Advanced high-iron coal fly ash zeolites for low-carbon emission catalytic combustion of VOCs

Abstract

Coal fly ash zeolites (CFAZs) with increased iron oxide content were prepared by double-stage alkaline conversion of high-iron lignite coal ash. Catalysts were modified by applying radiofrequency plasma treatment with trifluoromethane (RF CHF3) to increase their surface activity. Textural, compositional, and structural characterization of the parent and plasma-modified zeolites were performed by N2-physisorption, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) spectroscopy. Oxidation state and coordination of iron species were studied by Mössbauer spectroscopy, UV-Vis spectroscopy and temperature-programmed reduction method (TPR). Acidic properties were investigated by FT-IR spectroscopy of adsorbed pyridine. The initial and plasma-modified samples were studied under toluene oxidation, where toluene was used as a model VOC. The plasma-modified iron-enriched zeolite was found to exhibit high catalytic activity for VOC oxidation, reaching a 98% toluene conversion rate at 500 °C. Despite some structural degradation effects, the plasma-treated catalysts outperformed the untreated ones in catalytic activity. Furthermore, CO2 adsorption capacity of the plasma-modified zeolite was investigated under dynamic conditions. Although plasma-treated iron-enriched zeolite showed relatively low specific surface area (194 m2/g), high dynamic carbon capture capacity of 3.8 mmol/g was measured. It significantly exceeded the capacity of CFAZs with higher specific surface area, not to mention the pure zeolite phase.