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Metal oxide promoters for methanol electro-oxidation
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 x x x ( 2 0 1 4 ) 1 e9
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Metal oxide promoters for methanol electro-oxidation R.S. Amin a, K.M. El-Khatib a,*, S. Siracusano b, V. Baglio b, A. Stassi b, A.S. Arico b a b
Chem. Eng. & Pilot Plant Dept., National Research Center, El-Behous St., Dokki, Giza, Egypt CNR-ITAE, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy
article info
abstract
Article history:
Noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx and VOx) have been inves-
Received 10 January 2014
tigated as promoters of Pt electrocatalyst for methanol oxidation in acidic environment. Pt
Received in revised form
modification was made using low oxide content (5 wt%) in order to evaluate the possibility
8 April 2014
of using such oxide promoter in a multifunctional catalyst. At this low level of oxide
Accepted 14 April 2014
content, IrOx provided a larger promoting effect than RuOx. This occurred in the absence of
Available online xxx
specific alloying with Pt and also in the presence of lower catalyst dispersion. The electrocatalytic enhancement produced by the valve metal oxides was significantly lower than
Keywords:
IrOx and RuOx. These results are interpreted in terms of the different water displacement
Metal oxide promoter
mechanism for the various oxides. Such evidences seem to indicate that a multifunctional
Methanol oxidation
catalyst may represent a valid route to enhance methanol electro-oxidation.
Direct methanol fuel cell
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A large number of efforts are currently performed to develop highly active methanol electro-oxidation catalysts for application in direct methanol fuel cells (DMFCs) [1,2]. This technology is of relevant interest for the fuel cells early market and may compete in the near future with Lithium-batteries for portable applications [2,3]. Most of the studies in this field are addressed at improving the state-of-the-art PteRu electrocatalysts and in parallel multifunctional compounds of various formulations are actively investigated for this reaction [4e10]. Previous studies have underlined the importance of both nature and strength of adsorbed oxygen species in the reaction mechanism. Water discharging occurs at high potentials on Pt surface [11e14]. Since the role of Pt as catalyst
for methanolic species adsorption and dehydrogenation appears almost unique in the acid environment, there are several transition metals such Sn [15], W [16], Ni [17], beside Ru, which may promote water displacement at low potentials. Pd may represent an alternative to Pt for dehydrogenation but the reactions rates are significantly lower [18]. Most of the studies carried out on methanol oxidation especially those dealing with PteRu have been focused on the optimization of the composition e.g. the Pt/M atomic ratio and on a better understanding of the role of alloy and oxides on promoting the oxidation mechanism [19e22]. In this regard, the effect of lattice parameter, oxidation states, particle size and dispersion have been investigated [21,22]. Several reports not only have emphasised the need of an alloy formation to allow for an atomic mixing of the active catalytic sites but also the need to have specific
* Corresponding author. Tel.: þ20 1001074039; fax: þ20 233370931. E-mail address: [email protected] (K.M. El-Khatib). http://dx.doi.org/10.1016/j.ijhydene.2014.04.100 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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crystallographic planes on the surface [4,5,22e24]. Most of the studies that have been carried out in the last three decades clarify the fact that the unsupported or carbon supported PteRu alloys with equimolar composition provide the best performance, therefore these alloys represent the best anode catalysts for DMFCs [20,25]. The methanol electro-oxidation is a slow transfer reaction and represents the rate determining step of the DMFC process. Cross-over through the membrane can be significantly decreased if methanol is efficiently oxidised at the anode [26]. Accordingly, considerable efforts are made in developing new compositions and preparation methods in order to provide effective routes for methanol oxidation. There is an important analogy among the methanol oxidation and oxygen evolution; both processes require water discharging on the electrode followed by the adsorption of active oxygen species [2]. These give rise to a surface reaction with adsorbed methanolic residues in DMFCs or desorb as oxygen molecule in the electrolysis mode [2]. These processes occur in a quite different potential window. The catalysts were prepared using a simple polyol method involving precursor reaction in ethylene glycol. Metal oxide modifications of Pt at low contents, i.e. 5 wt%, to individuate a guideline for the development of a multifunctional catalyst. Structure, chemistry and morphology of these catalysts were analysed by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The aim of this work is to investigate the effect of adding noble metal oxides (e.g. IrOx, and RuOx) and valve metal oxides (e.g. SnOx and VOx) to Pt as promoters for electro-oxidation of methanol in acidic medium.
Experimental
for 6 h. The reason why the IrOx and VOx were not precipitated directly on carbon, as for PtRuOx and PtSnOx (see below), but dispersed directly in their oxide form, was due to the low yield achieved in this case using the precipitation procedure. The preparation of PteSnO2/C and PteRuO2/C nanoparticles supported on carbon (Vulcan XC-72R, Cabot Corp., USA) was carried out in two steps; first, an appropriate amount of M-precursors (SnCl2 and RuCl3) and carbon were mixed together with double distilled water. The metal oxide loading was fixed at 5 wt%. The solution pH was adjusted at 10 by using 1 M NaOH solution and the mixture was stirred constantly for 3 h to allow for a complete precipitation of the hydroxide. Thereafter, the mixture was filtered, washed at least for 6 times with double distilled water and dried in an oven at 80 C for 6 h. The dried mixture was then calcined at 400 C for 3 h in air to form oxides. The second step involved the dispersion of Pt nanoparticles over PteSnO2/C and PteRuO2/C via a microwave-polyol (M-P) process as described above.
Physico-chemical characterization The Pt/C and metal oxides catalyst were characterized by recording powder X-ray diffraction (XRD) patterns on a Philips X-pert 3710 X-ray diffractometer, using Cu Ka radiation, operating at 40 kV and 30 mA. The peak profile of the (2 2 0) reflection in the face centred cubic structure was obtained by using the Marquardt algorithm and used to calculate the crystallite size by using the DebyeeScherrer equation. The morphological characterization was carried out by transmission electron microscopy (TEM) analysis using a FEI CM12 microscope. Particle size distribution has been obtained by measuring the diameter of 200 particles assumed spherical in shape, in different regions of each sample. X-ray fluorescence was carried out with a Bruker S4 Explorer instrument.
Preparation of PtMOx electrocatalysts Electrochemical characterization The dispersion of V2O5 and IrO2 nanoparticles on carbon (Vulcan XC-72R, Cabot Corp., USA) was carried out via a solidstate reaction under intermittent microwave heating (IMH) method. In a first step, the V2O5 or IrO2 (99.9% purity) powder was well dispersed over Vulcan XC-72R carbon using a mixture of 50 ml 2-propanol þ 50 ml doubly distilled H2O. The V2O5 and IrO2 loadings fixed at 5 wt%, the mixture was stirred for 30 min, and then dried in oven at 80 C for 6 h. After drying the mixture was introduced into a household microwave (50 GHz, 1400 W) and heated six times, each time for 20 s followed by a 60 s pause. In a successive step, the Pt nanoparticles were supported over V2O5/C or IrO2/C via a microwave-polyol (M-P) process by using sodium borohydride and/or ethylene glycol as the reducing agent. The Pt loading was fixed at 25 wt%. An appropriate amount of the prepared powder and a Pt salt (H2PtCl6) were ultrasonically mixed in double distilled water. 15 ml of ethylene glycol and 0.4 M KOH (in ethylene glycol) was added dropwise to adjust the pH of the solution to about 10 in order to induce the formation of small and uniform Pt nanoparticles. The resulting mixture was subjected to microwave heating for 50 s (continuous), and then the resulting sample was filtered, washed at least for 6 times with double distilled water and dried in an oven at 80 C
The electrocatalytic activity towards methanol electrooxidation of the prepared electrocatalysts was investigated in 0.5 M H2SO4 solution using slow scan rate (5 mV s1) electrochemical polarization and cyclic voltammetry techniques. Voltamaster 6 potentiostat was employed for the electrochemical measurements; it was connected to a personal computer as data interface. The electrochemical tests were carried out in three electrode configuration; it consisted of a reference electrode (Hg/Hg2SO4/1.0 M H2SO4 (MMS)), a Pt wire (as the counter electrode), and a working electrode made of the electrocatalyst powder deposited on the surface of a commercial carbon rod with active surface area of 0.5 cm2. The surface of the electrode for each experiment was mechanically polished with emery papers of different grades. The polished surface was then rinsed with acetone followed by washing with double-distilled water. Prior to electrocatalyst deposition, an activation step was carried out by cycling the carbon electrode in 0.5 M H2SO4 solution in the potential range from 800 to þ1600 mV (MMS) for 50 cycles at a scan rate of 50 mV s1. 1.1 mg of the catalyst was then deposited onto the carbon surface using 5 wt% Nafion solution (dissolved in isopropyl alcohol). A uniform distribution of the
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Fig. 1 e X-ray diffraction patterns of Pt/C and Pt metal oxides electrocatalyst.
Nafion solution was achieved by adding appropriate doses using a micro-injector. Several minutes were allowed to elapse after each dose to evaporate the isopropyl alcohol. This modified electrode was then dried at room temperature and stored in a desiccator. All electrochemical measurements were conducted in de-aerated electrolytes at room temperature of 30 C 0.2.
Results and discussion X-ray diffraction patterns of different prepared catalysts are presented in Fig. 1. It can be observed that all diffraction patterns showed the three characteristic peaks of Pt [Pt(111), Pt(200) and Pt(220)], whereas the presence of crystalline oxide with tetragonal structure is observed for IrO2. Small Pt crystallite sizes are determined from the Scherrer equation. The mean values are 2.6, 2.1, 3.2, 3.6 and 3.9 nm for Pt/C, PtRuOx, PtIrOx, PtSnOx and PtVOx, respectively (as shown in Table 1). Valve metal oxides are often used to enhance the dispersion of noble metal catalysts in order to increase the surface area. However, they can also contribute to enhance the water displacement [2]. Moreover, the broadening the XRD peaks indicates that no specific effect is played by Sn and V in decreasing the crystallite size. Indeed the crystallite size is smaller for PtRuOx and PtIrOx. The occurrence of sharp peaks in the PtIrOx sample for the IrO2 phase indicates that IrO2 crystallizes with large particles compared to that of Pt. There is no evidence of diffraction peaks related to a crystalline phase is recorded for Sn, V and Ru. These oxides are generally present in the amorphous phase and in the
case of PtRuOx part of Ru is in metallic form alloyed with Pt as indicated from the shift of the diffraction peaks to higher Bragg angles compared to Pt. However, this not the case for the other catalysts where a very small shift is registered towards larger Bragg angles (as shown in Table 1). TEM analysis displays good homogeneity as shown in Fig. 2. There is no specific evidence of large metal or metal oxide particles. Micrographs show that particle size distribution is in agreement with the results obtained from XRD. The mean particle size derived by TEM analysis was in the range 3e4 nm; however, this analysis mainly included crystalline Pt particles. Histograms of the particle size distribution show a unimodal profile; this was asymmetrical essentially in the range of higher particle size as shown in Fig. 3. Linear sweep voltammetry curves of different catalysts in (0.3 and 0.6 M MeOH þ 0.5 M H2SO4) solutions at 5 mV s1 are displayed in Fig. 4(a) and (b). The highest mass activities are obtained for PteIrOx followed by PtRuOx, PtSnOx, and PteVOx, respectively. The activity of the valve metal oxide-based catalysts is two/three times lower than that achieved by RuOx and IrOx promoters. The highest mass activity is also accompanied by a small shift at lower values for the onset potential of the methanol oxidation process. The promoting effect of valve metals content for Pt with respect to bare Pt is not particularly evident for these samples; this may be due to the several effects: absence of alloying with Pt, coverage of Pt sites by the amorphous SnOx or VOx etc. There is an ability of such materials to promote water displacement assisting methanol oxidation on Pt, but their activity towards this process appears quite smaller than IrOx and RuOx. It seems that there are good analogies with the oxygen evolution, where the valve metal oxides are just used as diluting agents for the active phase. In principle, they may promote the process by creating synergisms due to their ability to contribute to water discharging [2]. In another words, valve metal oxides can help in discharging water thus promoting the oxidation of methanolic species. Generally, alloying Pt with Ru and using much larger content of Ru (i.e. an equimolar ratio with Pt) provides enhanced activity as well known; however, the results show that also moderate additions of oxide promoters can enhance the activity of Pt and may be the basis for the development of multifunctional catalysts where the Pt-alloy is mixed with oxide promoters to accelerate the different steps of methanol oxidation. Cyclic voltammetry analysis was carried out in a much larger potential window than linear sweep voltammetry. CV profiles show that at very high potentials, which are of less interest for practical fuel cell applications. The CV curves clarify that the valve metal oxides such as PtVOx may achieve
Table 1 e Crystallographic parameters for the PteMOx/C catalysts. Sample Pt/C PtRuO2/C(Pt:Ru ¼ 3.5:1) PtSnO2/C(Pt:SnO2 ¼ 4:1) PtV2O5/C(Pt:V2O5 ¼ 5:1) PtIrO2/C(Pt:IrOx ¼ 6:1)
Crystallografic phases
Crystallite size of Pt (nm)
Crystallite size of Pt (nm), TEM particle size
Crystallite size of oxides (nm)
Degree of alloy M% at. (M ¼ Ru,Sn,V and Ir)
fcc fcc fcc fcc fcc
2.6 3.2 3.6 3.9 2.1
3.5 3.34 3.75 3.37 3.0
Amorphous Amorphous Amorphous Amorphous 33.3
n.a. 11 n.a. n.a. 0
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Fig. 2 e TEM analysis of Pt/C and Pt metal oxides electrocatalyst.
peak current densities comparable to those of PtRuOx (Fig. 5). This indicates that the water displacement at metal valve materials and transfer of the adsorbed OH species to methanolic residues requires much larger energy than IrOx and RuOx phases conventionally used also for the oxygen evolution from water. However, the role of the metal valve materials can be that to bring about a synergism for the methanol oxidation as it occurs for oxygen evolution and to reduce the total noble metal content. The methanol oxidation reaction order at these electrocatalysts can be determined by plotting a logarithmic relation between the current density values of the forward peaks and methanol concentration as shown in Fig 6. The rate of methanol oxidation reaction is related to methanol concentration by the following equation:
Rate ¼ Ip ¼ k Cn
(1)
log Ip ¼ log k þ n log C
(2)
where: Ip is the peak current density, k is the reaction rate constant, C is the methanol concentration and n is the reaction order. Straight lines were obtained at all studied electrocatalysts. slope values of 1.153, 1.13, 1.12 and 0.91 are calculated for the oxidation peaks of Pt/C, PteVOx/C, PteRuOx/ C, and PteIrOx/C electrocatalysts, respectively. It is pointed out that the presence of different metal oxides does not affect significantly the methanol oxidation reaction order at the surface of different prepared electrocatalysts. PteVOx/C and PteRuOx/C show higher values for the reaction order of methanol oxidation. At high methanol concentration (0.6 M)
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30
50
Pt/C
Frequency / %
Frequency / %
60
40 30 20 10
20 15 10 5 0
0 0
1 2 3 4 Particle size / nm
18 16 14 12 10 8 6 4 2 0
2 2.5 3 3.5 4 4.5 5 5.5 6 Particle size / nm
5 25
PtRuOx/C
PtVOx/C 20 Frequency / %
Frequency / %
PtSnOx/C
25
15 10 5
1.5 2 2.5 3 3.5 4 4.5 5 5.5 Particle size / nm
0 2
3 4 5 6 Particle size / nm
7
35 PtIrOx/C
Frequency / %
30 25 20 15 10
5 0 2
3 4 5 Particle size / nm
6
Fig. 3 e Particle size distribution of Pt/C and Pt metal oxides electrocatalyst.
PtRuOx catalysts over-perform PtIrOx. This is in line with the literature results indicating PtRu as the best catalyst for current DMFCs [9]. At low methanol concentration and the presence of low level of oxide content, IrOx leads to achieving a higher current density for methanol oxidation at low overpotentials than RuOx. This cannot be explained neither in terms of an alloying effect for the present catalysts nor on the basis of dispersion. In fact, the degree of alloying was lower for Ir than Ru and the mean particle size of IrOx particle was not as small as that of RuOx. It is evident from XRD that crystalline IrO2 particles are present and possibly the presence of specific atomic arrangement may be useful for the reaction. However, this may be not
the major explanation of the electrocatalytic promotion at IrOx. In the case of the oxygen evolution process, although the crystalline IrO2 may show slightly better specific activity than amorphous IrO2, mass activity is larger for the latter due to the higher surface area and the number of active catalytic sites. The promotion of electrocatalytic activity is most probably related to the different characteristics of water displacement for these oxides which are known from the oxygen evolution process. Water displacement occurs on these oxides at low potentials through a sequence of elementary steps. These oxides favour water coverage on the surface which is followed by a dissociation step producing adsorbed hydroxyl species and hydride groups [27]. Hydride species are easily released as
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the electrochemical potential is high and it can promote migration and surface reaction among the oxygen species to evolve oxygen molecules. In this regard, the intrinsic activity of RuO2 for oxygen evolution is larger than that of IrO2 where the presence of more labile bonded species causes a lower coverage of hydroxyls on the surface. The overall oxidation process of methanol to carbon dioxide proceeds through a six electron donation process; yet, the rate determining step, derived from electrochemical steady-state measurements on Pt, through analysis of the Tafel slope, involves a one electron step [2,28]. On a pure Pt surface, the dissociative chemisorption of water on Pt is the rate determining step at potentials below w0.7 V vs. RHE, i.e. in the potential region that is of technical interest [2,28]. It is generally accepted that an active catalyst for methanol oxidation should give rise to water displacement at low potentials and to “labile” CO chemisorption. Moreover, a good catalyst for methanol oxidation should also catalyse the oxidation of carbon monoxide [2]. A useful approach is to add a second component (a “promoter”) to platinum in order to decrease the formation of poisoning species or to promote their oxidation at lower potential. Even if various theories have been put forward to
Fig. 4 e Linear Sweep Voltammogram of Pt/C and Pt metal oxides electrocatalyst a) 0.3 M methanol concentration, b) 0.6 M methanol concentration.
water coordinated protons together with one electron in the external circuit [27]. Thus, the enhanced activity of IrO2 must be thus related to the larger coverage of OH species or the more rapid migration on the surface especially when the oxide content is low. Probably, the fact that Ru-oxide may give rise under certain conditions to the growth of a thick oxide layer with composition RuOx (OH)y$zH2O may slow down the transfer of hydroxyl groups to Pt whereas this mechanism is much more efficient for metallic Ru alloyed to Pt. PteRu catalysts are very effective for methanol oxidation and the electrocatalytic activity seems to be directly related to the oxidation state of Ru [22]. A metallic state may favour the formation of a thin layer of adsorbed hydroxyl species, which, being labile-bonded to the surface, migrate more easily and more efficiently to Pt [2]. A similar explanation may be given for tin since the promotion of methanol and ethanol oxidation takes significant advance of the presence of PteSn alloys [2]. On the other hand, the presence of the Ru-oxide promotes the uptake of the hydroxyls to form a thick layer of strongly bonded species [22]. In the case of oxygen evolution, this effect is not critical because
Fig. 5 e Cyclic voltammogram of Pt/C and Pt metal oxides electrocatalyst a) 0.3 M methanol concentration, b) 0.6 M methanol concentration.
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the sites need to donate OHads species for the complete desorption of methanol residues. Although there are several stated mechanisms for methanol oxidation reaction, it is generally agreed that the most abundant surface intermediate is chemisorbed carbon monoxide (this is further illustrated in the following equations).
Fig. 6 e Logarithmic relation between the current density of oxidation peaks and methanol concentration of the prepared electrocatalysts and Pt/C for comparison.
explain the promoting effect of the additional elements (e.g. Ru, Sn, Ir etc.) [2,29e33], this subject remains controversial. Transition metal promoters and adatoms are seen as a means to improve the electrocatalytic behaviour of electrodes, either by minimizing the poisoning reaction or enhancing the main oxidation reaction. Since the CO oxidation process needs the presence of oxygenated species, numerous types of components have been explored, to generate O or OH species at lower potentials on the platinum surface. Historically, systematic screening of the possible metals show that only a few metals led to positive results [30]. Ruthenium, tin, molybdenum were investigated to determine the effects of alloying platinum with transition metals groups 4 to 6 with regard to the formation of OH adsorbed species [31]. Despite considerable efforts made over the last twenty years, the best alloying component known to enhance the electro-oxidation of methanol on platinum is ruthenium. By comparison with the mechanism discussed for pure platinum, the promoting effect of ruthenium can result from a bifunctional mechanism [28] as follows. The adsorbed OH species are formed at both Pt and Ru sites (Equations (5) and (7)) but in different potential ranges (Equation (7)). At suitable electrode potentials (0.2 V vs. RHE), water discharging occurs on Ru sites with formation of RueOH groups at the catalyst surface [28]. One of the rate determining step in the methanol oxidation process is water discharging (Equation (5)). This is similar to one of the rate determining steps for oxygen evolution (Equation (9)). This similarities make both IrO2 and RuO2 good promoters for both processes when these are in oxide form. In the case of methanol oxidation, the oxide promoter, e.g. IrO2, may be added to the PtRu alloy to produce a synergistic effect on the reaction process.
Methanol oxidation process Methanol oxidation is a slow reaction that requires multiple active sites for methanol adsorption and, at the same time,
4Pt þ CH3OH / 3 PtH þ PteCO þ 1e þ Hþ
(3)
3 PtH / 3 Pt þ 3Hþ þ 3 e
(4)
Pt þ H2O / Pt (OH)ads þ Hþ þ e
(5)
PteCO þ Pt (OH)ads / 2Pt þ CO2 þ Hþ þ e
(6)
An efficient catalyst favours CO adsorption on Pt and OH formation takes place on the second metal oxide as in the following equations. Accordingly, the OH adsorbed on the surface of the oxide (MOx: where M is Ir or Ru) may oxidize the CO present on the surface of Pt, as occurs in a bifunctional mechanism [34,35].
MOx þ H2O / MOxeOHads þ Hþ þ e
(7)
PteCOads þ MOxeOHads / Pt þ MOx þ CO2 þ Hþ þ e
(8)
Oxygen evolution reaction
2IrOx þ 2H2O / 2IrOxeOH þ 2Hþ þ 2e
IrOxeOH þ IrOxeOH / 2IrOx þ O2 þ 2Hþ þ 2e
(9)
(10)
It was interesting to observe that at this low level of oxide content and in the absence of large degree of alloying as usually observed in benchmark catalysts, IrOx provided a larger promoting effect than RuOx. As already discussed, this result cannot be explained neither in terms of an alloying effect (the degree of alloying was lower for Ir than Ru) nor on the basis of catalyst dispersion (the mean particle size of the IrOx particles was larger than RuOx). Regarding the valve metal oxides, the electrocatalytic enhancement was lower than IrOx and RuOx. However, these materials may be useful in reducing the noble metal loading in a multifunctional catalyst since they may act as dispersion phase for the noble metal oxide or may reduce the tendency of dissolution of Ru which give rise to the well known phenomenon of Ru ions cross-over through the membranes towards the cathode surface [2]. The electrocatalytic activity is thus related to the different characteristics of water displacement for these oxides which are known from the oxygen evolution process [27]. Such a parallelism may provide new routes to design multifunctional catalysts where Pt-alloys can be combined with oxide
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promoters to accelerate the multiple steps of the methanol oxidation process. [8]
Conclusions Methanol electrooxidation was studied at Pt electrodes modified with noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx and VOx) in acidic environment. Low metal oxide content (5% wt) was used to get insights on the promoting effect of the oxides in absence of significant alloying effects or surface enrichment phenomena. Under such conditions IrOx provided better electrocatalytic activity than RuOx. The valve metal oxides showed a modest promoting effect. The results are interpreted in terms of different water displacement mechanism for the oxides induced by the formation of strongly adsorbed hydroxyl species in the case of the Ruoxide. The parallelism with oxygen evolution helps in understanding the different promoting effects which are different in the two reactions methanol electro-oxidation and water splitting since they occur in a different potential range. The results may be useful in designing a multifunctional catalyst for methanol electro-oxidation.
Acknowledgements The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2011-2014) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement DURAMET no. 278054.
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performance of a direct methanol fuel cell. J Power Sources 2006;161(1):11e8. Gurau B, Smotkin E. Methanol crossover in direct methanol fuel cells: a link between power and energy density. J Power Sources 2002;112(2):339e52. Park K-W, Choi J-H, Ahn K-S, Sung Y-E. PtRu alloy and PtRuWO3 nanocomposite electrodes for methanol electrooxidation fabricated by a sputtering deposition method. J Phys Chem B 2004;108(19):5989e94. Profeti LPR, Simo˜es FC, Olivi P, Kokoh KB, Coutanceau C, Le´ger J-M, et al. Application of Pt þ RuO2 catalysts prepared by thermal decomposition of polymeric precursors to DMFC. J Power Sources 2006;158(2):1195e201. Cao D, Lu GQ, Wieckowski A, Wasileski SA, Neurock M. Mechanisms of methanol decomposition on platinum: a combined experimental and ab initio approach. J Phys Chem B 2005;109(23):11622e33. Kua J, Goddard WA. Oxidation of methanol on 2nd and 3rd row group VIII transition metals: applications to direct methanol fuel cells. J Am Chem Soc 1999;121:10928e41. Chen YX, Miki A, Ye S, Sakai H, Osawa M. Formate, an active intermediate for direct oxidation of methanol on Pt electrode. J Am Chem Soc 2003;125(13):3680e1. NM, Ross Jr PN. Surface science studies of model Markovic fuel cell electrocatalysts. Surf Sci Rep 2002;45(4e6): 117e229. Savadogo O, Lee K, Oishi K, Mitsushima S, Kamiya N, Ota KI. New palladium alloys catalyst for the oxygen reduction reaction in an acid medium. Electrochem Commun 2004;6(2):105e9. Lee K, Savadogo O, Ishihara A, Mitsushima S, Kamiya N, Ota K. Methanol-tolerant oxygen reduction electrocatalysts based on Pd-3D transition metal alloys for direct methanol fuel cells. J Electrochem Soc 2006;153(1):A20e4. Hu Y, Wu P, Yin Y, Zhang H, Cai C. Effects of structure, composition, and carbon support properties on the electrocatalytic activity of Pt-Ni-graphene nanocatalysts for the methanol oxidation. Appl Catal B e Environ 2012;111e112:208e17. Ruoshi L, Mao H, Zhang J, Huang T, Yu A. Rapid synthesis of porous Pd and PdNi catalysts using hydrogen bubble dynamic template and their enhanced catalytic performance for methanol electrooxidation. J Power Sources 2013;241:660e7. Sahin O, Kivrak H. A comparative study of electrochemical methods on PteRu DMFC anode catalysts: the effect of Ru addition. Int J Hydrogen Energy 2013;38:901e9. Li L, Xing Y. PtRu nanoparticles supported on carbon nanotubes as methanol fuel cell catalysts. J Phys Chem C 2007;111(6):2803e8. Nagle LC, Rohan JF. Aligned carbon nanotubeePt composite fuel cell catalyst by template electrodeposition. J Power Sources 2008;185(1):411e8. Arico` AS, Baglio V, Di Blasi A, Modica E, Antonucci PL, Antonucci V. Analysis of the high-temperature methanol oxidation behaviour at carbon-supported PteRu catalysts. J Electroanal Chem 2003;557:167e76. Steigerwalt ES, Deluga GA, Cliffel DE, Lukehart CM. A PtRu/ graphitic carbon nanofiber nanocomposite exhibiting high relative performance as a direct-methanol fuel cell anode catalyst. J Phys Chem B 2001;105(34):8097e101. Park K-W, Choi J-H, Kwon B-K, Lee S-A, Sung Y-E, Ha H-Y, et al. Wieckowski a chemical and electronic effects of Ni in Pt/Ni and Pt/Ru/Ni Alloy nanoparticles in methanol electrooxidation. J Phys Chem B 2002;106(8):1869e77. Chi C-F, Yang M-C, Weng H-S. A proper amount of carbon nanotubes for improving the performance of PteRu/C catalysts for methanol electro-oxidation. J Power Sources 2009;193(2):462e9.
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[26] Lin SD, Hsiao TC. Morphology of carbon supported PtRu electrocatalyst and the CO tolerance of anodes for PEM fuel cells. J Phys Chem B 1999;103(1):97e103. [27] Cruz JC, Baglio V, Siracusano S, Antonucci V, Arico` AS, Ornelas R, et al. Preparation and characterization of Ruo2 catalysts for oxygen evolution in a solid polymer electrolyte. Int J Electrochem Sci 2011;6(12):6607e19. [28] Chandrasekaran K, Wass JC, Bockris JO’M. The potential dependence of intermediates in methanol oxidation observed in the steady state by FTIR spectroscopy. J Electrochem Soc 1990;137(2):518e24. [29] Watanabe M, Motoo S. Electrocatalysis by ad-atoms: part II. Enhancement of the oxidation of methanol on platinum by ruthenium ad-atoms. J Electroanal Chem Interfacial Electrochem 1975;60(3):267e73. [30] Janssen MMP, Moolhuysen J. Platinumdtin catalysts for methanol fuel cells prepared by a novel immersion
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technique, by electrocodeposition and by alloying. Electrochim Acta 1976;21(11):861e8. Anderson AB, Grantscharova E, Seong S. Systematic theoretical study of alloys of platinum for enhanced methanol fuel cell performance. J Electrochem Soc 1996;143(6):2075e82. Mc Breen J, Mukerjee S. In situ X-ray absorption studies of a PteRu electrocatalyst. J Electrochem Soc 1995;142(10):3399e404. Kolla P, Kerce K, Fong H, Smirnova A. Metal oxides as catalyst promoters for methanol oxidation. MRS Proceedings 1446; 2012. http://dx.doi.org/10.1557/opl.2012.957 . Justin P, Charan PHK, Ranga Rao G. High performance PteNb2O5/C electrocatalysts for methanol electrooxidation in acidic media. Appl Catal B e Environ 2010;100(3e4):510e5. Scibioh MA, Kim S-K, Cho EA, Lim T-H, Hong S-A, Ha HY. PteCeO2/C anode catalyst for direct methanol fuel cells. Appl Catal B e Environ 2008;84:773e82.
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Metal oxide promoters for methanol electro-oxidation R.S. Amin a, K.M. El-Khatib a,*, S. Siracusano b, V. Baglio b, A. Stassi b, A.S. Arico b a b
Chem. Eng. & Pilot Plant Dept., National Research Center, El-Behous St., Dokki, Giza, Egypt CNR-ITAE, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy
article info
abstract
Article history:
Noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx and VOx) have been inves-
Received 10 January 2014
tigated as promoters of Pt electrocatalyst for methanol oxidation in acidic environment. Pt
Received in revised form
modification was made using low oxide content (5 wt%) in order to evaluate the possibility
8 April 2014
of using such oxide promoter in a multifunctional catalyst. At this low level of oxide
Accepted 14 April 2014
content, IrOx provided a larger promoting effect than RuOx. This occurred in the absence of
Available online xxx
specific alloying with Pt and also in the presence of lower catalyst dispersion. The electrocatalytic enhancement produced by the valve metal oxides was significantly lower than
Keywords:
IrOx and RuOx. These results are interpreted in terms of the different water displacement
Metal oxide promoter
mechanism for the various oxides. Such evidences seem to indicate that a multifunctional
Methanol oxidation
catalyst may represent a valid route to enhance methanol electro-oxidation.
Direct methanol fuel cell
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A large number of efforts are currently performed to develop highly active methanol electro-oxidation catalysts for application in direct methanol fuel cells (DMFCs) [1,2]. This technology is of relevant interest for the fuel cells early market and may compete in the near future with Lithium-batteries for portable applications [2,3]. Most of the studies in this field are addressed at improving the state-of-the-art PteRu electrocatalysts and in parallel multifunctional compounds of various formulations are actively investigated for this reaction [4e10]. Previous studies have underlined the importance of both nature and strength of adsorbed oxygen species in the reaction mechanism. Water discharging occurs at high potentials on Pt surface [11e14]. Since the role of Pt as catalyst
for methanolic species adsorption and dehydrogenation appears almost unique in the acid environment, there are several transition metals such Sn [15], W [16], Ni [17], beside Ru, which may promote water displacement at low potentials. Pd may represent an alternative to Pt for dehydrogenation but the reactions rates are significantly lower [18]. Most of the studies carried out on methanol oxidation especially those dealing with PteRu have been focused on the optimization of the composition e.g. the Pt/M atomic ratio and on a better understanding of the role of alloy and oxides on promoting the oxidation mechanism [19e22]. In this regard, the effect of lattice parameter, oxidation states, particle size and dispersion have been investigated [21,22]. Several reports not only have emphasised the need of an alloy formation to allow for an atomic mixing of the active catalytic sites but also the need to have specific
* Corresponding author. Tel.: þ20 1001074039; fax: þ20 233370931. E-mail address: [email protected] (K.M. El-Khatib). http://dx.doi.org/10.1016/j.ijhydene.2014.04.100 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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crystallographic planes on the surface [4,5,22e24]. Most of the studies that have been carried out in the last three decades clarify the fact that the unsupported or carbon supported PteRu alloys with equimolar composition provide the best performance, therefore these alloys represent the best anode catalysts for DMFCs [20,25]. The methanol electro-oxidation is a slow transfer reaction and represents the rate determining step of the DMFC process. Cross-over through the membrane can be significantly decreased if methanol is efficiently oxidised at the anode [26]. Accordingly, considerable efforts are made in developing new compositions and preparation methods in order to provide effective routes for methanol oxidation. There is an important analogy among the methanol oxidation and oxygen evolution; both processes require water discharging on the electrode followed by the adsorption of active oxygen species [2]. These give rise to a surface reaction with adsorbed methanolic residues in DMFCs or desorb as oxygen molecule in the electrolysis mode [2]. These processes occur in a quite different potential window. The catalysts were prepared using a simple polyol method involving precursor reaction in ethylene glycol. Metal oxide modifications of Pt at low contents, i.e. 5 wt%, to individuate a guideline for the development of a multifunctional catalyst. Structure, chemistry and morphology of these catalysts were analysed by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The aim of this work is to investigate the effect of adding noble metal oxides (e.g. IrOx, and RuOx) and valve metal oxides (e.g. SnOx and VOx) to Pt as promoters for electro-oxidation of methanol in acidic medium.
Experimental
for 6 h. The reason why the IrOx and VOx were not precipitated directly on carbon, as for PtRuOx and PtSnOx (see below), but dispersed directly in their oxide form, was due to the low yield achieved in this case using the precipitation procedure. The preparation of PteSnO2/C and PteRuO2/C nanoparticles supported on carbon (Vulcan XC-72R, Cabot Corp., USA) was carried out in two steps; first, an appropriate amount of M-precursors (SnCl2 and RuCl3) and carbon were mixed together with double distilled water. The metal oxide loading was fixed at 5 wt%. The solution pH was adjusted at 10 by using 1 M NaOH solution and the mixture was stirred constantly for 3 h to allow for a complete precipitation of the hydroxide. Thereafter, the mixture was filtered, washed at least for 6 times with double distilled water and dried in an oven at 80 C for 6 h. The dried mixture was then calcined at 400 C for 3 h in air to form oxides. The second step involved the dispersion of Pt nanoparticles over PteSnO2/C and PteRuO2/C via a microwave-polyol (M-P) process as described above.
Physico-chemical characterization The Pt/C and metal oxides catalyst were characterized by recording powder X-ray diffraction (XRD) patterns on a Philips X-pert 3710 X-ray diffractometer, using Cu Ka radiation, operating at 40 kV and 30 mA. The peak profile of the (2 2 0) reflection in the face centred cubic structure was obtained by using the Marquardt algorithm and used to calculate the crystallite size by using the DebyeeScherrer equation. The morphological characterization was carried out by transmission electron microscopy (TEM) analysis using a FEI CM12 microscope. Particle size distribution has been obtained by measuring the diameter of 200 particles assumed spherical in shape, in different regions of each sample. X-ray fluorescence was carried out with a Bruker S4 Explorer instrument.
Preparation of PtMOx electrocatalysts Electrochemical characterization The dispersion of V2O5 and IrO2 nanoparticles on carbon (Vulcan XC-72R, Cabot Corp., USA) was carried out via a solidstate reaction under intermittent microwave heating (IMH) method. In a first step, the V2O5 or IrO2 (99.9% purity) powder was well dispersed over Vulcan XC-72R carbon using a mixture of 50 ml 2-propanol þ 50 ml doubly distilled H2O. The V2O5 and IrO2 loadings fixed at 5 wt%, the mixture was stirred for 30 min, and then dried in oven at 80 C for 6 h. After drying the mixture was introduced into a household microwave (50 GHz, 1400 W) and heated six times, each time for 20 s followed by a 60 s pause. In a successive step, the Pt nanoparticles were supported over V2O5/C or IrO2/C via a microwave-polyol (M-P) process by using sodium borohydride and/or ethylene glycol as the reducing agent. The Pt loading was fixed at 25 wt%. An appropriate amount of the prepared powder and a Pt salt (H2PtCl6) were ultrasonically mixed in double distilled water. 15 ml of ethylene glycol and 0.4 M KOH (in ethylene glycol) was added dropwise to adjust the pH of the solution to about 10 in order to induce the formation of small and uniform Pt nanoparticles. The resulting mixture was subjected to microwave heating for 50 s (continuous), and then the resulting sample was filtered, washed at least for 6 times with double distilled water and dried in an oven at 80 C
The electrocatalytic activity towards methanol electrooxidation of the prepared electrocatalysts was investigated in 0.5 M H2SO4 solution using slow scan rate (5 mV s1) electrochemical polarization and cyclic voltammetry techniques. Voltamaster 6 potentiostat was employed for the electrochemical measurements; it was connected to a personal computer as data interface. The electrochemical tests were carried out in three electrode configuration; it consisted of a reference electrode (Hg/Hg2SO4/1.0 M H2SO4 (MMS)), a Pt wire (as the counter electrode), and a working electrode made of the electrocatalyst powder deposited on the surface of a commercial carbon rod with active surface area of 0.5 cm2. The surface of the electrode for each experiment was mechanically polished with emery papers of different grades. The polished surface was then rinsed with acetone followed by washing with double-distilled water. Prior to electrocatalyst deposition, an activation step was carried out by cycling the carbon electrode in 0.5 M H2SO4 solution in the potential range from 800 to þ1600 mV (MMS) for 50 cycles at a scan rate of 50 mV s1. 1.1 mg of the catalyst was then deposited onto the carbon surface using 5 wt% Nafion solution (dissolved in isopropyl alcohol). A uniform distribution of the
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Fig. 1 e X-ray diffraction patterns of Pt/C and Pt metal oxides electrocatalyst.
Nafion solution was achieved by adding appropriate doses using a micro-injector. Several minutes were allowed to elapse after each dose to evaporate the isopropyl alcohol. This modified electrode was then dried at room temperature and stored in a desiccator. All electrochemical measurements were conducted in de-aerated electrolytes at room temperature of 30 C 0.2.
Results and discussion X-ray diffraction patterns of different prepared catalysts are presented in Fig. 1. It can be observed that all diffraction patterns showed the three characteristic peaks of Pt [Pt(111), Pt(200) and Pt(220)], whereas the presence of crystalline oxide with tetragonal structure is observed for IrO2. Small Pt crystallite sizes are determined from the Scherrer equation. The mean values are 2.6, 2.1, 3.2, 3.6 and 3.9 nm for Pt/C, PtRuOx, PtIrOx, PtSnOx and PtVOx, respectively (as shown in Table 1). Valve metal oxides are often used to enhance the dispersion of noble metal catalysts in order to increase the surface area. However, they can also contribute to enhance the water displacement [2]. Moreover, the broadening the XRD peaks indicates that no specific effect is played by Sn and V in decreasing the crystallite size. Indeed the crystallite size is smaller for PtRuOx and PtIrOx. The occurrence of sharp peaks in the PtIrOx sample for the IrO2 phase indicates that IrO2 crystallizes with large particles compared to that of Pt. There is no evidence of diffraction peaks related to a crystalline phase is recorded for Sn, V and Ru. These oxides are generally present in the amorphous phase and in the
case of PtRuOx part of Ru is in metallic form alloyed with Pt as indicated from the shift of the diffraction peaks to higher Bragg angles compared to Pt. However, this not the case for the other catalysts where a very small shift is registered towards larger Bragg angles (as shown in Table 1). TEM analysis displays good homogeneity as shown in Fig. 2. There is no specific evidence of large metal or metal oxide particles. Micrographs show that particle size distribution is in agreement with the results obtained from XRD. The mean particle size derived by TEM analysis was in the range 3e4 nm; however, this analysis mainly included crystalline Pt particles. Histograms of the particle size distribution show a unimodal profile; this was asymmetrical essentially in the range of higher particle size as shown in Fig. 3. Linear sweep voltammetry curves of different catalysts in (0.3 and 0.6 M MeOH þ 0.5 M H2SO4) solutions at 5 mV s1 are displayed in Fig. 4(a) and (b). The highest mass activities are obtained for PteIrOx followed by PtRuOx, PtSnOx, and PteVOx, respectively. The activity of the valve metal oxide-based catalysts is two/three times lower than that achieved by RuOx and IrOx promoters. The highest mass activity is also accompanied by a small shift at lower values for the onset potential of the methanol oxidation process. The promoting effect of valve metals content for Pt with respect to bare Pt is not particularly evident for these samples; this may be due to the several effects: absence of alloying with Pt, coverage of Pt sites by the amorphous SnOx or VOx etc. There is an ability of such materials to promote water displacement assisting methanol oxidation on Pt, but their activity towards this process appears quite smaller than IrOx and RuOx. It seems that there are good analogies with the oxygen evolution, where the valve metal oxides are just used as diluting agents for the active phase. In principle, they may promote the process by creating synergisms due to their ability to contribute to water discharging [2]. In another words, valve metal oxides can help in discharging water thus promoting the oxidation of methanolic species. Generally, alloying Pt with Ru and using much larger content of Ru (i.e. an equimolar ratio with Pt) provides enhanced activity as well known; however, the results show that also moderate additions of oxide promoters can enhance the activity of Pt and may be the basis for the development of multifunctional catalysts where the Pt-alloy is mixed with oxide promoters to accelerate the different steps of methanol oxidation. Cyclic voltammetry analysis was carried out in a much larger potential window than linear sweep voltammetry. CV profiles show that at very high potentials, which are of less interest for practical fuel cell applications. The CV curves clarify that the valve metal oxides such as PtVOx may achieve
Table 1 e Crystallographic parameters for the PteMOx/C catalysts. Sample Pt/C PtRuO2/C(Pt:Ru ¼ 3.5:1) PtSnO2/C(Pt:SnO2 ¼ 4:1) PtV2O5/C(Pt:V2O5 ¼ 5:1) PtIrO2/C(Pt:IrOx ¼ 6:1)
Crystallografic phases
Crystallite size of Pt (nm)
Crystallite size of Pt (nm), TEM particle size
Crystallite size of oxides (nm)
Degree of alloy M% at. (M ¼ Ru,Sn,V and Ir)
fcc fcc fcc fcc fcc
2.6 3.2 3.6 3.9 2.1
3.5 3.34 3.75 3.37 3.0
Amorphous Amorphous Amorphous Amorphous 33.3
n.a. 11 n.a. n.a. 0
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Fig. 2 e TEM analysis of Pt/C and Pt metal oxides electrocatalyst.
peak current densities comparable to those of PtRuOx (Fig. 5). This indicates that the water displacement at metal valve materials and transfer of the adsorbed OH species to methanolic residues requires much larger energy than IrOx and RuOx phases conventionally used also for the oxygen evolution from water. However, the role of the metal valve materials can be that to bring about a synergism for the methanol oxidation as it occurs for oxygen evolution and to reduce the total noble metal content. The methanol oxidation reaction order at these electrocatalysts can be determined by plotting a logarithmic relation between the current density values of the forward peaks and methanol concentration as shown in Fig 6. The rate of methanol oxidation reaction is related to methanol concentration by the following equation:
Rate ¼ Ip ¼ k Cn
(1)
log Ip ¼ log k þ n log C
(2)
where: Ip is the peak current density, k is the reaction rate constant, C is the methanol concentration and n is the reaction order. Straight lines were obtained at all studied electrocatalysts. slope values of 1.153, 1.13, 1.12 and 0.91 are calculated for the oxidation peaks of Pt/C, PteVOx/C, PteRuOx/ C, and PteIrOx/C electrocatalysts, respectively. It is pointed out that the presence of different metal oxides does not affect significantly the methanol oxidation reaction order at the surface of different prepared electrocatalysts. PteVOx/C and PteRuOx/C show higher values for the reaction order of methanol oxidation. At high methanol concentration (0.6 M)
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30
50
Pt/C
Frequency / %
Frequency / %
60
40 30 20 10
20 15 10 5 0
0 0
1 2 3 4 Particle size / nm
18 16 14 12 10 8 6 4 2 0
2 2.5 3 3.5 4 4.5 5 5.5 6 Particle size / nm
5 25
PtRuOx/C
PtVOx/C 20 Frequency / %
Frequency / %
PtSnOx/C
25
15 10 5
1.5 2 2.5 3 3.5 4 4.5 5 5.5 Particle size / nm
0 2
3 4 5 6 Particle size / nm
7
35 PtIrOx/C
Frequency / %
30 25 20 15 10
5 0 2
3 4 5 Particle size / nm
6
Fig. 3 e Particle size distribution of Pt/C and Pt metal oxides electrocatalyst.
PtRuOx catalysts over-perform PtIrOx. This is in line with the literature results indicating PtRu as the best catalyst for current DMFCs [9]. At low methanol concentration and the presence of low level of oxide content, IrOx leads to achieving a higher current density for methanol oxidation at low overpotentials than RuOx. This cannot be explained neither in terms of an alloying effect for the present catalysts nor on the basis of dispersion. In fact, the degree of alloying was lower for Ir than Ru and the mean particle size of IrOx particle was not as small as that of RuOx. It is evident from XRD that crystalline IrO2 particles are present and possibly the presence of specific atomic arrangement may be useful for the reaction. However, this may be not
the major explanation of the electrocatalytic promotion at IrOx. In the case of the oxygen evolution process, although the crystalline IrO2 may show slightly better specific activity than amorphous IrO2, mass activity is larger for the latter due to the higher surface area and the number of active catalytic sites. The promotion of electrocatalytic activity is most probably related to the different characteristics of water displacement for these oxides which are known from the oxygen evolution process. Water displacement occurs on these oxides at low potentials through a sequence of elementary steps. These oxides favour water coverage on the surface which is followed by a dissociation step producing adsorbed hydroxyl species and hydride groups [27]. Hydride species are easily released as
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the electrochemical potential is high and it can promote migration and surface reaction among the oxygen species to evolve oxygen molecules. In this regard, the intrinsic activity of RuO2 for oxygen evolution is larger than that of IrO2 where the presence of more labile bonded species causes a lower coverage of hydroxyls on the surface. The overall oxidation process of methanol to carbon dioxide proceeds through a six electron donation process; yet, the rate determining step, derived from electrochemical steady-state measurements on Pt, through analysis of the Tafel slope, involves a one electron step [2,28]. On a pure Pt surface, the dissociative chemisorption of water on Pt is the rate determining step at potentials below w0.7 V vs. RHE, i.e. in the potential region that is of technical interest [2,28]. It is generally accepted that an active catalyst for methanol oxidation should give rise to water displacement at low potentials and to “labile” CO chemisorption. Moreover, a good catalyst for methanol oxidation should also catalyse the oxidation of carbon monoxide [2]. A useful approach is to add a second component (a “promoter”) to platinum in order to decrease the formation of poisoning species or to promote their oxidation at lower potential. Even if various theories have been put forward to
Fig. 4 e Linear Sweep Voltammogram of Pt/C and Pt metal oxides electrocatalyst a) 0.3 M methanol concentration, b) 0.6 M methanol concentration.
water coordinated protons together with one electron in the external circuit [27]. Thus, the enhanced activity of IrO2 must be thus related to the larger coverage of OH species or the more rapid migration on the surface especially when the oxide content is low. Probably, the fact that Ru-oxide may give rise under certain conditions to the growth of a thick oxide layer with composition RuOx (OH)y$zH2O may slow down the transfer of hydroxyl groups to Pt whereas this mechanism is much more efficient for metallic Ru alloyed to Pt. PteRu catalysts are very effective for methanol oxidation and the electrocatalytic activity seems to be directly related to the oxidation state of Ru [22]. A metallic state may favour the formation of a thin layer of adsorbed hydroxyl species, which, being labile-bonded to the surface, migrate more easily and more efficiently to Pt [2]. A similar explanation may be given for tin since the promotion of methanol and ethanol oxidation takes significant advance of the presence of PteSn alloys [2]. On the other hand, the presence of the Ru-oxide promotes the uptake of the hydroxyls to form a thick layer of strongly bonded species [22]. In the case of oxygen evolution, this effect is not critical because
Fig. 5 e Cyclic voltammogram of Pt/C and Pt metal oxides electrocatalyst a) 0.3 M methanol concentration, b) 0.6 M methanol concentration.
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the sites need to donate OHads species for the complete desorption of methanol residues. Although there are several stated mechanisms for methanol oxidation reaction, it is generally agreed that the most abundant surface intermediate is chemisorbed carbon monoxide (this is further illustrated in the following equations).
Fig. 6 e Logarithmic relation between the current density of oxidation peaks and methanol concentration of the prepared electrocatalysts and Pt/C for comparison.
explain the promoting effect of the additional elements (e.g. Ru, Sn, Ir etc.) [2,29e33], this subject remains controversial. Transition metal promoters and adatoms are seen as a means to improve the electrocatalytic behaviour of electrodes, either by minimizing the poisoning reaction or enhancing the main oxidation reaction. Since the CO oxidation process needs the presence of oxygenated species, numerous types of components have been explored, to generate O or OH species at lower potentials on the platinum surface. Historically, systematic screening of the possible metals show that only a few metals led to positive results [30]. Ruthenium, tin, molybdenum were investigated to determine the effects of alloying platinum with transition metals groups 4 to 6 with regard to the formation of OH adsorbed species [31]. Despite considerable efforts made over the last twenty years, the best alloying component known to enhance the electro-oxidation of methanol on platinum is ruthenium. By comparison with the mechanism discussed for pure platinum, the promoting effect of ruthenium can result from a bifunctional mechanism [28] as follows. The adsorbed OH species are formed at both Pt and Ru sites (Equations (5) and (7)) but in different potential ranges (Equation (7)). At suitable electrode potentials (0.2 V vs. RHE), water discharging occurs on Ru sites with formation of RueOH groups at the catalyst surface [28]. One of the rate determining step in the methanol oxidation process is water discharging (Equation (5)). This is similar to one of the rate determining steps for oxygen evolution (Equation (9)). This similarities make both IrO2 and RuO2 good promoters for both processes when these are in oxide form. In the case of methanol oxidation, the oxide promoter, e.g. IrO2, may be added to the PtRu alloy to produce a synergistic effect on the reaction process.
Methanol oxidation process Methanol oxidation is a slow reaction that requires multiple active sites for methanol adsorption and, at the same time,
4Pt þ CH3OH / 3 PtH þ PteCO þ 1e þ Hþ
(3)
3 PtH / 3 Pt þ 3Hþ þ 3 e
(4)
Pt þ H2O / Pt (OH)ads þ Hþ þ e
(5)
PteCO þ Pt (OH)ads / 2Pt þ CO2 þ Hþ þ e
(6)
An efficient catalyst favours CO adsorption on Pt and OH formation takes place on the second metal oxide as in the following equations. Accordingly, the OH adsorbed on the surface of the oxide (MOx: where M is Ir or Ru) may oxidize the CO present on the surface of Pt, as occurs in a bifunctional mechanism [34,35].
MOx þ H2O / MOxeOHads þ Hþ þ e
(7)
PteCOads þ MOxeOHads / Pt þ MOx þ CO2 þ Hþ þ e
(8)
Oxygen evolution reaction
2IrOx þ 2H2O / 2IrOxeOH þ 2Hþ þ 2e
IrOxeOH þ IrOxeOH / 2IrOx þ O2 þ 2Hþ þ 2e
(9)
(10)
It was interesting to observe that at this low level of oxide content and in the absence of large degree of alloying as usually observed in benchmark catalysts, IrOx provided a larger promoting effect than RuOx. As already discussed, this result cannot be explained neither in terms of an alloying effect (the degree of alloying was lower for Ir than Ru) nor on the basis of catalyst dispersion (the mean particle size of the IrOx particles was larger than RuOx). Regarding the valve metal oxides, the electrocatalytic enhancement was lower than IrOx and RuOx. However, these materials may be useful in reducing the noble metal loading in a multifunctional catalyst since they may act as dispersion phase for the noble metal oxide or may reduce the tendency of dissolution of Ru which give rise to the well known phenomenon of Ru ions cross-over through the membranes towards the cathode surface [2]. The electrocatalytic activity is thus related to the different characteristics of water displacement for these oxides which are known from the oxygen evolution process [27]. Such a parallelism may provide new routes to design multifunctional catalysts where Pt-alloys can be combined with oxide
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promoters to accelerate the multiple steps of the methanol oxidation process. [8]
Conclusions Methanol electrooxidation was studied at Pt electrodes modified with noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx and VOx) in acidic environment. Low metal oxide content (5% wt) was used to get insights on the promoting effect of the oxides in absence of significant alloying effects or surface enrichment phenomena. Under such conditions IrOx provided better electrocatalytic activity than RuOx. The valve metal oxides showed a modest promoting effect. The results are interpreted in terms of different water displacement mechanism for the oxides induced by the formation of strongly adsorbed hydroxyl species in the case of the Ruoxide. The parallelism with oxygen evolution helps in understanding the different promoting effects which are different in the two reactions methanol electro-oxidation and water splitting since they occur in a different potential range. The results may be useful in designing a multifunctional catalyst for methanol electro-oxidation.
Acknowledgements The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2011-2014) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement DURAMET no. 278054.
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