Heterogeneous catalytic decomposition of hydrazine in nitric acid solutions

Article abstract of DOI:10.1023/B:RACH.0000039116.09002.c8

The kinetics and stoichiometry of catalytic decomposition of hydrazine in 0.25-8 M HNO₃ in the presence of 1% Pt/SiO₂ were studied. The main reaction products are N₂, N₂O, and ammonium nitrate whose yield decreases with increasing [HNO₃]. It was found that catalytic decomposition of hydrazine in HNO₃ is a result of three different processes: heterogeneous catalytic disproportionation of hydrazine at the catalyst surface, oxidation of hydrazine with catalytically generated HNO₂, and heterogeneous catalytic oxidation of hydrazine with HNO₃. The contribution of these processes to the overall reaction is mainly governed by HNO₃ concentration and temperature. The role of platinum in the catalysis of hydrazine oxidation in nitric acid media and possible mechanisms of catalytic processes are discussed.

Full text of DOI:10.1023/B:RACH.0000039116.09002.c8

Radiochemistry, Vol. 46, No. 4, 2004, pp. 377 384. Translated from Radiokhimiya, Vol. 46, No. 4, 2004, pp. 348 355.
Original Russian Text Copyright 2004 by Anan’ev, Shilov.
Heterogeneous Catalytic Decomposition of Hydrazine
in Nitric Acid Solutions
A. V. Anan’ev and V. P. Shilov
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received July 23, 2003
Abstract The kinetics and stoichiometry of catalytic decomposition of hydrazine in 0.25 8 M HNO in
3
the presence of 1% Pt/SiO were studied. The main reaction products are N , N O, and ammonium nitrate
2
2
2
whose yield decreases with increasing [HNO ]. It was found that catalytic decomposition of hydrazine in
3
HNO is a result of three different processes: heterogeneous catalytic disproportionation of hydrazine at
3
the catalyst surface, oxidation of hydrazine with catalytically generated HNO , and heterogeneous catalytic
2
oxidation of hydrazine with HNO . The contribution of these processes to the overall reaction is mainly
3
governed by HNO concentration and temperature. The role of platinum in the catalysis of hydrazine oxida-
3
tion in nitric acid media and possible mechanisms of catalytic processes are discussed.
+
Thanks to its unique chemical and physical char-
acteristics, hydrazine is one of the most important and
well-studied nitrogen compounds. Hydrazine and its
derivatives are widely used in space technology as
propellant components or as monopropellants for
satellites and spacecrafts [1]. The other important field
of hydrazine application is chemistry and technology
of actinides. Having a pronounced reductive power,
hydrazine is used as nitrite scavenger in electrochemi-
cal reduction of U(VI) [2] or directly as a reducing
agent for stabilization of actinide ions in lower oxida-
tion states [3]. It was shown previously that the reduc-
ing activity of hydrazine with respect to actinide ions
increases significantly in the presence of some hetero-
geneous catalysts [4 6]. The kinetics of redox reac-
tions were studied, and attempts were made to eluci-
date the mechanism of heterogeneous catalytic reac-
tion of hydrazine with actinide ions in solutions [6 9].
However, some experimental results, such as contra-
dictory reaction orders with respect to hydrazine con-
centration, the influence of acidity on the process
rates, and superstoichiometric consumption of hy-
drazine, were not adequately explained.
hydrazinium ion N H . The kinetics and mechanism
2 5
of catalytic decomposition of hydrazine in perchloric
and sulfuric acid solutions in the presence of the
catalyst Pt/SiO were thoroughly studied previously
11]. It was found that, in acidic solutions containing
no nitrate ions, the catalytic decomposition of hydraz-
ine proceeds according to the stoichiometric equation
2
[
+
3
N H + H+ = 4NH +4 + N2.
(1)
2
5
The kinetics of hydrazine decomposition in nitric
acid solutions on the Pt/SiO catalyst was studied in
12]. A complicated dependence of the reaction rate
on the HNO concentration was found; however, no
assumptions on the reaction stoichiometry and mech-
anism were made. At the same time, there is no doubt
that these data are of scientific and practical interest.
2
[
3
Here we studied the features of heterogeneous
catalytic decomposition of hydrazine in nitric acid
solutions.
EXPERIMENTAL
In the experiments we used N H HNO and
2
4
3
The kinetics and mechanisms of catalytic decom-
position of hydrazine were thoroughly studied in the
presence of a great number of catalytic materials.
The results of these studies were summarized in a
monograph [10]. The above studies are related to the
catalytic decomposition of anhydrous N H or hydrate
HNO of pure and chemically pure grade, respectively.
3
The catalyst 1% Pt/SiO with the grain size of 0.5
2
1.0 mm was prepared by the standard procedures by
treatment with a hydrazine hydrate solution of silica
gel grains impregnated with H PtCl [11]. The actual
2
4
2
6
N H H O in gaseous or liquid state. The results ob-
content of platinum on silica gel, determined by
the atomic emission method (Perkin Elmer Optima
3000 RL) after dissolution of Pt from a weighed por-
tion of the catalyst in aqua regia, was 1.04%. The
2
4
2
tained cannot adequately explain the behavior of hy-
drazine in the presence of heterogeneous catalysts
in acidic solutions, in which it exists in the form of
1
066-3622/04/4604-0377 2004 MAIK Nauka/Interperiodica

378
ANAN’EV, SHILOV
The kinetic data were processed with the Table
Curve 2D software (Jandel Scientific). The rate con-
stants of the reaction were evaluated from the results
of three independent kinetic runs.
RESULTS AND DISCUSSION
Reaction Stoichiometry and Products
It is well known [14] that homogeneous oxidation
+
of hydrazine in HNO solution yields NH ions, HN ,
Fig. 1. Curves of potentiometric titration of the reaction
3
4
3
N O, and N .
mixture (I) before and (II) after catalytic decomposition of
2
2
1
hydrazine in the presence of 0.1 g ml of 1% Pt/SiO at
2
The preliminary qualitative analysis of the solution
and released gases for HN [a test with Fe(NO ) ] in
+
6
0
0 C. Composition of the initial solution: 0.43 M N H ;
.53 M HNO . Aliquot 5 ml. Titrant 1 M NaOH. Curve I:
2
5
3
3 3
3
catalytic decomposition of 0.25 M N H HNO in
initial pH 1.57, equipotential points: (1) pH 4.60, 2.629 ml;
2) pH 9.69, 4.770 ml; curve II: initial pH 1.81, equipoten-
tial points: (1) pH 5.66, 2.376 ml; (2) pH 10.49, 3.489 ml.
2
4
3
1
(
3 M HNO at 60 C in the presence of 0.1 g ml of
3
1
% Pt/SiO revealed no HN .
2 3
The mass-spectrometric analysis of the released
gases showed that the main gaseous products of cata-
specific surface area and pore volume (mesopores) of
the catalyst, according to the results of measurements
of nitrogen adsorption desorption at 77 K (Coulter
SA 3100), were 350 m g and 0.96 cm g , respec-
tively.
lytic decomposition of hydrazine in 3 M HNO were
3
2
1
3
1
N and N O. Trace amounts of NO and NO were
2
2
2
also found.
The experiments on the study of the reaction stoi-
chiometry were performed in 0.075 9.34 M HNO3
solutions containing fairly high (approximately 0.5 M)
The kinetic experiments were carried out in a tem-
perature-controlled glass reactor equipped with a
reflux condenser to prevent evaporation of working
solutions. The working solutions were prepared just
before experiments by mixing of the required amounts
of the initial solutions of HNO and N H HNO in
distilled water. The total volume of the working solu-
tions was 50 ml. Then, a weighed portion of the dry
catalyst preheated to the required temperature was
added into the reactor. The reaction mixture was
stirred by bubbling with argon. The stirring intensity
was monitored with a float-type flow meter. Aliquots
of the working solution were withdrawn for analysis
at regular intervals.
initial concentrations of hydrazine to provide accept-
able accuracy of potentiometric titration. Ammonium
and hydrazinium ions are indistinguishable in poten-
tiometric titration because of the close location of the
equivalence points in the titration curves. Therefore,
ammonium ions were determined after complete de-
composition of hydrazine, which was monitored spec-
trophotometrically using the color reaction with
DMAB. The typical curves of potentiometric titration
of the initial and resulting solutions are shown in
3
2
4
3
Fig. 1. The stoichiometric ratios [NH NO ] /[N H
4
3
2 5
NO ] and ([NO ]
[NO ] )/[N H NO ] , reflect-
3
0
3 0
3 2 5 3 0
The current concentration of hydrazine in the solu-
tion was determined spectrophotometrically using the
color reaction with DMAB (p-dimethylaminobenzal-
dehyde) [13]. The spectra were recorded on a Shimad-
zu-3110 spectrophotometer.
ing the accumulation of ammonium and the loss in the
total amount of nitrate at various initial concentrations
of HNO , are listed in Table 1.
3
Table 1 shows that an increase in the HNO con-
3
centration in the range 0.075 9.34 M results in a 45%
decrease in the amount of ammonium ions formed and
a 60% increase in the total nitrate consumption per
mole of hydrazine. Variation of the stoichiometric
ratios of ammonium accumulation and nitrate con-
The concentrations of acid and ammonium ions
in the working solutions were determined by poten-
tiometric titration (Metrohm 682 Titroprocessor) with
a standard NaOH solution.
The gases released during the reaction were ana-
lyzed in a separate series of experiments with a
Bakers QMS-200 mass spectrometer in the on-line
mode. Helium was used as a carrier gas. The gas flow
rate was monitored with an electronic flowmeter.
sumption with variation of [HNO ] suggests that the
3
overall reaction of catalytic decomposition of hy-
drazine in nitrate media is the sum of two or more
processes, whose contribution is governed by the
HNO concentration. Analysis of the concentration
3
RADIOCHEMISTRY Vol. 46 No. 4 2004

HETEROGENEOUS CATALYTIC DECOMPOSITION OF HYDRAZINE
379
dependences of the stoichiometric coefficients reveals
Table 1. Stoichiometry of catalytic decomposition of
hydrazine in nitric acid solutions at 60 C in the presence
of 0.1 g l 1 1% Pt/SiO2*
the concentration range 0.5 5 M HNO in which
3
the ratios [NH NO ] /[N H NO ] and ([NO ]
4
3
2
5
3 0
3 0
[
NO ] )/[N H NO ] vary insignificantly. Taking
3 2 5 3 0
into account the results of qualitative analysis of the
released gases, it can be suggested that in the above
[HNO ] , [N H NO ] , [NH NO ]
[NO ] [NO3]
4
3
3 0
3
0
2
5
3 0
M
M
[
N H NO ]
[N H NO ]
3 0
2
5
3 0
2
5
range of HNO concentration the catalytic decomposi-
3
tion of hydrazine nitrate mainly follows the stoichi-
ometric equation
0
0
0
1
3
6
7
9
.075
.25
.53
.05
.01
.13
.45
.34
0.51
0.50
0.43
0.54
0.56
0.54
0.68
0.56
0.65 0.01
0.57 0.01
0.52 0.01
0.54 0.01
0.55 0.02
0.44 0.01
0.40 0.01
0.36 0.03
0.40 0.01
0.58 0.02
0.55 0.03
0.59 0.03
0.65 0.03
0.70 0.03
0.97 0.04
3
6N H NO + 2HNO3 = 20NH NO + 32N2
2 5 3 4 3
+
3N O + 51H O.
(2)
2
2
Kinetics of Catalytic Decomposition of Hydrazine
The kinetics of hydrazine decomposition in nitric
*
Stoichiometric parameters were evaluated from the results
of three independent runs.
acid media on the catalyst 1% Pt/SiO were studied
2
2
+
under the following conditions: [N H ] = (0.4
5 0
+
hydrazine proceeds by zero-order and first-order reac-
tions with respect to [N H ], and the contribution of
each reaction to the overall process is governed by
the experimental conditions.
5
6
.82) 10 3 M; [H ] = 0.25 8 M HNO ; T = 25
3
+
1
0 C; S/L = 0.02 0.1 g ml . This low initial concen-
2 5
tration of hydrazine was chosen taking into account
the results of studying the kinetics of hydrazine de-
composition in other acid media. It was found [11]
Computer analysis of the kinetic data shows that
the rate of catalytic decomposition of hydrazine
+
that an increase in [N H ] above 0.01 M at S/L =
2
5 0
0
.1 g ml 1 results in saturation of active centers of the
catalyst, causing uncertainty in determination of the
reaction order with respect to hydrazine concentration.
Preliminary kinetic experiments showed that the
rate of catalytic decomposition of hydrazine increases
with increasing flow rate of argon in the range 50
5
00 ml min 1 (Fig. 2). This suggests that either the
reaction is diffusion-controlled or one of the volatile
products of hydrazine decomposition inhibits the ini-
tial process, blocking the active centers of the catalyst.
Therefore, all kinetic experiments were carried out at
a constant stirring intensity (WAr = 500 ml min 1
through the solution volume of 50 ml).
Fig. 2. Influence of the rate of argon bubbling W on the
Ar
3
kinetics of decomposition of 1.94 10 M hydrazine in
1
Analysis of the kinetic curves obtained at 27 C and
3.1 M HNO at 42 C in the presence of 0.06 g ml 1%
3
1
varied HNO concentration shows that the reaction
Pt/SiO . W , ml min : (1) 65, (2) 200, (3) 300, and
3
2
Ar
+
(
4) 500.
order with respect to [N H ] is not constant and
2
5
changes from 0 to 1 with variation of [HNO ]. There-
3
fore, to estimate tentatively variation of the rate of
catalytic decomposition of hydrazine, we chose the
parameter 1/ 50%, where
reaction completion. The dependence of 1/ 50% on
HNO ] presented in Fig. 3 shows that the process
is the time of 50%
50%
[
3
rate depends in a complex manner on the HNO con-
3
centration, passing through a minimum in the range
3
.0 3.7 M HNO3.
Fig. 3. Influence of HNO concentration on the rate of
3
3
It is evident that the fractional reaction order is an
catalytic decomposition of 1.94 10 M hydrazine at 27 C
1
apparent one. Actually, the catalytic decomposition of
in the presence of 0.06 g ml of 1% Pt/SiO .
2
RADIOCHEMISTRY Vol. 46 No. 4 2004

380
ANAN’EV, SHILOV
Table 2. Influence of experimental conditions on the rate constants* of catalytic decomposition of hydrazine in nitric acid
solutions in the presence of 1% Pt/SiO . Stirring: bubbling with argon, W = 500 ml min 1 through 50 ml of the reaction
2
Ar
mixture
3
S/L, g ml 1
k0 10 , mol l 1 min 1
5
k1, min 1
[
N H NO ] 10 , M
[HNO ], M
T,
42
43
42
36.5
36.5
36.5
36.5
36.5
42
42
42
42
42
42
42
42
42
42
42
42
42
42
42
30.5
50
60
69
22
31.5
41
52
28
36.5
50
C
2
5
3 0
3
5
3
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
.82
.88
.4
0.06
0.06
0.06
0.02
0.04
0.06
0.08
0.1
0.02
0.04
0.06
0.08
0.1
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
1.0
1.0
1.0
6.2
6.2
6.2
6.2
6.2
3.1
3.1
3.1
3.1
3.1
0.25
0.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.25
0.25
0.25
0.25
3.1
3.1
3.1
3.1
6.2
6.2
6.2
1.20
1.23
1.18
0.093
0.096
0.090
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
.94
1.9 0.6
2.7 0.8
4.1 1.7
5.6 1.7
7.8 1.4
2.92 0.07
6.85 0.05
11.1 0.2
14.4 0.2
20.1 0.4
0.9 0.5
1.0 0.2
1.1 0.3
2.4 0.3
8.9 0.8
13.6 1.8
8.5 1.2
4.0 3.9
0.105 0.008
0.23 0.01
0.31 0.02
0.43 0.03
0.55 0.02
0.110 0.007
0.106 0.003
0.096 0.004
0.066 0.004
0.11 0.03
0.18 0.01
0.29 0.05
0.477 0.006
0.69 0.01
0.065 0.003
0.15 0.01
0.218 0.006
0.30 0.01
0.023 0.003
0.038 0.004
0.05 0.01
0.06 0.04
0.17 0.02
0.31 0.02
0.62 0.06
0.4 0.2
0.8 0.7
1.9 0.4
0.9 0.7
0.4 0.2
1.1 0.2
7.7 0.8
30.6 0.4
3.3 1.3
4.1 1.7
4.9 4.2
*
The values of constants presented without experimental error were obtained from a single kinetic run.
throughout the studied range of experimental condi-
tions is adequately described by the equation
stants. In the boundary cases, when the process is the
purely zero-order or first-order reaction with respect to
hydrazine, the rate constants were evaluated using
the corresponding equations for approximation of the
dC/dt = k C + k ,
1
0
kinetic curves. The results of evaluation of k and
k0 for various experimental conditions are listed in
Table 2.
1
where k and k are the first-order and zero-order rate
constants, respectively. The integral form of this
equation is
1
0
It is seen from Table 2 that k and k do not notice-
1
0
ln[(C0 + k /k )/(C + k /k )] = k t,
ably depend on the initial concentration of hydrazine
0
1
0
1
1
in the range (0.4 6) 10 3 M. This allows k and
1
where C is the initial concentration of hydrazine.
k to be considered as true rate constants of zero-order
0
0
+
This equation was used for evaluating the rate con-
and first-order reactions with respect to [N H ].
2
5
RADIOCHEMISTRY Vol. 46 No. 4 2004

HETEROGENEOUS CATALYTIC DECOMPOSITION OF HYDRAZINE
381
The trends in variation of the rate constants of zero-
order and first-order reactions at 42 C with increasing
HNO ] (Fig. 4) show that heterogeneous catalytic
[
3
+
reactions in the system N H HNO Pt are complex.
The log log dependences of k and k on [HNO ] are
2
5
3
1
0
3
satisfactorily approximated by straight lines (Figs. 5
and 6), which allows estimation of the apparent reac-
tion orders with respect to HNO concentration. In the
3
range of low concentrations of HNO (0.25 2.0 M),
3
variation of the rate constant of the first-order reaction
(
0
k1) corresponds to negative fractional order ( 0.24
Fig. 4. Variation of the rate constants of zero-order (k ) and
0
.01) with respect to HNO concentration. At a higher
first-order (k ) reactions in catalytic decomposition of hy-
3
1
1
drazine at 42 C in the presence of 0.06 g ml of 1%
HNO concentration (4.0 8.0 M), the rate constant
k1 increases, with the apparent reaction order with
respect to HNO concentration being 2.6 0.2. The
3
Pt/SiO with increasing HNO concentration.
2
3
3
break in the plot (Fig. 5) at the point corresponding to
[
HNO ] = 3.0 M is caused by the fact that under these
3
conditions the decomposition of hydrazine is a zero-
order reaction with respect to [N H ]. The rate con-
stant of the zero-order reaction (k ) varies in ac-
cordance with the apparent orders of 1.96 0.3 and
+
2
5
0
3
.07 0.3 in the ranges 1.0 4.0 and 4.0 7.0 M
HNO , respectively.
3
It should be noted that the method used here for
determining the reaction orders with respect to HNO3
concentration is not quite rigorous in terms of formal
kinetics, since under conditions of variable ionic
strength of the solution we used the analytical concen-
Fig. 5. Influence of HNO concentration on the rate of the
3
+
first-order reaction with respect to [N2H5] in the range
1
0.25 8 M HNO at 42 C in the presence of 0.06 g ml of
3
1
% Pt/SiO .
2
tration of HNO rather than its thermodynamic activ-
3
ity. Nevertheless, this approach shows that catalytic
decomposition of hydrazine in HNO in the presence
3
of Pt/SiO2 is a superposition of three parallel in-
dependent first-order and zero-order reactions with
+
respect to N H , whose contribution to the overall
2
5
process is mainly governed by HNO concentration.
3
The temperature dependences of the rate constants
were studied at three different concentrations of
Fig. 6. Influence of HNO concentration on the rate of the
3
HNO : 0.25, 3.1, and 6.2 M.
3
+
zero-order reaction with respect to [N H ] in the range
2
5
1
In 0.25 M HNO , the catalytic decomposition of
1 8 M HNO at 42 C in the presence of 0.06 g ml of 1%
Pt/SiO .
2
3
3
hydrazine proceeds mainly by the first-order reaction
+
with respect to [N H ]. The contribution of the zero-
2
5
In 3.1 M HNO , the apparent activation energies
order reaction to the overall process is insignificant,
3
determined from the temperature dependences of k
which results in low accuracy in k determination
0
0
1
and k are 114.6 4 kJ mol (22 52 C) and 33.7
1
1
(
Table 2). Therefore, only k values were used for
1
2
kJ mol for the zero-order and first-order reactions
with respect to [N H ], respectively.
evaluation of the apparent activation energy. The
+
value of E* = 34.2 0.5 kJ mol 1 found in the tem-
2
5
perature range 30.5 69 C within the accuracy of the
experimental error coincides with the previously
determined apparent activation energy of catalytic de-
The apparent activation energy of the first-order
+
reaction with respect to [N H ] in 6.2 M HNO was
2
5
3
1
found to be 44.7 3 kJ mol in the temperature range
28 50 C. The zero-order component was not con-
sidered since its contribution to the overall process
composition of hydrazine in 0.2 M HNO , equal to
3
1
3
6.4 2 kJ mol [11].
RADIOCHEMISTRY Vol. 46 No. 4 2004

382
ANAN’EV, SHILOV
tion rate practically obeys the zero-order law with
respect to hydrazine concentration. It was shown
previously [11] that, at low hydrazine concentrations
(
less than 0.01 M) in the range of S/L ratio from 0.01
1
to 0.1 g ml , the catalyst surface is not saturated with
adsorbed hydrazine species. Therefore, appearance of
the zero-order component with respect to [N H ] is
+
2
5
not a manifestation of the adsorption nature of the
process. It is most probable that the zero order with
respect to hydrazine concentration is caused by its fast
reaction with HNO or with one of its derivatives
produced at the catalyst surface in contact with HNO3.
In this case, the rate of hydrazine loss is governed
2
Fig. 7. Influence of the S/L ratio on the rate of catalytic
decomposition of hydrazine in (1) 3.1 and (2) 6.2 M HNO
3
at 42 C.
by the rate of HNO formation.
2
under these conditions is extremely low and standard
Accelerated formation of HNO in nitric acid solu-
2
error in k determination reaches 90% (Table 2).
0
tions in contact of the latter with the catalyst Pt/SiO2
was observed previously [16, 17]. In this case, plati-
num appeared in the solution in the amount com-
parable with the concentration of the formed HNO2
[17]. Simple calculations using the thermodynamic
It should be noted that in 3.1 M HNO the con-
3
tribution of the zero-order reaction with respect to
+
[
N H ] to the overall catalytic decomposition of hy-
2
5
drazine increases with temperature, whereas in 6.2 M
HNO the trend is opposite.
activity coefficients of HNO at 25 C [18] and the
3
3
2+
2+
standard potentials of Pt /Pt (1.888 V) and PtO /Pt
The determined apparent activation energy is sig-
nificantly higher that the activation energy of diffu-
sion-controlled processes (approximately 19 kJ mol 1
2
(
0.837 V) couples [19] show that, for the range
of HNO2 concentration 10 6 10
4
M ( HNO2 is
[
15]). This suggests that all reactions involved in
taken equal to 1), the equilibrium potential of the
HNO /HNO couple in 3 M HNO varies in the range
heterogeneous catalytic decomposition of hydrazine in
nitric acid solutions are kinetically controlled.
3
2
3
1.14 1.08 V (vs. NHE). At the same time, taking by
2
+
6
convention [Pt ] = 10 M and
2+ = 1, we obtain
The log log dependences of the zero-order and
first-order rate constants on the catalyst content in
the reaction mixture (ratio S/L) are straight lines with
the slopes close to 1 (Fig. 7). This suggests that the
processes under consideration obey the first-order law
with respect to the effective concentration of active
catalytic centers.
Pt
2+
1
.011 and 1.077 V for the equilibrium couples Pt /Pt
and PtO /Pt in 3 M HNO , respectively (vs. NHE).
2+
2
3
Formal comparison of the resulting equilibrium oxida-
tion potentials shows that metallic platinum in nitric
acid solutions is oxidized to tetravalent state (PtO2).
This process is accompanied by reduction of HNO to
3
HNO . Platinum dioxide forms a protective layer at
2
Analysis of the results obtained allows some as-
sumptions on the nature of chemical transformations
occurring in the system N H HNO Pt.
the metal surface, inhibiting further oxidation of plati-
+
num. Formation of HNO in the reaction system in
2
2
5
3
hand is supported by trace amounts of NO and NO2
experimentally found in gaseous reaction products
removed with bubbling inert gas. Nitrogen oxide and
In the range of low HNO concentrations (0.25
3
+
2
.0 M), the first reaction order with respect to [N H ]
2 5
and negative fractional reaction order with respect to
acid concentration are apparently caused by hetero-
geneous disproportionation of hydrazine at the plati-
num surface by the mechanism described for solutions
of other acids [11]. This conclusion follows from the
formal coincidence of the reaction orders with respect
to acid concentration and the apparent activation
energy for perchloric and nitric acidic media.
nitrogen dioxide are genetically related to HNO by
the system of well-known chemical equilibria
2
3
HNO2
2NO + HNO3 + H O,
(3)
(4)
2
HNO2 + HNO3
2NO2 + H O.
2
The oxidative characteristics of HNO are usually
3
With increasing concentration of HNO to 1 M, the
considered in relation to the oxidative effect of nitro-
3
+
contribution of the zero-order reaction with respect to
nium ion NO present in HNO solution in a certain
2
3
+
+
[
N H ] becomes significant. In 3 M HNO , the reac-
equilibrium concentration [14, 20]. NO species can
2
5
3
2
RADIOCHEMISTRY Vol. 46 No. 4 2004

HETEROGENEOUS CATALYTIC DECOMPOSITION OF HYDRAZINE
arise in the fast equilibrium reaction
383
+
+
2HNO3
H NO + NO3
NO + NO + H O. (5).
2 3 2
2
3
Taking into account the aforesaid, we can assume that
decomposition of hydrazine in 1 7 M HNO in the
3
presence of the catalyst Pt/PtO is a result of the fol-
2
lowing reactions proceeding at the active centers of
the catalyst and in the solution. It is most likely that
the process begins from the step-by-step reaction of
active surface platinum atoms with nitronium ions
Pt + NO+
PtO + NO+* (slow),
PtO2 + NO+* (slow)
(6)
(7)
2
PtO + NO+
2
(
here and hereafter, the asterisk denotes species ad-
Fig. 8. Mass-spectrometric monitoring of N O in catalytic
2
sorbed at the catalyst surface). Adsorbed nitrinium
ions are the derivatives of HNO according to the
equilibrium
decomposition of 0.2 M hydrazinium nitrate in 3.0 M
1
HNO at 60 C in the presence of 0.1 g ml of 1% Pt/SiO .
Interval between scans 2 min.
2
3
2
NO+ + H O
+
HNO2 + H+
As seen from the above scheme, one of the prod-
ucts of isoterazene decomposition is HN3 [reac-
H NO
2
(8)
2
2
tion (10)]. However, as was mentioned above, HN3
and react with hydrazine at the catalyst surface by a
chain of fast reactions, which results in the zero reac-
tion order with respect to [N H ]. Among inter-
mediates of hydrazine oxidation, the initial product is
apparently diimide N H transforming into tetrazene
was not found in the solution and released gases. Most
+
likely, HN adsorbed at the catalyst surface is not
3
2
5
desorbed to the solution but is subjected to catalytic
decomposition at the platinum surface ([22], p. 627),
2
2
and isotetrazene [21], whose decomposition at the
catalyst surface yields N and N H in the first case
Pt
3
HN*3
NH3 + 4N2,
(15)
2
2 4
and NH and HN in the second case. Nitrous oxide
3
3
or is oxidized with HNO at the instant of its forma-
2
appears in the reaction products as a result of dimeri-
zation and decomposition of HNO. On the whole, the
mechanism of oxidative decomposition of hydrazine
can be tentatively presented by the following system
of chemical transformations:
tion. The mechanism of the latter process is unknown.
However, it can be assumed that this process follows
the stoichiometric equation ([22], p. 423)
HN *3 + HNO2
N2 + N O + H O.
(16)
2
2
NO+* + N H
+
N H + HNO* + 2H+,
*
(9)
HN + NH , (10)
2
5
2 2
This assumption is indirectly confirmed by the
results of mass-spectrometric determination of N O in
*
*
2
*
*
2
2
N H
HN=N NH NH
2
2
3
3
catalytic decomposition of hydrazine. The kinetics of
*
*
2
*
*
N O formation in decomposition of 0.2 M hydrazine
2
2
N H
H N N=N NH
N
+ N H ,
(11)
(12)
(13)
2
2
2
2
2
4
1
in 3.0 M HNO at 60 C in the presence of 0.1 g ml
3
HNO*
*
*
of 1% Pt/SiO is shown in Fig. 8. As was established
2
H N O
N O + H O,
2
2
2
2
2
2
in separate runs, the peak in the curve of N O release
2
coincides with the instant of completion of hydrazine
decomposition. A similar pattern is observed in mass-
spectrometric analysis of nitrogen evolution. It is
NH3 + HNO3
NH NO .
4 3
Oxidized active centers at the catalyst surface are
regenerated as a result of the reaction with hydrazine
evident that the adsorbed HN decomposes simultane-
3
ously with hydrazine decomposition. However, taking
into account that the rate of reaction of hydrazine with
*
PtO2 + N H
Pt + N2 + 2H O
(14)
2
4
2
HNO is higher than the rate of process (16) by a
2
and become accessible for the subsequent catalytic
reaction event.
factor of approximately 5 [23], one would expect that,
by the time of hydrazine disappearance, an excess of
RADIOCHEMISTRY Vol. 46 No. 4 2004

384
ANAN’EV, SHILOV
HN is accumulated at the catalyst surface. Its reac-
of Actinide Reactions), Moscow: Atomizdat, 1974.
4. Swanson, J.L., Platinum Catalyzed Hydrazine Reduc-
3
tion with catalytically generated HNO gives rise to
2
maxima in the curves of N O and N evolution.
tion of Plutonium(IV) and Uranium(VI), BNWL-1584,
2
2
1
971.
. Nemoto, S., Ikeda, S., and Kobayashi, H., US Patent
197274, 1980.
An increase in the HNO concentration in the range
3
5
6
4
8 M HNO results in an increase in the rate of cata-
3
4
lytic decomposition of hydrazine (Fig. 3). In parallel,
+
. Abdounnabi, H.M., Ananiev, A.V., and Krot, N.N.,
J. Radioanal. Nucl. Chem. Lett., 1994, vol. 186,
pp. 89 97.
. Abdounnabi, H.M. and Ananiev, A.V., Proc. Int.
Conf. on Fast Reactors and Related Fuel Cycles,
Kyoto (Japan), 1991, vol. 4, pp. 6.9 16.
. Tananaev, I.G. and Shilov, V.P., Radiokhimiya, 1989,
vol. 31, no. 6, pp. 52 55.
. Anan’ev, A.V., Shilov, V.P., Afonas’eva, T.V., et al.,
Radiokhimiya, 2001, vol. 43, no. 1, pp. 37 40.
the reaction order with respect to N H abruptly in-
2
5
creases from 0 to 1. Presumably, at high HNO con-
centrations catalytic oxidation of hydrazine with
3
7
HNO begins to play the decisive role. In this case,
3
the rate-determining stage of the process is the reac-
tion of hydrazine molecules adsorbed at the catalyst
surface with nitronium ions formed by equilibrium
reaction (5):
8
9
N H + NO+
*
NO+ + N H + H O.
(17)
2
4
2
2
2
2
1
0. Schmidt, E.W., Hydrazine and Its Derivatives, New
Apparently, the concentration of adsorbed hydrazine
molecules is in direct proportion to the hydrazine con-
centration in the bulk, which is responsible for the
York: Wiley, 1982.
11. Ananiev, A.V., Broudic, J.-C., and Brossard, Ph.,
Appl. Catal. A: Gen., 2003, vol. 242, no. 1, pp. 1 10.
12. Krot, N.N., Shilov, V.P., Dzyubenko, V.I., et al.,
Radiokhimiya, 1995, vol. 37, no. 1, pp. 23 27.
3. Dosage spectrophotometrique de l’hydrazine, Meth-
odes d’analyse 1968 du Commissariat a l’Energie
atomique, CETAMA, Raris: Presses Univ., Juin 1968,
no. 241.
+
first order with respect to [N H ]. Diimide and nitrini-
2
5
um ions formed in the latter reaction are subjected
to fast transformations (9) (13) proceeding both at the
catalyst surface and in the solution bulk.
1
The catalytic decomposition of hydrazine in 4 8 M
HNO resembles, in general features, the homogene-
3
1
1
4. Koltunov, V.S., Nikol’skii, V.A., and Agureev, Yu.P.,
ous oxidation of hydrazine with HNO [14]. Sig-
3
Kinet. Katal., 1962, vol. 3, no. 6, pp. 877 881.
nificant distinctions are decreased activation energy of
the heterogeneous reaction, characteristic of catalytic
5. Moelwyn-Hughes, E.A., The Chemical Statics and
Kinetics of Solutions, London: Academic, 1971.
Translated under the title Ravnovesiya i kinetika
reaktsii v rastvorakh, Moscow: Khimiya, 1975, p. 114.
processes, and the absence of HN , decomposed at the
3
platinum surface, in the final products of catalytic
oxidation of hydrazine. Increased yield of ammonium
ions in comparison with that in the homogeneous
process is due to heterogeneous decomposition of hy-
1
1
6. Ananiev, A.V., Broudic, J.-Ch., Brossard, Ph., and
Krot, N.N., Radiochim. Acta, 1997, vol. 78, pp. 142
1
45.
drazine [reaction (1)] and HN [reaction (15)] at the
3
7. Guenais-Langlois, S., Bouyer, C., Broudic, J.-Ch., and
Coq, B., Appl. Catal. B: Environ., 2000, vol. 27,
pp. 199 204.
catalyst surface. A decrease in the yield of ammonium
ions with increasing concentration of HNO (Table 1)
3
is caused by decreased contribution of these reactions
to the overall process and enhancement of the role of
oxidative transformations (9), (14), and (16) with
increasing [HNO3].
1
1
8. Davis, W., Jr. and Bruin, H.J., J. Inorg. Nucl. Chem.,
1
964, vol. 26, pp. 1069 1083.
9. Pourbaix, M., Atlas d’equilibres electrochimiques,
Paris: Gauthier-Villars, 1963, p. 379.
2
2
0. Abel, E., Helv. Chim. Acta, 1950, vol. 33, p. 785.
1. Browne, A. and Kirk, J., J. Am. Chem. Soc., 1928,
REFERENCES
vol. 50, p. 337.
2. Pascal, P., Nouveaux traité de chimie minérale, Paris:
1
2
3
. Urbanski, T., Chemistry and Technology of Explo-
sives, New York: Pergamon, 1967, vol. 3.
2
2
Masson, 1956, vol. 10.
. Finlayson, M.V. and Mowat, J.A.S., Electrochem.
Technol., 1965, vol. 3, p. 148.
3. Fitzpatrick, J., Meyer, T.A., O’Neill, M., and Wil-
liams, D.L.H., J. Chem. Soc., Perkin Trans. 2, 1984,
pp. 927 932.
. Koltunov, V.S., Kinetika reaktsii aktinoidov (Kinetics
RADIOCHEMISTRY Vol. 46 No. 4 2004