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Manganese-tuned chemical etching of a platinum–copper nanocatalyst with platinum-rich surfaces
Journal of Power Sources 304 (2016) 74e80
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Manganese-tuned chemical etching of a platinumecopper nanocatalyst with platinum-rich surfaces Y.Y. Huang, T.S. Zhao*, G. Zhao, X.H. Yan, K. Xu Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
h i g h l i g h t s PtCu particles with Pt-rich surfaces were used as precursors for etching. Manganese-tuned etching induced the formation of abundant Pt active sites. MOR activity of MnA-PtCu/C was ca. 4.0 times higher than that of Pt/C. Catalytic durability of MnA-PtCu/C towards MOR was improved significantly.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2015 Received in revised form 30 October 2015 Accepted 9 November 2015 Available online xxx
This work presents a modified chemical etching strategy to fabricate binary metal nanocatalysts with large active areas. The strategy employs PtCu alloy particles with Pt-rich outer layers as the precursor and manganese species to manipulate the acid leaching processes. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy techniques are used to analyze the catalyst structures and the tuning mechanism of manganese species during etching. It is found that the introduction of manganese species allows more Pt active sites to be formed onto the catalyst surface after etching, possibly due to reduction in the number of Pt atoms enclosed inside particles. The electrochemically active surface area of the synthetic MnA-PtCu/C catalyst increases by 90% relative to commercial Pt/C catalyst. As a result of the increase in active areas and the additional promotion effects by Cu, the MnAPtCu/C catalyst reveals a methanol oxidation activity 1.7 and 4.0 times higher than that of the synthetic PtCu/C and commercial Pt/C catalysts, respectively. © 2015 Elsevier B.V. All rights reserved.
Keywords: Fuel cells Methanol oxidation Catalyst Etching Platinumecopper Manganese
1. Introduction Carbon-supported Pt nanoparticles are currently the most widely used nanocatalysts in proton exchange membrane fuel cells (PEMFCs) [1e5]. These catalysts, however, are not only expensive, but have limited applications due to low natural reservation of Pt. Alloying Pt with other low-cost transition metals, e.g. Ni, Fe, Cu, Sn and Pb, was demonstrated as an effective approach to simultaneously reduce Pt loading and enhance catalytic activity [6e9]. Among these transition metals, Cu stands out for its ability to combine with Pt toward enhanced oxygen reduction reaction (ORR) for the cathode of PEMFCs, and its ease of promoting catalytic activities of Pt anodes toward electro-oxidation of small organic
* Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2015.11.038 0378-7753/© 2015 Elsevier B.V. All rights reserved.
molecules such as methanol, ethanol and formic acid [10e13]. Enhancement in catalytic properties of PtCu materials was generally attributed to modification in physical properties, e.g. electronic effects (ligand effect), geometric effects (compressive strain), surface roughness (facets and steps) and particle size effect [14e16]. Various bottom-up approaches, such as impregnation reduction, electrodeposition and hydrothermal reduction, were used to fabricate binary Pt-based nanocatalysts [17e20]. However, these approaches usually involve an excessive use of organic solvents and surfactants. Additionally, catalyst composition is generally difficult to control, particularly for the preparation of alloy nanoparticles with a high content of non-noble metals (over 50 at.%). Numerous recent studies have used dealloying of Pt-based materials, through which one component is selectively etched from the binary/ternary alloy, for the preparation of binary metal nanoparticles. Dealloyed materials usually possess the desirable characteristic of Pt-rich
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shells and nanoporous structure which contains concave areas [21e23]. For example, PtCu coreeshell nanoparticles were fabricated via electrochemically dealloying a bulk PteCu binary alloy by Ge's group [24]. They found that the dealloyed nanoporous PtCu material exhibited enhanced electrocatalytic activities toward formic acid oxidation reaction (FAOR) and ORR via a mechanism of tailoring the compressive strain of metal particles. Qiu et al. prepared a nanoporous PtCu catalyst with high Cu content via a onestep chemical dealloying process of bulk Pt4Cu21Mn75 precursor in an (NH4)2SO4 aqueous solution [25]. They revealed that the dealloying processes to the formation of coreeshell nanoparticles involved fast dissolution of Mn and slow dissolution of Cu. In comparison with electrochemical dealloying, chemical dealloying possesses the unique advantage of scalable production. With that being said, however, it has been commonly reported that dealloyed PtCu materials showed similar or smaller electrochemically active surface areas in comparison with commercial Pt/ C material. An investigation of the synthesis found that the Ptcontaining precursors for dealloying were compositionally uniform materials. For these uniform precursors, platinum may partially remain inside the particles even after dealloying, leading to a loss of Pt active sites. Thus, one solution is to use the precursors with Pt-rich outer layers for etching. This proposal is also supported by the fact that the driving force from the redox potential difference is easily powerful enough to reduce Pt ions and deposit them onto Cu surfaces. In addition, the PtCu etching processes are easily further optimized by introducing a third species on the precursor surface. To the best of our knowledge, however, very little exploration on this aspect has been conducted so far. In this work, Mn was selected to optimize the etching processes of PtCu precursor because it has a high redox potential at high valence, which makes it easy to deposit together with Pt on Cu surfaces through galvanic reactions. The deposited Mn oxides can be readily removed in an acid medium with reducing species. The PtCu composite can be prepared through this Mn-assisted etching approach, and the product is defined as MnA-PtCu/C. An initial Cu/ Pt atomic ratio was set as 50:1 in order to facilitate fragmentation of the precursor [10], maximizing the effective electrochemically active surface area (ECSA). The surface analysis of the precursor during the etching processes was collected by X-ray photoelectron spectroscopy; a possible tuning mechanism of Mn toward Pt atom assembly on the particle surfaces was proposed. Methanol oxidation reaction (MOR) was used to test the catalytic performance of the PtCu nanoparticles. 2. Experimental 2.1. Synthesis of catalysts Active carbon (Vulcan XC-72R) was pretreated with 5 M HNO3 at 110 C for 6 h. The mixture was subsequently neutralized with sodium hydroxide, filtered, rinsed with double distilled water and dried at 60 C for 24 h. Acid treating was used to produce surface oxygen-containing groups such as carbonyl, carboxyl and hydroxyl groups, which provide anchoring sites for metal particle deposition [26]. The MnA-PtCu/C alloy catalyst was prepared via surface substitution and acid leaching. During synthesis, 30 ml of 93.4 mM CuCl2 solution was diluted with 270 ml of double distilled water. 90 mg of the pretreated Vulcan XC-72R carbon was dispersed in 300 ml of double distilled water under ultrasonic stirring for 30 min; then 300 mg of NaBH4 was magnetically stirred with the carbon suspension. The CuCl2 solution was then dropped into the carbon/NaBH4 mixture and the reduction reaction was conducted for 1 h. The resultant suspension was filtered and rinsed to remove any residual NaBH4; then the filter residue was rapidly dispersed in
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200 ml of double distilled water again. 100 ml solution containing 3 ml of potassium permanganate (18.9 mM) and 3 ml of chloroplatinic acid (18.9 mM) was then added into this suspension. The galvanic substitution reactions between Mn (VII), Pt (IV) and Cu (0) were conducted for 2 h to obtain a PtMnCu/C precursor. Then, 2 ml of concentrated nitric acid (~65 wt.%) was slowly added into the mixture and the acid leaching reaction was conducted for 18 h to ensure complete dissolution of redundant metals. Finally, the mixture was filtered, rinsed with double distilled water and dried overnight at 60 C in air. Other two materials were prepared for comparison via the same method without the addition of potassium permanganate or chloroplatinic acid, and are defined as PtCu/ C and (MnCu)/C, respectively. 2.2. Physical and electrochemical characterization The contents of Pt, Mn and Cu in the catalytic materials were determined using an Ultima2 inductively coupled plasma optical emission spectroscopy (ICPeOES, Jobin Yvon). For the sample preparation, the carbon in the materials was removed at 700 C in air. Any residue was dissolved by aqua regia at room temperature for 24 h; the solution was transferred to a centrifuge and diluted with double distilled water for analysis. X-ray power diffraction (XRD) was conducted with a Philip X'Pert Pro MPP diffractometer using a Cu Ka (l ¼ 1.54 Å) radiation source. The morphology, dispersed state and size distribution of the metal particles were characterized using a JEOL JEM-2010 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The chemical valences of Pt, Mn and Cu in the materials during and after synthesis were analyzed by X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) with an Al Ka X-ray source at 1487 eV. The chamber pressure was kept below 3 1010 mbar during testing and specific correction was conducted the C 1s binding energy of 285 eV. Electrochemical measurements were performed using a CHI604 electrochemical work station (CH Instrument Inc.). The reference electrode was the Hg/Hg2SO4/0.5 M H2SO4 electrode (MMS) (0.62 V vs. SHE) and the counter electrode was the platinum net. A glassy carbon (0.1256 cm2) electrode covered by the catalytic material was used as the working electrode. For Pt/C working electrode preparation, 5 mg of the Pt/C (JM) catalyst (10 wt.% Pt) was ultrasonically stirred in 1 ml solution containing 50 ml of 5 wt.% Nafion solution (DuPont, USA) and 950 ml of ethanol. 4 ml of the slurry was coated on the polished glassy carbon electrode surface and the electrode was dried at room temperature in air for 30 min. The MnA-PtCu/C and PtCu/C working electrodes were prepared in the same way. The amount of other catalysts for preparation of the slurry depended upon their practical Pt loading, determined by ICPeOES analysis. A total Pt loading for each catalytic material on the electrode was set at 2 mg. Prior to taking measurements, the 0.5 M H2SO4 and 0.5 M H2SO4 þ 1 M CH3OH electrolytes were first deaerated with high purity N2. The working electrodes were pretreated in the 0.5 M H2SO4 solution by cyclic voltammetry (CV) at a scan rate of 50 mV s1 for several cycles in order to obtain a stable electrochemical response. CO stripping experiments were conducted using the following procedure: an H2SO4 solution was first deaerated with high purity N2. Subsequently, CO was admitted into the solution in the electrolytic cell and the adsorption process on the catalysts was maintained for 15 min. Excess CO in the solution was eliminated with high purity N2 before the stripping test. All electrochemical measurements were performed at room temperature. 3. Results and discussion Table 1 lists the contents of Pt, Cu and Mn in the as-prepared
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Table 1 Summary on the metal contents of the as-prepared catalysts in ICPeOES analysis. The samples for analysis were prepared by calcining the catalyst at 700 C in air, and then dissolving the residue with aqua regia at room temperature for 24 h and diluting with double distilled water. Catalyst
Element
Content (wt.%)
MnA-PtCu/C
Pt Cu Mn Pt Cu
9.3 4.5 6.5 102 11.2 1.8
PtCu/C
Fig. 1. XRD patterns of MnA-PtCu/C and PtCu/C.
catalysts. The practical Pt loading is 9.3 wt.% for MnA-PtCu/C with a Cu/Pt atomic ratio of 1.48:1 and 11.2 wt.% for PtCu/C with the Cu/Pt atomic ratio of 0.49:1, respectively. The Cu/Pt atomic ratio in the MnA-PtCu/C material is relatively high compared with that of numerous other dealloyed PtCu materials reported previously [10,11,21,24,25], suggesting that the Mn species-tuned etching processes using a precursor with Pt-rich outer layers, is an effective
approach to the synthesis of Pt-based nanocatalysts with a high content of non-noble metal. The Mn content in the MnA-PtCu/C material was determined to be 0.065 wt.% which is close to the error of the spectrometer, suggesting that Mn was removed completely after etching. The XRD patterns of MnA-PtCu/C and PtCu/C are shown in Fig. 1. The diffraction peak located at ca. 25.0 is assigned to the (002) plane of the hexagonal structure of carbon. Characteristic peaks for metal of the two catalysts are in the location between those of standard Pt and Cu, indicating that PtCu nanoparticles were crystallized in the face-centered cubic (fcc) phase along with lattice contraction relative to pure Pt. The peak ratio between Pt (111) and C (002) is higher on MnA-PtCu/C than on PtCu/C, which may be ascribed to high crystallinity and a high metal content in MnAPtCu/C. Diffraction peaks related to Mn are not found in the MnAPtCu/C catalytic material, in agreement with the result from the ICPeOES analysis. The morphology and particle size distribution of the asprepared MnA-PtCu/C and PtCu/C catalysts are displayed in Fig. 2. As shown in Fig. 2A1 and 2A2, most of the metal nanoparticles are uniformly dispersed on the carbon support surface. Based on the statistics of 800 particles for each catalyst, the average particle size of metal nanoparticles is around 3.2 nm for MnA-PtCu/C and 3.1 nm for PtCu/C as depicted in Fig. 2B1 and 2B2, respectively. The average particle sizes of the two materials are similar, thus performance difference attributed to particle size effect is negligible. Aside from single spherical particles, some particles were observed to consist of two or more spheres for both catalysts, as shown in Fig. 2C and D. These irregular particles possess atomic steps and concave areas on their surfaces, which were typically related to high-index facets and demonstrated to be highly active for MOR [14,27]. To reveal the origin of this structure, PtMn-decorated Cu particles before acid leaching were examined by TEM as shown in Fig. 3A. Numerous metal particle linkages were observed due to the extremely high Cu content in the precursor. Consequently, some particles maintained their consecutive structure during acid leaching, resulting in the morphology observed in high-magnification TEM images shown in Fig. 2C and D. The morphology of (MnCu)/C was also analyzed by TEM. However, metal particles were not found in this material. A
Fig. 2. The TEM images and the corresponding particle size distribution of MnA-PtCu/C (A1eD1) and PtCu/C (A2eD2).
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Fig. 4. XPS spectra of MnA-PtCu/C, PtCu/C and Pt/C (JM) in Pt 4f and Cu 2p regions.
Fig. 3. The TEM images of (A) PtMnCu/C precursor before acid leaching and (B) (MnCu)/C material.
typical TEM image of (MnCu)/C is shown in Fig. 3B. This phenomenon suggests that the Mn and Cu components were fully dissolved and removed from the carbon substrate after acid leaching, which also confirms the fact that only Pt atoms are able to prevent the Cu component from dissolution. Mn may be able to tune the PtCu etching processes during acid leaching. The XPS analysis was used to determine the electronic structure and valence state distribution of metals in the MnA-PtCu/C and PtCu/C catalysts. As shown in Fig. 4, a pair of asymmetric peaks constitutes the Pt 4f signal. The binding energy (BE) of Pt 4f7/2 (70.9 eV) on MnA-PtCu/C shifts about 0.2 eV toward the negative direction, relative to that of PtCu/C and Pt/C (JM) (71.1 eV), indicating an obvious electron-donating effect from Cu to Pt at a high Cu content in the MnA-PtCu/C catalyst. The high Cu content also causes a more negative Cu 2p peak signal in MnA-PtCu/C compared to PtCu/C. For both MnA-PtCu/C and PtCu/C materials, the Cu 2p peak fitting results indicate that trace amounts of Cu (II) component were found, suggesting that Cu (II) was fully removed by acid leaching. Remaining Cu in the catalysts mainly consisted of Cu (0)
species and some Cu (I) species may also exist in the catalyst. However, Cu (I) is difficult to distinguish from Cu (0) because their binding energies (BEs) are similar [28]. In order to obtain surface information on the etching processes of PtMnCu/C, XPS analysis by sampling aliquots of the reaction solution at various leaching times was conducted. Only the data of the leaching reaction in the first 30 min were listed because there was minimal change in metal contents after this period. At t ¼ 0 min, the peak fitting of the Cu 2p3/2 core level in Fig. 5 shows two BEs of 931.9 and 936.2 eV, which correspond to Cu (0) and Cu (II) species, respectively [29]. In addition, a satellite signal centered at around 944.0 eV is characteristic of Cu (II) species. The formation of abundant Cu (II) species was due to oxidation of Cu on the outer surfaces of particles by ambient air. Simultaneously, Mn (IV) was determined to be the dominant species in the precursor, which was produced from reduction of potassium permanganate by Cu (0). At 10 min, the peaks related to Cu (II) species become very small compared to that of Cu (0), suggesting that there was rapid dissolution of outer Cu by nitric acid. Meanwhile, the transformation from Mn (IV) to Mn (III) occurred simultaneously with Cu dissolution, likely driven by the standard redox potential difference between Mn (IV)/Mn (III) (0.95 V) and Cu (II)/Cu (0) (0.34 V) in an acid medium. As the leaching reaction proceeded, the ratios of Cu (0)/Cu (II) and Mn (III)/Mn (IV) increased gradually as shown in Fig. 6A. However, the atomic percentages of Cu and Mn in the material relative to their initial values at 0 min decreased (Fig. 6B). It is worth noting that the decreasing rate of Mn content is lower than that of Cu. During etching, abundant Cu in particles was dissolved by nitric acid and the Pt atoms on the exterior of particles underwent a process of surface rearrangement, resulting in the formation of nanoparticles with Pt-rich shells and PtCu alloy cores [11,25]. In this study, the existence of Mn species may prevent Pt atom insertion during surface atom rearrangement and keep additional Pt atoms on the exterior of particles. In this case, more Cu can be reserved with protection of Pt during acid leaching, which was confirmed by the ICPeOES analysis. Cyclic voltammograms (CVs) of the MnA-PtCu/C, PtCu/C and Pt/ C (JM) catalysts in 0.5 M H2SO4 solution are presented in Fig. 7A. The current was normalized with the loading of Pt. Anodic currents caused by the dissolution of Cu from the alloy lattice are not observed, revealing that Cu was located inside the nanoparticles or
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Fig. 5. XPS spectra of PtMnCu/C in Cu 2p and Mn 2p regions at the leaching time of 0, 10 and 30 min.
fully alloyed with Pt. The background information on the electrochemical processes occurring on the electrode surface such as hydrogen desorption/adsorption in around 0.6 ~ 0.35 V, doublelayer charging and discharging in around 0.35 ~ 0.05 V, surface oxidation and reduction processes in around 0.05e0.60 V, is clearly observed [30,31]. The ECSA was evaluated from hydrogen desorption/adsorption or carbon monoxide stripping peaks using the following formula:
ECSA ¼
Q ½Pt C
(1)
where [Pt] is the Pt loading (2 mg) in the electrode, Q is the charge for hydrogen desorption/adsorption or carbon monoxide oxidation (mC), and C equals to 210 mC cm2 for hydrogen conversion and to 420 mC cm2 for carbon monoxide stripping, respectively [32]. The ECSA calculated from hydrogen desorption/adsorption regions is 98.9, 68.1 and 52.6 m2 g1 for the MnA-PtCu/C, PtCu/C and Pt/C (JM) catalysts, respectively, suggesting that Mn species-tuned etching induced the formation of numerous Pt active sites. The surface oxidation/reduction at around 0.15 V was synchronously enhanced on MnA-PtCu/C, on the basis of its largest ECSA. This oxidation process is crucial to MOR because the oxygenated species generated on the catalyst surface facilitate the removal of strongly adsorbed intermediates via a bifunctional mechanism [33]. Methanol oxidation on the three catalysts was performed by cycling the electrode in 0.5 M H2SO4 þ 1 M CH3OH solution at a scan rate of 50 mV s1, as shown in Fig. 7B. It is widely accepted that the oxidation peak at the forward scan is associated with the electrooxidation of methanol to CH2OH, CHOH, COH, CO, HCHO and
HCOOH, etc, [34] while the peak at the backward scan is attributed to the reactivation of oxidized Pt [35]. The MnA-PtCu/C and PtCu/C catalysts have the mass activities of around 1.69 and 0.98 mA mg1 in terms of the forward peak current densities, respectively, which are 2.3e4.0 higher compared to that of Pt/C (JM) (0.42 mA mg1). The area activities relative to ECSA are 1.71, 1.44 and 0.80 mA cm2 for MnA-PtCu/C, PtCu/C and Pt/C (JM), respectively. The difference in area activities between MnA-PtCu/C and PtCu/C possibly originates from the different electronic structures and/or compressive strain [24,36]. These results also indicate that more active sites on MnA-PtCu/C and PtCu/C have significant contribution to their higher mass activities. The ratios between forward and backward peak currents (If/Ib) are 1.08, 1.06 and 0.80 for MnA-PtCu/C, PtCu/C and Pt/C (JM), respectively. This suggests that Cu in the catalysts can adjust the catalytic selectivity of MOR or enhance oxidative removal of residual intermediate species formed during methanol oxidation [37]. The constant potential tests in methanol solution were used to further evaluate the catalytic durability and tolerance toward strongly adsorbed intermediate poisoning. As shown in Fig. 7C, the initial stage within around 300 s showed that the currents for all catalysts fell quickly, which is attributed to accumulation of strongly adsorbed carbonaceous intermediates produced from methanol dehydrogenation on the catalyst surface. The surface catalyzed reactions subsequently reached an apparent steady state where the competitive adsorption of oxygenated species and carbonaceous species remained at a balance [38e40]. The MnAPtCu/C catalyst has the highest initial (0.75 mA mg1) and final (0.39 mA mg1) currents among the three catalysts, indicating that it exhibits the best catalytic durability. Tafel plots of the three
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Fig. 6. Surface valence state and content of Cu and Mn in PtMnCu/C as functions of the leaching time at 0, 10 and 30 min.
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catalysts were recorded at the scan rate of 2 mV s1 in methanol solution, as presented in Fig. 7D. The onset potential for MOR determined from the downward cusp is similar for all three catalysts. The curves from 0.13 to 0.10 V can be divided into two regions: the low potential region (0.13 V < E < 0.00 V) can be assigned to methanol adsorption and dehydrogenation processes, whereas the high potential region (0.00 V < E < 0.10 V) involves oxidative removal of strongly adsorbed species [41]. Linearly fitting these two regions shows Tafel slopes which are determined to be 37.4, 45.3 and 47.6 mV dec1 in the low potential region, and 211.7, 204.5 and 442.5 mV dec1 in the high potential region, for MnAPtCu/C, PtCu/C and Pt/C (JM), respectively. The similar Tafel slopes on MnA-PtCu/C and PtCu/C in the high potential region indicate their close catalytic rates on oxidative removal of strongly adsorbed species, higher than that of Pt/C. The dramatic slope change for all catalysts from low to high potentials indicates that the removal of strongly adsorbed species becomes the dominant processes for MOR. The MnA-PtCu/C catalyst has low slopes in the two potential regions, suggesting fast dehydrogenation and removal of strongly adsorbed species on its surface during MOR. Removal of strongly adsorbed species is a critical step in MOR, and CO is a species typically adsorbed strongly during MOR. Thus, the tolerance toward CO stripping is necessary to be evaluated. As shown in Fig. 8A, in comparison with commercial Pt/C catalyst, the PtCu/C catalyst exhibits slightly lower onset and peak potentials at 0.07 and 0.15 V for CO oxidation, respectively, suggesting that the addition of Cu in the catalyst promoted its anti-poisoning ability. However, higher content of Cu in MnA-PtCu/C leads to a slightly positive shift (0.01 V) on the peak potential compared to Pt/C, which does not benefit its tolerance towards CO poisoning. Meanwhile, a small amount of CO was oxidized at low potential (0.00 V) on MnA-PtCu/C. The ECSA calculated from CO stripping curves is 96.2, 66.7 and 60.1 m2 g1 for MnA-PtCu/C, PtCu/C and Pt/ C (JM), respectively. The result is in accordance with the calculated ECSA derived from hydrogen desorption/adsorption regions. Stability of the three catalysts was investigated by a long-term CV test in the N2-saturated sulfuric acid solution. Evolution of the ECSA
Fig. 7. Cyclic voltammograms of the MnA-PtCu/C, PtCu/C and Pt/C (JM) catalysts in (A) 0.5 M H2SO4 and (B) 0.5 M H2SO4 þ 1 M CH3OH solution. Scan rate: 50 mV s1. (C) Currentetime curves at 0.1 V and (D) Tafel plots of the catalysts at the scan rate of 2 mV s1 in 0.5 M H2SO4 þ 1 M CH3OH solution.
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types of catalytic reactions. Acknowledgments The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. 623313). References
Fig. 8. (A) CO stripping curves on the catalysts in H2SO4 solution; (B) Pt ECSAs of the MnA-PtCu/C, PtCu/C and Pt/C electrodes as functions of the number of CV cycles in N2saturated 0.5 M H2SO4. Inset in (B) shows the corresponding normalized ECSAs. Scan rate: 50 mV s1.
with the cycle number was plotted in Fig. 8B. It is found that the stability of the MnA-PtCu/C catalyst was not enhanced because the normalized ECSA drops to 28.0% of its biggest ECSA, slightly lower than the values of PtCu/C and Pt/C (32.4%). However, the MnA-PtCu/ C catalyst still keeps the largest ECSA than the other two catalysts during test. 4. Conclusions In summary, we have demonstrated a straightforward chemical etching approach by selectively etching less active Cu from PtCu precursors with Pt-rich outer layers, to the synthesis of small, highly dispersed PtCu alloy nanoparticles. The introduction of manganese species during synthesis allowed more Pt atoms to reside on the surfaces of nanoparticles, and simultaneously reserving more Cu component in the catalyst. The MnA-PtCu/C catalyst showed features of large ECSA, abundant surface steps and concave areas, modification in electronic structure and enhanced surface adsorption for oxygenated species. On the basis of the synergistic effect from these aspects, the MnA-PtCu/C catalyst exhibited significantly enhanced catalytic activity and durability toward MOR. The stability of MnA-PtCu/C was not obviously enhanced, which needs further improvement in future. The newly developed strategy in this study can also be applied to fabrication of other binary metal nanoparticles with large active areas for various
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Manganese-tuned chemical etching of a platinumecopper nanocatalyst with platinum-rich surfaces Y.Y. Huang, T.S. Zhao*, G. Zhao, X.H. Yan, K. Xu Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
h i g h l i g h t s PtCu particles with Pt-rich surfaces were used as precursors for etching. Manganese-tuned etching induced the formation of abundant Pt active sites. MOR activity of MnA-PtCu/C was ca. 4.0 times higher than that of Pt/C. Catalytic durability of MnA-PtCu/C towards MOR was improved significantly.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2015 Received in revised form 30 October 2015 Accepted 9 November 2015 Available online xxx
This work presents a modified chemical etching strategy to fabricate binary metal nanocatalysts with large active areas. The strategy employs PtCu alloy particles with Pt-rich outer layers as the precursor and manganese species to manipulate the acid leaching processes. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy techniques are used to analyze the catalyst structures and the tuning mechanism of manganese species during etching. It is found that the introduction of manganese species allows more Pt active sites to be formed onto the catalyst surface after etching, possibly due to reduction in the number of Pt atoms enclosed inside particles. The electrochemically active surface area of the synthetic MnA-PtCu/C catalyst increases by 90% relative to commercial Pt/C catalyst. As a result of the increase in active areas and the additional promotion effects by Cu, the MnAPtCu/C catalyst reveals a methanol oxidation activity 1.7 and 4.0 times higher than that of the synthetic PtCu/C and commercial Pt/C catalysts, respectively. © 2015 Elsevier B.V. All rights reserved.
Keywords: Fuel cells Methanol oxidation Catalyst Etching Platinumecopper Manganese
1. Introduction Carbon-supported Pt nanoparticles are currently the most widely used nanocatalysts in proton exchange membrane fuel cells (PEMFCs) [1e5]. These catalysts, however, are not only expensive, but have limited applications due to low natural reservation of Pt. Alloying Pt with other low-cost transition metals, e.g. Ni, Fe, Cu, Sn and Pb, was demonstrated as an effective approach to simultaneously reduce Pt loading and enhance catalytic activity [6e9]. Among these transition metals, Cu stands out for its ability to combine with Pt toward enhanced oxygen reduction reaction (ORR) for the cathode of PEMFCs, and its ease of promoting catalytic activities of Pt anodes toward electro-oxidation of small organic
* Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2015.11.038 0378-7753/© 2015 Elsevier B.V. All rights reserved.
molecules such as methanol, ethanol and formic acid [10e13]. Enhancement in catalytic properties of PtCu materials was generally attributed to modification in physical properties, e.g. electronic effects (ligand effect), geometric effects (compressive strain), surface roughness (facets and steps) and particle size effect [14e16]. Various bottom-up approaches, such as impregnation reduction, electrodeposition and hydrothermal reduction, were used to fabricate binary Pt-based nanocatalysts [17e20]. However, these approaches usually involve an excessive use of organic solvents and surfactants. Additionally, catalyst composition is generally difficult to control, particularly for the preparation of alloy nanoparticles with a high content of non-noble metals (over 50 at.%). Numerous recent studies have used dealloying of Pt-based materials, through which one component is selectively etched from the binary/ternary alloy, for the preparation of binary metal nanoparticles. Dealloyed materials usually possess the desirable characteristic of Pt-rich
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shells and nanoporous structure which contains concave areas [21e23]. For example, PtCu coreeshell nanoparticles were fabricated via electrochemically dealloying a bulk PteCu binary alloy by Ge's group [24]. They found that the dealloyed nanoporous PtCu material exhibited enhanced electrocatalytic activities toward formic acid oxidation reaction (FAOR) and ORR via a mechanism of tailoring the compressive strain of metal particles. Qiu et al. prepared a nanoporous PtCu catalyst with high Cu content via a onestep chemical dealloying process of bulk Pt4Cu21Mn75 precursor in an (NH4)2SO4 aqueous solution [25]. They revealed that the dealloying processes to the formation of coreeshell nanoparticles involved fast dissolution of Mn and slow dissolution of Cu. In comparison with electrochemical dealloying, chemical dealloying possesses the unique advantage of scalable production. With that being said, however, it has been commonly reported that dealloyed PtCu materials showed similar or smaller electrochemically active surface areas in comparison with commercial Pt/ C material. An investigation of the synthesis found that the Ptcontaining precursors for dealloying were compositionally uniform materials. For these uniform precursors, platinum may partially remain inside the particles even after dealloying, leading to a loss of Pt active sites. Thus, one solution is to use the precursors with Pt-rich outer layers for etching. This proposal is also supported by the fact that the driving force from the redox potential difference is easily powerful enough to reduce Pt ions and deposit them onto Cu surfaces. In addition, the PtCu etching processes are easily further optimized by introducing a third species on the precursor surface. To the best of our knowledge, however, very little exploration on this aspect has been conducted so far. In this work, Mn was selected to optimize the etching processes of PtCu precursor because it has a high redox potential at high valence, which makes it easy to deposit together with Pt on Cu surfaces through galvanic reactions. The deposited Mn oxides can be readily removed in an acid medium with reducing species. The PtCu composite can be prepared through this Mn-assisted etching approach, and the product is defined as MnA-PtCu/C. An initial Cu/ Pt atomic ratio was set as 50:1 in order to facilitate fragmentation of the precursor [10], maximizing the effective electrochemically active surface area (ECSA). The surface analysis of the precursor during the etching processes was collected by X-ray photoelectron spectroscopy; a possible tuning mechanism of Mn toward Pt atom assembly on the particle surfaces was proposed. Methanol oxidation reaction (MOR) was used to test the catalytic performance of the PtCu nanoparticles. 2. Experimental 2.1. Synthesis of catalysts Active carbon (Vulcan XC-72R) was pretreated with 5 M HNO3 at 110 C for 6 h. The mixture was subsequently neutralized with sodium hydroxide, filtered, rinsed with double distilled water and dried at 60 C for 24 h. Acid treating was used to produce surface oxygen-containing groups such as carbonyl, carboxyl and hydroxyl groups, which provide anchoring sites for metal particle deposition [26]. The MnA-PtCu/C alloy catalyst was prepared via surface substitution and acid leaching. During synthesis, 30 ml of 93.4 mM CuCl2 solution was diluted with 270 ml of double distilled water. 90 mg of the pretreated Vulcan XC-72R carbon was dispersed in 300 ml of double distilled water under ultrasonic stirring for 30 min; then 300 mg of NaBH4 was magnetically stirred with the carbon suspension. The CuCl2 solution was then dropped into the carbon/NaBH4 mixture and the reduction reaction was conducted for 1 h. The resultant suspension was filtered and rinsed to remove any residual NaBH4; then the filter residue was rapidly dispersed in
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200 ml of double distilled water again. 100 ml solution containing 3 ml of potassium permanganate (18.9 mM) and 3 ml of chloroplatinic acid (18.9 mM) was then added into this suspension. The galvanic substitution reactions between Mn (VII), Pt (IV) and Cu (0) were conducted for 2 h to obtain a PtMnCu/C precursor. Then, 2 ml of concentrated nitric acid (~65 wt.%) was slowly added into the mixture and the acid leaching reaction was conducted for 18 h to ensure complete dissolution of redundant metals. Finally, the mixture was filtered, rinsed with double distilled water and dried overnight at 60 C in air. Other two materials were prepared for comparison via the same method without the addition of potassium permanganate or chloroplatinic acid, and are defined as PtCu/ C and (MnCu)/C, respectively. 2.2. Physical and electrochemical characterization The contents of Pt, Mn and Cu in the catalytic materials were determined using an Ultima2 inductively coupled plasma optical emission spectroscopy (ICPeOES, Jobin Yvon). For the sample preparation, the carbon in the materials was removed at 700 C in air. Any residue was dissolved by aqua regia at room temperature for 24 h; the solution was transferred to a centrifuge and diluted with double distilled water for analysis. X-ray power diffraction (XRD) was conducted with a Philip X'Pert Pro MPP diffractometer using a Cu Ka (l ¼ 1.54 Å) radiation source. The morphology, dispersed state and size distribution of the metal particles were characterized using a JEOL JEM-2010 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The chemical valences of Pt, Mn and Cu in the materials during and after synthesis were analyzed by X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250) with an Al Ka X-ray source at 1487 eV. The chamber pressure was kept below 3 1010 mbar during testing and specific correction was conducted the C 1s binding energy of 285 eV. Electrochemical measurements were performed using a CHI604 electrochemical work station (CH Instrument Inc.). The reference electrode was the Hg/Hg2SO4/0.5 M H2SO4 electrode (MMS) (0.62 V vs. SHE) and the counter electrode was the platinum net. A glassy carbon (0.1256 cm2) electrode covered by the catalytic material was used as the working electrode. For Pt/C working electrode preparation, 5 mg of the Pt/C (JM) catalyst (10 wt.% Pt) was ultrasonically stirred in 1 ml solution containing 50 ml of 5 wt.% Nafion solution (DuPont, USA) and 950 ml of ethanol. 4 ml of the slurry was coated on the polished glassy carbon electrode surface and the electrode was dried at room temperature in air for 30 min. The MnA-PtCu/C and PtCu/C working electrodes were prepared in the same way. The amount of other catalysts for preparation of the slurry depended upon their practical Pt loading, determined by ICPeOES analysis. A total Pt loading for each catalytic material on the electrode was set at 2 mg. Prior to taking measurements, the 0.5 M H2SO4 and 0.5 M H2SO4 þ 1 M CH3OH electrolytes were first deaerated with high purity N2. The working electrodes were pretreated in the 0.5 M H2SO4 solution by cyclic voltammetry (CV) at a scan rate of 50 mV s1 for several cycles in order to obtain a stable electrochemical response. CO stripping experiments were conducted using the following procedure: an H2SO4 solution was first deaerated with high purity N2. Subsequently, CO was admitted into the solution in the electrolytic cell and the adsorption process on the catalysts was maintained for 15 min. Excess CO in the solution was eliminated with high purity N2 before the stripping test. All electrochemical measurements were performed at room temperature. 3. Results and discussion Table 1 lists the contents of Pt, Cu and Mn in the as-prepared
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Table 1 Summary on the metal contents of the as-prepared catalysts in ICPeOES analysis. The samples for analysis were prepared by calcining the catalyst at 700 C in air, and then dissolving the residue with aqua regia at room temperature for 24 h and diluting with double distilled water. Catalyst
Element
Content (wt.%)
MnA-PtCu/C
Pt Cu Mn Pt Cu
9.3 4.5 6.5 102 11.2 1.8
PtCu/C
Fig. 1. XRD patterns of MnA-PtCu/C and PtCu/C.
catalysts. The practical Pt loading is 9.3 wt.% for MnA-PtCu/C with a Cu/Pt atomic ratio of 1.48:1 and 11.2 wt.% for PtCu/C with the Cu/Pt atomic ratio of 0.49:1, respectively. The Cu/Pt atomic ratio in the MnA-PtCu/C material is relatively high compared with that of numerous other dealloyed PtCu materials reported previously [10,11,21,24,25], suggesting that the Mn species-tuned etching processes using a precursor with Pt-rich outer layers, is an effective
approach to the synthesis of Pt-based nanocatalysts with a high content of non-noble metal. The Mn content in the MnA-PtCu/C material was determined to be 0.065 wt.% which is close to the error of the spectrometer, suggesting that Mn was removed completely after etching. The XRD patterns of MnA-PtCu/C and PtCu/C are shown in Fig. 1. The diffraction peak located at ca. 25.0 is assigned to the (002) plane of the hexagonal structure of carbon. Characteristic peaks for metal of the two catalysts are in the location between those of standard Pt and Cu, indicating that PtCu nanoparticles were crystallized in the face-centered cubic (fcc) phase along with lattice contraction relative to pure Pt. The peak ratio between Pt (111) and C (002) is higher on MnA-PtCu/C than on PtCu/C, which may be ascribed to high crystallinity and a high metal content in MnAPtCu/C. Diffraction peaks related to Mn are not found in the MnAPtCu/C catalytic material, in agreement with the result from the ICPeOES analysis. The morphology and particle size distribution of the asprepared MnA-PtCu/C and PtCu/C catalysts are displayed in Fig. 2. As shown in Fig. 2A1 and 2A2, most of the metal nanoparticles are uniformly dispersed on the carbon support surface. Based on the statistics of 800 particles for each catalyst, the average particle size of metal nanoparticles is around 3.2 nm for MnA-PtCu/C and 3.1 nm for PtCu/C as depicted in Fig. 2B1 and 2B2, respectively. The average particle sizes of the two materials are similar, thus performance difference attributed to particle size effect is negligible. Aside from single spherical particles, some particles were observed to consist of two or more spheres for both catalysts, as shown in Fig. 2C and D. These irregular particles possess atomic steps and concave areas on their surfaces, which were typically related to high-index facets and demonstrated to be highly active for MOR [14,27]. To reveal the origin of this structure, PtMn-decorated Cu particles before acid leaching were examined by TEM as shown in Fig. 3A. Numerous metal particle linkages were observed due to the extremely high Cu content in the precursor. Consequently, some particles maintained their consecutive structure during acid leaching, resulting in the morphology observed in high-magnification TEM images shown in Fig. 2C and D. The morphology of (MnCu)/C was also analyzed by TEM. However, metal particles were not found in this material. A
Fig. 2. The TEM images and the corresponding particle size distribution of MnA-PtCu/C (A1eD1) and PtCu/C (A2eD2).
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Fig. 4. XPS spectra of MnA-PtCu/C, PtCu/C and Pt/C (JM) in Pt 4f and Cu 2p regions.
Fig. 3. The TEM images of (A) PtMnCu/C precursor before acid leaching and (B) (MnCu)/C material.
typical TEM image of (MnCu)/C is shown in Fig. 3B. This phenomenon suggests that the Mn and Cu components were fully dissolved and removed from the carbon substrate after acid leaching, which also confirms the fact that only Pt atoms are able to prevent the Cu component from dissolution. Mn may be able to tune the PtCu etching processes during acid leaching. The XPS analysis was used to determine the electronic structure and valence state distribution of metals in the MnA-PtCu/C and PtCu/C catalysts. As shown in Fig. 4, a pair of asymmetric peaks constitutes the Pt 4f signal. The binding energy (BE) of Pt 4f7/2 (70.9 eV) on MnA-PtCu/C shifts about 0.2 eV toward the negative direction, relative to that of PtCu/C and Pt/C (JM) (71.1 eV), indicating an obvious electron-donating effect from Cu to Pt at a high Cu content in the MnA-PtCu/C catalyst. The high Cu content also causes a more negative Cu 2p peak signal in MnA-PtCu/C compared to PtCu/C. For both MnA-PtCu/C and PtCu/C materials, the Cu 2p peak fitting results indicate that trace amounts of Cu (II) component were found, suggesting that Cu (II) was fully removed by acid leaching. Remaining Cu in the catalysts mainly consisted of Cu (0)
species and some Cu (I) species may also exist in the catalyst. However, Cu (I) is difficult to distinguish from Cu (0) because their binding energies (BEs) are similar [28]. In order to obtain surface information on the etching processes of PtMnCu/C, XPS analysis by sampling aliquots of the reaction solution at various leaching times was conducted. Only the data of the leaching reaction in the first 30 min were listed because there was minimal change in metal contents after this period. At t ¼ 0 min, the peak fitting of the Cu 2p3/2 core level in Fig. 5 shows two BEs of 931.9 and 936.2 eV, which correspond to Cu (0) and Cu (II) species, respectively [29]. In addition, a satellite signal centered at around 944.0 eV is characteristic of Cu (II) species. The formation of abundant Cu (II) species was due to oxidation of Cu on the outer surfaces of particles by ambient air. Simultaneously, Mn (IV) was determined to be the dominant species in the precursor, which was produced from reduction of potassium permanganate by Cu (0). At 10 min, the peaks related to Cu (II) species become very small compared to that of Cu (0), suggesting that there was rapid dissolution of outer Cu by nitric acid. Meanwhile, the transformation from Mn (IV) to Mn (III) occurred simultaneously with Cu dissolution, likely driven by the standard redox potential difference between Mn (IV)/Mn (III) (0.95 V) and Cu (II)/Cu (0) (0.34 V) in an acid medium. As the leaching reaction proceeded, the ratios of Cu (0)/Cu (II) and Mn (III)/Mn (IV) increased gradually as shown in Fig. 6A. However, the atomic percentages of Cu and Mn in the material relative to their initial values at 0 min decreased (Fig. 6B). It is worth noting that the decreasing rate of Mn content is lower than that of Cu. During etching, abundant Cu in particles was dissolved by nitric acid and the Pt atoms on the exterior of particles underwent a process of surface rearrangement, resulting in the formation of nanoparticles with Pt-rich shells and PtCu alloy cores [11,25]. In this study, the existence of Mn species may prevent Pt atom insertion during surface atom rearrangement and keep additional Pt atoms on the exterior of particles. In this case, more Cu can be reserved with protection of Pt during acid leaching, which was confirmed by the ICPeOES analysis. Cyclic voltammograms (CVs) of the MnA-PtCu/C, PtCu/C and Pt/ C (JM) catalysts in 0.5 M H2SO4 solution are presented in Fig. 7A. The current was normalized with the loading of Pt. Anodic currents caused by the dissolution of Cu from the alloy lattice are not observed, revealing that Cu was located inside the nanoparticles or
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Fig. 5. XPS spectra of PtMnCu/C in Cu 2p and Mn 2p regions at the leaching time of 0, 10 and 30 min.
fully alloyed with Pt. The background information on the electrochemical processes occurring on the electrode surface such as hydrogen desorption/adsorption in around 0.6 ~ 0.35 V, doublelayer charging and discharging in around 0.35 ~ 0.05 V, surface oxidation and reduction processes in around 0.05e0.60 V, is clearly observed [30,31]. The ECSA was evaluated from hydrogen desorption/adsorption or carbon monoxide stripping peaks using the following formula:
ECSA ¼
Q ½Pt C
(1)
where [Pt] is the Pt loading (2 mg) in the electrode, Q is the charge for hydrogen desorption/adsorption or carbon monoxide oxidation (mC), and C equals to 210 mC cm2 for hydrogen conversion and to 420 mC cm2 for carbon monoxide stripping, respectively [32]. The ECSA calculated from hydrogen desorption/adsorption regions is 98.9, 68.1 and 52.6 m2 g1 for the MnA-PtCu/C, PtCu/C and Pt/C (JM) catalysts, respectively, suggesting that Mn species-tuned etching induced the formation of numerous Pt active sites. The surface oxidation/reduction at around 0.15 V was synchronously enhanced on MnA-PtCu/C, on the basis of its largest ECSA. This oxidation process is crucial to MOR because the oxygenated species generated on the catalyst surface facilitate the removal of strongly adsorbed intermediates via a bifunctional mechanism [33]. Methanol oxidation on the three catalysts was performed by cycling the electrode in 0.5 M H2SO4 þ 1 M CH3OH solution at a scan rate of 50 mV s1, as shown in Fig. 7B. It is widely accepted that the oxidation peak at the forward scan is associated with the electrooxidation of methanol to CH2OH, CHOH, COH, CO, HCHO and
HCOOH, etc, [34] while the peak at the backward scan is attributed to the reactivation of oxidized Pt [35]. The MnA-PtCu/C and PtCu/C catalysts have the mass activities of around 1.69 and 0.98 mA mg1 in terms of the forward peak current densities, respectively, which are 2.3e4.0 higher compared to that of Pt/C (JM) (0.42 mA mg1). The area activities relative to ECSA are 1.71, 1.44 and 0.80 mA cm2 for MnA-PtCu/C, PtCu/C and Pt/C (JM), respectively. The difference in area activities between MnA-PtCu/C and PtCu/C possibly originates from the different electronic structures and/or compressive strain [24,36]. These results also indicate that more active sites on MnA-PtCu/C and PtCu/C have significant contribution to their higher mass activities. The ratios between forward and backward peak currents (If/Ib) are 1.08, 1.06 and 0.80 for MnA-PtCu/C, PtCu/C and Pt/C (JM), respectively. This suggests that Cu in the catalysts can adjust the catalytic selectivity of MOR or enhance oxidative removal of residual intermediate species formed during methanol oxidation [37]. The constant potential tests in methanol solution were used to further evaluate the catalytic durability and tolerance toward strongly adsorbed intermediate poisoning. As shown in Fig. 7C, the initial stage within around 300 s showed that the currents for all catalysts fell quickly, which is attributed to accumulation of strongly adsorbed carbonaceous intermediates produced from methanol dehydrogenation on the catalyst surface. The surface catalyzed reactions subsequently reached an apparent steady state where the competitive adsorption of oxygenated species and carbonaceous species remained at a balance [38e40]. The MnAPtCu/C catalyst has the highest initial (0.75 mA mg1) and final (0.39 mA mg1) currents among the three catalysts, indicating that it exhibits the best catalytic durability. Tafel plots of the three
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Fig. 6. Surface valence state and content of Cu and Mn in PtMnCu/C as functions of the leaching time at 0, 10 and 30 min.
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catalysts were recorded at the scan rate of 2 mV s1 in methanol solution, as presented in Fig. 7D. The onset potential for MOR determined from the downward cusp is similar for all three catalysts. The curves from 0.13 to 0.10 V can be divided into two regions: the low potential region (0.13 V < E < 0.00 V) can be assigned to methanol adsorption and dehydrogenation processes, whereas the high potential region (0.00 V < E < 0.10 V) involves oxidative removal of strongly adsorbed species [41]. Linearly fitting these two regions shows Tafel slopes which are determined to be 37.4, 45.3 and 47.6 mV dec1 in the low potential region, and 211.7, 204.5 and 442.5 mV dec1 in the high potential region, for MnAPtCu/C, PtCu/C and Pt/C (JM), respectively. The similar Tafel slopes on MnA-PtCu/C and PtCu/C in the high potential region indicate their close catalytic rates on oxidative removal of strongly adsorbed species, higher than that of Pt/C. The dramatic slope change for all catalysts from low to high potentials indicates that the removal of strongly adsorbed species becomes the dominant processes for MOR. The MnA-PtCu/C catalyst has low slopes in the two potential regions, suggesting fast dehydrogenation and removal of strongly adsorbed species on its surface during MOR. Removal of strongly adsorbed species is a critical step in MOR, and CO is a species typically adsorbed strongly during MOR. Thus, the tolerance toward CO stripping is necessary to be evaluated. As shown in Fig. 8A, in comparison with commercial Pt/C catalyst, the PtCu/C catalyst exhibits slightly lower onset and peak potentials at 0.07 and 0.15 V for CO oxidation, respectively, suggesting that the addition of Cu in the catalyst promoted its anti-poisoning ability. However, higher content of Cu in MnA-PtCu/C leads to a slightly positive shift (0.01 V) on the peak potential compared to Pt/C, which does not benefit its tolerance towards CO poisoning. Meanwhile, a small amount of CO was oxidized at low potential (0.00 V) on MnA-PtCu/C. The ECSA calculated from CO stripping curves is 96.2, 66.7 and 60.1 m2 g1 for MnA-PtCu/C, PtCu/C and Pt/ C (JM), respectively. The result is in accordance with the calculated ECSA derived from hydrogen desorption/adsorption regions. Stability of the three catalysts was investigated by a long-term CV test in the N2-saturated sulfuric acid solution. Evolution of the ECSA
Fig. 7. Cyclic voltammograms of the MnA-PtCu/C, PtCu/C and Pt/C (JM) catalysts in (A) 0.5 M H2SO4 and (B) 0.5 M H2SO4 þ 1 M CH3OH solution. Scan rate: 50 mV s1. (C) Currentetime curves at 0.1 V and (D) Tafel plots of the catalysts at the scan rate of 2 mV s1 in 0.5 M H2SO4 þ 1 M CH3OH solution.
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types of catalytic reactions. Acknowledgments The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. 623313). References
Fig. 8. (A) CO stripping curves on the catalysts in H2SO4 solution; (B) Pt ECSAs of the MnA-PtCu/C, PtCu/C and Pt/C electrodes as functions of the number of CV cycles in N2saturated 0.5 M H2SO4. Inset in (B) shows the corresponding normalized ECSAs. Scan rate: 50 mV s1.
with the cycle number was plotted in Fig. 8B. It is found that the stability of the MnA-PtCu/C catalyst was not enhanced because the normalized ECSA drops to 28.0% of its biggest ECSA, slightly lower than the values of PtCu/C and Pt/C (32.4%). However, the MnA-PtCu/ C catalyst still keeps the largest ECSA than the other two catalysts during test. 4. Conclusions In summary, we have demonstrated a straightforward chemical etching approach by selectively etching less active Cu from PtCu precursors with Pt-rich outer layers, to the synthesis of small, highly dispersed PtCu alloy nanoparticles. The introduction of manganese species during synthesis allowed more Pt atoms to reside on the surfaces of nanoparticles, and simultaneously reserving more Cu component in the catalyst. The MnA-PtCu/C catalyst showed features of large ECSA, abundant surface steps and concave areas, modification in electronic structure and enhanced surface adsorption for oxygenated species. On the basis of the synergistic effect from these aspects, the MnA-PtCu/C catalyst exhibited significantly enhanced catalytic activity and durability toward MOR. The stability of MnA-PtCu/C was not obviously enhanced, which needs further improvement in future. The newly developed strategy in this study can also be applied to fabrication of other binary metal nanoparticles with large active areas for various
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