Evolution and enhancement of the oxygen cycle in the catalytic performance of total toluene oxidation over manganese-based catalysts

Abstract

An investigation of the effect of oxygen and silver species on the oxidation of toluene has been launched to identify the reaction process and mechanism over MnOx/SBA-15 and Ag–MnOx/SBA-15 catalysts. Detailed knowledge of the effect and cycle of oxygen species under reaction is typically missing, and this study points at the behavior of oxygen species over MnOx/SBA-15 for toluene oxidation for the first time, using in situ mass spectrum techniques with the isotope 18O2. It is found that the Langmuir–Hinshelwood and Mars–van Krevelen mechanism coexist in toluene oxidation over MnOx/SBA-15 catalyst. In this study, surface and bulk lattice oxygen are shown to be vital active oxygen species in the whole reaction. The consumed oxygen species are replenished with gaseous oxygen, and the process follows the Mars–van Krevelen mechanism, as confirmed by detection of isotopic products. The adsorbed oxygen species with good mobility is also found to start up and participate in the reaction at lower temperatures, which follows the Langmuir–Hinshelwood reaction mechanism. The addition of Ag increases the amount of surface lattice oxygen species and improves the transformation rate and the migration of lattice oxygen, which effectively facilitates the low-temperature activity of toluene oxidation.