Dry Reforming of Methane over Ni-Al2O3 and Ni-SiO2 Catalysts: Role of Preparation Methods

摘要

The production of synthesis gas via the conversion of the two greenhouse gases CO2 and CH4 is an efficient process due to its dual industrial and environmental interest. The catalysts based on Ni-SiO2 and Ni-Al2O3 are typical and suitable for this reaction due to their mechanical strength, their good chemical and thermal stability in addition to their low cost and good availability. In this work, we have compared the catalytic performances of these two types of catalysts prepared by two different synthesis methods in dry reforming of methane (DRM).The results indicate that the catalytic performances are much more dependent on the support properties and that they are deeply influenced by the catalyst synthesis method. The textural properties as shown by N-2-physisorption analysis are strongly dependent on the support nature in the case of the catalysts prepared by the microemulsion (ME) method and the alumina-based Ni catalyst has a higher specific surface area and a higher pore volume compared to the SiO2 based one. The XRD, H-2-TPR and XPS results indicate that the preparation method has a significant influence on the state of NiO species. A Ni particle in the two Ni-SiO2-ME and Ni-Al2O3-ME catalysts prepared by microemulsion is much smaller. The strong metal support interaction promotes the formation of NiAl2O4 and Ni2SiO4 species respectively during the catalyst preparation process and makes the reduction of corresponding catalysts very difficult which may lead to a decrease in the content of active Ni species and give the Ni-Al2O3-ME catalyst a relatively low catalytic activity in DRM, especially when it is reduced under unfavorable conditions as is the case in this work. However, the strong metal support interaction between Ni and the support is also of beneficial to the formation and stabilization of small Ni particles well dispersed on the support after reduction of the Ni-SiO2-ME catalyst. In this system, the sintering and the carbon deposition are inhibited and the catalyst presents both better activity and stability. The Ni/Al2O3 catalyst exhibits a synergistic effect between the various phases NiO and NiAl2O4 formed during the synthesis process due to the different interactions strength between metal and support, which are in favor of the dispersion and stabilization of NiO species. As a result, Ni/Al2O3 provided with both proper textural properties and this synergistic effect, exhibits superior catalytic performances in term of activity, selectivity and stability in DRM. Despite the formation of carbon over this catalyst, it maintains its stability during a long-term test of more than 66 hours. This is due to the formation of active type of carbon and the delocalization of the Ni active sites on the latter to maintain their activity.