Thermogravimetric characterisation of alumina supported Co, Mo and Co-Mo catalysts for FT synthesis
Recent interest in the development of bimetallic catalysts for Fischer-Tropsch synthesis is due to the synergistic effects on activity and selectivity. Among others, the Co-Mo system has shown promise for enhanced olefin content [1]. However, since catalyst properties are a strong function of the thermal treatment, we have undertaken a detailed study of the calcination, reduction and re-oxidation, using thermogravimetric analysis, to relate solid-state kinetic parameters to catalyst intrinsic properties and hence tune preparation conditions to the desired catalytic performance.
Catalysts with different Co:Mo ratio were prepared by incipient wetness impregnation at pH 2, with 20% metal loading to ensure conservative monolayer coverage. Runs were performed on the ThermoCahn TGA and the transient conversion data were analysed in terms of mechanistically-based solid-state models which accommodate the complex processes of nucleation, agglomeration and re-dispersion. Parameter estimation and discrimination suggested that the reaction-controlled Avrami-Erofeev model as the most adequate representation
The spectra from temperature-programmed calcination revealed that the cobalt nitrate and ammonium molybdate decomposition took place between 170-210oC. Interestingly, the decomposition maxima of the bimetallic catalyst also fell within this range - signalling the promise chemical synergism. Additionally, successive TPR-TPO-TPR runs gave thermal profiles evincing facile catalyst regenerability and resilience to coking under FTS conditions. This study shows that the incorporation of Mo in a Co-Al2O3 catalyst has no deleterious effect on its calcination temperature and re-oxidative behaviour. It, however, increases the temperature required for similar reducibility, while an increase is observed in the strength of the acid sites in relation to FT synthesis.
[1] H. Chen, A.A. Adesina, Appl. Catal. A: General 112, p87