Oxidation mechanism of mesoxalic acid on a gold electrode

J. Electroanal. Chem., 178 (1984)271-279 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

271

OXIDATION M E C H A N I S M OF MESOXALIC ACID ON A G O L D E L E C T R O D E

J.M. FELl1], J. C L A R E T and C. M U L L E R

Departamento de Quimwa - Fisica, Fac. de Qulmwa, Umversldad de Barcelona, Barcelona (Spare) J.L. V A Z Q U E Z

Departamento de Qulmica General Fac. de Cwnctas, Umverstdad de Ahcante, Ahcante (Spare) A. A L D A Z

Departamento de Quimwa-Fisica, Fac. de Cwncms, Umverstdad de Ahcante, Ahcante (Spain) (Received 30th December 1983; in revised form 29th May 1984)

ABSTRACT Mesoxalic acid decarboxylates on a polycrystalline gold electrode giving CO 2 and H 3 0 + as the only products. A well-developed four-electron wave is obtained and a mechanism is given in agreement with the experimental results. Two different types of inhibition are detected. The first is caused by intermediate-poison adsorption and the second by oxide formation.

INTRODUCTION

The decarboxylation of organic acids is important not only from a theoretical point of view but also as a route in the synthesis of organic compounds [1]. However, as it normally takes place at potentials where the surface oxidation of the electrode and oxygen evolution occur simultaneously, the kinetic parameters are rather hard to obtain and it is difficult to discover the mechanism of the oxidation of the organic acid. For these reasons, we have undertaken a study of the oxidation of those organic acids whose structures allow the decarboxylation process to take place at more negative potentials (e.g. pyruvic acid [2], mesoxalic acid, this study; oxalic acid, the study of which is in progress; etc.). Mesoxalic acid is a strong organic acid (pK 1 = 2.50, pK 2 = 3.85-3.72 [3,4]) in which the carbonyl group is strongly hydrated [5,6] in 0.5 M H2SO 4. A study of the behaviour of this acid has only been carried out on a Pt electrode from an analytical point of view [7]. In this study, the decarboxylation process took place on an oxidized surface, so finally a more noble electrode, a gold one, was chosen by us for the study of the anodic behaviour of this organic acid. 0022-0728/84/$03.00

© 1984 Elsevier Sequoia S.A.

272 EXPERIMENTAL

Apparatus. The set-up consisted of an H - Q Belport potentiostat with iR correction, an H - Q Belport programmable function generator, an X - Y Houston recorder and a Pine rotating disc electrode. In the electrolysis experiments an Amel potentiostat and an Amel integrator were employed. Reagents and solutions. Mesoxalic acid from E G A was used as received; the H2SO 4 was Merck suprapur (in experiments with the rotating disc electrode the acid was p.a.). The water came from a Millipore-Milli Q system and its purity was controlled by means of the voltammetric curves obtained in Pt/0.5 M H2SO 4 or A u / 0 . 5 M H2SO 4. All the solutions were de-oxygenated in the usual way and an overpressure of N 2 was maintained within the cell. Electrodes. The working electrode was either a polycrystalline sphere of gold (99.998%) obtained by fusion of a wire (1 m m diameter) and placed on a Teflon holder or a gold disc of 0.8 cm diameter. The counter-electrode was a gold spiral wire. A saturated Na2SO 4 mercurous sulphate electrode (MSE) was used as the reference electrode with a Luggin capillary. All the potentials refer to this electrode. The temperature was maintained at 20 + 0.5 °C. Geometrical areas of the spherical electrodes were - 0.2 cm 2. GENERAL BEHAVIOUR

The oxidation of mesoxalic acid occurs on a free oxide gold surface giving an irreversible oxidation wave in the concentration range under study ( 1 0 - 5 - 1 0 -1 M) (Fig. 1). The peak current is diffusion-controlled at only low values of rotation speed, w, or sweep rate (Fig. 2). The peak current varies linearly with mesoxalic acid concentration in the range of 1 0 - 3 - 1 0 -2 M. At higher values of the concentration this relationship is not observed. For more positive potentials than the peak potential, the diffusion current (low values of sweep rate) overlaps with the surface gold oxidation current. At concentrations higher than 10 -2 M, the first gold oxide peak is reduced to a shoulder (Fig. 1), after which a sharp drop appears followed by stabilization of the current. The extrapolation [8] of the diffusion current (Fig. 1) shows that the oxidation process is inhibited by the surface oxidation. This inhibition is present for all concentrations. So, the voltammogram obtained at low concentration (when the surface oxidation current for a blank solution, 0.5 M H2SO 4, is subtracted from the experimental current) shows a shape similar to that obtained at higher concentrations. A reproducible wave is obtained at higher concentrations only if hydrogen evolution or surface oxidation take place during the sweep. All these facts show that an inhibition in the mesoxalic acid oxidation exists at all the concentrations studied. When the p H is increased at constant ionic strength, peak potentials shift to more negative values ( - 9 0 m V / p H ) whilst the peak current increases slightly, 10% ( p H interval 1-3.6). For more basic p H values the current diminishes slowly. This peak

273

current evolution is clearly different from that observed in the polarographic reduction of the acid [3]. The explanation could be that the hydrated form of mesoxalic acid is active only during the oxidation process. The presence of C1- ions in the solution leads to important modifications in the voltammogram. The peak shifts sharply to positive potentials and its current diminishes strongly (Fig. 3). In o

2

-0.'1-50I 0]00 ~~0"/'*00 ~.;/OE~v(vs.MOs9E~O

Fig. 1. Voltammetnc curves of 1.06×10 -3 M mesoxalic acid in 0.5 M H2SO4; v =100 mV s - I . ( m ) Experimental curve; (©) extrapolation of diffusion current of mesoxalic aod; ( ) experimental curve minus gold oxidation current, obtained with a solution of 0.5 M H2SO 4.

e

E

I

0.01 i

Fig. 2. (a) H2SO4.

i

0,;

¢(M)

0'1

03 i

vl/2/vl12s -112

Ip-c plot in 0.5 M H2SO4; v = 50 mV s -1. (b) Ip-v 1/2 plot of 10 -2 mesoxalic a o d in 0.5 M

274 1 M HC104 the peak potential shifts to more negative values, a few millivolts, with respect to the voltammogram obtained in H2SO 4 (Fig. 3) (the anionic adsorption order increases as follows: C10 4 < SO2 - < C1-). This behaviour can be explained by accepting that the mesoxalic acid a n d / o r an intermediate compound are adsorbed on the surface. REACTION PRODUCTSAND OVERALLREACTION The only reaction products were CO 2 and H3 O+ with a 100% yield in C O 2. The quantity of CO 2 was measured by means of the increase in the weight of the adsorbent employed, i.e. ascarite. The charge was measured on the basis of the integration of the current. No measurements of H + were made due to the low pH employed. The number of electrons transfered was four per molecule. Therefore, the overall reaction is" H O O C - C ( O H ) E - C O O H ~ 3 CO 2 + 4 H + + 4 e Electrolysis of 10- ~ M mesoxalic acid in 0.5 M H2SO 4 at potentials of 0.5 and 0.6 V (no surface oxidation exists) is inhibited after a few minutes, the current falling to 1% of its initial value. This inhibition can be eliminated by the application of a negative potential of at least - 0 . 6 V and may be caused by the adsorption of an intermediate species (the surface was not oxidized in order to avoid the formation of other products). •

\

I/t A/sup

1200

BOO

J

z,00

4

~J

SE) Fig. 3. Voltammelric curves of 10 ~2 mesoxalic acid; v = 100 mV s- 1. (a) ( _ _ )

l M HCIO4;(c) (

) 0.5 M H2SO4+2X10 -3 M KCI.

0.5 M H2SO4; (b) ( - - - )

275 O. <

~3oo

/

200

,00 j 0 0100 0.3oo

-0700

° ,V v MSEI

Fig. 4. V o l t a m m e t r i c c u r v e of 7,5 x 10 - 3 M mesoxalic acid in 0.5 M H2SO4; v = 8 m V s - 1 .

In order to obtain an appreciable current, electrolysis was carried out by means of cyclic sweeps between + 0.6 and - 0 . 6 V. The potential was stopped for 2 s at + 0.6 V before the next sweep. A blank solution of 0.5 M H2SO 4 electrolysed in this way gives a charge of 0.18 C per h. Partial electrolysis for 8 h of a 10-1 M mesoxalic acid solution gives a charge of more than 500 C; obviously the error in the measurement is less than 1%. The removal of electrode blocking is greater when the lower limit of the sweep is more negative. Hydrogen evolution activates the electrode and this fact differs from that observed in the oxidation of formic acid on a platinum electrode [9]. A strong interaction was observed between mesoxalic acid and gold oxides. This caused a decrease or event elimination of the gold oxides. So, Fig. 4 shows a practically no-existent reduction peak of gold oxides. The decrease of the area of this peak (the greater it is the slower the sweep rate) indicates a reaction between the oxides and mesoxalic acid. More work is in progress to study this very complex mechanism.

CALCULATION OF KINETIC PARAMETERS

In the intervals of the mesoxalic acid concentrations and for the pH studied, the Tafel slope was found to be identical for sweep rates of 1 and 2 mV s-~. The latter value was chosen to calculate the Tafel slope for obvious reasons. Kinetic parameters were calculated using the usual methods and the values obtained were as follows: order with respect to the mesoxalic acid concentration, interval 10-5-10 -2 M, one; order with respect to pH, interval 1-2, one; Tafel slope, 60 inV.

276 INHIBITION PROCESS Generally speaking, inhibition can be caused either by (a) a self-inhibitory process in which an intermediate poison inhibits the reaction; (b) the surface oxidation of the electrode which impedes the oxidation of the compound; (c) a change in the double layer structure which causes the potential available for the charge transfer to diminish appreciably. This later type of inhibition is normally present in all oxidation processes in which oxidation of the surface of the electrode takes place simultaneously, and will not be treated in this paper. The self-inhibition process in the oxidation of mesoxalic acid was demonstrated by a decrease in the current during the electrolysis experiments. This blocking may have been caused by the formation of an intermediate poison capable of inhibiting the electrochemical oxidation. To remove this inhibition it is necessary either to oxidize the surface or to discharge hydrogen at the electrode. However, in the study of reaction products the only activation procedure was the hydrogen evolution. In the potential interval comprised between the foot of the mesoxalic oxidation wave and the start of the oxide formation, this type of inhibition is indicated by a slow fall in the limiting current when a " h o l d " in the potential sweep is made (obviously a rotating disc electrode was used). The sudden decrease in the current at potentials corresponding to the beginning of the electrode oxidation points to the existence of another inhibition, an incomplete one, very different to that described previously and caused by oxide formation. This inhibition (very clear at higher concentrations of mesoxalic acid Fig. 3) is also present at lower concentrations as we have already pointed out (Fig. 1). This inhibition is the origin of the oxidation currents on the negative sweep (Fig. 4). These currents are higher than those obtained from the extrapolation of the theoretical diffusion current of mesoxalic acid for these potentials. This fact can be explained by the accumulation near the electrode of mesoxalic acid originating from the inhibition. This permits the diffusion process to level up the concentrations in the surface in relation to bulk values. The shape of the I - E curve during this inhibition changes at potentials more positive than 0.8 V and hence we think there must be a variation in the mechanism of the inhibition process. For the interval of potentials comprised between the beginning of the inhibition by oxide formation and the inversion potential, the current is partially governed by diffusion. This is proved by the influence of stirring on the value of the current. The influence of the degree of coverage of oxygen on the intensity of the current is very important. So, the current decreases sharply when the gold oxidation starts (Fig. 3, potentials near the first gold oxidation peak). A simple explanation would be that surface oxidation impedes the adsorption of mesoxalic acid or any intermediate compound adsorbed on more than one adsorption site per molecule. Inhibition by oxide was studied by assessing the influence of different time "holds" at three potentials, before, during and after the first gold oxidation peak

277

d u r i n g the positive sweep. To o b t a i n r e p r o d u c i b l e results we were o b l i g e d to cycle the electrode after each hold until it reached the p o t e n t i a l c o r r e s p o n d i n g to o x i d e f o r m a t i o n and the b e g i n n i n g of oxygen evolution. R e p r o d u c i b i l i t y was also p o s s i b l e b y cycling until h y d r o g e n evolution. T h e hold in the a b s e n c e of gold oxides, i.e. 0.6 V, b r i n g s a b o u t a c u r r e n t d e c r e a s e caused p r i n c i p a l l y b y the diffusion process. T h e restarting of the sweep gives a n o r m a l v o l t a m m o g r a m , (Fig. 5) with a lower c u r r e n t as the hold time increases. W h e n the h o l d is m a d e at potentials c o r r e s p o n d i n g to the first p e a k zone, i.e. 0.73 V, the current d r o p s m o r e quickly that when c o r r e s p o n d i n g to a diffusion process. T h e c o n t i n u a t i o n of the sweep shows the a p p e a r a n c e of a v o l c a n o - t y p e curve whose tail is identical in shape to the curve o b t a i n e d in the n o r m a l sweep. D u e to the high q u a n t i t y of charge involved, this curve is o b v i o u s l y n o t caused b y the o x i d a t i o n of a c o m p o u n d a d s o r b e d d u r i n g the hold. T h e r o t a t i o n of the electrode d i d not m o d i f y the shape of the curve b u t o n l y its height. This fact shows that this curve is not due to the o x i d a t i o n of a c o m p o u n d which has been f o r m e d in the reaction a n d which r e m a i n e d in the solution. It shows that the diffusion process plays a role in the value of the current intensity, b u t n o t in its shape (Fig. 6). D u r i n g the hold at 0.85 V, the c u r r e n t also d r o p s quickly a n d d u r i n g the c o n t i n u a t i o n of the sweep we o b t a i n curves of the same t y p e as those for the h o l d at 0.730 V. However, the currents are higher t h a n those o b t a i n e d in the c o n t i n u o u s sweep. This fact is due to restoration of the c o n c e n t r a t i o n g r a d i e n t of m e s o x a l i c acid

fpA/sup 3600 2800 2000 1200

/

400 O.DO

O.

0.700

0.900 1]00 E/V(vs.MSE)

Fig. 5. Voltammetric curves of 10 - 2 M mesoxalic acid in 0.5 M H 2 S O 4. Hold at 0.6 V for 15, 30 and 60 s; v = 100 mV s -1. Disc electrode, w = 0 rpm.

278

3600

[/}JA/sup

280O 2O0O 1200 40£

o.loo :o.3oo

'o7oo

o.9oo

tloo

E/V~vs.MSE) Fig. 6. V o l t a m m e t r i c curves of 10 - 2 M m e s o x a l i c acid in 0.5 M H 2 S O 4. H o l d at 0.73 V for s ( - - - - ) , 15 s (- • .), 30 s ( . . . . . ), 60 s ( - - - ) and 120 s. ( ). o = 100 m V s -1. Disc electrode, w = 0 rpm.

caused by the inhibition process. This allows diffusion to level off the concentrations. A noticeable fact is that in all these experiments the same quantity of oxide, measured by the reduction charge, is detected in the negative sweep. That is to say, during the sweep, with or without hold, the same degree of surface oxidation is found. The same holds were made for a blank solution, 0.5 M H2SO 4, and the curves obtained gave at 0.730 and 0.850 V the same shapes as those obtained in the presence of mesoxalic acid. These results can be explained by taking the shape of the curve to be the result of the surface oxidation process and the intensity of the current to be due to the oxidation process of mesoxalic acid. REACTION MECHANISM

All our results can be explained by accepting the following mechanism: (1)

M + C3H406

---~ M - C 3 H 4 0 6

(2) (3) (4)

M-C3H4C

6

~--~-,M - C 3 H 3 0 6

+ H+ + e -

M-C3H306

~ M-C2H304

+ CO 2

M-C2H304

~2CO 2+3H ÷+3e-+M

C3H406

~

3 CO 2 + 4 H ÷ + 4 e -

Rate-determining

step

Overall reaction

All the steps below the third one were joined in a partial reaction because the supposition of the third step as the rate-determining step makes the subsequent steps kinetically indistinguishable.

279 U s i n g the q u a s i - e q u i li b r i u m state a p p r o x i m a t i o n an d a d s o r p t i o n b e h a v i o u r with O --, 0, we obtain:

assuming a L a n g m u i r

i = 4 F K l K 2 k3c,,rgc H 1 exp( f E )

in which Corg is the mesoxalic acid c o n c e n t r a t i o n ; K, are the e q u i l i b r i u m constants, k 3 is the rate c o n s t a n t and f = F / R T . F o r this equation, the Tafel slope will be 60 m V and the o r d er with respect to p H and mesoxalic acid will be one, in total a g r e e m e n t with the e x p e r i m e n t a l results. ACKNOWLEDGEMENTS W e thank Dr. Jean Clavilier for his help a n d collaboration. W e are i n d e b t e d to C.I.R.I.T. for financial support. REFERENCES 1 2 3 4 5 6 7 8 9

L. Eberson m M. Baizer (Ed.), Organic Electrochemistry, Marcel Dekker, New York, 1972, Ch. Xlll. P. Gonzalo, A. Aldaz and J.L. Vazquez, J. Electroanal. Chem., 130 (1981) 209. S. Ono, M. Takagi and T. Wasa, Collect. Czech. Chem. Commun., 26 (1961) 141. A. Laptiskaya and B. Pirkes in R.V. Merstlin (Ed)., Isslead. Obl. Khim. Redkozomed Elem., Izd. Saratov Univ., 1966, p. 6. H. Strelow, Z. Elektrochem. Ber. Bunsenges. Phys. Chem., 66 (1962) 393. M.L. Ahrens, Ber. Bunsenges Phys. Chem., 72 (1968) 691. M. ignazak and A. Grzeidziak, Soc. So. Lodz. Acta Chim., 18 (1974) 135. M.A. Chment, J.L. Vfizquezand A. Aldaz, An. Qulm., 79 (1983) 660. H. Angerstein-Kozlowska, B. MacDougall and B.E. Conway, J. Electrochem. Soc., 120 (1973) 756.