Pd Supported on Titanium Nitride for Efficient Ethanol Oxidation

M. M. Ottakam Thotiyl, T. Ravi Kumar, and S. Sampath*Department of Inorganic Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India

ReceiVed: April 28, 2010; ReVised Manuscript ReceiVed: August 27, 2010

The excellent metal-support interaction between palladium (Pd) and titanium nitride (TiN) is exploited indesigning an efficient anode material, Pd-TiN, that could be useful for direct ethanol fuel cell in alkalinemedia. The physicochemical and electrochemical characterization of the Pd-TiN/electrolyte interface revealsan efficient oxidation of ethanol coupled with excellent stability of the catalyst under electrochemical conditions.Characterization of the interface using in situ Fourier transform infrared spectroscopy (in situ FTIR) showsthe production of CO2 at low overvoltages revealing an efficient cleaving of the C-C bond. The performancecomparison of Pd supported on TiN (Pd-TiN) with that supported on carbon (Pd-C) clearly demonstrates theadvantages of TiN support over carbon. A positive chemical shift of Pd (3d) binding energy confirms theexistence of metal-support interaction between Pd and TiN, which in turn helps weaken the Pd-CO synergeticbonding interaction. The remarkable ability of TiN to accumulate -OH species on its surface coupled withthe strong adhesion of Pd makes TiN an active support material for electrocatalysts.

1. Introduction

Fuel cells have been projected as promising alternate energydevices for the fast depleting conventional energy sources.1-3

There have been several studies on the use of various smallorganic molecules as fuels in direct fuel cells. In this direction,low molecular weight alcohols such as methanol and ethanolhave received considerable attention.4,5 Even though methanolis a promising fuel, the concern regarding its toxicity6 hasprojected ethanol as an excellent alternative material. Otheradvantages of ethanol include the ease of production fromagricultural products/fermentation of biomass and the projectedhigh energy density of ethanol-based fuel cells.7,8 The fuelcrossover that is often encountered in methanol-based fuel cellsis low in the case of ethanol fuel cells.9-11

However, there are many difficulties associated with ethanoloxidation and consequently in its use in the fuel cell mode.11-18

Complete oxidation of ethanol to CO2 involves 12 electrons andthe process involves the scission of a C-C bond thus demandinghigh activation energies to be overcome.11 Many of theintermediates (mainly CO and -CHO)15 produced during theoxidation reaction poison the anode electrocatalyst and in turnreduce the catalytic efficiency. This demands the presence ofanother metal such as Ru, Ni, Sn, and Ir to alleviate thepoisoning effect.13,16,17 The use of a second metal (M) has beenreported to promote water decomposition at low potentials toform, for example, M-OH groups which in turn assists in theremoval of CO and -CHO type species from the blockedcatalyst surface thereby freeing the active surface, by abifunctional-type mechanism.13,16,18 However, the alloy elec-trocatalysts achieve complete oxidation of ethanol to CO2 onlyat high anodic potentials making them less efficient for directethanol fuel cells (DEFCs). Though the poisoning effect isreduced, the combination of two precious metals makes thecatalyst cost very expensive. Hence, there is need to developcatalysts with high activity, high poison tolerance, and, ofcourse, low cost.

Pd is reported to be a very good catalyst for ethanol oxidationin alkaline medium and is 50 times more abundant than Pt.19,20

Very often, Pd is loaded on to carbon support and used in fuelcells. The activity of catalysts for fuel cell reactions dependson the judicious selection of supports as well. This aspect iswell-documented in the literature.21-23 Conventional carbonsupports are prone to undergo corrosion in aggressive electro-lytes that are very often encountered in fuel cells.24-30 Thecorroded carbon support cannot hold the catalyst on its surfaceleading to aggregation or sintering of noble metal particles(reduces electrochemically active surface area) and often result-ing in oxidation and subsequent leaching of the catalyst.24-30

Corrosion of the support/catalyst happens mainly because theyare exposed to aggressive electrolytes, high temperature andpressure, and high humidity. Carbon is known to undergocorrosion even at open circuit voltages of the fuel cell.

In the present paper, we report a highly efficient, remarkablystable Pd-supported titanium nitride (Pd-TiN) as a catalyst forefficient oxidation of ethanol in alkaline medium. Titaniumnitride (TiN) is a very hard, conducting ceramic material oftenused as an abrasive coating for engineering components.31,32 Inthe electronic industry, it is widely used as a “barrier metal”because of its excellent diffusion barrier properties.33,34 Itpossesses metal-like electronic conductivity35 with a veryreproducible surface for electron transfer. TiN is biocompatibleas well. Though TiN has been known as an anticorrosive, barriercoating material for decades, the available literature on itselectrochemical properties is scarce. TiN has been proposed asan electrode for supercapacitors;36 as a substrate for electrodepo-sition of metals such as Pt, Zn, Cu, and Ag;37-39 as a pHsensor;40 for the deactivation of marine bacteria;41 and inelectroanalysis.42 In a recent communication, we have high-lighted the use of platinized TiN for the electrochemicaloxidation of methanol in acidic medium.43 In a related study,Avasarala and co-workers have proposed Pt-TiN as a goodelectrode material for electrochemical reduction of oxygen insulfuric acid medium.44

The present paper explores the effect of TiN support for Pdcatalyst by using a variety of techniques such as in situ IR

* To whom correspondence should be addressed. E-mail: sampath@ipc.iisc.ernet.in. Phone: + 91 80 22933315. Fax: + 91 80 23601552.

J. Phys. Chem. C 2010, 114, 17934–1794117934

10.1021/jp1038514 2010 American Chemical SocietyPublished on Web 09/24/2010

spectroelectrochemistry, XPS, atomic force microscopy, Kelvinprobe microscopy, and electrochemistry. Further, the perfor-mance of Pd-TiN catalyst is compared with conventional Pdsupported on carbon. The support material TiN helps inalleviating CO poisoning of Pd and promotes ethanol oxidation.

2. Experimental Section

2.1. Chemicals and Materials. All the reagents and chemi-cals used were of analytical grade. K2PdCl4 (Ranbaxy, India),ethanol, KOH (Qualigens, India), Pd black (Aldrich, USA), TiNpowder (Aldrich, USA), and carbon powder (Vulcan XC 72)were used as received. Doubly distilled water was used for allexperiments. Prior to the experiments, all glass apparatus wascleaned with chromic acid, washed with double distilled water,rinsed with acetone, and dried.

2.2. Preparation of TiN Coating. TiN was coated on thinstainless steel sheets (SS-304) by cathodic arc depositiontechnique. A 20 in. Multi-Arc chamber was used for thedeposition. Before deposition, the coating plant was firstevacuated to a base vacuum of 10-6 Torr. During deposition,reactive nitrogen gas pressure and substrate bias voltage werekept constant at 10 mTorr and -200 V. The deposition timewas kept at 30 min and the thickness of the TiN coating was 3µm.

2.3. Preparation of TiN Working Electrode. The workingelectrodes were made by cutting the SS coated TiN into smallpieces and attaching a Cu wire to one end by spot-welding.The exposed Cu wire and stainless steel portions were insulatedwith epoxy resin. Prior to the electrochemical measurements,the TiN electrodes were degreased by wiping them with a tissuesoaked in absolute ethanol and thoroughly washed with doublydistilled water.

2.4. Palladium Deposition on TiN. Chronopotentiometrywas used for the deposition of Pd from a solution of 0.1MK2PdCl4 with TiN as the working electrode, a large area Ptfoil as the counter electrode, and saturated calomel electrode(SCE) as the reference electrode. The amount of deposited Pdwas controlled by varying the charge passed through the cell,at a current density of 4 mA/cm2. A similar procedure was earlierused for tuning loading of Pt on other substrates.45

The preparation of palladized TiN powder was carried outas follows. About 200 mg of TiN powder was mixed with anaqueous solution of PdCl2 to yield a certain weight percent ofPd on TiN. The solvent was allowed to evaporate overnightunder constant stirring. The resulting dry powder with adsorbedPdCl2 (on TiN) was then treated at 350 °C in H2 stream for 2 hin a tubular furnace to effect reduction of palladium ions to Pdon TiN. In certain experiments, TiN and carbon supportedelectrocatalysts were prepared by the polyol method as reportedearlier.44 It should be noted that even physical mixing of Ptpowder and transition metal oxide was reported to yield goodelectroactivity for methanol oxidation.46

2.5. Characterization. The supported catalysts Pd-TiN andTiN were characterized by scanning electron microscopy (FEI200 kV, Netherlands), X-ray diffraction (JEOL JDX 8030),X-ray photoelectron spectroscopy (XPS, Thermoscientific Mul-tilab 2000 instrument), atomic force microscopy (Digital Nano-scope 4A, USA), potentiostat/galvanostat (EG&G PARC, 263Aor CHI 660 A model), and spectroscopic techniques (Ther-monicolet 6700 FTIR with liquid N2 cooled HgCdTe detectorand Perkin-Elmer UV-vis spectrometers). Conductive AFM (C-AFM, Digital Nanoscope 4A, USA) and scanning kelvin probe(SKPM, Digital Nanoscope 4A, USA) images were acquiredwith a Co-Cr probe. A sample bias of 1 V with respect to thetip was applied for current measurements.

2.6. In Situ FTIR Spectroelectrochemistry. In situ FTIRspectroscopy under applied potential conditions was carried outas follows. A three-electrode cell consisting of Pd-TiN foil, Ptwire, and a Hg/HgO/1 M KOH (MMO) as working, counter,and reference electrode, respectively, was used in 0.5 M ethanolin 1 M KOH. The potential of the working electrode is scannedbetween -0.7 and 0.25 V and then reversed to -0.7 V at ascan rate of 10 mV/s and the spectra were recorded by holdingthe potential at desired values. An incident IR beam at an angleof about 65° with spectral resolution of 4 cm-1 was used. Thespectrum at -0.7 V was used as the reference throughoutthe experiment except for CO band measurements where thespectrum at 0.20 V (potential at which CO is completelyoxidized) was used as the reference. The spectra are presentedbased on the percentage reflectivity according to the followingrelation:

where RE represents the spectrum at the potential E V and R-0.7V

is the spectrum at -0.7 V. The spectra corresponding to theCO band are given based on

where R0.20V is the spectrum at 0.20 V. Accordingly positiveand negative bands indicate depletion and accumulation of thecorresponding species, respectively.

2.7. Electrochemical Oxidation of Ethanol with Pd-TiN.Electrochemical measurements were carried out in a three-electrode cell in a solution of 0.5 M ethanol + 1 M KOH. Cyclicvoltammograms were recorded in the potential range -0.7 to0.25 V. All solutions were deaerated with highly pure Ar for20 min prior to the experiment. The electrodes were kept indeaerated solution for 10 min for equilibration before themeasurements. All the experiments were carried out at 25 (0.2 °C.

3. Results and Discussion

3.1. Characterization of TiN. The TiN film deposited onSS surface is smooth, very adherent, and reflecting in nature.The SEM of bare TiN along with the EDS pattern (SupportingInformation, Figure S1) shows that the film is continuous anddoes not contain any pit or defect. The X-ray diffraction (XRD)pattern (Figure 1) reveals a highly oriented TiN (111) surface(2θ ) 36.1°) corresponding to cubic NaCl-type structure andthe lattice parameter is determined to be 0.4244 nm and is ingood agreement with the literature value (0.4241 nm; JCPDF38-1420).

Surface characterization with XPS of TiN film reveals threecomponents in the Ti(2p3/2) region (Figure 2) assigned to (a)TiN phase (455.01 eV), titanium oxynitride phase (456.45 eV),and TiO2 phase (458.41 eV).47 The Ti 2p1/2 observed in the range460-464 eV possesses two components, one at 460.85 andanother at 463.69 eV attributed to TiN and TiO2, respectively.This pattern is typical of TiN films prepared by the cathodicarc deposition technique.40,41 The N(1S) spectrum showscomponents corresponding to nitridic nitrogen (397.2 eV) andoxynitride (398.5 eV) phases.47,48 The presence of a low-energycomponent is reported to be due to atomic nitrogen-likespecies.49 On the basis of the XPS characterization, it isconcluded that there is a thin oxygen enriched layer on thesurface of TiN coexisting with the TiN phase.

∆R/R ) (RE - R-0.7V)/R-0.7V (1)

∆R/R ) (RE - R0.20V)/R0.20V (2)

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3.2. Characterization of Pd-TiN. Electrodeposition of Pdon TiN is carried out from a solution of 1 mM K2PdCl4 in 0.1M H2SO4 (Supporting Information, Figure S2). The bare TiNshows a featureless volatmmogram in the potential range, 0 to1 V in H2SO4 medium. Addition of 1 mM K2PdCl4 to theelectrolyte reveals well-defined redox peaks at 0.330 and a 0.680V vs SCE. The tail observed at the positive potential limit inFigure S2 (Supporting Information) is likely to be due to slowoxidation of TiN. Varying amounts of Pd are deposited on TiNsurface by controlling the charge passed through the cell at acurrent density of 4 mA/cm2 for different periods of time. Theas-deposited Pd particles have an average size around 25 nm(Supporting Information, Figure S3) and the AFM picture(Supporting Information, Figure S4) shows that the particleshave a tendency to agglomerate on the TiN surface. Since Pd

is more metallic than TiN, Kelvin probe measurements (SKPM)reveal lower work functions for Pd domains than that of TiNdomains (Supporting Information, Figures S5 and S6). Animportant observation is that the interface of Pd and TiN is foundto have a different work function as compared to the individualcomponents, confirming the existence of metal-supportinteraction.

Electrochemical characterization of Pd-TiN (Pd loading of 4µg/cm2) in 1 M KOH clearly shows hydrogen adsorption-desorption peaks in addition to Pd oxide formation and reduction(Supporting Information, Figure S7). Pd oxide formation isobserved around 0.270 V and the corresponding reduction peakis observed around -0.250 V. The H2 adsorption and desorptionfeatures are observed in the range -0.5 to -0.8 V as reportedfor Pd in alkaline media.8

3.3. Ethanol Oxidation. Bare TiN is inactive for ethanoloxidation in alkaline medium (Supporting Information, FigureS8). The voltammogram carried out on Pd-TiN in 1 M KOHcontaining 0.5 M ethanol shows a forward peak due to ethanoloxidation in the positive scan (Figure 3) and the negative scanshows a peak in the same direction. The oxidation currentsobserved in the reverse direction are due to the oxidation ofadsorbed intermediates as reported for Pd-based systemsearlier.20,50 However, the observations on Pd-TiN differ fromthe reported literature in certain aspects. First, ethanol oxidationcurrents grow as a function of number of cycles with a negativeshift in peak potentials. The oxidation currents increase up to15th cycle by ∼8 times and thereafter remain constant. Thisobservation points out that poisoning of Pd electrocatalyst whichis often encountered during ethanol oxidation is minimal on

Figure 1. X-ray diffraction pattern of titanium nitride thin film coatedon SS-304. The corresponding planes are marked in the figure. Thereflections marked with asterisks are from the stainless steel substrate.

Figure 2. X-ray photoelectron spectra of titanium nitride thin filmcoated on SS-304: (A) Ti(2p) and (B) N(1S) regions. The open circlesand solid red line represent the original and fitted data, respectively.Green dotted lines represent the deconvoluted bands.

Figure 3. Cyclic voltammograms of Pd-TiN electrode in the presenceof 0.5 M ethanol and 1 M KOH as a function of the number of cycles.The Pd loading is 4.4 µg/cm2 and the scan rate used is 20 mV/s. Thearea of TiN used is 0.25 cm2 (A). Plot of peak current and peak potentialvs cycle number (B).

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the Pd-TiN electrode. Palladium nanowires that have recentlybeen reported to be very active for ethanol oxidation also showa decreasing trend in the oxidation currents as a function of thenumber of cycles.51 The second difference observed with Pd-TiN is related to the shift of oxidation peak potentials from0.040 V for the first cycle to -0.090 V for the 10th cycle beforestaying constant at this value.

The increase in currents may be argued to be due to repetitivecycling of Pd surface in the oxide-formation region leading tochanges in surface area/activity of the electrode. However, Pddeposited on glassy carbon electrode using the same proceduredoes not result in an increase in currents when cycled in thesame potential range. Additionally, the Pd-TiN electrode showsexactly the same reversible redox behavior for ferrocyanide/ferricyanide couple (in 1 M KOH) with no change in peakcurrents, before and after ethanol oxidation in alkaline medium(Supporting Information, Figure S9). This suggests that theincrease in currents observed for ethanol oxidation is not dueto any reorganization of Pd domains on TiN surface duringpotential cycling. The diffused reflectance infrared spectra ofthe TiN surface before and after cycling in the same range inKOH solution shows an increase in -OH stretching intensitysupporting the formation of Ti-OH-type functional groups onthe TiN surface (Supporting Information, Figure S10). This issimilar to the growth of Ru-OH functional groups when Pt-Rucatalyst is used for oxidation of alcohols. Third, a controlexperiment where Pd-TiN is conditioned at a positive potentialof 0.25 V in KOH alone does not result in any change as far asthe effect of subsequent cycling in ethanolic KOH solution isconcerned. These observations point to the fact that theactivation of the Pd-TiN surface yielding high currents probablyhappens in situ during the oxidation of ethanol. The optimumloading of Pd is found to be 3-10 µg/cm2 of TiN.

To understand the role of the TiN surface, XPS has beencarried out before and after ethanol oxidation. The Ti (2p)spectra observed before and after the electrochemical oxidationof ethanol are shown in Figure S11 (Supporting Information).The deconvoluted spectrum shows contributions from TiON andTiO2 in addition to pure TiN phase before oxidation. Afterelectrochemical oxidation of ethanol, it is observed that thecontribution of oxynitride (456.56 eV) has increased relativeto that of th pure TiN phase. Avasarala and co-workers havehypothesized that the enhanced activity of TiN-based catalyststoward oxygen reduction in acid medium could be due to thepresence of oxynitride phase.44 We intentionally oxidized TiNbefore Pd deposition and used it for ethanol oxidation. Theefficiency and the poison tolerance of the catalyst is found tobe improved as compared to that of Pd deposited on as-preparedTiN surface (results not shown). Therefore, the growth ofoxynitride species during cycling may possibly play a role inthe enhanced currents and negative shift in peak potentialsobserved in the present study.

The oxidation state of Ti is 3+ in TiN and it is known thatTi3+ can be oxidized to TiO2+ in aqueous medium.52 Thevoltammogram of TiN in KOH shows a sharp peak around 1.5V due to the oxidation of the surface. However, it is very likelythat oxidation of TiN occurs at low voltages to a small extent.The TiO2+ species may aid the oxidation of ethanol by amediator-type mechanism thereby simultaneously getting re-duced to TiN though it is only speculative at present. Theformation of Ti-OH-type groups (as explained later in the sectionon in situ IR spectroelectrochemistry) improves the efficiencyfurther by removing the intermediate species formed during theoxidation of ethanol. Other possible reasons could be related

to the effect of metal-support interactions observed in othercatalytic reactions.53,54 It is already reported that the COdesorption temperature decreases when palladium is loaded ontoa support as compared to bare palladium.55-62 Similar metal-support interactions may play a role in the present studies aswell. The electronic nature of the Pd-TiN interface is differentfrom that of either Pd or TiN as revealed by the work functionmeasurements described earlier. The metal-support interactionspossibly introduce electron deficiency on Pd, which in turn maydecrease the Pd-CO synergetic bonding interactions remarkably.

3.4. In Situ Spectroelectrochemistry. In situ IR spectros-copy has been carried out in the reflectance mode as describedin the Experimental Section. Various regions of interest as afunction of applied potential are given separately in Figures 4-6.A distinct band in the negative direction observed at 3600 cm-1

(Figure 4) is assigned to be due to the symmetric stretching ofadsorbed water.63,64 The intensity of this band grows (accumula-tion of the species) with applied potential and it would help inthe formation of -OH-type functional groups useful for COalleviation. The presence of adsorbed water on Pd-TiN is furtherconfirmed based on growth in intensity of the band at 1600cm-1, which is due to the bending mode of adsorbed water(Figure 6B). The band positions for adsorbed water moleculesare different from those of bulk water in the three-dimensionalhydrogen-bonded state, but are rather similar to weakly hydrogen-bonded water species.60,61 Similar water bands have beenreported on PtRu systems and not for pure Pt demonstratingthat water is probably getting adsorbed on Ru under theelectrochemical conditions used.63,64 The ability of Ru to adsorbwater is responsible for the well-known bifunctional mechanismproposed for the oxidation of alcohols on Pt-Ru catalysts.63,64

Ethanol dehydrogenation on Pt sites poisons the surface withadsorbed CO (Pt-CO). Simultaneously, -OH groups formedon the Ru surface at low overpotentials help in the alleviationof CO poisoning.

It is reported that Pd-based electrocatalysts decorated withoxides such as CeO2, In2O3, NiO, MnO2, Co3O4, TiO2, andcarbon nanotubes65,66 have shown improved activity towardethanol oxidation in alkaline media over pure Pd. The improvedactivity of Pd-metal oxide catalysts has been proposed to bedue to a similar bifunctional-type mechanism.

On the Pd-TiN surface, the H2O/OH band intensities growas a function of potential and are very vital for the removal ofchemisorbed CO on the catalyst surface. Additionally, it is

Figure 4. In situ FTIR spectra in the range 2000 to 4000 cm-1 for theelectrooxidation of ethanol on the Pd-TiN electrode in the first cycle.The electrolyte is 0.5 M ethanol in 1 M KOH. The reference spectrumis obtained at -0.7 V. The spectra are obtained at a potential intervalof 0.1 V.

Efficient Oxidation of Ethanol in Alkaline Medium J. Phys. Chem. C, Vol. 114, No. 41, 2010 17937

reported that adsorption of water inhibits CO chemisorption.67

It is noteworthy that both molecular and dissociated forms ofadsorbed water have been reported on surfaces such as titaniumcarbide (TiC) and vanadium carbide (VC)68 with use of highresolution electron energy loss spectroscopy and XPS studies.

The ability of TiN to accumulate interfacial -OH-typefunctional groups has been probed with use of a bare TiNelectrode in 1 M KOH containing 0.5 M ethanol. The presenceof adsorbed water is easily detected by the accumulation ofspecies as revealed by growing intensities at 1600 and 3600cm-1 bands (Figure 5) as a function of applied potential. It isvery important to mention that the adsorbed -OH functionalgroups are observed at remarkably low overvoltages. This mayhave additional consequences in water electrolysis on TiNelectrodes at low overvoltages.

An intense CO2 band (O-C-O asymmetric stretch) isobserved at 2345 cm-1 (Figure 4) even in low potential rangessuggesting direct oxidation of ethanol.63,64,69,70 This is in goodagreement with the widely accepted parallel pathway for ethanoloxidation. According to the parallel pathway mechanism, ethanolelectrooxidation proceeds simultaneously through an acetalde-hyde/acetic acid (AA pathway) pathway where the C-C bonddoes not break and a CO pathway where the C-C bond iscleaved.69-71 The degree of domination of one pathway overthe other is potential specific. Accordingly, at low overpotentialsthe AA pathway will be dominant and at high potentials theCO pathway dominates. This happens mainly because theformation of CO requires the cleavage of the C-C bond and ithappens only at high overpotentials on most of the electrocata-lysts. The observation of CO2 band with considerable intensitiesat low overvolatges on Pd-TiN suggests that the CO pathwayis very dominant indicating its high efficiency toward ethanoloxidation.

The presence of adsorbed CO (2000-2100 cm-1) as afunction of potential is presented in Figure 6A. The bandposition is in good agreement with that observed on noble metalelectrodes.71,72 It is found to shift to higher wave numbers withanodic increase in potential and this observation is accountedfor, based on the Stark effect. The CO band intensity is foundto be maximum around -0.4 V and thereafter a decrease isobserved.

Hence, the observed variation in CO2 signal intensity can beexplained as follows. At low overpotentials adsorbed CO ismainly oxidized by the OH groups available on the TiN surface,while at higher anodic potentials it is also oxidized by the directelectrochemical route as well.

The bands around 1725 cm-1 (Figure 6B) could be due tocarbonyl stretching in acetaldehyde or acetate available at theinterface.72 However, the presence of acetaldehyde can beambiguously proved only by the C-C-O band at 930 cm-1,69,70

and it is not observed in the present studies. The presence ofacetate is confirmed by the appearance of the band at 1435cm-1.69 Since there is an intrinsic difficulty in detectingacetaldehyde due to its low absorptivity, it is assumed that bothacetaldehyde and acetate are formed in the present studies.73

Strong absorption of the acetate band at 1435 cm-1 makes theobservation of the carbonate band (at 1450 cm-1) difficult, asreported in the literature.69,71 The spectra in the range 800-2000cm-1 shows depletion in intensity of the band at 1060 cm-1,which is due to the C-O stretch in ethanol (Figure 6B).

3.5. Ex Situ UV-Visible Spectroscopy. Initial studies havebeen carried out to understand the products formed during theoxidation process, using UV-visible absorption spectroscopyon the electrolyte solution before and after cycling a large area

Figure 5. In situ FTIR spectra in the range of 1000 to 2000 cm-1 (A)and 2500 to 4000 cm-1 on the bare TiN electrode. The lectrolyte is0.5 M ethanol in 1 M KOH. The reference spectrum is obtained at-0.7 V.

Figure 6. In situ FTIR spectra in the range (A) 1800 to 2200 cm-1

(reference spectrum at 0.20 V) and (B) 800 to 2000 cm-1 (referencespectrum at -0.7 V) for the electrooxidation of ethanol at the Pd-TiNelectrode in the first cycle. The electrolyte is 0.5 M ethanol in 1 MKOH.

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Pd-TiN electrode for 200 cycles. The use of a large area andnumber of cycles is to ensure that enough products are formedin the bulk of the medium. Control experiments have beencarried out for possible products of ethanol oxidation in alkalinemedium such as acetaldehyde, acetic acid, and K2CO3. Twoshoulders, a large one around 275 nm and a small one around320 nm, are observed in addition to unreacted ethanol (Sup-porting Information, Figure S12) for the electrolyte used foroxidation of ethanol with Pd-TiN. The control UV-vis spectra(Figure S12, Supporting Information) reveals that one of thepossible products is acetaldehyde. Accumulation of acetate andK2CO3 may also result in the absorbance increasing around thisregion. Hence, the ex situ studies provide support for thepresence of acetaldehyde as observed in the in situ characteriza-tion given earlier.

3.6. TiN vs Carbon Support. A comparative study has beencarried out to decipher the effect of TiN as an active supportmaterial as opposed to state of the art carbon support (Vulcancarbon XC-72) that is used for anchoring Pd catalyst. The sameamount of palladium is loaded on to TiN and Vulcan carbonwith the polyol method (see Experimental Section) and subse-quently used for ethanol oxidation. The mass activity (A/mg)of the electrocatalysts are shown in Figure S13 (SupportingInformation). The mass activity (A/mg) on TiN support is higherthan that observed on carbon support. The onset potential andthe peak potential are found to be negatively shifted on TiNsupported catalyst. Table 1 gives the mass activity along withthe electrochemically active area and current density of thecatalysts used in the present studies. The electrochemicallyactive area has been found out by integrating the chargeassociated with PdO stripping and the details are given in thelast section of the Supporting Information. The inability of Pdto oxidize ethanol to CO2 (due to scission of the C-C bond) iswell documented in the literature and the oxidation stops at thegeneration of CH3CHO and CH3COOH.66,68,70 Therefore, theobserved high activity could be due to an efficient cleavage ofthe C-C bond when Pd is loaded on to TiN substrate. Similarmetal-support interactions modifying the electronic structureof the interface are well-documented in the literature.52-59

A good fuel cell catalyst should have high current densitiesat low overpotentials. The cyclic voltammograms clearly revealthat Pd-TiN exhibits higher currents than that observed on Pd-C, at low overpotentials. This is further confirmed from the Ivs t transients recorded at overpotential close to the onset value(Figure 7). The I vs t transients recorded at -0.5 V show thatthe currents on Pd-TiN are almost 10 times higher than thatobserved on Pd-C demonstrating the beneficiary effect of loadingPd on TiN. Enhanced currents are observed at other potentialsas well. The observations are similar and the currents are veryhigh when mass activity is used instead of EAA normalizedactivity (Figure 8). This observation demonstrates that the rateof the electron transfer and poison tolerance of the Pd-TiNelectrocatalyst is higher than that of Pd-C.

The ratio of the forward peak current (Ipf) to the reverse peakcurrent (Ipb) is often taken to be a benchmark of the performanceof a catalyst. However, this ratio depends on the reversal voltageand hence any comparison should be made under identical

conditions. The ratio (Ipf/Ipb) is observed to be 0.51 on Pd-TiN(Table 1) as compared to the value of 0.42 observed on Pd-C.Another method often used to assess the efficiency of the catalystis the potential separation (∆E) between the peaks in the forwardscan (Epf) and that of the reverse scan (Epb). The value observedfor Pd-TiN is 100 mV and that for Pd-C is 180 mV at the samescan rate and for the same loading of the catalyst, indicatingthe efficient and rapid removal of accumulated intermediateson the Pd-TiN surface. Steady state measurements to determinethe Tafel slopes are given in Figure S14 (Supporting Informa-tion). The Tafel slopes are found to be 146 and 251 mV/dec onPd-TiN and Pd-C, respectively. The value of teh Tafel slopeobserved on Pd-C is in good agreement with the value reportedin the literature.74,75 A low Tafel slope on Pd-TiN suggests anefficient charge transfer as compared to the Pd-C interface.74,76

The exchange current density (i0) is found to be 85 and 29 µA/cm2 for Pd-TiN and Pd-C, respectively. These observationsclearly manifest that TiN is an active support unlike theconventional carbon support.

The metal-support interactions between Pd and TiN arefurther investigated by XPS. The Pd (3d) spectra observed onPd-TiN and Pd-C are shown in Figure S15 (Supporting

TABLE 1: Voltammetric Characteristics of Pd-TiN and Pd-C Catalysts

material onset potential (mV) Ep (mV)a Ipf (mA/mg)b EAA (cm2/mg)c Ipf (mA/cm2)d Ipf/Ipb ∆E ) Epf - Epb(mV)

Pd-TiN -572 -76 59.2 20.6 2.87 0.51 100Pd C -465 4 21.5 13.23 1.63 0.42 180

a Peak potential. b Mass activity. c Electrochemically active area. d Current density normalized for electrochemically active area.

Figure 7. Current-time (I-t) transients recorded on Pd-TiN and Pd-Cat -0.25 (A) and -0.5 V (B) in 1 M KOH containing 0.5 M ethanol.Pd loading is 1 mg of Pd/cm2. The current densities are obtained bynormalizing it with the electrochemically active area. Note that thepotential of -0.5 V is close to the onset value while -0.25 V is in therising portion of the voltammogram.

Efficient Oxidation of Ethanol in Alkaline Medium J. Phys. Chem. C, Vol. 114, No. 41, 2010 17939

Information). The band center for Pd (3d) is found to be shiftedto higher binding energies by about 0.2 eV when Pd is loadedonto TiN as compared to carbon support. Additionally, the bandsare found to be broad on Pd-TiN clearly demonstrating the driftof electrons from Pd to TiN. Similar positive shift in the Pd(3d) binding energy has been reported when Pd is loaded on totransition metals60,62 and they have been shown to exhibitremarkably low CO desorption temperature.58-62 The electrondrift for Pd to the support is reported to create d-orbital vacancy(expansion of d-orbitals) resulting in decreased back-donationto CO, thereby weakening the Pd-CO bonding considerably. Asimilar shift observed with Pd- TiN suggests a weak Pd-CObonding thereby increasing the efficiency of CO removal fromits surface.

During the Fermi-level equilibration between Pd and TiN,Pd possibly develops a slight positive charge and TiN a slightnegative charge. In sulfated Pt/Al2O3 systems, the presence ofactive sites consisting of adjacent cationic Pt and anionic sulfateis reported to enhance the C-H bond activation in thecombustion of propane.77 Similar reasoning can be drawn hereas well that may enhance the C-H bond activation duringethanol electrooxidation on Pd-TiN resulting in high activitytoward ethanol oxidation.

On the basis of the above observations, the following schemeis suggested for ethanol electrooxidation on the Pd-TiN electrodesurface (Scheme 1). Ethanol dehydrogenates mainly on Pd sitesresulting in the formation of chemisorbed CO on Pd (Pd-CO).The presence of TiN leads to the formation of Ti-OH-typefunctional groups at low overvoltages whereas CO is ac-cumulated on Pd sites. The Pd sites will be regenerated by thebifunctional action of OH groups present on the TiN surface.

An important observation related to the strong adherence ofPd on to TiN support is given in Figure 9. The Pd oxideformation/reduction and hydrogen adsorption/desorption currentsobserved in the voltammograms on Pd-TiN in alkaline mediumreveal that the catalyst is in tact on the surface even after 1500cycles while Pd leaches off the carbon support under identicalconditions.

4. Conclusions

The electrochemical oxidation of ethanol has been carriedout in alkaline media on Pd-TiN electrode. The efficientmetal-support interactions reveal the superior performance ofPd-TiN in terms of high currents at low overpotentials and highexchange current density values. In situ FTIR studies revealthe presence of acetaldehyde/acetate, CO, and CO2 as thereaction products. Experiments with bare TiN confirm that theinterfacial water bands observed on Pd-TiN stem from the TiNsurface. This may provide the necessary -OH groups forscavenging adsorbed CO intermediates formed on the Pd surfaceduring ethanol oxidation. XPS analysis of Pd loaded onto TiNreveals a positive chemical shift for Pd (3d) binding energiesas compared to that on Pd loaded onto carbon supportconfirming the weakening of Pd-CO bonding on Pd-TiN. Cyclicvoltammetric studies demonstrate that Pd particles are firmlyheld by the TiN support suggesting that TiN can provide anexcellent platform for Pd catalyst.

SCHEME 1: Scheme of Ethanol Oxidation on thePd-TiN Electrode Surface

Figure 8. Current-time (I-t) transients recorded on Pd-TiN and Pd-Cat -0.25 (A) and -0.5 V (B) in 1 M KOH containing 0.5 M ethanol.Pd loading is 1 mg of Pd/cm2. The currents are normalized with respectto the mass of the catalyst. Figure 9. Electrochemical cycling of Pd-C (A) and Pd-TiN (B) in 1

M KOH at 100 mV/s. Pd loading is 83 µg/cm2.

17940 J. Phys. Chem. C, Vol. 114, No. 41, 2010 Thotiyl et al.

Acknowledgment. The authors wish to thank the DST, Indiafor financial support.

Supporting Information Available: The SEM/EDAX oftitanium nitride, cyclic voltammogram for Pd deposition, SEMof Pd particles on TiN, AFM and Kelvin probe images of Pd/TiN, voltammograms of Pd-TiN in KOH, voltammogram of TiNin KOH and in the presence of ethanol, voltammograms of Pd-TiN before and after ethanol oxidation, RAIR spectra of freshand used TiN, Ti (2p) XPS spectra of Pd-TiN before and afteroxidation of ethanol, voltammograms of Pd-TiN and Pd-C forethanol oxidation, Tafel plots and XPS spectra of Pd-TiN andPd-C, and table of voltammteric characteristics of Pd-TiN andPd-C for ethanol oxidation. This material is available free ofcharge via the Internet at http://pubs.acs.org.

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