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Catalytic Desulfurization of Carbon Black on a Platinum Oxide Electrode Karen E. Swider† and Debra R. Rolison* Code 6170, Surface Chemistry Branch, Naval Research Laboratory, Washington, D.C. 20375 Received March 31, 1998. In Final Form: February 16, 1999 Organically bound sulfur is a common impurity in carbon blacks, such as vulcanized carbon and coal. Whereas this organosulfur is usually removed by heating to high temperatures, we describe a low-temperature method to desulfurize solid carbon via oxidation (ultimately to SO42-) on a Pt catalyst in oxygenated aqueous media. The reaction is monitored on Vulcan carbon, which is the favored support for the Pt catalysts used in fuel-cell electrodes but which has a high concentration (nominally 5000 ppm) of organosulfur. Vulcan carbon is electrodesulfurized by dispersing it in a heated, dilute HNO3 electrolyte where it can physically contact a Pt mesh anode that is maintained as Pt oxide. After 40-180 min of electrolysis in oxygenated solution at +1.0 V versus Pd/H, sulfate is present in the electrolyte (as measured by precipitation of BaSO4). X-ray photoelectron spectroscopy of the dried, electrodesulfurized carbon shows that ∼30% of the near-surface organosulfur is removed. The remaining sulfur presumably does not react because it does not physically contact the hydrous platinum oxide (PtOxHy) at the electrode surface. Minimal desulfurization occurs at 0.2 V versus Pd/H, when the Pt mesh surface is composed of Pt0. These electrochemical results show that oxidized Pt is the catalyst necessary for oxidative desulfurization. In deoxygenated solution, electrodesulfurization still occurs for electrolyses at +1.0 V versus Pd/H, but SO42is not generated, indicating that SO2 is the partial oxidation product. The surface area and total oxygen content of the carbon are unaffected by the electrodesulfurization process. This simple, low-temperature method for desulfurization should be applicable to other forms of solid carbon containing organosulfur moieties.
Introduction Carbon is an important material for power generation,1,2 whether it is burned as fuel or used as a conductive solid to support the electrocatalysts that generate clean power (e.g., in proton-exchange membrane or phosphoric acid fuel cells).3,4 Supporting high-surface-area catalysts on carbon decreases the cost of fuel-cell electrodes without decreasing the active surface area of the electrocatalyst.5 Vulcan carbon is the preferred support for fuel-cell electrocatalysts because of its high electronic conductivity and favorable electrochemical properties.6 When carbon is used as a power source (i.e., a fuel) or as a catalyst support in power sources, its sulfur content must be considered. When coal is burned, the inherent organosulfur becomes SOx, which is a respiratory irritant and a precursor to acid rain.7 Sulfur is also introduced to carbon, usually via vulcanization, to alter the bulk and surface properties of the carbon.1,2 The surface organosulfur of carbon can affect the properties of supported fuel-cell electrocatalysts,8-12 and evidence is mounting that heteroatoms act as the anchor to support electrocatalyst nanoclusters on the carbon surface.9-13 Under certain * To whom correspondence should be addressed. E-mail: rolison@ nrl.navy.mil. † ASEE-ONR Postdoctoral Fellow, 1993-1996. (1) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (2) Carbon Black, 2nd ed.; Donnet, J. B., Bansal, R. C., Wang, M. J., Eds.; Marcel Dekker: New York, 1993. (3) Appleby, A. J.; Foulkes, F. R. Fuel Cell Handbook; Van Nostrand Reinhold: New York, 1989. (4) Dhar, H. P. J. Electroanal. Chem. 1993, 357, 237. (5) Kordesch, K.; Simader, G. Fuel Cells and their Application; VCH: Weinheim, 1996. (6) McBreen, J.; Olender, H.; Srinivasan, S.; Kordesch, K. V. J. Appl. Electrochem. 1981, 11, 787. (7) Encyclopedia of Chemistry, 4th ed.; Considine, D. M., Considine, G. D., Eds.; Van Nostrand Reinhold: New York, 1984; p 738.
conditions, the organosulfur in Vulcan carbon can poison the activity of Pt electrocatalysts.9,10 Whereas sulfur is removed from liquid feedstocks of carbonaceous fuels via catalytic hydrodesulfurization processes,8,14 it is usually removed from solid carbon via thermal desulfurization. The sulfur content of solid carbon is decreased by heating at temperatures between 500 and 1450 °C under inert atmospheres to form H2S and CS2.1,15,16 Sulfur can also be oxidatively removed from solid carbon via dispersion of a carbon slurry in 1 M NaOH under an applied potential in an electrochemical cell.17,18 In our previous research on the surface chemistry of fuel-cell catalysts supported on vulcanized carbon, we observed that Pt clusters can oxidize organosulfur to sulfate when the appropriate conditions are present (i.e., water, heat, and contact between the organosulfur and Pt).9 This adventitious, Pt-instigated desulfurization of its nearby carbon environment allows the electrocatalyst to operate in the fuel cell (at least initially) in the absence of an organosulfur poison. We now establish the Pt (8) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. The Chemistry of Catalytic Processes; McGraw-Hill: New York, 1978; p 390. (9) Swider, K. E.; Rolison, D. R. J. Electrochem. Soc. 1996, 143, 813. (10) Swider, K. E.; Rolison, D. R. Manuscript in preparation. (11) Roy, S. C.; Christensen, P. A.; Hamnett, A.; Thomas, K. M.; Trapp, V. J. Electrochem. Soc. 1996, 143, 3073. (12) Roy, S. C.; Harding, A. W.; Russell, A. E.; Thomas, K. M. J. Electrochem. Soc. 1997, 144, 2323. (13) Ye, S.; Vijh, A. K.; Dao, L. H. J. Electrochem. Soc. 1997, 144, 90. (14) Prins, R.; DeBeer, V. H. J.; Somorjai, G. A. Catal. Rev.sSci. Eng. 1989, 31, 1. (15) Krishnankutty, N.; Vannice, M. A. Chem. Mater. 1995, 7, 754. (16) Labib, M. E.; Thomas, J. H.; Embert, D. D. Carbon 1984, 22, 445. (17) Lalvani, S. B.; Ramaswami, K. J. Energy Resour. Technol. 1988, 110, 269. (18) (a) Wapner, P. G.; Lalvani, S. B.; Awad, G. Fuel Process. Technol. 1988, 18, 25. (b) Lalvani, S. B.; Weston, A. Fuel Process. Technol. 1989, 21, 117.
10.1021/la9803530 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/23/1999
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oxidation state that is responsible for the catalytic oxidation of organosulfur and use this knowledge to develop a method by which solid carbon can be catalytically desulfurized (or electrodesulfurized) under mild conditions. The oxidation state of the Pt is controlled electrochemically in a dilute aqueous acid electrolyte in which vulcanized carbon is maintained in a slurry where it is free to physically contact a high-surface-area Pt working electrode. The amount of sulfur oxidized out of the slurried carbon is determined as a function of Pt oxidation state, reaction time, and the gas used to sparge the electrolyte. Any organosulfur oxidized to sulfate is detected in the aqueous electrolyte using a Ba2+ precipitation test. The concentration of sulfur in the carbon and its oxidation state are monitored before and after electrodesulfurization using X-ray photoelectron spectroscopy (XPS). From the results of this laboratory-scale method, a future industrial process may be developed. Experimental Procedures Electrodesulfurization Procedure. Four ports of an electrochemical cell were fitted with a 9-mm2 (geometric area) Ptmesh working electrode, a Pt-mesh auxiliary electrode, a Pd/H reference electrode, and a water-cooled condenser. The fifth port of the cell was fitted with a fritted glass tube so that the electrolyte could be sparged with either air or argon. The cell was filled with a suspension of 0.5 g of Vulcan Carbon XC-72 (Cabot) in 60 mL of 0.05 N HNO3 (prepared from high-purity HNO3 (Baker Ultrex II) in 18 MΩ‚cm H2O (Barnstead Nanopure)). The acidified carbon suspension was heated to ∼95 °C19 and stirred while potentials from 0 to 1.3 V versus Pd/H were applied to the Pt working electrode using a potentiostat (EG&G PAR model 173) and a waveform sweep programmer (EG&G PAR model 175). After 1 h at 95 °C, the potential of the Pt-oxide stripping peak shifted -0.1 V, which is a consequence of the PtOxHy surface restructuring with time as the oxide layer grows.20 The oxide stripping peak shifted an additional -0.1 V to -0.2 V during 3-h electrolyses, presumably due to changes to both the PtOxHy surface and the Pd/H reference electrode. A freshly hydrided Pd/H reference was prepared for each electrodesulfurization experiment. At the completion of the electrodesulfurization process, the electrolyte was gravity filtered through porous paper and then pressure filtered through a 0.45-µm membrane (Acrodisk) to remove extraneous carbon. Several milligrams of Ba(OAc)2 were dissolved in this final filtrate to test for the presence of sulfate ions. While a clear solution resulted from Ba(OAc)2 dissolved in filtered 18 MΩ‚cm H2O, a visibly cloudy solution was observed in H2O having g8 ppm SO42- (i.e., g0.4 mg of SO42- (H2SO4: Ultrex, Baker Analyzed) in 50 mL of H2O) due to precipitation of water-insoluble BaSO4. Quantitative analysis of sulfate in the electrolyte was attempted using capillary electrophoresis but was not informative due to interference from the comparatively large concentration of nitrate ions in the electrolyte. Bulk and Surface Analyses. The carbon retained on the porous filter paper was air-dried for at least 24 h before being pressed into 99.99% In foil (Johnson Matthey) and analyzed by XPS (Fisons 220iXL, monochromatic Al KR X-rays, spot size ) 250 × 1000 µm). The energies of the photoelectron lines were referenced to the C 1s line of the Vulcan carbon, which is defined as graphitic carbon (C 1s ) 284.2 eV).9 The near-surface concentration of sulfur in the Vulcan carbon was monitored via measurement of the S 2p3/2 line at 163.5 eV (signal average of 40 scans/4 min). Analysis software (Spectrum Processing for Eclipse v2.1 Rev06) was used to determine the relative areas of the C 1s and S 2p3/2 elemental lines via a χ2-minimization routine (19) Electrodesulfurization at 25 °C was also attempted and desulfurization did occur by the criteria described below, but the kinetics of the process for the given times of electrolysis were improved at the higher temperature. (20) Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A. J. Electroanal. Chem. 1973, 43, 9.
Figure 1. Cyclic voltammetry of Pt-mesh working electrode in 50 mM HNO3 at 95 °C. with correction for cross-sectional differences and instrumental errors. A Shirley background was assigned to fit the C 1s and S 2p envelopes unless a visibly inappropriate assignment was made by the program, in which case a linear background was used. Although the entire C 1s and S 2p envelopes were deconvolved (with the binding energy difference between S 2p1/2 and S 2p3/2 constrained at 1.18 eV), the only quantitative information used was the concentration of sulfur as calculated from normalizing the weighted area of S 2p3/2 to that of graphitic C 1s. Final concentrations are determined from the average of measurements carried out in duplicate over at least two areas of each sample. The total sulfur content of the as-received and several desulfurized Vulcan carbon samples was determined by combustion analysis (Quantitative Technologies, Inc.). The surface area of the dried carbon was measured under 30% N2/He using single-point BET (Quantachrome Monosorb). Surface water was first desorbed by heating the carbon to 120 °C under the N2/He flow.
Results and Discussion The cyclic voltammetry (CV) of the Pt-mesh working electrode versus Pd/H in 0.05 M HNO3 at ∼95 °C is shown in Figure 1. The A region on the curve (between 0.2 and 0.5 V) is referred to as the double-layer region of the CV curve; this region is attenuated at 95 °C relative to room temperature. Region B to the right (or to more positive potentials) of this capacitive response is the oxide region of the Pt, where the surface/near-surface of the electrode is PtOxHy with 0 < x 1.2 V(Pd/H), region C, the anodic current increases, indicating O2 evolution. Oxide stripping peaks are observed in region D (ca. 0.5 V); H2 evolution occurs at potentials < 0.1 V (region E). The oxidation state of the Pt surface is easily controlled as a function of applied potential. Our XPS analysis of as-received Vulcan carbon indicates a near-surface sulfur concentration of 0.33 ( 0.02 atomic % (a/o) or ∼0.88 wt %.22 Figure 2a shows the S 2p region (21) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L., Jr. Electrochemistry for Chemists, 2nd ed.; Wiley-Interscience: New York, 1995; p 217. (22) The XPS-derived concentration of S reported in this paper is within error of that reported in our previous work (0.36 ( 0.02a/o);9 those data were derived on a different XPS instrument.
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Figure 3. Atom percent of sulfur in Vulcan carbon (as determined by XPS) as a function of the applied potential of the Pt-mesh electrode in 0.05 M HNO3. All electrodesulfurizations were run for 60 min at 95 °C with bubbling air. Open data points indicate that the electrolyte tested negative for sulfate.
Figure 2. X-ray photoelectron spectra of the S 2p region for (a) as-received Vulcan carbon, (b) Vulcan carbon after electrolysis as a dispersed solid contacting a Pt-mesh electrode at 1.0 V (vs Pd/H) for 40 min, and (c) Vulcan carbon after electrodesulfurizing at 1.0 V (vs Pd/H) for 180 min. Table 1. Sulfur Content before and after Electrodesulfurization of Vulcan Carbon at +1.0 V(Pd/H) in 0.05 M HNO3
as-received bubbling air no gas bubbling Ar
S/wt %a,b
S/ppm (wt)b
S/(a/o)b
S/(a/o) (XPS)
S/wt % (XPS)
S/ppm (XPS)
0.65 0.57 0.54 0.55
6500 5700 5400 5500
0.25 0.22 0.20 0.21
0.33 0.23 0.22 0.24
0.88 0.61 0.59 0.64
8800 6100 5900 6400
a Elemental analyses for C and S by combustion analysis. Weight percent of sulfur recalculated as normalized to the analyzed content of carbon. b
for the as-received Vulcan carbon; only one chemical state is observed with a binding energy (163.5 eV) consistent with thiophene-like sulfur.9,12,13 Combustion analysis of this as-received Vulcan carbon gives a bulk sulfur concentration of 0.65 wt % or 0.24a/o (see Table 1). The results from surface (XPS) and bulk (combustion) analyses quantitatively differ presumably because the sulfur impurities preferentially segregate to the carbon surface. The same trends are reflected by either analysis, however (see Table 1 for the entire analytical set of as-received and electrochemically treated Vulcan carbon), so the concentration of sulfur will be discussed using only the XPS data. Effect of Potential/Pt Surface State on Desulfurization. In Figure 3, the sulfur content of the carbon is compared for carbon reacted at different Pt oxidation states in the electrochemical cell. All electrolytes were sparged with air. When the Pt electrode is maintained at 1.0 V(Pd/ H), that is, when its surface is PtOxHy but before the evolution of O2 occurs, XPS analysis indicates that the
sulfur content of the carbon surface drops from 0.33 ( 0.02a/o to 0.23 ( 0.02a/o. XPS also shows (Figure 2b) that the speciation of the unreacted near-surface sulfur remains thiophene-like. The electrolyte from these experiments tests positive for SO42- after adding Ba(OAc)2. In comparison, under open-circuit conditions (∼0.8 V vs Pd/H), positive of the double-layer region, the sulfur content of carbon is 0.29 ( 0.01a/o, which indicates that the carbon has not been significantly desulfurized from the as-received carbon. The electrolyte from the opencircuit reaction tests negative for SO42-, indicating that there is
