O Promoter on the Activity and Stability of Nickel Catalysts Supported

Energy & Fuels 2008, 22, 2195–2202

2195

Effects of K2O Promoter on the Activity and Stability of Nickel Catalysts Supported on Mesoporous Nanocrystalline Zirconia in CH4 Reforming with CO2 Mehran Rezaei* Chemical Engineering Department, Faculty of Engineering, UniVersity of Kashan, Kashan, Iran

Seyed Mahdi Alavi Chemical Engineering Department, Iran UniVersity of Science and Technology, Tehran, Iran

Saeed Sahebdelfar Petrochemical Research & Technology Company (NPC-RT), Tehran, Iran

Zi-Feng Yan* State Key Laboratory for HeaVy Oil Processing, Key Laboratory of Catalysis, CNPC, China UniVersity of Petroleum, Dongying 257061, China ReceiVed February 16, 2008. ReVised Manuscript ReceiVed April 17, 2008

In this paper, the effects of K2O promoter on the activity, stability, and coke deposition of nickel catalysts supported on mesoporous nanocrystalline zirconia powder with high surface area and pure tetragonal crystallite phase were investigated in methane reforming with carbon dioxide. The samples were characterized by X-ray diffraction (XRD), BET surface area, temperature-programmed techniques (TPR, TPO, TPH, TPD), and scanning electron microscopy (SEM). The obtained results showed that the addition of K2O promoter affected the catalyst structure, increased the activity and stability, and also reduced the coke deposition on the catalysts.

Introduction Methane reforming with CO2 (dry reforming) has attracted more attention in the past 20 years due to its lower H2/CO ratio in product gas and its utilization of two greenhouse gases.1,2 At the same time, the high endothermicity of the dry reforming reaction makes it attractive as a choice of media in energy transmission systems.3 Noble metals such as Rh, Ru, and Ir exhibit high stability and are less sensitive to coke deposition,4–7 but high cost and restricted availability limited the use of noble metals as catalysts. Ni is another well-known catalyst for this reaction and it is popularly reported for its low price.4–6,8–11 Ni * To whom correspondence should be addressed. E-mail: rezaei@ kashanu.ac.ir. Fax: +98 361 5559930 (M.R.). E-mail: [email protected]. Fax: +86 546 8391971 (Z.-F.Y.). (1) Rostrup-Nielsen, J. R.; Hansen, J.-H.B. J. Catal. 1993, 144, 38. (2) Bradford, M. C. J.; Vannice, M. A. Catal. ReV. Sci. Eng. 1999, 41, 1. (3) Kodama, T.; Shimizu, T.; Kitayama, T. Energy Fuels 2001, 15, 60. (4) Choi, J. S.; Moon, K. I.; Kim, Y,G,; Lee, J. S.; Kim, C. H.; Trimm, D. L. Catal. Lett. 1998, 52, 43. (5) Nicho, N. N.; Casella, M. L.; Santori, G. F.; Ponzi, E. N.; Ferretti, O. A. Catal. Today 2000, 62, 231. (6) Hou, Z. Y.; Yokota, O.; Tanaka, T.; Yashima, T. Catal. Lett. 2003, 87, 37. (7) Hou, Z. Y.; Yokota, O.; Tanaka, T.; Yashima, T. Catal. Lett. 2003, 89, 121. (8) Ruckenstein, E.; Hu, Y.H. J. Catal. 1996, 162, 230. (9) Ruckenstein, E.; Wang, H.Y. J. Catal. 1999, 187, 151.

catalyst possesses high activity for this reforming reaction, while coke deposition, which deactivates Ni catalyst and blocks the reactor, is the main problem. It was found that coke formation is a structure-sensitive process which depends on the surface Ni species, particle size, and electron density. A lot of promoters to Ni catalyst and many kinds of supports were tried in order

Figure 1. XRD patterns of the samples at different calcination temperatures (a) 400, (b) 500, (c) 600, (d) 700, and (e) 800 °C.

10.1021/ef800114e CCC: $40.75  2008 American Chemical Society Published on Web 06/07/2008

2196 Energy & Fuels, Vol. 22, No. 4, 2008

Rezaei et al. Table 1. Structural Properties of the Samples DXRD (nm)

calcination conditions 250 400 500 600 700 800 a

°C/5 °C/5 °C/5 °C/5 °C/5 °C/1

h h h h h h

wt % tetragonal

BET area (m2 g-1)

Pore Volume (cm3 g-1)

t(101)

t(110)

t(112)

t(211)

421 337 281 261 174 78

0.44 0.350 0.282 0.253 0.234 0.119

1a 1.2a 5.4 7 11.2

6.8 6.3 10.3

5.4 6.1 10.5

5.6 6 9

amorphous amorphous amorphous 100 100 100

Corresponding to the peak at 2θ ) 30°.

Figure 3. TPR profile of the promoted catalysts (a) 5% Ni/ZrO2, (b) 5% Ni-1% K2O/ZrO2, (c) 5% Ni-3% K2O/ZrO2, and (d) 5% Ni-4% K2O/ZrO2.

shown a high thermal stability as a catalyst support.14 ZrO2 has three polymorphs: monoclinic (m-phase, below 1170 °C), tetragonal (t-phase, between 1170 and 2370 °C), and cubic (cphase, above 2370 °C).15 Among them, the tetragonal phase, (t-ZrO2), has both acid and basic properties,16 and the t-ZrO2 phase is active for some reactions.17 The use of zirconia requires a high specific surface area and suitable pore structure for catalysis applications. Since nanoscale supports usually possess more edges and corners, which can lead to higher performance of the catalyst, recently many methods have been explored in order to get nanocrystalline ZrO2 powders with high surface area for catalytic applications. Surfactant-assisted synthesis of nanosized inorganic materials has also attracted considerable interest because of its effective soft template effect, reproducibility, and simple maneuverability.18,19 In this paper, the surfactant-assisted route was employed to prepare mesoporous nanocrystalline zirconia powder with high surface area for catalysis applications and their potentials use as support for nickel catalysts in methane reforming with carbon dioxide were investigated. The effect of K2O promoter on the structural

Figure 2. (a) Pore size distributions and (b) XRD patterns of the reduced catalysts.

to eliminate the coke deposition.2,4–8 The use of supports with low concentrations of Lewis acid sites and/or the presence of basic sites, such as ZrO2, MgO, and La2O3, results in enhancing the catalyst activity, lower coke deposition rate, and therefore more stable catalysts.12,13 Among these supports, ZrO2 has

Table 2. Structural Properties of the Catalysts BET area (m2 g-1) K2O (wt %) 0 1 3 4

pore volume (cm3 g-1)

pore diameter (nm)

calcined

reduced

spent

calcined

reduced

spent

calcined

reduced

spent

134.6 127.6 111.3 82.4

130.7 113.6 109.6 79.8

122.6 118.3 108.5 75.3

0.166 0.180 0.166 0.167

0.171 0.169 0.164 0.161

0.215 0.211 0.194 0.157

3.78 4.13 4.19 5.70

4.01 5.97 4.26 5.41

7.01 5.64 5.22 5.66

K2O Promoter Effect on Nickel Catalysts

Energy & Fuels, Vol. 22, No. 4, 2008 2197

properties and catalytic activity and stability of nickel catalyst was also investigated. Experimental Details Materials. The starting materials were ZrO(NO3)2 · 6H2O, KNO3, and Ni(NO3)2 · 6H2O. Pluronic P123 block copolymer and ammonium hydroxide were used as surfactant and precipitation agent, respectively. Zirconia Preparation. In the surfactant-assisted route, aqueous ammonia (25 wt %) was added dropwise at room temperature to an aqueous solution containing metal precursors (0.03 molar) and Pluronic P123 block copolymer surfactant under rapid stirring by careful pH adjustment to 11. The P123 to zirconium molar ratio was chosen about 0.03. After precipitation, the slurry was stirred for another 30 min and then refluxed at 88 °C for 24 h under continuous stirring. The mixture was cooled to room temperature, filtered, and washed, first with deionized water and finally with acetone for an effective removal of the surfactant. The final product was dried at 110 °C for 24 h and calcined at different conditions. Catalyst Preparation. Supported nickel catalysts were prepared by impregnating of zirconia powder and an aqueous solution of nickel nitrate of the appropriate concentration to obtain a 5 wt % nickel content. After impregnation the powder was dried at 80 °C

Figure 5. Effect of K2O content on the (a) long-term stability and (b) H2/CO ratio of the 5%Ni/ZrO2 at 700 °C, GHSV ) 1.5 × 104 mL/ h · gcat.

Figure 6. XRD patterns of the spent catalysts after 50 h of reaction. Figure 4. Effect of K2O content on the (a) CH4 and (b) CO2 conversion of the 5%Ni/ZrO2, gas hourly space velocity (GHSV) ) 1.5 × 104 mL/h · gcat.

and calcined at 700 °C for 2 h in static air atmosphere. The potassium promoted catalysts were prepared by subsequent im-


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Figure 7. SEM analysis of the spent catalysts (a) 5% Ni/ZrO2, (b) 5% Ni-1% K2O/ZrO2, (c) 5% Ni-3% K2O/ZrO2, and (d) 5% Ni-4% K2O/ZrO2.

pregnation of KNO3 and nickel nitrate in the same procedure as described above. Characterization. The surface areas (BET) and pore size distributions were determined by nitrogen adsorption at -196 °C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). The XRD patterns were recorded on an X-ray diffractometer (PANalytical X’Pert-Pro) using a Cu KR monochromatized radiation source and a Ni filter in the range 2θ ) 10-80°. Temperature programmed reduction (TPR-H2) was performed in an automatic apparatus (ChemBET-3000 TPR/TPD, Quantachrome) equipped with thermal conductivity detector. The fresh catalyst (200 mg) was submitted to a heat treatment (10 °C/ min up to 800 °C) in a mixture gas flow (30 mL/min) of H2:Ar (10:90 vol %). Temperature programmed oxidation (TPO) of spent catalysts were performed in a similar apparatus by introducing a gas flow (30 mL/min) of the mixture O2/He (5:95 vol %) over 50 mg of spent catalysts, and the temperature was increased with a

heating rate of 10 °C/min up to 800 °C. Temperature programmed hydrogenation (TPH) of spent catalysts were performed in the same apparatus as described for TPR. The spent catalyst (25 mg) was submitted to a heat treatment (10 °C/min up to 800 °C) in a gas flow (30 mL/min) of the mixture H2:Ar (10:90 vol %). Prior to the TPR, TPO, and TPH experiments, the samples were outgassed under inert atmosphere, at 300 °C for 3 h. In CO2-TPD experiments, the sample was pretreated at 300 °C for 3 h in a He atmosphere, after cooled down to room temperature, the pretreated sample was exposed in CO2 for 30 min. After that, the sample was purged with He at room temperature for 30 min. CO2-TPD was carried out with a ramp of 10 °C/min from room temperature to a needed temperature under He stream. Catalytic Performance Evaluation. Activity measurements were carried out in a fixed-bed continuous-flow reactor made of a 7-mm i.d. quartz tube at atmospheric pressure. The reactor was charged with 200 mg of the prepared catalyst. Prior to the reaction,


Figure 8. Pore size distribution of the spent catalysts after 50 h of reaction.

the catalyst was reduced in situ at 650 °C for 4 h in H2 flow of 30 mL/min. Then, the reactant gas feed consisting of a mixture of CH4 and CO2 (CH4/CO2 ) 50/50 vol %) was introduced to the reactor at a total flow rate of 50 mL/min. The catalyst activity loss was monitored at 700 °C up to 50 h on stream. The gas composition of reactants and products were analyzed using a gas chromatograph equipped with a thermal conductivity detector (TCD) and a Carbosphere column.

Results and Discussion Structural Properties of the Zirconia. The XRD patterns of the zirconia calcined at different temperatures are shown in Figure 1. It can be seen that the samples calcined at 250, 350, and 500 °C are in amorphous form. Increasing the calcination temperature, the crystallite tetragonal phase was obtained. Of interest is that the tetragonal phase was stabilized at room temperature without addition of any dopants, which can be explained by a nanosize effect, which affects the crystallite phase composition and stabilizes of the tetragonal phase at room temperature.20 Increasing the calcination temperature, the crystallite sizes were increased, but the specific surface areas decreased, Table 1. Structural Properties of the Catalysts. The pore size distributions of the reduced catalysts are shown in Figure 2a. It is seen that all the catalysts showed a mesoporous structure with a narow pore size distribution. The obtained results showed that the addition of K2O increased the pore size distributions to larger sizes. Figure 2b shows the XRD patterns of the reduced catalysts with different potassium contents. As it can be seen, all the catalysts showed a very similar intensity for the peaks corresponding to metallic Ni. It is noted that the peaks corresponding to potassium oxide were not appeared, even in the catalyst with the highest potassium content, which could be due to the high dispersion of K2O on the catalyst surface. The structural properties of the catalysts with different K2O contents are reported in Table 2. The obtained results showed a decrease in BET area with increasing in K2O contents. In addition, the pore diameter of the catalysts was increased with increasing in K2O contents and the largest pore diameter was observed for the promoted catalyst with 4% K2O. Figure 3 shows the TPR profiles of the samples promoted with different potassium contents. The obtained results showed


that the potassium content has a significant effect on the reducibility of the catalyst. For all the catalysts several peaks were observed, which revealed that the nickel has to present in different species with different interaction with support. The first small peak at the lowest temperature around 340 °C could be related to reduction of NiO which has the lowest interaction with the support. The second peaks at temperature about 400-470 °C could be related to reduction of nickel oxide with higher interaction with support. The third and forth peaks at temperatures higher than 540 °C are related to reduction of nickel with the highest interaction with the support. As it can be seen, with increasing the potassium content, the peak at the highest temperature was shifted to higher temperature, which means higher metal-support interaction with increasing in potassium content. The obtained results also showed that the addition of potassium oxide to the nickel catalyst increased the metal-support interaction. Catalytic Performances. Figure 4 shows the CH4 and CO2 conversion of the catalysts promoted with various contents of potassium oxide. The obtained results indicated that the addition of potassium oxide increased both the CH4 and CO2 conversions in comparison to the unpromoted catalyst. The results also revealed that the increasing in potassium content decreased the methane conversion, which could be related to lower dispersion of nickel on the catalysts with higher loading of potassium oxide. The effect of potassium content on the long-term stability and H2/CO ratio are shown in Figure 5a and b, respectively. The obtained results showed a similar stability for the catalysts promoted with 1 and 3% K2O, while the promoted catalyst with 4% K2O showed a slightly decrease in CH4 conversion with time on stream, inspite of that all the promoted catalysts showed higher activity and stability in comparison to the unpromoted catalyst. The H2/CO ratio obtained for different catalysts showed a higher ratio for the unpromoted catalyst, while the lowest ratio was observed on the catalyst promoted with 4% of K2O. As can be seen, the H2/CO ratios are less than 1 because the reverse water gas shift reaction occurs simultaneously with the CO2 reforming of CH4. Structural Properties of the Spent Catalysts. Figure 6 shows the XRD patterns of the spent catalysts with various contents of potassium oxide after 50 h of reaction. The obtained results showed a graphite peak in all the catalysts except of the catalyst promoted with 4% K2O, which indicate a lower coke deposition on this catalyst. SEM analysis of the spent catalysts is shown in Figure 7. The results clearly showed a decrease in coke deposition with increasing in K2O content. The lowest carbon deposition was observed over the catalyst promoted with 4% K2O. The pore size distributions of the spent catalysts are also shown in Figure 8. It is seen that the mesoporosity was remained in all the catalysts, although the pore distributions were shifted to larger sizes, which indicates a high thermal stability for these catalysts. TPO Profiles of the Spent Catalysts. The TPO profiles of the spent catalysts are shown in Figure 9. It is seen that increasing in the potassium content decreased the intensity and the area of the observed peak in TPO profile, which means lower carbon deposition with increasing in potassium content. For the promoted catalyst with 4% K2O the intensity of the peak in TPO profile was very weak indicating a low degree of coke deposition on this catalyst. The results of TPO analysis confirmed the XRD results, as the XRD results showed the


Rezaei et al.

Figure 9. TPO profiles of the catalysts promoted with different potassium contents.

lowest intensity of the peak corresponding to graphite peak for the catalyst promoted with 4% K2O. TPH Profiles of the Spent Catalysts. The TPH profiles of the spent catalysts are shown in Figure 10. As it can be seen, for the spent catalysts promoted with 1 and 3% K2O two major peaks were observed in TPH profile. The first peak was observed at temperature about 300 °C and the second peak was observed at around 620 °C. The first peak is related to amorphous carbon and the second peak at higher temperature is related to whisker type carbon, which has a lower reactivity than amorphous carbon toward hydrogenation. Of interesting is that the increasing of the potassium content increased the fraction of amorphous carbon, especially for the catalyst promoted with 4% K2O. For the catalyst promoted with 4% K2O the peak corresponding to the whisker type carbon was disappeared. Temperature Programmed Desorption (TPD). Temperature programmed desorption of CO2 was performed to determine the basicity of the catalysts. Figure 11 shows the CO2-TPD profiles of the different catalysts. As it can be seen, all the catalysts showed a peak at about 140 °C which could be related to weakly bonded CO2. The obtained results indicated that with increasing of potassium content the intensity of the peak corresponding to CO2 was increased, which means the higher basicity of the catalyst promoted with higher potassium content. For the promoted catalyst, the K2O interacts with nickel in a way in which its activity is slightly lowered because of partial coverage of Ni active sites.22,21 Horiuchi et al.23 suggested that (10) Hu, Y. H.; Ruckenstein, E. Appl. Catal. 1997, 154, 185. (11) Wei, J. M.; Xu, B. Q.; Li, J. L.; Cheng, Z. X.; Zhu, Q. M. Appl. Catal. A: Gen. 2000, 196, L167.

the surface of the Ni catalyst with basic metal oxides was rich in adsorbed CO2, while the surface without them was rich in adsorbed CH4. This created an unfavorable condition for CH4 decomposition and, as a result, carbon deposition was suppressed during CO2-reforming on the promoted catalysts with basic metal oxides. For the Ni-K/ZrO2 catalysts as the potassium content increases, the enhanced rate of CO2 activation by K slowly offsets the effect of the partial blockage of nickel sites; hence, activity increases as K content increases. However, after a certain optimum K loading, partial blockage of nickel sites exceeds the increased CO2 activation and hence activity decreases slightly. Thus in Ni-K/ZrO2 catalysts, potassium (12) Lercher, J. A.; Bitter, J. H.; Hally, W.; Niessen, W.; Seshan, K. Stud. Surf. Sci. Catal. 1996, 101, 463. (13) Tomishige, K.; Chen, Y. G.; Fujimoto, K. J. Catal. 1999, 181, 91. (14) Clifford, Y. T.; Hsiao, B. Y.; Chiu, H. Y. Colloids Surf. A: Physicochem. Eng. Aspects 2004, 237, 105. (15) Chraska, T.; King, A. H.; Berndt, C. C. Mater. Sci. Eng., A 2000, 286, 169–178. (16) Yamaguchi, T. Catal. Today 1994, 20, 199–218. (17) Centi, G.; Cerrato, G.; D’Angelo, S.; Finardi, U.; Giamello, E.; Morterra, C.; Perathoner, S. Catal. Today 1996, 27, 265. (18) Kang, Z. H.; Wang, E. B.; Jiang, M.; Lian, S. Y. Nanotechnology 2004, 15, 55. (19) Kang, Z. H.; Wang, E. B.; Jiang, M.; Lian, S. Y.; Li, Y. G.; Hu, C. W. Eur. J. Inorg. Chem. 2003, 370. (20) Garvie, R. C. J. Phys. Chem. 1965, 69, 1298. (21) Garvie, R. C. J. Phys. Chem. 1978, 82, 218. (22) Juan-Juan, J.; Roman-Martinez, M. C.; Illan-Gomez, M. J. Appl. Catal. A: Gen. 2004, 264, 169. (23) Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Appl. Catal. A: Gen. 1996, 144, 111.


Figure 10. TPH profiles of the catalysts promoted with different potassium contents.

Figure 11. TPD profile of the catalysts promoted with different potassium contents.



works as a catalyst for the coke gasification without any structural modification of nickel. Conclusion The catalysts supported on mesoporous nanocrystalline zirconia showed an effective catalytic performance in methane reforming with carbon dioxide. The results showed that the addition of K2O promoter affected the catalyst structure and

Rezaei et al.

increased the activity and stability of the catalyst. An increasing in K2O content decreased the coke deposition and increased the fraction of amorphous carbon which has a high reactivity toward hydrogenation. Higher activity and stability of promoted catalysts could be related to higher basicity of the catalyst and enhanced capacity of CO2 adsorption. EF800114E

O Promoter on the Activity and Stability of Nickel Catalysts Supported

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