Unveiling the intrinsic role of water in the catalytic cycle of formaldehyde oxidation: a comprehensive study integrating density functional theory and microkinetic analysis

Abstract

Previous research is predominantly in consensus on the reaction mechanism between formaldehyde (HCHO) and oxygen (O₂) over catalysts. However, water vapor (H₂O) always remains present during the reaction, and the intrinsic role of H₂O in the oxidation of HCHO still needs to be fully understood. In this study, a single-atom catalyst, Al-doped C₂N substrate, Al₁/C₂N, can be adopted as an example to investigate the relationship and interaction among O₂, H₂O, and HCHO. Density functional theory (DFT) calculations and microkinetic simulations were carried out to interpret the enhancement mechanism of H₂O on HCHO oxidation over Al₁/C₂N. The outcome demonstrates that H₂O directly breaks down a surface hydroxyl group on Al₁/C₂N, considerably lowering the energy required to form crucial intermediates, thus promoting oxidation. Without H₂O, Al₁/C₂N cannot effectively oxidize HCHO at ambient temperature. During oxidation, H₂O takes the major catalytic responsibility, delaying the entrance of O₂ into the reaction, which is not only the product but also the crucial reactant to initiate catalysis, thereby sustaining the catalytic cycle. Moreover, this study predicts the catalytic behavior at various temperatures and presents feasible recommendations for regulating the reaction rates. The oxidation mechanism of HCHO is explained at the molecular level in this study, emphasizing the intrinsic role of water on Al₁/C₂N, which fills in the relevant studies for HCHO oxidation on two-dimensional carbon materials.