Sintering behavior of TiO2-supported model cobalt Fischer-tropsch catalysts under H2 reducing conditions and elevated temperature

Abstract

The sintering of model TiO2-supported cobalt catalysts has been studied. The TiO2 supports used were anatase, P25 (85% anatase-15% rutile) and rutile. The catalysts were characterized at each stage of treatment. These treatment stages were calcination, reduction and sintering. It was found that the TiO2 support does not influence the Co3O4 crystallite and particle size as shown by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM) respectively. The reduction of the cobalt catalysts and bare supports was studied by temperature programmed reduction (TPR). It was found that the bare supports were not as inert as expected. The supports showed minor reduction as seen in the H2 consumption. All the cobalt catalysts showed a two-step reduction. Sintering of anatase-supported cobalt was shown to be the most substantial with P25- and rutile-supported showing a lower sintering tendency; P25- supported cobalt being the most stable based on TEM measurements. Sintering kinetics based on the Generalized Power law Expression (GPLE) model, showed that sintering of anatase-supported cobalt is the most rapid with a large sintering rate constant. Sintering of P25-supported cobalt is the lowest, shown by the lowest sintering rate constant. The study has conclusively shown the effect of the catalyst support phase. The study has also shown that the use of high surface area supports is not necessarily the only answer to preventing sintering. The phase of the catalyst support is also important.