Ab initio molecular dynamics thermal decomposition of methane on Ni-Cu/γ-Al2O3 catalysts for CNT production: Effect of Cu promoter

The decomposition of a single CH4 molecule on the surface of Ni-Cu/γ-Al2O3 catalyst with 1:0, 1:1, and 0:1 Ni-Cu molar ratios for carbon nanotubes (CNT) production was investigated via ab initio molecular dynamics (AIMD) simulations with supporting empirical data. The simulation model was based on the formation of Ni-Cu alloy on γ-Al2O3 from empirical data. Structural relaxation and AIMD simulations were performed using the Local Density Approximation - Vosko-Wilk-Nusair (LDA-VWM) exchange correlation functional, Gaussian double zeta plus polarization function basis set (DNP) at 2 × 2 × 1 k-point calculation and orbital cut-off of 3.5 Å, with simulation parameters: T = 850 °C, time step = 1.9 fs (78.5 a.u.), total simulation time = 0.38 ps, with the canonical NVT (constant amount (N), volume (V) and temperature (T)) thermodynamic ensemble and Generalized Gaussian Moment (GGM) thermostat. Simulations predicted an increase in catalytic activity in the presence of Cu with the lowest C deposition on 0:1 Ni-Cu and is supported by the marked decrease in CNT yield in the absence of Ni. Simulation data also suggest a weak metal - γ-Al2O3 interaction in favor of tip-growth CNTs. Calculations predicted a decrease in crystal size with the addition of Cu to Ni/γ-Al2O3 indicative of the formation of a Ni-Cu alloy, and is supported by a corresponding decrease in CNT diameter. Calculations also predicted a decrease in the reactant-product transition state energy difference in 1:1 Ni-Cu supported by an increase in catalytic activity from empirical data.