Redox-Driven Reversible Structural Evolution of Isolated Silver Atoms Anchored to Specific Sites on γ‑Al2O3

• Published in: ACS catalysis 2022-01, Vol.12 (1), p.544-559
• Main Authors: Kubota, Hiroe, Mine, Shinya, Toyao, Takashi, Maeno, Zen, Shimizu, Ken-ichi
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.1c04924

Reversible structural transformation between atomic Ag­(I) and large Ag metal nanoparticles (NPs) on γ-Al2O3-supported Ag (Ag/Al2O3) catalysts for H2-assisted NO x selective catalytic reduction by NH3 was monitored by in situ X-ray absorption spectroscopy, ultraviolet–visible spectroscopy, infrared spectroscopy, and ex situ microscopy. Fresh Ag/Al2O3 was deactivated by H2 reduction at 800 °C owing to sintering of the atomic Ag­(I) species to form large (10–52 nm) Ag metal NPs. Reoxidation of the sintered catalyst using NO + O2 at 400 °C resulted in the redispersion of Ag metal NPs to the atomic Ag­(I) species, leading to the recovery of the catalytic activity. Sintering and dispersion occurred reversibly for 10 repetitive treatments of H2 ↔ NO + O2 at 600 °C. The structure of the anchoring site and the mechanism of oxidative dispersion were elucidated by kinetic studies, in situ spectroscopy, 27Al nuclear magnetic resonance spectroscopy, and density functional theory calculations. The isolated Ag­(I) cation was exchanged with H+ of the HO-μ1-AlVI site adjacent to the strong Lewis acid sites (unsaturated AlIV 3+) on γ-Al2O3, and, consequently, the anchored Ag­(I) species reduced the Lewis acid strength of the adjacent AlIV 3+ sites. During sintering under H2, the isolated AgO-μ1-AlVI species aggregated to form Ag metal NPs, regenerating the HO-μ1-AlVI sites on the support. During oxidative dispersion under the NO + O2 flow, the Ag metal NPs were oxidized to furnish mobile AgNO3 species that moved across the support surface and reacted with the anchoring site, HO-μ1-AlVI, to yield the original AgO-μ1-AlVI species and HNO3; the latter reacts with the γ-Al2O3 surface to yield the nitrate species. This study provides molecular level insights into the deactivation/reactivation of supported metal catalysts under reductive/oxidative conditions.