Revealing the unexpected promotion effect of diverse potassium precursors on α-MnO2 for the catalytic destruction of toluene

Abstract

The alkali metal potassium has the functions of structure promotion and electronic modulation in metal oxides. Herein, diverse potassium precursors (KOH, KNO₃, K₂SO₄, and KCl) were introduced to α-MnO₂ nanorods through a facile post-processing strategy. The presence of potassium species has a remarkable promotion effect on the catalytic performance of α-MnO₂. Amongst them, the KOH/MnO₂ sample has the highest activity and can destroy 90% toluene (1000 ppm) at just 226 °C with a reaction rate of 3.39 × 10⁻⁴ mol g_cat⁻¹ s⁻¹, which is over 20 times higher than that of pure α-MnO₂. Different anions in the potassium precursors bring a distinct mutation in the α-MnO₂ structure, promote the formation of MnO₆–K–MnO₆ bridging bonds in α-MnO₂, and exhibit obvious diverse abilities for balancing charge transfer. KOH is identified as the most promising precursor for alkali metal modification, which significantly improves the distribution of K species over the α-MnO₂ surface and strengthens the content and activity of lattice oxygen. It is confirmed that the lattice oxygen plays a key role in the catalytic oxidation of toluene over α-MnO₂, which follows the Mars–van Krevelen mechanism. Positive hole defects (Mn³⁺) caused by KOH treatment play an important role in the diffusion of O and enhance the reducibility of manganese oxide. In addition, the enhanced specific surface area, pore volume, and surface acidity are also conducive for the catalytic oxidation of toluene.