Dimethyl ether steam reforming to produce H2 over Ga-doped ZnO/γ-Al2O3 catalysts

Article abstract of DOI:10.1039/c6ra07940g

Herein, we report the performance of a series of gallium-doped zinc oxide (GDZ) catalysts mechanically mixed with γ-Al₂O₃ (GDZ/γ-Al₂O₃) for the dimethyl ether (DME) steam reforming (SR) to produce H₂. Compared with the ZnO catalyst mechanically mixed with γ-Al₂O₃, the doping of ZnO with gallium can significantly improve the conversion of DME and the yield of hydrogen. The catalyst with a Zn:Ga molar ratio of 9:1 has the highest conversion of DME (95.4%) and yield of hydrogen (95.0%) at 450 °C. The carbon dioxide selectivity of GDZ/γ-Al₂O₃ catalysts is higher than 95%, which is much higher than that of the CuZnAlO/γ-Al₂O₃ catalyst. Moreover, the GDZ/γ-Al₂O₃ catalysts have better stability than the CuZnAlO/γ-Al₂O₃ catalyst. Doping of ZnO with gallium generates a large number of oxygen vacancies on the catalyst, which is beneficial to the DME SR reaction. Our results indicate that HCOO- is an intermedium of DME SR over GDZ/γ-Al₂O₃ catalysts, and the transformation from HCOO- to CO₂ is the rate-controlling step. The results of conductivity indicate that the rate-controlling step is an n-type reaction.

Full text of DOI:10.1039/c6ra07940g

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Dimethyl ether steam reforming to produce H2 over
Ga-doped ZnO/g-Al2O3 catalysts†
Cite this: RSC Adv., 2016, 6, 52411
*
Shuang Zhou, Kui Ma, Ye Tian, Ming Meng, Tong Ding, Yuqing Zha, Tianyong Zhang
*
and Xingang Li
Herein, we report the performance of a series of gallium-doped zinc oxide (GDZ) catalysts mechanically
mixed with g-Al2O3 (GDZ/g-Al2O3) for the dimethyl ether (DME) steam reforming (SR) to produce H2.
Compared with the ZnO catalyst mechanically mixed with g-Al2O3, the doping of ZnO with gallium can
significantly improve the conversion of DME and the yield of hydrogen. The catalyst with a Zn : Ga molar
ratio of 9 : 1 has the highest conversion of DME (95.4%) and yield of hydrogen (95.0%) at 450 ꢀC. The
carbon dioxide selectivity of GDZ/g-Al2O3 catalysts is higher than 95%, which is much higher than that of
the CuZnAlO/g-Al2O3 catalyst. Moreover, the GDZ/g-Al2O3 catalysts have better stability than the
CuZnAlO/g-Al2O3 catalyst. Doping of ZnO with gallium generates a large number of oxygen vacancies
on the catalyst, which is beneficial to the DME SR reaction. Our results indicate that HCOO– is an
intermedium of DME SR over GDZ/g-Al2O3 catalysts, and the transformation from HCOO– to CO2 is the
rate-controlling step. The results of conductivity indicate that the rate-controlling step is an n-type
reaction.
Received 27th March 2016
Accepted 21st May 2016
DOI: 10.1039/c6ra07940g
www.rsc.org/advances
(CH3)2O + 3H2O (g) / 6H2 + 2CO2, DH298 ¼ 135 kJ molꢁ1 (1)
1. Introduction
As a H2 carrier material, DME possesses a lot of advantages,
The increasing consumption of fossil fuels inevitably leads to
both energy shortage and various environmental pollutions. So,
the exploration and application of new clean energies are
becoming more and more important. In recent years the fuel
cells using H2 as an energy source, such as the proton exchange
membrane fuel cells (PEMFCs), have attracted a lot of attention
due to their multiple advantages, e.g. high energy density, rapid
start response, compact size and zero pollution.1 To more
cleanly and efficiently use the limited fossil fuels, the traditional
fuels including hydrocarbons, alcohols and ethers are oen
converted to hydrogen through reforming processes. Among the
reported reforming techniques, steam reforming (SR) operating
at relatively low temperatures can generate more H2 due to the
efficient utilization of the hydrogen element in water. For
instance, the dimethyl ether steam reforming (DME SR) can
generate 6 times molar amounts of H2 per molar DME, as
described in eqn (1):
such as high H/C ratio, high energy density, no toxicity and no
C–C bonds in DME molecule. The absence of C–C bonds in
DME greatly decreases the carbon deposition on catalysts
during DME SR. All of these advantages make DME SR tech-
nique very promising in areas of onboard hydrogen produc-
tion. The DME SR reaction consists of two successive steps,
namely DME hydrolysis to methanol (DME H) and methanol
steam reforming (MSR) as expressed in eqn (2) and (3),
respectively.
(CH3)2O + H2O (g) / 2 CH3OH, DH298 ¼ 37 kJ molꢁ1 (2)
CH3OH + H2O / 3H2 + CO2, DH298 ¼ 49 kJ molꢁ1
(3)
The DME hydrolysis is catalyzed by solid acid catalysts, such
as bayerite-derived Al2O3, g-Al2O3, ZSM-5, H-mordenite and
TiO2. The alumina-based solid acid catalysts show higher
catalytic stability than the zeolite-based ones, since the strong
acidity of zeolites can readily lead to the formation of coke, and
decrease the activity and durability of the catalysts.2–4
The MSR reaction requires redox catalysts, such as copper
and zinc-based catalysts, especially the Cu-containing one.5–9
Our previous work has reported that the CuZnAl1ꢁxMxO (M ¼ Zr
or Ce) catalysts were active for MSR during DME SR.5,6 The
partial substitution of Al by Zr or Ce can increase the dispersion
of copper species, improve the reducibility of the catalysts, and
inhibit the sintering of Cu crystallites in the DME SR reaction.5,6
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of
Chemical Engineering, Tianjin University, Tianjin 300072, China. E-mail:
xingang_li@tju.edu.cn; tianye@tju.edu.cn
† Electronic supplementary information (ESI) available: TPD-MS-CH3OH with the
adsorption of CH3OH at 100 ꢀC on ZnO; Arrhenius plots for DME SR reaction over
GDZ/g-Al2O3 catalysts; XRD of the CuZnAlO/g-Al2O3 and Zn9Ga1O/g-Al2O3
catalysts before and aer stability test; TG of the CuZnAlO/g-Al2O3 and
Zn9Ga1O/g-Al2O3 catalysts before and aer stability test; catalyst
characterization of NH3-TPD and TG. See DOI: 10.1039/c6ra07940g
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Shimoda et al. reported that the CuFe2O4 spinel mixed with g-
Al2O3 exhibited high activity and stability for DME SR because
of formation of a new active phase of Cu–Fe–AlO4 for the MSR
step.7 Although the reported Cu-based catalysts displayed high
performance for DME SR, they also had some shortcomings,
such as high CO selectivity, low thermal stability and short
catalyst durability. Thus, it is necessary to develop copper-free
catalyst systems for DME SR.
2. Experimental
2.1 Catalyst preparation
The GDZ catalysts with different Zn/Ga atomic ratios were
prepared by co-precipitation method, using sodium carbonate
as precipitant. Zn(NO3)2$6H2O and Ga(NO3)3 solution
(purchased from Alfa Aesar) were used as source materials. A
Na2CO3 solution with [CO32ꢁ] ¼ 0.5 M and a mixed solution of
Zn and Ga nitrates with the total cations concentration of 1.0 M
were prepared, respectively. Then these two solutions were
simultaneously dropped into 100 mL deionized water by
keeping the pH around 7.5 under vigorous mechanical stirring
at the temperature of 60 ꢀC. Aer aged at the same temperature
for 2 h, the obtained precipitate was ltered and washed by
deionized water for three times, then dried overnight at 120 ꢀC
In methanol conversion reactions the CO2 selectivity was
highly related to the oxidation of formaldehyde. The litera-
ture reported that zinc oxide without copper was a good CO2-
selective catalyst in MSR, which gave the CO2 selectivity as
high as 95–99.6%.10 ZnO was benecial to oxidization of the
formaldehyde and the formaldehyde-derived intermediates
toward CO2, resulting in the high CO2 selectivity.10 However,
another study reported that considerable amounts of CH4
and CO could be identied during DME SR over ZnO–Al2O3
catalysts by a co-precipitation method. The presence of vast
Al3+ ions probably enhanced the direct decomposition of
methanol and DME, producing more CO and CH4.11 Our
previous work reported that the partial substitution of Al3+
(20%) by Ce4+ in ZnAlO mixed oxide catalyst can improve the
performance of ZnAlO catalyst for DME SR, and the modied
ZnAlCeO catalyst exhibited higher CO2 selectivity and better
start-off durability than ZnO and ZnAlO catalysts.12 The
introduction of Ce not only enhanced the dispersion of Zn
species but also inhibited the transformation of ZnO to
spinel ZnAl2O4. In addition, the interaction between ceria
and zinc oxide may produce new active sites, such as the
Ce4+–O–Zn2+ linkages.12
ꢀ
and nally calcined at 500 C for 4 h. According to the Zn/Ga
molar ratio, the prepared catalysts were denoted as ZnO,
Zn9.5Ga0.5O, Zn9Ga1O, Zn8Ga2O and Zn7Ga3O, respectively. For
comparison,
a conventional CuZnAlO catalyst was also
prepared, the details of which can be found elsewhere.6
Before used for the measurements of activity and durability,
the GDZ and CuZnAlO catalysts were mechanically mixed with
g-Al2O3 (supplied by Tianjin Chemical Research & Design
Institute, calcined at 600 ꢀC for 2 h, SSA ¼ 189 m2 gꢁ1) with the
mass ratio of 2 : 1 (GDZ or CuZnAlO to g-Al2O3), and the cata-
lysts were made into small pellets with the size of 40–60 mesh.
The total acidity of g-Al2O3 calculated by the temperature-
programmed desorption of ammonia (NH3-TPD) is 50 mmol
gꢁ1. The catalysts were denoted as GDZ/g-Al2O3 or CuZnAlO/g-
Al2O3.
Intrinsic ZnO is an n-type semiconductor.13 According to the
electron theory of catalysis on semiconductors, the catalytic
activity of semiconductor is determined by the position of the
2.2 Catalyst characterization
Fermi level on the surface of the crystal, which can be varied to The specic surface area (SSA) of the catalysts was measured by
some extent by doping the surface or the bulk of the crystal with nitrogen adsorption at ꢁ196 ꢀC on a Quantachrome Quad-
the properly chosen impurities in proper concentrations.14 Ga raSorb SI instrument. Before the measurements, the catalysts
atoms can substitutionally enter the lattice of ZnO acted as were degassed under vacuum at 300 ꢀC for 3 h. The specic
donors,15 and it can raise the Fermi level on the surface of ZnO. surface area was calculated using standard BET method in 0–0.3
Zinc oxide catalysts modied by gallium have been proved to be partial pressure range.
active for many reactions including water splitting,16,17 DME
SR18 and MSR reactions.19
The crystal structure of the catalysts was characterized by X-
ray diffraction (XRD). The tests were performed on Rigaku D/
Based upon the above analysis, we expected the doping of Max-2500 X-ray diffractometer operating at 200 mA and 40 kV,
gallium could improve the catalytic performance of ZnO-based using Cu Ka (l ¼ 0.15418 nm) as radiation source. The
catalysts for DME SR. In this work a series of gallium-doped ZnO diffraction data in the range of 2q from 10ꢀ to 90ꢀ were collected
(GDZ) catalysts physically mixed with g-Al2O3 (GDZ/g-Al2O3) at a speed of 8ꢀ minꢁ1, and the slow scan were collected at
with different zinc/gallium molar ratios were designed and a speed of 1ꢀ minꢁ1 from 30ꢀ to 40ꢀ.
prepared. Our results show that the GDZ/g-Al2O3 catalysts do
The temperature-programmed desorption of CH3OH (TPD-
display the remarkably improved catalytic performance, such as MS-CH3OH) was performed on a TPDRO instrument (Xian Quan
high DME conversion, high CO2 selectivity and long high- TP-5079) connected with a mass spectrometer (Hiden HPR20).
temperature durability, as compared with the traditional CuZ- 50 mg catalyst powder (40–60 mesꢀh) was put into the quartz
nAlO catalyst physically mixed with g-Al2O3. By using multiple tube reactor and pretreated at 400 C for 1 h under 8% H2/N2
techniques including XRD, TPD-MS-CH3OH, in situ FTIR of stream (30 mL minꢁ1), then the feed gas was switched to pure
CH3OH, X-ray photoelectron spectroscopy (XPS) and four-probe helium ow (99.999%, 30 mL minꢁ1) for 0.5 h at the same
direct current (DC) method, the catalysts were reasonably temperature to purge the gaseous and adsorbed H2. Aer the
characterized, and the structure–performance relationship and temperature cooled down to 60 ꢀC in the same helium ow,
the reaction pathways of CH3OH reforming were discussed, as pulses of CH3OH (1.0 mL) were injected into the catalyst bed
well.
until the MS signal got stable. Subsequently, the temperature-
52412 | RSC Adv., 2016, 6, 52411–52420
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programmed desorption was conducted ꢀfrom ambient H2 were analyzed by an Agilent 7890A chromatograph (GC).
ꢁ1
ꢀ
temperature to 700 C at a heating rate of 10 C min in the DME, CH4 and CH3OH were separated by a PoraPak Q capillary
same helium ow (99.999%, 30 mL minꢁ1).
column and analyzed by a ame ionization detector (FID), while
To evaluate the quantity of methanol adsorbed on the N2, CO and CO2 were separated by a PoraPak N column and
surface of the catalysts, the CH3OH pulse adsorption was also a MoleSieve 5A column, and analyzed by a thermal conductivity
performed on the TP-5079 with a thermal conductivity detector. detector (TCD). H2 was analyzed by TCD with N2 as carrier gas.
Similar to the test of TPD-MS-CH3OH, the catalysts were rst The DME conversion (XDME), H2 yield (yH ) and the selectivity
2
pretreated with 8% H2/N2 and puried with helium before (SC ) to Ci (including CO2, CO and CH4) are dened as follows:
i
measurement. Then, the pulses of CH3OH (1.0 mL) were rapidly
DMEin ꢁ DMEout
XDME
¼
ꢂ 100%
ꢂ 100%
(4)
(5)
(6)
injected into the tube every 0.5 h until the detector signal got
DMEin
ꢀ
stable at 60 C.
The in situ Fourier-transform infrared (FTIR) spectroscopies
of CH3OH were performed on a Nicolet Nexus spectrometer
equipped with a MCT detector cooled by liquid nitrogen. 20 mg
catalyst was ground into ne powder and pressed into a thin
wafer, which was then placed in a quartz IR cell connected to
a high vacuum system. The catalyst was in situ pretreated for 1 h
FH
1
6
2
yH
¼
ꢂ
2
FDME;in
FC
i
X
SC
¼
ꢂ 100%
FC
i
i
ꢀ
here, DMEin, and DMEout stand for the molar ow rate of DME
at 400 C in 5% H2/N2 mixture gas ow with a rate of 50 mL
minꢁ1. The background spectra were collected in vacuum.
Subsequently, the catalyst was exposed to CH3OH at 50 ꢀC in N2
ow to achieve adsorption saturation. The FTIR spectra were
2
in feed gas and product gas. FH represents the effluent molar
ow rate of H2, and FC is the molar ow rate of the Ci products.
i
ꢀ
recorded at the desired temperatures from 50 to 400 C.
3. Results and discussion
3.1 Catalysts characterization
The conductivities of the catalysts were measured with
a four-probe direct current (DC) method with an electro-
chemical workstation (VersaSTAT 3, Ametek). The powders were
pressed at 18 MPa and made into rectangular bars with a typical
3.1.1 Texture and bulk structure of the catalysts. The XRD
patterns of the catalysts are displayed in Fig. 1(a). The pattern of
the ZnO catalyst shows peaks matching with the zincite struc-
ture [JCPDS 36-1451]. The same peaks were also observed on the
GDZ catalysts, and no other phase was detected. Additionally,
compared with the pure ZnO catalyst, the intensity of the XRD
ꢀ
size of 25 ꢂ 7 ꢂ 1.5 mm and then calcined at 500 C for 3 h to
form dense bars. Four silver wires with silver paste were used as
electrodes. Before the test, the bars were reduced in H2 (50 mL
minꢁ1) at 400 C for 0.5 h, and then the conductivities of the
ꢀ
catalysts were measured in H2 atmosphere.
The X-ray photoelectron spectra were analyzed using an X-ray
photoelectron spectroscopy (XPS) (K-alpha X, Thermo Fisher
co., USA). The binding energies were referenced to the C1s line
at 284.6 eV from adventitious carbon.
The procedures of NH3-TPD and thermal gravimetric anal-
ysis (TG) can be obtained in ESI.†
2.3 Catalytic activity and durability tests
The evaluation of catalytic activity and durability of the catalysts
was performed in a continuous ow xed-bed reactor under
atmospheric pressure. Before activity evaluation, 500 mg cata-
lyst was loaded in the quartz reactor and in situ reduced at 400
ꢀC for 0.5 h in 10% H2/N2 (50 mL minꢁ1). The feedstock for
activity and durability tests is composed of N2 (40%), H2O vapor
(50%) and DME (10%) with the steam/carbon molar ratio (S/C)
ꢁ1
xed at 2.5, showing a space velocity of 12 000 mL hꢁ1 gcat
.
The external and internal mass transfer limitations have already
been excluded by variation of the linear ow rate over the
catalyst and the catalyst particle size, respectively. A mixture gas
of DME and N2 was directly fed to the reactor, and the water was
vaporized at 150 ꢀC before entering the reactor. For activity
measurements the reaction temperature varied in the range of
ꢀ
370–450 C, while for durability tests the reaction temperature
Fig. 1 XRD patterns in the region of (a) 10ꢀ to 75ꢀ with a speed of 8ꢀ
was xed at 440 ꢀC. The compositions of the inuent and
minꢁ1 and (b) 30ꢀ to 40ꢀ with a speed of 1ꢀ minꢁ1: (1) ZnO, (2)
effluent gases containing DME, CH4, CH3OH, N2, CO, CO2 and Zn9.5Ga0.5O, (3) Zn9Ga1O, (4) Zn8Ga2O and (5) Zn7Ga3O.
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peaks are noticeably reduced with the increase of the Ga content
in the catalysts. It suggests the gradually decreased particle size
and crystallinity. By applying the Scherrer equation, the average
crystallite size has been estimated to be 45.5 nm for the pure
ZnO, 11.7, 9.0, 4.4 and 3.9 nm for the catalysts Zn9.5Ga0.5O,
Zn9Ga1O, Zn8Ga2O and Zn7Ga3O, respectively.
Furthermore, Fig. 1(b) shows the slow scan XRD patterns.
Doping of ZnO with Ga causes the shi of the XRD peaks
towards small angles. The peak corresponding to (002) plane
shied from 34.48 to 34.18ꢀ, and the peak corresponding to
(101) plane shied from 36.31 to 36.15ꢀ. The shi of the
diffraction peaks indicates the GDZ catalysts still keep the
zincite structure and Ga is successfully incorporated into the
crystal lattice causing the lattice distortion. Wei et al. reported
that the interplanar spacing of the (101) plane in Ga-doped ZnO
catalysts shied from 0.2468 to 0.2473 nm.20 According to the
semiconductor theory, the Ga cations in the zincite structure
probably exist in the form of Ga3+$e, which can increase the
conductivity and the electron-donating ability of the catalyst. A
weak diffraction peak of gallium oxide (Ga2O3 [JCPDS 06-0509])
appears in the Zn8Ga2O pattern, and it becomes stronger in the
Zn7Ga3O pattern. It indicates that partial of Ga exists in the
form of Ga2O3 on the GDZ catalysts.
Fig. 2 The conductivity of the GDZ catalysts.
has unstable electrical properties.15 These results indicate that
Ga atoms have been doped into ZnO crystal lattice and lead to
the increase of conductivity, which is in accordance with the
XRD results.
3.1.3 TPD-MS-CH3OH. Methanol is an intermediate
product in DME SR, and its adsorption and activation on cata-
lyst surface is rather important. The methanol adsorption
behaviour on the catalyst surface was investigated by TPD-MS-
CH3OH. Table 1 lists the methanol adsorption amounts (MAA)
of the catalysts measured by pulse adsorption. In the TPD-MS-
CH3OH tests, the MS signals of the desorbed CH3OH, H2, CO2
and H2O were simultaneously monitored, as shown in Fig. 3(a–
d), respectively. Fig. 3(a) shows that the desorption of CH3OH
The results of the specic surface areas of the catalysts are
summarized in Table 1. The surface areas are enlarged with the
Ga contents increased.
3.1.2 Electrical conductivity of the catalysts. Doping of
semiconductor can change the position of the Fermi level on
the surface and in the bulk. The position of Fermi level will
affect both the electrical conductivity and the catalytic activity,
simultaneously.14 Therefore, there is a correlation, which is
related to the type of reaction (n-type or p-type reaction) and the
type of semiconductor (n-type or p-type), between the catalytic
activity and the electrical conductivity.14 ZnO is an n-type
semiconductor. Generally, Ga can be doped into the ZnO
crystal lattice as a donor impurity and be considered in the form
of Ga3+$e, which will increase the conductivity of GDZ catalysts.
However, excessive Ga will also lead to the formation of Ga2O3,
reducing the conductivity of GDZ catalysts. The conductivity
data of the catalysts are displayed in Fig. 2. The conductivity of
all of the GDZ catalysts increase with the rising temperature,
and the Zn9Ga1O catalyst shows the highest conductivity. The
data of the ZnO catalyst are not given, because the undoped ZnO
ꢀ
from the pure ZnO takes place at around 100 C with a small
peak area, suggesting that only small amount of CH3OH can be
adsorbed on ZnO, similarly to the previous report.12 The
desorption signals of CH3OH from the GDZ catalysts exhibits
two desorption peaks with much larger peak areas than that
from ZnO. The peak in the range of ꢃ60–200 ꢀC is attributed to
the desorption of molecularly adsorbed methanol, and the top
of the peak gradually increase from 98 to 115 ꢀC with the
increase of Ga in the catalyst, indicating doping of Ga raises the
ability for methanol adsorption on the catalyst. The signal of
desorption of CH3OH above 200 ꢀC mainly comes from the
rehydrogenation of methoxy species by hydroxyl groups.21 No
methanol was desorbed on ZnO above 200 ꢀC in Fig. 3(a).
Probably, there is no chemically adsorbed methanol on ZnO
under the present experimental conditions. It can also be
Table 1 Specific surface (SSA), average crystallite size (ACS), CH3OH adsorption amounts (MAA), apparent activation energy (Ea) and TOF of
different catalysts
Catalyst
SSA (m2 gꢁ1
)
ACS (nm)
MAAa (mmol gꢁ1
)
Eab (kJ molꢁ1
)
TOFc (sꢁ1
)
ZnO
9.2
57.0
68.8
80.2
93.2
45.5
11.7
9.0
4.4
3.9
0.05
0.31
0.34
0.33
0.35
—
—
Zn9.5Ga0.5
Zn9Ga1O
Zn8Ga2O
Zn7Ga3O
O
213.6 ꢄ 4.4
197.4 ꢄ 2.7
203.6 ꢄ 11.0
210.2 ꢄ 5.9
0.013
0.019
0.016
0.015
a
b
Methanol adsorption amounts (MAA) was calculated from the area of the peaks in the CH3OH pulse adsorption proles. Apparent activation
energy (Ea) was estimated from Arrhenius equation based on the activity over all the catalysts (50 mg) at 1 atm total pressure (space velocity of
12 000 mL hꢁ1 gcatꢁ1. 10% DME, 50% H2O and 40% N2). Turnover frequency (TOF) was estimated based on the MAA results and the activity
c
tests over all the catalysts (50 mg) at 430 ꢀC and 1 atm total pressure (space velocity of 12 000 mL hꢁ1 gcatꢁ1. 10% DME, 50% H2O and 40% N2).
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efficient way to enhance the chemical adsorption of methanol,
which occurs even at 60 ꢀC, as shown in Fig. 3. The desorption
proles of CO2 (Fig. 3(b)) show that the initial desorption
temperature of CO2 of the GDZ catalysts is at about 260–280 ꢀC.
With the increase of Ga in the catalyst, the temperature of the
desorption peak shied from 336 (Zn9.5Ga0.5O) to 326 ꢀC
(Zn9Ga1O) and then raises to 338 ꢀC (Zn7Ga3O). In Fig. 3(c), the
temperature for H2 desorption from the catalysts has a similar
trend with the CO2 desorption. The H2 desorption of the
Zn9Ga1O catalyst shows the lowest temperature. The initial
temperature of H2 desorption is at about 230 ꢀC for the GDZ
ꢀ
catalysts, which is lower than that of CO2 (about 260–280 C).
Moreover, the temperature of the H2 desorption peak is also
lower than CO2. Thus, it can be deduced that the formation or
desorption of CO2 might be the rate-controlling step of DME SR.
Two H2O desorption peaks are observed in Fig. 3(d). The peak
ꢀ
below 400 C is attributed to the desorption of molecular H2O
formed during CH3OH adsorption, and the peak above 400 ꢀC is
ascribed to the water produced from the interaction of surface
hydroxyls.12 The surface hydroxyls play a vital role in the reac-
tion from CH3OH to CO2 and H2. Chadwick and Zheng found
that over the dehydroxylated ZnO catalyst the dominant reac-
tion was the decomposition of methanol to form CO and H2
rather than the MSR to form CO2 and H2.22 In addition, the
formation of hydrogen oen takes place via tꢀhe interaction of
surface hydroxyl groups.23 Herein, above 400 C the GDZ cata-
lysts show the larger peak area of water desorption signals than
that of ZnO, indicating that the former may have higher cata-
lytic activities for DME SR.
3.2 Activity and stability
3.2.1 Activity on DME SR. Fig. 4 shows the conversion of
DME, the yield of hydrogen and the selectivity of CO and CO2 of
the catalysts as a function of the reaction temperature.
Zn9Ga1O/g-Al2O3 exhibits the best catalytic performance as
compared with others. At 400 ꢀC, the GDZ/g-Al2O3 catalysts
exhibit higher DME conversion (13–20%) than that of pure ZnO/
g-Al2O3 (ꢃ10%). With the temperature increasing to 450 ꢀC,
Zn9Ga1O/g-Al2O3 displays a DME conversion as high as 95%,
which is much higher than that for pure ZnO (65%). Several
assumptions to estimate the apparent activation energy of DME
SR over different catalysts were put up by Eguchi et al.24 namely:
(1) it is a pseudo-rst-order reaction, (2) the reactor is an
isobaric plug ow reactor, and (3) side reactions are neglected.
Based on these assumptions, corresponding Arrhenius plots for
DME SR over this series of catalysts are obtained (Fig. S2†). The
good linearity of Arrhenius plots indicates that the assumptions
Fig. 3 The signals of desorption of (a) CH3OH, (b) CO2, (c) H2 and (d)
H2O in TPD-MS-CH3OH over (1) ZnO, (2) Zn9.5Ga0.5O, (3) Zn9Ga1O, (4)
Zn8Ga2O and (5) Zn7Ga3O.
veried by the CO2 desorption (Fig. 3(b)) and H2 desorption are acceptable for the GDZ/g-Al2O3 catalysts. The obtained
(Fig. 3(c)) that there is also no CO2 and H2 desorption peak for apparent activation energy (Ea) and turnover frequency (TOF) of
ZnO. For comparison, we carried out a TPD-MS-CH3OH exper- different catalysts are listed in Table 1. Zn9Ga1O/g-Al2O3 shows
iment with adsorption of CH3OH on ZnO at 100 ꢀC. The cor- the lowest Ea (197.4 kJ molꢁ1) and highest TOF (0.019 sꢁ1
)
responding results are plotted in Fig. S1.† In Fig. S1,† the values, which is in accordance with the highest DME conver-
desorption signal of CH3OH, CO2 and H2 can be clearly sion. The hydrogen yield (yH ) exhibits the same trend with the
2
observed. It indicates that methanol is hardly chemically DME conversion as seen in Fig. 4(b), the values of the H2 yield
ꢀ
adsorbed over ZnO at 60 C until the adsorption temperature and the DME conversion are similar, indicating that DME
increases to 100 ꢀC. Nevertheless, doping Ga into ZnO is an almost transform to H2 and CO2. During DME SR, some side
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than 95% in the whole reaction temperature range, as shown in
Fig. 4(d).
With the increase of Ga content, the conversion of DME and
the yield of hydrogen increased and then decreased. The XRD
results showed that Ga was doped into the crystal lattice of ZnO.
As shown in Fig. 2, doping of Ga had caused the change of
electrical conductivity. The relationship between electrical
conductivity and catalytic performance is displayed in Fig. 5.
Compared with the change of the conductivity of GDZ catalysts,
the change of the conversion of DME and the yield of hydrogen
displayed a similar trend. The electron theory of catalysis on
semiconductor indicated in the case of an n-type reaction on an
n-type semiconductor, the electrical conductivity and the
activity in the same direction.14 Our results indicate that the
rate-controlling step of DME SR over GDZ/g-Al2O3 catalysts is an
n-type reaction. It can be accelerated by doping donor in the
lattice of n-type semiconductor of ZnO, which can enhance the
donor ability. The rate-controlling step may change with the
increase of donor. It indicates that a proper electrical conduc-
tivity is helpful to improve the activity and selectivity of the
catalyst. In this work, the highest electrical conductivity corre-
sponds to the highest catalytic activity. It indicates that
increasing the conductivity of GDZ catalysts probably further
enhance the activity of the catalyst. Moreover, as listed in Table
1, with the increase of the amount of gallium, the specic
surface areas of the GDZ catalysts signicantly enlarged. Thus,
although the conductivity of Zn7Ga3O and Zn8Ga2O was lower
than that of Zn9.5Ga0.5O, the formers still presented the higher
catalytic activity than the later, since the specic surface areas of
Zn8Ga2O (80.2 m2 gꢁ1) and Zn7Ga3O (93.2 m2 gꢁ1) are much
larger than that of Zn9.5Ga0.5O (57 m2 gꢁ1).
Understanding the formation and desorption of surface
species is helpful to the improvement of catalyst. According to
the results of TPD-MS-CH3OH, the initial desorption tempera-
ture of hydrogen on the GDZ catalysts is about 230 ꢀC, which is
Fig. 4 (a) Conversion of DME, (b) yield of H2, (c) selectivity of CO and
(d) selectivity of CO2 for the DME SR reaction.
reactions might occur,4–6 and the main by-products are CO and
CH4. As shown in Fig. 4(c), all catalysts exhibit the CO selectivity
less than 5% in the whole reaction temperature range, except
Zn7Ga3O. The selectivity of CO is much lower than that of our
previously reported CuZnAlO based catalysts.5,6 The small
amount of CO may come from some potential side reactions
such as reverse water gas shi reaction (r-WGS) and methanol
decomposition. Moreover, CH4 was not found in the product.
Thus, except Zn7Ga3O, all catalysts exhibit higher CO2 selectivity Fig. 5 The trend of conversion of DME, yield of H2 and conductivity.
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lower than that of CO2 (about 260–280 ꢀC). The top temperature doping into zinc oxide, X-ray photoelectron spectroscopy (XPS)
of the H2 desorption peak is also lower than that of CO2. We was employed. Fig. 7 shows the oxygen 1s (O 1s) peaks of ZnO
found that the trend of the conversion of DME, as well as the and Zn9Ga1O catalysts. The O 1s signals of the Zn9Ga1O catalyst
yield of hydrogen, is similar to the trend of the CO2 desorption pre-reduced in 5% H2/N2 for 0.5 h before XPS test is also given in
temperature. The lower desorption temperature of CO2 matches Fig. 7. The typical O 1s peak can be consistently tted by three
the higher activity and selectivity. Thus, we deduced that the nearly Gaussian, centered at 532.3, 530.8 and 531.5 eV,
formation of CO2 might be the rate-controlling step of DME SR respectively.25–27 The high binding energy component located at
using GDZ/g-Al2O3 as catalysts. The signals of CO2 and H2 532.3 eV is usually attributed to the adsorbed O2ꢁ
.
The
25
suggest that Zn9Ga1O has the highest activity to convert CH3OH component on the low binding energy side of O 1s spectrum
to H2 and CO2, as shown in Fig. 4(b and c). Enhancing the ability (530.8 eV) is attributed to the bulk O2ꢁ 26 The medium binding
of GDZ catalysts to convert CH3O– into CO2 will contribute to energy component centered at 531.5 eV is associated with O2ꢁ
the improvement of the catalytic activity of DME SR.
ions in the oxygen decient regions with the matrix of ZnO.27 So,
.
3.2.2 Durability test on the Zn9Ga1O/g-Al2O3 and CuZnAlO/ changes in the intensity of this component may be connected in
g-Al2O3 catalysts. To investigate the stability of this series of part to the variations in the concentration of oxygen vacancies.
catalysts for DME SR, the Zn9Ga1O/g-Al2O3 catalyst was selected In this work, Fig. 7 shows that the O 1s peaks of the Zn9Ga1O
for the prolonged durability test, which is carried out at 440 ꢀC catalyst slightly shied towards higher binding energy, as
for 48 h. Meanwhile, an identical experiment was performed on compared with ZnO. The peak shi is attributed to the differ-
the conventional CuZnAlO/g-Al2O3 catalyst for
a direct ence in the electronegativity of Zn and Ga. The valence electron
comparison. The results are shown in Fig. 6. For the CuZnAlO/g- density of Zn and O in the Zn–O–Ga bond in the Zn9Ga1O
Al2O3 catalyst, the DME conversion (around 98%) shows little catalyst becomes lower than that in the Zn–O–Zn bond in the
change in the rst 8 h, but aer this period it declines rapidly, ZnO catalyst because of the higher electronegativity of Ga than
giving the nal conversion of 62% at 48 h. For the catalyst that of Zn. As a result, the interaction between Zn and O is
Zn9Ga1O/g-Al2O3, it possesses an initial DME conversion of weakened, which leads to the increases in the binding energy of
ꢃ90%, a little lower than that of CuZnAlO/g-Al2O3, but it O 1s peaks.26,27
decreases more slowly, showing the conversion of 73% in the
In order to compare the cases of the GDZ and ZnO catalysts,
end. The total decreased DME conversion over the catalyst the relative intensities of the Zn9Ga1O, pretreated Zn9Ga1O and
Zn9Ga1O/g-Al2O3 during 48 h test is about 17%, much lower ZnO catalysts are calculated and are listed in Table 2. As
than that on the CuZnAlO/g-Al2O3 catalyst (36%). We deduce described in Table 2, the Zn9Ga1O and pretreated Zn9Ga1O have
that the good stability of the GDZ/g-Al2O3 catalysts probably higher values of the medium binding energy component than
originates from its high thermal stability and the ability of ZnO, conrming that the oxygen vacancy increased aer the Ga
resistance of coke deposition. As for the CuZnAlO/g-Al2O3 element incorporated into zinc oxide crystal lattice. Aer the
catalyst, the catalytic deactivation mainly results from the Zn9Ga1O catalyst was pretreated by H2, the percentage of lattice
readily sintered metallic Cu species and the coke deposition on oxygen decreased continuously, and the oxygen vacancy
the catalyst at the temperature above 300 ꢀC, as indicated by our increased. The oxygen vacancy plays an important role in the
XRD and TG results (Fig. S3 and S4†) in ESI.
adsorption of reactants and it is very important for DME SR.
To conrm the effect of oxygen vacancies, the catalytic tests
of the Zn9Ga1O/g-Al2O3 pretreated in different atmospheres
were also evaluated. As shown in Fig. 8, at 425 ꢀC the conversion
of DME over the pre-reduced Zn9Ga1O/g-Al2O3 catalyst can
reaches 67.8%, while it is only 48.5% and 44.3% over the
Zn9Ga1O/g-Al2O3 catalyst pretreated in pure N2 and the fresh
3.3 Mechanism of DME SR over GDZ/g-Al2O3 catalyst
3.3.1 XPS results. In order to identify the change of the
distribution and content of surface oxygen species caused by Ga
Fig. 7 The XPS spectra in the O 1s region of the ZnO and Zn9Ga1O
Fig. 6 The stability of DME SR over the Zn9Ga1O/g-Al2O3 and CuZ- catalysts: (1) ZnO, (2) Zn9Ga1O and (3) Zn9Ga1O pretreated in 8% H2/N2
nAlO/g-Al2O3 catalysts.
for 0.5 h before XPS test.
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Table 2 Relative intensity of O 1s peaks of ZnO and Zn9Ga1O catalysts,
evaluated by XPS
Fraction O state
Catalyst
532.3 eV (%)
531.5 eV (%)
530.8 eV (%)
ZnO
44.5
42.9
56.8
0
9.3
21.4
55.4
47.8
21.8
Zn9Ga1O
Zn9Ga1Oa
a
The Zn9Ga1O was reduced in 8% H2/N2 for 0.5 h before characterized
by XPS measurement.
one, respectively. Apparently, the pre-reduction treatment can
signicantly improve the catalytic activity of Zn9Ga1O/g-Al2O3.
So, it indicates that the increase of the number of oxygen
vacancies can improve the catalytic activity for DME SR over the
GDZ/g-Al2O3 catalyst.
3.3.2 In situ FTIR of CH3OH results. In order to infer the
reaction route, the thermal evolution of the adsorbed CH3OH
and other species on the Zn9Ga1O catalyst surface was moni-
tored by in situ FTIR spectroscopy. The temperature-dependent
IR spectra and the assignments of bands in the spectra are
shown in Fig. 9 and Table 3, respectively. At 50 ꢀC, a series of the
characteristic peaks corresponding to molecularly adsorbed
methanol can be observed. The peaks at 3709 cmꢁ1 and 3664
cmꢁ1 are assigned to the O–H stretching vibration [n (OH)] of
methanol molecules.28 The signal at 3006 cmꢁ1 belongs to the
C–H stretching vibration in the methyl group [n (CH3)] of
methanol molecules,29 and the band around 1032 cmꢁ1 is
attributed to the C–O stretching vibration [n (CO)] of CH3OH
molecule.21 Additionally, methoxy species are also detectable,
with the bands appearing at 3000–2800 cmꢁ1 (2961, 2926, 2868,
2821 cmꢁ1), ꢃ1453 cmꢁ1 and 1100–1000 cmꢁ1 (1070 and 1030
cmꢁ1), which correspond to n (CH),29,30 ds (CH3)29 and n (CO),21,30
respectively. The detected methoxy species at the low tempera-
Fig. 9 In situ FTIR spectra of adsorbed CH3OH at different tempera-
tures over the Zn9Ga1O catalyst: (1) 50 ꢀC, (2) 100 ꢀC, (3) 150 ꢀC, (4)
200 ꢀC, (5) 250 ꢀC, (6) 300 ꢀC, (7) 325 ꢀC, (8) 350 ꢀC, (9) 375 ꢀC and (10)
400 ꢀC.
weakened. A broad band in the region of 3600–3300 cmꢁ1
assigned to the n (OH) of adsorption H2O29 and an absorption
band at ꢃ1610 cmꢁ1 assigned to the d (HOH) of adsorption H2O
appeared.31 Negative signals between 3750 and 3580 cmꢁ1 also
appeared, indicating that a perturbation or a consumption of
the surface –OH groups on the catalyst occurred aer the
methanol adsorption. These results indicate that the hydroxyl
groups on the surface of GDZ catalyst react with methanol to
give methoxy and water molecules, which is in accordance with
the results of TPD-MS-CH3OH. With the temperature further
increased to 150 ꢀC or higher, the methoxy species gradually
decrease. When the temperature reaches 200 ꢀC, some new
ꢀ
ture of 50 C are mainly assigned to the methoxy in methanol
molecule. With the increase of temperature to 100 ꢀC, the
absorption intensity of adsorbed methanol molecule is
bands emerge at the positions of 2961, 1580, and 1369 cmꢁ1
,
which can be attributed to formate species. More specically,
the peak at 2961 cmꢁ1 is ascribed to stretching of carboxylate
and bending vibration of C–H mode (HCOO–).30 The bands at
about 1580 cmꢁ1 can be assigned to nas (COO) and 1369 cmꢁ1
assigned to ns (COO), respectively.32 With the temperature
ꢀ
increasing to 250 C the intensity of the absorption bands for
formate species is strengthened, suggesting the transformation
of more CH3O– to HCOO–. At this temperature the peaks of
HCOO– shi to lower wavenumbers, and wavenumber of the nas
(COO) shis from 1580 cmꢁ1 to 1568 cmꢁ1. Gaseous CO2 peaks
centered at 2372 and 2337 cmꢁ1 emerged at 300 ꢀC.31 This
means that the formate species and carbonate species turned
into carbon dioxide, and this is coincident with the TPD-MS-
CH3OH. The appearance of gaseous CO2 absorption bands
further demonstrates that the formed intermedium HCOO–
species prefers to turn into CO2. The amount of HCOO– species
Fig. 8 The conversion of DME over the Zn9Ga1O/g-Al2O3 catalyst
under different pretreatments: (1) pre-reduced in 8% H2/N2 mixture
gas, (2) pretreated in pure N2, (3) un-pretreated.
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that when the temperature reaches 100 ꢀC, methoxy species
and water appear. The signals of desorbed water also have
been observed in TPD-MS-CH3OH above 50 C. We infer that
Table 3 In situ FTIR assignments of surface species of adsorbed
CH3OH over Zn9Ga1O catalyst
ꢀ
Wavenumber
Groups
Vibrational mode
(cmꢁ1
)
Ref.
methanol molecules are adsorbed on the oxygen vacancies of
GDZ catalyst, and it dissociate to methoxy species and adsor-
bed hydrogen atoms (Ha) (eqn (7)), then the adsorbed
hydrogen atoms react with the surface hydroxyls to form water
molecules. The dissociation of methanol molecules to
methoxy species is a fast step since it can occur at a low
temperature. We infer that the adsorbed water molecules can
also dissociate to form the adsorbed hydroxyls (HOa) and
adsorbed hydrogen atoms (eqn (8)) on the oxygen vacancies,
and this process is a fast reversible process due to the easy
desorption of water. As shown in Fig. 4(d), we infer that the
adsorbed hydroxyls can further dissociate to form the adsor-
bed hydrogen atoms and adsorbed oxygen atoms (eqn (9)). The
adsorbed hydrogen atoms can combine to form the hydrogen
molecules (eqn (10)), which have been detected by TPD-MS-
H2O
–OH
d (H2O)
n (OH)
1610
31
28
29
21
3664, 3709
3600–3300
1032
CH3O–
n (CO)
1100–1000
3006
2961, 2926
2868
2821
1453
2961
1369
1580
2372, 2337
21 and 30
nas (CH3)
ns (CH3)
29
30
29
30
29
30
32
32
31
ds (CH3)
d (CH)
ns (COO–)
nas (COO–)
Physisorbed CO2
HCOO–
CO2
ꢀ
CH3OH above 220 C.
increases with the temperature elevated from 250 to 300 ꢀC.
Above 300 ꢀC, the amount of HCOO– species decreases with the
temperature further increasing, but the intensity of gaseous
CO2 is continuously strengthened, which suggests that more
and more HCOO– species are transformed into CO2.
CH3OH + , / CH3Oa + Ha
H2O + , / HOa + Ha
HOa / Oa + Ha
(7)
(8)
(9)
3.3.3 The pathway of DME SR over the GDZ/g-Al2O3 cata-
lyst. The GDZ catalysts mechanically mixed with g-Al2O3 have
been proved to be active for the DME SR reaction, especially the
extremely high CO2 selectivity. In order to understand DME SR
over the catalysts, we deduce a reaction mechanism according
to the results of XPS, in situ FTIR of CH3OH and the TPD-MS-
CH3OH. The proposed mechanism is shown in Fig. 10. It has
been proved that the DME SR reaction consists of two successive
steps, namely DME hydrolysis to methanol (DME H) (eqn (2))
and methanol steam reforming (MSR) (eqn (3)). In this work,
the DME hydrolysis is catalyzed by g-Al2O3, which acted as
a solid acid catalyst. We mainly focus on the MSR step. As
shown in XPS, the reduced GDZ catalyst has a lot of oxygen
vacancies under reducing atmosphere, and these oxygen
vacancies usually are considered as the active sites for the
chemisorption of methanol and water. The in situ FTIR shows
2Ha / H2
(10)
,: oxygen vacancy.
The appearance of signals of HCOO– in in situ FTIR at 200 ꢀC
proved that the CH3O– species transformed into HCOO–. We
infer the equations (eqn (11) and (12)) for this process.
(11)
(12)
The CO2 signals appeared in TPD-MS-CH3OH test at about
270 ꢀC. Moreover, at 300 ꢀC, new bands corresponding to
gaseous CO2 (2372, 2337 cmꢁ1) are observed in in situ FTIR
spectra. The appearance of the gaseous CO2 demonstrates that
the formed intermedium HCOO– prefers to turn into CO2 (eqn
(13)), which can provide a strong evidence for the high CO2
selectivity of DME SR on the GDZ/g-Al2O3 catalysts (Fig. 4(d)).
(13)
Fig. 10 The proposed mechanism for DME SR over the GDZ/g-Al2O3
catalyst.
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We found that CO2 was generated at the highest temperature
in all steps. Meanwhile, the temperature of CO2 desorption
showed in TPD-MS-CH3OH has a similar trend with the
conversion of DME. Thus, we deduce that the transformation
from HCOO– to CO2 is the controlling step for MSR over the GDZ
catalyst. As discussed in 3.2.1, the rate-controlling step of DME
SR over GDZ/g-Al2O3 catalysts is an n-type reaction. Therefore,
we deduce the transformation from HCOO– to CO2 is an n-type
reaction, which can be accelerated by doping donor in the lattice
of n-type semiconductor, and we have accelerated the trans-
formation from HCOO– to CO2 by the doping of ZnO with Ga.
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catalysts for DME SR to produce H2 was prepared. Compared
with the conventional catalyst, GDZ/g-Al2O3 catalysts exhibited
higher carbon dioxide selectivity and better stability. XRD
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of DME (95.4%) and yield of hydrogen (95%). The results of
conductivity indicate that the rate-controlling step of DME SR
over the GDZ catalysts is an n-type reaction. XPS results indi-
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conversion of DME, high selectivity of CO2 and high stability of
GDZ/g-Al2O3 catalysts makes it promising for DME SR both in
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Acknowledgements
This work is nancially supported by National Natural Science
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