Modification Of Ni/Al₂O₃-MgO Catalyst For CO₂ Reforming Of CH₄

CO2 reforming of CH4 has aroused the increasing interests because it can directly convert greenhouse gases CO2 and CH4 to valuable feedstock with a suitable ratio of H2/CO for Fischer-Tropsch. Ni-based catalysts are widely investigated in CO2 reforming of CH4 due to the low cost, easy availability and high catalytic activity. However, the easy deactivation caused by coke deposits or sintering of nickel particles limits their industrial utility. Therefore, it is the key to improve the anti-coke ability and anti-sintering ability of Ni-based catalyst for industrial standpoint.Ni-based catalysts prepared from MgAl layered double hydroxides for CO2 reforming of CH4 have high Ni dispersion and strong basicity. Therefore, in this paper, the MgO-Al2O3 mixed oxides calcined from commercial Mg-Al hydrotalcite was used as the support to prepare Ni-based catalyst for CO2 reforming of CH4 by an impregnation method. The resultant Ni/MgO-Al2O3 catalyst exhibited high catalytic activity, but worse stability when used in CH4 reforming of CO2. Therefore, it is the key to improve the catalytic stability of Ni/MgO-Al2O3 catalyst in this thesis.Compared with Ni-based catalysts with high catalytic performances and poor stability, carbon-based catalysts possess larger surface area, wider pore size distribution, and better resistance to sulfur poisoning and higher temperature, but lower catalytic activity. Based on the complementary properties of carbon catalysts and metal catalysts, in this work, carbon-Ni/MgO-Al2O3 composite catalysts were prepared using sucrose as carbon precursor and [Ni, Mg, Al]-layered double hydroxide (LDH) as metal catalyst precursor, respectively. Effects of preparation method, calcination temperature of LDH, carbonization temperature and sucrose/LDO (layered double oxide) mass ratio on the catalytic performance of resultant catalysts were investigated. N2 adsorption/desorption results showed that composition catalysts had larger specific surface area and wider pore size distribution than Ni/MgO-Al2O3 catalysts. The resultant composite catalysts displayed better stability during 10 h reaction, suggesting that the NiO in [Ni, Mg, Al]-layered double hydroxide can be in-situ reduced, which can avoid extra H2 reduction in traditional preparation process. The calcination temperature of LDH, carbonization temperature of composite catalyst, sucrose amount and pretreatment of composite catalyst precursor will obviously influence the catalytic performances of the resultant carbon-Ni/MgO-Al2O3 composite catalysts. When the calcination temperature, carbonization temperature and sucrose/LDO weight ratio are 350 ?, 800 ? and 1.2, respectively, composite catalyst exhibited the highest catalytic activity and best catalytic stability.Alkaline earth metal can improve the Ni dispersion and basicity of Ni-based catalyst. Therefore, Ni/MgO-Al2O3 catalyst was modified by alkaline earth metal Ca using microwave-assisted impregnation method. The effect of impregnation temperature, microwave power and Ca/Ni molar ratio on the catalytic properties of resultant catalyst were studied. The obtained catalysts displayed better stability in CO2 reforming of CH4 than Ni/MgO-Al2O3 catalyst. XRD results showed that Ni particle size of the catalysts modified by Ca was smaller than that of the unmodified catalyst. According to the TG result, catalyst with Ca/Ni molar ratio of 0.1 exhibited low coke deposition. When the impregnation temperature, microwave power and Ca/Ni molar ratio are 60 ?,600 W,0.1, respectively, the resultant catalyst showed better catalytic activity and stability.