Histidine-assisted synthesis of CeO2 nanoparticles for improving the catalytic performance of Pt-based catalysts in methanol electrooxidation

Article abstract of DOI:10.1039/c8nj03972k

His-CeO₂ nanoparticles were synthesized under urea hydrolysis conditions using histidine as the growth-directing agent. Characterization by transmission electron microscopy, powder X-ray diffraction spectroscopy, photoluminescence spectroscopy and X-ray photoelectron spectroscopy, indicated that the His-CeO₂ nanoparticles, which had a particle size of 12.2 nm, possessed a higher content of Ce³⁺ species and oxygen vacancies than bare CeO₂ nanoparticles. Pt/His-CeO₂-C electrocatalysts were constructed by using His-CeO₂ nanoparticles as co-catalysts and exhibited enhanced catalytic performance in the methanol oxidation reaction. Because His-CeO₂ co-catalysts supply sufficient OH_ads sites and show strong interactions with Pt species, Pt/His-CeO₂-C had better antipoisoning properties than Pt/CeO₂-C. In cyclic voltammetry tests, the mass activity for Pt/His-CeO₂-C was up to 1116.9 A g^-1, which is approximately 2.9 and 2.2 times greater than that of Pt/C-Z and commercial Pt/C-JM, respectively. These results pave the way for manufacturing Pt-based electrocatalysts for methanol electrooxidation with biomolecule-mediated modulation of the synergistic effects between the Pt species and metal oxides.

Full text of DOI:10.1039/c8nj03972k

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Histidine-assisted synthesis of CeO2 nanoparticles
for improving the catalytic performance of
Pt-based catalysts in methanol electrooxidation†
Cite this: New J. Chem., 2018,
42, 18159
Shengdong Dai, Jinli Zhang,
Yan Fu * and Wei Li
*
His-CeO2 nanoparticles were synthesized under urea hydrolysis conditions using histidine as the growth-
directing agent. Characterization by transmission electron microscopy, powder X-ray diffraction spectroscopy,
photoluminescence spectroscopy and X-ray photoelectron spectroscopy, indicated that the His-CeO2
nanoparticles, which had a particle size of 12.2 nm, possessed a higher content of Ce3+ species and oxygen
vacancies than bare CeO2 nanoparticles. Pt/His-CeO2-C electrocatalysts were constructed by using His-CeO2
nanoparticles as co-catalysts and exhibited enhanced catalytic performance in the methanol oxidation
reaction. Because His-CeO2 co-catalysts supply sufficient OHads sites and show strong interactions with
Pt species, Pt/His-CeO2-C had better antipoisoning properties than Pt/CeO2-C. In cyclic voltammetry
tests, the mass activity for Pt/His-CeO2-C was up to 1116.9 A gÀ1, which is approximately 2.9 and
2.2 times greater than that of Pt/C-Z and commercial Pt/C-JM, respectively. These results pave the way
for manufacturing Pt-based electrocatalysts for methanol electrooxidation with biomolecule-mediated
modulation of the synergistic effects between the Pt species and metal oxides.
Received 6th August 2018,
Accepted 1st October 2018
DOI: 10.1039/c8nj03972k
rsc.li/njc
intermediates, but also prevent the agglomeration of Pt nano-
particles on the carbon support.
1. Introduction
Direct methanol fuel cells (DMFCs) have emerged as promising
Owing to the intrinsic oxygen vacancies, the oxygen storage
power sources for future energy security owing to their advantages capacity, and the mutual conversion between Ce4+ and Ce3+,
of high energy conversion efficiency, low operation temperature CeO2 nanoparticles can provide sufficient OHads species at low
(o120 1C) and low environmental pollution.1,2 To date, Pt is the potentials to promote the efficient oxidation of poisoning
most commonly used electrocatalyst in DMFCs because of its intermediates formed during methanol electrooxidation.13–15
superior catalytic performance. However, Pt-based electrocatalysts Because the yield of the OHads species is highly dependent
are susceptible to poisoning by CO-like reaction intermediates upon the oxygen vacancy content, research has focused on
in acidic media,3,4 which may hinder its commercialization in increasing the intrinsic oxygen vacancies in CeO2 nanocrystals.16
DMFCs. To overcome this drawback, a variety of methods have For example, Chen and Sun17 synthesized shuttle-shaped CeO2
been developed, such as designing Pt-based alloys5–7 or using nanoparticles via a solvothermal process assisted by octadecyl-
metal oxides as co-catalysts.8–11 For example, Huang et al.12 amine and urea. Compared with CeO2 nanorods, the shuttle-
explored a convenient, scalable approach to fabricate 3D porous shaped CeO2 nanoparticles had a higher porosity and oxygen
Pt/RuO2/graphene catalysts. Due to the high dispersion of Pt vacancy content. The CO conversion over shuttle-shaped CeO2
nanoparticles and the OHads sites on RuO2 for eliminating COads
,
reached almost 100% at 250 1C, whereas only 50% conversion
Pt/RuO2/graphene exhibited enhanced electrocatalytic perfor- was accomplished at 290 1C for CeO2 nanorods. Moreover,
mance in the methanol oxidation reaction (MOR), and the a series of CuO-doped CeO2 catalysts was synthesized by
specific activity was determined to be 2.7 and 2.8 times higher Zheng et al.18 through a solvothermal method, in which the
than Pt/graphene and commercial Pt/C, respectively. Metal incorporation of Cu2+ into the CeO2 lattice contributed to
oxides not only improve the resistance of Pt species to reaction a higher oxygen vacancy content and a higher concentration
of the most active lattice oxygen. The CuO/CeO2 catalysts
Collaborative Innovation Center of Chemical Science and Chemical Engineering,
containing 10% CuO showed the highest CO conversion with
a T100 (temperature required for 100% conversion of CO) of
105 1C. These studies motivated us to explore new CeO2
School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300350, P. R. China. E-mail: fuyan@tju.edu.cn, liwei@tju.edu.cn;
Fax: +86-22-27404495; Tel: +86-22-27404495
nanomaterials with desirable oxygen vacancy contents as
co-catalysts in the MOR.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj03972k
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Amino acids, which contain diverse functional groups, such yellow powder. For the control experiments, CeO2 nanoparticles
as –COOH, –NH2, imidazole, guanidine, and thiol, coordinate were prepared without histidine using the same procedure.
with transition metal ions and subsequently regulate the
growth of inorganic materials, like ZnO,19 TiO2,20 and CuO.21 2.3 Synthesis of Pt/His-CeO2-C electrocatalysts
For example, Zhang et al.22 synthesized mesoporous CeO2 by
using a series of amino acids, including L-lysine, L-glutamic
His-CeO2 (4 mg) and carbon black (16 mg) were added to a
system containing EG (40 mL) and H2O (20 mL). The solution
acid, L-aspartic acid and L-glycine. The side groups of the amino
was ultrasonicated for 60 min to form a homogeneous suspen-
acids played a crucial role in determining the surface area and
sion. H2PtCl6 solution (663 mL, 38.616 mM) was added to the
suspension. After stirring for 3 h, the pH of the suspension was
adjusted to 12 with 0.5 M NaOH. The suspension was heated to
125 1C under reflux for 4 h. After filtration, the solid was
washed with water and anhydrous ethanol several times, and
then dried under vacuum at 60 1C for 12 h. Other Pt/His-CeO2-C
oxygen vacancy content of the hierarchical CeO2 nanostructures.
Atla et al.23 investigated the effects of different amino acids on
the physiochemical properties of CeO2 nanocrystals. CeO2
mediated by L-histidine contained a higher amount of surface
defects compared with nanocrystals templated with other amino
acids. These studies inspired us to used amino acids as growth-
electrocatalysts were synthesized using the same procedure
directing agents to increase the oxygen vacancy content of CeO2
with different His-CeO2 loadings with respect to carbon black
(10 and 40 wt%). For the control experiments, Pt/CeO2-C was
co-catalysts in the MOR.
In this study, we regulated the growth of CeO2 nanoparticles
prepared according to this procedure using bare CeO2 nano-
by using histidine under urea hydrolysis conditions. The functional
crystals, and Pt/C-Z was prepared without using CeO2.
groups –NH2, –COOH and imidazole, enriched the His-CeO2 nano-
particles with oxygen vacancies. The as-prepared Pt/His-CeO2-C
2.4 Electrochemical measurements
catalysts exhibited remarkable mass activity and anti-CO ability
The electrochemical measurements were conducted in a three-
in the MOR, with His-CeO2 as efficient co-catalysts. This study
electrode cell by using an electrochemical workstation (CHI
provides a promising method of manufacturing Pt-based electro-
660E, CH Instruments, China). A glassy carbon electrode was
catalysts for the MOR with facile modulation of the synergistic
used as the working electrode (diameter d = 3 mm), and a
platinum wire was used as the counter electrode. Moreover, a
saturated KCl, Hg/HgCl2 (SCE) reference electrode was used.
Typically, a mixture of the as-prepared catalyst (1.0 mg), iso-
propanol (200 mL), H2O (785 mL) and Nafion solution (15 mL,
effects between the Pt species and metal oxides by using bio-
molecules as growth-directing agents.
5.0 wt%) was sonicated for 30 min to form a homogeneous ink.
The ink (5.0 mL) was dropped onto the polished working
electrode and dried at room temperature for 60 min. The
2. Experimental section
2.1 Materials
Cerium nitrate (Ce(NO3)3Á6H2O) was obtained from Tianjin
working electrode was activated by cyclic voltammetry (CV)
between À0.2 and 1.0 V at 50 mV sÀ1 in N2-saturated H2SO4
Guangfu Fine Chemical Research Institute. L-Histidine was
purchased from Tianjin Heowns Technology Co., Ltd, China.
solution (0.5 M). CV for methanol oxidation was performed at
50 mV sÀ1 and room temperature in an N2-saturated solution
containing 0.5 M H2SO4 and 1.0 M CH3OH. Chronoamperometry
was performed at 0.60 V for 3600 s. For CO stripping, high-purity
CO was bubbled into 0.5 M H2SO4 for 30 min at an electrode
potential of 0 V vs. SCE. The dissolved CO was purged from the
electrolyte by bubbling with N2 for another 30 min. CV was
H2PtCl6Á6H2O (99.9%) was purchased from Sangon Biotech
(Shanghai) Co., Ltd, China. Carbon black (Vulcan XC-72) and
carbon-supported Pt (Pt/C-JM, 20 wt%) were obtained from
Hesen Electric Co., Ltd, China. Ethylene glycol (EG), sulfuric
acid and methanol were purchased from Jiangtian Chemical
Technology Co., Ltd, China. High-purity nitrogen and carbon
monoxide (Z99.99%) were provided by Tianjin Dongxiang Co.,
Ltd, China. The glassy carbon electrode (d = 3 mm) was
purchased from Tianjin Incole Union Tech. Co., Ltd, China.
performed on all the catalysts between À0.2 and 1.0 V at 50 mV sÀ1
.
2.5 Characterization
Transmission electron microscopy (TEM) was performed using
a Tecnai JEM-2100F electron microscope (JEOL, Japan) at an
2.2 Synthesis of His-CeO2 nanoparticles
His-CeO2 solids were synthesized by a modified solvothermal accelerating voltage of 200 kV. X-ray photoelectron spectro-
method.23 Ce(NO3)3Á6H2O (1.04 g) and urea (0.86 g) were scopy (XPS) was carried out on a PHI5000 Versaprobe spectro-
dissolved in water (80 mL). Then histidine was added to the meter (Ulvac-Phi, Japan). Powder X-ray diffraction (XRD)
solution to reach a final Ce3+ : histidine molar ratio of 1 : 2. After spectra were measured with a D/Max-2500 X-ray diffractometer
stirring for 30 min, the solution was transferred to a Teflon-lined (Rigaku, Japan) using a Cu Ka source (l = 0.154178 nm).
stainless steel autoclave (100 mL), and heated at 130 1C for 12 h Photoluminescence (PL) spectra were recorded on a Fluorolog
without stirring. After cooling, the white precipitate was filtered and 3-21 spectrometer (Jobin Yvon) with a 450 W Xe lamp as an
washed with water and anhydrous ethanol three times, and then excitation source at room temperature. The metal loadings
dried in the oven for 24 h at 60 1C. Finally, the as-prepared white were obtained by inductively coupled plasma-optical emission
powder was heated to 550 1C under air for 5 h to obtain straw spectrometry (ICP-OES; Thermo Jarrell-Ash Corp., USA).
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3. Results and discussion
3.1 Characteristics of His-CeO2 nanoparticles
fate of electron–hole pairs.27 Fig. 2d shows the PL spectra
of His-CeO2 and CeO2 excited at 325 nm. The PL intensity of
His-CeO2 is lower than that of bare CeO2, demonstrating a
lower electron recombination rate. This may be attributed to
the presence of surface defects or oxygen vacancies in His-CeO2
nanoparticles. Due to the higher Ce3+ species and oxygen
vacancy contents, a larger proportion of valence electrons can
be excited to the defect states or the oxygen vacancy states,
which would result in a lower PL signal compared with bare
CeO2.28,29
The His-CeO2 and bare CeO2 nanoparticles were characterized
by TEM, XRD, XPS and PL. As shown in Fig. 1 and Fig. S1 (ESI†), the
as-prepared His-CeO2 nanoparticles have an average diameter of
12.2 nm, and the lattice distance of 0.313 nm corresponds well to
the (111) lattice spacing of CeO2, suggesting that His-CeO2 nano-
particles mainly consist of (111) facets.24 In comparison, bare CeO2
nanoparticles are also composed of (111) facets, but have a smaller
mean size of 9.6 nm compared with His-CeO2.
Regarding the mechanism, histidine has –NH2, –COOH and
imidazole groups, which can coordinate with transition metal ions.
Under experimental conditions, NH4+ and CO32À from urea decom-
position can react with Ce3+, and histidine plays a crucial role in the
intermediate phase transformation and crystallinity and influences
the subsequent synthetic process.23 Consequently, CeO2 nano-
particles form with different physical properties compared with
CeO2 with no structure-directing agent.
3.2 Preparation and characterization of Pt/His-CeO2-C
The procedure for preparing the Pt/His-CeO2-C electrocatalyst
is shown in Scheme 1. Because CeO2 is negatively charged at
pH 4 7.8,14,30 His-CeO2 or bare CeO2 nanoparticles are
adsorbed on carbon black, which carries positive charges at
2À
pH 12. In addition, EG acts as the reductant to reduce PtCl6
ions to Pt(0). It is theoretically predicted that the noble metal
atoms are preferentially adsorbed at the anionic sites on the
CeO2 surface.31 In particular, Pt atoms are adsorbed on the
surface O-bridge sites.32 Therefore, the CeO2 surface also offers
strong interactions with the reduced Pt(0) species, contributing
to the deposition of Pt nanoparticles.
TEM images of as-prepared Pt/His-CeO2-C are shown in
Fig. 3. The Pt nanoparticles are uniformly distributed over the
whole surface of His-CeO2-C (Fig. 3a), with an average diameter
of 2.19 nm. Fig. 3b shows the lattice fringes of the Pt(111) facet
and the His-CeO2(111) facet. Pt nanoparticles are located at the
margin of the His-CeO2 deposited on the carbon support,
suggesting strong interactions between the Pt species and
His-CeO2. TEM images of Pt/CeO2-C and Pt/C-Z (Fig. S3, ESI†)
also show highly dispersed Pt nanoparticles with a size dis-
tribution of 2.2–2.3 nm.
In the XRD patterns (Fig. 2a), His-CeO2 shows identical
diffraction peaks to bare CeO2, which are indexed to the fluorite
cubic (f.c.c.) structure of CeO2.16 Thus, the addition of histidine has
little effect on the crystal form of CeO2. All the peak positions (2y) of
His-CeO2 are shifted toward lower angles relative to bare CeO2.
Moreover, the lattice parameters of His-CeO2 (5.42 Å) are higher
than those of bare CeO2 (5.4 Å) (Table S1, ESI†), which is attributed
to the local reduction of Ce4+ to Ce3+ and the subsequent increase
in oxygen vacancies.23 The average crystal sizes of His-CeO2 and
bare CeO2 calculated with the Scherrer equation from the diffrac-
tion peak of CeO2 (111) are 12.26 and 10.65 nm, respectively.
XPS spectra show that Ce4+ and Ce3+ species coexist on the
surface of His-CeO2 and bare CeO2 (Fig. 2b and Fig. S2, ESI†).
The proportion of Ce3+ (Ce3+/(Ce3+ + Ce4+)) on the surface of
His-CeO2 (19.76%) is higher than that of bare CeO2 (16.93%)
(Table S2, ESI†). The O 1s XPS spectra can be decomposed into two
peaks, which correspond to the lattice oxygen (OL) at 529.3 eV and
the chemisorbed surface oxygen (OC) at 531.2 eV. Compared with
bare CeO2, His-CeO2 exhibits stronger peak at 531.2 eV (Fig. 2c).
Therefore, the superior oxygen storage capacity could supply more
oxygen vacancies for the reaction.25,26
The loadings of Pt species in as-prepared Pt/His-CeO2-C
and commercial Pt/C-JM catalysts were measured by ICP-OES
(Table S3, ESI†) and were approximately 18% in Pt/C-Z, Pt/CeO2-C
and Pt/His-CeO2-C.
Next, the charge states of the Pt components in Pt/His-CeO2-C,
Pt/CeO2-C and Pt/C-Z were obtained by XPS (Fig. 4 and Fig. S4,
ESI†).33 As listed in Table S4 (ESI†), the proportions of Pt0
species in Pt/His-CeO2-C, Pt/CeO2-C and Pt/C-Z, were determined
as 67.87%, 66.16% and 60.35%, respectively. Compared with
Pt/C-Z, both Pt/His-CeO2-C and Pt/CeO2-C have higher fractions of
metallic Pt0 species, demonstrating the partial electron transfer
from CeO2 to the Pt species. The charge transfer could contribute to
the increased electron density of Pt, which weakens the s–p bond
and the CO adsorption strength on Pt, allowing CO desorb easily
from the Pt surfaces.34
PL spectroscopy is widely used to investigate the efficiency of
charge carrier migration and transfer properties as well as the
3.3 Electrochemical measurements of Pt/His-CeO2-C
Fig. 5a shows the CV curves of the as-prepared catalysts recorded
in 0.5 M H2SO4. Using His-CeO2 as a co-catalyst, Pt/His-CeO2-C
has a higher electrochemically active surface area (ECSA) than
those of Pt/CeO2-C, Pt/C-Z and commercial Pt/C-JM, suggesting
that a larger number of catalytically active sites are available
for electrochemical reactions. The ECSA of Pt/His-CeO2-C
Fig. 1 TEM images of (a) CeO2 and (b) His-CeO2. The scale bar represents
5 nm.
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Fig. 2 (a) XRD patterns of CeO2 and His-CeO2. XPS spectra for (b) Ce 3d core level regions of His-CeO2 and (c) O 1s core level regions of CeO2 and His-
CeO2. (d) Photoluminescence spectra of CeO2 and His-CeO2 excited at 325 nm.
Scheme 1 Illustration of the procedure for preparing Pt/His-CeO2-C.
Fig. 3 (a) TEM and (b) HRTEM images of Pt/His-CeO2-C. The inset in (a) is
the size distribution of the Pt nanoparticles.
(66.29 m2 gÀ1) is 1.53 and 1.28 times higher than those of Pt/C-Z
and Pt/C-JM, respectively.
which is 1.5 times than that of Pt/CeO2-C (765 A gÀ1), and about
The methanol electrooxidation activities are shown in 2.9 and 2.2 times that of Pt/C-Z (380 A gÀ1) and Pt/C-JM (510 A gÀ1),
Fig. 5b. In the CV curves, the peaks for the forward scan current respectively. Compared with other recently reported catalysts
density (If) are attributed to the oxidation of methanol, whereas (Table S5, ESI†), Pt/His-CeO2-C is a promising electrocatalyst
the peaks for the reverse scan current density (Ib) results from the for the MOR.
removal of intermediate carbonaceous species.35 The optimized
Fig. 5c shows the CO stripping curves of the Pt/His-CeO2-C,
loading of His-CeO2 on carbon black was considered first. Pt/CeO2-C, Pt/C-Z and Pt/C-JM catalysts. The oxidation peak
Comparing the electrocatalytic performance of Pt catalysts potentials of Pt/His-CeO2-C are lower than those of the other
supported on His-CeO2-C with different His-CeO2 loadings three catalysts, demonstrating that Pt/His-CeO2-C is more
(10–50% with respect to carbon black) (Fig. S5, ESI†), the effective for the electrooxidation of adsorbed CO. This enhance-
optimal loading of His-CeO2 in Pt/His-CeO2-C is 20 wt%. The ment is attributed to the synergistic effect between Pt and His-
mass activity for Pt/His-CeO2-C is up to 1116.9 A gÀ1 at 0.6 V, CeO2. His-CeO2 with sufficient OHads species promotes the
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Fig. 4 (a) Pt 4f XPS spectra of Pt/His-CeO2-C. The black, red, dark cyan, magenta, and dark yellow lines represent the raw, fitted, Pt0, Pt2+ and Pt4+
component curves, respectively. (b) Comparison of Pt 4f regions for Pt/C-Z, Pt/CeO2-C and Pt/His-CeO2-C.
Fig. 5 (a) CV curves of different catalysts in 0.5 M H2SO4 at 50 mV sÀ1; (b) mass activities of MOR recorded in 0.5 M H2SO4 + 1.0 M CH3OH at 50 mV sÀ1
;
(c) CO stripping curves of different catalysts in 0.5 M H2SO4 at 50 mV sÀ1 in the first forward scan. Inset: Magnified areas in the range of 0.4–0.75 V.
(d) Chronoamperometric curves of the catalysts tested at 0.6 V.
elimination of COads, releasing the active Pt sites and resulting
in the excellent antipoisoning properties of Pt/His-CeO2-C.
The generally accepted mechanism of the COads oxidation pro-
cess in CeO2-assisted Pt catalysts is thought to be bifunctional.15,24
Pt–CeO2 + xCO - Pt–CeO2Àx + xCO2
The OHads species generated on the CeO2 surface react with
COads on the Pt species when the Pt nanoparticles and CeO2 are
in contact with each other, and the active sites of Pt are
subsequently released for further MORs. Therefore, the catalytic
performance of CeO2-assisted Pt catalysts is strongly related to the
CeO2 OHads species and the interactions between the Pt species
and CeO2. In this work, histidine-mediated CeO2 provides
sufficient oxygen vacancies. The increased oxygen vacancies
CeO2 + H2O - CeO2–OHads + H+ + eÀ
Pt–COads + CeO2–OHads - Pt + CeO2 + CO2 + H+ + eÀ
or
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4. Conclusions
In this study, histidine was chosen as a growth-directing agent to
regulate the physicochemical properties of CeO2 nanoparticles
synthesized under urea hydrolysis conditions. Histidine-mediated
CeO2 nanoparticles with an average size of 12.2 nm had a larger
proportion of Ce3+ species and an increased oxygen vacancy content
compared with bare CeO2. Pt/His-CeO2-C electrocatalysts were
prepared by using His-CeO2 nanoparticles as co-catalysts, contain-
ing Pt nanoparticles with a particle size of around 2.2 nm and a
proportion of Pt0 species of 67.9%. CV showed that Pt/His-CeO2-C
had a mass activity of up to 1116.9 A gÀ1 and an activity approxi-
mately 1.5-, 2.9- and 2.2-fold greater than Pt/CeO2-C, Pt/C-Z and
Pt/C-JM, respectively. The higher activity of Pt/His-CeO2-C in the
MOR was attributed to the following factors. (i) His-CeO2 has more
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and can be oxidized by OHads species. (iii) The addition of His-CeO2
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Conflicts of interest
The authors declare no competing financial interest.
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