Experimental and DFT research on role of sodium in NO reduction on char surface under H2O/Ar atmosphere

•NO reduction characteristics on char surface in H2O differ from those in Ar.•Evolution pathways of NO reduction and CO release on char surface are studied by DFT.•The catalysis of Na intensifies the competition between the formation of CO and CO2.•Promotion ways of H2O atmosphere for NO reduction are elucidated at the molecular level.•The formation of H-C-O-Na groups is an important part of Na catalysis mechanism.