Catalytic reduction of U(VI) with formic acid in acid solutions on palladium catalysts

Article abstract of DOI:10.1007/s11137-008-1006-3

The kinetics of catalytic reduction of U(VI) with formic acid in H ₂SO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO₂ leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by catalytic reduction with formic acid.

Full text of DOI:10.1007/s11137-008-1006-3

ISSN 1066-3622, Radiochemistry, 2008, Vol. 50, No. 1, pp. 46 51. Pleiades Publishing, Inc., 2008.
Original Russian Text M.Yu. Boltoeva, V.P. Shilov, A.V. Anan’ev, 2008, published in Radiokhimiya, 2008, Vol. 50, No. 1, pp. 41 46.
Catalytic Reduction of U(VI) with Formic Acid
in Acid Solutions on Palladium Catalysts
M. Yu. Boltoeva, V. P. Shilov, and A. V. Anan’ev
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
Received November 30, 2006
Abstract The kinetics of catalytic reduction of U(VI) with formic acid in H2SO4 solutions in the presence
of Pd/SiO2 catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in
the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic
centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO2
leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the
contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of
active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the
energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd
nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by
catalytic reduction with formic acid.
PACS numbers: 82.20.Pm, 82.65.+r
DOI: 10.1134/S1066362208010062
Heterogeneous-catalytic processes can be success-
fully used for stabilization of the preset valence forms
of actinides in process solutions of radiochemical
plants.
EXPERIMENTAL
Preparation and characterization of catalysts.
Catalytic reduction of U(VI) was studied in the pres-
ence of various samples of Pd/SiO catalysts differing
2
One of important processes in the hydrometallurgy
of the nuclear fuel cycle is the reduction of U(VI) to
U(IV). One of the main chemical methods for prepar-
ing U(IV) is the reduction of U(VI) with hydrazine
and HCOOH in the presence of platinum catalysts
[1 6]. The major drawback of this method is the use
of expensive platinum as the active component of the
catalyst. In the course of natural wear and corrosion
processes, platinum gets into radioactive waste frac-
tions from which its recovery is impossible because of
specific features of radiochemical technology. There-
fore, search for and study of new catalysts for initiat-
ing redox reactions in aqueous media is an important
scientific and practical problem.
in the percent content of Pd on the support and par-
ticle size of the active metal. We used the same cata-
lyst samples as in [7]; their preparation and charac-
terization procedures are described in [7]. The charac-
teristics of the catalysts are given in Table 1. After
each series of kinetic experiments, the catalysts were
washed with water, dried at 100 C for 24 h, and then
reused.
Experimental procedure. Kinetic experiments
were performed in a temperature-controlled glass
reactor equipped with a reflux condenser to prevent
evaporation of the working solutions. In addition, the
Table 1. Characteristics of Pd/SiO2 catalysts
One of promising materials for preparing the cata-
lysts is Pd, which exhibits pronounced catalytic prop-
erties. In the previous paper [7] we showed that
Actual Mean particle
Dispersity
Catalyst
Pd con-
tent, %
diameter
DPd = 1.12/dvs
dvs, nm
Pd/SiO catalysts can be efficiently used for reducing
2
U(VI) with hydrazine in moderately acidic nitrate-free
solutions.
1% Pd/SiO2 (A) 0.89
1% Pd/SiO2 (B) 0.76
4% Pd/SiO2 (A) 3.92
4% Pd/SiO2 (B) 4.1
12.9
8.8
12.7
12.3
0.086
0.130
0.088
0.091
In this study we examined the kinetic relationships
of the catalytic reduction of U(VI) with formic acid
in acid solutions.
46

CATALYTIC REDUCTION OF U(VI) WITH FORMIC ACID
reaction vessel was protected from light to eliminate
47
possible contribution of photochemical processes. The
working solutions were prepared just before the exper-
iment by mixing the calculated amounts of solutions
of H SO , HCOOH, uranyl sulfate, and distilled
2
4
water. The total volume of the reaction mixture was
20 ml. After thermostating at a required temperature,
the catalytic reduction of U(VI) was initiated by intro-
ducing a weighed portion of a dry catalyst into the
reactor. The mixture was agitated by argon bubbling
at a definite rate monitored with a float-type flow
meter. To determine the current concentrations of
U(IV), we took at regular intervals 0.2-ml aliquots of
the working solution, mixed them with 2 ml of 2.0 M
H SO , and measured the optical density at
=
2
4
650 nm. The spectra were recorded with a Shimadzu
3100 spectrophotometer.
Fig. 1. (a) Kinetic curves of U(VI) reduction at its different
initial concentrations [U(VI)] and (b) initial reaction rate
0
as a function of [U(VI)] . Reaction system: 0.5 M H SO ,
The hydrogen content of gaseous reaction products
was determined with a Chrom 5 chromatograph (col-
umn length 2 m, column packed with 5A molecular
sieves, granule size 0.4 0.5 mm, thermal conductivity
0
2
4
1
0.5 M HCOOH, 0.05 g ml of 1% Pd/SiO (A); 60 C.
2
(a) [U(VI)] , M (1) 0.2, (2) 0.15, (3) 0.1, (4) 0.05, and
(5) 0.02; (b) n = 1.022, R = 0.999.
0
2
1
detector, carrier gas Ar, flow rate 30 ml min , room
temperature).
RESULTS AND DISCUSSION
Preliminary experiments showed that palladium
catalysts do not initiate the reduction of U(VI) with
formic acid in nitric acid solutions in a wide range of
experimental conditions, even after adding hydrazine
as antinitrite agent. In sulfuric acid, HCOOH starts to
1
reduce U(VI) in the presence of 0.1 g ml of 1%
Pd/SiO at a noticeable rate even at 30 C. At 60 C,
2
a 0.05 M solution of U(VI) in 0.5 M H SO is com-
pletely reduced with 0.5 M HCOOH within 1 h.
2
4
Fig. 2. (a) Kinetic curves of reduction of 0.1 M U(VI) in
the presence of various amounts of 1% PdSiO (A) and
2
(b) influence of the S : L ratio on the rate of catalytic re-
duction of U(VI) at [H SO ] = 0.5, [HCOOH] = 0.5 M;
To determine the order of the catalytic reaction
with respect to the U(VI) concentration, we performed
a series of experiments in the range of initial U(VI)
concentrations from 0.02 to 0.2 M (Fig. 1a). Analysis
of the dependence of the initial reaction rate on the
initial U(VI) concentration (Fig. 1b) shows that the
reduction of U(VI) follows a first-order law with
respect to [U(VI)]. Therefore, we calculated the first-
order rate constants k by the equation dC/d = k C.
2
4
1
2
60 C. (a) Amount of 1% Pd/SiO (A), g ml : (1) 0.2,
2
(2) 0.1, (3) 0.05, and (4) 0.02; (b) n = 1.03, R = 0.9992.
processes. In the coordinates ln[S : L] lnk , this
1
dependence is a straight line with a slope of 1.03
(Fig. 2b), which can be formally interpreted as the
apparent reaction order with respect to the volumetric
concentration of active catalytic centers in the reaction
system.
1
1
The kinetic curves of the catalytic reduction of
0.1 M U(VI) in 0.5 M H SO in the presence of 0.5 M
The values of k for various catalysts and experi-
2
4
1
HCOOH and various amounts (S : L ratios) of 1%
mental conditions are given in Table 2. The calcula-
tions, as a rule, were based on the results of three
independent kinetic experiments. The error in the
Pd/SiO (A) at 60 C are shown in Fig. 2a. An in-
2
1
crease in the catalyst amount from 0.02 to 0.2 g ml
leads to a proportional increase in the rate of U(VI)
reduction, which is typical of heterogeneous-catalytic
determination of k did not exceed 10%, which is
typical of kinetic measurements.
1
RADIOCHEMISTRY Vol. 50 No. 1 2008

48
BOLTOEVA et al.
Table 2. Catalytic reduction of U(VI) with formic acid in sulfuric acid solutions in the presence of palladium catalysts
1
1
[U(VI)]0, M
S : L, g ml
[HCOOH], M
[H2SO4], M
T,
C
k1, min
1% Pd/SiO2 (A)
2
0.1
0.1
0.1
0.1
0.02
0.05
0.1
0.15
0.2
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.05
0.1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.2
0.3
0.5
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.5
1
60
6.36 10
1.61 10
3.16 10
6.88 10
1.49 10
1.52 10
1.58 10
1.52 10
1.43 10
1.17 10
1.28 10
1.98 10
1.65 10
1.36 10
1.68 10
1.78 10
1.46 10
1.56 10
1.58 10
1.42 10
1.52 10
3.60 10
6.41 10
1.12 10
1.52 10
2.86 10
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
1
1
1
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
30
40
50
60
70
0.2
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
2
3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2
3
0.5
0.5
0.5
0.5
0.5
1% Pd/SiO2 (B)
2
2
2
1
1
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
30
40
50
60
70
4.17 10
6.79 10
9.25 10
1.24 10
1.83 10
4% Pd/SiO2 (A)
4% Pd/SiO2 (B)
2
2
2
1
1
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
32
40
50
60
70
5.34 10
7.13 10
1.22 10
1.52 10
2.14 10
2
2
1
1
1
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
32
40
50
60
70
6.32 10
8.48 10
1.04 10
1.48 10
1.8 10
The main kinetic relationships of the catalytic
reduction of U(VI) with formic acid in sulfuric acid
reduction on the HCOOH concentration in the range
0.1 3 M is complex. The reaction rate first increases
with an increase in [HCOOH] from 0.1 to 0.3 M,
after which it somewhat decreases and the remains
solutions were studied in the presence of 1% Pd/SiO
2
(A). The dependence of the rate constant of U(VI)
RADIOCHEMISTRY Vol. 50 No. 1 2008

CATALYTIC REDUCTION OF U(VI) WITH FORMIC ACID
virtually constant in the range of [HCOOH] 1.0
49
3.0 M (Fig. 3).
An increase in the H SO concentration in the
2
4
range 0.1 3 M at [U(VI)] = 0.05, [HCOOH] = 0.5 M
0
1
and 60 C in the presence of 0.1 g ml of 1% Pd/SiO
2
(A) does not lead to a noticeable change in k . This
1
fact indicates that hydrogen ions do not directly par-
ticipate in the rate-determining step of the reaction.
Fig. 3. Rate of catalytic reduction of U(VI) as a function of
Table 2 shows that all the catalysts exhibit similar
catalytic activity (per unit weight of the catalyst) in
the reduction of U(VI). Unexpectedly, the catalyst
activity does not increase with a fourfold increase in
the Pd content on the support.
HCOOH concentration at [U(VI)] = 0.05, [H SO ] = 0.5 M
0
2
4
1
and 60 C in the presence of 0.05 g ml of 1% Pd/SiO (A).
2
sults show that, under equal other conditions, the ap-
parent activation energy depends on the size of Pd
particles and metal content on the support.
The specific activity of the catalysts was evaluated
1
An important property of HCOOH is the capability
for catalytic decomposition to H and CO in the
by the turnover number r (min ), i.e., by the number
of catalytic events per surface atom of the catalyst in
2
2
presence of Pd metal. The kinetics and mechanism of
this reaction in aqueous solutions have been described
in detail in [8]. Therefore, to understand the mech-
anisms of catalytic reactions in the system U(VI)
unit time. The quantity r was calculated by the for-
mula r = V/N, where
is the reaction rate in the
1
1
mol l min units; V, volume of the reaction mixture
in liters; and N, number of moles of surface Pd atoms.
The number of surface atoms of the catalyst, involved
in the heterogeneous-catalytic process, was calculated
from the catalyst weight, percent content of Pd on the
support, and metal dispersity determined by electron
microscopy. Because the rate of U(VI) reduction [first
HCOOH Pd/SiO , it is very important to examine
2
whether the reduction of U(VI) and catalytic dehydro-
genation of HCOOH occur concurrently. To detect
possible formation of H in the course of catalytic
2
reduction of U(VI) with formic acid, we performed
a chromatographic analysis of the gaseous reaction
2+
order with respect to [UO ]) decreases with the reac-
2
products for the H content. Figure 4 shows how the
2
tion progress, as parameter we took the change in
the HCOOH concentration within the half-conversion
Table 3. Influence of the size of Pd nanoaggregates on
the specific activity of Pd/SiO2 catalysts in the reduction
of U(VI) with formic acid in sulfuric acid solutions at
[U(VI)]0 = 0.05, [HCOOH] = 0.5, [H2SO4] = 0.5 M and
T = 60 C
time
= ln2/k (k is the first-order rate constant),
1/2
1 1
normalized with respect to
, i.e., we took
=
1/2
C /2 [C is the initial concentration of U(VI)].
0
1/2
0
The turnover numbers calculated for [U(VI)] = 0.05,
0
[HCOOH] = 0.5, [H SO ] = 0.5 M and T = 60 C with
2
4
1
various catalysts are given in Table 3. As can be seen,
the specific (per surface atom) activity of palladium
catalysts depends not only on the size of nanoaggre-
gates of the active metal, but also on the percent con-
tent of the metal on the support, i.e., surface Pd atoms
located on coarse crystallites exhibit higher catalytic
activity. At the same time, an increase in the percent
Catalyst
dvs, nm
D
r, min
1% Pd/SiO2 (A)
1% Pd/SiO2 (B)
4% Pd/SiO2 (A)
4% Pd/SiO2 (B)
12.9
8.8
12.7
12.3
0.086
0.130
0.088
0.091
14.96
7.33
2.65
2.50
content of Pd on SiO decreases the activity of the
catalytic centers.
2
Table 4. Apparent activation energies of the reduction of
U(VI) with formic acid in sulfuric acid solutions in the
presence of various palladium catalysts at [U(VI)]0 = 0.05,
The temperature dependences of the rates of cata-
lytic reduction of U(VI) with HCOOH in sulfuric
acid solutions were measured in the presence of vari-
ous palladium catalysts. In the Arrhenius coordinates,
these dependences are straight lines in the temperature
range 30 70 C. The corresponding apparent activa-
tion energies are given in Table 4. With the catalyst
1
[HCOOH] = 0.5, [H2SO4] = 0.5 M and S : L = 0.05 g ml
1
Catalyst
dvs, nm
E*, kJ mol
1% Pd/SiO2 (A)
1% Pd/SiO2 (B)
4% Pd/SiO2 (A)
4% Pd/SiO2 (B)
12.9
8.8
12.7
12.3
43.3 3.5
30.8 1.5
32.0 2.3
24.0 1.0
4% Pd/SiO in the temperature range 30 50 C, an
2
induction period (about 6 min) is observed. Our re-
RADIOCHEMISTRY Vol. 50 No. 1 2008

50
BOLTOEVA et al.
reaction, or reversibly recombine with the evolution
of molecular hydrogen:
Pd(H) + Pd(H)
H2 + 2Pd,
(4)
with the rates of reactions (3) and (4) being compar-
able, as follows from the accumulation of H in gase-
2
ous reaction products.
Uranium(V) formed by reaction (3) rapidly dispro-
portionates into U(VI) and U(IV):
2UO+2 + 4H+
UO22+ + U4+ + 2H2O.
(5)
Fig. 4. Variation of the intensity of the chromatographic
peak of H with time in the course of (a) catalytic decom-
2
The above sequence of reactions (1) (5) describes
the catalytic reduction of U(VI) under the assumption
that the chemisorbed formate radical, which exhibits
pronounced reductive properties, decomposes by reac-
tion (2) faster than effects one-electron reduction of
uranyl ion. Otherwise, the fast radical reaction
position of HCOOH and (b) catalytic reduction of U(VI)
([U(VI)] = 0.05 M) with formic acid ([HCOOH] = 0.3,
[H SO ] = 0.5 M) on the catalyst 4% Pd/SiO (A) at 60 C.
0
2
4
2
intensity of the chromatographic peak of H varies
with time in the course of catalytic decomposition of
HCOOH and catalytic reduction of U(VI) with formic
acid on the catalyst 4% Pd/SiO (A) at 60 C. These
data show that both reactions are accompanied by the
evolution of gaseous H . Since H cannot be formed
in the oxidation of HCOOH with U(VI), we can con-
clude that the evolution of H in the course of catalyt-
ic reduction of U(VI) with formic acid on a Pd/SiO
2
UO22+ + Pd(HCOO)
UO+2 + H+ + CO2 + Pd (6)
2
would be possible. In this case, reaction (3) will also
be the slow, rate-determining step of the process,
because of the pronounced tendency of Pd to form
2
2
2
fairly stable compounds with H [9] and, hence, of the
2
2
catalyst is due to the side reaction of the catalytic
decomposition of HCOOH in the presence of Pd, oc-
curing concurrently with the main process.
decreased reactivity of the chemisorbed hydrogen
atom compared to the COOH radical.
The above reaction pathways are kinetically indis-
cernible. Summation of the reaction equations for both
schemes, taking into account the stoichiometric coef-
ficients, yields to the same overall reaction equation:
The results we obtained allow certain assumptions
concerning the mechanism of catalytic reduction of
U(VI) with HCOOH in sulfuric acid solutions in the
presence of a palladium catalyst. Most probably, as in
the case of catalytic decomposition of HCOOH on Pd
[8], the reaction starts with reversible chemisorption
of HCOOH molecules on the catalyst surface, with
two adjacent catalytic centers participating in the
process:
UO22+ + HCOOH + 2H+ = U4+ + CO2 + 2H2O, (7)
and therefore it is impossible to choose one of the two
reaction pathways on the basis of the available exper-
imental data.
The above-presented possible pathways of the
reduction of U(VI) with formic acid in the presence
2Pd + HCOOH
Pd(H) + Pd(HCOO).
(1)
of Pd/SiO are reasonably consistent with the kinetic
2
Ruthven and Upadhye [8] soundly believe that the
chemisorbed formate radical undergoes an intramolec-
ular transformation to form a chemisorbed H atom
parameters of the process. The first reaction order
with respect to [U(VI)] and the independence of the
+
reaction rate from the concentration of H ions direct-
and a free CO molecule:
2
ly follow from the suggested reaction mechanisms.
The complex dependence of the reduction rate on
[HCOOH] may be due to changes in the relative con-
tributions of concurrent reactions (3) and (4) to the
overall catalytic process with an increase in the cover-
age of the active surface of the catalyst as [HCOOH]
is increased. However, the origin of the statistically
significant maximum in the curve of the rate of U(VI)
reduction vs. [HCOOH] (Fig. 3) is unclear.
Pd(HCOO)
Pd(H) + CO2.
(2)
The H atoms bonded to catalytic centers can reduce
uranyl ions in a one-electron process:
Pd(H) + UO22+
UO+2 + H+ + Pd,
(3)
which is apparently the rate-determining step of the
RADIOCHEMISTRY Vol. 50 No. 1 2008

CATALYTIC REDUCTION OF U(VI) WITH FORMIC ACID
51
The dependence of the catalytic activity of surface
Pd atoms on the size of metal nanoaggregates (Table 3)
indicates that the catalytic reduction of U(VI) with
formic acid in sulfuric acid solutions is a structure-
sensitive process. The negative size effect is observed.
These results are also rationalized on the basis of the
suggested reaction mechanism. Indeed, a decrease in
the nanoaggregate size should lead to an increase in
the relative content of apical and edge coordination-
unsaturated atoms [10]. It is reasonable to expect that
the energy of the chemical bond of hydrogen atoms
with such low-coordinate surface Pd atoms will be
higher than with atoms located on crystallite faces.
As the ease of oxidation of such species with uranyl
ion will be determined by the Pd H bond enegry, its
increase will make the oxidation more difficult and
hence decrease the specific activity of the catalyst.
ments to a catalyst is its selectivity with respect to the
target process, i.e., the side reactions should be re-
duced to a minimum with the high catalytic activity in
the main process preserved. In the catalytic reduction
of U(VI) with formic acid in the presence of palladi-
um catalysts, the decomposition of HCOOH is such
a side reaction. As follows from [8], the catalytic
decomposition of HCOOH on Pd is a structure-inde-
pendent process. At the same time, as we showed, the
specific activity of the catalyst in the reduction of
U(VI) increases with an increase in the size of Pd
nanocrystallites on the support (negative size effect).
Thus, an increase in the size of Pd particles on the
support favors a decrease in the contribution of the
side reaction of HCOOH decomposition and hence an
increase in the relative yield of U(IV). Thus, the dis-
persity of the active component in Pd/SiO is an im-
2
portant selectivity factor in the catalytic reduction of
U(VI) with formic acid. Therefore, when preparing
U(IV) by catalytic reduction with formic acid, the use
of coarsely dispersed palladium catalysts is preferable.
The unexpected, at first glance, lack of noticeable
increase in the activity of palladium catalysts in the
reduction of U(VI) with formic acid at a fourfold
increase in the content of the active metal on the
support is also quite explicable. Actually, the point is
that the specific activity of the catalyst considerably
decreases with an increase in the Pd content (Table 3).
This trend is apparently caused by a considerable in-
crease in the contribution of the side reaction, catalyt-
ic decomposition of HCOOH, with an increase in the
number of active centers in the catalyst grain, at con-
stant concentrations of U(VI) and HCOOH in the
reaction system. Indeed, the chemisorbed hydrogen
atom Pd(H) is a common active participant of the
reduction of U(VI) and decomposition of HCOOH on
Pd, and the rates of the main (3) and side (4) reactions
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2
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4
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RADIOCHEMISTRY Vol. 50 No. 1 2008