Studies On Catalytic Conversion Of Syngas Over Supported Cobalt-based Bimetallic Catalysts

Aiming at controlling the products selectivity of cobalt-based catalysts in Fischer-Tropsch synthesis,the supported Co-based bimetallic catalysts were designed and synthesized.The relationship between the structure and properties of cobalt catalyst was studied by characterization.This dissertation investigated the effect of different promoters on the Co/CNT catalyst for the Fischer-Tropsch synthesis.It was found that the addition of Ir not only significantly improved the activity of Co/CNT catalyst,but also inhibited the formation of CH4 and increased that to C5+hydrocarbons.It was found that the crystal structure of Co/CNT catalyst gradually changed with the increase in Ir content.Co crystal changed from fcc phase to hep phase.According to the literature results,the hcp phase Co has a lower CO activation energy barrier,which we speculate that the hcp phase may be the main reason for the higher activity obtained in Fischer-Tropsch synthesis over Co-Ir/CNT catalyst.Co-Ir catalyst with different supports was studied and the same relationship between structure and activity was founcd.It is concluded that hcp phase is suitable for the Fischer-Tropsch synthesisThe hydrogenolysis ofn-hexadecane over Co-Ir/CNT catalysts with different Ir contents was studied.It was found that the hydrogenolysis activity of Co-Ir/CNT decreased with the increase of Ir.The hydrogenolysis reaction of Co-based catalyst has an important influence on the selectivity of CH4 in Fischer-Tropsch synthesis.We found that the selectivity of CH4 was reduced from 6.8%of 15Co/CNT to 2.7%of 15Co-2.0 Ir/CNT due to suppression of hydrogenolysis.However,the selectivity of CH4 increased gradually and the conversion of CO decreased over the Co-Ir/CNT catalyst with Ir content higher than 2%.Through TEM characterization of Co-Ir/CNT,it is found that Co particles decrease with the increase in Ir content.According to the literature,Co nanoparticles with sizes less than 4-6 nm,would show high selectivity of CH4.We speculate that this may be the reason of the increase in CH4 selectivity over Co-Ir/CNT with high Ir content.The stabilities of 15Co/CNT and 15Co-2.0Ir/CNT catalysts were tested and the results showed that,the 15Co-2.0Ir/CNT exhibited better stability than 15Co/CNT.During the test period of 1000 h,the conversion of CO and the selectivity of hydrocarbon didn't change siganificantly over 15Co-2.0Ir/CNT catalyst.In situ XPS characterization showed that 15Co-2.0Ir/CNT had strong antioxidant capacity.During the reaction,18.7%of Co0 was oxidized to Co2+ and Co3+ over 15Co-2.0Ir/CNT,while 46.3%of Co0 was oxidized over 15Co/CNT.The test of the vapor addition for the catalytic performances over these two catalysts was investigated,which further confirmed that the 15Co/CNT catalyst deactivated quickly after the introducing of water,while the 15Co/CNT catalyst was very stable.