Microscopic mechanism and kinetics of NO heterogeneous reduction on char surface: A density functional theory study

A quantum chemistry calculation with density functional theory at M06-2X/6-31G(d,p)//def-TZVP level is conducted to gain insights into the microscopic mechanism of NO heterogeneous reduction on char surface. Electronic properties of reactants based on Mulliken atomic charge and electrostatic potential reveal the attack sites and electron donation from char to NO during the chemisorption, which is consistent with HOMO and LUMO analysis. Two stages of NO secondary chemisorption, N2 and N2O formation are included in the evolution pathway of NO reduction, and the second NO chemisorption is determined as the rate-limiting step of NO reduction. Meanwhile, the migration of the oxygen atom is favorable for NO reduction, resulting in the preferred pathway of N2 formation than N2O in thermodynamics. According to the kinetic calculation, the reaction rate constant of NO reduction to release N2 is 1.438E+01 s−1 at 900 K, and the corresponding activation energies of N2 and N2O formation are 37.97 and 78.02 kcal/mol respectively, which is in coincidence with previous experiments. Mayer bond order analysis exhibits the C–N, N–O and N–N bond evolution of two rate-limiting steps, together with the bond strength between atoms. •Evolution pathway and microscopic mechanism of NO-char reaction are studied by DFT.•Electronic and orbital properties are analyzed to predict NO chemisorption sites.•Rate-limiting steps of two stages of NO reduction on char surface are determined.•Migration of surface oxygen atoms is favorable for NO reduction on char surface.•Contiguous active sites are not conducive to N2O detaching but to NO chemisorption.