Catalytic mechanism of bi-alkali-metal-doped char in heterogeneous reduction of NO: A density functional theory study

Bi-alkali-metal doping in char can significantly increase the heterogeneous reduction rate of NO. In this study, the effect of Na and K doping on heterogeneous reduction of NO by a char model with zigzag edge was systematically investigated by density functional theory. It was found with reduced density gradient and Mayer bond order analyses that alkali metals promoted the dissociation of the N-C and combination of N-Na or O-K. Moreover, bi-alkali-metal doping changed charge characteristics of the char, and promoted N-N formation and N-O breaking to shorten the reaction path. The simulation paths indicated that Na/K co-doping could reduce the energy barrier from 52 to 117 kJ/mol to less than 10 kJ/mol in the second and third steps of heterogeneous reduction. Both the mono- and bi-metals promoted reduction of NO on char. The kinetic calculation illustrated that the maximum fitted reaction activation energy is 179 kJ/mol for NO heterogeneous reduction at the edge of Na/K-doped char, whereas those for Na- and K-doped chars are 210 and 200 kJ/mol, respectively. This work provided a new and deep insight into the microscopic mechanism of NO-char reaction catalyzed by composite metal.