Surface Modified Ruthenium Nanoparticles: Structural Investigation

As expected from structural analysis, best results have been obtained with ... and SeO32- decorating the surface of the ruthenium particles is respons...
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J. Phys. Chem. C 2007, 111, 477-487

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Surface Modified Ruthenium Nanoparticles: Structural Investigation and Surface Analysis of a Novel Catalyst for Oxygen Reduction Sebastian Fiechter,* Iris Dorbandt, Peter Bogdanoff, Gerald Zehl, Hendrik Schulenburg, and Helmut Tributsch Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany

Michael Bron Technical UniVersity Darmstadt, Institute of Chemical Technology, Petersenstrasse 20, D-64287 Darmstadt, Germany

Jo1 rg Radnik Leipniz-Institut fu¨r Katalyse, Richard-Willsta¨tter-Strasse 12, D-12489 Berlin, Germany

Martin Fieber-Erdmann BESSY GmbH, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany ReceiVed: March 24, 2006; In Final Form: September 19, 2006

Ruthenium catalysts modified by selenium are of interest as a methanol insensitive oxygen reduction catalyst in a polymer electrolyte membrane fuel cell for mobile application. To elucidate the structural and chemical features of unsupported and carbon supported ruthenium nanoparticles prepared by thermolysis of Ru3(CO)12 in an organic solvent with and without the presence of dissolved selenium, different bulk and surface sensitive methods such as transmission electron microscopy, X-ray diffractometry, thermogravimetry coupled with mass spectrometry, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure analysis were performed. It was found that the as-grown catalytic particles prepared without Se and handled under ambient conditions are distinguished by a ruthenium core of 4 nm size, the surface of which is covered by an amorphous ruthenium oxide/hydroxide and metal organic residues from the process of synthesis. In the presence of Se, Ru-Se and Se-O bondings have additionally been found at the surface. After heat treatment at 900 °C under vacuum, organic residues and ruthenium oxides could be removed. The particles have grown to a size of about 10 nm, the surfaces of which are covered by Ru-Se and Ru-SeO3 units. The as-grown and heat-treated catalysts were characterized electrochemically by cyclovoltammetry, rotating disk electrode with and without methanol in the electrolyte, and rotating ring disk electrode measurements to quantify H2O2 production. As expected from structural analysis, best results have been obtained with heat-treated, Se-modified ruthenium catalysts. It is proposed that in the heat-treated samples an interaction between Se2-, [Se2]2-, and SeO32- decorating the surface of the ruthenium particles is responsible for the improved oxygen reduction process.

1. Introduction In the past decade, it was demonstrated that catalysts based on ruthenium-selenium can be used as cathode material in polymer electrolyte membrane fuel cells (PEM-FC) and direct methanol fuel cells (DMFC) due to a high selectivity in oxygen reduction.1 In contrast to metallic ruthenium that features a reaction via the hydrogen peroxide path,2 this catalyst yields oxygen reduction with a small contribution of peroxide formation of about 4%.3 Therefore, it has been concluded that the surfaces of the ruthenium particles are modified with the consequence of a change of kinetics. It has been shown that also a pure oxide such as RuO2 at the metallic ruthenium surface, which would be encountered at potentials of practical oxygen electrodes (0.7-0.9 V (NHE)), is inactive toward oxygen a Corresponding author. Telephone: +49-30-8062-2927. Fax: +49-038062-2434. E-mail: [email protected].

reduction.4 For this reason, the structure and the chemical composition of the ruthenium nanoparticles of our catalysts have been investigated in detail. Alonso-Vante was the first to describe the effect of selenium on the catalytic activity of ruthenium.5,6 Chatzitheodorou introduced pyrolysis of ruthenium carbonyl in a selenium containing xylene, toluene, or dichlorobenzene solution as preparation technique to yield a dispersions consisting of nanoscale, black particles in an organic solvent.7 Later, the same procedure was also employed by Solorza-Feria et al.1 and Trapp et al.8 In these early papers it was claimed that the catalyst samples possess a homogeneous composition that is in contrast to our findings and recent publications by Dassenoy et al., Alonso-Vante et al., and Malakhov et al.9-11 Employing a series of analytic instruments, the evidence is that the catalyst particles prepared by the same technique have an egg-shell-like structure characterized by a metallic core of 4 nm diameter,12 the surface

10.1021/jp0618431 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006

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of which is modified by selenium and oxygen, as will be shown in the following sections. 2. Experimental Section The catalysts, which were prepared by thermolysis of Ru3(CO)12 in, e.g., boiling xylene at different selenium concentrations with and without carbon support, were investigated by X-ray diffractometry (XRD), transmission electron microscopy (TEM), thermogravimetry coupled with mass spectrometry (TGMS), extended X-ray absorption fine structure (EXAFS) analysis, and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring disc electrode (RRDE) measurements were applied to characterize the catalyst electrochemically. Further details of the preparation procedures were described by Solorza-Feria1 and by Bron.3,18 The reference sample RuO2 was purchased from Alpha; RuSe2 was synthesized from highly pure elements (Alpha) in an evacuated and sealed quartz ampule13 and for reasons of comparison stored in air together with the catalytic materials. The samples were crystallographically analyzed employing a SIEMENS X-ray diffractometer (D500/5000) with Cu KR1 radiation and θ-2θ coupling. To confirm the particle size inferred from full width at half-maximum (FWHM) of the diffraction lines using Scherrer’s equation, TEM pictures were taken from ruthenium particles dispersed on a carbon film using a PHILIPS CM12 electron microscope. To study the thermal stability of our ruthenium catalysts, a 25 mg amount of substance was filled into an Al2O3 sample crucible, which then was placed on a ceramic holder in a furnace and heated under argon gas flow. The furnace was at one side attached to a balance and at the opposite side coupled to a quadrupole mass spectrometer via a skimmer (NETZSCH STA 403). EXAFS spectra were recorded in transmission mode at the X1.1 beam line at HASYLAB, Hamburg, Germany, equipped with a double crystal monochromator (Si (311) crystal). The catalyst samples were mixed with BN powder and pressed to pellets of 1 mm thickness and 10 mm diameter. Measurements were performed at the Ru K- and the Se K-edges, respectively. Subtraction of the background and normalization of the spectra were employed to isolate the EXAFS modulation χ(k) as a function of the photoelectron wave number k. The spectra were Fourier transformed in the k range of 2.5-12 Å-1 to R space. Evaluation and simulation of spectra were performed using the codes ATOMS,14 AUTOBK,15 FEFF,16 and FEFFIT.17 The XPS spectra were performed with a Fisons ESCALAB220 iXL system using monochromatic Al KR radiation. The peak resolution was about 0.5 eV. Due to the high conductivity of the samples charge compensation was not necessary. The binding energies were referred to the carbon peak at 284.7 eV. Binding energies and peak intensities were fitted by means of Lorentzian and Gaussian curves. Cross-section efficiencies were inferred from Scofield factors. The catalytic activity of the samples was determined via CV, RDE, and RRDE measurements. All experiments were performed in a conventional one-compartment electrochemical glass cell. A mercury sulfate electrode served as reference and a platinum wire as counter electrode. The catalyst powder was attached onto the working electrode, consisting of a PTFE surrounded glassy carbon rod with a diameter of 3 mm. A 1 mg amount of the catalyst sample to be characterized was ultrasonically suspended in 200 µL of a 0,2% Nafion solution (Aldrich). A precise amount of 5 µL of this suspension was then transferred onto the glassy carbon electrode and dried in

Figure 1. X-ray diffractogramms of a carbon supported catalyst, prepared by thermolysis of ruthenium carbonyl in a selenium saturated xylene solution in the presence of carbon black soot, as-prepared and after heat treatment at 900 °C under vacuum for 2 h. The vertical lines illustrate the line positions of hexagonal ruthenium (JCPDS no. 06663).

air at 60 °C. The CV and the (R)RDE measurements were performed at room temperature in 0.5 M H2SO4, saturated with nitrogen and with oxygen, respectively. In addition, RRDE measurements were performed to determine the amount of hydrogen peroxide formed during oxygen reduction. The catalyst suspensions were deposited onto the disc electrode in the same way as for RDE experiments. The collection efficiency of the rotating ring disc electrode was determined by employing 0.005 M K3[Fe(CN)6] solution in nitrogen saturated 0.01 M K2SO4 for each rotation rate with electrodes prepared in the same way as those for the oxygen reduction experiments and under similar experimental conditions. Prior to all measurements, the platinum ring electrode was activated by cyclic voltammetry in 0.5 M H2SO4 in the range from 0.1 to 1.1 V versus SHE at a sweep rate of 150 mV s-1. The catalyst was activated by cyclic voltammetry of the disc electrode in the range from 0 to 0.85 V versus SHE at a sweep rate of 50 mV s-1 until a constant CV was observed. During the RRDE measurements, the potential of the ring electrode was kept at 1.2 V versus SHE. At this potential, generated hydrogen peroxide is oxidized under diffusion control. 3. Results 3.1. XRD and TEM Analyses. XRD measurements have proven that the crystalline part of the catalysts consists of metallic ruthenium, as shown in Figure 1. Heating the catalyst under an inert gas or in a vacuum, the ruthenium particles coalesce, leading to larger entities, the mean size of which can be calculated from the FWHM of the ruthenium main peak at 2θ ) 44°. The result of increasing particle size, corroborated with a decrease in the FWHM of the XRD diffraction lines, can be seen from Figure 1 for an as-grown sample and for one after heat treatment at 900 °C under vacuum. Both diffractograms can be assigned to hcp ruthenium visualized by the position of black lines in Figure 1, which are related to the positions and intensities of hcp ruthenium diffraction lines according to JCPDS powder diffraction data file no. 06-0663. Catalyst particle sizes of 4 nm (as-grown material) and of 10 nm (heat-treated material) deduced from Scherrer’s equation were impressively confirmed by transmission electron micrographs of as-grown and heat-treated catalysts (Figure 2A-D).18 Preparing the catalysts without carbon support, agglomerates

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Figure 2. TEM pictures of ruthenium catalysts. Micrographs A and B show carbon supported catalysts as grown; graphs C and D, the same sample after heat treatment under vacuum at 900 °C (adapted from ref 18). Panel B illustrates that as-grown ruthenium particles sometimes consist of agglomerates of smaller individuals, coalescing at higher temperatures to facetted particles (truncated hexagonal dipyramids). Micrograph E shows particles with amorphous layers around the metallic ruthenium core which are attached to the carbon support (see magnification in the insert). TEM picture F is showing an unsupported RuxSey catalyst after heat treatment at 900 °C to visualize lattice plane arrays. Facetted particles with lattice planes were grown by coalescence. A typical powder spectrum is reproduced in the insert of panel F. A lattice plane distance of 2.35 ( 0.01 Å was obtained interpreted as the 100 hcp-ruthenium net plane.

J. Phys. Chem. C, Vol. 111, No. 1, 2007 479 were formed consisting of ruthenium particles, identified by net plane distances as hcp ruthenium. An example is shown in the TEM graph of Figure 2F, where ruthenium lattice planes become visible belonging to the 100 net plane array of ruthenium (d ) 2.34 Å) in a sample heat-treated at 900 °C. It turned out to be difficult to achieve high-resolution TEM showing lattice planes in as-grown samples. This finding can be explained by the presence of thick amorphous shells covering the particles. Such layers covering catalytic particles can be identified in Figure 2E by investigating a carbon supported ruthenium catalyst where the catalytic particles are separated from each other by the carbon support. To get insight into the chemical composition of the amorphous layers, desorption experiments were performed using TGMS. 3.2. TG-MS Measurements. It was evidenced by FT-IR measurements that, in the process of pyrolysis of Ru3(CO)12 in xylene, carbido carbonyl complexes such as Ru6C(CO)17 occur.3 Performing the process of pyrolysis in a closed vessel employing an autoclave, a cubane-type compound of composition Ru4Se4(CO)12 in the form of yellow orange crystallites of micrometer scale additionally appears in a black nanometer-sized ruthenium precipitate.19 Therefore, discussing desorption experiments, where volatile constituents of an amorphous layer are released by heat treatment in an argon gas flow, the presence of this type of phase attached to the surface of the ruthenium particle surface has to be taken into account. Parts A and B of Figure 3 show TG-MS curves of a selenium free and a selenium containing unsupported catalyst, respectively. The slight mass increase at the beginning of the experiments is due to buoyancy effects of the heated samples in a flowing argon atmosphere. Both catalysts experience a mass loss of 30%, which takes place in the case of the Se free material in two steps, a first one starting at 100 °C with a maximum release rate at 300 °C and a second at 1000 °C with a maximum release rate at 1200 °C. The first decomposition step is connected with the detection of H2O+ and CO2+ in the mass spectrometer, the second one with CO+ and CO2+. Comparing the peak intensities, the CO+ related peak in the first decomposition step can be interpreted as an ionization product of CO2+. From EXAFS and XPS measurements it can be concluded that the particle surface consists on the one hand of ruthenium oxide and hydroxide and one the other hand of fragments of a metal carbonyl (see below). The high amounts of CO2 released could point to a high concentration of metal organic remnants covering the ruthenium metal cores. Since the desorption experiments were performed in a furnace, equipped with a heated carbon furnace liner, the experiments were repeated by replacing the carbon by an Al2O3 tube. While the ionized fragment peaks belonging to the first step remained unchanged, in the second peak only O2+ and O+ fragments occurred, showing that at this temperature mainly ruthenium oxide is present. Studying the thermal decomposition of a Se bearing catalyst, a different behavior was observed. Four steps could be reproducibly identified: a first one starting at 90 °C with maximum mass release at 320 °C, a second small one at 500 °C, a third one starting at 780 °C with maximum at 870 °C, and a last one at 900 °C with maximum release at 1150 °C. The contribution of CO2 and H2O formed in the first step only amounted to 7 wt %. In the desorption steps above 700 °C release of selenium was notified. The first selenium release, detected as Se+ and Se2+, is correlated with CO2+ and CO+ peaks of the mass spectrometer, whereas in the high-temperature step above 900

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Figure 3. Thermogravimetric curves and related mass spectra of a ruthenium catalysts prepared by thermolysis of Ru3(CO)12 in (A) a selenium free xylene and (B) a selenium saturated xylene solution; (C) thermal stability of the catalysts in comparison with the reference substances Ru3(CO)12, RuO2, and RuSe2; (D) ln(∆m/dT)-1/T plot to visualize differences in the kinetic behavior of thermal decomposition.

°C exclusively selenium was released. The CO2+ and CO+ peaks can be explained by reaction of SeO2 molecules, released from the particles, with the hot carbon tube surface under formation of CO2. Repeating the experiment in a Al2O3 furnace core, SeO2+ and SeO+ mass peaks were exclusively found. In conformance with XPS spectra, which have proven the presence of selenide (Se2-) and selenite ((SeO3)2-) species in the catalysts, it is assumed that the first step of the release of selenium can be explained by a thermal decomposition of a ruthenium selenite- or oxiselenide-type bonding and the second one with the decay of a ruthenium selenide-type bond. To our knowledge, no crystallographically defined ruthenium selenite phase was described so far in the literature. The mass loss related to this step amounted to 5 wt % and that of decomposition of ruthenium selenide 20 wt %, respectively. The molar ratios of the four steps can be expressed in a first view by CO2:SeO2:Se ) 3:1:5. This allows the conclusion that the numbers of Ru-O and Ru-Se bonds in the layer covering the ruthenium particles are of comparable order of magnitude. In Figure 3C the thermal stabilities of Ru and Ru-Se catalyst as well as the reference substances RuO2, RuSe2, and Ru3(CO)12 are compiled. Ru3(CO)12 starts to decompose above 100 °C. The maximum of thermal decomposition (≈200 °C) is located at a temperature significantly smaller that those of all other curves. RuSe2 starts to decompose at 900 °C, which fits well with that of the Ru-Se catalyst, while the selenium free catalyst starts to decompose above 1000 °C into hcp ruthenium and

gaseous O2, which is higher than that of pure RuO2 (≈740 °C). In all cases XRD measurements exhibit the pattern of pure hcpruthenium. In Figure 3D the thermogravimetric curves (∆m vs T) of a pure ruthenium catalyst and the selenium modified counterpart are displayed in a ln ∆m vs 1/T plot. Studying the changes of slope in the course of the curves, it can be recognized that the thermal decomposition of both catalysts below 400 °C proceeds via different mechanisms, illustrating different constitutions of the particle surfaces. Investigating the thermal behavior of carbon supported catalysts, the related TG and MS curves showed analogous courses, which however were not as pronounced as in the case of unsupported material due to a smaller metal concentration (20 wt % Ru) of the catalyst. 3.3. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectra of selenium free and selenium containing catalysts are depicted in Figure 4A-D. Figure 4A shows the full range of binding energies measured with a pass energy of 150 eV. Bars in this figure denote Se LMM Auger peaks. Highly resolved measurements with a pass energy of 25 eV were performed for the C1sRu3d, O1s, and Se3d peaks (Figure 4B-D). In general, XPS spectra of selenium containing samples exhibit a smaller width for the Ru3d and the O1s peaks than Se free samples, indicating a higher chemical homogeneity of the related elements in the catalyst. The amounts of oxygen found in Se free samples are in accord with the results of the TG-MS measurements described above, where it was evidenced that the oxygen content

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Figure 4. XPS spectra of ruthenium catalysts with and without selenium: (A) overview spectrum (Se LMM Auger peaks are labeled with black solid lines) and spectra taken at the binding energies of (B) C1s Ru3d, (C) O1s, and (D) Se3d.

of selenium free samples was essentially higher than in a selenium containing one. On the basis of electrochemical work done by Lewerenz et al.,20 the binding energies of ruthenium found at EB ) 281.1 eV and EB ) 282.7 eV can be assigned as Ru-O bonds as known from RuO2 and RuO3 (Figure 4B). The binding energy at EB ) 281.7 eV can be interpreted as a metal organic phase according to a work published by Fachini and Cabrera studying ruthenium complexes such as Ru3(CO)9(CH3CN3)3.21 Smaller peaks at binding energies higher than 285 eV can be addressed as Ru3d5/2 peaks and C-O surface groups on carbon. From work published by Kim and Winograd a binding energy at EB ) 281.4 eV, found in ruthenium oxide catalysts, was correlated with RuOCO3 and hydrated RuO2.22 Comparing both the RuVOw with the RuxSeyOz spectrum, the peak at EB ) 281.1 eV belonging to Ru(IV) becomes dominant while all other structures decrease.

These results are in accord with TG-MS measurements, which exhibited a significant difference in the release of water and CO2 in RuVOw and RuxSeyOz catalysts. Measurements at the O1s peak in Figure 4C elucidated that in the case of a Se free catalyst the binding energy of oxygen at EB ) 531.0 eV is consistent with that in RuO3. The binding energy at EB ) 530.1 eV can be assigned to dissociatively adsorbed water on a ruthenium surface.20 Jaegermann and Ku¨hne23 assigned the peak at EB ) 530.1 eV to ruthenium oxide/ hydroxide species. In the case of Se containing samples, the binding energies at EB ) 530.1, 531.0, and 532.2 eV are assigned as RuO2, RuO3, and (SeO3)2-.23 As shown in Figure 4B in the case of the selenised catalyst, the oxidation state of ruthenium is again found to be shifted to lower oxidation. From the measurement at the Se3d peak it is obvious that selenium is present in two oxidation states, tentatively assigned

482 J. Phys. Chem. C, Vol. 111, No. 1, 2007 as Se2- (EB ) 54.7 eV) and Se4+ (EB ) 58.2 eV) (Figure 4D). Investigating air oxidized, polycrystalline RuSe2 powder Jaegermann assigned peak positions, found at EB ) 54.6 and 58.1 eV, as Se22- and SeO32-. 3.4. EXAFS Measurements. EXAFS is a unique method to elucidate next-neighbor distances even in cluster-like and even amorphous material as expected in our Ru-Se catalyst. In Figure 5A-C the radial distribution functions (RDFs) of asgrown and temperature annealed catalysts and of different crystallized reference substances (hcp-Ru, RuSe2, SeO2, RuO2) are depicted. The RDFs in Figure 5A,B show from the bottom to the top RuO2, Ru4Se4(CO)12, as-grown Ru, and Ru-Se catalyst and the same catalysts annealed at 450 and 900 °C as well as hcp-Ru and RuSe2 reference curves. The RDF curves of as-prepared catalysts (RuSe0.25 and Ru-Se as grown in Figure 5A-C) exhibit a broad peak at 1.4 Å assigned as Ru-O and Se-O distances, respectively. In the case of Ru-Se catalysts, however (Figure 5A,B), additional structures at 2.4 Å and above fit with the hcp-Ru reference. After annealing of the samples the Ru catalysts mainly show the pattern of hcp-Ru, while the Ru-Se catalyst develops a shoulder at the left side of the 2.4 Å peak which coincides with the main peak of RuSe2, indicating the closest Ru-Se distance at 2.47 Å. Measurements at the Se K-edge elucidate that the spectra only exhibit three peaks in the RDF curves, the first one at R ) 1.4 Å can be explained by a Se-O and the third at R ) 2.2 Å as a Se-Ru bond. The missing ones of further next-neighbor distances as known from crystalline SeO2 and RuSe2 are regarded as a hint that Se-O and Ru-Se bondings are accumalated at the surface of the catalytic particles. 3.5. Electrochemical Measurements. Parts A-E of Figure 6 show the electrochemical behavior of carbon supported ruthenium catalysts with and without selenium. Cyclovoltammetric curves illustrated in Figure 6A demonstrate the changed oxidation properties influenced by selenium. The selenium free material is characterized by reduction and oxidation peaks corroborated with an essentially higher capacity. In Figure 6B, the kinetic currents inferred from RDE measurements of a RuxSeyOz/C as-grown and after annealing at 900 °C are shown. At U ) 0.65 V the kinetic current of the annealed sample had been increased by a factor of 2.5. The influence of methanol can be read off from the jk-U curves in Figure 6C. Adding a 1 M ethanol solution to the 0.5 M H2SO4 electrolyte in the electrochemical cell the course of the curve of the electrode loaded with RuxSeyOz/C catalysts was totally inert toward the presence of methanol. The kinetic current of platinum at U ) 0.7 V, however, dropped from 100 A/cm2 to a value smaller than 1 × 10-3 A/cm2. The graph also demonstrates that the kinetic current of the RuxSeyOz/C electrode is at least by a factor of 100 smaller than that of platinum in a methanol free electrolyte. In Figure 6D two cyclovoltammetric curves of RuxSeyOz/C are depicted, confirming the small influence of methanol on the kinetic current. Studying H2O2 production on the catalytic centers of ruthenium catalysts, a reduction from 22 to 4% was found to be caused by the presence of selenium (see Figure 6D), demonstrating a dominant four-electron transfer in the oxygen reduction process. 4. Discussion The discussion is divided into two parts: initially the structural and chemical properties of the catalytic particles will be discussed, while in the second part the influence of the structure on the catalytic behavior is addressed. Finally some thoughts are presented on the nature of the catalytic center.

Fiechter et al. The catalysts prepared were yielded as black powders. By XRD and TEM it was found that the crystalline phase consists of hcp-ruthenium particles of typically 4-10 nm inferred from FWHM data using Scherrer’s equation and by analyzing particle size distribution in TEM pictures.12 Sometimes an amorphous shell covering the metallic nanoparticles could be visualized (Figure 2E). This amorphous part obviously has a complex composition: the presence of ruthenium oxide and hydroxide, selenide and selenate has been evidenced by XPS, TG-MS, and EXAFS measurements. Using the same preparation technique, Solorza-Feria1 and Alonso-Vante et al.24 described particle sizes of 3-4 nm in RuMo-Se catalyst of small Mo content by evaluating TEM pictures. Le Rhun et al.25 described intermediates such as Ru4Se2(CO)11 detected by 13C NMR and FT-IR spectroscopy. These intermediates occur during thermal decomposition using organic solutions when Ru3(CO)12 reacts with dissolved selenium. Crystalline phase formation of Ru4Se2(CO)11 was described by Layer et al.26 Another crystalline phase we obtained as byproduct growing Ru-Se catalyst particles under equilibrium pressure in an autoclave employing xylene as solvent was cubic Ru4Se4(CO)12.19 Alonso-Vante measured a Ru:Se ratio of about 2 using Rutherford backscattering.6 In our samples a Ru:Se ratio of 2.34 was inferred from TG-MS measurements. The highest electrochemical efficiency was yielded by lowering the selenium content in the precursor solution. An optimum was described by Bron et al.3 at Ru:Se ) 5.6. In TG-MS measurements, CO, CO2, and H2O were detected as main gas species, released upon heating in the temperature range from 250 to 350 °C (Figure 3A,B). This finding can be interpreted as the reaction of ruthenium oxide/hydroxide formed by contact of the catalyst with air after preparation with carbonyl groups attached at the surfaces of the ruthenium particles. The same reaction was described by Wang et al. studying desorption behavior of (0001)-oriented ruthenium surfaces covered by CO groups and RuO2.27 The step of release is corroborated with a weight loss of 5% in a Ru-Se catalyst and of 23% for a pure ruthenium catalyst stored in closed glass vessels under ambient conditions. Heating Ru-Se catalysts at temperatures above 500 °C under argon gas flow, the gas species SeO2 occurs above 700 °C and subsequently gaseous Se2 will be released above 900 °C. Heating of the catalyst up to 1400 °C leads to a total weight loss of 30% and the formation of ruthenium powder. The high mass loss of the catalysts suggests that the exposure of the catalysts to air leads to the formation of a layer exhibiting a few nanometers thickness in accord with TEM pictures (Figure 2E). The fact that the CO2 and water loss in an as-grown RuSe catalyst is essentially smaller than in a selenium free sample can be explained by the stabilizing influence of selenium on the surface of the ruthenium core against oxidation. Therefore, it is suggested that an amorphous layer protects the highly reactive ruthenium colloidal particles against corrosion. In this context, selenium plays an important role which is firmly bonded to ruthenium atoms at the surface of metallic ruthenium particles. Formation of amorphous ruthenium oxides when storing the catalyst samples in airswas confirmed in the work of Dassenoy et al.9 studying oxidation and reduction behavior of Rux and RuxSey cluster material using X-ray diffraction at wide angles (WAXS). A remarkably high stability of a RuxSey catalyst against oxidation heating the material up to 320 °C in an oxygen atmosphere was reported by them, although, using the same preparation method, the authors found an essentially smaller particle size ranging from 1.4 to 1.6 nm.

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Figure 5. Radial distribution functions from Fourier transformed EXAFS spectra: (A) ruthenium catalyst as-grown and heat-treated under vacuum at 450 and 900 °C, measured at the Ru K-edge. As reference the calculated RDF of RuO2 is given. (B) Selenium modified ruthenium catalyst as-grown and heat-treated under vacuum at 450 and 900 °C, measured at the Ru K-edge. For comparison, calculated and measured RDFs of RuO2, Ru4Se4(CO)12, hcp-Ru, and RuSe2 are given. (C) Selenium modified catalysts as-grown and heat-treated under vacuum at 450 and 900 °C, measured at the Se K-edge. In addition, RDFs of SeO2 and RuSe2 are shown differentiating between scattering of all neighbors in the bulk material and first neighbor scattering.

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Figure 6. (A) Cyclic voltammograms of as-grown RuVOw/C and RuxSeyOz/C catalysts, measured in an oxygen saturated 0.5 M H2SO4 electrolyte. (B) Tafel plots of a RuxSeyOz/C (Ru(20%)Se(14%)z/C) catalyst as-grown and after heat treatment at 900 °C for 2 h under vacuum. (C) Catalytic activity of a RuxSeyOz/C catalyst (Ru(20%)Se(14%)) as-grown and a platinum catalyst (Pt(20%)/C) before and after addition of a 1 M methanol solution. (D) RuxSeyOz/C catalyst (Ru(20%)Se(14%)) as-grown measured in an oxygen saturated 0.5 M H2SO4 electrolyte with and without methanol. (E) Potential dependent hydrogen peroxide production of RuVOw/C and RuxSeyOz/C (Ru(20%)Se(14%)) catalysts.

A small expansion of the mean metal-metal distance was inferred in comparison with that in bulk ruthenium. The best fit of the WAXS data was found by assuming a hcp structure of the ruthenium clusters as known from metallic ruthenium. Performing in-situ oxidation and reduction experiments ofsas the authors suggestedscluster-like Rux and RuxSey particles, they evidenced a very good resistance against oxidation in the metalselenium system. In the selenium free material they detected an oxide-like amorphous species RuVOw transformed to RuO2 at temperatures above 250 °C. From the FWHM of our XRD measurements a typical

ruthenium particle size of 4 nm was inferred for Ru-Se catalyst. Particles of the same size by preparing Ru and Ru-Se catalysts by the same method were also reported by Ramı´rez-Raya et al.28 and Solorza-Feria et al.29 Evaluating XPS data in as-grown Ru-Se catalysts, we suggest a covalent bonding of selenium to ruthenium and an ionic bonding of oxygen to selenium as found in (SeO3)2-. In RuOx catalyts, the emission lines at the O1s edge were interpreted as ruthenium oxides/hydroxides and water, dissociatively bound to ruthenium. In the RuxSeyOz catalyst the O1s peak at full width at half-maximum is smaller due to a shift

Surface Modified Ruthenium Nanoparticles from Ru(VI) to a ruthenium(IV) oxide. The shoulder at 532.2 eV is interpreted as a chemical shift typical for (SeO3)2- (Figure 4C). In general, the intensity of the O1s peak, diminished by a factor of 2-3, which becomes visible at the noise of the background, again points to the fact that selenium protects the catalyst against oxidation. This protecting behavior becomes especially obvious in analyzing the C1sRu3d3/2 lines comparing a RuVOw with a RuxSeyOz catalyst: while in a RuVOw catalyst Ru carbonyl remnants, hydrated RuO2, RuOCO3, and Ru(IV) are visible, in the RuxSeyOz catalyst a ruthenium peak position addressed as Ru(IV) is dominant. Considering the binding energies of selenium (see Figure 4D) in a RuxSeyOz catalyst, it is proposed to assign the peak positions to (SeO3)2- and Se2-, respectively. The intensity ratio I(Se2-)/ I[(SeO3)2-] ) 1.8 shows that at the surface of the catalyst particle Ru-Se bondings are dominant. In TG-MS measurements (Figure 3B) a ratio Se:SeO2 ) 5.4 was established. Since Jaegermann addressed the binding energy EB ) 54.7 eV (Figure 4D) as [Se2]2-, such units could also be bound to ruthenium at the surface of our ruthenium particles. These XPS results are confirmed by Solorza-Feria et al. studying (Ru1-xMox)ySez catalysts.1 In this paper the peak position of the O1s emission around 531 eV is attributed to oxides and hydroxides; the shoulder at 533 eV, to chemisorbed water. Measuring the Se3d emission line, a peak at EB ) 55 eV was addressed as [Se2]2-. Interestingly the peak at 58.7 eV, assigned as (SeO3)2-, was not as pronounced as in our samples. A Ru3d5/2 line at EB ) 280.5 eV was addressed as Ru2+. The authors arrived at the conclusion “that metal oxides are present on the first monolayers covering the particles”. In-situ and ex-situ EXAFS measurements at the Ru K-edge were described earlier by Alonso-Vante et al.,10 by Malakhov et al.,11 and by Kochubey et al..30 Ru-O, Ru-Ru, and Ru-Se distances could be analyzed. In ex-situ measurements of Se containing catalysts the Ru-Ru distance was dominant. Under anodic conditions oxidation occurred as expected.31 Measuring at the Ru K-edge, we found in our catalysts a broad peak at R ) 1.4 Å that has been assigned as Ru-O (1.96 Å) and/or Ru-CO (1.92 Å) bondings (Figure 5A,B, marked by arrow a). After heat treatment of the samples at 450 °C this strong feature has nearly vanished and the RDF pattern of hcpRu becomes dominant (see calculated and measured RDF of ruthenium in Figure 5A,B in comparison with the catalyst annealed at 450 and 900 °C). The dominant peak representing the smallest Ru-Ru distance is located at 2.4 Å (see arrow e in Figure 5B). In the case of a selenium modified catalyst, the peak at around 2 Å can be assigned as the nearest Ru-Se distance in the asgrown as well as in the heat-treated catalyst. For comparison, the main peak position of the reference materials RuSe2 and Ru4Se4(CO)12 are shown in Figure 5B, marked by arrows b,d. The position of this peak is slightly different from that attached to the left side of the main Ru-Ru peak at 2.33 Å, marked by arrow c in Figure 5B. No significant change of the RDF occurs when heating the catalysts from 450 up to 900 °C. Since ruthenium oxide cannot be evaporated up to this temperature in a vacuum or under an inert gas atmosphere (Figure 3C), purification of the surfaces of the ruthenium particles obviously has performed via reaction of oxide and carbonyl groups under formation of CO2. The volatility of gas species such as RuO3 is at temperatures below 900 °C too small to explain the disappearance of Ru-O bonding. It is thought that a relatively thick layer covers the ruthenium particles which

J. Phys. Chem. C, Vol. 111, No. 1, 2007 485 has to be of amorphous state due to the missing of further significant peaks as found in the RDF of reference sample RuO2. In contrast to selenium free Ru catalysts, this amorphous layer is thinner, evidenced by the additional appearance of the RDF pattern of hcp-ruthenium even in the as-grown material. Measurements at the Se K-edge give evidence that the RuSe bonding is restricted to the surface of the ruthenium particles. While at the Ru K-edge the RDF pattern of hcp-Ru is dominant interfering with the RDFs of selenium bearing phases, three atomic distances can be recognized at 1.4, 1.9, and 2.22 Å, the first of which was interpreted as Se-O bonding (Figure 5C, arrow a); the latter two are corroborated with Ru-Se and SeSe in [Se2]2- units bonding coinciding with the distances in the calculated RDF curve of RuSe2 (Figure 5C, arrows b,c). Since a nearly perfect fit could be obtained by taking into account only the next Se-Ru neighbors, it can be assumed that selenium is located close to the surface of the particles firmly bonded to ruthenium. Neither RuSe2 nor any other crystallographically defined species is present due to the missing of higher Se-Se or Ru-Se distances that are, e.g., typical for RuSe2 (arrows d,e), the only known stable compound in the system Ru-Se, crystallizing in the pyrite structure. Calculating the coordination number of selenium at R ) 2.22 Å (the Se-Ru distance with d ) 2.45 Å) from the fitted EXAFS function, a value of 2.0 was found, indicating the presence of Ru-Se-Ru units at the surface of the particles. Inferring the coordination number of Ru at the Ru K-edge (Ru-Se distance d ) 2.45 Å and Ru-Ru distance d ) 2.65 Å) a coordination number of 3.2 was found. In the reference materials RuSe2 and Ru coordination numbers of 6.2 and 12.0 were obtained as expected from the crystallographic structures. These findings are in accord with EXAFS fittings described by Alonso-Vante et al. reporting on a coordination of selenium by two ruthenium atoms.31 Annealing the catalyst at 450 and 900 °C under inert gas, a loss of selenium after heat treatment at 900 °C has to be expected, which becomes visible in the RDF curve by decreased peak intensities while the RDF at 450 °C interestingly shows a diminished Se-O peak. Inconsistent with the TG-MS curve, a weak peak related to Se-O distance occurs in the RDF after heat treatment at 900 °C. This behavior can be explained by an reoxidation of selenium at the surface of the ruthenium core of the RuxSey catalyst by handling the material again in air. From the analytical work described above it can be concluded that Rux and RuxSey nanoparticles oxidize in air, forming an amorphous layer consisting of ruthenium oxide/hydroxide and ruthenium selenide, but also of selenite, additionally containing residues of carbonyl bearing metal organic intermediates, formed in the organic solvent during synthesis. The core of the particles comprises hcp-Ru. The size of the hcp-Ru core of typically 4 nm diameter decreases with decreasing selenium content.3 A pure ruthenium catalyst exposed to air is completely oxidized after 40 days.9,25,32 In contrast, a RuxSey catalyst for x:y > 2.3 is chemically stable in ambient conditions after a small amount of oxide and selenite has been formed. In an idealized picture, the catalytic particles can therefore chemically also be described as RuxSeyOz.30 In the following section the active role of selenium enhancing catalytic activity in the oxygen reduction reaction will be discussed. It has been suggested that metal organic molecules that are additionally present on the surface of the Ru particles besides ruthenium oxide, selenide, and selenite play a role in the catalytic processes observed.33 Although in the presence of selenium the tendency to form a thick oxide and metal organic ruthenium

486 J. Phys. Chem. C, Vol. 111, No. 1, 2007 layer is diminished compared to a pure Ru/C catalyst (Figure 3A,B and Figure 6A), heating of RuxSeyOz catalysts in vacuum up to 900 °C leads to a further thinning of the amorphous layer correlated with an increased catalytic activity as shown in Figure 6B (see also refs 3, 12, and 18). The catalyst is now distinguished by hcp-Ru crystallites showing the shape of truncated hexagonal dipyramids of e10 nm diameter (see Figure 2D). Evaluating the TG curve of Figure 3B, a Ru:Se ratio ) 2.4 was obtained which is corroborated with a complete coverage of the ruthenium surface by Se2- and [Se2]2-: EXAFS has evidenced that selenium is covalently bonded to ruthenium (Figure 5C). On the basis of these results, some considerations can be made concerning the structure of the active center of the catalyst. Vogel suggested in his work RuxSey clusters of