Catalytic reduction of Np(VI) with formic acid in the presence of platinum nanoparticles

Article abstract of DOI:10.1134/S1066362206020032

The kinetics of catalytic reduction of Np(VI) with formic acid in the presence of Pt nanoparticles of different types ( brown colloid stabilized with sodium polyacrylate and nonstabilized gray colloid) was studied. In both cases in the examined range of conditions ([Np(VI)] ₀ = 2.80 × 10^-4-9.03 × 10^-4 M; [HCOOH] = 0.03-1.0 M; [Pt] = 4 × 10^-7-2 × 10^-5 M; T = 18-60°C) the reaction is zero-order with respect to [Np(VI)] and first-order with respect to [HCOOH]. The catalytic activity of the nonstabilized gray colloid exceeds by almost an order of magnitude that of the brown colloid, due to the blocking effect of stabilizing polyelectrolyte molecules on the active catalytic centers. The dependence of the reaction rate on the sodium polyacrylate concentration in the range 1 × 10^-4-1 × 10^-2 M is nonmonotonic, due to deflocculation of the nanoparticles. The mechanism of the catalytic reduction of Np(VI) with formic acid in the presence of Pt colloids is discussed; it involves a slow step of dissociative chemisorption of HCOOH molecules on the nanoparticle surface. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1066362206020032

ISSN 1066-3622. Radiochemistry, 2006, Vol. 48, No. 2, pp. 125 132. Pleiades Publishing, Inc., 2006.
Original Russian Text
No. 2, pp. 113 119.
A.V. Anan’ev, V.P. Shilov, N.L. Sukhov, E.V. Abkhalimov, B.G. Ershov, 2006, published in Radiokhimiya, 2006, Vol. 48,
Catalytic Reduction of Np(VI) with Formic Acid
in the Presence of Platinum Nanoparticles
A. V. Anan’ev, V. P. Shilov, N. L. Sukhov, E. V. Abkhalimov, and B. G. Ershov
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
Received April 7, 2005
Abstract The kinetics of catalytic reduction of Np(VI) with formic acid in the presence of Pt nanoparticles
of different types ( brown colloid stabilized with sodium polyacrylate and nonstabilized gray colloid) was
4
4
studied. In both cases in the examined range of conditions ([Np(VI)]0 = 2.80 10 9.03 10 M;
7
5
[HCOOH] = 0.03 1.0 M; [Pt] = 4 10
2
10 M; T = 18 60 C) the reaction is zero-order with respect to
[Np(VI)] and first-order with respect to [HCOOH]. The catalytic activity of the nonstabilized gray colloid
exceeds by almost an order of magnitude that of the brown colloid, due to the blocking effect of stabilizing
polyelectrolyte molecules on the active catalytic centers. The dependence of the reaction rate on the sodium
4
2
polyacrylate concentration in the range 1 10
1
10 M is nonmonotonic, due to deflocculation of the
nanoparticles. The mechanism of the catalytic reduction of Np(VI) with formic acid in the presence of Pt
colloids is discussed; it involves a slow step of dissociative chemisorption of HCOOH molecules on the
nanoparticle surface.
PACS numbers: 82.65.+r
DOI: 10.1134/S1066362206020032
Metal nanoparticles, primarily those of platinum-
group metals, actively and selectively catalyze many
redox processes [1 5]. Nanoparticles prepared by dif-
ferent methods noticeably differ in the catalytic activ-
ity [5]. Elucidation of the mechanism of the catalytic
effect and its correlation with the structure and nature
of nanocolloids is an urgent problem.
tion of actinide valence forms in real systems from
reprocessing of irradiated nuclear materials.
This study concerning the reduction of Np(VI) with
formic acid in the presence of Pt nanoparticles opens
a series of papers on the redox behavior of actinide
ions under catalysis with nanoparticles of platinum-
group metals.
The catalytic effect of platinum-group metal col-
loids on redox processes involving actinide ions was
observed for the first time in reduction of Pu(IV) with
formic acid on colloidal particles of Pd metal, formed
in the course of exhaustive denitration with formic
acid of process solutions from spent nuclear fuel re-
processing [6]. The observations were only qualita-
tive; no data on the kinetics and mechanism of the
process were reported. Catalytic reactions of actinide
ions with HCOOH in the presence of heterogeneous
supported platinum catalysts were studied in sufficient
detail. However, no data are available on the course
of these processes in the presence of Pt nanoparticles
occupying an intermediate position between hetero-
geneous and homogeneous catalytic systems. At the
same time, such data are of considerable scientific and
practical interest for elucidating the mechanism of
charge transfer between an actinide ion and a redox
reagent in the presence of platinum-group metal nano-
particles and for understanding regular trends in varia-
EXPERIMENTAL
Preparation and Characteristics of Pt Nanoparticles
The catalytic reduction of Np(VI) was studied in
the presence of two types of Pt colloids prepared by
different procedures.
The Pt colloid of the first type was prepared by
the standard procedure [5] involving the reduction of
2
H PtCl or K PtCl in 10 M NaOH with hydrogen
2
6
2
4
in the presence of a stabilizer, sodium polyacrylate
(NaPA). As a result, a brown colloidal solution stable
in time ( brown colloid) was obtained.
In the course of the studies we found that heating
on a water bath of a 0.1 0.5 M HCOOH solution con-
4
6
taining 10 10 M H PtCl causes the reduction of
2
6
Pt with the formation of a gray colloidal solution.
The gray colloid thus obtained remains stable (until
125

126
ANAN’EV et al.
to oxidize Np(V). Excess ozone was removed by air
bubbling. The Np concentration in the stock solution
was determined gravimetrically after drying a solution
aliquot and calcining the dry residue to NpO at
2
800 C. In the experiment we used chemically pure
grade chemicals and double-distilled water.
Kinetic experiments were performed in a tempera-
ture-controlled 1-cm quartz cell arranged in the cell
compartment of a Shimadzu UV-3110 spectrophotom-
eter. In experiments with the brown Pt colloid, solu-
tions of required compositions were prepared by mix-
ing in the cell the calculated amounts of solutions
(brought to the required temperature) of HCOOH,
4
Fig. 1. Optical spectra of 1 10 M solutions of Pt nano-
particles in 0.5 M HCOOH: (1) in the absence of a stabiliz-
ing polyelectrolyte ( gray colloid) and (2) in the presence
3
of 2 10 M sodium polyacrylate ( brown colloid).
precipitation of platinum black) at 25 60 C for sever-
al hours and can be used in kinetic experiments on
reduction of Np(VI).
4
3
2
1
10 M Pt + 2 10 M NaPA, and 2.8 10
M
Np(VI). The order of mixing the solutions, i.e., the
method of initiating the reaction, was varied if nec-
essary. In experiments with the gray Pt colloid, it
was prepared just before the kinetic experiment. To
do this, an aliquot of an H PtCl solution was injected
The size distribution of nanoparticles of the gray
and brown colloids was studied by atomic force
microscopy. The size distribution of the Pt particles in
the brown colloid is close to monodisperse, with
the mean nanoparticle size of 6 nm. The size distribu-
tion of the particles in the gray colloid is nonuni-
form. Along with a large amount of 4 8-nm particles,
there are relatively coarse aggregates 20 40 nm in
diameter. The gray color of the solution is caused by
the light scattering on the coarse particles.
2
6
into a test tube placed on a water bath at 80 90 C and
containing the required volume of HCOOH of the re-
quired concentration. After keeping for 10 min (time
required for complete reduction of Pt to zero oxidation
state, as determined in a separate series of preliminary
experiments), the solution was quickly cooled and
transferred into a cell kept at the experimental temper-
ature. The reduction of Np(VI) was initiated by inject-
The optical absorption spectra of both types of
colloids are shown in Fig. 1. The brown colloid
exhibits specific absorption in the near-UV range,
monotonically descending with increasing wavelength.
Such a pattern is characteristic of fine particles with
a size of up to 10 nm and is determined by intraband
electronic transitions [8]. The colloid prepared without
stabilizer ( gray colloid) exhibits increased optical
density in the long-wave range, indicative of the prev-
alent contribution of scattering on coarse particles to
the total light absorption of the solution. It should be
noted that the features of formation and mechanism of
stabilization of the gray Pt colloid in HCOOH solu-
tions are of independent scientific and practical inter-
est and will be considered in a separate paper.
2
ing into the cell an aliquot of a 2.8 10 M Np(VI)
solution. The kinetics of the Np(VI) reduction was
monitored by a decrease in the solution absorption at
= 300 nm [strong charge-transfer band of Np(VI)]
in the automatic mode at 0.1-min intervals. The rate
constants were calculated from the results of three
independent kinetic runs.
RESULTS AND DISCUSSION
Neptunium(VI), being quite stable in 0.1 1.0 M
HCOOH, starts to be reduced at a noticeable rate at
6
room temperature in the presence of 2 10 M Pt
nanoparticles stabilized with sodium polyacrylate
( brown colloid). The threshold concentration of the
7
Experimental Procedure
nonstabilized gray colloid is 10 M. In the pres-
ence of the brown Pt colloid, Np(VI) is reduced to
Np(V). With the nonstabilized gray colloid, the
reduction proceeds further. Upon prolonged (several
hours) storage of the reaction solution, small amounts
of Np(IV) appear, as judged from the spectrum. The
rate of the reduction of Np(V) to Np(IV), however, is
very low under the conditions used. Therefore, we
will not consider in this paper the kinetic features of
this process.
Neptunium-237 used in the study was purified by
an anion-exchange procedure. Perchloric acid solution
of Np(VI) was prepared as described in [9]. A solution
of Np in HNO from the ion-exchange column was
3
evaporated with HClO to dryness. The residue was
4
dissolved in double-distilled water. A spectrophoto-
metric analysis showed that the solution contained 1%
Np(V) and 99% Np(VI). The solution was ozonized
RADIOCHEMISTRY Vol. 48 No. 2 2006

CATALYTIC REDUCTION OF Np(VI) WITH FORMIC ACID
Catalytic Reduction of Np(VI)
127
in the Presence of the Brown Colloid
Preliminary experiments showed that the rate of
Np(VI) reduction with formic acid in the presence of
Pt nanoparticles depends on the order of mixing the
reactants, i.e., on the mode of the reaction initiation.
When the process is initiated by quick addition of an
aliquot of the catalyst solution to the spectrophoto-
metric cell, the reaction appears to be considerably
faster than when Np(VI) is added last. The rate
of the Np(VI) reduction is the lowest when HCOOH
is added last (Fig. 2). All the subsequent kinetic ex-
periments were performed with initating the reaction
by adding an aliquot of the Np(VI) solution to a solu-
tion containing HCOOH and the catalyst.
Fig. 2. Influence of the order of mixing the reactants on
4
the rate of catalytic reduction of 9.03 10 M Np(VI) in
5
0.33 M HCOOH in the presence of 1 10 M Pt ( brown
colloid) at 25 C. Component added last: (1) catalyst,
(2) Np(VI), and (3) HCOOH.
The catalytic reduction of Np(V) with formic acid
was studied in parametric kinetic experiments in
which we varied the concentrations of Np, HCOOH,
colloidal Pt, and stabilizer (NaPA) and the tempera-
4
ture in the following ranges: [Np(VI)] = 2.80 10
0
4
9.03 10 M; [HCOOH] = 0.1 0.9 M; [Pt] = 3
6
5
4
2
10
2
10 M; [NaPA] = 1 10
1
10 M;
and T = 18 40 C. The volume of the reaction solu-
tion was 3.0 ml.
Fig. 3. Kinetic curves of reduction of Np(VI) with formic
acid at 25 C in the presence of 1 10 M Pt ( brown
5
Typical kinetic curves of the catalytic reduction of
Np(VI), obtained at different HCOOH concentrations,
are shown in Fig. 3. The initial portions of the kinetic
curves can be roughly approximated by a zero-order
equation with respect to the Np(VI) concentration.
Then, as the running Np(VI) concentration decreases,
the reaction order grows, gradually approaching unity
by the end of the reaction. Computer processing of the
kinetic curves using the Table Curve 2D program
(Jandel Scientific) showed that the whole curves can
be satisfactorily described by a rate equation of the
order n with respect to the Np(VI) concentration:
colloid) at different HCOOH concentrations. [HCOOH], M:
(1) 0.9, (2) 0.7, (3) 0.5, (4) 0.33, and (5) 0.1.
n
d[Np(VI)]/dt = k [Np(VI)] . However, the experi-
n
Fig. 4. Influence of the initial concentration of Np(VI) on
mental reaction order with respect to [Np(VI)] is not
constant and varies depending on the Np(VI) concen-
tration in the solution. The fractional order with re-
spect to [Np(VI)] is an apparent quantity. It reflects
the adsorption nature of the catalytic process. Actually
the reaction occurs along zero- and first-order path-
ways with respect to [Np(VI)]. The contribution of
each of these pathways to the overall process is gen-
erally determined by the running concentration of
Np(VI) and amount of the catalyst in the solution.
To determine the true reaction order with respect to
[Np(VI)], we performed a series of kinetic experi-
ments with different initial concentrations of Np(VI)
the rate of its catalytic reduction in 0.33 M HCOOH in the
5
presence of 1 10
M Pt ( brown colloid) at 40 C.
4
4
[Np(VI)] , M: (1) 2.80 10 , (2) 4.60 10 , (3) 6.38
0
4
4
10 , and (4) 9.03 10
.
5
in 0.33 M HCOOH in the presence of 1 10 M Pt
at 40 C, determined from the initial linear portions of
the kinetic curves at different [Np(VI)], are as follows:
1
[Np(VI)], M
0, mol l 1 min
4
4
2.80 10
4.60 10
6.38 10
9.03 10
5.20 10
5.25 10
5.22 10
5.23 10
4
4
4
4
4
4
(Fig. 4). The initial rates
of the Np(VI) reduction
0
RADIOCHEMISTRY Vol. 48 No. 2 2006

128
ANAN’EV et al.
Table 1. Catalytic reduction of Np(VI) with formic acid in the presence of Pt nanoparticles ( brown colloid)
[Np(V)]0
[Pt]
[NaPA]
[HCOOH]
1
T,
C
k0,* mol l 1 min
M
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
5
6
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4
4
4
4
5
4
4
4
4
4
4
4
3
3
3
2
4
4
4
4
4
2.80 10
4.60 10
6.38 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
9.03 10
1 10
1 10
1 10
1 10
3 10
2 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
1 10
2.0 10
2.0 10
2.0 10
2.0 10
6.7 10
4.0 10
2.0 10
2.0 10
2.0 10
2.0 10
2.0 10
1.0 10
2.0 10
4.0 10
6.7 10
1.0 10
2.0 10
2.0 10
2.0 10
2.0 10
0.33
0.33
0.33
0.33
0.33
0.33
0.10
0.33
0.50
0.70
0.90
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
40
5.20 10
5.25 10
5.22 10
5.23 10
1.32 10
9.25 10
4
40
40
40
40
40
25
25
25
25
25
25
25
25
25
25
18
30
36
46
4
4
4
4
5
(5.0 0.1) 10
(1.62 0.05) 10
(2.39 0.04) 10
(3.6 0.2) 10
(4.94 0.08) 10
4
4
4
4
4
1.9 10
1.2 10
8.2 10
1.2 10
4
5
4
4
1.4 10
5
(6.34 0.04) 10
(2.64 0.07) 10
4
4
(5.0 0.1) 10
(7.3 0.1) 10
4
* The k values given without errors were obtained from a single kinetic experiment; the same for Table 2.
0
The fact that the initial reaction rate is essentially
independent of the Np(VI) concentration means that
the reaction is zero-order with respect to [Np(VI)].
of heterogeneous catalytic processes. The reaction rate
constant increases with an increase in the HCOOH
concentration in the range 0.1 0.9 M. In the logarith-
mic coordinates, this dependence is well approximated
by a straight line with a slope of 1 (Fig. 5a), indi-
cating that the reaction is first-order with respect to
[HCOOH].
This allows us to use the equation d[Np(VI)]/dt = k
0
for quantitative description of the kinetics of the
Np(VI) reduction with formic acid in the presence of
Pt nanoparticles. Data on the kinetics of catalytic
reduction of Np(VI) with formic acid in the presence
of the brown Pt colloid at varied experimental con-
ditions are listed in Table 1.
The temperature dependence of k in the Arrhenius
0
coordinates cannot be approximated by a straight line
throughout the examined range 18 46 C (Fig. 6a).
This fact reflects a transition from kinetic to diffusion
control of the process with increasing temperature
and is typical of heterogeneous catalytic reactions.
The apparent activation energy calculated from the
Table 1 shows that, with an increase in the catalyst
concentration in the reaction mixture, the reduction
rate virtually proportionally increases, which is typical
Fig. 5. Influence of the HCOOH concentration on the rate
Fig. 6. Temperature dependences of the rate constant of the
of the catalytic reduction of Np(VI) in the presence of
catalytic reduction of Np(VI) in 0.33 M HCOOH in the presence
5
5
7
(a) 1 10 M Pt ( brown colloid) at 25 C and (b) 6.44
of (a) 1 10 M Pt ( brown colloid) and (b) 4.13 10
M
7
10 M Pt ( gray colloid) at 40 C.
Pt ( gray colloid).
RADIOCHEMISTRY Vol. 48 No. 2 2006

CATALYTIC REDUCTION OF Np(VI) WITH FORMIC ACID
129
4
Table 2. Catalytic reduction of 9.03 10 M Np(VI) with formic acid in the presence of Pt nanoparticles ( gray colloid)
[Pt]
[NaPA]
M
[HCOOH]
1
T,
26
C
k0, mol l 1 min
7
7
7
6
6
6
5
7
7
7
7
7
7
7
7
5
5
5
5
5
5
5
4
4
4
4
4
5
4
4
4
4
4
4
4
4
4.13 10
6.44 10
8.20 10
1.60 10
4.13 10
8.20 10
1.0 10
6.44 10
6.44 10
6.44 10
6.44 10
6.44 10
4.13 10
4.13 10
4.13 10
1 10
0
0
0
0
0
0
0
0
0
0
0
0
0
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.033
0.10
0.33
0.67
1.0
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
(5.15 0.04) 10
26
26
26
26
26
22
40
40
40
40
40
40
50
60
22
22
22
22
22
7.95 10
1.0 10
2.3 10
5.9 10
8.2 10
8.15 10
(4.4 0.2) 10
(1.36 0.08) 10
(3.6 0.2) 10
(7.11 0.03) 10
(8.6 0.1) 10
(1.5 0.1) 10
(2.80 0.06) 10
(5.58 0.08) 10
0
0
4
4
3
3
3
4.0 10
8.0 10
2.0 10
4.0 10
6.0 10
5.3 10
4.7 10
4.2 10
4
4
4
4
1 10
1 10
1 10
1 10
(3.61 0.05) 10
4.7 10
linear portion of the Arrhenius plot in the range 18
36 C is 85.4 kJ mol .
( gray colloid) was studied under the following
1
4
conditions: [Np(VI)] = 9.03 10 M; [HCOOH] =
0
7
6
0.033 1.0 M; [Pt] = 4 10 8.2 10 M; [NaPA] =
Our experiments revealed a relatively unusual and
significant influence of the stabilizer (NaPA) con-
centration on the rate of the Np(VI) reduction with
HCOOH. Initially, an increase in [NaPA] in the range
4
3
1
10
8
10 M; and T = 25 60 C. The volume
of the reaction solution was 3.0 ml.
Typical kinetic curves of the catalytic reduction of
Np(VI) in 0.33 M HCOOH at 26 C, obtained at dif-
ferent concentrations of the nonstabilized Pt colloid,
are shown in Fig. 8. As in the case of the brown
colloid, the reaction is zero-order with respect to
4
(1.0 4.0) 10 M (Fig. 7) leads to the expected de-
crease in the reaction rate due to adsorption blocking
of active catalytic centers [10]. Further increase in the
2
stabilizer concentration to 10 M causes a noticeable
monotonic growth of the rate of Np(VI) reduction.
A control experiment showed that Np(VI) was stable
in the absence of HCOOH at 25 C in a solution con-
[Np(VI)]. The rate constants k of the catalytic reduc-
0
tion of Np(VI) with HCOOH in the presence of the
gray Pt colloid are given in Table 2.
2
5
taining 2 10 M NaPA and 1 10 M colloidal Pt,
i.e., NaPA did not reduce Np(VI) under these condi-
Comparison of the rates of the Np(VI) reduction
with HCOOH in the presence of gray and brown
tions. No reduction of Np(VI) was observed, either, in
2
a solution containing 0.33 M HCOOH and 2 10
M
NaPA in the absence of Pt nanoparticles. Thus, ac-
celeration of the Np(VI) reduction with formic acid
with increasing [NaPA] is not caused by certain im-
purities that could be present in commercial NaPA and
could initiate the reaction of Np(VI) with HCOOH
by a homogeneous catalytic mechanism.
Catalytic Reduction of Np(VI)
in the Presence of the Gray Colloid
Fig. 7. Influence of the sodium polyacrylate concentration
on the rate of the catalytic reduction of Np(VI) in 0.33 M
5
The catalytic reduction of Np(V) with formic acid
in the presence of nonstabilized Pt nanoparticles
HCOOH in the presence of 1 10 M Pt: (1) gray and
(2) brown colloid.
RADIOCHEMISTRY Vol. 48 No. 2 2006

130
ANAN’EV et al.
This significant difference between the apparent acti-
vation energies of the Np(VI) reduction in the pres-
ence of the gray and brown Pt colloids is obvious-
ly caused by the effect of the polyelectrolyte on the
heat of adsorption of the reacting species. It follows
from the Brønsted Polanyi equation [11] that the ap-
parent (experimentally determined) activation energy
of the heterogeneous catalytic process E* is related to
the true activation energy by E* = E
, where
true
is the heat of adsorption. As the catalyst surface is
energetically nonuniform, it can be expected that the
integral heat of adsorption will depend on the degree
of filling of the active centers. In the stabilized
brown colloid, NaPA is adsorbed on the surface of
Pt nanoparticles. Apparently, the polyelectrolyte mol-
ecules mainly occupy the most energetically favorable
active sites, which leads to a decrease in the heat of
adsorption of the reactants and increase in the appar-
ent activation energy. The polyelectrolyte also in-
creases the contribution of the diffusion constituent of
the reaction rate, which makes narrower the tempera-
ture range of the chemically controlled process in the
presence of the brown colloid and gives rise to an
inflection in the temperature dependence of the rate
constant (Fig. 6a).
Fig. 8. Kinetic curves of the catalytic reduction of Np(VI)
in 0.33 M HCOOH at 26 C in the presence of different con-
centrations of Pt nanoparticles ( gray colloid). [Pt], M:
6
6
6
(1) 8.20 10 , (2) 4.13 10 , (3) 1.60 10 , (4) 8.20
7
7
10 , and (5) 4.13 10
.
colloids (Tables 1, 2) reveals the following facts.
Despite smaller size of nanoparticles and hence more
developed surface of the brown colloid, its catalytic
activity is lower by almost an order of magnitude than
that of the gray colloid. A decrease in the catalytic
activity of the brown colloid is due to blocking of
the active surface of the catalyst with the stabilizer
(NaPA) molecules, hindering the access of the react-
ants to the catalyst surface. A similar pattern was
observed in experiments on catalytic reduction of
Methyl Viologen with molecular H on Pt nanopar-
As in the case of the brown colloid, addition of
NaPA to a solution of the gray colloid leads first to
a decrease in the rate of the Np(VI) reduction with
HCOOH and then to its increase. With both types of
2
ticles [10]. However, this phenomenon is not fully
understood. On the whole, the decreased catalytic
activity of the brown colloid may be attributed to
specific features of formation of the catalytic centers
on the nanoparticle surface in the presence of NaPA
and to the dependence of the concentration of these
centers on the size and structure of the colloids.
the nanocolloids, the minimum in the k [NaPA]
0
curves corresponds to the polyelectrolyte concentra-
3
tion of approximately 4.0 10 M (Fig. 7). It is
interesting that with the gray colloid the influence
of [NaPA] on the reaction rate is more pronounced
than with the brown colloid stabilized in advance.
As shown above, an increase in the Np(VI) reduction
rate with an increase in the NaPA concentration
As with the brown colloid, the rate of the cata-
lytic reduction of Np(VI) with HCOOH grows in pro-
portion with an increase in the concentration of the
nonstabilized Pt nanoparticles. The experimental reac-
tion order with respect to [HCOOH], found from the
3
above 4.0 10 M is associated neither with the
reducing effect of NaPA nor with the catalytic proper-
ties of the possible impurities. A reasonable explana-
tion is associated with the flocculation of Pt nano-
particles in aqueous solution. With addition of NaPA
and increase in its concentration, the polyelectrolyte
molecules are sorbed on open areas of the surface of
the colloidal particles, which results in blocking of
the catalytic centers and in the observed decrease in
the rate of the catalytic reaction. When the critical
concentration of NaPA is attained, the flocculation
gives way to deflocculation, i.e., breakdown of the
associates. This is accompanied by a decrease in the
size of the nanoparticles and hence in an increase in
the total surface area of the catalyst under the action
of the polyelectrolyte. A serious argument in favor of
dependence lnk ln[HCOOH] approximated by a
0
straight line, is 0.9 (Fig. 5b). Certain decrease in the
reaction order compared to the brown colloid is
apparently due to coarsening of the Pt nanoaggregates
with increasing [HCOOH] in the absence of the stabi-
lizing polyelectrolyte.
The temperature dependence of the rate of the
Np(VI) reduction in the presence of the gray colloid
in the coordinates 1/T lnk in the range 25 60 C is
0
a straight line (Fig. 6b) with a slope corresponding to
1
the apparent activation energy of 57.6 kJ mol , which
is appreciably lower than the value obtained for the
catalytic reduction of Np(VI) on the Pt nanoparticles
1
stabilized with sodium polyacrylate (85.4 kJ mol ).
RADIOCHEMISTRY Vol. 48 No. 2 2006

CATALYTIC REDUCTION OF Np(VI) WITH FORMIC ACID
this assumption is the trend in variation of the optical
131
spectra of the gray colloid in the presence of in-
creasing amounts of NaPA (Fig. 9). It is seen that an
4
increase in the NaPA concentration from 8 10
M
3
to the critical level of 4 10 M leads to charac-
teristic changes in the shape of the optical spectrum:
The absorption intensity increases in the UV range
and decreases in the long-wave region ( isobestic
point at about 750 nm). This trend is definitely indi-
cative of the breakdown of floccules and increase in
the number of separate nanoparticles. An increase in
the optical density in the UV range is caused by an
increase in the intrinsic absorption of platinum nano-
particles, and a decrease in the long-wave absorption
is indicative of a decrease in the contribution of the
light scattering factor due to breakdown of coarse
scattering particles (floccules). Thus, a decrease in
the catalytic activity of Pt nanoparticles due to block-
ing of the active centers with the polyelectrolyte can
be compensated by an increase in the total surface
area of the colloid. Starting from a certain concentra-
Fig. 9. Effect of NaPA additions on the absorption spectra
5
of 1 10 M gray Pt colloid. [NaPA], M: (1) 0, (2) 8.0
4
3
3
10 , (3) 4.0 10 , and (4) 6.0 10
.
is blocked by NaPA molecules. When the reaction is
initiated by adding Np(VI) to a solution containing
the stabilized colloid and a weak electrolyte, HCOOH,
an intermediate pattern is observed.
3
The above kinetic relationships allow certain as-
sumptions concerning the mechanism of the catalytic
reduction of Np(VI) with HCOOH in the presence of
Pt nanoparticles. The independence of the reaction
rate from the concentration of Np(VI) {zero reaction
order with respect to [Np(VI)]} indicates that Np(VI)
is reduced by a fast reaction with active species,
possibly radicals, generated by chemisorption of
HCOOH molecules on the catalytic centers of the
platinum colloid. Generation of the active species is
the rate-determining step of the overall process:
tion of the stabilizer (in our case, 4.0 10 M), the
latter trend becomes prevalent, leading to an increase
in the rate of the catalytic reduction of Np(VI) with
HCOOH.
The stability of Pt nanoparticles in the stabilized
brown colloid is determined not only by the pres-
ence of the stabilizing agent, but also by the ionic
strength of the medium. Addition of electrolytes (nep-
tunyl perchlorate, HCOOH) to such systems alters the
electrostatic equilibria in the colloidal solution, which
results in coarsening of the nanoparticles and, as a
consequence, in a decrease in the catalytic activity of
the colloid. In the nanosystem stabilized with the poly-
electrolyte, the colloid degrades relatively slowly, but
this process is the faster and its extent is the greater,
the higher the degree of dissociation of the electolyte
added. This trend is reflected in the dependence of
the rate of the catalytic reaction on the order of mix-
ing the reactants in the case when the reaction rate is
higher than the rate at which the colloid attains the
equilibrium state. Therefore, the catalytic reduction of
Np(VI) with HCOOH occurs faster when the reaction
is initiated by adding an aliquot of the pure stabilized
Pt colloid to a formate solution of Np(VI) (Fig. 2). If
the reaction is initiated by adding HCOOH to a solu-
tion containing Pt nanoparticles and Np(VI) perchlo-
rate, the Np(VI) reduction is considerably slower,
because an increase in the ionic strength of the col-
loidal solution due to addition of a strong electrolyte,
Np(VI) perchlorate, caused a decrease in the active
surface area of the colloid. Furthermore, access of
HCOOH molecules to active centers of the catalyst
HCOOHsol
(H COOH)ads
(slow step).
HCOOHads
,
(1)
HCOOHads
Ha*ds
+
COOHa*ds
(2)
The concentration of adsorbed HCOOH is propor-
tional to the concentration of HCOOH in the solution,
which determines the first order of the reaction with
respect to [HCOOH]. Then Np(VI) rapidly reacts with
the chemisorbed HCOOH molecule:
.
NpO22+ + (H COOH)ads
NpO+2 + COOHads + H+,
(3)
.
NpO22+ + COOHads
NpO+2 + CO2 + H+,
(4)
or with radicals:
.
NpO22+ + Hads
NpO+2 + H+,
(5)
(6)
.
NpO22+ + COOHads
NpO+2 + CO2 + H+.
RADIOCHEMISTRY Vol. 48 No. 2 2006

132
ANAN’EV et al.
Unfortunately, pathways (3), (4) and (5), (6) are
Foundation for Basic Research (project no. 03-03-
32239).
kinetically indiscernible. However, the latter step
seems preferable because of the following facts. It is
known that the catalytic dehydrogenation of HCOOH
in aqueous solution in the presence of palladium
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assistance in the chromatographic measurements.
The study was financially supported by the Russian
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RADIOCHEMISTRY Vol. 48 No. 2 2006