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Pyrolytic reforming of toluene over Ni-based catalysts loaded on nano-sized and micro-sized γ-Al 2 O 3 is investigated in a fixed bed reactor under the temperature from 500 °C to 800 °C. The physicochemical properties of γ-Al 2 O 3 and Ni/Al 2 O 3 catalysts were characterized by HRTEM, XPS, XRD and H 2 -pulse chemisorption. The metal Ni loaded on micro-sized γ-Al 2 O 3 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 H 2 yield from 5.2 mmol/g-toluene to 0.4 mmol/g-toluene over Ni/nano-Al 2 O 3 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-Al 2 O 3 were centered at 7.8 nm and 28.8 nm, respectively. The metastable NiC x crystallites were observed over the spent Ni/nano-Al 2 O 3 catalyst, while only metal Ni were obtained over the spent Ni/micro-Al 2 O 3 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 –CH 2 dehydrogenation over Ni 32 C, Ni 4 C and Ni 3 C 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-Al 2 O 3 with large Ni particle size, prolonging the catalytic endurance and promoting the CNTs (Carbon Nanotubes) growth without the deposition of unexpected amorphous carbon. • The influence of γ-Al 2 O 3 on Ni/Al 2 O 3 catalytic reforming performance was investigated. • Ni0+ particle size-dependent pyrolytic reforming for co-production of H 2 and CNTs was revealed. • The carbon solubility on Ni particles with different sizes was simulated. [ABSTRACT FROM AUTHOR]