The effect of lanthanum additives on the catalytic activities of Ni-Al₂O₃ coprecipitated catalysts for the methanation of carbon monoxide

A nickel-alumina coprecipitate, which on calcination and reduction under suitable conditions gave an active and stable catalyst for the methanation of carbon monoxide, was used to prepare a series of doped samples by impregnation with the nitrates of various cations. After calcination and reduction, these samples were examined for their activities in the methanation of carbon monoxide using differential scanning calorimetry (DSC). The poisoning effect of the Group I alkaline metals increased in the order: Li < Na < K < Cs, while the ions of the Group II cations tested had no major effect. Cu had a poisoning effect similar to that of Na, while the decrease in activity with Cr was smaller. However, La and Ce both gave slight increases in activity. The promotion by La was further investigated by preparing several series of catalysts with variable La contents, using both impregnation and coprecipitation techniques. An optimum activity for all these series was found at about 1 mol.% La; above this concentration, the activity decreased. This decrease was particularly marked for the impregnated samples, although it was less when the samples had been aged at high temperatures. It is suggested that low levels of La (probably present as La₂O₃) promote the active metallic sites on the catalyst surface but that higher levels block these sites; for the impregnated samples treated at higher temperatures, some of the La can diffuse away from the surface to give more homogeneous materials akin to those derived from the coprecipitates. The possible nature of the active site is discussed.