| Industrial Ammonia synthesis is still employed the Haber-Bosch process to fix N2 in the atmosphere,which still requires high temperature and high pressure conditions and consumes 1%of the world's electricity for production.Therefore,to explore a more moderate and more environmental friendly ammonia synthesis catalyst is the current research direction.The traditional ammonia synthesis is based on using nitrogen and hydrogen as feedstock,in which the operation condition and equipment was complicated.The ammonia producing from methane and nitrogen could be achieved under atmospheric pressure,which may avoid the mentioned defects of current ammonia synthesis.In our previous studies it was found that the?Fe-K?/AC had broad prospects for the ammonia synthesis activity of methane and nitrogen as raw materials.Methane and nitrogen were used as raw materials to carry out ammonia synthesis reaction on the basis of?Fe-K?/AC catalyst at atmospheric pressure.Although the ammonia production rate over?Fe–K?/AC was rather high and acceptable at initial stage,both catalytic activity and ammonia generation rate tended to drop rapidly during less than a half hour.It was found that carbon deposition is the main reason for the decrease of ammonia synthesis rates.Therefore,in order to solve the problem of carbon deposition in the above system,this paper has done with two solutions:i.Adding the third metal into?Fe-K?/AC catalyst to change the carbon morphology,such as?changing encapsulating carbon species to filamentous carbon?to improve catalytic performance;ii.Adding O2 to the feedstock?CH4&N2?to eliminate carbon deposition over active site of catalysts surface.The effects of loading amount,impregnation methods,reaction conditions and WHSVs to the catalytic performance were investigated.X-ray diffraction?XRD?,N2 isothermal adsorption-desorption?BET?,H2 temperature programmed reduction?H2-TPR?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?were used to study crystal phases,surface area,reduction temperature,carbon deposition morphology and the active metal dispersion of catalysts and so on.The main contents and results were as follows:?Fe-K?/AC catalyst doped with the third metal:By comparing ammonia synthesis rates of?Fe-K?/AC catalysts doped with different third metal from methane and nitrogen as feedstock at atmospheric pressure,the catalytic performances of?Fe-K-Ce?/AC was obviously superior to others,which was due to anti-sintering ability and dispersion improvement with the Ce addition.Different preparation methods were experimentally investigated,and the co-impregnation technique proved to have the best catalytic performance and new crystalline CeFeO3 was found,which may be the main reason for changing the carbon disposition species.The effect of Ce addition amounts on the activity of the catalyst was investigated and experimental results indicated that?Fe-K?/AC catalysts with 4%cerium addition have the well-dispersed active sites and highest catalytic activity.Also,experimental results show that The optimal operating conditions was 700? and WSHVs was 2760 mL/h,and the maximum ammonia production rate reached 9.5×10-8mol·g-1·s-1.Adding O2 to the feedstock?CH4&N2?to eliminate carbon deposition over the catalysts surface:with the O2 introduction,carbon deposition could be eliminated using oxidation of O2 and exhausted in the form of CO,CO2.The effects of impregnation sequence on the catalytic activity were also investigated.Ammonia synthesis rates on the?Fe-K?S/AC catalyst was higher than that of the co-impregnated catalyst?Fe-K?T/AC.On?Fe-K?S/AC catalyst Fe mainly exists in Fe3O4 crystal phase,while on?Fe-K?T/AC Fe is mainly Fe2O3crystal phase.The effect of O2 addition amount on the stability of ammonia synthesis was investigated and experimental results display that as feed gas ratio is CH4:N2:O2=40:16:4?N2:O2=16:4 ratio is closed to that in air?the ammonia synthesis stability is the best and the ammonia production rate is stabilized at 3.8×10-8mol·g-1·s-1. |
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