Narrowly dispersed, silica-supported Cu nanoparticles (ca. 2 nm) prepared via surface organometallic chemistry from a mesityl complex [Cu5Mes5] are highly active for the hydrogenation of a broad range of alkynes. High-throughput experimentation allows for identifying the optimal ligand and reaction conditions to turn these supported Cu nanoparticles into highly chemo- and stereoselective catalysts for the preparation of Z-olefins (overall, 23 examples). For instance, PCy3-modified Cu nanoparticles semihydrogenate 1-phenyl-1-propyne to cis-β-methylstyrene (20 bar H2, 40 °C) with turnover number and turnover frequency of ca. 540 and 1.9 min-1, respectively, and with 94% selectivity at full conversion.