physical chemistry chemical physics · 17078 phys. chem. chem. phys.,2011,13,1707717083 his journal...

8
ISSN 1463-9076 Physical Chemistry Chemical Physics www.rsc.org/pccp Volume 13 | Number 38 | 14 October 2011 | Pages 16869–17416 1463-9076(2011)13:38;1-P COVER ARTICLE Saykally et al. Electronic structure of aqueous borohydride: a potential hydrogen storage medium

Upload: others

Post on 11-Aug-2020

2 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

ISSN 1463-9076

Physical Chemistry Chemical Physics

www.rsc.org/pccp Volume 13 | Number 38 | 14 October 2011 | Pages 16869–17416

1463-9076(2011)13:38;1-P

COVER ARTICLESaykally et al. Electronic structure of aqueous borohydride: a potential hydrogen storage medium

Page 2: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 17077

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 17077–17083

Electronic structure of aqueous borohydride: a potential hydrogen

storage medium

Andrew M. Duffin,ab

Alice H. England,ab

Craig P. Schwartz,ab

Janel S. Uejio,ab

Gregory C. Dallinger,aOrion Shih,

aDavid Prendergast

cand

Richard J. Saykally*ab

Received 1st June 2011, Accepted 18th July 2011

DOI: 10.1039/c1cp21788g

Borohydride salts have been considered as good prospects for transportable hydrogen storage

materials, with molecular hydrogen released via hydrolysis. We examine details of the hydration

of sodium borohydride by the combination of X-ray absorption spectroscopy and first

principles’ theory. Compared to solid sodium borohydride, the aqueous sample exhibits an

uncharacteristically narrow absorption feature that is shifted to lower energy, and ascribed to the

formation of dihydrogen bonds between borohydride and water that weaken the boron–hydrogen

covalent bonds. Water also acts to localize the highly excited molecular orbitals of borohydride,

causing transitions to excited states with p character to become more intense and a sharp feature,

uncharacteristic of tetrahedral molecules, to emerge. The simulations indicate that water

preferentially associates with borohydride on the tetrahedral corners and edges.

Introduction

With large weight percent hydrogen capacity, stable reactants,

and benign reaction products, borohydride salts are exciting

candidates for transportable hydrogen storage materials.1–5

Sodium borohydride[NaBH4], in particular, has been closely

examined, but despite considerable promise, has only yielded

limited success.5–8 One problem is the relatively low solubility

of the product oxides.9 While solvation of these oxides is

important for applications, and has been addressed in a

companion study,9 it is the solvation and subsequent

hydrolysis of the borohydride ion itself that leads directly to

hydrogen production. Hence, details of hydration of sodium

borohydride are vital for fully understanding and exploiting

the hydrolysis reaction. Towards this end, we have investi-

gated aqueous sodium borohydride with a combination of

molecular dynamics simulations, near edge X-ray absorption

fine structure spectroscopy (NEXAFS), and calculations of

NEXAFS spectra generated with the excited electron and Core

Hole method (XCH).10

In aqueous solution, borohydride reacts with water to yield

molecular hydrogen and borate [B(OH)4�]:

BH4� + 4H2O - B(OH)4

� + 4H2 (1)

This hydrolysis reaction is catalyzed with acid to give boric

acid [B(OH)3] rather than borate as the reaction product:

BH4� + H+ + 3H2O - B(OH)3 + 4H2 (2)

In both cases, based on kinetic and isotopic data, it is reasoned

that the hydrolysis reaction proceeds via a 5-coordinate BH5

intermediate.11,12 Previous experiments13,14 and calculations15–17

have demonstrated the feasibility of the BH5 intermediate,

with analogies drawn to the well-established CH5+ carbo-

cation. However, in condensed phases, the reaction inter-

mediate is difficult to observe. More detailed information

about borohydride hydration could yield useful insight into

the hydrolysis mechanism.

Relatively few studies have been performed on borohydride

hydration and these have produced conflicting conclusions.

Based on Raman spectra in various solvents, Shirk et al.18

reasoned that no strong hydrogen bonds were formed between

borohydride and water. Strauss et al.19 reached the opposite

conclusion and suggested that the ‘‘solvation was tetra-

hedral. . . with the water protons pointing towards the faces

of the BH4� tetrahedron’’. Fig. 1 presents representations of

various configurations of borohydride solvation. More

recently, borohydride hydration has been studied as part of

a large group of molecules that form dihydrogen bonds.20–25

These more recent studies that dealt specifically with boro-

hydride confirmed the existence of a bond between a proton

donor and a borohydride hydrogen, but they did not specifi-

cally examine borohydride in water.24,25 With regards to

solvation, these experiments were interpreted as yielding

‘‘indirect evidence for [a] linear structure’’,20 referring to

aDepartment of Chemistry, University of California, Berkeley,CA 94720, USA. E-mail: [email protected];Tel: (510) 642-8269

bChemical Sciences Division, Lawrence Berkeley NationalLaboratory, Berkeley, CA 94618, USA

cMolecular Foundry, Lawrence Berkeley National Laboratory,Berkeley, California 94720, USA

PCCP Dynamic Article Links

www.rsc.org/pccp PAPER

Page 3: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

17078 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 This journal is c the Owner Societies 2011

solvation along the B–H bond or at the tetrahedral corner, see

Fig. 1. Ab initio Calculations on borohydrides evidenced a

linear X–H� � �H bond, but a bent H� � �H–B bond.23,24 Crystal

structure data from NaBH4�2H2O also give evidence for water

bonding to the hydride hydrogen.26

XAS spectra of solid sodium borohydride have been studied

previously; However, the spectrum was presented without

assignment of the salient spectral features.27 Boron-containing

minerals and glasses have also been studied and the spectral

features assigned.28–32 Trigonally coordinated boron exhibits a

sharp pre-edge feature around 194 eV, which is attributed to

the transition of an electron from the B 1s core state to

the unoccupied B 2pz orbital (oriented perpendicular to the

trigonal plane). There is also a higher energy peak above the

ionization potential (IP) that is attributed to transitions from

B 1s to s* orbitals. Tetrahedrally coordinated boron has one

main feature above the IP which is assigned to the transition of

electrons from B 1s to s* states.30

Here we investigate borohydride in basic solutions using

near edge X-ray absorption fine structure (NEXAFS) spectro-

scopy of liquid microjets.33 The unoccupied molecular orbitals

probed in NEXAFS experiments extend beyond the boron-

containing molecule itself and, as a result, are highly sensitive

to changes in local structure and environment, viz. aqueous

solvation. We interpret the experimental boron spectra with

simulated spectra generated using the recently developed first-

principles excited electron and core hole (XCH) method.10

Results and discussion

Fig. 2 shows experimental NEXAFS spectra of hydrated

sodium borohydride. It is interesting to compare these

aqueous sodium borohydride spectra to the spectrum of solid

sodium borohydride presented by Hallmeier et al.27 The

hydrated borohydride sample in Fig. 2 exhibits a relatively

sharp peak at 191 eV and a broader second feature at 198 eV.

The first peak in the solid sample spectrum (see Hallmeier et al.27)

is relatively broad and appears at higher energy, approxi-

mately 192.5 eV. The second peak in the solid sample is also

higher in energy, centered at 200 eV. There is a clear shift to

lower energy and narrowing of the first borohydride peak at

191 eV resulting directly from hydration effects.

Fig. 2 also plots the theoretical borohydride spectra. The

calculated spectrum matches well with experiment at the 191

eV feature, but misses the higher energy peak. Although the

hydrolysis reaction should be negligibly slow in 1 M NaOH,13

borohydride reacts with water to form borate, so it is possible

that the peak around 198 eV is due to B(OH)4�. However,

experiments with higher concentrations of NaOH, which

would entirely inhibit its production, gave identical results,

indicating that hydrolysis product is not responsible for the

second peak. Furthermore, the borate spectrum does not

cleanly subtract from the borohydride spectrum, also indicating

that borate is not responsible for the second peak. In addition,

Hallmeier et al.27 show a high energy feature at 200 eV in the

solid sample of sodium borohydride. Unfortunately, the origin

of the second peak in the borohydride spectrum remains

uncertain. However, we can speculate that the origin of the

second feature lies in configurations not sampled by the

molecular dynamics such as Na(BH4)+ or a BH5 intermediate.

The sharp peak at 191 eV in the hydrated borohydride

spectrum is uncharacteristic of tetrahedral boron molecules.

For example, NaB(OH)4,9,29,30 KBF4, and NH4BH4

27,34 all

have more typical tetrahedral boron K-edge NEXAFS spectra,

exhibiting a sharp rise at the absorption onset and a more

gradual decline in the post-absorption edge. The peak at

191 eV in the borohydride spectrum resembles a much

narrower and more symmetric transition to a p-state, similar

to the boric acid peak at 194 eV. Clearly, interaction with

water drastically alters the electronic structure of borohydride.

To illustrate the spectral effects of hydration, Fig. 1 also

presents a calculated NEXAFS spectrum from bare boro-

hydride anions. The spectrum of bare BH4� ions was calcu-

lated from the same snapshots as the fully hydrated system,

but with all the water molecules (and sodium ions) removed.

In this way, comparing the bare borate spectrum with the fully

hydrated spectrum reveals the spectral effects of the surrounding

Fig. 1 Idealized representations of water–borohydride interactions

(a) along the tetrahedral corner, (b) tetrahedral edge, (c) and tetra-

hedral face.

Fig. 2 Aqueous NaBH4 NEXAFS spectrum (thick line) compared

with theoretical calculations (dashed line) of the same system. The

calculations are in excellent agreement with experiment at the low

energy peak (191 eV) but fail to reproduce the high energy feature

(198 eV). Calculated spectra for bare BH4� (thin line) are inconsistent

with experiment and the main feature is much broader than for the liquid.

Page 4: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 17079

water molecules. The shifting to lower energy and narrowing

of the peak at 191 eV are reproduced in the bare vs. hydrated

borohydride calculations, confirming that water is directly

responsible for these spectral changes.

To further investigate borohydride hydration, spectra were

calculated for small hydrated clusters. Fig. 3 shows spectra

from a minimized BH4��5H2O cluster evolving to a BH4

��H2O

cluster by successively removing waters. The same shift in peak

energy and width observed in the fully hydrated and bare

calculations are observed in a stepwise fashion in the clusters.

The five-water cluster has a peak that is red-shifted and much

narrower compared to the one-water cluster. The shifting of

the peak in Fig. 3 is explained by the creation of dihydrogen

bonds. The borohydride hydrogens carry a partial negative

charge and interact favorably with the partial positive charges

on the hydrogens of water. As electron density is transferred to

the new H–H bond, the B–H bond is weakened. Consequently,

the overlap between the borohydride hydrogen and boron is

lessened and the antibonding orbitals shift to lower energy.

With each additional solvating water, antibonding states are

created at lower energy and the absorption feature shifts to

lower energy.

The fact that the 191 eV borohydride peak narrows upon

hydration is not expected. Schwarz et al.34 point out that when

a tetrahedral system is distorted, some of the spherical states

can adopt p character. Transitions from the B 1s state to

water-dependent p states may account for the relatively sharp

transition near the absorption edge. The sharp feature could

also be due to confinement of the wavefunction near the boron

atom. States above the LUMO can have local p character, but

transitions to these states remain weak due to poor spatial

overlap. The lower portion of Fig. 4 shows the LUMO + 2

state from one molecular dynamics snapshot with the waters

removed. While there is local p character around the boron,

the electron density is highly diffuse. As a result, overlap

between the core level and this diffuse state is small and there

is no sharp feature associated with this transition. In contrast,

Fig. 3 Depiction of a BH4��5H2O cluster and corresponding calcu-

lated spectrum. The spectra moving down were calculated after

successive removal of water from the 5-water cluster. The main

absorption feature moves to higher energy and broadens upon

removal of water. Dihydrogen bonds between the water and boro-

hydride are responsible for the shift in energy.

Fig. 4 Spectra and states from a single molecular dynamics snapshot

of aqueous NaBH4 (dashed line and top) and bare BH4� (solid line

and bottom). In both cases, the states show p character around the

boron, but the diffuse nature of the bare ion orbital makes transitions

to this state relatively weak. The arrow indicates the energy of the state

in the calculated spectrum. With water present, these excited state

orbitals are confined near the boron atom and the spatial overlap with

the core state is increased. Consequently, the spectrum shows a

sharper peak in water.

Page 5: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

17080 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 This journal is c the Owner Societies 2011

the upper portion of Fig. 4 shows some of the states respon-

sible for the sharp spectral feature in the solvated borohydride

spectrum. Upon hydration, the local p character remains

intact, but the wavefunction is more localized around the

boron and a few of the surrounding waters. The arrows in

the figure indicate the energy where the visualized states

contribute to the spectrum. When surrounded by water,

overlap between the core state and the spatially confined

excited states is increased and, consequently, a relatively

intense and narrow peak appears.

To illustrate the structure of water solvating borohydride,

Fig. 5 shows randomly chosen snapshots from the MD

calculations, suggesting that borohydride is hydrated at the

tetrahedral corners and edges. In fact, analysis of our MD

trajectory indicates that, on average, borohydride is tightly

bound to 2.42 waters with a HBH4–Hwat bond of less than

2.1 A; 43% of the water protons within 2.1 A were also closely

associated (o2.4 A) with a second borohydride hydrogen, i.e.

edge hydration. Only 6% of the bonded protons were within

2.4 A of three borohydride hydrogens. That is, very few waters

are bound to borohydride at the tetrahedral faces. That

borohydride is predominantly hydrated at the tetrahedral

corners and edge is consistent with previous conclusions that

the O–H� � �H angle would be fairly linear, while the B–H� � �Hangle is bent.23,35 This leads to a configuration ‘‘convenient for

the interaction with stronger proton doors in which the

hydride molecule is susceptible to proton transfer and side-on

dihydrogen ligand formation.’’23 In other words, hydration

along the tetrahedral edges is consistent with the formation of

a 5-coordinate hydrolysis transition state.

Radial distribution functions (RDFs) calculated from the

molecular dynamics simulations of the sodium borohydride

system are shown in Fig. 6. Given that the average B–H bond

length in borohydride is just less than 1.2 A, the peaks in the

B–Hwat and B–Owat RDFs clearly indicate that water is tightly

bound to and structured around the borohydride. The mole-

cular dynamics simulation also reveals that sodium ions are in

contact with the borohydride. This observation is consistent

with previous hydration studies18,19 and calculations done on

borohydride in methanol.36 Regardless of close association

with borohydride ions, the presence of sodium is spectro-

scopically inconsequential. Molecular dynamics simulations

and X-ray absorption spectra from the resulting snapshots

were also calculated for solvated borohydride in the absence of

Na+. Fig. 7 shows the hydrated borohydride spectrum in the

presence and absence of Na+. Consistent with the analysis

above, there is a slight red shift in the no-sodium case due to

the fact that more water can surround the borohydride. In

spite of this shift, within the error of the calculations, the two

spectra are identical. Sodium’s effect seems confined to the

exclusion of water around the borohydride.

There is evidence from the simulations that borohydride is

solvated at the tetrahedral corners (although skewed from

complete linearity), tending toward the tetrahedral edges. In

spite of this information, the nature of the second peak

(198 eV) in the experimental spectrum remains unclear.

Fig. 8 shows three idealized clusters used to compare hydra-

tion around borohydride. Static DFT calculations on these

clusters give the energy of the face centered water–borohydride

Fig. 5 MD snapshots from the QMMM simulation of the sodium

borohydride system. The snapshots are dominated by hydration along

the borohydride tetrahedral corners and edges. Hydration along

tetrahedral faces is less common. Green: boron, red: oxygen, white:

hydrogen, and blue: sodium.

Fig. 6 Boron to water–oxygen and boron to water–hydrogen radial

distribution functions. The peaks in the distribution functions show

clear structuring of the first solvation shell around borohydride and

are evidence for strong BH4–water interactions.

Fig. 7 Calculated spectra from the full NaBH4�118H2O system

(dashed line) and a BH4��118H2O system (solid line). Without Na+,

the spectrum has a slight shift to lower energy, however the two are

equal with in the error of the calculations (shaded area). The Na+

forms a contact ion pair with the BH4� and excludes water from

around the borohydride. Consequently, the red shift is consistent with

the observation that water shifts the spectrum to lower energy.

Page 6: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 17081

cluster at 1.5 eV above the energy of both the corner and edge

hydration. This energy difference for these three clusters is

consistent with the reduced frequency of face coordinating

configurations in the molecular dynamics simulations. How-

ever, Fig. 8 also shows the spectra associated with each idealized

cluster. It is interesting that the tetrahedral corner cluster

(and to a lesser extent the tetrahedral face cluster) exhibit a

higher energy feature B7 eV above the main feature, as

observed in the experiments. It is possible that these configura-

tions are under-sampled in the QM/MM simulations.

Only one of the 100 full system snapshots exhibited an

intense high energy feature. This snaphost was coordinated by

two water molecules: one associated with a borohydride

tetrahedral face and the other closely associated with the

opposite corner. Moreover, in the simulations done in the

absence of Na+, there were a greater number of snapshots

exhibiting high energy peaks; although this feature still did

not show up in the average. Again, these snapshots exhibited

coordination by two water molecules: one associated

with a tetrahedral face and the other at the opposite corner.

Because sodium excludes water from around the borohydride,

combination face and opposite corner hydration is more likely

in the absence of sodium ions. It is also possible that this

geometry of hydration leads most directly to the hydrolysis

transition state. Interestingly, lithium borohydride is not

stabilized in basic solution,18 probably because lithium is more

strongly solvated by water. Not only would Li+ be less likely

to form a contact pair with borohydride, but also,

with a higher hydrolysis constant,37 the water around Li+ is

more easily hydrolyzed. Protons from this hydrolysis

would catalyze borohydride’s reaction with water. Clearly,

the associated cation influences the formation of the hydrolysis

transition state.

While the QM/MM calculations sampled thermally accessible

borohydride vibrations, there remained a boundary between

the borohydride, calculated quantum mechanically, and the

water, calculated classically. DFT based molecular dynamics

simulations with subsequent XCH calculations (not shown)

gave results nearly identical to the QM/MM calculations.

Unfortunately, these calculations were unable to provide the

necessary information to properly identify all the features in

aqueous sodium–borohydride NEXAFS spectrum. Future

calculations with multiple solvated borohydride ions or calcu-

lations on NaBH4 solid may help in identifying the nature of

the high energy borohydride NEXAFS feature.

Conclusions

The boron K-edge NEXAFS spectra of aqueous sodium–

borohydride was measured and found to have a sharp peak

near the absorption edge at 191 eV. This relatively sharp peak

is a direct result of hydration, and this peak is red-shifted

compared to that of solid sodium–borohydride. This red-shift

is direct evidence for strong dihydrogen bonding between a

proton from water and a borohydride hydrogen. As a result of

this dihydrogen bond formation, the B–H bond in boro-

hydride is weakened and the antibonding states move to lower

energy. Concomitantly, the absorption feature moves to lower

energy. The peak at 191 eV is also uncharacteristically narrow

for a tetrahedral molecule. The calculations indicate that in the

absence of water transitions to states above the LUMO with p

character are relatively weak due to poor spatial overlap.

Water surrounding borohydride tends to confine these excited

state orbitals such that the overlap between the B 1s and the

excited state with p character is increased. In other words,

interaction with water increases the local p character of the

excited states around the boron atom. Consequently, the

solvated borohydride NEXAFS spectra exhibits an absorption

onset peak that is uncharacteristically sharp for a tetrahedral

molecule. QM/MM calculations favor borohydride hydration

at the tetrahedral corners and edges, but there is a high energy

peak in the spectrum (198 eV) that remains unaccounted for in

the calculations. The assignment of this high energy feature

may be key to gaining further insight into the hydrolysis

reaction mechanism. Calculations on borohydride solid

may give insight into the nature of the high energy NEXAFS

feature.

Methods

A. Samples

NaBH4 from Aldrich (99% pure) was added to a 1 M NaOH

solution to slow the hydrolysis reaction and evolution of

hydrogen gas. The water was Millipore filtered and had a

resistivity of 18 MO/cm.

B. NEXAFS spectroscopy

The boron K-edge (180–220 eV) is in the soft X-ray region,

accessible only in high vacuum environments. Volatile liquids,

such as an aqueous borohydride solution, are typically

incompatible with high vacuum; however, using liquid microjets,33

Fig. 8 Idealized one water–borohydride clusters and associated

NEXAFS spectra with the water in contact with a borohydride

tetrahedral corner (dotted line), edge (thin line), and face (bold line).

The tetrahedral corner and face hydration produce a peak at higher

energy that is observed in experiment, but not reproduced by

calculations.

Page 7: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

17082 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 This journal is c the Owner Societies 2011

we are able to inject a thin liquid filament into vacuum. With

this approach, and a few differential pumping sections, we are

able to windowlessly couple the liquid jet chamber to Beamline

8.0.1 at the Advanced Light Source at Lawrence Berkeley

National Laboratory. The soft X-rays from the undulator

beamline are focused to a spot size of B50 mm and a liquid

jet nominally 30 mm in diameter is positioned to intersect the

X-ray beam. The cascade of electrons from the X-ray absorption

process is collected on a biased copper electrode, placed perpendi-

cular to both the jet and the X-ray beam. Collected in this manner,

the total electron yield is a bulk-sensitive measurement.38,39

Because the sample volume is continually renewing,

contamination of the sample due to X-ray damage is mini-

mized. In the present experiment, the liquid jet is created by

using a Teledyne-ISCO syringe pump to pressurize solutions

behind a nozzle made from a small section of fused silica

capillary. Immediately after the X-rays interact with the liquid

in vacuum, the liquid passes through a skimmer and is

cryotrapped onto a liquid nitrogen cooled surface. The

cryotrapping, in conjunction with the use of turbomolecular

pumps, keeps the pressure in the interaction chamber in the

low 10�4 torr range, and allows for the windowless coupling to

the beamline.

X-Ray absorption spectra were collected by scanning the

X-ray energy from the beamline in 0.2 eV steps. The current

collected at the biased electrode was first amplified and

converted to a voltage with a picoammeter and then converted

into a frequency before being read into a beamline computer.

The spectra collected in this manner were normalized to the

current from a gold mesh located B3 meters upbeam. The

baseline was further corrected by subtracting a vapor

spectrum taken off the liquid jet. Multiple spectra were

collected, averaged, and area normalized for comparison.

The calibration of the beamline monochromator was checked

against a spectrum of solid boric acid. A more detailed

description of the experimental endstation and data collection

has been published elsewhere.33

C. Calculations

Because core level excitations are very fast compared to

nuclear motion (Frank-Condon principle) and due to the finite

dimensions of the sample volume exposed to X-rays, the

experimental spectra provide an instantaneous average of all

thermally populated vibrational degrees of freedom and

solvent structure. Consequently, to accurately simulate the

experimental spectra, it is necessary to account for both solute

configuration and structure of the solvent.40,41 To this end,

sodium borohydride was solvated in a periodic box with

TIP3P water, heated to 300 K (Langevin thermostat) and then

equilibrated to a final density under constant pressure

dynamics. Afterwards, a 10 ns constant volume simulation

was run and molecular configurations were collected from

100 snapshots sampled every 100 ps. The borohydride was

treated quantum mechanically with the empirical MNDO

Hamiltonian, and the surrounding water was treated classically

in a mixed quantum mechanics/molecular mechanics (QM/MM)

simulation. The nonbonding (Lennard-Jones) parameters for

boron were taken from Otkidach and Pletnev.42

Using the atomic positions from each snapshot, we calcu-

lated an X-ray absorption spectrum. The spectra from all 100

snapshots were averaged to produce a configurationally

averaged theoretical X-ray absorption spectrum. Details

concerning these X-ray absorption calculations have been

published previously.10 Briefly, the electronic structure of the

system was calculated using density functional theory.44 The

exchange–correlation energy was calculated with the Perdew–

Burke–Ernzerhof45 exchange–correlation functional within

the generalized-gradient approximation. A plane wave basis

set with periodic boundary conditions was employed to best

model both localized and delocalized states. To reduce calcu-

lation cost, pseudopotentials were used to mimic valence

electron scattering from all core electrons. To simulate the

experimental excitation of a boron 1s electron to an unoccu-

pied molecular orbital, the boron pseudopotential included a

core hole (electronic configuration 1s12s22p2) and an extra

electron was included with the valence electrons. That is, these

XCH calculations explicitly include both a full core hole and

an extra electron in the lowest energy valence state. Self

consistent energies and states were calculated with PWscf,

part of the Quantum-ESPRESSO package43 on the Franklin

supercomputer at NERSC. Transition amplitudes for the

calculated spectra were calculated within the single particle

and dipole approximations.10

Simulated spectra were based on numerically converged

sampling of the Brillouin zone (k-point sampling) using an

efficient interpolation scheme.46 The calculated spectra were

all uniformly broadened by Gaussian convolution of 0.2 eV

full width at half maximum. In order to obtain a meaningful

average spectrum, the ensemble of calculated spectra are

aligned to a reference spectrum based on total energy-

differences.40 The energy axis in this case is in reference to

the LUMO of the reference spectra. When compared to an

experimental spectrum the calculated spectrum is empirically

shifted in energy. To help correct for known bandgap under-

estimation in DFT, the energy axes for all calculated spectra

were stretched by 10%. Since only relative energies are

obtained, the ensemble-averaged spectra were empirically

aligned to experiment.

Acknowledgements

This work was supported by the Director, Office of Basic

Energy Sciences, Office of Science, U. S. Department of

Energy under Contract No. DE-AC02-05CH11231, through

the Lawrence Berkeley National Laboratory’s (LBNL) Che-

mical Sciences Division. First-principles simulations and the-

oretical guidance were provided by D. P. as part of a User

Project at the Molecular Foundry, LBNL, under the same

contract. High performance computing resources for NEX-

AFS simulations were provided by NERSC, a DOE Advanced

Scientific Computing Research User Facility. Additional com-

putational resources for molecular dynamics simulations were

provided by the Molecular Graphics and Computational

Facility at the College of Chemistry, University of California,

Berkeley. The authors would also like to thank Wanli Yang

for beamline support at the Advanced Light Source, LBNL.

Page 8: Physical Chemistry Chemical Physics · 17078 Phys. Chem. Chem. Phys.,2011,13,1707717083 his journal is c the Owner Societies 2011 solvation along the B–H bond or at the tetrahedral

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 17077–17083 17083

References

1 S. C. Amendola, et al., An ultrasafe hydrogen generator: aqueous,alkaline borohydride solutions and Ru catalyst, J. Power Sources,2000, 85(2), 186–189.

2 J. H. Kim, et al., Reversible hydrogen storage in calcium boro-hydride Ca(BH4)(2), Scr. Mater., 2008, 58(6), 481–483.

3 G. Severa, E. Ronnebro and C. M. Jensen, Direct hydrogenationof magnesium boride to magnesium borohydride: demonstrationof >11 weight percent reversible hydrogen storage, Chem. Commun.,2010, 46(3), 421–423.

4 U. B. Demirci and P. Miele, Sodium tetrahydroborate as energy/hydrogen carrier, its history, C. R. Chim., 2009, 12(9), 943–950.

5 B. H. Liu and Z. P. Li, A review: Hydrogen generation from boro-hydride hydrolysis reaction, J. Power Sources, 2009, 187(2), 527–534.

6 Go/No-Go Recommendation for Hydrolysis of Sodium Boro-hydride for On-Board Vehicular Hydrogen Storage, http://www.hydrogen.energy.gov/pdfs/42220.pdf. 2007.

7 U. B. Demirc, et al., Sodium Borohydride Hydrolysis as HydrogenGenerator: Issues, State of the Art and Applicability Upstreamfrom a Fuel Cell, Fuel Cells, 2010, 10(3), 335–350.

8 U. B. Demirci, O. Akdim and P. Miele, Ten-year efforts and ano-go recommendation for sodium borohydride for on-boardautomotive hydrogen storage, Int. J. Hydrogen Energy, 2009,34(6), 2638–2645.

9 A. M. Duffin, C. P. Schwartz, A. H. England, J. S. Uejio,D. Prendergast and R. J. Saykally, pH-Dependent X-RayAbsorption Spectra of Aqueous Boron-Oxides, J. Chem. Phys.,2011, 134, 154503.

10 D. Prendergast and G. Galli, X-Ray absorption spectra of waterfrom first principles calculations, Phys. Rev. Lett., 2006, 96(21).

11 R. E. Mesmer and W. L. Jolly, Hydrolysis of Aqueous Hydro-borate, Inorg. Chem., 1962, 1(3), 608.

12 R. L. Pecsok, Polarographic Studies on the Oxidation andHydrolysis of Sodium Borohydride1, J. Am. Chem. Soc., 1953,75(12), 2862–2864.

13 M. M. Kreevoy and J. E. Hutchins, H2bh3 as an Intermediate inTetrahydridoborate Hydrolysis, J. Am. Chem. Soc., 1972, 94(18), 6371.

14 T. J. Tague and L. Andrews, Reactions of Pulsed-LaserEvaporated Boron Atoms with Hydrogen-Infrared-Spectra ofBoron Hydride Intermediate Species in Solid Argon, J. Am. Chem.Soc., 1994, 116(11), 4970–4976.

15 O. A. Filippov, et al., Proton-transfer and H-2-eliminationreactions of main-group hydrides EH4- (E = B, Al, Ga) withalcohols, Inorg. Chem., 2006, 45(7), 3086–3096.

16 Y. Kim, J. Kim and K. H. Kim, Theoretical study for the potentialenergy surface of BH5 using the multicoefficient correlated quantummechanical methods, J. Phys. Chem. A, 2003, 107(2), 301–305.

17 M. S. Schuurman, W. D. Allen, P. von Rague Schleyer andH. F. Schaefer, The highly anharmonic BH5 potential energysurface characterized in the ab initio limit, J. Chem. Phys., 2005,122, 104302.

18 A. E. Shirk and D. F. Shriver, Solvent and Cation Dependence ofTetrahydroborate, Bh4-, Raman-Spectrum, J. Am. Chem. Soc.,1973, 95(18), 5901–5904.

19 I. M. Strauss, M. C. R. Symons and V. K. Thompson, SolvationSpectra-.53. Ir and Nuclear Magnetic-Resonance Studies of Tetra-hydroborate Anion in Various Pure Solvents and Binary AqueousMixtures, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 1253–1259.

20 N. V Belkova, E. S. Shubina and L. M. Epstein, Diverse world ofunconventional hydrogen bonds, Acc. Chem. Res., 2005, 38(8),624–631.

21 R. Custelcean and J. E. Jackson, Dihydrogen bonding: Structures,energetics, and dynamics, Chem. Rev., 2001, 101(7), 1963–1980.

22 O. A. Filippov, et al., Intermolecular HH vibrations of dihydrogenbonded complexes H3EH-center dot center dot center dot HOR inthe low-frequency region: Theory and IR spectra, J. Phys. Chem.A, 2008, 112(35), 8198–8204.

23 L. M. Epstein and E. S. Shubina, New types of hydrogen bondingin organometallic chemistry, Coord. Chem. Rev., 2002, 231(1–2),165–181.

24 L. M. Epstein, et al., Unusual hydrogen bonds with a hydride atomin boron hydrides acting as proton acceptor. Spectroscopic andtheoretical studies, Inorg.Chem., 1998, 37(12), 3013–3017.

25 E. S. Shubina, E. V. Bakhmutova, L. N. Saitkulova and L. M.Epstein, Intermolecular hydrogen bonds BH� � �HX in solution,Mendeleev Commun., 1997, 7, 83–84.

26 Y. Filinchuk and H. Hagemann, Structure and properties ofNaBH4�2H(2)O and NaBH4, Eur. J. Inorg. Chem., 2008, (20),3127–3133.

27 K. H. Hallmeier, et al., Investigation of Core Excited QuantumYield Spectra of High Symmetric Boron-Compounds, Spectro-chim. Acta, Part A, 1981, 37(12), 1049–1053.

28 R. Carboni, et al., Coordination of boron and phosphorous inBorophosphosilicate glasses, Appl. Phys. Lett., 2003, 83(21),4312–4314.

29 M. E. Fleet and X. Liu, Boron K-edge XANES of boron oxides:tetrahedral B–O distances and near-surface alteration, Phys. Chem.Miner., 2001, 28(6), 421–427.

30 M. E. Fleet and S. Muthupari, Boron K-edge XANES of borateand borosilicate minerals, Am. Mineral., 2000, 85(7–8), 1009–1021.

31 M. E. Fleet and S. Muthupari, Coordination of boron in alkaliborosilicate glasses using XANES, J. Non-Cryst. Solids, 1999,255(2–3), 233–241.

32 M. Kasrai, et al., Surface modification study of borate materialsfrom B K-edge X-ray absorption spectroscopy, Phys. Chem.Miner., 1998, 25(4), 268–272.

33 K. R. Wilson, et al., Investigation of volatile liquid surfaces bysynchrotron X-ray spectroscopy of liquid microjets, Rev. Sci.Instrum., 2004, 75(3), 725–736.

34 W. H. E. Schwarz, et al., K-Shell Excitations of Bf3, Cf4 andMbf4Compounds, Chem. Phys., 1983, 82(1–2), 57–65.

35 W. T. Klooster, et al., Study of the N–H center dot center dotcenter dot H–B dihydrogen bond including the crystal structure ofBH3NH3 by neutron diffraction, J. Am. Chem. Soc., 1999, 121(27),6337–6343.

36 Y. Suzuki, D. Kaneno and S. Tomoda, Theoretical Study on theMechanism and Diastereoselectivity of NaBH4 Reduction,J. Phys. Chem. A, 2009, 113(11), 2578–2583.

37 G. Wulfsberg, Principles of descriptive inorganic chemistry, Calif.University Science Books, Mill Valley, 1991.

38 C. D. Cappa, et al., Revisiting the total ion yield X-ray absorptionspectra of liquid water microjets, J. Phys.: Condens. Matter, 2008,20, 205105.

39 K. R. Wilson, et al., X-ray spectroscopy of liquid water microjets,J. Phys. Chem. B, 2001, 105(17), 3346–3349.

40 C. P. Schwartz, J. S. Uejio, R. J. Saykally and D. Prendergast, Onthe importance of nuclear quantum motion in near edge X-rayabsorption fine structure (NEXAFS) spectroscopy of molecules,J. Chem. Phys., 2009, 130, 184109.

41 J. S. Uejio, et al., Effects of vibrational motion on core-levelspectra of prototype organic molecules, Chem. Phys. Lett., 2008,467(1–3), 195–199.

42 D. S. Otkidach and I. V. Pletnev, Conformational analysis ofboron-containing compounds using Gillespie-Kepert version ofmolecular mechanics, THEOCHEM, 2001, 536(1), 65–72.

43 P. Giannozzi, et al., QUANTUM ESPRESSO: a modularand open-source software project for quantum simulations ofmaterials, J. Phys.: Condens. Matter, 2009, 21, 395502.

44 P. Hohenberg and W. Kohn, Phys. Rev., 1964, 136, B864;W. Kohn and L. J. Sham, Phys. Rev., 1965, 140, A1133.

45 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996,77, 3865.

46 D. Prendergast and S. G. Louie, Phys. Rev. B: Condens. MatterMater. Phys., 2009, 80, 235126.