The Effects Of Silver Promoter On Phase Transformation And Performance Of Fischer-Tropsch Iron-based Catalysts

The paper makes a systematic study of silver promoted iron-based catalysts for Fischer-Tropsch.We took the method of corprecipitation to prepared two series of catalysts,Fe-Ag catalysts and Fe-Si-Ag catalysts.Phases,BET,chemical bonds,morphology of crystal and reducibility of Fe-Ag catalysts were characterized respectively by XRD or Raman,low-temperature N2 absorption,FT-IR,SEM and TPR.And to make clear of the effects of content of silver and gas of reduction on performance of the Fe-Ag catalysts,reaction experiments were run on a stainless steel fixed bed reactor.The powder X-ray diffraction patterns showed that AgFeO2 emerged readily once Ag was added.Only peaks of AgFeO2 but not Fe2O3 were observed for the 100Fe50Ag catalyst and 100Fe100Ag catalyst, maybe, for the reason that a part of Fe2O3 crystals were too small (<2nm) to be detected by XRD or that there exist some other forms of silver-iron oxides whose diffractions peaks are the same as AgFeO2. BET surface area of Fe-Ag catalysts was in a range from 20 m2·g-1 to 30 m2·g-1.The pore-size curves of catalysts containing Ag were broader than 100Fe catalyst, maybe because that AgFeO2, having small size spheroidal crystals, dispersed bad and agglomerated leading to the change of pore size.Result of FT-IR demonstrated that large amount of Ag contained in iron based catalysts had an strong effect on the Fe-O bond. Raman characterization showed the same as XRD that Fe2O3 and AgFeO2 existed in Ag-Fe catalysts with ratio of Ag/Fe being smaller than 1/2 while only AgFeO2 was observed to be in the ones with ratio of Ag/Fe not smaller than 1/2. Due to the H spillover, silver benefited reduction of catalysts in H2-TPR. CO-TPR showed that Ag barely had effect on the reduction of catalysts with low silver content (0-20).While larger amount (50-100) of Ag made for the reduction from Fe3+ to Fe but not the carbonization of Fe in CO-TPR.The followings were found in reaction experiments. Selectivity of C5+ and CO2, ratio of olefin/alkane and the content of long chain hydrocarbon reduced but hydrogenation and WGS reaction enhanced and selectivity of CH4 increased once silver was contained.100Fe5Ag catalyst reduced by CO made a low ratio of olefin/alkane in products and was in favor of the production of CH3OH.However,100Fe5Ag catalyst reduced by H2 increased the ratio of olefin/alkane and holp produce C2H5OH. Otherwise, the kinds of gas for reduction didn’t effect the content of hydrocarbon obviously.