770
Langmuir 1986, 2, 770-773
model the fluorocarbon portion can shield the hydrocarbon portion due to overlap Since the sulfate ion has been removed, the calculation for the model in Figure 9b assumes the molecules are attached to the surface via the quaternary nitrogen and are bent near the thioether linkage. A plot of dispersion energy vs. n (calculated from the close-packed model of Figure 9b) is shown in Figure 10. The plot is similar to the experimental data and indicates that the model is not unrealistic. A detailed discussion of the model has been described p r e v i ~ u s l y ; ~ however, a few comments are relevant a t this time. (1) The model is calculated for tilt angles of 0-45". Beyond this value, the base of the molecule would become unshielded. Experimentally, this may not occur because the molecule is not as rigid as the model proposes but is free to move, creating a fluorocarbon umbrella that effectively shields the base of the molecule. ( 2 ) The model assumes the bent conformation, but there is no way of experimentally discerning this configuration vs. the one proposed in Figure 9a or other overlap con- formations.
Conclusions ESCA and surface energy measurements have been used to study the effect of CDT of PET on the adsorption characteristics of the cationc fluorosurfactant Zonyl FSC. Results indicate that oxidizing the surface via CDT causes the surfactant to uniformly and continuously distribute over the surface. Within a monolayer coverage regime, increasing the surfactant coverage causes reorientation of the surfactant molecules. On untreated PET, the surfactant coverage is incomplete. On CDT PET, the ESCA data suggest that the surfactant molecule is chemisorbed due to an ion exchange process. In the monolayer coverage regime two models have been proposed to predict the measured dispersion energy, Both models have no adjustable parameters and, in one case, a surface energy/surface population model is predicted that fits experimental data quite well. Results indicate that area average surface energies can be used to model observed effects. Registry No. PET, 25038-59-9; Zonyl FSC, 67479-85-0.
Investigation of Electrochemical Properties of Lead Ethyl Xanthate by Linear Potential Sweep Voltammetry I. C. Hamilton Chemistry Department, Footscray Institute of Technology, Footscray, Victoria 301 1, Australia
R. Woods* CSIRO Division of Mineral Chemistry, Port Melbourne, Victoria 3207, Australia Received M a y 6, 1986. I n Final Form: August 3, 1986 Electrochemical reactions of lead ethyl xanthate have been investigated by linear potential sweep voltammetry of the compound deposited on a gold electrode. Reduction resulted in the deposition of metallic lead with the release of xanthate ions. Oxidation resulted in the formation of dixanthogen with Pb2+or Pb(OH)2depending on solution pH. Lead xanthate hydrolyzed at pH values 110.2. The surface of the electrode was found to be hydrophobic when lead xanthate or its oxidation products were present. It is suggested that inhibition of galena flotation at high potentials with xanthate collectors results from surface oxidation of the mineral concurrent with oxidation of lead xanthate.
Introduction In the flotation of galena with ethyl xanthate as collector, lead ethyl xanthate is formed on the mineral surface as a result of the interaction between the collector, dissolved oxygen, and lead sulfide. As this process necessarily involves electrochemical steps, a knowledge of the electrochemical characteristics of lead xanthate is important in understanding the response of the galena/collector system to changes in the conditions of flotation. The reactions of alkali-metal xanthates have been studied a t galena ele~trodes.l-~These investigations have identified lead xanthate, chemisorbed xanthate, and dixanthogen as oxidation products. However, on the basis of these measurements, it is difficult to characterize further (1) Gardner, J. R.; Woods, R. Aust. J. Chem. 1977, 30, 981. (2) Pritzker, M. D.; Yoon, R. H. In Proceedings of the International Symposium o n Electrochemistry in Mineral and Metal Processing; Richardson, P. E., Srinivasan, S., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1984; p 26. (3) Lamache, M.; Lam, D.; Bauer, D. In Proceedings of the International Symposium o n Electrochemistry i n Mineral and Metal Processing; Richardson, P. E., Srinivasan, S., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1984; p 54.
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reactions of lead xanthate since they are not readily separated from other processes involving xanthate and from reaction of the sulfide mineral with the aqueous electrolyte. Guy and Trahar4 found the flotation of galena with ethyl xanthate to be inhibited a t high potentials and considered this to arise from oxidation of lead xanthate on the mineral surface by the reaction Pb(CzHSOCSJ2
+ 2H20
---*
HPbOZ- + (CzH,OCSz)z+ 3H+ + 2e (1)
Inhibition of flotation occurred close to the reversible potential of reaction 1 and displayed a similar pH dependence. oxidation of lead xanthate to dixanthogen could also result in Pb2+or Pb(OH)2depending on solution PH. Pb(CzH50CSz)z Pb2+ + (C2H50CSz)2+ 2e (2)
-
Pb(C2HsOCSZ)z + 2Hz0 Pb(OH),
-
+ (CzH,0CSz)2+ 2H+ + 2e
(4) Guy, P. J.; Trahar, W. J. I n t .
J. Miner. Process.
0 1986 American Chemical Society
(3)
1984, 12, 15.
-20
.
-30
1
Langmuir, Vol. 2, No. 6, 1986 771
Electrochemical Properties of Lead Ethyl Xanthate Studies have been made on the interaction of xanthate with a lead ele~trode.~ The potential of the lead/xanthate couple was determined (Eo = -0.609 V) and the solubility product of lead xanthate derived (K, = 4 X lo-"). The standard potentials of reactions 1,2, and 3, calculated from the solubility of lead xanthate, free energy data for Pb2+,Pb(OH)2,and HPb02-,6 and the redox potential for the oxidation of ethyl xanthate ions to dixanthogen (-0.057 V5), are 1.255, 0.427, and 0.830 V, respectively. The oxidation of lead xanthate to dixanthogen on a lead electrode at pH 9.25required an overpotential of more than 0.7 V above the reversible values for reactions 1 or 3. This could indicate that these reactions are highly irreversible. However, there are alternative explanations. For example, a potential drop could exist across a barrier layer of lead xanthate present on the lead electrode so that the measured potential does not represent the driving force for electrode reactions. This could also explain why the oxidation of xanthate ions in solution to dixanthogen was not evident on the voltammograms for a lead electrode in xanthate solution^.^ These considerations would suggest that the electrochemical reactions of lead xanthate would best be studied a t inert electrodes. Carbon paste electrodes have been applied by a number of workers to the quantitative and qualitative analysis of solid material^.^ We have found this electrode to be inappropriate for the investigation of lead xanthate; irreproducible and poorly defined currents were observed probably because lead xanthate and its oxidation product, dixanthogen, are soluble in the organic phase used to bind the carbon particles in the electrode. An alternative approach is to fix the solid to a noble metal substrate. In this paper we describe a linear potential sweep voltammetric examination of the reactions of lead ethyl xanthate deposited on a gold substrate.
Experimental Section Lead ethyl xanthate was precipitated from aqueous solution by the addition of potassium ethyl xanthate to lead nitrate. It was purified by recrystallization from solution in acetone. The electrode was prepared by spot welding gold foil, 11 X 12 mm in size, onto a platinum wire sealed into glass tubing. Lead xanthate was deposited on the gold surface by dipping the electrode in an acetone solution containing 1.5 mg cm-3 of this compound and evaporating the organic solvent. Electrolyte solutions were buffers of the following composition: (i) pH 5,0.059 M CH3COOH,0.1 M CH3COONa,(ii) pH 6.8,0.194 M H2B407,0.0015 M Na2B407,0.049 M Na2S04;(iii) pH 9.2,0.05 M Na2B,0,; (iv) pH 10.2, 0.018 M Na2B407,0.028 M NaOH; (v) pH 11.2, 0.017 M Na2B407,0.034 M NaOH. The electrode potential was controlled with a potentiostat designed and constructed in the CSIRO laboratories. It was programmed with a Utah Electronic Model 0151 sweep generator. Current-potential curves were recorded on a Yew Type 3086 X-Y recorder. Potentials were measured against a saturated calomel electrode (SCE) and converted to the standard hydrogen electrode (SHE) scale by assuming the SCE has a potential of 0.245 Vs on this scale. All potentials are reported against the SHE.
Results and Discussion Voltammograms for potential sweeps taken from the rest potential in the positive and in the negative directions are presented in Figure 1. At pH 5.0 and 6.8, an anodic peak is observed a t -0.4 V which can be assigned to reaction ~~
( 5 ) Woods, R. Aust. J. Chem. 1972, 25, 2329. (6) Pourbaix, M. Atlas D'Equilibres Electrochimiques; GauthierVillars: Paris, 1963. (7) Brainina, Kh. Z.; Vydrevich, M. B. J.Electroanal. Chem. 1981,121, 1.
( 8 ) Bates, R. G. Determination of pX; Wiley: New York, 1964; pp 458-483.
-06 -04-02 0 02 04 Potential ( V vs SHE)
Figure 1. Voltammograms for lead ethyl xanthate deposited on
gold. Linear potential sweeps at 1 mV s-l taken from the rest potential in the (-) positive and (- - -) negative direction.
5 % + 'B
2ot 0
0 m
E
-IO
E 5
0
-
-
, ' ,
i
0 0 2 04 Potentiol ( V vs SHE)
-04 - 0 2
Figure 2. Voltammograms for lead ethyl xanthate deposited on gold. Linear potential sweeps at 1 mV s-'. Solution of pH 5.0 containing (-) 0, (- - -) 1.4 X M, and (.-) 2.6 X M Pb2+.
2. This conclusion is substantiated by investigations of the influence of adding lead ions to the solution (Figure 2). The anodic wave shifts -30 mV to more positive potentials with each 10-fold increase in lead ion concentration. The potentials a t which the anodic waves commence in systems with defined lead ion concentrations are close to the reversible values for reaction 2, which suggests that these oxidation reactions are relatively fast. The position of the wave in the absence of added lead ions indicates that the effective lead concentration a t the electrode surface at the onset of observable current for the oxidation reaction is about 3 X M in the absence of stirring. This figure agrees with the value of 2.2 X lo4 M expected from the solubility product of lead ethyl xanthate.5 As the pH is increased into the basic region, Pb2+ is expected to be replaced by Pb(OH), in the oxidation of lead xanthate. The voltammogram recorded for 0.05 M borate (pH 9.2) was found to be unaffected by the addition of lead ions to the solution to ensure saturation with respect to solid species; this confirmed that Pb2+is not the reaction product a t this pH. However, the reaction in 0.05 M borate is more complex than that represented by reaction 3. Oxidation of lead electrodes in this medium results in the deposition of lead borate at a potential -0.15 V below that a t which Pb(OH), is p r o d ~ c e d . It ~ can be
772 Langmuir, Vol. 2, No. 6, 1986
Hamilton and Woods
seen from Figure 1 that the anodic oxidation of lead xanthate a t pH 9.2 commences a t a potential below the reversible value for reaction 3 (0.286 V), indicating that lead borate is a product in this borate solution. As in acid solutions, the oxidation of lead xanthate is relatively rapid. The reduction of lead xanthate gives rise to clearly def i e d peaks on negative-going scans from the rest potential (Figure 1). Pb(C2H50CS2)2+ 2e
-
Pb
+ 2C2H,0CS2-
(4)
At pH 6.8, 9.2, and 10.2, the cathodic current commences a t -0.38 V which is 0.09 V more negative than the reversible potential calculated by using the xanthate concentration from the solubility of lead xanthate. This could arise from the presence of some dixanthogen in the lead xanthate which will be reduced to xanthate before reaction 4 commences. The background current in the potential region positive to the reduction peak arising from reaction 4 could be due to this process. The formation of xanthate from dixanthogen will result in a shift in the reversible potential for reaction 4 t o more negative values than that for a saturated solution of lead xanthate. When the negative-going scans are reversed, anodic peaks appear on the subsequent positive-going scans due to the reverse of reaction 4 and to oxidation of lead. The latter contribution arises because some xanthate ions will have diffused away from the electrode in the time taken to reach potentials where the reverse of reaction 4 can occur. A t p H 5.0, hydrogen evolution occurs in the same POtential region as reaction 4 and obscures the lead xanthate wave (Figure 1). However, lead xanthate is reduced since the anodic oxidation of lead is apparent on the reverse scan. The voltammograms recorded a t pH 11.2 (Figure 1) show different features from those for lower pH values. First, the anodic wave is absent. This is due to hydrolysis of lead xanthate by Pb(C2HSOCS2)2 + 20Hand Pb(C2HSOCS2)2
+ 30H-
-+
-
Pb(OH)2 + 2CzHSOCS2(5)
HPb02-
+ 2C2H50CSz-
(6)
The xanthate ions diffuse away from the surface and hence do not become involved in subsequent electrode reactions. Second, the cathodic wave appears a t lower potentials. This is due to the operative process now being the reduction of Pb(OHI2to lead metal. The voltammogram for pH 10.2 suggests that partial decomposition of lead xanthate had taken place (Figure 1). The xanthate ion concentration in equilibrium with solid lead xanthate a t pH 10.2, calculated from reaction 5, from the known solubility of lead xanthate, and the free energies of formation of OH- and Pb(OH)2,is approximately 2 X M. Thus, significant hydrolysis is to be expected a t this pH. Reversal of the positive-going scans from the rest potential (Figures 1 and 2) resulted in a cathodic peak appearing on the negative-going scan. This is due to reduction of the products of anodic oxidation of lead xanthate formed on the preceding scan. If lead species are retained a t the interface, the process is the reformation of lead xanthate by the reverse of reactions 1-3. Otherwise, dixanthogen is reduced to xanthate ions, (C2HSOCS2)2 + 2e
-
2C2H50CS2-
(7)
The charge associated with the cathodic peaks in Figure (9) Fletcher, S.; Horne, M. D., private communication.
1 was -90% of that for the corresponding anodic peak, indicating that most of the dixanthogen remained a t the electrode surface. The increase in cathodic current a t potentials more negative than the cathodic peak is due to reduction of lead xanthate or, for the conditions in Figure 2 with added lead ions, to plating of metallic lead. Although the oxidation of lead xanthate is a rapid reaction, the reduction of the bulk of the products of oxidation requires a considerable overpotential. This implies that a nonelectrochemical step takes place following reactions 1-3. It is suggested that dixanthogen segregates at the surface into a separate phase. It is known'O that the rate of dissolution of dixanthogen is slow, and the transfer of dixanthogen back to the aqueous phase may be responsible for retarding the reduction. A t high overpotentials, the dixanthogen phase is probably reduced directly at the electrode surface. A gold electrode was coated with dixanthogen by dipping it in an ether solution of this compound and evaporating away the organic solvent. A cathodic peak, a t -0.3 V, was observed on a subsequent voltammogram in pH 9.2 buffer solution. This is the same value as the peak potential for the reduction of the products of oxidation of lead xanthate. This observation suggests that once a dixanthogen phase has formed, the reduction rate of these products is determined by the kinetics of reaction 7 at the gold surface. It is apparent from Figure 2 that the reduction reaction is accelerated to some extent by the presence of lead ions in solution. However, the influence of lead ions is small compared with the overall overpotential. The observation that the oxidation of lead xanthate occurs close to its reversible potential would appear to support the mechanism put forward by Guy and Trahar4 for the inhibition of galena flotation a t high potentials. However, the formation of dixanthogen, which is known to be highly hydrophobic,1°would be expected to enhance rather than depress flotation. The concomitant formation of Pb(OH)2,which is expected to be hydrophilic, might explain the decrease in overall hydrophobicity implied by inhibition of flotation. To test this hypothesis, bubble cling experiments were carried out to monitor the nature of the surface after oxidation of lead xanthate in basic solution. With 0.05 M borate, the oxidation reaction is complicated by the formation of lead borate. Thus, in order to ensure M sodium that reaction 3 was the operative process, a nitrate solution adjusted to a pTgof 9.2 was used for this experiment. Nitrogen bubbles passed through the cell were observed to attach to the electrode when it was coated with lead xanthate. Stepping the potential to values in the region where reaction 3 occurs did not change the hydrophobicity of the surface. The electrode became hydrophilic only when the potential was stepped to values a t which lead xanthate is reduced to lead metal. Thus, it would appear that oxidation of hydrophobic lead xanthate, according to equations of the type (1)-(3), cannot account on its own for the inhibition of galena flotation observed a t high pulp potentials. The potential range over which flotation efficiency drops sharply lies in the region where galena itself is known to oxidize."J2 The products of the surface oxidation of lead sulfide, under basic conditions, have been found to include lead hydroxide, sulfur-rich lead sulfide, and thi0su1fate.l~ (10) Hamilton, I. C.; Woods, R. Aust. J . Chem. 1979, 32, 2171. (11) Richardson, P. E.; Maust, E., Jr. In Flotation, A.M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; AIME: New York, 1976; p 364. (12) Gardner, J. R.; Woods, R. J . Electroanal. Chem. 1979,100,447.
Langmuir 1986,2, 773-776 The lead hydroxide film produced under these conditions is highly likely to be hydrated and will undoubtedly have a much lower density than the underlying mineral. Thus, relatively small percentages of surface reaction can lead to high coverages of hydrophilic lead hydroxide on the galena surface. The fact that collectorless flotation of galena, which is attributed to the hydrophobicity of the sulfur-rich oxidation product, is inhibited at potentials4 where only 10-20% of the surface reaction leads to the (13) Hamilton, I. C.; Woods, R. In Proceedings of t h e International Symposium on Electrochemistry in Mineral and Metal Processing; Richardson, P. E., Srinivasan, S., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1984; p 259.
773
production of the hydrophilic products, Pb(OH)2 and S2032-,12 gives some support to this view. Thus, the inhibition of the flotation of galena a t high potentials with xanthate collectors could result from oxidation of the mineral to form sufficient lead hydroxide to counteract the influence of dixanthogen. Furthermore, dixanthogen will be attached only weakly to the increasingly hydrophilic surface. In these circumstances, the turbulent hydrodynamic conditions in a flotation cell could result in stripping of dixanthogen and hence a diminution of the amount of this hydrophobic entity at the mineral surface. Registry No. Pb(C2H50CS2),,23810-93-7; KCzH,0CS2, 140-89-6;Pb(N03)2,10099-74-8;Pb(OH)Z,19783-14-3;Pb, 743992-1; Au, 7440-57-5;dixanthogen, 502-55-6;galena, 12179-39-4.
EXAFS Studies of Rh/NaY and RhCr/NaY Zeolite Catalysts: Evidence for Direct Bonding between Metal Particles and Anchoring Ions M. S. TZOU, B. K. Teo,t and W. M. H. Sachtlerh Ipatieff Laboratory, Chemistry Department, Northwestern University, Euanston, Illinois 60201 Received M a y 13, 1986. I n Final Form: August 15, 1986 The Rh K-edge EXAFS of Rh/NaY and RhCr/NaY zeolite catalysts, prepared by ion exchange of NaY with Cr(N03)3and Rh(NH3)5C13solutions, was measured. It was found that the Rh-Rh distance in these catalysts is 0.06 A shorter than in rhodium metal. Cr ions are found to interact directly with Rh particles; the Rh-Cr interatomic distance in Rh/CrNaY is 2.50 A. Particle size and coordination number of Rh are decreased by the chromium ions which act as chemical anchors.
Introduction Recently we found' that for Pt in NaY supports higher metal dispersion could be obtained by using transitionmetal ions such as Fe2+ or Cr3+ as "chemical anchors". When heated in hydrogen at 500 "C, the anchored samples also displayed a superior maintenance of the metal dispersion. In the present paper this work is extended to Rh in NaY supports using Cr3+ions as the chemical anchor. T o ascertain whether indeed a short-range interaction exists between Rh and Cr, we have used EXAFS which has been proven by several authors to be able to provide significant information for such i n t e r a ~ t i o n . ~We - ~ refer to published work on supported bimetallic clusters including Rh-Cu, Pt-Ir, Os-Cu, Ir-Rh,2 Rh-Co? and Rh-Fe.4 Also for highly dispersed monometallic catalysts, e.g., Rh/ y-A1203, the interfacial distance between rhodium and oxygen of the support was detected by EXAFS.5 Experimental Section Catalyst Preparation. For this study, two Rh-containing NaY zeolite catalysts were prepared by ion exchange of Linde NaY(LZY-52) and CrNaY, dispersed in water (1g/200 mL), with an aqueous solution containing 30 ppm Rh(NH3)&13(Strem Chemical) at 70 "C for 24 h. The CrNaY was obtained at room temperature by exchange of a 0.01 M Cr(N03)3.9H20solution with NaY (1 g/200 mL of water) which was previously adjusted by * T o whom correspondence should be addressed. +Current address: Department of Chemistry, University of Illinois-Chicago, Chicago, IL 60680.
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diluted HN03to near pH 4.0. As analyzed by atomic absorption and Galbraith Laboratory, Inc., the rhodium content is 4.0 wt YO in both catalysts and the chromium content is 1.43 wt % in Rh/CrNaY. The X-ray diffraction suggests that CrNaY retains a high degree of crystallinity. The Rh-containing zeolite powder for EXAFS measurements was placed in a specially designed aluminum-glass tubing cell containing two Kapton-film (500-wmthickness) windows. This allowed treatments at different temperatures under various atmospheres and v a c u ~ m .Both ~ catalysts were heated from room temperature to 360 or 550 "C in flowing 02,with a heating rate of 0.5 "C/min. This calcination was followed by purging with He for 1 h, reducing with H2for 2 h, and purging again with He for 1 h at the same temperature. The zeolite powder was then cooled to room temperature and transferred to the aluminum part of the cell. Finally, the glass arm was sealed off to keep the reduced rhodium in a pure He atmosphere. The X-ray adsorption measurements were performed at room temperature at the Cornel1 High Energy Synchrotron Source (CHESS)on the C-2 EXAFS beam line at Wilson Laboratory at Cornel1 University. Data Analysis. The raw EXAFS data in energy space (ln(lo/4 vs. E ) were reduced to photoelectron wavevector ( k ) space as described in the literature6 with E, = 23.205 eV. The resulting (1)Tzou, M. S.; Jiang, H. J.; Sachtler, W. M. H. Appl. Catal. 1986,20, 231. (2) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. Catal. Reu. Scc. Eng. 1984, 26, 81. (3) Van't Blik, H. F. J.; Koningsberger, D. C.; Prins, R. J. Catal. 1986, 97, 210.
(4) Ichikawa, M.; Fukushima, T.; Yokoyama, T.; Kosugi, N.; Kuroda, H. J . Phys. Chem. 1986,90, 1222. (5) Koningsberger, D. C.; van Zon, J. B. A. D.; van't Blik, H. F. J.; Vissez, G. J.;Prins, R.; Mansour, A. N.; Sayers, D. E.; Short, D. R.; Katzer, J. R. J . Phys. Chem. 1985,89, 4075.
0 1986 American Chemical Society
