Hydrotalcite-derived Co-containing mixed metal oxide catalysts for methanol incineration
Hydrotalcite-derived Co-containing mixed metal oxide catalysts for methanol incineration
Sylwia Basa?g 0 1 2
Frantis?ek Kovanda 0 1 2
Zofia Piwowarska 0 1 2
Andrzej Kowalczyk 0 1 2
Katarzyna Pamin 0 1 2
Lucjan Chmielarz 0 1 2
0 Department of Solid State Chemistry, University of Chemistry and Technology , Prague, Technicka ? 5, 166 28 Prague , Czech Republic
1 Faculty of Chemistry, Jagiellonian University , Ingardena 3, 30-060 Krako ?w , Poland
2 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences , Niezapominajek 8, Krako ?w , Poland
The Co-Mg-Al mixed metal oxides were prepared by calcination of co-precipitated hydrotalcite-like precursors at various temperatures (600-800 C), characterised with respect to chemical (AAS) and phase (XRD) composition, textural parameters (BET), form and aggregation of cobalt species (UV-vis-DRS) and their redox properties (H2-TPR, cyclic voltammetry). Moreover, the process of thermal decomposition of hydrotalcite-like materials to mixed metal oxide systems was studied by thermogravimetric method combined with the analysis of gaseous decomposition products by mass spectrometry. Calcined hydrotalcite-like materials were tested as catalysts for methanol incineration. Catalytic performance of the oxides depended on cobalt content, Mg/Al ratio and calcination temperature. The catalysts with lower cobalt content, higher Mg/Al ratio and calcined at lower temperatures (600 or 700 C) were less effective in the process of methanol incineration. In a series of the studied catalysts, the best results, with respect to high catalytic activity and selectivity to CO2, were obtained for the mixed oxide with Co:Mg:Al molar ratio of 10:57:33 calcined at 800 C. High activity of this catalyst was likely connected with the presence of a Co-Mg-Al spinel-type phases, containing easy reducible Co3? cations, formed during high-temperature treatment of the hydrotalcite-like precursor.
Hydrotalcite-like materials; Co-Mg-Al mixed oxide catalysts; Spinel formation; VOC total oxidation; Methanol incineration
Emissions of volatile organic compounds (VOCs) to the
atmosphere from various sources are one of the most
serious environmental problems . Majority of VOCs is
toxic and may take part in photochemical reactions in the
atmosphere with other pollutants (e.g. nitrogen oxides)
resulting in very dangerous secondary pollutants (e.g.
ozone, aldehydes, peroxyacyl nitrates). The best way to
reduce the VOCs emission is to eliminate them prior to
their discharge. Among various methods employed for
VOCs removal including thermal combustion, absorption,
adsorptive recovery and biofiltration , the
low-temperature catalytic incineration seems to be the most reliable
and effective post-treatment technology . The catalysts
for VOCs incineration should effectively operate at
relatively low temperatures to avoid additional heating of flue
gases as well as to limit the formation of NOx by thermal
mechanism . Noble metals, such as Pt [4?7], Pd [8?10],
Rh  and Au [12, 13], belong to the most active and
effective components of the catalysts for incineration of
broad range of various VOCs. On the other hand, high cost
of noble metals significantly limits commercial utilisation
of such catalysts. Cobalt belongs to the most promising
components of the non-noble metal-based catalytic systems
for VOCs incineration. Solsona et al.  reported the
supported and unsupported nanocrystalline cobalt oxides as
very effective catalysts for propane incineration (total
conversion of propane was achieved at about 250 C). The
catalytic activity was mainly dependent on the crystallite
size of cobalt oxides, decreasing with an increase in the
crystallite size. The Co3O4 oxide was identified as the most
catalytically active phase in the studied process.
Jira?tova? et al.  studied total oxidation of toluene and
ethanol over the hydrotalcite-derived Co?Mn?Al mixed
oxide catalyst (Co:Mn:Al molar ratio of 4:1:1) modified
with various amounts of potassium (0?3 mass%). The Co?
Mn?Al mixed oxide showed very good catalytic
performance in the studied processes and addition of low
amounts of potassium promoter additionally activated
mixed metal oxide catalyst mainly by modification of the
surface acid?base properties. Thus, it was shown that both
the nanocrystalline structure of cobalt oxide phases as well
as the surface acid?base properties of such catalytic
systems are very important parameters that determine the
catalytic performance in the processes of VOCs
incineration. These observations were inspiration for the present
study. Similarly to report of Jira?tova? et al.  mentioned
above, the hydrotalcite-like materials were used as
precursors of mixed metal oxide catalysts for VOCs
incineration. Thermal treatment of such precursors results in
nanocrystalline materials with homogenously distributed
active components. By tailoring of chemical composition
of hydrotalcite-like precursors and conditions of their
thermal treatment, mixed metal oxide systems with various
phase composition can be prepared [16?23]. The surface
acid?base properties of hydrotalcite-derived mixed metal
oxide catalysts can be controlled by changing of the Mg/Al
ratio in the hydrotalcite-like precursors.
The aim of the studies has been determination of the
influence of the Mg/Al ratio in the cobalt-containing
hydrotalcite-like precursors as well as calcination
conditions on their catalytic efficiency in the process of methanol
incineration. Methanol was used as a model VOC molecule
in the presented studies.
The hydrotalcite-like precursors with the intended Co/Mg/
Al molar ratios of 5/77/18, 10/72/18, 5/62/33 and 10/57/33
were synthesised by co-precipitation. The metal nitrates of
p.a. purity, namely Co(NO3)2 6H2O, Mg(NO3)2 6H2O and
Al(NO3)3 9H2O (all supplied by Sigma?Aldrich), NaOH
(POCh), Na2CO3 (POCh) and distilled water were used for
preparation of solutions. An aqueous solution (450 mL) of
Co, Mg and Al nitrates in appropriate molar ratio and total
metal ions concentration of 1.0 mol L-1 was added with
flow rate of 7.5 mL min-1 into 1000-mL batch reactor
containing 200 mL of distilled water. The flow rate of
simultaneously added alkaline solution of NaOH
(3.0 mol L-1) and Na2CO3 (0.5 mol L-1) was controlled
to maintain reaction pH of 10.0 ? 0.2. The co-precipitation
was carried out at vigorous stirring at 60 C. The resulting
suspension was stirred at 60 C for 60 min. The product
was filtered off, washed thoroughly with distilled water and
dried overnight at 60 C in air. Finally, the precursors were
calcined at various temperatures (600, 700 or 800 C) for
9 h in air. The calcined samples were kept in a desiccator
in order to avoid their contact with ambient atmosphere.
The prepared mixed oxide samples were labelled by
acronyms with molar ratios of cation constituents and
calcination temperature (e.g. Co10Mg57Al33-800).
The cation composition of the prepared hydrotalcite-like
precursors was determined by atomic absorption
spectroscopy. The samples were dissolved in a hydrochloric
acid solution (36%), and the cation concentration in
solutions was determined using 280FS AA instrument (Agilent
Technologies) at the following wavelengths: Co 240.7 nm,
Mg 285.2 nm and Al 309.3 nm.
The phase composition of the hydrotalcite-like
precursors and related mixed metal oxides obtained by their
calcination was determined by powder X-ray diffraction
(XRD). The powder XRD patterns were recorded with a
D2 Phaser diffractometer (Bruker) using Co Ka1 radiation
(k = 0.179 nm) in the 2H range from 8 to 80 , step size
0.02 . The qualitative analysis was performed with a
HighScore Plus 4.0 software package (PANanalytical). The
hydrotalcite lattice parameters a and c were determined
from XRD data using the following formulas: a = 2d110
and c = (3d003 ? 6d006)/2. The mean coherence length
L was calculated from the FWHM values (broadening at
half of the maximum intensity) of the diffraction lines
using Scherrer equation.
Thermal decomposition of the hydrotalcite-like
precursors was studied by thermogravimetric method coupled
with QMS analysis of evolved gases. The TG-DTG-QMS
measurements were carried out using a Mettler Toledo
851e instrument operated under a flow of air
(80 mL min-1) in the temperature range of 25?1000 C
with a heating rate of 10 C min-1. The gases evolved
during the thermal decomposition of the samples were
continuously monitored by quadrupole mass spectrometer
ThermoStar (Balzers) connected directly to microbalance
for the chosen mass numbers m/z (18-H2O?, 30-NO? and
The specific surface area of the calcined samples was
determined by the BET method using a 3Flex
(Micromeritics) automated gas adsorption system. Prior to
the nitrogen adsorption at -196 C all the samples were
outgassed under vacuum at 350 C for 24 h.
The UV?vis-DR spectra of the calcined samples were
recorded using an Evolution 600 (Thermo)
spectrophotometer. The measurements were performed in the range
from 190 to 900 nm with a resolution of 2 nm.
Cyclic voltammograms of the calcined samples were
recorded in a three-electrode cell using a graphite paste
electrode as the working electrode, platinum coil as the
auxiliary electrode and Ag|AgCl as the reference electrode.
Composite paste was prepared by mixing synthetic
graphite (100?150 mg) with Nujol (0.05 mL) and a small
amount of the mixed oxide catalyst (0.005?0.010 g). The
measurements were performed in acetate buffer
(pH = 4.6) as electrolyte at a scan rate of 50 mV s-1.
Before experiment, the solutions were pretreated with
argon to keep oxygen-free atmosphere during the
The reducibility of the calcined samples was studied by
temperature-programmed reduction method (H2-TPR).
Experiments were carried out in a fixed-bed flow
microreactor starting from room temperature to 950 C,
with a linear heating rate of 10 C min-1. H2-TPR runs
were carried out in a flow (10 mL min-1) of 5 vol.% H2
diluted in Ar (N5 quality, Messer). The evolution of
hydrogen was detected by microvolume TCD (Valco).
The mixed oxide samples obtained from hydrotalcite-like
precursors at 600, 700 and 800 C were tested as catalysts
for methanol incineration. Catalytic experiments were
performed in a fixed-bed flow quartz microreactor system
under atmospheric pressure in the temperature range from
125 to 425 C with an isothermal steps every 25 C. For
each test, 100 mg of catalyst was outgassed in a flow of air
at 500 C for 30 min. After cooling down to 125 C, the
gas mixture containing 3.7 vol% of methanol diluted in air
(total flow rate of 20 mL min-1) was supplied into the
reactor by the isothermal saturator (0 C). The reaction
products were analysed using a gas chromatograph SRI
8610C equipped with Hayesep D column as well as
methaniser-FID detection system.
Results and discussion
Formation of hydrotalcite-like materials after
co-precipitation reaction was proven by powder XRD results. In the
XRD patterns of the co-precipitated precursors (Fig. 1),
only diffraction lines characteristic for hydrotalcite-like
compounds were found, no other crystalline phase was
Fig. 1 Powder XRD patterns of the co-precipitated hydrotalcite-like
detected. The hydrotalcite lattice parameters a and c,
determined from XRD analysis, are compared in Table 1.
Slightly smaller a parameter observed for the samples with
increased aluminium content can be explained by smaller
ionic radius of Al3? (53 pm) in comparison with radius of
Mg2? (72 pm). The samples with increased aluminium
loading in the hydrotalcite-like samples exhibited also
slightly smaller d003 basal spacing, which resulted in a
decreased c parameter. Similar effect was observed by
authors for the Cu?Mg?Al hydrotalcite-like materials .
This effect could be related to the increased Al3? content in
the brucite-like layers, which causes the higher net positive
charge and tighter arrangement of the layers due to stronger
electrostatic interactions between the layers and interlayer
anions. The mean coherence length evaluated from XRD
data is related to structure ordering; it was evaluated from
broadening of hydrotalcite (hkl 003, 006, 110) diffraction
line using Scherrer equation; the L values varied in the
range of 9.1?12.8 nm and confirmed formation of
Cation composition of prepared hydrotalcite-like
precursors was determined by AAS after samples dissolution
in hydrochloric acid. The results presented in Table 2 show
Table 1 Lattice parameters and mean coherence length L (calculated
from FWHM of (003), (006), (110) line) of co-precipitated
that molar ratios of cations calculated from the measured
data are very close to the intended values, i.e. the molar
ratios of cations adjusted in nitrate solutions used in
The process of thermal decomposition of the
hydrotalcite-like samples into mixed oxides was studied by
thermogravimetry coupled with analysis of gaseous
products released during thermal treatment of the samples
(TG-QMS). Results of thermal analysis are presented in
Fig. 2. In general, thermal behaviour of hydrotalcite-like
compounds may be characterised by two main transitions:
(1) the loss of interlayer water without collapse of the
layered hydrotalcite structure and (2) subsequent loss of
hydroxyl groups accompanied by water release from the
brucite-like layers and decomposition of volatile
interlayer anions (e.g. carbonate or nitrate) at higher
temperatures . The temperature ranges of these two
transitions depend on cationic composition of the
hydroxide layers and interlayer anions. The first transition
related to the loss of interlayer water is represented by
DTG peaks and maxima of water evolution at
temperatures below 250 C. It should be noted that in the case of
the samples with higher Al/Mg ratio (Co5Mg62Al33 and
Co10Mg57Al33) these peaks were detected at temperatures
of about 20?25 C higher than in the samples with the
lower Al/Mg ratio (Co5Mg77Al18 and Co10Mg72Al18). It
could be explained by more effective stabilisation of
interlayer water in the samples with the increased content
of aluminium. Apart from interlayer water, very small
amounts of carbon dioxide were also released at
temperatures below 250 C, likely as a result of CO2
desorption from the samples surface or thermal
decomposition of unstable surface carbonates.
The second stage of the hydrotalcite-like samples
decomposition, including dehydroxylation of the
brucitelike layers as well as decomposition of interlayer anions,
was represented by DTG peaks and maxima of H2O, CO2
and NO evolution observed above 250 C. Release of
CO2 and H2O from the samples with the lower Al/Mg
ratio (Co5Mg77Al18 and Co10Mg72Al18) was represented
by single sharp maxima at about 355?375 C (Fig. 2). For
Table 2 Cation composition of the samples
Content of cations/mol%
the samples with the higher Al/Mg ratio (Co5Mg62Al33
and Co10Mg57Al33), the profiles of H2O and CO2
evolution were more complex and consisted of at least two
overlapping peaks of water release and at least three peaks
of carbon dioxide evolution. The low-temperature peak of
H2O release was located at about 310 C, while the
hightemperature peaks were located at 380 and 360 C for the
Co5Mg62Al33 and Co10Mg57Al33 samples, respectively.
Comparing positions of these peaks with temperatures of
pure Al(OH)3 and Mg(OH)2 dehydroxylation, which was
determined to be 295 C  and 377 C ,
respectively, it could be suggested that the low-temperature peak
was likely related to the release of OH- anions attached
to Al3?, while the high-temperature peak to
dehydroxylation of OH- attached to Mg2? cations. Such splitting of
the water evolution process was not observed for
Co5Mg77Al18 and Co10Mg72Al18 precursors due to lower
content of aluminium in these samples; however, the
asymmetry and positions of the water release peaks may
suggest that it is a superposition of the peaks related to
dehydroxylation of OH- bounded to Mg2? with the
smaller contribution of OH- attached to Al3?. The
profiles of CO2 evolution from Co5Mg62Al33 and
Co10Mg57Al33 were also much more complex comparing to
that observed for the Co5Mg77Al18 and Co10Mg72Al18
samples and consisted of at least four peaks. As it was
mentioned above, small peaks located below 250 C were
possibly related to CO2 desorption from the samples
surface or thermal decomposition of unstable surface
carbonates. Peaks centred at about 310 and 395?400 C
were assigned to decomposition of interlayer carbonates
and indicated the presence of CO32- anions stabilised in
the interlayer with various strengths. It could be explained
by different interaction and therefore also stabilisation of
interlayer anions by Al3? and Mg2? cations in hydroxide
layers. Moreover, CO2 release related to decomposition of
very stable carbonates was detected at temperature of
about 605?610 C. Similar effect was reported in our
previous studies for the Cu5Mg66Al29 and Mg71Al29
hydrotalcite-like samples, which released small amounts
of CO2 at temperatures as high as 637 and 588 C,
respectively . In the case of the samples with the
lower Al/Mg ratio, only one peak related to the thermal
decomposition of interlayer carbonates, centred at about
365 and 375 C, was detected for Co5Mg77Al18 and
Co10Mg72Al18, respectively. Release of small amounts of
NO was observed only for the samples with the lower Al/
Mg ratio and was represented by peaks located at about
355 and 470 C in the NO evolution profile of
Co5Mg77Al18 and maximum of NO evolution at about 500 C
for the Co10Mg72Al18 sample. These results may indicate
the presence of a small amount of nitrate anions in the
interlayer of the Co5Mg77Al18 and Co10Mg72Al18 samples,
Fig. 2 Results of thermogravimetry coupled with evolved gas analysis documenting thermal decomposition of the co-precipitated
but the NO evolution was more likely connected with
decomposition of residual nitrates, which were not
removed from the samples surface after washing.
Based on the results of thermogravimetric analysis,
temperatures of 600 C and higher were chosen for
calcination of the prepared hydrotalcite-like precursors. Such
temperatures should ensure a complete decomposition of
the hydrotalcite-like precursors and their transformation
into mixed metal oxides. The variation of calcination
temperatures should result in the preparation of the samples
with various phase composition, crystallinity and textural
Powder XRD patterns of the calcined samples are
presented in Fig. 3. The diffraction lines characteristic
for MgO (periclase) were found in the powder XRD
patterns of the mixed oxides obtained at 600 and 700 C
(the lines at approximately 43 , 50 and 74 2h). Phases
with spinel structure (represented by diffraction lines at
about 22 , 36 , 52 , 70 and 77 2h) were identified,
together with periclase-like oxides, in the powder XRD
patterns of the samples with the higher Al/Mg ratio
(Co5Mg62Al33 and Co10Mg57Al33) calcined at 800 C.
The spinel diffraction lines were not found in the
samples with the lower Al/Mg ratio (Co5Mg77Al18 and
Co10Mg72Al18). Comparing the powder XRD patterns of
the samples with the higher Al/Mg ratio (Co5Mg62Al33
and Co10Mg57Al33) calcined at 600 and 800 C, the
positions of periclase diffraction lines were shifted from
50.8 to 50.2 and 74.4 to 73.7 , respectively. The
corresponding slight increase in periclase lattice
parameter from about 0.4211 to 0.4225 nm could be explained
by incorporation of Al3? cations into MgO lattice in the
early stages of mixed oxide formation after thermal
decomposition of the hydrotalcite-like precursors.
Increasing calcination temperature enhanced
crystallisation of the MgO-like phase and formation of Mg?Al
spinel-type phase. The lattice parameter of MgO-like
phase formed at 800 (0.4225 nm) is close to that
reported for reference MgO (0.4223 nm, PDF No.
04-012-6481). Analogous changes in lattice parameters
of NiO-like phase were observed during calcination of
Ni?Al layered double hydroxides . For the samples
with the lower Al/Mg ratio (Co5Mg77Al18 and
Co10Mg72Al18), only negligible shift (below 0.2 ) in the
positions of periclase diffraction lines was detected.
Moreover, cobalt cations (both Co2? and even Co3?)
were very likely incorporated into the lattice of
periclase-like and spinel-type oxides. Radius of cobalt
cations is larger than that of Al3? ones and not very
different from the radius of Mg2? cations; therefore,
incorporation of small amounts of cobalt cations into the
lattice of Mg?Al mixed oxides is practically impossible
to detect by XRD.
The measured BET surface areas of the mixed oxide
samples obtained at various temperatures are compared in
Table 3. The results lead to the following conclusions:
(1) the increase in cobalt content in the samples decreased
the BET surface area; (2) the samples with higher Al/Mg
Table 3 BET surface area of the samples calcined at various
BET specific surface area/m2 g-1
ratio showed larger surface area and (3) the increase in
calcination temperatures resulted in gradual decrease in the
surface area of the samples.
The cobalt species formed during calcination of
hydrotalcite-like precursors were analysed by UV?vis-DR
spectroscopy (Fig. 4). The spectra consisted of the bands in
two regions characteristic of Co2? cations below 330 nm
and in the range 500?700 nm, as well as in the region
characteristic of Co3? cations in the range 330?480 nm
. The bands in the UV region below 330 nm were
assigned to a low-energy charge transfer between the
oxygen ligands and central Co2? ion in tetrahedral
symmetry . The absorption bands, centred at 360 nm, were
assigned to the 1A1g ? 1T2g transition in Co3? ions in
octahedral [31, 32]. The bands mentioned above were
present in the spectra of all the studied samples and
indicated that major part of cobalt cations was present in
the form of Co2?; a part of cobalt cations was oxidised
during calcination in air to Co3? species. Calcination of the
samples at high temperature of 800 C, especially in the
case of the samples with the higher Al/Mg ratio (Co5
Mg62Al33 and Co10Mg57Al33), resulted in the appearance
of the well-defined triplet at 545, 590 and 635 nm (inserts
in Fig. 4) corresponding to 4A2 ? 4T1 (P) transition in
Co2? ions in tetrahedral environment, characteristic for
CoAl2O4 spinel . The formation of such spinel was
reported to occur during thermal treatment of the samples
containing small amounts of uniformly distributed cobalt
species . On the other hand, the band characteristic of
Co3? cations in the range 330?480 nm could indicate the
presence of spinel phases containing trivalent cobalt, e.g.
MgCo2O4, MgCoAlO4 and Co3O4. However, the absence
of the band at about 690?700 nm corresponding to
1A1g ? 1T1g transition in Co3? ions in the octahedral
coordination implied that Co3O4 spinel was not formed or
was formed only in a very small amounts during
calcination of the hydrotalcite-like precursors . Moreover, it
should be noted that the highest intensity of the band
characteristic of Co3? cations (330?480 nm) was observed
for the samples with the higher cobalt content and lower
Al/Mg ratio (Co5Mg77Al18 series). Therefore, it seems
possible that this band represents mainly MgCo2O4 with
the inverse spinel structure .
The electrochemical properties of the mixed oxide
samples prepared at various temperatures were examined
by cyclic voltammetry. Figure 5 shows the cyclic
voltammograms obtained for the Co10Mg57Al33 sample calcined
at 600 and 800 C. The oxidation wave for the sample
calcined at 600 C is visible at Epa = 301 mV and
reduction wave can be observed at Epc = 8 mV. The
shape of the voltammogram profiles shows that the redox
process is irreversible and oxidation dominates over
reduction in the mixed oxide catalyst. In the Co10Mg57Al33
sample calcined at 800 C, the oxidation wave is present at
Epa = 301 mV, so exactly at the same position as for the
catalyst prepared at 600 C. On the other hand, significant
differences are observed in the reduction waves of the
Co10Mg57Al33 sample calcined at 600 and 800 C. In the
catalyst calcined at higher temperature, the reduction wave
was shifted to 21 mV and, moreover, additional
low-intensive reduction wave appeared at about 121 mV. Such
result may indicate that increase in calcination temperature
from 600 to 800 C generated more easily reducible cobalt
species in the Co10Mg57Al33 sample.
Figure 6 presents the results of the multicycle
voltammetry performed for the Co10Mg57Al33 sample calcined at
800 C. In the initial cycles, splitting of the oxidation wave
for two components was observed. This effect was less
significant in the subsequent redox cycles, and finally an
increase in the anodic current and the formation of one
intensive oxidation wave occurred. As it was mentioned
above, two reduction waves were observed in the profile of
the Co10Mg57Al33 sample calcined at 800 C. In the
subsequent redox cycles, the intensity of these waves,
0 200 400
Potential/mV vs Ag|AgCl
Fig. 5 Results of cyclic voltammetry measurements obtained for
Co10Mg57Al33 mixed oxides obtained at 600 and 800 C
Co10Mg57Al33 800 ?C
Increase in cycle numbers
0 150 300 450
Potential/mV vs Ag|AgCl
Fig. 6 Results of multicycle voltammetry measurements performed
for the Co10Mg57Al33 mixed oxide sample obtained at 800 C
especially that at higher positive potential, gradually
increased. It could be explained by the formation of the
more easily reducible cobalt species in the subsequent
redox cycles. Such effects were not observed for the
Co10Mg57Al33 sample calcined at 600 C, for which no
changes in the subsequent oxidation and reduction wave
profiles were found.
Figure 7 presents the results of
temperature-programmed reduction (H2-TPR) obtained for the
Co10Mg57Al33 sample calcined at 600 and 800 C. The reduction in
cobalt species proceeded in two steps. In the first step,
Co3? was reduced to Co2?, while in the second step Co2?
to Co0 [36, 37]. It should be noted that in the case of the
sample calcined at 800 C the reduction of Co3? to Co2?
took place at temperature lower by about 80 C in
comparison with the sample calcined at 600 C. Thus, the
Co10Mg57Al33 (600 ?C)
Fig. 7 Results of temperature-programmed reduction (H2-TPR)
performed for the Co10Mg57Al33 mixed oxide sample obtained at 600
and 800 C
Co10Mg57Al33 (800 ?C)
results of these studies are fully consistent with the results
of the cyclic voltammetry and support hypothesis that
calcination of Co10Mg57Al33 at 800 C generated cobalt
species containing easy reducible Co3?.
Mixed oxides obtained from hydrotalcite-like precursors
at various temperatures were tested as catalysts for
incineration of methanol, which was used as a model VOC. It
can be seen from Fig. 8 that catalytic performance of the
studied samples depended on chemical composition as well
as calcination temperature. In a series of the samples with
the higher Al/Mg ratio (Co5Mg62Al33 and Co10Mg57Al33),
an increase in calcination temperature resulted in catalytic
activation of the samples. Moreover, for the catalysts
calcined at elevated temperatures higher selectivity to CO2
was achieved (apart from CO2 only CO was detected as a
For series of the catalysts with the lower Al/Mg ratio
(Co5Mg77Al18 and Co10Mg72Al18), no simple correlation
between their catalytic performance and calcination
temperature was observed. In the Co5Mg77Al18 series, the
sample calcined at 800 C was significantly more active
and selective to CO2 than the catalysts calcined at lower
temperatures, which showed very similar catalytic activity
in methanol incineration. The activity of the catalysts of
Co10Mg72Al18 series was very similar and only slightly
depended on the calcination temperature. Among the
examined catalysts, the best results, with respect to activity
and selectivity to CO2, were obtained for the Co10Mg57Al33
sample calcined at 800 C. Over the presence of the latter
catalyst, methanol was completely oxidised to CO2 from
325 C. This sample belongs to the group of very effective
catalysts for methanol incineration (excluding noble
metalbased catalysts) reported in the literature [27, 38, 39]. Such
significant differences in the catalytic performance of the
studied samples can be explained by various phase
composition and redox properties of cobalt species present in
these phases. As it was shown by UV?vis-DRS and XRD
measurements, cobalt is present in the form of divalent and
Fig. 8 Results of catalytic tests of the obtained mixed oxide catalysts in the methanol incineration
trivalent cations in periclase-like and spinel-type mixed
oxides (due to low content of cobalt and its high dispersion
likely in the form of Mg1-xCoxO, Mg(1-x)CoxAl2O4 and
MgCoxAl(2-x)O4). Content of the spinel phases increased
with increasing calcination temperature. The samples
calcined at higher temperatures showed also better catalytic
activity in the methanol incineration, especially in the case
of the catalysts with the higher Al/Mg ratio. Thus, it could
be concluded that cobalt present in the spinel phases is
catalytically more active in comparison with cobalt in
periclase-like phase. It is well known that reduction of
Co3? to Co2? in the spinel phases (e.g. Co3O4 or
MgCoxAl(2-x)O4) occurs at temperatures significantly
lower in comparison to the reduction of Co2? to Co0 in
CoAl2O3, CoxMg(1-x)Al2O4 or CoO [36, 37]. Thus, cobalt
oxide phases with spinel structure containing easily
reducible Co3? species (Co3O4 or MgCoxAl(2-x)O4) could be
expected to be active components of the effective catalysts
for methanol incineration. The cyclic voltammetry and
H2TPR measurements showed the presence of easily
reducible phase (Fig. 5, the reduction of Co3? to Co2?) in the
most active catalyst (Co10Mg57Al33 sample calcined at
800 C). Moreover, this reduction process was intensified
in the subsequent voltammetric cycles (Fig. 6). Such
effects related to the presence of the easily reducible phase
as well as intensification of the reduction process were not
observed for other catalysts exhibiting lower catalytic
activity. Therefore, it could be suggested that this easily
reducible phases, formed during high-temperature
treatment (800 C) of hydrotalcite-like precursor, are
responsible for high catalytic activity in the process of methanol
incineration. Taking into account the fact that the most
active catalyst (Co10Mg57Al33 sample calcined at 800 C)
contained more than three times higher aluminium content
and nearly six times higher amount of magnesium in
comparison with cobalt content, the formation of the
MgCoxAl(2-x)O4 spinel phase is more favourable than
formation of Co3O4. Moreover, the presence of Co3O4 was
not confirmed by UV?vis-DR studies. Thus, it could be
suggested that the MgCoxAl(2-x)O4 spinel-type mixed
oxide is more catalytically active in comparison with other
cobalt-containing phases present in the Co10Mg57Al33
catalysts (e.g. Mg1-xCoxO periclase-like oxide) and is
responsible for the enhanced catalytic activity of the
sample calcined at high temperature (800 C).
The Co?Mg?Al mixed metal oxides were obtained from
hydrotalcite-like precursors with various cobalt contents
and different Mg/Al ratios by calcination at 600, 700 and
800 C. The prepared mixed oxides were tested as catalysts
in methanol incineration. It was shown that both cation
composition of hydrotalcite-like precursors and calcination
temperature strongly influenced the catalytic performance
of the mixed metal oxides. Among the studied samples, the
best catalytic results were obtained with the Co10Mg57Al33
sample having high cobalt content and lower Mg/Al ratio,
calcined at 800 C. High catalytic activity of the latter was
related to the presence of Co3?-containing spinel-type
phase, very likely the MgCoxAl(2-x)O4 mixed oxide; it was
probably more catalytically active in methanol incineration
than other cobalt-containing phases formed during
calcination of the hydrotalcite-like precursor. High catalytic
activity of the spinel phase was ascribed to the relatively
easy reducibility of Co3? to Co2? cations identified by
voltammetry and H2-TPR measurements.
Acknowledgements Part of the research was done with equipment
purchased in the frame of European Regional Development Fund
(Polish Innovation Economy Operational Program?Contract No.
Open Access This article is distributed under the terms of the Creative
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1. Zhang Z , Jiang Z , Shangguan W. Low-temperature catalysis for VOCs removal in technology and application: a state-of-the-art review . Catal Today . 2016 ; 264 : 270 - 8 .
2. Chiang YC , Chiang PC , Huang CP . Effects of pore structure and temperature on VOC adsorption on activated carbon . Carbon . 2001 ; 39 : 523 - 34 .
3. Toof LJ . A model for the prediction of thermal, prompt, and fuel NOx emissions from combustion turbines . Eng Gas Turbines Power . 1986 ; 108 : 340 - 7 .
4. Morales-Torres S , Maldonado-Ho?dar FJ, Pe?rez-Cadenas AF, Carrasco-Mar??n F. Design of low-temperature Pt-carbon combustion catalysts for VOC's treatments . J Hazard Mater . 2010 ; 183 : 814 - 22 .
5. Abdelouahab-Reddam Z , Mail ER , Coloma F , Sepu?lvedaEscribano A. Platinum supported on highly-dispersed ceria on activated carbon for the total oxidation of VOCs . Appl Catal A Gen . 2015 ; 494 : 87 - 94 .
6. Uso?n L , Colmenares MG , Hueso JL , Sebastia?n V , Balas F , Arruebo M , Santamar??a J. VOCs abatement using thick eggshell Pt/SBA-15 pellets with hierarchical porosity . Catal Today . 2014 ; 227 : 179 - 86 .
7. Mitu M , Razus D , Oancea D. Effect of CO2 dilution on propaneair isothermal catalytic combustion on Platinum . J Therm Anal Calorim . 2017 . doi:10.1007/s10973- 017 - 6167 -x.
8. Kim SC , Shim WG . Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds . Appl Catal B Environ . 2009 ; 92 : 429 - 36 .
9. Jab?on?ska M , Kro?l A, Kukulska-Zaja?c E, Tarach K , Girman V , Chmielarz L , Go?ra -Marek K. Zeolites Y modified with palladium as effective catalysts for low-temperature methanol incineration . Appl Catal B Environ . 2015 ; 166 - 167 : 353 - 65 .
10. Huang H , Ye X , Huang H , Zhang L , Leung DYC . Mechanistic study on formaldehyde removal over Pd/TiO2 catalysts: oxygen transfer and role of water vapor . Chem Eng J . 2013 ; 230 : 73 - 9 .
11. Kucherov AV , Sinev IM , Ojala S , Keiski R , Kustov LM . Adsorptive-catalytic removal of CH3OH, CH3SH, and CH3- SSCH3 from air over the bifunctional system noble metals/ HZSM-5. Stud Surf Sci Catal . 2007 ; 170 : 1129 - 36 .
12. Chen BB , Zhu XB , Crocker M , Wang Y , Shi C. FeOx-supported gold catalysts for catalytic removal of formaldehyde at room temperature . Appl Catal B: Environ . 2014 ; 154 - 155 : 73 - 81 .
13. Delannoy L , Fajerwerg K , Lakshmanan P , Potvin C , Me?thivier C , Louis C. Supported gold catalysts for the decomposition of VOC: total oxidation of propene in low concentration as model reaction . Appl Catal B: Environ . 2010 ; 94 : 117 - 24 .
14. Solsona B , Davies TE , Garcia T , Vazquez I , Dejoz A , Taylor SH . Total oxidation of propane using nanocrystalline cobalt oxide and supported cobalt oxide catalysts . Appl Catal B: Environ . 2008 ; 84 : 176 - 84 .
15. Jira?tova? K, Mikulova ? J, Klempa J , Grygar T , Bastl Z , Kovanda F. Modification of Co-Mn-Al mixed oxide with potassium and its effect on deep oxidation of VOC . Appl Catal A Gen . 2009 ; 361 : 106 - 16 .
16. Basa?g S , Piwowarska Z , Kowalczyk A , We?grzyn A , Baran R , Gil B , Michalik M , Chmielarz L. Cu-Mg-Al hydrotalcite-like materials as precursors of effective catalysts for selective oxidation of ammonia to dinitrogen-the influence of Mg/Al ratio and calcination temperature . Appl Clay Sci . 2016 ; 129 : 122 - 30 .
17. Jab?on?ska M , Chmielarz L , Wegrzyn A , Guzik K , Piwowarska Z , Witkowski S , Walton RI , Dunne PW , Kovanda F. Thermal transformations of Cu-Mg (Zn)-Al(Fe) hydrotalcite-like materials into metal oxide systems and their catalytic activity in selective oxidation of ammonia to dinitrogen . J Therm Anal Calorim . 2013 ; 114 : 731 - 47 .
18. Chmielarz L , Rutkowska M , Kus?trowski P , Drozdek M , Piwowarska Z , Dudek B , Dziembaj R , Michalik M. An influence of thermal treatment conditions of hydrotalcite-like materials on their catalytic activity in the process of N2O decomposition . J Therm Anal Calorim . 2011 ; 105 ( 161 ): 70 .
19. Kus?trowski P , We?grzyn A, Chmielarz L , Bronkowska A , Rafalska-?asocha A , Dziembaj R. Thermally induced transformations in polyoxometalate-pillared hydrotalcites . J Therm Anal Calorim . 2004 ; 77 ( 243 ): 51 .
20. Bankauskaite A , Baltakys K. Thermal , texture and reconstruction properties of hydrotalcites substituted with Na? or K? ions . J Therm Anal Calorim . 2015 ; 121 ( 227 ): 33 .
21. Smolakova L , Frolich K , Troppova I , Kutalek P , Kroft E , Capek L. Determination of basic sites in Mg-Al mixed oxides by combination of TPD-CO2 and CO2 adsorption calorimetry . When the same basic sites are reported from both techniques ? J Therm Anal Calorim . 2017 ; 127 : 1921 - 9 .
22. Meloni D , Monaci R , Cutrufello MG , Rombi E , Ferino I. Adsorption microcalorimetry characterization of K-doped MgAl mixed oxide catalysts for soybean oil transesterification synthesized by impregnation and ball milling techniques . J Therm Anal Calorim . 2015 ; 119 : 1023 - 36 .
23. Coriolano ACF , Alves AA , Araujo RA , Delgado RCOB , Carvalho FR , Fernandes VJ Jr, Araujo AS. Thermogravimetry study of the ester interchange of sunflower oil using Mg/Al layered double hydroxides (LDH) impregnated with potassium . J Therm Anal Calorim . 2017 ; 127 : 1863 - 7 .
24. Cavani F , Trifiro F , Vaccari A. Hydrotalcite-type anionic clays: preparation, properties and applications . Catal Today. 1991 ; 11 : 173 - 301 .
25. Taguchi M , Nakane T , Hashi K , Ohki S , Shimizu T , Sakka Y , Matsushita A , Abe H , Funazukuri T , Naka T. Reaction temperature variations on the crystallographic state of spinel cobalt aluminate . Dalton Trans . 2013 ; 42 : 7167 - 76 .
26. Li Y , Lu G , Ma J. Highly active and stable nano NiO-MgO catalyst encapsulated by silica with a core-shell structure for CO2 methanation . RSC Adv . 2014 ; 4 : 17420 - 8 .
27. Jab?on?ska M , Chmielarz L , We?grzyn A, Go?ra -Marek K , Piwowarska Z , Witkowski S , Bidzin?ska E, Kus?trowski P, Wach A , Majda D. Hydrotalcite derived (Cu, Mn)-Mg-Al metal oxide systems doped with palladium as catalysts for low-temperature methanol incineration . App Clay Sci . 2015 ; 114 : 273 - 82 .
28. Kovanda F , Rojka T , Bezdic?ka P , Jira?tova? K, Obalova? L , Pacultova ? K, Bastl Z , Grygar T. Effect of hydrothermal treatment on properties of Ni-Al layered double hydroxides and related mixed oxides . J Solid State Chem . 2009 ; 182 : 27 - 36 .
29. Zhang Q , Chen C , Wang M , Cai J , Xu J , Xia C. Facile preparation of highly-dispersed cobalt-silicon mixed oxide nanosphere and its catalytic application in cyclohexane selective oxidation . Nanoscale Res Lett . 2011 ; 6 : 1 - 7 .
30. Morpurgo S , Lojacono M , Porta P. Pillared hydroxycarbonates and mixed oxides. Part 1. Copper-zinc-cobalt-aluminum system . J Mater Chem . 1994 ; 4 : 197 - 204 .
31. Zayat M , Levy D. Blue CoAl2O4 particles prepared by the sol-gel and citrate-gel methods . Chem Mater . 2000 ; 12 : 2763 - 9 .
32. Liotta LF , Pantaleo G , Macaluso A , Di Carlo D , Deganello G . CoOx catalysts supported on alumina and alumina-baria: influence of the support on the cobalt species and their activity in NO reduction by C3H6 in lean conditions . Appl Catal A Gen . 2003 ; 245 : 167 - 77 .
33. van de Water LGA , Leendert Bezemer G , Versluijs-Helder JABM , Weckhuysen BM , de Jong KP . Spatially resolved UV-vis microspectroscopy on the preparation of alumina-supported Co Fischer-Tropsch catalysts: linking activity to Co distribution and speciation . J Catal . 2006 ; 242 : 287 - 98 .
34. Vakros J , Kordulis C , Lycourghiotis A. Cobalt oxide supported calumina catalyst with very high active surface area prepared by equilibrium deposition filtration . Langmuir . 2002 ; 18 : 417 - 22 .
35. Krezhov K , Konstantinov P. On the cationic distribution in MgxCo3-xO4 spinels . J Phys Condens Mater . 1992 ; 4 : L543 - 8 .
36. Han J , Jia L , Hou B , Li D , Liu Y , Liu Y. Catalytic properties of CoAl2O4/Al2O3 supported cobalt catalysts for Fischer-Tropsch synthesis . J Fuel Chem Technol . 2015 ; 43 : 846 - 51 .
37. Yu C , Zhou X , Weng W , Hu J , Chen X , Wei L. Effects of alkaline-earth strontium on the performance of Co/Al2O3 catalyst for methane partial oxidation . J Fuel Chem Technol . 2012 ; 40 : 1222 - 9 .
38. Chmielarz L , Piwowarska Z , Rutkowska M , Wojciechowska M , Dudek B , Witkowski S , Michalik M. Total oxidation of selected mono-carbon VOCs over hydrotalcite originated metal oxide catalysts . Catal Commun . 2012 ; 17 : 118 - 25 .
39. Dziembaj R , Molenda M , Chmielarz L , Zaitz MM , Piwowarska Z , Rafalska-?asocha A. Optimization of Cu doped ceria nanoparticles as catalysts for low-temperature methanol and ethylene total oxidation . Catal Today . 2011 ; 69 : 112 - 7 .