Properties of NiO-ThO2 mixed oxides from hydroxides and their hydrogen reducibility behaviour

Article abstract of DOI:10.1023/A:1013190701202

Some physico-chemical properties of NiO-ThO₂ mixed oxides of various compositions have been investigated. The presence of strongly bound constitutional water in the hydrogel of reactive forms of thorium dioxide, determined by their origin (thermal decomposition of mixed hydrogels) caused the different reduction behaviour as compared with other mixed oxide systems containing the only, thermodynamically less stabile reducible component. The significant effects of the thermal treatment in oxygen atmosphere, pre-irradiation by the gamma rays or accelerated electrons under various conditions (in air or in water suspension) as well as of surface chemical activation with a platinum complex on the reactivity of mixed oxides or reoxidized samples during their hydrogen reduction have been also proved.

Full text of DOI:10.1023/A:1013190701202

Journal of Thermal Analysis and Calorimetry, Vol. 66 (2001) 449–461
PROPERTIES OF NiO–ThO2 MIXED OXIDES FROM
HYDROXIDES AND THEIR HYDROGEN
REDUCIBILITY BEHAVIOUR
M. Pospíšil, R. Hošek and R. Silber
Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University,
115 19 Prague 1, Czech Republic
(Received December 10, 2000; in revised form May 2, 2001)
Abstract
Some physico-chemical properties of NiO–ThO2 mixed oxides of various compositions have been
investigated. The presence of strongly bound constitutional water in the hydrogel of reactive forms
of thorium dioxide, determined by their origin (thermal decomposition of mixed hydroxides) caused
the different reduction behaviour as compared with other mixed oxide systems containing the only,
thermodynamically less stabile reducible component. The significant effects of the thermal treat-
ment in oxygen atmosphere, pre-irradiation by the gamma rays or accelerated electrons under vari-
ous conditions (in air or in water suspension) as well as of surface chemical activation with a plati-
num complex on the reactivity of mixed oxides or reoxidized samples during their hydrogen
reduction have been also proved.
Keywords: hydrogen reduction, NiO–ThO2 mixed oxides, physico-chemical properties
Introduction
The hydrogen reduction of metallic oxides and of multicomponent oxide systems rep-
resents not only the fundamental process of powder metallurgy but also appears to be
one of the most important method for the catalysts preparation or their activation. De-
pending on the genesis, preparation conditions and composition, the character and
degree of mutual interaction of the individual components are significantly varied
which affects the reactivity of the mixed oxide systems during their reduction by the
gas. Different physico-chemical properties as well as differences in reduction kinet-
ics were observed with various two-component mixed oxides prepared either by ther-
mal decomposition of mixed crystalline salts of both metals or by calcination of co-
precipitated insoluble precursors [1]. In the latter case relatively scarce attention until
now has been paid to the mixed systems of hydroxide origin, where the presence of
constitutionally bound water in various concentrations may significantly affect their
reduction behaviour [2]. Therefore, the aim of the present contribution was to study
the properties and reducibility, tested by reduction with hydrogen, of the two-compo-
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© 2001 Akadémiai Kiadó, Budapest
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
nent mixed system, but contrarily to the work [2] containing the only reducible com-
ponent. Moreover, the influence of various factors modifying the reactivity of mixed
oxides in dependence of their composition was also investigated. The NiO–ThO2
mixed system was chosen for this study.
Experimental
The series of mixed oxides differing in their composition in the range 0–100 mass%
of each component were prepared by thermal decomposition of coprecipitated hy-
droxides for 4 h at 450°C in air (Table 1). Because of the low content of ThO2 in the
pure thorium dioxide (68.8 mass%) obtained under these conditions, the precursor of
this sample was repeatedly calcined at 950°C for the same time. The aqueous solu-
tions of thorium sulphate and nickel nitrate were mixed together in required ratios
and then coprecipitated during intensive agitation with a 10% stoichiometric excess
of a solution of NaOH. The concentration of all solutions was 1 mol L–1. Carefully
washed products and dried at 125°C were calcined and pulverized. The solution of
thorium sulphate was prepared from the starting thorium dioxide AR grade (Merck)
by acid smelting with KHSO4 and leaching from the melt.
Table 1 Notation, composition, specific surface areas (S) and size of coherent regions (L) of
NiO–ThO2 mixed oxides
Mass/%
ThO2
0.0
L/nm
Sample
S/m2 g–1
NiO
91.1
90.0
84.9
77.8
64.4
57.2
52.5
47.5
46.1
45.2
34.7
23.4
12.5
9.1
NiO+ThO2
91.1
95.2
91.7
92.6
90.4
90.9
91.2
95.4
94.2
94.3
90.7
94.2
92.4
92.6
68.8
93.8
NiO
47.5
33.8
25.3
26.8
25.5
24.9
24.8
31.1
29.5
25.7
ThO2
1
2
17.6
40.0
61.6
61.1
94.8
36.2
46.7
65.2
26.8
61.0
59.0
36.7
33.7
15.1
12.8
0.2
5.2
3
6.8
4
14.8
26.0
33.7
38.7
47.9
48.1
49.1
56.0
70.8
79.9
83.5
68.8
93.8
5
6
5.7
5.8
6.4
6.4
5.9
6.2
6.9
6.3
6.8
6.6
204
7
8
9
10
11
12
13
14
15
15*
0.0
0.0
*after recalcination at 950°C
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
451
The conditions for thermal decomposition were previously established by ther-
mal analysis of selected samples of precursors, performed on the Netzsch STA 490 –
thermoanalyser within the range 20–800°C at a heating rate of 10 deg min–1. The con-
tent of both metals in the mixed oxides was determined by atomic absorption spec-
troscopy and complexometrically with ethylenediaminetetraacetic acid. The micro-
structure of the samples was investigated by X-ray diffraction using a TUR M62 gen-
erator with a goniometer. Nickel-filtered CuKα radiation was used. Specific surface
areas were measured by low temperature selective adsorption of nitrogen from a
N2/H2 mixture and the morphology of the samples was studied using a JSM-840(A)-
Jeol scanning electron microscope. The surface oxidative abilities (content of iono-
genic chemisorbed oxygen O2–, normalized to unit surface of area) were determined
by iodometric titration.
Part of the samples prepared was, prior to use, irradiated at laboratory tempera-
ture with 60Co gamma rays in air or in suspension with distilled water using a dose of
1 MGy. In the latter case the samples were filtered after irradiation, dried in air at
110°C and immediately measured. Next portion of the mixed oxides was analogously
(in air at doses of 1 and 3 MGy and in water suspension at 1 MGy) irradiated by accel-
erated electrons with an average energy of 4 MeV from a high frequency linear accel-
erator. Selected samples of different composition were further heat treated for two
hours at 500 or 600°C in oxygen and argon atmospheres. Finally, some samples were
impregnated with 2% aqueous solution of H2PtCl6. After three-day storage of suspen-
sion the liquid phase was removed by drying at 110°C up to a constant mass. The
standard mass of these modified samples was either directly reduced with hydrogen
or the kinetics of the thermal decomposition of activator (H2PtCl6) in air and follow-
ing hydrogen reduction at the same temperature were measured. Moreover, the effect
of activation of the samples on the reduction kinetics of reoxidized mixed oxides was
also studied. In these experiments the activated samples after primary reduction were
oxidized in an oxygen flow up to a constant degree and then reduced again at the
same temperature. All above mentioned reactions proceeded at 320°C.
The reduction kinetics was partly studied by isothermal thermogravimetry in the
temperature range of 290–380°C and partly in the non-isothermal regime (tempera-
ture programmed reduction) at a heating rate of 10 deg min–1 up to 450°C. All experi-
ments were performed at a hydrogen flow of 60 mL min–1, using a 50 mg mass of the
sample. Under these conditions the kinetics of reduction at an arbitrary composition
of samples were found to be independent of the flow rate of the reducing gas.
Results and discussion
Physico-chemical properties of NiO–ThO2 mixed oxides
It follows from the results of chemical (Table 1) and X-ray analysis that the applied
conditions in the decomposition of the precursors yield a mixture of mixed oxides
NiO–ThO2 with a cubic structure as the major components of the system under study.
A significant deficit in the chemical analysis which was found in the whole range of
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452
POSPÍŠIL et al.: NiO–ThO2 OXIDES
composition may be due to the presence of non-metallic impurities, excess of oxygen,
which seems to be typical of the grey or black non-stoichiometric NiO oxide regard-
less of its origin [3]. The major part of this deficit appears to be connected with the
presence of water determined both by the origin of the samples under study and by
the conditions of their preparation. The temperature of decomposition of the starting
precursors plays the most important role. Analogously to the earlier studied system
based on the nickel oxide of hydroxide origin [2] the DTA curves of the precursors
with an excess of nickel show two endothermic effects (Fig. 1, curve 1). The first,
broad effect lies in the range 50–200°C and the second sharp one, the magnitude of
which decreases with increasing content of thorium, reaches a maximum at about
320°C. Similarly the thermogravimetric curve (Fig. 1, curve TG-1) gives evidence
that the decomposition proceeds in two consecutive steps. According to some pub-
lished data [4] at lower temperatures only the water adsorbed on the surface of parti-
cles is released (the first stage). Although the lattice of nickel hydroxide is disturbed
already under the temperature of 300°C [4, 5] the hydroxyl groups are fully splitted
off at higher temperatures lying in the range of 320–350°C (the second stage of de-
composition). This is in conformity with our results. As far as the character of the
DTA curves including the position of peaks, it is known that with the salts or com-
plexes of transition metals these parameters changes markedly depending on the
pre-history, age and conditions of preparation of the sample [6].
Fig. 1 DTA and TG curves of the precursors (mixed hydroxides) corresponding to
samples No. 3 (1) and 15 (2)
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
453
As can be seen from the thermogravimetric curve, the temperature used for the
preparation of the series (450°C) appears to be insufficient for the complete decom-
position of the precursor even though the lattice of nickel hydroxide is fully trans-
formed. The further increasing mass loss of the sample, observed also by other au-
thors [4], can be connected with the releasing of water strongly bound already in the
nickel oxide which is formed in situ during the transformation of the original hydrox-
ide lattice. According to some IR spectroscopic investigations it follows that the men-
tioned mass loss may be caused by the releasing of the solvated water [7] which is
bound by hydrogen bonds but the decomposition of various hydrated compounds of
nickel oxide [8] cannot be excluded, either. Only at the highest temperatures (about
800–900°C) the stoichiometric green and yellow nickel oxide is formed [3]. As can
be seen from Fig. 2 the hydroxide origin of final nickel oxide manifests itself by quite
different (needle shaped) morphology as compared with the same oxide prepared
from the basic carbonate or nitrate and decomposed at the same temperature. In the
latter case the grains are formed by spherical agglomerates being about 200–400 µm
in size without distinct porous texture [9, 10].
Fig. 2 Electron micrograph showing the needle-shaped morphology of the original
sample No. 1, magnification 10 000×
The continuous releasing of water connected with the mass loss during the ther-
mal treatment (Fig. 1, curve 2) gives evidence that the precursor of pure thorium di-
oxide is rather the hydrogel of reactive form of ThO2 (hydrated oxide ThO2·2H2O)
than Th(OH)4. In accordance with a high thermal stability of different hydrated ox-
ides or their hydrogels [11] the standard conditions of decomposition used for the
preparation of mixed oxides series appear to be sufficient for transformation of amor-
phous hydrogel to the cubic structure of ThO2, however with a high content of resid-
ual water (Table 1, sample 15). Table 1 also shows that the size of microcrystallites of
thorium dioxide (coherent regions) which lie in the interval 5.7–6.9 nm is consider-
ably smaller than that of nickel oxide and is virtually independent of the system com-
position. The pronounced increase of this parameter for sample 15 (Table 1) recalci-
nated at 950°C, is due to the sintering processes.
The mutual influence of both components in the system under study results in a
non-monotonous dependence of the specific surface areas on the composition
(Table 1). Relatively high absolute values of this quantity are in accordance with the
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454
POSPÍŠIL et al.: NiO–ThO2 OXIDES
genesis of these mixed oxides. In contrast to the mixed systems prepared by the ther-
mal decomposition of various crystalline salts (nitrates, chlorides etc.) where the melt
is created during their calcination, the same samples obtained by the decomposition
of amorphous precipitates (carbonates, hydroxides) possess, as a rule, higher surface
area values [12]. No significant difference in this parameter was found between the
original samples and those pre-irradiated under the above-mentioned conditions. This
is in accordance with published data [13]. In contrast with radioactive substances the
measurable changes of parameters such as microstructure, morphology and surface
areas of mixed oxides, can be induced as a result of pre-irradiation only by high en-
ergy corpuscular radiation at the higher doses then those applied in this work. Heat
treatment at 500 or 600°C in various atmospheres used led to the decrease in the spe-
cific surface areas by about 30%. In the region of the maximum value of surface area
and with the samples containing pronounced excess of ThO2 the system appears to be
the most sensitive to the treatment. It results in the decrease of this quantity up to 50%
of its original value for sample 5 and 70% for sample 13 (Table 1). In addition to the
grain sintering which was proved by microscopic investigation, releasing additional
portions of water bound in the system at temperatures lying over the standard decom-
position temperature may also affect the magnitude of specific surface area values.
As can be seen from Fig. 3 the small admixture of ThO2 leads to the decrease of
surface oxidative ability which gives evidence for the decrease of concentration of
Ni3+ ions in the nickel oxide. The non-monotonous dependence of this parameter on
the composition shows two sharp reproducible maxima, the first one in the region of
the middle of series and the second one with the sample of thorium dioxide contain-
ing a small admixture of the second component. Excluding the samples with the high-
est excess of nickel oxide (samples 1 and 2) the absolute values of this quantity as
well as the character of mentioned dependence were found to be preserved in the
samples after their long (two years) storage in air at room temperature. In contrast to
Fig. 3 Content of the ionogenic oxygen normalized to unit surface area Y vs. composi-
tion. 1 – freshly prepared samples; 2 – samples after two years storage
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
455
the stable mixed oxides the decrease of the oxidative ability of NiO oxide during its
storage (by about 45 and 33% with sample 2) may be connected with recrystallization
processes and establishment of a new equilibrium between surface and atmospheric
oxygen. This is supported by the fact that the heat treatment only in the oxygen atmo-
sphere caused the increase of the oxidative ability of pure NiO oxide by 50%. On the
contrary, no changes of this quantity were found with all samples including pure NiO
oxide treated in Ar atmosphere and with mixed oxides treated in oxygen.
Table 2 Changes in the surface oxidative ability (∆Y) of mixed oxides due to gamma irradiation
(dose 1 MGy) and accelerated electrons (doses 1 and 3 MGy) under different conditions
∆Y/%
e–, 1 MGy, ws
Sample
e–, 1 MGy, a e–, 3 MGy, a
γ, a
61.9
123.5
15.8
51.6
–3.8
–10.5
–10.8
15.3
20.4
2.8
γ, ws
146.0
100.0
25.9
1
14.9
43.6
9.0
3.7
87.5
38.7
2
20.5
3
15.0
17.8
4
6.9
39.1
–16.9
–11.2
–32.6
– 42.4
–31.6
–13.7
–37.1
–22.3
– 40.2
–15.8
– 46.4
0.0
– 47.5
–55.6
–52.6
–55.2
–14.1
–19.9
– 41.6
–36.3
–35.8
–37.8
– 46.2
0.0
5
–2.4
–15.1
–27.6
–23.0
–33.4
–20.9
–5.6
–7.8
–7.1
–13.1
0.0
–34.8
–26.1
–20.5
–20.1
–33.7
–15.6
–12.7
– 45.1
–7.1
6
7
8
9
10
11
–15.6
–18.5
–9.8
8.6
12
13
14
– 46.8
0.0
15
0.0
airradiated in air
wsirradiated in water suspension
The influence of pre-irradiation on the surface oxidative abilities of all samples,
expressed by the quantity ∆Y is summarized in Table 2. This quantity is defined by
the relation ∆Y (%)=100 (Y *–Y/Y), where Y * and Y represent the normalized content
of ionogenic oxygen of the irradiated and original samples, respectively. From the
given data the highest sensitivity towards irradiation only with the samples contain-
ing an extremely high amount of NiO oxide is evident, too. In this region of the com-
position the positive effect dominates, whereas for the mixed oxides containing a suf-
ficient amount of thorium dioxide the negative effect is typical. The increase of sur-
face oxidative ability is due to the radiation-induced oxidation of Ni2+ ions (more pre-
cisely Ni2+–Ni2+ centres) which was observed already in the earlier studied systems
containing nickel oxide [1, 14]. From this point of view no differences among the var-
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
ious morphological forms of nickel oxide were found. In water suspension probably
the radiolytic oxidation by strongly oxidizing OH radiacals takes place. Depending
on the composition the positive effect of radiation can be counteracted by the compet-
ing negative influence i. e. by reduction action of solvated electrons in the mixed
samples containing sufficient content of thorium dioxide. The cause of this may be
low concentration of centres which can be oxidized and the increase of the centres
(ions in higher valency states) which may be reduced. With regard to the experimen-
tal errors (±15%) the resulting zero effect or inexpressivedecrease of the surface oxi-
dative abilities are characteristic for these samples. The different values observed
with the samples irradiated by gamma rays as compared with those irradiated by ac-
celerated electrons at the same dose of radiation can be connected with the different
manner of energy deposition in the surface layers and in the bulk of the solid. More-
over the migration of activated oxygen from bulk to the surface may proceed in the
case of gamma irradiation.
Reactivity of NiO–ThO2 mixed oxides tested by hydrogen reduction
From the thermodynamic stability of both oxides it follows that under given condi-
tions the nickel oxide is the only reducible component in the studied system. How-
ever the experimental degrees of reduction achieved at 380°C exceeded the theoreti-
cal values calculated assuming the total reduction of nickel oxide in the region of its
prevailing content (Fig. 4). From the given dependences can be deduced that during
the reduction of NiO oxide and crystallochemical transformation of its lattice the re-
leasing of residual water present in the samples takes place, which increases overall
mass losses. At lower temperatures of reduction or with the samples containing an ex-
cess of second component its strong influence appears to be evident. The reactive in-
terface is blocked by non-reducible finely dispersed thorium dioxide so that the re-
Fig. 4 Dependence of the percentage reduction at different temperatures over the theo-
retical value corresponding to the total reduction of NiO oxide on the composi-
tion. 1 – 380°C; 2 – 320°C; 3 – 290°C
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
457
Fig. 5 Plot of maximum reduction rate Vmax. vs. composition at different temperatures
and reduction in non-isothermal regime. 1 – 380°C; 2 – 320°C; 3 – non-isother-
mal reduction; 4 – 290°C
duction of NiO oxide proceeds only to lower degrees. As can be seen from Fig. 5 the
thorium dioxide significantly affects not only the degree but also the kinetics of re-
duction of nickel oxide. This influence appears to be different in dependence on the
composition and temperature. Only at lower temperatures of reduction (curve 4) or
during the reduction proceeding in non-isothermal regime (curve 3) the maximum re-
duction rate monotonously decreases with increasing content of thorium dioxide.
This course is typical of homogeneous two-component systems containing a single
reducible component and has been observed with other various mixed oxides [15].
On the contrary, with heterophase-type mixed oxides the mutual interaction of the
two components may be still affected by steric and other factors. These determine not
only the retarding influence of the unreducible component but also positive (acceler-
ating) effects in dependence on temperature, actual degree of reduction and composi-
tion [9, 14]. With the samples containing an excess of NiO oxide its reduction was
found to be accelerated by the admixture of thorium dioxide at higher temperatures of
reduction (curves 1 and 2). As it was mentioned above, only under those conditions
the reduction of majority nickel oxide evokes the releasing of water strongly bound in
the stabile thorium dioxide hydrates. It may be the cause of the accelerating effect ob-
served. This is supported by the fact that the apparent activation energies of reduction
in this region of composition reached values around 55 kJ mol–1. This value lies be-
low that given in the literature for reduction of nickel oxide [10] (and generally ma-
jority of chemical reactions) and gives evidence for the participation of transport pro-
cesses. Moreover during the rapid reduction of nickel oxide which takes place at
higher temperatures the minority thorium dioxide probably hinders the formation of a
coherent metallic layer on the reduction interface. This effect leads also to the accel-
eration of reaction.
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
Fig. 6 Temperature-programmed reduction profiles of samples No. 2 (1) and 8 (2)
As can be seen from the temperature-programmed reduction profiles, i.e. from
the dependence of instantaneous reduction rate on the temperature (Fig. 6), various
portions of weakly bound (adsorbed) water appear to be released already at lower
temperatures in the whole range of composition. The proper reduction follows as a
consecutive process, which is connected with releasing both the reduction product-
water vapour and the water strongly bound in stabile hydrates of thorium dioxide.
The desorption of gaseous impurities and water adsorbed on the surface proceed also
during the thermal pre-treatment of the samples under above given conditions. The
activation of present or the creation of new reduction centres result in the acceleration
of the reduction, predominantly with the treated samples containing an excess of tho-
Fig. 7 Dependence of the quantity ∆Vmax. on composition of the samples irradiated by
gamma rays, 1 MGy in water suspension (1), accelerated electrons, 3 MGy in air
(2) and accelerated electrons, 1 MGy in water suspension (3)
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
459
rium dioxide. This effect can be expressed by the quantity ∆Vmax (%) defined analo-
gously to the quantity ∆Y, where Y and Y* are replaced by the maximum reduction rate
of the original sample (Vmax) and the same sample after treatment (Vm*ax ). This quantity
being in all cases higher for oxygen treatment changes in dependence on composition
from 30 to 95%.
Figure 7 shows the effect of pre-irradiation under various conditions on the re-
duction kinetics expressed by means of the quantity ∆Vmax. It follows from this figure
that the system under study appears to be the most sensitive to irradiation only in the
region of an excess of nickel oxide where the reduction of non-irradiated samples
proceeds with the maximum rate. The decrease in reduction rate of pure irradiated
nickel oxide, which was repeatedly observed earlier [16, 17], may be connected with
increasing over-stoichiometry of this oxide formed predominantly by strongly bound
ionogenic oxygen retarding the donor chemisorption of reducing gas. The presence of
water introduced to the system by the small admixtures of thorium dioxide results in
further drop of reduction rate in the irradiated samples. The fact that the negative ra-
diation effect appears to be in all cases markedly higher with the samples irradiated in
water suspension gives evidence for the retarding influence of the oxidizing products
created by the radiolysis of water. Similar behaviour was also observed with other
two-component mixed systems containing nickel oxide irradiated either in water or in
air [17]. Only small dependence on the dose applied was found with the samples irra-
diated by accelerated electrons in air and generally a lowering of radiation effects
with increasing temperature was observed. This indicates that with higher doses of
Fig. 8 Reduction with hydrogen of the original samples 6 (3) and 8 (4) followed by iso-
thermal thermogravimetry at 320°C. Reduction of sample 8 after impregnation
with H2PtCl6 and drying (5), its oxidation in the flow of oxygen (6) followed by
the secondary reduction with hydrogen (7). Thermal pre-decomposition of the
activator in air (sample 6, curve 2) and subsequent reduction of the activated
sample 6 with hydrogen (1)
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460
POSPÍŠIL et al.: NiO–ThO2 OXIDES
radiation and at higher temperatures of reduction the recombination and recovering
processes play increasingly their part.
Figure 8 demonstrates the influence of platinum on the reactivity of two samples
of different composition during their primary or secondary reduction by hydrogen.
Curves 3 and 4 represent the time course of reduction of the original samples 6 and 8,
curve 5 belongs to the reductive decomposition of the activator (H2PtCl6) and reduc-
tion of the activated sample 8. Both processes occurred in dry hydrogen with a high
rate. However the integral kinetic curve did not make possible to distinguish their
consecutivity. Curves 2 and 1 represent the preliminary thermal decomposition of the
activator in air (2) and the subsequent hydrogen reduction of the activated sample 6
(1). Both these reactions proceeded with an extremely high rate without measurable
induction period. Curve 6 belongs to the oxidation of activated and primarily reduced
sample 8 in the flow of oxygen. The course of subsequent secondary reduction of this
sample by hydrogen is demonstrated by curve 7. All above described processes took
place at 320°C. It is evident from these dependences that the activation leads to a pro-
nounced increase in the rate of reduction even in the case of reoxidized samples (sec-
ondary reduction) or the samples possessing a high content of non-reducible compo-
nent. From this it may be deduced that the non-reducible oxide can probably act as a
transmitter of active species (atomic hydrogen). Similar effects were observed with
other two component mixed systems activated by platinum, containing only one re-
ducible oxide [15].
Conclusions
The thermal decomposition of mixed nickel and thorium hydroxides leads to the forma-
tion of nickel oxide with a needle-shaped morphology in close epitaxial contact with tho-
rium dioxide containing a relatively high amount of residual strongly bound water. The
presence of thorium dioxide as well as the pre-irradiation of the samples by gamma rays
or accelerated electrons in air or in a water suspension significantly affects the surface ox-
idative ability of nickel oxide. The sign of the resultant effect depends on the composition
and appears to be a superposition of two competing processes.
Thermodynamically less stable nickel oxide is the only reducible component in
the system under study. The degree and the rate of its hydrogen reduction are mark-
edly influenced by the second component depending on the composition and temper-
ature of reduction. Only at lower temperatures the dependences of the reduction rate
on composition are similar those for the two-component mixed oxide systems con-
taining one stable non-reducible component. These differences are caused by the
presence of water released during the reduction at higher temperatures.
The action of the products of water radiolysis is probably the major cause for the
significant negative effect of pre-irradiation on the reduction kinetics for samples
with an excess of nickel oxide. The reduction kinetics may be positively influenced
(accelerated) by the admixture of platinum activator regardless of the composition.
Therefore the non-reducible component acts as a transmitter of active species (atomic
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POSPÍŠIL et al.: NiO–ThO2 OXIDES
461
hydrogen). The positive effect remains unchanged also for the secondary reduction of
reoxidized samples.
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