Co₂C-based Nanocatalysis For Direct Production Of Lower Olefins Via Syngas With High Selectivity

Direct production of lower olefins from syngas via Fischer-Tropsch synthesis as a non-oil-based process has attracted widespread attentions.In previous study,we discovered an outstanding CoMn-based catalyst for Fischer-Tropsch to olefins reaction and the active phase was Co₂C nanoprisms with specific exposed facets of?101?and?020?.On the basis of the results of DFT calculations,Co₂C nanostructures possessed strong facet effects during syngas conversion.The Co₂C?101?surface promoted the formation of lower olefins and the methane formation was inhibited on Co₂C?101?and Co₂C?020?.The CoMn-based catalysts for syngas conversion have been extensively studied for many years,while the Co₂C nanoprisms were unreported.In the present study,the effects of the combination method and concentrations of the different promoters on the formation and morphology control of the Co₂C nanoparticles were studied in detail.The connections were established between the catalyst structure and catalytic performance.These efforts were undertaken with the aim of clarifying the promoter effects and the formation mechanism of the Co₂C nanoprisms.In addition,the effects of the preparation methods on the active phase and performance were also investigated.The main contents of the thesis were as follows:?1?Formation mechanism of the Co₂C nanoprismsCharacterizations of the CoMn catalysts with different sodium concentrations by several techniques were performed to investigate the effect of the sodium promoter on the formation of the active phase and reveal the relationship between catalyst structure and FTO catalytic performance.The sodium promoter was found to induce the active phase transition of the CoMn catalysts from Co⁰ to Co₂C nanoprisms with specific exposed facets of?101?and?020?.As a result of the active phase transition from Co⁰ to Co₂C nanoprisms,the catalytic performance of the CoMn catalysts with different sodium concentrations changed from traditional FTS to FTO.The presence of the sodium promoter in the CoMn catalysts resulted in a sharp decrease in methane selectivity,a significant increase in lower olefins selectivity and the deviation of the methane from the classical ASF distribution.Very high selectivity to lower olefins?54.2 C%?and high Olefin/Paraffin ratio?23.9?with a very low CH₄ selectivity?5.9C%?were obtained over the CoMn-0.4Na catalysts.Furthermore,the carbonization rate and extent measurement and in situ IR studies revealed that the sodium promoter acted as an electronic donor to cobalt,led to stronger CO adsorption and enhanced CO dissociation.The sodium ions exposed at the edge of cobalt also acted as Lewis acids interacting with the oxygen atom of CO and enhanced the CO adsorption.Therefore,the addition of the sodium promoter to the CoMn catalysts promoted the formation and stabilization of the Co₂C nanoprisms with specifically exposed facets of?101?and?020?.?2?Effects of the alkali promoters on the formation and morphology of Co₂C nanoparticlesCharacterizations of the CoMn catalysts with different alkali promoters by several techniques were performed to investigate the effect of the alkali promoters on the formation and final morphology of Co₂C nanoparticles and reveal the relationship between catalyst structure and FTO catalytic performance.The alkali promoters were found to induce the active phase transition of the CoMn catalysts from Co⁰ to Co₂C nanoparticles which had two different morphologies of nanospheres and nanoprisms.The increase of the ionic potentials?IPs?of the alkali promoters resulted in the stronger polarization which may enhance the CO adsorption and benefit to the formation of the Co₂C phase.According to the morphology control effects of the potassium promoters on the?-Fe₅C₂ active phase,we speculated that the alkali promoters may modify the distributions of the exposed facets of the Co₂C nanoparticles in favour of the specific exposed facets of?101?and?020?and lead to the decrease of the Co₂C nanospheres and the increase of the Co₂C nanoprisms.As a result of the Co₂C morphology transition from nanospheres to nanoprisms,the catalytic performance of the alkali-promoted CoMn catalysts changed from deactivated cobalt-based FTS to FTO.With further increase of the IPs,the active phases of the CoMn-0.4Li sample included Co⁰ and Co₂C nanospheres because of the inhibition of the electronic promoter effects and the exposed facets control effects.The CoMn-0.4Li sample exhibited high methane selectivity,low selectivity to lower olefins and the product distribution obeyed the ASF distribution.According to the above results,the effects of the alkali promoters were related with the IPs and the unsuitable IPs led to the active phases of the Co⁰ or Co₂C nanospheres.The Na-promoted CoMn sample?IP=1.02?exhibited the best FTO performance.?3?Mechanism of manganese promoter for Co₂C morphology controlCharacterizations of the several metal oxide-promoted?Mn,Ce,La and Al?Co₂C-based catalysts by several techniques were performed to investigate the effects of the manganese promoter and the combination methods between the cobalt species and promoters on the final morphology of the Co₂C nanoparticles and reveal the relationship between catalyst structure and FTO catalytic performance.For the Mn/Co catalyst prepared via impregnation,Co₂C nanospheres were formed,which exhibited high methane selectivity,low C⁼_2-4 selectivity and low activity.The CoCe,CoLa and unpromoted samples also generated Co₂C nanospheres with similar catalytic performance as that of the Mn/Co sample prepared via impregnation.For the CoMn composite oxide catalyst prepared by co-precipitation,Co₂C nanoprisms with specifically exposed facets of?101?and?020?were obtained,which exhibited a promising FTO catalytic performance with high C⁼_2-4 selectivity,low methane selectivity and high activity under mild reaction conditions.In addition,due to the very high stability of the CoAl composite oxide,no Co₂C nanoparticles were obtained for the CoAl sample.For the CoMn sample,the manganese promoters not only acted as typical electronic promoters or structural promoters,but also had a strong control effect on the Co₂C morphology.With the synergy effects of the sodium promoter,the manganese promoter combined with cobalt species as composite oxides with suitable stability resulted in the formation of the Co₂C nanoprisms with specifically exposed facets of?101?and?020?and excellent FTO performance.?4?Supported CoMn catalysts for Fischer-Tropsch to olefinsThe support effects of alumina on the CoMn catalysts for the FTO reaction were investigated in details.The interactions between the cobalt species and the Al₂O₃supports played an important role on the final morphology of the Co₂C nanoparticles and the catalytic performance.In comparison with the CoMn catalyst prepared by co-precipitation,no CoMn composite oxides were obtainable for the CoMn/Al₂O₃catalysts because of the strong interactions between the cobalt species and the Al₂O₃supports.The Co₂C nanoparticles with irregular morphology were formed during reaction process and no Co₂C nanoprisms were obtained.The CoMn/Al₂O₃ catalysts exhibited high methane selectivity,low C⁼_2-4 selectivity and low activity,and the deviation of the methane selectivity from the classical ASF distribution was not observed.The formation of the CoMn composite oxide as catalyst precursor was necessary to obtain Co₂C nanoprisms.In order to form the CoMn composite oxide,it was suggested that the interaction between the cobalt species and the supports should be weak for the supported CoMn catalysts.