Ordered mesoporous carbon supported bifunctional PtM (M = Ru, Fe, Mo) electrocatalysts for a fuel cell anode

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1 Chinese Journal of Catalysis 37 (2016) 催化学报 2016 年第 37 卷第 1 期 available at journal homepage: Article (Special Column on New Porous Catalytic Materials) Ordered mesoporous carbon supported bifunctional PtM (M = Ru, Fe, Mo) electrocatalysts for a fuel cell anode Chin Te Hung a, Zih Hao Liou a,b, Pitchaimani Veerakumar a, Pei Hao Wu a, Tuan Chi Liu b, *, Shang Bin Liu a,c,# a Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan, China b Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan, China c Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan, China A R T I C L E I N F O A B S T R A C T Article history: Received 30 March 2015 Accepted 27 April 2015 Published 5 January 2016 Keywords: Ordered mesoporous carbon Platinum based electrocatalysts Methanol oxidation reaction X ray absorption spectroscopy Core shell alloy nanoparticles Carbon monoxide stripping voltammetry Fuel cells The deposition onto an ordered mesoporous carbon (OMC) support of well dispersed PtM (M = Ru, Fe, Mo) alloy nanoparticles (NPs) were synthesized by a direct replication method using SBA 15 as the hard template, furfuryl alcohol and trimethylbeneze as the primary carbon sources, and metal acetylacetonate as the alloying metal precursor and secondary carbon source. The physicochemical properties of the PtM OMC catalysts were characterized by N2 adsorption desorption, X ray diffraction, transmission electron microscopy, X ray absorption near edge structure, and extended X ray absorption fine structure. The alloy PtM NPs have an average size of 2 3 nm and were well dispersed in the pore channels of the OMC support. The second metal (M) in the PtM NPs was mostly in the reduced state, and formed a typical core (Pt) shell (M) structure. Cyclic voltammetry measurements showed that these PtM OMC electrodes had excellent electrocatalytic activities and tolerance to CO poisoning during the methanol oxidation reaction, which surpassed those of typical activated carbon supported PtRu catalysts. In particular, the PtFe OMC catalyst, which exhibited the best performance, can be a practical anodic electrocatalyst in direct methanol fuel cells due to its superior stability, excellent CO tolerance, and low production cost. 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Proton exchange membrane fuel cells (PEMFCs) are potential energy conversion power devices with the advantages not only of using renewable and eco friendly energy sources (e.g., hydrogen or methanol) as fuel but are also highly efficient. However, the majority of current PEMFCs use noble metals (e.g., Pt, Ru) supported on activated carbons as the electrocatalyst at both the anode and cathode, which limits mass production and cost and jeopardizes their commercialization [1 7]. Thus, the research and development R&D on the electrocatalyst for fuel cells directed towardsdecreasing or avoiding the utilization of a noble metal is a needed task. The electrocatalysts for the methanol oxidation reaction (MOR) at the anode of DMFCs normally utilize Pt based bifunctional noble catalysts supported on activated carbons or carbon porous materials (CPMs) [8 13].The incorporation of a second noble metal (e.g., Ru) was found to enhance the structural, electronic, and chemical properties of the supported catalyst. Indeed, supported PtRu/C catalysts were found to show im * Corresponding author. Tel: ; Fax: ; E mail: tcliu@mail.ntust.edu.tw # Corresponding author. Tel: , Fax: ; E mail: sbliu@sinica.edu.tw This work was supported by the Ministry of Science and Technology (NSC M MY3, NSC M MY3), Taiwan, China. DOI: /S (15) Chin. J. Catal., Vol. 37, No. 1, January 2016

2 44 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) proved stability and tolerance to CO poisoning, hence, exhibiting superior electrocatalytic activity for the MOR [10 13]. Nonetheless, severe Ru metal dissolution and the cross over of the fuel (methanol) through the polyelectrolyte membrane have been observed with most anodic PtRu/C catalysts during the longterm operation of DMFCs [14 16]. Thus, further investigation on the stability of the supported PtRu/C electrocatalyst remains an urgent task. In addition, from the viewpoints of the cost effectiveness of the membrane electrode assembly (MEA) [17 19] and overall performance of the PEMFC, the further reduction in production cost while maintaining a satisfactory MOR activity at the anode are critical issues that can benefit both fundamental research and practical industrial applications. Due to their unique high surface area, tunable meso scale pore size, and tailorable surface functionalities, which give good molecular transport and enhanced chemical properties, ordered mesoporous carbons (OMCs) synthesized by casting from a hard or soft silica template [20 24] have drawn considerable research attention. They have been widely applied for various applications such as catalyst support, adsorbent, sensors, and electrode matrices [20 26]. For examples, OMCsand CPMs have been exploited as the electrode support for Pt based electrocatalystsfor DMFCs/PEMFCs [9 11,27 34] and biomolecular sensors [25,35 37]. Aside from optimizing the performance of the Pt based electrocatalysts in fuel cells, intensive R&D attention has focused on reducing the loading of the precious metal or replacing it with a non noble metal [38 42] or metal free [43 49] catalyst. Previously, we reported a direct procedure to prepare well dispersed, highly stable mono (Pt) and bimetallic (PtRu) metal nanoparticles (NPs, 2 3 nm) on OMCs using SBA 15 as the template, furfuryl alcohol (FA) and trimethylbenzene (TMB) as the primary carbon source, and platinum and/or ruthenium acetylacetonate as the second metal and (secondary) carbon precursor [9,10]. These Pt and PtRu OMC catalysts were found to possess excellent electrocatalytic properties and stability that surpassed those of common commercial catalysts for the MOR, and can have practical and cost effective applications in PEMFCs/DMFCs. Here, the above direct replication method was employed to prepare a series of bifunctional Pt based PtM OMCs, where the second metal was Ru or a transitionmetal such as Fe or Mo. The structure, composition, morphology, and textural properties of the OMC supported PtM (M = Ru, Fe, Mo) alloy NP materials were characterized by transmission electron microscopy (TEM), X ray diffraction (XRD), and extended X ray absorption fine structure (EXAFS), and N2 adsorption desorption. Their electrocatalytic properties were assessed by cyclic voltammetry (CV) measurements. The PtM OMC electrodes showed excellent methanol oxidation activity and stability, especially the PtFe OMC catalyst, which has good perspective practical applications in DMFCs due to its cost effectiveness. 2. Experimental 2.1. Catalyst preparation The Pt based OMC supported bifunctional catalysts (PtM OMC; M = Ru, Fe, Mo) were prepared by a direct replication procedure reported elsewhere [9,10]. In brief, SBA 15 was first synthesized based on the receipt reported by Zhao et al. [50]. Then, the as synthesized SBA 15 sample was subjected to a dehydration treatment at 423 K under vacuum for 12 h before it was used as the hard template. The direct replication synthesis was carried out by mixing known amounts of the primary metal precursor, i.e., platinum (II) acetylacetonate (Pt(CH(COCH3)2)2, denoted as Pt(acac)2) and a second metal (M = Ru, Fe, Mo) precursor, viz., ruthenium (III) acetylacetonate (denoted as Ru(acac)3), iron (III)acetylacetonate (denoted as Fe(acac)3), or molybdenum (IV) dioxy acetylacetonate (denoted as MoO2(acac)2), in the primary carbon sources, namely, FA and TMB, under ultrasonication. To initiate the polymerization of the carbon precursors, a desired amount of oxalic acid was used as the catalyst. In this context, the two metal (Pt and M) acetylacetonates also served as the secondary carbon sources. Subsequently, the mixed solution was used to impregnate 0.5 g of dehydrated SBA 15 at room temperature by incipient wetness impregnation. This was followed by polymerization under ambient condition at 333 K, then at 353 K, both for 16 h. Typically, the carbonization procedure was performed under a N2 environment, first at 423 K for 2 h, followed by increasing the temperature to 573 K with a ramped rate of 1 K/min, then, increased to 1073 K with a rate of 5 K/min and maintained at the final temperature for an additional 4 h. The resulting silicon carbon (Si C) composite was etched by HF (1wt%) aqueous solution for 24 h to remove the silica template, then thoroughly washed with a mixture of distilled water and alcohol, and dried at 373 K to obtain the final products, which were denoted as Pt OMC (without the second metal) and PtM OMC (M = Ru, Fe, Mo) Catalyst characterization The structural integrity of the various samples was examined by small and large angle XRD measurements performed on a PANalytical X Pert PRO instrument. In addition, their textural properties were monitored by N2 adsorption desorption isotherms measured at 77 K using a Quantachrome Autosorb 1 physisorption apparatus. The specific surface area (SBET), total pore volume (Vtot), and pore size (dbjh) were derived using the conventional BET and BJH methods. TEM images of the catalyst samples were recorded by a JEOL JEM 2100 instrument operating at 200 kv. The metal composition in each sample wasdeterminedby inductively coupled plasma mass spectroscopy (ICP MS; PE SCIEX ELAN 6100 DRC). X ray absorption near edge structure(xanes) and EXAFS spectra of the metal supported OMCs were recorded at Beamline 01C1 at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The electron storage ring was operated at 1.5 GeV and 300 ma. A double Si(111) crystal monochromator was employed for energy selection with a resolution (ΔE/E) better than at the LIII edge for Pt and K edge for the second metal species (M = Ru, Fe, Mo). The calibration of the XANES and EXAFS spectra were made by recording the spectra

3 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) of standard compounds such as Pt, Ru, Fe, or Mo metal foil. The EXAFS function in energy space, (E), was obtained by subtracting the post edge background from the raw absorption data and followed by normalization with respect to the edge jump step. Then, the normalized (E) was transformed from energy space to k space (k denotes the photoelectron wave vector). All data in the (k) function were multiplied by a factor k 2 to compensate for the damping of EXAFS oscillations in the high k region. Subsequently, the k 2 weighted (k) data, which ranged from 3.6 to 12.5 Å 1 for the Pt LIII edge and 3.6 to 11.6 Å 1 for the Ru and Mo K edges, were Fourier transformed to r space to get the EXAFS contributions from the various coordination shells. A nonlinear least squares algorithm was applied to fit the EXAFS spectra in r space in the absence of phase correction in the range of Å for Pt and Å for Ru and Mo, respectively. All computer programs were implemented by a UWXAFS 3.0 package [51] with the backscattering amplitude and phase shift for the specific atom pairs calculated by the FEFF7 code [52] Electrochemical measurement To probe the electrocatalytic properties of the Pt OMC and PtM OMC catalysts, CV measurements were conducted on a galvanostat/potentiostat (Autolab, PGSTAT30) equipped with a rotating disk electrode (RDE). This was operated with three electrodes, namely, the Ag/AgCl reference electrode, the working electrode, and the counter electrode using a Pt wire. Prior to each run, the glassy carbon thin film electrode was polished to a mirror finish with a 5 m alumina suspension. The catalyst ink was prepared by adding 5 mg of Pt or PtM OMC sample into 2.5 ml of deionized water, followed by ultrasonic treatment for 0.5 h. Subsequently, 20 μl of the resulting suspension mixture was withdrawn and pipetted onto the glassy carbon electrode, followed by drying in air at 333 K for 0.5 h. Finally, 7.5 μlof 1% Nafion (DuPont) solution was added as a binder under a N2 environment. Electrooxidation of methanol (MeOH) was carried out at room temperature (298 K) in an electrolyte solution consisting of 0.5 mol/l H2SO4 and 1 mol/l MeOH. The data were recorded in the potential range between and 1.0 V with a typical scan rate of 10 mv/s. Prior to each CV measurement, the electrolyte solution was purged with N2 (99.9% purity) for at least 0.5 h to remove dissolved O2. Then, the experiment was conducted under purging N2 condition with an electrode rotating speed of 1600 r/min. For comparison, the CV measurement of a commercial PtRu/C catalyst (Johnson Matthey, 20 wt%pt and 10 wt%ru supported on Vulcan XC72 carbon black, hereafter denoted as PtRu/XC 72 ), were also recorded. 3. Results and discussion 3.1. Structural and textural properties of PtM OMC Figure 1 displays the small and large angle XRD patterns of the pristine OMC (without metal loading), monometallic Pt OMC, and various bimetallic PtM OMC catalysts. As shown in Fig. 1(a), the small angle XRD pattern of pristine OMC exhibited wellresolved diffraction peaks at 2 = 1.0, 1.6, and 1.9, which were indexed to the (100), (110), and (200) reflection, respectively, corresponding to the two dimensional (2D) hexagonal space group (p6mm) resembling the mesoporous CMK 3 carbon material [53,54]. Except for the marginal shift of the (100) peak towards lower 2 angles, similar lowangle XRD patterns were observed for the Pt OMC and other PtM OMC catalysts. This indicated that the mesoporous structure remained unchanged even in the presence of metal NPs. Unlike the large angle XRD pattern of the pristine OMC, which revealed two main diffraction peaks at 2 = 25 and 44 from amorphous carbon (Fig. 1), the Pt OMC sample showed additional peaks at 2 = 39, 47, 68, and 82, which were the (111), (200), (220) and (311) diffraction of crystalline Pt(0) metal with a face centered cubic (fcc) lattice. On the other hand, slight shifts in the diffraction peaks for most of the PtM OMCs were observed, indicating changes in their lattice constants from the alloyed PtM NPs in the OMC support. This assignment is supported by the fact that no characteristic diffraction peak for Ru, Fe, Mo, and their metal oxides [10,56 60] was observed. Due to the substitution of Pt by a smaller second metal (M = Ru, Fe, Mo), the PtRu, PtFe, and PtMo alloys as supported catalysts showed ananticipated reduction in particle size (Dp) [10,55], as deduced from the Scherrer formula and the data (Table 1). (100) (110) (200) (a) (111) (200) (220) (311) Intensity Pt-OMC OMC Intensity Pt-OMC OMC ` /( o ) Fig. 1. Small (a) and large angle XRD patterns of pristine OMC, Pt OMC, and various PtM OMC catalysts.

4 46 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) Table 1 Physical and textural properties of pristine OMC, Pt OMC, and PtM OMC samples. Sample Metal loading a (wt%) SBET dbjh b Vtot c Dp d Dp e Pt M (m 2 /g) (nm) (cm 3 /g) (nm) (nm) OMC (CMK 3) Pt OMC PtRu OMC PtFe OMC PtMo OMC a Metal loading ratio (wt%/wt%) deduced from ICP MS data. b BJH pore size derived from the adsorption branch of N2 isotherms. c Total pore volumes obtained from p/p0 = d Metal particle size calculated by Scherrer equation and XRD data. e Metal particle size determined from TEM images. In terms of their textural properties, all samples showed a well defined hysteresis loop in their N2 adsorption desorption isotherm (see Fig. S1(a) in the Supporting Information, hereafter denoted as SI ), which is typical for ordered mesoporous materials. All isotherms were Type IV according to IUPAC classification. Accordingly, their SBET, Vtot and dbjh deduced from the adsorption branch of the isotherms are depicted in Table 1 along with the metal loading ratio (M/Pt) deduced from ICP MS. Clearly, the pristine OMC, Pt OMC, and PtM OMC catalysts all possessed high surface areas (900 m 2 /g) and high pore volumes (0.70 cm 3 /g) with a uniform pore size distribution (PSD; see Fig. S1; SI ). However, notable decreases in the values of SBET, Vtot, and dbjh for Pt OMC and PtM OMCs compared to the pristine OMC were observed, which were due to the metal/alloy NPs in the mesopore channels of the carbon support. Nonetheless, the small variation in the pore sizes reflected that no serious pore blocking occurred in the support. We hypothesized that the well dispersed alloyed metal NPs were likely partially embedded or studded in the pore walls of the OMC [9,10], which may be responsible for the extraordinary stability observed (vide infra). The structure, morphology, and pore structure of the catalysts can be inferred from the TEM results, as shown in Fig. 2 (a) (d) (g) 200 nm µm µm (e) (h) Percentage (%) (c) Percentage (%) (f) Percentage (%) (i) Particle size (nm) Particle size (nm) Particle size (nm) Fig. 2. TEM images and histograms of particle size distribution of PtRu OMC (a c), PtFe OMC (d f), and PtMo OMC catalysts (g i).

5 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) and S1(c) (SI ). Large domains of ordered mesoporous channel sembedded with PtM alloy (or Pt) NPs with typical sizes of 2 3 nm were evident. Again, this was due to the direct replication synthesis method employed for the preparation of these supported catalysts. The nature and sizes of the metal/alloy NPs buried in the Si C composites remained practically unchanged after the removal of the silica hard templateby acid leaching. As a result, these metal/alloy NPs, which were confined in the pore channels of the carbon skeleton (average dbjh 3.0 nm; see Table 1), were highly dispersed and most likely partially embedded or studded on the inner pore surfaces of the OMC. These results obtained from the TEM analysis were in excellent agreement with those from the XRD and physisorption studies Surface and electronic properties of PtM OMC To study the electronic states and atomic environments of the alloyed PtRu, PtFe, and PtMo NPs in the OMCsupport, additional experiments by X ray absorption spectroscopy (XAS) were performed using a synchrotron radiation light source. The absorption spectra exhibited fine structures both above and below the corelevel absorption edges. In particular, the near edge (XANES) spectra provide information on the excitation of a core electron to quasi bound states, and the valence of the central atom can be determined from the edge position [9,61]. The normalized XANES spectra at the Pt LIII edge of Pt OMC and the PtM OMC catalysts are displayed in Fig. 3 along with that of the model compound (Pt foil). The absorption peaks near ev, namely, the white lines, were mainly the result of electron transition from the 2p3/2 band to the 5d band of the Pt metal. The corresponding absorption peak intensity reflected the d orbital occupancy. That the intensities of the absorption peak maxima in the white lines region for the PtM OMC catalysts were comparable with that of the Pt foil (Inset; Fig. 3) indicated their similar local electronic structure and oxidation state of Pt. This indicated the presence of the nearly fully reduced state of Pt in the supported alloy catalysts. Likewise, similar absorption peak intensities were observed near the white lines region of the normalized XANES spectra at the Ru K edge of PtRu OMC compared to that of the reference Ru foil Normalize absorbance (a.u.) (1) (2) (3) (4) (5) Energy (ev) Fig. 3. XANES spectra at the Pt LIII edge of Pt OMC (1), PtRu OMC (2), PtFe OMC (3), PtMo OMC (4), and Pt foil (5). Inset: blow up spectra near the white lines region. (Fig. S2(a); SI ),con irming the existence of reduced Ru in the alloyed NPs. On the other hand, notable differences in the absorption curves at the Mo K edge of PtRu OMC and Mo metal foil were observed (Fig. S2; SI ), revealing the occurrence of the partial oxidation state of Mo. There was no detectable XANES spectrum at the Fe K edge, probably due to the insufficient energy of the beam source. Based on the k 2 weighted EXAFS spectra at the Pt LIII edge of Pt foil, Pt OMC, and PtM OMC catalysts (Fig. 4) together with the Ru and Mo K edge spectra (not shown; also, the spectrum at the Fe K edge was not detected) and Fourier transforms, the parameters of the coordination number (N), bond distance (R), Debye Waller factor (σ 2 ), and inner potential shift (ΔEo) were deduced and summarized in Table 2. The weaker intensities in the EXAFS oscillations observed at the Pt LIII edge for the PtM OMCs compared to Pt OMC and Pt foil indicated a smaller cluster domain size of the second metal (M) compared to Pt. This may be inferred from the smaller coordination number (N) of the homometallic coordination shell (Pt Pt) observed with the PtM OMCs compared to Pt OMC (Table 2). In addition, a preferred bonding for homometallic (Pt Pt) over heterometallic (Pt M) coordination was also observed, as revealed by the larger differences in N, indicating the occurrence of aggregation of Pt atoms prior to the formation of alloy PtM NPs upon carbonization treatment. All the PtM OMC catalysts exhibited a lower sum of nearest neighbors around the absorbing Pt (ΣNPt i= NPt Pt + NPt M; Table 3) compared to the commercial PtRu/XC 72 catalyst (ΣNPt i = 8.9), in good agreement with the observed decreases in Dp for PtM OMCs compared to Pt OMC deduced from the XRD results (Table 1). The extent of alloying in the PtM NPs and their corresponding intra particle dispersion of Pt and M in the PtM OMC catalysts were further examined using a quantitative approach developed by Hwang and co workers [62,63]. The relative structural coordination and quantitative parameters, namely Pobs, Mobs, ΣNPt i, ΣNM i, JPt, and JM derived are summarized in Table 3. The value observed for JM being higher than JPt in each PtM OMC (M = Ru, Mo; corresponding values for Fe were unavailable) catalyst was consistent with a core shell structural model of a bimetallic nanocluster with Pt in the inner core and k 2 x(k) (1) (2) (3) (4) (5) k (Å 1 ) Fig. 4. K 2 weighted EXAFS spectra at Pt LIII edge of Pt OMC (1), PtRu OMC (2), PtFe OMC (3), PtMo OMC (4), and Pt foil (5).

6 48 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) Table 2 EXAFS parameters at the Pt metal (Pt; LIII edge) and second metal (M = Ru, Fe, Mo; K edge) for Pt OMC and PtM OMC catalysts. Sample Edge Shell N a Rj b (Å) ΔEo c (ev) σ d (Å 2 ) R factor e Pt OMC Pt L III Pt Pt PtRu OMC Pt L III Pt Pt Pt Ru Ru K Ru Ru Ru Pt PtFe OMC Pt L III Pt Pt Pt Fe Fe K * Fe Fe Fe Pt 19 PtMo OMC Pt L III Pt Pt Pt Mo Mo K Mo Mo Mo Pt PtRu/XC 72 Pt L III Pt Pt Pt Ru Ru K Ru Ru Ru Pt a Coordination number. b Bond distance. c Inner potential shift. d Debye Waller factor. e Standard deviation. * Not detected. M (second metal) in the outer shell [62]. This picture was supported by the results obtained from the above coordination number analysis Electrocatalytic activity of PtM OMC during MOR The electrocatalytic performance of the PtM OMCs as anodic catalysts for the MOR was evaluated by electrochemical analysis using CV with a RDE rotating speed of 1600 r/min 0.5 mol/l H2SO4 and 1.0 mol/l MeOH electrolyte solution at room temperature. The CV profiles observed for Pt OMC and the PtM OMC modified electrodes exhibited the typical voltammetric characteristics for the electrooxidation of methanol, as shown in Fig. 5(a). Typically, a methanol oxidation peak at 0.6 V was observed in the curve recorded during the forward scan, while an anodic peak at V was evident during the reverse scan. The latter can be attributed to the removal of the incompletely oxidized carbonaceous species formed during the forward scan [9,10,64 67]. As depicted in Table 4, a higher current density was observed for the supported bimetallic Table 3 Structural coordination and quantitative parameters for PtM OMC and the commercial PtRu/XC 72 catalysts. Sample Ptobs a Mobs b ΣNPt I c ΣNM I d JPt e (%) JM f (%) PtRu OMC PtFe OMC PtMo OMC PtRu/XC a Ptobs = NPt M/ΣNPt i. b Mobs = NM Pt/ΣNM i. c ΣNPt i = NPt Pt + NPt M. d ΣNM i = NM M + NM Pt. e JPt = Ptobs/Ptrandom 100%; Ptrandom values derived from ICP MS. f JM = Mobs/Mrandom 100%; Mrandom values derived from ICP MS. PtM OMC catalysts compared to the monometallic supported Pt OMC. This indicated that the incorporation of a second metal indeed led to significant enhancement in MOR activity. For example, the anodic peak current (If) observed for the PtRu OMC catalyst was higher than that of Pt OMC by nearly 2.5 times. Likewise, the If value observed for the PtFe OMC and PtMo OMC catalysts were about 1.5 times higher than that of Pt OMC. On the other hand, a lower onset potential (Vo) and anodic peak potential (Vap) were detected for all the PtM OMC catalysts compared to the Pt OMc (Table 4). Note that despite the higher metal loading (nominal Pt:Ru = 20 wt%:10 wt%) in the commercial PtRu/XC 72 catalyst over that of the PtM OMCs (nominal Pt:Ru = 10 wt%:10 wt%), a higher Vap value was observed for the former (0.71 V) than the latter (typically 0.60 V; see Table 4). This indicated that the PtM OMCs reported here possessed well dispersed alloy NPs with high metal surface areas [10] (vide infra). The catalytic performance of the electrocatalyst can also be assessed in terms of the ratio of the measured forward versus reverse anodic peak current (i.e., If/Ir). A high If/Ir ratio indicates a superior electrocatalytic activity during MOR and less catalyst deactivation due to the deposition of carbonaceous species on the active sites, hence, better CO tolerance [68]. The If/Ir values deduced from the Pt OMC and different PtM OMC catalysts are summarized in Table 4 together with literature data of other supported bimetallic catalysts using various carbon substrates. First, it is clear that the If/Ir ratios observed for the carbon supported bimetallic catalysts were larger than that of the monometallic catalyst. For examples, the If/Ir ratios obtained for the PtM OMCs (typically > 2.2) as well as from the commercial PtRu/XC 72 (2.8; but with a higher metal loading) were higher than that of the Pt OMC catalyst (2.0). This clearly indicates the benefit of incorporating a second metal (M) for CO

7 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) I (A/mg Pt) I (A/mg Pt) (a) Pt-OMC Potential (V vs Ag/AgCl) Time (s) Current density (a.u.) Pt-OMC PtRu/XC-72-83% -20% -26% -60% -67% Potential (V vs Ag/AgCl) Fig. 5. CV (a) and CA curves observed for Pt OMC and the PtM OMC catalysts during MOR. (c) CV curves of the catalysts recorded before (black curves) and after (red curves) adsorption of CO (500 ppm). Table 4 Comparison of electrochemical performance observed for the PtM supported catalysts in MOR. Catalyst Support a Metal Loading b Pt M If c (A/mg Pt) Vo d (V) Vap e (V) (c) SM f (m 2 /g Pt) Pt OMC This work PtRu OMC This work PtFe OMC This work PtMo OMC This work PtRu CS [8] PtRu XC [10] PtRu TFC [11] PtFe HCC [39] PtCo CNT [41] a OMC = ordered mesoporous carbon; CS = carbon shell; XC 72 = Vulcan XC72 carbon black; TFC = thin film carbon; HCC = hollowed carbon capsule; CNT = carbon nanotube. b Metal loading (in wt%) determined by ICP MS. M denotes secondary metal. c Current density. d Onset potential. e Peak potential. f Active Pt metal surface area was estimated assuming a monolayer of adsorbed CO and a Coulombic charge of 210 C/cm 2 for oxidation. If/Ir Ref. tolerance and overall MOR activity of the catalyst. From the viewpoint of material synthesis strategy, the PtRu OMC catalyst here prepared by the direct replication method gave rise to a much higher value of If/Ir (3.5) compared to that of PtRu/XC 72 (2.8) prepared by the conventional impregnation method even though the latter possessed a higher Pt loading. Clearly, the catalysts prepared by the direct method not only showed improved MOR activity but also less vulnerability to coke deactivation and better tolerance against CO poisoning. This gave an effective reduction in the polarization overpotential (Vo) [69] and a desirable increase in the current density (Fig. 5(a) and Table 4). This may be ascribed to the better alloying and dispersion of the metal NPs in the OMC support with a high BET surface area. In this regard, the PtFe OMC catalyst, which was synthesized by using a more cost effective Fe metal precursor to replace the noble (Ru) metal precursor, gave an overall performance in terms of Vo, Vap, and If/Ir ratio (cf., Table 4) that was superior or at least comparable to that of the commercial PtRu/XC 72 and other carbon supported bimetallic catalyst systems even though these other catalysts have much higher metal loadings (Table 4) Stability of electrocatalysts The electrochemical stability of the PtM OMC (M = Ru, Fe, Mo) catalysts during MOR was also evaluated by chronoam

8 50 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) perometric (CA) experiments at room temperature, as shown in Fig. 5. In general, when MeOH is oxidized on the surface of the catalyst, tenacious reaction intermediates such as COad also accumulate progressively if its rate of removal cannot keep pace up with the oxidation of MeOH. Thus, the catalyst s tolerance to CO poisoning can be assessed by monitoring the variation of current density with time (Fig. 5). Clearly, while the decay rates of the oxidation current density observed for the PtRu OMC and PtMo OMC samples were nearly the same, that obtained for the PtFe OMC catalyst was notably slower, indicating a better tolerance to the CO poisoning. To further assess the resistance to CO poisoning of the PtM OMCs, we performed additional CO stripping studies from which information on the interaction between the adsorbed CO and surface of the Pt based alloy NPs can be compared. The experiments were done by purging N2 containing 500 ppm of CO through the pre activated electrode in the presence of an electrolyte (0.5 mol/l H2SO4 and 1.0 mol/l MeOH) for 15 min while keeping a constant potential (5 V) at the rotating working electrode (1600 ppm). This allowed for complete adsorption of CO onto the catalyst surface prior to the CV measurement. The results obtained from the CO stripping experiments over the various catalysts are depicted in Fig. 5(c). It is obvious that a notable decrease in the electrooxidation current density of methanol was observed for the Pt OMC, PtMo OMC, and PtRu/XC 72 catalysts after they were exposed to CO, indicating an inferior MOR activity due to CO poisoning. On the other hand, only a marginal decrease in the current density was found for the PtRu and PtFe OMC catalysts during MOR after adsorption of CO. Clearly, these two catalysts exhibited superior methanol electrooxidation stability, which may be ascribed due to their tolerance for CO poisoning [12,70 73]. Moreover, in the presence of CO, a much lower MOR activity was observed for the monometallic Pt OMC compared to its bimetallic counterparts, indicating the role of the second metal on the electrocatalytic activity during MOR. Based on the results above, the PtFe OMC catalyst showed extraordinary electrocatalytic activity and CO tolerance during MOR which were comparable to that of PtRu OMC having a similar metal loading. In this context, this OMC supported Pt based catalyst which used the incorporation of a more cost effective second metal such as a transitionmetal, is a prospective material for practical applications in DMFC/PEMFCs as the anodeelectrode. 4. Conclusions A series of Pt based bifunctional metal (PtM, M = Ru, Fe, Mo) alloy catalysts supported on an ordered mesoscopic carbon (OMC) were synthesized by a one pot replication method. The PtM OMC catalysts possessed uniform and highly dispersed alloy nanoparticles (size 2 3 nm) in the pore channels of the OMC as evidenced by electron microscopy and X ray absorption spectroscopy (XAS). These supported bimetallic catalysts exhibited superior electrocatalytic performance and excellent tolerance to CO poisoning during methanol oxidation reaction (MOR) compared to a typical commercial PtRu/XC 72 catalyst. In particular, the PtFe OMC catalyst showed a comparable MOR activity to PtRu OMC, which would allow the use of a cost effective second metal and has prospective applications as the electrode material for fuel cells, sensors, and energy storage devices. 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10 52 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) 用于燃料电池阳极的有序介孔碳负载的双功能 Pt-M (M = Ru, Fe, Mo) 电催化剂 洪锦德 a, 刘子豪 a,b, 维拉库玛 a, 吴培豪 a, 刘端祺 a,* a,c,, 刘尚斌 a 中央 研究院原子与分子科学研究所, 台湾台北 b 国立 台湾科技大学化学工程系, 台湾台北 c 国立 台湾师范大学化学系, 台湾台北 摘要 : 质子交换膜燃料电池 (PEMFCs) 不仅采用可再生的和环境友好的能源 ( 如氢气和甲醇 ) 作为燃料, 而且转换效率高, 因此被认为是潜在的能量转换设备. 目前 PEMFCs 大多采用活性炭负载的贵金属作为其阴极和阳极的电催化剂, 因此制造成本高, 限制了它的实际应用. 燃料电池电催化剂研发的一个紧迫的任务就是避免使用采用贵金属. 直接甲醇燃料电池 (DMFCs) 阳极上进行的甲醇氧化反应 (MOR) 通常采用活性炭或多孔碳材料负载的 Pt 基双功能催化剂. 在此基础上复合第二贵金属 ( 如 Ru) 可改善其结构 电子和表面化学性质, 表现出较高的稳定性和抗 CO 中毒性能, 因而在 MOR 中具有优异的电催化活性. 然而, 在 DMFCs 长期运行期间, 该 Pt-Ru/C 阳极催化剂大多出现严重的金属 Ru 溶解流失, 以及燃料甲醇透过聚电解质膜的现象. 因此, 进一步提高 Pt-Ru/C 电催化剂的稳定性成为当务之急. 另外, 考虑到膜电极的成本和 PEMFCs 的整体效率, 在保持较好 MOR 活性的前提下, 减少贵金属用量或采用非贵金属, 甚至不用金属, 进一步降低其成本也尤为重要, 这也将十分有利于基础研究和实际的工业应用. 采用软模板或硬模板法制得的有序介孔碳 (OMCs) 因具有高的比表面积 可调的中孔尺寸和表面官能团性质而广受关注, 已经应用于催化剂载体, 吸附剂, 传感器和电极材料等领域. OMCs 可负载 Pt 制成电催化剂而用于 DMFCs/PEMFCs 中. 本课题组前期以介孔氧化硅 SBA-15 为硬模板剂, 糠醇和三甲苯为初始碳源, Pt 和 Ru 的乙酰丙酮化物为金属前驱体和次级碳源, 采用一步法直接制备了 OMC 负载的高分散 高稳定性的单 Pt 和 Pt-Ru 双金属纳米粒子, 在 MOR 中表现出优异的电催化活性和较高的稳定性性能, 超过常用的商用催化剂, 显示出较大的应用潜力. 因此, 本文采用类似的方法将高度分散的 PtM(M = Ru, Fe, Mo) 合金纳米粒子沉积于 OMCs 上, 从而制得 PtM-OMC(M = Ru, Fe, Mo) 催化剂 ; 同时采用 N 2 物理吸附 X 射线衍射 透射电镜 X 射线吸收近边结构 扩展 X 射线吸收精细结构谱等手段对其结构组成 形貌和织构等物化性质进行了表征. 结果表明, 合金化的 PtM 纳米粒子的平均粒径约 2 3 nm, 且高度分散于 OMC 载体孔道内. 另外, PtM 纳米粒子中第二金属 M(Ru, Fe, Mo) 大多以还原态形式存在, 形成了典型的核 (Pt)- 壳 (M) 结构. 循环伏安法测量结果表明, 在 MOR 反应中, 所制 PtM-OMC 电极表现出较高的电催化活性和抗 CO 中毒性能, 超过典型的活性炭负载的 Pt-Ru 催化剂. 尤其值得一提的是, Pt-Fe-OMC 催化剂不但具有非常高的稳定性, 优越的抗 CO 性能, 而且其催化 MOR 反应活性与 的相当, 因而具有更低的生产成本, 所以 Pt-Fe-OMC 催化剂在 DMFCs 阳极电催化剂具有很大的应用前景. 关键词 : 有序介孔碳 ; 铂基电催化剂 ; 甲醇氧化反应 ; X 射线吸收光谱 ; 核 - 壳合金纳米粒子 ; 一氧化碳溶出伏安法 ; 燃料电池 收稿日期 : 接受日期 : 出版日期 : * 通讯联系人. 电话 : ; 传真 : ; 电子信箱 : tcliu@mail.ntust.edu.tw # 通讯联系人. 电话 : ; 传真 : ; 电子信箱 : sbliu@sinica.edu.tw 基金来源 : 台湾科技支撑项目 (NSC M MY3, NSC M MY3). 本文的英文电子版由 Elsevier 出版社在 ScienceDirect 上出版 ( Supporting Information Volume adsorbed (ml/g) (a) OMC Relative pressure (p/p 0) dv/dd (cm 3 /(g nm)) OMC Pore diameter (nm) (c) Percentage (%) (d) Particle size (nm) Fig. S1. (a) N2 adsorption/desorption isotherms; BJH pore size distributions of pristine OMC, Pt OMC, and PtM OMC catalysts; (c) TEM image and (d) metal particle size histogram of the Pt OMC catalyst.

11 Chin Te Hung et al. / Chinese Journal of Catalysis 37 (2016) Normalized absorbance (a.u.) (a) Ru K-edge Ru foil Energy (ev) Normalized absorbance (a.u.) Mo K-edge Mo foil Energy (ev) I (ma) (c) Pt-OMC E vs Ag/AgCl (V) Fig. S2. XANES spectra at (a) Ru K edge of PtRu OMC; Mo K edge of PtMo OMC and their corresponding model compounds; (c) CV curves of Pt OMC and PtM OMC (M = Ru, Fe, Mo) catalystsrecorded under 1.0 mol/l H2SO4 electrolyte solution at room temperature.