Polyol synthesis of palladium hydride: Bulk powders vs. nanocrystals

Article abstract of DOI:10.1039/b902024a

Palladium is converted into palladium hydride, β-PdH_x, in polyol solution using NaBH₄ as a hydrogen source; nanocrystalline Pd reacts to form PdH_x faster and at lower temperatures than bulk Pd, and can be made to release h

Full text of DOI:10.1039/b902024a

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Polyol synthesis of palladium hydride: bulk powders vs. nanocrystalsw
Ting-Hao Phan and Raymond E. Schaak*
Received (in Berkeley, CA, USA) 30th January 2009, Accepted 8th April 2009
First published as an Advance Article on the web 1st May 2009
DOI: 10.1039/b902024a
Palladium is converted into palladium hydride, b-PdH
solution using NaBH as a hydrogen source; nanocrystalline Pd
reacts to form PdH faster and at lower temperatures than bulk
Pd, and can be made to release hydrogen to regenerate Pd.
x
, in polyol
merges the polyol process, one of the more desirable chemical
1
4
4
methods for nanocrystal synthesis, with metal hydrides for
applications in hydrogen storage.
x
x
In a typical synthesis, bulk PdH powder was formed by
dispersing 20 mg of commercially-available submicron Pd
powder (0.25–0.55 mm, Alfa-Aesar) in 20 mL tetraethylene
glycol (TEG), followed by heating to 90–210 1C and then
adding 3 mL of a fresh solution of NaBH in TEG (0.4 M)
4
15
with stirring for 2 min. Pd nanocrystals were synthesized by
simultaneously injecting separate ethylene glycol (EG) solu-
Palladium hydride, PdH
for hydrogen-based fuel systems. Because Pd readily dissociates
and absorbs hydrogen into its lattice, it can function as a
x
, is a well-studied prototype material
H
2
1
hydrogen storage material, a hydrogen dissociation catalyst
2
for activation of other hydrogen storage materials, and a
hydrogen-permeable membrane for transport, separation, and
3
purification. PdHx is also interesting for its composition-
2 4
tions of Na PdCl (3.3 mL, 0.14 M) and poly(vinylpyrrolidone)
(PVP, MW = 55 000; 3.3 mL, 0.21 M) into 7 mL of hot EG
(typically 110 1C). The Pd nanocrystals were isolated, washed,
and re-dispersed in TEG along with additional PVP, and this
solution was injected into heated TEG at 90–190 1C, followed
4
dependent superconducting properties, as well as its applica-
5
tions in sensing and catalysis. PdH
x
most commonly exists
in two forms: a-PdH
x
(fcc, 0 o x o 0.02) and b-PdH
(tetragonal, x = 1.33)
x
6
(
fcc, 0.58 o x o 1), although g-PdH
x
by 3 mL of a 0.4 M TEG solution of NaBH .
4
7
has also been claimed to form by ion implantation. Palladium
hydrides are typically formed by gas-phase hydrogen absorp-
tion, high-pressure hydrogen insertion, and electrochemical
Fig. 1 shows powder X-ray diffraction (XRD) data for bulk
(0.25–0.55 mm) and nanocrystalline (6–15 nm) Pd reacted with
NaBH in TEG for 2 min at different temperatures. The bulk
4
1
,8
reactions with palladium. Solution chemistry methods can
and nanocrystalline Pd samples were both found to have
˚
lattice constants of a = 3.89 A, which matches the literature
also produce PdH , for example during the aqueous reduction
x
2
+
9
16
values. It is known that a-PdH
˚
of Pd at 100 1C.
x
has a E 3.895 A and
˚
An alternative and mild solution chemistry method, intro-
duced by Murphy et al., is to react bulk Pd, as well as bulk-
scale hydrogen-absorbing intermetallic compounds, with
b-PdH
x
has a E 4.025 A, with concentration-dependent
6
˚
variation from B4.02 to 4.08 A. Because of the composition-
dependent lattice expansion, XRD is a useful tool for charac-
1
0
aqueous borohydride to form hydrides. This study very
effectively demonstrated the versatility and generality of low-
temperature ambient-pressure chemical methods for hydrogen
insertion into metals and intermetallics. While this earlier
work focused on bulk-scale materials, applying the boro-
hydride method to nanoscale metals and intermetallics, with
PdHx as a prototype system, is intriguing. For example,
nanoscale hydrides are known to have faster reaction kinetics
and lower desorption temperatures and activation energies
x
terizing the formation of PdH . Accordingly, as the reaction
temperature increases, the XRD pattern for bulk Pd changes
to a multi-phase fcc sample that is consistent with lattice
expansion due to hydrogen absorption. A GC with a thermal
conductivity detector confirmed the presence of hydrogen in
the PdH
x x
products: after heating bulk PdH in a closed
1
b,11
than their bulk analogs,
2
as well as size-dependent hydrogen
1
contents. Also, non-equilibrium phases can be stabilized
in nanocrystalline solids prepared through low-temperature
1
3
solution routes, which suggests that similar approaches for
hydrides could lead to the discovery of new hydrogen storage
materials. In this work, we describe a facile polyol-based
chemical route for converting Pd powders and nanocrystals
x
into PdH , experimentally demonstrating ambient-pressure
solution-phase hydrogen absorption, storage, and release in
a prototype metal hydride system. This study effectively
Fig. 1 Powder XRD patterns for (a) bulk Pd and (b) nanocrystalline
Pd reacted with NaBH in TEG at the temperatures indicated.
Complete conversion to PdH occurs at 190 1C for bulk Pd and at
20 1C for nano-Pd. Vertical dashed lines indicate the peak positions
for Pd.
Department of Chemistry and Materials Research Institute,
The Pennsylvania State University, University Park, PA 16802, USA.
E-mail: schaak@chem.psu.edu
w Electronic supplementary information (ESI) available: Complete
experimental details and GC data. See DOI: 10.1039/b902024a
4
x
1
3
026 | Chem. Commun., 2009, 3026–3028
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system, hydrogen gas was detected in the headspace, while
heating bulk Pd under identical closed-system conditions
showed no hydrogen. This confirms that the lattice expansion
observed by XRD is attributable to hydrogen absorption.
˚
At 190 1C, a nearly phase-pure fcc pattern with a = 4.040 A
is evident. This corresponds to b-PdH with x E 0.7, based on
6e
x
comparison with literature reports. Heating to higher
temperatures in solution begins to liberate hydrogen, returning
the sample to predominantly Pd. Nanocrystalline Pd shows a
similar trend, reacting with borohydride under identical con-
˚
x
ditions to form b-PdH (a = 4.037 A), followed by gradual
release of hydrogen as the temperature increases. However,
most of the nanocrystalline Pd is converted to PdHx at
Fig. 3 Powder XRD patterns showing hydrogen release for bulk and
temperatures as low as 90 1C, with phase-pure PdH
x
observed
nanocrystalline PdH
x
. For bulk PdH
x
(bottom), heating to 180 1C
with a smaller
releases some of the hydrogen, generating PdH
x
at 120 1C. Thus, nanocrystalline Pd reacts with NaBH4 in
solution to form a hydride at a significantly lower temperature
than bulk Pd under otherwise identical reaction conditions.
In addition to forming a hydride at lower temperatures,
nanocrystalline PdHx forms faster in solution than its bulk
analog. Fig. 2 shows powder XRD data for the reaction of
hydrogen content along with some Pd. For nano-Pd (top), heating
to 180 1C releases all of the hydrogen, generating only nano-Pd as a
product. Vertical dashed lines indicate the peak positions for Pd.
Nanocrystalline PdHx can also release the hydrogen to
re-form Pd at lower temperatures than bulk Pd. Fig. 3 shows
powder XRD data for the purest attainable samples of bulk
4
bulk and nanocrystalline Pd with NaBH in TEG at 90 1C.
After 10 min, bulk Pd had begun to absorb hydrogen, as
evidenced by the emergence of shoulders to the left of the 111
and 200 peaks of Pd. Based on quantitative analysis of
the phase fractions, only 43% of bulk Pd has converted to
and nanocrystalline PdH
80 1C for 4 h in a tube furnace under Ar. Under these
conditions, bulk Pd had begun to release hydrogen to form
x
, as well as these samples heated to
1
˚
a PdH
heating vs. a = 4.04 A before heating), as well as a small
amount of Pd. In contrast, nanocrystalline PdH has released
x
phase with lower hydrogen content (a = 4.00 A after
x
b-PdH . After 60 min, slightly more of the sample has been
˚
converted to b-PdH (54%). However, the reaction is not yet
x
x
complete, possibly because of decomposition and subsequent
lack of availability of the NaBH4 during the long reaction
time. The solution is replenished with a fresh solution of
almost all of the hydrogen under these conditions, as indicated
by the XRD pattern showing nearly phase-pure Pd without
evidence of PdH . The hydrogen can also be released in
x
NaBH
increase, reaching a maximum of 74%. However, conver-
sion to PdH is not complete at 90 1C. In contrast, 91% of
the nanocrystalline Pd has converted to b-PdHx after only
min of reaction with NaBH in TEG at 90 1C, with almost
phase-pure b-PdH formed within 10 min.
4
x
, and the phase fraction of b-PdH again begins to
solution by heating to higher temperatures, e.g. 270 1C in
1
-octadecene. Differences between PdH prepared using tradi-
x
x
1
tional methods and those made in polyol solution could arise
from some solvent decomposition and subsequent carbon
inclusion, either coating the surface or intercalated into the
structure, or interstitial boron from the borohydride.
1
4
x
Fig. 4a shows a TEM image and selected area electron
diffraction (SAED) pattern for the Pd nanocrystals that were
synthesized as described earlier. These nanocrystals were then
isolated, washed several times, and re-dispersed in fresh TEG
before adding NaBH and heating to 120 1C to form PdH .
4
x
The TEM image in Fig. 4b shows that the resulting PdHx
nanocrystals are similar in size and shape to the Pd nano-
crystal precursors. Quantitative analysis of the SAED patterns
for PdH
x
confirms that the nanocrystals correspond to
˚
the hydride: a = 3.9 A for the Pd nanocrystals in Fig. 4a
˚
and a = 4.1 A for the PdH
The general size and morphology of the Pd nanocrytals are
retained after absorbing hydrogen to form b-PdH , as well as
x
nanocrystals in Fig. 4b.
x
after releasing hydrogen to re-form Pd. Fig. 4c shows a TEM
image of a sample of Pd nanocrystals and Fig. 4d shows this
Fig. 2 Powder XRD patterns showing the time-dependent reactivity
same sample after conversion to b-PdH . The nanocrystals
x
of (a) bulk Pd and (b) nano-Pd. In (a), bulk Pd was reacted with
remain similar in size and morphology upon heating to 180 1C
for 4 h to release the hydrogen and regenerate Pd (Fig. 4e).
This indicates that the hydriding and dehydriding reactions
can be carried out on pre-formed Pd nanocrystals and proceed
with general retention of size and morphology. It is likely that
the PVP stabilizer helps to prevent significant aggregation, as
NaBH
added and reacted for an additional (iii) 10 min and (iv) 60 min. In (b),
nanocrystalline Pd (i) was reacted with NaBH in TEG for (ii) 1 min,
iii) 5 min, and (iv) 10 min, then (v) an additional 10 min after adding
4 4
in TEG for (i) 10 min and (ii) 60 min, then fresh NaBH was
4
(
4
fresh NaBH . Vertical dashed lines indicate the positions of the 111
and 200 peaks for Pd.
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This work was supported by the US Department of
Energy (DE-FG02-08ER46483), the Petroleum Research
Fund (administered by the American Chemical Society), a
DuPont Young Professor Grant, a Beckman Young Investi-
gator Award, a Sloan Research Fellowship, and a Camille
Dreyfus Teacher-Scholar Award. Electron microscopy was
performed at the Electron Microscopy Facility at the
Huck Institutes for the Life Sciences at Penn State. The
authors thank Seung-Hyun Anna Lee for help with GC data
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028 | Chem. Commun., 2009, 3026–3028
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