Study On Nickel-based Catalysts For Conversion Of Methane To Synthetic Gas

In this work, two catalyst series of Ni/MgO and NiO/γ-Al2O3 for two different routines of synthesis gas manufacture, the CH4-CO2 reforming and the partial oxidation of methane, were designed and prepared, and their physico-chemical properties and the catalytic performance were systematically investigated by means of BET, XRD, TPR, TEM, TG, XPS, exchanged pulse reaction and activity evaluation. As to the CH4-CO2 reforming catalyst system, the physico-chemical properties and catalytic performance of co-precipitated Ni/MgO were changed greatly by use of different precipitators. Compared with the catalyst NM2, which was prepared by use of K2CO3 as precipitator, the catalyst NM1 prepared by KOH as precipitator was of higher specific surface area and smaller size of nickel-magnesia solid solution crystallites, as well as smaller size of nickel crystallites after reduction. Therefore, NM1 showed higher catalytic activity and selectivity in methane reforming with carbon dioxide.The addition of rare earth oxide promoters La2O3 and CeO2 into the Ni/MgO catalysts (labeled as NLM and NCM)may affect their physico-chemical properties and catalytic performance. The presence of rare earth oxides could promote reduction of nickel species in nickel-magnesia solid solution, especially CeO2. Therefore, NLM and NCM both showed higher activity and selectivity than the Ni/MgO catalyst NM. However, La2O3, is mainly served as a structural promoter, which could enhanced resistance to sintering of nickel crystallites, but CeO2 is a kind of electronic promoter, which could benefit to eliminate carbon deposition on the catalyst. In addition, La2O3 and CeO2 both can promote the activation of CO2, and the effect of CeO2 was superior to that of La2O3. For the partial oxidation of methane to synthesis gas, the effects of MgO promoter on physico-chemical properties and catalytic performance of NiO/γ-Al2O3 catalyst were obvious. The presence of MgO could reduce the formation of NiAl2O4 spinel, and benefit the formation of NiO-MgO solid solution, which may improve the dispersion of NiO on γ-Al2O3, and also the catalytic performance for methane partial oxidation to synthesis gas. The catalyst Ni/γ-Al2O3 promoted with 7wt.% MgO showed better catalytic effect. The synergic effects of CeO2 and MO composed promoters on the improvement of physico-chemical properties and catalytic performances of NiO/MgO-γ-Al2O3 catalyst CMM1 were rather great. The composed catalyst with CeO2 and MO was showed better dispersion of NiO species, finer nickel crystallites and stronger ability for storage and release of oxygen, thus it exhibited much higher conversion of CH4, and the CO selectivity can be also maintained. The content of CeO2 and MO, and the CeO2 /MO ratio had great influence on the synergetic effect, With the increasing of promoter content, the dispersion of NiO species became worse, and CeO2 would aggregate on the surface of catalysts, which may accelerate the water-gas shift reaction, and result in the decrease of CO selectivity. The optimized catalyst NiO/CeO2-MO-MgO-γ-Al2O3 showed better catalytic reactivity for partial oxidation of methane to synthesis gas. At last, the impregnation sequence of catalyst components had also apparent influences. When Ni is impregnated firstly, and then promoters Ce and M, the prepared catalyst showed better dispersion of NiO, finer size nickel crystallites and ideal activity for conversion of CH4.