Oxygen Vacancy Defects in Hematite Enhance Maillard Reactions and Promote Recalcitrant Organic Carbon Formation

Mineral-catalyzed Maillard reactions are considered plausible abiotic pathways for humification and play an essential role in organic carbon (OC) transformation and sequestration. Oxygen vacancy defects (OVDs) are common in minerals and significantly influence their reactivity, yet their role in catalyzing the Maillard reaction remains unexplored. In this study, we systematically investigated the Maillard reaction between glucose and glycine, as model low-molecular-weight organic carbon compounds, catalyzed by hematite with varying OVD concentrations at the molecular level. High-resolution mass spectrometry revealed that OVDs enhance the Maillard reaction, facilitating the formation of recalcitrant geopolymers. Specifically, the relative abundance of lignin-like, tannin-like, and condensed aromatic compounds increased by 17.3% as OVD concentration increased by 0.49 mmol/g. Molecular reaction network analysis further indicated that OVDs increase molecular complexity, with dehydrogenation (-H₂) and dehydration (-H₂O) serving as key steps in molecular evolution. The presence of OVDs on hematite promoted the generation of Lewis acid sites. Spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) and density functional theory (DFT) calculations revealed that localized electron density at Lewis acid sites enriched reactant substrates, lowering the activation energy for the Schiff base reaction by 1.01 eV and thereby promoting the polymerization of aromatic compounds. Additionally, a portion of aromatic Maillard products were selectively localized at Lewis acid sites, enhancing OC stability. These findings provide mechanistic insights into OVD-catalyzed polymerization and underscore the importance of mineral defect structures in predicting soil OC persistence.