Ammonia borane decomposition in the presence of cobalt halides

In the present work we studied the effect of cobalt halides (CoF2, CoCl2, CoBr2 and CoI2; also denoted CoX2) on the thermal decomposition of ammonia borane NH3BH3 (AB) over the range 25–220 °C. The reaction was followed by thermogravimetric analysis and differential scanning calorimetry. The gaseous products analyzed by gas chromatography and mass spectrometry, and the solid by-products identified by elemental analysis: powder X-ray diffraction, infrared and X-ray photoelectron spectroscopies. Compared to pristine AB, the presence of CoCl2 and CoBr2 reduces both the induction period of the AB decomposition and the content of unwanted borazine in the H2 stream, whereas the presence of CoI2 or CoF2 has little or no effect, respectively. We propose that the positive effect of CoX2 comes from their electronic and steric effects, and that CoCl2 is the compound which shows the best properties relative to these effects. Herein, the roles of Co and X are discussed and a revised mechanism of the AB dehydrocoupling initiation is proposed.

► The effect of cobalt halides on thermal decomposition of ammonia borane has been studied. ► The presence of CoCl2 and CoBr2 reduces the induction period of the AB decomposition. ► The presence of CoCl2 and CoBr2 reduces the content of unwanted borazine in the H2 stream. ► The positive effect of CoX2 is proposed to be driven by electronic and steric effects. ► CoCl2 is the compound showing the best properties in terms of electronic and steric effects.