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Bifunctional Electrocatalysis in Pt-Ru Nanoparticle Systems C. Roth,*,† N. Benker,† R. Theissmann,‡ R. J. Nichols,§ and D. J. Schiffrin§ Centre for Nanoscale Science, Department of Chemistry, The UniVersity of LiVerpool, LiVerpool, L69 7ZD, United Kingdom, Institute for Materials Science, TU Darmstadt, D-64287 Darmstadt, Germany, and Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany ReceiVed June 1, 2007. In Final Form: October 4, 2007 Pt-Ru alloys are prominent electrocatalysts in fuel cell anodes as they feature a very high activity for the oxidation of reformate and methanol. The improved CO tolerance of these alloys has been discussed in relation to the so-called ligand and bifunctional mechanisms. Although these effects have been known for many years, they are still not completely understood. A new approach that bridges the gap between single crystals and practical catalysts is presented in this paper. Nanoparticulate model systems attached to an oxidized glassy carbon electrode were prepared by combining both ligand-stabilized and spontaneously deposited Pt and Ru nanoparticles. These electrodes showed very different voltammetric responses for CO and methanol oxidation. The cyclic voltammograms were deconvoluted into contributions attributed to Pt, Ru, and Pt-Ru contact regions to quantify the contribution of the latter to the bifunctional mechanism. Scanning transmission electron microscopy confirmed the proximity of Pt and Ru nanoparticles in the different samples.
Introduction Carbon-supported Pt-Ru alloy catalysts are employed at present in almost all polymer electrolyte fuel cell (PEMFC) anodes due to their large CO tolerance as compared to plain platinum.1-3 The onset of both methanol and CO oxidation is shifted to negative potentials by more than 100 mV4 when ruthenium is incorporated into the platinum catalyst. The optimum Pt/Ru ratio has been extensively investigated,5-7 and a 50:50 ratio has been found to give the best performance for the oxidation of CO-contaminated hydrogen, whereas a 90:10 Pt/Ru ratio is the most effective stoichiometry for methanol electro-oxidation. Two effects are considered for the superior activity of binary Pt-Ru catalysts: the ligand effect and the bifunctional mechanism.8-11 The ligand effect results from the modification of the electronic properties of platinum by ruthenium causing a decrease in the strength of the CO bond to the catalyst surface. In the bifunctional mechanism, ruthenium is believed to provide oxygen containing adsorbates at comparably negative potentials, which can oxidize CO at nearby platinum sites. Although these mechanisms have been known for many years, they are still not completely understood.12 * Corresponding author. Fax: +49 6151 16 6377; e-mail: c_roth@ tu-darmstadt.de. † TU Darmstadt. ‡ Karlsruhe Institute of Technology. § The University of Liverpool. (1) Diemant, T.; Hager, T.; Hoster, H. E.; Rauscher, H.; Behm, R. J. Surf. Sci. 2003, 541, 137. (2) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Electrochem. Solid-State Lett. 2001, 4, 217. (3) Bo¨nnemann, H.; Brinkmann, R.; Britz, P.; Endruschat, U.; Mortel, R.; Paulus, U. A.; Feldmeyer, G. J.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. New Mater Electrochem. Syst. 2000, 3, 199. (4) Kaiser, J.; Colmenares, L.; Jusys, Z.; Mo¨rtel, R.; Bo¨nnemann, H.; Ko¨hl, G.; Modrow, H.; Hormes, J.; Behm, R. J. Fuel Cells 2006, 6, 190. (5) Gasteiger, H. A.; Markovic, N. M.; Ross P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (6) Chu, D.; Gilman, S. J. Electrochem. Soc. 1996, 143, 1685. (7) Iwasita, T.; Hoster, H.; John-Annacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522. (8) Lu, C.; Rice, C.; Masel, R. I.; Babu, P. K.; Waszczuk, P.; Kim, H. S.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 9581. (9) Ishikawa, Y.; Liao, M. S.; Cabrera, C. R. Surf. Sci. 2000, 463, 66. (10) Liu, P.; Logadottir, A.; Norskov, J. K. Electrochim. Acta 2003, 48, 3731. (11) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W. Langmuir 1999, 15, 774.
During the past decade, bimetallic bulk and surface alloys, and plain and decorated single crystals, have been used in systematic mechanistic studies. Only recently, however, have non-alloyed systems received increased attention.13 Doubts regarding the true nature of the alloy systems were raised by Long et al.,14 who claimed that hydrous ruthenium oxides were the electro-active species and that a highly dispersed mixture of platinum and hydrous ruthenium oxides surpasses by far the activity of the standard Pt-Ru alloy catalyst. However, hydrous ruthenium oxides are rather ill-defined, highly amorphous compounds and, therefore, very difficult to investigate with conventional techniques (e.g., X-ray diffraction (XRD) or transmission electron microscopy (TEM)). Subsequent investigations using phase segregated Pt-Ru catalysts and even plain mixtures of Pt and Ru4,15-17 have indicated that direct contact between Pt and Ru might not be an essential prerequisite for an enhanced CO tolerance. Since the characterization of catalysts on the nanoscale is rather complex, it is very difficult to distinguish between bulk and surface alloys and surface-decorated materials. Indeed, even surface decoration may be enough to provide the improved CO tolerance reported.4 To study these effects, a different approach employing model systems to obtain a reproducible electrocatalytic behavior would be desirable. For this, it would be particularly interesting to control the separation between the Pt and the Ru phases as well as their sizes and the number of Pt-Ru contact sites. Nanoparticulate model systems recently have been prepared using ligand-stabilized Pt and Ru nanoparticles attached to an oxidized glassy carbon electrode.18 It was expected that mixtures of these nanoparticles would result in an increased electrocatalytic activity due to the (12) Roth, C.; Martz, N.; Hahn, F.; Leger, J.-M.; Lamy, C.; Fuess, H. J. Electrochem. Soc. 2002, 149, 433. (13) Zhang, J.; Lima, F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.; Hanson, J.; Adzic, R. R. J. Phys. Chem. B 2005, 109, 22701. (14) Long, J. W.; Stroud, R. M.; Swider-Lyons, K. E.; Rolison, D. R. J. Phys. Chem. B 2000, 104, 9772. (15) Tong, Y. Y.; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2002, 124, 468. (16) Dubau, L.; Hahn, F.; Coutanceau, C.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 2003, 407, 554. (17) Mazurek, M.; Benker, N.; Roth, C.; Fuess, H. Fuel Cells 2006, 6, 208. (18) Roth, C.; Papworth, A. J.; Hussain, I.; Nichols, R. J.; Schiffrin, D. J. J. Electroanal. Chem. 2005, 581, 79.
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bifunctional mechanism, but this was not observed, although TEM images confirmed the proximity of the Pt and Ru particles. It was proposed that either the presence of a residual stabilizing surfactant blocked the reaction active sites or that the two metallic phases were not in direct physical contact, their separation acting therefore as a barrier for oxidation following the bifunctional mechanism. To increase the metal-to-metal contact between the two phases, model systems employing a dispersion of a ligandstabilized metal in combination with the metal partner spontaneously deposited on an oxidized glassy carbon surface (i.e., without using the corresponding stabilized colloidal metal dispersion) are proposed in the present work for increasing the number of Pt-Ru contact sites. Spontaneous deposition appeared to be an ideal method for preparing improved model systems for establishing an electrocatalytic surface with a high level of contact between the components of the bimetallic catalyst. The spontaneous deposition of a foreign metal onto another metal surface19-24 is an attractive and simple route. Moreover, films formed by spontaneous deposition are very stable and do not appear to detach easily from the metal surface. For example, for Ru deposition onto a Pt single-crystal surface, scanning tunneling microscopy (STM) showed the formation of uniformly distributed 3 nm islands25 with a maximum coverage θ of 0.20 ML after 120 s of exposure. The island density increased with increasing ruthenium coverage. Best results were obtained with an aged RuCl3 solution,26 indicating that the hydrolysis of the ruthenium containing species led to the formation of hydrated ruthenium complexes. The coverage obtained is dependent on the sharing of oxygen atoms between ruthenium and platinum leading to their deactivation for further Ru deposition. Unlike the spontaneous deposition of Ru on Pt, multilayer Pt deposits can be obtained for Pt deposition onto Ru single-crystal surfaces.27 The morphology, coverage, and uniformity of the Pt deposits depend on the concentration of [PtCl6]2- and on immersion time. The Pt deposits obtained consisted mainly of 3-10 nm-sized Pt islands, whereas a roughness analysis showed that at least 10 Pt monolayers were deposited spontaneously.27 This is quite different from the results for the spontaneous deposition of Ru on Pt(hkl) and Pd on Pt(hkl),28,29 where only submonolayer coverages were achieved. The electrocatalytic properties for CO and methanol oxidation on Pt-Ru catalysts were investigated in the present work using both ligand-stabilized and spontaneously deposited Pt and Ru nanoparticles attached to an oxidized glassy carbon electrode. This approach may further complement single-crystal studies, as it easily addresses particle size, kinetics effects, and particle-support interactions. Although the polymeric stabilizing ligands employed are unlikely to be fully desorbed by the CO cleaning procedure followed, there is strong evidence that pristine metal sites are formed on nanoparticles following this procedure.30-32 (19) Frumkin, A. N.; Podlovchenko, B. I. Ber. Akad. Wiss. USSR 1963, 150. (20) Bockris, J. O. M.; Wroblowa, H. J. Electroanal. Chem. 1964, 7, 428. (21) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 259. (22) Petrii, O. A.; Kalinin, V. D. Russ. J. Electrochem. 1999, 35, 627. (23) Janssen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976, 21, 869. (24) Davies, J. C.; Hayden, B. E.; Pegg, D. J.; Rendall, M. E. Surf. Sci. 2002, 496, 110. (25) Crown, A.; de Moraes, I.; Wieckowski, A. J. Electroanal. Chem. 2001, 500, 333. (26) Gorstema, F. P.; Cobble, J. W. J. Am. Chem. Soc. 1961, 83, 4317. (27) Brankovic, S. R.; McBreen, J.; Adzic, R. R. J. Electroanal. Chem. 2001, 503, 99. (28) Attard, G. A.; Bannister, A. J. Electroanal. Chem. 1991, 300, 467. (29) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 5974.
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Experimental Procedures Nanoparticle Syntheses and Characterization. Colloidal solutions of Pt stabilized with polyvinylpyrrolidone (PVP-Pt33-35) and of Pt and Ru stabilized with sodium acrylate (SA-Pt and SA-Ru, respectively36) were prepared in water and characterized by elemental analysis and transmission electron microscopy (TEM). The PVPPt solution was prepared by the addition of 0.2 mL of 2% aqueous polyvinylpyrrolidone (Acros Organics) to aqueous H2PtCl6 under vigorous stirring in an ice bath and reduced by the slow addition of 20 mL of aqueous borohydride solution (34.4 mM, BDH) during a 4 h period. For the SA-Pt preparation, 2 mL of an aqueous solution of sodium acrylate (80 mM) was added to an aqueous solution of H2PtCl6 (25 mL, 2.67 mM, Aldrich, 99.9% purity) and reduced subsequently by dropwise addition of 12 mL of a freshly prepared aqueous sodium borohydride solution (20 mM) under vigorous stirring at room temperature for about 4 h. In both cases, the dark brown solutions of Pt nanoparticles were then filtered through a 0.45 µm Millipore syringe filter to remove any insoluble material. Colloidal Ru nanoparticles were prepared by adding 2 mL of a sodium acrylate solution (80 mM, Acros Organics) to aqueous ammonium hexachlororuthenate (25 mL, 1 mM, Aldrich) under vigorous stirring. The solution was placed in an ice bath, and aqueous NaBH4 (20 mL, 12 mM, BDH) was added dropwise. The nanoparticle preparations contained approximately 1 × 10-3 M metal as determined by elemental analysis. The platinum/stabilizer ratio was not determined due to analytical difficulties related to the separation of the nanoparticles from the preparation media containing an excess of the stabilizing polymer. It was probably lower for the SA-stabilized nanoparticles than for the PVP preparation, as the CO activation treatment had to be repeated in some cases for the PVPstabilized samples. The average particle size in the fresh preparations was determined by TEM. The diameters measured were 4 nm for PVP-Pt, 2 nm for SA-Pt, and less than 2 nm for the SA-Ru nanoparticles. The PVP preparation yielded a size and shape heterogeneous material, whereas for nanoparticles stabilized with sodium acrylate, a more homogeneous preparation of spherical particles was obtained. These showed a tendency to assemble into larger ordered agglomerates when left to dry on a TEM sample grid, indicating a stronger interaction between the SA molecules than between the PVP ligands. Further details of their synthesis and characterization can be found in refs 18 and 37. Electrode Preparation and Cyclic Voltammetry (CV). The electrochemical measurements were performed in a three-electrode cell using a Pt mesh counter electrode and a saturated calomel electrode (SCE) as a reference electrode, and all potentials are referred to this electrode. The potential was controlled with an Autolab PGSTAT20 potentiostat (Eco Chemie B. V.) operated using the Eco Chemie General Purpose Electrochemical System software, version 4.9. Cyclic voltammograms were recorded at a scan rate of 20 mV s-1. The positive potential limit was chosen to minimize bulk Ru oxidation, which occurs at potentials more positive than approximately 0.65 V. The working electrode was a glassy carbon disk (geometric area of 1.90 cm2) embedded in PTFE. The electrode was prepared by polishing with alumina powder (Buehler GmbH; down to 0.05 µm), rinsing with MilliQ water (Millipore system), and sonicating in water for 5 min. The electrode was rinsed again with ultrapure water and finally immersed into the deaerated electrolyte (0.1 M perchloric acid, puriss. p.a., ACS, Fluka). Three cyclic voltammograms of the polished glassy carbon electrode were recorded to verify that a clean (30) Solla-Gullon, J.; Montiel, V.; Aldaz, A.; Clavilier, J. J. Electrochem. Soc. 2003, 150, 104. (31) Solla-Gullon, J.; Montiel, V.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 2003, 491, 69. (32) Solla-Gullon, J.; Montiel, V.; Aldaz, A.; Clavilier, J. Electrochem. Commun. 2002, 4, 716. (33) Yu, W.; Liu, M.; Liu, H.; Zheng, J. J. Colloid Interface Sci. 1999, 210, 218. (34) Miyazaki, A.; Balint, I.; Nakano, Y. J. Nanopart. Res. 2003, 5, 69. (35) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938. (36) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831.
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Figure 1. Schematic diagrams of the different Pt-Ru systems investigated; gray is platinum and black is ruthenium. and reproducible surface was obtained. The electrode was then oxidized by stepping the potential to 2.0 V and holding this potential for 5 min to facilitate the attachment and immobilization of nanoparticles.38 After another three sweeps, the CV images looked identical, which was considered to be an indication of surface reproducibility. Three different model systems were prepared on these electrodes, and these are schematically illustrated in Figure 1. These consisted of the following: (a) Mixtures of PVP-Pt and SA-Ru18 as well as of SA-Pt and SA-Ru to compare the effect of different ligands (model system I): Identical volumes of 1 mL of the respective Pt and Ru preparation were mixed for 2 min in an ultrasonic bath, and then the polished electrode was immersed in this solution for 30 min. The particle separation and proximity of the Pt and Ru phase, respectively, appeared to depend on the ligands used for stabilization as well as immersion time. The excess solution was then rinsed off with Milli-Q water, and the electrode was transferred into the electrochemical cell. (b) SA-Pt attached onto spontaneously deposited Ru (model system II): Ruthenium was spontaneously deposited for 3 min onto the oxidized glassy carbon electrode from an acidic 10-5 M RuCl3 solution that had been aged for 14 days. After the electroless deposition process, the electrode was rinsed and immersed into the SA-Pt solution for 30 min. (c) SA-Ru attached onto spontaneously deposited Pt (model system III): Spontaneous deposition of platinum was carried out by immersion of the electrode in a slightly acidic 10-5 M H2PtCl6 solution for 15 min. The electrode was then rinsed with MilliQ water and immersed into the aqueous SA-Ru solution for approximately 30 min.39 The electrocatalytic activity of the different systems was studied for CO and methanol oxidation by CV in 0.1 M perchloric acid (puriss. p.a., ACS, Fluka). The electrolyte was first saturated with carbon monoxide for 5 min, and the potential was cycled 3 times between -0.4 and 0.75 V in this solution. No difference in the voltammetric response was observed. This served as an activation treatment to displace stabilizing ligands from the nanoparticle surface with strongly adsorbed CO. The electrolyte was then purged with N2 for 30 min, then adsorbed CO was stripped off from the surface in a positive going sweep, and the cell contents were replaced with fresh electrolyte. After recording the base voltammogram in 0.1 M HClO4, CO was adsorbed again at a potential of -0.2 V versus SCE for approximately 10 min. Nitrogen was then bubbled for at least 30 min, and adsorbed CO was stripped off from the surface by a positive going potential sweep to probe the electrode activity for CO oxidation. After two further sweeps to ascertain full oxidation of the adsorbed CO, methanol was added to the electrolyte to yield a 1 M solution, and three CV images were then recorded. All electrodes were subjected to the same procedure to guarantee similar (37) Roth, C.; Hussain, I.; Bayati, M.; Nichols, R. J.; Schiffrin, D. J. Chem. Commun. 2004, 13, 1532. (38) Cherstiouk, O. V.; Simonov, P. A.; Savinova, E. R. Electrochim. Acta 2003, 48, 3851. (39) Crown, A.; Johnston, C.; Wieckowski, A. Surf. Sci. 2002, 506, 268.
Figure 2. Cyclic voltammograms in 0.1 M perchloric acid for (a) a polished glassy carbon electrode (black dashed-dotted line), (b) a glassy carbon electrode after electro-oxidation at 2.4 V vs SCE for 9 min (gray solid line), (c) after attachment of PVP--Pt (black solid line), and (d) for CO stripping (gray dotted line). experimental conditions. Similarly to other ligand-stabilized nanoparticles used as electrocatalysts,30,37 all the model systems studied required CO stripping as an activation treatment.30-32 Without this, almost no activity for methanol oxidation could be observed, even though in model systems II and III, only one partner of the two nanoparticles was stabilized by a polymer. All the experiments were carried out at room temperature (22 ( 2 °C). Figure 2 shows the base voltammograms of the freshly polished glassy carbon electrode (a), the same electrode after oxidation (b), after attachment of the PVP-Pt nanoparticles (c), and the stripping of CO (d). A greatly enhanced pseudo-capacitance was observed after oxidation due to the large increase in surface area and in coverage by carbon functionalities caused by this pretreatment.38 Since the changes upon nanoparticle attachment and during CO stripping are small although highly reproducible, the background currents observed for the oxidized electrode were subtracted from the measured CV images. Background correction was carried out using the AUTOLAB software. All voltammograms were treated similarly to ensure consistency. The CV images were deconvoluted using Origin 6.1G software (OriginLab Corporation). TEM and STEM Measurements. The samples for electron microscopy were obtained by scraping off thin flakes from the particle-decorated surface of the glassy carbon and transferring them within a drop of MilliQ water onto a standard holey carbon film covered copper grid. Imaging was restricted to the thin edges of these flakes. A Philips CM200 transmission electron microscope with a LaB6 filament and an acceleration voltage of 200 kV was employed. In the standard transmission electron micrographs, however, Pt and Ru cannot be distinguished reliably since their size and shape did not differ to a large extent. The nanoEDX device of this equipment has a resolution limited by the beam diameter, and the nanoparticles are too close to each other to be resolved. The chemical analysis of the particles was carried out by ultrahigh spatial resolution EDX microanalysis. The scanning STEM instrument used
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Figure 3. Linear sweep voltammetry for the different Pt-Ru systems in 0.1 M HClO4: (a) CO stripping and (b) oxidation of methanol after cleaning the surface by the oxidation of adsorbed CO; sweep rate was 20 mV s-1. The electrode was cycled 3 times in all cases to maintain similar surface conditions. The first forward sweeps after background subtraction are shown in the figure. Further cycling has little influence on the CV images. for these measurements, a 200 kV FEI Tecnai F20 equipped with a super-twin lens and an integrated STEM unit, has a lateral resolution of less than 1 nm and thus can be used to study the proximity of nanoparticles and possible metallic contacts between the two elements in the particles investigated.
Results and Discussion Electrocatalytic Activity of the Different Model Systems. Large differences in the voltammetric responses were observed for the different electrode structures investigated (Figure 3). The CO stripping voltammogram for type I model systems consisting of particles stabilized with different polymers (PVP and SA) displays a narrow peak centered at approximately 0.65 V attributed to oxidation of CO adsorbed on Pt, simultaneously with a weak feature at approximately 0.4 V, typical of CO oxidation on ruthenium.4 For the mixtures of ligand-stabilized nanoparticles (model system I), a positive shift of the 0.65 V peak by approximately 30 mV is observed when SA-Pt is used instead of PVP-Pt. This is probably due to the different average particle size of PVP-Pt (4 nm) as compared to SA-Pt (2 nm), in good agreement with literature reports.40 In contrast, both model systems containing only one ligand-stabilized partner exhibit very broad anodic waves with current peaks with an onset at E > 0.2 V. The oxidation feature at ca. 0.4 V attributed to CO stripping from ruthenium is smaller for the system with spontaneously deposited Ru (model system II) than in the case when ligand-stabilized ruthenium is used (model system III). The same is observed when platinum is either spontaneously deposited or attached as ligand-stabilized nanoparticles (i.e., the (40) Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem. 2007, 599, 221.
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peak at 0.65 V increases). For methanol oxidation, a peak with a maximum at approximately 0.52 V was observed for model system I; this was significantly broader than the CO stripping peak. Model systems II and III show even wider oxidation waves with peak maxima, and onset potentials shifted negatively by approximately 100 mV, which might also be separated into two different contributions related to the stripping of CO adsorbed at two different adsorption sites. In addition, for model system II, an additional current increase is observed for E > 0.65 V. It is proposed that this is due to Ru oxidation occurring more easily on ruthenium particles obtained by spontaneous deposition than on ligand-stabilized Ru nanoparticles. Deconvolution of the Cyclic Voltammograms. A deconvolution procedure using Gaussian functions for convenience was employed for a more detailed analysis of the voltammograms.41,42 Constraints based on literature values4 for both the peak potential and the full width half-maximum (fwhm) of the contributions were employed for the fitting. The peak width of the Pt stripping peak was kept comparatively narrow, whereas for the CO oxidation peak on ruthenium, a larger fwhm was allowed. The deconvolution of the voltammograms into separate catalytic contributions is shown in Figure 4. The electrodes studied showed up to three distinct waves for CO and methanol oxidation. It is proposed that these can be attributed to catalytic activity at Ru, Pt, and/or Pt-Ru contact sites. The peak splitting in the CO stripping voltammograms is well-known and has been recently discussed by Maillard et al. (ref 43 and references cited therein). Ru-decorated Pt nanoparticles as well as Pt surfaces decorated by spontaneous Ru deposition were discussed in this review, and two types of CO species were identified: linear CO adsorbed on Ru atoms and linear adsorbed CO on Pt atoms, which can be consecutively electro-oxidized in a positive going sweep. The authors proposed that limited diffusion plays a key role; meaning either slow diffusion of CO between Pt and Ru sites or the slow reaction of CO with adsorbed OH. Similar to this work, Waszczuk et al.44 observed that the CV image of Ru-decorated Pt nanoparticles showed two peaks shifted to negative potentials (0.3 and 0.45 V vs SHE), while only one CO stripping peak at ca. 0.52 V versus SHE was observed for a clean carbon-supported Pt catalyst. These two peaks were consequently attributed to oxidation activity on Pt and Ru sites. The possibility of multiple peaks resulting from interparticle heterogeneity was also proposed by Maillard.40 This can be largely excluded in our systems since the ligand-stabilized nanoparticles, and in particular the SA preparations, have a narrow size distribution (about 2 ( 0.5 nm) and a predominantly spherical shape. For system I, the CO stripping voltammetric response for a surface containing SA-Pt and SA-Ru can be deconvoluted into two contributions with peak potentials at 0.42 and 0.68 V (Figure 4a). These correspond to independent electrocatalytic activities of Ru and Pt surface sites.4,44 For comparison, the CO stripping curve from a PVP-Pt electrode is shown in Figure 4b. This gives a satisfactory fit including only one contribution with a peak maximum at approximately 0.68 V. For model system I, it is proposed that the ligands prevent contact of the two metals, which then retain their individual electrocatalytic activity, and consequently, two separate voltammetric peaks are observed. In (41) Gallant, D.; Pezolet, M.; Jacques, A.; Simard, S. Corros. Sci. 2006, 48, 2547. (42) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438. (43) Maillard, F.; Lu, G.-Q., Wieckowski, A.; Stimming, U. J. Phys. Chem. B 2005, 109, 16230. (44) Waszczuk, P.; Solla-Gullon, J.; Kim, H.-S., Tong, Y. Y.; Montiel, V.; Aldaz, A.; Wieckowski, A. J. Catal. 2001, 203, 1.
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Figure 4. Examples of deconvolution of the voltammetric responses for CO and methanol oxidation into up to three distinct contributions: (a) system I; CO oxidation on a mixture of SA-Pt and SA-Ru nanoparticles, (b) CO stripping on Pt as a reference, (c) CO oxidation for spontaneously deposited Pt and attached SA-Ru (system III), (d) system I; methanol oxidation on a mixture of PVP-Pt and SA-Ru nanoparticles, and (e) system II; methanol oxidation on spontaneously deposited Ru and attached PVP-Pt. Black solid line is experimental curve, gray solid line is curve fit, and black dotted lines are individual contributions (at 0.45 V Ru sites and at 0.65 V Pt sites).
contrast, for model system III (Figure 4c), a third contribution at approximately 0.25 V is apparent. This is attributed to an enhanced activity at the Pt-Ru contact sites. The use of a partner that is not stabilized favors the formation of metal-to-metal contact sites, enabling synergistic effects between the two phases. In addition, these systems show broader CO stripping peaks, probably due to the wider particle size distribution obtained by spontaneous deposition as compared to the more uniform particle sizes obtained with ligand-stabilized preparations. For all the systems investigated, however, a much smaller current density for CO stripping from Ru is observed in comparison with CO oxidation on Pt. There are three possible reasons for this behavior: (a) The ruthenium activity for CO oxidation is in general smaller than for Pt, which according to Lin et al.45 should not be the case; (b) the CO activation procedure before measurement is less successful in removing the ligands from the ruthenium surface; and (c) the amount of ruthenium attached or deposited onto the electrode is much less than that (45) Lin, W. F.; Iwasita, T.; Vielstich, W. J. Phys. Chem. B 1999, 103, 3250.
for Pt. The latter assumption seems the most likely, especially in light of the methanol oxidation results discussed next. The results obtained for methanol oxidation are shown in Figure 4d for model system I with different stabilizing ligands (PVP-Pt and SA-Ru) and in Figure 4e for model system II. For model system I, a satisfactory fit can be obtained using only a contribution from Pt, although the quality of the fit can be improved by including a very small amount of active Pt-Ru contact sites. However, the voltammetric response of model system II requires three contributions to obtain a good fit. The current increase at 0.75 V is attributed to bulk ruthenium oxidation. The peak with a maximum at approximately 0.6 V is considered to be due to Pt, whereas the predominant feature at 0.35 V is attributed to an enhanced activity at Pt-Ru contact sites. The results for all the fits are summarized in Table 1 for CO stripping and in Table 2 for methanol oxidation. To quantify the results, the area below the peaks has been used to calculate the percentual share of each contribution. The percentages of Ru, Pt, and Pt-Ru contributions are compared for the three different model systems in Figure 5 by
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Table 1. Results of Deconvolution Routine Applied to Voltammetric Responses Obtained for CO Strippinga system I I* II III
attributed to Pt-Ru Ru Pt Pt-Ru Ru Pt Pt-Ru Ru Pt Pt-Ru Ru Pt
peak potential (mV)
peak area (µC/cm2)
percentage
0.27 0.44 0.72
2.9 15.7 79.4
3 16 81
0.42 0.68 0.30 0.47 0.62 0.29 0.42 0.65
6.2 97.8 8.5 27.8 84.7 10.6 41.2 81.0
6 94 7 23 70 8 31 61
Table 2. Results of Deconvolution Routine Applied to Voltammetric Responses Obtained in 1 M Methanola system I I*
a Systems: SA-Pt and SA-Ru (I) and mixture: PVP-Pt and SA-Ru (I*). Charge densities refer to geometric area.
Figure 5. Deconvolution results for (a) CO and (b) methanol oxidation. The results compare the percentages of Ru, Pt, and Pt-Ru activity for the three different model systems at the different peak potentials (see text for details).
considering that the different peaks from the deconvolution can be assigned to the different electrocatalytic centers described in the preceding paragraphs. For the CO stripping reaction, only minor differences are observed (Figure 5a). Pt shows the highest contribution to the overall activity, whereas that of the Pt-Ru contact sites is very small. The contribution of Pt-Ru contact sites is in all cases lower than 10%, although it appears to be higher when only one ligand-stabilized partner is present. For the methanol oxidation reaction (MOR), however, the behavior is clearly different (Figure 5b). Considerable differences in the contributions from the different model systems are observed andsapart from model system Isa significant contribution from the Pt-Ru contact sites is observed.
II
III
attributed to Ru Pt-Ru Pt Ru Pt-Ru Pt Ru Pt-Ru Pt bulk Ru oxidation Ru Pt-Ru Pt
peak potential (mV)
peak area (µC/cm2)
percentage
0.19 0.33 0.59
12.1 2.1 74.2
14 2 84
0.30 0.68
6.2 97.8
6 94
0.32 0.61 0.80b 0.16 0.30 0.56
47.8 25.8 18.4b 8.7 23.3 65.0
52 28 20 9 24 67
a Systems: SA-Pt and SA-Ru (I) and mixture: PVP-Pt and SA-Ru (I*). b Peak due to Ru oxidation.
Figure 6. Comparison of an electrode for the methanol oxidation reaction with attachment of ligand-stabilized SA-Pt nanoparticles first, then Ru deposition, with an electrode prepared in the opposite order in methanol (a) and summary of the results (b) for the different systems; black is SA-Ru, then Pt deposition; black striped is Pt deposition, then SA-Ru; gray is SA-Pt, then Ru deposition; and gray striped is vice versa.
In contrast with previous results,18 even a small Pt-Ru contribution is present for model system I when the same ligands are used for the stabilization of Pt and Ru (SA-Pt and SA-Ru). This behavior might result from the presence of a smaller number of ligand molecules present in the SA-nanoparticle solutions to prevent metal-to-metal contact, or ligand replacement might occur more easily when the same polymer is present in the surfactant shells. The relative Pt contribution is strongest for model systems I, for which either stabilized PVP-Pt or SA-Pt nanoparticles are attached to the oxidized glassy carbon surface. Model system II consisting of SA-Pt and spontaneously deposited ruthenium exhibits the highest activity at Pt-Ru contact sites both for CO
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Figure 7. Transmission electron micrographs of (a) SA-Pt, (b) SA-Ru (inset: at higher magnification), (c) model system III (SA-Ru, then Pt deposited), and (d) model system II (Ru deposited, then SA-Pt).
stripping and methanol oxidation. It is clear from the intense research described in the literature5,6 that a smaller proportion of ruthenium is needed for increased methanol oxidation activity, with an optimum ratio close to Pt/Ru 90:10.5 Interestingly, ruthenium shows a weak activity for the MOR (Figure 5b), which has not been observed before45 and that is strongly dependent on the electrode preparation conditions. Methanol oxidation does not take place significantly on Ru at room temperature, although a temperature-dependent activity was reported by Yang et al.,46 who recently observed methanol oxidation on Ru surfaces at room temperature at potentials very similar to the MOR potentials observed for commercial Pt-Ru (46) Yang, H.; Yang, Y.; Zou, S. J. Phys. Chem. B 2006, 110, 17296.
alloy electrocatalysts. The authors suggested that general surface oxidation and the lack of reactivity of ruthenium oxides prevent significant MOR activity. Surface oxidation should affect the ligand-stabilized and spontaneously deposited Ru nanoparticles used in this work much less than for bulk Ru, as both the surfactant shell and the presence of Pt at nearby ruthenium sites reduce the sensitivity toward surface oxide formation.47 This might explain the weak but clearly visible peak observed for the Ru sites. Influence of Deposition Order. Electrode preparation is very important to obtain reproducible results. This is true in particular for systems where ligand-stabilized nanoparticles are combined (47) Roth, C.; Benker, N.; Zils, S.; Chenitz, R.; Issanin, A.; Fuess, H. Z. Phys. Chem. 2007, 221, 1549.
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Roth et al.
Figure 8. Scanning transmission electron micrographs of model system III (a: SA-Ru, then Pt deposited) and model system II (b: Ru deposited, then SA-Pt).
with a non-ligand-stabilized partner obtained by spontaneous deposition (i.e., without prior attachment of a stabilizing polymer). In contrast to model system I, where the ligand-stabilized nanoparticle solutions are simply mixed and then attached to the carbon electrode, for model systems II and III, the order of the preparation steps plays an important role in the final activity since attached active metal clusters can be partially covered by particles deposited in a subsequent step. Figure 6a shows background-corrected voltammograms in methanol for model system III, with the attachment of SA-Pt before Ru deposition (solid line) and vice versa (dotted line). The CV images are different, although the maximum current density and the onset potentials stay essentially the same. Ru, Pt, and Pt-Ru contributions can be easily distinguished, but the ratio is significantly different depending on the deposition order. The effect of the electrode preparation on the different Pt, Ru, and Pt-Ru contributions for methanol electro-oxidation is summarized in Figure 6b. Spontaneous Pt deposition is a very effective method for obtaining high platinum loadings at the surface, but also, high coverage can be obtained by the attachment of ligand-stabilized Pt nanoparticles. However, the Pt peak intensity for methanol oxidation decreases sharply when ruthenium is deposited in the second step, as probably active Pt centers are covered by Ru islands. The highest activity of Pt-Ru contact sites is observed for model system II with SA-Pt attachment first and Ru deposition as the second step. This coincides with the lowest activity on Pt and the highest activity on Ru. The Ru contribution for methanol electro-oxidation ranges between 5 and 20%, and model system II with Ru deposition as the second step shows the highest MOR activity. In contrast, model system III when Pt is deposited first shows the lowest activities. It would be expected that ruthenium particles obtained by ligand stabilization show different activities from those Ru nanoparticles obtained via a spontaneous deposition route, as the former will have to be activated by CO stripping first to remove strongly attached ligand molecules from the surface. Thus, these particles might not be effective toward water activation (adsorption of OH groups as required in the bifunctional electrocatalysis mechanism) in the first place. In contrast, the latter might be more prone to ruthenium oxide formation or other deactivation processes than the ligand-stabilized nanoparticles since these are not protected
by a surfactant shell. The reasons for the observed behavior are not completely understood, but the reduction of Ru-OH species in the presence of Pt in proximity to Ru seems to play an important role here.47 Structure of the Electrocatalytic Layer. The SA-Pt and SA-Ru nanoparticle preparations in solution and the distribution of these nanoparticle preparations attached to a glassy carbon surface were analyzed by TEM after the electrochemical measurements, and the results are shown in Figure 7. The images presented in Figure 7a,b illustrate the different Pt and Ru nanoparticle preparations. Figure 7c,d demonstrates the proximity between nanoparticles, either platinum or ruthenium, for two different model systems. As an example, images of model system III (SA-Ru, then Pt deposited) and model system II (Ru deposited, then SA-Pt particles attached) are shown. Neither severe particle agglomeration nor other obvious degradation effects are visible after the electrochemical measurements. The nanoparticles appear mostly to be in contact with each other but not fused. In the case of the surfactant-stabilized particles, they are possibly separated by their organic ligand shell. The insets in Figure 7 show the presence of rather homogeneous and predominantly spherical particles with sizes of approximately 5 and 3 nm for model systems III and II, respectively, with the corresponding metallic lattice planes being visible. In the standard transmission electron micrographs, however, Pt and Ru nanoparticles cannot be clearly distinguished. Only in Figure 7d does it appear that the observed nanoparticle size and image contrast might be used to differentiate between SA-Pt and deposited Ru metal clusters. STEM Results. At least 10 different spots on each STEM grid were sampled, and representative images for model systems III and II are shown in Figure 8. It is clearly observed that the larger particles consist of platinum, whereas the smaller ones are, in most cases, ruthenium. The larger particle size of the platinum particles observed in the STEM images might be due to a slight particle growth either as a consequence of sample storage or by damage in the STEM electron beam. The STEM measurements reveal a homogeneous particle distribution over the several areas sampled and demonstrate the proximity of the Pt and Ru phases, if not the metallic contact between the elements. It appears that the number of Pt-Ru contact sites observed for samples containing model system II is slightly higher than that for system III, in good agreement with the electrochemical results.
Electrocatalysis in Pt-Ru Nanoparticle Systems
A comparison and validation of the deconvolution technique to establish the influence of Pt-Ru contact sites in the observed voltammograms using imaging techniques presents some difficulties since it is not possible to analyze reliably the fraction of the active catalytic surface containing the Pt-Ru contact sites. Techniques such as TEM or STEM give structural information only over a small fraction of the total electrode area, and in addition, it is not easy to define the point of contact between nanoparticles, at least with the resolution level available at present. This was the reason for choosing a peak deconvolution approach, which although not giving a quantitative assessment of Pt-Ru neighbor sites allows a comparison between different preparations.
Conclusion Model systems were prepared using a combination of ligandstabilized and spontaneously deposited nanoparticles attached to an oxidized glassy carbon surface to investigate synergisms in the Pt-Ru system for CO and methanol oxidation. The use of both ligand-stabilized and non-ligand-stabilized nanoparticles enabled the design of well-defined surfaces with a large structural variety rendering these novel systems appropriate for the investigation of reaction mechanisms. Significantly different voltammetric responses were obtained for the different surfaces,
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and these were analyzed by deconvolution in up to three distinct contributions attributed to stripping of CO populations from Pt, Ru, and Pt-Ru contact sites. The following three key observations have been made: (a) The presence of only one ligand-stabilized partner leads to an enhanced metal-to-metal contact, enabling bifunctional activity due to the larger number of Pt-Ru contact sites. (b) A weak but detectable MOR is observed on Ru at room temperature. It is proposed that the ligand-stabilized nanoparticles are less susceptible to surface oxidation, which has been reported to prevent bulk Ru activity. (c) Model system II, consisting of ligandstabilized Pt nanoparticles and spontaneously deposited ruthenium, shows the best performance for CO stripping and methanol oxidation, but the deposition order is important in determining activity. Acknowledgment. C.R. gratefully acknowledges a FeodorLynen fellowship. Thanks are due to I. Hussain, NIBGE, Faisalabad, for his help with the nanoparticle syntheses. Also, support from the European Union under the FP6 program (NENA project, FP6, Contract NMP3-CT-2004-505906) is acknowledged. LA7015929
