Electrochemical quartz crystal microbalance study of copper ad-atoms on gold and platinum electrodes Part I. Adsorption of anions in sulfuric acid

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ELSEVIER

Journal of Electroanalytical Chemistry 380 (1995) 255 260

Electrochemical quartz crystal microbalance study of copper ad-atoms on gold and platinum electrodes Part I. Adsorption of anions in sulfuric acid Masahiro Watanabe *, Hiroyuki Uchida, Nobuo Ikeda Laboratory of Electrochernical Energy Cont,ersion, Faculty of Engineering, Yamanashi Uni~.'ersity, Takeda 4-3, Kofu 400, Japan Received 21 March 1994; in revised form 13 June 1994

Abstract

The deposition and dissolution processes of copper ad-atoms on a gold or a platinum electrode in sulfuric acid electrolyte solution were investigated by using the electrochemical quartz crystal microbalance. It was found that the weight loss in the removal of the Cu-adlayer from the Au substrate was considerably larger than that expected from Faraday's law whereas the deviation for the Pt substrate was very small. The adsorption of bisulfate or sulfate anions both on Cu ad-atoms and on the electrode substrates was discussed quantitatively. It was demonstrated that higher coverage with Cu ad-atoms and lower adsorbability with bisulfate or sulfate anions were obtained on the Pt electrode than on the Au, and these effects could be ascribed to the difference in electronegativity between Pt and Au substrates.

Keywords: Quartz crystal microbalance; Copper ad-atoms; Adsorption; Anions; Sulfuric acid 1. Introduction Properties of metal electrodes are modified dramatically by the underpotential deposition (UPD) of a submonolayer of foreign ad-atoms on their surfaces under precisely controlled conditions [1-4]. In order to clarify the UPD, which is a basic electrochemical process, and to design high performance electrocatalysts, an in situ analysis of the surface of the ad-atom electrode at an atomic level of accuracy is quite important. The electrochemical quartz crystal microbalance (EQCM) [5] is one of the most useful in situ analytical techniques because it can detect momentary changes in mass of 10 - 9 g (ng) at the electrode surface. However, it has often been observed that the mass change monitored by the E Q C M is not in accord with that expected from the electric charge in the voltammogram for deposition/dissolution of ad-atoms [6-8] or even in that for oxidation/reduction of the electrode surfaces of gold [9] or platinum [10]. Such discrepancies have been explained by the electrosorption valency of metal ions [6,7], the adsorption of other species [8] and the water

* Corresponding author. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 4 ) 0 3 6 2 1 - 9

trapped at a roughened surface [9], etc., but the discussion is not always quantitative. In the present paper, we focus on the processes of deposition and dissolution of copper ad-atoms on a gold or a platinum electrode in sulfuric acid electrolyte solution using the EQCM, which has both high stability and high sensitivity. The adsorption of the electrolyte anions both on the ad-atoms and on the electrode substrates is discussed quantitatively. It is demonstrated that higher coverage with Cu ad-atoms and lower adsorbability with bisulfate or sulfate anions are obtained on a platinum electrode than on a gold electrode.

2. Experimental section Planar AT-cut quartz crystals (Torrane, CA) 2.54 cm in diameter were operated at the fundamental frequency of 5 MHz in all experiments. One side of the crystal surface was coated by RF-sputtering with a gold or a platinum film electrode. A gold film electrode was sputter-deposited onto the other face. A thin film of titanium was sputtered to improve the adhesion between the crystal and the above film electrodes. The

256

M. Watanabe et al. /Journal of Electroanalytical Chemistry 380 (1995) 255-260

crystal was mounted in an E Q C M sensor (Maxtek TPS500). Electrochemical measurements were carried out with a three-compartment Teflon cell. The Au- or P t - E Q C M sensor was screwed into the bottom of the cell. The projected surface area of the Au or Pt working electrode was 1.42 cm 2. A counter electrode of platinized platinum gauze was separated from the working electrode compartment by an ion exchange m e m b r a n e (Nafion 117). A reversible hydrogen electrode ( R H E ) was used as a reference electrode. All the electrode potentials in this p a p e r will be referenced to the RHE. The oscillating frequency of the E Q C M was recorded simultaneously with the electrode potential and the current by a personal computer interfaced to a frequency counter (HP 5334B) and a potentiostat (Hokuto Denko, HA-501). All the measurements were performed in a thermostat kept at 25°C. The sensitivity of the E Q C M determined by several calibration runs for electrodeposition of Ag was 17.8 ng H z - 1 cm-2, which equals the theoretical value [11]. The noise level of the present E Q C M system was _+0.1 Hz in frequency (i.e. ___2.5 ng) without any signal averaging. The electrolyte solution of 0.05 M H z S O 4 w a s prepared using a reagent grade chemical and a pure water distilled from permanganate solution and purified further by pre-electrolysis in a similar manner to that described previously [1]. The solutions were degassed by bubbling high purity argon gas for 30 rain prior to every measurement, and the cell was blanketed with Ar flow during the measurements. The purity of the electrolyte was checked from the reduction in hydrogen waves at the conventional voltammogram on an additional platinum wire electrode [1,12]. An electrochemical cleaning of the electrode surface was carried out by applying a potential-time sequence between 0.05 and 1.5 V in a similar manner to that described previously [1,12]. Copper ad-atoms were underpotentially deposited (UPD) on the gold or platinum electrode by keeping the potential at 0.40 or 0.35 V for various intervals (0-30 min) in 0.05 M H 2 S O 4 solution containing 10 -5 M CuSO 4. After the UPD, the potential was set at 0.05 V for 0.1 s, and the potential sweep was immediately started at a sweep rate of 0.1 V s - 1 with a reversal potential of 1.5 V.

3. Results and discussion

Fig. 1 shows a typical cyclic voltammogram (CV) of a gold film electrode on the face of the Q C M in the potential region between 0.05 and 1.5 V in 0.05 M H z S O 4 solution. The simultaneously recorded frequency change-potential response is also shown. Since the frequency at 0.05 V after the potential scan returns to the original value within an experimental error, it is

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not necessary to take into account the loss of mass due to dissolution of gold during the scan as described in reference [13]. The increase in mass between 1.2 and 1.5 V in the positive-going potential scan due to the formation of gold oxide layer was 25 ng, which is smaller than 29 ng calculated from Faraday's law for two electron oxidation on the charge for the oxidation peak in CV. It is considered that this deviation is attributed to the desorption of anions during the oxide formation, which is supported by the CV study on A u ( l l l ) [14], but the amount of desorbed anions is appreciably less than that estimated in Ref. [14]. The same quantity of weight loss as described above is seen for the oxide reduction in the negative-going scan. Therefore, in the oxide-region, the A u - E Q C M exhibits a very reversible response. A large and fairly reversible change in weight is seen at the double layer region between 0.05 and 1.2 V in the positive-going scan and 0.9 and 0.05 V in the negative-going scan. The increase in weight at 1 V was 131 ng. This is associated with changes in the structure of the double layer at a clean Au surface due to the adsorption of anions (bisulfate or sulfate ions) [14-20] or water molecules [17,21]. Recently, Magnussen et al. [22] reported the arrangement of adsorbed bisulfate H S O 4 ions on an A u ( l l l ) surface as observed by a scanning tunneling microscope (STM) and the coverage of HSO 4 at 1.0 V being ca. 0.4. In contrast, based on a voltammetric study, Shi et al. [18-20] claimed that the species adsorbed on A u ( l l l ) is the sulfate SO42ion and the coverage of SO 2 - is ca. 0.3-0.4. However, by F T I R [17], the adsorption of both H S O 4 and SO42on polycrystalline Au was observed. In Ref. [17], the adsorbability of sulfate and bisulfate is dependent on

M. Watanabe et al. /Journal of Electroanalytical Chemistry 380 (1995) 255-260

the potential, potential sweep rate, solution pH, and other factors, and SO 2- is the dominant adsorbed species at positive potential region. Since the E Q C M cannot distinguish between sulfate and bisulfate, we will then evaluate the amount of adsorbed species for each anion. The coverage of anion on our polycrystalline Au, 0~nl°n, was estimated to be 0.34 if we assume that the increase in mass up to 1.0 V is only due to the adsorption of H S O 4. However, this assumption is an oversimplification and the estimated 0~u'°n must be an upper limit because the anions may be accompanied by some water molecules [14]. Fig. 2 shows a cyclic voltammogram (CV) and a frequency change of a platinum film electrode on the face of the Q C M in 0.05 M H 2 S O 4 solution. The frequency changes for both formation and reduction of oxide on the platinum surface coincided with those expected from Faraday's law. However, in the positive-going scan in the hydrogen region between 0.05 and 0.35 V, an increase in mass commences in spite of desorbing hydrogen atoms from the surface, and it continues in the double layer region (0.35-0.8 V). The responses of P t - E Q C M in perchloric acid and in sulfuric acid reported by Shimazu and Kita [23] are similar to that in Fig. 2, but they concluded that the continuous increase in mass in the double layer region, AWDL, could be assigned predominantly to water adsorption on the Pt surface. An inspection of the relation between the ZlWDL and the charge in the double layer region indicates that the zlWDLfor the P t - E Q C M should be ascribed to a specific adsorption or co-adsorption of bisulfate a n d / o r sulfate ions, which have been clearly demonstrated by F T I R [24] and radio

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tracer experiments [25], rather than water. An upper anion limit of coverage of H S O 4 on the Pt electrode, Opt , was estimated to be 0.15 at 0.8 V assuming that the adsorption of pure, water-free H S O 4 is responsible for the increase in weight. Therefore, a smaller amount of bisulfate or sulfate anions adsorbs on Pt than on gold. The time courses of increases in mass during underpotential deposition of copper ad-atoms on the Au and Pt electrode are shown in Fig. 3. The frequency change of the A u - E Q C M levels off after about 600 s, indicating that both processes of the U P D and anion adsorption reached an equilibrium state in the solution. A slower but larger mass change is noted for the Pt substrate. Fig. 4 shows typical CVs on Au electrodes on which Cu ad-atoms were underpotentially deposited by keeping the potential at 0.4 V in 0.05 M H 2 S O 4 solution containing 10 -5 M CuSO 4 for various intervals. At the potential between approximately 0.5 and 0.6 V in the positive-going scan, a single anodic peak was observed in the CV, accompanied by a distinct mass loss in the E Q C M response. This peak corresponds to the dissolution of Cu ad-atoms which strongly bonded to the Au surface [12]. The U P D of Cu ad-atoms at a less positive potential (0.35 V) resulted in increased amount of Cu ad-atoms, and an additional small anodic peak at about 0.4 V (not shown in Fig. 4) as the coverage of Cu exceeded ca. 0.5 monolayer, which was assigned to Cu ad-atoms with weaker bond energy. The equilibrium coverages for the U P D at 0.4 and 0.35 V, which were determined by the conventional method based on the quantity of the anodic dissolution charge, could not exceed ca. 0.4 and 0.6, respectively. It must be noted that this calculation of the coverage contains an error due to a contribution of co-adsorbed anion to the measured charge [26,27]. An exact calculation is given later, and a very similar method appeared recently in Ref. [19].

M. Watanabe et al. /Journal of Electroanalytical Chemistry 380 (1995) 255-260

258

For a Pt substrate, as shown in Fig. 5, the anodic dissolution peak of Cu ad-atoms accompanied with a distinct mass loss in the E Q C M response is seen at a more positive potential region than that for the Au substrate. This suggests that Cu ad-atoms have stronger binding energy to the Pt substrate than to Au although they were deposited under the same U P D conditions. It is also observed for the Pt substrate that hydrogen waves decrease with increasing copper waves since adsorbed hydrogen atoms are replaced by Cu ad-atoms [12]. The quantity of charge for the dissolution of Cu was approximately twice the decrease in the charge required for the desorption of hydrogen adsorbed, Immediately after the dissolution of Cu ad-atoms, oxidation of the Pt electrode surface commences, accompanied with an increase in mass as shown by the corresponding EQCM. At a potential more positive than the dissolution peak of Cu ad-atoms, the A u - E Q C M result is almost parallel to that of the pure Au electrode, which shows an equal increase in mass attributed to the adsorption of anions. The P t - E Q C M result is also parallel to that of the pure Pt electrode in the potential region more positive than the dissolution peak of the ad-atoms. Since the CV and the frequency response were un-

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Potential / V vs. RHE Fig. 5. Cyclic voltammogram and simultaneously recorded frequency change of pure Pt-EQCM electrode (A; measured in 0.05 M H2SO4, reproduced from Fig. 2) and with Cu ad-atoms (B, C, D; measured in 0.05 M H 2 S O 4 + 10 5 M CuSO4). Sweep rate = 0.1 V s 1. Copper ad-atoms were underpotentially deposited on Pt by keeping the potential at 0.4 V in 0.05 M H2SO 4 + 10 -5 M CuSO 4 for 5 rain (B), 10 min (C) and 30 min (D).

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changed in the oxide-region regardless of whether Cu ad-atoms were deposited or not, the oxide formation a n d / o r the double layer structures must be the same as those for pure Au or pure Pt electrodes in that potential region. Then, the change in mass during the removal of the Cu-adlayer was determined by subtracting the E Q C M response of the substrate electrode without Cu ad-atoms in the positive-going scan (0.051.5 V) from that with Cu ad-atoms. In the same manner, the quantity of charge, Q, for the dissolution of Cu ad-atoms was also calculated by the subtraction of the charge without ad-atoms from that with ad-atoms in the same integration region. The mass loss in the removal of the Cu-adlayer from the Au and Pt electrode, W, is plotted as a function of the anodic charge, Q, in Fig. 6. It becomes clear that the W for Au substrate is considerably larger than that expected from Faraday's law (dotted line), i.e. more than twice, and the deviation is almost independent of the U P D potential. On the other hand, the relation between W and Q for Pt substrate is relatively close to that expected from Faraday's law. Deakin and Melroy [6] have reported an E Q C M response in 0.1 M HCIO 4 solution similar to that observed in this paper and claimed that an electrosorption valency 3' of 1.4 for Cu 2+ is responsible for the phenomenon; incomplete

M. Watanabe et al. /Journal of Electroanalytical Chemistry 380 (1995) 255-260

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discharge of Cu 2+. However, the y for C u 2+ onto Au summarized in reviews [28] is approximately equal to 2 or slightly less. In order to explain the large deviation of ca. 2 fold from the Faraday's law on our data shown in Fig. 6, the 3, should he 1.0 and this hypothesis is not tenable. Hence, we consider that the E Q C M response must be attributed to both faradaic dissolution of Cu ad-atoms and a desorption of anions which were adsorbed on Cu ad-atoms, and that a large amount of bisulfate or sulfate ions may specifically adsorb on copper ad-atoms at an Au substrate, while a small quantity of such anions adsorbs on Cu ad-atoms at a Pt substrate [26]. Borges et al. reported recently in a preliminary note [16] that the U P D of Cu from sulfuric acid solution onto a A u ( l l l ) - E Q C M surface is accompanied by the adsorption of sulfate or bisulfate. However, they did not take into account the charge arising from the a d s o r p t i o n / d e s o r p t i o n of anions. In the same issue of the journal [18,19], it was reported preliminarily that the apparent 3' for Cu 2+ on Au(111) depends on the amount of co-adsorbed SO 2-, based on a similar consideration to ours. We have studied independently the adsorption of anions on Cu ad-atoms of a series of coverages on Au and Pt substrates [26]. In order to estimate the amount of anions specifically adsorbed on the Cu ad-atoms, the following two simultaneous equations were solved. First, the mass loss, W, during the dissolution of the Cu-adlayer from the substrate is represented as, W = 63.55X + MY

259

(1)

where X and Y are the number of moles of copper ad-atoms dissolved and that of anions on Cu ad-atoms, respectively, and M is the molar mass of the anion.

(2)

where n is the absolute value of the valency of the anion and F is the Faraday constant. Eq. (2) assumes that a complete discharge of Cu 2+ (y = 2) gives an anodic faradaic current and that desorption of the anions on the Cu ad-atoms gives a cathodic capacitive current, so-called "double layer charging current". It has been reported that these anions are specifically adsorbed on the A u ( l l l ) surface with full or almost full charge transfer [14]. Although the y for SO42- in 0.1 M HC104 + 10 3 M K2SO 4 was reported to be 0.8 [18,20], they neglected the possibility of specific adsorption of perchlorate C10 4 ion. However, we detected a specific adsorption of C10 4 on the A u - E Q C M in 0.1 M HC10 4 [27]. Therefore, the actual 3, for SO42must be larger than that reported in Refs. [18,19]. We adopted values of full charge transfer of anions [14]. Then, the coverage of Cu ad-atoms on the substrate M, 0 Cu M , is determined as 0 cu

= X / N M (M = Au or e t )

(3)

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(4)

The calculated results both for Au and Pt substrates are shown in Fig. 7 which contains two extreme cases;

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260

M. Watanabe et al. / Journal of Electroanalytical Chemistry 380 (1995) 255-260

each assumes that only one anion species, either H S O [ ( n = l , M = 9 7 ) or SO42 ( n = 2 , M = 9 6 ) , specifically adsorbs on Cu ad-atoms without any accompanying water molecules. Two prominent features can be seen for the Pt and Au electrodes. For the Au substrate, the 0~'~i°n reaches a steady value at the O)~C~more than about 0.3, irrespective of the deposition potential of Cu ad-atoms (0.35 and 0.4 V). The coverages of anion on Cu ad-atoms assuming HSO 4 and SO42- are 0.55 and 0.4, respectively. These are larger than that on Au substrate without ad-atoms (0X"~i°n < 0.34) as described above, indicating that anions have higher affinity to copper ad-atoms than to the gold substrate. This is consistent with results given by in situ F T I R study [17]. A steep increase in weight during the UPD of Cu shown in Fig. 3 must respond to the formation of the Cu-adlayer on which anions specifically adsorb. Recently, a model structure of adsorbed bisulfate or sulfate ions on Cu ad-atoms underpotentially deposited on Au(111) has been demonstrated by an EXAFS study [29]. The present EQCM study is consistent with it and, moreover, gives the quantitative relationship between the amount of copper ad-atoms and that of adsorbed anions. On the other hand, for the Pt substrate, the 0 ~ ~°n is less than 0.1 over the whole 0pctu even when the coverage of Pt with Cu reaches one monolayer; the anions scarcely adsorb on Cu ad-atoms deposited as well as on Pt substrate (0~ti°"< 0.15 as described above). This also suggests that the adsorption of anions on Cu ad-atoms is strongly affected by the metal substrate. These results may be explained rationally by the difference either in the density of electronic states at the Fermi level, D(eF), or Pauling electronegativity, xeM, between gold and platinum [30,31]. Gold has lower D(e v) than platinum, and has larger XeM than platinum, i.e. electrons are strongly held within gold. Both the low D(e F) and the large x ~ of gold tend to attract negative species compared with Pt. If so, bisulfate or sulfate anions adsorb more specifically on Au than on Pt. The inverse situation is expected for Cu 2+ ions; both the coverage and the binding energy of Cu adatoms are higher for Pt substrate than for Au substrate. However, these results cannot be explained in terms of the work function (q)) of the metal substrate conventionally accepted, because @ of platinum is larger than that of gold. The 4) is essentially the same measure as x~. Trasatti [30] has demonstrated a good linear relationship between q~ and x M but gold falls far from such a relation. He has claimed that gold may have a higher effective q) than that widely accepted, and that the behaviour of gold remains an open quese

,

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The EQCM studies using various electrolyte solutions must contribute to the understanding of these problems. Detailed studies are in progress.

References [1] M. Watanabe and S. Motoo, J. Electroanal. Chem., 60 (1975) 259; M. Watanabe and S. Motoo, J. Electroanal. Chem., 60 (1975) 267; M. Watanabe, Denki Kagaku, 53 (1985) 671. [2] D.M. Kolb, in H. Gerisher and C.W. Tobias (eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Wiley, New York, 1978, p. 125 [3] R. Adzic, in H. Gerisher and C.W. Tobias (eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, Wiley, New York, 1984, p. 159. [4] B.E. Conway, Progress in Surface Science, 16 (1984) 1. [5] For recent reviews: R. Schumacher, Angew. Chem. (Intern. Ed.), 29 (1990) 329; D.A. Buttry and M.D. Ward, Chem. Rev., 92 (1992) 1355. [6] M.R. Deakin and O. Melroy, J. Electroanal. Chem., 239 (1988) 321. [7] O. Melroy, K. Kanazawa, J.G. Gordon II and D. Buttry, kangmuir, 2 (1986) 697. [8] M. Hepel and S. Bruckenstein, Electrochim. Acta, 34 (1989) 1499. [9] R. Schmacher, G. Borges and K. Kanazawa, Surf. Sci., 163 (1985) L621. [10] R. Raudonis, D. Plausinaitis and V. Daujotis, J. Electroanal. Chem., 358 (1993) 351. [11] G. Sauerbrey, Z. Phys., 155 (1969) 206. [12] N. Furuya and S. Motoo, J. Electroanal. Chem., 72 (1976) 165. [13] S. Bruckenstein and M. Shay, J. Electroanal. Chem., 188 (1985) 131. [14] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, J. Electroanal. Chem., 228 (1987) 429. [15] W. Stiickel and R. Schmacher, Ber. Bunsenges. Phys. Chem., 93 (1989) 600. [16] G.L. Borges, K.K. Kanazawa, J.G. Gordon II, K. Ashley and J. Richer, J. Electroanal. Chem., 364 (1994) 281. [17] D.B. Parry, M.G. Samant, H. Seki, M.R. Philpott and K. Ashley, Langmuir, 9 (1993) 1878. [18] Z. Shi and J. Lipkowski, J. Electroanal. Chem., 364 (1994) 289. [19] Z. Shi and J. Lipkowski, J. Etectroanal. Chem., 365 (1994) 303. [20] Z. Shi, J. Lipkowski, M. Gamboa, P. Zelanay and A. Wiekowski, J. Electroanal. Chem., 366 (1994) 317. [21] J.S. Gordon and D.C. Johnson, J. Electroanal. Chem., 365 (1994) 267. [22] O.M. Magnussen, J. Hageb6ch, J. Hotlos and R.J. Behm, Faraday Discuss., 94 (1992) 329. [23] K. Shimazu and H. Kita, J. Electroanal. Chem., 341 (1992) 361. [24] K. Kunimatsu, M.G. Samant and H. Seki, J. Electroanal. Chem., 258 (1989) 163. [25] G. Horanyi, Electrochim. Acta., 25 (1980) 43. [26] N. Ikeda, H. Uchida and M. Watanabe, The Fall Meeting of Chem. Soc. Jpn. (Sep. 28, 1993) Abstract No. 2A414; H. Uchida, N. lkeda, M. Suzuki, and M. Watanabe, The Fall Meeting of Electrochem. Soc. Jpn., (Oct. 14, 1993) Abstract No. C04. [27] H. Uchida, N. Ikeda, and M. Watanabe, The 61st Meeting of Electrochem. Soc. Jpn., (April 5, 1994) Abstract No. 3J24; to be submitted. [28] J.W. Schultze and K.J. Vener, J. Electroanal. Chem., 44 (1973) 63; J.W. Schultze and F.D. Koppita, Electrochim. Acta, 21 (1976) 327. [29] O.R. Melroy, M.G. Samant, G.L Borges, J.G. Gordon II, L. Blum, J.H. White, M.J. Albarelli, M. McMillan and H.D. Abruna, Langmuir, 4 (1988) 728. [30] S. Trasatti, in H. Gerisher and C.W. Tobias (eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 10, Wiley, New York, 1977, p.213; S. Trasatti, J. Electroanal. Chem., 33 (1971) 351. [31] M.V. Vojnovic and D.R. Sepa, J. Chem. Phys., 51 (1969) 5344.