Role of Alkali or Alkaline Earth Metals as Additives to Co/Al2O3 in Suppressing Carbon Formation during CO2 Reforming of CH4

Resumo

Effect of alkali or alkaline earth metals (Li, Na, K, Mg, Ca, and Sr) as additives to Co/Al₂O₃ on CH₄–CO₂ reforming and carbon formation was examined. It was shown that the alkali metals decreased the reforming rate, whereas the rate was scarcely influenced by the alkaline earth ones. Carbon deposition was also more markedly retarded by the alkali metals than the alkaline earth ones. It was revealed that one of the essential roles of the basic additives was to reduce the ability of cobalt for methane decomposition through a partial covering of the active sites. Another effect was to modify acid-base properties of the catalyst surface toward higher basicity thus increasing adsorption of acidic CO₂. Abundance of adsorbed CO₂ on the catalyst should be an unfavorable condition for decomposition of methane and consequently, carbon formation was further suppressed. It was also suggested that carbon gasification rate was not significantly influenced by the additives.