Zr Zn catalyst in methanol steam reforming for hydrogen

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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

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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

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 3

(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.


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




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


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




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.


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



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.


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


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



Fig. 3 e NH3-TPD profiles of the Pd-based catalysts.


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


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



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).


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


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




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


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



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).


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


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



Fig. 6 e Raman spectra of the reduced Pd-based catalysts.


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


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.




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).


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



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.


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,


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




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


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



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.


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


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.




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.


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Zr Zn catalyst in methanol steam reforming for hydrogen