Ligand Effects on the Hydrogen Evolution Reaction Catalyzed by Au13 and Pt@Au12: Alkynyl vs Thiolate

We studied the effects of the alkynyl and thiolate ligands (L) on the catalytic activities of Au-25(L)(18) and PtAu24(L)(18) for the hydrogen evolution reaction (HER): the ligands used were C CArF (Ar-F = 3,5-bis(trifluoromethyl)-phenyl) and SC2H4Ph. The onset potential for Au-25(C CArF)(18) was shifted to the positive side by similar to 70 mV with respect to that for Au-25(SC2H4Ph)(18), indicating that the HER activity of the pure Au-13 core was significantly enhanced when protected by alkynyl ligands. This remarkable ligand effect was explained by the acceleration of the protonation step to the more negatively charged Au-25(C CArF)(18) than the less negatively charged Au-25(SC2H4Ph)(18). In contrast, the HER activities of the doped PtAu24(C CArF)(18) and PtAu24(SC2H4Ph)(18) were comparable. The negligible ligand effect on the activity of the Pt@Au-12 core was ascribed to the change in the rate-determining step from the protonation step to the reductive regeneration step of the catalysts.