Unlocking Catalytic Efficiency: How Preparation Strategies and Copper Loading Enhance Hydroxyapatite Catalysts for NH₃ Oxidation

The selective catalytic oxidation of ammonia (NH3-SCO) is gaining attention due to the hazardous nature of NH3 and its inclusion in emission reduction frameworks such as the National Emission Ceilings Directive and the Gothenburg Protocol (1999). Copper-based hydroxyapatite (Cu/HAP) catalysts have emerged as a promising solution, offering high activity and cost-effectiveness. This study evaluated two preparation methods: a one-pot co-precipitation technique and post-synthesis copper deposition, varying both the contact time and copper concentration. The influence of copper loading and preparation method on catalyst performance in NH3-SCO was investigated in a continuous flow reactor over a temperature range of 200–500 °C, with a fixed gas hourly space velocity (GHSV) of 120,000 h−1 and an NH3/O2 ratio of 0.03. X-ray diffraction and DR-UV spectroscopy confirmed the high crystallinity of HAP and provided insights into copper speciation. X-ray photoelectron spectroscopy revealed that Cu/HAP catalysts prepared via one-pot co-precipitation predominantly contained isolated Cu2+ species, which were associated with high catalytic activity in selective NH3-SCO. Conversely, a higher degree of copper structuring was observed in catalysts prepared by post-synthesis deposition, particularly at higher Cu loadings. These findings highlight the potential to tailor Cu structuring on HAP to enhance performance in NH3-SCO through optimized preparation strategies.