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Surface Coordination of Ruthenium Clusters on Platinum Nanoparticles for Methanol Oxidation Catalysts E. R. Fachini,† R. Dı´az-Ayala,† E. Casado-Rivera,† S. File,‡ and C. R. Cabrera*,† Department of Chemistry, University of Puerto Rico, Rı´o Piedras Campus, P.O. Box 23346, San Juan, Puerto Rico 00931-3346, and Department of Biology, University of Puerto Rico, Rı´o Piedras Campus, P.O. Box 23346, San Juan, Puerto Rico 00931-3346 Received October 29, 2002. In Final Form: July 3, 2003 Inorganic surface modification was used to prepare Pt/Ru/Vulcan catalysts by coordinating a triruthenium cluster [ Ru3(CO)9(MeCN)3] on Pt nanoparticles. The method was used to provided a surface with catalytic activity for use in the direct methanol fuel cell. The cluster adsorptive process followed a Langmuir adsorption isotherm. The amount of Ru could be controlled by changing the experimental conditions of adsorption. The catalyst powder was characterized by energy-dispersive spectrocopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and electrochemical studies. The proposed methodology provides a way to place Ru atoms on a Pt surface while avoiding metal segregation. The optimum results were obtained with a catalyst that presented a Ru/Pt ratio of 0.07, as given by XPS analysis. The peak current for methanol oxidation in cyclic voltammetry scans was similar to that of commercial true alloy catalysts. On the basis of these results, some considerations about how ruthenium segregation interferes with methanol oxidation are addressed.
Introduction The direct methanol fuel cell (DMFC) is an attractive power source for mobile applications as a result of the high energy density of methanol, the portability of liquid rather than gaseous fuels, and the existence of a similar infrastructure that could be used for methanol distribution. In addition to being safe and renewable, methanol is also a combustible that exerts a low negative impact on the environment. The DMFC requires a catalyst. A platinum/ruthenium catalyst is, at this time, the alternative of choice because it strongly overcomes CO surface contamination. To reduce the amount of the expensive noble metal required, there have been considerable efforts to increase the dispersion of the metal on the support matrix. Most often, the bimetallic alloy is dispersed on a carbon support. However, the morphology, crystallography, and chemical environment of the particles alter the electronic properties of the catalyst. As a consequence, different manufacturing processes and catalytic pretreatment may result in a catalyst with different activities in methanol oxidation.1 The support material also influences catalytic activity. A common conductive support is Vulcan X-72, a carbon substrate with a graphitic character. The chemical characteristics of the support, its treatment, and cleanliness change the environmental conditions for the dispersion of the metal particles and their subsequent catalytic activity. This is principally due to problems of contamination such as platinum organosulfur poisoning.2 Therefore, a considerable amount of the catalyst manufacturing process does not result in a real bimetallic alloy but in a bimetallic surface with a certain amount of metal segregation.3-5 * Corresponding author. Email address: ccabrera@ cnnet.clu.edu. † Department of Chemistry, University of Puerto Rico. ‡ Department of Biology, University of Puerto Rico. (1) Stoyanova, A.; Naidenov, V.; Petrov, K.; Nikolov, I.; Vitanov, T.; Budevski, E. J. Appl. Electrochem. 1991, 29, 1197. (2) Swider, K. E.; Rolison, D. R. Electrochem. Solid-State Lett. 2000, 3, 4. (3) Richard, F.; Wohlmann, B.; Vogel, U.; Hoffschulz, H.; Wandelt, K. Surf. Sci. 1995, 335, 361.
The ruthenium segregation phenomenon had been the theme of fundamental studies by Weickowski’s group using spontaneous or electrochemical depositions on single platinum crystals or platinum black. They have used RuCl3 solutions to prepare ruthenium-decorated surfaces. The effect of Ru coverage, up to 0.65 Ru atoms per Pt surface atom, on methanol oxidation catalysis had been addressed.6-8 Ruthenium segregation occurs as twodimensional ruthenium islands as small as 2-5 nm9 on the platinum surface, if the proper experimental conditions are selected. The initial low coverage can be improved by repetitive adsorptions that generate tridimensional structures and overall more ruthenium on the surface.8 At this time it is thought that methanol oxidation occurs at the frontier of the ruthenium islands and is dependent on the valence state of ruthenium, the important factor being the presence of the metallic species and low valence ruthenium oxides.6 Many studies suggest that the latter species could be an activated water molecule on the ruthenium surface. Since the phenomena of ruthenium segregation has been proposed to be important in the enhancement of Pt/Ru catalytic activity, methodologies that can control, or avoid, the amount of segregation are highly desirable. To avoid segregation, one approach could be to protect the Ru atoms with ligands that are not completely lost during the adsorption process. Then, the organometallic clusters could be employed with reductive decomposition, when the ligands are removed, to create a bimetallic surface. (4) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Bo¨nnemann, H. J. Electrochem. Soc. 1998, 145, 925. (5) Page, T.; Johnson, R.; Hormes, J.; Noding, S.; Rambabu, B. J. Electroanal. Chem. 2000, 485, 34. (6) Kim, H.; Rabelo de Morais, I.; Tremigliosi-Filho, G.; Haasch, R.; Wieckowski, A. Surf. Sci. 2001, 474, L203. (7) Waszczuk, P.; Solla-Gullo´n, J.; Kim, H.-S.; Tong, Y. Y.; Montiel, V.; Aldaz, A.; Wieckowski, A. J. Catal. 2001, 203, 1. (8) Crown, A.; Johnston, C.; Wieckowski, A. Surf. Sci. 2002, 506, L268. (9) Friedch, K. A.; Geyzers, K.-P.; Henglein, F.; Marnann, A.; Stimming, U. Electrode processes VI. In ECS Proceedings; Wieckowski, A., Itaya, K., Eds.; The Electrochemical Society: Pennington, NJ, 1996; Vol. 96-8, p 119.
10.1021/la0267692 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/17/2003
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Table 1. Description of the Samples Prepared and Characterized in This Studya sample
description
[Ru3(CO)9(CH3CN)3] (mM; in CH2Cl2)
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8
20% Pt/Vulcan ETEK 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster reduction 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster reduction 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster reduction commercial 20% Pt/Ru/Vulcan (ETEK)
0.0 0.1 0.2 0.4 0.6 0.8 1.0
a
The reported concentration of the cluster was the concentration used during the adsorption step.
Despite the fact that impregnation methods have been used extensively to obtain fuel-cell catalysts, few published studies have employed the inorganic coordination of organometallic clusters to modify a substrate metal surface, to generate bimetallic catalysts. In this sense, Lee et al. used Ru(1,5-cyclooctadiene)(µ3-C3H5)2 hydrogenation in the presence of platinum black as a new way to build Pt/Ru catalysts for alcohol oxidation.10-12 The goal of the present study is to determine the electrochemical performance of platinum nanoparticles modified with highly dispersed ruthenium atoms obtained by ruthenium cluster adsorption phenomena. Mild conditions for the reductive cluster decomposition step were selected on the basis of previous thermometric studies because the use of a short time at moderate temperatures avoided the ruthenium segregation. The starting material was a commercial platinum carbon-supported catalyst modified with Ru3(CO)9(CH3CN)3, which can be adsorbed onto the Pt surface after its activation in dichloromethane solutions. The following presents the characterization and adsorptive considerations of the catalyst using powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy-dispersive spectroscopy (EDS). The electrochemical behavior was used as the criterion to compare the catalytic efficiencies of methanol oxidation. Considerations about the importance of the ruthenium segregation were addressed. Experimental Section Catalyst Powder Preparation. The starting support material used in the present experiments was a commercial 20% Pt/ Vulcan (ETEK) catalytic powder. All raw catalyst powders were heat-pretreated (up to 300 °C) in a tube furnace to remove surface contaminants, first in an atmosphere of 10% O2 in Ar and then in pure H2. The synthesis of the Ru cluster was performed following the procedure described by Aime et al.13 The adsorption of Ru3(CO)9(MeCN)3 from dichloromethane solutions at different concentrations was performed as follows:13 The cleaned raw catalyst powder was ultrasonically suspended in a solution of Ru3(CO)9(CH3CN)3 in dichloromethane for 30 s. After an adsorption time of 15 min, the suspension was centrifuged and the residual solid was washed three times with 10 mL of dichloromethane. The powder was dried under a vacuum and placed in a desiccator. Table 1 details the experimental conditions for each sample. Further reduction of the cluster was accomplished in the same system used for the heat treatment of the catalyst, in a pure H2 atmosphere at 230 °C. This temperature was selected because the total degradation of the unsupported cluster occurs at 227 °C, as was previously observed in a differential scanning thermometry experiment and confirmed with XPS analysis on the metallic residues after decomposition.14 (10) Lee, C. E.; Bergens, S. H. J. Phys. Chem. B 1998, 102, 193. (11) Lee, C. E.; Tiege, P. B.; Xing, Y.; Negandran, J.; Bergens, S. H. J. Am. Chem. Soc. 1997, 119, 3543. (12) Lee, C. E.; Bergens, S. H. J. Electrochem. Soc. 1998, 145, 4182. (13) Aime, S.; Dastru´, W.; Gobetto, R.; Krause, J.; Violano, L. Inorg. Chim. Acta 1995, 235, 357.
The Pt/Vulcan control sample received the same heat treatment, except that it was exposed to the solvent without the ruthenium cluster. EDS Analysis. EDS spectra were done using a JEOL JSM5800LV, operating at 20 kV. The catalyst powder was fixed on a sample holder with carbon tape and examined directly. The EDS results reported are an average of an analysis of three different large areas. The elemental standardless quantification analysis was performed using the EDAX DX-4 eDX ZAF system software, version 2.30. It should be noted that the reported values approximate the real values. The heights of principal Ru LR1 and Pt MR transition peaks were used to obtain the approximate elemental composition. The matrix effect was considered the same for all the samples. TEM. TEM was performed in a Zeiss EM-10 CA microscope working at 80 kV with an ultimate resolution of about 5 Å. Specimens were prepared using a sonicated (5 min) ethanol suspension of the catalyst powder. Drops of the suspension were deposited onto a carbon substrate type-B nickel grid, air-dried, and examined. The samples were examined at magnifications ranging from 20 000× to 230 000×. The diameters of particles were determined by measuring more than 350 particles for each sample in a random manner on micrographs enlarged 3× from the original negatives using a digital caliper (Ultra-Cal Mark III). XRD. The XRD analysis was carried out in a Bruker d5000 diffractometer system, equipped with a Cu KR source operating at 45 kV and 40 mA. The spectra were obtained at a scan rate of 12° min-1, with steps of 0.020° in the 2θ scans until acceptable spectra were obtained. Commercial software was used (EVA) for the treatment of the data to subtract the background and to measure the full width at half-maximum (fwhm) of selected reflections. The instrument resolution was less than 0.1°; this value is too low to contribute to any important features in the peaks. Therefore, the there was no peak correction based on instrument resolution. XPS. The XPS spectra were recorded in a PHI 5600 multisystem, using an Al monochromatic source, operating at 350 W and 15 kV, except for the Ru(3p)-region spectra, where the polychromatic Mg (400 W, 15 kV) was employed to improve the signal. The pressure in the main chamber was approximately 5 × 10-9 Torr during the analysis. Survey and multiplex spectra were obtained with pass energies of 93.9 and 5.85 eV, respectively. The catalyst powders were attached to a sample holder using a small piece of carbon tape. No significant charging effects were detected. The background was corrected using the Shirley method, and the fitting was performed based on asymmetrical (for platinum) and Gauss-Lorentzian (for the rest of the elements) distributions. All the binding energies reported were corrected using the signal for the principal peak of the carbonaceous matrix using 284.5 eV as an internal standard. The quantitative evaluation was done with the PHI software, based on atomic sensitivity factors for the specific transitions of certain elements. Electrochemical Studies. Electrode Preparation. Homemade glassy carbon (GC) disk electrodes (6 mm in diameter) were polished with 1-µm alumina powder, washed in an ultrasonic bath, and rinsed in Nanopure water. Then, a suspension of catalyst under ultrasonic homogenization, 0.0050 mg (around 10 µL) of catalytic paint (consisting of 1 mL of glycerol, 1 mL of (14) Fachini, E.; Cabrera, C. R. Unpublished data.
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Fachini et al. Table 2. Summary of Some Physical Characteristics of the Metal Nanoparticles
sample
average diametera (nm)
standard deviationa (nm)
range of particlesa (nm)
average diameterb (nm)
20% Pt/Vulcan S-2 S-5
2.2 1.8 2.5
0.8 0.8 0.9
1.1-4.7 0.7-4.1 1.0-4.8
4.0 2.7 4.0
a
From TEM analysis. b From XRD analysis.
in this work are referenced to the normal hydrogen electrode (NHE). The electrodes were first cleaned by applying many voltammetric cycles (20 mV/s, from +715 to -65 mV, 0.5 M H2SO4) until a reproducible response was reached and maintained at a hydrogen adsorption potential (-65 mV) for 2 min. Then, the first CV scan for methanol oxidation was recorded in a CH3OH/H2SO4 (1:1, 0.5 M) N2 degassed solution at 20 mV/s from -65 to +1165 mV. Then, the electrode was discharged; each electrochemical data reported is the mean of five or more readings, with dispersions ranging from 5 to 25%. Chronoamperommetry Studies. After similar CV pretreatment, the chronoamperograms were obtained by applying a potential step from 75 to 415 mV after stabilizing in the methanol solution for 5 min. The current responses were recorded for 40 min.
Results and Discussion
Figure 1. Typical TEM images of catalysts: (A) 20% Pt/Vulcan ETEK catalyst (as received); (B) sample S-5 (20% Pt/Vulcan ETEK after adsorption of Ru3(CO)9(CH3CN)3 from a 0.6 mM CH2Cl2 solution and cluster decomposition in a heated H2 atmosphere); (C) enlarged image of part B. a 4% Nafion solution in methanol, and 0.1 g of cleaned carbonsupported catalyst; after stirring for 24 h), was taken from the stirring plate, placed on the GC surface, and weighed. The electrodes with the paint were dried in a vacuum oven at 60 °C and, finally, were extensively rinsed with Nanopure water. Cyclic Voltammetry (CV) Studies. The electrochemical experiments were carried out in a common electrochemical cell, with a Pt counter and the Hg/HgSO4 as a reference electrode, using a BAS CV-50W voltammetric analyzer. All potentials reported
TEM Analysis. The TEM technique was used to characterize the cluster-modified nanoparticles. The three samples analyzed by TEM were the catalyst powder as received, one sample prepared with a low Ru3(CO)9(CH3)3 concentration (0.1 mM cluster in CH2Cl2 solution; sample named S-2), and another with a high ruthenium cluster concentration (0.6 mM; sample named S-5). Figure 1 shows typical images obtained by TEM. The average diameter, standard deviation, and approximated number of Pt and Pt/Ru particles by surface area are reported in Table 2 for the different samples. In addition, the histograms of three samples can also be seen in Figure 2. From the TEM results on these three samples, the dispersion of metal particles appears homogeneous on the support. As seen in Figure 1, the shape of Pt/Ru-modified particles was similar to pure Pt particles and quasispherical in all samples. In comparison, the commercially available Pt/Ru-alloyed particles usually had an elongated shape.15 In the present case, this shape was not observed. These observations lead us to think that the Ru atoms were well-distributed on the Pt surface and that few pure Ru particles, if they were present after thermal reductive treatment, were approximately spherical as well. However, we do not have any spectroscopic evidence of the existence of pure Ru particles or Ru adsorption of the Pt nanoaprticles. Similar effects were seen in the other samples. The absence of large aggregates in the TEM analysis shows that the mild conditions of thermal cluster degradation did not favor the coalescence of platinum nanoparticles. With the mild thermal conditions used in the present work, the mobility of noble metal atoms was not high enough to promote either platinum or ruthenium particle segregation.16 Therefore, the average particle size before and after heat treatment did not change too much, as is illustrated in the histograms presented in Figure 2. However, the comparison of different samples reveals that, after the Ru cluster adsorption and thermal treatment, there was some small change in the particle diameter in (15) Arico´, A. S; Cretı`, P.; Kim, T.; Mantegna, R.; Giordano, N.; Antonucci, V. J. Electrochem. Soc. 1996, 143, 3950. (16) Davies, J. C.; Hayden, B. E.; Pegg, D. J.; Rendall, M. E. Surf. Sci. 2002, 496, 110.
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Figure 3. Typical EDS spectra of the catalyst powders: (A) sample S-7 (20% Pt/Vulcan ETEK after adsorption of Ru3(CO)9(CH3CN)3 from a 1.0 mM CH2Cl2 solution and cluster decomposition in a heated H2 atmosphere); (B) sample S-1 (20% Pt/ Vulcan ETEK). Analysis condition: 20 kV.
Figure 4. Isotherm of adsorption of Ru3(CO)9(CH3CN)3 on 20% (w/w) Pt/Vulcan after 15 min of contact with the solution in CH2Cl2, at room temperature. The solid line represents the Langmuir isotherm. Figure 2. Histograms of particle diameters of representative samples: (A) 20% Pt/Vulcan ETEK catalyst (as received); (B) sample S-5 (20% Pt/Vulcan ETEK after adsorption of Ru3(CO)9(CH3CN)3 from a 0.6 mM CH2Cl2 solution and cluster decomposition in a heated H2 atmosphere); (C) sample S-2 (20% Pt/ Vulcan ETEK after adsorption of Ru3(CO)9(CH3CN)3 from a 0.1 mM CH2Cl2 solution and cluster decomposition in a heated H2 atmosphere).
both directions: toward smaller and larger diameters. The increase in size could be interpreted as the addition of metal material from the carbonaceous matrix to the platinum particles or to the addition of a new Ru layer. This size increase was greater in the sample S-5, which had the higher concentration of the ruthenium cluster than sample S-2 and, of course, than the catalyst powder without any treatment. To interpret the particles of smaller diameter, it is possible that more dispersed metal precursors from the matrix might have promoted new metal particles, thereby generating more small particles. In this sense, S-2, prepared with a lower ruthenium cluster concentration, promoted the formation of smaller ruthenium particles. Then, the decreasing of the particle diameter, more common in sample S-2, should be attributed to new pure and amorphous ruthenium particles that arose during the reduction of the adsorbed ruthenium cluster on the Vulcan substrate. Scanning Electron Microscopy EDS. Figure 3 shows a typical EDS spectrum for the analyzed catalyst powder.
The presence of carbon, oxygen, sulfur, platinum, and ruthenium could be observed. In some samples, additional peaks at 1.7 and 1.8 keV also revealed silicon contamination from glassware during catalyst preparation. From the EDS data, the adsorption isotherm of the Ru cluster on the Pt nanoparticles/Vulcan matrix (see Figure 4) was determined. Subsequent adsorption steps lead to an increase of the Ru surface concentration, even with an aged cluster, as is shown in Figure 5. The EDS analysis reveals similar results for all catalysts. Carbon, oxygen, sulfur, platinum, and ruthenium were the major components of the samples. The amount of ruthenium loading changed from one sample to another, increasing with the concentration of the ruthenium cluster solution in the adsorption step, agreeing with the expected behavior. The presence of sulfur is explained as a minor component of Vulcan. This carbon-bonded sulfur is very difficult to eliminate from the substrate. Despite this fact, it has been speculated that this kind of sulfur is not a severe contaminant of the platinum surface. The sulfur binds the graphitic network of the substrate in such a manner that no contamination of platinum was observed even though many months had passed between the preparation and the use of the catalysts.2 The peak of sulfur at 2.4 keV is due to its KR (2.31 keV) and Kβ (2.46 keV) transitions; these peaks were not resolved in the spectra but were present in the spectra of
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Figure 5. Relation between the amount of ruthenium adsorbed on the platinum surface (element percent) and the number of adsorption steps for a 3-month-aged cluster. Adsorption conditions: 1 mM dichloromethane solution, 15 min of adsorption in each step at room temperature.
all the samples that use the Vulcan substrate. For an electron beam of 10 kV, significant fluorescence for Pt occurring at 1.60 (Mz), 2.05 (MR), and 2.13 (Mβ) keV could be observed as was expected, being the last two poorly resolved peaks in the spectrum. The two most intense transitions for Ru, respectively LR1 and Lβ1, occurred at 2.56 and 2.68 keV. Finally, the presence of Si KR (1.74 keV) and Kβ (1.83 keV) transitions contributed to a broad peak at 1.75 keV. The presence of silicon came from the glassware used during the ruthenium cluster and catalyst synthesis. The noble metal loading of the raw material was close to the reported commercial values. No significant differences were found among the samples before and after the preparative treatments, except for the additional presence of Ru. The adsorption isotherm followed a Langmuir behavior, indicating that no or few interactions between the cluster molecules occurred during the adsorption. In addition, the adsorptive sites were active for just one cluster molecule, and the results should be a partial monolayer over carbon and Pt active sites. Specifically, for the spontaneous adsorption of RuCl3 from aqueous solutions, a limit of 0.2 monolayer has been established on the Pt(111) substrate.17 It was observed that the chemical behavior of the synthesized cluster changes with aging, as was noted by Foulds et al. for a very similar compound, Ru3(CO)10(MeCN)2.18 This was expected because it is known that Ru3(CO)9(MeCN)3 is stable only at freezing temperatures and under a nitrogen atmosphere. In solutions without acetonitriles, the cluster also decomposes. The triruthenium dodecacarbonyl [Ru3(CO)12] is not active for adsorption on platinum because it is not able to lose any ligand in dichloromethane solutions for short adsorption times, as in the present experiments.19 As a consequence, two recommendations are derived from these findings: first, the Ru3(CO)9(MeCN)3 must be used as soon as possible after its synthesis to obtain reproducible results. Aged Ru3(CO)9(MeCN)3 may be composed of a range of parent cluster fragments as a result of oxidation processes. Therefore, the aged cluster will behave differently in adsorption processes than fresh ones. Second, for identical adsorptive conditions, an aged cluster will promote a lower Ru concentration on the surface because it continuously (17) Herrero, E.; Feliu, J. M.; Wieckowski, A. Langmuir 1999, 15, 4944. (18) Foulds, G. A.; Johnson, B. F. G.; Lewis, J. J. Organomet. Chem. 1985, 296, 147. (19) Fachini, E. R.; Cabrera, C. R. Langmuir 1999, 15, 717.
Fachini et al.
Figure 6. XRD patterns of some analyzed samples: (A) pure Vulcan support; (B) commercial 20% Pt/Vulcan ETEK catalyst; (C) sample S-5 (20% Pt/Vulcan ETEK after adsorption of Ru3(CO)9(CH3CN)3 from a 0.6 mM CH2Cl2 solution and cluster decomposition in a heated H2 atmosphere). Patterns: solid line, Pt; dashed-dotted-dotted line, PtO; dashed-dotted line, PtO2; dotted line, Ru; dashed line, RuO2.
loses acetonitrile ligands during aging and degrades toward a more stable ruthenium content species. Despite this, repeated adsorptions with solutions of an aged cluster could give similar results compared to adsorptions done with freshly synthesized clusters. A good linearity between the amount of Ru and the number of adsorptive steps can be observed in Figure 5, using fresh dichloromethane solutions of a 3-month-old cluster stored at 3 °C under air. Recently, Crown et al., using spontaneous adsorption on platinum single crystals of different orientations and starting from RuCl3 solutions, have shown that multiple adsorption steps lead to threedimensional ruthenium structures on platinum surfaces with a higher coverage limit of 0.2 monolayer.8 XRD Analysis. Morphological information about the catalyst was obtained from XRD analysis. Figure 6 shows the diffraction pattern for the raw material (20% Pt/Vulcan ETEK) and a catalyst prepared by ruthenium cluster adsorption. The Vulcan pattern is shown for comparative purposes. The lines for Pt and the two more intense lines for the PtO, PtO2, Ru, and RuO2 reflections are shown in Figure 6 for clarity. The Vulcan substrate contributes to the XRD pattern with a reflection at a 2θ value of 25°, which is characteristic of graphitic species in this carbon matrix and common to all the samples analyzed.15 The XRD pattern for platinumsupported nanoparticles adds the characteristic peaks for Pt face-centered cubic (fcc) to the XRD pattern of Vulcan. All catalysts present similar patterns for Pt fcc, and the peaks correspond very well with the reported pattern for Pt.20 After the ruthenium cluster adsorption, the XRD pattern did not differ significantly from the XRD pattern for the catalyst as received (see Figure 6B,C). The tendency for higher angles in the Pt reflections, expected for alloys with Ru,15 was not observed. In other words, the metal particles are not true alloys. This fact could be expected, considering that the proposed methodology does not perturb the platinum fcc crystal structure with Ru atoms, as in an alloy; with the present technique, the Ru atoms are laid down on the Pt surface. Some ruthenium particles, whose presence could be suspected from the TEM analysis, must be amorphous. (20) Natl. Bur. Stand. Circ. (U.S.) 539 1953, 31.
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Figure 7. XPS spectra of the Ru(3p3/2), O(1s), Pt(4f7/2,5/2), and Ru(3d5/2,3/2)-C(1s) envelopes showing deconvolution treatment. (Sample S-7: 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition in a heated H2 atmosphere. Adsorption conditions: 15 min at room temperature; 1.0 mM Ru3(CO)9(CH3CN)3 in a CH2Cl2 solution.)
Some samples, including 20% Pt/Vulcan (ETEK) and Pt/Ru catalysts, present additional peaks at a 2θ value of 36°. Despite the difficult task of assigning these peaks to specific species, it is suspected from the data that they correspond to Pt oxidized species, such as PtO, PtO2, or Pt(OH)x. The present XPS results support this conclusion (see the following). The more intense reflections for pure Ru and RuO2 species rise at 2θ values of 44 and 28°, respectively. These values were not observed in the present patterns, and this agrees with the results of Arico´ et al.15 This means that either the Ru-adsorbed atoms followed the Pt pattern or, if Ru/RuO2 particles rise from adsorption phenomena as TEM analysis suggests, the particles were completely amorphous in the Vulcan substrate. In general, all Vulcan-supported catalysts present very broad peaks in their diffraction patterns. This is characteristic of very small particles. The average diameter for the Pt particles was calculated using the DebyeSherrer’s equation. Table 2 presents the diameter for the metal particles obtained from the 220 reflections. Despite that the 220 reflection has a lower intensity compared to those of the 111, 200, or 311 reflections, the error is minimized at 220 because the peak is better resolved than that of the other reflections. From this analysis, the Vulcan-supported metal particles were in the range of 2.0-5.0 nm. Moreover, XRD analysis showed that the heating step during cleaning and cluster thermal decomposition did not significantly change the crystalline behavior of the Pt particles or promote the coalescence of the Pt nanoparticles, in agreement with the TEM pictures. The average particle diameters calculated by the Debye-Sherrer’s equation were slightly greater than that observed in the TEM analysis (see Table 2). This could be explained by considering that TEM does not discriminate for Ru/RuO2 particles. Therefore, in TEM analysis, if the diameter of the Ru/RuO2 particles is lower than that of the Pt particles, the average diameter for all the metal particles will decrease. The diameters of the Pt particles, calculated from XRD data, remain almost constant after adsorption of Ru atoms. XPS Analysis. An XPS survey of the supported Pt/Ru catalyst (sample S-7) showed the presence of Pt, Ru, C, O, and small amounts of S, as can be expected for Vulcan substrates. The elemental compositions for each sample did not differ significantly, and they are presented in Table
Table 3. Elemental Composition of the Catalyst Surfaces by XPSa sample
Pt (%)
Ru (%)
O (%)
C (%)
S (%)
S-1 S-2 S-3 S-4 S-5 S-6 S-7
2.7 2.7 2.7 2.5 2.5 3.0 3.2
0.00 0.09 0.28 0.24 0.32 0.49 0.56
10 9.5 8.5 8.8 8.6 9.3 13
87 87 88 88 88 87 83
0.6 0.6 0.7 0.5 0.5 0.4 0.5
a The samples are 20% Pt/Vulcan ETEK + Ru (CO) (CH CN) 3 9 3 3 after cluster decomposition in a heated H2 atmosphere. Adsorption conditions: 15 min at room temperature; CH2Cl2 solution; S-1, 0.0 mM; S-2, 0.1 mM; S-3, 0.2 mM; S-4, 0.4 mM; S-5, 0.6 mM; S-6, 0.8 mM; S-7, 1.0 mM.
3. In Figure 7, multiplex spectra for Pt(4f), Ru(3p), O(1s), and C(1s), with the deconvolution curves, are presented as an example of typical results. From Figure 7, it is clear that thermal hydrogen treatment was not able to completely reduce the bimetal particles, emphasizing the importance of the electrochemical conditioning of the catalyst before determination of the catalytic activity. The Ru elemental composition by XPS was higher than the EDS results. This is not surprising considering that XPS is a true surface analysis (data from about 30 Å), but EDS probes deeper into the material. In general, XPS for the C region presented an enlargement at higher binding energies. The CsS of the Vulcan polymer and oxygenated functionalities, like CsOH, sCdO, and sCOOH, contributed to the spreading the C peak above 284.5 eV. Despite the high-energy resolution employed to observe the C region, they are not able to deconvolute the carbon envelope. Because the C peak is spread at a higher binding energy with no peaks clearly identified, much similar organic functionality should be present. Samples S-1 and S-7 were more oxidized during the thermal treatment than the other samples with an accent for oxygen organic functionalities. The adsorptive phenomena gave a small amount of Ru on the Pt/Vulcan substrate. In addition, the Ru(3d) photoelectrons, the more intense transition for Ru, arose together with the C(1s) peak. Consequently, the carbon peak suppresses the Ru envelope signal, and on many occasions, it is preferable to analyze the Ru(3p) transitions. Because of this, for the Pt/Ru/Vulcan samples, it was necessary to look at the Ru(3p) photoelectrons, even though
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Figure 8. Peak potential (versus NHE) for methanol oxidation for GC electrodes modified with Pt/Ru catalytic powders versus the solution cluster concentration employed during the adsorptive step. The catalysts are 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition in a heated H2 atmosphere. Adsorption conditions: 15 min at room temperature; 1.0 mM Ru3(CO)9(CH3CN)3 in a CH2Cl2 solution. CV conditions: CH3OH + H2SO4 (both 0.5 M), 20 mV s-1.
these transitions presented relatively poor S/N and large fwhm (see Figure 7). The metallic Ru(3p3/2) peak appears at 461.1-461.5 eV, and its presence has been correlated with higher catalytic activities toward methanol oxidation.7,21 The Ru species in our catalysts was almost RuO2, whose 3p peak rises near 463.5 eV. A smaller amount of RuO3 was also present. At this time, it is suspected that high valence states of ruthenium decrease the catalytic activity and may be related to the high peak potential for methanol oxidation (see comments that follow). However, ruthenium oxides can be electroreduced at lower potentials, for example, 0.30 V.6 Before the methanol oxidation sweeps, the reductive treatment of the electrodes (see Experimental Section) certifies at least the major presence of ruthenium metallic species and overcomes any problem associated with an excess of ruthenium oxides on the surface. The major signal for platinum was assigned to metallic Pt at 71.3 eV; as usual, the oxidized species as PtOx or Pt(OH)x were not clearly distinguished from Pt0. The Pt(IV) oxide arose at 73.6 eV. The O(1s) XPS envelope was complex as a result of the presence of a number of metal oxides and hydroxides and of organic functionalities due to the partial oxidation of the carbonaceous substrate. In addition, from the XPS S(2p) peaks, the formation of sulfones or sulfates was detected. The latter is even possible for the catalyst before sulfuric acid solution contact because of the presence of sulfur in the Vulcan substrate. It is well-known that Pt promotes the oxidation of S in the presence of O2 and traces of water. Electrochemical Analysis. CV analysis of methanol oxidation of the different prepared catalysts was done. The peak potential for methanol oxidation was measured. As can be seen in Figure 8, the peak potential reaches a minimum for some concentration around 0.5 mM. The peak potential range, 50 mV, was not spread was as expected; the lower value was 780 mV (0.6 mM), better than the peak potential found for pure platinum particles (830 mV) but higher than the peak potential for a commercial 20% Pt/Ru (ETEK) catalytic powder (680 mV). It should be noted that the bimetallic particles on the commercial ETEK catalyst are alloys, which are different from present Pt/Ru catalysts. It is suspected that the mild temperature conditions used during the reductive treatment are able to decompose the ruthenium cluster, but the ruthenium atoms remain separated. On Pt(111), the Ru bulk diffusion starts at temperatures higher than 450 (21) Koetz, R.; Lewrenz, H. J.; Stucki, S. J. Electrochem. Soc. 1983, 130, 825.
Fachini et al.
Figure 9. CV peak current for methanol oxidation for GC electrodes modified with Pt/Ru catalytic powders versus the solution cluster concentration employed during the adsorptive step. The catalysts are 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition in a heated H2 atmosphere. Adsorption conditions: 15 min at room temperature; 1.0 mM Ru3(CO)9(CH3CN)3 in a CH2Cl2 solution. The current was normalized for the platinum loading. CV conditions: CH3OH + H2SO4 (both 0.5 M), 20 mV s-1, versus NHE.
K, and this number could be even greater on graphitic substrates of Vulcan.16 In fact, for similar thermal reduction conditions, scanning tunneling microscopy analysis on highly oriented pyrolytic graphite revealed that the Ru3(CO)9(MeCN)3 preserves a well-ordered structure after reduction.14 Wieckowski’s group has proposed that the islands of ruthenium on the platinum substrate are necessary to activate the water molecule to effectively promote the complete oxidation of adsorbed methanol. Then, the methanol oxidation takes place at the interface between the platinum substrate and the ruthenium-ruthenium oxide islands. Our results show high peak potentials for methanol oxidation on platinum surfaces decorated with highly dispersed ruthenium atoms. Then, some amount of segregation leads to the catalytic activity and should be considered a very important parameter to fuel-cell catalysts design. In Figure 9, it is possible to identify a maximum oxidation current for the catalyst prepared from a 0.1 mM cluster solution. At higher Ru concentrations, the Ru atoms probably block the active platinum sites and the surface loses its catalytic power. For comparison, the value for the 20% Pt/Ru (ETEK) catalytic powder was 0.18 mA/ cm2, while the synthesized catalyst currents range from 0.07 to 0.26 mA/cm2. Considering the chronoamperommetric results presented in Figure 10, the best performance was obtained with the catalyst prepared with solutions of Ru cluster in the range of 0.2-0.4 mM (see Figure 11). This sample corresponds to 0.2% Ru on the surface, as was measured by EDS, or 0.1% by XPS. The residual current for the 20% Pt/Ru (ETEK) catalytic powder was 2.1 A/g, a value which is similar to that of the catalyst prepared with 0.4 mM ruthenium cluster. From the electrochemical results, there are some discrepancies that tend to demonstrate that the CV data and chronoamperometry data may differ. A decrease on the peak potential and an increase in the peak current does not necessarily mean a better catalyst. For fuel-cell applications, the chronoamperometry results tend to be a better tool because it is a longer methanol oxidation process. What is important from the CV result is that the Ru is modifying the Pt surface because we see a difference in the peak potential and current for methanol oxidation compared to pure Pt nanoparticles. However, we may have Ru nanoparticles present as well. Our group is moving toward characterizing single nanoparticles by high resolution TEM with X-ray fluorescence detection. Neverthe-
Ruthenium Clusters on Platinum
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Conclusions
Figure 10. Chronoamperograms for methanol oxidation on GC electrodes modified with Pt/Ru catalytic powders. The catalysts are 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition in a heated H2 atmosphere. Adsorption conditions: 15 min at room temperature; 1.0 mM Ru3(CO)9(CH3CN)3 solution in H2CCl2. Dashed line, sample S-1 (0.0 mM Ru3(CO)9(CH3CN)3 solution in H2CCl2); solid line, sample S-7 (1.0 mM Ru3(CO)9(CH3CN)3 solution in H2CCl2). Electrochemical conditions: CH3OH + H2SO4 (both 0.5 M), from 75 to 415 mV.
Figure 11. Residual current after 20 s (b) and 40 min (9) from chronoamperograms versus the solution cluster concentration employed during the adsorptive step. The GC electrodes were modified with Pt/Ru catalytic powders. The catalysts are 20% Pt/Vulcan ETEK + Ru3(CO)9(CH3CN)3 after cluster decomposition in a heated H2 atmosphere. Adsorption conditions: 15 min at room temperature, 1.0 mM Ru3(CO)9(CH3CN)3 solution in CH2Cl2. The current was normalized for the platinum loading. Chronoamperogram parameters: CH3OH + H2SO4 (both 0.5 M) solution potential step: from 75 to 415 mV.
less, we understand that the Ru X-ray fluorescence signal will be extremely small and difficult to detect.
The Ru3(CO)9(MeCN)3 can be adsorbed on 20% Pt/ Vulcan ETEK, modifying the usual behavior of the catalytic powder for methanol oxidation. The adsorption phenomenon follows a Langmuir adsorption isotherm as determined by X-ray fluorescence and XPS. This means that the amount of Ru on the surface can be controlled by changing the concentration of the Ru cluster/dichloromethane solutions. No segregation of the particles was observed with the proposed method. It is suspected that the ruthenium is highly dispersed on the platinum surface in a highly oxidized state, making reductive electrochemical pretreatment necessary to obtain metallic ruthenium. Neither XRD nor TEM analysis suggests the formation of any ruthenium species particles. Nevertheless, the electrochemical data support the platinum nanoparticle surface modification by ruthenium. The high potentials for methanol oxidation on those ruthenium-modified platinum particles were attributed to the absence of the ruthenium island formation: the fast and low-enough temperature conditions during cluster decomposition probably lead to a higher metal dispersion. Superior performance was achieved for the catalyst prepared from 0.4 mM Ru cluster solutions that presented the highest residual current and lower peak potential in methanol oxidation. Although the present work did not achieve the highest performance for a fuel-cell catalyst, the proposed methodology, which employs organometallic cluster adsorption to create decorated bimetal surfaces, could be extended for new catalytic applications. Acknowledgment. We gratefully acknowledge the financial support from the Army Research Office (DODEPSCoR), Grant DAAD19-00-1-0092, Department of Energy-HiCREST, Grant 3-49811-7840, Office of Naval Research (ONR), and the Materials Characterization Center (MCC) at UPR. E.R.F. had a fellowship from EPSCoR-NSF. LA0267692
