Room Temperature Synthesis of [Pd(cyclam)]₅{H₃Nb₆O₁₉}₂ ⋅ 26H₂O: a Suitable Precursor for the in‐situ Generation of a Highly Active Catalyst for Light‐Driven Hydrogen Evolution

Abstract

The first Pd2+ containing polyoxoniobate [Pd(cyclam)]5{H3Nb6O19}2 ⋅ 26H2O (I) was prepared at room temperature. In the structure, two hexaniobate clusters are surrounded by a bi‐capped cube formed by Pd2+ centered complexes. This motif is arranged in a layer‐like fashion with crystal water molecules located between cations and anions. Temperature resolved in‐situ X‐ray diffraction experiments demonstrate that successive removal of the crystal water molecules leads to formation of several crystalline intermediate phases. Water sorption investigations show that thermally removed H2O can be successfully reintegrated proceeding via two distinct steps. The catalytic performance for the light‐driven hydrogen evolution reaction (HER) was investigated in a sacrificial system and fluorescein Na+ salt as sensitizer yielding a high value of 90 μmol/h H2. Surprisingly, a composite Pd@Na7[HNb6O19] ⋅ 15H2O is in‐situ formed during the catalytic reaction by cation exchange with simultaneous reduction of Pd2+ nanoparticles decorating the hexaniobate support. The recovered catalyst was even more active producing 157 μmol/h H2 with an apparent quantum efficiency of 1.85 %. Photocatalytic experiments performed with ex‐situ generated Pd nanoparticles deposited on Na7[HNb6O19] ⋅ 15H2O show much lower activities indicating a synergistic effect of the in‐situ generated Pd@Na7[HNb6O19] ⋅ 15H2O catalyst.