ChemCatChem 2014, 6, 702 -710 702 CHEMCATCHEM MINIREVIEWS Scheme 1. Full and partial hydrogenation products formed from 1,3-butadiene, 1,3-cyclohexadiene, 1,3-cyclooctadiene and cyclopentadiene.Scheme 2. Single-site mechanism of full and partial hydrogenation of 1,3-cyclohexadiene catalysed by a single atom. Adapted from Ref. [13].Scheme 3. Multi-site mechanism of full and partial hydrogenation of 1,3-cyclohexadiene catalysed by an ensemble of surface atoms.