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CRC/TRR 247
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

Project C03

Properties of spinel-type metal oxides are affected by composition, morphology, bulk and surface structure as well as defect density. However, the mechanistic understanding of the impact of the “real surface structure” to activity/selectivity in elec­trocatalysis and thermal catalysis in solution is still insufficient. While the bulk structure of Co3O4 consists of cubic closed packed O2- ions and Co2+ ions on tetrahedral (⅛) and Co3+ on octahedral (½) sites, the surface distri­bution of Co2+/Co3+ is facet-dependent. Co3+ ions are enriched on {001} and {110} facets and Co2+ ions on (111) facets, however, the surface structure changes when exposed to H2/O2/H2O, i.e., the (111) surface is largely hydroxylated. 

To unambiguously identify structure-property relationships, the synthesis of nanoparticles (NPs) with precise composition, morphology (facets), electronic structure, and surface defects, i.e., oxygen vacancies (Ov), is mandatory. C03 aims to address the following issues in FP3:

i) Synthesis of metal-doped NPs. Binary (MxCo3-xO4) and ternary (Co1–xMIIx)(Co2–yMIIIy)2O4; MII = Fe, Ni; MIII = V, Cr, Fe; y = 0.1–0.3) NPs with tailored composition, size and morphology are targeted. Large metal cations (Ca2+, Sr2+, Ce3+, and Bi3+) are incorporated to increase (local) microstrain in the lattice and to affect phase transformations under operating conditions.

ii) Synthesis of N-doped NPs. Post-synthetic N2 or NH3 plasma processes are applied to generate surface-doped metal oxynitrides (MxCo3-xNyO4-y) with enhanced conductivity and modified electronic structures (band gap engineering).

iii) Engineering the NP defect structure. Post-synthetic O2 and H2 plasma treatment is used to systematically alter the oxygen vacency (Ov) density and hence the number of active sites, influencing the adsorption and dissociation of reactant molecules, which often is the rate-determining step in catalytic reactions, and the metal oxyhydroxide skin layer formation.

iv) Thermal and electrocatalytic properties. OER and alcohol oxidation properties of all materials synthesized in C03 are screened and promising NPs are provided to collaborators.

 

Figure: (A-D) SEM images / TEM-EDX mappings of Co3O4 and M0.1Co2.9O4 (M = Cr, Mn, Zn) cubes. (E) OER overpotentials (𝜂) at 5 / 10 mA cm−2.


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

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.