Enhanced oxidation of carbon monoxide on platinum in HClO4 via interaction with acetonitrile

Ekctmchksica Acta, Vol. 39, No. 1, P&X13-21.1994 Printed in Great Britain.

0

0013~4686/w $6.00 + o.oLl 1993. Pqamon Press Ltd

ENHANCED OXIDATION OF CARBON MONOXIDE ON PLATINUM IN HC104 VIA INTERACTION WITH ACETONITRILE X. H. XIA and W. VIELSTICH Institute of Physical Chemistry, Bonn University, WegelerstraBe 12, D-531 15 Bonn, Germany (Received 17 May 1993; in reoisedform 1 July 1993) Ahstraet-The influence of acetonitrile on both the adsorption and electro-oxidation of CO at smooth and rough platinum in 0.1 M HClO, has been investigated with the help of on-line electrochemical mass spectrometry (DEMS). It is shown that a small amount of acetonitrile (AN) in solution catalyzes strikingly the bulk electro-oxidation of CO. The catalytic effect is much higher on smooth than on rough platinum surfaces. The catalytic activity also depends upon the concentration of AN, for a smooth Pt surface below O.OlM. However, the electro-oxidation of CO adsorbates is seriously inhibited by the presence of AN in solution. The interaction between the surface species derived from AN and CO was studied in great detail. The results show that the adsorption of CO obviously can still proceed by removing parts of preadsorbed AN. On the other hand, the adsorption of AN is completely inhibited on the surface previously covered with a monolayer of CO,. When both, AN and CO, are present simultaneously in solution, co-adsorbed

surface species are formed at the electrode surface. We think that the enhanced electro-oxidation of CO is behaviour of AN.

due to a “third body” effect modified by a special adsorption/desorption Key words: DEMS, acetonitrile, CO, surface species, catalytic effect.

400mV in the negative direction. This effect was explained by a “bifunctional mechanism”[18]. In this mechanism the substrate surface sites adsorb organic residues and their adjacent sites are occupied by upd adatoms on which an oxygen-containing species is formed. The oxygen-containing species in sequence oxidize the poisoning organic residues. The model is in good agreement with the “reaction pair” mechanism proposed by Gilman[19] in which an oxidized platinum suface species is required to react with CO for oxidation to CO*. This bifunctional mechanism is also supported by the enhanced oxidation of CO on electrodes modified by S, Se and Te adatoms in acidic[20, 213 and basic[20] medium. In addition, Breiter[22] reported small catalytic effect of upd Cu on the oxidation of CO. It has been reported already in 1972 that a small amount of acetonitrile in solution is of high electrochemical activity[23-251. There is considerable amount of experimental evidence[26] that acetonitrile behaves as an amphiprotic solvent, exhibits extremely weak acidic and slightly stronger basic properties. Because pure acetonitrile is electrochemitally inert, it is normally used as an organic solvent to study the double layer behaviour of the electrode surfaceC27, 281. According to the literature[24], the adsorbed acetonitrile becomes progressively reduced. This was confirmed by means of radio-tracer experiments[29]. The products of acetonitrile reduction on platinum have been investigated recently by means of DEMS[30], the results are the same as already suggested in the literatureC23, 291. The influence of acetonitrile on the oxidation of HCOOH on Pt was studied primarily by Angerstein-Koxlowska et aI.[31]. A considerable

INTRODUCTION

The electro-oxidation of CO in acid solution has been extensively studied, because nowadays MW fuel cell units are supplied with Hz/CO gas mixtures [l]. However, as is generally known, the oxidation of CO is subjected to a high over-potential (cu. 0.9 V vs. rhe) on smooth platinum in acid solution, because of the formation of strongly bound CO absorbates[243. By in situ ir spectroscopy measurements[5-81, CO adsorbates in the mode of linear-[5-81, bridge[8] and multi-bound to the electrode surface were detected. Recently, ir reflection absorption spectroscopy studies of CO adsorption on single crystalline platinum were also undertakenC9, lo]. Similar results were found. Many methods have been attempted to improve the catalytic activity of the existent catalysts[ll]. With respect to the oxidation of organic compounds[12-151 and carbon monoxide[16-221, the most attractive improvement of the catalytic activity was obtained in the recent two decades. The modification of several noble metals with submonolayers of foreign metals by underpotential deposition (upd) produces, in some cases, a considerable increase in the electrocatalytic activity of the substrate. For example, the upd of lead adatoms can increase considerably the electro-oxidation of formic acid on platinum in acid solution[14, 151. Motoo and co-workers have reported the catalytic effects of foreign adatoms as ruthenium[16], tin and germanium[17] and arsenic[18] on the oxidation of CO on platinum. It is reported[lfl that the presence of less than half a monolayer of tin adatoms shifts the potential of the electro-oxidation of CO by u J&l-c

13

X. H. XIAand W. V~CH

14

increase in current for the lirst oxidation peak was observed by introducing acetonitrile into solution, as well as by using other active organic compounds, such as nitromethane[32], whereas dimethylsulphoxide and acetone[33] show an inhibition. The explanation of the catalytic effects of active organic compounds on the electro-oxidation of HCOOH is still ambiguous. In literature[31], the catalytic activity is interpreted by the so-called “third body” effect, in which the adsorbed organic compound prevents the formation of poisoning species derived from HCOOH. In another report[32], the authors studied the influence of nitromethane adsorption on the oxidation of formic acid at platinum. By comparing the results of nitromethane with those of acetonitrile and dimethylsulphoxide, they reached the conclusion that a competition for adsorption sites and modifications of adsorption energy can be used to explain the specific influences of organic modifiers on the oxidation rate of formic acid. In another paper[33], Zelenay and Sobkowski, by means of the radio-tracer technique, examined the interaction between the adsorbates derived from HCOOH and organic modifiers (acetonitrile, dimethylsulphoxide and acetone), but an explanation of the catalytic mechanism has not been presented. In order to explain the catalytic behaviour of acetonitrile on the oxidation of organic fuels, in this paper the influence of acetonitrile on the adsorption and oxidation of CO was investigated by means of a combination of electrochemical and on-line MS techniques[3,34]. EXPERIMENTAL Chemicals All the solutions were prepared with M&pore water (> 18 MR). In order to avoid effects of specific adsorption of anions and hydrolysis of acetonitrile, 0.1 M HClO, (>70% R. G. Riedel-deHaen) was used as the supporting electrolyte. Acetonitrile (>99.98% A.R., E. Merck, Darmstadt, Germany) and CO 4.7(99.997%, Messer Griesheim) were used without further purification. Argon 4.8(99.998%, Messer Griesheim) was used to deaerate the solution and displace the dissolved CO as well. Instrumentations A standard three-electrode flow cell controlled by a traditional three-electrode potentiostat was used in normal electrochemical experiments, in which the platinum sheets with an area of 1.6cm’ (real area is 3.25 cmz) and 4.0 cm’ were used as working electrode and counter electrode, respectively. For the determination of volatile products, eleo trochemical mass spectrometry (DEMS)[3, 341 was used. A special three-electrode flow all (total volume about Sml) is constructed in such a way that allows the performance of the replacement of solution under controlled potential. The working electrode was a layer of DODUCO PtS (average diameter of a platinum particle is 1-5~) mounted on the PTFE membrane (Gore Tex S 10570; thickness 75pm, poros size 0.02~ and with a porosity of 50%) which was connected directly to the inlet of the ion

source of the quadrupole mass spectrometer QMG 112A (Balzer). This catalyst layer was also used as the interface between the electrochemical cell and the mass spectrometer. The counter electrode was a Pt wire (diameter of 0.5 mm). A reversible hydrogen electrode was used as referena electrode in all experiments, and the potentials reported in this paper refer to this electrode in the same solution. All experiments were performed at room temperature. Prior to each measurement, the working electode was activated in 0.1 M HClO, by cycling the potential between the on-set of hydrogen and oxygen evolution at a scan rate of 1OOmVs-i until a reproducible voltammogram was obtained. During DEMS measurements, likewise, the simultaneous measurement of the mass signal for CO, (m/e = 44) showed a potential independent ground value which indicates the absence of organic impurities[ 15,343. RESULTS

AND DISCUSSION

1. E@cts of acetonitrile on the oxidation of CO at platinum in HClO,, solution 1.1. General features on smooth platinum. The potentiodynamic Z/E profile (Fig. 1 dotted curve) of the oxidation of CO at platinum shows a sharp peak at about 0.9OV in the anodic sweep (compare Ref. [2]). Below this potential, the anodic current approaches zero due to the formation of a strongly bound CO adsorbate which inhibits the bulk oxidation of CO; at potentials higher than 0.9V, the strongly bound CO will be oxidized due to the formation of surface oxide. In consequence, the oxidation of CO reaches its diffusion controlled level. During the cathodic scan, the diffusion current will persist to a potential at which nearly half the surface oxides are reduced. At this potential the I/E profile exhibits a maximum. If the potential sweeps to more negative values, the electrode surface will be covered quickly by CO adsorbates and the electrode loses again its activity. 1.2. The e&&s of acetonitrile on the bulk oxidation of CO. As can be seen in Fig. 1 (solid curve), the

0.6

0.4 2

a2

,

10

10

Q5

1.5

EIVvs. RHE

Fig. 1. Cyclic voltammograms of the electro-oxidation of bulk CO (saturated in solution) on smooth platinum in 0.1 M HCIO,, in the absence (dotted curve) and in the presence of 0.01 M acetonitrile (solid curve). The real area of the platinum surface is 3.3 cm’. Scan rate 20 mV s- I.

Enhanced oxidation of carbon monoxide

presence of 0.01 M acetonitrile in solution produces a significant increase in current at lower potentials during the potential positive going scan. Both the on-set and the total region of oxidation are markedly shifted towards negative potentials. The influence of the concentration of acetonitrile on the oxidation of CO were also observed. As shown in Fig. 2, the reduced on-set potential AE = E(AN = 0) - E(AN) (where E(AN) is delined as the potential at an oxidation current of lOficm_’ in the anodic polarization. E(AN) and E(AN = 0) are the potentials in presence and absence of acetonitrile in solution) and the increased current AI = Z(AN) - Z(AN = 0) (where Z(AN) and I(AN = 0) are the oxidation currents at 450mV in presence and absence. of acetonitrile in solution) depend strongly upon the concentration of acetonitrile. Up to [ANI = 0.01 M, these two parameters increase with the concentration of acetonitrile in solution. The maximum catalytic activity is reached at [ANI z 0.01 M. But even at a concentration of AN as high as 1 M the catalytic effect in the lower potential region is still significant. On the other hand, the promoting influence of acetonitrile on the electro-oxidation of CO is still maintained at a potential scan rate as low as 1 mVs- ‘. It is also observed that the catalytic activity of AN decreases by stopping the potential lower than 1OOmV for a certain time (eg 90s) during the continous cyclic potential scans. However, it shows an increase by setting the lower potential limit between 150 and 300mV. In the form of cyclic voltammograms and MSCV, the oxidation of CO on the rough electrode proceeds at more negative potentials than on smooth surface (Fig. 1, dotted curve). At more anodic potentials, the current per real surface, on the other hand, is much lower than in the case of the smooth surface. Again, the addition of acetonitrile into the solution produces an increase in current during the scan at lower potentials (Fig. 3(a), solid curves). Above 0.7 V, the catalytic effect of acetonitrile on the oxidation of CO on rough platinum is not as considerable as that on the smooth surface. The simultaneously registered mass signal for CO2 (Fig. 3b) shows the

- Jo0

0

0 -0

-5

-4

-3

-2

-I

0

logCANl/M

Fig. 2. Dependence of the increase in current AZ = Z(AN) - Z(AN = 0) and the reduced on-set potential AE = E(AN = 0) - E(AN) on the acetonitrile concentration, where Z is the current at 45OmV and E is the potential with an oxidation current of 1Oficn1-~ during the anodic sweep,see Fig. 1.

15

Fig. 3. Oxidation of bulk CO (saturated in solution) on rough platinum (real area: 60.1 cm2, roughness factor 81) in the absence (dotted curve) and in the presence of 0.05 M CH,CN in 0.1 A4 HClO, (solid curve). (a) Cyclic voltammograms (cu), and (b) simultaneously recorded mass signal for CO,, m/e = 44 (MSCV).Scan rate 1OmVs-l.

same behaviour as the current-potential profile. It clearly indicates that the increase in current corresponds to the formation of the iinal product CO,, regardless of the presence or absence of acetonitrile in solution. 2. Inhibition of acetonitrile on the electro-oxidation of the CO adsorbates 2.1. Potentiodynamic experiments. In order to make clear the mechanism of acetonitrile catalysis on the oxidation of CO, study of the effect of acetonitrile on the oxidation of CO adsorbates is necessary. The experiment was performed in a flow cell system as follows: A monolayer of CO adsorbate was formed at 400mV (stopped during the cathodic potential sweep) in 0.1 M HClO, during 3 min of CO bubbling. After the dissolved CO was removed by argon bubbling for gmin, the base electrolyte was replaced by an acetonitrile containing solution. Then the potential sweep was started. Figure 4 shows that the presence of acetonitrile in solution affects not only the peak potential of the oxidation of CO adsorbates, but also the shape of the oxidation peak. In the absence of acetonitrile in solution, a sharp peak at about 775mV (Fig. 4, dotted curve) is observed. However, if acetonitrile is added into the solution, the oxidation of strongly bound intermediates derived from CO is inhibited as is indicated by a distinct shift of the oxidation peak towards more positive potentials (Fig. 4, dashed and solid curves). The shifted potentials show a linear dependence on the

X. H. Xu and W. Vmtsncri

16

(a)

,, a2

‘\

\ \ \r

1

2 \

j ;:

- 0.1 00

‘2

10

a5

15

\

1

\

'.\

EIVvs. RHE

0.0. 0

Fig. 4. Inhibition of dissolved acetonitrile on the electrooxidation of CO absorbates (formed at a stop at 400mV during the cathodic sweep) on smooth platinum, 1OmVs-i. Concentration of acetonitrile: zero (dotted curve), 0.1 M (dashed curve) and 1.0 M (solid curve).

concentration of acetnitrile in solution (Fig. 5). The slope of this line is 24mV per decade. On the other hand, the presence of acetonitrile in solution makes the oxidation peak of CO adsorbates wider, and at the positive side of the oxidation peak a shoulder is also present. This shoulder becomes significant with an increase of the concentration of acetonitrile. These effects are similar to those observed in the presence of Cl- ions in solution for the oxidation of preoriented platinum adsorbates on co electrodes[35]. The authors emphasized that the specific absorption of Cl- inhibits the formation of surface oxides, which in turn is necessary to oxidize the strongly bound surface species of CO. 2.2. Potentiostatic experiments. The inhibition of AN in solution on the oxidation rate of CO adsorbates can be more clearly identified in the potentiostatic transient experiments. Figure 6 shows the influence of acetonitrile on the oxidation rate of the

-6

-5

-4

-3

-2

-1

\

\ A,

100

50 ,

1

t/s

Fig. 6. Oxidation of CO adsorbates (formed as in Fig. 4) on smooth platinum at 710mV in the presence of 0.001 M (curve 1) and 0.01 M (curve 2) CH,CN in 0.1 M HClO,. (a) Current/time transient, and (b) Coverage degree of CO as a function of time. Do-, was calculated from the change of the oxidation charge under the current/time curve.

strongly bound CO adsorbates under potentiostatic conditions at the oxidation potential of 0.71V. A well-shaped current peak, having the general characteristics of a surface oxidation process, is clearly observed. The maximum of the current (I,,,_) and the according time (t,,J strongly depend on the concentration of acetontrile. If the experiment is undertaken without any CO adsorbates on the surface, the transient current decays abruptly to the base signal. During the whole process, no current maximum can be observed. Therefore, the coverage degree of CO adsorbates can be calculated from the charge passed under the transient, corrected from the blank experiment in which there is no CO in solution. The results are presented in Fig. 6(b). It is very clear to see that the coverage follows a monotonic function with time. The higher the concentration of acetonitrile, the slower the decrease rate of @co with time. Hence, it can be concluded that the presence of acetonitrile in solution inhibits the rate of the oxidation of CO adsorbates, which is consistent with the conclusion in section 2.1. On the other hand, from the i-t transient experiments, quantative results can be obtained in terms of the reaction model, previously elaborated by Giian[19] that the oxidation of the strongly bound CO proceeds in terms of a so-called “reaction pair” mechanism, in which the molecules of adsorbed water also play a decisive role as a surface oxidant of organic surface species. The corresponding rate equation of this mechanism can be described as follows:

0

IqWWM

Fig. 5. Correlation between the shift of the peak potential for the oxidation of CO adsorbates and the concentration of acetonittile in solution. Data obtained from the curves like in Fig. 4 at scan rates of lOmVs-i.

I=

dO

-Q,-_= dt

kz, O(1 - 0) exp(aEF/RT),

(1)

where Q, is the charge for the oxidation of a monolayer of CO adsorbates, 8 is the surface coverage

Enhanced oxidation of carbon monoxide

17

degree of strongly bound species derived from CO, k& is the oxidation rate constant and a, E, R, and T have their usual meanings. Analogous to literature[35], it is suggested that acetonitrile dissolved in solution acts as known from Cl- ions. The presence of acetonitrile in solution will decrease the surface concentration of the surface oxide via competitive adsorption, and as a wnsequence will decrease the rate of oxidation. However, in the case with AN in solution, equation (1) should be written as follows: I=_&$

tAN3/M

Fig 7. Correlation between [AN] and log k(JQ for the oxidation of CO adsorbates (formed as in Fig. 4) at 690 mV c) and 710mV (+), respectively.

= k,, O( 1 - 0) exp(a’EF/R T) = k(E,J0(1-

O),

(2)

where k,, and a’ denote the oxidation rate constant and charge transfer coefficient in the presence of acetonitrile in solution. k(E,J is the potential dependent rate constant. The k,,, a’ and NE,,,) are functions of the concentration of AN. In equation (2), the condition for the appearance of a current maximum is :

(SF),_,. =($)._._(‘),_, =O f3) where O,, denotes the coverage degree at the current maximum. Because 8 shows a monotonic decrease with time, as shown in Fig. 6(b). So by wmbining equations (2) and (3), it follows O,, = 0.5. The correlation between k(E,J and the current maximum can be described as yEox) = 41,, . In terms of this formula and the current maximum of the transient, the potential dependent rate function k(Q) is calculated (Table 1). Table 1 and the plots of log k(E,J vs. [AM (Fig. 7), show, apparently, that the rate of the oxidation of CO adsorbates is reduced with [AN]. On the other hand, the potential dependent rate constant k(E_) can also be determined from a slope of ln(1 - @)/a vs. t (equation 2). The results are in good agreement with those derived from the maximum of the current. For comparison, the values derived here are also listed in Table 1 (in parentheses). These quantative results once more strongly sustain the fact that acetonitrile in solution behaves as an inhibitor of the oxidation of CO adsorbates by inhibiting the building of surface

oxide. However, it should be emphasized that the surface species derived from AN has almost no effect on the oxidation of CO adsorbate (see sections 3.3 and 3.4). 3. Interaction between adsorbates derivedfiom acetonitrile and CO The knowledge of the interaction between adsorbates derived from acetonitrile and CO, could deliver us more information on the catalytic action of acetot&rile. 3.1. Surface species derived from acetonitrile. Wasmus and Vielstich[30] had supposed that two surface species are formed during the adsorption of acetonitrile in associated and dissociated forms (for abbreviation, we name them surface species I and II). Their composition depends on the adsorption potential. Surface species I corresponds either to desorption of inact acetonitrile molecules in the double layer region or to ethane and ammonia at potentials less than 1OOmV. Regarding to surface species II, one has an oxidation products CO2 and NO, at potentials more positive than 0.7 and 1.2V, respectively. Figure 8 shows the I/E profile and the simultaneously recorded mass signals for m/e = 41 (acetonitrile) and 44 (carbon dioxide). In the cv [Fig. g(a)] there is a current peak appearing at the same potential as that of the desorption peak of adsorbed acetonitrile in Fig. 8(b). At this potential a formation of CO, is not observed. It is suggested that this current peak corresponds to the desorption of surface species I derived from acetonitrile. The other results are in good agreement with[30], except a maximum of m/e = 44 at about 150mV in the

Table 1. The potential dependent rate function k&J and the corresponding parameters obtained from transient experiments E,,== 710mV

E_ = 690mV

log[AN] IM)

t, f.9

-1 -2 -3 -4

202.4 78.0 25.4 10.5

[Ai,‘=

0

6.1 3.9

i_ 014 1.69 5.9 19.8 64.6 104.0 141.0

t

‘IIUX

k

01A)

x lO_‘(C s-1)

0.0676(0.116) 0.236 (0.246) 0.792 (0.802) 2.584 (1.567)

117.0 47.0 15.3 6.1

3.2 10.45 36.0 105.0

0.128(0.225) 0.418(0.499) 1.44 (1.284) 4.20 (4.33)

4.16 5.64 (4.410) (5.130)

:::5

162.0 158.5

6.48 6.34 (10.12) (6.585)

4%) x 10-4 (C s-‘)

4%)

X. H. X~Aand W. VIEUTTCH

18

(al

n

I (10

a5 E/W.

10 RHE

lb)

25

0.0 a0

R5

10 EIVvs.RHE

15

Fig. 8. Oxidation of the surface species derived from CH,CN at 200mV on rough platinum. (a) cu current/ potential of the first and second cycles, (b) MSCV for CH,CN (m/e = 41) showing the desorption of the intact adsorbed CH,CN surface species, and (c) MSCV for CO1 (m/e = 44). The real area of the platinum surface is 89cm (roughness factor 120). Scan rate lOmVs_‘.

cathodic sweep, which cannot be attributed to the formation of COz, but possibly to other reduction products. 3.2. Pt surj&e previously covered with a monolayer of CO. In this experiment, CO was adsorbed at 200mV over 10min. During this time a monolayer of CO is really formed, as indicated by the complete suppression of hydrogen adsorption-desorption. The solution was then purged with argon for 8min. Under controlled potential, the base electrolyte was replaced by 0.05 M AN + 0.1 M HC104. After 10min adsorption of acetonitrile, the solution was replaced again by 0.1 M HClO* several times. The I/E profile and the mass signals for m/e = 41 and 44 were simultaneously registered (Fig. 9). Figure 9(a) and (b) clearly shows that with or without the addition of AN, the main desorption peak of the CO adsorbate and the voltammetric characteristics in the potential region of the formation of surface oxide

I5

0.5

1.0 EIVvs.RtE

l.5

Fig. 9. Oxidation of a complete monolayer of CO adsorbate formed at 2OOmV (obtained following the anodic sweep) at rough platinum (roughness factor: 120). (a) cu for the base electrolyte (dotted curve), the oxidation of CO in supporting electrolyte (dashed curve), and the oxidation of CO adsorbates with the addition of 0.05 M a&o&rile at 400mV and adsorption for lOmin, (b) MSCV for CO, (m/ e = 44). and (c) MSCV for CH,CN (m/e = 41). Scan rate lOmVs_‘.

where the oxidation of surface species II from AN takes place (see Fig. 8), show the same behaviour. Only the predesorption peak of CO, at the potential between 0.4 and 0.6V appears smaller. This can be considered as a partial desorption of weakly bound surface species[36]. These facts demonstrate that the surface species II derived from acetonitrile cannot be formed on a surface previously covered with a monolayer of CO,,. In Fig. 9(c), one has only a potential-independent ground signal for m/e = 41, which indicates that surfaces species I of acetonitrile cannot be formed on a surface previously covered with a monolayer of CO. The CO adsorbate bound with the platinum suface is obviously stronger than in the case of acetonitrile. The preadsorbed CO cannot be removed by acetontrile in solution. 3.3. Platinum previously covered with acetonitrile to its saruration value. In another experiment, ace-

Enhanced oxidation of carbon monoxide

tonitrile was first adsorbed at 200mV (stopped from an anodic potential sweep) from a 0.05 M solution for 10min. Then the solution was replaced by 0.1 M HClO,, under controlled potential, several times in order to be certain that there is no more acetonitrile in solution. After the introduction of CO for lSmin, current and mass signals for acetonitrile (m/e = 41) were simultaneously recorded with time (Fig. 10). Then the dissolved CO was removed by argon bubbling for 8min, and a subsequent potential sweep was taken. The shape of the mass signal for acetonitrile [Fig lo(b)] is similar to that of the current [Fig. lo(a)]. There appears a maximum during the introduction of CO into the system. In order to identity that the mass signal peak for m/e = 41 results from the desorption of previously adsorbed a&o&rile. Experiments with either argon bubbling or base electrolyte displacing were also carried out. Just as expected, in these two cases, the curve of the registered mass signal for m/e = 41 vs. time has no maximum. The base signal shifts to slightly higher values, which clearly means that the perturbance of the system only accelerates the flow-rate of volatiles into the mass spectrometer. In other words, the mass signal for m/e = 41 [in Fig. 10(b)] is really from the desorption of previously adsorbed acetonitrile. It can be concluded that the bonding energy of species I derived from AN with platinum is not so high as that of CO. The place occupied by surface species I will be covered partially by CO. This suggestion will be supported by the subsequent MSCV for CH,CN, m/e = 41 (Fig. 11). According to literatures[23-25, 291, the surface species derived from acetonitrile at potentials lower than 0.7V are in their reduced state. They have to release electrons to the electrode to form the intact molecules before leaving from the platinum surface. The released electrons give a current (i,,,,,). On the other hand, it is also reported[31] that the maximum coverage by acetonitrile is about 0.7. On this platinum surface there is still free space for hydrogen adsorption. The adsorbed hydrogen will be displaced by CO in solution[3]. Therefore, the

19

1.5

la)

1.0

as < E

=

QO

: i j < Al_ L

00

05

l.0

15

E/V=RHE

!

z

1.0

j /.._... .?‘

0

6.0.

mle=41

Ol 00

a5

10 E/V vs.RHE

15

FiS 11. Oxidation of co-adsorbed surface species derived from CHsCN (firstly formed) and CO (secondly formed) on rough platinum (rou&nes.s factor 120). The formation of the surface species takes place at 2oOmV for IOmin. For comparison, the oxidation of CO adsorbates alone is also shown (dotted curve). (a) cu current/potential, (b) MSCV for CO, (m/e = 44), and (c) MSCV for CHsCN (m/e = 41). Scan rate lOmVs_‘.

current observed in Fig. 10(a) should include both the oxidation of reduced acetonitrile (iaN) and the desorption of adsorbed hydrogen (i,,) i=ilvlfiH.

tl min.

Fig. 10. Intluence of the introduction of CO into the system at 200 mV on the previously formed monolayer of CHsCN surface species on rough platinum (rotqhness factor 120). (a) Current/time transient, and (b) simultaneously registered mass signal for CH,CN (m/e = 41).

(4)

In the subsequent cu and MSCV for mass signal of AN and CO, (Fig. ll), it can be seen that a submonolayer of CO adsorbate is formed by occupying the remaining free surface sites and same of the sites previously occupied by AN. The peak potential of the oxidation of strongly bound CO adsorbate retains the same value as that of the oxidation of CO alone [compare the dotted and solid curves in Fig 11(a) and (b)]. This fact demonstrates that, different from the case of AN in solution, the preadsorbed AN

20

X. H. Xu and W. VIELSIICH

surface species does not affect the oxidation rate of the CO adsorbate, its existence on the surface only changes the surface composition of CO adsorbate, namely, the weakly bound CO surface species cannot be formed, as indicated by the disappearance of the predesorption peak of CO at potentials between 0.4 and 0.6 V (compare the continuous and dotted curves in Fig. 11). In Fig. 1l(a) and (b), at potentials higher than 1.2V, an oxidation current and the corresponding mass signal for CO, relating to the oxidation of the surface species II of AN (see Fig. 8) still exists. But the magnitude of this signal is smaller in comparison with those in Fig 8, which indicates that the surface species II derived fron AN is displaced partially by the introduction of CO into this system. For the case of surface species I derived from AN, the current potential profile and MSCV for CO, does not give any information. However, the simultaneously recorded mass signal for AN [Fig. 11(c)] shows clearly that the magnitude of the desorption peaks of the intact adsorbed acetonitrile surface species is reduced in comparison to Fig. 8(b). By analysis of the mass intensity for m/e = 41, it is shown that in this case about 60% of surface species I derived from AN is removed after the introduction of CO. In addition, the first desorption peak of surface species I of AN becomes sharp and the potential of the shoulder also shifts to a more negative value. Based on these facts, parallel to the results from the transient experiment (Fig. lo), it can be concluded that CO,, should be the stronger bound adsorbate and the previously adsorbed AN (both surface species I and II) can be removed partially by the introduction of CO. An interaction among the surface. species derived from acetonitrile and CO take place. The result described here, that the preadsorbed acetonitrile may be displaced partially by CO, is contrary to that previously reported[33]. By using the radio-tracer technique, the authors had found that the preadsorbed modifier (such as acetonitrile, dimethylsulphoxide or acetone) cannot be removed by formic acid in solution. This difference could be due to the different adsorption processes of formic acid and CO. 3.4. Co-adsorbed species derived simultaneously from CH,CN and CO. Figure 12 shows the oxidation of the co-adsorbed surface species formed from a solution of 0.05 M CH,CN + CO (saturated in solution) + 0.1 M HClO,, at 200mV for 15 min. The CD and the corresponding MSCV for CO, and CH,CN show similar behaviour to the curves in Fig. 11. In Fig. 12(a) and (c), the sharp and big peak centred at 775mV proves that the strongly bound CO adsorbate is the main component on this surface. The disappearance of the predesorption peak between 0.4 and 0.6V means that the weakly bound CO adsorbate cannot be formed due to the interaction with the adsorbed AN surface species. In agreement with the results obtained in Fig. 11, at potentials higher than 1.2V, the increased current [Fig. 12(a)] and the mass signal for CO, [Fig. 12(c)] demonstrate clearly the existence of a small amount of surface species II of AN. In addition, the existence of surface species I of AN can be identified by the simultaneously recorded mass signal for AN [Fig.

(a)

4

1s

(b) m/e=&7

0.0

I 0.0

(15

EIVvsRHE

1.0

1.5

IC 1

mle=44

E/VvsRHE

Fig. 12. Oxidation

of co-adsorbed surface species derived simultaneously from acetonitrile and CO at rough platinum (roughness factor 178). Adsorption at 2OOmV in 0.05 M AN + 0.1 M HClO, + saturated CO solution for 10min. The oxidation of CO adsorbates in supporting ekctrolyte is also presented (dashed curve). (aj-cu &rent/ potential; (b) MSCV for CH,CN, m/e = 41, and (c) MSCV for CO,, m/e = 44. Scan rate lOmVs_‘.

12(b)]. In this case, the amount of the surface species I of AN decreases to about 27% compared to the adsorption of AN alone. Taking into account the adsorption kinetics and thermodynamics of acetonitrile and CO, it can be concluded that CO competes adsorption with AN, and the co-adsorbed surface species derived from AN and CO are formed on the electrode surface when AN and CO are present simultaneously in solution. 4 CONCLUSIONS Acetonitrile in solution considerably catalyzes the direct oxidation of CO. This catalytic effect is higher

21

Enhanced oxidation of carbon monoxide

on smooth than on rough platinum. The presence of acetonitrile in solution shifts the on-set potential of the oxidation of dissolved CO in the negative direction by about 450 mV. The oxidation of CO adsorbates is strongly inhibited by the presence of acetonitrile in solution. Surface species derived from AN have almost no effect on the oxidation rate of CO adsorbates. The surface species derived from acetonitrile can be partially removed by CO. On the contrary, a previously adsorbed monolayer of CO adsorbates cannot be displaced by acetonitrile in solution. However, if both AN and CO are present in solution at the same time, co-adsorbed surface species are formed. The results obtained support in the tirst approximation the “third body” mechanism[31] in which the role of modifiers is to tailor the surface place in such a way as to inhibit the formation of poisons. In consequence, sites for the direct oxidation becomes more available. But only with this effect, the catalytic action of AN on the oxidation of CO cannot be explained. Because the surface sites needed by CO are less than those required by AN. We think that the promoting effect of AN on the oxidation of CO comes from a competitive adsorp tion of AN and CO which leads to an equilibrium state between the co-adsorbed surface species. In addition we propose a bifunctional effect in which a desorption of surface species I derived from AN gives rise to a formation of new surface sites (or new holes) where the adsorption of water molecules becomes available. The adsorbed water molecules can oxidize the CO adsorbates. In consequence, the oxidation current of CO in the lower potential region is increased markedly. Here, the function of surface species I of AN is similar to that of upd metals (eg Sn and Ru etc.) which adsorb oxygencontaining species at lower potentials than the substrate (eg platinum).

7. W. G. Golden, K. Kunimatsu and H. Seki, J. phys. Chem. 86,127s (1984). 8. T. Iwasita, F. C. Nart, B. Lopes and W. Vielstich, Elecrrochim. Acro 37,236l (1992). 9. F. Kitamura, M. Takeda, M. Takahashi and M. Ito, Chem. Phys. IAt. 142,318 (1987). 10. A. Rodes. E. Pastor and T. Iwasita J. electroanal. Chem. submitted. 11. R. Parsons and T. Vandemoot, J. electronal. Chem. 2S7,9 (1988). 12. R. R. Adzic, in Advances in Elecrrochemisrry aad Electrochemical Engineering (Edited by H. Gerischer and C. W. Tobias), Vol. 13, p. 159. Wiley-Interscience, New York (1984). 13. G. Kokkinidis, J. electroaaal. Chem. 201,217 (1986). 14. E. Schwatzer and W. Vielstich. Chem. Inc. Techn. 45. 201 (1973). 15. X. H. Xia and T. Iwasita, J. electrochem. Sot., 140.2559 (1993). 16. M. Watanabe and S. Motoo, J. electroanal. Chem. 60, 267 (1975); %, 203 (1979). 17. M. Watanabe and S. Motoo, J. electroanal. Chem. 69, 429 (1976); 110, 103 (1980). 18. S. Motoo and M. Watanabe, J. electroanal. Cheat. 111. 261(1980). 19. S. Gilman, J. Phys. Chem. aS, 70 (1964); 65, 2112 (1964). 20. M. Shibata and S. Motoo, J. electroad. Chem. 194, 261(1985). 21. M. Watanabe and S. Motoo, J. electroaaal. Chem. 194, 275 (1985). 22. M. W. Breiter, J. electroanal. Chem. 23, 173 (1969). 23. B. E. Conway, B. MacDougall and H. AngersteinKozlowska, J. Chem. Sot., Faraday Trans. I 6% 1566 fl972). 24. H. Ahgerstein-Kozlowska, B. MacDougall and B. E. Conway, J. electroaaal. Chem. 39,287 (1972). 25. B. MacDougall, B. E. Conway and H. AngersteinKozlowska, J. electroaaul. Chem. 32, App. 15-20 (1971). 26. I. M. Kocthoff, S. Bruckenstein and M. K. Chantooni, J. Am. them. Sot. 83,3927 (1961); 87,4428 (1965). 27. T. Davidson and B. S. Pons, J. electroaaal. Chem. lU, 237 (1981). 28. S. Pons, T. Davidson and A. Bewick, J. electroanal. Chem. 140,211(1982). 29. M. Szklarczyk and J. Sobkowski, Electrochim. Acta 2S, ”

1597 (1980).

REFERENCES 1. P. Stonehart, Ber. Buasenges. Phys. Chem. 94, 913 (1990). 2. W. Vielstich, Fuel cells, Revised English Edition, Interscience, New York (1970). 3. 0. Wolter and J. Heitbaum, Ber. Bunsenges. Phys. Chem. 88,6 (1984). 4. S. Wilhelm, W. Vielstich, H. W. Buschmann and T. Iwasita, J. electroanal. Chem. 229,377 (1987). 5. B. Beden, A. Bewick, K. Kunimatsu and C. Lamy, _ J. electroanal. Chem. 142,345 (1982). 6. N. K. Ray and A. B. Anderson, J. Phys. Chem. 86,485l (1982).

30. S. Wasmus and W. Vielstich, J. electroad. Chem. W, 323 (1993). 31. H. Angerstein-Kozlowska, B. MacDougall and B. E. Conway, J. electrochem. Sot. 128,756 (1973). 32. A. M. Baruzzi, V. Solis and M. Giordano, Electrochim. Acta 27,273 (1982).

33. P. Zelenay and J. Sobkowski, J. electroanal. Chem. 176, 209 (1984). 34. B. Bittins-Cattaneo, E. Cattaneo, P. Kiinigshoven and W. Vielstich. in Electroanalvtical Chemistrv (Edited bv A. J. Bard), Vol. 17, pp. 181-220. Marcel Dekker, New York (1991). 35. A. M. de Becdelievre, J. de Becdelievre and J. Clavilier, J. electroanal. Chem. 2!M,97 (1990).

36. T. Iwasita and U. Vogel, Electrochim. Acta 33, 557 (1988).