ARTICLE pubs.acs.org/JPCC
Synthesis, Crystal Structure, and Thermal Properties of the First Mixed-Metal and Anion-Substituted Rare Earth Borohydride LiCe(BH4)3Cl ‡ C. Frommen,*,† M.H. Sørby,† P. Ravindran,‡ P. Vajeeston,‡ H. Fjellvag, and B.C. Hauback† † ‡
Institute for Energy Technology, Physics Department, P.O. Box 40, NO-2027 Kjeller, Norway Center for Materials Science and Nano Technology, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
bS Supporting Information ABSTRACT: The mechanochemical reaction between LiBH4 and CeCl3 in a molar ratio of 3:1 has led to the formation of LiCe(BH4)3Cl. This compound is the first example of a mixedmetal and anion-substituted rare earth borohydride. The optimized structural parameters from total energy calculations based on DFT are in excellent agreement with the experimentally obtained data. The bonding interactions between constituents have been analyzed using charge density and partial density of states. The calculations suggest that LiCe(BH4)3Cl is an insulator. LiCe(BH4)3Cl crystallizes in the cubic space group I43m (Z = 8) with lattice constant a = 11.5916(1) Å. It starts to decompose at ∼220 °C and forms CeH2 and amorphous boron containing species during dehydrogenation. The activation energy for the thermal decomposition was determined as 106(6) kJ/mol, and the material shows partial reversibility. Under 80 bar of hydrogen, 13% of the initial hydrogen content could be rehydrogenated at 340 °C.
’ INTRODUCTION Borohydrides, also called tetrahydroborates, are a prospective class of materials for solid state hydrogen storage because of their high gravimetric hydrogen density of up to 20.8 wt % H for Be(BH4)2 and 18.5 wt % for LiBH4, respectively.13 They can be roughly divided into two main groups: (a) Materials that contain a metal ion with low electronegativity, like in the alkali metal borohydrides. This leads to an almost complete charge transfer from the metal to the BH4 group, resulting in a predominant ionic character in the metal-BH4 coordination. (b) Alkaline earth and transition-metal borohydrides that exhibit directionality, a higher electronegativity of the metal ion, and hence only partial charge transfer from the metal to the BH4 group. The latter leads to a destabilization of the BH4 unit by weakening of BH bonds and results in lower decomposition temperatures of transitionmetal borohydrides compared with alkali metal borohydrides. Since the discovery of LiK(BH4)24 and LiSc(BH4)4,5,6 the first crystalline mixed-metal (double cation) borohydrides, the combination of two metals with different electronegativity has been successfully used to tune the thermal stability of borohydrides. This in turn has made them more attractive as solid-state storage materials. The mixed-metal borohydrides often exhibit structures and components that have no common analogue among other known inorganic compounds. The discrete complex [Sc(BH4)4] anion has been observed for the first time in the series of MSc(BH4)4 (M = Li, Na, K) compounds.59 Similar complex anions like [Zn(BH4)3] r 2011 American Chemical Society
and [Zn2(BH4)5] were later also observed in the Zn-containing compounds MZn(BH4)3 and M2Zn(BH4)5 (M = Li, Na).9 The structures of the Zn-series feature two doubly interpenetrated 3D frameworks similar to those found in metal organic frameworks (MOFs), and observed for the first time in a metal hydride. Mechanochemistry, for example, ball-milling, has been successfully used to synthesize a variety of borohydrides, starting from a simple alkali borohydride and the corresponding metal chloride of the target material according to the metathesis reaction eq 1 nMBH4 þ M0 Cln f nMCl þ M0 ðBH4 Þn
ð1Þ
Using the above approach, compounds like Mn(BH4)2,10 the first crystalline 3D-metal borohydride, and several rare earth borohydrides RE(BH4)3 (RE = Y, Dy, Gd)1117 have been obtained recently. In addition, several bimetallic borohydrides were obtained via a more complex chemical reaction eq 2 mMBH4 þ M0 Cln f Mmn M0 ðBH4 Þm þ nMCl
ð2Þ
Among those are the series of Sc-containing MSc(BH4)4 (M = Li, Na, K) and Zn-containing compounds MZn2(BH4)5 and MZn(BH4)3 (M = Li, Na). Received: May 31, 2011 Revised: October 18, 2011 Published: October 18, 2011 23591
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The Journal of Physical Chemistry C
ARTICLE
In reality, the situation can be much more complex than that depicted by eqs 1 and 2 because the formation of ternary chlorides as well as that of mixed-cation and mixed-anion compounds are possible via addition reactions similar to eqs 3 and 4 2ScCl3 þ 4NaBH4 f NaScðBH4 Þ4 þ Na3 ScCl6
ð3Þ
KBH4 þ ZnCl2 f KZnðBH4 ÞCl2
ð4Þ
KZn(BH4)Cl218 and likewise NaY(BH4)2Cl2 are two prominent examples of a new class of mixed-cation and mixed-anion compounds. The combination of different ligands in metal borohydrides seems to be a feasible way to adjust their stability with respect to thermal decomposition, for example, hydrogen release temperature. Recently, two independent studies reported that ball-milling of LiBH4CeCl3 in a 3:1 molar ratio leads to the formation of cerium borohydride, Ce(BH4)3,19,20 with a cubic structure and lattice constant a = 11.64 Å. Because no further structural information was presented, however, we decided to repeat the synthesis procedure and to study the crystal structure of the product phase(s) by means of a combination of powder X-ray and neutron diffraction. To gain more insight into the structural stability and chemical bonding, additional ab initio calculations based on density functional theory (DFT) have also been performed. In this Article, we present our findings and show that the product of this synthesis procedure is in fact LiCe(BH4)3Cl, the first mixed-cation mixed-anion rare earth borohydride.
’ EXPERIMENTAL SECTION Sample Preparation. A mixture of Li11BD4 (>98%, Kat-
Chem) or LiBH4 (95% Sigma-Aldrich) and CeCl3 (99.99% Sigma-Aldrich) in a molar ratio of 3:1 was milled in a Fritsch Pulverisette 6 Monomill at 500 rpm for 6 h (ball-to-powder ratio 60:1). In a typical experiment, 6.162 mmol CeCl3 (1.519 g) and 18.486 mmol Li11BD4 (0.481 g) were used, resulting in a total powder mass of 2 g. Because of the strong neutron absorption of natural boron, the 11B isotope was used in the synthesis. Furthermore, deuterium was selected because of lower incoherent scattering than the H-isotope. All sample handling and preparations were performed under an inert Ar atmosphere in an MBraun Unilab glovebox fitted with a recirculation system and gas/humidity sensors. Oxygen and water levels were kept below 1 ppm during all operations. Initial Sample Characterization. All samples were initially investigated by powder X-ray diffraction (PXD) using a Bruker D8 Advance diffractometer (CuKα1,α2 radiation) equipped with a 1D LynxEye detector. The samples were mounted in 0.5 mm diameter glass capillaries sealed with glue. Powder Neutron Diffraction. Data at room temperature (RT) were collected with the PUS instrument21 at the JEEP II reactor at Kjeller, Norway. Neutrons with the wavelength λ = 1.5557 Å were obtained from a Ge (511) focusing monochromator. Data were collected from 10 to 130° in 2θ. The sample was contained in a rotating cylindrical vanadium sample holder with 6 mm diameter. The vanadium sample holder was closed with an indium wire O-ring in the glovebox before the data collection to avoid oxidation. Structure solution was performed by ab initio methods with the FOX program.22,23 Rietveld refinement was carried out using
the FULLPROF program.24 The background for both data sets was modeled by linear extrapolation between 15 manually chosen points. The ThompsonCoxHastings pseudo-Voigt profile function was selected for the global refinement of the PXD and powder neutron diffraction (PND) data sets. Unit cell parameters, zero-point, overall scale factors, peak shape mixing parameters, three Gaussian profile parameters (UVW), and two Lorentzian profile parameters were refined for each data set. Differential Scanning Calorimetry (DSC). Measurements were performed with a SensysDSC from SETARAM. Samples were placed either inside aluminum crucibles and sealed with pierced lids made of the same material or in airtight, highpressure capable containers made of stainless steel (pmax = 500 bar; Tmax = 600 °C). The data were collected under flowing argon (15 mL/min) using a constant heating rate of 2 °C/min. Simultaneous Thermogravimetric Analysis (TGA) with DSC. Experiments were performed with a Netzsch STA 449 F3 Jupiter instrument. Samples were measured in Al2O3 sample pans equipped with pierced lids made of the same material. The material was heated between 30 and 600 °C with a constant rate of 2 °C/min under argon gas (50 mL/min). The measurements were baseline corrected by means of the Proteus software package. Synchrotron Radiation Powder X-ray Diffraction (SRPXD). Time-resolved data were collected at the Swiss-Norwegian beamline (SNBL), station BM01A, at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The samples were mounted in 0.5 mm boronsilicaglass capillaries, kept in place by a glass rod and mounted in a Swagelok fitting. The capillary was then evacuated with a rotary pump and kept under dynamic vacuum. Two-dimensional powder data were collected using an imaging plate system (MAR345) with an exposure time of 30 s. The capillaries were rotated 30° during the exposure to reduce texture effects. For data readout and erasing, 90 s was needed, and thus complete data sets were collected every second minute. The wavelength was 0.70947 Å. The 2D data were integrated to yield 1D powder diffraction patterns with the program FIT2D25 and binned with a step size of 0.016°. High-resolution (HR) SR-PXD measurements were performed at SNBL, station BM01B, with a wavelength of 0.50123 Å between 2θ = 2 and 25° with a step-size of 0.003° (2θ) and binned at 0.006°. Temperature-Programmed Desorption (TPD) and Residual Gas Analysis (RGA). TPD and RGA were measured in an inhouse-developed setup under vacuum (105 mbar) between RT and 600 °C with a constant heating rate of 2 °C/min. RGA was measured with a MULTIVISON IP detector system coupled to a PROCESS Eye analysis package from MKS Instruments. Infrared Spectroscopy. Infrared spectroscopy was carried out on a BRUKER IFS 66v spectrometer equipped with a DTGS detector. The powders were embedded in KBr pellets (7 mm) with a 0.5% mass concentration. The transmission spectra of the KBr pellets were recorded in the 4008000 cm1 region with a resolution of 4 cm1. Scattering contribution to the spectra due to the presence of KBr is considered to be negligible in this region. Computational Methodology. First principles DFT calculations were performed using the Vienna ab initio simulation package (VASP)26 within the projector-augmented wave (PAW) method27 as implemented by Kresse and Joubert.28 The Kohn Sham equations29 were solved self-consistently using an iterative matrix diagonalization method. This is based on a band-by-band preconditioned conjugate gradient30 method with an improved 23592
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Figure 1. PXD pattern for a mixture of LiBD4-CeCl3 (3:1 ratio) after ball-milling and after postannealing at 165 °C and 60 bar of deuterium.
Pulay mixing31 to obtain efficiently the ground-state electronic structure. The forces on the atoms were calculated using the HellmannFeynman theorem, and they are used to perform a conjugate gradient relaxation. Structural optimizations were continued until the forces on the atoms had converged to
