The synthesis of metal phosphides: Reduction of oxide precursors in a hydrogen plasma

Article abstract of DOI:10.1002/anie.200801559

(Equation Presented) A NiPpy phosphide synthesis: Nickel phosphides such as Ni₂P and Ni₃P are synthesized stoichiometrically from oxide precursors in a nonthermal hydrogen plasma at atmospheric pressure and low temperature in 60 min (see scheme). This energy- and time-efficient approach can also be applied to the synthesis of other metal phosphides such as GaP, InP, MoP, and WP.

Full text of DOI:10.1002/anie.200801559

Communications
DOI: 10.1002/anie.200801559
Plasma Chemistry
The Synthesis of Metal Phosphides: Reduction of Oxide Precursors in a
Hydrogen Plasma**
Anjie Wang,* Minglei Qin, Jie Guan, Li Wang, Hongchen Guo, Xiang Li, Yao Wang, Roel Prins,
and Yongkang Hu
Phosphorus can react with most elements of the periodic table
to form different classes of phosphides, ranging from ionic for
the alkali and alkaline-earth elements to metallic or covalent
for the transition elements and covalent for the main-group
elements. Among them, InP and GaP (III–V semiconductors)
have various applications in telecommunications, optoelec-
oxide precursors to phosphides is neither thermodynamically
nor kinetically favorable. Since the formation of Ni P from
2
the oxides by means of the temperature-programmed reduc-
tion (TPR)method (Scheme 1)is thermodynamically unfav-
[
1,2]
tronic devices and solar cells,
while transition-metal
phosphides are attractive candidates for high-performance
catalytic, electronic, and magnetic applications.
[
3]
Scheme 1. A simplified equation for the synthesis of Ni Pfrom its
2
oxide precursors by H reduction.
A variety of methods for synthesizing bulk metal phos-
phides, including direct reaction of the appropriate elements
for prolonged periods at high temperature, reaction of
phosphine with metals or metal oxides, reduction of metal
phosphates by hydrogen, electrolysis of molten metal phos-
phate salts, solid-state metathesis, thermal decomposition of
single-source precursors, and self-propagation high-temper-
2
orable, the forward reaction has to be aided by high temper-
[
8]
ature and low water vapor pressure. Thus, Ni P can only be
2
À1
obtained at a low heating rate (e.g. 18Cmin )and a high H
2
flow velocity to purge the water (by-product)off the solid
surface. The forward reaction is slow because the H2
molecules must be split into hydrogen atoms, therefore the
metal oxide must first be reduced and then spilt-over
hydrogen atoms can reduce the phosphorus oxide, followed
by a solid-state reaction to form the metal phosphide. As a
[
4–6]
ature synthesis, are known.
These methods typically
require extremely high reaction temperatures (sometimes
above 10008C)and/or long reaction times. Metal phosphides
can be obtained under milder conditions in a solvothermal
approach, but this method is not feasible for the deposition of
metal phosphides on supports, and in cases where yellow
phosphorus and sodium are employed, care must be taken to
consequence, Ni P can only be prepared by means of the TPR
2
method above 5508C.
[
7]
ensure the rigorous absence of oxygen and water.
Only two methods, namely the reduction of metal
Herein we describe a new strategy for synthesizing metal
phosphides that uses nonthermal H plasma as the reduction
2
[
4]
phosphates by H2 and the phosphidation of metals or
medium instead of the H used in the TPR method. High-
2
[
5]
metal oxides with PH /H , are generally feasible for the
energy electrons collide inelastically with hydrogen molecules
in the plasma and transfer their energy to the latter, which
leads to the production of excited hydrogen species and ions
with a significantly higher reduction ability than molecular
3
2
preparation of supported transition-metal phosphides for use
as hydrotreating or hydrogenation catalysts. Supported metal
phosphides are usually prepared by the reduction method due
[
9]
to the high toxicity of PH . Nevertheless, the conversion of
hydrogen. These reactive species can reduce both the metal
oxide precursor and the phosphorus oxide, therefore oxide
precursors can be converted into metal phosphides in the H2
plasma under very mild conditions. Both bulk and supported
metal phosphides are accessible in this approach.
3
[*] Prof. Dr. A. Wang, M. Qin, J. Guan, L. Wang, Prof. H. Guo, X. Li,
Y. Wang, Prof. Y. Hu
State Key Laboratory of Fine Chemicals
Department of Catalytic Chemistry and Engineering
Dalian University of Technology
The XRD patterns of the bulk products synthesized from
nickel and phosphorus oxides at different reduction times
(Figure 1)indicate that Ni P crystals begin to form in the H
Dalian 116012 (P.R. China)
2
2
Fax: (+86)411-8899-3693
E-mail: ajwang@dlut.edu.cn
plasma within 10 min. This reaction time is very much shorter
than that used in the traditional TPR reduction method (over
[4]
Prof. Dr. R. Prins
Institut für Chemie- und Bio-Ingenieurwissenschaften
ETH Zürich
1
0 h).
No external heating was applied during the reduction.
Because a thermocouple cannot be used in the presence of
plasma, the temperature in the bed could not be measured
directly during the reduction. A temperature rise due to the
elastic collisions of high-energy electrons with the reactant
molecules will take place, but this rise is not likely to be
significant because the total power input is generally around
24–32 W in the steady state. A temperature-distribution
Wolfgang-Pauli-Strasse 10, 8093 Zürich (Switzerland)
[
**] This work was financially supported by the NSFC (20333030,
0503003, and 20773020), the Education Ministry of China
20030141026), the NCET, 111 Project, and the CNPC Innovation
2
(
Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801559.
6
052
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Angewandte
Chemie
Figure 1. XRD patterns of the bulk Ni Psamples synthesized by the H
2
2
plasma reduction method at various times. The atomic ratio of Ni/Pin
the precursors was 2.0. The total input of the AC power was
Figure 2. XRD patterns of the nickel phosphides synthesized from
oxide precursors with different Ni/Patomic ratios in H 2 plasma for
0 min. The total input of the AC power was 60 V”0.4 A. Gray:
7
0 V”0.35 A.
6
simulated patterns.
measurement across the reaction bed, using an IR imager,
showed that the bed temperature was around 1508C (Fig-
ure S2 in the Supporting Information). This indicates that the
enhanced reduction can be ascribed to irradiation by the cold
plasma.
activity than that prepared by the TPR method (Table 1).
Moreover, the PR-prepared catalyst remained active and
stable over a period of at least two days (Figure S3 in the
+
A large number of active hydrogen species, such as H, H ,
+
+
H , and H , exist in the H plasma, and these species possess
2
3
2
Table 1: CO chemisorption and HDS catalytic performance of Ni P/
[
9]
2
much higher reduction abilities than molecular H2. It is
therefore not necessary to heat the oxide precursors to
elevated temperatures during the reduction. Moreover,
plasmas can be used to clean the surface of solids, therefore
this surface cleaning ability may accelerate the desorption of
the by-product water molecules, thereby facilitating the
forward reaction in Scheme 1.
MCM-41 prepared by different reduction methods.
À1
Method
CO uptake
DBT conversion [%]
TOF [s
]
À1
[mmolg
]
[
10]
2808C
3008C
2808C
3008C
TPR
PR
21
29
44
62
68
89
0.0066
0.0068
0.01
0.01
The synthesis of metal phosphides by the TPR method
requires the use of excess phosphorus in the oxide precursors
because phosphorus loss occurs due to migration of volatile
Supporting Information). In agreement with the difference in
catalytic activity, the PR sample showed a significantly higher
CO chemisorption uptake, which is a measure of the number
[
11,12]
phosphorus species in the high-temperature reduction.
Both Ni P and Ni P are formed stoichiometrically in the H
2
2
3
plasma reduction, as evidenced by the XRD patterns of the
nickel phosphides synthesized from oxide precursors with
different Ni/P atomic ratios (Figure 2).
of accessible active HDS sites of the Ni P catalysts, than the
TPR sample (Table 1). Similar results were obtained for
2
[11]
bulk Ni P (Table S1 in the Supporting Information).
2
The very low heating rate that must be used in the
conventional TPR method implies that the reduction is the
rate-limiting step. The surplus volatile phosphorus species
generated in H may also be purged off in the high-velocity H
No XRD peaks were observed for either TPR- or PR-
prepared Ni P/MCM-41 with a low loading level (Figure S4 in
2
the Supporting Information), thus indicating that the Ni P
2
particles are small. The TEM images (Figure 3)confirmed
this but did not indicate a difference in the particle size
2
2
flow, thus leading to a loss of phosphorus. The reduction is
significantly enhanced in a H plasma, however, therefore it is
2
possible to synthesize the metal phosphides stoichiometrically
without appreciable phosphorus loss. Indeed, when phospho-
rus is used in excess it might form volatile phosphorus species,
which adsorb on the metal phosphide sites and block the
catalytic reaction.
Supported transition-metal phosphides are used as novel
[4,5]
hydrodesulfurization (HDS)catalysts.
We decided to use
mesoporous MCM-41 to support Ni P and tested its perfor-
2
mance in the removal of sulfur from dibenzothiophene
(
DBT)in the HDS reaction. The conventional TPR method
was used to prepare a Ni P/MCM-41 sample for comparison.
2
The Ni P/MCM-41 sample prepared by the plasma reduction
2
Figure 3. TEM images of Ni P/MCM-41 with a total loading of 20 wt%
2
(
PR)method was found to exhibit a substantially higher HDS
prepared by different reduction methods: a) PR, b) TPR.
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Communications
between the two supported catalysts. Distinct XRD peaks Experimental Section
were observed, however, for both TPR- or PR-prepared Ni P/
The bulk oxide precursors were prepared by co-precipitation from
Ni(NO and (NH HPO by adjusting the pH of the aqueous
2
MCM-41 when the loading was increased to 40 wt% (Fig-
ure S4). All these peaks seem to consist of a narrow peak
superposed on a broader peak and the peaks of the PR sample
are much broader. The TEM images (Figure S5 in the
Supporting Information)indicate that the narrower, more
)
3
2
4
)
2
4
solution, followed by calcination in air at 5008C for 2 h. The
supported oxide precursors were prepared by an incipient co-
impregnation and calcination method, using a mesoporous molecular
2
À1
sieve MCM-41 (specific surface area: 1046 m g ; pore volume:
3
À1
[13]
0
.95 m g ; average pore diameter: 3.8 nm)as the support.
oxide precursors were pelletized and sieved to 20–35 mesh. The
synthesis of Ni P was carried out in a dielectric barrier discharge
The
intense peaks in the XRD patterns of the TPR-prepared Ni P/
2
MCM-41 are due to the presence of a larger number of bulk
2
[
14]
(
DBD)reactor
consisting of a quartz tube and two electrodes
Ni P particles in the matrix. The relatively low CO chemis-
2
À1
(Figure S1 in the Supporting Information). The high-voltage elec-
trode was a stainless-steel rod with a diameter of 2 mm, which was
installed in the axis of the quartz tube and connected to an alternating
current supply. The grounding electrode was an aluminum foil, which
was wrapped around the quartz tube and linked to ground by a wire.
One gram of the precursor was charged into the quartz tube and pure
orption data (10 mmolg for TPR-prepared Ni P/MCM-41
and 11 mmolg for PR-prepared Ni P/MCM-41)show that
2
À1
2
blockage of the pore channels of MCM-41 may occur when
the larger bulk Ni P particles are present at a high loading
2
level.
To further prove the feasibility of the H plasma method in
synthesizing metal phosphides, we used this approach to
synthesize other metal phosphides. Figure 4 shows that pure
hydrogen from a gas cylinder was passed through the bed at
2
À1
150 mLmin during the synthesis. The voltage of the high-voltage
electrode was kept at around 12.8 kV and the discharge frequency at
2 kHz. For comparison, an MCM-41-supported Ni P sample was
1
2
prepared by the H TPR method. The obtained Ni P samples were
2
2
passivated with 0.5% O in He prior to exposure to air. The obtained
2
Ni P samples, bulk or supported, were characterized by XRD (Rigaku
2
D/Max 2400). The TEM images were recorded with a Philips Tecnai
G220 instrument operating at 200 kV. The CO adsorption was
measured with a chemisorption apparatus (Quantachrome Chem-
bet-3000)at 30 8C, according to the method described in the
[
15]
literature. The supported Ni P was tested in the HDS of DBT in
2
[
13]
a trickle-bed reactor.
Received: April 3, 2008
Published online: July 4, 2008
Keywords: hydrogen · nickel · plasma chemistry · pnictides ·
.
reduction
Figure 4. XRD patterns of WP, MoP, InP, and GaP synthesized from
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[
[
[
in optoelectronic devices and solar cells. When H is replaced
2
by NH and CH , it may also be possible to synthesize metal
3
4
nitrides and carbides by the plasma approach.
[
[
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6
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