Synthesis of Novel Pd/£-AlOOH Composites and Its Electrooxidationtoward Methanol in Alkaline Solution

Hongwei Hou1,+, You Zhu2 and Qingyuan Hu1,+

1China National Tobacco Quality Supervision & Test Center. No. 2 Fengyang Street, Zhengzhou High & New Technology IndustriesDevelopment Zone, Zhengzhou, 450001, People’s Republic of China2Shandong Tobacco Quality Supervision and Testing Station, Xinluo Street, Jinan High & New Technology IndustriesDevelopment Zone, Jinan, 250101, People’s Republic of China

Well-dispersed palladium nanoparticles on the surface of three-quarter-sphere-like £-AlOOH were prepared by an efficient, low-cost andfacile reductive process that utilizes 0.01mol/L NaBH4 solution. The morphology, structure and elemental composition of this Pd/£-AlOOHcomposite were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). The resultsshowed that the Pd nanoparticles were homogeneously dispersed and their size distributed from 6 to 8.5 nm. Electrochemical performance of Pd/£-AlOOH toward methanol in alkaline solution was examined using cyclic voltammetry (CV) and chronoamperometric techniques. The resultsindicated that this novel Pd/£-AlOOH composite had excellent electro-catalytic activity and long-term stability.[doi:10.2320/matertrans.M2013118]

(Received March 26, 2013; Accepted June 10, 2013; Published August 2, 2013)

Keywords: gamma-aluminum hydroxide, palladium nanoparticles, electrocatalysis, methanol oxidation

1. Introduction

Electrocatalytic oxidation of methanol is of great impor-tance in the electrochemistry of direct methanol fuel cell(DMFC) aimed at applications ranging from transportation toportable devices.1) Challenging issues, such as methanolcrossover, low catalytic activity of electrodes, high cost ofthe platinum-based electrocatalysts and susceptibility of thecatalysts to be poisoned by the CO-like intermediates, arethe main obstacles to the commercialization of DMFCtechnology.2) Therefore, it is essential to synthesize moreefficient and economical catalysts for methanol oxidation andto develop more suitable materials/technologies to reducemethanol crossover.3,4) Recently, in order to reduce the costof catalysts, various Pd nanostructures and Pd-based nano-composites with outstanding electrocatalytic properties havestimulated research interest.5,6)

It is generally accepted that the electrocatalytic perform-ance of Pd-based materials toward methanol oxidationis a very complex process and is strongly influencedby many factors that include noble metal size anddispersion, supporting material properties and the preparationmethods.7,8) A suitable supporting material plays a crucialrole in the performance of the electrocatalysts owing to theinteractions and surface reactivity.7) Pd carrier of variousmorphologies such as tube-like metal oxides (TiO2, ¢-MnO2,VOx),9­11) carbon network,12) hollow Fe3O4 nanospheres13)

were reported in the past. Supports that are superior in bothproperties and special morphologies are nontheless in greatdemand for seeking even advanced activity and stabilitytoward methanol electrooxidation.

Based on the study of tube-like £-AlOOH,14) three-quarter-sphere-like £-AlOOH15) and lamellar £-AlOOH architec-tures,16) in view of the fact that £-AlOOH is able to remove

hexavalent chromuim ions and hydrocyanic acid from wastewater and tobacco specific nitrosamines (TSNAs) fromtobacco smoke, the £-AlOOH has been selected to be asupport in this work. Herein, we described a reductionroute that immobilized palladium nanoparticles on thethree-quarter-sphere-like £-AlOOH. The electrocatalytic per-formance of the prepared Pd/£-AlOOH composites wasevaluated by methanol oxidation at the corresponding Pd/£-AlOOH modified glassy carbon (GC) electrode. The resultsindicated that the Pd/£-AlOOH composites had excellentelectrocatalytic activity for methanol oxidation and greatapplication potential in the fields of DMFCs and methanolsensors.

2. Experimental

2.1 Preparation of Pd/£-AlOOH compositesThree-quarter-sphere-like £-AlOOH was prepared follow-

ing our previous report.15) To synthesize Pd/£-AlOOHcomposites, a 2.0 © 10¹3mol/L tetrochloro palladium (II)ions (PdCl42¹) solution was firstly prepared by dissolving0.0355 g of deep brown palladium chloride (PdCl2) powder(Aldrich, 99%) and 0.876 g of sodium chloride (NaCl) in100mL of deionized water.17) 0.1 g of the as-obtained £-AlOOH samples was ultrasonically dispersed in 118mL ofaforementioned tetrochloro palladium (II) ions (PdCl42¹)solution. After stirring the mixture for about 3 h, 60mL0.01mol/L sodium borohydride (NaBH4) solution wasslowly dropped into the above mixture under vigorousstirring. After 3 h of aging, the resulting product was washedwith deionized water and ethanol, and dried under vacuumat 50°C for about 6 h. The metal loaded was normallycontrolled at around 20mass%. The accurate palladiumloading percentage was 17.6mass%, which was determinedby energy-dispersive X-ray spectroscopy (EDS).

+Corresponding author, E-mail: houhw@ztri.com.cn, huqy@ztri.com.cn

Materials Transactions, Vol. 54, No. 9 (2013) pp. 1686 to 1690©2013 The Japan Institute of Metals and Materials

2.2 Modification of GC electrode by Pd/£-AlOOHcomposites

Prior to the surface modification, the glass carbon (GC)electrode (3mm in diameter, 7mm2 active area) was polishedcarefully with 0.3 µm and then 0.05 µm alumina slurry, rinsedwith ethanol and sonicated in double distilled water. 5mg ofcatalyst, 50 µL of Nafion solution (5mass%, Aldrich) and1.0mL of ethanol were mixed via sonication. An aliquot(ca. 25 µL) of above mixture was transferred onto thepolished GC electrode via a springe, and allowed to dryunder an infrared heat lamp. The as-obtained Pd/£-AlOOHmodified GC electrode was employed as the workingelectrode in our experiments.

2.3 CharacterizationThe morphology, size and size distribution of the samples

were studied by field-emission scanning electron microscopy(JEOL JSM-6700F), transmission electron microscopy(JEOL JEM-7650F) and high-resolution transmission elec-tron microscopy (JEOL-2010). The samples for transmissionelectron microscopy analysis were prepared by placing a dropof the catalyst suspension (which has been sonicated for 1 hin acetone) on a 3mm copper grid and drying under ambientconditions. The phase identification was measured by X-raydiffraction (Philips X’pert X-ray diffractometer equippedwith Cu-K¡ radiation (­ = 0.154056 nm), scanning rate wasat 10°min¹1).

2.4 Electrochemical measurementElectrochemical measurements were performed on a

CHI660D electrochemical workstation (Chenhua, Shanghai)with a conventional three-electrode cell configuration.AlOOH support or palladium/£-AlOOH electrode (3mmdiameter) was the working electrode. The platinum wire andsaturated calomel electrode (SCE) were the counter electrodeand the reference electrode respectively. Cyclic voltammetry(CV) examinations were carried out by cycling the potentialbetween ¹0.8 and 0.4V (vs. SCE) at a scan rate of100mV s¹1. Chronoamperometric activity was measuredby keeping the fixed potential at ¹0.3V (vs. SCE). Theelectrolyte with 1.0mol/L methanol and 0.1mol/L sodiumhydroxide solution had been purged with nitrogen for at least30min prior to the experiments. All measurements wereperformed at room temperature.

3. Results and Discussion

The morphologies of the Pd/£-AlOOH were studied byfield-emission scanning electron microscopy and tansmissionelectron microscopy. As shown in Fig. 1(a), the structure ofthree-quarter-sphere-like £-AlOOH has not been impacted bythe synthetic process of palladium composite which has beenconfirmed by field-emission scanning electron microscopyimage (Fig. 1(b)). Although the £-AlOOH surface is notsmooth, its morphology is intact. Figure 1(c) suggests thatthe Pd nanoparticles are homogeneously deposited on the £-AlOOH. The size of the well dispersed palladium nano-particles ranges from 6 to 8.5 nm (high-resolution trans-mission electron microscopy images Fig. 1(d)). As demon-strated by the inset images of Fig. 1(d), the lattice spacing of

palladium (deep color) and £-AlOOH (light color) is about0.2275 and 0.3176 nm. These values are in accordance withthe d values from X-ray diffraction parrerns which provedpalladium nanoparticles are steadily loaded on £-AlOOHsupports. The elemental composition of the Pd/£-AlOOHsamples was determined by energy-dispersive X-ray spec-troscopy analysis. The result shown in Fig. 1(e), revealsthat the products contain aluminium, palladium, oxygen andcopper. In general, copper and oxygen are from the substrateor air.

X-ray diffraction patterns of the samples are shown inFig. 2. Figure 2(a) is the typical X-ray diffraction patternof the aluminum hydroxide (AlOOH) we prepared for thisexperiment. When compared with the standard pattern, all ofits diffraction peaks were easily indexed to the orthorhombicAlOOH (JCPDS Card No. 21-1307). The X-ray diffractionpattern of as-prepared palladium is shown in Fig. 2(b)(reference: JCPDS card 88-2335). The similarity betweenthe X-ray diffraction peaks of palladium/£-AlOOH compo-sites (Fig. 2(c)) and the standard spectrum states that face-centered cubic (fcc) phase palladium is attached to the £-AlOOH. Scherrer equation gives 7.5 nm, which matches theresults from high-resolution transmission electron microscopyimages, for the average size of the palladium nanoparticles.

All CVs of the palladium/£-AlOOH catalyst in both1.0mol/L methanol and 0.1mol/L sodium hydroxide

Fig. 1 FE-SEM and TEM images of Pd/£-AlOOH composites: (a) TEMimage, (b) FE-SEM image, (c) HRTEM image, (d) a magnified HRTEMimage, (e) EDS spectrum of Pd/£-AlOOH.

Synthesis of Novel Pd/£-AlOOH Composites and Its Electrooxidation toward Methanol in Alkaline Solution 1687

solution are shown in Fig. 3. The voltage was between ¹0.8and 0.4V and the scan rate was 100mV/s. Stable responseswere obtained after several cycles. Curve a displayed asmooth and flat curve suggested £-AlOOH had not anyelectrocatalytic activity for methanol oxidation. There weretwo apparent symmetric anodic peaks discovered in curve b,namely, ¹0.102V in the forward scan and the ¹0.288Vpeak in the reverse scan. The forward peak current is relatedto the oxidation of fresh chemisorbed species issued frommethanol adsorption while the one in the reverse scan canbe attributed to the removal of the incompletely oxidizedcarbonaceous species formed in the forward scan.18,19)

Investigating the catalytic properties of the palladium/£-AlOOH electrode, there were three aspects to compare.First, the more negative the anodic potential, the higherthe full cell voltage, when the cathodic potential is fixed.The peak potential of ¹0.102V in the forward scan ismore negative than the one on palladium/Fe3O4 electrode(Epa = 0.21V, estimated from the curve).13) Secondly, onsetpotential of the Pd/£-AlOOH electrode (ca. ¹0.55V) was

much lower than the one on /VOx-NTs electrode (ca.¹0.37V),11) Pd/C electrode (ca. ¹0.317V) and Pd/¢-MnO2

electrode (ca. ¹463V).10) Thirdly, the peak current densityappeared at 201 µA for Pd/£-AlOOH electrode was one foldhigher than that for Pd/VOx-NTs electrode (Ipa = 90 µA,estimated from the curve). These results pointed to the factthat the Pd/£-AlOOH composites had a different eclectro-catalytic activity toward methanol oxidation in sodiumhydroxide.

The dependence of peak currents on the square root ofscan rates is inserted in Fig. 4. The peak currents are linearlyproportional to the square root of scan rates within certainscan reate range. This suggests that the electrocatalyticoxidation of methanol on Pd/£-AlOOH/GC may becontrolled by a diffusion process under this scan rates.20,21)

A linear relationship between Epa (the forward scan) andlnðvÞ is revealed in Fig. 5. The peak potential (Epa) at theforward scan increases with the increase of v, indicatingthat the oxidation of methanol is an irreversible electrodeprocess.

Chronoamperometric is an effective technique in evalua-tion of the electrocatalytic stability of catalysts. A character-istic current­time curve of Pd/£-AlOOH toward methanoloxidation (in 0.1mol/L sodium hydroxide solution at¹0.3V vs. SCE) was obtained and displayed in Fig. 6.

(a)

(b)

Fig. 3 Cyclic voltammograms of different electrodes for the oxidation of1.0M methanol in 0.1M sodium hydroxide solution: (a) AlOOH/GCelectrode; (b) Pd/£-AlOOH/GC electrode; scan rate: 100mV/s.

Fig. 4 CV curves of Pd/£-AlOOH/GC electrode in 1.0M methanol and0.1M sodium hydroxide solution at different scan rates: (a) 30mVs¹1;(b) 50mVs¹1; (c) 80mV s¹1; (d) 100mVs¹1; (e) 125mV s¹1;(f ) 150mVs¹1; (g) 175mV s¹1; (h) 200mVs¹1; (i) 225mV s¹1;(j) 250mVs¹1 and (k) 275mVs¹1.

(a)

(b)

(c)

Fig. 2 XRD patterns of the as-prepared samples: (a) £-AlOOH, (b) Pd,(c) Pd/£-AlOOH composites.

Fig. 5 The relationship between Epa and lnðvÞ. CCH3OH = 1M.

H. Hou, Y. Zhu and Q. Hu1688

The polarization current decayed for the Pd/£-AlOOHcatalyst toward methanol electrooxidaion, similar to themethanol electrooxidation reaction on the platinum orpalladium-based electrocatalysts in alkaline media. Thedecayed current for the catalyst is possibly due to theformation of palladium-based oxides and/or absorbedintermediates during the methanol electrooxidation.22,23)

Interestingly, the values of current density of preparedcatalyst decrease gently. Of the initial value, about 21%remains after 960 s and 12% after 3600 s. Similar result hadbeen reported by Xu et al.22) Although they have not pursuedthe current value after 3600 s, their data showed that Pd/£-AlOOH catalyst are superior to both the platinum Pt/CMSand palladium/CMS that could keep only ³8 and ³7% ofthe initial value (estimated from the curves) after 900 s.This advantage illustrates that the Pd/£-AlOOH compositeshave better steady-state electrolysis for methanol oxidationin alkaline.

The long-term stability of palladium/£-AlOOH compo-sites modified GC electrode in 1.0mil/L methanol and0.1mol/L sodium hydroxide aqueous solution is shownin Fig. 7. It can be seen that the peak currents decreasegradually in successive scans, and the peak current of the500th scan is ³82% of the first scan. The loss of the peakcurrent may result from the consumption of methanol duringthe CV scan process. The poisoning and structure changeof the palladium nanoparticles as a result of potentialperturbation during the scanning in aqueous solution mayalso contribute to this loss, particularly in the presence ofthe intermediate organic compound during the methanoloxidation.10,13,19) Comparable catalytic activity of methanoloxidation was observed in long-term CV scans from thepalladium/£-AlOOH/GC electrode that was stored indeionized water for 7 days and reused. This indicates thatthe Pd/£-AlOOH composites prepared in our experimenthas good long-term stability and great application potentialin direct methanol fuel cells.

4. Conclusions

In this work, we first proposed using £-AlOOH as catalystsupporting material and successfully obtained Pd/£-AlOOHcomposites through a facile reductive process. The Pdparticles have been evenly immobilized on the surface of £-AlOOH and have relatively narrow particle size distributedfrom 6 to 8.5 nm. The results of CV show that the Pd/£-AlOOH catalyst has lower onset potential, higher peakcurrent density and better electrocatalytic activity. It impliesthat Pd/£-AlOOH composites have potential applications inthe fields of DMFCs and methanol sensor.

Acknowledgments

The authors are grateful for financial support by theNational Nature Science Foundation of China (No.20701040).

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