Influence of Na content on the catalytic properties of Pt-Ir/Al 2O3 catalysts for selective ring opening of decalin

Article abstract of DOI:10.1016/j.apcata.2014.04.036

The influence of the Na addition on Pt-Ir/Al₂O₃ on the reaction of selective ring opening of decalin and methyl cyclopentane was studied. The catalysts were evaluated by means of TPR, pyridine TPD, TEM, FTIR-CO and by the reaction tests of cyclohexane dehydrogenation, hydrogenolysis of cyclopentane, selective ring opening of decalin and methyl cyclopentane. It was found that Na strongly decreases the acidity and influences the metal properties of Pt-Ir/Al₂O₃ catalysts. The interaction between Pt and Ir is increased by the presence of Na, with larger metal crystals being obtained on the support modified by Na. As a consequence, the catalytic properties are also modified and the CP/CH ratio (parameter of demanding/not demanding reaction) is increased with sodium concentration. In the case of MCP reaction, Na strongly decreases the aromatic and n-C₆ formation and favors the selective reaction mechanism, thus favoring the formation of 2MP and 3MP. For the decalin reaction, it was found that the yield to the dehydrogenated product of the catalysts supported on Al₂O₃-Na is lower than that of the free Na-alumina supported catalyst, whereas the opposite occurs with the yield to C₁ due to the higher hydrogenolytic and lower dehydrogenation activity of the catalysts with Na. The yield to ring contraction (RC) products is higher on the catalyst without Na due to the higher acidity which favors the ring contraction reaction and the lower metallic activity; thus, the transformation of the RC product to ring opening products is decreased.

Full text of DOI:10.1016/j.apcata.2014.04.036

Applied Catalysis A: General 480 (2014) 42–49
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Influence of Na content on the catalytic properties of Pt-Ir/Al2O3
catalysts for selective ring opening of decalin
María A. Vicericha, Marcelo Oportusb, Viviana M. Beniteza, Patricio Reyesb,
Carlos L. Piecka,∗
a Instituto de Investigaciones en Catálisis y Petroquímica (INCAPE) (FIQ-UNL, CONICET), Santiago del Estero 2654, S3000AOJ, Santa Fe, Argentina
b Departamento de Físico Química, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of the Na addition on Pt-Ir/Al2O3 on the reaction of selective ring opening of decalin and
methyl cyclopentane was studied. The catalysts were evaluated by means of TPR, pyridine TPD, TEM, FTIR-
CO and by the reaction tests of cyclohexane dehydrogenation, hydrogenolysis of cyclopentane, selective
ring opening of decalin and methyl cyclopentane. It was found that Na strongly decreases the acidity and
influences the metal properties of Pt-Ir/Al2O3 catalysts. The interaction between Pt and Ir is increased
by the presence of Na, with larger metal crystals being obtained on the support modified by Na. As a
consequence, the catalytic properties are also modified and the CP/CH ratio (parameter of demanding/not
demanding reaction) is increased with sodium concentration. In the case of MCP reaction, Na strongly
decreases the aromatic and n-C6 formation and favors the selective reaction mechanism, thus favoring the
formation of 2MP and 3MP. For the decalin reaction, it was found that the yield to the dehydrogenated
product of the catalysts supported on Al2O3-Na is lower than that of the free Na-alumina supported
catalyst, whereas the opposite occurs with the yield to C1 due to the higher hydrogenolytic and lower
dehydrogenation activity of the catalysts with Na. The yield to ring contraction (RC) products is higher
on the catalyst without Na due to the higher acidity which favors the ring contraction reaction and the
lower metallic activity; thus, the transformation of the RC product to ring opening products is decreased.
Received 23 December 2013
Received in revised form 16 April 2014
Accepted 21 April 2014
Available online 30 April 2014
Keywords:
Selective ring opening
Pt-Ir/Al2O3 catalysts
Na influence
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
emissions of polyaromatic hydrocarbons. By reducing the tenor of
these compounds, a cleaner burning and increased cetane number
The diesel production process is very complex and involves the
selection and mixing of different oil fractions to meet certain spec-
ifications [1]. Diesel quality is measured by cetane number (CN).
Quality combustion occurs when there is rapid ignition followed
by a complete and uniform burning of fuel. The higher the CN,
the lower the ignition delay and the better the quality of com-
bustion. The incorporation of 0.5% additives (normally organic
nitrates) increases the CN by 10 units, improves cold-start per-
formance, reduces combustion noise and may reduce particulate
matter (PM) emissions. Unfortunately, the additives increase the
amounts of NOx emissions [2]. Diesel quality is affected by the
presence of pollutants such as sulfur, which contributes to the emis-
sion of SOx. The aromatics content also affects the quality of diesel
since it influences the combustion and particulate formation and
may be obtained [2,3]. The reduction of sulfur and aromatics can be
achieved with the use of technologies known as hydrotreating and
hydrocracking [3]. One alternative is to transform the light cycle
oil (LCO) stream in high quality diesel [4] because it represents
approximately 10–20% of the product of FFC (catalytic cracking)
and is sold as a product of little value for heating. The boiling range
of the LCO is similar to that of gas-oil. However, it has a large amount
of polycyclic aromatic compounds which makes its cetane number
unacceptably low (<30). Therefore, the opening of at least one of
the naphthene rings is needed to obtain a suitable cetane number.
Some noble metals such as Pt, Pd, Ir, Ru and Rh selectively
activate the ring opening of cyclic hydrocarbons to produce cor-
responding paraffins with the same number of carbon atoms [5,6].
The experiments carried out in this work indicate that supported
iridium catalysts are the most active ones since they have a low
contribution of exocyclic C C breaks, although the most suitable
distribution of the rings occurs on rings of 5 carbon atoms. One
solution is to promote ring contraction by means of a support
∗
Corresponding author. Tel.: +54 342 4533858; fax: +54 342 4531068.
E-mail address: pieck@fiq.unl.edu.ar (C.L. Pieck).
http://dx.doi.org/10.1016/j.apcata.2014.04.036
0926-860X/© 2014 Elsevier B.V. All rights reserved.

M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
43
having medium acidity. Industrial patents of SRO make use of irid-
ium catalysts supported on medium acidity mesoporous supports
controlled by the addition of alkali [7].
metallic salts on the support particles. The resulting mixture was
slowly dried at 70 ◦C until a dry solid was obtained. The drying
process was completed in an oven (overnight at 120 ◦C) and the
samples were then stabilized by calcination under flowing air for
4 h at 300 ◦C, and cooled down to ambient temperature under N2.
Samples were finally reduced under flowing H2 at 500 ◦C for 4 h. All
heating and cooling steps were set at 10 ◦C min−1. The concentra-
tion of aqueous solutions of metallic precursor salts was adjusted
to obtain the following metal loadings on the finished catalysts:
1.0%Pt-2%Ir-0.5%Na, 1.0%Pt-2%Ir-1.0%Na and 1.0%Pt-2%Ir-1.5%Na.
Electropositive promoters such as alkali and alkaline metals are
often used as additives in heterogeneous catalysis to increase the
catalytic activity, selectivity and/or stability. The ability of alkaline
additives to modify the catalytic activity and/or selectivity can be
understood by considering the effects induced on chemisorption
properties of the metal surface. These effects may be originated
from electronic interactions such as changing electron density
due to the formation of alkali-metal species, and/or electrostatic
interactions associated with alkali metal ions, and/or site blocking
[8–10]. It is widely recognized that the electron density of a sup-
ported noble metal is increased by the addition of alkali and alkaline
earth metals [11–15].
Other researchers also proposed different supports in order to
improve the performance of the catalysts for SRO. Djeddi et al. [16],
studying the SRO of MCP using Pt-Ir catalysts supported on TiO2 in
a wide reaction temperature range, found that all the catalysts have
low selectivity to cracking products. On the other hand, Fe, Mo and
Fe-Mo supported on silica mesoporous supports showed that the
highly dispersed isolated tetrahedral entities and highly dispersed
small FeOx nanoclusters have superior catalytic performance in the
ring opening of MCP. Moreover, the synergy between Fe and Mo was
not observed at low reaction temperatures (<200 ◦C) [17]. Tetralin
hydroconversion over supported iridium catalysts on silica, alu-
mina and amorphous silica-alumina supports has been investigated
[18]. It was reported that the intermediate concentration of silica
(40 wt%) leads to the highest activity and selectivity, in correla-
tion with the Brønsted acidity measured by infrared spectroscopy
of adsorbed pyridine. Indan ring opening was achieved with Ru-
Pd/Al2O3 catalysts where the catalyst with Ru/Pd = 4 atomic ratio
displayed the same high single cleavage selectivity and as low
light formation as iridium [19]. McVicker et al. [20] reported the
high activity and selectivity of the Ir/Al2O3 catalyst for RO of alkyl-
substituted cyclopentanes and bicyclic naphthenes of C5.
2.2. Analysis of metal contents
The Pt, Ir and Na contents of the catalysts were determined
by inductively coupled plasma-optical emission spectroscopy
equipment (ICP-OES) (Perkin Elmer, Optima 2100 DV) after acid
digestion.
2.3. Temperature-programmed reduction (TPR)
This technique allows gathering information about the inter-
action of the metal components by means of the measurement of
the hydrogen consumption during the reduction of the oxides at
constant heating rate. The temperature at which reduction occurs
and the number of reduction peaks depend on the oxidation state
of the metals, the interaction of the oxides among them and with
the support, and the possible catalytic action of Pt or other ele-
ments present or generated during reduction. These tests were
performed in an Ohkura TP2002 equipment provided with a ther-
mal conductivity detector. At the beginning of each TPR test, the
catalyst samples were pretreated in situ by heating under flowing
air at 350 ◦C for 1 h. Then, they were heated from room temperature
to 700 ◦C at 10 ◦C min−1 in a controlled gas stream of 5.0% hydrogen
in argon.
2.4. Temperature-programmed desorption of pyridine (TPD)
Resasco et al. [21] reported that in the case of the conversion
of decalin on acidic catalysts, the acid function alone is not able
to yield products with CN significantly higher than that of the
decalin feed. For example, from decalin (CN = 36) by acid mech-
anism the product with higher CN is 2,3,6 trimethyl heptanes
(CN = 30). Similarly, no significant gain in CN can be expected
with hydrogenolysis metal catalysts operating via the dicarbene
mechanism because the product with higher CN = 39 that could be
obtained is 1,2 dipropyl cyclohexane. Only in the case of selective
metal-catalyzed hydrogenolysis, with preferential cleavage at sub-
stituted C C bonds, the predicted products have CN substantially
higher than the decalin feed. For this reason, we study the influence
of sodium content on Pt-Ir/Al2O3 on the conversion and selectivity
of decalin. In other words, we try to perform the SRO of decalin
using catalysts with low acidity in order to promote only the metal
mechanism.
The amount and strength distribution of acid sites were assessed
by means of the temperature-programmed desorption of pyridine
(Py). 200 mg of catalyst was first immersed in a closed vial contain-
ing pure pyridine (Merck (99.9%)) for 4 h. Then the vial was opened
and the excess base was allowed to evaporate at room conditions
until apparent dryness. The sample was then loaded into a quartz
tube of 0.4 cm diameter over a quartz wool plug. A constant flow of
nitrogen (40 cm3 min−1) was kept through the sample. A first step
of desorption of weakly physisorbed base and sample stabilization
was performed by heating the sample at 110 ◦C for 2 h. Then, the
temperature of the oven was raised to a final value of 550 ◦C at a
heating rate of 10 ◦C min−1. The reactor outlet was directly con-
nected to a flame ionization detector. The detector signal (in mV
units) was sampled at 1 Hz and recorded in a computer device.
2.5. Transmission electron microscopy
2. Experimental
Transmission electron micrographs (TEM) and electron diffrac-
tion patterns (ED) were obtained in a Jeol JEM 1200 EXII microscope.
The supported catalysts were ground in an Agatha mortar and dis-
persed in ethanol. A diluted drop of each dispersion was placed on
a 150 mesh copper grid coated with carbon.
2.1. Catalyst preparation methods
␥-Al2O3 (Cyanamid Ketjen CK-300; pore volume = 0.5 cm3 g−1
,
Sg = 180 m2 g−1, 35–80 mesh) was calcined for 4 h at 500 ◦C under
flowing dry air. Firstly, a solution of NaOH on the support was
added in order to obtain a desired concentration of Na (0.5, 1.0 and
1.5 wt%) and it was left at rest for 1 h. Then, support samples were
left in contact with the amount of aqueous solutions of metallic pre-
cursors (H2PtCl6 and H2IrCl6) required for the desired metal loading
for 2 h at room temperature in order to get a uniform distribution of
2.6. Fourier transformed infrared (FTIR) spectroscopy of
chemisorbed CO
FTIR spectra of adsorbed CO were obtained in order to study
the effect of Ir deposition on the properties of the metal func-
tion. The spectra of chemisorbed CO for the prepared catalysts

44
M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
were recorded within the wavenumber range of 4000–1000 cm−1
.
A Shimadzu Prestige-21 spectrometer with a spectral resolution
of 4 cm−1 was used. Spectra were recorded at room temperature
and self-supported wafers with a diameter of 16 mm and a weight
of 20–25 mg were used. The experimental procedure was as fol-
lows: catalyst samples were reduced in a hydrogen flow at 400 ◦C
(reached at a 10 ◦C min−1 heating rate) for 30 min. Samples were
then outgassed at 2.7 × 10−3 Pa and 400 ◦C for 120 min. After an
initial (I) spectrum had been recorded, the samples were exposed
to a CO pressure of 4000 Pa for 5 min and then a second (II) FTIR
spectrum was recorded. The chemisorbed CO absorbance for each
sample was obtained by subtracting spectrum I from spectrum II.
2.7. Cyclopentane hydrogenolysis (CP)
The reaction was performed in a glass reactor (length 10 cm,
diameter 1 cm) under the following conditions: catalyst mass
150 mg, temperature 270 ◦C, pressure 0.1 MPa, H2 flow rate
40 cm3 min−1, cyclopentane flow rate 0.483 cm3 h−1. Cyclopentane
was fed to the reactor using a Sage Instruments 341B syringe pump.
Before the reaction was started, the catalysts were treated in H2
(60 cm3 min−1, 270 ◦C, 1 h). The reaction products were analyzed
by online gas chromatography using a ZB-1 capillary column. The
error in the conversion value in the CP hydrogenolysis test is about
8%. Only values of initial conversion at 5 min time-on-stream are
reported.
2.8. Cyclohexane dehydrogenation (CH)
The reaction was performed in a glass reactor (length 10 cm,
diameter 1 cm) under the following conditions: catalyst mass
50 mg, temperature 300 ◦C, pressure 0.1 MPa, H2 flow rate
80 cm3 min−1 and cyclohexane flow rate 1.61 cm3 h−1. Sage Instru-
ments 341B syringe pump was used to introduce the cyclohexane
into the reactor. Before the reaction was started, the catalysts were
treated in H2 (80 cm3 min−1, 500 ◦C 1 h). The reaction products
were analyzed by online gas chromatography using a ZB-1 capillary
column. All points reported are mean values obtained by averaging
12 consecutive measurements equally spaced along the run. No sig-
nificant catalyst deactivation was observed in any run. The average
error was less than 3%.
Fig. 1. TPR profiles of Pt and Ir monometallic catalysts with and without Na.
has a pronounced reduction peak at ca. 200 ◦C and a minor reduc-
tion band between 250 and 300 ◦C. This reduction pattern has been
frequently observed by several authors [22–24] and it is gener-
ally accepted that the first peak is due to the reduction of Pt oxide
species weakly interacting with alumina. The less important second
reduction band is assigned to the reduction of Pt oxychlorinated
species in strong interaction with the support [25]. It can be seen
that the monometallic Ir catalyst has a big peak centered about
240 ◦C as reported elsewhere [26]. This peak begins at 150 ◦C, fin-
ishes at 350 ◦C and has a shoulder at 260 ◦C. The broad reduction
zone evidences the different interactions between the Ir oxides
species and the support and the different particle sizes of the Ir
oxide agglomerates. The Na addition shifts the reduction peaks, and
the Pt-Na catalyst has a broad reduction peak which is centered at
240 ◦C. This could be due to the fact that Na inhibits the hydrogen
spillover interfering in the reduction of the metals [26]. However,
an opposite effect can be seen over the monometallic Ir catalyst.
Probably, Na produces a strong decrease of support acidity leading
to weak metal–support interaction.
2.9. Methylcyclopentane ring opening (MCP)
Before the reaction, samples were reduced at 250 ◦C for 1 h. The
conditions used were: mass = 135 mg, H2 flow rate = 36 cm3 min−1
,
MCP flow = 0.362 cm3 min−1, reaction temperature = 250 ◦C and
time = 2 h. The reaction products were analyzed on a Varian CX 3400
gas chromatograph equipped with a capillary column Phenomenex
ZB-1 connected online.
2.10. SRO of decalin
Fig. 2 shows the TPR profiles of Pt(1.0)-Ir(2.0)/Al2O3-Na(x) cata-
lysts. The bimetallic Pt-Ir/Al2O3 catalyst has a reduction peak close
to 200 ◦C due to a simultaneous reduction of Pt and Ir oxides making
clear the strong Pt-Ir interaction. Sodium addition shifts slightly the
reduction peak to a lower temperature which can be attributed to
a decrease of support acidity. As a consequence, the metal–support
interaction is decreased making it easier to reduce metal oxides.
On the other hand, as mentioned above, Na inhibits the hydrogen
spillover interfering in the reduction of the metals [26]. These two
opposite phenomena which lead to the addition of sodium cause
a slight displacement of the reduction peaks. In spite of the cat-
alyst with 1.5% sodium no peak shift was observed with respect
to the catalyst without sodium. It is important to note that in this
latter catalyst, the formation of a shoulder in the reduction peak
All SRO experiments were performed in a stainless steel,
autoclave-type stirred reactor. The reaction conditions were: tem-
perature = 300–325–350 ◦C, hydrogen pressure = 3 MPa, stirring
rate = 1360 rpm, volume of decalin = 25 cm3, catalyst loading = 1 g
and reaction time = 6 h. A sample was taken at the end of the
experiments and it was analyzed using a Varian 3400 CX gas chro-
matograph equipped with a capillary column (Phenomenex ZB-5)
and FID.
3. Results and discussion
Fig. 1 shows the TPR profiles of monometallic Pt and Ir cata-
lysts with and without 1 wt% of Na. The monometallic Pt catalyst

M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
45
Fig. 2. TPR profiles of bimetallic catalysts with different Na contents.
is observed, which intensifies by increasing the amount of Na. This
can be attributed to the reduction of Pt species in strong interaction
with Na, as shown in Fig. 1.
The pyridine thermodesorption technique provides valuable
information about the acidity and acid strength distribution of solid
catalysts. The area under the desorption curves is proportional to
total acidity. Table 1 shows the areas relative to the sodium-free cat-
alyst. As expected, the acidity decreases with increasing Na content
because they neutralize the acid sites of alumina. Furthermore, the
TPD profiles (no shown) revealed that the strength of the sites is
not significantly modified by Na addition because the temperature
desorption of pyridine is similar for all the catalysts. This indicates
that Na neutralizes the strengths and weaknesses of acid sites.
Fig. 3 shows the particle size distribution of the Pt (1.0)-Ir
(2.0)/Al2O3-Na(x) catalysts. It can be observed that with increas-
ing sodium concentration there is an increase in the metal particle
size. This can be explained considering that as sodium hydrox-
ide neutralizes the acid sites of the alumina support, it decreases
the metal–support interaction and, therefore, during the reduction
step, an enhancement in the metal particle size can occur. It can
be noted that the distribution of the particles of the catalyst with
1.5 wt% of Na has a significant fraction of particles in the 2–2.5 nm
range. This agrees with the TPR where the appearance of a shoulder
can be observed at a higher temperature reduction of the catalysts
with higher sodium content.
Fig. 3. Particle size distribution obtained by TEM of the bimetallic catalysts with
different Na contents.
The interaction between the metals was also studied by FTIR-
CO spectroscopy. Fig. 4 shows the IR spectra at 30 Torr in the
1850–2150 cm−1 wavelength range for the monometallic and
bimetallic catalysts under study. Fig. 4 shows only the wave num-
ber region corresponding to linear CO because the band due to CO
adsorbed in the bridge form, Pt2CO, was too small to be studied.
It can be seen that the monometallic Pt/Al2O3 catalyst shows an
absorption peak at 2085 cm−1 which corresponds to the adsorp-
tion of linear CO on Pt [27–29]. The adsorption peak shifts to a lower
frequency (2077 cm−1) on the Pt-Na/Al2O3 catalyst. Similar results
were reported by Shimizu et al. [30] on Pt-Na/SiO2 catalysts. They
attributed the shift to lower frequency to an increase in the electron
density of Pt by Na. They also proposed that the electron charge of
the Pt metal particles increases as the amount of electropositive
modifier (Na) increases as a result of the increased electron charge
of the oxygen atoms on the support. The increase in the electron
density of the Pt metal particles may be partially attributed to the
direct electron donation from Na to the Pt metal particles.
The monometallic Ir/Al2O3 catalyst shows two absorption bands
centered about at 2087 and 2020 cm−1. The incorporation of Na
shifts the absorption peak to 2064 cm−1 while the other peak at
2020 cm−1 disappears. Adsorption of CO on Ir/Al2O3 was studied
by Solymosi et al. [31]; they attributed a band at 2080–2050 cm−1
to the presence of Ir–CO complexes.
Table 1
Normalized pyridine TPD areas, average cyclohexane conversion and cyclopentane
conversion at 5 min time-on-stream for Pt(1)-Ir(2.0)/Al2O3-Na(x) catalysts.
Catalyst Na
(wt%)
Pyridine TPD
areas
CP conversion
(%)
CH conversion
(%)
CP/CH
Three bands have been identified in the 2100–1900 cm−1 range
on the FTIR-CO of Ir catalysts [32–38]. Those at 2087 and 2020 cm−1
correspond to linearly bonded CO adsorbed either on large and
small Ir particles [33] or over high- (planes) and low-coordinated
(edges, corners, etc.) Ir sites [39], respectively. The disappearance
0.0
0.5
1.0
1.5
1.00
0.74
0.36
0.23
94.0
97.9
88.9
96.2
44.7
37.4
32.0
25.4
2.10
2.62
2.78
3.79

46
M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
Fig. 5. FTIR spectra of CO adsorbed on bimetallic catalysts with and without Na at
30 Torr and the sum of the spectrum of monometallic Pt and Ir catalysts with and
without Na.
ensemble of neighboring metal atoms in order to form adsorbate
bonds with the proper strength. The geometric model has been
refined as “ensemble-size” model [43–45] on the assumption that
reaction rates are proportional to the probability of finding par-
ticular groups of neighboring atoms. Two classical test reactions
are used that have an entirely different behavior according to the
geometric factor theory: cyclopentane (CP) hydrogenolysis, which
is a demanding one [46], and cyclohexane (CH) dehydrogenation
which is non-demanding under the used experimental conditions
[41].
It can be seen that the conversion of cyclohexane decreases
as the Na concentration of the support increases whereas the
cyclopentane conversion does not change significantly. Conse-
quently, the CP/CH (parameter of demanding/not demanding
reaction) ratio increases with sodium concentration.
Both reactions are only catalyzed by the metal function; there-
fore, the variation can be attributed to the way in which the Na
interacts with the active phase (Pt-Ir), and how Na would alter the
interaction between the two metals. Sodium in interaction with
the active phase may, by a simple geometric effect, prevent the
adsorption of hydrocarbons. In this case, the hydrogenolysis reac-
tion should be the most affected one because it is a demanding
reaction. As a consequence, the CP/CH ratio should decrease with
increasing Na concentration. The interaction of sodium with the
active phase (Pt-Ir) should also produce a change in the electronic
structure of metals by changing the catalytic activity for both reac-
tions. Finally, Na could favor the formation of large ensembles of Pt
and Ir as it was asserted by TEM analysis.
Fig. 4. FTIR spectra of CO adsorbed on monometallic and bimetallic catalysts with
and without Na at 30 Torr.
of the absorption band at 2020 cm−1 due to Na addition can be
explained by an increase of crystal size while the shift to lower
frequencies by an increase of the electron density of the Ir metal
particles in a similar way to Pt/Al2O3 catalysts.
The influence of Na on bimetallic catalysts is apparently less
important because the shift to lower frequencies occurs only to a
slight extent. The absorption peak of the bimetallic catalysts (with
and without Na) appears at about 2075–2078 cm−1. It is interest-
ing to compare the absorption peaks of the bimetallic catalysts with
the corresponding spectrum obtained by the sum of monometal-
lic catalysts (Fig. 5). The absorption peak at 2020 cm−1 due to Ir
is not observed on the bimetallic Pt-Ir/Al2O3 catalyst. Moreover,
the absorption peak of Pt is shifted from 2087 to 2073 cm−1. In the
case of the promoted Na catalyst, there are shifts from 2069 cm−1
(spectrum due to the sum on monometallic Pt and Ir catalysts) to
2082 cm−1 (bimetallic Pt-Ir/Al2O3-Na(1.0%) catalyst). These results
indicate a strong interaction between Pt and Ir. In a previous work,
we confirmed the existence of a strong interaction between Pt and Ir
on alumina supported catalysts by TEM [40]. These results are con-
sistent with TPR experiments where a strong interaction between
Pt and Ir was observed. Moreover, they are in agreement with the
results of CO FTIR absorption.
Table 1 shows the conversion values of the reactions of
hydrogenolysis of cyclopentane and cyclohexane dehydrogenation.
Boudart et al. [41,42] classified catalytic reactions as “demanding”
(sensitive to morphological structure) and “facile” (structure-
insensitive), according to the requirement or not of a particular
The lower dehydrogenation activity as sodium increases is
explained by an increased particle size of Pt and Ir since the dehy-
drogenation of cyclohexane is not a demanding reaction whereas

M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
47
the hydrogenolysis reaction of CP would be benefited. Large crystals
of Pt and Ir are more active than small particles since hydrogenol-
ysis is a demanding reaction. The conversion of cyclopentane on
the hydrogenolysis reaction does not change. The increased size of
the metal particles causes an increase in the conversion by atom
exposed but the amount of superficial atoms is reduced.
The results obtained agree with those of TPR and TEM which
show that sodium favors the formation of large ensembles of Pt-Ir.
However, the electronic effect of sodium should not be discarded
because the increased size of the particles is not enough to explain
the observed behavior. Some authors reported a partial covering of
Pt surface by NaOx on Pt/TiO2 catalysts [47] and Pt/SiO2 catalysts
[30]. For K-loaded Pt/Al2O3, Busca et al. also proposed that Pt cen-
ters lie in close proximity of potassium oxide species [48]. Shimizu
et al. [30] found that the electron charge of the Pt atoms increases
as the amount of the electropositive modifier (Na) increases.
The ring opening reaction of methylcyclopentane produces
branched paraffins (2-methylpentane, 3-methylpentane) and lin-
ear paraffins (n-hexane). Differences in the distribution of the ring
opening products of MCP are attributed to the intrinsic nature of the
metals, metal dispersion, the nature of the support, the hydrocar-
bon adsorption mode on metals and experimental conditions [46].
In Pt/Al2O3 highly dispersed catalysts (d < 2 nm) breaking substi-
tuted C C bonds are favored since the ␲ allyl mechanism, which
requires a flat adsorption of three neighboring carbon atoms, inter-
acts with a single-metal site. Therefore, the products obtained
from the opening of the MCP in highly dispersed catalysts are 2-
methylpentane, 3-methylpentane and n-hexane. In the Pt/Al2O3
catalyst with lower dispersion (d > 2 nm), the preferential rupture
of unsubstituted C C bond takes place. This can be attributed to
a dicarbene mechanism where two endocyclic carbon atoms are
involved with adjacent metal atoms. In this case, the reaction
products are only 2-methylpentane and 3-methylpentane while
n-hexane is not formed. In the case of Ir catalysts the reaction
was found insensitive to the dispersion (only 2-methylpentane
and 3-methylpentane are obtained) [46,49]. Furthermore, Ir cat-
alysts showed a tendency to break endocyclic C C bonds of MCP
[30]. In this case, the ring opening of MCP takes place primarily
through a mechanism in which the intermediate dicarbenes are
adsorbed perpendicular to the metal surface. van Senden et al. [49]
using iridium catalysts with different dispersion reported that the
opening of endocyclic C C bond of MCP occurs when the cata-
lysts were covered with carbonaceous species [50]. Contrary to
Ir supported on alumina where the break occurs on substituted
endocyclic C C bonds, on Ir supported on SiO2 it was observed
that the predominant mechanism was dicarbene [51]. The liter-
ature remarks that the reactivity of iridium is much higher than
that of Pt [52] although the catalytic results did not confirm major
differences in the reaction rate [53]. Maire et al. [54] concluded
that the product distribution in RO reactions depends on the oper-
ating reaction mechanisms. According to a non-selective reaction
path that would occur on small metallic particles, with the same
breaking probability as any of the cyclic bonds, a product ratio n-
C6:2MP:3MP of 2:2:1 is expected. On the other hand, if a selective
mechanism where the breaking of substituted endocyclic bonds
is forbidden operates, such products ratio would be 0:2:1. Such
Fig. 6. Conversion values as a function of time obtained in the MCP reaction of
Pt(1.0)-Ir(2.0)/Al2O3-Na(x) catalysts.
mechanism would proceed on metallic aggregates of higher size.
An alternative mechanism, partially selective, with an intermediate
product distribution ratio, can also be possible.
Fig. 6 shows the conversion values as a function of time obtained
in the MCP reaction of Pt(1.0)-Ir(2.0)/Al2O3-Na(x) catalysts. It can
be seen that the conversion decreases with reaction time due to the
deposition of coke in all the catalysts. Moreover, it is observed that
the catalysts supported on Al2O3 with sodium have a higher activity
than the catalyst without sodium. However, the increased activity
is not directly related to the sodium content or to the cyclohexane
dehydrogenation as well as with acidity but it has a direct relation
with the cyclopentane hydrogenolysis activity reported in Table 1
for the catalyst without Na. That means the reaction is controlled
by the metal function mainly by the hydrogenolytic character of
the metal phase.
The values of selectivities towards reaction products taken at
15 min of time on stream (i.e. with very low amount of coke
deposited on catalyst surface) are shown in Table 2. It can be
observed that the Pt-Ir catalyst without Na have a high selectivity
towards benzene and n-C6 while the Na promoted catalysts pro-
duce a low amount of these compounds. The formation of aromatics
on noble metal supported catalysts was studied by Ponec et al. [55].
They were able to determine the contribution of metal and acid sites
of sulfur modified Pt/Al2O3, Pt-Co/Al2O3 and Pt-Re/Al2O3 catalysts
and found that cyclohexane, benzene and n-C6 required the pres-
ence of both metal and acid sites. Then, in the catalyst with Na the
reaction can be assumed to proceed through a bifunctional mech-
anism producing benzene and n-C6. On the other hand, the low
acidity of the catalysts with Na (Table 1) leads to a low formation
of n-C6 and benzene.
It is interesting to analyze the selectivity to C1 and C2–C5.
C1 is formed mainly by hydrogenolysis reaction on the metal
function while C2–C5 products are generated by cracking on
the acid function. The results clearly show that the addition of
Na decreases the cracking activity while the metal function is
improved. These results are in agreement with the lower acidity
Table 2
Selectivity to 2MP, 3MP, n-C6, C1–C5 and benzene (Bz) in the reaction of SRO of MCP at 15 min time on stream of Pt(1)-Ir(2.0)/Al2O3-Na(x).
Catalyst
Selectivity (%)
2MP
Molar ratio
Na (wt%)
3MP
n-C6
C1
C2–C5
Bz
RO
n-C6/3MP
2MP/3MP
0.0
0.5
1.0
1.5
25.6
60.1
58.9
58.2
16.4
26.2
22.8
23.0
12.8
3.3
2.3
0.4
2.6
4.6
3.0
18.0
7.1
10.3
12.4
26.8
0.7
1.1
81.6
90.3
85.1
85.6
0.78
0.13
0.10
0.16
1.56
2.29
2.58
2.53
2.6
0.8

48
M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
Table 3
Decalin conversion, percentage of cis and trans decalin and selectivity to different reaction products after 6 h of reaction for Pt(1)-Ir(2.0)/Al2O3-Na(x) catalysts.
Catalyst Na (wt%)
0.0
T (◦C)
X (%)
Trans (%)
Cis (%)
Yield (%)
CR
RO
RC
DH
300
325
350
7.1
10.1
28.5
82.5
77.9
61.2
10.4
12.0
10.3
2.9
5.0
9.8
23.2
40.6
28.6
60.9
35.6
23.8
13.0
18.8
38.8
0.5
1.0
1.5
300
325
350
11.6
30.4
40.4
78.2
60.7
48.1
10.2
8.9
11.5
11.3
19.5
22.2
31.8
45.0
38.6
44.3
20.0
20.5
12.6
15.5
18.7
300
325
350
10.8
17.7
37.1
78.7
72.6
54.2
10.5
9.7
8.7
11.0
10.8
14.9
36.2
30.4
32.3
42.5
44.2
32.0
10.3
14.6
20.8
300
325
350
13.4
16.6
43.9
78.1
72.3
48.7
8.5
11.1
7.4
14.5
11.2
20.0
28.2
32.4
38.8
46.0
44.0
14.5
11.3
12.4
26.7
T: reaction temperature; X: conversion.
and higher hydrogenolytic activity of the Na promoted catalysts
with respect to the catalyst without Na, as reported in Table 1.
It can be seen that all the catalysts supported on Al2O3-Na pro-
duce lower amounts of n-C6 than the catalyst without sodium. n-C6
should not be produced on large particles of metal according to the
selective model. Moreover, the 2MP/3MP ratio is close to 2 accord-
ing to the selective model. These results are in agreement with
the increased size of the metal particles of the catalysts supported
on Al2O3-Na, as shown in Fig. 3. Finally, it can be seen in Table 2
that catalysts supported on Al2O3-Na exhibit higher selectivity to
ring opening products (RO) than the Na-free counterpart. This can
be explained by taking into account the lower acidity of Pt and Ir
supported on Al2O3-Na. The suppressed cracking activity of these
catalysts reduces the rate of destruction of ring opening products.
The results of decalin conversion and the percentage of cis and
trans-decalin obtained after 6 h of reaction at different reaction
temperatures are shown in Table 3. The decalin used in the exper-
iments has a 37.5% cis, thus a trans/cis ratio = 1.63. It can be seen
in Table 3 that the trans/cis ratio is higher than the initial ratio
(1.63). Higher trans/cis ratios can be due to the higher reactivity
of the cis isomer [56,57] and isomerization reactions of cis/trans
[58]. It is known that cis-decalin in more selective to RO products
than trans-decalin which is converted to cracking products [56].
The cis/trans thermodynamic equilibrium composition (10–90 vs.
40–60 in the starting decalin) is rapidly reached, resulting in an
enrichment of the reactant mixture in trans-decalin. On the other
hand, as expected, the conversion increased as the reaction tem-
perature increased and the catalysts with Na are more active than
the catalyst without Na as found in the MCP reaction (Fig. 6).
These results demonstrate the importance of the metallic phase for
decalin transformation because the conversion is increased when
the acidity is decreased.
As decalin conversion leads to a complex mixture of more
than 200 compounds, the decalin reaction products were classified
according to the same criterion used in previous work [59]. The
products of reaction are classified into cracking products (C1–C9
products); ring opening (RO) C10 products; ring contraction (RC);
naphthalene and other products including heavy dehydrogenation
products.
Table 3 shows the selectivity values obtained from the reac-
tion of decalin SRO. The best results were obtained with catalyst
Pt(1.0)-Ir(2.0)-0.5% Na at the temperature of 325 ◦C. The formation
of cracking products increases with the reaction temperature in
all the catalysts studied because it is a reaction with high activa-
tion energy [60]. As expected, the increase in reaction temperature
also favors the formation of dehydrogenated products because the
dehydrogenation is an endothermic and reversible reaction [60].
The yield to ring opening products (defined as the selectiv-
ity to RO products × conversion/100) depends upon the reaction
temperature, the acidity of the support and the metal charge. An
increase of the temperature, acidity and metal charge promotes
the formation of ring opening compounds; however, it may also
favor cracking and dehydrogenation reactions that decrease the
product selectivity desired. The yield to dehydrogenated product
of the catalysts supported on Al2O3-Na is lower than that obtained
in free Na, in agreement with cyclohexane dehydrogenation con-
version reported in Table 1. The yield to cracking products (CR) is
higher on the catalysts promoted with Na. Taking into account that
cracking reactions are catalyzed by the strong acid sites of the sup-
port; the yield to CR products shown in Table 3 is opposed to the
expected one. This is indicating that CR products are also formed
by the metallic function, which is more active on the Na promoted
catalysts (Table 1).
The yield to RC products is higher in the catalyst without Na
because a higher acidity favors the ring contraction reaction and a
lower metallic activity suppresses the conversion of RC products
into RO ones.
4. Conclusions
The results of the catalysts characterization show that Na
strongly decreases the acidity and influences the metal prop-
erties of Pt-Ir/Al2O3 catalysts. The interaction between Pt and
Ir is increased by the presence of Na. Larger metal crystals
are obtained on the support modified by Na. As
a conse-
quence, the catalytic properties are also modified, the CP/CH
ratio (parameter of demanding/not demanding reaction) being
increased with sodium concentration. In the MCP reaction, Na
strongly decreases the aromatics yield and n-C6 formation and
favors the selective reaction mechanism favoring the formation of
2MP and 3MP. In the decalin reaction it was found that the yield
to dehydrogenated products of the catalysts supported on Al2O3-
Na is lower than that of the free Na-alumina supported catalyst
whereas the opposite occurs with the yield to C1 due to the higher
hydrogenolytic and lower dehydrogenation activity of the catalysts
with Na. The yield to ring contraction (RC) products is higher in the
catalyst without Na due to the higher acidity which favors the ring
contraction reaction and the lower metallic activity; thus, the trans-
formation of RC products into ring opening products is decreased.
Acknowledgment
Thanks are given to Elsa Grimaldi for the English language edit-
ing.

M.A. Vicerich et al. / Applied Catalysis A: General 480 (2014) 42–49
49
References
[30] K. Shimizu, K. Shimura, K. Kato, N. Tamagawa, M. Tamura, A. Satsuma, J. Mol.
Catal. A: Chem. 353–354 (2012) 171–177.
[31] F. Solymosi, E. Novak, A. Molnar, J. Phys. Chem. 94 (1990) 7250–7255.
[32] A.B. Dongil, B. Bachiller-Baeza, I. Rodríguez-Ramos, A. Guerrero-Ruiz, C. Mon-
delli, A. Baikerm, Appl. Catal. A: Gen. 451 (2013) 14–20.
[33] A. Erdohelyi, K. Fodor, G. Suru, Appl. Catal. A: Gen. 139 (1996) 131–147.
[34] M. Haneda, T. Fujitani, H. Hamada, J. Mol. Catal. A: Chem. 256 (2006) 143–148.
[35] F.J.C.M. Toolenaar, A.G.T.M. Bastein, V. Ponec, J. Catal. 82 (1983) 35–44.
[36] K. Tanaka, K.L. Watters, R.F. Howe, J. Catal. 75 (1982) 23–38.
[37] K.J. Lyons, J. Xie, W.J. Mitchell, W.H. Weinberg, Surf. Sci. 325 (1995) 85–92.
[38] M. Sushchikh, J. Lauterbach, W.H. Weinberg, Surf. Sci. 393 (1997) 135–140.
[39] A. Zecua-Fernández, A. Gómez-Cortés, A.E. Cordero-Borboa, A. Vázquez-Zavala,
Appl. Surf. Sci. 182 (2001) 1–11.
[40] S.A. D’Ippolito, V.M. Benitez, P. Reyes, M.C. Rangel, C.L. Pieck, Catal. Today 172
(2011) 177–182.
[41] M. Boudart, A. Aldag, J.E. Benson, V.A. Dougharty, C.G. Harkings, J. Catal. 6 (1966)
92–99.
[42] M. Boudart, Proceedings of the 6th International Congress of Catalysis, The
Chemical Society, London, 1976, pp. 1–9.
[43] W.M.H. Sachtler, R.A. van Santen, Adv. Catal. 26 (1977) 69–119.
[44] R. Coekelbergs, A. Frennet, G. Lienard, P. Resibois, J. Phys. Chem. 39 (1963)
585–591.
[45] J.A. Dalmon, G.A. Martin, J. Catal. 66 (1980) 214–221.
[46] F.G. Gault, Adv. Catal. 30 (1981) 1–95.
[47] X. Zhu, M. Shen, L.L. Lobban, R.G. Mallinson, J. Catal. 278 (2011) 123–132.
[48] T. Montanari, R. Matarrese, N. Artioli, G. Busca, Appl. Catal. B 105 (2011)
15–23.
[49] J.G. van Senden, E.H. van Broekhoven, C.T.J. Wreesman, V. Ponec, J. Catal. 87
(1984) 468–477.
[50] E. Fulop, V. Gnutzmann, Z. Paal, V. Vogel, Appl. Catal. 66 (1990) 319–336.
[51] P.T. Do, W.E. Alvarez, D.E. Resasco, J. Catal. 238 (2006) 477–488.
[52] J.H. Sinfelt, Adv. Catal. 23 (1973) 91–127.
[1] Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 17, 3rd ed., John Wiley
& Sons, Inc., USA, 1982, pp. 119–143.
[2] J.H. Gary, G.E. Handwerk, Petroleum Refining: Technology and Economics, Mar-
cel Dekker, Inc., New York, 1994.
[3] T.C. Kaufmann, A. Kaldor, G.F. Stuntz, M.C. Kerby, L.L. Ansell, Catal. Today 62
(2000) 77–90.
[4] V. Calemma, R. Giardino, M. Ferrari, Fuel Process. Technol. 91 (2010) 770–776.
[5] H. Du, C. Fairbridge, H. Yang, Z. Ring, Appl. Catal. A: Gen. 294 (2005) 1–21.
[6] P. Samoila, M. Boutzeloit, C. Especel, F. Epron, P. Marécot, J. Catal. 276 (2010)
237–248.
[7] K. Shimizu, T. Sunagawa, C.R. Vera, K. Ukegawa, Appl. Catal. A: Gen. 206 (2001)
79–86.
[8] J. Benzinger, R.J. Madix, Surf. Sci. 94 (1980) 119–153.
[9] D. Heskett, Surf. Sci. 199 (1988) 67–86.
[10] E.L. Garfunkel, J.E. Crowell, G.A. Somorjai, J. Phys. Chem. 86 (1982) 310–312.
[11] H. Yoshitake, Y. Iwasawa, J. Phys. Chem. 96 (1992) 1329–1334.
[12] H. Yoshitake, Y. Iwasawa, J. Catal. 131 (1991) 276–284.
[13] H. Yoshitake, Y. Iwasawa, J. Phys. Chem. 95 (1991) 7368–7372.
[14] T. Visser, T.A. Nijhuis, A.M.J. van der Eerden, K. Jenken, Y. Ji, W. Bras, S. Nikitenko,
Y. Ikeda, M. Lepage, B.M. Weckhuysen, J. Phys. Chem. B 109 (2005) 3822–3831.
[15] M. Lepage, T. Visser, F. Soulimani, A.M. Beale, A. Iglesias-Juez, A.M.J. van der
Eerden, B.M. Weckhuysen, J. Phys. Chem. C 112 (2008) 9394–9404.
[16] A. Djeddi, I. Fechete, F. Garin, Top. Catal. 55 (2012) 700–709.
[17] A. Boulaoued, I. Fechete, B. Donnio, M. Bernard, P. Turek, F. Garin, Microporous
Mesoporous Mater. 155 (2012) 131–142.
[18] S. Nassreddine, S. Casu, J.L. Zotin, C. Geantet, L. Piccolo, Catal. Sci. Technol. 1
(2011) 408–412.
[19] J. Shen, N. Semagina, ACS Catal. 4 (2014) 268–279.
[20] G.B. McVicker, M. Daage, M.S. Touvelle, C.W. Hudson, D.P. Klein, W.C. Baird Jr.,
B.R. Cook, J.G. Chen, S. Hantzer, D.E.W. Vaughan, E.S. Ellis, O.C. Feeley, J. Catal.
210 (2002) 137–148.
[21] R.C. Santana, P.T. Do, M. Santikunaporn, W.E. Alvarez, J.D. Taylor, E.L. Sughrue,
D.E. Resasco, Fuel 85 (2006) 643–656.
[22] S.M. Agustine, W.M.H. Sachtler, J. Catal. 116 (1989) 184–194.
[23] H. Lieske, G. Lietz, H. Spindler, J. Völter, J. Catal. 81 (1983) 8–16.
[24] G. Lietz, H. Lieske, H. Spindler, W. Hanke, J. Völter, J. Catal. 81 (1983) 17–25.
[25] L.S. Carvalho, C.L. Pieck, M.C. Rangel, N.S. Fígoli, J.M. Grau, P. Reyes, J.M. Parera,
Appl. Catal. A: Gen. 269 (2004) 91–103.
[26] C. Carnevillier, F. Epron, P. Marecot, Appl. Catal. A: Gen. 275 (2004) 25–33.
[27] J.A. Anderson, C.H. Rochester, Catal. Today 10 (1991) 275–282.
[28] F. Boccuzzi, G. Ghiotti, A. Chiorino, L. Marchese, Surf. Sci. 233 (1990) 141–152.
[29] J.A. Anderson, M.G.V. Mordente, C.H. Rochester, Chem. Soc. Faraday Trans. 1
(85) (1989) 2983–2990.
[53] M.J. Dees, V. Ponec, J. Catal. 115 (1989) 347–355.
[54] G. Maire, G. Plouidy, J.C. Prudhomme, F.G. Gault, J. Catal. 4 (1965) 556–569.
[55] M.J. Dees, M.H.B. Bol, V. Ponec, Appl. Catal. 64 (1990) 279–295.
[56] M. Santikunaporn, J.E. Herrera, S. Jongpatiwut, D.E. Resasco, W.E. Alvarez, E.L.
Sughrue, J. Catal. 228 (2004) 100–113.
[57] E.F. Sousa-Aguiar, C.J.A. Mota, M.L. Valle Murta, M. Pinhel da Silva, D. Forte da
Silva, J. Mol. Catal. A: Chem. 104 (1996) 267–271.
[58] R.C. Schucker, J. Chem. Eng. Data 26 (1981) 239–241.
[59] S.A. D’Ippolito, L.B. Gutierrez, C.L. Pieck, Appl. Catal. A: Gen. 445–446 (2012)
195–203.
[60] J.M. Parera, N.S. Fígoli, in: G.J. Antos, A.M. Aitani, J.M. Parera (Eds.), Catalytic
Naphtha Reforming: Science and Technology, Marcel Dekker Inc., New York,
1995 (Chapter 3).