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O3 modification effects on Pt–Pd/WO3-OMC electrocatalysts forormic acid oxidation
teeq ur Rehmana, Sk Safdar Hossainc, Sleem ur Rahmana,hakeel Ahmedb, Mohammad M. Hossaina,∗
Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi ArabiaCenter for Refining & Petrochemicals-Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi ArabiaDepartment of Chemical Engineering, King Faisal University, Al Hasa, Saudi Arabia
r t i c l e i n f o
rticle history:eceived 5 March 2014eceived in revised form 5 June 2014ccepted 7 June 2014vailable online 13 June 2014
eywords:O3
a b s t r a c t
This manuscript presents an experimental investigation of the promotional effects of WO3 on Pt/Pd/WO3-OMC electrocatalysts for oxidation of formic acid. Cyclic voltametry of Pd/WO 3-OMC exhibits 15% higheranodic current activity than that of Pd/OMC. The peak potential is also shifted more than 151 mV in thenegative direction. In the bimetallic catalyst samples, the addition of a small amount of Pt improves theperformance of PdPt/WO3-OMC significantly. The atomic Pd/Pt ratio of 2, displays the best current den-sity (56.8 mA/cm2), which is 1.2, 1.65 and 5.2 times higher than that of Pd/WO3-OMC, Pd1Pt1/WO3-OMCand Pd1Pt2/WO3-OMC, respectively. It also shows activity over an extended duration without any indica-
tion of CO poisoning effects. N2 adsorption/desorption analysis shows that WO3 modification enhancesthe surface characteristics of OMC with the pore size being slightly decreased from 3.84 nm to3.28 nm.Scanning electron microscopy, x-ray diffraction, transmission electron microscopy analyses reveal thatthe presence of WO3 enhances the uniform dispersion of active metal(s) on the support surface which isprimarily believed to be responsible for superior activity/stability of the Pd2Pt1/WO3-OMC.
Recently, formic acid has received considerable attention as aotential fuel in liquid fuel cell applications due to its easy han-ling/transportation, low toxicity, limited membrane crossovernd high open circuit potential as compared to methanol [1,2].lexibility of operating at high concentrations (5–12 M) also makesormic acid a more promising fuel than methanol which operateselatively at low concentrations (1–2 M) [3]. Large scale applicationf direct formic acid fuel cells (DFAFCs) can also offer an attractiveay of CO2 recycling; given that formic acid can be produced via
lectrochemical conversion of CO2. Thus, the integrated approachonsisting of (i) electrochemical conversion of CO2 to formic acidollowed by (ii) power generation using a DFAFC can provide notnly an efficient energy generation but also contribute to the global
fforts on CO2utilization/minimization [4].
In order to utilize the above mentioned advantages and for largecale applications, the present DFAFC technology requires further
improvement. The major issues with the available DFAFC systemsare, (i) the excessive use of noble metal based electrocatalysts toaccelerate the slow kinetics of the anodic electro oxidation of formicacid [5–7] and (ii) severe poisoning of precious active metals surfacedue to the strong adsorption of carbon monoxide [8]. Therefore, thedevelopment of low cost and stable catalysts for formic acid oxida-tion are important research goals for commercial scale applicationof DFAFCs. In the literature various transition metals have beenexplored as promoters to enhance the catalytic activity and stabilityof the noble metal based electrocatalysts. These metals also reducethe use of noble metals in the catalyst composition while main-taining or even improving the catalytic activity. The most commonstudied bimetallic catalysts include PtSn, PdFe, PdCo, PdNi, PdAu,PdPt and PtBi [9–14].
Conventionally, minimum amount of active metals/promotersare dispersed on a suitable support material to achieve highestpossible catalytic activity. High surface area, large pore volumeand superior electrical conductivity of the support material aredesirable to achieve high electrocatalytic activity of the supported
catalysts. Among the studied support materials, the large sur-face area carbon black is possibly the most commonly studiedsupport for DFAFC applications [11,13,15]. With some positiveaspects, there are some drawbacks of carbon black as a support
or electrocatalysts. For example, a fraction of the deposited metalarticles can be located within the deep cracks of the phase bound-ries and micropores of the carbon black support and they are nottilized efficiently in oxidation reaction [16]. In acidic media carbonlack also suffers serious corrosion problems in fuel cell opera-ion [17,18]. To avoid these problems, many other carbon materialsre also investigated as electrocatalyst support in DFAFC applica-ions, including carbon nanotubes (CNTs) [1,3,19–21], nanofibersCNFs)[7], ordered mesoporous carbon (OMCs) [22–27], graphene28] and metal carbides [29].
Previous studies showed that commonly used modifiers suchs ZrO2, CeO2, TiO2, SnO2, WO3, Fe2O3 and, NiOx [2,5,6,19,30–33]re beneficial to improve the activity of supported catalysts. Partic-larly, tungsten trioxide (WO3) showed promising characteristicss a support modifier for formic acid [30,31,34] and methanol fuelell [35–38] electro oxidation. Feng et al. showed improved perfor-ance of formic acid oxidation using a WO3-C hybrid support for
Pd based electrocatalysts [39]. In the above catalysts, WO3 actsoth as a support modifier and promoter of the noble metals. Hobbst al. reported that during oxidation reaction, WO3enhances theate of dehydrogenation in acidic medium due to hydrogen bronzeHxWO3) formation [37]. The oxophilic nature of WO3 assists inhe removal of adsorbed CO intermediates from the Pt metal sur-ace during the oxidation steps [36]. The presence of WO3 can alsoreate a barrier phase between the support and the active nobleetal slowing down the catalyst deactivation due to active metal
article agglomeration. Thus WO3 can play an important role inurther improvement of commercially available carbon black sup-orted catalysts by improving both the catalytic performance andO tolerance of fuel cell anodic catalyst electrodes [37]. As per thenowledge of the present authors, there are only a few reportsvailable in the open literature which have investigated WO3-OMCs a catalyst support for the electro oxidation.
Considering the advantages of WO3 as support modifier, theresent research has focused upon investigating WO3-OMC hybridaterial as a support for Pt–Pd bimetallic catalyst for electro oxi-
ation of formic acid in a DFAFC. The compositions of Pt andd were also varied to demonstrate their effects on the per-ormance of bimetallic catalysts. A hard template method wassed to prepare WO3-OMC through carbonization of sucrose intoesopores WO3-modified SBA-15. The corresponding PtPd/WO3-MC electrocatalysts were prepared using the sodium borohydride
eduction method. The morphology, structural properties andomposition of the PtPd/WO3-OMC catalysts were characterizedy using various characterization techniques including scanninglectron microscopy (SEM), thermo gravimetric analysis (TGA),-ray diffraction (XRD), transmission electron microscopy (TEM),2 adsorption/desorption isotherm and energy dispersive x-ray
pectroscopy (EDX). The electrocatalytic performances of theynthesized catalysts were examined by cyclic voltametry (CV),hronoamperometry (CA) and CO stripping voltammogram tests.
rom Loba Chemical (Pvt) Ltd. Hydrofluoric acid (HF, 40 wt.%),ulfuric acid (H2SO4, 97–98 wt.%), formic acid (HCOOH, 95 wt.%),thanol (C2H5OH, 99.8 wt.%), hydrochloric acid (HCl, 37 wt.%) andafion resin (5 wt.% solution in aliphatic alcohols and water) were
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purchased from Sigma–Aldrich. Millipore water was used for thepreparation of all aqueous solutions.
2.2. Preparation of PtPd/WO3-OMC electrocatalysts
In this investigation there are three major steps involved in thepreparation of the supported electrocatalysts. In the first step, SBA-15 was prepared by polymerization of TEOS using a hard templatemethod. In the second step, WO3 modified WO3-OMC support wassynthesized by carbonization of sucrose followed by hydrofluoricacid (HF) treatment to remove the unconverted silica traces com-pletely. In the third and final step, Pt and Pd were loaded on theWO3-OMC support using a sodium borohydride reduction method.The details of the above three steps are presented in the followingsubsections.
2.2.1. Synthesis of WO3-SBA-15The SBA-15 silica sample was synthesized by a hard-template
TEOS polymerization method as reported by Zhao et al. [40] andJun et al. [41] with slight modifications. The prepared SBA-15was then modified with WO3 by wetness impregnation methodusing H3PW12O40H2O as tungsten precursor. Phosphotungstic acid(PWA) hydrate solution was prepared in deionized water understirring at room temperature for 30 min. The solution was addedto the desired amount of preheated SBA-15 at 110 ◦C. The resultantsuspension was ultrasonicated for 24 h at room temperature andthen dried at 100 ◦C to remove the water completely. Finally, thesample was calcinated at 450 ◦C under argon flow for 4 h in orderto decompose PWA to WO3 by thermal means.
2.2.2. Synthesis of hybrid WO3-OMCWO3-OMC was prepared by carbonization of sucrose into meso-
pores of WO3-SBA-15 as reported by Wang et al. [42] with somemodifications. In this method, 1.0 g of WO3-SBA-15 was added toa solution containing 1.25 g of sucrose, 0.14 g of sulfuric acid and5.0 g of deionized water. The mixture was then placed in an ovenat 100 ◦C for 6 h, after that the oven temperature was increasedto 160 ◦C at a heating rate of 2 ◦C/min. The sample was kept at160 ◦C for another 6 h. The above steps were repeated adding 0.8 gof sucrose per cycle in order to fill the internal WO3-SBA-15 silicapores completely. The resultant material was pyrolyzed at 800 ◦Cunder N2 flow for 6 h to obtain the carbon–silica composite. Thecomposite was washed with 5 wt. % HF solution to remove thesilica template. Finally, the sample was filtered, washed with deion-ized water and dried at 100 ◦C for 4 h. Subsequently TGA analysisconfirmed the high temperature (25–400 ◦C) stability of supportmaterial.
2.2.3. Preparation of PtPd/WO3-OMC electro catalystBimetallic PtPd/WO3-OMC electrocatalysts were prepared by
the borohydride reduction method using NaBH4as the reducingagent. In this technique, the required amount of metal salts (palla-dium nitrate and hexachloroplatinic acid), were added dropwise toWO3-OMC support under constant stirring. The metal loaded WO3-OMC support was then added to 200 ml deionized water and stirredfor 3 h to form a homogeneous suspension. Appropriate amount ofsodium citrate solution was added to the suspension with vigor-ous stirring and followed by1 h ultrasonication. Freshly prepared120 mg (3 times molar ratio of active metals) NaBH4 solution wasslowly added to the suspension and set for another 12 h stirring toallow the complete reduction of the Pt and Pd precursors at room
temperature. The slurry was then centrifuged, washed with deion-ized water and dried at 110 ◦C for 4 h. In the final samples, totalmetal loading was 20 wt.% and the mass ratio of Pd to Pt was readilyadjusted by using different amounts of Pt and Pd precursor.
lysis A: General 482 (2014) 309–317 311
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.3. Catalyst characterization
BET surface areas and pore volume of the synthesized materialsere determined by N2 adsorption using a Micromeritics modelSAP 2010 analyzer. Prior to the measurements, the samples were
reated at 250 ◦C under nitrogen flow for 6 h in order to removeoisture completely. N2 adsorption was carried out in a liquid
itrogen bath maintained at 77 K.The XRD analysis was conducted to detect the crystalline phases
f catalysts and to measure the crystallite size. The XRD exper-ments were carried out using a Smart Lab (9 kW) Rigaku x-rayiffractometer, with a diffraction angle range 2� = 5–80◦ using Cu� radiation with a scan rate of 2◦ min−1.
Transmission electron microscopy (TEM) images were taken toetermine the metal dispersion on support material along with par-icle size of loaded metal. An ultra-high resolution FETEM (JEOL,EM-2100F) at an accelerating voltage of 200 kV was employed toapture the images of the solid samples. In sample preparationery small amount of finely grinded solid (catalyst/support) sam-le was first dissolved in 20 ml ethanol solvent and ultrasonicatedor 30 min. 5 �l of this solution was drawn in a micropipette andropped on a carbon/formvar (3 mm diameter) substrate grid. Theird was placed in a desiccator for drying. The dried sample is readyor analysis.
The morphologies of the support and catalysts were studied bysing a scanning electron microscope (JEOL JSM-6460LV) operatedt 20 kV equipped with energy dispersive x-ray (EDX). For SEM-DX analysis, 2–3 mg sample powder was dispersed on a copperape which was mounted on the sample holder. The sample washen coated with gold to make it conductive for electron beam.DX was carried out to find the composition of catalyst samples.
TGA was recorded on a Shimadzu TGA-60 between 25 ◦C and00 ◦C at a ramp rate of 10 ◦C/min under dry air for the determina-ion of oxidation temperature of the support material.
.4. Electrochemical measurements
The electrochemical oxidation of formic acid was studied using aiologic Potentiostat (VMP3 Biologic Science Instruments, France)t ambient temperature in a three electrode cell. A glassy carbon3 mm diameter) covered with a thin layer of Nafion-impregnatedatalyst (geometrical area of the electrode: 0.071 cm2) was useds the working electrode. A Pt grid (2.54 cm × 2.54 cm) connectedo a Pt wire (8 cm length, 1.23 mm dia.) and an Ag/AgCl electrode3.5 M KCl) were used as the counter and reference electrodes,espectively. All potentials reported as quoted versus the Ag/AgCleference. At first, 5 mg of electrocatalyst was dispersed in 1 mL ofthanol, 30 �L Nafion/aliphatic and water solution (5 wt.% Nafion)y sonication for 30 min to form a catalyst ink. 10 �L of this inkas transferred (by pipette) to the polished surface (aluminumowder of 0.3 � and 0.5 �) of the glassy carbon. For all the experi-ents, the metal loading on the working electrode was maintained
t 0.127 mg metal/cm2. CV data were recorded from −0.2 to 1.2 Vvs. Ag/AgCl) at a scan rate of 20 mV/s in 0.5 M H2SO4 solution withnd without 0.5 M HCOOH.CA at 0.3 V (vs. Ag/AgCl) in N2-saturated.5 M H2SO4 with 0.5 M CHOOH was also recorded.
Electrochemical active surface area (ECAS) and the CO poison-ng tolerance of the catalyst samples were tested by CO strippingoltammetry. In the stripping voltammogram, CO was bubbledhrough 0.5 M H2SO4 electrolyte solution for 30 min, keeping theorking electrode in the cell under constant applied electrodeotential of 0.2 V. The system was first purged with nitrogen and
hen the electrolyte solution was aerated with CO in order toissolve CO into the solution. The CO stripping voltammogramsere recorded from −0.2 to 1.2 V (vs. Ag/AgCl) at a scan rate of
0 mVs−1 to ensure the complete oxidation of adsorbed CO (COads).
Fig. 1. N2 adsorption–desorption isotherms & BJH pore size distribution of OMC,WO3-OMC, Pd/WO3-OMC, Pd1Pt1/WO3-OMC, Pd2Pt1/WO3-OMC and Pd1Pt2/WO3-OMC.
Finally, ECAS were calculated using 0.42 mC/cm2charge associatedfor COads monolayer [43–45].
3. Results and discussion
3.1. Physical characterizations
3.1.1. N2 adsorption isothermsFig. 1 displays the N2 adsorption–desorption isotherms and
the corresponding BJH (Barret–Joyner–Halenda) pore size distri-bution curves of the OMC, WO3 modified OMC support and theprepared catalyst samples. All these isotherms contain followingthree common phases: (i) monolayer adsorption, (ii) multilayeradsorption and (iii) capillary condensation. OMC and WO3-OMCsamples exhibit IV isotherms with a slightly sharp capillary conden-sation step between p/p0 = 0.42 and p/p0 = 0.95. This low pressurecapillary condensation indicates that OMC and WO3-OMC supportcontain smaller average pore sizes. Table 1 summarizes the BETsurface area, pore size and total volume of OMC and PtPd based cat-alysts as determined from the nitrogen adsorption isotherm data.The BET surface area of OMC is found to be 1005 m2/g, which is ingood agreement with a similar material reported by Zeng et al. [22].After WO3 modification, the specific surface area of OMC decreasedfrom 1005 m2/g to 861 m2/g. The deposition of WO3 inside themesopores of OMC is mainly responsible for the decrease in thespecific surface area of the WO3-OMC sample. From the BJH poresize distribution curve it is quite clear that pore size of all samplesare between 3 and 4 nm. The measured BJH pore size for OMC andWO3-OMC are 3.8 nm and 3.4 nm, respectively.
3.1.2. XRDFig. 2 displays the XRD patterns of the WO3-OMC support
and the Pt/Pd based catalysts. Catalysts with different Pd/Ptratios are denoted as Pd1Pt1/WO3-OMC, Pd2Pt1/WO3-OMC andPd1Pt2/WO3-OMC. In all samples, diffraction peak 2� = 19.18◦
corresponds to the (0 0 2) plane of the carbon support [5].ForWO3-OMC sample (pattern “a”, Fig. 2), WO3 reflections occurredat 21.7◦, 32.4◦, 40.1◦ and 46.7◦, which could be indexed to the
fcc-phase of tungsten oxide (JCPDS card no. 20-1324) indicatingthe framework growth of monoclinic WO3. In the Pd/Pt/WO3-OMCsamples, only the WO3 peaks at 21.7◦ remained as distinct peak(patterns “c, d, e and f”, Fig. 2). The WO3 reflections at 40.1◦ and
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Table 1Surface properties of OMC, WO3-OMC and catalysts samples.
6.7◦ in the Pd/Pt/WO3-OMC samples overlapped with Pd and Ptue to the close proximity of the fcc structures of W, Pd and Pt22,46,47]. The WO3 reflection at 32.4◦ was too weak to be detectedn the Pd/Pt/WO3-OMC samples. In fact, the WO3 peak at 32.4◦ inhe WO3-OMC sample was also very weak. Zeng et al. also reportedimilar observation for a Pt/WO3-OMC catalyst sample [22].
It is clear form Fig. 2 that the XRD patterns of both Pd andt are quite similar (patterns “c, d, e and f”, Fig. 2). For Pd/OMC,d/WO3-OMC and PtPd/WO3-OMC electrocatalysts, the peaks at9.4◦, 47.7◦and 68.0◦ correspond to the (1 1 1), (2 0 0) and (2 2 0)lanes of Pd, respectively, consistent with face-centered cubic (fcc)rystalline structure of palladium nanoparticles (JCPDS, Card No.5-6174). The XRD patterns of both the WO3-OMC and OMC sup-orted catalysts show that the support modification and decreasingd content ratio has no prominent effect upon the diffractioneaks except making it broader and slightly moving the peakso higher angle. In Pd1Pt2/WO3-OMC catalyst, the XRD patternhowed similar Pt diffraction peaks at 40.0◦, 47.9◦ and 69.3◦anglesorresponding to (1 1 1), (2 0 0) and (2 2 0), respectively. Thesebservations are consistent with the results reported by Sun et al.25].
The crystallite sizes of the metal particles (Pd and Pt) were cal-ulated using the Scherrer equation [26]. It was considered that themall amount and low peak intensity of WO3 has little contributiono the Pd/Pt (1 1 1) peak height [26]. The crystallite size obtainedor Pd/OMC, Pd/WO3-OMC, Pd1Pt1/WO3-OMC, Pd1Pt2/WO3-OMCnd Pd2Pt1/WO3-OMC samples were found to be 6.7 nm, 6.5 nm,
.2 nm, 6.6 nm and 6.0 nm respectively. It was noticed that the crys-al size of samples decreased with the increase of Pt to Pd ratio inhe catalyst.
ig. 2. Wide angle XRD pattern of WO3-OMC support, Pd/OMC and PtPd/WO3-OMCith varying ratios of Pt and Pd.
3.1.3. TEM analysisThe TEM images (Fig. 3B–D) of the WO3-OMC supported cata-
lysts show that noble metals are uniformly dispersed as nano sizedcrystals on the WO3 modified OMC support surface. The crystal sizeof Pd on the unmodified OMC support are larger (Fig. 3A) than thoseon the WO3-OMC support (Fig. 3B–D). This observation suggeststhat the presence of WO3 minimizes the agglomeration of Pd parti-cles on the WO3-OMC support surface. The presence of Pt also helpsfurther decreasing the Pd particle agglomeration and improves thePd dispersion on the WO3 modified support, as seen in Fig. 3C andD. In Fig. 3 the inserted histograms, constructed using 65 particles(4–9 nm) as reference, show the corresponding particle size distri-bution of each sample. The average particle size of Pd/WO3-OMC,Pd1Pt1/WO3-OMC, Pd2Pt1/WO3-OMC and Pt/WO3-OMC catalystsare found to be 7.2 nm, 6.9 nm, 6.7 nm and 6.2 nm, respectively.The measured particle sizes are good agreement with the valuescalculated applying the Scherrer equation to the XRD data.
3.1.4. SEM-EDX analysisFig. 4 shows the morphology of the support and electrocatalysts
using scanning electron microscopy analysis. SEM images clearlyevidence the aggregated rope-like structure with smooth surfacesof the WO3-OMC support. The lengths of the “ropes” are approx-imately 1–2 �m. There are broad interconnections between theropes, as seen in Fig. 4(A). Similarly, uniform Pt and Pd metal loadingon support material is observed in Fig. 4(B) for Pd2Pt1/WO3-OMCcatalyst sample. With WO3 modification, PtPd crystal sizes alsoappeared to be narrowly distributed. Within the resolution limitof SEM, it was not possible to differentiate any significant changesin shape between support and catalyst material except length ofrope appeared slightly reduced in Fig. 4(B).
The EDX spectra of WO3-OMC and Pd2Pt1/WO3-OMC samples(Fig. 4(C) and 4(D)) reveal the presence of carbon, tungsten, palla-dium and platinum. In EDX, the standard M� x-ray lines for W andPt metals lie at emission energies of 2.03 keV and 2.33 keV, respec-tively [48,49]. Consequently, the overlapping of the W and Pt peaksis observed between the energy range of 2–2.5 keV (Fig. 4(D)). Onthe contrary, clear Pd, Pt and W peaks are detected at the L� and L�lines given their clearly distinct values at these levels (L� and L�,respectively for W: 8.39, 9.61; Pt: 9.36, 11.07 and Pd: 2.83, 3.31,).The compositions of the each of the metals on OMC, measured fromthe EDX analysis, are presented in Table 1.
3.1.5. TGA analysisFig. 5 plots the weight loss of the WO3-OMC support, Pd/WO3-
OMC and Pd2Pt1/WO3-OMC samples against temperature in dryair. In order to ensure complete removal of mesoporous carbon (byoxidation) the samples were heated up to 800 ◦C at a heating rateof 10 ◦C/min [46]. The thermal stability of the OMC support wasdetermined between 25 ◦C to 460 ◦C range. The initial sharp mass
reduction (Fig. 5) relates to the evaporation of moisture presentin the samples. Due to the oxidation of OMC, the major weightlosses are found in the regions from 496 ◦C to 610 ◦C for WO3-OMC support, from 465 ◦C to 560 ◦C for Pd/WO3-OMC and from
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Fig. 3. TEM images and corresponding particle size distribution histograms of (A) Pd/OMC, (B) Pd/WO3-OMC, (C) Pd2Pt1/WO3-OMC and (D) Pd1Pt2/WO3-OMC.
Fig. 4. Low magnification SEM & EDX images of WO3-OMC ((A)and (C)), Pd2Pt1/WO3-OMC ((B)and (D)).
40 ◦C to 565 ◦C for Pd2Pt1/WO3-OMC catalysts samples. The resid-al mass greater than 20 wt.% (total Pt and Pd) suggests that araction of PtPd alloy might have oxidized at high temperature∼800 ◦C) [5].Overall, these observations confirm that actual andxpected compositions are quite consistent.
.2. Electrochemical characterization
.2.1. CO stripping analysisThe CO poisoning tolerance of PtPd based electrocatalysts are
erified by carbon monoxide (CO) stripping voltammetry analy-is. Fig. 6(A) and 6(B)show that the 2nd CV cycle (dotted lines)fter CO oxidation almost overlapped with pre adsorbed CO sta-le curve. This suggests that CO is completely oxidized in thest oxidation cycle and that the active surface area of all metals
ecome available for hydrogen adsorption/desorption after CO oxi-ation [22]. It is also clear from the figure that the onset potential432 mV) of Pd/WO3-OMC is shifted negatively by 108 mV com-ared to Pd/OMC (540 mV). The peak potential of Pd/WO3-OMC is
ig. 6. (A) 1st and 2nd cycles of CO stripping measurements for PtPd/WO3-supported eleO oxidation peaks: (a) Pd/OMC, (b) Pd/WO3-OMC, (c) Pd1Pt1/WO3-OMC, (d) Pd2Pt1/WO
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shifted 68 mV positively. Catalysts with lower onset potential arefound to be better for CO oxidation. Therefore, the addition of WO3enhances (onset potential) CO oxidation ability of the WO3-OMCsupported catalysts. For Pd2Pt1/WO3-OMC, the onset potential andCO peak intensity appeared at 358 mV and 16.3 mA/cm2, respec-tively, which is a 1.2 times negative shift in potential as comparedto Pd/WO3-OMC electrocatalyst. Similarly, the small addition of Ptin the Pd2Pt1/WO3-OMC catalyst shifted the peak potential neg-atively towards lower value (580 mV). Table 2 summarizes boththe ECAS of metals calculated by using 0.42 mC/cm2 [44,45] as thecharge associated to the monolayer on Pt and Pd nano particlesand the CO stripping measurements for all the studied catalysts.One can see form this table that both the intensity of CO oxida-tion peaks and the onset potential vary with increase of Pt content.Due to its lowest potential and maximum ECAS (62.4 m2/g) of thePd2Pt1/WO3-OMC catalyst is considered as a potential CO tolerancecatalyst.
3.2.2. Cyclic voltammetry analysisThe formic acid oxidation activity of the WO3modified electro-
catalysts was determined by CV (scan rate of 20 mV/s) in 0.5 MHCOOH and 0.5 M H2SO4 solution. Before measurements, N2 waspurged through the electrode and electrolyte solution in order todeaerate the system. The CV analysis of all the catalysts is presentedin Fig. 7(A) while Fig. 7(B) plots the maximum current for each cat-alyst. It is clear from these figures that in the forward scan, both theintensities and peak potential of PtPd/WO3-OMC catalysts changewith Pd to Pt ratios. Similarly, in the reverse scan, cathodic oxida-tion of formic acid and reduction of oxidized metal (Pd/Pt) oxidesaround 400 mV are also observed [51,52]. At the peak potential of−8.6 mV, the oxidation current density of Pd/WO3-OMC catalystisaround 48.8 mA/cm2 (3.41 A/mg) which is about 7 mA/cm2 higherthan that of the Pd/OMC (41.1 mA/cm2) catalyst. This improved cat-alytic performance of Pd/WO3-OMC is due to the good integrationof OMC and WO3 as observed in SEM and TEM analyses. Further-more, the current density of Pd/WO3-OMC is much greater thanthat of the previously reported commercially available differentcarbon supports with Pd [5,6,45–49]. As reported in Fig. 7(B), thecurrent density of the Pd2Pt1/WO3-OMC catalyst for the anodic scan
(56.8 mA/cm ) is 1.4, 1.2, 1.65 and 5.2 times higher than that ofthe Pd/OMC, Pd/WO3-OMC, Pd1Pt1/WO3-OMC (34.5 mA/cm2) andPd1Pt2/WO3-OMC (10.9 mA/cm2) catalysts, respectively. The peakpotential of the Pd2Pt1/WO3-OMC catalyst is also shifted positively
ctrocatalysts in 0.5 M H2SO4 at a scan rate of 20 mV/s, and (B): Enlargement of the3-OMC and (e) Pd1Pt2/WO3-OMC.
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Table 2Electrochemical properties of the studied catalysts on CO oxidation.
Catalyst Area of desorbed CO stripping ECASCOa Peak intensity
CO peak (mC/cm2) Eonset (mV) Epeak (mV) (m2/g metal) (mA/cm2)
a ECASCO = �cw∗0.42 , �c = mC/cm2, w = 0.127 mg/cm2, 0.42 mC/cm2
s compared to the Pd/OMC, Pd/WO3-OMCand Pd1Pt1/WO3-OMCatalysts. One can thus generally interpret that the formic acid elec-ro oxidation on the two Pd/OMC and Pd/WO3-OMC catalysts are
uch easier. However, due to the large ECAS (reported in Table 2)nd the smaller crystal size of Pt, electrochemical oxidation oformic acid was enhanced using thePd2Pt1/WO3-OMC catalyst [5].specially, with the decrease of particle size and Pd content ratio,
ig. 7. (A) CV patterns of Pd/OMC and PtPd/WO3-OMC electrocatalysts with varioust:Pd ratios in 0.5 M H2SO4 + 0.5 M HCOOH solution; (B) maximum currents duringV plots of Pd/OMC, Pd/WO3-OMC and PtPd/WO3-OMC electrocatalysts with varioust:Pd ratios.
the peak potential for formic acid electrooxidation shifted posi-tively.
Moreover, the oxidation peak of the Pd1Pt2/WO3-OMC cata-lyst at 677 mV is ascribed to go through multiple steps or anindirect oxidation pathway. CO species are intermediates whichstrongly adsorbed on the surface of the catalysts. Due to the poi-soning effects on the Pt surface oxidation current intensity reducedto 10.9 mA/cm2 [8], while in Pd/OMC, formic acid oxidation goesthrough a direct oxidation pathway [50].
From the above observations it can be concluded that the modi-fication of OMC with WO3 helps improving the peak current density(1.2 times higher than OMC), which positively affects the currentactivity of PtPd electrocatalysts. Also, the peak potential of thePd2Pt1/WO3-OMC catalyst was shifted positively ca. 387 mV butit did not significantly effect the catalytic activity towards formicacid oxidation. Previously, Zhang et al. suggested that the addi-tion of WO3 can inhibit the agglomeration of Pd nanoparticles inPd/C [31]. Their observation was further corroborated by the highECAS for Pd/C in presence of WO3. They indicated that the increasedactivity of the electrocatalysts upon adding WO3was not only dueto the uniform dispersion of Pd and smaller particle but also dueto hydrogen spillover effects [31]. Recently, Rutkowska et al. [53]also demonstrated electrochemical enhancement of WO3 on Pdnanoparticles during oxidation of formic acid. They concluded thatthe increase of the ECAS of such catalysts were mainly due to thesynergy effects of WO3 and Pd. The combined WO3/Pt enhances theavailability of fast and reversible redox transitions involving HxWO3and WO3 and also oxygen transfer between WO3 and WO3−y. In thecontext of the present work, by TEM analysis we confirmed thatWO3 assists by improving the dispersion of active species (Pd andPt) on the WO3 modified WO3-OMC support (Fig. 3(B), 3(C) and 3(D)with 3(A)). This played a positive role in addition to the hydrogenspillover effect to enhance the catalytic activity.
3.2.3. Chronoamperometry analysisChronoamperometry (CA) tests were carried out to measure
the stability and deactivation rate of the prepared electrocata-lysts. Additionally, CA also further confirmed the results obtainedfrom the cyclic voltammetry. As seen in Fig. 7A, the catalysts stud-ied in this investigation showed peak current densities in a widerange of potentials (−0.086 V to + 0.4 V). All the catalysts showedreasonably high current density (near about their peak values) at0.3 V. For this reason we selected 0.3 V to carry out CA to com-pare the stability of the electrocatalysts. Previously, Feng et al. alsoused a similar potential (0.3 V vs. Ag/AgCl [6]. Fig. 8 displays theresponse curves of chronoamperometry measurements determin-ing the performance stability and poisoning rate of tested catalystsin a solution of 0.5 M HCOOH and 0.5 M H2SO4 at 0.3V constantpotential. For all the catalysts, the current density initially fell
rapidly following a parabolic path and finally reached to a pseudosteady state at 1400 s. The bimetallic Pd2Pt1/WO3-OMC electro-catalyst showed higher activity in formic acid oxidation than thePd/OMC and the other PtPd/WO3-OMC electrocatalysts. The steady
316 A.u. Rehman et al. / Applied Catalysis A
Fig. 8. CA at 0.3 V (vs. Ag/AgCl) for Pd/OMC and PtPd/WO3-OMC electrocatalysts int
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tate current density 10.9 mA/cm2 recorded on the Pd2Pt1/WO3-MC catalyst was 1.6 times higher than the steady state currentensity with Pd/OMC (6.6 mA/cm2) catalyst. The current densitiesor Pd/WO3-OMC, Pd1Pt1/WO3-OMC and Pd1Pt2/WO3-OMC cata-ysts were 3.3 mA/cm2, 1.4 mA/cm2 and 6.1 mA/cm2, respectively.rom these results it is confirmed that WO3 based electrocat-lyst showed somewhat better performance/stability than thenpromoted in particular the Pd2Pt1/WO3-OMC catalyst exhibitsigh catalytic activity and stability for HCOOH electro oxida-ion.
. Conclusions
In this investigation a series of WO3-modified ordered meso-orous carbon (OMC) supported Pd/Pt based electrocatalysts wererepared, characterized and evaluated using different electro-hemical analysis techniques. After WO3 modification the specificurface area of the OMC support slightly decreased due to the block-ge of the pores. SEM, XRD and TEM revealed that the presence ofO3 enhanced the uniform metal dispersion on the support sur-
ace. The average crystal size decreased with increase of the Ptass fraction in bimetallic PtPd/WO3-OMC. Cyclic voltammetry
nd chronoamperometry measurements showed that Pd/WO3-MC electrocatalyst have higher electrocatalytic activity and long
erm stability for formic acid oxidation as compared to Pd/OMCatalyst. CO stripping analysis revealed that small addition oft on Pd/WO3-OMC enhanced the tolerance of CO poisoning ofd2Pt1/WO3-OMC electrocatalyst as compared to the Pd/OMCatalyst. The Pd2Pt1/WO3-OMC electrocatalyst exhibited superiorctivity and stability among all catalysts for oxidation reactions andlso possessed large ECAS (62.4 m2/g).
cknowledgements
The authors would like to acknowledge the support providedy King Abdulaziz City for Science and Technology (KACST) throughhe Science & Technology Unit at King Fahd University of Petroleum
Minerals (KFUPM) for funding pat of this work through projecto. 11-ENV1655-04 as part of the National Science, Technologynd Innovation Plan. The authors would also like to thank theentre of Research Excellence and Renewable Energy (CoRE-RE),
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research institute, KFUPM for the technical supports to carry outthis research. The assistance from Mr. M Latif Hashmi and Mr.Sadaqat support for XRD/SEM is highly appreciated.
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