Ce

10308

J. Phys. Chem. C 2008, 112, 10308–10315

Mechanism of Suppression of Carbon Deposition on the Pd-Ni/Ce(Sm)O2-La(Sr)CrO3 Anode in Dry CH4 Fuel Yuta Nabae,† Ichiro Yamanaka,*,† Masaharu Hatano,‡ and Kiyoshi Otsuka† Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan, and Technology Research Laboratory No. 1, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka-shi, Kanagawa 237-8523, Japan ReceiVed: February 19, 2008; ReVised Manuscript ReceiVed: April 10, 2008

Catalytic properties of Pd-Ni catalysts were studied to clarify the mechanism of suppression of carbon deposition on a Pd-Ni catalyst supported on a composite of Ce(Sm)O2 and La(Sr)CrO3, which is a promising anode material for direct oxidation of dry CH4 in solid oxide fuel cells (SOFCs). The catalytic activity of Pd-Ni catalysts for the carbon formation in the decomposition of CH4 was found to be strongly dependent on the support materials. Carbon deposition was remarkably suppressed on Pd-Ni/Ce(Sm)O2 and Pd-Ni/ La(Sr)CrO3 catalysts, whereas Pd-Ni/SiO2 and Pd-Ni/carbon catalysts were very active for the formation of carbon nanofibers. TEM analysis showed the morphology of the Pd-Ni particles and carbon deposit on the Pd-Ni/Ce(Sm)O2 catalyst was quite different from that on the Pd-Ni/C catalyst. XRD and TEM-EDS analysis indicate that Ce and Sm were doped into the Pd-Ni alloy from the Ce(Sm)O2 supports. High catalytic activity of the Pd-Ni/Ce(Sm)O2 catalyst for steam reforming indicates that the Pd-Ni catalysts modified with Ce still have high catalytic activity for the activation of CH4. A model for the mechanism of the suppression of the carbon deposition on the Pd-Ni catalyst is proposed focusing on the formation mechanism of the carbon deposit. The high catalytic activity of the Pd-Ni alloy for the activation of CH4 and the high tolerance to carbon deposition deriving from the doping are suitable for direct utilization of dry CH4 over SOFC anodes. Introduction Direct conversion of hydrocarbon fuels at the anodes in solid oxide fuel cells (SOFCs) has received a great deal of attention for efficient conversion of chemical energy to electric power. In particular, direct utilization of dry hydrocarbons is preferred to internal steam reforming, because of its high energy conversion efficiency and low operational costs.1,2 One major problem of this method is significant carbon deposition on the anodes. The Ni-Zr(Y)O2 cermet anode, which is a composite of Ni metal and yttria-stabilized zirconia (Zr(Y)O2), is the most common in SOFCs with H2 as a fuel.2 The large percentage of Ni helps the electrode to function as both an electric conductor and a catalyst for the oxidation of H2. However, it is not suitable for direct utilization of dry hydrocarbon fuels because Ni catalyzes the decomposition of hydrocarbons and causes serious carbon deposition.3–5 Despite this, direct utilization of dry methane has been performed with some Ni cermet anodes under limited reaction conditions. Ni cermet anodes can be used continuously for the oxidation of dry CH4 with high current density, which supplies sufficient O2- to the anode.6–8 However, the cells with the large percentage of Ni are easily damaged by the rapid carbon deposition at open-circuit condition.8,9 Cu cermet anodes10 and oxide-based anodes11 are more stable in dry hydrocarbon fuels, but their oxidation activity is not sufficient. The addition of small amounts of catalysts such as Rh,12 Ni,13 and Pd14 to these anodes is effective to increase the oxidation rate of dry hydrocarbons and current density. * Correspondingauthor.Phone/fax: +81-3-5724-2144.E-mail:yamanaka@ apc.titech.ac.jp. † Tokyo Institute of Technology. ‡ Nissan Research Center, Nissan Motor Co., Ltd., 1.

Recently, we have found that a Pd-Ni alloy catalyst supported on an oxide composite anode (La(Sr)CrO3 and Ce(Sm)O2) shows good performance with dry CH4 fuel at 1073-1173 K.15 Low loadings of Pd (11.7 µmol cm-2, 9 wt %) and Ni (11.7 µmol cm-2, 5 wt %) are a characteristic of this anode. La(Sr)CrO3 is responsible for electronic conduction and Ce(Sm)O2 is for ionic conduction.13 A strong synergy of Pd and Ni was observed in the electrochemical oxidation of CH4. The Pd-Ni/La(Sr)CrO3-Ce(Sm)O2 anode shows high tolerance to carbon deposition even at open-circuit condition.16 When the circuit was open, the catalytic decomposition of CH4 immediately stopped on the Pd-Ni/La(Sr)CrO3-Ce(Sm)O2 anode and there was no damage to the cell, whereas serious carbon deposition had been reported on some Ni cermet anodes.8,9 This high tolerance of the Pd-Ni/La(Sr)CrO3Ce(Sm)O2 anode to carbon deposition was unexpected, as Pd-Ni alloy catalysts had been reported as very active catalysts for the decomposition of CH4 to carbon nanofibers and H2.17 These results suggest that the Pd-Ni alloy on the La(Sr)CrO3-Ce(Sm)O2 support has a significant catalytic activity for the activation of CH4 but does not catalyze any serious carbon deposition. The purpose of this study is to explain clearly the high tolerance of the Pd-Ni/La(Sr)CrO3-Ce(Sm)O2 anode to carbon deposition. We herein report significant effects of the catalyst supports on the carbon deposition in the decomposition of CH4. A model for the mechanism of the suppression of carbon deposition is proposed based on the performance test and characterization of the Pd-Ni catalysts. The effect of the oxide materials such as cerium oxide in direct hydrocarbon SOFCs has been reported already.18,19 This effect has been thought of as the catalytic enhancement by the oxide materials in the oxidation of hydrocarbons or carbon spices. This paper, in

10.1021/jp801496v CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

Suppression of C Deposition in SOFCs

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10309

contrast, focuses on the mechanism of the formation of carbon deposit. Understanding the suppression of carbon deposition with active metal catalysts such as Pd and Ni would certainly contribute to the development of active anode materials for direct hydrocarbon SOFCs. Experimental Section Catalyst Preparation. The catalyst supports used in this work and their specific surface areas are as follows: SiO2 (Cab-OSil, 210 m2 g-1, Cabot Co.), carbon (Graphitized Carbon Fiber, 10 m2 g-1, Asahi Kasei Co.), Zr(Y)O2 ((YO1.5)0.08(ZrO2)0.92, 6 m2 g-1, Toso Co.), Ce(Sm)O2 (Ce0.8Sm0.2O1.9, 12 m2 g-1, Toshima Mfg. Co), and La(Sr)CrO3 (La0.8Sr0.2CrO3, 2 m2 g-1, Seimi Chemical Co). Pd-Ni catalysts were loaded on these supports by the conventional impregnation method. The catalyst support was dispersed in the aqueous solutions of (NH3)4PdCl2 · H2O (Aldrich Co.) and Ni(NO3)2 · 6H2O (Waco Co.). The mixture was stirred for several hours and then dried at 373 K. The powder of the catalyst precursor was reduced with H2 for 1 h at 573 K and was annealed in Ar for 3 h at 923 K. The annealing process enhances the formation of Pd-Ni alloy. The loadings of Pd and Ni were 850 µmol g-1 · catal., corresponding to 9 wt % of Pd and 5 wt % of Ni. Pd-Ni-M/Zr(Y)O2 and Pd-Ni-M/carbon catalysts were prepared by the coimpregnation method (M ) Zr, Y, Cr, Sr, La, Ce). ZrO(NO3)2 · 2H2O (Wako Co.), Y(NO3)3 · 6H2O (Soekawa Co.), Cr(NO3)3 · 9H2O (Wako Co.), Sr(NO3)2 (Wako Co.), La(NO3)3 · 9H2O (Soekawa Co.), or Ce(NO3)3 · 6H2O (Wako Co.) were added to the aqueous solutions of (NH3)4PdCl2 · H2O and Ni(NO3)2 · 6H2O. The impregnation and reduction procedures were the same as those for the Pd-Ni catalysts. The standard ratio of Pd, Ni, and the additive was Pd:Ni:M ) 10:10:1. Decomposition of CH4. Methane decomposition was carried out by using a flat-bottomed quartz reactor (diameter: 3 cm; length: 20 cm) and a conventional gas flow system under atmospheric pressure. Water vapor in the methane gas was removed with a cold trap (dry ice and ethanol, 203 K) placed before the reactor. The catalyst (100 mg) was placed on the flat bottom of the reactor. The catalyst was reduced with H2 (34 kPa, balanced with He) for 30 min at 573 K, and the reactor temperature was raised to 1073 K supplying He. The CH4 decomposition was started by introducing dry CH4 (101 kPa, 50 mL min-1) into the reactor at 1073 K and continued for 2 h. A molecule of CH4 generally generates two molecules of H2 and an atom of C:

CH4(g) f 2H2 + C(s)

(1)

The formation rate of H2 was determined by using a gas chromatograph (GC-8A, Shimadzu) with a thermal conductivity detector (TCD), an active carbon column (3φ × 2 m), and Ar carrier gas. The yield of the carbon deposit was evaluated by two methods: one involves the integration of the formation rate of H2, and the other involves the measurement of the powder weights before and after the decomposition. Steam Reforming of CH4. Steam reforming of CH4 was carried out in a conventional tubular quartz reactor (diameter: 2 cm) under atmospheric pressure. The catalyst (100 mg) was packed into the catalyst bed with quartz sand and quartz wool. The catalyst was reduced with H2 (34 kPa, balanced with He) for 30 min at 573 K, and the reactor temperature was raised to 1073 K supplying He. The steam reforming of CH4 was started by introducing a mixture of CH4 (50.7 kPa) and H2O (50.7 kPa) at 1073 K. The total flow rate was 100 mL min-1. The steam was supplied using a microfeeder (JP-S, Furue Science Co.)

Figure 1. XRD patterns of fresh (a) Pd-Ni/SiO2, (b) Pd-Ni/carbon, (c) Pd-Ni/Zr(Y)O2, (d) Pd-Ni/Ce(Sm)O2, and (e) Pd-Ni/La(Sr)CrO3 catalysts. Pd and Ni loadings: 850 µmol g-1 · catal.

and a vaporizer at 423 K. The formation rates of CO and CO2 were determined by an online gas chromatograph (Shimazu GC8A, TCD, He carrier gas) with two columns: Polapak Q (3φ × 2 m) for CO2 and AC (3φ × 2 m) for CO. H2 was analyzed by using another GC-8A with a TCD, Ar carrier gas, and an AC column. The deposition rate of carbon was evaluated from the material balance of the formation rates of CO, CO2, and H2:

r(C) ) [r(H2) - 3r(CO) - 4r(CO2)]/2

(2)

min-1)

Here, r is the formation rate (mol of each product. Characterization of the Catalyst. X-ray diffraction (XRD) analysis was performed with an X-ray diffractometer (RINT 2500V, Rigaku) with Cu KR radiation at room temperature. TEM images were obtained by using a field emission gun analytical transmission microscope (JEM-2010F, JEOL) equipped with a detector (Genesis, EDAX) for energy-dispersive X-ray spectroscopy (EDS). Results and Discussion CH4 Decomposition over the Pd-Ni Catalysts on Different Supports. To clarify how the supports affect the formation of carbon deposit, the decomposition of CH4 was performed with the Pd-Ni catalysts loaded on various supports: SiO2, carbon, Zr(Y)O2, La(Sr)CrO3, and Ce(Sm)O2. The Pd-Ni/SiO2 and Pd-Ni/carbon catalysts have been previously reported as very active catalysts for the decomposition of CH4 into multiwalled carbon nanotubes and pure H2.17 Zr(Y)O2 is a typical material for SOFC. Ce(Sm)O2 and La(Sr)CrO3 are the components of our composite anode, as mentioned in the Introduction.16 Figure 1 shows the XRD patterns in the range of 38-54° (2θ) for the fresh Pd-Ni catalysts supported on SiO2, carbon, Zr(Y)O2, Ce(Sm)O2, and La(Sr)CrO3. The Pd-Ni/SiO2 catalyst did not show any diffraction peaks assigned to Ni or Pd metals which should appear at 44.5°, 51.8°, 40.1°, and 46.7° respectively for Ni(111), Ni(200), Pd(111), and Pd(200). Instead, the diffraction peaks that can be assigned to Pd-Ni alloy were observed at 41.7° and 47.6°.20 The diffraction peaks of Pd-Ni alloy were also observed on Pd-Ni/carbon, Zr(Y)O2, Ce(Sm)O2, and La(Sr)CrO3, although the sharpness of the diffraction peaks

10310 J. Phys. Chem. C, Vol. 112, No. 27, 2008

Nabae et al. TABLE 1: Amounts of Carbon Deposit Evaluated from (a) the Formation Rates of H2 and (b) the Difference of Sample Weights before and after Decomposition of CH4 at 1073 Ka amounts of the deposited carbon/mg catalysts

(a) r(H2)

(b) weight

Pd-Ni/SiO2 Pd-Ni/carbon Pd-Ni/Zr(Y)O2 Pd-Ni/Ce(Sm)O2 Pd-Ni/La(Sr)CrO3

645.4 1784 1835 8.7 17.7

530.9 1683 1671

a

Figure 2. Formation rates of H2 in the CH4 decomposition over the Pd-Ni catalyst supported on (a) SiO2, (b) carbon, (c) Zr(Y)O2, (d) Ce(Sm)O2, and (e) La(Sr)CrO3 at 1073 K. The photographs show the catalysts before (left) and after (right) the reaction. Amount of catalyst: 100 mg. Pd and Ni loadings: 850 µmol g-1 · catal. CH4: 1 atm, 50 mL min-1.

differed from one another. The broad diffraction peaks indicate the wide distributions both of the Pd/Ni ratio and of the degree of crystallization. Other observed diffraction peaks can be assigned to each support. Figure 2 shows the time courses of the formation rates of H2 during the decomposition of CH4 with the supported Pd-Ni catalysts at 1073 K. The typical Pd-Ni/SiO2 catalyst showed a good formation rate of H2. The Pd-Ni/carbon catalyst showed a high and constant formation rate of H2 (2.5 mmol min-1), corresponding to 61% CH4 conversion. These results are in accordance with the report by Takenaka et al.17 The Pd-Ni/ Zr(Y)O2 catalyst showed as high catalytic activity as Pd-Ni/ carbon for the formation of H2. Photographs of the Pd-Ni catalysts (100 mg) before and after the CH4 decomposition are also shown in Figure 2. Significant amounts of carbon deposit were observed with the Pd-Ni/SiO2, Pd-Ni/carbon, and Pd-Ni/Zr(Y)O2 catalysts. As to the Pd-Ni/Ce(Sm)O2 and Pd-Ni/La(Sr)CrO3 catalysts, in contrast, the formation rates of H2 were very low at the beginning of the experiments, and immediately decreased to negligible rates. Any obvious volumetric change was not observed between the catalysts before and after the CH4 decomposition. Table 1 summarizes the amounts of carbon deposit after 2 h of CH4 decomposition. The column labeled a shows the values evaluated from the formation rates of H2 and the column labeled b shows those evaluated from the powder weights. As shown in the table, the two methods show almost identical results, although a small amount of hard carbon deposit could not be collected. The amounts of carbon deposit calculated from the formation rate of H2 are used for the rest of the discussion in this paper. The amount of the carbon deposit evaluated from the formation rate of H2 was 1.835 g/100 mg · cat. on the Pd-Ni/ Zr(Y)O2 catalyst. A lower amount of carbon deposition was

8.9

Reaction conditions are shown in the caption for Figure 2.

observed with the Pd-Ni/Ce(Sm)O2 (0.087 g/100 mg · cat.) and Pd-Ni/La(Sr)CrO3 (0.177) catalysts. The serious carbon deposition on the Pd-Ni/Zr(Y)O2 catalyst is not a surprising result as Pd-Ni catalysts have been known as active catalysts for the decomposition of CH4. If this catalyst is used as an anode material in a SOFC under dry hydrocarbon condition, it would result in serious damage for the cell because of the carbon deposition. This would be a similar phenomenon to what is observed for the conventional Ni-Zr(Y)O2 cermet anodes under dry hydrocarbon conditions.8 In contrast, the small formation of carbon deposit on the Pd-Ni/Ce(Sm)O2 and Pd-Ni/La(Sr)CrO3 catalysts corresponds to the high tolerance of the Pd-Ni/La(Sr)CrO3-Ce(Sm)O2 anode to carbon deposition.16 This catalytic property is suitable for direct utilization of dry hydrocarbon in SOFCs. It is interesting that two groups of catalysts showed completely different catalytic activities depending on the supports. It should be noted that the specific surface areas of the Zr(Y)O2 (6 m2 g-1), carbon (10 m2 g-1), Ce(Sm)O2 (12 m2 g-1), and La(Sr)CrO3 (2 m2 g-1) supports were not very different, except for the SiO2 (210 m2 g-1) support. This indicates that the chemical nature of the supports affect the catalysis of the Pd-Ni alloy for the decomposition of CH4. TEM Analysis of the Pd-Ni Catalysts after the CH4 Decomposition. The effects of the catalyst supports on the Pd-Ni particles and the carbon deposit were studied by TEM analysis after the decomposition of CH4. The Pd-Ni/carbon and Pd-Ni/Zr(Y)O2 catalysts after 2 h of CH4 decomposition were not suitable for the TEM analysis because the large amounts of carbon fully covered the Pd-Ni particles. The CH4 decomposition over the Pd-Ni/carbon and Pd-Ni/Zr(Y)O2 catalysts was, therefore, stopped after 8 min, and then the mixture of the catalyst and carbon was analyzed. Figures 3 and 4 show the TEM images of the Pd-Ni/carbon and Pd-Ni/Zr(Y)O2 catalysts after the CH4 decomposition for 8 min at 1073 K. A large number of carbon fibers with diameters of 10 to 1000 nm were observed. Multifaceted Pd-Ni particles were detached from the carbon and Zr(Y)O2 supports. Distorted and branched carbon fibers grew from the multifaceted Pd-Ni particles, which are in accordance with the reported studies,17.21 Figure 5 shows the TEM images of the Pd-Ni/Ce(Sm)O2 catalyst after the decomposition of CH4 for 2 h at 1073 K. A small amount of carbon deposit was observed around the Pd-Ni particles. The morphology of the Pd-Ni particles and the carbon was very different from that in Figures 3 and 4. There were many spherical Pd-Ni particles covered with thin carbon layers, and the particles were detached from the Ce(Sm)O2 support. To clarify the correlation between the particle size of the Pd-Ni alloy and the carbon formation activity, the particle sizes of the catalysts were measured on the TEM images. Figure 6 shows the distribution of the particle size of each catalyst. The


Figure 3. TEM images of Pd-Ni/carbon after 8 min of CH4 decomposition at 1073 K.

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10311

Figure 5. TEM images of Pd-Ni/Ce(Sm)O2 after 2 h of CH4 decomposition at 1073 K.

Figure 4. TEM images of Pd-Ni/Zr(Y)O2 after 8 min of CH4 decomposition at 1073 K.

number of particles counted for Pd-Ni/carbon, Pd-Ni/Zr(Y)O2, and Pd-Ni/Ce(Sm)O2 was respectively 61, 71, and 53. Most of the particle diameters were less than 200 nm in all three catalysts, although very large particles over 200 nm were observed in the Pd-Ni/carbon and Pd-Ni/Zr(Y)O2 catalysts. The average diameter of the Pd-Ni particles on the Pd-Ni/ carbon, Pd-Ni/Zr(Y)O2, and Pd-Ni/Ce(Sm)O2 catalysts was respectively 97.5, 138.2, and 53.1 nm. The dispersion of the Pd-Ni particle was in the order of Pd-Ni/Ce(Sm)O2 > Pd-Ni/ carbon > Pd-Ni/Zr(Y)O2. Indeed there is some difference among the dispersions of these three catalysts, but it does not seem to explain the significant difference in the catalytic activity for the decomposition of CH4. Many particles around 100 nm in the Pd-Ni/carbon and Pd-Ni/Zr(Y)O2 catalysts were found growing carbon nanofibers, while many with the same order of particle size in the Pd-Ni/Ce(Sm)O2 catalyst were found not growing any carbon nanofibers but were covered with thin carbon layers.

Figure 6. Distributions of the Pd-Ni particle size in (a) Pd-Ni/carbon, (b) Pd-Ni/Zr(Y)O2, and (c) Pd-Ni/Ce(Sm)O2 catalyts.

The morphology of the Pd-Ni particles and the carbon deposit seems more important rather than the dispersion of the catalyst to explain the different catalytic activities for the carbon formation. The Pd-Ni particles growing the carbon fibers (Figures 3 and 4) seem more faceted than those encapsulated by thin carbon layers (Figure 5). The faceted particle surface may be relevant to the growth of a particular plane on the crystal


Nabae et al.

Figure 7. Time courses of the formation rates of H2 in the decompositions of CH4 over the Pd-Ni-M/Zr(Y)O2 (M: Zr, Y, Cr, Sr, La, Ce) catalysts at 1073 K. Pd and Ni loadings: 850 µmol g-1 · catal. Catalyst: 100 mg. Total flow rate: 50 mL min-1.

of the Pd-Ni alloy. Some particular planes of the Pd-Ni alloy are known as the important surfaces for the continuous growth of the carbon nanofibers.20 CH4 Decomposition over the Pd-Ni-M/Zr(Y)O2 Catalysts. The similar particle sizes of the Pd-Ni catalysts showed very different catalytic activities for the CH4 decomposition depending on the catalyst supports: carbon and Zr(Y)O2 were active supports, whereas Ce(Sm)O2 and La(Sr)CrO3 were inactive. Note that the TEM analysis suggests that the Pd-Ni alloy particles were detached from the supports during the CH4 decomposition. The catalyst supports cannot affect the Pd-Ni alloy particles unless the catalyst particles contact the supports. We considered that the different catalytic activities were caused by a modification of the Pd-Ni particle before the CH4 decomposition. Some elements of the supports might have been doped into the Pd-Ni alloy during the catalyst preparation. Kepinski has reported that a Pd-Ce alloy was formed after the reduction of Pd/CeO2 at a high temperature and the alloy was inactive for the decomposition of ethylene.21 On the basis of our model, Pd-Ni-M/Zr(Y)O2 (M: Zr, Y, Cr, Sr, La, Ce) catalysts were prepared by the coimpregnation method and their catalytic activities for the decomposition of CH4 were investigated to clarify the effects of these elements. If these third elements can be doped into the Pd-Ni alloy and affect the catalysis, the carbon deposition would be suppressed. Figure 7 shows the time courses of the formation rate of H2 in the decomposition of CH4 by the Pd-Ni-M/Zr(Y)O2 catalysts at 1073 K. Note that the formation rate of H2 immediately decreased and H2 did not form after 10 min over the Pd-Ni-Sr/ Zr(Y)O2, Pd-Ni-La/Zr(Y)O2, and Pd-Ni-Ce/Zr(Y)O2 catalysts. Negligible amounts of carbon deposit were observed on these catalysts. The calculated carbon amounts for the Pd-Ni-Sr/ Zr(Y)O2, Pd-Ni-La/Zr(Y)O2, and Pd-Ni-Ce/Zr(Y)O2 catalysts were respectively 26.1, 4.1, and 13.5 mg. In contrast, the Pd-Ni-Zr/Zr(Y)O2, Pd-Ni-Y/Zr(Y)O2, and Pd-Ni-Cr/ Zr(Y)O2 catalysts showed high formation rates of H2, which were similar to those of the Pd-Ni/Zr(Y)O2 catalyst. The formation rates of H2 over the Pd-Ni-Y/Zr(Y)O2 and Pd-Ni-Cr/ Zr(Y)O2 catalysts decreased after 1 h. The amounts of carbon deposition on Pd-Ni/Zr(Y)O2, Pd-Ni-Zr/Zr(Y)O2, Pd-Ni-Y/ Zr(Y)O2, and Pd-Ni-Cr/Zr(Y)O2 were respectively 1784, 1722, 1630, and 1442 mg. These values are ca. 100 times larger than that on the Pd-Ni-Ce/Zr(Y)O2 catalyst. The Y or Cr addition to the Pd-Ni catalyst did not show any significant

Figure 8. XRD patterns of Pd-Ni-Ce/Zr(Y)O2. Pd:Ni:Ce ) (a) 10: 10:0, (b) 10:10:1, (c) 10:10:5, and (d) 10:10:10.

effect in suppressing the carbon deposition. These results clearly indicate that Sr, La, and Ce elements have specific roles in suppressing the carbon deposition on the Pd-Ni catalysts. Figure 8 shows the XRD patterns in the range of 38-54° (2θ) for the Pd-Ni-Ce/Zr(Y)O2 catalysts; the atomic ratios of Pd, Ni, and Ce are (a) 10:10:0, (b) 10:10:1, (c) 10:10:5, and (d) 10:10:10. The diffraction peaks of the Pd-Ni alloy at 41.7° and 47.6° (Figure 8a) shifted to lower angles and became broader by adding 10% of Ce relative to the Pd amount (Figure 8b). The diffraction peaks shifted to even lower angles and became even broader by adding more Ce (Figure 8c). These peak shifts to lower angles clearly indicate that the lattice parameter of the Pd-Ni alloy was increased by doping Ce, which has a larger atomic radius. When the Ce amount was equal to the amounts of Pd and Ni (Figure 8d), the diffraction peaks assigned to CeOCl were observed at 40.9°, 44.4°, 51.1°, and 51.8° (2θ), although any shift of the Pd-Ni alloy peak was not observed. This suggests that a small amount of Ce is able to be doped into the Pd-Ni alloy. CH4 Decomposition over the Pd-Ni-Ce/Carbon Catalyst. The TEM-EDS analysis for Pd-Ni/ Ce(Sm)O2 was attempted to determine the amount of Ce doped in the Pd-Ni alloy, but it was difficult because of the significant background in the Ce radiation by the Ce(Sm)O2 support. We therefore studied Pd-Ni-Ce/carbon catalyst to investigate the doping of Ce. The Pd-Ni-Ce/carbon catalyst was prepared by the coimpregnation method, and its catalytic activity for the CH4 decomposition was investigated. Figure 9 shows the time courses of the formation rates of H2 on the Pd-Ni/carbon and Pd-Ni-Ce/ carbon catalysts at 1073 K. As described above, the decomposition activity of the Pd-Ni/carbon catalyst was very high and showed a constant formation rate of H2. In contrast, Pd-Ni-Ce/ carbon showed very low catalytic activity for the decomposition of CH4. Any volumetric change was observed between the Pd-Ni-Ce/carbon catalysts before and after the reactions. The activity of the Pd-Ni-Ce/carbon catalyst for the decomposition of CH4 was as low as that of the Pd-Ni-Ce/Zr(Y)O2 catalyst. The effect of the Ce doping on the Pd-Ni catalyst was confirmed on the carbon support as well as the Zr(Y)O2 support. The components of the particles in the Pd-Ni-Ce/carbon catalyst were analyzed by TEM-EDS to determine the amount


J. Phys. Chem. C, Vol. 112, No. 27, 2008 10313 TABLE 2: Atomic Ratio of Pd, Ni, and Ce at Spots 1-8 on the Particles in the Pd-Ni-Ce/Carbon Catalsyt spot

atomic ratio/% Pd-L Ni-K

1 2 3 4 5 6 7

4.5 30.9 29.4 47.7 49.8 49.7 49.0

95.4 68.4 70.0 51.2 49.1 49.7 49.6

0.1 0.6 0.6 1.1 1.1 0.6 1.3

8

0.6

0.5

71.7

a

Figure 9. Formation rates of H2 in the decompositions of CH4 over the Pd-Ni/carbon and Pd-Ni-Ce/carbon catalysts at 1073 K. Pd:Ni: Ce ) 10:10:1; other reaction conditions are the same as for Figure 2.

Figure 10. TEM images of the Pd-Ni-Ce/carbon catalyst after 2 h of CH4 decomposition at 1073 K. Spots 1-8 were analyzed by EDS.

of Ce doped into the Pd-Ni alloy. Several spots with a narrow area within 5 nm on the catalyst particles were analyzed. Figure 10 shows the TEM images of the Pd-Ni-Ce/carbon catalyst after 2 h of CH4 decomposition at 1073 K. The spots analyzed by EDS are indicated in the TEM images. As the Pd-Ni-Ce/ carbon catalyst was inactive for the decomposition of CH4, any carbon fibers were not observed. Instead, Pd-Ni particles encapsulated by carbon layers were observed. This is similar to what was observed on the Pd-Ni/Ce(Sm)O2 catalyst (Figure 6). The atomic ratios of Pd, Ni, and Ce evaluated from the PdL, Ni-K, and Ce-L radiations are shown in Table 2. A strong

Ce-L

Cl-Ka

27.1

The Cl detection was carried out only at spot 8.

Ni-K radiation (95.4%) was observed at spot 1. The particles at spots 2-7 seem to be Pd-Ni-Ce compounds. Strong Ce-L radiation (71.7%) and Cl-K (27.1%) were observed at spot 8. The particles at spots 1 and 8 are probably Ni metal and CeOCllike compound, respectively. Apart from them, the majority of the particles (spots 2-7) observed in the TEM images are probably Ce-doped Pd-Ni alloy. The atomic percentages of Ce for spots 2-7 were 0.6 -1.3%. The average percentages of Pd, Ni, and Ce for spots 2-7 were respectively 42.9%, 56.2%, and 0.9%. For comparison, the EDS analysis for the three elements (Pd, Ni, and Ce) was performed on the Pd-Ni/carbon catalyst after 8 min of CH4 decomposition. The average percentages of Pd, Ni, and Ce evaluated from their radiations were respectively 48.9%, 50.9%, and 0.2%. This Ce content of 0.2% is presumably an error due to the background. The Pd-Ni particles in the Pd-Ni-Ce/carbon catalyst certainly showed higher Ce radiation than those in the Pd-Ni/ carbon catalyst, although the difference was quite small. Considering the significant effect of the Ce addition on the CH4 decomposition, the peak shifts observed in the XRD patterns of Pd-Ni-Ce/Zr(Y)O2, and the different morphology of the Pd-Ni particles detached from the supports, it could be concluded that the Pd-Ni-Ce/Zr(Y)O2 and Pd-Ni-Ce/carbon catalysts have the Pd-Ni alloy doped with a small amount of Ce (less than 1%). Similar discussion is possible as to the Pd-Ni/Ce(Sm)O2 catalyst. The Pd-Ni alloy could react with the Ce(Sm)O2 support during the catalyst preparation, and a small amount of Ce could be doped into the Pd-Ni particles. The doping of Ce into the Pd-Ni alloy is likely relevant to the suppression of the carbon deposition. Activation of the C-H Bond by the Pd-Ni Catalysts. The Pd-Ni/(Ce(Sm)O2-La(Sr)CrO3) anode is very active in the oxidation of dry CH4 to CO, CO2, and H2O in SOFCs at 1073-1173 K.16 We concluded that the activation of the C-H bond catalyzed by the Pd-Ni alloy was the key process to enhance the oxidation of CH4. In the present study, the modification of Pd-Ni catalyst by the Ce element was found to inhibit significantly the formation of carbon deposit. However, it is not clear whether the addition of Ce also inhibits the activation of C-H bonds. It is important to understand the correlation between the activities for the C-H bond activation and carbon formation regarding the Pd-Ni catalysts containing the Ce element. The comparison between the SOFC performances of the Pd-Ni/Ce(Sm)O2 anode and the Pd-Ni/Zr(Y)O2 anode might be helpful to discuss this issue, but it would be too complicated because the effect of electrical conductivity cannot be excluded from the discussion. The steam reforming of CH4 is rather a suitable reaction to discuss this issue.


Nabae et al. SCHEME 1: A Model for the Mechanism of the Suppression of Carbon Deposition on the Pd-Ni Catalysts Supported on Ce(Sm)O2 and La(Sr)CrO3

Figure 11. Formation rates of the products in the steam reforming of CH4 over (a) Pd-Ni/Zr(Y)O2 and (b) Pd-Ni/Ce(Sm)O2 catalysts at 1073 K. Amounts of catalyst: 50 mg. P(CH4): 50.7 kPa. P(H2O): 50.7 kPa. Total flow rate: 100 mL min-1.

Steam reforming of CH4 over the Pd-Ni/Ce(Sm)O2 and Pd-Ni/Zr(Y)O2 catalysts was carried out to discuss their catalytic activities for the activation of the C-H bond. Figure 11 shows the time courses of the conversion of CH4 and the formation rates of CO, CO2, H2, and C (carbon deposit) on (a) Pd-Ni/Zr(Y)O2 and (b) Pd-Ni/Ce(Sm)O2 catalysts. The Pd-Ni/ Ce(Sm)O2 catalyst showed a continuous catalytic activity for the CH4 steam reforming. The formation rates of products and the CH4 conversion (70%) were almost constant for at least 2 h. The deposition rate of carbon calculated by eq 2 was quite low during the experiment. In the case of the Pd-Ni/Zr(Y)O2 catalyst, the conversion of CH4 (70%) was good at the early stage of the experiment but gradually decreased over the course of the measuremt. The formation rates of the products also decreased over time. Note that the formation rate of the carbon deposit was not negligible. The Pd-Ni/Zr(Y)O2 catalyst was probably deactivated by the carbon deposition. Similar CH4 conversions at the early stage suggest that the Pd-Ni/Ce(Sm)O2 and Pd-Ni/Zr(Y)O2 catalysts have similar catalytic activities for the activation of the C-H bonds in CH4. The significant difference between the stabilities of these two catalysts presumably resulted from the low catalytic activity of the Pd-Ni/Ce(Sm)O2 catalyst for the carbon formation compared to the catalytic activity for the C-H bond activation. It can be concluded that the Pd-Ni catalyst containing the Ce

element has quite a high tolerance to carbon deposition, but still has active surfaces for the activation of the C-H bonds. Mechanism of the Suppression of Carbon Deposition. A model is proposed in Scheme 1 to explain the mechanism of the suppression of carbon deposition on the Pd-Ni catalysts. The growth mechanism of the carbon fibers during the CH4 decomposition over the supported Pd, Ni, and Pd-Ni catalysts has been reported as follows:3,4,20,21 (1) CH4 decomposes to H2 and carbon atoms on the (100) and (110) planes of the metal crystals; (2) the carbon atoms dissolve into the metal bulk phase and diffuse toward the (111) plane; (3) the carbon atoms segregate from the (111) plane forming a graphene layer; and (4) carbon fibers grow from the metal particles continuously by stacking the graphene layers. A similar growth mechanism is probably applicable for the carbon fibers produced over the Pd-Ni/SiO2, Pd-Ni/carbon, and Pd-Ni/Zr(Y)O2 catalysts studied in this work. The Pd-Ni particles observed in Figures 3 and 4 were quite multifaceted. The surface of the multifaceted particles could be one or some of the (111), (110), and (100) planes of the Pd-Ni alloy. In contrast, the Pd-Ni catalysts containing La, Ce, Sm, or Sr do not catalyze the formation of carbon nanofibers. These catalysts still probably have the active surfaces for the activation of the C-H bonds. The Pd-Ni particles in the Pd-Ni/ Ce(Sm)O2 (Figure 5) and Pd-Ni-Ce/carbon (Figure 10) catalysts had a spherical shape and were covered with thin carbon layers. Doping of La, Ce, Sm, and Sr may inhibit the formation of a particular plane that is essential for the growth of the carbon nanofibers. In such a condition, the carbon atoms cannot diffuse across the Pd-Ni bulk phase. They would cover the catalyst surface and finally deactivate the catalyst. If a sufficient amount of H2O is supplied, this carbon layer can be removed and the catalysis for the activation of the C-H bonds can be sustained. This reaction mechanism proposed for the Pd-Ni/Ce(Sm)O2 catalyst would explain the catalytic property of the Pd-Ni/ Ce(Sm)O2-La(Sr)CrO3 anode with dry CH4: high tolerance to carbon deposition and high catalytic activity for the electrochemical CH4 oxidation. The Pd-Ni catalysts modified by one or some of La, Ce, Sm, and Sr do not catalyze the formation of carbon nanofibers, but they are still active for the activation of CH4. If the cell is at an open-circuit condition, which supplies neither of H2O nor O2- to the anode, thin carbon layers would deactivate the Pd-Ni catalyst by covering the catalyst surface. Therefore, any serious carbon deposition would not occur on the anode at an open-circuit condition. If the cell is operated under a fuel cell condition, the carbon species on the Pd-Ni catalysts can be removed by reacting with H2O or O2- so that fresh catalyst surfaces can be maintained and CH4 can be oxidized continuously.

Suppression of C Deposition in SOFCs The novelty of our model for the suppression of carbon deposition is that it focuses on the mechanism of carbon formation on the metal particles. Some SOFC anodes containing rare earth oxides have been reported to have relatively high tolerance to carbon deposition. Iida et al. suggested that the effect was because the carbon species on the Ni/Ce(Sm)O2 anode could be removed easily.18 However, the present results of the CH4 decomposition, which does not have any oxidation process, suggest that the anode modified by La, Ce, Sm, and Sr could be simply effective for the suppression of carbon deposition. The effect of the additions of rare earth, alkaline, and alkaline earth elements on the suppression of the carbon deposition were readily observed in the reforming and partial oxidation of hydrocarbons to H2 and CO.22–26 So far, these effects have been discussed in terms of the interaction between the metal particles and supports, such as the reactivity of the lattice oxygen, the acidity of the supports, and the electron donation by the supports. The present model proposed in this study, in contrast, focuses on the mechanism of carbon formation on the metal particles detached from the support. The formation of carbon deposit is a common problem to all catalysts regarding the conversion of hydrocarbons, including direct utilization of hydrocarbons over the Ni cermet anode. If some of those catalysts have a similar mechanism for the formation of carbon deposit to the Pd-Ni catalyst, the model proposed in this study would be applicable to design Ni cermet anodes or other catalysts for hydrocarbons. Conclusion The mechanism of the suppression of carbon deposition on the Pd-Ni catalysts supported on the Ce(Sm)O2 and La(Sr)CrO3 supports was investigated. Significant suppression of carbon deposition was observed in the CH4 decomposition over the Pd-Ni catalysts containing Ce, Sm, La, and Sr elements. The XRD and TEM-EDS analysis suggested the doping of these elements into the Pd-Ni alloy. The doping to the Pd-Ni alloy probably inhibits the dissolution and diffusion of carbon atoms in the Pd-Ni bulk phase and suppresses the serious carbon deposition. The result of the steam reforming of CH4 over the Pd-Ni/Ce(Sm)O2 catalyst suggests that the Pd-Ni catalysts modified by Ce still have high catalytic activity for the activation of CH4. The proposed model explains well the catalytic property of the Pd-Ni/Ce(Sm)O2-La(Sr)CrO3 anode: high tolerance to carbon deposition and high catalytic activity for the direct utilization of dry CH4.

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10315 The high tolerance to carbon deposition deriving from the doping of rear earth and alkaline earth elements could be applied in modifying the conventional Ni-Zr(Y)O2 cermet anodes as well as in designing other new anode materials for direct utilization of dry hydrocarbons in SOFCs. Acknowledgment. We thank Akira Genseki (Center for Advanced Materials Analysis, Tokyo Institute of Technology) for TEM analysis. References and Notes (1) McIntosh, S.; Gorte, R. J. Chem. ReV. 2004, 104, 4845. (2) Atkinson, A.; Barnett, S.; Gorte, R. J.; Irvine, J. T. S.; Mcevoy, A. J.; Mogensen, M.; Singhal, S. C.; Vohs, J. Nat. Mater. 2004, 3, 17. (3) Baker, R. T. K. Carbon 1989, 27, 315(4) Rostrup-Nielsen, J. R. In Catalysis: Science and Technology; Anderson, J. R., Boudart M. , Eds.; Springer: Berlin, Germany, 1984; Vol. 5, p 1. (5) Zhang, T.; Amiridis, M. D. Appl. Catal., A 1998, 167, 161. (6) Aida, T.; Abudula, A.; Ihara, M.; Koiyma, H.; Yamada, K. In Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC IV); Dokiya, M., Yamanoto, O., Tagawa, H., Singhal, S. C., Eds.; The Electrochem Society: Pennington: NJ, 1995; p 801. (7) Murray, E. P.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649. (8) Liu, J. A.; Barnett, S. A. Solid State Ionics 2003, 158, 11. (9) Kim, H.; Lu, C.; Worrell, W. L.; Vohs, J. M.; Gorte, R. J. J. Electrochem. Soc. 2002, 149, A247. (10) Park, S. D.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265. (11) Tao, S. W.; Irvine, J. T. S. Nat. Mater. 2003, 2, 320. (12) Putna, E. S.; Stubenrauch, J.; Vohs, J. M.; Gorte, R. J. Langmuir 1995, 11, 4832. (13) Liu, J.; Madsen, B. D.; Ji, Z. Q.; Barnett, S. A. Electrochem. SolidState Lett. 2002, 5, A122. (14) McIntosh, S.; Vohs, J. M.; Gorte, R. J. Electrochem. Solid-State Lett. 2003, 6, A240. (15) Nabae, Y.; Yamanaka, I.; Takenaka, S.; Hatano, M.; Otsuka, K. Chem. Lett. 2005, 34, 774. (16) Nabae, Y.; Yamanaka, I.; Hatano, M.; Otsuka, K. J. Electrochem. Soc. 2006, 153, A140. (17) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otsuka, K. J. Catal. 2003, 220, 468. (18) Iida, T.; Kawano, M.; Matsui, T.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2007, 154, B234. (19) McIntosh, S.; Vohs, J. M.; Gorte, R. J. Electrochim. Acta 2002, 47, 3815. (20) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otsuka, K. J. Phys. Chem. B 2004, 108, 7656. (21) Kepinski, L. Catal. Today 1999, 50, 237. (22) Andrew, S. P. S. Ind. Eng. Chem., Prod. Res. DeV. 1969, 8, 321. (23) Matsumoto, H. Shokubai 1974, 16, 122. (24) Hu, Y.; Ruckenstein, E. Catal. Lett. 1997, 43, 71. (25) Wang, J. B.; Hsiao, S.; Huang, T. Appl. Catal., A 2003, 246, 197. (26) Hayakawa, T.; Suzuki, S.; Nakamura, J.; Uchijima, T.; Hamakawa, S.; Suzuki, K.; Shishido, T.; Takehira, K. Appl. Catal., A 1999, 183, 273.

JP801496V

Ce

Recommend Documents