Novel Pt/Mg(In)(Al)O catalysts for ethane and propane dehydrogenation

Article abstract of DOI:10.1016/j.jcat.2011.06.008

Catalysts for the dehydrogenation of light alkanes were prepared by dispersing Pt on the surface of a calcined hydrotalcite-like support containing indium, Mg(In)(Al)O. Upon reduction in H₂ at temperatures above 673 K, bimetallic particles of PtIn are observed by TEM, which have an average diameter of 1 nm. Analysis of Pt L_III-edge extended X-ray absorption fine structure (EXAFS) data shows that the In content of the bimetallic particles increases with increasing bulk In/Pt ratio and reduction temperature. Pt L_III-edge X-ray absorption near edge structure (XANES) indicates that an increasing donation of electronic charge from In to Pt occurs with increasing In content in the PtIn particles. The activity and selectivity of the Pt/Mg(In)(Al)O catalysts for ethane and propane dehydrogenation reactions are strongly dependent on the bulk In/Pt ratio. For both reactants, maximum activity was achieved for a bulk In/Pt ratio of 0.48, and at this In/Pt ratio, the selectivity to alkene was nearly 100%. Coke deposition was observed after catalyst use for either ethane or propane dehydrogenation, and it was observed that the alloying of Pt with In greatly reduced the amount of coke deposited. Characterization of the deposit by Raman spectroscopy indicates that the coke is present as highly disordered graphite particles <30 nm in diameter. While the amount of coke deposited during ethane and propane dehydrogenation are comparable, the effects on activity are dependent on reactant composition. Coke deposition had no effect on ethane dehydrogenation activity, but caused a loss in propane dehydrogenation activity. This difference is attributed to the greater ease with which coke produced on the surface of PtIn nanoparticles migrates to the support during ethane dehydrogenation versus propane dehydrogenation.

Full text of DOI:10.1016/j.jcat.2011.06.008

Journal of Catalysis 282 (2011) 165–174
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Novel Pt/Mg(In)(Al)O catalysts for ethane and propane dehydrogenation
Pingping Sun a, Georges Siddiqi a, William C. Vining a, Miaofang Chi b, Alexis T. Bell a,
⇑
a Department of Chemical and Bimolecular Engineering, University of California, Berkeley, CA 94720-1462, United States
b Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalysts for the dehydrogenation of light alkanes were prepared by dispersing Pt on the surface of a cal-
cined hydrotalcite-like support containing indium, Mg(In)(Al)O. Upon reduction in H2 at temperatures
above 673 K, bimetallic particles of PtIn are observed by TEM, which have an average diameter of
1 nm. Analysis of Pt LIII-edge extended X-ray absorption fine structure (EXAFS) data shows that the In
content of the bimetallic particles increases with increasing bulk In/Pt ratio and reduction temperature.
Pt LIII-edge X-ray absorption near edge structure (XANES) indicates that an increasing donation of elec-
tronic charge from In to Pt occurs with increasing In content in the PtIn particles. The activity and selec-
tivity of the Pt/Mg(In)(Al)O catalysts for ethane and propane dehydrogenation reactions are strongly
dependent on the bulk In/Pt ratio. For both reactants, maximum activity was achieved for a bulk In/Pt
ratio of 0.48, and at this In/Pt ratio, the selectivity to alkene was nearly 100%. Coke deposition was
observed after catalyst use for either ethane or propane dehydrogenation, and it was observed that the
alloying of Pt with In greatly reduced the amount of coke deposited. Characterization of the deposit by
Raman spectroscopy indicates that the coke is present as highly disordered graphite particles <30 nm
in diameter. While the amount of coke deposited during ethane and propane dehydrogenation are com-
parable, the effects on activity are dependent on reactant composition. Coke deposition had no effect on
ethane dehydrogenation activity, but caused a loss in propane dehydrogenation activity. This difference is
attributed to the greater ease with which coke produced on the surface of PtIn nanoparticles migrates to
the support during ethane dehydrogenation versus propane dehydrogenation.
Received 23 March 2011
Revised 5 June 2011
Accepted 7 June 2011
Available online 20 July 2011
Keywords:
Pt bimetallic catalyst
Mg(In)(Al)O
STEM
XAFS
Dehydrogenation
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
[1–9]. Bimetallic nanoparticles of Pt and the alloying element have
also been found to produceless coke [10–14], a productthat can con-
Ethylene, propylene, and butene are used extensively to pro-
duce rubber, plastics and many other products. The conventional
source of these light alkenes is steam cracking of alkanes, naphtha,
or gas oil. This process is typically carried out at 1123 K and is not
selective. For example, steam cracking of naphtha produces ethene
yields of 30% and propene yields of 20%, with substantial quantities
of methane and coke as byproducts. Catalytic dehydrogenation of
alkanes provides an alternative for producing light alkenes as well
as hydrogen, a product required for many refinery operations, most
notably heteroatom removal. While thermal dehydrogenation, like
steam cracking, is an endothermic process, it occurs at lower tem-
peratures, 973–1073 K, but can also produce methane and coke as
byproducts.
tribute to deactivation of the catalyst. Catalyst stability is further en-
hanced by using a metal oxide support free of acid sites that can
promote carbon deposition through polymerization of the product
alkene [14–19]. Recent work has shown that calcined hydrotal-
cite-like materials, referred to as Mg(Al)O, are ideal for this purpose
[11,20,21]. Moreover, the Al cations present at the support surface
help stabilize the dispersed metal particles against sintering [22].
Of the promoting elements listed earlier, the greatest attention
has been given to Sn [13,14,23–29]. This element can be intro-
duced by coimpregnation of Pt and Sn precursors into a metal
oxide support or by first forming a supported Pt catalyst and then
impregnating it with a solution containing the Sn precursor. The
properties of the catalyst have been found to depend on the com-
position of the Pt and Sn precursors and the catalyst preparation
procedure [14]. By contrast, the performance of PtIn catalysts is
less dependent on the composition of the Pt and In precursors
and the procedure used for catalyst preparation [30]. PtIn catalysts
have also been found to be more homogenous in composition than
PtSn particles [31] and to be stable for dehydrogenation of C4 and
C5 alkanes [3]. Consequently, PtIn catalysts could potentially be
effective catalysts for dehydrogenation of light alkanes in general.
Platinum is the most effective transition metal for promoting al-
kane dehydrogenation; however, in order to achieve high alkene
selectivity, it must be promoted with an element, such as Sn, Zn,
Ge, Ga, or In, which interacts with the Pt to form a bimetallic alloy
⇑
Corresponding author. Fax: +1 510 642 4778.
E-mail address: alexbell@uclink.berkeley.edu (A.T. Bell).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.06.008

166
P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
We have recently reported a novel approach for producing PtGa
then reduced in flowing 10% H2/Ar (50 cm3/min). The temperature
of the sample was raised at 5 K/min to 873 K and then maintained
at this level for 2 h. The sample was then flushed in flowing Ar for
90 min and then cooled down to 313 K. The uptake of chemisorbed
hydrogen was then measured by determining the uptake of H2
from pulses using a TCD detector. The pulse size was 50 cm3/g,
and the time between pulses was 10 min.
Electron micrographs of the support, with and without Pt, were
taken using an aberration-corrected FEI Titan 80/300-kV TEM/
STEM located at Oak Ridge National Laboratory. Bright-field and
high angle annular dark-field (HAADF) STEM images were recorded
simultaneously to get full information of the microstructure.
HAADF images were acquired with a 300-kV accelerating voltage
with a convergence angle of 30 mrad and a large inner collection
angle of 75 mrad. The contrast of the acquired HAADF images is
sensitive to atomic number. To minimize electron beam radiation
on the sample, all images were acquired in regions that had not
been previously illuminated, shortly after the electron beam had
been aligned in a neighboring region. Electron energy loss spectra
were acquired for 3 s with a convergence angle of 30 mrad and col-
lection angle of 35 mrad. The data were processed through
DigitalMicrograph.
bimetallic nanoparticles exhibiting high activity and selectivity for
ethane and propane dehydrogenation, as well as reduced carbon
deposition [32]. These catalysts were prepared by dispersing Pt
nanoparticles onto a calcined hydrotalcite-like support containing
Ga, Mg(Ga)(Al)O. Upon H2 reduction of Pt/Mg(Ga)(Al)O at 873 K, a
part of the Ga3+ cations at the surface of the support were reduced
and the resulting Ga atoms formed PtGa bimetallic particles by
alloying with the supported Pt particles. Since In can be introduced
into the support in a manner similar to Ga, we undertook an inves-
tigation of the properties of Pt/Mg(In)(Al)O catalysts for light al-
kane dehydrogenation. Here we report on the preparation,
characterization, and evaluation of such catalysts for ethane and
propane dehydrogenation. XANES and EXAFS were used to track
the alloy composition of the catalyst as a function of the bulk In/
Pt ratio and examine the effect of reduction temperature on alloy
formation. Information about metal particle size before and after
catalyst use was obtained by STEM, whereas Raman spectroscopy
was used to identify type of coke deposited following ethane or
propane dehydrogenation.
2. Experimental
Pt LIII- and In K-edge XAS data were collected at the Advanced
Photon Source at Argonne National Lab on beamline 10-BM with
a Si(1 1 1) monochromator crystal detuned by 40%. The sample
was located between two ionization chambers, and a reference foil
was placed after the second ionization chamber for energy calibra-
tion. Nitrogen and argon gas was used in the ionization chambers
for Pt LIII- and In K-edges, respectively, and all samples were inves-
tigated in transmission mode. All six samples with In/Pt ratios (In/
Pt = 0, 0.33, 0.48, 0.88, 1.7, 5.6) were prepared simultaneously un-
der three different pretreatment conditions: 723 K for 1 h in 4% H2/
He at 100 cm3/min, 873 K for 1 h in 4% H2/He at 100 cm3/min, and
873 K for 1 h in 10% O2/N2 after pretreatment at 873 K in hydrogen.
All samples were cooled to ambient temperature in He flowing at
100 cm3/min and then transferred to the beamline for immediate
EXAFS characterization.
2.1. Catalyst preparation
Mg(In)(Al)O was synthesized using the following procedure. A
mass of 58.31 g of Mg(NO3)2 6H2O (Alfa Aesar, 98–102%), appropri-
ate amounts (depending on the desired In loading) of Al(-
NO3)3Á9H2O (Alfa Aesar, 98–102%) and In(NO3)3ÁÂH2O (Alfa
Aesar, 99.9%) were dissolved in 250 ml deionized water, and 1.2 g
of Na2CO3 (EMD Chemicals Inc, 99.5%) and 11 g of NaOH (Fisher
Scientific, 98.3%) were dissolved in 250 ml deionized water. These
two solutions were then mixed dropwise with stirring, at about
333 K in about 20 min, and aged at room temperature for 18 h.
The resulting suspension was filtered, and the solid product dried
in air overnight at 383 K. The hydrotalcites-like material obtained
in this manner was heated in air to 973 K at 2 K/min and main-
tained at this temperature for 10 h to obtain the calcined support,
Mg(In)(Al)O. Mg(Al)O was prepared in a similar manner.
The same amount of Pt was dispersed onto 1 g of the calcined
support by incipient wetness impregnation, using a solution con-
taining 21 mg of Pt(acetylacetonate)2 (Sigma Aldrich, 99.99%) dis-
solved in 1.5 ml toluene. The resulting mixture was stirred until
powdery, then left at room temperature in air for 2 h, and finally
dried overnight at 383 K in an oven. After drying, the catalyst
was reduced at 723 K for 2 h (5 K/min temperature ramp) in 10%
H2/Ar (60 cm3/min). In this manner, a series of Pt/Mg(In)(Al)O cat-
alysts were prepared with different bulk In/Pt ratio.
EXAFS data were analyzed using the program Ifeffit and the
graphical user interfaces Athena and Artemis [33]. The incident
photon energy was calibrated to a reference Pt foil. The edge en-
ergy was set to the first inflection point on the rising part of the
absorbance edge. Data were normalized using a pre-edge line fit
from À150 to À50 eV below the edge and a quadratic polynomial
with a k-weight of two from 150 to 749 eV above the edge energy.
A spline was fit to the data 0 < k < 15 ÅÀ1 after the edge. The data
for 3.5 < k < 12.1 ÅÀ1 were fit from 1.8 to 3.2 Å for catalysts treated
in hydrogen at 873 K and from 1.4 to 3.2 Å for the other two pre-
treatments using k-weights of 1, 2, and 3. For all pretreatments,
the initial Pt and In paths were taken to be 2.8 Å. The same mean
square disorder,
r
2, and edge energy were used for both paths,
2.2. Catalyst characterization
but the path lengths were allowed to vary independently. For the
samples pretreated in hydrogen at 723 K, a third oxygen path
was added at a distance of 2 Å, and the bond length and coordina-
tion number were allowed to vary separately. The same structural
disorder parameter and edge energy for the Pt and In paths were
used in the oxygen path. Theoretical standards were constructed
using known crystal structures of PtIn alloys [34], as well as Pt oxi-
des [34].
The content of Pt, Mg, Al and In were determined by Galbraith
Laboratories (Knoxville, TN) using inductively coupled plasma
optical emission spectroscopy. The BET surface area of Pt/Mg(Al)(I-
n)O was determined by the multi-point method using an Autosorb-
1 instrument (Quantachrome Corporation). Prior to measuring the
adsorption isotherm, each sample was degassed at 573 K for 22–
24 h. The hydrotalcite-like material was characterized by X-ray
powder diffraction before and after calcination, using a Siemens
2.3. Catalyst testing
Diffractometer D 5000 with CuKa radiation (k = 1.5418 Å) at
20 kV and 5 mA. The samples were scanned from 2h values of 5°
to 70° with a step size of 0.02° and a dwell time of 1.0 s. The disper-
sion of Pt on Pt/Mg(Al)(In)O was determined by H2 chemisorption
using a AutoChem II 2920 (Micromeritics Instrument Corporation).
About 60 mg of Pt/Mg(In)(Al)O was loaded into a quartz cell and
Reactions were carried out in a quartz reactor with an inner
diameter of 7 mm. Prior to testing, the catalyst (0.025 g, 0.25–
0.5 mm particle size) was heated at 10 K/min to 873 K in 20% H2
in He and then maintained at this temperature for 1 h. The catalyst
bed was heated by a three-zone furnace (Applied Test System, Inc.)

P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
167
controlled by Watlow 988 controllers. The temperature of the cat-
alyst bed was measured by two thermocouples centered axially in-
side the reactor, one at the top and one at the bottom of the
catalyst bed. Brooks Mass Flow Controllers (MFC) were used to de-
liver the flow of each gas. Measurements of catalyst activity and
selectivity were carried out at 873 K, using 0.025 g of catalyst
and a feed consisting of 20% ethane or propane, 25% hydrogen,
and the balance He flowing at 60 cm3/min [32]. All experiments
were performed in the kinetic regime. Internal mass transport lim-
itations were not observed as evidenced by a linear Arrhenius plot
and satisfaction of the Weisz–Prater criterion [35].
a
b
2.4. Coke measurement
10
20
30
40
50
60
70
2θ (°)
Coke deposition occurred during the exposure of the catalyst to
ethane or propane. The amount of coke deposited was determined
by combustion of the deposited material. Prior to combustion, the
catalyst (0.025 g) was purged with flowing He (60 cm3/min) at
873 K for 5 min, and then exposed it to a mixture of 5% O2 in He
flowing at 60 cm3/min. The CO2 generated were monitored by on-
line mass spectrometry (MS). The amount of deposited coke was
calculated from the amount of generated CO2. Regeneration of
the catalysts was done by oxidizing the catalyst in a flow of 10%
O2 in He (100 cm3/min) for 20 min and then reducing the catalyst
in a flow of 20% H2 in He (100 cm3/min) for 30 min. Prior to chang-
ing the gas composition, the reactor was flushed with helium.
The Raman spectra of coke deposited during alkane dehydroge-
nation were performed with an epi-illumination, confocal Raman
microscope (LabRam HR, Horiba Jobin Yvon). A diode-pumped so-
lid-state 532-nm laser (Torus, Laser Quantum) was used as the
excitation source. The back-scattered light was filtered by a 532-
nm edge filter and channeled into a spectrograph/charge-coupled
device detector (Andor). The laser power was 3–9 mw, and the res-
olution of the apparatus was 1 cmÀ1. The data acquisition time was
between 4 and 15 s, and 8–20 scans were acquired depending on
the strength of the carbon signal.
Fig. 1. XRD of as-synthesized (a) and calcined hydrotalcites (b).
periclase MgO. The broad peaks of both materials are indicative
of small crystalline particles or a partially amorphous phase. Sim-
ilar changes in the XRD patterns were observed for all support
materials, independent of the In/Al ratio.
Table 2 lists the surface areas of the calcined mixed oxide mate-
rials. Mg(Al)O has a surface area of 176 m2/g and a pore volume of
0.6 cm3/g. Neither the surface area nor pore volume of Mg(In)(Al)O
changed much upon increasing the In content from 0–5.1 wt.%.
Minimal change in the surface area and pore volume occurred
upon the dispersion of Pt onto the support. The ratio of H atoms ad-
sorbed to the total number of Pt atoms in the catalyst is also pre-
sented in Table 2. For Pt/Mg(Al)O, this ratio is 0.84, which means
that the dispersion of the Pt is 84%. As discussed in the next para-
graph, the Pt particle size estimated from the dispersion, 1.3 nm, is
consistent with that determined from STEM images, about 1 nm,
independent of the bulk In/Pt ratio. The ratio of adsorbed H atoms
to total Pt atoms measured for the Pt/Mg(Al)(In)O samples is a
function of the In/Pt ratio. This ratio is 0.94 when In/Pt = 0.48
and decreases rapidly with increasing In/Pt ratio, reaching a value
of 0.06 when In/Pt = 10.7. Since, as discussed later, the size of the
PtIn particles remains essentially the same with increasing bulk
In/Pt ratio, the observed decrease in H/Pt ratio is ascribed to in-
crease coverage of the metal nanoparticles by In. This interpreta-
tion is consistent with previous research, showing that the
adsorption of H2 on Pt alloyed with Sn and Ga is reduced by the
presence of the alloying metal on the particle surface because the
alloying metal does not adsorb H2 [37,38].
3. Results and discussion
3.1. Catalyst characterization before reaction
A list of all samples studied is presented in Table 1. The Mg/
(Al + In) ratio was maintained close to 5 for the majority of the
samples, and the In/Al ratio was varied from 0 to 0.167. The Pt con-
tent was maintained between 0.7 wt.% and 0.8 wt.%, whereas the
In/Pt ratio varied from 0 to 10.7.
Fig. 1 shows the XRD pattern of the dried, as-synthesized sup-
port (top) and the same material after calcination (bottom). Prior
to calcination, the XRD pattern is characteristic of the layered dou-
ble hydroxide structure of hydrotalcite [36]. After calcination at
973 K, the structure of the solid changes from orthorhombic to cu-
bic, corresponding to the transformation of a two-dimensional lay-
ered structure to a three-dimensional structure, analogous to
A STEM image of Pt/Mg(In)(Al)O-0.88 is shown in Fig. 2. After
calcination, the support still exhibits a partially layered structure,
similar to that seen earlier for calcined Mg(Al)O [39]. Small metal-
lic particles about 1 nm in diameter are observed distributed
evenly over the support. Similar particle size distributions were
observed for all catalysts with In/Pt ratio from 0 to 10.7, suggesting
that the presence of In in the support does not change the size of
the metal particles relative to that observed in the absence of In.
Table 1
Composition of all catalysts.
Sample namea
Pt wt.%
In wt.%
Starting ratio Mg:Al:In
Mg/(Al + In)
In/Al
In/Pt
Pt/Mg(Al)O
0.70
0.89
0.71
0.83
0.67
0.76
0.81
0
100:10:0
5.09
5.13
4.82
4.90
4.60
5.31
5.02
0
–
Pt/Mg(Al)(In)O-0.33
Pt/Mg(Al)(In)O-0.48
Pt/Mg(Al)(In)O-0.88
Pt/Mg(Al)(In)O-1.7
Pt/Mg(Al)(In)O-5.6
Pt/Mg(Al)(In)O-10.7
0.171
0.201
0.428
0.674
2.49
5.12
100: 9.95:0.05
100: 9.9: 0.10
100: 9.86: 0.14
100: 9.75: 0.25
100: 9: 1
0.0047
0.0065
0.010
0.021
0.083
0.167
0.33
0.48
0.88
1.7
5.6
10.7
100: 8: 2
a
The letter refers to support name, the number afterward refers to In/Pt ratio of the catalyst.

168
P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
Table 2
Catalyst surface area (BET), pore volume, average pore radius and percent of exposed Pt sites.
Sample name
Exposed Pt sites (%)
Surface area (m2/g)
Pore volume (cm3/g)
Average pore radius (Å)
Pt/Mg(Al)O
84
84
94
65
35
12
6
176
148
161
167
191
158
161
0.60
0.55
0.61
0.75
0.58
0.49
0.61
70.0
73.7
75.9
89.8
60.3
62.1
75.9
Pt/Mg(Al)(In)O-0.33
Pt/Mg(Al)(In)O-0.48
Pt/Mg(Al)(In)O-0.88
Pt/Mg(Al)(In)O-1.7
Pt/Mg(Al)(In)O-5.6
Pt/Mg(Al)(In)O-10.7
CN of Pt-Pt bond
CN of Pt-In bond
CN of Pt-O bond
8
6
4
2
0
0
1
2
3
4
5
6
Bulk In/Pt
Fig. 3. Pt–Pt, Pt–In, Pt–O coordination numbers determined from Pt LIII edge. All
EXAFS data were acquired after reduction with 100 cm3/min of 4% H2/He at 723 K
for 1 h. Samples were cooled to ambient temperature in 100 cm3/min of He flow
before acquiring data. The error bars are the error in the coordination number
determined by the best fit for each sample.
5 nm
rises slightly. Since the metal nanoparticles are very small
(ꢀ1 nm), it is quite possible that the O atoms are present predom-
inantly on the surface of the particles, rather than in the bulk.
Pt LIII edge XANES data acquired after catalyst reduction at
723 K are shown in Fig. 4. All samples show similar edge energies,
but the intensity of the white line is higher for samples with In
present, than for the sample without In. This might be due to Pt–
O bonding in the In containing samples, in contrast to In-free sam-
ples, leading to an increase d-orbital vacancy and, hence, an in-
crease in the white line intensity. It has generally been observed
that the formation Pt alloys, such as PtSn [41], PtCu [42], PtIn
[30], PtAu [43], leads to a decrease in white line intensity of Pt LIII
edge. The white line intensity does not show a systematic change
with In/Pt bulk ratios, however, quite possibly because the white
line intensity is affected by both In and O. The oxygen effect may
dominate, since the white line intensity correlates with the Pt–O
coordination number.
Fig. 2. STEM of Pt/Mg(In)(Al)O 0.88 before reaction.
EXAFS spectra of Pt/Mg(Al)O and Pt/Mg(In)(Al) reduced in H2 at
723 K (see supplementary material) were analyzed to determine
the coordination numbers for Pt–Pt, Pt–In, and Pt–O nearest neigh-
bors. The results are presented in Table 3 and Fig. 3 for catalysts
prepared with bulk In/Pt ratio between 0 and 5.6. The data show
that with increasing In/Pt ratios, the Pt–Pt coordination number
decreases monotonically from 6.7 to 4.8, as the Pt–In and Pt–O
coordination numbers increase from 0 to about 0.5 and 1.4. The
appearance of Pt–In nearest neighbors as the bulk In/Pt ratio in-
creases indicates that catalyst reduction at 723 K is sufficient to en-
able some of the In3+ cations present at the support surface and
proximate to the Pt particles to be reduced, and resulting In atoms
to form PtIn alloys with the Pt particles. This interpretation is con-
sistent with the observation that In2O3 undergoes reduction in H2
above 673 K [40]. It is notable that in the absence of In, catalyst
reduction at 723 K is sufficient to remove all oxygen. However,
when In is present in the support, the Pt–O coordination number
Table 4 and Fig. 5 show that following hydrogen reduction at
873 K, the Pt–O coordination number is zero independent of the
bulk In/Pt ratio. However, as the bulk In/Pt ratio increases, the
Table 3
EXAFS fitting parameters for Pt/Mg(In)(Al)O after reaction in 4% H2/He for 1 h at 723 K. The data (3.5 < k < 12.1 ÅÀ1) were fit (1.4 < R < 3.2 Å) with one Pt and one In backscattering
path. The amplitude reduction factor, S20, was fixed to a value of 1 during the fit.
Catalyst
D
E0 (eV)
r2 (Å2)
CNPt–In
RPt–O (Å)
CNPt–In
RPt–In (Å)
CNPt–Pt
RPt–Pt (Å)
R-factor
Pt/Mg(Al)O
5.4 0.7
4.3 1.6
5.9 1.1
5.0 2.4
7.2 3.6
2.0 3.4
0.007 0.001
0.009 0.001
0.008 0.001
0.008 0.002
0.011 0.002
0.010 0.003
–
–
–
–
–
–
–
–
6.7 0.5
6.1 1.0
6.5 0.7
6.1 1.1
6.5 1.6
4.8 1.4
2.73 0.01
2.72 0.01
2.74 0.01
2.74 0.02
2.74 0.02
2.71 0.03
0.003
0.020
0.008
0.007
0.012
0.020
Pt/Mg(Al)(In)O-0.33
Pt/Mg(Al)(In)O-0.48
Pt/Mg(Al)(In)O-0.88
Pt/Mg(Al)(In)O-1.7
Pt/Mg(Al)(In)O-5.6
0.5 0.27
0.75 0.19
0.61 0.28
1.42 0.42
0.79 0.36
1.94 0.05
1.95 0.03
1.95 0.04
1.99 0.04
1.92 0.05
0.15 0.31
0.55 0.80
0.44 0.35
2.68 0.18
2.86 0.08
2.65 0.10

P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
169
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
9
8
7
6
5
4
3
2
1
0
CN of Pt-Pt bond
CN of Pt-In bond
CN of total
Pt/Mg(Al)O
Pt/Mg(In)(Al)O0.33
Pt/Mg(In)(Al)O0.48
Pt/Mg(In)(Al)O0.88
Pt/Mg(In)(Al)O1.7
Pt/Mg(In)(Al)O5.6
0
1
2
3
4
5
6
7
11550
11560
11570
11580
Energy (eV)
Bulk In/Pt
Fig. 4. Pt LIII XANES for Pt/Mg(Al)(In)O catalysts after treatment in 4% H2/He for 1 h
at 723 K. Samples were cooled to ambient temperature in He flow before data
acquisition. The inset is an enlargement of the normalized edge height.
Fig. 5. Pt–Pt and Pt–In coordination numbers determined from Pt LIII edge. All
EXAFS data were acquired after reduction with 100 cm3/min of 4% H2/He at 873 K
for 1 h. Samples were cooled to ambient temperature in 100 cm3/min of He flow
before acquiring data. The error bars are the error in the coordination number and
were determined by the best fit for each sample.
Pt–Pt coordination number decreases and the Pt–In coordination
number increases. This trend indicates that the In content of the
PtIn alloy particles formed upon reduction increases monotonically
with increasing In/Pt ratio. Since the total coordination number
(sum of CNPtPt and CNPtIn) is ꢀ6 for In/Pt ratios 0–5.6, it is con-
cluded that the diameters of the supported alloy particles do not
change significantly with increasing In content, consistent with
observations by STEM.
If it is assumed that In is uniformly distributed throughout the
alloy, then the fraction of In contained in the supported PtIn nano-
particles can be estimated from the relationship CNPt–In/(CNPt–
Pt + CNPt–In) assuming isostructural, spherical particles. The atomic
fraction of In in the alloy particles calculated this way is listed in
Table 5. It is apparent from these results that the fraction of the al-
loy consisting of In increases almost linearly with the bulk In/Pt ra-
tio, in Fig. 7.
Tables 3 and 4 show that the Pt–Pt bond distance for Pt/Mg(Al)O
reduced at both 723 K and 873 K is 2.73–2.74 Å, which is smaller
than that for bulk Pt, 2.77 Å. The decrease in metallic bond distance
with decreasing coordination is commonly observed [44]. This
phenomenon is due to lattice relaxation of smaller particles (1–
5 nm), which could result in 10% decrease in the Pt–Pt bond dis-
tance relative to that of bulk phase [45]. When In alloys with Pt,
the Pt–Pt bond distance in the PtIn nanoparticles increases by no
more than 0.03 Å. The calculated Pt–In bond distance exhibits
more variation from sample to sample than the calculated Pt–Pt
distance, but on average is 2.71 Å, which is comparable to the PtPt
distance is consistent. The calculated Pt–In distance is consistent
with those observed for PtIn alloys (2.73–2.95 Å) [34], but the
accuracy with which this distance can be determined does not al-
low assignment to a particular PtIn alloy composition.
Table 5
Estimated PtIn alloy compositions.
Sample name
Bulk
In/Pt
In/(Pt + In)
alloy (%)
In in reduced
state (%)
Pt/Mg(Al)(In)O-0.33
Pt/Mg(Al)(In)O-0.48
Pt/Mg(Al)(In)O-0.88
Pt/Mg(Al)(In)O-1.7
Pt/Mg(Al)(In)O-5.6
0.33
0.48
0.88
1.7
6
12
16
21
67
20
25
17
13
12
5.6
ratio. Pt LIII XANES characterizes dipole transitions from the 2p3/2
to the 5d5/2 orbital, since the 5d3/2 orbital is fully occupied
[46,47]. Therefore, changes in the intensity of the white line indi-
cate changes in the unoccupied states of the 5d5/2 orbital associ-
ated with metal oxidation and/or alloy formation. DFT
calculations demonstrate that the white line density can also
change with cluster size and cluster morphology because the den-
sity of states of nanometer-sized metal clusters is very different
from that of the bulk phase [48,49]. This trend is consistent with
experiments showing that the white line intensity for supported
metal particles decreases with decreasing metal cluster size
[44,50]. Since reduction at 873 K removes all oxygen from the sup-
ported nanoparticles and the size of the nanoparticles does not
change with increasing bulk In/Pt ratio, the effects of Pt oxidation
by O and changes in particle size with bulk In/Pt ratio on the inten-
sity of the white line can be ruled out. As a result, the observed
changes in the white line intensity of Pt LIII XANES spectra shown
in Fig. 6 can only be attributed to the formation of a PtIn alloy.
It is also observed that following reduction at 873 K, there is an
increase in the Pt LIII edge energy by about 1 eV with increasing
bulk In/Pt ratio. This shift in edge is a further indicator of alloy for-
mation. This trend is similar to that observed in XPS measurements
Fig. 6 shows Pt LIII XANES spectra for Pt/Mg(In)(Al)O catalysts
reduced at 873 K. The intensity of the white line decreases slightly
with increasing In amount but then increases at the highest In/Pt
Table 4
EXAFS fitting parameters of Pt/Mg(In)(Al)O after treatment in 4% H2/He for 1 h at 873 K. The data (3.5 < k < 12.1 ÅÀ1) were fit (1.8 < R < 3.2 Å) with one Pt and one In
backscattering path. The amplitude reduction factor, S20;, was fixed to a value of 1 during the fit.
Catalyst
D
E0 (eV)
r2 (Å2)
CNPt–In
RPt–In (Å)
CNPt–Pt
RPt–Pt (Å)
R-factor
Pt/Mg(Al)O
6.4 0.4
3.3 1.5
3.2 1.6
1.6 2.4
1.5 2.1
1.2 0.8
0.0065 0.0004
0.007 0.001
0.006 0.001
0.006 0.002
0.008 0.002
0.009 0.001
–
6.1 0.4
5.7 0.8
5.2 0.8
5.6 1.4
5.5 1.4
1.9 0.4
2.74 0.01
2.72 0.01
2.74 0.01
2.75 0.02
2.76 0.02
2.77 0.02
0.002
0.005
0.004
0.011
0.013
0.003
Pt/Mg(Al)(In)O-0.33
Pt/Mg(Al)(In)O-0.48
Pt/Mg(Al)(In)O-0.88
Pt/Mg(Al)(In)O-1.7
Pt/Mg(Al)(In)O-5.6
0.4 0.2
0.7 0.2
1.0 0.3
1.5 0.4
3.8 0.4
2.64 0.05
2.70 0.03
2.72 0.04
2.71 0.03
2.69 0.01

170
P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
Pt/Mg(Al)O
Pt/Mg(In)(Al)O0.33
Pt/Mg(In)(Al)O0.48
Pt/Mg(In)(Al)O0.88
Pt/Mg(In)(Al)O1.7
Pt/Mg(In)(Al)O5.6
1.2
1.0
0.8
0.6
0.4
0.2
0.0
100
90
80
70
60
11560
11565
0
2
4
6
8
10
12
11560
11580
Bulk In/Pt
Energy (eV)
Fig. 9. Dependence of ethane selectivity on the bulk In/Pt ratio. Reaction
conditions: T: 873 K, m = 25 mg, C2H6: 20%, total flow rate: 60 cm3/min, H2/
C2H6 = 1.25).
Fig. 6. Pt LIII XANES for Pt/Mg(Al)(In)O catalysts after treatment in 4% H2/He for 1 h
at 823 K. Samples were cooled to ambient temperature in He flow before data
acquisition. Inset is close-up of edge shift.
3.2. Catalyst performance
100
80
60
40
20
0
The activity of Pt/Mg(In)(Al)O catalysts for ethane dehydroge-
nation is shown in Fig. 8 as a function of the bulk In/Pt ratio. The
rate of C2H4 production per gram of catalyst (Fig. 8A) increases
with In/Pt ratio for In/Pt = 0–0.48, and then decreases sharply as
the In/Pt ratio is increased further. The turnover frequency for
dehydrogenation was determined on the basis of the H2 uptake
measurements reported in Table 2. This definition of TOFs is based
on the hypothesis that alkane dehydrogenation requires Pt sites
and that only these sites will adsorb H atoms. Both assumptions
have recently been confirmed by quantum chemical calculations
conducted in our group. Fig. 8B shows that the TOF for ethane
dehydrogenation increases by slightly more than a factor of two
as the bulk In/Pt ratio increases from 0 to about 2 and then de-
creases monotonically for higher In/Pt ratios. Fig. 9 shows that
the ethene selectivity increases rapidly with increasing In/Pt,
reaching a value of nearly 100% for In/Pt > 0.33. It is noted that
the only byproduct observed was methane. The amount of coke
formation was recorded after two hours of reaction is shown in
Fig. 10. As can be seen, the accumulation of coke decreases strongly
with increased alloying of In with Pt. The highest accumulation of
carbon was observed for Pt/Mg(Al)O. For this sample, the coke
accumulated on the catalyst after 2 h of time on stream constituted
only 0.3% of the total amount of the carbon contained in feed
passed over the catalyst during this time. By contrast, typically
10% of the feed was converted to gaseous products (ethene,
0
1
2
3
4
5
6
7
Bulk In/Pt
Fig. 7. Correlation between percent of alloy containing In with bulk In/Pt ratio.
when Pt is alloyed with Sn [25,51] or In [30]. The observed increase
in Pt binding energy in both cases can be attributed to charge
transfer from the alloying element to Pt since the electronegativity
of Pt (2.28 eV) is higher than that of Sn (1.8 eV) or In (1.7 eV). Sim-
ulation of the XANES spectrum of PtAu nanoparticles further sup-
ports the assignment of shifts in the white line position to charge
transfer occurring between the alloying elements [43].
(A)
(B)2.0
30
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
25
20
15
10
5
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Bulk In/Pt
Bulk In/Pt
Fig. 8. In/Pt effect on C2H4 formation rate for C2H6 dehydrogenation when normalized by catalyst weight (A) and exposed Pt sites determined by H2 chemisorption (B).
Reaction conditions: T: 873 K, m = 25 mg, C2H6: 20%, total flow rate: 60 cm3/min, H2/C2H6 = 1.25).

P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
171
Pt/Mg(In)(Al)O catalysts were also evaluated for propane dehy-
drogenation at 873 K. As seen in Fig. 12A, the rate of propane dehy-
drogenation passes through a maximum for a bulk In/Pt ratio of
0.48 and then rapidly declines to nearly zero. This behavior is sim-
ilar to that observed for ethane dehydrogenation (see Fig. 8A), but
the maximum rate for propane dehydrogenation is more than two-
fold higher than that for ethane dehydrogenation. Fig. 12B shows
that in a manner similar to that observed for ethane dehydrogena-
tion, the TOF for propane dehydrogenation passes through a max-
imum for In/Pt = 2. However, in contrast to what is seen for ethane
dehydrogenation (see Fig. 9), the selectivity to propene rises rap-
idly to nearly 100% for a bulk In/Pt ratio of 0.48, passes through a
maximum, and falls off for In/Pt ratios above about 2.0, shown in
Fig. 13. As shown in the inset of this figure, the selectivities to
CH4, C2H6 and C2H4 increases from nearly zero when the In/Pt ratio
is raised above 2.0. Fig. 14 shows that the amount of carbon accu-
mulated on the catalyst after 2 h of time on stream decreases sig-
nificantly as the bulk In/Pt ratio is increased from 0 to 0.48, and
decreases further but more slowly as the In/Pt ratio is raised
further.
120
100
80
60
40
20
0
0
2
4
6
8
10
12
Bulk In/Pt
Fig. 10. Dependence of C/Pt ratio on bulk In/Pt ratio after 2 h time on stream of
ethane dehydrogenation. Reaction conditions: T: 873 K, m = 25 mg, C2H6: 20%, total
flow rate: 60 cm3/min, H2/C2H6 = 1.25).
The propane dehydrogenation activity of Pt/Mg(In)(Al)O 0.48
with time on stream is shown in Fig. 15. In contrast to what was
seen for ethane dehydrogenation (see Fig. 13), the activity of this
catalyst decreases significantly during the first 2 h of its use for
propane dehydrogenation. On the other hand, as in the case of eth-
ane dehydrogenation, removal of accumulated carbon after 2 h of
time on stream and H2 reduction at 873 K fully restores the original
catalyst activity. Fig. 16 shows that the selectivity to propene rises
slightly with time on stream to 99% and that this change of selec-
tivity with time on stream is fully recovered each time the catalyst
is cycled. The only products other than C3H6 and H2 are CH4, C2H6
and C2H4.
35
30
Cycle 1
Cycle 2
Cycle 3
Catalysts used for ethane or propane dehydrogenation were
examined by STEM and Raman spectroscopy with the goal of
establishing whether metal particle sintering or carbon deposition
was the cause of a loss in catalytic activity with time on stream.
The average metal particle size after 2 h of reaction determined
from STEM images of Pt/Mg(Al)O and Pt/Mg(In)(Al)O were very
nearly the same as those determined prior to reaction, suggesting
that particle sintering did not occur during alkane
dehydrogenation.
Raman spectra of Pt/Mg(Al)O and Pt/Mg(In)(Al)O 0.88 taken
after ethane and propane dehydrogenation, respectively, are pre-
sented in Figs. 17 and 18. The appearance of these spectra is virtu-
ally the same independent of which catalyst or alkane was used.
Two broad bands are observed centered at 1332 cmÀ1 and
1585 cmÀ1. These features are best assigned to the D and G bands,
25
0
20
40
60
80 100 120 140
Time on stream (min)
Fig. 11. Effects of catalyst cycling on rate of ethene formation versus time on
stream. Reaction conditions: T: 873 K, m = 25 mg, C2H6: 20%, total flow rate:
60 cm3/min, H2/C2H6 = 1.25).
hydrogen, and methane); therefore, the selectivity to coke was 3%
for Pt/Mg(Al)O, and even lower for the In-promoted catalysts.
The stability of the most active catalyst, Pt/Mg(In)(Al)O 0.48,
was tested after coke had been removed from the catalyst by oxi-
dation, and the catalyst was then reduced in H2 at 873 K. As can be
seen from Fig. 11, the catalyst is very stable with both time on
stream and with repeated oxidation–reduction cycles.
3.5
(A)
(B)
80
3.0
2.5
2.0
1.5
1.0
0.5
0.0
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Bulk In/Pt
Bulk In/Pt
Fig. 12. In/Pt effect on C3H6 reaction rate for C3H8 dehydrogenation normalized by catalyst weight (A) and by exposed Pt sites determined by H2 chemisorption (B). Reaction
conditions: T: 873 K, m = 25 mg, C3H8: 20%, total flow rate: 60 cm3/min, H2/C2H6 = 1.25).

172
P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
CH4
C2H6
C2H4
95
90
85
80
75
70
65
20
16
12
8
Cycle 1
Cycle 2
Cycle 3
100
95
90
85
80
75
70
4
0
0
2
4
6
8
10 12
In/Pt
0
20
40
60
80 100 120 140
Time on stream (min)
0
2
4
6
8
10
12
Bulk In/Pt
Fig. 15. Effects of catalyst cycling on rate of propene formation versus time on
stream. Reaction conditions: T: 873 K, m = 25 mg, C3H8: 20%, total flow rate:
60 cm3/min, H2/C2H6 = 1.25).
Fig. 13. Dependence of propene selectivity on bulk In/Pt ratio. The selectivities of
CH4, C2H6 and C2H4 are also listed in the inset figure. Reaction conditions: T: 873 K,
m = 25 mg, C3H8: 20%, total flow rate: 60 cm3/min, H2/C2H6 = 1.25).
100
100
80
60
40
20
0
98
Cycle 1
96
94
92
90
Cycle 2
Cycle 3
0
20
40
60
80 100 120 140
Time on stream (min)
0
2
4
6
8
10
12
Bulk In/Pt
Fig. 16. Effects of catalyst cycling on selectivity of propene formation versus time
on stream. Reaction conditions: T: 873 K, m = 25 mg, C3H8: 20%, total flow rate:
60 cm3/min, H2/C2H6 = 1.25).
Fig. 14. Dependence of C/Pt ratio on bulk In/Pt ratio after 2 h time on stream of
propane dehydrogenation. Reaction conditions: T: 873 K, m = 25 mg, C2H6: 20%,
total flow rate: 60 cm3/min, H2/C3H8 = 1.25).
naceous deposit produced from ethane rather than propane could
migrate from the metallic particles where they are formed onto the
support. Consistent with this interpretation, it was observed that
following alkane dehydrogenation of propane, a greater loss in
CO chemisorption capacity was observed than following ethane
dehydrogenation. Given the strong similarities in the performance
of Pt/Mg(In)(Al)O and Pt/Mg(Ga)(Al)O, we suggest that the precur-
sors to carbon deposition produced during ethane dehydrogena-
tion occurring on Pt/Mg(In)(Al)O transfer more readily to the
support than do the carbon precursors produced during propane
dehydrogenation. A discussion of the mechanism by which carbon
deposition occurs on Pt/Mg(Al)O catalysts has been presented in
Ref. [54]. The authors of this study note that surface science studies
indicate that CAC bond cleavage of alkylidyne species formed on
the surface of Pt leads to the formation of surface carbon atoms
which then organize to form coke. It is also noted that propylidyne
species are more stable to hydrogenation than ethylidyne species
and, hence, more likely to lead to coke deposition.
A comparison of the performance of Pt/Mg(In)(Al)O 0.48 with
that of Pt/Mg(Al)O, PtSn/Mg(Al)O 0.6, and Pt/Mg(Ga)(Al)O 2.9 is
presented in Table 6. In each case, the catalyst exhibiting the high-
est activity was chosen. The M/Pt ratio given in this table is the
stoichiometric ratio of the promoting element to the total amount
of Pt in the catalyst. For Pt/Mg(Ga)(Al)O and Pt/Mg(In)(Al)O, the
promoting element, Ga or In, is added during the synthesis of the
support, whereas for PtSn/Mg(Al)O, the promoting element, Sn, is
added by grafting Sn onto the Pt particles of Pt/Mg(Al)O [53]. For
respectively, of highly disordered graphite particles [52]. In single
crystal graphite, the G band appears at 1575 cmÀ1 but shifts to
higher frequencies with decreasing crystal size. For finite crystal-
lites of graphite, the D band appears at 1355 cmÀ1 and is attributed
to zone-boundary phonon vibrations. The ratio of the band intensi-
ties ID/IG has been used to estimate the size of graphite domains.
For the spectra shown in Figs. 17 and 18, ID/IG = 1.3–1.5, from
which it is deduced that the graphite domains are about 30 nm.
The extent to which graphitic deposits produced during alkane
dehydrogenation are responsible for loss of catalyst activity with
time on stream needs to be examined. Comparison of Figs. 10
and 14 shows that the amount of carbon deposited on Pt/Mg(In)(A-
l)O 0.48 after 2 h of time on stream is roughly the same during eth-
ane and propane deposition, C/Pt ꢁ 25. At the same time, Figs. 11
and 15 show that while the loss in ethane dehydrogenation activity
is only 0.1%, the loss in activity under identical reaction conditions
is 20% in the case of propane dehydrogenation. This difference in
the extent of activity loss cannot be ascribed to the structure of
the carbon, since, as discussed earlier, both ethane and propane
produce carbonaceous deposits that are very similar in structure
and size. A higher loss in activity during propane versus ethane
dehydrogenation was also seen for Pt/Mg(Ga)(Al)O [32], but in that
case, the amount of carbon deposited was more than twofold
greater for ethane dehydrogenation. These observations were
attributed to the greater ease with which precursors to the carbo-

P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
173
l)O 2.9 > PtSn/Mg(Al)O 0.6 > PtGa/Mg(Al)O 3.0 > Pt/Mg(Al)O. How-
ever, the selectivity to propene is >98% for all of these catalysts
except Pt/Mg(Al)O, for which the selectivity is 88%. The loss of
activity with time on stream increases in the order Pt/Mg(In)(Al)O
0.48 < Pt/Mg(Ga)(Al)O 2.9 < PtSn/Mg(Al)O 0.6 < Pt/Mg(Al)O < PtGa/
Mg(Al)O 3.0, and the accumulation of carbon decreases in the order
Pt/Mg(Al)O
Pt/Mg(Ga)(Al)O
2.9 < PtGa/Mg(Al)O
3.0 ꢁ Pt/Mg(In)(Al)O
0.48 < PtSn/Mg(Al)O 0.6 < Pt/Mg(Al)O. Thus, Pt/Mg(In)(Al)O 0.48
gives the best performance for propane dehydrogenation based
on activity, propene selectivity, and particularly catalyst stability,
but not carbon accumulation.
Pt/Mg(In)(Al)O
It is evident from the preceding discussion that the performance
of Pt/Mg(In)(Al)O 0.48 and Pt/Mg(Ga)(Al)O 2.9 is very similar in
many respects and significantly superior to that of Pt/Mg(Al)O.
As shown here, and previously for Pt/Mg(Ga)(Al)O 2.9 [17], H2
reduction at 873 K results in the formation of PtIn and PtGa alloys,
respectively. The formation of such alloys increases the selectivity
of Pt for the alkane dehydrogenation and minimizes the formation
of lower molecular products, such as methane and ethane, and also
reduces the level of carbon deposition. The effects of alloy forma-
tion have also been reported for PtSn/Mg(Al)O and Sn-promoted
Pt supported on other metal oxides [6,11,20,21,54].
1000 1200 1400 1600 1800 2000
Wavenumber (cm-1)
Fig. 17. Raman spectra of Pt/Mg(Al)O and Pt/Mg(In)(Al)O 0.88 after 2 h of time on
stream for ethane dehydrogenation. Reaction conditions: T: 873 K, m = 25 mg, C3H8:
20%, total flow rate: 60 cm3/min, H2/C2H6 = 1.25).
Pt/Mg(Al)O
The results of the present investigation demonstrate the opti-
mal performance is achieved when the bulk In/Pt ratio is 0.48,
for which case, analysis of EXAFS data indicates that the PtIn alloy
formed contains only 12% In. This suggests that it is not necessary
to form a stoichiometric alloy phase such as Pt3In, which contains
25% In, in order to have a strong effect on catalytic properties of Pt.
The electronic effect of In on Pt is evidenced by XAFS. As seen in
Fig. 6, the incorporation of In into Pt nanoparticles results in an up-
ward shift of Pt Fermi energy and, hence, a decrease in the d band
vacancy. A similar shift has been also seen for PtSn alloys [55]. Both
experimental and theoretical studies have shown that PtSn adsorbs
ethene more weakly than pure Pt, and isotopic labeling studies
conducted on PtSn/Mg(Al)O have shown that readsorption of eth-
ene formed during ethane dehydrogenation is the primary cause
of methane formation and the deposition of carbon [53,56]. Thus,
it is reasonable to propose that In addition, similar with Sn addi-
tion, causes a decrease in alkene adsorption on Pt, leading to rapid
desorption of alkenes and therefore high selectivity toward alkene
production. Recent theoretical results for Pt3Sn and Pt3In alloys
support this hypothesis [43].
Pt/Mg(In)(Al)O
1000 1200 1400 1600 1800 2000
-1
Wavenumber (cm
)
Fig. 18. Raman spectra of Pt/Mg(Al)O and Pt/Mg(In)(Al)O 0.88 catalysts after 2 h of
time on stream for propane dehydrogenation.
ethane dehydrogenation, Pt/Mg(In)(Al)O 0.48, Pt/Mg(Ga)(Al)O 2.9,
and PtSn/Mg(Al)O 0.6 exhibit comparable activities and selectivi-
ties. The rate of ethane dehydrogenation for these three catalysts
is only slightly higher than that for Pt/Mg(Al)O, but the ethene
selectivity for these three catalysts is ꢀ37% higher than that for
Pt/Mg(Al)O. The amount of carbon accumulated after 2 h of time
on stream for Pt/Mg(In)(Al)O 0.48 and Pt/Mg(Ga)(Al)O 2.9 are com-
parable and significantly less than that for PtSn/Mg(Al)O and Pt/
Mg(Al)O. The most notable difference among the catalysts is their
stability with time on stream. The loss in activity during 2 h of time
on stream for Pt/Mg(In)(Al)O 0.48 is an order of magnitude lower
than that of Pt/Mg(Ga)(Al)O 2.9, PtSn/Mg(Al)O, and Pt/Mg(Al)O.
Thus, Pt/Mg(In)(Al)O 0.48 gives the best performance for ethane
dehydrogenation based on activity, ethene selectivity, accumula-
tion of carbon, and particularly catalyst stability.
4. Conclusion
Catalysts for the dehydrogenation of light alkanes have been
prepared by dispersing Pt nanoparticles onto a calcined hydrotal-
cite-like support containing In and Al, Mg(In)(Al)O. Upon reduction
at temperatures above 723 K, part of the In3+ cations present near
the support surface and proximate to Pt are reduced and the result-
ing In atoms form a PtIn bimetallic alloy with the supported Pt
nanoparticles. The average size of the Pt and PtIn particles is about
1 nm independent of the bulk In/Pt ratio of the catalyst. With
increasing bulk In/Pt ratios, the fraction of the alloy consisting of
Table 6 shows that for propane dehydrogenation, the catalyst
activity decreases in the order Pt/Mg(In)(Al)O 0.48 > Pt/Mg(Ga)(A-
Table 6
Comparison of catalyst properties.
Catalyst
M/Pt
Activity,
C2
l
mol/s/gcat
Selectivity, %
C2
Deactivation, %
C2
C/Pt, at/at
C2
C3
C3
C3
C3
Pt/Mg(Al)O
0
22
29
28
27
18
18
67
53
47
26
62
98
100
98
88
98
99
99
98
5
0.1
1
2
22
44
20
29
39
55
110
23
27
54
48
73
27
11
38
25
Pt/Mg(In)(Al)O
Pt/Mg(Ga)(Al)O
PtSn/Mg(Al)O
PtGa/Mg(Al)O
0.48
2.9
0.6
3
97

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P. Sun et al. / Journal of Catalysis 282 (2011) 165–174
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In increases linearly and the fraction of Pt atoms at the alloy sur-
face capable of adsorbing H2 decreases. Evidence is also found for
increasing In to Pt charge transfer with increasing bulk In/Pt ratio.
The activities of Pt/Mg(In)(Al) catalysts for ethane and propane
dehydrogenation reactions are strong functions of the bulk In/Pt
ratio. For both reactions, the maximum activity is achieved for a
bulk In/Pt ratio of 0.48, corresponding to a PtIn alloy containing
12% In. The selectivities to ethene and propene are nearly 100%
for Pt/Mg(In)(Al) 0.48. The alloying of In with Pt causes a signifi-
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significantly as the bulk In/Pt ratio increases from 0 to 0.48, and
more slowly for higher In/Pt ratios. Whereas only 0.1% of the initial
activity for ethane dehydrogenation is lost in 2 h on stream, 20% of
the initial activity for propane dehydrogenation is lost over the
same period. Since the size of the PtIn nanoparticles is unaffected
by catalyst use, catalyst deactivation is attributed to the accumula-
tion of carbon. While small domains (ꢀ30 nm) of graphitic carbon
are formed during ethane and propane dehydrogenation, the
location of their accumulation depends on the composition of
the alkane undergoing dehydrogenation. During ethane
dehydrogenation, carbon is accumulated primarily on the support,
whereas during propane dehydrogenation, a part of the carbon also
accumulates on the PtIn nanoparticles.
Acknowledgments
This work was supported by Chevron Energy and Technology
Company. A portion of this research was conducted at the SHaRE
User Facility, which is sponsored by the Division of Scientific User
Facilities, Office of Basic Energy Sciences, US Department of Energy.
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Appendix A. Supplementary material
ˆ
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2011.06.008.
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