Studies on the promotion of nickel—alumina coprecipitated catalysts: II. Lanthanum oxide

Two series of lanthanum promoted nickel—alumina catalysts have been prepared by coprecipitation of the metal nitrates, using potassium carbonate. The molar ratio between nickel and the sum of aluminium and lanthanum was kept constant at 2.5 or 9.0 within each series. The calcination and reduction of these samples were studied by thermogravimetry and their structures before and after calcination and reduction were examined by X-ray diffraction. The methanation activities of the final catalysts were determined by differential scanning calorimetry. The results showed clearly that the methanation of carbon monoxide over nickel—alumina catalysts is enhanced by the presence of La2O3. With low percentages of lanthanum, the promoter is built into the precursor structure during the coprecipitation process. This is a meta-stable situation; phase separation occurs during hydrothermal treatment. In both series there was an optimum amount of lanthanum at which the activity per gram of nickel reached a maximum. The optimum specific activity of a lanthanum promoted nickel—alumina catalyst was twice as large as that of the unpromoted material. Above these optimum values, the activity per gram of nickel decreased because of two effects: an increase in the nickel particle sizes and an increase in the amounts of potassium remaining from the precipitation step. Alumina is needed to stabilize the nickel crystallites against sintering. The promoting action of La2O3 is slightly higher after reduction at 400°C than after reduction at 600°C. Lanthanum increased the amount of carbon monoxide which was adsorbed slowly; the amount of carbon monoxide which was rapidly adsorbed, however, was not altered. The increase in activity was accompanied by an increase in the apparent activation energy