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

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 Co2+/3+ and Fe2+/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.