STRUCTURE AND ELECTRONIC EFFECTS OF COBALT FERRITES, CoxFe3-xO4, ON CATALYTIC DECOMPOSITION OF ISOPROPYL ALCOHOL

Article abstract of DOI:10.1135/cccc19941939

The cobalt ferrite spinel oxides CoxFe3-xO4 (0 < x < 3) have been prepared by coprecipitation.Their structure were characterized by DTA, X-ray diffraction and IR spectra, and their surface properties were determined from nitrogem adsorption isoterms at -196 deg C.Conductance of all the compositions was studied with and without isopropyl alcohol.The variation of E_? values was discussed in terms of oxide semiconducting properties and of ion distribution in the octahedral-tetrahedral sites of the spinel structures.The catalytic decomposition of isopropyl alcohol at 325 deg C in a flow system allowed to conclude that the inverse spinels formed in the iron-rich region ate active and selective sites for acetone formation, in contarst to the inverse spinels formed in the cobalt-rich region.On the other hand, the region of normal spinels showed low activity and selectivity to acetone formation.Correlations between the composition and electronic and catalytic properties of the catalysts are reported.

Full text of DOI:10.1135/cccc19941939

Catalytic Decomposition of Isopropanol
1939
STRUCTURE AND ELECTRONIC EFFECTS OF COBALT FERRITES,
CoxFe3−xO4, ON CATALYTIC DECOMPOSITION OF ISOPROPANOL
Khalf-Alla M. ABD EL-SALAAM, Abd El-Aziz A. SAID*, Ahmed M. EL-AWAD,
Ehsan A. HASSAN and Mohamed M. M. ABD EL-WAHAB
Chemistry Department,
Faculty of Science, Assiut University, 715 16 Assiut, Egypt
Received December 14, 1993
Accepted May 10, 1994
The cobalt–ferrite spinel oxides CoxFe3−xO4 (O < x < 3) have been prepared by coprecipitation. Their
structures were characterized by DTA, X-ray diffraction and IR spectra, and their surface properties
were determined from nitrogen adsorption isotherms at −196 °C. Conductance of all the compositions
was studied with and without isopropanol. The variation of Eσ values was discussed in terms of
oxide semiconducting properties and of ion distribution in the octahedral-tetrahedral sites of the spi-
nel structures. The catalytic decomposition of isopropanol at 325 °C in a flow system allowed to
conclude that the inverse spinels formed in the iron-rich region are active and selective sites for
acetone formation, in contrast to the inverse spinels formed in the cobalt-rich region. On the other
hand, the region of normal spinels showed low activity and selectivity to acetone formation. Correla-
tions between the composition and electronic and catalytic properties of the catalysts are reported.
Surface and bulk properties of ferrites have implications in several fields such as cata-
lysis and electrocatalysis, corrosion of metal alloys, as well as electronic applications.
The interesting physical and chemical properties of spinel ferrites arise from their activ-
ity to distribute cations among available tetrahedral (t) and octahedral (o) sites1.
Cobalt–ferrite spinels are of considerable industrial interest due to their catalytic activ-
ity2–5, structural stability, long life-time and resistance to sintering. The high activity of
the oxides was ascribed to the presence of Co2+ on octahedral sites which initiated a
cyclic electron transfer6. Stoichiometric CoFe2O4 contains only Co2+ and Fe3+. Semi-
conducting properties of CoxFe3−xO4 have been investigated with x < 1; the excess Fe2+
acts as an electron donor, causing n-type semiconductivity. When x > 1, Co3+ acts as an
electron acceptor, causing p-type semiconductivity. However, it was already found that
the wide variation of x in the cobalt–iron spinel system leads to a drastic change of its
catalytic and electrical properties. However, little attention has been paid to the cata-
* The author to whom correspondence should be addressed.
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1940
Abd El-Salaam, Said, El-Awad, Hassan,Abd El-Wahab:
lytic activity and electrical conductivity in relation to the surface structure and texture
of CoxFe3−xO4 in the 0 ≤ x ≤ 3 range.
The present paper reports the results of electrical conductivity measurements and of
the decomposition of isopropanol on CoxFe3−xO4 system. The physico-chemical proper-
ties of solid cobalt ferrites were characterized by DTA, IR spectra and XRD techniques.
The surface characteristics of various samples were obtained by N2 adsorption iso-
therms conducted at −196 °C. The catalytic activity and selectivity of different catalysts
were tested using isopropanol decomposition. Finally, semiconducting properties of the
catalysts were determined both in the presence and absence of the above alcohol.
EXPERIMENTAL
Materials
Analar grade chemicals were used. Mixed samples of iron and cobalt hydroxides (with the Co2+ mol
content x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0) were prepared by reported
methods6,7. Analytical grade isopropanol (BDH Ltd) was refluxed with sodium and distilled before
use in catalytic runs. Its purity was checked by gas chromatography. Nitrogen was purified by pas-
sing through a vanadium(II) solution and dried by passing through columns packed with anhydrous
calcium chloride.
Apparatus and Methods
Differential thermal analysis (DTA) of iron and cobalt hydroxides and their mixtures was made with
the use of an automatically recording thermobalance of the type 160 KS (Germany). The heating rate
was 10 °C/min, and 20 mg of each solid specimen was used in each experiment. α-Alumina powder
was applied as a DTA reference. X-Ray diffraction (XRD) analysis of the samples after their thermal
treatment was made with the use of Philips diffractometer (Model PW 2103, 35 kV and 20 mA) with
a source of CuKα radiation (Ni filtered). IR spectra of solid oxide catalysts were recorded in
1 600 – 200 cm−1 region with the spectrometer Perkin–Elmer, Model 599 B, using KBr technique.
Adsorption–desorption N2 isotherms were determined at −196 °C, after initial degassing of the cata-
lysts at 150 °C for 1 h, using conventional volumetric apparatus8. The SBET values were calculated
from adsorption data using the linear form of the BET equation9. The porosity of the samples was
assessed by the method of Brokhoff and de Boer10,11. The electrical conductivity measurements were
made as described previously12. The catalytic reactions were carried out in a conventional fixed-bed
flow type reactor. The exit feed was analyzed by direct sampling of the gaseous products into a Pye–
Unicam gas chromatograph, determining the amount of isopropanol and its decomposition products. All the
measurements were made after steady-state conditions were attained.
RESULTS AND DISCUSSION
Thermal Analysis
DTA curves of pure Fe(OH)3, Co(OH)2 and their mixtures are represented in Fig. 1.
Curve 1 shows that the decomposition of Fe(OH)3 proceeds in three steps with maxima
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Catalytic Decomposition of Isopropanol
1941
at 100, 240, and 500 °C, respectively. The first endothermic peak corresponds to the
loss of the adsorbed water, while the second, less pronounced endothermic peak can be
attributed to the hydroxide decomposition13 to amorphous γ-Fe2O3. The third exother-
mic peak corresponds to the phase transition14, i.e. γ-Fe2O3 → α-Fe2O3. On the addi-
tion of 0.2 or 0.4 mole % Co2+, the exothermic peak located at 500 °C shifts to higher
temperatures (577 and 604 °C, respectively), as shown by curves 2 and 3. For x = 0.6,
the just mentioned peak disappears while a small endothermic peak arises at around
660 °C (curve 4). This new peak is due to the solid–solid interaction betweeen α-Fe2O3
and Co3O4, forming the spinel structure in the low concentration. On increasing x up to
1.5, the decomposition of these samples leads to a new exothermic peak with maximum
at 410 °C and an endothermic peak located at 660 °C (curves 6 and 7 for x = 1.0 and
1.5, respectively). The former peak is caused by formation of cobalt–ferrite spinel13
while the latter can be explained on the basis of cation distribution15 via oxygen release.
Further increase in Co2+ content (curves 8 – 13) results in disappearance of two peaks
at 410 and 660 °C. The area of the new endothermic peak at 930 – 960 °C increases
with increasing x, reaching maximum for the pure Co(OH)2. It can be attributed16 to the
phase transition of Co3O4 to CoO. On the other hand, the endothermic peak with maximum
at 240 °C relates to the pure Co(OH)2, and curve 13 is due to the decomposition17 of
Co2O3 . H2O to Co3O4.
FIG. 1
DTA curves for iron hydroxide (1), cobalt hydroxide (13)
and their mixtures containing cobalt hydroxide at ratios x = 0.2
(2), 0.4 (3), 0.6 (4), 0.8 (5), 1.0 (6), 1.5 (7), 2.0 (8), 2.2
(9), 2.4 (10), 2.6 (11), and 2.8 (12)
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1942
X-Ray Diffraction
Abd El-Salaam, Said, El-Awad, Hassan,Abd El-Wahab:
To detect any chemical or structural change, all the prepared samples were subjected to
X-ray analysis. The values obtained for the d-plane spacing and their relative intensities
were matched with relevant ASTM cards18. The X-ray diffraction lines of the catalysts
calcined at 600 °C for 3 h are represented in Fig. 2. The figure shows that the predomi-
nant diffraction lines corresponding to α-Fe2O3 (x = 0) are located at d (Å) = 2.63 (100),
2.44 (68), 1.67 (36) and 3.54 (34). The intensity of the diffraction line at d (Å) = 2.63
decreases on increasing Co2+ content and disappears at x = 1.0 whereas the line at d (Å) = 2.46
disappears on the addition of Co2+ at x = 0.6. On the other hand, the new lines at d (Å) = 2.51
(100), 1.48 (32), and 2.94 (30) reach maximum at x = 1.0 and can be attributed to the
formation of CoFe2O4 lattice structure15,19. It is worth noting that the intensity of these
lines decreases on increasing Co2+ content up to x = 2.4, then they disappear. Moreover,
in the presence of Co2+ (x = 1.0), the lines corresponding to Co3O4 appear and become
predominant at x = 2.2. The most intense lines corresponding to Co3O4 are located
at d (Å) = 2.43 (100), 1.43 (53), 2.84 (45), 1.55 (44), 2.01 (30) and 4.62 (25). However,
XRD results allow to conclude that CoFe2O4 spinel is formed in higher amounts at x = 1.0.
Moreover, it predominates in the iron-rich region while its amount decreases sharply in
the cobalt-rich region.
FIG. 2
XRD lines for iron oxide (Fe2O3) (1), cobalt
oxide (Co3O4) (10) and their mixtures con-
taining Co3O4 at ratios x = 0.6 (2), 0.8 (3),
1.0 (4), 1.5 (5), 2.0 (6), 2.2 (7), 2.4 (8), and
2.6 (9) calcined at 600 °C for 3 h in air
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Catalytic Decomposition of Isopropanol
1943
IR Spectra
Examination of IR spectra of the catalysts calcined at 600 °C for 3 h (Fig. 3), indicates
that the absorption bands at 620 – 520, 460, 390, and 310 cm−1 correspond20 to
α-Fe2O3. On the other hand, the bands at 1 380, 660, 575 and 390 cm−1 could be as-
signed to Co3O4 lattice vibrations20. The absorption spectra of different spinel composi-
tions (Fig. 3) have two strong broad bands located at 590 and 400 cm−1 for x = 0.6, 0.8,
1.0, 1.5, and 2.0. The bands observed for Co2+ content at 0.6 > x > 2.0 correspond to the
pure Fe2O3 and Co3O4. The first band was attributed to the intrinsic vibration of tetrahe-
dral groups21, whereas the second one was reported to be due to vibrations of the bival-
ent metal ion–oxygen complex18.
Texture Assessment
The adsorption of nitrogen on the investigated catalysts proved to be rapid and revers-
ible, yielding type II of BDDT classification22. The variations in texture of the catalysts
calcined at 600 °C for 3 h were monitored for both pure and mixed oxides (Table I).
It is evident from Table I that SBET of Fe2O3 increases on increasing Co2+ up to x = 0.6,
then it decreases to x = 1.0. On the other hand, SBET of Co3O4 increases with increasing
Fe2O3 concentration up to x = 2.6, followed by continuous decrease to x = 2.0. The
increase in SBET of Fe2O3 on Co2+ addition can be attributed to the holes formed via
incorporation of Co2+ into Fe2O3 lattice23. It is known that Co3O4 has the spinel struc-
ture which contains both CoO and Co2O3, Co2+ ions are only effective when incorpor-
ated into Fe2O3 lattice. Thus, the following equation can be suggested,
FIG. 3
IR spectra of iron oxide (Fe2O3) (1), cobalt oxide
(Co3O4) (13), and their mixtures containing Co3O4 at
ratios x = 0.2 (2), 0.4 (3), 0.6 (4), 0.8 (5), 1.0 (6), 1.5
(7), 2.0 (8), 2.2 (9), 2.4 (10), 2.6 (11), and 2.8 (12)
calcined at 600 °C for 3 h in air
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1944
Abd El-Salaam, Said, El-Awad, Hassan,Abd El-Wahab:
.
1/2 O2 + 2 CoO → 2 Co| Fe |′ + Fe2O3 + 2 | e |
(A)
.
where Co| Fe | ′ denotes Co2+ ions that replace Fe3+ ions in the normal lattice and | e | is
a hole. Also, CoO could be partially incorporated in another way, such as
2 CoO → 2 Co| Fe | ′ + 1/2 Fe2O3 + Fe3+ + 1/2 O2 + e
(B)
where Fe3+ denotes interstitial ions leading to the lattice expansion accessible to ni-
trogen adsorption. On the other hand, the increase of SBET of Co3O4 on the addition of
Fe2O3 may be attributed to the Fe3+ incorporation into Co3O4 lattice, as shown in Eq.
(C) where Fe| Co |′ are Fe3+ ions substituting Co2+ ions in their normal sites.
.
Fe2O3 → 2 Fe| Co | + 2 CoO + 1/2 O2 + 2 e
(C)
TABLE I
Texture data for CoxFe3−xO4 system calcined at 600 °C for 3 h in air
Co2+ Ratio
SBET
St
Scum
VP
VPcum
St/SBET
x
m2/g
m2/g
m2/g
cm3/g
cm3/g
0a
7.06
16.8
22.7
25.8
22.9
9.1
7.0
16.7
22.6
23.8
22.8
8.3
8.1
20.8
21.6
18.3
31.0
10.9
17.1
13.7
26.6
30.4
36.4
25.7
21.0
0.02
0.06
0.06
0.06
0.10
0.02
0.06
0.04
0.09
0.08
0.08
0.07
0.06
0.05
0.11
0.09
0.12
0.13
0.03
0.10
0.07
0.15
0.12
0.12
0.12
0.10
0.991
0.994
0.991
0.992
0.996
0.912
0.991
0.990
0.976
0.996
0.918
1.000
0.964
0.2
0.4
0.6
0.8
1.0
1.5
2.0
2.2
2.4
2.6
2.8
3.0b
10.5
9.7
10.4
9.6
20.4
27.1
31.6
19.0
16.6
19.9
27.0
29.3
19.0
16.0
a Pure Fe2O3; b pure Co3O4.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Catalytic Decomposition of Isopropanol
1945
The decrease in SBET between two maxima may due to the starting formation of the
spinel structure, as detected by X-ray diffraction. The noticable little variation in SBET
values at x = 1.0, 1.5, and 2.0 may be attributed to the formation of the normal spinel,
i.e. CoFe2O4, which has stoichiometric structure6. However, the results suggest that
formation of the normal spinel is accompanied with the low surface area.
The examination of the porosity of the catalyst (Table I) shows that when comparing
Scum with SBET and VPcum with VP, then Scum > SBET and VPcum > VP. This indicates a
mesoporous nature. Similarly, also an upward deviation observed in the Va–t plots is
indicative of mesopores. Moreover, all the St/SBET values are higher than 91%, which
supports the validity of the standarded t-curve used.
Electrical Conductivity Measurements
Measurements of the electrical conductivity of Fe2O3, Co3O4 and their mixtures cal-
cined at 600 °C for 3 h were carried out in the absence and presence of gaseous isopro-
panol at 325 °C. The variation of log σ with composition is shown in Fig. 4. Curve 1
indicates that in the absence of the alcohol (N2 gas only), the addition of Co2+ into
Fe2O3 lattice leads to conductance increase up to x = 0.4, then a small increase up to x
= 2.2, followed by another increase up to x = 3.0. On the other hand, as the activation
energy provides the better information on conductivity changes, the log σ values were
plotted as a function of the reciprocal absolute temperature, and the activation energies
(Eσ) were determined from the slopes of the obtained lines24, where
σ = σ0 exp (−Eσ/RT) .
(1)
FIG. 4
The variation of log σ with composition of Fe2O3–Co3O4 system calcined at 600 °C for 3 h in the
absence (1) and presence of isopropanol (2) and the activation energy Eσ (3) in nitrogen
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1946
Abd El-Salaam, Said, El-Awad, Hassan,Abd El-Wahab:
The variation of the activation energy of electrical conduction with the catalyst compo-
sition is illustrated in Fig. 4, curve 3. The results show that the conductance of Fe2O3
increases while the activation energy decreases on the addition of Co3O4 up to x = 0.4.
It is known that the addition of Co3O4 (p-type, ref.25) to Fe2O3 (n-type, ref.26) should
decrease the number of charge carriers and thus to decrease the electrical conductance.
Our results indicate the opposite behaviour, which can be accounted for in terms of
Eq. (B) if incorporation of Co2+ into Fe2O3 leads to generation of electrons. On the other
hand, the addition of Fe2O3 to Co3O4 leads to a significant decrease in the conductance
and an increase in Eσ. This may be explained on the basis of Eq. (C), the free electrons
produced in p-type semiconductor decrease the number of charge carriers within the
solid matrix, which decreases the conduction and, subsequently, it increases the activa-
tion energy. The conductance behaviour for x increasing up to 2.2 corresponds to the
formation of the normal spinel CoFe2O4 spinel. Co2+ and Fe3+ cations are found to be
unequally distributed among the tetrahedral and octahedral lattice, and the distribution
is temperature-dependent27. As hopping between different cations on the octahedral
sublattice28 needs higher activation energy than for ions of the same metal, the conduc-
tion activation energy Eσ for the CoFe2O4 spinel is thus higher than that of the pure
oxides, as shown in curve 3. The higher values of Eσ may be due to the hopping sta-
bility between different valencies in the spinel structure. However, CoxFe3−xO4 spinels
have been studied by Jonker29 who pointed out that the compositions with 1 ≤ x ≤ 3 in
this series are of p-type, whereas those with 0 ≤ x ≤ 1 are n-type semiconductors.
Moreover, iron rich oxides of this system are the inverse spinels30, whereas the cobalt-rich
ones are the normal spinels31. From the Eσ values presented in Fig. 4, curve 3, one can
conclude that in the iron-rich region the conductance increases and Eσ decreases, while
the opposite behaviour is observed in the cobalt-rich region. Moreover, the region con-
taining CoFe2O4 shows parallel change in conductance and activation energy.
The electrical conductivity changes of the catalysts were also measured in the
presence of isopropanol vapours under the conditions of the catalytic decomposition of
the alcohol (see later). The changes of log σ with time for the catalyst calcined at 600
°C for 3 h are shown in Fig. 5. In the iron-rich region, the steady state is attained after
30 min, whereas in the cobalt-rich region, the steady-state attainment is fast (ca 10
min). On the other hand, the region of maximum formation of the CoFe2O4 spi-
nel (x = 1.0, 1.5, 2.0, and 2.2) shows little variation of conductance with time.
These results describe the way by which electron exchange between the alcohol and
the catalyst surface is realized. The log σ changes under steady-state conditions with
catalyst compositions are shown in Fig. 4, curve 2. They demonstrate the greater conduct-
ance during the reaction of isopropanol in the iron-rich region compared to the values
obtained in the absence of the alcohol. Contrarily, the conductance of the solids in the
cobalt-rich region and the region containing CoFe2O4 is lower than that obtained in the
absence of isopropanol (curve 1). The increase of conductance in the iron-rich region
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Catalytic Decomposition of Isopropanol
1947
lends further support to the n-type nature of the ferrite spinels and refers also to the
transfer of electrons of the alcohol to the catalyst surface. It was reported that32 the
catalytic reaction of isopropanol on n-type solids increases the number of charge car-
riers, causing thus the increase of conductivity and the decrease of the activation energy,
Eσ, while the opposite situation has been found for the conductivity of p-type. The
decrease in the conductance of the samples with x above 0.8 can be attributed to the
decrease of charge carriers via the consumption of the electrons released by isopro-
panol through the holes within the p-type semiconductors. This is in good agreement
with the results reported by Jonker29 and Dezsi30. In addition, the electronic theory of
chemisorption on semiconducting solid materials33 postulates a close relation between
the electronic properties of a catalyst and its catalytic activity. The width of the energy
gap is important in controlling the number of molecules which can be chemisorbed in
the course of a catalytic reaction and the nature of the chemical bond formed between
the molecule and the surface. These factors control at the same time the activity and
selectivity of the catalyst as well as mechanism of the catalytic reaction.
Catalytic Activity
The catalytic decomposition of isopropanol over pure Fe2O3, Co3O4 and their mixtures
calcined at 600 °C for 3 h was studied at 325 °C. The experimental conditions were
W/F = 1.87 g cat mol/l h, 1.5% isopropanol, total flow rate = 150 ml/min NTP N2 gas,
and the steady state was established after 1 h. The conversion, yield and selectivity of
the alcohol dehydration–dehydrogenation were calculated according to Agrawal et
FIG. 5
Variation of log σ/σ0 with time for iron oxide (Fe2O3) (1), cobalt oxide (Co3O4) (13) and their mix-
tures containing Co3O4 at ratios x = 0.2 (2), 0.4 (3), 0.6 (4), 0.8 (5), 1.0 (6), 1.5 (7), 2.0 (8), 2.2 (9),
2.4 (10), 2.6 (11) and 2.8 (12) calcined at 600 °C for 3 h during contact isopropanol at 325 °C
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1948
Abd El-Salaam, Said, El-Awad, Hassan,Abd El-Wahab:
al.34. The results are shown in Fig. 6. It was found that acetone and propylene are the
major products. The results can be divided into three groups: (i) when Fe2O3 contains
Co2+ up to x = 0.8, the conversion and yields of acetone and propylene all increase. This
increase, as already mentioned, is due to formation of the inverse spinel. The spinel35,36
with x < 1 has the cation distribution, [Fe3+]t [Fe2+Co2+Fe3+]oO4. Therefore, the existence of
a redox couple in the inverse spinel within the octahedron is possible and facilitates
chemisorption of isopropanol, and eventually also its decomposition. (ii) The great de-
crease in the conversion and yield of acetone on increasing Co2+ content up to x = 1.0
may be attributed to formation of the normal spinel CoFe2O4, accompanied with little
electron exchange within its structure as [Fe3+]t[Co2+Fe3+]oO4. It is of interest that the
great decrease in isopropanol conversion is accompanied by the great increase in
the selectivity of this spinel with respect to propylene formation. This suggests that the
reaction on CoFe2O4 proceeds mainly as dehydration. (iii) A steady increase in the
conversion and yield of acetone above minimum on increasing Co2+ content up to x = 2.6
may be attributed to the conversion of the normal spinel into the inverse one. For x > 1 the
chemical composition35,36 of ferrites can be written as [Fe3+]t[Co2+Co3+Fe3+]oO4. Con-
ductivity of these samples in the absence of isopropanol (Fig. 4, curve 1) indicates a
continuous increase of their conductance. The increase in the conductivity of ferrite
caused by replacement of Fe2+ ions in the octahedron with Co2+ cations in the spinel
ferrite lattice can be attributed to the distribution of the entering cations over different
sites in the spinel structure. Similar cation distribution in which Co2+, Co3+ and Fe2+
ions occupy octahedral sites is also proposed37 for the higher Co/Fe ratios, such as
[Fe3+]t[Co2+Co3+Fe2+]oO4.
FIG. 6
Variation of isopropanol conversion (❍), acetone yield (∆), propylene yield (❍) and selectivity to
acetone (❍) and to propylene (❍) (all in %) with the catalyst composition (Fe2O3–Co3O4 system cal-
cined at 600 °C for 3 h, the reaction carried out at 325 °C)
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Catalytic Decomposition of Isopropanol
1949
Such a distribution may lead to changes in both concentration and mobility of charge
carriers in the ferrite lattice. Finally, the decrease in the conversion and acetone yield
for the catalysts with x above 2.6 may be related to the activity of the cobalt oxide.
XRD measurements indicated that the formation of the ferrite spinel decreases on in-
creasing Co2+ content and disappears at x = 2.6. On the other hand, comparison of the
activity and selectivity of the catalysts with conduction activation energies shows that
on decreasing Eσ, the conversion, yield and selectivity towards acetone formation in-
creases and vice versa. The decrease in Eσ should thus facilitate electron exchange
between isopropanol and catalyst surface during the reaction.
In conclusion, the above results demonstrate that the dehydrogenation of isopropanol
proceeds easily on the inverse spinel catalysts. The redox couple so formed is respon-
sible for Eσ decrease, enhancing thus electron exchange between isopropanol and cata-
lyst surface. Moreover, the inverse spinel in the iron-rich region is more active and
selective with respect to acetone formation. On the other hand, the samples containing
normal spinel exhibit low activity and selectivity in isopropanol decomposition.
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Translation revised by J. Hetflejs.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)