Computational study of the formation, migration, and decomposition of epoxy groups during the combustion of char

Char is considered as one of the most promising base-load fuels for sustainable heat and power generation. Recent XPS studies have identified epoxides as some of the most abundant oxygen functional groups during the initial oxidation stage of raw char. These surface oxides are highly reactive and have been reported to migrate across the surface, converting into ethers, lactones, and other oxygenated species at temperatures above 500 ∘ C . The present study investigates the formation, surface mobility, and subsequent transformation of epoxides in char by means of molecular dynamics simulations that are applied to several molecular models, employing the GFN2-xTB method for the potential energy surface. To determine activation and reaction energies of the key reaction pathways we employ density functional theory with the TPSSh functional and the def-TZVP basis set. We identify two primary epoxide formation pathways: (i) the addition of molecular oxygen to edge radical sites, forming peroxo-bridges that subsequently cleave into alkoxy radicals and epoxides, and (ii) the direct addition of atomic oxygen to the char surface. We find low energy barriers for surface migration, indicating high mobility of epoxides. Additionally, in agreement with experimental observations, we find that structural defects can drive the conversion of epoxides into ethers and lactones. Our results highlight the role of epoxy groups as surface oxides in the heterogeneous combustion of raw char. • Two primary pathways for epoxide formation on char surfaces are revealed: via peroxo-bridge intermediates and direct atomic oxygen addition. • Density functional theory calculations show low energy barriers for epoxide migration, indicating significant surface mobility during char oxidation. • Structural defects on the char surface promote the transformation of epoxides into more stable oxygenated species like ethers and lactones, supporting experimental XPS findings.