Ind. Eng. Chem. Res. 2000, 39, 1891-1897
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Performance and Carbon Deposition Behavior of a NiO-MgO Solid Solution in Methane Reforming with Carbon Dioxide under Pressurized Conditions Keiichi Tomishige,* Yoshiyuki Himeno, Yuichi Matsuo, Yusuke Yoshinaga, and Kaoru Fujimoto* Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Activity and carbon deposition behavior were investigated on the catalyst for CO2 reforming of methane under pressurized conditions. A Ni0.03Mg0.97O solid solution catalyst with low surface area (∼4 m2/g) exhibited a lower carbon formation rate than other NiO-MgO solid solution and MgO-supported Pt catalysts. It is suggested that the suppression of Ni aggregation is important for the decrease of the carbon formation rate. The additive effect of Sn, Ge, and Ca to a Ni0.03Mg0.97O solid solution was investigated, and it was found that Sn is an effective component for the decrease of the carbon amount, which is especially formed by methane decomposition. Introduction Catalytic reforming of CH4 with CO2 to produce synthesis gas has gained a growing interest considering the chemical utilization of natural gas and CO2, which are substances intimately related to the environment and energy resource.1-4 So far, the most promising process for the chemical utilization of natural gas is its conversion to liquid fuels or valuable oxygenated chemicals via synthesis gas. Steam reforming of methane has been industrially operated under the partial pressure ratio of H2O/CH4 > 1.5 Excess H2O is used for the inhibition of the carbon deposition and the successive water-gas shift reaction is favored and, therefore, the H2/CO ratio in the product gas becomes higher than 3. Steam reforming is most suitable for hydrogen production. On the other hand, the catalytic reforming of methane with carbon dioxide is suitable for the production of CO-rich syngas, which can be used in FischerTropsch synthesis, methanol, and dimethyl ether. At the same time, this reaction can contribute to the utilization of natural gas fields containing a considerable amount of CO2.6 In this case, CO2 reforming of methane is convenient to be performed. The most serious problem is carbon deposition, which causes catalyst deactivation, plugging of the reactor, and breakdown of the catalyst.7,8 Methane decomposition [CH4 f 2H2 + C (∆H298 ) +75 kJ/mol)] and CO disproportionation [2CO f CO2 + C (∆H298 ) -173 kJ/mol)] are the main routes of the carbon formation.2 According to the thermodynamic calculation, CO2 reforming of CH4 is much more prone to cause carbon deposition than steam reforming because of its low H/C ratio in the reactant gas.5 Thus, because the catalysts
for steam reforming cannot be applied to CO2 reforming, it is necessary to develop new catalysts with high resistance to carbon deposition. Sulfur-passivated catalyst was effective for the carbon-free operation.9 Generally speaking, noble metal catalysts have higher resistance than Ni catalysts.10 However, considering the high cost and limited availability of these noble metals, it is more attractive to develop nickel catalyst with a high resistance to carbon deposition. Recently, we found that a reduced NiO-MgO solid solution catalyst with a low Ni content was a good catalyst.11,12 Ni0.03Mg0.97O solid solution catalyst reduced at 1123 K exhibited very high and stable activity without coke formation in both CO2 reforming of CH4 and steam reforming of CH4 under H2O/CH4 ) 1, while a large amount of carbon was deposited on the commercial steam reforming catalyst under 0.1 MPa and 1123 K conditions. Characterization results revealed that a Ni0.03Mg0.97O solid solution catalyst has highly dispersed nickel metal particles after reduction, which interact with the support surface.13,14 In terms of the utilization of syngas, the compressed syngas is more convenient. It has been proposed that researchers must appreciate that the key problem is catalyst life at high pressures because customers need H2 at >1.0 MPa and compression of large volumes of product gas is not often very acceptable.15 In this paper, it is investigated that catalytic properties and carbon deposition behavior in the CO2 reforming of methane under pressurized conditions on a NiOMgO solid solution catalyst and MgO-supported Ni and Pt catalysts. In addition, the additive effect of Sn, Ge, and Ca to NiO-MgO solid solution catalysts on catalytic properties and carbon deposition behavior was also investigated.
* To whom correspondence should be addressed. Tel.: +81-3-5841-7258. Fax: +81-3-5841-8578. E-mail: tomi@ appchem.t.u-tokyo.ac.jp.
Experimental Section Catalyst Preparation. NixMg1-xO solid solution catalysts were prepared by coprecipitating nickel
10.1021/ie990884z CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000
Table 1. BET Surface Area of Catalysts catalyst
BET (m2 g-1)
MgO(HSA) Ni0.01Mg0.99O(HSA) Ni0.03Mg0.97O(HSA) Ni0.10Mg0.90O(HSA) MgO(LSA) Ni0.01Mg0.99O(LSA) Ni0.03Mg0.97O(LSA) Ni0.10Mg0.90O(LSA)
18 15 18 14 5 5 4 3
acetate (>98.0%, Kanto Chemicals) and magnesium nitrate (>99.2%, Kanto Chemicals) aqueous solutions with potassium carbonate (>99.5%, Kanto Chemicals). After being filtered and washed with hot water, the precipitate was dried at 393 K for 10 h and then was calcined at 1223 K for 10 h. This is the preparation method of NixMg1-xO(HSA). The preparation method of NixMg1-xO(LSA) is as follows. After the precipitate was dried at 393 K for 10 h, it was precalcined at 773 K for 3 h. The sample was pressed (600 kg/cm2) into disks, and then they were calcined at 1223 K for 10 h. The Ni content was represented by a molar ratio of x ) Ni/ (Ni + Mg) and x ) 0.01, 0.03, and 0.10. Before use, all of these catalysts were pressed into tablets and crushed to 20-40 mesh particles. MgO(LSA)-supported Ni and Pt catalysts, which are denoted as Ni/MgO(LSA) and Pt/MgO(LSA), were prepared by impregnating homemade MgO with the acetone solution of Ni(C5H7O2)2‚H2O (>99%, Soekawa Chemicals) and Pt(C5H7O2)2 (>99%, Soekawa Chemicals), followed by drying at 393 K overnight. The preparation method of MgO(LSA) support was almost the same as that for NixMg1-xO(LSA). The loading of Ni and Pt catalysts was represented by the molar ratio of M/(M + Mg) ) 0.03 [M ) Ni, Pt]. Supported catalyst was not calcined at high temperature in order to suppress solid solution formation as much as possible during the pretreatment. The addition of Sn, Ge, and Ca to a Ni0.03Mg0.97O(LSA) solid solution catalyst was carried out by the impregnation method. Precursors were Sn(OC2H5)4, Ge(OC2H5)4, and Ca(C5H7O2)2‚2H2O, respectively. M/Ni0.03Mg0.97O(LSA) [M ) Sn, Ge, and Ca] was prepared by impregnating Ni0.03Mg0.97O(LSA) with an ethanol solution of Sn(OC2H5)4 (99%, High Purity Chemicals) and Ge(OC2H5)4 (99.999%, High Purity Chemicals) and an acetone solution of Ca(C5H7O2)2‚2H2O (>99.5%, Kanto Chemical). The concentration of the salt in an ethanol solution was rather dilute (50 mL of acetone/g of catalyst). This impregnation was carried out under atmosphere at room temperature. The catalyst was dried at 393 K for 12 h. The loading of the additive metals was represented by the weight percent. The surface area of the catalyst was measured by the Brunauer-EmmettTeller (BET) method with Gemini (Micromeritics). BET results are listed in Table 1. The BET surface area of supported catalysts was almost the same as that of MgO. Activity Test of CO2 Reforming of Methane. A catalytic activity test of methane reforming was carried out in a fixed-bed continuous-flow reaction system. The reactor was made of a quartz tube (4 mm i.d.) with stainless steal tube inside, and the quartz tube was sealed by an O-ring. The reaction temperature was controlled by a thermocouple, which was put at the outlet of the catalyst bed. The catalyst weight was about 0.2 g, and the catalyst bed was 20 mm long. After
the catalysts were reduced in flowing H2 (99.9995%, Takachiho) at 1123 K for 0.5 h, the reactant gas feed consisting of the mixture of CH4 (99.99%, Takachiho) and CO2 (99.99%, Takachiho) was introduced into the reactor under 1.0 and 2.0 MPa and W/F ) 0.4-2.4 (g h)/mol. In this case, F means the total flow rate. The reaction temperature was 1073-1173 K. The effluent gas was analyzed with an on-line thermal conductivity detector (TCD) gas chromatograph (GC) using 2 m active carbon as the separating column. An ice bath was set between the reactor exit and the GC sampling valve in order to remove water from the effluent gas. The methane conversion and H2/CO can be estimated to be 69% and 0.8 on the basis of thermodynamic calculation in CO2 reforming of methane (CH4/CO2 ) 1/1, total pressure 1.0 MPa, reaction temperature 1123 K) at equilibrium. Our results were not far from this calculation as shown below. Measurement of the Amount of Carbon Deposition. Thermogravimetric analysis (TGA) for the estimation of the carbon amount was carried out by using TGD-9600 (ULVAC, Shinku-riko, Inc.). After the catalytic reaction, a part of the catalyst (ca. 10 mg) was taken out from the inlet and outlet of the catalyst bed. A TGA profile was measured under air flowing (20 mL/ min) at the heating rate of 1000 K/h. Exothermic weight loss was observed at the temperature range between 800 and 1000 K. This can be assigned to the combustion of deposited carbon. It is possible to estimate the amount of carbon deposition on the basis of this weight loss. In-situ measurement of the amount of carbon deposition was carried out with the magnetic suspension balances (Rubotherm, Germany). It is possible to measure the change of the sample weight under high pressure (
