Mixed cobalt–iron oxide absorbents for low-temperature gas desulfurisation

Abstract

High surface area cobalt–iron oxides (with Co ∶ Fe ratios of [3 ∶ 1], [1 ∶ 1] and [1 ∶ 2]) have been prepared by the calcination of predominantly single-phase, coprecipitated hydrotalcite-type precursors prepared from mixed-Co²⁺–Fe³⁺ aqueous solutions. The oxide structures were all spinels; for the Co–Fe [3 ∶ 1] oxide, this was a Co₃O₄-type lattice, for the Co–Fe [1 ∶ 2] sample, it was a Fe₃O₄-type lattice and for Co–Fe [1 ∶ 1] an equal mix of the two was observed. The H₂S uptake of all the oxides was tested at 303 K and the data correlated with sorbent preparation and characterisation. The Co–Fe [1 ∶ 1] oxide absorbed the most H₂S before breakthrough and further studies into the preparation of this oxide showed that nitrate-free precursors produced at pH 7 and then aged for 30 minutes or more gave rise to the best oxide sorbents after calcination at 623 K rather than 723 K. These observations have been rationalised on the basis that these oxides had lower density and a greater mix of Co^2+/3+ and Fe^2+/3+ leading to more distortion of the parent oxide lattice. Increased H₂S uptake was ascribed to more rapid ion diffusion through the sorbent lattice leading to enhanced replenishment of surface oxide.