Factors affecting the hydrogen reduction kinetics of CsHSO4

Article abstract of DOI:10.1134/S0020168509010130

The hydrogen reduction of CsHSO₄, including in the presence of catalysts, is studied. The main factors affecting the rate of the process are determined. A possible reaction mechanism through the surface hydrated phase is discussed. Experiments

Full text of DOI:10.1134/S0020168509010130

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 1, pp. 85–89. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V. G. Ponomareva, G. V. Lavrova, 2009, published in Neorganicheskie Materialy, 2008, Vol. 45, No. 1, pp. 89–93.
Factors Affecting the Hydrogen Reduction Kinetics of CsHSO4
V. G. Ponomareva and G. V. Lavrova
Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences,
Novosibirsk, 630128 Russia
Received April 21, 2008
Abstract—The hydrogen reduction of CsHSO4, including in the presence of catalysts, is studied. The main fac-
tors affecting the rate of the process are determined. A possible reaction mechanism through the surface
hydrated phase is discussed. Experiments show that the compound is more stable in hydrogen without any trace
of moisture, including in the presence of a platinum catalyst.
DOI: 10.1134/S0020168509010130
INTRODUCTION
of only 0.4 wt % of CsHSO4. Reaction (1) is also
observed when Pt, Pt/C, and Pt/Rh are used as anodes;
at the same time, the compound CsHSO4 is chemically
stable in the presence of SnO2, Ni, and Pd [10, 11].
The family of superprotonic salts MxHy(AO4)z (M =
Cs, Rb, K, NH4; A = S, Se, P, As), which display high
proton conductivity (10–2–10–3 S/cm) in a temperature
range of 130 to 250°C, is of interest owing to the possi-
bility of its use in medium temperature fuel elements
(FEs) [1–7]. It is required in this case that a compound
be stable under hydrogen reduction conditions. In the
very first work [6] devoted to a new type of medium
temperature FC (called a solid acid FC), CsHSO4 was
used as an electrolyte. According to [6], when a salt is
hydrogen reduced on a Pt/C anode, hydrogen sulfide is
emitted:
The FC process of ç2S emission on platinum and its
effect on adsorptive and electrochemical anode charac-
teristics may depend on relative moisture, time, and the
value of the current passed, which requires a separate
study.
Thermodynamic calculations [12] show that reac-
tion (1) is an energetically advantageous process.
According to the authors' estimation, even with an air
pressure of 10 Pa, the inhibition of reaction (1) will
1/4
2CsHSO4 + 4H2 = ç2S + Cs2SO4 + 4H2O.
(1)
require the conditions pH O ⋅ p
> 104–106 MPa,
H2S
2
In a complete reaction, the CsHSO4 weight loss
should be ~22.6%. Indeed, subsequent thermogravi-
metric analyses showed that the reaction proceeded but
very slowly without catalysts: after 10 h in a hydrogen
flow at a temperature of 160°C, the ësHSO4 loss was
~0.03 wt % [8, 9].
According to [8, 9], catalysts such as Ir, Pt, Pd, and
WC efficiently accelerate the reaction of hydrogen sul-
fide formation: when ësHSO4 is kept with a platinum
catalyst at a temperature of 160°C (35 wt %), the loss
amounts to 6.8 wt % of the salt [8], while a ësHSO4 –
Pt/C (3 : 1) system kept for 48 h at 150°C results in a
loss of ~12.7 wt % [9].
which cannot be achieved in the course of the experi-
ment. However, a low reaction rate in the absence of
catalysts appears to be determined by a high energy
barrier at the elementary stages through which it pro-
ceeds. In particular, the reaction includes the stages of
H2 absorption, dissociation, a charge transfer reaction
(forming and breaking of hydrogen bonds, breaking of
S–O and Cs–O bonds), ion and water diffusion, and
water and H2S desorption. Thus, it seems relevant to
study the parameters affecting the kinetics of reaction
(1) in order to control it and find its inhibition mecha-
nisms.
This work is aimed at studying ësHSO4 reduction in
hydrogen and the dynamics of ç2S emission both under
the conditions of FC operation and in a catalyst + elec-
trolyte mixture with an extended contact surface. The
type of catalyst was varied (Pt, Pd, Ni), as well as the
amount of catalyst with respect to CsHSO4 (with
CsHSO4–metal contact surface), the concentration of
acid centers, the temperature (160–190°C), the water
vapor pressure in hydrogen (67–2900 Pa), and the
The formation of Cs2SO4 is supported by x-ray
phase analysis data. However, study [9] does not
describe the experimental conditions: sample moisture,
the partial pressure of water vapor, and the size of cata-
lyst particles, which can substantially affect the reac-
tion. The specific surface area of the catalyst (the size
of granules and agglomerates), porosity, and presence
of active centers are of great importance as well.
Our studies [10] show that a CsHSO4 + Pt (200 : 3)
mixture kept in hydrogen at 160°C for 7 h leads to a loss residual moisture content in the sample.
85

86
PONOMAREVA, LAVROVA
ousness, it is not complicated and is quite precise (error
is ~2%).
α, %
3.5
1
3.0
2.5
2.0
1.5
1.0
0.5
RESULTS AND DISCUSSION
When CsHSO4 was thermally kept for a long time in
hydrogen without a catalyst (more than 24 h), the
weight loss of CsHSO4 was no more than 0.004%,
which is consistent with the data of [8, 9]. The amount
of H2S emitted on a platinum anode under the condi-
tions of FC operation (at CsHSO4/Pt interface) in a
hydrogen flow for 24 h at 160°C corresponded to the
weight loss of CsHSO4 of ~0.001%. These values are
much smaller than the detection limit of the reverse
titration method. Therefore, to increase the amount of
H2S emitted, the CsHSO4 –metal contact surface was
enlarged through changing the ratio of components: at
the ratio of CsHSO4 : Pt = 80 : 1 and the size of Pt, Pd,
and Ni particles of ~20 nm, the contact surface was
increased by several orders of magnitude.
2
3
0
2
4
6
8
Time, h
Fig. 1. Dependence of the degree ofCsHSO conversion on
the time of hydrogen flow to the CsHSO + catalyst mixture
(80 : 1): (1) Pt, (2) Pd, (3) Ni ( pH O ~ 2900 Pa, T = 160°C).
4
4
2
In [14], it is shown that the exchange current density
in the H2, M/ CsHSO4 electrode reaction in hydrogen
EXPERIMENTAL
increases in the sequence Ni
Pt
Pd, exchange
Single crystals of cesium hydrosulfate were
currents being enhanced by an order of magnitude
when passing from one metal to another. This work pre-
sents studies of the kinetics of reaction (1) in the pres-
ence of these metals. Figure 1 shows that Pt signifi-
cantly catalyzes reaction (1), Pd is substantially less
active as a catalyst, and Ni hardly affects the reaction
rate. The smaller catalytic activity of Pd compared to Pt
along with its high electrochemical activity makes it a
promising material for anodes in FC, which was
observed in [11].
obtained by isothermal evaporation from aqueous solu-
tions containing equimolar amounts of ës2SO4 (chemi-
cally pure) and ç2SO4 at room temperature. Platinum
and palladium black were obtained by the formic acid
reduction of ç2[PtCl6] aqueous solutions and palladium
chloride with subsequent multiple water washings.
According to x-ray phase analysis and electron micros-
copy data, the size of catalyst particles did not exceed
20 nm. Hydrogen used for reduction was either
obtained by electrolysis and then saturated with water
vapors in a saturator at 25°C or supplied from a hydro-
gen cylinder. The rate of hydrogen flow to the system
was 0.05 ml/s.
The results obtained differ from the data in [8],
which can be due to a number of factors, in particular,
differences in the size of the specific surface area, the
number of active centers on the catalyst surface, and
sample prehistory and also different experimental con-
ditions.
In order to determine the amount of ç2S emitted in
the process of salt reduction, reverse iodometric titra-
tion [13] was applied. According to this method, emit-
ted hydrogen sulfide is passed through a zinc sulfide
solution (buffered with sodium acetate) that absorbs
H2S and forms ZnS by the reaction
It is known that the salt : catalyst ratio can substan-
tially affect the reaction kinetics. Apparently, a catalyst
affects the reaction solely on the contact surface with
the salt; thus, the maximum contact surface should be
observed when the surface ratio is 1 : 1. Under normal
conditions, it is hardly possible to grind CsHSO4 parti-
cles to less than 1 µm. With regard to the density of the
catalyst and cesium hydrosulfate and also the particle
size of catalysts used (platinum and palladium), it is
possible to estimate that the above surface ratio can be
achieved only when the salt : catalyst weight ratio is
~8 : 1 in the case of Pt and 15 : 1 in the case of Pd.
ZnCl2 + H2S
ZnS↑ + 2çël.
(2)
An excess amount of iodine aqueous solution, HCl (1 : 1),
and starch were added to the solution containing ZnS. This
blue solution was then titrated with sodium thiosulfate
until it became colorless according to the reactions
I2 + ZnS
ZnI2 + S↑,
(3)
(4)
I2 + 2Na2S2O3
2NaI + Na2S4O6.
Figure 2 shows the dependence of CsHSO4 conver-
The minimum amount of hydrogen sulfide calcu-
lated by the above method is 10–15 µg, which corre- sion in a CsHSO4 + Pt mixture at different ratios at a
sponds to a loss of 0.008–0.009 wt % in ësHSO4 reduc-
temperature of T = 160°C. It is seen that the dependence
tion. The method is intended for a direct determination is nearly linear, and if the ratio is multiplied by 4, the
of hydrogen sulfide in gas mixtures. Despite its labori- amount of emitted hydrogen sulfide increases ~32-fold.
INORGANIC MATERIALS Vol. 45 No. 1 2009

FACTORS AFFECTING THE HYDROGEN REDUCTION KINETICS OF CsHSO4
87
α, %
α, %
2.5
3.5
2
3.0
2.5
2.0
1.5
1.0
0.5
2.0
1.5
1.0
0.5
1
0
0
20 40 60 80 100 120 140 160
0
CsHSO : Pt
4
0
1
2
3
4
Time, h
Fig. 2. Dependence of the degree of CsHSO conversion on
Fig. 3. Dependences of the degree of CsHSO conversion
on the time of hydrogen flow at T = 160 (1), 190°C (2);
4
4
the ratio CsHSO : Pt in the mixture (T = 160°C, the time of
4
hydrogen flow 4 h, pH O ~67 Pa).
CsHSO : Pt = 80 : 1, p
~ 67 Pa.
4
H2O
2
α, %
With a temperature increase from 160 to 190°C, the
salt reduction reaction accelerates ~1.5-fold (Fig. 3),
which corresponds to the activation energy of reaction
(1) with the catalyst of ~50–60 kJ/mole. Note that this
activation energy value is typical of a dehydration reac-
tion of some crystalline hydrates [15].
The reaction performed in a hydrogen atmosphere
with different degrees of moisture shows that an
increase in the relative moisture of hydrogen speeds up
cesium hydrosulfate reduction in the presence of the
catalyst (Fig. 4). This effect is most vividly demon-
strated by the mixture of salt and palladium rather than
platinum. However, even in moist hydrogen, the degree
of CsHSO4 conversion does not exceed 3.5% at the ratio
of CsHSO4 : Pt = 80 : 1. The rate of reaction (1)
decreases with time at lower moisture levels, while at
higher moisture content the dependence of salt conver-
sion is almost linear.
The effect of moisture can be associated both with
better adhesion of the catalyst to electrolyte and with
the formation of new catalytically active centers, prob-
ably of an acid-base type. Moreover, water is likely to
participate in the reaction of sulfur reduction. For
instance, adsorption of water molecules on the CsHSO4
surface can lead to the formation of either CsHSO4 ·
nH2O crystalline hydrates with a lower energy of Cs–O
and S–O bonds or hydrated microlayers in which the
salt dissociates into ions, which accelerates sulfur
reduction. It is known that the surface properties of
MHSO4 (M = Cs, Rb, NH4) hydrosulfates vary consid-
erably from bulk ones. In particular, phase transitions
on the surface occur at much lower temperatures [16].
(a)
3.5
1
3.0
2.5
2.0
1.5
1.0
0.5
2
0
1
2
3
4
5
6
α, %
1.6
(b)
1
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2
0
1
2
3
4
5
6
7
8
9
Time, h
Fig. 4. Dependences of the degree of CsHSO conversion in
(a) CsHSO + Pt = 80 : 1 and (b) CsHSO + Pd = 80 : 1 on
the time of hydrogen flow at water vapor partial pressures in
hydrogen of (1) 2900 and (2) 67 Pa; T = 160°C.
4
This poses a question: Will the reaction proceed if
water adsorbed on the CsHSO4 surface is removed to
the maximum possible extent? For this purpose, the salt
4
4
INORGANIC MATERIALS Vol. 45 No. 1 2009

88
PONOMAREVA, LAVROVA
phase of CsHSO4. The presence of water first of all pro-
α, %
0.6
vides the breaking of the Cs–O bond, which is one of
the elementary acts of this multistage reaction. Hydro-
gen adsorption by cesium hydrosulfate seems to be
negligible. The presence of the catalyst accelerates the
adsorption and dissociation of hydrogen, facilitates
charge transfer, and reduces the formation energy of
intermediate reaction products. Nevertheless, without
any trace of moisture, cesium hydrosulfate can remain
stable for a long time in a hydrogen reducing atmo-
sphere even in the presence of catalysts.
1
0.5
0.4
0.3
0.2
0.1
2
0
0
CONCLUSIONS
1
2
3
4
5
The hydrogen reduction of ësHSO4 both with and
without catalysts (Pt, Pd, Ni) is studied. It is shown that
the reaction proceeds extremely slowly without a cata-
lyst. Catalytic activity increases in the sequence Ni
Pt. Palladium catalytic activity rises substan-
tially with increasing relative moisture.
Time, h
Fig. 5. Dependences of the degree of CsHSO conversion
for samples with different acidity: (1) CsHSO + H SO
(6 : 1); (2) CsHSO ; pressure of ~2900 Pa (without a cata-
lyst).
4
4
2
4
Pd
4
The factors affecting the degree of ësHSO4 reduc-
tion are determined. The degree of conversion increases
with increasing amount of a catalyst in its mixture with
the salt, with rising temperature, with increasing rela-
tive moisture or moisture content in ësHSO4, and with
increasing concentration of active acid centers on the
electrolyte surface. The latter two factors are close to
was preliminarily kept in a residual vacuum of ~104 Pa
at 160°C for 2 h, then mixed with the catalyst (Pt, Pd)
in a 4 : 1 ratio, pressed into a loose pellet (relative den-
sity of ~40%), and kept again under ~104 Pa at 160°C.
Then hydrogen ( pH ~ (53–93) × 103 Pa) was supplied
2
to the cell, and it was kept at 160°C. During the first 2 h, the action of a platinum catalyst in their nature and sig-
no weight loss was recorded, and after the next 6 h, the nificance.
degree of CsHSO4 conversion did not exceed 1%. When
Without any trace of moisture in hydrogen and on
CsHSO4 was kept without any catalyst and also in a
mixture with palladium in a 4 : 1 ratio, no weight loss
was observed under similar conditions. Contrary to the
data in [8], the formation of cesium sulfate, according
to x-ray phase analysis, was not proved in our case.
Hence, it is possible to conclude that the presence of
adsorbed water is essential for the CsHSO4 reduction
reaction to proceed, and when it is absent, the reaction
rate is extremely low. However, considering thermody-
namic data, an increase in the partial pressure of water
vapor should inhibit reaction (1). Therefore, the authors
of [6] tested FC in an atmosphere with 30 mol % H2O.
In this case, the effect of the kinetics of the reaction (its
mechanism) on the degree of conversion is more sub-
stantial than its thermodynamics.
the salt crystal surface, cesium hydrosulfate reduction
does not occur, and even in the presence of catalysts
(salt : catalyst = 4 : 1), the reaction rate is extremely
low.
The relatively low catalytic activity of palladium in
the cesium hydrosulfate reduction reaction along with
its high electrochemical activity makes it a promising
electrode material for medium temperature FEs.
ACKNOWLEDGMENTS
We are grateful to N.F. Uvarov, A.A. Matvienko,
and L.I. Brezhneva for the discussion of results.
This work was partially supported by the Russian
Foundation for Basic Research (grant nos. 05-03-
32278 and 07-03-12151).
Acidification of CsHSO4 with a small amount of sul-
furic acid also accelerates reaction (1). In this case, the
degree of CsHSO4 conversion increases even without
any catalyst (Fig. 5), which indicates that the reaction
proceeds faster in hydrated layers, apparently, with the
participation of acid centers. However, it should be
noted that, in this case, it is not excluded that an
increase in the amount of hydrogen sulfide emitted is
due to the direct reduction of sulfuric acid.
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