Cobalt–carbon nanocomposite catalysts of gas-phase hydrodechlorination of chlorobenzene

The catalytic efficiency of three different cobalt–carbon composites has been compared in the gas-phase hydrodechlorination (HDC) of chlorobenzene (CB) in a flow type fixed bed reactor. The Co@C composite was synthesized by evaporation of overheated liquid drop of Co in the flow of an argon–butene mixture; the Co/C composite – by pyrolysis of sawdust impregnated with Co(NO₃)₂ water solution, and the Co/CNT composite – by the impregnation of carbon nanotubes with Co(NO₃)₂ water solution. Different oxidation states of cobalt were observed in the studied composites according to XPS, TPR, and in situ vibrating sample magnetometry results: predominantly Co⁰ in Co@C, CoO in Co/C, and Co₃O₄ in Co/CNT. TPR showed the possibility of the reduction of Co₃O₄ to CoO and even to Co⁰ under conditions of HDC and established the temperatures of these transitions. TEM revealed that Co⁰ nanoparticles in the Co@C composite are encapsulated by the thin carbon shell, CoO nanoparticles in Co/C are immersed in the carbon matrix, and Co₃O₄ nanoparticles in Co/CNT are located both on the surface and inside the channels of CNTs. All the composites demonstrated activity in HDC of CB. The efficiency of not only Co⁰ but CoO in the CB HDC was shown. The CB conversion at 150–250 °C for Co/C and Co/CNT composites, containing predominantly cobalt oxides, was higher than that for Co@C one, containing Co⁰ nanoparticles coated with the thin carbon shell. The Co/C composite with the lowest Co content (1.3 wt.%) was more effective in HDC than Co/CNT (14.6 wt.% of Co) and Co@C (79 wt.% of Co) ones. High CB conversion for the Co@C composite at 350 °C resulted from the catalytic action of the graphene shell of Co particles, activated by subsurface Co⁰.