CHEMCATCHEM
FULL PAPERS
tungsten. With 5 wt.% tungsten loading, a highly effective cat-
alytic activity of the Pt-W/ZSM-5 catalyst can be obtained and
its TOF is more than 1 order of magnitude higher than that of
the Pt/ZSM-5 catalyst. The improvement in the catalytic activity
of the Pt-W/ZSM-5 catalyst should be related to the interaction
between platinum and tungsten. The presence of tungsten
can result in the presence of metallic platinum in Pt-W/ZSM-5
catalysts, and metallic platinum is scarcely found on the Pt/
ZSM-5 catalyst without tungsten. Therefore, the addition of
tungsten to Pt/ZSM-5 catalysts can increase the oxidation re-
sistance of platinum by an electron transfer from tungsten to
platinum at the W–Pt interface based on the interaction be-
tween platinum and tungsten oxide, which is responsible for
the improvement in the catalytic activity. In addition, the pres-
ence of platinum reduces the aggregation of tungsten oxides
and improves the reducibility of tungsten oxides.
tious carbon. The BE of the target elements (Pt4f, W4f, O 1s, and
C 1s) was determined at a pass energy of 25 eV. The XPS spectra
were fitted with XPS Peak 4.1 software using a properly weighted
sum of Lorentzian–Gaussian line shape after background subtrac-
tion according to Shirley’s method.
The platinum content in the catalyst was measured by using ICP
on a Vanan 710 instrument. Platinum dispersion was determined
through pulsed CO chemisorption at 358C, in which Ar (flow rate
À1
3
0 mLmin ) was used as a carrier gas and a pulse of 10.22 vol%
CO/Ar (0.5173 mL) was injected. Before CO uptake determination,
À1
all samples were pretreated in H (50 mLmin ) at 4008C for 2 h
2
À1
and then flushed with Ar (30 mLmin ) for 30 min. An adsorption
stoichiometry of Pt/CO=1 was assumed while determining plati-
num dispersion.
The TPR of the catalyst was performed in a conventional flow
system equipped with a thermal conductivity detector (TCD). The
catalyst (100 mg) was used. The reducing gas was consisted of
À1
The Pt-5W/ZSM-5 catalyst also demonstrates excellent stabil-
ity. No decrease in catalytic activity can be observed after the
online test for 50 h with temperature alternating between
10 vol% H in Ar, and its flow rate was 45 mLmin . The reactor
2
À1
was heated from RT to 8008C, with a heating rate of 108Cmin .
TPD of NH adsorbed on the catalyst was performed in a conven-
3
2008C for 10 h and 5008C for 10 h; after online aging at 7008C
tional flow system equipped with a TCD. The sample was pretreat-
for 30 h, the Pt-5W/ZSM-5 catalyst still demonstrates better
catalytic activity than the Pt/ZSM-5 catalyst.
ed in N at 6008C for 1 h before adsorbing NH . The NH -TPD ex-
2
3
3
periments were performed at 50–6008C under N
flow (flow rate
2
À1
À1
3
0 mLmin ). The heating rate was 108Cmin .
Experimental Section
Catalytic activity testing
Catalyst preparation
The catalytic activity of the catalyst was measured in a fixed-bed
laboratory microreactor system. The catalyst (200 mg, 40–60 mesh)
was placed in a 10 mm quartz reactor tube. The feed gas was con-
sisted of 2000 ppm C H +2% O /Ar balanced, and its flow rate
Ammonium tungstate hydrate (0.066, 0.195, 0.328, or 0.459 g; Sino-
pharm Chemical Reagent Co., Ltd., China) and oxalic acid (0.2 g; Si-
nopharm Chemical Reagent, China) were dissolved in deionized
water (40 mL) under stirring. Then, aqueous Pt(NO3)2 solution
3
8
2
À1
was 100 mLmin . The reactants and products were analyzed
online by using a GC equipped with a TCD. The catalytic activities
were characterized by propane conversion (%), T , T , and T ,
(
0.598 g, 15.05 wt.%; Heraeus Materials Technology Shanghai Ltd.,
China) was added to that mixture and the solution was heated to
08C under stirring. Next, H-ZSM-5 (6 g; Nankai University Catalyst
10
50
90
8
which are the reaction temperatures for the propane conversion of
0, 50, and 90%, respectively. All carbon balances were calculated
2
À1
Co. Ltd.; Si/Al=50; BET surface area=310 m g ) was added to
1
the heated solution and the solution was stirred continuously at
and were in the range of 100Æ5%. Reaction rates and TOFs were
8
08C until water had dried up. Finally, the catalysts were dried at
measured in a differential mode with the propane conversion of 5–
110 8C for 12 h and calcined at 5008C for 3 h under static air in
1
5% by controlling the W/F ratio. Both external and internal mass
a
1
muffle furnace. The as-prepared catalysts consisted of
.5 wt.% Pt and 1.0–7.0 wt.% W, which were labeled as Pt-xW/
transport limitations can be excluded by varying the particle size
and W/F ratio (the ratio of catalyst weight to total flow rate). All
data were obtained after the reaction stabilizes for 60 min.
ZSM-5 (x=1, 3, 5, 7). A similar method was used to prepare
.5 wt.%Pt/ZSM-5 and 5W/ZSM-5 catalysts.
1
Acknowledgements
Catalyst characterization
We acknowledge the financial support from the National Basic
Research Program of China (2010CB732300 and 2013CB933201),
the National Natural Science Foundation of China (21171055),
the National High Technology Research and Development Pro-
gram of China (2011AA03A406), the “Shu Guang” Project
The surface areas of the catalysts were determined from the N ad-
2
sorption measurements at À1968C with a Micromeritics ASAP 2400
instrument by using the BET method. The samples were degassed
at 1808C for 12 h before measurement. The XRD patterns of the
catalysts were recorded on a Rigaku D/Max-RC diffractometer
using CuKa radiation (l=1.541 ꢂ) and operating at 40 kV and
(
10GG23) of Shanghai Municipal Education Commission and
4
0 mA. The laser Raman spectra of the catalysts were recorded on
Shanghai Education, and the Development Foundation of the
Fundamental Research Funds for the Central Universities.
a Renishaw inVia Reflex spectrometer equipped with a charge-cou-
pled device detector. The excitation source used was an Ar ion
laser (l=514.5 nm) with a spot size of approximately 1 mm. The
power of the incident beam on the catalyst was 3 mW.
Keywords: oxidation · porous materials · platinum · tungsten ·
The XPS spectra were recorded at 298 K on a Thermo Scientific ES-
CALAB 250Xi X-ray photoelectron spectrometer using AlKa radia-
tion (1486.6 eV) and operating at 150 W. All BEs were determined
with respect to the C 1s line (284.8 eV) originating from adventi-
ZSM-5
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
9
&
ÞÞ
These are not the final page numbers!
Zhu, Zengzan
