Roles of niobium in the dehydrogenation of propane to propylene over a Pt/Nb-modified Al2O3catalyst

Article abstract of DOI:10.1039/d0nj04663a

The application of Pt-based catalysts in propane dehydrogenation (PDH) is restricted by low propylene selectivity, metal sintering and carbon deposition. It has been reported that the performance of catalysts is strongly related to the electronic and geometric properties of the Pt particles and the acidity of the support. Herein, we report that a niobium-modified alumina supported platinum catalyst exhibits high propylene selectivity and superior stability. A high conversion of 28.8% and a high propylene selectivity above 96% are achieved over the Pt/Nb1Al2-O catalyst for propane dehydrogenation at 600 C after 8 h in a fixed bed reactor. Consecutive dehydrogenation-regeneration operations showed that the stability of Pt/Nb1Al2-O was much higher than that of Pt/Al2O3. NH3-TPD revealed that the strong acidity is decreased, while the medium acidity is increased after introducing niobium into Al2O3, which may be attributed to the formation of new acid sites, probably Lewis acid sites from niobium oxide species. CO-adsorbed FTIR spectroscopy, CO chemisorption, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) results revealed the enhancement of electron density and the decrease of dispersion of Pt particles after the introduction of Nb, which is beneficial for inhibiting the by-reactions (carbon formation and C-C bond cleavage). This journal is

Full text of DOI:10.1039/d0nj04663a

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PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Roles of Niobium on the Dehydrogenation of Propane to
Propylene over Pt/Nb-modified Al2O3 Catalyst
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Tianhao Zhao§a, Shanshan Shen§a, Yanyan Jiab, Chih-Wen Paoc, Jeng-Lung Chenc, Yong Guoa, Xiaohui
Received 00th January 20xx,
Accepted 00th January 20xx
Liua, Sheng Dai*b, Yanqin Wang*a
DOI: 10.1039/x0xx00000x
The application of Pt-based catalyst in propane dehydrogenation (PDH) was restricted by low propylene selectivity, metal
sintering and carbon deposition. It is reported that the performance of catalyst is strongly related to the electronic and
geometric properties of the Pt particles and the acidity of the support. Herein, we report that niobium-modified alumina
supported platinum catalyst exhibits high propylene selectivity and superior stability. A high conversion of 28.8 % and a high
propylene selectivity above 96 % are achieved over Pt/Nb1Al2-O catalyst for propane dehydrogenation at 600 °C after 8 h
in a fixed bed reactor. Consecutive dehydrogenation-regeneration operations showed that the stability of Pt/Nb1Al2-O was
really higher than that of Pt/Al2O3. NH3-TPD reveals that the strong acidity is decreased, while the medium acidity is increased
after introducing niobium in Al2O3, which may be attributed to the formation of new acid sites, probably Lewis acid sites
from niobium oxide species. CO-adsorbed FTIR, CO chemisorption, X-ray photoelectron spectroscopy (XPS), and X-ray
absorption spectroscopy (XAS) reveal that the enhancement of electron density and the decrease of dispersion of Pt particles
after the introduction of Nb, which is beneficial for inhibiting the by-reactions (carbon formation and C−C bond cleavage).
1. Introduction
As mining shale gas technology matures, many efforts have stability of Pt particle is closely related to the support,18,19 and
been dedicated to find efficient catalysts for propylene in addition, the acidity of support also influences the
production via propane dehydrogenation (PDH).1-8 However, performance, because higher acidity will lead to serious coke
the PDH reaction is highly endothermic and high reaction formation.20, 21
temperature is essential, which would result in the major
Alumina is widely used as the carrier in Pt-based catalyst in
challenges of undesired reactions, such as hydrocarbon cracking PDH reaction due to its high thermal stability and large surface
and coke formation.9,10 Compared with other noble metals, Pt area.19,22 However, unsatisfactory catalytic performance was
is the only noble metal utilized in commercial PDH process due got over the solely Pt/Al2O3 catalyst because of the catalyst
to its superior activity in C−H bond activation and low activity acidity of Al2O3 and metal sintering. In order to reduce support
for C−C bond cleavage.11,12 However, the selectivity to acidity and suppress the sintering of Pt particles, Sn, In, Cu and
propylene still need to be enhanced and the coke formation as alkaline metals (Li, Na, and K) are introduced as auxiliary agents
well as the sintering of Pt particles need prevented.13-16 It is into Pt/Al2O3 and higher catalytic stability was got over these
found that the electronic state and geometric properties of Pt catalysts.21,23,24 So, suitable promoters are necessary for
have an effect on the reaction.9,12 The higher electron density traditional Pt/Al2O3 to get excellent catalytic performance.
of Pt is benefit to weakening the adsorption of olefins, thereby
Nb-containing catalysts show excellent performance in the
reducing coke formation. The small particle sized Pt favors the oxidative dehydrogenation (ODH) of alkanes.25-28 For example,
dehydrogenation reaction, but it is also beneficial to some Lemonidou et al. developed a kind of Ni-Nb-O oxide catalyst,
undesired side reactions, such as cracking of C-C bond, which is highly active and selective in ODH.26 MoVTeNbO
isomerization, and coke formation.17 So, there is still no general catalysts, prepared by López Nieto et al., showed high
consensus regarding the suitable dispersion and particle size of selectivity in the ODH of ethane.27 Lewis acid was proven to be
platinum for PDH process. It is reported that the dispersion and benefit for the dehydrogenation of isobutane via promoting the
C-H activation and accelerating H2 desorption, thus leading to
an improved reactivity.29,30 So, we propose that the Nb-
aShanghai Key Laboratory of Functional Materials Chemistry, Research Institute
containing catalysts, which were reported as catalysts with high
Lewis acidity,31 would be a promising candidate for achieving a
good performance in PDH process.
In this work, niobium was introduced into traditional Pt/Al2O3
catalyst by two-steps. First, the NbAl-O supports with different
Nb/Al ratios were synthesized by a co-precipitation method,
and then the Pt/NbAl-O catalysts were prepared by an incipient
wetness impregnation method. It was found that, the selectivity
of Industrial Catalysis, School of Chemistry and Molecular Engineering, East
China University of Science and Technology, 130 Meilong Road, Shanghai,
200237, P. R. China. E-mail: wangyanqin@ecust.edu.cn
bKey Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint
Research Center, Institute of Fine Chemicals, School of Chemistry & Molecular
Engineering, East China University of Science and Technology, Shanghai
200237, P.R. China. Email: shengdai@ecust.edu.cn
cNational Synchrotron Radiation Research Centre, Hsinchu, 30076, Taiwan.
§These two authors contributed equally.
† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/
x0xx00000x
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to C3H6 was improved greatly after introducing Nb into Al2O3. using an Agilent 725ES. A Micromeritics ASAP 2020 M sorption
After systematic characterizations, we find that, compared with analyzer was used for the nitrogen adsorption desorption
View Article Online
DOI: 10.1039/D0NJ04663A
Pt/Al2O3, the Pt/NbxAly-O catalysts have higher Pt electron measurements. Before measuring at −196 °C, the samples were
density and the smaller amount of low-coordinated Pt, which degassed at 200 °C for 6 h. The specific surface area was
could reduce coke formation, weaken the deep C-C bond calculated based on the Brunauer-Emmett-Teller (BET)
cracking and enhance the catalysts stability. In addition, the equation. The CO chemisorption was performed on a Huasi
decrease of the strong acidity of the catalyst also inhibits carbon DAS-7200 automatic chemisorption system at room
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deposition.
temperature (CO: Pt = 1:1).
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H2 temperature-programmed reduction (H2-TPR) was
conducted on a Huasi DAS-7200 automatic chemisorption
system. Before each measurement, to remove the physically
adsorbed species on the surface of the catalyst, the samples
were pretreated by using pure Ar at 150 °C for 1 h. After cooling
down to room temperature, the samples were heated to 800 °C
with a 10 °Cmin−1 ramp in 10% H2/Ar flow.
NH3-TPD was carried out in an apparatus (PX200, Tianjin
Golden Eagle Technology Co. Ltd.) equipped with a thermal
conductivity detector (TCD). The detailed experimental
procedures can refer to our previous work25 except to change
the weight of samples from 100 mg to 200 mg.
The diffuse reflectance infrared Fourier transform spectra
(DRIFTS) of CO-adsorption were measured on a Nicolet Model
iS-50 FT-IR spectrometer equipped with an MCT/A detector. For
all of the CO-adsorption DRIFTS measurements, the catalyst was
reduced in situ in a flow of 5 % H2/Ar at 600 °C and then flushed
in Ar flow for 10 min at 600 °C, followed by cooling to 25 °C.
Then, a flow of CO (30 mL/min) was introduced into the catalyst
cell, and the DRIFTS were collected.
X-ray photoelectron spectroscopy (XPS) was performed on a
Thermo Scientific Escalab 250 Xi system with Al Kα radiation
operated at 250 W. The C 1s level at 284.6 eV as an internal
standard was used to calibrate the binding energy. X-ray
adsorption fine structure measurements (XAFS) were
conducted at the beamline 44A of Taiwan Photon Source,
National Synchrotron Radiation Research Center in Taiwan.
XAFS measurements were conducted for the Pt L3-edge in
fluorescence mode. Before the measurements of XPS and
XANS, the samples were reduced at 600 °C for 1 h under 5 %
H2/Ar, cooled down to room temperature, then purged with N2
for 1h, and finally transferred to bottles and sealed in glove box
for use.
2. Experimental
2.1 Materials and methods
Materials
Propane, C3H8 (99.9%) was obtained from Shanghai Chunyu
Special Gas Co., Ltd. Ammonium niobate(V) oxalate hydrate was
bought from Dibo Reagent Co., Ltd. Aluminum nitrate
nonhydrate were purchased from Sinopharm Chemical Reagent
Co., Ltd. Pt (NO3)2 was bought from Heraeus Materials
Technology Shanghai Co., Ltd. NaOH was obtained from
Shanghai Titan Scientific Co., Ltd. All chemicals were of
analytical grade and used without further purification.
Synthesis of Pt/Al2O3 and Pt/NbAl-O and catalysts
The supports were prepared by precipitation method. In a
typical synthesis, Ammonium niobate(V) oxalate hydrate and
Al(NO3)3·9H2O were dissolved in 50 mL of water, respectively.
Then the solutions of aluminum nitrate and ammonium
niobium oxalate were simultaneously dropped into a 250 mL
beaker and the system was heated in a 70 °C oil bath. The pH of
the system was maintained at 7 by continuously dropping the
aqueous solution of NaOH. After stirring and heating for 1 h, the
solid formed was filtered, washed with distilled water and dried
at 50 °C for 12 h and 100 °C for 12 h, respectively. After calcining
at 600 °C, the NbxAly-O supports were obtained, where x/y
indicates the mass ratio of Nb/Al.
The γ-Al2O3 support was prepared in the same way by using
Al (NO3)3·9H2O as the only precursor.
Incipient wetness impregnation method was adopted for the
catalyst preparation. In a typical case, a certain amount of
deionized water was added to 0.4 mL of the aqueous solution
of Pt (NO3)2 (0.025 g/mL Pt). Subsequently, 1 g of support was
added to the above solution and stirred to obtain the precursor.
The Pt-based samples were obtained after drying (50 °C for 12
h, and 100 °C for 12 h) and further calcination at 450 °C for 5 h
in air.
The electron paramagnetic resonance (EPR) spectra were
measured at −196 °C by using a Bruker A300 spectrometer.
Thermogravimetric analysis (TG/DTA) was performed on a
PerkinElmer Pyris Diamond under air with a heating rate of 10
°Cmin−1.
C3H6-TPD was carried out on a Huasi DAS-7200 automatic
chemisorption system with MS to determine the desorption
energy of C3H6. Prior to C3H6 adsorption, the sample (0.2 g) was
reduced with 5 % H2/Ar at 600 °C for 1 h, and C3H6 (99.5%) was
adsorbed for 1 h after cooling to 50 °C. After purging He (30
mL/min) for 2 h, the temperature was ramped up to 800 °C with
a rate of 10 °C/min.
Material characterization
Powder X-ray diffraction (XRD) patterns were recorded with a
D8 Focus diffractometer by using Cu Kα1 (λ = 0.15406 nm)
radiation. The operation condition was 40 kV and 40 mA.
Scanning transmission electron microscopy (STEM)
characterization was performed using ThermoFisher Talos
F200X. High angle annular dark field (HAADF)-STEM images
were recorded using a convergence semi angle of 11 mrad, and
inner- and outer collection angles of 59 and 200 mrad,
respectively.
Catalytic reactions
The PDH reaction was conducted in a continuous flow fixed-bed
reactor with a 6 mm inner diameter. Prior to the test, 0.11 g of
catalyst with 40−60 mesh was in situ activated at 600 °C for 1 h
Inductively coupled plasma atomic emission spectrometry
(ICP-AES) was used to determine the Pt, Nb and Al content by
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with H2 (H2/N2=1:4) under atmospheric pressure. After
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
reduction, the reactant gas mixture (C3H8/H2/N2=1:1:3) was
introduced through the catalytic bed at 30 mLmin-1 and the
propane weight hourly space velocity (WHSV(C3H8), grams of
propane fed per gram of catalyst per hour) is 6 h-1. The PDH
process was conducted at 600 °C under atmospheric pressure,
and the reactor effluent was separated in a phase separator and
analyzed by online gas chromatography equipped with a flame
ionization detector (FID). The blank experiment was
investigated by running reactions at normal temperature, and
the influence could be neglected. The conversion of propane
(C3H8) and selectivity to the products (CxHy: C3H6, C2H6, C2H4, and
CH4), were calculated using the following equations:
 -Al2O3
 -b2O5





Nb2O5


Nb1Al1-O
Nb1Al2-O
Nb1Al4-O
Nb1Al8-O
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

Al2O3
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80
2 
3
8
3
8
[
]
[
]
퐹C H 푖푛 ― 퐹C H
Conv (%) =
표푢푡 ∗ 100
(1)
3
8
[
]
퐹C H 푖푛
Figure 1. XRD patterns of various supports.
[
]
푛푖 ∗ 퐹푖 표푢푡
Sel (%) = 푛 ∗ 퐹 표푢푡 ∗ 100
(2)
∑
[
]
푖
푖
where i represents the hydrocarbon product in the effluent gas,
ni is the number of carbon atoms of component i, and Fi is the
corresponding flow rate.
3. Result
The XRD diffraction was used to analyze the phase composition
of the samples with different Nb content, and the results are
shown in Figure 1. It is found that Al2O3 shows broad peaks at
39.5°, 45.8°, 66.8° and 85.0°, assigning to the γ-Al2O3 phase (PDF
No. 29-0063), while the Nb2O5 shows diffraction peaks centered
at 22.6°, 28.4°, 36.6°, 46.1° and 55.1°, which corresponds to TT-
Nb2O5 (pseudohexagonal Nb2O5) phase (PDF no. 30-0873).
Interestingly, the NbAl-O oxides show amorphous phases
except Nb1Al8-O, which is close to γ-Al2O3 phase. The absence
of the diffractions of γ-Al2O3 and TT-Nb2O5 suggests that the
coordination and crystallinity of oxides were changed after
mixing Nb and Al together. After loading with Pt, the XRD
patterns are also given and present in Figure S1. It is hardly seen
the diffraction peaks of Pt in Figure S1, indicating that very small
Pt particles are dispersed on all supports.
Figure 2. Typical HAADF-STEM images of (a) Pt/Al2O3, (b)
Pt/Nb1Al2-O. The insets show the size distribution of the two
catalysts.
The composition of various catalysts was measured by ICP
and the data are collected in Table 1. It shows that the mass
ratio of element of the resultant catalysts is in line with the
initial composition. N2 adsorption was used to measure the
surface area and the information were also summarized in
Table 1. It shows the specific surface area first increases and
then decreases. For Pt/Nb1Al2-O, the specific surface area is
157 m2g-1.
Table 1. Element contents, specific surface area and Pt dispersion.
Pt
(wt%)
Al
(wt%)
Nb
SBET
(m2g-1)
D.P.
(%)
da
(nm)
Catalysts
(wt%)
1%Pt-Al2O3
1%Pt-Nb1Al8-O
1%Pt-Nb1Al2-O
1%Pt-Nb1Al1-O
1%Pt-Nb2O5
0.94
0.93
0.92
0.89
0.93
53
38
29
24
-
-
247
270
157
100
25
68.4
52.3
54.7
35.2
27.9
1.65
2.16
2.06
3.21
4.05
4.9
16
22
68
Notes: a: d means the particle size of Pt estimated from dispersion. The calculation formula is d (nm)=1.13/dispersion.
This journal is © The Royal Society of Chemistry 20xx
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HAADF-STEM imaging was utilized to investigate the Pt particle (after 3 reaction cycles), which explainsDtOhIe: 10d.e10c3r9e/aDs0eNJo0f4C663H3A8
size and dispersion of the two representative catalysts. As shown in conversion and increase of C3H6 selectivity over Pt/Al2O3. For
Figure 2, tiny Pt nanoparticles showing higher Z-contrast were Pt/Nb1Al2-O, the Pt particle sizes are 7.70 nm and 9.53 nm after
observed over Pt/Al2O3 and Pt/Nb1Al2-O samples, indicating the high 1 reaction cycle and 3 cycles, respectively. The slower particle
dispersion of Pt particles in these two catalysts. The average size of aggregation means the particle stability is improved by
Pt particles of Pt/Al2O3 and Pt/Nb1Al2-O is 1.30 nm and 1.50 nm, introducing Nb. However, the anti-sintering ability of Pt/NbAl-O
respectively. To measure Pt dispersion, CO-pulse adsorption was still required to be strengthened by adding other promoters.
employed and the results were also collected in Table 1. It is found
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that the dispersion of Pt on Pt/Al2O3 catalyst was 68.4 %, while it is
1%Pt-Nb1Al1-O
54.7 % over Pt/Nb1Al2-O catalyst
.
The reduction of the supported Pt catalysts, as well as the
interaction of Pt with support was examined by using H2-TPR
technique, and the results were present in Figure 3. It can be
seen two reduction peaks appeared over all catalysts. The
former one appears at between 200 to 300 °C, while the latter
exists between 350 to 450 °C. These two peaks are assigned to
(i) the reduction of weakly interacted Pt species, and (ii) the
reduction of Pt species interacted strongly with alumina
support.32-35 Both two peaks slightly shift to higher temperature
with increasing Nb content, suggesting that interaction was
enhanced after introducing niobium, which would help to curb
the sintering of Pt particles and improve the catalyst stability.
The PDH reaction was carried out at 600 °C in a fixed reactor
and the results over above catalysts were shown in Figure 4 and
Figure S2. Figure 4a shows the time-on-stream of the propane
conversion over different catalysts. The highest conversion was
got over Pt/Al2O3 catalyst, which is around 32 % after 8 h.
However, low selectivity (< 80 %) to C3H6 and poor stability were
observed over Pt/Al2O3 catalyst (Figure 4b). It is reported that
the smaller particle sized Pt and higher metal dispersion favor
the dehydrogenation reaction, but they are also beneficial to
some undesired side reactions, such as the cracking of C-C bond
and coke formation. In addition, the strong acidity of Al2O3
would also lead to coke formation, this will be discussed later.
Hence, a high C3H8 conversion but low C3H6 selectivity was got
over Pt/Al2O3 catalyst. For Nb-containing catalysts, although the
conversion decreases with the increase of Nb content, the
selectivity is higher and reached 96 % over Pt/Nb1Al2-O catalyst.
So, we finally choose Pt/Nb1Al2-O as the best catalyst, because
of the high stability (deactivation rate parameter, κd=0.0520),
higher conversion (28.8 %) and highest selectivity (96.0 %)
(Table S1). The following characterizations are also focused on
Pt/Nb1Al2-O and Pt/Al2O3 catalysts. The selectivity to by-
products (CH4, C2H6 and C2H4) was given in Figure S3 have shown
that C-C bond cleavage is greatly reduced (CH4 yield from 10%
over Pt/Al2O3 to 3% over Pt/Nb1Al2-O) after the introduction of
Nb. The catalytic performance during consecutive
dehydrogenation-regeneration operations over Pt/Nb1Al2-O
and Pt/Al2O3 catalysts was also carried out and present in Figure
5. It is found that the activities both decreased, but compared
with Pt/Al2O3, better catalytic stability and higher C3H6
selectivity were still obtained over Pt/Nb1Al2-O catalyst. The
characterization performed on the spent catalyst (Figure S4-7)
indicated the particle sintering for both Pt/Al2O3 and Pt/Nb1Al2-
O. During the reaction, the Pt particles of Pt/Al2O3 gradually
aggregate to 8.65 nm (after 1 reaction cycle) and 12.17 nm
1%Pt-Nb1Al2-O
1%Pt-Nb1Al4-O
1%Pt-Nb1Al8-O
1%Pt-Al2O3
100
200
300
400
500
600
700
800
Temperature (0C)
Figure 3. H2-TPR profiles of Pt/Al2O3 and Pt/NbxAly-O catalysts.
Pt/Al2O3
50
Pt/Nb1Al8-O
Pt/Nb1Al2-O
Pt/Nb1Al1-O
40
Pt/Nb2O5
30
20
10
0
1
2
3
4
5
6
7
8
T (h)
100
90
80
70
60
50
Pt/Al2O3
Pt/Nb1Al8-O
Pt/Nb1Al2-O
Pt/Nb1Al1-O
Pt/Nb2O5
0
1
2
3
4
5
6
7
8
T (h)
Figure 4. The conversion of C3H8 (a) and the selectivity to C3H6 (b)
over various catalysts for PDH. Catalytic conditions: atmospheric pressure,
600 °C, C3H8/H2/N2 = 1/1/3, the total flow rate is 30 mL/min. WHSV of propane = 6
h−1 and 110 mg of catalyst.
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respectively.36 The NH3-TPD profiles in Figure 6 and the
calculated amount of each acid site listed in Table S2, both show
that the amounts of strong acids decrease, while the medium
acids increase significantly after introducing Nb into the Al2O3.
The increased medium acids sites is Lewis acid come from NbOx
species as shown in Py-FTIR (Figure S8). It is well accepted that
the strong acid sites of the catalyst can promote the formation
of coke.5,12 The conversion of propane looks higher over
Pt/Al2O3 catalyst, which would be due to the deep cracking to
smaller molecules and the coke formation. While Pt/Nb1Al2-O,
which has less strong acid sites and higher medium acid sites, is
benefit for C-H activation, but poor for carbon deposition and
C-C bond cracking, thus improving the selectivity to propylene
and enhancing the catalytic stability.
To further compare the carbon deposits on two catalysts, TG
was used to analyze the amount of coke after PDH reaction at
600 °C for 8 h (shown in Figure S10) and found less weight loss
in Pt/Nb1Al2-O (13.8 wt%) than that in Pt/Al2O3 (17.9 wt%),
indicating the doping of niobium in Al2O3 really suppressed
coke deposition.
The properties of Pt nanoparticles play key roles in PDH, so,
CO-adsorbed FTIR was used to investigate its coordinated
states. The DRIFTS spectra for CO adsorption on these three
catalysts (Pt/Al2O3, Pt/Nb1Al2-O and Pt/Nb2O5) are given in
Figure 7 and Figure S11, respectively. It can be seen that the CO
adsorption peaks are existed at 2000 cm−1 ~ 2100 cm−1 for all
three catalysts, which can be attributed to CO species linearly
absorbed on the Pt atoms. According to the previous literature,
the little peak of both two catalysts around 2080 cm-1 attributes
to the very high coordinated Pt sites, which was reported not
favor the dehydrogenation reaction.12,37 For Pt/Nb2O5, it could
be seen that there is a very large part of highly coordinated Pt
sites, which was reported not favor the dehydrogenation
reaction and the by-reactions (cracking of C-C bond,
isomerization, and coke formation), so even though the
selectivity to propylene is high, the conversion is very low. To
compare the differences between Pt/Al2O3 and Pt/Nb1Al2-O
catalysts, the linearly adsorbed CO curves were fitted and
separated into four peaks. (1): CO adsorption on very highly
coordinated Pt sites at some low index plane. (2): CO adsorption
on high index plane of small clusters. (3) and (4): CO adsorption
on low coordinated Pt sites, such as steps, edges, corners and
defect sites.9,38,39 It could be found that after introducing Nb
into the support, peak 4 (low coordinated Pt sites) decreased
from 26.6 % to 14.7 % with the amount of peak 2 (high index
plane of small clusters) increased. DFT calculations by Yang et
al. indicate that the low coordinated Pt favors the
hydrogenolysis and the side reactions (coke formation and
hydrocarbon cracking).37 These results mean that the doping of
niobium decreased the low coordinated Pt and would prevent
the hydrocarbon cracking and coke formation.
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C3H8 Conversion of Pt/Nb1Al2-O
C3H6 Selectivity of Pt/Nb1Al2-O
C3H8 Conversion of Pt/Al2O3
C3H6 Selectivity of Pt/Al2O3
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T (h)
Figure 5. The C3H8 conversion and C3H6 selectivity over
Pt/Nb1Al2-O and Pt/Al2O3 catalyst during consecutive
dehydrogenation-regeneration operations.
Reaction conditions: T=600 ℃, WHSV=6 h-1, N2:H2:C3H8=3:1:1. After each
PDH run, the catalyst was regenerated by burning coke at 600 ℃ in flowing
air for 2 h and then by H2 reduction at 600 ℃ for 1 h.
weak acid
medium acid
strong acid
Pt/Al2O3
Pt/Nb1Al2-O
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Temperature (0C)
Figure 6. NH3-TPD profiles of Pt/Al2O3 and Pt/Nb1Al2-O catalysts.
4. Discussion
It was reported that beside the properties of Pt, the acidity of
the catalyst also influences the catalytic performance, so the
influence of acidity was first discussed. NH3-TPD was carried out
to measure the amount of acid and acid distribution, which
were shown in Figure 6. It is well known that the peaks of NH3
desorption around 120-200 °C, 200-300 °C and above 300 °C are
regarded as the weak, medium, and strong acid sites,
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Pt foil
PtO2
(a)
1： 12.2%
2： 36.4%
3： 24.9%
4： 26.6%
1% Pt/Al2O3
2
Pt/Al2O3
Pt/Nb1Al2-O
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11540 11550 11560 11570 11580 11590 11600
2200
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Wavenumber (cm-1)
Energy (eV)
Figure 8. Normalized XANES spectra at Pt L3-edge of the Pt/Al2O3,
Pt/Nb1Al2-O, Pt foil and PtO2 reference.
2
(b) 1： 6.8%
2： 52.2%
1% Pt/Nb1Al2-O
may be due to the slight oxidation of the samples. Compared
with Pt/Al2O3, the intensities of Pt on Nb1Al2-O are lower,
which means the higher electron density. It indicates that NbAl-
O support denotes electron to Pt, which may be due to the
increased oxygen vacancies (Ov) with the introduction of
niobium. Therefore, EPR measurements were employed to
explore the Ov of the catalysts, and the results were shown in
Figure S14. For these two catalysts, there is an obviously signal
with a g value of 2.003, which is attributed to the single electron
of O2− radical trapped by Ov.40,41 The existence of Nb facilitated
the generation of Ov, which transfer electrons to Pt and also
generate more Lewis acid sites, finally enhanced propylene
selectivity and prevent coke formation.
In order to compare the adsorption strength of catalysts to
propylene, the adsorption of propylene on the catalysts were
examined by the means of C3H6-TPD, and the desorption
profiles were presented in Figure 9. The MS signal of TPD
measurements shows that the propylene adsorbed on both
Pt/Al2O3 and Pt/Nb1Al2-O can desorb at relatively low
temperature. But obviously, the desorption of C3H6 over
Pt/Nb1Al2-O catalyst shifts to a lower temperature compared
with that of Pt/Al2O3, which decreased from 99 °C to 80 °C.
More interestingly, the peak area of C3H6 desorption reduced a
lot on Pt/Nb1Al2-O, suggesting the weaker C3H6 adsorption. The
TPD characterization results are in line with the TG results.
Weaker interaction between the catalysts and C3H6 leads to the
less coke formation and higher catalyst stability.
3： 26.2%
4： 14.7%
3
1
4
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Wavenumber (cm-1)
Figure 7. CO-DRIFTs of the (a) Pt/Al2O3 and (b) Pt/Nb1Al2-O.
Besides support acidity and coordination state of Pt, previous
reports revealed that the electron density of the Pt-based
catalysts is the key for PDH process.9,36 To clarify the difference
of the surface states of Pt after introducing Nb, XPS analysis was
performed, but the signals of Al 2p and Pt 4f are overlapped
(Figure S12 and S13 and discussed therein). So XAS spectra were
used to identify the chemical states of Pt. Figure 8 shows the
normalized X-ray absorption near-edge structure (XANES)
spectra at Pt L3-edge for the catalysts and the reference spectra
of Pt foil and PtO2 powder. It can be seen that the intensities of
Pt in the catalysts are all slightly higher than that of Pt foil, which
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This work was supported financially by the NSFC of China (No.
21832002, 21872050, 21808063), the Fundamental Research
Funds for the Central Universities (222201718003) and the
Science and Technology Commission of Shanghai Municipality
(10dz2220500). Additional support was provided by the Feringa
Nobel Prize Scientist Joint Research Center.
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Pt/Al2O3
99 0C
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80 0C
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5. Conclusion
In summary, we prepared a novel catalyst by introducing Nb
into the traditional Al2O3 support for the PDH reaction, in which
the C3H8 conversion and the C3H6 selectivity reached 28.8% and
96.0% after 8 h, respectively. The function of Nb was
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the coke formation. CO-pulse adsorption and DRIFTS for CO
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coordinated Pt sites, which help to avoid the C-C bond cleavage
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increase of electron density of Pt particles which was proven by
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