A carbon nanodot modified Cu–Mn–Ce/ZSM catalyst for the enhanced microwave-assisted degradation of gaseous toluene

Abstract

In this work, carbon nanodots (CNDs) were firstly synthesized by hydrothermal treatment of natural biomass (dead leaves of a plane tree) without any synthetic chemicals. The concentration of the fabricated carbon nanodot solution was ca. 13 g L⁻¹ and the mass yield of CNDs was ca. 5.6%. The characterization results indicated that the prepared CNDs with sizes of 2–7 nm contained rich surface functional groups such as O and N-containing groups and possessed blue fluorescence properties. Using a conventional impregnation approach, the fabricated carbon nanodot solution was used to further modify a Cu–Mn–Ce/ZSM (CMCZ) catalyst. The CND modified Cu–Mn–Ce/ZSM (CND–CMCZ) catalyst was subsequently evaluated for the catalytic oxidation of gaseous toluene under microwave heating. The experimental results demonstrated that the catalyst modified with CNDs with the features of analogous humic acids clearly enhanced the adsorptive capacity of gaseous toluene and the rate of temperature increase of the catalyst during the catalytic reaction compared with the unmodified Cu–Mn–Ce/ZSM catalyst. The improved adsorptive capacity of gaseous toluene could be due to the enhanced specific surface area and the role of the surface functional groups of the composite catalyst, while the “hot spots” effect of the CNDs could significantly contribute towards the improved rate of temperature increase of the composite catalyst. Owing to these advantages, 75% of the gaseous toluene was degraded within 80 min at 150 °C using the CND–CMCZ catalyst, which was almost 1.9 times that of the unmodified catalyst. At a lower reaction bed temperature (e.g., 150 °C), the obtained catalytic performance could be due to a synergistic role between the carbon nanodots and Cu–Mn–Ce/ZSM. Although the degradation efficiency of gaseous toluene was further improved by increasing the reaction bed temperature (200 °C and 250 °C), the role of the CND modification for improving catalytic performance was obviously reduced. This could be ascribed to the structure and surface functional group damage of the carbon nanodots at higher catalytic temperatures. This work demonstrated the possibility of developing CND modified catalysts for low-temperature catalytic oxidation of volatile organic compounds (VOCs).