Elucidating the Key Role of pH on Light-Driven Hydrogen Evolution by a Molecular Cobalt Catalyst

Photochemical hydrogen generation from aqueous solutions can be accomplished with a combination of at least three molecular components: namely, a photosensitizer, a hydrogen-evolving catalyst, and an electron donor. A parameter that plays a key role in the light to hydrogen efficiency of such three-component systems is the solution pH. While this evidence has been usually observed in several works aiming at identifying catalysts and optimizing their performances, detailed studies capable of shining light on this issue have been extremely rare. Hence, the pH dependence of a reference three-component system based on Ru(bpy)32+ (where bpy = 2,2′-bipyridine) as the sensitizer, a cobaloxime HEC, and ascorbic acid as the sacrificial donor has been studied with care by merging photocatalytic hydrogen evolution kinetic data and detailed time-resolved spectroscopy results. The photocatalytic activity shows a bell-shaped profile as a function of pH which peaks at around pH 5. While at acidic pH (pH <5) the hydrogen-evolving activity is limited by the photogeneration of reduced sensitizer species, at neutral to basic pH (pH >5) the production of hydrogen is hampered by the disfavored protonation of the reduced Co(I) species. In this latter instance, however, hydrogen evolution is mainly slowed down rather than inhibited, as it is instead in the former case. This evidence affects the time scale of the photocatalysis and gives the opportunity to rationalize and correlate different results obtained with the same cobaloxime catalyst but under rather diverse experimental conditions.