Mechanism of N2O formation in catalytic after-treatment systems of ammonia/hydrogen-fuelled engines

Abstract

The combustion processes and catalytic after-treatment of ammonia/hydrogen-fueled engines, including NO_x storage and reduction (NSR) and noble-metal selective catalytic reduction (SCR), can produce the byproduct N₂O, a potent greenhouse gas that weakens the zero-carbon attribute of these fuels. Currently, the mechanism of N₂O formation on DeNO_x catalysts remains unclear due to limited research on catalytic after-treatment for such engines and the complexity of surface catalytic reactions. To elucidate the formation of N₂O on the DeNO_x catalysts of ammonia/hydrogen fuel engines, the impact factors on N₂O formation on platinum catalysts (typical catalysts in NSR and noble-metal SCR) were investigated using first-principles molecular dynamics (FPMD). By employing the blue-moon ensemble enhanced sampling method and the slow-growth approach for free energy surface exploration, together with density functional theory (DFT) for electronic structure analysis, a linear relationship between the spin splitting of the d states of Pt clusters and N₂O formation energy barriers was revealed, along with the increased structural sensitivity of Pt clusters with fewer atoms. It is highlighted that the energy barrier for N₂O formation is determined by the matching degree of energy levels between molecules and surfaces. These findings provide atomic-scale insights into N₂O formation on DeNO_x catalysts for ammonia/hydrogen-fueled engines, facilitating N₂O emission control for carbon-free engines.