Electrocatalytic Activity of Pd20-xAgx ... - ACS Publications

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Electrocatalytic Activity of Pd20-xAgx Nanoparticles Embedded in Carbon Nanotubes for Methanol Oxidation in Alkaline Media MORU SATYANARAYANA, Gaddam Rajeshkhanna, Malaya K. Sahoo, and Gangavarapu Ranga Rao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00544 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Applied Energy Materials

Electrocatalytic Activity of Pd20-xAgx Nanoparticles Embedded in Carbon Nanotubes for Methanol Oxidation in Alkaline Media M Satyanarayana, G Rajeshkhanna, Malaya K. Sahoo, G Ranga Rao∗ Department of Chemistry, Indian Institute of Technology Madras, Tamil Nadu 600036, India. Abstract A low Pd content and highly active bimetallic PdAg electrocatalytic nanoparticles supported on functionalised multiwalled carbon nanotubes (CNT) has been developed for electrochemical methanol oxidation reaction (MOR). Polyol synthesis procedure is used to substitute Ag into Pd particles in Pd/CNT matrix. The Pd20-xAgx/CNT samples (x wt% = 0, 5, 10 and 15) are characterized by XRD, XPS, Raman spectroscopy and microscopy methods. The electrocatalytic activities and stabilities of Pd20-xAgx/CNT catalysts were examined by cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP) measurements for electrochemical oxidation of methanol in alkaline media. The Pd20xAgx/CNT

with x = 10 wt% is found to be a semi-alloyed sample and shows the best catalytic

activity (731 mA mg-1) compared to Pd/CNT catalyst (408 mA mg-1) and other samples. This semi-alloyed sample also shows long stability and high resistance towards CO poisoning. The semi-alloyed Pd10Ag10 particles with optimised metal ratio on CNT matrix would be a promising catalyst material for direct methanol fuel cell applications. Keywords: Methanol oxidation, Pd nanoparticles, Carbon nanotubes, PdAg bimetallic, Electrocatalysis.

Introduction *

Corresponding author; Tel.: +91 44 2257 4226; Fax: +91 44 2257 4202; E-mail:

[email protected] (G. Ranga Rao).

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Fuel cell technology has been emerging as clean and alternate electrochemical energy supply route for electric vehicles and portable electronic devices because of the associated advantages such as low-to-zero pollutant emissions and high energy densities. Among various fuel cell categories, direct methanol fuel cells (DMFCs) are considered as unique energy sources with characteristic advantages which include excessive energy densities, low pollutant waste, low operational temperatures (60 - 100 °C) and liquid fuel feed. 1-3 Presently, platinum is an active catalytic component in the electrode material for methanol oxidation reactions (MORs), but it is expensive and strongly prone to CO poisoning during MOR [1]. Palladium is considered as an alternative to Pt to resolve these problems, since it has nearly equal electrocatalytic activity and less prone to reaction intermediates of MOR in alkaline media compare to Pt. Moreover, Pd is also a cost-effective choice as an electrocatalyst in fuel cells and about fifty times more abundant than Pt.4-6 The strategy to further improve the electrocatalytic performance of Pd or Pt-based elctrocatalysts is to combine them with synergistic secondary metals such as Sn,7, 8 Co,9, 10 Ni,11-13 Au,14 or Ag15 and metal oxides.16-19 The use of secondary metal or metal oxide is a promising method as they can attract oxygen-containing molecules at low potentials and help oxidize the adsorbed CO intermediate. This procedure can potentially reduce the poisoning of electrocatalyst and enhance its stability.17, 19 The Pd metal catalyst in the form of nanoparticles would be ideal to offer large active surface area and high electrocatalytic activity towards MOR. However, naked palladium nanoparticles tend to aggregate and tend to lose their electrocatalytic activity during MOR. High surface area and conductive carbon support is necessary as a catalyst backbone to prevent the aggregation of metal nanoparticles both in alkaline and acidic media. In recent years, a variety of carbon materials such as carbon nanotubes (CNTs),2, (Vulcan XC-72),14,

21

20

carbon black

carbon microspheres,22 carbon nanodots,23 carbon nanofibers,24 and 2 ACS Paragon Plus Environment

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graphene oxide (GO) nanosheets25 have been explored as supports for palladium-based electrocatalysts. Carbon nanotubes (CNTs) with unique structural properties have been used as support materials for metal nanoparticles to fabricate electrocatalysts.26-28 CNTs have tubular geometry of hundred micrometers of length, chemically inert, mechanically stable and high electrical conductivity. However, pristine CNTs are chemically passive and deficient in binding sites to anchor the precursor metal ions or metal nanoparticles. This generally leads to sluggish metal dispersion and agglomeration of metal nanoparticles. The binding sites are introduced in the form of hydroxyl/carboxyl functional groups by chemically oxidizing the CNTs under refluxing conditions using concentrated HNO3.29-31 Recent studies show that the performance of Pd/C catalysts in association with gold nanoparticles is far superior compared to mere Pd electrocatalysts. Reaction intermediates such as CO and hydrocarbon species are easily oxidized to CO2 in the presence gold particles and help freeing the active sites on Pd catalyst for further electrochemical reactions.21,

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Recently, Nguyen et al reported that impregnating Ag into the Pd/C matrix can enhance the bi-functional catalytic activity for ethanol oxidation reaction and resist CO poisoning compared to commercial Pt/C and Pd/C catalysts.33 It is therefore worthwhile to investigate Pd-based catalysts by incorporating Au and/or Ag as a viable alternative to enhance both electrocatalytic performance and catalyst stability.15,

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Recently, Ag is shown as a good

promoter in the form of PdAg alloy nanoparticles supported on carbon nanotubes for alcohol oxidation reaction.15 In the present study, we have investigated the synergistic effect of Ag substituted Pd nanoparticles anchored on functionalised CNT matrix. We have been able to reduce the Pd content and enhance the catalytic effect as well as the stability of the electrocatalyst for methanol oxidation in alkaline medium.



2. Experimental 2.1. Materials MWCNTs (Assay - min.95%, OD: 10-20 nm, Length: 10-30 µm) was obtained from Sisco Research Laboratories, India. Silver nitrate, palladium (II) chloride and ethylene glycol (EG) were purchased from Sigma Aldrich. All additional chemicals used were of analytical grade (> 99.5 % purity).

2.2. Preparation of PdAg/CNT Composites The PdAg/CNT electrocatalysts were prepared by a microwave-assisted polyol process.34, 35 MWCNTs are first purified and carboxyl functionalized by treating with 3 M HNO3 as reported earlier.26 Then, 80 mg of the functionalized MWCNTs was added to the mixture of 100 mL of ethylene glycol (EG) and isopropyl alcohol (IPA) (volume ratio of 4:1) and ultrasonicated until a homogeneous black suspension was obtained. Ethylene glycol (25 mL) was used to dissolve 0.443 g of PdCl2 salt by adding sufficient amount of 1.0 M HCl. The solution containing PdCl2-4 was used as precursor for depositing Pd. Similarly, 0.423 g of silver nitrate dissolved in 25 ml of EG solution was used as silver precursor. Both the precursor solutions containing desired weight ratios of Pd and Ag were added to the resultant MWCNT-EG homogeneous solution by fixing the total metal content at 20 wt% and sonicated for 30 min. The samples prepared have the Pd : Ag metal wt% ratios of 15:5, 10:10 and 5:15. The resultant solution was allowed to stir for 1 h to accumulate the Pd and Ag ions on the surface of carbon nanotubes. The pH of the stirring mixture was adjusted to ∼10 by dropwise addition of 1 M KOH-EG solution. Further, the mixture ink was exposed to microwave irradiation for 30 s two times with 30 s of interval in a domestic microwave oven (Sharp NN-S327 WF, 2450 MHz, and 1100W) for complete reduction of Pd and Ag metal ions. The resulting product was then allowed to cool to room temperature with continuous


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stirring. The pH of the microwave-treated solution mixture was then adjusted to ∼4 using 0.1 M HNO3 solution and was further stirred for 6 h before filtering out the residue. The resultant product was then repeatedly washed with Millipore water, followed by ethanol, until it became free of Cl− ion. The product was then dried overnight under vacuum at 60 °C. Schematic illustration of synthesis procedure is presented in Fig. S1 (Supporting information) 2.3. Characterization The X-ray diffraction (XRD) analysis of the synthesized nanocomposite and CNT were recorded by Brucker AXS D8 diffractometer in 2θ range of 10°-90° which was equipped with Cu (k = 1.5406 Å) as the x-ray source with a step size of 0.002o and a scan speed of 0.5 second per step. Raman spectra of CNT, functionalized CNT, Pd/CNT and PdAg/CNT samples were examined by using confocal spectrometer (MultiRAM Stand Alone FT-Raman Spectrometer) with Nd:YAG laser source (1064 nm). Morphology of CNT and PdAg/CNT hybrid composites were examined on an energy-dispersive x-ray analyser (EDXA) equipped field emission scanning electron microscope (FESEM) (FEG Inspect F) operating at 10 - 30 kV. The samples were smeared on a conducting carbon tape and a thin layer of gold coating was sputtered on the samples to avoid charging during analysis. Microstructural analysis of PdAg/CNT hybrid composite was carried out employing JEOL 3010 HRTEM fixed with UHR pole piece and operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were conducted using Omicron Nanotechnology instrument with Mg Kα (hν =1253.6 eV) radiation. 2.4. Electrochemical measurements Electrocatalytic activity measurements were carried out in a conventional three-electrode glass cell connected to CHI 7081C electrochemical workstation and employing Hg|HgO as a reference electrode, platinum foil (1 cm2 area) as a counter electrode and 6 mm glassy carbon



electrode (GCE) coated with catalyst sample as a working electrode. Prior to use, GCE was successively polished with 0.3 and 0.05 µm alumina powder slurries and rinsed with deionized water, followed by successive sonication in ethanol and deionized water, and allowed to dry. The catalytic ink was prepared by dispersing 5 mg of Pd/CNT or Pd20xAgx/CNT

catalyst powder in 1.8 mL of 1:1 IPA + H2O mixture and 0.2 mL of 5 wt% Nafion

in ethanol solution for 30 min. 20 µL of the composite ink was dropped onto the glassy carbon electrode by using a micropipette and dried at room temperature. The catalyst loading per unit geometric area is ca. 0.18 mg cm-2. The electrocatalytic activities of the catalysts towards methanol oxidation were determined by using high-purity-nitrogen-purged 1 M KOH and 0.5 M methanol test solution. The cyclic voltammetry measurements were done after attaining the stable current response of the working electrodes under continuous cycling at a scan rate of 20 mV s-1.

3. Results and discussion The powder XRD patterns of Pd20-xAgx/CNT (x = 0, 5, 10 and 15) catalysts are shown in Fig. 1a. The XRD shows that the Pd20-xAgx particles mostly exist as alloy phases on CNT matrix, except Pd10Ag10 sample where some metallic Ag phase can be inferred. The Bragg reflections due to crystalline silver at 38.5°, 44.7°, 64.8° and 77.7° representing (111), (200), (220) and (311) planes (JCPDS No. 04-0783). Similarly, Bragg reflections due to palladium nanoparticles at 39.7°, 46.2°, 67.5° represent the (111), (200), (220) planes of fcc structure (JCPDS No. 87-0638). The diffraction peak at ca. 26° is due to the carbon (002) plane. Using Scherrer method,36 the average crystallite sizes of Ag20 and Pd20 nanoparticles are estimated, respectively, as 15 nm and 6 nm. The structural changes in CNTs occurring during functionalization and after preparing the composite with Pd and Ag have been examined by Raman spectral features. Fig. 1b illustrates the Raman spectra of pristine CNT, functionalized-CNT, Pd/CNT and Pd10Ag10/CNT 6 ACS Paragon Plus Environment

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samples. All samples show the D-band (1283 cm-1) and G-band (1600 cm-1) doublet associated with structural disorders and graphitic nature of carbon materials, respectively. The ID/IG ratio is an indication of the structural defects caused by the chemical modifications of carbon. Functionalization of CNTs was confirmed by peak intensity ratios of D-band and G-band (ID/IG), whereas we could observe remarkable increase in ID/IG values from pristine CNT (1.17) to functionalized CNT (1.36). Further, no change is observed in the ID/IG values of the functionalized CNT in Pd/CNT (1.25) and Pd10Ag10/CNT (1.25) samples. This shows that there is partial reduction of functional groups which results in increased structural order in functionalized CNT in the metal loaded samples. In order to see the distribution of Pd10Ag10 nanoparticles on CNT matrix FESEM has been recorded. The FESEM images of Pd10Ag10/CNT (Fig. S2a & 2b) shows the existence of small clusters of PdAg semi-alloyed nanoparticles on CNT matrix. The chemical composition of the Pd10Ag10/CNT is examined by EDX measurements which shows the presence C, Pd and Ag elements in the sample (Fig. S2c). The EDX colour elemental mapping of Pd10Ag10/CNT composite is shown in Fig. 2(a-d), which shows the presence of Ag and Pd with uniform distribution on the CNT. Fig. 3(a-d) shows the TEM images of Pd10Ag10/CNT nanocomposite. These TEM images evidence that PdAg particles are attached to the surface of functionalised CNTs through strong interfacial interactions and which are not observed in the bulk space of the matrix. This represents that Pd and Ag nanoparticles are well connected with the highly conducting CNTs. The estimated average crystallite sizes of metal nanoparticles from TEM observations are found to be in the range of 12-16 nm. Fig. 4 shows the Pd3d XPS spectra of Pd/CNT and PdAg/CNT nanocomposites, the observed two spin-orbit splitting peaks at binding energy values (BE) of ~336.0 (3d5/2) and ~341.2 eV (3d3/2) are attributed to metallic Pd (Fig. 4a). The Ag3d XPS spectra of PdAg/CNT is shown in Fig. 4b, the two spin-orbit splitting peaks at BE values of 367.8 eV and 373.9 eV 7 ACS Paragon Plus Environment


respectively for 3d5/2 and 3d3/2 peak are ascribed to metallic Ag. The shift in BE values (∼0.6 eV) of the Pd 3d core level to higher bind energy were observed in the Pd10Ag10/CNT compare to the Pd/CNT catalyst attributed to the strong interaction between Pd and Ag atoms. Hence, this interactions leads to influence the adsorption of methanol or reaction intermediates to Pd active sites, resulting in the enhancement of the electrocatalytic activity towards MOR. In order to evaluate the MOR electrocatalytic of Pd/CNT and Pd20-xAgx/CNT (x = 5, 10 and 15) nanocomposite cyclic voltammetry (CV) analysis is conducted in 1.0 M KOH containing 0.5 M methanol and respective CVs are presented in Fig. 5. In Fig. 6 we show the CVs of all the samples examined in 1 M KOH under similar experimental conditions. Comparison of Figs. 5 and 6 reveals that all electrocatalysts are active for methanol oxidation under anodic conditions in 1 M KOH + 0.5 M CH3OH. Electrochemical oxidation of methanol at Pd/CNT and Pd20-xAgx/CNT nanocomposite electrodes shows two well defined oxidation current peaks (Fig. 5). The oxidation peak during forward scan represents the oxidation of freshly chemisorbed methanol molecules approaching the electrode surface from the bulk of the solution. The oxidation peak in reverse scan is attributed to the removal of CO and other residual carbonaceous species formed on the electrode during the forward sweep. It clearly seen that the methanol oxidation activity of Pd10Ag10 sample is much higher compared to other samples. The onset potential (Eop) of methanol oxidation, oxidation peak current density (Ip) and oxidation peak potentials (Ep) of all the electrode materials are presented in Table 1. From Fig. 5a and Table 1, we see that the electrocatalyst Pd10Ag10/CNT shows the lowest onset potential and the highest anodic peak current compared to other samples. Therefore Pd10Ag10 bimetallic composition is more active towards methanol oxidation. Addition of Ag atoms to


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Pd has greatly influenced the magnitude of the peak current as well as peak potential (Ep). The peak potential corresponding to the methanol oxidation is shifted to less positive side as the amount of Ag is increased in the bimetallic phase. The onset potentials (Eop) for methanol oxidation on Pd20-xAgx/CNT (x = 5, 10 and 15) nanocomposites are also found to be lesser than that of the Pd20/CNT electrode. However, the onset potentials for methanol oxidation on Pd10Ag10/CNT and Pd5Ag15/CNT electrodes are more negative compared to those of Pd20/CNT and Pd15Ag5/CNT electrodes. This result demonstrates an improved kinetics of methanol oxidation by introducing an optimum amount of Ag into Pd20/CNT composite. The order of electrocatalytic activity of Pd20-xAgx/CNT nanocomposites from the CV results is found to be Pd20/CNT ˂ Pd15Ag5/CNT ˂ Pd5Ag15/CNT ˂ Pd10Ag10/CNT. The Pd10Ag10/CNT electrode shows higher anodic peak current as well as lower onset potential compared to the other electrodes indicating that 10 wt% is an optimum replacement of Pd by Ag for higher electrocatalytic activity towards methanol oxidation. Further, in order to calculate the electrochemical active surface area (EASA) of all the electrode materials cyclic voltammetric analysis was carried out in 1 M KOH electrolyte. The EASA has been measured by determining the coulombic charge for the reduction of palladium oxide (QS). The Ag hydroxyl complex and PdO reduction peaks appear between 0.3 to 0.1 V (peak (i)) and ‒0.2 to ‒0.5 V (peak (ii)) in the cyclic voltammograms of all the electrodes recorded in 1 M KOH solution (Fig. 5b). The EASAs of all the electrodes have been estimated using the equation, EASA = QS / (QC × M Pd ) , where QC is the conversion factor and has been commonly taken as 405 µC cm-2 corresponding to reduction of PdO monolayer and M Pd is the catalyst loading in grams.38 The coulombic charge for the reduction of palladium oxide (QS) of all the electrodes is determined by integrating the palladium oxide reduction peak (ii) in Fig. 5b and used to calculate the EASA of the



electrodes. Calculated values of EASA for different electrodes are given in Table 1. It is clear from table 1 that Pd10Ag10/CNT catalyst shows higher EASA compared to other nanocomposites and the order of EASA of nanocomposites is Pd20/CNT ˂ Pd15Ag5/CNT ˂ Pd5Ag15/CNT ˂ Pd10Ag10/CNT which is agreeing well with the CV results obtained for methanol oxidation. We therefore conclude that the electrocatalytic activity of methanol oxidation is enhanced on Pd20-xAgx/CNT electrodes due to two factors, (i) increase in EASA and (ii) synergistic electronic contribution of optimum amount of Ag in the bimetallic particle. The influence of scan rate on the electrochemical oxidation of methanol on Pd10Ag10/CNT sample studied by the CV technique (Fig. S3a). The oxidation peak currents gradually increase with increasing scan rate, and there is a perfect proportionality observed between the oxidation peak current and the square root of the scan rate in the range of 5 to 50 mV s-1 (Fig. S3b). This behaviour indicates the diffusion limited electrochemical oxidation of methanol occurring on Pd10Ag10/CNT electrode. The stability and electrochemical activity of Pd20/CNT and Pd20-xAgx/CNT nanocomposite electrodes are investigated by chronoamperometry (CA) measurements. The typical chronoamperometry profiles of Pd/CNT and Pd20-xAgx/CNT composite electrodes are depicted in Fig. 6a. The CA profile of Pd10Ag10/CNT electrode shows the highest current and lowest current decay with polarization time during the methanol oxidation, which is consistent with the cyclic voltammetry results in Fig. 5a. Therefore, the Pd10Ag10/CNT electrode is much more efficient and tolerant to poisoning compared to other electrodes in the alkaline medium. Chronopotentiometric measurements give complimentary information on the stability and antipoisoning capability of the electrocatalysts for electrochemical oxidation of methanol. We have carried out steady state measurements at a constant current density of 5 mA cm−2 on Pd20/CNT and Pd10Ag10/CNT composite electrodes. Fig. 6b shows the change in electrode 10 ACS Paragon Plus Environment

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potential with time at a fixed bias current density. Both Pd20/CNT and Pd10Ag10/CNT composite electrodes show slow increase in electrode potentials with polarization time and after certain time a sharp increase in the potential to a limiting value. This sharp increase in polarization voltage is due to the poisoning effect associated with oxygen evolution or water electrolysis rather than electrochemical oxidation of methanol. The polarization potential is relatively lower for Pd10Ag10/CNT composite electrode which essentially indicates that Pd10Ag10 shows high catalytic activity and can withstand against poisoning species for longer time than Pd20/CNT. The Pd20/CNT catalyst electrode reaches ~0.6 V within 70 min while Pd10Ag10/CNT catalyst is more stable for much longer period of 142 min. The Pd10Ag10/CNT electrode maintains its electrocatalytic activity twice longer period than Pd20/CNT. Therefore addition of optimum amount of Ag to Pd is beneficial in order to improve the electrocatalytic activity, lowering the overpotential, improving the antipoisoning activity and providing longtime stability to Pd10Ag10/CNT catalyst. This electrode also exhibited higher electrocatalytic activity compared to the recently reported Pd-based catalysts, and a comparison of the activities is provided in Table 2. 2, 11, 12, 39-44 In MOR pathway, chemisorbed CO intermediate on the active sites of catalysts has been identified as a major poisoning species for MOR activity. CO stripping study could consider as a best method to examine the CO tolerance behaviour of catalysts. To observe this phenomenon, the CO anti-poisoning experiments were carried out at room temperature. In this process, initially the KOH solution (1.0 M) was deaerated by purging with nitrogen for 30 min and then allowed the catalyst to chemisorb the CO species by bubbling with CO gas for 15 min. Then, the residual CO in the solution was removed by again purging the solution with N2 for 30 min. Fig. 7 shows two consecutive CVs recorded at CO chemisorbed Pd/C (Fig. 7a) and Pd10Ag10/CNT (Fig. 7b) catalyst coated electrodes within the potentials between -0.8 V to 0.4 V at a scan rate of 50 mV s−1 in 1.0 M KOH. In the first forward scan, it is clear 11 ACS Paragon Plus Environment


to see that a CO oxidation peak appears. On the second forward scan, the CO oxidation peak disappears which infers the complete elimination of CO on the surface of catalyst, indicating regeneration of catalyst surface from CO poisoning. It is also observed that the onset potentials for the electrocatalytic oxidation of CO on Pd10Ag10/CNT (-0.53 V) is more negative than that on Pd/CNT (-0.32 V), indicating an increased CO oxidation activity and good CO anti-poisoning ability of Pd10Ag10/CNT. All the results further suggest that the Pd10Ag10/CNT demonstrate excellent electrocatalytic performance for MOR and superior CO tolerance ability. The plausible overall electrocatalytic pathways for MOR on Pd20-xAgx/CNT surface are shown in Fig. 8. The process of MOR involves 6-electron transfer reaction pathways with various organic intermediate species. Generally, Ag doesn’t act as the direct catalyst for MOR, Pd sites are solely responsible for the oxidation of methanol. However, numerous intermediates formed during the methanol oxidation process are prone to poison the active Pd catalyst sites for further oxidation.45, 46 Among various intermediates, CO is most stable and it has a more tendency to poison the active Pd sites, due to its high binding energy. Therefore, the addition of Ag can enhance the MOR activity due to synergistic effect. Because of oxophillic character of Ag, it promotes the -OH adsorption and thereby oxidation of CO intermediate to CO2 and H2O and generates free Pd sites for further adsorption and oxidation of MeOH as shown in Fig. 8. In addition, the combination of Ag with Pd can shift up the dband center of Pd and changes the electronic properties of the Pd, consequently enhances the charge transfer process and lowers the activation energy of MOR. The experimental observations show that the optimum amount of Ag in Pd20-xAgx/CNT is 10 (i.e Pd10Ag10/CNT). When x>10 (i.e Pd5Ag15/CNT) there is possibility of decrease in the Pd active sites for MOR thereby decrease in the catalytic activity, when x

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