Pt2Mo3N and PdPtMo3N: New interstitial nitrides prepared from freeze-dried precursors

Article abstract of DOI:10.1016/j.jssc.2004.06.006

The molybdenum bimetallic and trimetallic nitrides Pt₂Mo ₃N and PdPtMo₃N have been synthesized by ammonolysis of the stoichiometric amorphous precursor, obtained by freeze drying of aqueous solutions of the appropriate metal salts. These compounds have been characterized by elemental analysis, energy-dispersive analysis of X-rays, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermogravimetry analysis under an oxygen atmosphere. Pt₂Mo ₃N and PdPtMo₃N crystallize in the cubic space group P4₁32 (213), with lattice parameters of a=6.83586(4) and 6.82542(3)A, respectively, and form with the unusual filled β-manganese structure. These compounds are stable under air up to 580K, the oxidation being complete at 910K.

Full text of DOI:10.1016/j.jssc.2004.06.006

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 3219–3223
Pt2Mo3N and PdPtMo3N:new interstitial nitrides prepared from
freeze-dried precursors
Abdelouahad El-Himri, David Marrero-Lopez, and Pedro Nunez*
´ ´
Departamento de Quımica Inorganica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
Received 20 May 2004; received in revised form 4 June 2004; accepted 5 June 2004
Available online 15 July 2004
Abstract
The molybdenum bimetallic and trimetallic nitrides Pt2Mo3N and PdPtMo3N have been synthesized by ammonolysis of the
stoichiometric amorphous precursor, obtained by freeze drying of aqueous solutions of the appropriate metal salts. These
compounds have been characterized by elemental analysis, energy-dispersive analysis of X-rays, X-ray diffraction, scanning electron
microscopy, transmission electron microscopy and thermogravimetry analysis under an oxygen atmosphere. Pt2Mo3N and
PdPtMo3N crystallize in the cubic space group P4132 (213), with lattice parameters of a ¼ 6:83586ð4Þ and 6:82542ð3Þ A,
respectively, and form with the unusual filled b-manganese structure. These compounds are stable under air up to 580 K, the
oxidation being complete at 910 K.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Metallic nitrides; Freeze-dried precursor; Pt2Mo3N; PdPtMo3N; Molybdenum; Platinum; Palladium
1. Introduction
catalysts for important industrial processes [12]. One
possible way of further improving their catalytic
performance is to introduce other elements into the
simple binary material. Molybdenum nitrides constitute
examples of promising materials in this field due to their
competitive activity and selectivity for commercially
significant reactions including ammonia synthesis
[13–15], hydrodenitrogenation and hydrodesulfurization
[16–20].
Progress in nitride chemistry has allowed the identi-
fication of a set of favored structure types. Recently,
after the structure determination of Ni2Mo3N [21–23],
Ni2W3N [23] and Pd2Mo3N [24], a new structural type
came into existence in the area of nitride chemistry, the
filled b-Mn structure.
Several synthetic approaches have been used towards
preparing nitride catalysts. For example, first by the
direct reaction of a metal nitride with other metals or
nitrides; second by the reaction of a mechanical mixture
of two metal powders with nitrogen or ammonia; third
by the reaction between metal amides and nitrides; and
fourth by the ammonolysis of mixed metallic oxides.
A common feature of the newer synthesis methodol-
ogies is the use of alternative reaction paths through
The investigation of transition metal nitrides and
carbides is an emergent area in part due to the increasing
number of applications of these materials [1]. Thus, in
addition to traditional uses, based on mechanical
properties [2], their chemical and structural versatility
are leading to the discovery of new phases with exciting
and important optical, electrical, magnetic and chemical
properties [3–9].
Very few polymetallic nitrides have been synthesized
and characterized, chiefly due to the limitations imposed
by the preparation of these compounds. Only since the
1990s, with the development of new precursor synthesis
methods has the systematic study of polymetallic
nitrides been achievable.
Catalysis is a major driver in renewing interest in
applied transition metal nitrides chemistry [10,11]. The
rapidly expanding studies of monometallic interstitial
transition metal nitrides have demonstrated their
potential to replace expensive noble group metal
*Corresponding author. Fax:+34-922-318461.
E-mail address: pnunez@ull.es (P. Nunez).
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.06.006

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3220
which the cationic diffusion distances are reduced. This
provides a route to obtain new nitrides at relatively low
temperatures [25]. In this context, use of precursors
obtained by freeze-drying processing of the appropriate
aqueous metal salts has proved to be a very versatile
method for obtaining stoichiometrically controlled
complex polymetallic systems [26–30]. It is well known
that the preparative route plays a critical role in
governing material properties, controlling the structure,
morphology, grain size and surface area of the obtained
materials.
determined by thermogravimetric analysis with a TG/
DTA Perkin-Elmer instrument (mod. Pyris Diamond) at
a rate of 10 K/min up to 1173 K. The oxidation for both
metal nitrides begins at 580 K and is complete at 930 K.
2.2.2. X-ray diffraction and transmission electron
microscopy
X-ray powder diffraction patterns were obtained from
Philips X’Pert Pro automated diffractometer,
a
equipped with a primary monochromator and an
X’celerator detector. Routine patterns for phase identi-
fication were collected with a scanning step of 0.08ꢀ in 2y
over the angular range 2y 10–80ꢀ, with a collection time
of 5 s per step. The cell parameters of the products were
obtained by profile fitting of the pattern using the Le
Bail’s method [33] as implemented in the FULLPROF
program [34], from patterns collected with a scanning
step of 0.017ꢀ in 2y, over a wider angular range (2y
5–120ꢀ), and with a longer acquisition time (50 s per
step) in order to enhance the statistics. The fits were
performed using a pseudo-Voigt peak-shape function. In
the final runs, the usual profile parameters (scale factors,
background coefficients, zero-points, half-width, pseu-
do-Voigt and asymmetry parameters for the peak shape)
were refined. All graphical representations relating to
X-ray powder diffraction patterns were performed using
the WinPLOTR [35] program.
Here we report how the use of novel freeze-dried
polymetallic precursors has allowed us to synthesize the
new b-manganese-type bimetallic and trimetallic inter-
stitial nitrides, Pt2Mo3N and PdPtMo3N.
2. Experimental
2.1. Synthesis
Materials used as reagents in the current investigation
tetrahydrate,
were
ammonium
heptamolybdate
(NH4)6Mo7O24 Á 4H2O, (Aldrich, 99.98%), ammonium
hexachloropalladate (IV), (NH4)2[PdCl6] and ammo-
nium
hexachloroplatinate
(IV),
(NH4)2[PtCl6].
(NH4)2[PtCl6] and (NH4)2[PdCl6] were prepared follow-
ing the method described elsewhere [31].
Starting Pt-, Pd- and Mo-containing solutions were
prepared by dissolving their respective salts in distilled
water. The aqueous solutions were combined to obtain
Pt–Mo and Pd–Pt–Mo mixtures with nominal metal
molar ratios of 2:3 and 1:1:3, respectively. Droplets of
these solutions were flash frozen by projection on liquid
nitrogen and then freeze-dried at a pressure of 1–10 Pa
in a Heto Lyolab 3000 freeze-drier. In this way, dried
solid precursors were obtained as amorphous loose
powders (by powder X-ray diffraction). The procedure
for the ammonolysis of the precursor powders has been
described in a previous paper [32].
Samples for transmission electron microscope (TEM)
work were prepared by dispersion of fine-ground
material onto a holey carbon film. Electron diffraction
(SAED) patterns were obtained using a JEOL 2011
electron microscope, operating at an accelerating
voltage of 200 kV.
2.2.3. Microstructural characterization
The morphology of the freeze-dried precursors and
the resulting nitrides were observed using a scanning
electron microscope (Jeol JSM 6300) operating at an
accelerating voltage of 30 kV. All the preparations were
covered with a thin film of gold for better image
definition.
2.2. Characterization
2.2.1. Elemental analysis
Metal ratios in the solids were determined by energy-
dispersive analysis of X-ray (EDAX) on a Jeol JSM
6300 scanning electron microscope. Data were collected
with an Oxford detector, with quantification performed
using virtual standards on the associated Link-Isis
software. The operating voltage was 20 kV, and the
energy range of the analysis 0–20 keV. The nitrogen
content of the nitride was evaluated by standard
combustion analysis (EA 1108 CHNS-O); N2 and CO
were separated in a chromatographic column, and
measured using a thermal conductivity detector. The
thermal stability of Pt2Mo3N and PtPdMo3N in air was
3. Results and discussion
As mentioned in Section 1, we previously reported the
existence of single-phase Pd2Mo3N [24]. Our success in
the preparation of this nitride was based on the use of
freeze-drying procedure of aqueous solutions of the
appropriate metal salts and of careful control of the
experimental conditions. In the present work, we have
extended our synthetic approach to the synthesis of new
multimetallic interstitial molybdenum nitrides. Our goal
was then to investigate the optimal conditions for
obtaining pure Pt2Mo3N and PdPtMo3N cubic phases.

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Thus, we carried out ammonolysis procedures using
amorphous precursors under conditions similar to those
reported as optimal in our previous work [24]. In
preliminary experiments, both temperature and reaction
times were varied over wide ranges (1073–1273 K, 2–
24 h, slow vs. fast cooling). Regardless of the reaction
conditions with these limits, the ammonolysis of the
amorphous precursors resulted in black products. The
patterns indicate that the ammonolysis products consist
of quasi-single phases, isostructural with Ni2Mo3N and
Pd2Mo3N.
For Pt2Mo3N, a minor impurity of platinum metal
was consistently observed. By analogy to the synthesis
of Pd2Mo3N, to avoid the appearance of the platinum
phase, we subjected the amorphous Pt precursors to
thermal treatments at relatively low temperatures in air.
After treatment at 673 K for 12 h, the amorphous
precursors were transformed into brown solids. X-ray
diffraction revealed those to be a mixture of PtO and
MoO3. These solids were used as crystalline precursors
in the ammonolysis processes.
The ammonolysis of the crystalline precursor
(2PtO :3MoO 3) at 1173 K for 12 h leads to an almost
single-phase product isostructural with Ni2Mo3N and
Pd2Mo3N. The X-ray diffraction pattern of the product
displays also small peaks ascribed to Pt metal. From our
experiments, we conclude that the Pt2Mo3N is formed
from either amorphous or crystalline precursors and is
independent of the cooling rate. For simplicity, direct
ammonolysis of the amorphous precursor and a slow
cooling rate were finally adopted in our preparative
procedure. The new quaternary nitride PdPtMo3N
could be prepared by analogy to Pt2Mo3N (amorphous
freeze-dried precursor, Tfinal ¼ 1173 K, thold ¼ 12 h, slow
cooling). Similarly, a small quantity of platinum metal
was a common impurity. The contents of platinum
impurities determined from XRD data, using the
Fullprof program [34], were 1.8% and 2.3% for
Pt2Mo3N and PtPdMo3N, respectively.
The nitrogen content, 2.2% and 2.3%, determined by
combustion analysis is consistent with the Pt2Mo3N
(Calc. N(%)=2.02) and PdPtMo3N (Calc. N(%)=2.32)
stoichiometry, respectively.
The structures of Pt2Mo3N and PdPtMo3N were
refined in the space group P4132 from room temperature
powder X-ray diffraction data. The starting structural
model was that of Ni2Mo3N. In the structural refine-
ment, the presence of Pt metal impurities was also
considered. The results of the Rietveld refinement and
atomic parameter for Pt2Mo3N and PdPtMo3N are
shown in Table 1. Fig. 1 shows the observed and
calculated patterns of Pt2Mo3N.
The isostructurality of Pt2Mo3N and Pd2Mo3N and
the results of the structure refinement of PtPdMo3N
allow us an assumption that the latter is a part of the
20k
10k
0
20
40
60
80
100
120
2θ0/º]
Fig. 1. XRD pattern of Pt2Mo3N showing observed (circles) and
calculated (solid line) X-ray diffraction profiles for Pt2Mo3N. Tick
marks below the diffractograms represent the allowed Bragg reflec-
tions (top, Pt2Mo3N; bottom, Pt).
Table 1
Crystal parameters and selected bond distances for Pt2Mo3N and PtPdMo3N (space group no. 213, P4132)
Atom/Wyckoff site
x
y
z
Biso
Pt2Mo3N (a ¼ 6:83586ð3Þ A)
Pt (8c)
Mo (12d)
N (4b)
0.18759(11)
0.79773(17)
7/8
0.18759(11)
0.04573(17)
7/8
0.18759(11)
0.309
0.244
1.0
1/8
7/8
w2:4.02; Rp:9.96; Rexp:3.78; RB=5.94; RF=4.81
Pt–Pt=2.5639(10) A; Pt–Mo=2.7655(13); 2.8808(9); 2.9044(14) A; Mo–Mo=2.8776(11); 2.8588(15) A; Mo–N=2.1392(7)
PtPdMo3N (a ¼ 6:82542ð3Þ A)
M (8c)a
Mo (12d)
N (4b)
0.19057(9)
0.79751(13)
7/8
0.19057(9)
0.04751(13)
7/8
0.19057(9)
1/8
7/8
0.389
0.210
1.0
w2:5.34; Rp:9.32; Rexp:3.33; RB=5.55; RF=4.33
M–M=2.5738(9) A; M–Mo=2.7435(10); 2.8898(7); 2.8927(10) A; Mo–Mo=2.8614(13); 2.8842(12) A; Mo–N=2.1396(5)
a M=Pt or Pd.

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continuous solid solutions series between the ternary
compounds.
structure (space group P4132), there are two different
types of metallic atom sites which differ in their
electronic states and are inherently predisposed to
occupation by different atoms, due to their different
multiplicities. The result is that the Pt atoms are located
in a 12-fold, pseudo icosahedral coordination (Wyckoff
position 8c), surrounded by three Pt and nine Mo
atoms, giving Pt[Pt3Mo9] units. In turn, the Mo atoms
are located in 14-fold (Wyckoff position 12d), pseudo
tetradecahedron sites, surrounded by six Pt, six Mo and
two N atoms, giving Mo[Pt6Mo6N2] entities. Finally,
each N atom (Wyckoff position 4b) is octahedrally
coordinated by molybdenum, forming a distorted
octahedron. These [NMo6] octahedra share corners.
Pd and Pt show metal radii quite close due to the well-
known ‘‘lanthanide contraction’’, even they belong to
different d element series, 4d and 5d, respectively. For
this reason, the cubic a parameter changes only slightly
from Pd2Mo3N (a ¼ 6:81770ð3Þ A [24]) to PdPtMo3N
(a ¼ 6:82542ð3Þ A) and Pt2Mo3N (a ¼ 6:83586ð3Þ A), as
metal radii also change slightly from Pd (1.370 A) to Pt
(1.385 A) [36]. Consequently, there is no noticeable
variation in both metal–metal and metal–ligand bond
distances for Pd2Mo3N, PdPtMo3N and Pt2Mo3N (see
Table 1).
All SAED experiments revealed the cubic crystal
structure for Pt2Mo3N and no distortions or additional
superstructure spots were observed. Selected area
electron diffraction (SAED) patterns for Pt2Mo3N in
the [010] and ½120ꢁ zone axis are shown in Fig. 2. These
SAED patterns were refined using the results obtained
by XRD.
Since Pt2Mo3N and PdPtMo3N are isostructural, we
will only discuss the structure of Pt2Mo3N, in detail.
This structure consists of a b-Mn arrangement of
metallic atoms (Pt and Mo), with N atoms occupying
distorted octahedral interstitial sites. In the b-Mn
00011
00001
110000
21100
Fig. 3 shows the characteristic SEM images corre-
sponding to the freeze-dried precursors and the final
nitrides. The ammonolysis reaction induces significant
changes in the microstructure, affecting both the
morphology and the particle size.
[120]
[010]
Fig. 2. Electron diffraction patterns of Pt2Mo3N in the zone axis [010]
and ½120ꢁ: The SAED patterns obtained have been indexed with the
parameters found from X-ray diffraction.
Fig. 3. SEM micrographs showing the microstructure of amorphous freeze-dried precursors of metal ratios Pt:Mo 2:3 (a) and Pd:Pt:Mo 1:1:3 (b),
and the nitrides (c) Pt2Mo3N and PdPtMo3N (d).

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3223
In conclusion, we have prepared new molybdenum
bimetallic and tetrametallic interstitial nitrides,
Pt2Mo3N and PdPtMo3N. These nitrides are new
compounds in a singular b-Mn structural family in the
nitride chemistry.
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Acknowledgments
We wish to thank Jesus Canales-Vazquez for electron
diffraction measurements. This research was supported
by the Spanish Research Program MCyT (MAT-2001-
3334) and Canary Islands Goverment (COFI 2002-027).
D. Marrero-Lopez wishes to thank the Canary Islands
Goverment for fellowship grant.
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