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Development of active and stable nickel-magnesia solid solution catalysts for CO2 reforming of methane
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
375
Development of active and stable nickel-magnesia solid solution catalysts for CO 2 reforming of methane K. Tomishige, Y. Chen, X. Li, K. Yokoyama, Y. Sone, O. Yamazaki, and K. Fujimoto Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Nickel-magnesia solid solution catalyst with low Ni content had excellent stability for methane reforming with carbon dioxide at 1123 K without the formation of carbon deposition. Nickel-magnesia solid solution and magnesia supported nickel catalysts were characterized by means of FFIR, TEM, and the amount of H 2 and 0 2 adsorption. Ni metal particles on Ni0.o3Mg0.970have considerably high dispersion and the interaction with support surface. It is suggested that high resistance to carbon deposition is caused by supplying CO 2 or oxygen species through the interface between nickel metal and support surface.
1. INTRODUCTION CO 2 reforming of methane (equation 1) has been proposed as one of the most promising technologies for utilization of these two greenhouse gases, and this synthesis gas is suitable for Fischer-Tropsch synthesis and oxygenated chemicals. A serious problem is carbon deposition via Boudouard reaction (equation 2) and/or methane decomposition (equation 3). CH4+CO2--->2CO+2H2 2CO--->C+CO2 CHa--->C+2H2
AH=+247 kJ/mol AH=-173 kJ/mol AH=+75 kJ/mol
(1) (2) (3)
Carbon deposition has been reported to cause the catalyst deactivation, plugging the reactor. This has also been observed in steam reforming, but much more serious in CO 2 reforming of methane as expected by thermodynamic calculations [1]. Noble metals are found to be less sensitive to coking than nickel [2-4]. However, considering high cost and limited availability, it is more desirable to develop nickel-based catalysts which are resistant to carbon deposition. Recently we have reported that nickel-magnesia solid solution Ni0.03Mg0.970 has high and stable activity [5-9]. Our purpose is to elucidate catalytic active site and inhibition mechanism of carbon deposition by means of temperature programmed hydrogenation (TPH) of the sample after catalytic reaction and catalyst characterization.
2. EXPERIMENTAL Nickel-magnesia solid solution catalysts were prepared by coprecipitating nickel acetate
376 and magnesium nitrate aqueous solution with potassium carbonate. After being filtered and washed with hot water, the precipitate was dried overnight at 393 K, and then calcined in air at 1223 K for 10 h. Supported nickel catalysts (Ni/MgO and Ni/A1203)were prepared by impregnating MgO or A1203 with Ni(C5I-I702)2 acetone solution. MgO was prepared by the same method as solid solution catalyst. The loading was denoted as the molar ratio Ni/(Ni+Mg or A1). CH4-CO~ reaction was performed in a fixed bed flow reaction system equipped with gas chromatograph. Catalysts were reduced with H2at 1123 K before the reaction. Reaction condition was CH4/CO2=1/1, 773 K-1123 K, W/F=0.1-1.2 gh/mol. Carbon deposition was characterized by TPH method, in which pure hydrogen was introduced, and then temperature was raised from room temperature to 1123 K at a heating rate of 20 K/min. The sample after CH4-CO 2 reaction was quickly cooled down to room temperature under Ar flow, followed by replacing Ar with H 2. The signal corresponding to C H 4 formation was recorded continuously by FID detector without separating column. FYIR spectra were obtained in a transmission mode using in-situ IR cell connected to closed circulating system. The sample was reduced with hydrogen at 1123 K for 0.5 h. The samples for TEM observation were stored under vacuum until the measurements. Sample powder was dispersed in tetrachloromethane by supersonic waves and put on Cu grids under atmosphere. 100
3. RESULTS AND DISCUSSION
, %
o
.
80
Nio.=Mg0.9,O O
O
C~ - O
O O
O
{3
Figure 1 shows the catalytic activity on Ni0.03Mgo.97O' Ni/MgO, Ni/A12O3 .~ catalysts at 1123 K. Ni0.03Mgo.970was ~> 60 Ni(3 moi%)/MgO found to have very high and stable activity o for a long period (100 days). 3mo1% ~ 40 Ni/MgO did not so high activity, but rather -=_ stable. 3 mol% Ni/ml20 3 catalyst ~deactivated very rapidly and finally the 20 Ni(3 mol%)/AI203 reactor was plugged with the deposited carbon. We measured the amount of total 0 0 20 40 60 80 100 carbon species on the catalyst surface after Time on stream / day the reaction for 120 h. The results were 0.1 wt% on Ni 0.03Mg0.97O, 1.6 wt% Ni/MgO Figure 1. Reaction time dependence of and 10 wt% on Ni/A1203 catalysts. From methane reforming with carbon dioxide. Reaction condition: 1123 K, W/F=1.2 these results, it was found that Ni0.03Mgo.970 gh/mol, CHJCO2=I/1, 0.1 MPa, 0.1 g. catalyst has high resistance to carbon deposition in CO 2reforming of methane. Figure 2 shows the TPH results on Nio.03Mgo.970 and 3.0 mol% Ni/MgO after the reaction at 773 K, and the activity was listed in Table 1. Under this reaction condition, methane conversion is far from the thermodynamic equilibrium level. Two peaks were observed in the TPH profiles. One appeared at 550 K-700 K (a-carbon), and the other above 873 K (fl-carbon). It is found that the peak intensity of a-carbon was almost constant, while that of fl-carbon increased linearly with the time on stream. From the behavior and reactivity, fl-carbon is ascribed to deposited carbon, fl-carbon formation rate and selectivity were also in Table 1. Selectivity to carbon is much related to the dispersion of Ni metal particles. This suggested that carbon formation tended to proceed on the larger Ni particles. And carbon was formed on solid solution catalysts with higher Ni content. 9
O
O
O
i
i
i
i
377 Table 1. Catalyst properties of nickel-magnesia solid solution and supported catalysts. Catalyst BET O2 a) HEb) Ni'~ c) D , Rco~ fl-carbon rate ~ selP /mE/g /btmol/g-cat /% /% /btmol g~s -~ /bt C-mol g~s ~ /% 0.00 Nio.03Mg0.970 22 10.5 3.1 2.9 29.5 31 0.00 0.019 Nioa0Mgo.900 33 130.4 14.9 11.4 11.4 155 0.03 Ni/MgO h) 25 21.8 2.4 58.6 11.0 87 0.02 0.023 Ni/MgO i) 25 226.5 3.9 62.4 1.7 138 0.15 0.109 a: adsorption temperature 873 K, b: adsorption temperature 298 K, c: reduction degree of Ni was estimated by 2x(amount of 0 2 adsorption )/(total amount of Ni), d: dispersion of Ni was estimated by (amount of H 2 adsorption)/(amount of 0 2 adsorption), e: CO formation rate in the reforming of c n 4 with CO 2 under 773 K, 0.1 MPa, CHJCO2=I/1, W/F=0.1 gh/mol, catalyst weight: 0.05 g, f: r-carbon formation rate under the same reaction condition, g: r-carbon selectivity is estimated by ( r-carbon rate)/( r-carbon rate + CO formation rate), h, i: Ni loading of supported catalyst was 0.3 and 3.0 mol%, respecitively.
3 mol% N i / M g ~
Nio.03Mgo.970
9
_./x.__
3
9
s I
270
1
I
470 670 870 Temperature / K
I
1070
I
I
l
1
270 470 670 870 1070 Temperature / K
Figure 2. TPH profiles on the samples after being exposed to cn4-1-CO 2 for (1) 2, (2) 30, and (3) 60 min. Reaction conditions: 773 K, W/F=0.1 gh/mol, CH4/CO2=1/1, 0.1 MPa, 0.05 g. Figure 3 shows FUR spectra of CO adsorption on nickel magnesia catalysts. On Ni0a0Mg0.900 and Ni/MgO catalysts, linear (2100-2000 cma), bridge (2000-1850 cm -~) and physisorbed Ni(CO)4 (2057 cm -~) were mainly observed. In contrast, on Nio.03Mgo.970 nickel monomer and dimer carbonyl species which are interacted with MgO were mainly observed as previously reported[10]. These species were increased with the CO pressure, therefore they are found to be formed via CO induced structural change. On Nio.o3Mgo.970 solid solution, Ni metal particles seem to be highly dispersive. Figure 4 shows TEM image of reduced Ni 0.03Mg0.970. Large cube and small sphere on it were observed. Large cube is nickel-magnesia solid solution, small sphere is a nickel metal particle. The average size of small particle is about 4 nm. This is larger than that estimated by the dispersion as listed in Table 1. In addition, the number of metal particles are much smaller than that of solid solution cube, and that expected by reduction degree as listed in Table 1. This means that a lot of solid solution cubes with no metal particles were observed, and most nickel atoms can not be observed in TEM image. This is because the metal particle size is beyond the detection limit of TEM, or very small metal particle is oxidized during TEM sample preparation. It is strongly suggested that Ni o.o3Mgo.97O has more highly dispersive nickel metal particles than other three catalysts.
378
3.0 mol% Ni/MgO Figure 3. FTIR spectra of CO adsorption on the samples. Pco=13.3 kPa, 298 K.
:!;~
0.3 m o l % N i / M g O
,~'.~
,, ~ _
~: ..,;.
Ni 0.10Mgo.900
Nio.03 Mgo.97 O
100 nm Figure 4. TEM image of reduced Ni 0.03Mg0.97 O . c G ~,4
r
~ ~ ---.
c G --.
Wavenumber/cm-t
4. CONCLUSION
Ni0.03Mgo.970 solid solution catalyst has high resistance to carbon deposition in C O 2 reforming of methane. From the characterization results, this catalyst was found to have highly dispersive nickel metal particles with the interaction with support surface. The inhibition mechanism is suggested to be the activation of adsorbed CO 2 at the interface between metal and support surface and rapid supply of oxygen species to nickel surface. REFERENCES
1. J.R. Rostrup-Nielsen, Catalysis Science and Technology, J. R. Anderson, M. Boudart (eds), Germany, Berlin, Springer, 5 (1984) 3. 2. A.T. Ascchcroft, A. K. Vermon, M. L. H. Green and P. D. E Vermon, Science 352 (1991) 225. 3. J.R. Rostrup-Nielsen and J. H. B. Hansen, J. Catal., 144 (1993) 38. 4. J.T. Richardson and S. A. Paripatyadar, Appl. Catal., 61 (1990) 293. 5. O. Yamazaki, K. Omata, T. Nozaki and K. Fujimoto, Chem. Lett. (1992) 1953. 6. O. Yamazaki, K. Tomishige and K. Fujimoto, Appl. Catal. A:General 136 (1996) 49. 7. K. Tomishige, Y. Chen, K. Yokoyama, Y. Sone and K. Fujimoto, Shokubai, 39 (1997)70. 8. Y. Chen, K. Tomishige and K. Fujimoto, Appl. Catal. A:General, in press. 9. Y. Chen, O. Yamazaki, I~ Tomishige and K. Fujimoto, Catal. Lett., 39 (1996) 91. 10. A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1891.
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
375
Development of active and stable nickel-magnesia solid solution catalysts for CO 2 reforming of methane K. Tomishige, Y. Chen, X. Li, K. Yokoyama, Y. Sone, O. Yamazaki, and K. Fujimoto Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Nickel-magnesia solid solution catalyst with low Ni content had excellent stability for methane reforming with carbon dioxide at 1123 K without the formation of carbon deposition. Nickel-magnesia solid solution and magnesia supported nickel catalysts were characterized by means of FFIR, TEM, and the amount of H 2 and 0 2 adsorption. Ni metal particles on Ni0.o3Mg0.970have considerably high dispersion and the interaction with support surface. It is suggested that high resistance to carbon deposition is caused by supplying CO 2 or oxygen species through the interface between nickel metal and support surface.
1. INTRODUCTION CO 2 reforming of methane (equation 1) has been proposed as one of the most promising technologies for utilization of these two greenhouse gases, and this synthesis gas is suitable for Fischer-Tropsch synthesis and oxygenated chemicals. A serious problem is carbon deposition via Boudouard reaction (equation 2) and/or methane decomposition (equation 3). CH4+CO2--->2CO+2H2 2CO--->C+CO2 CHa--->C+2H2
AH=+247 kJ/mol AH=-173 kJ/mol AH=+75 kJ/mol
(1) (2) (3)
Carbon deposition has been reported to cause the catalyst deactivation, plugging the reactor. This has also been observed in steam reforming, but much more serious in CO 2 reforming of methane as expected by thermodynamic calculations [1]. Noble metals are found to be less sensitive to coking than nickel [2-4]. However, considering high cost and limited availability, it is more desirable to develop nickel-based catalysts which are resistant to carbon deposition. Recently we have reported that nickel-magnesia solid solution Ni0.03Mg0.970 has high and stable activity [5-9]. Our purpose is to elucidate catalytic active site and inhibition mechanism of carbon deposition by means of temperature programmed hydrogenation (TPH) of the sample after catalytic reaction and catalyst characterization.
2. EXPERIMENTAL Nickel-magnesia solid solution catalysts were prepared by coprecipitating nickel acetate
376 and magnesium nitrate aqueous solution with potassium carbonate. After being filtered and washed with hot water, the precipitate was dried overnight at 393 K, and then calcined in air at 1223 K for 10 h. Supported nickel catalysts (Ni/MgO and Ni/A1203)were prepared by impregnating MgO or A1203 with Ni(C5I-I702)2 acetone solution. MgO was prepared by the same method as solid solution catalyst. The loading was denoted as the molar ratio Ni/(Ni+Mg or A1). CH4-CO~ reaction was performed in a fixed bed flow reaction system equipped with gas chromatograph. Catalysts were reduced with H2at 1123 K before the reaction. Reaction condition was CH4/CO2=1/1, 773 K-1123 K, W/F=0.1-1.2 gh/mol. Carbon deposition was characterized by TPH method, in which pure hydrogen was introduced, and then temperature was raised from room temperature to 1123 K at a heating rate of 20 K/min. The sample after CH4-CO 2 reaction was quickly cooled down to room temperature under Ar flow, followed by replacing Ar with H 2. The signal corresponding to C H 4 formation was recorded continuously by FID detector without separating column. FYIR spectra were obtained in a transmission mode using in-situ IR cell connected to closed circulating system. The sample was reduced with hydrogen at 1123 K for 0.5 h. The samples for TEM observation were stored under vacuum until the measurements. Sample powder was dispersed in tetrachloromethane by supersonic waves and put on Cu grids under atmosphere. 100
3. RESULTS AND DISCUSSION
, %
o
.
80
Nio.=Mg0.9,O O
O
C~ - O
O O
O
{3
Figure 1 shows the catalytic activity on Ni0.03Mgo.97O' Ni/MgO, Ni/A12O3 .~ catalysts at 1123 K. Ni0.03Mgo.970was ~> 60 Ni(3 moi%)/MgO found to have very high and stable activity o for a long period (100 days). 3mo1% ~ 40 Ni/MgO did not so high activity, but rather -=_ stable. 3 mol% Ni/ml20 3 catalyst ~deactivated very rapidly and finally the 20 Ni(3 mol%)/AI203 reactor was plugged with the deposited carbon. We measured the amount of total 0 0 20 40 60 80 100 carbon species on the catalyst surface after Time on stream / day the reaction for 120 h. The results were 0.1 wt% on Ni 0.03Mg0.97O, 1.6 wt% Ni/MgO Figure 1. Reaction time dependence of and 10 wt% on Ni/A1203 catalysts. From methane reforming with carbon dioxide. Reaction condition: 1123 K, W/F=1.2 these results, it was found that Ni0.03Mgo.970 gh/mol, CHJCO2=I/1, 0.1 MPa, 0.1 g. catalyst has high resistance to carbon deposition in CO 2reforming of methane. Figure 2 shows the TPH results on Nio.03Mgo.970 and 3.0 mol% Ni/MgO after the reaction at 773 K, and the activity was listed in Table 1. Under this reaction condition, methane conversion is far from the thermodynamic equilibrium level. Two peaks were observed in the TPH profiles. One appeared at 550 K-700 K (a-carbon), and the other above 873 K (fl-carbon). It is found that the peak intensity of a-carbon was almost constant, while that of fl-carbon increased linearly with the time on stream. From the behavior and reactivity, fl-carbon is ascribed to deposited carbon, fl-carbon formation rate and selectivity were also in Table 1. Selectivity to carbon is much related to the dispersion of Ni metal particles. This suggested that carbon formation tended to proceed on the larger Ni particles. And carbon was formed on solid solution catalysts with higher Ni content. 9
O
O
O
i
i
i
i
377 Table 1. Catalyst properties of nickel-magnesia solid solution and supported catalysts. Catalyst BET O2 a) HEb) Ni'~ c) D , Rco~ fl-carbon rate ~ selP /mE/g /btmol/g-cat /% /% /btmol g~s -~ /bt C-mol g~s ~ /% 0.00 Nio.03Mg0.970 22 10.5 3.1 2.9 29.5 31 0.00 0.019 Nioa0Mgo.900 33 130.4 14.9 11.4 11.4 155 0.03 Ni/MgO h) 25 21.8 2.4 58.6 11.0 87 0.02 0.023 Ni/MgO i) 25 226.5 3.9 62.4 1.7 138 0.15 0.109 a: adsorption temperature 873 K, b: adsorption temperature 298 K, c: reduction degree of Ni was estimated by 2x(amount of 0 2 adsorption )/(total amount of Ni), d: dispersion of Ni was estimated by (amount of H 2 adsorption)/(amount of 0 2 adsorption), e: CO formation rate in the reforming of c n 4 with CO 2 under 773 K, 0.1 MPa, CHJCO2=I/1, W/F=0.1 gh/mol, catalyst weight: 0.05 g, f: r-carbon formation rate under the same reaction condition, g: r-carbon selectivity is estimated by ( r-carbon rate)/( r-carbon rate + CO formation rate), h, i: Ni loading of supported catalyst was 0.3 and 3.0 mol%, respecitively.
3 mol% N i / M g ~
Nio.03Mgo.970
9
_./x.__
3
9
s I
270
1
I
470 670 870 Temperature / K
I
1070
I
I
l
1
270 470 670 870 1070 Temperature / K
Figure 2. TPH profiles on the samples after being exposed to cn4-1-CO 2 for (1) 2, (2) 30, and (3) 60 min. Reaction conditions: 773 K, W/F=0.1 gh/mol, CH4/CO2=1/1, 0.1 MPa, 0.05 g. Figure 3 shows FUR spectra of CO adsorption on nickel magnesia catalysts. On Ni0a0Mg0.900 and Ni/MgO catalysts, linear (2100-2000 cma), bridge (2000-1850 cm -~) and physisorbed Ni(CO)4 (2057 cm -~) were mainly observed. In contrast, on Nio.03Mgo.970 nickel monomer and dimer carbonyl species which are interacted with MgO were mainly observed as previously reported[10]. These species were increased with the CO pressure, therefore they are found to be formed via CO induced structural change. On Nio.o3Mgo.970 solid solution, Ni metal particles seem to be highly dispersive. Figure 4 shows TEM image of reduced Ni 0.03Mg0.970. Large cube and small sphere on it were observed. Large cube is nickel-magnesia solid solution, small sphere is a nickel metal particle. The average size of small particle is about 4 nm. This is larger than that estimated by the dispersion as listed in Table 1. In addition, the number of metal particles are much smaller than that of solid solution cube, and that expected by reduction degree as listed in Table 1. This means that a lot of solid solution cubes with no metal particles were observed, and most nickel atoms can not be observed in TEM image. This is because the metal particle size is beyond the detection limit of TEM, or very small metal particle is oxidized during TEM sample preparation. It is strongly suggested that Ni o.o3Mgo.97O has more highly dispersive nickel metal particles than other three catalysts.
378
3.0 mol% Ni/MgO Figure 3. FTIR spectra of CO adsorption on the samples. Pco=13.3 kPa, 298 K.
:!;~
0.3 m o l % N i / M g O
,~'.~
,, ~ _
~: ..,;.
Ni 0.10Mgo.900
Nio.03 Mgo.97 O
100 nm Figure 4. TEM image of reduced Ni 0.03Mg0.97 O . c G ~,4
r
~ ~ ---.
c G --.
Wavenumber/cm-t
4. CONCLUSION
Ni0.03Mgo.970 solid solution catalyst has high resistance to carbon deposition in C O 2 reforming of methane. From the characterization results, this catalyst was found to have highly dispersive nickel metal particles with the interaction with support surface. The inhibition mechanism is suggested to be the activation of adsorbed CO 2 at the interface between metal and support surface and rapid supply of oxygen species to nickel surface. REFERENCES
1. J.R. Rostrup-Nielsen, Catalysis Science and Technology, J. R. Anderson, M. Boudart (eds), Germany, Berlin, Springer, 5 (1984) 3. 2. A.T. Ascchcroft, A. K. Vermon, M. L. H. Green and P. D. E Vermon, Science 352 (1991) 225. 3. J.R. Rostrup-Nielsen and J. H. B. Hansen, J. Catal., 144 (1993) 38. 4. J.T. Richardson and S. A. Paripatyadar, Appl. Catal., 61 (1990) 293. 5. O. Yamazaki, K. Omata, T. Nozaki and K. Fujimoto, Chem. Lett. (1992) 1953. 6. O. Yamazaki, K. Tomishige and K. Fujimoto, Appl. Catal. A:General 136 (1996) 49. 7. K. Tomishige, Y. Chen, K. Yokoyama, Y. Sone and K. Fujimoto, Shokubai, 39 (1997)70. 8. Y. Chen, K. Tomishige and K. Fujimoto, Appl. Catal. A:General, in press. 9. Y. Chen, O. Yamazaki, I~ Tomishige and K. Fujimoto, Catal. Lett., 39 (1996) 91. 10. A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1891.