Magnetic resonance and X-ray methods
Prof. Angelika Brückner
Operando electron paramagnetic resonance spectroscopy (EPR, ESR) is a unique tool for monitoring catalytic processes in which species with unpaired electrons are involved, e. g., supported and unsupported transition metal oxide catalysts and/or radical intermediates. For analyzing heterogeneous catalytic gas phase reactions, a home-made quartz plug-flow reactor (T ≤ 550 °C, p ≤ 20 bar) has been developed which is directly implemented in the cavity of an EPR X-band spectrometer. Currently, this setup is being modified also for heterogeneous liquid phase reactions (T ≈ 100 °C, p ≤ 20 bar). Major experience has been gained in the following applications:
- Selective oxidation and ammoxidation of aliphatic and aromatic hydrocarbons over metal oxide catalysts
- Selective catalytic reduction of nitrogen oxides over supported metal oxide catalysts and zeolites modified with transition metal ions
- Dehydrogenation and aromatization of paraffins
For the first time, the simultaneous coupling of operando EPR with UV-vis and Raman spectroscopy has been set up at LIKAT Berlin (see Figure). This novel technique revealed particular advantages for the study of Mo and V containing oxide catalysts during selective oxidation of propane and isobutane.
Additionally, modified setups have been recently developed by which operando-EPR studies of solid catalysts in flowing liquid reactant mixtures or of homogeneous catalytic systems at elevated temperature and pressure (up to 20 bar and 180 °C) can be performed. These novel techniques have been used for the first time to study supported Ni catalysts during the dimerization of butenes as well as of Cr complex catalysts during tetramerization of ethylene.
For performing operando NMR investigations of homogeneous catalytic processes, a new method has been developed ant LIKAT Rostock, which enables measurements under a controlled gas saturation pressure. Applications comprise hydrogenation, carbonylation, hydroformylation and olefin polymerization reactions.
In cooperation with the Federal Institute for Materials Research and Testing (BAM), a setup for simultaneous operando SAXS/WAXS/XAS spectroscopy is being built at BESSY II. This allows the simultaneous study of reaction-dependent changes of metal valence states and local structural modifications of metal ions by XAS together with changes of particle and crystallite sizes and phase compositions by SAXS/WAXS. Currently this setup is used to study supported Pd catalysts in the acetoxylation of toluene with acetic acid to elucidate reasons for activity differences. This knowledge will be used to develop more efficient preparation and conditioning protocols for these catalysts.
A new setup for monitoring catalyst synthesis (co-precipitation procedures) by simultaneous SAXS/WAXS/Raman spectroscopy has just been finished in cooperation with BAM at BESSY II. First experiments comprised the study of structural changes and crystallization of transition metal molybdate catalysts in dependence on the preparation conditions (temperature, pH, sequence of adding starting components).
Moreover, simultaneous XRD/Raman spectroscopy under normal laboratory conditions has been used recently to study the calcination of MoVNbTe mixed oxide precursors with the aim to optimize calcination conditions for the preferential formation of the target phases (M1, M2, Mo4O15). The coupling of the two techniques is particularly helpful since they show different sensitivity for the detection of the target phases and undesired MoO3 and MoO2 phases.
