Insights into the effects of steam on propane dehydrogenation over a Pt/Al2O3 catalyst

Article abstract of DOI:10.1039/c5cy00230c

Catalytic propane dehydrogenation over an alumina supported Pt catalyst in the presence of steam is carried out and it is found that the catalyst activity is increased and the apparent activation energy is lowered due to the presence of steam. Three possible mechanisms, i.e. co-adsorption, Langmuir-Hinshelwood and Eley-Rideal, of changes in energetics and pathways for propane dehydrogenation due to the presence of steam are explored by DFT calculation. The results show that co-adsorption of C₃ species with surface oxygenated species would elevate dehydrogenation energy barriers due to repulsive interactions between them. Surface -OH is more active than surface -O in activating the C-H bond in propane and propyl species through either the Langmuir-Hinshelwood or Eley-Rideal mechanism and plays an important role in propane dehydrogenation with steam. The Langmuir-Hinshelwood mechanism is kinetically favorable, in which the activations of the first H in propane by surface -OH are the rate determining steps, but the activation energies are higher than that on a clean Pt(111) surface. The observed enhanced catalyst's activity is ascribed to the lowered coking rates as well as the changes in surface coverage due to the co-adsorption of water and the surface oxygenated species.

Full text of DOI:10.1039/c5cy00230c

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DOI: 10.1039/C5CY00230C
Insights into the effects of steam on propane dehydrogenation
over Pt/Al2O3 catalyst
Yu-Ling. Shan1, Yi-An Zhu1, Zhi-Jun. Sui1,*, De Chen2 and
Xing-Gui Zhou1
1State Key Laboratory of Chemical Engineering, East China University of
Science and Technology, Shanghai 200237, P. R. China
2Department of Chemical Engineering, Norwegian University of Science and
Technology, N-7491, Trondheim, Norway
*Corresponding author:zhjsui@ecust.edu.cn
Abstract
Catalytic propane dehydrogenation over an alumina supported Pt catalyst in the
presence of steam is carried out and it is found that the catalyst activity is increased
and the apparent activation energy lowered due to the presence of steam. Three
possible mechanisms, i.e. co-adsorption, Langmuir-Hinshelwood and Eley-Rideal, of
changes in energetics and pathways for propane dehydrogenation due to the presence
of steam are explored by DFT calculation. The results show that co-adsorption of C3
species with the surface oxygenated species would elevate dehydrogenation energy
barriers due to repulsion interactions between them. Surface –OH is more active than
surface –O in activating C-H bond in propane and propyl species through either
Langmuir-Hinshelwood or Eley-Rideal mechanism and plays an important role in
propane dehydrogenation with steam. The Langmuir-Hinshelwood mechanism is
kinetically favorable, in which the activations of the first H in propane by surface -OH
are the rate determining steps, but the activation energies are higher than that on clean
Pt(111) surface. The observed enhanced catalyst’s activity is ascribed to the lowered
coking rates as well as the changes in surface coverage due to the co-adsorption of

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DOI: 10.1039/C5CY00230C
water and the surface oxygenated species.
Keywords: propane dehydrogenation; steam; DFT calculations; Pt catalyst; reaction
mechanism;
1 Introduction
Propene is an important building block for a wide range of commodity and
specialty chemicals1-4. Now, traditional ways of producing it can no longer meet the
ever growing need. As the natural gas supplies are increasing due to the rising
exploration of shale gas5, there has been growing interest in recent years in the
on-purpose production of propene from propane dehydrogenation (PDH) technologies.
Tens of PDH factories with total capacity of over 10 million of tons are building all
over the world6.
Supported platinum catalyst is used in several commercialized PDH
technologies7, e.g. Oleflex, STAR, etc. and proved by experimental and theoretical
studies to be the most active catalyst for PDH due to its high density of electronic
states close to the Fermi level8, 9,10. Large research efforts have been devoted to trying
to promote the performances of Pt catalysts by introducing promoters or using
different kinds of supports11-13. But coking and deactivating of this kind of catalyst are
still inevitable problems14.
In industrial practice, co-feed of hydrogen7,15-18 or steam19 with propane is used
to enhance the performance of platinum-based catalysts. The presence of hydrogen
can contribute to higher propene selectivity and lower coking rate15-17. Using DFT

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calculation, Hauser et al.20 and Yang et al.21 have proved that the presence of surface
hydrogen atom can shift the third C-H cleavage step to a higher value, making it less
competitive to the preferred alkene desorption step and thus improve the
dehydrogenation selectivity at the cost of elevating activation energies. However,
introducing hydrogen in the PDH process seem to be unfavorable from engineering
point of view since it would decrease the equilibrium conversion and increase the
compression energy consumption for the following separation process.
DOI: 10.1039/C5CY00230C
Steam has been widely used as energy carrier, coke remover and diluent in
dehydrogenation processes19, 22, 23. Introducing steam in the feed can strengthen PDH
processes by increasing propane equilibrium conversion and lowering the temperature
drop of the adiabatic dehydrogenation reactors19. Dong et al.24 found that under the
same propane partial pressure, steam can contribute to higher propene yield compared
with that in nitrogen atmosphere. However, the results reported by Fattahi et al.25 and
our group26 have shown that the promoting effect of steam on PDH over alumina
supported Pt-Sn catalyst is only valid in a certain range of steam amount.
For the purpose of taking the best advantage of those merits of steam, several
terms should be addressed. For one thing, platinum-based catalysts used in steam
atmosphere need to be designed carefully to avoid the sintering of Pt particles27,28
.
Beyond this, the critical role of steam on dehydrogenation kinetics over Pt surfaces
should be illustrated to help to determine the optimum operating conditions and
provide some insight into the basic principles for catalysts design. Unfortunately,
these questions are still poorly understood and consensus has hardly been reached. In

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the kinetic model for PDH in steam atmosphere developed by Luu et al.29 oDvOeIr: 1a0.1t0i3n9/C5CY00230C
modified Pt/γ -Al2O3, steam was involved in the dominator of their kinetic model
with a second order. But to interpret similar retarding effects of steam, some other
authors25,27,28 suggested that sintering of platinum particles was the reason for the
observed activity decrease. Several authors25, 28, 30 in literature also suggested that
surface hydroxyl generated from adsorption and dissociation of water may help to
remove hydrogen in the rate limiting step of propane dehydrogenation and thus
increase propene formation rate. Unfortunately, this hypothesis has not been verified
by either experimental or theoretical results.
In the present study, the effect of steam on PDH has been re-examined over a
monometallic Pt catalyst with narrow particle size distribution. To obtain additional
insights into the effect of steam on PDH over Pt nanoparticles, a computational study
has also been carried out, focusing on the influence of surface adsorbate, like H2O,
hydroxyl group (–OH) and oxygen atom ( –O), on the C-H bond activation.
2 Experimental detail
Alumina supported platinum catalyst was prepared by the colloid method31. The
preparation procedures were as follows. Firstly, 5 mL water and 5 mL ethylene glycol
were mixed in a flask. Then 2.0 g of γ-Al2O3 (Aldrich) and 0.1 g of H2PtCl6•6H2O
(99.9%, Sinopharm) was added and dispersed with the help of ultra-sonic wave. After
stirring overnight, the pH of the solution was adjusted to 13.0 by using sodium
hydroxide in non-aqueous ethylene glycol (2M). After that, the platinum was reduced
at 130 oC in argon for 4 h, then cooled to room temperature and the pH of the solution

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was adjusted to 3.0. After stirring another 2 h, the solids were obtainDeOdI: 1a0.f1t0e3r9/C5CY00230C
subsequent repeated centrifugation, washing and drying processes.
The metal loading of the catalyst was determined by ICP-AES and the metal
dispersion was characterized by H2-Chem and HRTEM techniques. The PDH reaction
was carried out in a μBenchCAT reactor (Altamira Instrument, USA) equipped with a
quartz reactor with an inner diameter of 6 mm. The reactor was placed in an electrical
o
furnace and the reaction temperature can be maintained within ±0.5 C of the desired
set point. The catalyst loading was 0.05g for all catalyst testing experiments. All the
experiments were conducted at WHSVpropane = 7.1 h-1 and a hydrogen to propane
molar ratio of 4. The water was introduced into the reactor system by using a pump and
vaporized before reaching the reaction zone. Argon was used as balance gas to get a
total flow of 200 ml/min. The mass transfer limitations were excluded at these
conditions. The gas composition (C1-C3 hydrocarbons, H2, CO and CO2) were
analyzed online with a 4-channel Micro-GC( INFICON3000, USA).
3. Computational details
Periodic density functional theory (DFT) calculations have been carried out
using the VASP package32-34. The generalized gradient approximation (GGA) with the
PBE functional was used for all the atoms. The interactions between valence electrons
and ion cores were represented by Blöchl’s all-electron-like projector augmented
wave method (PAW)35. A tight convergence of the plane-wave expansion was
obtained with a cut-off of 400 eV. Brillouin zone sampling was performed by using a
Monkhorst–Pack grid with respect to the symmetry of the system and the electronic

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occupancies were determined according to a Methfessel-Paxton scheme36 DOI: 10.1039/C5CY00230C
with an
energy smearing of 0.2 eV.
The Pt(111) surface was represented by a four-layer slab with a p(3×3) supercell,
as described previously37. A Monkhorst–Pack mesh of 7×7×1 k-points was used for
the 2D Brillouin zone integration. The bottom two layers of the slab were kept fixed
to their crystal lattice positions (Pt-Pt distance = 2.82 Å). Spin-polarization had been
considered for radical-containing systems. The ground-state atomic geometries of
bulk and surfaces were obtained by minimizing the Hellman–Feynman forces with the
conjugate-gradient algorithm until the total force on each ion was below 0.03 eV/Å.
The adsorption energy (Eads) was calculated according to the expression
Eads  Eadsorbate/substrate  Eadsorbate  Esubstrate
(1)
where EX is the DFT-total energy of the X system. A negative Eads indicates an energy
gain process.
Transition states (TS) were searched with the dimer method38-40. The most stable
configurations of the reactant on the surface were determined by the sta ndard DFT
minimization. These configurations were used as the initial state, from which the
dimer method was used to find the lowest curvature mode and to climb up the
potential energy surface from minima to saddle points. The convergence was regarded
to be achieved when the force on each atom was less than 0.03 eV/Å.
4. Results
4.1 Experimental results

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The physical properties of the prepared Pt/γ-Al2O3 catalyst are as follows. The
platinum loading of it is 1.5% and the mean particle size is 4.9 nm, based on H2-Chem
results (metal dispersion of 23.3 %), which is comparable to the statistics result of 5.5
nm obtained from HTEM images, shown in Fig. S1(Supporting Information).
Propane dehydrogenation over this catalyst was performed to investigate the
effect of steam. During the experiments, the conversion of propane was found to be
within 7.0%, thus the turnover frequency (TOF(propane, propylene)) to represent the
catalyst’s activity can be calculated as following :
mole of propane conversion
TOF

propane
mole of surface atoms of catalystreaction time
Fig. 1 Activity and selectivity to products for PDH over Pt(EG) catalyst at different H2O
partial pressure, recorded at 1.5minute later after feed in. Condition:
C3H8/H2/H2O/Ar=3/12/X/(100-X), X is shown in picture , 450 oC.
As is shown in Fig. 1(a), the catalyst’s activity (TOFpropane) increases with the
increase of steam partial pressure and reaches a plateau at higher steam concentrations,
which may indicate that the reaction is controlled by water adsorption equilibrium
under these water partial pressures. At the same time, the opposite trend is observed
for the propene formation rates and the gap between TOFpropane and TOFpropylene

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increases with the increase of steam partial pressure and finally reaches a plateau as
well. When looking into products distribution, it can be seen from Fig. 1(b) that the
loss of the selectivity to propylene mainly goes to the cracking products, CH4 and
C2H4, with a selectivity of around 60.0% at steam partial pressure of between 1.92
and 3.5 kPa. In the same steam partial pressure range, oxidizing products, CO and
CO2, are also detected with the selectivity to them of around 10.0%, indicating the
dissociation of water and the involvement of these oxygenated species in propane
dehydrogenation pathways.
Fig. 2 Apparent activation energies for PDH under different conditions. Condition: C3H8, 1.5
kPa; H2, 6 kPa, steam partial pressure is shown in picture, balance is nitrogen.
The apparent activation energies under different conditions were evaluated and
presented in Fig. 2. It is found that PDH in the presence of steam has lower activation
energies of 56.0 kJ/mol than that without steam introduction of 75.5 kJ/mol. The
activation energies obtained here fit in the range of 30-150 kJ/mol for PDH reported
in Ref41.
From results stated above, it can be seen that introduction of steam in the feed has
apparent and complicated influences on PDH over Pt catalyst, which testifies the

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essential role of water and surface oxygenated species in the PDH. But whether the
participation of these surface oxygenated species would alter the reaction route and
thus decrease the PDH apparent activation energy or not is still under debate. In the
following parts, DFT investigation of the effect of adsorption of steam and its
subsequent dissociated species on PDH reaction mechanism is conducted and the
factors contributing to changes in catalyst’s performance are discussed.
4.2 DFT results
We started with the co-adsorption of C3 species and water. Then, three possible
changes in dehydrogenation mechanisms were considered in the following parts: 1)
Dehydrogenation of C3 species over oxygenated species pre-covered Pt(111) surfaces
(Co-adsorption effects for short hereafter); 2) Dehydrogenation of C3 species over
Pt(111) surfaces with the participation of surface oxygenated species in the transition
states for dehydrogenation and hydrogen removal (Langmuir-Hinshelwood
mechanism); 3) Dehydrogenation of C3 species over the adsorbed surface oxygenated
species (Eley-Rideal mechanism).
4.2.1 Co-adsorption with water
Fig.3 Structures of the adsorbed water and propane in the presence of water

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The adsorptions of water monomer and co-adsorption of water and C3 species
were studied to explore possible existing interactions between them. The adsorption
configurations with lowest-energy for water monomer and water-propane complex
were presented in Fig. 3. A water monomer favorably adsorbs on atop sites of Pt(111)
surface via an O-Pt bond of 2.40 Å and ɑHO (HOH plane orientation over the surface )
of 97.2o, which are in consistent with DFT calculations in literatures42-44 and indicate
that water is nearly parallel to the surface. For the water-propane complex, the nearest
H-H bond between them is 1.94 Å. Changes in water structure are observed that the
O-Pt bond is shortened by 0.02 Å and the angle of ɑHO increases by 3.2o. But no
noticeable changes in configurations of propane compared with that on clean surface
could be observed.
The calculated adsorption energies are listed in table 1. To better understand the
interaction between the hydrocarbon species and water, the co-adsorption energies are
decomposed according to the following function:
where Eads_A/Eads_B is one-point adsorption energy of A or B by fixing their
geometries, Eint_AB is the interaction energy between A and B, Ecoads is the overall
co-adsorption energy of A-B complex, A and B represent hydrocarbon species and
water, respectively.
In the case of water-propane complex, no noticeable changes in adsorption
energy of propane (Eads_A) and water (Eads_B) is observed. The increased Ecoads is
ascribed to the existing of a weak attractive interaction between propane and water

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(0.05 eV in strength). But for n-propyl, i-propyl and propene, co-adsorption with
water result in a decrease of Ecoads (0.06 and 0.08 eV in strength, respectively) due to
repulsion interactions between them. It also should be noticed that, in the case of
propene-water co-adsorption, significant decrease of Eads_B was observed (decreased
by 0.05 eV), indicating changes of water adsorption configuration in addition to the
repulsion interaction.
Table 1. Co-adsorption energies of some intermediates for propane dehydrogenation in
the presence of surface H2O.
Eads_A
Eads_B
Ecoads
(eV)
Species
Eint_AB
Clean
-0.04
-1.98
-1.81
-1.12
coads clean coads
-0.04 -0.12 -0.12
-1.98 -0.12 -0.11
-1.80 -0.12 -0.10
-1.11 -0.12 -0.07
C3H8 & H2O
n-C3H7 & H2O
i-C3H7 & H2O
C3H6 & H2O
-0.21
-2.03
-1.82
-1.14
0.05
-0.06
-0.08
-0.04
4.2.2 Co-adsorption effects
The presence of carbon oxides as by-products indicates the dissociation of water
molecule on platinum surface. Therefore, surface -OH and -O are also considered in
the following parts to investigate the possible influences of them on dehydrogenation
mechanism. In this section, the co-adsorption effects are discussed.
The most favorable adsorption sites for surface -OH and -O are bridge and
hollow sites, respectively, as listed in table S1. To search for the transition states for
dehydrogenating steps, surface -OH or -O was put at different favorable adsorption

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sites adjacent to dehydrogenated species and the ones with the most stable
configurations were selected. The results were listed in Table 2.
Table. 2 Activation energies for dehydrogenation over clean and oxygenated species
pre-adsorbed Pt(111) surface.
Elementary Steps
Pt37
0.69
0.70
0.70
0.68
0.76
0.77
H2O*Pt
0.72
OH*Pt
0.75
0.74
0.76
0.76
0.79
0.83
O*Pt
0.74
0.74
0.75
0.73
0.87
0.86
CH3CH2CH3CH3CH2CH2 H
CH3CH2CH3CH3CHCH3 H
CH3CH2CH2CH3CHCH2 H
CH3CHCH3CH3CHCH2  H
CH3CHCH2CH3CHCH H
CH3CHCH2CH3CCH2  H
ES1
ES2
ES3
ES4
ES5
ES6
0.73
0.72
0.69
0.81
0.82
As can be seen from Table 2, co-adsorption of water and oxygenated species
with C3 species could elevate all the activation energies of different elementary steps
to some extent. For example, with the co-adsorption of water, the activation energies
of propane dehydrogenation to propene (ES1-ES4 in Table2) are elevated slightly by
0.01~0.03 eV; in the cases of co-adsorption with surface -OH and -O, the activation
energies of ES1-ES4 steps are lifted by 0.05~0.06 eV. The elevation of the activation
energies for dehydrogenation steps indicates that the catalyst activity may decrease for
the co-adsorption mechanism. Among all the three co-adsorption species,
co-adsorption of water has the least influence on PDH energy barriers, but the
discrepancies from the effect of the other two species is small. Considering the fact
that the structures of transition states remain almost unchanged compared with that on
clean surface (shown in Fig. S2), the repulsion between surface oxygenated species

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and C3 species would account for such increase in activation energy.
The activation energies of propene deep dehydrogenating steps (ES5 and ES6)
are elevated by 0.03-0.11 eV due to the presence of co-adsorption species indicating
the co-adsorption mechanism may decrease the catalyst activities. But, it should be
noticed that this may not mean the selectivity to propene cracking reactions is reduced.
Actually, for the co-adsorption of surface –OH, the difference between the ES5 and
ES1-ES4 is reduced, indicating the relative reaction rate of propene
dehydrogenation/propane dehydrogenation may increase and the selectivity to
propene will decrease due to the existence of this species.
4.2.3 Langmuir-Hinshelwood mechanism
The Langmuir-Hinshelwood mechanism is discussed in this section. For the
convenience of comparison, the related results of propane dehydrogenation on bare
Pt(111) surfaces37 are recalled.
Fig. 4 Transition states structures of propane dehydrogenation at a) clean Pt(111)
surface, b) OH*Pt(111) surface, c) O*Pt(111) surface.

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The structure configurations for propane dehydrogenation on bareDOPI:t(101.11013)9/C5CY00230C
surface are presented in Fig. 4(a). For the first C-H bond activation, through primary
H (TS1) or secondary H atom (TS2), the readily removing H atom is adsorbed on
bridge sites of Pt surface, with C-H bond length of 1.50 Å and 1.57 Å, respectively.
When an adsorbed hydroxyl is presented at atop of the adjacent Pt atom, it can be
involved in the TS (TS5 and TS6 in Fig. 4(b)) by interacting with the readily
removing H atom. These structures give shorter C-H bond length of 1.34 Å,
meanwhile, a water molecule is nearly formed with a H-O bond length of 1.50 Å.
Similar TS structures can be obtained in the case of surface oxygen atom (TS9 and
TS10 in Fig. 4(c)), in which oxygen atom is adsorbed at bridge site on Pt surface and
a surface hydroxyl is to be formed.

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Fig. 5 Reaction paths of propane dehydrogenation to propene: (1) at the clean platinum
surface Pt(111), (2) at the hydroxylated surface OH*Pt(111), (3) at the oxygenated surface
O*Pt(111). Energies are in eV.
The activation energies for these dehydrogenation steps are presented in Fig. 5.
For activation of the first H atom, the results show that the involvements of hydroxyl
in TS structures shift the energy barriers to higher values of 0.74 eV (TS5) and 0.77
eV (TS6) to produce n, i-propyl species, respectively, compared with those of 0.69 eV
(TS1) and 0.70 eV (TS2) on bare Pt(111) surface. Moreover, the surface -O involved
steps have even higher activation energies of 1.32 eV (TS9) and 1.37 eV (TS10),
suggesting the inactive nature of these surface oxygen atoms. The distance of H-Pt

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bond in these cases are elongated by around 0.3 Å, which indicates the decrease of
adsorption energy due to the repulsion interaction between the surface oxygenated
species and adsorbed C3 species and could partly account for the higher activation
energies of these steps.
Interesting phenomenon was found in the case of surface -OH assisted β-H
elimination steps. At the TS8, through which i-propyl dehydrogenates to propene, the
activated C-H bond length (1.53 Å) is similar to that on bare surface (1.54 Å) and the
H-O bond (1.86 Å) is longer than that in TS5-TS7 (1.40-1.50 Å). This configuration
gives an activation energy of 0.63 eV, lower than that on bare surface by a magnitude
of 0.05 eV. But, for surface -OH assisted n-propyl dehydrogenating to propene, the
activation energy is elevated to 0.88 eV. Surface -O is also inactive in the β-H
elimination steps. As can be seen from Fig. 6, the activation energies of these steps
are elevated to as high as 1.25~1.37 eV(TS 11 and TS12), which are much higher than
that of 0.68-0.70 eV(TS3 and TS4) on bare Pt(111) surfaces.
Table. 3 Decomposition of the activation energy for propane dehydrogenation (step 2) on
Pt(111) and OH*Pt(111)
Eads,TS
Eint,lat
Eads,TS
Eint,lat
Reaction Path
Eads,TS
Eads,TS (H)
(propene)
-0.79
-0.20
-0.82
-0.90
(H,propene)
0.48
(OH)
-
(OH,propene+H)
Through TS3
Through TS7
Through TS4
Through TS8
-2.93
-5.09
-2.96
-5.38
-2.62
-1.57
-2.2
-
-0.73
0.48
-2.14
-
-0.45
-
-2.61
0.54
-2.23
-0.18

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To provide a rational interpretation for the obtained results, the activation
energies for propene producing steps are decomposed as follows:
(3)
Eact  Eads,TS E
E
bond(C3H6 H)
ads,C3H7
where Eads,TS is the adsorption energy of the activated complex with respect to the
gaseous propene , OH and H species , Ebond(C3H6−H) is the C−H bond energy at the
methyl group of gaseous n-propyl or i-propyl and Eads,C3H7 is the adsorption energy of
propyl species. As the bond energy and propyl adsorption energy keep constant, the
variation in Eact depends solely on the changes in Eads, TS which can be further
decomposed into the following five terms:
(4)
Eads,TS  Eads,TS(propene) Eads,TS(H) Eads,TS(OH) Eint,lat(H,propene) Eint,lat(H+propene,OH)
where Eads,TS(propene), Eads,TS(H), Eads,TS(OH) are the adsorption energies of propene, H and
OH respectively with their geometries in the activated complex, Eint,lat(H,propene) and
Eint,lat(H+propene, OH) represents the interaction energy of H-propene and OH-(H+propene)
in the activated complexes respectively. From Eq 4, one can see that a more negative
Eads,TS(propene) (Eads,TS(H), Eads,TS(OH)) and a lower Eint,lat(H,propene) (Eint,lat(H+propene, OH)) lead
to a more negative Eads,TS, which contribute to a lower energy barrier.
The transition states of TS3 and TS4 on bare Pt(111) surface and their
counterparts of surface -OH assisted ones TS7 and TS8 are decomposed to analyze
the factors contributing to the elevated and lowered activation energies. The results
are shown in Table 3. In TS7, it is found that the adsorptions of propene and hydrogen
are weakened, hydroxyl can attract hydrogen with an attractive energy of -0.45 eV
and the interaction between hydrogen and propene is lowered. But the interactions are

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insufficient to compensate the weakened adsorption of hydrocarbons and shift the
activation energy to a higher value. In TS8, the attractive energy from surface -OH is
lower than that in TS7, but the adsorption energies for propene and hydrogen are
higher than those on bare Pt(111) surface and lower the activation energy. This means
that the adsorption energies of propene and hydrogen atom dominates in the activation
energies.
When the total energy of the initial propane and -OH (or –O) co-adsorption state
is set to zero, it can be seen that the surface -OH assisted dehydrogenation route is
thermodynamically favorable with heat releases of 0.59 eV and 0.77 eV for the two
consecutive C-H bond activation steps. While the situation is different for surface
oxygen atom involved reactions, in which the first step is a slightly endothermic
reaction and only the second step is an exothermic step with a releasing heat of 0.16
eV. Therefore, the surface oxygen atom is both kinetically and thermodynamically
unfavorable.
4.2.4 Eley-Rideal mechanism
It’s well known that catalytic oxidative dehydrogenation of propane over some
metal oxide catalysts follows Mars-van Krevelen mechanism45-49 and Eley-Rideal
mechanism50-53, for the later pathways, the surface oxygen species are involved in
activating and abstracting H from propane. To gain a better understanding of the effect
of oxygenated species on Pt(111) surface, this possible reaction pathway is also
considered here.

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Fig. 6 Transition states structures of propane oxidative dehydrogenation at a) OH*Pt(111)
surface, b) O*Pt(111) surface.
Fig. 7 Reaction pathways of oxidative dehydrogenation on: a) OH*Pt(111) surface, b)
O*Pt(111) surface.
Fig. 6 shows the transition states of H atom abstraction by surface oxygenated

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species and the corresponding energy profiles are shown in Fig. 7. In theDOcIa: s10e.10o39f/C5CY00230C
oxidation of propane by surface hydroxyl through H abstraction, the transition state
for the activation of the C-H bond in methyl group is successfully located and
presented in Fig.6 of TS13, in which the C-H bond is elongated from 1.10 Å to 1.54 Å
and a water molecule is readily formed with a H-O distance of 1.11 Å, which is quite
close to the H-O bond length of 0.99 Å in a gaseous H2O molecule. The reaction
barrier is calculated to be 1.08 eV. The activation of propane through the breaking of
C-H bond in methylene was also calculated and found to be more favorable with an
energy barrier of 0.88 eV. Shorter C-H bond length of 1.42 Å can be observed in the
geometry. From thermodynamic points of view, the C-H bond energy in methyl group
is stronger than that in the methylene group (420 kJ/mol and 401 kJ/mol54). Therefore,
the production of i-propayl radical is both thermodynamically and kinetically favored,
which is in consistence with the observations in the literature46, 54, 55
.
For oxidation of propane by atomic oxygen species, the energy barrier of
producing i-propyl radical is 0.22 eV lower than that of producing n-propyl radical
and kinetically favorable. However, compared with dehydrogenation by surface
hydroxyl, the activation of propane by atomic oxygen species is much harder since
producing i-propyl radical needs to conquer an energy barrier of as high as 1.64 eV.
The atop site for oxygen atom adsorption in transition states is one of the reasons
accounting for such high activation energy. The most favored adsorption
configuration for oxygen atom is hollow sites with adsorption energy of 4.41 eV and
adsorption on atop sites has adsorption energy of 3.13 eV. But the transition states

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cannot be successfully located when putting oxygen at either hollow or bridgeDsOiIt:e10s..1039/C5CY00230C
After the first H abstraction from propane, propyl radicals are formed in the gas
phase (as shown in Fig. 8). These radicals are close to the newly formed groups
(surface H2O or -OH) with bond lengths ranging from 1.99 Å to 2.18 Å. At the next
step, these radicals will adsorb on Pt(111) surface, and no transition states could be
located for the adsorption processes.
The β-hydrogen elimination steps can also proceed by the H abstracting reaction
in the presence of surface oxygenated species and this step follows surface reaction
mechanism. The transition states are successfully located and the specific
configurations are shown in Fig. 6 of TS15, TS16, TS19, and TS20. The activated
C-H bonds are shorter than these involved in the first hydrogen activation in propane, as
shown in Fig. 6, and the newly formed H-O bonds are longer accordingly. It’s apparent
that lower energies are needed to activate the second hydrogen, especially in the
surface hydroxyl involved reactions, and the activation energies are even lower than
that on clean Pt(111) surface.

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Fig. 8 Intermediates formed on hydroxylated and oxygenated platinum surfaces.
When dehydrogenation of propane by surface hydroxyl and oxygen atom are
compared, it is found that the first H abstraction reactions are the rate limiting steps in
both cases, which is in consistence with results obtained by DFT studies for oxidative
dehydrogenation of propane over vanadium oxide54-56 but differs from
dehydrogenation of propane over Pt(111) surface37. Surface hydroxyl is more active
than atomic oxygen species from the kinetic point of view since lower energy barrier
is needed to break C-H bond. Moreover, the consecutive two H abstracting reactions
by surface hydroxyl are all exothermic. So it is also thermodynamically more
favorable than dehydrogenation by surface O. But, among all the three mechanism
discussed above, the essential role of surface –OH is achieved most likely through
Langmuir-Hinshelwood mechanism, since the activation energies for this mechanism
have the relative low values.
5. Discussion
The increase of catalytic activity and decrease of apparent activation energies are

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observed in experiments when introducing steam in the PDH reaction. But the DFT
results stated above shows that co-adsorbing with oxygenated species can shift the
activation energies to higher values due to the repulsion among the co-adsorption
species. For the Langmuir-Hinshelwood mechanism, the activations of the first C-H
bonds in propane are not facilitated because of stereospecific blockade. In some
specific steps, e.g. i-propyl dehydrogenating to propylene, surface –OH can help to
activate C-H bond of methyl group due to the electronic interactions. For Eley-Rideal
mechanism, although the second step of C-H bond activation by surface –OH is
slightly kinetically favored, the first step of C-H bond abstraction gives high energy
barriers of 0.88 eV and 1.08 eV for i-propyl and n-propyl radical producing,
respectively. Therefore, the DFT results do not seem to support the conclusion of
alternation in reaction pathways for PDH in the presence of steam.
It should be noticed that adsorption of water on the Pt surface would change the
surface coverage of different species and therefore affect the reaction rate. For PDH
over Pt catalyst, Chen et al57. found the reaction order to hydrogen is -0.5, which
indicates that lower hydrogen partial pressure and surface coverage will lead to the
increase of the reaction rate. This phenomenon is also supported by the previous DFT
results21. But, the water adsorption on Pt(111) is weak and may have a low surface
coverage, which would limit the effect of steam in a certain range. As is shown in
Fig.9, hydrogen order to PDH reaction remains nearly unchanged in steam
atmosphere since the hydrogen order to PDH reaction can be expressed as follows31:
nH  H 0.5
2

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Fig. 9 Reaction order to hydrogen for PDH under different conditions. Condition:
C3H8, 1.5 kPa; H2, 6 kPa.
Table 4. Coking rates under different steam partial pressures
H2O Partial Pressure (KPa)
Reaction Time (h)
Coking Rate (gcoke/(gPt•s))
8.23e-5
0.00
1.31
1.95
2.63
3.28
1
1
1
1
1
7.29e-5
7.14e-5
6.86e-5
6.77e-5
Another advantages of steam for PDH is that steam could contribute to lower
coking rate25,58,59, which is justified by our TG analysis of the spent catalysts, as
shown in Table 4. The decrease of coking rate in the presence of steam contributes to
the increase of catalytic activity, since more Pt(111) surface may be free for the
reaction21. Furthermore, It should be noted that when steam is introduced, the
selectivity loss of propene mainly goes to cracking products, in accordance with other
authors28,60. Yang et al37,61 and y nen et al62 have shown that lower coordinated Pt
sites, like step sites, are highly active but are not selective for PDH reaction. On these

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sites, cracking products and coking reaction prevail. Therefore, mono-metallic Pt
catalysts suffer from rapid deactivation15, 63-66. When steam is introduced, the removal
of cokes will help recover and maintain these highly active Pt sites and increase the
catalyst’s activity and decrease the selectivity to propene at the same time. Since
propane dehydrogenation on these sites have lower activation energies37, 62, it can also
account for the experimentally observed decrease in activation energies67.
Finally, it should be noted that the change of propane dehydrogenation activities
is also related to the catalysts structures. It was found by Luu et al29 that steam was
involved in the dominator of their kinetic model with a second order over tin
promoted Pt/γ-Al2O3, indicating that H2O was adsorbed on active sites. But for their
Pt/γ-Al2O3 catalyst without tin addition, the dehydrogenating rates were hardly
dependent on the partial pressure of steam, revealing that the effects of steam might
depend on different support24,67 and catalysts’ composition24,29,67
.
6. Conclusion
A combined experimental and theoretical study of the effect of steam on
propane dehydrogenation was performed. The experimental results show that addition
of steam in the PDH reaction will increase the catalyst’s activities, lower
dehydrogenating activation energies and coking rate. Compared with dehydrogenation
over clean Pt(111) surface, the DFT results show that the energy barriers of different
dehydrogenation steps are elevated due to the adsorption of water and its dissociated
species. The only exception is for i-propyl dehydrogenating to propene step with the
involvement of surface -OH through both Langmuir-Hinshelwood and Eley-Rideal

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mechanism. For PDH under steam atmosphere, the role of surface –OH is essential,
most likely through Langmuir-Hinshelwood mechanism, in which the first C-H bond
activation is the rate determining step in the overall PDH reaction route. The
promoting effect of steam can be ascribed to the removal of cokes deposited on active
Pt surfaces and the changes of adsorption species coverage on Pt surface.
Acknowledgement
This work is supported by the National Basic Research Program of China (973
project, 2012CB720500), the Fundamental Research Funds for the Central
Universities of China (222201313006, WA1414043), National Science Foundation of
China (91434117, 21376076) and the 111 project(B08021).

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