Regeneration of atomic Ag sites over commercial γ-aluminas by oxidative dispersion of Ag metal particles

• Published in: Catalysis science & technology Vol. 13; no. 5; pp. 1459 - 1469
• Main Authors: Kubota, Hiroe, Mine, Shinya, Toyao, Takashi, Shimizu, Ken-ichi
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 06.03.2023
• Subjects: Aluminum oxide; Ammonia; Catalysts; Catalytic activity; Chemical reduction; Infrared spectra; Lewis acid; Metal particles; Nanoparticles; Oxidation; Selective catalytic reduction; Silver; Sintering; Sintering (powder metallurgy); Transitional aluminas; X ray absorption
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/D2CY01950G

Ag(3 wt%)-loaded γ-Al 2 O 3 (Ag/Al 2 O 3 ) 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 NH 3 or C 3 H 6 decreased in the order CTB > PUR > VGL > CFF. After sintering treatment (H 2 reduction at 800 °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/Al 2 O 3 at 500 °C, the in situ infrared (IR) spectra showed negative bands at 3762 cm −1 due to the HO-μ 1 -Al VI site, where the band intensity increased in the order CTB > PUR > VGL > CFF. IR study of pyridine adsorbed on Ag-free γ-Al 2 O 3 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/Al 2 O 3 sintered under NO + O 2 at 400 °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-μ 1 -Al VI site adjacent to the unsaturated Al IV 3+ site on γ-Al 2 O 3 is the anchoring site of the atomic Ag species, and the sintering resistance of Ag/Al 2 O 3 increases with the number of HO-μ 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-μ 1 -Al VI sites; hence, NO conversion increased with the number of HO-μ 1 -Al VI sites on the support. The present results provide molecular-level insights into the design of sintering-resistant Ag/Al 2 O 3 catalysts for SCR.