Toluene oxidation over ZrO2-based gasification gas clean-up catalysts: Part A. Effect of oxygen and temperature on the product distribution

Abstract

Biomass gasification combined with effective gas cleaning is an interesting way to produce energy or syngas for numerous applications. ZrO2-based catalysts have been proven to remove the undesired tar molecules when a convenient oxygen amount is added. The oxidation of toluene (a tar model compound) was addressed in this work by applying temperature-programmed surface reaction (TPSR) experiments with a continuous feed of toluene and oxygen. Complete toluene conversions were achieved over all zirconia-based catalysts (ZrO2, Y2O3-ZrO2 and SiO2-ZrO2) in toluene oxidation above 550°C (toluene WHSV 0.06–0.07 1/h). Toluene was oxidized into four products (CO2, H2O, CO and H2). The formation of incomplete oxidation products (CO and H2) suggests that these catalysts are able to convert undesired tar molecules also into valuable synthesis gas components. Increasing the oxygen amount increased the product ratio of CO2 and CO. Over pure ZrO2, the formation of CO and H2 was detected at 600°C even with the highest feed ratio of O2/TOL (≈3.5×theoretical toluene total oxidation ratio), while over the doped zirconias the formation of CO and H2 approached zero with increasing temperature and over-stoichiometric feeds. Separate CO oxidation experiments confirmed different but appreciable activities of different zirconia materials for that reaction. However, a very minor water-gas shift activity in the presence of oxygen was detected only over pure ZrO2 at the studied temperature range. The collected multiresponse reaction data was subsequently subjected to transient kinetic modeling established on hypotheses of surface reaction mechanisms (Part B: Kinetic modeling).