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J. Phys. Chem. C 2010, 114, 17934–17941
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 in designing an efficient anode material, Pd-TiN, that could be useful for direct ethanol fuel cell in alkaline media. The physicochemical and electrochemical characterization of the Pd-TiN/electrolyte interface reveals an 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) shows the production of CO2 at low overvoltages revealing an efficient cleaving of the C-C bond. The performance comparison of Pd supported on TiN (Pd-TiN) with that supported on carbon (Pd-C) clearly demonstrates the advantages of TiN support over carbon. A positive chemical shift of Pd (3d) binding energy confirms the existence of metal-support interaction between Pd and TiN, which in turn helps weaken the Pd-CO synergetic bonding interaction. The remarkable ability of TiN to accumulate -OH species on its surface coupled with the strong adhesion of Pd makes TiN an active support material for electrocatalysts. 1. Introduction Fuel cells have been projected as promising alternate energy devices for the fast depleting conventional energy sources.1-3 There have been several studies on the use of various small organic molecules as fuels in direct fuel cells. In this direction, low molecular weight alcohols such as methanol and ethanol have received considerable attention.4,5 Even though methanol is a promising fuel, the concern regarding its toxicity6 has projected ethanol as an excellent alternative material. Other advantages of ethanol include the ease of production from agricultural products/fermentation of biomass and the projected high energy density of ethanol-based fuel cells.7,8 The fuel crossover that is often encountered in methanol-based fuel cells is low in the case of ethanol fuel cells.9-11 However, there are many difficulties associated with ethanol oxidation and consequently in its use in the fuel cell mode.11-18 Complete oxidation of ethanol to CO2 involves 12 electrons and the process involves the scission of a C-C bond thus demanding high activation energies to be overcome.11 Many of the intermediates (mainly CO and -CHO)15 produced during the oxidation reaction poison the anode electrocatalyst and in turn reduce the catalytic efficiency. This demands the presence of another metal such as Ru, Ni, Sn, and Ir to alleviate the poisoning effect.13,16,17 The use of a second metal (M) has been reported to promote water decomposition at low potentials to form, for example, M-OH groups which in turn assists in the removal of CO and -CHO type species from the blocked catalyst surface thereby freeing the active surface, by a bifunctional-type mechanism.13,16,18 However, the alloy electrocatalysts achieve complete oxidation of ethanol to CO2 only at high anodic potentials making them less efficient for direct ethanol fuel cells (DEFCs). Though the poisoning effect is reduced, the combination of two precious metals makes the catalyst cost very expensive. Hence, there is need to develop catalysts with high activity, high poison tolerance, and, of course, low cost. * To whom correspondence should be addressed. E-mail: sampath@ ipc.iisc.ernet.in. Phone: + 91 80 22933315. Fax: + 91 80 23601552.
Pd is reported to be a very good catalyst for ethanol oxidation in alkaline medium and is 50 times more abundant than Pt.19,20 Very often, Pd is loaded on to carbon support and used in fuel cells. The activity of catalysts for fuel cell reactions depends on the judicious selection of supports as well. This aspect is well-documented in the literature.21-23 Conventional carbon supports are prone to undergo corrosion in aggressive electrolytes that are very often encountered in fuel cells.24-30 The corroded carbon support cannot hold the catalyst on its surface leading to aggregation or sintering of noble metal particles (reduces electrochemically active surface area) and often resulting in oxidation and subsequent leaching of the catalyst.24-30 Corrosion of the support/catalyst happens mainly because they are exposed to aggressive electrolytes, high temperature and pressure, and high humidity. Carbon is known to undergo corrosion even at open circuit voltages of the fuel cell. In the present paper, we report a highly efficient, remarkably stable Pd-supported titanium nitride (Pd-TiN) as a catalyst for efficient oxidation of ethanol in alkaline medium. Titanium nitride (TiN) is a very hard, conducting ceramic material often used as an abrasive coating for engineering components.31,32 In the electronic industry, it is widely used as a “barrier metal” because of its excellent diffusion barrier properties.33,34 It possesses metal-like electronic conductivity35 with a very reproducible surface for electron transfer. TiN is biocompatible as well. Though TiN has been known as an anticorrosive, barrier coating material for decades, the available literature on its electrochemical properties is scarce. TiN has been proposed as an electrode for supercapacitors;36 as a substrate for electrodeposition of metals such as Pt, Zn, Cu, and Ag;37-39 as a pH sensor;40 for the deactivation of marine bacteria;41 and in electroanalysis.42 In a recent communication, we have highlighted the use of platinized TiN for the electrochemical oxidation of methanol in acidic medium.43 In a related study, Avasarala and co-workers have proposed Pt-TiN as a good electrode material for electrochemical reduction of oxygen in sulfuric acid medium.44 The present paper explores the effect of TiN support for Pd catalyst by using a variety of techniques such as in situ IR
10.1021/jp1038514 2010 American Chemical Society Published on Web 09/24/2010
Efficient Oxidation of Ethanol in Alkaline Medium spectroelectrochemistry, XPS, atomic force microscopy, Kelvin probe microscopy, and electrochemistry. Further, the performance of Pd-TiN catalyst is compared with conventional Pd supported on carbon. The support material TiN helps in alleviating CO poisoning of Pd and promotes ethanol oxidation. 2. Experimental Section 2.1. Chemicals and Materials. All the reagents and chemicals used were of analytical grade. K2PdCl4 (Ranbaxy, India), ethanol, KOH (Qualigens, India), Pd black (Aldrich, USA), TiN powder (Aldrich, USA), and carbon powder (Vulcan XC 72) were used as received. Doubly distilled water was used for all experiments. Prior to the experiments, all glass apparatus was cleaned with chromic acid, washed with double distilled water, rinsed with acetone, and dried. 2.2. Preparation of TiN Coating. TiN was coated on thin stainless steel sheets (SS-304) by cathodic arc deposition technique. A 20 in. Multi-Arc chamber was used for the deposition. Before deposition, the coating plant was first evacuated to a base vacuum of 10-6 Torr. During deposition, reactive nitrogen gas pressure and substrate bias voltage were kept constant at 10 mTorr and -200 V. The deposition time was kept at 30 min and the thickness of the TiN coating was 3 µm. 2.3. Preparation of TiN Working Electrode. The working electrodes were made by cutting the SS coated TiN into small pieces and attaching a Cu wire to one end by spot-welding. The exposed Cu wire and stainless steel portions were insulated with epoxy resin. Prior to the electrochemical measurements, the TiN electrodes were degreased by wiping them with a tissue soaked in absolute ethanol and thoroughly washed with doubly distilled water. 2.4. Palladium Deposition on TiN. Chronopotentiometry was used for the deposition of Pd from a solution of 0.1 MK2PdCl4 with TiN as the working electrode, a large area Pt foil as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The amount of deposited Pd was controlled by varying the charge passed through the cell, at a current density of 4 mA/cm2. A similar procedure was earlier used for tuning loading of Pt on other substrates.45 The preparation of palladized TiN powder was carried out as follows. About 200 mg of TiN powder was mixed with an aqueous solution of PdCl2 to yield a certain weight percent of Pd on TiN. The solvent was allowed to evaporate overnight under constant stirring. The resulting dry powder with adsorbed PdCl2 (on TiN) was then treated at 350 °C in H2 stream for 2 h in a tubular furnace to effect reduction of palladium ions to Pd on TiN. In certain experiments, TiN and carbon supported electrocatalysts were prepared by the polyol method as reported earlier.44 It should be noted that even physical mixing of Pt powder and transition metal oxide was reported to yield good electroactivity for methanol oxidation.46 2.5. Characterization. The supported catalysts Pd-TiN and TiN were characterized by scanning electron microscopy (FEI 200 kV, Netherlands), X-ray diffraction (JEOL JDX 8030), X-ray photoelectron spectroscopy (XPS, Thermoscientific Multilab 2000 instrument), atomic force microscopy (Digital Nanoscope 4A, USA), potentiostat/galvanostat (EG&G PARC, 263A or CHI 660 A model), and spectroscopic techniques (Thermonicolet 6700 FTIR with liquid N2 cooled HgCdTe detector and Perkin-Elmer UV-vis spectrometers). Conductive AFM (CAFM, Digital Nanoscope 4A, USA) and scanning kelvin probe (SKPM, Digital Nanoscope 4A, USA) images were acquired with a Co-Cr probe. A sample bias of 1 V with respect to the tip was applied for current measurements.
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17935 2.6. In Situ FTIR Spectroelectrochemistry. In situ FTIR spectroscopy under applied potential conditions was carried out as follows. A three-electrode cell consisting of Pd-TiN foil, Pt wire, and a Hg/HgO/1 M KOH (MMO) as working, counter, and reference electrode, respectively, was used in 0.5 M ethanol in 1 M KOH. The potential of the working electrode is scanned between -0.7 and 0.25 V and then reversed to -0.7 V at a scan rate of 10 mV/s and the spectra were recorded by holding the potential at desired values. An incident IR beam at an angle of about 65° with spectral resolution of 4 cm-1 was used. The spectrum at -0.7 V was used as the reference throughout the experiment except for CO band measurements where the spectrum at 0.20 V (potential at which CO is completely oxidized) was used as the reference. The spectra are presented based on the percentage reflectivity according to the following relation:
∆R/R ) (RE - R-0.7V)/R-0.7V
(1)
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 the CO band are given based on
∆R/R ) (RE - R0.20V)/R0.20V
(2)
where R0.20V is the spectrum at 0.20 V. Accordingly positive and negative bands indicate depletion and accumulation of the corresponding species, respectively. 2.7. Electrochemical Oxidation of Ethanol with Pd-TiN. Electrochemical measurements were carried out in a threeelectrode cell in a solution of 0.5 M ethanol + 1 M KOH. Cyclic voltammograms were recorded in the potential range -0.7 to 0.25 V. All solutions were deaerated with highly pure Ar for 20 min prior to the experiment. The electrodes were kept in deaerated solution for 10 min for equilibration before the measurements. All the experiments were carried out at 25 ( 0.2 °C. 3. Results and Discussion 3.1. Characterization of TiN. The TiN film deposited on SS surface is smooth, very adherent, and reflecting in nature. The SEM of bare TiN along with the EDS pattern (Supporting Information, Figure S1) shows that the film is continuous and does 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 and the lattice parameter is determined to be 0.4244 nm and is in good agreement with the literature value (0.4241 nm; JCPDF 38-1420). Surface characterization with XPS of TiN film reveals three components 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 range 460-464 eV possesses two components, one at 460.85 and another at 463.69 eV attributed to TiN and TiO2, respectively. This pattern is typical of TiN films prepared by the cathodic arc deposition technique.40,41 The N(1S) spectrum shows components corresponding to nitridic nitrogen (397.2 eV) and oxynitride (398.5 eV) phases.47,48 The presence of a low-energy component is reported to be due to atomic nitrogen-like species.49 On the basis of the XPS characterization, it is concluded that there is a thin oxygen enriched layer on the surface of TiN coexisting with the TiN phase.
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Figure 1. X-ray diffraction pattern of titanium nitride thin film coated on SS-304. The corresponding planes are marked in the figure. The reflections marked with asterisks are from the stainless steel substrate.
Figure 3. Cyclic voltammograms of Pd-TiN electrode in the presence of 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. The area of TiN used is 0.25 cm2 (A). Plot of peak current and peak potential vs cycle number (B).
Figure 2. X-ray photoelectron spectra of titanium nitride thin film coated on SS-304: (A) Ti(2p) and (B) N(1S) regions. The open circles and solid red line represent the original and fitted data, respectively. Green dotted lines represent the deconvoluted bands.
3.2. Characterization of Pd-TiN. Electrodeposition of Pd on TiN is carried out from a solution of 1 mM K2PdCl4 in 0.1 M H2SO4 (Supporting Information, Figure S2). The bare TiN shows a featureless volatmmogram in the potential range, 0 to 1 V in H2SO4 medium. Addition of 1 mM K2PdCl4 to the electrolyte reveals well-defined redox peaks at 0.330 and a 0.680 V vs SCE. The tail observed at the positive potential limit in Figure S2 (Supporting Information) is likely to be due to slow oxidation of TiN. Varying amounts of Pd are deposited on TiN surface by controlling the charge passed through the cell at a current density of 4 mA/cm2 for different periods of time. The as-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 particles have 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 TiN domains (Supporting Information, Figures S5 and S6). An important observation is that the interface of Pd and TiN is found to have a different work function as compared to the individual components, confirming the existence of metal-support interaction. Electrochemical characterization of Pd-TiN (Pd loading of 4 µg/cm2) in 1 M KOH clearly shows hydrogen adsorptiondesorption peaks in addition to Pd oxide formation and reduction (Supporting Information, Figure S7). Pd oxide formation is observed around 0.270 V and the corresponding reduction peak is observed around -0.250 V. The H2 adsorption and desorption features are observed in the range -0.5 to -0.8 V as reported for Pd in alkaline media.8 3.3. Ethanol Oxidation. Bare TiN is inactive for ethanol oxidation in alkaline medium (Supporting Information, Figure S8). The voltammogram carried out on Pd-TiN in 1 M KOH containing 0.5 M ethanol shows a forward peak due to ethanol oxidation in the positive scan (Figure 3) and the negative scan shows a peak in the same direction. The oxidation currents observed in the reverse direction are due to the oxidation of adsorbed intermediates as reported for Pd-based systems earlier.20,50 However, the observations on Pd-TiN differ from the reported literature in certain aspects. First, ethanol oxidation currents grow as a function of number of cycles with a negative shift in peak potentials. The oxidation currents increase up to 15th cycle by ∼8 times and thereafter remain constant. This observation points out that poisoning of Pd electrocatalyst which is often encountered during ethanol oxidation is minimal on
Efficient Oxidation of Ethanol in Alkaline Medium the Pd-TiN electrode. Palladium nanowires that have recently been reported to be very active for ethanol oxidation also show a decreasing trend in the oxidation currents as a function of the number of cycles.51 The second difference observed with PdTiN is related to the shift of oxidation peak potentials from 0.040 V for the first cycle to -0.090 V for the 10th cycle before staying constant at this value. The increase in currents may be argued to be due to repetitive cycling of Pd surface in the oxide-formation region leading to changes in surface area/activity of the electrode. However, Pd deposited on glassy carbon electrode using the same procedure does not result in an increase in currents when cycled in the same potential range. Additionally, the Pd-TiN electrode shows exactly the same reversible redox behavior for ferrocyanide/ ferricyanide couple (in 1 M KOH) with no change in peak currents, before and after ethanol oxidation in alkaline medium (Supporting Information, Figure S9). This suggests that the increase in currents observed for ethanol oxidation is not due to any reorganization of Pd domains on TiN surface during potential cycling. The diffused reflectance infrared spectra of the TiN surface before and after cycling in the same range in KOH solution shows an increase in -OH stretching intensity supporting the formation of Ti-OH-type functional groups on the TiN surface (Supporting Information, Figure S10). This is similar to the growth of Ru-OH functional groups when Pt-Ru catalyst is used for oxidation of alcohols. Third, a control experiment where Pd-TiN is conditioned at a positive potential of 0.25 V in KOH alone does not result in any change as far as the effect of subsequent cycling in ethanolic KOH solution is concerned. These observations point to the fact that the activation of the Pd-TiN surface yielding high currents probably happens in situ during the oxidation of ethanol. The optimum loading of Pd is found to be 3-10 µg/cm2 of TiN. To understand the role of the TiN surface, XPS has been carried out before and after ethanol oxidation. The Ti (2p) spectra observed before and after the electrochemical oxidation of ethanol are shown in Figure S11 (Supporting Information). The deconvoluted spectrum shows contributions from TiON and TiO2 in addition to pure TiN phase before oxidation. After electrochemical oxidation of ethanol, it is observed that the contribution of oxynitride (456.56 eV) has increased relative to that of th pure TiN phase. Avasarala and co-workers have hypothesized that the enhanced activity of TiN-based catalysts toward oxygen reduction in acid medium could be due to the presence of oxynitride phase.44 We intentionally oxidized TiN before Pd deposition and used it for ethanol oxidation. The efficiency and the poison tolerance of the catalyst is found to be improved as compared to that of Pd deposited on as-prepared TiN surface (results not shown). Therefore, the growth of oxynitride species during cycling may possibly play a role in the enhanced currents and negative shift in peak potentials observed in the present study. The oxidation state of Ti is 3+ in TiN and it is known that Ti3+ can be oxidized to TiO2+ in aqueous medium.52 The voltammogram of TiN in KOH shows a sharp peak around 1.5 V due to the oxidation of the surface. However, it is very likely that oxidation of TiN occurs at low voltages to a small extent. The TiO2+ species may aid the oxidation of ethanol by a mediator-type mechanism thereby simultaneously getting reduced to TiN though it is only speculative at present. The formation of Ti-OH-type groups (as explained later in the section on in situ IR spectroelectrochemistry) improves the efficiency further by removing the intermediate species formed during the oxidation of ethanol. Other possible reasons could be related
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Figure 4. In situ FTIR spectra in the range 2000 to 4000 cm-1 for the electrooxidation of ethanol on the Pd-TiN electrode in the first cycle. The electrolyte is 0.5 M ethanol in 1 M KOH. The reference spectrum is obtained at -0.7 V. The spectra are obtained at a potential interval of 0.1 V.
to the effect of metal-support interactions observed in other catalytic reactions.53,54 It is already reported that the CO desorption temperature decreases when palladium is loaded onto a support as compared to bare palladium.55-62 Similar metalsupport interactions may play a role in the present studies as well. The electronic nature of the Pd-TiN interface is different from that of either Pd or TiN as revealed by the work function measurements described earlier. The metal-support interactions possibly introduce electron deficiency on Pd, which in turn may decrease the Pd-CO synergetic bonding interactions remarkably. 3.4. In Situ Spectroelectrochemistry. In situ IR spectroscopy has been carried out in the reflectance mode as described in the Experimental Section. Various regions of interest as a function 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 of adsorbed water.63,64 The intensity of this band grows (accumulation of the species) with applied potential and it would help in the formation of -OH-type functional groups useful for CO alleviation. The presence of adsorbed water on Pd-TiN is further confirmed based on growth in intensity of the band at 1600 cm-1, which is due to the bending mode of adsorbed water (Figure 6B). The band positions for adsorbed water molecules are different from those of bulk water in the three-dimensional hydrogen-bonded state, but are rather similar to weakly hydrogenbonded water species.60,61 Similar water bands have been reported on PtRu systems and not for pure Pt demonstrating that water is probably getting adsorbed on Ru under the electrochemical conditions used.63,64 The ability of Ru to adsorb water is responsible for the well-known bifunctional mechanism proposed for the oxidation of alcohols on Pt-Ru catalysts.63,64 Ethanol dehydrogenation on Pt sites poisons the surface with adsorbed CO (Pt-CO). Simultaneously, -OH groups formed on the Ru surface at low overpotentials help in the alleviation of CO poisoning. It is reported that Pd-based electrocatalysts decorated with oxides such as CeO2, In2O3, NiO, MnO2, Co3O4, TiO2, and carbon nanotubes65,66 have shown improved activity toward ethanol oxidation in alkaline media over pure Pd. The improved activity of Pd-metal oxide catalysts has been proposed to be due to a similar bifunctional-type mechanism. On the Pd-TiN surface, the H2O/OH band intensities grow as a function of potential and are very vital for the removal of chemisorbed CO on the catalyst surface. Additionally, it is
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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 is 0.5 M ethanol in 1 M KOH. The reference spectrum is obtained at -0.7 V.
reported that adsorption of water inhibits CO chemisorption.67 It is noteworthy that both molecular and dissociated forms of adsorbed water have been reported on surfaces such as titanium carbide (TiC) and vanadium carbide (VC)68 with use of high resolution electron energy loss spectroscopy and XPS studies. The ability of TiN to accumulate interfacial -OH-type functional groups has been probed with use of a bare TiN electrode in 1 M KOH containing 0.5 M ethanol. The presence of adsorbed water is easily detected by the accumulation of species as revealed by growing intensities at 1600 and 3600 cm-1 bands (Figure 5) as a function of applied potential. It is very important to mention that the adsorbed -OH functional groups are observed at remarkably low overvoltages. This may have additional consequences in water electrolysis on TiN electrodes at low overvoltages. An intense CO2 band (O-C-O asymmetric stretch) is observed at 2345 cm-1 (Figure 4) even in low potential ranges suggesting direct oxidation of ethanol.63,64,69,70 This is in good agreement with the widely accepted parallel pathway for ethanol oxidation. According to the parallel pathway mechanism, ethanol electrooxidation proceeds simultaneously through an acetaldehyde/acetic acid (AA pathway) pathway where the C-C bond does not break and a CO pathway where the C-C bond is cleaved.69-71 The degree of domination of one pathway over the other is potential specific. Accordingly, at low overpotentials the AA pathway will be dominant and at high potentials the CO pathway dominates. This happens mainly because the formation of CO requires the cleavage of the C-C bond and it happens only at high overpotentials on most of the electrocatalysts. The observation of CO2 band with considerable intensities at low overvolatges on Pd-TiN suggests that the CO pathway is very dominant indicating its high efficiency toward ethanol oxidation.
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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 (reference spectrum at -0.7 V) for the electrooxidation of ethanol at the Pd-TiN electrode in the first cycle. The electrolyte is 0.5 M ethanol in 1 M KOH.
The presence of adsorbed CO (2000-2100 cm-1) as a function of potential is presented in Figure 6A. The band position is in good agreement with that observed on noble metal electrodes.71,72 It is found to shift to higher wave numbers with anodic increase in potential and this observation is accounted for, based on the Stark effect. The CO band intensity is found to be maximum around -0.4 V and thereafter a decrease is observed. Hence, the observed variation in CO2 signal intensity can be explained as follows. At low overpotentials adsorbed CO is mainly oxidized by the OH groups available on the TiN surface, while at higher anodic potentials it is also oxidized by the direct electrochemical route as well. The bands around 1725 cm-1 (Figure 6B) could be due to carbonyl stretching in acetaldehyde or acetate available at the interface.72 However, the presence of acetaldehyde can be ambiguously 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 of acetate is confirmed by the appearance of the band at 1435 cm-1.69 Since there is an intrinsic difficulty in detecting acetaldehyde due to its low absorptivity, it is assumed that both acetaldehyde and acetate are formed in the present studies.73 Strong absorption of the acetate band at 1435 cm-1 makes the observation of the carbonate band (at 1450 cm-1) difficult, as reported in the literature.69,71 The spectra in the range 800-2000 cm-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 have been carried out to understand the products formed during the oxidation process, using UV-visible absorption spectroscopy on the electrolyte solution before and after cycling a large area
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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 Pd C
-572 -465
-76 4
59.2 21.5
20.6 13.23
2.87 1.63
0.51 0.42
100 180
a
Peak potential. b Mass activity. c Electrochemically active area. d Current density normalized for electrochemically active area.
Pd-TiN electrode for 200 cycles. The use of a large area and number of cycles is to ensure that enough products are formed in the bulk of the medium. Control experiments have been carried out for possible products of ethanol oxidation in alkaline medium such as acetaldehyde, acetic acid, and K2CO3. Two shoulders, a large one around 275 nm and a small one around 320 nm, are observed in addition to unreacted ethanol (Supporting Information, Figure S12) for the electrolyte used for oxidation of ethanol with Pd-TiN. The control UV-vis spectra (Figure S12, Supporting Information) reveals that one of the possible products is acetaldehyde. Accumulation of acetate and K2CO3 may also result in the absorbance increasing around this region. Hence, the ex situ studies provide support for the presence of acetaldehyde as observed in the in situ characterization given earlier. 3.6. TiN vs Carbon Support. A comparative study has been carried out to decipher the effect of TiN as an active support material as opposed to state of the art carbon support (Vulcan carbon XC-72) that is used for anchoring Pd catalyst. The same amount of palladium is loaded on to TiN and Vulcan carbon with the polyol method (see Experimental Section) and subsequently used for ethanol oxidation. The mass activity (A/mg) of the electrocatalysts are shown in Figure S13 (Supporting Information). The mass activity (A/mg) on TiN support is higher than that observed on carbon support. The onset potential and the peak potential are found to be negatively shifted on TiN supported catalyst. Table 1 gives the mass activity along with the electrochemically active area and current density of the catalysts used in the present studies. The electrochemically active area has been found out by integrating the charge associated with PdO stripping and the details are given in the last section of the Supporting Information. The inability of Pd to oxidize ethanol to CO2 (due to scission of the C-C bond) is well documented in the literature and the oxidation stops at the generation of CH3CHO and CH3COOH.66,68,70 Therefore, the observed high activity could be due to an efficient cleavage of the C-C bond when Pd is loaded on to TiN substrate. Similar metal-support interactions modifying the electronic structure of the interface are well-documented in the literature.52-59 A good fuel cell catalyst should have high current densities at low overpotentials. The cyclic voltammograms clearly reveal that Pd-TiN exhibits higher currents than that observed on PdC, at low overpotentials. This is further confirmed from the I vs t transients recorded at overpotential close to the onset value (Figure 7). The I vs t transients recorded at -0.5 V show that the currents on Pd-TiN are almost 10 times higher than that observed on Pd-C demonstrating the beneficiary effect of loading Pd on TiN. Enhanced currents are observed at other potentials as well. The observations are similar and the currents are very high when mass activity is used instead of EAA normalized activity (Figure 8). This observation demonstrates that the rate of the electron transfer and poison tolerance of the Pd-TiN electrocatalyst is higher than that of Pd-C. The ratio of the forward peak current (Ipf) to the reverse peak current (Ipb) is often taken to be a benchmark of the performance of a catalyst. However, this ratio depends on the reversal voltage and hence any comparison should be made under identical
Figure 7. Current-time (I-t) transients recorded on Pd-TiN and Pd-C at -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 by normalizing it with the electrochemically active area. Note that the potential of -0.5 V is close to the onset value while -0.25 V is in the rising portion of the voltammogram.
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 catalyst is the potential separation (∆E) between the peaks in the forward scan (Epf) and that of the reverse scan (Epb). The value observed for Pd-TiN is 100 mV and that for Pd-C is 180 mV at the same scan rate and for the same loading of the catalyst, indicating the efficient and rapid removal of accumulated intermediates on the Pd-TiN surface. Steady state measurements to determine the Tafel slopes are given in Figure S14 (Supporting Information). The Tafel slopes are found to be 146 and 251 mV/dec on Pd-TiN and Pd-C, respectively. The value of teh Tafel slope observed on Pd-C is in good agreement with the value reported in the literature.74,75 A low Tafel slope on Pd-TiN suggests an efficient 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 observations clearly manifest that TiN is an active support unlike the conventional carbon support. The metal-support interactions between Pd and TiN are further investigated by XPS. The Pd (3d) spectra observed on Pd-TiN and Pd-C are shown in Figure S15 (Supporting
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Figure 8. Current-time (I-t) transients recorded on Pd-TiN and Pd-C at -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 respect to the mass of the catalyst.
Information). The band center for Pd (3d) is found to be shifted to higher binding energies by about 0.2 eV when Pd is loaded onto TiN as compared to carbon support. Additionally, the bands are found to be broad on Pd-TiN clearly demonstrating the drift of electrons from Pd to TiN. Similar positive shift in the Pd (3d) binding energy has been reported when Pd is loaded on to transition metals60,62 and they have been shown to exhibit remarkably low CO desorption temperature.58-62 The electron drift for Pd to the support is reported to create d-orbital vacancy (expansion of d-orbitals) resulting in decreased back-donation to CO, thereby weakening the Pd-CO bonding considerably. A similar shift observed with Pd- TiN suggests a weak Pd-CO bonding thereby increasing the efficiency of CO removal from its surface. During the Fermi-level equilibration between Pd and TiN, Pd possibly develops a slight positive charge and TiN a slight negative charge. In sulfated Pt/Al2O3 systems, the presence of active sites consisting of adjacent cationic Pt and anionic sulfate is reported to enhance the C-H bond activation in the combustion of propane.77 Similar reasoning can be drawn here as well that may enhance the C-H bond activation during ethanol electrooxidation on Pd-TiN resulting in high activity toward ethanol oxidation. SCHEME 1: Scheme of Ethanol Oxidation on the Pd-TiN Electrode Surface
Thotiyl et al.
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.
On the basis of the above observations, the following scheme is suggested for ethanol electrooxidation on the Pd-TiN electrode surface (Scheme 1). Ethanol dehydrogenates mainly on Pd sites resulting in the formation of chemisorbed CO on Pd (Pd-CO). The presence of TiN leads to the formation of Ti-OH-type functional groups at low overvoltages whereas CO is accumulated on Pd sites. The Pd sites will be regenerated by the bifunctional action of OH groups present on the TiN surface. An important observation related to the strong adherence of Pd on to TiN support is given in Figure 9. The Pd oxide formation/reduction and hydrogen adsorption/desorption currents observed in the voltammograms on Pd-TiN in alkaline medium reveal that the catalyst is in tact on the surface even after 1500 cycles while Pd leaches off the carbon support under identical conditions. 4. Conclusions The electrochemical oxidation of ethanol has been carried out in alkaline media on Pd-TiN electrode. The efficient metal-support interactions reveal the superior performance of Pd-TiN in terms of high currents at low overpotentials and high exchange current density values. In situ FTIR studies reveal the presence of acetaldehyde/acetate, CO, and CO2 as the reaction products. Experiments with bare TiN confirm that the interfacial water bands observed on Pd-TiN stem from the TiN surface. This may provide the necessary -OH groups for scavenging adsorbed CO intermediates formed on the Pd surface during ethanol oxidation. XPS analysis of Pd loaded onto TiN reveals a positive chemical shift for Pd (3d) binding energies as compared to that on Pd loaded onto carbon support confirming the weakening of Pd-CO bonding on Pd-TiN. Cyclic voltammetric studies demonstrate that Pd particles are firmly held by the TiN support suggesting that TiN can provide an excellent platform for Pd catalyst.
Efficient Oxidation of Ethanol in Alkaline Medium Acknowledgment. The authors wish to thank the DST, India for financial support. Supporting Information Available: The SEM/EDAX of titanium nitride, cyclic voltammogram for Pd deposition, SEM of Pd particles on TiN, AFM and Kelvin probe images of Pd/ TiN, voltammograms of Pd-TiN in KOH, voltammogram of TiN in KOH and in the presence of ethanol, voltammograms of PdTiN before and after ethanol oxidation, RAIR spectra of fresh and used TiN, Ti (2p) XPS spectra of Pd-TiN before and after oxidation of ethanol, voltammograms of Pd-TiN and Pd-C for ethanol oxidation, Tafel plots and XPS spectra of Pd-TiN and Pd-C, and table of voltammteric characteristics of Pd-TiN and Pd-C for ethanol oxidation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kordesch, K. V.; Simader, G. R. Chem. ReV. 1995, 95, 191. (2) Diat, O.; Gebel, G. Nat. Mater. 2008, 7, 13. (3) Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.; Kobayashi, T. Angew. Chem., Int. Ed. 2007, 46, 8024. (4) Service, R. F. Science 2002, 296, 1222. (5) Kleiner, K. Nature 2006, 441, 1046. (6) Spinace, E. V.; Neto, A. O.; Linardi, M. J. Power Sources 2003, 124, 426. (7) Zhang, D. Y.; Ma, Z. F.; Wang, G. X.; Konstantinov, K.; Yuan, X. X.; Liu, H. K. Electrochem. Solid-State Lett. 2006, 9, A423. (8) Xu, C. W.; Wang, H.; Shen, P. K.; Jiang, S. P. AdV. Mater. 2007, 19, 4256. (9) Wang, C. Y. Chem. ReV. 2004, 104, 4727. (10) Paik, Y.; Kim, S. S.; Han, O. H. Angew. Chem., Int. Ed. 2008, 47, 94. (11) Song, S. Q.; Zhou, W. J.; Liang, Z. X.; Cai, R.; Sun, G. Q.; Xin, Q.; Stergiopoulos, V.; Tsiakaras, P. Appl. Catal., B 2005, 55, 65. (12) Xu, C. W.; Hu, Y. H.; Rong, J. H.; Jiang, S. P.; Liu, Y. L. Electrochem. Commun. 2007, 9, 2009. (13) Cao, L.; Sun, G. Q.; Li, H. Q.; Xin, Q. Electrochem. Commun. 2007, 9, 2541. (14) Mann, J.; Yao, N.; Bocarsly, A. B. Langmuir 2006, 22, 10432. (15) Vigier, F.; Coutanceau, C.; Hahn, F.; Belgsir, E. M.; Lamy, C. J. Electroanal. Chem. 2004, 563, 81. (16) Park, K. W.; Choi, J. H.; Kwon, B. K.; Lee, S. A.; Sung, Y. E.; Ha, H. Y.; Hong, S. A.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869. (17) Liu, F.; Lee, J. Y.; Zhou, W. J. J. Phys. Chem. B 2004, 108, 17959. (18) Russell, A. E.; Rose, A. Chem. ReV. 2004, 104, 4613. (19) Tsivadze, A. Y.; Tarasevich, M. R.; Andreev, V. N.; Bogdanovskaya, V. A. Russ. J. Gen. Chem. 2007, 77, 783. (20) Xu, C. W.; Cheng, L. Q.; Shen, P. K.; Liu, Y. L. Electrochem. Commun. 2007, 9, 997. (21) Wang, J. S.; Xi, J. Y.; Bai, Y. X.; Shen, Y.; Sun, J.; Chen, L. Q.; Zhu, W. T.; Qiu, X. P. J. Power Sources 2007, 164, 555. (22) Hyeon, T.; Han, S.; Sung, Y. E.; Park, K. W.; Kim, Y. W. Angew. Chem., Int. Ed. 2003, 42, 4352. (23) Yang, R. Z.; Qiu, X. P.; Zhang, H. R.; Li, J. Q.; Zhu, W. T.; Wang, Z. X.; Huang, X. J.; Chen, L. Q. Carbon 2005, 43, 11. (24) Korovin, N. V. Electrochim. Acta 1994, 39, 1503. (25) Taniguchi, A.; Akita, T.; Yasuda, K.; Miyazaki, Y. J. Power Sources 2004, 130, 42. (26) Reiser, C. A.; Bregoli, L.; Patterson, T. W.; Yi, J. S.; Yang, J. D.; Perry, M. L.; Jarvi, T. D. Electrochem. Solid-State Lett. 2005, 8, A273. (27) Knights, S. D.; Colbow, K. M.; St-Pierre, J.; Wilkinson, D. P. J. Power Sources 2004, 127, 127. (28) Antolini, E. J. Mater. Sci. 2003, 38, 2995. (29) Shao, Y.; Wang, J.; Kou, R.; Engelhard, M.; Liu, J.; Wang, Y.; Lin, Y. Electrochim. Acta 2009, 54, 3109. (30) Shao, Y.; Yin, G.; Gao, Y. J. Power Sources 2007, 171, 558. (31) Schintlmeister, W.; Pacher, O.; Pfaffinger, K.; Raine, T. J. Electrochem. Soc. 1976, 123, 924. (32) Cho, J. S.; Nam, S. W.; Chun, J. S. J. Mater. Sci. 1982, 17, 2495. (33) Ting, C. Y. Thin Solid Films 1984, 119, 11. (34) Wittmer, M.; Studer, B.; Melchior, H. J. Appl. Phys. 1981, 52, 5722. (35) Rostlund, T.; Thomsen, P.; Bjursten, L. M.; Ericson, L. E. J. Biomed. Mater. Res. 1990, 24, 847.
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