Cobalt ferrite nanoparticles for the preferential oxidation of CO

Article abstract of DOI:10.1016/j.apcata.2016.03.024

In the present study cobalt ferrite (CoFe₂O₄) nanoparticles have been successfully prepared through the glucose-assisted hydrothermal method. We verified that the use of glucose is highly advantageous for the synthesis in order to prevent particle agglomerates as well as to obtain single-phase CoFe₂O₄ nanoparticles. The H₂-TPR results showed that the CoFe₂O₄ indicates the reduction of CoO to Co° and of Fe₂O₃ to Fe₃O₄ followed by the reduction of Fe₃O₄ to metallic iron Fe°. XPS observations evidenced the presence of Fe³⁺ and Co²⁺ and practically exclude the existence of Fe²⁺ and Co³⁺ species at the surface. In situ DRIFTS-MS showed formate species at 100 °C which increased with increasing temperature due to the reaction between CO gas and hydroxyl groups of cobalt ferrite. The as-prepared material exhibited high CO conversion and good stability on stream at 250 °C during 30 h. In addition, it has been observed that the cobalt ferrite catalyst is inhibited by the presence of H₂O and CO₂, although a totally reversible process, but have no influence on the chemical structure of the surface active sites.

Full text of DOI:10.1016/j.apcata.2016.03.024

Accepted Manuscript
Title: Cobalt ferrite nanoparticles for the preferential oxidation
of CO
Author: Carlos Alberto Chagas Eugenio F. de Souza Marta
C.N.A. de Carvalho Ruth L. Martins Martin Schmal
PII:
S0926-860X(16)30147-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.03.024
APCATA 15816
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29-1-2016
16-3-2016
20-3-2016
Please cite this article as: Carlos Alberto Chagas, Eugenio F.de Souza,
Marta C.N.A.de Carvalho, Ruth L.Martins, Martin Schmal, Cobalt ferrite
nanoparticles for the preferential oxidation of CO, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.03.024
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Highlights
•
•
•
•
•
Cobalt ferrite prepared by glucose-assisted hydrothermal method
Cobalt ferrite nanostructure
High catalytic activity on deep CO removal in hydrogen-rich stream
Good stability in CO-PROX reaction during 30 h
Promising potential in further application in PEMFCs
1

Graphical Abstract
Catalytic performance of the cobalt ferrite for CO-PROX in hydrogen-rich stream.
100 CO(g) + O2(g) + H2(g)
CO2(g) + H2(g)
80
60
CoFe2O4
T50%CO
40
155
20
0
50 75 100 125 150 175 200 225 250
Temperature (°C)
2

Cobalt ferrite nanoparticles for the preferential
oxidation of CO
Carlos Alberto Chagas, Eugenio F. de Souza, Marta C. N. A. de Carvalho, Ruth L.
Martins, Martin Schmal*
Federal University of Rio de Janeiro - Núcleo de Catálise, Programa de Engenharia
Química, COPPE, Rio de Janeiro/RJ, Brazil.
*Email: schmal@peq.coppe.ufrj.br
3

Abstract
In the present study cobalt ferrite (CoFe2O4) nanoparticles have been successfully
prepared through the glucose-assisted hydrothermal method. We verified that the use of
glucose is highly advantageous for the synthesis in order to prevent particle
agglomerates as well as to obtain single-phase CoFe2O4 nanoparticles. The H2-TPR
results showed that the CoFe2O4 indicates the reduction of CoO to Co0 and of Fe2O3 to
Fe3O4 followed by the reduction of Fe3O4 to metallic iron Fe0. XPS observations
evidenced the presence of Fe3+ and Co2+ and practically exclude the existence of Fe2+
and Co3+ species at the surface. In situ DRIFTS-MS showed formate species at 100 °C
which increased with increasing temperature due to the reaction between CO gas and
hydroxyl groups of cobalt ferrite. The as-prepared material exhibited high CO
conversion and good stability on stream at 250 °C during 30 h. In addition, it has been
observed that the cobalt ferrite catalyst is inhibited by the presence of H2O and CO2,
although a totally reversible process, but have no influence on the chemical structure of
the surface active sites.
Keywords: Cobalt ferrite, Stability, CO-PROX, Hydrogen, Fuel cells
4

1. Introduction
Due to the ever-growing importance of technologies involving energy storage
and conversion, many efforts have been focused on the development of more efficient
fuel cells (FC’s). Among the most important FC’s, proton exchange fuel cells (PEMFC)
represent an interesting alternative for power generation in mobile and stationary
applications, combining high power density, efficiency and environmentally
friendliness [1,2]. However, it has been demonstrated that the ideal fuel for PEMFC
usually requires, besides elevated hydrogen (H2) concentrations, extremely low amounts
of carbon monoxide (CO), which in turn represents one of the major challenges of the
field [3].
The conversion of CO to CO2 also known as preferential (or selective) oxidation
(CO-PROX) is generally performed by supported noble-metal catalysts. As an example,
Au-based catalysts are commonly employed in CO-PROX reactions because of its
ability to oxidize CO even at ambient temperatures, especially when dispersed as
nanoparticles on oxide supports [4,5]. Catalysts of the Pt-group also present good
activity for CO removal and relatively lower H2 consumption, besides presenting an
good tolerance to carbon deposition [6,7]. Despite these facts, the relative limited
resources associated with high costs has impeded the widespread use of noble metal
catalysts in large scale [8]. For that reason, the search for low cost and well-performing
alternative catalysts for CO-PROX reactions is also an important area of research in
energy technology.
The most important requirements for an ideal CO-PROX catalyst are: (i) high
CO oxidation rates at low temperatures; (ii) high selectivity to remove even traces of
CO from the H2-rich gas mixture, but with no H2 consumption and; (iii) tolerance to
5

CO2 and H2O possibly present in the feed stream [9]. Accordingly, some transition
metal oxides (TMO) have received considerable attention in recent years because they
were found to be active in a great number of important reactions, including CO
oxidation in H2-rich stream [1]. Moreover, TMOs are known for being much less costly
than the noble metal-based catalysts, being therefore very interesting alternatives for
CO-PROX reaction.
Particularly, cobalt ferrite (CoFe2O4) has been attracting attention because of its
interesting electronic, magnetic, optic and catalytic properties [10]. However, it is
known that these properties usually depend on the method employed in the preparation
of the material [11]. Consequently, a number of different synthesis methods have been
proposed for preparing CoFe2O4, including: ceramic [12], co-precipitation [13],
hydrothermal [14], thermal decomposition [15], sol-gel [16] and combustion methods
[17]. Among them, the hydrothermal method has afforded excellent results in the
synthesis of transition metal oxide nanoparticles [14]. For example, Meng et al. [18]
synthesized CoFe2O4 as hollow spheres through the hydrothermal treatment of an
aqueous solution containing glucose, ammonium iron (II) sulfate hexahydrate and
cobalt (II) sulfate heptahydrate for applications as high-density magnetic recording
materials and drug carriers. This method involves the formation of a carbon sphere with
metal ions incorporated into the hydrophilic shell with subsequent removal of the
carbon cores via calcination.
In the present work, cobalt ferrite nanoparticles have been prepared via glucose-
assisted hydrothermal method and characterized by a number of experimental
techniques, including XRF, XRD, BET, MEV and TPR. Then the catalytic activity and
the stability of the CoFe2O4 nanoparticles were determined for the CO-PROX reaction
using various feed components. Furthermore, the surface species formed under reaction
conditions were also examined using in situ DRIFTS spectroscopy and on line MS gas
analyses.
2. Experimental
Synthesis of the cobalt ferrite
6

CoFe2O4 nanoparticles were prepared by hydrothermal process [14]. In a typical
procedure, appropriate amounts of Fe(NO3)3.6H2O and Co(NO3)36H2O were
individually dissolved in deionized water until the formation of clear solutions and then
mixed together. The amount of nitrate was calculated in order to achieve the same
Co:Fe atomic ratio present in CoFe2O4. The resulting solution was mixed drop wise
with an aqueous solution of glucose (considering a glucose/metal ratio of 5:1) in order
to prevent agglomeration. The resulting mixture was placed in a Telfon-linked autoclave
and maintained at 180 °C for 24 h. Then, the obtained powder was filtered, washed
several times with distilled water and ethanol and finally dried in a vacuum oven at
60 °C for 5 h. Subsequently, the metal oxide-carbon composite was calcined in air at
550 °C (heating rate of 4 °C min-1) for 4 h in order to remove the carbon core, leading to
the formation of cobalt ferrite nanoparticles.
Characterization techniques
The elemental composition of the as-prepared cobalt ferrite nanoparticles was
determined by X-ray fluorescence (XFR) using a Rigaku spectrometer RIX 3100 with
Rh tube as source of radiation. The measurement was performed onto pressed pellets
containing 300 mg of sample. BET surface area measurements were carried out at
-196 °C using a Micromeritics ASAP2010 instrument. Before the BET analysis, the
samples were degassed at 300 °C overnight under vacuum. Powder X-ray diffraction
(XRD) experiments were performed using a Rigaku Miniflex diffractometer operated at
30 kV and 15 mA. The angular interval of 10 to 70° was varied in 0.01° steps using a 1
s counting time per step. The crystallite size was calculated from the X-ray data using
Scherrer’s equation. Then, the diffraction data were refined using the FullProfSuite®
software. Temperature programmed reduction (TPR) was performed in order to evaluate
the reduction properties of the nanoparticles. The TPR analysis was performed with a
mixture of 1.53 vol.% H2/Ar (30 mL min-1) and the temperature was increased from
ambient to 1000 °C at 10 °C min-1. Prior to the TPR experiment, the sample was treated
by heating from ambient to 150 °C for 30 min under pure argon flow, in order to clean
the sample surface. The surface chemical state of the atoms as well as their relative
abundance were evaluated by X-ray photoelectron spectroscopy (XPS) using an
ESCALAB 250 spectrometer (Thermo Scientific), employing monochromatic Al Kα
7

(1486.6 eV) as the X-ray source. The C 1s signal at 284.6 eV was the binding energy
reference. Spectra were analyzed using a Gaussian-Lorenzian peak shape obtained from
CasaXps software.
Catalytic tests
The catalytic activity was evaluated at atmospheric pressure and at different
temperature (from 50 to 250 °C) in a fixed bed quartz micro-reactor. The feed gas
consisting of CO (1 vol.%) and O2 (1 vol.%), H2 (60 vol.%) and balance He was
allowed to pass through the catalyst bed (150 mg) at a flow rate of 100 mL min-1. Prior
to the catalytic test, the samples were pretreated in He flow (60 mL min-1) at 200 °C for
60 min. The effect of H2O vapor and CO2 on the catalytic performance was also
investigated under isothermal conditions at 250°C; the feed composition was constituted
of CO (1 vol.%) and O2 (1 vol.%), H2 (60 vol.%), CO2 (10 vol.%), H2O (3 vol.%) and
He balance. The outlet gases were analyzed by an on-line gas chromatograph (GC)
(Trace GC Ultra, Thermo Electron Corp.) equipped with TCD and FID detectors; He
was employed as carrier gas. The CO and O2 conversion and CO2 selectivity were
calculated as reported in our previous study [1].
In situ DRIFTS-MS catalytic studies
In situ diffuse reflectance infrared spectroscopy (DRIFTS) measurements were
performed on a Nicolet spectrometer (Nexus 470 model) equipped with a MCT detector
cooled by liquid nitrogen. Before the measurement has been performed, the sample
powder was treated in situ under He up to 200°C for 1 h in order to clean up the surface.
After cooling down to room temperature, a background spectrum was collected for
spectra correction. Subsequently, the reaction mixture (1% CO + 1% O2 + 60% H2 in
He) was introduced inside the DRIFTS cell passing over the catalysts at atmospheric
pressure. The spectra were collected at 100, 150, 200 and 250 °C in the 4000-800 cm-1
range. The exit lines from the DRIFTS cell were connected to Pfeiffer-Vacuum Mass
Spectrometer (MS) in order to monitor the gaseous effluent. The signal intensity
8

corresponding to m/z=2 (H2), 16 (CH4), 18 (H2O), 28 (CO), 32 (O2), 44 (CO2) ions were
monitored constantly.
3. Results
XRD and SEM results
SEM micrographs and XRD measurements of the systems before and after
calcination are presented in Figure 1. The non-calcined system showed a spherical
shape and smooth morphology along with uniform size and dimension. As seen, the
crystalline peaks observed in the diffractogram before calcinations are insignificant,
indicating that the material was predominately amorphous.
On the other hand, it can be seen that after the calcination process the system
exhibited high degree of crystallinity with well-defined XRD peaks, which are
consistent with the CoFe2O4 structure (JCPDS22-1086). No peaks from others phases or
impurities were detected, suggesting high purity of the cobalt ferrite prepared through
the present glucose-assisted hydrothermal method. Furthermore, the Rietveld
refinement (Fig. 2) showed an excellent agreement between the observed and calculated
patterns, which may be confirmed by observing the difference line (blue).
Some crystallographic parameters obtained by refinement are summarized in
Table 1. The lattice parameter (a=0.8380 nm) agrees well with the standard value
reported for stoichiometric CoFe2O4 (0.8392 nm, JCPDS-22-1086). The crystallite size
estimated by Scherrer’s equation was found to be 17 nm, indicating that the
hydrothermal method used in the present study is appropriated for the synthesis of pure
cobalt ferrite nanostructure with particle size located in the nanometer range, usually
less than 100 nm. Table 2 shows some of the physical-chemical properties of the as-
prepared CoFe2O4. Accordingly, XFR data indicate a Fe/Co bulk atomic ratio of 1.8,
9

which in turn is in good agreement with the nominal value (= 2) expected for pure
(stoichiometric) cobalt ferrite.
TPR and XPS studies
TPR analyses were carried out in order to examine the influence of reducing
conditions (as used in our catalytic tests) on the structure of cobalt ferrite. As seen in
Fig. 3, the H2-TPR profile can be decomposed into two main reduction peaks: the first
one formed around 405 °C, associated with the reduction of CoO to metallic cobalt
(Co0), in conjunction with the reduction of Fe2O3 to Fe3O4, both present in the ferrite
phase [19,20]; followed by a subsequent peak positioned at 460 °C, which might be
interpreted as a reduction of Fe3O4 to FeO and Fe0 [21,22]. The reduction process can be
described by the following equations:
CoFe2O4 + H2 → Fe2O3 + Co0 + H2O
3Fe2O3 + H2 → 2Fe3O4 + H2O
Fe3O4 + 4H2 → 3Fe + 4H2O
(1)
(2)
(3)
Importantly, the TPR result also suggests that the as-prepared (non-reduced)
cobalt ferrite is stable within the temperature range used in our catalytic tests (50-
250 °C). This is a very interesting feature, assuming that reduction of cobalt during the
reaction would probably lead to undesirable reactions such as methanation and H2
oxidation [23]. The H2 consumption for cobalt ferrite was 140 µmol, which is lower
than the theoretical value (172 µmol) required for complete reduction, indicating that
not all cobalt and/or iron were reduced. The theoretical H2 consumption was calculated
based on the total reduction of cobalt ferrite, according with Equation 4.
CoFe2O4 + 4H2 → Co + 2Fe + 4H2O
(4)
The XPS spectra related to the Co 2p, Fe 2p, O 1s and C 1s core level regions
are presented in Fig. 4. The corresponding Co 2p spectrum exhibited two main peaks
around 780.4 and 795.6 eV associated with the doublet Co 2p3/2 and Co2p1/2 regions,
respectively, besides two relatively less intense shake-up satellite peaks around 786.3
10

and 802.3 eV. These observations evidence the presence of Co2+ species and practically
exclude the existence of Co3+ species at the surface [24,25]. The Fe 2p region of the
XPS spectrum shows two main peaks located at 710.9 and 724.03 eV, mostly
attributable to spin-orbit components, namely Fe 2p3/2 and Fe 2p1/2, respectively, being
in good agreement with the commonly assigned values for Fe3+ [26,27]. Furthermore,
the presence of two satellite peaks at 731.9 and 717.7 eV confirm the presence of Fe3+
and also discard the existence of Fe2+ species. The O 1s spectrum can be decomposed
into three components: a main peak located at 529.1 eV assigned to lattice oxygen, a
less intense peak at 531. 4 eV attributed to the presence of hydroxyl groups, and the
least intense peak at 533.1 eV characteristic of carbonate structures [28,29]. The surface
relative Fe/Co atomic ratio calculated from XPS data was of 1.6, which is slightly lower
than the bulk ratio (Fe/Co=1.8 see Table 2), suggesting an enrichment of cobalt species
at the surface.
Catalytic tests
The corresponding activity and selectivity measured at different temperatures are
shown in Fig. 5. The catalytic activity is usually associated with the temperature
corresponding to 50% conversion of CO to CO2 (T50%CO), which can be obtained from
the light-off curve [30]. In this work, we observed that the cobalt ferrite catalyst
exhibited good activity for CO-PROX reaction, being characterized by a T50%CO = 155
°C and a CO2 selectivity of 100 %. Moreover, these results were found to be better than
those observed for LaCoO3 catalysts in our previous studies [1], under similar feed
composition and space time velocity, where the observed T50%CO was 240 °C and the
CO2 selectivity of 82%.
The stability test at 250 °C is shown in Figure 6. As can be seen, no significant
decrease in the catalytic activity and selectivity was observed during 30 h. These results
can confirm the high stability presented by the cobalt ferrite catalyst in CO-PROX
reaction for more than 30 hours of reaction.
Effect of CO2 and water
11

The effect of CO2 and H2O in the reaction mixture was studied in order to
evaluate the catalytic performance under more realistic conditions, since conventional
reformed gases usually contain both. The reaction was performed by adding separately
and simultaneously CO2 and/or H2O in the feed stream at 250 °C. The results are
displayed in Fig. 7 and show that the addition of 3% vol. of H2O in the feed mixture
leads to a slight loss in the CO conversion (around 7%). On the other hand, by adding
10% vol. of CO2 the CO conversion decayed significantly from 76% to slightly less
than 43%. Similarly, the CO conversion drops substantially after simultaneous addition
of CO2 and H2O in the feed mixture (from 75% to 37%). Noteworthy is that, when CO2
and/or H2O are removed from the feed gas, the initial CO conversion was almost totally
recovered. These results show that the presence of CO2 and H2O can influence
negatively the reaction, affecting the CO conversion and poisoning the active sites.
Since these are products of the reaction they might inhibit the reaction kinetics for
concentrations above the feed level, but have no influence on the chemical structure of
the surface active sites since the conversion is fully restored after the removal of H2O
and CO2 feeds.
In situ DRIFTS-MS
The nature of the adsorbed and gas phase reaction intermediates was also
monitored by in situ DRIFTS-MS. Figures 8 and 9 shows the spectral DRIFTS and
Mass Spectra (MS) results, respectively, at different reaction temperatures under the
CO-PROX feed mixture. The spectral zone between 2000-800 cm-1 (Figure 8b) displays
bands mostly attributable to carbonate-type species. The most intense bands in this zone
are detected at 1218, 1410 and 1621 cm-1. Bands at 1218 and 1621 cm-1 could be
assigned to bicarbonate species adsorbed on the surface [31], whereas the band at
1410 cm-1 has been attributed to polydentate carbonate species [32]. In turn, a band at
1359 cm-1 (attributed to formate species) can be evidenced at 100 °C and higher
temperatures possibly due to the reaction between CO gas and hydroxyl groups
adsorbed on the cobalt ferrite surface [33]. The other spectral zone (between 4000 and
2000 cm-1, Figure 8a) shows bands located at 2175 and 2115 cm-1 which are important
signals corresponding to the gaseous and weakly adsorbed CO [34,35]. However, it can
be seen that along with increasing temperatures these bands tend to decrease, which is
12

also consistent with MS results (see Fig. 9). The doublet located at 2328 and 2361 cm-1
are associated with the stretching vibration of the O=C=O bond, suggesting the presence
of CO molecules entrapped in the pores of the oxides [36]. The intensity of CO2
increased up to 150 °C and decreased above 200 °C. A band located at 3018 cm-1 (only
observed above 200 °C) is related to the presence of gaseous CH4 [37]. Accordingly, the
methane formation at higher temperatures is also confirmed by MS (Fig. 9), suggesting
a methanation reaction. The appearance of bands at 2952 and 2872 cm-1 also confirm
the existence of CH2(ad) species [37].
Jackson et al. [38] and Aranda and Schmal [39] employed TPD of CO with mass
spectrometry and demonstrated that during the CO desorption on Pt or Pd supported on
Al2O3, that H2 and CO2 are released. This was attributed to a surface reaction between
adsorbed CO on the metal and their neighboring hydroxyls on the support. One can
describe this reaction as a modified water gas shift reaction where the support hydroxyls
play the role of water. This symmetric release of CO2 and H2 during the TPD of CO was
also observed in our work. Figure 8 shows that band at 2361 cm-1 corresponding to CO2
increased till 150 °C, was constant up to 200 °C and then decreased at 250 °C,
confirmed through Mass spectroscopy, as shown in Fig. 9. Figure 8 also shows a band
at 1359 cm-1 which can be attributed to formate species. This band increases with
increasing heating. Indeed, the hydroxylation (CO + OH → CO2 + 0.5H2) prevailed up
to 150 °C, but then a different mechanism may occur. Due to the H2 in the feed, the
cobalt ferrite can be reduced at the surface to metallic cobalt or iron, according with Eq.
4. In fact, it is confirmed in Figure 8 on bands at 2175 and 2015 cm-1 which evidences
the CO adsorption on the metal and/or ionic species, respectively, according the
literature [38,39]. Therefore, we assume that the H2 is dissociated over metallic sites and
then combined with the oxygen in the lattice forming hydroxyls groups. On the other
hand, the oxygen in vacancies is replaced by the molecular oxygen, recycling the
process at the surface, as shown in scheme bellow:
(Dissociation)
H2 + 2* → 2H(a)
[2OH-]
(Hydroxyls)
(Oxygen in vacancies)
2O-
O2 → 2O-
(Replace oxygen in the lattice)
13

The hydroxyls combine with CO adsorbed on the active site lead to the
formation of a formate species (CO(a) + OH(a) → HCOO). On the other hand, water was
also produced during the reduction of the ferrite, also seen in Fig. 9. In fact, resulting
water can also be dissociated with the formation of hydroxyls and subsequently forming
formate species, according to Tanaka et al. [40].
The CO and O2 conversions decrease with increasing temperature (Figure 8).
Note that CO2 content is low at lower temperature and increases around 150-200 °C, but
decreases in some extent above 200 °C. Therefore, in this temperature range there is
methane and water formation, according to reactions:
CO + 3H2 → CH4 + H2O
CO2 + 4H2 → CH4 + 2H2O
ΔH°(298 K) = -217 kJ/mol
ΔH°(298 K) = -178 kJ/mol
Besides, there are other reactions due to the formation of water and CO2: the
shift and methane reforming reactions.
CO + H2O → CO2 + H2
CH4 + H2O → CO + 3H2
CH4 + CO2 → 2CO + 2H2
ΔH°(298 K) = -41 kJ/mol
ΔH°(298 K) = 206 kJ/mol
ΔH°(298 K) = 247 kJ/mol
The last two and the reverse shift reaction are endothermic reactions. In fact, the
CO2 selectivity decreases because the reverse shift reaction in this temperature range. In
general, the literature reported that cobalt for this reaction either as bulk or supported
catalysts exhibit low stability favoring undesirable reactions that are associated to the
reducibility of cobalt species in the H2-rich stream. Yung et al. [41] used CoOx/ZrO2
catalyst and found also CO conversions as high as 90% at 175 °C, but above this
temperature occurred H2 oxidation and methanation reactions. Ko et al. [42] showed
that the methanation was dominant over CoO leading to high methane yield from
180 °C. On the other hand, cobalt-based perovskites are more resistant to methanation
(T > 350 oC for LaCoO3) and present better stability [43]
Our findings on a similar
.
LaCoO3 catalyst showed that the presence of H2O and CO2 in the feed stream presented
similar behavior [43]. In fact, the adsorptions are reversible influencing the reaction
14

kinetics. However, blocking of surface sites and formation of carbonate are the only
possibilities that inhibited the reaction rates which are reversible by desorption or
carbonate decomposition.
5. Conclusions
In summary, cobalt ferrite nanoparticles were successfully synthesized by
glucose-assisted hydrothermal method. The structural characterization revealed the
formation of a single phase and confirmed the nanocrystalline nature of the material.
The as-prepared material exhibited high CO conversion and good stability on stream at
250 °C during 30 h. In situ DRIFTS-MS showed formate species at 100 °C which
increased with increasing temperature due to the reaction between CO gas and hydroxyl
groups of cobalt ferrite. The results also showed that the presence of CO2 and H2O
affect the CO conversion and might inhibit the reaction kinetics for concentrations
above the feed level, but have no influence on the chemical structure of the surface
active sites since the conversion is fully restored after the removal.
.
Acknowledgements
The authors gratefully acknowledge Prof. Fabio B. Passos (Recat - UFF) for the
XPS analysis, and CNPq, CAPES, FAPERJ and FINEP for financial support.
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17

Table caption
Table 1: Some physical chemical properties of the cobalt ferrite
Table 2. Some crystallographic parameters and mean crystallite size of the cobalt ferrite.
18

Figure caption
Figure 1. SEM images and XRD measurements of the non-calcined and calcined
systems.
Figure 2. Rietveld refined XRD diffractogram. The red circles represent experimental
points while the solid black line represents Rietveld refined values. The difference
between the experimental and refined curves is represented by the lowest blue line. The
marked positions are the allowed Bragg peaks.
Figure 3. H2-TPR profile of the cobalt ferrite.
Figure 4. XPS spectra of Co 2p, Fe 2p, O 1s and C 1s of the cobalt ferrite.
Figure 5. CO conversion (A), O2 conversion (B) and CO2 selectivity (C) as a function of
the reaction temperature.
Figure 6. Stability test with time on stream for cobalt ferrite under isothermal conditions
at 250 °C.
Figure 7. Effect of the 3 vol.% H2O and 10 vol.% CO2 addition in the gas feed at
250 °C.
Figure 8. (a) High wavenumber and (b) low wavenumber in situ DRIFTS spectra at 100,
150, 200 and 250 °C under CO-PROX conditions (1% CO, 1% O2, 60% H2 and balance
He).
Figure 9. MS data from in situ DRIFTS analysis as function of reaction temperature.
19

Table 1
Cell parameters (nm)
a=b=c (CeFe2O4)
0.8380
Volume
(nm3)
FWHM
(°)
Crystallite size
(nm)
Phase
CoFe2O4
0.5821
0.4836
17.0
Table 2
Catalyst
Bulk Co (wt%) Bulk Fe (wt%) Bulk Fe/Co SBET* (m2 g-1)
24.1 43.4 1.8 22
Cobalt ferrite
20

Figure 1
Calcined
Fe/Co = 2.1
(EDS)
Calcined
Non-calcined
10
20
30
40
50
60
70
Non-calcined
Bragg's angle 2θ (deg.)
Figure 2
21

Y observed
Y calculated
Yobs. - Ycalc.
Bragg position
CoFe2O4
10
20
30
40
50
60
70
80
90
Bragg's angle 2θ (deg.)
Figure 3
CoFe2O4
460
CoO to Co and Fe2O3 to Fe3O4
Fe3O4 to FeO + Fe0
405
100
200
300
400
500
600
700
800
900 1000
Temperature (°C)
Figure 4
22

2p1/2
2p3/2
2p1/2
795.6
2p3/2
780.4
Fe 2p
Co 2p
724.03
710.9
ΔE = 13.2
ΔE = 15.2
731.9
786.3
717.7
802.3
Satellite
Satellite
Satellite
Satellite
740 735 730 725 720 715 710 705
805 800 795 790 785 780 775
Binding Energy (eV)
Binding Energy (eV)
529.6
O 1s
C 1s
284.6
531.4
287.8
533.2
538
536
534
532
530
528
526 296 294 292 290 288 286 284 282 280
Binding Energy (eV)
Binding Energy (eV)
Figure 5
100
C
80
100 A
100 B
80
80
60
40
20
0
155
60
40
20
0
60
T50%CO
40
20
0
50 75 100 125 150 175 200 225 250
Temperature (°C)
50 75 100 125 150 175 200 225 250
Temperature (°C)
50 75 100 125 150 175 200 225 250
Temperature (°C)
Figure 6
23

100
90
80
70
60
50
40
30
20
O2 Conv.
CO Conv.
250 °C
CO2 Sel.
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Time (h)
Figure 7
24

100
80
60
40
20
CO + O2
H2
CO + O2
H2
CO + O2
H2
CO + O2
H2
CO + O2
H2 + CO2
H2O
CO + O2
H2 + H2O
CO + O2
H2 + CO2
250 °C
1
2
3
4
5
6
7
8
Time (h)
Figure 8
200 °C
150 °C
100 °C
150 °C
100 °C
25 °C
25 °C
2000 1800 1600 1400 1200 1000 800
4000 3600 3200 2800 2400 2000
Wavenumber (cm-1)
-1
Wavenumber (cm )
Figure 9
25

CO
O2
CO2
H2O
CH4
100
150
200
250
Temperature (°C)
26