Fig. 1. (a) UV-vis spectra of the reaction solution measured every 10 min. Reaction conditions: II_ox (0.5 mM), I (0.05 mM), sodium acetate buffer (pH 4, 25 mM, 3 mL), 298 K, under Ar (0.1 MPa), the reaction was initiated by bubbling hydrogen gas for 1 min. (b) Proposed catalytic mechanism for reduction of II_ox with I by hydrogen gas. The atoms of POMs are represented by polyhedra; [WO₆]⁶⁻ and [WO₆]⁷⁻: light green, [PO₄]³⁻: gray.