Noble metal-free POM@MOFs photocatalytic systems : in-depth structural characterization and development of thin films

Post-doctoral allowance, 12 mois, AAP 2018-1

Team : Laboratoire de Chimie des Processus Biologiques, Collège de France

Project leader : Caroline Mellot-Draznieks

Abstract :

Scientific context: Recently, we have combined for the first time the three realms of porphyrins antenna, polyoxometalates and metal-organic frameworks, which had remained unexplored so far, to provide a proof-of-concept “three-in-one” photocatalysts: the Co-POM@MOF-545, where the catalytically active Co-POM is immobilized into the porphyrin-based MOF-545 possesses remarkable water oxidation activities and recyclability (G. Paille et al. J. Am. Chem. Soc. 2018, 140, 3613). Our unique noble metal-free photosystem benefits from the confinement of the catalytic POMs into the pores, making further structural characterizations a crucial while challenging issue.

Aim of the post-doc project: Along a first line, our ambition is to bring the atomic scale understanding and structure-properties rationale of such complex systems through a major step further. Indeed, the direct structural characterization of cat@MOFs composites possessing immobilized catalytic species has been neglected in all studies reported since it is not accessible using conventional diffraction techniques due to their positional/statistical disorder of encapsulated/immobilized species. Here, PDF (pair distribution function) techniques are appropriate for locally probing the POM and its environment by collecting POM@MOF pair distribution functions. In depth structural characterization of cat@MOF composite photosystems has never been achieved so far at the atomic level. This project thus aims to leverage that limit by precisely identifying and locating POMs of interest for WOC or HER within MOFs pores thanks to last generation PDF techniques.

Along a second line of innovative character, thin films technology applied to MOF-based photosystems is indeed a burgeoning area. We control the thin film synthesis of Co-POM@MOF-545 to produce a versatile easy-to-use photochemical heterogeneous setup. EXAFS/XANES runs at SOLEIL, Samba line, will allow us gaining further local structural details, using our initial existing structural models derived from DFT calculations.

Methods for POM@MOFs synthesis and Standard Characterizations : The selected MOF is the porphyrinic MOF-545. For PDF studies on powders, the immobilized POM will be the sandwich-type POM [(PW₉O₃₄)₂Co₄(H₂O)₂]^10- (known as a photocatalyst for WOC) and its analog with Mn (known as a photocatalyst for HER). The preparation of the POM@MOF powder composite is already well handled by impregnation. The POM@MOFs will be characterized by usual physical techniques (EDX, elemental mapping, BET, ICP-MS, UV-vis, IR spectroscopy). For the thin films preparation, the MOF powders prepared will be fixed on the surface of a transparent FTO support using electrophoresis. Once the MOF is deposited on FTO, the POM will then be impregnated inside the pores. The above characterization techniques will be used when appropriate. The visible-light driven activity for WOC or HER of the (Co, Mn)-POM@MOF materials (powders and thin films) will be assessed and compared in terms of catalytic performances, stability and recyclability. The Collège de France is well-equipped for photochemical reactions and the quantification of gas with a unique analytical platform.

Required Skills: experience in Pair Distribution Function data collection and analysis, Experience in EXAFS/XANES would be welcome.

Collaborations :