CrystEngComm
Paper
measured by ICP, was 9.9%, 14.9% and 19.1%, respectively.
It seems that the lower V/Ti molar ratio favored the doping of
vanadia into the lattice of TiO2, whereas more vanadia spe-
cies existed on the surface of the samples at higher V/Ti
molar ratios, which is in accordance with the XRD patterns
in Fig. 5.
bipyramids were active for the selective reduction of NO by
ammonia.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21025312 and 21103177) and the
National Key Basic Research Program of China
(2013CB933100).
Fig. 8 shows the Raman spectra of the VOx/TiO2 samples.
The intensity of the symmetric stretching vibration of O–Ti–
O, typically shown at 144 cm−1 in anatase TiO2,24 gradually
decreased with increasing content of vanadia. This is caused
by the enhanced exposure of the {001} facet with increasing
VOx doping. The {001} facet of TiO2 is terminated with five-
fold Ti atoms (Ti5C) and two-fold O atoms (O2C), and such an
atomic configuration decreases the number of symmetric
stretching vibrations of O–Ti–O and hence weakens their
intensity.14,24 The broad peaks at 813 and 938 cm−1 were
assigned to the asymmetric stretching modes of V–O–V and
terminal VO.34–38 This means that surface vanadia exists
mainly as polymeric species. However, an additional peak at
993 cm−1 over the VOx/TiO2-18 sample appeared, indicating
the presence of crystalline V2O5.39–41 That is, most vanadia
species were incorporated into the TiO2 lattice for the
VOx/TiO2-10 sample, whereas more surface vanadia and a
small amount of crystalline V2O5 were formed on TiO2 trun-
cated bipyramids at higher vanadia contents.
Fig. 9 shows the NO conversions on the VOx/TiO2 trun-
cated bipyramids in NH3-SCR. The VOx/TiO2-10 catalyst
showed an NO conversion of 50% at 643 K and 80% at 703 K.
With further increasing vanadia content, the catalytic activity
increased significantly; the VOx/TiO2-18 catalyst provided an
NO conversion of 50% at 433 K and 90% at 476 K. The reac-
tion rate measured at 473 K (Table 1) increased from 2.3 ×
10−8 mol g−1 s−1 over the VOx/TiO2-10 sample to 1.7 × 10−6
mol g−1 s−1 on the VOx/TiO2-18 catalyst, while the correspond-
ing turnover frequency, based on the actual loading of VOx,
increased remarkably from 2.1 × 10−5 to 8.9 × 10−4 s−1. Since
crystalline V2O5 is usually less active for NH3-SCR,42 the
higher activity of the VOx/TiO2-18 catalyst might be related to
the presence of surface vanadia species on the TiO2 trun-
cated bipyramids. The relatively lower activity of the
VOx/TiO2-10 catalyst was caused by the incorporation of
most vanadia species into the TiO2 lattice.
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Anatase TiO2 truncated bipyramids with preferential exposure
of the reactive {001} facet were successfully synthesized using
only vanadia as the structure-directing agent. The exposure of
the {001} facet approached 53% upon increasing the amount
of vanadia, which strongly adsorbed on the reactive facet and
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