Chemical interactions of pyridine and cyanide with silver

J. Electroanal Chem., 112 (1980) 127--135

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CHEMICAL INTERACTIONS OF PYRIDINE AND CYANIDE WITH SILVER

G. BLONDEAU and J. ZERBINO *

Laboratoire Physique des Liquzdes Electrochimie, 4, place Jussieu 75230 Parzs Cedex 05 (France) N. J A F F R E Z I C - R E N A U L T

Laboratoire Ptrre Sffe C.N.R.S.--C.E.N./Saclay B.P. No. 2, 91190 Gtf sur Yvette (France)

ABSTRACT From a quantitative determination of pyridine and cyanide adsorbed on a silver electrode, by a.radlochemlcal technique, we have shown that the two adsorbate-~ilver systems are different. After a dissolution--redeposition electrochemical cycle the quantity of pyridine adsorbed depends on the charge transfer. For low charge transfer (<50 mC cm -~ ) the quantity increases from three to nine monolayers and depends on the nature of the supporting electrolyte, which suggests the formation of new bonds between pyridine, Ag and the anion of the supporting electrolyte. For high charge transfers the quantity of pyridine increases, the rate of increase depending on the supporting electrolyte (KI > KCl > KC104); in our opinion this is due to a trapping of pyridine in the salt formed between the support electrolyte anion and silver. The quantity of pyridine adsorbed at the sdver electrode which can be as large as 100 equivalent monolayers can explain part of the enhancement of the Raman signal observed for this system. After a dissolution--redeposltion electrochemical cycle the quantity of cyanide adsorbed remains constant, the cyanide--silver system is reversible and the Raman enhancement observed at the rest potential, is due only to Ag--CN interactions.

Recently, a very large number of papers have dealt with the interaction of adsorbates containing delocalized electrons with silver electrode. These studies started with the observation of an anomalously intense Raman spectra for pyridine [1--5] and cyanide [6--7] adsorbed at the silver electrode. N o w the emphasis is on the interpretation and explanation of these enhanced spectra [8--17]. However, except in very few papers [17--18], the chemical interaction of the adsorbate with silver has been ignored. A specific electrochemical preparation o f the silver surface is needed in order to obtain this enhanced effect which results in an "activation" of the surface. We have already shown [18] that the amount o f pyridine adsorbed on silver in the KC1 supporting electrolyte depends on the charge transferred during the activation. This increase of pyridine adsorbed can be as large as one and a half orders o f magnitude, giving, incidentally half o f the explanation for the Raman enhancement. This increase of pyridine adsorbed could be partly explained in * On leave from La Plata University, Conicet, Argentina.

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terms of trapped pyridine when AgC1 is formed during the dissolution deposition process. The pyridine--silver system possesses peculiar characteristics, therefore the model for the interpretation o f the Raman spectra enhancement will be complicated. In order to test a simpler system, we have chosen the adsorption of cyanide [6] that does not involve the interaction of the supporting electrolyte. In this paper, we have studied the influence of the potential, charge transfer and supporting electrolyte on the quantity of adsorbed species. We have used a radiochemical technique to determine this quantity. Pyridine is potassium halide solutions and potassium cyanide in Na:SO4 solutions were chosen because they are the most frequently encountered in Raman studies. EXPERIMENTAL

A polycrystalline silver foil (area 10 cm:, purity 99.99%) was used. In order to prevent any fl-count emanating from the mounting material the silver foil was n o t embedded. The surface was first mechanically (abrasive paper 800) and then chemically polished in a chromic chloride acid mixture. It was previously checked that this procedure ensures crystalline perfection and non-contamination of the surface [191. For pyridine experiments, the solutions were prepared with pyridine labelled with ~4C (provided b y Amersham, France), its specific activity being 1 mCi mo1-1. We prepared a solution with pyridine concentration 50 mM and KX (X = Cl-, C10;~, I-) concentration 0.1 M. Cyanide experiments were carried out using KCN labelled with ~4C (provided by C.E.A./Saclay), its specific activity being 40 mCi mo1-1. The electrolyte was 0.01 M KCN and 0.1 M Na2SO4. Standard electrochemical equipment was used to regulate the working electrode potentials. All potentials were measured against the saturated calomel electrode (SCE). All experiments were carried o u t at r o o m temperature. After electrochemical treatment the silver electrode were removed, rinsed with twice-distilled water and alcohol and dried in air at ambient temperature. The H-radioactivity was then measured using a proportional counter. After drying, some experiments were carried o u t with collodion evaporated on the silver surface in order to prevent any loss of radioactive material. Experiments made without collodion gave the same results. Therefore, the amount of pyridine left on the surface after drying was stable, even after several days. RESULTS

The enhancement is most generally obtained when the silver electrode has been cycled in the anodic region. But, depending on the adsorbed species, the current--potential curves are different. It is therefore important, for b o t h pyridine and cyanide, to follow the influence of the potential at which the measurements are made.

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~+400mV Fig. 1. Dissolution--redepos, tion e l e c t r o c h e m i c a l cycle for Ag in a 50 m M p y r i d i n e - - 0 . t M KC104 solution (charge transfer = 100 mC cm -2),

Influence of the potential Pyridine In Fig. 1, we present the Ag current--potential curve for a 50 mM pyridine-0.1 M KC104 solution. Three sweeps (--300 mV to + 200 mV) were carried o u t before any measurement in order to obtain a reproducible current--potential curve. The total a m o u n t of charge passed through the Ag electrode never exceeded 1 pC cm -2 (equivalent to 3 X 10 -3 monolayers of Ag redeposited). The curve shown in Fig. 1 corresponds to the fourth cycle. Point A (--6 mV) is the rest potential of the Ag electrode in this solution, the amount of pyridine detected corresponding to 3 X 10 is molecules cm -2, above the detectable limit of our m e t h o d which is 2 X 10 is molecules cm -2. If a monolayer corresponds to 9 X 1014 molecules cm -2 [20], the measured quantity is equivalent to three monolayers on the apparent surface (whatever the roughness of the electrode). From point A, towards positive potentials, the silver electrode dissolves; at point B (+360 mV) we take the electrode o u t of the solution just before redeposition of Ag ÷, and we find exactly the same quantity o f pyridine adsorbed as at the rest potential (Point A). In spite of a dissolution corresponding to 100 mC cm -2 (300 monolayers of Ag) the surface newly created at point B is equivalent to the surface at the rest potential. After redeposition of silver, carried o u t at a constant sweep rate of 90 mV min -1 at point C (--20 mV), we n o w measure 14 X 10 Is molecules cm -2 (equivalent to 17 monolayers of pyridine adsorbed). This result implies that pyridine is trapped during the redeposition process o f Ag +, as already mentioned in ref. 18.

Cyanide In Fig. 2 we present the Ag current--potential curve for a 0.01 M KCN-0.1 M Na2SO4 solution. In this case it is not necessary to cycle the electrode because the first cycle is identical to any following one. This sweep is limited to --1000 + 300 mV in order to avoid the dissolution--deposition of Ag which takes place at more positive potential. According to Lebedev et al. [ 21], the different peaks observed on the curve can be attributed to the following:

130 :E

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Fig. 2. Sliver c u r r e n t - p o t e n t i a l curve for a 0.01 M K C N - - 0 . 1 M Na2SO 4 solution.

peak I, the formation of the silver cyanide complex Ag(CN)~; peak II, the formation o f the oxide Ag20; peak III, the reduction of Ag20; peak IV, the dissociation of Ag(CN)~. This interpretation, based on the thermodynamical stability of Ag:O in these conditions, agrees with that of. Otto et al. [22]. However, it has been shown recently through Auger measurements that peak II could be related to the presence of AgCN on the electrode [23]. The amounts of cyanide adsorbed at different points of the cycle are as follows: at the rest potential A (--560 mV), 3 X 10 ~s molecules cm -2 which corresponds to two monolayers of adsorbed CN- if we refer to the coverage of 1.5 X 10 ~s molecules cm-: for a monolayer of CN- [24]. This result is in good agreement with an earlier work b y Cabane-Brouty and Oudar [25]. At point B (--95 mV), after the complex formation, the a m o u n t of cyanide adsorbed does n o t change. At point C (+250 mV), afteI the oxide formation, [21,22] or AgCN formation [23], we obtain 4 X 10 ~7 molecules cm -2 corresponding to a very large quantity adsorbed which is more easily understandable b y the formation of AgCN than b y the formation of Ag:O. At point D (--130 mV), when the c o m p o u n d is reduced the quantity of cyanide returns to the two-monolayer value. Looking n o w at the same cycle b u t going far enough towards positive potentials to obtain the dissolution--deposition of silver (+500 to --800 mV} (which is the electrochemical treatment for Furtak's and Otto's experiments [6,22]), we obtain a current--potential curve which is represented in Fig. 3 where peaks V and VI correspond, respectively, to the dissolution and deposition of silver. Since the starting point is identical to that of Fig. 2, the a m o u n t of CNmeasured at points A, B and C are the same than in Fig. 2. But at point E (+440 mV) just after the end of the dissolution (25 mC cm-2), we obtain 1018 molecules cm -2, equivalent to 650 monolayers; this simple result compared to the pyridine results show h o w these two adsorbates are different (at the same point for pyridine experiments we obtained only three monolayers of pyridine). It seems that the process o f silver deposition increases the quantity adsorbed since we obtain 2 × 10 is molecules cm -2 at point F (+125 mV) just

131 ::2:

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Fig. 3. D i s s o l u t i o n - - r e d e p o s l t i o n e l e c t r o c h e m i c a l cycle f o r Ag in a 10 -2 M KCI--0.1 M Na2SOa s o l u t i o n (charge t r a n s f e r = 25 m C c m -2).

after redeposition. When the surface c o m p o u n d is reduced at point G (--130 mV) most o f the CN- is released in solution and the quantity is only 3 × 1016 molecules cm -2. The desorption of the silver complex releases the excess of CN- on the electrode and at point H (--800 mV) we are back to the t w o monolayers we found before any electrochemical treatment. Very recently the same measurement has been performed on an Ag(111) single-crystal electrode after one cycle which roughly corresponds to the cycle described here [26]. These workers have found 1.4 X 10 Is molecules cm -2, which is half of our coverage. The discrepancy can be explained in terms o f different roughness and grain boundaries of our electrode which is polycrystalline. However, it must be pointed o u t that the reversible stage is reached only when < 5 0 mC cm -2 have been passed during the ariodic dissolution. For higher charge transfer the interface is much t o o disturbed and more than t w o monolayers are left on the surface (geometric). For example, after a charge flow o f 150 mC cm -2, after peak V, 1016 molecules cm -2 are left (~6 monolayers).

Influence of the charge transfer These experiments were carried o u t using 50 mM pyridine solutions with three different supporting electrolytes -- KC104, KC1 and KI at 0.1 M concentration. Details of the electrochemical "activation" o f the silver surface are as follows: (1) Immediately after polishing, the electrode was immersed in the solution, the rest potential being --6 mV (vs. SCE) in the double-layer range. (2) Three sweeps were imposed to the electrode from --330 mV to +200 mV (vs. SCE) starting towards the anodic potentials in order to obtain a reproducible current--potential curve. (3) The "activation" of the silver electrode was performed at +400 mV (vs.

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Fig. 4. Variation of the amount of adsorbed pyridine at the silver electrode function of the charge transfer. 50 mM pyridine. (e) 0.1 M KC1, (0) 0.1 M KC104, (v) 0.1 M KI.

SCE) and the a m o u n t of charge was measured with a coulometer. (4) After this "activation" the potential was decreased to --6 mV (vs. SCE) at a constant sweep rate and the electrode was removed from the solution at this potential, the current being always <5 #A cm -2. Then the electrode was rinsed and dried and its H-radioactivity was measured. All the results are shown in Fig. 4 where the a m o u n t of adsorbed pyridine is presented as a function of the charge transfer. In order to standardize our results with Raman experiments, we have computed the a m o u n t of pyridine in equivalents of monolayers using the Barradas and Conway results [20] and the charge transfer in equivalent of monolayers of silver dissolved using the Pettinger et al. results [ 5 ]. When 0.1 M KC104 is used as supporting electrolyte we measure the equivalent of three monolayers adsorbed up to 0.5 mC cm -2 (~1.5 monolayers of Ag dissolved and redeposited). Afterwards the quantity adsorbed increases linearly with the logarithm of the charge transfer. The behavior observed when 0.1 M KC1 is used as supporting electrolyte is different: three stages are observed; at the lowest charge transfer we measure three monolayers of pyridine above 50 pC cm -2 (0.15 monolayers o f Ag dissolved); a constant value of about nine monolayers of pyridine is observed up to 50 mC cm -2 (150 monolayers of Ag dissolved) and above that point the quantity o f pyridine increases with the charge transfer at a higher rate than in the KC104 supporting electrolyte. Only a few points have been measured in the 0.1 M KI supporting electrolyte owing to the non-adherence o f the deposit. But it can be noticed t h a t the quantities of adsorbed pyridine in these conditions are always much higher than in the two other supporting electrolytes. For instance, at 600 mC cm -2 we obtain 1.4 × 10 '7 molecules cm -2.

133 DISCUSSION

Pyridine The Raman enhancement is due to the accumulation of pyridine on the silver surface and to the electronic interaction of Ag with pyridine. The results we have reported deal only with the a m o u n t of pyridine at the surface due to the electrochemmal cycle. After dissolving t h e equivalent of 300 monolayers of Ag, we have measured exactly the same quantity of pyridine adsorbed than before the dissolution (points A and B in Fig. 1). This shows that the pyridine accumulation occurs only after Ag deposition. Before cycling the electrode or just after Ag dissolution only three monolayers are observed at the surface, b u t even in this case of a very small deposition (50 pC cm -2 -~ 0.15 monolayer of Ag) the quantity of pyridine increases to an equivalent of nine monolayers. This effect is related to the formation of new bonds between pyridine, Ag and the anion of the supporting electrolyte. This idea is confirmed b y results in literature [17] which show b y a careful analysis of the position and structure of the Raman signal that intermediates species are formed. Other experiments using the surface plasmon technique also show the presence of a new specms at the pyridine--silver surface [27]. It is important to point out that this mechanism is true only for low charge transfers where no roughness appears [ 5]. The Ag deposition is the indispensable first step to create active species, b u t if we strongly increase the charge transfer a new p h e n o m e n o n occurs which is certainly a trapping of pyridine in the salt formed between the supporting electrolyte anion and silver. This p h e n o m e n o n would correspond to the continuous increase of the a m o u n t of pyridine adsorbed wi~h charge transfer observed above 0.5 mC cm -2 for KC104, 50 mC cm -2 for KC1 and 5 mC cm -2 for KI. In the curves presented in Fig. 4, one sees that the slope corresponding to KI is higher than that corresponding to KC1 which is higher in KC104 solutions, in the same order than the solubility products of AgI,. AgC1 and AgOH, since most probably in KC104 solution the product present at the electrode is AgOH. This trapping mechanism m a y explain the influence of the support electrolyte on Raman signals which is more intense in I- than in C1- solutions [2]. It also explains the increase of the Raman signal observed by Albrecht and Creighton [4] when they increased the charge transfer above 30 mC cm -2. It is known from optical measurements [5], that in KC1 solutions there is no significant increase of the roughness of the silver surface until a charge transfer equivalent to 100 monolayers has been passed through the electrode. In Fig. 4, the region from 5 pC cm -~ to 50 mC cm -2 is not affected by the roughness. The increase above 50 mC is partly due to change in the surface structure (roughness, sponginess) the true surface is impossible to evaluate. In these conditions, it makes no sense t o calculate the quantity of pyridine in the equivalent monolayer and only the gross quantity is significant. We can point out that Raman experiments made in these conditions were not corrected for the true surface and the enhancement observed was therefore truly related to the increase of t h e amount of pyridine.

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Cyanide As we have shown, pyridine is certainly not the ideal case since the pyridine--Ag system is quite complex. Some Raman experimentalists were thinking of using CN- to reduce the system to a simple adsorption case [6]. This idea is confirmed by our results: at the rest potential, we have always found two monolayers of CN- adsorbed even after a dissolution--deposition cycle if not t o o much charge is flowed through the silver electrode. It is known [22] that there is not coadsorption of the supporting electrolyte (Na2SO4) with cyanide. In the cyanide-silver system there is no accumulation and no influence of the supporting electrolyte, the Raman enhancement is due only to the interaction of CN- with silver (electronic transfer) and to the roughness of the electrode. This conclusion must be taken carefully because if one starts Raman measurement along the silver electrochemical cycle, at specific potentials (points C, E, F in Fig. ,3) a large signal will be found due only to an accumulation of CN-. CONCLUSION

We have shown that in the case of pyridine the electrochemical cycle of dissolution-deposition leads to the formation of an Ag--pyridine complex system and that the supporting electrolyte anion influences the quantity of pyridine trapped in the surface c o m p o u n d . Depending on the charge flowed through the silver electrode we have found two different types o f behaviors: after small charge transfer we have the creation of a new species between Ag, pyridine and the anion of the supporting electrolyte; for large charge transfer, pyridine is trapped in the silver salt. In the last case the Raman enhancement is due to the nature of the new species created and to the accumulation of pyridine at the surface. By using labelled Ag, halide and pyridine we hope to elucidate this surface c o m p o u n d . The adsorption mechanism o f cyanide is completely different from that of pyridine. We have shown that before or after a dissolution--deposition electrochemical cycle the CN- quantity remains equal. In that case the Raman enhancement is due only to Ag--CN interactions. This sytem is simpler and its study in Raman spectroscopy should be more promising. REFERENCES 1 2 3 4 5 6 7 8 9 10

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