Nickel and cobalt hexamethylentetramine complexes (NO3) 2Me(H2O)6(HMTA)2·4H 2O (Me = Co2+, Ni2+): New molecular precursors for the preparation of metal dispersions

Article abstract of DOI:10.1021/ic7013013

Nickel and cobalt hexamethylenetetramine (HMTA) complexes (NO ₃)₂Me(H₂O)₆(HMTA) ₂·4H₂O were prepared and characterized structurally by single-crystal X-ray diffraction. Both compounds crystallize in the triclinic P1 space group with the same structure. The structures are three-dimensional hydrogen-bonded supramolecular frameworks containing two-dimensional cationic assemblies connected with proton acceptors, which are noncoordinated anionic species (nitrate) and neutral HMTA molecules. Thermal decomposition of these compounds under an inert atmosphere leads to the high-surface-area metal-carbon foams containing nickel and cobalt nanoparticles embedded within onionlike carbon shells. The decomposition process as studied by thermal analysis and in situ X-ray absorption spectroscopy (XAS) measurements occurs as a rapid loss of oxygen in the form of CO, beginning at temperatures as low as 323 K. As the in situ XAS study demonstrated, the evolution of nickel and cobalt coordination spheres occurs with intermediate formation of amorphous metal carbides, which subsequently decompose to the metal nanoparticles covered with carbon shells.

Full text of DOI:10.1021/ic7013013

Inorg. Chem. 2008, 47, 2303-2311
Nickel and Cobalt Hexamethylentetramine Complexes
2+
2+
(
NO ) Me(H O) (HMTA) ·4H O (Me ) Co , Ni ): New Molecular
3
2
2
6
2
2
Precursors for the Preparation of Metal Dispersions
,
†
†
‡
Pavel Afanasiev,* Sandra Chouzier, Tivadar Czeri, Guillaume Pilet, Christophe Pichon,
§
‡
‡
Magalie Roy, and Michel Vrinat
†
Institut de Recherche sur la Catalyse et EnVironnement, UniVersité de Lyon 1, 2 AVenue A.
Einstein, 69626 Villeurbanne Cédex, France, Institut Français du Pétrole, IFP-Lyon, BP 3,
6
9390 Vernaison, France, and Groupe de Cristallographie et Ingénierie Moléculaire, Laboratoire
des Multimatériaux, UMR 5615 CNRS, UniVersité de Lyon 1, Campus de la Doua,
9622 Villeurbanne Cédex, France
6
Received July 2, 2007
3 2 2 6 2 2
Nickel and cobalt hexamethylenetetramine (HMTA) complexes (NO ) Me(H O) (HMTA) ·4H O were prepared and
characterized structurally by single-crystal X-ray diffraction. Both compounds crystallize in the triclinic P 1¯ space
group with the same structure. The structures are three-dimensional hydrogen-bonded supramolecular frameworks
containing two-dimensional cationic assemblies connected with proton acceptors, which are noncoordinated anionic
species (nitrate) and neutral HMTA molecules. Thermal decomposition of these compounds under an inert atmosphere
leads to the high-surface-area metal-carbon foams containing nickel and cobalt nanoparticles embedded within
onionlike carbon shells. The decomposition process as studied by thermal analysis and in situ X-ray absorption
spectroscopy (XAS) measurements occurs as a rapid loss of oxygen in the form of CO, beginning at temperatures
as low as 323 K. As the in situ XAS study demonstrated, the evolution of nickel and cobalt coordination spheres
occurs with intermediate formation of amorphous metal carbides, which subsequently decompose to the metal
nanoparticles covered with carbon shells.
3,4
1
. Introduction
biomass gasification or hydrodechlorination. In the last few
years, considerable research activity was targeted on the
preparation of nanoscopic nickel and cobalt, using various
Porous materials with hierarchical pore systems attract
1
,2
great interest in researchers.
Spongelike or foamlike
5
–8
physical or chemical methods.
structures consisting of nanoparticles are eminently appropri-
ate for applications where a large surface area is needed,
such as electrochemistry or catalysts. Polymodal pore
structure allows optimization of the relationship between the
chemical reaction and the mass transfer from the solid surface
to the volume of the reactants and vice versa. Nickel and
cobalt are well-known magnetic materials, whereas nano-
particles of these metals are widely used in the actual
catalytic studies on newly developed processes such as
Hexamethylenetetramine (HMTA) easily forms stoichio-
metric molecular compounds with transition metals, which
can be conveniently used for the preparations of inorganic
materials by decomposition. Recently, a molybdate HMTA
complex has been prepared, whose decomposition in an inert
9
atmosphere leads to the dispersed Mo
2
N nitride and then is
extended to the ternary (Ni) Co-Mo nitrides. In the course
of these studies, we observed that decomposition of mono-
*
To whom correspondence should be addressed. E-mail:
(3) Furusawa, T.; Tsutsumi, A. Appl. Catal. 2005, 278, 195.
(4) Keane, M. A.; Patterson, P. M. Catal. Lett., 2005, 99, 33.
(5) Yamamoto, K.; Sunagawa, Y.; Takahashi, H.; Muramatsu, A. Chem.
Commun. 2005, 348.
pavel.afanasiev@ircelyon.univ-lyon1.fr.
†
Institut de Recherche sur la Catalyse et Environnement, Université de
Lyon 1.
‡
Institut Français du Pétrole, IFP-Lyon.
UMR 5615 CNRS, Université de Lyon 1.
(6) Ekstrand, A.; Jansson, K.; Westin, G. Chem. Mater. 2005, 17, 199.
(7) Hou, Y.; Kondoh, H.; Ohta, T. Chem. Mater. 2005, 17, 3994.
(8) Tsang, S. C.; Caps, V.; Paraskevas, I.; Chadwick, D.; Thompsett, D.
Angew. Chem., Int. Ed. 2004, 43, 5645.
§
(
1) Kuang, D.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126,
0534.
2) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772.
1
(
(9) Afanasiev, P. Inorg. Chem. 2002, 41, 5317.
1
0.1021/ic7013013 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 7, 2008 2303
Published on Web 03/05/2008

Afanasiev et al.
a
Table 1. Crystallographic Data for (NO3)2Me(H2O)6(HMTA)2 ·4H2O
Table 2. Important Bond Lengths and Distances for Compound 1
Cobalt Environment
Co1-O2 2.104(2)
Hydrogen Bonds
+
(2) Me ) Ni2+
(
1) Me ) Co2
refined formula
C12H44Co1N10O16
643.5
triclinic
C12H44Ni1N10O16
643.2
triclinic
Co1-O3
2.044(2)
Co1-O1
2.116(2)
-
1
fw (g mol
cryst syst
)
unit cell constants
a (Å)
b (Å)
c (Å)
R (deg)
donor-H-acceptor
H-acceptor
9.1231 (3)
9.4137 (5)
9.6718 (4)
88.214 (2)
75.600 (3)
61.618 (2)
704.13 (6)
P 1j (No. 2)
1
293
1.52
0.695
0.0421
0.0480
1.12
0.59
-0.36
2430
209
9.0976 (6)
9.3448 (6)
9.6909 (5)
87.779 (3)
75.679 (3)
61.255 (3)
696.78 (8)
P 1j (No. 2)
1
293
donor
H
acceptor
angle (deg)
distance
O2
O1
O2
O3
O4
O3
O4
O1
O5
O5
H20
N5
N2
O8
O4
N4
N3
O5
O7
O6
O8
154.7
170.4
156.3
166.2
172.6
172.7
173.4
174.7
171.7
169.3
2.016
2.002
2.009
1.811
1.940
1.918
1.923
2.005
1.928
2.041
H11
H21
H31
H40
H30
H41
H10
H50
H51
ꢀ
(deg)
γ (deg)
3
V (Å )
space group
Z
T (K)
density (g cm-3)
1.53
0.782
0.0453
0.0475
1.15
0.51
-0.33
1724
208
-
1
µ (mm
)
b
R
c
Table 3. Important Bond Lengths and Distances for Compound 2
Rw
S
∆
∆
Nickel Environment
-
3
Fmax (e Å )
Fmin (e Å-3)
Ni1-O3
2.022(2)
Ni1-O2
2.063(2)
Ni1-O1
2.063(3)
no. of reflns used
Hydrogen Bonds
no. of param
a
b
All data collected with Mo KR radiation (λ ) 0.710 69 Å). R ) ∑(|Fo
donor-H-acceptor
H-acceptor
distance
c
2
2
1/2
-
Fc|)/∑|Fo|. Rw ) {∑[w(|Fo - Fc|) ]/∑[w|Fo| ]} .
donor
H
acceptor
angle (deg)
O2
O1
O2
O3
O4
O3
O4
O1
O5
O5
H20
N5
N2
O8
O4
N4
N3
O5
O7
O6
O8
155.8
162.3
160.1
166.4
173.9
165.2
177.1
167.8
170.1
165.8
2.011
2.013
1.997
1.797
1.931
1.944
1.945
2.000
1.909
2.030
metallic Ni- and Co-HMTA complexes led to the corre-
sponding metals. A more detailed study of these systems
revealed that both the initial compounds and decomposition
products possess interesting properties, namely, the former
are three-dimensional (3-D) supramolecular networks and
the latter are metallic nanoparticles embedded within a porous
carbon matrix. The present paper deals with the structural
characterization of Ni- and Co-HTMA complexes and the
properties of their decomposition products.
H11
H21
H31
H40
H30
H41
H10
H50
H51
were anisotropically refined. The H atoms of water molecules were
found by Fourier differences and their positions refined.
The decomposition of compounds of cobalt (1) and nickel (2)
was carried out in the temperature range 573–973 K under a
2
. Experimental Section
nitrogen flow of 3 L h- with a heating rate of 5 K min
1
-1
.
Ni- and Co-HMTA complexes were obtained by aqueous reaction
of the corresponding nitrates salts with HMTA. In a typical
synthesis, 5.8 g of nickel (0.02 mol) or cobalt nitrate hexahydrate
Decomposition products were collected and kept under argon. The
gaseous products evolved upon heating of the samples were studied
using a mass spectrometer gas trace A (Fison Instruments) equipped
with a quadrupole analyzer (VG analyzer) working in a faraday
mode. The ionization was done by electron impact with an electron
energy of 65 eV. The samples (ca. 0.1 g) were heated in a quartz
cell at a heating rate of 5 K min- . A silica capillary tube heated
at 453 K continuously bled off a proportion of the gaseous reaction
products. Several signals were continuously registered, correspond-
(
0.02 mol) dissolved in 25 mL of distilled water was added to 25
mL of an aqueous solution containing 14 g of HMTA (0.1 mmol).
Macroscopic platelike crystals (rose and light-green color for cobalt
(1) and nickel (2), respectively) were filtered, washed with acetone,
1
and dried in air (synthesis yield about 85%, not optimized).
Structure determination was carried out on a Nonius Kappa CCD
diffractometer. The experimental details for the structure determi-
2+
2+
nation of (NO
3
)
2
Me(H
2
O)
6
(HMTA)
2
·4H
2
O (Me ) Co , Ni ) are
2 2 2 2
ing to molecular ions and fragments of H O, NO, N , CO, O , CO ,
given in Table 1. Once the data processing was performed by the
and NO
2
.
10
Kappa CCD analysis softwares, the lattice constants were refined
by least-squares refinement using 2490 reflections (1.0° < θ < 28°)
for the cobalt compound and 2554 reflections (1.0° < θ < 28°) for
the nickel compound. No absorption correction was applied to the
data sets. The structures were solved by direct methods (SIR97
To characterize solid decomposition products, the X-ray diffrac-
tion (XRD) patterns were obtained on a Bruker diffractometer with
Cu KR emission. The diffractograms were analyzed using the
standard JCPDS files. Chemical analyses were realized using the
atomic emission method. Scanning electron microscopy (SEM)
images were obtained on a Hitachi S800 device at the CMEABG
Center of Lyon I Claude Bernard University. Transmission electron
microscopy (TEM) micrographs were obtained on a JEOL 2010
device with an accelerating voltage of 200 keV. Specific surface
areas and porosities were measured by low-temperature nitrogen
adsorption and calculated by means of Brunauer-Emmett-Teller
11
program ) combined with Fourier difference syntheses and refined
12
against F using reflections with I/σ(I) > 3 (CRYSTALS program ).
All important bond lengths and angles are summarized in Tables 2
and 3. For all of the non-H atoms, the agitation thermal factors
(10) Nonius. COLLECT, DENZO, SCALEPACK, SORTAV; Kappa CCD
Program BV: Delft, The Netherlands, 1999.
(11) Cascarano, G.; Altomare, A.; Giacovazzo, C.; Guagliardi, A.; Moliterni,
A. G. G.; Siliqi, D.; Burla, M. C.; Polidori, G.; Camalli, M. Acta
Crystallogr. 1996, A52, C-79.
(12) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
CRYSTAL Issue 11; Chemical Crystallography Laboratory: Oxford,
U.K., 1999.
2304 Inorganic Chemistry, Vol. 47, No. 7, 2008

Nickel and Cobalt Hexamethylentetramine Complexes
Figure 1. Illustration of the labeling for compounds 1 and 2. Thermal
ellipsoids are represented with a probability of 30%.
and Barrett-Joyner-Halenda equations. Adsorption measurements
were performed on a Micromeritics ASAP2020 volumetric adsorp-
tion apparatus. Prior to measurement, the samples were degassed
at 473 K for 4 h.
Extended X-ray absorption fine structure (EXAFS) experiments
were carried out at the X1 beamline of the HASYLAB facility
(Hamburg, Germany) using synchrotron radiation from the DORIS
III ring. The Ni and Co K edges were monitored in the transmission
mode, using a double Si(111) crystal monochromator. To character-
ize the metal environment evolution during heat treatments, a cell
for in situ measurements was used. The cell was heated in an oven
and crossed by the X-ray beam, while a nitrogen flow was
1
3
maintained. The EXAFS data were treated with FEFF and
14
VIPER programs. Then the edge background was extracted using
Bayesian smoothing with a variable number of knots. The curve
fitting was done alternatively in the R and k spaces, and the fit was
accepted only in the case of simultaneous convergence in k and R
spaces (absolute and imaginary parts for the latter). Coordination
Figure 2. Projection of the structural packing for both structures along
the c axis of the unit cell. For clarity, H atoms have been omitted.
numbers (CNs), interatomic distances (R), Debye-Waller param-
2
eters (σ ), and energy shifts (∆E
0
) were used as fitting variables.
Constraints introduced relating to the fitting parameters were to
obtain values at physically reasonable intervals. The quality of the
fit was evaluated using the values of variance and goodness.
3
. Results and Discussion
.1. Structure of the Precursor Complexes. Macroscopic
3
platelike crystals of Ni- and Co-HTMA complexes were
spontaneously grown from aqueous solutions. The rose and
light-green colors of the crystals for cobalt and nickel,
respectively, were similar to the colors of the corresponding
nitrates, indicating that no change of metal coordination
occurred as a result of HMTA introduction. The results of
Figure 3. ORTEP graph showing the 16-membered hydrogen-bonded ring
of the cationic network with participation of coordinated water. Nitrate
groups and noncoordinated water molecules are removed for the presentation
clarity.
chemical analysis suggest the stoichiometry (NO
3
)
2
Me-
2
+
2+
(
H
2
O)
6
(HMTA)
2
2
·4H O (with Me ) Ni , Co ), which fits
are located in a perfect octahedral environment built from six
water molecules (Table 2), with no more atoms being in special
positions. As follows from the geometric parameters of the
structure, HMTA is not bound to the metal, remaining outside
perfectly with the single-crystal XRD results.
Both substances crystallize in the triclinic system, exhibit
very close unit cell parameters (Table 1), and then present
exactly the same structure. For clarity of the structural
description of both compounds, the same labeling was used
2 6
the coordination shell. Adding to the Me(H O) complex (with
Me ) Co or Ni) and HTMA, four free water molecules were
found in the structure. Then, structural cohesion is assumed by
hydrogen bonds building a 3-D network (Figures 2-4 and
Tables 2 and 3). HMTA remains in the outer sphere probably
because of its relatively low basicity.
(
Figure 1). Metal ions are placed in the inversion center and
(
(
13) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. ReV.
B 1998, 58, 7565.
14) Klementiev, K. V. J. Phys. D: Appl. Phys. 2001, 34, 209. www.
desy.de/∼klmn/viper.html, VIPER for Windows, freeware.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2305

Afanasiev et al.
and O3) and another in which noncoordinated water participates
(O4) together with coordinated water (O3) (Figure 4).
Nitrate anion molecules from different layers are connected
through the coordinated water molecules, which creates
chains interconnecting the adjacent HMTA layers along the
z direction (-N1-O7-H10-O1-Co-O1- chain). At the
2
same time, nitrate anions form standalone R
4
(12) rings
within the anionic layers with participation of only nonco-
ordinated water (-N1-O6-H50-O5-H51-O8- chain).
Interpenetration of these hydrogen-bonded patterns creates
a 3-D framework. It would be interesting to study the nature
of anionic and cationic moieties on the hydrogen-bonding
trends in such HMTA-based supramolecular networks. Our
preliminary tests (2001, unpublished work by P.A.) showed
that, besides cobalt and nickel, other metals such as tin,
cadmium, iron, and zinc are able to form well-crystallized
complexes of the described type, whereas sulfate, acetate,
chloride, and perchlorate may be involved in the anionic
networks. Without much doubt, this list can be extended even
more. It might be interesting to study how the metal
coordination ability and/or anion properties influence the
hydrogen bonding in such structures. Another possibility to
construct supramolecular assemblies with participation of
HMTA is to associate it with some metallic oxoanions. In
this last case, ammonium or alkylammonium (but not
tetraalkylammonium) is a convenient species to build up the
cationic part of the network. An example of such a complex
was the earlier described ammonium heptamolybdate-HMTA
compound for which the unit cell was indexed but the
Figure 4. ORTEP graph representing a R16 ring of hydrogen bonds with
participation of noncoordinated water (O4).
While HMTA can formally be considered as an aliphatic
amine (tetraazaadamantane), its complexing properties are
much weaker than those of other polyamines such as
ethylenediamine. At the same time, it belongs to the family
of good proton acceptors for installing hydrogen bonds, as
1
5
defined earlier by Donohue.
The presence of four symmetrically placed N atoms and
the overall cubic symmetry of the HMTA molecule allow
the formation of up to four hydrogen bonds, thus generating
hydrogen-bonded supramolecular frameworks. Indeed, su-
pramolecular adducts of HMTA with organic and inorganic
hydrogen-bonding donors have been described, such as
16
layered bis(hexamethylenetetramine) diiodozinc(II) or mul-
tiple helices and strings in the adducts with bisphenols and
1
7
trisphenols.
With the preparation technique being extremely simple,
the complexes of HMTA with nickel and cobalt sulfate were
prepared yet in 1982 by Ahuja et al., but no structural
9
structure is still not resolved. Unfortunately, a more detailed
discussion of this interesting topic is beyond the scope of
the present work.
18
characterization was done. Thermal analysis of the HMTA
adducts to hydrated L Cl compounds L Cl · 0.2HMTA ·
nH O (L ) La, Pr, Nd, Sm, Dy, Er; n ) 8, 10, 12) was
n
3
n
3
3
.2. Decomposition Mechanism. As follows from the
2
structural characterization, in the 3-D frameworks of com-
pounds 1 and 2, hydrated Co cations, and nitrate anions are
separated in the layers by HTMA molecules. Therefore,
decomposition upon heating is expected to proceed differ-
ently compared to that of cobalt nitrate (which gives Co O
3 4
as a solid product at ca. 500 K). We observed that
1
9
reported by Zalewicz, focused on the low-temperature
dehydration processes. These nonexhaustive examples dem-
onstrate a high ability of HMTA to produce supramolecular
compounds with various H donors, including organic acids
and metals of main and transition groups.
As shown in the analysis of hydrogen-bond connectivity
in the structures of 1 and 2, all N atoms of HMTA are
involved in the hydrogen bonds, this creating a 3-D su-
pramolecular assembly. Viewed along the [001] direction,
the structure shows a hexagonal-like packing motif in which
every Co(H O) octahedron is surrounded by six HTMA
2 6
molecules, which forms a trigonal antiprism. Perpendicular
to [001], the structure can be alternatively represented as
decomposition of compounds 1 (cobalt complex) and 2
(nickel complex) in the temperature range 573-1073 K
produced abundant black powder products. XRD patterns
of the products indicate the formation of corresponding
metals mixed with amorphous carbon. The differential
thermal analysis curves showed somewhat different decom-
position curves for nickel and cobalt compounds. The cobalt
complex produced a very strong and narrow exothermic peak
around 500 K, followed by a plateau of almost constant mass,
whereas the nickel complex decomposed in several ill-
resolved steps with total mass losses of 69 and 70 wt %, as
can be seen in Figure 5. In both cases, the mass spectra of
released gases indicate the formation of CO, ammoniac, some
being lamellar, cationic layers and anionic layers of nitrates
4
separated by HMTA (Figure 2). In the cationic layer, R
4
(16)
20
rings are formed by two hydrated Co ions and two HMTA
molecules (Figure 3). Two types of such rings could be
identified: one containing only coordinated water donors (O2
2
NO, and CH O (Figure 5a).
(
(
(
15) Donohue, J. J. Phys. Chem. 1952, 56, 502.
16) Pickardt, J.; Droas, P. Acta Crystallogr. 1989, C45, 360.
17) Coupar, P. I.; Glidewell, C.; Ferguson, G. Acta Crystallogr. 1997,
B53, 521.
To explain high decomposition rates at relatively low
temperatures, two factors should be taken into account. First
of all, the presence of oxidizing nitrate and reducing HMTA
may lead to ignition of the compound and its spontaneous
decomposition as it occurs in the organic nitrates, although
(
(
(
18) Ahuja, I. S.; Yadava, C. L.; Singh, R. J. Mol. Struct. 1982, 81, 229.
19) Zalewicz, M. Thermochim. Acta 1989, 149, 133.
20) Etter, M. C. Acc. Chem. Res. 1990, 23, 120.
2306 Inorganic Chemistry, Vol. 47, No. 7, 2008

Nickel and Cobalt Hexamethylentetramine Complexes
Figure 6. In situ XAS spectra of the cobalt (a) and nickel (b) compounds
upon their heating under a nitrogen flow.
Figure 5. Mass spectra of gases evolved upon heating of a cobalt-containing
complex in flowing argon, 5 K min- (a), and thermogravimetric curves of
decomposition at the same conditions for nickel- and cobalt-containing
complexes (b).
1
edge structures (XANES) region of compounds 1 and 2
heated under nitrogen at different temperatures are shown
in Figure 6. For both solids, the edge shape was changed in
the same way from the initial substance to the products of
heating. The characteristic white line attributed to cobalt and
nickel in the oxygen environment was present in the initial
solids but decreased upon heating, which indicates an
increase of the electron population in the cobalt (nickel) third
level. Thus, we infer from the XANES part of the spectra
that the cobalt and nickel coordination changed from the
oxygen octahedra in the 1 and 2 precursors to the disordered
and partially reduced states already at relatively low
temperatures.
much less violently, because of the presence of water and
the very negative oxygen balance of these complexes.
Second, cobalt is known to strongly accelerate decomposition
2
1,22
of nitrate;
this might further increase the autocatalytic
nature of the reactions involved.
The mass loss and the nature of gases produced allow for
the proposal of the main decomposition pathway as follows:
(
NO ) Me(H O) (C H N ) · 4H O f Me + 6C +
3 2 2 6 6 12 4 2 2
1
0H O + 6CO + N + 8NH (1)
2 2 3
Besides this reaction, some secondary processes surely occur
during decomposition, leading to some NO formation. As
follows from the stoichiometry, all carbon cannot leave from
the solids as CO, even if all oxygen is used to produce carbon
oxide during decomposition. Indeed, in all specimens
considerable amounts of residual carbon are detected. The
calculated amount of carbon in the solid residual formed
according to reaction (1) is 56%, whereas its real amount
varied from 45 to 50% according to the specimen. Moreover,
some oxygen is consumed to produce NO, unambiguously
detected by mass spectrometry. At last, heavy organic
residuals observed on the cold parts of the reactor after
decomposition indicate that part of HMTA probably leaves
the complex without reaction (by sublimation), which
explains the deviation from the stoichiometry of reaction (1).
However, reaction (1) seems to provide a good general
description of the decomposition of complexes 1 and 2.
The Co and Ni K edges in the X-ray adsorption near-
Analysis of the in situ EXAFS spectra as a function of
the heating temperature reveals fine details of the decom-
position mechanism. For both metals, evolution is similar,
but this issue can be better illustrated by Ni K spectra because
in the case of nickel the evolution is smoother. In the initial
1
and 2 compounds, the most important peaks on the Fourier
transform (FT) of EXAFS spectra correspond to those
expected from the structures of six oxygen neighbors (Figure
7). Because there are no close-standing heavy neighbors in
the structure, the EXAFS spectra can be fairly well described
2
+
2 6
by these first six oxygen shells of the hydrated M(H O)
ions (Table 4), with the contribution from HMTA and nitrate
being less important. After heating to 373 K, the position of
this main peak virtually does not change but the intensity
decreases twice (losing crystallinity). At the same time, some
new and far-standing heavy (nickel and cobalt) neighbors
begin to appear, which are very positionally disordered (see
Debye–Waller factor estimations in Table 4). This situation
corresponds to the loss of crystallographic water and the solid
amorphization. At this stage of 10–20% weight loss, the onset
(
(
21) Xu, R.; Zeng, H. C. Chem. Mater. 2003, 15, 2040.
22) Ehrhardt, C.; Gjikaj, M.; Brockner, W. Thermochim. Acta 2005, 432,
3
6.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2307

Afanasiev et al.
23
the Ni atom, whereas we observe about four C atoms. Even
for the relatively low precision of CN determination from
EXAFS, this is a very significant difference. Therefore, we
cannot postulate any structural relationship between the
known nickel and cobalt crystalline carbides and the
amorphous carbide that we observe by EXAFS. It seems
rather that Ni and Co atoms are embedded into the
amorphous carbonaceous matrix. This can be expected from
the large excess of carbon, in both the precursor and the
decomposition products.
In any case, both experimental and theoretical data suggest
the instability of existing nickel and cobalt carbides and their
decomposition to carbon and the corresponding metals.
Figure 7. FT module of the EXAFS spectra, measured in situ upon heating
of compound 2 under nitrogen at different temperatures.
2
4
Indeed, Goodman et al. studied the kinetics of Ni
transformations using Auger spectroscopy and observed very
rapid decomposition of Ni C with th eformation of graphite
3
C
Table 4. Evolution of the Coordination State of Nickel during Heating
of Compound 2 under Nitrogen, as Inferred from the Ni K Edge
EXAFS Fit Results
3
2
2
atom type
R (Å)
N
σ (Å )
∆E 0 (eV)
at 700-800 K. Density functional theory calculations also
corroborated the metastability of nickel and cobalt carbides.
Initial 2
5.7(4)
O
2.05(1)
0.007(1)
-1.0(4)
The formation energy of Ni
nickel or cobalt and graphite is positive; i.e., the M
are metastable or unstable at ambient conditions. These
results are in line with our observations.
3
C calculated with respect to bulk
C phases
3
73 K
O
Ni
Ni
2.06(2)
3.14(3)
3.64(5)
4.2(5)
5.2(5)
4.2(5)
0.009(2)
0.030(5)
0.022(7)
-1.0(4)
-1.0(4)
-1.0(4)
3
2
5
4
93 K
C
O
Ni
1.81(2)
2.21(2)
2.82(2)
4.5(3)
2.3(2)
3.5(5)
0.008(1)
0.008(1)
0.008(1)
-2.0(4)
-2.0(4)
-2.0(4)
3.3. Characterizations of the Decomposition Prod-
ucts. Core–shell magnetic particles are presently a hot
research topic. Indeed, metal-carbon composite materials
are attractive for several applications including magnetic
5
53 K
C
Ni
1.83(2)
2.42(2)
4.1(5)
2.1(5)
0.015(5)
0.021(7)
-3.2(4)
-3.2(4)
2
6–28
29
30
materials,
electrochemical applications.
supercapacitors, and catalysts and for
7
73 K
31,32
For some of these applica-
C
Ni
1.82(3)
2.42(3)
0.2(1)
5.5(2)
0.020(5)
0.008(1)
-3.0(5)
-3.0(5)
tions, carbon-encapsulated metal particles represent the most
desirable form of a metal-carbon composite material. These
include magnetic materials and magnetically separable ad-
sorbents and/or catalysts in which the carbon surface is
exposed while magnetic metal nanoparticles are protected
by the carbon shell. Because of such a core–shell structure,
of structure collapse is evident and probably Co (Ni) atoms
become closer.
A very important change of the metal environment occurs
suddenly near the main weight loss temperature, 493 K. At
the level of ca. 40% weight loss, the oxygen peak near 2.0
Å disappears and leaves in its place two new types of
neighbors at 1.81 and 2.21 Å. The first one can be attributed
to carbon because among the elements involved only carbon
may provide such a short interatomic distance with nickel.
The second type of neighbor at 2.21 Å is presumably the
residual oxygen, which completes the coordination shell of
the metal, its CN remaining close to six. While this drastic
change occurs, the far-standing metal neighbor shifts to 2.8
Å; i.e., it continues to come closer to the central atom.
Soon after the main weight loss (523 K), the remaining
oxygen leaves and the metal-metal distance appears, close
to that in the bulk metal. Carbon still remains the main
element in the coordination shell. However, upon further
heating (573 K and further), the coordination rapidly evolves
toward that of the bulk metal. At 777 K, no presence of
carbon neighbors could be reliably inferred from the EXAFS
spectrum fitting.
3
3
these materials possess high chemical stability, while
nonprotected carbon-supported metals particles are easily
3
4
oxidized even at low temperatures.
Owing to these advantageous properties, various methods
for synthesis of the core–shell carbon-encapsulated nano-
(
(
23) Nagakura, S. J. Phys. Soc. Jpn. 1958, 13, 1005.
24) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; White, J. M. J. Catal.
1980, 64, 479.
(
25) Shein, I. R.; Medvedeva, N. I.; Ivanovskii, A. L. Physica B 2006,
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71, 126.
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J. Appl. Phys. 2005, 97, 044313.
(
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27) Wang, H.; Wong, S. P.; Cheung, W. Y.; Ke, N.; Chiah, M. F.; Liu,
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602.
(
(
29) Li, W.; Reichenauer, G.; Fricke, J. Carbon 2002, 40, 2955.
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Zhecheva, E.; Mateos, J. M. J. N. J. Electrochem. Soc. 2004, 151,
A2113.
(
Overall, the in situ EXAFS experiments demonstrate that
the decomposition occurs via the intermediate amorphous
product in which nickel (cobalt) has mostly carbide-like
coordination. However, the crystalline structure of the only
(32) Alcántara, R.; Lavela, P.; Ortiz, G. F.; Tirado, J. L.; Stoyanova, R.;
Zhecheva, E.; Merino, C. Carbon 2004, 42, 2153.
(
(
33) Lu, A. H.; Li, E. C.; Matoussevitch, N.; Spliethoff, B.; Bönnemann,
H.; Schüth, F. Chem. Commun. 2005, 98.
34) Chernavskii, P. A.; Pankina, G. V.; Chernavskii, A. P.; Peskov, N. V.;
Afanasiev, P.; Perov, N. S.; Tennov, V. A. J. Phys. Chem. C 2007,
111, 5576.
known carbide Ni
3
C predicts only two carbon neighbors for
2308 Inorganic Chemistry, Vol. 47, No. 7, 2008

Nickel and Cobalt Hexamethylentetramine Complexes
particles have been developed, including high-temperature
3
5,36
techniques such as arc discharge
depositions.
of chemical vapor
3
7,38
Although these methods have succeeded in
synthesizing carbon-encapsulated transition metals, the high-
temperature process of the methods hinders the preparation
of the carbon-encapsulated nanoparticles of the metastable
phase and low-melting-point metals.
To decrease the preparation temperature, decomposition
of molecular precursors has been studied, for example,
carbon-encapsulated iron by pyrolysis of ferrocene in pure
39
hydrogen. Other examples include the preparation of cobalt
nanoparticles from the [bis(salicylidene)cobalt(II)]-oleylamine
4
0
complex by thermal decomposition and acetylide decom-
position, leading to encapsulated cobalt, nickel, and pal-
Figure 8. Nitrogen adsorption–desorption isotherms for the cobalt (1) and
nickel (2) solids after heating at 773 K. For presentation convenience, the
curve of 2 was shifted down by a value of 20 cm /g.
4
1
ladium. Combined techniques were also reported such as
two-step preparations including the glycothermal preparation
3
4
2
of metal nanoparticles followed by carbonization.
Investigation of the morphology of the decomposition
products using electron microscopy and surface area mea-
surements reveals that these are solids with open pores and
2
2
high specific surface areas (190 m /g for cobalt and 165 m /g
for nickel, both obtained at 823 K). A spongelike morphology
is characteristic for all of the specimens studied. A similar
morphology was found for nickel- and cobalt-containing
specimens, whatever the final temperature, provided it was
higher than 673 K. Probably, the morphology is determined
by decomposition, which occurred rapidly at relatively low
temperature. In this case, the heating rate and the final
temperature should be of less importance.
Like any foams, the solids obtained are fractal structures
with polymodal pore distributions. Low-temperature nitrogen
adsorption–desorption isotherms showed very broad pore-
size distributions with pronounced mesoporosity and macro-
porosity features (Figure 8). At the same time, the nitrogen
desorption isotherms suggest a relatively small share of the
microporosity in the total pore volume, though the latter is
often observed in the carbon-based materials obtained from
Figure 9. SEM images of the metal–carbon composite foam obtained by
the decomposition of the nickel complex (2) at 823 K. Images were taken
at magnification 6000 (left, a) and 40 000 (right, b).
pressure 0.7–0.8, the isotherms demonstrate sharp slopes,
indicating that they contain macrospores. The micropore
volume percentage of the total pore volume in both samples
was less than 15–20%.
43
pyrolysis of organics. High surface area and large pore size
for the preparations reported here are probably related to the
large amount of gases produced during the decomposition.
From the six kinds of adsorption isotherms by IUPAC
At the same time with mesopores accessible by nitrogen
adsorption, spherical open macropores of different size
ranging approximately from 100 nm to 1 µm were observed
straightforwardly by SEM (Figure 9). The energy-dispersive
X-ray (EDX) analyses coupled with SEM and TEM observa-
tions confirm that the grains correspond to metals, whereas
the surrounding matter is carbonaceous. Metal particle sizes
are similar for cobalt and nickel, with nickel grains slightly
larger and more anisotropic. Visual inspection of TEM
images gives the value between 10 and 20 nm (Figures 10
and 11). Statistical treatment of representative images for
the specimens heated at 773 K (Figure 12a) shows rather
narrow particle size distribution centered at 12 nm. The
distribution is not symmetric, which is expected for a
kinetically controlled process.
4
4
classification, the solids are meso- and macroporous
showing type II shape. In the low-pressure region, the
isotherms have inflection points at relative pressures of less
than 0.05. At these points, samples have absorbed only about
3
2
0 cm /g of nitrogen. Then, in the intermediate domain, the
adsorbed volume increases almost linearly. Over the relative
(
(
(
35) Scott, J. H. J.; Majetich, S. A. Phys. ReV. B 1995, 52, 12564.
36) Huo, J.; Song, H.; Chen, X. Carbon 2004, 42, 3177.
37) Flahaut, E.; Agnoli, F.; Sloan, J.; O’Connor, C.; Green, M. L. H. Chem.
Mater. 2002, 14, 2553.
(38) Wang, Z. H.; Choi, C. J.; Kim, B. K.; Kim, J. C.; Zhang, Z. D. Carbon
2
003, 41, 1751.
(
(
39) Sano, N.; Akazawa, H.; Kikuchi, T.; Kanki, T. Carbon 2003, 41, 2159.
40) Salavati-Niasari, M.; Davar, F.; Mazaheri, M.; Shaterian, M. J. Magn.
Magn. Mater. 2008, 320, 575.
The final temperature does not strongly influence the
particles size. This was inferred from the comparison of two
XRD patterns of cobalt-containing solids, heated at 773 and
1073 K, respectively (not shown). As follows from Scherrer
calculations, an increase of the heating temperature by 300
K led only to a modest particle size increase from 6.6 to 8
(
(
(
(
41) Nishijo, J.; Okabe, C.; Oishi, O.; Nishi, N. Carbon 2006, 44, 2943.
42) Zhu, G. X.; Wei, X. W.; Jiang, S. J. Mater. Chem. 2007, 17, 2301.
43) Steel, K. M.; Koros, W. J. Carbon 2005, 43, 1843.
44) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Perotti,
R. A. Pure Appl. Chem. 1985, 57, 603.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2309

Afanasiev et al.
TEM yielded real size distribution statistics for each sample,
the XRD Scherer analysis led to a “mean” particle size. This
discrepancy is well documented and was attributed to the
4
5
different weightings inherent in the use of XRD.
Inspection of the SEM and TEM images for the 1 and 2
decomposition products leads to that conclusion that for any
heating temperature the pore walls have a very broad
distribution of thicknesses between 10 and 500 nm, with the
maximum of distribution at ca. 60 nm (Figure 12b).
Compared to the particle size, the mean wall thickness is
significantly greater. The metal particles are arranged within
the pore walls, and analysis of the tilted images suggests
that there are almost no contacts between the particles (Figure
Figure 10. TEM images of the metal–carbon composite foam obtained
from the nickel complex (2) at magnification 50 000 (a) and a stand-alone
particle encapsulated within lamellar carbonaceous matter (b).
10b). This partially explains the high stability of the particle
size to the heating. At the same time, high-resolution TEM
shows that the metal particles are covered with onionlike
carbon shells. These carbon shells probably have a stabilizing
effect on the particles size, which remained nearly constant
when the final decomposition temperature was increased. Our
preliminary studies show that the materials have a saturation
magnetization of about 50 emu/g in the case of cobalt. The
materials withstand heating in air to 500 K without a
significant magnetization drop. As with the oxidation stability
under ambient conditions, it is worth noting that these
materials, first prepared by one of the authors in 2001 (see
ref 9) and stored in air, still keep virtually the same
magnetization values as freshly prepared solids.
Figure 11. Representative TEM images of solids 1 (a) and 2 (b) after
heating at 773 K, used for the statistical determination of the particle size.
4
. Conclusions
Cobalt and nickel are very important metallic materials,
with a wide range of applications related to their catalytic
and magnetic properties. However, there are quite a few
efficient methods to produce porous solids of high surface
area. Modifications of the Raney technique remained until
recently the most frequently used approach to the preparation
of these metals in a highly dispersed state appropriate for
adsorption and catalysis. However, in the last few years, a
boost of research activity was observed on the preparation
of nanoscopic nickel and cobalt, using various physical or
4
6–48
chemical methods.
For the preparations described here, the important point
is that the newly synthesized molecular precursors are simple
compounds with definite structure. This allows avoidance
of any uncertainty in the processing stages prior to the
preparation of metal nanoparticle foams. HMTA is an easily
available compound produced by the reaction of ammonia
with formaldehyde. HMTA is very cheap and nontoxic unlike
many other precursors such as aliphatic (di)amines. This
makes our approach as environmentally friendly as possible
for these types of materials (any nickel and cobalt compounds
(
45) Ashcroft, A. T.; Cheetham, A. K.; Harris, P. J. F.; Jones, R. H.;
Figure 12. Size distribution histogram for particles obtained from the
decomposition of 2 at 773 K (a) and distribution of the shell thickness in
the same sample (b).
Natarajan, S.; Sankar, G.; Stedman, N. J.; Thomas, J. M. Catal. Lett.
1
994, 24, 47.
(
46) Sergiienko, R.; Shibata, E.; Zentaro, A.; Shindo, D.; Nakamura, T.;
Qin, G. Acta Mater. 2007, 55, 3671.
nm. Note that calculation of the particle size from the XRD
patterns using the Scherrer formula gave an average size
somewhat smaller than the results of the TEM study. While
(47) Singjai, P.; Wongwigkarn, K.; Laosiritaworn, Y.; Yimnirun, R.;
Maensiri, S. Curr. Appl. Phys. 2007, 7, 662.
(
48) Baranauskas, V. V.; Zalich, M. A.; Sanders, M.; St. Pierre, T. G.;
Riffle, J. S. Chem. Mater. 2005, 17, 5246.
2310 Inorganic Chemistry, Vol. 47, No. 7, 2008

Nickel and Cobalt Hexamethylentetramine Complexes
are toxic by definition). The precursor′s preparation occurs
in one step in an aqueous solution at ambient conditions.
Therefore, this method using simple and cheap precursors
seems suitable for large-scale production of catalysts or
magnetic materials.
Because of the presence of a high amount of nitrogen and
some oxygen in the precursor, large amounts of gaseous
water, CO, and nitrogen are formed during the decomposi-
tion. Exothermal interaction of nitrate with amine leads to a
strong gas dilatation, and the foam is formed. The foam
produced possesses a highly porous structure.
The precursors are highly soluble in water (though less
than the initial nitrate), and the solution is stable. Evaporation
leads to the reprecipitation of the precursor. Therefore, the
magnetic foam can be obtained within any template with
appropriate cavities by simple heating of an impregnated
solid.
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC7013013
Inorganic Chemistry, Vol. 47, No. 7, 2008 2311