Silver nanoparticles (NPs) immobilized by core–shell Co@SiO₂ nanospheres (Co@SiO₂/Ag) have been in situ generated during the hydrolysis of ammonia borane (AB). The as-synthesized Co@SiO₂/Ag nanocomposites (NCs) were characterized by XRD, SEM, TEM, EDX, SAED, ICP, XPS and the N₂ adsorption–desorption techniques. It is found that well dispersed silver NPs (∼2 nm) were successfully immobilized on core–shell Co@SiO₂ nanospheres. The resulting Co@SiO₂/Ag NCs exhibited a superior catalytic activity for hydrogen generation from hydrolysis of AB, as compared to that of Co@SiO₂, free Ag, and the physical mixture of Co@SiO₂ and Ag NPs. The hydrolysis activation energy for Co@SiO₂/Ag NCs was measured to be approximately 25.6 kJ mol⁻¹, which is lower than most of the reported values for the same reaction by other catalysts, indicating the superior catalytic performance of the Co@SiO₂/Ag NCs.