)

3

2

“undisturbed”

Pt(lOO) from a 0.05 M

I (b) “distwbed”

potential Fig. 6. Electrodesorption

vs. RHE /V

of methanol intermediate adsorbed on Pt(1OO) single crystal surfaces from a solution; (a) “undisturbed” (Ead = 450 mV, tad= 600 s) and (b) “dis(Earl = 400 mV, tad = 660 s); (- ~ -) base electrolyte; o = 20 mV s- I.

0.05 M HCIOI + 0.01 M CH,OH turbed”

hydrogen sites are blocked to a larger extent than the weakly bonded hydrogen sites. The oxidation of the adsorbate takes place in a single, very sharp peak at 0.73 V for “undi?turbed” Pt(100) and in two distinct, more cathodic peaks at 0.68 V and 0.70 V for “disturbed” Pt(100). Whereas the degree of coverage did not exceed values of 0 = 0.4 on “undisturbed” Pt (1 00), even for long adsorption times (tad = 1380 s), the cycling caused activation both for adsorbate formation (see Fig. 8, compare the coverage for “disturbed” and “undisturbed” Pt(lO0)) and adsorbate oxidation (see Fig. 6, compare the onset and the maximum of adsorbate oxidation). The results of equivalent measurements on “undisturbed” and “disturbed” Pt( 111) are given in Fig. 7. In contrast to the behaviour on Pt(100) surfaces the blocking of the hydrogen sites by the adsorbate on Pt(l1 I), surfaces affects all available sites nearly equally. -t‘he oxidation of the adsorbate on these surfaces happens in a broad peak at 0.67 V for “undisturbed” Pt(111) and 0.63 V for “disturbed” Pt(lll). In the case of the “undisturbed” crystal the adsorbate oxidation

overlaps with the desorption process of the very strongly bonded hydrogen. Exhibiting similar behaviour to that of Pt(lOO), cycling of well-ordered Pt( 111) facilitates both adsorbate formation and adsorbate oxidation.

DISCUSSION From the viewpoint of electrocatalysis it is important that cycling into the oxide region improves the Pt( 100) and Pt( 1 I 1) single crystal surfaces promoting methanol oxidation: the potentials of onset and maximum current for bulk and adsorbate oxidation are shifted to more negative values. This can be related to the fact that the formation of the oxide layer (from which the additional O-atom for the final oxidation is obtained) starts at more negative product CO, potentials on the cycled surfaces. From our experiments the following charges involved in formation and desorption of methanol adsorbate on the respective Pt single crystal surfaces

Methanol adsorbates on Pt single crystal surfaces

1503

Pt(ll1) (b) “disturbed”

(a) “undisturbed”

potential

vs. RHE /V

Fig. 7. Electrodesorption of methanol intermediate adsorbed on Pt(ll1) single crystal surfaces from a 0.05 M HClO& + 0.01 M CH,OH solution; (a) “undisturbed” (Ead = 400 mV, tad= 660 s) and (b) “disturbed” (Ead = 400 mV, tad= 600s); (- - -) base electrolyte; u = 10 mV s-‘.

can be determined: (i) the charge delivered during adsorption of methanol adsorbate, Qad; (ii) the charge delivered during oxidation of methanol adsorbate, Qox; (iii) the charge for hydrogen desorption on the uncovered Pt surface Qi; (iv) the charge for hydrogen desorption on the Pt-surface covered with methanol adsorbate Qph. As lower potential limit for charge integration in (iii) and (iv) the value 0.08 V was taken. According to Biegler et a/.[351 at this potential a hydrogen coverage of only 0.77 is present on a polycrystalline Pt surface. The values for Qg and QH MeChhave also to be corrected for the respective charges of double layer formation (linear extrapolation). From these charge values some characteristic quantities can be calculated: (i) the coverage degree with adsorbate: O=Q:-Qph_~~H.

QFI

(ii) the number of electrons per site involved oxidation process of methanol adsorbate:

eps =

Q

AQ,$.77

towards

only,

whereas QJQ,,

1 means an increasing amount

Table 1. Characteristic charges measured during formation and oxidation of methanol adsorbate on Pt(100) single crystal surfaces and calculated quantities 0 (coverage with adsorbate), Q&,_ and eps (electrons per site involved in the oxidation process). AQd0.77 flc

WlW

9.82 12.27 12.27 15.95

Pt(100) “disturbed”

32.21 49.32 57.37 68.44

“undisturbed”

’

(iii) the ratio QJQ,, of charges released during adsorption and oxidation of methanol adsorbate. Some characteristic results for the methanol adsorbate on “undisturbed” and “disturbed” Pt(100) and Pt( 111) single crystal electrodes in perchloric acid are shown in Tables 1 and 2. If one assumes, as demonstrated by ECTDMS measurements on annealed polycrystalline Pt electrodes[lSj, that methanol adsorbate consists of COH and CO particles[ 151. one can calculate the ‘respective mole fractions from the measured values QdQ,,[lS]. For Q_,lQ, = 2 the absorbate consists of ,,C,& or YO-particles

Q; ’

in the

Qad PC

Q Jlc”

8

17.07 22.98 32.72 34.51

14.20 15.30 20.32 18.24

64.47 122.53 154.21 211.76 f 3.5 0%

QJQ,,

eps

0.24 0.30 0.30 0.39

1.20 1.50 1.61 1.89

1.4 1.2 1.6 1.2

43.14 81.08 48.75 116.35

0.32 0.49 0.57 0.68

1.50 1.51 1.82 1.82

1.3 1.6 1.5 1.7

f 3.5 y0

t 5%

+ 5%

f 5%

The respective charges for hydrogen desorption on the uncovered Pt(100) single crystal surfaces are: Qz/O.77 = 40.90& for “undisturbed” Pt(100): QL/0.77 = 100.65 PC for “disturbed” Pt(lO0).

decreasing of ,y,OH-

1504

B. BITTINS-CATTANEO et al. Table 2. Characteristic charges measured during formation and oxidation of methanol adsorbate on Pt(l11) single crystal surfaces and calculated quantities 0 (coverage with adsorbate), Qad/Qpxand ep.7 (electrons per site involved in the oxidation process)

AQ$o.77 PC Pt(lll) “undisturbed” Pt(ll1) “disturbed”

4.18 10.36 12.60

Q

Qad

ptF

PC 4.84 18.86 27.03

5.10 14.01 18.01

e

Q.dQm

eP=

0.14 0.37 0.45

0.95 1.35 1.50

1.2 1.4 1.4

13.25

34.63

19.48 28.44

48.34 96.70

23.29

0.34

30.21 48.35

0.50 0.73

1.49

I.7

1.60 2.00

1.6 1.7

* 3.5 %

* 3.5 %

+ 3.5 %

+ 5%

+ 5%

k 5%

The respective charges for hydrogen desorption on the uncovered Pt( 111) single crystal surfaces are: Q$/O.77 = 28.00 PC for “undisturbed” Pt(ll1); Q&/O.77 = 38.96 PC for ‘disturbed” Pt(ll1). particles present on the surface, until fpr QJQ,, = 1 the adsorbate consists of ,y,OH-particles only[lS]. Similarly the eps values can reflect characteristics of the methanol adsorbate: adsorbates consisting of triple bonded ,fl\OH or bridge bonded /c\O yield eps = 1 whereas linear bonded

70

yields eps = 2.

From our measurements we calculate eps values ranging between 1.2 and 1.7 for the different Pt surfaces. This shows that the methanol adsorbate consists of a mixture of,y.H//C\O (eps = 1) and 70 (eps = 2) particles. The extreme values eps = 1 or 2 are reached in no case. Moreover, the eps values do not show a significant dependence of coverage degree 8. nor are rhey characteristic for a given surface structure. In the discussion of eps values one has to consider that the calculation of eps is hampered by the difficulty of giving a value/for AQH: there is an assumption necessary about the potential dependence of the hydrogen coverage. In our case the value was taken from polycrystalline PI (see above). In a recent publication[36] Sun and Clavilier report eps values for the methanol adsorbate on low index and stepped Pt single crystal surfaces. In 0.1 M HCIO, they find eps = 2.06 for Pt(lO0) (“undisturbed”) and eps = 2.05 for Pt(ll1) (“undisturbed”) with coverages 0 = 0.57 and 0.37 respectively. These values are calculated from niethanol adsorptiondesorption measurements performed in a technique where potential control and exclusion of air contamination cannot be granted at each stage of the experiment. In addition to the difticulties with the determination of eps mentioned above, this may cause the discrepancy with our results. Figures 8 and 9 show the results for the calculated ratio Q,d ./Q,, of charges released during adsorption and oxidation of methanol adsorbate as function of the coverage degree 0 oft he surface on Pt( 100) and Pt( 111). The trends towards CO- and COH-particles are indicated too. In contrast to the charge measurements reported by Wilhelm et aI.[15] for smooth, annealed and porous polycrystalline Pt electrodes, the present measurements show a considerably smaller scattering of data for all types of Pt single crystal electrodes. This may be interpreted in terms of a very stable and very smooth

surface state of the Pt single crystal electrodes here and in the uniform adsorption procedure in all experiments.

studied applied

Pt( 100) For “undisturbed” Pt(100) the scale of adsorbed particles lies between nearly only COH-type and only CO-type in a small coverage range between 6, = 0.2 and 0.4 (see Fig. 8). As in the case of polycrystalline Pt[l5] the adsorbate composition seems to be determined by geometric requirements: low coverages preferring the formation of,U$H-particles and high coverages preferring/C?-

or CO-particles.

This

means

that

the

square lattice of Pt( 100) (in contact with the electrolyte present as unreconstructed Pt(lOO) (1 x l)([lS] and references therein)) does not favour totally the formation of the triple bonded COH-particles even at /I\ coverages lower than 0.3. Moreover, -the observation that the maximum adsorbate coverage for this surface is lower than B = 0.5 (even for adsorbate formation out of more concentrated methanol solutions) shows that the CO-species are not adsorbed in a dense layer, but seem to repel each other on this flat (up to the atomic scale) surface. For the “disturbed” Pt(100) surface the situation is quite different. Upon cycling into the oxide region, the single crystal surface is roughened (enhancement by a factor of 2.5) by introducing O-atoms or OH-particles into the upper Pt layer, thereby displacing the Pt atoms[37 391. After reduction of the oxide layer these Pt atoms do not retain their old sites but stay in the new surface states[39]. As was shown by LEED analysis of a Pt(lO0) surface, random steps and terraces have been formed with a certain loss of long range order[39]. This means that other than the fourfold Pt(lOO) sites have been introduced, namely threefold (111) and (110) sites[39], which change the adsorption characteristics for hydrogen and methanol adsorption considerably. (In contact with electrolyte the Pt(ll0) surface reconstructs to a (110) (2 x 1) structure, which is actually a 3(111) x 2( 111) surface[40].) In the adsorbate desorption spectrum for “disturbed” Pt(1CO) (Fig. 6) two distinct

Methanol adsorbates on Pt single crystal surfaces

1505

= 1380 s) means that the surface with step and terrace sites offers more adsorption possibilities than a flat one.

Pt(100)

Pt(ll1)

Fig. 8. Ratio of charges released during adsorption and oxidation of methanol adsorbate formed on Pt( 100) from a 0.05 M HClO, + 0.01 M CHIOH solution as function of coverage: a “undisturbed” (Ee,, = 450 mV, tad = 6&l 380 s) and x “disturbed” (I&,, = 4OOmV, tad = t&tssos).

Pt(1111

Figure 9 shows the measured values Qad/QX as function of coverage for the Pt(lt1) surfaces. For “undisturbed” Pt( I II) theadsorbed particlesareCOH for the lowest coverages and a mixture of COH and CO in a ratio 1: 1 for the highest coverage 0 = 0.46. From Fig. 9 it is obvious that for the “disturbed” Pt( Ill) surface the measured values Qad/Qox tit quite well into the data for the “undisturbed” Pt(lt1) continuing them to higher coverages. This is in contrast to the behaviour of the Pt(lO0) surface (Fig. 8). The result is a linear decrease of the amount of adsorbed COH-particles (at 0 - 0.1 only COHparticles are adsorbed) until their total disappearance are at high coverages (at 0 - 0.73 only CO-particles adsorbed). This linear change in the adsorbate composition together with the close similarity between the desorption spectra of “undisturbed” and “disturbed” Pt( 1 t 1) (see Fig. 7) reflect that only minor modifications of the surface structure occur upon cycling Pt( 111) electrodes into the oxide region. The surface of “undisturbed” Pt( 111) is increased by a factor 1.4 only. The “undisturbed” Pt(lll) surface in contact with electrolyte has the unreconstructed Pt( 1 I I)(1 x I) structure[38,40]. Upon repetitive cycling into the oxide region the surface reconstructs, (110) like sites are introduced and a stepped surface of the form n(l11) x (111) is formed. This. has been shown by underpotential deposition of Cu[33] and H[41] and by LEED analysis[32]. In contrast to the situation with Pt(lO0) for the “disturbed” state of Pt(ll1) only sites of the same symmetry, namely (111) like sites are introduced. Thus the desorption spectrum (one broad peak) is not altered (except for a shift in cathodic direction) and the adsorbate composition is determined by space requirements on the same type of adsorption sites only. CONCLUSIONS

coverage

0

Fig. 9. Ratio of charges released during adsorption and oxidation of methanol adsorbate formed on Pt( 111) from a 0.05 M HCIO,, +O.Ol M CH,OH solution as function of coverage; a “undisturbed” (E, = 400 mV, fad = 6%660 s) and x “disturbed” (gad = 400 mV, tad= 30-600 s).

characteristic for two distinct peaks appear adsorptiondesorption sites for methanol adsorbate. These are probably the former (100) sites (now with somewhat lower desorption energy) and the new (11 l)like sites (the peak potential approaches that of the (It I) desorption peak in Fig. 7). As can be concluded from Fig. 8 the new surface geometry does not faVOUr an adsorbate with three bonds, probably because of the small length of the terraces. On the other hand the obvious increase in surface coverage (the measurements for the highest coverages 0 = 0.39 and 0 = 0.78 for “undisturbed” and “disturbed” Pt(lOO), respectively were taken for equal adsorption times tad

The surface structure present on Pt single crystal electrodes depends on the electrode pretreatment. Moreover, the oxidation of methanol adsorbate or bulk methanol is a surface sensitive process. Our results on well-ordered (“undisturbed”) and on cycled (“disturbed”) single crystal electrodes show that the adsorbate composition (COH- and CO-particles) and the respective desorption profiles are determined by the structure of unreconstructed and reconstructed surfaces. Acknowledgements-This work was supported by the European Communities under contract EN3EOO72D. Partial financial support from Siemens AG is gratefully acknowledged. REFERENCES 1. W. Vielstich, Fuel Cells, Wiley, New York (1970). 2. M. W. Breiter, Z. phys. Chem. 98, 23 (1975). 3. B. I. Podlovchenko and E. P. Gorgonova, Do&l. Akad. Nauk SSSR 156, 673 (1964).

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