Journal of Alloys and Compounds 448 (2008) 122–127
On the feasibility of phosphide generation from phosphate reduction:
The case of Rh, Ir, and Ag
Christina M. Sweeney, Kimber L. Stamm, Stephanie L. Brock ∗
Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202, USA
Received 17 August 2006; received in revised form 7 October 2006; accepted 10 October 2006
Available online 9 November 2006
Abstract
The feasibility of reductive hydrogen annealing of metal phosphates as a pathway to phosphides has been explored for Rh, Ir and Ag. Phosphate
precursors were prepared by combination of metal salts with ammonium monohydrogen phosphate in water. Rh and Ir precursors were isolated by
solvent evaporation, whereas the reaction with silver acetate yielded Ag3 (PO4 ) as a precipitate. Thermogravimetric analysis was employed using
5% H2 in Ar to ascertain appropriate reduction conditions for phosphide formation. Rh2 P was formed as the sole crystalline phase from slow heating
to 375 ◦ C, although energy dispersive spectroscopy suggests amorphous phosphorus may be present as an impurity. Ir2 P is best prepared from
ramping to 600 ◦ C in an inert atmosphere, followed by hydrogen annealing. Iridium metal is also present as an impurity, apparently arising from
the iridium reagent or in the process of preparing the phosphate precursor. Ag3 (PO4 ) does not yield a phosphide upon treatment with hydrogen,
instead reducing directly to the metal at temperatures as low as 140 ◦ C.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Inorganic materials; Chemical synthesis; Gas–solid reactions; Thermal analysis; X-ray diffraction
1. Introduction
Metal-rich binary transition-metal phosphides are of fundamental and technical importance due to their unique magnetic
and catalytic properties. For example, Rh2 P is a superconductor with a Tc = 1.3 K, whereas Fe3 P is a robust ferromagnet
with a Curie temperature of 716 K [1–3]. Nickel, cadmium, and
rhodium phosphides have been shown to catalyze the hydrogenation of acetylene to ethylene [4], while supported phosphides
of iron (Fe2 P), cobalt (Co2 P), molybdenum (MoP), tungsten
(WP) nickel (Ni2 P), and mixed ternary phases thereof, have
been employed as catalysts in the hydrodesulfurization (HDS)
and hydrodenitrogenation (HDN) of petrochemicals [5–7], with
dispersed Ni2 P exhibiting higher catalytic activity than commercially used supported sulfided molybdenum catalysts [5]. Since
the properties of metal-phosphides are wide ranging and phase
dependent, the methodology used to synthesize these phosphides
must permit formation of single-phase material of targeted stoichiometry. Additionally, for catalytic applications supported
∗
Corresponding author. Tel.: +1 313 577 3102; fax: +1 313 577 8822.
E-mail address: [email protected] (S.L. Brock).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.10.035
phases are often desired, thus the chosen methodology should
be amenable to formation of dispersed nanoscale phosphides.
The synthesis of transition-metal phosphides has been
accomplished utilizing a number of methodologies including
reactions of pure elements, solid-state metathesis, solvothermal
synthesis, and organometallic decomposition routes. Solid state
reactions of the elements tend to be time consuming (days to
weeks) and are heavily dependent on diffusion distances in the
solid state; thus, multiple heating and grinding cycles must be
employed [8,9]. The use of molten media or exploitation of vapor
transport processes using multizoned furnaces can decrease the
heating time and result in well-formed crystals [10]. Solidstate metathesis [11,12] and solvothermal synthesis [13–15] are
faster methods for generating transition-metal phosphides and
can be tuned to prepare materials that are microcrystalline or
nanocrystalline. Finally, the decomposition of organometallic
precursors has been used extensively for the formation of dispersed phosphide nanoparticles within silica matrices [16,17],
or in coordinating solvents [18–23].
A relatively facile method for phosphide formation that is also
employed for generating dispersed supported nanoparticulate
materials is the hydrogen reduction of phosphates to phosphides at elevated temperatures. This methodology has been
C.M. Sweeney et al. / Journal of Alloys and Compounds 448 (2008) 122–127
123
Table 1
Typical phases of selected transition metal phosphides and their physical properties (taken from Ref. [1], unless otherwise noted)
dation state of the metal. An excess of between 10% and 50% phosphate was
employed in each case.
Metal
Phase
Characteristics
Superconductor, Tc = 1.3 K
Metallic
Rh
Rh2 P
Rh4 P3
Rh3 P2 a
RhP2
RhP3
2.2.1. Rhodium phosphate precursor
A suitable precursor for reduction was synthesized by mixing stoichiometric
amounts of hydrated RhCl3 (0.155 g, 7.4 × 10−4 mol, calculated based on the
formula weight of RhCl3 only) and (NH4 )2 HPO4 (0.107 g, 8.1 × 10−4 mol) in
∼40 mL of distilled water for 30 min. The solution was transferred to a roundbottom flask and heated to around 100 ◦ C under dynamic vacuum for 2 h to
produce an orange/red sticky substance.
Ag
AgP2
AgP3
Ag3 P11 b
Semiconductor, diamagnetic
Semiconductor, diamagnetic
Diamagnetic
Ir
Ir2 P
IrP
IrP2
IrP3
Metallic
Antiferromagnetic, TN = 0.35 K
Semiconductor, diamagnetic
Semiconductor, diamagnetic
a
b
Semiconductor, diamagnetic
Semiconductor, diamagnetic
Ref. [32].
Ref. [33].
successfully employed for a wide range of metals, and can also
be used to make heavier pnictides (arsenides, antimonides) [24].
The reaction takes place at temperatures ranging from ca. 400
to 1100 ◦ C and generally takes only a few hours to go to completion. At these temperatures, the H2 is postulated to reduce
phosphorus via initial metal reduction, and the associated protons combine with the oxygen of the pnictate to form water
as a byproduct [24]. The relative ease of preparing supported
phosphate materials (via incipient wetness, or nanoparticle precursor techniques) has made this method the most suitable for
the preparation of catalysts, and the product formed is dependent on both the reduction temperature as well as the chemical
nature of the precursor [5–7,25–27].
In addition to metals for which the phosphates are proven to
reduce to phosphides, Gopalakrishnan has also predicted transition metals for which this approach is likely to be applicable
based on the trend that metals capable of having their metal oxide
phases reduce to the element with hydrogen should also undergo
phosphate precursor reduction to the phosphide upon hydrogen annealing [24]. As a test of this premise, we report herein
the application of the hydrogen reduction methodology towards
phosphides of Rh, Ir, and Ag; all of which meet the criterion proposed by Gopalakrishnan. These metals are employed in a wide
range of catalytic processes, suggesting that the corresponding
phosphides might also show catalytic promise, in addition to
exhibiting a range of compositionally dependent magnetic and
electronic properties, as indicated in Table 1.
2.2.2. Iridium phosphate precursor
A suitable iridium phosphate precursor was prepared by mixing IrCl3 ·3H2 O
(0.456 g, 1.29 × 10−3 mol) and (NH4 )2 HPO4 (0.249 g, 1.89 × 10−3 mol) in
∼40 mL of distilled water. The mixture was concentrated to a tacky green
material by the same methodology applied to the rhodium phosphate
precursor.
2.2.3. Silver phosphate precursor
Ag3 PO4 was prepared by bench top precipitation from the reaction
of stoichiometric amounts of Ag(CH3 COO) (3.651 g, 2.19 × 10−2 mol) and
(NH4 )2 HPO4 (0.999 g, 7.57 × 10−3 mol) in ∼40 mL of distilled water. The
reactants immediately precipitated to form a fine yellow powder that was
vacuum filtered, washed with distilled water, and allowed to dry at room
temperature.
2.3. Physical measurements
2.3.1. Power X-ray diffraction
Powder X-ray diffraction (PXRD) characterization of all precursors and
products was performed using a Rigaku RU 200 B 12 kW rotating anode diffractometer with Cu K␣ radiation (1.54056 Å) at 40 keV and 150 mA. Samples
were ground with a mortar and pestle in a small amount of petroleum jelly and
applied to a quartz (0 0 0 1) sample holder. Using the JADE program, patterns
were compared to known PDF (powder diffraction file) patterns from the ICDD
(International Center for Diffraction Data) database, release 2000.
2.3.2. Thermogravimetric analysis—reduction to phosphide
Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Pyris
1 TGA. Analysis was carried out under 5% flowing H2 in Ar gas using 20–50 mg
of sample in an Al2 O3 pan. Data were acquired up to 900 ◦ C with ramp-rates
varying from 5 to 40 ◦ C/min.
2.3.3. Energy dispersive spectroscopy
Compositional data were gathered on the products of reduction for Rh using
energy dispersive spectroscopy (EDS) in a Hitachi S-2400 scanning electron
microscope (SEM) equipped with an EDAX model detector. After reduction, a
small chunk of the product was isolated for analysis and mounted on an Al stub
using conductive carbon tape.
3. Results
3.1. Rhodium phosphate precursor synthesis and reduction
2. Experimental
2.1. Materials
All chemicals were used as received: silver(I) acetate (Fisher,
Ag(CH3 COO)), rhodium(III) chloride (40.03% Rh Assay, Alfa Aesar,
RhCl3 ·xH2 O), iridium(III) chloride (99.9%, Alfa Aesar, IrCl3 ·3H2 O), and
ammonium monohydrogen phosphate (Fisher, (NH4 )2 HPO4 ).
2.2. Synthesis
Reaction stoichiometries were chosen to reflect the actual (RhPO4 , Ag3 PO4 )
and hypothetical (IrPO4 ) phosphate phases expected based on the starting oxi-
The rhodium phosphate precursor was prepared by mixing
rhodium chloride and ammonium monohydrogen phosphate in
a minimum amount of water, producing an orange/red solution with no evident precipitate. This was condensed under
vacuum and heat until only a tacky substance remained. Powder X-ray diffraction (PXRD) yielded a complex pattern with
a large number of peaks (supporting information) from which
no definitive rhodium or phosphate containing phases could be
identified from the ICDD (International Center for Diffraction
Data) search/match database.
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C.M. Sweeney et al. / Journal of Alloys and Compounds 448 (2008) 122–127
(supporting information). A fast ramp (20–40 ◦ C/min) to 400 ◦ C
yielded a mixture of Rh2 P and Rh (total weight loss: 50%,
supporting information).
The product of the 375 ◦ C reduction was analyzed by energy
dispersive spectroscopy (EDS) in conjunction with scanning
electron microscopy (supporting information). From evaluation
of several spots within the sample, the ratio of Rh:P is found
to vary from 0.80 to 0.98:1. In some cases, the EDS analyses
indicated the presence of a small amount of aluminum, whose
presence is attributed to contamination arising from scraping
the sample from the alumina TGA pan. No other elements were
detected.
3.2. Iridium phosphate synthesis and reduction
Fig. 1. Thermogravimetric analysis of the rhodium precursor with a 5 ◦ C/min
ramp rate to approximately 375 ◦ C, conducted in 5% H2 in Ar with a 1 min
isothermal step at 375 ◦ C.
Analysis of the reduction conditions required for phosphide synthesis was explored by thermogravimetric analysis (TGA). After several attempts to optimize the reduction
conditions, the precursor was reduced using a temperature
scan from 50 to 375 ◦ C at a rate of 5 ◦ C/min followed by a
1 min isothermal step. The data (Fig. 1) show that the precursor undergoes four distinct mass losses with a total loss of
69.3%. The products of reduction were dark grey powders. The
diffraction pattern (Fig. 2) was consistent with the PDF pattern for Rh2 P and there was no evidence of any crystalline
impurities.
Both the ramp rate and the final temperature are found to be
crucial for successful generation of Rh2 P as a single crystalline
phase. Ramping to a final temperature of 250 ◦ C at 10 ◦ C/min
(total weight loss: 25%) yielded a largely amorphous pattern
with weak, broad peaks that could be attributed to Rh metal
Fig. 2. Powder X-ray diffraction pattern of the product from the reduction of the
rhodium phosphate precursor at 375 ◦ C using a ramp rate of 5 ◦ C/min followed
by a 1 min isothermal step at 375 ◦ C. The dotted lines represent the diffraction
pattern of Rh2 P (PDF# 02-1299) and the solid line, Rh metal (PDF# 05-0685).
The iridium phosphate precursor was synthesized analogously to the rhodium material by mixing iridium chloride and
ammonium monohydrogen phosphate in a minimum amount
of water. The mixture formed a green transparent solution,
which was condensed under vacuum until only a sticky material remained. Analysis of the product by PXRD (Fig. 3, bottom trace) revealed a largely amorphous phase with a hump
around 40◦ 2θ, which corresponds to the main peak for iridium
metal.
The precursor was initially reduced in six different trials
from 50 ◦ C to final temperatures ranging from 200 to 700 ◦ C,
at 100 ◦ C increments. The studies were all conducted with a
ramp rate of 5 ◦ C/min by TGA under an atmosphere of 5% H2
in Ar. These analyses of a wide range of temperatures were
conducted in order to observe any trend in mass loss and deter-
Fig. 3. Powder X-ray diffraction pattern of the iridium phosphate precursor and
reduction products from treatment at several different temperatures (ramp rate:
5 ◦ C/min). The dotted lines represent the diffraction pattern of Ir2 P (PDF# 021291) and the solid line, Ir metal (PDF# 06-0598).
C.M. Sweeney et al. / Journal of Alloys and Compounds 448 (2008) 122–127
Fig. 4. Powder X-ray diffraction pattern of the products of iridium phosphate
thermal gravimetric analysis conducted by ramping to temperature in an inert
atmosphere (Ar), followed by treatment in flowing H2 /Ar at temperature for 1 h.
The dotted lines represent the diffraction pattern of Ir2 P (PDF# 02-1291) and
the solid line, Ir (PDF# 06–0598).
125
Fig. 5. Powder X-ray diffraction patterns of Ag3 PO4 and the reduction products
obtained from conducting thermal gravimetric analysis in flowing H2 /Ar to 140,
180, and 200 ◦ C. The solid line represents the diffraction pattern of Ag3 PO4
(PDF # 84-0510), the dashed line, Ag (PDF# 87-0720). The unmarked peaks at
140 and 180 ◦ C can be indexed to Ag2 HPO4 (PDF# 75-1075) and/or Ag4 P2 O7
(PDF# 40-0058).
3.3. Silver phosphate synthesis and reduction
mine the optimum temperature for phosphide formation. Fig. 3
shows the PXRD results for several representative reductions.
The reduction conducted at 200 ◦ C (10% total weight loss),
300 ◦ C (30% total weight loss), and 400 ◦ C (47% total weight
loss) produces crystalline products consistent with the pattern for
iridium metal. The products from the 200 ◦ C reduction are somewhat amorphous (i.e., the peaks are broad) while the products
from higher temperature reactions show a marked sharpening of
the diffraction peaks, consistent with a more crystalline product. The PXRD of the products of the reductions conducted at
500, 600, and 700 ◦ C (all ca. 55% total weight loss) had peaks
consistent with the known pattern of Ir2 P as well as Ir metal,
but they all had additional peaks near 33◦ and 50◦ 2θ. These
peaks were not consistent with any known phases in the ICDD
database. The appearance of the peaks also changes, becoming
narrower with increasing temperature, suggesting an increase in
crystallinity.
In an attempt to eradicate Ir metal and the unknown phase
from the final phosphide, pre-reaction in an inert atmosphere
(Ar) was employed at 500 ◦ C (ramp rate, 40 ◦ C min, 44% loss,
supporting information). Once at temperature, 5% H2 in Ar was
introduced and allowed to reduce the precursor isothermally for
1 h, resulting in an additional weight loss of 21%. The product
was grey to light grey, mostly dull in appearance, and brittle,
with just a few particles taking on a metallic silver color. A
similar methodology was utilized with a final temperature of
600 ◦ C, with a similar overall weight loss, TGA profile, and
visual product characteristics.
PXRD analyses of both of these trials (Fig. 4) show that the
peaks at 33◦ and 55◦ are not present and that the pattern is
consistent with the PDF patterns of Ir2 P and Ir metal and no
other crystalline phases. However, we were unable to find any
conditions that would yield Ir2 P exclusive of Ir.
Silver phosphate was synthesized by a simple bench-top
precipitation reaction between silver acetate and ammonium
monohydrogen phosphate in water. A fine yellow powder was
produced, filtered from solution by suction filtration, and dried at
room temperature. The product was identified by PXRD analysis (Fig. 5, bottom trace) and found to match the pattern for
Ag3 PO4 .
Reduction of the precursor was explored by TGA and the
data are displayed in Fig. 6. The initial reduction was performed
under 5% H2 in Ar from 50 to 900 ◦ C with a ramp rate of
Fig. 6. Thermogravimetric analysis of silver phosphate (Ag3 PO4 ) conducted in
flowing H2 /Ar to 900 ◦ C by 10 ◦ C/min.
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C.M. Sweeney et al. / Journal of Alloys and Compounds 448 (2008) 122–127
10 C/min. These reducing conditions were used to scan over
a range of temperatures within which reduction of the precursor
to phosphide is likely to occur and to use these results to help
pin-point the optimal reduction parameters. The data consist of
three substantial mass losses with an overall mass loss of 59%.
The product of the reduction was a sticky clump of silver-colored
shiny material, and yielded a PXRD pattern consistent with the
pattern for elemental silver.
Loss of oxygen to form a hypothetical composition “Ag3 P”
implies a weight change of ca. 20–30% (assuming modest
amounts of water in the Ag3 PO4 product). Based on the TGA
(Fig. 6), this suggests that a reduction temperature in the region
of 130–180 ◦ C should be optimal. Thus, reductions were conducted by TGA with a temperature that ranged from 50 ◦ C to
the targeted temperatures (140 and 180 ◦ C) using a ramp rate of
10 ◦ C/min, as well as a reduction at a slightly higher temperature
(200 ◦ C). Fig. 5 shows the PXRD results of the final products
of these reductions. At 140 ◦ C, the product was a mixture of
fine grey powder and silver-colored, shiny particles. The PXRD
pattern was clean and consistent with the known patterns of
the silver phosphate precursor (Ag3 PO4 ), silver hydrogen phosphate (Ag2 HPO4 ), as well as silver metal. At 180 ◦ C the product
was similar in color and consistency to the 140 ◦ C product, but
analysis of the product by PXRD suggested the presence of silver pyrophosphate, Ag4 P2 O7 , plus some degree of Ag2 HPO4 ,
as well as silver metal. Finally, the reduction at 200 ◦ C produced
a clump of shiny, silver colored material and its powder pattern
was consistent with elemental silver. No evidence of any silver
phosphide phases was seen in any of the PXRD patterns.
4. Discussion
Ideally, we sought stoichiometric single-phase crystalline
metal phosphates of Rh, Ir, and Ag as precursors so that it
would be possible to correlate specific weight losses in the
TGA with the transformation from crystalline phosphates to
phosphides. However, there are no reported phosphates of iridium, and rhodium phosphate synthesis is typically performed
via chemical vapor transport reactions at elevated temperatures
[28]. Accordingly, we adopted for Ir and Rh a co-isolation strategy to prepare well-mixed precursor phases, reasoning that these
might be sufficiently reactive to form phosphides. In fact, this
is very similar to the impregnation route to supported phosphides employed in catalytic studies [5–7]. Precursors were
prepared by combining Rh(III) or Ir(III) salts in water with
ammonium monohydrogen phosphate in a 1:1 molar ratio to
make a transparent solution, and then evaporating the water at
100 ◦ C in vacuo. Although a range of pHs were explored for
the Rh and Ir solutions, at no point could a precipitate be recovered from the aqueous solution. In contrast, reactions with Ag(I)
complexes (without pH modification) resulted in the immediate
precipitation of Ag3 PO4 as a single-phase, highly crystalline
material.
The as-prepared phosphate precursors were reduced in the
TGA using 5% H2 in Ar and a variety of temperature programs. In the case of Rh, rapid heating (40 ◦ C/min) to 200 or
400 ◦ C resulted in Rh metal formation; whereas the use of a
much slower ramp rate (5 ◦ C/min) and a slightly lower temperature (375 ◦ C) produced Rh2 P as the only crystalline phase,
from which we reasoned that rapid heating may not permit time
for Rh and P to diffuse through the solid and induce phosphide
formation, instead resulting in Rh0 . Despite the promise of the
powder diffraction pattern, chemical analysis by energy dispersive spectroscopy suggests that the product is phosphorus rich,
exhibiting a stoichiometry approaching 1:1 Rh:P instead of 2:1.
Thus, it is likely that amorphous phosphorus is also present in
the product, consistent with the starting stoichiometry (Rh:P
1:1.1) of the precursor. Since red phosphorus sublimes at 417 ◦ C,
post-treatment at higher temperatures for longer times (several
minutes) should be sufficient to produce material that is single
phase Rh2 P in both structure and composition. This may also be
achievable with continued hydrogen reduction (removing phosphorus as PH3 ), but caution must be taken to avoid reduction of
the metal. It should be noted that although Cl is present in the
initial phosphate precursor (from Rh(III) chloride), it appears to
be volatilized during reduction, since it does not appear in the
EDS spectrum of the product. Likewise, we expect that the phosphate counterion (NH4 + ) is removed as NH3 under the reduction
conditions. The loss of NH4 + and Cl− and the transformation of
the precursor to a material of stoichiometry “RhP” is consistent
with the observed weight loss by TGA (65% predicted versus
69% observed).
Based on our experience with Rh, the reduction of the iridium
precursor was initially conducted utilizing a very slow ramp rate
of 5 ◦ C/min, with final temperatures ranging from 200 to 700 ◦ C.
Nevertheless, Ir metal appeared in the pattern for all temperatures explored, and was the only crystalline product for samples
heated to ≤400 ◦ C. When heated to 500, 600, and 700 ◦ C, the
distinctive pattern of Ir2 P becomes apparent, and the peaks of
both Ir2 P and Ir sharpen with increasing temperature, suggesting
improved crystallinity. Concomitant with the appearance of Ir2 P,
a series of unidentified peaks appeared in the regions around 33◦
and 50◦ 2θ.
We reasoned that isolated Ir(III) in the co-precipitate may be
reducing to Ir metal at relatively low temperatures, and so performed subsequent thermal gravimetric analyses by ramping to
temperature in an inert atmosphere, followed by an isothermal
step under reducing conditions. Heating to 500 or 600 ◦ C under
these modified conditions continued to result in Ir metal formation; however, the relative intensity of the Ir peaks to those of Ir2 P
has significantly decreased relative to ramping to temperature in
a reducing temperature, and the 600 ◦ C treated sample shows
a dramatic improvement over the 500 ◦ C treated sample, both
with respect to crystallinity and ratio of Ir to Ir2 P. Furthermore,
heating under an inert atmosphere prior to introducing reducing
gas has virtually eliminated the unknown byproduct: only a very
weak hump can be distinguished in the region around 50◦ 2θ.
Attempts to avoid Ir formation by adding excess (NH4 )2 HPO4
at the outset (Ir:P ratio 1:1.5) were also unsuccessful.
The inability to eradicate all of the elemental Ir may be a
consequence of its presence from the beginning in the iridium
phosphate precursor. A closer look at the X-ray diffraction pattern reveals the likely presence of some amorphous or nanocrystalline Ir (broad feature near 40◦ 2θ). The origin of this impurity
C.M. Sweeney et al. / Journal of Alloys and Compounds 448 (2008) 122–127
is unknown, but upon heating, even under inert conditions, the
elemental Ir can be expected to crystallize.
In contrast to the rhodium and iridium phosphate precursors,
Ag3 (PO4 ) does not reduce to form a phosphide. The presence
of elemental silver is found upon reduction at temperatures as
low as 140 ◦ C, and appears along side Ag3 (PO4 ) and Ag2 HPO4 ,
suggesting that the transformation is limited only by the rapidity of the TGA experiment. At 180 ◦ C, elemental silver is the
dominant phase, accompanied by a small quantity of silver
pyrophosphate, whereas by 200 ◦ C, silver is the only crystalline
product. Thus, it appears that the method of phosphide synthesis
by hydrogen annealing of precursor phosphates is inapplicable
to silver, despite the fact that silver satisfies the condition set
forth by Gopalakrishnan et al.: the oxide, Ag2 O, can be reduced
to the metal using hydrogen annealing [24]. It would appear that
the reaction proceeds via metal reduction, in lieu of phosphate
reduction, as indicated in Eqs. (1) and (2). A possible explanation for the inconsistency can be found upon examination of the
kinds of products that have been achieved by this methodology.
In all cases, the products are either monopnictides or phases still
more metal rich [24]. This is consistent with the data presented
here for Rh and Ir, with the most metal rich phase (M2 P) being
formed in each case, despite the fact that the stoichiometry of the
precursor would be expected to favor MP (M = Rh, Ir). However,
as indicated in Table 1, the phase predicted from the stoichiometry of the silver precursor (Ag3 P) does not exist, nor are there
any reported metal rich or monophosphides of silver, perhaps
explaining the failure of the reaction. It should be noted that
Ag-rich arsenide [29] and antimonide [30,31] phases are well
documented, and may therefore be accessible by the pnictate
reduction route.
Ag3 PO4 + 21 H2 → Ag0 + Ag2 (HPO4 )
(1)
2Ag3 PO4 + H2 → 2Ag0 + Ag4 P2 O7 + H2 O.
(2)
5. Conclusion
The ability to form transition metal phosphides by hydrogen reduction of phosphate precursors has been validated for
rhodium and iridium, and the formation of a discrete, single
phase crystalline phosphate precursor does not appear to be a
prerequisite for phosphide formation. Additionally, it has been
shown that silver phosphate fails to produce silver phosphide
under the same reduction methodology. This conclusion yields
an exception to the premise that metals capable of metal oxide
reduction to elemental metal are also capable of phosphide synthesis by the studied method.
Acknowledgements
Support was provided by the National Science Foundation
(CAREER award DMR-0094273 and IGERT-970952). Pala-
127
niappan Arumugum and Keerthi Senevirathne are acknowledged for reproducing some of the data presented here.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.jallcom.2006.10.035.
References
[1] F. Hulliger, Struct. Bond. 4 (1968) 83–229.
[2] C.J. Raub, W.H. Zachariasen, T.H. Geballe, B.T. Matthias, J. Phys. Chem.
Solids 24 (1963) 1093.
[3] P.P.J. van Engelen, K.H.J. Buschow, J. Less-Common Met. 159 (1990)
L1–L4.
[4] E.L. Muetterties, J.C. Sauer, J. Am. Chem. Soc. 96 (1974) 3410–3415.
[5] S.T. Oyama, J. Catal. 216 (2003) 343–352.
[6] S.J. Sawhill, D.C. Phillips, M.E. Bussell, J. Catal. 215 (2003) 208–219.
[7] V. Zuzaniuk, R. Prins, J. Catal. 219 (2003) 85–96.
[8] H.G. von Schnering, W. Hönle, Chem. Rev. 88 (1988) 243–273.
[9] C.N.R. Rao, J. Gopalakrishnan, Acc. Chem. Res. 20 (1987) 228–235.
[10] R. Gruehn, R. Glaum, Angew Chem. Int. Ed. 39 (2000) 692–716.
[11] E.G. Gillian, R.B. Kaner, Chem. Mater. 8 (1996) 333–343.
[12] R.F. Jarvis Jr., R.M. Jacubinas, R.B. Kaner, Inorg. Chem. 29 (2000)
3243–3246.
[13] S.L. Brock, S.C. Perera, K.L. Stamm, Chem. Eur. J. 10 (2004) 3364–3371,
and references therein.
[14] J.A. Aitken, V. Ganzha-Hazen, S.L. Brock, J. Solid State Chem. 178 (2005)
970–975.
[15] H. Hou, Q. Yang, C. Tan, G. Ji, B. Gu, Y. Xie, Chem. Lett. 33 (2004)
1272–1273.
[16] C.M. Lukehart, S.B. Milne, S.R. Stock, Chem. Mater. 10 (1998) 903–908.
[17] F. Schweyer-Tihay, P. Braunstein, C. Estournés, J.L. Guille, B. Lebeau,
J.-L. Paillaud, M. Richard-Plouet, J. Rosé, Chem. Mater. 15 (2003) 57–62.
[18] S.C. Perera, P.S. Fodor, G.M. Tsoi, L.E. Wenger, S.L. Brock, Chem. Mater.
15 (2003) 4034–4038.
[19] S.C. Perera, G. Tsoi, L.E. Wenger, S.L. Brock, J. Am. Chem. Soc. 125
(2003) 13960–13961.
[20] C. Qian, F. Kim, L. Ma, F. Tsui, P. Yang, J. Liu, J. Am. Chem. Soc. 126
(2004) 1195–1196.
[21] J. Park, B. Koo, Y. Hwang, C. Bae, K. An, J. Park, M. Park, T. Hyeon,
Angew Chem. Int. Ed. 43 (2004) 2282–2285.
[22] J. Chen, M. Tai, K. Chi, Chem. Commun. 14 (2004) 296–298.
[23] K.A. Gregg, S.C. Perera, G. Lawes, S. Shinozaki, S.L. Brock, Chem. Mater.
18 (2006) 879–886.
[24] J. Gopalakrishnan, S. Pandey, K.K. Rangan, Chem. Mater. 9 (1997)
2113–2116.
[25] J.A. Rodriquez, J. Kim, J.C. Hanson, S.J. Sawhill, M.E. Bussell, J. Phys.
Chem. B 107 (2003) 6276–6285.
[26] K.L. Stamm, J.C. Garno, G. Liu, S.L. Brock, J. Am. Chem. Soc. 125 (2003)
4038–4039.
[27] P. Arumugam, S.S. Shinozaki, R. Wang, G. Mao, S.L. Brock, Chem. Commun. (2006) 1121–1123.
[28] P. Rittner, R. Glaum, Z. Kristallogr. 209 (1994) 162–169.
[29] M.R. Baren, Bull. Alloy Phase Diag. 11 (1990) 113–116.
[30] C. Cipriani, M. Corazza, G. Mazzetti, Eur. J. Mineral. 8 (1996) 1347–1350.
[31] J.T. Vaughey, L. Fransson, H.A. Swinger, K. Edstrom, M.M. Thackeray, J.
Power Sources 119–121 (2003) 64–68.
[32] E.H. El Ghadraoui, R. Guerin, M. Sergent, Acta Crystallogr. C 39 (1983)
1493–1494.
[33] M.H. Moller, W. Jeitschko, Inorg. Chem. 20 (1981) 828–833.