Environ. Sci. Technol. 1988, 22, 1499-1505
Electrogenerative Oxidation of Dissolved Sulfur Dioxide with Packed-Bed Anodes John C. Card, Michael J. Foral, and Stanley H. Langer" Chemical Engineering Department, University of Wisconsin, Madison, Wisconsin 53706
H An operating electrogenerative cell to oxidize dissolved
sulfur dioxide at a packed-bed anode was designed and constructed. I t incorporated an oxygen cathode and was used to study generated current from a representative group of catalytic electrodes and electrode systems. Included in the study were a packed bed composed of Teflon-platinum composite electrodes of the gas diffusion type without backing, platinum deposited on a graphite packed bed, a graphite packed bed, and a graphite packed bed in which iodide ion was dissolved in the flowing sulfur dioxidesulfuric acid solution to serve as an electron-transfer mediator. Both the platinum-graphite packed-bed electrode and the iodide-mediated electrode provided currents in the range of 100 mA/cm2. The uncatalyzed graphite electrode gave currents of -15 mA/cm2. It would appear that all of these electrodes might be modified for improved performance in the future. Teflon is not particularly advantageous, and catalytic platinum does not seem to be necessary at the anode for effective operation of electrogenerative cells utilizing dissolved sulfur dioxide. Because of the substantial currents that were generated, variations of the anodes studied here could be incorporated in the processing of waste effluents in the future if economics warrant. The most promising appear to be graphite-supported platinum or perhaps another electrocatalyst or the iodide-mediated-graphite system.
Introduction The possibility of reacting sulfur dioxide with oxygen to form sulfuric acid in the electrogenerative mode has been of growing interest in recent years in connection with environmental concerns about waste effluents (1,2). Reactions at catalytic electrodes can be presented by anode: cathode:
-
E", V (SHE) 2S02 + 4Hz0 2H2S04+ 4HC + 4e 0.16 (1) Oz + 4e + 4H+ 2Hz0 1.229 (2) 2SOz + 0 2 + 2Hz0 2HzS04 1.07 (3)
-
-+
while electrons are carried through an external circuit and protons are transported through a barrier electrolyte. The favorable free energy of the reaction and the utilization of catalytic electrodes for oxgyen reduction and sulfur dioxide oxidation make it possible to carry out the overall electrochemical oxidation of eq 3 to produce sulfuric acid together with the generation of electricity. With the subsequent recognition of the efficacy of electrogenerative reduction of nitric oxide and possibilities for removing it from dilute gaseous feed streams (3-5) in a separate cell, opportunities arise for devising integrated flue gas treatments that involve conversion of sulfur dioxide to sulfuric acid and nitric oxide to reduction products for other applications in electrocatalytic systems that require no external power sources (6). There is the possibility, for instance, of combining the sulfuric acid with ammonia from nitric oxide reduction or other sources to produce ammonium sulfate fertilizer. Most of the earlier work on electrogenerative SO2 oxidation involved vapor-phase sulfur dioxide, often in conjunction with the use of gas diffusion 0013-936X/88/0922-1499$01.50/0
electrodes. While procedures based on direct electrogenerative reaction of SO2-containinggas streams are compatible with some flue gas treatments (2-7), possibilities exist for SO2 removal through contact with various scrubbing systems. With these latter systems it may be possible to use a scrubbing solution containing SO2directly in a packed-bed electrogenerative reactor, to produce sulfuric acid with some regeneration of the sulfuric acid based scrubbing solution (6, 8). Because systems containing dissolved sulfur dioxide also might be applied in processing pyrometallurgical effluents (I) and could have advantages in integrated flue gas treatments (6) including the use of less costly catalysts, we initiated this investigation of several types of packed-bed anodes for electrogenerative oxidation of sulfur dioxide solutions in an operating cell with an efficient gas diffusion type oxygen cathode. This paper presents results for initial comparison from a study of operating electrogenerative oxidation cells involving packed-bed anodes of several designs. The materials investigated here include a packed bed composed of Teflon-platinum composite electrodes of the gas diffusion type without backing, platinum deposited on a graphite packed bed, an untreated graphite packed bed, and one in which iodide ion is added to the sulfur dioxide solution to serve as an electron-transfer mediator. The latter is of special interest because it is illustrative of a possible means for achieving low-cost electrogenerative type systems with minimal use of catalytic precious metals. Several of the materials investigated here show promise for utilization in operative electrogenerative reactors. Previous Work. Sulfur dioxide electrooxidation has been studied in both electrolytic (9-14) and electrogenerative ( I , 2,6,15-19) modes. The references contain more extensive bibliographies. Electrolytic studies include potentiometric (9,12,20,21) and voltammetric (10-14) work on the kinetics and mechanism of the oxidation. The SO2oxidation is a very irreversible process on Pt electrodes (20,21). Voltammetric studies show two oxidation peaks, one at -0.5 V vs SCE and the other at 1.0 V vs SCE. The first peak has been attributed (13)to direct electron-transfer reaction between SOz and the electrode and the second to enhancement of surface oxide formation by the chemical reaction of SO2 and surface oxygen species (10-13):
-
+ HzO 2H+ + PtO + 2e PtO + SO2 + HzO Pt + H2S04 Pt
--*
(4) (5)
Thus, two parallel paths can be proposed. At less anodic potentials (ca. 0.3 V vs SCE) SO2can be oxidized by direct electron transfer (22):
0 1988 American Chemical Society
SOz + * SOz*+
-
-
+ 2H20
HS03*
+ eHS03* + H30+
SOz*+
HS03*+ + e-
HS03*++ 2H20
H2S04+ H30+
Environ. Sci. Technol., Vol. 22, No. 12, 1988
(6)
(7) (8)
(9) 1499
where * represents an adsorption site. At higher anodic potentials ( E > 1.2 V vs SCE) surface oxide species evidently block the surface so that overall SO2oxidation drops off sharply (10, 11, 14, 21). Lu et al. (23,24)described an electrolytic reactor study of gas-phase SO2 oxidation connected with hydrogen production. Few references to cells for SOz oxidation with flow-through anodes have been reported in the literature. Recently, Struck et al. (25)described a three-compartment electrolytic cell to oxidize SO2at the anode and produce hydrogen at the cathode where a third compartment between the anode and cathode flowing electrolytes served to minimize migration of SO2 to the cathode. Another flue gas desulfurization process has been suggested by Kreysa et al. (26). In the central anodic compartment of the cell, graphite particles act as an electrochemical adsorption column while SO2is sparged through and continuously electrochemicallyoxidized. In a second, adjacent anode compartment containing copper particles, SO2is catalytically oxidized by oxygen to produce sulfuric acid and copper sulfate. The cathode reaction taking place in the third compartment is the reduction of copper sulfate to copper metal on a bed of copper particies. Possibilities for electrogenerative sulfur dioxide oxidation to H2S04 were recognized during the modern surge of fuel cell related investigations (15, 16, 19) with gas diffusion electrodes (27-29) providing marked improvements in cell performance. Wiesener et al. (16-18) patented cells using platinum anodes activated by aluminum-vanadium or aluminum-molybdenum spinels. In work from our laboratories ( 1 , 2) the electrogenerative oxidation of SO2 on American Cyanamid LAA-2 platinum black/Teflon composite gas diffusion electrodes provided closed circuit current densities of up to 145 mA/cm2. In a "flow-past" electrogenerative mode (6),current densities of -30 mA/cm2 were achieved. The catalytic effect of iodide ion (I-)on electrochemical SO2 oxidation has drawn attention and an iodine-based catalyzed system has been used as a cell depolarizer in electrolytic studies (31). Schulten and Behr (32)patented an ion exchange membrane divided cell in which sulfur dioxide was electrolytically oxidized to H2S04with iodide catalytic mediation:
I2+ H2S03+ H20
-
H2SO4
+ 2HI
(10)
using an anode of platinum supported on porous graphite. Operation was at 15-60 "C under elevated pressure up to 100 atm. Struck et al. (33) confirmed a catalytic effect of HI on a vitreous carbon electrode in an electrolytic system. The effect of KI on SO2 oxidation on bulk platinum was studied by Matveeva et al. (34) using voltammetric techniques. They concluded that, depending on the ratio of the KI and SO2 concentrations, sulfur dioxide oxidation can occur homogeneously (see below) or heterogeneously where adsorbed iodine acts as an electrocatalyst. Recently, Cho et al. (35) studied the iodide-mediated oxidation of SO2on smooth platinum and graphite electrodes in 0.5 M H2S04using a potentiodynamic method. At low electrode potentials, iodide ions are postulated to oxidize to iodine which reacts homogeneously with sulfur dioxide: 21-
-
I2 + 2e
I2 + SO2 + 2H20
Eo = 0.54 V (SHE)
(11)
H2S04+ 21-
(12)
-
+ 2H+
Generally, iodide ions are regenerated near the anode surface in batch reactors (eq. 12) eliminating need for more iodide ion from the bulk solution. At higher potentials iodate forms and participates in the oxidation. 1500 Envlron. Sci. Technol., Vol. 22, No. 12, 1988
I-
+ 3H20
-
IO3-
+ 6H' + 6e
E" = 1.09 V (SHE) (13) This range is not accessible in electrogenerative cells with oxygen cathodes. Experimental Section Materials. Commercial porous graphite sheet was purchased from the Electrosynthesis Co. (no. SG-13; 50% porosity, 3.2-mm thick). In all electrogenerative reactor performance experiments, the cathode was an American Cyanamid LAA-2 (9 mg of Pt/geometric cm2)gas diffusion electrode. One of the packed-bed anode designs incorporated American Cyanamid AA-1 (9 mg of Ptlgeometric cm2) platinum-Teflon composite electrodes. AA-1 electrodes are 6-7 mils in thickness with an open structure and have been described (27, 36). Type LAA-2 and AA-1 electrodes are similar except that the former are manufactured with a porous, hydrophobic Teflon backing. Both are supported on 50-mesh expanded tantalum screen. Some operational properties of LAA-2 electrodes have been described (37,38). With the porous Teflon backing, the electrode (overall thickness 10 mils) is gas permeable and electrolyte impermeable. Electron microscopy shows this Teflon backing to have a porous structure with pore size of -1 pm. An American Cyanamid graphite LSE gas diffusion electrode was used in conjunction with the AA-1 packed bed. Hydrogen hexachloroplatinate(1V) hydrate crystals (99.995%) for platinum deposition were obtained from the Aldrich Chemical Co. Catalytic Anode Preparation. The platinum-treated graphite sheet electrode was 51 mm X 13 mm. Platinum was deposited on the graphite by impregnation and reduction of aqueous H2PtCl,y4H20.The graphite sheet was saturated with a 50 wt % aqueous solution of H2PtC1,y 4H20 to incipient wetness and allowed to dry; after atmosperic exposure for 12 h at room temperature it was heated in an oven for 45 min at 100 "C. Reduced platinum was then obtained at 250 "C after 3 h in flowing hydrogen. The final platinum loading on the electrode was found to be 17.9 mg of Pt/geometric cm2 of electrode. Cell Design. Teflon-Platinum Composite Electrode in Packed Bed. For this work, a hybrid electrogenerative cell was constructed incorporating a packed-bed anode, with a well-characterized (27, 37, 38) efficient oxygen cathode. Anolyte (3 mm in thickness) and catholyte (4 mm in thickness) chambers were separated by an RAI Research RAIF'ORE R-4010 membrane. Oxygen was supplied to the cathode. This packed-bed anode consisted of five American Cyanamid AA-1 (9 mg/cm2 Pt each) platinum black-Teflon composite gas diffusion type electrodes separated with tantalum 80-mesh screens (Unique Wire Weaving Co., Hillside, NJ) to maintain conductivity and facilitate contact between dissolved reactant and electrocatalyst. This packed bed (3 mm in thickness) was mounted inside a circular electrolyte compartment in a cell of a type described previously (37,38). A porous American Cyanamid LSE graphite electrode for conduction was located at the exterior of the packed bed. This porous, Teflon-backed electrode provided for gas-phase permeability while being liquid-phase impermeable. An inert nitrogen gas stream was passed over this exterior electrode during the experiment. A &-mesh platinum current collector was positioned between the graphite anode gas diffusion electrode and the electrocatalytic packed bed. Cell Design for Packed-Bed Operation. For other test work, an altered geometry was used in a rectangular cell with electrodes of matching rectangular design (see Figure 1). The cell design incorporated a catalytic
. . e. 1.
Flgure 1. Schematic representation of electrogenerafiecell with packed-bedanode. Inlets and ouibts are staggered. Key: (a)anode face plate with provision lor reference electrode (doned line) as required, (b) anolyte chamber, 3.2 mm mick which hc?dsthe pad& bed: (c) catholyte chamber 4 mm thick with static catholyte: (d) cathode face plate 6.4 mm thick wlth provision for oxygen gas flow: (e) rectangular gas diffusion cathode (American Cyanamid pt LAA-2. 51 X 13 mm); (I)RAI 4010 cation-exchange membrane; (g) replaceable graphfiesheet packed-bed anode (51 X 13 X 3.2 mm): (s) dissolved
S02-aqueous H2S0. anolyte feed. Phtinum screen current collectors used at the front of each electrode are not shown. Teflon gasket spacer (5 mils) separates anode from membrane. Cell is constructed of stress-relieved polypropylene.
H
Flgure 2. Reactant and electrolyte flow system for packed-bed SO2 Oxidation. Key: (a)cell (see Figure 1 for detail): (b) omet for flowing anolyte: (c)eWciyie brfdge fw reference electrode, br@e in antact with packed-bed anode: (d) anolyte chamber containing packed-bed electrode: (e) reservoirs for static catholyte:(1)Teflon needle-valve fw
control of flowing anolyte from upper resewoir: (g) saturated caiomal reference electrode; (h) catholyte chamber.
packed-bed electrode through which flowed sulfur dioxide saturated 3 M aqueous sulfuric acid electrolyte. The inlets and outlets of the two electrolyte chambers and the oxygen gas face plate were designed in a staggered configuration. This made construction of this thin cell possible and facilitated connection to the flow system (see Figure 2) with Swagelok Teflon tube fittings. The anolyte entered at the bottom of the packed-bed electrode through a spreader. The packed-bed anode was 3.2 mm thick, as was the packed-bed chamber. Provision was made for measurement of the anode potential versus a calomel reference electrode containing saturated sodium chloride solution (SSCE) in an arrangement similar to that described by Jashki et al. (39). A thin Teflon gasket spaced the packed bed away from the membrane. The cathode electrolyte chamber was 4 mm in thickness and the catholyte was generally stagnant. The oxygen cathode incorporated an LAA-2 type gas diffusion electrode, which was designed and placed so that 51 X 13 mm of area was exposed opposite the packed bed. The dimensions of the graphite and platinum-treated graphite electrodes follow the recommendations of Risch and Newman (40) and Alkire and Ng ( 4 0 , who compared models for the performance of perpendicular and parallel electrode geometries. In the perpendicular geometry utilized here, the upward electrolyte flow through the packed-bed anode is orthogonal to the current flow. This design features an aspect ratio [length in electrolyte flow
direction (51 mm)/thickness in current flow direction (3.2 mm)] significantly greater than unity (16 for the present case). Procedures. Cell Performance. Electrogenerative cells were operated with the graphite anodes described above as well as the packed-bed anode composed of AA-1 electrodes; all cells u t i l i a well-characterized (37,38,42) LAA-2 oxygen cathode. Platinum screen (45-mesh) current collectors were used for both electrodes. The catholyte was 3 M HzS04(25 w t %) and the anolyte was sulfur dioxide saturated 3 M HzS04. The saturated electrolyte was passed a t 0.78-0.97 mL/min through the anode compartment while cell polarization (current and potential) data were obtained (see figures). Performance or polarization curves were performed by a standard procedure (I, 2 , 3 8 ) . A high-impedance voltmeter was connected across the cell in parallel with a series circuit composed of an ammeter and variable resistor. Two polarization experiments were performed for each set of operating conditions, after enough time at open circuit to establish a stable potential, typically l(t15 min. For the first experiment, the external load resistor was changed every 4 min, while 5 min was allowed to establish a stable current and cell potential for the second experiment. At this time, values of cell potential, anode potential versus the reference, and current were noted and the load resistor was changed for the next point. The effeds of platinum deposition could be determined by noting the characteristics of the cell with an untreated graphite anode. The feasibility of iodide-mediated electrogenerative sulfur dioxide oxidation with an untreated 51 X 13 mm graphite sheet placed in the anode compartment was also studied. Polarization curves were obtained with 2 g/L KI present in the sulfur dioxide saturated sulfuric acid anolyte. Internal resistance of the cell was measured before the performance experiments with a Keithley A.C. milliohmmeter where feasible. Prior to this measurement, a constant current supply was connected across the cell while hydrogen gas was flowed past the cathode. The current source (negative terminal to anode) was adjusted to force the cell potential to -0 V. Reduction pretreatment was maintained under these conditions for -1 h. The cell resistance was then measured. The resistance measurement was performed before SOz was introduced at the anode because of a tendency for reduction to a form of sulfur that severely inhibits electrocatalysis. With the cell utilizing AA-1 electrodes, hydrogen was passed over both electrodes before measuring cell resistance (38). Characterization of Active Catalytic Platinum Surface. A cyclic voltammetric technique was employed to measure platinum surface area on the basis of coulometry for the oxidation of adsorbed hydrogen atoms. The graphite sheet electrode was placed in a standard threeelectrode cell filled with ultrapure 0.5 M H,S04 electrolyte (prepared from Ultrex concentrated HzS04 and triply distilled water). A platinum spiral and a standard calomel electrode (SCE) filled with saturated NaCl were used as the counterelectrode and reference electrode, respectively. A standard voltammetry experimental array was assembled (54) and the electrode cycled a t 0.5 V/min between 0.21 and -0.25 V vs SCE. The voltammetric sweep with hydrogen adsorption/desorption currents is shown in Figure 5. Anodic currents were integrated to provide a measure of the effective platinum surface area (55). The electrode was supported in the cell with a length of 0.2mm-diameter Pt wire which, since it was wrapped tightly around the graphite electrode, also served as the current Environ. Sci. Technol., Vol. 22.
No. 12. 1988 1501
l-d-2-A 20 eo
0
loo
I (mA/cm2)
Flgure 3. Performance curves, cell polarlzation for electrogeneratlve sulfur dloxide oxidation with oxygen cathode. Curve A: gas-phase operation at 21 'C with LAA-25 anode; SOp flow rate, 53 mL/min; 3 M H2S0, anoiyte flow rate, 0.6 mL/min; R,,, = 0.27 0. Curve B: liquid-phase operation at 25 'C with AA-1 packed-bed anode (see Experimental Section); flow rate of SO,-saturated 3 M H$04 anolyte through packed bed is 0.94 mL/min; RI, = 0.32 Q;3 M H2S04catholyte. Both curves are corrected for IR losses. (Electrode area is 5.1 cm2.)
feeder. Control experiments showed that there was negligible hydrogen adsorption on this Pt wire or on the untreated graphite sheet. The significant peak separation between corresponding reversible hydrogen deposition and oxidation peaks in Figure 5 arises from uncompensated IR losses in the voltammetric cell (43). Such effects are quite pronounced in this experiment because of the relatively high currents generated. Details of the voltammetric procedure are presented elsewhere (5). Surface Area Measurement (Nitrogen Adsorption). Physical surface areas of the electrodes were obtained by a BET method based on nitrogen adsorption under flow conditions. The BET technique is well established and described in detail in the literature (44-46). For this work, a single-point BET method was used with a flow type surface area analyzer (MicromeriticsFlowsorb I1 2300). A mixture of 30% nitrogen in helium at atmospheric pressure flowed over the graphite or platinum-graphite sample in a U-tube sample holder. Nitrogen was adsorbed onto the surface at liquid nitrogen temperature, by immersion of the sample tube in liquid nitrogen. After adsorption, rapid heating of the sample holder resulted in nitrogen desorption in a sharp peak. Matched thermal conductivity detectors located in the gas line before and after the sample tube allowed adsorption and desorption peaks to be monitored and plotted. Integration of the desorption peak was used as a basis for determining the surface area of the sample. Integration was internally calibrated by the instrument. For the porous graphite sheet, the surface area was found to be 0.26 m2/g, or 1443 cm2true/cm2 geometric for the 3.2-mm-thick piece (51 X 13 mm). For the platinum deposited on porous graphite sheet (same size electrode), the total surface area was 0.4 m2/g, or 1600 cm2true/cm2 geometric. Results and Discussion
Since gas diffusion Teflon-platinum fuel cell type electrodes have been effective in many electrogenerative systems (37, 38), it was desirable to test them with dissolved substrate systems. A packed-bed anode composed of representative American Cyanamid AA-1 electrodes, which do not have Teflon backing, was tested for oxidation of dissolved SOz in aqueous sulfuric acid in a cell configuration resembling earlier ones (38,47). Cell performance results are presented in Figure 3 (curve B). Results of a comparable high-performance gas-phase SO2 oxidation using Teflon-backed electrodes are also shown in Figure 3 (curve A) for comparison. For ease of comparison, 1502 Environ. Sci. Technol., Vol. 22, No. 12, 1988
I (mA/cm2)
Figure 4. Performance curve (A) for electrogenerative sulfur dioxide oxidation with R-graphite packed-bed anode and gas diffusion type oxygen cathode. Anoiyte is SO,-saturated 3 M H2S04flowing at 0.76 mL/min through the packed-bed anode. Cathoiyte is 3 M H2S04. Curve B: cell potentials are corrected for IR losses; Rlnt= 0.37 0. Operating temperature, 28 'C. (Electrode area, 6.45 cm2.)
correction has been made for internal voltage loss (IR correction) in the electrolyte for both cells. Gas-phase operation with American Cyanamid LAA-25 electrodes (25 mg of Pt/geometric cm2)yielded a current density as high as 110 mA/geometric cm2. Although the catalyst loading of the AA-1 packed bed was much higher (45 mg of Pt/ geometric cm2),the observed current density of only -30 mA/geometric cm2 indicates limited effectiveness of the AA-1 electrodes for this dissolved substrate reaction system. A previously reported (5) platinum surface area analysis of the porous Teflon-backed American Cyanamid LAA-2 electrode, which incorporates an electrocatalyst quite similar to the AA-1 electrodes employed here, yielded a platinum surface area of 1500 cm2true/cm2 geometric. At closed circuit, an effectiveturnover frequency of -0.01 s-l is calculated for the platinum electrocatalyst of the packed bed of AA-1 electrodes. It would appear then that the hydrophobic treatment so effective for vapor-phase oxidation is not only not necessary for reaction of the dissolved substrate in the electrolyte, but may well be detrimental. With these results and earlier experience, further pursuit of the utilization of the AA-1 electrodes for dissolved SO2 oxidation did not seem worthwhile. To improve the efficacy of testing and operation of other candidate materials as packed-bed anodes, the cell configuration of Figure 1 was devised. Provision was made for incorporating a variety of electrodes and systems for testing the soluble substrate oxidation. It more nearly conforms to a geometry suited for packed-bed operation with soluble substrates in that the electrodes used here are of a unique one piece rectangular integral design. Based on other analyses (40,41),provisions are made for a rectangular electrode of dimensions that appear to be suitable for obtaining some operating data. Performance or polarization data for a cell employing a graphite-supported platinum packed-bed catalytic anode in the test cell with an LAA-2 gas diffusion oxygen cathode are shown in Figure 4. Both IR corrected and uncorrected performance curves are shown in this figure for later comparisons. It might be remembered that there is some control of this voltage loss through adjustment of cell design and parameters. The anode was fabricated after several experiments and was devised for high performance for dissolved SOz oxidation as well as for other dissolved substrate oxidations. As can be seen from Figure 4, current densities of up to 80 mA/geometric cm2were obtained at room temperature with a platinum loading of 17.9 mg/geometric cm2. With lower external resistance during performance testing it might have been driven to even higher current densities. For this study, however, it was decided not to use the electrode at higher anode potentials (where it might be oxidized) so that it might be used for other comparison experiments with other substrates. Thus, polarization
40c
20
I
: A
-20
-40
I (mA/cm2)
u 0.2 0.0 -0.2
E (V vs. SCE) Flgure 5. Hydrogen adsorption and desorption currents on Pt-graphite electrode using cyclic voltammetry. Scan rate is 0.5 V/min and electrolyte is ultrapure 0.5 M H2S04. Electrode had geometric area of 6.45 cm2 and was not pretreated prior to scans. (See Experimental Section for details.)
toward higher potentials (see Figure 7 later) was limited. For purposes here, it was important to demonstrate operation of packed beds in a current and potential range of interest and to show that performances of 100 mA/cm2 can be approached and is quite feasible. A number of parameters including loading remain for future study. Other precious metals also may be useful as electrocatalytic materials under special operating conditions. For instance, we have obtained currents -40% lower than with platinum using palladium gas diffusion electrodes. To work toward maximum performance, however, platinum is emphasized here. Another reason is because the presence of nitric oxide (from flue gases) was found to cause dissolution of palladium electrocatalysts into sulfuric acid electrolytes. The platinum-graphite type of electrode presents some problems in characterization. However, an electrochemical method of slow-sweep voltammetry was adapted. Its use for electrocatalyst characterization is described elsewhere (5). It is based on coulometric measurement in the hydrogen adsorption region (-0.2 to 0.2 V vs SCE) where surface hydrogen is oxidized or produced. The results obtained for these voltammetric hydrogen adsorption/ desorption experiments are shown in Figure 5. They provide a means of estimating reliably the active platinum surface area of the platinum-catalyzed graphite sheet electrode. The integration of either the positive (desorption) or negative (adsorption) sweep currents provides a measure of hydrogen adsorbed on the platinum surface. By using the total hydrogen desorption charge of 0.665 C, a true Pt surface area of 3170 cm2 [assuming a standard value of 210 &/true cm2 (55)]can be calculated. Assuming a 1:l hydrogen:Pt surface site ratio, which is customary, a Pt site density of 6.44 X lo" surface Pt sites/geometric cm2is obtained. Combination of this result with the known weight of the deposited platinum gives a calculated platinum dispersion of 0.2%. Thus at closed circuit with this electrode, a turnover frequency of 0.39 s-l is calculated assuming all SO2oxidation reactions are due to platinum (this ignores any graphite catalysis). Conversion of SO2 to H2SO4 occurs in this reactor (of 6.45 cm2apparent area) mol/min. The SO2 concentration at a rate of 1.6 X in the SO2-saturated 3 M H2SO4 anolyte is estimated to be 1.1M (f15%) based on the data of Cupr (48) and the estimation method of Millett (49).This would indicate that the single-pass conversion of SO2to HzS04is -20% under the specified operating conditions. While the performance of these electrodes will undoubtedly be improved
Figure 6. Performance curves for electroaenerative sulfur dioxlde oxidation with a graphite packed-bed anode 6 d a gas diffusion oxygen cathode. Curve A with added KI; anolyte flow rate, 0.97 mL/mln. Curve B without added KI; anolyte flow rate, 0.99 mL/min. Curve A has 2 g/L K I added to the S02-saturated3 M HzS04anolyte. Catholyte is 3 M HzSO,. Cell voltages are the observed values at Indicated current densities. Operating temperature, 26 O C .
in the future, the results above are quite encouraging. Catalytic type carbons alone might be candidates for oxidation of sulfur dioxide in a variety of schemes, since many have been shown to have electrocatalytic properties (for instance, see ref 50). Because of this and for comparison with the above, the untreated graphite sheet used as a support was tested alone as a packed-bed electrode opposite the LAA-2 oxygen gas diffusion cathode in the cell of Figure 1. Performance without IR correction is shown as curve B in Figure 6. Because of limited reversibility of hydrogen oxidation/reduction on graphite, the procedure normally employed for measuring cell resistance is not feasible. Though an estimate could be made on the basis of the cell of Figure 4, these corrections were not used in favor of examining actual cell performance. As can be noted from Figure 6, graphite alone does exhibit small but significant catalytic effects for SO2oxidation giving current densities of up to 15 mA/cm2 (operation with the platinum anode current collector alone produced negligible current, -2 mA/cm2). However, in view of other high-performance operations described here, graphite use alone was not pursued further. In many discussions of the economics of utilization of electrogenerative methods, reservations regarding the use of expensive precious metal catalysts have arisen. Among alternate catalytic schemes appearing to be worthy of consideration (50) are mediated systems. Because of the use of mediator iodide ion in conjunction with acid manufacture and other applications with electrolytic schemes (see literature review and key references there), it appeared to be worthy of consideration (33, 35). For this investigation the graphite packed bed of curve B in Figure 6 was examined again with an iodide mediator in the test cell at the anode and with the LAA-2 oxygen cathode. For exploratory purposes, 0.2% potassium iodide was added to the flowing anolyte. The performance for this operating cell is presented as curve A of Figure 6. While the pure graphite support is only moderately active toward direct SOp oxidation (curve B), the addition of 2 g/L (12 mM) potassium iodide to the electrolyte as an electron-transfer mediator results in currents of up to 100 mA/cm2. In terms of generated current, the performance of the pure graphite sheet now compares favorably with the platinum-containing anode. While cell resistance again can be estimated so that current-resistance corrections can be applied, actual operating cell results as shown in Figure 6 still seem most appropriate for assessing possibilities. In Figure 7, the polarization of the graphite packed-bed anode with potassium iodide added to the anolyte (curve A) and the platinum-graphite packed-bed anode (curve B) are presented. Anode polarization data shown in Figure Environ. Sci. Technol., Vol. 22, NO. 12, 1988
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800
400
at elevated temperatures could then become a consideration. However, concentrating SOz by other means might not require higher operating temperatures (52). The problem of iodide separation and recycling when utilized in catalytic mediation appears to be surmountable since subsequent separation seems quite feasible. Some possibilities include distillation (32) or precipitation (53) of special compounds. Summary
Figure 7. Measured polarization (vs RHE) of two forms of packed-bed anode as a function of cell current. Correction of 0.2 V applied to measured value for calomel electrode. Curve A: graphite sheet packed-bed anode in SO2-saturated 3 M H2SO4 anolyte containing 2 g/L K I . Curve B: Pt-graphite sheet packed-bed anode in SO,-saturated 3 M H,SO,. Cathode is Pt LAA-2 electrode to which pure O2is fed. Conditions presented in Figures 4 and 6.
7 were measured against the calomel reference electrode (SSCE) corrected to a reversible hydrogen electrode (RHE) based on the measured SSCE vs RHE potential difference of 0.2 V. At open circuit, both anodes exhibit potentials around 0.36 V vs RHE, which is probably established by reaction of adsorbed sulfur dioxide. As current is drawn from the cell, however, both types of anode polarize rapidly, although the graphite-iodide anode system (curve A) polarizes more than the platinum-graphite anode (curve B). Even at low current densities, the graphite-iodide anode system exhibits a potential near the iodide oxidation (see eq 11)potential of 0.54 V vs SHE. This suggests that iodide ion may indeed be the electroactive species in this electrode system. Iodide would be electrooxidized to iodine which subsequently reacts chemically with dissolved SO2 (an electrochemical followed by a chemical step or EC mechanism), as represented by eq 11 and 1 2 (35). Tafel plots over a limited range were prepared by using the first few (high-potential) points of Figures 4 and 6, correcting for the oxygen electrode polarization. Tafel slopes of 36 mV/decade were found for both the graphite-supported platinum and the pure graphite electrodes. The slopes of 36 mV suggest a complex reaction mechanism on platinum and graphite surfaces. It is difficult to interpret these at present. However, these data can be compared to the slopes of 44 mV determined by other workers for smooth bulk platinum (13, 30). The Tafel slope for the graphite sheet anode with potassium iodide added to the anolyte is 30 mV. The Tafel slope for iodide oxidation on platinum, where iodide adsorption with electrooxidation is rate controlling, is 59.1 mV/an. However, the value of a is 0.78 for iodide oxidation on platinum (51). This would give a slope of 38 mV, or perhaps lower with a high a value, close to what might be expected for a situation where a 2-electron oxidation of iodide to iodine would be rate limiting, such as at high cell potentials (low overpotentials). This is consistent with the mechanism of eq 11 and 12. The results above show that packed-bed anodes can be successfully used to convert dissolved sulfur dioxide to sulfuric acid in the electrogenerative mode, even at room temperature. The high geometric current densities (80-100 mA/cm2) reported in these preliminary studies bode well for refinement and development of even more efficient catalysts. The iodide-mediated graphite system is exceptionally promising. In addition to high overall activity, it could eliminate use of platinum or other precious metals. Though iodide itself can be relatively expensive for large-scale use, it may be more effective at lower concentrations (below 2 g/L) and higher temperatures. Of course, the decreased solubility of SOz in sulfuric acid electrolyte 1504
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A continuous-flow-throughcell has been devised to study operation of dissolved substrate electrogenerativesystems. A variety of catalytic electrodes and schemes have been investigated to bring about electrogenerative oxidation of dissolved sulfur dioxide to sulfuric acid for possible use in effluent treatment processes. The iodide-mediated graphite system with high overall activity is particularly interesting, since the need for expensive noble metals is reduced. Teflon-platinum composite electrodes do not appear advantageous. Other systems examined here present possibilities for improvement. The results suggest that incorporation of packed-bed electrodes for operations with other electrogenerative oxidation systems may be feasible. Acknowledgments We thank Steve Lyke for the surface area measurements and other experimental support as well as Liliana Concari and Milton Stewart who participated in these experiments. Electron microscopic data were obtained through Richard Noll. Registry NO.SOZ, 7446-09-5; H$04,7664-93-9;I-, 20461-54-5; Pt, 7440-06-4; graphite, 7782-42-5; Teflon, 9002-84-0.
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Received for review October 13,1987. Accepted June 14,1988. We thank the National Science Foundation and the University of Wisconsin for support of this work.
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