Nonthermal plasma catalysis enhances simultaneous removal of toluene and ozone over TiO2@ZIF-8

Abstract

Ultrasmall TiO2 nanoparticles (average diameter of 6.6 nm) were encapsulated inside zeolitic imidazolate framework-8 (ZIF-8) via a self-assembly method, and the synthesized TiO2@ZIF-8 catalyst was applied for the removal of toluene using nonthermal plasma (NTP). Physicochemical characterization was performed to investigate the structure and morphology of TiO2@ZIF-8. The results revealed the high dispersion of TiO2 nanoparticles in ZIF-8 and the formation of a Z-type heterojunction between TiO2 and ZIF-8, which efficiently utilized the UV light generated during plasma discharge to produce •OH and •O2‾. Upon treating toluene exhaust gas of 100 mg·(m3) −1 at 92.5% relative humidity, the toluene conversion efficiency was significantly improved to 91.17% with a COX selectivity of 76.45% by TiO2@ZIF-8 in NTP-catalysis at a specific energy density (SED) of 507 J L−1, which was improved by 45.96% and 52.27%, respectively. This was mainly due to the good hydrophobicity of ZIF-8, which helped maintain efficient catalytic performance under high humidity. The maximum ozone decomposition efficiency of 99.22% was achieved for the accelerated charge transfer efficiency and nearly formed NTP discharge behavior in the porous structures of the catalyst. In the 36-h stability test, high efficiency of toluene conversion, COX selectivity, and ozone decomposition were maintained with no significant decay observed. Discharge characteristic analysis and electron paramagnetic resonance (EPR) characterization revealed the mechanism of toluene degradation in which electrons, ·OH, ·O2 ⁻, and O3 participated in toluene degradation. The possible pathways of toluene degradation were proposed based on the intermediates detected by chromatography–mass spectrometry. This work provides a new approach for anchoring and dispersing nanoparticles by ZIF-8 to avoid the reunion of nanoparticles and a new direction for synthesizing high-performance catalysts for volatile organic compound (VOC) degradation in NTP-catalysis.