L. Matachowski et al. / Applied Catalysis A: General 469 (2014) 239–249
245
In turn, the irreversible ammonia uptake on the Cs2(SS) salt
followed by partial removal of the water molecules hydrating pro-
tons. From the beginning of drying process the lattice parameter
of the H PW12O40·29H O decreases from 2.327 to 1.217 nm, char-
equal to 2.04 mol NH /mol Keggin unit is much higher than those
3
for the Cs2(SL) and K2(SL) samples, 1.19 and 1.18 mol NH /mol,
3
3
2
respectively. Alike, the molar heat of NH sorption of 54.4 kJ/mol for
acteristic for H PW
O
12 40
·6H O acid. Next, the partial removal of
3
3
2
the Cs2(SS) salt is considerably lower compared to those obtained
for the Cs2(SL) and K2(SL) samples, 97.1 and 97.8 kJ/mol, respec-
tively. However, these latter values closely correspond to the heat
water hydrating protons adjusts the lattice parameter of the HPW
to that of the Cs3 core. It has been shown previously [49] that
at the temperature range of 413–453 K the lattice parameter of
H PW12O40·3H O is equal to 1.176 nm. In turn, the lattice param-
+
+
of [N H7] adducts formation [45]. The [N H7] adducts are formed,
2
2
3
2
+
when ammonia molecules interact with already existing NH4
eter of the parent Cs3 salt of 1.187 nm, after 7 weeks of ageing,
slightly increased to 1.192 nm in the Cs2(SL) salt. The compari-
son of lattice parameter of trihydrated HPW (1.176 nm) and that
of the Cs2(SL) salt (1.192 nm) shows that the transformation of
cations via hydrogen bond. It may be suggested that, in both Cs2(SL)
and K2(SL) salts, ammonia interact with the protons, which are eas-
ily accessible because of their location in the surface layer. It seems
+
ꢀ
that the [N H7] adducts on the surface of the Cs2(SL) sample form
the ‘HPW + Cs3 mixture into the Cs2(SL) salt with the ‘core–shell’
2
+
a barrier, which effectively prevents, existing inside the bulk, Cs
or K cations from the interaction with the NH3 molecules. As the
structure by the self-organizing process is possible. This may also
indicate that the protons, in the surface layer of the HPW covering
the Cs3 core, are hydrated by, at least, one molecule of water. This
is consistent with the results obtained by the DRIFT and DSC/TG
methods.
+
result, the composition of the core (Cs3 or K3) does not influence the
process of ammonia sorption. It seems to be true since the average
molar heat of ammonia sorption on the Cs3 and K3 salt are almost
the same, 66.0 and 66.9 kJ/mol NH , respectively. On the other hand,
The adjustment process, which undergoes during the formation
of the Cs2(SL) secondary structure should be easier than that in the
K2(SL) salt. In the latter case, the difference between lattice param-
eters of the K3 salt (1.159 nm) [4] and the HPW acid (1.217 nm)
is larger than that between those of the Cs3 (1.187 nm) and the
HPW, but nevertheless the ‘core–shell’ secondary structure of the
3
the similar molar heats of the NH sorption obtained for the Cs2(SL)
3
and K2(SL) samples suggest, by analogy, the ‘core–shell’ structure
of Cs2(SL) salt. In turn, lower molar heat obtained for the Cs2(SS)
+
+
salt can be an average, of very weak Cs –NH3 and strong H –NH3
interactions, resulting from much lower accessibility of protons for
ammonia molecules compared to that of cesium ions [24].
Thus, it can be concluded that the XRD, DRIFTS, BET, DSC/TG and
ammonia sorption results consistently show that the our method
K HPW12O40 salt is formed, irrespective of applied method of the
2
preparation. Thus, it can be suggested that the our method leads to
the formation of the ‘core–shell’ secondary structure for the Cs2(SL)
and K2(SL) salts by a self-organizing process. This conclusion can be
expanded to the mixture of other heteropolyacids and their neutral
salts. In fact, in our patent [50] it was demonstrated that the mixing
of the H PMo12O40·29H O acid with the Ag PMo12O40 salt in the
of preparation effectively leads to the Cs HPW12O40 salt exhibiting
2
the ‘core–shell’ secondary structure. The formation of the Cs2(SL)
sample in the solid state under the influence of water present in air
atmosphere can be expressed by the equation:
3
2
3
same conditions caused the formation of the Ag HPMo12O40 salt
2
H PW12O40 (the shell)
Cs PW12O40 (the core)
3
3
H PW12O40 + 2Cs PW12O40 → 2
= Cs2(SL)
with a ‘core–shell’ secondary structure.
3
3
In order to confirm the similarity of the secondary structures of
the Cs2(SL) and K2(SL) salts, the site occupancy factor (SOF) using
the Rietveld refinement was calculated for the Cs2(SS), Cs2(SL) and
K2(SL) samples. It turned out that the SOFCs in the Cs2(SS) and
Cs2(SL) samples was significantly different, 0.71 and 0.93, respec-
tively. The obtained values are close to the theoretical values, of 0.67
for the ‘solid solution’ and 1.0 for the ‘core–shell’ structure, respec-
tively. In turn, the SOFK for the K2(SL) salt with the ‘core–shell’
structure was calculated to be 0.91 near to that for the Cs2(SL) sam-
ple. These results also clearly indicate the ‘core–shell’ structure of
the Cs2(SL) sample.
The mechanism of the self-organizing formation of the Cs2(SL)
salt can be represented in three steps:
ꢀ
1
2
3
. The hydration of HPW present as a component in the ‘HPW + Cs3
mixture, to form H PW12O40·29H O.
3
2
. The covering of the Cs3 core by the fully hydrated heteropoly-
acid.
. The adjustment of lattice parameter of the HPW to that of the
Cs3 core by the loss of water molecules hydrating protons and
simultaneously, slightly adjustment of lattice parameter of the
Cs3 core to that of the surface layer of HPW. It seems that both
processes undergo during the drying of the sample.
To confirm the influence of heteropolyacid on the morphology of
obtained Cs2(SL) sample the SEM method was applied. Fig. 7 shows
ꢀ
the morphology of the Cs3 salt before its use in the ‘HPW + Cs3 mix-
The first and the second steps are possible because the humid-
ture and the morphology of the Cs2(SL) sample, which was obtained
after 7 weeks in contact with air of 20% relative humidity. As it is
seen, under the influence of the HPW in humid atmosphere, the Cs3
aggregates of ca. 0.5 m fragmented to primary particles of diam-
eter lower than 50 nm. It can be expected that this fragmentation
process proceeds because of covering the particles by the HPW. If
the HPW would be supported on the surface of the Cs3 core in the
bulk form, that could not be the reason of complete fragmentation
of the aggregates of initial Cs3 salt.
ity influences the water content in the heteropolyacids, leading to
the formation a ‘pseudoliquid phase’ [46]. It caused that the solid
heteropolyacids behaves as a highly concentrated solution [46,47],
which can cover the Cs3 core. It can be underlined that both steps
strongly depend on the level of humidity of air atmosphere. This
can be a reason why the self-organizing process proceeds much
faster in air atmosphere with higher relative humidity. Thus, the
present data demonstrate that apart from the hydration level in
the HPW [23] also the presence of water in the surrounding atmo-
sphere of the ‘Cs3 + HPW’ mixture plays an important role because
it remarkably facilitates the mobility of the HPW.
A possibility of the formation of the ‘core–shell’ secondary struc-
ture was shown in our previous paper concerning the K2(SL) salt
[4]. The K3 core was covered by approximately one monolayer of
the HPW, assuming equal exposition of (1 0 0) and (1 1 0) planes.
The third step can be explained on the basis of lattice parame-
ters of both components, the HPW acid and Cs3 salt. The lattice
2
−1
The surface area of the K3 core was 114.3 m g , whilst the Cs3
ꢀ
2
−1
parameter of the Cs PW12O40 salt is 1.187 nm whereas that of
salt used to the ‘HPW + Cs3 mixture had 113.2 m g (Table 1).
Thus, it can be suggested that similar calculation is also valid for
the Cs2(SL) salt.
3
the H PW12O40·29H O is equal to 2.327 nm [48] and that of the
3
2
H PW12O40·6H O is determined to be 1.217 nm [4]. Thus, it seems
3
2
that the adjustment of the lattice parameter of the HPW to that of
the Cs3 core could proceed by the removal of crystallization water
With the aim to check out the homogeneity of the surface com-
position of the Cs2(SL) and Cs3 samples the EDS measurements