Formation of N2O during thermal decomposition of d-metal hydrates nitrates

Article abstract of DOI:10.1016/j.tca.2006.02.006

It was found that during thermal decomposition of hydrates of d-metal nitrates nitrous oxide (N₂O) is always present among the other gaseous products: H₂O, HNO₃, NO, NO₂ and O₂. In all studied decomposition reactions the nitrous oxide formed contains approximately 5-8% of total nitrogen coming from nitrate. Two new mechanisms of N₂O formation, based on the reaction between NO or NO₂ and nitrate ion, have been proposed. The discussed third possible mechanism: (a) 2NO = N₂O₂ and (b) N₂O₂ + NO = N₂O + NO₂ is unlikely.

Full text of DOI:10.1016/j.tca.2006.02.006

Thermochimica Acta 446 (2006) 113–116
Formation of N2O during thermal decomposition
of d-metal hydrates nitrates
Andrzej Małecki∗, Barbara Małecka
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. Mickiewicza 30, 30-059 Krako´w, Poland
Received 30 November 2005; received in revised form 23 January 2006; accepted 3 February 2006
Available online 23 March 2006
Abstract
It was found that during thermal decomposition of hydrates of d-metal nitrates nitrous oxide (N2O) is always present among the other gaseous
products: H2O, HNO3, NO, NO2 and O2. In all studied decomposition reactions the nitrous oxide formed contains approximately 5–8% of total
nitrogen coming from nitrate. Two new mechanisms of N2O formation, based on the reaction between NO or NO2 and nitrate ion, have been
proposed. The discussed third possible mechanism: (a) 2NO = N2O2 and (b) N2O2 + NO = N2O + NO2 is unlikely.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Thermal decomposition; d-Metal nitrate; Nitrous oxide
ꢀ
to Men +, what can be described by the total equation:
1. Introduction
ꢀ
Generallythermaldecompositionof d-metalhydratesnitrates
can be described by the following equation [1]:
−
Men+ + NO3 → Men + + yNO + (1 − y)NO2
ꢀ
ꢁ
ꢀ
ꢁ
1 + 2y − ꢀn
ꢀn + 1
+
O2 +
O2−
(3)
Me(NO3) · qH2Oh−ea→tingMexOy + (HNO3, NO, NO2, O2, H2O)
4
2
n
(1)
where ꢀn = nꢀ − n > 0.
The whole process of decomposition consists of several consec-
utivereactionswhichwerepublishedin [2]intheformofscheme
given in Fig. 1. The mutual ratio of gaseous products: HNO3,
NO, NO2, O2 and H2O, depend on the metal nitrate as well as
on the experimental conditions of decomposition (i.e. mass of
the sample, heating rate, gas−over decomposing sample).
The degradation of NO3 ions proceeds according to the
following formal equation:
In fact, due to low probability of direct electron transport
from cation to anion, Eq. (3) should be considered as a sum of
elementary reactions. One elementary reaction is degradation of
NO3− described by Eq. (2) and the second—oxidation of Men+
described by Eq. (4):
ꢀn
ꢀ
ꢀn
Men+
+
O2 → Men +
+
O2−
(4)
4
2
2x + 1
1
O2 + O2−
2
−
NO3 → xNO + (1 − x)NO2 +
(2)
4
Thus, the oxidation of metal cation is a result of a sec-
ondary process in which oxygen produced during NO3− degra-
dation takes part. For instance, decomposition of Ni(NO3)2 in
inert atmosphere (Ni2+ does not change its oxidation state) is
described by the summary equation:
The oxidation state of metal Me in oxide MexOy is the same
as in metal nitrate only when cation Men+ can not be oxidized
(for example, Zn2+ or Cd2+). Otherwise, cation Men+ oxidizes
1 + y
∗
Corresponding author.
E-mail address: malecki@agh.edu.pl (A. Małecki).
Ni(NO3)2 → NiO + yNO + (2 − y)NO2 +
O2
2
0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.02.006

114
A. Małecki, B. Małecka / Thermochimica Acta 446 (2006) 113–116
Table 1
m/z with corresponding ions
m/z
Ion
12
14
16
17
18
28
30
32
C+
N+, N2
O+, O2
OH+
2+
2+
H2O+
CO+, N2
NO+
+
+
O2
Fig. 1. The main routes of thermal decomposition of hydrates of nitrates of
d-electron metals.
44
46
63
CO2+, N2O+
+
NO2
HNO3
+
while decomposition of Mn(NO3)2 in inert atmosphere (Mn2+
changes its oxidation state) is given by the following equation:
y
2
+
+
To distinguish between ICO and IN O the ionic current I12
Mn(NO3)2 → MnO2 + yNO + (2 − y)NO2 + O2
2
was recorded. The ionic current I12 o2f C+ which is a result of
fragmentation of CO2 molecule during its ionization by electron
beam in mass spectrometer can be used to extract the signal of
CO2 from the total current I44. When energy of electrons which
ionized CO2 molecule is 0.16 aJ (70 eV—standard in mass spec-
Unfortunately a significant number of papers devoted to ther-
mal decomposition of metal nitrates present results obtained
either without the analysis of gaseous products of decomposi-
tion or with the analysis of only main gases. The usefulness of
those results is limited from the point of view of determination
the reaction routes.
+
+
trometry) then the ratio IC /ICO is 0.025 [8]. This value for
our spectrometer was confirmed by independent measurements.
2
In this paper we show that, besides HNO3, NO, NO2, O2 and
H2O also nitrous oxide, N2O, is present among gaseous prod-
ucts of d-metal nitrates decomposition. N2O formation was not
discussed in earlier papers devoted to d-metal nitrates decom-
position. Only in paper [3], N2O is mentioned among gaseous
products of Mn(NO3)2 decomposition. The mechanism of N2O
formation will be discussed.
+
Taking this into account the ionic current IN O can be calcu-
lated as:
2
+
IC
+
IN O = I44
−
(5)
2
0.025
3. Results and discussion
The detailed TG/DTA/EGA-MS studies of thermal
decomposition of several hydrates of d-metal nitrates:
Cr(NO3)3·9H2O [4], Zn(NO3)2·6H2O [5], Mn(NO3)6·6H2O
[2], Co(NO3)2·6H2O [6], Ni(NO3)2·6H2O [2,7], Hg2(NO3)2
[9], Hg(NO3)2 [9] carried out by us showed that nitrous oxide
N2O is always observed among gaseous products together with
HNO3, NO, NO2, O2 and H2O. For all studied nitrates nitrous
oxide contains approximately 5–8% of total nitrogen coming
from nitrate. It is necessary to emphasize, that formation of
N2O during thermal decomposition is characteristic not only
for hydrates of d-metal nitrates. N2O was observed by us as a
product of decomposition of NaNO3, Ca(NO3)2, Al(NO3), etc.
[9]. Fig. 2. presents ionic currents of chosen gaseous products
(NO, N2O and O2) recorded during thermal decomposition of
mentioned d-metal nitrates. An example of N2O formation dur-
ing thermal decomposition of p-metal nitrate, Al(NO3)3·6H2O,
was showed as well. For clarity we do not present ionic currents
2. Experimental
All the experiments concerning thermal decomposition of
hydrates of nitrates of different metals were carried out with
commercial compounds in wide range of experimental condi-
tions taking into account isothermal and non-isothermal mea-
surements (heating rates varied in the range 0.1–20 K min−1),
mass of the samples (1–20 mg), gaseous atmosphere surround-
ing decomposing samples (helium, synthetic air with control
of humidity). During measurements TGA and DTA signals
were recorded (SDT 2960 TA INSTRUMENTS) simultaneously
with MS spectra of gaseous products of decomposition using
quadrupole mass spectrometer QMS 300 Thermostar, Balzers.
The m/z values (m—molar mass of Xz+ ion, z—charge of the
ion) for which the ionic currents were recorded by mass spec-
trometer as well as the species corresponding to individual m/z
are collected in Table 1.
+
for NO2. It is enough to remember that the course of I
The resolution of quadrupole mass spectrometer is approx-
imately equal to 1 amu, thus the ionic current Im/z = I44 can
correspond to N2O+ and as well to CO2+. It is necessary to
remember that before measurement the samples had contact
with CO2 always present in atmosphere and small amount of
it could be absorbed by the sample. Due to very high sensitivity
of mass spectrometer this CO2 is detected during the analysis
of evolving gases thus the total ionic current I44 is the sum of
NO2
+
with temperature is nearly the same as for INO and differences
between those two currents concern the values not the shape.
It seems that the general scheme of thermal decomposition of
Me(NO3)n·qH2O given in Fig. 1 should be completed with the
another way leading to nitrous oxide formation. Formally this
way could be described by the following reaction:
−
2NO3 = N2O + 2O2 + O2−
(6)
+
+
the currents coming from CO2 and N2O: I44 = I
+ IN O
.
CO2
2

A. Małecki, B. Małecka / Thermochimica Acta 446 (2006) 113–116
115
Fig. 2. Ionic currents of NO+, O2 and N2O+ for decomposition of some metal nitrates as a function of temperature (β = 2 ◦C min−1, sample mass 12 mg, helium).
+
However, N2O in contradiction to NO and NO2, cannot form
during thermal degradation of single NO3− anion and some kind
of synthesis is necessary. This synthesis in N2O formation is
required because of N N bond existing in nitrous oxide. Two
resonance forms of N2O molecule structure are:
nitrogen. The detailed quantum mechanics calculations show yet
that as a result of nitrogen atom hybridi+zation in NO molecule
−
and free electrons pair at N atom (: N = O·) the electron density
··
··
in NO is higher at nitrogen atom which is negative pole of dipole
[10–14]. In the symmetric, flat NO3− ion (sp2 hybridization) the
negative charge is delocalized between four atoms forming this
ion. Due to symmetry, electron density at oxygen atoms have to
be the same, and as a result of differences in electronegativity
excess of negative charge is observed at these atoms (Fig. 3).
That is why in all nitrates Me(+) (−)O NO2 bonds are observe−d
[15]. Theoretical studies of aqueous solutions containing NO3
ions confirm that negative charge is shifted to oxygen atoms
··
··
··
··
··
··
N N O ↔: N N O :
Additionally, the direct reaction given by Eq. (6) is unlikely
due to the negative charge of nitrate ions which makes difficult
contact between them.
We propose a hypothesis of N2O formation based on the
charge distribution in the dipole formed by NO molecule. Using
concept of electronegativity one can expect that electron density
in NO molecule is rather higher at oxygen atom because the
electronegativity of oxygen is greater than electronegativity of
−
[16]. Thus, the formal charge of nitrogen atom in NO3 ion in
relation to oxygen atoms should be positive [17]. Taking into
account the dipole structure of NO and charge distribution in

116
A. Małecki, B. Małecka / Thermochimica Acta 446 (2006) 113–116
and CO or H2 which are apparently similar to reaction (9b):
N2O2 + CO = N2O + CO2
N2O2 + H2 = N2O + H2O
(10a)
(10b)
are described in chemical kinetics handbooks. The carbon
monoxide and hydrogen are strong reducing agents contrary to
the nitric oxide which has weak reducing properties. This shows
that probability of the reaction (9b) is rather low. Additionally
oxygen formed during thermal decomposition of nitrates can
react with N2O2 according to well known reaction:
Fig. 3. Hybrid structure of the nitrate ion.
N2O2 + O2 = 2NO2
This decreases the possibility of occurring the reaction (9b).
In result of above considerations mainly the proposed reac-
tion (7) and possible reaction (8) should be considered as sources
of N2O during thermal decomposition of nitrates.
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the nitrate ion we can point at possibility of reaction given by
the following equation:
−
5
1
NO + NO3 = N2O + O2 + O2−
(7)
4
2
Formation of N N bond in reaction (7) results directly from
interaction between N(−) (NO) and N(+) (NO3−).
The nitrogen atom in NO2 molecule has the valency electrons
which do not take part in chemical bonds N O (Fig. 4). There-
fore, the react−ion similar to (7) seems to be possible between
NO2 and NO3
:
−
7
1
NO2 + NO3 N2O + O2 + O2−
(8)
4
2
The occurrence of the reactions (7) and (8) leads to almost the
same type of changes of concentration NO and N2O in gaseous
products of decomposition which is observed in experiments.
The other possibility of N2O formation during nitrate decom-
position is based on the two consecutive reactions:
2NO ꢀ N2O2
(9a)
(9b)
N2O2 + NO = N2O + NO2
Reaction (9a) is well documented as occurring in gaseous and
liquid phase. The structure of dimer N2O2, containing chain of
atoms O N N O was described for instance in paper [18]. The
second reaction (9b) was mentioned by Melia [19] and to our
knowledge it was never reported later. Reactions between N2O
[17] K. Lenore McEwen, J. Chem. Phys. 34 (1961) 547.
[18] A. Snis, I. Panas, Chem. Phys. 221 (1997) 1.
[19] T.P. Melia, J. Inorg. Nucl. Chem. 27 (1965) 95.