Effects of Ni substitution on active oxygen species and electronic interactions over La0.8Ce0.2MnO3/mesoporous ZSM-5 for oxidizing C6H14

Abstract

Based on fly ash, mesoporous ZSM-5 (MZ) molecular sieves were prepared using a combination of hydrothermal treatment and fractional calcination. The La0.8Ce0.2Mn1-yNiyO3/MZ(y≤0.5) obtained using an impregnation load method was studied regarding the hexane oxidation, which was chosen as the probe molecule of volatile organic compounds (VOCs) at 120−390 °C. Additionally, the physicochemical characteristics of the catalysts were characterized by XRD, SEM, TEM, BET, XPS, NH3-TPD, H2-TPR and O2-TPD to investigate the electronic interactions over La0.8Ce0.2MnO3/MZ with Ni substitution and the active oxygen species, the main influence factors and the reaction mechanism for hexane oxidation. The results showed that there was a strong electronic interaction between LaMnO3 and MZ, and Ni substitution at Mn sites in La0.8Ce0.2MnO3 could promote the electronic transfer among the La, Ce, Ni, Mn and MZ, which could improve the low temperature reducibility, as well as facilitate the migration and distribution of the surface oxygen species. Additionally, La0.8Ce0.2Mn0.5Ni0.5O3/MZ demonstrated a higher catalytic activity for hexane oxidation due to its higher migration capacity of the outer lattice oxygen species (O2−), indicating that the outer lattice oxygen species as the active oxygen species were the main factor for hexane oxidation corresponding to the Mars-Van Krevelen (MvK) reaction mechanism. However, the specific surface area, the low temperature reducibility of high valence state cation ions (Mn4+ and/or Ni4+/Ni3+) and the surface acidity could be key but not main factors in hexane oxidation. Furthermore, La0.8Ce0.2Mn1-yNiyO3/MZ (y≤0.5) showed good catalytic stability, coke deposition resistance and H2O resistance. Based on the results of in situ DRIFTS and GC–MS, two reaction routes of hexane oxidation could be proposed over the La0.8Ce0.2Mn1-yNiyO3/MZ (y≤0.5) catalysts in air, therein, the decomposition of carbonate species could be one of the main rate- controlling steps at ≤ 320 °C.