Modified zinc oxide absorbents for low-temperature gas desulfurisation

Abstract

The hydrogen sulfide absorption capacity of zinc oxide doped with first-row transtion-metal oxides (ca. 5% metal oxide loading) has been determined using a pulse reactor. The doped oxides were prepared either by impregnation of ZnO with the transition-metal nitrates or by comprecipitation of the transition-metal and zinc nitrates with ammonium/sodium carbonate. These absorbent precursors were then calcined to give the mixed oxides.

Transmission electron microscopy studies of the impregnated and calcined absorbents revealed that the transition-metal oxides were finely dispersed over the ZnO as γ-Fe₂O₃, Co₃O₄ and CuO from the respective nitrate salts of these metals. The basal planes were the predominat exposed faces of the hexagonal ZnO in all the absorbents, irrespective of whether they were prepared by the coprecipitation or impregnation route. CuO and Co₃O₄ were not seen as separate phases in the respective calcined coprecipitated absorbents, but the particle morphology was noticeably changed after sulfidation and crystalline ZnS was detected by electron diffraction. Oxides prepared by the coprecipitation route had higher surface areas and a greater capacity for H₂S removal than their impregnated counterparts. Doping with iron salts had little effect on the H₂S uptake of ZnO, irrespective of whether an impregnation or a coprecipitation route had been used, but doping with copper or cobalt salts resulted in a marked enhancement in the H₂S uptake in each case.

The reaction of the mixed oxides with H₂S was restricted to ca. 0.6 monolayers on average, based on calculations from surface areas, and is therefore largely confined to the surface of the oxides. However, the resulting sulfide is not present entirely as an adsorbed layer; some reaction occurs locally in depth to form ZnS crystallites sufficiently large to give an electron diffraction pattern. The main role of the transition-metal oxide is to increase the total surface area available for reaction with H₂S.