Upcycling Pond Coal Ash for CO Oxidation: An Operando Spectroscopic Study

Abstract

Pond coal ash (PCA) is an abundant industrial waste whose disposal poses significant environmental challenges; however, its intrinsic mineral complexity offers opportunities for sustainable catalytic upcycling. In this work, pond coal ash collected from a thermal power plant in Mongolia was upcycled into functional catalysts for CO oxidation through wet magnetic separation, yielding magnetic (M-PCA) and non-magnetic (NM-PCA) fractions. Comprehensive characterization using X-ray fluorescence, X-ray diffraction with Rietveld refinement, N2 physisorption, and TEM-EDX mapping revealed comparable bulk elemental compositions but pronounced differences in crystalline phase assemblies, spatial elemental distribution, and surface heterogeneity between the two fractions. While both materials are dominated by oxide-silicate and Ca-rich phases after oxidative pretreatment, NM-PCA uniquely exhibits a higher degree of chemical and structural heterogeneity, including localized iron-rich domains and a broader distribution of defect-associated surface environments. Temperature-programmed oxidation demonstrated enhanced oxygen uptake for NM-PCA, indicating superior oxygen activation capacity despite its lower bulk iron content. Steady-state CO oxidation experiments showed that NM-PCA consistently outperforms M-PCA, achieving ∼44% higher CO conversion at 400 °C and ∼31% higher conversion at 450 °C. For the first time, operando temperature-programmed DRIFTS coupled with GC-MS was employed to directly elucidate the CO oxidation mechanism over pond coal ash-derived catalysts, revealing temperature-dependent CO adsorption on aluminosilicate oxides containing surface sites, transient stabilization of CO2 as surface carbonate species on Ca-rich phases, and high-temperature carbonate decomposition contributing to CO2 release. These findings demonstrate that catalytic performance is governed by the mineralogical organization and microstructural distribution of the constituent phases, rather than by the bulk iron content alone, establishing a sustainable pathway for upcycling pond coal ash into low-cost catalysts for environmental remediation.