Oxidation of 4-methoxy-1-naphthol on promoted platinum catalysts

Article abstract of DOI:10.1134/S0023158417040103

Oxidative coupling of naphthols is a useful method for the formation of new carbon-carbon bonds in organic synthesis. In the presence of hydrogen peroxide, platinum supported on activated carbon catalyses this reaction. The outcome is influenced by the solvent, the reaction temperature and the physical structure of the catalyst. The catalyst structure is determined by the synthesis method and the modifier used (Bi or Sb). Within 40 min 4-methoxy-1-naphthol can be converted to 4,4'-dimethoxy-2,2'-binaphthalenyl-1,1'-diol with a yield of up to 94%, or to 4,4'-dimethoxy-2,2'-binaphthalenylidene-1,1'-dione with a yield of 92%. High amounts of quinoid byproducts (≤22%) are observed in nitromethane as the solvent.

Full text of DOI:10.1134/S0023158417040103

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 4, pp. 441–447. © Pleiades Publishing, Ltd., 2017.
Published in Russian in Kinetika i Kataliz, 2017, Vol. 58, No. 4, pp. 465–472.
Oxidation of 4-Methoxy-1-Naphthol on Promoted Platinum Catalysts1
M. V. Maphorua, J. Hevelinga, b, *, and S. Kesavan Pillaic
aDepartment of Chemistry, Tshwane University of Technology, Pretoria 0001, South Africa
bInstitute for Nano-Engineering Research, Tshwane University of Technology, Pretoria 0001, South Africa
cNational Centre for Nano-Structured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa
*e-mail: hevelingj@tut.ac.za
Received October 24, 2016
Abstract⎯Oxidative coupling of naphthols is a useful method for the formation of new carbon-carbon bonds
in organic synthesis. In the presence of hydrogen peroxide, platinum supported on activated carbon catalyses
this reaction. The outcome is influenced by the solvent, the reaction temperature and the physical structure
of the catalyst. The catalyst structure is determined by the synthesis method and the modifier used (Bi or Sb).
Within 40 min 4-methoxy-1-naphthol can be converted to 4,4'-dimethoxy-2,2'-binaphthalenyl-1,1'-diol
with a yield of up to 94%, or to 4,4'-dimethoxy-2,2'-binaphthalenylidene-1,1'-dione with a yield of 92%.
High amounts of quinoid byproducts (≤22%) are observed in nitromethane as the solvent.
Keywords: oxidative coupling, 1-naphthol, platinum, promoter, solvent effect, selectivity
DOI: 10.1134/S0023158417040103
INTRODUCTION
AC supported metal catalysts can be prepared with
the help of chemical and physical methods. Chemical
methods include impregnation and electroless (reduc-
tive) deposition. Physical methods make use of lasers,
sonication, microwave (MW) and UV irradiation, as
well as plasma technology and supercritical fluids [8].
The method of preparation has a decisive influence on
the structure and the morphology of the final product
[9]. Most deposition methods result in an extended par-
ticle-size distribution, accompanied by agglomeration
[8]. In contrast, use of MW irradiation during synthesis
can be expected to create homogeneous nucleation
sites, resulting in a narrowed particle-size distribution,
high metal dispersion, and uniform particle shapes [10,
11]. In addition, MW irradiation offers shorter reaction
times and reduced energy consumption [10, 12]. In
combination with ethylene glycol it is a very convenient,
fast and efficient method of metal deposition onto sup-
ports [13], as glycol can act both as the solvent and as
the reducing agent for metal precursors [14].
Aryl-aryl bond formations are of great importance in
synthetic organic chemistry. The oxidative coupling of
naphthols can be used for this purpose. Takeya et al. [1,
2] found that 4-methoxy-1-naphthol (1) couples in the
presence of oxygen and semiconductors, e.g. SnO2 and
ZrO2, to give the binaphthol 2, the binaphthone 3 and
the coupled naphthoquinone 4 (see the scheme). These
compounds are of importance for the pharmaceutical
industry [1, 2]. They are also useful intermediates for
the preparation of naturally occurring substances, such
as diosindigo B, biramentaceone, and violet-quinone
[1]. Based on an earlier patent [3], we recently reported
the effective use of Bi-promoted Pt supported on acti-
vated carbon (AC, 5%Pt–5%Bi/AC) as a heteroge-
neous catalyst for the oxidative coupling of substituted
1-naphthols that contain electron-donating groups in
ortho- or para-position to the hydroxyl group [4]. Pt–
Bi/AC is a commercially available catalyst for electro-
catalysis and selective oxidation reactions. With respect
to the latter application, it is speculated that Bi exerts a
geometric blocking effect on Pt, thus decreasing the
active site ensembles of Pt. These minimised sites still
allow for the desired chemical reaction to proceed, but
they are inactive for unwanted side reactions [5, 6].
Lately, we demonstrated that antimony can be used to a
similar effect, and 5%Pt–5%Sb/AC was shown to have
superior selectivity for the oxidation of cinnamyl alco-
hol to cinnamaldehyde in comparison to corresponding
Bi-containing catalysts [7].
In this study, the catalytic properties of 5%Pt–
5%Bi/AC and 5%Pt–5%Sb/AC catalysts prepared by
electroless (reductive) deposition (ED), including MW
assisted glycol reduction, are described. The nano-
structures obtained have an influence on the catalytic
activity and selectivity for the oxidative coupling of
4-methoxy-1-naphthol, which is used as the substrate.
EXPERIMENTAL
Materials
Antimony chloride (99%) was purchased from
Sigma-Aldrich and ethylene glycol (99.5%) from Pro-
1
The article is published in the original.
441

442
MAPHORU et al.
mark Chemicals. All other materials were as listed in a gen physisorption (BET) was carried out using a Micro-
previous publication [15]. The purity of 4-methoxy-1-
naphthol was 97% (Sigma-Aldrich).
metrics ASAP 2020 surface area and porosity analyser.
Approximately 0.05 g of each sample was degassed for 6 h
and analysed at –196°C (77 K) for 5 h using N2 as the
adsorbent. Sample morphologies and elementary com-
positions were analysed by means of a JEOL JSM-7500F
field-emission scanning electron microscope with an
electron accelerating voltage of 15.0 kV. All samples
were sputter coated with carbon. High-resolution trans-
mission electron microscopy (HRTEM) was performed
on a JEOL JEM-2100 HRTEM instrument. The elec-
tron acceleration voltage was 200 kV. The samples were
dispersed in ethanol using an ultrasonic bath, and sup-
ported on carbon-coated copper grids (200 mesh). Par-
ticle sizes were measured with the help of the ImageJ
software. Approximately 100 particles were measured
per sample. The full characterization of the organic
compounds was described previously [15].
Synthesis of Catalysts
The preparation of 5%Pt–5%Bi on AC by ED was
carried out as described earlier [15]. By testing with
AgNO3, the final aqueous filtrate was found to be free of
chloride. Vacuum-dried (VD) samples were dried at
room temperature (r. t.) under vacuum for 4 h, oven-
dried (OD) samples in an oven at 140°C for 24 h. The
same procedure was used to prepare VD 5%Pt/AC, as
well as VD and OD 5%Pt–5%Sb/AC; SbCl3 (0.1228 g,
0.4108 mmol) was the Sb precursor.
Microwave-assisted loading (MW): for the prepara-
tion of MW 5%Pt–5%Bi/AC, H2PtCl6 · 6H2O
(0.1328 g, 0.2564 mmol) and Bi(NO3)3 · 5H2O (0.1161 g,
0.2393 mmol) were transferred into a 100 mL beaker
containing ethylene glycol (20 mL), followed by ultra-
sonication for 10 min to completely dissolve the salts. To
this mixture, AC (0.9001 g) was added, followed by ultra-
sonication for 1 h prior to MW irradiation. The MW-
assisted metal loading was conducted in a Mars Xpress
MW system at 600 W and 190°C in a closed Teflon ves-
sel. The time to reach the final temperature was set at
5 min, and irradiation continued for 10 min at 190°C to
allow for the complete reduction of Pt(IV) to Pt(0).
Samples were centrifuged, washed with deionised water,
Oxidation Reactions and Product Separation
Oxidative coupling reactions (at r. t. or under reflux)
were carried out in the same manner as described earlier
[15]. That is, for each reaction 6.34 mmol starting mate-
rial (SM) and 0.0401 g catalyst were used in 25 mL of
solvent, to which 22.1 mmol H2O2 was added as a
and left to air-dry (AD) for 24 h. The same method was 30% aq. solution over 40 min. The reaction mixture was
followed to prepare MW 5%Pt–5%Sb/AC; SbCl3
(0.1228 g, 0.4111 mmol) was used as the precursor for Sb.
separated by column chromatography. Product selec-
tivities were calculated as mole percentages based on the
moles of SM incorporated into each product. Turn-
over frequencies (TOFs) are reported as moles of H2O2
required for the formation of all identified products
(compare Scheme 1) per moles of platinum present in
Characterization of Nanocomposites
and Organic Compounds
Elemental analyses (Pt, Bi and Sb) were performed
on a Spectro Arcos FHS ICP-OES instrument. Nitro- the catalyst over the total reaction time (min–1).
OMe
O
O
O
+H2O2
O
OMe
3
−2H2O
OMe
O
OH
−3H2O
+2H2O2
+H2O2
OMe
OH
5
6
OMe
O
+H2O2
+H2O2
OH
−H2O
−MeOH
−2H2O
2
OMe
−H2O
−MeOH
1
OH
4
OMe
O
O
Scheme 1. Oxidation of 4-methoxy-1-naphthol (1) over Pt catalysts; H2O2 is the oxidant.
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OXIDATION OF 4-METHOXY-1-NAPHTHOL
443
Table 1. Preparation methods, drying conditions, metal loadings, surface areas and average particle sizes of samples
Pt
Bi
Sb
APSa,
nm
S,
m2 g–1
Sample Preparation Drying
Drying
T, °C
Sample
Drying t, h
designation method
method
wt %
AC
VD 5%Pt/AC
AC
VD Pt
–
–
–
–
4
4
24
24
4
24
24
–
–
–
4.8
4.9
4.9
–
–
–
–
–
–
4.8
4.8
4.9
1195
865
655
535
860
625
620
855
–
39.7
8.75
3.26
6.96
–
ED
ED
ED
MW
ED
ED
MW
VD
VD
OD
AD
VD
OD
AD
r. t.
r. t.
140
r. t.
r. t.
140
r. t.
5.0
4.9
4.9
5.0
5.0
4.8
5.0
VD 5%Pt–5%Bi/AC VD Bi
OD 5%Pt–5%Bi/AC OD Bi
MW 5%Pt–5%Bi/AC MW Bi
VD 5%Pt–5%Sb/AC VD Sb
OD 5%Pt–5%Sb/AC OD Sb
MW 5%Pt–5%Sb/AC MW Sb
–
–
–
8.32
a
Determined by HRTEM.
RESULTS AND DISCUSSION
similar and a chain-like morphology emerges by
HRTEM (Fig. 1e); as for VD Sb, the nanochains were
also found to contain both Pt and Sb. Due to this
Catalyst Characterization
The sample designations, the preparation meth- peculiar morphology, particle-size distributions could
ods, the drying conditions, the surface areas and the not be determined (Table 1). For MW Sb (Fig. 1f) uni-
average particle sizes (APS) measured for the different form crystalline cubic particles containing Pt and Sb
catalysts are shown in Table 1. Independent of the syn- are seen, and an APS of 8.32 nm was calculated.
thesis method used, the metal loadings are always near
to the targeted values. The BET surface area of the AC
Oxidative Coupling of 4-Methoxy-1-Naphthol
support is close to 1200 m2/g. Lower total surface areas
are found after metal loading, probably due to some
pore blockage. Some restructuring of AC must also be
taken into consideration. Zhu and others [16]
observed the exfoliation of graphite by MW treatment,
accompanied by a considerable increase in surface
area. Similarly, our composites prepared by the MW
method distinguish themselves by high remaining sur-
face areas (855–860 m2/g).
Hydrogen peroxide was used as the oxidant. Ide-
ally, the addition rate of the oxidant should match the
rate of its consumption [18], as any excess of H2O2
must be expected to decompose on the catalyst surface
[19, 20] and may lead to the over-oxidation of the cat-
alytically active metal (Pt) [6]. For each test,
3.5 equivalents of the oxidant were added in a drop-
wise manner over 40 min, which was sufficient to
achieve total conversions over the catalyst with the
highest Pt dispersion (OD Bi) at r. t. and under reflux.
This is of significance, because the catalyst with the
highest Pt surface area must also be expected to exhibit
the highest decomposition rate for H2O2. Anderson
et al. [18] needed more than 5 equivalents of H2O2 to
HRTEM images of the Bi- and Sb-promoted cata-
lysts are shown in Fig. 1. From these the APS (Table 1)
were determined. The APS for VD Bi (Fig. 1a) is
8.75 nm. The nanoparticles of OD Bi (Fig. 1b) are
highly dispersed and have an APS of 3.26 nm. Small
particles are possibly more effective in blocking some
of the accessible pores of the AC support. Accordingly, completely convert 1-octanol over a 5%Pt–1%Bi/C
catalyst. Remarkably, the need for excess H2O2 is not a
OD Bi has the lowest surface area (535 m2/g) of all the
materials seen in Table 1. The APS of MW Bi (Fig. 1c) peculiarity of precious metal catalysts, e.g. Kholdeeva
is 6.96 nm. The unpromoted VD Pt has by far the et al. [21] used a 2.5-fold excess for an oxidation reac-
highest APS (39.7 nm) and also the highest total sur- tion catalysed by Ti-silicate.
face area of all metal-loaded samples.
Under our reaction conditions 4,4-dimethoxy-
For VD Sb (Fig. 1d) agglomerated chain-like 2,2'-binaphthalenyl-1,1'-diol (binaphthol 2) and 4,4'-
structures with a diameter of ~11.6 nm are observed. dimethoxy-2,2'-binaphthalenylidene-1,1'-dione (Rus-
No fringes were detectable. HRTEM-EDX analysis sig’s blue or binaphthone 3) are always the major
(Fig. 2) indicates that the nanochains contain both Pt products, accompanied by variable amounts of qui-
and Sb. This was also confirmed by SEM “point and nones, namely 1'-hydroxy-4'-methoxy-2,2'-binaph-
shoot” EDX analysis (not shown). The measurements thalenyl-1,4-dione (coupled naphthoquinone 4),
suggest the presence of almost amorphous Pt embed- 4-methoxy-1,2-naphthoquinone 5, and 1,4-naphtho-
ded in amorphous Sb or vice versa, thus forming a quinone 6 (Scheme 1). The binaphthol must be
nearly amorphous matrix. For carbon-supported regarded as the intermediate to 3 and 4 [4]. The con-
Pt‒Sb, also Yu and Pickup [17] found that Pt and Sb versions and selectivities obtained over our catalysts in
are always associated. The morphology of OD Sb is different solvents (at r. t. or under reflux) are listed in
KINETICS AND CATALYSIS Vol. 58 No. 4 2017

444
MAPHORU et al.
(а)
(b)
(c)
200 nm
(d)
100 nm
200 nm
(e)
(f)
10 1/nm
100 nm
200 nm
200 nm
Fig. 1. HRTEM images of VD Bi (a), OD Bi (b), MW Bi (c), VD Sb (d), OD Sb (e), and MW Sb (f).
Table 2. The TOFs are plotted in Fig. 3. These are tively converted further to the binaphthone 3. In this
based on the total amount (mol) of Pt present on each regard, OD Sb (entry 13) is the only exception
catalyst. Under comparable reaction conditions, this (93% conv.); it is also the only catalyst that does not
should lead to TOFs reflecting the relative activities of form any 3 and 6. As a rule, high amounts of quinones
the catalysts [22, 23].
are observed in MeNO2.
In THF at 60°C the reaction is very selective and
no quinones are formed (entries 15–21). Depending
on the catalyst composition, the conversion increases
up to 100% and increasing amounts of 2 are converted
further to 3. The TOFs (Fig. 3) improve in the order
OD Sb < VD Sb < VD Bi < MW Sb < OD Bi ≅
MW Bi ≅ VD Pt.
Judged by the TOFs, the catalyst activity in MeNO2
at r. t. increases in the order OD Sb < VD Sb < VD Pt <
MW Bi ≅ MW Sb = VD Bi < OD Bi (Fig. 3). Only over
the most active catalyst is 6 observed as a byproduct
(Table 2, entry 3). Under reflux (entries 8–14) conver-
sions are complete for all catalysts, and 2 is quantita-
In CH2Cl2 at 38°C (entries 22–28) OD Bi appears
to be the most active catalyst (entry 24); it gives 100%
conversion of 1, and 2 is completely converted to 3.
However, some 4 (17.4%) and a high amount of
unidentified side products (15.6%) are also found.
Therefore, based on the calculation of the TOFs, the
order of reactivity is as follows: OD Sb < VD Sb < VD
Bi ≅ VD Pt < OD Bi < MW Sb ≅ MW Bi. Like in THF,
the naphthoquinones 5 and 6 are not formed.
Figure 3 clearly demonstrates that VD and OD Sb
under all conditions are the two least active catalysts,
while VD Bi, MW Bi and MW Sb show a rather similar
performance. At least in MeNO2 at r. t. (first set of
plots in Fig. 3) OD Bi stands out as the most active
sample. In this set all catalysts are compared at partial
conversion with respect to 1 and 2, a condition that is
preferable for the meaningful comparison of catalysts.
600
Sb
500
400
O
Sb
300
200
100
Cu
Pt
Pt
C
Cu
Sb
Pt
Cu
Pt
Pt
Pt
Pt
0
1
2
3
4
5
6
7
8
9
10 11
Energy, keV
The structures of VD and OD Sb differ profoundly
from all the other samples. Both consist of agglomer-
ated Pt–Sb nanochains (Figs. 1d, 1e) and have similar
Fig. 2. HRTEM-EDX spectrum of the VD Sb nanochains
seen in Fig. 1d.
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OXIDATION OF 4-METHOXY-1-NAPHTHOL
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Table 2. Oxidation of 4-methoxy-1-naphthol catalysed by AC supported 5%Pt, 5%Pt–5%Bi and 5%Pt–5%Sb catalysts
prepared by different methods
Selectivity, mol %
Entry
Catalyst
Solvent
T, °C
Conv., %
2
36.9
37.3
7.2
3
4
–
5
–
6
–
1
2
VD Pt
MeNO2
r. t.
r. t.
r. t.
r. t.
r. t.
r. t.
r. t.
96
61.3
59.9
55.5
73.8
–
33.1
54.1
100
47.8
31.4
3.4
VD Bi
OD Bi
MW Bi
VD Sb
OD Sb
MW Sb
VD Pt
VD Bi
OD Bi
MW Bi
VD Sb
OD Sb
MW Sb
VD Pt
VD Bi
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
THF
–
–
–
3a
4
–
–
16.9
–
25.3
76.8
61.8
35.3
Trace
–
–
–
5b
6
–
–
–
–
–
–
–
7
64.0
74.0
88.4
67.1
83.1
63.4
–
–
–
–
53.3
100
100
100
100
100
93.2
100
97.0
8c
9
–
–
15.8
7.0
20.7
12.8
18.0
–
96
–
Trace
1.3
Trace
Trace
7.4
Trace
–
10a
11
96
–
–
96
–
–
12c
96
–
–
13a
14
96
74.2
–
–
96
82.0
95.2
64.4
–
11.7
–
15c
60
2.3
20.5
Trace
–
16c
17
THF
60
–
–
94.0
100
OD Bi
MW Bi
VD Sb
OD Sb
MW Sb
VD Pt
VD Bi
OD Bi
THF
60
60
60
60
60
38
38
38
14.0
15.0
93.5
93.7
35.2
41.2
81.0
84.9
Trace
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
18
19
20
21
22
23
THF
100
THF
≤100
6.2
–
THF
–
THF
100
63.1
58.1
41.3
67.0
–
CH2Cl2
CH2Cl2
CH2Cl2
–
92.7
100
58.1
Trace
–
24a
25
26
27
28
17.4
100
100
61.0
6.8
MW Bi
VD Sb
OD Sb
MW Sb
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
38
38
38
38
8.1
91.6
–
–
–
–
–
Trace
–
–
–
–
–
98.9
92.5
18.1
–
–
81.6
Trace
100
a
b
c
13
A violet-blue compound that appears to be a polymer was detected ( C NMR), together with other byproducts.
Polymeric compounds and black deposits formed.
Two unknown compounds (purple green) with similar retention factors could not be separated by column chromatography.
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MAPHORU et al.
15.0
13.8
16
14
12
10
8
14.8
14.8
14.4
14.4
14.2
14.2
14.0
13.8
13.3
12.6
12.4
12.0
11.3
11.0
11.1
7.8
7.2
6.7
6.7
6.4
6
4
2
0
4.7
4.1
1.9
0.5
0.5
0.2
Catalyst, solvent
Fig. 3. TOFs based on the productive consumption of H O , calculated for the catalysts used in different solvents (r. t. or reflux).
2
2
total surface areas (Table 1). Fringes could not be tionship could be expected for an oxidation reaction,
detected by HRTEM, suggesting that amorphous Sb but Takeya et al. [25] noticed that for SnCl4-mediated
covers Pt. However, some exposed Pt metal was
detected by SAED (insert of Fig. 1d). It is conceivable,
that all the materials other than VD and OD Sb are
catalytically more active because of a higher presence
of surface Pt. The differences in activity and selectivity
of the catalysts, as seen in Table 2 and Fig. 3, can
therefore be attributed to variations in the nanostruc-
ture, as well as variations in the metal distribution and
exposure. The APS of VD Bi, MW Bi and MW Sb are
relatively close to each other (7.0–8.8 nm, Table 1), an
observation that may significantly contribute to their
similarity in catalytic performance. In contrast, on
OD Bi platinum is highly dispersed (APS = 3.3 nm),
and this catalyst exhibits by far the highest TOF
(12 min–1) when compared to the other samples under
the condition of partial conversion in MeNO2 at r. t.
(Fig. 3). As should be expected, the activity of VD Pt
(APS = 39.7 nm) is rather low, and at r. t. in MeNO2
oxidative coupling reactions of 1-naphthol, solvents
with a low DN were more favourable. Nevertheless, in
our case the selectivity to the coupled products 2 and 3
was found to correlate negatively with the solvent accep-
tor number; no other correlations between solvent
properties and reaction data could be established [15].
CONCLUSIONS
Platinum is an efficient catalyst for the oxidative
coupling of 1. At complete conversion of the SM, high
selectivities to the binaphthone 3 can be obtained over
MW Bi in THF (85%) and in CH2Cl2 (92%). Over the
less active VD Sb catalyst the intermediate binaphthol 2
is formed as the principal product in THF (94%). No
quinoidal side products are found in THF. The highest
amounts of quinones are seen in MeNO2: over OD Bi
17% of 6 are formed at r. t. and 21% under reflux.
it shows a TOF of only 4.1 min–1.
The activity and selectivity of the catalysts is strongly
influenced by their structural and morphological char-
acteristics. These vary with the catalyst synthesis
method and the promoter element used (Bi or Sb). VD
and OD Sb are characterized by a chain-like morphol-
ogy of Sb and Pt with little exposure of Pt crystals.
Under all reaction conditions, these two samples are of
low activity. The highest activity is indicated for OD Bi,
which contains Pt in a well dispersed state. This sample
shows always maximum conversions of 1 and 2.
It was shown earlier [15] that the (most active) OD
Bi catalyst can be recycled without any significant
decline in activity, but metal losses of 2.6% (Pt) and
5.1% (Bi) occurred during the first run. In a control
test no catalytic activity of leached metal was found.
The results obtained are in agreement with the pre-
viously proposed mechanism, by which the reaction
proceeds on metallic-state platinum [4].
As to the effect of the solvent, it can be stated that
THF does not promote the formation of quinones,
and high yields of 2 can be obtained (93.5%). How-
ever, in MeNO2 quinones are formed in significant
quantities. In a dedicated study [15], we could not
establish a positive correlation between reactivity data
The results obtained are in agreement with a previ-
ously proposed mechanism, by which the reaction
proceeds on metallic-state platinum.
Ongoing work involves further characterization of
and the solvent donor number (DN) [24]. Such a rela- the catalysts and testing of other SMs.
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OXIDATION OF 4-METHOXY-1-NAPHTHOL
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12. Ko, F., Lee, C., Ko, C., and Chu, T., Carbon, 2005,
ACKNOWLEDGMENTS
M.V. Maphoru is grateful for a DAAD-NRF bur-
vol. 43, p. 727.
13. Li, H., Zhang, S., Yan, S., Lin, Y., and Ren, Y., Int. J.
sary.
Electrochem. Sci., 2013, vol. 8, p. 2996.
14. Kawasaki, H., Nanotechnol. Rev., 2013, vol. 2, p. 5.
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