pubs.acs.org/Langmuir © 2010 American Chemical Society
Nanoporous PtRu Alloys for Electrocatalysis Caixia Xu,*,† Lin Wang,‡ Xiaolei Mu,† and Yi Ding*,‡ †
School of Chemistry and Chemical Engineering, University of Jinan, Jinan, China, and ‡School of Chemistry and Chemical Engineering, Shandong University, Jinan, China Received November 1, 2009. Revised Manuscript Received January 12, 2010
We describe a facile route to the straightforward fabrication of nanoporous (NP) PtRu alloys with predetermined bimetallic compositions. Electron microscopy and X-ray diffraction characterizations demonstrate that selective etching of Al from ternary PtRuAl source alloys generates three-dimensional bicontinuous NP-PtRu alloy nanostructures with a single-phase face-centered-cubic crystalline structure. X-ray photoelectron spectroscopy shows a slight electronic structure modification of Pt by alloying with Ru as well as uniform surface and bulk bimetallic ratio. With characteristic structural dimensions less than 5 nm, these high surface area bimetallic nanostructures show distinct electrocatalytic performance as the Ru content varies within the structure. Among all samples, NP-Pt70Ru30 shows the highest specific activity as well as the most negative onset potential toward methanol oxidation reaction. NP-Pt50Ru50 was found to possess a similar specific activity to the commercial E-TEK Pt50Ru50/C catalyst, but its onset and peak potentials are about 70 mV more negative. CO stripping experiments demonstrate that the adsorption of CO is the weakest on NP-Pt70Ru30, and further increasing the Ru content actually shifts the CO stripping peak to a more positive potential. Thus, the overall sequence for CO-tolerance is NP-Pt70Ru30 > NP-Pt50Ru50 ≈ Pt50Ru50/C > NPPt30Ru70 > Pt/C.
Introduction Direct methanol fuel cells (DMFCs) are promising power sources for portable electronics due to their attractive characteristics of high energy density, low operation temperature, and low pollution for environment.1 PtRu bimetallic catalysts are deemed as the most promising anode catalysts in DMFCs due to their superior activity and CO tolerance in methanol oxidation reaction (MOR) as compared with pure Pt.2-4 Two possible mechanisms have been proposed to explain the catalytic enhancement of Pt by alloying with Ru for MOR. The most accepted one is the bifunctional mechanism where Ru surface sites allow the formation of oxygenated species to oxidize the dissociative intermediates produced on nearby Pt sites.3-5 As to the ligand-effect mechanism, it is based on a concept of electronic structure modification of Pt through the interaction with Ru.6,7 In recent years significant efforts have been paid to the development of new synthetic routes to the fabrication of various PtRu nanostructures, aimed at improving their catalytic properties by optimizing the composition and structure. Currently, PtRu alloy nanostructures are mostly in a state of supported nanoparticle
form8,9 and can be prepared by colloidal,10 impregnation,11 microemulsion,12 coreduction,13 microwave polyol,14,15 or sonochemical method.16 Despite the great advances for the exploration of these PtRu nanostructures,17,18 the above-mentioned methods typically involve multistep operation and are consequently difficult to simultaneously achieve a good control to the composition, morphology, and structure uniformity of the resulted nanocatalysts. Recently, nanoporous (NP) metallic structures have attracted great attention in the application of electrocatalysis, sensing, and optics due to their bicontinuous nanoscale skeletons and interconnected hollow channels, which are favorable for easy mass transport and high electron conductivity.19-21 For example, Koczkur et al.22 have prepared various porous alloy network structures, including NP-PtRu using a hydrothermal coreduction technique, that showed enhanced catalytic properties as compared with the bulk material. However, it is noted that the porous network structures generated through this wet-chemical procedure are actually in a form of nanoparticle aggregations, and the typical structure dimension is of order 100 nm. It is thus quite
*Corresponding authors: e-mail
[email protected], Ph þ86-53182767367, Fax þ86-531-82767367 (C.X.); e-mail
[email protected], Ph þ86-531-88366513, Fax þ86-531-88366280 (Y.D.).
(11) Jeon, M. K.; Won, J. Y.; Lee, K. R.; Woo, S. I. Electrochem. Commun. 2007, 9, 2163–2166. (12) Zhang, X.; Chan, K. Y. Chem. Mater. 2003, 15, 451–459. (13) Teng, X.; Maksimuk, S.; Frommer, S.; Yang, H. Chem. Mater. 2007, 19, 36–41. (14) Tsuji, M.; Kubokawa, M.; Yano, R.; Miyamae, N.; Tsuji, T.; Jun, M. S.; Hong, S.; Lim, S.; Yoon, S. H.; Mochida, I. Langmuir 2007, 23, 387–390. (15) Liu, Z. L.; Lee, J. Y.; Chen, W. X.; Han, M.; Gan, L. M. Langmuir 2004, 20, 181–187. (16) Basnayake, R.; Li, Z. R.; Katar, S.; Zhou, W.; Rivera, H.; Smotkin, E. S.; Casadonte, D. J.; Korzeniewski, C. Langmuir 2006, 22, 10446–10450. (17) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795–1803. (18) Tong, Y.; Kim, H.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2002, 124, 468–473. (19) Erlebacher, J.; Aziz, M. J.; Karama, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450–453. (20) Ding, Y.; Chen, M. W. MRS Bull. 2009, 34, 569–576. (21) Rolison, D. R. Science 2003, 299, 1698–1701. (22) Koczkur, K.; Yi, Q. F.; Chen, A. C. Adv. Mater. 2007, 19, 2648–2652.
(1) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133–161. (2) Wang, X. G.; Liao, J. H.; Liu, C. P.; Xing, W.; Lu, T. H. Electrochem. Commun. 2009, 11, 198–206. (3) Park, K. W.; Sung, Y. E. J. Phys. Chem. B 2005, 109, 13585–13589. (4) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267–273. (5) Liu, Z.; Lee, J. Y.; Chen, W.; Han, M.; Gan, L. M. Langmuir 2004, 20, 181– 187. (6) Babu, P. K.; Kim, H. S.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. B 2003, 107, 7595–7600. (7) Frelink, T.; Visscher, W.; Van Veen, J. A. R. Langmuir 1996, 12, 3702–3708. (8) Wang, H. S.; Alden, L. R.; DiSalvo, F. J.; Abr€una, H. D. Langmuir 2009, 25, 7725–7735. (9) Zhou, C. M.; Wang, H. J.; Peng, F.; Liang, J. H.; Yu, Hao.; Yang, J. Langmuir 2009, 25, 7711–7717. (10) Vidakovic, T.; Christov, M.; Sundmacher, K.; Nagabhushana, K. S.; Fei, W.; Kinge, S.; Bonnemannd, H. Electrochim. Acta 2007, 52, 2277–2284.
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Figure 1. SEM (a), TEM (b, c), and HRTEM (d) images of NP-Pt70Ru30 alloy. (e, f ) SEM images of NP-Pt50Ru50 and NP-Pt30Ru70 alloys, respectively.
unlikely that these structures can compete with the commercially available nanoparticle catalysts in view of a lower surface area for nearly 2 orders of magnitude. Notably, it was reported by several groups that alloy corrosion, under appropriate conditions, can be used to make nanoporous metals that show some intriguing structural properties.23,24 The most known example is nanoporous gold (NPG) made by dealloying AuAg alloys, which shows interesting catalytic and optical performances.24,25 While less attention has been paid to develop multicomponent alloy systems,24,26,27 in the present work, we focus on the fabrication of NPPtRu alloys with an emphasis on their promising electrocatalytic properties. In contrast to the known preparation methods for nanoparticles, dealloying involves only benchtop processing in aqueous solutions at room temperature, and is applicable to largescale preparation of alloy nanostructures with predetermined compositions at nearly perfect yield. Detailed electrochemical characterizations demonstrate that these NP-PtRu bimetallic nanostructures possess distinct electrocatalytic activities toward methanol and CO oxidation, which are markedly better than the commercial E-TEK PtRu/C catalyst.
Experimental Section Ternary PtRuAl alloy foils with different compositions were made by refining high-purity (>99.99%) Pt, Ru, and Al at high temperatures under the protection of high-purity argon in an arc furnace, followed by melt-spinning. The Al atomic content in all source alloys was controlled to be the same at 80 at. %. Dealloying of Al was carried out in 2 M NaOH solution for 48 h. The E-TEK Pt50Ru50/C (atomic ratio) catalyst was purchased from Aldrich
(23) Xu, C. X.; Su, J. X.; Xu, X. H.; Liu, P. P.; Zhao, H. J.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 12680–12681. (24) Ge, X.; Yan, X.; Wang, R.; Tian, F.; Ding, Y. J. Phys. Chem. C 2009, 113, 7379–7388. (25) Ding, Y.; Kim, Y. J.; Erlebacher, J. Adv. Mater. 2004, 16, 1897–1900. (26) Xu, C.; Wang, R.; Chen, M.; Zhang, Y.; Ding, Y. Phys. Chem. Chem. Phys. 2010, 12, 239–246. (27) Snyder, J.; Asanithi, P.; Dalton, A. B.; Erlebacher, J. Adv. Mater. 2008, 20, 4883–4890.
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with total metal loading at 28 wt % on carbon powder, which are denoted as PtRu/C in the following descriptions. Powder X-ray diffraction data were collected on a Bruker D8 advanced X-ray diffractometer using Cu KR radiation (λ = 1.5418 A˚) at a scan rate of 0.04° s-1. The microstructures of all samples were characterized on a JEOL JSM-6700F field emission scanning electron microscope (SEM) and a JEM-2100 highresolution transmission electron microscope (TEM and HRTEM). Surface composition and property of NP-PtRu alloys were analyzed with an ESCALab250 X-ray photoelectron spectroscopy (XPS). The surface area and pore size distribution were measured with a Quadrasorb SI-MP (Quantachrome Instruments) using the Brunauer-Emmett-Teller (BET) method. All electrochemical measurements were performed on a CHI 760C potentiostat in a standard three-electrode cell with Pt foil served as a counter electrode. Mercury sulfate electrode (MSE) was selected as the reference electrode. All potentials were provided according to the RHE scale for clarity. The catalyst suspensions were made by mechanically mixing 4.0 mg of catalysts powder, 1.0 mg of carbon powder, 300 μL of isopropanol, and 100 μL of Nafion solution (5 wt %). All mixtures were sonicated for 30 min to form a uniform suspension. Catalyst ink was placed on a polished 4 mm diameter glassy carbon electrode as the working electrode for electrochemical testing. Electrolyte solutions were deoxygenated by bubbling with high-purity N2 for at least 30 min prior to measurements. All current densities of Pt/Ru catalysts were normalized by integrating the CO stripping charge recorded on catalyst surface.28 The electrocatalytic activities of MOR on all catalysts were detected in 0.5 M H2SO4 þ 1.0 M CH3OH solution by using cyclic voltammetry. CO stripping experiments were carried out by first holding the thus-made electrodes at 0.15 V (vs RHE) in a 0.5 M H2SO4 solution with continuous CO bubbling for 20 min. The electrode was then transferred into a 0.5 M N2-purged H2SO4 solution to record the CO stripping profiles. Methanol adsorption/dehydrogenation was conducted by holding the thus-made electrodes at 0.20 V in 0.5 M H2SO4 þ 1.0 M CH3OH solution for 1 h at room temperature. The excess methanol on electrodes was removed by rinsing with ultrapure water, and the amount of dissociative (28) Jiang, J. H.; Kucernak, A. Electrochem. Commun. 2009, 623–626.
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Figure 3. XRD patterns of NP-PtRu alloys. The standard patterns of Pt (JCPDS 65-2868) and Ru (JCPDS 65-1863) are attached at the bottom for comparison.
Figure 2. SEM images of the resulted structures after dealloying Pt14Ru6Al80 alloy at room temperature for (a) 24 and (b) 72 h in 2 M NaOH solution and in (c) 0.5 and (d) 5 M NaOH solution for 48 h, respectively. intermediates was detected in 0.5 M N2-purged H2SO4 solution to record the stripping profiles.
Results and Discussion The nominal compositions of the source ternary alloys can be predetermined by controlling the initial feed ratio, which are Pt14Ru6Al80, Pt10Ru10Al80, and Pt6Ru14Al80, with different Pt/ Ru compositions, and these values were later on confirmed by compositional analysis with EDS. The phase structure of PtRuAl source alloys was first examined by XRD. As shown in Figure S1, all three source alloys show similar diffraction patterns. A majority of diffraction peaks can be ascribed to a PtAl6-type alloy (JCPDS 47-0891), indicative of possible formation of PtRuAl alloys. However, the rather complicated diffraction patterns cannot exclude the possible existence of small amount of individual phases of Pt, Ru, Al, or their alloys of other type. Interestingly, as discussed below, the possible existence of these minor phases does not seem to affect the formation of a uniform single-phase PtRu alloy after dealloying. Compared with Pt and Ru, the Al component is much more reactive and thus can be easily leached out in acid or alkaline solution at room temperature. In view of the great success of Raney’s method in making Raney metals such as Ni,29 we employed dilute NaOH solution as the electrolyte to selectively leach away Al. Figure 1a shows a representative SEM image of the sample after dissolving Al from Pt14Ru6Al80 alloy. The resulted Pt70Ru30 alloy is characterized by an open bicontinuous network structure with narrow pore and ligament size at ∼5 nm. TEM images provide more details for this structure. As illustrated in Figure 1b,c, the dark skeletons further confirm the formation of three-dimensional (3D) interconnected network structure at nanoscale. The inner bright regions represent the bicontinuous hollow channels embedded in the solid nanoarchitectures. Figure 1d provides a typical high-resolution TEM (HRTEM) image for this structure, where the highly ordered lattice fringes were well resolved with the lattice spacing calculated to (29) Raney, M. Ind. Eng. Chem. 1940, 32, 1199–1203.
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be ∼0.218 nm, corresponding to the (111) crystal plane of a PtRu alloy structure. It is interesting to find that the lattice fringes extend across several ligaments along (111) planes, indicating a singlecrystalline grain nature for the porous structure, which is similar to that observed in NPG.20,25 As show in Figure 1e,f, dealloying of Pt10Ru10Al80 and Pt6Ru14Al80 alloys also resulted in similar 3D porous structures with ligament size averaging at 5 and 7 nm, respectively. Nitrogen adsorption-desorption isotherms were used to further characterize the pore structure of the resulted nanoalloys. As shown in Figure S2, the pore size for NP-Pt70Ru30 mainly distributes around 4 nm, which is in good agreement with the TEM analysis. The surface area was measured to be ∼32 m2 g-1. NP-Pt70Ru30 alloy was selected as an example to monitor the structure evolution upon treating under different dealloying conditions. Figure 2a,b shows the resulted structures after dealloying Pt14Ru6Al80 alloy for 24 and 72 h in 2 M NaOH solution, respectively. It can be clearly found that the typical size of the nanoporous structure upon dealloying for 24 h locates around 3 nm. After dealloying for 48 h, the size mainly distributes at 5 nm (Figure 1a), while it is around 10 nm for 72 h sample. The resulted structures were also examined under different concentration of NaOH at a same dealloying time of 48 h. From Figure 2c, the nanoporous structure dealloyed in 0.5 M NaOH solution shows a uniform spongy morphology with pore/ligament size at ∼3 nm. When 2 or 5 M of NaOH solution was used, the resulted porous alloys show slightly coarsened structures (Figures 1a and 2d). On the basis of these experimental observations, it can be concluded that the structure size can be tuned by employing different concentrations of NaOH solution or different etching times. XRD was used to analyze the crystalline structure of NP-PtRu samples. It is known that Pt possesses a face-centered cubic (fcc) structure, while Ru has a hexagonal close-packed structure.30 Interestingly, the diffraction peaks for all NP-PtRu samples in Figure 3 could be ascribed to an fcc structure, which indicates a random distribution of Ru atoms on the cubic lattice sites of Pt.31 With an increase in Ru content the diffraction peaks shift to higher angles, indicating a lattice contraction due to the substitution of smaller Ru atoms. It should be noted that the diffraction patterns for all samples are quite similar to each other with a strong broad peak around 41.0° (2θ). The most pronounced diffraction peak can be assigned to the (111) reflection, while reflections corresponding to (200) and (220) planes are so weak that they are barely discernible in the XRD patterns. The very (30) Kim, Y.; Nama, S.; Shim, H.; Ahn, H.; Anand, M.; Kim, W. Electrochem. Commun. 2008, 10, 1016–1019. (31) Liu, Z. L.; Ling, X. Y.; Su, X. D.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234–8240.
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Figure 4. XPS spectra of Pt 4f core levels for NP-PtRu alloys. The dashed line represents location of the Pt 4f7/2 line for pure Pt.
broad diffraction mainly features the large surface stress developed during dealloying, because the grain size of our alloy samples is typically of order a few micrometers. The higher (111) reflection intensity reveals that all NP-PtRu alloys have preferentially oriented crystalline structure with (111) planes parallel to the supporting substrate, which is generally in good agreement with HRTEM observations. XPS is sensitive to examine the surface composition and the electronic structure of Pt for these alloy nanostructures. Figure 4 presents the XPS spectra for the Pt 4f core level region for all samples, which are characterized by two peaks corresponding to Pt 4f7/2 and Pt 4f5/2 photoemission lines. The binding energies (BE) of Pt 4f7/2 for all NP-PtRu alloys locate at ∼71.4 eV, and a slight right shift to higher BE can be observed as the Ru content increases. Compared with 71.2 eV for pure Pt,32 the positive shift of Pt 4f core level suggests a lowering of the Fermi level or an increase of the d-vacancy.33 Igarashi et al. have reported that CO chemisorption on Pt surface with an increased d-vacancy can lead to a smaller contribution of back-donation of Pt 5d electrons to the 2π* orbital of CO, resulting in a lowered CO coverage or in another words enhanced CO tolerance on Pt alloys.33 In our experiments, the positive shifts of BE indicate that the electronic structure of Pt was effectively modified due to the alloying with Ru, which may greatly improve the catalytic activity of Pt in terms of the methanol adsorption/dehydrogenation, activation of water, and CO tolerance. The surface compositions of NP-PtRu alloys estimated from XPS analysis were determined to be Pt72Ru28, Pt51Ru49, and Pt31Ru69, respectively, which is in excellent agreement with the initial feed ratios for bulk alloys. The uniform surface and bulk bimetallic compositions in NP-PtRu alloys provide a substantial basis to explore the effect of Ru content in the following electrochemical measurements. On the basis of a series of structure characterizations as described above, it can be concluded that NP-PtRu alloys can be easily fabricated at nearly absolute yield with controllable structure, uniform length scale, and uniform bimetallic composition. NP-PtRu alloys represent a class of particularly desirable catalysts because the interconnected skeletons are much favorable for the electron transport, while the open porous channels extending in all three dimensions facilitate the unblocked transport of medium molecules. It is thus interesting to explore the electrocatalytic performance of NP-PtRu alloys toward small organic molecules such as methanol. Figure 5 presents the (32) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Physical Electronics, Inc., Eden Prairie, MN, 1995; p 181. (33) Igarashi, H.; Fujino, T.; Zhu, Y.; Uchida, H.; Watanabe, M. Phys. Chem. Chem. Phys. 2001, 3, 306–314.
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Figure 5. Forward voltammetric curves of (a) specific activity and (b) the mass current density for NP-PtRu alloys and PtRu/C catalyst for MOR in mixed 0.5 M H2SO4 þ 1.0 M CH3OH solution, respectively. Scan rate: 20 mV/s.
forward voltammetric curves for NP-PtRu alloy samples and PtRu/C catalyst (the complete profiles are provided in Figure S3). It can be clearly found that these catalysts exhibit markedly different features for onset potential, peak potential, and specific activity. With the increase in electropotential, the oxidation current increases rapidly and reaches a maximum where the kinetics are optimized by a balance between the methanol dehydrogenation and subsequent oxidation of dissociative intermediates.34 The fast deactivation thereafter is ascribed to the formation of high surface coverage of OHads, which inhibits the availability of active sites for methanol dehydrogenation.34 The lower peak potential for MOR implicates the facilitated reaction kinetics for methanol dehydrogenation and the formation of OHads. From Figure 5, the peak on the NP-Pt70Ru30 sample centered at 0.81 V, whereas for NP-Pt50Ru50 sample, it slightly shifts to a lower potential at 0.79 V. The peak potential further decreases to ∼0.77 V when the Ru content increases to 70%. In view of the gradual negative shift of peak potential with an increase in Ru content, it is considered that a higher Ru content in the structure provides sufficient active sites to oxidize the COads intermediates by the oxygenated species on Ru atoms, which promote the reaction kinetics for more complete methanol oxidation. Notably, the peak potentials on all NP-PtRu alloys are evidently lower than that of PtRu/C (0.862 V) catalyst,13 as characterized by an evident negative shift about 50-80 mV. The more negative peak potential aslo indicates that methanol molecules can be more easily oxidized on these alloy nanostructures. As to the specific activity (Figure 5a), NP-Pt70Ru30 shows the highest current density with a value of ∼2.2 mA cm-2, and for NP-Pt50Ru50 it is ∼1.7 mA cm-2, both of which are higher than (34) Tripkovic, A. V.; Popovic, K. D.; Grgur, B. N.; Blizanac, B.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2002, 47, 3707–3714.
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that of PtRu/C catalyst (∼1.5 mA cm-2). For NP-Pt30Ru70 sample, it dramatically decreases in specific activity due to the incorporation of more Ru atoms into the structure, which leads to the dilution of catalytic Pt surface sites.35 Previous studies have proposed that several neighboring Pt sites are required in order to achieve the decomposition of methanol,36 while monomers (single atoms), dimers, or other isolated surface Pt clusters (so-called ensemble effect) are much less capable to provide an effective dissociative adsorption for methanol molecules and consequently will show low or no activity for MOR.37 As to the mass efficiency (Figure 5b), it was found that NP-Pt70Ru30 sample possesses the highest mass activity (150 mA/mg Pt) among all catalysts, which is much higher than that of PtRu/C catalyst (90 mA/mg Pt).13 Notably, NP-Pt50Ru50 and NP-Pt30Ru70 alloys also show higher mass activity than PtRu/C catalyst. Based on the observations above, higher electrocatalytic performance on NP-Pt70Ru30 alloy in terms of lower oxidation potential and higher current density can be ascribed to the optimum alloy composition among these NP-PtRu alloy catalysts because three nanoporous samples possess analogous morphology and similar length scale. Meanwhile, the unique bicontinuous network structure also contributes to the enhanced performance compared with PtRu/C catalyst because it favors the electron and mass transport across the entire electrode. Onset potential of MOR is also an important parameter to evaluate the catalytic activity at low potentials for electrocatalysts. From Figure 5a, it can be clearly observed that NP-PtRu alloys show an onset potential around 0.35 V, which is about 100 mV more negative than that of PtRu/C catalyst (∼0.45 V), a further evidence for the enhanced activity for methanol dehydrogenation at lower potentials as well as a possible activation of water.38 The comparison of specific activity at given potentials also demonstrates this viewpoint which is illustrated by the solid and dotted lines. Between 0.5 and 0.6 V, NP-Pt70Ru30 and NPPt50Ru50 samples exhibit similar specific activities, which are nearly 4 times the activity on the PtRu/C catalyst. It is noted interestingly, at potentials below 0.7 V, NP-Pt30Ru70 also performs better than PtRu/C catalyst. Methanol dehydrogenation and the corresponding stripping were carried out to further evaluate the low-potential catalytic activity on all PtRu alloy nanocatalysts. Markovic et al.39 have studied the methanol dehydrogenation on PtRu catalyst by in situ IR spectroscopy and have found that the formation of CO intermediates occurred at around 0.1 V via methanol dehydrogenation, while the onset of methanol oxidation to form CO2 begins at 0.45 V accompanied by a decrease of COads. Their observations indicate that oxygenated species that can oxidize methanol and/or its dissociative intermediates are not produced at lower potentials (
