Insights into the electrochemical behaviour of composite materials: Monovacant polyoxometalates @ porous metal-organic framework

摘要

The first electrochemical investigation of composites materials based in polyoxometalates (POMs) encapsulated in the porous Metal-Organic Framework (MOF) MIL-101 is reported. The electrochemical behaviour of the porous MIL-101 and composite materials based in two monovacant Keggin-type POMs {K_7[PW_(11)O_(39)]·10H_2O (PW_(11)) and K_8[SiW_(11)O_(39)]·13H_2O (SiW_(11))} encapsulated in MIL-101 (respectively, PW_(11)@MIL-101 and SiW_(11)@MIL-101) were studied by cyclic voltammetry. Initially, to investigate the influence of the support MIL-101 in the electrochemical behaviour of the two monovacant POMs, the compounds were individually immobilized on the surface of a pyrolytic graphite (PG) electrode. A reversible surface-controlled redox signal attributed to the chromium metal centres (Cr~(3+)/Cr~(2+)) was identified in MIL-101, and this represents the first electrochemical study of this MOF material. The monovacant compounds PW_(11) and SiW_(11) also revealed surface-controlled processes corresponding to W~(VI) <-> W~V reductions. PW_(11) showed two reversible signals (involving one or two electrons), whereas SiW_(11) showed a quasi-reversible one-electron and a reversible two-electron process for the firstand second W reductions, respectively. For the latter, an extra process was detected and attributed to the formation of β-isomer (β-SiW_(11)). When the composites materials PW_(11)@MIL-101 and SiW_(11)@MIL-101 were immobilized on the surface of the electrode, the support MIL-101 maintained its characteristic electrochemistry and the redox potentials of the POMs were not significantly changed. However, when encapsulated inside the cages of the support material the POMs revealed a distinct electrochemical behaviour, given that their redox processes became diffusion-controlled, indicating that the POMs have some degree of mobility inside the support framework.