Combined experimental and computational study of CO oxidation promoted by Nb in manganese oxide octahedral molecular sieves

Framework-substituted Mn oxide octahedral molecular sieves with different Nb concentrations (2-20mol% Nb-K-OMS-2) were synthesized via a single-step reflux method allowing for direct incorporation of the dopant into the mixed-valent Mn structure. Their specific surface areas ranged from 75 to 199m²g^-1 with modified composition, size, morphology, porosity, thermal stability, and redox properties depending on the extent of substitution. Catalytic testing showed that the Nb-K-OMS-2 materials were active for CO oxidation and that the presence of Nb significantly enhanced the activity of pure K-OMS-2. For example, the conversions of 1% CO at 100°C using 0%, 2%, 5%, 10%, 15%, and 20% Nb-K-OMS-2 were 4%, 10%, 25%, 62%, 59%, and 41%, respectively. When the O₂ concentrations increased from 1% to 10% at 120°C, the activities of 10% and 15% Nb-K-OMS-2 materials were improved by as much as 61% and 69%, respectively. These catalysts were also stable and less prone to deactivation by moisture (~3% H₂O) at temperatures >100°C than pure K-OMS-2. Theoretical calculations revealed that the substitution of Mn by Nb was a thermodynamically-favorable process and produced electrophilic centers, which can provide favorable sites for strong CO adsorption on the Nb-K-OMS-2 surface. The interaction of CO at these sites exhibited the beneficial effect of Nb substitution in the K-OMS-2 materials.