Plasma-Modified Coal Fly Ash Zeolites with Enhanced Catalytic Efficiency toward the Total Oxidation of Volatile Organic Compounds as Low-Cost Substitutes for Platinum Group Metals Catalysts

The initial coal fly ash zeolites (CFAZ) are obtained from raw coal fly ash with two SiO2/Al2O3 ratios of 2.0 and 2.2 by hydrothermal activation or by two-stage process. Microwave plasma surface treatment of CFAZ is performed to modify their surface and therefore their catalytic activity in total VOC oxidation. CHF3 and SF6 are used as plasma agents at two different treatments times-30 s and 60 s. The obtained hierarchal CFAZ are characterized by X-ray diffraction, N-2-physisorption, temperature programmed reduction experiments, UV-vis spectroscopy and X-ray photoelectron spectroscopy to clarify the effect of plasma treatment on the surface and reduction properties, morphology and structure of the modified samples. The catalytic performance of the modified CFAZ is investigated in a flow system toward toluene oxidation in the temperature range of 250-520 degrees C. It has been observed that CHF3 favors the catalytic activity of CFAZ at shorter treatment duration, while SF6 inhibits their catalytic properties. The development of efficient low-cost catalyst will contribute to the saving of critical raw materials such as platinum group metals, and to the utilization of the enormous resource of coal ash instead of its landfilling in accordance to the development of circular economy and smart waste management.