Elucidation of the surface pathways over MnOX based 3D printed monoliths for the catalytic oxidation of toluene: In-situ DRIFTS measurements

This study provides a pioneering analysis of the surface reaction pathway during the catalytic oxidation of toluene on MnO_X-based 3D-printed monolithic catalysts. Four monoliths, including green monolith (support), CeO_X, MnO_X and MnCeO_X, were evaluated by DRIFT spectroscopy. These in-situ measurements allow us to elucidate the reaction pathways as a function of its surface compositions. Indeed, all monoliths favored the adsorption of toluene at low temperatures and the formation of several surface intermediates such as benzyl alcohol and some carboxylate species but, complete oxidation to CO₂, was only observed over the MnCeO_X at 300 °C. MnCeO_X monolith showed a mixture of Mn₃O₄ and CeO₂ crystalline phases identified by XRD and Raman with higher concentration of lattice oxygens according to XPS measurements. Thus, the presence of abundant lattice oxygen and reduced Mn and Ce species on the surface of MnCeO_X seems to be crucial for the complete oxidation of toluene. A reaction pathway has been proposed for this monolith involving the adsorption of toluene via the methyl group, followed by the sequential formation of benzyl alcohol, benzaldehyde and benzoate. The cleavage of the aromatic ring produced formate species, which were further converted to CO and CO₂. In addition, this work demonstrates that water not only competes for the same adsorption sites as toluene molecules, but also participates in the generation of reactive species, which significantly influences the overall oxidation process and the formation of intermediate species produced by the interactions of water species with organic compounds. Finally, it is important to mention that the monoliths evaluated in this work open up new opportunities for using 3D printing as a rapid and cost-effective method to enhance catalyst activity in real-world applications and decrease VOC emissions.