The Study On Ni-CeO₂ Catalysts For Water-gas Shift And Reverse Water-gas Shift Reaction

In this work, a series of Ni–CeO₂ catalysts were prepared by different methods, and the catalytic performance for high temperature water-gas shift (WGS) and reverse water-gas shift (RWGS) reaction was investigated. Meanwhile,the catalysts were characterized by means of different characterization methods, to study the relations between catalytic activity and preparation method, loading amount of nickel, reductive property, reaction conditions. The physical and chemical natures of catalyst active sites were also discussed. The main experimental results and conclusions are as follows:The catalytic performance over M–CeO₂ (M=Fe,Co,Ni,Zn) for RWGS was evaluated and compared, and it is found that Ni–CeO₂ catalyst is the most active one among them. Ni–CeO₂ catalysts with different Ni content were prepared by co-corporation method, and used for RWGS reaction. The results indicate that nickel content in catalysts effect catalytic performance markedly and the 2% (weight percent of Ni in NiO-CeO₂) Ni–CeO₂ is the most selective and active catalyst for RWGS reaction. The catalysts were characterized by TG-DTA,BET,XRD,TEM and H2-TPR methods, etc. The results show that, in Ni–CeO₂ catalysts, there are three kinds of nickel species, nickel ions in ceria lattice, highly dispersed NiO and bulk NiO particles. The formation of the solid solution of CexNi1-xO2 by the incorporation of Ni2+ ions into ceria lattice would result in the generation of oxygen vacancies in ceria lattice. At low nickel contents (≤2% Ni), nickel is in the state of Ni2+ in the solid solution of Ce1-xNixO2 and NiO phase highly dispersed on CeO₂. At high nickel contents (≥5%Ni), crystalline nickel oxide, solid solution and highly dispersed NiO phase on CeO₂ co-existed. Aggregated NiO on the surface of CeO₂ dominated at high Ni content (≥5% Ni). Combining with their catalytic performance, it is proposed that oxygen vacancies and highly dispersed Ni are the key active components for RWGS, and bulk Ni is the key active component for methanation.The highly dispersed NiO and the bulk NiO can be dissolved in nitric acid, thus Ni–CeO₂ catalysts were prepared by coprecipitation-nitric-acid-leaching method, in which the"nitric-acid-leaching"is to remove NiO particles. Ni–CeO₂ prepared by co-precipitation-nitric-acid-leaching method was found to be highly active and selective for high temperature water-gas shift reaction in reformate. H₂-TPR and XRD results suggest that the high activity for water-gas shift reaction was attributed to high amounts of oxygen vacancies formed by the incorporation of Ni2+ ions into ceria lattice in the catalyst.The preparation method, the kind of precipitant, calcination temperature and Ni content of Ni–CeO₂ catalysts have great effects on catalyst structure, reducibility of catalysts and catalytic performance for WGS and RWGS reaction. The preparation method of the co-precipitation with mixed Na2CO3: NaOH=1:1 as precipitant favors the formation of CexNiyO solid solution. In Ni–CeO₂ catalyst prepared by co-precipitation-nitric-acid-leaching method, the part of the Ni2+ in the CeO2 lattice could migrate from lattice to surface to form highly dispersed NiO and bulk NiO, when it was calcined at temperature of 600℃or under WGS reaction at 600℃; while most Ni species could be translated to the bulk NiO at high temperature of 800℃.