Modulation of paired acid centers for the α-, β-, γ- and δ-MnO2 for the NH3-SCR: A comparative density functional theory (DFT) study

The selective catalytic reduction with ammonia (NH3-SCR) has been the state-of-the-art technology for NOx pollutant control, in which the catalyst plays a key part. Along with the urgent academic pursuit of higher SCR activity at lower temperatures, manganese oxides are considered one of the most promising components to construct a strong oxidation center for the catalyst surface. However, the rough use of manganese oxides has only a limited promotion effect on lowering the temperature window of the catalyst. Their SCR performance will be largely determined by the physicochemical properties of manganese oxides of different crystal phases (α, β, γ, and δ-MnO2) and the modulation methods of acid centers, which are still controversial and lack systematic and reliable arguments. In this paper, we conducted comprehensive research on their physicochemical properties and proposed a reliable evaluation of their potential for SCR reaction via density functional theory (DFT) calculation. W and Mo were introduced as new acid centers to pair with redox centers. Among the α-, β-, γ-, and δ-MnO2, γ-MnO2 showed the strongest NH3 adsorption capacity with relatively high NO capture ability. However, when Mn atoms coordinated with W or Mo atoms to form Mn-O-W or Mn-O-Mo bonds on the α-MnO2 surface, the altered electronic distribution characteristics resulted in the maximum NH3 and NO adsorption energies with the minimum dehydrogenation barrier. Mo 4d had better bonding interaction with N 2p compared with W 5d, showing a higher NH3 capture ability. The electron transfer results indicated that the high catalytic activity of MnO2 originated from the strong interaction between H and the O2c site on the surface, while the O2c site easily forms an oxygen vacancy. Additionally, W or Mo doping may change the rate-determining steps.