An Investigation On The Application Of Ni-based Perovskites In Dry Reforming Of Methane

Dry reforming of methane(DRM)is a clean and efficient way of energy conversion.It has following advantages:(1)It consumes CH4 and CO2 so that no CO2 separation is needed;(2)It doesn’t require steam as compared with steam reforming of methane,thus DRM can reduce operational costs and can be applied where water is not available;(3)Safer as compared with partial oxidation of methane;(4)It has theoretical H2/CO ratio of 1,which is suitable for the Fischer-Tropsch synthesis.However,the Ni-based catalysts whose industrialization is most likely suffer from severe carbon deposition.Thus,many researches focus on improving the carbon resistance as well as the catalytic activity of Ni-based catalysts.In order to improve the catalytic performance of Ni-based catalysts,based on perovskite materials,we have focused on two aspects in this thesis,namely,(1)studying the effects of Cu substitution at the B-site of La2(NiCu)O4 catalyst precursors on the catalytic performance of NiCu bimetallic catalysts and(2)studying the effects of Ce substitution at the A-site of(LaCe)Ni0.5Fe0.5O3 on the catalytic performance.According to our researches,the main results are obtained as follows:The effects of Cu substitution at the B-site of La2(NiCu)O4 catalyst precursors on the catalytic performance of NiCu bimetallic catalysts are investigated.It is found that Cu substitution at the B-site of La2NiO4 can significantly improve the carbon resistance of the catalysts.The carbon deposition rate of the reduced La2NiO4(Ni/La2O3)is as high as 0.4 gc/gcath,while the reduced La2(Ni0.8Cu0.2)O4 shows optimal performance with negligible carbon deposition as well as high catalytic activity(the conversions of CH4 and CO2 are 73%and 80%,respectively.).The remarkably improved carbon resistance of the reduced La2(Ni0.8Cu0.2)O4 is attributed to:(1)the smaller metallic particles of the reduced perovskite as compared with those of the impregnated catalysts;(2)the cage-like structure of surface segregated Cu,which divides the surface of Ni ensembles into small areas and suppresses the formation of stable graphite nucleus.The effects of Ce substitution at the A-site of(LaCe)Ni0.5Fe0.5O3 on the catalytic performance are examined.Fe substitution at the B-site of LaNiO3 is proved to be an effective way to suppress carbon deposition,however,the improved carbon resistance is obtained at the expense of a sacrifice in the catalytic activity due to the decrease in the active Ni content.To solve this problem,we substituted La by Ce in(La1-xCex)Ni0.5Fe0.5O3 and found that catalysts with high catalytic activity as well as high carbon resistance can be obtained by Ce substitution.The optimal catalytic performance is achieved when x=0.4-0.6.The used catalysts consist of Ni,CeO2,and(LaCe)(NiFe)O3.The metallic Ni phase provides the primary catalytic activity,while(LaCe)(NiFe)O3 perovskite dominates the enhanced catalytic activity.We propose that the Ce3+ cation at the A-site of(LaCe)(NiFe)O3 improves the catalytic activity of the perovskite(1)by introducing more oxygen vacancies by activating the B-site cations,and/or(2)through a redox mechanism,in which Ce3+/Ce4+ cations reversibly shuttle between(LaCe)(NiFe)O3 and CeO2 depending on the local redox fluctuations.The large difference in the solubility of Ce in the perovskite between the oxidizing and relatively reducing environments provides the essential driving force for the exchange of the Ce element between CeO2 and the perovskite.