Photosynthetic Water Splitting: In situ Photoprecipitation of Metallocatalysts for Photoevolution of Hydrogen and Oxygen

Abstract

Using the technique of in situ photoprecipitation, a comparative study of the structurally homologous ionic species hexachloroplatinate ([PtC&12-), hexachloroosmiate ( [OsC&12-), and hexachloroiridates was conducted for photoconversion to metallocatalysts for photosynthetic hydrogen evolution. As with earlier [PtC1612-studies, [osc16l2-can be photoconverted to a metallocatalyst at photosystem I (PSI), although at a rate about 50% slower than that of [PtC&]2-. However, once photoprecipitated, its catalytic action for H2 production was 3 times as high as that of metallic platinum. Simultaneous photoevolution of 02 and H2 was observed in [osc16l2--photoprecipitated thylakoids. Maximum hydrogen evolution rate was 113 nmol-h-'.mg chl-'. Surprisingly, neither [IrC&12--nor [IrC&13-treated thylakoids were able to produce H2. Analysis indicated that [IrC1612-was able to accept only one electron by transformation to [IrC&]3-which was completely unable to acquire subsequent electrons from PSI. The inability of [IrC&l" to be reduced to metallic iridium is presumably due to a high energy level barrier of [IrC&]3-reduction.

11 Re-routing redox chains for directed photocatalysis

“…This ensures that an electron is transferred within the time constraints of the system, which is ~65 ms before the electron on F B recombines with P 700 + . The same phenomenon has been observed for hexachloroosmiate ([OsCl 6 ] 2− ) [18] and hexachlororuthenate ([RuCl 6 ] 2− ) [19]. Upon illumination of PSI reaction centers, electrons are promoted to the F B iron-sulfur cluster, where they are capable of reducing ferric ions to ferrous ions in what is known as the Hill reaction of photosynthesis .…”

“…Colloidal platinum can be directly deposited onto thylakoid membranes and isolated PSI complexes in an approach pioneered by Greenbaum and coworkers [16][17][18]. This ensures that an electron is transferred within the time constraints of the system, which is ~65 ms before the electron on F B recombines with P 700 + .…”

Z-scheme solar water splitting via self-assembly of photosystem I-catalyst hybrids in thylakoid membranes

“… a Sacrificial electron donor: 100 mM sodium ascorbate. b Estimated from P700 + signal ratios determined by EPR (see ESI Fig. S1). c Ref. 28. d Ref.…”

10 Photosynthesis and hydrogen metabolism revisited. On the potential of light-driven hydrogen production in vitro

“…Hill and Bendall proposed the Z-scheme in 1960 and agreed on PSI as the final reducing site in photosynthetic electron transport [22]. The 'Hill Reaction ' can be followed alternatively using hexachloro palatinate [PtCl 6 ] -2 on thylakoid membranes and isolated PSI [92][93][94]. In a process referred to as photoprecipitation, metallic Pt particles form on the stromal ridge of PSI, shuttling electrons from F B to protons in the immediate vici nity of the catalytic Pt surface [95].…”