In order to promote process intensification in syngas production from biomass gasification, our research team has already considered the integration of transition metal-based nanocatalysts in the biomass feedstock through its impregnation with metal salt aqueous solutions. The purpose of this work is to provide new insights into the complex physicochemical and catalytic mechanisms involved in this catalytic pathway from nickel salt. Applying a primary vacuum during impregnation allowed the rate of nickel insertion to be optimized and the generation of strong interactions between the metal cations and the lignocellulosic matrix. During biomass pyrolysis, Ni⁰ nanoparticles (NPs) form in situ below 500 °C through carbothermal reduction and provide the active sites for adsorption of aromatic hydrocarbons and subsequent catalytic conversion. In order to test whether it was possible to improve the catalytic efficiency of Ni⁰ NPs by making them available right from the pyrolysis onset, some preformed Ni⁰ NPs were inserted into the biomass prior to pyrolysis. The in situ generated Ni⁰ NPs exhibit higher catalytic efficiency, particularly for aromatic tar conversion, than preformed Ni⁰ NPs. The high decrease in hard-to-destroy aromatic hydrocarbons formation during pyrolysis is of particular interest in the overall gasification process. The proposed catalytic strategy reveals promising for simplifying the cleaning up of the producer gas.