Mimicking the active site and microenvironment of hydrogenase in a heterometallic MOF with enhancing stability and high efficiency for aerobic solar hydrogen production

Developing efficient and robust artificial hydrogenases with sophisticated structure as catalysts is potential for practical solar hydrogen application, withstanding challenges proposed by oxygen generated in situ by water splitting. Based on Pearson's hard/soft acid/base (HSAB) theory and compartmental ligand strategy, we prepared a 3D heterometallic MOF, [Ni3Pr2(6-mna)6(H2O)4]center dot(H2O)5 (1) (6-mna = 6-mercaptonicotinate divalent anion), with alternately arranged redox-active NiS cluster-based nodes and redox-inactive PrO cluster-based nodes to mimic simultaneously both the active center and the surrounding polypeptide microenvironment in natural hydrogenase. Despite numerous negative effects caused by oxygen, with the sophisticated structure, 1 achieved both enhanced stability and efficient hydrogen producing activity of 40.3 mmol/g in air atmosphere. The influence of oxygen on photocatalytic hydrogen production was investigated by contrasting mechanisms of both photo-induced electron transferring step and chemical catalyzing step under aerobic and anerobic conditions. The reductive mechanism is promoted under anerobic conditions, whereas oxidative mechanism is favored in aerobic conditions due to the suppressed production of radical intermediate Fl3-center dot in the presence of oxygen, as evidenced by in-situ electron spin resonance spectroscopy and in-situ ultraviolet-visible absorption spectrum. This paper presents a simple method for MOFs to closely simulate both the active center and surrounding protein of biological enzymes, and offers a new pathway for developing stable and efficient catalyst for solar hydrogen production.