Mixed Metal Oxide Clusters: Model Systems for Catalytically Active Materials

• Applicants: Professor Dr. Knut R. Asmis; Professor Dr. Joachim Sauer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 430942176

Our initial project (1/2020-12/2022) combines state-of-the-art mass spectrometric and IR spectroscopic experiments with quantum chemical calculations to gain molecular-level insights into C–H bond activation by supported metal oxide catalysts. We identified Co-, Fe- and Ni-substituted Al8O12+ and Al3O4+ clusters as gas-phase model systems for active sites in such catalysts and studied their structure and reactivity towards methane. The Al8O12+ and Al3O4+ clusters showed an opposite behavior on substitution of Fe for Al, both with respect to structure and reactivity. In Al8O12+, substitution is isomorphous, but accompanied by a change of the Fe oxidation state, which converts the reactive terminal Al−O•− bond into a non-reactive terminal Fe=O bond. In Al3O4+, on the contrary, a change of the Fe oxidation state induces a structural change and creates a highly reactive terminal Al−O•− radical site. We demonstrated that Kohn-Sham density functional theory (DFT) is not predictive with regard to the relative stability of such valence isomers, even not for singly substituted systems, like Al2FeO4+. Multi-reference wave-function calculations are required to correctly predict the lowest energy structure. Here, we propose to continue this fruitful line of research with the shift of the reactivity focus from methane in the first period to water for the present proposal. In addition to the homolytic splitting of the C-H and O-H bonds of methane and water, respectively, water may also undergo heterolytic splitting. We study the reactivity of mixed metal oxide clusters that are models for (i) solid simple metal oxides such as Al2O3 doped with transition metal atoms (Fe, Co, Ni, Cu, Zn), and (ii) Fe-doped nickel oxyhydroxide (NiOOH). In particular, we would like to find out, which role O•− radical sites in these clusters play. We make us of mass spectrometric experiments together with structure determination using vibrational action spectroscopy, and combine these experiments with quantum chemical reactivity calculations and quantum chemical predictions of structures and vibrational spectra.

• DFG Programme: Research Grants