Understanding the “seesaw effect” of interlayered K+ with different structure in manganese oxides for the enhanced formaldehyde oxidation

Although the potassium-promoted catalysts exhibit improved catalytic activities, the role of potassium with different structural sites still needs to be studied by more experiments and theory calculation. Herein, potassium (K+) formed via different binding sites (isolated and localized) over layered MnO2 was investigated. The isolated K+ dissociated between layers via weak chemical bond while localized K+ coordinated with oxygen atoms at vacancy site. K+ with localized type (L-MnO2) exhibited the higher activity and the lower Ea (45.2 kJ/mol), compared with isolated one (58.1 kJ/mol) under 200 ppm HCHO, 120,000 mL/g h GHSV and ∼45% relative humidity. Evidenced by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), hydrogen temperature programmed reduction (H2-TPR) and density functional theory (DFT) simulation, the isolated K+ in interlayer are energetically favourable for dissociation of O2 as well as H2O by charge transfer from potassium to oxygen. As a result, the desorption of H2O was inhibited, thus, became the kinetic barriers during the whole reaction process. K+ with localized form possessed suitable ability for O2 activation and weak adsorption of H2O, exhibiting enhanced catalytic activity. Inspired by the seesaw effect, a clear understanding of K+ effect on layered MnO2 towards O2 adsorption and H2O desorption was illustrated.