Abstract
Pt/SiO(2)-Al(2)O(3) 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/SiO(2)-Al(2)O(3)-IM (Pt/SiO(2)-Al(2)O(3)-IM-fresh), Pt species were dispersed unselectively on SiO(2)-Al(2)O(3), 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/SiO(2)-Al(2)O(3)-IM-aged) increased significantly, especially Pt particles on SiO(2) showed obvious agglomeration and some even increased to 40 nm. Conversely, for the catalyst prepared through the SEA process, Pt species of Pt/SiO(2)-Al(2)O(3)-SEA-fresh were selectively absorbed on Al(2)O(3), 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/SiO(2)-Al(2)O(3)-SEA presented better catalytic activity and thermal stability than Pt/SiO(2)-Al(2)O(3)-IM, i.e., the temperatures of 50% NH(3) conversion for the fresh and aged Pt/SiO(2)-Al(2)O(3)-SEA catalysts were 216 and 223 °C, respectively, much lower than those for Pt/SiO(2)-Al(2)O(3)-IM-fresh (228 °C) and Pt/SiO(2)-Al(2)O(3)-IM-aged (250 °C).