- Email: [email protected]
Pt-Au Film Catalyst for Formic Acid Oxidation
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000 Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 215 (2017) 211–218
9th International Conference on Materials for Advanced Technologies (ICMAT 2017) 9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th
Pt-Au Film Catalyst for Formic Acid Oxidation Pt-Au Film Catalyst for Formic Acid Oxidation a,b Cian McKeowna,b, a,b,* and Fernando M. F. Rhena,b Cian McKeown * and Fernando M. F. Rhen a
Department of Physics, University of Limerick, Limerick V94 T9PX, Ireland b Department of Physics, University of Limerick, Limerick T9PX, Ireland Bernal Institute, University of Limerick, Limerick V94V94 T9PX, Ireland b Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland
a
Abstract Abstract Direct formic acid fuel cells (DFAFCs) have shown promise as efficient and renewable power sources for portable electronics. Direct formic fuel cellsexhibit (DFAFCs) have shown promise efficient andreaction renewable powerdue sources portableofelectronics. However, pureacid Pt catalysts poor activity for the formic as acid oxidation (FAOR) to thefor formation poisoning However, pure slow Pt catalysts exhibit poor activity Here, for thewe formic reaction (FAOR) due to the formation of poisoning species, which down the rate of oxidation. reportacid on oxidation the novel synthesis of a bimetallic, Pt-Au film electrocatalyst species, which slow down the rate of oxidation. Here, we report on the of novel synthesis Pt-Auwas filmdeposited electrocatalyst for the oxidation of formic acid (HCOOH). A bilayer stack composed individual Pt of anda bimetallic, Au metal layers using for the oxidation of formic acid (HCOOH). A surface bilayer stack composed of individual and Au metal layers wastemperature, deposited using magnetron sputtering, resulting in a smooth consisting of tightly packed Pt nanocrystallites. At room the magnetron in a smooth surface consisting tightly packed nanocrystallites. At roomwas temperature, bilayer stacksputtering, shows theresulting electrochemical characteristics of a pure of Pt surface. A post-synthesis heat treatment carried outthe to bilayer stack shows theconsisting electrochemical pure the Pt surface. A post-synthesis treatment was carried out of to form a bimetallic film of the characteristics two metals. Asofa aresult, heat-treated film showedheat electrochemical characteristic form a bimetallic film consisting of thesurface. two metals. As a result, thesurface heat-treated filmgreatly showedenhanced electrochemical of both metals, indicating a mixed metal The Pt-Au catalyst exhibited activity characteristic for formic acid both metals, indicating mixed metalbulk surface. The Pt-Au surface greatly surface enhanced activitythe for oxidation formic acid oxidation compared to a commercial Pt catalyst. Thecatalyst presence of Auexhibited on the catalyst promotes of oxidation commercial bulk Pt catalyst. The of Au effect. on theTherefore, catalyst surface promotes theofoxidation of formic acidcompared through to theadirect dehydrogenation pathway, viapresence the ensemble the heat treatment a sputtered formic direct method dehydrogenation pathway, via the ensemble effect. with Therefore, the heatfortreatment of a of sputtered bilayer acid stackthrough providesthe a simple for forming stable bimetallic Pt-Au catalysts high activity the oxidation formic bilayer acid. stack provides a simple method for forming stable bimetallic Pt-Au catalysts with high activity for the oxidation of formic acid. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Selection Selection and/or and/or peer-review peer-review under under responsibility responsibility of of the the scientific scientific committee committee of of Symposium Symposium 2017 2017 ICMAT. ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. Keywords: Formic Acid Oxidation; Platinum; Gold; Bimetallic; Catalyst Keywords: Formic Acid Oxidation; Platinum; Gold; Bimetallic; Catalyst
* Corresponding author. Tel.: +00353 61 23 4824 * E-mail Corresponding Tel.: +00353 [email protected] 23 4824 address:author. [email protected]; E-mail address: [email protected]; [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 © 2017 The Authors. Published by Elsevier Selection and/or peer-review under responsibility of theLtd. scientific committee of Symposium 2017 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2017.11.011
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
212 2
1. Introduction Direct formic acid fuel cells (DFAFCs) have attracted much attention in recent years as efficient power sources for portable electronics [1,2]. They have several advantages over direct methanol fuel cells (DMFCs) including lower toxicity [3], lower onset potential [4], higher theoretical electromotive force (EMF) [5,6], and vastly lower fuel crossover through Nafion [7] (a proton conducting polymer film widely used in polymer electrolyte membrane (PEM) fuel cells). Fuel crossover is detrimental to fuel cell efficiency as it reduces fuel utilization and overall performance. Although neat formic acid has a lower energy density than neat methanol (2103 Wh L -1 versus 5897 Wh L-1), much higher concentrations of formic acid can be used (10 M versus 3M methanol) due to its low Nafion crossover, hence favoring formic acid [8]. At the DFAFC anode, liquid HCOOH is adsorbed and oxidized to form CO 2. The formic acid oxidation reaction (FAOR) proceeds through a ‘dual pathway mechanism’ on Pt-based electrocatalysts [9,10]. One pathway involves the direct dehydrogenation of HCOOH, producing CO2 without the formation of CO or any other poisoning species. The second pathway involves the indirect dehydration of HCOOH on the catalyst surface. This pathway produces CO2 using surface bound CO as a reaction intermediate. CO adsorbs strongly to Pt catalysts, poisoning the surface and reducing the efficiency of the fuel cell and is an unwanted by-product of the FAOR. The parallel pathways can be described as follows: Direct (Dehydrogenation) Pathway: HCOOH → CO2 + 2H+ + 2e-
(1)
Indirect (Dehydration) Pathway: HCOOH → COads + H2O +
H2O → OHads + H + e OHads + COads → CO2 + H+ + e-
(2) -
(3) (4)
On a pure Pt catalyst, the FAOR proceeds predominantly through the indirect pathway with reduced kinetics due to CO poison formation [11]. Therefore the development of novel electrocatalysts is needed to enhance the activity and durability of Pt for the FAOR. The catalytic activity of pure metals has been shown to be enhanced though strain engineering [12,13] or by alloying with a second metal. This has been shown to enhance the catalytic activity of pure Pt for a number of catalytic processes including oxygen reduction [14,15], ethanol oxidation [16,17] and methanol oxidation [18, 19]. These alloys enhance the catalytic properties by inducing electronic changes (ligand effect) or geometric changes (ensemble effect) to the pure metal catalyst. Bimetallic Pt-Au catalysts have shown promising activity as electrocatalysts for the oxidation of formic acid [20]. However, due to the thermodynamic immiscibility of Au and Pt, stable alloys cannot be easily formed [21,22]. In this study, we report on a novel synthesis method using sputtered Au and Pt films, followed by heat treatment for the formation of electrocatalysts for formic acid oxidation. The electrocatalytic activity of the Pt-Au films was investigated using cyclic voltammetry and linear sweep voltammetry and the dominant reaction pathway for formic acid oxidation was determined. 2. Materials and Methods 2.1. Catalyst synthesis Magnetron sputtering was used to deposit the metal catalyst layers. In order to produce uniform films with controlled thickness, careful calibration of the magnetron sputtering parameters was required. The deposition rate of
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
213 3
each metal was dependent on their material parameters, namely the Z-factor and density, and the power supplied to the target. In this work, three individual metal layers were deposited on a polished Si substrate. A thin (5 nm) Ti was deposited to act as an adhesion layer for Au deposition. A 20 nm Au film was deposited, followed by a 10 nm Pt film. The bilayer was annealed in a quartz tube furnace at a range of temperatures in a forming gas atmosphere. The tube was flushed with the forming gas for 1 hour before annealing to ensure no oxygen was present in the quartz tube. 2.2. Physical characterization The deposited surfaces were characterized by scanning electron microscopy (SEM, Hitachi SU-70) in order to investigate the film’s morphology. The thickness of the metal layers was measured using cross sectional transmission electron microscopy (TEM, JEOL 2010F). The crystallographic structure of the Pt-Au films was investigated using X-ray diffraction (XRD, Analytical X’Pert MPD Pro). The X-ray radiation used was Cu K-alpha radiation (λ = 1.5418 Å). 2.3. Electrochemical characterization Electrochemical characterizations were carried out to investigate the catalyst’s activity for the oxidation of formic acid. A single compartment glass cell was used to carry out the cyclic voltammetry using a standard three electrode configuration. The sputtered films were used as planar working electrodes and a small active electrode area was created using a varnish. A large-area Pt wire and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. The electrolyte was purged with pure N 2 gas for 15 minutes before each experiment. Cyclic voltammetry measurements were measured between -0.2 and 1.3 V vs. SCE for the Pt surfaces and between -0.2 and 1.5 V vs. SCE for the Au-containing surfaces in N2-saturated 0.5 M H2SO4 at a scan rate of 100 mV s-1. The electrochemical surface area (ECSA) was determined by calculating the charge due to hydrogen adsorption in the hydrogen region (between -0.2 and 0.15 V vs. SCE) and relating that with the charge density of Pt (210 µC cm-2). To investigate the activity of the Pt-Au and bulk Pt electrodes for the oxidation of formic acid, linear sweep voltammetry (LSV) was carried out between -0.2 and 1.0 V vs. SCE at 50 mV s -1 in N2-saturated 0.5 M H2SO4 + 0.5 M HCOOH. 3. Results and Discussion 3.1. Physical characterization The morphology of the thin film catalysts was investigated using scanning electron microscopy (SEM). Figure 1(a) shows the surface of the thin film electrode at room temperature (RT), consisting of tightly packed nanocrystallites. After annealing for 1 hour at 200 oC, a similar platinum surface is seen in Figure 1(b). A slight growth in nanocrystallite size was observed. Figure 1(c) shows a vastly different surface after annealing the films at 400 oC for 1 hour. The surface consists of two separate metals, as shown by the contrasting bright and dark regions on the SEM image.
4214
Author / Procedia 00 (2017) 000–000 Cian name McKeown et al. Engineering / Procedia Engineering 215 (2017) 211–218
Fig. 1. Scanning electron microscopy (SEM) images of the thin film catalyst surface at (a) room temperature and after annealing for 1 hour in a quartz tube furnace at (b) 200 oC and (c) 400 oC.
The cross sectional TEM images in Figure 2 show the effect of annealing on the sputtered metal layers. An assputtered film is shown in Figure 2(a), with visibly distinct interfaces between the Si substrate, Ti adhesion layer, Au layer and the top-most Pt film. The thickness of the metal layers was consistent with their nominal values. Figure 2(b) shows the formation of a bimetallic Pt-Au film after annealing at 400 oC. The Si substrate and Ti adhesion layer appear unchanged by the heat treatment but there is no longer a clear interface between the Pt and Au layers. The combined thickness of the individual Pt and Au layers matched the thickness of the Pt-Au film formed after annealing.
Fig. 2. Cross sectional transmission electron microscope (TEM) images of thin film catalysts at (a) room temperature and (b) after annealing at 400 oC for 1 hour.
Author name / Procedia Engineering 00 (2017) 000–000 Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218
5 215
The crystal structure of the thin film catalyst was investigated using X-ray diffraction (XRD). Figure 3 shows the Xray diffractammograms of the catalyst layers at (a) room temperature, (b) 200 oC and (c) 400 oC. The three plots share common diffraction peaks at 38.18 o and 81.72o which are attributed to the Au (111) and Au (222) diffraction peaks, respectively, from the Au layer (ICSD # 98-002-1538). Diffraction peaks were also observed at 39.41 o and 85.33o which correspond to the Pt (111) and Pt (222) peak positions of a face centered cubic (FCC) crystal structure (ICSD # 98-002-2003). Peaks from the underlying Ti adhesion layer were also observed and are denoted by a ♦ symbol. The crystallographic investigation reveals that magnetron sputter deposition of the metal films onto a Si substrate resulted in a (111) textured nanocrystalline films. The XRD plots of the room temperature and annealed structures contain matching diffraction peaks, confirming that an alloy had not been formed, due to the immiscibility of the two metals [21,22].
Fig. 3. X-ray diffraction (XRD) patterns of the thin film catalysts at (a) room temperature and after annealing under inert gas in a quartz tube furnace for 1 hour at (b) 200 oC and (c) 400 oC. Characteristic FCC (111) and (222) peaks were observed for Au and Pt. Peaks from the Ti adhesion layer are denoted by a ♦ symbol.
3.2. Electrochemical characterization The results of cyclic voltammetry on the thin film catalysts are shown in Figure 4. The Pt surfaces exhibit characteristic regions of hydrogen adsorption and desorption, double layer capacitance and formation and reduction of surface oxides. Above the upper limit of 1.3 V vs. SCE, oxygen evolution takes place on the pure Pt surfaces. A Pt oxide reduction peak was observed at about 0.5 V vs. SCE for the as-sputtered and 200 oC annealed films. An upper limit of 1.5 V vs. SCE was used for characterizing the Pt-Au surface in order to fully observe the behavior of oxide formation on Au, which occurs between 1.1 and 1.5 V vs. SCE [23]. The scan shows characteristics of both Au and Pt. Two peaks were observed on the reverse scan, one matching Au oxide reduction at 0.93 V vs. SCE [23] and a second peak at 0.39 V vs. SCE corresponding to the Pt oxide reduction. The Pt oxide reduction peak was shifted to a lower potential than that seen for the bulk Pt. This shift in potential is due to Pt-Au having diminished
216 6
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
activity as an electrocatalyst for the oxygen reduction reaction due to the presence of Au on the surface, which is inactive towards oxygen reduction [24].
Fig. 4. Cyclic voltammograms measured on thin film catalyst after room temperature sputtering (solid line) and subsequent annealing at 200 oC (dashed line) and 400 oC (dotted line). A scan rate of 100 mV s-1 was used for the CVs in N2-saturated 0.5 M H2SO4.
The activity of the Pt-Au film catalyst was investigated for the oxidation of formic acid. Linear sweep voltammetry (LSV) was carried out in N2-saturated 0.5 M H2SO4 + 0.5 M HCOOH. Figure 5 shows the linear sweep voltammograms obtained for pure Pt and the Pt-Au film electrocatalysts. The inset graph shows a magnified image of the bulk Pt electrode. Two oxidation peaks were observed during the LSV measurements on both catalysts. The first peak (P1 at 0.37 V vs. SCE) corresponds to the direct oxidation of formic acid through the direct pathway while the second peak (P2 at 0.7 V vs. SCE) corresponds to the oxidation of CO ads through the indirect pathway. The Pt-Au catalysts exhibit a maximum peak current density of 7.86 mA cm-2 at 0.37 V vs. SCE, while the activity for FAOR was much lower on the bulk Pt electrode, which reached a maximum of 0.17 mA cm -2 at 0.7 V vs. SCE. The ratio between the peak current densities (j P1 / jP2) is an indicator of which reaction pathway is dominant in the oxidation of formic acid [25]. If (jP1 / jP2) < 1, the oxidation of formic acid occurs through the indirect dehydration pathway, which results in the formation of surface bound CO, a poisoning intermediate during the oxidation process [26]. This pathway is undesirable as it is detrimental to the operation of the direct formic acid fuel cell. If (j P1 / jP2) > 1, the oxidation of formic acid occurs through the direct dehydrogenation pathway which avoids poisoning of the electrode surface and thus extends the lifetime of the fuel cell. A (jP1 / jP2) value of 2.24 was measured on a thin film Pt-Au electrode, indicating that the oxidation of formic acid on Pt-Au proceeds predominantly through the direct pathway. From analysis of the LSV on bulk Pt in the inset of Figure 5, the (j P1 / jP2) value was 0.77. This shows us that the oxidation of formic acid occurs through the indirect pathway for polycrystalline Pt.
Cian McKeown et al. / Procedia Engineering (2017) 211–218 Author name / Procedia Engineering 00 (2017)215 000–000
2177
Fig. 5. Linear sweep voltammogram of Pt and Pt-Au electrocatalysts showing their activity for the oxidation of formic acid in a 0.5 M H 2SO4 + 0.5 M HCOOH solution. A scan rate of 50 mV s-1 was used. The inset graph shows a magnified image of the LSV on the bulk Pt electrode.
The XRD investigation in Figure 3 revealed that an alloy had not been formed between Pt and Au after annealing at 400 oC. Because of this, we can conclude that the enhancement in FAOR is not due to the electronic or ligand effect as the electronic structure of Pt is not modified by the formation of a bimetal with Au. The presence of Au on the surface of the catalyst shown in Figure 1 leads to the conclusion that the ensemble effect played a key role in the enhancement of the FAOR. The results of the electrochemical characterization agree with this, as the LSV in Figure 5 showed that the reaction proceeded through the direct pathway on Pt-Au. The direct pathway involves the dehydrogenation of formic acid and only requires one Pt site. On bulk Pt, the FAOR proceeds through the indirect pathway which involves the removal of H2O and forms COads as shown in Equation (2). This dehydration step requires three contiguous Pt sites [27]. When Au nanocrystallites are present the catalyst surface, the three Pt sites are separated, which facilitates dehydrogenation and the direct pathway for formic acid oxidation is favored. 4. Conclusion Bimetallic Pt-Au catalysts have been synthesized by annealing individually sputtered metal layers. The presence of two separate surface metals was observed by electron microscopy, indicating the formation of a bimetallic Pt-Au catalyst. The Pt-Au catalyst showed enhanced activity for the oxidation of formic acid, which proceeded through the direct dehydrogenation pathway. The enhanced activity was a result of the presence of Au on the catalyst surface, separating contiguous Pt sites. The post-synthesis heat treatment of sputtered nanocrystalline films provides a simple and effective method for forming stable bimetallic Pt-Au catalysts with high activity for the oxidation of formic acid. Acknowledgements This research is supported by Science Foundation Ireland grant number 12/IP/1692 and the HEA PRTLI4 programme (INSPIRE). The authors are thankful to the Microscopy Society of Ireland, The Institute of Physics Energy Group and the Institute of Physics for providing travel grant support through the Conference Travel Award, Research Student Conference Fund and C.R. Barber Trust Fund, respectively.
218 8
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
References [1] C. Rice, S. Ha, R. I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, Direct Formic Acid Fuel Cells, J. Power Sources 111 (2002) 83-89. [2] X. W. Yu, P. G. Pickup, Recent Advances in Direct Formic Acid Fuel Cells (DFAFC), J. Power Sources 182 (2008) 124-132. [3] S. Ha, R. Larsen, R. I. Masel, Performance Characterization of Pd/C Nanocatalyst for Direct Formic Acid Fuel Cells, J. Power Sources 144 (2005) 28-34. [4] J. Willsau, J. Heitbaum, Analysis of Adsorbed Intermediates and Determination of Surface Potential Shifts by Dems, Electrochim. Acta 31 (1986) 943-948. [5] Y. M. Zhu, S. Y. Ha, R. I. Masel, High Power Density Direct Formic Acid Fuel Cells, J. Power Sources 130 (2004) 8-14. [6] P. Hong, Y. L. Zhong, S. J. Liao, J. H. Zeng, X. Y. Lu, W. Chen, A 4-Cell Miniature Direct Formic Acid Fuel Cell Stack with Independent Fuel Reservoir: Design and Performance Investigation, J. Power Sources 196 (2011) 5913-5917. [7] X. Wang, J. -M. Hu, I. -M. Hsing, Electrochemical Investigation of Formic Acid Electro-Oxidation and Its Crossover through a Nafion® Membrane, J. Electroanal. Chem. 562 (2004) 73-80. [8] G. L. Soloveichik, Liquid Fuel Cells, Beilstein J. Nanotechnol. 5 (2014) 1399-1418. [9] J. D. Lović, A. V. Tripković, S. L. J. Gojković, K. D. Popović, D. V. Tripković, P. Olszewski, A. Kowal, Kinetic Study of Formic Acid Oxidation on Carbon-Supported Platinum Electrocatalyst, J. Electroanal. Chem. 581 (2005) 294-302. [10] R. Larsen, S. Ha, J. Zakzeski, R. I. Masel, Unusually Active Palladium-Based Catalysts for the Electrooxidation of Formic Acid, J. Power Sources 157 (2006) 78-84. [11] S. Park, Y. Xie, M. J. Weaver, Electrocatalytic Pathways on Carbon-Supported Platinum Nanoparticles: Comparison of Particle-SizeDependent Rates of Methanol, Formic Acid, and Formaldehyde Electrooxidation, Langmuir 18 (2002) 5792-5798. [12] V. A. Sethuraman, D. Vairavapandian, M. C. Lafouresse, T. A. Maark, N. Karan, S. H. Sun, U. Bertocci, A. A. Peterson, G. R. Stafford, P. R. Guduru, Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt, J. Phys. Chem. C 119 (2015) 19042-19052. [13] F. M. F. Rhen, C. McKeown, Enhanced Methanol Oxidation on Strained Pt Films, J. Phys. Chem. C 121 (2017) 2556-2562. [14] J. Greeley, I. E. L. Stephens, A. S. Bondarenko, T. P. Johansson, H. A. Hansen, T. F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J. K. Nørskov, Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts, Nat. Chem. 1 (2009) 552-556. [15] U. A. Paulus, A. Wokaun, G. G. Scherer, T. J. Schmidt, V. Stamenkovic, V. Radmilovic, N. M. Markovic, P. N. Ross, Oxygen Reduction on Carbon-Supported Pt-Ni and Pt-Co Alloy Catalysts, J. Phys. Chem. B 106 (2002) 4181-4191. [16] L. H. Jiang, G. Q. Sun, S. G. Sun, J. G. Liu, S. H. Tang, H. Q. Li, B. Zhou, Q. Xin, Structure and Chemical Composition of Supported Pt–Sn Electrocatalysts for Ethanol Oxidation, Electrochim. Acta 50 (2005) 5384-5389. [17] F. Colmati, E. Antolini, E. R. Gonzalez, Effect of Temperature on the Mechanism of Ethanol Oxidation on Carbon Supported Pt, PtRu and Pt3Sn Electrocatalysts, J. Power Sources 157 (2006) 98-103. [18] Z. L. Liu, X. Y. Ling, X. D. Su, J. Y. Lee, Carbon-Supported Pt and PtRu Nanoparticles as Catalysts for a Direct Methanol Fuel Cell, J. Phys. Chem. B 108 (2004) 8234-8240. [19] P. K. Shen, A. C. C. Tseung, Anodic Oxidation of Methanol on Pt/WO3 in Acidic Media, J. Electrochem. Soc. 141 (1994) 3082-3090. [20] M. Mirdamadi-Esfahani, M. Mostafavi, B. Keita, L. Nadjo, P. Kooyman, H. Remita, Bimetallic Au-Pt Nanoparticles Synthesized by Radiolysis: Application in Electro-Catalysis, Gold Bull. 43 (2010) 49-56. [21] X. N. Xu, G. W. Qin, Y. P. Ren, B. Shen, W. L. Pei, Experimental Study of the Miscibility Gap and Calculation of the Spinodal Curves of the Au–Pt System, Scr. Mater. 61 (2009) 859-862. [22] X. N. Xu, Y. P. Ren, C. F. Li, S. Li, G. W. Qin, Thermodynamic Assessment of Au-Pt System, Trans. Nonferrous Met. Soc. China 22 (2012) 1432-1436. [23] J. P. Hoare, A Cyclic Voltammetric Study of the Gold‐Oxygen System, J. Electrochem. Soc. 131 (1984) 1808-1815. [24] J. Luo, L. Wang, D. Mott, P. N. Njoki, Y. Lin, T. He, Z. Xu, B. N. Wanjana, I. -I. S. Lim, C. -J. Zhong, Core/Shell Nanoparticles as Electrocatalysts for Fuel Cell Reactions, Adv. Mater. 20 (2008) 4342-4347. [25] M. S. El-Deab, Electrocatalysis by Nanoparticles: Oxidation of Formic Acid at Manganese Oxide Nanorods-Modified Pt Planar and Nanohole-Arrays, J. Adv. Res. 1 (2010) 87-93. [26] G. A. El-Nagar, A. M. Mohammad, M. S. El-Deab, B. E. El-Anadouli, Electrocatalysis by Design: Enhanced Electrooxidation of Formic Acid at Platinum Nanoparticles–Nickel Oxide Nanoparticles Binary Catalysts, Electrochim. Acta 94 (2013) 62-71. [27] A. Cuesta, M. Escudero, B. Lanova, H. Baltruschat, Cyclic Voltammetry, FTIRS, and DEMS Study of the Electrooxidation of Carbon Monoxide, Formic Acid, and Methanol on Cyanide-Modified Pt(111) Electrodes, Langmuir 25 (2009) 6500-6507.
ScienceDirect ScienceDirect Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000 Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 215 (2017) 211–218
9th International Conference on Materials for Advanced Technologies (ICMAT 2017) 9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th
Pt-Au Film Catalyst for Formic Acid Oxidation Pt-Au Film Catalyst for Formic Acid Oxidation a,b Cian McKeowna,b, a,b,* and Fernando M. F. Rhena,b Cian McKeown * and Fernando M. F. Rhen a
Department of Physics, University of Limerick, Limerick V94 T9PX, Ireland b Department of Physics, University of Limerick, Limerick T9PX, Ireland Bernal Institute, University of Limerick, Limerick V94V94 T9PX, Ireland b Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland
a
Abstract Abstract Direct formic acid fuel cells (DFAFCs) have shown promise as efficient and renewable power sources for portable electronics. Direct formic fuel cellsexhibit (DFAFCs) have shown promise efficient andreaction renewable powerdue sources portableofelectronics. However, pureacid Pt catalysts poor activity for the formic as acid oxidation (FAOR) to thefor formation poisoning However, pure slow Pt catalysts exhibit poor activity Here, for thewe formic reaction (FAOR) due to the formation of poisoning species, which down the rate of oxidation. reportacid on oxidation the novel synthesis of a bimetallic, Pt-Au film electrocatalyst species, which slow down the rate of oxidation. Here, we report on the of novel synthesis Pt-Auwas filmdeposited electrocatalyst for the oxidation of formic acid (HCOOH). A bilayer stack composed individual Pt of anda bimetallic, Au metal layers using for the oxidation of formic acid (HCOOH). A surface bilayer stack composed of individual and Au metal layers wastemperature, deposited using magnetron sputtering, resulting in a smooth consisting of tightly packed Pt nanocrystallites. At room the magnetron in a smooth surface consisting tightly packed nanocrystallites. At roomwas temperature, bilayer stacksputtering, shows theresulting electrochemical characteristics of a pure of Pt surface. A post-synthesis heat treatment carried outthe to bilayer stack shows theconsisting electrochemical pure the Pt surface. A post-synthesis treatment was carried out of to form a bimetallic film of the characteristics two metals. Asofa aresult, heat-treated film showedheat electrochemical characteristic form a bimetallic film consisting of thesurface. two metals. As a result, thesurface heat-treated filmgreatly showedenhanced electrochemical of both metals, indicating a mixed metal The Pt-Au catalyst exhibited activity characteristic for formic acid both metals, indicating mixed metalbulk surface. The Pt-Au surface greatly surface enhanced activitythe for oxidation formic acid oxidation compared to a commercial Pt catalyst. Thecatalyst presence of Auexhibited on the catalyst promotes of oxidation commercial bulk Pt catalyst. The of Au effect. on theTherefore, catalyst surface promotes theofoxidation of formic acidcompared through to theadirect dehydrogenation pathway, viapresence the ensemble the heat treatment a sputtered formic direct method dehydrogenation pathway, via the ensemble effect. with Therefore, the heatfortreatment of a of sputtered bilayer acid stackthrough providesthe a simple for forming stable bimetallic Pt-Au catalysts high activity the oxidation formic bilayer acid. stack provides a simple method for forming stable bimetallic Pt-Au catalysts with high activity for the oxidation of formic acid. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Selection Selection and/or and/or peer-review peer-review under under responsibility responsibility of of the the scientific scientific committee committee of of Symposium Symposium 2017 2017 ICMAT. ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. Keywords: Formic Acid Oxidation; Platinum; Gold; Bimetallic; Catalyst Keywords: Formic Acid Oxidation; Platinum; Gold; Bimetallic; Catalyst
* Corresponding author. Tel.: +00353 61 23 4824 * E-mail Corresponding Tel.: +00353 [email protected] 23 4824 address:author. [email protected]; E-mail address: [email protected]; [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 © 2017 The Authors. Published by Elsevier Selection and/or peer-review under responsibility of theLtd. scientific committee of Symposium 2017 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2017.11.011
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
212 2
1. Introduction Direct formic acid fuel cells (DFAFCs) have attracted much attention in recent years as efficient power sources for portable electronics [1,2]. They have several advantages over direct methanol fuel cells (DMFCs) including lower toxicity [3], lower onset potential [4], higher theoretical electromotive force (EMF) [5,6], and vastly lower fuel crossover through Nafion [7] (a proton conducting polymer film widely used in polymer electrolyte membrane (PEM) fuel cells). Fuel crossover is detrimental to fuel cell efficiency as it reduces fuel utilization and overall performance. Although neat formic acid has a lower energy density than neat methanol (2103 Wh L -1 versus 5897 Wh L-1), much higher concentrations of formic acid can be used (10 M versus 3M methanol) due to its low Nafion crossover, hence favoring formic acid [8]. At the DFAFC anode, liquid HCOOH is adsorbed and oxidized to form CO 2. The formic acid oxidation reaction (FAOR) proceeds through a ‘dual pathway mechanism’ on Pt-based electrocatalysts [9,10]. One pathway involves the direct dehydrogenation of HCOOH, producing CO2 without the formation of CO or any other poisoning species. The second pathway involves the indirect dehydration of HCOOH on the catalyst surface. This pathway produces CO2 using surface bound CO as a reaction intermediate. CO adsorbs strongly to Pt catalysts, poisoning the surface and reducing the efficiency of the fuel cell and is an unwanted by-product of the FAOR. The parallel pathways can be described as follows: Direct (Dehydrogenation) Pathway: HCOOH → CO2 + 2H+ + 2e-
(1)
Indirect (Dehydration) Pathway: HCOOH → COads + H2O +
H2O → OHads + H + e OHads + COads → CO2 + H+ + e-
(2) -
(3) (4)
On a pure Pt catalyst, the FAOR proceeds predominantly through the indirect pathway with reduced kinetics due to CO poison formation [11]. Therefore the development of novel electrocatalysts is needed to enhance the activity and durability of Pt for the FAOR. The catalytic activity of pure metals has been shown to be enhanced though strain engineering [12,13] or by alloying with a second metal. This has been shown to enhance the catalytic activity of pure Pt for a number of catalytic processes including oxygen reduction [14,15], ethanol oxidation [16,17] and methanol oxidation [18, 19]. These alloys enhance the catalytic properties by inducing electronic changes (ligand effect) or geometric changes (ensemble effect) to the pure metal catalyst. Bimetallic Pt-Au catalysts have shown promising activity as electrocatalysts for the oxidation of formic acid [20]. However, due to the thermodynamic immiscibility of Au and Pt, stable alloys cannot be easily formed [21,22]. In this study, we report on a novel synthesis method using sputtered Au and Pt films, followed by heat treatment for the formation of electrocatalysts for formic acid oxidation. The electrocatalytic activity of the Pt-Au films was investigated using cyclic voltammetry and linear sweep voltammetry and the dominant reaction pathway for formic acid oxidation was determined. 2. Materials and Methods 2.1. Catalyst synthesis Magnetron sputtering was used to deposit the metal catalyst layers. In order to produce uniform films with controlled thickness, careful calibration of the magnetron sputtering parameters was required. The deposition rate of
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
213 3
each metal was dependent on their material parameters, namely the Z-factor and density, and the power supplied to the target. In this work, three individual metal layers were deposited on a polished Si substrate. A thin (5 nm) Ti was deposited to act as an adhesion layer for Au deposition. A 20 nm Au film was deposited, followed by a 10 nm Pt film. The bilayer was annealed in a quartz tube furnace at a range of temperatures in a forming gas atmosphere. The tube was flushed with the forming gas for 1 hour before annealing to ensure no oxygen was present in the quartz tube. 2.2. Physical characterization The deposited surfaces were characterized by scanning electron microscopy (SEM, Hitachi SU-70) in order to investigate the film’s morphology. The thickness of the metal layers was measured using cross sectional transmission electron microscopy (TEM, JEOL 2010F). The crystallographic structure of the Pt-Au films was investigated using X-ray diffraction (XRD, Analytical X’Pert MPD Pro). The X-ray radiation used was Cu K-alpha radiation (λ = 1.5418 Å). 2.3. Electrochemical characterization Electrochemical characterizations were carried out to investigate the catalyst’s activity for the oxidation of formic acid. A single compartment glass cell was used to carry out the cyclic voltammetry using a standard three electrode configuration. The sputtered films were used as planar working electrodes and a small active electrode area was created using a varnish. A large-area Pt wire and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. The electrolyte was purged with pure N 2 gas for 15 minutes before each experiment. Cyclic voltammetry measurements were measured between -0.2 and 1.3 V vs. SCE for the Pt surfaces and between -0.2 and 1.5 V vs. SCE for the Au-containing surfaces in N2-saturated 0.5 M H2SO4 at a scan rate of 100 mV s-1. The electrochemical surface area (ECSA) was determined by calculating the charge due to hydrogen adsorption in the hydrogen region (between -0.2 and 0.15 V vs. SCE) and relating that with the charge density of Pt (210 µC cm-2). To investigate the activity of the Pt-Au and bulk Pt electrodes for the oxidation of formic acid, linear sweep voltammetry (LSV) was carried out between -0.2 and 1.0 V vs. SCE at 50 mV s -1 in N2-saturated 0.5 M H2SO4 + 0.5 M HCOOH. 3. Results and Discussion 3.1. Physical characterization The morphology of the thin film catalysts was investigated using scanning electron microscopy (SEM). Figure 1(a) shows the surface of the thin film electrode at room temperature (RT), consisting of tightly packed nanocrystallites. After annealing for 1 hour at 200 oC, a similar platinum surface is seen in Figure 1(b). A slight growth in nanocrystallite size was observed. Figure 1(c) shows a vastly different surface after annealing the films at 400 oC for 1 hour. The surface consists of two separate metals, as shown by the contrasting bright and dark regions on the SEM image.
4214
Author / Procedia 00 (2017) 000–000 Cian name McKeown et al. Engineering / Procedia Engineering 215 (2017) 211–218
Fig. 1. Scanning electron microscopy (SEM) images of the thin film catalyst surface at (a) room temperature and after annealing for 1 hour in a quartz tube furnace at (b) 200 oC and (c) 400 oC.
The cross sectional TEM images in Figure 2 show the effect of annealing on the sputtered metal layers. An assputtered film is shown in Figure 2(a), with visibly distinct interfaces between the Si substrate, Ti adhesion layer, Au layer and the top-most Pt film. The thickness of the metal layers was consistent with their nominal values. Figure 2(b) shows the formation of a bimetallic Pt-Au film after annealing at 400 oC. The Si substrate and Ti adhesion layer appear unchanged by the heat treatment but there is no longer a clear interface between the Pt and Au layers. The combined thickness of the individual Pt and Au layers matched the thickness of the Pt-Au film formed after annealing.
Fig. 2. Cross sectional transmission electron microscope (TEM) images of thin film catalysts at (a) room temperature and (b) after annealing at 400 oC for 1 hour.
Author name / Procedia Engineering 00 (2017) 000–000 Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218
5 215
The crystal structure of the thin film catalyst was investigated using X-ray diffraction (XRD). Figure 3 shows the Xray diffractammograms of the catalyst layers at (a) room temperature, (b) 200 oC and (c) 400 oC. The three plots share common diffraction peaks at 38.18 o and 81.72o which are attributed to the Au (111) and Au (222) diffraction peaks, respectively, from the Au layer (ICSD # 98-002-1538). Diffraction peaks were also observed at 39.41 o and 85.33o which correspond to the Pt (111) and Pt (222) peak positions of a face centered cubic (FCC) crystal structure (ICSD # 98-002-2003). Peaks from the underlying Ti adhesion layer were also observed and are denoted by a ♦ symbol. The crystallographic investigation reveals that magnetron sputter deposition of the metal films onto a Si substrate resulted in a (111) textured nanocrystalline films. The XRD plots of the room temperature and annealed structures contain matching diffraction peaks, confirming that an alloy had not been formed, due to the immiscibility of the two metals [21,22].
Fig. 3. X-ray diffraction (XRD) patterns of the thin film catalysts at (a) room temperature and after annealing under inert gas in a quartz tube furnace for 1 hour at (b) 200 oC and (c) 400 oC. Characteristic FCC (111) and (222) peaks were observed for Au and Pt. Peaks from the Ti adhesion layer are denoted by a ♦ symbol.
3.2. Electrochemical characterization The results of cyclic voltammetry on the thin film catalysts are shown in Figure 4. The Pt surfaces exhibit characteristic regions of hydrogen adsorption and desorption, double layer capacitance and formation and reduction of surface oxides. Above the upper limit of 1.3 V vs. SCE, oxygen evolution takes place on the pure Pt surfaces. A Pt oxide reduction peak was observed at about 0.5 V vs. SCE for the as-sputtered and 200 oC annealed films. An upper limit of 1.5 V vs. SCE was used for characterizing the Pt-Au surface in order to fully observe the behavior of oxide formation on Au, which occurs between 1.1 and 1.5 V vs. SCE [23]. The scan shows characteristics of both Au and Pt. Two peaks were observed on the reverse scan, one matching Au oxide reduction at 0.93 V vs. SCE [23] and a second peak at 0.39 V vs. SCE corresponding to the Pt oxide reduction. The Pt oxide reduction peak was shifted to a lower potential than that seen for the bulk Pt. This shift in potential is due to Pt-Au having diminished
216 6
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
activity as an electrocatalyst for the oxygen reduction reaction due to the presence of Au on the surface, which is inactive towards oxygen reduction [24].
Fig. 4. Cyclic voltammograms measured on thin film catalyst after room temperature sputtering (solid line) and subsequent annealing at 200 oC (dashed line) and 400 oC (dotted line). A scan rate of 100 mV s-1 was used for the CVs in N2-saturated 0.5 M H2SO4.
The activity of the Pt-Au film catalyst was investigated for the oxidation of formic acid. Linear sweep voltammetry (LSV) was carried out in N2-saturated 0.5 M H2SO4 + 0.5 M HCOOH. Figure 5 shows the linear sweep voltammograms obtained for pure Pt and the Pt-Au film electrocatalysts. The inset graph shows a magnified image of the bulk Pt electrode. Two oxidation peaks were observed during the LSV measurements on both catalysts. The first peak (P1 at 0.37 V vs. SCE) corresponds to the direct oxidation of formic acid through the direct pathway while the second peak (P2 at 0.7 V vs. SCE) corresponds to the oxidation of CO ads through the indirect pathway. The Pt-Au catalysts exhibit a maximum peak current density of 7.86 mA cm-2 at 0.37 V vs. SCE, while the activity for FAOR was much lower on the bulk Pt electrode, which reached a maximum of 0.17 mA cm -2 at 0.7 V vs. SCE. The ratio between the peak current densities (j P1 / jP2) is an indicator of which reaction pathway is dominant in the oxidation of formic acid [25]. If (jP1 / jP2) < 1, the oxidation of formic acid occurs through the indirect dehydration pathway, which results in the formation of surface bound CO, a poisoning intermediate during the oxidation process [26]. This pathway is undesirable as it is detrimental to the operation of the direct formic acid fuel cell. If (j P1 / jP2) > 1, the oxidation of formic acid occurs through the direct dehydrogenation pathway which avoids poisoning of the electrode surface and thus extends the lifetime of the fuel cell. A (jP1 / jP2) value of 2.24 was measured on a thin film Pt-Au electrode, indicating that the oxidation of formic acid on Pt-Au proceeds predominantly through the direct pathway. From analysis of the LSV on bulk Pt in the inset of Figure 5, the (j P1 / jP2) value was 0.77. This shows us that the oxidation of formic acid occurs through the indirect pathway for polycrystalline Pt.
Cian McKeown et al. / Procedia Engineering (2017) 211–218 Author name / Procedia Engineering 00 (2017)215 000–000
2177
Fig. 5. Linear sweep voltammogram of Pt and Pt-Au electrocatalysts showing their activity for the oxidation of formic acid in a 0.5 M H 2SO4 + 0.5 M HCOOH solution. A scan rate of 50 mV s-1 was used. The inset graph shows a magnified image of the LSV on the bulk Pt electrode.
The XRD investigation in Figure 3 revealed that an alloy had not been formed between Pt and Au after annealing at 400 oC. Because of this, we can conclude that the enhancement in FAOR is not due to the electronic or ligand effect as the electronic structure of Pt is not modified by the formation of a bimetal with Au. The presence of Au on the surface of the catalyst shown in Figure 1 leads to the conclusion that the ensemble effect played a key role in the enhancement of the FAOR. The results of the electrochemical characterization agree with this, as the LSV in Figure 5 showed that the reaction proceeded through the direct pathway on Pt-Au. The direct pathway involves the dehydrogenation of formic acid and only requires one Pt site. On bulk Pt, the FAOR proceeds through the indirect pathway which involves the removal of H2O and forms COads as shown in Equation (2). This dehydration step requires three contiguous Pt sites [27]. When Au nanocrystallites are present the catalyst surface, the three Pt sites are separated, which facilitates dehydrogenation and the direct pathway for formic acid oxidation is favored. 4. Conclusion Bimetallic Pt-Au catalysts have been synthesized by annealing individually sputtered metal layers. The presence of two separate surface metals was observed by electron microscopy, indicating the formation of a bimetallic Pt-Au catalyst. The Pt-Au catalyst showed enhanced activity for the oxidation of formic acid, which proceeded through the direct dehydrogenation pathway. The enhanced activity was a result of the presence of Au on the catalyst surface, separating contiguous Pt sites. The post-synthesis heat treatment of sputtered nanocrystalline films provides a simple and effective method for forming stable bimetallic Pt-Au catalysts with high activity for the oxidation of formic acid. Acknowledgements This research is supported by Science Foundation Ireland grant number 12/IP/1692 and the HEA PRTLI4 programme (INSPIRE). The authors are thankful to the Microscopy Society of Ireland, The Institute of Physics Energy Group and the Institute of Physics for providing travel grant support through the Conference Travel Award, Research Student Conference Fund and C.R. Barber Trust Fund, respectively.
218 8
Cian McKeown et al. / Procedia Engineering 215 (2017) 211–218 Author name / Procedia Engineering 00 (2017) 000–000
References [1] C. Rice, S. Ha, R. I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, Direct Formic Acid Fuel Cells, J. Power Sources 111 (2002) 83-89. [2] X. W. Yu, P. G. Pickup, Recent Advances in Direct Formic Acid Fuel Cells (DFAFC), J. Power Sources 182 (2008) 124-132. [3] S. Ha, R. Larsen, R. I. Masel, Performance Characterization of Pd/C Nanocatalyst for Direct Formic Acid Fuel Cells, J. Power Sources 144 (2005) 28-34. [4] J. Willsau, J. Heitbaum, Analysis of Adsorbed Intermediates and Determination of Surface Potential Shifts by Dems, Electrochim. Acta 31 (1986) 943-948. [5] Y. M. Zhu, S. Y. Ha, R. I. Masel, High Power Density Direct Formic Acid Fuel Cells, J. Power Sources 130 (2004) 8-14. [6] P. Hong, Y. L. Zhong, S. J. Liao, J. H. Zeng, X. Y. Lu, W. Chen, A 4-Cell Miniature Direct Formic Acid Fuel Cell Stack with Independent Fuel Reservoir: Design and Performance Investigation, J. Power Sources 196 (2011) 5913-5917. [7] X. Wang, J. -M. Hu, I. -M. Hsing, Electrochemical Investigation of Formic Acid Electro-Oxidation and Its Crossover through a Nafion® Membrane, J. Electroanal. Chem. 562 (2004) 73-80. [8] G. L. Soloveichik, Liquid Fuel Cells, Beilstein J. Nanotechnol. 5 (2014) 1399-1418. [9] J. D. Lović, A. V. Tripković, S. L. J. Gojković, K. D. Popović, D. V. Tripković, P. Olszewski, A. Kowal, Kinetic Study of Formic Acid Oxidation on Carbon-Supported Platinum Electrocatalyst, J. Electroanal. Chem. 581 (2005) 294-302. [10] R. Larsen, S. Ha, J. Zakzeski, R. I. Masel, Unusually Active Palladium-Based Catalysts for the Electrooxidation of Formic Acid, J. Power Sources 157 (2006) 78-84. [11] S. Park, Y. Xie, M. J. Weaver, Electrocatalytic Pathways on Carbon-Supported Platinum Nanoparticles: Comparison of Particle-SizeDependent Rates of Methanol, Formic Acid, and Formaldehyde Electrooxidation, Langmuir 18 (2002) 5792-5798. [12] V. A. Sethuraman, D. Vairavapandian, M. C. Lafouresse, T. A. Maark, N. Karan, S. H. Sun, U. Bertocci, A. A. Peterson, G. R. Stafford, P. R. Guduru, Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt, J. Phys. Chem. C 119 (2015) 19042-19052. [13] F. M. F. Rhen, C. McKeown, Enhanced Methanol Oxidation on Strained Pt Films, J. Phys. Chem. C 121 (2017) 2556-2562. [14] J. Greeley, I. E. L. Stephens, A. S. Bondarenko, T. P. Johansson, H. A. Hansen, T. F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J. K. Nørskov, Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts, Nat. Chem. 1 (2009) 552-556. [15] U. A. Paulus, A. Wokaun, G. G. Scherer, T. J. Schmidt, V. Stamenkovic, V. Radmilovic, N. M. Markovic, P. N. Ross, Oxygen Reduction on Carbon-Supported Pt-Ni and Pt-Co Alloy Catalysts, J. Phys. Chem. B 106 (2002) 4181-4191. [16] L. H. Jiang, G. Q. Sun, S. G. Sun, J. G. Liu, S. H. Tang, H. Q. Li, B. Zhou, Q. Xin, Structure and Chemical Composition of Supported Pt–Sn Electrocatalysts for Ethanol Oxidation, Electrochim. Acta 50 (2005) 5384-5389. [17] F. Colmati, E. Antolini, E. R. Gonzalez, Effect of Temperature on the Mechanism of Ethanol Oxidation on Carbon Supported Pt, PtRu and Pt3Sn Electrocatalysts, J. Power Sources 157 (2006) 98-103. [18] Z. L. Liu, X. Y. Ling, X. D. Su, J. Y. Lee, Carbon-Supported Pt and PtRu Nanoparticles as Catalysts for a Direct Methanol Fuel Cell, J. Phys. Chem. B 108 (2004) 8234-8240. [19] P. K. Shen, A. C. C. Tseung, Anodic Oxidation of Methanol on Pt/WO3 in Acidic Media, J. Electrochem. Soc. 141 (1994) 3082-3090. [20] M. Mirdamadi-Esfahani, M. Mostafavi, B. Keita, L. Nadjo, P. Kooyman, H. Remita, Bimetallic Au-Pt Nanoparticles Synthesized by Radiolysis: Application in Electro-Catalysis, Gold Bull. 43 (2010) 49-56. [21] X. N. Xu, G. W. Qin, Y. P. Ren, B. Shen, W. L. Pei, Experimental Study of the Miscibility Gap and Calculation of the Spinodal Curves of the Au–Pt System, Scr. Mater. 61 (2009) 859-862. [22] X. N. Xu, Y. P. Ren, C. F. Li, S. Li, G. W. Qin, Thermodynamic Assessment of Au-Pt System, Trans. Nonferrous Met. Soc. China 22 (2012) 1432-1436. [23] J. P. Hoare, A Cyclic Voltammetric Study of the Gold‐Oxygen System, J. Electrochem. Soc. 131 (1984) 1808-1815. [24] J. Luo, L. Wang, D. Mott, P. N. Njoki, Y. Lin, T. He, Z. Xu, B. N. Wanjana, I. -I. S. Lim, C. -J. Zhong, Core/Shell Nanoparticles as Electrocatalysts for Fuel Cell Reactions, Adv. Mater. 20 (2008) 4342-4347. [25] M. S. El-Deab, Electrocatalysis by Nanoparticles: Oxidation of Formic Acid at Manganese Oxide Nanorods-Modified Pt Planar and Nanohole-Arrays, J. Adv. Res. 1 (2010) 87-93. [26] G. A. El-Nagar, A. M. Mohammad, M. S. El-Deab, B. E. El-Anadouli, Electrocatalysis by Design: Enhanced Electrooxidation of Formic Acid at Platinum Nanoparticles–Nickel Oxide Nanoparticles Binary Catalysts, Electrochim. Acta 94 (2013) 62-71. [27] A. Cuesta, M. Escudero, B. Lanova, H. Baltruschat, Cyclic Voltammetry, FTIRS, and DEMS Study of the Electrooxidation of Carbon Monoxide, Formic Acid, and Methanol on Cyanide-Modified Pt(111) Electrodes, Langmuir 25 (2009) 6500-6507.