La effect on the long-term durability of Ni-Mg-Al^sub 2^0^sub 3^ catalysts for syngas methanation

Various contents of La (1-10 wt.%) were added to Ni-Mg-Al2O3 catalyst to improve its catalytic activity and long-term stability for CO methanation. The optimum amount of La for the highest CO conversion is obtained at 1 wt.% La addition for fresh catalysts, whereas at 5 wt.% La for aged catalyst. During a long term ageing test at a high severity condition (850 °C, space velocity = 250,000 h-1 and 40% steam in the feed), catalysts having 0-3 wt% La rapidly lose their activity at the initial 10 h period due to the sintering of Ni particles, and this is followed by slower degradation due to the thermal sintering of the support alumina. Catalysts having higher La contents (5-10 wt.%) exhibit an increase in the CO conversion at a certain period, which is attributed to the uncovering effect of La on the Ni surface through the agglomeration of dispersed La into aggregates. For fresh catalysts, the CO conversion and turnover frequency are highest at 1 wt.% La and 7 wt.% La, respectively. For aged catalysts, both of these are highest at 5 wt.% La.