Recently, a widely used method for recycling fly ash waste of coal collected from the electrostatic precipitators has been the synthesis of zeolites from this waste material. Subsequently, zeolites can be used for a variety of purposes, but one that is gaining a growing amount of interest is the use of their porous structure for the detection of volatile organic compounds. In this study, fly ash of lignite coal collected from the electrostatic precipitators of one of the largest TPPs in Bulgaria was used as a raw material for the synthesis of zeolites of Na–X type via ultrasonic-assisted double-stage fusion–hydrothermal alkaline conversion.
In order to reduce the size of the synthesized zeolites and, therefore, improve the quality of the composite thin films, as well as to study the influence of zeolites’ sizes on the sensing properties, the synthesized zeolites were wet-milled at three different durations. All zeolite powders were studied from the viewpoint of their surface morphology and structure via scanning electron microscopy and X-ray diffraction, respectively. The porosity and particle size of Na–X zeolites prior to and after milling were investigated using N2-physisorption and dynamic light scattering, respectively.
Zeolites thus obtained (milled and not-milled) were used to produce composite thin films based on sol–gel niobium pentoxide (Nb2O5). A complete optical characterization of the thin films was made, and their sensing properties with respect to acetone vapor were studied. The change in the reflection coefficient ∆R of the films was calculated from measured reflectance spectra of the films prior to and after exposure to the selected vapors.
Author Contributions
Conceptualization, K.L., T.B. and S.B.; methodology, K.L. and T.B.; software, K.L. and T.B.; validation, K.L., T.B. and S.B.; formal analysis, K.L., D.Z.-F. and M.V.; investigation, K.L., D.Z.-F. and M.V.; resources, S.B. and T.B.; writing—original draft preparation, K.L.; writing—review and editing, K.L., T.B. and S.B. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Acknowledgments
Research equipment of Distributed Research Infrastructure (INFRAMAT), part of the Bulgarian National Roadmap for Research Infrastructures, supported by the Bulgarian Ministry of Education and Science was used in this investigation.
Conflicts of Interest
The authors declare no conflict of interest.
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