Excellent Durability of Substoichiometric Titanium Oxide As a Catalyst

Article pubs.acs.org/IECR

Excellent Durability of Substoichiometric Titanium Oxide As a Catalyst Support for Pd in Alkaline Direct Ethanol Fuel Cells Son Truong Nguyen, Jong-Min Lee, Yanhui Yang, and Xin Wang* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459 ABSTRACT: In this study, substoichiometric titanium oxide, TinO2n−1, was compared with carbon black and TiO2 in the role of catalyst supports for Pd in alkaline direct ethanol fuel cell. 10%Pd/C, 10% Pd/commercial TiO2 and 10%Pd/TinO2n−1 catalysts were successfully prepared by a polyol method. The supports and electrocatalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) techniques. Excellent durability was found for TinO2n−1 with holding potential and multiscan tests in N2-purged 1 M KOH solution. Among the three catalysts, Pd/TinO2n−1 showed the best activity and stability for ethanol electrooxidation in alkaline media.

1. INTRODUCTION The depletion of fossil fuels has forced the world to find alternative energy sources and effective ways for energy conversion. Fuel cell technology has been shown as an attractive solution for producing electricity from chemical energy with high efficiency. Among various types of fuel cells, alkaline direct ethanol fuel cell (ADEFC) has recently attracted more and more attention due to the nontoxicity and high energy density of ethanol, the abundance of agricultural products for ethanol production, and the improved kinetics of electrode reactions in alkaline media.1,2 As far as catalyst utilization in low temperature fuel cell is concerned, support material is normally used for the dispersion of catalyst particles to increase the interfacial surface area. Possessing a high surface area and high electronic conductivity, Vulcan XC-72 carbon black has been widely used as the support for electrocatalysts in proton exchange membrane fuel cells (PEMFCs). However, it has been found that carbon black is oxidized during the operation of PEMFCs. The corrosion of the support will cause the detachment of metal particles from the support and facilitate their aggregation. This will lead to a significant loss of electrochemical surface area of the catalysts, which has a negative effect on the performance and working efficiency of the fuel cells.3−11 Therefore, it is necessary to find new catalyst supports that have good corrosion resistance under the working environment of PEMFCs. Titanium-based oxides with good mechanical resistance and high stability have attracted researchers’ attention for their application as catalyst supports for low temperature fuel cell. TiO2 was combined with carbon black as catalyst supports for oxygen reduction reaction (ORR) in acidic media.12,13 It was found that the addition of TiO2 helped to enhance the ORR activity of the catalysts. TiO2 was also found to improve the stability of cathodic catalysts in PEMFCs.14,15 The combination of TiO2 with carbon nanotube (CNT) and Pt showed higher ethanol oxidation activity and better CO tolerance compared to Pt/C and Pt/CNT in acidic media.16 Apart from TiO2, Magnéli-phase titanium oxides TinO2n−1 (4 ≤ n ≤ 9) with good electrical conductivity and high oxidation-resistivity have been © 2012 American Chemical Society

explored as electrode materials and catalyst supports in electrochemical systems. A commercial product of TinO2n−1, known as Ebonex, has been found to be electrochemically stable in acidic and basic solutions.17−21 PtCo/Ebonex showed better catalytic efficiency for oxygen evolution than Pt/Ebonex and unsupported PtCo. The electronic interaction between the metals and the support was assumed to play a big role in the activity enhancement.22 Vracar et al. investigated the activity of Ebonex-supported Pt toward oxygen reduction reaction (ORR) in 0.5 M HClO4 solution. A significant improvement in the kinetics of ORR on the Pt/Ebonex was observed, compared with that on polycrystalline Pt.23 Ti4O7, a member of substoichiometric titanium oxides possessing a high electrical conductivity,19−21,24 has been examined as an effective support for electrocatalysts in PEMFCs.4,25 Ioroi et al. found that Ti4O7-supported Pt catalysts had similar catalytic activities for ORR and hydrogen oxidation but higher stability in comparison with those of carbon-supported Pt catalysts in acidic condition. As mentioned above, ADEFC is a promising type of PEMFCs. However, to our best knowledge, there have been few studies about using TinO2n−1 and TiO2 as catalyst supports for ADEFC. Therefore, in this research, we carried out an investigation for the utilization of TiO2 and TinO2n−1 as supports for palladium for ethanol oxidation reaction and compared them with the convenient support, Vulcan XC-72 carbon black, in alkaline media.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial anatase TiO2 was purchased from Alfa Aesar. Carbon black Vulcan XC-72 was bought from Gashub (Singapore). Ethylene glycol (EG), Pd(NO3)2·xH2O, and KOH were purchased from Sigma-Aldrich. All the chemicals were used without further purification. Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 9966

November 21, 2011 January 27, 2012 February 1, 2012 February 1, 2012 dx.doi.org/10.1021/ie202696z | Ind. Eng. Chem. Res. 2012, 51, 9966−9972

Industrial & Engineering Chemistry Research

Article

2.2. Synthesis of Catalysts. Briefly, TinO2n−1 was synthesized by heating the commercial anatase TiO2 with a hydrogen flow at 1050 °C for 6 h.4,26 The catalysts were prepared by a polyol reducing method: 100 mg of a support (carbon black, commercial TiO2, TinO2n−1) was added into 40 mL of ethylene glycol (EG)−deionized (DI) water (1/1, v/v) solution and dispersed by sonication and stirring. Pd(NO3)2·xH2O (25.5 mg) was mixed with 10 mL of DI water by sonication and stirring. Then, the second solution was added dropwise into the first one, and the mixture was stirred for 15 min. KOH (50 mL, 1 M) in EG-water (1/1, v/v) was added into the mixture, and the solution was stirred and refluxed at 140 °C for 3 h. After reflux, the mixture was filtered or centrifuged, and the solid was washed several times with DI water. Finally, it was dried at 60 °C overnight. 2.3. Sample Preparation for Conductivity Measurement. Solid powder was dried at 105 °C for overnight and then was pressed into pellets (diameter of 6.1 mm) using a Carver hydraulic press (Model 4128) with a pressure of 457 MPa. All the pellets were dried again at 105 °C overnight before conductivity measurement. 2.4. Characterization. X-ray diffraction (XRD) measurement was carried out with a D8 Bruker AXS X-ray diffractometer (Cu Kα radiation, 40 kV, 20 mA, 2θ range of 20−90°, scan rate 0.025°/s). Electrical conductivity measurement was carried out at room temperature by a two-Pt probe DC technique with an Autolab PGSTAT302 potentiostat (Eco Chemie). To eliminate the effect of contact resistance between the samples and the probes, the conductivity of each material was calculated from the measurement of samples with different thickness. The electrical resistance of the samples was determined from the slopes of linear voltage vs current curves. N2 adsorption/desorption isotherms were measured at −196 °C with an Autosorb 6B (Quanta Chrome). Prior to the measurement, the samples were degassed at 250 °C overnight. The specific surface area was calculated by the Brunauer− Emmett−Teller (BET) method. Pore size was calculated by the Barrett−Joyner−Halenda (BJH) method. Electrochemical investigation was performed with the Autolab PGSTAT302 potentiostat using a three-electrode cell with a Pt wire and a Hg/HgO electrode as the counter and reference electrode, respectively. To prepare the working electrode for electrochemical tests, 3.3 mg of each catalyst was mixed with 1 mL of ethanol. From this mixture, 10 μL was dropped onto a glassy carbon electrode (GCE), and the GCE was dried in air. The catalyst layer was fixed to the electrode by 5 μL of 0.5% Nafion solution. N2 gas was bubbled into the test solutions for air removal. All current values in electrochemical results were normalized by the electrochemical active surface area (ECSA) of the catalysts determined by the PdO reduction peak of the blank cyclic voltammetry (CV) test in 1 M KOH solution with the assumption that a value of 405 mC/cm2 is needed for the reduction of PdO monolayer.1 2.5. Durability Tests. Durability tests were carried out with a PAR VMP2Multichannel Potentiostat (Princeton Applied Research) using two different methods. (1) Potentiostatic measurement: The catalysts were kept at 0.3 and 0.7 V in N2 purged 1 M KOH solution for 1 h. After each test, the electrochemical active surface area (ECSA) values of the catalysts were determined by CV technique. (2) Multiple CV sweeps: In this test, 500 and 1000 CV scans in N2 purged 1 M

KOH solution were performed for each catalyst with a potential range from −0.8 V to +0.3 V and a scan rate of 50 mV s−1.

3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns of the commercial TiO2 and the partially reduced product of the commercial TiO2 at 1050 °C,

Figure 1. XRD profiles of commercial TiO2, TinO2n−1 and standard XRD spectra of anatase TiO2 and different Magnéli phase titanium oxides.

as well as standard XRD spectra of anatase TiO2 and different Magnéli phase titanium oxides (JCPDS 21-1272, 50-0787, 510641, 50-0788, 50-0789, 50-0790, and 50-0791 for anatase TiO2, Ti4O7, Ti5O9, Ti6O11, Ti7O13, Ti8O15, and Ti9O17, respectively). The XRD profile of the commercial TiO2 displays typical peaks of anatase structure as reported in the literature27,28 and displayed in the standard XRD spectrum of anatase TiO2. These peaks are absent in the XRD spectrum of TinO2n−1, indicating that the anatase structure of TiO2 was completely converted into the structure of TinO2n−1, which is validated by its black−blue color appearance, in contrast to the white color of the commercial TiO2. A weight loss of ca. 7% was also observed for TinO2n−1 compared to TiO2. These phenomena are consistent with what was obtained in previous studies.4,19,20,25,29 Compared with the standard XRD patterns of substoichiometric titanium oxides, the product mainly consists of Ti9O17, Ti8O15, Ti7O13, and Ti6O11. These compounds were formed from the reaction of TiO2 and H2 at high temperature: nTiO2 + H2 → TinO2n−1 + H2O.21 Figure 2 presents XRD results of 10%Pd/commercial TiO2, 10%Pd/ Ti nO2n−1 and 10%Pd/C in comparison with those of commercial TiO2 and TinO2n−1. The XRD profile of 10%Pd/ C displays typical peaks of face-centered cubic (FCC) structure of Pd, including (111), (200), (220), and (311) lattices.1 The XRD spectra of 10%Pd/commercial TiO2 and 10%Pd/TinO2n−1 show two small peaks at 40° and 46°, which belong to (111) and (200) lattice of Pd structure, respectively. This indicates the presence of Pd nanoparticles on the commercial anatase TiO2 and TinO2n−1 support. Other FCC peaks of Pd are overlapped by the peaks of TiO2 and TinO2n−1. The support materials were compressed into small pellets and the electrical conductivity was measured by a DC method. The results shown in Table 1 indicate that TinO2n−1 has a 9967

dx.doi.org/10.1021/ie202696z | Ind. Eng. Chem. Res. 2012, 51, 9966−9972


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values are 255, 266, and 45 m2/g for carbon black, TiO2 and TinO2n−1, respectively. Two strategies were used to investigate the durability of Pd/ C, Pd/commercial TiO2, and Pd/TinO2n−1 in this work. In the first one, fixed potential values of 0.3 and 0.7 V were applied to the working electrode for 1 h in 1 M KOH solution to measure the corrosion currents, and CVs were recorded before and after the tests to determine Pd ECSAs of the catalysts. The chronoamperometric curves in Figure 4a show that Pd/ TinO2n−1 has a smaller corrosion current than that of Pd/ commercial TiO2 and Pd/C at 0.3 V. Corrosion current values of 1.15, 2.44, and 2.86 μA/cm2 were observed for Pd/TinO2n−1, Pd/commercial TiO2, and Pd/C after 1 h, respectively. This trend was also found in Figure 4b. At 0.7 V, Pd/TinO2n−1 shows a corrosion current of 0.013 mA/cm2 at 1 h, which is only 25% and 42% of those obtained for Pd/C (0.052 mA/cm2) and Pd/ commercial TiO2 (0.031 mA/cm2), respectively. With much higher values of corrosion current at 0.7 V, it is clear that the higher holding potential has a stronger effect on the degradation of the catalysts. Pd ECSAs of the catalysts normalized with initial ECSA values are plotted versus the holding potentials in Figure 5. The Pd ECSA losses again confirm the above trend. Pd/TinO2n−1 lost 5 and 7% of its Pd ECSA at 0.3 and 0.7 V, respectively, while Pd/C lost 18 and 35% of its Pd ECSA value. Pd/commercial TiO2 shows a better result than Pd/C but worse than Pd/TinO2n−1, with a Pd ECSA loss of 13 and 22% in the two fixed potential tests. In the second durability test strategy, multiple CV cycles were performed on the working electrode with a potential range of −0.8 to 0.3 V. As shown in Figure 6, Pd/TinO2n−1 exhibits a slower degradation rate of Pd ECSA than Pd/commercial TiO2 and Pd/C. After 1000 cycles, only a loss of 39% of Pd ECSA was found for Pd/TinO2n−1 while 58% and 51% of Pd ECSA were lost for Pd/C and Pd/commercial TiO2, respectively. As reported above, all the results of the two durability test methods showed that TinO2n−1 possesses a better stability compared to commercial TiO2 and carbon black in alkaline media. This is in agreement with the results by Ioroi’s group for acidic media.4,25 In that work, it was found that carbon black was oxidized beyond 0.9 V (vs RHE) while Ti4O7 still showed a stable state up to 1.8 V (vs RHE) in a single cell test. Electrocatalytic activity of the catalysts for ethanol oxidation in ADEFC was investigated in 1 M KOH + 1 M C2H5OH solutions. Linear sweep voltammograms of the catalysts for ethanol oxidation are displayed in Figure 7. Pd/C and Pd/TiO2 shows an onset potential for ethanol oxidation at ca. −0.50 V, while a more negative value of −0.55 V is seen for Pd/TinO2n−1. Furthermore, the highest peak current of 2.10 mA/cm2 is observed for Pd/TinO2n−1, while 1.05 and 1.85 mA/cm2 are found for Pd/C and Pd/TiO2, respectively. These data imply that Pd/TinO2n−1 has the greatest activity for ethanol oxidation among the three catalysts. With similar onset potential and higher peak current density, Pd/commercial TiO2 shows a better oxidation activity for ethanol in alkaline media compared to the convenient Pd/C catalyst. The excellent activity of Pd/ TinO2n−1 for ethanol oxidation possibly results from an electronic interaction between the support and the metal nanoparticles.19,38 According to Hammer and Nørskov’s d-band center theory, the lattice constant of small metal particles on an oxide support suffers a compression or expansion due to an interaction with the lattice of the support, which causes a change in the d-band center of the metal and results in its adsorption capability change.39−41 As shown in Table 2, Pd has

Figure 2. XRD profiles of the supports and catalysts.

Table 1. Measured Properties of Different Supports conductivity (S·cm−1) support carbon black TiO2 TinO2n−1

literature value 4.0, 5.4, 7.4 −13

−6

10 , 10 1035, 1995 (Ti4O7) 631 (Ti5O9) 63 (Ti6O11) 25 (Ti8O15)

meas. value 6.9 −8

4.9 × 10 1.7 × 10−7

BET surface area (m2/g)

BJH pore diam. (Ǻ )

255

23.4

266 45

11.2 43.1

higher conductivity than commercial TiO2 but lower than that of carbon black. The measured conductivities of carbon black and commercial TiO2 are 6.9 and 4.9 × 10−8 S·cm−1, respectively, which are consistent with those in the literature (4.0, 5.4, 7.4 S·cm−1 for carbon black and 10−13, 10−6 S·cm−1 for TiO2).30−35 However, the conductivity value measured for TinO2n−1 (1.7 × 10−7 S·cm−1) is different from those of Ti4O7, Ti5O9, Ti6O11, and Ti8O15 reported in the literature (1035, 1995 S·cm−1 for Ti4O7, 631 S·cm−1 for Ti5O9, 63 S·cm−1 for Ti6O11, and 25 S·cm−1 for Ti8O15; no conductivity reports for Ti7O13 and Ti9O17 in the literature).21,24,35,36 The experiments were repeated several times and double checked by AC impedance measurement to ensure the validity of the obtained conductivity values. As analyzed above, the XRD pattern of TinO2n−1 confirmed that the substoichiometric titanium oxide is a mixture of Ti9O17, Ti8O15, Ti7O13, and Ti6O11. The low electrical conductivity may also be attributed to the mixed polycrystalline phase of the substiochiometric titanium oxides. As examined below, the substoichiometric titanium oxides synthesized all show porous and polycrystalline structure, while the reported values are based on nonporous single crystal, which may be the primary reason for the discrepancy. Besides, with different compression pressures, the pellets will have different interparticle porosities, which may result in the variation in measured conductivities.33,37 To investigate the intraparticle porosity of the supports, nitrogen adsorption/desorption tests were performed, and the results are displayed in Table 1 and Figure 3. The mean pore size of TinO2n−1 (43.1 Ǻ ) is larger than that of commercial TiO2 (11.2 Ǻ ) due to the broadening effect of the partial reduction with H2 at high temperature. As a result, the BET surface area of TinO2n−1 is smaller than that of TiO2. The BET surface area 9968



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Figure 3. Nitrogen adsorption isotherms of (a) carbon black, (b) commercial TiO2, and (c) TinO2n−1.

Figure 4. Corrosion curves of the catalysts in fixed potential tests.

center of Pd. This improves the adsorption of OH− onto the catalyst surface and helps to remove ethoxy species out of active sites of the catalyst.1,39,43 As a consequence, the kinetics of the ethanol oxidation is enhanced. Moreover, as shown in Table 1,

a lattice constant of 0.389 nm while unit-cell parameters of Ti6O11, Ti7O13, Ti8O15, and Ti9O17 are larger.1,42 The lattice constant mismatch between Pd and TinO2n−1 causes a tensile strain in the structure of Pd particles and shifts up the d-band 9969



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Table 2. Unit Cell Parameters of Palladium, Anatase Titanium Oxide, and Some Magnéli Phase Titanium Oxides phase

a (nm)

anatase TiO2 Ti6O11 Ti7O13 Ti8O15 Ti9O17 palladium

0.374 0.555 0.554 0.553 0.552 0.389

b (nm)

c (nm)

structure

0.713 0.713 0.713 0.714

0.951 3.223 3.815 4.406 5.003

tetragonal triclinic triclinic triclinic triclinic face-centered cubic

the mean pore size of TinO2n−1 is 43.1 Ǻ , which is larger than those of carbon black and commercial TiO2 (23.4 and 11.2 Ǻ , respectively). From the XRD data of Pd/C and Scherrer’s equation, mean Pd particle size was found to be 28 Ǻ . This size is smaller than the pore size of TinO2n−1 but larger than those of carbon black and commercial TiO2. Therefore, Pd nanoparticles can be deposited not only on the external surface of TinO2n−1 but also in the pores of the support. This will help to prevent the agglomeration of the Pd nanoparticles and therefore, improve the ethanol oxidation process. In the case of Pd/TiO2, the anatase TiO2 has lattice constants of 0.374 and 0.951 nm,44,45 which are smaller than those of Ti6O11, Ti7O13, Ti8O15, and Ti9O17 (Table 2). As a result, the d-band center of Pd is less shifted up and thus, TiO2 is less effective than TinO2n−1 in improving the ethanol oxidation activity of Pd. On the other hand, TinO2n−1 and TiO2 are hypo-d-electron oxides while Pd is a hyper-d-electron metal.23,46−53 According to Brewer’s theory, the stronger the hypo-hyper-d-interelectronic bonding between the support and the metal, the weaker the adsorptive bonding of poisoning intermediates (ethoxy CH3CO), and consequently, the higher the electrocatalytic activity for ethanol oxidation.23,46−53 The stability of the catalysts for ethanol oxidation was investigated by using chronoamperometry technique at an anodic potential of −0.25 V in the 1 M KOH + 1 M C2H5OH solution (Figure 8). At the initial stage, current densities of all

Figure 5. Normalized Pd ECSAs of the catalysts after fixed potential tests.

Figure 6. Normalized Pd ECSAs of the catalysts after multiscan tests.

Figure 8. CA results of the catalysts at −0.25 V (vs Hg/HgO) in 1 M C2H5OH + 1 M KOH solution.

the catalysts degrade rapidly as a result of the adsorption of poisoning intermediates. Then, a slow decay was observed after 100 s. At the second stage, the highest current was found for Pd/TinO2n−1, while the lowest one was observed for Pd/

Figure 7. LSV plots of the catalysts in 1 M C2H5OH + 1 M KOH solution with a scan rate of 50 mV s−1. 9970



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commercial TiO2. These results suggest that among the three electrocatalysts, Pd/TinO2n−1 has the best tolerance to the poisoning species.

4. CONCLUSIONS The application of TinO2n−1 and commercial TiO2 as catalyst supports for Pd in ADEFC was investigated in this work. TinO2n−1 showed a higher electronic conductivity than that of TiO2 but lower than that of carbon black. From fixed potential and multiple scan tests, excellent durability was found for TinO2n−1 compared with carbon black and commercial TiO2 in alkaline media. Among the three catalysts, Pd/Ti nO2n−1 displayed the highest activity and stability for the electrooxidation of ethanol. The study showed that TinO2n−1 is a promising support for the catalysts in ADEFC.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (65) 6316 8866. Fax: (65) 6794 7553. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are deeply grateful to the Ministry of Education (Singapore) for the academic research fund ARC 2/10 (MOE2009-T2-2-024) and the National Research Foundation (Singapore) for the Competitive Research Program (2009 NRF-CRP 001-032). One of the authors (S.T.N.) would like to thank AUN/SEEDNet-JICA for giving a scholarship.

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REFERENCES

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