Wheel-shaped icosanuclear Cu-containing polyoxometalate catalyst: Mechanistic and stability studies on light-driven hydrogen generation

In this paper, we report the synthesis and characterization of a wheel-shaped icosanuclear-containing polyoxometalate (POM), K₁₂Li₁₃[Cu₂₀Cl(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]·22H₂O (K₁₂Li₁₃-Cu₂₀P₈W₄₈). The resulting cation-exchanged tetrabutylammonium salt of the polyoxoanion Cu₂₀P₈W₄₈ (TBA-Cu₂₀P₈W₄₈) exhibits high efficiency for visible-light-driven H₂ production in the presence of an [Ir(ppy)₂(dtbbpy)][PF₆] photosensitizer and a triethanolamine electron donor. Under optimal conditions, the turnover number for H₂ production reaches ∼2892 after 5 h of photocatalysis and thereafter continuously increases to ∼13400 in a long-term 120 h reaction, representing the best performance among all reported transition-metal-substituted POM catalysts. Mechanistic studies confirm the existence of reductive and oxidative quenching processes, of which the reductive quenching pathway is dominant. Various stability tests demonstrate that the TBA-Cu₂₀P₈W₄₈ catalyst slowly dissociates Cu ions under turnover conditions; however, both the starting TBA-Cu₂₀P₈W₄₈ and its molecular decomposition products are dominant active species for efficient and long-term H₂ production.