Cu-based Polyoxometalate Catalyst for Efficient Catalytic Hydrogen Evolution

Copper-based complexes have been
largely neglected as potential water reduction catalysts. This article
reports the synthesis and characterization of a tetra-copper-containing
polyoxotungstate, Na3K7[Cu4(H2O)2­(B-α-PW9O34)2]·30H2O (Na3K7-Cu4P2). Cu4P2 is a water-compatible catalyst
for efficient visible-light-driven hydrogen evolution when coupled
to (4,4′-di-tert-butyl-2,2′-dipyridyl)-bis­(2-phenylpyridine­(1H))-iridium­(III) hexafluorophosphate ([Ir­(ppy)2(dtbbpy)]­[PF6]) as a light absorber and triethanolamine
(TEOA) as sacrificial electron donor. Under minimally optimized conditions,
a turnover number (TON) of ∼1270 per Cu4P2 catalyst is
obtained after 5 h of irradiation (light-emitting diode; λ =
455 nm; 20 mW); a photochemical quantum efficiency of as high as 15.9%
is achieved. Both oxidative and reductive quenching pathways are observed
by measuring the luminescence intensity of excited state [Ir­(ppy)2(dtbbpy)]+* in the presence of Cu4P2 or TEOA,
respectively. Many stability studies (e.g., UV–vis absorption,
FT-IR, dynamic light scattering, transmission electron microscopy,
and scanning electron microscopy/energy-dispersive X-ray spectroscopy)
show that catalyst Cu4P2 undergoes slow decomposition under turnover
conditions; however, both the starting Cu4P2 as well as its molecular
decomposition products are the dominant catalytically active species
for H2 evolution not Cu or CuOx particles. Considering the high abundance and low cost of copper,
the present work provides considerations for the design and synthesis
of efficient, molecular, water-compatible Cu-based water reduction
catalysts.

铜基配合物作为潜在的水还原催化剂（water reduction catalysts）长期以来被极大忽视。本文报道了一种含四铜多钨酸盐（tetra-copper-containing polyoxotungstate）Na3K7[Cu4(H2O)2(B-α-PW9O34)2]·30H2O（简称Na3K7-Cu4P2）的合成与表征。当以六氟磷酸(4,4'-二叔丁基-2,2'-联吡啶)双(2-苯基吡啶(1H))合铱(III)（[Ir(ppy)2(dtbbpy)][PF6]）作为光吸收剂（light absorber）、三乙醇胺（triethanolamine, TEOA）作为牺牲电子给体（sacrificial electron donor）时，Cu4P2可作为水相容性催化剂高效实现可见光驱动产氢（visible-light-driven hydrogen evolution）。在经初步优化的反应条件下，经过5小时光照（发光二极管，λ=455 nm，功率20 mW），每分子Cu4P2催化剂的周转数（TON, turnover number）可达约1270，光化学量子效率（photochemical quantum efficiency）最高可达15.9%。通过分别检测Cu4P2或TEOA存在下激发态[Ir(ppy)2(dtbbpy)]+*的发光强度（luminescence intensity），证实了氧化淬灭路径与还原淬灭路径均存在。多项稳定性表征（如紫外-可见吸收光谱（UV–vis absorption）、傅里叶变换红外光谱（FT-IR）、动态光散射（dynamic light scattering）、透射电子显微镜（transmission electron microscopy）以及扫描电子显微镜/能量色散X射线光谱（scanning electron microscopy/energy-dispersive X-ray spectroscopy））结果表明，Cu4P2催化剂在周转条件下会发生缓慢分解；然而，无论是原始Cu4P2还是其分子级分解产物，均为产氢反应的主要催化活性物种，而非金属铜或氧化铜颗粒。鉴于铜元素储量丰富且成本低廉，本研究为高效、分子级、水相容性铜基水还原催化剂的设计与合成提供了重要参考。