Study On Preparation And Catalytic Performance Of Nickel Catalysts For Methanation Reaction

Syngas methanation process and catalyst have been recognized as the most important part in the process of coal to Synthetic Natural Gas (SNG). Aiming at the core scientific issues of the traditional methanation catalyst such as, poor activity at low temperature, easily sintering, coking, and deactivating of the catalysts at high temperature, we carried out systematic research in the following aspects:(1) Ni-V2O3/Al2O3 catalysts with various V contents were prepared by co-impregnation method for CO methanation. The influence of vanadium oxide addition on catalyst structure, morphology, surface characteristics, distribution and reducibility of Ni species as well as catalytic activity and stability, was investigated in detail. Compared to the catalyst without vanadium, the Ni-V2O3/AI2O3 catalysts showed significant improvement in the activity, stability, and resistance to coke formation in CO methanation. It was found that Ni3V2O8 was formed during the calcination of the Ni-V2O3/Al2O3 catalysts, which led to the formation of smaller Ni particle sizes (ca.3.0 nm) as compared to the case without vanadium oxide addition. The higher catalytic activity of the Ni-V2O3/Al2O3 catalysts for CO methanation was mainly due to the larger H2 uptake, the higher Ni dispersion as well as the smaller metallic Ni nanoparticles. The oxidation-reduction cycle of V2O3 could increase the oxygen vacancies, which enhanced the dissociation of CO2 by-product and generated surface oxygen intermediates, thus preventing carbon deposition on the Ni particles in CO methanation. In addition, these catalysts also showed high activities for CO2 methanation at both atmospheric and 2.0 MPa pressures.(2) Based on the above results, a series of ternary Ni-V-Al metal oxide composites with ordered mesoporous structure and various V contents were rationally designed and facilely synthesized via a facile one-pot evaporation-induced self-assembly (EISA) strategy for CO methanation. The results showed that the Ni-V-Al catalysts with ordered mesopores had better catalytic properties than the impregnation-derived Ni catalysts supported on alumina supports with ordered mesopores or with unordered-mesopores. The addition of proper amount of vanadium oxide in the ordered mesoporous Ni-V-Al catalysts further promoted the catalytic activity towards CO methanation. In a 110 h atmospheric-pressure lifetime test, the ordered mesoporous Ni-V-Al catalyst showed significant improvement in both anti-coking and anti-sintering, mainly because of the smaller Ni particle size (ca.3 nm), the strong metal-support interaction as well as the appropriate channel diameter (ca.10 nm).(3) A group of ternary Ni-Cr-Al metal oxide composites with ordered mesoporous structure and varied Cr and Ni contents via the one-pot EISA method were prepared and tested for CO methanation to produce SNG. It was found that the Cr species were incorporated in the framework of the ordered mesoporous alumina (OMA) while Ni particles were distributed in the OMA channels, and this long-range ordered mesoporous structure could be well maintained when the total loading of nickel and chromium oxides was below 22 wt%. In general, the optimized ordered mesoporous Ni-Cr-Al catalysts showed remarkable enhancement in both anti-coking and anti-sintering, and much higher catalytic activity and excellent stability as compared with the conventional method-derived catalysts. The improved catalytic performances were attributed to the much reduced Ni particle size embedded in the OMA channels, the confinement effect of the mesopores as well as the promotion effect of Cr, which could significantly increase their H2 and CO uptakes.(4) A group of ternary Ni-Fe-Al metal oxide composites were prepared via an EISA method and tested for CO methanation to produce SNG. It was found that the Fe species were in the state of Fe3O4 and FeO rather than metallic Fe over the reduced Ni-Fe-Al catalysts. The addition of proper amount of Fe species in the ordered mesoporous Ni-Fe-Al catalysts could promote the catalytic activity due to the improved H2 uptake. In addition, Ni-Fe-Al catalysts also showed enhanced both anti-coking and anti-sintering properties.(5) The Ni-CeO2/Al2O3 catalysts with a NiO loading of 40 wt% were prepared by a modified deposition-precipitation (DP) method. Compared with the Ni catalysts prepared by co-impregnation and sequential impregnation methods, the Ni catalysts synthesized by DP method showed the enhanced catalytic performance as well as resistance to both coking and sintering for CO methanation. It was found that CeO2 nanoparticles were selectively deposited on the surface of NiO rather than on Al2O3 due to the electrostatic interaction during the DP process, effectively restraining Ni particles from sintering during the reduction and reaction at the high temperatures, and inhibiting coke formation by increasing supply of active oxygen species on the nickel surface.(6) The production of low-temperature methanation catalyst Ni-V2O3/Al2O3 was scaled up in laboratory and the shaping process was also investigated. The shaped catalysts showed similar overall performances (morphology, strength, activity and stability) with the commercial ones, and were very stable during 100 h lifetime test in laboratory scale.