Bimetallic Cluster Provides a Higher Activity Electrocatalyst for Methanol Oxidation*

Transcription

1 Journal of Cluster Science, Vol. 18, No. 1, March 2007 (Ó 2007) DOI: /s Bimetallic Cluster Provides a Higher Activity Electrocatalyst for Methanol Oxidation* Brenda L. Garcı a, 1 Burjor Captain, 2 Richard D. Adams, 2,4 Ana B. Hungria, 3 Paul A. Midgley, 3 Sir John Meurig Thomas, 3 and John W. Weidner 1 Received September 28, 2006; accepted September 28, 2006; published online January 10, 2007 The cluster complex Pt 2 Ru 4 (CO) 18 was used as a precursor to prepare a 60 wt% 1:2 Pt:Ru nanoparticles on carbon (PtRu/C) for use as an electrocatalyst for methanol oxidation. This bimetallic carbonyl cluster complex was found to provide smaller, more uniform bimetallic nanoparticle that exhibited higher electrocatalytic activity than a 60 wt% 1:1 Pt:Ru commercial catalyst from E-Tek. Using bimetallic cluster precursors simplifies the synthetic procedures by reducing the need for high temperature reduction and assures a more intimate mixing of the two different metals. Transmission electron microscopy (TEM) images of the catalyst obtained from the cluster precursor showed bimetallic nanoparticles having a narrow size range of 2 3 nm that were dispersed uniformly over the surface of the support. Images of the commercial catalyst showed particles 3 4 nm in diameter that tended to agglomerate near the edges of the carbon support particles. Cyclic voltammograms of methanol oxidation from the two catalysts showed significantly higher activity for the cluster-derived catalyst. The onset potential for methanol oxidation for the cluster-derived catalyst was approximately 170 mv lower than that of the commercial catalyst at 100 A/g Pt, and approximately 250 mv lower at 400 A/g Pt. KEY WORDS: PtRu nanoclusters; catalyst; methanol; fuel cell. * This report is dedicated to Prof. Gu nter Schmid on the occasion of his 70th birthday. 1 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA. 2 Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA. 3 Department of Materials Science, University of Cambridge, Cambridge, CB2 3QZ, UK. 4 To whom correspondence should be addressed. Adams@mail.chem.sc.edu /07/ /0 Ó 2007 Springer Science+Business Media, LLC

2 122 García et al. INTRODUCTION Fuel cells are attracting considerable attention due to their highly efficient energy conversions. In particular, there is much interest in the Direct Methanol Fuel Cell (DMFC) because it uses the alternative fuel methanol [1, 2], which has a very high volumetric energy density. Unfortunately, this fuel cell currently suffers from a high overpotential that severely limits its efficiency. Considerable efforts have been made to develop catalysts to decrease the overpotential at the anode of the DMFC where the methanol is oxidized. Bimetallic platinum ruthenium catalysts have emerged as the most promising to date [1 3]. First principles calculations [4, 5] have shown that the advantage of Pt-Ru catalysts over simple Pt catalysts for methanol oxidation is due to a lower energy barrier provided by a bifunctional mechanism in which water is activated at a ruthenium site and the methanol is oxidized at a platinum site. Gasteiger et al. [6] showed that the optimal surface Ru composition was about 10 mole% when using polished bulk alloys of Pt and Ru with a well-defined crystal structure. However, they noted that the results could be significantly different when testing nanoparticle catalysts due to the irregular shapes and metal segregation effects in the nanoparticles. Watanabe et al. [7] and Arico et al. [8] found that for Pt/Ru nanoparticles on carbon supports (PtRu/C), the maximum methanol oxidation activity was found in catalysts with 1:1 Pt:Ru molar ratio. However, the reasons for the large discrepancies in the methanol oxidation activities between well-defined crystal surfaces and electrodes formed by nanoparticles are not fully understood. It is possible that non-uniform distributions of Pt and Ru atoms in nanoparticles contributes to the differences. Two common methods for synthesizing Pt-Ru nanoparticle catalysts for fuel cells are co-precipitation of separate Pt and Ru precursors and colloidal cluster deposition onto a carbon support in the presence of hydrogen at a high temperature [3, 9]. Co-precipitation methods do not allow good control over the size, shape, or composition of the individual nanoparticles that are formed and colloidal methods may have problems in reduction of the metal salts and completely removing particle stabilizers. Accordingly, some recent efforts have focused on the use of bimetallic cluster complexes as precursors for methanol electrocatalysts, but incomplete removal of the ligands can complicate and/or contaminate the bimetallic catalysts that are formed [10 12]. The advantage of using molecular clusters as precursors is that they can offer a greater control of the Pt:Ru ratio and more intimate association of the two metals [13]. They also allow synthesis of more uniform sized nanoparticles. Studies of catalysts created from cluster precursors may also allow researchers to bridge the gap

3 Higher Activity Electrocatalyst for Methanol Oxidation 123 in understanding between methanol oxidation on well-defined crystal surfaces and methanol oxidation electrodes formed by nanoparticles. Here we report some preliminary results of the use of the molecular cluster complex Pt 2 Ru 4 (CO) 18 [14], 1 as a catalyst precursor for the oxidation of methanol. In the structure of 1, the two platinum atoms are sandwiched between two Ru 2 (CO) 8 groups, see Fig. 1. Compound 1 contains only CO ligands that can be readily removed by heating. Recent studies have shown that compound 1 is a good precursor to Pt Ru bimetallic nanoparticles when supported on carbon [15]. Although the 1:2 Pt:Ru ratio found in 1 is not the ideal 1:1 ratio for DMFC electrocatalysts [7, 8], our work shows that this cluster precursor does provide a superior electrocatalyst for methanol oxidation when compared with a commercially prepared 1:1 PtRu/C catalyst (E-Tek). The catalyst also appears to be superior to bimetallic catalysts prepared by physically mixing platinum and ruthenium carbonyl compounds [16]. EXPERIMENTAL SECTION Cluster Synthesis of a 60 wt% 1:2 PtRu/C catalyst The bimetallic carbonyl cluster complex Pt 2 Ru 4 (CO) 18 was prepared according to the previously published procedure [14]. The catalyst precursor Fig. 1. A diagram showing the molecular structure of Pt 2 Ru 4 (CO) 18 1.

4 124 García et al. was weighed under an Ar atmosphere (50 mg) and transferred to a vial containing 25 ml of methylene chloride. The color of the catalyst precursor in solution was deep pink. Ketjenblack carbon (20.4 mg) was then weighed and added to the solution. The mixture was sonicated at room temperature for 1 hour.the vial containing the sample was immersed in a water bath and the solvent was evaporated using a gentle stream of nitrogen. The carbonyl ligands were removed by heating the sample at 200 C for 4 hours in an atmosphere of 4% H 2 in N 2 to yield the PtRu/C catalyst. Electrode Preparation A suspension of 6.0 mg of the PtRu/C catalyst in 3.0 ml of H 2 O and 3.0 ml of isopropyl alcohol (IPA) was sonicated for 15 min. This suspension was then pipetted (using several aliquots, with a maximum aliquot size of 5 ll, to ensure film uniformity and integrity) onto a glassy carbon electrode so that the catalyst film contained approximately 37.5 nmol of Pt. To this catalyst film, a 5 ll aliquot of binding solution (prepared by diluting a 5 wt % Nafion Ò in IPA with more IPA 1:20 by volume) was pipetted onto the electrode and allowed to dry. Three-Electrode Cyclic Voltammetry Testing Cyclic Voltammetry (CV) was used to measure the catalytic activity of PtRu/C films in a solution of 0.5 M H 2 SO 4 and 1 M methanol. The glassy carbon electrode containing the electrode film was submerged in a reaction flask filled with the 0.5 M H 2 SO 4 and 1 M methanol mixture. The solution was bubbled with N 2 for 15 min prior to testing. The counter electrode was a platinum wire electrode and the reference was an Hg/HgSO 4 electrode with a Luggin capillary. The CV was conducted in a potential range between )0.7 V and 0.6 V versus the Hg/HgSO 4 electrode at a scan rate of 5 mv/s using a Princeton Applied Research 263A potentiostat controlled by using the software package Corrware. The potential scans started at open circuit (0.24 V), swept down to )0.7 V, swept up to 0.6 V, and swept back down to )0.7 V. Electron Microscopy High angle annular dark field (HAADF) images were recorded on a 200 kv FEI Tecnai F20 STEM/TEM electron microscope. Specimens were prepared by depositing the particles of the samples to be investigated onto a copper grid supporting a perforated carbon film. Deposition was achieved by dipping the grid directly into the powder of the samples to avoid contact

5 Higher Activity Electrocatalyst for Methanol Oxidation 125 with any solvent. X-ray Energy Dispersive Spectra (XEDS) were acquired at 15 kv on a JEOL 5800 LV Scanning Electron Microscope equipped with a UTW X-Ray detector. The probe size for XEDS analysis 100 nm and the X-ray spectra were corrected for atomic number (Z), absorption (A) and fluorescence (F). Bulk Analyses Inductively Coupled Plasma (ICP) Analyses of the catalyst samples were performed by Galbraith Laboratories, Knoxville, TN. For the clusterderived catalyst the percentage metal loading was found to be 44.0% and the Pt/Ru ratio was For the E-tek sample the percentage metal loading was found to be 51.6% and the Pt/Ru ratio was RESULTS AND DISCUSSION A representative HAADF image of the resultant carbon supported PtRu nanoparticles is shown in Fig. 2 [17, 18]. The white spots represent the metallic nanoparticles. The images show a large number of small nanoparticles, 2 3 nm in diameter dispersed uniformly over the surface of the support. The particle sizes are approximately double the size of similar particles prepared on carbon by Hills et al. [15]. However, the loadings in this previous study were 1 2 wt% metal on the carbon support. The 44 wt% catalyst used for this work is typical of loadings used in commercial DMFC Fig. 2. STEM image of a 44 wt% 1:2 PtRu/C catalyst. The cluster complex Pt 2 Ru 4 (CO) 18 was used as a precursor to prepare the catalyst.

6 126 García et al. electrodes. For comparison, representative images of the nanoparticles on the E-Tek commercial catalyst are shown in Fig. 3. The metal particles on the commercial catalyst are fewer in number and generally have a larger size. The particles show a significant tendency to aggregate near the edges of the carbon particles. In these edge regions islands of nanoparticles up to 10 5 nm are seen frequently. Figure 4 shows the plots of the composition of the PtRu electrocatalyst nanoparticles on carbon prepared from the cluster complex Pt 2 Ru 4 (CO) 18 (4a), and electrocatalyst obtained from the commercial E-Tek (4b). The horizontal dotted lines represent the theoretical and actual Pt/Ru compositions of the two materials. The compositions were determined by EDX spectroscopy using a SEM with a probe size > 100 nm (see supporting information). The SEM analysis shows that the catalyst prepared from the cluster has an average Pt/Ru ratio of 0.42±0.16 which is very close to the theoretical value of 0.5 based on the stoichiometric ratio of the two metals indicated by the formula of the cluster complex. The Pt/Ru ratio of this sample as determined by Inductively Coupled Plasma (ICP) analysis was The commercial catalyst (E-TEK) has an average Pt/Ru ratio of 0.96±0.25 which is also very close to the theoretical value of 1.0. The Pt/Ru ratio of this sample as determined by Inductively Coupled Plasma analysis was The cyclic voltammograms (CV) for the catalyst prepared from 1 and the E-Tek catalyst are compared in Fig. 5. The CVs are normalized according to the grams of platinum on the electrode as determined by the Fig. 3. STEM image of a 52 wt% 1:1 PtRu/C commercial catalyst from E-Tek.

7 Higher Activity Electrocatalyst for Methanol Oxidation 127 Fig. 4. Plots showing the composition of PtRu nanoparticles on carbon determined by EDX using an SEM (probe size > 100 nm): (a) electrocatalyst prepared from the cluster complex Pt 2 Ru 4 (CO) 18 ; and (b) electrocatalyst obtained from E-Tek. The horizontal dotted lines represent the theoretical and actual Pt/Ru compositions of the two materials. ICP analyses. The methanol oxidation peak current for the commercial catalyst was 480 A/g Pt and that for the cluster-derived catalyst was 2000 A/ g Pt. The closest CV comparison on a commercial catalyst in terms of reaction conditions (i.e., temperature, methanol concentration) and electrode type (i.e., composite electrode on a disk electrode) is from Liao et al. [19]. In Liao et al. [19], the activity of a similar PtRu/C catalyst from Johnson Matthey at 25 C is reported to be as 590 A/g Pt at a scan rate of 50 mv/s. The significantly higher peak height for the same amount of Pt 2400 Current (A/g Pt) M H 2 SO 4 1 M Methanol Scan Rate: 5 mv/s 1:2 Pt-Ru/C (Cluster) 1:1 Pt-Ru/C (E-Tek) Voltage (V) vs RHE Fig. 5. Cyclic Voltammograms (CVs) for catalyst derived from cluster complex 1 and the commercial E-Tek catalyst. The electrodes were loaded with 37.5 nmol of Pt.

8 128 García et al. loading generally indicates that the catalyst has a higher dispersion and thus more active sites. This analysis is supported by the difference in catalyst particle size seen in the STEM images in Figs. 2 and 3. Not only does 1 provide a catalyst with more active sites, but more importantly these sites are more active, as evident from the substantial reduction in the onset potential for methanol oxidation. This enhanced activity is more clearly seen in Fig. 6, which is a plot of the forward scan in Fig. 5, but corrected for background and rotated to show voltage versus current. The potential for methanol oxidation by cluster-derived catalyst is approximately 170 mv lower than that of the commercial catalyst at 100 A/ g Pt and approximately 250 mv lower at 400 A/g Pt. This is significantly better than the approximately 120 mv at 10 A/g Pt enhancement that Boxall et al. [11] achieved for the 1:1 microwave irradiated synthesis method over a 60 wt% 1:1 E-Tek electrode. In addition, the 80 A/g Pt at 400 mv versus Reversible Hydrogen Electrode (RHE) achieved for the clusterderived catalyst at room temperature is comparable to the catalysts in the review article by Liu et al. [3] even though most of those were run between C. The increased catalytic activity we achieved indicates that the anode overpotential for methanol oxidation in a DMFC is lowered substantially by using the molecular cluster catalyst without increasing the Pt loading. The higher catalytic activity is probably attributable to more intimate contact between the Ru and Pt atoms in the cluster catalyst, which enables more particles to take part in the bifunctional mechanism of methanol oxidation :1 PtRu/C (E-Tek) Voltage vs. RHE (V) :2 PtRu/C (Cluster) M H 2 SO 4 1 M Methanol Scan Rate: 5 mv/s Current (A/g Pt) Fig. 6. Methanol oxidation overpotential from the CV experiments for the catalyst derived from cluster complex 1 and the commercial E-Tek catalyst.

9 Higher Activity Electrocatalyst for Methanol Oxidation 129 The increase in the peak methanol oxidation current and the lowering of the onset potential for the cluster catalyst compared to the E-Tek catalyst indicate that the ability to disperse the catalyst well and control the proximity of Pt and Ru atoms is critical to the improvement in catalyst performance. In addition, the 1:1 Pt:Ru ratio for nanoparticle catalysts reported in the DMFC literature [7, 8] may not give the optimum performance. CONCLUSIONS In conclusion, the cluster complex Pt 2 Ru 4 (CO) 18,1 was used as a precursor to prepare 44 wt% 1:2 Pt:Ru nanoparticles on carbon (PtRu/C) for use as an electrocatalyst for methanol oxidation. This bimetallic carbonyl cluster complex was found to provide smaller, more uniform bimetallic nanoparticles that exhibited higher electrocatalytic activity than a 52 wt% 1:1 commercial catalyst from E-Tek. This enhanced performance was achieved even though the Pt:Ru ratio in the cluster-derived catalyst had less than the accepted optimum value used in the commercial catalyst. ACKNOWLEDGEMENTS This research was supported by the Office of Basic Energy Sciences of the U. S. Department of Energy under Grant No. DE-FG02-00ER REFERENCES 1. M. P. Hogarth and T. R. Ralph (2002). Platinum Met. Rev. 46, M. P. Hogarth and G. A. Hards (1996). Platinum Met. Rev. 40, H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, and D. P. Wilkinson (2006). J. Power Sources 155, S. Desai and M. Neurock (2003). Electrochim. Acta 48, S. K. Desai and M. Neurock (2003). Phys. Rev. B 68, H. A. Gasteiger, N. Markovic, P. N. Ross, and E. J. Cairns (1993). J. Phys. Chem. 97, M. Watanabe, M. Uchida, and S. Motoo (1987). J. Electroanal. Chem. 229, A. S. Arico, P. L. Antonucci, E. Modica, V. Baglio, H. Kim, and V. Antonucci (2002). Electrochim. Acta 47, K. Y. Chan, J. Ding, J. W. Ren, S. A. Cheng, and K. Y. Tsang (2004). J. Mater. Chem. 14, J. T. Moore, J. D. Corn, D. Chu, R. Jiang, D. L. Boxall, E. A. Kenik, and C. M. Lukehart (2003). Chem. Mater. 14, D. L. Boxall, G. A. Deluga, E. A. Kenik, W. D. King, and C. M. Lukehart (2001). Chem. Mater. 13, 891.

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