The present study demonstrates that the silica-encapsulated Ni and Ru nanoparticles (nano-Ni@SiO2 and nano-Ru@SiO2) are catalysts more active and stable than the currently known catalysts for the production of COx-free hydrogen through ammonia decomposition. The effects of SiO2 encapsulation are closely related to the nature of the enwrapped core elements. The core size and exposure can be modified upon acid treatment, and the core surface property can also be modified by La or Ce doping. Both approaches result in activity enhancement. The superior performance and stability of the core-shell structures are originated from the unique feature of microcapsular-like reactor. (C) 2010 Elsevier B.V. All rights reserved.