Using STA-MS and Pulse-TA for the study of metal oxide catalysts
Ursula Bentrup Institut für Angewandte Chemie Berlin-Adlershof e.V., Richard-Willstätter-Str. 12, D-12489 Berlin, Germany
The catalytic performance of metal oxide catalysts using in partial oxidation and de-hydration reactions is influenced by their redox behaviour and adsorption properties for reactants and products. Different specific methods, e.g. XRD, TPD, FTIR, UV-VIS, EPR, ESCA, are used to characterise surface and phase composition, phase transi-tions, surface acidity, dispersion of catalytic active species, and redox properties. It will be shown, that thermal analytical methods coupled with gas analysis, especially STA-MS and PTA, are additional suitable tools to study the thermal and redox behav-iour of metal oxide catalysts. Various catalyst systems used for particular reactions have been selected to illustrate the potential of thermal analytical investigations.
Alkali sulfate modified V2O5 catalysts were used in the partial oxidation of methyl aromatics to their corresponding aldehydes. During catalytic reaction, structural changes and phase transitions take place, especially under reducing conditions. It was shown, that the strength of educt adsorption and its desorption behaviour de-pends on the catalyst phase composition. The favoured formation of bronze-like phases also enhances the release of SO2 at lower temperatures. The admixture of pyridine to the feed improves the aldehyde selectivity. It was found by STA-MS that pyridine has been incorporated into the bronze structure during reaction that de-creases the surface acidity and stabilises the V4+ oxidation state.
MnO2/NaY egg-shell composite catalysts operate with good performance in the SCR of NOx at low temperatures (T < 200°C). The manganese component preferably acts as oxidising agent and the zeolite stores the NOx species as nitrite and nitrate ions in the solid. It was found by FTIR measurements that in the presence of SO2 the nitrate species are replaced by adsorbed SO2 and bulk sulfate is formed. This effect hinders the storage of NOx. Because of the superposition of nitrite and sulfate modes we tried to check the amount and kind of the produced sulfates by investigation of their de-composition. Considering the thermal behaviour of pure manganese and alkali sul-fates, it could be shown that sulfate formation takes place at both, the manganese component and the zeolite.
Pulse thermal analysis is a useful method to study the redox behaviour of CrOx- and VOx-based catalysts applied in oxidation and dehydration reactions of low alkanes. Pulsing of propane causes reduction of the CrOx- and VOx-components, respectively, which is accompanied by mass loss and strong thermal effects. The extent of the ef-fects depends on the concentration of the catalytic active component. Using alternat-ing pulses of propane and oxygen both processes - reduction and reoxidation - can be investigated. It has been found that the redox behaviour is influenced by the sup-port and by the applied preparation method. Whereas the CrOx catalysts were not totally reoxidised, the VOx catalysts reoxidise very fast. Additionally, the coking of the catalyst during reaction with propane has been studied.
Getting more insight into the reaction by application of PTA experiments, selection of suitable reaction conditions and pulse regimes is necessary.
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