Elucidating the promotional effects of niobia on SnO2 for CO oxidation: Developing an XRD extrapolation method to measure the lattice capacity of solid solutions

Article abstract of DOI:10.1039/c5cy01870f

A series of Sn-Nb binary catalysts have been prepared by using a co-precipitation method and used for CO oxidation. All the catalysts show much higher specific surface areas than the individual oxides, indicating their improved thermal stability. It was found that Nb⁵⁺ cations can be doped into the lattice of tetragonal rutile SnO₂ to replace a portion of the Sn⁴⁺ to form a solid solution structure. Using an XRD extrapolation method, the SnO₂ lattice capacity for Nb₂O₅ has been quantified, which proves that only 25% of the Sn⁴⁺ in the SnO₂ lattice can be replaced by Nb⁵⁺ to form a stable solid solution. For the samples with Nb contents below the capacity, the formed solid solution structure can induce the formation of a large quantity of stable and active surface deficient oxygen species due to charge imbalance and lattice defects, which improve the CO oxidation activity remarkably. For the samples with Nb contents above the capacity, the excess Nb is present as free Nb₂O₅ in the catalysts, which is harmful to their activity. It is concluded that Nb, as an effective promoter for SnO₂, must be incorporated into its lattice as Nb⁵⁺ to form a solid solution. A Sn-Nb solid solution without excess Nb₂O₅ is not only a promising catalyst itself, but also a good support for precious metals to prepare catalysts for CO oxidation.

Full text of DOI:10.1039/c5cy01870f

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ARTICLE
Elucidating the Promotional Effects of Niobia on SnO for CO
2
Oxidation: Developing XRD Extrapolation Method to Measure the
Lattice Capacity of Solid Solutions
Cite this: DOI:
1
0.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Xianglan Xu, Fang Liu, Xue Han, Yuanyuan Wu, Wenming Liu, Rongbin Zhang, Ning Zhang and
Xiang Wang ∗
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of Sn-Nb binary catalysts have been prepared by co-precipitation method and used for CO oxidation. All the
catalysts show much higher specific surface areas than the individual oxides, indicating their improved thermal stability. It
5
+
4+
was found that Nb cations can be doped into the lattice of tetragonal rutile SnO
2
to replace part of the Sn to form solid
has been quantified, which proves
2
lattice can be replaced by Nb to form stable solid solution. For the samples with Nb
solution structure. With XRD extrapolation method, the SnO lattice capacity for Nb O
2
2 5
4
+
5+
that only 25% Sn in the SnO
contents below the capacity, the formed solid solution structure can induce the formation of a large quantify of stable and
active surface deficient oxygen species due to charge imbalance and lattice defects, which improves the CO oxidation
activity remarkably. For the samples with Nb contents above the capacity, the excess Nb is present as free Nb
catalysts, which is harmful to their activity. It is concluded that Nb, as an effective promoter for SnO , must be
is not only a
2 5
O in the
2
5
+
incorporated into its lattice as Nb to form solid solution. Sn-Nb solid solution without excess Nb
2 5
O
promising catalyst itself, but also a good support for precious metals to prepare catalysts for CO oxidation.
1
2-19
as CO oxidation, CH
4
deep oxidation
and NOx selective
2
reduction . It is revealed that with simple co-precipitation or
0
1
. Introduction
8
, 9, 21-23
13, 16
17
hydrothermal method, Fe, Cr, Mn
, Ce
cations can be doped into the crystal lattice of SnO
solid solution structure, which improves the activity as well as
thermal stability of the obtained catalysts. In addition, if SnO
is prepared into special morphologies, such as nano-rod, it
, Ta and Cu
Catalytic CO oxidation is an important reaction both for
fundamental understanding and air pollution control, which
has been studied intensively over the past decades. For room
temperature CO oxidation, Hopcalite catalyst, which consists
of Cu and Mn oxides, is an active and applicable catalyst, but
1
5
2
to form
2
1
4, 19
1
shows reaction behaviour similar to noble metals
.
suffers from quick moisture deactivation . Supported noble
Particularly, Sn-MFI catalysts, in which the Sn cations were
incorporated into the framework of MFI matrix, exhibits not
only significantly improved NOx-SCR activity, but also superior
metals, such as Pt, Pd, Ru and Au supported on metal oxides
are reported to be active for CO oxidation and resistant to
2
-4
water vapour deactivation . However, it is well known that
noble metals have limited resource and hence having high
price. Therefore, there is still strong motivation for people to
develop non-noble metals catalysts or suitable supports to
reduce the amount of noble metals for the catalysts but with
still competitive performance for CO oxidation.
2
0
resistance to both water and sulphur deactivation . The
kinetics of CO oxidation on Cu, Fe, Mn, Co-doped SnO
2
2
4
catalysts were also investigated by Madras and co-workers
.
The research on the catalytic application of niobium
compounds and materials has started to attracted people’s
attention since 1980s, and have been used as catalysts for
2
SnO is an n-type semiconductor with tetragonal rutile
2
5-27
5
various reactions of industrial importance
. In most cases,
structure and has a high melting point of 1630 °C . It
possesses both abundant metastable surface deficient oxygen
niobium oxide has been used as a catalyst additive, which can
generally improve the activity, selectivity and stability of the
modified catalysts. Jardim and co-workers reported that Nb
6
-9
and active lattice oxygen species Therefore, it is a potential
1
0, 11
catalytic material for air emission control
. Over past five
can dissolve into CeO
2
lattice matrix to form solid solutions,
years, our group has performed a series of systematic work to
understand its catalytic properties for various reactions such
which are better supports for Pt for CO preferential oxidation
Institute of Applied Chemistry, College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, P. R. China
Corresponding author. Tel: +86 15979149877, E-mail address: xwang23@ncu.edu.cn ( X. Wang )
∗
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28
in excess of H
2
than CeO
2
and Nb
2
O
5
. Moreover, Nb cations around 8.0. The mixture was then vacuum filtered. Afterwards,
were also found to be able to incorporated into the framework the precipitates were washed thoroughly with distilled de-
-1
of different molecular sieves to achieve catalysts with ionized (DDI) water until Cl free (detected by 0.1M AgNO
enhanced activity for CO oxidation or thioethers oxidation solution) and its total dissolved solids (TDS) were less than 20
3
2
9, 30
than the unmodified materials
. In addition, niobium oxide ppm. The achieved solids were dried at 110 °C overnight (~12
itself was also adopted as support for metals and metal oxides h), and then calcined at 500 °C in air atmosphere for 6 h, with
2
5-27
−1
to prepare solid catalysts
. In particular, the acidic property a heating rate of 10 °C min h to the target calcination
catalysts are
. Nakajima and co-workers used it as a catalyst named as SnNb5-1, SnNb4-2, SnNb3-3 and SnNb2-4, SnNb1-5,
of hydrated niobium oxide, niobic acid, has attracted much temperature. The finalized Nb-doped SnO
2
3
1, 32
attention
for several organic reactions generally requiring acidic sites, according to the Sn/Nb molar ratios. For clarification, SnNb5-1
and found that niobic acid is a catalyst with water-tolerant denotes a catalyst with a Sn/Nb molar ratio of 5/1.
3
2
Lewis acidic sites . In addition, niobium oxide has certain
redox property and photosensitivity, which are used to of 5/1, was also prepared with impregnation and physical
For SnNb5-1, the optimal sample with a Sn/Nb molar ratio
3
3
34
prepare oxidation catalysts
respectively.
or photo catalysts
,
mixing method for comparison study, which were named as
SnNb5-1-IMP and SnNb5-1-PHY, respectively. In detail, SnNb5-
Up to date, only a few publications can be found on using 1-IMP was prepared by impregnating NbCl
modified by Nb as heterogeneous catalysts. Li and co- onto a SnO support calcined at 500 °C with incipient wetness
workers reported that with the combination of SnO and method, which was followed by drying at 110 °C and final
Nb , catalysts with improved acidity as well as better calcination at 500 °C for 6 h in air atmosphere. For the
performance can be achieved for ethylene oxide hydration
5
ethanol solution
SnO
2
2
2
2 5
O
3
5,
preparation of SnNb5-1-PHY, Nb
Furthermore, Cruz and co-workers studied the preparation °C were mixed together first and then grounded for about 60
of foordite SnNb and pyrochlore Sn Nb , and identified min. Afterwards, the solid mixture was calcined at 500 °C again
their structures with different characterization methods
2 5 2
O and SnO calcined at 500
3
6
.
2
O
6
2
2 7
O
3
7
.
.
in air atmosphere for 6 h to get the final catalyst.
3
8, 39
Their photocatalytic activity has also been investigated
However, to the best of our knowledge, SnO
for environmental catalytic applications, such as CO oxidation, precursor. PdCl
has not yet been reported. Our previous work testified that ratio of 80 mg Pd metal in 1mL DDI water to prepare stable
with suitable preparation method, Ta cations can be solution. Afterwards, certain amounts of SnO , SnNb5-1,
incorporated into the lattice matrix of SnO to form solid SnNb4-2 supports prepared in the last step were added into
solution structure, thus obtaining catalysts with significantly this PdCl solution. The mixture was then dried at 110 °C with
The supported Pd catalysts were prepared by traditional
2
modified by Nb impregnation method with aqueous PdCl
2
solution as the Pd
2
was first dissolved into DDI water with the
2
2
2
enhanced activity and water resistance for CO oxidation. As Nb constant stirring, and calcined again at 500 °C with the heating
and Ta are in the same group in the periodic table, Nb has rate is 10 °C min−1 in air atmosphere for 6 h to get the final Pd
some similar chemical properties to Ta. Therefore, with the catalysts with 2wt%Pd loading. The catalysts were named as
2
expectation to prepare even better catalysts for CO oxidation, 2% Pd/SnO , 2% Pd/ SnNb5-1, 2% Pd/SnNb4-2.
and investigate the modification effects of Nb on SnO , a series
2
of Sn-Nb solid solution catalysts with different Sn/Nb molar
ratios have been prepared. It is revealed that the optimal Sn-
Nb catalyst SnNb5-1, has higher CO oxidation activity than the
best Sn-Ta catalyst reported in our former work under the
same condition. In addition, to clarify the modification effects
2
.2. Activity evaluation
The catalysts were evaluated for CO oxidation with a U-shaped
quartz tube (ID = 6 mm) reactor in a down flow over 100 mg
catalyst. Typically, 0.3 - 0.4 mm catalyst particles were used for
activity evaluation. A K-type thermocouple was placed on top
of the catalyst bed with the thermocouple head point touching
the catalyst to monitor the reaction temperature. To measure
the light-off behaviours of the catalysts, all data were collected
with increasing the temperature. The volume composition of
2
of Niobia on SnO , an XRD extrapolation method has been
developed in this study to measure the lattice capacity of
metal oxide solid solutions. With various characterization
methods, the reasons accounting for the activity improvement
have been investigated and elucidated.
the feed gas was 3% CO, 3% O
2 2
and balanced by high purity N
-1
，with a flow rate of 30 mL min , which corresponds to a gas
hourly space velocity (GHSV) of 18,000 mL/(g cat h). The
reactants and products were analysed on-line on a GC9310 gas
chromatograph equipped with a TDX-01 column and a TCD.
2
. Experimental
2.1. Catalysts preparation
2
Nb-modified SnO samples with different Sn/Nb molar ratios Before analysis, the reaction at each temperature over the
were prepared with co-precipitation method. In detail, catalysts was stabilized at least 30 min to get steady state
-
1
-
aqueous ammonia solution (15 mol L ) was dripped slowly reaction data. The flow rate of the carrier gas H is 30 mL min
2
-1
1
into the NbCl
5
(99.9%, 0.5 mol L ) and SnCl
4
5H
2
O (99.5%, 0.5
.
-1
mol L ) ethanol solution mixtures under constant stirring until
the precipitation was complete, at which point the pH was
2
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.3. Catalyst characterization
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ARTICLE
2
conversion profiles of SnNb5-1 and SnNb4-2, the two Sn rich
samples, to lower temperature regions. The highest activity
was obtained on SnNb5-1, on which the temperatures
corresponding to 10%, and 100% CO conversion are about 50
Nitrogen adsorption-desorption of the samples were carried
out at 77 K on Micromeritics ASAP2020 instrument. The
specific surface areas of the samples were calculated using
Brunauer–Emmett–Teller (BET) method in the relative
2
and 70 °C lower than that on pure SnO , respectively. In
contrast, the CO conversion profile of SnNb3-3, the sample
with an equal amount of Sn and Nb, overlaps nearly with that
pressure (p/p
of the samples were calculated with Barrett-Joyner-Halenda
BJH) method. The average pore sizes of the samples were
0
) range of 0.05-0.25. The pore size distributions
of pure SnO
2
, indicating that it has a similar activity to
. Further increasing the amount of Nb has
(
individual SnO
2
obtained from the peak positions of the distribution curves.
Powder XRD patterns were recorded on a Bruker AXS
D8Focus diffractometer instrument operating at 40 kV and 30
mA with Cu target Kα-ray irradiation. Scans were taken with a
obviously negative effects on the activity of the achieved
catalysts, as testified by the degraded activity of the two Nb
rich samples, SnNb2-4 and SnNb1-5. However, it is noted here
that the activity of these two samples is still much better than
2
θ range from 10 to 90 ° and with a step of 0.03 °/s.
2 5
that of pure Nb O . This indicates that too much Nb in the Sn-
TGA-DSC experiments were carried out on a TA Q600
-1
Nb composite oxides is harmful to the activity of the resulted
catalysts.
instrument in an air flow of 100 mL min . The pure and Nb
modified samples, which were dried at 110 °C overnight after
precipitation, were directly heated from room temperature to
9
00 °C with a heating rate of 10 °C /min.
-TPR experiments were carried out on FINESORB 3010C
auto chemisorption instrument with a 10% H /Ar gas mixture
H
2
2
flow, with the temperature being increased from room
temperature to 850 °C at a rate of 10 °C /min. Generally, 20 mg
catalysts were used for the test. A thermal conductivity
detector (TCD) was employed to monitor the H
For H consumption quantification, CuO (99.99%) was used as
the calibration standard according to the method described in
2
consumption.
2
1
8, 40
detail in references
.
XPS experiments were carried out on a PerkinElmer
PHI1600 system using a single Mg-Kα X-ray source operating at
300 W and 15 kV of voltage. The spectra were obtained at
ambient temperature with an ultrahigh vacuum. The binding
energies were calibrated using the C1s peak of graphite at
284.5 eV as a reference.
Fig. 1. CO oxidation on Sn-Nb catalysts with different Sn/Nb molar ratios
To further clarify the modification effects of Nb on SnO
and R , the differential reaction rates normalized by catalyst
weight and surface area, were collected on the samples at 200
C with CO conversions below 20% and compared in Table 1.
Both the R and R of SnO are much higher than those of
Nb , indicating further that SnO contains more active
catalytic sites, and is much more reactive than Nb for CO
oxidation. With the addition of a small amount of Nb, the R of
SnNb5-1 is significantly improved, which is more than four
times of pure SnO . In contrast, its R is also higher than that of
2
,
3
. Results and discussion
R
w
s
3
.1. Activity evaluation
To explore Nb doping effects on the catalytic performance of
SnO , CO conversion versus temperature plots on the Sn-Nb
composite oxides catalysts with different Sn/Nb molar ratios
are illustrated in Fig. 1. Pure SnO itself shows certain activity
°
w
s
2
2
2
O
5
2
2 5
O
2
w
for the reaction, on which CO complete conversion was
achieved at 340 °C. Previous works found that both surface
2
s
deficient oxygen and bulk lattice oxygen species of SnO
2
are
. In comparison, bulk
shows very low activity for the reaction, on which only
60% CO conversion was obtained even at 500 °C. This is due
to the lack of any active oxygen species on Nb , which will
be discussed in more detail in the following H -TPR studies.
is much more active than Nb for CO
1
3, 17
the pure SnO , indicating more active catalytic sites have also
2
active and contribute to CO oxidation
Nb
been formed in this catalyst, possibly due to the synergetic
2 5
O
interaction between Sn and Nb oxides. While SnNb4-2 shows
~
still higher R
lower than that of SnO
addition, the activation energies on SnNb5-1, SnNb4-2 and
SnNb3-3 samples are close to that of pure SnO . In contrast,
w
, the R
w
of SnNb3-3, SnNb2-4 and SnNb1-5 are
2
O
5
2
, especially for the latter two. In
2
Apparently, pure SnO
oxidation.
2
2 5
O
2
the activation energies on SnNb2-4 and SnNb1-5, the Nb rich
catalysts, are evidently higher than those on the Sn-rich
Interestingly, the addition of a small amount of Nb into
SnO can improve the activity of the obtained catalysts
2
samples and pure SnO
2
. These results demonstrate again that
significantly, as evidenced by the apparent shifting of the CO
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the presence of too much Nb in the samples is in fact binary samples, both individual SnO
detrimental to the active sites of the samples. On the other smaller pore volumes, which are composed of relatively larger
hand, these facts also prove that SnO should be the active pores. With the combination of SnO and Nb , it is evident
2 2 5
and Nb O have much
2
2
2 5
O
component for Sn-Nb binary catalysts, and Nb acts mainly the that the pore volumes of all the composite oxide catalysts
role as a promoter. Therefore, with the addition of too much become larger, and the pore sizes become smaller. Apparently,
Nb
destroyed by the excess inert Nb
activity of the Nb rich catalysts. On the contrary, with the be favourable for the diffusion of the reactants and products.
addition of a small amount of Nb , the activity of the initial However, we believe this texture structure change is still not
active sites can be promoted remarkably, as evidenced by the the major reasons accounting for the activity improvement.
reaction rates on SnNb5-1. In conclusion, Nb is a good
promoter for SnO to prepare catalyst for CO oxidation if the
2
O
5
, the initial active sites based on SnO
2 2 2 5
could be the combination of SnO and Nb O resulted in catalysts with
2 5
O
, hence damaging the larger amount of smaller pores in the structure, which could
2 5
O
2 5
O
2
addition amount is not excess.
Table 1 Physicochemical properties of Sn-Nb catalysts with different Sn/Nb
molar ratios
R
w
R
s
E
a
Pore
Pore
Size
Samples
S
BET
2
[mmol (s [mmol (s
(kJ/mol)
[d]
[a]
Volume
-1
2
-1
(
m /g)
3
gcat)
m cat)
(
cm /g)
(nm)
-
4
[b]
-6 [c]
×
10 ]
×10 ]
SnO
2
28
113
142
99
0.087
0.13
0.13
0.14
0.13
0.13
0.062
9.7
4.1
3.7
5.0
4.9
4.4
6.1
2.66
9.51
11.01
3.01
40.4
40.5
40.3
39.2
58.9
57.4
74.2
SnNb5-1
SnNb4-2
SnNb3-3
SnNb2-4
SnNb1-5
12.44
4.27
1.67
1.69
88
0.069
0.020
0.079
0.020
0.0053
96
2
Nb O
5
20
0.0011
[
[
[
a]
:
Calcined at 500 °C for 6 h in air.
b]
Differential reaction rate measured at 200 °C and normalized by catalyst weight.
Differential reaction rate measured at 200 °C and normalized by catalyst surface
c]
area.
[
d]
a
E : apparent activation energy.
3
.2. N
To clarify the reasons for the promotional effects, the texture
properties of the catalysts were subjected to N adsorption-
2
adsorption-desorption measurements
2
desorption measurements, with the isotherms and pore
diameter distribution profiles shown in Fig. S1, and the
quantification results in Table 1. The specific surface areas of
2
calcined at 500 °C are 28 and 20 m /g,
pure SnO
2
and Nb
2
O
5
respectively. With the addition of Nb into the SnO
2
, the surface
Fig. 2. XRD patterns of the Sn-Nb catalysts with different Sn/Nb molar ratios. (a)
calcined at 500 °C, (b) calcined at 1000 °C (except for SnO and Nb at 500 °C)
areas of all the resulted samples are significantly improved,
which are much higher than that of the two individual oxides.
SnNb5-1, the most active catalyst in this study, has a surface
2
2 5
O
2
2
area of 113 m /g, which is about 30 m /g lower than that of
3
.3. XRD analysis
SnNb4-2. In addition, although the surface areas of SnNb2-4
2
and SnNb1-5 are 60 - 70 m /g higher than that of SnO
2
, their XRD patterns of the samples, which were calcined at 500 °C,
, only the diffraction
phase can be observed, as
catalyst under general condition, it is not the determining indicated by the three strongest peaks at 2θ of 26.60 °, 33.78 °
CO oxidation activity is much lower. Therefore, it is concluded are shown in Fig. 2(a). For pure SnO
that although surface area is important for the activity of a feature of tetragonal rutile SnO
2
2
4
1
reason for the activity of Sn-Nb composite oxides. The tailoring and 51.78 ° . For pure Nb
of the active sites on the sample surface by Nb addition could of 22.60 °, 28.52 ° and 36.58 ° prove the formation of the
2 5
O , the three strongest peaks at 2θ
play a more critical role here.
crystalline tetragonal phase of Nb
4
The pore sizes and pore volumes of the samples were also
measured and listed in Table 1. In comparison with Sn-Nb individual oxides are well crystallized during the calcination,
2
O
5
(JCPDS card n. 28-0317)
2
. The sharp diffraction peaks testify that both of the
4
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which is in agreement with their low surface areas. However, the high temperature calcination process, which is in line with
for all the Sn-Nb binary samples, only largely broadened their much improved surface areas than that of the individual
diffraction peaks of rutile SnO
in the XRD patterns of SnNb2-4 and SnNb1-5, the two Nb rich diffraction peaks of rutile SnO
samples, no any diffraction peaks related to niobium oxide can partly owing to the less contents of SnO
be seen, indicating that Nb cations could be incorporated into Obviously, the combination of Sn and Nb oxides can obtain
the lattice of SnO to form solid solution structure, or the catalysts with significantly improved thermal stability, which is
2
phase can be detected. Even if SnO
2
and Nb
2
O
5
. With the increase of Nb amount, the typical
phase become less intensive,
in the samples.
2
2
2
excess niobium oxide could be highly dispersed on the surface important for an air emission control catalyst, especially for
of the resulted catalysts, thus escaping the detection by XRD. those high temperature processes.
2
Obviously, niobium, as the additive for SnO , impedes the
crystallization of the Sn-Nb binary catalysts effectively during
Table 2 Physicochemical properties of Sn-Nb catalysts calcined at 1000 °C
Mean
(
110)
(101)
2θ (°) d (Å)
S
BET
2
Phase
Samples
Crystallite
Size (nm)
a
(
m /g)
Composition
I/I
0
2
θ (°)
d (Å)
SnO
2
26.60. 3.349 33.78 2.651
16.2
13.9
13.2
14.7
18.4
6
Rutile SnO
2
-
SnNb5-1 26.64 3.343 33.98 2.636
SnNb4-2 26.68 3.340 34.00 2.635
SnNb3-3 26.65 3.342 34.02 2.633
SnNb2-4 26.66 3.341 33.94 2.634
30
30
26
19
Rutile Sn-Nb solid solution
0.000
0.168
0.825
1.973
2 5
Rutile Sn-Nb solid solution+Nb O
Rutile Sn-Nb solid solution +Nb
2
O
O
5
5
Rutile Sn-Nb solid solution +Nb
2
a
I/I
0
: the ratio of the peak intensity of Nb
O
2 5
(001) at 22.60 ° to that of rutile SnO
2
(110) at 26.60 ° in Figure 2b.
samples increase, while the
d
values decrease. This
To further identify the structure and phase compositions demonstrates clearly that Nb cations have been doped into
of the samples with different Sn/Nb molar ratios, the samples the matrix of tetragonal SnO in the samples to replace part of
2
4
+
were thus calcined at 1000 °C for 4 h to get better the Sn cations to form stable solid solution structure. Since
crystallization, and then subjected to XRD measurements, with the most stable oxide for Nb under general conditions is
5
+
the results shown in Fig. 2(b). Again, for SnNb5-1, only Nb O , there is high possibility that Nb is present as Nb in the
2
5
tetragonal rutile SnO phase can be observed but with better lattice. In this study, XPS results have indeed testified that in
2
5
+
crystallinity, indicating Nb cations can be doped into the lattice the Sn-Nb samples, Nb is predominantly present as Nb
of SnO to form solid solution structure. In contrast, starting cations in the SnO lattice or as Nb O on the surface of the
2
2
2 5
from SnNb4-2, Nb O phase can also be observed, whose peak catalysts, which will be discussed in more detail later.
5
2
43, 44
intensity increases also with the Nb amount. Considering the
According to former publications
, for two cations to
fact that all the Sn-Nb catalysts were calcined at the same effectively form solid solution, they must obey the following
temperature, it is reasonable to deduce that the amount of rules:
free Nb O crystallite increases. Based on the change of the
2
(R -R )/R <30% ( R1 and R2 are the radii of the two
1 2 1
5
diffraction features of the samples with different Sn/Nb molar cations, and R >R )
1
2
ratios, there is the possibility that Nb cations first dissolve into
the matrix lattice of rutile SnO2 to form solid solution two elements)
structure. After reach the capacity, the excess Nb O could In the history, tolerance factor, a calculation method
X -X ≤0.4
(X and X are the electronegativity of the
1 2
1
2
2
5
2
-
then disperse finely in the catalysts, thus evading the XRD based on the ionic radii of the metal cations and O anion in
detection when calcined at 500 °C. However, with the the structure has been proposed to identify the possibility of
increasing of the calcination temperature to 1000 °C, the finely them to form solid solution, or the stability of the formed solid
4
5, 46
dispersed Nb O became better crystallized, and then captured solution
5
.
2
4
+
by the XRD measurements.
Sn cation has a CN of 6 in tetragonal rutile SnO , whose
2
5
If Nb cations were doped into the lattice of SnO to form radius is 0. 69 Å. It is very close to 0. 64 Å of Nb cation with a
+
2
3
+
4+
solid solution structure, either as Nb , Nb or Nb
5+
5
CN of 6 . The electronegativity of Sn and Nb elements are
，
5
generally, 2θ shift and d value change should be observed X =1.96 and X =1.60 , respectively. Therefore, the two cations
1
2
4
+
since the radius of Sn with a coordination number (CN) of 6 match the basic condition to form solid solution. However,
3
in the structure of rutile SnO is 0.69 Å, while that of Nb , Nb
+
4+
since the initial crystal structure and valence states of the two
2
5
+
5
and Nb with a CN of 6 is 0.72 Å, 0.68 Å and 0.64 Å . For cations are different, there is high possibility for them to form
clarification, the 2θ and d values of peak (110) and (101) of low degree solid solution with certain capacity.
SnO phase for the modified and un-modified samples are also
2
identified carefully and compared in Table 2. Apparently,
compared with pure SnO , the 2θ values of all the Sn-Nb binary
2
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2
2
than the surface area of 6 m /g for individual SnO calcined at
the same temperature. This demonstrates further that
niobium oxide, either in the lattice or on the surface of the
catalyst, is of great help for the thermal stability of the Sn-Nb
binary catalysts.
3
.4. TGA-DSC measurements for the phase change and thermal
stability
To gain deeper understanding of the modification effects of Nb
on SnO
TGA-DSC techniques, with the results shown in Fig. 4. Pure
SnO shows majorly two weight loss stages. The first stage
with 4% weight loss and accompanied by an endothermic peak
at 90 °C is due to the loss of physically adsorbed H O. The
2
, several typical samples dried at 110 °C were tested by
2
2
second 8% weight loss stage accompanied by a weak
exothermic peak at 405 °C could be due to the crystallization
Fig. 3. The lattice capacity of rutile SnO
2
for Nb cations in Sn-Nb solid solution.
of SnO
1
2
, the loss of chemically bound H
2
O and hydroxyl groups
7
.
2 5
In contrast, pure Nb O shows two weight loss stages
accompanied by two endothermic peaks at ~98 and ~175 °C in
sequence, which can be assigned to the loss of physically
adsorbed (~2%) water and part of the crystal lattice water
Although former literatures have provided reliable
knowledge to discern if two metal cations can form solid
solution or not, it seems that no easily applicable experimental
method has been developed to identify the accurate lattice
capacity, which is defined as the maximum amount of guest
cations or anions doped into a host’s lattice to form stable
solid solution. Therefore, in this study, the XRD extrapolation
method, which has been developed by Xie and co-workers to
measure the monolayer dispersion capacity of a support for a
(~4%), respectively. Interestingly,
a strong and sharp
exothermic peak is evidently observed at 572 °C without any
49
weight loss, which is ascribed to the crystallization of Nb
As proved by XRD results, Nb cations have been
completely dissolved into the crystal lattice of SnO for SnNb5-
. Therefore, its TGA-DSC profile is very similar to that of pure
2
O
5
.
2
1
4
7, 48
2
SnO , which shows also two weight loss stages. Again, the first
supported metal oxide active component
adopted to quantify the SnO lattice capacity for Nb
detail, the intensity of the strongest diffraction peak of Nb
, has been
~
3% weight loss stage is due to the loss of physically adsorbed
O. The second ~7% weight loss stage could also mainly due
to the crystallization of Sn-Nb solid solution and loss of
chemically bound H O and hydroxyl groups, which gives an
2
2 5
O
. In
H
2
2 5
O
(001) at 22.60 ° (I) was normalized by the intensity of the
strongest peak of rutile SnO
2
(110) at 26.60 °, which is used as
ratios for the Sn-Nb
contents. The I/I ratios are then
contents of the catalysts to get Fig.
, in which the title of x-axis is the Nb content in per gram
in the samples. Based on this, the extrapolated lattice
in 1.0 g SnO , which equals to a Sn/Nb
molar ratio of 3/1. In other words, in the crystal lattice of rutile
2
overall weak exothermic effect. Interestingly, for SnNb2-4
sample, besides the typical weak exothermic peak at ~345 °C
an internal standard (I
catalysts with different Nb
plotted against the Nb
0 0
) to get I/I
2
O
5
0
2
for SnO or Sn-Nb solid solution, another evident exothermic
2
O
5
peak at 677 °C, which is not accompanied by any weight loss, is
observed. Apparently, this peak can be assigned to the
3
2 5
O
of SnO
2
crystallization of the excess Nb
2 5
O in the catalyst. However,
capacity is 0.3 g Nb
2
O
5
2
2 5
compared with pure Nb O , the crystallization temperature is
4
+
5+
about ~100 °C higher, possibly due to the influence from the
formed Sn-Nb solid solution. After all, this demonstrates
SnO
2
, only 25% Sn cations can be replaced by Nb . As
can be
mentioned above, starting from SnNb4-2, free Nb
2 5
O
2 5
further that on this sample, excess and inert Nb O is present
observed in the Sn-Nb binary samples, and its diffraction
intensity increases with Nb amount. Therefore, it is reasonable
on in the catalyst, thus diluting the active sites based on the
Sn-Nb solid solution. The same thing should occur for SnNb1-5,
which contains even more Nb. This is in agreement with the
XRD results, and provides additional evidence to account for
the fact that too much Nb has negative effect on the CO
oxidation activity of the Sn-Nb binary catalysts.
to deduce that after the amount of Nb cations in SnO
2
lattice
2 5
reaches the 25% capacity, free Nb O will be present on the
surface of Sn-Nb solid solution, which could block the active
sites based on Sn-Nb solid solution, and damage the activity of
this catalyst. For clarification, the phase compositions of the
samples with different Sn/Nb ratios are summarized in Table 2.
The surface areas of the samples calcined at 1000 °C are
2
also listed in Table 2. The highest surface area of 30 m /g is
achieved with SnNb5-1 and SnNb4-2, which is much higher
6
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Fig. 4. TGA-DSC profiles of Sn-Nb catalysts with different Sn/Nb molar ratios.
its inert lattice oxygen species starts to get reduction at 800 °C
2
, which explains its low activity for CO oxidation.
9, 31
3
.5. H -TPR and XRD analysis of the redox behaviours
2
Compared with individual SnO , SnNb5-1, SnNb4-2 and
2
To understand the impact of Nb modification on the redox
SnNb3-3, the three Sn rich samples, show more complicated
reduction behaviour in the high temperature region (500-800
properties of the catalysts, H -TPR experiments were
2
performed, with the profiles shown in Fig. 5. For clarification,
°
C) with multiple peaks. Whereas, SnNb2-4 and SnNb1-5, the
the quantitative H consumption results are also listed in Table
2
two Nb rich samples, display only a much smaller reduction
peak at 595 °C. Since Nb-O bonds are too potent to be reduced
below 800 °C, it is believed that all these reduction peaks
should be contributed by the reduction of tin oxides to
3
.
The major reduction of pure SnO occurs at 646 °C, which
2
4
0 50
has been assigned to the reduction of SnO to metallic Sn
2
.
The quantification results in Table 3 also reveal an O/Sn atomic
metallic Sn in the samples. Therefore, the H uptake amounts
2
ratio of 2.0/1.0, which fits to the stoichiometry of SnO and
2
of the samples are quantified and the O/Sn atomic ratios of
the samples are calculated by subtracting the Nb amount. As
shown in Table 3, with the increasing of Nb content, the H2
also confirms this assignment. Pure Nb O shows no reduction
2
5
peak in the tested temperature region, indicating the lack of
any active oxygen species on its surface. It was reported that
Institute of Applied Chemistry, College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, P. R. China
Corresponding author. Tel: +86 15979149877, E-mail address: xwang23@ncu.edu.cn ( X. Wang )
∗
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consumption amount of the catalysts keeps on decreasing.
To elucidate this, SnNb5-1, the typical sample in this
Moreover, the O/Sn atomic ratios become lower and deviate study, was reduced at different temperatures and measured
more and more from 2.0/1.0, the stoichiometry of SnO . This by XRD, with the results shown in Fig. 6. For the sample
implies that part of the Sn cations in the Sn-Nb binary reduced at 500 °C for 30 min, besides the initial SnO , a new
catalysts become more difficult to be reduced and is non- crystalline phase, pyrochlore Sn Nb , is evidently observed
, indicating Sn cations can only be reduced to Sn at this
2
4
+
2
2
2 7
O
3
6
4+
2+
reducible below 800 °C.
temperature and forms pyrochlore tin niobates. For the
sample reduced at 600 °C for 30 min, while a certain amount
of SnO can still be detected, the amount of pyrochlore
2 7
Sn Nb O becomes obviously larger, and a small amount of
2
2
metallic Sn starts to appear. For the sample reduced at 850 °C
for 30 min, the initial SnO disappears completely, and only
metallic Sn and pyrochlore Sn Nb phases can be observed
by XRD. Obviously, the presence of Nb cations in the Sn-Nb
2
2
2 7
O
4
+
samples can impede the complete reduction of Sn to metallic
Sn by forming pyrochlore Sn Nb . Therefore, it is not difficult
2
2 7
O
to understand that all the Sn-Nb catalysts have O/Sn molar
ratios lower than 2.0/1.0, and the ratio keeps on decreasing
with the increasing of Nb contents.
2
Fig. 6. XRD patterns of SnNb5-1 after H -TPR at different temperatures
Fig. 5. H
2
-TPR profiles of Sn-Nb catalysts with different Sn/Nb molar ratios.
It is particularly noted here that SnNb5-1 and SnNb4-2,
together with pure SnO , exhibit a low temperature peak at
300 °C, which can be assigned to the reduction of the surface
2
~
6
, 7
deficient oxygen species on SnO
temperature peaks were enlarged and shown separately in Fig.
(b). The H consumption amount of this peak for each sample
is quantified and listed in Table 3. It was reported previously
that for pure SnO , this surface deficient oxygen species can be
mostly depleted if the calcination temperature is above 300 °C
2
. For clarification, the low
Table 3 H
2
-TPR quantification results of Sn-Nb catalysts
5
2
Low Temperature High Temperature
2
00-350 °C
500-800 °C
Consumption
(mmol/g)
13.20
O/Sn
Samples
H
2
Consumption
H
2
Atomic Ratio
2
(
mmol/g)
6
, 7
SnO
2
0.23
1.21
0.73
0.21
-
2.0/1.0
1.9/1.0
1.7/1.0
1.2/1.0
0.9/1.0
0.9/1.0
-
. Apparently, the incorporation of a small amount of Nb
cations into the matrix lattice of SnO to form solid solution
structure can stabilize this deficient oxygen species effectively,
as evidenced by the much larger H consumption amount for
SnNb5-1
SnNb4-2
SnNb3-3
SnNb2-4
SnNb1-5
11.10
2
7.80
4.40
2
2.10
SnNb5-1 and SnNb4-2. However, further increasing the Nb
amount is detrimental to this surface deficient oxygen species,
as testified by the results of SnNb3-3, SnNb4-2 and SnNb5-1,
the three Nb rich samples.
-
0. 62
Nb
2
O
5
-
-
8
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On Sn-Nb binary oxide catalysts, CO oxidation should
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follow Mars-van Krevelen mechanism, which involves the
consumption and regeneration of the surface and lattice active
oxygen species. The presence of a large amount of this active
surface deficient oxygen species in SnNb5-1 and SnNb4-2,
indeed, could be the major reasons accounting for their
enhanced CO oxidation activity. In contrast, SnNb3-3 has
similar amount of this surface deficient oxygen species to
2
individual SnO , therefore, they have close CO oxidation
activity. SnNb2-4 and SnNb5-1, the two Nb rich samples, lack
this surface deficient oxygen species, thus displaying much
2
lower activity than individual SnO .
To further clarify the intrinsic reasons influencing the
activity, the Rw (Table 1) versus the amount of surface
deficient oxygen species is plotted and shown in Fig. S2. It is
obvious that the differential rates are nearly proportional to
the surface deficient oxygen amount.
In summary, CO oxidation activity on the catalysts
correlates very well with the amount of the surface deficient
oxygen species. We thus believe it plays a critical role for the
CO oxidation activity of the catalysts.
3.6. XPS measurements of the surface compositions for the
catalysts
2
To further understanding the doping effects of Nb on SnO and
the surface composition of the Sn-Nb catalysts, XPS
experiments were performed for SnNb5-1, SnNb4-2, SnNb2-4,
2 2 5
together with individual SnO , Nb O and illustrated in Fig. 7.
The quantification results are also summarized in Table 4. It is
noted here that all the binding energies shown here were
calibrated by C1s internal standard.
Fig. 7(a) compares the O1s signals of all the samples. The
2
un-symmetric O1s peaks (~530 eV) of SnO and Sn-Nb catalysts
suggest the presence of multiple oxygen species with different
chemistry environments in the samples. In contrast, pure
Nb
suggests the only presence of surface lattice oxygen species.
Therefore, the un-symmetric O1s peaks of SnO and Sn-Nb
2 5
O displays one very symmetric peak at ~530 eV, which
2
catalysts are deconvoluted, fitted and quantified in Table 4.
According to former publications, the major O1s peak at ~530
2
eV can be assigned to the surface lattice oxygen of SnO (Olat)
5
1, 52
.
However, the smaller shoulder O1s peak at ~531 eV can
be assigned to the loosely bonded, surface oxygen species
5
1
(O
ads
)
. Based on this, the Oads/Olat ratios for the samples
were calculated and listed in Table 4. For pure SnO , the
ads/Olat ratio is 0.61. However, for SnNb5-1 and SnNb4-2, the
2
O
two Sn rich catalysts with improved CO oxidation activity, the
ratios increased evidently to 0.89 and 0.78, which is in
2
agreement with the H -TPR results, proving the formation of
more surface deficient oxygen species in the samples due to
the formation of solid solution structure. On the contrary, for
SnNb2-4, the sample with a degraded CO oxidation activity,
the ratio is 0.52, less than that of individual SnO
conclusion, XPS measurements provide additional evidence to
-TPR to demonstrate that the CO oxidation activity of the Sn-
2
. In
H
2
Fig. 7. XPS analysis of the catalysts
Nb binary catalysts relates closely to the amount of the surface
deficient oxygen species.
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Fig. 7(b) compares the Sn 3d signals of all the samples. According to literatures, this set of Nb3d doublet peaks can be
5
+ 55, 56
The ~495 eV and ~486 eV doublet peaks are ascribed to Sn assigned to Nb
53, 54
. Therefore, XPS provides strong evidence
4
3/2 and 3d5/2 for Sn , respectively
+
3d
. In addition, the peak to testify that Nb cations are in fact incorporated into the
5
as Nb to replace part of the
+
shapes are symmetric, which indicates the chemistry crystal lattice of rutile SnO
4
environment of Sn cations is uniform and excludes the original Sn to form solid solution structure. Due to charge
possibility of the presence of lower valence state Sn, such as imbalance and lattice defects, there is no doubt that more
2
+
2
+
Sn or metallic Sn. The results here are in line with the XRD surface deficient oxygen species will be produced. As a
and H -TPR results, indicating in the Sn-Nb binary catalysts, the consequence, those Sn rich samples, in which the Nb contents
2 5
Sn species is initially fully oxidized and present as Sn . are less than 25% lattice capacity, and without excess Nb O ,
2
4
+
Fig. 7(c) compares the Nb3d signals of all the samples. show significantly improved CO oxidation activity.
Typically, a set of doublet peaks at ~210 eV and ~207 eV are
2 5
observed for all the Sn-Nb samples and the individual Nb O .
Table 4 Quantified XPS Results of the Catalysts
O1s Binding
Energy (eV)
Sn3d Binding
Energy (eV)
Sn
Nb3d Binding
Energy (eV)
Sn/Nb
ΔE
ΔE
Molar Ratio
Samples
O
ads/ Olat
a
b
(eV)
(eV)
O
ads
O
lat
Sn 3d5/2
Nb 3d3/2
Nb 3d5/2
3d
3/2
SnO
2
531.33 530.63
0.61
0.89
0.78
0.52
-
495.24 486.74
495.24 486.79
495.40 486.94
495.19 486.73
8.50
8.45
8.46
8.46
-
–
–
–
-
SnNb5-1 531.43 530.61
SnNb4-2 531.34 530.54
SnNb2-4 531.29 530.55
209.90
209.84
209.79
209.66
207.10
207.04
206.99
206.96
2.80
2.80
2.80
2.70
6.0/1.0
0.7/1.0
11.0/1.0
-
2
Nb O
5
-
-
-
-
a
The binding-energy difference between the Sn3d3/2 and Sn3d5/2 peaks.
The binding-energy difference between the Nb3d3/2 and Nb3d5/2 peaks
b
As shown in Table 4, ΔE, the binding-energy difference oxide, both the surface composition and element distribution
between the Sn3d3/2 and Sn3d5/2 peaks, of the Sn-Nb binary of the prepared catalysts can be significantly modified, which
2
catalysts are smaller than that of the individual SnO , proving can alter the reaction performance of the catalysts evidently.
4
+
the change of the chemical environment of the Sn cations
after Nb introduction. This provides additional evidence to
2
show that Nb cations could enter the lattice of SnO to form
3
.7. The comparison study for samples prepared by different
solid-solution structure. In comparison, the binding-energy
difference between the Nb3d5/2 and Nb3d3/2 peaks for the Sn-
Nb samples is ~2.8 eV, which is about 0.1 eV larger than that of
methods
As discussed above, for those Sn-Nb samples with Nb
content higher than the capacity but not exceeding the
amount of Sn, the excess Nb is present as free Nb on
5
+
pure Nb
2 5
O , indicating the chemical environment of Nb
4
cations is also altered by the neighbouring Sn cations.
+
2 5
O
To understand the surface composition of the catalysts surface of the of the formed Sn-Nb solid solution, which is
with different Sn/Nb ratios, the surface Sn/Nb molar ratios of detrimental to the active sites and decrease the activity of the
the catalysts are also calculated and listed in Table 4. SnNb5-1, catalysts. To confirm this, two additional catalysts with a
the best catalyst in this study, has a Sn/Nb molar ratio of 6/1, Sn/Nb molar ratio of 5/1, the best formulation found in this
which is similar to the initial ratio for catalyst preparation. study, were intentionally prepared by impregnation (SnNb5-1-
5
+
However, due to the incorporation of Nb cations into the IMP) and physical mixing method (SnNb5-1-PHY). It is noted
+
4
SnO
2 2
lattice, Sn cations are thus slightly enriched on the here that the SnO used for sample preparation was first
catalyst surface. Interestingly, the surface Sn/Nb molar ratio of calcined in air atmosphere at 500 °C for 4 h.
SnNb4-2 is extremely low, which is only 0.73/1. According to
The phase compositions of the catalysts were first
the XRD results, in this catalyst, the Nb content has already measured by XRD, with the results shown in Fig. S3. Different
been over the lattice capacity of 25%, and microcrystalline from SnNb5-1 prepared by co-precipitation method, Nb
Nb particles are detected. Apparently, as we expected, the crystallites can be evidently detected in both SnNb5-1-IMP and
excess Nb should mainly disperse on the surface of the SnNb5-1-PHY, indicating niobium oxide is dispersed on the
formed Sn-Nb solid solution support. However, the surface surface of the SnO support. H -TPR results shown in Fig. S4
Sn/Nb molar ratio of SnNb2-4 increases significantly to 11/1. also prove that the redox behaviours of SnNb5-1-IMP and
Obviously, for this sample with much excess Nb , it seems SnNb5-1-PHY are different from that of SnNb5-1. Above 400
the large amount of Nb is now acted as the support, and °C, both SnNb5-1-IMP and SnNb5-1-PHY show a single
the formed Sn-Nb solid solution disperses onto its surface, reduction peak at ~660 °C, which is similar to that of individual
2 5
O
2 5
O
2 5
O
2
2
2
O
5
2 5
O
4
+
hence the Sn species is enriched on the catalyst surface. In SnO
2
and assigned to the reduction of Sn to metallic Sn. It is
summary, with the addition of different amount of niobium noted that the specific surface areas of SnNb5-1-IMP and
1
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2
SnNb5-1-PHY are 27 and 26 m /g in sequence, which are close supports than the individual SnO
to that of SnO
that both SnNb5-1-IMP and SnNb5-1-PHY exhibit lower CO rival performance could be obtained. In conclusion, Sn-Nb
2
. With these Sn-Nb supports,
2
. As a consequence, it is not surprised to find catalysts containing less amount of precious metals but with
oxidation activity than SnNb5-1, which is close to that of SnO
as displayed in Fig. 8.
2
,
solid solution oxides are not only better catalysts for CO
oxidation, but also better supports for precious metals.
After all, these results testify strongly that, Nb, as a
promoter for SnO , must be doped into the lattice of SnO as
2
2
5
+
4
. Conclusions
Nb to form solid solution, thus inducing the formation of
more surface deficient oxygen species and in turn improving
In this study, a series of Sn-Nb composite oxide catalysts have
been prepared by co-precipitation method and used for CO
oxidation. All the Sn-Nb binary catalysts show much higher
specific surface areas than the individual oxides, proving their
improved thermal stability. It was found by XRD and TGA-DSC
2 5
the activity of the catalysts. The presence of Nb O on the
catalyst surface could damage the active sites, and eventually
degrade the activity of the catalysts.
5
+
techniques that Nb cations can be doped into the lattice of
4
+
tetragonal rutile SnO
solution structure. With XRD extrapolation method, the SnO
lattice capacity for Nb has been quantified, which is 0.3 g
Nb /1.0 g SnO , equalling to a Sn/Nb molar ratio of 3/1. This
indicates that the original Sn cations in the rutile SnO
2
to replace part of the Sn to form solid
2
2 5
O
2
O
5
2
4
+
2
can
5
+
only be maximally replaced by 25% Nb to form stable solid
solution structure. For the samples with Nb content less than
the capacity, such as SnNb5-1, the formed solid solution
structure has induced the formation of a large quantify of
stable and active surface deficient oxygen species because of
2
charge imbalance and lattice defects, as evidenced by H -TPR
and XPS. As a consequence, the catalysts demonstrate
remarkably improved CO oxidation activity than the individual
SnO
the capacity but not exceeding the amount of Sn, the excess
Nb is present as free Nb on the surface of the catalysts,
2
. For those Sn-Nb samples with Nb content higher than
Fig. 8. CO oxidation activity on SnNb5-1 catalysts prepared by different methods.
2 5
O
which can damage the active sites and decrease the activity of
the catalysts. Comparison study on SnNb5-1 samples prepared
with impregnation and physical mixing methods, on which
3.8. The application potential tests of the catalysts
Nb
2 5
O crystallites are observed on the surface, also proves
For an air emission control catalyst, its stability is a critical
this. It is concluded that Nb, as an effective promoter for SnO
2
,
issue to determine its application potential. Therefore, SnNb5-
, the best catalyst in this study, was subjected to a stability
5+
must be incorporated into its lattice as Nb to form solid
solution structure. If it is present on the surface of SnO , it is
inversely harmful to its CO oxidation activity. Last but not the
least, Sn-Nb solid solution without excess Nb is not only a
1
2
test at 240 °C with a CO conversion around 80%. As shown in
Fig. S5, no deactivation is observed for the catalyst during a
2 5
O
1
00 hours’ test, indicating this catalyst is stable, and have the
potential for CO oxidation in real emission control. It was
formerly reported that SnO is a better support for precious
metals than those rigid supports, such as SiO or Al , to
, mainly due to the
strong interaction between precious metals and SnO support.
With SnO as the support, supported Pt and Pd catalysts with
improved activity and water resistance can be obtained.
However, individual SnO generally suffers from low surface
promising catalyst itself, but also a good support for precious
metals to prepare catalysts for air emission control.
2
2
2 3
O
1
0, 11, 57
prepare catalysts for CO oxidation
Acknowledgements
2
This work was supported by the Chinese Natural Science
Foundation (21263015, 21203088, 21567016), the Education
Department of Jiangxi Province (KJLD14005) and the natural
science foundation of Jiangxi Province (20142BAB213013),
which is greatly acknowledged by the authors.
2
2
area, which may restrict the activity of the prepared catalysts.
As reported above, Sn-Nb binary oxides, despite the Sn/Nb
molar ratio, always have much higher surface area than the
2
individual SnO . With the expectation to achieve catalysts with
1
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DOI: 10.1039/C5CY01870F
TOC
Elucidating the Promotional Effects of Niobia on SnO for CO Oxidation:
2
Developing XRD Extrapolation Method to Measure the Lattice Capacity
of Solid Solutions
Xianglan Xu, Fang Liu, Xue Han, Yuanyuan Wu, Wenming Liu, Rongbin Zhang, Ning
Zhang, Xiang Wang∗
With XRD extrapolation method, SnO lattice capacity for Nb O is quantified. Sn-Nb
2
2 5
2 5
solid solution without excess Nb O is promising for CO oxidation.
∗
Corresponding author. Tel: +86 15979149877, E-mail address: xwang23@ncu.edu.cn ( X. Wang )