Facile one-pot synthesis of Pt nanoparticles /SBA-15: an active and stable
material for catalytic applications†
Junjiang Zhu,*
a
Xiao Xie,
a
S
onia A. C. Carabineiro,
b
Pedro B. Tavares,
c
Jos
e L. Figueiredo,
b
Reinhard Schom€
acker
a
and Arne Thomas*
a
Received 14th January 2011, Accepted 12th March 2011
DOI: 10.1039/c1ee01040a
Pt/SBA-15 with an enhanced surface area but unchanged pore
diameter (compared to pure SBA-15) and a Pt average particle size
of 9 nm shows a high and stable activity for both gas-phase CO
oxidation and liquid-phase cyclooctadiene hydrogenation. No
intrinsic change in the structure of the catalyst occurs after several
reaction cycles, suggesting that the Pt/SBA-15 presented here is an
active and stable catalyst.
Supported noble metal catalysts are widely used in industrial appli-
cations due to their excellent catalytic performances, either for high
temperature gas-phase reactions (e.g. purification of exhaust gas)
1–3
or for moderate temperature liquid-phase reactions (e.g. synthesis of
organic chemicals)
4–6
or others.
7,8
However, as supported noble
metals are prone to aggregation and leaching during reactions,
deactivation often occurs quickly, which obviously should be avoided
considering the high costs of these catalysts. Therefore, preparation
of noble metal catalysts with good catalytic performance, but also
high stability, is essential for practical applications.
Bulk noble metals are expensive and have low surface areas thus
show none or low catalytic activity. Therefore, supports are frequently
used on which small nanoparticles of the noble metals are immobi-
lized, yielding an increased number of accessible active sites and thus
decreasing the cost and increasing the catalytic performance.
9
However, for the preparation of such catalysts some issues have to be
addressed, e.g. finding suitable preparation methods, the selection of
supports, the choice of the metal precursors, etc., as different materials
or technologies used can lead to diverse catalytic properties.
Porous silicas are one of the most commonly used supports for
noble metals. In this respect SBA-15 type silica
10–13
has received a lot
of attention since it was first reported, and, at least in the scientific
literature, it is now one of the most widely used supports in catalysis.
This is due to the straightforward synthesis and its highly defined
textural properties, such as the high surface area, ordered pore
structure, controllable pore size, etc. As the surface of SBA-15 is
relatively inert, it is difficult to directly graft metal nanoparticles
(NPs) on it. Hence, additives or surfactants are often used to func-
tionalize the SBA-15 surface before grafting the metal NPs, and their
deposition on the surface is usually carried out by a post-synthesis
method.
14–20
This often leads to formation of NPs lacking uniformity
in size and shape. Furthermore, the interaction between the NPs and
the support is often not strong enough, and therefore agglomera-
tion
20–24
or leaching
24–29
of the catalyst during the reaction is often
observed. This is true in particular for noble metal catalysts.
a
Institut f€
ur Chemie, Technische Universit€
at Berlin, Englische Str. 20,
b
Laboratory of Catalysis and Materials (LCM), Associate Laboratory
(LSRE/LCM), Department of Chemical Engineering, Faculty of
Engineering, University of Porto, Rua Dr Roberto Frias, 4200-465
Porto, Portugal
c
Centro de Qu
ımica, Universidade de Tr
as-os-Montes e Alto Douro,
Apartado 1013, 5001-801 Vila Real, Portugal
† Electronic supplementary information (ESI) available: Experimental
details and more characterization results. See DOI: 10.1039/c1ce01040a
Broader context
Supported noble metals are active catalysts for various reactions in heterogeneous catalysis. However, there are also several
problems in practical applications, such as aggregation and leaching. Preparation of highly active and stable nano-scale noble metals
is an important goal of catalysis and materials scientists, as an efficient catalyst can not only reduce cost but also save energy of
chemical processes. In developing the supported metal catalysts, one problem, however, is always unavoidable: the surface area and
the pore size of the support decrease obviously after metal loading. This leads to a negative effect on catalytic reactions, where the
surface area or the pore size plays an important role. Here, we report a new method for the preparation of a SBA-15 supported
platinum catalyst, Pt/SBA-15, which shows an enhanced surface area and unchanged pore diameter comparing to pure SBA-15, and
a Pt average particle size of 9.3 nm. Catalysis studies indicate that this catalyst can show a high and stable activity for both gas-phase
CO oxidation and liquid-phase cyclooctadiene hydrogenation, and no intrinsic change in the structure of the catalyst occurs after
several reaction cycles, suggesting that the Pt/SBA-15 presented here is a promising material for environmental catalysis use.
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Recently, some efforts were made for in situ incorporation of noble
metal NPs into the SBA-15 structure:
30–33
Richards et al.
30
succeeded
to confine Au NPs on the pore walls of SBA-15 by adding HAuCl
4
solution directly to the synthesis process. However, the sample
obtained showed irregular shapes and pores; Somorjai et al.
32
reported that Pt with particle size ranging from 1.7 nm to 7.0 nm can
be deposited on SBA-15 in the presence of poly(vinylpyrrolidone)
(PVP), but the surface area and/or the pore size of the samples
significantly decreased when compared to the pure SBA-15.
In a recent work, we have shown that the introduction of poly
(vinylalcohol) (PVA) to the synthesis process of SBA-15 can create
additional mesopore connectivities in the structure, leading to an
overall increased surface area and pore volume, while keeping the
mesopore size and structure unchanged.
34
Using the same strategy,
the introduction of PVA stabilized Pt nanoparticles to SBA-15 during
synthesis was tested in this work. It was found that the obtained
SBA-15 material with incorporated Pt NPs, Pt/SBA-15, also show an
increased surface area and unchanged mesopore size when compared
to the parent SBA-15. This is essentially different from other Pt
loaded porous silicas,
32,33,35–37
which in general showed a decrease of
surface area of SBA-15 after addition of Pt, regardless of the used
additional agent. Moreover, catalytic tests indicated that the Pt/SBA-
15 material prepared by the method presented here shows a high and
stable activity either for gas-phase CO oxidation or for liquid-phase
cyclooctadiene hydrogenation, suggesting that this material has great
potential for practical applications.
The catalyst was synthesized using a similar procedure as reported
for SBA-15,
10–13
except that addition of a PVA stabilized Pt sol was
included. The obtained catalysts were named based on the initial
weightpercentofPtinSBA-15,for example, 0.5% Pt/SBA-15 means
that the weight ratio of Pt (calculated from H
2
PtCl
6
)toSiO
2
(calculated from tetraethoxysilane (TEOS)) is 0.005 to 0.995. The Pt
loading was confirmed by Inductively Coupled Plasma (ICP)
measurements and is shown in Table S1†.
Fig. 1 shows the results obtained from X-Ray Diffraction (XRD)
measurements at small angles, showing that all samples exhibit an
intense peak at 2q¼0.8–0.9 that corresponds to the (100) diffraction
of the SBA-15 2D-hexagonal structure, indicating that its ordered
pore structure remains unchanged after Pt addition. The well-
resolved (110) and (200) diffraction peaks also indicate that there is
no substantial change in the pore structure of samples after Pt
addition. A slight shift of the (100) peak position after Pt incorpo-
ration to higher angles indicates a small decrease of the unit cell.
N
2
sorption isotherms are depicted in Fig. 2 and show that all
materials exhibit a well defined step with a hysteresis loop corre-
sponding to the filling of mesopores with narrow pore size distribu-
tions, confirming that the addition of Pt does not change the
mesoporous structure of SBA-15, as already indicated by XRD.
Interestingly, it was found that the pore size distribution of SBA-15,
with or without the addition of Pt, remains unchanged, with a value
of 9.5 nm. On the other hand, the surface area of SBA-15 increased
significantly after the addition of Pt, from 877 m
2
g
1
for the pure
SBA-15 to, for example, 1300 m
2
g
1
for 0.5 wt% Pt/SBA-15 (the
other porosity values can be found in Table S1†; calculations were
made as described in the literature
38
). The increased surface area is
due to the additional mesopore connectivities created by PVA, which
therefore seems to have the ability to prevent aggregation of Pt NPs
as well as to create new pores in the SBA-15 structure. Indeed, the
surface area of SBA-15–PVA sample (with no Pt addition) also
increasedincomparisontothatof pure SBA-15. Nevertheless, it
should be pointed out that the surface area and pore volume of SBA-
15 also increased after Pt incorporation. To the best of our knowl-
edge, this is the first report showing that noble metal incorporation on
SBA-15canbeaccompaniedwithanincreaseinsurfacearea,while
keeping the mesopore size unchanged at the same time. Although in
several previous works it was shown that addition of supplementary
agents can increase the surface area of SBA-15, no report exists on the
simultaneous improvement of the surface area, after the incorpora-
tion of metal NPs, independently of the supplementary agent. This
method also works for other metals such as gold (i.e. Au/SBA-15,
see Fig. S2†). These unexpected properties suggest that the current
Fig. 1 XRD patterns of pure SBA-15 and that after incorporation of
several amounts of Pt.
Fig. 2 N
2
sorption isotherms (A) and pore size distributions (B) of pure
SBA-15 and after addition of PVA and various Pt contents.
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Pt/SBA-15 material would exhibit additional interesting perfor-
mances in practical use, where the influence of the surface area or the
pore size of the catalyst needs to be considered.
Transmission Electron Microscopy (TEM) images were obtained
in order to check the ordered structure of Pt/SBA-15 and to measure
the particle size of Pt NPs. The results presented in Fig. 3 show that
the 0.5% Pt/SBA-15 samples have a 2D hexagonal structure as that of
pure SBA-15, confirming that the addition of PVA or Pt did
not change the ordered structure of SBA-15. An overview image
(Fig. 3(A)) shows that the Pt particles were highly dispersed in the
SBA-15 structure and that the Pt particle size varies between 4.1 nm
and 15.5 nm (average ca. 9.3 nm, Fig. 3(B)), and does not change
much with the Pt loading (TEM images of the other samples can be
found in Fig. S3†), suggesting that this method is favorable for
preparing a highly dispersed Pt/SBA-15 material. Although it cannot
be excluded that Pt NPs are located on the outer surface of SBA-15,
careful observation indicates that at least some of the Pt NPs are
incorporated into the pores of SBA-15 (Fig. 3(C)).
Fig. 4(A) shows the catalytic activities of the Pt/SBA-15 materials
for gas-phase CO oxidation reaction. Although no substantial change
in the activity of pure SBA-15 and 0.2% Pt/SBA-15 was observed,
a significant decrease in the temperature for complete conversion of
CO was observed with 0.5% Pt/SBA-15, from 400 C for pure SBA-
15 to 300 C for 0.5, 1.0 and 2.0% Pt/SBA-15, indicating that Pt
incorporation can decrease the temperature of total CO conversion.
The ignition temperature (300 C) of the Pt/SBA-15 for CO
oxidation observed here is in agreement with that of Pt/mSiO
2
reported by Somorjai et al.
39
This temperature lies between that of Pt
(100) (227 C) and Pt(111) (347 C) single crystals, thus suggesting
that the Pt NPs on SBA-15 are mostly composed by cubic and
cuboctahedron shapes, which expose mostly the (100) and (111)
surfaces
39
(see the HRTEM image in Fig. 3(D)). The jump in the
ignition temperature from 0.2% Pt/SBA-15 to 0.5% Pt/SBA-15
implies that there is a threshold Pt loading for CO oxidation reaction,
below which value the catalyst cannot work efficiently. However, no
significant increase in the activity was observed above this value
(i.e. from 0.5% to 2.0% Pt/SBA-15), indicating that the Pt loading in
0.5% Pt/SBA-15 is enough. Long-time activity tests in Fig. 4(B) show
that the CO oxidation activity (81% CO conversion) of sample
0.5% Pt/SBA-15 remains unchanged, even after being carried out at
290 C for 15 hours, indicating a very stable activity for the reaction.
For comparison, an additional catalyst prepared by the post-
synthesis method (1 wt% Pt/SBA-15—post) was also tested, but it
was found that Pt was not easily loaded on the SBA-15 and, there-
fore, its activity was similar to that of pure SBA-15.
Based on the results shown in CO oxidation, sample 0.5% Pt/SBA-
15 was chosen as a test catalyst for liquid phase reaction and the
results are shown in Fig. 5, where cyclooctadiene (COD) hydroge-
nation was used as a model reaction. The COD conversion reached
90% within 35 min at 70 C, indicating that the catalyst is very
active for this reaction. The change of the activity with time was not
monitored due to the short reaction time, and thus only the final
activity was measured. The reusability of the catalyst was tested by
first removing the used reactant, and then washing the catalyst with
heptane 3 times (the removal of the used reactant and the washed
heptane was done with the help of a pump), after that the reactor was
refilled with fresh COD and the new reaction started. The obtained
results can be found in Fig. 5, which shows that the catalyst can work
up to 6 runs (the maximum tested) with no appreciable loss in the
Fig. 3 TEM images of 0.5% Pt/SBA-15. (A) Overview image; (B)
particle size distribution of Pt NPs; (C) image showing small particles are
in the pore of SBA-15; (D) high resolution TEM image.
Fig. 4 (A) CO conversion of SBA-15 with or without the incorporation
of Pt at different temperatures; (B) long-time stability test for 0.5%
Pt/SBA-15 in CO oxidation at 290 C.
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activity, suggesting that the Pt/SBA-15 sample is stable and has good
reusability for liquid phase reaction.
Hence, it can be concluded that the current Pt/SBA-15 material is
active and can show stable activity either for gas-phase CO oxidation
or for liquid-phase COD hydrogenation, satisfying the requirements
of a catalyst for industrial application. To check if any changes
occurred in the catalyst after usage, studies on the catalyst after reac-
tion were carried out, and the results are shown in Fig. 6. N
2
sorption
isotherms and pore size distributions (Fig. 6(A)) show that there is no
intrinsic change in the catalyst before and after CO oxidation, and the
minor difference found in the surface area is within the experimental
error (<5%). However, a slight decrease in the surface area and pore
size was detected after the hydrogenation reaction, which could be due
to: (1) coke accumulation on the pore wall of the catalyst and (2)
shrinkage of the material during the reaction. Indeed, the XRD
measurements shown in Fig. 6(B) show a small shift in the (100) peak
position to higher angles after COD hydrogenation reaction (curve
‘‘U2’’). However, TGA tests show that there was no appreciable
difference in the weight loss of the catalyst before and after the reaction
(see Fig. S6†). Hence, it can be inferred that the decrease in the surface
area and pore size after 6 runs in COD hydrogenation reaction is due to
the shrinkage of the porous structure of the sample. Still, the
unchanged COD conversion after those runs suggests that this
shrinkage does not significantly influence the catalytic performances.
TEM images of the catalyst after CO oxidation and COD hydroge-
nation reaction are shown in Fig. 6(C) and (D), respectively, showing
that the well ordered structure of the materials is maintained during the
reactions. Furthermore, no intrinsic change in the particle size before
and after the reaction was observed, suggesting that the Pt NPs are
very stable and that no aggregation occurs during the reaction, even for
the gas phase CO oxidation performed at 300 C. Metal leaching is
another factor which is often observed to lead to catalyst deactivation.
Therefore the Pt loading after the COD reaction was also analyzed,
and no appreciable loss was detected (see Table S1†), suggesting that
Pt was well stabilized to the SBA-15 surface. As a result, it can be
concluded that the here reported Pt/SBA-15 material is not only active
but also highly stable for catalysis (as no aggregation at high
temperatures and no leaching in liquid phase reaction occurred).
The high stability of the Pt/SBA-15 catalyst might come from its
efficient synthesis method. That is, the Pt NPs were first prepared and
protected by PVA, and then in situ incorporated into the SBA-15
structure. This way the Pt NPs are highly dispersed and well
embedded in the SBA-15 structure, avoiding aggregation and/or
leaching during the reaction.
In summary, we showed in this work that platinum nanoparticles
can be simply incorporated into the SBA-15 structure by a facile one-
pot synthesis method. Unlike what was reported in previous works,
the results shown here indicate that the surface area of SBA-15 can
increase significantly after Pt incorporation, while the mesopore size
is kept unchanged. This is due to the presence of PVA in the Pt sol.
Catalytic tests indicate that the prepared Pt/SBA-15 material is active
either for gas-phase CO oxidation or for liquid-phase cyclooctadiene
hydrogenation. Furthermore, the material is highly stable in the
reaction and no appreciable loss in the activity was observed in the
long-term activity tests, suggesting that the present Pt/SBA-15 is
ahighlyactiveandstablematerialfor catalytic uses, satisfying the
requirements of a catalyst for industrial application.
Acknowledgements
Dr Yilmaz Aksu from the Technical University of Berlin is
acknowledged for the measurement of TGA curves. Financial
support from the German Research Foundation (DFG, grant
No. TH 1463/5-1) and the Cluster of Excellence ‘‘Unifying Concepts
in Catalysis’’ (EXL 31411) is gratefully acknowledged.
References
1 W. Tang, Z. P. Hu, M. J. Wang, G. D. Stucky, H. Metiu and
E. W. McFarland, J. Catal., 2010, 273, 125–137.
2 A. C. Gluhoi, N. Bogdanchikova and B. E. Nieuwenhuys, Catal.
Today, 2006, 113, 178–181.
3 F. Tao, S. Dag, L. W. Wang, Z. Liu, D. R. Butcher, H. Bluhm,
M. Salmeron and G. A. Somorjai, Science, 2010, 327, 850–853.
Fig. 5 Activity and reusability of 0.5% Pt/SBA-15 for cyclooctadiene
hydrogenation; reaction conditions: 1 g catalyst + 1.13 mL cyclooctadiene
+ 60 mL heptane (solvent), 70 C, P
H
2
¼1.1 bar. (Weight loss of the
catalyst during the pumping process was taken into consideration when
calculating the activity at each batch.) Fig. 6 Characterizations of 0.5% Pt/SBA-15 sample after reaction; (A)
N
2
sorption isotherms and the corresponding pore size distributions
(inset); (B) XRD patterns at small angles (Note: ‘‘U1’’ is the used catalyst
after CO oxidation and ‘‘U2’’ is the used catalyst after cyclooctadiene
hydrogenation); (C) TEM images of U1; (D) TEM images of U2.
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 2020–2024 | 2023
Published on 15 April 2011. Downloaded by TU Berlin - Universitaetsbibl on 31/03/2016 12:34:43.
View Article Online
4 J. J. Zhu, S. A. C. Carabineiro, D. Shan, J. L. Faria, Y. J. Zhu and
J. L. Figueiredo, J. Catal., 2010, 274, 207–214.
5 C. E. Chan-Thaw, A. Villa, P. Katekomol, D. S. Su, A. Thomas and
L. Prati, Nano Lett., 2010, 10, 537–541.
6 D. Wang, F. Ammari, R. Touroude, D. S. Su and R. Schlogl, Catal.
Today, 2009, 147, 224–230.
7 J. S. chen, C. P. Chen, J. Liu, R. Xu, S. Z. Qiao and X. W. Lou, Chem.
Commun., 2011, 47, 2631–2633.
8 J. Liu, S. Z. Qiao, S. B. Hartono and G. Q. Lu, Angew. Chem., Int.
Ed., 2010, 49, 4981–4985.
9 J. J. Zhu, Y. C. Wei, W. K. Chen, Z. Zhao and A. Thomas, Chem.
Commun., 2010, 46, 6965–6967.
10 D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson,
B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548–552.
11 D. Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky,
J. Am. Chem. Soc., 1998, 120, 6024–6036.
12 D. Y. Zhao, J. Y. Sun, Q. Z. Li and G. D. Stucky, Chem. Mater.,
2000, 12, 275.
13 A. Galarneau, N. Cambon, F. Di Renzo, R. Ryoo, M. Choi and
F. Fajula, New J. Chem., 2003, 27, 73–79.
14 Z. J. Wang, Y. B. Xie and C. J. Liu, J. Phys. Chem. C, 2008, 112,
19818–19824.
15 Z. L. Zheng, H. F. Li, T. F. Liu and R. Cao, J. Catal., 2010, 270, 268–
274.
16 W. Huang, J. N. Kuhn, C. K. Tsung, Y. Zhang, S. E. Habas, P. Yang
and G. A. Somorjai, Nano Lett., 2008, 8, 2027–2034.
17 B. Lee, Z. Ma, Z. T. Zhang, C. Park and S. Dai, Microporous
Mesoporous Mater., 2009, 122, 160–167.
18 X. Y. Liu, A. Q. Wang, X. D. Wang, C. Y. Mou and T. Zhang, Chem.
Commun., 2008, 3187–3189.
19 C. Y. Ma, B. J. Dou, J. J. Li, J. Cheng, Q. Hu, Z. P. Hao and
S. Z. Qiao, Appl. Catal., B, 2009, 92, 202–208.
20 Y. S. Cho, J. C. Park, B. Lee, Y. Kim and J. H. Yi, Catal. Lett., 2002,
81, 89–96.
21 M. N. Timofeeva, S. H. Jhung, Y. K. Hwang, D. K. Kim,
V. N. Panchenko, M. S. MeIgunov, Y. A. Chesalov and
J. S. Chang, Appl. Catal., A, 2007, 317, 1–10.
22 C. P. Huang and Y. H. Huang, Appl. Catal., A, 2008, 346, 140–
148.
23 P. Han, H. M. Zhang, X. P. Qiu, X. L. Ji and L. X. Gao, J. Mol.
Catal. A: Chem., 2008, 295, 57–67.
24 M. N. Timofeeva, O. A. Kholdeeva, S. H. Jhung and J. S. Chang,
Appl. Catal., A, 2008, 345, 195–200.
25 D. P. Serrano, J. Aguado and C. Vargas, Appl. Catal., A, 2008, 335,
172–179.
26 J. Demel, J. Cejka and P. Stepnicka, J. Mol. Catal. A: Chem., 2010,
329, 13–20.
27 P. Y. Wang, Q. S. Lu and J. G. Li, Mater. Res. Bull., 2010, 45, 129–
134.
28 F. B. Su, L. Lv, F. Y. Lee, T. Liu, A. I. Cooper and X. S. Zhao, J. Am.
Chem. Soc., 2007, 129, 14213–14223.
29 H. Essa, E. Magner, J. Cooney and B. K. Hodnett, J. Mol. Catal. B:
Enzym., 2007, 49, 61–68.
30 J. C. Hu, L. F. Chen, K. K. Zhu, A. Suchopar and R. Richards, Catal.
Today, 2007, 122, 277–283.
31 D. Tian, G. P. Yong, Y. Dai, X. Y. Yan and S. M. Liu, Catal. Lett.,
2009, 130, 211–216.
32 H. Song, R. M. Rioux, J. D. Hoefelmeyer, R. Komor, K. Niesz,
M. Grass, P. D. Yang and G. A. Somorjai, J. Am. Chem. Soc.,
2006, 128, 3027–3037.
33 R. M. Rioux, R. Komor, H. Song, J. D. Hoefelmeyer, M. Grass,
K. Niesz, P. D. Yang and G. A. Somorjai, J. Catal., 2008, 254, 1–11.
34 J. J. Zhu, K. Kailasam, X. Xie, R. Schomaecker and A. Thomas,
Chem. Mater., 2011, DOI: 10.1021/cm1028639.
35 C. M. Yang, P. H. Liu, Y. F. Ho, C. Y. Chiu and K. J. Chao, Chem.
Mater., 2003, 15, 275–280.
36 Z. Konya, E. Molnar, G. Tasi, K. Niesz, G. A. Somorjai and
I. Kiricsi, Catal. Lett., 2007, 113, 19–28.
37 J. Liu, Q. Yang, X. S. Zhao and L. Zhang, Microporous Mesoporous
Mater., 2007, 106, 62–67.
38 J. Liu, Q. H. Yang, L. Zhang, H. Q. Yang, J. S. Gao and C. Li, Chem.
Mater., 2008, 20, 4268–4275.
39 S. H. Joo, J. Y. Park, C. K. Tsung, Y. Yamada, P. D. Yang and
G. A. Somorjai, Nat. Mater., 2009, 8, 126–131.
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