Abstract
Polyoxometalates (POMs) such as K10[(PW9O34)2Ni4(H2O)2].32 H2O (Ni4P2) represent a rare class of noble-metal-free catalysts for the hydrogen evolution reaction (HER), but their homogeneous nature limits practical application due to issues with recyclability and structural stability. Prior attempts to heterogenize Ni4P2 in metal organic frameworks (MOFs) either reduced catalytic activity or resulted in leaching. Meanwhile, Na10[Mn4(H2O)2(VW9O34)2].26 H2O (Mn4V2)—another scarcely explored noble-metal-free HER catalyst—remains unheterogenized and suffers similar limitations. In this study, we report the successful encapsulation of both Ni4P2 and Mn4V2 within the cavities of robust MIL-100-Fe to form heterogeneous, leaching-proof composites: Ni4P2@MIL-100-Fe (1) and Mn4V2@MIL-100-Fe (2). Confining POMs to a solid matrix improved their recyclability and structural stability. This heterogenization strategy also directly resulted in superior catalytic performance over the homogeneous POMs. The turnover number (TON) for the Ni4P2-based system increased from 6500 to 6725 in composite 1, while the TON for the Mn4V2 system rose from 535 to 587 in composite 2. These values represent the highest TON reported to date for these POMs. Advanced mechanistic studies reveal an efficient electron-transfer cascade, from MIL-100-Fe to POMs, facilitating proton reduction potential. Crucially, both catalysts maintained structural integrity and reusability over multiple cycles.