ENHANCED AND SYNERGISTIC CATALYSIS OF ONE-POT SYNTHESIZED PALLADIUM-NICKEL ALLOY NANOPARTICLES FOR ANODIC OXIDATION OF METHANOL IN ALKALI

Accepted Manuscript Title: ENHANCED AND SYNERGISTIC CATALYSIS OF ONE-POT SYNTHESIZED PALLADIUM-NICKEL ALLOY NANOPARTICLES FOR ANODIC OXIDATION OF METHANOL IN ALKALI Authors: Sreya Roy Chowdhury, Srabanti Ghosh, Swapan Kumar Bhattachrya PII: DOI: Reference:

S0013-4686(17)31688-2 http://dx.doi.org/doi:10.1016/j.electacta.2017.08.050 EA 30054

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

5-4-2017 21-7-2017 8-8-2017

Please cite this article as: Sreya Roy Chowdhury, Srabanti Ghosh, Swapan Kumar Bhattachrya, ENHANCED AND SYNERGISTIC CATALYSIS OF ONE-POT SYNTHESIZED PALLADIUM-NICKEL ALLOY NANOPARTICLES FOR ANODIC OXIDATION OF METHANOL IN ALKALI, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.08.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ENHANCED AND SYNERGISTIC CATALYSIS OF ONE-POT SYNTHESIZED NANOPARTICLES

PALLADIUM-NICKEL FOR

ANODIC

ALLOY

OXIDATION

OF

METHANOL IN ALKALI

Sreya Roy Chowdhury a , Srabanti Ghosh b , Swapan Kumar Bhattachrya a *

a

Physical Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata – 700032,

India. b

Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for

Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 098, India.

*Email – [email protected] Tel.: +919831699643, Fax: +913324146584

ABSTRACT In search for cost effective catalysts capable to oxidize methanol efficiently, nearly mono dispersed PdxNiy binary electro catalysts of varying mutual composition have been synthesized using classical wet chemical protocol in a single pot in absence of any capping agent. The obtained PdxNiy nanoalloys are stable in dispersion and powder form. The X-ray diffraction, spectroscopic and microscopic studies reveal crystallites with diameter of ca 5nm are agglomerated in nearly spherical shape on the base carbon electrode. The synthesized

1

material shows enhanced synergistic effect in catalysis of methanol oxidation reaction in alkali and the alloy containing 20 atom % of Ni seems to be the best for fuels like methanol, formaldehyde unlike sodium formate whose oxidation is catalysed at best by C/Pd electrode. The C/PdxNiy catalysts are significantly protected from poisoning, leading to the excellent electrocatalytic activity. Cyclic voltammograms of possible intermediates like formaldehyde, sodium formate and ex-situ FTIR, chromatographic (HPLC) studies of reaction products reveal that Ni accelerates formation of carbonate rather than formate elucidating the plausible mechanism of the reaction.

Keywords: Alloy nanoparticles, Methanol oxidation, Mechanistic pathway, Synergistic effect, HPLC Analysis of MOR product.

INTRODUCTION Energy crisis has become the most vital challenge of modern civilization due to continuous increase of global energy demand and depletion of natural resources of fuels. Potential researches have been carried out in the past two decades for the development of innovative energy technologies, [1] still the problem is a serious issue today. Fuel cell is one of the finest solutions to the problem, which can provide energy security as well as environmental sustainability [2]. So, different types of cells with fuels like hydrogen [3], alcohol [4, 5], formate [6], and solid oxide [7] etc is developed. Among the various alcohols, methanol seems to be one of the most promising chemical power sources. Low molar mass, high energy density (6.1KWhkg-1), advantage of lacking C-C bond, zero or low exhaust, ease of recharging etc make methanol to be a most valuable fuel ingredient in fuel cells [8, 9, 10]. Moreover, availability of methanol is high and it can be generated and obtained from 2

different sources e.g. natural gas, oil, coal, etc. For direct methanol fuel cell (DMFC), there is no need of reformer; therefore the system is compact and suitable for portable and mobile power generation [9, 10]. However the large scale commercialisation of is restricted due to low activity, poor durability, reliability, high manufacturing cost etc. Moreover DMFC anode catalysts of DMFC are currently exclusively Pt based; these suffer from surface poisoning by strongly bound CO intermediates, less abundance of Pt and high construction cost. For these reasons, catalysts with increased CO tolerance are desirable and area of further research interest [11]. The cost of catalyst can be reduced by replacing Pt with more abundant palladium and using thin film of catalyst. To decrease the noble metal loading and improving the mass rationalised catalytic activity, incorporation of another oxophilic metal to the noble metal matrix has become a successful practice [12, 13]. The activity of multi component catalyst is decided not only by the intrinsic properties of the metal components but also by their synergistic interactions [12, 13]. Initial studies have indicated that the synergistic effect on the performance of hetero metallic nanocatalysts are subject to surface electronic states, which are greatly affected by changes in the geometric parameters of the catalysts, in particular related to local strain and effective atomic coordination number on the surface. According to the density functional theory of Norskov and co-worker [14-16] the d-band shift and the changes of segregation energy of added metals forming alloy, would influence the capability of Pd based bi metallic catalysts. Bimetallic alloys of Pd with various metals such as Pd-Co [17], Pd-Cu [18, 19], Pd-Ni [20], Pd-Au [21], Pd-Ag [22] etc have exhibited higher capability of alcohol oxidation compared with monometallic Pd catalyst. Several researchers reported that Pd and its alloy with Ni [20, 23, 24] could act as effective catalysts for the oxidation of ethanol. Lower % d band character enhances absorption process during methanol oxidation along with high electro catalytic ability of Pd, made PdxNiy alloy a good choice of catalyst for fuel cell oxidation. But the path of oxidation of methanol 3

molecule on Pd-Ni nano composite and the optimum bimetallic composition of catalyst still remain unresolved and hence need investigation. On this view point we developed PdxNiy bimetallic catalyst using classical wet chemical synthesis protocol. Another factor observed in our early studies [25-27], is that the presence of capping agent with the nanocatalyst often reduces the catalytic activity. Stronger the ligand or capping agent and greater the concentration of the capping agent, worse is the capability of a definite amount of the catalyst. On the other hand, nanometal catalysts and particularly nano noble metals can be obtained also in absence of capping agent; the required stability comes from the solvent coordination. In such cases synthesized particles are free from unwanted adsorbed molecules of capping agent and therefore show greater catalytic activity. Oxidations of formaldehyde and formate on pure Pd and PdxNiy nanoalloy surface are also investigated to understand the mechanism of MOR and a plausible mechanistic path is proposed after analysing the probable products of MOR based on cyclic voltammetric, ex-situ FTIR and HPLC studies.

1. EXPERIMENTAL DETAILS 2.1. Reagents PdCl2 and Nafion (10 mass %) were purchased from Arora Matthey Ltd and Sigma-Aldrich respectively. NiCl2, sodium hydroxide, sodium borohydride, sodium formate, formaldehyde and methanol of analytical reagent grade from Merck, and deionised water (DW) purified through Millipore system were used. Other reagents were commercially available and analytical reagent grade. 2.2. Synthesis of bimetallic nanoparticle Classical wet chemical strategy is employed in a single pot for the synthesis of PdxNiy nanoalloy. For this, 0.01M K2PdCl4 and 0.01M NiCl2 solutions were taken in a round bottom

4

flask with different definite ratio of volume and mixed well by constant stirring for 15 minutes. Then the mixed solution was heated with stirring to 60oC in oil thermostat followed by reduction using excess sodium borohydride as reducing agent. Within a minute of addition, the reaction mixture became black, the instantaneous change of colour indicated quick generation of nanoparticles. Finally the black precipitate was separated from solution by centrifuging at a speed of 10,000 rpm for 5 minute. Then the precipitate was washed several times with Millipore water and taken in a watch glass and then heated in vacuum to dry and collected as powder. For comparison, pure Pd and Ni nanoparticles were also synthesized under the same reaction conditions by taking the corresponding precursor. 2.3. Structural characterization X-ray powder diffraction (XRD) study was carried out using a (Bruker D8 Advance) diffractometer equipped with a CuKα radiation source (λ=1.5418Ǻ generated at 40kV and 40mA). Transmission electron microscopic (TEM) observations were performed using an FEI (Technai S-Twin, operating at 200kV) instrument and with a JEOL 100CXII transmission electron microscope at an accelerating voltage of 100kV. The HRTEM micrographs were recorded using the FEI instrument. Field emission scanning electron microscopy (FESEM, QUANTA FEG 250) investigations were performed by applying a diluted drop of material on a silicon wafer. 2.4. Fabrication of electrodes The electro-catalytic responses of palladium nanoparticle and palladium-nickel nanoparticle were verified using cyclic voltammetric (CV) and fixed potential chronoamperometric (CA) studies. The graphite rod was used as a support for the electro catalyst. Before applying the catalytic powder on the working electrode, carbon surface was polished with emery papers in different grades and cleaned with ethanol. Then mid portion of the rod was wrapped with 5

teflon tape keeping both ends bare. Chemical deposition of palladium and Pd-Ni alloy nanoparticles were executed on one flat end of the rod and other end was kept bare for electrical connection. Electrodes were prepared by “drop and dry” technique of chemical solution deposition. In all cases of the construction of the electrodes, 2mg solid sample per 1mL of water was taken in a stoppard conical flask followed by sonication for at least 20 minute. After sonication 10µL of solution was taken and dropped on bare portion of a previously treated and polished graphite disc electrode and dried for about 1 hour. 5µl of 1(w/v) % nafion was dropped on it covering solid deposit and dried for about 1 hour. The nafion can prevent the catalyst for detachment without affecting the transport of the reactants and products during the electro oxidation of methanol. Finally the catalyst-coated electrodes were dried at ambient temperature for 24hour. 2.5. Electrochemical characterization Electro-catalytic responses were studied at room temperature, (250C) using a conventional three electrode cell with computer aided potentiostat/galvanostat instrument (AUTOLAB company). Freshly prepared nitrogen (99.99%) purged solution was used in each experiment. The working electrodes were Pd, Ni, and PdxNiy nanoalloy of different composition having geometrical surface area 0.0717cm2. The reference electrode was Hg/HgO/OH- (1M) (MMO), whose equilibrium electrode potential was ~0.1V with respect to standard hydrogen electrode (SHE). In each measurement, a large Pt-foil (1cm x1cm) was used as a counter electrode and the potential data were recorded against that of MMO. Numerous CV cycles were executed in the potential range of -0.7V to +0.7V with a scan rate of 1mVs-1 and 50mVs-1 in 1M aqueous NaOH solution with and without 0.5M methanol. The study was also extended to different concentrations like 12, 36, 100mM of methanol, formaldehyde and sodium formate solution to understand the mechanism of the reaction. CA experiment was carried for 7200s at -0.3V for the study of relative stability of the electrodes. The current 6

density values were calculated based on the geometrical surface area of different electro catalysts. 2.6. Studies of the products Following our previous studies [19, 22, 28, 29], a current density of 30µAcm-2 is drawn from 0.5M methanol in 1M NaOH solution kept in N2 atmosphere for 72hour using constructed electrodes like C/Pd, C/Pd4Ni and C/Pd3Ni2 as anodes in separate experiments and a large Pt electrode is used as cathode in each case. A part of the resulting anode solution was used as sample solution for chromatography study. Another part was dried in vacuum and the obtained semi solid product was used in FTIR study. Ex-situ FTIR study was carried out with semi solid products using FTIR spectrophotometer (Perkin Elmer, SN-74514, Spectrum RX1, resolution 4cm-1). Chromatographic determination of formate and formaldehyde was executed using a high-performance liquid chromatography (HPLC) (Shimadzu Corporation, Japan). 20μL of sample solution was injected into a Phenomenex C18 column, provided with dilute sulphuric acid (5mM) as eluent at 300C. Flow rate was 0.5mLmin−1. Each experimental solution was injected three times to check the reproducibility of data. Formate and formaldehyde are detected and separated as product compounds with a UV-absorbance detector. Integration of the chromatographic peaks registered at 254nm was performed by the peak fitting software Origin 8.5. The relative amount of the formate and formaldehyde was calculated by considering the relative peak area corresponding to each component calculated from the chromatogram.

3. RESULTS AND DISCUSSIONS 3.1. Size and morphology of synthesized nanomaterials

7

The formation of face centred cubic (fcc) crystalline structure of Ni, Pd and Pd xNiy nanoalloy of different composition synthesized under similar experimental condition was confirmed by XRD pattern presented in Fig. 1a. The profile 1(a) ‘i’ and 1(a) ‘vi’ correspond to the synthesized pure Pd and Ni nanoparticles. Each pattern exhibits at least four different planes of fcc lattice indexed as (111), (200), (220) and (311). The corresponding 2 theta values for pure Pd are 39.810, 46.090, 67.610 and 80.880 respectively vide from JCPDS data file no 870720, while for pure Ni the corresponding 2 theta values of the first three planes are 44.470, 51.830, 76.350 respectively vide from JCPDS data file no. 87-0712. Similar pattern of XRD peaks are exhibited in profiles 1(a) ‘ii’- ‘v’ for PdxNiy nanoalloys of different compositions and the extent of nickel increases from ‘ii’ to ‘v’. Planes for the synthesized PdNi nanoparticles shift towards higher 2θ values with increasing atom % of Ni in PdxNiy alloy. Similar nature of shift in 2θ values for PdxNiy alloy was found by others [23, 30] in their syntheses of PdNi nanoalloy. The slight shift of peak positions towards higher angle proves that the materials are PdNi alloy with decreased d-spacing and shrink of the lattice constant, due to the incorporation of the Ni atoms into the Pd fcc lattice. This result further indicates that all phases present in the system are bimetallic not a physical mixing of Pd-Ni nanoparticle. Lattice parameter of both samples was calculated by least square method. This result also indicates that composition of nanoalloy can easily varied by change in molar ratio of the reactants. Similar kind of work was also done by our group with Pd-Cu nanoalloy [19]. The binary composition of the Pd-Ni is verified using Vegard’s law [31, 32], which illustrates that the molar compositions of Pd and Ni in the alloy nanoparticles could be obtained from the shifts of lattice parameter calculated from the angular position of the (111), (200) and (220) diffractions. The mutual binary composition of Pd and Ni in PdxNiy alloy nanoparticles is presented in (Table1) and found almost similar to that of precursor solution. According to

8

the broadening of the peak of the X-ray profile, the mean crystallite size of the samples can be evaluated using the classical Debye Scherrer equation [33],

Bhkl 

K T cos hkl

where βhkl is the crystallite size contribution to the peak width (integral or full width at half maximum) in radians, K is a constant near unity (here, 0.9 was used), λ is the wavelength of X-ray (here, 1.5418Ǻ), θhkl is the peak position of the reflection plane (hkl), and T is the average thickness of the crystal in a direction normal to the diffracting plane (hkl). In the present work, the PdNi (111) reflection was used and the mean size was calculated to be nearly 5nm (Table1) for all the alloys of varying composition indicating that the subsequent change in catalytic capability of the synthesized alloys is due to the change of the composition of catalyst instead of their sizes of the crystallites.

Long range XPS study of the catalyst is presented in Fig. 1(b) which reveals the presence of carbon, oxygen, palladium and nickel at the surface. XPS spectra of Pd 3d and Ni 2p regions [34, 35] are illustrated in figures 1(c) and 1(d) respectively. Fig. 1(b) reveals the presence of 3d5/2 and 3d3/2 peaks at binding energy of 335 and 340eV respectively. The curve fittings with the experimental 3d peaks of Pd nucleus show that the zero valent Pd metal is almost solely present in the synthesized alloy. Pd2+ species is almost absent and Pd is formed and unaltered its valence state in presence of Ni in the synthesized alloy nanoparticle. This indicates the uniqueness of the synthesised alloy and the synthetic procedure. The Ni 2P3/2,1/2 peaks at 852.7 and 870.5 eV along with corresponding satellite peaks at 858 and 875 eV are composed of various surface chemical states like 3% of metallic Ni, 40% of NiO and 57% of Ni(OH)2. It seems that these oxide species of Ni directly help in the progress of the MOR reaction subsequently. The computation from the peak-areas of Pd and Ni suggests that the ratio of

9

number of molecules of Pd and Ni at the surface is 53:47 for the alloy Pd80Ni20. This signifies that the formation of oxides of Ni during environmental exposure helps Ni to come out to the surface preferentially.

The TEM images further authenticate the microstructure of the synthesized Pd and representative Pd4Ni alloy nanoparticles. Fig. 2(a) and representative Fig. 2(b) reveal almost spherical microstructure of both Pd and Pd4Ni nanoparticles. Fig. 2(c) and 2(d) reveal the well resolved fringes with lattice spacings of 2.26Ǻ and 2.20Ǻ which can be ascribed to the (111) plane of pure Pd and Pd4Ni respectively. Notably, the fringes of Pd4Ni are located between the standard peaks of pure Ni and Pd. This result again confirms the formation of PdxNiy alloy phase in the prepared nanoparticles. Formation of nanoparticle of alloys was further proved from SEM and EDX studies. The representative SEM image Fig. 3(a) of Pd4Ni alloy reveals closely spaced nanosphere on electrode (surface) with particle size in nano meter range. The elemental distribution of Ni and Pd in the Pd4Ni nano particles was measured through compositional line profiles and EDX mapping analysis. As shown in Fig. 3(c-d), the Pd and Ni traces showed very good match in almost all sites. The elemental mapping results indicate that both Pd and Ni are homogeneously distributed throughout the matrix further confirming the alloy formation at least in statistical sense. 3.2. Cyclic voltammetric study of PdxNiy electrode in alkaline solution The steady cyclic voltammograms of the differently constructed electrodes immersed in N2–saturated 1M NaOH solution were taken. These were executed within a potential range of -0.7V to +0.7V vs. MMO at a scan rate 50mVs-1, and are depicted in Fig. 4(a). There was no peak for nafion coated graphite carbon electrode indicating that the supporting material for electrochemical study is inactive throughout the potential region investigated. As 10

evidenced from the inset of Fig. 4(a), one shoulder and one peak arise for C/Ni electrode at ca -0.207V and +0.514V respectively during anodic scan which can be assigned to conversion of Ni to Ni(OH)2 and Ni(OH)2 to NiOOH respectively following equations (1 and 2) [25, 27, 36]. The corresponding shoulder and peak during reverse scan were observed at the potential of -0.477V and +0.360V due to the opposite reaction as reported in other studies [27, 36]. Ni + 2OH- = Ni(OH)2 + 2e-

(1)

Ni(OH)2 + OH- = NiOOH + H2O + e-

(2)

The CV profiles (Fig.4(a)) for C/Pd electrode show one forward peak during anodic sweep at 0.081V corresponding to the OH- adsorption on the electrode surface following equation (3) and one backward peak around -0.321V for reduction of PdO to Pd following equation (4).

(3) The CV profiles (Fig.4 (a)) for C/PdxNiy electrodes clearly show two distinct regions during anodic sweep corresponding to the electrochemical processes occurring due to electrooxidation of Pd and Ni on the electrode surface. Peak appears around +0.3V is due to formation of Pd-O from Pd-OHads following the reaction (4) The electrochemical active surface area (EASA) of C/Pd and C/PdxNiy electrocatalysts has been measured by calculating the Coulombic charge (Q) for the reduction of palladium oxide according to the following equation: EASA=Q/SI, where Q is the Coulombic charge for the reduction of palladium oxide, S is the proportionality constant used to relate charge with area and I is the catalyst loading in gm-2. A charge value of 405mCcm-2 is assumed for the reduction of PdO monolayer [37, 38]. The roughness factor, Rf = θ/apparent geometrical area, (Table2) reveals that the effective surface area is increased many fold with respect to

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geometrical area indicating advantage of using nanoparticle based constructed thin-film electrode. 3.3. Cyclic voltammetric study of methanol oxidation in alkali Fig. 4(b) compares the steady CVs of methanol oxidation reaction on C/Pd and different C/PdxNiy electrodes in alkaline methanol solution between the potential regions of -0.7V to +0.7V at a scan rate of 50mVs-1. For comparative study, different parameters like anodic peak current densities in forward and backward scan (iF and iB) the corresponding peak potentials (EF and EB), and the ratio of forward and backward peak current density (i F/iB) were evaluated from the CVs (Fig. 4 (b)) and are presented in Table 2. The profiles show typical behaviour observed for methanol oxidation [19] with two anodic peaks. The CV profile of C/Ni suggests that bare Ni does not show any significant oxidation of alkaline methanol in the potential region studied vide from inset of Fig.4(b). On the other hand, C/Pd and C/PdxNiy electrode show profound oxidation of methanol as it appears from large forward and backward peak current densities (i F and iB). A large peak for methanol oxidation appears around +0.03V to -0.063V during anodic scan and the negatively shifted peak potential for the alloy electrodes indicates that these are good catalyst for MOR. The oxidation peaks in the reverse (iB) scans are due to the removal of carbonaceous species that are not completely oxidised on the forward scan, and the oxidation of methanol by fresh adsorption [39].

The study reveals that the presence of Ni weakens CO adsorption seemingly because of decreased back-donation from Pd to anti-bonding CO orbital’s [38] due to decreased % d band character of Pd on alloying. Moreover Ni is less electronegative (1.91) than Pd (2.20), hence bonding electrons are seemed to be more shared by Pd than Ni. This might decrease the Pd-CO binding energy, improve the oxidation of CO-like intermediates from methanol

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dehydrogenation and enhance the adsorption and oxidation of methanol molecules [35, 40]. The bifunctional mechanism could also play an important role in improving the catalytic performance of C/PdxNiy electrocatalyst. Ni is more oxophilic than Pd and has the capacity to generate surface Ni-OH moieties at a low potential value [40]. This hydroxyl species could oxidise COads poisoning intermediates to liberate active Pd sites for further oxidation reaction. The decreased particle size of C/PdxNiy electrocatalyst in comparison to that of C/Pd and C/Ni could also increase the exposed surface area for the oxidation reaction, thus enhancing the catalytic performance. The mass normalized peak current densities (iF) show that C/Pd4Ni is the best catalyst among the electrodes studied. Fig. 4(c) presents the profiles of iF and iB with atom % of Ni, indicating greater values of iB in reference to that with pure Pd at most of the compositions containing Ni. The stability of the best electrode C/Pd4Ni is checked by performing a multi-scan CV experiment with it, on application of 100 consecutive triangular sweeps of potential. Fig. 5(a) reveals that in the 100th cycle the peak current is reduced by 4.3% which indicates that the catalytic capability of the electrode is highly stable in the reaction condition of MOR. The XRD profile (Fig. 5 (b)) of the thin film made on FTO consisting the catalyst particles along with graphite dust and nafion after subjecting to 100 cycle of voltammetric operation for MOR, reveals a very small decrease of the diameter of spherical nanoparticle from 5nm to 4nm. The shifting of the peak position of (111) plane of the catalyst before and after 100th cycle of voltammetric operation reveals the change of alloy compositions from 80% of Pd to 88% Pd indicating loss of Ni from the surface. The effects of changes of decrease of diameter and increase of atom % of Pd in the alloy composition on the catalytic capability of the electrodes are opposite in nature and hence the catalytic capability of the electrodes does not change much in the 100th cycle. 3.4. Potentiodynamic polarization study

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Fig. 6(a) depicts the Tafel plots for oxidation of methanol on the surface of the constructed electrodes. The required potential (E) and current density (i) values were obtained from potentiodynamic current measurements of 0.5M methanol in 1M NaOH solution in the potential range of -0.7V to +0.7V at a scan rate of 1mVs-1 [41]. The obtained data are presented in Table 3 for all the electrodes. Tafel polarization analysis was executed following linear Tafel relation according to the equation

To calculate exchange current density, equilibrium potential, Ee is set as -0.9566 V vs. MMO, which represents the equilibrium potential of oxidation of methanol to carbonate after considering pH effect and potential shift between Hg/HgO and NHE in 1M NaOH. The exchange current density obtained by extrapolating the linear fitted Tafel line to where overpotential equals to zero. The Table 3 shows that the Tafel slope also passes through a maximum with increasing atom % of Ni in the system of alloys, signifying the reverse order in the change of number of electron transfer per molecule of methanol in the reaction. The data of the table also reveals that the exchange current density of C/PdxNiy electrodes varies in the order C/Pd4Ni>C/Pd9Ni>C/Pd>C/Pd3Ni2. The variations of exchange current density of different electrodes are also presented at the inset of Fig. 6(a). 3.5. Chronoamperometric study For further evaluation of the activity of the C/Pd and C/PdxNiy nanoalloy for methanol oxidation reaction at constant potential, chronoamperometric (CA) measurements were carried out at a potential of -0.3V for 7200s in a solution of 0.5M methanol in 1M NaOH as depicted in Fig.6 (b). All catalyst shows the typical current density-time responses for methanol electro-oxidation, representing the initial high current density, which is attributed to the double layer charging and numerous available active sites on the catalyst surface. After that a gradual current decay is observed, implying formation of intermediate carbonaceous 14

species, such as CO-like species and their accumulation on the surface active sites. A pseudo steady state is also observed and all catalyst shows a consistent steady current density after 7200s. Among all catalyst, C/Pd4Ni shows the maximum current density (49.99mAmg-1) further establishing its superiority over other similarly synthesized catalyst. The order of current density in mAmg-1 is given in parenthesis: C/Pd3Ni2 (1.38) < C/Pd (9.31) < C/Pd9Ni (15.38) < C/Pd7Ni3 (35.54) < C/Pd4Ni (49.99). The % retention of current density at the 2nd hour with respect to that of the 1st hour(i2hx100/i1h), for different electrodes vary in the same order with the values 24.4, 33.3, 49.2, 68.2, 82.2 respectively, indicating the stability and synergistic catalytic ability of the C/Pd4Ni electrode. The order of the catalytic behaviour (of the different electrodes studied) measured by iF, iB of CV study, steady current density of chronoamperometric study and i0 etc follow the same trend. 3.6. Cyclic voltammetric study of oxidation of methanol, formaldehyde and formate in alkali To elucidate the path of anodic oxidation of methanol, cyclic voltammetric study was carried out using C/Pd and C/Pd4Ni electrodes immersed in 1M NaOH with and without differently concentrated (12, 36, and 100mM) sodium formate , formaldehyde and methanol fuels in the potential range of -0.7V to +0.7V at a scan rate of 50mVs-1. For each fuel, the increment in the current density with the concentration of the fuels helps recognition of characteristic peaks for formate, formaldehyde and methanol as evident from Fig. 1(a-c) and 2(a-c) of C/Pd of ‘Supporting Document’ for C/Pd and C/Pd4Ni electrodes respectively. Fig. 7(a) and 7(b) reveal that each fuel exhibits characteristic features with respect to each other and the blank CV obtained for 1M NaOH without fuels for both the electrodes C/Pd and C/Pd4Ni respectively. The potential (Volt) of the most intense forward peak in the CV’s of the fuels follow the order: HCOONa (-0.4011)
oxidation of methanol and formate suggests that methanol does not form carbonate through formation of formate. Moreover similar observation (in Volt) is also found for C/Pd4Ni electrode where the respective order is HCOONa (-0.469)
3.7. FTIR and chromatographic studies of the products Assignments of main FTIR bands observed from spectra of the products of methanol oxidation in alkaline medium are presented in Table 4. It is observed from Fig. 8(a) and Table 4 that the peak (P4) at ca 872cm-1 arises due to CO32- ion. For each electrode the most intense broad peak (P3) appears at ca 1463cm-1 due to both CO32- and HCOO- ions. Peak for formate appears at 1634cm-1 (P2). Appearance of weak peak (P1) at 1759 cm-1 confirms the presence of C=O stretching (without H-bonding). These peaks indicate the presence of aldehyde (-CHO group) which reveals that methanol is oxidized through the formation of formaldehyde as one of the intermediates. Moreover it is observed that the ratio of absorbance corresponding to (P4) and (P2) decreases in the order (values given within the parenthesis): C/Pd4Ni (2.396)> C/Pd (1.530) > C/Pd3Ni2 (1.042) respectively. The chromatographic study [42, 43] (Fig. 8b) also reveals that both the maximum intensity and the area (presented within the parenthesis in minute) under the absorbance–time curves for the formate ion (formic acid) different electrodes vary in the order: C/Pd3Ni2 16

(495.09) < C/Pd4Ni (1442) < C/Pd (2696). On the other hand the lower catalytic capability of C/Pd3Ni2 is also reflected by the area under the curve for intermediate formaldehyde which accumulates in the solution due to oxidation of methanol (Scheme 1) and follows the order C/Pd4Ni (12401) > C/Pd (10288) > C/Pd3Ni2 (8328). Notably, the standard curves for known concentrations of sodium formate and formaldehyde have been presented in Fig. 3 of ‘Supporting Document’. It indicates that the concentration of formate as a product in the solution after drawing current decreases on introducing Ni into the Pd lattice. The decrease in concentration of formate for C/Pd3Ni2 electrode in respect to that for C/Pd and C/Pd4Ni, is mainly due to its lower catalytic capability than the other electrodes as evident from CV and CA studies. On the other hand such decrease for C/Pd4Ni than that for C/Pd indicates the preferential formation of other product than formate, for C/Pd4Ni electrode. Since formate and formaldehyde are two products as revealed from FTIR study, it is evident that the other product mentioned above is carbonate. 3.8. Mechanism of methanol oxidation In the mechanism, (Scheme 1) methanol is converted first either to formaldehyde by two electron transfer, followed by formation of adsorption intermediate (A) or to formation of formate by four electron transfer pathway. The intermediate, (A) subsequently follows transfer by two parallel pathways. In one pathway (I) the intermediate (A) forms linearly bonded PdCO which subsequently slowly transforms to CO32- by adsorbed M-OH and OH- ions. In another path (II), adsorbed formate ion is formed followed by either fast transfer to CO2 or formation of metal formate which subsequently undergoes slow transformation to bicarbonate and carbonate. The different pathways are carried simultaneously with varying degree for all the electrodes used 17

as catalyst. In presence of enough Ni in the bi-metallic alloy constituting the electrodes, path (IIa) is mainly followed because significant extent of Ni(HCOO)2 is formed in comparison to Pd(HCOO)2, as supported by Ni-O and Pd-O bond lengths (of 111 planes) which are 1.88Ǻ and 2.16Ǻ respectively [44]. For electrodes containing less mol % of Ni, formate undergoes fast decarboxylation reaction following path (IIb) which requires no involvement of adsorbed MOH. The other pathway, (I) is also followed by the electrodes when significant number of MOH is available at the surface. In this respect, it must be noted that the formation of Ni(OH)x (x=1, 2) is easy but its reduction is more difficult than that of Pd(OH)x. This is supported by the reduction potential of M(OH)2/M couple for Pd and Ni, which are 0.07V and -0.24V respectively. So, formation of free HCOO- would be more for C/Pd electrode and electrodes constituted with PdxNiy alloys containing less mol % of Ni. Moreover, the formation of free HCOO- is immediately followed by formation of nickel formate which is more stable than Pd-formate and hence formation of CO32- from HCOO(M/2) is much slower in case of C/Pd4Ni as electrode. Thus for this electrode the reaction follows mainly path (I) where formation of Ni(OH)x helps C/Pd4Ni electrodes to remove the poisonous intermediate PdCO from the surface effectively more than C/Pd electrode. The apparent anomaly that the peak current density of formate oxidation and accumulation of free formate ion in the products of methanol oxidation with C/Pd electrode are both greater than that with C/Pd4Ni electrode, can be resolved by considering that oxidation of methanol follows mainly path (I) where formation of greater PdOH from Ni(OH)x on the Ni containing electrodes by the following reaction is favoured: Pd + Ni(OH)x = PdOH + Ni(OH)x-1, x=1,2.

(5)

Thus, Ni(OH)x may act as a store-house of PdOH. This extra PdOH helps in greater oxidation of methanol through the paths (I and IIa) which require involvement of PdOH. Thus, the electrodes constituted with lower mol % (upto 30) of Ni, sufficient MOH is formed as well as 18

enough surface of Pd remains for methanol adsorption. Here, the optimum composition of these two factors is reached at 20 mol% of Ni. Thus, C/Pd4Ni is the best electrode studied for methanol oxidation. On the other hand, since the path (IIb) of oxidation of formate does not require such involvement of Pd-OH, therefore C/Pd is kinetically better electrode than C/Pd4Ni for oxidation of formate.

4. CONCLUSIONS In exploration of less expensive efficient anode-catalyst of direct methanol fuel cell, syntheses of low-dispersed nanoparticles of Pd, Ni and PdxNiy alloy of different compositions have been carried out using classical wet chemical synthetic protocol with sodium borohydride as reducing agent. The studies of anodic oxidation of methanol in alkali on as synthesized Pd and PdxNiy alloy nanoparticles supported on carbon electrode reveal improved, stable and synergistic catalysis of Pd4Ni compared to Pd nanoparticle counterpart. The electrode shows the highest mass normalised peak current density (673.83mAmg-1 of Pd), the highest exchange current density (6.83x10-3mAmg-1) for methanol oxidation reaction. The lower and higher catalytic activity of C/Pd4Ni than C/Pd for oxidising intermediates sodium formate and formaldehyde respectively, indicate that the oxidation reaction does not proceed mainly through formate, elucidating the plausible mechanism of the reaction.

Acknowledgements The authors like to thank Jadavpur University (UGC CAS program) for all financial and instrumental support.

19

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23

Figure Legends Figure1. (a) XRD patterns for the synthesized (i) Pd, (ii) Pd9Ni (iii) Pd4Ni (iv) Pd7Ni3, (v) Pd3Ni2, (vi) Ni nanoparticles. (b) Long range XPS among different cores and core level XPS of (c) Pd 3d and (d) Ni 2p in the Pd4Ni alloy.

Figure2. (a) and (b) TEM images, (c) and (d) HRTEM images of pure Pd and Pd 4Ni nanoparticle respectively.

Figure3. (a) FESEM images showing the microstructure,(b) EDX spectra of Pd4Ni and (c) and (d) EDX mapping images of Pd, and Ni nanoparticles. Figure4. Cyclic voltammograms (in mA mg-1) in 1M NaOH of bare carbon, C/Pd, C/Pd9Ni, C/Pd4Ni, C/Pd7Ni3, C/Pd3Ni2. Inset: CV profiles for C/Ni and bare graphite electrode, at room temperature. The scan rate of potential was 50mVs-1. (b) The CV plot in current density (in mAmg-1of Pd) of C/Pd, C/Pd9Ni, C/Pd4Ni, C/Pd7Ni3, C/Pd3Ni2 alloy electrodes in alkaline 1M NaOH oxidation of methanol 0.5M at room temperature. The scan rate of potential was 50mVs-1. (c) Profile of iF and iB with varying % of Ni.

Figure5. (a) Cyclic voltammograms of C/Pd4Ag electrode up to 100th cycle in 0.5M methanol in 1M NaOH at 50mVs-1 scan rate. (b) XRD profiles of the used C/Pd4Ni electrode in comparison to that of as prepared Pd4Ni.

Figure6. (a) Tafel behaviours of C/Pd, C/Pd9Ni, C/Pd4Ni, C/Pd7Ni3, C/Pd3Ni2 electrode at the slowest scan rate of 1mVs-1. Inset: Variation of exchange current density with atom % of Ni. (b) Chronoamperometric profiles for C/Pd and different C/PdxNiy electrode for 0.5M methanol in 1M aqueous NaOH solution at potential of -0.3V upto 7200s. Inset: Variation of current density with atom % of Ni. 24

Figure7. (a) Cyclic voltammograms for C/Pd catalyst for MeOH, HCHO, HCOONa fuels each of concentration 100mM in 1M aqueous NaOH scan rate 50mVs-1. (b) Cyclic voltametric profiles for C/Pd4Ni catalyst for (a) MeOH, HCHO, HCOONa fuels each of concentration 100mM in aqueous NaOH at scan rate 50mVs-1.

Figure8. (a) Ex-situ FTIR profiles of MOR products for C/Pd, C/Pd4Ni and C/Pd3Ni2catalysts. (b) Absorbance versus retention time profile for MOR oxidation product formate and formaldehyde using C/Pd, C/Pd4Ni, C/Pd3Ni2 electrodes.

25

Figure1

(b)

(a) (111)

(220)

Intensity (a. u.)

Absorbance (a.u)

PdNi

(v) (iv) (iii) (ii)

(i)

20

40

50 60 70 2 Theta (degree)

90

1400 1300

Experimental Fittings

Pd 3d5/2

1200 1100

Pd3d

O 1s

20000 15000

C 1s

10000

400

600

800

Binding Energy (eV)

(d)

Pd 3d

Intensity (a. u.)

80

25000

5000 200

450 400

Intensity (a. u.)

(c)

30

Ni 2p

30000

(200)

(vi)

35000

Pd 3d3/2

1000 900 800 700

Ni 2p

Experimental Fittings

2p3/2

350 300 NiO Ni(OH)2

250

Ni

200

2p3/2

2p1/2 2p1/2

satellite

satellite

150 100 50

600

0

330

335

340

345

350

840

Binding Energy (eV)

850

860

870

Binding Energy (eV)

26

880

Figure2

27

Figure3

28

Figure4 (a)

25

Current density/mAcm-2

Current density/mAmg-1

300 250 200 150 100

20 15 10

shoulder

5 0 -5 -10

shoulder -0.5

50

0.0 Potential/V

0.5

0 -50

C/Pd

-100 Carbon C/Pd7Ni3 C/Pd9Ni

C/Pd4Ni

-150

-0.5

C/Pd3Ni2

0.0

0.5

Potential / V

(c)

(b)

450

8

700

C/Pd4Ni

Methanol oxidation on C/Ni electrode

6 4

Current density/mAmg-1

600

Current density/mAcm-2

Current Density/mAmg-1 of Pd

750

C/Pd7Ni3

2 0

C/Pd

-2 -0.5

0.0 Potential / V

0.5

C/Pd9Ni

300

C/Pd3Ni2

150 0

iF iB

600 500 400 300 200 100

-150 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0

Potential / V

29

10

20 % of Ni

30

40

Figure5

Current density / mAmg-1of Pd

(a) 700 600

1st 10th 20th 30th 50th 70th 80th 90th 100th

500 400 300 200 100 0 -100 -200 -0.5

0.0

0.5

Potential / V

(b)

Intensity/ a.u

Before Catalysis After Catalysis

35

40

45

50

55

60

65

2 Theta/degree

30

70

75

80

Figure6

Exchange Current density/mAmg-1

(a)

Potential/V

-0.1

-0.2

0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000 -0.001 0

10

20

30

40

% of Ni

C/Pd C/Pd9Ni

-0.3

C/Pd4Ni C/Pd7Ni C/Pd3Ni2

-0.4 -0.5

0.0

0.5

1.0

1.5

logi

(b) 50

Current density/mAmg-1

Current density/ mAmg-1

120 100 80

40

30

20

10

0 0

10

20

30

40

% Ni

60

C/Pd4Ni C/Pd7Ni3

40

C/Pd9Ni

20

C/Pd

C/Pd3Ni2

0 0

2000

4000 Time/s

31

6000

Figure7

Current density/mAmg-1of Pd

(a) 200 MeOH

100 0 -100 HCHO

NaOH

-200

HCOONa

-300 -400 -0.5

0.0

0.5

Potential / V

Current density/mAmg-1ofPd

(b) 200 100 0 MeOH

-100

HCHO NaOH

-200 HCOONa

-300 -0.5

0.0

Potential/V

32

0.5

Figure8

(b)

(a)

4200 24

HCHO

2800

C/Pd3Ni2

HCOONa

18 1400 12 (P4)

0 C/Pd3Ni2

Absorbance

6

(P1) (P2) (P3)

%T

78 (P1) (P2)

52 26 (P4)

C/Pd4Ni

(P3)

0

7800 HCHO HCOONa

5200 2600 0

12300

72 (P1) (P2)

48 24

1000

HCHO

4100 C/Pd

0

(P3)

0

2000

C/Pd

HCOONa

8200

(P4)

0

C/Pd4Ni

3000

0

4000

5

10

Time / min

-1 Wave No (cm )

33

15

20

Scheme1: The proposed methanol oxidation reaction pathway in alkaline medium on C/Pd and C/PdxNiy electrodes.

Scheme1

34

Table1. Diffraction peaks, crystallite size, lattice parameters and composition of the asprepared nanoparticles

Nanoparticles

Pd

Pd9Ni

Pd4Ni

Pd7Ni3

Position of d-spacing (Å) 2θ (From XRD) (degree)

Crystallite size (nm)

39.81

2.264

9

3.92

100

46.09

1.969

40.17

2.245

5

3.89

92.28

46.27

1.962

40.79

2.212

5

3.83

78.97

46.89

1.938

41.31

2.185

5

3.78

67.81

47.73

1.905

41.65

2.168 5

3.76

60.52

47.89

1.899

44.47

2.037 29

3.53

0

51.83

1.764

Pd3Ni2

Ni

Cell parameter a (Å)

35

Evaluated atomic ratio of Pd

Table 2. The peak potentials (EF and EB), peak current density and other related parameters obtained from cyclic voltammetric studies of C/Pd, different C/PdxNiy alloy electrodes immersed in 0.5M methanol in 1M NaOH solution at room temperature.

Electrodes

ECSA

Rf

(cm2mg1 ofPd)

EF

iF

(V)

(mA/cm-2)

iF (mA/mg)

EB

iB

(V)

(mA/cm-2)

iB (mA/m g)

iF/iB

C/Pd

2136.84

596.05

+0.033

104.59

375.93

-0.335 26.75

93.20

4.03

C/Pd9Ni

4131.21

1085.52

-0.030

83.84

319.14

-0.300 40.38

157.53

2.03

C/Pd4Ni

4384.80

1074.85

-0.035

103.15

673.83

-0.325 29.55

191.99

3.51

C/Pd7Ni3

3498.58

789.01

-0.006

96.75

395.06

-0.296 129.28

32.03

3.06

C/Pd3Ni 2

5326.97

812.79

-0.063

28.31

125.98

-0.305 7.17

31.74

3.97

36

Table3 Comparative Tafel slope and exchange current density data from potentiodynamic studies of C/Pd and C/PdxNiy electrodes for oxidation of methanol at a scan rate of 1 mVs-1.

Electrodes

Intercept/ V

Slope/V dec-1

Adj.R-Square

Exchange Current Density/ mAmg-1

C/Pd

-0.308

+0.126

0.987

2.44x10-5

C/Pd9Ni

-0.348

+0.145

0.986

2.41x10-4

C/Pd4Ni

-0.423

+0.192

0.969

6.83x10-3

C/Pd7Ni3

-0.348

+0.145

0.986

2.84x10-4

C/Pd3Ni2

-0.324

+0.121

0.983

3.02x10-5

37

Table4.Assignments of main FTIR bands observed from spectra of the products of methanol oxidation in alkaline medium.

C/Pd

C/Pd4Ni

C/Pd3Ni2

electrode

electrode

electrode

Wave number (cm-1)

Possible assignments

Wave number(cm-1)

Possible assignment

Wave number(cm-1)

2979

-CH symmetrical stretching

2979

-CH 2972 symmetrical stretching

-CH symmetrical stretching

1768

Carbonyl

1768

Carbonyl

Carbonyl

1759

Possible assignments

(C=O) stretching

(C=O) stretching

(C=O) stretching

1616

Asymmetric 1679 stretching of formate

Asymmetric 1643 stretching of formate

Asymmetric stretching of formate

1454

Symmetric 1463 stretching of Formate /carbonate

Formate / 1445 carbonate

Formate carbonate

1383

Formate

-

-

-

-

872

CO32-

872

CO32-

872

CO32-

692

CO32-

692

CO32-

692

CO32-

38

/