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Ethanol Oxidation on Carbon-Supported Pt, PtRu, and PtSn Catalysts Studied by Operando X-ray Absorption Spectroscopy Julia Melke,† Alexander Schoekel,‡ Ditty Dixon,‡ Carsten Cremers,§ David E. Ramaker,| and Christina Roth*,‡ Fraunhofer-Institute for Solar Energy Systems ISE, Freiburg, Germany, Institute for Materials Science, Technische UniVersita¨t Darmstadt, Darmstadt, Germany, Fraunhofer-Institute for Chemical Technology ICT, Pfinztal, Germany, and Department of Chemistry, The George Washington UniVersity, Washington, D.C. 20052 ReceiVed: September 29, 2009; ReVised Manuscript ReceiVed: February 1, 2010
Operando X-ray absorption spectroscopy (XAS) has been used to study the adsorbates and structural changes and their dependence on potential, existing during the ethanol oxidation reaction (EOR) on carbon-supported Pt, PtRu, and PtSn anode catalysts. Conventional EXAFS was applied to identify nanoparticle structure and particle size. The ∆µ-XANES technique was used to investigate adsorbed species with potential. On pure Pt, an overall increase in ∆µ amplitude exists under EOR compared to that existing during the methanol oxidation reaction (MOR). This increased amplitude was attributed mainly to the C1 species on the surface during the EOR; these C1 species and CO become oxidized when O(H) come down on the surface. On PtRu catalysts, the O(H) formation and C-species oxidation begins at lower potentials compared to Pt. The ligand effect from oxidized RuOx islands is operative in PtRu and responsible for the performance enhancement. On PtSn, we observe O(H) at nearly all potentials, which may be explained by a very strong ligand effect involving SnOx. The operando ∆µ and EXAFS results enable the determination of relative active surface areas, particle structure, and adsorbate coverages with potential of C species, OH, and O providing new insights into the role of OH in the EOR. Introduction Fuel cells that directly convert liquid fuels, such as methanol or ethanol, at low temperatures are probably the most viable way for powering portable devices. Ethanol instead of methanol offers a number of advantages, such as a lower toxicity, a higher energy density, and a higher boiling point. Furthermore, the infrastructure for its distribution already exists. The most critical obstacle for the use of ethanol in direct ethanol fuel cells, DEFC, is the lack of a high performance anode catalyst with high selectivity toward the complete oxidation to CO2. A simplified overview of the ethanol oxidation reaction (EOR) is given in Figure 1.1-6 The main reaction products besides CO2 are acetic acid and acetaldehyde on carbon-supported Pt catalysts.1,2 Acetaldehyde can be further oxidized to CO2 or acetic acid,3-5 whereas acetic acid is found to be an end product.6 Further details on the EOR mechanism are given in Figure 2. It has been shown by radio-labeled differential electrochemical mass spectroscopy (DEMS) that the oxidation of ethanol to acetaldehyde on polycrystalline Pt takes place by splitting off a hydrogen atom from the CR atom and from the hydroxyl group at potentials as low as about 0.3 V.7 During the EOR, strongly bound species are observed by different spectroscopic methods (mainly IR) on polycrystalline electrodes5,6,8-10 and on single crystal electrodes.11,12 Two studies have identified C1 species as the dominant adsorbates.5,8 When subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was used, only CO was detected as adsorbate.8 By surface enhanced Raman spectroscopy (SERS) both linearly bonded CO * Corresponding author. E-mail:
[email protected]. † Fraunhofer-Institute for Solar Energy Systems ISE. ‡ Technische Universita¨t Darmstadt. § Fraunhofer-Institute for Chemical Technology ICT. | The George Washington University.
Figure 1. Simplified overview of the different possible reaction paths and products during EOR.
and CHx species were found for the case of ethanol, while for the oxidation of acetaldehyde adsorbed acetyl was observed in addition to CO and CHx.5 Other studies have identified significant amounts of acetyl and adsorbed acetaldehyde and acetic acid as adsorbed intermediates during EOR, besides the CO and CHx species.5,6,9,8 It is very likely that different surface structures are responsible for the inconsistencies in adsorbate and intermediate distributions, because the EOR is extremely structure-sensitive. This has been shown by numerous studies on single-crystal electrodes.11-14 Furthermore, anion adsorption on the Pt electrode from the electrolyte affects the reaction,15 and therefore, experimental conditions could be another source of inconsistencies in the literature. It has been shown by calculations16 and experiments13,11 that on the Pt(111) surface the reaction to acetaldehyde and acetic acid is preferred (left side of Figure 2) and that this reaction is disturbed by adsorbed acetate. Colmati et al.13 suggested that the C-C bond splitting on the Pt(111) surface is only possible in the presence of surface defects. Consistently, Tarnowski et al.14 have found that C-C bond breaking is more facile if the
10.1021/jp909342w 2010 American Chemical Society Published on Web 03/10/2010
Ethanol Oxidation on Pt, PtRu, and PtSn Catalysts
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Figure 3. Schematic illustration of the EXAFS and ∆µ techniques. Figure 2. Summary of the ethanol oxidation reaction (EOR) mechanism on Pt. Path on the left is preferred in Pt(111) and at high surface coverages and the path on right side is dominant at low coverages on stepped surfaces and defects. Adsorbates pictured in black were found experimentally and molecules in blue are present in solution.
surface step density is large. On a Pt(110) and a Pt(100) surface, it was shown for potentials below the water oxidation potential that linear and bridge-bonded CO exists. Acetaldehyde and acetic acid formation start only if some area on the surface becomes cleared by oxidation of such blocking CO species.13 In addition, it was shown theoretically that, on the open Pt(100) and on the stepped Pt(211) surface, the pathway toward CO2 is preferred at low adsorbate coverages, while at high coverage the reaction to acetaldehyde is enhanced.16 As summarized from literature data in Figure 2,11,13,14,16,17 two main reactions exist: the reaction to acetaldehyde and acetic acid takes place at Pt(111) or, in the case of high surface coverage, at all sites (left side of Figure 2). The part of this reaction toward acetaldehyde has been found to be reversible.17 Here, the adsorbed intermediates are acetate and acetyl and further dehydrogenation of acetyl is prohibited at Pt(111). The full oxidation to CO2 takes place only at open or stepped surfaces like Pt(110), Pt(100), and Pt(211) (right side of Figure 2). The dehydrogenation of the CR atom is preferred, which avoids the formation of acetaldehyde and further dehydrogenation and C-C bond breaking is possible. Bond breaking takes place either within adsorbed CHCO or CH2CO species.16 The strongly adsorbed intermediates are CHx and CO. It has been found that Pt-M (M ) Sn, Ru) catalysts are significantly more active for the EOR than Pt alone.18-21 For PtRu, it was found that the CO oxidation occurs at lower potentials due to a bifunctional mechanism (oxidation by OH on the M islands) and ligand effect (oxidation by OH formed on Pt atoms near the M islands and weakening of the Pt-CO bond), enabling the reaction with the O(H) groups at lower potentials.22,23 The EOR selectivity is shifted toward the production of acetic acid21 with increasing Ru content.24 This suggests that the bifunctional mechanism (enabling reaction of C intermediate with OH) and the ligand effect (weakening of the Pt-C adsorbate intermediate bond) could both be responsible for the performance enhancement in DEFCs. For PtSn catalysts, a higher turnover of ethanol can be achieved, but the selectivity is shifted strongly toward the undesired products of acetic acid and acetaldehyde.18,25,20 For PtSn catalysts, a strong connection between the activity for the EOR and the Sn/Pt composition has been reported.25-27 This can be explained by a change in the degree of alloying26,28 or equivalently by a difference in oxidation state of the Sn,25,29
because with less alloying the oxidation level increases. It was found that a nonalloyed PtSnO2/C catalyst enhances the acetic acid production and a bifunctional mechanism of the SnO2 phase was suggested. In contrast, a high degree of alloying enhances the turnover rate toward acetaldehyde.25 Furthermore, adsorbed Sn atoms on Pt single crystals,29 as well as Pt/C or Pt3Sn/C electrodes,30 led to an increase in catalytic activity. In the latter reference, the PtSn electrode with adsorbed Sn atoms exhibits the highest activity for the EOR. It was suggested that the underpotential-deposited (upd) Sn atoms adsorb at the edges and thus inhibit the strong adsorption of CO. Mukerjee et al.31 investigated the structure of a Pt/C catalyst with upd Sn and could not find any Sn-Pt interaction using X-ray absorption spectroscopy (XAS). This was explained by a very disordered tin layer. However, it was shown that the Sn-O coordination and the oxidation state of the upd Sn are altered with change in applied potential. The literature clearly shows that the catalysts morphology is crucial for the EOR and a better understanding of the interplay between morphology and reaction is necessary to create optimized catalysts. In this work, we have studied commercial carbon-supported Pt and Pt-based alloy catalysts by operando XAS. The investigations were carried out on catalyst-coated membranes (CCM) in an operating fuel cell as a function of potential, measured versus a dynamic hydrogen electrode (DHE). At the same time, XAS provides information about morphology and geometric structure of the catalysts as well as changes in electronic structure due to surface adsorbates. The X-ray absorption near edge structure (XANES) was used to study the coverage of poisoning species and the extended X-ray absorption near edge structure (EXAFS) was employed to determine the catalyst structure. The XANES analysis was done by utilizing the ∆µ-XANES technique, which generates differences between two spectra µ(E), as shown in Figure 3. A comparison of the resulting ∆µ(E) with simulated or known signatures can provide information about the kind of adsorbate and their configuration on the surface. Conventional EXAFS analysis (also shown in Figure 3) yields information about coordination number and bond length by fitting a structure model to experimental data. The fit procedure is usually done in R-space after background removal and transformation. From the coordination number the particle size32 is assessable. Experimental Section Preparation of the Catalyst-Coated Membrane (CCM). Commercially available carbon-supported Pt (40 wt % Pt), PtRu (40 wt % Pt, 20 wt % Ru on Vulcan XC-72) purchased from
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Johnson Matthey and PtSn (30 wt % Pt, 10 wt % Sn on Vulcan XC-72) purchased from E-Tek Inc. were used as anode catalysts. Carbon-supported Pt (40 wt % on Vulcan XC-72) was used as cathode catalyst in all cases. The CCMs were prepared by hotspraying an ink, composed of the dispersed catalyst, Nafion and high purity water, on a Nafion115 membrane. The metal loading of the CCMs was 4.1940 mg cm-2 for the Pt, 4.9612 mg cm-2 for the PtSn and 5.8852 mg cm-2 for the PtRu catalyst. The electrode area was 25 cm2. Ex Situ Measurements. The PtSn catalyst mentioned above was characterized by XAS and X-ray diffraction (XRD) measurements on the as-received powder and a powder scraped off from a used CCM. For these measurements the powder was placed between two glass-fiber sheets. XAS data were recorded at the Sn K edge and Pt L3 edge in transmission at the Beamline X1, Hasylab Hamburg. The XRD measurements were carried out with a STOE STADI-P with germanium monochromized Cu KR radiation (λ ) 1.5406 Å) and a position-sensitive detector with 40° aperture 2 θ in transmission mode. Electrochemical Operando XAS Measurement. XAS measurements were performed at the Pt L3 edge utilizing the XAS beamline at ANKA, Karlsruhe, Germany. The measurements were carried out in transmission mode with the beam passing through a sample and a reference foil simultaneously, the latter to provide energy calibration. The electrochemical characterization was carried out in a fuel cell during half cell tests, when the normal cathode side was fed with hydrogen instead of oxygen and thus serving as a dynamic hydrogen electrode (DHE). Following the measurement of the blank catalyst in its dry state, the investigated Pt/C electrode was reduced with hydrogen. Subsequently, water was introduced into the fuel cell and potentials of 0.45 and 0.95 V were applied. This was followed with 1 M ethanol or 1 M methanol aqueous solutions introduced into the cell. Several potentials were applied for each solution. The fuel cell was brought back to open circuit potential, OCP, between each potential step only for the Pt catalyst in ethanol. Between each different alcohol solution, water was fed into the cell and held at a potential of 0.95 V for at least 1.5 h to oxidize off any carbon containing adsorbates remaining on the surface. Several (up to 10) quick-EXAFS spectra (recording time about 8 min) were recorded from E ) 11200 up to 12500 eV at ambient temperature for each potential. The procedures utilized with hydrogen, water, and ethanol fuels on all the anode catalysts were the same, except that for the PtSn and PtRu catalysts, the potentials were applied successively without going to OCP in between. For the XAS measurements the CCMs were sandwiched between Au-coated stainless steel end plates with integrated, interdigitated flow fields and X-ray transparent Kapton foil windows. Below the X-ray transparent window, a part of the cathode catalyst layer was removed. Between flow field and electrode a Toray TGP H 90 gas diffusion layer was placed. Hydrogen (N 5.0) was supplied at 50 mL min-1 by a flow controller (Bronkhorst, Netherlands). The alcohol liquids were supplied at 1.2 mL min-1 by a membrane pump (HPLH PF, CAT Ingenieurbu¨ro M. Zipperer GmbH, Germany) and a separate pump identical in construction was used for water. Potentiostatic E/i curves were recorded using a commercial potentiometer (Wenking Model HP 88). Data Processing. XANES Analysis. The XANES region of the XAS data were analyzed using the ∆µ technique.23,33,34 The absorption coefficient µ was obtained from the raw data with the ATHENA code,35 which uses the AUTOBK algorithm for background removal described in detail elsewhere.36 For the
Melke et al.
Figure 4. (A) ∆µ signatures for Pt in its blank state, in water at 0.95 V vs DHE and in hydrogen and 1 M aqueous ethanol solution at OCP condition. (B) ∆µ signatures for Pt/C in 1 M aqueous ethanol at several potentials vs DHE. ∆µ signatures are obtained with ∆µ(x)H2O,0.45V ) µ(x) - µ(clean,0.45V).
XANES analysis, the normalization was carried out between 20-150 eV relative to the edge. The normalized data were averaged and further aligned using the reference foil data. The ∆µ were obtained by subtracting the measurement for water at 0.45 V versus DHE from the appropriate spectra at a certain condition x,
∆µ(x)H2O,0.45V ) µ(x) - µ(clean, 0.45V)
(1)
Here the subscript “H2O,0.45 V” indicates the reference utilized to obtain the ∆µ and argument, “clean,0.45 V” or “x” indicates what adsorbate the spectrum represents. For pure Pt at 0.45 V, H, OH, and O adsorption are at a minimum, so at this potential the surface should be the most free of adsorbates and represent the “clean” surface. In the case of Pt, only O(H) species are adsorbed at higher potentials, and this allows a straightforward determination of the experimental signature for O(H) (here indicating OH and O), and likewise for H at very low potentials (
