Catalytic behavior and surface species investigation over γ-Al 2O3 in dimethyl ether hydrolysis

Article abstract of DOI:10.1016/j.apcata.2013.04.011

The catalytic behavior and surface species over γ-Al ₂O₃ in hydrolysis of dimethyl ether (DME) was examined by experimental and theoretical studies. It was experimentally observed that γ-Al₂O₃ substantially catalyzed DME hydrolysis producing methanol at 250-350 C with high stability at this temperature range with respect to carbon formation. Other carbon-containing species yielded from side reactions were present in trace amounts at temperatures below 375 C. The Density Functional Theory calculations results suggested that DME hydrolysis is thermodynamically favorable on the hydroxylated γ-Al₂O ₃(1 1 0) but it is not favorable on (1 0 0) surfaces at this reaction temperature range. The DME hydrolysis on γ-Al₂O₃ is more likely to occur at a hydroxyl surface group which has relatively high acidity.

Full text of DOI:10.1016/j.apcata.2013.04.011

Applied Catalysis A: General 460–461 (2013) 99–105
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic behavior and surface species investigation over ␥-Al2O3 in
dimethyl ether hydrolysis
Pussana Hirunsit, Kajornsak Faungnawakij∗, Supawadee Namuangruk,
Chuleeporn Luadthong
National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Klong Laung, Pathumthani 12120, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic behavior and surface species over ␥-Al2O3 in hydrolysis of dimethyl ether (DME) was exam-
ined by experimental and theoretical studies. It was experimentally observed that ␥-Al2O3 substantially
catalyzed DME hydrolysis producing methanol at 250–350 ◦C with high stability at this temperature
range with respect to carbon formation. Other carbon-containing species yielded from side reactions
were present in trace amounts at temperatures below 375 ◦C. The Density Functional Theory calcu-
lations results suggested that DME hydrolysis is thermodynamically favorable on the hydroxylated
␥-Al2O3(1 1 0) but it is not favorable on (1 0 0) surfaces at this reaction temperature range. The DME
hydrolysis on ␥-Al2O3 is more likely to occur at a hydroxyl surface group which has relatively high
acidity.
Received 18 January 2013
Received in revised form 13 March 2013
Accepted 6 April 2013
Available online 19 April 2013
Keywords:
␥-Al2O3
Hydroxylated ␥-Al2O3
Hydrolysis
Dimethyl ether
Methanol
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
It was reported that DME SR (Eq. (1)) comprises two moderately
endothermic reactions in sequence: DME hydrolysis to methanol
Dimethyl ether (DME) has been considered a promising chem-
ical platform for utilization in various applications, including
automotive fuels, generation of electric power, and residential
applications such as heating and cooking [1,2]. In particular, DME
is a suitable compound as a source of hydrogen for fuel cell appli-
cations since it has high energy density, high hydrogen to carbon
ratio, and is easily stored and transported. Like LPG, it is a gas at nor-
mal temperature and pressure, and can readily be transformed into
a liquid when it is subjected to modest pressure. Given these simi-
larities, the infrastructure used for LPG distribution can be readily
adapted to DME. DME is preferable to methanol for those applica-
tions because it is non-toxic and non-corrosive. In addition, it can
be catalytically reformed at a lower temperature than ethanol [3,4]
and methane [5,6].
In recent years, the catalytic reactions of DME including com-
bustion [7,8], hydrolysis [9,10], and hydrocarbon synthesis [11–14]
have been widely investigated. Steam reforming (SR), partial oxi-
dation (PO), and autothermal reforming (ATR) of DME are three
major routes to produce hydrogen. Among them, DME SR provides
the greatest quality of reformates to be fed to fuel cells [15–18].
(CH3OH) (Eq. (2)) and methanol steam reforming to hydrogen and
carbon dioxide (Eq. (3)).
DME SR : (CH3)2O + 3H2O(g)→6H2 + 2CO2 ꢀH0r = 122 kJ · mol−1
(1)
DME hydrolysis : (CH3)2O + H2O(g) → 2CH3OH(g)
ꢀH0r = 24 kJ · mol−1
(2)
MeOH SR : CH3OH(g) + H2O(g) → 3H2 + CO2 ꢀH0r = 49 kJ · mol−1
(3)
Generally, DME hydrolysis is catalyzed by acid catalysts, such as
zeolites and alumina [19], while methanol steam reforming is cat-
alyzed by Cu–and Pd–based catalysts [20,21]. It was reported that
DME hydrolysis (Eq. (2)) is a rate-determining reaction in DME SR
and would determine the overall reaction of the reforming reac-
tions (Eq. (1)) [22]. Recently, we comparatively studied the catalytic
performances and the kinetic parameters of solid acids (␥-Al2O3,
H-ZSM-5, H-mordenite, and TiO2) in DME hydrolysis. Although
zeolites, such as, ZSM-5 and mordenite, and WO3 provided better
∗
Corresponding author. Tel.: +66 2 564 7100x6638; fax: +66 2 564 6981.
E-mail addresses: kajornsak@nanotec.or.th, jiw037@yahoo.com
(K. Faungnawakij).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.04.011

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P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
Fig. 1. (a) Dry ␥-Al2O3(1 0 0), and (b) ␥-Al2O3(1 1 0).
activity than alumina, they were rapidly deactivated due to cok-
ing problems [23–25]. Given these limitations, alumina is the most
suitable and commonly used catalyst for DME hydrolysis. Al2O3
typically showed an excellent DME hydrolysis activity at temper-
atures above 300 ◦C due to its moderate acidity. Nonetheless, it
was not clear how the acid strength on alumina affects the activity
of DME hydrolysis and how the acid catalyst hydrolyzes DME to
methanol.
In the present work, we have examined DME hydrolysis over
␥-Al2O3 using a combination of experimental studies and density
functional theory (DFT) studies. We attempted to gain insight into
the reaction phenomenon on ␥-Al2O3, as this would be beneficial
for understanding the nature of active sites on ␥-Al2O3 used in
the reactions, and would provide guidance for the future design
of catalysts and the optimization of reforming conditions.
2. Experimental
2.1. Catalyst preparation and characterization
␥-Al2O3 (JRC-ALO8) was provided by the Catalysis Society of
Japan. The catalyst samples were treated in air at 700 ◦C for 30 min
prior to their use in hydrolysis reaction tests. A nitrogen sorp-
tion system (BEL Japan Bellsorp–max) was employed to determine
adsorption–desorption isotherms. The Brunauer–Emmett–Teller
(BET) approach was employed to determine the surface area of
the samples. The elemental composition (Si, Al, Na) of the sam-
ples was measured by wavelength dispersive X-ray fluorescence
(WDXRF) analysis using a Philips PW-2404. The crystalline phase
of the samples was measured by powder X–ray diffraction (XRD)
technique using a Bruker D8 Advance coupled with a Cu K␣ radi-
ation source. Temperature-programmed desorption of ammonia
(NH3–TPD) was carried out for measurement of the acidity of
1000
adsorption
desorption
800
80
70
60
50
40
30
600
20
10
400
0
0
20 40 60 80 100
200
0
Pore diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
100
200
300
400
500
600
700
800
Temperature (oC)
Relative pressure (P/Po)
Fig. 2. NH3-TPD curve over ␥-Al2O3.
Fig. 3. N2 sorption curves and pore size distribution of ␥-Al2O3.

P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
101
60
(a)
12
10
8
Equilibrium
Experimental
No catalyst
50
40
30
20
10
0
DME
6
4
Methanol
Other C-containing
species
2
0
0.0
0.5
1.0
1.5
2.0
2.5
200
250
300
350
400
Temperature (oC)
S/C (mole ratio)
Fig. 5. The effect on S/C mole ratios on DME hydrolysis. Reaction tempera-
ture = 300 ◦C; GHSV = 4000 h−1
1.0
0.8
0.6
0.4
0.2
0.0
.
(b)
H2
CO
CH4
CO2
thermal conductivity detector (VARIAN, CP-4900). A PoraPLOT Q
column was used for separation of DME, CH3OH, and CO2 and a
molecular sieve 5A column for separation of H2, O2, N2, CH4, and
CO. DME conversion is defined as follows:
ꢀ
ꢁ
FDMEin − FDMEout
DME conversion (%) = 100
FDMEin
where FDMEin and FDMEout stand for the molar flow rate of DME into
and out from the conventional flow reactor, respectively.
2.3. Computational details
The simulation methodology employed fully periodic plane-
wave Density Functional Theory (DFT) calculations as implemented
in the Vienna ab initio Simulation Program [26,27]. The calculations
utilized the Perdew-Wang GGA functional [28,29] implemented
with Projector Augmented Wavefuction (PAW) [30,31] method
for representing the non-valence core electrons. For all calcula-
tions reported herein, we have used 400 eV cutoff energy and the
Methfessel-Paxton smearing [32] of order 1 with a value of smear-
ing parameter ␴ of 0.2 eV.
200
250
300
350
400
Temperature (oC)
Fig. 4. Temperature dependence of reactants and products during DME hydrolysis
catalyzed by ␥-Al2O3: (a) effluent flow of DME, methanol, and C-containing gases,
(b) effluent flow of H2, CO, CH4 and CO2. The dotted lines (. . .) in (a) represent the
equilibrium data. Reaction conditions: S/C = 1.5; GHSV = 2200 h−1
.
catalysts. A sample was heated up to 500 ◦C and kept for 1 h in He
flow for degassing. The catalyst sample (200 mg) was cooled down
to 100 ◦C, followed by adsorption of NH3 in pure NH3 flow until sat-
uration. Consequently, NH3–TPD was initiated in a heating process
at a rate of 10 ◦C min−1 up to 800 ◦C. The NH3 desorbed from the
catalyst surface was monitored by an on-line mass spectrometer.
Temperature-programmed oxidation (TPO) was employed to ana-
lyze the carbon deposition on catalyst surfaces. The catalyst sample
(100 mg) was oxidized in 5% O2/He flow in a heating process at a
rate of 10 ◦C min−1. The product gases were monitored by on-line
mass spectrometer.
The ␥-Al2O3 bulk model structure was taken from Ref. [33,34]
shown in Fig. S1 in the supplementary material. The experimen-
tal results found the alumina to be in gamma crystalline phase
or partially in an amorphous form. Attempts to characterize the
structure of the alumina in the amorphous phase were however
unclear. Thus, the (1 0 0) and (1 1 0) crystalline surfaces were chosen
because the (1 0 0) and (1 1 0) dominates in ␥-alumina nanocrys-
tallites [33]. Nevertheless, the hydroxyl group which presents on
(1 1 0) is similar to the predominant surface hydroxyl group type
found on an amorphous alumina surface obtained by the atom-
istic molecular dynamics simulation by Curtiss and co-workers
[35]. The slab models of ␥-Al2O3(1 0 0) and (1 1 0) surfaces con-
˚
2.2. Evaluation of catalytic performance
tain ten Al2O3 molecular units and ∼12 A of the vacuum region
excluding adsorbates. The side and top view of dry ␥-Al2O3(1 0 0)
and (1 1 0) surfaces are shown in Fig. 1a and b, respectively. For ␥-
Al2O3(1 0 0), the slab is composed of 9 atomic layers, where the top
5-layers are relaxed while the bottom fixed 4-layers were set to ␥-
Al2O3 bulk bond distances according to their optimized structural
model as determined from bulk calculations. For ␥-Al2O3(1 1 0),
the relaxed atoms are located at ∼45% of the top slab excluding
the vacuum region. The p (1 × 1) unit cell is applied for hydrox-
alted surface study and the bigger p (2 × 1) unit cell is appled when
DME and DME dissociation products adsorbed on hydroxylated
The evaluation of catalytic activity was carried out using a
conventional flow reactor under atmospheric pressure. Mass flow
controllers were used to adjust the gas feed rate and a low pressure
gradient pump was used to control the water feed rate. A mixture
of DME, steam, and nitrogen at a steam-to-carbon ratio (S/C) and a
gas hourly space velocity (GHSV) was supplied to the catalyst bed
at reaction temperature.
The analysis of influent and effluent gaseous compositions was
carried out using online gas chromatographs equipped with a

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P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
Fig. 6. Most stable geometry of (a) hydroxylated ␥-Al2O3(1 0 0) with OH coverage of 8.8 OH·nm−2 and (b) hydroxylated ␥-Al2O3(1 1 0) with OH coverage of 11.8 OH·nm−2
.
surfaces to prevent adsorbate-adsorbate interactions between unit
cells. A Monkhorst-Pack [36] mesh k-point sampling in the surface
Brillouin zone of 3 × 3 × 1 was used. The results were checked for
convergence with respect to energy cutoff and number of k-points.
The convergence criterion for electronic self-consistent iteration
was set to 10−7 eV and the ionic relaxation loop was limited for
methanol deviated from the equilibrium values. This indicated the
higher degree of side reactions at the higher temperatures. In this
high temperature range of 375–400 ◦C, other carbon-containing
species were produced, for example, CH4, CO2, and CO. Fig. 4b
shows the volumetric flow rate of H2, CH4, CO, and CO2 during
DME hydrolysis over ␥-Al2O3. The production of all gaseous species
was very minor (below 1 cm3 min−1). Hydrogen was the major
component, while CO and CH4 were also largely detected. It was
considered that formation of H2, CH4, CO and CO2 would be ascrib-
able to the following reaction:
˚
all forces smaller than 0.04 eV/A for free atoms. We mainly focus
on thermodynamic point of view in the caluculations due to high
reaction temperature and high steam to carbon feeding ratio. Thus,
the kinetics aspect is not expected to be limited in this case.
DME decomposition : (CH3)2O → CH4 + CO + H2
ꢀH0r = −1 kJ · mol−1
(4)
(5)
3. Results and discussion
3.1. Experimental work
Direct CO2 formation : (CH3)2O + H2O(g) → CH4 + CO2 + 2H2
ꢀH0r = −42 kJ · mol−1
The alumina stays in gamma crystalline phase (XRD analysis
in Fig. S2). It has a specific surface area of 137 m2 g−1 with total
pore volume of 1.2 cm3 g−1. Based on NH3-TPD analysis in Fig. 2,
there are two major NH3 desorption curves; one in a tempera-
ture range of 150–500 ◦C and another in a temperature range of
500–800 ◦C. The curves at high and low temperature ranges were
considered as strong and weak/medium acid sites, respectively. The
strong and weak/medium acid amounts were determined as 10
and 40 ␮mol g−1, respectively. The N2 sorption analysis revealed
the mesostructure of this material, while its pore size distribu-
tion was ranging from ca. 10–90 nm in diameter, and peaked at
40 nm (Fig. 3). It can be seen that the pore size of the alumina partly
covered the macroporous region.
Fig. 4a and b shows the volumetric flow rate of DME, methanol,
H2 and other gaseous carbon species during DME hydrolysis over
␥-Al2O3. ␥-Al2O3 catalyzed DME hydrolysis from temperature of
ca. 250 ◦C. In Fig. 4a, the DME flow rate decreased as the tempera-
ture increased, and it approached the DME hydrolysis equilibrium
at the temperature range of 325–350 ◦C. The formation of methanol
corresponded well with the decrease of DME via DME hydrolysis.
However, at temperatures above 350 ◦C the amounts of DME and
MeOH decomposition : CH3OH(g) → 2H2+CO ꢀH0r =91 kJ · mol−1
(6)
The major side-reactions over ␥-Al2O3 were remarkedly differ-
ent from those of ZSM-5 which produced significant amounts of
CH4, C3H6, i-C4H8, and n-C4H8. It should be noted that methanol
was not formed under the reaction conditions without steam co-
feed (S/C = 0). The effect of S/C in DME hydrolysis over ␥-Al2O3
is depicted in Fig. 5. The observed DME conversion under steam-
free condition was trace. Typically, the higher S/C ratios gave the
improved hydrolysis activity, suggesting the important role of
steam. Although the stoichiometric ratio of S/C was 0.5 for DME
hydrolysis reaction, the hydrolysis conversion was extremely low
under this condition. When S/C increased from 0.5 to 1.5, the con-
version increased monotonously. This result should be describable
to the enhanced surface concentration of OH groups proportional

P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
103
Fig. 7. (a) DME adsorption on hydroxylated ␥-Al2O3(1 0 0) and (b) 2CH3OH adsorption on hydroxylated ␥-Al2O3(1 0 0) upon DME dissociation.
to steam content. The high S/C ratio above 1.5 provided a high DME
conversion, nearly comparable to the equilibrium data. The effect
of steam on the methanol formation became insignificant when S/C
was above 2, due to the thermodynamic equilibrium control.
The stability of ␥-Al2O3 was promising in a reaction temperature
range of 350–375 ◦C, while ZSM-5 rapidly degraded at temper-
atures above 300 ◦C. From TPO analysis, carbon formation over
␥-Al2O3 spent for 100 h was not clearly observed (results not
shown). It should be noted that DME was not converted to other
products in a homogenous reaction. When ␥-Al2O3 (DME hydrol-
ysis catalyst) was mixed with copper-based catalyst (methanol
reforming catalyst), the resultant composite was highly active
in reforming DME to hydrogen. It was proved that the cop-
per and alumina composite catalyst provided excellent activity
and stability for longer than 1000 h in DME SR at 350–375 ◦C
[25].
3.2.1. Hydroxylated ꢁ-Al2O3 (1 0 0) and (1 1 0)
The hydration of the ␥-Al2O3 surface is crucial as it modifies
the number of active surface sites and determines the nature of
catalytic properties. The water and hydroxyl groups adsorbed on
␥-Al2O3, which are the products of water dissociation on the sur-
face, proceeded on the DME adsorption sites of interest. The models
of the hydroxylated ␥-Al2O3 (1 0 0) surface and (1 1 0) surface are
firstly developed. The relationship between the hydroxyl coverage
and temperature has been computed previously by Digne et al. [33]
Applying this relationship, the hydroxylated ␥-Al2O3 (1 0 0) and
(1 1 0) surfaces with the OH coverage corresponding to the experi-
mental temperature range of 250–350 ◦C, in which methanol is the
dominant product (Fig. 4a), were constructed. The hydroxylated ␥-
Al2O3 (1 0 0) has OH coverage of 8.8 OH·nm−2 and 11.8 OH·nm−2
for hydroxylated ␥-Al2O3 (1 1 0).
The adsorption energies per water molecule are calculated using
the following equation;
ꢂ
ꢃ
3.2. DFT calculations
Esurf+adsorbates − Esurf − n ∗ EH
O
2
(g)
Ead
=
(7)
n
In order to better understanding the DME hydrolysis reac-
tion on ␥-Al2O3, we performed DFT calculations to elucidate
the active sites of the hydroxylated ␥-Al2O3 on DME hydroly-
sis to yield CH3OH. We used the hydroxylated ␥-Al2O3 surface
models corresponding to specific reaction temperatures to study
DME and CH3OH adsorption on hydroxylated ␥-Al2O3 surfaces
in order to examine the thermodynamics of DME hydrolysis on
␥-Al2O3.
where Esurf+adsorbates is the calculated total energy of the adsorbed
molecules on the surface, Esurf the calculated total energy of the
bare ␥-Al2O3 surface. EH
gas-phase, and n are the number of adsorbed water molecules. A
negative Ead value refers to exothermic adsorption, with stronger
adsorption as the value becomes more negative. A positive Ead value
refers to endothermic adsorption. In this case, we are focusing on
is the calculated energy of the isolated
O
2
(g)

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P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
Fig. 8. (a) DME adsorption on hydroxylated ␥-Al2O3(1 1 0) and (b) 2CH3OH adsorption on hydroxylated ␥-Al2O3(1 1 0) upon DME dissociation. The arrow indicates the
location of a dissociated H atom which forms CH3OH with CH3O after DME dissociation.
the most stable hydroxylated surface configurations and we neglect
zero-point energies and thermal effect corrections.
(1 1 0). The deviation may arise from details in calculations involv-
ing applied functional and specified cut off energy.
In ␥-Al2O3(1 0 0), the OH coverage of 8.8 OH·nm−2 corresponds
to two water molecules adsorbed on a ␥-Al2O3(1 0 0) supercell. The
hydroxylated structural models are shown in Fig. 6a. The calculated
value of the first water adsorption occurring in a dissociative mode
on ␥-Al2O3(1 0 0) is exothermic with Ead=−77.63 kJ mol−1 (the
structure is shown in supplementary material, Fig. S3). The water
molecule is not fully dissociated, forming a strong hydrogen bond
3.2.2. Dimethyl ether and its dissociation products adsorption
In this section, the molecular adsorption of DME on hydroxy-
lated surfaces and its dissociation products interaction with H2O
and/or hydroxyl groups forming CH3OH are investigated. The DME
adsorption energy is calculated by the following equation;
˚
between the hydroxyl groups with a distance of 1.68 A. The second
Ead/DME = Ehydroxylated surf+DME − Ehydroxylated surf − EDME(g)
(8)
adsorbed water molecule is not dissociative, forming a hydrogen
bond with the hydroxyl group formed by the first dissociated water
molecule featuring adsorption with Ead=−76.63 kJ mol−1 per water
molecule, which is comparable to the first water molecule’s adsorp-
tion energy. The relatively weak water adsorption on (1 0 0) surface
reflects in the low thermal stability and the (1 0 0) surface was
found to be completely dehydrated approximately above 310 ◦C
[33], while the (1 1 0) surface remain hydroxylated approximately
up to 800 ◦C [33].
where Ead/DME is the calculated DME adsorption energy,
Ehydroxylated surf+DME is the calculated total energy of DME adsorbed
on hydroxylated surface, Ehydroxylated surf is the calculated total
energy of the hydroxylated surface structure, and EDME(g) is the
calculated energy of the isolated gas-phase DME. A positive Ead/DME
value refers to endothermic adsorption, and a negative value refers
to exothermic adsorption with stronger adsorption as the value
becomes more negative. We are focusing on the most stable
configurations of DME interacting with the hydroxylated surface
and we neglect zero-point energies and thermal effect corrections
in Ead/DME calculations.
In ␥-Al2O3(1 1 0), the OH coverage of 11.8 OH·nm−2 corresponds
to four water molecules adsorbed on a ␥-Al2O3(1 1 0) supercell. The
hydroxylated structural models are shown in Fig. 6b. The first three
water molecules adsorbed in dissociative modes and the fourth
water molecule does not dissociate. Water adsorption on (1 1 0) is
considerably stronger than on (1 0 0) due to strong intrinsic Lewis
acidity of a highly unsaturated Al 3-fold site while the unsaturated
Al on (1 0 0) is all 4-fold sites. It also provides a greater variety of
OH groups than on the (1 0 0) surface. Ead are decreasing (weaker
adsorption) while the coverage increases from 1 to 4 molecules;
−225.95, −194.97, −176.98 and −148.81 kJ mol−1, respectively.
The configurations of the first three water molecules adsorption
are shown in supplementary material (Fig. S4). Digne et al. [33]
reported −240 kJ mol−1 of the first dissociative water adsorption on
˚
In ␥-Al2O3(1 0 0), DME forming a hydrogen bond (O H = 2.10 A)
with OH adsorbed on the surface is the most stable configu-
ration (shown in Fig. 7a). The adsorption is exothermic with
Ead/DME = −14.77 kJ mol−1. The structure is even less stable by
24.65 kJ mol−1 when DME binds with the H2O on the surface. The
most stable structure of 2CH3OH adsorbed on (1 0 0) upon DME dis-
sociation (Fig. 7b) indicate that a water molecule also dissociates.
The two CH3OH forms a hydrogen bond cyclic between CH3OHs and
the hydroxyl groups on the surface. The reaction free energy which
2CH3OH adsorbed on (1 0 0) (Fig. 7b) is the final states and DME
adsorbed on (1 0 0) (Fig. 7a) is the initial state is calculated. The

P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
105
reaction free energy includes the zero-point energies correction,
be different from that on (1 0 0) surface, which a water dissociation
possibly involves in the pathway on (1 0 0) surface.
ꢄ
enthalpic temperature correction ( CpdT), and entropy contribu-
tion (TS) of the adsorbates at each state calculated from adsorbed
species vibrations by standard methods [37]. They are used to
convert the electronic energies into free energies at 250 ◦C and
350 ◦C which are the temperatures of the DME hydrolysis reac-
tion (Fig. 4a). The values of these correction terms are shown in
the supplementary material (Tables S1 and S2). The reaction free
energy on (1 0 0) is +11.21 kJ mol−1 at 250 ◦C, and +12.75 kJ mol−1 at
350 ◦C indicating that DME hydrolysis on (1 0 0) is thermodynami-
cally unfavorable at this temperature range. Furthermore, the low
thermal stability of the active adsorbed water and hydroxyl sites
on (1 0 0) surface above 310 ◦C may reduce the reactivity on (1 0 0)
as the temperature increases.
Acknowledgements
The authors acknowledged the financial support from the
National Nanotechnology Center (NANOTEC) and the Thailand
Research Fund (TRF), and the computing resource from the National
Electronics and Computer Technology Center (NECTEC).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2013.04.011.
In ␥-Al2O3(1 1 0), DME adsorption is favorable to form hydro-
gen bonds with one of the hydroxyl groups (shown in Fig. 8a) and
the adsorption is exothermic and more favorable than on the (1 0 0)
with Ead/DME = −38.26 kJ mol−1 DME binding with H2O is less favor-
able with Ead/DME = −18.16 kj mol−1 DME binds most stably with the
hydroxyl group connected with two Al atoms that are free from OH.
This OH group is also the predominant group in hydroxylated amor-
phous alumina surfaces [35]. The higher reactivity of this bridging
OH group that is connected with two Al atoms free from OH may
result from higher Lewis acidity than other OH groups which bind to
a single Al atom or bind to Al atoms not free from OH. DME adsorp-
tion on the (1 1 0) surface is more favorable than on (1 0 0) surface.
A water dissociation involves in DME dissociation on (1 0 0) sur-
face may lead to different DME hydrolysis pathways on (1 0 0) and
(1 1 0) surfaces. Upon DME dissociation producing CH3 and CH3O,
CH3 binds with an OH group adsorbed on a single Al atom to form
CH3OH and CH3O combines with a dissociated H atom from one of
the bridging OH group (shown in Fig. 8b with an arrow) to form
CH3OH which binds on Al surface atom. The first CH3OH forms a
hydrogen bond with H2O and the latter CH3OH forms hydrogen
bonds with other OH groups. The calculated reaction free energy
on (1 1 0) surface including ZPE and thermal correction at 250 ◦C is
−57.49 and −38.26 kJ mol−1 at 350 ◦C. This suggests DME adsorp-
tion and DME hydrolysis on a hydroxylated ␥-Al2O3(1 1 0) surface is
thermodynamically favorable and the reaction can be spontaneous
at this temperature range.
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The catalytic hydrolysis of dimethyl ether (DME) over ␥-Al2O3
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