New insight on the ZnO sulfidation reaction: Evidences for an outward growth process of the ZnS phase

Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fisher-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the different steps involved in the solid transformation. Various ZnO materials with well-determined characteristics have been synthesized to perform this study. The kinetics of ZnO sulfidation by H2S has been investigated using isothermal and isobaric thermogravimetry. In view of the kinetic curves appearance and in agreement with literature, the rate-determining step is more likely to be a diffusion step. However a kinetic blocking has been noticed for some ZnO particles. As expected, characterizations of the reaction products using electronic microscopies indicate that a polycrystalline ZnS layer is formed around the monocrystalline ZnO particles, but surprisingly, cavities in the bulk of ZnO have been noticed at partial conversion. At total conversion, a void is observed in the core of the sulfided particles, which proves that ZnS is formed through an outward growth mechanism. Regarding the ZnO sulfidation reaction, the relevance of the widely used shrinking core based models has thus to be reconsidered. The formation of two ZnS polymorphs (sphalerite Zn1-xS and wurtzite ZnS1-x) has also been evidenced by XRD characterizations of sulfided solids, which may be explained in the operating conditions according to the literature by considering the nanometric size of ZnS crystals formed. As sphalerite is a zinc deficient phase, whereas wurtzite is sulfur deficient, different scenarios can be postulated for the mechanisms of zinc and oxygen diffusion occurring through the ZnS layer and involving the various related ZnS point defects. Beyond a critical value of fractional conversion, cavities that are formed at the internal ZnO/ZnS interface may cause the kinetic blocking of the sulfidation reaction by preventing the zinc atoms from diffusing into the ZnS layer. (C) 2011 Elsevier B.V. All rights reserved.