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Chemical state of ruthenium submonolayers on a Pt(1 1 1) electrode
Surface Science 474 (2001) L203±L212
www.elsevier.nl/locate/susc
Surface Science Letters
Chemical state of ruthenium submonolayers on a Pt(1 1 1) electrode H. Kim a, I. Rabelo de Moraes a, G. Tremiliosi-Filho b, R. Haasch c, A. Wieckowski a,* a
Department of Chemistry, Fritz Seitz Materials Research Laboratory, School of Chemical Sciences, University of Illinois, 600 Mathews Avenue, Urbana, IL 61801, USA b Instituto de Quõmica de S~ ao Carlos/USP, 13560-970 S~ ao Carlos, SP, Brazil c Material Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 5 October 2000; accepted for publication 2 November 2000
Abstract Oxidation states of ruthenium on a Pt(1 1 1)/Ru electrode were investigated by X-ray photoelectron spectroscopy. Ruthenium was added to platinum by electrochemical deposition methods: spontaneous deposition and electrolysis. Depending on the electrode potential, deposition conditions, and presence/absence of methanol in solution, metallic ruthenium (3d5=2 core-level binding energy of 280.3 eV), RuO2 (280.9 eV), and RuO3 (282.8 eV) were found on the surface. After correlating ruthenium valence states to methanol oxidation reactivity, we concluded that the presence of a Ru metallic phase ± covered by a weakly bonded Ru oxidation precursor ± was a prerequisite for eective methanol oxidation electrocatalysis. This precursor was most likely ``activated'' water supplying the oxygen needed for transformation of surface CO to CO2 at the edge of ruthenium islands on the Pt substrate. Ó 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Alcohols; Oxidation; Electrochemical methods; Catalysis; Platinum; Ruthenium; Single crystal surfaces; X-ray photoelectron spectroscopy
1. Introduction This study is related to methanol electrooxidation catalysis in a direct oxidation methanol fuel cell (DMFC) [1]. The best currently used DMFC anode is obtained by deposition of carbon sup-
* Corresponding author. Tel.: +217-333-7943; fax: +217-2448068. E-mail address: [email protected] (A. Wieckowski).
ported (or unsupported) platinum/ruthenium nanoparticles on a proton exchange membrane which serves as a solid polymer electrolyte of the cell [2±4]. Ruthenium present on the electrode surface signi®cantly promotes the methanol oxidation reaction, and the nature of the promotion mechanism is under intense scrutiny [5]. Laboratory model surfaces for the practical Pt/Ru catalyst are platinum electrodes containing controlled amounts of ruthenium added either by electrodeposition [5] or vacuum deposition [6]. If the support is a platinum single crystal surface of speci®c
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 0 5 5 - 4
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surface crystallography, the catalytic activity of such Pt/Ru surfaces can be tested as a function of both surface composition and surface structure [7]. Previous data indicate that among low index Pt(hkl) substrates containing ``optimized'' amounts of surface ruthenium [7] the highest catalytic activity is displayed with Pt(1 1 1)/Ru. It was also found that ruthenium was distributed as a network of nano-dispersed islands, and methanol oxidation occurred at the edge of such islands (Refs. [8,9] and references therein). Despite major progress in our knowledge about methanol oxidation electrocatalysis on Pt/Ru surfaces, outstanding and yet unresolved issue is the chemical state of ruthenium involved in methanol oxidation promotion. In previous work, carbon supported (or unsupported) Pt/Ru electrodes were investigated using X-ray photoelectron spectroscopy (XPS) [10±13]. Ruthenium oxides were usually found as the main catalyst components [11, 12]; however, metallic ruthenium (at Ru 3d5=2 at 280.2 eV) appeared after the electrode was used for methanol oxidation for an extended period of time [11]. In a series of experiments [14,15] using Ru 3p3=2 intensity at 461.4 eV [16], the ruthenium species electrodeposited on platinum at a submonolayer coverage was reported to be purely metallic in a broad electrode potential range (from 0.2 to 0.9 V vs. reversible hydrogen electrode (RHE)). However, in commercial nanoparticle Pt/ Ru alloy catalysts more recent studies [17±19] indicate that the fraction of ruthenium metal (at 280.1 eV) is small, and the prevailing near-surface ruthenium component is anhydrous RuO2 at 281.2 eV (Ru 3d5=2 ). A slightly up-shifted feature at the binding energy above 282 eV was assigned to hydrous ruthenium oxide (RuO2 x H2 O). Our work with a commercial Johnson±Matthey catalyst [9] supports the experimental data [17,18], although more metallic ruthenium was observed, re¯ecting most probably dierences in sample composition provided by fuel cell catalyst manufacturers. In situ measurements by the use of surface-enhanced Raman spectroscopy (SERS) [20] were indicative of the presence of RuO2 and RuO3 as well as RuO4 , and X-ray absorption near-edge structure data indicate that addition of ruthenium to platinum increases d-vacancies of the Pt substrate, and
ruthenium appears oxidized in the double layer and hydrogen range of electrode potentials [21]. Since a broadly acknowledged conclusion has not been reached concerning the oxidation state(s) of ruthenium in Pt/Ru mixed metal electrode systems, we are interrogating ruthenium electrodeposited on a Pt(1 1 1) substrate by the use of XPS. The XPS instrument hosts an electrochemical cell, and sample exposure to air is avoided the electrode transfer between the cell and the ultrahigh vacuum (UHV) chamber [5]. Controlled amounts of ruthenium were added by spontaneous deposition processes [22], or by electrolysis [7]. Studies of methanol oxidation were conducted by chronoamperometry [7,23]; and a Pt/Ru catalyst preparation was characterized by cyclic voltammetry [23]. 2. Experimental 2.1. Electrochemical conditions for deposition of ruthenium on Pt(1 1 1) The real surface area of the Pt(1 1 1) electrode was obtained using hydrogen adsorption±desorption charges from cyclic voltammograms [24±26]. Spontaneous deposition was carried out in 0.5 mM RuCl3 x H2 O in 0.1 M HClO4 for 2 min [7,22] at open circuit, roughly 0.90 V. (All potentials were measured against Ag/AgCl reference, and are reported versus the (RHE).) The Pt(1 1 1)/Ru surface was removed from the deposition bath, transferred to a cell containing the clean supporting electrolyte, and treated by two cyclic voltammetric scans from 0.00 to 0.88 V in order to break the chemisorbed Ru-containing anions to simpler fragments (see further), and to con®rm the cleanness and crystallographic orientation of the mixed metal surface [5,7]. This procedure yields surfaces with 0.20 ML Ru coverage (the coverage, ML, is a packing density, the number of Ru adatoms to the number of platinum sites on the Pt(1 1 1) substrate). In parallel experiments, constant potential electrolysis was conducted at 0.30 V from a 2 mM Ru(NO)(NO3 )3 (ruthenium nitrosylnitrate) solution in 0.5 M H2 SO4 for 1 min to yield a ruthenium coverage of 0.20 ML [23,27]. The coverage value of 0.20 ML is the optimum coverage for methanol
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oxidation on Pt(1 1 1)/Ru surfaces, as demonstrated before [5,8]. 2.2. Measurements in ultrahigh vacuum A disk-shaped platinum crystal oriented in the (1 1 1) direction was mounted inside the UHV chamber. The base pressure of the system was 10 10 Torr. The surface was cleaned by Ar bombardment and vacuum annealed at 800°C in the presence of oxygen (10 8 Torr), and was characterized by low energy electron diraction (LEED) and Auger electron spectroscopy (AES). Details of the LEED/AES measurements and UHV experimental conditions have been reported previously [5]. For the XPS work, a second UHVelectrochemistry system [28] was equipped with an ESCA M-Probe high resolution, multi-channel hemispherical electron analyzer (Surface Science Instruments). Samples were excited using a monochromatic AlKa source hm 1486:6 eV operated at 110 W. The analysis was carried out under ®xed analyzer transmission (constant pass energy) mode, with a constant pass energy of 25 eV. The size of the incident X-ray beam was 800 lm, and the resolution of the detector was 0.027 eV. Wide range XPS spectra (from 30 to 540 eV), and narrow range, high-resolution spectra (from 273 to 293 eV) were obtained from the Pt surfaces and Pt/ Ru surfaces (transferred to UHV from electrolytic media, see below) with the Pt 4f7=2 core-level energy of 71.2 eV selected as the binding energy reference. For nonlinear least-square curve ®tting± spectral deconvolution procedures, an S-Probe version of the 1.36 ESCA software (Fison Instruments) was used. Spectral peaks were ®tted by using mixed Gaussian±Lorentzian lineshape and Shirley baselines [29±31]. For electrochemical experiments in the UHV instrument [5], the antechamber was ®lled with ultrahigh purity argon (99.999%) to atmospheric pressure, and ruthenium deposition was carried out in an electrochemical cell attached to the antechamber. The recently reviewed procedure [5] was applied where an exposure of the crystal to air at any phase the experiment was stringently avoided. A predetermined electrode potential was applied upon the electrode removal from solution
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and transfer to UHV. The Ru deposits obtained by either spontaneous deposition or by electrolysis were characterized by XPS, and ruthenium chemical states obtained under various electrochemical conditions are reported. 2.3. Methanol oxidation rates measured by chronoamperometry Methanol oxidation measurements were carried out in a freestanding electrochemical cell previously reported [5,27]. A platinum single crystal, Pt(1 1 1) of 0.2 cm in diameter, was used as a working electrode in a meniscus con®guration. The electrode was annealed in hydrogen±air ¯ame at 800°C for 2 min, cooled to a moderate temperature, and immersed in Millipore water [5,7, 22,23,25]. The electrode was next transferred to the cell, with the surface protected by a water drop adhering to the surface. After ruthenium deposition [7], the electrode was removed from the solution, rinsed with Millipore water, treated by two voltammetric cycles in the potential range speci®ed above (in 0.1 M H2 SO4 ). Next, the electrode was transferred to a cell containing a methanol solution (0.1 M H2 SO4 0.1 M CH3 OH). The methanol oxidation reaction was conducted on the Pt(1 1 1)/Ru electrode, under chronoamperometric conditions [7]. Immediately before recording the oxidation current at a chosen potential, the electrode was held for 1 s at a potential 100 mV more positive than the onset of hydrogen evolution. The chronoamperometric current for methanol oxidation was measured between 0.40 and 0.70 V (vs. RHE) for 20 min. When the experiment with a given Pt/Ru surfaces ended, ruthenium was removed from platinum by several negative- and positive-going scans at 100 mV s 1 in the potential range determined by the onset of hydrogen and oxygen evolution. 2.4. Chemicals and other conditions The reagents used were Millipore water (18 MX cm), HClO4 and H2 SO4 distilled from Vycor (GFS Chemicals), Ru(NO)(NO3 )3 , RuCl3 x H2 O from Aldrich, and CH3 OH (Spectrophotometric Grade, Mallinckrodt Spectrar). The electrode
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potential was controlled via a PAR Model 283 potentiostat (EG&G Princeton Applied Research). All measurements were carried out at room temperature. 3. Results 3.1. X-ray photoelectron spectroscopy measurements Ruthenium standard for XPS measurements for all Pt/Ru samples was a multi-layer ®lm of ruthenium obtained by electrolysis from a ruthenium nitrosylnitrate solution at a potential slightly more negative than that for hydrogen evolution. After 0.5 h of electrodeposition, a thick Ru ®lm was formed that virtually eliminated XPS platinum intensities (following the sample transfer from the cell to UHV, as described above). Peak ®tting of such a standard reveals a spin±orbit-split doublet of Ru 3d5=2 and Ru 3d3=2 at 280.3 eV (0.1 eV) and 284.5 eV (0.1 eV), respectively (Fig. 1a), which are characteristic of metallic ruthenium [29]. The peak area ratio between Ru 3d5=2 and Ru 3d3=2 was 3:2, which closely corresponds to the theoretical value due to spin±orbit interactions of delectrons. The spin±orbit splitting of 4.18 eV also coincides with the theoretical value. A full width at half maximum of the 3d5=2 component was 1.0 eV (0.05 eV), which is smaller than that of Ru 3d3=2 by 0.43 eV due to the Coster-Kronig decay eect [32]. The curve-®tting parameters were used as ®xed values for the XPS studies of the submonolayer ruthenium adsorbates discussed below. The same Gaussian±Lorentzian natural line width and asymmetry parameters were applied, and the deviation of ®tting parameters was allowed within 95% of the con®dence limit, v2 . The ®rst spectrum obtained at a submonolayer ruthenium coverage is shown in Fig. 1b. Approximately 0.20 ML of ruthenium was added to platinum by electrolysis at 0.30 V for 1 min in a 0.5 mM RuCl3 0:1 M HClO4 solution in. The electrode and the cell were rinsed with Millipore water to eliminate all bulk ruthenium species from the cell. The electrode potential was then stepped to 50 mV (for 1 min) located in the hydrogen under-
potential deposition range, near the threshold of hydrogen evolution. The bands at 280.3 and 284.5 eV correspond to metallic ruthenium (cf. with Fig. 1a), which is in accordance with Pourbaix diagrams predications [33]. This demonstrates that a submonolayer metallic ruthenium deposited on platinum from aqueous media can be transferred to UHV without oxidation by water and mineral acid vapors even if the electrode potential control is terminated upon the sample removal from the cell. This observation is a benchmark for further studies reported below. Another set of data was obtained using the spontaneous deposition process from chloride media. As mentioned in the experimental section, the ruthenium coverage under such deposition conditions is also 0.20 ML, and the deposition open circuit potential (OCP) is close to 0.90 V [22]. A typical Ru core-level 3d photoelectron spectrum from such Pt(1 1 1)/Ru samples removed from the electrolyte at the OCP is shown in Fig. 2a. Peak ®tting of the Ru 3d region reveals three spin±orbit doublets, showing that the composition of the deposit is not uniform. The sample contains highvalency ruthenium 3d5=2 states: RuO2 (281.0 eV), and RuO3 (283.0 eV), but also metallic ruthenium (280.3 eV). (3d3=2 peaks, as weaker intensities, are not discussed). Auger electron spectroscopic analyses conducted under conditions very similar (or identical) to those reported in this article demonstrate a large amount of surface oxygen present [34], and the XPS data shown in Fig. 2a identify the oxidized species as ruthenium oxides, or hydroxides. However, these spontaneously formed ruthenium oxo-species and/or Ru-containing chemisorbed anions [35] decompose to simpler Ru forms under voltammetric conditions (see Section 2), or under potentiostatic electrode treatment at suciently negative potentials, as shown in Fig. 2b. Three vertical dashed lines highlight the 3d5=2 states, including that for metallic ruthenium on the surface. Assignment of ruthenium oxidation states to the binding energies around 282 eV (in Fig. 2) is not straightforward [17,18,20,36±40]. Cox et al. concluded the peak at 282.2 eV could be assigned to RuO2 with the binding energy shift induced by ®nal-state screening by Ru 3d electrons, and the
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Fig. 1. (a) X-ray photoelectron spectrum of a thick, multi-layer ruthenium ®lm obtained by electrolysis from 2 mM Ru(NO)(NO3 )3 solution in 0.1 M H2 SO4 on a platinum substrate, and at 50 mV vs. RHE. Peak deconvolution of the Ru 3d core-level energy region yields two ruthenium intensities at 280.3 eV (3d5=2 ) and 284.4 eV (3d3=2 ), and a small graphitic carbon intensity at 284.5 eV (1s). (b) Xray photoelectron spectrum of a submonolayer of ruthenium deposited on a Pt(1 1 1) electrode from 0.5 mM RuCl3 solution in 0.1 M HClO4 at 0.30 V, for 1 min. Before transfer to UHV, the electrode was held in 0.1 M HClO4 at 50 mV for 30 min. The intensity is normalized to the 3d3=2 peak height in (a). Other details are the same as in (a).
one at 282.2 eV to an unscreened RuO2 [40]. On the other hand, the intensity centered at 282.8 eV may be due to RuO3 , as a defective structure of RuO2
with ``multiple layer'' thickness [36]. The existence of RuO3 was also indicated by Ru 3d and O 1s XPS [36,37]. Weaver et al. have observed the presence of
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Fig. 2. (a) X-ray photoelectron spectrum of ruthenium on a Pt(1 1 1) electrode after spontaneous deposition from 0.5 mM RuCl3 solution in 0.1 M HClO4 (for 1 min). Peak ®tting and other spectral details as in Fig. 1 and in the text. (b) As in (a), followed by voltammetric sweep to the negative electrode potential (0.30 V) and held at 0.30 V for 1 min. Vertical dashed lines highlight all 3d5=2 states, including that for the metallic ruthenium.
RuO3 by the use of SERS, as mentioned above [20]. SERS detected a band at 800 cm 1 , as well as the bands of RuO2 at 510 and 630 cm 1 , and RuO4 at 875 cm 1 , indicating a presence of an intermediate oxidation state between Ru(IV) and Ru(VIII). Some other groups associated the peak at 282.8 eV to hydroxide formation on RuO2 , which should yield the XPS intensity at 280.9 eV [17,18]. Notably, the data in Fig. 3a demonstrate that ruthenium adspecies deposited from nitrosyl-
nitrate media at 0.30 V contains predominantly metallic ruthenium, and that at higher potentials a progressive oxidation of ruthenium is observed (see data for 0.40±0.60 V in Fig. 3(b)±(d). From all the measurements it appears that ruthenium oxides present at the surface at relatively high potentials, say above 0.4 V, can be reduced back to metallic ruthenium at lower potentials, and vice versa. Basically the same surface ruthenium reduction/oxidation pattern as in Fig. 3 was ob-
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prepared in this study. Spectral assignments for ruthenium oxides are basically in agreement with spectral data presented by other authors, as discussed above. 3.2. Methanol oxidation reactivity Fig. 4 shows a block diagram for methanol oxidation currents (current densities) at the Pt(1 1 1) and Pt(1 1 1)/Ru surfaces at various electrode potentials measured after 20 min of the oxidation reaction [5]. At all potentials, the Pt(1 1 1)/ Ru surface displays higher currents than the clean Pt(1 1 1) surface, and the current increases as the electrode potential increases. However, the ruthenium enhancement (the ratio of the current density corresponding to methanol oxidation with and without ruthenium on the Pt(1 1 1) surface [23]) signi®cantly decreases between 0.4 and 0.5 V, and remains stable at a relatively low level at higher potentials (Table 2). This is the key reactivity result that will be discussed below in the context of the chemical state data obtained by XPS (Fig. 3).
4. Discussion
Fig. 3. (a) X-ray photoelectron spectrum of the Ru 3d region after deposition from 2 mM Ru(NO)(NO3 )3 in 0.5 M H2 SO4 at 0.30 V for 1 min. (b) As in (a), but following the deposition at 0.30 V the electrode potential was stepped to 0.40 V for 1 min. (c) As in (a) following the deposition at 0.30 V the electrode potential was stepped to 0.50 V for 1 min. (d) As in (a) following the deposition at 0.30 V the electrode potential was stepped to 0.60 V for 1 min. (No further change was found in the range from 0.60 to 0.9 V.)
tained with Pt(1 1 1)/Ru samples obtained by spontaneously deposited ruthenium from chloride media. Table 1 shows binding energies for various oxidation states of ruthenium deposited on the Pt(1 1 1) surface. The binding energy of submonolayer amounts of metallic ruthenium is consistent both with the literature data [16,17,20,36] (see also Table 1) and with the thick ®lm Ru standard
Our data (Fig. 4 and Table 2, also Ref. [41]) indicate that the Pt(1 1 1)/Ru surface is markedly more active than a clean Pt(1 1 1) surface only in the potential range negative of a threshold value located at the beginning of the double layer range for the platinum electrode. For instance, the enhancement is very large at and below 0.4 V, and quite small at higher potentials. We now look at the distribution of ruthenium valencies as the electrode potential varies from 0.30 to 0.60 V (Fig. 3). Clearly, the contribution of the elemental ruthenium adphase to the overall surface population decreases signi®cantly as the potential increases. The decrease in the presence of metallic ruthenium in UHV with the potential is concomitant with the increase in surface coverage of RuO2 and, to some extent, by RuO3 . Since a reverse is true for methanol reactivity (Fig. 4); (that is, the magnitude of the catalytic enhancement decreases with potential, we conclude that the presence of Ru surface
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Table 1 Experimental binding energies for oxidation states of ruthenium added to a Pt(1 1 1) surface by electrolysis and spontaneous deposition (®rst two rows under spin±orbit notations)
Electrolysis Spontaneous deposition Literature energies
Ru 3d5=2 =eV
Ru 3d3=2 =eV
RuO2 3d5=2 =eV
RuO2 3d3=2 =eV
RuO3 3d5=2 =eV
RuO3 3d3=2 =eV
280.3 280.3
284.5 284.5
280.9 281.1
285.1 285.3
282.8 283.0
287.0 287.2
279.9±280.3a±f
284.0±284.5a±f
280.7±281.0a±f
284.9±285.2a±f
282.4±282.6a±e
286.6±286.8a±e
The binding energies were obtained from nonlinear least-square curve-®tting±spectral deconvolutions, see text. For comparison, binding energy intervals from literature for speci®c spin±orbits are given in the last row. a Ref. [36]. b Ref. [16]. c Ref. [10]. d Ref. [20]. e Ref. [46]. f Ref. [18].
Fig. 4. Block diagrams of current density due to methanol oxidation under chronoamperometric conditions at dierent potentials for Pt(111)/clean and Pt(111)/Ru (after 2 min of deposition). Current density measured after 10 min of polarization in 0.1 M HClO4 0:1 M CH3 OH at 25°C.
species in situ, which appears in UHV as metallic ruthenium is a prerequisite for eective Pt/Ru methanol electrooxidation catalysis. From the observation that when ruthenium oxides partially
replace metallic ruthenium the surface activity is reduced, we conclude that the ruthenium oxides act as a catalytic de-enhancer rather than as a promoter. Notably, the exposure of the Pt/Ru surface to a methanol containing solution at a ®xed potential causes some ruthenium oxide reduction, and promotes the appearance of more metallic ruthenium in UHV (not shown, see also Ref. [11]). This most likely re¯ects the physical phenomena that occur when Pt/Ru catalysts are ``activated'' in real-world methanol oxidation fuel cell devices. The quantitative correlation between the surface and reactivity data between Figs. 3 and 4 should be highlighted. Metallic ruthenium is present in the full potential range causing the Pt(1 1 1)/Ru surface to be overall more active than clean platinum. However, increasing the amount of ruthenium oxides is prohibitive for sustaining the high catalytic enhancement. At this point it remains unknown whether the de-enhancement is due to steric reasons (blocking access of surface CO to the active Ru-metal island edges on the Pt substrate) and/or due to electronic deactivation of the Pt(1 1 1)/Ru catalyst by the oxides.
Table 2 Ruthenium enhancement factors calculated from data in Fig. 4 (see caption of Fig. 4) Potential/V (vs. RHE)
0.30
0.40
0.50
0.60
0.70
Ru enhancement
1
81
5.9
3.4
5.4
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The experiment conducted here is a typical ex situ experiment in which a substrate is transferred to UHV after electrochemical characterization and/ or surface modi®cation by electrochemical deposition. The strength of this approach ± besides the access to UHV surface science techniques [5] ± is in its capability of discriminating between weakly and chemically bonded species on the surface. That is, when metallic ruthenium appears in UHV, it indicates that there is no chemically bound oxygen to ruthenium in UHV. However, the experiment reveals very little about the species that are weakly interacting with the surface in situ, such as surface water, or some other weakly bound, water derived surface products, collectively termed here (and below) as activated water. Notably, while molecular water desorbs from Ru(1 1 1) below 220 K, and from Pt(1 1 1) to UHV below 170 K, it interacts chemically with the two surfaces [42]. It has already been postulated that water chemisorbed on a pure platinum catalyst at suciently high electrode potentials, (Pt(H2 O)act ), is the source of oxygen needed for catalytic transformation of surface CO to carbon dioxide [24]. Accordingly, we may reformulate the bifunctional mechanism for enhanced methanol electrooxidation [43], and propose that the transformation of surface CO to CO2 on Pt(1 1 1)/Ru occurs through the activated water molecules present on the metallic ruthenium adphase, and that such water serves as the oxygen donor, for instance: Pt±CO Ru H2 Oact ! Pt Ru CO2 2H 2e
1
Reaction (1) is followed by methanol decomposition on the free Pt sites to create new surface bound CO molecules, which results in sustaining the methanol steady-state oxidation cycle, as frequently reported [1].
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by the results of recently published investigations of a commercial highly dispersed (nanoscale) Pt/ Ru alloy catalyst [9], which also showed that the gain in the methanol oxidation activity could be correlated to the increase in preponderance of metallic ruthenium. However, even after hydrogen evolution from such highly dispersed alloy surfaces, a noticeable amount of ruthenium oxides remained in the surface zone of the catalyst (within the mean free path of photoelectrons), together with the metallic ruthenium phase. In contrast, a smooth bulk Pt/Ru alloy surface is prone to a much more facilitated reduction to the elemental metallic constituent [44]. Therefore, at this point, we conclude that the nanoparticle Pt/Ru alloy composite, once oxidized (e.g. during the catalyst manufacturing process) resists reduction to a larger extent that the bulk Pt/Ru alloy does (see Fig. 2 in Ref. [44]). This dierence is an important issue that deserves further attention. In additional support of this work, a signi®cant amount of metallic ruthenium was identi®ed in the commercial Pt/Ru alloy catalyst by the use of CO stripping voltammetry [45]. It was therefore concluded that ``an important prerequisite for higher direct methanol oxidation fuel cell anodic activity of such catalysts is to achieve a catalyst surface of highly metallic character''. While some ruthenium oxides may remain in the surface zone of the Pt/Ru catalyst under operating fuel cell conditions [19], our data put primary emphasis on the active role of the metallic ruthenium catalyst in the overall methanol oxidation reaction. Namely, when ruthenium is deposited (this work) or alloyed with platinum [44], the methanolic C±H bond breaks on the Pt surface, water is activated on the surface of metallic ruthenium, and CO is oxidized at the edge of ruthenium islands [8].
Acknowledgements 5. Conclusions Our data indicate that the presence of metallic ruthenium is essential for eective Pt/Ru methanol electrooxidation catalysis. The data are supported
The authors thank Ms. Svetlana Mitrovski for her technical assistance. This work is supported by the National Science Foundation under grant no. CHE 9985837, and by the Department of Energy grant DEFG02-99ER14993. Dr. I. Rabelo de
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Moraes gratefully acknowledges FAPESP±Brazil for a postdoctoral fellowship. References [1] A. Hamnett, in: A. Wieckowski (Ed.), Interfacial Electrochemistry: Experimental, Theory and Applications, Section: Electrocatalysis, Marcel Dekker, New York, 1999, p. 843. [2] X.M. Ren, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 143 (1996) L12. [3] S.J.C. Cleghorn, X. Ren, T.E. Springer, M.S. Wilson, C. Zawodzinski, T.A. Zawodzinski, S. Gottesfeld, Int. J. Hydrogen Energy 22 (1997) 1137. [4] B.D. McNicol, D.A.J. Rand, K.R. Williams, J. Pow. Sour. 83 (1999) 15. [5] W. Chrzanowski, A. Wieckowski, in: A. Wieckowski (Ed.), Interfacial Electrochemistry: Experimental, Theory and Applications, Section: Electrocatalysis, Marcel Dekker, New York, 1999, p. 937. [6] T.D. Jarvi, T.H. Madden, E.M. Stuve, Electrochem. SolidState Lett. 2 (1999) 224. [7] W. Chrzanowski, A. Wieckowski, Langmuir 14 (1998) 1967. [8] E. Herrero-Rodriguez, J.M. Feliu, A. Wieckowski, Langmuir 15 (1999) 4944. [9] A. Crown, H. Kim, G.Q. Lu, I. Moraes, C. Rice, A. Wieckowski, J. New Mat. Electrochem. Systems 3 (2000) 275. [10] A. Hamnett, B.J. Kennedy, F.E. Wagner, J. Catal. 124 (1990) 30. [11] J.B. Goodenough, A. Hamnett, B.J. Kennedy, R. Manoharan, S.A. Weeks, J. Electroanal. Chem. 240 (1988) 133. [12] A. Hamnett, B.J. Kennedy, Electrochim. Acta 33 (1988) 1613. [13] C.T. Hable, M.S. Wrighton, Langmuir 9 (1993) 3284. [14] S. Cramm, K.A. Friedrich, K.-P. Geyzers, U. Stimming, R. Vogel, Fresenius J. Anal. Chem. 358 (1997) 189. [15] K.A. Friedrich, K.-P. Geyzers, A. Marmann, U. Stimming, R. Vogel, Z. Phys. Chemie 208 (1999) 137. [16] R. K otz, H.J. Lewerenz, S. Stucki, J. Electrochem. Soc. 130 (1983) 825. [17] L. Hagans, K.W. Swider, D.R. Rolison, in: J. McBreen, S. Srinivasan (Eds.), Electrode Materials and Processes for Energy Conversion and Storage IV, PV 97-13, Electrochemical Society, Pennington, NJ, 1997, p. 86. [18] R. Rolison, P.L. Hagans, K.E. Swider, J.W. Long, Langmuir 15 (1999) 774. [19] J.W. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, J. Phys. Chem. B 104 (2000) 9772. [20] H.Y.H. Chan, C.G. Takoudis, M.J. Weaver, J. Catal. 172 (1997) 336.
[21] J. McBreen, S. Mukerjee, J. Electrochem. Soc. 142 (1995) 3399. [22] W. Chrzanowski, A. Wieckowski, Langmuir 13 (1997) 5974. [23] W. Chrzanowski, H. Kim, G. Tremiliosi-Filho, A. Wieckowski, B. Grzybowska, P. Kulesza, J. New Mat. Electrochem. Systems 1 (1998) 31. [24] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Phys. Chem. 98 (1994) 5074. [25] J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [26] E. Herrero, W. Chrzanowski, A. Wieckowski, J. Phys. Chem. 99 (1995) 10423. [27] G. Tremiliosi-Filho, H. Kim, W. Chrzanowski, A. Wieckowski, B. Grzybowska, P. Kulesza, J. Electroanal. Chem. 467 (1999) 143. [28] M. Han, P. Mrozek, A. Wieckowski, Phys. Rev. B 48 (1993) 8329. [29] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Eden Prairie, Minnesota, 1995. [30] S. Doniach, M. Sunjic, J. Phys. C: Solid St. Phys. 3 (1970) 285. [31] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [32] N. Martensson, R. Nyholm, Phys. Rev. B 24 (1981) 7121. [33] N. De Zoubov, M. Pourbaix, in: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutions, Section 13.1, National Association of Corrosion Engineers, Houston, 1974, p. 343. [34] W. Chrzanowski, H. Kim, A. Wieckowski, Cat. Lett. 50 (1998) 69. [35] T. Iwasita, H. Hoster, A. John-Anacker, W.F. Lin, W. Vielstich, Langmuir 16 (2000) 522. [36] K.S. Kim, N. Winograd, J. Catal. 35 (1974) 66. [37] Lj. Atanasoska, R.T. Atanasoski, F.H. Pollak, W.E. OÕGrady, Surf. Sci. 230 (1990) 95. [38] C. Malitesta, G. Morea, L. Sabbatini, P.G. Zambonin, Ann. Chim. (Rome) 78 (1988) 473. [39] H.J. Lewerenz, S. Stucki, R. K otz, Surf. Sci. 126 (1983) 463. [40] P.A. Cox, J.B. Goodenough, P.J. Tavener, D. Telles, R.G. Egdell, J. Solid State Chem. 62 (1986) 360. [41] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Electroanal. Chem. 361 (1993) 269. [42] P.A. Thiel, T.E. Madey, Surf. Sci. Rep. 7 (1987) 211, see page 264. [43] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 275. [44] R. Liu, H. Iddir, Q. Fan, G. Hou, A. Bo, K.L. Ley, E.S. Smotkin, Y. Sung, H. Kim, S. Thomas, A. Wieckowski, J. Phys. Chem. B 104 (2000) 3518. [45] H.N. Dinh, X. Ren, F.H. Garzon, P. Zelenay, S. Gottesfeld, J. Electroanal. Chem. in press. [46] Y.J. Kim, Y. Gao, S.A. Chambers, Appl. Surf. Sci. 120 (1997) 250.
www.elsevier.nl/locate/susc
Surface Science Letters
Chemical state of ruthenium submonolayers on a Pt(1 1 1) electrode H. Kim a, I. Rabelo de Moraes a, G. Tremiliosi-Filho b, R. Haasch c, A. Wieckowski a,* a
Department of Chemistry, Fritz Seitz Materials Research Laboratory, School of Chemical Sciences, University of Illinois, 600 Mathews Avenue, Urbana, IL 61801, USA b Instituto de Quõmica de S~ ao Carlos/USP, 13560-970 S~ ao Carlos, SP, Brazil c Material Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 5 October 2000; accepted for publication 2 November 2000
Abstract Oxidation states of ruthenium on a Pt(1 1 1)/Ru electrode were investigated by X-ray photoelectron spectroscopy. Ruthenium was added to platinum by electrochemical deposition methods: spontaneous deposition and electrolysis. Depending on the electrode potential, deposition conditions, and presence/absence of methanol in solution, metallic ruthenium (3d5=2 core-level binding energy of 280.3 eV), RuO2 (280.9 eV), and RuO3 (282.8 eV) were found on the surface. After correlating ruthenium valence states to methanol oxidation reactivity, we concluded that the presence of a Ru metallic phase ± covered by a weakly bonded Ru oxidation precursor ± was a prerequisite for eective methanol oxidation electrocatalysis. This precursor was most likely ``activated'' water supplying the oxygen needed for transformation of surface CO to CO2 at the edge of ruthenium islands on the Pt substrate. Ó 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Alcohols; Oxidation; Electrochemical methods; Catalysis; Platinum; Ruthenium; Single crystal surfaces; X-ray photoelectron spectroscopy
1. Introduction This study is related to methanol electrooxidation catalysis in a direct oxidation methanol fuel cell (DMFC) [1]. The best currently used DMFC anode is obtained by deposition of carbon sup-
* Corresponding author. Tel.: +217-333-7943; fax: +217-2448068. E-mail address: [email protected] (A. Wieckowski).
ported (or unsupported) platinum/ruthenium nanoparticles on a proton exchange membrane which serves as a solid polymer electrolyte of the cell [2±4]. Ruthenium present on the electrode surface signi®cantly promotes the methanol oxidation reaction, and the nature of the promotion mechanism is under intense scrutiny [5]. Laboratory model surfaces for the practical Pt/Ru catalyst are platinum electrodes containing controlled amounts of ruthenium added either by electrodeposition [5] or vacuum deposition [6]. If the support is a platinum single crystal surface of speci®c
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 0 5 5 - 4
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surface crystallography, the catalytic activity of such Pt/Ru surfaces can be tested as a function of both surface composition and surface structure [7]. Previous data indicate that among low index Pt(hkl) substrates containing ``optimized'' amounts of surface ruthenium [7] the highest catalytic activity is displayed with Pt(1 1 1)/Ru. It was also found that ruthenium was distributed as a network of nano-dispersed islands, and methanol oxidation occurred at the edge of such islands (Refs. [8,9] and references therein). Despite major progress in our knowledge about methanol oxidation electrocatalysis on Pt/Ru surfaces, outstanding and yet unresolved issue is the chemical state of ruthenium involved in methanol oxidation promotion. In previous work, carbon supported (or unsupported) Pt/Ru electrodes were investigated using X-ray photoelectron spectroscopy (XPS) [10±13]. Ruthenium oxides were usually found as the main catalyst components [11, 12]; however, metallic ruthenium (at Ru 3d5=2 at 280.2 eV) appeared after the electrode was used for methanol oxidation for an extended period of time [11]. In a series of experiments [14,15] using Ru 3p3=2 intensity at 461.4 eV [16], the ruthenium species electrodeposited on platinum at a submonolayer coverage was reported to be purely metallic in a broad electrode potential range (from 0.2 to 0.9 V vs. reversible hydrogen electrode (RHE)). However, in commercial nanoparticle Pt/ Ru alloy catalysts more recent studies [17±19] indicate that the fraction of ruthenium metal (at 280.1 eV) is small, and the prevailing near-surface ruthenium component is anhydrous RuO2 at 281.2 eV (Ru 3d5=2 ). A slightly up-shifted feature at the binding energy above 282 eV was assigned to hydrous ruthenium oxide (RuO2 x H2 O). Our work with a commercial Johnson±Matthey catalyst [9] supports the experimental data [17,18], although more metallic ruthenium was observed, re¯ecting most probably dierences in sample composition provided by fuel cell catalyst manufacturers. In situ measurements by the use of surface-enhanced Raman spectroscopy (SERS) [20] were indicative of the presence of RuO2 and RuO3 as well as RuO4 , and X-ray absorption near-edge structure data indicate that addition of ruthenium to platinum increases d-vacancies of the Pt substrate, and
ruthenium appears oxidized in the double layer and hydrogen range of electrode potentials [21]. Since a broadly acknowledged conclusion has not been reached concerning the oxidation state(s) of ruthenium in Pt/Ru mixed metal electrode systems, we are interrogating ruthenium electrodeposited on a Pt(1 1 1) substrate by the use of XPS. The XPS instrument hosts an electrochemical cell, and sample exposure to air is avoided the electrode transfer between the cell and the ultrahigh vacuum (UHV) chamber [5]. Controlled amounts of ruthenium were added by spontaneous deposition processes [22], or by electrolysis [7]. Studies of methanol oxidation were conducted by chronoamperometry [7,23]; and a Pt/Ru catalyst preparation was characterized by cyclic voltammetry [23]. 2. Experimental 2.1. Electrochemical conditions for deposition of ruthenium on Pt(1 1 1) The real surface area of the Pt(1 1 1) electrode was obtained using hydrogen adsorption±desorption charges from cyclic voltammograms [24±26]. Spontaneous deposition was carried out in 0.5 mM RuCl3 x H2 O in 0.1 M HClO4 for 2 min [7,22] at open circuit, roughly 0.90 V. (All potentials were measured against Ag/AgCl reference, and are reported versus the (RHE).) The Pt(1 1 1)/Ru surface was removed from the deposition bath, transferred to a cell containing the clean supporting electrolyte, and treated by two cyclic voltammetric scans from 0.00 to 0.88 V in order to break the chemisorbed Ru-containing anions to simpler fragments (see further), and to con®rm the cleanness and crystallographic orientation of the mixed metal surface [5,7]. This procedure yields surfaces with 0.20 ML Ru coverage (the coverage, ML, is a packing density, the number of Ru adatoms to the number of platinum sites on the Pt(1 1 1) substrate). In parallel experiments, constant potential electrolysis was conducted at 0.30 V from a 2 mM Ru(NO)(NO3 )3 (ruthenium nitrosylnitrate) solution in 0.5 M H2 SO4 for 1 min to yield a ruthenium coverage of 0.20 ML [23,27]. The coverage value of 0.20 ML is the optimum coverage for methanol
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oxidation on Pt(1 1 1)/Ru surfaces, as demonstrated before [5,8]. 2.2. Measurements in ultrahigh vacuum A disk-shaped platinum crystal oriented in the (1 1 1) direction was mounted inside the UHV chamber. The base pressure of the system was 10 10 Torr. The surface was cleaned by Ar bombardment and vacuum annealed at 800°C in the presence of oxygen (10 8 Torr), and was characterized by low energy electron diraction (LEED) and Auger electron spectroscopy (AES). Details of the LEED/AES measurements and UHV experimental conditions have been reported previously [5]. For the XPS work, a second UHVelectrochemistry system [28] was equipped with an ESCA M-Probe high resolution, multi-channel hemispherical electron analyzer (Surface Science Instruments). Samples were excited using a monochromatic AlKa source hm 1486:6 eV operated at 110 W. The analysis was carried out under ®xed analyzer transmission (constant pass energy) mode, with a constant pass energy of 25 eV. The size of the incident X-ray beam was 800 lm, and the resolution of the detector was 0.027 eV. Wide range XPS spectra (from 30 to 540 eV), and narrow range, high-resolution spectra (from 273 to 293 eV) were obtained from the Pt surfaces and Pt/ Ru surfaces (transferred to UHV from electrolytic media, see below) with the Pt 4f7=2 core-level energy of 71.2 eV selected as the binding energy reference. For nonlinear least-square curve ®tting± spectral deconvolution procedures, an S-Probe version of the 1.36 ESCA software (Fison Instruments) was used. Spectral peaks were ®tted by using mixed Gaussian±Lorentzian lineshape and Shirley baselines [29±31]. For electrochemical experiments in the UHV instrument [5], the antechamber was ®lled with ultrahigh purity argon (99.999%) to atmospheric pressure, and ruthenium deposition was carried out in an electrochemical cell attached to the antechamber. The recently reviewed procedure [5] was applied where an exposure of the crystal to air at any phase the experiment was stringently avoided. A predetermined electrode potential was applied upon the electrode removal from solution
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and transfer to UHV. The Ru deposits obtained by either spontaneous deposition or by electrolysis were characterized by XPS, and ruthenium chemical states obtained under various electrochemical conditions are reported. 2.3. Methanol oxidation rates measured by chronoamperometry Methanol oxidation measurements were carried out in a freestanding electrochemical cell previously reported [5,27]. A platinum single crystal, Pt(1 1 1) of 0.2 cm in diameter, was used as a working electrode in a meniscus con®guration. The electrode was annealed in hydrogen±air ¯ame at 800°C for 2 min, cooled to a moderate temperature, and immersed in Millipore water [5,7, 22,23,25]. The electrode was next transferred to the cell, with the surface protected by a water drop adhering to the surface. After ruthenium deposition [7], the electrode was removed from the solution, rinsed with Millipore water, treated by two voltammetric cycles in the potential range speci®ed above (in 0.1 M H2 SO4 ). Next, the electrode was transferred to a cell containing a methanol solution (0.1 M H2 SO4 0.1 M CH3 OH). The methanol oxidation reaction was conducted on the Pt(1 1 1)/Ru electrode, under chronoamperometric conditions [7]. Immediately before recording the oxidation current at a chosen potential, the electrode was held for 1 s at a potential 100 mV more positive than the onset of hydrogen evolution. The chronoamperometric current for methanol oxidation was measured between 0.40 and 0.70 V (vs. RHE) for 20 min. When the experiment with a given Pt/Ru surfaces ended, ruthenium was removed from platinum by several negative- and positive-going scans at 100 mV s 1 in the potential range determined by the onset of hydrogen and oxygen evolution. 2.4. Chemicals and other conditions The reagents used were Millipore water (18 MX cm), HClO4 and H2 SO4 distilled from Vycor (GFS Chemicals), Ru(NO)(NO3 )3 , RuCl3 x H2 O from Aldrich, and CH3 OH (Spectrophotometric Grade, Mallinckrodt Spectrar). The electrode
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potential was controlled via a PAR Model 283 potentiostat (EG&G Princeton Applied Research). All measurements were carried out at room temperature. 3. Results 3.1. X-ray photoelectron spectroscopy measurements Ruthenium standard for XPS measurements for all Pt/Ru samples was a multi-layer ®lm of ruthenium obtained by electrolysis from a ruthenium nitrosylnitrate solution at a potential slightly more negative than that for hydrogen evolution. After 0.5 h of electrodeposition, a thick Ru ®lm was formed that virtually eliminated XPS platinum intensities (following the sample transfer from the cell to UHV, as described above). Peak ®tting of such a standard reveals a spin±orbit-split doublet of Ru 3d5=2 and Ru 3d3=2 at 280.3 eV (0.1 eV) and 284.5 eV (0.1 eV), respectively (Fig. 1a), which are characteristic of metallic ruthenium [29]. The peak area ratio between Ru 3d5=2 and Ru 3d3=2 was 3:2, which closely corresponds to the theoretical value due to spin±orbit interactions of delectrons. The spin±orbit splitting of 4.18 eV also coincides with the theoretical value. A full width at half maximum of the 3d5=2 component was 1.0 eV (0.05 eV), which is smaller than that of Ru 3d3=2 by 0.43 eV due to the Coster-Kronig decay eect [32]. The curve-®tting parameters were used as ®xed values for the XPS studies of the submonolayer ruthenium adsorbates discussed below. The same Gaussian±Lorentzian natural line width and asymmetry parameters were applied, and the deviation of ®tting parameters was allowed within 95% of the con®dence limit, v2 . The ®rst spectrum obtained at a submonolayer ruthenium coverage is shown in Fig. 1b. Approximately 0.20 ML of ruthenium was added to platinum by electrolysis at 0.30 V for 1 min in a 0.5 mM RuCl3 0:1 M HClO4 solution in. The electrode and the cell were rinsed with Millipore water to eliminate all bulk ruthenium species from the cell. The electrode potential was then stepped to 50 mV (for 1 min) located in the hydrogen under-
potential deposition range, near the threshold of hydrogen evolution. The bands at 280.3 and 284.5 eV correspond to metallic ruthenium (cf. with Fig. 1a), which is in accordance with Pourbaix diagrams predications [33]. This demonstrates that a submonolayer metallic ruthenium deposited on platinum from aqueous media can be transferred to UHV without oxidation by water and mineral acid vapors even if the electrode potential control is terminated upon the sample removal from the cell. This observation is a benchmark for further studies reported below. Another set of data was obtained using the spontaneous deposition process from chloride media. As mentioned in the experimental section, the ruthenium coverage under such deposition conditions is also 0.20 ML, and the deposition open circuit potential (OCP) is close to 0.90 V [22]. A typical Ru core-level 3d photoelectron spectrum from such Pt(1 1 1)/Ru samples removed from the electrolyte at the OCP is shown in Fig. 2a. Peak ®tting of the Ru 3d region reveals three spin±orbit doublets, showing that the composition of the deposit is not uniform. The sample contains highvalency ruthenium 3d5=2 states: RuO2 (281.0 eV), and RuO3 (283.0 eV), but also metallic ruthenium (280.3 eV). (3d3=2 peaks, as weaker intensities, are not discussed). Auger electron spectroscopic analyses conducted under conditions very similar (or identical) to those reported in this article demonstrate a large amount of surface oxygen present [34], and the XPS data shown in Fig. 2a identify the oxidized species as ruthenium oxides, or hydroxides. However, these spontaneously formed ruthenium oxo-species and/or Ru-containing chemisorbed anions [35] decompose to simpler Ru forms under voltammetric conditions (see Section 2), or under potentiostatic electrode treatment at suciently negative potentials, as shown in Fig. 2b. Three vertical dashed lines highlight the 3d5=2 states, including that for metallic ruthenium on the surface. Assignment of ruthenium oxidation states to the binding energies around 282 eV (in Fig. 2) is not straightforward [17,18,20,36±40]. Cox et al. concluded the peak at 282.2 eV could be assigned to RuO2 with the binding energy shift induced by ®nal-state screening by Ru 3d electrons, and the
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Fig. 1. (a) X-ray photoelectron spectrum of a thick, multi-layer ruthenium ®lm obtained by electrolysis from 2 mM Ru(NO)(NO3 )3 solution in 0.1 M H2 SO4 on a platinum substrate, and at 50 mV vs. RHE. Peak deconvolution of the Ru 3d core-level energy region yields two ruthenium intensities at 280.3 eV (3d5=2 ) and 284.4 eV (3d3=2 ), and a small graphitic carbon intensity at 284.5 eV (1s). (b) Xray photoelectron spectrum of a submonolayer of ruthenium deposited on a Pt(1 1 1) electrode from 0.5 mM RuCl3 solution in 0.1 M HClO4 at 0.30 V, for 1 min. Before transfer to UHV, the electrode was held in 0.1 M HClO4 at 50 mV for 30 min. The intensity is normalized to the 3d3=2 peak height in (a). Other details are the same as in (a).
one at 282.2 eV to an unscreened RuO2 [40]. On the other hand, the intensity centered at 282.8 eV may be due to RuO3 , as a defective structure of RuO2
with ``multiple layer'' thickness [36]. The existence of RuO3 was also indicated by Ru 3d and O 1s XPS [36,37]. Weaver et al. have observed the presence of
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Fig. 2. (a) X-ray photoelectron spectrum of ruthenium on a Pt(1 1 1) electrode after spontaneous deposition from 0.5 mM RuCl3 solution in 0.1 M HClO4 (for 1 min). Peak ®tting and other spectral details as in Fig. 1 and in the text. (b) As in (a), followed by voltammetric sweep to the negative electrode potential (0.30 V) and held at 0.30 V for 1 min. Vertical dashed lines highlight all 3d5=2 states, including that for the metallic ruthenium.
RuO3 by the use of SERS, as mentioned above [20]. SERS detected a band at 800 cm 1 , as well as the bands of RuO2 at 510 and 630 cm 1 , and RuO4 at 875 cm 1 , indicating a presence of an intermediate oxidation state between Ru(IV) and Ru(VIII). Some other groups associated the peak at 282.8 eV to hydroxide formation on RuO2 , which should yield the XPS intensity at 280.9 eV [17,18]. Notably, the data in Fig. 3a demonstrate that ruthenium adspecies deposited from nitrosyl-
nitrate media at 0.30 V contains predominantly metallic ruthenium, and that at higher potentials a progressive oxidation of ruthenium is observed (see data for 0.40±0.60 V in Fig. 3(b)±(d). From all the measurements it appears that ruthenium oxides present at the surface at relatively high potentials, say above 0.4 V, can be reduced back to metallic ruthenium at lower potentials, and vice versa. Basically the same surface ruthenium reduction/oxidation pattern as in Fig. 3 was ob-
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prepared in this study. Spectral assignments for ruthenium oxides are basically in agreement with spectral data presented by other authors, as discussed above. 3.2. Methanol oxidation reactivity Fig. 4 shows a block diagram for methanol oxidation currents (current densities) at the Pt(1 1 1) and Pt(1 1 1)/Ru surfaces at various electrode potentials measured after 20 min of the oxidation reaction [5]. At all potentials, the Pt(1 1 1)/ Ru surface displays higher currents than the clean Pt(1 1 1) surface, and the current increases as the electrode potential increases. However, the ruthenium enhancement (the ratio of the current density corresponding to methanol oxidation with and without ruthenium on the Pt(1 1 1) surface [23]) signi®cantly decreases between 0.4 and 0.5 V, and remains stable at a relatively low level at higher potentials (Table 2). This is the key reactivity result that will be discussed below in the context of the chemical state data obtained by XPS (Fig. 3).
4. Discussion
Fig. 3. (a) X-ray photoelectron spectrum of the Ru 3d region after deposition from 2 mM Ru(NO)(NO3 )3 in 0.5 M H2 SO4 at 0.30 V for 1 min. (b) As in (a), but following the deposition at 0.30 V the electrode potential was stepped to 0.40 V for 1 min. (c) As in (a) following the deposition at 0.30 V the electrode potential was stepped to 0.50 V for 1 min. (d) As in (a) following the deposition at 0.30 V the electrode potential was stepped to 0.60 V for 1 min. (No further change was found in the range from 0.60 to 0.9 V.)
tained with Pt(1 1 1)/Ru samples obtained by spontaneously deposited ruthenium from chloride media. Table 1 shows binding energies for various oxidation states of ruthenium deposited on the Pt(1 1 1) surface. The binding energy of submonolayer amounts of metallic ruthenium is consistent both with the literature data [16,17,20,36] (see also Table 1) and with the thick ®lm Ru standard
Our data (Fig. 4 and Table 2, also Ref. [41]) indicate that the Pt(1 1 1)/Ru surface is markedly more active than a clean Pt(1 1 1) surface only in the potential range negative of a threshold value located at the beginning of the double layer range for the platinum electrode. For instance, the enhancement is very large at and below 0.4 V, and quite small at higher potentials. We now look at the distribution of ruthenium valencies as the electrode potential varies from 0.30 to 0.60 V (Fig. 3). Clearly, the contribution of the elemental ruthenium adphase to the overall surface population decreases signi®cantly as the potential increases. The decrease in the presence of metallic ruthenium in UHV with the potential is concomitant with the increase in surface coverage of RuO2 and, to some extent, by RuO3 . Since a reverse is true for methanol reactivity (Fig. 4); (that is, the magnitude of the catalytic enhancement decreases with potential, we conclude that the presence of Ru surface
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Table 1 Experimental binding energies for oxidation states of ruthenium added to a Pt(1 1 1) surface by electrolysis and spontaneous deposition (®rst two rows under spin±orbit notations)
Electrolysis Spontaneous deposition Literature energies
Ru 3d5=2 =eV
Ru 3d3=2 =eV
RuO2 3d5=2 =eV
RuO2 3d3=2 =eV
RuO3 3d5=2 =eV
RuO3 3d3=2 =eV
280.3 280.3
284.5 284.5
280.9 281.1
285.1 285.3
282.8 283.0
287.0 287.2
279.9±280.3a±f
284.0±284.5a±f
280.7±281.0a±f
284.9±285.2a±f
282.4±282.6a±e
286.6±286.8a±e
The binding energies were obtained from nonlinear least-square curve-®tting±spectral deconvolutions, see text. For comparison, binding energy intervals from literature for speci®c spin±orbits are given in the last row. a Ref. [36]. b Ref. [16]. c Ref. [10]. d Ref. [20]. e Ref. [46]. f Ref. [18].
Fig. 4. Block diagrams of current density due to methanol oxidation under chronoamperometric conditions at dierent potentials for Pt(111)/clean and Pt(111)/Ru (after 2 min of deposition). Current density measured after 10 min of polarization in 0.1 M HClO4 0:1 M CH3 OH at 25°C.
species in situ, which appears in UHV as metallic ruthenium is a prerequisite for eective Pt/Ru methanol electrooxidation catalysis. From the observation that when ruthenium oxides partially
replace metallic ruthenium the surface activity is reduced, we conclude that the ruthenium oxides act as a catalytic de-enhancer rather than as a promoter. Notably, the exposure of the Pt/Ru surface to a methanol containing solution at a ®xed potential causes some ruthenium oxide reduction, and promotes the appearance of more metallic ruthenium in UHV (not shown, see also Ref. [11]). This most likely re¯ects the physical phenomena that occur when Pt/Ru catalysts are ``activated'' in real-world methanol oxidation fuel cell devices. The quantitative correlation between the surface and reactivity data between Figs. 3 and 4 should be highlighted. Metallic ruthenium is present in the full potential range causing the Pt(1 1 1)/Ru surface to be overall more active than clean platinum. However, increasing the amount of ruthenium oxides is prohibitive for sustaining the high catalytic enhancement. At this point it remains unknown whether the de-enhancement is due to steric reasons (blocking access of surface CO to the active Ru-metal island edges on the Pt substrate) and/or due to electronic deactivation of the Pt(1 1 1)/Ru catalyst by the oxides.
Table 2 Ruthenium enhancement factors calculated from data in Fig. 4 (see caption of Fig. 4) Potential/V (vs. RHE)
0.30
0.40
0.50
0.60
0.70
Ru enhancement
1
81
5.9
3.4
5.4
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The experiment conducted here is a typical ex situ experiment in which a substrate is transferred to UHV after electrochemical characterization and/ or surface modi®cation by electrochemical deposition. The strength of this approach ± besides the access to UHV surface science techniques [5] ± is in its capability of discriminating between weakly and chemically bonded species on the surface. That is, when metallic ruthenium appears in UHV, it indicates that there is no chemically bound oxygen to ruthenium in UHV. However, the experiment reveals very little about the species that are weakly interacting with the surface in situ, such as surface water, or some other weakly bound, water derived surface products, collectively termed here (and below) as activated water. Notably, while molecular water desorbs from Ru(1 1 1) below 220 K, and from Pt(1 1 1) to UHV below 170 K, it interacts chemically with the two surfaces [42]. It has already been postulated that water chemisorbed on a pure platinum catalyst at suciently high electrode potentials, (Pt(H2 O)act ), is the source of oxygen needed for catalytic transformation of surface CO to carbon dioxide [24]. Accordingly, we may reformulate the bifunctional mechanism for enhanced methanol electrooxidation [43], and propose that the transformation of surface CO to CO2 on Pt(1 1 1)/Ru occurs through the activated water molecules present on the metallic ruthenium adphase, and that such water serves as the oxygen donor, for instance: Pt±CO Ru H2 Oact ! Pt Ru CO2 2H 2e
1
Reaction (1) is followed by methanol decomposition on the free Pt sites to create new surface bound CO molecules, which results in sustaining the methanol steady-state oxidation cycle, as frequently reported [1].
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by the results of recently published investigations of a commercial highly dispersed (nanoscale) Pt/ Ru alloy catalyst [9], which also showed that the gain in the methanol oxidation activity could be correlated to the increase in preponderance of metallic ruthenium. However, even after hydrogen evolution from such highly dispersed alloy surfaces, a noticeable amount of ruthenium oxides remained in the surface zone of the catalyst (within the mean free path of photoelectrons), together with the metallic ruthenium phase. In contrast, a smooth bulk Pt/Ru alloy surface is prone to a much more facilitated reduction to the elemental metallic constituent [44]. Therefore, at this point, we conclude that the nanoparticle Pt/Ru alloy composite, once oxidized (e.g. during the catalyst manufacturing process) resists reduction to a larger extent that the bulk Pt/Ru alloy does (see Fig. 2 in Ref. [44]). This dierence is an important issue that deserves further attention. In additional support of this work, a signi®cant amount of metallic ruthenium was identi®ed in the commercial Pt/Ru alloy catalyst by the use of CO stripping voltammetry [45]. It was therefore concluded that ``an important prerequisite for higher direct methanol oxidation fuel cell anodic activity of such catalysts is to achieve a catalyst surface of highly metallic character''. While some ruthenium oxides may remain in the surface zone of the Pt/Ru catalyst under operating fuel cell conditions [19], our data put primary emphasis on the active role of the metallic ruthenium catalyst in the overall methanol oxidation reaction. Namely, when ruthenium is deposited (this work) or alloyed with platinum [44], the methanolic C±H bond breaks on the Pt surface, water is activated on the surface of metallic ruthenium, and CO is oxidized at the edge of ruthenium islands [8].
Acknowledgements 5. Conclusions Our data indicate that the presence of metallic ruthenium is essential for eective Pt/Ru methanol electrooxidation catalysis. The data are supported
The authors thank Ms. Svetlana Mitrovski for her technical assistance. This work is supported by the National Science Foundation under grant no. CHE 9985837, and by the Department of Energy grant DEFG02-99ER14993. Dr. I. Rabelo de
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