Promoting effect of H₂S on the performance of ZrO₂ and La₂O₃-ZrO₂ catalysts in biomass gasification gas clean-up

Use of the syngas produced by biomass gasification is rendered unviable because of impurity compounds, whose efficient removal is key to the feasibility of gasification concepts. Tars present in the biomass gasification gas can be efficiently oxidized by ZrO₂ catalysts. H₂S has been shown to increase the oxidation activity on ZrO₂ and most notably on La₂O₃-ZrO₂. Gasification gas clean-up experiments were performed at 600–800 °C to study the beneficial effect of H₂S on naphthalene and toluene conversions. During time-on-stream with repeated doses of H₂S on ZrO₂, cumulative and strong H₂S adsorption was observed as the conversion of toluene and naphthalene increased during the first H₂S doses and stabilized towards the end. Therefore, a limited capacity of the catalyst to adsorb H₂S was suggested, which is the origin of sulfur tolerance. Activity enhancement with H₂S occurs most notably on La₂O₃-ZrO₂ and towards naphthalene oxidation. XPS characterization of the used catalysts showed sulfur retention on the La₂O₃-ZrO₂ but not on pure ZrO₂, indicating that La₂O₃-ZrO₂ holds more sulfur on its surface or binds it stronger. The surface properties of La₂O₃-ZrO₂ were thus characterized with in situ DRIFTS using CO₂ and CH₃OH as probe molecules. The surface of La₂O₃-ZrO₂ was suggested to have more low coordination sites than ZrO₂, where sulfur is suggested to be bound and alter the reactivity of the surface oxygen.