Mechanism of thermal decomposition of d-metals nitrates hydrates

Article abstract of DOI:10.1023/A:1010155931266

A series of six nitrates(V) hydrates of 4d-metals as well as mercury and cadmium thermal decomposition was examined by DTA, TG and EGA techniques. It was found that thermal decomposition of d-metals nitrate(V) hydrates proceeds in three stages: partial dehydration, oxo-nitrates and hydroxide nitrates formation and metal oxides formation. General chemical equations for all decomposition stages were proposed. It was found that dehydration of hydrated salts is accompanied by partial decomposition of nitrate(V) groups.

Full text of DOI:10.1023/A:1010155931266

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 17–23
MECHANISM OF THERMAL DECOMPOSITION OF
d-METALS NITRATES HYDRATES
A. MaÓecki, R. Gajerski, S. ˜abuí, B. Prochowska-Klisch and
K. T. Wojciechowski
Faculty of Materials Science and Ceramics, University of Mining and Metallurgy
Al. Mickiewicza 30, 30–059 Cracow, Poland
Abstract
A series of six nitrates(V) hydrates of 4d-metals as well as mercury and cadmium thermal decompo-
sition was examined by DTA, TG and EGA techniques. It was found that thermal decomposition of
d-metals nitrate(V) hydrates proceeds in three stages: partial dehydration, oxo-nitrates and hydrox-
ide nitrates formation and metal oxides formation. General chemical equations for all decomposition
stages were proposed. It was found that dehydration of hydrated salts is accompanied by partial de-
composition of nitrate(V) groups.
Keywords: DTA, EGA, d-metals, nitrates, thermal decomposition
Introduction
d-Metal nitrates hydrates are the class of substances commonly used for preparation
of high surface area materials. From this point of view the detail microscopic mecha-
nism as well as the kinetics of thermal decomposition of these salts is important to
know. There are some reports concerning decomposition of nitrates but general
mechanism of this process has not been found so far [1–3].
The goal of the work undertaken in our laboratory is to find regularities concern-
ing these reactions. In the study reported here, a series of six nitrates(V) hydrates of
4
d metals as well as mercury and cadmium were investigated. Special attention will
be paid to the influence of cation ability to change its oxidation state on the mecha-
nism of thermal decomposition.
Experimental
Results presented in this work are based on earlier investigations on decomposition of
Cr(NO ) ·9H O, Co(NO ) ·2H O and Ni(NO ) ·2H O [4, 5]. Additionally preliminary
investigations on decomposition of the following nitrates were made: Fe(NO ) ·6H O,
3
3
2
3 2
2
3 2
2
3
3
2
Cu(NO ) ·2.5H O, Zn(NO ) ·6H O, Cd(NO ) ·4H O and Hg(NO ) ·nH O [6–8]. Com-
3 2
3
2
2
2
3 2
2
3 2
2
pounds used in experiments were commercial, pure for analysis.
1
418–2874/2000/ $ 5.00
2000 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
©
Kluwer Academic Publishers, Dordrecht

1
8
MA£ECKI et al.: d-METALS NITRATES HYDRATES
Studies of thermal decomposition processes were pursued using thermogravi-
metric analysis (TG) and differential thermal analysis (DTA) on SDT 2960 TA IN-
STRUMENTS and evolved gas analysis (EGA) of volatile products with quadrupole
mass spectrometer QMS ThermoStar by Balzers. The QMS was operated with elec-
tron energy of 700 eV.
Additionally, nitrate oxides evolved during decomposition were oxidized by
ozone, then absorbed in solution and analyzed as nitrate ions using ion-selective elec-
trode.
Solid products of decomposition were identified by X-ray diffraction analysis as
well as by chemical analysis (gravimetric and volumetric). Absorption spectrophoto-
metry was used to determine a ratio of metal ion contents in different oxidation states.
Results and discussion
It is characteristic of all studied metal nitrates that they crystallize from water solu-
tions as ionic hydrates and that their dehydration cannot be achieved by heating, be-
cause dehydration proceeds together with partial decomposition of nitrate groups.
The only exception is cadmium nitrate hydrate. Two causes of this can be pointed out.
–
Nitrate ion NO has got flat crystallographic structure, with three-fold axis of
symmetry, which goes through the metal atom and is perpendicular to the nitrate ion
3
–
plane. In the simplest case oxygen in NO has formal charge equal 1/3 elementary
3
charge unit. For small cations of d-metals, the typical configuration in the crystallo-
graphic lattice of nitrates(V) with coordination number 6 provides neutralization of
cation charge at its oxidation state equal +2. In this case anhydrous nitrates could be
obtained by thermal dehydration. In fact, it is possible only for Cd(NO ) , other dis-
3 2
cussed nitrates can be produced only by synthesis in non-water medium [9].
For the present, the crystallographic structure of the anhydrous nitrates(V) under
discussion is not known in detail though they are supposed to form van der Waals
crystals in solid state. If so, producing anhydrous salts by dehydration require com-
plete reconstruction of crystallographic lattice accompanied by partial decomposition
of nitrate(V) ions.
For small cations with oxidation state higher than +2, the coordination number
required for neutralization of electric charge by nitrate(V) ions (with formal charge
on oxygen equal 1/3) is too high in respect to geometrical conditions, such as is not
noticed in real compounds. This geometric problem vanishes if we consider
k+
hydration of cation which leads to formation of large ion M(H O) (k>2) with elec-
2
n
tric charge spread on its surface. The hydrated ion can coordinate adequate number of
k+
nitrate(V) ions to neutralize M charge. In that case dehydration of metal nitrate(V)
hydrate proceeds with partial decomposition of nitrate(V) ions.
–
Formally also alternate structure of NO ion exists which allows formation of
3
anhydrous nitrates(V). In this structure two oxygen ions have formal electric charge
equal 1/2 and the whole nitrate(V) ion acts as bidentate or bridge ligand. With such a
structure, high coordination number must not realize neutralization of charge of
structure unit. However anhydrous metal nitrates(V) which exhibit this alternate
J. Therm. Anal. Cal., 60, 2000

MA£ECKI et al.: d-METALS NITRATES HYDRATES
19
–
structure of NO ion form rather van der Waals crystals in solid state. Thus the mech-
3
–
anism of dehydration with changing NO ion structure from this with electric charge
3
uniformly distributed on oxygen ions to that with electric charge equal 1/2 only on
two oxygen ions is unlikely.
Another cause of forming hydrates by d-metals nitrates(V) is their ability to form
m+
aqua-complexes [M(H
2
O)
p
]
in water solutions. Thus the correct formula of hydrated
nitrate(V), M(NO ) ·nH O, should be as follows: [M(H O) ](NO ) ·(n–p)H O. This no-
3
m
2
2
p
3 m
2
tation shows that in the single molecule of salt differently bounded water molecules
semipolar, van der Waals and hydrogen bond) can exist, the consequence of which is
that dehydration proceeds by stages.
(
–1
3 2 2
Fig. 1 Non-isothermal decomposition of Zn(NO ) ·6H O (in helium, β=5°C min )
Figure 1 shows typical TG and DTA curves for d-metals nitrates(V) hydrates de-
composition by example of Zn(NO ·6H O.
Results obtained show that the first stage of thermal decomposition of ni-
)
3
2
2
trates(V) hydrates under investigation is partial dehydration frequently preceded by
melting:
o
3
0–60
C
M(NO ) ⋅nH O   →M(NO ) ⋅(n–q)H O+qH O
(1)
3
m
2
3
m
2
2
and (n–q) 2 (only for cadmium nitrate(V) (n–q)=0). In the next stage decomposition
proceeds with further dehydration accompanied by decomposition of nitrate(V) ions.
The image of this phenomenon depends on the cation nature, first of all on its ability
3
+
2+
to undergo oxidation. If cation can not be oxidized (e.g. Fe or Zn ), the second
stage of nitrates(V) decomposition leads to formation of nitrates(V) hydroxides ac-
cording to the following scheme:
o
≈
60–180
C
M(NO ) ⋅(n–q)H O   →M(OH) (NO ) +rHNO +(n−q−r)H
2
O
(2A)
3
m
2
r
3
m–r
3
J. Therm. Anal. Cal., 60, 2000

2
0
MA£ECKI et al.: d-METALS NITRATES HYDRATES
In fact (n–q) moles of H O means only summary content of water in the mole-
2
cule, not only crystallization water but also water bounded by van der Waals forces.
In Fig. 1 it can be noticed that increasing of H O signal corresponds to appearing
2
of HNO signal. On DTA curve, below 50°C an endothermic effect can be observed
3
without a change of mass on TG curve corresponding to melting of the sample. For-
mation of metal hydroxides nitrates(V) accompanied by dehydration solves the prob-
lem of neutralization of cation charge by surrounding anions discussed before. For-
mal charge on oxygen atom in OH group is –1 in respect to –1/3 in nitrate(V) ion.
Analogous situation occurs during Fe(NO
) ·6H O thermal decomposition.
3 3 2
m+
In the third stage of decomposition of M(NO ) ·nH O when M cannot be oxi-
3
m
2
–
dized, degradation of NO groups occurs in both those connected to cation and those
3
from HNO , formed in the second stage of decomposition. Degradation proceeds
3
with formation of NO, NO , N O and O . NO, NO and O were reported as decom-
2
2
2
2
2
position products in previous works, but significant amount of N O observed in pres-
2
ent investigations has not been reported yet. Generally, reactions in third decomposi-
tion stage can be written as:
o
150–108 C
3m–3r–2α
r
>
M(OH) (NO )    →MO +(m–r–α )NO+αNO +
O + H O
2
r
3
m–r
m/2
2
2
2
4
o
m–r
r
>
150–180
C
M(OH) (NO )    →MO
+
N O+(m–r)O + H O
2
r
3
m–r
m/2
2
2
2
2
o
70–200 C
2s+1
1
≈
HNO    →sNO+(1–s)NO +
O + H O
2
(3A)
3
2
2
2
4
In reactions (3A) two parallel processes of conversion of nitrate(V) ion were
taken into account because formation NO, NO and N O seems to be unlikely in re-
spect to N–N bond existing in N O but absent in other nitrogen oxides.
2
2
2
m+
3+
When cation M can not be oxidized (e.g. Fe ) during the reaction and m>2,
oxo-nitrates can be formed in the second stage of decomposition of d-metal ni-
trates(V), besides hydroxides nitrates:
o
60–180 C

r 
2 
≈
M(NO ) ⋅(n–q)H O   →MO (NO3 )m–r +rHNO + n–r– H O (3A’)
3
m
2
r /2
3
2

Oxo-nitrates decompose to oxides like hydroxide nitrates do. It is worth to em-
phasize that this way of decomposition provides easy neutralization of cation charge
2
–
in the lattice for O ion appears.
The mechanism of nitrates decomposition described above changes when cation
m1 +
m2 +
M
can become oxidized to M
Co(NO ) ·2H O and Cr(NO ) ·9H O (Figs 2 and 3). The first stage of decomposition
. Good examples of this can be decomposition of
3
2
2
3 3
2
of this type nitrates is analogous to mechanism of decomposition of nitrates described
above with cations which can not change their oxidation state. The second stage is
more complex than reaction (2A) and consists of additional reactions of electron
J. Therm. Anal. Cal., 60, 2000

MA£ECKI et al.: d-METALS NITRATES HYDRATES
21
–1
3 2 2
Fig. 2 Non-isothermal decomposition of Co(NO ) ·2H O (in helium, β=5°C min )
–1
3 2 2
Fig. 3 Non-isothermal decomposition of Cr(NO ) ·9H O (in helium, β=5°C min )
transport between anionic and cationic sub-lattice resulting in oxidation of cation and
reduction of nitrates(V) groups. Generally, it can be written as:
o
≈
60–180
C
m1 +
m2 +
MNO ) ⋅(n–q)H O   →M Mx O (OH) (NO ) +
3
m1
2
1– x
y
z
3 v

z+m –v–α –β 
1
αNO+βΝO +(m –v–α –β)HNO + n–q–
H O
(2B)
2
1
3
2
2


A feature characteristic of the reaction (2B) is a lack of oxygen in the products of
reaction. Elemental oxygen appears only in the third stage, in which further decom-
position of oxo-nitrates and hydroxide nitrates(V) occurs, according to one of two
possible reaction schemes:
J. Therm. Anal. Cal., 60, 2000

2
2
MA£ECKI et al.: d-METALS NITRATES HYDRATES
o
m1 +
m2 +
>120–150
C
M1– x Mx O (OH) (NO )    →
y
z
3 v
m1 +
m2 +
M1– x' Mx' O +α 'NO+β'NO +γ'O +δ'H O
y
2
2
2
(
3B)
o
>120–150 C
m1 +
m2 +
M1– x Mx O (OH) (NO )    →
y
z
3 v
MOm1 /2 +α ''NO+β''NO +γ''O +δ''H O
2
2
2
.
..+α ''NO+β''NO +γ''O +δ''H O
2
2
2
Another parallel way of reaction leading to formation of N O instead of NO and
2
NO is also observed. It is possible too, that in third stage, along with reactions (3B),
2
the following reaction takes place:
o
>150 C
 y' m1 
m1 +
m2 +
M1– x' Mx' O   →MO +
–
O
(3B’)
y'
m1 /2
2
 2
4

Some oxygen is formed during thermal decomposition of HNO like it is in case
3
of Co(NO ) ·2H O decomposition (Fig. 2). This is not the case for m >2, when oxy-
3
2
2
1
gen does not arise (or amount of oxygen is so small that hard to detect). If there is no
oxygen in decomposition products but HNO is present, reaction proceeds through
3
m1 +
m2 +
–
oxidation of M
to M
and NO ions are reduced without elemental oxygen lib-
3
eration. This can be illustrated by comparison of thermal decomposition of cobalt ni-
trate(V) hydrates and chromium nitrate(V) hydrates given in Figs 2 and 3.
The above presented series of chemical equations corresponding to thermal decom-
position of d-metals nitrate hydrates gives general view of reactions occurring during the
process of decomposition. However, a few questions remain without answers:
1
which NO ion being in one configuration undergoes or is it a result of processes
. Is formation of both NO and NO a consequence of two different processes
2
–
3
–
which undergo NO ions being in different configuration?
2
. What determines the [NO]/[NO ] ratio? It is only known that [NO]/[NO ] de-
2 2
pends somehow on the way the decomposition is carried out and the thermal history
of the sample investigated.
3
formed? It seems that formation of N O is only slightly dependent on the cation of the
. Another question concerns the mechanism of N–N bond creation while N O is
2
2
–
decomposed nitrate. Is N O a product of reaction between nitrate ions: 2NO →
2
3
2
N O+2O +O ? If so, oxidation of cation cannot be related to N O formation.
–
2
2
2
4
mal stability of NO ion in d-metals nitrates(V) in relation to alkali metals ni-
. The basic question remains without answer: what is a reason of lowering ther-
–
3
trates(V). For instance, KNO even at 600°C does not exhibit any decomposition
3
–
symptoms. May be the cause is that NO ions are the ligands in complex ions
3
(m1 +b)+
[
M(H O) (NO ) ]
or that a hydrogen bond exists in hydrates H..ONO . The
2
2
a
3 b
answer can be connected with the role of d-electrons in the mechanism of thermal de-
composition of nitrates(V) hydrates.
A full set of results concerning mechanism of thermal decomposition of all
d-metal nitrates(V) hydrates can give the answers to these questions.
J. Therm. Anal. Cal., 60, 2000

MA£ECKI et al.: d-METALS NITRATES HYDRATES
23
o
Polish State Committee for Scientific Research supported this work under project N 11.11.160.94.
References
1
2
3
4
J. Mu and D. D. Perlmutter, Thermochim. Acta, 56 (1982) 253.
B. V. L’vov and A. V. Novichikhin, Spectrochim. Acta Part B, 50 (1995) 1427.
J. G. Jackson, R. W. Fonsca and J. A. Holcombe, Spectrochim. Acta Part B, 50 (1995) 1449.
A. MaÓecki, A. MaÓecka, R. Gajerski, B. Prochowska-Klisch and A. Podgórecka, J. Thermal
Anal., 34 (1988) 203.
5
6
A. MaÓecki, R. Gajerski, S. ˜abuí and B. Prochowska-Klisch, J. Thermal Anal., 39 (1993) 545.
th
R. Gajerski, S. ˜abuí, A. MaÓecki and K. T. Wojciechowski, Proc. of 7 Conf. on Calorimetry
and Thermal Analysis CETTA’97, Poland, ed. University of £ód¿, 1997, p. 309.
th
K. T. Wojciechowski, R. Gajerski, S. ˜abuí and A. MaÓecki, Proc. of 7 Conf. on Calorimetry
7
8
9
and Thermal Analysis CETTA’97, Poland, ed. University of £ód¿, 1997, p. 80.
th
K. T. Wojciechowski, R.Gajerski, S. ˜abuí and A. MaÓecki, Proc. of 7 Conf. on Calorimetry
and Thermal Analysis CETTA’97, Poland, ed. University of £ód¿, 1997, p. 79.
A. F. Wells, Structural inorganic chemistry, Polish ed., WNT Warszawa 1993.
J. Therm. Anal. Cal., 60, 2000