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Precise Identification and Chraracterization of Catalytically Active Site on the surface of g-Alumina.pdf (3.1 MB)
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Precise Identification and Characterization of Catalytically Active Sites on the Surface of γ-Alumina

preprint
submitted on 10.02.2020 and posted on 11.02.2020 by Konstantin Khivantsev, Nicholas R. Jaegers, Ja-Hun Kwak, János Szanyi, Libor Kovarik

γ-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization and over a hundred years of active research, the nature of the specific sites on the surface of γ-alumina which are responsible for its unique catalytic properties has remained obscure and controversial. Herein, we identify for the first time using combined infrared spectroscopy, electron microscopy and solid-state nuclear magnetic resonance measurements that the octahedral amphoteric (O)5Al(VI)-OH sites on the massively restructured (110) facets on typical rhombus-platelet γ-alumina as well as (100) segments of irrational surfaces (invariably always present in all γ-alumina samples) are largely responsible for its unique catalytic activity. Such (O)5Al(VI)-OH sites are also present on macroscopically defined (100) facets γ-alumina of different, more elongated/rod-like geometry. They anchor organometallic fragments and isolated metal atoms. The exact mechanism by which these sites lose -OH group upon thermal dehydroxylation is clarified, resulting in coordinatively unsaturated penta-coordinate Al+3O5 sites that can activate nitrogen. These coordinatively unsaturated Al penta-coordinate sites demonstrate new chemistry, producing the first well-defined thermally stable Al-carbonyl complexes. Our findings reunite and successfully clarify the contradictory findings in the literature over the last century, illuminating the true nature and the relationship between the catalytically active and coordinatively unsaturated Al sites on the surface of γ-alumina.

Funding

The research at PNNL was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences Catalysis Program (DE-AC05-RL01830, FW-47319). Experiments were conducted in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RL01830

History

Email Address of Submitting Author

Konstantin.Khivantsev@pnnl.gov

Institution

Pacific Northwest National Laboratory

Country

United States

ORCID For Submitting Author

0000-0002-4810-586X

Declaration of Conflict of Interest

No conflict of interest to declare

Version Notes

Version 1.0

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