Adsorption and desorption of lead at polycrystalline gold electrode and its effect on the superimposed reduction of nitrate anion

J. Electroanal. Chem., 159 (1983) 223--227

223

Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary note

ADSORPTION AND D E S O R P T I O N OF LEAD AT P O L Y C R Y S T A L L I N E GOLD E L E C T R O D E AND ITS E F F E C T ON THE SUPERIMPOSED R E D U C T I O N OF N I T R A T E ANION

J. GARCIA-DOMENECH, M.A. CLIMENT and A. ALDAZ*

Departamento de Qu[mica-F~sica e Instituto de Interfaces, Facultad de Ciencias, Universidad de Alicante, Alicante (Spain) J.L. VAZQUEZ

Departamento de Qu(mica General, Facultad de Ciencias, Universidad de Alicante, Alicante (Spain) J. CLAVILIER

Laboratoire d'Electrochimie Interfaciale, C.N.R.S., 92195 Meudon Principal cedex (France) (Received 7th September 1983}

Reduction of nitrate anion has been observed when a monolayer of lead is formed or removed on Ag single crystal electrodes [ 1 ]. No such effect seems to have been reported for gold substrate, however, an anomalous voltammetric reduction peak was observed during the formation of Pb monolayer at Au(100) in presence of nitrate anion which has been described as "an irreversible change" [ 2]. The present communication deals with the conditions for observation of this unusual reduction process of nitrate at partially lead-covered gold substrate. EXPERIMENTAL

Reagents. Special care was taken in relation with the purity of the reagents. The HC104 used was Merck "suprapur" and the Pb(NO3)2 was Merck p.a and Probus p.a. The Merck p r o d u c t was employed with and without recrystallization (twice). The water used was obtained using a Millipore-MiUi Q system with and without an Organex cartouche. The gas e m p l o y e d in the deoxygenation procedure was N48 from S.E.O. Analysis gives a concentration in 02 <~ 3 ppm. Electrodes. The working electrode was a gold polycrystalline sphere prepared by fusion of a wire (99.999%) in a gas--oxygen flame, and m o u n t e d in a Teflon holder. The normal electrochemical activation procedure was used for the preparation of the surface (potential sweeps between H2 and 02 evolution). During the experiments different electrodes were used. The counter electrode was a gold cylinder in the centre of which the working electrode was placed. The reference electrode was a saturated sulfate electrode, SSE, and the potentials are given in *To w h o m correspondence should be addressed. 0022-0728/83/$03.00

© 1983 Elsevier Sequoia S.A.

224 relation to this reference. A Luggin capillary was used for the connection to the cell. All measurements were carried out at room temperature, 19--20°C. Before proceeding with the study of underpotential deposition (upd) of Pb 2÷ the quality of the reagents were checked, using as control m e t h o d the voltammetric curves obtained in 0.5 M Au/H2SO4 (suprapur) or 0.7 M Au/HC104 (suprapur). These curves were compared with the results obtained in other work. RESULTS Figure 1 shows the voltammogram of the system Au + 0.7 M HC104 + 10 -3 M Pb(NO3)2. The appearance of the curve (the number of peaks, heights and potentials) is similar to that obtained by other authors [3--5] (ref. 5 is a review). The reproducibility of the curves is very good and this can be taken as another proof of the quality of the experiments. However, for Pb 2+ concentrations > 10 -2 M, a new reduction peak appears (Fig. 2, peak A) during the positive sweep at - 6 4 0 mV. This new peak A is correlated to peak B as it appears when peak B is present. Moreover, the presence of peak A always produces a wave (wave C) at 500 mV; this wave C is produced by oxidation of species formed in A. To make sure that wave C was due to oxidation of species formed in peak A, different experiments were carried out. Figure 3 shows the influence on wave C of a stop at E = - 6 2 5 mV during 50 s with and without stirring. The stop with stirring does not produce wave C while that without stirring produces a wave higher than the wave obtained during a continuous sweep. This clearly shows the relation between A and C and moreover that the wave C is governed by diffusion. Peak A is very sensitive to the electrode surface preparation. Figure 4 shows the influence of the sweeps on the development of A. For this experiment the gold electrode was initially "electrochemically" activated in a 1 M HC104 solution and held at 0 V while sufficient Pb(NO3)2 solid was added to give 3 × 10 -2 M 5

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Fig. 3. Voltammogram for Au + 0.7 M HC104 + 2.3 x 10 -2 M Pb2+; v = 50 mV/s. ( - - ) Continuous sweep without stirring; ( ..... ) with a stop at E = -625 mV during 50 s without stirring; ( . . . . . ) continuous sweep with stirring; ( - - - - - - ) sweep following curve ( ..... ). Pb 2+. W h e n t h e n u m b e r o f s w e e p s increases, t h e p e a k d e v e l o p s b e t t e r . P e a k A a p p e a r e d irrespective o f w h e t h e r t h e gold e l e c t r o d e h a d b e e n o x i d i z e d o r n o t . T h e b u l k d e p o s i t i o n o f Pb, Fig. 5 did n o t m o d i f y p e a k A a p p r e c i a b l y . T h e d i f f e r e n t t y p e s o f w a t e r u s e d (Millipore w a t e r o b t a i n e d w i t h or w i t h o u t an O r g a n e x c a r t o u c h e ) did n o t a p p e a r t o h a v e a n y i n f l u e n c e o n p e a k A. A d d i t i o n o f C1- u p to 10 - s M gives rise t o an e n h a n c e m e n t in p e a k s B a n d A a n d wave C w h i c h t h e n decrease f o r higher c o n c e n t r a t i o n in C1-; f o r [C1-] t> 5 X 10 - 4 M p e a k s B a n d A a n d c o n s e q u e n t l y wave C d i s a p p e a r , Fig. 6. T h e presence o f dissolved o x y g e n d o e s n o t a f f e c t p e a k A. T h e p e a k s are n o t o b s e r v e d w i t h o u t n i t r a t e anion.

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From all the above observations it may be reasonably assumed that peak B and A result of the catalytic reduction of NO3 anion and wave C corresponds to the wave recorded when NOT is oxidized. The existence of an heterogeneous electrocatalytic mechanism for NO~- reduction is suggested by the following experiment: if, in the course of Pb desorption, the positive sweep is stopped for few seconds at a coverage slightly higher to that corresponding to the negative foot of peak A, this peak disappears when the posi-

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Fig. 6. Influence of CI- on peaks A and B. Au + 0.7 M HC104 + 3 X 10 - 2 M Pb2+; ( - - ) no C1-; (. . . . . . ) 10 -4 M Cl-; ( - - - - - - ) 5 X 10 - 4 M C1-.

tive sweep is triggered again, giving a normal partial voltammogram for the desorption of the remaining adsorbed lead. A stop at slightly higher coverage does not affect peak A. The experiments indicate that there is a critical arrangement of the adlayer at a critical potential at which the reduction takes place and that probably a rearrangement of the adlayer occurs during the stop near this critical coverage. It seems that this reduction process is a significant example of an electrocatalytic reaction sensitive to the distribution of electrode sites surrounding the reducible species. More work is in progress on this theme. ACKNOWLEDGEMENT

This study was supported by the Commission asesora de Investigacion Cientifica y Tecnica of Spain.

REFERENCES 1 C. Mayer, K. Jiittner and W.J. Lorenz, J. Appl. Electrochem., 9 (1979) 161. 2 P. Hagans, A. Homa and E. Yeager, Proceedings of the 155th Electrochem. Soc. Meeting, Boston, 1979, p. 896. 3 R. Ad~i~, M. Spasojevi~ and A. Despi~, Electrochim. Acta, 24 (1979) 577. 4 R. Ad~.i~, E. Yeager and B. Cahan, J. Electrochem. Soc., 121 (1974) 474. 5 D.M. Kolb in H. Gerischer and C. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Wiley, New York, 1978, Ch. 2.