W.-J. Ji, C.-T. Au et al.
sites, may be helpful to further understand the reaction
mechanism.
Powder X-ray diffraction (XRD) was performed (2q: 10 to 808) on a Phi-
lips X’Pert Pro diffractometer with CuKa radiation (0.1541 nm). The size
and morphology of the as-synthesized samples were determined with a
JEOL JEM-1010 TEM at an accelerating voltage of 80 kV. HRTEM
images were taken using a JEOL JEM-2100 electron microscope operat-
ed at 200 kV.
Conclusion
The BET surface area was measured on a NOVA-1200 material physical
structure determinator based on the N
therms at 77 K. Prior to measurements, all samples were degassed at
73 K for 4 h. Elemental analysis of Au was performed using ICP-AES
on a J-A1100 Versa Probe spectrometer. The samples were dissolved in a
mixture of concentrated HCl and HNO3 with a volumetric ratio of 3:1
prior to analysis.
2
adsorption and desorption iso-
The present study demonstrates that the catalytic activities
of (101), (200), and (103) facets of Mn O nanocrystallities
3
4
5
in benzene combustion are clearly different. When Au is
loaded on these surfaces, the activities are notably enhanced
due to the synergetic effects arising from intense metal–sup-
port interactions between the electronically modified Au
nanoclusters and the substrate surfaces, resulting in the
weakening of MnÀO bonds at the Au–Mn O boundary. Re-
2 2 2
H -TPR measurements using a H –Ar mixture (4.98% H by volume)
were carried out with the sample installed in a U-shaped quartz reactor
connected to a thermal conduction detector (TCD); a set amount of
sample (50 mg) was used for each measurement. Before switching to the
3
4
action path A (Scheme 1), in which benzene on Mn O is
3
4
2
H –Ar stream, the sample was treated in an Ar stream at 1208C for 1 h
oxidized by the released oxygen from Mn O through a
and then cooled to RT. Temperature programming was performed in the
3
4
À1
5
0–5008C range at a rate of 108Cmin . Temperature-programmed de-
MVK mechanism as well as by the weakly adsorbed oxygen
species, is important for Au-free Mn O substrates, while re-
sorption (TPD) of O
actor. The catalyst (200 mg) was pretreated in an Ar stream at 5008C for
h, and oxygen adsorption proceeded with exposure to pure O at 2008C
2
was also carried out using the U-shaped quartz re-
3
4
action path B, in which benzene is oxidized by the lattice
oxygen around the Au–Mn O boundary, becomes more crit-
1
2
3
4
for 0.5 h. After cooling to room temperature, the sample was purged with
À1
ical for the Au-deposited Mn O . The oxygen species acti-
Ar (60 mLmin ) for 1 h. The sample was then heated to 8008C at a rate
3
4
À1
À1
of 108Cmin in the Ar flow (60 mLmin ), and the effluent gases were
vated on the Au NPs can spillover onto the Au–Mn O
3
4
analyzed by a TCD.
boundary and react with the neighboring benzene mole-
cules. This can be regarded as a modified version of path B.
À1
In situ IR spectra were recorded (in the 1400–3500 cm region, resolu-
À1
tion: 4 cm ) on a Bruker VERTEX70 spectrometer using the DRIFT
H -TPR, O -TPD and in situ FTIR characterizations were
2
2
technique. Before IR measurements, the sample was swept with air at
2008C for 1 h. Then, a gas mixture of 1000 ppm benzene plus balanced
dry air was continuously passed through the sample cell. The spectra
were collected at the designated temperatures.
used to elucidate the main reaction pathways described
above and further clarified the early presumption that the
metal–substrate boundary is the key region for the reaction.
Catalyst activity tests and kinetic measurements
The catalysts were evaluated at atmospheric pressure using a fixed-bed
quartz reactor. The sample (400 mg) was diluted with chemically inert
À1
SiC. Before reaction, the sample was kept in a flow of air (40 mLmin
)
Experimental Section
at 2008C for 1 h. Benzene vapor was carried by an air stream bubbling in
a benzene saturator stationed in an ice bath. The benzene-saturated air
was further diluted by another air stream before reaching the catalyst
bed. The concentration of benzene in the feed was controlled by adjust-
ing the flow rate of the air stream passing through the saturator. The
Catalyst preparation
Synthesis of Mn
(
3 4 3 2 2 2
O CPs: In a typical procedure, Mn ACHTUNGRTNEUN(G CH COO ) ·4H O
1.25 g) was dissolved in distilled water (100 mL). Then, ammonium hy-
droxide (2 mL) and distilled water (58 mL) were mixed and added to the
manganese acetate solution under continuous stirring. After that, NaOH
1m, 10 mL) was added dropwise to the above solution under stirring for
0 min. Then, the mixture was transferred into a 200 mL Teflon-lined
stainless steel autoclave and heated at 1608C for 12 h. The brown precipi-
tate was collected by centrifugation, washed several times with distilled
water and absolute ethanol, dried at 808C under vacuum for 3 h, calcined
at 3008C under air atmosphere for 3 h, and cooled to RT under nitrogen.
À1
total flow rate was approximately 40 mLmin . The temperature was
raised stepwise with an interval of 108C from 1508C to a temperature at
which complete conversion of benzene was achieved. The data presented
at each temperature are the averaged values of three measurements. The
unreacted benzene and products were analyzed online using a gas chro-
matograph equipped with a HP FFAP column (0.32 mm ꢂ 25 m) and a
flame-ionization detector. The permanent gas products were analyzed
online by another gas chromatograph equipped with a packed column of
Alltech Hayesep D HP and a TCD. The differences between the inlet
and outlet concentrations of benzene were used to calculate conversion
(
3
Synthesis of Mn
of Mn CPs, except without the addition of ammonium hydroxide and
with more NaOH solution (1m, 70 mL).
Synthesis of Mn OPs: The procedure is similar to that of the synthesis
of Mn HPs, except that a NaOH solution (1m, 70 mL) was replaced
by an ammonium hydroxide solution (1m, 70 mL).
Preparation of Au/Mn samples
3 4
O HPs: The procedure is similar to that of the synthesis
3
O
4
data. No carbon-containing product other than CO
carbon balance based on benzene and CO
out the use of a catalyst, the reactivity was negligible under the condi-
tions adopted in the present work.
2
was detected
with Æ3% accuracy). With-
(
2
3
O
4
3
O
4
3
O
4
Gold NPs were deposited on the as-synthesized manganese oxides of dif-
ferent morphologies with a nominal Au loading of 2 wt% by means of
Acknowledgements
[
18]
deposition–precipitation (DP), originally adopted by Haruta et al. In a
typical procedure, the Mn substrates were dispersed in an aqueous
solution of HAuCl at a fixed pH of 10. The suspension was aged at
33 K for 2 h. The solid was filtered out, washed several times with distil-
led water, and dried at 373 K in air for 10 h. Then the samples were cal-
cined at 573 K for 4 h in flowing N . The Au-containing catalysts are de-
noted herein as Au/Mn HPs, Au/Mn CPs, and Au/Mn OPs.
Characterization of catalysts
3
O
4
Financial support from the Ministry of Science and Technology of China
(2013AA031703) is greatly appreciated.
4
3
2
[
[
3
O
4
3
O
4
3 4
O
6486
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6480 – 6487
Fei, Zhao-Yang
