Enhanced electrocatalytic performance for methanol oxidation with a Magnéli phase molybdenum oxide/Pt-black composite

Article abstract of DOI:10.1016/j.molcata.2015.02.001

A composite electrocatalyst composed by prepared Magnéli phase Mo₄O₁₁ and Pt-black (Pt/Mo₄O₁₁) was found with highly effective for the methanol oxidation reaction (MOR). Cyclic voltammetry (CV) result showed that the mass activity for the forward peak (I_f) of Pt/Mo₄O₁₁ was determined to be 722.20 mA mg^-1_Pt, e.g., about twice of the mass activity of the stand-alone Pt-black. This suggested that the electrocatalytical reactivity is improved when the crystallised Mo₄O₁₁ assists for methanol oxidation. It is also found that the peak current ratio of the forward versus the reverse (I_f/I_b) is 1.21 for Pt/Mo₄O₁₁ and 0.81 for Pt/C, respectively. X-ray photoelectron spectroscopy (XPS) results indicated that different valence states of molybdenum coexist in Mo₄O₁₁; this promotes both hydrogen spillover and oxygen buffering effects. The synergistic effects of Pt and Mo₄O₁₁ on the electrochemical properties represent a critical advantage of the Pt/Mo₄O₁₁ catalyst for anti-poisoning towards the intermediates in the MOR.

Full text of DOI:10.1016/j.molcata.2015.02.001

Accepted Manuscript
Title: Enhanced Electrocatalytic Performance for Methanol
´
Oxidation with a Magneli Phase Molybdenum Oxide/Pt-black
Composite
Author: Fan Yang Fan Li Yan Wang Xin Chen Dingguo Xia
Jingbo Liu
PII:
S1381-1169(15)00049-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.02.001
MOLCAA 9417
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
2-9-2014
25-1-2015
1-2-2015
Please cite this article as: Fan Yang, Fan Li, Yan Wang, Xin Chen, Dingguo Xia,
Jingbo Liu, Enhanced Electrocatalytic Performance for Methanol Oxidation with a
´
MagneliPhaseMolybdenumOxide/Pt-blackComposite, JournalofMolecularCatalysis
A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.02.001
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Enhanced Electrocatalytic Performance for Methanol Oxidation with
a Magnéli Phase Molybdenum Oxide/Pt-black Composite
Fan Yanga, Fan Lia*, Yan Wang a,b, Xin Chena, Dingguo Xiac*，Jingbo Liud*
(a. Department of Chemistry and Chemical Engineering, Beijing University of Technology, Ping le yuan 100,
100124, Beijing, China. E-mail: vanadiumli@bjut.edu.cn;
b. National Engineering Laboratory for Advanced Coatings Technology of Metal Material, China Iron and Steel
Research Institute Group, 100081, Beijing, China
c. College of Engineering, Peking University, Beijing 100871, China. E-mail: dgxia@pku.edu.cn
d. Department of Chemistry, Texas A&M University-Kingsville, Kingsville, Texas 78363, E-mail: Highlights
►
jingbo.liu@chem.tamu.edu)
Highlights
Enhanced Electrocatalytic Performance for Methanol Oxidation with a Magnéli Phase
Molybdenum Oxide/Pt-black Composite
1. Crystallised Magnéli phase Mo4O11 used as a co-catalyst support to
enhance MOR
2. Multiple Mo valence states coexist in Mo4O11
3. These promote hydrogen spillover and oxygen buffering in the Pt/Mo4O11
catalyst.
4. The catalyst shows improved poisoning tolerance and electrocatalytic activity.
Graphical abstractEnhanced Electrocatalytic Performance for Methanol
Oxidation with a Magnéli Phase Molybdenum Oxide/Pt-black
Composite
Abstract
A composite electrocatalyst composed by prepared Magnéli phase Mo4O11 and
Pt-black (Pt/Mo4O11) was found with highly effective for the methanol oxidation
reaction (MOR). Cyclic voltammetry (CV) result showed that the mass activity for the
forward peak (If) of Pt/Mo4O11 was determined to be 722.20 mA mg-1Pt, e.g., about
twice of the mass activity of the stand-alone Pt-black. This suggested that the
electrocatalytical reactivity is improved when the crystallised Mo4O11 assists for
methanol oxidation. It is also found that the peak current ratio of the forward versus
the reverse (If/Ib) is 1.21 for Pt/Mo4O11 and 0.81 for Pt/C, respectively. X-ray
photoelectron spectroscopy (XPS) results indicated that different valence states of
molybdenum coexist in Mo4O11; this promotes both hydrogen spillover and oxygen
buffering effects. The synergistic effects of Pt and Mo4O11 on the electrochemical
properties represent a critical advantage of the Pt/Mo4O11 catalyst for anti-poisoning
towards the intermediates in the MOR.

Keywords: crystallised Magnéli phase, Mo4O11, methanol oxidation reaction,
poisoning tolerance
1 Introduction
Diversified energetic resources, growth in generation capacity, and feasible strategies
are required to meet the increasing energy needs of our society [1-2]. One promising
alternative strategy is to use fuel cells as green energy sources to spontaneously
convert chemical energy into electricity, releasing heat and water when the
electrochemical reactions occur [3-4]. Direct methanol fuel cells (DMFCs), which use
liquid and renewable methanol as fuel supplies, have recently received much attention
by the scientific community. The advantages of DMFCs over hydrogen-fed fuel cells
include convenient storage and transportation of fuels, high specific energy density at
low operating temperature, and flexible design of the fuel cell devices [1-3]. However,
several issues hinder their commercialization, such as durability, efficiency, and costs.
One of the major difficulties is the sluggish kinetics for the methanol oxidation
reaction (MOR) of the most commonly used Pt-anode catalysts in DMFCs; this is the
result of the self-poisoning caused by strongly adsorbed intermediates such as CHO
and CO, ect.. [4-8]. In order to address this critical issue, in the last decade different
kinds of co-catalytic elements, such as Ru, Ce, and W, have been incorporated into the
Pt-based catalysts [9-14]. The presence of second and/or third elements in the
Pt-based catalysts may improve the MOR kinetics via a bifunctional mechanism or a
synergistic effect.
Numerous studies have shown that Mo-modified Pt electrocatalysts possess effective
poisoning tolerance towards MOR. Ross and his co-workers [15,16] confirmed that
PtMo alloys have exceptional low-temperature CO-tolerance in bulk electrodes.
However, synthesis of PtMo nanoparticles is particularly challenging, because of the
large negative redox potential of the Mon+/Mo0 couple and the low miscibility of Pt
and Mo. Yet, Pt catalysts modified by molybdenum oxides (instead of simple Mo)
displayed high tolerance toward CO [17-19]. Although MoOx is an effective
co-catalyst owing to the several accessible valence states of Mo (IV, V, and VI), its

amorphous characteristics lead to poor conductivity [20]. To solve this issue, MoOx
particles have been deposited on carbon-based materials such as carbon black, carbon
nanotubes, or carbon nanofibers [21-23]. To date, co-depositions [24] and consecutive
impregnations [25] have been developed to prepare Pt/MoOx/C catalysts. However, a
certain amount of Pt is embedded and capped as a result of these methods; this
problem reduces their applicability. New approaches that improve the conductivity of
Mo oxides are thus highly desired. For instance, it was found that a sub-stoichiometric
compound in MoO systems (e.g., the Magnéli phase) has a suitable electronic
conductivity. Among these Mo oxides, Mo4O11 has received much attention because
of its quasi-two-dimensional [26,27]; for instance, electrical resistivity of γ-Mo4O11
single crystals is about 10-6 Ω·m at 300 K [28]. Based on the results of our previous
study on Magnéli phase Ti4O7 as a novel durable Pt catalyst support [29], we further
showed in this study that the electron conductor Magnéli phase Mo4O11 can be
employed as a co-catalyst support, with a high poisoning tolerance. In particular,
electrochemical tests indicated that crystallised Mo4O11 mixed with Pt-black shows
improved anti-poisoning for the intermediates in the MOR. Results based on X-ray
photoelectron spectroscopy (XPS) also suggested that a strong metal-support
interaction occurs between Pt and Mo4O11, and that different valence states of
molybdenum (IV, V, and VI) coexist in Mo4O11. This is a favourable feature that can
promote hydrogen spillover and oxygen buffering effects, which may contribute to
improve the anti-poisoning properties and the electrocatalytic activity of the Pt/
Mo4O11 catalyst. The synergistic effect of the Pt/ Mo4O11 catalyst and a possible
mechanism of the catalytic process were also studied and discussed in detail.
2 Experimental
2.1 Synthesis of the Mo4O11 support
All reagents were of analytical grade and used as received without further purification.
In a typical procedure, the Mo4O11 support was synthesised via two steps. Firstly, the
MoO3 precursor was obtained as follows: 15 g of (NH4)6Mo7O24 4H2O (AMT) was
dispersed in an alumina ceramic boat and placed in a tube furnace in ambient

atmosphere; the temperature was then raised to 873 K with a rate of 5 K min-1, and
kept constant for 5 h; finally, the system was naturally cooled to room temperature.
Secondly, 5 g of the MoO3 precursor was reduced in a H2 (5 vol%)-Ar mixing-gas
flow at 873 K for 4 h to synthesise the Mo4O11 support. To load the Pt nanoparticles
on the Mo4O11 support, 20 mg of Pt-black and 80 mg of the as-synthesised Mo4O11
support were mechanically mixed for 0.5 h.
2.2 Characterizations of materials
The crystalline phases of the MoO3 precursor and Mo4O11 were characterised by
X-ray powder diffraction (XRD) analysis (D8 Advance, Bruker, Karlsruhe,
Deutschland). Morphologies were observed using a scanning electron microscope
(SEM, Hitachi S-4300, Tokyo, Japan). Transmission electron microscopy (TEM) and
high-resolution transmission electron-microscopy (HRTEM) images of Pt/Mo4O11
were taken by a JEOL JEM2100 (TEM, JEOL JEM2100, Tokyo, Japan). Surface
properties and the electronic structure of the catalysts were evaluated by XPS with Al
Kα radiation (Kratos AXIS Ultra DLD, Manchester Britain). The binding energy was
calibrated with the C1s position of the carbon support at 284.80 eV. The composition
of the catalysts was determined by inductively coupled plasma atomic-emission
spectrometry (ICP-AES, Thermo Fisher, IRIS Intrepid ER/S, Waltham, USA).
2.3 Electrochemical measurements
Glassy carbon (GC) electrodes were polished with alumina powder, ultrasonic
washing and blow drying, before dropping the catalyst ink. Based on the ICP-AES
measurements, Pt/Mo4O11 and Pt-black (Hispec 1000, Johnson Matthey) were diluted
to 1.3 mg mL-1 and 0.8 mg mL-1 with 0.05 wt% ethanol diluted Nafion solution,
respectively. Then, 10 µl of the two suspensions were transferred onto the polished
GC electrode substrates. Before testing, the electrodes were solidified for 0.5 hours
under ambient conditions.
Electrochemical measurements were performed by cyclic voltammogram (VMP3,
Bio-Logic SA). The GC electrode (Ø5 mm, with an area of 0.196 cm2) was set as the
working electrode. A Pt foil (10×10×0.3 mm) and a reversible hydrogen electrode
(RHE) were used as the counter electrode and the reference electrode, respectively.

The RHE was filled with 0.1 M HClO4 solution. Cyclic voltammetry (CV)
measurements were carried out in an Ar-purged 0.1 M HClO4 solutions at a sweep
rate of 100 mV s-1. The electrochemically active surface area (ECSA) was calculated
based on equation (1), as follows:
Q
H
0.21[Pt]
ECSA =
(1)
where the correlation constant of 0.21 (mC cm-2) represents the charge required for
the monolayer adsorption of hydrogen on a Pt surface. [Pt] is the Pt loading
(mg cm-2) on the electrode,
is the amount of charge exchanged during the
Q
H
adsorption of hydrogen atoms on Pt (mC cm-2), which measured under the
electro-adsorption curves between 0.05 V and 0.4 V.
MOR measurements were performed in a 0.1 M HClO4 and 0.1 M CH3OH solution at
a sweep rate of 100 mV s-1under ambient atmosphere. Durability tests (DTs) were
performed in an Ar-purged 0.1 M HClO4 solution at a sweep rate of 100 mV s-1 for
1500 cycles.
3 Results and discussion
3.1 Crystalline phase analysis
Morphologies and crystal structures of the MoO3 precursor and of the synthesised
Mo4O11 support are shown in Fig. S1. In particular, XRD data (Fig. S1B) indicated
that MoO3 has an orthorhombic crystalline structure and that is well-indexed with the
standard profiles (PDF 35-0609). The molybdenum suboxide Mo4O11 (Fig. S1D) was
formed in the reducing atmosphere at the temperature of 873 K, and it consists of a
mixture of monoclinic and orthorhombic phases (at 2θ of 22°). The principal phase of
orthorhombic was well-indexed with the standard profiles (PDF 05-0338). In line with
previous studies [30,31], about 8.7 (wt.) % of MoO2 was found in the as-synthesised
of Mo4O11 (Fig. 1A). It was also found that the orthorhombic phase transforms into
the monoclinic phase when kept for a relatively long time at high temperatures (such
as 873 K), while the orthorhombic phase is thermodynamically stable at temperatures

higher than 1000 K. In order to prevent agglomeration of MoO3 particles during the
high-temperature reduction and to improve the purity of the Mo4O11 product will be
systematically investigated in future. As shown in Fig. 1B, both Mo4O11 and Pt were
detected when the electrocatalyst Pt/Mo4O11 was investigated with XRD. Pt displayed
a face-centred cubic and was indexed with the standard (PDF 04-0802). Because the
size of Pt-black is significantly smaller than that of micro-sized Mo4O11, Pt/Mo4O11
shows the electronic-conductor Magnéli phase Mo4O11 decorated by Pt-black (Fig.
1B).
Fig. 1 (A) Quantitative fit analysis of Mo4O11 by MDI Jade program; phases content
is shown in the inset. (B) X-ray powder diffraction (XRD) patterns of Pt/Mo4O11 and
the atomic schematic graph shown in the inset.
3.2 Morphological analysis
The TEM micrograph (Fig. 2) confirmed that Pt-black is deposited on the surface of
the Magnéli phase Mo4O11. Orderly dispersed points were observed by the selected
area electron diffraction (SAED) pattern of the blue core region (Fig. 2A), indicating,
in line with the XRD data, the formation of a monoclinic Mo4O11. In contrast, ring
patterns of the small particles were collected in the SAED (Fig. 2A), which are
indexed as (111), (200), (220), and (311) planes of the face-centred-cube (fcc) of the
Pt crystal. This finding suggested that the core Mo4O11 is decorated by the Pt-black
particles, in agreement with the XRD results (Fig. 1B). To further investigate the
bonding between Mo4O11 and Pt-black, high-magnification TEM and HRTEM were
employed. Fig. 2B displays an enlarged image of the area marked in Fig. 2A, which
clearly shows that the Pt-black nanoparticles are dispersedly attached on the surface

of Mo4O11; Fig. 2C displays the HRTEM image of the marked Mo4O11 area (region
“a” in Fig. 2B), which indicates that the Mo4O11 (211) lattice plane is exposed. The
Fourier transform (FT) pattern (inset in Fig. 2C) further proved this observation. Fig.
2D shows the HRTEM image of marked Pt-black area (region “b” in Fig. 2B),
showing that the Pt (111) lattice plane is exposed. Previous studies showed that the Pt
(111) lattice plane of Pt exhibits highly electrocatalytic activity [32,33].
Fig. 2 (A) Transmission electron microscopy (TEM) image of Pt/Mo4O11 and the
selected area election diffraction (SAED) pattern are shown in the inset, up-right (blue
area) is the monoclinic Mo4O11 and down-right (red area) is the Pt-black,
respectively; (B) high-magnification TEM image of the area marked with a red square
in Fig. 2A; (C) high-resolution transmission electron-microscopy (HRTEM) image of
the marked Mo4O11 area (a in Fig. 2B), with a Fourier transform (FT) pattern shown
in the inset; (D) HRTEM image of the marked Pt-black area (b in Fig. 2B), with a FT
pattern shown in the inset.
3.3 Oxidation state analysis

XPS measurements were performed to examine the composition of Pt/Mo4O11 and
Pt-black as well as the valence state of the metal elements. The Pt 4f spectra of
Pt/Mo4O11 and Pt-black are shown in Fig. 3A. Comparison of these spectra suggested
that the binding energy of Pt 4f for Pt/Mo4O11 depends on the blue shifts (71.52eV),
indicating that an interaction between Pt and the Mo4O11 support exists. This shift
may be regarded as a strong metal-support interaction effect [19,29]. This strong
interaction may promote the mass transfer between Pt-black and Mo4O11, which in
turn may enhance the catalytic activity of the catalyst. Fig. 3B displays the results of
the deconvolution of Mo 3d peaks of the Mo4O11 support. The Mo 3d5/2 signals at
230.11, 232.66, and 233.11 eV are assigned to Mo(IV), Mo(V), and Mo(VI),
respectively. Our results thus indicate that different states of Mo 3d are coexisting in
Mo4O11.The presence of multiple valence could promote hydrogen spillover and
oxygen buffering effects, which may contribute to the improvement of the
intermediate-tolerance and electroactivity of the Pt/Mo4O11 catalyst. Although peaks
of the binding energy and intensities of peaks of Mo 3d for Pt/Mo4O11 significantly
changed before (Fig. 3C) and after (Fig. 3D) the durability tests, these spectra were
found to correlate with Mo(IV), Mo(V), Mo(VI) states. The deconvolution statistics
(Table S1) confirmed that synergistic effects occurred in the Pt/Mo4O11 composite.

Fig. 3 (A) X-ray photoelectron spectroscopy (XPS) spectra of Pt 4f for Pt/Mo4O11
(red) and Pt-black (black); (B) Mo 3d spectra for the Mo4O11 support; Mo 3d spectra
of Pt/Mo4O11 before (C) and after (D) the durability tests.
3.4 Electrochemical behaviour of Pt/Mo4O11
According to previous studies [15,18], metallic molybdenum or molybdenum
oxide-modified Pt catalysts have shown enhanced CO-tolerance for the MOR. Fig. 4A
displays CV curves of Pt/Mo4O11 and Pt-black recorded at ambient conditions in an
Ar-purged 0.1 M HClO4 solution. The ECSA of the Pt/Mo4O11 catalyst was calculated
via equation (1) to be 40.05 m2 g-1 (Fig. S2), e.g., 1.62 times larger than that of
Pt-black (24.73 m2 g-1). However, Mo4O11 has no obvious absorption and desorption
of hydrogen region (Fig. S3A), the ECSA of Mo4O11 is negligible compared with that
of Pt/Mo4O11 (Fig. S3B). These results indicated that the main role of Mo4O11 is that
of improving the distribution and the activity of Pt: On one hand, Mo4O11 acts as the
carbonaceous support to attach Pt-black nanoparticles and to increase the
electrochemically active sites, on the other hand, the hydrogen spillover effects may
occur on the Mo4O11 support, due to the presence of the metal, which serves as a
lower-energy pathway for H2 to firstly adsorb onto the metal and then diffuse on the
support surface. As shown in Fig. 4, clear redox peaks of MoOx between 0.35 V and
0.55 V were detected. These were attributed to the Faradaic process of
intercalation/de-intercalation of hydrogen into MoOx, forming hydrogen molybdenum
bronzes, e.g., HaMoOx (0 < a < 2) [34-37]. The stoichiometric formula can be depicted
as MoO11/4, where the hydrogen spillover occurs on the surface of the HaMoOx, as in

equation (2).
(a −b)H+ + (a −b)e− + HbMoO11/4
→
(2)
HaMoO11/4 (0 < b < a < 2)
←
We suggest for steps for the hydrogen spillover effect (Fig. 4A): i), reactant molecules,
such as H2 and CH3OH, are chemically adsorbed on the Pt surface and then
dissociated to hydrogen atoms (H); ii), hydrogen atoms are oxidised to cation,
providing an extra electron for MoO11/4; iii), H+ is transferred to the MoO11/4 surface
by a spillover mechanism; iv) H+ diffuses into the MoO11/4 lattice, reacting with the
electron and the matrix to neutralise the charge, this leading to the formation of
HaMoO11/4. Thus, both the enhanced dispersion by the Mo4O11 support and the
hydrogen spillover on the surface of HaMoO11/4 improved the ECSA of Pt/Mo4O11
catalyst.
Fig.4 Comparison of the electrocatalytic activities of Pt/Mo4O11 and Pt-black. Pt
loading was 2.6 and 8 µg for Pt/Mo4O11 and Pt-black, respectively. (A) CV curves
recorded in an Ar-purged 0.1 M HClO4 solution with a sweep rate of 100 mV s-1 of
Pt/Mo4O11 (red) and Pt-black (black), as well as a schematic illustration of hydrogen
spillover effect shown in inset, the green ball (MoO11/4) with the violate ball (H)

together to form hydrogen molybdenum bronzes, HaMoO11/4; (B) CV curves recorded
in a 0.1 M HClO4 and 0.1 M CH3OH mixed solution at a sweep rate of 100 mV s-1
under ambient atmosphere, and a schematic of methanol adsorption shown in the inset;
(C) enlarged images of the purple squares in Fig. 4A (left) and Fig. 4B (right),
respectively; (D) enlarged images of the CVs (solid) and MOR CVs (dash) between 0
V and 0.7 V.
3.5 Comparison of Pt/Mo4O11 and Pt-black behaviour
MoOx materials have recently received much attention, due to their high oxygen
storage capacity, similar to that of CeOx [38]. Enhancement of the utilization and
stability of Pt catalyst activity with these series of oxides is critical for MOR.
Crystallised MoO11/4 becomes an option because of its intrinsic properties, i.e., the
high electronic conductivity and the oxygen buffering effect [22,39]. Redox couples
in different valence states of molybdenum can facilitate oxygen transfer to the
interface between Pt and MoO11/4. The CV curves of MOR for Pt/Mo4O11 and Pt-black
(Fig. 4B) showed that the forward peak current (If) of Pt/Mo4O11 (722.20 mA mg-1
)
Pt
is 2.03 times larger than that of Pt-black (356.10 mA mg-1Pt). The forward peak
potential (Fp) of Pt/Mo4O11 was found to be shifted to a lower value (0.87 V),
compared with the potential of Pt-black (0.93V). The ratio of the forward peak current
to the back peak current (If/Ib) is 1.21 for Pt/Mo4O11, 0.94 for Pt-black, and 0.81 for
Pt/C (Fig. S4),. These data indicated that Pt-black mixed with crystallised Mo4O11 has
a higher poisoning tolerance than that of the Pt-black and Pt/C catalysts. The peak
potentials and currents of the MOR on the electrodes (Table S2) suggested that the
Pt/Mo4O11 can provide comparable electrochemical reactivity as that of the Pt/C
catalyst. Thus, it can be concluded that the Pt/Mo4O11 catalysts are characterised by an
improved tolerance to CO poison.
Fig. 4D shows the comparison between the CVs (solid) and MOR CVs (dash) for
Pt/Mo4O11 and Pt-black, with an applied voltage ranging between 0.0 V and 0.7 V.
Although the onset potential of Pt/Mo4O11 is equivalent to that of Pt-black, the If value
of Pt/Mo4O11 is higher than that of Pt-black, this suggests that the kinetics of MOR
reaction catalysed by Pt/Mo4O11 are more effective; in contrast, the If potential of

Pt/Mo4O11 is lower than that of Pt-black, which shows that Pt/Mo4O11 catalytic
dehydrogenation of methanol are thermodynamically easier. This observation may be
attributed to the intense dehydrogenation from methanol occurring after
de-intercalation on the surface of HaMoO11/4. In particular, the If value results from the
methanol adsorption on the surface of Pt, followed by the dehydrogenation reaction.
Hydrogen intercalation/de-intercalation on the surface of Mo4O11, (HbMoO11/4
↔
HaMoO11/4) plays an essential role in the removal of a certain amount of produced
intermediate poisonous substances from the surface of Pt; more hydrogen
de-intercalation from HaMoO11/4 results in more hydrogen dissociated from methanol
adsorbed on Pt. This process can be described with the following equation (3), its
schematic demonstration being shown in Fig. 5I:
ꢀꢀ⇀
HbMoO β Pt −[CHyO]ads +δ HaMoO11/ 4 + (α − β )Pt
11/ 4 ↽ꢀꢀ
α
Pt −[CHxO]ads
+
δ
(3)
(
β < α ,0 < b < a < 2,0 < y < x < 4)
The dissociated hydrogen can be embedded into HbMoO11/4 to form HaMoO11/4 by the
hydrogen spillover effect, releasing (α-β) Pt active sites. As the reaction proceeds,
oxygen rapidly diffuses and binds to the surface of the (α-β) Pt active sites to form
Pt-Oads species, via an oxygen buffering effect of the Mo4O11 support. The formation
of Pt-Oads in turn promotes the oxidization of β Pt-[CHyO]ads to H2O and CO2, and
regeneration of free Pt sites for further oxidation of methanol. This also explains the
highly anti-poisoning (removal the adsorbed intermediate product) property of
Pt/Mo4O11. This process can be described with the following equation (4), its
schematic demonstration being shown in Fig. 5II:
Mo4O11
(α
−
β
)Pt −[OH]ads
+
β
Pt −[CHxO]ads
→α Pt + (β + xβ )H2O + β CO2
(4)
(α
= 3β + xβ )

Fig. 5 Schematic illustration of methanol oxidation reaction on the Pt/Mo4O11catalyst;
I and II represent the hydrogen spillover effect, and the oxygen buffering effect,
respectively.
3.6 Stability of the electrocatalysts
Stability is a critical criterion to evaluate the performance of an electrocatalyst. The
DTs were estimated by CV sweeps at 100 mV s-1 in an Ar-purged 0.1 M HClO4
solution at room temperature (Fig. S5). The ECSAs of Pt-black and Pt/Mo4O11 are
reduced with the increment of the CV circles, as shown in Figures S5A and S5B,
respectively. After 500 cycles, the loss rate of Pt-black was found to be 16.91% (to
compare with 8.93% of Pt/Mo4O11). The ECSA loss of the Pt-black was attributed to
the aggregation of the nanoparticles during the test and it may be explained as follows:
the Pt is separated from the surface of support during preparation, because of the
mechanical mixing and the solubility of hydrogen molybdenum bronzes after Mo4O11
intercalation of hydrogen. The molybdenum dissolution can be detected via the colour
change of the supernatant (composed of HaMoO11/4) to blue (Fig. S6). To further
illustrate the presence of soluble molybdenum bronzes, we performed accelerated DTs
by increasing the CV scanning potential range, between 0.0 V and 1.2V (Fig. S7). The
XRD results (Fig. S8) showed that the relative intensity ratio of Pt to Mo4O11
increases significantly after 1500 cycles of the DT. This may result from the loss of
Mo4O11 during the process. Therefore, we suggest that the degeneration of ECSA of
Pt/Mo4O11 is related to the instability of Mo4O11.
Although the mechanical mixing showed some limitations in the effective
dispersion of Pt on the surface of Mo4O11, the ECSA of Pt/Mo4O11 is 1.62 times larger

than that of Pt-black. Even after 1500 cycles of DT, the ECSA obtained from the CV
curve of Pt/Mo4O11 (26.28 m2 g-1) hardly changed from that of Pt-black (24.73 m2 g-1)
before DT. This demonstrates the evident advantages of using the co-catalyst support
composed of crystallised Mo4O11 over traditional catalysts. Although methods for an
efficient Pt loading need to be developed, our data showed that Pt/Mo4O11 displays a
reasonably high electrochemical activity for MOR. Despite the formation of hydrogen
molybdenum bronzes accelerated the ECSA loss of Pt/Mo4O11, it also promoted
hydrogen spillover and oxygen buffering effect, which may contribute to improve the
poisoning tolerance and electrocatalytic activity of the Pt/Mo4O11 catalyst towards
MOR.
4 Summary
In this work, crystallised Magnéli phase Mo4O11 as a quasi-two-dimensional
conductor has been used as a co-catalyst support, mixed with Pt-black to enhance
MOR. XPS results revealed a strong metal-support interaction between the Pt and
Mo4O11; in addition, various valence states of Mo (IV, V, and VI) were found to
coexist in Mo4O11, which may promote hydrogen spillover and oxygen buffering.
These effects together contributed to the improvement of the poisoning tolerance and
the electrocatalytic activity of the Pt/Mo4O11 catalyst for MOR. Thus, the results
collected in this work demonstrated that the crystallised Magnéli phase Mo4O11 may
be regarded as a promising co-catalyst support for MOR. Further studies are needed to
improve the stability of the crystallised Magnéli phase Mo4O11, which is a critical
property for the performance of an electrocatalyst.
Acknowlegment
The authors gratefully acknowledge Prof. Jiwu Huang (Central South University) for
his assistance in the quantitative analysis by JADE. This work was financially
supported by National Natural Science Foundation of China (51172007, 11179001,
50974006), Beijing Municipal Natural Science Foundation (2120001, 2110001,
2102004), and the Funding Project for Academic Human Resources Development in
Institutions of Higher Learning under the jurisdiction of the Beijing Municipality
(PHR 201107104).

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Supporting information (SI)
Enhanced Electrocatalytic Performance for Methanol Oxidation with
a Magnéli Phase Molybdenum Oxide/Pt-black Composite
Fig. S1 (A) Scanning electron microscope (SEM) image and (B) XRD pattern of
MoO3 precursor; (C) SEM image and (D) XRD pattern of Mo4O11.

Fig. S2 Histogram of the specific ECSA of the Pt-black (black) and Pt/Mo4O11 (red).
Fig. S3 CV curves of Mo4O11 and Pt/Mo4O11 recorded in an Ar-purged 0.1 M HClO4
solution at a sweep rate of 100 mV s-1. Mo4O11 loading were 10 and 10.4 µg for
Mo4O11 and Pt/ Mo4O11, respectively. (A) Mo4O11, (B) Pt/Mo4O11 (red) and Mo4O11
(black).
Fig. S4 CV curves of Pt/Mo4O11 (red) and Pt/C (blue) recorded in a 0.1 M HClO4 and
0.1 M CH3OH mixed solution at a sweep rate of 100 mV s-1 under ambient
atmosphere. Pt loading were 2.6 µg and 2.4 µg for Pt/Mo4O11 and Pt/C, respectively.

Fig. S5 CV curves recorded in an Ar-purged 0.1 molL-1 HClO4 solution with a sweep
rate of 100 mV s-1. Pt loading were 2 and 13 µg for Pt/Mo4O11 and Pt-black,
respectively. (A) CV curves of Pt-black before and after a 500-cycle durability test, (B)
CV curves of Pt/Mo4O11 before and after a 500-cycle durability test.
Fig. S6 Photograph of the aqueous of Pt/Mo4O11 and Pt-black in 0.1molL-1 HClO4
solution before and after standing for one week under hydrogen atmosphere.
Fig. S7 CV curves recorded in an Ar-purged 0.1molL-1 HClO4 solution with a sweep
rate of 100 mV s-1. Pt loading were 8 µg and 2.6 for Pt-black and Pt/Mo4O11,
respectively. (A) CV curves of Pt-black before and after a 1500-cycle durability test,
(B) CV curves of Pt/Mo4O11 before and after a 1500-cycle durability test.

Fig. S8 XRD patterns of Mo4O11 and Pt/Mo4O11 before and after a 1500-cycle
durability test, respectively.
The change in the Pt-crystal size was calculated from the Pt (111) peak in XRD
analysis based on Scherrer equation, as follows:
K
γ
D =
Bcos
θ
Where K is a dimensionless shape factor, K=0.89. γ is X ray wavelength, γ=0.154056
nm. β is the line broadening at half the maximum intensity (FWHM). θ is the Bragg
angle. D is the mean size of the ordered (crystalline) domains, which may be smaller
or equal to the grain size.
Fig. S9 Pt (111) peak in the XRD of (A) Pt/Mo4O11 and (B) Pt/Mo4O11-CV,
respectively. The red line is the fitting curve.
The average crystallite sizes of Pt nanoparticles on the Mo4O11 support before and
after a 1500-cycle durability test are ca. 7 and 8 nm, respectively. This shows that
there was a slight aggregation of Pt nanoparticles after the durability test, which

corresponds to the reduction of electrocatalyst activity.
Table S1. Binding energy (3d, eV) and atomic content (%) of different Mo valence
states [Mo(IV), Mo(V), Mo(VI)] obtained from the Mo3d XPS spectra.
Sample
Binding energy
Mo4+
230.11
7.09
Mo5+
232.66
70.74
Mo6+
233.11
22.17
Mo4O11
Content
Binding energy
Content
231.80
3.99
232.80
80.01
233.23
16.00
Pt/Mo4O11
Binding energy
Content
229.93
1.47
233.00
44.94
233.39
53.59
Pt/Mo4O11-CV
In the mechanical mixing of Pt-black and Mo4O11 support, Mo4O11 would occur
comproportionation (Mo+4 + Mo+6 → 2Mo+5). The content of Mo(IV) specie in
Pt/Mo4O11 catalyst (3.99%) was decreased compare to Mo4O11 support (7.09%). In
theory, Mo+4 and Mo+6 should be equal to the amount of reduction in the same
comproportionation. But Mo+6 (6.17%) decreased much more than that of Mo+4
(3.1%), which proves that a strong interaction between Pt and Mo4O11. Comparison of
Fig. 3A suggested that the binding energy of Pt 4f for Pt/Mo4O11 depends on the blue
shifts (71.52eV). This phenomenon shows that the oxidation state of Pt increases in
Pt/Mo4O11 catalyst. That is to say, the electronic of Pt will delocalize to Mo4O11 to
reduce Mo+6 (Mo+6 + e-1 → Mo+5). This strong interaction between Pt and Mo4O11
leads to the unequal reduction of Mo+4 and Mo+6, naturally.
Table S2 Comparison of the electrochemical activities of Pt/Mo4O11, Pt-black and
Pt/C.
If potential
(V)
If current
Ib potential Ib current
sample
If/Ib
(mA mg-1
)
(mA mg-1
(V)
)
Pt
Pt
0.93
0.90
356.10
823.65
0.81
0.72
380.43
0.94
0.81
Pt-black
Pt/C
1019.81

0.87
722.20
0.77
598.29
1.21
Pt/ Mo4O11
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