CO oxidation on SnO2 surfaces enhanced by metal doping

Article abstract of DOI:10.1039/c7cy02108a

Doping metal atoms into a host metal oxide lattice can enhance its catalytic activity by modulating the properties of surface oxygen. Here, Pt-doped antimony-tin oxide (Pt/Sb-SnO₂) was compared with Pt-deposited tin oxide (Pt/SnO₂) and Pt-deposited silica (Pt/SiO₂) for the oxidation of CO and propylene. 0.1 wt% Pt was deposited in all three cases. High angle annular dark field scanning transmission electron microscopy images, diffuse reflectance infrared Fourier transform spectra, pulsed H₂ chemisorption results, and X-ray photoelectron spectra indicated that Pt/Sb-SnO₂ has atomically doped Pt inside the SnO₂ lattice while Pt/SnO₂ has Pt nanoparticles covered with SnO₂ layers and Pt/SiO₂ has Pt nanoparticles exposed at the SiO₂ surface. Pt/Sb-SnO₂ showed the best activity for CO oxidation but the poorest activity for propylene oxidation. Propylene oxidation occurred the least on Pt/Sb-SnO₂ due to the lack of surface Pt sites. CO temperature-programmed reduction and O₂ temperature-programmed desorption results revealed that surface oxygen is the most active on Pt/Sb-SnO₂. The formation of carbonates during CO oxidation was monitored, and Pt/Sb-SnO₂ showed the least amount of surface carbonates with enhanced activity and durability. Doping a minimal amount of precious metal can be an efficient strategy to control the properties of metal oxide catalysts.

Full text of DOI:10.1039/c7cy02108a

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CO oxidation on SnO2 surfaces enhanced by metal
doping†
Cite this: DOI: 10.1039/c7cy02108a
*
Junemin Bae, Jiwhan Kim, Hojin Jeong and Hyunjoo Lee
Doping metal atoms into a host metal oxide lattice can enhance its catalytic activity by modulating the
properties of surface oxygen. Here, Pt-doped antimony–tin oxide (Pt/Sb–SnO2) was compared with Pt-
deposited tin oxide (Pt/SnO2) and Pt-deposited silica (Pt/SiO2) for the oxidation of CO and propylene. 0.1
wt% Pt was deposited in all three cases. High angle annular dark field scanning transmission electron
microscopy images, diffuse reflectance infrared Fourier transform spectra, pulsed H2 chemisorption results,
and X-ray photoelectron spectra indicated that Pt/Sb–SnO2 has atomically doped Pt inside the SnO2 lattice
while Pt/SnO2 has Pt nanoparticles covered with SnO2 layers and Pt/SiO2 has Pt nanoparticles exposed at
the SiO2 surface. Pt/Sb–SnO2 showed the best activity for CO oxidation but the poorest activity for propyl-
ene oxidation. Propylene oxidation occurred the least on Pt/Sb–SnO2 due to the lack of surface Pt sites.
CO temperature-programmed reduction and O2 temperature-programmed desorption results revealed
that surface oxygen is the most active on Pt/Sb–SnO2. The formation of carbonates during CO oxidation
was monitored, and Pt/Sb–SnO2 showed the least amount of surface carbonates with enhanced activity
and durability. Doping a minimal amount of precious metal can be an efficient strategy to control the prop-
erties of metal oxide catalysts.
Received 14th October 2017,
Accepted 20th December 2017
DOI: 10.1039/c7cy02108a
rsc.li/catalysis
metal oxides.25 The doped metal atoms can affect the proper-
ties of surface oxygen on the host metal oxide, resulting in
1. Introduction
Catalytic CO oxidation is a major solution for CO removal in
automotive exhaust after-treatment and is often used as a
monitoring reaction to investigate the catalytic properties of
heterogeneous catalysts.1 As the regulations for exhaust pol-
lutants from industrial and transport sources have been
strengthened, highly efficient catalysts are in great demand to
remove CO and other pollutants, such as hydrocarbons.2–5
Platinum group metals (PGM) have been widely studied in
catalytic oxidation due to their high catalytic activity.6–14 How-
ever, the usage of PGM catalysts should be minimized due to
their high cost and they often suffer from CO poisoning.15–19
Alternatively, various transition metal oxides have been used
for catalytic oxidation due to facile oxygen transfer at the
surface.20–24 Controlling the properties of surface oxygen on
the metal oxide catalysts is very important to improve their
catalytic activity.
enhanced oxidation on the surface. Tang et al. studied Pt-
doped CeO2 for methane oxidation showing that the Pt atom
activates the oxygen atoms on the CeO2 surface and the acti-
vation energy for methane dissociative adsorption de-
creases.26 Luo et al. showed that doping of Pd into mixed
oxides MOx–CeO2 (M = Mn, Fe, Co, Ni, Cu) has enhanced CO
oxidation by generating active oxygen species on the MOx–
CeO2 support.27 Doping of Pt into TiO2 was also reported to
have enhanced the photocatalytic activity of TiO2 for NO oxi-
dation or indoor air purification.28,29
Recently, we have reported the preparation of Pt single-
atom catalysts dispersed on an antimony–tin oxide support.30
Density functional theory (DFT) calculations showed that Pt
favorably replaces the Sb sites in the bulk phase or the sub-
surface of the antimony–tin oxide. Herein, a 0.1 wt% Pt-doped
antimony–tin oxide (Pt/Sb–SnO2) catalyst was prepared and its
catalytic activity for CO and propylene oxidation was investi-
gated. Pt-deposited tin oxide (Pt/SnO2) and Pt-deposited silica
(Pt/SiO2) catalysts with 0.1 wt% Pt were also prepared for com-
parison. Whereas the Pt/Sb–SnO2 has Pt dopants replacing
the SnO2 lattice sites, the Pt/SnO2 has Pt nanoparticles cov-
ered with thin SnO2 layers and the Pt/SiO2 has Pt nano-
particles with exposed Pt surfaces. They show distinct catalytic
activity for CO and propylene oxidation. The effect of Pt-
doping on the properties of surface oxygen and the formation
The catalytic activity of metal oxides can be promoted by
doping a small amount of precious metal atoms into the host
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute
of Science and Technology, Daejeon 34141, South Korea.
E-mail: azhyun@kaist.ac.kr; Tel: +82 42 350 3922
† Electronic supplementary information (ESI) available: Fig. S1–S8. See DOI:
10.1039/c7cy02108a
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of surface carbonates during CO oxidation were investigated
using various techniques.
to determine the surface state, and the binding energies were
calculated using the maximum intensity of the adventitious
C 1s signal at 284.8 eV as a reference.
CO temperature-programmed reduction (CO-TPR), O2
temperature-programmed desorption (O2-TPD), and pulsed
H2 chemisorption were performed using a BELCAT-B (BEL)
equipped with a thermal conductivity detector and the prod-
2. Experimental
2.1 Catalyst synthesis
Pt-doped antimony–tin oxide (Pt/Sb–SnO2) was synthesized by
a conventional incipient wetness impregnation method.30
Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Sigma-Al-
drich) was dissolved in 15 μl of anhydrous ethanol (Sigma-Al-
drich) with an adjusted concentration. The Pt precursor solu-
tion was mixed with 50 mg of antimony–tin oxide (Alfa Aesar)
and dried in a vacuum oven at 50 °C overnight. Then, the
sample was reduced at 400 °C in a tube furnace under 10
vol% H2 balanced with N2 for 1 h. Pt-deposited tin oxide (Pt/
SnO2) and Pt-deposited silica (Pt/SiO2) were synthesized using
the same procedure. The Pt precursor solutions were mixed
with 50 mg of tin oxide (Sigma-Aldrich) or silica (Sigma-Al-
drich) and dried in a vacuum oven at 50 °C overnight. Then,
the samples were reduced at 400 °C under 10 vol% H2 bal-
anced with N2 for 1 h. The bare supports were also reduced
at 400 °C under 10 vol% H2 balanced with N2 for 1 h prior to
oxidation reactions or characterization.
ucts were also monitored using
a mass spectrometer
(BELMASS, BEL). For CO-TPR, the catalyst (50 mg) was pre-
treated in a He stream at 100 °C for 1 h and cooled down to
room temperature. Then, the catalyst was exposed to 10% CO
in He with a flow rate of 50 ml min−1 and heated from room
temperature to 1000 °C with a ramping rate of 10 °C min−1.
For O2-TPD, the catalyst (200 mg) was pre-treated under 5%
O2 balanced with a He flow at 500 °C for 1 h, cooled down to
room temperature, and purged with He for 30 min. Then, the
catalyst was heated in He with a flow rate of 50 ml min−1
from room temperature to 1000 °C with a ramping rate of 10
°C min−1. For pulsed H2 chemisorption, the catalyst (50 mg)
was pre-treated under 5% H2 balanced with an Ar flow at 400
°C for 1 h, purged with Ar at the same temperature for 1 h,
and cooled down to room temperature. Then, H2 was pulsed
every 1 min in a stream of Ar until the adsorption of H2 onto
the sample was saturated.
Diffuse reflectance infrared Fourier-transform spectro-
scopy (DRIFTS) measurements were carried out using a Nico-
let iS-50 (Thermo Scientific). CO chemisorption was moni-
tored using 4 mg of catalyst ground with 196 mg of KBr. The
mixed powder was placed into a cell and pre-treated with Ar
at 100 °C for 1 h to remove any water or adsorbed residues
on the catalyst surface. Then, 2% CO balanced with Ar was
charged for 20 min at room temperature. The CO gas was
purged out using a vacuum pump after 20 min, and then
DRIFT spectra were collected under vacuum. The formation
of carbonates during CO oxidation was monitored by in situ
DRIFTS. CO oxidation was performed with an inlet gas flow
consisting of 1% CO and 1% O2 balanced with Ar at elevated
temperatures. After exposing to the reaction gases for 10
min, the catalyst was purged with an Ar flow for 10 min, and
then IR spectra were collected.
2.2 Catalytic oxidation reactions
CO oxidation was carried out in a quartz fixed-bed reactor
with 50 mg of the catalyst. The catalyst was pre-treated at 100
°C under He flow for 1 h to remove any H2O, then the sample
was cooled to room temperature. CO oxidation was
performed with an inlet gas flow consisting of 1% CO and
1% O2 balanced with He. The total flow rate was 100 ml
min−1 (space velocity (SV) = 120 000 ml g−1 h−1). The reactor
temperature was increased by 10 °C until CO conversion
approached 100%. At each temperature, the sample was held
in a continuous feed stream to obtain steady-state signals.
Propylene oxidation was also carried out in the same reactor
with an inlet gas flow consisting of 0.1% C3H6 and 1% O2
balanced with He. The total flow rate was 100 ml min−1 (SV =
120 000 ml g−1 h−1). The product gases were analysed using
an on-line gas chromatograph (GC, Younglin GC 6500)
equipped with Carboxen 1000/GS-GASPRO columns and a
thermal conductivity detector (TCD)/flame ionization detector
(FID). All experiments were conducted at atmospheric
pressure.
3. Results and discussion
3.1 Characterization of Pt/Sb–SnO2, Pt/SnO2, and Pt/SiO2
catalysts
HR-TEM and HAADF-STEM images of the 0.1 wt% Pt/Sb–
SnO2, Pt/SnO2, and Pt/SiO2 catalysts are shown in Fig. 1. No
noticeable Pt nanoparticles are observed on the Pt/Sb–SnO2,
instead only Sb–SnO2 domains are observed. HR-TEM images
of the bare Sb–SnO2 support are shown in Fig. S1(a and b).†
The HAADF-STEM image of the 0.1 wt% Pt/Sb–SnO2, how-
ever, shows that single Pt atoms are located on the lattice
fringe of SnO2, which are denoted by white circles in
Fig. 1(b). The lattice spacing is consistent with the values pre-
viously reported.31–33 Additional HAADF-STEM images show-
ing the single Pt atoms on SnO2 are shown in Fig. S2(a and
2.3 Characterization
The actual metal contents of the catalysts were measured by
inductively coupled plasma-optical emission spectrometry
(ICP-OES 720, Agilent). The BET surface areas of the catalysts
were measured by N2 adsorption at −196 °C using a BET in-
strument (Tristar II 3020). The catalysts were observed by
transmission electron microscopy (TEM; TF30 ST, Tecnai)
and high angle annular dark field-scanning TEM (HAADF-
STEM; Titan cubed G2 60-300, FEI). X-ray photoelectron
spectroscopy (XPS; K-alpha, ThermoVG Scientific) was used
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Fig. 2 (a) DRIFT spectra for CO chemisorption on the 0.1 wt% Pt/Sb–
SnO2, Pt/SnO2, Pt/SiO2, and their corresponding bare supports, Sb–
SnO2, SnO2, and SiO2. (b) XPS Pt 4f spectra of 0.1 wt% and 1 wt% Pt/
Sb–SnO2, Pt/SnO2, and Pt/SiO2 catalysts.
wt% Pt catalysts as shown in Fig. 2(a). A strong peak at 2057
cm−1 and a weak peak at 1893 cm−1 for Pt/SiO2 are assigned
to linear and bridge-bonded CO on the metallic Pt sites,
respectively.35–38 The shoulder at 1998 cm−1 might be caused
by CO adsorbed on the interface between the Pt nano-
particles and silica.35,39 For Pt/SnO2, a very weak peak at 2041
cm−1 was observed and the same peak was also observed for
the bare SnO2 support. No peaks for CO adsorbed on the Pt
sites were observed. It was reported that CO was not
adsorbed in the Pt/SnO2 catalyst reduced at 400 °C.40,41 The
SnO2 shell would inhibit CO adsorption on the top surface of
the Pt nanoparticles, indicating that Pt nanoparticles in the
Pt/SnO2 catalyst are present inside the SnO2 layers, not on
the SnO2 surface. The Pt/Sb–SnO2 catalyst also showed no
DRIFTS peak, indicating that the Pt atoms where CO can be
adsorbed do not exist on the surface. The Pt atoms are doped
inside the Sb–SnO2 surface, not exposed at the surface. Addi-
tionally, the size and dispersion of Pt on the 0.1 wt% Pt cata-
lysts were estimated by pulsed H2 chemisorption as shown in
Table 1. The Pt/SiO2 showed a Pt dispersion of 27.6% and a
Pt domain size of 4.1 nm. However, H2 uptakes on the Pt/Sb–
Fig. 1 HR-TEM images of the 0.1 wt% Pt catalysts: (a) Pt/Sb–SnO2, (c)
Pt/SnO2, and (e) Pt/SiO2. HAADF-STEM images of the 0.1 wt% Pt cata-
lysts: (b) Pt/Sb–SnO2, (d) Pt/SnO2, and (f) Pt/SiO2. White circles in (b) in-
dicate single Pt atoms on SnO2.
b).† The Pt/SnO2 in Fig. 1(c and d) shows Pt nanoparticles
covered with a thin SnO2 layer. The core–shell structure was
also previously reported for a 1 wt% Pt/SnO2 catalyst exposed
to air after the reduction treatment at 400 °C.34 This structure
would be formed by phase separation of oxidized Sn species
from the metallic PtSn in a mild oxidizing atmosphere. An
HR-TEM image of the bare SnO2 support and additional
HAADF-STEM images of the Pt/SnO2 can be found in Fig.
S1(c and d) and S2(c and d),† respectively. Fig. 1(e and f)
show typical images of Pt nanoparticles deposited on the sil-
ica support in Pt/SiO2.
Table 1 BET surface area, Pt loading, Pt dispersion and estimated diame-
ter for 0.1 wt% Pt/Sb–SnO2, Pt/SnO2, Pt/SiO2, and their corresponding
bare supports Sb–SnO2, SnO2, and SiO2 reduced at 400 °C
c
Catalyst
SBET (m2 g−1
)
Pta (wt%)
DPtb (%)
dPt (nm)
The phase composition and crystalline structure of the 0.1
wt% Pt catalysts and bare supports were observed using XRD
patterns as shown in Fig. S3.† The Pt/Sb–SnO2 and Pt/SnO2
have dominant SnO2 and metallic Sn phases, and a SnSb
phase was additionally observed on the Pt/Sb–SnO2.30 No dif-
fraction peaks of the Pt species were observed in all 0.1 wt%
Pt catalysts due to low Pt contents.
Sb–SnO2
SnO2
SiO2
Pt/Sb–SnO2
Pt/SnO2
Pt/SiO2
23
18
197
23
13
180
—
—
—
0.104
0.103
0.098
—
—
—
n. d.
n. d.
27.6
—
—
—
n. d.
n. d.
4.1
a
b
The actual Pt loading was measured by ICP-OES. Pt dispersion
was measured by pulsed H2 chemisorption; for Pt/Sb–SnO2 and Pt/
SnO2, H2 chemisorption was not detected. Pt nanoparticle size was
estimated from the Pt dispersion.
c
DRIFTS for CO chemisorption was performed to reveal the
chemical state and presence of Pt on the surface of the 0.1
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SnO2 and Pt/SnO2 were not detected, indicating that there
are no Pt atoms at the surface where H2 can be adsorbed.
This supports that Pt is present beneath the surface of Pt/Sb–
SnO2 and Pt/SnO2.
The Pt oxidation states of the catalysts were further inves-
tigated by XPS as shown in Fig. 2(b). The XPS measurements
were carried out for the 0.1 wt% Pt catalysts, but Pt 4f peaks
were hardly observed because the Pt content was too low.
Thus, 1 wt% Pt catalysts were alternatively used. The XPS Pt
peaks for Pt/Sb–SnO2 are shifted to a higher binding energy
compared to Pt/SnO2 and Pt/SiO2. The obtained Pt 4f peaks
were deconvoluted into Pt0 and Pt2+. Whereas Pt/SiO2 and Pt/
SnO2 showed Pt 4f7/2 peaks at 71.5 eV and 71.4 eV, respec-
tively, indicating that the Pt has a mainly metallic state, Pt/
Sb–SnO2 showed a Pt 4f7/2 peak at 72.3 eV indicating that the
Pt has a mainly oxidic state. The Pt in Pt/Sb–SnO2 exists as
Pt2+, supporting that the Pt is atomically doped into the SnO2
lattice. XPS for Sb 3d was further performed on 0.1 wt% Pt/
Sb–SnO2 and bare Sb–SnO2 as shown in Fig. S4.† Surface Sb
peaks are rarely observed for the 0.1 wt% Pt/Sb–SnO2 and
Sb–SnO2 reduced at 400 °C, whereas the Sb peak was clearly
observed for fresh Sb–SnO2. This indicates that the Sb atoms
move inward because of the reduction treatment at 400 °C,
and the Pt is doped by substituting the Sb sites existing in-
side the surface.30 As evidenced by HAADF-STEM images, CO-
DRIFT spectra, and XPS results, the 0.1 wt% Pt catalysts have
distinct surface structures; Pt/Sb–SnO2 has doped Pt inside
the SnO2 lattice while Pt/SnO2 has Pt nanoparticles covered
with SnO2 layers and Pt/SiO2 has Pt nanoparticles with ex-
posed Pt surfaces.
Fig. 3 (a) CO oxidation and (b) C3H6 oxidation results for 0.1 wt% Pt/
Sb–SnO2, Pt/SnO2, Pt/SiO2, and their corresponding bare supports Sb–
SnO2, SnO2, and SiO2. CO oxidation was performed at 120 000 ml g−1
h−1 with an inlet gas flow of 1% CO and 1% O2 balanced with He. C3H6
oxidation was performed at the same space velocity with an inlet gas
flow of 0.1% C3H6 and 1% O2 balanced with He.
3.2 Catalytic oxidation of CO and propylene
CO oxidation was performed for the 0.1 wt% Pt catalysts and
bare supports as shown in Fig. 3(a). Pt/Sb–SnO2 presented
the highest activity for CO oxidation by achieving 100% CO
conversion at 240 °C, whereas Pt/SnO2 or Pt/SiO2 showed
100% conversion at 270 °C. While the bare SnO2 achieved
100% CO conversion at 410 °C, the bare Sb–SnO2 showed an
enhanced CO oxidation activity by achieving 100% CO con-
version at 310 °C. The improvement of the CO oxidation ac-
tivity of Sb–SnO2 compared to SnO2 is caused by the incorpo-
ration of Sb into the SnO2 lattice. Interestingly, after 0.1 wt%
Pt-doping, the temperature at which 100% CO conversion
was attained was reduced significantly further to 240 °C. This
indicates that the 0.1 wt% Pt-doping enhances the CO oxida-
tion activity of Sb–SnO2. Fig. S5† shows the effect of Pt load-
ing or the kind of deposited metal. When 0.05, 0.1, 0.2, and 1
wt% Pt were deposited onto the Sb–SnO2 support as shown
in Fig. S5(a),† all Pt/Sb–SnO2 catalysts showed better activity
than the Sb–SnO2. Interestingly, the 0.1 wt% Pt/Sb–SnO2
showed the best activity, and a higher Pt loading showed no
further enhancement. HR-TEM images of the 0.1, 1, and 4
wt% Pt/Sb–SnO2 catalysts are shown in Fig. S6.† No notice-
able Pt nanoparticles were observed in all Pt/Sb–SnO2 cata-
lysts. DRIFTS for CO chemisorption was further performed
on the 0.1, 1, and 4 wt% Pt/Sb–SnO2 catalysts as shown in
Fig. S7(a).† All Pt/Sb–SnO2 catalysts showed no peaks for CO
adsorbed on the Pt sites. Fig. S7(b)† shows the XPS Pt 4f
spectra of the catalysts with various Pt wt%. All Pt/Sb–SnO2
catalysts showed a Pt 4f7/2 peak at 72.3 eV. This indicates that
the Pt in Sb–SnO2 has a mainly Pt2+ state even at high Pt
loading. When 0.1 wt% Pt, Pd, Rh, and Ru were deposited
onto Sb–SnO2 as shown in Fig. S5(b),† the activity for CO oxi-
dation was enhanced in all cases while that doped with Pt
showed the largest enhancement. While the Pt in the 0.1
wt% Pt/Sb–SnO2 catalyst was not exposed at the surface, the
Pt/Sb–SnO2 showed better activity than the Pt/SiO2 with ex-
posed Pt surfaces. With Pt/Sb–SnO2 and Pt/SnO2, CO oxida-
tion would occur on the SnO2 surface as opposed to Pt/SiO2
where the active sites are the Pt nanoparticles. CO oxidation
on SnO2 surfaces has been reported to follow a Mars–van
Krevelen mechanism.42–45 CO is initially adsorbed on the
SnO2 surface and then oxidized by the surrounding less-
coordinated surface oxygen atoms forming CO2 and oxygen
defects. The gaseous O2 then reacts with the reduced surface
to regenerate the surface oxygen atoms. Thus, this property
of surface oxygen is important for enhancing CO oxidation
on the SnO2 surface. The enhanced CO oxidation on the Pt/
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Sb–SnO2 seems to originate from the change in the proper-
ties of surface oxygen on the Sb–SnO2 by 0.1 wt% Pt-doping.
Propylene oxidation was also performed for the 0.1 wt% Pt
catalysts and bare supports as shown in Fig. 3(b). In contrast
to the CO oxidation, the 0.1 wt% Pt/SiO2 showed the best ac-
tivity by achieving 100% propylene conversion at 220 °C while
the Pt/SnO2 and Pt/Sb–SnO2 showed 100% conversion at 260
°C and 340 °C, respectively. Complete oxidation of propylene
requires initial C–H and C–C bond cleavage on the active
sites,46,47 and typically occurs at much lower temperatures on
platinum than on metal oxides.48,49 Propylene oxidation is be-
lieved to occur through a reaction with electrophilic oxygen,
which is produced by adsorption of gaseous O2 on the catalyst
surface.50,51 The lattice oxygen on metal oxides typically plays
a lesser role in propylene oxidation. C–H and C–C bonds
would be cleaved more easily on the exposed Pt surface of the
Pt/SiO2 than the Sb–SnO2 surface into which Pt is doped. The
absence of Pt sites on the Sb–SnO2 surface also supports the
poor activity of Pt/Sb–SnO2 for propylene oxidation.
lattice oxygen in each catalyst. The peaks at 210 °C in the
magnified figure indicate that CO reacts with the surface oxy-
gen of the catalysts to form CO2, which is generally the first
step of the oxidation reaction on metal oxide catalysts follow-
ing the Mars–van Krevelen mechanism.52,53 The CO2 forma-
tion peak of Pt/Sb–SnO2 starts at 100 °C, which is about 70
°C lower than that of Pt/SnO2. Also, the peak for Pt/Sb–SnO2
was the largest, and only a small peak was observed for Pt/
SnO2, indicating that the surface oxygen of Pt/Sb–SnO2 is the
most active. The Pt/SiO2 catalyst showed no CO reduction
peak in the entire temperature range, indicating that the
SiO2 support does not participate in the CO oxidation. It was
also confirmed that CO2 formation on the Sb–SnO2 surface is
significantly promoted after doping Pt atoms. This indicates
that the surface oxygen on Pt/Sb–SnO2 can be detached more
easily than that on the bare Sb–SnO2.
The O2-TPD results obtained by monitoring the desorbed
O2 with a mass spectrometer (m/z 32) are shown in Fig. S8.†
The peaks at 920 °C correspond to the desorption of bulk lat-
tice oxygen while the peaks at 500–800 °C indicate the de-
sorption of surface oxygen. Pt/Sb–SnO2 showed the surface
oxygen peak at 540 °C while the Sb–SnO2 support showed the
peak at 670 °C. The surface oxygen is more active on Pt/Sb–
SnO2 than on the bare Sb–SnO2, implying that the doped Pt
activates the Sn–O bonds on the Sb–SnO2 surface. For Pt/
SnO2 and bare SnO2, oxygen desorption peaks were not ob-
served in the entire temperature range. It was reported that
no oxygen desorption peaks were observed in the O2-TPD re-
sults for the reduced SnO2.54,55 Clearly, Pt-doping facilitated
the transfer of surface oxygen on Sb–SnO2 as evidenced by
CO-TPR and O2-TPD.
3.3 Surface oxygen and carbonates
The properties of surface oxygen of the 0.1 wt% Pt catalysts
and bare supports were investigated using CO-TPR and O2-
TPD techniques. Fig. 4 shows the CO-TPR results obtained by
monitoring the produced CO2 with a mass spectrometer (m/z
44). The peaks above 600 °C indicate the reduction of bulk
In situ DRIFTS was performed for CO oxidation at elevated
temperatures to investigate the formation of carbonate spe-
cies on the surface. The carbonate species are easily formed
on the catalyst surface during CO oxidation,56,57 and they of-
ten poison surface active sites resulting in degraded
activity.58–60 Schumacher et al. reported that carbonate spe-
cies are accumulated on the surface of a Au/TiO2 catalyst in-
ducing deactivation.61 Jansson et al. reported that carbonate
species on the Co3O4 surface lead to deactivation.62,63 Fig. 5
shows the DRIFT spectra of the 0.1 wt% Pt catalysts. For Pt/
Sb–SnO2 in Fig. 5(a), no carbonate species are observed at
low temperatures, then weak peaks appear at 1340, 1393,
1416, 1445, and 1488 cm−1, which correspond to free carbon-
ate or monodentate carbonate formed on the Sb–SnO2 sur-
face.64,65 For Pt/SnO2 in Fig. 5(b), large peaks at 1353, 1400,
and 1456 cm−1 are observed corresponding to the carbonate
species formed on the SnO2 surface, and additional peaks are
observed at 1541 and 1607 cm−1 corresponding to bidentate
carbonate species.64,65 For Pt/SiO2 in Fig. 5(c), weak peaks are
observed at 1317, 1417, and 1593 cm−1 corresponding to free
carbonate or bidentate carbonate formed on the silica sur-
face, but other large peaks appear at 1798 and 1945 cm−1
,
which correspond to the carbonates formed on the Pt nano-
particles or CO adsorbed on the Pt nanoparticles with a
bridge mode.36,66 The Pt/SnO2 or Pt/SiO2 catalysts suffer from
Fig. 4 CO-TPR results by monitoring CO2 (m/z 44) formation with a
mass spectrometer for 0.1 wt% Pt/Sb–SnO2, Pt/SnO2, Pt/SiO2, and their
corresponding bare supports Sb–SnO2, SnO2, and SiO2.
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Fig. 6 Long-term stability test of the 0.1 wt% Pt catalysts including Pt/
Sb–SnO2, Pt/SnO2, and Pt/SiO2 for CO oxidation at 220 °C under an
inlet gas flow consisting of 1% CO and 1% O2 balanced with He with a
SV of 120 000 ml g−1 h−1
.
on the Sb–SnO2 surface, resulting in enhanced activity and
durability for CO oxidation.
Conclusions
Pt/Sb–SnO2, Pt/SnO2, and Pt/SiO2 catalysts with the same Pt
weight percentages of 0.1 wt% were prepared. HAADF-STEM
images, CO-DRIFT spectra, and XPS results showed that the
Pt was atomically doped inside the SnO2 lattice in Pt/Sb–
SnO2, Pt nanoparticles overlaid with a thin SnO2 layer were
deposited on the SnO2 support in Pt/SnO2, and Pt nano-
particles with exposed Pt surfaces were deposited on the SiO2
support in Pt/SiO2. These catalysts were used for CO oxida-
tion or propylene oxidation. Pt/Sb–SnO2 showed the best ac-
tivity for CO oxidation but the poorest activity for propylene
oxidation. The surface oxygen would be used the most easily
for CO oxidation on Pt/Sb–SnO2, but the absence of Pt sites
at the surface inhibited propylene oxidation. CO-TPR and O2-
TPD results confirmed that surface oxygen on Pt/Sb–SnO2 is
the most active. In situ DRIFT spectra were obtained for CO
oxidation, and they showed that surface poisoning species
such as carbonates or strongly adsorbed CO were observed
the least on the Pt/Sb–SnO2 catalyst. Pt/Sb–SnO2 showed en-
hanced activity and durability for CO oxidation due to facile
oxygen transfer at the surface and less formation of surface
carbonates. This work would provide a useful strategy for pro-
moting the activity and durability of metal oxide catalysts.
Fig. 5 In situ DRIFT spectra of the 0.1 wt% Pt catalysts for CO
oxidation at elevated temperatures: (a) Pt/Sb–SnO2, (b) Pt/SnO2, and (c)
Pt/SiO2. The catalysts were exposed to the reaction gases consisting of
1% CO and 1% O2 balanced with Ar for 10 min and purged with Ar for
10 min; IR spectra were then collected at each temperature.
the formation of carbonates on the SnO2 or Pt surface. The
surface poisoning species such as carbonate or CO are
formed much less on the Pt/Sb–SnO2 with enhanced activity
for CO oxidation.
Long-term stability tests of the 0.1 wt% Pt catalysts were
carried out by running CO oxidation at 220 °C. Fig. 6 shows
that CO conversion for Pt/SiO2 rapidly decreased from 4% to
zero at 24 h of the reaction, and CO conversion for the Pt/
SnO2 catalyst decreased from 15% to 9% at 24 h. This might
be attributed to a large amount of CO and carbonate species
accumulated on the Pt nanoparticles and SnO2 surface on
Pt/SiO2 and Pt/SnO2, respectively. However, Pt/Sb–SnO2
maintained high CO conversion without deactivation for 80
h. The facile oxygen transfer and less carbonate formation on
the surface would enhance the durability of the Pt/Sb–SnO2
catalyst. Pt-doping changed the properties of surface oxygen
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the National Research
Foundation of Korea (NRF-2015R1A2A2A01004467; NRF-
2016R1A5A1009592).
Catal. Sci. Technol.
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References
29 Y. Hu, X. Song, S. M. Jiang and C. H. Wei, Chem. Eng. J.,
2015, 274, 102–112.
1 H. J. Freund, G. Meijer, M. Scheffler, R. Schlogl and M.
Wolf, Angew. Chem., Int. Ed., 2011, 50, 10064–10094.
2 Y. F. Y. Yao, J. Catal., 1984, 87, 152–162.
3 M. Ozawa, M. Kimura and A. Isogai, J. Alloys Compd.,
1993, 193, 73–75.
4 G. W. Graham, H. W. Jen, O. Ezekoye, R. J. Kudla, W. Chun,
X. Q. Pan and R. W. McCabe, Catal. Lett., 2007, 116, 1–8.
5 I. Heo, M. H. Wiebenga, J. R. Gaudet, I. S. Nam, W. Li and
C. H. Kim, Appl. Catal., B, 2014, 160, 365–373.
6 D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed.,
2005, 44, 7852–7872.
30 J. Kim, C.-W. Roh, S. K. Sahoo, S. Yang, J. Bae, J. W. Han
and H. Lee, Adv. Energy Mater., 2018, 1701476.
31 D. F. Zhang, L. D. Sun, J. L. Yin and C. H. Yan, Adv. Mater.,
2003, 15, 1022–1025.
32 N. Wan, X. Lu, Y. S. Wang, W. F. Zhang, Y. Bai, Y. S. Hu and
S. Dai, Sci. Rep., 2016, 6, 18978.
33 A. Biaggi-Labiosa, F. Sola, M. Lebron-Colon, L. J. Evans, J. C.
Xu, G. W. Hunter, G. M. Berger and J. M. Gonzalez,
Nanotechnology, 2012, 23, 455501.
34 N. Kamiuchi, K. Taguchi, T. Matsui, R. Kikuchi and K.
Eguchi, Appl. Catal., B, 2009, 89, 65–72.
7 W. T. Yu, M. D. Porosoff and J. G. G. Chen, Chem. Rev.,
2012, 112, 5780–5817.
8 M. M. Schubert, S. Hackenberg, A. C. van Veen, M. Muhler,
V. Plzak and R. J. Behm, J. Catal., 2001, 197, 113–122.
9 L. Q. Liu, F. Zhou, L. G. Wang, X. J. Qi, F. Shi and Y. Q.
Deng, J. Catal., 2010, 274, 1–10.
35 B. T. Qiao, A. Q. Wang, X. F. Yang, L. F. Allard, Z. Jiang, Y. T.
Cui, J. Y. Liu, J. Li and T. Zhang, Nat. Chem., 2011, 3, 634–641.
36 M. Moses-DeBusk, M. Yoon, L. F. Allard, D. R. Mullins, Z. L.
Wu, X. F. Yang, G. Veith, G. M. Stocks and C. K. Narula,
J. Am. Chem. Soc., 2013, 135, 12634–12645.
37 K. Ding, A. Gulec, A. M. Johnson, N. M. Schweitzer, G. D.
Stucky, L. D. Marks and P. C. Stair, Science, 2015, 350,
189–192.
10 P. Bera, K. C. Patil, V. Jayaram, G. N. Subbanna and M. S.
Hegde, J. Catal., 2000, 196, 293–301.
11 T. Bunluesin, H. Cordatos and R. J. Gorte, J. Catal.,
1995, 157, 222–226.
12 J. H. Park, J. H. Cho, S. E. Kang, K. H. Cho, T. W. Lee, H. S.
Han and C. H. Shin, Korean J. Chem. Eng., 2013, 30,
598–604.
13 C. Kim, J. G. Oh, Y. T. Kim, H. Kim and H. Lee, Electrochem.
Commun., 2010, 12, 1596–1599.
14 S. Yang and H. Lee, ACS Catal., 2013, 3, 437–443.
15 K. Liu, A. Q. Wang and T. Zhang, ACS Catal., 2012, 2,
1165–1178.
38 L. DeRita, S. Dai, K. Lopez-Zepeda, N. Pham, G. W. Graham,
X. Q. Pan and P. Christopher, J. Am. Chem. Soc., 2017, 139,
14150–14165.
39 O. Pozdnyakova, D. Teschner, A. Wootsch, J. Krohnert, B.
Steinhauer, H. Sauer, L. Toth, F. C. Jentoft, A. Knop-Gericke,
Z. Paal and R. Schlogl, J. Catal., 2006, 237, 1–16.
40 T. Okanishi, T. Matsui, T. Takeguchi, R. Kikuchi and K.
Eguchi, Appl. Catal., A, 2006, 298, 181–187.
41 N. Yamaguchi, N. Kamiuchi, H. Muroyama, T. Matsui and K.
Eguchi, Catal. Today, 2011, 164, 169–175.
16 C. H. Lee and Y. W. Chen, Ind. Eng. Chem. Res., 1997, 36,
1498–1506.
17 K. Tikhomirov, O. Krocher, M. Elsener and A. Wokaun, Appl.
Catal., B, 2006, 64, 72–78.
18 S. S. Lee, C. Y. Fan, T. P. Wu and S. L. Anderson, J. Am.
Chem. Soc., 2004, 126, 5682–5683.
19 Z. L. Wu, M. J. Li and S. H. Overbury, J. Catal., 2012, 285,
61–73.
42 M. Batzill and U. Diebold, Prog. Surf. Sci., 2005, 79, 47–154.
43 X. Wang, L. H. Xiao, H. G. Peng, W. M. Liu and X. L. Xu,
J. Mater. Chem. A, 2014, 2, 5616–5619.
44 Z. S. Lu, D. W. Ma, L. Yang, X. B. Wang, G. L. Xu and Z. X.
Yang, Phys. Chem. Chem. Phys., 2014, 16, 12488–12494.
45 W. J. Kim, S. W. Lee and Y. Sohn, Sci. Rep., 2015, 5, 13448.
46 A. M. Gabelnick, A. T. Capitano, S. M. Kane, J. L. Gland and
D. A. Fischer, J. Am. Chem. Soc., 2000, 122, 143–149.
47 J. Koop and O. Deutschmann, Appl. Catal., B, 2009, 91, 47–58.
48 A. S. Kota, R. K. Dadi, D. Luss and V. Balakotaiah, Chem.
Eng. Sci., 2017, 166, 320–333.
20 S. Royer and D. Duprez, ChemCatChem, 2011, 3, 24–65.
21 Y. Ren, Z. Ma, L. P. Qian, S. Dai, H. Y. He and P. G. Bruce,
Catal. Lett., 2009, 131, 146–154.
22 Q. Fu, F. Yang and X. H. Bao, Acc. Chem. Res., 2013, 46,
1692–1701.
23 M. Baldi, V. S. Escribano, J. M. G. Amores, F. Milella and G.
Busca, Appl. Catal., B, 1998, 17, 175–182.
49 J. J. Spivey, Ind. Eng. Chem. Res., 1987, 26, 2165–2180.
50 J. Haber and W. Turek, J. Catal., 2000, 190, 320–326.
51 A. J. Binder, T. J. Toops, R. R. Unocic, J. E. Parks and S. Dai,
Angew. Chem., Int. Ed., 2015, 54, 13263–13267.
24 X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta and W. J. Shen, Nature,
2009, 458, 746–749.
25 E. W. McFarland and H. Metiu, Chem. Rev., 2013, 113,
4391–4427.
26 W. Tang, Z. P. Hu, M. J. Wang, G. D. Stucky, H. Metiu and
E. W. McFarland, J. Catal., 2010, 273, 125–137.
27 J. Y. Luo, M. Meng, J. S. Yao, X. G. Li, Y. Q. Zha, X. Wang
and T. Y. Zhang, Appl. Catal., B, 2009, 87, 92–103.
28 B. K. Vijayan, N. M. Dimitrijevic, J. S. Wu and K. A. Gray,
J. Phys. Chem. C, 2010, 114, 21262–21269.
52 D. W. v. K. P. Mars, Chem. Eng. Sci., 1954, 3, 41–59.
53 J. Gao, C. Jia, L. Zhang, H. Wang, Y. Yang, S.-F. Hung, Y.-Y.
Hsu and B. Liu, J. Catal., 2016, 341, 82–90.
54 B. Mirkelamoglu and G. Karakas, Appl. Catal., A, 2005, 281,
275–284.
55 S. Saukko, U. Lassi, V. Lantto, M. Kroneld, S. Novikov, P.
Kuivalainen, T. T. Rantala and J. Mizsei, Thin Solid Films,
2005, 490, 48–53.
56 H. H. Kung, M. C. Kung and C. K. Costello, J. Catal.,
2003, 216, 425–432.
This journal is © The Royal Society of Chemistry 2017
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
57 A. A. Herzing, C. J. Kiely, A. F. Carley, P. Landon and G. J.
Hutchings, Science, 2008, 321, 1331–1335.
62 J. Jansson, J. Catal., 2000, 194, 55–60.
63 J. Jansson, A. E. C. Palmqvist, E. Fridell, M. Skoglundh, L.
Osterlund, P. Thormahlen and V. Langer, J. Catal.,
2002, 211, 387–397.
64 Z. Y. Sun, L. Jin, S. He, Y. F. Zhao, M. Wei, D. G. Evans and
X. Duan, Green Chem., 2012, 14, 1909–1916.
65 H. G. Li, X. Jiao, L. Li, N. Zhao, F. K. Xiao, W. Wei, Y. H. Sun
and B. S. Zhang, Catal. Sci. Technol., 2015, 5, 989–1005.
66 M. A. Newton, D. Ferri, G. Smolentsev, V. Marchionni and
M. Nachtegaal, Nat. Commun., 2015, 6, 8675.
58 M. Stamatakis, M. A. Christiansen, D. G. Vlachos and G.
Mpourmpakis, Nano Lett., 2012, 12, 3621–3626.
59 J. Saavedra, T. Whittaker, Z. F. Chen, C. J. Pursell, R. M.
Rioux and B. D. Chandler, Nat. Chem., 2016, 8, 585–590.
60 T. A. Ntho, J. A. Anderson and M. S. Scurrell, J. Catal.,
2009, 261, 94–100.
61 B. Schumacher, V. Plzak, M. Kinne and R. J. Behm, Catal.
Lett., 2003, 89, 109–114.
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