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) is used, which is positioned directly in the cavity of an EPR X-band spectrometer. Apart from this, for the first time a dedicated probehead has been developed to conduct in-situ EPR studies at elevated temperatures (T ≤ 550 °C) in Q-band [see J. Am. Chem. Soc. 132 (2010) 9873]. This is especially useful for enhancing the resolution of complicated superimposed spectra. Significant experience has been gained in the following applications:
- Selective oxidation and ammoxidation of aliphatic and aromatic hydrocarbons by means of metal oxide catalysts
- Selective catalytic reduction of nitrogen oxides by means of supported metal oxide catalysts and zeolites modified with transition metal ions
- Dehydrogenation and aromatization of Paraffins
- Dehydration of glycerol
- Photo(electro)catalytic water splitting
This research group was the first in the world to set up the simultaneous coupling of operando EPR with UV-vis and Raman spectroscopy; this gave rise to particular advantages for the study of Mo and V containing oxide catalysts during selective oxidation and ammoxidation reactions. Most recent developments include the first setup of operando-EPR/UV-vis/ATR-FTIR spectroscopy [see Angew. Chem. Int. Ed. 53 (2015) 11791-11794].
In recent years, modified set-ups have been developed making it possible to perform operando-EPR studies of solid catalysts in flowing liquid reactant mixtures, as well as of homogeneous catalytic systems at elevated temperature and pressure including feeding gaseous reactants (up to 20 bar and 180 °C). These novel techniques have been used for the first time to study supported Ni catalysts during the dimerization of butenes, as well as Cr complex catalysts during tetramerization of ethylene.
Moreover, in situ-EPR spectroscopy has been adapted to perform mechanistic studies of photocatalytic reactions, such as water reduction to hydrogen. With the help of a new set-up for simultaneous operando EPR/Raman studies, it proved to be possible to identify almost all intermediates passed during homogeneous light-driven water reduction by an iridium photosensitizer and an iron carbonyl catalyst [see Angew. Chem. Int. Ed. 50 (2011) 10246]. A similar experimental set-up has been used to identify electron transfer pathways during water reduction on solid photocatalysts consisting of plasmonic metal particles supported on semiconductors [see Angew. Chem. Int. Ed. 52 (2013) 11420]. Currently we are developing in-situ EPR cells for monitoring solid catalyst layers deposited on electrodes during water (photo)electrolysis.
In-situ and operando X-ray absorption spectroscopy (XAS) measurements are performed at different synchroton facilities, e. g. at BESSY in Berlin in cooperation with the Federal Institute for Materials Research and Testing (BAM), at ESRF in Grenoble and at SOLEIL in Paris. Accordingly, simultaneous in-situ SAXS/WAXS/XAS measurements were performed at BESSY with modified supported metal catalysts (e.g. Pd/TiO2) during treatment in an inert, reducing and oxidizing atmosphere. The aim was to study changes of metal valence states and local structural modifications of metal ions by XAS, and to analyze changes of particle and crystallite sizes and phase compositions by SAXS/WAXS. Investigations using rapid XANES and EXAFS are currently being performed at ESRF and SOLEIL with supported Pd and Ni catalysts during catalytic reactions (e.g. during acetoxylation of toluene with acetic acid); this allows geometrical and electronic changes to be detected with a time resolution of about one minute under reaction conditions. These investigations provide new insights into the interaction between modifiers, active metal particles and the support which are not easily accessible by other methods under reaction conditions.
Simultaneous XRD/Raman spectroscopy under normal laboratory conditions was used some years ago to study the calcination of MoVNbTe mixed oxide precursors; the aim in this was 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 are characterized by different levels of sensitivity in terms of detection of the target phases and unwanted MoO3 and MoO2 phases. Currently, a new experimental set-up is being built for such studies, providing for the recording of diffractograms and spectra at shorter time intervals as well as in a reactor cell with a smaller dead volume.