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C nanonetwork catalysts for glycerol electrooxidation by small amounts of Pd
Journal of Electroanalytical Chemistry 847 (2019) 113225
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
The enhanced activity of AuAg/C nanonetwork catalysts for glycerol electrooxidation by small amounts of Pd
T
S. Yongprapata, , A. Therdthianwonga, S. Therdthianwongb ⁎
a
Fuel Cells and Hydrogen Research and Engineering Center, Pilot Plant, Development and Training Institute (PDTI), King Mongkut's University of Technology Thonburi (KMUTT), 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand b Department of Chemical Engineering, Faculty of Engineering, King Mongkut's University of Technology Thonburi (KMUTT), 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand
ARTICLE INFO
ABSTRACT
Keywords: Glycerol electrooxidation Trimetallic catalyst AuAgPd Direct glycerol fuel cell, nanonetwork catalyst
The catalytic properties of the Au/C catalyst toward glycerol electrooxidation in alkaline medium have been tremendously improved by an incorporation of Au with Ag. In this work, the catalytic performance of the AuAg/ C was further improved by introducing small amount of a third metal, Pd. The catalysts were synthesized by a facile synthesis using the co-reduction of metal precursors in the presence of polyvinyl alcohol. Two series of catalysts with Au:Ag atomic ratios of 1:4 and 1:8 were prepared. Each series consisted of 4 catalysts with various ratios of Au:Pd ranging from 1:0.1 to 1:0.2 and a catalyst without Pd. The resulting metal nanoparticles were formed as nanonetworks with Ag-rich shell. The catalytic activity of the Pd-promoted AuAg-based catalysts was superior over the unpromoted one except for the AuAgxPd0.2/C in which containing too high Pd amount. The most active catalyst was AuAg4Pd0.1/C yielding 2.5 times higher current density than did the AuAg4/C catalyst. Hence, the proposed catalyst model makes use of the surface modifications of the segregated surface, which may influence further catalyst designs for other applications.
1. Introduction Au-based catalysts are the most active catalysts for glycerol electrooxidation in alkaline, in which the reaction rate of glycerol on Au is extraordinarily high [1–8]. In addition, Au-based catalysts show superior electrocatalytic stability for glycerol electrooxidation in half-cell tests [6,9], making them favorable as an anode catalyst for alkaline direct glycerol fuel cells using anion exchange membranes. There are several attempts, which include using metal oxides [7,9] or alloying with other metals, such as Pt [10,11], Ag [12–16] or Pd [17–20], to improve the catalytic activity of Au catalysts toward glycerol electrooxidation. Among these metals, Ag is one of the most suitable promoters of Au. In our previous work, AuAg/C prepared by the galvanic displacement of Ag by Au3+ was highly active for glycerol electrooxidation, as well as for other smaller alcohols [21]. During cyclic voltammetry, the catalytic activity of the galvanic-displaced AuAg/C catalyst was gradually increased. It was anticipated that the alcohol adsorption induced surface segregation and the more active Au would be drawn toward the surfaces of the nanoparticles via the interaction with alcohol [22]. On the other hand, Ag could be kept on the surface by the interaction with the hydroxide ion in the solution because of its higher affinity with hydroxide
⁎
[23]. Consequently, the activity of the AuAg catalyst depended on the concentration of both alcohol and hydroxide in an electrolyte. The aim of this work was to further improve the catalytic properties of the AuAg catalyst with low Au content by adding an appropriate third metal. Pd, another metal which provides an improved catalytic activity toward electrooxidation of glycerol and smaller alcohol molecules while alloying with Au was selected. The synergistic between Pd and Au such as electronic modification and bifunctional mechanism was reported [17,18]. In this work, the promotional effects of small amounts of Pd on AuAg catalysts were investigated. The AuAgPd catalysts with Au:Pd ratios of 1:0.1, 1:0.15, and 1:0.2 by mol were synthesized by a co-reduction method using polyvinyl alcohol as a protecting agent. The electrocatalytic activities and stabilities of the synthesized catalysts toward glycerol electrooxidation were studied by cyclic voltammetry and chronoamperometry, respectively. 2. Experimental procedure 2.1. Electrocatalyst preparation The AuAg/C and AuAgPd/C catalysts were prepared by the co-
Corresponding author. E-mail address: [email protected] (S. Yongprapat).
https://doi.org/10.1016/j.jelechem.2019.113225 Received 22 October 2018; Received in revised form 1 June 2019; Accepted 11 June 2019 Available online 12 June 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 847 (2019) 113225
S. Yongprapat, et al.
reduction of their metal precursors. A PdCl2 precursor was pre-dissolved in KCl solution to form a dissolved KPdCl4 complex. Calculated amounts of HAuCl4, AgNO3, and KPdCl4 solution were mixed together in the presence of polyvinyl alcohol (PVA). The overall metal concentration in solution was kept constant at 100 μg L−1. After mixing well, the metal precursors were reduced by excess amount of 0.1 M NaBH4 in one portion under vigorous stirring. Vulcan XC-72 as a dispersed media was added after 15 min of reduction. The mixture was stirred for another 30 min to immobilize the metal nanomaterials on Vulcan XC-72. Finally, the AuAg/C or AuAgPd/C catalysts were filtered, washed with de-ionized water and ethanol, then dried in a vacuum oven. Two series of catalysts were prepared with Au:Ag molar ratios of 1:4 and 1:8. Each series consisted of 4 catalysts with different Au:Pd ratios ranging from no Pd to 1:0.2. The catalysts obtained were designated as AuAg4Pdx/C and AuAg8Pdx/C (x = 0.1, 0.15 and 0.2) for both catalyst series. The total metal loading on all the catalysts was fixed at 20 wt%.
Hence it can be expected that the peak position of AuAg bimetallic remains unchanged. The broad peaks from 20° to 50° and a tiny peak at about 54° were the characteristic of the sample grid [8]. No sign of the Pd peak appeared (according to JCPDS No. 89–4897 at 2θ of 40.1, 46.5, and 68.1) even in the catalyst with the highest Pd loading (AuAg4Pd0.2/C), implying the absence of Pd crystal formation. All the metals appeared in metallic form as no evidences of AgO, AuO, Au2O3 and PdO peaks (JCDPS No. 41-1104, 23-0278, 43-1039 and 41-1107) were observed. The crystal size of the nanonetwork was roughly estimated using Scherrer's equation with the shape factor of spherical particle (k = 0.94). The calculated crystal sizes based on the (220) reflecting plane are shown in Table 1. The crystal sizes of AuAg4 and AuAg8 were around 6.0 nm while that of the Pd-containing catalysts were slightly larger in range of 6.1–6.4 nm. The binding energies of Au 4f7/2 and Ag 3d5/2 in all of the catalysts are summarized in Table 1. The peak intensity of Pd was too low to deconvolute. Au and Ag only appeared in metallic form. All the binding energies of both Au and Ag were the same with very small variations regardless of the change in composition. However, it should be noted that the binding energies of Au 4f7/2 and Ag 3d5/2 on Au/C and Ag/C were 84.7 and 368.5 eV, respectively [12]. The binding energy of Ag in all the prepared catalysts shifted to higher values, indicating electron transfer from Ag to Au, as was observed in the AuAg bimetallic catalyst [13]. The metal compositions characterized by XPS and EDS are shown in Table 1. Pd was detected by both XPS and EDS, however, at the significant level too low to appropriately use for calculating Pd:Au ratio. XPS is accomplished by exciting a sample surface using the Al Kα X-ray. This process produces photoelectron which was used to identify and quantify the element along with its chemical state. This technique was surface sensitive with a typical analysis depth of several nanometers. In contrast, the SEM/EDS detects the X-rays emitted from the electronexcited specimen. The depth of analysis depends on the accelerating voltage of electrons with a typical surface depth of several micrometers. For the nanonetworks with about 10 nm diameter, the XPS provides the surface composition while EDS provides the overall composition of the metal. From EDS results, the overall Ag:Au ratio were close to the intended ratio at around 4:1 and 8:1 for electrocatalysts in AuAg4- and AuAg8series. The Ag:Au ratio from XPS was significantly variated from the bulk ratio. Most of them have higher Ag:Au ratio than that in the bulk. These results suggested the formation of mixed AueAg phase in the form of Au-rich core and Ag-rich surface.
2.2. Catalyst characterization The morphology of the catalysts was examined by Transmission Electron Microscopy (TEM). The TEM images were taken at a voltage of 200 kV by a JOEL JEM 2010. The electronic states of Au and Ag were studied by using X-ray photoelectron spectroscopy (XPS), which was performed by an AXIS Ultra DLD (Kratos Analytical) with Al K-alpha monochromatic X-ray radiation (1486.68 eV) at 150 W. The crystallite sizes and lattice parameters of the as-prepared catalysts were characterized by an X-ray diffractometer (XRD). A Bruker AXS: D8DISCOVER with Goebel Mirror Cu Ka radiation (l = 0.1540 nm) using a 40-mA filament current and 40-kV tube voltage was employed to collect XRD patterns. The metal loading and Ag:Au ratio was estimated by the Energy dispersive spectroscopy (EDS) equipped with Scanning electron microscopy (SEM). The overall component analysis was performed with an aperture of 30 μm at 200 kV. The average element composition was calculated from three points of the collected data. The electrochemical measurements were conducted using a SI 1287 potentiostat (Solartron). To form the working electrode, the well-mixed catalyst ink containing the catalyst (2.5 mg), 5%wt Nafion solution (21.7 μl), and isopropanol (2 ml) were drop-cast onto a glassy carbon electrode. A mercury‑mercury oxide (1 M NaOH, MMO) and a 1-cm2 Au gauze were used as a reference electrode and a counter electrode, respectively. All the reported potentials were taken versus the MMO. 3. Results and discussion
3.2. Electrochemical activity
3.1. Physicochemical characterization
Fig. 4 shows the CVs of the catalysts in the AuAg4 series in the KOH electrolyte. For comparison, the CVs at a slow scan rate of 10 mV s−1 of Ag/C, Au/C and Pd/C prepared by using the same method are shown in Fig. 5. In KOH electrolyte, only the peaks associated with oxide formation and reduction were observed. The positions of these peaks on Au/C and Ag/C took place at around the same potentials while those of Pd/C occurred at more negative potential. All the AuAg4Pdx catalysts exhibited only the peaks similar to the peaks associated with Ag oxide formation and reduction [24,25]. The peak positions were unchanged by the addition of Pd. However, the current density of the Ag oxide reduction peak was lowered as the amount of Pd increased, indicating a lower electroactive surface area for Ag. All the catalysts prepared in this work were required to undergo multiple cycles of cyclic voltammetry in a solution containing glycerol and KOH for activation. The catalyst was almost inactive during the first cycle. Then, the current density sharply increased until it reached its maximum value in several cycles. This occurrence was similar to that of the AuAg/C catalysts prepared by the galvanic displacement method,
The TEM images of the catalysts in the AuAg4/C series and AuAg8/C are shown in Fig. 1. The metal nanoparticles on all catalysts appeared as a large nanoparticle network. Neither the ratio of Au:Ag nor the presence of Pd had affected the sizes of the nanowires connected to a larger network. The average diameter of the nanoparticle network was about 10 nm on all the catalysts. The TEM images of AuAg4/C and AuAg4Pd0.2/C (the catalyst with the highest Pd loading), as shown in Fig. 2, displayed the lattice fringes of Au(111) or Ag(111) with a distance of 2.35 Å, which was consistent regardless of the Pd ratio. No separated lattices of Pd(111) appeared (distance = 2.24 Å). The amount of Pd in the nanoparticles may have been too small to cause a noticeable change in the morphologies of the AuAgPd catalysts. The XRD patterns of the AuAg4/C and AuAg8/C series, as shown in Fig. 3, exhibit the fcc lattices identical to those of the Au/C prepared by PVA protection method [8] regardless of their compositions. Similar results were also observed for the AuAg8/C series (results not shown). The alloying formation of AuAg cannot be observed from the XRD results, since both Au and Ag have the same lattice constant of 4.080. 2
Journal of Electroanalytical Chemistry 847 (2019) 113225
S. Yongprapat, et al.
Fig. 1. TEM images of (a) AuAg4/C, (b) AuAg4Pd0.1/C, (c) AuAg4Pd0.15/C, (d) AuAg4Pd0.2/C, and (e) AuAg8/C.
in which the catalysts exhibited the same behaviors [12]. Fig. 6 shows the CVs of the glycerol electrooxidation in the fifth scanning cycle of all the prepared catalysts. The glycerol electrooxidation on these catalysts was similar to that on the Au surface [6]. The only main electrooxidation peak began at −0.2 V. After that, the current density increased until reaching the maximum at around 0.25 V, depending on the composition of the catalyst. Beyond this potential, the metal surface was covered by oxide, then the catalytic activity declined and was regained in the reverse scan once the surface oxide was reduced at approximately 0.15 V. The glycerol electrooxidation on the Pd surface that normally occurs at a more negative potential than on Au-based catalysts [26] was not observed in the voltammogram. The glycerol electrooxidation activities of the AuAg-based catalysts
were greatly improved by the addition of Pd. The catalysts with low Pd content, i.e., with Pd0.1 and Pd0.15, were more active than their respective AuAg catalysts. AuAg4Pd0.1/C and AuAg8Pd0.15/C were the most active catalysts in their series, yielding about 2.5 and 1.5 times higher current density, respectively, than those on the unpromoted catalysts, as shown in Table 2. However, the catalyst became less active with higher Pd content. Fig. 7(a) and (b) display the chronoamperograms of all the prepared catalysts for glycerol electrooxidation. Each CA was recorded after the catalyst was cycled in 2 M KOH electrolyte for 5 cycles between −0.8 and 0.6 V to obtain the constant CV. The current density obtained was very low during the very first second, followed by a sharp increase in current density. This result was different from that was obtained with the Au/C catalyst, for which the
Fig. 2. TEM images displaying the lattice fringes of (a) AuAg4/C and (b) AuAg4Pd0.2/C. 3
Journal of Electroanalytical Chemistry 847 (2019) 113225
S. Yongprapat, et al.
Fig. 3. X-ray diffraction patterns of catalysts in AuAg4/C and AuAg8/C series. Table 1 The physicochemical properties obtained from XRD, XPS and EDS results. Electrocatalyst
AuAg4/C AuAg4Pd0.1/C AuAg4Pd0.15/C AuAg4Pd0.2/C AuAg8/C AuAg8Pd0.1/C AuAg8Pd0.15/C AuAg8Pd0.2/C a
Crystal sizea
Binding energy (eV)
Ag:Au ratio
(nm)
Au 4f7/2
(XPS)
(EDS)
5.85 5.92 4.93 3.65 8.43 9.41 10.53 9.61
4.41 4.39 4.56 4.67 7.83 8.77 8.89 7.76
5.98 6.40 6.16 6.29 6.09 6.38 6.47 6.43
84.7 84.7 84.7 84.7 84.7 84.7 84.7 84.6
Ag 3d5/2 368.7 368.7 368.7 368.7 368.8 368.8 368.7 368.7
± ± ± ± ± ± ± ±
0.04 0.20 0.21 0.70 0.55 0.67 0.93 0.35
Fig. 6. CVs at the fifth cycle of the catalysts in the (a) AuAg4-series and (b) AuAg8-series in 1 M glycerol and 2 M KOH at a scan rate of 50 mV s−1.
Estimated from XRD patterns.
Table 2 Maximum current densities from CV and final current densities from CA results of glycerol electrooxidation obtained from all the prepared catalysts. Electrocatalyst
Imax (A mgAu−1)
Final current density (A mgAu−1)
AuAg4/C AuAg4Pd0.1/C AuAg4Pd0.15/C AuAg4Pd0.2/C AuAg8/C AuAg8Pd0.1/C AuAg8Pd0.15/C AuAg8Pd0.2/C
10.78 26.49 17.39 8.68 14.98 20.04 22.06 11.85
0.77 2.07 1.57 1.13 0.69 1.66 2.06 0.96
current density was high at the beginning due to double layer charging, then slowly decayed due to catalyst poisoning [6]. The addition of Pd resulted in a change in the CA pattern. The CA of the Pd-containing catalysts took a longer time to reach the highest current density. AuAg4/C and AuAg8/C took only about 1 min while the Pd-promoted catalysts required at least 5–10 min. After achieving the maximum value, the current densities of the unpromoted catalysts suddenly dropped with a high decay rate and followed by a slower decreased after 30 min. On the Pd-promoted catalysts, however, the current density was sustained for few minutes, and dropped with a steadier current decay rate than the unpromoted catalysts. As a result, all of the Pd-promoted catalysts yielded higher final current densities than that of the unpromoted catalysts, as summarized in Table 2. The enhancement by Ag of the activity and stability of the Au-based catalysts was reported elsewhere [12]. The promotion effects of Ag included the increase in the adsorption capability of glycerol on the Au surface by the reduction of the 5d band density of Au [13] and by the bifunctional mechanism by which the Ag supplies oxygenated species to
Fig. 4. CVs of catalysts in AuAg4/C series in 2 M KOH at a scan rate of 50 mV s−1.
Fig. 5. CVs of AuAg4Pd0.1/C, Ag/C, Au/C and Pd/C in 2 M KOH at a scan rate of 10 mV s−1. 4
Journal of Electroanalytical Chemistry 847 (2019) 113225
S. Yongprapat, et al.
under Grant No. TRG5880131 and by King Mongkut's University of Technology Thonburi (KMUTT) through the “KMUTT 55th Anniversary Commemoration Fund”. References [1] Y. Kwon, S.C.S. Lai, P. Rodriguez, M.T.M. Koper, Electrocatalytic oxidation of alcohols on gold in alkaline media: base or gold catalysis? J. Am. Chem. Soc. 133 (2011) 6914–6917. [2] J.H. Zhang, Y.J. Liang, N. Li, Z.Y. Li, C.W. Xu, S.P. Jiang, A remarkable activity of glycerol electrooxidation on gold in alkaline medium, Electrochim. Acta 59 (2012) 156–159. [3] L. Xin, Z. Zhange, Z. Wang, W. Li, Simultaneous generation of mesoxalic acid and electricity From glycerol on a gold anode catalyst in anion-exchange membrane fuel cells, ChemCatChem 4 (2012) 1105–1114. [4] J. Qi, L. Xin, D.J. Chadderdon, Y. Qiu, Y. Jiang, N. Benipal, C. Liang, W. Li, Electrocatalytic selective oxidation of glycerol to tartronate on Au/C anode catalysts in anion exchange membrane fuel cells with electricity cogeneration, Appl. Catal. B Environ. 154-155 (2014) 360–368. [5] D. Padayachee, V. Golovko, B. Ingham, A.T. Marshall, Influence of particle size on the electrocatalytic oxidation of glycerol over carbon-supported gold nanoparticles, Electrochim. Acta 120 (2014) 398–407. [6] S. Yongprapat, A. Therdthianwong, S. Therdthianwong, Au/C catalyst prepared by polyvinyl alcohol protection method for direct alcohol alkaline exchange membrane fuel cell application, J. Appl. Electrochem. 42 (2012) 483–490. [7] S. Yongprapat, S. Therdthianwong, A. Therdthianwong, RuO2 promoted Au/C catalysts for alkaline direct alcohol fuel cells, Electrochim. Acta 83 (2012) 87–93. [8] S. Yongprapat, A. Therdthianwong, S. Therdthianwong, Au/C catalysts promoted with metal oxides for ethylene glycol electro-oxidation in alkaline solution, J. Electroanal. Chem. 697 (2013) 46–52. [9] D. Padayachee, V. Golovko, A.T. Marshall, The effect of MnO2 loading on the glycerol electrooxidation activity of Au/MnO2/C catalysts, Electrochim. Acta 98 (2013) 208–217. [10] L. Lertthahan, S. Yongprapat, A. Therdthianwong, S. Therdthianwong, Pt-modified Au catalysts for direct glycerol electro-oxidation in an alkaline medium, Int. J. Hydrog. Energy 42 (14) (2017) 9202–9209. [11] C. Jin, C. Sun, R. Dong, Z. Chen, Electrocatalytic activity of PtAu/C catalysts for glycerol oxidation, J. Nanosci. Nanotechnol. 12 (2012) 324–329. [12] S. Yongprapat, A. Therdthianwong, S. Therdthianwong, Improvement of catalytic performance of AuAg/C catalysts prepared by galvanic displacement technique for glycerol electrooxidation in alkaline medium, J. Appl. Electrochem. 48 (2018) 317–328. [13] A.C. Garcia, P.P. Lopes, J.H. Gomes, C. Pires, E.B. Ferreira, R.G.M. Lucena, L.H.S. Gasparotto, G. Tremiliosi-Filho, Eco-friendly synthesis of bimetallic AuAg nanoparticles, New J. Chem. 38 (2014) 2865–2873. [14] A.C. Garcia, J. Caliman, E.B. Ferreira, G. Tremiliosi-Filho, J.J. Linares, Promotional effect of Ag on the catalytic activity of Au for glycerol electrooxidation in alkaline medium, ChemElectroChem 2 (2015) 1036–1041. [15] J.F. Gomes, A.C. Garcia, L.H.S. Gasparotto, N.E. de Souza, E.B. Ferreira, C. Pires, Influence of silver on the glycerol electro-oxidation over AuAg/C catalysts in alkaline medium: a cyclic voltammetry and in situ FTIR spectroscopy study, Electrochim. Acta 144 (2014) 361–368. [16] C. Jin, J. Zhu, R. Dong, Z. Chen, Modification of Ag nanoparticles/reduced graphene oxide nanocomposites with a small amount of Au for glycerol oxidation, Int. J. Hydrog. Energy 41 (38) (2016) 16851–16857. [17] W. Hong, C. Shang, J. Wang, E. Wang, Synthesis of dendritic PdAu nanoparticles with enhanced electrocatalytic activity, Electrochem. Commun. 48 (2014) 65–68. [18] H. Xu, B. Yan, J. Wang, S. Li, C. Wang, Y. Shiraishi, Y. Du, P. Yang, Ultrasonic assisted synthesis of N-doped graphene-supported binary PdAu nanoflowers for enhanced electrooxidation of ethylene glycol and glycerol, Electrochim. Acta 245 (2017) 227–236. [19] H. Xu, J. Wang, B. Yan, S. Li, C. Wang, Y. Shiraishi, P. Yang, Y. Du, Facile construction of fascinating trimetallic PdAuAg nanocages with exceptional ethylene glycol and glycerol oxidation activity, Nanoscale 9 (2017) 17004–17012. [20] B.T.X. Lam, M. Chiku, E. Higuchi, H. Inoue, Preparation of PdAg and PdAu nanoparticleloaded carbon black catalysts and their electrocatalytic activity for the glycerol oxidation in alkaline medium, J. Power Sources 297 (2015) 149–157. [21] S. Yongprapat, A. Therdthianwong, S. Therdthianwong, The activity of AuAg/C prepared by galvanic displacement of Ag on carbon by Au3+ towards alcohol electrooxidation, SEE 2016 on “Energy & Environment Change: Innovating for a Sustainable Future, Bangkok, Thailand”, 2016. [22] H. Liao, A. Fisher, Z.J. Xu, Surface segregation in bimetallic nanoparticles: a critical issue in electrocatalyst engineering, Small 11 (27) (2015) 3221–3246. [23] C. Yang, B. Huang, L. Xiao, Z. Ren, Z. Liu, J. Lu, L. Zhuang, Activating Ag by even more inert Au: a peculiar effect on electrocatalysis toward oxygen reduction in alkaline media, Chem. Commun. 49 (2013) 11023–11025. [24] S.T. Mayer, R.H. Muller, Nucleation of silver (I) oxide investigated by spectroscopic ellipsometry, J. Electrochem. Soc. 135 (9) (1988) 2133–2142. [25] D. Hecht, R. Frahm, H.H. Strehblow, Quick-scanning EXAFS in the reflection mode as a probe for structural information of electrode surfaces with time resolution: an in situ study of anodic silver oxide, J. Phys. Chem. 100 (1996) 10831–10833. [26] E. Habibi, H. Razmi, Glycerol electrooxidation on Pd, Pt and Au nanoparticles supported on carbon ceramic electrode in alkaline media, Int. J. Hydrog. Energy 37 (2012) 16800–16809. [27] M. Mougenot, A. Caillard, M. Simoes, S. Baranton, C. Coutanceau, P. Brault, PdAu/C catalysts prepared by plasma sputtering for the electro-oxidation of glycerol, Appl. Catal. B Environ. 107 (2011) 372–379.
Fig. 7. CAs of (a) AuAg4 and (b) AuAg8 catalyst series in a solution of 1 M glycerol and 2 M KOH at 0.25 V.
the surface-Au for glycerol electrooxidation [12]. This improvement was probably due to the synergistic effects between Pd and Au, which had been observed by several studies [17,18,27]. PdAu is more active for alcohol electrooxidation than Au or Pd alone. The proposed synergistic effects between Pd and Au were the modification of the electronic structure of Pd [17] and the bifunctional mechanism by which the Au supplies oxygenate species for alcohol electrooxidation on Pd [18]. 4. Conclusions In this work, the remarkable improvement of the catalytic activity of the AuAg-based catalyst was illustrated. The catalytic activity of low Au loading catalysts was greatly enhanced. Two series of AuAgPd/C catalysts with small Pd portions were prepared by the co-reduction of the metal precursors in the presence of PVA. The obtained catalysts were in the form of nanonetworks with average diameters of about 10 nm. The physicochemical characterizations were found only in the metallic phases of Au and Ag. The composition analysis using XPS and EDS suggested the formation of Au-rich core and Ag-rich shell. Despite the absence of Pd due to its small amount in the catalysts, the catalytic activity of the Pd-containing catalysts at Au:Pd ratios of 1:0.1 and 1:0.15 had been improved greatly from the unpromoted catalyst. AuAg4Pd0.1/C and AuAg8Pd0.15/C were the best performers in their series. The results of this work showed that the activity of the segregated catalyst may be controlled by the use of an appropriate type and amount of third metal. Acknowledgments This work was supported by the Thailand Research Fund (TRF)
5
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
The enhanced activity of AuAg/C nanonetwork catalysts for glycerol electrooxidation by small amounts of Pd
T
S. Yongprapata, , A. Therdthianwonga, S. Therdthianwongb ⁎
a
Fuel Cells and Hydrogen Research and Engineering Center, Pilot Plant, Development and Training Institute (PDTI), King Mongkut's University of Technology Thonburi (KMUTT), 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand b Department of Chemical Engineering, Faculty of Engineering, King Mongkut's University of Technology Thonburi (KMUTT), 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand
ARTICLE INFO
ABSTRACT
Keywords: Glycerol electrooxidation Trimetallic catalyst AuAgPd Direct glycerol fuel cell, nanonetwork catalyst
The catalytic properties of the Au/C catalyst toward glycerol electrooxidation in alkaline medium have been tremendously improved by an incorporation of Au with Ag. In this work, the catalytic performance of the AuAg/ C was further improved by introducing small amount of a third metal, Pd. The catalysts were synthesized by a facile synthesis using the co-reduction of metal precursors in the presence of polyvinyl alcohol. Two series of catalysts with Au:Ag atomic ratios of 1:4 and 1:8 were prepared. Each series consisted of 4 catalysts with various ratios of Au:Pd ranging from 1:0.1 to 1:0.2 and a catalyst without Pd. The resulting metal nanoparticles were formed as nanonetworks with Ag-rich shell. The catalytic activity of the Pd-promoted AuAg-based catalysts was superior over the unpromoted one except for the AuAgxPd0.2/C in which containing too high Pd amount. The most active catalyst was AuAg4Pd0.1/C yielding 2.5 times higher current density than did the AuAg4/C catalyst. Hence, the proposed catalyst model makes use of the surface modifications of the segregated surface, which may influence further catalyst designs for other applications.
1. Introduction Au-based catalysts are the most active catalysts for glycerol electrooxidation in alkaline, in which the reaction rate of glycerol on Au is extraordinarily high [1–8]. In addition, Au-based catalysts show superior electrocatalytic stability for glycerol electrooxidation in half-cell tests [6,9], making them favorable as an anode catalyst for alkaline direct glycerol fuel cells using anion exchange membranes. There are several attempts, which include using metal oxides [7,9] or alloying with other metals, such as Pt [10,11], Ag [12–16] or Pd [17–20], to improve the catalytic activity of Au catalysts toward glycerol electrooxidation. Among these metals, Ag is one of the most suitable promoters of Au. In our previous work, AuAg/C prepared by the galvanic displacement of Ag by Au3+ was highly active for glycerol electrooxidation, as well as for other smaller alcohols [21]. During cyclic voltammetry, the catalytic activity of the galvanic-displaced AuAg/C catalyst was gradually increased. It was anticipated that the alcohol adsorption induced surface segregation and the more active Au would be drawn toward the surfaces of the nanoparticles via the interaction with alcohol [22]. On the other hand, Ag could be kept on the surface by the interaction with the hydroxide ion in the solution because of its higher affinity with hydroxide
⁎
[23]. Consequently, the activity of the AuAg catalyst depended on the concentration of both alcohol and hydroxide in an electrolyte. The aim of this work was to further improve the catalytic properties of the AuAg catalyst with low Au content by adding an appropriate third metal. Pd, another metal which provides an improved catalytic activity toward electrooxidation of glycerol and smaller alcohol molecules while alloying with Au was selected. The synergistic between Pd and Au such as electronic modification and bifunctional mechanism was reported [17,18]. In this work, the promotional effects of small amounts of Pd on AuAg catalysts were investigated. The AuAgPd catalysts with Au:Pd ratios of 1:0.1, 1:0.15, and 1:0.2 by mol were synthesized by a co-reduction method using polyvinyl alcohol as a protecting agent. The electrocatalytic activities and stabilities of the synthesized catalysts toward glycerol electrooxidation were studied by cyclic voltammetry and chronoamperometry, respectively. 2. Experimental procedure 2.1. Electrocatalyst preparation The AuAg/C and AuAgPd/C catalysts were prepared by the co-
Corresponding author. E-mail address: [email protected] (S. Yongprapat).
https://doi.org/10.1016/j.jelechem.2019.113225 Received 22 October 2018; Received in revised form 1 June 2019; Accepted 11 June 2019 Available online 12 June 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 847 (2019) 113225
S. Yongprapat, et al.
reduction of their metal precursors. A PdCl2 precursor was pre-dissolved in KCl solution to form a dissolved KPdCl4 complex. Calculated amounts of HAuCl4, AgNO3, and KPdCl4 solution were mixed together in the presence of polyvinyl alcohol (PVA). The overall metal concentration in solution was kept constant at 100 μg L−1. After mixing well, the metal precursors were reduced by excess amount of 0.1 M NaBH4 in one portion under vigorous stirring. Vulcan XC-72 as a dispersed media was added after 15 min of reduction. The mixture was stirred for another 30 min to immobilize the metal nanomaterials on Vulcan XC-72. Finally, the AuAg/C or AuAgPd/C catalysts were filtered, washed with de-ionized water and ethanol, then dried in a vacuum oven. Two series of catalysts were prepared with Au:Ag molar ratios of 1:4 and 1:8. Each series consisted of 4 catalysts with different Au:Pd ratios ranging from no Pd to 1:0.2. The catalysts obtained were designated as AuAg4Pdx/C and AuAg8Pdx/C (x = 0.1, 0.15 and 0.2) for both catalyst series. The total metal loading on all the catalysts was fixed at 20 wt%.
Hence it can be expected that the peak position of AuAg bimetallic remains unchanged. The broad peaks from 20° to 50° and a tiny peak at about 54° were the characteristic of the sample grid [8]. No sign of the Pd peak appeared (according to JCPDS No. 89–4897 at 2θ of 40.1, 46.5, and 68.1) even in the catalyst with the highest Pd loading (AuAg4Pd0.2/C), implying the absence of Pd crystal formation. All the metals appeared in metallic form as no evidences of AgO, AuO, Au2O3 and PdO peaks (JCDPS No. 41-1104, 23-0278, 43-1039 and 41-1107) were observed. The crystal size of the nanonetwork was roughly estimated using Scherrer's equation with the shape factor of spherical particle (k = 0.94). The calculated crystal sizes based on the (220) reflecting plane are shown in Table 1. The crystal sizes of AuAg4 and AuAg8 were around 6.0 nm while that of the Pd-containing catalysts were slightly larger in range of 6.1–6.4 nm. The binding energies of Au 4f7/2 and Ag 3d5/2 in all of the catalysts are summarized in Table 1. The peak intensity of Pd was too low to deconvolute. Au and Ag only appeared in metallic form. All the binding energies of both Au and Ag were the same with very small variations regardless of the change in composition. However, it should be noted that the binding energies of Au 4f7/2 and Ag 3d5/2 on Au/C and Ag/C were 84.7 and 368.5 eV, respectively [12]. The binding energy of Ag in all the prepared catalysts shifted to higher values, indicating electron transfer from Ag to Au, as was observed in the AuAg bimetallic catalyst [13]. The metal compositions characterized by XPS and EDS are shown in Table 1. Pd was detected by both XPS and EDS, however, at the significant level too low to appropriately use for calculating Pd:Au ratio. XPS is accomplished by exciting a sample surface using the Al Kα X-ray. This process produces photoelectron which was used to identify and quantify the element along with its chemical state. This technique was surface sensitive with a typical analysis depth of several nanometers. In contrast, the SEM/EDS detects the X-rays emitted from the electronexcited specimen. The depth of analysis depends on the accelerating voltage of electrons with a typical surface depth of several micrometers. For the nanonetworks with about 10 nm diameter, the XPS provides the surface composition while EDS provides the overall composition of the metal. From EDS results, the overall Ag:Au ratio were close to the intended ratio at around 4:1 and 8:1 for electrocatalysts in AuAg4- and AuAg8series. The Ag:Au ratio from XPS was significantly variated from the bulk ratio. Most of them have higher Ag:Au ratio than that in the bulk. These results suggested the formation of mixed AueAg phase in the form of Au-rich core and Ag-rich surface.
2.2. Catalyst characterization The morphology of the catalysts was examined by Transmission Electron Microscopy (TEM). The TEM images were taken at a voltage of 200 kV by a JOEL JEM 2010. The electronic states of Au and Ag were studied by using X-ray photoelectron spectroscopy (XPS), which was performed by an AXIS Ultra DLD (Kratos Analytical) with Al K-alpha monochromatic X-ray radiation (1486.68 eV) at 150 W. The crystallite sizes and lattice parameters of the as-prepared catalysts were characterized by an X-ray diffractometer (XRD). A Bruker AXS: D8DISCOVER with Goebel Mirror Cu Ka radiation (l = 0.1540 nm) using a 40-mA filament current and 40-kV tube voltage was employed to collect XRD patterns. The metal loading and Ag:Au ratio was estimated by the Energy dispersive spectroscopy (EDS) equipped with Scanning electron microscopy (SEM). The overall component analysis was performed with an aperture of 30 μm at 200 kV. The average element composition was calculated from three points of the collected data. The electrochemical measurements were conducted using a SI 1287 potentiostat (Solartron). To form the working electrode, the well-mixed catalyst ink containing the catalyst (2.5 mg), 5%wt Nafion solution (21.7 μl), and isopropanol (2 ml) were drop-cast onto a glassy carbon electrode. A mercury‑mercury oxide (1 M NaOH, MMO) and a 1-cm2 Au gauze were used as a reference electrode and a counter electrode, respectively. All the reported potentials were taken versus the MMO. 3. Results and discussion
3.2. Electrochemical activity
3.1. Physicochemical characterization
Fig. 4 shows the CVs of the catalysts in the AuAg4 series in the KOH electrolyte. For comparison, the CVs at a slow scan rate of 10 mV s−1 of Ag/C, Au/C and Pd/C prepared by using the same method are shown in Fig. 5. In KOH electrolyte, only the peaks associated with oxide formation and reduction were observed. The positions of these peaks on Au/C and Ag/C took place at around the same potentials while those of Pd/C occurred at more negative potential. All the AuAg4Pdx catalysts exhibited only the peaks similar to the peaks associated with Ag oxide formation and reduction [24,25]. The peak positions were unchanged by the addition of Pd. However, the current density of the Ag oxide reduction peak was lowered as the amount of Pd increased, indicating a lower electroactive surface area for Ag. All the catalysts prepared in this work were required to undergo multiple cycles of cyclic voltammetry in a solution containing glycerol and KOH for activation. The catalyst was almost inactive during the first cycle. Then, the current density sharply increased until it reached its maximum value in several cycles. This occurrence was similar to that of the AuAg/C catalysts prepared by the galvanic displacement method,
The TEM images of the catalysts in the AuAg4/C series and AuAg8/C are shown in Fig. 1. The metal nanoparticles on all catalysts appeared as a large nanoparticle network. Neither the ratio of Au:Ag nor the presence of Pd had affected the sizes of the nanowires connected to a larger network. The average diameter of the nanoparticle network was about 10 nm on all the catalysts. The TEM images of AuAg4/C and AuAg4Pd0.2/C (the catalyst with the highest Pd loading), as shown in Fig. 2, displayed the lattice fringes of Au(111) or Ag(111) with a distance of 2.35 Å, which was consistent regardless of the Pd ratio. No separated lattices of Pd(111) appeared (distance = 2.24 Å). The amount of Pd in the nanoparticles may have been too small to cause a noticeable change in the morphologies of the AuAgPd catalysts. The XRD patterns of the AuAg4/C and AuAg8/C series, as shown in Fig. 3, exhibit the fcc lattices identical to those of the Au/C prepared by PVA protection method [8] regardless of their compositions. Similar results were also observed for the AuAg8/C series (results not shown). The alloying formation of AuAg cannot be observed from the XRD results, since both Au and Ag have the same lattice constant of 4.080. 2
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Fig. 1. TEM images of (a) AuAg4/C, (b) AuAg4Pd0.1/C, (c) AuAg4Pd0.15/C, (d) AuAg4Pd0.2/C, and (e) AuAg8/C.
in which the catalysts exhibited the same behaviors [12]. Fig. 6 shows the CVs of the glycerol electrooxidation in the fifth scanning cycle of all the prepared catalysts. The glycerol electrooxidation on these catalysts was similar to that on the Au surface [6]. The only main electrooxidation peak began at −0.2 V. After that, the current density increased until reaching the maximum at around 0.25 V, depending on the composition of the catalyst. Beyond this potential, the metal surface was covered by oxide, then the catalytic activity declined and was regained in the reverse scan once the surface oxide was reduced at approximately 0.15 V. The glycerol electrooxidation on the Pd surface that normally occurs at a more negative potential than on Au-based catalysts [26] was not observed in the voltammogram. The glycerol electrooxidation activities of the AuAg-based catalysts
were greatly improved by the addition of Pd. The catalysts with low Pd content, i.e., with Pd0.1 and Pd0.15, were more active than their respective AuAg catalysts. AuAg4Pd0.1/C and AuAg8Pd0.15/C were the most active catalysts in their series, yielding about 2.5 and 1.5 times higher current density, respectively, than those on the unpromoted catalysts, as shown in Table 2. However, the catalyst became less active with higher Pd content. Fig. 7(a) and (b) display the chronoamperograms of all the prepared catalysts for glycerol electrooxidation. Each CA was recorded after the catalyst was cycled in 2 M KOH electrolyte for 5 cycles between −0.8 and 0.6 V to obtain the constant CV. The current density obtained was very low during the very first second, followed by a sharp increase in current density. This result was different from that was obtained with the Au/C catalyst, for which the
Fig. 2. TEM images displaying the lattice fringes of (a) AuAg4/C and (b) AuAg4Pd0.2/C. 3
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Fig. 3. X-ray diffraction patterns of catalysts in AuAg4/C and AuAg8/C series. Table 1 The physicochemical properties obtained from XRD, XPS and EDS results. Electrocatalyst
AuAg4/C AuAg4Pd0.1/C AuAg4Pd0.15/C AuAg4Pd0.2/C AuAg8/C AuAg8Pd0.1/C AuAg8Pd0.15/C AuAg8Pd0.2/C a
Crystal sizea
Binding energy (eV)
Ag:Au ratio
(nm)
Au 4f7/2
(XPS)
(EDS)
5.85 5.92 4.93 3.65 8.43 9.41 10.53 9.61
4.41 4.39 4.56 4.67 7.83 8.77 8.89 7.76
5.98 6.40 6.16 6.29 6.09 6.38 6.47 6.43
84.7 84.7 84.7 84.7 84.7 84.7 84.7 84.6
Ag 3d5/2 368.7 368.7 368.7 368.7 368.8 368.8 368.7 368.7
± ± ± ± ± ± ± ±
0.04 0.20 0.21 0.70 0.55 0.67 0.93 0.35
Fig. 6. CVs at the fifth cycle of the catalysts in the (a) AuAg4-series and (b) AuAg8-series in 1 M glycerol and 2 M KOH at a scan rate of 50 mV s−1.
Estimated from XRD patterns.
Table 2 Maximum current densities from CV and final current densities from CA results of glycerol electrooxidation obtained from all the prepared catalysts. Electrocatalyst
Imax (A mgAu−1)
Final current density (A mgAu−1)
AuAg4/C AuAg4Pd0.1/C AuAg4Pd0.15/C AuAg4Pd0.2/C AuAg8/C AuAg8Pd0.1/C AuAg8Pd0.15/C AuAg8Pd0.2/C
10.78 26.49 17.39 8.68 14.98 20.04 22.06 11.85
0.77 2.07 1.57 1.13 0.69 1.66 2.06 0.96
current density was high at the beginning due to double layer charging, then slowly decayed due to catalyst poisoning [6]. The addition of Pd resulted in a change in the CA pattern. The CA of the Pd-containing catalysts took a longer time to reach the highest current density. AuAg4/C and AuAg8/C took only about 1 min while the Pd-promoted catalysts required at least 5–10 min. After achieving the maximum value, the current densities of the unpromoted catalysts suddenly dropped with a high decay rate and followed by a slower decreased after 30 min. On the Pd-promoted catalysts, however, the current density was sustained for few minutes, and dropped with a steadier current decay rate than the unpromoted catalysts. As a result, all of the Pd-promoted catalysts yielded higher final current densities than that of the unpromoted catalysts, as summarized in Table 2. The enhancement by Ag of the activity and stability of the Au-based catalysts was reported elsewhere [12]. The promotion effects of Ag included the increase in the adsorption capability of glycerol on the Au surface by the reduction of the 5d band density of Au [13] and by the bifunctional mechanism by which the Ag supplies oxygenated species to
Fig. 4. CVs of catalysts in AuAg4/C series in 2 M KOH at a scan rate of 50 mV s−1.
Fig. 5. CVs of AuAg4Pd0.1/C, Ag/C, Au/C and Pd/C in 2 M KOH at a scan rate of 10 mV s−1. 4
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under Grant No. TRG5880131 and by King Mongkut's University of Technology Thonburi (KMUTT) through the “KMUTT 55th Anniversary Commemoration Fund”. References [1] Y. Kwon, S.C.S. Lai, P. Rodriguez, M.T.M. Koper, Electrocatalytic oxidation of alcohols on gold in alkaline media: base or gold catalysis? J. Am. Chem. Soc. 133 (2011) 6914–6917. [2] J.H. Zhang, Y.J. Liang, N. Li, Z.Y. Li, C.W. Xu, S.P. Jiang, A remarkable activity of glycerol electrooxidation on gold in alkaline medium, Electrochim. Acta 59 (2012) 156–159. [3] L. Xin, Z. Zhange, Z. Wang, W. Li, Simultaneous generation of mesoxalic acid and electricity From glycerol on a gold anode catalyst in anion-exchange membrane fuel cells, ChemCatChem 4 (2012) 1105–1114. [4] J. Qi, L. Xin, D.J. Chadderdon, Y. Qiu, Y. Jiang, N. Benipal, C. Liang, W. Li, Electrocatalytic selective oxidation of glycerol to tartronate on Au/C anode catalysts in anion exchange membrane fuel cells with electricity cogeneration, Appl. Catal. 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Linares, Promotional effect of Ag on the catalytic activity of Au for glycerol electrooxidation in alkaline medium, ChemElectroChem 2 (2015) 1036–1041. [15] J.F. Gomes, A.C. Garcia, L.H.S. Gasparotto, N.E. de Souza, E.B. Ferreira, C. Pires, Influence of silver on the glycerol electro-oxidation over AuAg/C catalysts in alkaline medium: a cyclic voltammetry and in situ FTIR spectroscopy study, Electrochim. Acta 144 (2014) 361–368. [16] C. Jin, J. Zhu, R. Dong, Z. Chen, Modification of Ag nanoparticles/reduced graphene oxide nanocomposites with a small amount of Au for glycerol oxidation, Int. J. Hydrog. Energy 41 (38) (2016) 16851–16857. [17] W. Hong, C. Shang, J. Wang, E. Wang, Synthesis of dendritic PdAu nanoparticles with enhanced electrocatalytic activity, Electrochem. Commun. 48 (2014) 65–68. [18] H. Xu, B. Yan, J. Wang, S. Li, C. Wang, Y. Shiraishi, Y. Du, P. Yang, Ultrasonic assisted synthesis of N-doped graphene-supported binary PdAu nanoflowers for enhanced electrooxidation of ethylene glycol and glycerol, Electrochim. Acta 245 (2017) 227–236. [19] H. Xu, J. Wang, B. Yan, S. Li, C. Wang, Y. Shiraishi, P. Yang, Y. Du, Facile construction of fascinating trimetallic PdAuAg nanocages with exceptional ethylene glycol and glycerol oxidation activity, Nanoscale 9 (2017) 17004–17012. [20] B.T.X. Lam, M. Chiku, E. Higuchi, H. Inoue, Preparation of PdAg and PdAu nanoparticleloaded carbon black catalysts and their electrocatalytic activity for the glycerol oxidation in alkaline medium, J. Power Sources 297 (2015) 149–157. [21] S. Yongprapat, A. Therdthianwong, S. Therdthianwong, The activity of AuAg/C prepared by galvanic displacement of Ag on carbon by Au3+ towards alcohol electrooxidation, SEE 2016 on “Energy & Environment Change: Innovating for a Sustainable Future, Bangkok, Thailand”, 2016. [22] H. Liao, A. Fisher, Z.J. Xu, Surface segregation in bimetallic nanoparticles: a critical issue in electrocatalyst engineering, Small 11 (27) (2015) 3221–3246. [23] C. Yang, B. Huang, L. Xiao, Z. Ren, Z. Liu, J. Lu, L. Zhuang, Activating Ag by even more inert Au: a peculiar effect on electrocatalysis toward oxygen reduction in alkaline media, Chem. Commun. 49 (2013) 11023–11025. [24] S.T. Mayer, R.H. Muller, Nucleation of silver (I) oxide investigated by spectroscopic ellipsometry, J. Electrochem. Soc. 135 (9) (1988) 2133–2142. [25] D. Hecht, R. Frahm, H.H. Strehblow, Quick-scanning EXAFS in the reflection mode as a probe for structural information of electrode surfaces with time resolution: an in situ study of anodic silver oxide, J. Phys. Chem. 100 (1996) 10831–10833. [26] E. Habibi, H. Razmi, Glycerol electrooxidation on Pd, Pt and Au nanoparticles supported on carbon ceramic electrode in alkaline media, Int. J. Hydrog. Energy 37 (2012) 16800–16809. [27] M. Mougenot, A. Caillard, M. Simoes, S. Baranton, C. 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Fig. 7. CAs of (a) AuAg4 and (b) AuAg8 catalyst series in a solution of 1 M glycerol and 2 M KOH at 0.25 V.
the surface-Au for glycerol electrooxidation [12]. This improvement was probably due to the synergistic effects between Pd and Au, which had been observed by several studies [17,18,27]. PdAu is more active for alcohol electrooxidation than Au or Pd alone. The proposed synergistic effects between Pd and Au were the modification of the electronic structure of Pd [17] and the bifunctional mechanism by which the Au supplies oxygenate species for alcohol electrooxidation on Pd [18]. 4. Conclusions In this work, the remarkable improvement of the catalytic activity of the AuAg-based catalyst was illustrated. The catalytic activity of low Au loading catalysts was greatly enhanced. Two series of AuAgPd/C catalysts with small Pd portions were prepared by the co-reduction of the metal precursors in the presence of PVA. The obtained catalysts were in the form of nanonetworks with average diameters of about 10 nm. The physicochemical characterizations were found only in the metallic phases of Au and Ag. The composition analysis using XPS and EDS suggested the formation of Au-rich core and Ag-rich shell. Despite the absence of Pd due to its small amount in the catalysts, the catalytic activity of the Pd-containing catalysts at Au:Pd ratios of 1:0.1 and 1:0.15 had been improved greatly from the unpromoted catalyst. AuAg4Pd0.1/C and AuAg8Pd0.15/C were the best performers in their series. The results of this work showed that the activity of the segregated catalyst may be controlled by the use of an appropriate type and amount of third metal. Acknowledgments This work was supported by the Thailand Research Fund (TRF)
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