| Popis: |
Ag(3 wt%)-loaded gamma-Al2O3 (Ag/Al2O3) catalysts were prepared using four types of commercially available alumina powders (CTB, PUR, VGL, and CFF). Based on the support, the activity of these catalysts for the H-2-assisted selective catalytic reduction (SCR) of NO by NH3 or C3H6 decreased in the order CTB > PUR > VGL > CFF. After sintering treatment (H-2 reduction at 800 degrees C), the particle size of the Ag metal nanoparticles (NPs) changed and was found to be correlated with the catalytic activity (CTB < PUR < VGL < CFF). After re-oxidation of H-2-reduced Ag/Al2O3 at 500 degrees C, the in situ infrared (IR) spectra showed negative bands at 3762 cm(-1) due to the HO-mu(1)-Al-VI site, where the band intensity increased in the order CTB > PUR > VGL > CFF. IR study of pyridine adsorbed on Ag-free gamma-Al2O3 showed that the number of strong Lewis acid sites (unsaturated Al-IV(3+)) increased in the same order, CTB > PUR > VGL > CFF, and the number of strong Lewis acid sites decreased when Ag was loaded on the supports. In situ X-ray absorption near-edge structure (XANES) and UV-vis studies of Ag/Al2O3 sintered under NO + O-2 at 400 degrees C showed oxidative redispersion of the Ag metal NPs to regenerate atomic Ag(i) sites. The amount of redispersed Ag metal and the initial rates of redispersion estimated from the in situ UV-vis results changed in the following order: CTB > PUR > VGL > CFF. These results suggest that the HO-mu(1)-Al-VI site adjacent to the unsaturated Al-IV(3+) site on gamma-Al2O3 is the anchoring site of the atomic Ag species, and the sintering resistance of Ag/Al2O3 increases with the number of HO-mu(1)-Al-VI sites. During H-2-assisted SCR, where both H-2 and NO + O-2 were co-fed to the catalysts, the number of highly dispersed Ag species (active sites) increased with the number of HO-mu(1)-Al-VI sites; hence, NO conversion increased with the number of HO-mu(1)-Al-VI sites on the support. The present results provide molecular-level insights into the design of sintering-resistant Ag/Al2O3 catalysts for SCR. |
