Highly efficient Mn-Fe bimetallic oxides for simultaneous oxidation of NO and toluene: Performance and mechanism

Abstract

A series of PEG modified Mn-Fe bimetallic oxides with various of Mn/Fe molar ratio were synthesized by co-precipitation method to investigate the performance and mechanism of simultaneous oxidation of NO and toluene. The Mn2Fe1 catalyst with Mn/Fe molar ratio of 2:1 obtained good low-temperature catalytic oxidation performance with 87 % NO oxidation efficiency at 230 °C and 100 % CO2 selectivity for toluene at 200 °C. The excellent catalytic performance benefited from the synergistic interaction between Mn and Fe, which increased the specific surface area, enhanced the dispersion and low-temperature reducibility, and produced abundant adsorbed oxygen as well as Mn3+ and Fe3+. The generated oxygen vacancies promoted the activation of oxygen and the adsorption-oxidation of NO and toluene. The activity results showed that NO had promoting effect on toluene removal in the presence of O2, while toluene could inhibit NO removal within 200 °C. However, the inhibitory effect of toluene on NO disappeared above 200 °C. Additionally, NO showed negative effect on toluene removal within 140 °C in the absence of O2 for Mn2Fe1 catalyst, but this negative effect of NO on toluene disappeared with the increase of temperature. Toluene oxidation took the superiority during the simultaneous oxidation of NO and toluene. It was also found that O2 significantly promoted the NO and toluene deep oxidation, besides, NO also played a positive role in the deep oxidation of toluene to a certain extent due to the formation of strong oxidizing NO2. While the presence of toluene showed negative effect on NO deep oxidation, especially in the absence of O2. In addition, SO2 acted a negative role for NO and toluene removal due to the accumulation of sulfate on catalyst surface. Interestingly, NO was inhibited first, followed by toluene. Furthermore, in situ DRIFTS was performed to study the intermediates in the oxidation of NO and toluene, and a possible reaction mechanism for simultaneous oxidation of NO and toluene over Mn2Fe1 was proposed.