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ethanol oxidation
Materials Chemistry and Physics xxx (2015) 1e9
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Nanoporous Pd/TiO2 composites prepared by one-step dealloying and their electrocatalytic performance for methanol/ethanol oxidation Yanyan Song, Caihua Wei, Xiaolong Zhang, Xin Wei, Xiaoping Song, Zhanbo Sun* School of Science, Key Laboratory of Shaanxi for Advanced Functional Materials and Mesoscopic Physics, Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, PR China
h i g h l i g h t s Novel nanoporous Pd/TiO2 composites are directly prepared from the melt-spun AlePdeTi ribbons by a one-step dealloying strategy. The nanoporous Pd/TiO2 composites exhibit a bicontinuous interpenetrating ligament-pore structure. The best electrocatalytic performances of the composites are nearly triple and double as high as that of nanoporous Pd toward methanol and ethanol oxidation, respectively. The triple enhancement is attributed to the synergistic effect between Pd and TiO2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2014 Received in revised form 30 April 2015 Accepted 9 May 2015 Available online xxx
A facile one-step dealloying strategy for the preparation of nanoporous Pd/TiO2 composites from the melt-spun AlePdeTi precursor alloys is reported. The structure analysis reveals that the nanoporous Pd/ TiO2 composites exhibit a bicontinuous interpenetrating ligament-pore structure. The electrochemical measurements show that the composites observably enhance the electrocatalytic performance towards methanol/ethanol oxidation when the Ti content in the precursor alloys is 0.3e0.7 at%. Among these composites, the nanoporous Pd/TiO2 composites dealloyed from Al84.5Pd15Ti0.5 exhibits the best catalytic activity, which is triple and double times for methanol and ethanol oxidation, respectively, compared with nanoporous Pd. The enhancement is attributed to the synergistic effect between Pd and TiO2. However, when the Ti content is 1 at% in the precursor alloys, the catalytic activity will obviously decline. © 2015 Published by Elsevier B.V.
Keywords: Composite materials Nanostructures Chemical synthesis Electrochemical properties
1. Introduction Noble metal nanomaterials with unique physical and chemical properties have received enormous attention due to their high catalytic performance [1,2]. The noble metal nanomaterials with various morphology, such as nanoporous structure [3], nanoparticles [4], nanoplate arrays [5], nanotrees [6], nanowires [7], nanoflowers [8], etc, have been synthesized. In comparison with other nanostructures, the nanoporous (np) structures of the prepared noble metal materials show excellent electrocatalytic performance due to their inherent special morphology and characteristics, such as the open porosity, high specific surface area, high structural stability, superior connectivity, and so on [9e13]. Such morphology and characteristics endow them with the ability
* Corresponding author. E-mail address: [email protected] (Z. Sun).
to interact with atoms, ions and molecules not only at their surfaces, but also throughout the bulk of the materials. In addition, the partitions between particles by the pore structures greatly inhibit particle growth and reduce their aggregation [11e13]. Dealloying is a common process to obtain nanoporous materials, during which an alloy is “parted” by the selective dissolution of the most electrochemically active of its elements. This process results in the formation of the nanoporous structure composed almost entirely of the more noble alloy constituents [14,15]. Recently, np-noble metals, such as Ag, Au, Pd, and Pt have been widely studied [10,16e18]. Among them, np-Pd with high resistance to CO poisoning has aroused increasing interest owing to its superior electrocatalytic activities towards alcohol oxidation and formic acid oxidation [9,19,20]. For instance, Yu et al. [21] synthesized np-Pd through electrochemically dealloying the ternary Pd30Ni50P20 metallic glass and found that the as-prepared np-Pd exhibited the enhanced catalytic activity towards formic acid
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oxidation. Zhang et al. [18] reported that np-Pd obtained by dealloying the AlePd alloys is an excellent electrocatalyst for the oxidation of methanol and formic acid due to its large specific surface area and high catalytic activity. For Pd-based electrocatalysts, transition metal oxides as a kind of effective promoters or appropriate supporting materials have been reported [22e29]. These transition metal oxides which promote Pd electrocatalysts show an improved performance than pure Pd. TiO2 is a noteworthy promoter for noble metal electrocatalysts due to its unique advantages including long-term stability in acidic and alkaline solutions and strong interaction with metals. The strong interaction can improve the activity of the catalysts [27,30e34]. Wang et al. [27] found that Pd supported on TiO2 nanotube composite catalysts displayed a superior methanol oxidation performance compared with pure Pd and other TiO2 substrates in acid medium. Xi et al. [35] reported that Pt/C catalysts with TiO2 exhibit excellent activity toward alcohol oxidation because TiO2 in the catalyst can absorb OHads species and promote the oxidation reaction on the electrode. At present, Pd/TiO2 composites with high electrocatalytic performance are generally synthesized by impregnation and precipitation methods, and can serve as high-efficient anode catalysts for applications in fuel cells [27,36e38]. In most studies, Pd in the composites is loaded on the TiO2 supports. However, these strategies have some shortcomings. In particular, the agglomeration of Pd particles easily occurs on the TiO2 supports and makes the active surface area lower, resulting in a reduction of catalytic efficiency. Moreover, as a semiconductor, TiO2 supports will lead to the performance decline due to the decrease of the electrocatalyst conductivity. In this work, the novel np-Pd/TiO2 composites were successfully prepared by a simple method of one-step dealloying the melt-spun AlePdeTi ribbons in an alkaline solution, and their structures and electrochemical properties were characterized by XRD, SEM, TEM, XPS, CV, CA and EIS. Owing to the enhancement of TiO2, the composites show enhanced electrocatalytic performance towards methanol/ethanol oxidation in alkaline media compared with np-Pd. 2. Experimental 2.1. Preparation of the AlePdeTi precursor alloys
2100 High-Resolution Transmission Electron Microscope (HRTEM, JEOL Ltd.) and a JSM-7000F Scanning Electron Microscope (SEM, JEOL Ltd.) equipped by an Energy Disperse Spectroscopy (EDS). An Axis Ulra Kratos X-ray Photoelectron Spectrometer (XPS) was employed to measure the valence states of Pd and Ti using Al Ka Xray source (1486.68 eV). 2.4. Electrochemical measurements The electrochemical measurements were performed in a standard three-electrode cell using a VersaSTAT MC workstation. The preparation process of the working electrode was described below: 2 mg fine ground dealloyed samples, 0.5 mg acetylene black, 200 mL isopropanol, and 200 mL Nafion solution (0.5 wt%) were ultrasonically mixed. Then, 2 mL of the homogeneously mixed catalyst ink was placed on a freshly polished glassy carbon (GC) electrode with a diameter of 3 mm, and the dealloyed sample loading was kept at 10 mg. The counter electrode was a pure Pt net with an area of 1.0 cm2, and an Ag/AgCl (sat. KCl) electrode was used as the reference electrode. Voltammetric behavior was characterized in a 0.5 M KOH solution, and the electrocatalytic activity measurements were carried out in a 0.5 M KOH þ 0.5 M methanol/ethanol solution. All the measurements were conducted at room temperature under the protection of ultrapure N2. 3. Results and discussion 3.1. Microstructural characterization of the np-Pd/TiO2 composites Fig. 1 shows the XRD patterns of the melt-spun Al85-xPd15Tix (x ¼ 0, 1, at%) ribbons and the corresponding dealloyed samples. The experiments reveal that the diffraction peaks of a-Al and an intermetallic compound Al3Pd, as shown in Fig. 1(a) and (b), can be detected in all ribbons, and the diffraction peaks related to Ti do not appear, suggesting a dissolution of Ti in a-Al and Al3Pd (unknown structure, PDF No. 44-1021). After the ribbons were dealloyed, as shown in Fig. 1(c) and (d), only the diffraction peaks of fcc-Pd (PDF No. 46-1043) can be detected. The microstructure of the dealloyed Al85Pd15 and AlePdeTi ribbons has been observed using SEM, and typical images are shown in Fig. 2. The surface of the dealloyed Al85Pd15 ribbons exhibits an open, bicontinuous and interpenetrating ligament-pore
Al85-xPd15Tix (x ¼ 0, 0.3, 0.5, 0.7, 1.0, at%) alloy ingots with nominal compositions were prepared from Al (99.90%), Pd (99.99%) and Ti (99.99%) by arc-melting in a vacuum arc furnace at an argon atmosphere. And then, the 3 g ingots were reheated to a necessary temperature by high frequency induction and solidified into ribbons by single roller melt spinning at a speed of 33 m s1. The thickness of the melt-spun ribbons was approximately 30 mm and the width was about 3 mm. 2.2. Preparation of the nanoporous composites The melt-spun ternary ribbons were dealloyed in a 20 wt% NaOH solution at room temperature until no obvious bubbles emerged. Subsequently, the dealloying was continuously carried out in the same solution at 60 ± 5 C for 48 h. The dealloyed samples were washed with distilled water and dried at 50 C in a drying oven box. 2.3. Microstructural characterization The phase structures of the melt-spun ribbons and the dealloyed samples were analyzed with a Bruker D8 advanced X-ray Diffractometer (XRD). The microstructures were characterized by a JEM-
Fig. 1. XRD patterns of the Al85Pd15 (a), Al84Pd15Ti1 (b) ribbons, dealloyed Al85Pd15 (c) and Al84Pd15Ti1 (d) ribbons.
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Fig. 2. Plan-view SEM images showing the microstructures of the dealloyed Al85Pd15 (a) and Al84.5Pd15Ti0.5 (c) ribbons. (b) and (d) show the corresponding section-view. Inset in (c) shows the EDS spectrum of the dealloyed AlePdeTi ribbons.
structure, as shown in Fig. 2(a). Where, the surface consists of small concave and convex. This uneven surface should result from corrosion flaw. A large number of pores spread on the uneven surface. The estimated size of most fine pores is less than 10 nm. Fig. 2(b) shows a typical section-view of the dealloyed Al85Pd15 sample. A three-dimensional ultrafine ligament-channel structure can be seen, and the pore channels run throughout the whole ribbons. Fig. 2(c) shows the surface morphology of the dealloyed Al84.5Pd15Ti0.5 ribbons. Compared to that of the dealloyed Al85Pd15 sample, its surface image becomes nebulous. This may be because the surface is covered by the oxide due to the addition of Ti in the precursor alloys. Even so, the ultrafine nanoporous structure surface is still faintly visible. A three-dimensional ultrafine ligamentchannel structure is kept after the addition of Ti, and the bicontinuous interpenetrating ligament-channel structure exhibits a uniform tendency, as shown in Fig. 2(d). The EDS spectrum reveals that the dealloyed AlePdeTi ribbons are mainly composed of Pd, Ti, O and residual Al, as shown in the inset of Fig. 2(c). Additionally, the EDS results of all the as-dealloyed samples are presented in Table 1. The data show that the atomic ratios of Pd: Ti in all the as-dealloyed samples are approximately in agreement with those in the precursor alloys. The less difference results from the inevitable mass loss of Pd or Ti atoms during the dealloying process and the measurement error of the EDS. XPS spectrum for Pd 3d and Ti 2p of the dealloyed Al85-xPd15Tix (x ¼ 0, 0.5, at%) ribbons are shown in Fig. 3. In order to confirm the possible oxidation state of Pd and Ti, the splitting spectrum of Pd 3d
Table 1 The EDS results of the dealloyed Al85-xPd15Tix (x ¼ 0, 0.3, 0.5, 0.7, 1 at%) ribbons. Samples Dealloyed Dealloyed Dealloyed Dealloyed Dealloyed
Al85Pd15 Al84.7Pd15Ti0.3 Al84.5Pd15Ti0.5 Al84.3Pd15Ti0.7 Al84Pd15Ti1
Pd (at%)
Ti (at%)
Residual Al (at%)
91.27 89.41 88.52 86.71 87.25
0 2.06 3.21 4.33 5.46
8.73 8.53 8.27 8.96 7.29
and Ti 2p has been measured. The Pd 3d spectrum of the dealloyed Al85Pd15, as shown in Fig. 3(a), exhibits a low-energy (Pd 3d5/2) peak at ~ 335.5 eV and a high-energy (Pd 3d3/2) peak at ~ 341.0 eV, which is attributed to the metallic Pd according to the literature [39,40]. However, when Ti is added into the precursor alloys, the Pd 3d spectrum of the dealloyed Al84.5Pd15Ti0.5 ribbons presents a slightly negative shift to the lower binding energy. The Pd peaks are located at 335.1 eV (Pd 3d5/2) and 340.5 eV (Pd 3d3/2), which also belong to the metallic Pd (Pd0) species according to the literature [41,42]. It is noticed that the Ti 2p spectrum, as shown in Fig. 3(b), can be detected in the dealloyed AlePdeTi samples, and the doublet peaks occur at 458.4 eV (Ti 2p3/2) and 464.3 eV (Ti 2p1/2), indicating the existence of TiO2 according to the studies [43,44]. No metallic Ti peaks can be measured, which means all the Ti atoms are oxidized into TiO2. It is deduced that TiO2 is in amorphous state or the content is low, resulting in the absence of the diffraction peaks of TiO2 for the dealloyed AlePdeTi samples in Fig. 1. Besides that, it should be indicated that there exist two states of Pd (Fig. 2(a)) in all samples. The Pd 3d spectrum is located at ~336.9 eV and ~342.4 eV, which can be assigned to the PdO (Pd2þ) on the basis of the binding energy (BE), according to the research of the Ref. [40,45]. This means that partial Pd atoms are oxidized to PdO (Pd2þ). Interestingly, the content proportion of PdO is clearly reduced after the addition of Ti into the precursor alloys according to the results of the splitting peak. Detailed information about the microstructures of the dealloyed Al85Pd15 and Al84.5Pd15Ti0.5 ribbons was investigated via TEM. The typical results are shown in Fig. 4. A typical bicontinuous interpenetrating ligament-pore structure clearly exhibits in the dealloyed Al85Pd15 sample, as shown in Fig. 4(a). The size of most pores is less than 10 nm, which agrees with the result of SEM observation (Fig. 2). The corresponding select area electron diffraction (SAED) pattern demonstrates that the connecting ligaments of the nanoporous structure are composed of randomly oriented fcc Pd nanocrystals, as shown in the inset of Fig. 4(a). The SAED pattern can be indexed as Pd (111), (200), (220), (311) and (2 2 2) reflections from the inner ring to the outer one, respectively. The HRTEM image
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Fig. 3. XPS spectra of Pd 3d (a) and Ti 2p (b) for the dealloyed Al85-xPd15Tix (x ¼ 0, 0.5, at%) ribbons.
Fig. 4. TEM and HRTEM images of the dealloyed Al85Pd15 (a, b) and Al84.5Pd15Ti0.5 (c, d) ribbons.
further confirms the ultrafine ligament-channel structure, as shown in Fig. 4(b). The ligaments are composed of intercommunicated and mutually overlapped Pd particles. Moreover, the lattice fringes are well resolved with the lattice spacing calculated to be ~0.224 nm, corresponding to the (111) crystal plane of Pd. A similar nanoporous structure can be observed on the dealloyed Al84.5Pd15Ti0.5 ribbons, as shown in Fig. 4(c). The corresponding SAED pattern is shown in the inset of Fig. 4(c), which also confirms the polycrystalline nature of the connecting ligaments composed of Pd particles. The HRTEM observation demonstrates that the structure and size of ligament-channel and Pd particles don't vary obviously after the addition of Ti into the precursor alloys, as shown in Fig. 4(d). It indicates that although the dealloyed Al84.5Pd15Ti0.5 ribbons are composed of Pd and a small amount of TiO2 (Figs. 2(c) and 3(b)), it is difficult to identify the morphology of phase contained Ti through the TEM observation. The above results reveal that the np-Pd/TiO2 composites have been obtained by dealloying Al85-xPd15Tix (x ¼ 0.3, 0.5, 0.7, 1, at%) precursor alloys. The formation mechanism of the nanoporous Pd
prepared by the dealloying of melt-spun AlePd alloys in alkaline aqueous solution has been extensively studied [9,18,46]. In this report, for the melt-spun AlePdeTi alloy, Al is easy to react with NaOH aqueous solution to produce soluble NaAlO2. As a-Al (Pd, Ti) and Al3Pd (Fig. 1) are decomposed, the released Pd atoms accumulate and reorganize into the nanoporous structure with bicontinuous interpenetrating ligaments/channels. The corresponding chemical reaction equations are described as follows: 2Al þ 2NaOH þ 2H2O ¼ 2NaAlO2 þ 3H2[
(1)
Al3Pd þ NaOH þ H2O / NaAlO2 þ Pd þ H2[
(2)
The reaction rate increases with the content of NaOH increasing in alkali solutions, but much more of NaOH could induce coarsening of ligaments/pores in the nanoporous structure. According to the research of Ref. [46], 20wt% NaOH is the optimal value for dealloying AlePd-based alloys. Moreover, the released highly active Ti atoms from the decomposed a-Al remain in the nanoporous
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structure and can undergo a natural oxidation with aqueous solution under long time condition to form TiO2. This process is similar to the report of the reference [47]. As a result, the np-Pd/TiO2 composites are formed. For convenient description, the dealloyed Al85Pd15, Al84.7Pd15Ti0.3, Al84.5Pd15Ti0.5, Al84.3Pd15Ti0.7 and Al84Pd15Ti1 ribbons are designated as np-Pd, np-Pd/TiO2-1, np-Pd/ TiO2-2, np-Pd/TiO2-3 and np-Pd/TiO2-4, respectively.
3.2. Electrocatalytic performance of the np-Pd/TiO2 composites Fig. 5 shows the cyclic voltammograms (CVs) of the np-Pd/TiO2 composites in a 0.5 M KOH solution at a scan rate of 20 mV s1. The CV of np-Pd is presented simultaneously in Fig. 5 for comparison. For the np-Pd and np-Pd/TiO2 composites, the characters of the CVs are similar, indicating that the introduction of TiO2 has a little effect on the CV character of Pd. The density of charge associated to the reduction of a monolayer for Pd oxides can be used to calculate the accurate electrochemically active specific surface areas (EASA) according to the Ref. [48,49]. In this report, the voltage (E) of the characteristic reduction peak is about 0.35 V in the back scan. The calculated EASA values are 52.0, 80.9, 86.7, 65.5 and 54.4 m2 g1 for np-Pd, np-Pd/TiO2-1, np-Pd/TiO2-2, np-Pd/TiO2-3 and np-Pd/TiO24, respectively. This result reveals that the utilization of Pd in the np-Pd/TiO2 composites is very high. The catalytic activity and the intrinsic activity can be evaluated by mass specific current (MSC) and surface specific current (SSC), respectively. For the Pd-based catalyst, the anodic peak current density (jp) in the forward scan is normally used to evaluate the electrocatalytic activities of catalysts. In addition, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation [50e52]. Fig. 6 shows the CVs of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ0.5 M methanol solution at a scan rate of 20 mV s1 jp (MSC) of the np-Pd/TiO2-1, np-Pd/TiO2-2 and np-Pd/TiO2-3 samples, as shown in Fig. 6(a), is higher than that of np-Pd. Among them, jp (201.8 mA mg1) of np-Pd/TiO2-2 is the highest and nearly 3 times as high as that of np-Pd (69.7 mA mg1). Nevertheless, for np-Pd/TiO2-4, jp is reduced below that of np-Pd. SSCs of the np-Pd/ TiO2 composites, as shown in Fig. 6(b), increase with the increasing TiO2 content but decrease again when jp reaches a maximum value (~0.225 mA cm2). These results indicate that the catalytic activity
Fig. 5. CV curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH solution. The scan rate is 20 mV s1.
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of the np-Pd/TiO2 composites increases obviously due to the addition of appropriate TiO2. Moreover, the inset of Fig. 6(b) shows that the values of the onset potential (Eop) for the np-Pd/TiO2 composites except np-Pd/TiO2-4 are more negative than that of npPd. This variation presents an improvement in the reaction kinetics of the np-Pd/TiO2 composites according to the view of the Ref. [53,54]. The If/Ib ratios of the composites (If/Ib ¼ 3.7, 4.0, 4.2, 5.0 for np-Pd/TiO2-1, np-Pd/TiO2-2 np-Pd/TiO2-3 and np-Pd/TiO2-4, respectively) are higher than that of np-Pd (If/Ib ¼ 3.3), indicating that the more intermediate carbonaceous species are oxidized to carbon dioxide in the forward scan on the np-Pd/TiO2 surface. In other words, the np-Pd/TiO2 composites possess better poisoningtolerance. The dependence of methanol oxidation on the np-Pd/TiO2-2 composite upon the potential scan rate is shown in Fig. 7. jp (MSC) for methanol oxidation becomes larger with increasing scan rates from 5 to 150 mV s1. The relation between jp and the square root of the scan rate (v1/2) is shown in Inset 1 of Fig. 7. jp is linearly proportional to v1/2, which suggests that the electrocatalytic oxidation methanol on the np-Pd/TiO2-2 composite may be controlled by a diffusion process according to the work of the literature [55,56]. Furthermore, Ep in the forward scan increases with the increase of v, and a linear relationship can be obtained between Ep and ln(v), as shown in Inset 2 of Fig. 7. It indicates that the methanol oxidation of the np-Pd/TiO2 samples is an irreversible electrode process, based on the research [57,58]. In order to evaluate the electrochemical stability, the chronoamperometric measurements of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ 0.5 M methanol solution at a potential of 140 mV vs. Ag/AgCl (sat. KCl) were investigated, with the results illustrated in Fig. 8. The current density decreases over time, indicating the poisoning of the electrocatalysts, which may be due to the formation of intermediates and some poisoning species during the methanol oxidation reaction according to the Ref. [59]. Moreover, the current densities on the np-Pd/TiO2 composites except npPd/TiO2-4 are higher than that on np-Pd after 3000 s elapses, indicating that the np-Pd/TiO2 composites are much stabler and more poisoning-tolerant compared to np-Pd, and the best behavior can be observed from np-Pd/TiO2-2. The electrocatalytic activity of np-Pd/TiO2 and np-Pd for ethanol oxidation was examined. The results of the CVs and the chronoamperometric measurements in a 0.5 M KOH þ 0.5 M ethanol solution are shown in Fig. 9. Where, jp (MSC) of np-Pd/TiO2-1, npPd/TiO2-2 and np-Pd/TiO2-3 is much higher than that of np-Pd. For np-Pd/TiO2-2, jp (313.0 mA mg1) is the highest and about 2 times as high as that of np-Pd (158.3 mAmg1), seen in Fig. 9(a). With increasing scan rate from 10 to 200 mV s1, jp of np-Pd/TiO2-2 gradually increases and the peak potential shifts slightly to the positive position, as shown in Fig. 9(b). It is obvious that the relation between jp and v1/2 is similar to that of methanol oxidation (Inset 1 in Fig. 7(b)), indicating that ethanol oxidation is also a diffusion controlled process. Meanwhile, the good linear correlation between Ep and ln(v) shows the irreversible electrode process too. Moreover, the chronoamperometric measurements for ethanol reveal clearly that the current densities on the np-Pd/TiO2 composites are higher than that on np-Pd after 3000 s elapses, which can be seen from Fig. 9(c). However, the activity of np-Pd/TiO2-4 is significantly lower than that of np-Pd. The above results indicate that the scenario of ethanol oxidation is similar to that of methanol oxidation when the np-Pd/TiO2 composites are compared with np-Pd. The np-Pd/TiO2 composites have been successfully achieved by simply dealloying the melt-spun AlePdeTi ribbons. The composites exhibit higher anodic current, better stability, and better poisoning-tolerance, compared to np-Pd. Among them, the np-Pd/ TiO2-2 sample exhibits the best catalytic activity for methanol/
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Fig. 6. CV curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ 0.5 M methanol solution at a scan rate of 20 mVs1: mass specific current (a) and surface specific current (b).
ethanol oxidation. The improvement can be attributed to the synergistic effect between the Pd and TiO2, which includes the electronic interaction and the interfacial effect: 1. Electronic interaction between Pd and TiO2
Fig. 7. CV curves of the np-Pd/TiO2-2 composite in a 0.5 M KOH þ 0.5 M methanol solution at different scan rates. Inset 1: the plot of jp vs. v1/2. Inset 2: the plot of Ep vs. ln (v/mVs1).
Fig. 8. Chronoamperometry curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ0.5 M methanol solution at a potential of 140 mV vs. Ag/AgCl (sat. KCl).
According to the XPS results, the Pd 3d spectrum of the np-Pd/ TiO2 composites presents a slightly negative shift to lower binding energy compared to np-Pd (Fig. 3(a)). Some studies [60e62] consider that the negative shift of the binding energy is caused by the increase of the particle size or electron density. In this report, the particle size of np-Pd/TiO2 is not larger than that of np-Pd (Figs. 2 and 4). Consequently, the decline of the binding energy is mainly due to the increase of electron density on Pd. These results imply that an electronic interaction between Pd and TiO2 exists in the np-Pd/TiO2 composites. The reduction of binding energy decreases the chemisorptions ability for the adsorbate during electrochemical reactions. In other words, it will reduce the adsorption of CO-like intermediate species and result in a superior poisoningtolerance (Fig. 6, 8 and 9). The experiments (Figs. 6 and 9) reveal that the peak currents of the methanol/ethanol oxidation are obviously higher than that of np-Pd, which means the decline of the chemisorptions ability doesn't decrease too much for methanol/ ethanol chemisorptions, indicating that the np-Pd/TiO2 composites not only improve poisoning-tolerance, but also maintain excellent electrocatalytic performance for methanol/ethanol oxidation. The results are similar to the report [49,63,64]. 2. Interfacial effect between Pd and TiO2 (1) It has been well proved that the interfacial effect between noble metals and metal oxides can increase the catalytic activity of the composites [9,65,66]. Moreover, because of the formation of interfacial effect between Pd and TiO2, the oxygen-containing species (OHads) can more easily form on the surface of TiO2, and reaction with CO-like intermediate species on the np-Pd surface will produce CO2 or other dissolvable products and release the active sites for further electrochemical reaction. (2) The experiments reveal that the np-Pd/TiO2 composites possess enhanced EASA (Fig. 5) compared to np-Pd. On the basis of the analysis of XPS (Fig. 3(a)), the content proportion of PdO in the np-Pd/TiO2 composites is clearly lower than that of the np-Pd. PdO doesn't possess electrocatalytic activity toward methanol/ethanol oxidation; therefore, the
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Fig. 9. CV curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ 0.5 M ethanol solution at a scan rate of 20 mVs1 (a). CV curves of the np-Pd/TiO2-2 composite in a 0.5 M KOH þ 0.5 M ethanol solution at different scan rates (b). Inset 1 in (b): the plot of jp vs. v1/2. Inset 2 in (b): the plot of Ep vs. ln(v/mVs1). (c) shows the chronoamperometry curves of the np-Pd and np-Pd/TiO2 composites in 0.5 M KOH þ 0.5 M ethanol solution at a potential of 140 mV vs. Ag/AgCl (sat. KCl).
decreasing content of PdO is beneficial to retaining more Pd active sites. These indicate that the form of TiO2 has a positive effect on the electrocatalytic reaction. More active sites are available on the np-Pd/TiO2 composites due to the interaction between Pd and TiO2 as well as the concerted effect. The high utilization of Pd in the np-Pd/TiO2 composites will enhance the electrocatalytic activity of the npPd/TiO2 samples. However, when the Ti content is 1 at% in the precursor alloys, the electrocatalytic performance of the np-Pd/TiO2 composites will obviously decline. Such a decrease may be related to two reasons: the conductivity for the np-Pd/TiO2 composites may be significantly decreased when the TiO2 content is excess, which is against electronic transmission. The overmuch TiO2 may block the bicontinuous hollow channels, cause the decrease of active sites, and reject the transport of methanol/ethanol molecules and diffusion of the intermediates in the catalytic process.
interpenetrating ligament-pore structure is formed in the composites. The np-Pd/TiO2 composites exhibit enhanced electrocatalytic activities toward methanol/ethanol oxidation in the alkaline media. The highest anodic current density (MSC) appears for the np-Pd/TiO2-2 sample dealloyed from Al84.5Pd15Ti0.5 ribbons, which is nearly three and two times as high as that of np-Pd for methanol and ethanol oxidation, respectively. The enhancement is due to the synergistic effect between Pd and TiO2. However, when the Ti content is 1 at% in the precursor alloys, the overmuch TiO2 in the composites may cause lower conductivity, blocking of the bicontinuous hollow channels and significant decrease of the electrocatalytic activity. Acknowledgments The authors sincerely acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51371135). References
4. Conclusions The novel np-Pd/TiO2 composites are successfully prepared from the melt-spun AlePdeTi ribbons by a simple one-step dealloying method in the alkaline solution. A bicontinuous
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Nanoporous Pd/TiO2 composites prepared by one-step dealloying and their electrocatalytic performance for methanol/ethanol oxidation Yanyan Song, Caihua Wei, Xiaolong Zhang, Xin Wei, Xiaoping Song, Zhanbo Sun* School of Science, Key Laboratory of Shaanxi for Advanced Functional Materials and Mesoscopic Physics, Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, PR China
h i g h l i g h t s Novel nanoporous Pd/TiO2 composites are directly prepared from the melt-spun AlePdeTi ribbons by a one-step dealloying strategy. The nanoporous Pd/TiO2 composites exhibit a bicontinuous interpenetrating ligament-pore structure. The best electrocatalytic performances of the composites are nearly triple and double as high as that of nanoporous Pd toward methanol and ethanol oxidation, respectively. The triple enhancement is attributed to the synergistic effect between Pd and TiO2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2014 Received in revised form 30 April 2015 Accepted 9 May 2015 Available online xxx
A facile one-step dealloying strategy for the preparation of nanoporous Pd/TiO2 composites from the melt-spun AlePdeTi precursor alloys is reported. The structure analysis reveals that the nanoporous Pd/ TiO2 composites exhibit a bicontinuous interpenetrating ligament-pore structure. The electrochemical measurements show that the composites observably enhance the electrocatalytic performance towards methanol/ethanol oxidation when the Ti content in the precursor alloys is 0.3e0.7 at%. Among these composites, the nanoporous Pd/TiO2 composites dealloyed from Al84.5Pd15Ti0.5 exhibits the best catalytic activity, which is triple and double times for methanol and ethanol oxidation, respectively, compared with nanoporous Pd. The enhancement is attributed to the synergistic effect between Pd and TiO2. However, when the Ti content is 1 at% in the precursor alloys, the catalytic activity will obviously decline. © 2015 Published by Elsevier B.V.
Keywords: Composite materials Nanostructures Chemical synthesis Electrochemical properties
1. Introduction Noble metal nanomaterials with unique physical and chemical properties have received enormous attention due to their high catalytic performance [1,2]. The noble metal nanomaterials with various morphology, such as nanoporous structure [3], nanoparticles [4], nanoplate arrays [5], nanotrees [6], nanowires [7], nanoflowers [8], etc, have been synthesized. In comparison with other nanostructures, the nanoporous (np) structures of the prepared noble metal materials show excellent electrocatalytic performance due to their inherent special morphology and characteristics, such as the open porosity, high specific surface area, high structural stability, superior connectivity, and so on [9e13]. Such morphology and characteristics endow them with the ability
* Corresponding author. E-mail address: [email protected] (Z. Sun).
to interact with atoms, ions and molecules not only at their surfaces, but also throughout the bulk of the materials. In addition, the partitions between particles by the pore structures greatly inhibit particle growth and reduce their aggregation [11e13]. Dealloying is a common process to obtain nanoporous materials, during which an alloy is “parted” by the selective dissolution of the most electrochemically active of its elements. This process results in the formation of the nanoporous structure composed almost entirely of the more noble alloy constituents [14,15]. Recently, np-noble metals, such as Ag, Au, Pd, and Pt have been widely studied [10,16e18]. Among them, np-Pd with high resistance to CO poisoning has aroused increasing interest owing to its superior electrocatalytic activities towards alcohol oxidation and formic acid oxidation [9,19,20]. For instance, Yu et al. [21] synthesized np-Pd through electrochemically dealloying the ternary Pd30Ni50P20 metallic glass and found that the as-prepared np-Pd exhibited the enhanced catalytic activity towards formic acid
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oxidation. Zhang et al. [18] reported that np-Pd obtained by dealloying the AlePd alloys is an excellent electrocatalyst for the oxidation of methanol and formic acid due to its large specific surface area and high catalytic activity. For Pd-based electrocatalysts, transition metal oxides as a kind of effective promoters or appropriate supporting materials have been reported [22e29]. These transition metal oxides which promote Pd electrocatalysts show an improved performance than pure Pd. TiO2 is a noteworthy promoter for noble metal electrocatalysts due to its unique advantages including long-term stability in acidic and alkaline solutions and strong interaction with metals. The strong interaction can improve the activity of the catalysts [27,30e34]. Wang et al. [27] found that Pd supported on TiO2 nanotube composite catalysts displayed a superior methanol oxidation performance compared with pure Pd and other TiO2 substrates in acid medium. Xi et al. [35] reported that Pt/C catalysts with TiO2 exhibit excellent activity toward alcohol oxidation because TiO2 in the catalyst can absorb OHads species and promote the oxidation reaction on the electrode. At present, Pd/TiO2 composites with high electrocatalytic performance are generally synthesized by impregnation and precipitation methods, and can serve as high-efficient anode catalysts for applications in fuel cells [27,36e38]. In most studies, Pd in the composites is loaded on the TiO2 supports. However, these strategies have some shortcomings. In particular, the agglomeration of Pd particles easily occurs on the TiO2 supports and makes the active surface area lower, resulting in a reduction of catalytic efficiency. Moreover, as a semiconductor, TiO2 supports will lead to the performance decline due to the decrease of the electrocatalyst conductivity. In this work, the novel np-Pd/TiO2 composites were successfully prepared by a simple method of one-step dealloying the melt-spun AlePdeTi ribbons in an alkaline solution, and their structures and electrochemical properties were characterized by XRD, SEM, TEM, XPS, CV, CA and EIS. Owing to the enhancement of TiO2, the composites show enhanced electrocatalytic performance towards methanol/ethanol oxidation in alkaline media compared with np-Pd. 2. Experimental 2.1. Preparation of the AlePdeTi precursor alloys
2100 High-Resolution Transmission Electron Microscope (HRTEM, JEOL Ltd.) and a JSM-7000F Scanning Electron Microscope (SEM, JEOL Ltd.) equipped by an Energy Disperse Spectroscopy (EDS). An Axis Ulra Kratos X-ray Photoelectron Spectrometer (XPS) was employed to measure the valence states of Pd and Ti using Al Ka Xray source (1486.68 eV). 2.4. Electrochemical measurements The electrochemical measurements were performed in a standard three-electrode cell using a VersaSTAT MC workstation. The preparation process of the working electrode was described below: 2 mg fine ground dealloyed samples, 0.5 mg acetylene black, 200 mL isopropanol, and 200 mL Nafion solution (0.5 wt%) were ultrasonically mixed. Then, 2 mL of the homogeneously mixed catalyst ink was placed on a freshly polished glassy carbon (GC) electrode with a diameter of 3 mm, and the dealloyed sample loading was kept at 10 mg. The counter electrode was a pure Pt net with an area of 1.0 cm2, and an Ag/AgCl (sat. KCl) electrode was used as the reference electrode. Voltammetric behavior was characterized in a 0.5 M KOH solution, and the electrocatalytic activity measurements were carried out in a 0.5 M KOH þ 0.5 M methanol/ethanol solution. All the measurements were conducted at room temperature under the protection of ultrapure N2. 3. Results and discussion 3.1. Microstructural characterization of the np-Pd/TiO2 composites Fig. 1 shows the XRD patterns of the melt-spun Al85-xPd15Tix (x ¼ 0, 1, at%) ribbons and the corresponding dealloyed samples. The experiments reveal that the diffraction peaks of a-Al and an intermetallic compound Al3Pd, as shown in Fig. 1(a) and (b), can be detected in all ribbons, and the diffraction peaks related to Ti do not appear, suggesting a dissolution of Ti in a-Al and Al3Pd (unknown structure, PDF No. 44-1021). After the ribbons were dealloyed, as shown in Fig. 1(c) and (d), only the diffraction peaks of fcc-Pd (PDF No. 46-1043) can be detected. The microstructure of the dealloyed Al85Pd15 and AlePdeTi ribbons has been observed using SEM, and typical images are shown in Fig. 2. The surface of the dealloyed Al85Pd15 ribbons exhibits an open, bicontinuous and interpenetrating ligament-pore
Al85-xPd15Tix (x ¼ 0, 0.3, 0.5, 0.7, 1.0, at%) alloy ingots with nominal compositions were prepared from Al (99.90%), Pd (99.99%) and Ti (99.99%) by arc-melting in a vacuum arc furnace at an argon atmosphere. And then, the 3 g ingots were reheated to a necessary temperature by high frequency induction and solidified into ribbons by single roller melt spinning at a speed of 33 m s1. The thickness of the melt-spun ribbons was approximately 30 mm and the width was about 3 mm. 2.2. Preparation of the nanoporous composites The melt-spun ternary ribbons were dealloyed in a 20 wt% NaOH solution at room temperature until no obvious bubbles emerged. Subsequently, the dealloying was continuously carried out in the same solution at 60 ± 5 C for 48 h. The dealloyed samples were washed with distilled water and dried at 50 C in a drying oven box. 2.3. Microstructural characterization The phase structures of the melt-spun ribbons and the dealloyed samples were analyzed with a Bruker D8 advanced X-ray Diffractometer (XRD). The microstructures were characterized by a JEM-
Fig. 1. XRD patterns of the Al85Pd15 (a), Al84Pd15Ti1 (b) ribbons, dealloyed Al85Pd15 (c) and Al84Pd15Ti1 (d) ribbons.
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Fig. 2. Plan-view SEM images showing the microstructures of the dealloyed Al85Pd15 (a) and Al84.5Pd15Ti0.5 (c) ribbons. (b) and (d) show the corresponding section-view. Inset in (c) shows the EDS spectrum of the dealloyed AlePdeTi ribbons.
structure, as shown in Fig. 2(a). Where, the surface consists of small concave and convex. This uneven surface should result from corrosion flaw. A large number of pores spread on the uneven surface. The estimated size of most fine pores is less than 10 nm. Fig. 2(b) shows a typical section-view of the dealloyed Al85Pd15 sample. A three-dimensional ultrafine ligament-channel structure can be seen, and the pore channels run throughout the whole ribbons. Fig. 2(c) shows the surface morphology of the dealloyed Al84.5Pd15Ti0.5 ribbons. Compared to that of the dealloyed Al85Pd15 sample, its surface image becomes nebulous. This may be because the surface is covered by the oxide due to the addition of Ti in the precursor alloys. Even so, the ultrafine nanoporous structure surface is still faintly visible. A three-dimensional ultrafine ligamentchannel structure is kept after the addition of Ti, and the bicontinuous interpenetrating ligament-channel structure exhibits a uniform tendency, as shown in Fig. 2(d). The EDS spectrum reveals that the dealloyed AlePdeTi ribbons are mainly composed of Pd, Ti, O and residual Al, as shown in the inset of Fig. 2(c). Additionally, the EDS results of all the as-dealloyed samples are presented in Table 1. The data show that the atomic ratios of Pd: Ti in all the as-dealloyed samples are approximately in agreement with those in the precursor alloys. The less difference results from the inevitable mass loss of Pd or Ti atoms during the dealloying process and the measurement error of the EDS. XPS spectrum for Pd 3d and Ti 2p of the dealloyed Al85-xPd15Tix (x ¼ 0, 0.5, at%) ribbons are shown in Fig. 3. In order to confirm the possible oxidation state of Pd and Ti, the splitting spectrum of Pd 3d
Table 1 The EDS results of the dealloyed Al85-xPd15Tix (x ¼ 0, 0.3, 0.5, 0.7, 1 at%) ribbons. Samples Dealloyed Dealloyed Dealloyed Dealloyed Dealloyed
Al85Pd15 Al84.7Pd15Ti0.3 Al84.5Pd15Ti0.5 Al84.3Pd15Ti0.7 Al84Pd15Ti1
Pd (at%)
Ti (at%)
Residual Al (at%)
91.27 89.41 88.52 86.71 87.25
0 2.06 3.21 4.33 5.46
8.73 8.53 8.27 8.96 7.29
and Ti 2p has been measured. The Pd 3d spectrum of the dealloyed Al85Pd15, as shown in Fig. 3(a), exhibits a low-energy (Pd 3d5/2) peak at ~ 335.5 eV and a high-energy (Pd 3d3/2) peak at ~ 341.0 eV, which is attributed to the metallic Pd according to the literature [39,40]. However, when Ti is added into the precursor alloys, the Pd 3d spectrum of the dealloyed Al84.5Pd15Ti0.5 ribbons presents a slightly negative shift to the lower binding energy. The Pd peaks are located at 335.1 eV (Pd 3d5/2) and 340.5 eV (Pd 3d3/2), which also belong to the metallic Pd (Pd0) species according to the literature [41,42]. It is noticed that the Ti 2p spectrum, as shown in Fig. 3(b), can be detected in the dealloyed AlePdeTi samples, and the doublet peaks occur at 458.4 eV (Ti 2p3/2) and 464.3 eV (Ti 2p1/2), indicating the existence of TiO2 according to the studies [43,44]. No metallic Ti peaks can be measured, which means all the Ti atoms are oxidized into TiO2. It is deduced that TiO2 is in amorphous state or the content is low, resulting in the absence of the diffraction peaks of TiO2 for the dealloyed AlePdeTi samples in Fig. 1. Besides that, it should be indicated that there exist two states of Pd (Fig. 2(a)) in all samples. The Pd 3d spectrum is located at ~336.9 eV and ~342.4 eV, which can be assigned to the PdO (Pd2þ) on the basis of the binding energy (BE), according to the research of the Ref. [40,45]. This means that partial Pd atoms are oxidized to PdO (Pd2þ). Interestingly, the content proportion of PdO is clearly reduced after the addition of Ti into the precursor alloys according to the results of the splitting peak. Detailed information about the microstructures of the dealloyed Al85Pd15 and Al84.5Pd15Ti0.5 ribbons was investigated via TEM. The typical results are shown in Fig. 4. A typical bicontinuous interpenetrating ligament-pore structure clearly exhibits in the dealloyed Al85Pd15 sample, as shown in Fig. 4(a). The size of most pores is less than 10 nm, which agrees with the result of SEM observation (Fig. 2). The corresponding select area electron diffraction (SAED) pattern demonstrates that the connecting ligaments of the nanoporous structure are composed of randomly oriented fcc Pd nanocrystals, as shown in the inset of Fig. 4(a). The SAED pattern can be indexed as Pd (111), (200), (220), (311) and (2 2 2) reflections from the inner ring to the outer one, respectively. The HRTEM image
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Fig. 3. XPS spectra of Pd 3d (a) and Ti 2p (b) for the dealloyed Al85-xPd15Tix (x ¼ 0, 0.5, at%) ribbons.
Fig. 4. TEM and HRTEM images of the dealloyed Al85Pd15 (a, b) and Al84.5Pd15Ti0.5 (c, d) ribbons.
further confirms the ultrafine ligament-channel structure, as shown in Fig. 4(b). The ligaments are composed of intercommunicated and mutually overlapped Pd particles. Moreover, the lattice fringes are well resolved with the lattice spacing calculated to be ~0.224 nm, corresponding to the (111) crystal plane of Pd. A similar nanoporous structure can be observed on the dealloyed Al84.5Pd15Ti0.5 ribbons, as shown in Fig. 4(c). The corresponding SAED pattern is shown in the inset of Fig. 4(c), which also confirms the polycrystalline nature of the connecting ligaments composed of Pd particles. The HRTEM observation demonstrates that the structure and size of ligament-channel and Pd particles don't vary obviously after the addition of Ti into the precursor alloys, as shown in Fig. 4(d). It indicates that although the dealloyed Al84.5Pd15Ti0.5 ribbons are composed of Pd and a small amount of TiO2 (Figs. 2(c) and 3(b)), it is difficult to identify the morphology of phase contained Ti through the TEM observation. The above results reveal that the np-Pd/TiO2 composites have been obtained by dealloying Al85-xPd15Tix (x ¼ 0.3, 0.5, 0.7, 1, at%) precursor alloys. The formation mechanism of the nanoporous Pd
prepared by the dealloying of melt-spun AlePd alloys in alkaline aqueous solution has been extensively studied [9,18,46]. In this report, for the melt-spun AlePdeTi alloy, Al is easy to react with NaOH aqueous solution to produce soluble NaAlO2. As a-Al (Pd, Ti) and Al3Pd (Fig. 1) are decomposed, the released Pd atoms accumulate and reorganize into the nanoporous structure with bicontinuous interpenetrating ligaments/channels. The corresponding chemical reaction equations are described as follows: 2Al þ 2NaOH þ 2H2O ¼ 2NaAlO2 þ 3H2[
(1)
Al3Pd þ NaOH þ H2O / NaAlO2 þ Pd þ H2[
(2)
The reaction rate increases with the content of NaOH increasing in alkali solutions, but much more of NaOH could induce coarsening of ligaments/pores in the nanoporous structure. According to the research of Ref. [46], 20wt% NaOH is the optimal value for dealloying AlePd-based alloys. Moreover, the released highly active Ti atoms from the decomposed a-Al remain in the nanoporous
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structure and can undergo a natural oxidation with aqueous solution under long time condition to form TiO2. This process is similar to the report of the reference [47]. As a result, the np-Pd/TiO2 composites are formed. For convenient description, the dealloyed Al85Pd15, Al84.7Pd15Ti0.3, Al84.5Pd15Ti0.5, Al84.3Pd15Ti0.7 and Al84Pd15Ti1 ribbons are designated as np-Pd, np-Pd/TiO2-1, np-Pd/ TiO2-2, np-Pd/TiO2-3 and np-Pd/TiO2-4, respectively.
3.2. Electrocatalytic performance of the np-Pd/TiO2 composites Fig. 5 shows the cyclic voltammograms (CVs) of the np-Pd/TiO2 composites in a 0.5 M KOH solution at a scan rate of 20 mV s1. The CV of np-Pd is presented simultaneously in Fig. 5 for comparison. For the np-Pd and np-Pd/TiO2 composites, the characters of the CVs are similar, indicating that the introduction of TiO2 has a little effect on the CV character of Pd. The density of charge associated to the reduction of a monolayer for Pd oxides can be used to calculate the accurate electrochemically active specific surface areas (EASA) according to the Ref. [48,49]. In this report, the voltage (E) of the characteristic reduction peak is about 0.35 V in the back scan. The calculated EASA values are 52.0, 80.9, 86.7, 65.5 and 54.4 m2 g1 for np-Pd, np-Pd/TiO2-1, np-Pd/TiO2-2, np-Pd/TiO2-3 and np-Pd/TiO24, respectively. This result reveals that the utilization of Pd in the np-Pd/TiO2 composites is very high. The catalytic activity and the intrinsic activity can be evaluated by mass specific current (MSC) and surface specific current (SSC), respectively. For the Pd-based catalyst, the anodic peak current density (jp) in the forward scan is normally used to evaluate the electrocatalytic activities of catalysts. In addition, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation [50e52]. Fig. 6 shows the CVs of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ0.5 M methanol solution at a scan rate of 20 mV s1 jp (MSC) of the np-Pd/TiO2-1, np-Pd/TiO2-2 and np-Pd/TiO2-3 samples, as shown in Fig. 6(a), is higher than that of np-Pd. Among them, jp (201.8 mA mg1) of np-Pd/TiO2-2 is the highest and nearly 3 times as high as that of np-Pd (69.7 mA mg1). Nevertheless, for np-Pd/TiO2-4, jp is reduced below that of np-Pd. SSCs of the np-Pd/ TiO2 composites, as shown in Fig. 6(b), increase with the increasing TiO2 content but decrease again when jp reaches a maximum value (~0.225 mA cm2). These results indicate that the catalytic activity
Fig. 5. CV curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH solution. The scan rate is 20 mV s1.
5
of the np-Pd/TiO2 composites increases obviously due to the addition of appropriate TiO2. Moreover, the inset of Fig. 6(b) shows that the values of the onset potential (Eop) for the np-Pd/TiO2 composites except np-Pd/TiO2-4 are more negative than that of npPd. This variation presents an improvement in the reaction kinetics of the np-Pd/TiO2 composites according to the view of the Ref. [53,54]. The If/Ib ratios of the composites (If/Ib ¼ 3.7, 4.0, 4.2, 5.0 for np-Pd/TiO2-1, np-Pd/TiO2-2 np-Pd/TiO2-3 and np-Pd/TiO2-4, respectively) are higher than that of np-Pd (If/Ib ¼ 3.3), indicating that the more intermediate carbonaceous species are oxidized to carbon dioxide in the forward scan on the np-Pd/TiO2 surface. In other words, the np-Pd/TiO2 composites possess better poisoningtolerance. The dependence of methanol oxidation on the np-Pd/TiO2-2 composite upon the potential scan rate is shown in Fig. 7. jp (MSC) for methanol oxidation becomes larger with increasing scan rates from 5 to 150 mV s1. The relation between jp and the square root of the scan rate (v1/2) is shown in Inset 1 of Fig. 7. jp is linearly proportional to v1/2, which suggests that the electrocatalytic oxidation methanol on the np-Pd/TiO2-2 composite may be controlled by a diffusion process according to the work of the literature [55,56]. Furthermore, Ep in the forward scan increases with the increase of v, and a linear relationship can be obtained between Ep and ln(v), as shown in Inset 2 of Fig. 7. It indicates that the methanol oxidation of the np-Pd/TiO2 samples is an irreversible electrode process, based on the research [57,58]. In order to evaluate the electrochemical stability, the chronoamperometric measurements of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ 0.5 M methanol solution at a potential of 140 mV vs. Ag/AgCl (sat. KCl) were investigated, with the results illustrated in Fig. 8. The current density decreases over time, indicating the poisoning of the electrocatalysts, which may be due to the formation of intermediates and some poisoning species during the methanol oxidation reaction according to the Ref. [59]. Moreover, the current densities on the np-Pd/TiO2 composites except npPd/TiO2-4 are higher than that on np-Pd after 3000 s elapses, indicating that the np-Pd/TiO2 composites are much stabler and more poisoning-tolerant compared to np-Pd, and the best behavior can be observed from np-Pd/TiO2-2. The electrocatalytic activity of np-Pd/TiO2 and np-Pd for ethanol oxidation was examined. The results of the CVs and the chronoamperometric measurements in a 0.5 M KOH þ 0.5 M ethanol solution are shown in Fig. 9. Where, jp (MSC) of np-Pd/TiO2-1, npPd/TiO2-2 and np-Pd/TiO2-3 is much higher than that of np-Pd. For np-Pd/TiO2-2, jp (313.0 mA mg1) is the highest and about 2 times as high as that of np-Pd (158.3 mAmg1), seen in Fig. 9(a). With increasing scan rate from 10 to 200 mV s1, jp of np-Pd/TiO2-2 gradually increases and the peak potential shifts slightly to the positive position, as shown in Fig. 9(b). It is obvious that the relation between jp and v1/2 is similar to that of methanol oxidation (Inset 1 in Fig. 7(b)), indicating that ethanol oxidation is also a diffusion controlled process. Meanwhile, the good linear correlation between Ep and ln(v) shows the irreversible electrode process too. Moreover, the chronoamperometric measurements for ethanol reveal clearly that the current densities on the np-Pd/TiO2 composites are higher than that on np-Pd after 3000 s elapses, which can be seen from Fig. 9(c). However, the activity of np-Pd/TiO2-4 is significantly lower than that of np-Pd. The above results indicate that the scenario of ethanol oxidation is similar to that of methanol oxidation when the np-Pd/TiO2 composites are compared with np-Pd. The np-Pd/TiO2 composites have been successfully achieved by simply dealloying the melt-spun AlePdeTi ribbons. The composites exhibit higher anodic current, better stability, and better poisoning-tolerance, compared to np-Pd. Among them, the np-Pd/ TiO2-2 sample exhibits the best catalytic activity for methanol/
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Fig. 6. CV curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ 0.5 M methanol solution at a scan rate of 20 mVs1: mass specific current (a) and surface specific current (b).
ethanol oxidation. The improvement can be attributed to the synergistic effect between the Pd and TiO2, which includes the electronic interaction and the interfacial effect: 1. Electronic interaction between Pd and TiO2
Fig. 7. CV curves of the np-Pd/TiO2-2 composite in a 0.5 M KOH þ 0.5 M methanol solution at different scan rates. Inset 1: the plot of jp vs. v1/2. Inset 2: the plot of Ep vs. ln (v/mVs1).
Fig. 8. Chronoamperometry curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ0.5 M methanol solution at a potential of 140 mV vs. Ag/AgCl (sat. KCl).
According to the XPS results, the Pd 3d spectrum of the np-Pd/ TiO2 composites presents a slightly negative shift to lower binding energy compared to np-Pd (Fig. 3(a)). Some studies [60e62] consider that the negative shift of the binding energy is caused by the increase of the particle size or electron density. In this report, the particle size of np-Pd/TiO2 is not larger than that of np-Pd (Figs. 2 and 4). Consequently, the decline of the binding energy is mainly due to the increase of electron density on Pd. These results imply that an electronic interaction between Pd and TiO2 exists in the np-Pd/TiO2 composites. The reduction of binding energy decreases the chemisorptions ability for the adsorbate during electrochemical reactions. In other words, it will reduce the adsorption of CO-like intermediate species and result in a superior poisoningtolerance (Fig. 6, 8 and 9). The experiments (Figs. 6 and 9) reveal that the peak currents of the methanol/ethanol oxidation are obviously higher than that of np-Pd, which means the decline of the chemisorptions ability doesn't decrease too much for methanol/ ethanol chemisorptions, indicating that the np-Pd/TiO2 composites not only improve poisoning-tolerance, but also maintain excellent electrocatalytic performance for methanol/ethanol oxidation. The results are similar to the report [49,63,64]. 2. Interfacial effect between Pd and TiO2 (1) It has been well proved that the interfacial effect between noble metals and metal oxides can increase the catalytic activity of the composites [9,65,66]. Moreover, because of the formation of interfacial effect between Pd and TiO2, the oxygen-containing species (OHads) can more easily form on the surface of TiO2, and reaction with CO-like intermediate species on the np-Pd surface will produce CO2 or other dissolvable products and release the active sites for further electrochemical reaction. (2) The experiments reveal that the np-Pd/TiO2 composites possess enhanced EASA (Fig. 5) compared to np-Pd. On the basis of the analysis of XPS (Fig. 3(a)), the content proportion of PdO in the np-Pd/TiO2 composites is clearly lower than that of the np-Pd. PdO doesn't possess electrocatalytic activity toward methanol/ethanol oxidation; therefore, the
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Fig. 9. CV curves of the np-Pd and np-Pd/TiO2 composites in a 0.5 M KOH þ 0.5 M ethanol solution at a scan rate of 20 mVs1 (a). CV curves of the np-Pd/TiO2-2 composite in a 0.5 M KOH þ 0.5 M ethanol solution at different scan rates (b). Inset 1 in (b): the plot of jp vs. v1/2. Inset 2 in (b): the plot of Ep vs. ln(v/mVs1). (c) shows the chronoamperometry curves of the np-Pd and np-Pd/TiO2 composites in 0.5 M KOH þ 0.5 M ethanol solution at a potential of 140 mV vs. Ag/AgCl (sat. KCl).
decreasing content of PdO is beneficial to retaining more Pd active sites. These indicate that the form of TiO2 has a positive effect on the electrocatalytic reaction. More active sites are available on the np-Pd/TiO2 composites due to the interaction between Pd and TiO2 as well as the concerted effect. The high utilization of Pd in the np-Pd/TiO2 composites will enhance the electrocatalytic activity of the npPd/TiO2 samples. However, when the Ti content is 1 at% in the precursor alloys, the electrocatalytic performance of the np-Pd/TiO2 composites will obviously decline. Such a decrease may be related to two reasons: the conductivity for the np-Pd/TiO2 composites may be significantly decreased when the TiO2 content is excess, which is against electronic transmission. The overmuch TiO2 may block the bicontinuous hollow channels, cause the decrease of active sites, and reject the transport of methanol/ethanol molecules and diffusion of the intermediates in the catalytic process.
interpenetrating ligament-pore structure is formed in the composites. The np-Pd/TiO2 composites exhibit enhanced electrocatalytic activities toward methanol/ethanol oxidation in the alkaline media. The highest anodic current density (MSC) appears for the np-Pd/TiO2-2 sample dealloyed from Al84.5Pd15Ti0.5 ribbons, which is nearly three and two times as high as that of np-Pd for methanol and ethanol oxidation, respectively. The enhancement is due to the synergistic effect between Pd and TiO2. However, when the Ti content is 1 at% in the precursor alloys, the overmuch TiO2 in the composites may cause lower conductivity, blocking of the bicontinuous hollow channels and significant decrease of the electrocatalytic activity. Acknowledgments The authors sincerely acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51371135). References
4. Conclusions The novel np-Pd/TiO2 composites are successfully prepared from the melt-spun AlePdeTi ribbons by a simple one-step dealloying method in the alkaline solution. A bicontinuous
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Please cite this article in press as: Y. Song, et al., Nanoporous Pd/TiO2 composites prepared by one-step dealloying and their electrocatalytic performance for methanol/ethanol oxidation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.030