Comparison of One-Pot Synthesis and Incipient Wetness Impregnation for Nb-Mn/MCM-41 Catalysts in Plasma-Catalytic Oxidation of Toluene

Niobium-manganese co-loaded MCM-41 catalysts were synthesized via one-pot synthesis (Nb-Mn/MI) and incipient wetness impregnation (Nb-Mn/MII) for the plasma-catalytic degradation of toluene. BET, XRD, TEM, XPS, and O2-TPD results showed that increasing metal loading decreased surface area, pore volume, and both the Mn4+/(Mn2++Mn3+) ratio and Olat/Oads ratio across all samples. At 600 J/L, 3%Nb-Mn/MII gave the highest toluene removal efficiency of 96.5%, CO2 selectivity of 55.0%, and carbon balance of 85.4%. Voltage-current measurements indicated nearly identical discharge characteristics for both synthesis routes. These observations suggest that the performance differences may be related to the distinct dispersion states and redox properties of the catalysts rather than to plasma modification. The impregnated samples appeared to benefit from highly dispersed surface nanoparticles at lower metal loading (3%Nb-Mn/MII), whereas the one-pot samples seemed to retain relatively better structural integrity at higher metal loading (7%Nb-Mn/MI), presumably due to stronger metal-support interactions. This work highlights the potential of tailoring synthesis strategy to metal loading for improved plasma-catalytic VOC abatement.