Nanocrystalline ZnCO_3: A novel sorbent for low-temperature removal of H_2S

摘要

The reactivity of a nanocrystalline ZnCO_3 toward H_2S (0.2 vol% in N_2/H_2 mixture) at 140-180℃ was characterized by thermal gravimetric analysis and by breakthrough curves measurements. We have found that under used conditions transformation of ZnCO_3 into ZnS is complete and the rate determining step of the sulfidation is the surface reaction. Such behavior is in strike contrast with that of ZnO whose sulfidation is severely limited by diffusion. The higher reactivity of ZnCO_3 in comparison with ZnO is attributed to the different microstructure of ZnS layer formed in these materials after a partial sulfidation. As in ZnO-ZnS transformation the molar volume increases (from 14.5 to 23.8cm~3/mol), a continuous protective ZnS layer is formed hampering the access of H_2S to the non reacted ZnO core. By contrast, in ZnCO_3-ZnS transformation the molar volume decreases (from 27.9 to 23.8 cm~3/mol), which produces a discontinuous non-protective ZnS layer enabling a complete transformation of ZnCO_3 even at 140℃. The higher reactivity of ZnCO_3 results in a considerable increase of the breakthrough sulfur capacity of the carbonate in comparison with oxide. The material has therefore a good potential for being used as a disposable sorbent for H_2S capture at low temperature.