High-loaded nickel–alumina catalyst for direct CO2 hydrogenation into synthetic natural gas (SNG)

The methanation of carbon dioxide was carried out over a high-loaded Ni–Al mixed oxide catalyst (ca. molar ratio Ni/Al = 5), prepared by conventional coprecipitation of the metal precursors. This route makes possible to obtain multimetallic mixed oxides upon calcination with high metal loading and high surface area. X-ray powder diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscope (TEM) and thermogravimetric analysis were used for studies of the precursors and catalysts. Calcined samples were activated under H2 at 500 °C, leading to the formation of small metallic nickel particles (ca. 6 nm) dispersed on a high surface area Ni(Al)Ox matrix. Activity tests were conducted using different H2/CO2 molar ratios (3–5), WHSV (0.2–1.0 molCO2/(gcat h), temperatures (250–500 °C), and pressures (5–20 bar). Despite the high nickel loading (ca. 70 wt.%), which is theoretically thought to be counter-productive for the nickel-based catalyst performance, our Ni–Al activated catalyst exhibited high CO2 conversion, and rendered a CH4 selectivity very close to 1. This is originated by the formation of small metallic nickel crystallites (ca. 6 nm) dispersed over NiO–alumina upon partial reduction of the mixed oxide. The catalyst experiences complete reduction during reaction, which slightly increases the Ni crystallite size, but preserves the high activity in ca. 500 h lifetime tests even at high space velocity.