Hydrogen adsorption/desorption properties of polymer-derived Ni nanocrystallites embedded within amorphous silicon nitride
ASAKUMA N. 1, TADA S. 1,2, HONDA S. 1, ASAKA T. 1, TAMURA T. 3, BERNARD S. 4, IWAMOTO Y. 1
1 Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan; 2 Ceramic Technologies for Futuristic Mobility (CTFM) Center, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, India; 3 Department of Applied Physics, Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan; 4 CNRS, IRCER, UMR 7315, University of Limoges, Limoges, France
Electronic metal-support interaction (EMSI) has a significant influence on the chemisorption ability and subsequent catalytic performance of transition metals (TMs) since the interaction gives rise to the interface with an electronic state different from the balk regime . Silicon nitride (Si3N4) is an attractive material as catalytic support because of its surface amino groups expected to electrically interact with the loaded TMs . However, the Si3N4 easily reacts with the water, resulting in the formation of the oxidized surface layer on Si3N4 . Therefore, a catalyst with Si3N4 support should be favored to be prepared under anhydrous conditions.
In our previous study, the successful formation was achieved under anhydrous conditions for the Ni nanocrystallites embedded within the amorphous Si3N4 (Ni/a-Si3N4) through the polymer-derived ceramics (PDCs) route . Moreover, the embedment structure of TMs within catalytic support generally facilitates the EMSI because the structure has a larger contact area between TMs and support matrices.
Herein, to investigate the effect of amorphous Si3N4 matrix on hydrogen adsorption/desorption properties of Ni, we successfully prepared two different composites systems: Ni nanocrystallites embedded within amorphous Si3N4 (Ni/a-Si3N4) and those within amorphous silicon dioxide (Ni/a-SiO2) having the Ni average crystallite sizes of 7.6 and 7.0 nm, respectively. For the fabrication of two different composites under anhydrous conditions, firstly, a system of Ni embedded within the amorphous silicon carbonitride (a-SiCN) matrix was synthesized through the PDCs route using the poly(vinylmethyl-co-methyl)silazane (HTT1800) and nickel hexaammine chloride (Ni(NH3)6Cl2 with the atomic ratio of Ni/Si to 0.05. Then, the polymer-derived Ni/a-SiCN was successfully converted into Ni/a-Si3N4 and Ni/a-SiO2 samples by the individual anneal treatments.
XRD analysis for the heat-treated samples revealed that the matrix in all the samples was X-ray amorphous due to the absence of the periodic structure. TEM observation supported both Ni/a-Si3N4 and Ni/a-SiO2 samples preserved the embedment structure of Ni crystallites within the matrix.
Elemental analysis revealed that the anneal conditions performed in this study were found to be effective for the nitridation and oxidation of Ni/a-SiCN to afford Ni/a-Si3N4 and Ni/a-SiO2, respectively. Moreover, the residual chlorine could be efficiently reduced to below 0.2 wt% in both Ni/a-Si3N4 and Ni/a-SiO2 samples.
Regardless of the ramping rate, the H2-TPD profiles for the Ni/a-Si3N4 and Ni/a-SiO2 samples showed the H2 desorption peak from the Ni surface at about 100 to 130 °C. Based on the Redhead analysis , it was revealed that the Ni/a-Si3N4 sample had lower activation energy for hydrogen desorption, compared with the Ni/a-SiO2 sample. XPS analysis detected a higher electron density of the Ni nanocrystallites in the as-synthesized Ni/a-Si3N4 sample. Therefore, it was strongly suggested that the embedment structure allowed the electronic interaction between the Ni nanocrystallites and a-Si3N4 matrix, which lead to facilitating H2-desorption.
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