PtPdRh Mesoporous Nanospheres: an Efficient Catalyst for Methanol

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PtPdRh Mesoporous Nanospheres: an Efficient Catalyst for Methanol Electro-Oxidation Kai Deng, You Xu, Chunjie Li, Ziqiang Wang, Hairong Xue, Xiaonian Li, Liang Wang, and Hongjing Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03656 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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PtPdRh Mesoporous Nanospheres: an Efficient Catalyst for Methanol Electro-Oxidation Kai Deng, You Xu,* Chunjie Li, Ziqiang Wang, Hairong Xue, Xiaonian Li, Liang Wang,* and Hongjing Wang* State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, P. R. China

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ABSTRACT: Porous multi-metallic alloyed nanostructures possess unique physical and chemical properties to generate promising potential in fuel cells. However, the controllable synthesis of this kind of materials still remains challenging. Herein, we report a facile method for the one-pot, high-yield synthesis of tri-metallic PtPdRh mesoporous nanospheres (PtPdRh MNs) under mild conditions. The resultant PtPdRh MNs possess the features of uniform shape, a narrow size distribution, plenty of well-defined mesopores, highly open structure, and multi-component effects, which impart advantages such as large surface area, favorable mass diffusion, high utilization of electrocatalysts, and synergy among the various metal components. Benefitting from the synergetic effects originating from the multi-metallic composition and unique mesoporous structure, the as-prepared PtPdRh MNs exhibit remarkably enhanced electrocatalytic performance for methanol oxidation reaction (MOR) relative to bi-metallic PtPd MNs and commercial Pt/C catalyst.

KEYWORDS: tri-metallic PtPdRh; mesoporous materials; nanostructures; electrocatalysts; methanol oxidation reaction


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INTRODUCTION Direct methanol fuel cells (DMFCs) could convert the chemical energy of methanol fuel directly into electricity and thus are considered as a promising energy conversion technology for powering portable electronics.1-9 Up to now, platinum (Pt) is still the most satisfactory catalyst for the anodic methanol oxidation reaction (MOR) in DMFCs owing to its excellent catalytic activity.10-13 Unfortunately, Pt is easily poisoned by COad, which is an intermediate of the MOR.14-17 In order to enhance CO tolerance and decrease Pt consumption, much attention has been paid to alloying Pt with other noble metals like Pd18, Au19, Ag20, Ru21, Rh22 or less expensive 3d-transition metals like Mo23, Fe24, Co25, Ni26-28, or Cu29-30. It is well recognized that the unique strain and electronic effects resulting from these Pt-based alloy systems could improve the catalytic performance.3132

In particular, multi-metallic Pt-based alloys, when compared to mono-metallic or bi-

metallic system counterparts, often exhibit more greatly improved catalytic activity for MOR due to the synergic effect of multi-components.33-40 It has been extensively shown that the MOR is a structure-sensitive reaction on Ptbased catalyst surfaces.41 Previous related studies regarding the dependence of MOR


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performance on particle shape of catalysts have underscored the importance of surface structure control in the design and synthesis of catalysts.42-43 Therefore, controllable synthesis of Pt-based catalyst materials with desire morphology and structure represents another powerful approach for boosting the catalytic performance.44-48 Among various nanostructures, unsupported metallic mesoporous structures can provide higher surface areas and larger pore volumes relative to its solid counterparts.49-51 Inspired by these unique properties and functions, researchers have been making efforts for the development of this field, with various kinds of mesoporous metallic materials have been reported.52-53 Nevertheless, in contrast with mono-metallic or bi-metallic systems, mesoporous Pt-based multi-metallic alloyed nanostructures still remain insufficiently explored.54-55 In generally, the controllable synthesis of multimetallic nanoalloys with desire well-defined mesoporous structure through wet-chemical approach is more difficult to be accomplished compared to their unitary or binary counterparts due to their complicated components and reaction kinetics.56-57 Especially, the synthesis of Rh-containing Pt-based multi-metallic alloy mesoporous nanostructures are particularly challenging because Rh has a higher surface energy compared with


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other noble metals such as Pt, Au, and Pd. Hence, facile and efficient strategies are required for controllable synthesis of Rh-containing Pt-based multi-metallic alloys with well-defined mesoporous structures. Herein, we report a simple yet effective, one-pot chemical-reduction method for the fabrication of tri-metallic PtPdRh mesoporous nanospheres (PtPdRh MNs). The asmade PtPdRh MNs possess the features of uniform shape, a narrow size distribution, plenty of ultra-large mesopores, highly open structure, and multi-component effects, which impart advantages such as large surface areas, favorable mass diffusion, high utilization of electrocatalysts, and synergy among the various metals. Electrochemical investigations revealed that these as-made PtPdRh MNs exhibited enhanced catalytic activity toward MOR over bi-metallic PtPd MNs and commercial Pt/C catalyst due to unique compositional (i.e., muitl-metallic alloy) and geometrical (i.e., mesoporous structure) properties of the material. Moreover, our present approach can be accomplished without the need for any seeds, hard templates and organic solvents, and facilitates the large-scale preparation of catalysts for practical applications.


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EXPERIMENTAL SECTION Materials and chemicals All the chemicals were used as received without further purification. H2PtCl6, K2PtCl4, Na2PdCl4, RhCl3, and Pluronic F127 were purchased from Sigma-Aldrich. Pt/C (20 wt%) catalyst was obtained from Alfa Aesar. L-ascorbic acid (AA) was supplied from Aladdin Industrial Corporation (Shanghai, China). All the aqueous solutions were prepared with deionized water. Synthesis of PtPdRh MNs First, K2PtCl4 solution (1.2 mL, 20.0 mM), H2PtCl6 solution (1.8 mL, 20.0 mM), Na2PdCl4 solution (0.5 mL, 20.0 mM), RhCl3 solution (0.5 mL, 20.0 mM) and F127 (60.0 mg) were mixed together and sonicated to make F127 completely dissolved. Then, AA solution (3.0 mL, 0.1 M) was added, followed by continuously sonicating for 20 min in a 35 ℃ water bath. The final product was collected through centrifugation, washed with ethanol and water for three times. Synthesis of PtPd MNs


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The procedure for the synthesis of PtPd MNs is the same with that of PtPdRh MNs except that there is no RhCl3 solution was added. Characterization Scanning electron microscopy (SEM) imaging was conducted on a ZEISS SUPRA 55 microscopy at 10 kV. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained from a JEOL JEM-2100F microscopy at 200 kV. Highangle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and elemental mapping measurements were carried out on a FEI Tecnai G2 F20 microscope built as an accessory on the JEOL JEM-2100F microscopy. The powder Xray diffraction (XRD) patterns were acquired on an X-ray diffractometer (PANalytical X’Pert PRO) with Cu Ka radiation (λ = 0.1542 nm). Inductively coupled plasma mass spectrometry (ICP-MS) analysis was carried out using an Elan DRC-e instrument. Electrochemical characterization Electrochemical investigations were performed in a conventional three-electrode cell by using an electrochemical workstation (CHI 760E, Chenhua Co., Shanghai, China). A catalyst-modified glassy carbon electrode (GCE, 3 mm in diameter) served as the


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working electrode. The GCE was modified by dropping 5 μL of homogeneous suspension of catalyst onto the electrode (catalyst loading amount: 10 μg) and dried under ambient condition. Then 5 μL of Nafion (0.5 wt%) was added to catalyst-modified GCE and left to fully dried again. A Ag/AgCl (saturated KCl) electrode and a Pt wire were used as the reference electrode and counter electrode, respectively. All potentials, if not specified, were recorded according to reversible hydrogen electrode (RHE) in this work. MOR measurements were performed in a solution of 0.5 M H2SO4 solution containing 1.0 M methanol (CH3OH). Cyclic voltammetry (CV) curves were recorded at a scan rate of 50 mV s-1. Chronoamperometric (CA) measurements were performed at 0.8 V (vs. RHE). The specific and mass activities were obtained by normalizing the MOR currents to the electrochemically active surface area (ECSA) and loading amount of Pt of the catalysts, respectively. RESULTS AND DISCUSSION Figure 1a is the typical SEM image, from which a large number of uniform, welldispersed sphere-like nanostructures can be observed. These nanospheres have a narrow size distribution and the average diameter was determined to be approximately


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85 nm (Figure S1). Upon closer examination by higher magnification SEM image, it was found

that

well-defined

mesoporous

structures

are

deeply

embedded

and

interconnected over the entire of nanospheres (Figure 1b). TEM characterization further confirms the mesoporous spherical shape of the samples, as evidenced in Figure 1c and Figure 1d. Moreover, the pore walls in these PtPdRh MNs are composed of numerous interconnected ultra-small nanoparticle subunits (Figure 1d), which are favorable for providing a high density of active sites for catalytic applications. As shown in the inset of Figure 1c, the selected-area electron diffraction (SAED) pattern indicated that these PtPdRh MNs are polycrystalline. The diffraction rings in the SAED pattern can readily be indexed to the (111), (200), (220) and (311) planes of face-centered cubic (fcc) alloy structure, respectively. HRTEM images (Figure 1e and Figure 1f) taken from the edge part of the typical PtPdRh MNs reveal that the lattice spacing distance is 0.216 nm, corresponding to (111) crystal plane of metallic fcc structure. This is also evidenced by the fast Fourier transform (FFT) in the inset of Figure 1f. The crystalline phases of the resultant products were analyzed by XRD. As shown in Figure 2, the XRD pattern of PtPdRh MNs shows five diffraction peaks


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centered at 2θ values of 39.9, 46.5, 67.8, 81.7, and 86.2, corresponding to the (111), (200), (220), (311), and (222) reflections of fcc PtPdRh alloyed crystal structure, respectively. When compared with standard diffraction patterns of pure Pt (JCPDS no. 65-2868), the corresponding diffraction peaks of PtPdRh MNs show positively slight shift of 2θ values, implying that the partial substitution of larger Pt atoms by smallersized Rh atoms. Moreover, no other diffraction peaks of mono-component Pt, Pd or Rh were observed, indicating the probable formation of ternary alloyed structure. In order to understand the distribution of Pt, Pd and Rh in the MNs, we further performed elemental analysis for a representative MN by using HAADF-STEM. EDX maps obtained during STEM measurements reveal the uniform spatial distribution of Pt, Pd, and Rh elements in the MN (Figure 3). The atomic ratio of Pt/Pd/Rh in the typical PtPdRh MNs was detected to be 6/2/1 according to ICP-MS analysis. These results further imply that the as-obtained PtPdRh MNs are indeed an alloy structure. As for the formation of the mesoporous spherical nanostructures, the presence of F127 with a suitable concentration is believed to play an essential role in such a surfactant-directed synthesis process. Control experiments showed that high-quality


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MNs with uniform shape and size can only be generated in the presence of sufficient F127 under the typical synthetic conditions; while only non-uniform aggregates were obtained without the addition of F127 or any other surfactants (Figure S2a). Moreover, non-porous sphere-shaped nanoparticles were formed when F127 was replaced by Brij 58 (Figure S2b). These results demonstrated the bi-functional roles of F127 as both pore-directing agent and protecting agent in our current synthetic system.58-59 Regarding the tri-metallic composition of the PtPdRh MNs, a balance between nucleation and crystal growth resulting from the precise control of the reduction kinetics of various metal precursors also largely determines the shape and microstructure of the final products. The reduction kinetics of multiple metal precursor species influences by not only their redox potentials but also their complexation effect with surfactant F127.55 Control experiments show that various mono-metallic samples (Pt, Pd) and bi-metallic samples (PtPd, PtRh, PdRh ) with different morphologies can be produced by reducing the corresponding metal precursors using AA (Figure S3 and Figure S5a). In contrast, the reduction of the Rh precursor cannot occur at 35 oC without mixing with a Pt and/or Pd precursor (Figure S4a), while the co-reduction of two-components (Pt- and Rh-


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precursors) and three-components (Pt-, Pd- and Rh-precursors) can be observed from the changes in solution color (Figure S4b and Figure S4c). These results indicate that the preformed Pd/Pt seeds are necessary for inducing/catalyzing the co-reduction synthesis of tri-metallic PtPdRh nanocrystals. As for the Pt precursor species, the combination of K2PtCl4 ([PtCl4]2-/Pt: +0.76 V vs. SHE) with H2PtCl6 [[PtCl6]2-/[PtCl4]2-: +0.68 V vs. SHE] is favorable for the generation of high-quality PtPdRh MNs with an average particle of about 85 nm. When only using K2PtCl4 as Pt precursor, the average particle size of the resultant products was decreased to around 40 nm due to its faster reduction rate that the combination of K2PtCl4 with H2PtCl6 (Figure S6a). In contrast, the average particle size was increased to approximately 105 nm in the case of using only H2PtCl6 as Pt precursor due to lower tendency toward reduction than [PtCl4]2- (Figure S6b). Moreover, the adding amount of Pd precursor and Rh precursor can also affect the final shape and microstructure of the as-formed Pt-based tri-metallic alloy. When the reaction is performed in the absence of Pd, nanoporous structure PtRh particles are generated with parts of non-porous aggregates owing to the slow reduction rate of the reaction (Figure S5a). The synthesis with 0.5 mL of Na2PdCl4 precursor solution could


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generate our typical PtPdRh MNs (Figure 1a). Further increasing the amount of Pd precursor to 1.5 mL could yield mesoporous particles with a wide size distribution (Figure S5b). On the other hand, when the reaction is performed in the absence of Rh, nonuniform particles yielded due to the fast reduction rate of Pt- and Pd-precursors by AA (Figure S3c), indicating that the introduction of Rh3+ ions can optimize the complicated reduction rate and further manipulate reduction kinetics. By varying the amount of Rh precursor, the composition of the PtPdRh NPs can be adjusted without affecting the morphology (Figure S7). Therefore, the precise control of the reduction kinetics of multiple metal precursor species plays a key role in achieving the high-quality PtPdRh MNs. The electrochemical catalytic activity of the typical PtPdRh MNs for the MOR was investigated. For comparison, the performance of bi-metallic PtPd MNs and commercial Pt/C catalyst was also tested. In order to calculate the ECSAs of the electrocatalysts, CV measurements were conducted in N2-purged 0.5 M H2SO4 solution. Figure S8 shows the CVs of the three catalysts, from which the ECSAs were calculated to be 49.6 m2 g-1 for PtPdRh MNs, 48.7 m2 g-1 for PtPd MNs and 50.4 m2 g-1 for Pt/C, respectively.


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With the information in mind, we carried out electrochemical evaluation of MOR on these three catalysts. Figure 4a compared the CV curves of various catalysts for MOR recorded in N2-purged 0.5 M H2SO4 solution containing 1.0 M CH3OH. The specific activities of the catalysts were obtained by normalizing the MOR current to the ECSAs of the catalysts. Typical characteristic peaks of MOR can be observed on these three Pt-based catalysts. In the positive scan, the current peaks are attributed to oxidation of methanol molecules. The current peaks in the reverse scan correspond to further oxidation of the incompletely oxidized carbonaceous species. As shown in Figure 4a, relatively weak oxidation peaks are observed on bi-metallic PtPd MNs and Pt/C catalyst. In contrast, our as-synthesized PtPdRh MNs result in an increase in the activity for the electro-oxidation of methanol compared to PtPd MNs and Pt/C. The onset potential of the forward anodic peak of mesoporous PtPdRh MNs is more negative than that of PtPd MNs and Pt/C (Figure 4b), suggesting the MOR on the PtPdRh MNs is more favorable. As shown in Figure 4d, the peak current density in the positive scan for PtPdRh MNs (1.41 mA cm-2) is 1.6 and 3.3 times higher than PtPd MNs (0.88 mA cm-2) and Pt/C (0.43 mA cm-2), respectively. The peak specific activity of our PtPdRh MN catalysts is


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also higher than some recent reported Pt-based MOR catalysts, such as core-shell Au@Pd@Pt (1.17 mA cm-2)60, dendrites PtPdCu (1.15 mA cm-2)37, nanoporous PtPdCu (1.38 mA cm-2)54, and porous PtPdCu (0.86 mA cm-2)61. For further comparing the intrinsic catalytic activity of various electrocatalysts, the MOR current was normalized with the mass of Pt loading. As shown Figure 4c and Figure 4d, the corresponding forward current peak on PtPdRh MNs (1.56 mA μg-1Pt) is 1.9 and 3.5 folds higher than those of PtPd MNs (0.808 mA μg-1Pt) and Pt/C (0.45 mA μg-1Pt), respectively. The mass activity of PtPdRh MNs is also superior to the PtPd MNs and Pt/C when dividing the current density with overall mass of noble metals (Figure S9). Such a peak mass activity on the PtPdRh MNs is also better than a few other Ptbased MOR catalysts, such as PtPdCu nanowires (1.50 mA μg-1Pt)34, PtPdCu dendrites(0.69 mA μg-1Pt)37, nanoporous PtPdCu (0.43 mA μg-1Pt)54, concave PtPdCu (0.53 mA μg-1Pt)62, and porous PtPdCu (0.85 mA μg-1Pt)61. The electrocatalytic stability of the PtPdRh MNs was further evaluated by accelerated durability test (ADT). After cycling the potential between 0 and 1.2 V (vs. RHE) in 0.5 M H2SO4 solution for 1,000 cycles, the typical PtPdRh MNs show 21%


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degradation in their initial ECSA compared to 60% degradation of commercial Pt/C (Figure 5a and Figure S10a), revealing a better durability for PtPdRh MNs. Similarly, the catalytic stability of PtPdRh MNs for MOR is also carried out in a 0.5 M H2SO4 solution containing 1.0 M CH3OH. The PtPdRh MNs reserve 71% of their initial mass activities compared to 36% of commercial Pt/C (Figure 5b, Figure 5c and Figure S10b). Furthermore, the chronoamperometry measurements for 3600s shows that the as-made PtPdRh MNs exhibit a higher specific current density and a slower current decay than that of commercial Pt/C catalyst during the entire process (Figure 5d). These results suggest an enhanced catalytic durability and stability for MOR, which is mainly attributed to excellent structural stability. The enhanced electrocatalytic performance of the PtPdRh MNs toward OER may be ascribed to their tri-metallic component and well-defined mesoporous structure (Figure 6). On the one hand, the introduction of Pd and Rh components into Pt structure can modify the electronic structure of Pt atoms and the electronic effect originated from the synergistic effect between Pt, Pd and Rh favors enhanced intrinsic MOR activity. On the other hand, the well-fined mesoporous structure and stable architectonics could


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expose abundant catalytic active centers, facilitate the diffusion of reactant and product species, and effectively suppresses the agglomeration of the particles, leading to improved catalytic stability and durability. CONCLUSIONS In summary, we have successfully synthesized ternary PtPdRh MNs with controllable composition and morphology by means of a simple, one-pot chemical-reduction strategy. The introduction of F127 surfactant with a suitable concentration and the precise control of the reduction kinetics of various metal precursors are essential for the high-yield synthesis of PtPdRh MNs. In comparison with bi-metallic PtPd MN counterparts and commercial Pt/C, these as-synthesized PtPdRh MNs show higher electrochemical activities toward MOR. The enhanced MOR performance can be attributed to the synergetic effects of the tri-metallic alloyed composition and unique mesoporous structure. This work should open a new avenue to synthesize highly active porous Pt-based multi-metallic alloy materials for various catalytic applications.


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ASSOCIATED CONTENT

Supporting Information Extra characterization results and electrochemical data (Figure S1-Figure S10). AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21601154, 21776255, 21701141), and Natural Science Foundation of Zhejiang Province (No. LQ18B010005), and China Postdoctoral Science Foundation (No. 2018M632500).


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Figure 1. (a-b) SEM and (c-d) TEM images of the PtPdRh MNs. The inset in (c) is the SAED patterns of the PtPdRh MNs. (e) HRTEM image of the edge part of a single PtPdRh MN. (f) The lattice fringes in the square area in (e). The inset in (f) displays the corresponding FFT pattern.


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Figure 2. XRD pattern of the PtPdRh MNs. Inset image shows the slow scanning XRD profile at 2θ =80-85o


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Figure 3. (a) HAADF-STEM and (b-d) elemental mapping images of a single PtPdRh MN.


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Figure 4. (a) Specific activities, (b) linear-sweep voltammograms and (c) mass activities of MOR recorded in 0.5 M H2SO4 with 1 M CH3OH at a scanning rate of 50 mV s-1. (d) Comparisons of the specific and mass activities at 0.9 V.


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Figure 5. CV curves of PtPdRh MNs before and after the durability tests in (a) 0.5 M H2SO4 and (b) 0.5 M H2SO4 with 1 M CH3OH, respectively, at a scanning rate of 50 mV s-1. (c) Comparisons of mass activities at 0.9 V. (d) Chronoamperometric curves for PtPdRh MNs and commercial Pt/C.


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Figure 6. Schematic illustration of the compositional and structural advantages of the PtPdPh MN catalyst for MOR.


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PtPdRh Mesoporous Nanospheres: an Efficient Catalyst for Methanol

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