Heterogeneous Oxidation Catalysis in the Liquid Phase

C03: Synthesis of Spinel-Type Nanoparticles with Tailored Morphology and Molecular Metal-Oxo Cluster Model Systems

Prof. Dr. Stephan Schulz

The development of a mechanistic understanding of low temperature (electro)catalytic transformations at solid-liquid interfaces often lacks from the exact knowledge of the chemical composition and local (surface) structure of the active catalyst as well as the identification of molecular reaction intermediates. Nanoparticles with well-defined size and shape – and hence surface structure – as well as molecular clusters with defined structure, which serve as (soluble) model systems for the rather complex geometrical surface structure of a heterogeneous (insoluble) catalyst and which can be studied by in situ and in operando spectroscopic methods, are promising candidates to reduce the complexity of the complex surface structure of powdered solids.

This project develops solvothermal routes for the synthesis of binary (CoxFe3-xO4, x = 0-3), ternary ((MxCo1-x)Fe2O4, M = Mg, Mn, Ni, Zn, x = 0-1; Co(M’xFe2-x)O4, M’ = V, Al, x = 0-2) and quaternary (MxCo1-x)(M’xFe2-x)O4 mixed-metal spinel-type oxides in solution. The chemical composition, size and morphology of the nanoparticles are systematically varied and the decisive role of the surface structure/faceting on the electrocatalytic activity is evaluated in comparative ensemble measurements in the electrocatalytic oxygen evolution reaction (OER) in the liquid phase. We will furthermore develop molecular metal oxide clusters with defined chemical composition and molecular (atomistic) structure, i.e., multinuclear complexes with heterocubane-type M4O4 core, which may serve as structural cutouts of surface structure of the nanoparticles. Such clusters also serve as molecular models for investigating solvation processes and chemical reactions, i.e., alcohol oxidation reactions, to identify relevant organic species (adsorbents, reaction intermediates) and catalytically active molecular species. The studies provide valuable information including spectroscopic fingerprints, which may help to identify organic species and structural motifs of the catalytically active nanoparticle surface. The activity/selectivity of the molecular catalysts is furthermore studied by analyzing the reactions product, while first mechanistic insights are gained from in situ NMR and IR spectroscopy.

(Figure: Structure/crystals of Co4O4(OAc)4(py)(MeCN)2).