Intermetallic compound PtMny-derived Pt?MnOx supported on mesoporous CeO2: Highly efficient catalysts for the combustion of toluene

Article abstract of DOI:10.1016/j.apcata.2020.117509

Intermetallic compounds are a kind of important materials in heterogeneous catalysis. In this work, we first synthesized the PtMn_y intermetallic nanocrystals using the polyvinyl pyrrolidone-assisted ethylene glycol reduction method, and then loaded them on the surface of mesoporous CeO₂ (meso-CeO₂) derived from a KIT-6-templating route, generating the mPt?nMnO_x/meso-CeO₂ (m = 0?0.39 wt%, n = 0?1.21 wt%) catalysts after calcination at 500 °C in air. It is found that the as-obtained catalysts displayed an ordered mesoporous architecture with surface areas of 95?108 m²/g. The 0.37Pt?0.16MnO_x/meso-CeO₂ sample exhibited the best catalytic performance for toluene combustion (T50 % =162 °C and T90 % =171 °C at space velocity = 40,000 mL/(g h)). Kinetic analysis reveals that the apparent activation energy (57 kJ/mol) obtained over the best-performing 0.37Pt?0.16MnO_x/meso-CeO₂ sample was lower than those (63?75 kJ/mol) obtained over the other samples. Furthermore, the 0.37Pt?0.16MnO_x/meso-CeO₂ sample possessed good thermal stability and water-resistant performance. Benzyl alcohol, benzoic acid, and maleic anhydride were proven to be the main intermediates of toluene combustion, hence, toluene combustion might take place through a sequence of toluene → benzyl alcohol and benzoic acid → maleic anhydride → carbon dioxide and water, which might obey the Eley?Rideal reaction mechanism. It is concluded that loading of Pt and MnO_x enhanced the adsorbed oxygen and Mn²⁺ species concentration and low-temperature reducibility, thus promoting toluene combustion over 0.37Pt?0.16MnO_x/meso-CeO₂.

Full text of DOI:10.1016/j.apcata.2020.117509

Journal Pre-proof
Intermetallic compound PtMny-derived PtnullMnOx supported on
mesoporous CeO2: Highly efficient catalysts for the combustion of
toluene
Xiaohang Fu, Yuxi Liu, Jiguang Deng, Lin Jing, Xing Zhang,
Kunfeng Zhang, Zhuo Han, Xiyun Jiang, Hongxing Dai
PII:
S0926-860X(20)30102-2
DOI:
https://doi.org/10.1016/j.apcata.2020.117509
APCATA 117509
Reference:
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Applied Catalysis A, General
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2 February 2020
1 March 2020
2 March 2020
Please cite this article as: Fu X, Liu Y, Deng J, Jing L, Zhang X, Zhang K, Han Z, Jiang X, Dai
H, Intermetallic compound PtMny-derived Pthorbar;MnOx supported on mesoporous CeO2:
Highly efficient catalysts for the combustion of toluene, Applied Catalysis A, General (2020),
doi: https://doi.org/10.1016/j.apcata.2020.117509
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Intermetallic compound PtMny-derived PtMnOx supported on
mesoporous CeO2: Highly efficient catalysts for the combustion of
toluene
Xiaohang Fu, Yuxi Liu*, Jiguang Deng, Lin Jing, Xing Zhang, Kunfeng Zhang, Zhuo
Han, Xiyun Jiang, Hongxing Dai*
Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing
on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials,
Education Ministry of China, Laboratory of Catalysis Chemistry and Nanoscience,
Department of Chemistry and Chemical Engineering, College of Environmental and
Energy Engineering, Beijing University of Technology, Beijing 100124, China
* Corresponding authors.
E-mail addresses: yxliu@bjut.edu.cn (Dr. Y. Liu); hxdai@bjut.edu.cn (Prof. H. Dai)
Tel. No.: +8610-6739-6118; Fax: +8610-6739-1983.
Graphical abstract
PtMnOx/meso-CeO2
Loading the PtMny intermetallic nanocrystals derived from the PVP-assisted EG

reduction route on meso-CeO2 and after calcination in air generates the PtMnOx/meso-
CeO2 catalysts. The good performance for toluene combustion of
0.37Pt0.16MnOx/meso-CeO2 is related to its high adsorbed oxygen and Mn2+ species
concentration and good low-temperature reducibility.
Highlights

PtMny intermetallic nanocrystals are fabricated using the PVP-assisted EG
reduction method.

mPtnMnOx/meso-CeO2 is prepared via adsorption of PtMny NCs on meso-
CeO2.



0.37Pt0.16MnOx/meso-CeO2 perform the best for toluene combustion.
0.37Pt0.16MnOx/meso-CeO2 exhibits good water-resistant performance.
Oads and Mn2+ concentration, low-temp. reducibility, and toluene adsorption
ability govern activity.
Abstract
Intermetallic compounds are a kind of important materials in heterogeneous catalysis.
In this work, we first synthesized the PtMny intermetallic nanocrystals using the
polyvinyl pyrrolidone-assisted ethylene glycol reduction method, and then loaded them
on the surface of mesoporous CeO2 (meso-CeO2) derived from a KIT-6-templating
route, generating the mPtnMnOx/meso-CeO2 (m = 00.39 wt%, n = 01.21 wt%)
o
catalysts after calcination at 500 C in air. It is found that the as-obtained catalysts

displayed an ordered mesoporous architecture with surface areas of 95108 m2/g. The
0.37Pt0.16MnOx/meso-CeO2 sample exhibited the best catalytic performance for
toluene combustion (T50% = 162 oC and T90% = 171 oC at space velocity = 40,000 mL/(g
h)). Kinetic analysis reveals that the apparent activation energy (57 kJ/mol) obtained
over the best-performing 0.37Pt0.16MnOx/meso-CeO2 sample was lower than those
(6375 kJ/mol) obtained over the other samples. Furthermore, the
0.37Pt0.16MnOx/meso-CeO2 sample possessed good thermal stability and water-
resistant performance. Benzyl alcohol, benzoic acid, and maleic anhydride were proven
to be the main intermediates of toluene combustion, hence, toluene combustion might
take place through a sequence of toluene  benzyl alcohol and benzoic acid  maleic
anhydride  carbon dioxide and water, which might obey the EleyRideal reaction
mechanism. It is concluded that loading of Pt and MnOx enhanced the adsorbed oxygen
and Mn2+ species concentration and low-temperature reducibility, thus promoting
toluene combustion over 0.37Pt0.16MnOx/meso-CeO2.
Keywords: Mesoporous ceria; PtMn intermetallic nanocrystal; Supported PtMnOx
catalyst; Volatile organic compound; Toluene combustion.
1. Introduction
Volatile organic compounds (VOCs) are the important precursors of photochemical
smog and fine particulate matter (PM2.5), which are harmful to the atmosphere and
human being's health [1]. Accordingly, it is highly required to abate VOCs emissions.
Among the methods of VOCs removal, catalytic combustion is considered to be the
most effective because of its low cost and high efficiency [2,3]. The key to catalytic

combustion of VOCs is generation of the catalysts that are cheap in cost, high in activity,
and good in stability.
Although supported Pt catalysts exhibit excellent activity for VOCs combustion at
low temperatures [4], they are expensive and easily sintered at high temperatures, which
greatly limit their wide applications. To solve these problems, it is a better choice to
dope a base metal in Pt nanoparticles (NPs) to reduce the Pt amount and improve the
thermal stability. As we know, support plays a remarkable role in improving catalytic
activity [5,6]. Owing to excellent oxygen storage and release ability, ceria-based
materials have attracted much attention in recent years [7,8]. A higher surface area is
favorable for enhancement in activity of a material, and the good strategy to increase
surface area is to fabricate the material with an ordered mesoporous architecture.
Intermetallic compounds are the materials comprising two or more elements located
left and around the Zintl line in the periodic table [9], and their crystal structures are
completely or at least partly ordered and different from those of the constituent elements.
Heterogeneous catalysis of intermetallic compounds has been regarded as a quickly
developing field [10]. The peculiar combination of the crystal and electronic structures
of intermetallic compounds results in their unique adsorption and thus catalytic
properties, in which covalent bonding is necessary to stabilize the specific crystal and
electronic structure of an intermetallic compound. Such a site preference can provide
the catalytic stability under reaction conditions. This makes intermetallic compounds
be highly interesting catalytic materials. For example, Armbrüster et al. [11] adopted a
co-reduction of the palladium and gallium precursors in organic solvents to prepare the

GaPd or GaPd2 NPs, and observed that the activity per Pd atom of these materials
reached those of the commercially available supported Pd catalysts while preserving
the excellent selectivity of the unsupported materials. Osswald et al. [12] reported that
nearly no deactivation or loss in selectivity was observed over the Ga7Pd3, GaPd or
GaPd2 intermetallic compound as compared with the 5 wt% Pd/Al2O3 sample within 20
h of reaction (hydrogenation of acetylene). The high selectivity of the GaPd
compounds prevented formation of the carbonaceous deposits and hence these
intermetallic compounds showed good stability. To the best of our knowledge, there
have been rare works related to the utilization of intermetallic compounds in the
application of VOCs combustion. For example, Willis et al. [13] studied promotional
effect of the transition metal on methane oxidation over the palladium catalysts, and
found that some metals (Fe, Co, and Sn) inhibited sintering of the active Pd metal phase,
while the others (Ni and Zn) increased its intrinsic activity as compared with a
monometallic Pd catalyst. Therefore, intermetallic compounds are promising materials
in heterogeneous catalysis. In recent years, VOCs combustion over the supported
bimetallic catalysts has been studied intensively and extensively. For instance, we
previously [14] synthesized AuPd alloys supported on three-dimensionally ordered
macroporous (3DOM) Co3O4, and found that the bimetallic catalysts were more active
and stable than the single metal counterparts for toluene oxidation. After investigating
the one-step seeding growth-derived core-shell Au@Co NPs, Yan et al. [15] pointed
out that the Au@Co catalyst was highly active for hydrolytic dehydrogenation of
ammonia borane. Ho et al. [16] used the boraneamine reduction method to obtain the

bimetallic MPd (M = Co or Cu) NPs, and claimed that the MPd NPs showed good
catalytic performance for formic acid electrooxidation.
In the past several years, our group has prepared 1.67 wt% Mn3O42 wt% Au/3DOM
La0.6Sr0.4CoO3 [17], 0.25 wt% Pt1/meso-Fe2O3 [18], and 8.5 wt% Co3O4/MnO2 [19] via
the polyvinyl alcohol-protecting reduction route, and observed that most of the
materials performed well in catalyzing combustion of the typical VOCs (e.g., toluene,
benzene, and o-xylene). Herein, we first synthesized the PtMny intermetallic
nanocrystals (NCs) using the polyvinyl pyrrolidone (PVP)-assisted ethylene glycol (EG)
reduction method, and then loaded them on the surface of ordered mesoporous CeO2
(meso-CeO2), characterized their physicochemical properties of the as-prepared
mPtnMnOx/meso-CeO2 (m = 00.39 wt%, n = 01.21 wt%), and evaluated their
catalytic activities for toluene combustion.
2. Experimental
2.1. Catalyst preparation
The ordered mesoporous silica (KIT-6) template was fabricated according to the
procedures reported in the literature [20]. Ordered mesoporous CeO2 (meso-CeO2) was
fabricated by the KIT-6-templating strategy. 2.0 g of the KIT-6 template was added to
40 mL of ethanol containing 4.0 g of Ce(NO3)36H2O. The mixture was first dried at
room temperature (RT) and calcined at a ramp of 1 oC/min from RT to 600 oC and kept
o
o
at 600 C for 6 h. The obtained powders were twice treated in a hot (60 C) NaOH
aqueous solution (concentration = 2.0 mol/L) for the removal of the SiO2 template, and
washed with deionized water and ethanol several times, hence obtaining the meso-CeO2

support.
The PtMny NCs and Pt NPs were synthesized using the PVP-protected EG reduction
strategy with an ethanol solution of NaBH4 as reducing agent. The typical synthesis
processes are as follows: The desired amounts of chloroplatinic acid (H2PtCl6),
manganese acetate (theoretical Mn/Pt molar ratio (y) = 0, 2.0, 7.0, and 11.6, which were
aimed to control the actual loadings of MnOx and Pt in the samples), PVP (Pt/PVP mass
ratio = 1 : 1.5), and 20 mL of EG were added to 100 mL of two-necked round-bottom
o
flask at 50 C under stirring. After that, a NaBH4 ethanol solution (0.75 mol/L,
PtMny/NaBH4 molar ratio = 1 : 10) was fast added to generate a black suspension after
vigorous stirring for 15 min, hence obtaining the PtMny NCs (the actual Mn/Pt molar
ratio (y) = 1.5, 7.7, and 11.6 (Table1)). The Pt NPs and Mn NPs were fabricated using
the above same method in the absence of manganese acetate and chloroplatinic acid,
respectively.
The meso-CeO2-supported PtMny NCs (PtMny/meso-CeO2) and Pt NPs (Pt/meso-
CeO2) were prepared via an adsorption route. A desired amount of meso-CeO2 was
added to the above obtained suspension of PtMny NCs or Pt NPs (theoretical Pt loading
= 0.5 wt%). The suspension was ultrasonically (60 kHz) treated for 15 min and
vigorously stirred for 8 h, the wet solid was first filtered and then dried at 50 oC for 12
h, obtaining the PtMny/meso-CeO2 samples. Finally, the PtMny/meso-CeO2 samples
were calcined in air at a ramp of 5 oC/min from RT to 500 oC and kept at this temperature
for 2 h, thus generating the mPtnMnOx/meso-CeO2 (m and n are the weight
percentages of Pt and MnOx, respectively) samples. Using the inductively coupled

plasmaatomic emission spectroscopic (ICPAES) technique, we can realize that (i)
the actual Pt and MnOx loadings in the Pt/meso-CeO2 and MnOx/meso-CeO2 samples
were 0.39 and 0.16 wt%, respectively; and (ii) the actual Pt (m) and MnOx (n) loadings
in the mPtnMnOx samples were 0.37 and 0.16 wt%, 0.36 and 0.78ꢀwt%, and 0.37 and
1.21 wt% (Table 1), respectively. The corresponding samples were denoted as
0.39Pt/meso-CeO2, 0.16MnOx/meso-CeO2, 0.37Pt0.16MnOx/meso-CeO2 (derived
o
from PtMn1.5/meso-CeO2 after calcination at 500 C in air), 0.36Pt0.78MnOx/meso-
o
CeO2 (derived from PtMn7.7/meso-CeO2 after calcination at 500 C in air), and
0.37Pt1.21MnOx/meso-CeO2 (derived from PtMn11.6/meso-CeO2 after calcination at
500 oC in air).
All of the chemicals were A.R. in purity, which were purchased from China National
Medicines Corporation Ltd.
2.2. Catalyst characterization
Physicochemical properties of meso-CeO2, 0.16MnOx/meso-CeO2, 0.39Pt/meso-
CeO2, and mPtnMnOx/meso-CeO2 were measured by means of the ICPAES, XRD,
BET, TEM, XPS, H2-TPR, O2-TPD, toluene-TPD, and in situ DRIFTS techniques. The
detailed characterization procedures are stated in the Supplementary material.
2.3. Catalytic activity measurement
Catalytic activities of the samples were evaluated in a continuous-flow fixed-bed
quartz tubular microreactor (i.d. = 6.0 mm). The sample (50 mg, 4060 mesh) was
diluted with quartz sand (250 mg, 4060 mesh) for minimizing the effect of possible
hot spots. Before the test, the sample was treated in O2 (20 mL/min) at 250 oC for 1 h.

The reactant gas mixture contained 1000 ppm toluene, 20 vol% O2, and N2 (balance),
and the total flow was 33.4 mL/min (space velocity (SV) = ca. 40,000 mL/(g h)). 1.0,
3.0, and 5.0 vol% water vapor was introduced to the reaction system through a water
saturator at 17, 35, and 45 oC, respectively. 5.0 vol% CO2 was introduced from a CO2
cylinder (N2 as balance gas). Reaction mixtures were online analyzed on a gas
chromatograph (GC-2010, Shimadzu) equipped with a flame ionization detector and a
thermal conductivity detector, in which a Stabilwax column (30 m in length) and a
Carboxen 1000 column (3 m in length) were employed. Only CO2 was detected in the
outlet gas mixture. The balance of carbon in the catalytic system was 99.5  1.5 %.
Toluene conversion (X %) was calculated according to the formula: X = (cinlet
−
coutlet)/cinlet × 100 %, in which the cinlet and coutlet are the inlet and outlet toluene
concentrations in the feed stream, respectively.
3. Results and discussion
3.1. Crystal phase composition, pore structure, and surface area
Fig. 1Ashows XRD patterns of the meso-CeO2, 0.39Pt/meso-CeO2, 0.16MnOx/meso-
CeO2, and mPtnMnOx/meso-CeO2 samples. After referring to XRD pattern (JCPDS
PDF# 34-0394) of the standard ceria sample, we can know that CeO2 in all of the
samples possessed a cubic crystal structure, and their crystal planes are shown in Fig.
1A(e). No diffraction signals assigned to the MnOx and/or Pt phases were observed, a
result possibly owing to their low loadings and good dispersion on the surface of meso-
CeO2. In other words, loading of Pt and/or MnOx did not induce an obvious change in
crystal phase of meso-CeO2. The observation of a weak diffraction peak at 2θ = ca. 1 o

in small-angle XRD pattern of each sample (Fig. 1B) means generation of an ordered
mesoporous architecture. The calculated grain sizes of meso-CeO2 in the samples were
ca. 10 nm (Table 1).
Shown in Fig. 2A are N2 sorption isotherms of the meso-CeO2, 0.39Pt/meso-CeO2,
0.16MnOx/meso-CeO2, and mPtnMnOx/meso-CeO2 samples. Each sample displayed
a IV-typed isotherm with a hysteresis loop in the relative pressure range of 0.11.0,
indicating formation of mesopores. As shown in the pore-size distribution (Fig. 2B) of
each sample, a peak appeared in the range of 3–6 nm. Textural parameters of the
samples are listed in Table 1. There was no significant drop in surface area of the
supported Pt, MnOx or mPtnMnOx sample. Surface areas, average pore sizes, and pore
volumes of the as-prepared samples were 95108 m2/g, 4.0 nm, and 0.190.22 cm3/g,
respectively.
Fig. 3 shows TEM images of the samples. The meso-CeO2 support possessed a good-
quality ordered mesopore architecture (Fig. 3a). The lattice spacing (d value) of the
(200) crystal plane of meso-CeO2 was measured to be ca. 0.276 nm (Fig. 3b), which
was similar to that of the standard CeO2 sample. After loading the Pt, MnOx or
mPtnMnOx NPs, the ordered mesoporous structure was well retained (Fig. 3c, e, and
g). Due to low loadings, we did not observe the Pt, MnOx or mPtnMnOx NPs in the
TEM images, but the Pt, MnOx, and CeO2 with Pt and MnOx highly distributed on the
surface of meso-CeO2 were detected in the HADDFSTEM image and elemental
mappings of the 0.37Pt0.16MnOx/meso-CeO2 sample (Fig. 4). Fig. 5 shows TEM
images and particle-size distribution of PtMn1.5 intermetallic NCs. Obviously, the

obtained PtMn1.5 NCs were monodispersive and uniform in particle size (ca. 2.2 nm,
which was estimated by statistically analyzing particle sizes of 150 NPs). It can be
deduced from Figs. 4 and 5 that there was formation of a PtMn1.5 intermetallic
compound and PtMnOx NPs in the 0.37Pt0.16MnOx/meso-CeO2 sample. That is to
say, the adopted preparation methods were effective for generation of the PtMny
intermetallic compounds and mPtnMnOx/meso-CeO2 samples.
3.2. Catalytic performance and effects of moisture and carbon dioxide on activity
Toluene combustion was chosen to evaluate catalytic activities of the samples.
Toluene conversions at various temperatures, reaction rate normalized per gram of
catalyst, and Arrhenius plots are presented in Fig. 6. When only quartz sand was loaded
o
in the microreactor, no significant toluene conversions (< 2 %) below 300 C at SV =
40,000 mL/(g h), indicating that no obvious gas-phase reaction took place. In other
words, toluene was catalytically oxidized over the samples. When reaction temperature
rose, toluene conversion increased monotonously (Fig. 6A). Pt loading resulted in
enhancement in activity of the meso-CeO2 and its supported MnOx samples. Loading a
small amount (0.16 wt%) of MnOx could improve the catalytic activity, but an excessive
amount (0.78 or 1.21 wt%) of MnOx caused the catalytic activity to decrease. For the
sake of better comparison on catalytic performance of the samples, we use the reaction
temperatures (T10%, T50%, and T90%), at which 10, 50, and 90 % conversions of toluene
were achieved, respectively, as listed in Table 2. Obviously, the
0.37Pt0.16MnOx/meso-CeO2 sample performed the best, with the T10%, T50% , and T90%
o
being 127, 162, and 171 C, respectively. Compared to the T50% and T90% over the

0.39Pt/meso-CeO2 sample, those over the 0.37Pt0.16MnOx/meso-CeO2 sample
o
decreased by 19 and 18 C, respectively. The changing trend in toluene reaction rate
normalized per gram of catalyst versus temperature was the same as that in toluene
conversion versus temperature (Fig. 6B). Apparent activation energies (Ea) for toluene
combustion over the samples (Table 2) were calculated from their Arrhenius plots (Fig.
6C). The Ea obtained over 0.37Pt0.16MnOx/meso-CeO2 was 57 kJ/mol, much lower
than those (6375 kJ/mol) obtained over meso-CeO2, 0.16MnOx/meso-CeO2,
0.39Pt/meso-CeO2, 0.36Pt0.78MnOx/meso-CeO2, and 0.37Pt1.21MnOx/meso-CeO2.
It is well known that the lower the Ea, the better the performance of a catalyst. Hence,
the 0.37Pt0.16MnOx/meso-CeO2 sample exhibited the best activity. These results
demonstrate that loading Pt and MnOx with proper amounts favored the augmentation
in catalytic performance of toluene combustion.
Catalytic activities for toluene combustion of the 0.37Pt–0.16MnOx/meso-CeO2
sample and the catalysts reported in the literature are summarized in Table S1 of the
Supplementary material. Apparently, 0.37Pt0.16MnOx/meso-CeO2 (T90% = 171 oC at
SV = 40,000 mL/(g h); toluene reaction rate at 160 oC = 0.339 mol/(gcat s)) exhibited
much better activity than 1 wt% Pt/SBA-15 (T90% = 189 oC at SV = 10,000 h–1; toluene
reaction rate at 160 oC = 0.067 mol/(gcat s)) [21], 0.27 wt% Pt/3DOM 26.9CeO2Al2O3
o
o
(T90% = 198 C at SV = 20,000 mL/(g h); toluene reaction rate at 160 C = 0.061
mol/(gcat s)) [4], 1.3 wt% Au/CeO2 (T90% = 250 oC at SV = 35,000 h–1; toluene reaction
rate at 160 oC = 0.029 mol/(gcat s)) [22], and 15 wt% Mn/ClinoptiloliteCeO2 (T90%
=
320 oC at SV = 6000 h–1; toluene reaction rate at 160 oC = 0.003 mol/(gcat s)) [23], but

o
was inferior in activity to 0.2 at% Pt/CeO2-nanorod (T90% = 150 C at SV = 48,000
o
mL/(g h); toluene reaction rate at 160 C = 0.976 mol/(gcat s)) [24] and 6.5 wt%
Au/meso-Co3O4 (T90% = 138 oC at SV = 20,000 mL/(g h) ; toluene reaction rate at 160
oC = 0.956 mol/(gcat s)) [25].
In order to investigate catalytic stability of the 0.37Pt0.16MnOx/meso-CeO2 sample,
we carried out on-stream toluene combustion at 160 and 180 oC and SV = 40,000 mL/(g
h), and the results are shown in Fig. 7. There was no apparent loss in activity within 30
h
of on-stream reaction at 160 or 180 oC, demonstrating that the
0.37Pt0.16MnOx/meso-CeO2 sample possessed a good catalytic stability under the
adopted conditions.
Since H2O and CO2 are the main products in toluene combustion, it is necessary to
probe their effects (Fig. 8) on activity of the best-performing 0.37Pt0.16MnOx/meso-
CeO2 sample at 180 oC and SV = 40,000 mL/(g h). It is obviously seen that introduction
of 1.05.0 vol% water vapor to the reaction system did not induce considerable changes
in activity (Fig. 8A), which demonstrates that the 0.37Pt0.16MnOx/meso-CeO2
sample exhibited good water-resistant performance. Owing to competitive adsorption
of carbon dioxide and toluene molecules on active sites of the sample, toluene
conversion decreased from 96 to 88 % in the presence of 5.0 vol% CO2, but it was
slowly recovered after cutting off the provision of CO2 (Fig. 8B). This result suggests
that partial deactivation of 0.37Pt0.16MnOx/meso-CeO2 caused by CO2 was reversible.
3.3. Low-temperature reducibility
The low-temperature reducibility of a catalyst is important in the low-temperature

removal of VOCs [26]. H2-TPR technique was employed to measure reducibility of the
samples, and their profiles are illustrated in Fig. 9A. For the meso-CeO2 sample, the
o
peak at 515 C was owing to reduction of Ce4+ to Ce3+. After doping MnOx on the
o
surface of meso-CeO2, the peak at 511 C originated from reduction of Ce4+ to Ce3+,
o
and the one at 255 C was ascribable to reduction of MnO2 and/or Mn2O3 to Mn3O4.
After loading of Pt on the surface of meso-CeO2, the peak assignable to reduction of
o
Ce4+ to Ce3+ was considerably shifted to a lower temperature (228 C), which was a
result due to promotional effect of the hydrogen chemically adsorbed on the Pt NPs
[27]. Hence, loading of Pt NPs could greatly improve low-temperature reducibility of
meso-CeO2. When PtMnOx was loaded on the surface of meso-CeO2, the reduction
band was significantly shifted to a low temperature (100120 oC), which was owing to
reduction of the oxidized Pt2+ to Pt0 as well as the removal of adsorbed oxygen species.
It can been seen from Table 3 that H2 consumption of the meso-CeO2 and
0.16MnOx/meso-CeO2 samples was 10.3 and 10.6 mmol/gcat, respectively, whereas that
(10.912.5 mmol/gcat) of the mPtnMnOx/meso-CeO2 samples was increased. Based on
the surface Ce4+, Mn3+, and Pt2+ contents obtained after quantitatively analyzing their
XPS spectra, we can see that theoretical H2 consumption of the meso-CeO2,
0.16MnOx/meso-CeO2, and 0.39Pt/meso-CeO2 samples was in the range of 14.515.0
mmol/gcat, but it increased from 15.2 to 19.6 mmol/gcat when the doped MnOx content
rose from 0.16 to 1.21 wt% (Table S2). Obviously, theoretical H2 consumption of the
samples was higher than their corresponding actual H2 consumption, which was due to
the fact that surface element compositions of the samples were different from their bulk

compositions. Fig. 9B shows the initial H2 consumption rate as a function of inverse
temperature of the samples, from which one can realize that the low-temperature
reducibility increased in the order of meso-CeO2 < 0.16MnOx/meso-CeO2
<
0.39Pt/meso-CeO2 < 0.37Pt1.21MnOx/meso-CeO2 < 0.36Pt0.78MnOx/meso-CeO2 <
0.37Pt0.16MnOx/meso-CeO2, with the 0.37Pt0.16MnOx/meso-CeO2 sample
possessing the best low-temperature reducibility (coinciding with its highest catalytic
activity for toluene combustion). Therefore, catalytic activity of the sample was closely
related to its low-temperature reducibility.
3.4. Surface property
We used the XPS technique to investigate the element compositions, metal oxidation
states, and adsorbed oxygen species on the surface of the samples, and their Ce 3d, O
1s, Pt 4f, and Mn 2p3/2 XPS spectra are presented in Fig. 10. Two sets of the Ce 3d3/2
and Ce 3d5/2 signals (Fig. 10A) were recorded: (i) The 3d3/2 final states at binding energy
(BE) = 901.7, 904.2, 908.8, and 917.3 eV; and (ii) the 3d5/2 final states at BE = 882.5,
885.0, 889.9, and 898.5 eV. The components at BE = 885.0 and 904.2 eV were the
signals from the surface Ce3+ species, while the others originated from the surface Ce4+
species [28,29]. The result clearly indicates that the Ce on the surface of each sample
was present in Ce3+ and Ce4+. The Ce3+/Ce4+ molar ratios (0.250.26) of meso-CeO2
and 0.16MnOx/meso-CeO2 were lower than those (0.290.34) of 0.39Pt/meso-CeO2,
0.37Pt0.16MnOx/meso-CeO2,
0.36Pt0.78MnOx/meso-CeO2,
and
0.37Pt1.21MnOx/meso-CeO2 (Table 2), with the 0.37Pt0.16MnOx/meso-CeO2
sample possessing the highest Ce3+/Ce4+ molar ratio (0.34). In meso-CeO2, a higher

Ce3+ content means a more amount of oxygen vacancies. It is well known that gas-
phase oxygen molecules can be activated at oxygen vacancies to be the active oxygen
adspecies, which would be favorable for the combustion of VOCs. Hence, the
0.37Pt0.16MnOx/meso-CeO2 sample performed the best, a result due to its highest
oxygen vacancy density. As shown in Fig. 10B, the asymmetrical O 1s spectrum of each
sample was decomposed into three components at BE = 529.6, 531.9, and 533.6 eV,
corresponding to the surface lattice oxygen (Olatt), adsorbed oxygen (Oads), and
adsorbed water or carbonate species [30], respectively. As can be seen from Table 2,
the 0.37Pt0.16MnOx/meso-CeO2 sample possessed the highest Oads/Olatt molar ratio
(0.85) among all of the samples, again confirming that a high Oads concentration favored
toluene combustion. Illustrated in Fig. 10C are Pt 4f XPS spectra of the 0.39Pt/meso-
CeO2,
0.37Pt0.16MnOx/meso-CeO2,
0.36Pt0.78MnOx/meso-CeO2,
and
0.37Pt1.21MnOx/meso-CeO2 samples. The components at BE = 72.8 and 76.1 eV
were owing to the surface Pt0 species, whereas the ones at BE = 74.2 and 78.1 eV
originated from the surface Pt2+ species [31]. Usually, the oxidized noble metal can
facilitate the improvement in activity of a supported catalyst for VOCs oxidation. As
shown in Table 2, the Pt2+/Pt0 molar ratio (2.6) on the surface of
0.37Pt0.16MnOx/meso-CeO2 was the highest, rendering this sample to show the best
catalytic activity for toluene combustion. Fig. 10D presents Mn 2p3/2 XPS spectra of
the Mn-containing samples. Due to the low doping of MnOx, intensity of the Mn 2p3/2
signal was weak. Each of the Mn 2p3/2 spectra was asymmetrical and could be
decomposed into two components at BE = 642.1 and 640.6 eV as well a weak satellite

at BE = 644.3 eV, which were ascribed to the surface Mn3+ and Mn2+ species [32,33],
respectively. The surface Mn2+/Mn3+ molar ratio decreased from 2.5 to 1.7 with the
MnOx loading increased from 0.16 to 1.21 wt% (Table 2), indicating that the surface
Mn2+ concentration was the highest on the surface of the 0.37Pt0.16MnOx/meso-CeO2
sample. Previously, we demonstrated that CoO was more active than Co3O4 (in which
Co2+ and Co3+ co-existed) for the combustion of o-xylene [34]. Similarly, MnO would
be more active than MnOx (in which Mn2+ and Mn3+ co-existed (Mn 2p3/2 XPS results).
That is to say, a higher Mn2+ concentration on the surface of 0.37Pt0.16MnOx/meso-
CeO2 would favor the combustion of toluene.
2

Generally speaking, Oads species (for example, O2 , O2  or O) have an important
role to play in the reductionoxidation reactions [35]. A higher Oads concentration is
usually favorable for the combustion of VOCs. The toluene reaction rate (normalized
per gram of noble metal), TOFPt (normalized per mole of Pt) and TOFMn (normalized
per mole of Mn) at 160 oC as a function of surface Oads/Olatt molar ratio of the samples
are shown in Fig. 11. Obviously, the toluene reaction rate and TOFPt or TOFMn at 160
oC increased with the rise in Oads/Olatt molar ratio. The results demonstrate that catalytic
performance was closely related to the Oads species concentration. Moreover, PtO and
MnO was the main active sites for toluene activation, and higher surface PtO and MnO
concentrations would render the catalyst to show a better activity. Based on the Oads,
PtO, and MnO species concentrations on the sample surface (Table 2), it is
understandable that the 0.37Pt0.16MnOx/meso-CeO2 sample performed the best in
toluene combustion.

3.5. O2-TPD, toluene-TPD, and in situ DRIFTS
To probe oxygen and toluene adsorption ability, we used the O2- and toluene-TPD
techniques to determine desorption behaviors of oxygen and toluene from the
0.39Pt/meso-CeO2 and mPtnMnOx/meso-CeO2 samples. As can be seen from the O2-
o
TPD profiles (Fig. 12), the first peak at low temperature (< 200 C) originated from
desorption of the chemisorbed oxygen species as well as decomposition of surface PtO,
o
the second one between 200400 C was due to desorption of the oxygen from the
MnOx (Mn2+1yMn3+ Ox  Mn2+O), while the last one at high temperature (> 500 oC)
y
corresponded to desorption of the lattice oxygen from meso-CeO2 [36]. After
quantitative analysis on the peaks in O2-TPD profiles of the samples, one can see that
the amount (0.97 mmol/gcat) of the chemisorbed oxygen species as well as the surface
oxygen species due to PtO decomposition (the first peak) was the highest on
o
0.37Pt0.16MnOx/meso-CeO2, although the total oxygen desorption below 400 C
increased with the rise in MnOx doping (Table 3), which might be a result owing to the
increased Mn3+ concentration in the enhanced MnOx-doped sample. In general, toluene
adsorption ability is one of the factors influencing activity of a catalyst. As shown in
the toluene-TPD profiles (Fig. 13), each sample exhibited a toluene desorption peak
o
centered at 7583 C, assignable to desorption of the physically and/or weakly
chemisorbed toluene; and toluene desorption also increased with the rise in MnOx
doping (Table 3). This result suggests that doping of MnOx was beneficial for the
enhancement in toluene adsorption. The temperature (ca. 75 oC) of the desorption peak
over the 0.39Pt/meso-CeO2 or 0.37Pt0.16MnOx/meso-CeO2 sample was lower than
o
that (ca. 83 C) over the 0.37Pt0.78MnOx/meso-CeO2 or 0.37Pt1.21MnOx/meso-
CeO2 sample, indicating that adsorption of toluene on the latter samples was stronger
than that on the former samples. Since the best catalytic activity was achieved over

0.37Pt0.16MnOx/meso-CeO2 with the lowest toluene adsorption (33.8 mol/gcat),
whereas 0.37Pt1.21MnOx/meso-CeO2 with the highest toluene adsorption (72.8
mol/gcat) showed the lowest catalytic activity among the mPtnMnOx/meso-CeO2
samples, it can be reasonably deduced that toluene adsorption was not responsible for
catalytic activity of the mPtnMnOx/meso-CeO2 samples. In other words, toluene
combustion might undergo via the interaction of gas-phase toluene with the Oads species
to produce CO2 and H2O (i.e., the EleyRideal reaction mechanism might dominate the
combustion of toluene) or there were other factors (e.g., surface Oads and Mn2+ species
concentrations and low-temperature reducibility) influence toluene combustion. Based
on the above characterization results, we can realize that the enhanced adsorbed oxygen
and Mn2+ species concentration and improved low-temperature reducibility induced by
loading of Pt and MnOx were responsible for the good toluene combustion performance
of 0.37Pt0.16MnOx/meso-CeO2.
In order to identify the intermediate species during toluene combustion, we used the
in situ DRIFTS technique to monitor the possibly formed intermediates on
0.39Pt/meso-CeO2 and 0.37Pt0.16MnOx/meso-CeO2. The in situ DRIFTS spectra of
the two samples at different temperatures are shown in Fig. 14, and their absorption
band assignments can be seen from Table S3. The absorption bands at 1494 and 1602
1
1
cm originated from ring vibrations of toluene [37,38], the one at 29723072 cm was
carbonhydrogen stretching vibration of the aromatic rings in toluene, and the one at
1
2925 cm was the carbonhydrogen stretching vibration of the alkyl group in toluene
[39]. By comparing band intensity of the two samples, we can realize that a higher
amount of aromatic ring species and a lower amount of alkyl groups were formed over

the 0.37Pt0.16MnOx/meso-CeO2 sample at the same temperature. It can be reasonably
deduced that the alkyl group in toluene was first oxidized and the aromatic ring was
then oxidized. When the temperature increased, the two bands at 2348 (belonging to
1
CO2) and 3684 cm (belonging to H2O) appeared over both samples. This result
demonstrates that the aromatic ring was further oxidized to carbon dioxide and water
1
at high temperatures. In addition, benzyl alcohol (11801310 cm ), benzoic acid (1531
1
1
and 1452 cm ), and maleic anhydride (18111966 cm ) were also detected as
intermediates [40,41]. Therefore, toluene combustion might proceed according to the
following steps: toluene  benzyl alcohol and benzoic acid  maleic anhydride 
carbon dioxide and water.
4. Conclusions
The PtMny intermetallic NCs were synthesized via the PVP-assisted EG reduction
route. Loading of PtMny NCs on the surface of meso-CeO2 (derived by the KIT-6-
o
templating method) and after calcination at 500 C in air led to generation of the
mPtnMnOx/meso-CeO2 catalysts. All of the samples were cubic in crystal structure
and displayed an ordered mesoporous architecture with surface areas of 95108 m2/g.
Loading of Pt and MnOx resulted in improved low-temperature reducibility and
increased Oads and Mn2+ species concentration (which were beneficial for toluene
combustion), and doping of MnOx also enhanced toluene adsorption ability. Among all
of the samples, 0.37Pt0.16MnOx/meso-CeO2 performed the best for toluene
combustion (T50% = 162 oC and T90% = 171 oC at SV = 40,000 mL/(g h)). Moreover, the
0.37Pt0.16MnOx/meso-CeO2 sample possessed low activation energy and good

thermal stability and water-resistant performance. The competitive adsorption of CO2
and toluene molecules on the surface of 0.37Pt0.16MnOx/meso-CeO2 made CO2
introduction exert a negative effect on toluene combustion, but its induced partial
deactivation was reversible. Toluene combustion might undergo via a sequence of
toluene  benzyl alcohol and benzoic acid  maleic anhydride  carbon dioxide and
water.
Credit author statement
Mr. Wenbo Pei prepared the catalysts, evaluated the activities, and made characterization of XRD,
SEM, TEM, HAADFSTEM, BET, O2-TPD, and H2-TPR; Prof. Jiguang Deng and Lin Jing examined the
effect of treatment temperature on activity of the typical samples; Mr. Kunfeng Zhang, Zhiquan
Hou, Zhuo Han and Ali Rastegarpanah did the XPS and FT-IR characterization and examined the
effects of SV, H2O, CO2, and SO2; Prof. Hongxing Dai and Dr. Yuxi Liu was responsible for the whole
work.
Acknowledgements
This work was supported by the NSF of China (21677004, 21876006, and 21976009),
National Natural Science Committee of ChinaLiaoning Provincial People's
Government Joint Fund (U1908204), and Foundation on the Creative Research Team
Construction Promotion Project of Beijing Municipal Institutions (IDHT20190503),
and Natural Science Foundation of Beijing Municipal Commission of Education
(SQKM201710005004).
Appendix A. Supplementary Data
Supplementary data associated with this article can be found, in the online version, at

http://dx.doi.org/10.1016/j.apcata.2020.XX.XXX.
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Table 1. BET surface areas, pore volumes, average pore diameters, average crystallite
sizes (Dsupport), actual Pt or MnOx contents, and Mn/Pt molar ratios of the samples.
Table 2. Catalytic activities at SV = 40,000 mL/(g h), TOFs and toluene combustion
rates at 160 oC, apparent activation energies (Ea), and surface element compositions of
the samples.
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Fig. 1. (A) Wide- and (B) small-angle XRD patterns of (a) meso-CeO2, (b)
0.37Pt0.16MnOx/meso-CeO2,
(c)
0.36Pt0.78MnOx/meso-CeO2,
(d)
0.37Pt1.21MnOx/meso-CeO2, (e) 0.39Pt/meso-CeO2, and (f) 0.16MnOx/meso-CeO2.
Fig. 2. (A) Nitrogen adsorptiondesorption isotherms and (B) pore-size distributions of
(a) meso-CeO2, (b) 0.37Pt0.16MnOx/meso-CeO2, (c) 0.36Pt0.78MnOx/meso-CeO2,
(d) 0.37Pt1.21MnOx/meso-CeO2, (e) 0.39Pt/meso-CeO2, and (f) 0.16MnOx/meso-
CeO2.
Fig. 3. TEM images of (a, b) meso-CeO2, (c, d) 0.39Pt/meso-CeO2, (e, f)
0.37Pt0.16MnOx/meso-CeO2, and (g, h) 0.16MnOx/meso-CeO2.
Fig. 4. HADDFSTEM image and elemental mappings of the 0.37Pt0.16MnOx/meso-
CeO2 sample.
Fig. 5. (a, b) TEM images and (c) particle-size distribution of PtMn1.5 NPs.
Fig. 6. (A) Toluene conversion as a function of reaction temperature, (B) reaction rate
normalized per gram of catalyst, and (C) Arrhenius plots of (a) meso-CeO2, (b)

0.37Pt0.16MnOx/meso-CeO2,
(c)
0.36Pt0.78MnOx/meso-CeO2,
(d)
0.37Pt1.21MnOx/meso-CeO2, (e) 0.39 Pt/meso-CeO2, and (f) 0.16MnOx/meso-CeO2
at SV = 40,000 mL/(g h).
Fig. 7. Toluene conversion as a function of on-stream reaction time over
0.37Pt0.16MnOx/meso-CeO2 at 160 and 180oC at SV = 40,000 mL/(g h), respectively.
Fig. 8. Effects of (A) H2O and (B) CO2 on catalytic activity of 0.37Pt0.16MnOx/meso-
CeO2 at 180 oC and SV = 40,000 mL/(g h).
Fig. 9. (A) H2-TPR profiles and (B) initial H2 consumption rate as a function of inverse
temperature of the samples.
Fig. 10. (A) Ce 3d, (B) O 1s, (C) Pt 4f, and (D) Mn 2p3/2 XPS spectra of (a) meso-CeO2,
(b) 0.16MnOx/meso-CeO2, (c) 0.39Pt/meso-CeO2, (d) 0.37Pt0.16MnOx/meso-CeO2,
(e) 0.36Pt0.78MnOx/meso-CeO2, and (f) 0.37Pt1.21MnOx/meso-CeO2.
Fig. 11. O2-TPD profiles of (a) 0.39Pt/meso-CeO2 (b) 0.37Pt0.16MnOx/meso-CeO2,
(c) 0.36Pt0.78MnOx/meso-CeO2, and (d) 0.37Pt1..21MnOx/meso-CeO2.
Fig. 12. (A) Toluene reaction rate versus Oads/Olatt molar ratio, (B) TOFPt versus Oads/Olatt
molar ratio, (C) TOFMn versus Oads/Olatt molar ratio of the samples for toluene
combustion at 160 oC and SV = 40,000 mL/(g h).
Fig. 13. Toluene-TPD profiles of the samples.
Fig. 14. In situ DRIFTS spectra of (A, B) 0.39Pt/meso-CeO2 and (C, D)
0.37Pt0.16MnOx/meso-CeO2 during toluene combustion at different temperatures
(reaction conditions: 1000 ppm toluene + 20 vol% O2 + N2 (balance); SV = 40,000
mL/(g h)).