We report that both surface-bound H atoms and bulk H atoms, upon moving out from the bulk of a Ni single crystal to its surface of a (111) orientation, are reactive with adsorbed C2H2, but the two kinds of H atoms have unique product distributions. Both bulk H and surface-bound H react with C2H2 to produce adsorbed ethylidyne, CCH3, while only bulk H hydrogenates C2H2 to gas-phase ethylene and ethane, the products of interest in acetylene hydrogenation catalysis for the purification of ethylene streams. Their distinct reactivities arise from both their different directions of approach to the Π orbitals of the unsaturated hydrocarbon and their substantially different energetics. These observations demonstrate that H embedded in the metal catalyst is a reactant in alkyne hydrogenation and is not solely a source of surface-bound H which then reacts with acetylene, as proposed from correlations between the hydrogenation activity of Raney Ni and Pd catalysts and the amount of H absorbed in these catalysts. The reactivities of these two kinds of H atoms are clearly distinguished in this experiment because of the capability to synthesize either bulk H or surface-bound H cleanly in an ultrahigh vacuum environment.