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

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. Water accelerates the oxidation cycle of formaldehyde on Al1/C2N, and plays a greater catalytic role than oxygen, but cooperation can exert greater influence.