Unveiling the Effects of Alkali Metal Ions Intercalated in Layered MnO2 for Formaldehyde Catalytic Oxidation

Two-dimensional (2D) layered MnO2 materials, composed of exotic electronic properties and accessible active sites with alkali metal ions, provide a comprehensive platform for developing catalysts with chemical modification. Significantly, K+-contained layered MnO2 catalysts have been verified as strong candidates toward catalytic oxidation of formaldehyde (HCHO). Unveiling the effects of alkali metal ions on active sites is critical to understand the interaction between reactants and active centers. Through a combination of analytical tools with periodic computational density functional theory modeling, the surface structures and the exposing specific defects of alkali metal ions affiliated to oxygen vacancies (Vo) are figured out by comparing three typical alkali metal ion-intercalated (Na+, K+, and Cs+) layered MnO2 materials. These materials have been synthesized via a molten salt method, with high yield, large lateral size, and nanometer thickness in a few moments. We demonstrate that the alkali metal ions could remarkably alter the formation energy of Vo by the sequence of CsMnO (1.94 eV) < KMnO (1.97 eV) < NaMnO (2.07 eV) < ideal MnO2 surface without the intercalated ion (2.23 eV). As a result, CsMnO with the most surface Vo sites could achieve efficient HCHO oxidation to CO2, with a HCHO consumption rate of about 0.149 mmol/(g·h) at 40 °C in 200 ppm HCHO/humid air [gas hourly space velocity = 80,000 mL/(g·h)]. Different from the Mars–van-Krevelen process, quantum chemical calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy revealed that the main reaction pathway might be HCHO­(ad) + [O]­(ad) → DOM → [HCOO–]s → CO2 via a Langmuir–Hinshelwood (L–H) mechanism. Alkali metals remarkably promoted the HCHO conversion by trapping oxygen through Vo sites and accelerating the facile reaction among adsorbed oxygen with adsorbed HCHO to deep degradation products (CO2 and H2O).