CATALYST LOSS AND RETENTION DURING ALKALI-CATALYZED CARBON GASIFICATION IN CO2

Characterization of the catalyst/carbon samples by outgassing yielded reproducible and consistent results, provided the amounts of CO2 and CO released are related to the amount of alkali metal actually present. The catalytically active cluster is anchored to the carbon by phenolate groups and is capable of chemisorbing one CO2 molecule per three to five alkali metal atoms. Between K/C = 0.02 to 0.04 the alkali cluster size increases, but the specific gasification rate, r(K), remains unaffected. During potassium catalysed CO2 gasification initially 20% to 40% of the potassium was lost; the rest remained present throughout the gasification process. The amount of catalyst that can be stabilised on the carbon increases in the order Na < K < Cs and depends on the amount of oxygen in the carbon. The latter is reduced by heat treatment of the applied carbon, and hence a smaller amount of catalyst can be stabilised. The differences between the alkali metals arise from their ability to migrate into the carbon matrix and their interaction with lattice oxygen at which they are trapped. This migration of Cs and K into the carbon matrix explains the burn-off profiles of catalyst/carbon samples after TPD treatment. Initially only part of the catalyst is active for gasification, but gradually more catalyst becomes available, resulting in an increasing gasification rate. This behaviour is absent for sodium.