New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from11B NMR measurements

Article abstract of DOI:10.1021/jp063362n

To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH₄ in alkaline media, the kinetics of the reaction has been investigated by ¹¹B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH₄^- concentration in the range 0.05-0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03-0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH₄ and its perdeuterated analogue NaBD₄ in H₂O, in D ₂O and H₂O/D₂O mixtures. When the process was carried out in D₂O, deuterium incorporation in BH₄ ^- afforded BH_4-nD_n^- (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hydrolysis have been evaluated. Such a regression analysis was performed on a reaction scheme wherein the starting reactant BH₄^- is allowed both to reversibly exchange hydrogen with deuterium atoms of D₂O and to irreversibly hydrolyze into borohydroxy species B(OD)₄^-. In contrast to acid-catalyzed hydrolysis of sodium borohydride, our results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions.

Full text of DOI:10.1021/jp063362n

17024
J. Phys. Chem. B 2006, 110, 17024-17033
New Insights on the Mechanism of Palladium-Catalyzed Hydrolysis of Sodium Borohydride
1
1
from B NMR Measurements
G. Guella,* C. Zanchetta, B. Patton, and A. Miotello
Dipartimento di Fisica, UniVersit a` degli studi di Trento, Via SommariVe 14, I-38050 PoVo, Trento, Italy
ReceiVed: May 31, 2006; In Final Form: June 22, 2006
To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH
4
in alkaline media,
1
1
the kinetics of the reaction has been investigated by B NMR (nuclear magnetic resonance) measurements
-
taken at different times during the reaction course. Working with BH
4
concentration in the range 0.05-0.1
M and with a [substrate]/[catalyst] molar ratio of 0.03-0.11, hydrolysis has been found to follow a first-
order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We
followed the reaction of NaBH
When the process was carried out in D
) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed.
4
and its perdeuterated analogue NaBD
4
in H
2
-
O, in D
afforded BH4-n
2
O and H
2 2
O/D O mixtures.
-
2
O, deuterium incorporation in BH
4
D
n
(n ) 1, 2, 3,
4
By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the
elementary steps involved in the palladium-catalyzed borohydride hydrolysis have been evaluated. Such a
-
regression analysis was performed on a reaction scheme wherein the starting reactant BH
to reversibly exchange hydrogen with deuterium atoms of D
species B(OD)
4
is allowed both
2
O and to irreversibly hydrolyze into borohydroxy
4
. In contrast to acid-catalyzed hydrolysis of sodium borohydride, our results indicate that in
-
the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the
corresponding hydrolysis reactions.
1
. Introduction
As opposed to other chemical hydrides, such as LiAlH4 or
NaH, only a small amount of hydrogen is liberated after
The hydrolysis of sodium borohydride (NaBH4 + 4H2O f
borohydride dissolution in water (solid NaBH4 and molar excess
of water). In fact, the formation of strongly basic metaborate
NaB(OH)4 + 4H2) is of great practical significance in connection
with hydrogen production to be used in fuel cells for energy
supply in new portable and cellular technologies. This reaction
is extremely efficient, because the total produced H2 stems both
from NaBH4 (one-half) and from H2O (the other half). More-
over, the H2 thus generated is almost free of impurities, and it
may be used in proton exchange membrane (PEM) fuel cells
without further cleanup. To have practical significance in
hydrogen production and power generation, the hydrolysis
process of NaBH4 must occur in such a way as to permit a
controlled stability in hydrogen release.
-
ion (B(OH)4 ) leads to a fast pH increase of the solution and,
thereafter, to a quenching of the overall hydrolytic process.3
This outcome has been exploited to prepare concentrated
aqueous solutions of NaBH4 in strongly alkaline media where
such a hydride is stable over a period of 6 months when stored
under nitrogen at room temperature (half-life of 426 days at
pH 14 and T ) 298 K). Alkaline solutions of NaBH4 can,
however, be promptly hydrolyzed by using selected metal (or
metal salts) catalysts, such as the transition metals Pt, Ru, Co,
and Ni, eventually dispersed on different solid supports. In such
a system, hence, the presence/absence of a suitable metal catalyst
acts as an on/off switch for generation of H2.
Even if sodium borohydride is known as a hydrogen source
via its hydrolysis, it can also be oxidized directly in an
electrochemical cell when the eight-electron anodic process
Even if the rate dependence on pH and other mechanistic
-
-
-
-
BH4 + 8OH f BO2 + 6H2O + 8e is coupled to a suitable
-
details of the BH4 acid-catalyzed hydrolysis were deeply
-
-
cathodic half-reaction such as 2O2 + 4H2O + 8e f 8OH .
investigated by Schlesinger et al. more than 50 years ago,3
the structures of true intermediates along the reaction pathway
and the reaction variables controlling the competition between
hydrolysis and exchange are still not fully understood.
-5
The calculated standard electromotive force of the overall
-
-
working reaction BH4 + 2O2 f BO2 + 2H2O is 1.64 V,
suggesting a strongly exergonic process (calculated Gibbs free
energy energy is -303 kcal mol ). Until now, however, the
development of such a fuel cell is hampered by the poor
1
-
1
The state of knowledge is even worse for metal-catalyzed
-
-
BH4 hydrolysis where the true chemical nature of the catalyzing
utilization efficiency of the BH4 electroxidation due to the
species (metal, metal boride, metal in oxidized form) is not
definitely established, and more generally, the involved mech-
anism has been largely overlooked.
competitive hydrolysis leading to products which, in turn, are
oxidizable at somewhat more negative potentials than the parent
ion. Hydrogen evolution is considered here to be a major
problem affecting the Coulombic efficiency of the system and
the number of utilized electrons.2
-
The overall hydrolysis reaction of borohydride, BH4 + 3H2O
+ H+ f B(OH)3 + 4H2, takes place stepwise. Besides the
6
7
work of Mesmer and Jolly (water solutions, pH range 3.8-
8
1
4) and that of Wang and Jolly (methanol-water solutions,
*
guella@science.unitn.it. Tel: +39 0461 88 1536. Fax: +39 0461 88
+
1
696.
low pH value: [H3O ] ) 0.1-1.1 M, T ) 195 K), most kinetic
1
0.1021/jp063362n CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/10/2006

Pd-Catalyzed Hydrolysis of NaBH4
J. Phys. Chem. B, Vol. 110, No. 34, 2006 17025
studies were performed in alkaline or strongly alkaline solutions.
There is a general agreement that, in these media, the loss of
the first of four hydrogen atoms is the rate-determining step
for the overall process. In alkaline media, the attack on BH4
by the protic solvent (H2O) is thought to afford a pentacoordinate
2. Experimental Methods
.1. Reagent and Sample Preparation. An alkaline solution
of sodium borohydride (pH 13, (0.055 ( 0.001) M; Carlo Erba
Reagents), was prepared for GC measurements. Alkaline
solutions (10 mL, pH 13) of sodium borohydride and sodium
borodeuteride (Sigma Aldrich Reagents) were prepared for each
of four sets of NMR experiments: solution 1, NaBH4/H2O
2
9
-
“
BH5” intermediate, the existence of which has been recently
10,11
demonstrated by both theoretical
tigations. This short-lived intermediate may eventually evolve
in two different directions, either relaxing back to borohydride
and experimental inves-
12
(0.073 ( 0.002) M; solution 2, NaBH4/D2O (0.057 ( 0.002)
M; solution 3, NaBD4/H2O (0.088 ( 0.002) M; and solution 4,
NaBD4/D2O (0.085 ( 0.002) M; by dissolving the suitable
amount of solid sample in 0.10 M NaOH or 0.10 M NaOD.
The latter was obtained by deuterium exchange of dry solid
NaOH and repeated treatments with D2O (99.9%, Merck). The
titer values of reagents were independently measured through
(with H/D scrambling if the hydrolysis is carried in a deuterated
solvent) or decomposing irreversibly to molecular hydrogen and
-
B(OH)4 , possibly via a BH3 intermediate. According to
13
Kreevoy and Hutchins, the values of the specific rate constants
-
5
at T ) 298 K for BH4 hydrolysis are kH+ ) (9.9 ( 0.3) × 10
-
1
-1
-7 -1
L mol
s
and kH O ) (2.2 ( 0.2) × 10 s , while the ratio
4
2
iodometric method.
between the specific rate of the two competitive processes
causing the disappearance of the presumed intermediate BH5
Kinetic measurements, aimed at establishing the rate law of
-
the BH4 /H2O, in Pd/C 10 wt %, were carried out by following
-
1
is (2.3 ( 0.3) L mol . When the hydrolysis is carried out in
deuterated water (D2O), the corresponding values are kD+ )
the H2 evolution rate by GC analysis, adding, respectively, 79.82
(0.0015 M), 159.63 (0.0030 M), 239.45 (0.0045 M), and 319.26
(0.0060 M) mg of commercially available Pd/C (Sigma-Aldrich,
Milano) to 50 mL samples of the above solutions ((0.055 (
0.001) M) with 150 mL of distilled water.
5
-1 -1
-8
(
5.3 ( 0.4) × 10 L mol
s
and kD O ) (2.4 ( 0.3) × 10
2
-1
s , and the ratio between the processing responsible for the
disappearance of BH4D is (1.09 ( 0.04) L mol- leading to
proton-specific isotope effect of 1.9 and to a water isotope effect
of 9.0.
1
NMR measurements were carried out by adding, respectively,
6.0, 6.4, 7.5, 6.3, and 5.9 mg of commercially available Pd/C
(
1
10 wt % Sigma-Aldrich, Milano) to the borohydride solutions
-4 prepared as previously described. To avoid systematic
In principle, heterogeneous catalysis presents many advan-
tages over homogeneous acid-catalyzed hydrolysis, because it
is expected to be almost independent from the pH of the reaction
medium, the catalyst would be reusable and, more important
for application in hydrogen production, it could act as an on/
errors, all the NMR measurements were carried out at least in
duplicate.
For each set of NMR measurements, 500 µL of NaBH4 or
14
NaBD alkaline solutions (pH 13) were put into a 5 mm NMR
test tube. In the NaBH4/H2O and NaBD4/H2O runs, the samples
were added with 50 µL of D2O to provide an internal lock
frequency control. All the NMR instrumental parameters were
optimized for both H and B detection, and a reference
spectrum was always acquired before the addition of the catalyst.
2.2. GC Measurements. The rate of hydrogen emission was
evaluated by GC measurements carried out with Agilent 3000
Micro GC (Agilent Technologies) equipped with a Plot-U (3
m) and a molecular sieve (10 m) columns.
off device. Kaufman and Sen have studied the effect of some
transition metals (Cu, Ni, and Co) and their salts by concluding
that the overall hydrogen production in the presence of metal
4
+
salts is both metal- and acid-catalyzed due to the fast H
1
11
production in their reduction by BH4- species. These results
were in good agreement with the mechanism previously
proposed by Holbrook and Twist,15 which still represents a
milestone in the metal-catalyzed hydrolysis of the borohydride
ion.
We report here the kinetic results of the metal-catalyzed BH4-
An appropriate reactor with a thermostatic bath was used for
the reaction study. The system was set up to permit the
connection with other devices used in the experiment (pressure
sensor, inlet gas, catalyst insertion device) and to collect emitted
hydrogen to the GC instrument. In any run, the catalyst was
placed on the appropriate device inside the reactor, and the
system was sealed. Nitrogen was used as a feed gas (20 kPa),
acting also as the solution stirrer. This configuration permitted
a response time of GC of approximately 1 min. To avoid gas
condensation in the inlet tube of GC, a water vapor trap was
placed on the gas evolved line; argon was used as the gas carrier
hydrolysis (rate law and activation energy) obtained by using
10 wt % Pd/C as catalyst. A preliminary investigation on such
a system was carried out, by measuring the hydrogen evolution
rate with volumetric and/or gas-chromatographic (GC) tech-
niques at different temperatures. Such data proved to be very
useful to plan a complete nuclear magnetic resonance (NMR)
kinetic analysis that allowed us to follow the time dependence
of all the long-living species involved in the reaction, not only
-
within the BH4 /H2O reacting system but also its deuterated
-
-
analogues, such as BD4 /H2O or BH4 /D2O reaction systems.
By best fitting of the time dependence of the 11B NMR signals,
for the first time we report here the rate constants of the
deuterium incorporation processes into initial borohydride and
the corresponding hydrolysis specific rates. In fact, by using
NMR, the expected competition between hydrolysis and hydrogen/
deuterium exchange processes is promptly detected by the
appearance, at different times, of all the deuterated hydrides
(0.55 MPa).
To follow the hydrogen evolution during reaction, only a
small portion of the produced gas must enter the GC device
about 1% of the total flow) which provided the rate of reaction
(
in arbitrary units. Since a constant volume of the sampling gas
is injected into the Micro GC at different times, the time
dependence of the amount of detected hydrogen provides the
evolution rate (in arbitrary units) of the reaction. With the
concentration of the borohydride running solution before and
after the end of reaction, a correct quantitative scale unit can
be obtained and therefore the total amount of the produced
hydrogen.
-
BH4-nDn (n ) 0, 1, 2, 3, 4). The adopted simulation procedure
involves the numerical solutions of the appropriate differential
equations to account for a given reaction scheme in which the
-
starting reactant BH4 is either allowed to reversibly exchange
its hydrogen with deuterium atoms of D2O and to irreversibly
hydrolyze (together with the above-mentioned partially deuter-
The amount of borohydride present in the system was
-
4
ated hydrides) to borohydroxy species B(OD4) .
determined by the use of the iodometric method by adding a

17026 J. Phys. Chem. B, Vol. 110, No. 34, 2006
Guella et al.
molar excess of IO3- to the reaction medium and back-titrating
former was evaluated before the addition of the catalyst and
for the latter at the end of the reaction. They were, hence,
consistently used for all the spectra collected throughout. After
the catalyst particle precipitation, the solution contained in the
main volume sampled by the RF coil becomes almost homo-
geneous.
-
2-
the remaining IO3 with buffered S2O3
.3. NMR Measurements. NMR spectra were taken at 298.2
K with an Avance 400 Bruker spectrometer operating at 400.13
.
2
1
11
MHz for H and at 128.38 MHz for B and equipped with a 5
mm inverse broad band (BBI) probe. δB values were recorded
in parts per million (ppm) relative to BF3‚Et2O ( )0.0 ppm in
2
.3.2. Data Analysis. To fit the time dependence of all the
chemical species detectable by NMR, the disappearance of the
CDCl3) and δH values relative to the solvent residual signals
δH 4.72 ppm). Optimized π/2 pulse width for H (8 µs, at a
transmission power of 0.0 dB) and B (12 µs, at a transmission
power -3.0 δB) were obtained on the same samples by the
Bruker subroutine. The length of the π/2 pulse was checked at
the beginning of any measurement session, and the temperature
-
1
hydride BH4 (and of its deuterated analogues) and the increase
(
-
1
1
of the borate B(OH)4 were followed and analyzed. The major
-
work was carried out on the BH4 /D2O system wherein all the
intermediate species BH4-nDn (n ) 0, 1, 2, 3, 4) and the
-
hydrolysis end product B(OD)4 were detected.
1
To follow the evolution of whole system, only the central
peak of any species was considered. Due to the spin quantum
was controlled throughout the experiments to (0.1 K. H NMR
spectra were taken by using water suppression pulse sequence,
-
11
1
1
number of deuterium (I ) 1), BH4 appears in the B-proton-
while B NMR spectra were acquired either as fully coupled
or as proton-decoupled with 5 s time delay: 32 scans. The
phase-cycled Hahn echo pulse sequence16 was also used with a
recycle time of 2 s, delay between pulses was 100 ms, sweep
width was 38.46 kHz, and acquisition time was 0.104 s. In all
-
decoupled spectrum as a singlet, BH3D as a 1:1:1 triplet (JB-D
as 1:2:3:2:1 quintet, BHD as a 1:3:6:
:6:3:1 septet, and BD4 as a 1:4:10:16:19:16:10:4:1 nonet (see
-
-
) 12.2 Hz), BH D
2
2
3
-
7
Figure 1). Therefore, to normalize the integrated signals, the
area was multiplied for a suitable factor (by referring to the
species BH4-nDn, such a factor is 1, 3, 3, 27/7, and 81/19 for n
1
11
H and B NMR measurements, 32k data points were collected
leading to a digital resolution of 0.28 Hz/point (SWH ) 9260
Hz) and 0.25 Hz/point (SWB ) 8280 Hz), respectively. Data
were collected with XWINNMR (version number 1.3) and
)
0, 1, 2, 3, and 4, respectively).
The integration of single peaks involved signal deconvolution
due to the peak overlap. Every spectrum was phase-optimized
in a frequency domain nearby the interested peak.
processed using both Bruker TOPSPIN (version number 1.3)
and MestReC data processing software.17
1H, 11B- H coupled, or B- H decoupled experiments could
1
11
1
2.3.3. Simulation and Fitting. All NMR data have been used
as input data for the Gepasi program, a free software for the
in principle be used for our purpose, but we decided to rely on
the latter hardware configuration and to use the Hahn echo pulse
sequence for reducing rolling baselines of the NMR spectra.
1
8-20
simulation of the dynamics of chemical processes,
using a
Levenberg-Marquardt fitting optimization model. The software
converts the whole set of equations defined by a given reaction
scheme in a matrix system of differential equations.
Gepasi uses the sum of squares of residuals
-
With such a pulse sequence, the multiplicity of all the BH4-nDn
(n ) 1, 2, 3, 4) species could be better detected, and the area
of the corresponding signal at any given time more easily
integrated.
/
i
2
yi - y
Due to the low magnetic moment of Pd atoms (-0.642 µ/µn),
the relaxation times of 1H and 11B NMR signals are not
ssq )
(1)
∑
(
)
i
w
i
significantly affected by the presence of metal in the analyzed
sample. Moreover, even if at the beginning of the reaction the
magnetic field homogeneity is lowered by H2 bubbling off and
by the inhomogeneous character of the sampled solution, NMR
spectra with acceptable resolution can be taken through kinetic
runs.
where the sum is over all the experimental data, yi are the
measured values of the variables, y are the simulated values
/
i
of the variables, and wi are weighting constants used to scale
all variables to similar values such that all have equal weight
on the fit. The global minimum of the sum of squares represents
the best that the model can do to explain the experiment.
In borohydrides and their hydroxy derivatives, the majority
11
of the chemical shifts of B (H coupled) signals occur between
0 and -50 ppm (assuming BF3‚Et2O as reference at 0.0 ppm),
which represent a span of about 8000 Hz at the operating
2
.3.4. Ab Initio Calculations. Ab initio quantum chemical
1
calculations were performed by using the Gaussian 03 (revision
2
1
1
1
1
C.02) package. Geometry optimizations were carried out at
the density functional theory level by using the hybrid functional
instrumental frequency, whereas the J values of the B- H
1
1
coupling are about 80 Hz. Since the line width of B NMR
signals is quite narrow and the chemical shift of the hydrolysis
22
DFT-B3LYP and the 6-311++G(d,p) basis set. All structures
were characterized by frequency calculations at the same level
of theory in order to find stationary points as true minima
11
intermediates BHn(OH)m is expected around 0-20 ppm, the
B
spectra provide sufficient information for the determination of
individual structures.
(
number of imaginary frequency ) 0), and zero point energy
(ZPE) corrections were scaled by 0.9806, as usually performed
2
.3.1. Procedure During Kinetics Measurements. The initial
2
3
at such a level of theoretical approach.
time of the reaction was taken after addition, through a
homemade minifunnel, of a suitable amount of the catalyst
powder (∼10% on molar ratio). The sample tube was then
strongly shaken and introduced into the NMR probe, the field
homogeneity rapidly optimized, and a multiple acquisition
experiment launched. The elapsed time between catalyst addition
and the first NMR spectrum acquisition was always in the range
3
. Results and Discussion
3.1. GC Measurements. Commercial 10 wt % Pd/C powders
were found to be efficient catalysts for the hydrolysis of sodium
borohydride as shown in Figure 2, where the percentage of H2
volume generated during the catalytic hydrolysis of 0.055 M
NaBH4 solution (pH 13) with different catalyst concentrations
at T ) 296 K is reported. The hydrogen production starts
immediately without any induction time and shows a first-order
kinetic dependence on Pd concentration.
9
0-110 s. Since the spin-lattice (T1) relaxation times of the
1
1
B nucleus in the borohydrides (no matter their H/D label) are
-
significantly longer (about 12 s) than T1 of borates B(OH)4 /
B(OD)4 (about 1 s), the empirical integration constant for the
-

Pd-Catalyzed Hydrolysis of NaBH4
J. Phys. Chem. B, Vol. 110, No. 34, 2006 17027
Figure 2. Hydrogen production versus time at different molar
concentrations of metal catalyst.
constant evaluated according to Kreevoy and Hutchins at pH
1
3
+
-7
1
3 would be khyd ) kH+[H3O ] + kH O[H2O] ) 2.3 × 10
2
-
1
s . Hence, with our present catalyst amount, the overall
catalytic effect is estimated to be about 4 orders of magnitude
greater.
To obtain a rough estimate of the activation energy involved
in the H2 production, the values of the rate constant were
determined at various temperatures. The slope of ln(k) versus
the reciprocal absolute temperature (1/T) leads to an activation
-
1
energy of 28 kJ mol . This value is similar to the activation
-
1 24
energy found by Amendola (47 kJ mol ) at higher NaBH4
14
and NaOH concentrations and by Kaufman and Sen with
-
1
different bulk metal catalysts: 75 kJ mol for cobalt, 71 kJ
-
1
-1
mol for nickel, and 63 kJ mol for Raney nickel.
-1
Recently, an activation energy of 28.5 kJ mol has been
25
reported using Ru(0) nanoclusters as catalyst. However, it must
be emphasized that in such investigation a zero-order reaction
process is observed as a function of the catalyst concentration.
-
This reaction order is probably due to the BH4 -induced
dynamic saturation of the active sites of the catalyst surface
during the reaction. Saturation effects are expected when
hydride/catalyst molar ratio is high (always greater than 100)
as in the case of Ru(0). On the contrary, in our measurements,
such a ratio was never less than 40.
3
.2. NMR Measurements. 3.2.1. NMR Kinetic Measurements
11
with NaBH4 in Alkaline H2O. In a typical B-proton-decoupled
NMR spectrum, obtained during hydrolysis (solution 1), aside
from the upfield signal at -41.8 ppm, due to the unreacted
Figure 1. 11B NMR proton-decoupled spectra taken at different times
during the Pd/C catalyzed hydrolysis of solution 2. (a) Before the
-
BH4 , a broad signal at 1.76 ppm attributable to borate species
-
catalyst addition (singlet for BH
4
species); (b) after 550 s (triplet for
-
-
B(OH)4 was present. However, no intermediates of HnB(OH)4-n
(n ) 1, 2, 3) type were detected. In principle, the lack of
detection of such species could be due to the low symmetry of
their structures leading to a significant broadening of 11B signals.
We must emphasize, however, that we were able to detect them
during the acid-catalyzed hydrolysis of sodium borohydride (data
not shown). In particular, the species BH3OH was there found
as a sharp 1:3:3:1 quartet (at -12.9 ppm, J( H, B) ) 87.50
Hz) in the fully coupled B NMR spectrum. Since such a
-
-
BH
880 s (septet for BHD
species). Arrows show the central peak of any multiplet.
3
D species); (c) after 1050 s (quintet for BH D species); (d) after
2
2
- -
3 4
species); and (e) after 4020 s (nonet for BD
1
Hydrogen evolution data can be nicely fitted in any run by a
single exponential of the form [H2]i(t) ) [H2]max(1 - e -kobs,i
t
)
-
where [H2]max is assumed from the reaction stoichiometry to
be [H2]max ) 4[BH4 ]0 and kobs,i represents the actual value of
the rate constant observed for a given initial condition. In all
-
1
11
1
1
-
the measurements carried out with an initial molar ratio of
species (and/or any of the type HnB(OH)4-n ) was not detected
-
[
BH4 ]/[Pd] within the range 0.03-0.11, we obtain first-order
in the corresponding NMR spectra of the palladium-catalyzed
hydrolysis, we conclude that they are not long-living intermedi-
ates along the coordinate of the Pd-catalyzed reaction. The
observed rate constants of such a process were evaluated by
kinetics both in catalyst and in borohydride. We are well aware
that the catalyst’s activity depends on the Pd active sites on the
surface of the C powder, but in a well-stirred dispersion of Pd/C
catalyst, we can consider them to be proportional to the catalyst’s
-
-
following the time dependence of both BH4 and B(OH)4
“
concentration”, here written as [Pd]. To give an example, the
NMR signals and by fitting such experimental data with the
Levenberg-Marquardt model as implemented in the Gepasi
program. By fitting the NMR curves describing the time
dependence, for a given amount of added catalyst, we found
value of kobs in the run where Pd/C was present at 5.5% molar
ratio was estimated to be kobs ) (2.4 ( 0.3) × 10 s .
-
4
-1
We must emphasize that in the absence of catalyst the rate

17028 J. Phys. Chem. B, Vol. 110, No. 34, 2006
Guella et al.
-
two single-exponential functions of the form [BH4 ](t) )
TABLE 1: Kinetic Constants, at 298 K, Obtained from the
Simplified Model, According to the Reported Reaction
Schemea
-
-k
t
-
-
-k
t
[
BH4 ]0 e obs and [B(OH)4 ](t) ) [BH4 ]0[1 - e obs ]. Such
functions stem by integration, with suitable boundary conditions,
-
-3
of the first-order rate equations of the form -d[BH4 ](t)/dt )
rate
constant
values × 10
-1 -2
-
-
-
-
reaction scheme
[s
M ]
kobs[BH4 ](t) and d[B(OH)4 ](t)/dt ) kobs([BH4 ]0 - [BH4 ](t)).
-
5
-1
-1
-
-
-
Thus, the value of kobs,NMR ((3.6 ( 0.2) × 10
s
M )
k
k
k
k
1,hyd
BH
BH
4
+ D
+ D
2
O f B(OD)
4
+ HD
n
(1.46 ( 0.04)
(2.37 ( 0.07)
(11.7 ( 0.9)
-
2,exc
4
2
O f ∑BH4-n
D
+ HDO
-
(
solution 1) can be straightforwardly obtained by simulation of
the experimental curves. Such a value of kobs,NMR includes not
only the true hydrolytic constant khyd but also the concentrations
of catalyst (0.010 M). The true hydrolytic rate constant khyd (s
-
-
-2,exc
3,hyd
∑BH4-n
∑BH4-n
D
D
n
+ HDO f BH
4
2
+ D O
4
-
n
+ D
2
O f B(OD)
+ HD (0.18 ( 0.02)
-1
a
2
[D O] ) 52.7 M; [HDO] ) 2.5 M.
-2
-
M ) of the BH4 /H2O reacting system is therefore evaluated
-
3
-1
-2
to be 3.6 × 10
s
M . The first-order dependence of the
-
rate from the BH4 concentration was confirmed by t1/2 NMR
measurements taken on samples containing different initial
concentration of the hydride evaluated by the crossing point of
-
-
the BH4 (t) and B(OH)4 (t) curves, observed at (340 ( 5) s.
No matter how the hydrolytic rate constant is evaluated, its
value represents the true rate constant of the hydrolysis in pure
water only if we could assume that the overall kinetic effect of
the deuterated water (10 vol %), present in the NMR tube, is
negligible. Since during the reaction course we do not observe
-
any deuterium incorporation into BH4 species and the rate
-
constant of the BH4 hydrolysis in D2O is expected to be lower
-
than in H2O, the contribution of the second term (kD O[BH4 ]-
2
[D2O][Pd]) in the appropriate differential equation (see Sup-
porting Information, eq 1) would be negligible. In fact, kD O is
2
about 4 times lower than kH O, as will be discussed in the next
2
Figure 3. Consecutive irreversible first-order reaction. Dashed lines:
fitting curves. Symbols: experimental data.
section, and [D2O] here is 10 times lower than H2O. Thus, the
-
value of the true khyd of BH4 in H2O/D2O ) 9:1 at any given
initial ratio of borohydride/catalyst must be nearly the same,
This assumption was roughly confirmed by comparison of the
-
within the estimated relative error, as that of BH4 in pure H2O
1
-
area of the HDO H NMR signal in our BH4 /D2O reacting
system with respect to the area of the same signal in the same
volume of the commercial sealed ampule. Such measurements
lead to an estimation of HDO concentration in our sample of
-
at the same [BH4 ]/[catalyst] ratio.
3
.2.2. NMR Kinetic Measurements with NaBH4 in Alkaline
D2O. a. Experiments. As expected and already anticipated
Figure 1), things are much more complicated when the
(
2
.5 M (and thus, D2O is estimated to be 52.7 M). Both such
hydrolysis is carried out in deuterated water (solution 2). In
fact, in the upfield region of 1 B-proton-decoupled NMR
spectrum, many other signals arise. In particular, deuterium
-
concentrations are so high with respect to the initial BH4
concentration to ensure a pseudo-first-order kinetics for all the
processes where they are involved. Since the equilibrium
constant of the reaction H2O + D2O h 2HDO is about 1, the
residual [H2O] is expected to be absolutely negligible. Both in
hydrolytic and in exchange processes, HDO may act either as
a proton donor molecule (affording hydrolysis and exchange
1
-
-
incorporation in BH4 leads to BH4-nDn (n ) 1, 2, 3, 4) species
which appear on the scene one after another. Besides the
expected signal of unreacted BH4 and B(OD)4 (singlets at
-
-
δB -41.8 and 1.76, respectively), we detected here the presence
-
-
of the deuterated species BH3D (triplet at δB -42.0), BH2D2
-
on all the scrambled hydrides but not on the initial BH4 species)
-
(
quintet at δB -42.2), BHD3 (septet at δB -42.4), and even
or as a deuteron donor (affording hydrolysis and exchange on
-
traces of BD4 (nonet at δB -42.6). As already found in
-
all the scrambled hydrides but not on the end products B(OH)4
hydrolysis of solution 1, the scrambled intermediates HnDmB-
species). However, because [D2O] is about 25 times higher than
-
(
OD)p (n + m ) 4 - p; p ) 1, 2, 3, 4) were not detected.
[HDO] and the corresponding hydrolytic rate constants khyd,D2O
To estimate the rate constants of all the processes leading to
are not more than 5 times lower than khyd,HDO, the contribution
to hydrolysis of the terms such as khyd,HDO[HDO] should be
-
scrambled hydrides and to the final B(OD)4 species, the initial
conditions must be accurately defined. This is not a simple
matter when working with D2O solutions, since heavy water in
contact with air containing light water vapor rapidly becomes
contaminated with it. Kobayashi26 investigated the speed of this
process and found that a sample of D2O open to the air, at 294.5
K and 70% humidity, went from an initial 97.7% D to 13% in
about 10 h. There is also an apparently rapid equilibrium, H2O
about 5 times lower than terms such as khyd,D2O[D O]. Concern-
2
ing the effect on the H/D exchange, we can assume kexc,HDO to
be almost the same as kexc,D2O in all the processes where HDO
acts as a deuteron donor, while kexc,HDO must be evaluated in
all the processes where HDO acts as a proton donor.
b. Results for Nonlinear Regression Analysis. A nonlinear
least-squares regression technique was used to compare the
experimental data with a simplified kinetic model (reaction
scheme in Table 1) wherein we considered only the starting
+
D2O h 2HDO, so that, after a few hours, only an equilibrated
mixture among different isotopically substituted water molecules
is formed. It follows that there is some uncertainty in the
definition of D2O and HDO initial concentrations. Since our
experiments were carried out just after the preparation of
solution 2, which was kept at T ) 277 K in a closed vial, we
can safely assume that the D2O titer could be about 4% lower
than that contained in the commercial sealed ampule (99.5%).
-
-
reactant (BH4 ), the end product (B(OD)4 ), and the global
change of all the intermediate scrambled hydrides (BH4-nDn
with n ) 1, 2, 3, 4). The “best fit” of the NMR data (see Figure
3) with the solutions of the differential equations (see Supporting
Information, eqs 5-10) led to the best values of the hydrolysis
(khyd) and exchange rate constants (kexc) as reported in Table 1.

Pd-Catalyzed Hydrolysis of NaBH4
J. Phys. Chem. B, Vol. 110, No. 34, 2006 17029
TABLE 2: Kinetic Constants at 298 K Obtained from
Optimized Modela
enthalpies and Gibbs free energies. In the last column of Table
3
is reported the sum of electronic and thermal enthalpies and
-
3
rate
values × 10
the sum of electronic and thermal Gibbs free energies for every
minimized chemical species.
-
1
-2
constant
reaction scheme
[s
M ]
-
-
-
The adequacy of our theoretical approach is granted by the
comparison of the calculated standard free energy (-277 kJ
k
k
k
k
k
k
k
k
k
k
k
k
k
1,hyd
BH
BH
BH
BH
BH
BH
BH
BH
4
4
3
3
3
2
2
2
+ D
2
O f B(OD)
4
+ HD
(1.29 ( 0.09)
(2.9 ( 0.1)
(27 ( 2)
-
2,exc
+ D
2
O f BH D + HDO
3
-
-
D + HDO f BH
4
+ D
2
O
-1
-
-
-2,exc
3,hyd
4,exc
mol ) of the hydrolysis reaction BH4 + 4H2O f B(OH)4
-
-
D + D
2
O f B(OD)
4
-
+ HD
+ HDO
(0.4 ( 0.3)
(1.2 ( 0.1)
-1
+
4H2 with its experimental value (-320 kJ mol ) if we take
-
D + D
2
O f BH
2
D
2
-
-
b
into account that calculations are referred to a virtual process
occurring in a pure gas phase. If we compare the calculated
zero-point energy difference (an observable not significantly
affected by solvent interactions) at T ) 298 K between H2O
-4,exc
5,hyd
6,exc
D
D
D
2
2
2
+ HDO f BH
3
D + D
2
O
10
-
-
-
+ D
+ D
2
O f B(OD)
4
+ HD
+ HDO
(0.3 ( 0.6)
-
2
O f BHD
3
(1.6 ( 0.4)
-
-
-
-
b
-6,exc
7,hyd
8,exc
BHD
BHD
BHD
3
3
3
-
-
+ HDO f BH
2
D
2
+ D
+ HD
2
O
9
-
b
+ D
+ D
2
O f B(OD)
4
0.8
1
10
-1
and D2O (15 kJ mol ) with the reported experimental value
-
b
2
O f BD
4
+ HDO
-
1
(
14.9 kJ mol ), the agreement is far better. Since the H/D
-
b
-8,exc
9,hyd
BD
BD
4
+ HDO f BHD
+ D
3
+ D
2
O
2
-
-
b
exchange mechanism plays a main role in the BH4 /D2O system
and no experimental data, to the best of our knowledge, are
reported on the thermodynamic parameters of deuterium scram-
bling of borohydride, we decided to calculate them for every
process of the type BH4-nDn + D2O ) BH3-nDn+1 + HDO
with n ) (0, 1, 2, 3). As we can appreciate in Table 3, since
the calculated ZPE difference between BH4-nDn and BH3-n-
4
2
O f B(OD)
4
+ D
0.8
a
The reaction scheme is reported in second column. [D O] ) 52.7
M, [HDO] ) 2.5 M. Only indicative value.
2
b
-
-
The most interesting outcome is the prevalence of the
exchange routes on the net hydrolytic reaction, at least in the
early part of its evolution (0-20 min). Thus, in Figure 3, there
is clear-cut evidence that the rate of overall formation of the
scrambled intermediates is higher than the overall rate of
formation of the hydrolytic end product B(OD)4 , which in turn
is determined by D2O attack on both BH4 and intermediate
hydrides. In particular, the rate constants of the backward
exchange reactions (k-2,exc) have been found to be 5 times higher
than that of the corresponding forward processes (k2,exc); the
latter, in turn, is about twice the hydrolytic rate constant of the
-
-
Dn+1 is lower than the calculated ZPE difference between D2O
and HDO, the calculated Gibbs free energy of all the above
-
-1
processes is slightly positive (about +2.5 kJ mol ). At T )
-
298 K, Keq,exc, the associated thermodynamic equilibrium
constant would be therefore just a little less than 1 (about 0.4)
when all the products and reactants are at their standard
concentration (1.0 M). The equilibrium position is expected to
be strongly shifted toward the products (deuterated hydrides)
due to the initial conditions of our reaction where the solvent,
D2O, is present in large molar excess in the reaction medium.
Moving along similar lines as those described above for the
simplified reaction scheme, the solutions of the differential
equations (see Supporting Information, eqs 11-19) were fitted
to the NMR data (Figure 4) obtaining the “best” values of all
the involved hydrolysis (khyd) and exchange rate constants
(k(i,exc) as reported in Table 2.
-
BH4 species (k1,hyd) and about 1 order of magnitude higher
than the hydrolytic rate constants of all the deuteron-containing
BHnDm species (k3,hyd).
-
The values of the rate constants obtained in such a simplified
model were used as initial trial parameters in a more detailed
reaction scheme (second column of Table 2) where we
considered the reactions involving all the NMR-detected boron
species. We assumed reversible exchange reactions and irrevers-
ible hydrolysis reactions on the grounds of the corresponding
thermodynamic parameters. In fact, whereas the hydrolysis is
a strongly favored process (Gibbs free energy variation, ∆G )
These results confirm those already outlined in the simplified
model; thus, exchange rate constants are again higher (in
particular, those relative to the backward processes) than
hydrolytic rate constants and indicate a slight decrease of both
exchange and hydrolytic rate constants with the increase of
deuterium content on the reaction intermediates. The only
exception is represented by the rate constants of the processes
-
1
-
320 kJ mol ), the free energy of any H/D exchange process
is expected to be near zero. Ab initio calculations on the relevant
chemical species involved in such reactions (Table 3) allowed
us to generate thermochemical information, such as reaction
TABLE 3: Absolute Energies (Es) and ZPEs at 298 K Scaled by 0.98 at DFT-B3LYP/6-311++G(d,p) Level
H° sum of
∆
∆G° sum of
electronic and
thermal free energies
[au]
zero-point
electronic and
thermal enthalpies
[au]
molecular
species
correction
∆ZPE
Ea [au]
[kJ mol
-1
]
[kJ mol-1]
-
BH
BH
BH
4
3
2
-27.275299
90.9
85.8
80.4
74.9
70.8
62.0
54.4
46.9
167.9
136.9
33.1
31.0
26.8
116.0
90.4
31.4
0.0
5.1
10.5
16.0
20.1
0.0
7.6
15.1
0.0
31.0
0.0
4.2
6.3
0.0
-27.238204
-27.240301
-27.242426
-27.244576
-27.246752
-76.433014
-76.435936
-76.438903
-328.419188
-328.431426
-1.166414
-27.259688
-27.262218
-27.264758
-27.267308
-27.269868
-76.453267
-76.457911
-76.46104
-328.453355
-328.467169
-1.181200
-1.183364
-1.185718
D-
D
-
2
-
BHD
3
-
4
BD
H
2
O
HDO
O
B(OH)
B(OD)
-76.458531
D
2
-
-
4
4
-328.486674
-1.1795715
H
2
HD
-1.167733
D
2
-1.169297
-
BH
BD
BO
3
3
2
OH
-102.561549
-175.521600
-102.516417
-102.526176
-175.508277
-102.544083
-102.555328
-175.53344
-
OD
25.6
0.0
-
a
au ) 2624 kJ mol-1
1
.

17030 J. Phys. Chem. B, Vol. 110, No. 34, 2006
Guella et al.
reacting systems. In particular, in the former system we have
highlighted through a detailed kinetic analysis and by high-
level theoretical calculations a significant isotope effect on the
hydrolysis. If, for the sake of reliability, we consider only the
kinetic parameters of the reaction steps involved in the first half
of the reaction course showed in Figure 4, it is evident that (i)
-
the hydrolytic rate constant in D2O of the starting BH4 species,
-
3
-1
-2
1
.3 × 10
s
M , is about one-third of the corresponding
-
rate constant in H2O, and the major contribution to BH4
disappearance is given by the forward exchange process leading
-
-
-
to BH3D species; (ii) the hydrolytic rate constant of BH3D
-3
-1
-2
in D2O is about one-third (1.3 × 10
s
M ) of that of BH4
in the same solvent, and again, the main contribution to its
disappearance is given by the forward process leading to
-
-
BH2D2 ; and (iii) the exchange rate constant in the BH3D /
BH2D2 system is, in turn, about one-half of that evaluated in
BH4 /BH3D system. In our opinion, the latter outcome can
be explained on statistical grounds by taking into account that
deuterium incorporation can be obtained by the exchange with
-
-
Figure 4. Optimized model. Dashed lines: fitting curves. Solid lines:
experimental data.
-
any one of the four H atoms in the BH4 species, while it occurs
only by exchanging with three H of BH3D , thus introducing a
-
involving the BD4- species, but our experimental NMR
measurements do not allow us to obtain reliable values, since
its NMR signal was not significantly higher than the instrumental
noise. Therefore, the values of such rate coefficients (k8,exc,
k-8,exc, and k9,hyd reported in Table 2) can be considered just
indicative estimates.
further activation entropy contribution into the rate constant of
the latter reaction. It must be emphasized that the rate of D-H
-
exchange in acid-catalyzed hydrolysis of BH4 is much slower
than the rate of hydrolysis, as demonstrated by hydrolysis of
-
-
BH4 in D2O, or BD4 in H2O, in which mainly HD (90-95%)
2
7
is evolved. This outcome is in sharp contrast with reactivity
of NC-BH3 and Me3N-BH3 wherein the rates of D-H
exchange are much greater than their rates of hydrolysis being
3
.2.3. NMR Kinetics Measurements with NaBD4 in Alkaline
-
H2O and in D2O. Deuterium/hydrogen exchange was also
observed when the reaction was carried out in solution 3.
However, using NaBD4, the initial multiplet extends over so
wide a frequency domain (more than 100 Hz in the early part
of the process) as to hinder the analysis of the single scrambled
-28
1
5-fold in the case NC-BH3
and even more for Me3N-
29
-
BH3, allowing the preparation of NC-BD3 and R3N-BD3
derivatives by treatment of hydrogenated hydride reagents in
+
D3O /D2O at relatively low pD values. The above examples
-
H/D hydrides BHnD4-n (n ) 1, 2, 3, 4); even if they appeared
demonstrate that any change in the coordination sphere of boron
results in a dramatic change in the kinetics of both hydrolysis
and D-H exchange in borohydride complexes. In the palladium-
catalyzed hydrolysis discussed here, borohydride is thought to
interact with the metal surface producing a surface-linked
1
1
in the B NMR spectra one after another, their NMR signals
were heavily overlapped. Hence, the evaluation of the kinetic
parameters of all the exchange processes was not practicable,
but nevertheless, the hydrolytic rate constant was obtained by
nonlinear least-squares regression fitting of the rate of disap-
pearance of the overall unresolved multiplet belonging to the
borohydride complex.
Through a reversible dissociative chemisorption of BH4-,
-
BHnD4-n species and from the rate of formation of the end
palladium atoms should produce the surface intermediate species
-
product borate B(OH)4 . The rate constant thus obtained was
-
Pd-BH3 and Pd-H hydride, as shown in Figure 5 and eqs
-
-3 -1
-2
khyd(BD4 /H2O) 3.3 ( 0.5) × 10
s
M , which is about
2
-6
-
the same as the above-discussed khyd(BH4 /H2O). In such a
system, the t1/2 was estimated by the crossing point between
hydrides and borate curves to be (300 ( 10) s. Isotope effect
due to B-H(D) bond breaking seems therefore not effective.
BH4- + 2Pd h BH Pd + PdH
-
(2)
(3)
3
PdH + D O h PdD + HDO
2
-
Finally, the rate constant of the BD4 /D2O reacting system
-
(
solution 4) was evaluated to be khyd(BD4 /D2O) ) (7.7 ( 0.2)
-
-
BH Pd + PdD h BH D + 2Pd
(4)
3
3
-
4
-1
-2
×
10
s
M
with a t1/2 of (1490 ( 20) s, leading to an
estimated solvent isotope effect of about (4.5 ( 0.4).
-
-
BH Pd + D O f BH OD + PdD
(5)
(6)
3
2
3
3
.2.4. Mechanistic Considerations. So far, we have demon-
strated that the hydrolysis of sodium borohydride on Pd/C obeys
PdH + PdD f 2Pd + HD
-
the rate law -d[NaBH4]/dt ) kobs[BH4 ], with an activation
-
1
energy of 28.5 kJ mol . Moreover, the observed rate constant
for the hydrolysis of the corresponding perdeuterated analogue
Addition of the species involved in the steps in eqs 2-4 leads
-
-
to the overall exchange reaction BH4 + D2O h BH3D
+
-
BD4 in H2O has been found, within experimental error, to
HDO, while considering steps 2, 5, and 6, we obtain the
hydrolysis reaction BH4 + D2O f BH3OD + HD. The
intermediate BH3(OH) undergoes further hydrolysis with a
-
-
-
assume the same value as that of BH4 in H2O. This suggests
-
that the cleavage of the B-H bonds in all species along the
reaction coordinate does not contribute to the rate-determining
step. Moreover, the nondetectability in our NMR spectra of any
-
faster rate than BH4 itself.
-
Whereas the boron atom in Pd-BH3 undergoes irreversible
-
partially hydrolyzed intermediates of the type BH4-n(OH)n
reductive attack by H2O leading to the monohydroxy borohy-
-
indicates that their lifetime is short on the NMR time scale.
dride BH3OH and to the metal hydride Pd-H, the quenching
-
Finally, H/D exchange among BHnD4-n (n ) 0, 1, 2, 3, 4)
of Pd-H affords H2 and a free palladium active site. On the
other hand, when the reaction is carried out in D2O, the Pd-D
-
-
has been detected both in the BH4 /D2O and in the BD4 /H2O

Pd-Catalyzed Hydrolysis of NaBH4
J. Phys. Chem. B, Vol. 110, No. 34, 2006 17031
Figure 5. Proposed mechanism for the Pd-catalyzed hydrolysis of BH -
4
2
in D O. Numbers 1, 2, 3, and 4 refer to eqs 2-5.
species (that can be produced both by nucleophilic attack of
the oxygen atom on the boron atom of the palladium-coordinated
PdBH3 species (eq 5) and/or by H/D exchange of D2O (eq 3)
significance and implications for many metal-catalyzed hy-
drolysis of sodium borohydride and its analogues. From such
an energy profile, we can appreciate that under thermodynamic
constraints (for example, in the case of high temperature or low
activation energy barriers) the strong exothermicity of the
hydrolysis route would drive the reaction irreversibly toward
borate and hydrogen evolution, whereas H/D exchange could
not be observed. If, however, the reaction is carried under kinetic
conditions, a fast equilibration among scrambled hydrides could
occur without opening the hydrolysis channel, since the activa-
tion barrier of exchange Ea(exc) could be lower than Ea(hyd).
-
on the PdH species) can either introduce the D isotope into the
-
-
Pd-BH3 species affording PdBH2D and/or lead to HD via
reaction with PdH, thus regenerating the required free form of
the catalyst. A primary kinetic solvent isotope effect will be
observed if a proton (deuteron) is transferred from the solvent
in the rate-determining step of the reaction.
Since a significant solvent isotope effect on the hydrolytic
rate constant has been observed when the reaction is carried
out in D2O (k(H2O)/k(D2O) ≈ 4.5), the rate-determining step
must involve the breaking of the O-D bond both in the
hydrolysis and in the exchange route. On the other hand, no
isotope effect on the hydrolysis rate has been detected within
-
In such conditions, it would be possible to obtain pure BD4
by exploiting the strong mass law effect of the deuterated solvent
-
(D2O). The cyanoborohydride BH3CN fulfills such a require-
ment, and we can anticipate here that, by using the same reaction
-
-
the reacting system BD4 /H2O, indicating that a fast chemi-
conditions as for the BH4 /D2O system, no hydrolytic products,
11
sorption of the hydride into the palladium surface is occurring
and that the stronger B-D bond breaking does not occur in the
rate-determining step of the hydrolysis.
but only H/D exchanged hydrides, can be detected in B NMR
-
measurements of the BH3CN /D2O system. As already dis-
-
cussed, the Pd-catalyzed hydrolysis of BH4 in D2O is far from
All this evidence seems to suggest a reaction course as
such an extreme state; it behaves as an intermediate case
affording in the reaction course both hydrolysis and exchanged
products but with slightly higher rates for the exchange
elementary steps.
-
showed in Figure 5 where the intermediates PdBHnDn (n + m
)
3; n ) 0, 1, 2, 3) represent the active substrates for both the
hydrolytic and exchange routes playing the same role of the
neutral BHnDm (m + n ) 5; n ) 0, 1, 2, 3, 4, 5) species in the
Whatever the true mechanism, palladium hydride species are
thought to play a main role both in the exchange and in the
hydrolytic routes. Although the properties of transition metal
hydrides have been (and are) of great interest, there is still a
lack of basic information on metal-hydrogen (in particular on
Pd-H) energetics. The simplest systems would be the diatomic
MH molecules, which however are formed only in the gas phase
and are usually transient species. By analyzing measured
homolytic bond energies of such transition metal hydrides, it
has been found that the average M-H bond energy is higher
for metals with high electronegativity, with low promotion
energy from the ground state to the reactive valence state, and
with high M-M bond energy. The latter is often unknown
but can be estimated from the heat of metal atomization
enthalpy, which represents the strength of the corresponding
metal-metal bond. Following these lines, it is expected that
Pd-H bond energy is lower of that of Ni-H, and even much
lower than Pt-H. Another important chemical property that can
be accounted for by the above-cited thermodynamic functions
is the hydridic-protonic preference of a given transition metal
hydride in a given solvent, for example, water. A metal hydride
has a relevant hydridic character only if its electronegativity is
very low, and thus, for example, the Cu-H bond is considered
hydridic, whereas Au-H is essentially protonic. On these
grounds, we can evaluate that, whereas the Pd-H bond is
essentially hydridic (therefore, PdH + H2O leads to reversible
equilibrium with PdOH + H2 products and PdH + D2O to
PdOD + HD through H/D exchange), Pt-H is expected to be
-
acid-catalyzed process. Since in the BH4 /D2O reacting system
we have observed that the H/D scrambling is faster than the
corresponding hydrolytic process, the activation energy of the
latter must be higher than that of the former.
-
A schematic energy profile for the BH4 /D2O reacting system
that summarizes all our data is reported in Figure 6.
The thermochemical values used to place the scrambled
-
intermediates relative to starting BH4 are calculated from
theoretical ab initio calculations (Table 3) as enthalpy differ-
-
1
ences. Thus, for instance, the reported +8.7 kJ mol (endo-
-
thermic process) in correspondence to BD4 species is the
3
0
calculated thermally averaged enthalpy difference of the reaction
-
-
BH4 + 4D2O a BD4 + 4HDO. The energy levels of other
scrambled intermediates have also been placed according to the
-
enthalpy difference value of the reactions BH4 + nD2O a
-
-1
-
BH4-nDn + nHDO leading to +2.1 kJ mol (BH3D
+
-1
-
-1
-
HDO), 4.2 kJ mol (BH2D2 + 2HDO), 6.6 kJ mol (BHD3
-
1
-
+
3HDO), and 8.7 kJ mol (BD4 + 4HDO). Concerning the
hydrolysis products, the same approach has been used to
-
-1
calculate the relative energy of BH3OD (-29 kJ mol ,
exothermic process) as the enthalpy difference of the reaction
-
-
BH4 + D2O a BH3OD + HD and of the end products
-
-1
B(OD)4 (-292 kJ mol ) as the enthalpy difference of the
-
-
overall hydrolytic reaction BH4 + 4D2O a B(OD)4 + 4HD.
The relative position of intermediates and transition states are
instead purely hypothetical but somehow account for all kinetics
observations. We believe that such a scenario has wide

17032 J. Phys. Chem. B, Vol. 110, No. 34, 2006
Guella et al.
4 2 n 4-n
Figure 6. Schematic reaction coordinate diagram for the BH O reacting system. The thermochemical values used to locate products (BH D
-/D
-
-
+
nHDO) relative to starting reagent BH
starting materials, intermediates, transition states, and products are purely hypothetical but in agreement with all kinetic observations.
4
+ nD O are from DFT-B3LYP/6-311++G(d,p) ab initio calculations. The broken lines connecting
2
protonic due to the higher electronegativity of Pt with respect
to Pd atoms. Our preliminary results on Pt/C-catalyzed hydroly-
sis of NaBH4 not only suggest that the rate constant for the H2
production is higher than that measured here for the Pd/C
catalyst but also that the hydrolysis rate constant is at least 1
order of magnitude higher than the H/D scrambling processes.
Our investigations will be focused on such a topic in the future
in order to better understand the atomic/molecular aspects
affecting the overall catalysis.
whereas B-H coupling constants are about 12 Hz) but also
reliable kinetic data could be obtained. We can anticipate here
that such a case is not restricted to palladium-supported
catalyzed reactions. It is worthy of note that the opportunity to
follow the reaction course of any chemical process with NMR
detection offers great advantages with respect to other analytical
techniques , since it is possible to have both quantitative data
and precise structural identification of eventual intermediates
such as, in our case, the partially deuterated borohydride species.
By fitting the kinetics NMR data with the numerical solutions
of the appropriate differential equations describing a given
reaction system, we have evaluated the rate constants of the
elementary steps of the palladium-catalyzed borohydride hy-
Conclusions
We have exploited the NMR-suitable properties of palladium
atoms (low magnetic moment) in order to develop a very
effective methodology for analyzing kinetics aspects of the Pd-
-
drolysis. Our results highlight the following: (i) BH4-nDn but
-
not HnB(OD)4-n (n ) 1, 2, 3) are the only long-living
catalyzed hydrolysis of NaBH4. Such a methodology mainly
relies on 11B NMR on the heterogeneous mixture given by solid
intermediates (at least on the NMR time scale), thus indicating
that the latter undergo hydrolysis at a faster specific rate than
catalyst dispersed in the hydride’s aqueous alkaline solution
-
without recourse to any workup. In particular, 1 B- H NMR
1
1
BH4 itself; (ii) B-H bond cleavage does not contribute to the
rate-determining step, since no substrate isotope effect can be
observed; (iii) since solvent isotope effect is present, the rate-
determining step must involve the breaking of the water O-H
bond both in the hydrolysis and in the exchange route; and (iv)
decoupled spectra acquired with Hahn spin-echo pulse se-
quence have been proven here to be a vast source of information
on the reaction path and very useful for a complete kinetic
analysis substantiated by substrate and solvent isotope effects.
NMR spectra have been taken continuously during the catalytic
-
since in the BH4 /D2O reacting system we have observed that
-
the H/D scrambling is faster (at least in the early part of reaction
course) than the corresponding hydrolytic process, the activation
energy of the latter must be slightly higher than that involved
in the exchange process.
conversion of the BH4 hydrogen/deuterium mixtures in order
to follow the kinetics of the reaction by recording all the detected
NMR signals as a function of time.
To the best of our knowledge, such an approach has never
been attempted, possibly due to the common (and correct)
opinion that high-resolution NMR measurements can be per-
formed only on homogeneous samples and in the absence of
paramagnetic species. As a matter of fact, we were quite
surprised to realize that, at least when the metal catalyst does
not possess a significant magnetic moment, not only NMR
spectra with good resolution (line width in the range 2-4 Hz,
More generally, the role played by the metal in the hydrolysis
+
seems quite different from the role played by H in the acid-
catalyzed process. Whereas in the latter the H2 (and Borax)
production is induced by a rate-determining step wherein a
proton attacks the boron atom to give the neutral intermediate
BH5, which in turn affords H2 and reactive BH3 species, in
alkaline solutions the activation of the substrate toward hy-

Pd-Catalyzed Hydrolysis of NaBH4
J. Phys. Chem. B, Vol. 110, No. 34, 2006 17033
drolysis is caused by electron-withdrawing and electron-
releasing effects of the palladium atoms on the catalyst surface
via the putative chemiadsorbed species Pd-BH3- and Pd-H.
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Acknowledgment. We acknowledge Damiano Avi for Micro
GC measurements. This work was granted by the Department
of Physics, University of Trento, in the framework of the italian
hydrogen-FISR program.
9
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Supporting Information Available: A complete description
of the differential equations corresponding to each reaction
scheme. This material is available free of charge via the Internet
at http://pubs.acs.org.
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