Influence of thiourea on hydrogen evolution at a silver electrode as studied by electrochemical and SERS methods

41

Journal of Electroanalytical Chemistry, 367 (1994) 41-48

Influence of thiourea on hydrogen evolution at a silver electrode as studied by electrochemical and SERS methods J. Bukowska and K. Jackowska Department of Chemistry, University of Warsaw, Pasteur Street 1, 02-093 Warsaw (Poland)

(Received 19 March 1993; in revised form 22 June 1993)

Abstract The inhibiting (in acidic medial and catalytic (in neutral media) effects of thiourea 0’I.J) on H, evolution at a silver electrode has been studied by linear sweep voltammetry. The influence of the nature and concentration of anions present in TU solutions on this process has been determined. The results of electrochemical measurements are discussed in terms of the coadsorption of anions with TU and the different orientations of TU molecules with respect to the electrode surface. This orientation, as determined from surface-enhanced Raman spectra, depends on the TU concentration and the type of anion. It is postulated that in neutral solutions NH, groups H-bonded to H,O or HsO+ molecules mediate in proton discharge. In acidic media an interaction of NH bonds with anions is responsible for the inhibiting effect of TU on H, evolution.

1. Introduction

2. Experimental

A number of organic compounds containing nitrogen and sulphur (e.g. diphenylamine, pyridine, and thiourea (TU) and its N-substituted compounds) are known to accelerate the electrochemical evolution of H, at the mercury electrode [l-4]. It is known that TU molecules can catalyze the electroreduction of cations such as Zn*+ and Cd*+ [5,6] and that the process is influenced by anions adsorbed at the mercury electrode. The catalytic effect of TU on H, evolution at a Ga electrode in both acid and base media has also been reported [7]. In contrast, TU catalyzes H, evolution at Ag electrodes only in neutral solutions and inhibits it in acid media. No clear explanation of the difference in TU activity has been given. However, Tian et al. [Sl, discussing surface-enhanced Raman spectroscopy (SERS) results, have recently suggested that changes in TU orientation have a substantial effect on H, evolution at Ag electrode. They also suggested that the coadsorption of TU with anions takes place through the NH: group in acid media and through the NH, group in neutral solutions. More extensive studies of the coadsorption of TU with anions (particularly the influence of the nature and concentration of anions) and its effect on H, evolution at a Ag electrode are presented in this paper.

A polycrystalline Ag disk (99.999% assay; geometric area, 0.2 cm*> was used as the working electrode. Before all electrochemical and spectroscopic measurements, the Ag electrode was roughened by exposure to three oxidation-reduction cycles (ORCs) in 0.1 M KC1 (the potential was swept from -0.3 to +0.3 V/SCE and back) at a sweep rate of 5 mV s-l. The real area of the electrode was not determined but the charge passing through the cell during the ORC was checked to enable comparison of the results in successive experiments. The spectroscopic set-up for SERS experiments and the electrochemical equipment for cyclic voltammetry have been described elsewhere [9]. The spectra were obtained using a Spectra Physics Ar+ ion laser (514.5 nm). The spectral bandpass was 5 cm-’ and each spectrum was recorded at a constant potential maintained by a potentiostat. Measurements were carried out in aqueous solutions of 0.1 M LiClO,, 0.1 M KCl, 1.0 M LiClO, and 1.0 M KC1 at pH 1 and pH 4.4 containing different amounts of TU (5 X 10m5 M, 5 X 10m4 M, 5 X 10e3 M, 5 X lo-* M and 0.1 M). The solutions were prepared from crystallized analytical reagents and triple-distilled water. Before the measurements, the solutions were deoxygenated with argon. All electrode potentials are

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J. Bukowska,R Jackowska /Influence of thioureaon H, evolution

reported with respect to the saturated trode (SCE).

calomel elec-

3. Results and discussion 3.1. Electrochemical results

Figure 1 shows the typical current-voltage (I-E) curves recorded on a roughened Ag electrode in the potential range from - 0.3 to -0.9 V at a sweep rate of 40 mV s- ’ in solutions containing 0.1 M HClO, + 0.1 M LiClO, and 0.1 M HCIO, + 1.0 M LiClO, with different TU concentrations varying from 5 x 10v5 M to 0.1 M. Similar I-E curves were obtained in solutions containing 0.1 M HCl + 0.1 M KCl, 0.1 M HCl + 1.0 M KCI. We found no evidence of any influence of the sweep rate on the I values at H, evolution potentials. Tafel plots (E log I> were obtained for all the measured systems. The linearity of the Tafel plots is limited to the current range from 10e5 to 10s3 A. In this range, the slope b of the lines is equal to 130 + 5 mV. A deviation from linearity in the Tafel region was observed for solutions containing 0.1 M HCI + 1.0 M KCl+c TU. The values of b and the relatively low values of the heat of adsorption of H, on Ag [lo] suggest a Heyrovsky mechanism for H, evolution. As can be seen (Fig. 11, TU inhibits H, evolution. The changes Aq in the H, evolution overvoltage, defined as the difference between the potentials E obtained at different concentrations of TU and the potential E obtained in the

solution without TU, at constant I are presented in Table 1. In all cases the inhibition of H, evolution increases with increasing concentration of TU up to 5 x lo-* M. The effect is manifested more strongly in the presence of Cloy ions. In both systems (ClO;, Cl-) the increase in electrolyte concentration results in a decrease in Aq. As can be seen, the decrease in Aq is much greater in Cl- media. Figure 2 shows the influence of TU on H, evolution in acidified solutions containing LiClO, at pH 4.4. Similar Z-E curves were obtained in a solution containing KC1 at pH 4.4. The I-E curves were recorded in the potential range from -0.8 to - 1.6 V at a sweep rate of 40 mV s-i. The Tafel plots were drawn but a clear Tafel region was found only for the 0.2 M LiCIO, + TU system. Moreover, we obtained very high values for the slopes (about 240-300 mV). TU has a catalytic effect upon H, evolution in both 0.2 M LiClO, and 0.2 M KC1 solutions. The effect increases with increasing TU concentrations and is manifested more strongly in 0.2 M LiClO, solution. When the concentration of electrolyte increases, the Z-E curves become more complex. A region of prewaves appears in 1.0 M LiClO, solutions with TU, which is not observed in 1.0 M KCI. In both solutions the curves cross the line for pure solutions at potentials of - 1.38 and - 1.45 V for 1.0 M LiClO, and 1.0 M KC1 solutions respectively. The results indicate that the catalytic effect of TU decays at

E/V vs. SCE -0.9

E/V

(0)

-0.7

-0.5

-0.3

vs. SCE

(b)

Fig. 1. Z-E CIKWSfor H, evolution on a roughened Ag electrode in (a) 0.1 M HC104 + 0.1 M LiClO, and (b) 0.1 M HClO, + 1 M LiCIO, containing various concentrations c of TU: c=0;.5x10-5M;X5x10-4M;05x10-3M,05x10-2M;~0.1M.

J. Bukowska, K Jackowska / Influence of thiourea on H, evolution

43

TABLE 1. Changes in the overvoltage Aq = JY,(~ - EcrTuj_,, for measured systems at constant current I = 3 x 10e4 A 103c (TU)/mol 1-l

Aq/mV pH 4.4

pH1 0.1 M HCIO, + 0.1 M LiClO, 60 110 190 230 190

0.05 0.1 5 50 100

0.1 M HC104 + 1.0 M LiClO, 50 90 130 130 140

0.1 M HCl + 0.1 M KC1

120 150 190 190

0.1 M HCl + 1.0 M KC1

0.2 M LiClO,

1.0 M LiClO,

-40 -70 -70 - 120 -90

0 30 50 80 70

0.2 M KC1 1.0 M KCI

16 8 -16 -25 -40

-16 -16 -70 -60 -40

a

b

-50 -30 -16

16 30 16 8 -30

0 0

aZ=3x10-4A,bZ=5x10-4A

lower TU concentrations and at potentials more negative than the crossing point. The An values for these systems are given in Table 1. Negative values of Aq imply acceleration of H, evolution. The 87 values, for 1.0 M KC1 solutions are presented at two constant current values (3 x 10e4 and 5 X 10e4 A) before and after the crossing point. The different effects of TU on H, evolution which we have observed is in good agreement with results published by Tian et al. [S]. However, the catalytic effect observed here is smaller because of the lower pH values of the solution studied. Some points must be taken into consideration when the influence of TU on H, evolution is discussed.

(i) The inibiting effect is observed at potentials more positive than the potential of zero charge Epzc. The catalytic effect was found at potentials more negative than E,,. For a smooth polycrystalline Ag electrode E pzc in the presence of weakly adsorbed F- ions is equal to -0.97 V/SCE [ll] and is shifted by about - 60 mV in 0.1 M KC1 solution [12]. (ii> An increase in the TU concentration causes an increase in the absolute value of Aq in both acidic (pH 1) and acidified (pH 4.4) solutions. (iii) Both inhibiting and catalytic effects are manifested more strongly in the presence of LiClO, than in the presence of KC1 at constant TU concentration. (iv) An increase in electrolyte concentration results

-1.6

-1.6

(a)

E/V vs. SCE -1.2

-0.8

E/V vs. SCE -1.2

-0.0

I

lOOpA

(b) Fig. 2. Z-E curves for H, evolution on a roughened Ag electrode in acidified solutions (pH 4.4) of (a) 0.2 M LiCIO, and (b) 1.0 M LiClO, containing various concentrations c of Tu: c=O,. c=5X1005 M; X ~=5X10-~ M, 0 c=5X1003 M; 0 c=5XlO-* M; A c = 0.1 M.

44

J. Bukowska, K Jackowska / Influence of thiourea on H2 evolution

in a decrease in both the inhibiting and catalytic effects at constant TU concentration. (v) In acidic media, H, evolution potentials are shifted to more positive values in the presence of KCI; in acidified solutions this effect is observed only for 1.0 M KC1 electrolyte. TU can adsorb strongly at Hg, Bi and Ag electrodes because of the highly specific interaction between the sulphur and the metal [13-161 Adsorption of TU can affect H, evolution by changing the potential profile in the diffuse part of the double layer and also by decreasing the amount of free electrode surface at which the H+ ions can discharge. Both effects increase with the TU concentration and can explain the inhibiting effect, i.e. the increase of An for H, evolution with concentration in acidic media. However, it is also known that TU exists on Hg and Ag at potentials much more negative than E,,, and desorbs at highly negative potentials. In this case the increase in TU concentration should result in a decrease in the catalytic effect. Our experimental results for the acidified media cannot be explained in this way. It has been reported in the literature [6,17] that TU can induce adsorption of cations. Thus we can assume that TU induces the adsorption of H+ ions which increases with TU concentration, facilitating H, evolution. The influence of anions on H, evolution in acidic media can be explained in terms of changes in the potential & at the outer Helmholtz plane. In the case of strong specific adsorption of Cl- ions, the & values

decrease and the process of discharge of H+ ions is accelerated in comparison with the medium containing ClO; ions. Therefore the inhibiting effect is lower in the presence of Cl- than in the presence of CIO; . The influence of electrolyte concentration can also be explained by the changes in & values. At u > 0 (E > E,), an increase in electrolyte concentration at constant TU concentration results in a decrease in & and Aq. At u < 0 (E
Several reports of SERS studies of TU adsorption on a Ag electrode have been published [8,18-241. Fleischmann and coworkers [8,19] have reported the coadsorption of TIJ with anions and suggested that anions may be coadsorbed through the NH, group in neutral solutions but through the NH: group in acidic media. SER spectra of TU solutions in acidic media containing ClO; and Cl- anions are presented in Fig. 3.

%s

3500

3300

3100

1500

Fig. 3. SER spectra of TU solutions in the presence of ClO; (TU + 0.1 M supporting electrolyte).

1100

:I

SCNNout of

1

A

700

), Cl- (---_)

plane

G/cm-’ and SOi-

(.-.-.)

anions at E = -0.8 V (5

x

lo-’

M

J. Bukowska, K Jackowska / Injluence of thiourea on H, evolution

ISCNN

Its

-1.0

-1.2

-1.4

E/V

Fig. 8. IsCm/ZsC vs. electrode potential for neutral solutions of 5 x lo-’ M TU containing chloride (0) and perchlorate (X) anions (0.1 M).

range - 0.6 to -0.8 V the NH groups of TU are H-bonded to Cl- anions simultaneously interacting with Ag atoms. There is no evidence for TU reorientation in the perchlorate medium. This conclusion is opposite to that of Tian et al. [8]. TU molecules are oriented more perpendicularly in the perchlorate medium than in the chloride medium. When all the spectroscopic observations described above are taken into account, the coadsorption of TU

Fig. 9. Model of the madsorption

of TU with ClO;

47

with ClO; and Cl- anions can be presented as shown in Fig. 9. We now consider the influence of anion concentration on the ratio Z,,,,/Zc, (Fig. 7). As can be seen (Fig. 7(b)), the ClO; anion has no effect on these ratios, but in Cl- solutions the higher relative intensity of the 615 cm-’ band is found at the same potential in more concentrated solutions. This means that more TU molecules are oriented flat to the surface at higher concentrations of Cl- anions. , Rather different results were found for neutral TU solutions. The relative intensities of the SCNN band at various electrode potentials are plotted in Fig. 8 for Cl- and ClO; media. In contrast with the acidic solutions, the values are higher for ClO; than for Clsolutions at all potential values, indicating a more parallel orientation of TU molecules in the presence of ClO; ions. The increase in the ZSCNN/ZCSratio with increasing negative potential for Cl- solutions is much smaller than that observed for acidic media. In neutral solutions containing Cl07 ions TU molecules are oriented more parallel to the surface than they are in acidic media. The above results for the orientation of the TU molecule with respect to the electrode surface correlate with the inhibiting effect of TU on the H, evolution in acidic media: (i) a flatter orientation is observed at lower TU concentrations, favoring H, evolution;

and Cl- anions on a Ag electrode.

48

J. Bukowska, K Jackowska / Influence of rhiourea on Hz evolution

(ii> the orientation of the TU molecule is more perpendicular in the presence of ClO; anions than in the presence of Cl- anions, and this inhibits H, evolution more effectively. In neutral solutions ClO; anions induce more parallel orientation of TU than is the case with Clanions, and this corresponds to the facilitation of H, evolution. Thus it is concluded that H, evolution is faster at a more parallel orientation of TU molecules to the Ag electrode surface. However, the orientation of TU is not the only factor influencing this process. Our SE&3 data indicate that H, evolution is not catalyzed as long as NH, groups of TU are engaged in interactions with anions. After desorption of the anions, NH, groups can form H bonds with H,O or H30+, thus mediating proton discharge on the electrode surface. 4. Conclusions We believe that the inhibiting (in acidic media) or catalytic (in acidified and neutral media) effects of TU on H, evolution at Ag electrodes can be correlated with different orientations of TU to the surface in the potential range of proton discharge. The NH, groups of TU hydrogen bonded to H,O or H,O+ can mediate in this process. This mediation is most effective for almost parallel orientation of TU molecule. The inhibiting effect of TU in acidic media follows from the interaction of both NH, groups with anions. In neutral solutions in the potential range of H, evolution anions are not coadsorbed with TU and NH, groups are H-bonded to H,O (H,O+), facilitating discharge of proton. Acknowledgments This work was financially supported by the Warsaw University program BW-662/12/92.

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