Highly Active and Dispersed Ni/Al2O3 Catalysts for CO Methanation Prepared by the Cation–Anion Double-Hydrolysis Method: Effects of Zr, Fe, and Ce Promoters

CO Methanation over NiO-CeO2 Mixed-Oxide Catalysts Prepared by a Modified Co-Precipitation Method: Effect of the Preparation pH on the Catalytic Performance

In this study, a series of NiO-CeO₂ mixed-oxide catalysts have been prepared by a modified co-precipitation method similar to the one used for the synthesis of hydrotalcites. The syntheses were carried out at different pH values (8, 9 and 10), in order to determine the influence of this synthetic variable on the properties of the obtained materials. These materials were characterized by using different techniques, such as TGA, XRD, ICP, N₂ adsorption-desorption isotherms, H₂ temperature-programmed reduction (H₂-TPR), and electron microscopy, including high-angle annular dark-field transmission electron microscopy (HAADF-TEM) and EDS. The characterization results revealed the influence of the preparation method, in general, and of the pH value, in particular, on the textural properties of the oxides, as well as on the dispersion of the Ni species. The catalyst prepared at a higher pH value (pH = 10) was the one that exhibited better behavior in the CO methanation reaction (almost 100% CO conversion at 235 °C), which is attributed to the achievement, under these synthetic conditions, of a combination of properties (metal dispersion, specific surface area, porosity) more suitable for the reaction.

Interaction between Nickel Oxide and Support Promotes Selective Catalytic Reduction of NOx with C3H6

Selective catalytic reduction of NO x by C 3 H 6 (C 3 H 6 -SCR) was investigated over NiO catalysts supported on different metaloxides. A NiAlO x mixed oxide phase was formed over NiO/γ-Al 2 O 3 catalyst, inducing an immediate interaction between NiO x and AlO x species. Such interaction resulted in a charge transfer from Ni to Al site and the formation of Ni species in high oxidation state. In comparison to other NiO-loaded catalysts, NiO/γ-Al 2 O 3 catalyst exhibited the highest NO x conversion at temperature higher than 450 °C, but a poor C 3 H 6 oxidation activity due to the decreased nucleophilicity for surface oxygen species. By temperatureprogramed NO oxidation, it is indicated that nitrate species were rapidly formed and stably maintained at high temperature over NiO/γ-Al 2 O 3 catalyst. In situ transient reactions further verified the LangmuirHinshelwood mechanism for C 3 H 6 -SCR, where both gaseous NO and C 3 H 6 were adsorbed and activated on catalyst surface and reacted to generate N 2 . Due to the strong metal-support interaction over NiO/γ-Al 2 O 3 catalyst, both nitrate and C x H y O z intermediates were well preserved to attain high C 3 H 6 -SCR activity.