Influence of the chemical activation of carbon nanofibers on their use as catalyst support

Three nickel catalysts supported on carbon nanofibers (CNFs) have been prepared by deposition–precipitation (ca. 3%, w/w). The CNFs were prepared by catalytic chemical vapour decomposition of ethylene over a Ni/SiO2 catalyst, and chemically activated with RbOH (CNFRbOH) and KOH (CNFKOH). The materials were characterized by transmission electron microscopy (TEM), N2 adsorption–desorption, temperature-programmed oxidation, X-ray diffraction (XRD), acid–base titrations and temperature-programmed decomposition in helium. KOH was the most effective activating agent, developing carbon porosity, decreasing the crystalline character and increasing surface acidity to a higher extent. After nickel introduction, the catalysts were characterized by temperature-programmed reduction, XRD, TEM, N2 adsorption–desorption and acid–base titrations. Surface-area weighted mean Ni particle diameters (post reduction at 400 °C) followed the sequence: Ni/CNFKOH (4.9 nm) < Ni/CNFRbOH (8.6 nm) < Ni/CNF (25.5 nm), where a further activation of the support led to smaller/better dispersed nickel particles. The surface acidity of the catalysts followed the sequence: Ni/CNFRbOH < Ni/CNF < Ni/CNFKOH. The gas phase hydrogenation of butyronitrile was used as a surface characterization test, where bigger nickel particles consistently developed higher catalytic activity. Selectivity, however, was not sensitive to metal particle size, but increased towards condensation products with increasing catalyst acidity.

Figure optionsDownload high-quality image (161 K)Download as PowerPoint slideResearch highlights▶ Chemical activation by RbOH and KOH impacted on CNF porosity, acidity and crystallinity. ▶ CNF chemical activation impacted on size and morphology of deposited Ni particles. ▶ Ni particles deposited on activated CNFs promoted butyronitrile hydrogenation to lower extent. ▶ Selectivity to amines was not sensitive to metal particle size, but it was to catalyst acidity.