Low Temperature Gas Desulphurisation Using Mixed Cobalt-Zinc Oxides

[摘要] The hydrogen sulphide absorption capacity of a range of mixed cobalt-zinc oxides was determined using a continuous flow microreactor. The mixed oxides were prepared by the thermal decomposition of hydroxycarbonate precursors. The precursors were synthesized by the coprecipitation method using the mixed metal nitrates and ammonium or sodium carbonate. X-Ray diffraction studies of the precursors synthesized from ammonium carbonate revealed that at low cobalt loadings hyrozincite, Zn5(C03)2(OH)6, was present as the predominant phase and spherocobaltite, COCO3, as the minor phase. At cobalt loadings ≥50% spherocobaltite was observed as the major phase. However, the Co/Zn 100/0 precursor revealed only the presence of a metastable phase, Co(CO3)0.5(OH)1.0.0.1H2O which had been previously identified by Porta et al. UV-VIS-NIR diffuse reflectance spectroscopy revealed that the cobalt was present in the 2+ oxidation state in an octahedral environment in all the precursors. When sodium hydrogen carbonate was used as the precipitating agent sodium zinc carbonate, Na2Zn3(CO3)4!3H2O, was formed. This was believed to be an intermediate in the formation of hydrozincite. For the Co/Zn 30/70 loading the pH used during the coprecipitation altered the predominant phase structure found in the final precursor. The major phase obtained on calcination of the precursors with a Co/Zn ratio ≤30/70 was ZnO with the a 'cobalt oxide type' phase (taken to be CO3O4) present as a minor phase. As the cobalt loading was increased this situation was reversed and CO3O4 became the major phase. Varying the temperature at which the precursor was calcined to the oxide revealed that the highest surface area was achieved when the precursor was calcined at ca. 200°C. An X-Ray photoelectron study of the oxides calcined at 350°C revealed that only Co3+ and Zn2+ ions were present at the surface suggesting the presence of a 'surface spinel', ZnCo2O4. Segregation of cobalt to the surface was observed. This segregation was believed to be determined during the synthesis of the precursor and was carried through to the oxide. The reaction of the mixed oxides with H2S was restricted to ca. 3 monolayers on average, based on calculations from surface areas, and is therefore largely confined to the surface of the oxides. For the oxides calcined at ≥350°C the linear relationship observed between the surface area and the sulphur uptake suggested that lattice diffusion played a major role in the rate determining step, the main role of the cobalt being to increase the surface area. The oxides calcined ≤250°C showed no correlation between surface area and sulphur uptake. It is likely that both pore and lattice diffusion contributed to the rate of sulphur uptake in these oxides. The oxides with a Co/Zn ratio of 100/0 were found to be the best sulphur absorbents. Calculations indicated that a bulk reaction had taken place in these oxides. Transmission electron micrographs of the mixed oxides before and after sulphidation revealed the presence of a sheet-like material which became more prevalent the more the mixed oxide was sulphided. Electron energy loss spectroscopy indicated that these sheet-like regions were predominantly zinc in composition.

[发布机构] University:University of Glasgow

[关键词] Chemical engineering