Catalytic decomposition of hydrazine in weakly alkaline solutions on platinum nanoparticles

Article abstract of DOI:10.1007/s11137-005-0031-8

The kinetics and stoichiometry of catalytic decomposition of hydrazine in 0.01 M NaOH solutions in the presence of unstabilized ("gray" colloid) and stabilized with sodium polyacrylate ("brown" colloid) platinum nanoparticles were studied. The main decomp

Full text of DOI:10.1007/s11137-005-0031-8

Radiochemistry, Vol. 46, No. 6, 2004, pp. 578 582. Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 531 535.
Original Russian Text Copyright 2004 by Anan’ev, Boltoeva, Sukhov, Bykov, Ershov.
Catalytic Decomposition of Hydrazine in Weakly Alkaline
Solutions on Platinum Nanoparticles
A. V. Anan’ev, M. Yu. Boltoeva, N. L. Sukhov, G. L. Bykov, and B. G. Ershov
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received December 25, 2003
Abstract The kinetics and stoichiometry of catalytic decomposition of hydrazine in 0.01 M NaOH solutions
in the presence of unstabilized ( gray colloid) and stabilized with sodium polyacrylate ( brown colloid)
platinum nanoparticles were studied. The main decomposition products are ammonia and N with H impurity
2
2
(
up to 1.5%), i.e., hydrazine decomposition predominantly follows the stoichiometric equation 3N H =
2 4
4
NH + N . The catalytic activity was studied as influenced by nanoparticle size distribution. Despite higher
3 2
nanoparticle dispersion, the catalytic activity of the stabilized brown colloid is lower than that of the gray
colloid. The reaction mechanism is discussed.
Owing to high reduction activity, hydrazine is
widely used in wet processes of the nuclear fuel cycle
as stabilizer of actinide ions in lower oxidation states
In the first procedure, colloidal platinum was pre-
pared directly in the reaction system by injection of
an aliquot of H PtCl solution into a temperature-
2
6
[1] or as nitrite scavenger [2]. The presence of hydraz-
controlled working solution of sodium hydroxide
2
ine in aqueous waste is undesirable, because it often
complicates treatment of liquid radioactive waste, in
particular, in the course of chemical denitration [3].
It was found [4 6] that hydrazine is effectively de-
composed in the presence of finely dispersed platinum
catalyst supported on silica. However, this Pt/SiO2
catalyst can be used only in acidic solutions because
of low chemical stability of the support (silica) in
alkaline and carbonate solutions. Hence, search for
new catalytic materials stable in the above media is of
particular importance.
(10 NaOH) containing excess hydrazine. In this
case, platinum was rapidly reduced to the finely dis-
persed metal and the solution became gray due to pla-
tinum colloid; the gray colloid was stable within
the entire experimental time. This colloid catalyzed
decomposition of excess hydrazine.
In the second case, platinum nanoparticles were
prepared according to the standard procedure [10] by
2
reduction of H PtCl or K PtCl in 10 M NaOH
2
6
2
6
solution with molecular hydrogen in the presence of
sodium polyacrylate stabilizer. As a result, stable
brown colloidal solution was prepared ( brown col-
loid).
Colloids of platinum group metals are highly selec-
tive catalysts for many chemical reactions [7, 8]. Plati-
num nanoparticles exhibit catalytic properties both
in oxidation (e.g., of 2-propanol [9]) and reduction
The size distributions of platinum nanoparticles in
gray and brown colloids, determined by the
atomic-force microscopy, are shown in Fig. 1. The
particle size distribution in the gray colloid is non-
uniform (Fig. 1a): it contains significant amount of
(
[
reaction of molecular hydrogen with methylviologen
10]) processes. It was found [10] that the catalytic
activity of platinum nanoparticles prepared by various
procedures is considerably different.
4
8-nm nanoparticles along with fairly coarse ag-
In this work we continued a study of the catalytic
properties of platinum nanoparticles, with hydrazine de-
composition in weakly alkaline solutions as example.
gregates of 20 40 nm size. The gray color of the
solution is determined by light scattering on these
large aggregates. The brown colloid is virtually
monodisperse (Fig. 1b), and the average particle size
is about 4 nm.
EXPERIMENTAL
Preparation of Platinum Nanoparticles
and Their Properties
The absorption spectra of both colloids are shown
in Fig. 2. The brown colloid exhibits specific ab-
sorption in the near-UV region, which monotonically
decreases with increasing wavelength. Such a spec-
trum, typical of fine colloids with the particle size less
Catalytic decomposition of hydrazine was studied
using two types of colloids prepared by different
procedures.
1
066-3622/04/4606-0578 2004 MAIK Nauka/Interperiodica

CATALYTIC DECOMPOSITION OF HYDRAZINE IN WEAKLY ALKALINE SOLUTIONS
579
Fig. 1. Atomic-force microscopic patterns and size distribution of platinum nanoparticles. (a) Gray colloid and (b) brown
colloid.
than 10 nm, is determined by intraband electronic tometrically
transitions [11]. There is no absorption maximum (DMAB) [12].
in the case of stabilizer-free gray colloid, and the
smooth shape of the spectrum indicates that the total
absorption of the colloid solution is predominantly
due to the light scattering on the coarse particles.
with
p-dimethylaminobenzaldehyde
The stoichiometry of catalytic decomposition of
hydrazine in the presence of platinum colloids was
studied by analysis of the gaseous reaction products,
performed on a Chrom 5 chromatograph equipped
with a 2-m column packed with zeolite 5A (0.4
Experimental Procedure
0
.5 mm particle size) and a katharometer (argon carrier
1
gas, 30 ml min flow rate, room temperature).
The kinetic experiments were performed in a tem-
perature-controlled glass reactor equipped with a
reflux condenser to prevent evaporation of the work-
ing solutions. The reaction system was stirred with
a magnetic stirrer with a controllable stirring rate. The
catalytic decomposition of hydrazine was initiated
either by addition of an aliquot of H PtCl solution
2
6
into the working solution ( gray colloid) or by injec-
tion of the required amount of N H into the brown
2
4
colloid solution. Aliquots (0.1 ml) of the working
solutions were sampled at certain intervals in the
course of the experiment. The current concentration
of hydrazine in solution was determined spectropho-
4
Fig. 2. Absorption spectra of colloid solutions (1 10
Pt): (1) gray colloid and (2) brown colloid.
M
RADIOCHEMISTRY Vol. 46 No. 6 2004

580
ANAN’EV et al.
tional component of the first order is observed in
the kinetic curves. These changes in the reaction order
with respect to the hydrazine concentration are prob-
ably due to the adsorption mechanism of the hetero-
geneous-catalytic decomposition of this compound.
The zero reaction order is realized when the concen-
tration of the reacting species in solution is much
higher than the volume concentration of the active
catalytic centers. Under these conditions, the reaction
rate is independent of the current concentration of
hydrazine and is determined only by the concentration
of platinum nanoparticles in solution.
Fig. 3. (a) Kinetic curves of catalytic decomposition of
hydrazine in the presence of platinum nanoparticles ( gray
colloid of various concentrations) and (b) rate of catalytic
To exclude uncertainty due to the variable reaction
decomposition of hydrazine as influenced by the concentra-
3
order, we performed a series of experiments at low
tion of the gray colloid at [N H ] = 2 10
M,
2
4 0
platinum concentrations ( 10 4 M), when the reaction
[
(
NaOH] = 0.01 M, T = 74 C. (a) Colloid concentration, M:
1) 5 10 , (2) 1 10 , (3) 5 10 , and (4) 1 10
6
5
5
4
.
order with respect to the hydrazine concentration was
mainly zero. The reaction rate constants were calcu-
lated using the zero-order equation d[N H ]/d = k .
2
4
0
With increasing catalyst concentration in the 5
6
1 10 4 M range at 74 C, the rate of hydrazine
10
decomposition increases:
[
Pt], M
k , mol l 1 min 1
0
5
1
5
1
10 6
1.03 10 5
6
5
10
10
1.92 10
5
9.04 10 5
1.63 10 4
4
10
In the ln[Pt] lnk0 coordinates, this dependence
can be plotted as a straight line with a slope of 0.93
(Fig. 3b), which can be formally interpreted as an
apparent reaction order with respect to the concentra-
tion of the active catalytic centers in the reaction
system.
Fig. 4. Kinetic curves of catalytic decomposition of hy-
5
drazine in the presence of
gray colloid (5 10 ) at
3
[
N H ] = 2 10 M, [NaOH] = 0.01 M, and temperature,
2
4 0
C: (1) 30, (2) 40, (3) 56, and (4) 74.
RESULTS AND DICUSSION
The effect of temperature on the kinetics of catalyt-
ic decomposition of hydrazine upon addition of
H PtCl is unusual. With decreasing temperature of
Catalytic Decomposition of Hydrazine in the Presence
of Gray Colloid
2
6
the reaction mixture, the reaction order deviates from
zero. Moreover, temperature changes in the 30 74 C
range do not affect the rate of hydrazine decomposi-
tion (Fig. 4). Even for diffusion-controlled reactions,
Our preliminary experiments showed that hy-
drazine completely decomposes within 2.5 h upon
6
addition of H PtCl (5 10 M) to the solution of
2
6
the temperature dependence of the reaction rate should1
3
N H OH (2 10 M) at 80 C. This is probably due
2
5
correspond to the activation energy E* = 19 kJ mol
to its catalytic decomposition on the Pt nanoparticles
[13]. In the case of catalytic decomposition of hy-
arising from stoichiometric reduction of PtCl2 ions
6
drazine in the presence of H PtCl , we failed to reveal
2
6
with excess hydrazine.
a statistically significant effect of temperature on the
reaction rate. This phenomenon is probably due to
a decrease in the rate of platinum reduction to metal
with decreasing temperature. The catalytic activity of
hydrated platinum dioxide and oxide forming succes-
The kinetic curves of the catalytic decomposition
of hydrazine (2 10 3 M) in 0.01 M NaOH solution in
the presence of platinum (5 10 1 10 4 M) at
6
7
4 C are shown in Fig. 3a. The kinetic dependences
are linear, which suggests the zero reaction order with
respect to hydrazine concentration. With increasing
platinum concentration over the above range, an addi-
sively as the intermediates after addition of H PtCl6
2
into the alkaline solution of hydrazine may be higher
than the catalytic activity of metallic particles of the
RADIOCHEMISTRY Vol. 46 No. 6 2004

CATALYTIC DECOMPOSITION OF HYDRAZINE IN WEAKLY ALKALINE SOLUTIONS
581
gray colloid, which is the final reduction product.
These processes strongly affect the reaction kinetics,
and thus the reliable interpretation of the experimental
data obtained under given experimental conditions
is impossible. At least, this unusual phenomenon
requires additional study.
The catalytic activity of the gray colloid de-
creases at its reuse. This is probably caused by coar-
sening of platinum nanoparticles due to coagulation
and by decrease in the active surface area of the cata-
lyst in the absence of stabilizer. Certain fraction of
the gray colloid precipitates as platinum black
within several hours.
Fig. 5. (a) Kinetic curves of catalytic decomposition
of hydrazine in the presence of platinum nanoparticles
brown colloid of various concentrations) and (b) rate of
(
catalytic decomposition of hydrazine as influenced by the
concentration of the brown colloid at [N H ] = 1.77
Catalytic Decomposition of Hydrazine in the Presence
of Brown Colloid
2
4 0
2
10
M, [NaOH] = 0.01 M, T = 74 C. (a) Colloid concen-
4
4
4
tration, M: (1) 1 10 , (2) 2 10 , and (3) 3 10
.
The kinetics of the catalytic decomposition of
hydrazine in the presence of brown platinum colloid
(
[Pt] = 1 10 4 3 10 4 M) stabilized with sodium
polyacrylate (NaPA) was studied at 74 C, at [N H
2
4
2
H O] = 1.77 10 M and [NaOH] = 0.01 M. The
2
0
Pt/NaPA molar ratio was constant (1 : 2) in all the
experiments. Under these experimental conditions,
hydrazine decomposition is a zero-order reaction with
respect to the hydrazine concentration (Fig. 5a). With
4
increasing catalyst concentration in the 1 10
3
4
1
0
M range, the rate of hydrazine decomposition
increases:
Fig. 6. Catalytic activity of the brown colloid at reuse,
as influenced by the catalyst aging. (1) Freshly prepared
colloid, (2) second run in 24 h, (3) third run in 72 h, and
(4) colloid prepared in advance.
1
[
Pt], M
k0, mol l min 1
4
5.22 10 5
1
2
3
10
10
10
4
4
4
1.06 10
1.66 10 4
ic reduction of methylviologen with molecular hy-
drogen on platinum nanoparticles [14]. It was found
that the reaction rate decreases linearly with increasing
ln[NaPA].
In the ln ln coordinates, this dependence is linear
Fig. 5b). Similarly to the case of gray colloid, the
(
slope corresponding to the apparent reaction order
with respect to the concentration of the active centers
is close to 1 (1.05), which is common for heterogene-
ous catalytic reactions in aqueous media.
The brown colloid solution can be prepared
in situ by addition of H PtCl aliquot to the working
2
6
solution containing NaPA stabilizer and excess hy-
drazine. The catalytic activity of this freshly prepared
brown colloid is significantly higher than that of
the catalyst prepared by the procedure [10] involving
reduction of platinum with molecular hydrogen. As
seen from Fig. 6, the reused catalyst prepared in situ
exhibits lower catalytic activity, which gradually de-
creases to the activity of the stabilized colloid pre-
pared accoding to [10]. This decrease in the catalytic
activity is probably caused by a decrease in the specif-
ic surface area due to the particle coarsening at colloid
aging. This fact is confirmed by changes in the ab-
sorption spectra recorded at reuse of the colloids in
Now, let us compare the rates of hydrazine decom-
position in the presence of gray and brown col-
loids (Figs. 3 and 5). Despite smaller particle size and
thus more developed surface area of the brown col-
loid, its catalytic activity is lower than that of the
gray colloid by a factor of approximately 3. This
decrease in the catalytic activity is probably due to
blocking of the catalyst active surface with the sta-
bilizer molecules (NaPA), which decreases the acces-
sibility of the catalyst surface for the reacting com-
pounds. Similar phenomenon was observed in catalyt-
RADIOCHEMISTRY Vol. 46 No. 6 2004

5
82
ANAN’EV et al.
followed by recombination of H atoms:
the catalytic reaction; the intensity of absorption in the
UV region decreases, whereas in the visible spectral
range it increases. These spectral changes suggest that
the colloid particle size and, thus, the contribution
of the light dispersion increase (Fig. 2).
*
*
*
NH
NH
+ H
,
2
ads
ads
ads
*
2H
H .
2
ads
Chromatographic analysis of the gaseous reaction
products was performed with the brown colloid
prepared in situ. The main reaction product is nitrogen
with 1 1.5% of hydrogen. It should be noted that hy-
drogen is liberated only in the first reaction stage
The rate of this process is significantly lower than
the rate of reaction of NH* radicals with hydrazine
2
molecules; this explains low yield of molecular hy-
drogen in the course of catalytic decomposition of
hydrazine.
(
(
up to 30% conversion). Sodium hydroxide solution
0.01 M) containing 0.01 M N H in the presence of
2
4
ACKNOWLEDGMENTS
4
4
platinum (1 10 M) and NaPA stabilizer (2 10
2
3
10
M) liberates 0.30 0.06 mol of N2 and
The study was financially supported by the Russian
Foundation for Basic Research (project no. 03-03-
3
(
4.8 3) 10 mol of hydrogen per mole of decom-
posed hydrazine. These data suggest that the catalytic
decomposition of hydrazine in weakly alkaline solu-
tions in the presence of platinum nanoparticles fol-
lows the stoichiometric equation
3
2239).
REFERENCES
. Koltunov, V.S., Kinetika reaktsii aktinoidov (Kinetics
1
3
N H = 4NH3 + N2.
of Actinide Reactions), Moscow: Atomizdat, 1974.
2
4
2
3
. Finlayson, M.V. and Mowat, J.A.S., Electrochem.
Technol., 1965, vol. 3, p. 148.
. Radioactive Waste Management Series. Denitration of
Radioactive Liquid Waste, Cecille, L. and Halaszo-
vich, S., Eds., London: Graham, 1986.
Certain amount of hydrogen in the gaseous prod-
ucts of hydrazine decomposition indicates that, initial-
ly, the following reaction also takes place:
2
N H = 2NH3 + N2 + H2.
2 4
4. Krot, N.N., Shilov, V.P., Dzyubenko, V.I., et al.,
Radiokhimiya, 1995, vol. 37, no. 1, pp. 23 27.
However, the contribution of this reaction to the
overall process of hydrazine decomposition does not
exceed 1.5%.
5
. Ananiev, A.V., Broudic, J.-C. and Brossard, Ph.,
Appl. Catal. A: Gen., 2003, vol. 242, no. 1, pp. 1 10.
. Anan’ev, A.V. and Shilov, V.P., Radiokhimiya, 2004,
6
With certain approximation, we assumed that, simi-
larly to catalytic decomposition of hydrazine in acidic
solutions, the limiting stage of the process is the dis-
sociation of hydrazine molecules adsorbed on the
catalyst surface, followed by subsequent rapid reaction
vol. 46, no. 4, pp. 348 355.
7. Lewis, L.N., Chem. Rev., 1993, vol. 93, p. 2692.
. Gates, B.C., Chem. Rev., 1995, vol. 95, p. 511.
. Henglein, A., Ershov, B.G., and Malow, M., J. Phys.
Chem., 1995, vol. 99, p. 14129 14136.
8
9
of sorbed NH* radicals with hydrazine molecules
2
10. Ershov, B.G., Izv. Ross. Akad. Nauk, Ser. Khim.,
001, no. 4, pp. 600 605.
in solution [5].
2
1
1. Ershov, B.G. and Sukhov, N.L., Zh. Fiz. Khim., 2001,
*
2NH ads,
N H
2
4
2
vol. 75, no. 8, pp. 1430 1434.
ads
1
2. Dosage spectrophotometrique de l’hydrazine. Méth-
odes d’analyse 1968 du Commissariat a l’Energie
atomique, Paris: CETAMA, 1968, no. 241.
3. 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.
*
*
NH
+ N H
NH3ads + N H sol.
2 3
2
2
4
ads
sol
In turn, N H* radicals decompose to form am-
monia and nitrogen:
2
3
1
*
2
N H
N2 + 2NH3.
2
3
sol
1
1
4. Ershov, B.G. and Sukhov, N.L., Mendeleev Commun.,
The rate of this reaction is very high (k > 3
(
in press).
01 cm mol s ) [15].
2
3
1
1
1
5. Friswell, N.J. and Govenlock, B.G., Advances in Free
Radical Chemistry, Williams, G.H., Ed., London:
Academic, 1967, no. 2, pp. 1 45.
Appearance of hydrogen in the reaction products is
probably due to dissociation of sorbed NH* radicals
2
RADIOCHEMISTRY Vol. 46 No. 6 2004