Interaction of H2S with ZrO2 and its influence on reactivity of surface oxygen

ZrO2 catalysts can be efficiently applied to convert tars into less harmful compounds in biomass gasification gas clean-up, also in the presence of H2S. In fact, H2S has even been observed to enhance naphthalene (a model compound for tar) oxidation activity on ZrO2. Sulfur binding and its effect on the reactivity of surface oxygen species, including OH groups as well surface lattice oxygen, on ZrO2 was studied using temperature-programmed methods. At 30 °C approx. half of the adsorbed H2S was physisorbed and the other part dissociated titrating terminal OH groups. This dissociated H2S was strongly adsorbed. Moreover, it was found that after treatment with H2S until 400 °C half of the adsorbed species were irreversibly adsorbed. H2S adsorbed inducing desorption of water at two distinct temperatures (∼170 and 280 °C) during temperature ramp. Thus, H2S adsorption at elevated temperatures produced surface sulfur by replacement of surface lattice oxygen at two types of minority sites (max. 5% of a monolayer). Therefore we suggest that multicoordinated OH groups and surface lattice oxygen at defective sites are involved. CO-TPR revealed increased reactivity of surface lattice oxygen on ZrO2 with increasing amount of sulfur on the surface. In addition, strong adsorption of carbonates was suppressed by the presence of sulfur. The observed H2S-induced enhancement of naphthalene oxidation in gasification gas clean-up is suggested to be caused by the increased reactivity of surface oxygen species in the vicinity of sulfur species on the surface