Optimized Design Method for Pt/SiO₂-Al₂O₃ with High NH₃-SCO Activity and Thermal Stability

Pt/SiO₂-Al₂O₃ catalysts were prepared by the traditional impregnation method (IM) and the strong electrostatic adsorption (SEA) process. Differences in particle size, surface chemical state, Pt adsorption site, ammonia oxidation activity, and thermal stability of Pt species were studied systematically. For the fresh catalyst of Pt/SiO₂-Al₂O₃-IM (Pt/SiO₂-Al₂O₃-IM-fresh), Pt species were dispersed unselectively on SiO₂-Al₂O₃, and the large average size (6.6 nm) of Pt species could be observed in a bimodal distribution (ranges of 5.5-6.5 and 8.5-9.5 nm). After the hydrothermal treatment, the Pt size of the aged catalyst (Pt/SiO₂-Al₂O₃-IM-aged) increased significantly, especially Pt particles on SiO₂ showed obvious agglomeration and some even increased to 40 nm. Conversely, for the catalyst prepared through the SEA process, Pt species of Pt/SiO₂-Al₂O₃-SEA-fresh were selectively absorbed on Al₂O₃, the Pt particle size was in the range of 1.5-6.0 nm, and the average particle size was only 2.7 nm. After hydrothermal aging, Pt species did not show obvious agglomeration (the average particle size was 3.2 nm). Above all, Pt/SiO₂-Al₂O₃-SEA presented better catalytic activity and thermal stability than Pt/SiO₂-Al₂O₃-IM, i.e., the temperatures of 50% NH₃ conversion for the fresh and aged Pt/SiO₂-Al₂O₃-SEA catalysts were 216 and 223 °C, respectively, much lower than those for Pt/SiO₂-Al₂O₃-IM-fresh (228 °C) and Pt/SiO₂-Al₂O₃-IM-aged (250 °C).