Novel hydrogen chemisorption properties of polymer-derived alkali meal cation-modified amorphous Si-B-N-based ceramic material
TADA S. 1,2, TERASHIMA M. 2, SHIMIZU D. 2, NORIFUMI A. 2, BERNARD S. 3, IWAMOTO Y. 2
1 Ceramic Technologies for Futuristic Mobility (CTFM) Center, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, India; 2 Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan; 3 University of Limoges, CNRS, IRCER, UMR 7315, Limoges, France
We report on unique hydrogen (H2) chemisorption properties of polymer-derived alkali metal cation (Amc)-modified amorphous silicon boron nitride (SiBN) ceramic materials. Since 2010, the concept of “main-group elements as transition metals” has been raised to receive attractive attention to develop novel catalysts composed only of main-group elements.1 The frustrated Lewis pair (FLP) system—a simple combination of Lewis acidic and basic components but with steric hindrance to prevent the formation of classical Lewis acid-base adducts—can be recognized as one of the findings based on the novel concept and has drawn increasing interest because of its exceptional functionality to activate small molecules in both homogeneous and heterogeneous catalysis.2,3 Recently, we have explored novel functions based on the novel transition metal-free catalysis, and a polymer-derived silicon-aluminum-nitrogen (SiAlN)-based system has been found to show novel hydrogen chemisorption properties.4 In this study, we have successfully synthesized micro-/mesoporous amorphous Amc-modified SiBN ceramic via the polymer-derived ceramics (PDCs) route: sodium (Na) and boron (B)-modified polysilazane (PSZ), as a single-source precursor, was pyrolyzed at 1000 °C under flowing ammonia (NH3) to afford X-ray amorphous ceramic. Elemental analyses for the pyrolyzed samples confirmed the successful incorporation of Na and B components into the X-ray amorphous silicon nitride matrix at the controlled atomic ratios, Na/Si and B/Si, respectively. The textural properties of the as-pyrolyzed samples were evaluated by N2 adsorption-desorption isotherm measurements at –196 °C. The Na-SiBN ceramic exhibited micro-/mesoporosity formed in situ during the pyrolysis under flowing NH3, while the Amc-free X-ray amorphous SiBN ceramic indicated non-porous nature. The structure-property relationships were deeply investigated by the local structural analysis performed by the solid-state MAS-NMR measurements and H2 adsorption-desorption measurements (H2-TPD measurement): (i) reversible H2 adsorption and desorption property (H2 adsorption temperature, TH2 = 150 °C) was successfully demonstrated on the Na-SiBN sample; (ii) the calculated activation energy for H2 desorption (ΔEde,H2) based on the Redhead analysis was 117 kJ mol–1, which suggesting the detection of chemisorbed H2, (iii) H2 storage property was affected by the specific surface area, i.e., the number of active sites within the Na-SiBN surface network, and (iv) local structural analysis by the 11B-MAS NMR spectroscopy measurement suggested that the B modified amorphous silicon nitride network via B–N bond formation, and local structural changes of the Na-SiBN sample before and after H2 treatment and/or X-ray irradiation—three-fold coordinated boron species (sp2 moiety) converted into four-fold coordinated boron species (sp3 moiety) under the specific conditions. At the Ecers 2023 symposium, the relationship between the local structure having both Na and B, and the unique H2 chemisorption property will be further discussed based on the novel concept of transition and noble metal-free catalysis.
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