Toluene oxidation over ZrO₂-based gasification gas clean-up catalysts: Part B. Kinetic modeling

Energy, liquid biofuels and chemicals can be produced from biomass via gasification followed by gas cleaning over ZrO₂-based catalysts. When a small amount of oxygen is added, ZrO₂-based catalysts can be utilized in tar decomposition reactions (tar molecules need to be removed before further use). Oxidation of toluene as tar model compound was studied by temperature-programmed experiments over ZrO₂, Y₂O₃-ZrO₂ and SiO₂-ZrO₂. In the first part of this study (Part A: Effect of oxygen on the formation of primary products), special attention was given to the effect of oxygen amount on the formation of synthesis gas components (CO and H₂) from toluene. The data obtained in Part A was extended and subjected to kinetic modeling. As a result, the reaction mechanism of toluene oxidation over ZrO₂-based catalysts was discovered. The first step in toluene oxidation is the adsorption of toluene as a benzyl species. Next, the surface benzyl species is oxidized, possibly via superoxide surface species (O₂ ^-), into four products in a single net primary reaction. Oxidation of CO and H₂ are the secondary reactions in the model. Furthermore, the present study clearly demonstrates the strength of temperature programming as a transient technique for comprehensive mechanistic investigations, kinetic model development and parameter estimation.