Revealing the unexpected promotion effect of diverse potassium precursors on α-MnO2 for the catalytic destruction of toluene

Article abstract of DOI:10.1039/c9cy02347j

The alkali metal potassium has the functions of structure promotion and electronic modulation in metal oxides. Herein, diverse potassium precursors (KOH, KNO₃, K₂SO₄, and KCl) were introduced to α-MnO₂ nanorods through a facile post-processing strategy. The presence of potassium species has a remarkable promotion effect on the catalytic performance of α-MnO₂. Amongst them, the KOH/MnO₂ sample has the highest activity and can destroy 90% toluene (1000 ppm) at just 226 °C with a reaction rate of 3.39 × 10^-4 mol g_cat^-1 s^-1, which is over 20 times higher than that of pure α-MnO₂. Different anions in the potassium precursors bring a distinct mutation in the α-MnO₂ structure, promote the formation of MnO₆-K-MnO₆ bridging bonds in α-MnO₂, and exhibit obvious diverse abilities for balancing charge transfer. KOH is identified as the most promising precursor for alkali metal modification, which significantly improves the distribution of K species over the α-MnO₂ surface and strengthens the content and activity of lattice oxygen. It is confirmed that the lattice oxygen plays a key role in the catalytic oxidation of toluene over α-MnO₂, which follows the Mars-van Krevelen mechanism. Positive hole defects (Mn³⁺) caused by KOH treatment play an important role in the diffusion of O and enhance the reducibility of manganese oxide. In addition, the enhanced specific surface area, pore volume, and surface acidity are also conducive for the catalytic oxidation of toluene.

Full text of DOI:10.1039/c9cy02347j

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Q. Zhu, Z. Jiang,
M. Ma, C. He, Y. Yu, X. Liu and R. K. AlBilali, Catal. Sci. Technol., 2020, DOI: 10.1039/C9CY02347J.
Volume
Number
21 January 2017
Pages 313-534
7
2
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Catalysis
Science &
Technology
rsc.li/catalysis
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
rsc.li/catalysis

Please do not adjust margins
Page 1 of 12
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C9CY02347J
ARTICLE
Revealing the unexpected promotion effect of diverse potassium-
precursors on α-MnO2 for toluene catalytic destruction
Qing Zhu,1,† Zeyu Jiang,1,† Mudi Ma,1,† Chi He,1,2,* Yanke Yu,3 Xiaohe Liu,1,* Reem Albilali,4
Received 00th January 20xx,
Accepted 00th January 20xx
Alkali metal potassium has function of structure promoter and electronic modulation for metal oxides. Herein, diverse
potassium precursors (KOH, KNO3, K2SO4 and KCl) were introduced to nanorods α-MnO2 through a facile post processing
strategy. The presence of potassium species has a remarkable promotion effect on catalytic performance of α-MnO2.
Amongst, KOH/MnO2 sample has the highest activity and can destruct 90% of toluene (1000 ppm) at just 226 °C with a
reaction rate of 3.39 × 10-4 mol gcat-1 s-1, over 20 times higher than that of pure α-MnO2. Different anions in potassium
precursors bring a distinct mutation in α-MnO2 structure, promotes the formation of MnO6-K-MnO6 bridging bonds in α-
MnO2 and exhibit obvious diverse ability for balancing charge transfer. KOH is identified as the most promising precursor
for alkali metal modification, which significantly improves the distribution of K species over α-MnO2 surface and
strengthens the content and activity of lattice oxygen. It is confirmed that the lattice oxygen plays a key role in toluene
catalytic oxidation over α-MnO2, which follows the Mars-van Krevelen mechanism. Positive hole defects (Mn3+) caused by
KOH treating play an important role in the diffusion of O and enhance the reducibility of manganese oxide. In addition, the
enhanced specific surface area, pore volume and surface acidity is also conductive to catalytic oxidation of toluene.
DOI: 10.1039/x0xx00000x
the industrial application of these catalysts have, however,
reduced their widespread utilization. In recent years, transition
Introduction
Volatile organic compounds (VOCs) produced from production
and use of industrial products are seen as main factors causing
air pollution.1 Reducing the emission of volatile organic
compounds is one of the biggest challenges facing the chemical
industry in the 21st century. Toluene, as a kind of typical
aromatic compounds, is widely used as industrial raw materials
and solvents.2 The catalytic oxidation of VOCs into CO2 and
water is a relevant approach and helpful to remove low
concentration of VOCs (< 0.5 vol%) in waste gases. The
rational designation of efficient catalysts is the key point of
toluene catalytic decomposition.3,
catalysts and supported noble metal are two types of generally
studied materials in VOC destruction.5, Attributing to the
metal oxide catalysts have been widely used in VOC catalytic
oxidation, because of their economic, stable and adjustable
properties.7, 8 Among them, manganese oxide is considered to
be a promising catalyst for toluene decomposition, which
possesses the multiple coordination numbers and oxidation
states. The diverse manganese oxides have been utilized in
VOC removal, such as propane, n-hexane, benzene, and
10
formaldehyde.9,
In our previous work, 100% methyl ethyl
ketone was converted to CO2 at 195 °C under a gas hourly
space velocity of 37,200 h-1 over highly exposed {101} facets
of Mn3O4.9 Wu and co-workers reported that α-MnO2 prepared
by redox-precipitation method could completely decompose o-
xylene to CO2 at 220 °C under a gas hourly space velocity of
8,000 h-1.10
It has been illustrated that different crystal structures,
morphologies or oxidation states of manganese oxides have a
significantly impact on their catalytic property.11-14 The
catalytic efficiency for toluene combustion over manganese
oxide catalysts is related to the oxygen mobility of catalysts,
decreasing with the order of Mn3O4 > Mn2O3 > MnO2.11 Zhang
et al. claimed that MnO2 catalysts exhibit outstanding catalytic
efficiency for formaldehyde oxidation. They illustrated that
tunnel structure and active lattice oxygen primarily lead to
prominent performance.12 Compared to β-MnO2, γ-MnO2 and
δ-MnO2, the α-MnO2 catalysts displays the best catalytic
stability and activity for propane oxidation based on different
crystal phases.13 Recently, Wang and co-workers verified that
rod-α-MnO2 catalyst performs better than the tube-α-MnO2,
4
Transition metal oxide
6
excellent catalytic performance, noble metal-based catalysts are
extensively investigated by researchers for toluene combustion.
As reported by Wang and co-workers, the Pd-Pt/SiO2 catalyst
can decompose 98% of toluene (1000 ppm) at just 160 °C.6 The
economic limitations and easily to be poisoned associated with
1Department of Environmental Science and Engineering, State Key Laboratory of
Multiphase Flow in Power Engineering, School of Energy and Power Engineering,
Xi'an Jiaotong University, Xi'an 710049, Shaanxi, P.R. China
2National Engineering Laboratory for VOCs Pollution Control Material & Technology,
University of Chinese Academy of Sciences, Beijing 101408, P.R. China
3Department of Chemical Engineering, Columbia University, New York 10027, United
States
4Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal
University, P.O. Box 1982, Dammam 31441, Saudi Arabia
*To whom correspondence should be addressed:
Tel./Fax: +86 29 8266 3857; E-mail: chi_he@xjtu.edu.cn (C. He)
Tel./Fax: +86 29 82668169; E-mail: xiaohe-liu@outlook.com (X.H. Liu)
† Electronic supplementary information (ESI) available: FE-SEM images; EDX-
mappings; Raman spectra.
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 2 of 12
ARTICLE
Journal Name
flower-Mn2O3 and wire-α-MnO2, achieving total toluene Then obtained mixture was mixed for another 2 hVcieownAtritniculeoOunslilnye
conversion at lower temperatures.14
before transformed to 250 mL Teflon-lined stainless steel
DOI: 10.1039/C9CY02347J
As a kind of widely studied manganese oxide, α-MnO2 cover autoclave, and heated at 120 °C for 4 h later. Then the products
corner-shared MnO6 octahedron and form a (2 × 2) cavity, as a were cleaned using deionized water for three times and
result it has been widely applied to soot combustion and NO desiccated overnight at 80 °C to obtained the final materials,
16
removal.15,
A plenty of studies have been conducted to respectively denoted as KNO3/MnO2, K2SO4/MnO2, KCl/MnO2,
improve the catalytic efficiency through a modification method, and KOH/MnO2.
18
such as acid treatment and alkali treatment.17,
Some 2.2. Catalyst characterizations
conclusions illustrate that alkali metal elements, such as K+ and X-ray diffraction (XRD) was performed over an X’Pert Pro
Na+ species, can enter the cavity of α-MnO2 and exert a powder diffraction system (PANalytical, Netherlands) under
positive effect of stabilizing structure and electronic modulation Cu-Kα radiating process in the 2θ of 10-80° with a scan rate of
on the catalyst.19 Moreover, the physical and chemical 10°·min-1, tube voltage of 40 kV, and current of 40 mA. N2
properties of alkali metal doped α-MnO2 depend on the nature sorption isotherms were conducted with an SSA-6000 gas
of alkali species inserted. Liu et al. concluded that K+ can sorption analyzer at 77 K for measuring the particular surface
promote the treatment of VOCs over manganese oxides greatly area (SBET) of synthesized samples. The samples underwent the
ascribed to its size fit better with the cavity of α-MnO2.20 As a pre-treatment at 200 °C for 2 h at the start of test. The
result, researchers tend to choose K+ to modify the α-MnO2.21, morphology was observed on a Hitachi S-4800 (Japan) field-
22
Hao and co-workers changed the K+ existent condition by emission scanning electron microscopy (FE-SEM) at 15 kV
introducing it in different methods, the results verified that K+ acceleration voltage. High resolution transmission electron
in the tunnels is able to stably coordinated with eight nearby O microscopy (HR-TEM) was detected on the Tecnai G2F30 (FEI,
sp3 atoms.21 Specifically, Hou et al. confirmed that the America) at 300 kV acceleration voltage. The Fourier transform
introducing of K+ to α-MnO2 enhances the ratio of Mn3+/Mn4+ infrared spectra (FTIR) were detected on a Bruker Tensor 37
and demonstrated that oxygen defects play a significant role on spectrometer (Germany). Thermo-gravimetric analysis was
the catalytic activity of α-MnO2.22 Several conclusions have conducted on an HCT-2TGA/DSC-1 analyzer (Beijing
been made considering the promotion effect of potassium; Permanent, China) heating at a rate of 10°C·min-1 in a N2
however, the in-depth analysis of the positive effect of diverse stream, from 30 to 800 °C. Laser Raman spectra were obtained
potassium precursors on α-MnO2 have been somewhat on a JobinYvon S.A.S. LabRAM Aramis spectrometer (Horiba,
overlooked, which is crucial for its further application in VOC Japan) at room temperature, using the 532 nm line of Ar+ ion
elimination. Herein, we adopted a post processing method to laser as the source for exciting process. The K content of
induce K species from different potassium precursors, including catalyst was detected by inductively coupled plasma/optical
KOH, KNO3, K2SO4 and KCl, into α-MnO2 nanorods, and emission spectroscopy (ICP-OES) with
a Perkin-Elmer
catalytic performance of all prepared samples on toluene OPTIMA 7000 DV. X-ray photoelectron spectroscopy (XPS)
decomposition was extensively determined. Furthermore, the analyses of samples were conducted with an AXIS ULtrabld
toluene combustion mechanism over prepared catalysts was instrument (Kratos, UK) under Mg Kα radiation (hν = 1253.6
revealed. This work can provide a fundamental foundation for eV). All binding energies (BEs) were corrected with C 1s of
the designation of considerable K-modified oxide catalysts and 284.8ev, and the peaks were fitted with CASA XPS software.
expand their application in VOC decomposition.
The element distribution was determined by XRF (X-ray
Fluorescence Spectrometer) test.
H2-TPR experiments were performed on a PCA-1200
(Builder, China). 100 mg of catalyst was pre-treatment at 200
°C (10 °C·min-1) for 30 min under N2 stream (30 mL·min-1).
Then, with a mixture of 5 vol. % H2/Ar (30 mL·min-1), the
samples were heated from ambient temperature to 800 °C at 10
°C·min-1. Hydrogen consumptions were measured online using
Experimental
2.1. Catalyst preparation
The nanorod-like α-MnO2 material was prepared by a
hydrothermal method. Typically, 3.375 g MnSO4·H2O, 4.575 g
(NH4)2S2O8, and 5.278 g (NH4)2SO4 were dissolved in 80 mL
of purified water and underwent the stirring process for 2h at
ambient temperature. Subsequently, the mixture solution was
placed in 250 mL Teflon-lined stainless steel autoclave and
underwent the heating process for 24 h at 120 °C. Through
filtration, the product was harvested and cleaned using
deionized water for several times. Finally, the obtained sample
was desiccated overnight at 80 °C and then annealed in air at
300 °C for 2 h to get nanorod-like α-MnO2.
Potassium from common precursors (KNO3, K2SO4, KCl,
and KOH) was respectively introduced into the prepared α-
MnO2 via a simple hydrothermal process. To be specific, 8.694
g of α-MnO2 material was mixed with 80 mL of potassium-
containing solution (1 M) and ultrasonic dispersed for 20 min.
a
thermal conductivity detector (TCD). Temperature
programmed desorption of NH3 (NH3-TPD) was performed on
the same device. In particular, 75 mg of catalyst was pre-
treatment in a pure N2 flow (40 mL·min-1) at 200 °C for 30 min
and subsequently cooled to 100 °C before treating with 2.01 vol.
% NH3/N2 (40 mL·min-1) for 30 min. The catalyst saturated
with NH3 was treated with a pure N2 flow (40 mL·min-1) for 1 h
at 100 °C. NH3-TPD profiles were recorded online from 100 to
800 °C with a heating rate of 10 °C·min-1.
Initial H2 consumption rate (mmol g-1) of prepared materials
is calculated based on the fitting peak area of H2 consumption
from H2-TPR profiles, as follows:
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 12
Catalysis Science & Technology
Scat nCuO 1000
SCuO mcat
Ea
View Article Online
DOI: 10.1039/C9CY02347J
(1)
(5)
H2 consumption rate =
lnr
 
 C
toluene
RT
where Ea is the activation energy (kJ mol-1); R is the gas
constant (kJ mol-1 K-1); and T is the reaction temperature (K).
Ea can be obtained from the slope of the linear curve between
lnr and 1/T.
where Scat is the fitting peak area of hydrogen consumption for
prepared catalysts; SCuO represents the fitting peak area of
hydrogen consumption for CuO standard sample; nCuO is the
amount of CuO standard sample (mol); mcat represents the
weight of catalyst (g).
2.3. In situ DRIFTS study
Results and discussion
In situ DRIFTS of toluene combustion was carried out on a
Bruker Tensor 37 infrared spectrometer with a mercury
cadmium telluride (MCT) detector which is cooled by liquid
nitrogen. The sample was pre-treated at 250 °C for 1 h in a flow
of N2 (60 mL/min) to remove the surface impurities and then
cooled down to room temperature to collect the background
spectrum. Later, the reaction spectra were collected at 180-260
°C, in the presence of 1000 ppm toluene and 20% O2/N2. All
spectra were collected in the range of 4000-1000 cm-1, with 100
scans, at an instrument resolution of 4 cm-1.
3.1. Structural and textual property
X-ray diffraction patterns of α-MnO2 and K-contained MnO2
oxides were displayed in Fig. 1A. The diffraction peaks from
all samples exhibit the typical tetragonal cryptomelane-type
MnO2 crystal structure (PDF #44-0141), indicating that
crystallizations of prepared α-MnO2 samples maintained well
during process of potassium incorporation.23 However, the peak
intensity of KOH/MnO2 sample slightly decreased compared
with the pure α-MnO2 samples, and the worse crystallization
may have some reason with alkaline condition of KOH
solution. Furthermore, the crystal sizes (CS) of prepared
catalysts were valued using the Scherrer equation,8 based on the
half-peak width of XRD (211) diffraction peak, showing the
results in Table 1. As calculated, the grain sizes decreased in
most K-contained samples except KNO3/MnO2. That is 14, 11,
9, 9 and 21 nm corresponding to MnO2, KOH/MnO2,
KCl/MnO2, K2SO4/MnO2 and KNO3/MnO2, respectively. It is
reported that the smaller crystal size is conducive to promote
the exposure of active phase and enhance the active particular
surface area.8
FE-SEM and HR-TEM were further adopted to discuss the
influences of the post-treatment process on the morphology of
α-MnO2. As displayed in Figs. 1C-L and S3, all synthesized
samples present a typical nanorod-like morphology. The lattice
spacing of exposed (200) plane was counted (as shown in Fig.
1F-G), which has no significance variation between K-
contained materials (KOH/MnO2 (5.06 Ǻ), KNO3/MnO2 (5.02
Ǻ), K2SO4/MnO2 (5.13 Ǻ) and KCl/MnO2 (5.04 Ǻ)) and MnO2
(5.04 Ǻ).24 Furthermore, the EDX-mapping images in Figs. 1H-
L indicated the uniform distribution of K in KOH/MnO2
material after the hydrothermal treatment.
2.4. Catalytic performance
The catalytic performance of toluene was performed in a
continuous flow fixed-bed steel reactor composed by a steel
tube (I.D. = 10 mm) at air pressure. The temperature of reactor
was regulated with a K-type thermocouple embedded into the
catalyst bed. The concentrations of toluene and reaction
products were detected online with an online gas
chromatograph (GC9890, Linghua, China) equipped with a
flame ionization detector (FID) and HT-Wax column in 30 m ×
0.32 mm (ID) × 0.5 μm. In each test, 0.3 g of sample was
placed at the center of reactor and a gas mixture (20 vol. %
O2/N2) including around 1000 ppm of toluene was maintained
at 250 mL/min (gas hourly space velocity (GHSV) of 15,000 h-
1). Stability test was performed at 230 °C for 30 h after the
stabilization of the reaction.
The conversion of toluene (Xtoluene) was calculated as
Equation (2),
[Cin ]-[Cout
[Cin ]
]
(2)
Xtoluene (%) 
100%
where [Cin] and [Cout] are toluene concentrations in the outlet
and inlet gases, respectively.
The CO yield (
) was obtained by Equation (3),
Y
Fig. 1B shows FTIR spectra from prepared materials. The
transmission peaks centered at around 3420 and 1632 cm-1 can
be assigned to stretching vibrations of hydroxyl group and
surface absorbed water, respectively. Meanwhile, Mn-O lattice
vibrations in all prepared samples give rise to the bands in the
range of 400 and 800 cm-1(468, 523 and 717 cm-1). The band at
1130 cm-1 represents Mn3+-O vibration (suggested the presence
of Mn3+), and disappeared after modified with K+.20
2
CO2
[CO2 ]out
7[Cin ] Xtoluene
(3)
YCO (%) 
100%
2
where [CO2]out denotes the concentration of CO2 in the outlet
gas.
The reaction rate (rtoluene, mol·gcat-1·s-1) was obtained by
Equation (4),
The N2 sorption and pore size distribution properties of
prepared catalysts were also analysed, and results are shown in
Figure 2. All materials exhibit type IV adsorption-desorption
isotherms with a type H3 hysteresis loop in relative pressure
(P/P0) between 0.8-1.0, indicating coexistence of mesopores
and macropores.25 As listed in Fig. 2B, samples exhibit
dispersive pore distribution. A different Hysteresis loop can be
detected over KOH/MnO2 catalyst. The changes may come
Xtoluene Vtoluene
(4)
r

toluene
Wcat
where Wcat denotes the catalyst weight (g); Vtoluene is the toluene
gas flow rate (mol·s-1).
When the toluene conversion is less than 20%, the empirical
kinetic expression of toluene oxidation rate equation can be
described as Equation (5),
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3

Please do not adjust margins
Catalysis Science & Technology
Page 4 of 12
ARTICLE
Journal Name
from the structural failure as discussed in the XRD results (Fig. with the decreased results of the crystal size as mentioned
View Article Online
DOI: 10.1039/C9CY02347J
2A). The surface area (SBET), listed in Table 1, show an above (Table 1). Meanwhile, the total pore volume (VP) also
increasing trend in the K-modified α-MnO2 catalysts (74.5-85.6 increased,
m2 g-1), compared to the pure α-MnO2 (66.5 m2 g-1), consistent
Table 1 Textural properties of synthesized catalysts.
h
Eai
a
b
c
SBET
VP
DP
Csd
Olatt
K content (wt.%)
T90
Samples
(m2/g)
66.5
84.8
85.6
75.0
83.4
(cm3/g)
0.32
(nm)
19.5
34.2
23.8
28.7
22.0
(nm)
ratiog
XPSe
/
ICPf
(°C)
321
(kJ mol-1)
79.5
α-MnO2
14
11
9
/
0.59
1.13
1.03
0.88
0.82
226
243
236
319
62.3
68.7
79.1
79.8
KOH/MnO2
KCl/MnO2
K2SO4/MnO2
0.89
4.00
1.27
2.21
1.02
10.4
5.88
4.77
3.96
0.52
0.59
9
KNO3/MnO2
0.46
21
a
b
c
Note: Specific surface area obtained at P/P0 = 0.05-0.30; Total pore volume estimated at P/P0 = 0.99; BJH pore diameter
d
e
calculated from the desorption branch; Crystal size obtained from XRD patterns calculated by the Debye-Scherrer formula;
content over prepared samples determined by XPS; K content detected by ICP-OES; The ratio of Olatt/Oads over materials
estimated by O1s XPS; h The temperature at which 90% toluene was destructed; i Apparent activation energy.
K
f
g
A
B
C
D
H
100 nm
200 nm
G
E
I
F
d200=5.06 Å
d200=5.04 Å
FFT
FFT
10 nm
10 nm
10 nm
200 nm
J
K
L
Overlay
Mn
K
O
Fig. 1 (A) X-ray diffraction patterns of prepared materials; (B) FTIR spectra of all prepared materials; (C) FE-SEM images of
KOH/MnO2; (D,F) HR-TEM images of α-MnO2; (E,G) HR-TEM images of KOH/MnO2; (H-L) EDS-mapping images of
KOH/MnO2.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 12
Catalysis Science & Technology
A
B
As displayed in Fig. 3A, there is no parent KVi2ewpArptieclaekOsnliinne
DOI: 10.1039/C9CY02347J
pure α-MnO2 sample. For K-contained MnO2, binding energies
of K 2p3/2 and K 2p1/2 are centered around 291.7-292.1 and
294.5-294.9 eV, respectively, indicating the successful insertion
of K+. Specifically, the K+ species could enter the tunnel of
MnO2 and form a strong interaction of K-O-Mn. As reported by
Chen and coworkers, a d-sp orbital hybridization existed in K-
contained samples leads to different electronic states of K.27
Fitting analysis of these spectra reveals that different precursor
K sources have different bonding abilities with oxides and
various capabilities to provide electrons. KOH/MnO2 shows a
much higher binding energy, indicating it is a superior electron
source for potassium.
Fig. 2 (A) N2 adsorption-desorption isotherms and (B) pore size
distribution of prepared materials.
from 0.32 cm3 g-1 of pure α-MnO2 to 0.52-0.89 cm3 g-1 of K+-
modified materials (KOH/MnO2 possesses the highest VP of
0.89 cm3 g-1). Generally, promoted specific area and pore
volume of oxide catalysts are conducive to the catalytic
oxidation of toluene.22
The size of Mn 3s multiple splitting (ΔE) can be applied to
acquire average oxidation state (AOS) of Mn, which follows
equation (6)28:
(6)
AOS  8.95-1.13E(eV)
The contents of potassium element in all materials were
determined by ICP-OES and XPS (Table 1), showing that K
contents in prepared materials are significantly influenced by
potassium precursors. The KOH/MnO2 catalyst possesses the
highest K contents of 10.4 wt. % (bulk) and 4.00 wt. % (surf),
and the loading efficiency changes with the order of
KOH/MnO2 > K2SO4/MnO2 > KCl/MnO2 > KNO3/MnO2. The
anions in different potassium precursors affect the content and
distribution of K in MnO2, in consistent with previous report.26
In order to eliminate the influence of anions from different
potassium precursors on the catalytic performance, the presence
of anions was further determined by XRF (X-ray fluorescence
spectrometer) experiment and the results were shown in Table
S1. Negligible contents of anions such as Cl-1 were determined.
We deduced that anions were nearly washed off completely
during the preparation process.
The multiple splitting of α-MnO2 is 4.75 eV, and the AOS is
3.6. The Mn 3s ΔE becomes lower (4.65 eV for KOH/MnO2)
when K is incorporated in α-MnO2. The results show that the
AOS of Mn tends to increase. As expected, K cations would
have the function of electronic modulation and the insertion of
K species could reduce the Mn4+ content, leading to an
increasing atomic ratio of Mn3+/Mn4+.22 The unexpected
increase of AOS could be explained by the leaching of Mn3+
during the post-modification process29 as confirmed by the
FTIR results.20 Meanwhile, it is suggested that the formation of
positive hole defects (Mn3+) can do favor to the diffusion of O,
resulting enhanced reducibility of manganese oxides. Chen et
al. claimed that the extrinsic defects caused by doping can
enhanced the dissolution rate extensively.30
A
B
A
A
3.2. Surface chemical properties and enhanced active oxygen
species
The chemical states of K, Mn and O on the surface of the
material were studied by XPS. The corresponding spectrums
are shown in Figs. 3 and 4.
A
C
B
A
A
A
C
D
A
D
A
A
Fig. 4 (A) O 1s XPS spectra of all prepared materials; (B) The
relative ratio of distinct O species over all catalysts determined
by the O 1s XPS spectra; (C,D) Laser Raman spectra of
prepared materials.
The O 1s spectrum can be divided into three sub-peaks
centred at BEs of 529.7, 531.9 and 533.5 eV, assigned to the
lattice oxygen, adsorbed oxygen and surface adsorbed water,
respectively (Fig. 4A).9 However, in K-contained samples, the
Fig. 3 (A) K 2p and (B) Mn 3s XPS spectra of prepared
materials; (C,D) Laser Raman spectra of prepared materials.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5

Please do not adjust margins
Catalysis Science & Technology
Page 6 of 12
ARTICLE
Journal Name
deconvoluted O 1s peaks tend to shift to lower binding energy. 700 °C and the weight loss declines in the order of KCl/MnO2
View Article Online
As reported, the introduced K+ could interact with O atoms and (11.2%) > K2SO4/MnO2 (10.8%) > KOH/MnO2 (10.5%) >
DOI: 10.1039/C9CY02347J
form the K-O-Mn bond; as a result, the O species gain electrons KNO3/MnO2 (9.8%) > MnO2 (7.6%).
and the binding energy of Olatt shift to a lower value.27
A
B
Meanwhile, different K-contained samples exhibited different
shifts which reflected various intensity of K-O-Mn bond.
In general, oxygen species play a key role in toluene
28
combustion,26,
and the atomic ratio of Olatt/Oads of all
prepared samples was counted as shown in Table 1 and Fig. 4B.
It was enhanced remarkably after the insertion of potassium,
following the sequences of KOH/MnO2 (1.13) > KCl/MnO2
(1.03) > K2SO4/MnO2 (0.88) > KNO3/MnO2 (0.82) > MnO2
(0.55).
Fig. 5 (A) H2-TPR profiles and (B) initial H2 consumption rate
The structural characteristics of synthesized materials were
further investigated by Raman spectroscopy, as shown in Figs.
3C, D and Fig. S3. The Raman spectrum of α-MnO2 features
four contributions at 347, 580, and 640 cm-1, in good agreement
with previous vibrational studies on α-MnO2. In detail, band at
347 cm-1 is attributed to the Mn-O bending and bands at 580
and 640 cm-1 corresponds to the symmetric Mn-O stretching
vibrations of manganese octahedral.31 Interestingly, the K-
contained samples exhibit different degree of blue shifts at
around 640 cm-1, indicating that the Mn-O was strengthened,
consistent with the AOS results given above.
The XPS and Raman results are further confirmed by H2-
TPR. Reducibility of all materials was analyzed by H2-TPR, as
shown in Fig. 5 and Table S2. The reduction peaks centered at
397 and 573 °C can be found for pure α-MnO2 sample,
ascribing to the successive reduction processes of MnO2 to
Mn3O4 and Mn3O4 to MnO, respectively.32 However, these
reduction peaks shift to lower temperatures for all K-contained
materials, in which the MnO2 species may reduce to MnO in a
narrow temperature range with Mn2O3 and Mn3O4 as
intermediates. It shows that the formation of MnO6-K-MnO6
bridging bonds makes the energy of Mn-O bond relatively
uniform.33 The temperature of initial reduction peak increases
in the order of KOH/MnO2 (346 °C) < KNO3/MnO2 (373 °C) <
KCl/MnO2 (385 °C) < K2SO4/MnO2 (401 °C), indicating the
best reducibility of KOH/MnO2 material. To further comparing
the reducibility, the initial H2 consumption rates of all samples
were calculated, as presented in Fig. 5B.7 Obviously,
KOH/MnO2 (2.55 × 10-7mol g-1 s-1) exhibits a much better
reducibility at 240 °C compared with the prepared KNO3/MnO2
(0.84 × 10-7 mol g-1 s-1), KCl/MnO2 (0.42 × 10-7 mol g-1 s-1),
K2SO4/MnO2 (0.24 × 10-7 mol g-1 s-1), and MnO2 (0.19 × 10-7
mol g-1 s-1) catalysts.
as a function of inverse temperature of all materials.
A
B
C
E
D
Fig. 6 Thermos-gravimetry (TG) analysis of (A) MnO2, (B)
KOH/MnO2, (C) K2SO4/MnO2, (D) KCl/MnO2 and (E)
KNO3/MnO2 materials.
Temperature-programmed desorption of NH3 was applied to
investigate the density and strength of acidic site in the
synthesized materials. The NH3-TPD profiles of materials
showed in Fig. S5 can be deconvolved into several peaks
(Table S4), corresponding to the physically-adsorbed NH3 (<
220 °C), ammonia-adsorbed on Brønsted acid sites (240-380
°C) and ammonia-adsorbed on Lewis acid sites (> 400 °C).35
Accordingly, the quantitative assessment of different acid
species was evaluated by counting the peak area of NH3-TPD
(Table S4). Obviously, the modification of potassium decreases
the Brønsted acid sites content except KOH-MnO2. The
abundant Brønsted acid sites promote VOC molecules
adsorption and accelerate the regeneration of surface hydroxyl,
which is benefit for the decomposition of intermediates during
VOC oxidation.36
Fig. 6 and Table S3 illustrate the TGA-DTG results of
prepared materials. The initial weight loss under 200 °C is
attributed to desorption of physically and chemically adsorbed
water. The following losses between 200 and 700 °C represent
the release of oxygen. The peak between 200-300 °C is
attributed to desorption of surface hydroxyl radial. The peaks
between 300-700 °C correlate to the evolution of oxygen in
lattice structure. As reported previously, the weight loss in 200-
700 °C correlates to the activity of chemical oxygen, being
critical to oxidation reactions.34 As displayed, K-contained
MnO2 sample shows lower deoxidization temperatures in 200-
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 12
Catalysis Science & Technology
A
B
A
View Article Online
DOI: 10.1039/C9CY02347J
B
D
C
C
D
Fig. 8 (A) Catalytic stability, (B) three-cyclic stability, and effect of
Fig. 7 (A)The light-off curves of toluene combustion over prepared (C) CO2 and (D) H2O on the toluene combustion over KOH/MnO2.
materials; (B) The reaction rates of toluene oxidation over catalysts;
(C) CO2 selectivity of different samples for toluene decomposition;
(D) Apparent activation energy (Ea) of different catalysts for toluene
oxidation.
CO2 yield of active samples was further studied, as displayed
in Fig. 7C. It is displayed that 100% of CO2 yield can be
realized below 345 °C, over K-modified α-MnO2 catalysts; in
comparison, only 55% of CO2 selectivity can be reached over
pure α-MnO2. The temperature for 100% CO2 yield over
3.3. Catalytic performance
prepared materials increases with order of KOH/MnO2 (258
All prepared catalysts were adopted in catalytic destruction of
°C) < K2SO4/MnO2 (268 °C) < KCl/MnO2 (308 °C) <
toluene, as displayed in Fig. 7. Obviously, the introducing of
KNO3/MnO2 (345 °C). These results further demonstrate that
potassium species significantly improves the catalytic
doping K is a promising method for improving the catalytic
performance of α-MnO2 in toluene decomposition (Fig. 7A).
Particularly, KOH/MnO2 sample exhibits the best catalytic
performance of α-MnO2, and KOH is confirmed to be the most
suitable precursor. The introducing of K species balance the
efficiency, over which 90% of toluene (1000 ppm) can be
charge transfer and weaken the Mn-O bond in MnO2 materials,
decomposed at just 226 °C, much lower than of K2SO4/MnO2
which significantly enhance the activation of active oxygen
(236 °C), KCl/MnO2 (243 °C), KNO3/MnO2 (319 °C), and
species; therefore, the depth of mineralization of intermediates
MnO2 (321 °C) catalysts. In addition, the reaction rate was
subsequently assessed for comparing the low-temperature
can be strengthened.38
The stability is a critical parameter for catalysts since their
activity of synthesized catalysts (Fig. 7B).5 The calculated
susceptibility to experience thermal deactivation usually
reaction rates for KOH/MnO2 at 205 and 226 °C can
accounts for their limited applications in industrial fields. The
respectively reach at 2.25 × 10-4 and 4.98 × 10-4 mol gcat-1 s-1,
cyclic performance and stability in different atmospheres of the
remarkably higher than that of KCl/MnO2 (1.28 × 10-4 and 3.39
most active KOH/MnO2 material were tested.39 As shown in
× 10-4 mol gcat-1 s-1), K2SO4/MnO2 (0.33 × 10-4 and 2.70 × 10-4
Fig. 8A, a small loss in catalytic activity can be observed
mol gcat-1 s-1), MnO2 (0.11 × 10-4 and 0.20 × 10-4 mol gcat-1 s-1)
during the 30 h of stability test, indicating that KOH/MnO2
and KNO3/MnO2 (0.09 × 10-4 and 0.12 × 10-4 mol gcat-1 s-1). By
material exhibits excellent stability in toluene oxidation.
calculating the apparent activation energy (Ea), the low
Besides, the catalyst shows good toluene combustion
temperature activity of the synthesized samples can be
performance in three repeated runs (Fig. 8B), indicating its
compared as catalyst with a lower Ea value can make toluene
more easily oxidized.9 As shown in Fig. 7D and Table 1, Ea
superior cyclic stability. 3 vol.% and 5 vol.% of CO2 was added
to the toluene mixture respectively, and both shows negligible
values for toluene oxidation increase in the following order of
effect on toluene conversion over KOH/MnO2 (Fig. 8C). The
KOH/MnO2 (62.3 kJ mol−1) < KCl/MnO2 (68.7 kJ mol−1) <
addition of H2O (5 vol.%) has a small inhibition effects on the
KNO3/MnO2 (79.1 kJ mol−1) < MnO2 (79.5 kJ mol−1) <
conversion of toluene (decreased about 4%) due to the
K2SO4/MnO2 (79.8 kJ mol−1). The results is consistent with the
competition of water and oxygen adsorption on the catalyst
expectation that the values of activation energy (Ea) of the total
surface. When the addition of H2O was stopped, it takes a
oxidation of toluene ranging from 42-145 kJ mol-1.37 Above
period time to partially recover the activity of the catalyst,
results indicate that the introduction of potassium species
suggesting that H2O was adsorbed strongly to active sites (Fig.
positively affects the performance of α-MnO2 in toluene
oxidation and the promotion effect is influenced by potassium
8D).8
Table 2 shows the comparison between the reported catalytic
precursors.
results in the literature and the catalytic results obtained in this
work. Obviously, KOH/MnO2 prepared in this work shows
excellent performance in toluene catalytic combustion among
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7

Please do not adjust margins
Catalysis Science & Technology
Page 8 of 12
ARTICLE
Journal Name
different reported manganese oxide catalysts. It is also more Furthermore, the promoter effect of K is comparable to
View Article Online
DOI: 10.1039/C9CY02347J
active than most of single phase transition metal oxide catalysts. synergism of various transition metals.
Table 2 Toluene oxidation over different catalysts.
Samples
Reaction conditions
T90 (°C)
Ea (kJ mol-1)
References
This work
KOH/MnO2
1000 ppm, 50000 mL g-1 h-1
226
62.3
MnO2
1000 ppm, 50000 mL g-1 h-1
1000 ppm, 100 cm3 min-1
1000 ppm, 100 cm3/ min-1
1000 ppm, 100 cm3 min-1
1000 ppm, 100 cm3 min-1
1000 ppm, 60000 mL g-1 h-1
1000 ppm, 20000 mL g-1 h-1
500 ppm, 19200 mL g-1 h-1
600 ppm, 50000 h-1
321
375
295
270
250
320
/
79.5
This work
[40]
MnO2
/
Mn2O3
/
[40]
Mn3O4
/
[40]
0.5 wt% K/Mn3O4
LaMnO3
Fe2O3
/
[40]
123.3
195.4
/
[37]
[41]
sc-NiO
300
300
297
235
224
256
210
[42]
CuO
120 ± 16
/
[43]
Co3O4
1000ppm, 60 000 mL g-1 h-1
1000 ppm, 60000 mL g-1 h-1
1000 ppm, 60000 mL g-1 h-1
1000 ppm, 60000 mL g-1 h-1
500 ppm , 22500 mL g-1 h-1
[44]
Mn0.3Zr0.7O2
Mn3-xFexO4
Mn3-xCuxO4
CoMn2O4
87.2
70
[45]
[46]
127
35.5
[46]
[47]
3.4. Correlation between structure and catalytic performance
enlarged, the grain size decreased, and the specific surface area
Aimed at revealing the effect of potassium precursors on the increased. The abundant specific surface area is helpful for the
catalytic activity of α-MnO2, all prepared catalysts were active reaction, and larger pore volume facilitates mass transfer
adopted in catalytic destruction of toluene. Results show that during toluene oxidation.26
the presence of potassium species remarkably enhanced
As determined by the ICP and XPS test, the anions of diverse
catalytic performance of α-MnO2, and KOH/MnO2 possessed potassium precursors make a difference on the loading content
an excellent catalytic performance for toluene oxidation at just and distribution of potassium in the matrix.48 By the way the
226 °C. It is well-accepted that toluene catalytic oxidation over content of potassium exhibits a positive relationship with the
manganese oxide generally follows the Mars-van Krevelen catalytic performance in toluene oxidation. Specifically, the K+
(MvK) mechanism, which is further confirmed by in situ species entered the tunnel of MnO2 and formed the MnO6-K-
DRIFTS results in this work. That is, toluene molecules are first MnO6 bridging bonds.31 As reported by Chen and coworkers, a
adsorbed on the catalyst surface and then oxidized by the lattice d-sp orbital hybridization existed in K-contained samples leads
oxygen, then lattice oxygen is supplemented by active oxygen to the different electronic states of K.27 The K 2p fitting
and the reduced oxide catalyst returns to its original state.36
analysis of these spectra reveals that different precursor K
During this process, parameters (e.g., the specific surface sources have different bonding abilities with oxides and various
area, chemical states of elements, mobility and activity of capabilities to provide electrons. KOH/MnO2 shows a much
oxygen species) of catalysts are correlated to its catalytic higher binding energy, indicating it is a superior electron source
performance. In this work, hydrothermal treatment has for potassium.
successfully introduced the K+ into the tunnel of α-MnO2, as a
As mentioned before, both surface oxygen and lattice oxygen
result, the lattice spacing expanded slightly, pore volume are involved in this process and may have impacts on the
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 12
Catalysis Science & Technology
catalytic activity. The XPS test of the used KOH/MnO2 catalyst,
View Article Online
DOI: 10.1039/C9CY02347J
which has undergone a 30 h stability test, was applied to further
determine the roles of oxygen species. The O 1s spectra (Figure
4C) can be decomposed into three sub-peaks centered at BEs of
529.7, 531.9 and 533.5 eV roughly, assigned to the lattice
oxygen, adsorb oxygen, and surface adsorbed water,
respectively.23 Results (Figure 4D) show that the content of
lattice oxygen decreased extensively while that of adsorbed
oxygen has no obvious variation, indicating that the former may
play a more important role in the oxidation of toluene.49 The
ratios of the two oxygen species (Olatt/Oads) in all samples were
also counted, following the sequences of KOH/MnO2 (1.13) >
KCl/MnO2 (1.03) > K2SO4/MnO2 (0.88) > KNO3/MnO2 (0.82)
> MnO2 (0.55). Higher lattice oxygen content was acquired
after the doping of K+, and the distribution is consistent with
the catalytic performance. KOH/MnO2 sample exhibits the best
catalytic efficiency, over which 90% of toluene (1000 ppm) can
be decomposed at just 226 °C, much lower than that of
K2SO4/MnO2 (236 °C), KCl/MnO2 (243 °C), KNO3/MnO2 (319
°C), and MnO2 (321°C) catalysts. It is proposed that the lattice
oxygen in manganese oxide plays a key role in toluene
combustion.
The insertion of K+ into the empty sites in the tunnels also
enhanced the reducibility of α-MnO2. Two reduction peaks of
pure α-MnO2 are attributed to the continuous reduction process
from MnO2 to Mn3O4 and from Mn3O4 to MnO.32 However,
they shift to lower temperatures and integrated together for all
K-contained materials. It shows that the formation of MnO6-K-
MnO6 bridge bond makes the energy of Mn-O bond relatively
uniform.33 As inferred above, the unexpected increase of AOS
can be explained by leaching of Mn3+ during the post-
modification process29 as confirmed by the FTIR results.20
Hence, we believe that hydrothermal impregnation leads to the
formation of positive pore defects (Mn3+), which will play an
important role in the diffusion of O and enhance the reducibility
of manganese oxides.29
Fig. 9 In situ DRIFTS spectra of active catalysts for toluene
oxidation as a function of reaction temperature in flowing
toluene + N2 + O2: (A) MnO2 and (B) KOH/MnO2.
According to the in situ DRIFTS results, it can be inferred
that the toluene oxidation over KOH/MnO2 and α-MnO2
materials obeys a similar mechanism, as proposed in Scheme 1.
Toluene molecule firstly adsorbs on the surface of catalysts,
and transforms into benzylic, aldehydic, benzoate and maleic
anhydride species consecutively. These produced intermediates
are oxidized into harmless H2O and CO2 eventually.51 In this
dynamic oxidation process, the lattice oxygen is the active
species and adsorption site for the oxidation of toluene. The
redox cycle of Mn4+ ↔ Mn3+ takes place easily in the course of
surface lattice oxygen removal and replenishment.28 The
benzoate specie is included as the primary intermediate in
toluene combustion over KOH/MnO2 material attributing to its
intense peaks compared with that of pure α-MnO2. Meanwhile,
the existence of alkoxide species represents the adsorption of
toluene is insignificant and the maleic anhydride species could
be transform into CO2 easily in low temperature range over
KOH/MnO2, further indicating its considerable activity and
green ways for toluene elimination.
3.5. Mechanism study
The catalytic behavior of toluene combustion over KOH/MnO2
and α-MnO2 materials was explored by in-situ DRIFTS.
Obvious peaks at 1130, 1311, 1399, 1601, 3029, 3611 and 3742
cm-1 can be observed over α-MnO2 sample (Fig. 9A), while the
peaks at 1130, 1298, 1399, 1537, 1601, 3029 and 3742 cm-1 can
be detected over KOH/MnO2 material (Fig. 9B). In detail, the
negative peaks around 3742 cm-1 is ascribed to the surface
hydroxyl groups, representing the acid sites over the α-MnO2
surface. Furthermore, the broad positive peaks detected at
3060-3550 cm-1 corresponds to desorption of H2O mainly
coming from toluene combustion, which is gradually increased
as the increasing of reaction temperature. The enhanced band at
1601 cm-1 belongs to the vibration of benzaldehyde species.
Scheme 1 Proposed mechanism of toluene catalytic oxidation
over KOH/MnO2 catalyst.
Conclusions
The detected peaks around 1298-1311 and 1399-1537 cm-1 on In summary, the introducing of K species enhances the
both samples are attributed to the maleic anhydride and performance of α-MnO2 in toluene oxidation and different
benzoate species, respectively. Notably, the strong peak at 1130 potassium precursors play an important role on it. The best
cm-1 can be only detected over α-MnO2, which is assigned to activity can be achieved by KOH/MnO2, over which 90% of
the characteristic of alkoxide species vibration from toluene toluene can be converted at 226 °C with a relative considerable
adsorption.50
stability and CO2 selectivity. Different anions in the potassium
precursor cause obvious mutation of α-MnO2 structure and
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9

Please do not adjust margins
Catalysis Science & Technology
Page 10 of 12
ARTICLE
Journal Name
12 J. Zhang, Y. Li, L. Wang, C. Zhang and H. He, Catal. Sci.
promote the formation of MnO6-K-MnO6 bridging bond and
exhibit obvious diverse ability for balancing charge transfer.
KOH is identified as the most promising precursor for alkali
metal modification, which significantly improves the
distribution of K species over α-MnO2 surface and strengthens
the content and activity of lattice oxygen. It is suggested that
lattice oxygen plays an important role in the oxidation of
toluene catalyzed by α-MnO2, which follows the Mars-van
Krevelen mechanism. In addition, the positive hole defects
(Mn3+) caused by the KOH treatment plays an important role in
the diffusion of O, and enhances the reducibility of manganese
oxide. The enhanced surface area, pore volume and surface
acidity, as well, favour of the depth of the mineralization of
toluene.
View Article Online
Technol., 2015, 5, 2305-2313.
DOI: 10.1039/C9CY02347J
13 Y. Xie, Y. Yu, X. Gong, Y. Guo, Y. Guo, Y. Wang and G.
Lu, CrystEngComm, 2015, 17, 3005-3014.
14 F. Wang, H. Dai, J. Deng, G. Bai, K. Ji and Y. Liu, Environ.
Sci. Technol., 2012, 46, 4034-4041.
15 F. Gao, X. Tang, H. Yi, C. Chu, N. Li, J. Li and S. Zhao,
Chem. Eng. J., 2017, 322, 525-537.
16 Z. Shang, M. Sun, X. Che, W. Wang, L. Wang, X. Cao, W.
Zhan, Y. Guo, Y. Guo and G. Lu, Catal. Sci. Technol., 2017,
7, 4710-4719.
17 L. T. Tseng, Y. Lu, H. M. Fan, Y. Wang, X. Luo, T. Liu, P.
Munroe, S. Li and J. Yi, Sci. Rep., 2015, 5, 9094.
18 J. Luo, H. T. Zhu, J. K. Liang, G. H. Rao, J. B. Li and Z. M.
Du, J. Phys. Chem. C., 2010, 114, 8782-8786.
19 Q. Feng, H. Kanoh, Y. Miyai and K. Ooi, Chem. Mater.,
1995, 7, 148-153.
20 J. Liu, V. Makwana, J. Cai, S. L. Suib and M. Aindow, J.
Phys. Chem. B, 2003, 107, 9185-9194.
21 Z. Hao, Z. Shen, Y. Li, H. Wang, L. Zheng, R. Wang, G. Liu
and S. Zhan, Angew. Chem., Int. Ed., 2019, 58(19), 6351-
6356.
Conflicts of interest
There are no conflicts to declare.
22 J. Hou, Y. Li, L. Liu, L. Ren and X. Zhao, J. Mater. Chem. A,
2013, 1, 6736-6741.
23 S. Rong, P. Zhang, F. Liu and Y. Yang, ACS Catal., 2018, 8,
3435-3446.
24 L.-T. Tseng, Y. Lu, H. M. Fan, Y. Wang, X. Luo, T. Liu, P.
Munroe, S. Li and J. Yi, Sci. Rep. 2015, 5, 9094.
25 C. Chen, X. Feng, Q. Zhu, R. Dong, R. Yang, Y. Cheng and
C. He, Inorg. Chem., 2019, 58, 2717-2728.
26 J. Hou, L. Liu, Y. Li, M. Mao, H. Lv and X. Zhao, Environ.
Sci. Technol., 2013, 47, 13730-13736.
27 J. Gao, Z. Huang, Y. Chen, J. Wan, X. Gu, Z. Ma, J. Chen
and X. Tang, Chem. - Eur. J., 2018, 24, 681-689.
28 V. Santos, M. Pereira, J. Órfão and J. Figueiredo, Appl.
Catal., B, 2010, 99, 353-363.
29 V. Santos, O. Soares, J. Bakker, M. Pereira, J. Órfão, J.
Gascon, F. Kapteijn and J. Figueiredo, J. Catal., 2012, 293,
165-174.
30 H. Chen, L. Jin, M. Wang, D. Xiao, D. Reisner, Sci. Technol.
Adv. Mater. 2007, 8, 63-66.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21677114, 21876139, 21922606),
the Key R&D Program of Shaanxi Province (2019SF-244,
2019ZDLSF05-05-02), the Shaanxi Natural Science
Fundamental Shaanxi Coal Chemical Joint Fund (2019JLM-14),
the National Key R&D Program of China (2016YFC0204201),
the Fundamental Research Funds for the Central Universities
(xjj2017170), and the Innovation Capability Support Program
of Shaanxi (2018PT-28, 2017KTPT-04). The authors gratefully
acknowledge the support of K.C. Wong Education Foundation
and also appreciate the editor and reviewers for their
professional work and valuable comments.
31 A. S. Poyraz, J. Huang, C. J. Pelliccione, X. Tong, S. Cheng,
L. Wu, Y. Zhu, A. C. Marschilok, K. J. Takeuchi and E. S.
Takeuchi, J. Mater. Chem. A, 2017, 5, 16914-16928.
32 M. Wang, L. Zhang, W. Huang, T. Xiu, C. Zhuang and J. Shi,
Chem. Eng. J., 2017, 320, 667-676.
33 C. Wang, L. Sun, Q. Cao, B. Hu, Z. Huang and X. Tang,
Appl. Catal., B, 2011, 101, 598-605.
34 B. Zhao, R. Ran, X. Wu and D. Weng, Appl. Catal., A, 2016,
514, 24-34.
Notes and references
1
2
3
4
5
C. He, J. Cheng, X. Zhang, M. Douthwaite, S. Pattisson and
Z. Hao, Chem. Rev., 2019, 119, 4471-4568.
X. Zhang, Y. Yang, L. Song, J. Chen, Y. Yang and Y. Wang,
J. Hazard. Mater., 2019, 365, 597-605.
M. Ma, H. Huang, C. Chen, Q. Zhu, L. Yue, R. Albilali and
C. He, Mol. Catal., 2018, 455, 192-203.
C. He, Z. Jiang, M. Ma, X. Zhang, M. Douthwaite, J.-W. Shi
and Z. Hao, ACS Catal., 2018, 8, 4213-4229.
35 X. Tang, F. Gao, Y. Xiang, H. Yi, S. Zhao, X. Liu and Y. Li,
Ind. Eng. Chem. Res., 2015, 54, 9116-9123.
36 K. Jirátová, J. Mikulová, J. Klempa, T. Grygar, Z. Bastl and
F. Kovanda, Appl. Catal., A, 2009, 361, 106-116.
37 Z. Sihaib, F. Puleo, J. Garcia-Vargas, L. Retailleau, C.
Descorme, L. Liotta, J. Valverde, S. Gil and A. Giroir-
Fendler, Appl. Catal., B, 2017, 209, 689-700.
38 S. Rong, K. Li, P. Zhang, F. Liu and J. Zhang, Catal. Sci.
Technol., 2018, 8, 1799-1812.
39 X. Wang, Y. Liu, T. Zhang, Y. Luo, Z. Lan, K. Zhang, J.
Zuo, L. Jiang and R. Wang, ACS Catal., 2017, 7, 1626-1636.
40 S. C. Kim and W. G. Shim, Appl. Catal., B, 2010, 98, 180-
185.
Z. Jiang, C. He, N. F. Dummer, J. Shi, M. Tian, C. Ma, Z.
Hao, S. H. Taylor, M. Ma and Z. Shen, Appl. Catal., B, 2018,
226, 220-233.
6
7
8
H. Wang, W. Yang, P. Tian, J. Zhou, R. Tang and S. Wu,
Appl. Catal., A, 2017, 529, 60-67.
M. Tian, C. He, Y. Yu, H. Pan, L. Smith, Z. Jiang, N. Gao, Y.
Jian, Z. Hao and Q. Zhu, Appl. Catal., A, 2018, 553, 1-14.
Y. Jian, T. Yu, Z. Jiang, Y. Yu, M. Douthwaite, J. Liu, R.
Albilali and C. He, ACS Appl. Mater. Interfaces, 2019, 11,
11369-11383.
9
Y. Jian, M. Ma, C. Chen, C. Liu, Y. Yu, Z. Hao and C. He,
Catal. Sci. Technol., 2018, 8, 3863-3875.
41 J. Chen, X. Chen, W. Xu, Z. Xu, J. Chen, H. Jia and J. Chen,
Chem. Eng. J., 2017, 330, 281-293.
42 Q. Meng, J. Liu, X. Weng, P. Sun, J. A. Darr and Z. Wu,
Catal. Sci. Technol., 2018, 8, 1858-1866.
10 Y. Wu, Y. Lu, C. Song, Z. Ma, S. Xing and Y. Gao, Catal.
Today, 2013, 201, 32-39.
11 S. C. Kim and W. G. Shim, Appl. Catal., B, 2010, 98, 180-
185.
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 12
Catalysis Science & Technology
43 D. Delimaris and T. Ioannides, Appl. Catal., B, 2009, 89,
View Article Online
295-302.
DOI: 10.1039/C9CY02347J
44 A. Rokicińska, P. Natkański, B. Dudek, M. Drozdek, L.
Lityńska-Dobrzyńska and P. Kuśtrowski, Appl. Catal., B,
2016, 195, 59-68.
45 X. Yang, X. Yu, M. Jing, W. Song, J. Liu and M. Ge, ACS
Appl. Mater. Interfaces, 2018, 11, 730-739.
46 L. Liu, J. Sun, J. Ding, Y. Zhang, T. Sun and J. Jia, Inorg.
Chem., 2019, 58, 13241-13249.
47 C. Dong, Z. Qu, Y. Qin, Q. Fu, H. Sun and X. Duan, ACS
Catal., 2019, 9(8), 6698-6710.
48 G. Zhu, J. Zhu, W. Li, W. Yao, R. Zong, Y. Zhu and Q.
Zhang, Environ. Sci. Technol., 2018, 52, 8684-8692.
49 X. Feng, M. Tian, C. He, L. Li, J.-W. Shi, Y. Yu and J.
Cheng, Appl. Catal., B, 2020, 264, 118493.
50 B. Zhao, Y. Jian, Z. Jiang, R. Albilali and C. He, Chin. J.
Catal., 2019, 40, 543-552.
51 X. Yang, X. Yu, M. Jing, W. Song, J. Liu and M. Ge, ACS
Appl. Mater. Interfaces, 2018, 11, 730-739.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11

Catalysis Science & Technology
Page 12 of 12
View Article Online
DOI: 10.1039/C9CY02347J
Graphical abstract