The Effects Of Pre-treatment On The Structure And Property For Cobalt-based Fischer-Tropsch Catalyst

It is a significant approach to convert synthesis gas?H₂ and CO?into high quality clean energy and high value-added chemical products though Fischer-Tropsch synthesis?FTS?.Generally,cobalt-based catalyst and iron-based catalyst are used for the FTS.Cobalt catalyst has advantages in high CO conversion and low water-gas-transformation?WGS?.Although,FTS has been studied for many years,there still exists room for improving the efficiency of this process by understanding of working mechanism on the cobalt catalyst.Since the catalytic activity is proportional to the particle size of the supported cobalt nanoparticles,the strategy of re-activating catalyst is to re-disperse the large cobalt nanoparticles resulting from sintering/agglomeration,which is of great benefit to not only the theoretical research but also the industrial application.In this paper,cobalt-based catalyst was prepared by incipient wetness impregnation method.And then,the catalyst was pre-treated by reduction-oxidation-reduction?ROR?cycle or reduction-carbonization-reduction?RCR?cycle.We used the XRD?BET?TEM and H₂-TPR characterization to study the influence of pre-treatment on the structure of the catalyst.FTS reaction was carried out in a tubular flow fixed-bed reactor.The conclusion was shown as followed:?1?To discuss the influence on cobalt particle size for different pre-treatments,including reduction,reduction-oxidation?RO?and reduction-oxidation-reduction?ROR?cycle,we prepared various catalyst with different initial size by using different cobalt precursors and diverse calcined conditions,in static air or flowing N₂.The results show that these pre-treatments had no effect on the structure of catalyst.However,the cobalt particle size had significantly changed.Though the RO treatment,we found that the cobalt particle size decreased and the decrease extent of cobalt particle strongly dependent on the initial particle size,and the catalyst with larger original cobalt oxide size has a larger decrease extent.For example,the cobalt particle size of Co/SiO₂ and Co/SiO₂-ROR was 35.2 and 24.7 nm,respectively.?2?The influence of reduction-oxidation-reduction?ROR?pretreatment on 20Co/SiO₂ catalyst has been investigated using different oxidizing gases including water vapor and oxygen in the oxidation step.The water vapor was introduced by a carrier of N₂ bubbling into a sealed triangular flask containing water that was set in the oil bath,the flow of water vapor was determined by adjusting the oil temperature.It was found that the water vapor had no effect on the textural properties,however,the concentration of Si-OH groups on the surface of SiO₂ significantly increased with the increase of oil temperature.These Si-OH groups had strong interaction with the silicate support and reacted with oxidized cobalt?CoO?,which increased the dispersion of the cobalt-based catalyst.On the other hand,the ROR treatment though water vapor promoted the formation of irreducible cobalt silicate,it was ascribed to the deactivation of Co/SiO₂-RO?H₂O?R catalyst.?3?To discuss the influence of cobalt crystal face of 15Co/SiO₂ catalyst during the reduction-carbonization-reduction?RCR?treatment,we designed several experiments and some characterization.It was shown that,after the reduction step,the phase state of cobalt was metallic cobalt,and the cobalt crystal face was 111.And then,after the reduction-carbonization?RC?step,the phase state of cobalt was cobalt carbide,and the cobalt carbide crystal face was also 111.Finally,when the catalyst was treated with the reduction-carbonization-reduction?RCR?,the cobalt phase was also metallic cobalt,however,the cobalt crystal face had changed.After RCR treatment,the cobalt crystal face was 101 and 002.These differences directly contribute to the catalytic performance,the catalyst activity was very low after RC treatment,and methane was the manly product.On the other hand,after RCR treatment the catalyst activity was a bit lower than the catalyst with reduction,however,the selectivity of olefins was great increased.