Effect of the platinum content on the microstructure and micropore size distribution of Pt/alumina-pillared clays

Article abstract of DOI:10.1021/jp054954t

The aim of this work is to study the effect of the platinum content (0-1.8 wt % Pt) on the microstructure of an alumina-pillared clay. For this purpose, the nitrogen physisorption data at -196°C, the micropore size distributions of the supported platinum catalysts, and the hydrogen chemisorption results at 30°C have been analyzed and compared. The preparation of the catalysts has modified the textural properties of the Alpillared clay support, giving rise to a loss of surface area and micropore volume. After reduction at 420°C, the presence of dispersed metallic platinum with mean crystallite size in the 22-55 Arange has been found by hydrogen adsorption. Comparison of all results reveals that the platinum species block the micropore entrances by steric hindrance to nitrogen access as the platinum content increases.

Full text of DOI:10.1021/jp054954t

J. Phys. Chem. B 2005, 109, 23461-23465
23461
Effect of the Platinum Content on the Microstructure and Micropore Size Distribution of
Pt/Alumina-Pillared Clays
M. Barrera-Vargas,† J. Valencia-Rios,‡ M. A. Vicente,§ S. A. Korili, and A. Gil*,
Department of Chemistry, Materials and Catalysis Laboratory, UniVersity of Cordoba, Monteria, Colombia,
Department of Chemistry, Heterogeneous Catalysis Laboratory, National UniVersity of Colombia, Bogota,
Colombia, Department of Inorganic Chemistry, Faculty of Chemical Sciences, UniVersity of Salamanca,
Plza. de la Merced, E-37008 Salamanca, Spain, and Department of Applied Chemistry, Building Los Acebos,
Public UniVersity of NaVarre, Campus of Arrosadia, E-31006 Pamplona, Spain
ReceiVed: September 1, 2005; In Final Form: October 17, 2005
The aim of this work is to study the effect of the platinum content (0-1.8 wt % Pt) on the microstructure of
an alumina-pillared clay. For this purpose, the nitrogen physisorption data at -196 °C, the micropore size
distributions of the supported platinum catalysts, and the hydrogen chemisorption results at 30 °C have been
analyzed and compared. The preparation of the catalysts has modified the textural properties of the Al-
pillared clay support, giving rise to a loss of surface area and micropore volume. After reduction at 420 °C,
the presence of dispersed metallic platinum with mean crystallite size in the 22-55 Å range has been found
by hydrogen adsorption. Comparison of all results reveals that the platinum species block the micropore
entrances by steric hindrance to nitrogen access as the platinum content increases.
1. Introduction
The microporous structure of the pillared clays is character-
ized by the interlayer spacing, which is the distance between
the clay layers, and the interpillar spacing, which is the distance
between the intercalating species. These spacings can be
controlled to a great extent by adjusting the various parameters
involved in the synthesis process.7 The incorporation of specific
cations into the pillared structure can improve the adsorption
and catalysis properties of the final pillared solids.
The development in the 1970s of inorganic pillared interlay-
ered clays (in short PILCs), an important category of mi-
croporous materials, has created remarkable new opportunities
in the field of the synthesis and applications of clay-based
solids.1-5 Materials with larger pore sizes and stronger acid
properties than zeolites have been the object of this research.
These materials are prepared by exchanging the charge-
compensating cations present in the interlamellar space of the
swelling smectitic clays with hydroxy-metal polycations. When
the smectite clays are dispersed in water, the layers swell
because of the hydration of the interlamellar cations which act
as counterions to balance the negative charges of clay layers.
Therefore, polyoxocations in aqueous solutions can be interca-
lated into the interlayer space by cation exchange. On calcining,
the inserted polycations yield rigid, thermally stable oxide
species, named pillars, which prop apart the clay layers and
prevent their collapse. This process results in an interesting two-
dimensional porous structure of molecular dimensions.
Compared to other microporous solids, pillared clays have
several advantages. First, clays are abundant natural materials,
and pillared clays are synthesized under conditions milder than
those of the synthesis of aerogels, heteropolyoxometalates, and
zeolites.6 Another feature is that pillared clays have pore
openings of about 10 Å, larger than those of zeolites and
activated carbons, that facilitate the incorporation of metal ions,
either at the stage of pillar formation or in the postpillaring
treatment.3 Thus, pillared clays have a potential to be used as
molecular sieves and shape-selective catalysts for a wide range
of molecular sizes.
Several authors have reported various techniques of doping
pillared clays with controlled amounts of cations without causing
significant damages to the layered structures,8-10 although the
micropore structure and surface nature of the solids are altered
by the introduced cations, this resulting in a significant
modification in adsorption properties of the doped clays.11 The
microstructure developed in the pillaring process, in particular
the micropore size distribution (MPSD), determines and limits
the potential use of pillared clays in catalytic, purification, and
sorption-based separation processes. Therefore, the characteriza-
tion and quantitative evaluation of the microstructure is the
object of a considerable research effort,12-20 with experimental
and theoretical studies based on gas adsorption being the
approach most widely considered.
Malla and Komarneni21 improved the water adsorption
properties of pillared clays by introducing Ca2+ into the
interlayer spacing. Molinard and Vansant22 showed N2/O2
selectivities by cation-modified pillared clays with ion-
exchanged alkaline earth ions and reported no adsorption for
N2 and O2 at room temperature. Cheng and Yang23 also studied
the N2 and O2 adsorption capacities of alkali metal ion-
exchanged pillared clays showing for Li+/Zr-PILC better results
than those obtained by Molinard and Vansant. The incorporation
of different-sized charge-compensating alkali and alkaline-earth
cations into the structure of pillared clays has been examined
by Hutson et al.,11 finding only a change in the pore volumes
and a corresponding shift in the percentage of total micropore
† University of Cordoba.
‡ University of Colombia.
§ University of Salamanca.
Public University of Navarre.
* Corresponding author: e-mail andoni@unavarra.es.
10.1021/jp054954t CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/19/2005

23462 J. Phys. Chem. B, Vol. 109, No. 49, 2005
Barrera-Vargas et al.
volume from the micropores and supermicropores to the
ultramicropores. Zhu et al.24 studied the effect of heat treatments
on the pore and adsorption characteristics of sodium-doped Al-
PILCs. They observed that the adsorption capacity is clearly
related to the cation content and the calcination temperature.
Recently, the modification of the textural properties of Al-, Zr-,
and Ti-PILCs upon doping with copper has been investigated
by Bahranowski et al.,25 indicating that the effect of the copper
addition on the texture of pillared matrices depends on the nature
of pillars and on the method of doping, affecting the micropore
and the mesopore systems.
Platinum-supported catalysts have been widely studied and
used in various applications, alumina being the most used
support and several compounds being employed as source of
the metal. We have recently reported the incorporation of
platinum into pillared clays by means of impregnation with [PtII-
(NH3)4]Cl2, the resulting solids being used in the complete
oxidation of acetone and methylethyl ketone, two important
volatile organic compounds (VOCs).26,27 Another interesting
coordination compound of platinum which can be used as
precursor is [PtII(acac)2], acac being an acetylacetonate ligand,
having the advantage that only organic moieties are bonded to
platinum in the coordination compound, giving a good disper-
sion of platinum on the surface of the pillared clay.28 Thus, the
aim of this work is to contribute to the evaluation of the
microstructure developed by an alumina-pillared clay used as
support of platinum for the catalytic combustion of CO.
Figure 1. Nitrogen adsorption at -196 °C, in a semilogarithmic plot,
starting from low pressures: (O) Al-PILC, (0) 0.4-Pt/Al-PILC, (])
1.1-Pt/Al-PILC, (4) 1.8-Pt/Al-PILC.
Hydrogen (Agafano-Colombian, 99.90%) chemisorption ex-
periments were performed at 30 °C by a dynamic pulse method
using a Quantachrome ChemBET-3000 TPR/TPD. The samples
were reduced at 420 °C, then purged with argon (Agafano-
Colombian, 99.90%) for 0.2 h at 420 °C, and finally cooled to
room temperature under an argon flow. Hydrogen pulses were
injected at 30 °C until the area of consecutive eluted pulses
was constant.
2. Experimental Section
The natural clay mineral used in this work was a saponite
from Yunclillos deposit (Toledo, Spain). The fraction with
particle size smaller than 2 µm was obtained by aqueous
decantation of the natural clay and used for the intercalation
experiments.
3. Results and Discussion
The basal spacings of the parent saponite and of its pillared
modification with alumina, determined by XRD analysis, are
15.2 and 17.9 Å, respectively.
An aluminum polycation solution29,30 was prepared by slow
addition of a 0.1 mol/dm3 solution of Al(NO3)3‚9H2O (Merck,
95.0%) to a 0.2 mol/dm3 solution of NaOH (Merck, 99.0%)
under vigorous stirring, with an OH-/Al3+ mole ratio equal to
2.4 (pH ) 4.0). The hydrolyzed solution was allowed to age
for 48 h at 40 °C under continuous stirring. An Al/clay ratio of
6.6 mmol/g was used in the intercalation process. The clay
suspensions were kept in contact with the solution at 40 °C for
2.5 h, washed by centrifugation and dialysis until absence of
chloride, dried at 80 °C for 4 h, and calcined at 500 °C for 2 h.
X-ray powder diffraction (XRD) patterns of the solid were
obtained using a Philips PWO 1150/90 diffractometer with Ni-
filtered Cu KR radiation.
Supported platinum catalysts were prepared by wet impregna-
tion of the alumina-pillared clay with a solution of platinum-
(II) acetylacetonate (Pt(C5H7O2)2, Aldrich, 97%) in dioxane
(Merck, 99.5%). The resulting slurries were evaporated slowly
under reduced pressure in a rotavapor, and the obtained solids
were calcined in air at 500 °C for 5 h to give the Pt/Al-PILC
catalysts. The platinum content was determined by atomic
absorption spectroscopy using a Varian spectrophotometer
model A10. The samples will be referred to in wt %-Pt/Al-
PILC notation.
The study of the surface and the porous network of the
pillared clays using nitrogen adsorption data can provide useful
information about the location of the impregnated cations. The
nitrogen adsorption of the support and of the impregnated solids
starting from low pressures is shown in Figure 1, where it can
be seen that the adsorption isotherms of Al-PILC and 0.4-Pt/
Al-PILC were of type I in the Brunauer, Deming, Deming and
Teller (BDDT) classification,31 while the adsorption isotherms
of the rest of samples were of type II. The textural properties
are presented in Table 1. The Langmuir surface area (SLang) was
calculated from adsorption data in the relative pressure range
between 0.01 and 0.05, considering a nitrogen molecule cross-
sectional area of 0.160 nm2.31 The total pore volume (Vp) was
assessed from the amount of nitrogen adsorbed at a relative
pressure of 0.99, assuming that the density of the nitrogen
condensed in the pores is equal to that of liquid nitrogen at
-196 °C (0.81 g/cm3).31 The cumulative mesopore volume
(∑Vp) was determined for pores in the range between 15 and
500 Å using the Barrett-Joyner-Halenda (BJH) method.31 The
surface area of the samples varied between 92 and 253 m2/g
and their pore volumes between 0.142 and 0.229 cm3/g,
depending on the platinum content (see Table 1).
Several methods and models have been applied to the nitrogen
adsorption data for various pore geometries in order to
characterize the microporosity of the solids.19,32 In the present
work, the micropore size distributions (MPSD) derived from
the models proposed by Horvath and Kawazoe33 (HK), Cheng
and Yang14 (ChY), and Saito and Foley34 (SF) have been used
to study the microporous region of the solids. The physico-
chemical properties of the adsorbate-adsorbent system required
Nitrogen (Agafano-Colombian, 99.9990%) physisorption
experiments were performed at -196 °C using a Quantachrome
Autosorb-1-MP adsorption analyzer. The samples were previ-
ously degassed at 300 °C for 12 h at a pressure lower than 50
µmHg. The weight of each sample was 0.2 g. Only the nitrogen
adsorption data up to a relative pressure of 0.2 were taken into
account for the micropore characterization.

Pt/Alumina-Pillared Clays
J. Phys. Chem. B, Vol. 109, No. 49, 2005 23463
TABLE 1: Textural and Metallic Properties Derived from Nitrogen Adsorption at -196 °C and Hydrogen Adsorption at 30 °C
sample
Al-PILC
0.4-Pt/Al-PILC
1.1-Pt/Al-PILC
1.8-Pt/Al-PILC
SLanga (m2/g)
Sextb (m2/g)
Vpc (cm3/g)
∑Vpd (cm3/g)
SMete (m2/g)
DPtf (%)
dPtg (Å)
253 (Kh ) 486)
175 (K ) 468)
103 (K ) 462)
92 (K ) 453)
47
38
45
48
0.229
0.184
0.142
0.158
0.112
0.065
0.102
0.105
0.502
0.547
0.991
48.3
19.5
21.3
22.3
55.2
50.5
a Specific surface areas from the Langmuir equation (0.01 < p/p° < 0.05, interval of relative pressure). b Specific external surface areas obtained
from the t-plot method. c Specific total pore volumes at p/p° ) 0.99. d Cumulative pore volumes from the Barrett-Joyner-Halenda (BJH) method
f
(for pores in the 15-500 Å range). e Metal surface area. Platinum dispersion. g Mean size of the platinum particles. h Langmuir K values, characteristic
of the intensity of the adsorbate-adsorbent interactions.
Figure 2. Micropore size distributions derived from the Horvath-
Kawazoe model: (O) Al-PILC, (0) 0.4-Pt/Al-PILC, (]) 1.1-Pt/Al-
PILC, (4) 1.8-Pt/Al-PILC.
Figure 4. Micropore size distributions derived from the Saito-Foley
model: (O) Al-PILC, (0) 0.4-Pt/Al-PILC, (]) 1.1-Pt/Al-PILC, (4) 1.8-
Pt/Al-PILC.
summarized in Table 2. For comparison purposes, the micropore
volumes have been calculated by means of the Dubinin-
Astakhov (DA) equation35 in the relative pressure range between
0.01 and 0.05 (Vµp(DA)), and the results are also included in Table
2.
When the results of the Al-PILC and the platinum-doped
materials are compared, a continuous decrease of the adsorbed
nitrogen volume is observed for the supported solids in the
whole relative pressure range. This fact shows that the impreg-
nation with platinum produces a continuous loss of the textural
properties of the Al-PILC as the Pt content increases up to 1.1
wt %. The textural properties remain almost unchanged when
the platinum content increases from 1.1 to 1.8 wt % (see Table
1). If the specific mesopore volume is estimated by subtracting
the micropore volume from the total pore volume, it is revealed
that the mesopore volume remained almost unchanged and that
the pore volume loss suffered by the materials almost exclusively
affected the micropores of the Al-PILC. This suggests that some
platinum species may block the entrance to the porous network
and/or occupy the inner pores of Al-PILC during the preparation
process.
It has been reported that the structural heterogeneities of
materials, which can arise from networking effects related to
interconnected pores of various sizes and shapes, can be
described by the fractal geometry from theoretical and experi-
mental studies of adsorption. The fractal dimension, which
quantitatively evaluates the fractal geometry,36 is considered as
a measure of the surface and structural irregularities of a solid.
The value of this dimension can vary from 2 to 3,37 the lower
limiting value of 2 corresponding to a perfectly regular smooth
surface, whereas the upper limiting value of 3 is related to the
maximum allowed complexity of the surface. The fractal
dimensions (DF) of the solids calculated by the method proposed
Figure 3. Micropore size distributions derived from the Cheng-Yang
model: (O) Al-PILC, (0) 0.4-Pt/Al-PILC, (]) 1.1-Pt/Al-PILC, (4) 1.8-
Pt/Al-PILC.
by these models were estimated as proposed by Gil and
Grange.16 The MPSD obtained in this way are shown in Figures
2-4. As can be seen, the distributions are bimodal and do not
depend a lot on the particular model. It seems that the
microstructure of these materials can be equally described by
either slitlike or cylindrical pore geometries when nitrogen is
used as adsorbate.19 When the MPSD of the various samples
are compared, no differences are observed. The distributions
show a first maximum at about 4.9 Å and a second maximum
at 6.4 Å. The pore diameter ranges of the second maximun of
the distributions are summarized in Table 2.
The micropore volumes (Vµp(HK)), calculated according to the
HK model and the method proposed by Gil and Grange,16 are

23464 J. Phys. Chem. B, Vol. 109, No. 49, 2005
Barrera-Vargas et al.
TABLE 2: Microporous Properties Derived from Nitrogen Adsorption at - 196 °C
slitlike model
DA equation
Ee (kJ/mol)
g
sample
Al-PILC
0.4-Pt/Al-PILC
1.1-Pt/Al-PILC
1.8-Pt/Al-PILC
Vµp(HK)a (cm3/g)
dpHKb (Å)
dpChYc (Å)
Vµp(DA)d (cm3/g)
nf
DF
0.103
0.072
0.042
0.038
5.5-12.5
5.4-12.6
5.5-12.5
5.5-12.3
5.5 - 9.9
5.3-10.4
5.5-10.7
5.5-10.2
0.097
0.069
0.035
0.035
19.2
19.8
13.7
19.7
2.2
1.9
6.7
2.1
2.79
2.77
2.76
2.74
a Specific micropore volumes derived from the Horvath-Kawazoe (HK) model. b Pore diameter range of the second maximum of the Horvath-
Kawazoe micropore size distributions. c Pore diameter range of the second maximum of the Cheng-Yang micropore size distributions. d Specific
micropore volumes derived from the Dubinin-Astakhov (DA) equation. e Characteristic energy from the Dubinin-Astakhov equation. f Exponent
of the Dubinin-Astakhov equation. g Fractal dimensions calculated from Avnir-Jaroniec method in the relative pressure range 0.08-0.2.
by Avnir and Jaroniec38 in the range of relative pressures
between 0.08 and 0.239 are included in Table 2. The values
obtained for the materials of this study, 2.74-2.79, are close
to the upper limiting value of 3, thus indicating a high
heterogeneity of the solids, caused by structural and/or surface
factors.
croporous network of the alumina-pillared clay after the catalyst
preparation process.
4. Conclusions
The platinum doping produces a continuous loss of the
textural properties of the alumina-pillared clay, as a function
of the quantity of the metal. This effect can have a strong
influence on the accessibility of the molecules of adsorbates
and reactants to active sites in the micropores when the solids
are used as adsorbents or catalysts. The variation of the
accessibility with the metal content can be utilized to control
or improve the performance of the solids for certain chemical
reactions.
The results from nitrogen physisorption data at -196 °C
indicate that the mesopore volume remains almost unchanged
and that the pore volume loss suffered by the materials almost
exclusively affected the micropores of the pillared clay. Several
approaches have been considered and compared in order to
characterize the microporous structure of the pillared clays. The
results obtained suggest that some platinum species occupy the
inner porous network of the alumina-pillared clay after the
preparation process, blocking the pore entrances. The complexity
of these materials is confirmed by the structural analysis from
the fractal dimension.
All the DF values are very similar, suggesting a high structural
similarity of the solids. The fractal dimension is estimated from
the adsorption data in the range of relative pressures 0.08-0.2
where the total micropore filling takes place and only the
supermicropores are characterized. More information about the
structural heterogeneity of the materials can be obtained from
the MPSD, the specific micropore volumes, and the n exponent
of the Dubinin-Astakhov equation. The comparison of the
specific micropore volumes and of the n exponents, also given
in Table 2, suggests that there is a substantial portion of
micropores with steric hindrance for the accessibility of nitrogen
in the samples 1.1-Pt/Al-PILC and 1.8-Pt/Al-PILC. In the case
of the sample 1.1-Pt/Al-PILC, this loss of accessibility can be
due to the presence of platinum species that block the pore
entrances generating fine pores, as indicates the high value of
n. An increase of the platinum content, sample 1.8-Pt/Al-PILC,
reduces the accessibility to the fine pores, diminishing the value
of n.
The measurement of the metallic surface of the platinum
pillared clays using hydrogen adsorption data provides informa-
tion about the location of the impregnated cations. The catalyst
properties related to the metal phase are shown in Table 1. The
metal dispersion, DPt, was calculated from the chemisorption
results using the formula D (%) ) 0.039U/W, where U is the
µmol of gas chemisorbed per gram of catalyst and W the metal
weight fraction, assuming a stoichiometry of one hydrogen
molecule adsorbed per two surface platinum atoms.40,41 An
average cross section area per surface platinum atom of 8.41
Å2, as proposed by Lemaitre et al.,41 has been used for the
calculation of the metal surface area, SMet. The mean surface
particle size was calculated from the metal dispersion values,
assuming that the metallic particles on the surface of the catalysts
are spherical and wholly exposed to adsorption42 and that the
density of platinum is equal to 21.45 g/cm3.41 The metal surface
area of the samples varied between 0.502 and 0.991 m2/g and
the mean particle size between 22.3 and 55.2 Å, depending on
the platinum content (see Table 1).
The metal properties evaluated by hydrogen chemisorption
at 30 °C and after reduction of the Pt/Al-pillared saponite at
420 °C can also explain the textural properties obtained from
nitrogen adsorption.
Acknowledgment. Financial support by the Spanish Ministry
of Education and Science and FEDER (MAT2003-01255 and
MAT2002-03526) is gratefully acknowledged. S.A.K. acknowl-
edges financial support by the Ministry of Education and Science
through the Ramon-y-Cajal program.
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