Study of the adsorption of p-nitrophenol on platinized platinum electrodes

Electroanalytical Chemistry and Interracial Electrochemistry, 43 (1973) 441-446

441

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

STUDY OF THE ADSORPTION OF p-NITROPHENOL ON PLATINIZED PLATINUM ELECTRODES

G. HORANYI and G. VI~RTES

Central Research Institute for Chemistry, Hungarian Academy of Sciences, Budapest (Hungary) (Received 25th September 1972, in revised form 5th December 1972)

In a preliminary note 1 the study of the adsorption of p-nitrophenol from 1.0 M HC10 4 solution on platinized platinum electrodes by tracer methods was reported. The main results may be summarized as follows (i) Between 300-800 mV (RHE) practically no change of adsorption occurs on varying the electrode potential. (ii) At these potentials and concentrations above 10 -4 mol 1-1 the extent of adsorption is not affected by concentration. (iii) The desorption rate of adsorbed species is very low. From these statements the conclusion was drawn that the adsorption properties of p-nitrophenol cannot be treated by adsorption equilibrium theories. It is well known from earlier work 2' 3 that the hydrogenation and electrochemical hydrogenation of p-nitrophenol occurs at a significant rate on platinum electrodes in both acidic and alkaline media at potentials more negative than 300400 inV. The assumed mechanism of hydrogenation is an ionic one i.e. the process takes place without participation of adsorbed hydrogen atoms. The role of p-nitrophenol adsorption in the reaction is not );et clarified, however. Similar problems arise in connection with the behaviour of phenol compounds at extreme positive potentials where the oxidation or the oxidative adsorption of these compounds must be taken into consideration. A number of papers appeared recently4-6 on the problem of selective inhibition of electrode reactions by phenol compounds at potentials more positive than10001200 inV. For the explanation Of the inhibition phenomena the potential and concentration dependent adsorption of phenols, including p-nitrophenol, was supposed. The aim of the present paper is, as an extension of our previous work, to examine the adsorption phenomena in connection with these problems. EXPERIMENTAL

The experimental techniques and procedure applied in tracer measurements are described elsewhere 7. 14C-labelled p-nitrophenol of 1 mCi mmol- 1 molar activity was used with 1.0 M HC10 4 and 1.0 M NaOH as supporting electrolytes. For the spectrophotometric measurements a Unicam S.P. 800B spectrophotometer was used. The electrochemical measurements were carried out in a conventional three compartmer/t cell.

442

G. HORANYI, G. VI~RTES

A D S O R P T I O N AND REACTION AT POTENTIALS M O R E NEGATIVE THAN 300-400 mV

The study of the potential dependence of the adsorption was done in a wider potential range than previously 1. The results of these measurements are shown in Fig. 1. Adsorption decreases at potentials more negative than 200 mV and more positive than 800 mV. The increase of the apparent adsorption observed above 1300 mV is in accordance with the conclusion drawn by Gileadi e t al. 4-6 from their inhibition measurements. Questions concerning the adsorption between 300800 mV are discussed in ref. 1. "T

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In this part the phenomena observed at potentials more negative than 300400 mV will be discussed. In the case of hydrogenation (reduction) a connection may be observed between the extent of adsorption and the reaction rate. This may be seen from Fig. 2. Stirring of the solutior~ is stopped and restarted, respectively, at the times indicated by the arrows. The count rate and current (rate of hydrogenation) change simultaneously. By stopping the stirring the rate of transport is lowered. Taking into account that the adsorption of p-nitrophenol is irreversible the simultaneous decrease of the adsorption and reaction rate cannot be explained unless the assumption is made that the reaction proceeds via adsorbed species. It may be shown, however, that there is no simple relation between the extent of adsorption and the reaction rate. A significant part of the adsorbed species does not take part in the reaction. This is shown by Fig. 3. The surface of the electrode was covered with labelled p-nitrophenol. A large excess of nonlabelled p-nitrophenol was added to the solution at the time indicated by the arrow. At 250 and 200 mV the reaction takes place at a considerable rate but the amount of adsorbed labelled species does not decrease significantly. This means that at these potentials the major part of the adsorbed species is not affected by the reaction. A similar conclusion may be drawn from Fig. 2. To the significant relative change in the current corresponds a slight change in adsorption. The immobile component of the adsorbed species may be eliminated by cathodic treatment but this is a slow process as shown in Fig. 4. In alkaline medium the kinetic behaviour of p-nitrophenol differs from that in acidic 3, but this is not reflected in the adsorption. Figure 5 shows the potential dependence of the adsorption from 1.0 M N a O H solution starting at 500 mV.

ADSORPTION OF p-NITROPHENOL ON Ptz-Pt

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444

G. HORANYI, G. VI~RTES

The main difference between the curves plotted for 1.0 M HCIO4 and 1.0 M NaOH is that at 0 mV there is no significant adsorption in the latter. This does not mean, however, that the adsorption in alkaline medium is a mobile one. Exchange experiments show similarity to the previous observation (Fig. 6). The experiments yielding the results shown in Fig. 6 were carried out in the same way as those for Fig. 3. Summarizing our observations it must be stated that there is no unambiguous relation between the extent of adsorption and the rate of the hydrogenation. ADSORPTION ABOVE 1300 mV

Our studies on the adsorption at potentials more positive than 1200-1300 mV were restricted to acidic media. In acidic media the steady state oxidation rate for phenol at these potentials is negligible as shown in refs. 4 and 5. In alkaline media, however, a reaction may be observed in the course of which an insoluble resin-like film forms on the surface of the electrode s' 9. No attempt was made, therefore, to study the adsorption in alkaline media at potentials where the formation of insoluble products must be taken into account. In the case of 1.0 M HC104 solutions at potentials more positive than 800 mV the adsorption decreases following the increase of oxygen adsorption i.e. oxygen replaces the species previously adsorbed. Above 1300 mV adsorption increases again. The extent of adsorption is significantly greater at 1500 mV than at 400-500 mV (Fig. 1). Accepting the assumption made previously that at 400-500 mV a monolayer is formed, the phenomenon observed above 1300 mV cannot be regarded as a simple adsorption process. The nature of the adsorption is quite different from that observed at more negative potentials. This is shoWn by the study of the adsorption on preoxidized surfaces. On oxidized surfaces the extent of adsorption is small at potentials more negative than 1200-1300 mV as demonstrated by Fig. 7. Following the reduction of the oxide layer adsorption increases. This means that this type of adsorption takes place on bare metal surface centres and the oxide layer inhibits adsorption.

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ADSORPTION OF p-NITROPHENOL ON Ptz-Pt

At extreme positive potentials adsorption occurs on a surface covered by oxygen. The increase of adsorption at low concentrations is a slow process; the rate of the adsorption depends on the potential as shown in Fig. 8. The adsorbed species cannot be removed from the surface at a significant rate by either cathodic or anodic polarization. (These experiments were carried out after washing the cell.) The conclusion to be drawn from these observations is that this type of adsorption is irreversible, too. On immersing the electrode in 1.0 M NaOH solution a rapid decrease of adsorption may be observed. On the basis of the above statements a further electrochemical study of the adsorption was carried out. A platinized platinum electrode (geometrical surface area 4 cm z, roughness factor ~ 500) was maintained for 5 min at a given potential (1400 mV) in a 1.0 M HC10 4 solution containing 5 x 10- 2 mol 1-1 p-nitrophenol. After the electrode was rinsed in distilled water it was immersed in a 1.0 M HC104 solution for 5 rain. Galvanostatic charging curves were taken following this procedure. First, the charging curve was taken in the cathodic direction and after arriving at 0 mV the direction of the current was reversed.

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The results of these measurements are summarized in Figs. 9 and 10. From the curves it follows that the hydrogen adsorption capacity of the electrode decreases significantly following the adsorption of p-nitrophenol at 1400 mV. This shows the presence of adsorbed particles on the surface. On immersing the electrode in NaOH solution the colourless solution turns yellow i.e. after such a pretreatment a significant amount of adsorbed species remains on the surface. Comparison of the u.v. spectrum of this solution with that of p-nitrophenol solutions shows that the adsorption was presumably accompanied by some chemical changes. Similar results may be obtained without taking the charging curves. Typical spectra of p-nitrophenol and the adsorbed species are shown in Fig. 11. Supposing that the peaks observed in the case of adsorbed species are of the

446

G. HORANYI, G. VI~RTES

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REFERENCES 1 G. Hor~inyi,J. Electroanal. Chem., 31 (1971) App. 1. 2 I. Telcs and F. Nagy, Ma#y. Kern. Foly., 67 (1971) 496. 3 F. Nagy, I. Telcs and G. Hor~nyi, Acta Chim. (Budapest), 37 (1963) 295. 4 T. Bejerauo, Ch. Forgach and E. Gileadi, J. Electroanal. Chem., 27 (1970) 69. 5 E. Zeigerson and E. Gileadi, J. Electroanal. Chem., 28 (1970) 421. 6 T. Bejerano and E. Gileadi, J. Electroanal. Chem., 38 (1972) 137. 7 G. Horhnyi, J. Solt and F. Nagy, J. Electroanal. Chem., 31 (1971) 87. 8 J. F. Hedenburg and H. Freiser, Anal. Chem., 25 (1953) 1355. 9 J. N. Ginzburg, Zh. Fiz. Khim., 33 (1959) 1504.