Effect of N-doping on NO2 adsorption and reduction over activated carbon: An experimental and computational study

Due to high adsorption capacity, NO2 adsorption using activated carbon has become a potential denitration technology. A correlation between N-doping and NO2 adsorption process on activated carbon is pointed out by previous studies but lacks a clear explanation. The main objective of this work is to clarify the influencing mechanism of N-doping on NO2 adsorption and reduction over activated carbon. N-doped activated carbons were synthesized by melamine impregnation followed by calcination and tested as NO2 adsorbents. Experimental results showed that AC600 with highest N content possessed highest NO2 adsorption capacity and lowest NO release percentage. NO2 adsorption on activated carbon is a complex process, including NO2 adsorption, NO2 reduction to NO, and NO release. Density functional theory calculations results showed that N-doping affected all steps mentioned above. Firstly, N-doping increased NO2 physisorption energies, meaning that NO2 was easier to be adsorbed. Thus, NO2 adsorption capacities increased with the increase of N content experimentally. Secondly, N-doping increased the energy barrier of the rate-determining step of NO2 reduction. Thus, the reduction of NO2 to NO was harder. Thirdly, N-doping increased NO desorption energies, implying that the release of generated NO was harder. The inhibition of NO2 reduction and NO desorption results in the decrease of NO release percentage due to N-doping jointly. This work elucidates the influencing mechanism of N-doping on NO2 adsorption and NO2 reduction on theoretical level, which fills up the research gap and guides the design of activated carbon for efficient NO2 adsorption.