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A Kinetic Model for the Dissolution Mechanism of Copper in Acidic Sulfate Solutions

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  Eleclrochimica Acta, Vol. 38, No. 14, pp. 2121-2127. 1993 0013- 4686/93 $6.00 + 0.00 Printed in Great Britain. f; 1993. Pergamon Press Ltd. A KINETIC MODEL FOR THE DISSOLUTION MECHANISM OF COPPER IN ACIDIC SULFATE SOLUTIONS DANNY K. Y. WONG,* BRUCE A. W. C~LLER~ nd DOUGLAS R. MacFaRLaNEt School of Chemistry, Macquarie University, Sydney, New South Wales 2109, Australia; TDepartment of Chemistry, Monash University, Clayton, Victoria 3168, Australia Received 10 March 1993) Abstract--Steady-state polarization curves and impedance data have been obtained for the electrochemi- cal dissolution of copper in 1.0 M Na,SO, solutions at pH 1, 2, 4 and 5. The steady-state polarization curves display only active dissolution in the potential range investigated (from the open circuit potential up to + 1OOmV vs. see) and exhibit a Tafel range of 54.&55.6mV. Two time constants, over a wide spectrum of frequencies (10-3-1@H~), have been observed in all complex impedance plots. The experi- mental results have heen quantitatively fitted by a reaction mechanism model which provides an excellent fit to the steady state data and is in qualitative agreement with the impedance data. Key words: impedance electrochemistry, copper dissolution, kinetic and mechanism, rotating copper elec- trode, acidic sulfate solutions. INTRODUCTION Anodic dissolution of copper is involved in electrorefining[l], in electropolishing[2], in corro- sion phenomena[2] and in maintaining the supply for electroless deposition processes[l]. Many funda- mental studies have been concerned with the mecha- nism of corrosionC2, 33 and deposition[6-131 of copper in various media, but very little has been reported concerning the anodic dissolution of copper at potentials other than the corrosion potential. Mattsson and Bock&[61 and others[7, 81 employed a galvanostatic transient polarization method to study the kinetics of deposition and disso- lution of copper in CuSO, solution. According to these authors, the dissolution of copper takes place in a step-wise route in which Cu+ ion is the interme- diate species (Model I). Model I kt cu F Cu+ + e- k-1 kz cu+ \ Cu*+ + e-. (If4 k-z This dissolution mechanism was proposed because the Tafel plot was found to be linear with a Tafel slope of 40mV at 25°C. In this model, the redox process between Cu+ and Cu2+ (Ib) is therefore regarded as the rate-controlling step and the reac- tion steps relating to k,/k_, are supposed to equili- brate very quickly compared to those relating to k2/k_2. Impedance results of a copper electrode at the equilibrium potential in a 0.56M CuSO, + 0.5 M H,SO, solution obtained by Dmitriev et aL[14] also implied the slow electron exchange reac- Author to whom correspondence should be addressed. tion (Cu’ + Cuz+ + e-) on the electrode, the steady rate of which was itself limited by diffusion of Cu+. Brown and Thirsk[9] have found results support- ing Model I using a potentiostatic transient tech- nique with a rotating disk electrode in 0.3 M CuSO, + 0.2M H,SO,. The anodic current was observed to increase with increased electrode rotation speed in the overpotential range +5 to + lOmV, and the local mass transport of Cu+ is considered in addi- tion to the diffusion of Cu2+ from the electrode. However, Glarum and Marshall[lS], reported an admittance study of a copper rotating disk electrode during deposition and dissolution processes in 0.25 M CuSO, + 1.75 M H,SO,, agreed that Model I is applicable near the equilibrium potential if allowance is made for the diffusion of Cu+ and Cu’+ ions. The admittance results were used to infer that Cu+ concentration is substantially decreased at more anodic potentials due to a disproportionation reaction (2Cu+ + Cu+ + Cu) and the growth of metallic clusters in solution, while frequency disper- sion was found at cathodic potentials due to a double layer relaxation. Slaiman and Lorenz[16] and Stankovic[ 171 have carried out studies of copper dissolution in a solu- tion of H,SO, + CuSO, using a galvanostatic method. Slaiman and Lore&s results suggested that step (Ia) in Model I be split in order to accommo- date the presence of an adsorbed Cu+ species after comparison with an equivalent circuit consisting of a parallel arrangement of a double layer capacitance with two polarization resistances in series, and one of these two polarization resistances in turn, in parallel with an adsorption capacitance. Slaiman et al. found that, under their experimental conditions, no significant evidence for diffusion polarization was detected. Stankovic observed that the pseudocapaci- tance increases with increase in potential and this was suggested to be due to the adsorption of Cu+ 2121  2 22 D. K. Y. WONG et al. ions before the exchange of electrons as in step (Ib). Several research groups[l&221 have provided evi- dence for the presence of Cu(1) in solution during the dissolution process. Hence, these authors have pro- posed a model for copper dissolution represented by Model II. Model II kl cu \ Cu(I),, + e- k-1 kz Cu(I) d = cut1) k-z h cum Cu(II) + e-. k-3 More recently, Balakrishnan and Venkatesan[23] have investigated the anodic behaviour of copper in various media including Na,SO,, NaCl, NaNO, , NH&l and (NH&IO4 solutions using rotating disk and ring-disk electrodes under potentio-dynamic conditions (potential scanning rate of 200mV min-‘). These authors showed anodic polarization curves of copper electrodes in acidic sulfate solutions and observed that no passivation process occurred in these systems. An anodic Tafel slope of 46 mV was obtained in neutral sulfate solution and 40mM in acidic solution (0.05 M HzSO,, + 0.2 M Na,SO,, pH 1). Also, these authors reported an increased dis- solution rate of copper with increased sulfate ion concentration especially at high potentials. However, no detailed discussion of a possible reaction mecha- nism was given. Awad er ~I.[241 carried out dissolution studies in acidified aqueous solutions of various anions using a potentiostatic method. Tafel plots were constructed for the systems studied. The Tafel slopes (ranging from 55 to 72mV in chloride, nitrate and sulfate solutions) were found to depend on the nature of the anion, indicating the possibility of anion partici- pation in the anodic dissolution of the metal. Hence, these authors proposed a reaction mechanism involving the general anion, A-, based on the forma- tion of intermediate Cu+ compounds (a fast reaction). This was then followed either by a slow electron exchange reaction or by a slow dispro- portionation reaction (Model III). Model III kI Cu+A-- _ CuA+e- k-1 either CuA A Cu2+ + A- + e- or 2CuA A Cu + Cu2+ + 2A-. In the present work[25], the electrochemical dis- solution of copper in acidic sulfate solutions has been studied using ac impedance techniques. By comparison with earlier work, results presented here cover not only a wider range of pH (pH l-5), but also a wider spectrum of frequencies (10-3-104H~). Extending the range of H+ concentration to five orders of magnitude allows investigation of the dependence of the rate of reaction on pH and hence of the role of hydrogen ions in the dissolution process. Results obtained from dc measurements generally only indicate the overall reaction rate which depends on the slowest reaction path for a series process, or on the fastest reaction path for a parallel process in a particular reaction mechanism. On the other hand, the uc impedance measurements provide additional information arising from the intermediates involved in the reaction pathway, espe- cially in the low frequency region where slow relax- ation processes are more easily observed. The low frequency region is here studied to frequencies an order of magnitude lower than in any previous work. The present study has also covered potentials ranging from the vicinity of the potential of zero current up to about + 1OOmV vs. see, in comparison to earlier work in which attention has only been centred around the potential of zero current poten- tial (ie the corrosion potential). Hence, it is hoped that more complete information can be obtained through the present study and thereby the disso- lution mechanism of copper in acidic sulfate solu- tions will be clarified. EXPERIMENTAL Analytical reagent grade Na,SO, (anhydrous, granular) and H2S04, purchased from Ajax Chemi- cal, were used without further purification. Solutions of l.OM Na,SO, and H2SO4 were prepared in doubly distilled water. Electrolyte solutions of differ- ent pH were prepared by mixing various proportions of l.OM Na,SO, and H,SO,. All electrolyte solu- tions were purged of oxygen by degassing with puri- fied nitrogen for 16h[26] before steady state measurements were commenced. The nitrogen was purified by passing through a column of BASF R3-11 copper catalyst and saturated by passage through doubly distilled water. After deaeration a blanket of nitrogen was kept over the solutions. All experiments were performed in the poten- tiostatic mode using a three-electrode cell. The refer- ence electrode was a saturated calomel electrode (see) and the counter electrode was a large piece of platinum metal. A rotating copper (specpure grade from Johnson & Matthey Chemicals Limited) disk electrode (0.0194 f 0.0002 cm2) was employed as the working electrode. The copper electrode was etched in 20% (vol/vol) HNO, solution for 2min[27] and rinsed with electrolyte solution before steady-state measurements were commenced. Steady-state measurements were made via a Solar- tron 1186 Electrochemical Interface. Current mea- surements were made when, at a set potential, the current had reached a steady state. Impedance mea- surements were performed using a Solartron 1250 Frequency Response Analyzer. The system was oper- ated under automated control by a 6809 microprocessor-based computer system. RESULTS AND DISCUSSION The steady-state polarization curves of the disso- lution of copper in l.OM Na,SO, solutions at pH 1.0, 2.0, 3.0, 4.0 and 5.0 are depicted in Fig. 1. In each case, the steady state was obtained within 5- 10min at a given potential and the Tafel slopes  Electrochemical dissolution of copper 2123 pH 4.0 Fig. 1. Experimental (dashed lines) and simulated (solid lines) steady-state polarization curves of disso- lution of copper in a mixture of various proportions of l.OM Na,SO, and 1.0 M H,SO,. Electrode rotation speed = 25OOrpm. Marked points indicate the polarization potentials at which the impedance was measured. The kinetic parameters used in simulations are tabulated in Table 1. observed range from 54.8 to 55.6mV (compared to 55-72 mV reported previously[24]). In general, all five polarization curves exhibit only active dissolution behaviour in the potential range investi- gated, in agreement with those reported by Balakrishnan[23] and Awad[24]. Impedance spectra for solutions of pH 1.0, 2.0, 3.0, 4.0 and 5.0 have been obtained at different polariza- tion potentials. Two capacitive arcs were found in all cases. Hence, only impedance spectra for solutions of pH 1.0 and 5.0 at different polarization potentials, labeled on each steady-state polarization curve, are displayed in Figs 2 and 3, respectively. In general, the sixes of the arcs, and hence the corresponding scales of impedance, decrease as the potential becomes more anodic at each pH. In addition, the lower frequency arc increases in size when the pH of the solution becomes much more basic, eg pH 5.0, as shown in Fig. 3. Also, at higher pH (pH4.0 and 5.0), m3---;~ E 0 100 200 300 400 SW600 6 k- I 811 1 :_, 1 0 100 200 300 400 5 I REAL PART, Ohm the size of this low frequency arc increases as the electrode rotation speed decreases, indicating the involvement of a diffusion process in determining the ac response. On the other hand, no such electrode rotation speed dependence has been observed for the lower frequency arcs in low pH experiments (pH 1.0, 2.0 and 3.0). Therefore, while the high frequency arc corresponds to a faradaic impedance due to a charge transfer resistance in parallel arrangement with the double layer capacitance[28], the lower frequency arc probably arises from a charge transfer interme- diate adsorption at low pH and a process involving diffusion at higher pH. Polarization curves and impedance spectra do not allow direct determination of the chemical nature of reaction intermediate species, but do enable ideas to be developed about the potential and time depen- dence of the concentrations and surface coverages of each species. Several hypothetical mechanisms for REAL PART, Ohm Fig. 2. Experimental (left) and simulated (right) complex impedance plots in pH 1.0 solution. Polarization potentials at which the impedance was measured are marked on the pH 1.0 polarization curve in Fig. 1 by the corresponding letters. Parameter is frequency in Hz  2124 D. K. Y. WONG et al. Fig. 3. I/ ‘.“I\.’ 0 200 400 600 800 1'000 E 6 2oo1 B5 1 1, f-f?yk~ 0 100 200 300 400 REAL PART, Ohm REAL PART, Ohm Experimental (left) and simulated (right) complex impedance plots in pH 5.0 solution. Polarization potentials are. marked on the pH 5.0 curve in Fig. 1. copper in acidic sulfate solutions have been pro- posed and computer simulated in the present study to generate theoretical steady-state polarization curves and impedance spectra. The validity, though not the uniqueness, of a reaction mechanism can only be established when agreement is observed with the polarization curve and a large number of imped- ance measurements at different points on the curve, in terms of the magnitude of the impedance and fre- quency distribution. In this case, a model that accounts for the dissolution mechanism of copper must explain the various features in the steady-state polarization curves and in the impedance spectra. The simplest mechanism found to provide a sulll- cient explanation of the major features of the experi- mental data presented in this paper is as follows: cu + Cu(II),, t4 Cu(II),, + Cu(II),, + 2e- k-, lk, kI cu - Cu(I),, k-1 k-3 tlk, Cu(I)zd d f ,, . In this model, the reaction step (l/-l) represents the electrochemical conversion of copper metal to an adsorbed Cu(1) species on the electrode surface. The latter is further oxidized to Cu(II), also adsorbed on the electrode surface The reaction step relating to k, is a self-catalytic process and no consumption of Cu(II), takes place. Such a catalytic step allows the separation to the Cu(II),, relaxation time constant from the dc current through the electrode interface. Alternatively, Cu(I)*, probably anion-associated, which can then be desorbed and diffuse into the bulk of the solution. A large number of simpler models derived from this model, including Models I-III, were also investigated, but in each case a serious dis- crepancy was observed between the simulated and observed results. Computer simulations were performed by tran- scribing the proposed model into mathematical expressions based on several assumptions described previously[28]. For model IV, the current density expression and mass balance expressions are written aS: ; = k, - Bl(kl + k-, - k2 - (k, + k_, - 2kJ B de, dt = k, - f+(k, + k_, + k, + k3) - 8,(k, - k_,) + (Ck_,) p 2 = lk2 - e2 k_ 2) dC y dt = t k, + Ck-, - flux, (1) (2) (3) (4) where Or denotes the fractional electrode coverage by Cu(I), & the fractional electrode coverage by Cu(II), and C the surface contact concentration of Cu(I)*. /i and y represent the maximum number of molecules adsorbed per unit electrode surface area and the perpendicular distance from the electrode interface occupied by a species in solution making contact with the surface, respectively. Note that k,
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