γ-Al2O3, an important catalyst and catalyst support, is widely used in various industrial applications. The five-coordinated aluminum (Al(V)) on the surface of γ-Al2O3, which is claimed as "super-five", can affect the catalytic performances of γ-Al2O3.
Due to the low crystallinity of γ-Al2O3 and the low content of Al(V) on the surface, which is only about 3% of the total aluminum content, it's difficult to experimentally characterize the structure of Al(V). All the reported structural models of Al(V) on the surface of γ-Al2O3 were based on theoretical calculations.
Recently, a research team led by Prof. HOU Guangjin from the State Key Laboratory of Catalysis(SKLC), in collaboration with Dr. GAN Zhehong from the National High Magnetic Field Laboratory, for the first time observed the structure of Al(V) on the surface of γ-Al2O3 using ultrahigh-field (1.5GHz) solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
This study was published in ACS Central Science and was selected as the Supplementary Cover Article on May 23.
Amorphous Al(V) domains on the surface of γ-Al2O3, liable to structural reconstruction and origins of distinct catalytic properties. (Image by WANG Rui and ZHAO Zhenchao)
The researchers comparatively investigated the structural properties of commercial γ-Al2O3 and amorphous alumina nanosheets (Al2O3-NS) rich in Al(V) by ultrahigh-field multinuclear and multi-dimensional Magic Angle Spinning (MAS) NMR.
They quantitatively analyzed the aluminum species in both aluminas and found the flexible structural features on the surface of Al2O3-NS. And they demonstrated the hydroxyl groups on the surface of γ-Al2O3 with close spatial proximity that were able to be removed under high-temperature dehydration, resulting in surface structure reconstruction.
Moreover, by using ultrahigh-field 27Al-27Al double-quantum NMR, the researchers for the first time revealed that most Al(V) species tended to aggregate into Al(V) domains on the surface of γ-Al2O3 like Al2O3-NS, rather than tetragonal pyramid coordination on (100) surface previously predicted from theoretical models.
"These new insights into surface Al(V) species would help us to better understand the structure and function relationship of γ-Al2O3 when used as catalysts and catalyst supports", said Prof. HOU.
The study was supported by the National Natural Science Foundation of China, the National Key R&D Program, the Liaoning Revitalization Talents Program, and the Dalian Youth Science and Technology Star. (Text by WANG Rui and ZHAO Zhenchao)
