Methanol Electrooxidation on PtRu Bulk Alloys and Carbon-Supported

pubs.acs.org/Langmuir © 2009 American Chemical Society

Methanol Electrooxidation on PtRu Bulk Alloys and Carbon-Supported PtRu Nanoparticle Catalysts: A Quantitative DEMS Study Hongsen Wang, Laif R. Alden,† F. J. DiSalvo, and H
ector D. Abru~na* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301. † Current address: BASF Catalysts LLC, Iselin, NJ Received January 28, 2009. Revised Manuscript Received April 27, 2009 Methanol electrooxidation on smooth Pt and PtRu bulk alloys and carbon-supported Pt and PtRu nanoparticle catalysts has been studied using cyclic voltammetry and potential step chronoamperometry combined with differential electrochemical mass spectrometry (DEMS). The current efficiencies for generated CO2 and methyl formate were calculated from Faradaic current (coulometric charge) and mass spectrometric currents (charges) at m/z = 44 and m/z = 60. The effects of Ru content in PtRu catalysts, catalyst loading/roughness, and the concentration of sulfuric acid as supporting electrolyte on the reaction kinetics and product distribution during methanol electrooxidation have been investigated. The results indicate that Pt-rich PtRu alloys and carbon-supported PtRu catalysts with ca. 20 atom % Ru content exhibit the highest catalytic activity for methanol electrooxidation, that is, the highest Faradaic current and the highest current efficiency for CO2 generation at low applied potentials. As the catalyst loading/roughness increases, the current efficiency for CO2 formation increases due to the further oxidation of soluble intermediates (formaldehyde and formic acid). At high concentrations of sulfuric acid, the electrooxidation of methanol was suppressed; both the oxidative current and the current efficiency of CO2 decreased, likely due to sulfate/bisulfate adsorption.

1. Introduction The electrochemical oxidation of methanol on platinum and platinum-based catalysts has been extensively studied in the last decades due to its potential application as a fuel in direct alcohol fuel cells (DAFCs).1-17 For fuel cells to be economically viable, the turnover rate for complete oxidation of methanol must be higher than 0.1 s-1 between +0.2 and +0.4 V versus the reversible hydrogen electrode (RHE).18 It is widely recognized that, as a single component catalyst, platinum is the only element that exhibits significant electrocatalytic activity toward methanol oxidation. The catalytic activity of platinum, however, is still much too low to consider direct methanol fuel cells (DMFCs) *Corresponding author. Telephone: 1-607-255-4720. Fax: 1-607-255-9864. E-mail: [email protected]. (1) Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735. (2) Smotkin, E. S.; Dı´ az-Morales, R. R. Annu. Rev. Mater. Res. 2003, 33, 557. (3) Solla-Gull
on, J.; Vidal-Iglesias, F. J.; L
opez-Cudero, A.; Garnier, E.; Feliu, J. M.; Aldaz, A. Phys. Chem. Chem. Phys. 2008, 10, 3689. (4) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (5) Watanabe, M.; Motto, S. J. Electroanal. Chem. 1975, 60, 259. (6) Gasteiger, H. A.; Markovi
c, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (7) Dubau, L.; Hahn, F.; Coutanceau, C.; L
eger, J.-M.; Lamy, C. J. Electroanal. Chem. 2003, 554-555, 407. (8) Casado-Rivera, E.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; V
azquezAlvarez, T.; Angelo, A. C. D.; DiSalvo, F. J.; Abru~na, H. D. J. Am. Chem. Soc. 2004, 126, 4043. (9) Wang, H.; Baltruschat, H. J. Phys. Chem. C 2007, 111, 7038. (10) Korzeniewski, C.; Childers, C. L. J. Phys. Chem. B 1998, 102, 489. (11) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967. (12) Wasmus, S.; Ku¨ver, A. J. Electroanal. Chem. 1999, 461, 14. (13) Jusys, Z.; Kaiser, J.; Behm, R. J. Electrochim. Acta 2002, 47, 3693. (14) Housmans, T. H. M.; Koper, M. T. M. J. Phys. Chem. B 2003, 107, 8557. (15) Madden, T. H.; Arvindan, N.; Stuve, E. M. J. Electrochem. Soc. 2003, 150, E1. (16) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522. (17) Basnayake, R.; Li, Z.; Katar, S.; Zhou, W.; Rivera, H.; Smotkin, E. S.; Casadonte, D. J.; Korzeniewski, C. Langmuir 2006, 22, 10446. (18) Lipkowski, J.; Ross, P. N. Electrocatalysis: Frontiers in Electrochemistry; Wiley: New York, 1998.

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as practical power sources. In order to improve the performance of DMFCs, platinum-based binary catalysts such as PtRu,6,11,16,17,19-26 PtSn,24,27-29 and PtMo30-32 have been examined toward the oxidation of methanol. PtRu binary catalysts, in particular, have proven to exhibit stable catalytic activity toward methanol electrooxidation. At present, the effects of a second metal as alloy or adlayer element on platinum, toward methanol oxidation, have been ascribed to the following reasons: (a) Bifunctional mechanism. Oxygen-containing species prefer to adsorb at Ru sites at potentials 0.2-0.3 V lower than at a pure platinum surface, and the carbonaceous species adsorbed on platinum sites are preferentially oxidized by oxygen-containing species formed on neighboring Ru atoms.6,19 (b) Electronic effect or ligand effect. The second metal modifies the electronic nature of the surface of the base metal.4,9,26 Such an effect was observed, for example, where the Pt-CO bond strength was found to be weakened by the presence of Ru or Sn.33 (19) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (20) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795. (21) Wang, H.; Wingender, C.; Baltruschat, H.; Lopez, M.; Reetz, M. T. J. Electroanal. Chem. 2001, 509, 163. (22) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994, 379, 307. (23) Tremiliosi-Filho, G.; Kim, H.; Chrzanowski, W.; Wieckowski, A.; Grzybowska, B.; Kulesza, P. J. Electroanal. Chem. 1999, 467, 143. (24) Morimoto, Y.; Yeager, E. B. J. Electroanal. Chem. 1998, 444, 95. (25) Kabbabi, A.; Faure, R.; Durand, R.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 41. (26) Waszczuk, P.; Wieckowski, A.; Zelenay, P.; Gottesfeld, S.; Coutanceau, C.; L
eger, J.-M.; Lamy, C. J. Electroanal. Chem. 2001, 511, 55. (27) Haner, A. N.; Ross, P. N. J. Phys. Chem. 1991, 95, 3740. (28) Wang, K.; Gasteiger, K. A.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 1996, 41, 2587. (29) Bittins-Cattaneo, B.; Iwasita, T. J. Electroanal. Chem. 1987, 238, 151. (30) Samjesk
e, G.; Wang, H.; Lo¨ffler, T.; Baltruschat, H. Electrochim. Acta 2002, 47, 3681. (31) Kita, H.; Nakajima, H.; Shimazu, K. J. Electroanal. Chem. 1988, 248, 181. (32) Nakajima, H.; Kita, H. Electrochim. Acta 1990, 35, 849. (33) Iwasita, T.; Nart, F. C.; Vielstich, W. Ber. Bunsen Ges. Phys. Chem. 1990, 94, 1030.

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Up to now, PtRu is generally accepted to be the best anode catalyst for DMFCs. Nevertheless, the optimal alloy composition and the mechanistic role of the cocatalysts are still a subject of ongoing research and discussion.34 Earlier, Watanabe and Motto reported that a Pt-Ru “black” alloy with 50 atom % ruthenium exhibited the maximal current toward methanol oxidation at 40 °C, and presented an explanation based on the theory of the bifunctional mechanism.19 At present, it is well-known that the bifunctional mechanism can be used to explain the more efficient oxidation of CO on Pt-Ru. However, unlike COads oxidation, methanol dehydrogenation to form adsorbed CO on Pt has been found to require at least three adjacent sites.6,35 Hence, a Pt-Ru alloy surface with 50 atom % Ru would not appear to be the optimal catalyst for methanol oxidation. Gasteiger et al. suggested an optimal PtRu alloy surface composition of ca. 10 atom % Ru for the electrooxidation of methanol at ambient temperatures. However, the optimal composition of PtRu alloys also appeared to depend on the temperature, the methanol concentration, and the potential range chosen for comparison.6,20 Other groups have reported that Pt-rich PtRu catalysts with a Ru content ranging from 10 to 40 atom % exhibit the highest activity toward methanol oxidation.11,13,16,25,36-39 Recently, Lamy et al. reported that PtRu (80:20)/C exhibits the highest activity and that a Pt + Ru/XC72 catalyst (a mixture of colloidal Pt, colloidal Ru, and carbon powder) is more active than a PtRu/XC72 (alloy) due to its higher ability to dehydrogenate methanol.7 Because of these inconsistent results about the optimal atomic ratio of PtRu catalysts for methanol oxidation, we have systematically examined the activity of different PtRu catalysts (including bulk PtRu alloy electrodes with different Ru content (from 10% to 50%) and carbon-supported Pt and PtRu nanoparticle catalysts (E-TEK) toward the eletrooxidation of methanol. In addition to the total oxidative current, the current efficiencies of generated CO2 and methyl formate during methanol electrooxidation were also determined via differential electrochemical mass spectrometry (DEMS). They were used for evaluating the electrocatalytic activity of PtRu electrodes. For methanol eletrooxidation, the maximal catalytic activity was observed for Pt0.8Ru0.2 among all Pt and PtRu alloy bulk electrodes investigated. PtRu/C electrodes exhibited a more negative onset potential toward methanol oxidation, and also higher Faradaic current and current efficiencies for CO2 generation in the low potential region (e0.65 V), when compared to Pt/C. We also found that the activity of PtRu/C electrodes can be further enhanced by partial dissolution of surface Ru atoms to create a Pt-rich surface. Adsorbed anions can affect the electrocatalytic activity of processes of practical importance. Methanol oxidation is a surface sensitive reaction. In contrast to perchloric acid, sulfate/ bisulfate anions (in sulfuric acid solution) can strongly adsorb to platinum and PtRu alloy surfaces and thus affect methanol adsorption, leading to the suppression of methanol oxidation.9 In this paper, we have also examined the effects of sulfuric acid concentration on the activity of Pt/C and PtRu/C catalysts toward methanol oxidation, and current efficiencies for CO2 generation. (34) Petrii, O. A. J. Solid State Electrochem. 2008, 12, 609. (35) Cuesta, A. J. Am. Chem. Soc. 2006, 128, 13332. (36) Entina, V. S.; Petry, O. A. Elektrokimiya 1968, 4, 678. (37) I
udice De Souza, J. P.; Iwasita, T.; Nart, F. C.; Vielstich, W. J. Appl. Electrochem. 2000, 30, 43. (38) El-Shafei, A. A.; Hoyer, R.; Kibler, L. A.; Kolb, D. M. J. Electrochem. Soc. 2004, 151, F141. (39) Solla-Gull
on, J.; Vidal-Iglesias, F. J.; Montiel, V.; Aldaz, A. Electrochim. Acta 2004, 49, 5079.

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2. Experimental Section DEMS Setup and Electrochemical Instruments. The DEMS setup has been described in detail in previous publications.40 In short, the DEMS setup consists of two differentially pumped chambers and a quadrupole mass spectrometer (Leybold Inficon Transpector H-100M). The main chamber was pumped by a Pfeiffer 65 L/s turbomolecular pump backed by a Pfeiffer dry diaphragm pump, in order to avoid contamination from oil vapors. The mass spectrometer analysis chamber was pumped by a Varian 250 L/s turbomolecular pump, which was backed by a Varian Triscroll dry pump, again to avoid contamination from oil vapor. The quadrupole mass spectrometer (Leybold Inficon Transpector H-100M) was connected to the analysis chamber and contained a Channeltron Electron multiplier/Faraday cup detector with a sensitivity of 100 A/Torr. The time constant of the mass spectrometer was in the millisecond regime. A dual thin-layer flow cell made of Kel-F was connected to the main chamber via an angle valve for the DEMS experiments. The construction of this cell was described in detail in previous papers.21,40,41 There are two compartments: the upper one for electrochemical reactions and the lower one for mass spectrometric detection, which are connected through six capillaries. In the upper compartment, the working electrode was placed against a ∼100 μm thick Teflon gasket with an inner diameter of 6 mm. This leaves an exposed area of 0.28 cm2 and results in an electrolyte volume of ∼3 μL. In the lower compartment, a porous Teflon membrane (Gore-Tex) was supported on a stainless steel frit and served as the interface between the electrolyte and vacuum. It was pressed against a ∼100 μm thick Teflon gasket with an inner diameter of 6 mm. The Gore-Tex Teflon membrane had a mean thickness of 75 μm, a mean pore size of 0.02 μm, and a porosity of 50%. Two Pt wires at the inlet and outlet of the thinlayer cell, which were connected through an external resistance (0.2-3 MΩ), were used as counter electrodes. A reversible hydrogen electrode (RHE), connected to the outlet of the DEMS cell through a Teflon capillary, served as reference electrode. The electrolyte flow was driven by the hydrostatic pressure in a supply bottle (flow rate about 10 μL/s), which ensured fast transport of species formed at the electrode to the mass spectrometric compartment, where the volatile products were evaporated into the vacuum system of the mass spectrometer (time constant, ca. 1 s) through the bare porous Teflon membrane. Electrochemical experiments were carried out with an EG&G model 173 potentiostat/galvanostat, a model 175 universal programmer, and homemade data acquisition software (CV_EI400) combined with a NI-DAQ card. All potentials are referred to a RHE (0.1 M H2SO4). Preparation of Pt and PtRu Bulk Electrodes. PtRu bulk alloys were synthesized by arc melting and subsequent annealing, as described in detail in previous publications.8,42 The final, pure Pt and PtRu alloys, pellets were cut into cylinders with a diameter of 3 mm and were press-fitted into Teflon rods with a diameter of 13 mm. Electrical contact was made through a graphite felt plug connected to a stainless steel holder. Contact resistances between the stainless steel holder and the surface of the electrode were less than 4 Ω. Once mounted, the electrodes were successively polished with 400, 600, 800, and 1200 grid emery paper (Buehler). Prior to measurements, they were subsequently polished with 0.3 and 0.05 μm Micro Polish Alumina (Buehler). Afterward, the electrode surfaces were cleaned by sonication in 5 M KOH, dipped briefly in 5 M H2SO4, and finally rinsed with ultrapure water.43 The Pt electrode had a roughness factor of 2.5-3 as determined from the charge associated with hydrogen adsorption (HUPD). (40) Wang, H.; Alden, L.; DiSalvo, F. J.; Abru~na, H. D. Phys. Chem. Chem. Phys. 2008, 10, 3739. (41) Wang, H.; Lo¨ffler, T.; Baltruschat, H. J. Appl. Electrochem. 2001, 31, 759. (42) Casado-Rivera, E.; G
al, Z.; Angelo, A. C. D.; Lind, C.; DiSalvo, F. J.; Abru~na, H. D. ChemPhysChem 2003, 4, 193. (43) Markovi
c, N. M.; Gasteiger, H. A.; Ross, P. N., Jr.; Jiang, X.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91.

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All PtxRuy electrodes were assumed to have the same roughness factor as the Pt electrode. We also attempted to use the CO stripping charge to determine the real surface area of PtxRuy electrodes.44 However, it appears that the CO saturation coverage decreases significantly with an increase in Ru content, likely due to the low adsorption energy of CO on PtRu bulk alloys and OH adsorption. This is different from the case of Ru modified Pt surfaces, in which the CO coverage on Pt increases slightly after modification with Ru.45 Therefore, current densities were calculated based on geometric surface areas. Chemicals and Solutions. All solutions were prepared with ultrapure Millipore water (18.2 MΩ cm). Sulfuric acid (J. T. Baker Ultrapure Reagent) solutions were used as supporting electrolyte; methanol (99.9+%) from Burdick&Jackson and methyl formate (anhydrous, 99%) from Sigma-Aldrich were also used in this work. All solutions were deaerated with high purity argon for at least 20 min and measurements were conducted at room temperature (22 ( 1 °C).

Preparation of Thin-Film Carbon-Supported Pt and PtRu Electrodes. Catalyst suspensions (inks) of Pt/C and PtRu/C (E-TEK Inc.) were prepared by mixing 8 mg (for 50 wt % metal loading on carbon) or 10 mg (for 20 wt % metal loading on carbon) of catalyst, 3980 μL of water, 1000 μL of isopropyl alcohol, and 20 μL of 5% Nafion solution and sonicating for 10 min. Thin-film Pt/C and PtRu/C electrodes were prepared by pipetting and drying 20 μL of the appropriate catalyst suspension on a mirror-polished glassy carbon disk (Sigradur G from Hochtemperatur Werkstoffe GmbH, 6 mm in diameter) electrode. The above procedure leads to a noble metal loading of 56 μg/cm2 (for 50 wt % metal loading on carbon) or 28 μg/cm2 (for 20 wt % metal loading on carbon). Pt/C (50 wt %) and Pt/C (20 wt %) had mean particle sizes of 3.3 and 3.7 nm, respectively. PtRu/C(50 wt %) and PtRu/C (20 wt %) had mean particle sizes of 2-3 nm. The electrochemically active surface areas of Pt/C (50 wt %) and Pt/C (20 wt %) electrodes were 12 ( 1 and 6 ( 0.5 cm2, respectively, as determined from the hydrogen adsorption charge (HUPD). The electrochemically active surface areas of PtRu/C (50 wt %) and PtRu/C (20 wt %) electrodes were 14 ( 1 and 13 ( 1 cm2, respectively, as determined by CO stripping charge, assuming the same CO coverage for Pt/C and PtRu/C surfaces.13,44

Calibration of DEMS Setup for CO2 and Methyl Formate. The calibration of the DEMS setup has been described in

detail in previous publications.40 For quantitative detection of the CO2 generated during methanol oxidation, a calibration experiment, involving formic acid oxidation, was carried out every day to obtain the calibration constant K*(44): K
ð44Þ ¼ 2QMS ð44Þ=QF

ð1Þ

where QF is the Faradaic charge corresponding to the oxidation of formic acid to CO2, QMS(44) is the integrated mass spectrometric current for CO2, and 2 is the number of electrons for formic acid oxidation to CO2. The calibration constant K*(60) for methyl formate was determined as follows: We prepared a methyl formate aqueous solution (10 mM) saturated with CO2, flowed this solution through the DEMS cell, and simultaneously monitored m/z = 44 and 60. The solution was made by initially saturating it with CO2 and subsequently adding methyl formate.  K
ð60Þ ¼ K
ð44Þ ðIMS ð60Þ=CHCOOCH3 Þ= ðIMS ð44Þ=CCO2 Þ ¼ 4%K
ð44Þ

ð2Þ

(44) Nagel, T.; Bogolowski, N.; Baltruschat, H. J. Appl. Electrochem. 2006, 36, 1297. (45) Massong, H.; Wang, H.; Samjesk
e, G.; Baltruschat, H. Electrochim. Acta 2000, 46, 701.

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where IMS(60) and IMS(44) are the intensities of m/z = 60 and 44, respectively, CHCOOCH3 and CCO2 are the concentrations of methyl formate (10 mM) and CO2 (37 mM), respectively, and K*(44) is the calibration constant of CO2. An average value for the current efficiency of CO2 or methyl formate for methanol oxidation was obtained by integrating the Faradaic current (QF) and mass spectrometric current of m/z = 44 (QMS(44)) or m/z = 60 (QMS(60)) in one potential sweep cycle. The average current efficiencies of CO2 and methyl formate were calculated using eqs 3a and 3b, respectively: ηQ ¼ ð6QMS ð44Þ=K
ð44ÞÞ=QF

ð3aÞ

ηQ ¼ ð4QMS ð60Þ=K
ð60ÞÞ=QF

ð3bÞ

in which ηQ is the average current efficiency, 6 and 4 are the number of electrons for methanol oxidation to CO2 and methyl formate, respectively, and K*(44) and K*(60) are the calibration constants for CO2 and methyl formate, respectively. Similarly, the current efficiencies of CO2 and methyl formate at a constant potential can also be determined using eqs 4a and 4b, respectively: ηI ¼ ð6IMS ð44Þ=K
ð44ÞÞ=IF ð4aÞ ηI ¼ ð4IMS ð60Þ=K
ð60ÞÞ=IF

ð4bÞ

where ηI is the current efficiency, IF is the total Faradaic current, IMS(44) and IMS(60) are the mass spectrometric currents for CO2 and methyl formate, respectively; 6 and 4 are the numbers of electrons for methanol oxidation to CO2 and methyl formate, respectively.

3. Results 3.1. Methanol Electrooxidation on PtRu Bulk Alloys. Cyclic Voltammograms of PtRu Bulk Alloys with Different Ru Content in 0.1 M H2SO4 Solution. Figure 1 shows the cyclic voltammograms (CVs) of different PtRu bulk alloys in 0.1 M H2SO4 solution. The CVs recorded for different PtRu bulk alloys are similar to those presented in the literature.6,46 With an increase in Ru content, the hydrogen adsorption/ desorption peaks diminish in amplitude. Compared to pure platinum, the double layer region of PtRu becomes broader due to the formation and reduction of Ru oxide at the PtRu alloy surface. Methanol Electrooxidation on PtRu Bulk Alloys with Different Ru Content in 0.2 M CH3OH + 0.1 M H2SO4 Solution. Figure 2 shows the Tafel plots for different PtRu bulk alloys in 0.2 M methanol + 0.1 M H2SO4 solution. The electrooxidation of methanol on pure platinum onsets at around +0.5 V, where adsorbed CO starts to be oxidized. Compared to pure platinum, the onset potential for methanol electrooxidation is shifted negatively on all PtRu bulk alloys. In particular, the Pt0.8Ru0.2 bulk alloy exhibits the highest activity toward methanol oxidation, that is, over 100 mV negative shift in the onset potential, and the highest current density at potentials below +0.7 V. In order to more fully evaluate the activity of a catalyst, in addition to the current density, the current efficiency or yield of CO2 is also an important point to consider. To monitor the formation of CO2, we performed DEMS experiments. For small organic molecules (SOMs) whose oxidation overlaps the oxidation of the electrode surface, DEMS offers the advantage that the (46) Hoster, H.; Iwasita, T.; Baumgartner, H.; Vielstich, W. J. Electrochem. Soc. 2001, 148, A494.

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Figure 1. Cyclic voltammograms of smooth polycrystalline Pt and PtRu bulk electrodes in 0.1 M H2SO4 solution. Scan rate: 50 mV/s.

Figure 2. Tafel plots of methanol oxidation on smooth Pt and PtRu alloy electrodes in 0.2 M methanol + 0.1 M H2SO4 solution in DEMS cell. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s.

mass spectrometric current can rule out the pseudocapacitance interference from surface (Ru, in this case) oxidation. Figure 3 shows typical cyclic voltammograms (CVs) and mass spectrometric cyclic voltammograms (MSCVs) at m/z = 44 (corresponding to CO2) for methanol electrooxidation at a pure platinum electrode and a Pt0.8Ru0.2 alloy electrode. The formation of CO2 on the Pt0.8Ru0.2 alloy starts at +0.4 V, that is, over 100 mV more negative than on pure platinum. After calibrating the DEMS setup, the current efficiencies of CO2 were calculated, and they are presented in Figure 4. The current efficiency of CO2 generation increases with an increase in Ru content with a maximum efficiency of about 50% at a Ru content of 20 atom % and subsequently decreases with further increases in Ru content. Therefore, the Pt0.8Ru0.2 alloys exhibit the highest activity, among all studied PtRu alloys, for complete oxidation of methanol to CO2. 3.2. Methanol Electrooxidation on Carbon-Supported Pt and PtRu Catalysts. Cyclic Voltammograms of CarbonSupported Pt and PtRu Catalysts in 0.1 M H2SO4 Solution. Figure 5 shows the CVs of carbon-supported Pt and PtRu (47) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Bo¨nnemann, H. J. Electrochem. Soc. 1998, 145, 925.

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Figure 3. Simultaneously recorded CVs (a) and MSCVs of CO2 at m/z = 44 (b) on smooth polycrystalline Pt and Pt0.8Ru0.2 electrodes in 0.2 M methanol + 0.1 M H2SO4 solution. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s.

Figure 4. Current efficiencies (η) of CO2 generation during methanol electrooxidation on smooth polycrystalline Pt and PtRu alloy electrodes plotted versus Ru content. “Average” means the average current efficiency of CO2 generation in one potential cycle; “0.7 V” indicates current efficiency of CO2 generation at +0.7 V.

catalysts in 0.1 M H2SO4. The CVs of Pt/C and PtRu/C are consistent with those from the previous publications.39,47-49 Similarly to polycrystalline platinum, the two carbon-supported nanoparticle platinum (Pt/C) electrodes also exhibited two pairs of hydrogen adsorption/desorption peaks between +0.05 and +0.35 V in the CVs. However, the double layer region was relatively large likely due to the high surface area of the carbon support. From the charge for oxidation of a full monolayer of adsorbed hydrogen, the electrochemically active surface areas for Pt/C (50 wt %) and Pt/C (20 wt %) electrodes were estimated to be 12 ( 1 and 6 ( 0.5 cm2, respectively, which are equivalent to ca. 0.75 cm2/μg Pt. For the two carbon-supported nanoparticle PtRu (PtRu/C) electrodes, the hydrogen adsorption/desorption peaks could not be observed, in a manner similar to the bulk Pt0.5Ru0.5 electrode. In the case of PtRu/C electrodes, HUPD could not be used to estimate the real surface area due to OH (48) Korzeniewski, C.; Basnayake, R.; Vijayaraghavan, G.; Li, Z.; Xu, S.; Casadonte, D. J., Jr. Surf. Sci. 2004, 573, 100. (49) Savinova, E. R.; Hahn, F.; Alonso-Vante, N. J. Phys. Chem. C 2008, 112, 18521.

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Figure 5. Cyclic voltammograms of carbon-supported Pt and PtRu catalyst (E-TEK, 50 wt % and 20 wt %) electrodes in 0.1 M H2SO4 solution. Scan rate: 10 mV/s.

adsorption, even below 0.2 V.39,44 For these two PtRu/C electrodes, we used the CO stripping charge to estimate their electrochemically active surface areas, which were 14 ( 1 cm2 for PtRu/C (50 wt %) and 13 ( 1 cm2 for PtRu/C (20 wt %).13,44 It should be noted that, for CO stripping on PtRu/C, the non-Faradaic charge could constitute 50% of total electrochemical charge in cyclic voltammograms, even after background subtraction.44,50 Therefore, we used the mass spectrometric charge at m/z = 44 to estimate the real surface area, assuming that the saturated CO coverage on PtRu/C was the same as on Pt/C.13,39,44 Methanol Electrooxidation on Carbon-Supported Pt and PtRu Catalysts in 0.2 M CH3OH + 0.1 M H2SO4 Solution. Methanol electrooxidation on carbon-supported Pt and PtRu catalysts (50 wt %) was studied by cyclic voltammetry combined with mass spectrometry, and the results are presented in Figure 6 (left-hand panel). Similar to polycrystalline platinum, methanol oxidation on the Pt/C electrode starts at around +0.5 V, where the formation of CO2 and methyl formate also onsets. After calibrating the DEMS setup, the average current efficiencies for generation of CO2 and methyl formate over the potential region between +0.05 and +0.75 V were ca. 65% and 1%, respectively. For the PtRu/C electrode, methanol oxidation starts at +0.35 V, that is, over 150 mV more negative than on the Pt/C electrode. The formation of CO2 also starts around +0.35 V, and, in contrast to this, the formation of methyl formate occurs around +0.5 V. The average current efficiencies of CO2 and methyl formate formation over the potential range between +0.05 and +0.75 V were ca. 87% and 1%, respectively. These results are consistent with the previous finding that Ru addition to Pt can promote the complete oxidation of methanol to CO2.9,21,41,51 Methanol oxidation on PtRu/C (20 wt %) was also studied (not shown). The average current efficiency of CO2 over the potential range between +0.05 and +0.75 V was ca. 73%, which is less than that for PtRu/C (metal loading: 50 wt %), likely due to lower catalyst loading/lower surface area.21,41,51 (50) Jusys, Z.; Massong, H.; Baltruschat, H. J. Electrochem. Soc. 1999, 146, 1093. (51) Wang, H.; Baltruschat, H. Proc. Electrochem Soc. 2001, 4, 50.

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To compare the catalytic activity of Pt/C and PtRu/C (50 wt %) catalysts at steady state, potential step measurements were also performed. Figure 6 (right-hand panel) shows the simultaneously recorded transients of Faradaic current and mass spectrometric current for CO2 at m/z = 44 on Pt/C and PtRu/C electrodes in 0.2 M methanol + 0.1 M H2SO4 solution after stepping the potential from +0.05 to +0.6 V. Immediately following the potential step, a very high oxidative current and a very high rate of CO2 formation were observed on the Pt/C electrode. However, the Faradaic current and the rate of CO2 formation decayed very fast, ostensibly due to poisoning by CO. Although the PtRu/C exhibited a relatively low initial oxidative current, the current decayed much more slowly. Ten minutes after the potential step, the Faradaic currents and mass spectrometric currents of CO2 were used to calculate the current efficiencies of CO2 generation. In the low potential region, such as +0.6 V, PtRu/C exhibited a higher current than Pt/C toward methanol oxidation, and the current efficiencies of CO2 generation were ca. 58% and 81% for the Pt/C and PtRu/C catalysts, respectively. Methanol oxidation on the PtRu/C (20 wt %) was also performed at +0.6 V (not shown). The current efficiency of CO2 generation at +0.6 V was ca. 74%, which is lower than the 81% determined for the PtRu/C (50 wt %), again due to lower catalyst loading/ lower real surface area. Sulfuric Acid Concentration Effect on Methanol Electrooxidation on Carbon-Supported Pt and PtRu Catalysts. In numerous previous reports on methanol electrooxidation, sulfuric acid is often used as the supporting electrolyte. However, the effect of sulfuric acid concentration on methanol electrooxidation, especially product distribution, has not been investigated. In this section, we present findings on the effects of sulfuric acid concentration on methanol electrooxidation on carbonsupported Pt and PtRu catalysts, and focus on the variation of activity and current efficiency of CO2 generation with the concentration of sulfuric acid. Figure 7 (left-hand panel) shows CVs (a) and MSCVs for CO2 at m/z = 44 (b) and methyl formate at m/z = 60 (c) for methanol oxidation on carbon-supported Pt in 0.2 M methanol solution at different concentrations of sulfuric acid as supporting electrolyte. It is evident that the concentration of sulfuric acid can affect the oxidative current. While changing the concentration of sulfuric acid from 0.1 to 0.5 M did not affect the onset potential, and the peak current only increased slightly, when the concentration of sulfuric acid was further increased to 2 M, the oxidation of methanol was significantly suppressed; that is, there was a significant decrease in the oxidative current and partial suppression of CO2 formation. After calibrating the DEMS setup, the average current efficiencies for CO2 generation over the potential region between +0.05 and +0.75 V were ca. 65%, 63%, and 44%, respectively, when the concentrations of sulfuric acid were 0.1, 0.5, and 2.0 M. The average current efficiency for methyl formate increased from ca. 1% to 1.3% with changing the sulfuric acid from 0.1 to 2.0 M. The effect of the concentration of sulfuric acid on methanol oxidation at a constant potential of +0.6 V is presented in Figure 7 (right-hand panel). With an increase in the sulfuric acid concentration, both the Faradaic current and the formation of CO2 decreased at steady state. After 10 min of oxidation reaction at +0.6 V, the current efficiencies of CO2 generation in 0.1, 0.5, and 2.0 M sulfuric acid as supporting electrolyte were ca. 58%, 47%, and 39%, respectively. The effects of sulfuric acid concentration on methanol oxidation on PtRu/C electrodes are presented in Figure 8 (lefthand panel). It is clear that, with an increase in sulfuric acid DOI: 10.1021/la900305k

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Figure 6. (Left-hand panel) Simultaneously recorded CVs (a) and MSCVs of CO2 at m/z = 44 (b) and methyl formate at m/z = 60 (c) on carbon-supported Pt and PtRu catalyst (E-TEK, 50 wt %) electrodes in 0.2 M methanol + 0.1 M H2SO4 solution. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s. (Right-hand panel) Simultaneously recorded Faradaic currents (a) and mass spectrometric currents of CO2 at m/z = 44 (b) on carbon-supported Pt and PtRu catalyst (E-TEK, 50 wt %) electrodes in 0.2 M methanol + 0.1 M H2SO4 solution at +0.6 V (RHE). Electrolyte flow rate: 10 μL/s.

Figure 7. (Left-hand panel) Simultaneously recorded CVs (a) and MSCVs of CO2 at m/z = 44 (b) and methyl formate at m/z = 60 (c) on carbon-supported Pt catalyst (E-TEK, 50 wt %) electrode in 0.2 M methanol solution with different concentrations of H2SO4. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s. (Right-hand panel) Simultaneously recorded Faradaic currents (a) and mass spectrometric currents of m/z = 44 (b) on carbon-supported Pt catalyst (E-TEK, 50 wt %) electrode in 0.2 M methanol solution with different concentrations of H2SO4 at +0.6 V (RHE). Electrolyte flow rate: 10 μL/s.

concentration, the oxidative current and the rate of CO2 formation decrease. The average current efficiencies of CO2 over the potential region between +0.05 and +0.75 V in 0.1, 0.5, and 2.0 M sulfuric acid, as supporting electrolyte, were ca. 87%, 79%, and 52%, respectively, which are much higher than those on Pt/C electrodes. Average current efficiencies for methyl formate for all cases were about 1%. Similar to the results from cyclic voltammetry, at a constant potential of +0.6 V, the oxidative current and the rate of CO2 formation also decreased with an increase in sulfuric acid concentration, as shown in Figure 8 (right-hand panel). After 10 min of oxidation reaction at +0.6 V, the current efficiencies for CO2 generation in 0.1, 0.5, and 2.0 M sulfuric acid as supporting electrolyte were ca. 81%, 78%, 7730 DOI: 10.1021/la900305k

and 68%, respectively, which are also higher when compared to those on Pt/C electrode. The effects of sulfuric acid concentration, catalyst loading/ roughness and Ru content on the current efficiency of CO2 during methanol oxidation are summarized in Figure 9. Compared to Pt catalysts, PtRu catalysts exhibit higher current efficiencies for CO2 generation from methanol electrooxidation. With an increase in the catalyst loading/roughness, higher current efficiencies of CO2 are observed due to the higher likelihood of further oxidation of intermediates formed during methanol oxidation. With an increase in sulfuric acid concentration, both the current and CO2 formation are suppressed during methanol electrooxidation on Langmuir 2009, 25(13), 7725–7735

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Figure 8. (Left-hand panel) Simultaneously recorded CVs (a) and MSCVs of CO2 at m/z = 44 (b) and methyl formate at m/z = 60 (c) on carbon-supported PtRu catalyst (E-TEK, 50 wt %) electrode in 0.2 M methanol solution with different concentrations of H2SO4. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s. (Right-hand panel) Simultaneously recorded Faradaic currents (a) and mass spectrometric currents of m/z = 44 (b) on carbon-supported PtRu catalyst (E-TEK, 50 wt %) electrode in 0.2 M methanol solution with different concentrations of H2SO4 at +0.6 V (RHE). Electrolyte flow rate: 10 μL/s.

Figure 9. Comparison of current efficiency for CO2 generation during methanol oxidation on different electrodes at +0.6 V (RHE) for different concentrations of sulfuric acid. Error bar: 10 %.

Pt and PtRu catalysts, likely due to increased sulfate/bisulfate adsorption. Activation of PtRu/C Electrodes for Methanol Oxidation. As previously mentioned, the Pt0.8Ru0.2 bulk electrode was found to be more active than the Pt0.5Ru0.5 bulk electrode toward methanol oxidation at ambient temperatures. However, commercial carbon-supported PtRu catalysts normally have an atomic ratio of 1:1. Here, we wanted to determine if PtRu/C electrodes could be activated by partially dissolving some surface Ru atoms. In the DEMS cell, we scanned the potential between +0.05 and +1.15 V at a scan rate of 10 mV/s and at an electrolyte flow rate of 10 μL/s for 35 cycles in 0.1 M H2SO4 solution. The final CV of the activated PtRu/C electrode is shown in Figure 10. The hydrogen adsorption/desorption peaks on Pt become visible and very similar to those of the Pt0.8Ru0.2 bulk electrode, indicating that this surface is enriched in Pt (and based on the voltammetric response, we estimate that the ratio of Ru/Pt is close to 20:80). First, we characterized the activated surface by CO stripping, as shown in Figure 11. It should be noted that, for CO stripping on PtRu surfaces, non-Faradaic charges can reach about 50% of the total charge even after background subtraction due to Ru hydroxyl formation and anion readsorption after CO removal.44,50 Therefore, a much higher CO stripping peak was observed for PtRu/C, when compared to Pt/C and activated Langmuir 2009, 25(13), 7725–7735

Figure 10. CVs of carbon-supported PtRu catalyst (E-TEK, 50 wt %) electrode in 0.1 M H2SO4 solution before and after activation by dissolving part of surface Ru atoms. Scan rate: 50 mV/s.

PtRu/C, although the CO2 mass spectrometric charges were comparable. (This, again, underscores the value of DEMS measurements). The COads oxidation on PtRu/C exhibited a peak at ca. +0.52 V, which is over 200 mV negative of the value for Pt/C. The COads stripping peak is further negatively shifted DOI: 10.1021/la900305k

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Figure 11. Simultaneously recorded CVs (a) and MSCVs of CO2 at m/z = 44 (b) for CO stripping on carbon-supported Pt and PtRu catalyst (E-TEK, 50 wt %) electrodes in 0.1 M H2SO4 solution. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s. (dotted black line) Pt/C; (solid black line) PtRu/C before activation; (solid gray line) PtRu/C after activation.

to +0.48 V following activation. While this result is in agreement with some recent work,7,39 it is inconsistent with previous earlier reports in which Pt0.5Ru0.5 bulk alloys exhibited the highest activity toward the oxidation of adsorbed CO.52,53 This discrepancy could be due to the difference between a bulk alloy and a nanoparticle alloy. After CO stripping measurements, we examined the activity of this electrode toward methanol oxidation, and the results are shown in Figure 12. Compared to a PtRu/C electrode, the activated surface is more active, although the onset potential remained invariant. The current for methanol oxidation and the rate of CO2 generation at the peak potential increased by a factor of over 2.5, while the current efficiency of CO2 generation remained virtually constant. This result also suggests that, for carbon-supported PtRu catalysts, a Pt-rich surface is more active than one with an atomic ratio of Pt/Ru of 50:50. This result is consistent with findings from a number of groups.7,54 If the potential is scanned to an even more positive potential limit (e.g., +1.15 V), the PtRu/C electrode loses most of its electrocatalytic activity in the reverse scan due to the blocking effect of the oxide film formed. However, the activated PtRu/C electrode still exhibits catalytic activity in the reverse scan (Figure 12, right-hand panel).

4. Discussion It is generally accepted that PtRu catalysts are the most active for methanol electrooxidation. However, the optimal Ru/Pt ratio is still not definitively established. The electrocatalytic activity of PtRu toward methanol electrooxidation dates back to the 1960s.34,55 Early on, Watanabe et al. reported that PtRu, with a Ru content of 0.5, exhibited the highest activity toward methanol oxidation, and assumed that the bifunctional mechanism played (52) Kabbabi, A.; Faure, R.; Durand, R.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 41. (53) Gasteiger, H. A.; Markovic, N. A.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (54) Dubau, L.; Coutanceau, C.; Garnier, E.; Leger, J.-M.; Lamy, C. J. Appl. Electrochem. 2003, 33, 419. (55) Petry, O. A.; Podlovchenko, B. I.; Frumkin, A. N.; Lal, H. J. Electroanal. Chem. 1965, 10, 253.

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the dominant role in the enhancement.19 Later on, considering that methanol decomposition to CO requires three neighboring Pt atoms, Ross’ group proposed a different model for the catalytic effect of Ru on methanol oxidation, in which a surface structure consisting of one Ru atom neighboring three Pt sites represented the optimal geometry for methanol oxidation.6 According to this, an optimal alloy surface composition with ca. 10 atom % Ru for the electrooxidation of methanol was predicted. However, numerous experimental results did not support such a prediction. Wieckowski and Chrzanowski studied the electrocatalytic activity of Ru-modified low-index platinum single-crystal faces, Pt(111), Pt(100), and Pt(110), toward methanol electrooxidation and found that the maximum reactivity coverage of Ru varied with the platinum surface structure. They reported values of 0.2, 0.3, and 0.15 for Pt(111), Pt(100), and Pt(110), respectively.11 I
udice De Souza et al. found that at room temperature a Pt-Ru (θRu = 0.25) layer electrodeposited on gold substrates had the highest activity for methanol oxidation.37 Iwasita et al. studied the electrooxidation of methanol on Pt(111)/Ru electrodes with different Ru coverages, and a maximum in catalytic activity was observed between 10% and 40% of Ru coverage.16 Kolb et al.38 examined the electrocatlytic behavior of three preferentially oriented “Pt(111)”, “Pt(110)”, and “Pt(100)” surfaces modified by Ru submonolayers toward methanol oxidation and found that the enhancement factor depended on the surface crystallography and on the Ru coverage. They reported that the optimal Ru coverage at room temperature was 30-40% of a monolayer for all three surfaces and that the catalytic activity at optimal Ru coverage decreased in the order: “Pt(111)” > “Pt(110)” > Pt(111) > “Pt(100)”. Jusys et al. found that unsupported PtRu alloy nanoparticles with 15 atom % Ru exhibited the highest activity.13 Solla-Gullon et al. found that Pt80Ru20 nanoparticles are the best electrocatalysts for both COads and methanol oxidation.39 Recently, Lamy et al. reported that carbon-supported PtRu with 20 atom % Ru was the most active catalyst toward methanol oxidation and that a simple mixture of Pt and Ru nanoparticles supported on Vulcan was superior to Vulcansupported PtRu alloy catalysts.7 Thus, it seems that the optimal ratio of Ru/Pt varies with the preparation methods, that is, codeposition, adatom, alloy, and mixtures of Pt and Ru nanoparticles. From these results, it appears that the Ru distribution and segregation on the surface play key roles in determining the optimal Ru/Pt ratio.48,56 Recently, Cuesta reported that an atomic ensemble of at least three contiguous Pt atoms is required for the decomposition of methanol to yield adsorbed CO on Pt(111) surfaces.35 Extending this concept to PtRu systems, the distribution of Pt and Ru atoms on PtRu surfaces could play a major role in the measured electrocatalytic activity. Friedrich et al. found that, for Ru modified Pt(111) surfaces, the distribution of Ru adatoms is not homogeneous and that Ru adatoms form 2-5 nm islands on Pt(111) surfaces.57 However, in PtRu alloys, Ru appears to be uniformly distributed. In the case of PtRu nanoparticles, the distribution of Ru could be different from that in bulk PtRu alloys; even for different synthesis methods.48 Korzeniewski et al. found that PtRu nanoparticles, with separated Pt and Ru phases, were more active than PtRu alloy nanoparticles.48 Lewera et al. reported that all bimetallic PtRu nanoparticles in the reduced state were more active toward methanol oxidation than partially or fully oxidized nanoparticles and that the activity decreased in (56) Lewera, A.; Zhou, W. P.; Vericat, C.; Chung, J. H.; Haasch, R.; Wieckowski, A.; Bagus, P. S. Electrochim. Acta 2006, 2006, 3950. (57) Friedrich, K. A.; Geyzers, K.-P.; Linke, U.; Stimming, U.; Vogel, R. Z. Phys. Chem. 1999, 208, 137.

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Figure 12. Simultaneously recorded CVs (a) and MSCVs of CO2 at m/z = 44 (b) on carbon-supported PtRu catalyst electrodes with and without activation in 0.2 M methanol + 0.1 M H2SO4 solution. Scan rate: 10 mV/s. Electrolyte flow rate: 10 μL/s. Potential region: (left-hand panel) +0.05 to +0.75 V; (right-hand panel) +0.05 to +1.15 V.

the sequence: Ru modified Pt > PtRu alloy > core-shell Pt-on-Ru.56 Long et al. also found that a PtRu alloy nanoparticle catalyst has orders of magnitude less activity, for methanol oxidation, than a mixed-phase electrocatalyst containing Pt metal and hydrous ruthenium oxides (RuOxHy). They proposed that significant quantities of electron-proton conducting RuOxHy are required to achieve high activity for methanol oxidation.58 The PtRu particle interconnection, related to catalyst loading per unit area of carbon support, could also influence the activity of PtRu. Gavrilov et al. reported that as the amount of metal per unit surface area of carbon increased, the specific catalytic activity toward CO monolayer oxidation and methanol oxidation was significantly enhanced.49,59 They proposed that intergrain boundaries, connecting crystalline domains in nanostructured PtRu catalysts, produced at high metal-on-carbon loadings, provide active sites for electrocatalytic processes. According to the above discussions, the distribution and segregation of Ru in PtRu catalysts needs to be further systematically studied. However, such a study is beyond the scope of this work. Here, we compared the effects of Ru content on the activity of PtRu alloy bulk catalysts and PtRu alloy nanoparticle catalysts (E-TEK)60,61 toward methanol oxidation on the basis of DEMS measurements. In general, it requires the consideration of some key factors to balance the decomposition of methanol to CO and its further oxidation to CO2. In Ross’ model, it is assumed that methanol oxidation proceeds only via adsorbed CO and that the dissociative adsorption of methanol is the rate-determining step for methanol oxidation on PtRu electrodes. On the other hand, the electronic effects of Ru, on methanol adsorption, were not considered. In fact, at low potentials, a low Ru content can promote methanol decomposition to form adsorbed CO,9,51 and the further oxidation of methanol adsorbates could also control the overall process. While Ru can promote the oxidation of adsorbed CO on Pt, it can also block the Pt surface, which would appear to be (58) Long, J. W.; Stroud, R. M.; Swider-Lyons, K. E.; Rolison, D. R. J. Phys. Chem. B 2000, 104, 9772. (59) Gavrilov, A. N.; Savinova, E. R.; Simonov, P. A.; Zaikovskii, V. I.; Cherepanova, S. V.; Tsirlina, G. A.; Parmon, V. N. Phys. Chem. Chem. Phys. 2007, 9, 5476. (60) Radmilovic, V.; Gasteiger, H. A.; Ross, P. N. J. Catal. 1995, 154, 98. (61) Vogel, W. J. Phys. Chem. C 2008, 112, 13475.

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Figure 13. Schemetic representation of PtRu(111) alloy surface with 25 atom % Ru.

unfavorable for COads formation from methanol decomposition without considering the electronic effect of Ru, since methanol adsorption does not occur on pure Ru at room temperature.6 As a result, on PtRu catalysts, there would be a competition between the rate of formation of adsorbed CO and the rate of oxidative removal, which would, in turn, depend on Ru coverage. However, on PtRu surfaces with very small amounts of Ru, COads formation from methanol decomposition is not inhibited. In fact, it is even enhanced due to electronic effects.9,51 Considering electronic effects and the bifunctional mechanism, a Ru coverage of 25 atom % would appear to be the optimal composition, as shown in Figure 13 (note that here only the (111) surface is considered). Around each Ru atom, there are three ensembles of three contiguous Pt atoms, which, as mentioned previously, would be required for methanol decomposition.35 Each Pt atom has two neighboring Ru atoms, which change the electronic properties of the Pt atoms and thus enhance methanol decomposition,9,51 and also help oxidize adsorbed CO via the bifunctional mechanism.19 Among all the PtRu samples that we studied, Pt0.8Ru0.2 and Pt0.7Ru0.3 alloys exhibited the best performance, that is, the lowest onset potentials and the highest oxidation currents at low potentials. These observations are consistent with the above viewpoint. As we mentioned before, except for Watanabe and Motoo’s result,19 most other published results indicate that the most active PtRu catalysts are Pt-rich.6,7,13,16,37,38 The apparent discrepancy of Watanabe and Motoo’s results19 could arise from an overestimation of the Ru coverage which was based on the adsorption charge of OH on Ru atoms. In addition, the formation of RuO2 could also affect the Ru coverage values.62 (62) Rose, A.; Crabb, E. M.; Qian, Y.; Ravikumar, M. K.; Wells, P. P.; Wiltshire, R. J. K.; Yao, J.; Bilsborrow, R.; Mosselmans, F.; Russell, A. E. Electrochim. Acta 2007, 52, 5556.

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Article Scheme 1. Simplified Scheme of the Pathways for Methanol Oxidation on Platinum Electrodes9,51,a

a x = 1-3, “sur” indicates the electrode surface, “bulk” refers to bulk solution, “diff” indicates the diffusion of species, and “ad” denotes an adsorbed species.

As discussed in previous papers,9,51 methanol oxidation on Pt and PtRu catalysts proceeds via a complex parallel and consecutive reaction mechanism, which is described in Scheme 1. In one pathway, methanol is oxidized via COads to form CO2, while in another methanol oxidation proceeds via soluble intermediates, that is, formaldehyde and formic acid. The extent to which these soluble intermediates are further oxidized to CO2 depends on the convection/diffusion conditions. At high flow rates, high concentration of methanol, and on smooth electrode surfaces, most of the soluble intermediates are transported away from the electrode without further oxidation to CO2. For methanol oxidation on smooth Pt surfaces, formation of soluble intermediates (formaldehyde and formic acid) dominates over CO2 generation. The low current efficiency of CO2 generation (ca. 20%) supports this viewpoint. It is well-known that, on pure Pt, the oxidative removal of adsorbed CO formed from methanol decomposition is rate-determining in the first consecutive set of reactions. Since the adsorption of oxygen-like species on pure Pt does not occur to any appreciable extent below ∼ +0.7 V, the oxidative removal of adsorbed CO takes place very slowly over the low potential range, resulting in methanol oxidation mainly via soluble intermediates. For smooth, Pt-rich PtRu alloy surfaces, the rate of the first reaction pathway is increased due to electronic effects and the bifunctional mechanism,9,21,26,51 leading to an increase in the current efficiency of CO2 generation. As the ratio of Ru/Pt reaches ca. 0.2 in PtRu alloys, the rate of formation of COads from methanol decomposition and the rate of its oxidative removal are likely to reach an optimal balance. As a result, the maximum current efficiency of CO2 generation as well as Faradaic current for methanol oxidation are obtained over the low potential range. However, as the Ru content increases to reach values as high as 50%, methanol decomposition to COads on Pt sites is likely inhibited due to lack of adsorption sites. Thus, the rate-determining step changes from the oxidative removal of COads to formation of COads. Methanol oxidation via adsorbed CO becomes slow again, relative to methanol oxidation via soluble intermediates that can occur on the ensembles of less than three adjacent Pt atoms. As a result, this leads to a drop in the current efficiency of generated CO2 as well as the Faradaic current. For carbon-supported nanoparticle Pt and PtRu electrodes, a higher current efficiency for CO2 generation is obtained during methanol oxidation, when compared to smooth bulk electrodes. As mentioned in previous papers,21,41 since Pt/C and PtRu/C electrodes have a higher specific surface area compared to smooth electrodes, the soluble intermediates formed would have a higher probability of being further adsorbed and oxidized to CO2 before they are transported away from the electrode surface due to the continuous flow of electrolyte. Therefore, with an increase in total metal loading, the current efficiency for CO2 generation would be expected to increase as was, indeed, observed. 7734 DOI: 10.1021/la900305k

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Compared to Pt bulk electrodes, methanol oxidation on Pt0.5Ru0.5 bulk electrodes exhibits a very low current efficiency for CO2 generation. However, a higher current efficiency of CO2 generation was observed on PtRu/C electrodes relative to Pt/C electrodes, which is opposite to the behavior observed for smooth bulk electrodes. While largely speculative on our part, we believe that this could arise from the fact that PtRu nanoparticles have more edges, defects, and vertices than smooth PtRu bulk alloys and thus are more active toward methanol decomposition to form adsorbed CO, which could be further oxidized to CO2, leading to a higher yield of CO2. PtRu/C electrodes can be further activated by dissolving part of the surface Ru atoms. This activation procedure results in an enhancement of both COads and methanol oxidation. According to the bifunctional mechanism, a PtRu (50:50) alloy should be the most active toward COads oxidation. However, the electronic effects of Ru cannot be ignored. In fact, Ru can significantly modify the electronic properties of Pt, as indicated by the fact that the CO adsorption strength on Pt (as determined by in situ FTIR) can be weakened by Ru.33 For methanol oxidation on PtRu/C electrodes, Pt-rich surfaces also favor methanol oxidation, which is consistent with the results for PtRu bulk alloys. Adsorbed anions can also affect electrocatalytic processes. Methanol oxidation is a surface sensitive reaction. In contrast to perchloric acid, sulfate/bisulfate anions in sulfuric acid solution can be strongly adsorbed on platinum and PtRu alloy surfaces and thus affect methanol adsorption and its oxidation rate. It is generally believed that strongly adsorbed anions can inhibit methanol oxidation by blocking surface sites. However, Schell and Kumara Swamy reported that the rate of methanol and formic acid oxidation could be substantially increased by replacing a small amount of the supporting electrolyte (perchloric acid) with either sulfuric acid or tetrafluoroboric acid.63,64 In their works, cyclic voltammotry was used to study anion effects. However, potential step chronoamperometry was not employed. In our work, we also observed that, in CVs, a higher current peak for methanol oxidation on Pt/C occurred in 0.5 M H2SO4 relative to 0.1 M H2SO4. However, the steady-state current for methanol oxidation decreased with an increase in H2SO4 concentration. It has been previously reported9 that, in perchloric acid solution, the Faradaic current for methanol oxidation and the formation of CO2 and methyl formate are about twice those in sulfuric acid solutions. Here, we find that the concentration of sulfuric acid has an appreciable effect on methanol oxidation for all the catalysts studied. With an increase in sulfuric acid concentration, the rates of both methanol oxidation and CO2 formation were suppressed. Moreover, the current efficiency of CO2 generation also decreased. These results indicate that adsorption of sulfate/bisulfate anions hinders methanol oxidation on both Pt and PtRu catalysts. The current efficiency of CO2 generation with increasing sulfuric acid concentration suggests that the further oxidation of formed intermediates to CO2 and/or the first pathway via COads are also suppressed, likely due to suppression of hydroxyl adspecies formation by the adsorption of sulfate/bisulfate anions. Thus, a lower concentration of sulfuric acid solutions should be employed as long as it does not compromise performance due to lower solution conductivity.

5. Conclusions We have employed the DEMS technique in conjunction with electrochemical methods such as cyclic voltammetry and (63) Kumara Swamy, B. E.; Schell, M. J. Phys. Chem. B 2006, 110, 5139. (64) Schell, M.; Kumara Swamy, B. E. J. Electroanal. Chem. 2005, 584, 157.

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potential step chronoamperometry to study methanol electrooxidation on smooth Pt and PtRu bulk alloys and carbonsupported Pt and PtRu nanoparticle catalysts. The effects of Ru contents, catalyst loading/roughness and the concentration of sulfuric acid were investigated based on Faradaic currents and the product distributions. From these studies, the following conclusions can be drawn: Among Pt and all studied PtRu bulk alloy electrodes, PtRu with a Ru content of ca. 0.2 exhibited the highest catalytic activity toward methanol oxidation, that is, the highest oxidative current and highest current efficiency of CO2 generation. Pt0.5Ru0.5 bulk electrodes did not show high catalytic activity likely due to suppression of methanol dehydrogenation/adsorption. A higher current efficiency of CO2 generation was observed on carbon-supported Pt and PtRu nanoparticle catalysts relative to smooth bulk electrodes, due to the higher catalyst loading/surface roughness. For high catalyst loadings and/or rough electrodes, the intermediates (formaldehyde and formic acid) can be further oxidized before they are transported away by convection/ diffusion. Ru can promote the oxidation of methanol via the

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adsorbed CO pathway due to fast oxidative removal of CO to CO2. For carbon-supported nanoparticle PtRu catalysts, Pt-rich surfaces also appear to be more active, consistent with Pt-rich PtRu bulk electrodes. For methanol oxidation on PtRu catalysts, both the bifunctional mechanism and electronic effects play primary roles. To design active PtRu catalysts, one should consider the proper atomic ratio of Ru/Pt to balance the methanol dehydrogenation step and subsequent oxidative removal of adsorbed CO. Under ambient conditions, PtRu catalyst surfaces with a Ru content of about 0.2 rather than 0.5 appear to be optimal toward methanol oxidation. With an increase in the sulfuric acid concentration, the electrooxidation of methanol is suppressed; that is, both the oxidative current and the current efficiency of CO2 generation decrease. This indicates that sulfate/bisulfate adsorption competes with methanol adsorption and oxidation. Acknowledgment. This work was supported by the U.S. Department of Energy and NYSTAR.

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