Mechanistic and electronic structure investigation of K⁺-catalyzed supercritical water gasification of coal: A combined experimental and DFT study

The potassium ion (K⁺) is widely recognized as an effective catalyst for enhancing coal gasification efficiency and H₂ yield during supercritical water gasification (SCWG). However, a comprehensive understanding of its catalytic mechanism spanning from macroscopic phenomena to microscopic electronic structures in SCWG is still lacking. Therefore, experiments and density functional theory (DFT) calculations were integrated to clarify the intrinsic catalytic mechanism of K⁺ in SCWG. Experimental results showed that potassium salt accelerated the decomposition of liquid-phase aromatic intermediates, promoted the evolution of solid residues toward highly porous structures, and drove the enrichment and rearrangement of oxygen-containing functional groups in coal, thereby improving carbon gasification efficiency and H₂ yield. DFT analysis of electronic structure and weak interactions for key structures along the phenol and naphthalene reaction pathways revealed that K⁺ polarized the local electrostatic field and induced π-electron redistribution in aromatic rings. Upon deprotonation of hydroxyl groups to form O^– sites, K⁺ migrated to the oxygen centers and formed electrostatically coordinated C_ar–O–K⁺ complexes, which substantially lowered the Gibbs free energy barriers of the key steps and increased the rate constants of the rate-determining steps (RDS) by four orders of magnitude for phenol and two orders of magnitude for naphthalene at 500 °C. As oxygen-containing functional groups were subsequently removed from the aromatic framework, K⁺ was released from the C_ar–O–K⁺ coordination environment and regenerated, completing a full catalytic cycle that provided an electronic structure basis for the C–O functional group enrichment observed by FT-IR and XPS. This study provides experimental guidance and theoretical support for understanding the catalytic mechanism of alkali metals in SCWG.