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C catalysts with different alloying degrees for ethanol oxidation in alkaline media
Electrochimica Acta 144 (2014) 50–55
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pd-Au/C catalysts with different alloying degrees for ethanol oxidation in alkaline media Yuan-Hang Qin a,b, *, Yunfeng Li b , Ren-Liang Lv a , Tie-Lin Wang a , Wei-Guo Wang a , Cun-Wen Wang a,c, * a b c
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430074, China Department of Chemical Engineering, University of Missouri-Columbia, Missouri 65211, USA Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430074, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 July 2014 Received in revised form 15 August 2014 Accepted 16 August 2014 Available online 2 September 2014
High alloyed Pd-Au/C catalyst is prepared through a rate-limiting strategy in water/ethylene glycol solution. Pd/C and low alloyed Pd-Au/C catalysts are prepared with trisodium citrate and sodium borohydride as stabilizing and reducing agents, respectively. Transmission electron microscopy (TEM) shows that the synthesized Pd(Au) particles are well dispersed on the catalysts. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show that the high alloyed Pd-Au/C catalyst presents a relatively homogenous structure while the low alloyed Pd-Au/C catalyst presents a Pd-rich shell/Au-rich core structure. Electrochemical characterization shows that the low alloyed Pd-Au/C catalyst exhibits the best catalytic activity for ethanol oxidation reaction (EOR) in alkaline media, which could be attributed to its relatively large exposed Pd surface area as compared with the high alloyed Pd-Au/C catalyst due to its Pd-rich shell structure and its enhanced adsorption of OHads as compared with Pd/C catalyst due to its core-shell structure. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: ethanol oxidation alloy palladium gold
1. Introduction Recently, direct ethanol fuel cells (DEFCs), which are considered as promising future power sources for electric vehicles and small portable electronics, have drawn increasing attention because ethanol is less toxic compared to methanol and can be produced in large quantities by the fermentation of sugar-containing biomass [1,2]. However, the sluggish ethanol oxidation kinetics is still a great challenge to the commercialization of DEFCs. It is therefore of great interest to improve the ethanol oxidation kinetics by suitable means. One means to achieve this goal is by operating DEFCs in alkaline environment, where the electrocatalytic activities of catalysts for ethanol oxidation reaction (EOR) could be significantly improved. Moreover, in alkaline media, the less expensive Pd catalyst has electrocatalytic performance comparable to, or even better than the Pt catalyst for EOR [3,4]. Although Pd has shown marked superiority compared to Pt in terms of catalytic activity, stability and cost, more efforts still needed to further improve its electrocatalytic performance so as to
* Corresponding authors. Tel.: +86 27 87194882; Fax: +86 27 87194882. E-mail addresses: [email protected] (Y.-H. Qin), [email protected] (C.-W. Wang). http://dx.doi.org/10.1016/j.electacta.2014.08.078 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
meet the application requirements of DEFCs. Substantial research efforts have therefore been devoted to the systematic manipulation of the chemical composition and structure of Pd-based catalysts to further improve the catalytic EOR performance [1,5]. Introducing a second metal into Pd to prepare bimetallic catalysts has become a widely employed approach to improve the catalytic performance of Pd catalyst for EOR. Among the various bimetallic Pd-based catalysts, Pd-Au bimetallic catalyst has attracted particular attention, probably because Au can catalyze many reactions such as CO oxidation and partial oxidation of hydrocarbons with high efficiencies as well as stabilize the neighboring metal catalyst due to its unique electron-withdrawing effect on neighboring metal atoms [5–13]. Although the prepared Pd-Au bimetallic catalysts generally exhibited enhanced electrocatalytic activities as compared with monometallic Pd catalysts for EOR in alkaline media, the structureactivity relationships of Pd-Au bimetallic catalysts have not been reliably established. One of the unestablished relationships is how the alloying degree of Pd-Au bimetallic catalyst affects its catalytic performance for EOR. He et al. prepared a low alloyed Pd-Au/C catalyst, which demonstrated better catalytic activity and stability for EOR in alkaline media as compared with Pd2.5Sn/C and Pt/C catalysts [5]. Huang et al. prepared reduced graphene oxide sheets supported low alloyed Pd-Au bimetallic nanoparticles, which
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demonstrated excellent catalytic activity and good stability for EOR in alkaline media [12]. Feng et al. prepared high alloyed Pd-Au/C catalysts, which demonstrated better catalytic activity and stability for EOR in alkaline media as compared with Pd/C catalyst [7]. It appears that the alloying degree of Pd-Au bimetallic catalysts has little influence on their catalytic EOR performance, as evidenced by their enhanced catalytic performance as compared with Pd catalysts. However, Xu et al. prepared a series of carbon-supported Pd-Au alloy catalysts and found that their catalytic activity for EOR in alkaline media decreased in the sequence Pd/C > Pd3Au/C > Pd7Au/C > PdAu/C, while their stability decreased in the sequence PdAu/C > Pd3Au/C > Pd7Au/C > Pd/C [14]. As is well known, the arrangement of the constitute metals in the bimetallic nanoparticles may have a profound influence on the catalytic performance of catalysts [15]. Therefore, it is of great importance to find the relationship between the alloying degree and the catalytic performance of the Pd-Au nanoparticles for EOR in alkaline media. In this work, high and low alloyed Pd-Au/C catalysts as well as Pd/C catalyst were prepared and their catalytic activities and stabilities for EOR in alkaline media were evaluated. 2. Experimental Carbon black (CB, Vulcan XC-72, Cabot) supported high alloyed Pd-Au nanoparticles with different Pd/Au ratios (from 9:1 to 1:1) were prepared by using a rate-limiting strategy proposed by Suo et. al [16], and the Pd-Au/C catalyst with a Pd/Au ratio being 4:1 exhibited the best compromise between high catalytic activity and good catalytic stability for EOR in alkaline media. In this work, the high alloyed Pd-Au/C catalyst with a Pd/Au ratio being 4:1 was used as the reference catalyst and its preparation procedure was as follows. Firstly, 294.1 mg of trisodium citrate was dissolved into 50 mL of water/ethylene glycol (v/v = 1:1) solution, and then 50 mg of CB was added into the above solution. After 2 h of ultrasonic stirring, the CB slurry was heated under reflux for 5 min and then a precursor solution with 26.1 mg of K2PdCl4 and 7.9 mg of HAuCl4 3H2O (with an atomic ration of Pd: Au = 4:1) dissolved in 10 mL of water/ethylene glycol (v/v = 1:1) solution was added into the CB slurry in a dropwise manner. Another 40 mL of water/ethylene glycol (v/v = 1:1) solution was added into the reaction system and the reflux was continued for an additional 3 h. The resultant solution was filtered, washed with deionized water and then dried at 80 C overnight. The obtained catalyst with a total noble metal loading of 20 wt. % is denoted as Pd-Au/C-H. CB supported low alloyed Pd-Au nanoparticles were prepared by a modified Turkevich method [17]. Firstly, 294.1 mg of trisodium
Fig. 1. XRD patterns of the synthesized Pd/C, Pd-Au/C-H and Pd-Au/C-L catalysts.
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citrate was dissolved into 10 mL of water and then 26.1 mg of K2PdCl4 and 7.9 mg of HAuCl4 3H2O were added into the trisodium citrate solution. After the complete dissolution of K2PdCl4 and HAuCl4 3H2O under sonication, the above solution was added dropwise into a stirred 10 mL of 5 mg mL1 CB slurry. After 12 h of stirring, 10 mL of 0.5 M freshly prepared ice-cold NaBH4 solution was added into the above solution in a dropwise manner. After an additional 12 h of stirring, the solution was filtered, washed with deionized water and then dried at 80 C overnight. The obtained catalyst with a total noble metal loading of 20 wt. % is denoted as Pd-Au/C-L. The modified Turkevich method was also used for the preparation of Pd/C catalyst with a Pd loading of 20 wt. %. The noble metal loadings of the catalysts determined by direct current plasma atomic emission spectroscopy analyses were consistent with the nominal values. X-ray diffraction (XRD) patterns of the catalysts were recorded on an X-ray diffractometer (Philips X-Pert) using Cu Ka as radiation source at a scanning rate of 0.026 s1. The morphologies of the particles on the catalysts were characterized by a transmission electron microscope (TEM, JEM-1400). Surface information of the catalysts was recorded by X-ray photoelectron spectroscopy (XPS, Kratos Axis 165) using Al monochromatic X-ray. Electrochemical characterization was performed on a PGSTAT 302 N electrochemical workstation (Eco Chemie B.V., The Netherlands). All experiments were conducted in a three-electrode system at room temperature. The working electrode was prepared as follows. A suspension of Pd-Au/C (Pd/C) ink was prepared by ultrasonically dispersing Pd-Au/C (Pd/C) in ethanol for 30 min and 5 mL of the above solution (2 mg mL1) was pipetted onto a glassy carbon (GC) electrode with 5 mm diameter (0.196 cm2). After
Fig. 2. XPS spectra of Pd3d (a) and Au4f (b) of Pd-Au/C-H and Pd-Au/C-L catalysts.
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drying at room temperature, 5 mL of Nafion solution (0.05 wt. %) was pipetted onto the surface of the GC electrode and allowed drying again at room temperature. The loading of PdAu (Pd) on the working electrode was 2 mg. A Pt foil and an Ag/AgCl (3 M NaCl) electrode were used as counter and reference electrodes, respectively. 3. Results and discussion Fig. 1 shows the XRD patterns of the Pd/C and Pd-Au/C catalysts. Positions of the diffraction peaks of Pd/C catalyst fits well with the characteristic face centered cubic (fcc) patterns of Pd. It can be seen that the Pd-Au particles in the Pd-Au/C-L catalyst exhibit XRD patterns with superimposed patterns of Pd and Au, indicating low alloyed Pd-Au particles. While the Pd-Au particles in the Pd-Au/C-H catalyst exhibit only a single phase with the diffraction peak positions located between those of pure Pd and Au, indicating that Pd and Au are highly alloyed, forming high alloyed Pd-Au particles. The crystallite sizes of Pd/C and Pd-Au/C-H catalysts calculated based on the diffraction peaks of Pd(Au) (111) are 3.7 and 4.3 nm, respectively. The crystallite size of Pd-Au/C-L catalyst was not determined here because its superimposed XRD patterns may cause great error in calculation. XPS characterization was carried out to get surface information of Pd-Au/C-H and Pd-Au/C-L catalysts. Fig. 2 shows the Pd3d and Au4f peaks of Pd-Au/C-H and Pd-Au/C-L catalysts. The surface atomic ratio of Pd to Au in Pd-Au/C-L is ca. 24:1, which is much larger than the stoichiometric value (4:1), indicating the
enrichment of Pd element on the surface of the nanoparticles [16]. While the surface atomic ratio of Pd to Au in Pd-Au/C-H is ca. 5:1, which is close to the stoichiometric value (4:1). The above XRD and XPS results have strongly suggested that most of the Pd-Au nanoparticles in Pd-Au/C-H catalyst form high alloyed structure, while those in Pd-Au/C-L catalyst form low alloyed structure. The low alloyed Pd-Au/C-L catalyst was synthesized because the reducing agent was added dropwise to the metallic ion solutions in excess, which creates a synthesis environment for faster deposition of Au than Pd due to the higher reduction potential of Au ions than that of Pd ions (0.591 V for PdCl42/Pd vs. 1.0 V for AuCl4/Au) [16,18] and thus leads to a Pd-rich shell/Au-rich core structure. On the other hand, the environment of a limited supply of Pd and Au precursors (supplied in drop-by-drop) and an abundant reducing reagents at high temperature, as in the rate-limiting strategy, accelerates the reduction rates of both Pd and Au ions and drives the synthesis process to be controlled by the mass transport of Pd and Au precursors. Therefore, the drop-by-drop arrival of Pd and Au ions determines catalyst synthesis condition and simultaneous reduction of Pd and Au ions to form alloy structure is thus achieved [16]. Fig. 3 shows the TEM images and the corresponding size distribution histograms of Pd/C (Fig. 2(a)), Pd-Au/C-H (Fig. 2(b)) and Pd-Au/C-L (Fig. 2(c)) catalysts. It can be seen that the particles on the three catalysts are well dispersed, indicating that both the modified Turkevich method and the ethylene glycol method are powerful to synthesize particles with good dispersion, which may result from the powerful stabilizing effect of trisodium citrate used
Fig. 3. TEM images and the corresponding size distribution histograms of Pd/C (a), Pd-Au/C-H (b) and Pd-Au/C-L (c) catalysts.
Y.-H. Qin et al. / Electrochimica Acta 144 (2014) 50–55
in the two methods. The average mean particle sizes of Pd/C and Pd-Au/C-H catalysts determined from TEM are in agreement with those determined from XRD. Compared with the particles in Pd-Au/C-H, the particles in Pd/C and Pd-Au/C-L exhibit smaller mean particle size and better dispersion. Fig. 4(a) shows the cyclic voltammograms of ethanol oxidation on the three catalysts in deaerated 1 M KOH + 1 M ethanol solution. It can be seen that two well-defined oxidation peaks can be clearly observed. In the forward scan, the oxidation peak is corresponding
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to the oxidation of freshly chemisorbed species coming from ethanol adsorption. In the backward scan, the oxidation peak is primarily associated with the removal of carbonaceous species not completely oxidized in the forward scan. Thus, the oxidation peak during the forward scan is usually used to evaluate the catalytic activity of the catalyst [19]. It can be seen that among the three catalysts, Pd-Au/C-L exhibits the highest electrocatalytic activity for EOR while Pd-Au/C-H exhibits the lowest one. The different electrocatalytic activities exhibited by Pd-Au/C-H and Pd-Au/C-L suggest that the structure of Pd-Au bimetallic particles plays an important role in determining the catalytic performance of Pd-Au/C catalyst. The generally accepted mechanism for EOR in alkaline media can be summarized as follows [7,20,21]. Ethanol oxidation starts with the dissociative adsorption of ethanol (Eq. (1)). CH3CH2OH + 3 OH $ CH3COads + 3H2O + 3 e
(1)
When the scanning potential goes higher, the oxidation rate is enhanced because the adsorption of OH occurs (Eq. (2)), and then the adsorbed CH3COads reacts with the electrochemically adsorbed OHads generating CH3COOH (Eq. (3)). OH $ OHads + e
r:d:s:
CH3 COads þ OHads ! CH3 COOH
(2)
(3)
In alkaline media acetic acid exists as an acetate ion (Eq. (4)). fast
CH3 COOH þ OH ! CH3 COO þ H2 O
Fig. 4. Cyclic voltammograms of Pd/C, Pd-Au/C-H and Pd-Au/C-L catalysts in 1 M KOH with (a) and without (b) 1 M ethanol solution and the corresponding specific activity in 1 M KOH + 1 M ethanol solution (c) with a scanning rate of 50 mV s1 at room temperature.
(4)
It has been suggested that the rate-determining step in EOR is Eq. (3). The incorporation of Au into Pd catalyst can promote the adsorption of OHads onto the catalyst surface and thus help to enhance the EOR process, which could be explained based on the d-band center theory. According to d-band theory, the d-band center of Pd will shift upward when it is combined with Au because the lattice constant of Au (4.08 Å) is larger than that of Pd (3.89 Å) [22]. The upward shift of d-band center could promote the adsorption of OH, which could facilitate the oxidation of CH3COads and thus help to enhance the EOR process. However, in alkaline media there are sufficient OH groups which can weaken the adsorption of CH3COads species for the relief of catalyst poisoning [1,23] and the excess adsorption of OH may cause a competition with the adsorption of ethanol [19]. Thus, too much adsorption of OH on Pd by the incorporation of Au may not adequately fulfill its effect on EOR in alkaline media. In addition, Pd acts as primary active sites for EOR and the incorporation of Au could lead to the less-exposed surface Pd atoms, which may have a counterproductive effect for EOR. Since Pd acts as primary active sites for EOR, it is important to evaluate the utilization of Pd on catalyst. Fig. 4(b) shows the cyclic voltammograms of the three catalysts in de-oxygenated 1 M KOH solution in the potential range from -0.8 to 0.2 V. It can be seen from Fig. 4(b) that a well-defined cathodic peak, which can be attributed to the reduction of Pd oxide to Pd during the cathodic sweep because Au has not yet been oxidized at the potential of 0.2 V [7,24], presents in all the three cyclic voltammograms. The cathodic peak of Pd/C catalyst is much larger than that of Pd-Au/C catalyst, due to the facts that the Pd loading on the former is larger than that on the latter (2 mg vs. 1.37 mg) while the mean particle size of the former is smaller than that of the latter. In addition, the incorporation of Au, which could cause less-exposed Pd surface atoms, may also lead to the small cathodic peak presented by Pd-Au/C catalyst. Since the coulombic charge consumed during the reduction of Pd oxide to Pd is proportional to the electrochemically
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presents the best one, as evidenced by its catalytic superiority over almost the entire time scale even though its relatively poor electrocatalytic activity for EOR as compared with the other two catalysts. The relatively good catalytic stability presented by Pd-Au/ C-H may be attributed to its relatively large adsorption of OHads. 4. Conclusions In summary, high and low alloyed Pd-Au/C catalysts synthesized in this work exhibit decreased and increased electrocatalytic activities, respectively, for EOR in alkaline media as compared with Pd/C catalyst. The decreased activity presented by the high alloyed Pd-Au/C catalyst could be attributed to its larger mean particle size and fewer exposed Pd surface atoms. The increased activity presented by the low alloyed Pd-Au/C catalyst could be attributed to the enhanced adsorption of OHads results from the geometric strain effect. Fig. 5. Chronoamperograms of Pd/C, Pd-Au/C-H and Pd-Au/C-L catalysts in 1 M KOH + 1 M ethanol solution at -0.2 V (vs. Ag/AgCl).
active surface area (EASA) of Pd on the catalyst, the coulombic charge, which can be determined by integrating the well-defined peak, could be used to estimate the EASA of Pd and evaluate the utilization of Pd on catalyst [25,26]. The EASAs based on the coulombic charge are 1.19, 0.688 and 0.850 mC cm2 for Pd/C, Pd-Au/C-H and Pd-Au/C-L, respectively. It can be seen that the EASA of Pd-Au/C-H is much smaller than that of Pd-Au/C-L, which results from two reasons. First, Pd-Au/C-H has a larger mean particle size as compared with Pd-Au/C-L. Second, Pd-Au/C-H have fewer exposed Pd surface atoms as compared with Pd-Au/C-L because the latter catalyst presents a Pd-rich shell structure. The above two reasons may also account for the better catalytic activity presented by the low alloyed Pd-Au/C-L catalyst as compared with the high alloyed Pd-Au/C-H catalyst. Compared with Pd/C, Pd-Au/C-L exhibits a better electrocatalytic activity for EOR, although the former catalyst presents a smaller mean particle size and a larger EASA. The enhanced catalytic activity may be attributed to the enhanced adsorption of OHads onto the Pd-Au/C-L catalyst surface, which results from the geometric strain effect. In the Pd-Au/C-L catalyst, a tensile strain forms in the Pd-enriched shell supported on the Au-rich core with a larger lattice parameter. The expansion in the shell could upshift the d-band center of Pd [27], and thereby promotes the adsorption of OHads compared to the unstrained Pd in Pd/C catalyst and results in an increase in the catalytic activity. In addition, the ligand effect between Pd and Au as well as the ensemble effect results from the proper ensemble of Pd atoms on the catalyst surface may also contribute to the enhanced catalytic activity. To facilitate the comparison of the “intrinsic” electrocatalytic activities of the three catalysts, the currents in Fig. 4 (a) were re-normalized by EASAs of Pd of the catalysts and the results are shown in Fig. 4 (c). It can be seen that Pd/C presents a lower “intrinsic” activity as compared with Pd-Au/C-H and Pd-Au/C-L, which exhibit close “intrinsic” activity, as evidenced by their close peak current densities. It is important to note that the mass activity remains a good indicator of the effectiveness of metal utilization in a catalyst system, while the specific activity demonstrates the actual value of the intrinsic activity of the Pd metal. In terms of practical applications of a catalyst, we should do our best to improve the noble metal utilization, namely the mass activity [28]. The stability test for the three catalysts for EOR have been carried out by the chronoamperometry technique at a potential of -0.2 V in 1 M KOH + 1 M ethanol solution and the corresponding results are shown in Fig. 5. It is can be seen that the Pd/C catalyst presents the poorest stability for EOR, while the high alloyed Pd-Au/C-H catalyst
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pd-Au/C catalysts with different alloying degrees for ethanol oxidation in alkaline media Yuan-Hang Qin a,b, *, Yunfeng Li b , Ren-Liang Lv a , Tie-Lin Wang a , Wei-Guo Wang a , Cun-Wen Wang a,c, * a b c
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430074, China Department of Chemical Engineering, University of Missouri-Columbia, Missouri 65211, USA Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430074, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 July 2014 Received in revised form 15 August 2014 Accepted 16 August 2014 Available online 2 September 2014
High alloyed Pd-Au/C catalyst is prepared through a rate-limiting strategy in water/ethylene glycol solution. Pd/C and low alloyed Pd-Au/C catalysts are prepared with trisodium citrate and sodium borohydride as stabilizing and reducing agents, respectively. Transmission electron microscopy (TEM) shows that the synthesized Pd(Au) particles are well dispersed on the catalysts. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show that the high alloyed Pd-Au/C catalyst presents a relatively homogenous structure while the low alloyed Pd-Au/C catalyst presents a Pd-rich shell/Au-rich core structure. Electrochemical characterization shows that the low alloyed Pd-Au/C catalyst exhibits the best catalytic activity for ethanol oxidation reaction (EOR) in alkaline media, which could be attributed to its relatively large exposed Pd surface area as compared with the high alloyed Pd-Au/C catalyst due to its Pd-rich shell structure and its enhanced adsorption of OHads as compared with Pd/C catalyst due to its core-shell structure. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: ethanol oxidation alloy palladium gold
1. Introduction Recently, direct ethanol fuel cells (DEFCs), which are considered as promising future power sources for electric vehicles and small portable electronics, have drawn increasing attention because ethanol is less toxic compared to methanol and can be produced in large quantities by the fermentation of sugar-containing biomass [1,2]. However, the sluggish ethanol oxidation kinetics is still a great challenge to the commercialization of DEFCs. It is therefore of great interest to improve the ethanol oxidation kinetics by suitable means. One means to achieve this goal is by operating DEFCs in alkaline environment, where the electrocatalytic activities of catalysts for ethanol oxidation reaction (EOR) could be significantly improved. Moreover, in alkaline media, the less expensive Pd catalyst has electrocatalytic performance comparable to, or even better than the Pt catalyst for EOR [3,4]. Although Pd has shown marked superiority compared to Pt in terms of catalytic activity, stability and cost, more efforts still needed to further improve its electrocatalytic performance so as to
* Corresponding authors. Tel.: +86 27 87194882; Fax: +86 27 87194882. E-mail addresses: [email protected] (Y.-H. Qin), [email protected] (C.-W. Wang). http://dx.doi.org/10.1016/j.electacta.2014.08.078 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
meet the application requirements of DEFCs. Substantial research efforts have therefore been devoted to the systematic manipulation of the chemical composition and structure of Pd-based catalysts to further improve the catalytic EOR performance [1,5]. Introducing a second metal into Pd to prepare bimetallic catalysts has become a widely employed approach to improve the catalytic performance of Pd catalyst for EOR. Among the various bimetallic Pd-based catalysts, Pd-Au bimetallic catalyst has attracted particular attention, probably because Au can catalyze many reactions such as CO oxidation and partial oxidation of hydrocarbons with high efficiencies as well as stabilize the neighboring metal catalyst due to its unique electron-withdrawing effect on neighboring metal atoms [5–13]. Although the prepared Pd-Au bimetallic catalysts generally exhibited enhanced electrocatalytic activities as compared with monometallic Pd catalysts for EOR in alkaline media, the structureactivity relationships of Pd-Au bimetallic catalysts have not been reliably established. One of the unestablished relationships is how the alloying degree of Pd-Au bimetallic catalyst affects its catalytic performance for EOR. He et al. prepared a low alloyed Pd-Au/C catalyst, which demonstrated better catalytic activity and stability for EOR in alkaline media as compared with Pd2.5Sn/C and Pt/C catalysts [5]. Huang et al. prepared reduced graphene oxide sheets supported low alloyed Pd-Au bimetallic nanoparticles, which
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demonstrated excellent catalytic activity and good stability for EOR in alkaline media [12]. Feng et al. prepared high alloyed Pd-Au/C catalysts, which demonstrated better catalytic activity and stability for EOR in alkaline media as compared with Pd/C catalyst [7]. It appears that the alloying degree of Pd-Au bimetallic catalysts has little influence on their catalytic EOR performance, as evidenced by their enhanced catalytic performance as compared with Pd catalysts. However, Xu et al. prepared a series of carbon-supported Pd-Au alloy catalysts and found that their catalytic activity for EOR in alkaline media decreased in the sequence Pd/C > Pd3Au/C > Pd7Au/C > PdAu/C, while their stability decreased in the sequence PdAu/C > Pd3Au/C > Pd7Au/C > Pd/C [14]. As is well known, the arrangement of the constitute metals in the bimetallic nanoparticles may have a profound influence on the catalytic performance of catalysts [15]. Therefore, it is of great importance to find the relationship between the alloying degree and the catalytic performance of the Pd-Au nanoparticles for EOR in alkaline media. In this work, high and low alloyed Pd-Au/C catalysts as well as Pd/C catalyst were prepared and their catalytic activities and stabilities for EOR in alkaline media were evaluated. 2. Experimental Carbon black (CB, Vulcan XC-72, Cabot) supported high alloyed Pd-Au nanoparticles with different Pd/Au ratios (from 9:1 to 1:1) were prepared by using a rate-limiting strategy proposed by Suo et. al [16], and the Pd-Au/C catalyst with a Pd/Au ratio being 4:1 exhibited the best compromise between high catalytic activity and good catalytic stability for EOR in alkaline media. In this work, the high alloyed Pd-Au/C catalyst with a Pd/Au ratio being 4:1 was used as the reference catalyst and its preparation procedure was as follows. Firstly, 294.1 mg of trisodium citrate was dissolved into 50 mL of water/ethylene glycol (v/v = 1:1) solution, and then 50 mg of CB was added into the above solution. After 2 h of ultrasonic stirring, the CB slurry was heated under reflux for 5 min and then a precursor solution with 26.1 mg of K2PdCl4 and 7.9 mg of HAuCl4 3H2O (with an atomic ration of Pd: Au = 4:1) dissolved in 10 mL of water/ethylene glycol (v/v = 1:1) solution was added into the CB slurry in a dropwise manner. Another 40 mL of water/ethylene glycol (v/v = 1:1) solution was added into the reaction system and the reflux was continued for an additional 3 h. The resultant solution was filtered, washed with deionized water and then dried at 80 C overnight. The obtained catalyst with a total noble metal loading of 20 wt. % is denoted as Pd-Au/C-H. CB supported low alloyed Pd-Au nanoparticles were prepared by a modified Turkevich method [17]. Firstly, 294.1 mg of trisodium
Fig. 1. XRD patterns of the synthesized Pd/C, Pd-Au/C-H and Pd-Au/C-L catalysts.
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citrate was dissolved into 10 mL of water and then 26.1 mg of K2PdCl4 and 7.9 mg of HAuCl4 3H2O were added into the trisodium citrate solution. After the complete dissolution of K2PdCl4 and HAuCl4 3H2O under sonication, the above solution was added dropwise into a stirred 10 mL of 5 mg mL1 CB slurry. After 12 h of stirring, 10 mL of 0.5 M freshly prepared ice-cold NaBH4 solution was added into the above solution in a dropwise manner. After an additional 12 h of stirring, the solution was filtered, washed with deionized water and then dried at 80 C overnight. The obtained catalyst with a total noble metal loading of 20 wt. % is denoted as Pd-Au/C-L. The modified Turkevich method was also used for the preparation of Pd/C catalyst with a Pd loading of 20 wt. %. The noble metal loadings of the catalysts determined by direct current plasma atomic emission spectroscopy analyses were consistent with the nominal values. X-ray diffraction (XRD) patterns of the catalysts were recorded on an X-ray diffractometer (Philips X-Pert) using Cu Ka as radiation source at a scanning rate of 0.026 s1. The morphologies of the particles on the catalysts were characterized by a transmission electron microscope (TEM, JEM-1400). Surface information of the catalysts was recorded by X-ray photoelectron spectroscopy (XPS, Kratos Axis 165) using Al monochromatic X-ray. Electrochemical characterization was performed on a PGSTAT 302 N electrochemical workstation (Eco Chemie B.V., The Netherlands). All experiments were conducted in a three-electrode system at room temperature. The working electrode was prepared as follows. A suspension of Pd-Au/C (Pd/C) ink was prepared by ultrasonically dispersing Pd-Au/C (Pd/C) in ethanol for 30 min and 5 mL of the above solution (2 mg mL1) was pipetted onto a glassy carbon (GC) electrode with 5 mm diameter (0.196 cm2). After
Fig. 2. XPS spectra of Pd3d (a) and Au4f (b) of Pd-Au/C-H and Pd-Au/C-L catalysts.
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drying at room temperature, 5 mL of Nafion solution (0.05 wt. %) was pipetted onto the surface of the GC electrode and allowed drying again at room temperature. The loading of PdAu (Pd) on the working electrode was 2 mg. A Pt foil and an Ag/AgCl (3 M NaCl) electrode were used as counter and reference electrodes, respectively. 3. Results and discussion Fig. 1 shows the XRD patterns of the Pd/C and Pd-Au/C catalysts. Positions of the diffraction peaks of Pd/C catalyst fits well with the characteristic face centered cubic (fcc) patterns of Pd. It can be seen that the Pd-Au particles in the Pd-Au/C-L catalyst exhibit XRD patterns with superimposed patterns of Pd and Au, indicating low alloyed Pd-Au particles. While the Pd-Au particles in the Pd-Au/C-H catalyst exhibit only a single phase with the diffraction peak positions located between those of pure Pd and Au, indicating that Pd and Au are highly alloyed, forming high alloyed Pd-Au particles. The crystallite sizes of Pd/C and Pd-Au/C-H catalysts calculated based on the diffraction peaks of Pd(Au) (111) are 3.7 and 4.3 nm, respectively. The crystallite size of Pd-Au/C-L catalyst was not determined here because its superimposed XRD patterns may cause great error in calculation. XPS characterization was carried out to get surface information of Pd-Au/C-H and Pd-Au/C-L catalysts. Fig. 2 shows the Pd3d and Au4f peaks of Pd-Au/C-H and Pd-Au/C-L catalysts. The surface atomic ratio of Pd to Au in Pd-Au/C-L is ca. 24:1, which is much larger than the stoichiometric value (4:1), indicating the
enrichment of Pd element on the surface of the nanoparticles [16]. While the surface atomic ratio of Pd to Au in Pd-Au/C-H is ca. 5:1, which is close to the stoichiometric value (4:1). The above XRD and XPS results have strongly suggested that most of the Pd-Au nanoparticles in Pd-Au/C-H catalyst form high alloyed structure, while those in Pd-Au/C-L catalyst form low alloyed structure. The low alloyed Pd-Au/C-L catalyst was synthesized because the reducing agent was added dropwise to the metallic ion solutions in excess, which creates a synthesis environment for faster deposition of Au than Pd due to the higher reduction potential of Au ions than that of Pd ions (0.591 V for PdCl42/Pd vs. 1.0 V for AuCl4/Au) [16,18] and thus leads to a Pd-rich shell/Au-rich core structure. On the other hand, the environment of a limited supply of Pd and Au precursors (supplied in drop-by-drop) and an abundant reducing reagents at high temperature, as in the rate-limiting strategy, accelerates the reduction rates of both Pd and Au ions and drives the synthesis process to be controlled by the mass transport of Pd and Au precursors. Therefore, the drop-by-drop arrival of Pd and Au ions determines catalyst synthesis condition and simultaneous reduction of Pd and Au ions to form alloy structure is thus achieved [16]. Fig. 3 shows the TEM images and the corresponding size distribution histograms of Pd/C (Fig. 2(a)), Pd-Au/C-H (Fig. 2(b)) and Pd-Au/C-L (Fig. 2(c)) catalysts. It can be seen that the particles on the three catalysts are well dispersed, indicating that both the modified Turkevich method and the ethylene glycol method are powerful to synthesize particles with good dispersion, which may result from the powerful stabilizing effect of trisodium citrate used
Fig. 3. TEM images and the corresponding size distribution histograms of Pd/C (a), Pd-Au/C-H (b) and Pd-Au/C-L (c) catalysts.
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in the two methods. The average mean particle sizes of Pd/C and Pd-Au/C-H catalysts determined from TEM are in agreement with those determined from XRD. Compared with the particles in Pd-Au/C-H, the particles in Pd/C and Pd-Au/C-L exhibit smaller mean particle size and better dispersion. Fig. 4(a) shows the cyclic voltammograms of ethanol oxidation on the three catalysts in deaerated 1 M KOH + 1 M ethanol solution. It can be seen that two well-defined oxidation peaks can be clearly observed. In the forward scan, the oxidation peak is corresponding
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to the oxidation of freshly chemisorbed species coming from ethanol adsorption. In the backward scan, the oxidation peak is primarily associated with the removal of carbonaceous species not completely oxidized in the forward scan. Thus, the oxidation peak during the forward scan is usually used to evaluate the catalytic activity of the catalyst [19]. It can be seen that among the three catalysts, Pd-Au/C-L exhibits the highest electrocatalytic activity for EOR while Pd-Au/C-H exhibits the lowest one. The different electrocatalytic activities exhibited by Pd-Au/C-H and Pd-Au/C-L suggest that the structure of Pd-Au bimetallic particles plays an important role in determining the catalytic performance of Pd-Au/C catalyst. The generally accepted mechanism for EOR in alkaline media can be summarized as follows [7,20,21]. Ethanol oxidation starts with the dissociative adsorption of ethanol (Eq. (1)). CH3CH2OH + 3 OH $ CH3COads + 3H2O + 3 e
(1)
When the scanning potential goes higher, the oxidation rate is enhanced because the adsorption of OH occurs (Eq. (2)), and then the adsorbed CH3COads reacts with the electrochemically adsorbed OHads generating CH3COOH (Eq. (3)). OH $ OHads + e
r:d:s:
CH3 COads þ OHads ! CH3 COOH
(2)
(3)
In alkaline media acetic acid exists as an acetate ion (Eq. (4)). fast
CH3 COOH þ OH ! CH3 COO þ H2 O
Fig. 4. Cyclic voltammograms of Pd/C, Pd-Au/C-H and Pd-Au/C-L catalysts in 1 M KOH with (a) and without (b) 1 M ethanol solution and the corresponding specific activity in 1 M KOH + 1 M ethanol solution (c) with a scanning rate of 50 mV s1 at room temperature.
(4)
It has been suggested that the rate-determining step in EOR is Eq. (3). The incorporation of Au into Pd catalyst can promote the adsorption of OHads onto the catalyst surface and thus help to enhance the EOR process, which could be explained based on the d-band center theory. According to d-band theory, the d-band center of Pd will shift upward when it is combined with Au because the lattice constant of Au (4.08 Å) is larger than that of Pd (3.89 Å) [22]. The upward shift of d-band center could promote the adsorption of OH, which could facilitate the oxidation of CH3COads and thus help to enhance the EOR process. However, in alkaline media there are sufficient OH groups which can weaken the adsorption of CH3COads species for the relief of catalyst poisoning [1,23] and the excess adsorption of OH may cause a competition with the adsorption of ethanol [19]. Thus, too much adsorption of OH on Pd by the incorporation of Au may not adequately fulfill its effect on EOR in alkaline media. In addition, Pd acts as primary active sites for EOR and the incorporation of Au could lead to the less-exposed surface Pd atoms, which may have a counterproductive effect for EOR. Since Pd acts as primary active sites for EOR, it is important to evaluate the utilization of Pd on catalyst. Fig. 4(b) shows the cyclic voltammograms of the three catalysts in de-oxygenated 1 M KOH solution in the potential range from -0.8 to 0.2 V. It can be seen from Fig. 4(b) that a well-defined cathodic peak, which can be attributed to the reduction of Pd oxide to Pd during the cathodic sweep because Au has not yet been oxidized at the potential of 0.2 V [7,24], presents in all the three cyclic voltammograms. The cathodic peak of Pd/C catalyst is much larger than that of Pd-Au/C catalyst, due to the facts that the Pd loading on the former is larger than that on the latter (2 mg vs. 1.37 mg) while the mean particle size of the former is smaller than that of the latter. In addition, the incorporation of Au, which could cause less-exposed Pd surface atoms, may also lead to the small cathodic peak presented by Pd-Au/C catalyst. Since the coulombic charge consumed during the reduction of Pd oxide to Pd is proportional to the electrochemically
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presents the best one, as evidenced by its catalytic superiority over almost the entire time scale even though its relatively poor electrocatalytic activity for EOR as compared with the other two catalysts. The relatively good catalytic stability presented by Pd-Au/ C-H may be attributed to its relatively large adsorption of OHads. 4. Conclusions In summary, high and low alloyed Pd-Au/C catalysts synthesized in this work exhibit decreased and increased electrocatalytic activities, respectively, for EOR in alkaline media as compared with Pd/C catalyst. The decreased activity presented by the high alloyed Pd-Au/C catalyst could be attributed to its larger mean particle size and fewer exposed Pd surface atoms. The increased activity presented by the low alloyed Pd-Au/C catalyst could be attributed to the enhanced adsorption of OHads results from the geometric strain effect. Fig. 5. Chronoamperograms of Pd/C, Pd-Au/C-H and Pd-Au/C-L catalysts in 1 M KOH + 1 M ethanol solution at -0.2 V (vs. Ag/AgCl).
active surface area (EASA) of Pd on the catalyst, the coulombic charge, which can be determined by integrating the well-defined peak, could be used to estimate the EASA of Pd and evaluate the utilization of Pd on catalyst [25,26]. The EASAs based on the coulombic charge are 1.19, 0.688 and 0.850 mC cm2 for Pd/C, Pd-Au/C-H and Pd-Au/C-L, respectively. It can be seen that the EASA of Pd-Au/C-H is much smaller than that of Pd-Au/C-L, which results from two reasons. First, Pd-Au/C-H has a larger mean particle size as compared with Pd-Au/C-L. Second, Pd-Au/C-H have fewer exposed Pd surface atoms as compared with Pd-Au/C-L because the latter catalyst presents a Pd-rich shell structure. The above two reasons may also account for the better catalytic activity presented by the low alloyed Pd-Au/C-L catalyst as compared with the high alloyed Pd-Au/C-H catalyst. Compared with Pd/C, Pd-Au/C-L exhibits a better electrocatalytic activity for EOR, although the former catalyst presents a smaller mean particle size and a larger EASA. The enhanced catalytic activity may be attributed to the enhanced adsorption of OHads onto the Pd-Au/C-L catalyst surface, which results from the geometric strain effect. In the Pd-Au/C-L catalyst, a tensile strain forms in the Pd-enriched shell supported on the Au-rich core with a larger lattice parameter. The expansion in the shell could upshift the d-band center of Pd [27], and thereby promotes the adsorption of OHads compared to the unstrained Pd in Pd/C catalyst and results in an increase in the catalytic activity. In addition, the ligand effect between Pd and Au as well as the ensemble effect results from the proper ensemble of Pd atoms on the catalyst surface may also contribute to the enhanced catalytic activity. To facilitate the comparison of the “intrinsic” electrocatalytic activities of the three catalysts, the currents in Fig. 4 (a) were re-normalized by EASAs of Pd of the catalysts and the results are shown in Fig. 4 (c). It can be seen that Pd/C presents a lower “intrinsic” activity as compared with Pd-Au/C-H and Pd-Au/C-L, which exhibit close “intrinsic” activity, as evidenced by their close peak current densities. It is important to note that the mass activity remains a good indicator of the effectiveness of metal utilization in a catalyst system, while the specific activity demonstrates the actual value of the intrinsic activity of the Pd metal. In terms of practical applications of a catalyst, we should do our best to improve the noble metal utilization, namely the mass activity [28]. The stability test for the three catalysts for EOR have been carried out by the chronoamperometry technique at a potential of -0.2 V in 1 M KOH + 1 M ethanol solution and the corresponding results are shown in Fig. 5. It is can be seen that the Pd/C catalyst presents the poorest stability for EOR, while the high alloyed Pd-Au/C-H catalyst
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