Surface synergy of Fe-doped Ni-Mo alloys enabling controlled carbon nanotubes growth from oil palm waste

The catalytic conversion of lignocellulosic biomass into carbon nanomaterials remains fundamentally constrained by the instability of active metal phases and the poor regulation of carbon flux derived from complex pyrolysis atmospheres. Herein, a dual-stage catalytic pyrolysis strategy is developed to elucidate how catalyst lattice engineering and biomass-derived gas composition cooperatively govern carbon nanotube (CNT) nucleation and growth over Fe-doped Ni/Mo bimetallic catalysts. Characterization studies revealed that moderate Fe incorporation (i.e., 6 wt%) leads to partial substitution of Ni by Fe at octahedral sites in the NiO lattice, as supported by Rietveld refinement showing reduced Ni site occupancy together with changes in crystallite size and microstrain, without the formation of bulk Fe oxide phases. The NiMoFe-6 catalyst operating at a catalytic reactor temperature of 600 °C and a pyrolysis reactor temperature of 800 °C, achieved CNTs with the highest yield and the narrowest outer diameter (20.8 ± 0.5 nm). The gas composition of oil palm fiber (OPF) pyrolysis demonstrates that CNT growth is governed by a mixture of small hydrocarbon molecules (i.e., C₃H₆ and C₄H₁₀) rather than a single carbon source. Density functional theory calculations successfully elucidated that Mo incorporation homogenizes facet-dependent adsorption energies on NiMoFe alloys and strongly adsorbing facets activate olefins for nucleation. Overall, this study presents a nexus relationship between catalyst lattice engineering, the gas composition of OPF, and facet-dependent carbon adsorption, which is essential for the stability of the alloy, carbon flux regulation, and the structural evolution of CNTs.