Effects of Added Molybdenium and Chromium on the Thermal Stabilities and Activities of Raney-type Nickel Catalysts for High Temper ature Methanation of Carbon Monoxide

Abstract

The preparative methods of the Raney-type Ni-Al₂O₃, Ni-Cr-Al₂O₃, Ni-Mo-Al₂O₃ and Ni-Mo-Cr-Al₂O₃ catalysts were examined with emphasis on their thermal stabilities for high temperature methanation of CO. The catalysts were prepared by oxidation and calcination of Raney nickel in air. Addition of (NH₄)₂Cr₂O₇ and (NH₄)₆Mo₇O₂₄ into sodium hydroxide aq. soln. upon leaching Ni-Al alloy were found to be effective for the homogeneous distribution of alumina and promoters in the catalysts (Figs.1, 3). Interactions between nickel oxide and other constituent oxides in the precursors increased with increasing the temperature and period of calcination, except the case of the single addition of Cr (Table 2). For the precursors of a Ni-Mo-Cr-Al₂O₃ catalyst, interaction between NiO and the constituent oxide increased markedly in the calcination temperature range from 600 to 650°C. The fraction of free NiO attained below 30%, and a decrease in the crystallite size of NiO was observed (Table 2). Catalysts containing Mo showed higher methane selectivity than those free from Mo (Fig.4). The carbon content of the catalysts after the thermal stability test decreased in the following order: Ni-Mo-Al₂O₃>G-65 (commercial Ni-Al₂O₃)>Ni-Al₂O₃≈Ni-Mo-Cr-Al₂O₃>Ni-Cr-Al₂O₃, as shown in Table 4. The relationship between the thermal stabilities of the catalysts and the catalyst properties, such as the amount of free nickel, homogenuity of the catalyst components, and the species of promoter were discussed. Simultaneous addition of Cr and Mo as promoter was found to be effective in improving the thermal stabilities of catalysts.
Ni-Mo-Cr-Al₂O₃ calcined at 650°C for 16 h exhibited high thermal stability (Fig.6) due to the depressed nickel sintering and carbon deposition (Table 3, 4).