Cobalt nanoparticles containing both Co²⁺ and Co⁰ species supported on carbon nitride can function as heterogeneous nanocatalysts for a general transfer hydrogenation reaction in aqueous ammonia-borane solution at room temperature. The conversions of nitroarenes, olefins, imines, aldehydes, ketones and cyanobenzene are high with superior selectivity under mild conditions. This noble-metal-free catalyst is also cheap and reusable.