Dry Reforming Of Methane Over Perovskite-type Catalysts

The catalytic reforming of CH₄ with CO₂ is industrially attractive not only because it yields syngas with H₂/CO ratio suitable for Tischer-tropsch synthesis, but also because it can be used in some plant favorable circumstances to transform effluent containing CO₂ into valuable feedstock. Among the numerous catalysts reported, nickel-based catalysts show good performance in terms of methane conversion and syngas selectivity. However, the main problem to this kind of catalyst is the coke formation that leads to catalysts deactivation. In order to inhibit carbon deposition, one must keep the size of the metal clusters smaller than the critical size needed for coke formation. One of the promising solutions is to use the oxide support itself as a source of small metal particlesThe perovskite type oxides LaNiO₃ and La₂NiO₄ used as catalysts for CO₂ reforming of CH₄ have been studied these years, since it can prepare catalyst with well-dispersed nickel particles-Ni precipitating to the surface under reducing atmosphere. In this paper, we used the perovskite type oxides La₂NiO₄ as catalyst precursor for CO₂ reforming of CH₄. We studied the catalysts behavior of the catalyst and try to give some possible mechanisms here.In chapter 3, we further investigated CO₂ reforming of CH₄ catalyzed by pre-reduced LaBO₃ and La₂BO₄ (B= Ni) compounds prepared by Pechini method. Among them, the Ni catalyst from K₂NiF₄-type perovskite oxides La₂NiO₄ exhibit high activity and a rather high stability. It has been found that the catalytic reforming over the series of perovskite type oxides showed the orderliness as follows. As for the rate of CH₄ conversion, it was La₄Ni₃O₁₀3Ni₂O₇2NiO₄3NiO₅4NiO₆; LaNiO₃2NiO₄3NiO₅4NiO₆; and for carbon deposition, it was: La₄Ni₃O₁₀(8.4%)3Ni₂O₇(10.8%)2NiO₄(12.3%); and for the peak temperature to remove the carbon with oxygen, it was: La₄Ni₃O₁₀ (598℃)> La₃Ni₂O₇(593℃) >La₂NiO₄(570℃). All these facts indicate that rate of CH₄ conversion and carbon deposition are deeply effected by the metal nickel content in the catalysts. And there is a quick equilibrium between the elimination and production of CH_x and graphitic carbon on the surface of working catalysts.In chapter 4, a series of Co-doped perovskite-type mixed-oxideswere prepared by citric acid complexing method and characterized by XRD and TPR techniques. At x<0.4, the catalytic activities of the catalysts increased with the increase of x-value. However, at 1.0>x>0.4, the effect of x-value on the catalytic activities was not so obvious. Thus we conclude that only a limited amount of Ni is effective for the titled reaction and excessive amount of Ni was useless on one hand, and resulted in accommodation of the active species on the catalyst surface on the other hand.In chapter 5, pulse reactions of CH4, CO2 and mixed gas (CrVCC^l) were systematically conducted over La2NiO4 catalyst to investigate the behavior of the catalyst with different surface state, e.g. fresh, and well reduced. In the experiments, we found that the perovskite-type complex oxides may be used as the methane partial oxidation catalysts. CO2 can be adsorbed and decomposed on the well-reduced La2NiO4 catalysts. The complete oxidation activity of La2NiO4 will be improved after pretreated with CO2. We proposed the hypothesis of mechanism for the CH4 reforming of CO2 over La2NiC>4 as the following process: the adsorption > activation and decomposition of CCV the decomposition of CH^ the removing of carbon deposition and the partially and well oxidation of CH4. Among this, the I^ChCCh specie produced by interaction between CO2 and L^Cb played an important role in carbon elimination and the re-production of La2...