@I Copyright 1991 American Chemical Society
JULY 1991 VOLUME 7, NUMBER 7
Letters Electrocatalytic Oxidation of Methanol by Assemblies of Platinum/Tin Catalyst Particles in a Conducting Polyaniline Matrix Christopher T. Hable and Mark S. Wrighton’ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received December 6, 1990. I n Final Form: April 26,1991 Oxidation of MeOH at electrodes modified with polyaniline and Pt/Sn particles has been studied in aqueous HzSO, electrolyte at 298 K. The Pt/Sn particles have been deposited into the polyaniline by electrochemical deposition from aqueous HzSO4 containing Pt(1V) and Sn(1V) from addition of KzPtCls and SnC405Hz0, respectively. The activity for MeOH oxidation is higher than that of polyaniline-coated electrodes modified with Pt alone. The highest catalytic activity is observed for Pt/Sn particles that are codeposited from solutionscontaining7 mM Sn(1V)when a 3 mM Pt(1V)solution is used. Characterization of the electrodes by SEM and scanning Auger spectroscopy shows that the codeposition procedure yields roughly spherical catalyst particles containing both Pt and Sn. Significantly the two metals are found together, as determined by high lateral resolution Auger electron spectroscopy. Samples analyzed by X-ray photoelectron and Auger spectroscopy show variability in the Pt:Sn ratio for samples prepared in the same fashion, but the most active catalysts typically show surface ratios of Pt:Sn between 8 1 and 2:l. For catalyst loading of -0.5 mg/cmz we find anodic currents of -5 mA/cm2 at +0.3 V vs SCE at 298 K for MeOH oxidation in 0.5 M HzS04 containing 20% (by volume) MeOH. The onset of MeOH oxidation is near 0 V vs SCE.
Introduction We wish to report that electrochemical codeposition of Pt and Sn into polyaniline yields Pt/Sn catalyst particles having higher activity for MeOH oxidation than Pt particles alone. Electrocatalytic properties of Pt/Sn have been reported previously for modified Rh, Ir, and Pt but the conducting polymer provides the possibility of higher surface areas. Pt and Sn have previously been codeposited into solid polymer electrol y t e ~ .The ~ use of a conductive polymer such as polyaniline, however, might allow use of thicker coatings of polymer and higher catalyst loadings, owing to the higher
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(1) Cathro, K. J. J. Electrochem. SOC.1969, 116, 1608. (2) (a) Janssen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976,21, 861. (b) Andrew, M. R.; Drury, J. S.; McNicol, B. D.; Pinnington, C.; Short, R. T. J. Appl. Electrochem. 1976,6,99. (3) Katayama, A. J. Phys. Chem. 1980,84, 376. (4) Aramata, A.; Kodera, T.; Masuda, M. J.Appl. Electrochem. 1988, 18, 577.
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conductivity of the polyaniline. Redox including polyaniline,*ll have been used as supporta for catalysts for the reduction of H20,5v6 C02,7 or 0z8as well as the oxidation of Hz8peand small organic molecules such as MeOHgJoand HCOOH.ll Polyaniline is a particularly attractive material for a catalyst support because of ita (5) Bruce, J. A.; Murahaahi, T.; Wrighton, M. S. J. Phys. Chem. 1982, 86, 1552. (6) Dominey,R. N.; Lewis, N. &;Bruce, J. A,; Bookbinder, D. C.; Wrighton, M. S. J. Am. Chem. SOC.1982,104,467. (7) (a) Stalder, C. J.; Chao, S.;Summers, D. P.; Wrighton, M. S. J.Am. Chem. SOC.1983,105,6318. (b) Chao, 5.;Stalder, C. J.; Summers, D. P.; Wrighton, M. S. J. Am. Chem. SOC.1984,106, 2723. (c) Stalder, C. J.; Chao, S.; Wrighton, M. S. J. Am. Chem. SOC.1984,106,3673. (8)Thackeray, J. W.; Wrighton, M. S. J.Phys. Chem. 1986,90,6674. (9) Kost, K. M.; Bartak, D. E.; Kazee, B.;Kuwana, T. A m l . Chem. 1988,60,2379. (10) Ocon Esteban, P.; Leger, J.-M.; Lamy, C.; Genies, E. J. Appl. Electrochem. 1989, 19, 462. (11) (a) Gholamian, M.; Sundaram, J.; Contractor, A. Q. Langmuir 1987,3,741. (b) Gholamian, M.;Contractor, A. Q. J.Electroaml. Chem. Interfacial Electrochem. 1990, 289, 69.
0 1991 American Chemical Society
Letters
1306 Langmuir, Vol. 7, No.7, 1991 Scheme I. Incorporation of Catalyst into a Polyaniline Matrix
Polyaniline
\ ' I
Pt\Sn Particlea
high surface area,I2J3 high condu~tivity,'~ and durability under conditions relevant to the operation of MeOH fuel cells employing aqueous acidic ele~trolytes.~6J6 In addition to fuel cell applications, incorporation of metal particles into conducting polymers shows promise in electrochemical sensors when used in combination with microelectrode arrays.aJ7 Scheme I represents an electrode modified first with polyaniline and then with Pt/Sn particles. The catalyst assemblies have been characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning Auger spectroscopy (AES), and the illustration in Scheme I is in accord with the structure actually found. Most significantly, the overpotential for MeOH oxidation is lowered by the Pt/Sn catalyst in comparison to when Pt alone is ~sed.~JOAlso, it is important that electrocatalytic oxidation of MeOH occurs in a potential region where peak activity for the oxidation of MeOH corresponds to the potential region where polyaniline is most cond~ctive.'~ Experimental Section Electrochemistry. Electrochemical experiments were run on a Pine Instruments Model RDE 4 potentiostat or a PAR Model 173 potentiostat with a PAR Model 175 programmer. Current voltage curves were recorded on a Kipp and Zonen 90B XY or XYY' recorder. Cyclic voltammetry and short duration polarization studies were done in a one-compartment cell using a saturated calomel (SCE) reference electrode and a Pt gauze counter electrode. Glassy carbon (GC) electrodes (3 mm diameter) were obtained from Bioanalytical Systems. GC electrodes were polished initially with 9-1 pm diamond paste. Before each experiment electrodes were polished again with 0.3-pm alumina, followed by sonication in H2O. Substrates for SEM, XPS, and AES were either 0.1 mm thick Au foil or 1 pm thick e- beam evaporated Au on a polished Si wafer with a Cr (50 A) adhesion layer. K2PtCb (Alfa), SnC4.5H20 (Baker Chemical Co.), HzSO,,HClOd (Mallinckrodt), and HPLC grade H20 (Omnisolve) were used as received. Aniline (Aldrich) was distilled from CaH2. Electrode Modification. Electrodes were derivatized with polyaniline by elect~opolymerization~~ of 0.1 or 0.5 M aniline in 1.0 M HzSO4 or HC104. Electrodes were cycled repeatedly from -0.2 to +0.9 V at either 20 or 100 mV/s until the desired coverage of polymer was obtained. Coveragewas measured by determining the charge passed corresponding to the oxidation and reduction of the polyaniline in 0.5 M Has04 for scans between -0.2 and +0.9 V vs SCE. Typical amounts of charge were 2 X 1VZto 10-l (12) Carlin, C. M.; Kepley, L. J.; Bard, A. J. J. Electrochem. SOC.1986, 132, 363. (13) Huang,W.-S.;Humphrey, B.D.;McDiarmid,A. G. J. Chem. SOC., Faraday Trans. 1 1986,82,2386. (14) Paul,E.W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1986, 89, 1441. (15) McNicol, B. D. J. Electrooanal. Chem. Interfacial Electrochem. 1981, 118, 71.
(16) Hampson, N. A.; Willars, M. J.; McNicol, B. D. J. Power Sources
1979,4, 191. (17) Lofton, E.P.Ph.D. Thesis, M.I.T., 1987.
C/cm2 of electrode area. Catalyst particles were incorporated into the polyaniline matrix by cycling the polyaniline-modified electrodes in a solution containing Pt(1V) and Sn(1V) between +0.5 and -0.3 V at 50 mV/s. The catalyst was deposited from solutions of 0.5 M H2SO4 containing 3 mM K2PtCb and SnC4-5H20. The Sn(1V) concentration was varied from 0 to 11 mM. For large area electrodes solutions with higher concentrations of KzPtC& and SnC4.5H20 gave the highest catalytic activity more consistently. From measurements of cathodic charge passed corresponding to reduction of Pt(1V) to Pt(O),we can estimate the amount of Pt deposited into the polyaniline film; typical values were 0.1-0.6 mg of Pt/cm2 of electrode area. Microscopy. SEM was done on a Hitachi Model 5-800 electron microscope. Samples were sputter coated with a thin layer of8O% Au/20% Pdalloytominimizechargingofthesample. Spectroscopy. XPS spectra were obtained on a Surface Science Instruments Model SSX-100 spectrometer using monochromatic A1 K a radiation and operating at
