Effect of genesis on the properties and hydrogen reducibility of NiO-ZnO mixed oxides

Article abstract of DOI:10.1023/B:JTAN.0000017326.19595.0f

The physico-chemical properties and reactivity tested by hydrogen reduction have been studied for two series of NiO-ZnO mixed oxides of various composition. The solid nickel oxide or zinc oxide in interaction with the solution of nitrate of the second com

Full text of DOI:10.1023/B:JTAN.0000017326.19595.0f

Journal of Thermal Analysis and Calorimetry, Vol. 75 (2004) 35–50
EFFECT OF GENESIS ON THE PROPERTIES AND
HYDROGEN REDUCIBILITY OF NiO–ZnO MIXED
OXIDES
*
M. Pospíšil , V. ‡uba and D. Poláková
Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, 115 19
Prague 1, Czech Republic
(
Received June 17, 2003; in revised form September 19, 2003)
Abstract
The physico-chemical properties and reactivity tested by hydrogen reduction have been studied for two
series of NiO–ZnO mixed oxides of various composition. The solid nickel oxide or zinc oxide in interac-
tion with the solution of nitrate of the second component were used as the precursors in each series. The
differences in some physico-chemical parameters of the samples in both series were correlated with their
reduction behaviour, followed both in iso- and non-isothermal regime. Moreower, the influence of vari-
ous factors modifying the reactivity of mixed oxides was also investigated and the results were compared
with those obtained from earlier studied analogous systems of quite different origin.
Keywords: hydrogen reduction, NiO–ZnO mixed oxides, physico-chemical properties
Introduction
Within the framework of systematic research of the effects of genesis and various treat-
ments on the reactivity of two-component oxide catalysts tested by their reduction with
hydrogen the NiO–ZnO mixed system has been investigated. Different physico-
chemical properties as well as differences in reduction kinetics were observed with
mentioned mixed oxides of various composition prepared by thermal decomposition of
crystalline nitrates of both metals [1], by calcination of coprecipitated basic carbonates
65
without [2] or with incorporated radionuclide Zn [3] or by decomposition of precur-
sors in different combinations. In the last case the properties and reduction behaviour of
two series of NiO–ZnO mixed oxides were compared [4]. The precursor of the first se-
ries (denoted as series N) arised by evaporation of suspension of the basic nickel car-
bonate with Zn(NO ) solution to dryness and vice versa the precipitates of basic zinc
3
2
carbonate were mixed with nickel nitrate solution during the preparation of the second
series (series Z). In all above mentioned cases the high degree of mutual interaction of
both components in the final mixed oxide systems was proved.
*
Author for correspondence: E-mail: Milan.Pospisil@fjfi.cvut.cz
1
388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
©
2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
It was found that in the series N containing basic nickel carbonate as a compo-
nent of the precursor, its decomposition leads to the formation of higher nickel oxides
with variable composition in the range from NiO1.33 to NiO1.50. Their presence is con-
nected with variable content of the different bound residual water [5]. The influence
of basic amorphous carbonate in mixed precursors manifests itself by higher values
of surface areas of final oxides i. e. with pure nickel oxide of series N including
mixed samples with its prevailing content and with pure zinc oxide of series Z includ-
ing mixed samples containing an excess of ZnO [4]. Analogously the content of
chemisorbed oxygen was found to be substantially higher with the mixed samples of
series N containing an excess of nickel oxide including pure NiO as compared with
the same samples of series Z. This finding is in full accordance with p-semiconduct-
3
+
ing character of NiO. The presence of Ni ions is compensated by ionogenic forms of
+
super-stoichiometric oxygen. With zinc oxide the defects formed by interstitial Zn
ions and by oxygen vacancies determine its n-semiconductivity. Therefore the con-
tent of chemisorbed oxygen with the samples of both series containing an excess of
ZnO including pure zinc oxide was found to be zero [4].
The samples with nickel nitrate precursor (series Z) contain practically stoichio-
metric nickel oxide. Therefore the sum of mass% of both oxides of stoichiometry NiO
and ZnO achieves nearly 100% over the whole range of composition. Contrary to se-
ries N with these samples the changes in the amount of chemisorbed oxygen induced
by irradiation appear to be negligible. For both series the formation of solid solution
during the decomposition of precursors appears to be characteristic but the dependen-
cies of NiO lattice parameter (aNiO) on composition are quite different. For the samples
of series N this quantity intensely increases up to 30% content of ZnO. With its further
increasing content the rise of aNiO is very slow. With series Z aNiO achieves at 30% of
ZnO a maximum and then decreases. The increase of the lattice parameter of metallic
nickel (a ) with increasing content of ZnO and the formation of a new cubic structure
Ni
of Ni–Zn alloy as a result of partial reduction of ZnO were observed with the reduced
samples of both series N and Z [4].
The samples of series N containing a high excess of nickel oxide assign the high-
est reactivity whereas the reduction rate of the samples of series Z is substantially
lower and monotonously decreases with increasing content of ZnO. The degree of
partial reduction of zinc oxide achieves its maximum at about 20% of ZnO for se-
ries N, for series Z this maximum lies at 80% of ZnO. In the present paper the analo-
gous systems precursors of which consist of the pure oxide of one component in inter-
action with the salt of the second component have been investigated. Therefore one
series of NiO–ZnO mixed oxides of various composition was prepared from the pre-
cursors containing nickel oxide of carbonate origin and solution of zinc nitrate (de-
noted series A), whereas the precursors of the other series consist of zinc oxide (car-
bonate origin) and solution of nickel nitrate (series B). In addition to the physico-
chemical properties the influence of various factors modifying the reactivity of both
systems was also studied.
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
37
Experimental
Two series of mixed oxides with various proportions of the components in the
range 0–100% were prepared by thermal decomposition of the precursors for 4 h
–
1
at 450°C in air. The starting solutions of AR grade nickel or zinc nitrate (1.5 mol L )
were precipitated during intensive agitation with a 15% stoichiometric excess of
aqueous Na CO solution. A carefully washed and dried precipitates of basic nickel
2
3
(
zinc) carbonates were calcinated for 4 h at 450°C and pulverized by grinding. For the
preparation of the precursors of series A the powdery nickel oxide was then mixed
with required volumes of the solution of zinc nitrate. These suspensions formed were
evaporated on the water bath to dryness at 120°C. Before the final decomposition the
products were homogenized by grinding in an agate mortar. An analogous procedure
was used for the preparation of the precursors of series B in which the powdery ZnO
oxide was mixed with the solution of nickel nitrate.
The content of both metals in the mixed oxides was determined chelato-
metrically in combination with atomic absorption spectroscopy. The microstructure
of the samples was investigated by X-ray diffraction using a TUR M62 generator
with goniometer. Nickel-filtered CuK radiation was used. Specific surface areas
α
were measured by low temperature selective adsorption of nitrogen from a N /H2
2
mixture and the morphology of the mixed oxides and the products of their reduction
was investigated using a JSM 840A (Jeol) scanning electron microscope. The content
of chemisorbed oxygen (its ionogenic forms expressed as the surface concentration
2
–
of O ions) was determined by iodometric titration.
Selected samples of different composition were heat-treated for 4 h at 600, 750
and 900°C in air and at 600°C for 2 h in oxygen or helium atmospheres. The samples
treated in this manner were, prior to use, cooled down to room temperature in the same
60
atmosphere. Part of the samples prepared were further irradiated with Co gamma rays
in air and by accelerated electrons of an average energy of 4 MeV from a high-
frequency linear accelerator using a dose of 3 MGy. The chemical modification of the
reactivity was performed by impregnation of various samples from both series with 2%
aqueous solution of H PtCl . After three-day storage of suspension the liquid phase was
2 6
removed by drying at 80°C up to a constant mass. The activated samples were either di-
rectly reduced with hydrogen or preliminary heat-treated in air and then reduced with
hydrogen at the same temperatures. The reduction kinetics were studied partly by iso-
thermal thermogravimetry in the range of 350–440°C and partly in the non-isothermal
–1
regime (temperature programmed reduction) at a heating rate of 10°C min up to
–1
4
40°C. The reduction took place in hydrogen flowing at 60 mL min using a 50 mg
mass of the sample.
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Results and discussion
Physico-chemical properties of mixed nickel and zinc oxides
Structure, morphology
From the results of chemical and X-ray analyses follows that both systems under
study consist of the separate phases of cubic nickel oxide and hexagonal zinc oxide.
Contrary to the earlier studied series Z some of first samples of both series A and B
contain non-stoichiometric nickel oxide. With further increasing content of ZnO the
sum of both oxides of stoichiometry NiO and ZnO approaches 100% (Table 1). In
contrast with the samples of pure nitrate or carbonate origin [1, 2] as well as with the
series N or Z [4] no changes in the lattice parameter of NiO with increasing content of
the second component were observed. The absence of the solid solution of both ox-
ides in the whole range of composition demonstrates the lower degree of the mutual
influence of components determined by the origin of both systems. Analogously the
size of NiO microcrystallites in the present studied series A and B appears to be inde-
pendent of composition (Table 1) and this parameter is by two orders of magnitude
higher than that with the series N or Z [4].
The interaction of both components in the systems under study results in a
non-additivity of the specific surface areas and non-monotonous dependence of this
parameter on the composition (Fig. 1), especially with the samples of series A. These
dependencies are in accordance with the results of morphological investigation of the
samples and correlate with the different origin of both series. During the preparation
only one precursor (corresponding nitrate) undergoes the decomposition which pro-
ceeds very rapid under temperature used. Therefore the last stage of this operation ap-
Fig. 1 Dependences of the specific surface areas (S) on composition. 1 – series A; 2 – series B
J. Therm. Anal. Cal., 75, 2004

POSP͞IL et al.: NiO–ZnO MIXED OXIDES
39
Table 1 Notation, composition and size of coherent regions (L) of NiO–ZnO mixed oxides of the
series A and B
Mass/%
ZnO
L/nm
Sample
NiO
93.95
94.93
91.35
90.63
86.66
86.13
82.60
81.08
77.53
75.67
69.83
63.78
60.45
60.24
51.14
48.03
40.23
36.77
31.80
33.18
21.62
20.01
14.17
11.85
10.53
9.59
NiO+ZnO
93.95
94.93
96.33
95.09
97.06
94.72
96.17
96.11
97.37
96.65
98.78
96.43
98.73
98.62
99.06
99.56
99.36
99.21
99.90
99.83
98.98
99.94
97.87
97.63
97.65
99.48
97.69
99.67
98.49
99.50
NiO
ZnO
1
A
0.00
350.7
165.3
295.8
150.8
427.2
189.6
450.4
190.8
340.5
120.4
1
B
0.00
2
A
4.98
2
B
4.46
3
A
10.40
8.59
3
B
4
A
13.57
15.03
19.84
20.98
28.95
32.65
38.28
38.38
47.92
51.53
59.13
62.44
68.10
66.65
77.36
79.93
83.70
85.78
87.12
89.89
92.14
96.49
98.49
99.50
4
B
5
A
5
B
6
A
6
B
130.5
120.2
140.9
146.4
7
A
7
B
8
A
8
B
9
A
9
B
1
0A
0B
1A
1B
2A
2B
3A
3B
4A
4B
5A
5B
1347.5
242.4
1285.9
198.5
1250.6
294.5
1087.4
205.4
900.5
320.8
831.5
194.3
1
1
1
1
1
1
1
1
5.55
1
3.18
1
0.00
1
0.00
pears to be only the heat treatment of final mixed oxides determining their morpho-
logical properties. The samples of series A are more resistant towards sintering pro-
cesses owing to the lower dispersity of their microcrystallites (Table 1) as compared
with the series B. Therefore the resulting grains are formed by smaller agglomerates
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
with rugged surface so that the overall surface areas appear to be higher than those
with series B, virtually in the whole range of composition. The abrupt decrease of this
parameter with the samples containing more than 20% of zinc oxide distinctly shows
the influence of increasing concentration of nitrate precursor. During its decomposi-
tion a melt is created so that the final oxides possess very low surface areas. As can be
seen from Table 2, the additional heat treatment when the temperatures exceeded the
calcination temperature (450°C) leads to the pronounced decrease of the surface ar-
eas with the samples of series A. On the contrary with the series B, where the major-
ity portions of microcrystallites and agglomerates were sintered already during the
preparation, the decrease of this parameter and the effect of sintering processes at the
additional heat treatment is substantially lower. With the earlier studied series N and
Z the degree of mutual interaction of both components is higher owing to the simulta-
neous decomposition of mixed salts (amorphous basic carbonate with crystalline ni-
trate). Hence the effect of nitrate, decreasing the final surface areas appears to be still
higher [4].
Table 2 Decrease of the specific surface areas with the samples of series A and B after their heat
treatment at different temperatures
Decrease of S/%
Series
A
Mass/% ZnO
6
00°C
750 and 900°C
1
2
5
8
0.4
8.9
9.1
3.7
80
90
86
75
50
70
60
50
50
80
70
50
8.6
50
2
3
6
0.9
8.4
2.4
44
B
47
50
Chemisorbed oxygen, effect of pre-irradiation
In the preparations of all series the different dependencies of the content of ionogenic
chemisorbed oxygen normalized to unit surface area of the samples on composition
were also compared. From the presence of a maximum of the freshly prepared sam-
ples in the region of an excess of nickel oxide with the series B (Fig. 2, curve 1) it can
be deduced, similarly to the series N or Z, that the dissociative chemisorption of mo-
2
+
2+
lecular oxygen takes place both on the dominant Ni – Ni centres and on the mixed
2
+
+
Ni –Zn centres. In comparison with the series N the second maximum in the middle
of series was not observed and the amount of oxygen further monotonously decreases
with increasing content of zinc oxide. From Fig. 3, curve 1, it can be seen that with
the freshly prepared samples of series A no additional mixed centres enhancing the
chemisorption of oxygen are created so that the content of surface oxygen monoto-
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
41
Fig. 2 Dependence of the content of superstoichiometric oxygen normalized to unit
surface area X (series B) on the composition. 1 – freshly prepared samples;
2
4
– samples after two years storage; 3 – samples after three-years storage;
– samples irradiated by accelerated electrons, 3 MGy; 5 – samples irradiated
by gamma rays, 3 MGy
Fig. 3 As in Fig. 2 but for the series A. 1 – original samples; 2 – samples after two
years storage; 3 – samples irradiated by gamma rays; 4 – samples irradiated by
accelerated eletrons
nously decreases with decreasing content of nickel oxide in the whole range of com-
position. This behaviour is in accordance with above mentioned semiconducting
character of both pure oxides. Curves 2, 3 (Fig. 2) and 2 (Fig. 3) also show that the
dynamic equilibrium between surface and atmospheric oxygen changes with time
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
during storage of the samples. These changes appear to be positive or negative, but
the character of the dependence remains unchanged. Analogously as it was found ear-
lier [4], that the higher content of super-stoichiometric oxygen in pure nickel oxide of
series A compared with the same sample of series B which may be probably deter-
mined predominantly by the different morphologic character of decomposed precur-
sors – basic nickel carbonate or crystalline nickel nitrate. On the contrary, the distinc-
tive influence of the temperature of precursor decomposition, mentioned in paper [6]
is out of question in our case due to the same values of this parameter.
The pre-irradiation both by gamma rays and accelerated electrons results in a pro-
nounced increase in the amount of chemisorbed oxygen with the samples containing an
excess of nickel oxide in both series (Fig. 2, curves 4 and 5, Fig. 3, curves 3 and 4). The
unchanged character of the dependence for the series B (Fig. 2) gives evidence, that in
this case and in this region of composition the increase in concentration of surface oxy-
gen can be attributed predominantly to the shift of equilibrium between various forms of
oxygen sorbed on the already existing centres in favour of the strongly bound ionogenic
form. In addition to this effect with the series A a new maximum of given dependence af-
ter irradiation (Fig. 3) proves that the subsequent chemisorption of oxygen from the at-
mosphere on the new centres created by ionization takes place. Figure 4 represents the
dependence of the quantity ∆X on composition for both series. This quantity is expressed
*
*
by the relation ∆X(%) =100(X –X)/X, where X and X denote the normalized content of
chemisorbed oxygen of the irradiated and original samples, respectively. Whereas no
considerable differences between the effect of gamma rays or accelerated electrons on
the samples of series A were found, the mixed oxides in the middle of series B appear to
Fig. 4 Plot of quantity ∆X(%), defined in the text, vs. composition for the samples of
both series. 1 – series A gamma irradiated; 2 – series A irradiated by accelerated
electrons; 3 – series B irradiated by accelerated electrons; 4 – series B gamma
irradiated. Doses 3 MGy
J. Therm. Anal. Cal., 75, 2004

POSP͞IL et al.: NiO–ZnO MIXED OXIDES
43
be much more sensitive to the irradiation with gamma rays (curve 4). Owing to the rela-
tively high content of zinc oxide in this region of composition, the enhanced adsorption
2+
+
3+
2+
2–
of atmospheric oxygen on mixed centres Ni –Zn +O →Ni –Zn +O induced by
2
gamma radiation can be assumed. In addition to that, this radiation may evoke, in contrast
with accelerated electrons the migration of activated oxygen from bulk to the surface of
particles [7].
The accelerated electrons induce in the central region of composition the opposite
3+
2+
effect i.e. the radiation reduction of ions in higher valency states (Ni , Zn ) which takes
place also with the samples of series A containing an excess of zinc oxide regardless of
the type of radiation. At the doses applied in this work such reduction may be caused by
hydrated electrons resulting in radiolysis of the trace amounts of moisture [8]. The same
effect was observed with the different mixed oxide systems irradiated directly in suspen-
sion with distilled water [9], or during the radiolytic reduction of metal ions in aqueous
systems [10] already at substantially lower doses. The above mentioned role of genesis in
connection with mixed centres for chemisorption of oxygen can be supported by the find-
ings, that the high temperature treatment in air leads to the increase of normalized content
of chemisorbed oxygen only with the mixed samples of series B containing an excess of
NiO oxide. Up to 35% of zinc oxide the increase of this quantity averages 20% (treatment
at 750°C) or 75% (treatment at 900°C). With further increasing ZnO content the amount
of oxygen monotonously decreases up to zero. With the mixed samples of series A under
the same conditions only the decrease of this quantity was observed regardless of the at-
mosphere used. The high temperature treatment of pure nickel oxide leads to the gradual
loss of super-stoichiometric oxygen and two processes may be assumed. With increasing
temperature the neutral weakly bound oxygen may be desorbed [6] or transformed to the
–
2
–
2–
ionogenic forms like O , O , O owing to the facilitation of electron transfer [11]. These
ionogenic forms then may be partially incorporated into the lattice due to its increased
mobility or desorbed from the surface. At about 800–900°C fully stoichiometric nickel
oxide is formed [9].
Reduction of mixed oxides
Kinetics and degree of hydrogen reduction
As it follows from Fig. 5 the maximum reduction rate (Vmax) monotonously and
non-linear decreases with increasing content of thermodynamically more stable zinc
oxide in the whole range of composition for both series. A similar course was found
also for the dependence of the same quantity normalized to the unit surface area (spe-
cific maximum reduction rate, Vmax, s) on composition. This course appears to be dif-
ferent from that in the earlier studied systems N and Z [4], where the strongest mutual
influence of both components was observed in the region of occurrence of solid solu-
tion. Above the concentration level of 25–30 mass% the zinc oxide formed a separate
phase and its effect on the quantity Vmax was practically negligible. Moreover with the
series N the extremely high reactivity of the system manifested itself by a local maxi-
mum of the dependence under discussion at about 10% of ZnO oxide. It was found
that this behaviour is connected with the presence of very finely dispersed nickel ox-
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
Fig. 5 Dependence of the maximum reduction rate Vmax on composition at different
temperatures. 1 – series A, 440; 2 – series B, 440; 3 – series A, 350°C
ide (microcrystallite size about 3 nm) of super-stoichiometry which was created pre-
dominantly by weakly bound forms of oxygen making very rapid sorption of hydro-
gen and subsequent surface nucleation of the reduction product possible [4]. Even
though the analogous effect was not observed with the systems A and B, the higher
reactivity of the samples of series A in the whole range of composition is also evident
from Fig. 5 as well as from the dependencies of the apparent activation energy on
composition. With the series A this quantity increases monotonously wits increasing
–
1
amount of ZnO up to its 20% content from 40 to 100 kJ mol and then remains un-
changed. With the series B in the region of 0–30% ZnO this quantity rises from 60
–
1
to 150 kJ mol . The lower values of the apparent activation energies of reduction cal-
culated for the samples containing an excess of nickel oxide including pure NiO ox-
ide give evidence for the participation of transport processes. Owing to the higher
surface areas as well as resistance to the sintering and therefore more porous texture
with the samples of series A the transport of water vapour may be facilitated. This
leads to the acceleration of reduction and lowering of the activation energy [12].
The different reactivity of the samples of both series A and B follows also from
Fig. 6. This figure shows that the degree of reduction over the theoretical value, cor-
responding to the total reduction of NiO, increases monotonously with increasing
content of ZnO oxide in the series A only, containing the nickel oxide of carbonate
origin. A sharp maximum of this dependence was reached at 80 mass% of zinc oxide.
At the same reduction temperature the excess of reduction with the series B reaches
only 30% in the region of prevailing content of nickel oxide. The X-ray analysis of
the samples reduced at this temperature proved that the partial reduction of the sec-
ond component (ZnO) takes place in both series up to the different degree depending
on their origin and composition. The same dependences for the earlier studied se-
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
45
Fig. 6 Dependence of the percentage reduction at different temperatures over the theo-
retical value corresponding to the total reduction of NiO on the composition.
1
– series B, 440; 2 – series B, 380; 3 – series A, 440; 4 – series A 380°C
Fig. 7 Dependence of rate constants k of the reduction on composition for the original
and irradiated samples of a – series A reduced at 440°C and b – series B reduced
at 380°C. 1 – original samples; 2 – irradiated by gamma rays, dose 3 MGy;
3
– irradiated by accelerated electrons, dose 3 MGy
ries N and Z showed more distinctive differences [4]. This effect may also be related
to the different size distribution of grains and their agglomerates [13]. The kinetics of
reduction of both series A and B can be quantitatively described in conformity with
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
1
/3
the shrinking unreacted-core model by the equation: 1– (1–α) =kt, where α is the
degree of reduction in time t and k is the rate constant. As can be seen from Fig. 7 the
rate constant of the reduction for both series decreases monotonously with increasing
content of ZnO similarly as the quantity Vmax
.
Effect of pre-irradiation
In contrast with the series B, the samples of series A appear to be sensitive towards
pre-irradiation. This manifests itself by the decrease of the reduction rate especially
with irradiated pure nickel oxide and the mixed samples containing its distinctive ex-
cess. Analogous effect was observed with the earlier studied series N [4]. However in
contrast with this series no inversion of negative radiation effect to the positive one
(
acceleration of the reduction) was proved with the irradiated samples of series A
containing an excess of zinc oxide. This gives evidence for very low concentration of
lattice perturbations and local boundaries of interacting phases making possible the
capture and stabilization of radiation-induced charge defects especially quasi-free
electrons accelerating the decomposition processes [14]. As it follows from Fig. 8,
both quantities characterizing the reduction rate (Vmax, respectively, Vmax, s and k) may
be affected in an opposite manner. With the selected mixed samples of series B con-
taining an excess of nickel oxide the value of rate constant decreases with increasing
temperature of heat pre-treatment whereas the quantity Vmax, s increases. This gives
evidence for the activation of surface centres by removal of bound water and
Fig. 8 Plot of specific maximum reduction a – rate Vmax, s and b – rate constant k vs.
composition for the selected samples of series B, heat treated at different tem-
peratures and reduced at 440°C. 1 – original non-treated samples; samples
treated at 2 – 600; 3 – 750 and 4 – 900°C
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
47
Fig. 9 Dependence of the maximum reduction rate Vmax for the samples of a – series A
measured in non-isothermal regime on composition and b – tempera-
ture-programmed reduction profile of sample 4, series A
desorption of gaseous impurities. In addition to the observed increase in ionogenic
forms of oxygen retarding the donor chemisorption of reducing hydrogen, the en-
hanced adsorption of weakly bound, easily reducible neutral oxygen can be assumed
during the cooling of the samples treated in air. In contrast with the rate constant, the
quantity Vmax, s characterizes the reaction on the surface or in the near sub-surface lay-
ers of the grain. From Fig. 9b showing the dependence of the instantaneous reduction
rate on the temperature, measured in a non-isothermal regime (temperature pro-
grammed reduction profile) it is evident that in accordance with the above mentioned
kinetic model the maximum reduction rate is attained in the initial stages of the reduc-
tion. The same character exhibit the dependences of the instantaneous reduction rate
on the reduction degree, measured in an isothermal regime for both series. However
with increasing content of zinc oxide the quantity Vmax is shifted towards higher val-
ues of reduction degree. As it follows from the comparison of Fig. 5, curves 1 and 3
(
samples of series A) with Fig. 9a the courses of the dependencies of quantity Vmax on
composition appear to be similar both for the non-isothermal and for the isothermal
regime of measurement.
Effect of chemical modification of surface
Reactivity of the samples in both series may be substantially enhanced by surface im-
pregnation with a platinum activator. Figure 10 shows the dependence of rate con-
stants of the reduction on composition for the samples of series B impregnated with
the solution of H PtCl and measured by different ways. Selected samples were ther-
2 6
mally pre-treated in air at 350°C up to the constant mass loss and then reduced by hy-
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POSP͞IL et al.: NiO–ZnO MIXED OXIDES
Fig. 10 Dependence of the rate constants of reduction on composition for selected samples
of series B activated with platinum and reduced at 350°C. 1 – original samples;
2
3
– hydrogen reduction after pre-decomposition of activator (H
– reductive decomposition of the impregnated samples directly in hydrogen
2 6
PtCl ) in air;
drogen at the same temperature (curve 2). In subsequent experiments the same im-
pregnated samples were directly reduced by hydrogen (reductive decomposition,
curve 3). Both processes took place without observable induction period. As com-
pared with the original non-modified samples (curve 1) the reduction is accelerated,
however the activation appears to be much more effective in the case of reductive de-
composition. In the former case it may be assumed that the thermal decomposition of
the activator takes place according to the equation
H
2
PtCl
6
+1/2O
2
→H O+Pt+3Cl
2 2
In addition to that the pre-treatment in air leads to the reaction in surface layers
NiPtCl +1/2O →NiO+Pt+3Cl
6
2
2
Therefore the following reduction appears to be facilitated predominantly by plat-
inum due to the spill-over effect. More reactive species (atomic hydrogen) can migrate
by surface diffusion from Pt catalyst to reduced oxide and the reaction is accelerated
even though both phases (Pt/oxide) are gradually separated by creating product-
metallic nickel. As it was found [15], only the fraction of platinum which is formed by
the latter reaction substantially increases the rate of reduction whereas the catalytic ac-
tivity of the platinum fraction originated according to the former reaction appears to be
slight. During the reductive decomposition in hydrogen probably the reaction
NiPtCl
6
+NiO+4H
2
→2Ni+Pt+6HCl+H O
2
J. Therm. Anal. Cal., 75, 2004

POSP͞IL et al.: NiO–ZnO MIXED OXIDES
49
takes place so that the positive effect of platinum simultaneously superimposes with
the autocatalytic effect of reduced metallic nickel. The efficiency of the positive ef-
fect increases with increasing proportion of ZnO which probably acts as a transmitter
of active hydrogen. Moreover in contrast with the non-impregnated samples in both
cases the slow reduction of pure zinc oxide as well as its reduction in mixed samples
was observed at low temperatures. The literature indicates that the temperature of re-
duction may be markedly reduced by small admixtures of platinum with various
hardly reducible metal oxides, however in the limits of their thermodynamical stabil-
ity [16]. Similar results were found with the impregnated samples of series A. In com-
parison with the series B the only difference lies in lower accelerating effect predomi-
nantly with the samples preliminary thermally treated and then reduced. It may be
connected with different epitaxial contact of activator determined by different mor-
phology and dispersity [15, 16].
Conclusions
The two series of NiO–ZnO mixed oxides of various composition prepared by calci-
nation from solid nickel oxide of carbonate origin suspended in solution of zinc ni-
trate (series A) and from solid zinc oxide of carbonate origin mixed with solution of
nickel nitrate (series B) consist of separate phases of both oxides. Although the de-
gree of mutual interaction of the two components appears to be lower than that with
earlier studied analogous systems precursors which were formed by mixed unsoluble
or soluble salts, the series A and B compared widely differ both in their physico-
chemical properties and chemical reactivity tested by reduction with hydrogen.
The preparation of mixed oxides of both series resulted in differences in the
dispersity of microcrystallites, their morphology, thermal stability, absolute values of
specific surface areas and normalized content of surface chemisorbed oxygen as well
as in character of the dependences of these parameters on composition.
The presence of thermodynamically more stable zinc oxide affects not only the re-
duction kinetics but also the overall degree of reduction achieved. Depending on the
temperature the partial reduction of zinc oxide also takes place however with both se-
ries up to different degrees and in different regions of composition. The reduction ki-
netics of both series can be quantitatively described by the shrinking unreacted-core
model. However the oxides of series A show the higher reactivity as well as the higher
sensitivity towards pre-irradiation both by gamma rays and by accelerated electrons.
The reduction can be substantially accelerated by the addition of platinum acti-
vator due to the spill-over effect and its superposition with autocatalytical influence
of reduced nickel. The efficiency of the positive effect increases with increasing con-
tent of zinc oxide up to the maximum, however in the case when the initial form of ac-
tivator (H PtCl ) impregnated on the surface was reductive decomposed in the flow
2 6
of hydrogen. The preliminary decomposition of the activator in air leads only to the
slight acceleration of following hydrogen reduction. Owing to the different epitaxial
contact this positive effect appears to be higher with the samples of series B. In the
J. Therm. Anal. Cal., 75, 2004

5
0
POSP͞IL et al.: NiO–ZnO MIXED OXIDES
presence of activator the thermodynamically more stable zinc oxide undergoes reduc-
tion at lower temperatures.
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