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Preparation and Electrochemical Characterization of PtRuO2-Ce0.5Pr0.5O2-δ/C Catalysts for Methanol Electrooxidation Wei Du, Xiaoying Xie, Dexia Xu, and Chengde Huang* Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China ReceiVed March 19, 2008. ReVised Manuscript ReceiVed June 27, 2008
The methanol electrooxidation reaction was studied on carbon-dispersed Pt-rare earth oxide nanocatalysts with RuO2 nanorod in H2SO4/CH3OH solutions. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize PtRuO2-Ce0.5Pr0.5O2-δ/C catalysts. The electrochemical activities for the methanol electrooxidation reaction were measured in a powder microelectrode by cyclic voltammetry. The results showed that PtRuO2-Ce0.5Pr0.5O2-δ/C exhibited the highest electroactivity and PtRuO2-Pr6O11/C exhibited extremely small current density. Interpretation of trial results was based on the fact that, in contrast to PtRu/C catalysts, PtRuO2-Pr6O11/C catalysts had better ability to dehydrogenate methanol and enhance the effect of the bifunctional mechanism in the methanol oxidation reaction.
1. Introduction In recent years, electrochemical oxidation reactions of small, oxygenated organic molecules (SOMs) have received considerable attention because of their potential to produce electricity with high efficiency in fuel cells. At present, platinum is the best-known catalyst for the adsorption and dissociation of SOMs. However, poison COad,1 which is often an intermediate of such SOM electrooxidation reactions on the Pt surface, tends to be strongly adsorbed at bridging and 3-fold hollow sites and blocks the surface-active sites for further catalysis, leading to a dramatic decrease in the efficiency of the SOM electrooxidation. The approach to solve this problem is to add other materials into platinum. According to the bifunctional mechanism,2 these added metals can activate water molecules and provide preferential sites for -OHad3adsorption, which can be combined with poison CO to produce CO2, making the platinum poisoned by CO regenerate. Among these materials, PtRu,4-6 PtSn,7 and PtRuIr8 alloys have been recommended as suitable catalysts. Recent studies of developing catalysts with transition-metal oxides and rare earth oxides9-11 have shown that these oxides can improve electrocatalytic properties in methanol and ethanol * To whom correspondence should be addressed. E-mail: cdhuang@ tju.edu.cn. (1) Junhua, J.; Kucernak, A. J. Electroanal. Chem. 2003, 543, 187– 199. (2) Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E. S.; Sung, Y. E.; Kim, H.; Thomas, S.; Wieckowski, A. J. Phys. Chem. B 2000, 104 (15), 3518–3531. (3) Waszczuk, P.; Solla-Gullo´n, J.; Kim, H. S.; Tong, Y. Y.; Montiel, V.; Aldaz, A.; Wieckowski, A. J. Catal. 2001, 203 (1), 1–6. (4) Jiang, L.; Sun, G.; Zhao, X.; Zhou, Z.; Yan, S.; Tang, S.; Wang, G.; Zhou, B.; Xin, Q. Electrochim. Acta 2005, 50 (12), 2371–2376. (5) Deivaraj, T. C.; Lee, J. Y. J. Power Sources 2005, 142 (1-2), 43– 49. (6) Solla-Gullo´n, J.; Vidal-Iglesias, F. J.; Montiel, V.; Aldaz, A. Electrochim. Acta 2004, 49, 5079–5088. (7) Wang, H.; Zhao, Y.; Jusys, Z.; Behm, R. J. J. Power Sources 2006, 155 (1), 33–46. (8) Liao, S.; Holmes, K.; Tsaprailis, H.; Birss, V. I. J. Am. Chem. Soc. 2006, 128 (11), 3504–3505.
oxidation reactions. It has also been reported that CeO2 supported on Pt particles12 possesses a large electrochemically active surface area for methanol electrooxidation because CeO2 can release oxygen reversibly. In this study, we report the synthesis, physical, and electrochemical characterizations of PtRuO2-Ce0.5Pr0.5O2-δ/C catalysts. Their activity toward the electrooxidation of methanol is investigated, and their reaction mechanism is also discussed. 2. Experimental Section 2.1. Preparation of Anode Electrocatalysts. The PtRuO2Ce0.5Pr0.5O2-δ/C catalyst was prepared as follows. First, the Ce0.5Pr0.5O2-δ was synthesized by the sol-gel method according to the literature.13 A total of 0.05 mol of Ce(NO3)3 (99.9%) and 0.05 mol of Pr(NO3)3 (99.5%) were dissolved in 150 mL of H2O, and then 0.10 mol of citrate (99.5%) dissolved in 100 mL of H2O was added to the above mixture solution. The mixture was then heated under stirring at 70 °C until it became transparent gel. The gel was further dried at 100 °C overnight and finally calcined at 500 or 900 °C for 4 h. The co-reduced PtRuO2-Ce0.5Pr0.5O2-δ/C (PtRu(1:1)/[PtRu(1: 1) + C] ) 20 wt %) catalysts were synthesized by the conventional borohydride reduction method. First, the Vulcan XC-72 carbon (Cabot Com.) and Ce0.5Pr0.5O2-δ powder were dispersed with an ultrasonic wave in Millipore water (18.2 MΩ), and then an adequate amount of Pt salt (H2PtCl6 · 6H2O) and Ru salt (RuCl3) was added to the solution by maintaining the molar ratio of Ce0.5Pr0.5O2-δ/ platinum and ruthenium (PtRu/Ce0.5Pr0.5O2-δ) at 1:1. After mixing for 1 h at 60 °C, the metal salts were reduced by NaBH4. At the same time, the solution was stirred vigorously by constantly adding the ammonium hydroxide to maintain its pH. The resulting (9) Park, K. W.; Ahn, K. S.; Nah, Y. C.; Choi, J. H.; Sung, Y. E. J. Phys. Chem. B 2003, 107 (18), 4352–4355. (10) Lin, C.; Xin-dong, W.; Min, G. Acta Phys. Chim. Sin. 2006, 22 (2), 19–23. (11) Chaojie, S.; Mohammad, K.; Peter, G. P. J. Appl. Electrochem. 2006, 36 (3), 339–345. (12) Bai, Y.; Wu, J.; Qiu, X.; Xi, J.; Wang, J.; Li, J.; Zhu, W.; Chen, L. Appl. Catal., B 2007, 73 (1-2), 144–149. (13) Luo, M.; Yan, Z.; Jin, L.; He, M. J. Phys. Chem. B 2006, 110 (26), 13068–13071.
10.1021/ef800200u CCC: $40.75 2008 American Chemical Society Published on Web 08/06/2008
Catalysts for Methanol Electrooxidation precipitates were washed with deionized water and dried in the vacuum oven at 80 °C overnight. The 20 wt % PtRu/C, PtRuO2-CeO2/ C, and PtRuO2-Pr6O11/C catalysts, by maintaining the molar ratio of platinum/ruthenium (Pt/Ru) at 1:1 in ammonium hydroxide solution, were also prepared for comparison. The subsequent preparative procedures were similar to that of the PtRuO2Ce0.5Pr0.5O2-δ/C catalysts as mentioned above. Finally, all of the catalysts were annealed at 400 °C in air for 2 h. 2.2. X-ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HR-TEM). The crystallographic structures of supported catalysts were examined by XRD, using a Bruker D8 ADVANCE X-ray diffractometer operated with a Cu KR source (λ ) 1.541 Å) at 40 kV and 40 mA. The 2θ angular region between 5° and 110° was explored at a scan rate of 0.02°/ step. The HR-TEM analysis was conducted by a Tecnai G2 F20 microscope to obtain the microcosmic images and shapes of the dispersed nanoparticles on the carbon material surface. 2.3. Electrochemistry. 2.3.1. Preparation of Electrodes. The microelectrode was used in this study. The working electrode was prepared by sealing a 100 µm diameter Pt wire in a glass tube and grinding its end to form a plane perpendicular to the axis of the tube. Then, the disk microelectrode was etched with aqua regia to create a cavity at its tip. The depth of the cavity (60 µm) was controlled by a duration of etching. After clearing and drying, the etched tip was then ground on a flat plate (such as a glass slide) with the PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst until the microcavity was filled with catalyst. In this paper, we determined by measuring the volume (diameter, 1 mm; length, 1.2 mm) in all of the weight of catalyst to estimate the weight of catalyst in the microcavity (PtRu/C, 2.21 × 10-4 mg; PtRuO2-Ce0.5Pr0.5O2-δ/C, 2.29 × 10-4 mg). 2.3.2. Electrochemical Measurements. A conventional onecompartment glass cell with a Luggin capillary was used in the electrochemical experiments. A large area platinum foil served as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. All of the experiments were carried out in 2.5 mol L-1 H2SO4 + 2 mol L-1 CH3OH, prepared using high-purity reagents and purified water in a Milli-Q (Millipore) system. The electrochemical experiments were performed with a CHI660B potentiostat. The potential was scanned between -0.13 and 1.07 V, at scan rate of 5 mV s-1. Before recording, the potential was repeatedly scanned in the same range to remove residual impurities. The impedance spectra were obtained at frequencies between 10 kHz and 0.01 Hz. The amplitude of the sinusoidal potential signal was 5 mV. During each experiment period, the surface cleanliness of the electrode and its catalytic activity for the methanol oxidation were examined and recorded by cyclic voltammetry.
3. Results and Discussion 3.1. Structural and Morphological Features. Figure 1 shows the XRD patterns of all of the synthesized PtRuO2-oxide/C catalysts. It can be seen that the diffraction peaks at 2θ ) 39.8° (111), 46.2° (200), 67.5° (220), and 81.3° (311) can be indexed to face-centered cubic platinum, which is consistent with the standard powder diffraction file of Pt (JCPDS 04-0802). The strong diffraction peaks at the Bragg angles of 35.1°, 54.3°, 59.4°, and 74.1° correspond to the (011), (121), (002), and (022) facets of RuO2 (JCPDS 43-1027). The other main observable peaks can be attributed to the (111), (200), (220), and (311) planes of CeO2 (28.5°, 33.0°, 47.4°, and 56.2°, cubic CeO2, JCPDS 81-0792), to the (110) and (201) plane of Ru crystal (69.02° and 85.56°), and to the (111), (200), (222), and (422) planes of Pr6O11 (located at 2θ values of 28.0°, 32.4°, 58.0°, and 87.7°, respectively, cubic Pr6O11, JCPDS 42-1121). There is no shift in any of diffraction peaks of platinum in PtRuO2-oxide/C catalyst, indicating that the addition of oxide has no effect on the crystalline lattice of platinum. To examine
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Figure 1. XRD patterns of all of the synthesized PtRuO2-oxide/C catalysts: (a) PtRuO2-CeO2/C, (b) PtRuO2-Pr6O11/C, (c) PtRuO2Ce0.5Pr0.5O2-δ/C (500 °C), and (d) PtRuO2-Ce0.5Pr0.5O2-δ/C (900 °C).
the effects of adding oxide on the Pt particle size of PtRuO2-oxide/C catalysts, The Pt (111) peaks were used to calculate the particle size of Pt according to Scherrer’s equation d)
kλ β1/2 cos θ
where d is the mean particle size in Å, k is a coefficient taken here as 0.9, λ is the wavelength of the X-rays used (1.540 56 Å), β is the width of the diffraction peak at half-height in radians, and θ is the angle at the position of the peak maximum. The average Pt nanoparticle sizes of PtRuO2-CeO2/C, PtRuO2-Pr6O11/C, PtRuO2-Ce0.5Pr0.5O2-δ/C (Ce0.5Pr0.5O2-δ calcined at 500 °C), and PtRuO2-Ce0.5Pr0.5O2-δ/C (Ce0.5Pr0.5O2-δ calcined at 900 °C) is estimated to be 8.2, 15.1, 8.2, and 9.4 nm, respectively. The results show that the addition of CeO2 and Ce0.5Pr0.5O2-δ solid solution has a greater effect on the Pt nanoparticle sizes than that of Pr6O11. It is also observed that, after high-temperature calcinations of Ce0.5Pr0.5O2-δ, the characteristic peaks of CeO2 and Pr6O11 in PtRuO2-Ce0.5Pr0.5O2-δ/C become stronger and sharper. This result reveals that the calcinations can alter the crystalline degree of the mixed oxides. Figure 2a shows the TEM image of PtRuO2-Ce0.5Pr0.5O2-δ/C (Ce0.5Pr0.5O2-δ calcined at 900 °C) in detail and clearly demonstrates that these catalysts consist of a nanorod with tube diameters between 6 and 12 nm. To determine the nanorod composition, the atomic compositions were examined by the energy-dispersive X-ray (EDX) technique equipped with a nanoprobe. The EDX spectrum of the nanorod in Figure 2a indicates clearly the presence of Ru and O in the nanorod. By evaluating the spectra, we found out that the approximate atomic ratio of Ru/O was 1:2 (matching RuO2) and deduced that the nanorod was a RuO2 nanorod. It can be seen that a few particles with size more than 5 nm are supported on the RuO2 nanorod. These particles probably are platinum and Ce0.5Pr0.5O2-δ. In Figure 2c, the transmission electron micrographs of the sample show that catalyst nanoparticles appear to be uniformly distributed on the nanorod support and there is no evidence for agglomeration of the catalyst crystallites. Because RuO2 nanorods are assembled, therefore, the catalyst assembly can be observed in the photo. However, the catalyst disperses very well in a small scale (see Figure 2a). A good dispersion of metal
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Figure 3. Cyclic voltammograms of samples in 2.5 M H2SO4 plus 2.0 M CH3OH solution: (a) PtRuO2-CeO2/C, (b) PtRuO2-Pr6O11/C, and (c) PtRuO2-Ce0.5Pr0.5O2-δ/C (500 °C).
Figure 2. (a) TEM image of PtRuO2-Ce0.5Pr0.5O2-δ/C (Ce0.5Pr0.5O2-δ calcined at 900 °C). (b) EDX spectrum of the nanorod in Figure 2a. (c) Panorama of PtRuO2-Ce0.5Pr0.5O2-δ/C.
particles on support materials effects the electrocatalytic activity. The activity of the catalyst is greater when there is a better dispersion of particles and a more available specific area. 3.2. Effect of Catalyst Composition. Figure 3 shows the cyclic voltammograms (CV) of the various PtRuO2-oxide/C electrodes in 2.5 M H2SO4 plus 2.0 M CH3OH solution. The potential was swept between -0.13 and 1.07 V, at a rate of 5 mV s-1. The forward scan peak potential of the methanol electrooxidation is located at 0.68 V versus SCE for all
PtRuO2-oxide/C electrodes, and the backward scan peak potential is located at 0.56, 0.6, and 0.52 V for PtRuO2-Ce0.5Pr0.5O2-δ/C, PtRuO2-CeO2/C, and PtRuO2-Pr6O11/C, respectively. The peak current density, i.e., mass activity, which is defined as the ratio of peak current obtained from the forward CV scans to the mass of Pt in the electrode, is used to evaluate the activity of catalysts. As shown in Figure 3, the current peak density of methanol electrooxidation on PtRuO2-Pr6O11/C displays the lowest peak current density. The PtRuO2Ce0.5Pr0.5O2-δ/C catalysts, which Ce0.5Pr0.5O2-δ calcined at 500 °C, display a higher peak current density of methanol electrooxidation. This result indicates that, adding agent for the high metal content anode electrocatalysts, the effect of various rare earth oxides is different because the many factors determine the surface characteristic of rare earth oxides. It is seen that the PtRuO2-Ce0.5Pr0.5O2-δ/C catalysts display higher current density than PtRuO2-CeO2/C and PtRuO2-Pr6O11/C, which can be explained by the fact that, with the number of hydroxyl groups increasing on the surface of Ce0.5Pr0.5O2-δ composite oxide and with the interaction of CeO2 and Pr6O11 producing Ce0.5Pr0.5O2-δ solid solution, an array of oxygen atoms in the lattice is more uniform and tighter because of the synergistic effect. 3.3. Effect of Calcination Temperature. Figure 4 summarizes the cyclic voltammograms of the methanol in two electrocatalysts at a scan rate of 5 mV s-1. These voltammograms show such characteristics during forward potential sweep: a current peak appears at about 0.68 V, and during backward potential sweep, a current peak appears at about 0.57 V. From Figure 4, we can find that the effect of calcination temperature on the peak current density is very different. Li et al.14 reported that the calcinations would play an important role for further catalytic reaction because the chemical properties of CeO2 and Pr6O11 changed as a result of high-temperature calcinations; for example, the hydroxyl group on the surface of oxide was activated, or the active sites increased. It is seen that the PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst, which Ce0.5Pr0.5O2-δ calcined at 900 °C, exhibited stronger catalytic activity for methanol oxidation than the PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst, which Ce0.5Pr0.5O2-δ calcined at 500 °C. 3.4. Comparison between PtRuO2-Ce0.5Pr0.5O2-δ/C and PtRu/C Catalysts. Figure 5 compares CV curves for methanol electrooxidation on PtRu/C and PtRuO2-Ce0.5Pr0.5O2-δ/C. It (14) Li, C.; Jiang, Z.; Xin, Q. J. Chin. Rare Earth Soc. 1994, 1, 24–29.
Catalysts for Methanol Electrooxidation
Figure 4. Cyclic voltammograms of the PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst as a function of the calcining temperature of Ce0.5Pr0.5O2-δ: (a) PtRuO2-Ce0.5Pr0.5O2-δ/C (500 °C) and (b) PtRuO2-Ce0.5Pr0.5O2-δ/C (900 °C).
Figure 5. Cyclic voltammograms for methanol electrooxidation on Pt-Ru/C and PtRuO2-Ce0.5Pr0.5O2-δ/C.
can be seen that the peak current density values on the modified composite catalyst are much larger than on PtRu/C. Figure 6 shows a comparison of current-time transients of methonal oxidation at 0.3 V (potential selected from cyclic voltammogram) on PtRu/C and PtRuO2-Ce0.5Pr0.5O2-δ/C at room temperature. Interestingly, the current density on PtRuO2-Ce0.5Pr0.5O2-δ/C is significantly higher compared to PtRu/C. AC impedance spectroscopy was used to evaluate the activity of the two catalysts. Nyquist plots for the PtRu/C and PtRuO2-Ce0.5Pr0.5O2-δ/C catalysts at 30 °C at various bias potential are presented in parts a and b of Figure 7, respectively. When the potential is lower than 0.3 V, big open arcs can be observed, revealing a slow reaction rate of methanol oxidation caused by the COad poisoning effect. With the increase of the potential, the diameter of the arc decreases sharply, indicating the increasing driving force for the methanol oxidation process. In comparison to PtRu/C, the PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst performs better because its semicircle is smaller than in the first case. The PtRu/C catalyst exhibites the larger semicircle, which means very large charge-transfer resistance. It is observed that the addition of composite rare earth oxides can improve the COad tolerance of catalyst.
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Figure 6. Chronoamperometry for methanol electrooxidation on Pt-Ru/C and PtRuO2-Ce0.5Pr0.5O2-δ/C.
Ceria-supported noble-metal catalysts are capable of storing oxygen under oxidizing conditions and releasing oxygen under reducing conditions via the facile conversion between Ce4+ and Ce3+ oxidation states.15 Oxygen anion vacancies in ceria are considered to play an essential role in catalytic reactions.16 To improve both the stability and oxygen storage capacity of cerium oxide, zirconium and/or praseodymium atoms were added to CeO2.17 It is noteworthy that Ce-Pr mixed oxides exhibit excellent redox properties and enhance the efficiency of oxygen exchange. Praseodymium atoms incorporated in ceria ensure that the resulting materials have high oxygen mobility within a large range of temperatures. Praseodymium ions allow for the creation of oxygen vacancies in ceria at room temperature. These vacancies can even be observed in an oxidizing atmosphere. In the present investigation, we employed Ce0.5Pr0.5O2-δ as a cocatalytic material along with the PtRuO2/C catalyst based on the fact that Ce0.5Pr0.5O2-δ has a higher oxygen storage capacity (OSC).18 The oxygen atoms in the lattice of Ce0.5Pr0.5O2-δ can either be directly or indirectly involved in promoting the bifunctional mechanism of the methanol oxidation reaction on the PtRuO2/C catalyst. The bifunctional mechanism that has been well-established for methanol oxidation on PtRu alloys3 can be written for PtRuO2-Ce0.5Pr0.5O2-δ/C as follows: CH3OH + Pt f Pt-CO + 4H+ + 4e- (in multiple steps) RuO2 + H2O f RuO2(OH) + H+ + eCe0.5Pr0.5O2-δ + H2O f Ce0.5Pr0.5O2-δ(OH) + H+ + ePt-CO + RuO2(OH) f Pt + RuO2 + CO2 + H+ + ePt-CO + Ce0.5Pr0.5O2-δ(OH) f Pt + Ce0.5Pr0.5O2-δ + CO2 + H+ + eThus, Ce0.5Pr0.5O2-δ species on the Pt surface promote the formation of OH groups adjacent to CO-poisoned Pt sites, combine with the adsorbed CO, and strip CO from the surface (15) No¨renberg, H.; Briggs, G. A. D. Surf. Sci. 1999, 424 (2-3), 352– 355. (16) Zonglan, Y.; Xia, L.; Jianhai, L.; Guanqun, X.; Mengfei, L. J. Inorg. Mater. 2005, 20 (3), 143–148. (17) Rodriguez, J. A.; Hanson, J. C.; Kim, J.; Liu, G. J. Phys. Chem. B 2003, 107 (15), 3535–3543. (18) Bernal, S.; Blanco, G.; Cauqui, M. A.; Marty´n, A.; Pintado, J. M.; Galtayries, A.; Sporken, R. Surf. Interface Anal. 2000, 30, 85–89.
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Figure 7. (a) AC impedance spectroscopy of the PtRu/C catalyst at 30 °C at various bias potentials. (b) AC impedance spectroscopy of the PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst at 30 °C at various bias potentials.
to form CO2. Then, the clean Pt surface becomes available for further methanol oxidation. 4. Conclusions Developing a catalyst with rare earth oxides is an effective method to improve the activity of PtRu/C electrocatalysts for methanol oxidation. However, the improvement of the electrocatalytic activity is found to be bigger only when Ce0.5Pr0.5O2-δ calcined at 900 °C. The electrocatalytic activity for methanol oxidation, evaluated by cyclic voltammetry, follows the order: PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst (Ce0.5Pr0.5O2-δ calcined at 900 °C) > PtRuO2-Ce0.5Pr0.5O2-δ/C catalyst (Ce0.5Pr0.5O2-δ calcinedat500°C)>PtRuO2-CeO2/C>PtRu/C>PtRuO2-Pr6O11/ C. The impedance measurement results confirmed the effect of composite rare earth oxide and RuO2 in methanol oxidation
because of the lower charge-transfer resistance in this system in comparison to that in the PtRu/C system. The enhanced reaction activity for the CH3OH oxidation by adding rare earth oxides and RuO2 nanorode is likely because Ce0.5Pr0.5O2-δ and RuO2 species on the Pt surface promote the formation of OH groups adjacent to CO-poisoned Pt sites. Acknowledgment. The authors thank Tianjin University (China) and the 973 Project of China (No. 2008CB617502) for financial support. Supporting Information Available: Figure S1 with the EDS spectrum of the catalyst in Figure 2c. This material is available free of charge via the Internet at http://pubs.acs.org. EF800200U
