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CeO2-ZrO2 catalysts for methanol decomposition
Journal of Energy Chemistry 23(2014)755–760
Co-modified Pd/CeO2-ZrO2 catalysts for methanol decomposition Ming Zhaoa,b ,
Hailong Zhanga ,
Xue Lia , Yaoqiang Chena∗
a. College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China; b. Sichuan Provincial Environmental Protection Environmental Catalytic Materials Engineering Technology Center, Chengdu 610065, Sichuan, China [ Manuscript received March 19, 2014; revised May 6, 2014 ]
Abstract Pd/Ce0.8 Zr0.2 O2 catalysts modified by cobalt were prepared by a sequential impregnation method and characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption (Brunauer-Emmet-Teller), oxygen storage capacity (OSC), CO-chemisorption, H2 -temperature-programmed reduction (H2 -TPR) and X-ray photoelectron spectroscopy (XPS). The effect of Co on the performance of methanol decomposition was evaluated at a fixed-bed microreactor. The results showed that the addition of Co can improve the oxygen storage capacity of the catalyst and the dispersion of Pd. XPS results indicated that Pd was in a partly oxidized (Pdδ+ , 1<δ<2) state and Co2+ was present in Pd catalysts modified by Co. A 90% conversion of methanol was achieved at around 280 ◦ C over Pd-Co/Ce0.8 Zr0.2 O2 catalyst which was 20 ◦ C lower than that over Pd/Ce0.8 Zr0.2 O2 , indicating that both Pdδ+ and Co2+ play an important role in improving the catalytic activity of methanol decomposition. Key words palladium; cobalt; methanol decomposition; CeO2 -ZrO2
1. Introduction In order to solve the energy crisis and environmental pollution problems, methanol is considered as an alternative fuel for automobiles. The decomposition of methanol into H2 and CO has received attention as a method to increase fuel efficiency for methanol powered vehicles [1]. At cold start, the methanol decomposition catalysts need to be active at low temperatures. The activity and stability are two major challenges in methanol decomposition reaction. Recently, an attempt has been made to tackle this problem and to further improve the catalytic activity by the addition of metal ions. In the past decades, many metals such as Cu, Ni and Mn have been extensively investigated. However, the catalysts still suffer a slow deactivation, and a by-product is usually observed [2]. Noble metal-containing catalysts, especially the Pd-containing catalysts, show high activity and selectivity for methanol decomposition at low temperature and the activity is largely affected by the supports [3,4]. Cex Zr1−xO2 solid solution is a good support for noble metals in methanol decomposition [5,6]. The catalytic property can be modified by additives, however, the effects of additives seem to be less systematically investigated. Generally, it is recognized that electron density of the metal is increased by the addition of electropho-
bic alkaline and alkaline earth groups. Palladium exists more or less in oxidative state compared with original Pt/Al2 O3 because of electronegativity of additives, and the oxidative state of palladium can be maintained by the electrophilic property of additive [7]. Bimetallic Co-Pd nanoparticles supported on MgO have been investigated in methanol decomposition [8]. In this work, Pd-based catalysts can be influenced by adding Co as a second metal to the system in methanol decomposition. Special attention is paid to the metal-support interaction and the electronic surroundings of Pd sites during the catalytic process. 2. Experimental 2.1. Catalyst preparation Ce0.8 Zr0.2 O2 samples were prepared by co-precipitation method from the corresponding chemicals: Ce(NO3 )3 ·6H2 O, ZrOCO3 at a nominal composition. The precursors were mixed in an aqueous solution respectively, and an appropriate amount of fresh H2 O2 was added. Then the mixed salt solution was added dropwise into a mixed aqueous solution of ammonia to reach a pH value of 9.0 in stirring. Finally the
Corresponding author. Tel: +86-028-85418451; Fax: +86-028-85418451; E-mail: [email protected]; [email protected] This work was supported by the National Natural Science Foundation of China (No. 21173153) and Sichuan Province Science and Technology Support Projects(2012FZ0008). ∗
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60209-6
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precipitates were filtered, thoroughly washed, dried and calcined at 600 ◦ C for 4 h to achieve the Ce0.8 Zr0.2 O2 powder. The Ce0.8 Zr0.2 O2 powders were impregnated with the solution of Co(NO3 )2 with CoO2 content of 0.5 wt%. Afterwards, the materials were dried at 105 ◦ C overnight and calcined at 500 ◦ C for 3 h in air to achieve the Co/Ce0.8 Zr0.2 O2 powder. The prepared Ce0.8 Zr0.2 O2 and Co/Ce0.8 Zr0.2 O2 powders were impregnated with the solution of Pd (NO3 )2 . Afterwards, the materials were dried at 105 ◦ C overnight and calcined at 500 ◦ C for 3 h in air, mixed with some water, ground and finally formed slurry. The resulting slurry was coated on a honeycomb cordierite (1.5 cm3 , Corning, America) and the excessive slurry was blown away with compressed air, then dried and calcined at 500 ◦ C for 3 h in air. Finally, a monolithic catalyst with 1.0 wt% Pd was obtained. 2.2. Determination of methanol decomposition activity The reaction of gas phase methanol decomposition was performed in a fixed-bed continuous flow reactor operating under atmospheric pressure. The catalysts were placed in a conventional tubular quartz reactor. Reduction of the catalysts was performed in a flow of 1.8 dm3 /h 5% H2 in N2 at 400 ◦ C for 1 h before catalytic test. Then methanol diluted with argon (MeOH, 15%; GHSV, 3400 h−1 ) was fed at 160−300 ◦ C. The outlet reaction gas was analyzed with an on line gas chromatograph equipped with a TDX-1 column (2 m) and Porapak-Q column (2 m). Column switching was applied to optimize the separation of H2 , CO, CH4 and CO2 as well as CH3 OH, CH3 OCH3 and HCOOCH3 . 2.3. Catalyst characterization The BET specific surface area and pore size measurement were performed on an automated surface area and pore size analyzer (Quantachrome). The BET specific surface area was determined by a pure N2 adsorption-desorption technique at liquid nitrogen temperature. Prior to the measurements, the samples were degassed for 1 h at 350 ◦ C under vacuum condition. X-ray diffraction (XRD) patterns were obtained on a D/MAX-Ra rotatory diffractometer, using Cu Kα radiation (λ = 0.1541 nm), 50 kV and 180 mA. Samples were scanned from 2θ equal to 20o up to 90o . The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a spectrometer (XSAM-800, KRATOS Co) with Mg Kα radiation under UHV. All the samples were pretreated in a stream of 5% H2 -95% N2 at 400 ◦ C for 1 h, and cooled to room temperature. The binding energy was determined by reference to C 1s binding energy of 284.8 eV. Temperature-programmed reduction (H2 -TPR) experiments were carried out in a conventional system equipped with a thermal conductivity detector (TCD). All the samples (100 mg) were pretreated in a quartz U-tube in a flow of pure
N2 at 400 ◦ C for 45 min, then cooled. The reduction reaction was carried out in a flow of 5% H2 in N2 from 50 ◦ C to 900 ◦ C with a linear heating rate of 8 ◦ C/min. Oxygen storage capacity (OSC) was estimated as follows. A certain amount of catalysts were heated in a quartz reactor in a flow of 40 mL/min H2 at 550 ◦ C for 45 min. Then the catalysts were switched to heat in a flow of 20 mL/min N2 at 200 ◦ C and iteratively pulsed at a desired temperature until no more loss of O2 injected was detected. Pd dispersion was measured after reducing the sample (200 mg) at appropriate temperature (400 ◦ C) under 5% H2 95% N2 flow for 1 h. The remnant H2 was driven off with argon at 420 ◦ C for 30 min, then the sample was cooled down to room temperature under argon flow. Finally, pulse of CO was injected up to the breakthrough point. Pd dispersion was evaluated from CO consumption. 3. Results and discussion 3.1. Catalytic activity evaluation The catalytic performance of the prepared catalysts is shown in Figure 1. Compared with Pd/Ce0.8 Zr0.2 O2 , PdCo/Ce0.8 Zr0.2 O2 showed a higher conversion in methanol decomposition at the same reaction temperature. PdCo/Ce0.8 Zr0.2 O2 achieved a 70% conversion at 260 ◦ C which was higher than that of Pd/Ce0.8 Zr0.2O2 (50.3%). This could be attributed to the fact that Co additive can affect the chemical state of palladium and create more Co2+ and Pdδ+ species which are conducive to methanol decomposition into H2 and CO. At the same time, a little CH4 at different temperatures was detected. For example, 5.4% CH4 was detected at 260 ◦ C for Pd/Ce0.8 Zr0.2 O2 , but only 2.6% CH4 for PdCo/Ce0.8 Zr0.2 O2 , indicating that the selectivity of CH4 decreases when Co is added to catalysts. In other word, the addition of Co favors the formation of H2 and CO. After used at 320 ◦ C for 72 h, Pd-Co/Ce0.8 Zr0.2 O2 still maintained high activity and 80% conversion, indicating that this catalyst has good stability.
Figure 1. Conversion of methanol over two catalysts at different temperatures
Journal of Energy Chemistry Vol. 23 No. 6 2014
3.2. Specif ic surface area measurements The BET surface area and pore volume of catalysts are shown in Table 1. Pd/Ce0.8 Zr0.2 O2 had 62.0 m2 /g specific surface area but Pd-Co/Ce0.8Zr0.2 O decreased slightly to 58.0 m2 /g. It may be attributed to that the cobalt oxides cover the catalyst surface and block the partial pores. On the other hand, for Pd-Co/Ce0.8 Zr0.2 O2 , there was one more impregnation and calcination step, leading to a slight decrease of specific surface area. However, Pd-Co/Ce0.8 Zr0.2 O2 had higher activity in methanol decomposition. This indicates that the surface area is not the main factor to affect the catalytic activity. There is a weak correlation between methanol decomposition and surface area. Then we inferred that a new active center may be presence. Table 1. Textural properties of catalysts Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2
SBET (m2 /g) 62.0 58.0
Pore volume (cm3 /g) 0.10 0.08
3.3. Oxygen storage capacity and Pd dispersion The Pd dispersion and particle size estimated from CO chemisorption as well as the oxygen storage capacity of the two catalysts are listed in Table 2. The OSC of Pd/Ce0.8 Zr0.2 O2 was 437 µmol/g but that of PdCo/Ce0.8 Zr0.2 O2 increased to 717 µmol/g, indicating that Co addition greatly promotes the oxygen transfer from bulk to support surface and from support surface to Pd particles [9] helping Pd with more active Pdδ+ state. They can promote the decomposition of methanol because the cleavage of methanol involves the oxygen transfer and C–H break. Generally, the dispersion of active ingredient is very important for the catalyst performance. As shown in Table 2, Pd-Co/Ce0.8 Zr0.2O2 catalyst possessed 72% of Pd dispersion which was higher than that of Pd/Ce0.8 Zr0.2 O2 (53%), and Pd-Co/Ce0.8 Zr0.2 O2 catalyst formed smaller Pd particles (1.28 nm) than Pd/Ce0.8 Zr0.2O2 (1.73 nm) catalyst, implying that more palladium exposed on the surface. This is resultant in good catalytic activity of methanol decomposition (Figure 1). Based on the above analysis, one can conclude that Co can enhance the OSC of Pd/Ce0.8 Zr0.2 O2 and Pd dispersion, which play an important role in methanol decomposition reaction.
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which is generally belonged to the reduction of PdO involving Ce0.8 Zr0.2O2 . When Co was added to Pd/Ce0.8 Zr0.2O2 , the reduction temperature of PdO shifted to high temperature and the area of reduction peak became larger compared with Pd/Ce0.8 Zr0.2 O2 . This phenomenon may derive from the interaction between Co oxides and Ce0.8 Zr0.2 O2 as well as Co oxides and Pd oxide species, and the presence of Co oxides in a catalyst would inhibit the growth of noble metal and easily form smaller palladium particles, resulting in the decrease of reduction temperature and the increase of reduction area. At the same time, the introduction of Co oxide promotes the oxygen transfer and mobility, which can facilitate the reduction of CeO2 and maintain Pd with more cationic Pdδ+ state [10]. Pdδ+ ions are possibly formed through Ce4+ +Pd0 →Ce3+ +Pdδ+ . When palladium is at a semi-oxidized state, it will lead to the decrease of reduction temperature and reduction area. The peak at 390 ◦ C of Pd-Co/Ce0.8 Zr0.2O2 is attributed to the simultaneous reduction of Co and Ce oxides [11], indicating that there are strong Pd–Co and Co–Ce interactions. The strong interaction greatly influences the reduction behavior, causing that the reduction of Co3 O4 to CoO slightly shifts to higher temperature and that of surface ceria shifts to lower temperature [12−14]. The unreduced cobalt species (mainly Co2+ ) of PdCo/Ce0.8 Zr0.2 O2 could interact with CeO2 -ZrO2 or Pd and create new active sites. Therefore, we can speculate that both Co2+ and Pd&+ are the active sites for methanol decomposition which favor the decomposition of methanol to CO and H2 .
Table 2. Oxygen storage capacity and Pd dispersion of catalysts Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2
Pd dispersion (%) 53.70 72.61
Pd particle size (nm) 1.73 1.28
OSC (µmol/g) 473 717
Figure 2. H2 -TPR profiles of two catalysts
3.5. NH 3 -TPD measurements 3.4. H 2 -TPR measurements TPR profiles of the two catalysts are shown in Figure 2. Both of them had the sharp reduction peaks at about 100 ◦ C,
NH3 -TPD profiles of Pd-Co/Ce0.8Zr0.2 O2 and Pd/Ce0.8 Zr0.2 O2 are shown in Figure 3. Two catalysts had different NH3 desorption peaks. The peak at low temperature (LT<210 ◦ C region) is assigned to the desorption of
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ammonia, which is bound via Pdδ+ bonds to NH3 . For PdCo/Ce0.8 Zr0.2 O2 , the desorption diagram showed two well separated peaks at 181 and 226 ◦ C. The middle temperature (about 226 ◦ C) region is assigned to the free electron pair orbital of nitrogen with the empty d2z orbital of Co2+ , Ce3+ and Zr4+ . In a word, Pd-Co/Ce0.8 Zr0.2O2 catalyst possesses more acid sites than Pd/Ce0.8 Zr0.2 O2 , which is in favor of methanol decomposition into CO and H2 because acid sites can promote the adsorption of methanol on the catalyst surface.
Figure 4. XRD patterns of two catalysts
3.7. XPS measurements
Figure 3. NH3 -TPD profiles of two catalysts
3.6. XRD measurements XRD patterns of the samples at 550 ◦ C for 3 h are shown in Figure 4. No diffraction peaks of palladium (2θ = 40.0o ) and PdO (2θ = 34.0o) or PdO2 (2θ = 54.6o ) were observed, indicating that Pd species is highly dispersed on the surface of carriers. The high dispersion of Pd species improved the catalytic activity, which was coincided with the results of the activity testing. The major peak of the XRD patterns was consistent with the characteristic peaks of cubic CeO2 , indicating the formation of CeO2 -ZrO2 solid solutions. There were no CoOx peaks, implying that Cox+ enters into CeO2 lattice or well disperses on the surface of the supports. The lattice parameters of Pd-Co/Ce0.8 Zr0.2 O2 were similar to those of Pd/Ce0.8 Zr0.2 O2 , revealing that small part of Co cations are incorporated into CeO2 -ZrO2 lattice and more are finely dispersed on the surface of the solid solution. The former cause larger particle size of CeO2 -ZrO2 , and the latter can decrease the specific surface area (Table 3). This was consistent with the results of TPR and BET. Table 3. Crystal cell parameter and crystalline size Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2
Crystalloid parameter (nm) 0.5347 0. 5345
Particle size (nm) 6.2 6.5
In order to investigate the effect of Co on Pd-based catalysts, XPS was used to characterize the electronic state of Pd and the surface composition of catalysts. As shown in Figure 5, Pd 3d5/2 of the two catalysts only had a relatively symmetrical peak, and no shoulder peak appeared, showing that only a palladium species exists on the catalyst surface. For Pd/Ce0.8 Zr0.2 O2 , the peak of Pd 3d5/2 appeared at 336.8 eV, suggesting that the palladium on the surface is Pd2+ . The binding energy of Pd 3d5/2 for Pd-Co/Ce0.8 Zr0.2 O2 catalyst was 335.9 eV. According to the standard manual [15], the binding energies of Pd 3d5/2 for Pd0 and PdO are 335.0 and 340.0 eV, respectively. This may be inferred that the palladium in Pd-Co/Ce0.8Zr0.2 O2 catalyst is a partly oxidized state. The valence of Pd may be between 0 and +2 in PdCo/Ce0.8 Zr0.2 O2 catalyst. Compared with Pd/Ce0.8 Zr0.2O2 , the doping of CoOx made Pd 3d5/2 peak shift to lower binding energy by 0.9 eV, indicating that the addition of Co affects the chemical state of palladium, which may be due to the interaction between Pd and Co. On the other hand, the electronegativity of Co and Pd is 1.8 and 2.2, respectively, and the electrons of Co move around Pd which can increase the electron density around Pd. The strong interaction between palladium particles and support causes the transfer of electron from palladium to ceria. Generally, the partly oxidized state of Pd is more active than Pd0 and Pd2+ for methanol decomposition. The abstraction of C–H bond in the adsorbed methoxy group CH3 O (CH3 O(ad)+H(ad) = CH2 O(ad)+H2 (g)) is the rate controlling step. When Pd becomes a partly oxidized state in the catalysts, an electron is withdrawn from the methoxy group to Pdδ+ and the C–H bond of methoxy group will be weakened, resulting in the acceleration of the abstraction of C–H bond [16]. In Table 4, the content of Pd on the catalyst surface was greater than the theoretical value, and the dispersion of palladium on the surface of Pd-Co/Ce0.8 Zr0.2 O2 (0.94%) was higher than that of Pd/Ce0.8 Zr0.2 O2 (0.74%). This may be due to the impregnation method used in the experiment.
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In the preparation process, the impregnated Co firstly occupied a part of the channel and then diffused into the depths, which can influence Pd entering into the pore. No same peaks were observed in the XRD patterns of the samples after reaction, suggesting that palladium particles are highly dispersed. The chemical bonding such as Pd–O–Ce and Pd–Co can be
formed in the interface of palladium, resulting in the high activity of Pd-Co/Ce0.8 Zr0.2O2 . Therefore, it can be speculated that methanol decomposition reaction is related to the number of active centers and the chemical states, and Pdδ+ is more beneficial than Pd2+ and Pd0 for methanol decomposition to H2 and CO.
Table 4. Data from XPS analysis of reduced catalysts Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2 a
EB (eV) Pd 3d5/2 336.8 335.9
Co 2p3/2 – 780.8
Pd 0.74(0.7)a 0.94(0.6)a
– 786.0
Surface concentration (%) Co Co/Ce – – 1.46(3.09)a 15.57(12.24)a
Theoretical atomic percentage of each atom
a distance of 5−6 eV. Co3 O4 has a weak satellite peak at 10−11 eV, and Co0 does not have a satellite peak. Co 2p3/2 binding energy at 780.8 eV and a satellite peak at 786.5 eV were observed, indicating that the state of Co in Pd-Co/Ce0.8Zr0.2 O2 is Co2+ . This result shows that Co3+ in Pd-Co/Ce0.8Zr0.2 O2 can be reduced to Co2+ which promotes the oxidation of Pd to Pdδ+ species because of the interaction between Pd and Co. They are conducive to methanol decomposition to H2 and CO. 4. Conclusions
Figure 5. XPS spectra of Zr 3p and Pd 3d regions for reduced catalysts
The catalytic activities of Pd/Ce0.8 Zr0.2 O2 and PdCo/Ce0.8 Zr0.2 O2 prepared by impregnation method are mainly dependent on the dispersion of palladium on the catalyst surface and the numbers of acid sites, Co2+ and Pdδ+ species. The catalysts with a small particle size, Co2+ and Pdδ+ species are more efficient in methanol decomposition. Co as a second metal to Pd/CeO2 -ZrO2 catalyst considerably improves the activity of methanol decomposition. Acknowledgements This work was in part supported by the National Natural Science Foundation of China (No. 21173153) and Sichuan Province Science & Technology Support Program (2012FZ0008).
References
Figure 6. XPS spectra of Co 2p3/2 region for reduced Pd-Co/Ce0.8 Zr0.2 O2 catalyst
As shown in Figure 6, Co 2p3/2 binding energies and satellite peak size changed because of Co chemical state. According to the literatures [17,18], Co 2p3/2 binding energy ranges of Co0 , CoO, Co(OH)2 and Co3 O4 are 777.5−778.3, 779.7−781.7, 780.7−781.1 and 779.6−780.6 eV, respectively. CoO and Co(OH)2 has a strong satellite peak at
[1] Vancoillie J, Demuynck J, Sileghem L, Van de Ginste M, Verhelst S. Int J Hydrog Energy, 2012, 37(12): 9914 [2] Cheng W H, Shiau C Y, Liu T H, Tung H L, Lu J F, Hsu C C. Appl Catal A, 1998, 170(2): 215 [3] Usami Y, Kagawa K, Kawazoe M, Matsumura Y, Sakurai H, Haruta M. Appl Catal A, 1998, 171(1): 123 [4] Zhao M, Li X, Zhang L H, Zhang C, Gong M C, Chen Y Q. Catal Today, 2011, 175(1): 430 [5] Li X, Wang X W, Zhao M, Liu J Y, Gong M C, Chen Y Q. Chin J Catal (Cuihua Xuebao), 2011, 32(11): 1739 [6] Wang H R, Chen Y Q, Zhang Q L, Zhu Q C, Gong M C, Zhao M. J Nat Gas Chem, 2009, 18(2): 211 [7] Yoshida H, Yazawa Y, Hattori T. Catal Today, 2003, 87(1-4): 19 [8] Borchert H, J¨urgens B, Nowitzki T, Behrend P, Borchert Y, Zielasek V, Giorgio S, Henry C R, B¨aumer M. J Catal, 2008,
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256(1): 24 [9] Kapoor M P, Raj A, Matsumura Y. Microporous Mesoporous Mater, 2001, 44: 565 [10] Bensalem A, Bozon-Verduraz F, Perrichon V. J Chem Soc Faraday Trans, 1995, 91(14): 2185 [11] Wang L, Li D L, Koike M, Watanabe H, Xu Y, Nakagawa Y, Tomishige K. Fuel, 2013, 112: 654 [12] Liotta L F, Ousmane M, Di Carlo G, Pantaleo G, Deganello G, Boreave A, Giroir-Fendler A. Catal Lett, 2009, 127(3-4): 270 [13] Liotta L F, Ousmane M, Di Carlo G, Pantaleo G, Deganello G, Marci G, Retailleau L, Giroir-Fendler A. Appl Catal A, 2008,
347(1): 81 [14] Lin S S Y, Kim D H, Engelhard M H, Ha S Y. J Catal, 2010, 273(2): 229 [15] Vincent Crist B. Handbook of Monochromatic XPS Spectra. Kawasaki: XPS International, Inc, 1999 [16] Sun K P, Lu W W, Wang M, Xu X L. Appl Catal A, 2004, 268(12): 107 [17] Hagelin-Weaver H A E, Hagelin W, Hoflund G B, Minahan D M, Salaita G N. Appl Surf Sci, 2004, 235(4): 420 [18] Ernst B, Hilaire L, Kiennemann A. Catal Today, 1999, 50(2): 413
Co-modified Pd/CeO2-ZrO2 catalysts for methanol decomposition Ming Zhaoa,b ,
Hailong Zhanga ,
Xue Lia , Yaoqiang Chena∗
a. College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China; b. Sichuan Provincial Environmental Protection Environmental Catalytic Materials Engineering Technology Center, Chengdu 610065, Sichuan, China [ Manuscript received March 19, 2014; revised May 6, 2014 ]
Abstract Pd/Ce0.8 Zr0.2 O2 catalysts modified by cobalt were prepared by a sequential impregnation method and characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption (Brunauer-Emmet-Teller), oxygen storage capacity (OSC), CO-chemisorption, H2 -temperature-programmed reduction (H2 -TPR) and X-ray photoelectron spectroscopy (XPS). The effect of Co on the performance of methanol decomposition was evaluated at a fixed-bed microreactor. The results showed that the addition of Co can improve the oxygen storage capacity of the catalyst and the dispersion of Pd. XPS results indicated that Pd was in a partly oxidized (Pdδ+ , 1<δ<2) state and Co2+ was present in Pd catalysts modified by Co. A 90% conversion of methanol was achieved at around 280 ◦ C over Pd-Co/Ce0.8 Zr0.2 O2 catalyst which was 20 ◦ C lower than that over Pd/Ce0.8 Zr0.2 O2 , indicating that both Pdδ+ and Co2+ play an important role in improving the catalytic activity of methanol decomposition. Key words palladium; cobalt; methanol decomposition; CeO2 -ZrO2
1. Introduction In order to solve the energy crisis and environmental pollution problems, methanol is considered as an alternative fuel for automobiles. The decomposition of methanol into H2 and CO has received attention as a method to increase fuel efficiency for methanol powered vehicles [1]. At cold start, the methanol decomposition catalysts need to be active at low temperatures. The activity and stability are two major challenges in methanol decomposition reaction. Recently, an attempt has been made to tackle this problem and to further improve the catalytic activity by the addition of metal ions. In the past decades, many metals such as Cu, Ni and Mn have been extensively investigated. However, the catalysts still suffer a slow deactivation, and a by-product is usually observed [2]. Noble metal-containing catalysts, especially the Pd-containing catalysts, show high activity and selectivity for methanol decomposition at low temperature and the activity is largely affected by the supports [3,4]. Cex Zr1−xO2 solid solution is a good support for noble metals in methanol decomposition [5,6]. The catalytic property can be modified by additives, however, the effects of additives seem to be less systematically investigated. Generally, it is recognized that electron density of the metal is increased by the addition of electropho-
bic alkaline and alkaline earth groups. Palladium exists more or less in oxidative state compared with original Pt/Al2 O3 because of electronegativity of additives, and the oxidative state of palladium can be maintained by the electrophilic property of additive [7]. Bimetallic Co-Pd nanoparticles supported on MgO have been investigated in methanol decomposition [8]. In this work, Pd-based catalysts can be influenced by adding Co as a second metal to the system in methanol decomposition. Special attention is paid to the metal-support interaction and the electronic surroundings of Pd sites during the catalytic process. 2. Experimental 2.1. Catalyst preparation Ce0.8 Zr0.2 O2 samples were prepared by co-precipitation method from the corresponding chemicals: Ce(NO3 )3 ·6H2 O, ZrOCO3 at a nominal composition. The precursors were mixed in an aqueous solution respectively, and an appropriate amount of fresh H2 O2 was added. Then the mixed salt solution was added dropwise into a mixed aqueous solution of ammonia to reach a pH value of 9.0 in stirring. Finally the
Corresponding author. Tel: +86-028-85418451; Fax: +86-028-85418451; E-mail: [email protected]; [email protected] This work was supported by the National Natural Science Foundation of China (No. 21173153) and Sichuan Province Science and Technology Support Projects(2012FZ0008). ∗
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60209-6
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precipitates were filtered, thoroughly washed, dried and calcined at 600 ◦ C for 4 h to achieve the Ce0.8 Zr0.2 O2 powder. The Ce0.8 Zr0.2 O2 powders were impregnated with the solution of Co(NO3 )2 with CoO2 content of 0.5 wt%. Afterwards, the materials were dried at 105 ◦ C overnight and calcined at 500 ◦ C for 3 h in air to achieve the Co/Ce0.8 Zr0.2 O2 powder. The prepared Ce0.8 Zr0.2 O2 and Co/Ce0.8 Zr0.2 O2 powders were impregnated with the solution of Pd (NO3 )2 . Afterwards, the materials were dried at 105 ◦ C overnight and calcined at 500 ◦ C for 3 h in air, mixed with some water, ground and finally formed slurry. The resulting slurry was coated on a honeycomb cordierite (1.5 cm3 , Corning, America) and the excessive slurry was blown away with compressed air, then dried and calcined at 500 ◦ C for 3 h in air. Finally, a monolithic catalyst with 1.0 wt% Pd was obtained. 2.2. Determination of methanol decomposition activity The reaction of gas phase methanol decomposition was performed in a fixed-bed continuous flow reactor operating under atmospheric pressure. The catalysts were placed in a conventional tubular quartz reactor. Reduction of the catalysts was performed in a flow of 1.8 dm3 /h 5% H2 in N2 at 400 ◦ C for 1 h before catalytic test. Then methanol diluted with argon (MeOH, 15%; GHSV, 3400 h−1 ) was fed at 160−300 ◦ C. The outlet reaction gas was analyzed with an on line gas chromatograph equipped with a TDX-1 column (2 m) and Porapak-Q column (2 m). Column switching was applied to optimize the separation of H2 , CO, CH4 and CO2 as well as CH3 OH, CH3 OCH3 and HCOOCH3 . 2.3. Catalyst characterization The BET specific surface area and pore size measurement were performed on an automated surface area and pore size analyzer (Quantachrome). The BET specific surface area was determined by a pure N2 adsorption-desorption technique at liquid nitrogen temperature. Prior to the measurements, the samples were degassed for 1 h at 350 ◦ C under vacuum condition. X-ray diffraction (XRD) patterns were obtained on a D/MAX-Ra rotatory diffractometer, using Cu Kα radiation (λ = 0.1541 nm), 50 kV and 180 mA. Samples were scanned from 2θ equal to 20o up to 90o . The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a spectrometer (XSAM-800, KRATOS Co) with Mg Kα radiation under UHV. All the samples were pretreated in a stream of 5% H2 -95% N2 at 400 ◦ C for 1 h, and cooled to room temperature. The binding energy was determined by reference to C 1s binding energy of 284.8 eV. Temperature-programmed reduction (H2 -TPR) experiments were carried out in a conventional system equipped with a thermal conductivity detector (TCD). All the samples (100 mg) were pretreated in a quartz U-tube in a flow of pure
N2 at 400 ◦ C for 45 min, then cooled. The reduction reaction was carried out in a flow of 5% H2 in N2 from 50 ◦ C to 900 ◦ C with a linear heating rate of 8 ◦ C/min. Oxygen storage capacity (OSC) was estimated as follows. A certain amount of catalysts were heated in a quartz reactor in a flow of 40 mL/min H2 at 550 ◦ C for 45 min. Then the catalysts were switched to heat in a flow of 20 mL/min N2 at 200 ◦ C and iteratively pulsed at a desired temperature until no more loss of O2 injected was detected. Pd dispersion was measured after reducing the sample (200 mg) at appropriate temperature (400 ◦ C) under 5% H2 95% N2 flow for 1 h. The remnant H2 was driven off with argon at 420 ◦ C for 30 min, then the sample was cooled down to room temperature under argon flow. Finally, pulse of CO was injected up to the breakthrough point. Pd dispersion was evaluated from CO consumption. 3. Results and discussion 3.1. Catalytic activity evaluation The catalytic performance of the prepared catalysts is shown in Figure 1. Compared with Pd/Ce0.8 Zr0.2 O2 , PdCo/Ce0.8 Zr0.2 O2 showed a higher conversion in methanol decomposition at the same reaction temperature. PdCo/Ce0.8 Zr0.2 O2 achieved a 70% conversion at 260 ◦ C which was higher than that of Pd/Ce0.8 Zr0.2O2 (50.3%). This could be attributed to the fact that Co additive can affect the chemical state of palladium and create more Co2+ and Pdδ+ species which are conducive to methanol decomposition into H2 and CO. At the same time, a little CH4 at different temperatures was detected. For example, 5.4% CH4 was detected at 260 ◦ C for Pd/Ce0.8 Zr0.2 O2 , but only 2.6% CH4 for PdCo/Ce0.8 Zr0.2 O2 , indicating that the selectivity of CH4 decreases when Co is added to catalysts. In other word, the addition of Co favors the formation of H2 and CO. After used at 320 ◦ C for 72 h, Pd-Co/Ce0.8 Zr0.2 O2 still maintained high activity and 80% conversion, indicating that this catalyst has good stability.
Figure 1. Conversion of methanol over two catalysts at different temperatures
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3.2. Specif ic surface area measurements The BET surface area and pore volume of catalysts are shown in Table 1. Pd/Ce0.8 Zr0.2 O2 had 62.0 m2 /g specific surface area but Pd-Co/Ce0.8Zr0.2 O decreased slightly to 58.0 m2 /g. It may be attributed to that the cobalt oxides cover the catalyst surface and block the partial pores. On the other hand, for Pd-Co/Ce0.8 Zr0.2 O2 , there was one more impregnation and calcination step, leading to a slight decrease of specific surface area. However, Pd-Co/Ce0.8 Zr0.2 O2 had higher activity in methanol decomposition. This indicates that the surface area is not the main factor to affect the catalytic activity. There is a weak correlation between methanol decomposition and surface area. Then we inferred that a new active center may be presence. Table 1. Textural properties of catalysts Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2
SBET (m2 /g) 62.0 58.0
Pore volume (cm3 /g) 0.10 0.08
3.3. Oxygen storage capacity and Pd dispersion The Pd dispersion and particle size estimated from CO chemisorption as well as the oxygen storage capacity of the two catalysts are listed in Table 2. The OSC of Pd/Ce0.8 Zr0.2 O2 was 437 µmol/g but that of PdCo/Ce0.8 Zr0.2 O2 increased to 717 µmol/g, indicating that Co addition greatly promotes the oxygen transfer from bulk to support surface and from support surface to Pd particles [9] helping Pd with more active Pdδ+ state. They can promote the decomposition of methanol because the cleavage of methanol involves the oxygen transfer and C–H break. Generally, the dispersion of active ingredient is very important for the catalyst performance. As shown in Table 2, Pd-Co/Ce0.8 Zr0.2O2 catalyst possessed 72% of Pd dispersion which was higher than that of Pd/Ce0.8 Zr0.2 O2 (53%), and Pd-Co/Ce0.8 Zr0.2 O2 catalyst formed smaller Pd particles (1.28 nm) than Pd/Ce0.8 Zr0.2O2 (1.73 nm) catalyst, implying that more palladium exposed on the surface. This is resultant in good catalytic activity of methanol decomposition (Figure 1). Based on the above analysis, one can conclude that Co can enhance the OSC of Pd/Ce0.8 Zr0.2 O2 and Pd dispersion, which play an important role in methanol decomposition reaction.
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which is generally belonged to the reduction of PdO involving Ce0.8 Zr0.2O2 . When Co was added to Pd/Ce0.8 Zr0.2O2 , the reduction temperature of PdO shifted to high temperature and the area of reduction peak became larger compared with Pd/Ce0.8 Zr0.2 O2 . This phenomenon may derive from the interaction between Co oxides and Ce0.8 Zr0.2 O2 as well as Co oxides and Pd oxide species, and the presence of Co oxides in a catalyst would inhibit the growth of noble metal and easily form smaller palladium particles, resulting in the decrease of reduction temperature and the increase of reduction area. At the same time, the introduction of Co oxide promotes the oxygen transfer and mobility, which can facilitate the reduction of CeO2 and maintain Pd with more cationic Pdδ+ state [10]. Pdδ+ ions are possibly formed through Ce4+ +Pd0 →Ce3+ +Pdδ+ . When palladium is at a semi-oxidized state, it will lead to the decrease of reduction temperature and reduction area. The peak at 390 ◦ C of Pd-Co/Ce0.8 Zr0.2O2 is attributed to the simultaneous reduction of Co and Ce oxides [11], indicating that there are strong Pd–Co and Co–Ce interactions. The strong interaction greatly influences the reduction behavior, causing that the reduction of Co3 O4 to CoO slightly shifts to higher temperature and that of surface ceria shifts to lower temperature [12−14]. The unreduced cobalt species (mainly Co2+ ) of PdCo/Ce0.8 Zr0.2 O2 could interact with CeO2 -ZrO2 or Pd and create new active sites. Therefore, we can speculate that both Co2+ and Pd&+ are the active sites for methanol decomposition which favor the decomposition of methanol to CO and H2 .
Table 2. Oxygen storage capacity and Pd dispersion of catalysts Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2
Pd dispersion (%) 53.70 72.61
Pd particle size (nm) 1.73 1.28
OSC (µmol/g) 473 717
Figure 2. H2 -TPR profiles of two catalysts
3.5. NH 3 -TPD measurements 3.4. H 2 -TPR measurements TPR profiles of the two catalysts are shown in Figure 2. Both of them had the sharp reduction peaks at about 100 ◦ C,
NH3 -TPD profiles of Pd-Co/Ce0.8Zr0.2 O2 and Pd/Ce0.8 Zr0.2 O2 are shown in Figure 3. Two catalysts had different NH3 desorption peaks. The peak at low temperature (LT<210 ◦ C region) is assigned to the desorption of
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ammonia, which is bound via Pdδ+ bonds to NH3 . For PdCo/Ce0.8 Zr0.2 O2 , the desorption diagram showed two well separated peaks at 181 and 226 ◦ C. The middle temperature (about 226 ◦ C) region is assigned to the free electron pair orbital of nitrogen with the empty d2z orbital of Co2+ , Ce3+ and Zr4+ . In a word, Pd-Co/Ce0.8 Zr0.2O2 catalyst possesses more acid sites than Pd/Ce0.8 Zr0.2 O2 , which is in favor of methanol decomposition into CO and H2 because acid sites can promote the adsorption of methanol on the catalyst surface.
Figure 4. XRD patterns of two catalysts
3.7. XPS measurements
Figure 3. NH3 -TPD profiles of two catalysts
3.6. XRD measurements XRD patterns of the samples at 550 ◦ C for 3 h are shown in Figure 4. No diffraction peaks of palladium (2θ = 40.0o ) and PdO (2θ = 34.0o) or PdO2 (2θ = 54.6o ) were observed, indicating that Pd species is highly dispersed on the surface of carriers. The high dispersion of Pd species improved the catalytic activity, which was coincided with the results of the activity testing. The major peak of the XRD patterns was consistent with the characteristic peaks of cubic CeO2 , indicating the formation of CeO2 -ZrO2 solid solutions. There were no CoOx peaks, implying that Cox+ enters into CeO2 lattice or well disperses on the surface of the supports. The lattice parameters of Pd-Co/Ce0.8 Zr0.2 O2 were similar to those of Pd/Ce0.8 Zr0.2 O2 , revealing that small part of Co cations are incorporated into CeO2 -ZrO2 lattice and more are finely dispersed on the surface of the solid solution. The former cause larger particle size of CeO2 -ZrO2 , and the latter can decrease the specific surface area (Table 3). This was consistent with the results of TPR and BET. Table 3. Crystal cell parameter and crystalline size Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2
Crystalloid parameter (nm) 0.5347 0. 5345
Particle size (nm) 6.2 6.5
In order to investigate the effect of Co on Pd-based catalysts, XPS was used to characterize the electronic state of Pd and the surface composition of catalysts. As shown in Figure 5, Pd 3d5/2 of the two catalysts only had a relatively symmetrical peak, and no shoulder peak appeared, showing that only a palladium species exists on the catalyst surface. For Pd/Ce0.8 Zr0.2 O2 , the peak of Pd 3d5/2 appeared at 336.8 eV, suggesting that the palladium on the surface is Pd2+ . The binding energy of Pd 3d5/2 for Pd-Co/Ce0.8 Zr0.2 O2 catalyst was 335.9 eV. According to the standard manual [15], the binding energies of Pd 3d5/2 for Pd0 and PdO are 335.0 and 340.0 eV, respectively. This may be inferred that the palladium in Pd-Co/Ce0.8Zr0.2 O2 catalyst is a partly oxidized state. The valence of Pd may be between 0 and +2 in PdCo/Ce0.8 Zr0.2 O2 catalyst. Compared with Pd/Ce0.8 Zr0.2O2 , the doping of CoOx made Pd 3d5/2 peak shift to lower binding energy by 0.9 eV, indicating that the addition of Co affects the chemical state of palladium, which may be due to the interaction between Pd and Co. On the other hand, the electronegativity of Co and Pd is 1.8 and 2.2, respectively, and the electrons of Co move around Pd which can increase the electron density around Pd. The strong interaction between palladium particles and support causes the transfer of electron from palladium to ceria. Generally, the partly oxidized state of Pd is more active than Pd0 and Pd2+ for methanol decomposition. The abstraction of C–H bond in the adsorbed methoxy group CH3 O (CH3 O(ad)+H(ad) = CH2 O(ad)+H2 (g)) is the rate controlling step. When Pd becomes a partly oxidized state in the catalysts, an electron is withdrawn from the methoxy group to Pdδ+ and the C–H bond of methoxy group will be weakened, resulting in the acceleration of the abstraction of C–H bond [16]. In Table 4, the content of Pd on the catalyst surface was greater than the theoretical value, and the dispersion of palladium on the surface of Pd-Co/Ce0.8 Zr0.2 O2 (0.94%) was higher than that of Pd/Ce0.8 Zr0.2 O2 (0.74%). This may be due to the impregnation method used in the experiment.
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In the preparation process, the impregnated Co firstly occupied a part of the channel and then diffused into the depths, which can influence Pd entering into the pore. No same peaks were observed in the XRD patterns of the samples after reaction, suggesting that palladium particles are highly dispersed. The chemical bonding such as Pd–O–Ce and Pd–Co can be
formed in the interface of palladium, resulting in the high activity of Pd-Co/Ce0.8 Zr0.2O2 . Therefore, it can be speculated that methanol decomposition reaction is related to the number of active centers and the chemical states, and Pdδ+ is more beneficial than Pd2+ and Pd0 for methanol decomposition to H2 and CO.
Table 4. Data from XPS analysis of reduced catalysts Catalysts Pd/Ce0.8 Zr0.2 O2 Pd-Co/Ce0.8 Zr0.2 O2 a
EB (eV) Pd 3d5/2 336.8 335.9
Co 2p3/2 – 780.8
Pd 0.74(0.7)a 0.94(0.6)a
– 786.0
Surface concentration (%) Co Co/Ce – – 1.46(3.09)a 15.57(12.24)a
Theoretical atomic percentage of each atom
a distance of 5−6 eV. Co3 O4 has a weak satellite peak at 10−11 eV, and Co0 does not have a satellite peak. Co 2p3/2 binding energy at 780.8 eV and a satellite peak at 786.5 eV were observed, indicating that the state of Co in Pd-Co/Ce0.8Zr0.2 O2 is Co2+ . This result shows that Co3+ in Pd-Co/Ce0.8Zr0.2 O2 can be reduced to Co2+ which promotes the oxidation of Pd to Pdδ+ species because of the interaction between Pd and Co. They are conducive to methanol decomposition to H2 and CO. 4. Conclusions
Figure 5. XPS spectra of Zr 3p and Pd 3d regions for reduced catalysts
The catalytic activities of Pd/Ce0.8 Zr0.2 O2 and PdCo/Ce0.8 Zr0.2 O2 prepared by impregnation method are mainly dependent on the dispersion of palladium on the catalyst surface and the numbers of acid sites, Co2+ and Pdδ+ species. The catalysts with a small particle size, Co2+ and Pdδ+ species are more efficient in methanol decomposition. Co as a second metal to Pd/CeO2 -ZrO2 catalyst considerably improves the activity of methanol decomposition. Acknowledgements This work was in part supported by the National Natural Science Foundation of China (No. 21173153) and Sichuan Province Science & Technology Support Program (2012FZ0008).
References
Figure 6. XPS spectra of Co 2p3/2 region for reduced Pd-Co/Ce0.8 Zr0.2 O2 catalyst
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