Three-dimensional ordered macroporous Mn–Ce composite oxide catalysts with excellent low-temperature toluene oxidation performance: synergistic effect and reaction mechanism

Abstract

A set of Mn–Ce composite catalysts with three-dimensionally ordered macroporous (3DOM) structures were synthesized via the PMMA colloidal crystal templating approach. Among them, 3DOM-Mn1Ce2 exhibited superior toluene oxidation performance, achieving 90% conversion at 244 °C, along with exceptional stability, reusability, and water resistance. XRD and SEM results confirmed the formation of a solid solution with a well-defined 3DOM structure. 3DOM-Mn1Ce2 possessed the largest BET surface area (61.02 m² g⁻¹) and an optimal pore structure. The intense synergistic effect between manganese and cerium accelerated the redox cycling between Mn³⁺/Mn⁴⁺ and Ce³⁺/Ce⁴⁺ pairs, and significantly enhanced the mobility of lattice oxygen and the low-temperature reducibility of the catalyst. The relative contributions of physicochemical properties to the catalytic activity for toluene oxidation were quantified by multiple linear regression (MLR) combined with principal component analysis (PCA), revealing that the Mn⁴⁺/Mn ratio exerted the most significant influence. The comparison of toluene-TPD and toluene-TPSR results on 3DOM and bulk Mn1Ce2 catalysts revealed that the 3DOM structure significantly enhances toluene adsorption and activation. In situ DRIFTS results revealed that the catalytic oxidation of toluene over 3DOM-Mn1Ce2 followed the Mars–van Krevelen (MvK) mechanism. The catalytic reaction pathway was proposed, identifying the cleavage of aromatic CC bonds in the benzoate intermediate as the rate-determining step.