New Insights into the NH ₃ Decomposition Mechanism Promoted by Sewage Sludge Char: The Synergistic Effect of Oxygen-Containing Functional Groups and Fe ₂ P/Fe

Ammonia (NH3) generated during sewage sludge gasification adversely affects syngas quality and causes catalyst poisoning in downstream processes. Previous studies indicate that sewage sludge-derived char holds potential for in situ NH3 removal. However, the underlying mechanism, particularly the synergistic effect between surface oxygen-containing functional groups and the main ash phases, remains unclear. In this study, fixed-bed experiments, multicharacterization analyses, and theoretical calculations were employed to elucidate the catalytic decomposition mechanism of NH3 over char at 200–900 °C. At low temperatures (200–500 °C), NH3 conversion is limited to 16.4% due to insufficient activation of the active sites. In contrast, at 900 °C, active sites are adequately activated, achieving a peak conversion of 99.2%. It was revealed that the formation of double hydrogen bonds between NH3 and the oxygen-containing functional groups inhibits the loss of these oxygen-containing functional groups, thereby promoting NH3 adsorption and subsequent decomposition. This effect is further strengthened by Fe2P/Fe, with Fe2P primarily facilitating adsorption and Fe contributing to catalytic decomposition. During the decomposition step mediated by Fe2P/Fe, electrons from N–H cleavage transfer to *H via the functional groups and carbon substrate, thereby promoting H2 formation. These findings provide theoretical support for optimizing the NH3 removal process in gasification.