Visible-Light-Driven Photoproduction of Hydrogen Using Rhodium Catalysts and Platinum Nanoparticles with Formate

Photochemical hydrogen production
is carried out using molecular
Rh complexes and sodium formate in the presence of platinum nanoparticles
(PtNPs) in aqueous buffer solution. Visible-light-driven photocatalytic
reactions for hydrogen production with and without nicotinamide adenine
dinucleotide (NAD+) follow two different pathways. Complex
[Cp*Rh­(bpy)­(OH2)]2+ selectively reduces NAD+ to generate NADH using formate as a proton and electron donor
and the chemically generated NADH is sequentially used by PtNPs upon
photoactivation of eosin Y to produce hydrogen. However, hydrogen
is also produced in photoreactions of the Rh catalyst and PtNPs with
formate in the absence of NAD+ and eosin Y. The second
pathway for hydrogen production was performed under the conditions
without NAD+ and eosin Y and derived from a direct electron
transfer from in situ generated rhodium­(III)-hydride
species to photoexcited PtNPs. The direct electron transfer from the
rhodium-hydride species to visible-light-driven PtNPs was first observed
in this study. These two pathways for hydrogen production showed different
rate-limiting steps based on a Hammett plot using Rh catalysts containing
electron-donating and electron-withdrawing groups. Kinetic isotope
effects as well as Hammett plot supported the rate-limiting step of
the NADH generation for the first pathway of hydrogen production and
the Rh–H formation for the second pathway.