The Study On Catalytic Performances Of Ni-Based Catalysts For Water Gas Shift Reaction

In recent years,with the rapid development of fuel cells,the demand for hydrogen has increased significantly.However,hydrogen comes mainly from steam reforming,so that hydrogen contains a small amount of CO.CO has toxic effect on Pt electrode in fuel cells,which affects the commercial application of fuel cells.Water gas shift?WGS?reaction is one of the main methods to solve this problem because it can not only reduce the content of CO in hydrogen but also produce the same volume of hydrogen.Due to various shortcomings of industrial WGS catalysts,it is necessary to develop new highly efficient catalysts.In the present work,the activity of Ni/Al catalyst was modified by adding other metal components.The effects of various factors such as the pretreatment atmosphere and temperature,preparation method,and metal loading on the WGS activity of catalysts were investigated in detail.1.The catalytic activities of Ni/Al catalysts was studied by adding different contents of Mg additive.The Ni-Mg catalysts supported on?-Al₂O₃ were synthesized by co-impregnation method and deposition-precipitation method.The effects of pretreatment atmosphere and temperature on the catalytic activity of Ni-Mg catalyst were investigated.It was found that the activity of Ni-Mg catalyst supported on?-Al₂O₃ after reduction at 400 ^oC in H₂ was higher than other samples,in which the CO could be completely transformed at 260 ^oC.X-ray diffraction?XRD?and Transmission electron microscpope?TEM?characterizations illustrate that the special MgAl₂O₄ spinel appears in the catalyst reduced at 400 ^oC.The formation of MgAl₂O₄may increase the specific surface area of the catalyst,facilitate the dispersion of Ni species on the surface of the support,and reduce the difficulty of NiO to Ni⁰.This may be the main reason for the high activity of the catalyst in the WGS reaction.X-ray photoelectron spectroscopy?XPS?results reveal that the molar ratio of Ni/Al decreases with the increase of reduction temperature,implying the content of NiO in the surface layer decreases,and the reduction degree of the sample increases.Therefore,the pretreatment atmosphere and temperature have a great effect on the formation of Ni⁰ in the catalyst,and further influence the reaction performance.2.Ni-Co catalysts were synthesized by co-impregnation method and deposition precipitation method.The effects of metal loading,pretreatment atmosphere and temperature etc.on the catalytic activity of Ni-Co catalysts were investigated.The results show that the activity of Ni-Co/?-Al₂O₃ catalyst calcined with 300 ^o C N₂ and reduced at 300 ^oC in H₂ was higher than other samples,in which 100%conversion of CO was achieved at 280 ^oC.Compared with Ni/Al catalyst,the addition of Co improved the stability of bimetallic catalyst in WGS reaction.The structures of Ni-Co catalysts were characterized by XRD,TEM,and XPS.It was found that with the increase of the reduction temperature,the particle sizes of Ni and Ni-Co alloy increase obviously.The active sites of the surface metals have decreased because of the aggregation of the particles.This may explain why the activity of catalyst calcined with 300 ^oC N₂ and reduced at 300 ^oC was higher than other samples.Ni-Co alloy in the active sample may cause the synergy effect between Ni and Co,resulting in the higher activity and stability of Ni-Co/Al catalyst compared with Ni/Al catalyst.