Electro-oxidation of coal on NiO and/or Co3O4 modified TiO2/Pt electrodes

Article abstract of DOI:10.1016/j.electacta.2011.05.039

A TiO₂/Pt based electrode exhibited better activity for the oxidation of coal in a basic system compared to Ti/Pt, TiO₂-Cu/Pt and pure metal electrodes. The surface morphologies and composition of the electrodes were studied by SEM and XRD, respectively. Linear sweep voltammetry was employed to investigate the catalytic effects of electrodes, and the product of coal oxidization was determined by a gas collection test. The TiO ₂/Pt electrodes that were modified with NiO and/or Co ₃O₄ exhibited higher average currents and a lower decrease in mass during electrolysis compared to the other electrodes; this finding indicated that NiO and Co₃O₄ play important roles as catalysts.

Full text of DOI:10.1016/j.electacta.2011.05.039

Electrochimica Acta 56 (2011) 6299–6304
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electro-oxidation of coal on NiO and/or Co3O4 modified TiO2/Pt electrodes
Chao Wang, Yonggang Zhao, Wei Zhou∗, Huaiyou Liu, Renhe Yin∗
Department of Chemistry, Shanghai University, Shanghai 200444, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A TiO2/Pt based electrode exhibited better activity for the oxidation of coal in a basic system compared to
Ti/Pt, TiO2–Cu/Pt and pure metal electrodes. The surface morphologies and composition of the electrodes
were studied by SEM and XRD, respectively. Linear sweep voltammetry was employed to investigate the
catalytic effects of electrodes, and the product of coal oxidization was determined by a gas collection test.
The TiO2/Pt electrodes that were modified with NiO and/or Co3O4 exhibited higher average currents and
a lower decrease in mass during electrolysis compared to the other electrodes; this finding indicated that
NiO and Co3O4 play important roles as catalysts.
Received 20 January 2011
Received in revised form 11 May 2011
Accepted 11 May 2011
Available online 19 May 2011
Keywords:
Coal
Electro-oxidation
TiO2/Pt electrodes
Basic electrolyte
Metal oxide catalyst
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
anode gas consists of CO2 [10]. The mechanism of coal oxidation
has been proposed as follows [11–13]:
Electro-oxidation of coal is considered a highly efficient way to
utilize fossil fuel, namely clean coal technology (CCT) [1]. The elec-
trolysis of coal, which can be used as a depolarization agent [2],
is an alternative for hydrogen production. At 25 ◦C, the theoretical
potential of coal electrolysis is 0.21 V [3], which is much lower than
the water electrolysis potential (1.23 V) [4]. The actual energy con-
sumption for coal electrolysis is only one-third to one-half of the
energy for water electrolysis. CO2 and H2, collected at the anode
and cathode, respectively, are pure enough for use without further
purification.
Anode : C + 2H2O → CO2 + 4H+ + 4e−
Cathode : 4H++ 4e− → 2H2
(1)
(2)
(3)
Total : C + 2H2O → CO2 + 2H2
The catalytic effect of various electrodes has been investigated,
and Pt is commonly used as a working electrode [3–6]. Sathe and
Botte plated noble metals (Pt, Rh, Pt–Rh, Pt–Ir, and Pt–Ir–Rh) on car-
bon fibers and investigated their electrolytic efficiency of producing
CO2 from coal; they found that Pt–Ir showed the best resistance
to erosion and weight loss consisted of only 1%. All the electrodes
exhibited a similar performance, and the energy consumption was
50% lower compared to hydrogen production by the electrolysis of
water under similar operating conditions [7]. Our group investi-
gated Ti/Pt–Fe [8], Ti/TiO2–Pt and Ti/TiO2–Pt–Ru [9] electrodes for
the electro-oxidation of coal in sulphuric acid, and we found that
Ti/Pt–Fe exhibited the best activity. A less expensive Pt–Fe alloy was
generated on the Ti sheet surface. The metal oxide modified elec-
trodes (Ti/IrO2–RuO2, Ti/IrO2) also perform well because 99.98% of
Thus far, most work has focused on the electro-oxidation in
acidic systems. However, the oxidation in a basic system could liq-
uefy coal in high efficiency. Humic acid, traditionally produced by
an electrical method, is the main product in basic systems [14–16].
It is different from the oxidization in acidic systems, in which coal is
used as a depolarization agent. Senftle et al. reported that the oxida-
tive consumption of coal to produce alkali-soluble compounds was
found to proceed preferentially at the edges of aromatic planes [17].
Organic sulfur removal from coal by electrolysis is also performed
in alkaline media [18]. A basic system enables the utilization of
electrodes modified with metal oxides, which have shown their
catalytic activity in organic electrochemistry because metal oxides
may dissolve in acidic systems [19].
In the present work, several electrodes were investigated for
the electro-oxidation of coal. We determined that TiO2/Pt based
electrodes modified by NiO and/or Co3O4 showed the best catalytic
effect.
∗
Corresponding authors. Tel.: +86 021 66133517; fax: +86 021 66133517.
E-mail addresses: zhouw@shu.edu.cn (W. Zhou), yinrh@staff.shu.edu.cn (R. Yin).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.05.039

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C. Wang et al. / Electrochimica Acta 56 (2011) 6299–6304
Fig. 1. SEM images of the electrodes: (a) TiO2/Pt; (b) TiO2/Pt–Co3O4; (c) TiO2/Pt–NiO; (d) TiO2/Pt–Co3O4–NiO; (e) TiO2/Pt; and (f) TiO2–Cu/Pt. Co3O4 and NiO were electrode-
posited; (a–d) Pt was electrolessly deposited onto TiO2 and (e and f) Pt and Cu were electrodeposited.
2. Experimental
2.2. Electro-oxidation of coal
2.1. Preparation of electrodes
A potentiostat (CHI660B, Shanghai Chenhua Instrument Com-
pany) was used for the electrolysis experiments. The cell consisted
of an H-type cell, whose compartment was separated by a porosity
Ti substrates (2 cm × 2 cm) modified by porous TiO2 layers were
obtained as previously described [9]. Pt particles were electro-
lessly plated in a plating solution (0.2 g/L of H2PtCl6·6H2O, 12 g/L of
hydrazine hydrate, 5 g/L of hydroxylamine hydrochloride, pH = 4.5,
T = 45 ◦C). The substrate was covered by Pt particles in 10 min.
Under a constant current density of 2.1 mA/cm2 and pH = 4.5,
the TiO2/Pt electrodes were electroplated with a different metal
oxide catalyst and in different electroplate solutions in 5 min as
shown in Table 1. Finally, the prepared electrodes were heated to
500 ◦C to form the dense phase and then cooled to room tempera-
ture.
Table 1
Electroplating solutions for different electrodes.
preparation.
X denotes the agent used in
CoSO4·7H2O
NiSO4·6H2O
H3BO3
(30 g/L)
NaCl
(15 g/L)
(20 g/L)
(200 g/L)
TiO2/Pt–Co3O4
TiO2/Pt–NiO
TiO2/Pt–Co3O4–NiO
X
X
X
X
X
X
X
X
X
X

C. Wang et al. / Electrochimica Acta 56 (2011) 6299–6304
6301
Fig. 3. LSV results of TiO2 based electrodes: (a) TiO2/Pt; (b) TiO2/Pt–Co3O4; (c)
TiO2/Pt–NiO; and (d) TiO2/Pt–Co3O4–NiO.
Fig. 2. XRD analysis of the electrodes: (a) TiO2/Pt; (b) TiO2/Pt–Co3O4; (c)
TiO2/Pt–NiO; (d) TiO2/Pt–Co3O4–NiO; (᭹) Ti; (ꢀ) TiO2; (ꢀ) Pt; (ꢁ) Co3O4; and (ଝ)
same as Pt (1.90 mg/cm2). However, the XRD peaks of Co3O4 were
quite weak (Fig. 2(b) and (d)), indicating that most of the Co3O4
existed in an amorphous phase.
NiO.
frit. The solution used for the cathode was 1 M NaOH, and the anode
solution consisted of 6 g of coal suspended in 100 mL of NaOH (1 M),
which was stirred at a constant rate. Pt was used as a counter elec-
trode, while a saturated calomel electrode (SCE) was used as the
reference electrode.
3.2. Linear-sweep voltammetry (LSV) and chronoamperometry
(i–t curve) analysis
The LSV results of different electrodes are shown in Fig. 3.
An increase of current from 0.7 V indicated that the coal electro-
oxidation occurred. We performed the electrolysis study at 1.2 V for
the gas-collection test and determined the existence of the reaction.
2.3. SEM, XRD and ICP analyses
Scanning electron microscopic (SEM) images were taken with
a JSM-6700F microscope (Shimadzu Ltd.). X-ray diffraction (XRD)
measurements were performed with a DLMAX-2200 diffractome-
ter (JEOL Ltd.), and an IRIS Advantage 1000 Atomic Emission
Spectrometer (Thermo Elemental Ltd.) was used for Inductive Cou-
pled Plasma Emission Spectrometer (ICP) measurements.
At 0.8 V, the modified TiO2/Pt electrodes showed
a
higher current compared to the TiO2/Pt electrode, and the
TiO2/Pt–Co3O4–NiO electrode performed the best. Its average
current was 16.20 mA during the electrolysis study at 1.2 V for 1 h
(Table 2). The good performance should be ascribed to the high
content of catalyst on the TiO2/Pt–Co3O4–NiO electrode. According
to the ICP analysis, 3.42 mg/cm2 of catalyst (1.86 mg/cm2 Co3O4
and 1.56 mg/cm2 NiO) was modified on the substrate. The catalyst
contents for TiO2/Pt–Co3O4 and TiO2/Pt–NiO were 1.46 mg/cm2
(Co3O4) and 1.72 mg/cm2 (NiO), respectively. Under the same
conditions (0.21 mA/cm2, 5 min), a high efficiency (3.42 mg/cm2)
for the deposition of catalyst could be achieved if the active metals
were codeposited.
The current contribution per gram (CCG) of these catalysts was
calculated to evaluate the catalytic efficiency. Values of the CCG for
TiO2/Pt–Co3O4, TiO2/Pt–NiO and TiO2/Pt–Co3O4–NiO, were 2.1 A/g,
2.3 A/g and 1.2 A/g, respectively. Both NiO and Co3O4 exhibited
high catalytic activities. A low CCG for TiO2/Pt–Co3O4–NiO could
possibly be due to its relatively low surface area.
3. Results and discussion
3.1. Morphology and composition analysis
Fig. 1 shows the SEM images of the electrodes modified by
nanoparticles of different catalysts. Compared to the TiO2 with
electrodeposited Pt (Fig. 1(e)), TiO2 with electrolessly deposited
Pt showed a homogenous surface consisted of nano-Pt particles
(Fig. 1(a)). The difference was due to the low conductivity of the
TiO2 substrate. When Cu was electroplated to enhance the conduc-
tivity of the substrate, relatively homogenous Pt particles could be
electrodeposited onto the TiO2–Cu substrate, as shown in Fig. 1(f).
Particles on the surface of the TiO2/Pt–Co3O4 electrode con-
sisted of small pieces, while those on the TiO2/Pt–Co3O4–NiO and
TiO2/Pt–NiO electrodes consisted of small balls. After the electroly-
sis of coal (120 h), 11.6% of the Co3O4 was lost on the TiO2/Pt–Co3O4,
while only 6.4% of the NiO on the TiO2/Pt–NiO was lost, which will
be discussed in Section 3.4. These secondary units (small balls and
pieces) that constituted particles were responsible for the mass
decrease of the electrodes. The balls reinforced particles on the
surface.
Fig. 4 shows the i–t curves of different electrodes. The TiO2/Pt
electrode showed a much higher current compared to the pure
metal electrodes, suggesting that the porous surface of TiO2 should
be responsible for the current enhancement. However, due to the
low conductivity of the TiO2, Pt cannot be electrodeposited firmly
and homogeneously on TiO2, as shown in Fig. 1(e), while the elec-
troless plated Pt performed much better (Fig. 1(a)).
3.3. Gas collection
The compositions of deposited catalysts on the electrode sur-
faces were confirmed by XRD analysis (Fig. 2). The XRD spectra of
TiO2/Pt–Co3O4–NiO showed peaks of Co3O4 (2 2 0) and NiO (0 1 2,
1 0 4 and 1 1 0). There were no peaks found for the alloy phases of
Co3O4 and NiO. According to the ICP results, the mass density of
Co3O4 on TiO2/Pt–Co3O4 was 1.86 mg/cm2, which was almost the
Due to its complexity, the reaction of coal is usually investigated
by indirect methods, e.g., as a gas collection test. We collected gas
generated from both the anode and cathode, and the gas volume
after 0.5 h was calculated as shown in Fig. 5; the main gas products
consisted of O2 and H2. A NaOH solution was used for the blank test,

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Table 2
Catalytic activity of prepared electrodes (electrolyzed for 1 h).
TiO2/Pt
TiO2/Pt–Co3O4
TiO2/Pt–NiO
TiO2/Pt–Co3O4–NiO
Current density (A/g)
Total current (mA)
H2 (L/(g h))
Electrolytic efficiency of H2 (%)
Gas from anode (L/(g h))
1.2
2.1
2.3
1.2
9.12
0.54
97.93
0
12.44
0.95
98.21
0
15.72
1.02
98.13
0
16.20
0.53
98.57
0
O2 generation was increased by the coal that was oxidized instead
of OH−. Thus, different from reaction (1), the half reaction on the
anode could be described as follows:
Coal + OH− → oxidized coal + H2O
(4)
In addition, the coal particles enhanced the resistance of the solu-
tion, leading to a decrease in the total current. Therefore, less H2
was obtained from the cathode compared to the blank test.
The activities of H2 production on different electrodes are
listed in Table 2. The TiO2/Pt–Co3O4–NiO electrode showed the
best performance with a high columbic efficiency (98.57%) of
H2 production. The amount of generated H2 per gram from
TiO2/Pt–NiO (1.02 L/(g h)) was two times more than from TiO2/Pt
and TiO2/Pt–Co3O4–NiO, which confirmed the CCG results.
3.4. ICP analysis of the electrodes
The ICP analysis was used to examine the mass decrease of metal
oxides modified on the electrodes and to investigate the stability
of different morphologies (Fig. 6). The catalysts were more stably
attached on the TiO2 substrate compared to the Ti substrate. A loss
of 54.2% Pt was determined for the Ti/Pt–Co3O4 electrode after
120 h of electrolysis, while 8.1% Pt was lost on a TiO2/Pt–Co3O4.
As shown in Fig. 6(a), all the TiO2 based electrodes show a smaller
Fig. 4. i–t curves of different electrodes in the coal slurry at 1.2 V.
and a gas volume ratio of H2 (cathode) to O2 (anode) was constant at
2:1 as shown in Fig. 5; the gas was due to water splitting. However,
the situation with the coal slurry was quite different. First, little
gas was collected on the anode because the overpotential of the
Fig. 5. Volume of collected gas at 1.2 V: (a) TiO2/Pt; (b) TiO2/Pt–Co3O4; (c) TiO2/Pt–NiO; and (d) TiO2/Pt–Co3O4–NiO.

C. Wang et al. / Electrochimica Acta 56 (2011) 6299–6304
6303
As p-type semiconductors, both NiO and Co3O4 can produce
hydroxyl radical to oxidize the coal at a high enough potential
[24,25]. Metal oxide hydroxide undergoes the following process
when used as an anode material:
OH−M−O→OH•OH + e−
(7)
It was reasonable to assume that MOOH (M = Ni, Co) was respon-
sible for the oxidization of coal. The reaction of coal can be started by
hydroxyl radicals as shown in reaction (8). Reaction (9) could occur
in the blank solution, in which oxygen is obtained from hydroxyl
radicals.
With coal : •OH + coalM−O→OHoxidized coal
Blank : •OH + •OH → (1/2)O2 + H2O
(8)
(9)
The effect of Co3O4 was determined by gas collection curves in
the blank test. Comparing the results from Fig. 5(b) and (c), more
gas was generated on TiO2/Pt–Co3O4 compared to TiO2/Pt–NiO.
This finding indicated that the overpotential of oxygen evolution
could be decreased by Co3O4 in a basic system [19]. Co3O4, which
competed with coal in affecting the overpotential of O2, facilitated
reaction (9) because of its high ability to generate the hydroxyl rad-
ical. However, curve a in Fig. 5(b) indicated its lower activity for coal
oxidation compared to TiO2/Pt–NiO, and it could be attributed to
the relatively low ability of Co3O4 to introduce the hydroxyl radical
into coal (reaction (8)).
An interesting phenomenon is that TiO2/Pt–Co3O4–NiO had the
highest current density (4.05 mA/cm2) despite its low CCG value.
There could be a possible cooperation effect on the surface of NiO
and Co3O4, which is currently under further investigation.
4. Conclusion
A thin layer of porous TiO2 could be used as an electrode sub-
strate for coal electrolysis because its conductivity can be enhanced
by metal deposition. An electroless deposit of Pt nanoparticles onto
a TiO2 substrate showed a homogeneous, stable surface. A NiO or
Co3O4 modified TiO2/Pt electrode showed the highest activity for
the oxidization of coal, which was confirmed by both gas collec-
tion and an LSV test. Both metal oxides could generate hydroxyl
radicals, which enhance the electro-oxidation of coal.
Fig. 6. Mass decrease of catalyst on the following: (a) TiO2/Pt–Co3O4, Ti/Pt–Co3O4
and (b) TiO2/Pt–Co3O4–NiO, Ti/Pt–Co3O4–NiO.
mass decrease. The porous surface of the TiO2 substrate was greatly
responsible for the high stability of the modified catalyst because
the catalysts could insert themselves into the porous surface.
The NiO enhanced the adhesion of Pt and Co3O4 on both Ti
and TiO2. Compared to Ti/Pt–Co3O4, on which 54.2% Pt was lost,
Ti/Pt–Co3O4–NiO had 92.6% Pt remaining on the substrate after
electrolysis for 120 h. With the addition of NiO, the mass loss of
Co3O4 decreased from 11.6% to 9.1%. According to the SEM results,
TiO2/Pt–Co3O4–NiO and TiO2/Pt–NiO almost had the same mor-
phology, suggesting that NiO was responsible for the formation
of the ball-like secondary units that enhanced the stability of the
electrodes.
Acknowledgements
The present work was supported by the National Science Foun-
dation of China (Nos. 20673071 and 20873083), the Shanghai
Education Commission (fifth key disciplines; No. J50102) and
the State Key Laboratory of Chemical Engineering (No. SKL-ChE-
08A01).
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(5)
(6)

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