We have studied the decomposition pathway and products of Mg-borohydride using density functional theory (DFT) calculations of free energy (including vibrational contributions) in conjunction with a Monte Carlo-based crystal structure prediction method, the prototype electrostatic ground state (PEGS) search method. We find that a recently proposed Mg(B 3H 8) 2 intermediate (Chong et al. Chem. Commun.2011, 47, 1330) is energetically highly unfavorable with respect to decomposition into MgB 12H 12 and hence is not a thermodynamic reaction product. We systematically search for low-energy structures of Mg-triboranes [Mg(B 3H 8) 2, MgB 3H 7, and Mg 3(B 3H 6) 2], closo-borane MgB nH n (n = 6, 7, 8, 9, 10, 11), and Mg(B 11H 14) 2 compounds using PEGS simulations, refining the resulting structures with accurate DFT calculations. We find that none of these compounds break the previously determined thermodynamically stable decomposition path: Mg(BH 4) 2 - 1/ 6MgB 12H 12 + 5/ 6MgH 2 + 13/ 6H 2 - MgB 2 + 4H 2. However, the reaction [Mg(BH 4) 2 - 1/ 3Mg 3(B 3H 6) 2 + 2H 2] involving a Mg 3(B 3H 6) 2 product has an enthalpy close to that of the MgB 12H 12 pathway and falls within the desired enthalpy window for near-ambient reversibility [20-50 kJ/(mol H 2)]. This indicates that (1) if MgB 12H 12 is kinetically hindered in the decomposition of Mg(BH 4) 2 such as in the aforementioned reference (Chong et al. Chem. Commun.2011, 47, 1330), Mg 3(B 3H 6) 2 might be yielded as a metastable intermediate, and (2) Mg 3(B 3H 6) 2 could possibly be rehydrided back to Mg(BH 4) 2 under modest H 2(T,p) conditions. We suggest that the observed intermediate is not [B 3H 8] but could be another triborane such as [B 3H 6].
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films