Literature Survey on Cobalt Aluminates

Literature Survey on Cobalt AluminatesWhat is Cobalt Aluminate?Alumina supported cobalt readily forms surface compounds that only reduce at elevated temperaturesCompound formation between cobalt and the support Alumina, may occur during pre-treatment and/or FT reaction.Water vapor added during reduction with hydrogen enhances formation of aluminate type species.Substitution of Co3+ ions in Co3O4 spinel structure by Al3+ ions causes difficult reducibility of cobalt catalysts calcined at higher temperatures.

During typical reduction of supported cobalt at low temperatures (e.g., 350400 C), a sizeable fraction of the cobalt remains in the oxide form, mainly as CoONonreducible, surface, or subsurface cobalt aluminate-like compounds are likely to form when Co0 or surface Co2+ ions are well dispersed on the alumina surfaceHydrated alumina stabilize these cobalt oxidic species in strong interaction with the support During prolonged treatment at temperature, they may diffuse into subsurface regions resulting in nonreducible cobalt aluminate formation.

Problems Caused by Co Aluminates in the FT ProcessAt low loadings (e.g., < 5%), 60 to 80% of the cobalt is present as CoO strongly bound to the support surface, i.e., a surface cobalt aluminate, CoO*Al2O3, which requires very high temperatures to reduce It is well known that in CoAl2O4, Co2+ ions sit in a spinel structure. The term nonreducible is applied because this cobalt aluminate can only be reduced above 800900C The properties of alumina make it difficult to control the temperature of the catalyst-bed by forming hot spots due to the lower thermal conductivity of alumina.Prevention of formation of Cobalt Aluminates