Tada, Shotaro ; Asakuma, Norifumi ; Ando, Shiori ; Asaka, Toru ; Daiko, Yusuke ; Honda, Sawao ; Haneda, Masaaki ; Bernard, Samuel ; Riedel, Ralf ; Iwamoto, Yuji (2021)
Novel hydrogen chemisorption properties of amorphous ceramic compounds consisting of p-block elements: exploring Lewis acid–base Al–N pair sites formed in situ within polymer-derived silicon–aluminum–nitrogen-based systems.
In: Journal of Materials Chemistry A, 9 (5)
doi: 10.1039/d0ta10271g
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

This paper reports the relationship between the H2 chemisorption properties and reversible structural reorientation of the possible active sites around Al formed in situ within polymer-derived ceramics (PDCs) based on an amorphous silicon–aluminum–nitrogen (Si–Al–N) system. Al-modified polysilazane, as a ceramic precursor, was first pyrolyzed at 1000 °C under flowing ammonia to generate a Si–Al–N-based ceramic. XRD and HRTEM analyses confirmed the amorphous state of the titled ceramics. N2 adsorption–desorption isotherm measurements and HAADF-STEM observation of amorphous SiAlN indicated that Al-incorporation in the early step of the process led to the generation of micro/mesoporosity in the amorphous ceramic with nanopores of 1 to 4 nm in size. XPS and pyridine sorption infra-red spectroscopy analyses revealed the in situ formation of Lewis acidic Al sites within the amorphous Si–Al–N surface network. As a result, the Si–Al–N compound was highly moisture sensitive. Then, to investigate the intrinsic properties of the highly reactive Al sites, the Si–Al–N compound was pretreated at 400–800 °C under an inert atmosphere. Temperature-programmed-desorption (TPD)-mass spectroscopy analysis of the pre-treated sample after H2 treatment above 100 °C resulted in the detection of a broad H2 desorption peak at around 100 to 350 °C. The H2 desorption peak intensity apparently increased when H2 treatment was performed at 150 °C, and the activation energy for H2 desorption was determined to be 44 kJ mol−1. 27Al MAS NMR spectroscopic analysis for the pre-treated sample showed reversible local structure reorientation around reactive Al nuclei, and formation and deformation of 5-fold coordinated Al by H2 chemisorption and desorption, respectively. In addition, the CO2 hydrogenation reaction on the pre-treated sample was successfully demonstrated by TPD measurements after exposure to a mixed gas of H2 and CO2 with a 4 : 1 ratio at 400 °C. These results suggest that highly distorted 4-fold coordinated Al serves as a Lewis acid–base Al–N pair site to promote H2 chemisorption at T > 100 °C followed by formation of a hydrogenated 5-coordinated Al unit where CO2 hydrogenation proceeds at T = 400 °C.

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