Low-temperature abatement of toluene over Mn-Ce oxides catalysts synthesized by a modified hydrothermal approach

Highlights: • Mn-Ce solid solution forms by combining redox-precipitation and hydrothermal method. • Mn_0.6Ce_0.4O₂ shows the highest activity with toluene complete conversion at 210 °C. • Abundant Ce³⁺ and Mn³⁺ in catalyst facilitate the formation of oxygen vacancies. • Benzyl alcohol, carbonate and formate species are the crucial intermediates. Mn-Ce oxides catalysts were synthesized by a novel method combining redox-precipitation and hydrothermal approach. The results indicate that the ratio between manganese and cerium plays a crucial role in the formation of catalysts, and the textual properties as well as catalytic activity are remarked affected. Mn_0.6Ce_0.4O₂ possesses a predominant catalytic activity in the oxidation of toluene, over 70% of toluene is converted at 200 °C, and the complete conversion temperature is 210 °C. The formation of Mn-Ce solid solution markedly improves the surface area as well as pore volume of Mn-Ce oxide catalyst, and Mn_0.6Ce_0.4O₂ possesses the largest surface area of 298.5 m²/g. The abundant Ce³⁺ and Mn³⁺ on Mn_0.6Ce_0.4O₂ catalyst facilitate the formation of oxygen vacancies, and improve the transfer of oxygen in the catalysts. Meanwhile, it is found that cerium in Mn-Ce oxide plays a key role in the adsorption of toluene, while manganese is proved to be crucial in the oxidation of toluene, the cooperation between manganese and cerium improves the catalytic reaction process. In addition, the reaction process is investigated by in situ DRIFT measurement, and it is found that the adsorbed toluene could be oxidized to benzyl alcohol as temperature rises around 80–120 °C that can be further be oxidized to benzoic acid. Then benzoic acid could be decomposed to formate and/or carbonate species as temperature rises to form CO₂ and H₂O. In addition, the formed by-product phenol could be further oxidized into CO₂ and H₂O when the temperature is high enough.