Ni0+ particle size-dependent pyrolytic reforming of toluene over Ni/Al2O3 for the co-production of hydrogen and CNTs

Abstract

Pyrolytic reforming of toluene over Ni-based catalysts loaded on nano-sized and micro-sized γ-Al2O3 is investigated in a fixed bed reactor under the temperature from 500 °C to 800 °C. The physicochemical properties of γ-Al2O3 and Ni/Al2O3 catalysts were characterized by HRTEM, XPS, XRD and H2-pulse chemisorption. The metal Ni loaded on micro-sized γ-Al2O3 gave the steady toluene conversion of more than 80% and graphite yield of 87.3% after 1 h of reforming reaction. The rapid decrease of H2 yield from 5.2 mmol/g-toluene to 0.4 mmol/g-toluene over Ni/nano-Al2O3 was obtained during the 1 h of reforming reaction at 500 °C. The particle size of metal Ni over nano-sized and micro-sized γ-Al2O3 were centered at 7.8 nm and 28.8 nm, respectively. The metastable NiCx crystallites were observed over the spent Ni/nano-Al2O3 catalyst, while only metal Ni were obtained over the spent Ni/micro-Al2O3 catalyst. Ni0+ particle size-dependent crystalline phase was evidenced by MD simulations, where incorporation of more carbon atoms was achieved on small Ni particles. The energy barrier for –CH2 dehydrogenation over Ni32C, Ni4C and Ni3C were estimated to be 90.2, 105.1 and 116.0 kJ/mol through the DFT simulation, respectively. It can be concluded that the crystalline Ni phase was maintained over Ni/micro-Al2O3 with large Ni particle size, prolonging the catalytic endurance and promoting the CNTs (Carbon Nanotubes) growth without the deposition of unexpected amorphous carbon.