Iron-based nanocatalysts were synthesized by a green and facile one-pot hydrothermal carbonization (HTC) process. Precursory iron salts with different concentrations were dissolved together with the lignocellulosic matrix (i.e., pinewood sawdust) in the solution, and the mixtures were heated to 200 degrees C for 1 h in a sealed autoclave. After hydrothermal treatment, carbon spheres were formed with the iron precursors tightly embedded in the microsphere/mesopheres of the amorphous carbon matrix. By contrast, the impregnation of the iron salts onto the hydrothermal carbons (hydrochars) derived from HTC of sawdust were conducted as a control method. The nanomaterials prepared by these two approaches were evaluated comparatively. Compared to the impregnation approach, a nanocatalyst with more uniformly dispersed Fe nanoparticles (NPs) immobilized on the hydrochars was produced by the one-pot synthesis. Furthermore, the particle size of the Fe NPs and surface area of the nanocatalysts via the one-pot synthesis can be manipulated. The catalytic abilities of the Fe-based nanomaterials prepared by the two approaches were then tested in a fixed bed tubular reactor for biomass tar using phenol as the model compound. The nanocatalysts prepared by the one-pot synthesis exhibited higher catalytic activity in the thermal decomposition of the phenol at mild temperatures and resistance to coke deposition. The present study showed that the simple and efficient one-pot synthesis assisted uniform dispersion of Fe NPs on the surface of hydrochar through a surface-mediated chemistry. In addition, such facile one-pot synthesis may be applied to synthesize other metal or metal oxide NPs embedded in the hydrochars with more complex structures and unique chemical/physical properties. (C) 2016 Elsevier B.V. All rights reserved.