Effect of alkali and alkaline earth metal dopants on catalytic activity of mesoporous cobalt oxide evaluated using a model reaction

Article abstract of DOI:10.1016/j.apcata.2018.01.028

Herein we report the synthesis of mesoporous cobalt oxides in pure (Co₃O₄) and alkali and alkaline earth metal doped form (Li-, Ca-, Cs-, and Na-, K-, and Mg/Co₃O₄) via the inverse micelle method. The as-prepared materials were characterized by powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen sorption (BET), hydrogen-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). Characterization results suggested that the as-synthesized materials are of amorphous and mesoporous nature. Their catalytic activity was investigated using a model reaction, namely the liquid-phase morin oxidation. Results revealed pure cobalt oxide to be the better catalyst compared to its doped counterparts. The stability of Li/Co₃O₄ material was investigated exemplarily by recycling and reusing the catalysts for as many as four catalytic cycles. Conversion of morin was complete in all runs and no significant metal leaching could be detected by the use of inductively coupled plasma mass spectrometry (ICP-MS).

Full text of DOI:10.1016/j.apcata.2018.01.028

Accepted Manuscript
Title: Effect of alkali and alkaline earth metal dopants on
catalytic activity of mesoporous cobalt oxide evaluated using a
model reaction
Authors: Ndzondelelo Bingwa, Semakaleng Bewana,
Matumuene Joe Ndolomingo, Naphtaly Mawila, Batsile
Mogudi, Phendukani Ncube, Emanuela Carleschi, Bryan P.
Doyle, Marco Haumann, Reinout Meijboom
PII:
S0926-860X(18)30045-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.01.028
APCATA 16535
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
17-7-2017
15-1-2018
26-1-2018
Please cite this article as: Bingwa N, Bewana S, Ndolomingo MJ, Mawila N,
Mogudi B, Ncube P, Carleschi E, Doyle BP, Haumann M, Meijboom R, Effect
of alkali and alkaline earth metal dopants on catalytic activity of mesoporous
cobalt oxide evaluated using a model reaction, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.01.028
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Effect of alkali and alkaline earth metal dopants on catalytic activity of
mesoporous cobalt oxide evaluated using a model reaction
Ndzondelelo Bingwaa, Semakaleng Bewanaa, Matumuene Joe Ndolomingoa, Naphtaly Mawilaa,
Batsile Mogudia, Phendukani Ncubea, Emanuela Carleschib, Bryan P. Doyleb, Marco Haumannc,
Reinout Meijbooma*
aDepartment of Chemistry, University of Johannesburg, PO Box 524, Auckland Park 2006,
Johannesburg, South Africa
bDepartment of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006,
Johannesburg, South Africa
c Lehrstuhl für Chemische Reaktionstechnik (CRT), Friedrich-Alexander-Universität Erlangen-
Nürnberg (FAU), Egerlandstrasse 3, 91058 Erlangen, Germany
Graphical Abstract
1

Highlights
1. Synthesis of Co3O4 and doped-Co3O4 catalysts is described.
2. The Co3O4 catalysts are active in oxidation of morin.
3. The H2-TPR and XPS analyses confirm electron structure change.
Abstract
Herein we report the synthesis of mesoporous cobalt oxides in pure (Co3O4) and alkali and
alkaline earth metal doped form (Li-, Ca-, Cs-, and Na-, K-, and Mg/Co3O4) via the inverse
micelle method. The as-prepared materials were characterized by powder X-ray diffraction
(pXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM),
nitrogen sorption (BET), hydrogen-temperature-programmed reduction (H2-TPR), and X-ray
photoelectron spectroscopy (XPS). Characterization results suggested that the as-synthesized
materials are of amorphous and mesoporous nature. Their catalytic activity was investigated
using a model reaction, namely the liquid-phase morin oxidation. Results revealed pure cobalt
oxide to be the better catalyst compared to its doped counterparts. The stability of Li/Co3O4
material was investigated exemplarily by recycling and reusing the catalysts for as many as four
catalytic cycles. Conversion of morin was complete in all runs and no significant metal leaching
could be detected by the use of inductively coupled plasma mass spectrometry (ICP-MS).
Keywords: Morin oxidation; reducible metal oxides; metal dopants, effective kinetics, transport
limitation.
Introduction
Model reactions have been successfully applied to study catalytic systems [1-4]. Over the past
decades several reports emerged describing kinetics of model reactions such as e.g. morin
oxidation and 4-nitrophenol reduction [5-7]. Most of these studies employed metallic
nanoparticles as catalysts [8] and results suggested that these model reactions take place on the
2

surface of the catalyst [9, 10]. Not surprisingly, surface properties such as surface area and
electronic structure of the catalyst are of significant importance in heterogeneous catalysis [11].
Previous studies have indicated that an increase in the surface area of the catalyst results in
increased activity, and changes in the electronic structure of the catalyst impact the activity either
in a positive or negative way [12]. Such changes of the catalytic system can be induced in several
ways, including, but not limiting to the immobilization of catalytically active species [13, 14],
reduction or oxidation of the catalyst [15], or the use of doping agents [16-18]. In catalytic
oxidation with oxygen-rich catalysts such as reducible transition metal oxides, it is believed that
the lattice oxygen participates in the oxidation process according to a Mars van Krevelen
mechanism [19, 20]. Thus, the extent of catalyst reduction or oxidation is of significant
importance.
Metal oxides have been successfully employed as catalysts in a wide range of oxidation reactions
and the electronic structure is believed to be one of the defining properties of these oxide
catalysts. Doped metal oxide catalysts are reported to be better catalysts in many oxidation
reactions. Studies on NiO system doped with Zr proved the positive effect of doping with Zr in
oxidative dehydrogenation of ethane at high temperatures [21]. In this case, the improved
selectivity indicate a change in chemical environment around the active Ni centers. Such a
change in chemical environment of the active metal can be brought about by changes in acidity
and basicity of the transition metal and the lattice oxygen, respectively. This then lead to altered
strength of chemisorption of the substrate and intermediates, and the rate of product desorption.
Depending on the electronic properties of the valence band of the dopant, it can either bind
strongly or weakly to the lattice oxygen, thus, the Lewis acidity of the metal cation changes.
Other materials with tuneable properties were reported e.g. by Tian et al. [22]. These materials
have mixed phases of manganese oxides in the lattices such as Mn2O3, and Mn3O4. With these
materials, catalytic conversions of 8 and 10% were achieved for the oxidation of cyclohexane
and n-hexane, respectively. Recently, ternary oxides of Mn and Co were reported to have
different catalytic activities due to different interactions with Ru which was used as the third
metal [23].
Based on these observations, were became interested in the effects of different alkali and alkaline
earth metal doping agents on mesoporous cobalt oxide in oxidation reactions. Morin oxidation
3

was chosen by virtue of being a model reaction and because of the ease at which the reaction can
be monitored using fast and reliable spectroscopic techniques such as UV-Visible
spectrophotometry. Scheme 1 shows the oxidation of morin to morin oxide and its overoxidation
to multiple phenolic acids.
Scheme 1. Oxidation of morin to morin oxide and overoxidation to multiple products.
In the past, information gained from morin oxidation has been used to elucidate the mechanism
of oxidation of similar flavonoids such as e.g. quercetin. It was reported that the presence of
H2O2 as an oxidant is essential for generation of characteristic fragments and the positions of the
hydroxyl groups determine the types of products formed [24]. Later, it was then suggested that
biomimetic chemical oxidations of naturally occurring polyphenols may help elucidate oxidation
mechanisms in e.g. food commodities [25].
Experimental
Materials and chemicals
The cobalt(II) nitrate hexahydrate (99 %), morin monohydrate (≥99 %), poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol) (P-123), hydrogen peroxide (H2O2)
(30 % in water), 1-butanol (≤99 %)were all purchased from Sigma-Aldrich and used without
further purification. Nitrate salts of cesium (99 %), magnesium (≤99 %), sodium, and lithium
nitrate (Reagent Plus) were also purchased from Sigma-Aldrich and used without further
purification. Anhydrous sodium carbonate (99 %), sodium hydrogen carbonate (99.5 %), calcium
nitrate (≥98 %) and potassium nitrate (99 %) salts were purchased from Associated Chemical
Enterprises. Milli-Q water (18 MΩ·cm) was used in all experiments.
Catalyst synthesis
4

The preparation of the catalysts was achieved with the use of a soft-template approach. In a
typical experiment, 8 g of templating polymer P-123 was dissolved in 60 ml of 1-butanol. A 20 g
of the metal precursor, cobalt(II) nitrate hexahydrate, was added and allowed to dissolve. At this
stage, the dopants were added and allowed to react for 20 minutes under vigorous stirring. Then,
10.3 ml of 55 % nitric acid was added and the mixture was allowed to react for 3 hours. The
resulting pink gel was placed in an oven set at 60 °C for 12 hours to evaporate the solvent. Then,
a heating cycle followed. The material was placed in an oven at 120 °C for 5 hours. The powder
obtained was washed with ethanol and centrifuged and dried in a vacuum oven for 12 hours. The
dried powder was further calcined at 150 °C for 12 hours.
Catalyst characterization
Prior to catalytic evaluation, the catalysts were characterized using different physical techniques.
For the pXRD analysis, the materials were carefully ground and analyzed with a Rigaku
MiniFlex 600 powder diffractometer equipped with Cu Kα1 (λ = 0.1542 nm) radiation source.
Low angle measurements were performed between 0.5 and 10° 2θ at a step rate of 0.1 °/min and
high angle measurements were performed between 10 and 90° 2θ with a step rate of 0.4 °/min.
The resulting p-XRD pattern was matched using the Match! 2 software [26].
BET measurements were performed using a Micromeritics ASAP 2460 surface area and porosity
analyzer. Prior to the analysis, the samples were degassed with N2 gas at 90 °C for 12 hours. All
the measurements were carried out at -196 °C and the pore volume and pore sizes were
calculated from the adsorption curves using the BJH model.
The TEM and HRTEM analyses were performed using a JEOL-JAM 2100F electron microscope
operating at 200 kV. The catalytic materials were dispersed in small amounts of ethanol and
sonicated for 30 minutes. Thereafter, they were placed on top of carbon-coated Cu-grids.
The SEM analysis was performed on a Tescan Vega 3LMH scanning electron microscope with
the samples carbon-coated on copper grids using an Agar Turbo Carbon Coater.
The amount of dopants in the catalysts was determined by Spectro Acros inductively coupled
plasma omission electron spectroscopy (ICP-OES). The materials were digested using aqua regia
5

(1:3 v/v nitric acid and hydrochloric acid). Catalyst leaching test was done on a Shimadzu
ICPMS-2030 spectrometer.
The H2-TPR measurements were carried out using an AutoChemII from Micromeritics. The
analysis was performed between 25 and 900 °C with 10:90 H2-He ratio at a temperature ramping
rate of 10 °C/minute. Furthermore, CO2-TPD analyses was carried out using the same
instrument. A mixture with a ratio of 10:90 carbon dioxide and helium was used as a probe gas.
Analysis was carried out with a ramping rate of 3 °C/min in the temperature range of 30 to
550 °C.
XPS data were acquired at room temperature with a SPECS Phoibos 150 electron energy
analyser, using a monochromatised Al K photon source (h = 1486.71 eV). The overall energy
resolution was set to 0.5 eV for all the spectra. The binding energy was calibrated with respect to
the main peak in the C 1s core level (284.6 eV). To counteract the effect of surface charging,
samples were irradiated with a low energy electron flood gun during measurements (electron
energy: 3 eV, emission current: 20 mA).
Catalytic evaluation
All catalytic runs were performed in the dark in a 50 ml round-bottom flask covered with silver
foil. Briefly, a 25 × 10-3 M morin stock solution was prepared in 5 × 10-3 M carbonate buffer. In
a typical reaction, 1 × 10-3 M of morin and 10 × 10-3 M of H2O2 were used. The catalyst
concentration was varied between 3.46 × 10-6 M and 34.7 × 10-6 M. Prior to the catalytic
reaction, the solid catalysts were dispersed in de-ionized water (0.05 g in 10 ml de-ionized
water). Sampling was performed at three minutes intervals. Monitoring of the catalytic reaction
progress was performed using a Shimadzu UV-1800 UV-Vis spectrophotometer at λ 410 nm.
Catalysts stability was investigated by recycling the catalysts and reusing it for as many as four
cycles. The fine Li/Co3O4 catalyst powder was filtered over a 5 µm frit and the supernatant was
analyzed for Li and Co ions using Shimadzu ICPMS-2030 to investigate leaching of the material
during the reaction.
Results
Catalyst characterization
6

The powder X-ray diffraction patterns obtained in low angle measurements for the pure and
doped-cobalt oxides (Figure 1(a)) indicate that the materials have long-range mesoporous
structures. This is evidenced by the fact that diffraction took place below 2° 2θ range. The high
angle measurements shown in Figure 1(b) confirm a spinel structure of the synthesized cobalt
oxide (JCPDS 090418). Furthermore, the crystallite system was found to be cubic by Match !2
software. Usually in low angle measurements by conventional diffractometer set-ups, similar
features are observed. The low angle features observed herein are characteristic of the as-
synthesized materials as there is a clear distinction between them and a non-porous cobalt oxide
standard, Figure S1 in the ESI.
Figure 1: Diffraction pattern of pure and doped cobalt oxides. Graphs in (a) show low angle
diffraction between 0.5° and 2° 2θ range and in (b) is the wide angle diffraction patterns
7

measured between 20° and 65° 2θ. The N2 sorption results showing (c) type IV hysteresis loops
and (d) the pore size distribution of pure and doped cobalt oxides.
Figure 1(c-d) shows the type IV hysteresis loops and pore size distributions. The surface
properties such as the surface area, pore volume, and pore size were determined through nitrogen
sorption experiments. All the synthesized catalysts appeared to be mesoporous in nature and type
IV hysterisis loops, which suggest the mesoporous nature, were obtained from adsorption and
desorption branches. The pure cobalt oxide has the largest surface area compared to all the doped
catalysts. It is important to note that the average pore diameter of cobalt oxide increased upon
doping, thus, leaving the pure cobalt oxide with the highest pore volume compared to all other
catalysts. Table 1 show the surface properties of the synthesized pure and doped cobalt oxide.
Table 1: Surface properties of pure and doped cobalt oxide obtained from N2 sorption
experiments.
a
b
c
Catalyst
SBET
Vpore
Dpore
(nm)
12
Crystallite sized % dopinge
Basicity
(m2/g)
32.9
10.7
17.3
4.7
(cm3/g)
0.93
0.04
0.05
0.03
0.06
0.08
0.11
(nm)
(µg CO2)
Co3O4
22
48
86
15
36
40
32
-
56.1
14.5
75.7
-
Li/Co3O4
Na/Co3O4
K/Co3O4
Cs/Co3O4
15
2.4
2.1
2.3
2.4
2.6
2.4
13
18
17.0
13
145.1
224.3
441.5
Mg/Co3O4 22.1
15
Ca/Co3O4 20.8
19
a BET surface area, b pore volume calculated from desorption branch (BJH model), c pore diameter
calculated from the desorption branch (BJH model), and d Crystallite sizes calculated using Scherrer
equation. e dopants %loading as determined by ICP-OES.
TEM images revealed the porous nature of the synthesized cobalt oxide particles and the
morphology of the cobalt oxide particles did not change after doping. A wide range of shapes
were observed for both the pure and doped cobalt oxide particles. From the scanning electron
microscope images, the distribution of size varied from mostly larger to smaller particles. Figure
8

2 shows TEM and SEM micrographs of pure and Li/Co3O4 (also see Figure S2 and S3 in ESI for
the remaining materials).
Figure 2: TEM images of (a) pure cobalt oxide and (b) lithium doped cobalt oxide showing
mesoporous nature. SEM images for (c) pure cobalt oxide and (d) lithium-doped cobalt oxide
showing the morphology of the catalysts.
In oxidation reactions it is important to investigate reducibility of the catalysts as it relates to
different catalytic activities [18]. The H2-TPR revealed the temperatures at which the catalysts
can be reduced. Based on the results obtained, the catalysts were reduced at fairly low
temperatures compared to other catalysts such as Ni-Zr-O catalysts [21]. The reduction peaks of
pure mesoporous oxide shifted slightly upon doping with alkali and alkaline earth metals.
Although cesium and lithium did not affect the reduction temperature of pure cobalt oxide
significantly, there were significant shifts to higher temperatures for most dopants. This shifting
9

to higher temperatures of reduction peaks upon doping is consistent with literature reports [27].
Figure 3a illustrates graphically the temperature-programmed reduction results.
Figure 3: (a) H2-TPR for pure and doped-Co3O4 catalysts and (b) the O 1s core level electrons
XPS spectrum for undoped and doped Co3O4 catalysts.
In Figure 3b, XPS data of the O 1s core level electrons show three components. Component 1 is
the one on the higher binding energy side of the main peak, and it is due to oxygen vacancies in
the main Co3O4 matrix, component 2 (or the main peak) originates from stoichiometric oxygen
in Co3O4, and component 3 only appears in the doped catalysts, and can be attributed to charge
transfer from the dopant to the oxygen ions.
For the transition metal component, Co changes in the chemical environments of the Co 2p1/2 and
Co 2p3/2 peaks upon doping were observed. A slight shift towards lower binding energies is
observed upon doping, Figure 4 and Figure S4 in ESI. However, XPS analysis did not pick up all
10

the dopants. Most notable, is the absence of the Li peak. This is due to the low % loading and
due to Li being a light element. The peaks for other dopants are shown in Figure S5 and the
survey spectra is shown in Figure S6 in ESI. The influence of doping on the binding energy is
evident in all doped catalysts. The line shape of the core level is composed of two sets of peaks
corresponding to 2p3/2 and 2p1/2 spin-orbit states, whose centroids are located at binding energies
of ~779 and ~794 eV, respectively, separated by a spin-orbit splitting of 15.1 eV. The Co 2p3/2
binding energy region is located between 773 eV and 791 eV, while the Co 2p1/2 binding energy
region is located between 791 eV and 807 eV. The Co 2p line shape of the undoped catalyst can
be fitted with three spin-orbit doublets, corresponding to Co2+ and Co3+ states. The dashed spin-
orbit doublet, Figure 4 (bottom), which is not present in the undoped catalyst, consistently
appears in the Co 2p core level spectra of all doped catalysts. It is located at about +1.8 eV
relative to the main Co2+ component. It can attributed to charge transfer from the dopant to
cobalt. It is either an additional oxidation state for Co (i.e. Co1+ in this case), or a “shifted” Co2+
state due to the charge transfer mentioned above.
Figure 4: Changes in binding energy for pure and doped metal oxides determined by XPS
analysis. (a) Pure Co3O4 and (b) Cs/Co3O4.
11

From the CO2-TPD experiments, the dopants increased the basic sites of the catalysts. One
interesting observation is that the basic sites increase with increase in the valence of the alkali
metal. The same trend is observed for alkaline earth metals. Furthermore, alkaline earth metals
appeared to increase the total basicity of the catalyst more than the alkali counterparts. The TPD
plots can be seen in Figure S7 in the supplementary information.
Catalytic experiments
First the reaction was conducted in the absence of any catalyst but appropriate amounts of
hydrogen peroxide. There was no activity observed as shown in Figure S8. In the presence of
catalyst, the conversion of morin was observed by the disappearance of the peak at λ = 410 nm
and appearance of a new absorption peak around λ = 320 nm over time (see Figure 5(a) and (b)).
Kinetic measurements revealed pseudo-first order kinetics of the catalytic oxidation of morin
with excess peroxide with respect to the concentration of morin for both the pure cobalt oxide
and the doped catalysts. This assumption seemed justified by the linearity of the natural
logarithmic plots against time shown in Figure 5(c). Interestingly, all dopants had a negative
effect on the catalytic activity of the cobalt oxide as the apparent rates decreased upon doping.
Amongst the dopants, potassium proved to be the worst as apparent rates decreased significantly.
On the other hand, cesium was a better dopant, showing a high rate compared to other dopants.
The apparent rates increased with increasing concentrations of the catalysts, as shown in Figure
5(d) and in Figure S9. However, the slope from a differential analysis (see Figure S10 in ESI)
revealed a fractional order with respect to active metal in the range of 0.2 (for pure Co3O4) to 0.7
(for Mg/Co3O4). This rather low value for the dependency of effective rate with concentration of
active metal could be a hint for the mass transport of morin to the active sites limiting the overall
reaction.
12

Figure 5: (a) Time-based spectral evolution of morin oxidation measured at time intervals of
three minutes. (b) Kinetic traces for catalyzed oxidation of morin monitored at λ = 410 nm. (c)
Natural logarithmic plots for the determination of reaction order. (d) Dependence of apparent
rate constants on the Co3O4 concentration monitored at λ = 410 nm. Reaction conditions: 1 × 10-3
M of morin, 10 × 10-3 M of H2O2 and 6.9 × 10-6 M catalyst (In graph d the concentration of the
catalyst was varied)
Such mass transport limited reactions are often encountered in heterogeneously catalyzed
reactions [28]. A-priori criteria are a common technique to estimate the influence of mass
transport based on only a few preliminary experimental data. Here, we used the well-established
Weisz-Prater criterion to assign the influence of pore diffusion limitations.
r  R2
eff
(1)
 
cs  Deff
13

As indicated in Equation (1), the criterion requires the effective reaction rate reff, the known
catalyst pellet radius R, substrate concentration cs (assuming no film diffusion limitation) and the
effective Diffusion coefficient Deff. The effective diffusion coefficient was estimated from
literature data of the molecular diffusion coefficient of morin in water to be 5.45 × 10-10 m2 s-1
(see SI for details). With particle pellet sizes in the range of 5 µm, the Weisz-Prater criterion
resulted in value well below 0.1 for all catalyst tested (see Supporting Information for details).
This is in agreement with the fractional order of catalyst concentration. This indicates that the
metal oxide catalysts are highly active for the oxidation reaction and although the pellet size is
small, this high intrinsic activity results in the diffusion through the pores of the catalyst to be the
overall rate limiting step.
Lithium doped cobalt oxide was chosen for catalyst recyclability and reusability tests because it
allowed the detection of both Co and Li via ICP-MS to account for possible metal leaching. The
lithium doped cobalt oxide was recycled using a centrifuge and reusability tests revealed only a
minor decrease in conversion from the second to the fourth run as shown in Figure 7.
Interestingly, the first catalytic run appeared to be slower than the second to fourth runs. This can
be attributed to surface restructuring of the catalyst during the first catalytic cycle resulting in a
slower reaction. To further account for this, the second , third and fourth catalytic runs gave
reactions rate constants of 8.9, 9.2 and 8.8 × 10-4 min-1, respectively. These values do not differ
significantly from each other compared to 6.61 × 10-4 min-1 from the first catalytic cycle. The
similar reaction rate constants up to the fourth catalytic run are an indication of good catalytic
stability. It even seems that the catalyst required some induction period through the first run to
become fully active. The small difference in activity correlated with the leaching test using ICP-
MS, as they indicated that there was no detectable leaching of either Li nor Co ions from the
catalytic material (ICP-MS detection limit of 1 ppm). This illustrates the good stability of the
material in catalytic conditions set out in this work.
This behavior was also reported in literature for the reduction of 4-nitrophenol with similar
catalysts [7].
14

Figure 7: Catalyst reusability study using the Li/Co3O4 catalysts showing slight changes in
apparent reaction rate constants with each catalytic cycle. Reaction conditions: 2 × 10-3 M of
morin, 10 × 10-3 M of H2O2 and 21 × 10-6 M catalyst.
Discussions
The surface properties of the synthesized cobalt oxide and the alkali- and alkaline-earth metal
doped cobalt oxide show the mesoporous nature as depicted by the type IV hysteresis loops. The
type IV hysteresis loops are similar to the one reported by Wu et al. [29]. This mesoporous
nature of the materials is brought about by the structure-directing inverse micelles that form by
the P-123 surfactant. Also the use of nitric acid to induce acidic media played a crucial role.
Literature results indicate that the NOx species formed by decomposition of nitrate ions are
adsorbed on the surface of oxo-clusters, and thus, prevent condensation of the clusters [18].
The effect of the dopants on Co3O4 is significant on the surface properties such as surface area
and pore characteristics. The alkaline earth metals induce a relatively small reduction of the
surface area compared to the alkali counterparts. Also changes in pore volume of the undoped
Co3O4 are consistent with the above trend. The alkaline earth metal dopants reduced the pores of
Co3O4 considerably less than the alkali earth metals. These results are consistent with literature
and are mostly due to increases in crystallite sizes upon doping, see table 1 [30].
Furthermore, the sol-gel method used in the synthesis of the cobalt oxide yielded spinel cobalt
oxide structure. The presence of Co3+ and Co2+ in the cobalt oxide structure is indicative of
15

different reactive sites and thus different binding energy between the cations with different
oxidation states and lattice oxygen of the oxide. The binding of oxygen to different cobalt
species is evidenced by two reduction peaks in temperature-programmed reduction. The first
peak that appeared is assigned to the most electropositive cobalt cation due to ease of reduction.
Changes in electronic structure upon doping changed the electronic environment around the
cationic centers, evidenced by shifting of reduction peaks in H2-TPR measurements, Figure 3a,
and shifting binding energy in XPS (see Table S2 in supplementary information). These two
methods are complementing each other. In XPS analysis, an increase of electron density around
the cationic center is observed. This phenomenon is also observed in H2-TPR analysis.
The slight reduction in catalytic activity of Co3O4 upon doping is not surprising. The decreased
surface area of the Co3O4 upon doping is the major contributing factor because changes in the
electronic structure of the original oxide as depicted by H2-TPR measurements and XPS are
minimal. However, even minimal changes in chemical and physical environment of the pure
Co3O4, can play a role in the catalytic activity of this material. The shifting of reduction peaks to
higher temperatures, indicative of a change in electronic environment on the catalyst surface,
does not lead to improved catalytic activity in oxidation reactions. The less reducibility of the
catalyst is an indication of a poor oxidation catalyst [18]. Also the increase in basicity of the
catalyst upon doping is an indication of less adsorption of morin on the catalysts surface. Morin
is a polyphenolic compound, thus it has many basic hydroxyl moieties which adsorb better on
acid sites. Furthermore, we recently showed that mesoporous cobalt oxide is a good reduction
catalysts using the reduction of 4-nitrophenol as a model reaction [31]. In our recent publication,
we also reported other active transition metal oxides including cobalt oxide in 4-nitrophenol
reduction [32]. Changes in the electronic structure of the catalytically active metal oxide supports
upon immobilization of AuPd nanoalloys was the main reason for improved catalytic activity.
Herein, this change in electronic structure is also evident in the XPS analysis, Figure 3b, where
there is an extra component on the O 1s core level electrons indicative of a different electron
environment compared to the undoped catalyst.
It is important to note that the negative effect the alkaline earth metal dopants have in the
electronic structure, as shown by significant shifts to higher temperature compared to the alkali
counterparts in the H2-TPR measurements, Figure 3, is countered by the better surface areas they
16

posses. While on the other hand, the smaller surface areas of the alkali doped Co3O4 catalyst is
countered by the better reducibility when compared to the alkaline earth metal doped Co3O4
catalysts. This might be one of the reasons for similar activities observed for the doped catalysts.
It is, however, not the first time that doping with alkali and alkaline earth metals decrease
catalytic activity in oxidation reaction. Elkins et al. reported decrease in activity of MgO catalyst
in oxidative coupling of methane upon doping with Li [33].
Conclusion
The synthesis, characterization and catalytic activity of pure and alkali- and alkaline earth metal
doped-Co3O4 was successfully described. Furthermore, the incorporation of doping agents did
not change the morphology of the mesoporous cobalt oxide. The catalytic activity of the as-
synthesized catalyst decreased upon doping and based on the H2-TPR and XPS analysis
electronic properties were altered. Textural properties also play a significant role in determining
the catalytic activity.
Furthermore, comparison of the doped catalysts amongst themselves illustrate that the basicity of
the dopant plays a crucial role in the activity as more basic dopants result in higher reaction rates
in the oxidation of morin. All the catalyst are very active for oxidation of morin. In addition, Co
is a less expensive metal compared to noble metals such as Pt and Au. Therefore, these catalysts
have a potential to replace noble-metal-based catalysts.
Acknowledgements
This work is based on the research supported in part by the National Research Foundation of
South Africa (Grant 85386 and 93205). We would also like to thank the University of
Johannesburg for funding, and Shimadzu South Africa for the use of their instruments.
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