Anal. Chem. 1993, 85, 1805-1807
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TECHNICAL NOTES
Electrochemical Measurement of Platinum Surface Areas on Particulate Conductive supports Tri D. Tran and Stanley H. Langer* Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706
INTRODUCTION In the course of our investigations of platinum-graphite electrocatalysts for packed beds1 we found the need for a convenient, reproducible means of measuring deposited electrocatalyst surface areas. The electrochemical method and apparatus described here based on the coulometry of hydrogen adsorption and desorption is a modification of earlier methods used for measurements on bulk metal surfaces.2 It utilizes a slow-scan sweep in combination with a special electrocatalyst holder to accommodate electrolyte conductivity complications and other effects.3~4The method has the advantage of being versatile and applicable to low surface area supported catalysts and avoids higher temperature hydrogen treatment (>250 "C)where sintering occurs in a number of instances.116 It also eliminates a procedure involving the use of Teflon to bind carbon-supported electrocatalysts for surface area measurements.4~~ To accommodate the conducting catalyst particle packing, a new working electrode assembly was designed and fitted into the three-electrode cell system used earlier.* Here the modified packed-bed, working electrode assembly is described together with suitable methodology for measuring platinum surface areas for catalyst particles on conductive supports. Results are in good agreement with those obtained from conventional hydrogen chemisorption experiments.
Flgure 1. Schematic representatlonof cell working conflguration(not to scale): A, electrode holder assembly; B, end view of Luggin p r o k C, piatlnumsplrai counterelectrode;D, working electrodelead. Provision for electrolyte purging capabiilty (sparging tube and gas outlet) not shown for ciarlty.
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EXPERIMENTAL SECTION Cyclic voltammetric type measurements were performed in a standard three-electrode cell constructed from a modified 250mL glass flask.' The cell contained the new working electrode holder designed for packed-bed catalyst samples, a 20-cm platinum spiral counterelectrode, and a standard calomel reference electrodef i e d withsaturated NaClsolution (SSCE)which commonly exhibited a constant potential of 262 mV vs a reversible hydrogen electrode in the same solution. The reference electrode is connected with a bridging solution to a Luggin capillary sealed into the side of the flask. A cell schematic is shown in Figure 1. Standard provisions for sparging and a gas outlet were also built into the cell, but they are not shown in the figure. To whom correspondence should be addressed.
(1) Tran, T. D.; Londner, I.; Langer, S. H. Electrochim. Acta 1993,38, 221-234. (2) (a) Woods, R. InEkctroanalytical Chemistry,A Seriesof Advances; Bard, A. J., Eds.; Marcel Dekker: New York, 1976; Vol. 9, pp 48-57. (b) Woode, R. J. Electroanal. Chem. 1974,49, 217-226. (3) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentab and Applications; Wiley: New York, 1980; pp 220-221. (4) Foral, M. J.; Langer, S. H. J. Electroaml. Chem. 1988,246,193205 ---.
(5) Card, J. C.; Lyke, S. E.; Langer, S. H. J. Appl. Electrochem. 1990, 20,26*280. (6) Connolly, J. F.; Flannery, R. J.; Aronowitz, G. J. Electrochem. SOC. 1966,113,577-580. (7) Bett, J.; Kinoshita, K.; Routsis, K.; Stonehart, P. J. Catal. 1973, 29, 160-168. (8)Kinoshita,K.; Lundquiet, J.; Stonehart,P. J. Catal. 1973,31,326334. 0003-2700/93/0365-1805$04.00/0
Flgure 2. Schematic of electrode holder assembly: (I) jolhtmounted (cf. Figure 1) ll4-in.o.d. glass tube consisting of a, worklng electrode lead (1.3-mm diameter), connected to current collector by a 0.08mm-thick Pt tab (welded to Pt wlre at comer of (11)) and b, 1/4-ln. Swageldc fltting. (ii) L-shaped Teflon arm. (ill) cylindrical, threaded (20 tpi) Teflon electrode holder head (2.8 cm diameter X 2.5 cm) consisting of c, threaded (20 tpi, not shown) adjustable backpkce (3 mm thlck) with slot for screwdriver adJustmentwlthin the threaded(not shown)electrodeholderhead,d, 1.2-cm-dlametersamplecomparbnent (1 cm deep) and packed catalysts: e, 45-mesh platinumscreen current collector; and f, threaded (20 tpi) Teflon electrode retainer (3.4-cm outsidedlameter)taperedtoprovklea1.2-cm-diameterclrcukropenhrg.
The working electrode holder shown in Figure 2 was designed to accommodate working samples of either packed-bed particles or three-dimensional porous sheets. It consisted of three major @ 1993 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 85, NO. 13, JULY 1, 1993
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E / V vs SSCE Flgurs 3. (a)Cyclic voltammogram for graphke support (dashed line) and Pt-graphke catalyst particle (solid line) in ultrapure 0.5 M H2S04. Working electrode exposed area, 1.15 cm2. Platinum surface area determined from integrated adsorbedhydrogen oxldatbn charge (shaded area)between -0.21 and 0.2 V (SSCE). (b) Slow potentlaiscan Drwram . at 0.5 V/min.
assemblies: (i) a 1/4-in.-o,d.glass tube sealed into the glass lid of a 60/50 standard glass tapered joint, (ii) an L-shaped, machined Teflon rod with a hollow (1.5-mm diameter) conduit, and (iii) the Teflon electrode holder. The long end of (ii) was attached to the glass tube with a l/4-in. standard Teflon Swagelok fitting while the short end, which was threaded, could be screwed into the threaded back of the electrode holder. The electrode holder itself contained a threaded hollow, cylindrical sample compartment (1.2-cm diameter) fitted with a threaded Teflon backpiece which could be positioned by screwingforward or backward within the compartment to accommodate samples of varying thickness. In the working configuration most used by us, 0.15 g of the graphite-supported electrocatalyst particles were loaded into the compartment (sample thickness -2 mm) and secured in place with the 45-mesh platinum screen current collector (2.5-cm diameter), which also served to give contact. The collector was held in place by a threaded Teflon electrode retainer with a 1.15cm2 circular opening. The tip of the mounted rigid Luggin capillary was positioned in the center of the exposed face of the packed-bed electrode through adjustment of the holder assembly (Le,, accomplished by varying the arm length of (ii)). Sieved 20-30-mesh graphite particles (Superior Graphite Co., Desulco 9012) were used as supports. They were pretreated as described earlier.' The 0.5 M HzSO4 electrolytewas prepared from Fisher Optima Sulfuric acid and HPLC grade water, an alternative to the triply Voltammograms distilled water used earlier in our lab~ratory.~~' were comparable to those obtained with triply distilled water and reproducible. The electrical system included a Princeton Applied Research PAR Model 175 function generator, a PAR Model 173 potentiostat, and a Bascom-Turner 4110 X-Yrecorder and integrator. Working electrode potentials were monitored using a Keithley 177 multimeter. The procedure for electrode pretreatment and surface area measurement resembled that used earlier by us for gas diffusion electrodes.' After cell assembly, the cell with the electrolyte was sparged (5hrough a sparging tube) with high-purity grade NZgas (Matheson) for 30 min. During this period, the working electrode potential was held at 0.2 V vs SSCE. This was followed by an activation procedure at the working electrode involving potential cycling at 5 V/min between -0.2 to 1.0 V vs SSCE for 20 min. This procedure also served to remove surface oxides and impurities. The actual voltammogram for measuring surface area was then obtained at a slower scan rate of 0.5 V/min. An illustrative voltammogram for a platinum-graphite catalyst and one for the uncoated graphite support (dashed line) are shown in Figure 3. For this illustrative situation no charge was ascribed to the blank support since support alone showed negligible charge (i.e., Pt areas) in the hydrogen adsorption/desorption region. The total charge corresponding to a monolayer of adsorbed hydrogen (shaded area in Figure 3) was calculated by integrating over the hydrogen desorption region (-0.21 to 0.2 V vs SSCE)
Table I. Catalyst Dispersion Data from HZChemisorption and Cyclic Voltammetrv - Exmriments % Pt surface catalysta' (expt), specific Pt (Pt loading, Wt 7%/ aiea,b mz/ rAyMrull samgraphite pretreat./ depositef Hz Hz catalyst redctn method) chemisorptne (CV) ple 1 0.44/EtOH (T114) 0.084 8 7.9 2 l.O1/Sn-Pdd/Hz (T116) 0.130 5 5.2 3 0.44/Sn-Pdd/EtOH (T115) 0.116 12 11.0 aGraphite support surface (BET) area, 0.90 m2/g. Graphite pretreatment and catalyst reduction methods described in detail in refs 1 and 10. * Specific platinum area calculated on the basis of voltammetric data. Chemisorption experiments were furnished by Mobil Research Lab. Ten minutes of support pretreatment.10 and subtracting the double-layer chargee as indicated. This integrated charge was used for calculating platinum surface areas, assuming monolayer hydrogen atom coverage and a hydrogen charge density of 210 pClcrn2 of platinum surface.2
RESULTS AND DISCUSSION The relatively slow scan rate utilized here was intended to accommodate electrolyte conductivity effects and any related complications.3 As is apparent in Figure 3, this approach produces only partially resolved peaks in the hydrogen region but reproducible coulometric measurements for hydrogen coverages. Charging currents in the double-layer region were linear with scan rates in the range between 0.2 and 5 V/min.3 The ratio of integrated desorption to adsorption charges was 0.8 with a 0.5 V/min scan rate, which was the scan rate selected for measurements reported here. This is also the scan rate used earlier.114 Some illustrative data on percentage platinum atoms exposed (dispersion) from voltammetric surface measurements and hydrogen chemisorption for several graphitesupported platinum catalysts are shown in Table I. Specific platinum areas for these catalysts are relatively low (less than 1m2/gof deposited catalyst). They are representative of the graphite-supported catalysts being studied in our laboratory. Agreement between the results from hydrogen chemisorption studies and those from cyclic voltammetric experiments described here is quite satisfactory. With this electrochemical technique, measurements on samples with relatively low platinum areas could be obtained (e.g., example 1 in Table I, 0.084 m2/g on a sample containing 0.44 w t 5% Pt). Results for catalysts supported on a tin-palladium pretreated graphite10 are also shown in Table I, The tinpalladium pretreated graphite tended to stabilize platinum surfaces deposited from chloroplatinic acid solution either from conventional impregnation and hydrogen reduction6 or through direct ethanol reduction.' Base voltammograms for hydrogen on the bare pretreated support before platinum deposition indicated hydrogen adsorption equivalent to less than 1.5% of the area for examples 2 and 3. The ratio of integrated desorption to adsorption charges was closer to unity (0.92) with a 2 V/min scan and with higher scan rates. Some reasons for this have been discussed elsewhere,2.6 and the explanation may well involve some hydrogen evolution during adsorption or even some reduction of a small amount of oxides which might be formed a t the positive end of the anodic sweep. However, the 0.5-V scan rate selected for use here was generally satisfactory: and the hydrogen desorption charge was only -10% less than that with a 2 V/min scan. Scan rates did affect the symmetry of the hydrogen adsorption and desorption regions about the (9) Liu, R.; Her, W.; Fedkiw, P. S. J. Electrochem. SOC. 1992, 139, 15-23. (10)Tran, T. D.; Langer, S. H. Electrochim. Acta, in press.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1, 1993
potential axis. The adsorptionldesorption peak separation (which may be associated with ir effects; cf. ref 3)did increase with increasing scan rates (e.g., 20 and 60 mV for 0.5 and 2 V/min sweep rates, respectively). However, surface area measurements are the issue here and the 0.5-V scan is satisfactory for measurements of the type being considered here which are based on a number of assumptions.2~3J1 This electrode holder can be used with a variety of working samples ranging from electrocatalysts supported on conductive particles to graphite sheets6 since the sample compartment depth can be adjusted to accommodate electrodes of varying thickness. Gas diffusion electrode surface areas can also be measured.4 The agreement of results for the method described here with hydrogen chemisorption measurements is reasonable, considering the many assumptions that are made in interpreting data for both types of measurement.a.2aJ1
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Electrochemical methods of this type have been used by other investigators (e.g., refs 9 and 12). They do appear to be more versatile than hydrogen chemisorption measurements and convenientfor the study of platinum-based catalysts deposited on conductive supports which are used in solution, especially those which might sinter under conventional gas-phase hydrogen chemisorption procedures.1.5
ACKNOWLEDGMENT We thank the National Science Foundation, Mobil Corp. and the University of Wisconsin Graduate School for support. We also thank Dr. T. F. Degnan, Jr. (Mobil Research Laboratory, Paulsboro, NJ) for results for the hydrogen chemisorption experiments, Dr. Michael Foral for helpful discussion, and the reviewers for helpful comments.
(11) Boudart, M.; Djega-Mariadaesou, G. Kinetics of Heterogeneous Catalytic Reactions; Princeton University Press: Princeton, NJ, 1984, pp 20-26.
(12) Giordano, N.;Pasealacqua, E.; Pino, L.; Arico, A. S.; Antonucci, V.; Vivaldi, M.; Kinoehita, K. Electrochim. Acta 1991, 36, 1979-1984.
RECEIVEDfor review January 28, 1993. Accepted March
17,1993.
