A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution

Abstract

A tetra-nickel-containing polyoxotungstate, Na6K4[Ni4(H2O)2(PW9O34)2]·32H2O (Na6K4-Ni4P2), has been synthesized in high yield and systematically characterized. The X-ray crystal structure confirms that a tetra-nickel cluster core [Ni4O14] is sandwiched by two trivacant, heptadentate [PW9O34](9-) POM ligands. When coupled with (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate [Ir(ppy)2(dtbbpy)][PF6] as photosensitizer and triethanolamine (TEOA) as sacrificial electron donor, the noble-metal-free complex Ni4P2 works as an efficient and robust molecular catalyst for H2 production upon visible light irradiation. Under minimally optimized conditions, Ni4P2 catalyzes H2 production over 1 week and achieves a turnover number (TON) of as high as 6500 with almost no loss in activity. Mechanistic studies (emission quenching, time-resolved fluorescence decay, and transient absorption spectroscopy) confirm that, under visible light irradiation, the excited state [Ir(ppy)2(dtbbpy)](+)* can be both oxidatively and reductively quenched by Ni4P2 and TEOA, respectively. Extensive stability studies (e.g., UV-vis absorption, FT-IR, mercury-poison test, dynamic light scattering (DLS) and transmission electron microscopy (TEM)) provide very strong evidence that Ni4P2 catalyst remains homogeneous and intact under turnover conditions.

Nickel-based cocatalysts for photocatalytic hydrogen production

“…In fact, many kinds of Ni-based molecular complexes have been reported as excellent electrocatalysts for H 2 evolution and as catalysts for photochemical H 2 production in combination with molecular dyes [111,[136][137][138][139][140][141]. For example, Xu and co-workers reported the use of a nickel-thiolate complex, which was assembled from Ni(II) salt and 2-mecaptoethanol in one step in water, Source: Reprinted with permission from Ref.…”

Photochemical Water Oxidation Using {PMo12O40@Mo72Fe30}n Based Soft Oxometalate

“…39,54,55 which in turn can improve the catalytic activity. Among all these, the most prominent POM based water oxidation catalyst is the one as mentioned earlier: Na 10 [Co 4 [56][57][58][59] In photosynthesis system II (PS-II), oxygen evolving complex's active center is a tetra nuclear hetero valence manganese cluster which generates oxygen from water in the presence of light. 60 In PS-II, tetra nuclear manganese center is linked with one calcium ion via carboxylate ligand of the protein residue that provides the°exibility around the coordination sites.…”

Nickel‐Containing Keggin‐Type Polyoxometalates as Hydrogen Evolution Catalysts: Photochemical Structure–Activity Relationships

“…[29] However,p reviously reported POM WRCs have been tested under very differentc atalytic conditions (Table 1), often in the presenceo fc ocatalysts, such as platinum-containing compounds,a dditional photosensitizers, or specifically in organic solvents. [13] As reported earlier forPOM WOCs, [30] catalyst stability is also ac riticali ssue in hydrogen evolution catalysis.…”