J. Electroanal. Chem., 127 (1981) 157-167 157 Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands

EFFECT OF 0 2 REDUCTION ON THE COVERAGE WITH COae AT PLATINUM IN ACID SOLUTION AND REACTION MECHANISMS

M.W. BREITER

General Electric Corporate Research and Development, Schenectady, N Y 12345 (U.S.A.)

(Received 16th January 1981; in revised form 23rd April 1981)

ABSTRACT

The coverage of carbon monoxide on platinized platinum decreases only slightly during the reduction of molecular oxygen at 0.1, 0.2 and 0.3 V, while it disappears within the initial 500 s on smooth platinum in 0.5 M H2SO 4 at room temperature. The removal of COad during the stirring of 02 is due to intermediately formed hydrogen peroxide. The removal rate differs so much on smooth and platinized platinum because the total formation of H202, referred to the real surface area, is considerably larger on the smooth electrode than the platinized electrode. In contrast to the oxidation of molecular hydrogen, the reduction of molecular oxygen is not affected by COad on the platinized electrode. The 02 reduction remains a predominant four-electron process in the presence of COad- The results are discussed in terms of different pathways for the 02 reduction.

INTRODUCTION

The effect of chemisorbed carbon monoxide on the cathodic evolution [1] and anodic oxidation [2] of molecular hydrogen was previously studied at two smooth platinum electrodes of different shape in 0.5 M H2SO 4 at room temperature. At constant potential, the rate of each of these two reactions was found to decrease markedly with increasing coverage of carbon monoxide > 0.6. In contrast, the inhibition was small at coverages < 0.6. Mass transport processes remained rate- determining in the latter region.

A study of reactions involving molecular oxygen is reported for smooth and platinized platinum, covered with COad , in this communication. The investigations were restricted to potentials of the so-called hydrogen region [3]. Chemisorbed carbon monoxide is anodically oxidized [4] at these electrodes at potentials > 0.3 V, referred to a hydrogen electrode in the same acid solution. Hydrogen is evolved at potentials < 0.05 V in solutions, stirred with an inert gas or 02. Thus, the potential range of the study was limited to a region in which the limiting current of convective diffusion of molecular oxygen is reached [5,6]. The work was carried out to obtain information of the following type:

(a) To what extent is the coverage with COaa affected by 02 in the said potential range?

(b) If the coverage does not decrease too rapidly with time on one of the

0022-0728/81/0000-0000/$02.50 © 1981 Elsevier Sequoia S.A.

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electrodes, is the net rate of oxygen reduction the same at constant potential in the absence and presence of COad?

(c) Does 'the formation [7-9] of H202 depend upon the coverage with COad on one of the electrodes?

(d) How does the net reaction

GOad + ½ 02 = CO2 (1)

proceed under the given conditions? The effect of aging of smooth platinum electrodes on the 0 2 reduction was

previously studied [10] in sulfuric acid solutions of different concentrations after an initial "activation" procedure. A transition from a four-electron process to a two-electron reduction was observed by a galvanostatic technique. The 0 2 reduction was inhibited [11] in sulfuric acid solution contaminated by air. On the rotating ring-disc electrode both the ring and disc current decreased at constant potential of the disc. It was stated [11] that a change in the mechanism did not occur.

EXPERIMENTAL

The reader is referred to ref. 12 for details concerning the preparation of the electrolyte (0.5 M H2SO4), the purification of the solution and the Pyrex glass vessel. The large platinum foil with a geometric surface area A = 83 cm 2 and the charge equivalent sQH = 0.28 mC cm -2 for a monolayer of H atoms in the absence of adsorbed impurities was used as the smooth electrode. A platinized platinum foil opposite the smooth foil served as the counter-electrode in this case. Since it turned out that the coverage with COaa decreased too rapidly with time on the foil for a successful investigation of question (b) of the Introduction, more experiments were made on the aged platinized cylinder with A = 50 cm 2 and sQn = 64 mC cm -2. The absence of adsorbed impurities was verified by the characteristic shape of an anodic charging curve [12] in the beginning of every run.

A run involved the following steps: (a) The test electrode was brought to 0.1 V under nitrogen stirring at 1 cm 3 s

and kept there potentiostatically. (b) The nitrogen stirring was replaced for 600 s by stirring with CO of CP grade

to form a monolayer of COaa. (c) The electrode was stirred with nitrogen for 1000 s to remove the dissolved

carbon monoxide from the solution. (d) The electrode potential was moved to 0.05 V by a cathodic current pulse. Then

the electrode was left at open circuit while the electrolyte was replaced once with fresh 0.5 M H2SO4, saturated previously with nitrogen. Air did not have access to the compartment of the test electrode during this procedure. Replacement of the electrolyte is necessary, especially for the platinized platinum electrode, since a small amount of soluble organic compound is formed [13] by a reductive process in the presence of CO at potentials of the hydrogen region.

(e) An anodic charging curve was recorded from 0.05 to about 0.4 V, avoiding the

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oxidation of COad. The coverage with COad was determined in the usual way [12] from the change of the saturation coverage of Had:

0co = 1 --sQH/sQH (2)

Here sQn and HQ~ denote the charge equivalent of hydrogen monolayer coverage in the absence and presence of COad.

(f) The electrode was potentiostated at the desired potential U. The nitrogen stirring was replaced by stirring with molecular oxygen of CP grade at 1 cm 3 s - i . The current I was recorded as a function of time t until a steady state was achieved.

(g) The oxygen stirring was replaced by nitrogen stirring and the current was recorded at the same potential as in step (f) as a function of time up to 3500 s.

(h) The potentiostatic circuit was disconnected. An anodic charging curve up to about 1.2 V was taken subsequently.

After replacing the electrolyte with fresh electrolyte a new run was started with step (a). To obtain information about the formation of H202 in the absence of COad, steps (f)-(h) were carried out.

The charging curve of step (h) displayed only a short arrest, owing to the anodic oxidation of COad to CO 2, for the smooth platinum foil. Most of the chemisorbed carbon monoxide disappeared during step (f) at potentials of 0.1, 0.2 and 0.3 V. Information about the rate of removal of COad by the net reaction (1) was gained at 0.2 V during step (f). A rapid cathodic potential sweep (6 V s ~), starting at 0.2 V, was repeatedly applied at different times during 1500 s of 02 stirring. The voltam- metric I -U curves were photographed from the screen of the Tektronix 502 oscilloscope.

All the experiments were carried out at room temperature (22 + 1 °C).

RESULTS

The traces of cathodic i-U curves obtained from 0.2 V at 6 V s - 1 on the smooth platinum foil at different times during step (f) are replotted in Fig. 1. The initial coverage with COad was 1 for curve (a). Trace (e) coincides with trace (f), taken from 0.2 V in the absence of COati.

The coverages 0CO,i n at the beginning of step (f) and 0CO,end at the end of step (g), are given for four different potentials for the platinized electrode in Table 1.

The cathodic current due to 02 reduction during step (f) is given as a function of time for the platinized electrode at 0.1 V in Fig. 2. The initial coverage with COad

TABLE 1

Coverage with COaa at the beginning of the 02 stirring of step (f) and at the end of N 2 stirring of step (g) on platinized platinum

U/V 0.1 0.2 0.3 0.4

0co,i n 0.95 0.95 0.94 0.91

0CO,end 0.91 0.90 0.77 0.70

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u/v -0.4 -0.2 0 0.2 I

~ Y . . . . ~6 0 . . . . . lOgO0 . . . . . ~500 I t/S

Fig. 1. Voltammetric current-potential curves taken from 0.2 V at 6 V s - ~ on smooth platinum in 0.5 M H2SO 4 after different times of 02 stirring. Initial coverage with COad was 1 for curves (a)-(e) and 0 for curve (f). (a) 0 s; (b) 90 s; (c) 150 s; (d) 300 s; (e) 1000 s; (f) 0.

Fig. 2. Cathodic current as a function of time during the 02 stirring on the platinized platinum electrode at 0.1 V: (©) initially 0co =0; (A) initially 0co =0.95.

0.5 &

I - - 4 m

0.1

\ \

t /$ Fig. 3. Decay of the cathodic current on platinized platinum as a function of time during step (g) at different potentials in the absence of COaa: (O) 0.1 V; (A) 0.2 V; (V) 0.3 V; ([]) 0.4 V.

161

50

.~ 0,5 E

0.1

O.OI \ \ \

\

I I ~ , I I000 2000 3000

I/S

Fig. 4. Decay of the cathodic current on platinized platinum as a function of time during step (g) at different potentials. Coverages 0coxn d after 3500 s are given in Table 1. (©) 0.1 V; (A) 0.2 V; (V) 0.3 V; ([]) 0.4 V.

was 1 or 0. Similar curves were obtained on the smooth electrode. There was a difference in the final value of the limiting diffusion current density: 1.6 mA cm -2 for platinized platinum and 0.62 mA cm -2 for the smooth electrode. It is likely that the smaller limiting current results from the less effective stirring of the foil.

The decrease of the cathodic current with time during the nitrogen stirring of step (g) is shown for the platinized electrode in Figs. 3 and 4 for different potentials. The coverage with COad was zero for the measurements in Fig. 3. It varied between 0co,i" and 0CO,end, given in Table 1, for the measurements in Fig. 4. The decrease of the current on the smooth platinum electrode was studied only at 0.2 V. The log I- t curve on the smooth electrode was parallel to that of the platinized electrode during the first 500 s. After 1200 s the current densities at 0.2 V were 7 × 10 -6 A cm -2 for the platinized electrode and 5.8 × 10 -6 A cm 2 for the smooth electrode.

DISCUSSION

Change of carbon monoxide coverage with time during 02 stirring

The i-U curve (a) in Fig. 1 is characteristic for smooth platinum covered by a nearly complete monolayer of COad. There is no cathodic wave between 0.2 and 0 V,

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resulting from the deposition of Had. The adsorption sites are blocked by COaa. The rise of l i ] with potential, starting at about - 0.1 V, is due to hydrogen evolution. The H 2 evolution reaction is strongly inhibited [1]. Curve (b) taken after 90 s of 02 stirring displays a relatively small wave between 0.2 and 0 V. The height of this wave increases with the time of 02 stirring. Simultaneously, the beginning of H 2 evolution is observable at less negative potentials. Finally, the i -U curve (d) differs only slightly from curve (f), measured in the absence of rOad. The results in Fig. 1 demonstrate that the coverage with rOad disappears with the time of O; stirring during step (f) on smooth platinum. The average rate of rOad removal was estimated from the change of 0co and the charge equivalent sQco = 0.45 mC cm -2 of the saturation coverage of carbon monoxide: 0.75 X 10 6 A c m -2 for 90 s; 1.2 X 10 6 A cm -2 for 150 s; 1.5 × 10 6 A cm -2 for 300 s. The latter rate represents the average rate of removal of the saturation coverage, since curve (d) in Fig. 1 is very close to curve (f). For the estimate, the value of 0co was approximately obtained from the change of the charge under the cathodic wave between 0.2 and about 0 V, reflecting the increase of the hydrogen coverage with time (compare eqn. 2).

In contrast to smooth platinum, the coverage with GOad decreases only slightly on the platinized electrode at 0.1 and 0.2 V during the sequence of steps (f) and (g). The decrease becomes larger with increasing potential of execution of steps (f) and (g), but the coverage remains above about 0.7 (see Table 1). It is suggested that a large part of this decrease of 0co is due to the oxidation of the so-called type II species [4] which are formed by the interaction of carbon monoxide with platinum, covered with Had. The oxidation of these species starts at potentials of about 0.3 V in 0.5 M H2SO 4 at room temperature. At 0.1 and 0.2 V where the oxidation of type II species is negligibly small [4], the average rate of rOad removal during steps (f) and (g) amounts to 1 × 1 0 6 A r m 2.

The average rate of removal of a small portion of COaa is close (0.75 × 10 6 vs. 1 )< 1 0 - 6 A cm -2) for smooth and platinized platinum. This rate is much smaller than the limiting current densities of convective diffusion of molecular oxygen. A similar statement holds for the average rate of total removal of rOad on smooth platinum at 0.2 V. It is concluded on the basis of the above results that:

(a) Net reaction (1) is not controlled by mass transport of 02. (b) Mass transport of another soluble compound of a reducing nature is rate-

determining. Conclusion (b) follows from the fact that the average rate of rOad removal is practically the same for the two electrodes when it is referred to the geometric surface area and not to the real area. The real area of the platinized electrode is about 230 times that of the smooth electrode, as judged from the ratio of the respective sQn values.

Evidence for the formation of H202 during the 02 reduction will be presented in the next section. Since HzO 2 is produced, it is suggested that the removal of CO~d occurs under the given conditions by the reaction

COad q- H202 = CO 2 -- H20 (3)

Since the efficiency of H202 formation was found to be larger at short times after

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the start of the 02 reduction at constant potential than at long times on smooth platinum [5,7], hydrogen peroxide should be immediately available for the occur- rence of reaction (3) under the present experimental conditions.

Note that the conclusion in the preceding paragraph is restricted to operation at constant potential in the hydrogen region. Since the potential does not stay constant during the COaa removal by 02 at open circuit, the mechanism is more complicated in this case. The occurrence of reaction (3) under certain experimental conditions is an additional feature in electrochemical studies, not common to gas-phase studies of net reaction (1).

Formation of hydrogen peroxide

The current-time curves, given in a semilogarithmic plot in Figs. 3 and 4, consist of two linear parts with different slopes. The first part extends to 1000 s in Fig. 3 and to about 500 s in Fig. 4. The current decreases rapidly with time, Independent of potential, the current has the same value at a given time there. This implies that the limiting current of O 2 reduction is measured. It decreases with time since the bulk concentration of O 2 becomes smaller during the N 2 stirring by step (g). The extrapolation of the first linear part, shown in Figs. 3 and 4 by a dashed line, indicates that the bulk concentration of 02 would be reduced to a very small level after about 2300 s if the further access of traces of O 2 could be kept low.

After a curved part, the I-t curves become linear again at all the potentials in Fig. 3 and at 0.1, 0.2 and 0.3 V in Fig. 4. The second linear part which has a smaller slope than the first part is attributed to the reduction of hydrogen peroxide formed mainly during step (g), and to a small extent during the first 500 s of step (f). It might be argued that the second linear part still represents 02 reduction, since traces of oxygen get into the solution even under N 2 stirring. Two arguments are in favor of H202 reduction:

(a) The second linear parts are located higher with decreasing potential in Figs. 3 and 4. The position should be independent of potential if 02 reduction is involved.

(b) A linear curve is expected if the bulk concentration of H202 decreases at a constant rate [5] (here limiting current of H202 reduction). For the O 2 reduction the curves should be parallel to the abscissae after 3500 s, since a steady state is approached with regard to the bulk concentration of 02 .

By extrapolating the linear portion of the I-t curves in the semilogarithmic plot to the time of origin (about 50 s in this case), an estimate of the charge (2FbcH2o2V) required for the formation of H202 during step (f) may be obtained. The slope of the second linear part is proportional [5] to DA/rV. Here D designates the diffusion coefficient of H202, A the geometric surface area, 8 the diffusion-layer thickness and V the volume of the solution. At 0.1 V the value of 2FbCH2oViS 1 C at 0co = 0 and 8 C at 0CO,end = 0.91. The total charge consumed during step (f) was 122 C. Thus only a small fraction of the total current was used for the H202 formation at 0.1 V. The amount is still smaller at 0.2, 0.3 and 0.4 V. The O 2 reduction is mainly a

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four-electron process in the presence and absence of GOad at platinized platinum:

0 2 + 4 H + + 4 e - = 2 H 2 0 (4)

The results obtained here at 0co = 0 are in agreement with those [5] on platinized platinum in solutions of different pH.

The I - t curves in Fig. 3 demonstrate that the extent of H202 formation during step (f) becomes larger with decreasing potential in the hydrogen region. It is suggested that the reaction

Ha d + 02 = UO2a d (5)

contributes as the initial step in a mechanism [14] involving adsorbed hydrogen atoms. The contribution of this mechanism becomes larger between 0.2 and 0.1 V where the loosely bonded hydrogen atoms are adsorbed in the absence of COad. The mechanism is only effective in the hydrogen region. The increase of the H202 production in the hydrogen region during voltammetric sweeps on the smooth platinum disc [11] may be interpreted in the same fashion.

A similar dependence of the H20 a formation upon the potential in the hydrogen region is found in the presence of large coverages of GOad (compare Fig. 4). The free energy of hydrogen adsorption in the Temkin model for the heterogeneous surface of platinum electrodes was found to decrease [9] in absolute values with increasing values of 0co > 0.6. Hydrogen atoms are less tightly bonded in the presence than in the absence of COad. The rapid increase of the H202 formation with potential and the greater extent in the presence of CO~d are correlated to the strength of the interaction between hydrogen atoms and the platinum surface. Loosely bonded hydrogen appears to facilitate reaction (5).

Effect of COaa on the 0 2 reduction at platinized platinum

Figure 2 shows the increase of the limiting current of 02 reduction at 0.1 V in the presence and absence of COaa as a function of time during the 02 stirring of step (f). The current reaches a plateau after about 1000 s when the equilibrium value of the bulk concentration bCo: is reached. The curves which are not given here for 0.2, 0.3 and 0.4 V are similar to the respective curves in Fig. 21 A difference in the shape of the l - t curves in the presence and absence of COad could not be detected. These results suggest a negligible effect of COad on the net reaction (4).

The influence of COad on the oxidation of molecular hydrogen and the reduction of molecular oxygen at potentials of the hydrogen region are compared for the same platinized electrode in Fig. 5. For the H 2 oxidation, the ratio of the current at 0.18 V to the limiting diffusion current I~,u2 is plotted vs. 0co. The curve is similar to that [2] on smooth platinum. The ratio is practically constant up to 0¢o = 0.6. Then it decreases rapidly with COad. In contrast, the ratio I/I~,o~ is practically independent of 0co. There is some spread in the experimental data, but this does not change the statement. The current at different values of 0co was measured at 0.1, 0.2, 0.3 and 0.4 V. The value of II,o2 was determined at 0co = 0. The coverage was taken as

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1.0

0.5 I-t

O O---=O

0 I 1 0 0.5 1.0

ec0

Fig. 5. Plot of the ratio of current to limiting diffusion current of species j: (O) 02 reduction; (R) H 2 oxidation.

1/2(0co,i n + 0CO,end ). The latter approximation does not affect the results because I/I1,o2 is independent of 0co.

The results in Fig. 5 confirm the preceding conclusion that the 02 reduction does not depend upon the coverage with carbon monoxide up to 0.95. In another type of study [15] it was found that adsorbed layers of different ad-atoms did not affect the 02 reduction on platinum or exerted a slightly inhibiting effect. The inhibition of the 02 reduction in air-contaminated acid solution amounted to about 8% on smooth platinum [11]. Silver ad-atoms on smooth platinum [16] represent the exception among the ad-atoms studied so far. A monolayer of Agad led to a strong inhibition of the 02 reduction.

If it is assumed that only sites free of COad are involved in the 02 reduction, a strong inhibition might be expected for the following reasons:

(a) Under mass transport control, the effective area for diffusion of 0 2 should be small at large coverages of COad, leading to a reduction of I~,o2"

(b) If one of the kinetic steps involving adsorbed intermediates becomes rate- controlling, a strong inhibition should be observed. The H 2 oxidation represents an example (compare Fig. 5). The results in Figs. 2 and 5 are best explained by the assumption that it does not matter whether the 0 2 molecule hits a site free of, or covered with, COaa. Two interpretations are compatible with this assumption:

(c) The molecules are reduced wherever they hit the surface in a random fashion. (d) Surface diffusion is sufficiently rapid on top of COad that the 0 2 molecules

reach a site free of 0 2. Weak adsorption of 0 2 or subsequent intermediates is involved so that an influence of the large coverage with CO~d is not recognizable. The mechanism does not require dual sites. A distinction between possibilities (c) and (d) cannot be made on the basis of the

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present results. It is pointed out that the number of separate conditions which have to be fulfilled simultaneously for possibility (c) is great.

Pathways for 02 reduction

Attempts have been made [11,17] recently to establish a correlation between the type of 02 adsorption and the path of the reduction. If the molecule is bonded to either two neighbored Pt atoms (bridge model), or parallel to the surface to one Pt atom (Griffin model), the 02 reduction was proposed [17] to be a four-electron process. On the other hand, if end-on adsorption (Pauling model) existed, the formation of H202 was favored (two-electron process).

The bonding of 02 molecules to the surface should be strongly affected by the chemisorption of other species which do not participate to a large extent in reactions (4) or (5). Let us consider two limiting cases:

(a) Reaction (4) occurs only on free sites. (b) Reaction (4) takes place wherever the 02 molecules hit the surface, e.g. mainly

on sites covered by the other species. In case (a) sites with a relatively low heat of adsorption for the other chemisorbed

species are free. These sites should also possess a low heat of adsorption for 02 in the bridge model or the Griffin model. Adsorption according to the latter two models is unlikely for 02 on top of a chemisorbed layer of other species (case b). An effect, probably of an inhibiting nature, should be seen in either of the two limiting cases.

Chemisorbed H atoms in the absence of COad are one example for the other species mentioned above. The hydrogen coverage at a given potential was found [18] to be the same on smooth platinum under voltammetric conditions for N 2 and 02 stirring. Impedance measurements by voltammetry with superimposed ac did not reveal a change of the capacitive component in the hydrogen region when the Stirring was switched from N 2 to 02. On platinized platinum the rate of net reaction (4) is controlled by mass transport at 0.4V ( 0 H = 0 ) and at 0.1V (O H =0.8). Chemisorbed carbon monoxide represents a second example. At 0.3 V it is O H = 0, 0.94 ~< 0co ~< 0.77. Reaction (4) remains controlled by mass transport.

The above considerations suggest that an adsorption of 02 molecules according to the bridge or Griffin model is unlikely for platinized platinum. This conclusion is in agreement with earlier work in which the dissociative adsorption of 02 was ruled out for the mechanism of 02 reduction. The mechanism for 02 reduction proposed [18] there is in agreement with the present results.

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