The effect of CO2 on a cobalt-based catalyst for low temperature Fischer-Tropsch synthesis

A series of Fischer-Tropsch synthesis (FTS) experiments, which entailed repeatedly switching between a CO (CO/H-2/N-2) and a CO2 (CO2/H-2/N-2) feed, were conducted in a fixed bed reactor over a cobalt-based catalyst. It is worth noting that the effect of the CO2 on the properties of a cobalt-based catalyst was very small under the reaction conditions we chose. There was no apparent catalyst deactivation at reaction temperatures of 180 degrees C and 200 degrees C when we continually alternated between the CO and CO2 feeds. We observed dramatic changes in the catalyst activity and product selectivity for CO2 hydrogenation before and after the initial FTS for CO feed at 180 degrees C. In addition, during the initial CO hydrogenation on the cobalt catalyst, both the olefin and paraffin formation rates suddenly changed from one pseudo-stable state to another. These differences may have been caused by liquid products, whether deposited on the catalyst surface or in the catalyst pores during CO FTS. A mild catalyst deactivation was observed at the operating temperatures of 210 degrees C and 220 degrees C, respectively. According to the comparison we made between the conversion of the feed gases and the product formation rates for paraffin and olefin, and our speculations concerning possible side reactions, we conclude that the catalyst deactivation is possibly attributable to the re-oxidation by water. (C) 2012 Elsevier BM. All rights reserved.