Non-thermal plasma in honeycomb catalyst for the high-throughput removal of dilute styrene from air

This study focused on the removal of styrene using a metal-coated monolith (MCM) catalyst integrated with non-thermal plasma. The MCM catalyst was prepared by depositing a ZSM-5 support impregnated with a combination of three metals (Co, Pd, La) on a practical-scale honeycomb monolith. A comparative study with a commercial metal-coated monolith (CMCM) of the same dimensions was conducted to evaluate the effectiveness of the MCM with respect to styrene removal. The plasma discharge in the monolith was characterized by measuring the voltage and current waveforms and optical emission spectroscopy (OES). The electric power of the plasma discharge largely depended on the metal loading, flow rate, and humidity of the feed gas. The removal efficiency of styrene (inlet concentration: 20 parts per million, volumetric (ppm)) was examined using both plasma catalytic oxidation (PCO) and thermal catalytic oxidation (TCO), to enable the effect of plasma to be clearly assessed. PCO with the MCM catalyst achieved styrene removal efficiency of nearly 100% at a specific energy input (SEI) of 40 J/L, whereas the CMCM catalyst achieved 92% removal for identical SEI. In comparison, TCO with the MCM and CMCM catalysts required high temperatures of 420 °C and 480 °C, respectively, for the complete removal of styrene. In addition, the CO2 selectivity of the MCM catalyst was higher at 83% compared with 75% for the CMCM at SEI of 40 J/L. The optimal silica binder content for the deposition of a robust washcoat layer was found to be approximately 5%.