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XPS and reactivity study of bimetallic nanoparticles containing Ru and Pt supported on a gold disk
Electrochimica Acta 51 (2006) 3950–3956
XPS and reactivity study of bimetallic nanoparticles containing Ru and Pt supported on a gold disk A. Lewera a , W.P. Zhou a , C. Vericat a,1 , J.H. Chung a , R. Haasch a , A. Wieckowski a,∗ , P.S. Bagus b,∗∗ a
Department of Chemistry and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA b Department of Chemistry, University of North Texas, Denton, TX 76203-5070, USA Received 4 August 2005; received in revised form 26 October 2005; accepted 3 November 2005 Available online 22 December 2005
Abstract We report a new method of immobilization of catalytic metal/alloy nanoparticles on a gold disk for transfer from an electrochemical cell to UHV (without sample exposure to air) for XPS analyses. Using this immobilization approach, several samples were examined: a core-shell Pt-on-Ru catalyst prepared from Ru black onto which Pt was spontaneously deposited, commercial Pt/Ru 50:50 nanoparticle alloy, as well as single metal Ru and Pt nanoparticle samples. The catalysts were characterized for the Ru oxidation state and for the methanol electrooxidation activity (as Pt was always metallic). For all bimetallic samples, we found that the reduced nanoparticles were more active towards methanol oxidation than the fully or partially oxidized samples. Regardless the Ru oxidation state however, the activity was lower than that previously reported for Ru decorated Pt nanoparticle catalysts (Ru-on-Pt). Possible reasons for the reactivity differences are discussed. © 2005 Elsevier Ltd. All rights reserved. Keywords: Bimetallic; Catalysis; Electrochemistry; XPS; Pt; Ru; Methanol
1. Introduction The direct methanol fuel cell (DMFC) is a promising candidate for a wide variety of energy applications, e.g., for the electronics, recreational and transportation use [1–4]. One of the limiting factors in the DMFC technology is the unsatisfactory catalytic behaviour of the cell anode, which usually consists of platinum/ruthenium nanoparticles as the oxidation catalyst. A major effort has been made to improve the anode performance and advance the understanding of the mechanism of methanol oxidation on bimetallic Pt-Ru surfaces, and great progress has been made. Still, more work is needed to improve the overall understanding of both the catalyst properties and function, and also to bring the DMFC operations to more effective, stable and robust levels. ∗
Corresponding author. Tel.: +1 217 333 7943; fax: +1 217 244 8068. Corresponding author. Fax: +1 940 565 4318. E-mail addresses: [email protected] (A. Wieckowski), [email protected] (P.S. Bagus). 1 Present address: Instituto de Investigaciones Fisicoqu´ımicas Te´ oricas y Aplicadas (INIFTA) CC 16 Suc, 4 (1900) La Plata, Argentina. ∗∗
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.11.009
In this report a core-shell, Ru-Pt nanoparticle catalyst (Pton-Ru) was prepared [5] and characterized for structure and reactivity in the methanol oxidation process. For comparison, the commercial bimetallic Pt/Ru alloy catalyst, and single metal Pt and Ru samples were used [6–11]. The nanoparticles were immobilized on a surface of gold for transfer to UHV for Xray photoelectron spectroscopy (XPS) characterization [12]. The electrochemical characterization included chronoamperometric methanol oxidation studies in an electrochemical cell that was an integral part of the transfer system. Overall, with the bimetallic platinum-ruthenium catalysts, it is well known that Pt surface sites break the methanol molecule to fragments, which leads predominantly to chemisorbed CO formation on the Pt sites of the catalysts. The CO is next oxidized to CO2 on the Ru sites [12–24]. While these issues have been broadly recognized, there is no agreement on what is the oxidation state of ruthenium involved in the CO removal process. That is, either stable Ru oxides or metallic Ru is the reactive catalysts constituents [9,10,25]. In the latter case, the metallic Ru sites still need to be covered by some surface oxygen forms, either surface OH or an “activated” water [18]. Also, there is much less information about platinum deposits on ruthenium as catalysts for methanol
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oxidation [5] in contrast to ruthenium deposits on platinum, and the former is the main focus of this paper. 2. Experimental Chemicals used were: Ru black (Johnson and Matthey), Pt black (Johnson and Matthey), Pt/Ru 50:50 alloy nanoparticles catalyst (Johnson and Matthey), K2 PtCl4 (Johnson and Matthey), H2 SO4 (GFS, double distilled from Vycor), methanol (Fisher Scientific, ACS grade) and Millipore water (>18 M). Ultra high purity quality argon and hydrogen gases were supplied by S.J. Smith Welding Supply. Cyclic voltammetric measurements (all in 0.5 M sulfuric acid electrolyte) were carried out using a PAR 263 A potentiostat and associated auxiliaries. Potentials were measured versus an Ag/AgCl/3 M NaCl electrode (Bioanalytical Systems) but are given versus RHE. All experiments were carried out at ambient temperature (25 ± 2 ◦ C). All measurements were carried out inside our EC-XPS instrument either under ultra-pure Ar atmosphere (in EC chamber) or in UHV. The instrument used, consist of two chambers, EC, and UHV coupled together, in which an exposure of a sample to air during the sample transfer between them is avoided [12]. The ESCA M-Probe high resolution, multi-channel hemispherical electron analyzer (Surface Science Instruments) equipped with a monochromatic Al K␣ line (hν = 1486.6 eV), and operated at 110 W, was used [12]. The base pressure of the UHV-XPS apparatus (after a fresh bake-out) was in the 10−11 Torr range. Photoelectron energy was measured using a fixed analyzer transmission (constant pass energy) mode with the constant pass energy of 25 eV, and the size of the incident X-ray beam was 800 m. The M-Probe ESCA software Version 1.36 (Fisons Surface Science) was used, and spectral peaks were fitted using a mixed Gaussian–Lorentzian line shape and Shirley baselines. Linearity of the BE scale of the detector was assured using four Au peaks, namely 4f7/2 (84.0 eV), 4d5/2 (335.2 eV), 4p3/2 (546.4 eV) and 4s (762.2 eV) [26]. For the Ru 3d and Pt 4f region, the exact BE corrections were determined by comparison with cleaned but not annealed Ru (0 0 0 1) or Pt (1 1 1) samples (to maintain surface porosity), assuming Ru 3d5/2 BE equal to 280.1 eV and Pt 4f7/2 BE equal to 71.1 eV for such surfaces [26,27]. To obtain a typical Pt-on-Ru nanoparticle sample using single spontaneous deposition [5], ca. 30 mg of Ru nanoparticles was reduced by hydrogen gas at 100 ◦ C as reported before [5]. After cooling to room temperature under hydrogen, deoxygenated Millipore water was injected (under argon) to the preparative apparatus in order to protect the Ru surface by water. The wet Ru nanoparticle sample was transferred to a beaker, and ca. 60 mL of 10 mM K2 PtCl4 /0.1 M H2 SO4 solution was admitted. The suspension of the Pt-on-Ru nanoparticles was stirred for 1 h and the nanoparticles were isolated after rinsing with 0.1 M H2 SO4 and Millipore water. Platinum packing density on the Pt-on-Ru catalysts was calculated by using inductively coupled plasma (ICP) and the data treatment previously reported [5,19]. The surface area was calculated using CO stripping peaks; it was assumed that a 420 C of
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the CO stripping charge is consumed per 1 cm2 of the electrode surface [5,19]. The packing density of Pt on Ru in the measurements reported in this communication was 0.46 (the Pt/Ru ratio of Pt atoms to Ru atoms on the nanoparticle sample). Before electrochemical processes, the commercial Pt/Ru 50:50 alloy nanoparticle catalyst (Johnson and Matthey), as well as single metal Pt and Ru nanoparticle samples were used “as received”. 3. Results and discussion As pointed out above, the nanoparticles of interest to this study are: core-shell Pt-on-Ru (obtained by a single spontaneous deposition of Pt to Ru nanoparticles [5]), the Johnson and Matthey 50:50 Pt/Ru alloy catalyst and single metal Ru and Pt nanoparticles. However, the main focus in this paper is on Pt-on-Ru. The particles were physically immobilized on the inert surface of gold (Fig. 1) for electrochemical and XPS characterization studies. Notice that the schematic representation of a nanoparticle monolayer in Fig. 1 refers to a thin film of nanoparticles that are electrically shorted to each other either by neighbour-to-neighbour contacts and/or via the conducting gold support. The proposed nanoparticle catalyst immobilization method is an improvement of the physical deposition by solution evaporation method (in air), already being widely used [8,19,28–34]. Its advantage is that the nanoparticles are deposited in the antechamber of the UHV system and are ready to transfer to the main chamber for the XPS analyses. Notice that the nanoparticle suspension in water is made dry by clean pumping in a controlled atmosphere of argon (and water vapour) thus preventing any conceivable contaminations to get to the nanoparticle surface. Several steps are involved in the nanoparticle immobilization process. The nanoparticles are first dispersed in water by sonication and bubbling with argon for 30 min. This yields a homogeneous looking solution of suspended particles. Using argon overpressure, the solution is introduced to the electro-
Fig. 1. Details of nanoparticles immobilization on a surface of the gold disk. The schematic representation of a nanoparticle monolayer refers to a thin film of nanoparticles that are electrically shortened to each other either by neighbourto-neighbour contacts and/or via the conducting gold support.
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Fig. 2. A representative XPS spectrum of the Ru 3d region for fully oxidized Pt-on-Ru nanoparticle sample. The Ru doublets are fitted with the 3d5/2 intensities at 280.2 eV (metallic Ru, solid line), 281.4 eV (RuO2 , dashed line) and 282.8 eV (most likely RuO3 , dashed–dotted line) [26,27,35–38] (The dotted line is assumed to contribute from graphitic carbon). Note: In this study, the Pt component of the Pt-on-Ru nanoparticle sample is always metallic. The fitting components are described in the figure.
chemical cell in the EC chamber, which is coupled with UHV chamber [12]. Brief immersion and removal of the gold disk to and from the solution results then in formation of a hanging solution droplet and simultaneous (partial) deposition of the nanoparticles on the gold surface. Drying the surface in argon and rinsing the disk surface with water (or with the supporting electrolyte) completes the immobilization cycle. The immersion procedure takes less than 30 s; longer time causes a poor nanoparticle adhesion to the surface. After the disk exposure to the flow of argon, usually for 1 h, the surface is coated by a thin nanoparticle film (see above) with the dark shade due to the nanoparticle agglomeration in the disk centre. The substrate is then transferred to UHV for the XPS analysis with the X-ray beam pointing at the shaded area. We found that during the XPS measurements, the signal from gold (4f) was practically negligible. Each disk composition was examined four times and all data presented below represent the average of such four independent measurements. All samples after deposition on gold substrate show strong oxidation of Ru component (Fig. 2). In the XPS spectra reported in Figs. 2–4, the peak fitting was carried out with: (i) the smallest number of 3d doublets, (ii) the binding energies, and the theoretical intensity ratio between 3d5/2 and 3d3/2 for these three doublets kept constant for all fits and (iii) and with the 3d5/2 intensities at 280.2 eV (metallic Ru), 281.4 eV (RuO2 ) and 282.8 eV (most likely RuO3 ) [26,27,35–38]. An unavoidable but small amount of carbon was also detected. We found that in the electrode potential range of methanol oxidation the Pt component in any of the platinumruthenium samples studied in this project was always metallic. (Notice, Fig. 4, an excellent resolution between XPS peaks for the Ru metal and the Ru oxide identified, based upon the previous standards [26,38] as RuO2 .) In contrast, without very drastic electrochemical reduction (see below), Ru was fully or partially oxidized, Figs. 2–4 and captions. Beginning with the fully oxidized sample in air (Fig. 2), further stepwise reduction of Ru
Fig. 3. A representative XPS spectrum of the Ru 3d5/2 region for a partially reduced Pt-on-Ru nanoparticle sample. The first phase of reduction was carried out by holding the electrode at +50 mV (vs. RHE) for 1 h, see text. The Ru doublets are fitted as in Fig. 2: metallic Ru: solid line, RuO2 : dashed line and RuO3 : dashed–dotted line. The fitting components are described in the figure.
was accomplished by holding the sample at 50 mV for a few 1-h time periods (Figs. 3 and 4). However, a complete reduction was not obtained using the 50 mV reduction potential. Instead, the reduction had to be carried out at −100 mV for at least 2 h, Fig. 5. Slow hydrogen evolution was then observed. Interestingly, reduction of ruthenium oxides in the Ru-on-Pt samples was achieved much easier. E.g., the complete Ru reduction in the Ru-on-Pt samples was obtained at +50 mV for 1 h [39]. No reoxidation of Ru was observed after methanol oxidation experiment. Each investigated sample was also characterized in EC chamber by cyclic voltammetry in 0.5 M H2 SO4 , scan rate 50 mV/s (Figs. 6 and 7). It can be seen that oxidized samples show higher capacitive currents, as expected for Ru oxides [25,40,41].
Fig. 4. A representative XPS spectrum of the Ru 3d region for a partially reduced Pt-on-Ru nanoparticle sample; a higher extent of reduction vs. Fig. 3, see text. The reduction was carried out by holding the electrode at +50 mV for 2 h. The Ru doublets are fitted as in Fig. 2: metallic Ru: solid line, RuO2 : dashed line and RuO3 : dashed–dotted line. The increase in the metallic Ru participation vs. Fig. 3 should be seen (the solid line). Also notice a clear experimental resolution between Ru metallic and RuO2 peaks. The fitting components are described in the figure.
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Fig. 5. A representative XPS spectrum of the Ru 3d region for fully reduced (Ru metallic) Pt-on-Ru sample. The sample was reduced by holding at −100 mV vs. RHE for 2 h, leading to undetectable oxide level (metallic Ru: solid line). The fitting components are described in the figure. Fig. 8. (A) Representative XPS spectra in the Ru 3d region of a fully oxidized Pt-on-Ru sample, and a fully oxidized Pt/Ru alloy sample (see Section 2). The electron core level binding energy for the standard metallic Ru 3d5/2 peak at 280.1 eV [12,26] is marked using the vertical line. The spectra correspond to the methanol oxidation currents shown in Fig. 9. (B) Representative XPS spectra in the Ru 3d region of a fully reduced Pt-on-Ru sample (reduction at −100 mV for 2 h), and a fully reduced Pt/Ru alloy sample. The electron core level binding energy for the standard metallic Ru 3d5/2 peak at 280.1 eV [12,26] is marked using vertical line. The spectra correspond to methanol oxidation currents shown in Fig. 9.
Fig. 6. Representative voltammograms of oxidized and reduced Pt-on-Ru nanoparticles. Solution used was 0.5 M H2 SO4 . Scan rate: 50 mV/s.
Once the Ru surface redox state (see Fig. 8) was confirmed by XPS, either the Pt-on-Ru sample or the other samples (all deposited on gold) were transferred back to the electrochemical cell, and used as a catalyst for methanol electrooxidation (Fig. 9; also Fig. 11 for single metal samples). Specific XPS spectra that
Fig. 7. Representative voltammograms of oxidized and reduced Pt/Ru alloy nanoparticles. Solution used was 0.5 M H2 SO4 . Scan rate: 50 mV/s.
correspond to catalyst oxidation states for the methanol oxidation data (Fig. 9) are shown in Fig. 8A and B. The oxidation was carried out at 0.4 V in 0.5 M CH3 OH in 0.5 M H2 SO4 solution for 1 h, holding the disk with the catalyst in a meniscus configuration. Both current densities and the current density ratios are summarized in Table 1. From the current density ratios (Table 1) we conclude that the activity towards methanol oxidation for the bimetallic catalysts increases when Ru is reduced, which is in agreement with some earlier data obtained by different groups, using a range of Pt-Ru compositions [11,14,18,25,41,42].
Fig. 9. Methanol oxidation current densities measured in a 0.5 M CH3 OH in 0.5 M H2 SO4 solution at 0.4 V. The currents shown correspond to the oxidation states of catalysts confirmed by XPS in Fig. 8A and B.
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Table 1 Measurements of current density for methanol oxidation (Fig. 9) for the representative bimetallic samples studied in this project Sample
Current, Iox (A/cm2 ; Ru fully oxidized)
Current, Ired (A/cm2 ; Ru completely reduced)
Current density ratio (Ired /Iox )
Pt-on-Rua,b Pt/Ru (50:50) alloyb Ru-on-Ptc
0.6 1.9 3.2
1.0 2.8 4.1
1.7 1.5 1.3
The current density ratios are summarized in the last table column. The ruthenium oxidation state is indicated in the table, platinum was always metallic. a Packing density of Pt on Ru 0.46 (see Section 2 and Ref. [5]). b Present data. c Data from Ref. [19].
In Fig. 9 the activity of the Pt-on-Ru catalyst is compared to that of the Johnson and Matthey 50:50 Pt/Ru alloy catalyst. It is clear that the alloy catalyst is always more active that the decorated Pt-on-Ru catalyst. This is in contrast to what we reported with the Ru-on-Pt core-shell catalyst [19], which catalytic activity per the real surface area (turnover) was higher than that of the alloy catalyst. To conclude, the order of activity for the methanol oxidation is: Ru-on-Pt [19] > Pt/Ru alloy > Pt-on-Ru. Notice, as reported before [5], that low amounts of Pt added to Ru (lower than the packing density of 0.46) contributed to less methanol oxidation catalysis; the higher catalytic activity was only obtained with addition of platinum beyond the 0.46 Pt/Ru packing density level. The latter increased the 3D character of the Pt-on-Ru catalyst [5]. Building the stocked 3D Pt structure on a Pt interlayer (supported on a Ru nanoparticle surface) was not a desireable effect in the present XPS study, and the higher Pt-load samples than 0.46 Pt/Ru were not investigated. Samples with lower than 0.46 Pt/Ru packing density are now being studied so that we can also obtain core level binding energy insights into the electronic structure of these samples. The data from this project will be reported soon. The data in Figs. 10 and 11 show the XPS and reactivity data of the single metal Pt and Ru nanoparticle catalysts in the
Fig. 10. (A) A representative XPS spectrum of the Pt 4f region of a single platinum metal sample (platinum black). The figure provides evidence for fully metallic character of the platinum sample used for the reactivity measurements shown in Fig. 11. (B) A representative XPS spectrum of the Ru 3d region of a single ruthenium metal sample (ruthenium black). The sample was reduced by holding at −100 mV vs. RHE for 2 h. The figure provides evidence for fully metallic character of the ruthenium sample used for the reactivity measurements shown in Fig. 11.
methanol electrooxidation reaction. The conditions used were the same as for the Ru/Pt nanoparticles. The current (Fig. 11) associated with the XPS spectra in Fig. 10 is negligible for the Ru nanoparticles [43] and very low for Pt nanoparticles (although still measurable at 0.4 V after an hour of measurements [17,23,44]). The reason for the current decay is CO chemisorption that passivates (poisons) Pt surface sites against farther decomposition of methanol and/or its oxidation to carbon dioxide [42,44]. The data reiterate that the Pt nanoparticles alone are a poor catalyst for methanol oxidation at 0.4 V (and lower) electrode potential, and assign the metallic state of platinum (Fig. 10A) to this low activity observation. It also shows that the Ru sample, even if fully reduced (Fig. 10B), cannot activate methanol even if the surface is free of chemisorbed CO (as the current is very low in the full time scales of the current for Pt in Fig. 11). This is again as expected [43]. Now, compare the completely reduced bimetallic samples, Fig. 9. As mentioned above, the comparison shows the higher methanol oxidation activity for the alloy versus the Pt-on-Ru sample. Can this difference in activity be caused by a different ratio between Pt and Ru atoms on the catalyst surfaces? The platinum packing density in the Pt-on-Ru sample is 0.46 Pt/Ru (Section 2). However, there is strong evidence in the literature that some of the Pt deposits on the Ru surface has a 3D character [45], so the Pt coverage will be smaller, probably lower than 0.3
Fig. 11. Methanol oxidation current densities taken in a 0.5 M CH3 OH in 0.5 M H2 SO4 solution at 0.4 V. The current densities correspond to the single metal Pt and Ru samples described and confirmed by XPS in Fig. 10A and B, respectively. The data prove a complete lack of reactivity of the Ru sample, even if fully reduced.
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ML. The nominal elemental ratio between Pt and Ru on the Johnson and Matthey 50:50 Pt/Ru alloy catalyst is 50:50; that is, the Pt packing density on the catalysts surface should be close to 0.5 (not far off from the nominal Pt packing density 0.46 value in the Pt-on-Ru sample). Again though, 195 Pt ECNMR measurements indicate that there is a significant surface enrichment of Pt atoms in the Pt-Ru alloy nanoparticles at the expense of the amount of surface Ru [6]. Therefore, in the first approximation, the Pt-on-Ru catalyst is less active that the alloy catalyst because of the deficiency of Pt catalytic surface sites, which is largely remedied by further addition of Pt [5]. It is not clear why the Pt-on-Ru catalyst and the alloy catalyst are not as active as that the Ru-on-Pt catalyst [5,19]. Here, however, it is well-known ([23,46] that alloying of Pt with Ru reduces the total density of states (DOS) at the Pt sites, and the resulting electronic alteration is the basis for the ligand field contribution to, so called “Ru enhancement” [6,47]. Therefore, we believe that our data demonstrate that the electronic environment of CO obtained from methanol decomposition on Pt deposits on Ru is not the same as on the Pt nanoparticle surface covered by Ru (on the Ru-on-Pt catalyst). Furthermore, this environment is also different for the Pt/Ru alloy. In addition, the adsorbed OH will be “different” among the various systems. This is to say that the electronic environment for the OH adsorbed at the Pt/Ru edge [48] in the Pt-on-Ru samples is not the same as for the Pt/Ru edge in the Ru-on-Pt sample, and is different at the alloy nanoparticle surface [6]. These important issues need further investigations as strongly related to the role of electronic surface structure of Pt/Ru electrodes in methanol oxidation electrocatalysis, also in reference to the direct methanol oxidation fuel cells (DMFC). Overall, the above considered aspect of research described in this communication, and in Refs. [5,19,49,50] suggest that the electrocatalytic activity towards methanol oxidation is strongly dependent on the Pt and Ru surface atoms arrangement (and on the related electronic-level consequences, see above). This is in agreement with the results of the recent study by Korzeniewski et al. [8] who has demonstrated that different atomic distributions on bimetallic nanosized catalysts of the same total composition show different electrocatalytic activity. The Korzeniewski study [8] strongly pertains to the observations reported above. 4. Conclusions Immobilization of metal/alloy catalytic nanoparticle samples containing platinum and ruthenium on gold in the UHVelectrochemistry instrument made possible the sample transfer from an electrochemical cell to UHV for XPS measurements. A definite link was established between the oxidation state of Ru in the bimetallic catalysts and the catalyst activity for methanol oxidation in 0.5 M H2 SO4 solution (at a constant potential of 0.4 V versus RHE). The main focus was on the core-shell (decorated) Pt-on-Ru catalyst and on comparisons with the Pt/Ru alloy and Ru-on-Pt catalysts. We confirm some of the previous data that the catalytic activity toward methanol oxidation is the highest when ruthenium is in the metallic state, in contrast to the oxidized Ru samples. (Apparently, in such measurements, platinum is always metallic.) With the fully reduced catalysts, the order of activity
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for methanol oxidation is: Ru-on-Pt [19] > Pt/Ru alloy > Pt-onRu. While the higher activity of the alloy catalyst versus the Pt-on-Ru sample (at 0.46 Pt/Ru packing density) can be caused by a not fully optimized ratio between Pt and Ru atoms on the Pton-Ru surface, the explanation of the difference between the two core-shell catalysts is more involved. Namely, we believe that the electronic environment of CO obtained from the methanol decomposition on Pt deposits on Ru (that is, on the Pt-on-Ru catalyst) is not the same as on the Pt nanoparticle surface covered by Ru (the Ru-on-Pt catalyst). However, the electronic environment of the OH, needed to react CO to CO2 , may also be different in the three bimetallic samples examined. Overall, these data suggest that the electrocatalytic activity towards methanol oxidation is strongly dependent on the Pt and Ru surface atoms arrangement and on the related electronic-level consequences, in agreement with some of the results reported recently [8]. In some simpler instances, a single metal Pt sample with a clear XPS signature of the metallic state was moderately active at 0.4 V and, as expected, subject of CO poisoning (that gives rise to the rapidly decaying current–potential curve). The XPSconfirmed, completely reduced metallic Ru nanoparticles are entirely inactive, as also expected from previous data. Acknowledgements This project is supported by the U.S. Department of Energy under grant DEFG02-91-ER45439, by the National Science Foundation under grant NSF CHE03-4999 and by the Army Research Office. The authors thank S.T. Kuk for sample preparation and M. Strawski for assistance. References [1] Inc. Science Applications International Corporation EG&G Technical Services, Fuel Cell Handbook, sixth ed., Morgantown, West Virginia 26507-0880, 2002. [2] A. Wieckowski, E. Savinova, C. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Dekker, New York, 2003. [3] J. Lipkowski, P.N. Ross (Eds.), Electrocatalysis, Wiley-VCH, New York/Chichester/Weinheim/Brisbane/Singapore/Toronto, 1998. [4] A. Wieckowski (Ed.), Interfacial Electrochemistry. Theory, Experiment, and Applications, Marcel Dekker Inc., New York, Basel, 1999. [5] S.T. Kuk, A. Wieckowski, J. Power Sources 141 (2005) 1. [6] P.K. Babu, H.S. Kim, E. Oldfield, A. Wieckowski, J. Phys. Chem. B 107 (2003) 7595. [7] H.N. Dinh, X.M. Ren, F.H. Garzon, P. Zelenay, S. Gottesfeld, J. Electroanal. Chem. 491 (2000) 222. [8] C. Korzeniewski, R. Basnayake, G. Vijayaraghavan, Z.R. Li, S.H. Xu, D.J. Casadonte, Surf. Sci. 573 (2004) 100. [9] J.W. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, J. Phys. Chem. B 104 (2000) 9772. [10] D.R. Rolison, P.L. Hagans, K.E. Swider, J.W. Long, Langmuir 15 (1999) 774. [11] A. Crown, H. Kim, G.Q. Lu, I.R. de Moraes, C. Rice, A. Wieckowski, J. New Mater. Electrochem. Syst. 3 (2000) 275. [12] C. Vericat, M. Wakisaka, R. Haasch, P.S. Bagus, A. Wieckowski, J. Solid State Electrochem. 8 (2004) 794. [13] M.T.M. Koper, Surf. Sci. 548 (2004) 1. [14] R. Viswanathan, G.Y. Hou, R.X. Liu, S.R. Bare, F. Modica, G. Mickelson, C.U. Segre, N. Leyarovska, E.S. Smotkin, J. Phys. Chem. B 106 (2002) 3458.
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XPS and reactivity study of bimetallic nanoparticles containing Ru and Pt supported on a gold disk A. Lewera a , W.P. Zhou a , C. Vericat a,1 , J.H. Chung a , R. Haasch a , A. Wieckowski a,∗ , P.S. Bagus b,∗∗ a
Department of Chemistry and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA b Department of Chemistry, University of North Texas, Denton, TX 76203-5070, USA Received 4 August 2005; received in revised form 26 October 2005; accepted 3 November 2005 Available online 22 December 2005
Abstract We report a new method of immobilization of catalytic metal/alloy nanoparticles on a gold disk for transfer from an electrochemical cell to UHV (without sample exposure to air) for XPS analyses. Using this immobilization approach, several samples were examined: a core-shell Pt-on-Ru catalyst prepared from Ru black onto which Pt was spontaneously deposited, commercial Pt/Ru 50:50 nanoparticle alloy, as well as single metal Ru and Pt nanoparticle samples. The catalysts were characterized for the Ru oxidation state and for the methanol electrooxidation activity (as Pt was always metallic). For all bimetallic samples, we found that the reduced nanoparticles were more active towards methanol oxidation than the fully or partially oxidized samples. Regardless the Ru oxidation state however, the activity was lower than that previously reported for Ru decorated Pt nanoparticle catalysts (Ru-on-Pt). Possible reasons for the reactivity differences are discussed. © 2005 Elsevier Ltd. All rights reserved. Keywords: Bimetallic; Catalysis; Electrochemistry; XPS; Pt; Ru; Methanol
1. Introduction The direct methanol fuel cell (DMFC) is a promising candidate for a wide variety of energy applications, e.g., for the electronics, recreational and transportation use [1–4]. One of the limiting factors in the DMFC technology is the unsatisfactory catalytic behaviour of the cell anode, which usually consists of platinum/ruthenium nanoparticles as the oxidation catalyst. A major effort has been made to improve the anode performance and advance the understanding of the mechanism of methanol oxidation on bimetallic Pt-Ru surfaces, and great progress has been made. Still, more work is needed to improve the overall understanding of both the catalyst properties and function, and also to bring the DMFC operations to more effective, stable and robust levels. ∗
Corresponding author. Tel.: +1 217 333 7943; fax: +1 217 244 8068. Corresponding author. Fax: +1 940 565 4318. E-mail addresses: [email protected] (A. Wieckowski), [email protected] (P.S. Bagus). 1 Present address: Instituto de Investigaciones Fisicoqu´ımicas Te´ oricas y Aplicadas (INIFTA) CC 16 Suc, 4 (1900) La Plata, Argentina. ∗∗
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.11.009
In this report a core-shell, Ru-Pt nanoparticle catalyst (Pton-Ru) was prepared [5] and characterized for structure and reactivity in the methanol oxidation process. For comparison, the commercial bimetallic Pt/Ru alloy catalyst, and single metal Pt and Ru samples were used [6–11]. The nanoparticles were immobilized on a surface of gold for transfer to UHV for Xray photoelectron spectroscopy (XPS) characterization [12]. The electrochemical characterization included chronoamperometric methanol oxidation studies in an electrochemical cell that was an integral part of the transfer system. Overall, with the bimetallic platinum-ruthenium catalysts, it is well known that Pt surface sites break the methanol molecule to fragments, which leads predominantly to chemisorbed CO formation on the Pt sites of the catalysts. The CO is next oxidized to CO2 on the Ru sites [12–24]. While these issues have been broadly recognized, there is no agreement on what is the oxidation state of ruthenium involved in the CO removal process. That is, either stable Ru oxides or metallic Ru is the reactive catalysts constituents [9,10,25]. In the latter case, the metallic Ru sites still need to be covered by some surface oxygen forms, either surface OH or an “activated” water [18]. Also, there is much less information about platinum deposits on ruthenium as catalysts for methanol
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oxidation [5] in contrast to ruthenium deposits on platinum, and the former is the main focus of this paper. 2. Experimental Chemicals used were: Ru black (Johnson and Matthey), Pt black (Johnson and Matthey), Pt/Ru 50:50 alloy nanoparticles catalyst (Johnson and Matthey), K2 PtCl4 (Johnson and Matthey), H2 SO4 (GFS, double distilled from Vycor), methanol (Fisher Scientific, ACS grade) and Millipore water (>18 M). Ultra high purity quality argon and hydrogen gases were supplied by S.J. Smith Welding Supply. Cyclic voltammetric measurements (all in 0.5 M sulfuric acid electrolyte) were carried out using a PAR 263 A potentiostat and associated auxiliaries. Potentials were measured versus an Ag/AgCl/3 M NaCl electrode (Bioanalytical Systems) but are given versus RHE. All experiments were carried out at ambient temperature (25 ± 2 ◦ C). All measurements were carried out inside our EC-XPS instrument either under ultra-pure Ar atmosphere (in EC chamber) or in UHV. The instrument used, consist of two chambers, EC, and UHV coupled together, in which an exposure of a sample to air during the sample transfer between them is avoided [12]. The ESCA M-Probe high resolution, multi-channel hemispherical electron analyzer (Surface Science Instruments) equipped with a monochromatic Al K␣ line (hν = 1486.6 eV), and operated at 110 W, was used [12]. The base pressure of the UHV-XPS apparatus (after a fresh bake-out) was in the 10−11 Torr range. Photoelectron energy was measured using a fixed analyzer transmission (constant pass energy) mode with the constant pass energy of 25 eV, and the size of the incident X-ray beam was 800 m. The M-Probe ESCA software Version 1.36 (Fisons Surface Science) was used, and spectral peaks were fitted using a mixed Gaussian–Lorentzian line shape and Shirley baselines. Linearity of the BE scale of the detector was assured using four Au peaks, namely 4f7/2 (84.0 eV), 4d5/2 (335.2 eV), 4p3/2 (546.4 eV) and 4s (762.2 eV) [26]. For the Ru 3d and Pt 4f region, the exact BE corrections were determined by comparison with cleaned but not annealed Ru (0 0 0 1) or Pt (1 1 1) samples (to maintain surface porosity), assuming Ru 3d5/2 BE equal to 280.1 eV and Pt 4f7/2 BE equal to 71.1 eV for such surfaces [26,27]. To obtain a typical Pt-on-Ru nanoparticle sample using single spontaneous deposition [5], ca. 30 mg of Ru nanoparticles was reduced by hydrogen gas at 100 ◦ C as reported before [5]. After cooling to room temperature under hydrogen, deoxygenated Millipore water was injected (under argon) to the preparative apparatus in order to protect the Ru surface by water. The wet Ru nanoparticle sample was transferred to a beaker, and ca. 60 mL of 10 mM K2 PtCl4 /0.1 M H2 SO4 solution was admitted. The suspension of the Pt-on-Ru nanoparticles was stirred for 1 h and the nanoparticles were isolated after rinsing with 0.1 M H2 SO4 and Millipore water. Platinum packing density on the Pt-on-Ru catalysts was calculated by using inductively coupled plasma (ICP) and the data treatment previously reported [5,19]. The surface area was calculated using CO stripping peaks; it was assumed that a 420 C of
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the CO stripping charge is consumed per 1 cm2 of the electrode surface [5,19]. The packing density of Pt on Ru in the measurements reported in this communication was 0.46 (the Pt/Ru ratio of Pt atoms to Ru atoms on the nanoparticle sample). Before electrochemical processes, the commercial Pt/Ru 50:50 alloy nanoparticle catalyst (Johnson and Matthey), as well as single metal Pt and Ru nanoparticle samples were used “as received”. 3. Results and discussion As pointed out above, the nanoparticles of interest to this study are: core-shell Pt-on-Ru (obtained by a single spontaneous deposition of Pt to Ru nanoparticles [5]), the Johnson and Matthey 50:50 Pt/Ru alloy catalyst and single metal Ru and Pt nanoparticles. However, the main focus in this paper is on Pt-on-Ru. The particles were physically immobilized on the inert surface of gold (Fig. 1) for electrochemical and XPS characterization studies. Notice that the schematic representation of a nanoparticle monolayer in Fig. 1 refers to a thin film of nanoparticles that are electrically shorted to each other either by neighbour-to-neighbour contacts and/or via the conducting gold support. The proposed nanoparticle catalyst immobilization method is an improvement of the physical deposition by solution evaporation method (in air), already being widely used [8,19,28–34]. Its advantage is that the nanoparticles are deposited in the antechamber of the UHV system and are ready to transfer to the main chamber for the XPS analyses. Notice that the nanoparticle suspension in water is made dry by clean pumping in a controlled atmosphere of argon (and water vapour) thus preventing any conceivable contaminations to get to the nanoparticle surface. Several steps are involved in the nanoparticle immobilization process. The nanoparticles are first dispersed in water by sonication and bubbling with argon for 30 min. This yields a homogeneous looking solution of suspended particles. Using argon overpressure, the solution is introduced to the electro-
Fig. 1. Details of nanoparticles immobilization on a surface of the gold disk. The schematic representation of a nanoparticle monolayer refers to a thin film of nanoparticles that are electrically shortened to each other either by neighbourto-neighbour contacts and/or via the conducting gold support.
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Fig. 2. A representative XPS spectrum of the Ru 3d region for fully oxidized Pt-on-Ru nanoparticle sample. The Ru doublets are fitted with the 3d5/2 intensities at 280.2 eV (metallic Ru, solid line), 281.4 eV (RuO2 , dashed line) and 282.8 eV (most likely RuO3 , dashed–dotted line) [26,27,35–38] (The dotted line is assumed to contribute from graphitic carbon). Note: In this study, the Pt component of the Pt-on-Ru nanoparticle sample is always metallic. The fitting components are described in the figure.
chemical cell in the EC chamber, which is coupled with UHV chamber [12]. Brief immersion and removal of the gold disk to and from the solution results then in formation of a hanging solution droplet and simultaneous (partial) deposition of the nanoparticles on the gold surface. Drying the surface in argon and rinsing the disk surface with water (or with the supporting electrolyte) completes the immobilization cycle. The immersion procedure takes less than 30 s; longer time causes a poor nanoparticle adhesion to the surface. After the disk exposure to the flow of argon, usually for 1 h, the surface is coated by a thin nanoparticle film (see above) with the dark shade due to the nanoparticle agglomeration in the disk centre. The substrate is then transferred to UHV for the XPS analysis with the X-ray beam pointing at the shaded area. We found that during the XPS measurements, the signal from gold (4f) was practically negligible. Each disk composition was examined four times and all data presented below represent the average of such four independent measurements. All samples after deposition on gold substrate show strong oxidation of Ru component (Fig. 2). In the XPS spectra reported in Figs. 2–4, the peak fitting was carried out with: (i) the smallest number of 3d doublets, (ii) the binding energies, and the theoretical intensity ratio between 3d5/2 and 3d3/2 for these three doublets kept constant for all fits and (iii) and with the 3d5/2 intensities at 280.2 eV (metallic Ru), 281.4 eV (RuO2 ) and 282.8 eV (most likely RuO3 ) [26,27,35–38]. An unavoidable but small amount of carbon was also detected. We found that in the electrode potential range of methanol oxidation the Pt component in any of the platinumruthenium samples studied in this project was always metallic. (Notice, Fig. 4, an excellent resolution between XPS peaks for the Ru metal and the Ru oxide identified, based upon the previous standards [26,38] as RuO2 .) In contrast, without very drastic electrochemical reduction (see below), Ru was fully or partially oxidized, Figs. 2–4 and captions. Beginning with the fully oxidized sample in air (Fig. 2), further stepwise reduction of Ru
Fig. 3. A representative XPS spectrum of the Ru 3d5/2 region for a partially reduced Pt-on-Ru nanoparticle sample. The first phase of reduction was carried out by holding the electrode at +50 mV (vs. RHE) for 1 h, see text. The Ru doublets are fitted as in Fig. 2: metallic Ru: solid line, RuO2 : dashed line and RuO3 : dashed–dotted line. The fitting components are described in the figure.
was accomplished by holding the sample at 50 mV for a few 1-h time periods (Figs. 3 and 4). However, a complete reduction was not obtained using the 50 mV reduction potential. Instead, the reduction had to be carried out at −100 mV for at least 2 h, Fig. 5. Slow hydrogen evolution was then observed. Interestingly, reduction of ruthenium oxides in the Ru-on-Pt samples was achieved much easier. E.g., the complete Ru reduction in the Ru-on-Pt samples was obtained at +50 mV for 1 h [39]. No reoxidation of Ru was observed after methanol oxidation experiment. Each investigated sample was also characterized in EC chamber by cyclic voltammetry in 0.5 M H2 SO4 , scan rate 50 mV/s (Figs. 6 and 7). It can be seen that oxidized samples show higher capacitive currents, as expected for Ru oxides [25,40,41].
Fig. 4. A representative XPS spectrum of the Ru 3d region for a partially reduced Pt-on-Ru nanoparticle sample; a higher extent of reduction vs. Fig. 3, see text. The reduction was carried out by holding the electrode at +50 mV for 2 h. The Ru doublets are fitted as in Fig. 2: metallic Ru: solid line, RuO2 : dashed line and RuO3 : dashed–dotted line. The increase in the metallic Ru participation vs. Fig. 3 should be seen (the solid line). Also notice a clear experimental resolution between Ru metallic and RuO2 peaks. The fitting components are described in the figure.
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Fig. 5. A representative XPS spectrum of the Ru 3d region for fully reduced (Ru metallic) Pt-on-Ru sample. The sample was reduced by holding at −100 mV vs. RHE for 2 h, leading to undetectable oxide level (metallic Ru: solid line). The fitting components are described in the figure. Fig. 8. (A) Representative XPS spectra in the Ru 3d region of a fully oxidized Pt-on-Ru sample, and a fully oxidized Pt/Ru alloy sample (see Section 2). The electron core level binding energy for the standard metallic Ru 3d5/2 peak at 280.1 eV [12,26] is marked using the vertical line. The spectra correspond to the methanol oxidation currents shown in Fig. 9. (B) Representative XPS spectra in the Ru 3d region of a fully reduced Pt-on-Ru sample (reduction at −100 mV for 2 h), and a fully reduced Pt/Ru alloy sample. The electron core level binding energy for the standard metallic Ru 3d5/2 peak at 280.1 eV [12,26] is marked using vertical line. The spectra correspond to methanol oxidation currents shown in Fig. 9.
Fig. 6. Representative voltammograms of oxidized and reduced Pt-on-Ru nanoparticles. Solution used was 0.5 M H2 SO4 . Scan rate: 50 mV/s.
Once the Ru surface redox state (see Fig. 8) was confirmed by XPS, either the Pt-on-Ru sample or the other samples (all deposited on gold) were transferred back to the electrochemical cell, and used as a catalyst for methanol electrooxidation (Fig. 9; also Fig. 11 for single metal samples). Specific XPS spectra that
Fig. 7. Representative voltammograms of oxidized and reduced Pt/Ru alloy nanoparticles. Solution used was 0.5 M H2 SO4 . Scan rate: 50 mV/s.
correspond to catalyst oxidation states for the methanol oxidation data (Fig. 9) are shown in Fig. 8A and B. The oxidation was carried out at 0.4 V in 0.5 M CH3 OH in 0.5 M H2 SO4 solution for 1 h, holding the disk with the catalyst in a meniscus configuration. Both current densities and the current density ratios are summarized in Table 1. From the current density ratios (Table 1) we conclude that the activity towards methanol oxidation for the bimetallic catalysts increases when Ru is reduced, which is in agreement with some earlier data obtained by different groups, using a range of Pt-Ru compositions [11,14,18,25,41,42].
Fig. 9. Methanol oxidation current densities measured in a 0.5 M CH3 OH in 0.5 M H2 SO4 solution at 0.4 V. The currents shown correspond to the oxidation states of catalysts confirmed by XPS in Fig. 8A and B.
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Table 1 Measurements of current density for methanol oxidation (Fig. 9) for the representative bimetallic samples studied in this project Sample
Current, Iox (A/cm2 ; Ru fully oxidized)
Current, Ired (A/cm2 ; Ru completely reduced)
Current density ratio (Ired /Iox )
Pt-on-Rua,b Pt/Ru (50:50) alloyb Ru-on-Ptc
0.6 1.9 3.2
1.0 2.8 4.1
1.7 1.5 1.3
The current density ratios are summarized in the last table column. The ruthenium oxidation state is indicated in the table, platinum was always metallic. a Packing density of Pt on Ru 0.46 (see Section 2 and Ref. [5]). b Present data. c Data from Ref. [19].
In Fig. 9 the activity of the Pt-on-Ru catalyst is compared to that of the Johnson and Matthey 50:50 Pt/Ru alloy catalyst. It is clear that the alloy catalyst is always more active that the decorated Pt-on-Ru catalyst. This is in contrast to what we reported with the Ru-on-Pt core-shell catalyst [19], which catalytic activity per the real surface area (turnover) was higher than that of the alloy catalyst. To conclude, the order of activity for the methanol oxidation is: Ru-on-Pt [19] > Pt/Ru alloy > Pt-on-Ru. Notice, as reported before [5], that low amounts of Pt added to Ru (lower than the packing density of 0.46) contributed to less methanol oxidation catalysis; the higher catalytic activity was only obtained with addition of platinum beyond the 0.46 Pt/Ru packing density level. The latter increased the 3D character of the Pt-on-Ru catalyst [5]. Building the stocked 3D Pt structure on a Pt interlayer (supported on a Ru nanoparticle surface) was not a desireable effect in the present XPS study, and the higher Pt-load samples than 0.46 Pt/Ru were not investigated. Samples with lower than 0.46 Pt/Ru packing density are now being studied so that we can also obtain core level binding energy insights into the electronic structure of these samples. The data from this project will be reported soon. The data in Figs. 10 and 11 show the XPS and reactivity data of the single metal Pt and Ru nanoparticle catalysts in the
Fig. 10. (A) A representative XPS spectrum of the Pt 4f region of a single platinum metal sample (platinum black). The figure provides evidence for fully metallic character of the platinum sample used for the reactivity measurements shown in Fig. 11. (B) A representative XPS spectrum of the Ru 3d region of a single ruthenium metal sample (ruthenium black). The sample was reduced by holding at −100 mV vs. RHE for 2 h. The figure provides evidence for fully metallic character of the ruthenium sample used for the reactivity measurements shown in Fig. 11.
methanol electrooxidation reaction. The conditions used were the same as for the Ru/Pt nanoparticles. The current (Fig. 11) associated with the XPS spectra in Fig. 10 is negligible for the Ru nanoparticles [43] and very low for Pt nanoparticles (although still measurable at 0.4 V after an hour of measurements [17,23,44]). The reason for the current decay is CO chemisorption that passivates (poisons) Pt surface sites against farther decomposition of methanol and/or its oxidation to carbon dioxide [42,44]. The data reiterate that the Pt nanoparticles alone are a poor catalyst for methanol oxidation at 0.4 V (and lower) electrode potential, and assign the metallic state of platinum (Fig. 10A) to this low activity observation. It also shows that the Ru sample, even if fully reduced (Fig. 10B), cannot activate methanol even if the surface is free of chemisorbed CO (as the current is very low in the full time scales of the current for Pt in Fig. 11). This is again as expected [43]. Now, compare the completely reduced bimetallic samples, Fig. 9. As mentioned above, the comparison shows the higher methanol oxidation activity for the alloy versus the Pt-on-Ru sample. Can this difference in activity be caused by a different ratio between Pt and Ru atoms on the catalyst surfaces? The platinum packing density in the Pt-on-Ru sample is 0.46 Pt/Ru (Section 2). However, there is strong evidence in the literature that some of the Pt deposits on the Ru surface has a 3D character [45], so the Pt coverage will be smaller, probably lower than 0.3
Fig. 11. Methanol oxidation current densities taken in a 0.5 M CH3 OH in 0.5 M H2 SO4 solution at 0.4 V. The current densities correspond to the single metal Pt and Ru samples described and confirmed by XPS in Fig. 10A and B, respectively. The data prove a complete lack of reactivity of the Ru sample, even if fully reduced.
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ML. The nominal elemental ratio between Pt and Ru on the Johnson and Matthey 50:50 Pt/Ru alloy catalyst is 50:50; that is, the Pt packing density on the catalysts surface should be close to 0.5 (not far off from the nominal Pt packing density 0.46 value in the Pt-on-Ru sample). Again though, 195 Pt ECNMR measurements indicate that there is a significant surface enrichment of Pt atoms in the Pt-Ru alloy nanoparticles at the expense of the amount of surface Ru [6]. Therefore, in the first approximation, the Pt-on-Ru catalyst is less active that the alloy catalyst because of the deficiency of Pt catalytic surface sites, which is largely remedied by further addition of Pt [5]. It is not clear why the Pt-on-Ru catalyst and the alloy catalyst are not as active as that the Ru-on-Pt catalyst [5,19]. Here, however, it is well-known ([23,46] that alloying of Pt with Ru reduces the total density of states (DOS) at the Pt sites, and the resulting electronic alteration is the basis for the ligand field contribution to, so called “Ru enhancement” [6,47]. Therefore, we believe that our data demonstrate that the electronic environment of CO obtained from methanol decomposition on Pt deposits on Ru is not the same as on the Pt nanoparticle surface covered by Ru (on the Ru-on-Pt catalyst). Furthermore, this environment is also different for the Pt/Ru alloy. In addition, the adsorbed OH will be “different” among the various systems. This is to say that the electronic environment for the OH adsorbed at the Pt/Ru edge [48] in the Pt-on-Ru samples is not the same as for the Pt/Ru edge in the Ru-on-Pt sample, and is different at the alloy nanoparticle surface [6]. These important issues need further investigations as strongly related to the role of electronic surface structure of Pt/Ru electrodes in methanol oxidation electrocatalysis, also in reference to the direct methanol oxidation fuel cells (DMFC). Overall, the above considered aspect of research described in this communication, and in Refs. [5,19,49,50] suggest that the electrocatalytic activity towards methanol oxidation is strongly dependent on the Pt and Ru surface atoms arrangement (and on the related electronic-level consequences, see above). This is in agreement with the results of the recent study by Korzeniewski et al. [8] who has demonstrated that different atomic distributions on bimetallic nanosized catalysts of the same total composition show different electrocatalytic activity. The Korzeniewski study [8] strongly pertains to the observations reported above. 4. Conclusions Immobilization of metal/alloy catalytic nanoparticle samples containing platinum and ruthenium on gold in the UHVelectrochemistry instrument made possible the sample transfer from an electrochemical cell to UHV for XPS measurements. A definite link was established between the oxidation state of Ru in the bimetallic catalysts and the catalyst activity for methanol oxidation in 0.5 M H2 SO4 solution (at a constant potential of 0.4 V versus RHE). The main focus was on the core-shell (decorated) Pt-on-Ru catalyst and on comparisons with the Pt/Ru alloy and Ru-on-Pt catalysts. We confirm some of the previous data that the catalytic activity toward methanol oxidation is the highest when ruthenium is in the metallic state, in contrast to the oxidized Ru samples. (Apparently, in such measurements, platinum is always metallic.) With the fully reduced catalysts, the order of activity
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for methanol oxidation is: Ru-on-Pt [19] > Pt/Ru alloy > Pt-onRu. While the higher activity of the alloy catalyst versus the Pt-on-Ru sample (at 0.46 Pt/Ru packing density) can be caused by a not fully optimized ratio between Pt and Ru atoms on the Pton-Ru surface, the explanation of the difference between the two core-shell catalysts is more involved. Namely, we believe that the electronic environment of CO obtained from the methanol decomposition on Pt deposits on Ru (that is, on the Pt-on-Ru catalyst) is not the same as on the Pt nanoparticle surface covered by Ru (the Ru-on-Pt catalyst). However, the electronic environment of the OH, needed to react CO to CO2 , may also be different in the three bimetallic samples examined. Overall, these data suggest that the electrocatalytic activity towards methanol oxidation is strongly dependent on the Pt and Ru surface atoms arrangement and on the related electronic-level consequences, in agreement with some of the results reported recently [8]. In some simpler instances, a single metal Pt sample with a clear XPS signature of the metallic state was moderately active at 0.4 V and, as expected, subject of CO poisoning (that gives rise to the rapidly decaying current–potential curve). The XPSconfirmed, completely reduced metallic Ru nanoparticles are entirely inactive, as also expected from previous data. Acknowledgements This project is supported by the U.S. Department of Energy under grant DEFG02-91-ER45439, by the National Science Foundation under grant NSF CHE03-4999 and by the Army Research Office. The authors thank S.T. Kuk for sample preparation and M. Strawski for assistance. References [1] Inc. Science Applications International Corporation EG&G Technical Services, Fuel Cell Handbook, sixth ed., Morgantown, West Virginia 26507-0880, 2002. [2] A. Wieckowski, E. Savinova, C. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Dekker, New York, 2003. [3] J. Lipkowski, P.N. Ross (Eds.), Electrocatalysis, Wiley-VCH, New York/Chichester/Weinheim/Brisbane/Singapore/Toronto, 1998. [4] A. Wieckowski (Ed.), Interfacial Electrochemistry. Theory, Experiment, and Applications, Marcel Dekker Inc., New York, Basel, 1999. [5] S.T. Kuk, A. Wieckowski, J. Power Sources 141 (2005) 1. [6] P.K. Babu, H.S. Kim, E. Oldfield, A. Wieckowski, J. Phys. Chem. B 107 (2003) 7595. [7] H.N. Dinh, X.M. Ren, F.H. Garzon, P. Zelenay, S. Gottesfeld, J. Electroanal. Chem. 491 (2000) 222. [8] C. Korzeniewski, R. Basnayake, G. Vijayaraghavan, Z.R. Li, S.H. Xu, D.J. Casadonte, Surf. Sci. 573 (2004) 100. [9] J.W. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, J. Phys. Chem. B 104 (2000) 9772. [10] D.R. Rolison, P.L. Hagans, K.E. Swider, J.W. Long, Langmuir 15 (1999) 774. [11] A. Crown, H. Kim, G.Q. Lu, I.R. de Moraes, C. Rice, A. Wieckowski, J. New Mater. Electrochem. Syst. 3 (2000) 275. [12] C. Vericat, M. Wakisaka, R. Haasch, P.S. Bagus, A. Wieckowski, J. Solid State Electrochem. 8 (2004) 794. [13] M.T.M. Koper, Surf. Sci. 548 (2004) 1. [14] R. Viswanathan, G.Y. Hou, R.X. Liu, S.R. Bare, F. Modica, G. Mickelson, C.U. Segre, N. Leyarovska, E.S. Smotkin, J. Phys. Chem. B 106 (2002) 3458.
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