A Study On The Promoters For Modification Of Silica-supported Nickel-based Methanation Catalysts

In the 21 st century, extensive consumption of fossil energy lead to sereval sevious issues, including energy shortage and environmental pollution（eg. Fog and haze）, which has been the bottle-neck factor restricting the sustainable development of human society. Thus, it becomes the focus of research to develop the clean and efficient energy. Based on the properties of energy production and consumption structure in china, developing coal to SNG becomes an important way to realize clean utilization of coal, and methanation reaction process and catalyst synthesis have been considered as the core part in this technology. Ni-based catalysts are vulnerable to sintering and coking under the bed overheating, further leading to the catalyst deactivation. According to the above problem, therefore, in this paper, the refractory oxide, SBA-15 and SiO₂-TiO₂（ST） are applied as supports to prepare the Ni catalyst. The influence of adding V, Ce, Zr to Ni/SBA-15 catalyst on the catalytic performance in CO methenation has been investigated; The mechanism of improving the anti-sintering and anti-coking, which benefit from the TiO₂ in Ni/SiO₂-Ti O₂ catalyst has been discussed. The main research contents and results of the work are as follows:（1） The Ni-M（V, Ce, Zr）/SBA-15 catalysts were prepared by ultrasonic impregnation. The influences of different Ni loadings, V, Ce, and Zr addition and SBA-15 channel confined effect to the catalytic activity, selectiviey, anti-sintering for catalysts are investigated. The catalytic tests were examined under the reaction conditions of 180–400°C, 1.0 MPa, 15000 mL?g-1?h-1, and the results show that the addition of V, Ce, and Zr improve the catalytic performance of catalysts. The 10Ni-5V/SBA-15 displays the best CO conversion and CH4 seclectivity（99.9% CO conversion,95.5% CH4 seclectivity）, and the catalyst has the high stability after 60 h long time stability test at 550 °C.（2） Characterization using H₂-TPR, H₂-TPD, XPS, TEM and XRD measurements revealed that addition of V, Ce, Zr not only enhanced the Ni dispersion of 10Ni/SBA-15 catalyst, which generated more exposed Ni active sites and improved the ability of adsorption and dissociation for H₂, but also V, Ce promoters could facilitate the CO dissociation via electronic effect, Zr promoter species lead to the formation of oxygen vacancies which increase the ability of CO adsorb and dissociate; these behaviors reduced the reaction energy barriers and then increased the catalytic activity and CH4 selectivity. Moreover, it was confirmed that the addition of promoter and SBA-15 channel confined effect could restain Ni agglomeration and sintering, and then enhance the stability of catalyst.（3） A series of composite support SiO₂-TiO₂（ST） were prepared by free template sol–gel method, and the Ni/ST catalysts were obtained through ultrasonic impreganation. The effects of catalysts with different Si/Ti atomic ratio on the catalytic activity, CH4 selectiviey, anti-sintering and anti-coking were investigated. The experimental results suggested that 10Ni/STx（x= 50, 10, 5, 1） diaplayed better catalytic performances relative to Ni/SiO₂、Ni/TiO₂ and Ni/Al2O3, among these catalysts, 10Ni/ST5 catalyst has the best catalytic activity, under the conditions of 370 oC、1.0 MPa、15000 mL?g-1?h-1, the CO conversion can reach 99.9% and the CH4 selectivity is 95.6%, meanwhile, the catalyst exhibits excellent anti-sintering and anti-coking during 144 h long time stability test.（4） The composite support ST in Ni/STx catalyst could provide suitable metal-support interaction, and then facilitate Ni dispersion, which makes these catalysts have more suitable Ni active sites for methanation. The phenomenon that proper amount of the isolated TiO₂ phase appeared in ST5 composite support can serve as the role of electronic promoter was first discovered, illustrating that isolated TiO₂ not only could effectively facilitate the Ni dispersion of catalyst, but also trigger the electron transfer and enhance the electron cloud density of Ni0（Ni0 �' Niδ-）, which could weaken the C-O bond of Ni-C-O bond and enhance CO dissociation ability, and then improve the catalyic activity, selectivity and anti-sintering and anti-coking of catalyst. In addition, the experimental results revealed that three deposited carbon species at least formed on the used catalysts: active carbon species, carbon fiber species（carbon whiskers, carbon filaments）, and graphite carbon species; comparing to other used catalysts, 10Ni/ST5 catalyst has the lowest amount of deposited carbon species and the best catalytic stability.