Study On The Stability Of Catalytic Reforming Of CH₄ With CO₂ Over Ni-Mg-Al₂O₃ Catalysts

Methane reforming with carbon dioxide synthetically utilizes CH4 and CO2, which are both greenhouse gases, and produces syngas (H2/CO-1) which is used in industry widely. The syngas can be directly used be the feedstock for methanol, dimethyl ether and Fischer-Tropsch (F-T) synthesis. What’s more important, this reaction does not require water; the major gas producing area in our country is the west where water is not available. Thus this reaction process has a very important practical significance for our country. Ni-based catalyst is the most widely used for the reaction because of its high activity, but it is deactivated easily due to the metal sintering and carbon deposition, which is the major obstacle for the reaction process to achieve industrial application.The oxides obtained from hydrotalcite are widely used for various catalytic reactions due to many interesting properties such as basic properties, homogeneous mixtures of oxide with very small crystal size. In this work, a series of Ni-Mg-Al2O3 hydrotalcite-derived catalysts was prepared by carbonate co-precipitation method and the effect of calcination temperature on chemical, morphological and catalytic properties was investigated. A second metal (Mo) was introduced into Ni-Mg-Al hydrotalcite to improve the stability of catalyst. Dual-bed catalysts containing SBA-15 and hydrotalcite were prepared to improve the catalytic stability further.The catalytic performance of these catalysts was investigated under the condition of 800℃, atmospheric pressure. The catalysts were characterized by power X-ray diffraction, N2-physisorption, H2-TPR, CO2-TPD, TG, O2-TPO-MS and TEM. Some conclusions can be drawn by the analysis of the results of activity, stability and characterization.The evaluation results of activity and stability indicated that Ni-MgO-Al2O3 catalysts have high activity and stability; the catalysts keep their high activity after 2000 h with the CH4 and CO2 conversion over 95% under GHSV= 8000 mL/gcat·h. CH4 conversion keeps at 90% after the reaction of 1000 h by the catalysts added Mo under GHSV= 12000 mL/gcat·h and the dual-bed catalysts also show high activity and stability for 500 h under GHSV= 12000 mL/gcat-h.The catalysts obtained from hydrotalcite show small crystal size, strong metal-support interaction and excellent anti-carbon deposition ability. For the catalysts calcined at 600℃,700℃ and 800℃, the catalytic activity is slightly affected by the sintering of metal particles and carbon deposition but is seriously affected by the spinel-like phase. The activity undergoes a drop in the beginning of reaction due to that Ni0 is covered with MgO and the CH4 conversion keeps at 85% for 500 h. Then with the increase of reaction time, the amount of formed Mg-Al spinel-like phase increases. As the result, more and more Ni0 expose to the surface, the activity returns to the initial level and can maintain it throughout the reaction of 1500 h.Mo can be fully integrated into the structure of hydrotalcite even the added quantity to 9wt%. In addition, very stable Mo species is formed after calcination due to that Mo is blended in NiO-MgO solid solution. The Mo species cannot be reduced to Mo0 so that the addition of Mo does not improve the catalytic performance. Instead, when the Mo addition is 5%, the catalytic stability declined sharply because the active site Ni is covered by formed MoO3.The mixed states of SBA-15 and hydrotalcite play decisive roles in the properties and catalytic performance of Ni-MgO-Al2O3/SBA-15 catalysts. The catalyst whose metal oxides undergo twice calcination shows excellent anti-carbon deposition and catalytic stability. The stability is attributed to the strong metal-support interaction and the channel local effect of SBA-15.