Crystals in crystals-nanocrystals within mesoporous zeolite single crystals

Article abstract of DOI:10.1021/ja050380u

A major factor governing the performance of catalytically active particles supported on a zeolite carrier is the degree of dispersion. It is shown that the introduction of noncrystallographic mesopores into zeolite single crystals (silicalite-1, ZSM-5) may increase the degree of particle dispersion. As representative examples, a metal (Pt), an alloy (PtSn), and a metal carbide (β-MO₂C) were supported on conventional and mesoporous zeolite carriers, respectively, and the degree of particle dispersion was compared by TEM imaging. On conventional zeolites, the supported material aggregated on the outer surface of the zeolite particles, particularly after thermal treatment. When using mesoporous zeolites, the particles were evenly distributed throughout the mesopore system of the zeolitic support, even after calcination, leading to nanocrystals within mesoporous zeolite single crystals.

Full text of DOI:10.1021/ja050380u

Published on Web 05/13/2005
Crystals in CrystalssNanocrystals within Mesoporous Zeolite
Single Crystals
Claus H. Christensen,†,‡ Iver Schmidt,† Anna Carlsson,† Kim Johannsen,*,† and
Konrad Herbst†
Contribution from Haldor Topsøe A/S, NymølleVej 55, DK-2800 Lyngby, Denmark, and Center
for Sustainable and Green Chemistry, Department of Chemistry, Technical UniVersity of
Denmark, DK-2800 Lyngby, Denmark
Received January 20, 2005; E-mail: kjo@topsoe.dk
Abstract: A major factor governing the performance of catalytically active particles supported on a zeolite
carrier is the degree of dispersion. It is shown that the introduction of noncrystallographic mesopores into
zeolite single crystals (silicalite-1, ZSM-5) may increase the degree of particle dispersion. As representative
examples, a metal (Pt), an alloy (PtSn), and a metal carbide (â-Mo2C) were supported on conventional
and mesoporous zeolite carriers, respectively, and the degree of particle dispersion was compared by
TEM imaging. On conventional zeolites, the supported material aggregated on the outer surface of the
zeolite particles, particularly after thermal treatment. When using mesoporous zeolites, the particles were
evenly distributed throughout the mesopore system of the zeolitic support, even after calcination, leading
to nanocrystals within mesoporous zeolite single crystals.
Introduction
However, if these zeolite-supported materials are to have the
desired catalytic properties, two conditions have to be met:
(1) The acidic site and the supported material generally have
to be in close proximity.
(2) The material has to be well-dispersed throughout the
zeolite.
Zeolites are naturally occurring or artifically created com-
pounds consisting of tetrahedral SiO4 units linked together in
intricate crystalline frameworks. These frameworks contain
crystallographically determined micropores with diameters
between 3 and 10 Å, giving the zeolite molecular-sieving
capabilities. By now more than 145 types of zeolites, each with
a unique crystalline structure and micropore system, have been
identified.1
Zeolites can be chemically modified in a number of ways,
of which metal substitution is the most common. Different
metals, most notably Al, can replace some of the Si atoms in
the framework, in which case the framework acquires a negative
charge for each AlO4 unit. The negative charges are countered
by cations. If this cation is H+, the zeolite becomes acidic. In
the last few decades, examples of other metals substituted into
the framework have been investigated, such as Ti,2 V3 (giving
the zeolite oxidative properties), and several other metals.
Alternatively to chemical modification of the framework, the
use of zeolites as carrier material for catalytically active
components such as metals, alloys, carbides, and sulfides has
been investigated. The catalytic properties of these materials
when supported on zeolites can be drastically different to the
bulk materials due to the dispersion of these materials in the
zeolite matrix. Furthermore, the acidity of the zeolite is
combined with the catalytic properties of the supported com-
ponent, giving rise to bifunctional catalysts.
These conditions are often difficult to achieve, since the
supported material in almost all cases remains on the outer
surface of the zeolite as bulk material, while the acidic sites
are dispersed throughout the zeolite single crystal.4
Indeed, only a few examples of supported material within
the micropores are known, in which case the preparation
procedure is more delicate than the usual impregnation proce-
dures. The catalytic activity of the supported material in the
micropores seems to be low, probably due to spatial restraints
in the micropores.5 Thus, there is a need to develop bifunctional
zeolitic catalysts with high dispersion, in which the catalytically
active component is located neither at the outer surface nor the
micropore system.
One possibility is to increase the external surface area.
Different techniques for production of nanosized zeolites have
been developed, one example being the confined space synthe-
sis.6 Unfortunately, nanosized crystals exhibit problems of
handling.
As an alternative, different mesoporous materials have been
presented. Most notable is the M41S family of mesoporous
materials,7 but they lack the crystallinity which is seen in zeolite
† Haldor Topsøe A/S.
(4) Knapp, C.; Obuchi, A.; Uchisawa, J. O.; Kushiyama, S.; Avila, P.
Microporous Mesoporous Mater. 1999, 31, 23.
‡ Technical University of Denmark.
(1) Database of zeolite structures: http://www.iza-structure.org/databases/.
(2) Taramasso, M.; Perego, G.; Notari, B. US Patent 4,410,501, 1983.
(3) Centi, G.; Perathoner, S.; Trifiro, F.; Aboukais, A.; Aissi, C. F.; Guelton,
M. J. Phys. Chem. 1992, 96, 2617.
(5) Hensen, E. J. M.; van Veen, J. A. R. Catal. Today 2003, 86, 87.
(6) Madsen, C.; Jacobsen, C. J. H. Chem. Commun. 1999, 673.
(7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.
Nature 1992, 359, 710.
9
8098
J. AM. CHEM. SOC. 2005, 127, 8098-8102
10.1021/ja050380u CCC: $30.25 © 2005 American Chemical Society

Nanocrystals within Mesoporous Zeolite Crystals
A R T I C L E S
Scheme 1. Principles of Synthesis Procedures for Conventional
and Mesoporous Zeolites
Scheme 2. Comparison of Impregnation and Thermal Activation
Steps on Conventional and Mesoporous Zeolites
crystals, severely limiting the potential of these materials. Also,
the delaminated zeolites of the ITQ-type have been presented
as an example of zeolitic materials with increased surface area,8
but they exhibit the same difficulty in handling as nanosized
crystals as well as a delicate preparation procedure, which seems
to limit their application.
Recently, a new family of crystalline zeolitic materials was
presented, the so-called mesoporous zeolite single crystals. This
new material features individual zeolite single crystals with an
additional noncrystalline mesopore system interconnected with
the usual micropore system of the zeolite, resulting in a bimodal
pore distribution. These materials are produced by crystallizing
the zeolite around a removable matrix, most often carbon (see
Scheme 1). By adjusting the carbon matrix, mesopores with
different pore size and tortuosity can be produced.9,10 Other
schemes have been presented as well, such as assembling zeolite
seeds with organic structure directors present,11 by resin
macrotemplating,12 or by using carbon aerogels.13
it is possible to have increased activity, selectivity, and lifetime
of the catalyst in several processes.17-19
Thus, hierarchical zeolites such as the mesoporous zeolites
are of significant interest both in materials science and catalysis.
But mesoporous zeolites do not only offer advantages as pure
compounds. They also seem to be suitable candidates for
bifunctional catalysts.16 The increased surface area would not
only decrease transport limitations in the zeolite but also be
beneficial in the effort to disperse the catalytic phase in the
zeolite to a higher degree than what has been seen in
conventional zeolite single crystals. This will result in smaller
crystals of the supported material in closer proximity to the
acidic sites than previously seen with conventional zeolites
(Scheme 2).
The introduction of this noncrystalline mesopore system leads
to a number of different positive effects on the performance of
mesoporous zeolites in catalysis. Two examples, which have
previously been shown, are better resistance to deactiVation,14
which is of major importance in many industrial catalytic
applications (one important example being Mo on HZSM-5 for
dehydroaromatization of methane15) and a significant decrease
in transport limitation.16 Both of these factors have a marked
influence on the catalytic properties, and it has been shown that
Experimental Section
Preparation of Zeolite Materials. Conventional zeolite single
crystals (silicalite-1 and H-ZSM-5) were prepared according to the
literature.20 The mesoporous zeolite single crystal samples (silicalite-1
and H-ZSM-5) were prepared as described previously using carbon
(8) Corma, A.; Forne´s, V.; Mart´ınez-Triguero, J.; Pergher, S. B. J. Catal. 1999,
186, 57.
(9) Jacobsen, C. J. H.; Madsen, C.; Houzˇvicka, J.; Schmidt, I.; Carlsson, A. J.
Am. Chem. Soc. 2000, 122, 7116.
(10) Janssen, A. H.; Schmidt, I.; Jacobsen, C. J. H.; Koster, A. J.; De Jong, K.
P. Microporous Mesoporous Mater. 2003, 65, 59.
(11) Liu, Y.; Pinnavaia, T. J. J. Mater. Chem. 2004, 14, 1099.
(12) Naydenov, V.; Tosheva, L.; Sterte, J. Microporous Mesoporous Mater.
2000, 35-36, 621.
(17) Christensen, C. H.; Johannsen, K.; Schmidt, I.; Christensen, C. H. J. Am.
Chem. Soc. 2003, 125, 13370.
(18) Schmidt, I.; Krogh, A.; Wienberg, K.; Carlsson, A.; Brorson, M.; Jacobsen,
C. J. H. Chem. Commun. 2000, 2157.
(13) Tao, Y.; Kanoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 10974.
(14) Janssens, T. V. W.; Dahl, S.; Jacobsen, C. J. H. EP 1,479,662.
(15) Su, L.; Liu, L.; Zhuang, J.; Wang, H.; Li, Y.; Shen, W.; Xu, Y.; Bao, X.
Catal. Lett. 2003, 91, 155.
(19) Houzˇvicka, J.; Jacobsen, C. J. H.; Schmidt, I. Stud. Surf. Sci. Catal. 2001,
135, 158.
(20) Robson, H.; Lillerud, K. P. Verified Synthesis of Zeolitic Materials, 2nd
ed.; Elsevier: Amsterdam, 2001.
(16) Hartmann, M. Angew. Chem., Int. Ed. 2004, 43, 2-4.
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A R T I C L E S
Christensen et al.
Table 1. Metal Loading of the Samples As Determined by ICP
black pearls (particle size 18 nm) as carbon matrix.10,18 This results in
samples with mesopore volumes in the range of 0.4-1.2 mL/g.
Preparation of Supported Samples. Three sets of supported
samples were prepared, each on mesoporous and conventional zeolite.
Pt/Silicalite-1. Pt(norbornene)3 (norbornene ) bicyclo[2.2.1]hept-
2-ene) was prepared according to the literature.21 The Pt/silicalite-1
sample was prepared by conventional Schlenk line techniques. Zeolite
silicalite-1 (3.5 g) was dried at 200 °C for 2 h and cooled to ambient
temperature in N2 atmosphere. The sample was impregnated by a
solution of Pt(norbornene)3 (172 mg) in CH2Cl2 (2.8 mL). After standing
for 1 h, the solvent was evaporated in a vacuum. The evacuated sample
was heated to 90 °C for 2 h to decompose Pt(norbornene)3. After
cooling, the sample was washed with pentane and dried in a vacuum.
Another sample on mesoporous silicalite-1 was prepared similarly
by impregnation of 78 mg Pt(norbornene)3 dissolved in CH2Cl2
(1.8 mL) on 1.6 g mesoporous silicalite-1.
sample
metal load (wt %)
Pt/mesoporous silicalite-1
2.0
Pt/conventional silicalite-1
1.9
Pt/mesoporous silicalite-1, calcined
Pt/conventional silicalite-1, calcined
PtSn/mesoporous silicalite-1
PtSn/conventional silicalite-1
â-Mo2C/mesoporous ZSM-5
â-Mo2C/conventional ZSM-5
2.1
2.1
1.3 (Pt), 1.1 (Sn)
1.5 (Pt), 0.7 (Sn)
10.6
10.2
The EDS line scans and maps were calculated from the integrated
intensities of the relevant peaks after background subtraction.
Results and Discussion
Zeolites and General Sample Characterization. All zeolite
samples were 100% crystalline, as judged from XRPD. The
crystal size of the two H-ZSM-5 samples were equal (1 µm),
whereas the crystals of the mesoporous silicalite-1 sample were
smaller (0.5 µm) than the conventional silicalite-1 crystals
(2 µm). The two H-ZSM-5 samples had Si/Al ratios of 40. The
metal loadings of the samples prepared on the zeolites are
displayed in Table 1.
TEM confirmed that the two mesoporous samples indeed
consisted of mesoporous single crystals and not agglomerates
of nanosized crystals. Using TEM images it is difficult to
determine the location of metal particles, since TEM images
are 2D projections of 3D objects; thus, it is not straightforward
to determine whether metal particles are located in the intra-
crystalline mesopores as well as at the geometrical external
surface or exclusively on the geometrical external surface.
Recently, stereo-TEM characterization of mesoporous zeolites
showed that the metal particles present in the samples are located
in the intracrystalline mesopores as well as the external surface
of the mesoporous zeolite single crystals.23 However, in these
samples the metal was introduced unintentionally as an impurity
in the carbon matrix during synthesis, and furthermore, no
conclusions about the distribution between micro- and meso-
pores were reached. But the results in ref 23 indicate that the
metal particles distributed on mesoporous zeolites in general
are most likely located in the intracrystalline mesopores as well
as on the external surface of the zeolite crystals.
Pt/Silicalite-1. Supported Pt catalysts are extensively used
for a number of different reactions, both in research and
industry.24 Due to their excellent hydrogenating properties for
several functional groups, Pt supported on Al2O3 or SiO2 has
found widespread application in a number of refinery processes.
Large-scale applications of Pt catalysts supported on zeolite Y
are also found for skeletal isomerization processes of C4-C8
paraffin hydrocarbon streams in refineries.
Despite the very similar Pt loadings on both the conventional
and mesoporous silicalite-1 samples, significant differences of
Pt distribution on the carrier particles are apparent from the TEM
images with corresponding EDS line scans (Figure 1). In the
conventional silicalite-1 sample (Figure 1a,c) the twinned shape
of the silicalite-1 particle is recognized as an increase of the Si
signal at 320 nm and a corresponding decrease at 720 nm. Pt is
located as a thin layer around the outer surface of the zeolite
Both Pt/silicalite-1 samples were calcined at 500 °C (heating ramp
1 °C/min) for 1 h with airflow over the sample.
PtSn/Silicalite-1. Both 0.36 g of SnCl2‚2H2O and 0.24 g of
H2PtCl6 were dissolved in 10 mL of 0.01 M hydrochloric acid, and
5.0 g of silicalite-1 was suspended in the solution with stirring for 2 h.
Water was evaporated in a vacuum, and the sample was dried overnight
at 110 °C. The sample was calcined in air at 400 °C for 4 h, cooled,
reduced in H2 at 400 °C for 2 h, cooled, and passivated in 3% O2/He
at room temperature for 1 h. Another sample was prepared similarly,
using mesoporous silicalite-1.
â-Mo2C/ZSM-5.22 A slurry was formed by mixing 50 g of HZSM-5
with 75 mL of distilled water. An aqueous molybdate solution to obtain
a 10 wt % Mo loading was prepared by dissolving 10.7 g of ammonium
heptamolybdate (Aldrich, +99.5%, AHM) in 75 mL of distilled water
by gently heating to 80 °C while being stirred for 15 min. The hot
AHM solution was slowly added to the zeolite slurry with stirring.
Then the temperature was increased to 80 °C, whereby a yellow slurry
was formed, and the water was evaporated with stirring for ap-
proximately 2 h.
The dried material was pressed into tablets, crushed, and sieved into
a 250-425 µm fraction, which was loaded into a flow reactor.
Calcination was carried out in a flow (1.0 NL/h) of dry air by increasing
the temperature to 400 °C at a rate of 5 °C/min and keeping it there
for 1 h before cooling.
After flushing the reactor with He, the preactivation feed consisting
of a 1:9 n-butane/hydrogen mixture was introduced. Then the temper-
ature was increased to 350 °C at 15 °C/min and kept at this temperature
for 10 h. After changing the feed gas to methane, the temperature was
increased to 700 °C at 5 °C/min and held for 1 h before cooling to
room temperature.
Characterization. The crystal sizes of the zeolite samples were
determined by conventional scanning electron microscopy (SEM) and
(scanning) transmission electron microscopy (TEM/STEM), the crystal-
linity of the zeolites was analyzed by X-ray powder diffraction (XRPD),
and the elemental composition was determined by inductively coupled
plasma spectroscopy (ICP). The particle sizes of the active phases were
determined by (S)TEM. It was not possible to obtain reliable data on
metal dispersion using chemisorption techniques or to measure particle
sizes using XRPD due to the low metal loading of the samples.
In TEM experiments, the samples were characterized using either
bright field TEM or dark field STEM, depending on the nature of the
sample. About 100 crystals were investigated for each sample, and
representative examples are shown here. The TEM was coupled with
energy dispersive spectroscopy (EDS) mapping to determine the
distribution of different elements in the individual single crystals. The
acquisition time for the EDS acquisition was adjusted so that clear peaks
were obtained also for the low-concentration elements in the spectra.
(23) Boisen, A.; Schmidt, I.; Carlsson, A.; Dahl, S.; Brorson, M.; Jacobsen, C.
J. H. Chem. Commun. 2003, 958.
(24) Cornils, B., Herrmann, W. A., Schlo¨gl, R., Wong, C.-H., Eds.; Catalysis
from A to Z; Wiley-VCH: Weinheim (Germany) 2000; p 451f.
(21) Crascall, L. E.; Spencer, J. L. Inorg. Synth. 1990, 28, 127.
(22) Bouchy, C.; Schmidt, I.; Anderson, J. R.; Jacobsen, C. J. H.; Derouane, E.
G.; Derouane-Abd Hamid, S. B. J. Mol. Catal. A: Chem. 2000, 163, 283.
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Nanocrystals within Mesoporous Zeolite Crystals
A R T I C L E S
Figure 1. TEM images of Pt/conventional silicalite-1 (left column) and
Pt/mesoporous silicalite-1 (right column) before calcination. Element-
sensitive EDS line scans (position indicated by lines in images) giving
relative atomic concentrations for Pt (bottom) and Si (top) are also shown.
Figure 2. TEM images and EDS scans (Pt bottom and Si top) of
Pt/conventional silicalite-1 (left column) and Pt/mesoporous silicalite-1 (right
column) after calcination in air at 500 °C.
a formation of Pt particle “islands” on the surface of the
conventional silicalite-1. In the line scan it is clearly seen that
Pt atoms are aggregated, most likely at defects and steps on
the outer surface. In the calcined mesoporous silicalite-1 sample,
only a few large Pt particles were detected. A random distribu-
tion of the Pt particles throughout the carrier was kept after the
calcination.
PtSn/Silicalite-1. Alloy catalysts are another interesting group
of catalysts, due to the bifunctional nature of the active phase.
Best known are probably Pt-Re, Pt-Ir, and Pt-Sn, which are
used for reforming of naphtha in refineries or the “three-way
catalyst” containing alloys of Pt metals for treatment of engine
exhausts gases.25 Alloy catalysts are also attracting increasing
academic attention for different reactions, one example being
NiAu for steam reforming.26 PtSn/silicalite-1 is an example of
an alloy catalyst, which is currently being regarded for reactions
such as hydrogenation/dehydrogenation reactions.27 The pres-
ence of Sn decreases the size of Pt ensembles and thus reduces
hydrogenolysis.
particle; note the significant increase in the Pt signal at the outer
edges and twin edges (50, 320, 720, and 980 nm) where the
beam direction is parallel to the edges. Several large Pt particles
(ca. 30 nm) can be seen on the surface of the conventional
zeolite sample in Figure 1 and other images taken during TEM
investigation. The mesoporous silicalite-1 (Figure 1b,d) has a
high intracrystalline porosity seen both directly in TEM and as
fluctuations of the Si signal in EDS. In contrast to the Pt
distribution on the surface of the conventional silicalite-1
particles, the Pt particles on the mesoporous silicalite-1 are well-
dispersed throughout the entire mesopore system. As a conse-
quence, the particle size of Pt is very small. From the line scan
it can be seen that the Pt concentration is closely following the
Si concentration, and large Pt particles were not detected. Thus,
the presence of the mesopore system allows for a larger fraction
of the zeolite volume to participate in dispersing the metal,
resulting in smaller particle sizes in the mesoporous zeolite. The
reason for this is most likely the high amount of lattice
terminations along the mesopore system, which gives a high
amount of lattice defects available for the crystallization of Pt.
Pt/Silicalite-1 Samples after Calcination at 500 °C. Many
catalytic processes of industrial relevance involving Pt catalysts
are carried out at elevated temperatures. An important parameter
for the performance of the catalysts is the stability of the metal
particles against sintering. To investigate the effect of a thermal
treatment on metal dispersion, both Pt/silicalite-1 samples were
subjected to calcination at 500 °C. TEM images (Figure 2) of
the calcined conventional silicalite-1 sample show the expected
increase of the Pt particle size. The sintering procedure led to
Again, two samples where prepared using either conventional
or mesoporous silicalite-1 as support. Analysis of the PtSn/
silicalite-1 samples showed that the metal loadings of the two
samples are similar: 1.5 wt % Pt and 0.7 wt % Sn in the
conventional silicalite-1 and 1.3 wt % Pt and 1.1 wt % Sn in
the mesoporous silicalite-1. From EDS element mapping in the
TEM of selected crystals it can be seen that the Pt and Sn are
(25) Olivas, A.; Jerdev, D. I.; Koel, B. E. J. Catal. 2004, 222, 285.
(26) Ponec, V. Appl. Catal. A 2001, 2001, 31.
(27) Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek,
A. M.; Nørskov, J. K.; Stensgaard, I. Science 1998, 279, 1913.
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A R T I C L E S
Christensen et al.
Figure 5. Bright field TEM images of â-Mo2C/conventional ZSM-5 (left)
and â-Mo2C/mesoporous ZSM-5 (right), both after thermal treatment at
700 °C.
of the problems with this catalyst is the difficult preparation. It
has to be carbonized at high temperatures, causing phase
transformations and sintering, which lower the surface area of
the â-Mo2C phase.
XRPD confirmed the presence and purity of the â-Mo2C
phase after the preparation. No trace of MoO3 was seen. The
Mo loadings were 10.6 and 10.2 wt % on mesoporous and
conventional zeolite ZSM-5, respectively.
The same trends are seen for the two samples of supported
nonmetallic â-Mo2C as with metallic Pt and PtSn. In the sample
with conventional zeolite, the â-Mo2C crystal agglomerates are
of 100 nm size and located at the outer surface of the zeolite
crystals, whereas using mesoporous zeolite single crystals the
â-Mo2C crystals are dispersed in the mesopore system with a
size of <20 nm, even though both samples were prepared at
700 °C (Figure 5).
Figure 3. TEM image and element map of PtSn/conventional silicalite-1
particles. The element map is showing the local EDS signals of Si, Sn and
Pt, respectively.
Conclusions
For several different examples of nanocrystals, i.e., pure
metals, alloys, and carbides, deposited onto conventional zeolite
crystals, our detailed TEM investigations show that the nano-
crystals decorate the external surface of the zeolite. Additionally,
the nanocrystals are clearly seen to preferentially deposit at
defect sites at the surface. Thus, to obtain high concentrations
of well-dispersed nanocrystals at zeolite surfaces, a high density
of defect sites is required. This provides a new guideline for
design of zeolite catalysts. With our recently discovered family
of mesoporous zeolite single crystals, which are among the most
porous materials known, the difficulty of site separation in
bifunctional zeolite catalysts can be overcome. In this case,
introduction of the nanocrystals is conveniently performed even
at high concentrations, since the extensive mesopore system
present in each crystal allows introduction of substantial amounts
of precursor solutions. Upon transformation of the precursors
into nanocrystals, the mesoporous zeolite single crystals provide
a sufficient number of defect sites throughout the entire zeolite
crystal to efficiently disperse nanocrystals in very close proxim-
ity to all the active sites in the micropores that are responsible
for shape selectivity. In these materials, access to the nano-
crystals is possible through both the micropores and the
mesopores, and such materials show significant promise as
heterogeneous catalysts. Accordingly, with the present meth-
odology it is possible to distribute nanocrystals inside meso-
porous zeolite single crystals.
Figure 4. TEM images of PtSn/conventional silicalite-1 (left) and PtSn/
mesoporous silicalite-1 (right) after calcination and reduction.
located in the same metal clusters at the same relative amounts
throughout all clusters examined (Figure 3 and other images).
This was seen for both samples. These element mappings thus
indicate that Pt and Sn in both samples are alloyed and not two
segregated metallic phases.
The TEM images of the two PtSn/silicalite-1 samples show
the same trends as were seen with the Pt/silicalite-1 samples
above: On the sample with conventional silicalite-1, the PtSn
particles are located at the outer surface as a mixture of small
particles (<10 nm) and large agglomerates (>100 nm). On the
sample with mesoporous silicalite-1, PtSn is distributed through-
out the zeolite crystal as small (<20 nm) particles, and no large
particle agglomerates can be detected (Figure 4).
Thus, once more it has been shown that the mesoporous
silicalite-1 sample allows the metals to disperse throughout the
zeolite single crystal mesopore system as nanocrystals, whereas
the use of a conventional silicalite-1 as support for PtSn gives
a mixture of nanocrystals and large agglomerates that most likely
are all located on the outer surface.
â-Mo2C/ZSM-5. Zeolite-supported â-Mo2C is an example
of a catalyst that is not metallic in nature. It is currently being
investigated as a catalyst for methane dehydroaromatization. One
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