Nitrogen defect structure and NO+ intermediate promoted photocatalytic NO removal on H2 treated g-C3N4

•The photocatalytic performance of g-C3N4 is tuned via effect of N-Defects.•Experimental and theoretical calculations were highly combined.•The effects of nitrogen defects on the electronic structure were investigated.•The NO+ reaction intermediate was formed on the N defects sites.•The pivotal roles of N-defects in g-C3N4 on reaction mechanism were revealed.