Theoretical prediction of metastable intermediates in the decomposition of Mg(BH 4) 2

Yongsheng Zhang; Eric Majzoub; Vidvuds Ozolinš; C. Wolverton

doi:10.1021/jp302303z

Theoretical prediction of metastable intermediates in the decomposition of Mg(BH ₄) ₂

Yongsheng Zhang^*, Eric Majzoub, Vidvuds Ozolinš, C. Wolverton

^*Corresponding author for this work

Materials Science and Engineering

Research output: Contribution to journal › Article › peer-review

55 Scopus citations

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(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].

Original language	English (US)
Pages (from-to)	10522-10528
Number of pages	7
Journal	Journal of Physical Chemistry C
Volume	116
Issue number	19
DOIs	https://doi.org/10.1021/jp302303z
State	Published - May 17 2012

ASJC Scopus subject areas

Electronic, Optical and Magnetic Materials
General Energy
Physical and Theoretical Chemistry
Surfaces, Coatings and Films

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@article{b73dfcdac38041ae82c45695e36cb54f,

title = "Theoretical prediction of metastable intermediates in the decomposition of Mg(BH 4) 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(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].",

author = "Yongsheng Zhang and Eric Majzoub and Vidvuds Ozolin{\v s} and C. Wolverton",

year = "2012",

month = may,

day = "17",

doi = "10.1021/jp302303z",

language = "English (US)",

volume = "116",

pages = "10522--10528",

journal = "Journal of Physical Chemistry C",

issn = "1932-7447",

publisher = "American Chemical Society",

number = "19",

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TY - JOUR

T1 - Theoretical prediction of metastable intermediates in the decomposition of Mg(BH 4) 2

AU - Zhang, Yongsheng

AU - Majzoub, Eric

AU - Ozolinš, Vidvuds

AU - Wolverton, C.

PY - 2012/5/17

Y1 - 2012/5/17

N2 - 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].

AB - 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].

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JO - Journal of Physical Chemistry C

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ER -

Theoretical prediction of metastable intermediates in the decomposition of Mg(BH ₄) ₂

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