Modification of MOF catalysts by manipulation of counter-ions: experimental and theoretical studies of photochemical hydrogen production from water over microporous diruthenium (II, III) coordination polymers

Abstract

In this study we investigated the photochemical production of hydrogen from water using three heterogeneous microporous ruthenium coordination polymers [Ru₂(p-BDC)₂X]_n (p-BDC = 1,4-benzenedicarboxylate, X = Cl, Br and BF₄) in the presence of multi-component systems. The order of catalytic performances is [Ru₂(p-BDC)₂Br]_n>[Ru₂(p-BDC)₂BF₄]_n>[Ru₂(p-BDC)₂Cl]_n. The most active catalyst, [Ru₂(p-BDC)₂Br]_n, caused the evolution of 46.7 μmol hydrogen molecules with a turn-over number of 18.7 based on [Ru₂(p-BDC)₂Br]_n under visible light irradiation for 4 h. We ascertained that the differences in catalytic activities originated from (1) the efficiency of the quenching of methyl-viologen radicals by [Ru₂(p-BDC)₂X]_n and (2) the durability of the structure in the reaction. In order to examine the catalytic reaction mechanism, we performed theoretical calculations for neutral model structures [Ru₂(HCOO)₄X(H₂O)] (X = Cl, Br), one-electron reduction model complexes [Ru₂(HCOO)₄X(H₂O)]⁻  and deduced intermediate model structures [H–Ru₂(HCOO)₄X] using broken-symmetry hybrid density functional theory methods.

Keywords:

• water splitting
• coordination polymers
• metal organic frameworks

Acknowledgements

One of the authors (Y. Kataoka) expresses his special thanks to the Global center of excellence (COE) Program ‘Global Education and Research Center for Bio-Environmental Chemistry’ of Osaka University. This work was supported by a Grant-in-Aid for Specially Promoted Research (No. 19350077) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.