New PtSn structured catalysts with ZnAl2O4 thin film for n-butane dehydrogenation reaction

Article abstract of DOI:10.1016/j.apcata.2019.117315

Starting from structured supports based on compact spheres coated with a thin and porous layer of ZnAl₂O₄, mono and bimetallic catalysts were prepared. These catalysts were applied for the n-butane dehydrogenation reaction to produce light olefins. The structured supports were synthesized by coating with: bohemite-nitrate purified method (BP) and citrate-nitrate method (C). From the characterization results, the existence of strong Pt-Sn interaction in both bimetallic catalysts with probable alloys formation was found. In the PtSn/Sp-Zn-C catalyst, where higher metallic Pt-Sn interactions were observed, most of its metallic particles have sizes between 1 and 1.5 nm, which indicates a good metallic dispersion. These properties led to a catalyst with the best catalytic behavior, thus showing high yields to butenes, and a very good stability. Moreover, the presence of a structured support improves the mass and heat transference in this reaction carried out at high temperature.

Full text of DOI:10.1016/j.apcata.2019.117315

Journal Pre-proof
New PtSn structured catalysts with ZnAl2O4 thin film for n-butane
dehydrogenation reaction
Sergio de Miguel, Adriana Ballarini, Sonia Bocanegra
PII:
S0926-860X(19)30470-3
DOI:
https://doi.org/10.1016/j.apcata.2019.117315
APCATA 117315
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
2 September 2019
10 October 2019
22 October 2019
Please cite this article as: de Miguel S, Ballarini A, Bocanegra S, New PtSn structured
catalysts with ZnAl2O4 thin film for n-butane dehydrogenation reaction, Applied Catalysis A,
General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117315
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New PtSn structured catalysts with ZnAl2O4 thin film for n-
butane dehydrogenation reaction
Sergio de Miguel, Adriana Ballarini and Sonia Bocanegra*
Instituto de Investigaciones en Catálisis y Petroquímica “Ing. José Miguel Parera”
(INCAPE), Fac. de Ingeniería Química, Univ. Nac. del Litoral, CONICET, CCT-Santa
Fe, Colectora Ruta 168 Km0, Santa Fe C.P.3000, Argentina. E-Mail:
sbocane@fiq.unl.edu.ar
Highlights
 Novel structured catalysts with thin and stable layer on ZnAl2O4 were synthesized
 Excellent capacities for dispersions and stability of metallic phase were achieved
 Very good performance of the PtSn/Sp-Zn-C catalyst for butenes production
Graphical Abstract
1

Abstract
Starting from structured supports based on compact spheres coated with a thin and porous
layer of ZnAl2O4, mono and bimetallic catalysts were prepared. These catalysts were applied
for the n-butane dehydrogenation reaction to produce light olefins. The structured supports
were synthesized by coating with: bohemite-nitrate purified method (BP) and citrate-nitrate
method (C). From the characterization results, the existence of strong Pt-Sn interaction in
both bimetallic catalysts with probable alloys formation was found. In the PtSn/Sp-Zn-C
catalyst, where higher metallic Pt-Sn interactions were observed, most of its metallic particles
have sizes between 1 and 1.5 nm, which indicates a good metallic dispersion. These
properties led to a catalyst with the best catalytic behavior, thus showing high yields to
butenes, and a very good stability. Moreover, the presence of a structured support improves
the mass and heat transference in this reaction carried out at high temperature.
KEYWORDS: ZnAl2O4 structured support; PtSn catalysts; thin films coating; butenes
production
Introduction
Catalysts obtained by coating of thin layers (with active species) on inert substrates, named
structured catalysts, have recently gained importance due to the improvement in mass and
heat transfer coefficients [1,2,3,4], compared to conventional catalysts, in rapid reactions
carried out at high temperatures like n-alkanes dehydrogenation. With regard to substrates,
various geometries such as tubes, platelets, honeycombs, and foams have been used. In
addition, different coating materials like alumina, zirconium oxide, cupric oxide, alumina
with ceria, lanthanum oxide or magnesium oxide have been employed in such procedures [2].
2

The direct catalytic reaction of alkanes dehydrogenation subjects the catalysts to very severe
reaction conditions, since it is carried out at high temperatures, favoring the formation of
coke, which deactivates the catalysts [5,6]. For this reason, it is necessary to periodically
regenerate the catalysts with processes of oxidation [7,8]. The requirements for a good active
phase for these catalysts are high selectivity to alkenes and catalytic stability. The most
common metals used in dehydrogenation reactions are Pt or Pd accompanied by metallic
promoters such as Sn, In, Ga and Ge [6,8,9,10,11,12]. The supports of the catalysts used in
dehydrogenation processes must have low acidity and good interaction with metals. The first
property is to avoid the cracking of alkanes and the second one is to ensure catalytic stability
in reaction-regeneration cycles [5,7,8]. The materials most used as supports for
dehydrogenation reactions are alumina [11,13,14], alumina with alkaline and alkaline-earth
metals [15,16], silica [12,13], hydrotalcite [8,17,18] and MgAl2O4 [4,6,9,18]. Our research
group and others studied PtSn catalysts supported on ZnAl2O4 powders for dehydrogenation
reactions [5,19,20,21,22], however they cannot be used industrially in this way, in powder
form, due to operation problems like pressure drop. Hence, an innovative way is to use
structured supports with ZnAl2O4 coating for PtSn catalysts.
In this research work we study Pt catalysts with Sn as metallic promoter for n-butane
dehydrogenation reaction. These metals were supported on a thin and porous layer of
ZnAl2O4, which coated compact spheres of alpha-alumina as inert substrate. The originality of
this work lies on two main factors: the spherical geometry of the material to be coated, very
little studied [4,11,23,24], and the coating material, ZnAl2O4, without any previous evidence
for this application.
Materials and Methods
Preparation of supports
3

The supports were prepared by coating of ZnAl2O4 on α-Al2O3 spheres. The spheres (2 mm
diameter), provided by SASOL company, had very low specific surface (Sg = 5.0 m2g-1).
Materials employed for the coating process were: commercial bohemite, AlO(OH) (Disperal
P2) provided by SASOL company, Zn(NO3)2.6H2O (Fluka, purity 99.5 %), Al(NO3)3.9H2O
(Sigma Aldrich, purity 99.5 %) and citric acid, C6H8O7.H2O (Sigma Aldrich, purity 99.5 %).
a) Acid pre-treatment of α-Al2O3 spheres
The spheres were pretreated with HCl 1 M solution in order to develop roughness in the
compact material, which would improve the further adhesion of the coating layer.
a) Coating by bohemite-nitrate purified method (BP)
Three suspensions of 5, 10 and 15 wt % of bohemite were prepared with the addition of nitric
acid as stabilizing agent. After 48 h in the different suspensions, the gels were formed. A first
layer with bohemite was obtained by immersion in the gel (5 wt %) of the spheres placed in a
basket subjected to rotation. They were later dried in hot air flow. This process was repeated
with the gels of 10 wt % and 15 wt %. Finally, the spheres were dried at 110 ºC during 24 h.
The bohemite-coated spheres were immersed in a beaker with a volume of a Zn(NO3)2.6H2O
solution 0.5 M in order to achieve an Al/Zn molar ratio equal to 2. Afterwards, the beaker
with the spheres was placed in furnace at 110ºC for 24 h and finally the coated spheres were
calcined at 850 ºC for 4 h in air flow. The XRD analysis of the layers material showed the
ZnAl2O4 spinel formation and, what is more, the presence of ZnO as impurity (product of an
incomplete reaction), Therefore, they were submitted to a purification treatment with
(NH4)2CO3 solution, as in previous studies [19]. The characterization of these materials by
SEM displayed that the spheres were uniformly coated with a layer of about 16 µm of
thickness. The synthesized support was called Sp-Zn-BP, being Sp the α-Al2O3 spheres, Zn
the ZnAl2O4, and BP the coating method. The synthesis and characterization of this structured
4

material were analyzed previously [25], and two microphotographies of the coated sphere are
shown in Figure 1.
Figure 1: SEM microphographies of Sp-Zn-BP support: coated sphere (a) and cut of coated
sphere (b).
b) Coating by citrate-nitrate method (C)
A precursor solution of ZnAl2O4 was prepared by using Zn (NO3)2.6H2O, Al(NO3)3.9H2O and
C6H8O7.H2O. In this solution, the amounts of the reagents were the necessaries to obtain a
Al/Zn molar ratio equal to 2 and C6H8O7.H2O/nitrate ions molar ratio equal to 0.5. The
spheres were placed in a beaker and a certain volume of the solution was added in order to
achieve a [solution volume/weight spheres] ratio of 60 mL.g-1. Then, the solution (together
with spheres) was heated with soft stirring up to boiling point, and was maintained under
these conditions for 30 min in order to form a gel. The gel with the spheres were dried in
vacuum furnace at 100 ºC for 24 h. Then, the coated spheres were subjected to a complex
thermal treatment that include heating in N2 flow until 500 ºC, calcination in a flow of O2
(5%)-N2(95%) up to 700 ºC and finally calcination in air flow at 800 ºC. The diffractogram of
the layer material showed the ZnAl2O4 spinel formation and only traces of the ZnO impurity.
In this case, the purification treatment was not applied in order to determine the incidence of
ZnO traces on the catalytic behavior. SEM microphotographs of this structured material
showed a thin and rough layer coating the spheres with a uniform thickness of about 14 µm.
5

The obtained support was called Sp-Zn-C. The synthesis and characterization of this
structured material was previously studied by us [26], and two microphotographies of the
coated sphere are displayed in Figure 2.
Figure 2: SEM microphographies of Sp-Zn-C support: coated sphere (a) and cut of coated
sphere (b).
Catalysts preparation
The Sp-Zn-C and Sp-Zn-BP supports were used for the preparation of Pt (0.3 wt%) and Pt
(0.3 wt%) Sn (0.3 wt%) catalysts by incipient impregnation technique. The precursor
solutions were chloroplatinic acid (H2PtCl6) and stannous chloride (SnCl2) with HCl (1 M).
The [solution volume/support weight] ratio used for the impregnation process was 1.4 mL.g-1.
The supports were immersed in a H2PtCl6 solution with the adequate concentration to obtain
0.3 wt % of Pt during 6 h at room temperature. Then, the precursors were dried 12 h at 110 ºC
in furnace and finally they were calcined in air flow (100 mL.min-1) for 3 h. For PtSn
catalysts, the process described above was repeated until the drying step in furnace, after
which the supports impregnated with Pt were immersed in the SnCl2 hydrochloric solution (6
h at room temperature) with the suitable concentration to achieve a Sn content of 0.3 wt%.
Later, the samples were dried for 12 h at 110 ºC and then, calcined 3 h in air flow at 500 ºC.
Characterization of catalysts
6

The hydrogen chemisorption (H) experiments were carried out at room temperature in a static
volumetric equipment. The procedure consisted of high vacuum degassing of the sample,
reduction with hydrogen at 500 °C, and further degassing at 500 °C under vacuum. Then a
hydrogen measurement was carried out at room temperature. The adsorption isotherms were
linear in the pressure range used (50-250 mm Hg) and the chemisorbed hydrogen monolayer
was calculated by extrapolating the isotherm to pressure equal to 0.
The temperature programmed reduction (TPR) experiments were carried out by heating the
samples from room temperature up to 600 °C with a 6 °C min-1 heating rate. The reducing
mixture (5 %v / v H2-N2) was fed to the reactor with a flow rate of 9 mL.min-1. The catalysts
were previously calcined in air at 500 °C for 3 h.
The X-ray photolectronic spectroscopy (XPS) spectra were carried out in a Multitechnique
Specs equipment with a dual X-ray source of Mg/Al (XR50 model) and a hemispheric
PHOIBOS 150 analyzer in FAT mode. The spectra were obtained with step energy of 30 eV
and a Mg anode operated at 100 W. The pressure during the measurement was less than 2.10-8
mbar. Samples (previously reduced) were supported on double-sided Cu tape and evacuated
in ultra high vacuum prior to experiments.
The transmission electron microscopy (TEM) observations were performed on a JEOL 100
CX II microscope with a nominal resolution of 6 Å. The operation conditions were
acceleration voltage of 100 KV and magnification 45,000x. The samples obtained by
scrapping of the spheres layers were suspended in ethanol, which was then evaporated on the
grid. A significant number of metal particles were measured in order to obtain the size
distributions.
The test reactions of metallic phase: cyclohexane dehydrogenation (CHD) and cyclopentane
hydrogenolysis (CPH) were carried out in a differential flow reactor using a volumetric flow
of 6 mL.h-1. Prior to the reactions, the catalysts were reduced "in situ" under H2 flow for 2 h at
7

500 °C and the reaction products were analyzed by gas chromatography. The CPH was
carried out at 500 °C using a H2/CP molar ratio equal to 22. The CHD was carried out at 300 °
C (or 400 ºC) using a H2/CH molar ratio equal to 26. The activation energy in CHD for
different catalysts was obtained by Arrhenius plot, measuring the initial reaction rate at three
temperatures close to 300 ºC (or 400 ºC). The conversion was lower than 7 % for all samples.
Temperature programmed oxidation (TPO) profiles of the used catalysts (after n-butane
dehydrogenation reaction) were determined by using the TGA technique. Experiments were
carried out in a SDTA Mettler STARe equipment. Both the samples and references (fresh
catalysts) were stabilized under flowing N2 at 250 ºC before TGA experiments. TGA
experiments were carried out with a heating rate of 5 ºC min-1 from 250 to 500 ºC under
flowing air.
Catalytic tests in n-butane dehydrogenation reaction
The experiments were carried out at 530 °C for 2 h in a quartz flow reactor. The reactor (with
a catalyst weight of 0.200 g) was fed with 18 mL min–1 of the reactive mixture (n-butane +
hydrogen, H2/n-C4H10 molar ratio = 1.25). All gases, n-butane, N2 (used for purge), and H2
were high purity ones (> 99.99 %). Prior to the reaction, catalysts were reduced in situ at 530
°C under flowing H2 for 3 h. The reactor effluent was analyzed in a GC-FID equipment with a
packed column (1/8× 6 m, 20 wt.% BMEA on Chromosorb P-AW 60/80), which was kept at
50 °C during the analysis. With this analytical device, the amounts of methane, ethane,
ethylene, propane, propylene, n-butane, 1-butene, cis-2-butene, trans-2-butene and 1,3
butadiene were measured. The n-butane conversion was calculated as the sum of the
percentages of the chromatographic areas of all the reaction products (except H2) corrected by
the corresponding response factor. The selectivity to the different reaction products (i) was
defined as the ratio:
8

mol of product i/Σ mol of all products (except H2). Taking into account the high temperatures
used for the reaction, it was necessary to determine the contribution of the homogeneous
reaction. For this purpose, a blank experiment was performed by using a quartz bed and the
results showed a negligible n-butane conversion (<< 1 %).
Results
Figure 3: Results of n-butane dehydrogenation reaction: conversion and TOF values vs the
reaction time for all catalysts.
Figure 4: Results of n-butane dehydrogenation reaction: selectivity to butenes vs the reaction
time for all catalysts.
9

Figure 5: Results of n-butane dehydrogenation reaction: yield to butenes (product of
conversion x selectivity) vs the reaction time for all catalysts.
Figures 3, 4 and 5 show the results of n-butane dehydrogenation reaction for mono and
bimetallic catalysts. Figure 3 displays differences in the behavior of the monometallic
catalysts, the Pt/Sp-Zn-BP catalyst have higher values of n-butane conversion through the
reaction time than the Pt/Sp-Zn-C catalyst. The addition of Sn to the Pt catalysts improves the
conversion values of the monometallic catalysts, especially the PtSn/Sp-Zn-C catalyst, which
showed conversion values that practically duplicated those of the Pt/Sp-Zn-C catalyst.
Besides the PtSn/Sp-Zn-C catalyst displayed the best performance of all catalysts in n-butane
conversion. With regard to the selectivity to butenes (Figure 4), all catalysts showed high
values between 94 to 98 %. However, there are small differences in favor of the bimetallic
catalysts compared to their respective monometallic ones. Besides, it should be noted that the
selectivity values of the catalysts supported in Sp-Zn-C were slightly higher than those
supported in Sp-Zn-BL. Figure 5 displays the yields to butenes (which is defined as the
product of the n-butane conversion and the selectivity to butenes) for all catalysts. This figure
shows very similar curves to those of Figure 3. The highest yield to butenes values
corresponded to the PtSn/Sp-Zn-C catalyst, however, this sample showed a significant drop in
yield values (Y = 23 %) along the reaction time. This means that the PtSn/Sp-Zn-C catalyst
10

displayed a low catalytic stability. The PtSn/Sp-Zn-BP catalyst showed lower yield values
than that PtSn/Sp-Zn-C catalyst. The yield to butenes values for PtSn/Sp-Zn-BP catalyst
ranged between 20.6 and 19.5 % (Figure 5) showing good catalytic stability (Y = 5 %),
which is better than the one from the corresponding monometallic catalyst (Y = 12 %). With
respect to the amount of deposited coke during the reaction, Temperature Programmed
Oxidation (TPO) experiments for the samples after reaction were performed, observing that
the carbon contents of both bimetallic catalysts were negligible (less than 0.1 wt%).
Table 1: Results of: hydrogen chemisorption (H) and calculated metallic dispersion (D), initial
reaction rate (R0) and activation energy (Ea) of cyclohexane dehydrogenation reaction (CHD) and
initial rate (R0) of cyclopentane hydrogenolysis reaction (CPH).
b
c
d
Ha
D
R0
Ea
R0
Catalizador
(%)
CHD
CHD
CPH
Pt/E-Zn-BP
5,7
74
11,0
36,3
1,2
PtSn/E-Zn-BP
Pt/E-Zn-C
0,1
--
n.d.
26,3
1,9
--
1,1
<1
<1
3,6
46
--
17,7
28,0
PtSn/E-Zn-C
a µmol H2 g cat.-1
0,8
c Kcal mol CH-1
The results of hydrogen chemisorption and test reactions (cyclohexane dehydrogenation,
CHD and cyclopentane hydrogenolisis, CPH) are shown in Table 1. There is a sharp decrease
of the hydrogen chemisorption values in the bimetallic catalysts compared to the
corresponding monometallic catalysts. This fact can be attributed to geometric and/or
electronic effects of the Sn promoter on the Pt sites, or to a significant increase in the size of
the metallic particles of Pt in bimetallic catalysts. However, when presenting TEM results,
this last hypothesis will be discarded. With regard to the CHD reaction, it is a structure-
insensitivity one [27], whose activation energy can be changed by electronic effects, derived
from the presence of a second metal or from the interaction with the support. In this reaction,
it can be observed a marked decrease of the initial reaction rate value of PtSn/Sp-Zn-C
11

catalyst respect to monometallic one. This fact, together with the hydrogen chemisorption
results, indicates that in this bimetallic catalyst, there would be blocking effects of the active
sites of Pt by the Sn atoms. On the other hand, the PtSn/Sp-Zn-BP catalyst is practically
inactive at 300 ºC in CH reaction, while the corresponding monometallic one has an initial
rate value of 11 mol CH g Pt-1 h-1. This would also indicate the presence of blocking effects of
the promoter on Pt sites in PtSn/Sp-Zn-BP sample. Concerning the values of activation energy
of cyclohexane dehydrogenation reaction (ECH), the PtSn/Sp-Zn-C catalyst shows an
important increase with respect to the Pt/Sp-Zn-C catalyst (Table 1), which indicates the
presence of electronic effects of Sn on the actives sites of Pt in this bimetallic catalyst. Since
the PtSn/Sp-Zn-BP catalyst is not active in the reaction at 300ºC, its activation energy could
not be determined therein. For this reason, experiments were carried out at 400 ºC for mono
and bimetallic catalysts supported in Sp-Zn-BP, which yielded the following results: ECH
PtSn/Sp-Zn-BP = 31.3 Kcal mol CH-1 and ECH Pt/Sp-Zn-BP = 7.5 Kcal mol CH-1. These
results point in the direction of possible electronic modifications of the Sn promoter on the Pt
atoms in PtSn/Sp-Zn-BP catalyst. Regarding the results of the CPH reaction, as shown in
Table 1, all samples show low hydrogenolytic activity. This phenomenon has already been
observed [19] and can be explained by the structure of metallic particles deposited on
ZnAl2O4. In fact, Aguilar-Ríos et al. [28] studied Pt/ZnAl2O4 catalysts by using TEM and
found that the metallic particles would have a rounded shape with very low density of steps
and edges, which are the active sites for the hydrogenolysis reactions [27].
In conclusion, according to the results showed in Table 1, both bimetallic catalysts present
geometric and electronic effects of the Sn promoter on Pt sites, indicating strong Pt-Sn
interactions.
12

Figure 6: TPR profiles of Pt/Sp-Zn-C and PtSn/Sp-Zn-C catalysts.
Figure 7: TPR profiles of Pt/Sp-Zn-BP and PtSn/Sp-Zn-BP catalysts.
Figures 6 and 7 display the profiles of temperature programmed reduction (TPR) for mono
and bimetallic catalysts. The Pt/Sp-Zn-C catalyst showed a main reduction peak located at 93
ºC and a shoulder at 150 ºC (Figure 6 a). These signals can be attributed to the reduction of
the different oxidized Pt species [29]. The main peak would correspond to the reduction of
PtO2, while the shoulder at higher temperature can be assigned to the reduction of
oxychlorinated Pt species.
13

The PtSn/Sp-Zn-C catalyst showed a main reduction peak located at 235 ºC, a shoulder at 110
ºC and reduction area between 450 – 550 ºC (Figure 6 b). The main peak would correspond to
a co-reduction of Pt and Sn or of intermetallic species, since this peak is shifted to higher
temperatures and has a higher area than that observed in the monometallic catalyst. These
facts point to an important interaction between platinum and tin with probable alloy formation
or intermetallic compound conformation. The shoulder at 110 ºC could correspond to a small
fraction of free Pt species since it appears at a temperature close to the monometallic catalyst
reduction peak. The small reduction zone at high temperatures observed in Figure 6 b can be
attributed to the reduction of oxidized species of Sn in contact with the support [19].
Figure 7 displays TPR profiles corresponding to Pt and PtSn catalysts supported on Sp-Zn-C.
Figure 7 a shows three peaks, which can be also attributed to the reduction of different
oxidized platinum species. It must be highlighted that this phenomenon was not observed in
other studies about Pt/ZnAl2O4 powder catalysts [19,20]. The peaks in Figure 7 a are located
at temperatures of 117, 209 and 235 ºC. The first signal would correspond to the reduction of
PtO2 compound. The peaks located at 209 and 235 ºC can be attributed to two different
PtOxCly species. In relation to the bimetallic catalyst (Figure 7 b), the same three peaks are
observed as in the case of the Pt catalyst, but at larger scale. This fact also points in the
direction of a Pt-Sn co-reduction phenomenon. In conclusion, both bimetallic catalysts
showed strong Pt-Sn interactions, but the PtSn/Sp-Zn-C catalyst seems to have a higher
degree of interaction due to the important increase in the main reduction peak area compared
to the corresponding monometallic catalyst, and also, due to the significant temperature shift
of this reduction peak.
14

Figure 8: Analysis by XPS of Pt 4f7/2 signal for PtSn/Sp-Zn-BP catalyst.
Figure 9: Analysis by XPS of Sn 3d5/2 signal for PtSn/Sp-Zn-BP catalyst.
Figure 10: Analysis by XPS of Pt 4f7/2 signal for PtSn/Sp-Zn-C catalyst.
15

Figure 11: Analysis by XPS of Sn 3d5/2 signal for PtSn/Sp-Zn-C catalyst.
XPS spectra for bimetallic catalysts are shown in Figures 8, 9, 10 and 11. The Pt 4f signal, the
most intense for platinum, overlaps with two Al 2p signals coming from the support.
However, it is possible to separate them and make the deconvolution for both signals. Figure
8 shows the Pt 4f and Al 2p signals for PtSn/Sp-Zn-BP catalyst. The presence of the Pt 4f5/2
and 4f7/2 doublet, placed at 71.2 and 74.5 eV of binding energy, indicates the existence of
zerovalent Pt [30] in this reduced catalyst. In the same way, in the PtSn/Sp-Zn-C catalyst the
Pt 4f5/2 and 4f7/2 doublet, located at 71.7 and 75.0 eV of binding energy is shown in Figure 10.
In both samples, only the presence of Pt (0) was observed. This was confirmed by analyzing
the Pt 4d signal, which was much less intense than Pt 4f one and therefore had more noise
than the main signal.
Figure 9 shows the Sn 3d5/2 signal for PtSn/Sp-Zn-BP catalyst. This signal was deconvoluted
in two peaks located at 484.4 eV and 486.2 eV. The first peak might correspond to Sn alloyed
with Pt [12], while the second one can be attributed to tin oxides [12,30]. Figure 11 also
shows two peaks derived from the deconvolution of the Sn 3d5/2 signal, which might
correspond to alloyed tin (485.3 eV) and tin oxides (486.8 eV), for PtSn/Sp-Zn-C catalyst.
The difference between both catalysts is the proportion of alloyed tin in respect to total
surface tin species. In the PtSn/Sp-Zn-BP catalyst, the percentage of alloyed tin is 13%, while
16

in the PtSn/Sp-Zn-C catalyst it is 39%. This difference can be associated to the phenomenon
observed in TPR experiments, where higher interaction between Pt and Sn metals in the
PtSn/Sp-Zn-C catalyst was observed. This process could lead to the Pt-Sn alloys formation.
Figure 12: Distribution of metallic particle sizes of Pt/Sp-Zn-C catalyst measured by TEM.
Figure 13: Distribution of metallic particle sizes of PtSn/Sp-Zn-C catalyst measured by TEM.
17

Figure 14: Distribution of metallic particle sizes of Pt/Sp-Zn-BP catalyst measured by TEM.
Figure 15: Distribution of metallic particle sizes of PtSn/Sp-Zn-BP catalyst measured by
TEM.
Figure S1: TEM image of Pt/Sp-Zn-C catalyst.
18

Figure S2: TEM image of Pt/Sp-Zn-BP catalyst.
Figure S3: TEM image of PtSn/Sp-Zn-C catalyst.
19

Figure S4: TEM image of PtSn/Sp-Zn-BP catalyst.
Figures 12, 13, 14 and 15 show the results of the measurement of metallic particle sizes
obtained by TEM. Both the distribution and the average particle size are similar for Pt and
PtSn catalysts supported in Sp-Zn-C, as shown in Figures 12 and 13. The size distribution is
narrow with most metallic particles between 1.0 and 1.5 nanometers, and the mean diameter
(D) is 1.4 nm (see TEM images in Supplementary Figures S1 and S3). For the catalysts
supported in Sp-Zn-BP, the particle sizes distribution of Pt catalyst was different to that from
PtSn catalyst. In the first sample, there is a larger amount of metallic particles of sizes
between 1.5 and 2.0 nm (Figure 14) compared to the size distribution of the bimetallic
catalyst displayed in Figure 15. In consequence, the PtSn/Sp-Zn-BP catalyst have a mean
particle diameter slightly smaller than that of Pt/Sp-Zn-BP catalyst (see TEM images in
Supplementary Figures S2 and S4). In both bimetallic catalysts, the influence of Sn addition
to Pt catalysts in the mean diameter of metallic particle was not significant. To sum up, the
20

catalysts have very small metal particles and a narrow size distribution, with predominance of
particles between 1.0 and 1.5 nanometers.
Discussion
Taking into account the TEM results it is observed that both monometallic catalysts have
equal particle mean diameter, however, the metallic dispersion derived from hydrogen
chemisorption results showed differences between these catalysts. The calculation of metallic
dispersion (D) from TEM measurements by using the expression D = 1.13/d [31], where d is
the average particle size in nm, results in 80 % for Pt/Sp-Zn-BP and Pt/Sp-Zn-C catalysts.
This estimation is close to the D measured by hydrogen chemisorption for the Pt/Sp-Zn-BP
catalyst (see Table 1). The discrepancy with Pt/Sp-Zn-C catalyst might be the result of the
presence of ZnO as an impurity in the Sp-Zn-C material [26], which can form Pt-Zn alloys,
thus affecting the properties of the metallic phase of this catalyst. Taking into consideration
that alloys of platinum decrease the amount of chemisorbed hydrogen [8,13,32,33], the low
value of H2 chemisorption of Pt/Sp-Zn-C catalyst showed in Table 1 can be attributed to the
presence of Pt-Zn alloys in this sample, as found by Boccuzzi et al. in Pt/ZnO catalysts [34].
This phenomenon would also explain the low activity of the Pt/Sp-Zn-C catalyst in the n-
butane dehydrogenation reaction (see Figure 3). In this sense, Pakhomov et al. [35] found that
Pt/ZnAl2O4 catalysts, with ZnO impurity in the support, had low activity in the n-butane
dehydrogenation reaction.
Taking into account the dispersion values calculated from the particle size (which do not have
the disadvantage of being distorted by electronic effects in the metallic phase as discussed
above), the dispersions of the bimetallic catalysts were calculated, giving values of 80% for
PtSn/Sp-Zn-C catalyst and 86% for PtSn/Sp-Zn-BP catalyst. Based on these metal
dispersions, the TOF values in n-butane dehydrogenation reaction for all catalysts were
21

calculated. These results are presented in Figure 3. The TOF values show the same tendency
that the n-butane conversion values, that is the PtSn/Sp-Zn-C catalyst has the highest TOF
values of all catalysts.
Concerning the CPH, a structure sensitive reaction [27], both monometallic catalysts present
very low hydrogenolytic activity, as seen in the very low values of R0 CP in Table 1. This
property of the Pt catalysts supported on ZnAl2O4 was previously observed by Aguilar-Ríos et
al. [28] and Bocanegra et al.[19]. The first work found, through TEM measurements, that the
Pt particles supported on ZnAl2O4 showed a rounded shape with very low density of steps and
edges. These sites are the most active for hydrogenolysis reactions [27]. These results
referring to the structure of the metallic particles of Pt and the formation of Pt-Zn alloys show
that the Pt supported on ZnAl2O4 has different characteristics to those of the Pt supported on
MgAl2O4, which was studied in a previous work [4].
The CHD test reaction is considered structure insensitive [27], and its rate-limiting step on Pt
surfaces is the benzene desorption [36]. Table 1 shows that the initial reaction rate (R0 CH) of
the Pt/Sp-Zn-C catalyst is higher than that of the Pt/Sp-Zn-BP catalyst. This behavior could
be explained taking into account the foregoing information about the presence of Pt-Zn alloys
in the Pt/Sp-Zn-C catalyst. According to Consonni el al. [37], the Pt sites, when alloying to
Zn, form Pt-–Zn+ entities where the Pt atom is electron-rich and repels the electrons of the
C=C bond, facilitating the benzene desorption.
With respect to PtSn/Sp-Zn-C bimetallic catalyst, a marked increase in the activity of this
sample respect to corresponding monometallic one was observed (see Figure 3). The Sn
addition to the Pt/Sp-Zn-C catalyst produced an important change in the catalytic behavior,
which could be associated to a modification of the metallic phase. In this sense, Pakhomov et
al. [21] studied PtSn/ZnAl2O4 catalysts and did not find PtZn alloys in the samples with Sn/Pt
molar ratio > 1. Moreover, there is an increase in catalytic stability in PtSn/Sp-Zn-C catalyst
22

respect to the corresponding monometallic one, as shown in Figure 5. Considering the other
bimetallic catalyst (PtSn/Sp-Zn-BP), the Sn addition to the monometallic catalyst also
produced both an increase in n-butane conversion and in the catalytic stability. The role of the
Sn addition on the catalytic properties of PtSn catalysts supported on different materials and
used in the hydrocarbons dehydrogenation reactions has been discussed in many previous
works [12,14,18,19,33,38]. Agreement has been reached on the fact that a strong Pt-Sn
interaction favors catalytic behavior in dehydrogenation. Although it has been reported that
the presence of metallic tin can decrease the catalytic activity [14,21,38], it is also true that the
presence of electronic effects of the Sn atoms on the Pt sites, with possible alloys formation,
favors both the selectivity to olefins and the stability of the metallic phase [12,16,19,33,39].
However, the presence of tin in ionic form has been highlighted by other researchers as
playing an important role in improving the catalytic behavior of PtSn catalysts [14,39,40]. A
complex metallic phase has been proposed for the PtSn catalysts, where Pt sites coexist with
small PtSn alloy clusters and ionic tin at the platinum-support interface.
This work confirms the existence of strong Pt-Sn interaction in both bimetallic catalysts
through the results of TPR experiments, CHD reaction (test reaction of the metallic phase),
hydrogen chemisorption and XPS analysis. TPR results of bimetallic catalysts showed an
increase of reduction area and shifts to higher temperatures of the main reduction peaks,
indicating co-reduction of PtSn with alloy formation or intermetallic compound conformation.
The important decrease of initial rate values of CHD points towards the existence of
geometric effects of the Sn atoms on Pt sites. The marked increase in activation energy values
(ECH) indicates the presence of strong electronic effects of tin on platinum sites, with possible
alloys formation. The drop in hydrogen chemisorption values of bimetallic catalysts regard to
the corresponding Pt catalysts also indicates a clear influence of tin on the platinum sites
through geometric and/or electronic effects. Moreover, XPS results show that Sn is in two
23

oxidation states, a minor proportion alloyed with Pt, and a higher proportion as ionic tin,
probably close to Pt-support interface.
The catalyst with higher proportion of alloyed tin (39 %) was the PtSn/Sp-Zn-C one. In this
catalyst, 80 % of their metallic particles had sizes between 1 and 1.5 nm according to TEM
results, which indicates a very good metallic dispersion. These properties led to a catalyst
with the best catalytic behavior, thus showing yields to butenes between 26.5 % and 20.4 %.
Another important factor for the improvement of the catalytic performance of these samples is
the presence of a structured support consisting of a porous external layer of small thickness
(between 15 and 20 µm). Hence, the active phase is deposited on the external surface of the
catalyst, improving the mass and heat transference. This would allow the rapid diffusion of
the reactant (n-butane) to the active centers located in the layer, the fast reaction, and the
immediate desorption of the formed olefins, thus avoiding the formation of undesirable
products and increasing the selectivity to butenes.
Conclusions
Structured supports composed of thin, uniform and stable porous layers of ZnAl2O4 coating
alpha-alumina spheres were obtained by two synthesis methods. These supports showed a
good ability to disperse metals. In fact, the catalysts displayed very small metallic particles
with a predominance of sizes between 1 and 1.5 nm and a narrow size distribution. In
addition, the particles seems to have a structure with low concentration of edges, corners and
steps, which explain the low hydrogenolytic activity and the high dehydrogenating selectivity
to butenes. Characterization results show strong intermetallic interactions, with possible PtSn
alloys formation. The addition of Sn to Pt catalysts improves the behavior in n-butane
dehydrogenation reaction, especially the stability of its catalytic properties. This improvement
24

is more marked in the case of the PtSn/Sp-Zn-C catalyst, which exhibits very good yields to
butenes. With respect to the presence of ZnO in the Sp-Zn-C support, this impurity decreased
the activity of the monometallic catalyst. However, the Sn addition to Pt catalyst produced an
important modification of the metallic phase, enhancing the catalytic performance.
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