Low-temperature (553 K) catalytic growth of highly ordered carbon filaments during hydrodechlorination reactions

In the gas-phase hydrodechlorination of chlorobenzene to benzene over Ni/Si O2, a pretreatment of the catalyst with HCl or HBr gas induced the growth o f highly ordered carbon filaments from the catalyst surface. This filamento us growth was observed to occur at 553 K, i.e., at least 150 K less than th at reported previously in the literature for the catalytic formation of gra phitic carbon. The hydrogen halide pretreatment also resulted in an appreci able increase in the average nickel particle size and a suppression of hydr odechlorination activity. In the absence of this pretreatment the catalyst exhibited stable hydrodechlorination activity and there was no evidence of any filamentous carbon growth. Analysis of the halide-treated samples revea led the presence of potassium that was not evident in the freshly activated catalyst but was introduced as a result of a corrosive attack of the hydro gen halide gas on the microreactor unit. The source of our low-temperature ordered carbon growth must be electronic in nature and is attributed to an intimate combination of halogen/alkali metal on the catalyst surface that f acilitates a restructuring of the active sites where a destructive chemisor ption of chlorobenzene precedes the dissolution of carbon and precipitation in an ordered fashion.