Pressure dilution, a new method to prepare a stable Ni/fumed silica catalyst for the dry reforming of methane

Abstract

A novel method of catalyst preparation, using pressure dilution, which increased the dispersion and stability of a Ni/fumed SiO2 catalyst is described. We first studied the effect of carbon formation on the activity and stability of Ni/fumed SiO2 catalysts at different times on stream (TOS) during the dry reforming of methane (DRM). The catalysts were characterized by SEM/TEM and BET, XRD, XPS, TGA/DSC, and CO chemisorption. Catalysts prepared by impregnation of Ni(NO3)2·6H2O onto fumed SiO2, were subjected to various pretreatments which yielded high initial activities at 600 °C. The activation energy was measured to be 91 kJ/mol and the initial rate was of 8.5 [mol of CH4/gNi h], comparable with Pt-Ni alloy catalysts. Depending on the pretreatment, however, the catalysts deactivated at different rates due to carbon formation. The carbon structure was studied by SEM/TEM and its amount measured by a carbon analyzer. We found that carbon accumulates mostly in the form of carbon nanotubes (C-NT) with Ni crystallites at the top, where the reaction takes place without affecting the activity during first hours of TOS. We also found formation of clumps of entangled C-NT with Ni crystallites encapsulated by the entangled C-NT, leading to loss of active area and deactivation.A newly discovered method of catalyst preparation, referred as pressure dilution, resulted in a significant increase in Ni dispersion from 19% to 61%, when supported on fumed SiO2. Moreover, Ni dispersion can be controlled by the pressure applied during catalysts’ preparation. The higher dispersion resulted in a higher catalyst activity and increased stability towards carbon formation. A hypothesis of the re-dispersion process occurring during p-dilution is proposed.