Oxygen-Nonstoichiometric YBaCo4O7+δ as a Catalyst in H2O2 Oxidation of Cyclohexene

Article abstract of DOI:10.1007/s10562-014-1408-0

Here we present oxygen-nonstoichiometric transition metal oxides as highly prominent candidates to catalyze the industrially important oxidation reactions of hydrocarbons when hydrogen peroxide is employed as an environmentally benign oxidant. The proof-of-concept data are revealed for the complex cobalt oxide, YBaCo₄O_7+δ (0 < δ < 1.5), in the oxidation process of cyclohexene. In the 2-h reaction experiments YBaCo₄O_7+δ was found to be significantly more active (>60 % conversion) than the commercial TiO₂ catalyst (<20 %) even though its surface area was less than one tenth of that of TiO₂. In the 7-h experiments with YBaCo₄O_7+δ, 100 % conversion of cyclohexene was achieved. Immersion calorimetry measurements showed that the high catalytic activity may be ascribed to the exceptional ability of YBaCo₄O_7+δ to dissociate H₂O₂ and release active oxygen to the oxidation reaction. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1408-0

Catal Lett (2015) 145:576–582
DOI 10.1007/s10562-014-1408-0
Oxygen-Nonstoichiometric YBaCo4O7+d as a Catalyst in H2O2
Oxidation of Cyclohexene
•
Outi Parkkima Ana Silvestre-Albero
•
•
Joaquin Silvestre-Albero Maarit Karppinen
Received: 20 September 2014 / Accepted: 21 October 2014 / Published online: 31 October 2014
Ó Springer Science+Business Media New York 2014
Abstract Here we present oxygen-nonstoichiometric tran-
sition metal oxides as highly prominent candidates to catalyze
the industrially importantoxidation reactions ofhydrocarbons
when hydrogen peroxide is employed as an environmentally
benignoxidant. The proof-of-concept dataare revealed for the
complex cobalt oxide, YBaCo4O7?d (0\ d \ 1.5), in the
oxidation process of cyclohexene. In the 2-h reaction exper-
iments YBaCo4O7?d was found to be significantly more
active ([60 % conversion) than the commercial TiO2 catalyst
(\20 %) even though its surface area was less than one tenth
chemistry (e.g. removal of VOCs) and fine chemical
industry [1–3]. These oxidation processes involve the con-
version of the corresponding chemicals into intermediates
(e.g. epoxides, alcohols, etc.) and/or into carbon dioxide
and water as a final product. In particular—from the eco-
nomical point of view—partial oxidation of alkenes to
produce epoxides is one of the momentous industrial reac-
tions since epoxides are synthetic precursors for numerous
fine chemicals and polymer syntheses [4], e.g. production of
propylene oxide via epoxidation of propylene using tert-
butyl hydroperoxide [5]. The epoxide intermediates are
catalytically produced from the corresponding alkenes via
oxygen transfer reactions using a peroxy acid as an oxidant
and a transition metal compound as a catalyst [6, 7]. The
role of the active metal in such oxidation reactions is to
activate the oxygen–oxygen bond in the peroxo acid
(g2-coordinated peroxide) in order to supply electrophilic
oxygen able to react with the alkene molecule [6, 8]. Among
the different transition metal catalysts, Ti-based systems
have been the most widely investigated. Besides the Shell
catalyst based on Ti(IV)-SiO2, there have been numerous
efforts to design variously substituted Ti(IV)-framework
materials, e.g. TS-1, Ti-MCM41, etc., with excellent cata-
lytic performance in the aforementioned peroxo-acid oxi-
dized reactions [9–12]. X-ray absorption spectroscopy
analysis has revealed that four-coordinated Ti(IV) centers
indeed are the active sites for the oxidation reactions [13].
While peroxo acids are by far the most commonly
employed oxidants in the epoxidation reactions, there are
certain drawbacks commonly associated with their use
such as a low reaction rate and the large amounts of these
acids typically remaining as an unwanted residue in the
product. Hence, alternative strong and green oxidants such
as hydrogen peroxide are urgently searched for. Hydrogen
peroxide is easy to handle and it yields an environmentally
of that of TiO2. In the 7-h experiments with YBaCo4O7?d
,
100 % conversion of cyclohexene was achieved. Immersion
calorimetry measurements showed that the high catalytic
activity may be ascribed to the exceptional ability of
YBaCo4O7?d to dissociate H2O2 and release active oxygen to
the oxidation reaction.
Keywords Nonstoichiometric oxide Á Catalyst Á
Hydrocarbon oxidation Á H2O2 oxidant
1 Introduction
Catalytic oxidation of alcohols and hydrocarbons is an
important industrial process in both environmental
O. Parkkima Á M. Karppinen (&)
Laboratory of Inorganic Chemistry, Department of Chemistry,
Aalto University, 00076 Aalto, Finland
e-mail: maarit.karppinen@aalto.fi
A. Silvestre-Albero Á J. Silvestre-Albero (&)
Laboratorio de Materiales Avanzados, Departamento de
´
´
Quımica Inorganica-Instituto Universitario de Materiales,
Universidad de Alicante, Ap. 99, 03080 Alicante, Spain
e-mail: joaquin.silvestre@ua.es
123

Oxygen-Nonstoichiometric YBaCo4O7?d as a Catalyst
577
benign by-product, i.e. water. Unfortunately, aqueous H2O2
exhibits a poor catalytic performance on Ti(IV) centers
supported on mesoporous silica due to the competitive
adsorption between H2O2 and H2O on the hydrophilic
Ti(IV) sites. Consequently, new catalytic systems are
desired which would efficiently decompose H2O2 and yield
active electrophilic oxygen.
Y
Ba
Co
O
Besides titanium, other transition metals such as cobalt
have also been investigated as potential catalysts in the
oxidation of alkenes [14–16]. Interestingly, partial substi-
tution of tetrahedrally-coordinated Al(III) and P(V) in
aluminophosphates by Co(III) and Ti(IV) resulted in
excellent catalysts for the oxidation of olefins when acetyl
peroxyborate was used as the oxidant [15]. The redox-
active cobalt centers apparently provided the location for
the initiation and generation of free-radical intermediates
for the catalytic oxidation. Unfortunately, these Co-based
catalysts were found inactive when H2O2 was used as the
oxidant [16].
Fig. 1 Schematic illustration of the crystal structure of YBaCo4O7?d
will report our highly promising results on the catalytic
performance of YBaCo4O7?d in the oxidation of cyclo-
hexene in combination with H2O2 as the green oxidant. The
performance is discussed in comparison to those of a
simple binary cobalt oxide powder and commercial
titanium oxide (P25) powder.
In recent years several families of oxygen-nonstoichio-
metric transition metal oxide materials have been inten-
sively investigated—largely due to their intrinsic redox
properties—for a variety of modern materials technologies
ranging from superconducting, thermo-electric, ferroelec-
tric and magnetic devices to batteries, fuel cells, sensors
and oxygen-storage devices. These materials have however
remained rather unexplored in catalysis. Cerium oxide is
the most widely-used oxygen-storage material; it is also
known to be an active catalyst in various reactions [17].
Recently it was also shown to be active in epoxidation
reactions [18]. Here our hypothesis is that in addition to
ceria, other oxygen-nonstoichiometric and thereby redox-
active transition metal oxides would be highly potential
material candidates for next-generation catalyst systems
involving the catalytic oxidation of hydrocarbons with
H2O2. As the first candidate material to be investigated we
selected the complex cobalt oxide, YBaCo4O7?d
(0 \ d \ 1.5). It has a significantly larger oxygen-storage
capacity (2,600 lmolO/g [19]) than the best ceria deriva-
tive CeO2–ZrO2 (1,500 lmolO/g [20]). In addition,
YBaCo4O7?d releases all the stored oxygen in air at
appreciably low temperatures below 400 °C whereas ceria
requires a harsh reducing atmosphere and a much higher
temperature for the oxygen release. As shown in Fig. 1, the
crystal structure of YBaCo4O7?d contains corner-sharing
CoO4 tetrahedra of mixed-valent Co(II/III) [21]. Despite its
unique properties—the large low-temperature oxygen
mobility, the wide range of oxygen-nonstoichiometry and
the presence of tetrahedrally coordinated Co(II/III) centers
[19, 22–26]—and the fact that it has already been consid-
ered as a promising oxygen buffer in various other appli-
cations (for a review, see [27]), the YBaCo4O7?d phase has
not yet been exploited in heterogeneous catalysis. Here we
2 Experimental
2.1 Catalyst Sample Preparation
The YBaCo4O7?d sample was prepared through a solid
state synthesis route from appropriate amounts of the
starting material powders, Y2O3 (99.9 %), BaCO3 ([99 %)
and Co3O4 (99.7 %), thoroughly ground with ethanol and
calcined in air at 900 °C for 12 h. The calcined sample
powder was pressed into a pellet of *1 g and fired in air at
1,150 °C for 30 h, followed by furnace-cooling down to
room temperature. After the synthesis the sample was
pulverized, and the oxygen content of the powder was
adjusted to d & 0 by annealing it in a tube furnace in an Ar
gas flow at 500 °C for 10 h after which the furnace was
slowly cooled down.
Additionally a simple binary cobalt oxide sample was
prepared for reference by heat-treating 1 g of commercial
cobalt oxide (Co3O4, 99.7 %, Merck) at 1,150 °C in air for
15 h. For the sake of comparison the catalytic activity of a
commercial titanium oxide material (P25, Aldrich) was
investigated too.
2.2 Basic Characterization of Sample Powders
Phase purity/composition of the samples was confirmed by
´
X-ray powder diffraction (XRD; Panalytical XPert Pro
MPD, CuKa1 radiation, PixCel detector) measurements.
Specific surface area was determined for the sample pow-
ders through the BET equation from the N2 adsorption data
obtained at -196 °C using an in-house designed and
developed volumetric equipment. Before the adsorption
experiments the samples were degassed at 250 °C for 4 h.
123

578
O. Parkkima et al.
The precise oxygen content of the YBaCo4O7?d powder
CoO
was analyzed by iodometric titration [28]. About 20 mg of
finely ground sample powder was dissolved in oxygen-free
1 M HCl in an air-tight cell under constant N2 gas
(99.999 %) bubbling, aided with a significant excess of
potassium iodide (ca. 1.5 g). The iodine formed in the
redox reaction between trivalent cobalt and iodide was
titrated with standard 0.010 M Na2SO3 solution (Merck) in
the presence of starch indicator. The experiment was
repeated three times with an excellent reproducibility. The
oxygen content of the cobalt oxide reference powder could
not be determined by titration because the material was
insoluble in 1 M HCl. For this reason, the precise stoi-
chiometry of our binary cobalt oxide sample was investi-
gated by thermogravimetric (TG; Pyris 1 TGA) reduction.
A sample specimen of ca. 20 mg was heated to 800 °C
with a heating rate of 5 °C/min in an H2 (5 %)/Ar gas flow.
During this reductive TG run the oxide is reduced to Co
metal, and the initial amount of oxygen in the sample can
be calculated from the overall weight loss. The same
thermobalance was used to confirm the oxygen absorption/
desorption characteristics of the YBaCo4O7.05 sample
powder in O2 gas flow. In these experiments a sample
specimen of *20 mg was slowly (1 °C/min) heated from
room temperature to 500 °C in an O2 gas flow (40 ml/min).
The chemical state of the catalyst surface was investigated
by X-ray photoelectron spectroscopy (XPS; VG-Microtech
Multilab 3000 spectrometer, hemispherical electron ana-
lyzer, Mg–Ka (h = 1,253.6 eV; 1 eV = 1.6302 9 10-19 J)
300-W X-ray source). The sample powder was pressed into a
small Inox cylinder and mounted on a sample rod placed in a
pretreatment chamber before being transferred to the ana-
lysis chamber. Before recording the spectrum, the sample
was maintained in the analysis chamber until a residual
pressure of ca. 5 9 10-7 N/m2 was reached. The spectra
were collected at pass energy of 50 eV. All binding energies
were referenced to the C 1s line at 284.6 eV, which provided
binding energy values with an accuracy of 0.2 eV.
Co3O4
YBaCo4O7+δ
20
30
40
50
60
70
2θ
Fig. 2 XRD patterns for the YBaCo4O7?d sample and the
cobalt-oxide reference powder
gas-chromatography (Shimadzu GC-2010, flame ionization
detector FID) and a capillary column HP-5. Blank exper-
iments were performed using the same experimental con-
ditions without a catalyst.
To understand the nature of the catalytic reaction,
immersion calorimetry measurements into H2O2 (30 wt%
in water, Aldrich) were performed in a Setaram Tian-
Calvet C80D calorimeter working at 30 °C. Prior to the
experiment, the sample specimen was degassed in glass-
made vacuum equipment down to 10-3 Pa at 250 °C for
4 h. After degassing, the glass bulb containing the sample
was sealed into vacuum and inserted into the calorimetric
chamber containing the immersion liquid. Once thermal
equilibrium is reached, the brittle tip of the glass bulb is
broken and the heat of interaction is recorded as a function
of time. The total enthalpy of immersion (-DHimm) is
obtained by integration of the signal, after appropriate
corrections for the breaking of the tip (exothermic) and the
heat of evaporation of the immersion liquid necessary to fill
the empty volume of the bulb with the vapor at the cor-
responding vapor pressure (endothermic).
2.3 Catalytic Evaluation
Catalytic studies were performed using cyclohexene
(99 %, Aldrich) as the substrate and hydrogen peroxide as
the primary oxidant. In a typical reaction, 2 mmol of
cyclohexene, 2 mmol of H2O2 (30 wt% in water, Aldrich),
40 mg of the catalyst powder, 0.25 ml of octane as an
internal standard and 1-propanol as a solvent up to 5.8 ml
final volume were added to a 50 ml round-bottom flask
fitted with a reflux condenser. The reaction mixture was
stirred at 70 °C for 2 h (in some experiments also for a
longer period of time). The reaction was terminated by
quenching with water. The catalyst was separated by fil-
tration and the reaction products were analyzed by on-line
3 Results and Discussion
3.1 Chemical Characteristics of the Catalyst Powders
Figure 2 shows XRD patterns for the two cobalt-oxide
powders investigated. The binary cobalt oxide reference
sample is a mixture of two phases, CoO as the main phase
and Co3O4 with a lesser amount [29, 30]. The average
oxygen content of the sample was determined to be
CoO1.04 on the basis of TG analysis. For the YBaCo4O7?d
sample, XRD confirms the presence of a single phase. The
lattice parameters were readily refined from the XRD
123

Oxygen-Nonstoichiometric YBaCo4O7?d as a Catalyst
579
1.4
1.2
YBaCo4O7+δ
1.0
0.8
0.6
0.4
0.2
CoO1+δ
0.0
100 150 200 250 300 350 400 450 500
534
532
530
528
526
Temperature (oC)
Binding Energy (eV)
Fig. 3 TG curves for the YBaCo4O7?d sample and the cobalt-oxide
reference powder recorded in an O2 gas flow (heating rate 1 °C/min)
Fig. 4 O 1s core level spectra for YBaCo4O7.05. The measured data
are presented with the thick black line and the convoluted peaks are
presented with thinner lines
˚
pattern in space group P63mc, with a = 6.3033 A and
˚
c = 10.2414 A, as expected from our previous work [19].
hydroxide [34]. The smallest peak at the highest energy
(which could probably be convoluted into several smaller
peaks) is most likely due to traces of organic compounds
(alcohols, esters etc.) or adsorbed water [34]. The binding
energy values are somewhat lower than those of the binary
Co(II,III) oxide Co3O4 (530.0–530.8–532.7) [33], which is
consistent with the fact that the other cation species (Y and
Ba) in this lattice are much more electropositive than
cobalt. Corresponding values for lattice O 1s in Y2O3 and
BaO are 529.0 eV [35] and 528.2 eV [36], respectively.
Actually, for mixed oxides containing different metal
species somewhat average binding energy values are
expected from those for the individual binary oxides [36].
The XPS spectrum for Y 3d was found to have two
peaks corresponding to the spin orbital splitting of 3d5/2
(156.7 eV) and 3d3/2 (158.7 eV): the binding energy values
are similar to those reported for Y(III) [37]. For samples
containing both Co and Ba, XPS is not the optimal analysis
tool due to the overlap of Co 2p and Ba 3d spectra in the
energy range of 770–800 eV [38, 39]. Luckily, from the
oxygen content analysis we know that in the YBaCo4O7.05
sample and also in the CoO1.04 reference cobalt exists with
a mixed II/III valence.
Iodometric titration experiments gave the oxygen content
for the YBaCo4O7?d sample at d = 0.05. Hence, in both of
our cobalt-oxide samples cobalt exists with a mixed II/III
valence.
Figure 3 shows the TG curves recorded for the YBaCo4
O7.05 sample and also for the CoO1.04 reference upon
heating in an O2 gas flow up to 500 °C. For the reference
cobalt oxide no oxygen absorption/desorption is seen in
this temperature range. The oxygen content of YBaCo4
O7.05 also remains unchanged upon heating up to ca.
200 °C, but above this temperature the material exhibits a
sudden weight increase, indicating a substantial increase in
the oxygen content up to d & 1.4, followed by a weight/
oxygen loss upon further heating around 400 °C. Hence
our YBaCo4O7?d sample indeed possesses the remarkable
oxygen absorption/desorption or redox capability as
expected from previous works [19, 22–27].
The surface area of the YBaCo4O7.05 sample powder
was determined by the BET method to be very low
(*2 m2/g) as the sample was synthesized through
solid-state synthesis at the relatively high temperature of
1150 8C. For comparison, the surface area of the com-
mercial TiO2 reference powder is *50 m2/g.
3.2 Catalytic Activity
The chemical nature of the surface species in the
YBaCo4O7.05 sample was evaluated by XPS; the O 1s core
level spectrum is presented in Fig. 4. There are three
spectral components found at 529.4 eV (33 % of total O),
531.4 eV (59 %) and 532.7 eV (6.6 %). The pattern cor-
responds to a typical transition metal oxide surface like that
of NiO [31], Cr2O3 [32] or Co3O4 [33]. Only the first peak
at 529.4 eV is probably due to the lattice oxygen. The
major peak in the middle has been proposed to be due to
defective sites within the crystal, adsorbed oxygen or
The catalytic behavior was studied for the two cobalt-oxide
powders, YBaCo4O7.05 and CoO1.04, in the oxidation pro-
cess of cyclohexene with the H2O2 oxidant. The epoxida-
tion of cyclohexene is a well-known reaction with two
different reaction mechanisms, the radical route giving rise
to 2-cyclohexen-1-ol and cyclohexene oxide via allylic
oxidation through cyclohexenyl hydroperoxide, and the
direct oxidation pathway to cyclohexene oxide and
1,2-cyclohexane-diol, after a subsequent ring-opening by
123

580
O. Parkkima et al.
solution and the decomposition of H2O2. Apparently,
allylic oxidation and ring-opening of the epoxide, the two
major side reactions in the epoxidation of cyclohexene that
are responsible for the low selectivity to the epoxide, are
inhibited in the presence of the YBaCo4O7?d catalyst.
To evaluate the catalytic performance of the different
catalysts, Fig. 5a compares the catalytic conversion for the
cyclohexene oxidation at 70 °C after a reaction time of 2 h.
As it can be observed, not only the conventional transition-
metal catalyst TiO2 but also the binary cobalt-oxide
CoO1.04 exhibit a poor catalytic conversion efficiency (not
much enhanced compared to the blank experiment). This is
most likely due to the difficulties of these systems to
activate/dissociate the O–O bond in H2O2, like also
observed earlier with similar binary oxides [16]. Quite
remarkably, for our complex cobalt oxide, YBaCo4O7?d, a
nearly three-fold increase in the catalytic conversion effi-
ciency from 20 % to nearly 60 % was achieved. Moreover,
as it can be observed in Fig. 5b, completely conversion of
cyclohexene was achieved for YBaCo4O7?d in 7 h. From
these results it can be envisaged that YBaCo4O7?d is an
extraordinarily active catalyst for cyclohexene oxidation,
and apparently even when H2O2 is used as the oxidant,
with comparable values in terms of conversion and selec-
tivity towards the epoxide to the best catalysts reported in
the literature for transition metal based catalysts [40–42].
To further confirm the exceptional behavior of YBaCo4
O7?d as an oxidation catalyst, and more specifically in the
activation of the H2O2 molecule, immersion calorimetry
measurements were performed. Immersion calorimetry is a
powerful technique to estimate the enthalpy of immersion
of a liquid molecule on a solid surface; it depends on the
surface area available to the liquid, the porosity of the solid
and the specific interactions of the liquid molecule with the
solid surface [43]. Taking into account that in heteroge-
neous catalysis the degree of the interaction between the
reactants and the active sites determines the catalytic
behavior, e.g. the activation of the oxidant [44], immersion
Table 1 Product distribution for the catalytic oxidation of
cyclohexene
Sample
Product distribution %
Epoxide
Ketone
Ether
Alcohol
Diol
YBaCo4O7?d
P25
71.1
26.7
0
5.0
1.3
0
10.9
27.1
32.2
0
12.9
42.9
40.7
1.9
27.1
CoO1?d
H2O present in the reaction media. For both the cobalt-
oxide catalysts, the main reaction products were 2-cy-
clohexen-1-ol (alcohol), 2-cyclohexene-1-one (ketone),
1,2-cyclohexane-diol (diol) and its corresponding mono
ether (ether), and cyclohexene oxide (epoxide), although
the relative amounts of these components somewhat varied
depending on the catalyst material and the catalytic con-
ditions (reaction temperature and time). In Table 1, we
summarize the composition of the reaction product for
selected representative experiments.
The product distribution obtained for the different cat-
alysts evaluated in the oxidation of cyclohexene suggests
that the direct epoxidation mechanism must prevail over
the radical mechanism, except for the CoO1?d catalyst
where the large proportion of 2-cyclohexen-1-ol suggests
some contribution from the radical mechanism. Concerning
the selectivity to the most valuable reaction product, the
epoxide, YBaCo4O7?d catalyst exhibits an exceptional
behavior with a selectivity value as high as 71 %. This
selectivity achieved using 1-propanol as a solvent and
H2O2 as an oxidant highly overpasses the selectivity for the
reference sample, P25, with a value around 25 % and that
of CoO1?d with a value close to 0. The poor selectivity of
the last two catalysts must be attributed to their high ability
to hydrolyze the epoxide into the diol (high selectivity to
1,2-cyclohexane-diol), a process assisted by H2O intro-
duced into the reaction media by aqueous 30 % H2O2
60
100
(a)
(b)
50
80
60
40
20
0
1:1
40
30
20
10
0
Blank
TiO2
YBaCo4O7+δ
CoO1.04
1
2
3
4
5
6
7
Time(h)
Fig. 5 Cyclohexene conversion a for the different catalysts evaluated at 70 °C and after 2-h reaction time, and b for the YBaCo4O7?d catalyst
with increasing reaction time
123

Oxygen-Nonstoichiometric YBaCo4O7?d as a Catalyst
581
catalytic activity was achieved with YBaCo4O7?d for the
oxidation process of cyclohexene with H2O2. In the 2-h
reaction experiments it was three times more active (60 %
conversion) than TiO2 (\20 %), despite its much smaller
surface area, and in the longer experiments essentially
complete conversion of cyclohexene to reaction products
was achieved with a high selectivity to the epoxide.
Immersion calorimetry measurements showed that the high
catalytic activity may be ascribed to the exceptional ability
of YBaCo4O7?d to activate H2O2 such that electrophilic
oxygen is efficiently released; this active oxygen then
provides the excellent oxidation capacity for hydrocarbons.
Most importantly, we foresee that our concept to employ
an oxygen-nonstoichiometric redox-active transition-metal
oxide as a catalyst in the environmentally benign oxidation
reactions of hydrocarbons with hydrogen peroxide should
be applicable to a range of nonstoichiometric transition
metal oxide materials, thus opening new horizons in the
science and technology of catalytic hydrocarbon oxidation
processes.
Table 2 Enthalpy of
immersion for H2O2 on the
different oxide catalysts
evaluated
Sample
2DHimm (J/g)
TiO2
17
CoO1.04
213
YBaCo4O7.05
[3137
calorimetry measurements into H2O2 were considered
highly useful.
Our immersion calorimetric measurement data summa-
rized in Table 2 show that commercial TiO2 exhibits a
small enthalpy of immersion into H2O2, thus suggesting a
rather physisorption phenomena without specific liquid–
solid interactions. The binary cobalt-oxide powder syn-
thesized as a reference material exhibits a little higher
enthalpy of immersion, thus reflecting the presence of
specific interactions between H2O2 and the Co(II/III) spe-
cies, in close agreement with the better catalytic behavior
of Co versus Ti in oxidation reactions when using H2O2 as
an oxidant. Most interestingly, for the complex cobalt
oxide YBaCo4O7?d the enthalpy of immersion exhibits a
dramatic increase above the detection limit of the system.
This extremely high enthalpy value clearly reflects the
presence of associated reaction processes during the calo-
rimetric analysis. At this point it is also important to
mention that hydrogen peroxide decomposition measure-
ments performed outside the calorimetric chamber con-
firmed the aforementioned findings: whereas the CoO1.04
oxide produced essentially no reaction in the presence of
hydrogen peroxide, incorporation of a small amount of
YBaCo4O7?d resulted in a vigorous exothermic reaction
with a large oxygen evolution. After the reaction, the
YBaCo4O7?d powder was dried and measured with XRD
to verify that it had not decomposed in the process.
Moreover, the same catalyst powder sample was also
repeatedly made in contact with a fresh batch of H2O2
which resulted in a similar formation of gas and heat as in
the initial experiment; this confirmed the reusability of
YBaCo4O7?d as a catalyst in the oxidation process.
Acknowledgments This work was supported by Academy of
Finland (No. 255562) and Generalitat Valenciana (PROMETEO/
2009/002).
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We have successfully demonstrated that the complex
cobalt oxide, YBaCo4O7?d, that is currently strongly
emerging as a low-temperature oxygen-storage material
works also as an exceptionally efficient catalyst in the
oxidation of cyclohexene when hydrogen peroxide is
employed as the oxidant. There would be several advan-
tages related with environmental concerns if the commonly
used peroxo acid oxidants could be replaced by the green
hydrogen peroxide. Our results revealed that compared to
e.g. the commercial TiO2 catalyst, remarkably enhanced
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