The core-shell nanostructures (M@SiO(2), Al(2)O(3), MgO; M=Fe, Co, Ni, Ru) are synthesized and applied in the production of CO(x)-free H(2) through NH(3) decomposition. The characterization results indicate that the dimension and dispersion of the core particles is dependent on the core constitution, and the shell encapsulation effect (thickness of shell, single or hierarchical enwrapping) is determined by the nature of cores as well as the core-shell interaction. The core property can also be influenced by core starting material and nominal core/shell ratio. The obtained core-shell catalysts show not only superior activity but also better stability than the naked nanoparticles or the supported counterparts. The presence of stable shells provides the unique environment around cores, functioning as microcapsular-like reactors in which adsorption and catalytic reaction are enhanced on the cores. The stable shells effectively prevent the core particles from aggregation during reaction. (C) 2010 Elsevier B. V. All rights reserved.