Adsorption and anodic oxidation of methanol on iridium and rhodium electrodes

Elcctroch~ca

Acta. 1971. Vol. 16. pp. 913 lo 938. Persamon Press. Printed in Northem Ireland

ADSORPTION AND ON IRIDWM V. S. BAGOTZICY, Yu.

ANODIC OXIDATION OF METHANOL AND RHODIUM ELECTRODES* B. VASSILIEV, 0.

A.

KHAZOVA and S. S. SEDOVA

Institute of Electrochemistry, Academy of Sciences of the U.S.S.R., Moscow, U.S.S.R. Abstract-Adsorption and electro-oxidation of methanol on smooth iridium and rhodium electrodes have been studied. The regularities obtained were compared with the results of previous measurements on smooth platinum. The adsorption of methanol on iridium has been established as characterized by regularities peculiar to a surface with an exponentially distributed inhomogenity of adsorption sites (Freundlich isotherm, linear change of the activation energy of adsorption with logarithm of surface coverage). The adsorption regularities for rhodium are more complex. The character of the isotherms on iridium and rhodium, as well as on platinum, does not depend on the nature of adsorbed neutral particles (methanol, hydrogen erc) and is apparently determined by the electrode surface properties. As follows from kinetic regularities (influence of potential, concentration and pH of solution, surface coverage) the rate-determining step of steady-state methanol electro-oxidation on iridium and rhodium is the oxidation of carbonaceous chemisorbed particles by adsorbed OH radicals. R&sum&-On a 6tndie l’adsorption et l’oxydation du methanol sur des electrodes d’iridium et de rhodium poli. Les r&sultats obtenus ont &e compare avec les r&mltats de recherches analogues sur le platme poli. L’adsorption du m&hanol sur l’iridium peut etre p&se& par des equations valables pour des surfaces avec une distribution exponentielle des 6n&gies d’adsorption (hquation Freundlich, changement lin&tire de l’&n&gie d’activation avec le logarithme du degre de recouvrement). Les lois d’adsorption du m&hanol sur le rhodium sont plus compliquees. Sur l’iridium et le rhodium, ainsi que sur le platine, la forme de l’isotherme d’adsorption ne change pas pour l’adsorption de divers substances neutres (methanol, hydrogene etc) et est probablement determinee par les propri&t% de la surface des electrodes. Les r$@aritb cinetiques pour l’oxydation du methanol (influence du potentiel, concentration et pH de la solution, degre de recouvrement) indiquent l’oxydation des particles chimisorbes par des radicaux OH comme stade limitatif de la reaction globale sur l’iridium et le rhodium. Zusammenfassnng-Die Adsorption und Oxydation von Methanol wnrde auf glatten Iridium- und Rhodiumelektroden untersucht. Die erhaltenen Gesetzmiissigkeiten wurden mit den Ergebnissen frtlherer analoger Messungen an glatten Platinelektroden verglichen. Die Methanoladsorption auf Iridium llsst sich durch Gleichungen beschreiben, die ftir OberflZchen mit emer exponentiellen Verteihmg der Adsorptionsenergie charakteristisch ist (Freundlich-Isotherme, lineares Ansteigen der Aktivationsenergie mit dem Logarithmus des Besetzungsgrades). Fur die Methanoladsorption auf Rhodium lassen sich keine einfache Gesetzmsissigkeiten ableiten. An Iridium und Rhodium, sowohl als such an Platin, Pndert sich die Form der Isotherme bei der Adsorption verschiedener neutraler Adsorbate (Methanol, Wasserstoff u.s.w.) nicht; wahrscheinlich ist die Form der Isotherme in erster Linie durch die OberlIlcheneigenschaften der Elektrode bedingt. Die kinetischen Gesetzmsssigkeiten fll die Methanoloxydation auf Iridium und Rhodium (Einfluss von Potential, Konzentration und pH der LSsung, Oberll~chenbesetzungsgrad) deuten auf die Oxydation chemosorbierter Teilchen durch OH Radikale als den geschwindigkeitsbestimmenden Schritt der Reaktion. INTRODUCTION

A LARGE number of experimental data have been accumulated at present on adsorption and anodic oxidation of methanol and other organic substances on platinum.1-4 However, very little is known about the other metals of the platinum group.6-12 Breiter was the first to conduct a comparative study of methanol electro-oxidation on the platinum-group metals, using pulse techniques.6 Recent studies often compare catalytic activities of various platinum-group metals in the methanol oxidation reactions.‘,* The general trend of these works is aimed, however, at the choice of the * Manuscript received 3 December 1969. 5

913

914

V. S. BAGOTZKY, Yu. B. VAWLIEV,0. A. KHAZOVA

and S. S. SEDOVA

most active catalyst for the methanol fuel cell, but it provides no insight into the mechanism and kinetics of the electro-oxidation. This paper is devoted to a systematic study of adsorption and anodic oxidation of methanol on smooth iridium and rhodium. Measurements were conducted under conditions analogous to those employed in previous investigations on smooth platinum.1~1sS14That allowed the direct comparison of the behaviour of iridium, rhodium and platinum. EXPERIMENTAL

TECHNIQUE

Methanol adsorption on the smooth iridium and rhodium electrodes was studied by the method of fast potentiodynamic pulses. Preliminary study allowed the choice of the general parameters for pre-treatment of the electrode surface and for measurement procedure itself that assured good reproducibility of results. The electrodes were primarily subjected to a cathodic-anodic polarization by imposing a sweep from ~l1= -0.1 to 15 V(he) for 1 h (all potentials are referred to the hydrogen electrode in the same solution). The rhodium electrode was subjected to a 2-min activation repeated before each pulse. Then a complex potentiodynarnic pulse was imposed on the electrode. The latter was first kept at 1.0 V for 30 s to clean the surface from adsorbed organic substances. This potential was lower than that on platinum (qr = 1.2 V), because of the earlier oxygen adsorption on rhodium and iridium. Oxides formed on iridium at ~)r= 1-OV can be rapidly reduced by sweeping the potential to the desired value at qp < 0.5 V. At q$‘* > 0.5 V, analogously to platinum a supplementary reducing pulse was imposed at q’r = O-05V for 30 s. A similar pulse was employed for rhodium, since, even with longer exposure to the potentials of the double-layer region, oxygen adsorbing at 1-OV cannot be reduced on rhodium. Methanol adsorption on the electrode surface begins after sweeping the potential to q.+?. A measuring pulse was imposed in the electrode after a strictly definite adsorption time 7aaa. Cathode pulses with a sweep rate 40 V/s were imposed for obtaining quantitative dependencies. No change in the amount of chemisorbed substance was observed in all solutions during the period of the measuring pulse. Actually, when the next pulse was imposed just after the first one, the two i/q curves entirely coincided. Thus, hydrogen adsorbs only on the sites free of organic substances; on the other hand, there is no additional adsorption of organic substance from solution during the measuring pulse. To elucidate the nature of the chemisorbed particle, anodic pulses (160 V/s) were imposed and the hydrogen generated during dehydrogenation of the methanol molecule was determined as in the previous investigations on platinum.l*le In kinetic measurements the same pulse technique was used. Maximal (at 8,’ = 0) and steadystate currents corresponding to steady-state 6,’ values were registered at given concentrations and potentials (13,’ is the degree of surface coverage with organic particles related to the total number of adsorption sites). All the reagents employed were of high purity grades. Solutions were prepared using bi-distilledwater, and prior to each run were additionally purified by many-hour electrolysis on large platinized platinum nets. The solution purity was determined by the hydrogen adsorption drop (A&J with time in a 1 N H,SO, supporting solution. For pure solutions A& did not exceed 5% in the longest experiment.

Adsorptionandanodicoxidationof methanolon iridiumandrhodium Hydrogen

adrorptian

and the character

915

of surface inhomogeneity

Figure 1 shows the potentiodynamic i/p curves of hydrogen adsorption on platinum, iridium and rhodium measured at a 40 V/s potential sweep starting from O-4 V. I zo-

5-

FIG.

I.

Potentiodynamic i/q curves of hydrogen adsorption on platinum (l), iridium (2) and rhodium (3). Potential sweep from vr O-4 V at 40 V/s in 1 N H,SO, solution.

All the current values are referred to one surface atom. The iridium and rhodium hydrogen adsorption curves differ markedly from those obtained on platinum. Hydrogen adsorption on iridium starts at the same potentials as on platinum. However there is a lower quantity of strongly bonded hydrogen (adsorbing within the 0-4-O-2 V potential range) on iridium in comparison with platinum. Most of the hydrogen atoms are weakly bonded with the iridium surface and adsorb within the O-2-0+0V range. A first weak peak (at qr = O-17V) and a second sharp one (at CJ+ = O-07V) are observed. Still less hydrogen is adsorbed on rhodium within the O-I-O-2 V range (cu 25 %). The hydrogen adsorption curve has a single peak at q+ = O-1V. On rhodium and iridium, hydrogen adsorption was entirely reversible at all sweep rates investigated. These results are in agreement with published data.lssm By integrating the differential curves shown in Fig. 1, the hydrogen adsorption isotherms were calculated taking 8, = 1 at vr = O-0 for these metals.17 To a first approximation, the hydrogen adsorption isotherm on platinum in both acid and alkaline solutions is represented by a linear 8,/logpn, plot (the Temkin isotherm),21

916

V. S. BAGOTZKY,Yu. B. VASSILIEV,0. A. KHAZOVA

and S. S. SEDOVA

where, either in 1 N H,S04 or 1 N KOH, the inhomogeneity coefficient f m 14. This equation is characteristic of a surface with uniform distribution of inhomogeneity. As seen from Fig. 2 a and b, in the large range of hydrogen pressures from mu, = 1O-s to 1 atm and the medium coverage region (O-15 -=c 13~ < O-9) the isotherm of hydrogen adsorption on iridium can be approximated by a linear dependence of log OHon log Pn, either in acid or alkaline solution. Thus to a first approximation, O--l +2--07 -05 -0.4-

. . -6 I

-10 I o-3

-4

-6 1 0.2 -3

-g

logpv Pr

01 -I

-2

atm ’

V

0 1ogc,,

-12 I

-a

-10

-6

-4

0.2

03

I. 0

-I

-2

-2 0.1 I. I

0

iOgpnz.

Qr *

0

ioqc,,

0 -I

0m V

L

ok -2

M

I

M

‘pr.v bqcM,

M

RQ. 2. Isotherms of adsorption of hydrogen (0) and methanol (0) on smooth iridium. (a, b) and smooth rhodium (c): (a) in 1 N H&30,, (b) in 1 N KOH, 20°C methanol at rp 0.4 V, 26°C: (c) in 1 N HISOa. methanol at vr 0.3 V, 40°C).

Adsorption and anodic oxidationof methanolon iridiumand rhodium

this adsorption

obeys the Freundlich

917

isotherm

8, = k . cg” = k’(&;)l”

(2)

where n = 55-6-5 in 1 N H,SOd and 4-65 in 1 N KOH. Such an isotherm is specific for an exponential distribution of the adsorption energies of the surface sites.8a As seen from Fig. 2c, the isotherm of hydrogen adsorption on rhodium at 4O”C, within the accuracy of experiment, can be represented as two linear parts in BJlogp,, co-ordinates. The first region at OH < 0.25 has a coefficient f = 25 (this part may be also well described by an exponential Freundlich equation for the isotherm, testifying to an equivalence of relations for both exponential and uniform distributions at large values of n) ;= for the second part (0, > O-25), f = 6. Methanol adsorption on iridium and rhodium

Addition of methanol causes an almost regular decrease of hydrogen adsorption in the whole potential range of adsorption. As the bulk concentration of methanol increases, there is a point when the electrode surface coverage by chemisorbed particles does not rise further. On iridium the surface coverage with methanol attains the limit Orx = 0.63-0.69, ie about 35 % of the surface sites capable of hydrogen adsorption cannot be occupied by organic particles, due to steric hindrance. On rhodium Orx M O-7. Thus on rhodium and iridium the steric hindrance to methanol adsorption is somewhat greater than on platinum (ey = 0*75-O-8). For all the metals investigated in methanol solutions the limiting surface coverage by chemisorbed particles is independent of potential, until there are no other particles adsorbed on the surface. Isotherms of the steady-state methanol adsorption on the iridium electrode in 1 N H,SOa solution are shown in Fig. 3. At the potentials investigated (from 0.05 to O-7 V) in the medium coverage region the isotherms are represented by linear log &‘/log c dependencies and can be described by the Freundlich equation I&’ = K&l’*. O-

-02-

-04-

‘&

-o.fi-

4 -0,8-

-IO-

FIG. 3. Isotherms of methanol adsorption on iridium under steady-state conditions at different potentials in 1 N HtSOl; 26°C 1, 0-2; 2, 0.3; 3, 0.35; 4. 0.4; 5, 0.45; 6, 0.5; 7, 0.55; 8, 0.6; 9, O-7 V.

(3)

918

V.

S. BAGOTZKY,

Yu.

B. VASSILIEV, 0. A. WWVA

and S. S. SEDOVA

At potentials above 0.15 V the adsorption isotherms are parallel ; the exponent n’ = 55-6-5 coincides with the value for the hydrogen adsorption isotherm. In 1 N KOH solution the isotherms of methanol adsorption on iridium are of the same form and n’ = 4.6 (Fig. 2b). The adsorption isotherms of methanol on rhodium (at constant potential) are similar to those of hydrogen adsorption, ie they also have two linear parts in the (&‘/log c coordinates, the first at OR’ below 0.25 and the second at OR above 0.25. In Fig. 2 a-c, the isotherms of methanol and hydrogen adsorption on platinum, iridium and rhodium are superimposed by shifting the curves along the log c axis. It can be seen that the hydrogen and methanol adsorption on all these metals is entirely analogous. Hence one may conclude that the electrode surface properties in general determine the character and shape of the isotherms in adsorption of such neutral compounds as methanol and hydrogen on platinum, iridium and rhodium. Probably, the repulsion forces between particles of neutral substances adsorbing on the platinum-group metalsas are rather weak, and display a very slight effect on the character of the adsorption isotherm, This, perhaps, can explain the fact that the slope coefficients for the logarithmic isotherms of adsorption of a wide number of neutral substances on platinum coincide very well, amounting approximately to I= 13-14.l~~~ On the other hand, the repulsion forces between charged particles adsorbing on platinum are more significant; thus the slope of their adsorption isotherms changes sharply. In spite of a changing character of the methanol adsorption isotherm when passing from platinum to iridium and rhodium there is a only slight change in methanol adsorption at medium concentrations. For example, a steady state coverage 0%’ = O-5 in 1 N H,SO* at y= = 0.4 V is attained at bulk concentrations of methanol = 5 x 1W2, 5 x 1O-2 and 1 x 10-l M on platinum, iridium and rhodium %H,OH respectively. Dependence of methanol adsorption upon the electrode potential and pH of solution

As in experiments with platinum, the isotherms of methanol adsorption on iridium shift parallel to each other with changing potential. The dependence of the methanol adsorption on platinum, iridium and rhodium upon the potential at a constant bulk concentration of methanol is represented in Fig. 4 a-e. In analogy with platinum, the methanol adsorption potential dependence on iridium and rhodium is described by a dome-shaped curve with a maximum in the potential region where the hydrogen and oxygen adsorption on the given electrode is minimal. Such a dependence is in agreement with the Frumkin theory of adsorption of organic substances on metals capable of hydrogen adsorption.2s In acidic solutions the maximal adsorption of methanol is observed on iridium at q+ = 0*3-O-4 V, on rhodium at O-2-O-4 V, and on platinum at 0.35-0.55 V. With a potential shift to negative values the methanol adsorption decreases as the electrode surface gets covered with hydrogen; the character of such a On’ decrease is determined by the shape of the hydrogen-adsorption/potential plot. It was found that to the Erst approximation the dependence of log (ok- 0,‘) upon potential of the iridium electrode (and correspondingly on log 13,) is a linear function at q= = O-35-O-0 V. On rhodium at the potentials more negative than 0.2 V, the coverage decreases

Adsorption and anodic oxidationof methanol on iridiumand rhodium

0.6 c

I

I

04

0.2

06

I

O-8

-06

I

1

0

I

cl.2

I

I

0.4

0.6

FIG. 4. Dependence of methanol adsorption on potential of platinum (a), iridium (b) and rhodium k): (a) l-0, 1 M CH,OH + 1 N HIS0 4; 2-0,1 M CH,OH + I N KOH; dashed line, calculated; points, experimentfrom CH,OH -I- 1 N HISOI, 2O”C, (b) l-l M CH,OH -I- 1 N HsSO,; 2-l M CH,OH + 1 N KOH; 3-Q 1 M CH,OH f 1 N H&30,, 26°C: (c) 0.5 M CH,OH f 1 N H.SOa, 40°C. dashedlie. calculated.

919

920

V. S. BAWTZKY, Yu. B. VASSILJEV,0. A. KI-LGOVA and S. S. SHWVA

sharply with potential, a major quantity of hydrogen being adsorbed within a narrower potential region. The adsorption of oxygen on rhodium and iridium starts at more negative potentials than on platinum. On iridium the oxygen adsorption is noticeable at vr > O-5 V. But a decrease in the methanol surface coverage on iridium with increasing anodic potential is observed even at qr > 0.4 V. An analogous picture is found for rhodium. In log &‘/971: co-ordinates the decrease observed on iridium is a linear function, log 8,’ = const -

5 &= .

Q)~,

(4)

where y = O-82, ie close to unity. An earlier decrease of coverage on iridium and rhodium (compared to platinum) is explained by the close rates of methanol adsorption and electro-oxidation of the carbonaceous particles. The descending branch of this curve at positive potentials may be calculated accounting for the steady-state conditions, ie from equality of adsorption rate and that of removal of the chemisorbed particles as a result of their For this range of potentials one may ignore the electro-oxidation and desorption. desorption rate as compared to that of the particle removal during oxidation. Then Substituting an expression combining the rates of adsorption and v,ds = i,te&. oxidation with the coverage and potential, the steady-state coverage may be estimated at different potentials. Thus, in Fig. 4 a dashed line shows the dependence on’ ZJSq= calculated for rhodium and platinum. There is a good coincidence between the experimental and calculated curves. A shift of the adsorption region of organic substances at changing pH of solution is a specific feature of platinum and other electrodes showing considerable hydrogen adsorption. 25 Experiment shows that on the platinum-group metals the dependence of methanol adsorption shifts with changing pH in the same manner as the potential of the equilibrium hydrogen electrode. Figure 4 compares the 0,‘/~,, curves on platinum and iridium in 1 N H,SO., and 1 N KOH. Their cathode branches practically superimpose. Either on platinum

o-0.2-

-0.4‘s" F

-oa-

-oa-

FIG. 5. Dependence of surface coverage of smooth iridium electrode with methanol (1 M CH,OH) on pH of buffer solution (I&PO,, H3S0,, KOH, ctot 1 N). l,gJr=0*4V; 2, f$%=0*5V.

Adsorption and anodic oxidation of methanol on iridium and rhodium

921

or iridium the anodic parts are shifted to less positive potentials. It will be shown that the more earlier decrease in the electrode surface coverage in the alkaline media is associated with the fact that in comparison with acidic solutions, the former show a much lower adsorption rate and a somewhat higher rate of oxidation. Figure 5 shows the dependence of the iridium surface coverage with chemisorbed methanol upon pH of solution (at constant potential us to the hydrogen electrode in the same solution). When passing from pH 3-5 the electrode surface coverage markedly reduces and then it increases again with rising pH_ On iridium the dependence of the coverage upon pH is analogous to that on platinum, which also shows a minimum of methanol adsorption at pH w 4. Kinetics of methanol adsorption

Methanol adsorption on smooth iridium and rhodium electrodes is a slow reaction, and in all solutions investigated not diffusion but the kinetics of adsorption defines the increase of surface coverage with time. As seen from Figs. 6 and 7, the kinetic isotherms of methanol adsorption on

FIG. 6. Kinetic isotherms of methanol adsorption on iridium at different bulk concentrations of methanol in 1 N H,SOc solution, g.+ = 0.4 V; 26°C.

1, 10-S; 2, IO-~; 3, 5 x 10-a; 4, 10-l;

5.5

x 10-l;

6, 1; 7,5;

8,lO M.

iridium at different concentrations and potentials in the region of medium coverage (0.15 < OR’ Q Opd) have the form log 0,’ = const + -L log 7, ovll

(5)

corresponding to the Bangham-Burt equation.% At different bulk concentrations and potentials (besides 0.1 and O-05 V) the kinetic isotherms on iridium are parallel. Their slope gives l/an = 0.36. Taking n’ = 6, the transfer coefficient can be found from adsorption isotherm, a = 0.46 (ie close to 0.5). As follows from Fig. 5, the adsorption rate is the reciprocal of coverage in fractional

A.

V. S. BAOOTZKY, Yu. B. VASSILIW, 0.

KHAZOVA

and S. S. SEDOVA

l

I

I

I

I

2

wr,

L4

3 s

FIG. 7. Kinetic isotherms of methanol adsorption (1 M CH,OH, 26°C) on iridium at different potentials. 1, O-05; 2, O-1; 3, 0.2; 4,0-35; 5.0-4; 6, O-45; 7, O-55 V.

exponent. Moreover, experiment shows that at constant surface coverage the adsorption rate is proportional to the bulk concentration of methanol. Thus the rate of methanol adsorption on the smooth iridium electrode obeys Uads

=

k&, .cB .tFan'),

(6)

analogous to the Kwan relation for adsorption on a surface with exponential distribution of adsorption energies.27 The dependence of adsorption rate on potential (at 8n’ = const.) has a domeshaped form (Fig. 8) showing a maximum at 0*3-O-4 V, ie in the region of minimal adsorption of hydrogen and oxygen on iridium. only a slight change in the methanol adsorption rate is observed within the potential range from 0.35 to O-15 V, where the coverage of the iridium surface with hydrogen is negligible. But at potentials

I

1

I

0

02

04 'p.3

Fm.

I

0.6

v

8, Dependence of methanol adsorption rate on iridium on potential at different coverages. 1, e,’ o-1 ; 2, O-25; 3, o-4.

Adsorption aad anodic oxidation of methanol on iridium and rhodium

923

from 0.15 to O-0 V, as the surface coverage with hydrogen shows a rapid increase, the adsorption rate falls sharply. Under the same conditions, the adsorption rate of methanol is by about an order of magnitude lower on iridium than on platinum. A rapid increase of the adsorption rate along with a changing slope of the kinetic isotherms are observed at increasing temperature. The activation energy of methanol adsorption on iridium was determined by investigating the kinetics at various temperatures. With increasing logarithm of coverage, the calculated activation energy of adsorption increases linearly from 8.4 (at on’ = 0.16) to 12.3 kcal/mol (at f3,’ = O-5), Fig. 9, according to

E, = E* + ad In 8,‘.

I

I

-0.8

I

I

L

-0.6

log

(7)

-0.4

-0.2

e;

FIG. 9. Change of activation energy of methanol adsorption on iridium with the coverage of surface with chemisorbed particles at q+ = O-3 V in 0.1 M CHaOH +

1 N HISO

solution.

From the slope of this plot it follows that an’ ti 3.04 or at a w O-5 n’ w 6.1, which is in agreement with the value of n’ obtained from the adsorption isotherms at constant temperature. Activation energies of methanol adsorption on iridium are close to those found on platinum. Pecularities of methanol adsorption on rhodium influence also its kinetics. As seen from Fig. 10, the kinetic isotherm of adsorption consists of two linear parts on the &‘/log r relation. Transition from one to another part occurs at en’ M 0.25. In each part an increase of coverage with the time can be satisfactory described by 8,’ = const + 1 In 7. Qf’

(8)

The slope of the first part, afi’ = 18-4. Employing fr’ = 35, estimated from the slope of the steady state methanol adsorption isotherm we obtain a = 0.52. The second part has af&’ = 3-6, substituting the value frr’ = 6 found earlier, we obtain

V. S. BAOOTZKY, Yu. B. VASSELXEW, 0. A.

924

I

I

I

2

tog I-, FIG.

10.

O(= 062. for the rate

KHAZOVA

and S. S. SEDOVA

I

3

s

Kinetic isotherm of methanol adsorption on rhodium in 1 M CHtOH 1 N HaSOr solution at q+ = O-3 V, 20°C.

Thus, transition coefficients have reasonable values. of methanol adsorption on rhodium is

+

A general expression

for and

ka

= &a ’ c exp(-cr.’

. O-25) exp(-arfIi(&

-

0.25))

(10)

for O-25 < eR Q 0.7, where k& is the rate constant of adsorption at OR’+ 0 and the given potential. Comparing the kinetic isotherm of methanol adsorption on rhodium with the data found on pIatinum, one may conclude that the adsorption on rhodium is slower (10 times slower at 20”) and the steady state coverage values are lower than on platinum. Methanol

dehydrogenation

and the nature of chemisorbedparticles

Introduction of methanol in a contact with iridium and rhodium electrodes at qr w O-3-0.4 V produces a non-steady current of hydrogen ionization. Thus, analogously to platinum the methanol adsorption on iridium and rhodium, involves a C-H bond cleavage and a far-reaching molecular destruction. The carbonaceous residues are strongly bounded with the surface atoms of the metal. In general, the methanol adsorption can be represented by the scheme CH,OH =

Me,(CH,,OH)

+ n H,,.

(11)

If the adsorption occurs at open circuit, the formation of hydrogen should lead to a shift of potential. At closed circuit and qr = const > 0 V the hydrogen atoms formed undergoes immediate ionization. To elucidate the nature of adsorbed particle the quantity of electricity expended in the oxidation of chemisorbed particles (QJ, and that consumed by the oxidation of hydrogen, produced in the dehydrogenation (Qeby”, were compared, with the

Adsorption and anodic oxidation of methanol on

iridium and rhodium

925

number of adsorption sites (measured from the diminution of hydrogen adsorption) occupied by molecular residues on the surface AQ&. As was shown earlier, on platinum in acidic solutions Q, = Q”,“y” = AQ=, and assuming that these conditions provide an oxidation to CO, with a release of six electrons, the chemisorbed particle should have a structure of C-OH. If however the final products contain those of incomplete oxidation: CH,O and HCOOH (as for example on platinum in alkaline solutions), then a certain quantity of particles would be present on the surface, which were sorbed via fission of one or two hydrogen atoms and released one or two electrons during oxidation to the corresponding products_ The over-all composition of the chemisorbed particles is determined by the composition of the products, since in all cases there is the same relation between Q,, Qghydr and AQE. Comparison of Q,, Qghrdr and AQH for rhodium was made at elevated temperature, 40°C. It was found that Q, = Qghy” = 1.3A QH. Assuming that the methanol oxidation on rhodium in acidic solutions is analogous to that on platinum, involving a release of six electrons, the relation obtained should indicate that a methanol molecule loses three hydrogen atoms and the remaining particle releases three electrons during its oxidation. The number of sites occupied on the surface, ie the number of bonds with the surface of the metal, is less than the number of split C-H bonds. Methanol adsorption on rhodium at open circuit obeys the same relation.= As follows from Table 1, AQH M Q $fhyk for different times of adsorption on smooth iridium in 1 N H,SO, + 0.1 M C&OH, ie adsorbing on the iridium surface from acidic solutions a methanol molecule should occupy as many adsorption sites as the number of hydrogen atoms it loses during dehydrogenation. TABLB 1.

Tads,

s

QH dehydrP c 8

AQ&,

PC

Q$=h’d=/AQE

dehyar COMPARISON OF QE OF ADSORPTION AT @” A

AQ,

AM)

8’ ON

IRIDIUM

O-4 V IN 0-l M Cl&OH

FOR DIPPERENT

TIMES

+ 1 N H,SO,

8

12

16

20

24

32

40

60

140

197 o-14 224 O-88

243 O-17 272 O-9

293 0.2 320 O-92

344 0.21 336 l-03

392 0.23 375 l-04

465 0.26 415 l-22

523 o-31 495 1.06

650 0.39 625 l-04

753 049 780 O-97

Average o-995

We failed to obtain the Q, value on the smooth iridium electrode, since it was difficult to determine separately the quantities of electricity expended in the oxidation of chemisorbed particles and in the adsorption of oxygen and due to a strong methanol effect upon the oxygen adsorption on iridium. Moreover, there is no information for the iridium electrode on the analysis of oxidation products that would allow us to determine the average number of electrons released by a methanol molecule in oxidation. The present authors and Volfkovich have employed a direct electrochemical method, which, on a porous iridium electrode, allowed the determination of (1) the number of electrons released in the oxidation of hydrogen generated in dehydration of a methanol molecule, (2) the number of electrons released in the oxidation of a chemisorbed residue and (3) the average number of electrons released in the oxidation of a methanol molecule.

926

V. S. BAGOTZIW, Yu. B. V~ssu.rsv, 0. A. KHAZOVA and S. S. SEDQVA

The measurement technique is based on a slow exchange between the solution in the pores of a porous electrode and outside of the electrode and can be accomplished in two versions : the adsorption and the diffusion methods. In the adsorption method, the porous electrode is impregnated with a diluted methanol solution at vr = 0.02 V (under conditions excluding any adsorption), after which the solution in the bulk is rapidly replaced by a supporting electrolyte. Knowing the open porosity of the electrode and the initial methanol concentration, we can calculate the total quantity of methanol molecules present in the pores of the electrode. At a potential jump to pad” = O-25 V all the methanol molecules from the solution in the pores are adsorbed ofi the electrode surface, and the quantity of electricity passed is determined from the i/7 curves. After that the potential was allowed to jump to qrox = 05-0+7 V and the quantity of electricity spent on the oxidation of chemisorbed residues was measured. The diffusion version differs from that of adsorption by a direct jump of the potential to yrox after which the i/T curve corresponding to a complete oxidation of methanol molecules inside the porous electrode into the final products was obtained. The average number of electrons lost by a methanol molecule in the oxidation was equal to 4 (by the diffusion method) and did not vary as the concentration increased 20-fold. The average number of electrons released in the dehydrogenation of one methanol molecule and in the oxidation of a chemisorbed residue, according to the adsorption procedure, was in each case 2. From data of electrochemical measurements on smooth iridium electrodes in acidic solutions, A& = Qghgdr, and it may be concluded that adsorbing on iridium a methanol molecule loses as an average two hydrogen atoms. The carbonaceous particle formed occupies two adsorption sites on the surface and releases two electrons in its oxidation to the final products. Non-steady currents in methanol oxidation, and dependence of dehydrogenationrate upon potential The non-steady currents observed when a methanol solution is brought into contact with pure surfaces of platinum, iridium and rhodium at O-l-O-6 V decrease as the surface coverage with organic matter increases. The relation between A& and a”,“‘” observed at all adsorption times, indicates that in acidic media no methanol can be chemisorbed on iridium and rhodium avoiding a dehydrogenation step. Thus, the non-steady current of hydrogen oxidation is related to the adsorption rate as iu = n, . I;. t’,&, (12) where n, is the number of H atoms splitting from a methanol molecule (see also2@). It was shown that on iridium in acidic solution the logarithm of the non-steady current falls linearly with increasing logarithm of adsorption time (with slope m = 06-0-8) at all potentials and concentrations, const

(13)

IH=-.

Tm

Substituting relation (6) for the methanol adsorption iE=k-

dWR = nlf. d7

rate into (12) we obtain

k& . c, . 13(l--=~‘).

Adsorption and anodic oxidation of methanol on iridium and rhodium

927

Figure 11 shows the adsorption rate (determined by a cathode pulse method from coverage changing with time) plotted in electric units against the non-steady current at 0.4 V, and the dependence on the non-steady current on coverage, coneming the validity of (14). As follows from the drop of the non-steady current measured at the same potentials and various methanol concentrations, this value is directly proportional to the bulk methanol concentration at 0’n = const (very high methanol concentratrons, ~o~,~n > 1 M, however, showed a deviation).

-5-

"E P a -6x8

Fro. 11. Change of non-steady current with time in 1 N H&SO, + 0.1 M CHLOH solution on iridium at different potentials, 26T. 1, ~lr 0.25 V; 2,0.4; 3,0*485 V; 4,055 V; 5,0.65

V.

From (14) we obtain iH =

const ,(l-l/cm') *

WI

Taking l/an’ = O-36 from the methanol-adsorption kinetic data on iridium, it can be seen that the experimental (13) entirely fits (15). Substituting equation (9) for the methanol-adsorption rate on rhodium into (12) gives . JH

= nl Fk T$‘.

c . exp(-ctarf’t&‘),

(16)

or replacing en’ with its expression through time r we arrive at the non-steady current/ time relation const . z==-. I-

07)

This recalls the relation obtained for the non-steady current on platinum. When passing from 8,’ < O-25 to t9n’ > O-25, the value of the constant should change. Figure 12 shows an experimental plot of the non-steady current in log i/log T co-ordinates. It can be seen that a linear part may be isolated with an almost unit slope, which corresponds to (17). Figure 13 shows the maximal non-steady-current/potential dependency for iridium in 1 M CH,OH + 1 N H$O,. At potentials from O-05 to O-3 V this dependence

V. S. BAGOTZKY, Yu. B. VASSILIEV,0. A. KHAZOVAand S. S. SEDOVA

928

FIG. 12. Dependence of logarithm of non-steady current at qr = 0.4 V on the logarithm of time in O-5 M CH,OH + 1 N HaSOd solution ondrhodium, 40°C.

-7-

I -0.2

04

O-B

0.6 pra

I

"

13. Dependence of the steady current (1) of methanol anodic oxidation, and the maximal non-steady current (2) on smooth iridium electrode on potential in 1 N HISO, + O-1 M CHIOH solution, 26°C. FIG.

Adsorption and anodic oxidation of me-01

on iridium and rhodium

929

obeys the Tafel equation with a slope O-1 15 V,

where ar = 0.5. As in experiments with platinum, the strong potential effect on the rates of methanol adsorption and dehydrogenation on iridium observed in this region is a consequence of the surface coverage with adsorbed hydrogen. Since in the potential range investigated the logarithm of the electrode surface coverage with adsorbed hydrogen is a linear function of potential, we obtain from (18) ‘IIWX

%I

= K.

e,af?

(19

In the double-layer region the non-steady current changes only very slightly with potential, increasing approximately twice in the region from O-35 to &65 V. At v= > 0.65 V the non-steady current and hence the maximal rate of adsorption with dehydrogenation decreases with potential as a result of surface coverage with oxygen. Figure 14 plots the non-steady and steady currents vs potential on rhodium in O-5 M CH,OH + 1 N H,SO,. The maximal non-steady current and hence the methanol adsorption rate on rhodium, as well as on iridium and platinum, show an increase in the potential region from 0.15 to O-3 V as hydrogen desorbes from the surface. In the double-layer region the adsorption rate shows a slighter change with potential.

IrL* 02

0.4

0.6

0.6

FIG. 14, Dependence of logarithm of the maximal non-steady current (1) and steady current (2) of methanol anodic oxidation on smooth rhodium electrode upon potential in 03 M CH,OH + 1 N HISOl solution, 40°C.

The phenomena in alkaline solutions are much more complex. A study of adsorption on iridium in 1 N KOH showed that Qghy” > Q,, ie the hydrogen quantity formed during the dehydrogenation step far exceeds the amount of chemisorbed particles produced. An analogous situation was observed on platinum in concentrated alkaline solutions of methanol, and also during oxidation of formic acid on active platinum and palladium catalysts. In the last case, along with adsorption and dehydrogenation, formic acid has been found to undergo a catalytic decomposition into Hz and COz.@ 6

V. S. BAOOTWCU, Yu. B. VASS~IEV,0. A. KHAZOVA and S. S. SEDOVA

930

methanol also undergoes Apparently, on iridium, along with chemisorption, catalytic decomposition into molecular hydrogen and stable oxidation products, OH /

CH,OH

wads

a

zw co2

i,*

IrmW%-,OH),~ + n Hads

/

\

gas phase

“cll”CH,O

nH++ne

+ H2

Thus, a parallel catalytic process produces a large quantity of hydrogen per chemisorbed particle. Indeed, as evident from Fig. 15, the non-steady current of hydrogen ionization is by about two orders of magnitude higher than the adsorption rate. The drop of the logarithm of the non-steady current again, as in acidic solutions, changes linearly with the logarithms of time and of coverage.

0

2

I -1.2 I

- 6.5 I

-I

-0.8

-7

-7.5 I

log 4

T.

s

1oge&,=o4vl

l.cqvakp,‘o4v)*

A/an2

FIG. 15. Change of non-steady current with time on smooth iridium electrode in 1 N KOH + 1 M CH,OH solution at differentpotentials,26°C. 1, q’r o-25 v; 2, o-4 v; 3, o-55 v; 4, o-65 v.

Figure 16 shows a dependence of the maxima1 non-steady current corresponding to the rates of catalytic decomposition and adsorption on iridium at 6n’ M 0, upon the potential in a methanol solution in 1 N KOH. In the potential region from O-05 to O-3 V this dependence obeys the Tafel equation with a coefficient a log ~/&JJ= 0.07 V, that is, almost twice as small as the value found in acid solutions. In the double-layer region the non-steady current varies very little, increasing five-fold in the potential region sweep from 0.25 to 0.65 V. At q.~~ > O-65 V the non-steady current and hence the rate of catalytic decomposition falls, because of oxygen adsorption on the iridium surface. At vr = 0.65-1.0 V, log i/v= is a linear plot with a slope -0+140 V. Steady-state

rate of methanoi oxidation

As seen from Fig. 13, the steady-state polarization curve of methanol oxidation on a smooth iridium electrode in 1 M CH,OH + 1 N H2S0,, within the potential

Adsorption and anodic oxidation of methanol on iridium and rhodium

931

FIG. 16. (1) Dependence of the steady current (1) of methanol anodic oxidation and of the maximal non-steady current at &’ = 0 on iridium upon potential in 1 N KOH + 1 M CH,OH solution, 26°C.

rangeO-35to O-55V, is described by the Tafel equation with a slope O-120 V; at positive potentials it deviates from the Tafel line, passing a maximum at 0.7538 V. As the potential increases further the current falls. The figure also shows the maximal adsorption rate (at OR’ w 0) potential dependence for the same conditions. A direct comparison of the two rates gives evidence that at’ qr < 035 V in acid solutions the maximal adsorption rate exceeds that of electro-oxidation, and under steady-state conditions adsorption cannot be the limiting step of methanol electro-oxidation. In the range from 0.3 to 0.7 V the maximal adsorption rate very slightly depends on potential, while the rate of electro-oxidation increases rapidly. As a result, they become commensurate above O-5 V, and the electro-oxidation rate starts to depend on the slower rate of methanol adsorption (at en’ FY 0): electro-oxidation leads within this potential range to a drop of the steady-state coverage of the iridium surface with chemisorbed organic particles, due to their oxidation. Polarization curves of methanol oxidation on iridium in 1 N H.$O, at methanol concentrations from 1O-2 to 2 M are shown in Fig. 17. Change of methanol concentration does not alter the general shape of the curve. Only the Tafel slope shows some increase (up to O-140 V) at the lower methanol concentration, and a more pronounced limiting adsorption current is observed. Above O-7 V, the oxidation rate decreases exponentially with rising potential or increasing coverage of the electrode surface with adsorbing oxygen. In the last case oxygen inhibits not the rate of electrochemical oxidation itself but rather the rate of adsorption. Figure 14 presents the steady current of methanol oxidation US potential on rhodium. The shape ofthe polarization curve on rhodium is entirely reminiscent of the corresponding dependencies on iridium and platinum. At v* < O-6 V the adsorption rate exceeds that of oxidation under steady-state conditions. The Tafel slope for the steady current of methanol oxidation on rhodium is 120 mV over the more

932

V. S. BAOOTZKY,Yu. B. VAEEZLIEV, 0. A. KHAZWA and S. S. SIIDOVA

I

I

o-2

0.4

0,8

0.6 pr‘

I I

V

FIG. 17. Dependence of steady current of methanol anodic oxidation upon potential of smooth iridium electrode in 1 N H&O, solution at different methanol concentrations. 6, 2 M. 1, 10-p; 2, 2 x lo-%; 3, 5 x 1O-p; 4, 10-l; 5, 5 x lo-‘;

O-5 to 0.65 V. At 0.6547 V the rates of adsorption and oxidation equalize, the steady state coverage of the electrode surface falls to zero, and the adsorption becomes a limiting step of the oxidation process. Thus the general character of the polarization curves as well as the relation of the adsorption and oxidation rates do not change when passing from platinum to iridium and rhodium. On iridium and rhodium, the potential at which the electro-oxidation rate begins to be dependent on the slower adsorption step is shifted to a less positive region than on platinum. The current maximum and the beginning of the drop on the polarization curve are also shifted, since adsorption of oxygen on rhodium and iridium begins at less positive potentials than on platinum. In alkaline solutions the polarization curves of methanol oxidation are analogous. For example, Fig. 16 shows the maximal non-steady current corresponding to the rates of adsorption and catalytic decomposition of methanol on iridium at en’ = 0, and the steady-state rate of methanol electro-oxidation plotted against potential in 1 M CHaOH + 1 N KOH. In the potential range O-3-0.6 V it is a line with a Tafel slope 0,135 V. At v* = O-65 V a maximum is observed, after which the current On iridium in alkaline solutions, as on falls, because of the oxygen adsorption. platinum, such behaviour is observed at potentials more negative than those in acidic media. In the region of medium concentrations the steady state rate of anodic oxidation of methanol on iridium shows itself as a function of fractional power of bulk range

TABLE 2

qr. V

0.4

0.45

0.55

*w O-6

0.7

B

044

0.52

0.54

O-78

0.9

Adsorption and anodic oxidation of methanol on iridium and rhodium

933

-5-

% Y ¶ -

-6

B

-7.

, -I

-2

I

d

0

I

RG. 18. Dependence of the rate of methanol anodic oxidation on iridium in 1 N H,SO‘ upon the bulk concentration of methanol at different potentials. 1, e)r O-4 V; 2,0.45 V; 3, 0.55 V; 4, 0.6 V; 5, 0.7 V; 6, 0.75 V.

concentration,

Fig. 18, ist = K . cs’.

(20)

The value of 0’ is close to O-5 at q+ < 0.6 V, whereas at vr > O-6 V it increases, approaching unit (at yr > O-7 V) when the adsorption is already a limiting step. With increasing methanol concentration, at a certain concentration the reaction rate reaches a limiting value and even decreases. Analogous dependencies were observed on platinum electrodes. Figure 19 shows the methanol electro-oxidation rate on iridium LWthe surface (instead of bulk) concentration of organic substance. At medium coverages the logarithm of the oxidation rate increases linearly with the rising logarithm of coverage, ie for q3r= const is* = k.Bgi (21) n

c-3 ”

0

-5-

.

4 0 a .,‘

3

-6-

0”

7-

(<

I 42

I

I

-m

-0.6

-0,6

-0-4

-0-2

Log 9;

19. Change of the rate of methanol anodic oxidation on iridium with the electrode surface coverage by organic particles in 1 N H,SO, solution at different potentials. 1, Qr 0.4 v; 2, o-45 v; 3, 055 v; 4, 0% v; 5, 0.7 v.

FIG.

934

Yu. B. VASSLIW,

V. S. BAOOTZKY,

0.

A.

KHAZOVA

and S. S. SEDOVA

= 26-2.8; thus if n’ = 6, /?’ N D5. Equation (21) for the oxidation current is analogous to that for the desorption rate from an exponentially inhomogeneous surface. Thus, the steady-state oxidation reaction of methanol is limited by the subsequent oxidation of the chemisorbed molecular fragments leading to their removal from the surface. When passing from one to another point in the polarization curve, not only the potential but also the surface concentration of the reactant changes. Hence, at a constant methanol bulk concentration the polarization curves do not account for the real effect of potential on the rate of electro-oxidation. To elucidate the real effect of potential on the methanol electro-oxidation rate on iridium, polarization curves were plotted at OR’ = const. As seen from Fig. 20, they are straight Iines within the range of potentials from O-4 to O-7 V, with slope O-12 V, ie where rri

ist =

W)

where 8” % 0.5.

-7-

1 . D2

I

I

1

0.4

0.6 Qr.

0.6

V

RG. 20. True polarization curves of methanol anodic oxidation on iridium in

1 N H,SO, solution at constantsurfacecoveragesof the electrodeby organic particles. 1, 0,’ = O-14; 2, O-19; 3, 0.23; 4,0-3; 5,0,36.

Combining (21) and (22) we obtain for the anodic smooth iridium electrode

iat = K . f3p’ . exp B”r;’ ( RTq ) =

oxidation

of methanol

on (23)

Analogously, the simultaneous effect of potential on the coverage and oxidation rate of the adsorbed particles affects the polarization curves on rhodium, shown in Fig. 14. The dependence known for 8 R’ on potential (Fig. 4c) allows us to correct the current values and obtain the oxidation current/potential plot, at 8, = const. The calulated plot has a slope 50 mV.

Adsorption and anodic oxidation of methanol on iridium and rhodium.

93s

The anodic oxidation of methanol was investigated at different pH. As follows from Fig. 13 and 16, on rhodium, iridium and platinum, to the first approximation, the slope of the polarization curve does not change with pH, and the curve follows the potential shift of the equilibrium hydrogen electrode. In an analogous manner, the methanol adsorption region is shifted at varying pH. Rates of methanol electro-oxidation on iridium (at v= = const) trs pH of solution are shown in Fig. 21. Such a change in the oxidation rate with pH at a constant

-4 t

-6-

-7

, 0

I 2

I 4

6

8

IO

; 12

14

PH Fro.

21. Dependence of the rate of methanol (0.1 M) anodic oxidation on iridium upon pH in phosphate buffer solutions at different potentials. 1, qr o-4 v; 2, O-5 v; 3, o-7 v.

potential (in respect to the hydrogen electrode in the same solution) is analogous to that on platinum and may result from the &’ variation with pH. To a first approximation, the oxidation rate constant of chemisorbed carbon-containing particles is independent of pH,

In some cases, a catalytic decomposition of methanol, depending on the pH, superimposes on this process. On platinum in concentrated alkaline methanol solutions, the superposition of a catalytic process may be considerable. On iridium (see Fig. 16) even in 1 M CH,OH + 1 N KOH at 9 M 025 V, the polarization curve shows a catalytic wave with a Tafel slope 0.07 V at qr < O-25 V. Even in acid solutions, methanol oxidation on a porous iridium electrode prepared by pressing iridium powder, showed a considerable catalytic wave (Fig. 22) with Tafel slope O-065 V. Probably the same reason accounts for the unusual shape of the polarization curve of methanol oxidation on a rhodium-plated electrode (Fig. 23). Within the potential range from O-45 to O-55 V the polarization curve is linear with slope 0.035 V. Unlike smooth rhodium, on the rhodium-plated electrode an earlier (by 150 mV) and sharper current drop is observed. However, all these changes observed on smooth electrodes owing to the imposition of a catalytic process are effects of the second order, and are beyond the scope of the present paper.

V. S. BAWTZKY, Yu. 3. VASSUIW, 0. A. KHAZOVA and S. S. SEDOVA

936

FM.

22.

Polarization curve of methanol anodic oxidation on (1) porous (2) smooth iridium electrodes in 1 N HISOb + 01 CH,OH.

I

1

0.2

0.6

0.4 Pr

ho.

’

V

23. Polarization curve of methanol anodic oxidation on a rhodium-plated rhodium electrode in O-5 M CHIIOH + I N HISO,. 40°C.

Mechanism of anodic oxidation of methanol From the kinetic dependencies obtained, we can conclude that, in general, the mechanism of methanol electro-oxidation on iridium and rhodium is the same as that proposed earlier for platinum. 1*2e*80 At ~)r -=cO-7 V a removal of chemisorbed fragment of a methanol molecuIe from the surface during its oxidation is the slow step of the process. The dependence of the oxidation rate on pH indicates that the oxidation of such particles should involve participation of adsorbed OH particles or some other active form of oxygen resulting on the surface from a discharge of water molecules (in acid solutions) or OH- ions (in alkaline solutions).

Adsorption and anodic oxidation of methanol on iridium and rhodium The mechanism suggested for explains all the experimental facts rhodium as well. Account should steps. The methanol dehydrogenation of three hydrogens but through a

937

the methanol oxidation on platinum adequately observed not only on platinum but on iridium and be taken, however, of some possible intermediate should undoubtedly proceed not via direct fission consecutive splitting of each hydrogen atom, ie

CHsOH -%

$H20H

CHzOH LXCXHOH

+ Heds, + Had_

X

CHOH -+;OH xx There occur two other simultaneous

+ H,,.

(10

(110

processes on the electrode,

Had+H++e Hz0 5

0)

(Iv) OH,,,

+ H+ + e.

0

The OH,, particles generated in the discharge of water may oxidize not only the C-OH particles but may also interact with the intermediately formed particles resultxxx ing after incomplete dehydrogenation,

C&OH X

+ OHads -%

CHsO + HsO,

CHOH xx

+ 2 OHad,, R,

HCOOH

+ H,O,

C-OH xxx

+ 3 OHads -%-

COs + 2 HsO.

(VI)

(VII) WW

As a result, the formation of the incomplete oxidation products CHsOH and HCOOH will be observed along with COs evolution. The relative yields of oxidation products will depend on the rates of the separate stages. Oxidation on platinum in acid solutions leads usually to CO,, ie it is the COH xxx fragment which mainly undergoes oxidation. In alkaline solution, however, HCOOis a main product. Hence, there is a predominant oxidation ofxCxH-OH (or $JFO-) particles instead of their subsequent

dehydrogenation

fox $Z;OH, ie k, > k3.

If at first adsorption alone is carried out at more negative potentials, the dehydrogenation then should probably proceed to the end (to C-OH) in both acidic and xxx alkaline solutions. If the generated particles are subsequently oxidized at higher potentials, then CO, should be the single oxidation product.31 Thus there is a clear difference between these experiments and oxidation under steady-state conditions, or, in other words, a difference between such expressions as “adsorbed” and “to be oxidized” particles (under steady-state conditions). The mechanism of methanol oxidation on iridium is entirely analogous to that described above, but with dehydrogenation restricted to the splitting off of two hydrogen atoms (either in separate dehydrogenation and oxidation or in oxidation under steady-state conditions).

938

V. S. BAOOTZKY, Yu. B. VASSILIEV, 0. A. KHAZOVA and S. S. SEDOVA

On iridium, oxygen adsorption is more reversible in respect to platinum and starts at lower potentials. Hence methanol oxidation on iridium should probably occur in the coverage region 8 cH > O-1; thus we have a true Tafel slope of O-120 V, On smooth rhodium the true Tafel slope is only slightly lower than on pIatinum and the dehydrogenation proceeds up to the cleavage of three hydrogen atoms. Thus, in spite of quantitative differences, there is a far-reaching qualitative identity for methanol adsorption and oxidation on platinum, iridium and rhodium. REFERENCES 1. V. S. BAG~TZKY and Yu. B. VASSILIEV,Electrochim. Actu 9, 869 (1964); II, 1439 (1966); 12, 1323 (1967). 2. B. B. DAMASKIN, 0. A. PETRY and V. V. BATRAKOV, Adsorbtsiu orgunicheskih soedineny M eIectrodXh, p. 243. Izd. Nauka, Moscow (1968). 3. B. PIERSMAand E. GILEADI, in Modern Aspects ofElectrochemistry, No. 4, ed. J. O’M. B~CKRI~, p. 47. Plenum, New York (1966). 4. B. PIERSMA, in Electrosorption, ed. E. GILEADI, p. 19. PIenum, New York (1967). 5. M. W. BREITER,Electrochim. Actu 8,973 (1963). 6. A. T. KUHN, H. WROBL~WA and J. O’M. BOCKRIS, Trans. Faraday. Sot. 63,1458 (1967). 7. Ii. BINDER, A. K~~I~LINOand G. SANDSTEDE,in Hydrocarbon Fuel Cells, ed. B. S. BAKER, p. 91. Academic, New York-London (1965). 8, C. E. HEATH, Proceedings of the Symposium on Materials Associated with Direct Energy Conversion, p. 53. A.I.Ch.E.-L Chem. E. Symposium Series No. 5, London (1965). A. G. P~HENI~HNIKOV and R. KH. BURSHTEIIN,Elektrokhimica4,508 (1968). 9. A. A. MI-, 10. V. S. ENTINA, 0. A. PETRY and I. V. SHELEPIN,Eiektrokhimiu 2,457 (1966); V. S. ENTINA and 0. A. PETRY, Elektrokhimia 3, 1237 (1967); 0. A. PETRY and N. L~KHANAI, Elektrokhimiu 4, 514, 656 (1968). 11. 0. A, PETRY, B. I. PODLOVCHENKOand A. N. FRUMKIN, in Sovremennye Problemyfizicheskoi Khimii, Vol. 2, p. 196. Izd. MGU, Moscow (1968). 12. K. J. CATHRO, Efectrochem. Tech. 5,441 (1969). 0. A. KHAZOVA, Yu. B. VASSIIJ~Vand V. S. BAOOTZKY, Electrokhimiu 1,84 (1965). ::: S. S. BESKOROVAYNAYA,Yu. B. VASSILIEVand V. S. BACIOTZKY, Electrokhimia 1, 1029 (1965); 2,167 (1966); J. WEBER, Yu. B. VASSILIEVand V. S. BAOOTZKY, Electrokhimiu 2,515.522 (1966). 15. M. W. BREITER,C. A. KNORR and W. VGLKE, Z. Eiektrochem. 59,681 (1955). 16. F. WILL and C. A. KNORR, 2. Elektrochem. 64, 270 (1960). R. V. MARVET and 0. A, PETRY, EIektrokhimia 3,1445 (1967). :8’ M. W. BREITER,Z. phys. Chem. N.F. 52,73 (1967). 19: Yu. M. ‘IkzIRIN, Dokl. Akad. Nauk SSSR 126,827 (1959). M. R. T ARASEVLCH,K. A. RADWSHKINA and R. KH. BURSHTEIN,Efektrokhimia 3,455 (1967). ;:: M. I. TEMKM, Zh.$z. Khim. 14, 1153 (1940); 15,296 (1941). 22. S. Z. ROGINSKY, Adsorbtsia i kataliz na neodnorodnikhpoverkhnostiakh. Izd. Akad. Nauk SSSR, Moscow (1948); JA. B. ZELDOVICH, Acta Phys.-Chim. USSR 1,961 (1935). 23. S. B. BRUMMEICand R. CAHILL, Discuss. Faraday Sot. 45, (1968). S. TRASA~ and L. FORMARO,.7.electroanal. Chem. 17, 343 (1968). g: A. N. FRUMKIN, Dokl. Akud. ZVuukSSSR l&l,1432 (1964). D. H. BANGHAM and F. F. BURT, Proc. R. Sot. A105,481 (1924). ;4 T. KWAN, J.phys. Chem. 60,1033 (1956). 28: T. BIE~LER and D. F. A. KOCH, J. efecfrochem. Sot. 114,904 (1967). 29. Yu. B. VASSILIEVand V. S. BAOOTZKY. in Toplivnye elemnty: Kinetika e1ektrodnykhprotsessov, p. 280. Izd. Nauka, Moscow (1968). 30. V. S. BAIXTZKY, Yu. B. VASSILIEV,0. A. KHAZOVA and S. S. BESKOROVAINAIA,in Toplivnye elementy: Kinetika elektrodnykh protsessov, p. 198. Izd. Nauka, Moscow (1968). 31. B. I. PODLOVCHENKO,A. N. FRUMKIN and V. F. STENIN. EIektrokhimiu 4,339 (1968).