Perovskite Oxides Supported Nickel Catalysts For Steam Reforming Of Ethanol To Produce Hydrogen

The presently critical challenge faced to the steam reforming of ethanol (SRE) is the poor stability for SRE catalyst. Besiedes, the cost of SRE catalyst still need to be reduced further. Ni based catalyst is promising for SRE reaction to produce hydrogen, because its high catalystic ativity and low price as compared to the noble metal. However, the problem of sintering and coke for SRE catalyst need to be further solved. Perovskite oxides have high thermal and chemical durability. The interation between perovskite oxides and metal could improve the metal dispersion. The vacancies in perovskite oxides facilitate the elimination of surface carbon. In this work, perovskite supported Ni catalysts prepared by one-step citrate complexing method were applied to SRE. The correlationship between catalyst properties including the active component, support and their interaction, and catalystic performance was investigated.LaFeyNi1-yO3 perovskite-type oxides supported highly dispersed NiO catalysts were prepared by one-step citric-complexing method, and applied to the steam reforming of ethanol for hydrogen production. The XRD and TEM results indicate that one-step citric-complexing method is a simple as well as an effective way for producing well dispersed NiO particles. The dispersive NiO particles tend to interact with the perovskite oxide and partially incorporate into the perovskite structure, leading to the formation of LaFeyNi1-yO3 and some resultantly separated Fe ions onto the perovskite surface. The catalystic performance tests showed that the high activities of NiO/ LaFeyNi1-yO3 were attributed to the dispersed Ni. The CH4 selectivity was sensitive to the particle sizes of supported Ni, and the small nickel particle depresses the methane formation. The LaFeyNi1-yO3 supported nickel catalysts exhibited very good carbon deposition resistance, which could be ascribed to the highly dispersed Ni and the oxygen vacancies in LaFeyNi1-yO3.The perovskite La-St-Fe-O (St=Ca, Sr) supported Ni catalysts were applied to SRE. The perovskite structure before substitution is LaFe1-yNiyO3. For the samples substituted by Sr or Ca, as indicated by the XRD results, the Ca and Sr were successfully introduced into the La site of the LaFe1-yNiyO3. The Ca in the perovskite leads to the enrichment of oxygen vacancies. The correlation between the oxygen vacancies and the stability for SRE indicates that the surface oxygen vacancies and the promoted bulk oxygen species restrain the carbon formation and facilitate the carbon elimination. The surface and lattice oxygen vacancies are also beneficial for the reaction between water and hydrocarbon species, reducing carbon containing intermediates and accelerating carbon elimination. It is found that the Sr in the perovskite lowers the dispersion of metallic Ni, leading to a poor SRE activity. The A-site doped perovskite La_1-xCa_xFe_1-yNi_yO₃ supported nickel catalysts exhibit very good stability for SRE, due to the surface and bulk oxygen vacancies.The La_1-xCa_xFe_0.7Ni_0.3O₃ prepared by using citrate complexation was applied to steam reforming of ethanol and oxidative SRE. A reduction-oxidation cycle was proposed and then evidenced by TPR, XRD and TEM measurements: reducing of nickel ions from the perovskite structure to form nano-size metallic nickel particles and then oxidating the nickel nano particles back into the perovskite structure lattice. It has been verified by experiments that the highly dispersed nickel particles are detected on reduced La_1-xCa_xFe_0.7Ni_0.3O₃ and that these particles are indeed restorable. Generally, oxygen addition in the steam reforming of ethanol will accelerate the sintering of metal particles. However, owing to the existence of this segregation-incorporation cycle of nickel species in the perovskite oxides lattice, the sintering of nickel particles under oxidative steam reforming of ethanol is depressed effectively.