Trans. Nonferrous Met. Soc. China 28(2018) 676-686

Facile synthesis of 3D nanoporous Pd/Co₂O₃ composites with enhanced catalytic performance for methanol oxidation

Yan-yan SONG^1,2,3, Dong DUAN^1,2,3, Wen-yu SHI^1,2,3, Hai-yang WANG^1,2,3, Zhan-bo SUN^1,2,3

1. School of Science, Xi’an Jiaotong University, Xi’an 710049, China;

2. Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter of Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China;

3. Key Laboratory of Shaanxi for Advanced Functional Materials and Mesoscopic Physics, Xi’an Jiaotong University, Xi’an 710049, China

Received 8 November 2016; accepted 27 May 2017

Abstract: To simultaneously reduce noble metal Pd usage and enhance electrocatalytic performance for methanol oxidation, Pd/Co₂O₃ composites with ultrafine three-dimensional (3D) nanoporous structures were designed and synthesized by simple one-step dealloying of a melt-spun Al-Pd-Co alloy with an alkaline solution. Their electrocatalytic activity in alkaline media was determined by a Versa-STAT MC workstation. The results indicate that the typical sizes of the ligaments and pores of the composites were approximately 8-9 nm. The Co₂O₃ was uniformly distributed on the Pd ligament surface. Among the as-prepared samples, the nanoporous Pd/Co₂O₃ composite generated from dealloying of the Al_84.5Pd₁₅Co_0.5 alloy had the best electrocatalytic activity, and its activity was enhanced by approximately 230% compared with the nanoporous Pd from dealloying of Al₈₅Pd₁₅. The improvement of the electrocatalytic performance was mainly attributed to the electronic modification effect between Pd and Co as well as the bifunctional mechanism between Pd and Co₂O₃.

Key words: nanoporous Pd/Co₂O₃; dealloying; Al-Pd-Co alloy; electrocatalytic performance; methanol oxidation

1 Introduction

Recently, nanoporous (np) noble metals, such as  np Pt [1,2] and np Au [3,4], have attracted considerable attention due to their large surface area, ultralow density, cost reduction, excellent electrical and thermal conductivities, and catalytic recyclability [5,6], which hold great promise for electrocatalysis [7-9], sensing [10,11], energy storage devices [12], and fuel cells [13]. The dealloying method in previous reports has been demonstrated to be a powerful and facile method to prepare nanoporous metallic materials [14-16]. During this process, the reactive atomic content in source alloys can be controlled to be a suitable level, and then, the source alloys can be etched through chemical or electrochemical methods to selectively dissolve one or more components from the alloy, leading to the formation of a bicontinuous open nanoporous structure [6,17,18].

It is well known that catalysis at the electrode is a surface phenomenon and that the catalytic activity depends on the dispersion or exposed percentage of active atoms in the catalyst [19]. Therefore, the design of hybrids that contain more active atoms on the outer surface of the material is being pursued to make the most efficient use of noble metals [20]. For now, loading noble metal nanoparticles on different transition metal oxide supporting materials is a conventional synthesis  strategy [21,22]. Unfortunately, these noble metal-based catalysts are prone to noble metal particle agglomeration and their stability becomes poor during long-term catalysis. Therefore, catalysts are generally subjected to a poison that is generated from carbon monoxide adsorption during the catalytic process, resulting in lower utilization of noble metals and reduction of the catalytic efficiency [23]. Moreover, oxide supports, such as semiconductors, will lead to a performance decline due to the reduction of the conductivity of the electrocatalyst.

However, what would happen if nanoporous noble metals are used as supporting materials and transition metal oxides are used as promoters to decrease the utilization of noble metals? Nanoporous noble metals with three-dimensional (3D) bicontinuous inter- penetrating structures have high structural stability with no agglomeration. This feature allows nanoporous noble metal supporting materials to not only minimize the amount of noble metal used, reduce the cost and result in full utilization of noble metals, but also enable the maximum exposure of active sites, leading to absorption of small organic molecules in the electrolyte, providing a short transportation path for electrons and ions, and generating faster reaction kinetics and higher electrocatalytic reactivity [24]. Furthermore, noble metals that have outstanding electron conductivity can serve as electronic conductive supports and facilitate electron transfer, which would lead to a high electrocatalytic performance. If a transition metal oxide is used as a promoter on the surface of a np noble metal, oxygen-containing species (OH_ads) can form more easily, so the interaction between the np noble metal and transition metal oxide would be more efficient. In a series of previous studies, we have demonstrated that composites designed for the above strategy possess outstanding catalytic activity, including the Ag-MO₂ (M=Ce, Ti, Zr) catalyst [25-27].

Among the np noble metals, nanoporous Pd is of special interest due to its relative abundance in nature, low cost, and good resistance to CO poisoning, which has caused it to have been widely used as a superior catalyst for a variety of research fields ranging from organic synthesis and fuel cells to environmental protection [28-30]. Thus, in the present work, 3D np-Pd/Co₂O₃ composites with a controllable composition were fabricated through one-step dealloying of a ternary Al-Pd-Co alloy in an alkaline solution, which has not been previously reported.

2 Experimental

2.1 Preparation of nanoporous composites

Al-Pd-Co alloys were produced by arc-melting pure elements (Al: 99.9%, Pd: 99.9%, Co: 99.9%) in a high-frequency induction furnace in a pure argon atmosphere. This process yielded alloys with nominal compositions of Al_85-xPd₁₅Co_x (x=0, 0.3, 0.5, 0.7, 1). From the alloys, ribbons with a thickness of 30 μm and width of 3 mm were fabricated using the melt-spinning method under a pure argon atmosphere as reported in our previous work [31]. Ribbon dealloying was performed in an aqueous 20% (mass fraction) NaOH solution at 60 °C for 48 h. The dealloyed ribbons were rinsed with distilled water, dried at 50 °C in a drying oven, and then stored in a vacuum chamber prior to further testing. Note that the reaction rate increased with increasing concentrations of NaOH solution, but excessive concentrations resulted in an undesirable coarsening of the ligaments [32]. Experiments have shown that 20% (mass fraction) NaOH can be used to dealloy Al-Pd based alloys.

2.2 Characterization

The structures, morphologies and compositions of all samples were characterized by a Bruker D8 advanced X-ray diffractometer (XRD), JEM-2100 high-resolution transmission electron microscope (HRTEM, JEOL Ltd.), Fei Titan Themis 200 TEM, a JSM-7000F scanning electron microscope (SEM, JEOL Ltd.) equipped with an energy disperse spectroscope (EDS), and axis Ulra Kratos X-ray photoelectron spectrometer (XPS) using an Al K_α X-ray source (1486.68 eV).

2.3 Electrochemical measurements

A Versa-STAT MC workstation with a classical three-electrode cell was used for the electrochemical measurements. The preparation process of the working electrode was as follows: First, 2 mg of a finely-ground dealloyed sample and 0.5 mg of an acetylene black were dispersed in a mixing solution of 200 μL of isopropanol and 200 μL of Nafion (0.5%, mass fraction) to form an evenly distributed catalyst suspension liquid by sonication. Then, 2 μL of the suspension liquid was spread on a polished glassy carbon electrode (GC) that had a geometric area of 0.071 cm² (which acted as the working electrode), and finally, the electrode was dried at room temperature in air. A platinum net (1 cm²) and Ag/AgCl (saturated KCl) electrode served as the counter electrode and reference electrode, respectively. The electrocatalytic activity and stability of the dealloyed samples for methanol oxidation were measured in 0.5 mol/L CH₃OH and 0.5 mol/L KOH. The electrolyte solutions were deaerated with ultrahigh purity nitrogen for 30 min prior to any measurement. All of the electrochemical experiments were conducted at ambient temperature.

3 Results and discussion

3.1 Microstructural characterization of np-Pd/Co₂O₃ composites

The XRD patterns of the dealloyed Al-Pd and the Al-Pd-Co samples are presented in Fig. 1. It can be seen that only the diffraction peaks of fcc-Pd (PDF #46-1043) were detected in the dealloyed Al-Pd sample, and no additional diffraction peaks related to Al (or its oxides) were observed. The pattern of the dealloyed Al-Pd-Co was similar to that of the dealloyed Al-Pd sample, and the diffraction peaks of pure Co or its oxides could   not be readily detected. The broadness of the diffraction peaks in all the samples may arise from the large surface strain that formed during dealloying because of the smaller ligament size and smaller grain size.

Fig. 1 XRD patterns of dealloyed Al_85-xPd₁₅Co_x (x=0, 0.5, 1) ribbons

Figure 2 illustrates representative SEM and EDS images of the dealloyed Al-Pd and Al-Pd-Co samples. From the plane-section and cross-section images, as shown in Figs. 2(a) and (b), a typical bicontinuous nanoporous structure with ultrafine nanoscale ligaments and open pores was successfully generated, and interconnected hollow channels extended throughout the sample, indicating that the dealloyed Al-Pd sample had a 3D nanoporous structure. After adding a certain amount of Co to the precursor alloy, as shown in Figs. 2(c)-(f), the micromorphology and structure of the dealloyed Al-Pd-Co samples maintained an excellent 3D nanoporous structure. However, with the addition of a very small amount of Co, the nanoporous structure of the dealloyed Al_84.5Pd₁₅Co_0.5 sample exhibited a refinement phenomenon (Figs. 2(c) and (d)) compared with the dealloyed Al-Pd. This may lead to a larger electrochemically-active surface area, and may produce more active sites for the electrocatalytic process. As the amount of Co is further increased (Figs. 2(e) and (f)), the nanoporous structure of the dealloyed Al₈₄Pd₁₅Co₁ becomes coarser.

Fig. 2 SEM images showing typical microstructures of dealloyed ribbons

The typical EDS pattern and resulting data of the dealloyed Al-Pd-Co samples are shown in Fig. 3 and Table 1. The results indicate that the dealloyed ribbons were mainly composed of Pd, Co, O and residual Al. Most of the Al atoms were removed during the dealloying process. The Pd and Co contents (experimental values) were basically consistent with the initial feeding ratios of Pd to Co in the ternary Al-Pd-Co alloy (theoretical values). This illustrates that the composition of the resulting sample was well controlled by the dealloying method. Considering that the Co content is low in the composites, the related diffraction peaks are hard to be detected by XRD.

Fig. 3 Typical EDS pattern of dealloyed Al₈₄Pd₁₅Co₁ ribbon

The BET surface areas and mean pore diameters of the dealloyed Al_85-xPd₁₅Co_x (x=0, 0.3, 0.5, 0.7, 1) samples are provided in Table 2. It can be seen that with the addition of elemental Co to the precursor alloy, the surface areas and pore volumes of the dealloyed Al-Pd-Co samples became noticeably larger and their mean pore diameters became distinctly smaller than those of the dealloyed Al₈₅Pd₁₅ sample. Furthermore, it should be noted that when the Co content is too great (x＞0.5), the surface areas and pore volumes of the dealloyed Al-Pd-Co samples will be reduced and their pore sizes will be increased. That is to say, the nanoporous structures of the dealloyed Al_84.3Pd₁₅Co_0.7 and Al₈₄Pd₁₅Co₁ samples present a coarsening trend, which is consistent with the SEM images (Fig. 2). This represents an adverse phenomenon for catalytic activity toward methanol oxidation. Therefore, the dealloyed Al_84.5Pd₁₅Co_0.5 sample had the best morphology.

The TEM and HRTEM images and corresponding selected area electron diffraction (SAED) pattern of the dealloyed Al_84.5Pd₁₅Co_0.5 sample, as representative of a typical pattern for the dealloyed Al-Pd-Co samples, are shown in Fig. 4. The results demonstrate that the ligaments of the dealloyed sample interconnect with each other and that a bicontinuous ligament-channel structure is generated, which is in agreement with the SEM observation. The SAED and HRTEM images indicate that the samples possess a polycrystalline structure that is composed of randomly oriented fcc Pd and that one of the lattice fringe spacings is 0.224 nm, which is consistent with the distance of the dominant (111) planes for Pd. The typical sizes of the ligaments and pores are approximately 8-9 nm.

Table 1 EDS data results of dealloyed Al_85-xPd₁₅Co_x (x= 0.3, 0.5, 0.7, 1) ribbons

Table 2 BET surface areas and mean pore diameters of dealloyed Al_85-xPd₁₅Co_x (x=0, 0.3, 0.5, 0.7, 1) ribbons

The elemental distribution of the nanoporous structure from the dealloyed Al_84.5Pd₁₅Co_0.5 sample is provided by TEM-EDS mapping. From Fig. 5, the dealloyed samples are mainly composed of Pd, Co and  O. By combining Fig. 4 and Fig. 5, the results indicate that the ligaments in the nanoporous structure are composed of Pd atoms, as shown in Fig. 5(b) and that the Co and O atoms are uniformly distributed on the ligament surfaces of the nanoporous structure, as shown in Figs. 5(c) and 5(d).

Fig. 4 TEM image of dealloyed Al_84.5Pd₁₅Co_0.5 ribbons (a), corresponding SAED pattern (b), and HRTEM image (c)

Fig. 5 TEM image (a) of dealloyed Al_84.5Pd₁₅Co_0.5 ribbons and corresponding EDS element mapping of Pd (b), Co (c) and O (d)

The surface compositions of the dealloyed Al-Pd-Co ribbons were determined by XPS. The spectra of Pd 3d and Co 2p are shown in Fig. 6, and their related splitting spectra were also collected to determine the valence states of Pd and Co. Both Pd and PdO peaks are present on the surface of the dealloyed sample, as seen in Fig. 6(a). The Pd 3d_5/2 peak at 335.4 eV and 3d_3/2 peak at 340.4 eV are attributed to metallic Pd (Pd⁰) [33,34]. The doublet peaks located at 336.3 and 342.4 eV are assigned to PdO (Pd²⁺) according to Refs. [35,36], indicating that a small amount of PdO is generated during the dealloying process. Compared with the Pd 3d spectrum of the dealloyed Al₈₅Pd₁₅ [31], the Pd 3d spectrum of the dealloyed Al_84.5Pd₁₅Co_0.5 ribbons presents a slightly negative shift to a lower binding energy. In addition, it should be noted that the form of the Co atoms on the ligament surfaces of the samples is Co₂O₃ and no metallic Co (Co⁰) is found, as shown in Fig. 6(b). This is due to the fact that the primary peaks located at 781.3 eV and 797.1 eV in the Co 2p orbital belong to Co 2p_3/2 and Co 2p_1/2 of Co₂O₃, respectively, according to Ref. [37]. The peaks located at 787.6 eV and 804.8 eV are their corresponding satellite signals.

The above results reveal that np-Pd/Co₂O₃ composites were fabricated by simple one-step dealloying of the ternary Al-Pd-Co precursor alloy. The composites possess a 3D nanoporous structure, and their compositions can be controlled well. The formation mechanism is shown in Fig. 7. Because Al is leached away, the remaining Pd and Co atoms can interdiffuse at the solution/solid interface and self-assemble to form 3D bicontinuous nanoporous structures. Meanwhile, highly active Co atoms undergo natural oxidation in aqueous solutions over an extended period of time (48 h) to form Co₂O₃. The formation process of Co₂O₃ is similar to that reported in Ref. [38]. As a result, 3D np-Pd/Co₂O₃ samples are formed.

3.2 Electrocatalytic performance of np-Pd/Co₂O₃ composites

For convenience, the dealloyed Al₈₅Pd₁₅, Al_84.7Pd₁₅- Co_0.3, Al_84.5Pd₁₅Co_0.5, Al_84.3Pd₁₅Co_0.7 and Al₈₄Pd₁₅Co₁ ribbons are designated as np-Pd, np-Pd/Co₂O₃-1, np-Pd/Co₂O₃-2, np-Pd/Co₂O₃-3 and np-Pd/Co₂O₃-4, respectively.

As seen in Fig. 8, background cyclic voltammograms (CVs) of the samples were collected in a 0.5 mol/L KOH solution at a scan rate of 20 mV/s. The typical CV profiles of the np-Pd/Co₂O₃ composites are similar to those of np-Pd [31]. Pd oxides are formed on the surface of the composites at the high potential end during the positive sweep, leading to a reduction of the characteristic peak at approximately -0.37 V during the negative scan (Fig. 8(a)) [39,40]. The electrochemically  active surface area (EASA) of a noble metal is usually used to evaluate the number of catalytically-active sites. Generally, the EASA of Pd or Pd-based catalysts can be estimated according to the following equation 1 [41]:

                                   (1)

where Q_s is the coulombic charge (μC) obtained by integrating the current peak of Pd oxide reduction, Q_c is the conversion factor and is commonly taken as 405 μC/cm², and m is the mass (mg) of Pd. Though the measurement of EASA in alkaline media may deviate from the true value because complex oxide multilayer rather than a monolayer is present on the surface of Pd, it can be used when comparing a series under identical conditions. The EASAs of np-Pd, np-Pd/Co₂O₃-1, np-Pd/Co₂O₃-2, np-Pd/Co₂O₃-3 and np-Pd/Co₂O₃-4 composites are 52.0, 56.2, 78.9, 72.9 and 65.1 m²/g, respectively, as shown in Fig. 8(b). The EASA gradually increases with the increase in Co₂O₃ content and then decreases with a further increase in Co₂O₃ content. The EASA value of the np-Pd/Co₂O₃ composites is all higher than that of the np-Pd.

Fig. 6 XPS spectra of Pd 3d (a) and Co 2p (b) of dealloyed Al_84.5Pd₁₅Co_0.5 ribbons

Fig. 7 Formation mechanism of 3D np-Pd/Co₂O₃ composites

Figure 9 shows the CVs of np-Pd/Co₂O₃ and np-Pd in 0.5 mol/L KOH + 0.5 mol/L methanol. The oxidation peak can be assigned to the oxidation of a freshly- chemisorbed species that comes from methanol adsorption. The peak current densities (J_p) of methanol oxidation are regarded as primary parameters to evaluate the electrocatalytic activities of the catalysts [42]. The catalytic activity of the composites is evaluated by the mass-normalized current density, which intuitively reflects the synergistic effect between Co₂O₃ and Pd, as shown in Fig. 9(a). The calculated mass activities of the np-Pd/Co₂O₃-1, np-Pd/Co₂O₃-2, np-Pd/Co₂O₃-3 and np-Pd/Co₂O₃-4 composites reach 150.3, 227.2, 207.2 and 195.8 mA/mg, respectively, as shown in Fig. 9(b). By comparison, these activities are 2.2, 3.3, 3.0 and 2.8 times that of np-Pd (69.7 mA/mg), respectively. Thus, all of the composites have higher electrocatalytic performances than np-Pd, and np-Pd/Co₂O₃-2 has the best activity for methanol oxidation. Simultaneously, the EASA normalized current density is also examined for all samples to further compare their intrinsic activity, as shown in Fig. 9(c). The of the np-Pd/Co₂O₃ composites in the methanol solution is at least twice as large as that of np-Pd, as shown in Fig. 9(d). In addition, regarding the onset potential (φ_op), np-Pd/Co₂O₃ composites possess more negative φ_op values compared with np-Pd. The values of φ_op for np-Pd/Co₂O₃ composites are similar (approximately -0.55 V), which is approximately 0.2 V less than that of np-Pd (approximately -0.35 V), as indicated by the arrows in Fig. 9. This result demonstrates that the np-Pd/Co₂O₃ composites require a lower potential than np-Pd catalysts to perform methanol oxidation, that is, electrocatalytic methanol oxidation occurs more easily [43].

Fig. 8 CV curves of np-Pd and np-Pd/Co₂O₃ composites in 0.5 mol/L KOH solution at scan rate of 20 mV/s (a) and corresponding calculated EASA values of samples (b)

Fig. 9 CV curves of np-Pd and np-Pd/Co₂O₃ composites in 0.5 mol/L KOH + 0.5 mol/L methanol solution at scan rate of 20 mV/s

The long-term stability of electrocatalysts for methanol oxidation is further investigated according to the chronopotentiometric (CA) curves in a solution of  0.5 mol/L KOH + 0.5 mol/L CH₃OH at -0.14 V vs Ag/AgCl. Figure 10 shows the CA curves normalized by the equivalent mass of Pd (J ^mass) and EASA (J ^EASA) for the samples. The initial current density drops according to the carbonaceous intermediates generated on the surface of the electrode during methanol oxidation [44]. The CA curves indicate that all of the np-Pd/Co₂O₃ composites are more durable than np-Pd, which can be attributed to the minimization of the accumulation of poisoning species on the Pd active sites by the addition of Co₂O₃. In sum, the np-Pd/Co₂O₃-2 composite has the optimum electrocatalytic performance and stability for methanol oxidation, which means that the np-Pd/Co₂O₃-2 composite achieves a balance between catalytic activity and poison resistance when the appropriate form of Co₂O₃ is used. Additionally, the order of the stabilities of the samples for methanol oxidation is in good agreement with the CV results.

Fig. 10 Chronopotentiometric curves of 0.5 mol/L methanol in 0.5 mol/L KOH solution on np-Pd/Co₂O₃ and np-Pd samples  at potential of -0.14 V vs Ag/AgCl

The transportation characteristics of the np-Pd/ Co₂O₃-2 composite were investigated by varying the scan rate in 0.5 mol/L KOH and 0.5 mol/L CH₃OH solutions, and the results are shown in Fig. 11. J ^mass increases with the increasing scan rate, as shown in Fig. 11(a), indicating that the electrocatalytic oxidation of methanol on the composites may be a diffusion- controlled process. The relationship between J ^mass and v^1/2 is linear (Fig. 11(b)), confirming that the electrocatalytic oxidation of methanol on np-Pd/Co₂O₃-2 is a diffusion-controlled process [45,46]. Additionally, the peak potential (φ_p) increases with the increase of v, and a linear relationship is obtained between φ_p and ln v, as shown in Fig. 11(c). This indicates that the oxidation of methanol on the np-Pd/Co₂O₃-2 composite is an irreversible electrode process [41].

Fig. 11 CV curves of 0.5 mol/L methanol in 0.5 mol/L KOH solution on np-Pd/Co₂O₃-2 composite

According to the experimental results, np-Pd/Co₂O₃ composites have excellent electrochemical activity, long-term stability and low φ_op. The primary reasons that the components have these properties can be rationalized as follows. First, an electronic modification effect is present between Pd and Co. After adding Co to the precursor alloy, the XPS peaks of Pd 3d undergo a slightly negative shift to lower binding energies compared with those of np-Pd (Fig. 6). The negative  shift of the Pd peaks in the composites is due to the small electron transfer from Co to Pd because the electronegativity of Co (1.88) is much smaller than that of Pd (2.2) [47], which indicates that the addition of Co to the precursor alloy modifies the electronic structure of Pd and produces an electronic modification effect between Pd and Co. In addition, electron transfer can influence the d-band density of Pd. According to previous research [47,48], the Pd d-band center will shift to a lower energy after adding Co to Pd, resulting in weaker Pd-adsorbate bonds, that is to say, the adsorption of the adsorbates is weaker at the Pd sites. In this way, Pd poisoning by CO-like species is decreased because these species are more weakly adsorbed on Pd, and more Pd sites are released for continuous methanol oxidation, which leads to enhancement of the activity. Second, there is a bifunctional mechanism between Pd and Co₂O₃. As some intermediates, especially CO, are formed at Pd active sites, some intermediates accumulate on the active sites surface, blocking adsorption of methanol molecules. Therefore, catalytic activation is restricted. The addition of Co to the precursor can effectively relieve the above problem because the Co atoms in the precursor alloy are completely oxidized into Co₂O₃ after undergoing the extensive dealloying process. Co₂O₃ can provide the necessary OH species, which are permanently be available at the Co₂O₃ surface sites. The formation of OH_ads can remove intermediates from the Pd surface by transforming the CO-like poisoning species on the surface of Pd into carbon dioxide or other cleansing products. The bifunctional mechanism occurs as  follows [49-51]:

Co₂O₃+OH^-Co₂O₃-OH_ads+e                                                  (2)

Co₂O₃-OH_ads+Pd-(CO)_ads+OH^-Pd+Co₂O₃+CO₂↑+H₂O+e                   (3)

Co₂O₃-OH_ads+Pd-(CO)_ads+3OH^-Pd++Co₂O₃+2H₂O+e                (4)

Third, the 3D nanoporous structure with more exposed Pd atoms can offer a large amount of active sites in electrolyte solutions; thus, methanol molecules can be adsorbed. The structure is favorable for the unlimited transport of molecules and electron conductivity and also favors the diffusion of intermediates, therefore enhancing the stability of the catalyst and poison resistance. In addition, compared with np-Pd, np-Pd/Co₂O₃ composites have a more refined 3D nanoporous structure and more active sites (Fig. 2, Table 2, Fig. 4 and Fig. 8), which are beneficial to enhancing the electrochemical activity.

With the increase of the Co content in the precursor alloy, the BET surface areas (Table 2) and EASAs   (Fig. 8) of the np-Pd/Co₂O₃ composites made from the dealloyed Al-Pd-Co precursors (with Co existing as Co₂O₃ in composites) become larger than those of np-Pd, which is beneficial to the improvement of the catalytic activity. Furthermore, the electronic modification effects between Pd and Co as well as the synergistic effects between Pd and Co₂O₃ are also favorable for methanol oxidation, as previously mentioned. Thus, as the Co (Co₂O₃) content increases, the electrochemical activity of the composites increases. When the Co molar fraction x in the precursor is 0.5, the electrochemical activity of the np-Pd/Co₂O₃-2 composite made from dealloyed Al_84.5Pd₁₅Co_0.5 alloy reaches its maximum value (Figs. 9 and 10). When the Co content increases too much (x＞0.5) in the precursor alloy, the nanoporous structures of the composites tend to slightly coarsen, resulting in the reduction of the BET surface area (Table 2) and EASA (Fig. 8), which inhibits the enhancement of catalytic activity (Figs. 9 and 10). Moreover, excessive Co₂O₃ may aggregate and cover the active surface of the nanoporous structure. Under those circumstances, methanol molecules are unable to access the active sites located in the interior nanopores. As a result, the best additive content of Co in the Al_85-xPd₁₅Co_x precursor alloy is x=0.5.

4 Conclusions

1) 3D nanoporous Pd/Co₂O₃ composites are fabricated from the ternary Al-Pd-Co precursor alloy by facile one-step dealloying in an alkaline solution, and their composition can be controlled well.

2) All of the composites have electrocatalytic activity and long-term stability for methanol oxidation that is superior to those of np-Pd. Among these composites, the np-Pd/Co₂O₃-2 composite made from dealloying of the Al_84.5Pd₁₅Co_0.5 alloy possesses the best electrocatalytic activity, and its activity is enhanced by approximately 230% compared to np-Pd.

3) Enhancement of the electrocatalytic activity of the composites is due to the 3D nanoporous structure, electronic modification effect between Pd and Co, and bifunctional mechanism between Pd and Co₂O₃.

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对甲醇氧化具有增强效应的三维纳米多孔Pd/Co₂O₃复合材料的简单制备

宋衍滟^1,2,3，段 栋^1,2,3，时文宇^1,2,3，王海洋^1,2,3，孙占波^1,2,3

1. 西安交通大学 理学院，西安 710049；

2. 西安交通大学 教育部物质非平衡合成与调控重点实验室，西安 710049；

3. 西安交通大学 陕西省先进功能材料及介观物理重点实验室，西安 710049

摘  要：为降低贵金钯的使用量同时提高甲醇氧化的催化性能，设计了超细三维纳米多孔(np)Pd/Co₂O₃复合材料，并通过在碱液中对熔体快淬Al-Pd-Co合金带进行简单的一步脱合金法合成。利用Versa-STAT MC工作站检测了所制复合材料在碱液中的甲醇氧化电催化活性。结果表明，所得样品的韧/孔尺寸约为8~9 nm，Co₂O₃均匀分布在Pd韧带表面。其中，通过脱合金Al_84.5Pd₁₅Co_0.5得到的np-Pd/Co₂O₃-2样品性能最佳，与脱合金Al₈₅Pd₁₅所得的np-Pd相比，性能提高约230%。这主要归因于Pd和Co之间存在的电子改性效应及Pd和Co₂O₃之间的双功能机理。

关键词：纳米多孔Pd/Co₂O₃；脱合金；Al-Pd-Co合金；电催化性能；甲醇氧化

(Edited by Bing YANG)

Foundation item: Project (51371135) supported by the National Natural Science Foundation of China

Corresponding author: Zhan-bo SUN; Tel/Fax: +86-29-82665995; E-mail: szb@mail.xjtu.edu.cn

DOI: 10.1016/S1003-6326(18)64700-4