Experiments on the thermal activation of hydrogen release of NaBH₄-sodalites characterized by IR- and MAS-NMR spectroscopy

authored by

Claus Henning Rüscher, Lars Schomborg, Thomas Bredow

Abstract

New results on the reactions of the (BH₄)-anion enclathrated in the cages of sodalites are reported. Hydrothermally synthesized NaBH₄-sodalites (ǀNa₈(BH₄)₂ǀ[SiAlO₄]₆) always contain hydro-sodalite type cages (ǀNa₃(H₂O)₄ǀ[SiAlO₄]₃). With increasing temperature dehydration occurs. Above 250 °C a limited reaction of the residual water is going on with (BH₄)-cage fillings releasing hydrogen and the appearance of certain borate specifications enclosed in the sodalite cages. The effect of a reaction of oxygen with the (BH₄)-anions in the sodalite-cages at temperatures above 400 °C is also shown. The degree of (BH₄)-conversion using wet and dry N₂ stream is further followed by IR and MAS NMR investigations. External supply of water largely enhances the degree of reaction, e.g. at 400 °C from 16% to 44% loss of (BH₄)-absorption intensity. However, ¹¹B MAS NMR shows 8% and 22% of a conversion of (BH₄)-cage fillings into new borate species in dry and wet N₂ stream, respectively. These lower values are explained by a loss of B-ions via formation and evaporation of BH₃ from the sodalite. Further evaluation of the ¹¹B MAS NMR spectra could resolve the formation of (B(OH)₃)^-, (BO(OH)₂)^-, (B(OH)₄)^- along with unreacted (BH₄)-species in the cages. ¹H-MAS NMR shows a −3.8 ppm signal related to cage isolated (OH)^-, which suggests an initial reaction step via (H⁺ + BH₄⁻) to (BH₃ + H₂). The formation of (B(OH)₃), however, also indicated sufficient water for a reaction of BH₃ releasing further hydrogen. The formation of (BH₃) could be observed in temperature dependent IR investigations at temperatures above 400 °C.

Organisation(s)

Institute of Mineralogy

External Organisation(s)

University of Bonn

Type

Article

Journal

International Journal of Hydrogen Energy

Volume

47

Pages

36175-36189

No. of pages

15

ISSN

0360-3199

Publication date

15.10.2022

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Renewable Energy, Sustainability and the Environment, Fuel Technology, Condensed Matter Physics, Energy Engineering and Power Technology

Sustainable Development Goals

SDG 7 - Affordable and Clean Energy

Electronic version(s)

https://doi.org/10.1016/j.ijhydene.2022.05.302 (Access: Closed)