Colloidal platinum nanoparticles with 2.2 nm were immobilized on gamma-iron oxide (gamma-Fe2O3) powder to make a supported catalyst (Pt/gamma-Fe2O3) with controlled Pt nanoparticle size and size distribution. This catalyst was used to catalyze the selective hydrogenation of o-chlorobenzene to o-chloroaniline without using any solvent; 99.9% selectivity at 99.95% conversion was achieved. (C) 2012 Elsevier B.V. All rights reserved.