SiO2-surface-assisted controllable synthesis of TaON and Ta 3N5 nanoparticles for alkene epoxidation

Article abstract of DOI:10.1002/anie.201107216

Touching the surface: Well-defined TaON and Ta₃N₅ nanoparticles were fabricated by a SiO₂-surface-assisted urea method, in which silica catalyzes CN_x formation from urea. Such controlled nitridation endows the nanoparticles with improved activity and tunable selectivity for alkene epoxidation because of the increased electron density around Ta and surface basicity. Copyright

Full text of DOI:10.1002/anie.201107216

Angewandte
Chemie
DOI: 10.1002/anie.201107216
Functional Materials
SiO2-Surface-Assisted Controllable Synthesis of TaON and Ta3N5
Nanoparticles for Alkene Epoxidation**
Qingsheng Gao,* Sinong Wang, Yuchun Ma, Yi Tang, Cristina Giordano, and Markus Antonietti
Metal nitrides are promising materials because of their
prominent properties originating from metal–nitrogen
bonds and noble-metal-like electron features, such as thermal
stability, mechanical hardness, superconductivity, and high
catalytic performance in hydrocarbon conversion.[1] More
importantly, their bulk and surface properties are significantly
associated with the nitridation levels, which can be adjusted as
such. Taking tantalum (oxy)nitrides as the current example,
introducing N atoms into Ta2O5 brings a remarkably nar-
rowed and tunable band-gap energy, thus enabling novel
visible-light photocatalysts.[2] Meanwhile, the catalytic prop-
erties of Ta in tantalum (oxy)nitrides for nonphotocatalytic
reactions are also expected to be improved because of the
tailored nitridation and easier electron transfer from N (vs. O)
to Ta centers. However, there are still no reports on these
effects to the best of our knowledge. Significantly, such
modulation of material properties by controlled nitridation
provides a new strategy for designing novel catalysts.[2a,3]
The controllable synthesis of TaON and Ta3N5 nano-
particles (NPs) with tailored chemical composition is there-
fore an attractive goal,[2e,3a] which is, however, difficult to
achieve by current strategies because of easy overreactions of
nitridation.[4] For example, by-products of Ta3N5, Ta4N5, and
even TaN will be easily generated from further nitridation of
TaON during calcination. The traditional method using NH3
is limited for the danger associated with using NH3 at high
temperature and the complexity related to the rigid control of
gas composition, flow rate, and pressure.[4,5] Moreover,
although urea and cyanamide have been reported as safe
nitrogen sources for preparing tantalum nitrides, only impure
and deeply reduced TaN could be obtained, as an effective
control on reactions is absent.[6]
herein a new SiO2-surface-assisted strategy for the controlled
fabrication of TaON and Ta3N5 NPs, which successfully avoids
the disadvantage mentioned above. Assisted by SiO2, the
production of TaON over Ta3N5 NPs with tailored composi-
tion was achieved through calcination of Ta–urea (TaU) gels
with suitable urea/Ta ratios (RU/Ta). A mechanism is proposed
that urea is converted into carbon–nitride (CNx) species on
SiO2-surface at mild temperature,[8] which further acts as a
slow release N source for controlled nitridation. The elec-
tronic properties of Ta are tuned by the different nitridation
levels in TaON and Ta3N5 NPs, which significantly improves
the activity for alkene epoxidation, as compared to Ta2O5
NPs. This is the first time to discover that introducing N into
Ta2O5 can remarkably improve epoxidation activity because
of the easier electron transfer from N (vs. O) to Ta.
Furthermore, the controlled nitridation endows catalysts
with tunable surface basicity, noticeably influencing epoxida-
tion selectivity. Owing to the facile synthesis and prominent
catalytic behavior associated with tailored nitridation, we
believe that our effort will pave the way for designing
nanostructured catalysts of metal–nitrides.
As the key assisting agent, SiO2 NPs (5–15 nm, SBET
=
527.3 m2 gꢀ1) were added into TaU precursor, resulting in a
homogenous gel denoted as TaU/SN. After calcination under
N2 flow and following treatment with NaOH solution, highly
crystalline TaON and Ta3N5 NPs were obtained at RU/Ta values
of 1.5 and 3.0, respectively. These products were confirmed as
g-TaON (ICDD: 01-076-3258) and Ta3N5 (ICDD: 04-004-
4564) by their X-ray diffraction (XRD) patterns (Figure 1a).
Scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM) images (Figure 1b–e) show that the
products are NPs with size of 20–25 nm, which can be easily
dispersed in EtOH or water by ultrasonic agitation. The size
nicely fits with the calculated results from XRD analysis,
which reveal values of approximately 20.6 nm for TaON and
18.9 nm for Ta3N5. g-TaON can be well identified in high-
resolution TEM images (Figure 1c) by the clear lattice fringes
of (110) and (001), while for Ta3N5 the planes of (200) and
(110) are well observed (Figure 1e).
Recently, we discovered that the controlled release of N
source from urea is significant to control metal nitridation.[7]
As a further controlled version of urea pyrolysis, we present
[*] Dr. Q. S. Gao, Dr. C. Giordano, Prof. M. Antonietti
Department of Colloid Chemistry
Max Planck Institute of Colloids and Interfaces
Research Campus Golm, 14424 Potsdam (Germany)
E-mail: qingsheng.gao@mpikg.mpg.de
The colors of samples give further clear evidence about
their structures, as yellow TaON and red Ta3N5 NPs show
absorption bands around 500 and 600 nm, respectively, in UV/
Vis spectra (Figure S1 in the Supporting Information). This
result proves the narrowing of the band gap compared with
Ta2O5 as a result of nitridation. Moreover, NPs composed of
tunable TaON and Ta3N5 phases are easily obtained (Fig-
ure S2) by switching the RU/Ta value from 1.5 to 3.0, further
indicating the potential for their controlled synthesis.
S. N. Wang, Dr. Y. C. Ma, Prof. Y. Tang
Department of Chemistry and Shanghai Key Laboratory of Molec-
ular Catalysis and Innovative Materials
Fudan University, Shanghai 200433 (P.R. China)
[**] We thank the Max Planck Society, BMBF (035F0353A-E), the
National Science Foundation of China (20873028, 21073041), and
the Major State Basic Research Development Program
(2009CB623506) for financial support.
Several control experiments were carried out to examine
the mechanism underlying the controllable synthesis of TaON
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107216.
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be taken into account for the controlled generation of TaON
and Ta3N5 NPs.
It is well known that upon fast heating from 300 to 4208C,
urea decomposes to ammonia and isocyanic acid [Eq. (1)].[8]
Catalyzed by OH groups on silica, isocyanic acid is converted
into cyanamide, which can trimerize to form melamine
[Eqs. (2) and (3)].[8] The overall reaction is given in Equa-
tion (4).
300ꢀ420 ꢁ
C
ðNH Þ CO
HNCO þ NH
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ƒƒƒƒƒ!
2
2
3
½SiO2ꢂꢀOH
2 O¼C¼NꢀH
3 H NꢀCꢃN
H NꢀCꢃN þ CO
2
ƒƒƒƒƒ!
2
400 ꢁC
400 ꢁ
C
C N ðNH Þ
ƒƒƒ!
2
3
3
2 3
400 ꢁC=SiO2
6ðNH Þ CO
C N ðNH Þ þ 6 NH þ 3 CO
ƒƒƒƒƒƒ!
2
2
3
3
2
3
3
2
With further thermal treatment, as-formed melamine
polycondenses to CNx species with yellow melon and even s-
triazines.[9] The formation of CNx species from urea during
our calcination can be well confirmed by FTIR investigation.
After treating TaU/SN precursor at 5008C, the FTIR
spectrum (Figure 2a) clearly displays the characteristic
ꢀ1
ꢃ
bands associated with the vibrations of C N (2053 cm ), C-
N(-C)-C (1363 cmꢀ1) and s-triazine (1624 and 1556 cmꢀ1).[8a,10]
Similar observations for the process without SiO2 suggest that
ꢀ
the conversion from urea to CNx can be also achieved by Ta
OH. However, the above absorption bands in the presence of
SiO2 are blue-shifted relative to those without SiO2, which
implies that the interaction between the SiO2 surface and CNx
stabilizes the CNx species.[11] Moreover, the large surface area
of SiO2 NPs remarkably enhances such function. Because CNx
is available for metal nitridation through decomposition to
active N species,[12] its delayed decomposition on SiO2 surface
is significant for controlled nitridation. The different CNx
decompositions against heating with and without SiO2 can be
well reflected by the evolution of N content in TaU/SN and
TaU (RU/Ta = 3.0) treated from 500 to 7008C (Figure 2b). CNx
decomposition slows down with the assistance of SiO2. The
content of N is kept at the same level around 8.0 wt% under
6508C, and remarkably decreases to 6.97 wt% at 7008C,
indicating major decomposition around 7008C. However,
without SiO2, the N content is drastically and continuously
decreased from 500 to 7008C. The mole ratios of N/Ta also
present the consistent evolution. It should be pointed out that
the easier CNx decomposition in TaU will induce inhomoge-
neous N species atmosphere and consequent overreactions.[4]
Thus, the formation and stabilization of CNx on large SiO2
surface is important for TaON and Ta3N5 controlled synthesis
because of the suppression of N source release (Figure 2c).
Figure 1. a) XRD patterns of I) TaON and II) Ta3N5 NPs obtained by
the SiO2-surface-assisted urea method with RU/Ta values of 1.5 and 3.0.
The simulated patterns for g-TaON and Ta3N5 are also shown. b) SEM
and c) TEM images of TaON NPs, and d) SEM and e) TEM images of
Ta3N5 NPs. Insets of (b) and (d) shows photos of TaON and Ta3N5
NPs dispersed in EtOH.
and Ta3N5 NPs in the presence of SiO2. It was found that SiO2
is indispensable. Without SiO2, only bulky mixtures of TaON,
Ta3N5, and even Ta4N5 were obtained, indicating that the
controlled nitridation and nucleation relies on the large-
surface SiO2 template (Figure S3). Because of the easy
overreactions during nitridation, mixtures of TaON and
Ta3N5 were obtained with RU/Ta values ranging from 1.5 to
4.0 in the absence of silica. Ta3N5 becomes predominant when
the RU/Ta value is increased to 6.0, but unfortunately it is
accompanied by TaON and Ta4N5 as by-products. Further
experiments reveal that the specific surface area of SiO2 is
significant to its function to avoid overreactions. When bulky
SiO2 (ca. 500 nm) with a small surface area of SBET = 5.6 m2 gꢀ1
was used, the overreactions could not be avoided, and only
the mixture of TaON and Ta3N5 was received at RU/Ta values of
1.5 and 3.0, respectively (Figure S4). Therefore, the interac-
tions and reactions between the SiO2 surface and urea should
In this mechanism, the surface area of silica is the key
factor. If the same amount of mesoporous SBA-15 with larger
surface (SBET = 675.4 m2 gꢀ1) than SiO2 NPs is employed, pure
TaON NPs are obtained at a lower RU/Ta value of 1.0
(Figure S6), which suggests that more CNx can be stored on
the larger surface of SBA-15. When the RU/Ta ratio is increased
to 4.0 and 5.0, the deep nitridation product, Ta4N5, is
generated as a result of excessive CNx within SBA-15,
compared with pure Ta3N5 NPs obtained at the same ratios
using SiO2 NPs (see Figure S2). This result further validates
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Figure 3. Ta 4f XPS spectra of a) Ta2O5, b) TaON, and c) Ta3N5 NPs.
The Ta 4f peaks are shifted to lower energies and have increased N/O
ratio in TaON and Ta3N5 NPs relative to those of Ta2O5. The structures
in the insets of (a–c) present the octahedral coordination environment
of a) Ta2O5, b) TaON, and c) Ta3N5. Ta small black ball, O gray ball, N
large black ball.
introduction of N, these peaks are shifted to lower binding
energies of 25.7 and 27.6 eV in TaON, and 25.0 and 26.9 eV in
Ta3N5, respectively (Figure 3b,c). The noticeable red shifts of
about 0.8 and 1.5 eV in respective TaON and Ta3N5 are
believed to be due to the easier electron transfer from N than
O.[3a] Remarkably, the increased electron density around Ta
will endow such nanoparticles with improved catalytic
properties.
The oxides of group V are promising alkene oxidation
catalysts,[14] and available Ta catalysts for epoxidation have
recently attracted much attention because epoxides are
important intermediates in fine chemical production and
polymer synthesis.[15] Herein, we employed cyclooctene
epoxidation as a probe reaction for our TaON and Ta3N5
NPs, as well as Ta2O5 NPs (Table 1). All of these NPs possess
similar sizes (ca. 20 nm) and surface areas (ca. 20 m2 gꢀ1,
Table S1). Ta2O5 NPs exhibit a low conversion of 57.1%,
whereas, improved activity with higher conversion of 96.0%
and 96.5% was presented on TaON and Ta3N5 NPs, respec-
tively (Table 1, entries 1–3). In view of their similar surface
area, the improved activity of Ta in TaON and Ta3N5 should
be ascribed to the increased electron density around Ta
resulting from N introduction, as ascertained by XPS results
in Figure 3. As reported, the epoxidation activity of Ta is
associated with the step of [Ta·O*] reacting with alkene to
epoxides, and can be improved by effective electron-donor
ligands around Ta.[15c,d] In TaON and Ta3N5 NPs, the N atom
can serve as an electron donor and promotes the O* transfer
from [Ta·O*] to cyclooctene, consequently improving the
activity (Step I in Scheme 1). According to XPS results, Ta3N5
NPs with lower Ta 4f binding energies than TaON is expected
to be more active, and, correspondingly, its conversion of
65.4% is observed to be higher than that on the latter
(59.1%) when reaction time is shortened (Table 1, entries 4
and 5). Furthermore, the epoxidation selectivity is also
Figure 2. a) FTIR spectra of products obtained after treating I) TaU/SN
and II) TaU at 5008C. b) N content and N/Ta mole ratio evolutions of
TaU/SN and TaU with increasing temperatures. c) Illustration of SiO2-
surface-assisted mechanism for controllable synthesis of TaON and
Ta3N5 NPs through CNx-intermediate process catalyzed by SiO2.
the mechanism of the SiO2 surface for achieving controlled
nitridation. Additionally, such a mechanism was also found to
be general for other metals such as Zr, in which Zr2N2O NPs
are well obtained and overreaction ZrN is suppressed by SiO2
(Figure S7). We believe this will open up opportunities to
design various nanostructures of functional metal–nitrides.
Changing the nitridation level of tantalum (oxy)nitrides
provides a new approach to tune the electronic properties of
Ta atom, because of the lower electronegativity of N atoms
than O atoms. This situation is already indicated by X-ray
photoelectron spectroscopy (XPS, Figure 3). In Ta2O5 NPs,
the Ta 4f7/2 and 4f5/2 peaks appear at 26.5 and 28.3 eV
(Figure 3a), which correspond to Ta5+ in Ta2O5.[13] With the
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Table 1: Epoxidation of cyclooctene over Ta2O5, TaON, and Ta3N5 NPs[a].
selectivity. For example, the Ta3N5 NPs treated by 1m HCl
show the increased selectivity of up to 85.8% (Table 1,
entry 6). Conversely, the selectivity on Ta2O5 NPs cannot be
obviously varied by acid or base treatment (Table 1, entries 7
and 8), supporting again the surface base character derived
from N atom.
Entry
Catalyst
Conversion
[%]
Selectivity
[%]
The consistent performances of TaON and Ta3N5 NPs
obtained by another Ca-assisted urea method[7] (Table 1,
entries 9–11) further confirm the improved activity and
tunable selectivity significantly associated with their tailored
nitridation level. It should be noted that Ta3N5 NPs from Ca-
assisted processes are received after HCl washing to remove
Ca species, and they deliver epoxide selectivity of as high as
85.4%, which can be decreased to 74.9% after treating with
1m NH3·H2O (Table 1, entry 11). From the above results, it
can be concluded that introducing N significantly promotes
the Ta activity for epoxidation, and also generates tunable
basicity important to tailor the epoxide selectivity. Controlled
nitridation plays a key role in epoxidation, and, herein, TaON
NPs with suitable nitridation degree present high activity and
selectivity.
In summary, a novel SiO2-surface-assisted urea method
was introduced for the controlled synthesis of TaON and
Ta3N5 NPs. By varying the urea/Ta ratio in the precursor gel,
both TaON and Ta3N5 NPs with defined structure and size can
be achieved. Moreover, a novel mechanism employing SiO2
surfaces is proposed whereby silica catalyzes and stabilizes
CNx formation to achieve a slow release N source for
controlled nitridation. The as-synthesized TaON and Ta3N5
NPs show improved activity and tunable selectivity for
cyclooctene epoxidation. This behavior is explained by the
increased electron density around Ta and surface basicity
based upon the controlled introduction of N atom. The
nitridation level is important to achieve highly active and
selective metal catalysts for epoxidation, and our method is
believed to be broadly applicable to the design of novel
nanostructured catalysts.
[b]
1
2
3
4
5
6
7
8
Ta2O5
57.1
96.0
96.5
59.1
65.4
96.9
57.9
54.9
96.8
99.0
97.5
12.8
95.9
95.6
78.4
97.9
79.8
85.8
97.9
99.3
96.1
85.4
74.9
99.0
TaON[c]
[c]
Ta3N5
TaON[d]
[d]
Ta3N5
Ta3N5–H[e]
Ta2O5-H[e]
Ta2O5–OH[f]
TaON[g]
9
[g]
10
11
12
Ta3N5
Ta3N5–OH[h]
blank
[a] Typical reaction conditions: 0.92 mL of cyclooctene, 0.04 g of catalyst,
4.0 mL CH3OH, and 3.0 mL H2O2 (50 wt%), 608C for 18 h. [b] Ta2O5 NPs
were obtained through traditional reverse homogeneous precipitation
(Figure S8). [c] TaON and Ta3N5 NPs were obtained by the SiO2-surface
assisted method. [d] The reaction time was decreased to 10 h. [e] Cata-
lysts were used after treating with 1m HCl for 1 day. [f] Catalyst was used
after treating with 1m NH3·H2O for 1 day. [g] TaON and Ta3N5 NPs were
obtained by Ca-assisted urea method. [h] Ta3N5 NPs obtained through
Ca-assisted urea method were used after treating with 1m NH3·H2O for
1 day.
Received: October 12, 2011
Published online: December 8, 2011
Scheme 1. I) Epoxidation of cyclooctene by TaON and Ta3N5 NPs, and
II) overreaction of hydration to diol products catalyzed by the basic
site on Ta3N5.
Keywords: epoxidation · nanoparticles · surface chemistry ·
.
tantalum
significantly influenced by N introduction. Ta2O5 and TaON
NPs present the selectivity as high as 95.9% and 95.6%,
respectively, whereas only a lower selectivity of 78.4% is
found on Ta3N5 (Table 1, entries 1–3). In the GC–MS results
of products catalyzed by Ta3N5 NPs, considerable amounts
(ca. 21.3%) of cyclooctane–(di)ol are detected (Figure S9),
indicating the hydration of epoxide to (di)ol products as
overreactions.[16] As enough N is introduced, a strong basic
site will be generated on the catalysts,[17] such as Ta3N5, which
is validated by their temperature programmed desorption
(TPD) curves of CO2 (Figure S10). Thus, catalyzed by base
sites on Ta3N5 NPs, the as-formed epoxide is hydrated with
water from H2O2 solution, resulting in decreased epoxide
selectivity (Step II in Scheme 1). Moreover, such basicity
originating from N is tunable by acid or base treatment
because of reversible protonation, generating an altered
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