Filamentous carbon growth on nickel/silica: potassium and bromine as catalyst promotors

In a previous Communication, we reported the catalytic growth of highly ordered carbon filaments during the hydrodechlorination of chlorobenzene over nickel/silica at the remarkably low temperature of 553 K. This low temperature growth was attributed to the presence of bromine and potassium on the catalyst surface, which facilitated a restructuring of the active sites leading to a destructive chemisorption of the reactant that preceded the dissolution and precipitation of carbon in an ordered fashion. In this paper, we examine the effect of bromine and potassium built into (by impregnation with KOH, HBr and KBr) a Ni/SiO2 parent sample on filamentous carbon production through ethylene decomposition. The parent catalyst delivered a low yield of carbon (0.2 g(C)g(cat)(-1)) and promoted the competing hydrogenation (to ethane) step to the same degree; hydrogenolysis to methane represented the only other significant reaction. The nature of the carbon deposition has been characterised by high-resolution transmission electron microscopy (HRTEM) and temperature-programmed oxidation (TPO). Impregnation with KOH resulted in little change in carbon yield but the hydrogenolysis step was completely suppressed. A combination of CO chemisorption/temperature programmed desorption has been employed to probe the electronic structure of the nickel sites where the presence of potassium limited CO uptake in contrast to surface bromine which served, through electron withdrawal, to strengthen the CO-Ni interaction. Incorporation of bromine into the catalyst resulted in a marked increase in nickel particle size and a dramatic enhancement of carbon yield; the sample treated with KBr delivered the appreciably higher carbon yield of 7.6 g(C)g(cat)(-1)). While the presence of bromine on the surface served to enhance carbon deposition, the additional incorporation of potassium raised the degree of order in the carbon growth.