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Investigation of the role of Nb on Pd¡Zr¡Zncatalyst in methanol steam reforming for hydrogenproduction
Fufeng Cai a, Peijing Lu a, Jessica Juweriah Ibrahim b, Yu Fu a,Jun Zhang a,*, Yuhan Sun a,c,**
a CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute,
Chinese Academy of Sciences, Shanghai 201210, Chinab Key Laboratory of Bio-based Material, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy
of Sciences, Qingdao 266101, Chinac School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
a r t i c l e i n f o
Article history:
Received 16 November 2018
Received in revised form
3 March 2019
Accepted 17 March 2019
Available online xxx
Keywords:
Methanol steam reforming
Hydrogen production
Nb-modified Pd�Zr�Zn catalysts
Pd�Zn alloy
Oxygen vacancies
* Corresponding author.** Corresponding author. CAS Key Laboratorytute, Chinese Academy of Sciences, Shangh
E-mail addresses: [email protected] (J. Zhhttps://doi.org/10.1016/j.ijhydene.2019.03.1250360-3199/© 2019 Hydrogen Energy Publicati
Please cite this article as: Cai F et al., Investproduction, International Journal of Hydrog
a b s t r a c t
Methanol steam reforming is regarded as a very promising process to generate H2 suitable
for fuel cells. Typically, the Pd-based catalysts can catalyze efficiently methanol steam
reforming for hydrogen production. But their high selectivity to CO, a byproduct of
methanol reforming reaction, severely limits their potential application. In this work, a
series of Nb-modified Pd�Zr�Zn catalysts with different Nb loadings were prepared to
study their catalytic activities with more focus on the role of Nb on Pd�Zr�Zn catalyst for
methanol steam reforming. The prepared catalysts were fully analyzed by using various
characterization techniques, for example, ICP, BET, SEM, XRD, H2-TPR, NH3-TPD, HRTEM,
CO chemisorption, XPS, and Raman. The experimental results showed that an increase in
Nb loading for the Nb-modified Pd�Zr�Zn catalysts led to a decrease of the methanol
conversion and H2 production rate. This was probably due to the decrease in the amount of
oxygen vacancies on the catalyst surface. However, introduction of Nb into Pd�Zr�Zn
catalyst increased the acid strength on the catalytic surface. The aldehyde species derived
from methanol decomposition were readily transformed to HCOOH, thus yielding high
selectivity to CO2 for the Nb-modified Pd�Zr�Zn catalysts. Significantly, the addition of Nb
to Pd�Zr�Zn catalyst facilitated the incorporation of Pd into the ZnO lattices, which led to
the formation of Pd�Zn alloy. Consequently, the Nb-modified Pd�Zr�Zn catalysts
exhibited significantly lower CO selectivity and production rate than the Pd�Zr�Zn cata-
lyst. From the results, this work offers a new way to the rational design of selective
methanol steam reforming catalysts to decrease the formation of byproduct CO.
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Insti-ai 201210, China.ang), [email protected] (Y. Sun).
ons LLC. Published by Elsevier Ltd. All rights reserved.
igation of the role of Nb on Pd�Zr�Zn catalyst in methanol steam reforming for hydrogenen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x2
Introduction
As energy and environmental challenges become increasingly
prominent, it is imperative to exploit and use clean and
renewable energy resource [1]. In view of this, proton ex-
change membrane fuel cells (PEMFCs), which use hydrogen to
produce electricity for different mobile applications, have
attracted a great deal of attention in recent years because of
their high energy efficiency and environmental friendliness
[2]. However, the major obstacle for the wide application of
PEMFCs is the difficulties related to the storage and trans-
portation of hydrogen, because it is highly diffusive, inflam-
mable and explosive. Currently, the use of liquid fuels that are
efficiently converted to H2 onboard by the catalytic reforming
is one of the most promising approaches to solve the above
stated challenge [3,4]. In this regard, hydrogen production
routes by methane and ethanol reforming [5e7], and the
methanol steam reforming (CH3OH þ H2O / CO2 þ 3H2) have
been extensively investigated in the literature [8e10]. This is
because methanol in liquid state at room temperature, has
high H/C ratio, extremely low content of sulfur, and relatively
low reaction temperature [8].
In the past decade, the Cu-based catalysts, represented
mainly by the Cu/ZnO/Al2O3 catalyst, are the most commonly
applied for methanol steam reforming since they usually
possess high reaction activity and good selectivity for hydrogen
production at relatively low temperature (200e350 �C) [11e15].But due to the thermal instability and pyrophoric character-
istic, the active sites for this kind of catalyst are very easy to
reunite and become inactivated at high temperature steam
environment. This has prompted researchers to search for
other alternative catalyst systems. Accordingly, many other
kinds of catalysts are developed and studied, among which the
PdZn-containing catalysts have received a great deal of interest
and attention because of their good stability and high selec-
tivity to CO2 and H2 [16,17]. Since 1993, Iwasa and co-workers
[18,19] first tested the catalytic activity for methanol steam
reforming on different supported Pd catalysts, for example, Pd/
ZrO2, Pd/A12O3, Pd/SiO2, Pd/ZnO, and so on. They found that Pd
supported on ZrO2, Al2O3 or SiO2 catalyst was highly selective
to methanol decomposition, thus producing large amounts of
CO, which should be decreased as much as possible because it
can poison the electrode of PEMFCs severely. On the contrary,
Pd supported on ZnO catalyst exhibited anomalous high
methanol steam reforming activity and selectivity to CO2 and
H2, attributable to the formation of Pd�Zn alloy under high
temperature reducing conditions.
After the study of Iwasa et al. [19], the activities of PdZn-
containing catalysts for methanol steam reforming were
widely examined. Chin et al. [20] systematically investigated
the effect of the Pd/ZnO catalyst preparation and pretreatment
on methanol steam reforming. They found that the prepara-
tion and pretreatment of Pd/ZnO catalyst were critical to the
formation of Pd�Zn alloy and thus significantly affected its
catalytic performance. Matsumura et al. [21] noted that intro-
duction of a small amount of Al to the Pd/ZnO catalyst facili-
tated the formation of PdeZn alloy particles, which resulted in
the increased catalytic activity for methanol steam reforming.
Similarly, Baltan�as et al. [22] found that Pd supported on ZnO-
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
CeO2 mixed oxides exhibited higher activity and stability than
Pd supported ZnO, whichwas due to the formation of bulk and
surface PdeZn alloy in the presence of CeO2. In recent years,
researchers have focused more on the inner effect mecha-
nisms of PdeZn alloy and/or ZnO support on the selectivity for
methanol steam reforming. Wang et al. [23] reported that the
presence of more polar ZnO facets in Pd/ZnO catalyst favored
the formation of stable PdeZn alloy phase, thus achieving high
CO2 selectivity. Li et al. [24] claimed that the ZnO support
exhibited strong ability to activate water, while the formed
PdeZn alloy caused the formaldehyde to react with water to
form HCOOH, which together led to the high CO2 selectivity on
PdZn/ZnO catalyst. Slightly different from the report of Li et al.
[24], Armbruster et al. [25] established that the presence of
large interface between PdeZn nanoparticles and ZnO patches
was responsible for high CO2 selectivity of PdZn/ZnO catalyst
for methanol steam reforming.
Although much work has been done as stated above, there
still exists some controversies regarding the roles of Pd�Zn
alloy and/or ZnO in the activity of PdZn-containing catalysts.
For example, Datye et al. [26] reported that the Pd/ZnO catalyst
containing small Pd�Zn alloy particles (<2 nm) often exhibited
lower selectivity to CO2, and larger Pd�Zn alloy particles did
not adversely affect the catalytic activity for methanol steam
reforming. However, Wang and co-workers [27] in their study
claimed that the particle sizes of Pd�Zn alloy had a great
impact on determining the selectivity to CO2 in methanol
steam reforming reaction, and that the Pd/ZnO catalyst treated
with a high reduction temperature (650 �C, ~12 nm of Pd�Zn
alloy particles) exhibited higher CO2 selectivity than the sam-
ple reduced at low temperature (425 �C) with Pd�Zn alloy
particle sizes of ~5 nm. Obviously, further research on the role
of Pd�Zn alloy in the activity of PdZn-containing catalysts is
very important. Recently different kinds of metal-doped PdZn-
containing catalysts were used to study the influence of Pd�Zn
alloy onmethanol steam reforming, because the characteristic
of Pd species can be modulated by the appropriate dopant
[21,22]. Based on the research ideas in the literature [21e25],
and in order to gainmore insight into the effect of Pd�Zn alloy,
a series of Nb-modified Pd�Zr�Zn catalysts with different Nb
loadings were prepared in this work to study their catalytic
activities with more focus on the role of Nb on Pd�Zr�Zn
catalyst for methanol steam reforming. The influences of
experimental parameters such as reaction temperature, N2
flow rate and feed rate of methanol-water mixture on meth-
anol steam reforming were tested. Furthermore, the reaction
pathway for hydrogen production by methanol steam
reforming on Nb-modified Pd�Zr�Zn catalyst was also inves-
tigated. In sum, through in-depth study of the effects of Nb on
Pd�Zr�Zn catalyst structure and catalytic performance, this
work offers a way to the design of selective methanol steam
reforming catalysts to decrease the formation of byproduct CO.
Experimental
Materials
In this study, palladium nitrate dihydrate (Pd(NO3)2$2H2O,
Pd � 39.5 wt%), zirconium nitrate pentahydrate
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(Zr(NO3)4$5H2O, �99.5 wt%), zinc nitrate hexahydrate
(Zn(NO3)2$6H2O,�99.0 wt%) and niobiumoxalate (C10H5NbO20,
�98.0 wt%), purchased from Sinopharm Chemical Reagent
Co., Ltd., were applied as precursors for Pd, Zr, Zn and Nb,
respectively. In addition, methanol (CH3OH, �99.7 wt%), also
from Sinopharm Chemical Reagent Co., Ltd., was used for
steam reforming reaction. Potassium hydroxide (KOH, �90 wt
%) and potassium carbonate (K2CO3, �99.0 wt%), both from
Aladdin Industrial Corporation, were applied as precipitation
agents. Nitrogen gas (N2, 99.999%) and 5 vol% H2/Ar mixture
gas were purchased from Shanghai Pujiang Special Gas Co.
Ltd. The standard gases with various concentrations of H2, N2,
CO2, CO and CH4 were supplied from Shanghai Weichuang
Standard Gas Analysis Technology Co. Ltd. Ultrapure water
(18.2 MU*cm), acquired from an ultrapure water system
(HHitechMaster-S) in the laboratory, was used throughout the
experiment.
Catalysts preparation
A co-precipitation method was used to prepare the
Pd�Zr�Zn catalyst. In a typical synthesis, calculated
amounts of palladium nitrate dihydrate (0.075 g), zirconium
nitrate pentahydrate (1.41 g) and zinc nitrate hexahydrate
(5.48 g) were dissolved in 300 mL of ultrapure water. The
mixture was dispersed with an ultrasonic dispersion in-
strument for 2 h at 40 �C and stirred (800 rpm) vigorously with
a mechanical stirrer for 2 h at 40 �C. Subsequently, an
aqueous solution containing potassium hydroxide
(0.5 mol L�1) and potassium carbonate (0.2 mol L�1) was
slowly added into the above solution until the pH measured
by a pH meter (Mettler Toledo FE20) reached 8.5. The slurry
was stirred continuously for 48 h at 40 �C, then filtered by a
Buchner funnel, and washed thoroughly with ultrapure
water (500 mL � 5). The resulting precipitate was dried at
100 �C overnight in an oven and calcined at 400 �C with a
heating rate of 5 �C$min�1 in a stationary air for 4 h to achieve
the Pd�Zr�Zn catalyst. The Nb-doped Pd�Zr�Zn catalysts
with different Nb mass loadings were prepared by impreg-
nating uncalcined Pd�Zr�Zn sample (�100 mesh) with a
calculated amount of aqueous solution of niobium oxalate.
After impregnation, the slurry was evaporated to dry by
heating and stirring at 50 �C using a magnetic heated stirrer.
Next, the resulting precursor underwent drying at 100 �Covernight and calcination at 400 �C with a heating rate of
5 �C$min�1 for 4 h to obtain the Nb-modified Pd�Zr�Zn cat-
alysts. The prepared Nb-modified Pd�Zr�Zn catalysts were
identified as xNb/Pd�Zr�Zn, in which x referred to the
nominal weight percentage of niobium on the ZnO support.
In the case of Nb-modified and unmodified Pd�Zr�Zn cata-
lysts, the nominal mass loading of Pd and Zr on the ZnO
support were fixed at 2 wt% and 20 wt%, respectively. For
comparison, the Pd�Zn and Pd�Zr catalysts, both with 2 wt%
loading amount of Pd, were also prepared by a co-
precipitation method using the aforementioned procedure.
Finally, all the prepared Pd-based catalysts were ground into
powder (�100 mesh) and pressed to form granules with sizes
of 40e60 meshes.
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
Characterization methods
The actual contents of Pd and Nb in the prepared catalysts
were measured with inductively coupled plasma atomic
emission spectroscopy (Optima 8000 DV, PerkinElmer). In
order to ensure complete dissolution, a certain amount of
aqua regia was used to digest the samples prior to the ICP
determination. The physical properties of the Pd-based cata-
lysts like specific surface area (SBET) and pore volume (Vp) were
determined by nitrogen physisorption at �196 �C in a Micro-
metrics Tristar II 3020 apparatus. Before the measurement,
the test sample was degassed under high vacuum at 200 �C for
6 h to remove the moisture from the surface and pore. Ac-
cording to the nitrogen adsorption-desorption isotherms, the
specific surface areas of the Pd-based catalysts were calcu-
lated by means of Brunauer�Emmett�Teller (BET) equation.
The surface morphologies of the reduced Pd-based cata-
lysts were characterized with a Zeiss Supra 55 scanning
electron microscope operating at a low accelerating voltage of
5 kV. A small quantity of test sample was uniformly attached
to a conductive carbon tape on a round aluminum slab. The
microstructures of the reduced Pd-based catalysts were
investigated by high-resolution transmission electron micro-
scopy (HRTEM) using a JEM 2100F scanning transmission
electron microscope with an acceleration voltage of 200 kV.
This transmission electron microscope was equipped with a
high-angle annular dark field detector and an energy disper-
sive spectrometer. In a typical measurement, a small amount
of test sample was mixed thoroughly with anhydrous ethanol
by using ultrasonic dispersion (100 kHz) for half hour, and a
drop or two of the suspension was dripped on a holey copper
grid.
The phase compositions of the Pd-based catalysts after
calcination and reduction were studied by power X-ray
diffraction technique with a Rigaku Ultima IV X-ray diffrac-
tometer using Cu Ka as radiation at 40 kV and 30mA. The data
for XRD patterns of the test samples were collected in the
range of 2q ¼ 10e90� with a scanning rate of 4�$min�1. The
acquired diffraction patterns of the samples were analyzed by
MDI Jade 5.0 and compared with those of standard database.
In addition, the particle sizes of ZnO crystals and Pd�Zn alloy
in the reduced Pd-based catalysts were estimated from the
XRD patterns using the Debye-Scherrer equation.
In order to evaluate the reducibility of the Pd-based cata-
lysts, the H2-TPR experiment was conducted using a Micro-
metrics Autochem II 2920 apparatus equipped with a thermal
conductivity detector. Typically, 50 mg of test sample was
held by some high-purity quartz wool and fixed in a U-shaped
quartz tube reactor that was mounted inside an electric
heating furnace. After pretreating at 300 �C for 1 h and then
cooling at room temperature in a flow of high-purity He
(30 ml min�1), the test sample was heated from 50 to 750 �C at
a heating rate of 10 �C$min�1 with a flow of 5 vol% H2/Ar
mixture gas (30 ml min�1). The signal of hydrogen consump-
tion was followed by the thermal conductivity detector during
the continuous increase of the temperature.
The Pd dispersion of the Pd-based catalysts was deter-
mined by CO chemisorption with the same instrument as
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x4
above. In a typical measurement, 100 mg of test sample was
first outgassed at 300 �C for 1 h in a flow of high-purity He
(30 ml min�1), and cooled down to room temperature, and
then in situ reduced at 350 �C for 2 h under a flow of 5 vol% H2/
Ar mixture gas (30 ml min�1). Following the reduction, the U-
shaped quartz tube reactor was purged with flowing He
(30 ml min�1) at the same temperature for 0.5 h and cooled
down to 50 �C. The CO chemisorption measurement was
performed by injecting numbers of CO pulses (0.5 mL every
time) until saturation with CO for the test sample at 50 �Cunder atmospheric pressure. The Pd dispersion of the Pd-
based catalysts was calculated by assuming that the CO:Pd
stoichiometry on the catalyst surface was one.
To measure the acidity of the Pd-based catalysts, the NH3-
TPD analysis was carried out by using a TP-5080 dynamic
adsorption instrument, from Tianjin Xianquan Co. Ltd., which
was equipped with a thermal conductivity detector. Prior to
the NH3 adsorption, 100 mg of test sample was placed in a
quartz tubular reactor and pretreated at 300 �C for 1 h in a flow
of high-purity He (30 ml min�1). Subsequently, the NH3
(30 ml min�1) was adsorbed on the sample at 100 �C for 0.5 h
and switched to flowing He (30 ml min�1) for 1.5 h to remove
the physically adsorbed NH3 from the sample surface. Finally,
the test samplewas heated from 100 to 700 �C at a heating rate
of 10 �C$min�1 and the signal of NH3 desorption was moni-
tored by the thermal conductivity detector.
The surface chemical state of the Pd-based catalysts was
investigated by a Thermo Scientific™ K-Alpha X-ray photo-
electron spectrometer with an Al Ka radiation source. Before
themeasurement, the test samples were reduced at 350 �C for
2 h in a flow of 5 vol% H2/Ar mixture gas (70 ml min�1). The
binding energies of all elements in the Pd-based catalysts
were calibrated by using the C1s peak at 284.8 eV from neutral
carbon as the reference. The processing and curve-fitting for
the obtained XPS spectra were carried out by means of
Avantage software. The intensity ratios of different kinds of
oxygen species on the catalyst surface were estimated by
using the areas of the binding energy peaks of corresponding
oxygen species. The Raman spectra of the Pd-based catalysts
were recorded at room temperature by using a Thermo Sci-
entific™ DXR2xi Raman imaging microscope with 532 nm
laser excitation. Prior to the measurement, the test samples
were reduced at 350 �C for 2 h in a flow of 5 vol%H2/Armixture
gas (70 ml min�1). The laser beam was focused on the test
sample by adjusting the location of sample platform through
the laser safety goggles, and the scanning range was from 80
to 1300 cm�1.
Catalytic activity tests
The activity of the Pd-based catalysts was measured for
methanol steam reforming by using a self-built reactor sys-
tem. Fig. S1 displayed the schematic diagram and the side
view of experimental apparatus that contained the reactor,
vaporizer, condenser, metering pump, micro gas chroma-
tography (GC), etc. As shown in Fig. S1, methanol steam
reforming was performed in a quartz tubular fixed-bed
reactor with a length of 350 mm and an inside diameter of
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
6 mm at atmospheric pressure. In a typical measurement,
400 mg of calcined Pd-based catalysts (40e60 mesh) and
800 mg of quartz sand (40e60 mesh) were fully mixed and
loaded in the central part of the quartz tube, corresponding
to the constant temperature section of reactor. Then, the
quartz tube was vertically fixed inside an electric heating
furnace, and the temperature of reactor was controlled by
means of a thermocouple inside the furnace. Meanwhile, the
temperature of catalyst bed was monitored by a K-type
thermocouple inserted into the catalyst bed. Before the cat-
alytic activity test, the Pd-based catalysts were in situ
reduced in the quartz tubular fixed-bed reactor at 350 �C for
2 h with a flow of 5 vol% H2/Ar mixture gas (70 ml min�1).
Following the reduction, the temperature of the reactor was
cooled to 200 �C under the flowing H2/Ar mixture gas. Next,
the piping system was flushed with a flowing nitrogen gas
(70 ml min�1) for half hour to remove the H2/Ar mixture gas
inside the reactor. A pre-mixture feed of water andmethanol
in a molar ratio of 1.2:1 was continuously fed into the
vaporizer operating at 160 �C with a flow rate of 3 mL h�1 by
using a metering pump. The resulting vapor mixture was
introduced into the reactor by flowing nitrogen gas
(70 ml min�1). The effects of reaction temperature, nitrogen
gas flow rate and feed rate of water-methanol mixture on
methanol steam reforming were studied respectively by
changing the reaction variables in the ranges of 200e350 �C,20e90 ml min�1 and 0.6e8.4 mL h�1.
After a certain time of operation (ca. 40 min), the product
steam mixture was cooled sufficiently by a chilled condenser
to trap the residual methanol and water. After drying, the
gaseous products were analyzed on line by an Inficon 3000
micro GCwith nitrogen gas as an internal standard substance.
This micro GC was equipped with two thermal conductivity
detectors, and the chromatographic columns were respec-
tively Plot Q (analysis of CO2) and molecular sieve (analysis of
H2, N2, CH4 and CO) that were used in parallel connection
mode. The carrier gas for the GC was high-purity Ar, and the
operational conditions were as follows: the temperatures of
the automatic injector and detector were 80 �C, and those of
the Plot Q and molecular sieve columns were 50 and 70 �Crespectively. A standard curve was achieved from the GC
analysis of standard gases with various concentrations of H2,
N2, CO2, CO and CH4, which permitted the quantitative ana-
lyses of the gas component concentrations. After 2 h time on
steam, a stable state of the catalytic performance was reached
by the GC analysis (sampling every 20 min) of the gaseous
products at a given reaction temperature, and data acquisition
was performed after 3 h of reaction time. The achieved cata-
lytic activities of the Pd-based catalysts were averages of 3e4
data points at the same reaction conditions. It is worthy to
mention that the H2, CO and CO2 are detected in the gaseous
products, but no methane. The methanol conversion and
products selectivity were calculated by using Eqs. (1)e(3),
where FCH3OH,in, FCO,out and FCO2,out were themolar flow rate of
CH3OH feed in, CO feed out and CO2 feed out, respectively. The
values of carbon balance for methanol steam reforming
(based on methanol conversion) were higher than 98% for
each set of data. The uncertainties of methanol conversion
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g en en e r g y x x x ( x x x x ) x x x 5
and products selectivity for the micro GC analysis were less
than 1%.
Methanol conversionð%Þ ¼ FCO;out þ FCO2 ;out
FCH3OH;in� 100 (1)
CO2 selectivityð%Þ ¼ FCO2 ;out
FCO;out þ FCO2 ;out� 100 (2)
CO selectivityð%Þ ¼ FCO;out
FCO;out þ FCO2 ;out� 100 (3)
Besides the aforementioned activity test, the temperature
programmed surface reaction (TPSR) of methanol and water
on the Pd-based catalysts was also studied in a quartz micro
reactor coupled with a ThermoStar™ GSD 320 T series mass
spectrometer. Typically, 100 mg of Pd-based catalysts was
loaded and in situ reduced in the reactor at 350 �C for 2 h with
a flow of 5 vol% H2/Ar mixture gas (70 ml min�1). After the
reduction, the temperature of the reactor was decreased to
room temperature and the system was purged with a flowing
Ar (50 ml min�1) for half hour. Subsequently, the saturated
methanol and water vapors (molar ratio z 1.2:1) were carried
into the reactor by a flowing Ar (50mlmin�1) for 20min from a
bubbler. The CH3OH�H2O�TPSRwas carried out by increasing
the temperature from 40 to 500 �C at a heating rate of
5 �C$min�1 under a flowing Ar (50 ml min�1). The reaction
products were on linemonitored bymass spectrometer on the
basis of signal intensities of H2 (m/z ¼ 2), CH4 (m/z ¼ 15), H2O
(m/z ¼ 18), CO (m/z ¼ 28), HCHO (m/z ¼ 30), CH3OH (m/z ¼ 31),
CO2 (m/z ¼ 44), HCOOH (m/z ¼ 46), and HCOOCH3 (m/z ¼ 60).
Results and discussion
Catalysts characterization
As shown in Table 1, the physical and chemical properties of
the Pd-based catalysts derived from different characterization
methods are given. The actual contents of Pd in the catalysts
determined by ICP were slightly lower than the nominal
values andmaintained at about 1.5 wt% loading. Similarly, the
actual loadings of Nb in the catalysts were also less than the
Table 1 e Physicochemical properties of the Pd-based catalyst
Catalyst Pdcontenta
Nbcontenta
SBETb
(m2$g�1)Vp
b (cm3$
Pd�Zn 1.64 e 39 0.301
Pd�Zr 1.73 e 106 0.084
Pd�Zr�Zn 1.59 e 70 0.305
0.5Nb/Pd�Zr�Zn 1.42 0.38 68 0.310
1Nb/Pd�Zr�Zn 1.45 0.86 67 0.333
4Nb/Pd�Zr�Zn 1.50 3.22 65 0.313
7Nb/Pd�Zr�Zn 1.48 6.12 63 0.302
10Nb/Pd�Zr�Zn 1.44 8.79 60 0.240
a Measured from ICP.b Measured from BET.c Estimated from XRD.d Measured from CO chemisorption.e Estimated from XPS.
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
nominal values, which was probably attributable to the
metals loss in the process of catalyst preparation. In Fig. S2,
the Pd�Zn and Pd�Zr catalysts showed two different types of
the N2 adsorption�desorption isotherms. The Pd�Zn catalyst
possessed relatively small specific surface area, large pore size
distribution and pore volume, whereas the Pd�Zr catalyst was
the opposite (Fig. S2 and Table 1). This indicated that the
Pd�Zn catalyst had mesoporous structure, whilst the struc-
ture of Pd�Zr catalyst was more compact. Similar to the
Pd�Zn catalyst, the N2 adsorption�desorption isotherm of
Pd�Zr�Zn catalyst displayed a typical type IV isotherm and
H1 hysteresis loop. However, the Pd�Zr�Zn catalyst
possessed high specific surface area and small pore size dis-
tribution, showing its good structure. When different Nb
loadings were added, the N2 adsorption�desorption isotherm
of Nb-modified Pd�Zr�Zn catalysts remained the same shape.
However, the pore diameter distribution, specific surface area
and pore volume decreased in the modified Pd�Zr�Zn cata-
lysts containing a high loading of Nb,most probably due to the
blockage of channel by the excessive amounts of Nb species.
Fig. S3 illustrates the SEM images of the Pd-based catalysts. It
was evident that a loose structure existed in the Pd�Zn
catalyst, whereas the Pd�Zr catalyst showed a dense
massive structure with a large number of fine particles
deposited on the surface. For the Pd�Zr�Zn catalyst, a loose
structure with many small granules attached to the surface
was observed. However, with an increase in the Nb loading,
the microstructures of Nb-modified Pd�Zr�Zn catalysts were
changed from initially loose into compact, in line with the
results of BET measurements.
Fig. 1 displays the XRD patterns of the Pd-based catalysts
upon calcination and reduction. In the case of calcined Pd�Zr
catalyst (Fig. 1a), only weak diffraction peaks at 2q ¼ 30.2� and50.4� were ascribed to the presence of tetragonal phase of ZrO2
(t-ZrO2, PDF#17-0923), suggesting a poor crystallization of ZrO2
[28]. In contrast, the existence of ZnO phase (PDF#36-1451)
gave rise to significant diffraction peaks for calcined Pd�Zn
catalyst at 2q ¼ 31.7�, 34.4�, 36.2�, 47.5�, 56.6�, 62.8�, 66.4�,67.9�, 69.1�, 72.6�, 76.9�, 89.6� [29], and the ZnO phase was
present in all PdZn-containing catalysts. But for the un-doped
and Nb-doped Pd�Zr�Zn catalysts, the strength and width of
diffraction peaks for ZnO phase were weakened and
s.
g�1) dZnOc
(nm)dPdZn
c
(nm)Pd dispersiond (%) Oads/Olatt
molar ratioe
20.7 19.9 4.2 0.52
e e 29.5 0.69
19.4 e 24.1 0.60
17.5 12.8 7.9 0.43
17.2 12.5 7.6 0.43
16.7 12.0 7.1 0.41
17.7 11.2 6.7 0.41
17.0 10.4 6.5 0.40
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 1 e XRD patterns of the Pd-based catalysts upon calcination (a) and reduction (bed).
Fig. 2 e H2-TPR profiles of the Pd-based catalysts.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x6
broadened respectively with the increase of Nb content. This
meant that the introduction of Nb was favorable for the
dispersion of ZnO phase to a certain extent. Therefore, smaller
crystallite sizes (Table 1) of ZnO measured by the diffraction
peak at 2q ¼ 56.6� were present in the Nb-doped Pd�Zr�Zn
catalysts [21]. It should be emphasized, however, that no
characteristic peaks of PdO (PDF#41-1107, 2q ¼ 33.8�) [20] andNb species were detected in the calcined Pd-based catalysts,
which might be due to their homogeneous distribution on the
catalyst surface.
After reduction at 350 �C for 2 h with a flow of 5 vol% H2/Ar
mixture gas (Fig. 1b�d), the diffraction peaks of t-ZrO2 at
2q ¼ 30.2� and 50.4� were still evident in the Pd�Zr catalyst.
Similarly, the characteristic peaks of ZnO phase were also
present in all PdZn-containing catalysts. As displayed in
Fig. 1c, the diffraction peaks that can be assigned to Pd�Zn
alloy (PDF#06-0620, 2q ¼ 41.2� and 44.1�) were clearly
observed on Pd�Zn and Nb-modified Pd�Zr�Zn catalysts [26].
But surprisingly, such characteristic peaks corresponding to
Pd�Zn alloy were not detected in Pd�Zr�Zn catalyst. This
indicated that no Pd�Zn alloy phase was formed in the
Pd�Zr�Zn catalyst and metallic Pd (PDF#46-1043, 2q ¼ 40.1�
and 46.6�) [30] remained after the reduction process, consis-
tent well with the results of TEM and XPS (see below). In Table
1, the particle sizes of the Pd�Zn alloy on the Pd�Zn and Nb-
modified Pd�Zr�Zn catalysts, estimated from the Debye-
Scherrer equation, were found to be about 20 nm and 12 nm.
This suggested that the addition of Nb facilitated the forma-
tion of fine Pd�Zn alloy particles. Additionally, it was
observed that the patterns of ZnO for un-doped and Nb-doped
Pd�Zr�Zn catalysts shifted slightly to a higher 2q angle with
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
the introduction of Nb (Fig. 1d). A possible explanation for this
was that it could probably have been due to substitution of
Zn2þ ions with other metal species such as Nb and/or Pd into
the ZnO lattices [31,32]. This is consistent with the results of
XPS and Raman (see below).
As depicted in Fig. 2, the H2-TPR measurement was per-
formed to probe the reducibility of the Pd-based catalysts.
Only a weak negative peak at about 65 �C and a small positive
peak at about 105 �C were present in the Pd�Zr catalyst. In
contrast, the Pd�Zn catalyst exhibited aweaknegative peak at
about 70 �C, and three significant positive peaks in the range of
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 3 e NH3-TPD profiles of the Pd-based catalysts.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g en en e r g y x x x ( x x x x ) x x x 7
85e200 �C, 220e470 �C and 470e650 �C respectively. According
to the literature [30,33], the weak negative peak for the Pd�Zr
and Pd�Zn catalysts at 50e80 �C was due to the decomposi-
tion of PdHx produced from the reduction of PdO species at
room temperature, while the hydrogen consumption peak at
85e200 �C was because of the reduction of the residual frac-
tion of PdO species. In addition, the large hydrogen con-
sumption peaks for the Pd�Zn catalyst at 220e650 �C were
because of the reduction of ZnO in the presence of Pd [33,34].
That is, the ZnO species in the Pd�Zn catalyst are reduced to
Zn and transformed into the Pd�Zn alloy. Similar to the
Pd�Zn catalyst, the Pd�Zr�Zn catalyst also displayed a weak
negative peak and three obvious positive peaks. But compared
with Pd�Zn catalyst, the hydrogen consumption peak (labeled
a) at 85e270 �C for the Pd�Zr�Zn catalyst was increased. Two
obvious hydrogen consumption peaks (labeled g and d) at
400e720 �C for the Pd�Zr�Zn catalyst moved significantly to
higher temperature when compared to the Pd�Zn catalyst.
These results indicated that higher temperature of hydrogen
treatment was required for the reduction of PdO and ZnO
species in the Pd�Zr�Zn catalyst, which might mean that the
addition of Zr to Pd-Zn catalyst changed the contact state of
PdO and ZnO species (see below).
In comparison to the Pd�Zr�Zn catalyst, the strength of
the hydrogen consumption peak a corresponding to the
reduction of PdO for the Nb-modified Pd�Zr�Zn catalysts at
about 130 �C decreased dramatically or even disappeared. A
possible explanation for this could be that the PdO species in
the Nb-modified Pd�Zr�Zn catalysts has been completely
reduced with a flowing hydrogen atmosphere at room tem-
perature [34]. Furthermore, the weak negative peak and the
high temperature peaks (i.e., g and d) for the Nb-modified
Pd�Zr�Zn catalysts at 50e120 �C and 410e750 �C respec-
tively were shifted to higher temperature when compared
with the Pd�Zr�Zn catalyst. But interestingly, a weak
hydrogen consumption peak b at about 310 �C was observed
on the Nb-modified Pd�Zr�Zn catalysts. According to litera-
ture and TPR analysis of ZnO, ZrO2 and Nb2O5 (Fig. S4), the
hydrogen consumption peak b for the Nb-modified Pd�Zr�Zn
catalysts at about 310 �C was attributable to the reduction of
ZnO [34], which led to the formation of Pd�Zn alloy. Table S1
lists the amounts of consumed hydrogen in the course of TPR
measurement. As can be seen, introduction of Nb into the
Pd�Zr�Zn catalyst decreased the amounts of consumed
hydrogen at 85e200 �C and produced a new hydrogen con-
sumption peak at 270e400 �C, which were ascribed to the
reduction of PdO and ZnO species respectively. Based on the
above analysis, addition of Nb to the Pd�Zr�Zn catalyst
favored the reduction of ZnO species and the formation of
Pd�Zn alloy. However, the ZnO species in the Pd�Zr�Zn
catalyst could not reduce to Zn and convert to the Pd�Zn alloy
with a flowing hydrogen atmosphere at 350 �C. This is in
agreement with the result of XRD.
As indicated in Fig. 3, the NH3-TPD technique was used to
elucidate the difference in the acidity of the Pd-based cata-
lysts. According to the literature [35,36], the surface acid
strength of catalyst can be classified as weak acid, medium
acid, and strong acid that correspond to the NH3 desorption
peak at 120e250, 250e420 and 420e700 �C respectively. In
Fig. 3, the Pd�Zr catalyst only exhibited a significant NH3
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
desorption peak at 120e400 �C, indicating that the acidity of
the Pd�Zr catalyst was mostly a weak-medium one. In
contrast, two obvious NH3 desorption peaks in the tempera-
ture range of 240e310 and 450e540 �C were observed in the
Pd�Zn catalyst, showing the existence of some strong acid
sites on the catalyst surface. Similarly, the Pd�Zr�Zn catalyst
also possessed two significant NH3 desorption peaks at
120e400 and 430e590 �C respectively. But the strength of the
NH3 desorption peak at 430e590 �C for the Pd�Zr�Zn catalyst
was greatly higher than that for the Pd�Zn catalyst, suggest-
ing that more strong acid sites existed in the Pd�Zr�Zn
catalyst surface. Interestingly, with the addition of Nb, a sig-
nificant new NH3 desorption peak for the Nb-modified
Pd�Zr�Zn catalysts was observed in the range of
270e360 �C. Furthermore, the intensity of NH3 desorption
peak at 460e650 �C for the Nb-modified Pd�Zr�Zn catalyst
improved with the increase in Nb content. This indicated that
the introduction of Nb into the Pd�Zr�Zn catalyst increased
both the acid quantity and strength. To illustrate this point,
the acidities of the Pd-based catalysts are summarized in
Table S1. It is apparent that the addition of Nb species
increased the acid strength of Pd�Zr�Zn catalyst, and the
10Nb/Pd�Zr�Zn catalyst possessed the highest concentration
of acid sites.
Fig. 4 shows the TEM images of the reduced Pd-based cat-
alysts. As can be seen, evident agglomerates were present in
the Pd�Zn catalyst (Fig. 4a). In contrast, the Pd�Zr�Zn and
1Nb/Pd�Zr�Zn catalysts exhibited dispersed particles with no
obvious agglomeration (Fig. 4g and n), reflecting their superior
structure. Owing to the similar morphologies for the Pd spe-
cies and ZnO species, the distinguishing work is difficult. For
this reason, the EDS analysis was also carried out along with
the TEM measurement. As displayed in Fig. S5, trace amounts
of Pdwere detected in the selected areas that were assigned to
the Pd species and marked with red circles in Fig. 4. The par-
ticle sizes of Pd species obtained from TEM measurement in
the reduced Pd-based catalysts were estimated to be 6e9 nm,
which were less than those obtained from XRD analysis (see
Table 1). This discrepancy might be due to the inherent dif-
ference in detecting mechanisms of TEM and XRD [26]. To
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 4 e TEM images (a, b), HAADF-STEM image (c) of the reduced Pd¡Zn catalyst and the corresponding EDX element
mapping images (def); TEM images (g, h), HAADF-STEM image (i) of the reduced Pd¡Zr¡Zn catalyst and the corresponding
EDX element mapping images (jem); TEM images (n, o), HAADF-STEM image (p) of the reduced 1Nb/Pd¡Zr¡Zn catalyst and
the corresponding EDX element mapping images (qeu).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x8
investigate the Pd species in the reduced Pd-based catalysts,
HRTEM measurement was performed. According to related
literature [26,29], the lattice spacings of Pd�Zn alloy (111) and
metallic Pd (111) in the reduced PdZn-containing catalysts are
2.19 �A and 2.246 �A respectively. As displayed in Fig. 4h, a
particle with 2.25 �A lattice spacing was observed in the
reduced Pd�Zr�Zn catalyst. In contrast, the lattice spacings of
the particles in the reduced Pd�Zn and 1Nb/Pd�Zr�Zn cata-
lysts were found to be 2.2 �A (Fig. 4b and o). This indicated that
the Pd species in the reduced Pd�Zr�Zn catalyst were iden-
tified as metallic Pd, whilst the Pd�Zn alloy was exhibited in
the reduced Pd�Zn and 1Nb/Pd�Zr�Zn catalysts, in line with
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
the result of XRD. However, it should be noted that the dif-
ference (<3%) in the lattice spacings of Pd�Zn alloy (111) and
metallic Pd (111) was very small.
In order to get a deeper understanding of the Pd species in
the reduced Pd-based catalysts, the HAADF-STEM images of
the reduced Pd-based catalysts and the corresponding EDX
element mapping images are also displayed in Fig. 4. Gener-
ally, the brighter the areas in the HAADF-STEM image, the
bigger the atomic number is [37,38]. Therefore, in this study,
the brighter areas in the HAADF-STEM images (blue circles in
Fig. 4) were Pd-rich. Obviously, compared with the reduced
Pd�Zn and 1Nb/Pd�Zr�Zn catalysts, the bright areas of
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g en en e r g y x x x ( x x x x ) x x x 9
reduced Pd�Zr�Zn catalyst were more concentrated. But
interestingly, the bright areas of reduced Pd�Zr�Zn catalyst
exhibited low density of Pd distribution and high density of Zr
distribution (white circles in Fig. 4), though all the reduced
PdZn-containing catalysts showed homogeneous distribu-
tions of O and Zn. In contrast, relatively low density of Zr
distribution and homogeneous distribution of Pd were
observed in the reduced 1Nb/Pd�Zr�Zn catalyst (Fig. 4s and
u). Similarly, a homogeneous distribution of Pd was also
exhibited in the reduced Pd�Zn catalyst (Fig. 4f). The main
factor that caused the above different bright areas and
element distributions was most probably due to the presence
of different Pd species in the reduced PdZn-containing cata-
lysts [38]. As verified by XRD analysis, the Pd species in the
reduced Pd�Zn and Pd�Zr�Zn catalystswere Pd�Zn alloy and
metallic Pd respectively. A possible explanation for this was
that the addition of Zr into the Pd�Zn catalyst enhanced the
interaction between Pd and Zr species and decreased the
probability of contact of Pd and Zn species (see Fig. 4k�m),
which inhibited the formation of Pd�Zn alloy. In contrast, the
introduction of Nb into the Pd�Zr�Zn catalyst diminished the
effect of Zr species and increased the intimate contact be-
tween Pd and Zn species (see Fig. 4reu and Raman analysis),
which favored the formation of Pd�Zn alloy.
Besides the measurement of EDX element mapping, CO
chemisorption was carried out to evaluate the Pd dispersion
on the reduced Pd-based catalysts. It can be observed from
Table 1 that the Pd dispersion of Pd�Zn catalyst was only 4.2%,
whereas the Pd�Zr and Pd�Zr�Zn catalysts exhibited rela-
tively high Pd dispersions that were 29.5% and 24.1% respec-
tively. But unexpectedly, the Nb-modified Pd�Zr�Zn catalysts
showed smaller adsorbed amounts of CO and only about 7% of
Pd dispersionwas achieved. In a series of studies on PdZn/ZnO
catalysts with different ZnO precursors, Mendes et al. [29]
found that the presence of Pd�Zn alloy in the reduced PdZn/
ZnO catalyst significantly reduced the amount of chemisorbed
CO and therefore obtained smaller Pd dispersion. Similar
behavior was also reported elsewhere for the reduced PdZn-
containing catalysts such as Pd/ZnO and Zn�Pd/C catalysts
[21,22]. In this work, the Pd�Zr and Pd�Zr�Zn catalysts pre-
sented similar dispersions that were higher than 20%. How-
ever, obviously different from the Pd�Zr and Pd�Zr�Zn
catalysts, very low Pd dispersions (<8%) were obtained in the
Pd�Zn and Nb-modified Pd�Zr�Zn catalysts. This suggested
that the Pd species in the reduced Pd�Zn and Nb-modified
Pd�Zr�Zn catalysts existed in the state of Pd�Zn alloy,
which agreed with the result of XRD.
As shown in Fig. 5, XPS measurement was conducted to
identify the surface chemical state of reduced Pd-based cata-
lysts. In Fig. 5a, it was clear that the peaks derived from the Zn,
O, C, Zr, and Nb elements were detected in the XPS survey
spectra of Pd-based catalysts. But possibly because of its weak
signal, the peak corresponding to Pd element was not found in
the survey spectra. Based on previous reports [22,30], the
characteristic peaks for Pd 3d5/2 signal at about 335.1, 335.6
and 336.9 eV were assigned to the presence of metallic Pd,
Pd�Zn alloy and Pd2þ species respectively. As illustrated in
Fig. 5b, only two peaks at about 335.6 and 340.8 eV respectively
corresponding to Pd 3d5/2 and Pd 3d3/2 were observed in the
reduced Pd�Zn catalyst. This suggested that the Pd species in
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
the reduced Pd�Zn catalyst was identified as Pd�Zn alloy, in
line with the result of XRD. Different from the Pd�Zn catalyst,
the Pd 3d5/2 peak for the reduced PdZr-containing catalysts
overlappedwith the Zr 3p3/2 peak [21]. To deepen investigation
on the Pd species, the Pd 3d5/2 peak for the reduced PdZr-
containing catalysts was deconvoluted from the Zr 3p3/2
peak by assuming that the energy separation between the Pd
3d5/2 and Pd 3d3/2 was maintained at 5.25 eV [21]. It was found
that the binding energies for the Pd 3d5/2 and Pd 3d3/2 peaks in
the reduced Pd�Zr and Pd�Zr�Zn catalysts were about 335.1
and 340.3 eV respectively. But for the Nb-modified Pd�Zr�Zn
catalysts, the Pd 3d5/2 and Pd 3d3/2 peaks showed a slight shift
to higher binding energies that were about 335.6 and 340.8 eV
respectively. These results proved that the Pd species in the
reduced Nb-modified Pd�Zr�Zn catalysts was Pd�Zn alloy,
whilst the metallic Pd remained in the reduced Pd�Zr and
Pd�Zr�Zn catalysts. Additionally, the intensities of Pd 3d
peaks for Nb-modified Pd�Zr�Zn catalysts were greatly lower
than those for Pd�Zn catalyst and decreased with the
enhancement of Nb loading. A possible explanation for this
was that the introduction of Nb favored the formation of fine
Pd�Zn alloy (see Table 1).
Fig. 5 presents the Zn 2p, Nb 3d, Zr 3d, and O 1s spectra of
the reduced Pd-based catalysts. In Fig. 5c, all reduced PdZn-
containing catalysts exhibited two strong peaks centered at
about 1021.3 and 1044.4 eV and were assigned to Zn 2p3/2 and
Zn 2p1/2 respectively. This indicated that the zinc species in
the reduced PdZn-containing catalysts weremainly presented
in a state of Zn2þ [29]. In Fig. 5d, the doublet peaks at about
207.1 and 210.0 eV respectively ascribed to Nb 3d5/2 andNb 3d3/
2 of Nb5þ were evident in the reduced Nb-modified Pd�Zr�Zn
catalysts [39]. The Nb 3d peaks for the reduced Nb-modified
Pd�Zr�Zn catalysts slightly shifted to higher binding energy
with an increase in the Nb loading, whilst the corresponding
Zn 2p peaks moved to lower binding energy. The explanation
for this could be that, due to Nb-doping in the ZnO lattices,
resulted in the intimate contact between the Nb and Zn spe-
cies, thereby producing charge transfer from the niobium ion
to the zinc species [32]. In Fig. 5e, two peaks centered at about
182.1 and 184.5 eV for the reduced PdZr-containing catalysts
were assigned to the Zr 3d5/2 and Zr 3d3/2 of Zr4þ respectively
[40]. Similar to theNb 3d peaks, the Zr 3d peaks for the reduced
PdZr-containing catalysts also displayed a slight shift to
higher binding energy, whichmight be due to the fact that the
incorporation of Nb into the Pd�Zr�Zn catalyst decreased the
amount of oxygen vacancies in the ZrO2 lattices [28,41]. In
Fig. 5f, the asymmetric O 1s spectrum for the reduced Pd-
based catalysts was deconvoluted into two peaks centered at
about 530.2 and 531.7 eV respectively, indicating the presence
of two different kinds of oxygen species on the catalyst sur-
face [29]. The lower binding energy peak marked as Olatt was
ascribed to the lattice oxygen connected to metal ions in the
ZnO and/or ZrO2 phases, whilst the higher binding energy
peak labeled as Oads was considered to be the adsorbed oxygen
species on the catalyst surface [42]. Documented evidence
showed that the surface adsorbed oxygen speciesweremainly
derived from the surface hydroxyl-like groups and oxygen
vacancies of metal oxides such as ZnO and/or ZrO2 [29,43].
Accordingly, the changes in the intensity ratio of Oads/Olatt, to
a certain extent can reflect the variation in the relative
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 5 e X-ray photoelectron spectroscopy profiles of the reduced Pd-based catalysts: survey spectra (a); Pd 3d (b); Zn 2p (c);
Nb 3d (d); Zr 3d (e); O 1s (f).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x10
abundance of oxygen vacancies on the catalyst surface [29]. To
compare the amount of oxygen vacancies on the catalyst
surface, Table 1 lists the intensity ratio of Oads/Olatt for the
reduced Pd-based catalysts. As can be seen, the introduction
of Nb into Pd�Zr�Zn catalyst reduced the intensity ratio of
Oads/Olatt, indicating that the addition of Nb to Pd�Zr�Zn
catalyst decreased the concentration of oxygen vacancies on
the catalyst surface.
To further investigate the structural information, Raman
spectra of the reduced Pd-based catalysts were recorded at
80�1300 cm�1. As displayed in Fig. 6, the reduced Pd�Zn
catalyst exhibited five obvious Raman peaks centered at
about 100, 330, 437, 580, and 638 cm�1. The peaks centered at
around 100 and 330 cm�1 were assigned to the vibrations of
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
heavy Zn sub-lattice and Zn�O bonds in the ZnO lattices
respectively [44,45]. In addition, the peak centered at about
437 cm�1 was ascribed to the ZnO nonpolar optical phonon
mode that derived from the characteristic peaks of wurtzite
structure, whilst the peak at around 580 cm�1 was considered
to be the longitudinal optical phonon mode caused by the
oxygen vacancies in the ZnO lattices [46,47]. Slightly different
from the above mentioned peaks, the Raman peak at about
638 cm�1 for the reduced Pd�Zn catalyst was attributable to
the Pd-doping in the ZnO lattices [31,48], which caused an
intimate contact between the Pd and Zn species, and thus
easily produced the Pd�Zn alloy. In the case of the reduced
Pd�Zr catalyst, only a broad Raman peak centered at about
570 cm�1 was observed, suggesting the existence of the
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 6 e Raman spectra of the reduced Pd-based catalysts.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g en en e r g y x x x ( x x x x ) x x x 11
amorphous ZrO2 [49], which was in agreement with the result
of XRD. For the reduced Pd�Zr�Zn catalyst, the characteristic
peaks at 437 and 100 cm�1 respectively corresponding to the
wurtzite structure and the vibrations of Zn�O bonds in the
ZnO lattices were still evident. In addition, the reduced
Pd�Zr�Zn catalyst exhibited a significant Raman peak at
around 580 cm�1, indicating the presence of large amount of
oxygen vacancies. However, the Raman peak at about
638 cm�1 arising from the Pd-doping in the ZnO lattices was
not observed in the reduced Pd�Zr�Zn catalyst. This indi-
cated that the Pd species in the reduced Pd�Zr�Zn catalyst
cannot easily access the ZnO lattices, which might be due to
the fact that the addition of Zr to Pd�Zn catalyst decreased the
Fig. 7 e Activity results for methanol steam reforming performe
H2 production rate (b); CO selectivity (c); CO production rate (d).
mixture, 0.05 mL min¡1; molar ratio of water to methanol, 1.2:1
of quartz sand, 800 mg; temperature of vaporizer, 160 �C; press
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
interaction between Pd and Zn species. When different Nb
loadings were introduced into the Pd�Zr�Zn catalyst, the
peak at about 580 cm�1 corresponding to the presence of ox-
ygen vacancies in the ZnO lattices decreased and disappeared
rapidly. But interestingly, in the case of reduced Nb-modified
Pd�Zr�Zn catalysts, the intensities of peaks centered at
about 850, 638 and 437 cm�1 that were respectively assigned to
the stretching mode of Nb�O bonds, Pd-doping in the ZnO
lattices and characteristic peaks of wurtzite structure gradu-
ally enhanced with the increase in Nb loading [39,48,50]. This
indicated that the addition of Nb to Pd�Zr�Zn catalyst facili-
tated the incorporation of Pd into the ZnO lattices, but
decreased the amount of oxygen vacancies on the catalyst
surface. This agrees with the results of XRD and XPS.
Methanol steam reforming study
Catalytic activity measurementFig. 7 displays the activity results for methanol steam
reforming carried out on different Pd-based catalysts at the
atmospheric pressure and at the temperature range of
200e350 �C. For comparison, the catalytic activities of pure
supports, ZnO, ZrO2 and Nb2O5, were also tested under the
same experimental conditions. But the pure supports were
found to be completely inactive (not shown in Fig. 7), indi-
cating that the Pd species as the active sites were essential for
hydrogen production by methanol steam reforming. As illus-
trated in Fig. 7, the methanol conversion and H2 production
rate for all the Pd-based catalysts gradually enhancedwith the
increase in reaction temperature. This could be due to the fact
that methanol steam reforming was an endothermic reaction
[51]. Among the tested Pd-based catalysts, the Pd�Zr catalyst
achieved a relatively low catalytic activity, exhibiting 5e85%
d on different Pd-based catalysts: methanol conversion (a);
Experimental parameters: feed rate of methanol-water
; N2 flow rate, 70 mL min¡1; mass of catalyst, 400 mg; mass
ure of reactor, 1 bar.
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x12
methanol conversion and 15.4e238.3 mmol h�1$gcat�1 H2 pro-
duction rate in the temperature range ofmeasurement (Fig. 7a
and b). However, the CO selectivity and production rate for the
Pd�Zr catalyst were very high, and they remained in the range
of 64e93% and 5.2e66.3 mmol h�1$gcat�1 respectively at the
investigated temperatures (Fig. 7c and d). This suggested that
the Pd active sites of Pd�Zr catalyst tended to promote the
methanol decomposition instead of the methanol steam
reforming, thus producing large amounts of CO [30]. Similarly,
a high concentration of CO (32e49% selectivity and
4.4e43.4 mmol h�1$gcat�1 production rate) was detected on
Pd�Zr�Zn catalyst. In contrast, the Pd�Zn catalyst showed
lower selectivity to CO than the Pd�Zr and Pd�Zr�Zn cata-
lysts at each reaction temperature. This could be attributed to
the presence of Pd�Zn alloy [33]. However, it should be
pointed out that the methanol conversion of Pd�Zr�Zn
catalyst was higher than that of the Pd�Zn and Pd�Zr cata-
lysts. This might be due to the existence of synergetic effects
between the Pd and ZnO and/or ZrO2 species [21].
In the case of Nb-modified Pd�Zr�Zn catalysts, the
methanol conversion and H2 production rate were gradually
decreased with the increase in Nb loading, indicating that the
addition of Nb species was not favorable for the conversion of
methanol. A possible explanation for this was that incorpo-
ration of Nb into Pd�Zr�Zn catalyst decreased the amount of
oxygen vacancies on the catalyst surface, as confirmed by XPS
and Raman, and thereby reduced the adsorption and activa-
tion of methanol steam on the active sites of the catalyst
[40,52]. But interestingly, the Nb-modified Pd�Zr�Zn catalysts
exhibited significantly lower CO selectivity and production
rate than the Pd�Zr�Zn catalyst at each reaction
Fig. 8 e CH3OH¡H2O¡TPSR profiles on different Pd-based cataly
(d).
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
temperature. This showed that the introduction of Nb into
Pd�Zr�Zn catalyst greatly inhibited the production of CO
from methanol reforming. According to the literature [26,27],
the formation of Pd�Zn alloy was indispensable to attain high
CO2 selectivity when methanol steam reforming was per-
formed on Pd-based catalysts. Furthermore, it has been re-
ported that the ZnO supported Pd catalysts with higher acid
strength favored the production of CO2 in methanol steam
reforming, whilst the more basic CeO2 supported Pd catalysts
obtained high selectivity to CO [34]. As verified by XRD and
NH3-TPD, the addition of Nb to Pd�Zr�Zn catalyst facilitated
the formation of Pd�Zn alloy and increased the acid strength.
Therefore, it was understood that the addition of Nb to
Pd�Zr�Zn catalyst significantly decreased the selectivity to
CO in methanol steam reforming. Among the examined Pd-
based catalysts, the 1Nb/Pd�Zr�Zn catalyst showed the
lowest CO selectivity and production rate. For example, when
the reaction was performed at 300 �C, 82.1% methanol con-
version and 6.5% CO selectivity were obtained on the 1Nb/
Pd�Zr�Zn catalyst, which was superior to the 3PdZnAl/ZrO2
catalyst with 36%methanol conversion and 12%CO selectivity
in the literature [21]. Given its good catalytic performance for
methanol steam reforming (in terms of methanol conversion
and CO selectivity), the 1Nb/Pd�Zr�Zn catalyst was selected
for further study.
Temperature programmed surface reaction testIn Fig. 8, the temperature programmed surface reaction (TPSR)
test was performed on different Pd-based catalysts to inves-
tigate the transformation of methanol and water under a dy-
namic process. As can be seen, the signals of H2, CO2 and CO
sts: Pd¡Zn (a), Pd¡Zr (b), Pd¡Zr¡Zn (c), and 1Nb/Pd¡Zr¡Zn
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 9 e Activity results for methanol steam reforming
performed on 1Nb/Pd¡Zr¡Zn catalyst in different N2 flow
rates (a) and feed rates of methanol-water mixture (b).
Experimental parameters: feed rate of methanol-water
mixture, 0.05 mL min¡1; molar ratio of water to methanol,
1.2:1; N2 flow rate, 70 mL min¡1; mass of catalyst, 400 mg;
mass of quartz sand, 800 mg; temperature of vaporizer,
160 �C; temperature of reactor, 270 �C; pressure of reactor,
1 bar.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g en en e r g y x x x ( x x x x ) x x x 13
for all the Pd-based catalysts were first detected when the
temperature reached about 160 �C. With further increase of
temperature, the signals of H2, CO2 and CO for the Pd-based
catalysts were increased in varying degrees. However, no
obvious signals corresponding to HCHO and HCOOCH3 for all
the Pd-based catalystswere detected in the temperature range
of measurement. A possible explanation for this was that the
HCHO and HCOOCH3 as the intermediates were short-lived
and rapidly decomposed to produce H2, CO and CO2 [53].
Therefore, the HCHO and HCOOCH3 were not captured by the
mass spectrometer. Similarly, no noticeable HCOOH signal
was observed in Pd�Zn, Pd�Zr and Pd�Zr�Zn catalysts, but
the 1Nb/Pd�Zr�Zn catalyst exhibited a very weak HCOOH
signal at the temperature range of 320e470 �C. This suggested
that the aldehyde species derived from the dehydrogenation
of methanol were much easier to convert into formic acid on
the 1Nb/Pd�Zr�Zn catalyst surface. Therefore, there existed
small amounts of unconverted formic acid that can be
detected.
Besides the difference in HCOOH signal, it was clear from
Fig. 8 that the major difference for the CH3OH�H2O�TPSR
profiles on the Pd-based catalysts was the signal intensities of
H2, CO, CO2 and CH4. In the case of Pd�Zn catalyst, the value of
CO2 signal was moderately identical with that of CO signal at
the temperature range of 160e270 �C. But with further in-
crease of temperature, the value of CO2 signal for the Pd�Zn
catalyst was significantly higher than that of CO signal. This
indicated that the production rate of CO2 mainly derived from
the decomposition of formic acid was higher than that of CO
produced from the decomposition of aldehydes species at
higher temperature. In addition, an obvious CH4 signal was
observed in Pd�Zn catalyst when the temperature was above
350 �C, an indication that methanation activity was enhanced
with the increase in temperature. In contrast, no noticeable
signal corresponding to CH4 was detected in Pd�Zr catalyst.
Meanwhile, the value of CO2 signal for the Pd�Zr catalyst was
far lower than that of CO signal when the temperature was
higher than 160 �C. This demonstrated that the Pd�Zr catalyst
tended to promote themethanol decomposition instead of the
methanol steam reforming, in line with the result of catalytic
activity. Similarly, the value of CO2 signal for the Pd�Zr�Zn
catalyst was also lower than that of CO signal when the
temperature exceeded 180 �C. However, the Pd�Zr�Zn cata-
lyst displayed a noticeable CH4 signal at the temperature
range of 380e500 �C, implying the presence of methanation
reaction at higher temperature.
Different from the above-mentioned three catalysts, the
value of CO2 signal for the 1Nb/Pd�Zr�Zn catalyst was always
higher than that of CO signal in the temperature range of
measurement. Meanwhile, the 1Nb/Pd�Zr�Zn catalyst
exhibited the highestmethanation activity at the temperature
range of 330e500 �C. This showed that the decomposition of
aldehydes species to produce COwas significantly suppressed
on the 1Nb/Pd�Zr�Zn catalyst surface. In order to clarify this
point further, the ratio of CO2 and CO signals profiles derived
from CH3OH�H2O�TPSR test on the Pd-based catalysts are
displayed in Fig. S6. For the 1Nb/Pd�Zr�Zn catalyst, the value
of ratio of CO2 and CO signals was always greater than one at
the temperature range of 160e500 �C, and reached the
maximum when the temperature was at 280 �C. In contrast,
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
the values of ratio of CO2 and CO signals for the other three
catalysts were significantly less than one, especially for the
Pd�Zr catalyst. Interestingly, when the temperature was
increased from 200 �C to 300 �C, the value of ratio of CO2 and
CO signals for four catalysts decreased in the order 1Nb/
Pd�Zr�Zn > Pd�Zn > Pd�Zr�Zn > Pd�Zr, which was well in
agreement with their CO2 selectivity shown in Fig. 7. This
proved again that the 1Nb/Pd�Zr�Zn catalyst exhibited lower
CO selectivity, most probably due to its strong suppression
ability for the direct decomposition of aldehydes species.
Effect of experimental parametersAs shown in Fig. 9, the experiments were carried out at
different N2 flow rates and feed rates of methanol-water
mixture to investigate the effects of experimental parame-
ters on methanol steam reforming over 1Nb/Pd�Zr�Zn cata-
lyst at 270 �C and atmospheric pressure. In Fig. 9a, it was clear
that the increase in N2 flow rate (20e90mLmin�1) resulted in a
significant decline in methanol conversion from 72% to 43%
and H2 production rate from 262 mmol h�1$gcat�1 to
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x14
158mmol h�1$gcat�1 respectively. A possible explanation for this
was that the residence time for the methanol and steam
molecules on the active sites of the catalyst was shortened
with the enhancement of the N2 flow rate, which led to limited
contact between the methanol-steam molecules and active
sites of the catalyst. Accordingly, the production of CO from
the side reactions such as reversewater-gas shift reactionwas
suppressed to some extent because of the decreased contact
time [52]. For this reason, the CO selectivity and production
rate decreased respectively from 7.4% to 6.6 mmol h�1$gcat�1 at
20 mL min�1 of N2 flow rate to 6.0% and 3.2 mmol h�1$gcat�1 at
90mLmin�1 of N2 flow rate. Similar to the effect of N2 flow rate
on methanol steam reforming, the increase in feed rate of
methanol-water mixture (0.01e0.14 mL min�1) also led to a
noticeable decline in methanol conversion from 82% to 30%,
though the H2 production rate was increased from
60 mmol h�1$gcat�1 to 258 mmol h�1$gcat
�1 (Fig. 9b). The main
reason for the above phenomenon was because of the
decrease of catalyst-to-methanol ratio in the reaction system
with the increase in feed rate of methanol-water mixture.
Additionally, the increasing feed rate of methanol-water
mixture will not only decrease the contact time of methanol
and steam on the active sites of the catalyst, but will reduce
the adsorption of methanol on the catalyst surface because it
exhibited weaker absorption ability when compared with
steam [24]. Therefore, the decrease in the methanol conver-
sion with the increasing feed rate of methanol-water mixture
can be understood. Similarly, owing to the decreased contact
time of methanol and steam on the catalyst surface, the CO
selectivity was slightly decreased from 7.3% to 5.2% when the
feed rate of methanol-water mixture was raised from
0.01 mL min�1 to 0.14 mL min�1, though relatively high CO
production rate, up to 5.3 mmol h�1$gcat�1 , was obtained at high
feed rate of methanol-water mixture [48]. This indicated that
the 1Nb/Pd�Zr�Zn catalyst still exhibited stronger suppres-
sion ability for the production of CO at the condition of high
feed rate of methanol-water mixture.
Effect of Nb on the structural feature of Pd�Zr�Zn catalystFor the Pd�Zr and Pd�Zr�Zn catalysts prepared in this work,
it was clear that CO and H2 were predominantly produced in
the methanol reforming reaction system (see Fig. 7). In
contrast, the Pd�Zn and Nb-modified Pd�Zr�Zn catalysts
exhibited relatively high selectivity to CO2 and H2. The dif-
ference in the catalytic performance of these catalysts can be
explained by the following reasons. According to the literature
[26,27], it is generally accepted that the formation of Pd�Zn
alloy is essential to obtain high CO2 selectivity during meth-
anol steam reforming. Furthermore, numerous studies sug-
gested that the structure of aldehyde species derived from the
dehydrogenation of methanol on Pd�Zn alloy was signifi-
cantly different from that on metallic Pd [18,19]. That is, the
aldehyde species exist as a h1(O)�structure on Pd�Zn alloy,
whereas the h2(C,O)�aldehyde species are preferentially
adsorbed on metallic Pd (see Fig. 10). It is worthy to state that
the h1(O)ealdehyde species preserved its molecular identity,
and then easily transformed to HCOOH by a nucleophilic
addition of water, which was finally converted to CO2 and H2.
In contrast, the h2(C,O)ealdehyde species adsorbed on
metallic Pd were rapidly decarbonylated to CO and H2 during
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
methanol reforming. This was due to the strong back donation
of electrons from the metallic Pd into the p*CO antibonding
orbital of the h2(C,O)ealdehyde species [30]. As confirmed by
XRD and XPS, the Pd species in the reduced Pd�Zr and Pd�Zn
catalysts were in the state of metallic Pd and Pd�Zn alloy
respectively, but no Pd�Zn alloy phase was formed in the
reduced Pd�ZreZn catalyst. Interestingly, as verified by XRD
and Raman, the addition of Nb to Pd�Zr�Zn catalyst facili-
tated the incorporation of Pd into the ZnO lattices, which
favored the formation of Pd�Zn alloywith small particle sizes.
Therefore it was understandable that large amounts of CO
were produced on Pd�Zr and Pd�Zr�Zn catalysts during
methanol reforming, whilst the Pd�Zn and Nb-modified
Pd�Zr�Zn catalysts exhibited relatively high selectivity to
CO2 (Fig. 10).
However, it seems the difference in the activity of the
studied catalysts cannot be attributed entirely to a difference
in Pd species, because the Pd�Zn and Nb-modified Pd�Zr�Zn
catalysts presented significantly different catalytic perfor-
mance (in terms of CO selectivity and methanol conversion)
despite the presence of Pd�Zn alloy. In 2005, Thompson and
co-workers [34] prepared a series of ZnO and CeO2 supported
catalysts with different Pd loadings for methanol steam
reforming. They claimed that the ZnO supported Pd catalysts
with higher acid strength favored the production of CO2 and
the more basic CeO2 supported Pd catalysts easily facilitated
the decomposition of aldehyde species, yielding high selec-
tivity to CO. Similar to the report of Thompson et al., the
addition of Nb to Pd�Zr�Zn catalyst of this work increased the
acid quantity and strength as shown in NH3-TPD, thus
significantly reducing the production of CO. In addition, as
confirmed by XRD and CO chemisorption, the Nb-modified
Pd�Zr�Zn catalysts possessed more dispersed Pd�Zn alloy
as active sites for methanol steam reforming when compared
with the Pd�Zn catalyst. This can inhibit the production of CO
to some extent [22]. Recently Mendes et al. [29] systematically
examined the effects of the calcination atmospheres (H2, N2,
air and O2) of the ZnO precursor on the catalytic performance
of PdZn/ZnO catalyst in methanol steam reforming. They
found that the PdZn/ZnO catalyst with ZnO precursor calcined
in a H2 atmosphere exhibited higher catalytic activity (mainly
in terms of methanol conversion). This was primarily due to
the presence of large amounts of oxygen vacancies on the
catalyst surface, which served as the active sites for methanol
steam adsorption and activation. As confirmed by XPS and
Raman, the introduction of Nb into Pd�Zr�Zn catalyst of this
study significantly decreased the amount of oxygen vacancies
on the catalyst surface. So from this evidence, the decrease in
themethanol conversion for Nb-modified Pd�Zr�Zn catalysts
with the increasing Nb loading can be understood.
Proposed reaction pathwayIn recent years, the reaction pathway for methanol steam
reforming over catalyst has been vastly studied by using
different technologies such as TPSR, DRIFTS and DFT
[10,34,53]. For the Pd-based catalysts, there are two reaction
pathways for methanol steam reforming widely reported in
the literature [30,33]. The first is the decomposition of meth-
anol to HCHO, which then rapidly decomposes to H2 and CO
that is further converted to CO2 and H2 by the water-gas shift
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
Fig. 10 e The schematic of aldehyde species conversion for methanol steam reforming performed on Pd¡Zn (a), Pd¡Zr (b),
Pd¡Zr¡Zn (c), and 1Nb/Pd¡Zr¡Zn (d) catalysts.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g en en e r g y x x x ( x x x x ) x x x 15
reaction. The second is the methanol decomposition to HCHO
followed by a nucleophilic addition of water and/or methanol
to produce HCOOH and/or HCOOCH3 respectively, which
finally transforms to CO2 and H2. It should be pointed out that
these two reaction pathways for methanol steam reforming
over the Pd-based catalyst are greatly affected by the type and
the nature of support. In 2003, Iwasa et al. [30] systematically
investigated the catalytic activity and reaction pathway for
methanol steam reforming on different supported Pd cata-
lysts. They found that the HCHO existing as h1(O)eHCHO
species was produced first from the dehydrogenation of
methanol and then attacked by water to form HCOOH on the
catalysts containing Pd alloy, and finally converted to CO2 and
H2. In contrast, the HCHO in the state of h2(C,O)ealdehyde
species derived from the methanol dehydrogenation was
rapidly decomposed to CO and H2 on the catalysts containing
metallic Pd. Slightly different from the study of Iwasa et al.
[30], Thompson and co-workers [34] focused on the charac-
teristics effect of the catalyst support on the reaction pathway
for methanol steam reforming, and thus tested the catalytic
performance using methanol and its potential intermediates,
HCHO, HCOOH, HCOOCH3, and CO, as reactants on the ZnO
and CeO2 supported Pd catalysts. They reported that the
aldehyde species produced from the methanol
Fig. 11 e Proposed reaction pathway for methanol steam
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
dehydrogenation can either decompose to CO and H2 or
convert to HCOOH depending on the nature of catalyst sup-
port. With the aid of different characterization techniques,
Thompson et al. [34] in their study established that the Pd/ZnO
catalyst with higher acidity was inclined to produce HCOOH,
whereas more basic Pd/CeO2 catalyst favored the decompo-
sition of HCHO to CO and H2.
As displayed in Fig. 11, the possible reaction pathway for
methanol steam reforming performed on 1Nb/Pd�Zr�Zn
catalyst was presented based on the literature and our
experimental results. As verified by XRD, XPS, and Raman, the
addition of Nb to Pd�Zr�Zn catalyst favored the incorporation
of Pd into the ZnO lattices, which facilitated the formation of
Pd�Zn alloy with small particle sizes. Therefore on the 1Nb/
Pd�Zr�Zn catalyst, the methanol was first dehydrogenated to
the h1(O)eHCHO species through the O�H bond breaking fol-
lowed by a nucleophilic addition of water to produce HCOOH,
which finally decomposed to CO2 and H2 (Fig. 11a). Further-
more, as confirmed by the NH3-TPD, the 1Nb/Pd�Zr�Zn
catalyst possessed a higher acid strength, favoring the trans-
formation of methanol into HCOOH that was also detected by
TPSR of this study, thus yielding high CO2 selectivity. This is in
agreement with the report of Thompson et al. [34]. However,
the h2(C,O)eHCHO species derived from the methanol
reforming performed on 1Nb/Pd¡Zr¡Zn catalyst.
n Pd�Zr�Zn catalyst in methanol steam reforming for hydrogen/10.1016/j.ijhydene.2019.03.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( x x x x ) x x x16
dehydrogenation may also exist on the catalyst surface since
the reverse water gas shift reaction cannot contribute to the
production of CO frommethanol steam reforming on the 1Nb/
Pd�Zr�Zn catalyst. Consequently, a certain amount of CO
produced from the decarbonylation of h2(C,O)eHCHO species
was also detected in the course of the reaction (Fig. 11b). This
indicated that the reaction pathway for methanol steam
reforming, CH3OH / HCHO / HCOOH / CO2 þ H2, occurred
competitively with the decomposition of HCHO to CO and H2
on the 1Nb/Pd�Zr�Zn catalyst. But obviously, the reaction
rate of formerwasmuch higher than that of the latter because
the selectivity to CO2 for methanol steam reforming per-
formed on 1Nb/Pd�Zr�Zn catalyst was significantly higher
than that to CO. In addition, the CO2 and/or CO formed in the
reaction can react with H2 to produce CH4 through the
methanation reaction at high temperature (>320 �C) on 1Nb/
Pd�Zr�Zn catalyst, as verified by TPSR of this study.
Conclusions
In this work, a series of Nb-modified Pd�Zr�Zn catalysts with
different Nb loadings prepared by the impregnation method
were thoroughly characterized and evaluated for the pro-
duction of hydrogen from methanol steam reforming. The
catalytic activity results showed that the methanol conver-
sion and H2 production rate for the Nb-modified Pd�Zr�Zn
catalysts were gradually decreased with the increase in Nb
loading, which was attributed to the decrease in the amount
of oxygen vacancies on the catalyst surface. However, the
addition of Nb to Pd�Zr�Zn catalyst increased the acid
quantity and strength on the catalytic surface. The aldehyde
species produced from the decomposition of methanol were
readily converted to HCOOH by a nucleophilic addition of
water, thus obtaining high CO2 selectivity for the Nb-modified
Pd�Zr�Zn catalysts. More importantly, the characterization
results indicated that the introduction of Nb into Pd�Zr�Zn
catalyst facilitated the incorporation of Pd into the ZnO lat-
tices, which favored the formation of Pd�Zn alloy. Therefore,
the Nb-modified Pd�Zr�Zn catalysts presented much higher
CO2 selectivity and production rate than the Pd�Zr�Zn cata-
lyst. Further studies on the effect of experimental parameters
and temperature programmed surface reaction test demon-
strated that the Nb-modified Pd�Zr�Zn catalyst exhibited
high selectivity for methanol steam reforming.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21805302) and Shanghai
Sailing Program (No. 18YF1425800). The authors gratefully
acknowledge these grants.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.ijhydene.2019.03.125.
Please cite this article as: Cai F et al., Investigation of the role of Nb oproduction, International Journal of Hydrogen Energy, https://doi.org
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