Optical spectroscopic and thermoanalytical methods
Dr. Ursula Bentrup
For the characterisation of catalysts and the study of their behaviour under reaction-like conditions, different in situ and operando spectroscopic methods (FTIR, Raman, UV-vis-DRS) are used for the investigation of heterogeneously as well as homogeneously catalyzed reactions.
While infrared spectroscopy is applied to investigate the formation, stability and reactivity of adsorbates on the surface of solid catalysts or of different species in liquid phase, UV-vis spectroscopy is helpful in studying the changes (oxidation states, coordination) of the active sites, especially in the case of catalysts containing transition metals. Raman spectroscopy is useful for studying metal-nonmetal vibrations in the low frequency range.
The measurements of UV-vis spectra are carried out in reflectance/transition mode using various quartz optical fiber sensors (AVASPEC, Avantes) which are directly positioned in the catalyst bed or in the solution, respectively. For measuring the Raman spectra, the laser beam of the fiber optical Raman spectrometer (RXN, Kaiser Optical Systems) is focussed through the quartz reactor onto the catalyst bed or the reaction solution. Additionally, a Raman immersion probe is available for measurements in liquid phase. Furthermore, operando spectroscopic studies of gas-solid reactions can be carried out on a Renishaw inVia Raman Microscope equipped with a special reaction cell (LINKAM).
FTIR investigations of solids are carried out in transmission mode using self-supporting wafers or in diffuse reflection mode using powder samples. Special heatable and evacuable reaction cells embedded in the spectrometer (Bruker Tensor 27, Nicolet 6700, Nicolet iS10) are used for investigations under reaction-like conditions. A special setup was developed for adsorption studies at low temperatures.
For measurements in liquid phase, a fiber optical diamond ATR probe (infrared fiber sensors) is used, coupled to the FTIR spectrometer (Nicolet Avatar 370); here the probe is directly immersed into the reaction solution. A special homemade ATR-FTIR flow-through cell enables photocatalytic reactions to be monitored. Liquid phase reactions at higher temperatures and pressures can be studied using a special ATR reaction cell.
By means of the available fiber optical techniques simultaneous measurements of FTIR, UV-vis and Raman spectra are conducted in a single experiment. In this way e.g. reactions in liquid phase (including reactions under pressure) can be followed real-time and in situ. LIKAT was first to develop and implement combined DRIFTS/UV-vis for operando spectroscopic investigations of gas-solid reactions.
Thermoanalytical methods (TG, DSC) - also coupled with MS - (Setaram Sensys TG-DSC-MS) - are applied to investigate the thermal behavior of catalyst precursor compounds in various gases, and to study the redox properties by means of pulse thermoanalysis using reactive gases.
LIKAT applies these methods to the following reactions:
- Selective oxidation of hydrocarbons on mixed metal oxide catalysts and on zeolite catalysts doped with transition metals
- Selective catalytic reduction of nitrogen oxides on supported catalysts
- Metathesis reactions of olefins on supported metal oxide catalysts
- Heterogeneously catalyzed hydrogenation of CO2
- Homogeneously catalyzed reactions (e.g. hydrogenations, cyclizations, oxidations)
- Photocatalytic water splitting
- In situ monitoring of inorganic preparation processes