Theoretical Prediction of Metastable Intermediates in the Decomposition of Mg(BH4)2

Abstract

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(B3H8)2 intermediate (Chong et al. Chem. Commun.2011, 47, 1330) is energetically highly unfavorable with respect to decomposition into MgB12H12 and hence is not a thermodynamic reaction product. We systematically search for low-energy structures of Mg-triboranes [Mg(B3H8)2, MgB3H7, and Mg3(B3H6)2], closo-borane MgBnHn (n = 6, 7, 8, 9, 10, 11), and Mg(B11H14)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(BH4)2 → 1/6MgB12H12 + 5/6MgH2 + 13/6H2 → MgB2 + 4H2. However, the reaction [Mg(BH4)2 → 1/3Mg3(B3H6)2 + 2H2] involving a Mg3(B3H6)2 product has an enthalpy close to that of the MgB12H12 pathway and falls within the desired enthalpy window for near-ambient reversibility [20–50 kJ/(mol H2)]. This indicates that (1) if MgB12H12 is kinetically hindered in the decomposition of Mg(BH4)2 such as in the aforementioned reference (Chong et al. Chem. Commun.2011, 47, 1330), Mg3(B3H6)2 might be yielded as a metastable intermediate, and (2) Mg3(B3H6)2 could possibly be rehydrided back to Mg(BH4)2 under modest H2(T,p) conditions. We suggest that the observed intermediate is not [B3H8] but could be another triborane such as [B3H6].