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Carbon supported Pt, Ru and Mo catalysts for methanol electrooxidation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 7 6 1 e1 4 7 6 8
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Carbon supported Pt, Ru and Mo catalysts for methanol electrooxidation Erika Teliz a,c,*, Vero´nica Dı´az a,b, Ignacio Pe´rez a,c, Mariana Corengia a,b, Carlos Fernando Zinola a,c a
Facultad de Ciencias, UdelaR, Igua´ 4225, 11400 Montevideo, Uruguay Facultad de Ingenierı´a, Udelar, J. Herrera y Reissig 565, 11300 Montevideo, Uruguay c Nu´cleo Interdisciplinario Ingenierı´a Electroquı´mica, J.E. Rodo´ 1843, 11200 Montevideo, Uruguay b
article info
abstract
Article history:
Methanol electrochemical oxidation on carbon supported electrocatalysts was studied on
Received 12 September 2011
platinum, ruthenium and molybdenum as active phases. Pt/C, PtRu/C, PtMo/C and PtRuMo/
Accepted 12 December 2011
C catalysts were synthesized with 20% metal loading by chemical reduction. These cata-
Available online 9 January 2012
lysts were physical and electrochemical characterized by Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), cyclic
Keywords:
voltammetry and CO anodic stripping voltammetry. Chronoamperometry was used to
Electrocatalysis
analyze and compare the catalysts activities after an electrochemical surface activation.
Platinum
The platinum active area was determined by anodic stripping CO voltammetry, exhibiting
Molybdenum
a different electrochemical profile for each catalyst. PtMo/C CO oxidation profile exhibited
Ruthenium
two peaks and clearly depicted the lowest onset potential value.
Catalytic poisons
The electrochemical methods revealed an enhanced performance of PtMo/C catalysts for methanol oxidation in comparison with the others catalysts studied. After the integration of chronoamperometric plots over 20 min in methanol acid media at 450 mV, PtMo/ C catalyst presented charge densities values three times greater than PtRu/C and PtRuMo/ C. It was not found any catalytic activity for the Pt/C at this potential value. According with our results PtMo/C can be considered more tolerant to the formation of catalytic poisons. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells systems are promising energy conversion devices because of their high efficiency, low polluting emissions and low operating temperatures [1,2]. They can operate with pure hydrogen in the laboratories. However, it is a practical use the choice of hydrogen rich synthetic gas produced from the reforming of hydrocarbons, as well as the use of methanol [3]. The anode should be able to tolerate fuel contaminants and partial oxidation products, which are a main cause of anodic voltage loss [4].
Many efforts have been conducted to improve the lifetime of the methanol fuel cell without increasing cost or losing performance, exploring binary and ternary anode catalysts [5e7]. The problem of ternary alloys formation is the incessant changes in the morphology during operation that yields new surface compositions with different electrocatalytic abilities towards alcohols oxidations. So far, many Pt-based catalysts have been proposed as alternatives to PtRu [8,9] Among them, bimetallic PtMo alloy catalyst has attracted considerable attention due to their high catalytic performance [10e13]. Carbon supported PtRuMo electrocatalysts have attracted
* Corresponding author. Facultad de Ciencias, UdelaR, Igua´ 4225, 11400 Montevideo, Uruguay. E-mail address: [email protected] (E. Teliz). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.084
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2. 2.1.
60 50
j/ µ Acm-2
more attention than binary systems in recent years [14e16], since promising results have been attained for polymer electrolyte fuel cells with reformate gas and alcohols. In a previous work [17], we have found that surface electrochemical pretreatments usually lead to surface and structure modifications producing large electrocatalytic improvements of current and power efficiencies when using methanol as fuel [17]. The surface pretreatment involves a net cathodic polarization in the hydrogen evolution potential region. The purpose of this work is to study the performance of Pt/ C, PtRu/C, PtMo/C and PtRuMo/C electrocatalysts towards methanol electrooxidation.
All platinum, ruthenium and molybdenum/platinum catalysts were supported on Vulcan XC-72 carbon with a designed metal loading of 20 wt % using the borohydride method [18]. The carbon slurry was prepared by dispersing 0.04 g of carbon in 40 mL of Millipore-MilliQ plus water (18.2 MUcm resistivity) after 40 min sonication. In all cases, chloroplatinic acid hexahydrate, molybdenum(V) chloride and ruthenium(III) chloride were used as platinum, molybdenum and ruthenium precursors, respectively. To obtain the desired proportion at the catalyst the metal precursors were added to the carboncontaining solution to the 1:1 atomic ratio for PtRu/C and PtMo/C and to the 1:1:1 for PtRuMo/C. The powder was obtained by the chemical reduction of the previous solution (mother solution) with an excess of sodium borohydride in an ultrasonic
30 20 10 0
-10 0.6
Experimental Electrode preparation and pretreatments
40
0.7
E/V vs RHE
0.8
0.9
Fig. 2 e First positive-going potential scan run at 0.10 V sL1 for CO stripping from Pt/C (black line), PtRu/C (red line), PtMo/C (blue line), PtRuMo/C (green line) after 20 min of adsorption at 0.05 V in CO-saturated supporting electrolyte. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bath during 80 min. The precipitate was isolated by centrifugation, washed with water and dried at 80 C overnight. The ternary catalyst was synthesized changing the order that carbon was impregnated with the metallic salt precursors. The electrode was prepared mixing, in an ultrasonic bath, 2.0 mg of the synthesized catalyst with 15 mL of Nafion 5% and 485 mL of Millipore water. In all cases the activation is completed after a careful cathodization program, in order to properly select the applied potential and time (0.10 V for 30 min) required for a net hydrogen evolution process. After then, the electrode surface is stabilized by ten cycles at 0.10 V s1 within the entire potential range (0.05 and 1.45 V). This activation pretreatment ensures the complete reduction of the salts precursors, as well as the eventual surface restructuration [10].
2.2.
Electrodes characterization
2.2.1.
Electrochemical characterization
Electrochemical runs were performed using a three-electrode compartment cell with the studied carbon supported catalysts deposited on a gold disc as working electrode. A large area smooth platinum counter electrode and a reversible hydrogen
Fig. 1 e Cyclic voltammetry of carbon-supported electrodes activated by cathodization at L0.10 V for 30 min followed by ten cycles between 0.05 and 1.45 V. The potentiodynamic contour was run at 0.10 V sL1 between 0.05e1.45 V in 1 M sulfuric acid solution at room temperature. Pt/C (black line), PtRu/C (red line), PtMo/C (blue line), PtRuMo/C (green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 e Mean particle size calculated from XRD powder patterns using the Debye-Scherrer equation. Catalyst
Mean particle size (nm)
Pt/C PtRu/C PtMo/C PtRuMo/C
3.5 5.0 3.8 2.3
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Fig. 3 e Transmission electron micrographs of carbon supported catalysts; (A) Pt/C, (B) PtRu/C, (C) PtMo/C, (D) PtRuMo/C.
reference electrode with a Luggin-Haber capillary tip completed the electrochemical system. Potential values in the text are given on the hydrogen reference electrode (RHE) scale. 1 M sulphuric acid is used as supporting electrolyte, which was prepared using Millipore water.
The electrochemical characterization of the catalysts was performed by cyclic voltammetry in the supporting electrolyte at 0.10 V s1 between 0.05 and 1.45 V. Surface active area was determined after the integration of the complete first positive-going CO stripping profile. We performed the
Fig. 4 e Mapping and SEM micrographs for Pt/C catalyst.
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Fig. 5 e Mapping and SEM micrographs for PtRu/C catalyst.
electrochemical experiments using a PGZ 301 Voltalab potentiostat-galvanostat-impedance with the Voltamaster 4 software.
2.2.2.
Physical characterization XRD, TEM and EDS
X-ray powder diffraction data were collected using a Rigaku ULTIMA IV, 285 mm radius, Powder Diffractometer operating ˚) in Bragg-Brentano geometry. CuKa radiation (l ¼ 1.5418 A monochromatized with a diffracted beam bent germanium crystal was used to collect data over the 39e90 2q range in steps of 0.02 and 0.01 deg using a scintillation detector. Peak positions were extracted from the diffractograms using the program POWDERX [19]. Multiple datasets were collected for each sample in order to obtain representative values. We calculated the mean particle size of each catalyst using the DebyeeScherrer formula from XRD patterns. In order to check the dispersion states and particle sizes we employed Transmission Electron Microscopy (TEM) with relative medium acceleration energy of 80e100 KV and 300,000 and 600,000 magnifications. The relative atomic ratio percentage and the atomic surface distribution for each catalyst was estimated from EDS micrographs using the Scanning Electron Microscope JEOL JSM 5900.
2.3. Electrocatalytic performance towards methanol and CO electrooxidation In order to study the electrocatalytic performance of the catalysts three methodologies were conducted in oxygen-free 0.1 M methanol þ 1 M sulphuric media, namely, cyclic voltammetry and chronoamperometric transients. Linear sweep voltammetry was conducted on each surface starting from 0.05 V and scanning towards positive values up to 0.90 V run at 0.10 V s1. Chronoamperometric curves were performed until 20 min at the same potentials of the impedance spectra (from 0.45 to 0.60 V). The charge density values under the chronoamperometric plots were calculated until 20 min only for comparison purposes.
3.
Results and discussion
3.1.
Surface characterization
Fig. 1 depicts the voltammetric profile of the catalysts after the electrochemical pretreatment. It seems that the electroreduction process induced by cathodization produces
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 7 6 1 e1 4 7 6 8
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Fig. 6 e Mapping and SEM micrographs for PtMo/C catalyst.
a cleanness of the electrode and an ulterior restructuration [17]. The effect is important on binary and ternary platinum alloys because it seems to produce a complete reduction of salts precursors. In this sense, the double layer capacity decreased as a consequence of the lower number of charge carriers in the outer Helmholtz plane (OHP). For PtMo/C we can also observe the superimposed redox couple of MoO3/ MoO2 species. On the other hand, PtRu/C shows the formation of RuO2 at 0.6 V and its reduction near 0.5 V (during the cathodic scan) superimposed with the platinum oxide reduction peak. In all the cases the hydrogen adsorption/ desorption peaks are very well defined due to the cathodization program. This electrochemical pretreatment [20] produces a surface rearrangement leading to (111) e stepped surfaces. The CO anodic stripping profiles are depicted in Fig. 2. It is clear that the different profiles exhibited by each catalyst are strongly dependent on the nature of the catalyst. Besides, PtMo/C shows noticeable lower onset potential values than the other catalyst surfaces. On one hand, we obtained the expected profile for CO oxidation in acid media on Pt/C catalyst. It is important to state that in contrast to Pt/C conventional CO oxidation profile, that on PtMo/C shows a first anodic charge density contribution clearly larger than the
second. In the case of PtRu/C, the single peak is shifted towards lower potential values than Pt/C. In the ternary alloy we clearly observed the influence of both metals on Pt/C profile. Moreover, the carbon supported electrodes were characterized by XRD patterns to check the particle mean size. The latter, obtained using the (111) diffraction line broadening analysis, showed a remarkable agreement between PtMo/C and Pt/C. The mean particle size was calculated from the Debye-Scherrer equation, showing that the PtRu/C exhibits the largest value and PtRuMo/C the smallest one (Table 1). The surface characterization was completed by TEM images for each catalyst. The less dispersed catalyst was PtMo/C (Fig. 3).
3.1.1. SEM analysis 3.1.1.1. Pt/C catalyst. Pt/C mapping in Fig. 4 depicts a uniform Pt concentration distribution. C and O concentration patterns are very similar. Besides, we can appreciate negligible concentration zones for those elements. On the other side, the Pt atomic percentage is near 1.04%, which is associated with a Pt catalysts mass/mass percentage of 17%.
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Fig. 7 e Mapping and SEM micrographs for PtRuMo/C catalyst.
3.1.1.2. Binary alloys. 3.1.1.2.1. PtRu/C catalyst. We can observe the same behavior for C, O and Pt in this catalyst (Fig. 5). In contrast to Pt, Ru concentration shows a qualitative different pattern which is similar to that depicted by O. As a result, it is possible the presence of a ruthenium oxide in the catalyst surface. The Pt/Ru atomic ratio is near 1.
3.1.1.2.2. PtMo/C catalyst. C, O and Pt present similar concentration patterns as it is shown in the Pt/C catalyst (Fig. 6). Mo and Pt concentration patterns are both similar and depict a uniform concentration distribution. The Pt/Mo atomic ratio is 2.3/1. We expected a 1:1 ratio between Pt and Mo, but some molybdenum species is volatilized during the weighting process in the presence of oxygen leading to hydrochloric acid. 3.1.1.3. Ternary alloy. 3.1.1.3.1. PtRuMo/C catalyst. Pt concentration pattern is the same as those depicted in the other catalysts. As it just was stated in the catalysts above, C and O present a similar concentration distribution in the selected area. Ru concentration pattern is qualitatively different
from that on Pt but similar to that on O. This fact was just observed on the PtRu/C catalyst. In contrast to what is stated in the PtMo/C catalyst, now, Mo concentration pattern is similar to that of Ru but it differs from the Pt concentration sample (Fig. 7). This fact could explain the different electrocatalytic behavior between the different catalysts surfaces. Finally, the atomic ratio for this alloy is Pt/Ru/Mo:0.7/1.11/0.63.
3.2.
Electrocatalytic performance
On the other hand, methanol electrocatalytic oxidation was studied by different electrochemical techniques. Fig. 8 shows the methanol current transients performed at 0.50 and 0.60 V, during 20 min. After integration of the current densities, we obtained the surface oxidation charge densities for each catalyst at the studied potential values after correcting the results by subtracting the charge corresponding to the oxidation in the supporting electrolyte in absence of methanol (Table 2). At 0.45 V, Pt/C supporting electrode does not present any catalytic activity, meanwhile PtMo/C shows the best performance towards methanol electrooxidation. The latter,
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with CO attached to Pt sites. The reaction mechanism is shown as follows:
PteCO þ RueOH —> Pt þ Ru þ CO2 þ Hþ þ e
Fig. 8 e Current densities transients in 0.1M methanol D 1M sulfuric acid for Pt/C (black line), PtRu/C (red line), PtMo/C (blue line), PtRuMo/C (green line); (A) at 0.50 V and (B) at 0.60 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
presents the largest charge densities values for all the studied potential. PtRu/C presents a good performance, similar to that observed on PtMo/C but only at 0.60 V. At this potential it is possible that water discharge favors the bifunctional mechanism. PtMo/C seems to be the most tolerant catalyst to catalytic poisons. In order to study the charge transfer process in the surface catalysts, a linear potential sweep was performed by analyzing the quasi-steady state stationary polarization curve for methanol electrooxidation on carbon supported electrode [24]. In a previous work we have observed that catalysts containing Mo always presents the greatest peak current densities. On the other hand, PtRuMo/C depicted the lowest onset potential value.
Considering the electrochemical spectrum to be very close to that of PtMoRu, which is originally Ru being reduced from higher oxidation states as a consequence of cathodic treatments (Fig. 2), it is likely that the 4 þ valence state is present at the beginning of the experiment. When the voltage slowly increased to 1.0 V it moves from RuO2 that is the oxidation state of Ru in the catalyst at this potential to a more likely 2 þ valence state. Thus, in the course of the oxidation process, Ru surface is maintained between 0 and 2þ and the Pt surface is not disturbed at all in this potential range. The anodic oxidation of Ru at lower potentials may result in the formation of oxygen adsorbed species on the surface of Ru. At the same time methanol oxidized on Pt surface by breaking CeH bond results in the formation adsorbed CO on Pt sites of the catalyst. Larger potential values are necessary to desorb CO from the pure Pt surface. But the adsorbed species on Ru surface oxidizes CO to CO2 [25]. Therefore, in multistep processes, the clean Pt surface is retained for methanol oxidation as well as oxidized Ru surface is available for CO oxidation. In the case of Pt/C a main process attributed to the oxidation of linear-bonded CO is observed at 0.78 V. The same process is seen when the cell scan rate is reduced to lower values, but with a shift to less positive potentials (0.65 V), implying that the CO oxidation is favored. The oxidation of bridge and linear attached CO are evidenced by the peaks at 0.42 and 0.48 V, respectively. The absence of large anodic charges involved CO oxidation processes at low potentials for PtMo/C is a confirmation that the CO tolerance observed for this catalyst is substantially achieved by the lowering of the CO concentration promoted by a heterogeneous chemical step, well-known water gas shift reaction.
CO þ H2O ——> CO2 þ H2 Since the Mo(IV)/Mo(VI) transition takes place at around 0.46 V [26], and the potential of the fuel cell anode is always below 0.4 V, it is probable that Mo(IV) is the active catalyst for the heterogeneous chemical step.
Table 2 e Methanol oxidation charge densities values for Pt/C, PtRu/C, PtMo/C and PtRuMo/C after 20 min at different potentials. Charge density values (mC/cm2)
4.
Discussion
The results analyzed above support the mechanism of bimetallic catalytic oxidation of adsorbed species in which Ru centers play the role for providing oxidant species to react
E/mV
Pt/C
PtRu/C
PtMo/C
PtRuMo/C
450 500 550 600
0.00 5.57 20.25 74.34
3.82 5.31 30.48 91.23
13.59 29.36 52.40 107.86
3.67 3.50 17.55 54.48
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An important point to be observed in the CO stripping voltammograms for Mo/C, is a shift of Mo(IV)/Mo(VI) process for more positive potentials after the CO adsorption (not shown). This behavior is an evidence of an interaction between CO and the Mo sites. A shift of the potentials to less positive with temperature is observed, but this is less remarked than for Pt/ C, because they refer to different processes. As seen from comparisons with the results in Figs. 2 and 8, the Mo features at PtMo/C and PtRu/C must also refer to different processes, because they appear at different electrode potentials. For PtMo/C at room temperature, two anodic CO oxidation contours (0.68 and 0.78 V) related to the Mo(IV)/Mo(VI) and CO-Mo surface process and the CO oxidation at Pt sites, respectively, are observed. These features do not correlate with the LangmuireHinshelwood mechanism that PtRu/C surfaces obey.
5.
Conclusions
PtRuMo/C shows the highest current density peak, as well as the lowest onset potential value in linear potential sweep, for methanol electrooxidation than PtMo/C and PtRu/C. However, it depicts as the less tolerance surface to catalytic poisons. We can conclude that PtMo/C catalysts always exhibited the largest activity for methanol oxidation. It presents the highest charge density values due to their greatest tolerance to catalytic poisons. Besides, PtMo/C shows noticeable lower onset potential value than the other catalysts surfaces for CO electrooxidation. The values of the Tafel slopes by performing the quasistationary polarization curves exhibited a net Tafel line, potential vs. logarithm of current between 0.45 and 0.65 V (not shown). In the case of Pt/C and PtMo/C the Tafel slopes are equal to 0.083 V dec1. Values of Tafel slopes as high as 3RT/2F were reported some time ago [21e23] for carbon monoxide oxidation in conditions where the proposed mechanism denote a first fast single electron transfer, followed by a slow surface electrochemical step as rate determinant. This path is that of an adsorbed carbon monoxide converted to adsorbed formiate, previous to the surface oxidation to carbon dioxide. The presence of ruthenium in the PtRuMo/C catalysts retains almost the same Tafel slope (87 mV dec1) because of the presence of molybdenum. On the other hand, PtRu/C exhibits Tafel slopes of 0.120 V dec1, so the classical Tafel slope value of 2RT/F is obtained where the rate determining step of a single and first electron transfer occurs. In the case of the ternary catalyst a slightly higher Tafel slope value is observed than on Pt/C and PtMo/C, however at potentials larger than 0.65 V a similar behavior than that on PtRu/C is observed. The ternary catalysts exhibit an electrochemical behavior between those of PtRu/C and PtMo/C. Therefore, binary catalysts produce hydroxyl species at lower potentials than platinum and they also are more tolerant towards the presence of poisoning intermediates than pure platinum. In fact, this can also be due to the electronic effect caused by molybdenum that can produce less strongly anchoraged adsorbates. This is a consequence of the electron injection of molybdenum atoms to the platinum surface. The result is an increase of the methanol oxidation rate since the intermediates are being much more labile, in spite of having the same rate determining step as platinum.
Acknowledgments The authors are greatly acknowledged to the CSIC Projects of the UdelaR. V. Dı´az and C.F. Zinola are researchers at the Chemistry Area of PEDECIBA United Nations. The authors are grateful obliged to the XRD and EDS measurements of carbon-supported catalysts to Dr. R. Faccio and Msc. S. Villar, respectively.
references
[1] Fuel cell handbook. Morgantown, West Virginia: EG&G Services. Parson, Laboratory, National Energy Technology; 2004. [2] Barbir F. In: Vielstich W, Lahm A, Gesteiger HA, editors. Handbook of fuel cells-fundamental technology and applications, vol. 4. Chichester: Wiley; 2003. [3] Olah G, Goeppert A, Surya Prakash GK. Beyond oil and gas: the methanol economy. Weinheim: Wiley-VCH Verlag GmbH & Co; 2006. [4] Lima A, Coutanceau C, Leger JM, Lamy C. J Appl Electrochem 2001;31:379. [5] Rashidi R, Dincer I, Naterer GF, Berg P. J Power Sources 2009; 187:509. [6] Antolini E. Appl Catal B Environ 2007;74:324. [7] Liu H, Song C, Zhang L, Zhang J, Wang H, Wilkinson DP. J Power Sources 2006;155:95. [8] Watanabe M, Motoo S. J Electroanal Chem 1975;60:275. [9] Oetjen HF, Schmidt VM, Stimming U, Trila F. J Electrochem Soc 1996;143:3838. [10] Cooper JS, McGinn PJ. J Power Sources 2006;163:330. [11] Mukerjee S, Lee SJ, Ticcianelli E, McBreen J, Grur BN, Markovic NM, et al. Electrochem Solid State Lett 1999;2:12. [12] Mukerjee S, Urian RC, Lee SJ, Ticcianelli E, McBreen J. J Electrochem Soc 2004;151:A1094. [13] Urian RC, Gulla´ AF, Mukerjee S. J Electroanal Chem 2003; 554e555:307. [14] Song CJ, Khanfar M, Pickup PG. J Appl Electrochem 2006; 36:339. [15] Hou Z, Yi B, Yu H, Lin Z, Zhang H. J Power Sources 2003; 123:116. [16] Papageorgopoulos DC, Keijzer M, de Bruijn FA. Electrochim Acta 2002;48:197. [17] Dı´az V, Zinola CF. J Colloid Int Sci 2007;313:232. [18] Salgado JR, Antolini E, Gonza´lez ER. J Power Sources 2004; 138:56. [19] Dong CJ. Appl Cryst 1999;32:838. [20] Dı´az V, Real S, Te´liz E, Zinola CF, Martins ME. Int J Hydrogen Energy 2009;34:3519. [21] Bergelin M, Herrero E, Feliu´ JM, Wasberg M. J Electroanal Chem 1999;467:74. [22] Love B, Lipkowski J. Effect of surface crystallography on electrocatalytic oxidation of carbon monoxide on Pt electrodes. ACS Symp Ser 1988;378:484. [23] Palaikis L, Zurawski D, Hourani M, Wieckowski A. Surf Sci 1988;199:183. [24] Camargo A, Corengia M, Dı´az V, Martı´nez S, Te´liz E, Zinola CF. Electrocatalysis of Molybdenum e containing substrates for fuel cells applications. Nova publishers, “Molybdenum: characteristics, production and applications”, in press. [25] Park KW, Choi JH, Lee S-A, Pak C, Chang H, Sung YE. J Catal 2004;224:236. [26] Zhang H, Wang Y, Fachini ER, Cabrera CR. Electrochem Solid State Lett 1999;2:437.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Carbon supported Pt, Ru and Mo catalysts for methanol electrooxidation Erika Teliz a,c,*, Vero´nica Dı´az a,b, Ignacio Pe´rez a,c, Mariana Corengia a,b, Carlos Fernando Zinola a,c a
Facultad de Ciencias, UdelaR, Igua´ 4225, 11400 Montevideo, Uruguay Facultad de Ingenierı´a, Udelar, J. Herrera y Reissig 565, 11300 Montevideo, Uruguay c Nu´cleo Interdisciplinario Ingenierı´a Electroquı´mica, J.E. Rodo´ 1843, 11200 Montevideo, Uruguay b
article info
abstract
Article history:
Methanol electrochemical oxidation on carbon supported electrocatalysts was studied on
Received 12 September 2011
platinum, ruthenium and molybdenum as active phases. Pt/C, PtRu/C, PtMo/C and PtRuMo/
Accepted 12 December 2011
C catalysts were synthesized with 20% metal loading by chemical reduction. These cata-
Available online 9 January 2012
lysts were physical and electrochemical characterized by Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), cyclic
Keywords:
voltammetry and CO anodic stripping voltammetry. Chronoamperometry was used to
Electrocatalysis
analyze and compare the catalysts activities after an electrochemical surface activation.
Platinum
The platinum active area was determined by anodic stripping CO voltammetry, exhibiting
Molybdenum
a different electrochemical profile for each catalyst. PtMo/C CO oxidation profile exhibited
Ruthenium
two peaks and clearly depicted the lowest onset potential value.
Catalytic poisons
The electrochemical methods revealed an enhanced performance of PtMo/C catalysts for methanol oxidation in comparison with the others catalysts studied. After the integration of chronoamperometric plots over 20 min in methanol acid media at 450 mV, PtMo/ C catalyst presented charge densities values three times greater than PtRu/C and PtRuMo/ C. It was not found any catalytic activity for the Pt/C at this potential value. According with our results PtMo/C can be considered more tolerant to the formation of catalytic poisons. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells systems are promising energy conversion devices because of their high efficiency, low polluting emissions and low operating temperatures [1,2]. They can operate with pure hydrogen in the laboratories. However, it is a practical use the choice of hydrogen rich synthetic gas produced from the reforming of hydrocarbons, as well as the use of methanol [3]. The anode should be able to tolerate fuel contaminants and partial oxidation products, which are a main cause of anodic voltage loss [4].
Many efforts have been conducted to improve the lifetime of the methanol fuel cell without increasing cost or losing performance, exploring binary and ternary anode catalysts [5e7]. The problem of ternary alloys formation is the incessant changes in the morphology during operation that yields new surface compositions with different electrocatalytic abilities towards alcohols oxidations. So far, many Pt-based catalysts have been proposed as alternatives to PtRu [8,9] Among them, bimetallic PtMo alloy catalyst has attracted considerable attention due to their high catalytic performance [10e13]. Carbon supported PtRuMo electrocatalysts have attracted
* Corresponding author. Facultad de Ciencias, UdelaR, Igua´ 4225, 11400 Montevideo, Uruguay. E-mail address: [email protected] (E. Teliz). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.084
14762
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 7 6 1 e1 4 7 6 8
2. 2.1.
60 50
j/ µ Acm-2
more attention than binary systems in recent years [14e16], since promising results have been attained for polymer electrolyte fuel cells with reformate gas and alcohols. In a previous work [17], we have found that surface electrochemical pretreatments usually lead to surface and structure modifications producing large electrocatalytic improvements of current and power efficiencies when using methanol as fuel [17]. The surface pretreatment involves a net cathodic polarization in the hydrogen evolution potential region. The purpose of this work is to study the performance of Pt/ C, PtRu/C, PtMo/C and PtRuMo/C electrocatalysts towards methanol electrooxidation.
All platinum, ruthenium and molybdenum/platinum catalysts were supported on Vulcan XC-72 carbon with a designed metal loading of 20 wt % using the borohydride method [18]. The carbon slurry was prepared by dispersing 0.04 g of carbon in 40 mL of Millipore-MilliQ plus water (18.2 MUcm resistivity) after 40 min sonication. In all cases, chloroplatinic acid hexahydrate, molybdenum(V) chloride and ruthenium(III) chloride were used as platinum, molybdenum and ruthenium precursors, respectively. To obtain the desired proportion at the catalyst the metal precursors were added to the carboncontaining solution to the 1:1 atomic ratio for PtRu/C and PtMo/C and to the 1:1:1 for PtRuMo/C. The powder was obtained by the chemical reduction of the previous solution (mother solution) with an excess of sodium borohydride in an ultrasonic
30 20 10 0
-10 0.6
Experimental Electrode preparation and pretreatments
40
0.7
E/V vs RHE
0.8
0.9
Fig. 2 e First positive-going potential scan run at 0.10 V sL1 for CO stripping from Pt/C (black line), PtRu/C (red line), PtMo/C (blue line), PtRuMo/C (green line) after 20 min of adsorption at 0.05 V in CO-saturated supporting electrolyte. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bath during 80 min. The precipitate was isolated by centrifugation, washed with water and dried at 80 C overnight. The ternary catalyst was synthesized changing the order that carbon was impregnated with the metallic salt precursors. The electrode was prepared mixing, in an ultrasonic bath, 2.0 mg of the synthesized catalyst with 15 mL of Nafion 5% and 485 mL of Millipore water. In all cases the activation is completed after a careful cathodization program, in order to properly select the applied potential and time (0.10 V for 30 min) required for a net hydrogen evolution process. After then, the electrode surface is stabilized by ten cycles at 0.10 V s1 within the entire potential range (0.05 and 1.45 V). This activation pretreatment ensures the complete reduction of the salts precursors, as well as the eventual surface restructuration [10].
2.2.
Electrodes characterization
2.2.1.
Electrochemical characterization
Electrochemical runs were performed using a three-electrode compartment cell with the studied carbon supported catalysts deposited on a gold disc as working electrode. A large area smooth platinum counter electrode and a reversible hydrogen
Fig. 1 e Cyclic voltammetry of carbon-supported electrodes activated by cathodization at L0.10 V for 30 min followed by ten cycles between 0.05 and 1.45 V. The potentiodynamic contour was run at 0.10 V sL1 between 0.05e1.45 V in 1 M sulfuric acid solution at room temperature. Pt/C (black line), PtRu/C (red line), PtMo/C (blue line), PtRuMo/C (green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 e Mean particle size calculated from XRD powder patterns using the Debye-Scherrer equation. Catalyst
Mean particle size (nm)
Pt/C PtRu/C PtMo/C PtRuMo/C
3.5 5.0 3.8 2.3
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Fig. 3 e Transmission electron micrographs of carbon supported catalysts; (A) Pt/C, (B) PtRu/C, (C) PtMo/C, (D) PtRuMo/C.
reference electrode with a Luggin-Haber capillary tip completed the electrochemical system. Potential values in the text are given on the hydrogen reference electrode (RHE) scale. 1 M sulphuric acid is used as supporting electrolyte, which was prepared using Millipore water.
The electrochemical characterization of the catalysts was performed by cyclic voltammetry in the supporting electrolyte at 0.10 V s1 between 0.05 and 1.45 V. Surface active area was determined after the integration of the complete first positive-going CO stripping profile. We performed the
Fig. 4 e Mapping and SEM micrographs for Pt/C catalyst.
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Fig. 5 e Mapping and SEM micrographs for PtRu/C catalyst.
electrochemical experiments using a PGZ 301 Voltalab potentiostat-galvanostat-impedance with the Voltamaster 4 software.
2.2.2.
Physical characterization XRD, TEM and EDS
X-ray powder diffraction data were collected using a Rigaku ULTIMA IV, 285 mm radius, Powder Diffractometer operating ˚) in Bragg-Brentano geometry. CuKa radiation (l ¼ 1.5418 A monochromatized with a diffracted beam bent germanium crystal was used to collect data over the 39e90 2q range in steps of 0.02 and 0.01 deg using a scintillation detector. Peak positions were extracted from the diffractograms using the program POWDERX [19]. Multiple datasets were collected for each sample in order to obtain representative values. We calculated the mean particle size of each catalyst using the DebyeeScherrer formula from XRD patterns. In order to check the dispersion states and particle sizes we employed Transmission Electron Microscopy (TEM) with relative medium acceleration energy of 80e100 KV and 300,000 and 600,000 magnifications. The relative atomic ratio percentage and the atomic surface distribution for each catalyst was estimated from EDS micrographs using the Scanning Electron Microscope JEOL JSM 5900.
2.3. Electrocatalytic performance towards methanol and CO electrooxidation In order to study the electrocatalytic performance of the catalysts three methodologies were conducted in oxygen-free 0.1 M methanol þ 1 M sulphuric media, namely, cyclic voltammetry and chronoamperometric transients. Linear sweep voltammetry was conducted on each surface starting from 0.05 V and scanning towards positive values up to 0.90 V run at 0.10 V s1. Chronoamperometric curves were performed until 20 min at the same potentials of the impedance spectra (from 0.45 to 0.60 V). The charge density values under the chronoamperometric plots were calculated until 20 min only for comparison purposes.
3.
Results and discussion
3.1.
Surface characterization
Fig. 1 depicts the voltammetric profile of the catalysts after the electrochemical pretreatment. It seems that the electroreduction process induced by cathodization produces
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Fig. 6 e Mapping and SEM micrographs for PtMo/C catalyst.
a cleanness of the electrode and an ulterior restructuration [17]. The effect is important on binary and ternary platinum alloys because it seems to produce a complete reduction of salts precursors. In this sense, the double layer capacity decreased as a consequence of the lower number of charge carriers in the outer Helmholtz plane (OHP). For PtMo/C we can also observe the superimposed redox couple of MoO3/ MoO2 species. On the other hand, PtRu/C shows the formation of RuO2 at 0.6 V and its reduction near 0.5 V (during the cathodic scan) superimposed with the platinum oxide reduction peak. In all the cases the hydrogen adsorption/ desorption peaks are very well defined due to the cathodization program. This electrochemical pretreatment [20] produces a surface rearrangement leading to (111) e stepped surfaces. The CO anodic stripping profiles are depicted in Fig. 2. It is clear that the different profiles exhibited by each catalyst are strongly dependent on the nature of the catalyst. Besides, PtMo/C shows noticeable lower onset potential values than the other catalyst surfaces. On one hand, we obtained the expected profile for CO oxidation in acid media on Pt/C catalyst. It is important to state that in contrast to Pt/C conventional CO oxidation profile, that on PtMo/C shows a first anodic charge density contribution clearly larger than the
second. In the case of PtRu/C, the single peak is shifted towards lower potential values than Pt/C. In the ternary alloy we clearly observed the influence of both metals on Pt/C profile. Moreover, the carbon supported electrodes were characterized by XRD patterns to check the particle mean size. The latter, obtained using the (111) diffraction line broadening analysis, showed a remarkable agreement between PtMo/C and Pt/C. The mean particle size was calculated from the Debye-Scherrer equation, showing that the PtRu/C exhibits the largest value and PtRuMo/C the smallest one (Table 1). The surface characterization was completed by TEM images for each catalyst. The less dispersed catalyst was PtMo/C (Fig. 3).
3.1.1. SEM analysis 3.1.1.1. Pt/C catalyst. Pt/C mapping in Fig. 4 depicts a uniform Pt concentration distribution. C and O concentration patterns are very similar. Besides, we can appreciate negligible concentration zones for those elements. On the other side, the Pt atomic percentage is near 1.04%, which is associated with a Pt catalysts mass/mass percentage of 17%.
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Fig. 7 e Mapping and SEM micrographs for PtRuMo/C catalyst.
3.1.1.2. Binary alloys. 3.1.1.2.1. PtRu/C catalyst. We can observe the same behavior for C, O and Pt in this catalyst (Fig. 5). In contrast to Pt, Ru concentration shows a qualitative different pattern which is similar to that depicted by O. As a result, it is possible the presence of a ruthenium oxide in the catalyst surface. The Pt/Ru atomic ratio is near 1.
3.1.1.2.2. PtMo/C catalyst. C, O and Pt present similar concentration patterns as it is shown in the Pt/C catalyst (Fig. 6). Mo and Pt concentration patterns are both similar and depict a uniform concentration distribution. The Pt/Mo atomic ratio is 2.3/1. We expected a 1:1 ratio between Pt and Mo, but some molybdenum species is volatilized during the weighting process in the presence of oxygen leading to hydrochloric acid. 3.1.1.3. Ternary alloy. 3.1.1.3.1. PtRuMo/C catalyst. Pt concentration pattern is the same as those depicted in the other catalysts. As it just was stated in the catalysts above, C and O present a similar concentration distribution in the selected area. Ru concentration pattern is qualitatively different
from that on Pt but similar to that on O. This fact was just observed on the PtRu/C catalyst. In contrast to what is stated in the PtMo/C catalyst, now, Mo concentration pattern is similar to that of Ru but it differs from the Pt concentration sample (Fig. 7). This fact could explain the different electrocatalytic behavior between the different catalysts surfaces. Finally, the atomic ratio for this alloy is Pt/Ru/Mo:0.7/1.11/0.63.
3.2.
Electrocatalytic performance
On the other hand, methanol electrocatalytic oxidation was studied by different electrochemical techniques. Fig. 8 shows the methanol current transients performed at 0.50 and 0.60 V, during 20 min. After integration of the current densities, we obtained the surface oxidation charge densities for each catalyst at the studied potential values after correcting the results by subtracting the charge corresponding to the oxidation in the supporting electrolyte in absence of methanol (Table 2). At 0.45 V, Pt/C supporting electrode does not present any catalytic activity, meanwhile PtMo/C shows the best performance towards methanol electrooxidation. The latter,
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with CO attached to Pt sites. The reaction mechanism is shown as follows:
PteCO þ RueOH —> Pt þ Ru þ CO2 þ Hþ þ e
Fig. 8 e Current densities transients in 0.1M methanol D 1M sulfuric acid for Pt/C (black line), PtRu/C (red line), PtMo/C (blue line), PtRuMo/C (green line); (A) at 0.50 V and (B) at 0.60 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
presents the largest charge densities values for all the studied potential. PtRu/C presents a good performance, similar to that observed on PtMo/C but only at 0.60 V. At this potential it is possible that water discharge favors the bifunctional mechanism. PtMo/C seems to be the most tolerant catalyst to catalytic poisons. In order to study the charge transfer process in the surface catalysts, a linear potential sweep was performed by analyzing the quasi-steady state stationary polarization curve for methanol electrooxidation on carbon supported electrode [24]. In a previous work we have observed that catalysts containing Mo always presents the greatest peak current densities. On the other hand, PtRuMo/C depicted the lowest onset potential value.
Considering the electrochemical spectrum to be very close to that of PtMoRu, which is originally Ru being reduced from higher oxidation states as a consequence of cathodic treatments (Fig. 2), it is likely that the 4 þ valence state is present at the beginning of the experiment. When the voltage slowly increased to 1.0 V it moves from RuO2 that is the oxidation state of Ru in the catalyst at this potential to a more likely 2 þ valence state. Thus, in the course of the oxidation process, Ru surface is maintained between 0 and 2þ and the Pt surface is not disturbed at all in this potential range. The anodic oxidation of Ru at lower potentials may result in the formation of oxygen adsorbed species on the surface of Ru. At the same time methanol oxidized on Pt surface by breaking CeH bond results in the formation adsorbed CO on Pt sites of the catalyst. Larger potential values are necessary to desorb CO from the pure Pt surface. But the adsorbed species on Ru surface oxidizes CO to CO2 [25]. Therefore, in multistep processes, the clean Pt surface is retained for methanol oxidation as well as oxidized Ru surface is available for CO oxidation. In the case of Pt/C a main process attributed to the oxidation of linear-bonded CO is observed at 0.78 V. The same process is seen when the cell scan rate is reduced to lower values, but with a shift to less positive potentials (0.65 V), implying that the CO oxidation is favored. The oxidation of bridge and linear attached CO are evidenced by the peaks at 0.42 and 0.48 V, respectively. The absence of large anodic charges involved CO oxidation processes at low potentials for PtMo/C is a confirmation that the CO tolerance observed for this catalyst is substantially achieved by the lowering of the CO concentration promoted by a heterogeneous chemical step, well-known water gas shift reaction.
CO þ H2O ——> CO2 þ H2 Since the Mo(IV)/Mo(VI) transition takes place at around 0.46 V [26], and the potential of the fuel cell anode is always below 0.4 V, it is probable that Mo(IV) is the active catalyst for the heterogeneous chemical step.
Table 2 e Methanol oxidation charge densities values for Pt/C, PtRu/C, PtMo/C and PtRuMo/C after 20 min at different potentials. Charge density values (mC/cm2)
4.
Discussion
The results analyzed above support the mechanism of bimetallic catalytic oxidation of adsorbed species in which Ru centers play the role for providing oxidant species to react
E/mV
Pt/C
PtRu/C
PtMo/C
PtRuMo/C
450 500 550 600
0.00 5.57 20.25 74.34
3.82 5.31 30.48 91.23
13.59 29.36 52.40 107.86
3.67 3.50 17.55 54.48
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An important point to be observed in the CO stripping voltammograms for Mo/C, is a shift of Mo(IV)/Mo(VI) process for more positive potentials after the CO adsorption (not shown). This behavior is an evidence of an interaction between CO and the Mo sites. A shift of the potentials to less positive with temperature is observed, but this is less remarked than for Pt/ C, because they refer to different processes. As seen from comparisons with the results in Figs. 2 and 8, the Mo features at PtMo/C and PtRu/C must also refer to different processes, because they appear at different electrode potentials. For PtMo/C at room temperature, two anodic CO oxidation contours (0.68 and 0.78 V) related to the Mo(IV)/Mo(VI) and CO-Mo surface process and the CO oxidation at Pt sites, respectively, are observed. These features do not correlate with the LangmuireHinshelwood mechanism that PtRu/C surfaces obey.
5.
Conclusions
PtRuMo/C shows the highest current density peak, as well as the lowest onset potential value in linear potential sweep, for methanol electrooxidation than PtMo/C and PtRu/C. However, it depicts as the less tolerance surface to catalytic poisons. We can conclude that PtMo/C catalysts always exhibited the largest activity for methanol oxidation. It presents the highest charge density values due to their greatest tolerance to catalytic poisons. Besides, PtMo/C shows noticeable lower onset potential value than the other catalysts surfaces for CO electrooxidation. The values of the Tafel slopes by performing the quasistationary polarization curves exhibited a net Tafel line, potential vs. logarithm of current between 0.45 and 0.65 V (not shown). In the case of Pt/C and PtMo/C the Tafel slopes are equal to 0.083 V dec1. Values of Tafel slopes as high as 3RT/2F were reported some time ago [21e23] for carbon monoxide oxidation in conditions where the proposed mechanism denote a first fast single electron transfer, followed by a slow surface electrochemical step as rate determinant. This path is that of an adsorbed carbon monoxide converted to adsorbed formiate, previous to the surface oxidation to carbon dioxide. The presence of ruthenium in the PtRuMo/C catalysts retains almost the same Tafel slope (87 mV dec1) because of the presence of molybdenum. On the other hand, PtRu/C exhibits Tafel slopes of 0.120 V dec1, so the classical Tafel slope value of 2RT/F is obtained where the rate determining step of a single and first electron transfer occurs. In the case of the ternary catalyst a slightly higher Tafel slope value is observed than on Pt/C and PtMo/C, however at potentials larger than 0.65 V a similar behavior than that on PtRu/C is observed. The ternary catalysts exhibit an electrochemical behavior between those of PtRu/C and PtMo/C. Therefore, binary catalysts produce hydroxyl species at lower potentials than platinum and they also are more tolerant towards the presence of poisoning intermediates than pure platinum. In fact, this can also be due to the electronic effect caused by molybdenum that can produce less strongly anchoraged adsorbates. This is a consequence of the electron injection of molybdenum atoms to the platinum surface. The result is an increase of the methanol oxidation rate since the intermediates are being much more labile, in spite of having the same rate determining step as platinum.
Acknowledgments The authors are greatly acknowledged to the CSIC Projects of the UdelaR. V. Dı´az and C.F. Zinola are researchers at the Chemistry Area of PEDECIBA United Nations. The authors are grateful obliged to the XRD and EDS measurements of carbon-supported catalysts to Dr. R. Faccio and Msc. S. Villar, respectively.
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