Thermal solid-solid interactions and physicochemical properties of NiO/Fe2O3 system doped with K2O

Article abstract of DOI:10.1016/j.tca.2008.01.012

The effects of calcination temperature and doping with K₂O on solid-solid interactions and physicochemical properties of NiO/Fe₂O₃ system were investigated using TG, DTA and XRD techniques. The amounts of potassium, expressed as mol% K₂O were 0.62, 1.23, 2.44 and 4.26. The pure and variously doped mixed solids were thermally treated at 300, 500, 750, 900 and 1000 °C. The catalytic activity was determined for each solid in H₂O₂ decomposition reaction at 30-50 °C. The results obtained showed that the doping process much affected the degree of crystallinity of both NiO and Fe₂O₃ phases detected for all solids calcined at 300 and 500 °C. Fe₂O₃ interacted readily with NiO at temperature starting from 700 °C producing crystalline NiFe₂O₄ phase. The degree of reaction propagation increased with increasing calcination temperature. The completion of this reaction required a prolonged heating at temperature >900 °C. K₂O-doping stimulates the ferrite formation to an extent proportional to its amount added. The stimulation effect of potassium was evidenced by following up the change in the peak height of certain diffraction lines characteristic NiO, Fe₂O₃, NiFe₂O₄ phases located at d spacing 2.08, 2.69 and 2.95 A?, respectively. The change of peak height of the diffraction lines at 2.95 A? as a function of firing temperature of pure and doped mixed solids enabled the calculation of the activation energy (ΔE) of the ferrite formation. The computed ΔE values were 120, 80, 49, 36 and 25 kJ mol^-1 for pure and variously doped solids, respectively. The decrease in ΔE value of NiFe₂O₄ formation as a function of dopant added was not only attributed to an effective increase in the mobility of reacting cations but also to the formation of potassium ferrite. The calcination temperature and doping with K₂O much affected the catalytic activity of the system under investigation.

Full text of DOI:10.1016/j.tca.2008.01.012

Available online at www.sciencedirect.com
Thermochimica Acta 470 (2008) 18–26
Thermal solid–solid interactions and physicochemical properties of
NiO/Fe2O3 system doped with K2O
W.M. Shaheen
Physical Chemistry Department, Catalysis and Surface Laboratory, National Research Center, Dokki, Cairo, Egypt
Received 19 April 2007; received in revised form 15 January 2008; accepted 16 January 2008
Available online 21 January 2008
Abstract
The effects of calcination temperature and doping with K2O on solid–solid interactions and physicochemical properties of NiO/Fe2O3 system
were investigated using TG, DTA and XRD techniques. The amounts of potassium, expressed as mol% K2O were 0.62, 1.23, 2.44 and 4.26. The
pure and variously doped mixed solids were thermally treated at 300, 500, 750, 900 and 1000 ◦C. The catalytic activity was determined for each
solid in H2O2 decomposition reaction at 30–50 ◦C. The results obtained showed that the doping process much affected the degree of crystallinity
of both NiO and Fe2O3 phases detected for all solids calcined at 300 and 500 ◦C. Fe2O3 interacted readily with NiO at temperature starting from
700 ◦C producing crystalline NiFe2O4 phase. The degree of reaction propagation increased with increasing calcination temperature. The completion
of this reaction required a prolonged heating at temperature >900 ◦C. K2O-doping stimulates the ferrite formation to an extent proportional to
its amount added. The stimulation effect of potassium was evidenced by following up the change in the peak height of certain diffraction lines
˚
characteristic NiO, Fe2O3, NiFe2O4 phases located at “d” spacing 2.08, 2.69 and 2.95 A, respectively. The change of peak height of the diffraction
˚
lines at 2.95 A as a function of firing temperature of pure and doped mixed solids enabled the calculation of the activation energy (ꢀE) of the
ferrite formation. The computed ꢀE values were 120, 80, 49, 36 and 25 kJ mol−1 for pure and variously doped solids, respectively. The decrease
in ꢀE value of NiFe2O4 formation as a function of dopant added was not only attributed to an effective increase in the mobility of reacting cations
but also to the formation of potassium ferrite. The calcination temperature and doping with K2O much affected the catalytic activity of the system
under investigation.
© 2008 Elsevier B.V. All rights reserved.
Keywords: TG–DTA; XRD; NiO/Fe2O3; NiFe2O4; Catalytic activity
1. Introduction
manganese ferrite were normally employed as active catalysts in
a big variety of catalytic reactions. These reactions include oxi-
dation, decomposition of alcohols and decarboxylation of some
organic acids. Therefore, preparation of ferrite composites with
certain specific properties as well as the mechanism of their for-
mation became the object of several investigations [9–13]. On
the other hand, ferrites crystallize in three crystal types, namely,
spinel, garnet and magnetoplumbite [14]. These spinel ferrites
can be obtained by solid–solid interactions between ferric and
transition metal oxides or carbonates. The produced ferrites are
much influenced by the prehistory of parent solids, their ratio
and also by addition of small amount of certain foreign oxides
[15–17].
A great deal of fundamental research has been done on mixed
catalysts in the field of heterogeneous catalysis [1–3]. These
mixed systems may be more active than their individual compo-
nents [4,5]. The so-called mixed catalysts are often produced by
thermal decomposition of mixed components of transition met-
als [6]. The thermal treatment of mixed solid components may
lead to the formation of new compounds as a result of solid–solid
interactions between thermal products [7,8].
Most of divalent metal oxides (MO) interact with Fe2O3
yielding the corresponding ferrite with the formula MFe2O4.
These ferrites are important materials that found wide uses in
many industrial applications. Cobalt, nickel, copper, zinc and
The present work reports the results of a study on the effects
of calcination temperature and doping with different amounts
of K2O on solid–solid interactions and physicochemical prop-
erties of NiO/Fe2O3 system. Pure and variously doped mixed
E-mail address: waleed shaheen@hotmail.com.
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.01.012

W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
19
solids were calcined at 300, 500, 750, 900 and 1000 ◦C. The
3. Results and discussion
thermal products obtained at various treatment temperatures
were characterized using thermal analyses (TG–DTA) and X-ray
diffraction (XRD) techniques. The catalytic activity of all solids
was measured by using catalytic decomposition of hydrogen
peroxide at 30, 40 and 50 ◦C.
3.1. Thermal behavior of pure and variously doped mixed
solids
Fig. 1a shows the (TG–DTA) curves of pure mixed solids hav-
ing the formula 1/3NiCO3·2Ni(OH)3·4H2O/FeCO3·Fe(OH)2.
The thermograms obtained from the mixed solids treated with
small percentage of K2O were similar to each other and that of
pure mixed solids. The DTA curve of pure mixed solids shows
threeendothermicpeakswiththeirminimalocatedat56, 228and
302 ◦C. In addition, an exothermic peak its maximum located
at 800 ◦C was detected. The TG curve consisted of three weight
lossprocessesattemperaturerangedbetween55and110 ◦C, 180
and 250 ◦C, and 280 and 400 ◦C. These processes were accom-
panied by total weight losses of 7.5, 22.5 and 29.5%. The first
process corresponds to the departure of physisorbed water from
both salts and the removal of water of crystallization of basic
nickel carbonate [17]. The second process represents the simul-
taneous thermal decomposition of the NiCO3 and Ni(OH)2 of
theanhydrousbasicnickelcarbonateandformationofFe(OOH).
The last process indicates the complete thermal decomposition
2. Experimental
2.1. Materials
The starting materials used in this investigation were solids
of basic nickel carbonate [NiCO3·2Ni(OH)3·4H2O], basic iron
carbonate, FeCO3·Fe(OH)3 and potassium nitrate, KNO3. The
chemicals employed were of analytical grade and were supplied
by Prolabo Company.
A series of mixed oxide catalysts of NiO/Fe2O3 system hav-
ing equal molar ratio were prepared by mechanical mixing of
their corresponding salts. The mixed oxides were obtained by
firing the mixed solids in air at 300, 500, 750, 900 and 1000 ◦C
for 4 h.
Samples of the doped mixed solids were prepared by impreg-
nation method by treating a given mass of the prepared mixed
solids of nickel and iron carbonates with solutions containing
different proportions of potassium nitrate dissolved in the least
amount of distilled water. The extents of doping expressed in
mol% K2O were 0.62, 1.23, 2.44 and 4.26. The pure and doped
solids were dried at 100 ◦C then calcined in air at 300, 500, 750,
900 and 1000 ◦C for 4 h.
2.2. Techniques
Thermal analyses, i.e., thermogravimetry (TG) and differen-
tial thermal analysis (DTA) were carried out using a Shimadzu
Dt-40 thermal analyzer. The sample was placed in platinum cru-
cible (0.1 cm3). The samples were studied under air atmosphere
with flowing rate of 30 ml/min. Constant weights of sample
(62–69 mg) were used in order to avoid the effect of variation in
sample weight on peak shape and temperature. Alpha alumina
was used as a reference material in this technique. The rate of
heating was fixed at 10 ◦C min−1
.
An X-ray investigation of pure and variously doped solids
preheated in air at 300, 500, 750, 900 and 1000 ◦C was con-
ducted using a Philips diffractometer (Goniometer PW 1390),
employing iron Ka radiation as the X-ray source. The X-ray
tube was operated at 36 kV and 16 mA. The samples were finely
grounded and packed in plastic holder. The diffraction angle 2θ
was scanned at a rate of 2◦ min−1
.
The catalytic activity of pure and doped solids was deter-
mined using H2O2 decomposition in aqueous solution, as a
model reaction, at 30, 40 and 50 ◦C, using 0.5 ml volume of
H2O2 of known concentration diluted to 20 ml with distilled
water. The mass of catalyst sample taken in each kinetic exper-
imental was fixed at 10 mg for all samples. The reaction was
followed up throughout a gasmetric measurment of oxygen lib-
erated at different time interval. Details of experimental method
have been given elsewhere [18].
Fig. 1. (a) Thermal analysis of pure mixed solids with the for-
mula [1/3NiCO3·2Ni(OH)3·4H2O/FeCO3·Fe(OH)2]. (b) Thermal analysis
of treated solids with the formula [1/3NiCO3·2Ni(OH)3·4H2O/0.4KNO3/
FeCO3·Fe(OH)2].

20
W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
of both salts into NiO and Fe2O3 [17,19]. The weight of this
sample did not undergo any change on heating at ≥500 ◦C with
the detection of an exothermic peak with its maxima located at
800 ◦C. This might reflect a probability of solid–solid interaction
between the produced oxides to form nickel ferrite according to
the following equation [17,20]:
tion lines of nickel ferrite increased by increasing the amount
of dopant up to certain extent as well as with increasing the cal-
cination temperature to 1000 ◦C. (vi) The X-ray diffractograms
of pure solids heavily doped with K2O and calcined at 750,
900 and 1000 ◦C show the detection of new diffraction lines
(d = 5.94, 2.97, 2.83, 2.57 and 2.54) characteristic to potassium
ferrite (KFe11O17) phase beside NiO and NiFe2O4 phases with
the disappearance of all diffraction lines of Fe2O3 for the solids
heavily doped with K2O. (vii) Furthermore, increasing the cal-
cination temperature of all solids to 1000 ◦C was resulted in
decreasing of the peak height of NiO and Fe2O3 phases. In con-
trast, the degree of crystallinity of both nickel and potassium
ferrite phases increased.
Inspection of the above results indicates that: (i) no
solid–solid interaction between NiO and Fe2O3 takes place
at calcination temperature below 700 ◦C. (ii) The presence of
diffraction lines of free oxides in the case of pure mixed solids
preheated at 1000 ◦C showed that the complete conversion of
these oxides into NiFe2O4 requires a prolonged heating at tem-
perature >1000 ◦C [21]. It seems that the ferrite produced at
750 ◦C covers the surfaces of grains of the free oxides hindering
their further diffusion. So, the nickel ferrite formed initially acts
as an energy barrier against the completion of the solid–solid
NiO + Fe2O3 → NiFe2O4
The TGA and DTA curves of the heavily K2O treated sam-
ple having the formula [1/3NiCO3·2Ni(OH)3·4H2O/0·4KNO3/
FeCO3·Fe(OH)2] are illustrated in Fig. 1b. The TGA curve of
this sample showed four weight loss processes at temperature
ranged between 50 and 110 ◦C, 190 and 250 ◦C, 260 and 300 ◦C,
and 670 and 750 ◦C. These processes were accompanied by total
weightlossesof7, 20, 32and33%. Theseprocesseswereaccom-
panied with the detection of three endothermic peaks whose
minima were located at 70, 225, 283 and 750 ◦C. In addition, an
exothermic peak with its maxima located at 799 ◦C.
The comparison of DTA curves of pure and doped solids
revealed that potassium-doping enhanced the thermal decom-
position of both basic nickel and iron carbonates into their
corresponding oxides. The endothermic peak relative to this
process shifted from 300 to 280 ◦C. Furthermore, the complete
conversion of basic nickel carbonate, potassium nitrate and basic
iron carbonate solids into the corresponding NiO, K2O and
Fe2O3 takes place at about 280 ◦C with a total weight loss of
32% which is very close to the value found theoretically. The
endothermic peak located at 750 ◦C and the exothermic peak
located at 799 ◦C may correspond to the formation of potassium
and nickel ferrite compounds, KFe11O17 and NiFe2O4, respec-
tively. The identification of these compounds will be discussed
later in the XRD section of the present work.
3.2. XRD investigation of pure and doped mixed solids
precalcined at different temperatures
Figs. 2–6 represent the X-ray diffractograms of pure and var-
ious K2O-doped NiO/Fe2O3 system calcined at 300, 500, 750,
900 and 1000 ◦C. In addition, Table 1 shows the effect of K2O-
doping and calcination temperature of NiO/Fe2O3 system on
the peak height of main diffraction lines of detected phases. The
examination of these figures and Table 1 shows that: (i) pure and
doped solids calcined at 300 and 500 ◦C consisted only of the
diffraction lines of NiO and Fe2O3 phases. (ii) The peak heights
and the degree of crystallinity of both NiO and Fe2O3 phases
progressively increased with increasing the calcination tempera-
ture. (iii)TheadditionofincreasingamountsofK2Ofollowedby
calcination at 300 and 500 ◦C resulted in a progressive decrease
in the peak heights and the degree of crystallinity of the detected
phases with the absence of any diffraction lines of potassium
oxide as a separate phase. Furthermore, the diffraction lines of
NiO completely disappeared in the heavily K2O-doped sample
calcined at 300 ◦C. (iv) The X-ray diffractograms of pure and
variously doped solids calcined at 750 and 900 ◦C consisted of
the diffraction lines of free oxides as major phase together with
the appearance of new diffraction peaks characteristic of nickel
ferrite (NiFe2O4) phase. (v) The peak height of the main diffrac-
Fig. 2. X-ray diffractograms of pure mixed solids and that doped with different
amounts of K2O followed by calcination at 300 ◦C.

W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
21
of the reacting oxides and consequent easily solid-state reaction
process. It can also be seen from Table 1 that the presence of
K2O brought about a regular decrease in the peak heights of the
˚
diffraction lines at 2.08 and 2.69 A characteristic to NiO and
Fe2O3 with a subsequent increase in the peak height of the line
˚
at 2.95 A characteristic to NiFe2O4 phase. These results showed
clearly that K2O-doping enhances the ferrite formation to an
extent, proportional to its amount present.
3.3. Catalytic activity measurements of pure and doped
mixed solids precalcined at different temperatures
The experimental results obtained revealed that the decompo-
sition of H2O2 over pure and variously K2O-doped NiO/Fe2O3
system calcined at 300 and 500 ◦C followed first-order kinetics.
The kinetics of the catalyzed reaction being monitored for reac-
tions carried out at 30, 40 and 50 ◦C by measuring the volume
liberated oxygen at different time intervals until no further O2
was liberated. Table 2 depicts the values of reaction rate constant
k (min−1) measured at different reaction temperatures. The val-
Fig. 3. X-ray diffractograms of pure mixed solids and that doped with different
amounts of K2O followed by calcination at 500 ◦C.
interaction between NiO and Fe2O3. It has been reported by El-
Shobaky et al. [16,17] that this energy barrier could be overcome
by doping the reacting mixed solids with certain foreign oxides
as Li2O, Al2O3 or ZnO.
In this work, it can be seen that the doping of the system under
investigation with K2O resulted in stimulation of nickel ferrite
formation. The observed enhancement of nickel ferrite forma-
tion as a result of doping with K2O and consequent increase in
the mobility of the diffusion of nickel ions through the growing
NiFe2O4 phase can be investigated by determining the activation
energy of nickel ferrite formation (ꢀE) for pure and K2O-doped
mixed solids. This has been achieved from the results given in
Table 1 by assuming that the height of the characterized diffrac-
˚
tion line at 2.95 A is a measure of the amount of NiFe2O4 present
in given mixed solids at definite temperature (T). By plotting the
˚
peak height of the diffraction line at 2.95 A versus 1/T, a straight
line is obtained whose slope determines (ꢀE) value by direct
application of the Arrhenius equation (see Fig. 7). The com-
puted (ꢀE) values 120, 80, 49, 36 and 25 kJ mol−1 for pure and
those solids doped with 0.62, 1.23, 2.44 and 4.26 mol% K2O,
respectively. The decrease in (ꢀE) values, due to potassium-
doping to an extent proportional to the amount of K2O present,
reflects an effective increase in the mobility of thermal diffusion
Fig. 4. X-ray diffractograms of pure mixed solids and that doped with different
amounts of K2O followed by calcination at 750 ◦C.

22
W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
the (k) values reaching to a maximum limit in the presence of
1.23 mol%K2OthendecreasedbyincreasingtheamountofK2O
added above this limit. The maximum increase in the catalytic
activity measured at 30 ◦C attained 41%. The XRD investiga-
tion of pure and variously K2O-doped solids calcined at 300 and
500 ◦C showed clearly that this treatment decreased the crystal-
lite size of both NiO and Fe2O3 phases. This finding might lead
to an effective increase in the external surface area of both NiO
and Fe2O3 with subsequent increases in the concentration of
active sites taking part in H2O2 decomposition.
On the other hand, it was found by many authors that
alkali-doping of transition metal oxide catalysts is normally
accompanied by an increase in its catalytic activity [22–24].
Thisexpectationcomesfromthefactthatthealkalinitytreatment
of solid catalysts increases the (pH) value of the solution. The
results of this work show that K2O treatment of the investigated
system was thought to increase its catalytic activity towards
H2O2 decomposition due to a possibly induced increase in the
pH value of H2O2 solution. Preliminary experiments showed
that the pH value of the solution resulting from treating a given
Fig. 5. X-ray diffractograms of pure mixed solids and that doped with different
amounts of K2O followed by calcination at 900 ◦C.
ues of rate constants were obtained from the plots of ln(a/a − x)
against time, where a is the initial concentration of H2O2 and x
is the amount of evolved oxygen (see Fig. 8a and b). In addition,
the effect of doping process and calcination temperature on the
catalytic activity of NiO/Fe2O3 system under investigation is
represented in Fig. 9a and b.
Inspection of the data given in Table 1 and Figs. 8–9 shows
that: (i) individual NiO or Fe2O3 obtained by thermal treatment
of their salts at 300 and 500 ◦C exhibit a very small catalytic
activity in H2O2 decomposition. (ii) The catalytic activity of
mixed NiO–Fe2O3 obtained by thermal treatment of mixed
solids at the same calcination temperatures was found to be
greater than those of single oxides. This can be interpreted in
terms of the concept of bivalent catalytic centers [6,22]. In other
words, the higher catalytic activity of two component oxides
may be attributed to the fact that besides the one-component
sites Ni+2 or Fe+3 there will also be the mixed sites (Ni+2–Fe+3).
(iii) The catalytic activities of pure and variously doped solids
progressively increased with increasing the reaction tempera-
ture from 30 to 50 ◦C. In addition, it decreased on increasing
their calcination temperatures from 300 to 500 ◦C. (iv) Treating
the system investigated with an increasing amount of K2O fol-
lowed by calcination at 300 resulted in a progressive increase in
Fig. 6. X-ray diffractograms of pure mixed solids and that doped with different
amounts of K2O followed by calcination at 1000 ◦C.

W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
23
Table 1
Effect of calcination temperature and K2O-doping of NiO/Fe2O3 system on the peak height of main diffraction lines of NiO (d = 2.08, 100%), Fe2O3 (d = 2.69,
100%), NiFe2O4 (d = 2.95, 28%) and KFe11O17 (d = 2.65, 71%)
Pure and variously
doped solids
Calcination
temperature (◦C)
NiO (a.u.) (d = 2.08)
(100%)
Fe2O3 (a.u.)
(d = 2.69) (100%)
NiFe2O4 (a.u.)
(d = 2.95) (28%)
KFe11O17 (a.u.)
(d = 2.65) (71%)
NiO/Fe2O3
45
39
32
22
–
63
–
–
–
–
–
–
–
–
–
–
+0.62 mol% K2
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
56
46
42
22
300
500
750
900
1000
NiO/Fe2O3
77
51
49
48
47
105
60
54
49
46
–
–
–
–
–
–
–
–
–
–
+0.62 mol% K2O
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
NiO/Fe2O3
122
168
72
63
–
24
33
41
42
44
–
–
20
29
32
+0.62 mol% K2O
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
88
81
80
61
–
NiO/Fe2O3
124
122
97
95
84
106
59
48
–
47
52
53
52
50
–
–
24
31
33
+0.62 mol% K2O
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
–
NiO/Fe2O3
124
112
95
92
90
85
56
43
–
80
74
67
60
56
–
–
27
33
35
+0.62 mol% K2O
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
–
weight (10 mg) of heavily K2O-doped solid samples precalcined
at 300 and 500 ◦C with a given volume of distilled water (20 ml)
at 30 ◦C for about 1 h increases slightly from 6.90 to 7.7. This
increase in the pH value of the test solutions might suggest that
most of K2O added was effectively retained in the investigated
solids. If most of the dopant added remained as a separate phase,
it would dissolve easily in distilled water leading to a measurable
increase in its pH value, which has not been verified experimen-
tally. The observed increase in the catalytic activity of mixed
solids due to the treatment with K2O might reflect an effective
increase in the concentration of catalytically active constituents
of the system under investigation. (v) Increasing the amount of
dopant above 1.23 mol% resulted in a measurable decrease in
the (k) value. The addition of 4.76 mol% K2O per mol catalyst
followed by calcination at 300 ◦C led to a maximum decrease
of 57% in (k) value measured at 30 ◦C. (vi) K2O-doping of the
Table 2
Effect of K2O-doping of NiO/Fe2O3 system precalcined at 300, 500 ◦C on the reaction rate constant (k), activation energies (ꢀEa, ꢀEa∗) and frequency factor (log A)
for H2O2 decomposition conducted at 30, 40 and 50 ◦C
Dopant concentration
(mol%)
Calcination
temperature
(◦C)
30 ◦C k × 10 (min−1
)
40 ◦C k × 10 (min−1
)
50 ◦C k × 10 (min−1
)
(ꢀEa)
kJ mol−1
log A
(ꢀEa∗)
kJ mol−1
NiO/Fe2O3
0.81
0.94
1.14
0.60
0.35
0.20
0.06
1.36
1.50
1.83
1.14
0.73
0.36
0.15
1.84
2.45
3.02
2.19
1.55
0.70
0.40
34
40
40
54
61
62
79
12.25
14.61
14.70
19.90
22.40
22.40
28.67
34
34
34
34
35
–
+0.62 mol%K2O
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
Fe-salt
300
500
Ni-salt
–
NiO/Fe2O3
0.71
0.66
0.54
0.36
0.26
0.08
0.03
1.20
1.09
0.81
0.67
0.51
0.17
0.08
2.19
2.04
1.60
1.22
0.96
0.36
0.22
47
54
45
51
55
62
82
17.22
16.03
16.28
18.51
19.94
22.17
29.56
47
47
47
48
48
–
+0.62 mol%K2O
+1.23 mol% K2O
+2.44 mol% K2O
+4.76 mol% K2O
Fe-salt
Ni-salt
–

24
W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
mechanism of the catalyzed reaction and hence gives useful
information about possible changes in the concentration and
nature of the catalytically active constituents. The values of k
measured at 30, 40 and 50 ◦C over pure and variously doped
solids enable ꢀE to be calculated by direct application of Arrhe-
nius equation. The calculated values of ꢀE are given in Table 2,
which also include the values of pre-exponential factor A of the
Arrhenius equation. It is shown that log A changes from 12.25
to 22.40 for pure and variously doped solids preheated at 300 ◦C
and from 16.03 to 19.94 for the catalysts investigated calcined at
500 ◦C. Thechangesinthevalueoflog AduetodopingwithK2O
are normally followed by corresponding changes in the magni-
tude of ꢀE. The magnitudes of ꢀE were recalculated (ꢀE*)
for the different treated solids preheated at 300 and 500 ◦C. The
computed (ꢀE*) values are given in the last column of Table 2.
The results given in Table 2 clearly show that the values of ꢀE
and ꢀE* are identical for the untreated solids, but ꢀE* remains
virtually unchanged for pure and doped solids calcined at 300
˚
Fig. 7. Change of ln peak height of the diffraction line of NiFe2O4 (2.95 A) as
a function of precalcination temperature for pure and K2O-doped mixed solids.
system investigated followed by calcination at 500 ◦C resulted
in a significant decrease in its catalytic activity. The maximum
decrease in the (k) value measured at 30 ◦C reaches 64%. The
observed decrease in the catalytic activity as a result of increas-
ing the dopant concentration above 1.23 mol% K2O might be
attributed to a possible location of potassium ions on top sur-
face layers of the treated catalyst blocking some of active sites
that contribute in H2O2 decomposition. (vii) Increasing of cal-
cination temperature from 300 to 500 ◦C was accompanied by a
progressive decrease in the catalytic activity of pure and doped
samples under investigation. This is because of increasing the
degree of crystallinity of catalytically active sites of nickel of
ferric oxide phases which was accompanied by increasing their
crystallite size calculated by using Scherrer equation [25]. (viii)
No measurable catalytic activity was observed for all pure and
doped mixed solids preheated at 750, 900 and 1000 ◦C (not
given). This behavior may be attributed to the formation of
nickel and potassium ferrites (NiFe2O4 and KFe11O17). The
poor performance of mixed solids preheated at 750, 900 and
1000 ◦C might be explained in terms of the restricted redox cou-
ple represented by (Mn/Mn−1) in the electronic composition of
the catalysts and, possibly, the absence of Mn on the octahe-
dral lattice site which may initiate the cyclic electron-transfer
process on the catalyst surface [23,26].
Fig. 8. (a) Effect reaction temperature and K2O-doping on the reaction rate
constant (k) of pure NiO/Fe2O3 system heated at 300 ◦C. (b) Effect reaction
temperature and K2O-doping on the reaction rate constant (k) of pure NiO/Fe2O3
system heated at 500 ◦C.
Determination of the apparent activation energy (ꢀE) for
the catalysis of H2O2 decomposition over pure and variously
doped samples shed some light on the possible change in the

W.M. Shaheen / Thermochimica Acta 470 (2008) 18–26
25
(NiFe2O4) phase whose degree of crystallinity increased by
increasing the calcination temperature within 700–1000 ◦C.
4. Doping of the system investigated with K2O enhanced the
formation of crystalline nickel ferrite phase up to certain
extent of amount of the dopant added. The computed value
of activation energy of formation of nickel ferrite was 120,
80, 49, 36 and 25 kJ mol−1 for pure mixed solid samples and
for those doped with 0.62, 1.23, 2.44 and 4.26 mol% K2O,
respectively.
5. The samples heavily doped with K2O resulted in the for-
mation of potassium ferrite (KFe11O17) phase starting from
750 ◦C. The degree of crystallinity of this phase increased
with increasing the amount of K2O as well as with increasing
the calcination temperature up to 1000 ◦C.
6. The catalytic activity of mixed oxides, NiO–Fe2O3, which
decreased by increasing the treatment temperature from 300
to 500 ◦C was found to be bigger than those of single oxides.
In addition, there is no measurable catalytic activity for all
the solids preheated at 750, 900 and 1000 ◦C.
7. The catalytic activity of NiO/Fe2O3 system preheated at
300 ◦C increased by doping with K2O up to a certain extent.
The addition of 4.76 mol% K2O affected a decrease of 57%
and 63% in the value of reaction rate constant over the doped
catalysts calcined at 300 and 500 ◦C, respectively.
8. The doping process did not modify the mechanism of cat-
alytic reaction, but changed the concentration of catalytically
active components.
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