Synergistic photo-thermal catalytic NO purification of MnO x /g-C 3 N 4 : Enhanced performance and reaction mechanism

Abstract Both MnO x and g-C 3 N 4 have been proved to be active in the catalytic oxidation of NO, and their individual mechanisms for catalytic NO conversion have also been investigated. However, the mechanism of photo-thermal catalysis of the MnO x /g-C 3 N 4 composite remains unresolved. In this paper, MnO x /g-C 3 N 4 catalysts with different molar ratios were synthesized by the precipitation approach at room temperature. The as-prepared catalysts exhibit excellent synergistic photo-thermal catalytic performance towards the purification of NO in air. The MnO x /g-C 3 N 4 catalysts contain MnO x with different valence states on the surface of g-C 3 N 4 . The thermal catalytic reaction for NO oxidation on MnO x and the photo-thermal catalytic reaction on 1:5 MnO x /g-C 3 N 4 were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy ( in situ DRIFTS). The results show that light exerted a weak effect on NO oxidation over MnO x , and it exerted a positive synergistic effect on NO conversion over 1:5 MnO x /g-C 3 N 4 . A synergistic photo-thermal catalytic cycle of NO oxidation on MnO x /g-C 3 N 4 is proposed. Specifically, photo-generated electrons (e − ) are transferred to MnO x and participate in the synergistic photo-thermal reduction cycle (Mn 4+ → Mn 3+ → Mn 2+ ). The reverse cycle (Mn 2+ → Mn 3+ → Mn 4+ ) can regenerate the active oxygen vacancy sites and inject electrons into the g-C 3 N 4 hole (h + ). The active oxygen (O − ) was generated in the redox cycles among manganese species (Mn 4+ /Mn 3+ /Mn 2+ ) and could oxidize the intermediates (NOH and N 2 O 2 − ) to final products (NO 2 − and NO 3 − ). This paper can provide insightful guidance for the development of better catalysts for NO x purification.