It is reported that catalytic hydrogenation of cinnamaldehyde to cinnamyl alcohol is a structural sensitive reaction dependent on size and type of metal doper of unsupported platinum nanocrystals used. Smaller sizes of platinum nanocrystals are found to give lower selectivity to cinnamyl alcohol, which suggests the high index Pt sites are undesirable for the terminal aldehyde hydrogenation. A plot of reaction selectivity across the first row of transition metals as dopers gives a typical volcano shape curve, the apex of which depicts that a small level of cobalt on platinum nanocrystals can greatly promote the reaction selectivity. The selectivity towards cinnamyl alcohol over the cobalt doped Pt nanocrystals can reach over 99.7%, following the optimization in reaction conditions such as temperature, pressure and substrate concentration. Detailed studies of XRD, CO chemisorption (for FTIR), TEM, SEM, AES and XPS of the nanostructure catalyst clearly reveal that the decorated cobalt atoms not only block the high index sites of Pt nanocrystals (sites for Co deposition) but also exert a strong electronic influence on reaction pathways. The d-band centre theory is invoked to explain the volcano plot of selectivity versus metal doper.