Low-temperature NH3-SCO over CuOx/Al2O3 enhanced by precursor-derived Cl-bonded Cu species

We examined how the choice of copper precursor influences the catalytic performance of CuO_x/Al₂O₃ catalysts prepared via impregnation. Four types of catalysts were synthesized using different copper precursors: copper acetylacetonate, copper nitrate, copper sulfate, and copper chloride. Their catalytic activity in NH₃ oxidation was evaluated alongside their structures and surface properties to elucidate the relationships between precursor selection, surface species formation, and catalytic performance. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) revealed the formation of distinct surface species originating from different anionic components of precursors. Particularly, surface Cl-bonded Cu species such as CuCl₂ from copper chloride markedly enhanced the catalytic activity, while the CuSO₄ species formedi with copper sulfate precursor significantly reduced the activity. The catalyst prepared with an optimal amount of copper chloride precursor achieved 100 % NH₃ conversion over a broad temperature range (230–360 °C). Furthermore, it maintained complete NH₃ removal at 250 °C for 510 min, with N₂ selectivity exceeding 70 %. New Cl-bonded Cu species such as Cu₂(OH)₃Cl species was formed upon prolonged annealing at 600 °C, which promoted NH₃ oxidation at low temperatures (< 160 °C). These findings highlight the crucial role of precursor-derived surface species, particularly the promotional effect of Cl-bonded Cu species, in enhancing the NH₃-SCO activity of CuO_x/Al₂O₃ catalyst.