Methanol steam reforming catalysts derived by reduction of perovskite-type oxides LaCo1-x-yPdxZnyO3±δ

Article abstract of DOI:10.1039/c5cy01410g

Methanol steam reforming (MSR) catalysts are derived from perovskite-type oxides LaCo_1-x-yPd_xZn_yO_3±δ by reductive pretreatment. The unsubstituted LaCoO_3±δ (LCO) and LaCo_1-x-yPd_xZ

Full text of DOI:10.1039/c5cy01410g

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Methanol steam reforming catalysts derived
by reduction of perovskite-type oxides
Cite this: DOI: 10.1039/c5cy01410g
LaCo1
Pd Zn O
±
†
−x−y
x
y 3 δ
a
b
c
a
Jagoda Kuc, Matthias Neumann, Marc Armbrüster, Songhak Yoon,
d
d
e
a
Yucheng Zhang, Rolf Erni, Anke Weidenkaff and Santhosh Kumar Matam*
Methanol steam reforming (MSR) catalysts are derived from perovskite-type oxides LaCo1−x−yPd
Zn
reductive pretreatment. The unsubstituted LaCoO3±δ (LCO) and LaCo1−x−yPd 3±δ (Co substituted with
x y
Zn O
3±δ by
x
y
O
Pd and/or Zn) are synthesized by a citrate method and characterized by different techniques. The
perovskite-type oxides exhibit a rhombohedral crystal structure and a comparable surface area (≈8.5 (±2)
2
−1
m
g ). The temperature-programmed reduction (TPR) shows low (100 °C < T < 450 °C) and high (T >
50 °C) temperature reduction events that correspond to partial and complete reduction of the non-rare-
earth metal ions, respectively. At high temperatures, Pd–Zn alloy nanoparticles are formed exclusively on
Pd- and Zn-containing LaCo1−x−yPd Zn
3±δ, as evident from high angular annular dark-field scanning
transmission electron microscopy (HAADF-STEM). The CO -selective MSR performance of the catalysts
strongly depends on the reductive pretreatment temperature, catalyst composition (i.e., the Pd: Zn molar
ratio and the degree of Co substitution) and reaction temperature. Only LaCo1−x−yPd Zn
3±δ catalysts
show a low-temperature CO selectivity maximum between 225 and 250 °C, while all catalysts present
similar high-temperature selectivity maxima at T > 400 °C. The former is missing on LCO, LaCo1−xPd
4
x
y
O
2
x
y
O
2
O
3±δ
x
Received 26th August 2015,
Accepted 22nd September 2015
or LaCo1 Zn O
±
δ. Pd–Zn nanoparticles facilitate ZnIJOH)2 and CoIJOH)2 formation exclusively on
−y
y
3
LaCo1−x−yPd Zn
x y
O
3±δ, as evident from in situ XRD under steam atmosphere. This indicates the important
selectivity, which is improved from 0 to 76% at
3±δ, respectively. The high-temperature CO
selectivity is
role of Pd–Zn nanoparticles in the low-temperature CO
25 °C on LCO and LaCo0.75Pd0.125Zn0.125
2
DOI: 10.1039/c5cy01410g
2
O
2
www.rsc.org/catalysis
governed by the bulk catalyst composition and the occurrence of reverse water gas shift reaction.
existing infrastructure and safer to handle than compressed
H . The MSR reaction proceeds according to eqn (1).
2
1
. Introduction
1
,6
Catalytic methanol steam reforming (MSR) has great potential
to supply H for fuel cells, especially for mobile applications
such as proton exchange membrane fuel cells (PEMFCs).
−
1
2
CH
3
OH + H
2
O ⇌ CO
2
+ 3H
2
ΔH° = 49 kJ mol
(1)
1–3
Methanol (CH OH) as a fuel has several advantages: (i) it can
be a carbon-neutral renewable feedstock, (ii) its gravimetric
However, the main challenge with the MSR reaction is to
avoid the formation of CO as a by-product due to CH OH
3
4
3
3
,7,8
H density is higher than those of either compressed H gas
decomposition (eqn (2)).
CO is a poison to the fuel cell
2
2
9
,10
or liquid H2 (ref. 5) and (iii) it is easier to distribute with
electrode.
a
−1
Empa, Swiss Federal Laboratories for Materials Science and Technology,
CH OH ⇌ CO + 2H ΔH° = 91 kJ mol
(2)
3
2
Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
E-mail: Santhosh.matam@empa.ch, matamsanthosh@yahoo.com
Besides, the reverse water gas shift reaction (rWGS) can con-
tribute to CO formation (eqn (3)).
b
Max-Planck-Institut für Chemische Physik fester Stoffe, D-01187 Dresden,
Germany
c
Materials for Innovative Energy Concepts, Institute of Chemistry, Faculty of
−
1
Natural Sciences, Technische Universität Chemnitz, D-09107 Chemnitz, Germany
CO
2
+ H
2
⇌ CO + H
2
O
ΔH° = 41 kJ mol
(3)
d
Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials
Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
These two undesired side reactions can be thermodynami-
cally limited by conducting the MSR reaction at lower tem-
peratures with suitable catalysts. The most promising cata-
lysts reported so far have been based on either Cu or Pd
e
Materials Chemistry, Institute for Materials Science, University of Stuttgart,
D-70569 Stuttgart, Germany
1
1
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy01410g
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1
2
supported typically on ZnO and/or Al O . Pd-based catalysts
dissolved in 150 mL of deionized water and subsequently,
citric acid was added in a metal-to-acid molar ratio of 1 : 1.1.
The resulting aqueous solution was carefully evaporated in a
rotary evaporator at 70 °C and 100 mbar to obtain a viscous
solution, which was then dried overnight at 80 °C in a vac-
uum oven operated between 50 and 70 mbar. The resulting
solid foam-like material was ground in a mortar and
subjected to calcination in synthetic air at 800 °C for 2 h.
With this method, perovskite-type oxide catalysts with varying
Pd and Zn molar ratios (0.09, 0.38, 1.0 and 1.15) were pre-
2
3
exhibit better thermal stability than Cu-based ones with a
1
3
comparable CO selectivity of ≥98%. The CO
Pd/ZnO catalysts is attributed to the intermetallic compound
IMC) ZnPd that forms upon reductive pretreatment at
around 400 °C. A recent review addresses the role of the
intermetallic compound ZnPd in MSR.
unsupported ZnPd reveal a strong dependence of the CO2
selectivity on the chemical composition of the compound.
The highest CO selectivity of around 99% is achieved when
the Pd : Zn ratio is lower than one. It is further demon-
strated that ZnPd in combination with ZnO is responsible for
the high CO selectivity. The synthesis of supported ZnPd
nanoparticles by reductive decomposition of ternary
hydrotalcite-like compounds (HTlcs) is reported. The struc-
tural flexibility of HTlcs allows a large number of elemental
reducible and non-reducible) combinations in which all
metal cations are in close interaction making HTlcs potential
precursors for the formation of intermetallic nanoparticles.
This method provides a basis for deriving well-dispersed
2
2
selectivity of
(
7
,8
3
Studies on
2
1
0
pared. Also, LaCo1 Pd Zn O
catalysts having different
−x−y
x
y
3±δ
degrees (x and y = 0.05, 0.15 and 0.25) of Co substitution
with Pd and Zn (at a 1 : 1 Pd : Zn molar ratio) were synthe-
sized. For comparative purposes, reference perovskite-type
oxides of unsubstituted LaCoO3±δ (LCO), palladium-
substituted LaCo0.873Pd0.127O3±δ (LCPO) and zinc-substituted
LaCo0.89Zn0.11O3±δ (LCZO) were also prepared. For the sake of
clarity, the catalyst compositions are abbreviated. For exam-
ple, LCPZO-1.15 indicates the presence of La, Co, Pd and Zn,
while the number defines the Pd : Zn molar ratio in the cata-
lyst. Similarly, the second number in a catalyst like LCPZO-1-
0.15 indicates a Pd and Zn (at a 1 : 1 ratio) substitution level
1
4
2
5
(
ZnPd nanoparticles that show a CO
2
selectivity of 61% (at
5
2
50 °C, 14% CH OH conversion).
3
In this study, we report perovskite-type oxides as novel
of 15 mol% in LaCo0.85Pd0.075Zn0.075O3±δ. The catalyst compo-
precursors for deriving MSR catalysts. To this end, well-
defined single-phase perovskite-type oxides with the general
sition, Pd and/or Zn content and BET surface area are
reported in Table 1.
3
formula ABO are employed. A and B represent a rare earth
metal cation coordinated to 12 oxygen atoms and a transition
metal cation surrounded by 6 oxygen atoms in octahedral
2
.2 Characterization
1
5
coordination, respectively. The physical and chemical prop-
erties of the materials can be tailored for specific applica-
tions by partial substitution of A and/or B site cations with
suitable ones, whilst preserving the perovskite crystal struc-
Inductively coupled plasma optical emission spectrometry
(ICP-OES). The chemical composition of the perovskite-type
oxide catalysts was determined by ICP-OES analysis (Vista RL,
Varian). For each experiment, around 10 mg of catalyst pow-
der was dissolved in aqua regia using an ultrasonic bath and
subsequently analyzed. For each catalyst, an average of three
values was taken.
1
6,17
ture.
These metal cations are in close interaction due to
the covalent nature of the bonds in the compounds that
exhibit remarkable catalytic properties, for example,
1
8
LaFe0.57Co0.38Pd0.05
O
3
as a three-way catalyst. This study
2
N physisorption. Nitrogen adsorption and desorption iso-
concludes that Pd located in the crystal structure emerges
from the crystal to the surface under reducing environment,
and reintegrates into the crystal upon oxidation. Based on
the former concept, the novel MSR catalysts are derived for
the first time by reductive pretreatment of perovskite-type
therms at −196 °C were obtained using a Micromeritics ASAP
2020 instrument. Prior to the experiments, the catalysts were
pretreated at 300 °C for 2 h at 133.3 mbar. The total surface
area of the catalysts was evaluated by the Brunauer–Emmett–
2
0
Teller (BET) method.
LaCo1−x−yPd
x
Zn
sensitive to the LaCo1 Pd Zn O
y 3±δ
y
O
3±δ. The CO
2
selectivity of the catalysts is
X-ray diffraction (XRD). Powder XRD patterns of the mate-
rials were obtained on a PANalytical X'Pert PRO MRD θ–2θ
scan system using a Johansson monochromator and an
composition and the
−x−y
x
degree of Pd–Zn alloy formation.
1
X'Celerator linear detector with Cu Kα radiation (λ = 1.5405
Å, 45 kV and 40 mA). XRD data were collected in the 2θ range
of 20–80 degrees. XRD patterns were analyzed by the Le Bail
2
. Experimental
2
.1 Catalyst preparation
21
22
method integrated into the program FullProf to deter-
mine the lattice parameters. The crystallite sizes are semi-
A series of single-phase perovskite-type oxides with the gen-
eral formula LaCo1 Pd Zn O
amorphous citrate method.
LaIJNO ) ·6H O (Alfa Aesar, 99.9%), CoIJNO ) ·6H O (Alfa
2
3
were synthesized by the
Metal salt precursors
quantitatively calculated based on the Scherrer equation
which gives information on the crystallite size of the sample.
−x−y
x
y
3±δ
7,19
1
2
4
The Thompson–Cox–Hastings pseudo-Voigt function was
chosen as the profile function.
3
3
2
3 3
2
Aesar, 98%), ZnIJNO ) ·6H O (Sigma Aldrich, 98%) and
3
3
2
PdIJNO
3
)
3
(Alfa Aesar, 4.42% w/w Pd cont.) were used. As a
In situ X-ray diffraction (in situ XRD). A PANalytical X'Pert
PRO MPD X-ray diffractometer with a gas-tight Anton Paar
XRK 900 heating chamber composed of heating and gas
chelating agent, citric acid monohydrate (Alfa Aesar, 99%)
was utilized. Required amounts of metal precursors were
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Table 1 Catalyst composition and physical properties
Metal content (wt%)
MSR performance at 225 °C
2
CH OH con. CO selec. H concentration
BET
(m g
3
2
2
−1
Material
Chemical composition
Pd
Zn
)
(%)
(%)
(ppm)
a
LCO
LCPO
LCZO
LaCo0.987IJ±0.024)
O
3±δ
—
—
—
8.9
10.5
0.4
12.0
2.0
0
7.6
2.5
1500
4200
—
a
a
a
LaCo0.862IJ±0.017)Pd0.123IJ±0.003)
LaCo0.874IJ±0.015)Zn0.106IJ±0.002)
O
O
3±δ
3±δ
5.10 (±0.05)
—
a
2.75 (±0.01) 10.5
LCPZO-0.09
LCPZO-0.38
LaCo0.823Pd0.014Zn0.163
O
3±δ
0.60
2.35
4.31
3.80
8.4
8.6
1.5
4.3
2.0
8.3
10.6
11.0
26.0
35.0
35.3
53.3
76.0
24.0
—
—
2800
4600
7200
—
LaCo0.8Pd0.055Zn0.145
O
3±δ
a
a
a
LCPZO-1-0.05 LaCo0.939IJ±0.036)Pd0.024IJ±0.001)Zn0.024IJ±0.001)
LCPZO-1-0.15 LaCo0.830IJ±0.005)Pd0.073IJ±0.001)Zn0.072IJ±0.001)
LCPZO-1-0.25 LaCo0.735IJ±0.030)Pd0.124IJ±0.005)Zn0.123IJ±0.004)
O
O
O
3±δ 1.02 (±0.02)a 0.61 (±0.02)a 6.5
a
3±δ 3.11 (±0.02) 1.88 (±0.02)
a
7.5
7.0
7.0
a
a
3±δ 5.07 (±0.06) 3.09 (±0.01)
5.35 2.85
LCPZO-1.15
LaCo0.763Pd0.127Zn0.11O
3±δ
a
Determined by ICP-OES analysis; other values are nominal values.
feeding (5850 TR, Brooks Instrument) accessories was used
to analyze the structural transformation of the catalysts dur-
ing a temperature-programmed redox cycle (in reducing and
oxidizing atmospheres) and steam treatment. The gases used
Transmission electron microscopy (TEM). High angular
annular dark-field scanning transmission electron micros-
copy (HAADF-STEM) analyses were performed on a JEOL JEM
2200fs microscope operating at 200 kV. The materials were
dispersed in deionized water and deposited on carbon film-
coated copper grids, which were then dried in air. The parti-
cle size distribution was determined by counting around 100
nanoparticles using Digital Micrograph software (Gatan Inc.).
2 2
were N (99.999%, Messer) and H (99.999%, Messer). XRD
data were collected in the 2θ range of 20–80° with a step size
of 0.026° using Cu Kα1/2 radiation (λaverage = 1.5418 Å, 45 kV
and 40 mA). Temperature-programmed reduction (TPR)
experiments were performed while heating from room tem-
−
1
perature to 800 °C (5 K min ) in 5 vol% H in N (total flow:
1
2
2
−
1
2.3 Methanol steam reforming (MSR)
00 ml min ), followed by cooling to room temperature in
the same gas mixture. Subsequently, the reaction cell was
flushed with N2 for 10 min followed by temperature-
Catalytic MSR activity experiments were carried out at ambi-
ent pressure in a plug flow quartz glass reactor (d = 6 mm,
i
programmed oxidation (TPO) conducted in 5 vol% O
99.999%, Messer) in N (99.999%, Messer) (total flow: 100
2
l = 400 mm) equipped with flow-bus operated gas manifold
(Bronkhorst) and gas analyzing systems (GC and MS). The
reactor was loaded with 100 mg (sieve fraction of 150–200
μm) of catalyst that was diluted with quartz glass particles of
the same size in a 1 : 1 volume ratio to improve mass and
heat transport, and firmly packed between two quartz wool
plugs. The reactor was positioned vertically in a programma-
ble tube furnace (controlled with Controltherm software,
Nabertherm), and a K-type thermocouple was coaxially
inserted into the catalyst bed to measure the temperature.
Prior to the MSR reaction, the catalysts were reduced at either
(
2
−
1
ml min ). Diffraction patterns were recorded at a constant
temperature during heating at a regular interval of 25 K.
Similarly, in situ XRD during temperature-programmed
reaction with steam (4 vol% H O in N , total flow: 100 ml
2
2
−
1
min ) was conducted between 100 and 550 °C to monitor
the phase evolution as a function of temperature. The diffrac-
tion patterns were collected every 15 K. Prior to the experi-
ments, the catalysts were reductively pretreated in 5 vol% H
in N at 600 °C (5 K min ) for 1 h and cooled to 100 °C. It
2
2
−
1
−
1
should be mentioned that CH OH was not included in the
300 or 600 °C in 25 vol% H in He (total flow: 50 ml min
2
3
feed due to technical limitations of the in situ reaction
chamber.
He) for 1 h. After reductive pre-treatment, the catalysts were
cooled to 100 °C. At this temperature, the desired MSR reac-
tion mixture was introduced. The effect of the H O-to-CH OH
H temperature-programmed reduction (H -TPR). H -TPR
2
2
2
2
3
measurements were carried out on
a
Quantachrome
volume ratio on the CO selectivity was assessed by varying
2
CHEMBET-3000 instrument to study the effect of Co substitu-
tion with Pd and/or Zn on the reduction properties of
2 3
the ratio between 1 and 1.3. The H O and CH OH streams
were produced by passing He through two saturators con-
LaCo1−x−yPd
x
Zn
y
O
3±δ. A U-shaped quartz glass reactor was
taining deionized H O and 99.997% CH OH (SeccosSolv®,
2
3
−
1
loaded with 50 mg of the catalyst and the catalyst bed tem-
perature was measured through the reactor wall by inserting
a K-type thermocouple into the reactor cavity. The TPR pro-
files of the catalysts were obtained by heating the reactor
Merck), respectively. The resulting gas stream (100 ml min )
with 4 vol% H O and 3 vol% CH OH was fed to the reac-
2
3
tor. The reaction was carried out between 100 and 550 °C
−
1
3 2
(2 K min ), and the CH OH conversion (%), H O conversion
−
1
from room temperature to 900 °C (5 K min ) in a 5 vol% H
(%), CO2 selectivity (%) and H2 concentration (ppm) were
studied as a function of temperature. Additionally, an
isothermal MSR test was performed for 80 h at 245 °C over
LCPZO-1-0.15. The reactor outlet was connected to a gas chro-
matograph (3000A microGC, Agilent Technologies, equipped
2
−
1
in N
tion was measured with a thermal conductivity detector
TCD). The H O formed during the reduction was prevented
2
mixture (total flow: 80 ml min ). Hydrogen consump-
(
2
from passing through the detector by adsorption.
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with PoraPLOT Q and Molecular Sieve 5A columns in combi-
nation with a thermal conductivity detector) to analyze the
3 2
unconverted reactants (i.e., CH OH and H O) and reaction
products (CO and CO). H was detected and quantified with
2
2
a mass spectrometer (Pfeiffer Omni).
Under all reaction conditions tested in the study, the
observed reaction products were H , CO and CO. Other by-
2
2
products such as CH or H CO were not detected. Thus, the
4
2
CH OH conversion (XCH OH (%)) is defined as:
3
3
(
4)
where nIJCH OH) is the initial concentration at the reactor
3
in
3
inlet and nIJCH OH)out is the outlet concentration at a given
temperature.
Fig. 1 XRD patterns of the catalysts recorded at ambient atmosphere.
The CO selectivity (S
(%)) is defined as:
CO
2
2
(
5)
of the catalysts strongly depends on the degree of Co substi-
tution. The phase purity is retained up to an x and y substitu-
tion level (in LaCo1 Pd Zn O3±δ) of around 0.25. Above this,
−x−y
x
y
where nIJCO
the products at the reactor outlet.
2
) and nIJCO) correspond to the concentrations of
additional reflections assignable to PdO and ZnO are
observed (not shown); hence such catalysts are not consid-
ered further.
The effect of substitution of Co with Pd and/or Zn on the
x y
reduction behavior of LaCo1−x−yPd Zn O3±δ is investigated by
3
. Results and discussion
3
.1 Characterization
The composition and the surface area (SBET) of the catalysts
are shown in Table 1. The ICP-OES data confirm the nominal
Pd and Zn contents in the catalysts, indicating the efficiency
temperature-programmed reduction (TPR) with hydrogen.
The TPR profiles of the catalysts are shown in Fig. 2 and S2.†
The unsubstituted LCO exhibits two main reduction events
centered around 400 and 585 °C. The low-temperature reduc-
tion event consists of two signals indicating the reduction of
two distinct Co oxide species with different reduction behav-
of the synthesis method. The S
of the unsubstituted LCO
BET
2
−1
2
−1
is 8.9 m g which increases slightly (10.5 m g ) upon sub-
stitution of Co with either Pd or Zn. This is in line with the
2
5
30
literature which indicates that partial substitution of Co
iors. Based on the literature, the low-temperature events are
3
+
n+
with Pd in LaCo0.95Pd0.05O decreases the perovskite crystal-
assigned to the partial reduction of Co to Co (0 < n+ < 3),
3
lite size and hence increases the specific surface area. Accord-
ingly, the crystallite size of 80 nm is determined for LCO.
This size is decreased to 30 nm for LCPO. Contrarily, it
appears that the substitution of Co with both Pd and Zn
whereas the high-temperature peak can be assigned to the
n+
0
reduction of Co
to Co which is in line with
2
−1
slightly decreases the SBET to around 7.5 (±1) m
Table 1).
The XRD patterns of the catalysts are depicted in Fig. 1
and S1.† According to the PDF reference code 01-084-0848
ref. 26), both unsubstituted LCO and substituted
Zn 3±δ exhibit a single-phase rhombohedral
perovskite crystal structure with the space group R ¯3 c.
g
(
(
LaCo1−x−yPd
x
y
O
2
7,28
These results indicate that the perovskite structure of
LaCoO3±δ is preserved upon partial substitution of Co cations
with Pd and/or Zn ions in LaCo
Pd Zn O3±δ, in line with
x y
1−x−y
2
8
our previous observations. The XRD reflections of the
substituted LaCo1−x−yPd Zn 3±δ are located at slightly lower
θ angles compared to those of LCO. The slight shift in the
θ angles indicates the unit cell expansion of the perovskite
x
y
O
2
2
crystal due to the substitution of Co cations with Pd and/or
2
9
Zn ions that have larger ionic radii than the Co ions. The
phase purity of the rhombohedral perovskite crystal structure
Fig. 2
2
H -TPR profiles of the catalysts.
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1
6
thermogravimetric analysis. These results suggest that the
reduction of Co oxide in LaCoO3±δ is a two-step process.
Interestingly, Pd-containing LCPO exhibits a low-temperature
reduction peak between 100 and 300 °C with a maximum
centered at 150 °C that contains a shoulder at 200 °C. The
2
+ 28,31
peak at 150 °C is attributed to the reduction of Pd ,
while the shoulder can be assigned to the partial reduction
3
+
of Co . The high-temperature reduction peak is located at
20 °C and can be ascribed to the reduction of bulk cobalt
oxide to elemental Co. These reduction events indicate that
5
3
+
Pd decreases the reduction temperature of Co in the perov-
skite. This can be anticipated from the hydrogen spillover
3
1
effect. In contrast, Zn-containing LCZO shows two reduc-
tion events at relatively higher temperatures as compared to
LCO and LCPO (Fig. 2). The low-temperature reduction peak
is centered at 425 °C. The high-temperature peak exhibits
two unresolved peaks at 620 and 650 °C. The latter peak
could be due to the reduction of ZnO species. These reduc-
tion events suggest that Zn increases the reduction tempera-
3
+
32
ture of Co in the perovskite. Based on these results, it can
readily be seen in Fig. 2 that Pd and Zn-containing LCPZO-
1
.15, LCPZO-1-0.05, LCPZO-1-0.15 and LCPZO-1-0.25 catalysts
also show the low- and high-temperature reduction events.
The low-temperature signal is located between 170 and 240
°C, which is closer to the reduction event observed for Pd-
containing LCPO at 150 °C than that for LCO. The high-
temperature reduction event of the catalysts also matches the
event observed for LCPO at 520 °C. The observed slight differ-
ences in the reduction events of the catalysts can be tenta-
tively attributed to the variation in the substituent
distribution.
Fig. 3 XRD patterns of LCO during TPR (above) and subsequent TPO
For better understanding of the reduction behavior, the
materials are monitored during TPR by XRD to follow the
evolution of phases as a function of temperature. The XRD
patterns of the unsubstituted LCO during TPR are shown in
Fig. 3. The rhombohedral crystal structure of LaCoO3±δ
remains detectable up to 275 °C, indicating that the reduc-
tion of LCO does not take place. This is in good agreement
(below).
the formation of such phases cannot be ruled out. These
results confirm that the low-temperature reduction event (at
around 400 °C) observed on LCO during H -TPR (Fig. 2) is
2
with the corresponding H
2
-TPR data which show that the
due to the partial reduction of cobalt cations. Above 500 °C,
3
6
reduction begins only above 275 °C (Fig. 2). Above this tem-
perature, noticeable changes in the XRD pattern of LCO can
be observed. The changes include peak broadening and a
shift in the peak positions to lower 2θ angles (i.e., larger
d-spacing) due to lattice expansion caused by thermal effects.
The former effect can be caused by the partial reduction of
LCO, which in turn creates oxygen vacancies in the lattice. In
order to maintain the overall charge neutrality in the crystal,
the formation of cubic (PDF reference code: 03-065-3185)
3
7
and hexagonal La O (PDF reference code: 01-074-2430) as
2
3
3
8
well as elemental Co (PDF reference code: 00-015-0806) is
observed. Cubic La O disappears between 575 and 600 °C,
2
3
while hexagonal La O and Co are sustained up to 800 °C
2
3
2
(Fig. 3). These results are again in line with the H -TPR pro-
file of LCO indicating that complete reduction of the non-
rare-earth metal ions is attained between 550 and 625 °C.
Subsequently, the material is cooled from 800 °C to room
3
+
reduction of Co to a lower valence state (<3+) occurs. As a
result, the average ionic radius of cobalt cations increases
temperature in the same gas mixture, flushed with N for 10
2
3
3
leading to CoO
6
octahedral expansion. Further increase in
minutes and subjected to temperature-programmed oxidation
(TPO) between room temperature and 800 °C. The XRD pat-
terns of LCO obtained during TPO are shown in Fig. 3. It is
interesting to note that during re-oxidation, elemental Co is
oxygen vacancies leads to the formation of vacancy-ordered
perovskite-related structures such as La Co O (PDF reference
3
3 8
3
4
35
code: 01-089-1319) observed between 325 and 475 °C.
Besides, the formation of other structures like La Co
2
2
O
5
3 4
sustained up to 225 °C. At 275 °C, the formation of the Co O
3
9
could not be categorically determined due to the overlap of
multiple XRD reflections at the same 2θ angles. Therefore,
phase (PDF reference code: 01-076-1802) is observed. Fur-
ther heating leads to the disappearance of Co O at around
3
4
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6
00 °C; however, the hexagonal La O phase is still sustained.
lower temperatures, as compared to the unsubstituted LCO.
Similarly, the onset temperature required for the successive
phase transitions is significantly lower for the substituted
materials compared to that for LCO. For example, the rhom-
bohedral crystal structure of LCPZO-1-0.15 is sustained only
up to 100 °C, while that of the unsubstituted LCO is
maintained up to 275 °C. The low stability of the perovskite
structure is attributed to the H2 spillover effect due to Pd
(ref. 31) that enhances the reduction of Co . The XRD reflec-
tions of LCPZO-1-0.15 shift towards lower angles at around
125 °C, indicating the occurrence of detectable reduction.
2
This is again in agreement with the corresponding H -TPR
data which show that the reduction starts at around 125 °C
2
3
Above this temperature, simultaneous disappearance of
La and appearance of the rhombohedral LCO phase are
observed. The XRD reflections attributable to the single-
phase perovskite are completely restored above 700 °C, which
is close to the calcination temperature of the catalysts (i.e.,
2 3
O
8
00 °C). These results suggest the occurrence of bulk struc-
1
8
ture reversibility of LaCoO3±δ upon a redox cycle.
The in situ XRD patterns of Pd and Zn-substituted
LaCo1 Pd Zn O catalysts during H -TPR are also
recorded. The behavior of the catalysts during TPR is similar
as evident from the XRD patterns of the catalysts (data not
presented). As an example, the in situ XRD patterns of
LCPZO-1-0.15 are shown in Fig. 4. However, the XRD patterns
3
+
−x−y
x
y
3±δ
2
(Fig. 2). The formation of the La Co O phase is observed at
3
3 8
of Pd and Zn-substituted LaCo1−x−yPd
x
Zn
y
O
3±δ are different
175 °C and is visible up to 500 °C. The formation tempera-
ture is much lower than that observed for unsubstituted
LCO. Above 500 °C, elemental Co is formed. Between 475 and
from those of the unsubstituted LCO. The main differences
observed are in the phase transition temperature and compo-
sition. In general, the reduction temperature of substituted
2 3
550 °C, the formation of cubic and hexagonal La O phases is
LaCo1 Pd Zn O decreases with increasing Pd content in
observed. Above 575 °C, the cubic La O phase disappears
−x−y
x
y
3±δ
2 3
the catalyst, in line with the H
2
-TPR data. The thermal expan-
and the XRD pattern is dominated by hexagonal La
2 3
O and
sion of the substituted perovskite lattice occurs at relatively
elemental Co. This is in line with the H -TPR profile of
2
LCPZO-1-0.15 that shows complete reduction of the non-rare-
earth ions at around 600 °C.
2 3
During subsequent TPO, hexagonal La O and elemental
Co are sustained up to 200 °C as evident from Fig. 4. The par-
tial oxidation of Co to Co
00 °C, and hexagonal La
3
O
4
is observed between 225 and
is visible up to 650 °C. Above
6
2
O
3
this temperature, the rhombohedral crystal structure of the
perovskite appears and thereafter dominates the XRD pat-
tern. In general, the effect of Co substitution with Pd/Zn on
the re-oxidation during TPO appears to be minimal as evi-
dent from the phase composition and its transition tempera-
tures that are comparable for both unsubstituted LCO and
LCPZO-1-0.15. In line with this, TPO completely restores the
rhombohedral perovskite crystal structure of the
unsubstituted LCO and LCPZO-1-0.15 (Fig. S3†) without alter-
ing the phase composition of the material, indicating the
self-regeneration of the catalyst, at least in the bulk, during a
redox cycle.
To summarize the TPR investigation, the reduction of the
unsubstituted and substituted materials is a two-step process
involving low- and high-temperature reduction events. In situ
XRD reveals that the phase composition is temperature
dependent. Oxygen-deficient perovskite, cubic/hexagonal
2 3
La O and elemental Co phases are formed during TPR. The
oxygen-deficient perovskite phases are observed between 175
and 500 °C. The complete reduction of the catalysts occurs,
3
+
except for La as expected, at around 600 °C. The literature
review indicates that upon reductive treatment of classical
catalysts like Pd/ZnO, the intermetallic compound ZnPd is
3
formed. The temperature required for the formation of such
species strongly depends on the nature of the catalyst. The
formation of ZnPd is typically identified by XRD reflections at
13,40
2
θ of 41.2° and 44.2° in the case of Cu radiation.
the formation of ZnPd (also elemental Pd and Zn as well as
ZnO) in substituted LaCo1−x−yPd Zn 3±δ catalysts during TPR
Thus,
Fig.
4 XRD patterns of LCPZO-1-0.15 during TPR (above) and
subsequent TPO (below).
x
y
O
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is carefully analyzed. However, the identification of ZnPd
reflections is not straightforward from the XRD patterns of
the materials due to reflections arising from other phases
derived from the perovskite-type oxides such as oxygen-
deficient perovskite-type oxides, Co and La O that overlap at
the same 2θ angles. Therefore, the materials are further ana-
lyzed by HAADF-STEM to assess the state of Pd and/or Zn
species on the catalyst surface.
The HAADF-STEM images of fresh and reduced LCPZO-1-
0.15 are shown in Fig. 5. The fresh catalyst shows only perov-
skite crystals that can be seen as bright large particles
(Fig. 5A). These crystals are around 50 nm in size and are a
solid solution of perovskite-type oxide LCPZO-1-0.15 that con-
tains Pd and Zn within the crystal sites of Co. Any precipita-
tion of Pd and/or Zn from the crystal as nanoparticles is not
observed, which is in excellent agreement with the XRD data
2
3
Fig. 5 High angular annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the catalysts: fresh LCPZO-1-0.15 –
after calcination at 800 °C for 2 h (A); reduced LCPZO-1-0.15 – after reduction at 600 °C for 1 h (B); re-oxidized LCPZO-1-0.15 – after re-oxidation
at 800 °C for 1 h (E); reduced LCPO – after reduction at 600 °C for 1 h (F). EDX data obtained by a line scan on a Pd–Zn nanoparticle (inset) on the
reduced LCPZO-1-0.15 (C) and particle size distribution on the reduced LCPZO-1-0.15 (D).
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that reflect only the rhombohedral crystal phase (Fig. 1). The
reductive pretreatment at 600 °C results in the formation of
Pd–Zn nanoparticles as evident from the bright spots
(
Fig. 5B). The composition of the nanoparticles is confirmed
by an EDX line profile (Fig. 5C). These Pd–Zn nanoparticles
are homogeneously dispersed on the hexagonal La sub-
2 3
O
strate and the size distribution of the particles is narrow as
evident from the histogram (Fig. 5D). The average particle
size is determined to be around 3.5 (±1) nm, which is indeed
below the detection limit of XRD (Fig. 4). Such Pd–Zn nano-
particles are not observed after reductive pretreatment at 300
°C (data not shown). Interestingly, these nanoparticles disap-
pear upon re-oxidation as evident from Fig. 5E indicating the
dissolution of Zn and Pd into the perovskite crystal. These
observations infer the occurrence of a self-regeneration pro-
cess within the perovskite crystal under a redox cycle, in line
1
8
with in situ XRD (Fig. S3†) and the literature. In compari-
son, reduction of LCPO at 600 °C also yields Pd nanoparticles
as evident from Fig. 5F. However, these Pd nanoparticles are
larger than the Pd–Zn nanoparticles observed on LCPZO-1-
0
.15 and their size distribution is broader (between 4 and 10
nm). These results may suggest that the presence of Zn
restricts not only the particle size but also the size distribu-
4
1,42
tion, which is similar to the effect of Sn or P on Pt,
indi-
cating that base elements promote better dispersion of noble
metals. Based on these observations, it can be concluded that
the reductive pretreatment at 600 °C leads to the formation
of highly dispersed Pd and Pd–Zn nanoparticles on LCPO
and LCPZO-1-0.15, respectively.
Fig. 6 The effect of reductive pretreatment temperature on the MSR
performance of LCO (■, □) and LCPZO-1-0.15
pretreatment at 300 °C (open symbols) and 600 °C (closed symbols) is
studied with an H O : CH OH ratio of 1.3.
( , ). Reductive
2
3
3
.2 Methanol steam reforming (MSR)
The MSR performance of the perovskite-type oxide-derived
materials are evaluated as function of reductive
pretreatment temperature, H O : CH OH ratio, Co substitu-
a
significantly improves after reductive pretreatment at 600 °C,
while conversion begins at around 240 °C and is completed
at around 340 °C. Due to the significant difference in the
2
3
tion with Pd and/or Zn, Pd : Zn molar ratio, Pd and Zn con-
centration (at 1 : 1 Pd : Zn molar ratio) and time on stream
(stability).
3 2
CH OH conversion activity of the catalyst, the CO selectivity
The effect of reductive pretreatment temperature. It is evi-
is not comparable below 400 °C (Fig. 6). Above this tempera-
ture, the selectivity of the catalyst is similar regardless of the
reductive pretreatment temperature.
Differently, the substituted LCPZO-1-0.15 exhibits very
interesting results after the two reductive pretreatments.
Reduction at 300 °C results in slightly higher overall activity
dent from H -TPR and in situ XRD data that the reduction
2
behavior and phase composition of the catalysts vary as a
function of temperature. Based on this finding, two reductive
pretreatment temperatures are selected: (i) 300 °C and (ii)
3
+
6
00 °C. At 300 °C, the surface or partial reduction of Co
2
+
and the complete reduction of Pd occur, while at 600 °C,
bulk reduction of cobalt and zinc cations takes place and
well-dispersed Pd or Pd–Zn nanoparticles are formed as evi-
dent from HAADF-STEM. Temperature-programmed MSR is
studied after two different reductive pretreatments, and the
corresponding CH OH conversion and CO selectivity profiles
than that at 600 °C, while the overall CO
catalyst is lower after pretreatment at 300 °C. At a compara-
ble CH OH conversion of around 25% at 250 °C, CO selectiv-
2
selectivity of the
3
2
ities of around 25 and 65% are observed after pretreatment
at 300 and 600 °C, respectively (Fig. 6). The activity and selec-
tivity data corroborate two important aspects of the catalyst
that are derived from the characterization results: (i) reduc-
tive pretreatment at 300 °C results in the formation of ele-
3
2
of selected catalysts are shown in Fig. 6. The MSR perfor-
mance of the catalysts is found to be strongly dependent on
the reductive pretreatment temperature. After pretreatment
mental Pd (as evident from H
known to decompose methanol with high CH
2
-TPR, see Fig. 2) which is
OH conversion
at 300 °C, the unsubstituted LCO shows detectable CH
3
OH
3
conversion only at around 350 °C, and complete conversion
is observed at around 500 °C. The activity of the catalyst
activity and (ii) pretreatment at 600 °C forms highly dis-
persed Pd–Zn nanoparticles and enhances the methanol
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steam reforming selectivity. The activity data reflect the litera-
ture, concluding that elemental Pd is active but not selective
4
3
13,44
for the MSR reaction, while ZnPd is more selective.
It is
also suggested that complete alloy formation is not a prereq-
uisite for achieving high CO2 selectivity, but elimination of
small monometallic Pd particles (<2 nm) formed during
4
5
reductive pretreatment is essential. In line with these obser-
vations, the CO selectivity profile of the LCPZO-1-0.15 cata-
2
lyst consists of two distinct maxima. The low-temperature
maximum centered at around 240 °C, which is not present in
the profile of the unsubstituted LCO, indicates the important
2 2
role of Pd–Zn in CO selectivity. The high-temperature CO
selectivity maximum assigned to above 400 °C is identical
over all the catalysts studied here and is tentatively attributed
to the bulk catalyst composition. The contribution of rWGS
4
3
(
eqn (3)) cannot be excluded. Based on these observations,
the reductive pretreatment temperature of 600 °C is
implemented for further studies.
The effect of the H O : CH OH ratio. An important reac-
2
3
tion parameter that can influence CO
2
selectivity is the H
2
O-
O-
to-CH OH ratio in the MSR feed. According to eqn (1), H
3
2
to-CH OH ratio of one should yield selective MSR perfor-
3
mance giving rise to CO
formation of CO is a result of CH
depends on the catalyst composition and its H O activation
2
and H
2
, without CO formation. The
3
OH decomposition which
2
2 3
capability. Thus, the effect of the H O-to-CH OH ratio on the
MSR performance of the catalysts is studied by employing
two different ratios, namely 1 and 1.3 (not shown). The com-
position with excess H
tion) in the feed yields better CH
selectivity in the whole temperature range than the stoichio-
metric feed composition. At 240 °C, around 16% CH OH con-
version and 64% CO selectivity are obtained with the feed
ratio of 1.3, while they become significantly lower at the stoi-
chiometric feed ratio (6% conversion and 50% CO selectiv-
ity). The superior CO selectivity of the catalyst appears to be
constant in the whole temperature range studied. These
results suggest that the H O-to-CH OH ratio of 1.3 yields bet-
selectivity, and hence is
2
O (i.e., decreased CH
3
OH concentra-
Fig. 7 The effect of Co substitution with Pd and/or Zn on the MSR
performance of LCO (■, □), LCPO (
.15 ( ). Reductive pretreatment at 600 °C and an H
ratio of 1.3 are employed.
,
), LCZO (
,
), and LCPZO-
3
OH conversion and CO
2
1
,
2
O : CH OH
3
3
2
2
and LCPZO-1.15 exhibits around 11% conversion. Despite
their comparable activity, LCPO and LCPZO-1.15 present sig-
nificantly different CO2 selectivities of 7 and 23%, respec-
tively. The CO selectivities of LCZO and LCO are 4 and 0%,
2
respectively. These results suggest that the unsubstituted
2
2
3
ter MSR performance, especially CO
utilized for further studies.
2
LCO mainly promotes CH OH decomposition (eqn (2)) at this
3
The effect of Co substitution with Pd and/or Zn. In order
to assess the role of the Pd–Zn particles in the CO selectivity,
temperature. The partial substitution of Co with either Pd or
Zn does not prevent the detrimental decomposition reaction.
2
6
,10,13,46
the effect of Co substitution with Pd, Zn and Pd/Zn is stud-
ied. The MSR performance of the unsubstituted LCO is com-
pared with those of substituted LCPO, LCZO and LCPZO-1.15
in Fig. 7. The overall MSR activity slightly increases upon par-
tial substitution of Co with Pd (see LCPO). Contrarily, the
activity decreases considerably upon fractional substitution
of Co with Zn (LCZO). The MSR activity of LCPZO-1.15
containing both Pd and Zn is comparable to that of LCPO.
This behavior is known for Pd but not for Zn.
influence of Zn on the reaction might have been masked by
the La and CoO/Co in LCZO. However, partial substitution
of Co with both Pd and Zn considerably suppresses the decom-
position reaction as evident from the CO selectivity of LCPZO-
1.15. This could be assigned to the improved H O activation
capability of the catalyst and is discussed later in the effect
of Pd/Zn concentration section. The effect of Pd–Zn on CO
The
2 3
O
2
2
4
7
2
These results indicate that Pd is more active for CH
version than Zn, which is in line with previous reports.
The CO selectivity of the catalysts is strongly dependent on
3
OH con-
selectivity can only be observed below 400 °C and is the
3
,6,10,46
highest at around 250 °C. Above 400 °C, the CO selectivity of
2
2
the substituted catalysts coincides well with that of the unsub-
the catalyst composition, especially below 400 °C as
2
stituted LCO. These observations suggest that the CO selectiv-
discussed earlier. At 250 °C, LCO and LCZO show CH OH
ity of LCO can be improved well below 250 °C by substitution
3
conversions of around 0.4 and 1.5%, respectively, while LCPO
of Co with Pd and Zn. To this end, the effect of the Pd: Zn
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molar ratio (in the catalyst precursor LaCo1 Pd Zn O3±δ) on
is identical as noted above. Interestingly, the low-temperature
−x−y
x
y
the CO selectivity is methodically studied.
CO selectivity maxima shift from 250 to 243 and 225 °C with
2
2
The effect of the Pd : Zn molar ratio. The Pd and Zn molar
ratio in LaCo1 Pd Zn O is varied between 0 and 1.15
increasing Pd and Zn concentration in LCPZO-1-0.05, LCPZO-
1-0.15 and LCPZO-1-0.25, respectively. The MSR performance
of the catalysts at 225 °C is compared in Fig. 9. At this tem-
−x−y
x
y 3±δ
1
3,44
(
Table 1), which is based on the literature
and Pd–Zn
4
8
phase diagram. The MSR performance of the catalysts is
3
perature, the CH OH conversion is much lower over LCO
compared with that of the unsubstituted LCO at 225 °C in
(0.5%) than that over LCPZO-1-0.05 (2.5%). Despite this, the
Fig. 8. The CH OH conversion increases from 0.5 to 10.5%
CO selectivity of LCO is around 0% while that of LCPZO-1-
3
2
with increasing Pd : Zn molar ratio from 0 to 1. Upon further
increasing the ratio from 1 to 1.15, the conversion remains
the same. The CO2 selectivity also increases significantly
from 0 to 57% with increasing Pd : Zn ratio up to 1. Above
0.05 is 35.3%. By increasing the Pd/Zn concentration from
LCPZO-1-0.05 to LCPZO-1-0.15, not only CH OH conversion
3
(8.4%) but also CO2 selectivity (53.3%) improves. The CO2
selectivity further increases from 53.3% to 76.0% with the
increase in the Pd/Zn content in LCPZO-1-0.25, while the con-
version improves slightly to 10.7%. The MSR performance of
this level, however, the CO selectivity significantly drops to
2
2
3% for LCPZO-1.15. These observations indicate the impor-
tance of the Pd : Zn molar ratio in the LaCo1−x−yPd Zn
x
y
O
3±δ
2 2
the catalysts is in line with the H O conversion and H con-
precursor for CO -selective MSR performance, in line with
centration as shown in Fig. 9. At 225 °C, H O is not converted
2
2
previous reports on unsupported ZnPd (ref. 10, 49) and
supported Pd/ZnO. Such studies conclude that catalysts
on the unsubstituted LCO, hence CO2 formation does not
also occur, indicating the absence of the MSR reaction (eqn
(1)) as well as rWGS (eqn (3)). Thus, a H2 concentration of
1500 ppm at this temperature over LCO can be attributed to
1
3
with a Pd : Zn ratio close to one present CO -selective MSR
2
3
activity and excess Pd promotes the detrimental CH OH
decomposition reaction (eqn (2)), which is also evident from
the LCPO performance as discussed above. Based on this
3
the CH OH decomposition reaction (eqn (2)). This trend is
altered for Pd and Zn-containing catalysts. By increasing the
amount of Pd/Zn in the LCPZO-1-0.05, LCPZO-1-0.15 and
finding, an attempt is made to further improve the CO
tivity of the catalysts by increasing the total Pd and Zn con-
tent in LaCo1 Pd Zn O while keeping the molar ratio
2
selec-
2
LCPZO1-0.25 catalysts, H O conversion increases linearly
confirming the involvement of Pd/Zn in the H O activation
−x−y
x
y
3±δ
2
(Pd : Zn) of 1 and the phase purity (single-phase rhombohe-
and hence in the selective MSR reaction. This is further
dral crystal structure) of the perovskite-type oxide catalyst
precursor.
The effect of Pd and Zn (Pd/Zn) concentration (at 1 : 1 Pd :
Zn molar ratio). The effect of three different degrees of Co
supported by the H
increases also linearly with the amount of Pd/Zn in the cata-
lysts. The H formation rate is 1.9, 3.0 and 4.8 times higher
on LCPZO-1-0.05, LCPZO-1-0.15 and LCPZO-1-0.25, respec-
tively, than that on the unsubstituted LCO. These results con-
firm that the low-temperature MSR performance of the
unsubstituted LCO is very poor, which can be greatly
improved by substitution of Co with Pd/Zn. This can be
2
concentration at 225 °C (Fig. 9) which
2
substitution (see Table 1) with Pd and Zn on the CO selectiv-
2
ity is studied. The overall MSR performance of the catalysts
increases significantly with increasing concentration of
Pd/Zn, especially the low-temperature CO2 selectivity (see
Fig. S4†). Above 400 °C, the MSR performance of the catalysts
2
attributed to the improved H O activation ability of the
Fig. 9 The effect of Pd/Zn concentration (at 1 : 1 Pd : Zn molar ratio)
on the MSR performance of the catalysts is compared at 225 °C: LCO
(squares), LCPZO-1-0.05 (triangles), LCPZO-1-0.15 (stars) and LCPZO-
Fig. 8 The effect of the Pd : Zn molar ratio on the MSR performance
of the catalysts is compared at 225 °C: LCO (■, □), LCPZO-0.09 (
LCPZO-0.38 ), LCPZO-1-0.15 and LCPZO-1.15
CH OH conversion (closed symbols) and CO
,
,
),
).
1-0.25 (diamonds). CH
conversion (half-filled symbols), CO
concentration (symbols with cross).
3
OH conversion (closed symbols),
2
H O
(
,
(
,
)
(
2 2
selectivity (open symbols), and H
3
2
selectivity (open symbols).
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catalysts. However, the high-temperature (>400 °C) MSR per-
formance of the catalysts is comparable and can be ascribed
appears at 130 °C indicating the transformation of the hex-
agonal La O phase to LaIJOH) . Simultaneously, Co dimin-
2
3
3
to the bulk catalyst composition (i.e., La
phases may promote temperature-dependent rWGS (eqn (3))
2
O
3
and Co). These
ishes between 130 and 205 °C as evident from the XRD
reflection at 2θ = 44.5° that disappears with temperature.
This is accompanied by the emergence of the CoO phase at
2θ = 36.4° and 42.3°. The intensity of the reflections
increases with temperature, suggesting the re-oxidation of
elemental Co by steam. Besides these two phases, the
LaOOH phase appears at around 350 °C and is visible up to
and/or CH OH decomposition (eqn (2)) reactions. An attempt
3
is made to shed some light on the origin of CO
the catalysts by in situ XRD under steam atmosphere.
2
selectivity of
3
.3 In situ XRD under steam atmosphere
5
00 °C (not shown). Above this temperature, La CoO
2 4
In situ XRD under steam atmosphere is studied over the
unsubstituted LCO and substituted LCPZO-1-0.15 (Fig. 10).
Prior to the experiments, the catalysts are reductively
pretreated at 600 °C for 1 h followed by cooling down to 100
(besides CoO) is observed, suggesting the formation of
oxygen-deficient perovskite-type oxides under steam atmo-
sphere. Based on these results, it can be suggested that
LaIJOH) , Co, CoO and LaOOH phases are present between
3
°C, which is identical to the pretreatment protocol utilized
130 and 400 °C.
for MSR activity tests (section 3.2). At this temperature, hex-
After reductive pretreatment at 600 °C, LCPZO-1-0.15
yields a mixture of cubic and hexagonal La O and Co (see
agonal La O (2θ = 26.3° and 30.1°) and Co (2θ = 44.5°) are
2
3
2 3
identified on LCO. Upon introduction of H
2
O vapor (steam)
Fig. 10), in agreement with in situ XRD during TPR (Fig. 4). It
is worth noting that during cooling from 600 to 100 °C,
at 100 °C, the hexagonal La O phase gradually diminishes
2
3
and disappears at around 150 °C. Subsequently, a new phase
assignable to LaIJOH) (2θ = 27.4°, 28°, 39.6° and 48.7°)
2
LCPZO-1-0.15 reacts below 250 °C with H O that forms dur-
ing the reductive pretreatment (see the bottom four patterns
in Fig. 10). As evident from Fig. 10, the XRD reflections attrib-
3
utable to hexagonal La
intensity, while reflections assignable to cubic La
6.5° and 43.8°) grow in intensity which is accompanied by a
shift in the XRD reflections to slightly lower 2θ angles as
O
2 3
(2θ = 26.2° and 30°) diminish in
2 3
O
(2θ =
2
36
compared to the reference La
2 3
O .
This is different from that
observed for LCO which does not show any hydroxide phase
formation while cooling from 600 to 100 °C after reductive
pretreatment. Upon introduction of steam, LCO forms La-
IJOH) at the expense of hexagonal La O which is again dif-
3
2 3
ferent from that observed on LCPZO-1-0.15. Besides this, two
new phases, ZnIJOH) (2θ = 30.5°) and CoIJOH) (2θ = 37.7°),
are identified on LCPZO-1-0.15, which are not present (espe-
cially CoIJOH) ) on LCO. The formation of hydroxide phases
2
2
2
indicates the reactivity of the catalysts towards
molecules.
2
H O
Upon introduction of steam at 100 °C, the hexagonal
phase starts to diminish (2θ of 30°) and disappears at
around 160 °C, while the ZnIJOH) phase grows in intensity
2 3
La O
2
along with CoIJOH)
arising from cubic La
2
. However, the XRD reflection at 26.5°
is hardly affected. Unlike on LCO,
2 3
O
the LaIJOH)3 phase is not observed on LCPZO-1-0.15. These
results suggest different sensitivities/reactivities of the cata-
lysts towards H
C. Above this temperature (not shown), the following impor-
tant phase transformations are observed: (i) the ZnIJOH)
2
O. The observed phases are present up to 430
°
2
phase disappears with simultaneous appearance of the ZnO
phase that reflects at 2θ of 36.1°, 42.3° and 61.4° (PDF refer-
5
0
ence code: 03-065-0523), and (ii) CoIJOH)
2
diminishes and is
perhaps transformed into CoO (PDF reference code: 01-071-
5
1
1
178) which overlaps at the same 2θ of ZnO. Above 500 °C,
La CoO , cubic La , ZnO and CoO are present.
By comparing LCO and LCPZO-1-0.15, it is evident that the
2
4
2 3
O
Fig. 10 In situ XRD patterns of LCO (above) and LCPZO-1-0.15
below) during steam activation. For the sake of clarity, selected XRD
(
evolution of phase composition under steam atmosphere is
different and is a function of temperature. For example, at
reflections up to either 340 °C (LCO) or 430 °C (LCPZO-1-0.15) are
shown. Refer to section 2.2 for experimental conditions.
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25 °C, the LaIJOH) and CoO phases are mainly present on
3
LCO, while ZnIJOH) , CoIJOH) and cubic La O are observed
2
2
2 3
on LCPZO-1-0.15. However, the phase composition, such as
oxygen-deficient perovskite-type oxides La CoO and CoO, is
2
4
similar over the two catalysts above 500 °C which can explain
the identical high-temperature (>400 °C) CO selectivity pat-
2
tern of the catalysts. The low-temperature (<400 °C) CO2
selectivity of the substituted LCPZO-1-0.15 can be attributed
to Pd–Zn nanoparticles that promote hydroxide phases such
as ZnIJOH) which is not observed on the unsubstituted LCO.
2
This is anticipated due to the fact that LCO does not contain
Zn. Interestingly, both LCO and LCPZO-1-0.15 contain Co but
the formation of CoIJOH)2 is only observed on the latter
catalyst.
4
. Structure–activity relationships
x y
The perovskite-type oxides LaCo1−x−yPd Zn O3±δ exhibit a
single-phase rhombohedral crystal structure after calcination
Scheme 1 Schematic representation of the self-regeneration process
upon a redox cycle: the rhombohedral crystal phase of a perovskite-
as evident from XRD (Fig. 1 and S1†). The surface area of the
2
−1
catalysts varies between 6.5 and 10.5 m g , indicating simi-
lar physical and textural properties of the catalysts. The
reduction behavior of the materials significantly depends on
the nature of the substituent (i.e. Pd or Zn) of Co as evident
type oxide LaCo1
Pd Z O
±
δ 2 3
transforms into hexagonal La O , cubic
−x−y
x
y
3
2
Co and Pd–Zn alloy nanoparticles upon reduction at 600 °C in H .
These phases reassemble as rhombohedral LaCo1−x−yPd
re-oxidation at 800 °C.
x y
Z O
3±δ upon
from H -TPR data (Fig. 2 and S2†). Compared with the TPR
2
profile of LCO, the profiles of LCPO and LCZO evidently show
that Pd decreases while Zn increases the reduction tempera-
ture, respectively. In the presence of both Pd and Zn, a mod-
erate reduction behavior of the catalysts is observed which
falls between the reduction temperatures of LCPO and LCZO.
In any case, these catalysts are reducible in two steps as evi-
dent from the low- (<450 °C) and high-temperature (>450
particles are not observed after reductive pretreatment at 300
°C. Based on this data, reductive pretreatment temperatures
of 300 and 600 °C are investigated for the MSR reaction. The
results indicate that the higher the reductive pretreatment
temperature, the better the MSR performance of the catalysts,
2
especially the CO selectivity which is temperature and com-
°
C) reduction events. In situ XRD during TPR shows that the
position (Pd : Zn ratio and Pd and/or Zn concentration)
dependent. The selectivity profile comprises a low- (between
225 and 250 °C) and a high-temperature (>400 °C) maxi-
mum. The former is catalyst composition dependent, while
the latter is not and is comparable over all the catalysts. The
low-temperature reduction step is due to the partial reduc-
tion of cobalt cations, while the high-temperature reduction
event at 600 °C is attributable to the complete reduction of
the perovskite-type oxide (except for LCZO) to its constituents
like hexagonal La O and elemental Co. The formation of nei-
activity data indicate that the low-temperature CO
is directly related to the H O conversion activity of the cata-
lysts, which is proportional to the Pd/Zn content of the cata-
lyst. The ZnIJOH) and CoIJOH) phases exclusively form on Zn
and Pd-containing LaCo1−x−yPd Zn 3±δ catalysts as evident
2
selectivity
2
3
ther elemental Pd nor Pd–Zn during TPR could be confirmed
categorically from the dominant XRD reflections of La O
2
2
3
and elemental Co. However, reduction of Pd and Zn-
containing catalysts at 600 °C results in the formation of
homogeneously distributed Pd–Zn nanoparticles of around
3
0
size of Pd or Pd–Zn nanoparticles might have been below the
XRD detection limit. Based on these results, the following
scheme can be envisaged to explain the increased MSR selec-
tivity of catalysts derived from their precursor perovskite-type
2
2
x
y
O
from in situ XRD under steam atmosphere. These two phases
are not observed on either unsubstituted LCO or mono-metal
Pd- or Zn-substituted LCPO and LCZO. The hydroxide phases
are present between 100 and 400 °C, and above this tempera-
ture, they transform into the corresponding oxide and
oxygen-deficient lanthanum cobaltite phases. The presence of
hydroxide phases below 400 °C coincides well with the low-
.5 (±1) nm as evident from the HAADF-STEM of LCPZO-1-
.15. Likewise, Pd nanoparticles are formed on LCPO. The
oxides LaCo1 Pd Zn O
y 3±δ
by reduction. According to
temperature CO
the catalyst composition. Though the role of CoIJOH)
in the CO selectivity is not clear, the role of ZnO is well doc-
2
selectivity maxima, which is dependent on
−x−y
x
Scheme 1, the rhombohedral crystal structure of the
perovskite-type oxides transforms into its constituents, hexag-
onal La O and Co, and highly dispersed Pd or Pd–Zn nano-
2
species
2
2
3
2 3
umented. The dissociation of H O and CH OH is one of the
5
2
particles precipitate upon reductive treatment at 600 °C.
These phases appear to be largely reversible upon TPO as evi-
dent from XRD (Fig. S3†) and HAADF-STEM (Fig. 5E). Such
initial steps in the MSR reaction and is promoted on ZnO,
especially on the unsaturated surface. The remaining reaction
might proceed over the Pd surface which is in close vicinity
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the size and distribution of the nanoparticles. The Pd–Zn
nanoparticles significantly improve the CO -selective metha-
2
nol
steam
reforming
(MSR)
performance
of
LaCo1 Pd Zn O3±δ, while Pd nanoparticles mainly promote
−x−y
x
y
CO-selective methanol decomposition. Thus, the CO -selec-
2
tive MSR performance of the catalysts strongly depends on
the reductive pretreatment temperature and catalyst composi-
tion. The unsubstituted LaCoO3±δ (LCO) merely promotes the
3
CH OH decomposition reaction. Partial substitution of Co
with either Pd (LCPO) or Zn (LCZO) does not improve the
MSR performance. This is altered by Co substitution with
both Pd and Zn. Furthermore, the MSR activity improves with
increasing Pd/Zn molar ratio up to 1. The higher the degree
of Co substitution with Pd : Zn (1 : 1), the better the MSR per-
formance. Accordingly, the best MSR performance is achieved
over LaCo0.75Pd0.125Zn0.125O3±δ with a CO2 selectivity of 76%
at 225 °C. The CO2 selectivity profile of the substituted
3
Fig. 11 The stability of LCPZO-1-0.15 during MSR at 245 °C: CH OH
conversion (closed symbols) and CO
was interrupted for a few minutes during 58.5 h of time on stream.
Reductive pretreatment at 600 °C and an H O : CH OH ratio of 1.3 are
employed. Refer to section 2.3 for experimental details.
2
selectivity (open symbols). GC
x y
LaCo1−x−yPd Zn O3±δ catalysts consists of two distinct max-
ima. The low-temperature (225–250 °C) CO2 selectivity is
attributed to Pd–Zn nanoparticles which are responsible for
2
3
the formation of ZnIJOH)
sphere, as evident from in situ XRD. Neither the low-
temperature CO selectivity maximum nor these hydroxide
2 2
and CoIJOH) under steam atmo-
5
3,54
to the Zn oxide and/or hydroxide.
These results are in
2
line with a recent study which concludes that the low-
phases are observed on unIJor mono-metal)-substituted LCO
temperature CO selectivity crucially depends on the presence
(LCPO or LCZO). The high-temperature (>400 °C) CO selec-
2
2
1
4,46
of oxidized Zn species in contact with ZnPd during MSR.
Therefore, the low-temperature CO selectivity of the catalysts
in the present study is attributed to Zn-based species, includ-
tivity maximum is identical on all catalysts studied and is
attributed, in general, to the bulk catalyst composition (i.e.,
La O and Co) and the occurrence of rWGS reaction. The
2
2
3
5
5
ing Pd–Zn nanoparticles. The temperature-dependent CO
2
substituted LaCo1−x−yPd
stability as evident from LaCo0.85Pd0.075Zn0.075
after 80 h of time on stream which shows a relative activity
loss of 35% and increased CO selectivity from 40 to 60%.
x y
Zn O3±δ catalysts exhibit promising
selectivity profiles below 400 °C over the catalysts are tenta-
tively assigned to the dynamic behavior of the active/selective
O3±δ at 245 °C
phase(s) as a function of temperature. The CO
2
selectivity of
2
the catalysts, regardless of the composition, is similar above
4
3
4
00 °C, suggesting the occurrence of rWGS reaction. There-
Acknowledgements
fore, the stability of the LCPZO-1-0.15 catalyst during the
MSR reaction is assessed at 245 °C over 80 h (Fig. 11). The
reaction temperature corresponds to the low-temperature
The authors are thankful to Staatssekretariat für Bildung und
Forschung (SBF project no. C11.0034) for financial support.
The COST Action – CM0904 is also greatly appreciated. The
authors acknowledge the Swiss National Science Foundation
for funding the Micromeritics ASAP 2020c instrument
through R'Equip (project no. 206021 128741/1).
2 3
CO selectivity region of the catalyst. An initial CH OH con-
version of around 20% is observed which remains the same
up to 30 h of time on stream. After that, the conversion
slightly decreases and a conversion of around 13% is
detected after 80 h of time on stream, which accounts to an
overall loss of 35%. Interestingly, the CO selectivity continu-
2
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