The influence of La-doping on the activity and stability of Cu/ZnO catalyst for the low-temperature water-gas shift reaction

Article abstract of DOI:10.1016/j.jcat.2010.05.004

The influence of lanthanum (La) doping on the performance of 37 wt% Cu/ZnO catalysts for the low-temperature water-gas shift (LT-WGS) reaction was investigated. A 2.3 wt% La loading improved catalyst activity compared to the neat Cu/ZnO and Cu/ZnO/Al₂O₃ systems and was accompanied by a lowering of the activation energy. Higher La loadings promoted the adsorption of H₂O at the expense of CO, resulting in a decrease in LT-WGS activity. Additionally, 2.3 wt% La acted to stabilise catalyst activity compared with the neat Cu/ZnO. XPS and H₂-TPR assessment demonstrated a strong interaction between Cu and La components, while data from CO and H₂O-TPD studies favoured the associative WGS mechanism in this instance. Activity and stability findings also suggested metallic Cu was not responsible for LT-WGS activity.

Full text of DOI:10.1016/j.jcat.2010.05.004

Journal of Catalysis 273 (2010) 73–81
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
The influence of La-doping on the activity and stability of Cu/ZnO catalyst for the
low-temperature water–gas shift reaction
Rothman Kam a, Cordelia Selomulya a, Rose Amal b, Jason Scott b,
*
a Department of Chemical Engineering, Clayton Campus, Monash University, Australia
b ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Sciences and Engineering, The University of New South Wales, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of lanthanum (La) doping on the performance of 37 wt% Cu/ZnO catalysts for the low-tem-
perature water–gas shift (LT-WGS) reaction was investigated. A 2.3 wt% La loading improved catalyst
activity compared to the neat Cu/ZnO and Cu/ZnO/Al2O3 systems and was accompanied by a lowering
of the activation energy. Higher La loadings promoted the adsorption of H2O at the expense of CO, result-
ing in a decrease in LT-WGS activity. Additionally, 2.3 wt% La acted to stabilise catalyst activity compared
with the neat Cu/ZnO. XPS and H2-TPR assessment demonstrated a strong interaction between Cu and La
components, while data from CO and H2O-TPD studies favoured the associative WGS mechanism in this
instance. Activity and stability findings also suggested metallic Cu was not responsible for LT-WGS
activity.
Received 22 February 2010
Revised 10 May 2010
Accepted 11 May 2010
Available online 8 June 2010
Keywords:
Water–gas shift
Cu/ZnO
Lanthanum
H2O-TPD
Ó 2010 Elsevier Inc. All rights reserved.
CO-TPD
1. Introduction
catalyst stability is also a major challenge for the Cu/ZnO system
for LT-WGS reactions. It is well documented that Cu/ZnO is suscep-
The water–gas shift (WGS) is a reversible, exothermic chemical
reaction for conditioning product streams from the steam reform-
ing of hydrocarbons for industrial hydrogen production. It de-
scribes the reaction between carbon monoxide and water vapour
to produce carbon dioxide and hydrogen (Eq. (1)).
tible to sintering during reaction [6,7], resulting in a loss of active
sites. Cu/ZnO deactivation has been reported to derive from car-
bonate build-up on the surface which acts as a reaction-inhibiting
spectator specie [8].
Improved LT-WGS catalyst performance may be achieved by
doping the Cu/ZnO with additional metal cations. This is the case
for commercial Cu/ZnO/Al2O3 catalyst where alumina is employed
to stabilise the ZnO and prolong catalyst life [1,2,5,9]. Alkaline-type
cations have also been reported to improve Cu/ZnO LT-WGS activ-
ity. Shishido et al. [6] found doping Cu/ZnO with alkaline earth
metals (Mg, Ca, Sr and Ba) increased catalyst activity but did not
improve stability. Lanthanum (La) is an alkaline-type metal that
has been demonstrated by Schaper et al. [10] to provide structural
CO þ H2O $ CO2 þ H2
D
H ¼ ꢀ41:1 kJ molꢀ1
ð1Þ
Commercially, the WGS reaction is performed in two stages to
overcome the thermodynamic and kinetic limitations of the reac-
tion. The two steps involve a high-temperature (350–500 °C) shift
(HT-WGS) reaction typically over Fe2O3/Cr2O3 catalysts [1,2], at
which the favourable kinetics can be exploited and the volume of
the catalyst minimised, followed by a low-temperature (200–
250 °C) shift (LT-WGS) reaction over a Cu/ZnO-based catalyst
which takes advantage of the thermodynamic equilibrium at low
temperature [1,3]. Currently, the LT-WGS typically requires a large
reactor volume due to its slow reaction kinetics [1]. The LT-WGS
has also been observed to occur in small reactors at low tempera-
tures [4], although under these conditions the reverse reaction (i.e.
CO generation) was favoured. Hence, it remains desirable to devel-
op catalysts with higher activity to reduce steam production
requirements [5], reduce the LT-WGS catalyst volume and ulti-
mately reduce the costs associated with reactor size. Improving
stability for a methanation catalyst support (c-Al2O3).
Even though Cu/ZnO catalysts are well studied, uncertainties re-
main regarding the precise nature of the WGS mechanism. These
mechanisms are generally believed to be either associative
[11,12] or regenerative [6,13,14]. Salient features of the proposed
WGS mechanisms involve the adsorption of CO and H2O onto the
catalyst surface [15]. Some hold the view that during the WGS
reaction, a bifunctional synergistic effect exists between the copper
and zinc oxide [16,17] where the copper species activates the CO
molecule while the ZnO activates the water molecule [11,18].
The main difference between the two mechanisms concerns the
formation of formate-type (HCOO*) and/or carboxyl-type (OCOH*)
intermediates that subsequently decompose into the reaction
products (Eq. (2)) during the associative route [12,15,19].
* Corresponding author. Fax: +61 2 9385 5966.
E-mail address: jason.scott@unsw.edu.au (J. Scott).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.05.004

74
R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
CO þ H2O $ ðHCOOꢁ or OCOHꢁÞ þ Hꢁ $ CO2 þ H2
ð2Þ
2.3. Temperature-programmed reduction, desorption and oxidation
The regenerative route consists of surface oxidation of a oxygen va-
cancy site (ꢁ) by water vapour (Eq. (3)) followed with surface reduc-
tion by carbon monoxide (Eq. (4)) [7,13].
Temperature-programmed reduction (TPR), H2O and CO
desorption (TPD) and oxidation (TPO) experiments were per-
formed on a Micromeritics Autochem 2920 instrument interfaced
with a thermal conductivity detector (TCD) with the results nor-
malised to the mass of the sample used in the experiments. An iso-
propanol cooling trap was placed between the sample and the TCD
to retain the water formed during the reduction process. Down-
stream to the TCD, gases were analysed with an online mass spec-
trometer (Balzers Thermostar Quadrupole). For TPR experiments,
each sample (50 mg) was pre-treated at 120 °C under He
(50 ml minꢀ1) for 1 h then cooled to ambient temperature. TPR
was then performed under 5% H2/He (50 ml minꢀ1) at a ramp rate
of 5 °C minꢀ1 to 400 °C with the results recorded by the TCD.
Before each H2O-TPD experiment, the catalyst (50 mg) was re-
duced with 10% H2/He (50 ml minꢀ1) at 230 °C for 1 h. The catalyst
was then flushed for 1 h in He (50 ml minꢀ1) for the remainder of
H2O þ ꢁ ! H2 þ Oꢁ
ð3Þ
ð4Þ
CO þ Oꢁ ! CO2 þ ꢁ
The objective of this study is to explore the effect of lanthanum
(La) as a promoter in the Cu/ZnO catalytic system during WGS
reaction. In particular, the influence of La on Cu/ZnO catalyst activ-
ity and stability were investigated to gain greater insights into the
mechanisms governing catalyst performance of La promoted Cu/
ZnO catalyst. The catalysts were benchmarked against a commer-
cial Cu/ZnO/Al2O3 formulation for the LT-WGS reaction.
2. Experimental
2.1. Catalyst preparation
the TPD experiment. Deionised water (1.0 lL) was injected into
the sample cell using a micro-syringe. Pulses were repeated five
times with a 1-min interval between each injection, and the cata-
lyst bed was then flushed with He for a further 30 min. Finally, the
catalyst bed was ramped to 600 °C at a rate of 10 °C minꢀ1. The
outlet gas was monitored by mass spectrometry (Balzers Thermo-
star) when H2O was pulsed into the sample cell housing the re-
duced sample. No hydrogen was generated during this period.
During CO-TPD, the catalyst was reduced with 5% CO/He
(50 ml minꢀ1) at 230 °C for 1 h followed by flushing under He
(50 ml minꢀ1) for 1 h to remove physisorbed CO before returning
to ambient temperature. The CO-TPD proceeded by ramping to
600 °C at a rate of 10 °C minꢀ1. In all TPD experiments, oxidative
pre-treatment of catalysts to remove residue carbonaceous species
derived during FSP synthesis was not employed. The temperature
requirement to oxidize these species was too high (400–450 °C)
[24] which would likely have invoked catalyst sintering and influ-
enced the results.
A flame spray pyrolysis (FSP) reactor [20] was used to synthe-
sise CuO, ZnO, Cu/ZnO, Cu/ZnO/La2O3 and Cu/ZnO/Al2O3 particles.
In all cases, the Cu loading was maintained at approximately
37 wt%, while the Zn and La loadings were varied. The Cu/ZnO/
Al2O3 catalyst (F-COM) was fabricated based on a typical commer-
cial catalyst composition (weight ratio 37:55:8, respectively [21]).
Precursor solutions containing predetermined amounts of metal
oxides with a total molar concentration of 0.5 M were prepared
by mixing zinc 2-ethylhexanoate (Alfa, purity > 99%), lanthanum
(III) 2-ethylhexanoate (10 wt% in hexane solution, Aesar), alumin-
ium s-butoxide (Strem, purity > 98%) and copper 2-ethylhexanoate
(Aldrich, purity > 99.9%) in xylene (Riedel de Haen, 96%). During
FSP synthesis, the liquid precursor was fed (rate: 5 ml minꢀ1) to
the flame by a syringe pump (Inotech R232). The generated parti-
cles were collected on a glass fibre filter (Whatmann GF/D) with
the aid of a vacuum pump (Alcatel). Detailed descriptions on other
operating conditions can be found elsewhere [20]. La2O3 (Aldrich,
99.9%) was used as the La reference in this study.
During TPO, the catalyst bed was ramped to 500 °C at
10 °C minꢀ1 under 5% O2/He (50 mL minꢀ1). No catalyst pre-treat-
ment was conducted prior to TPO experiments.
2.2. Catalyst characterisation
2.4. Catalyst activity
X-ray diffraction (XRD) spectra were collected on a Philips
X’Pert MPD instrument using Cu K
a (k = 1.542 Å) with scan range
Activity testing was performed using a 6 mm I.D. stainless steel
packed bed reactor with the as-prepared catalyst sample (100 mg)
from 20° to 90° at a scan rate of 0.22° minꢀ1 and step size of
0.026°. Surface analysis of the catalysts was performed by X-ray
photoelectron spectroscopy (XPS) on ESCALab220i-XL (VG Scien-
diluted with
a-Al2O3 (70 lm, 500 mg). Prior to each activity test,
catalysts were reduced under a total flow of 100 ml minꢀ1 (10%
H2/N2) for 1 h at 230 °C (ramp rate of 5 °C minꢀ1) and flushed with
N2 for 30 min before being returned to ambient temperature. The
H2 concentration and temperature ramping rate were selected to
avoid thermal sintering of the catalyst due to the exothermic nat-
ure of the reduction from CuO to Cu metal reaction (Eq. (5)) [3].
tific) using a monochromatised Al K
a radiation at a pass energy
of 20 eV and at P < 2 ꢂ 10ꢀ9 mbar. The energy scale was calibrated
and corrected for charging using the C1s (285.0 eV) line as the
binding energy reference. Transmission electron microscope
(TEM) imaging of the catalysts was performed using a Phillips
CM-200 to observe size and morphology. Inductively coupled plas-
ma optical emission spectrometry (ICP-OES) was used to deter-
mine the metal content in each synthesised sample. The
measurements were taken on a Perkin-Elmer OPTIMA DV3000
apparatus, and the sample was dissolved in nitric acid (3 M) before
measurement. Copper metal surface area (CSA) was determined by
the N2O decomposition method at 90 °C using the same experi-
mental methodology reported by Jensen et al. [22], assuming a
reaction stoichiometry of two Cu atoms per N2 atom and a Cu sur-
face density of 1.46 ꢂ 1019 atom mꢀ2 [23]. Prior to the measure-
ment, 30 mg of sample was reduced at 230 °C for 30 min and
flushed with He (50 ml minꢀ1) for a further 30 min. The specific
surface areas (SSA) of the prepared catalysts were analysed by
nitrogen adsorption at 77 K, on the Micromeritics Tristar 3000,
using the BET model.
CuO þ H2 ! Cu0 þ H2O
D
H ¼ ꢀ80:8 kJ molꢀ1
ð5Þ
The reactant gas stream typically comprised 7% CO, 8.5% CO2, 23%
H2O, 37.5% H2 and 25% N2 (Coregas) at a flow rate of 100 ml minꢀ1
and was discharged at atmospheric pressure. Deionised water was
added via a vapourizer fed by a high-precision syringe pump (ISCO
Inst., model 260D). Operating temperatures ranged from 150 to
300 °C with 50 °C increments ramped at 10 °C minꢀ1. The product
stream, after passage through an ice trap to remove water, was ana-
lysed using two gas chromatographs (Shimadzu GC8A) fitted with
thermal conductivity detectors. One chromatograph was operated
with helium as the carrier gas through a 1.8-m CTR I column (All-
tech Associates, Inc) for CO2, CO, N2, O2 and CH4 analysis. The sec-
ond chromatograph employed argon as the carrier gas through a

R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
75
2-m molecular sieve 13X column (Alltech Associates, Inc.) for H2
analysis. The CO conversion was far from thermodynamic equilib-
rium [25] due to design of the experimental set-up which induced
low residence times to avoid mass transfer limitations on the
results.
Catalyst stability was continuously evaluated at an operating
temperature of 300 °C for 25 h. This temperature is higher than
standard LT-WGS conditions (200–250 °C) [2,26] and was selected
to accelerate the rate of any deactivation. Catalysts samples were
the particles are nonporous, as frequently observed for flame-made
particles [29].
The bulk weight ratio of Cu, Zn and La determined by ICP-OES
(Table 1) indicated actual cation loadings were close to the nomi-
nal values. Cu0 surface area increased from 7.6 m2/g for the neat
material to 27.4 m2/g when mixed with Zn; however, the presence
of La served to decrease the Cu0 surface area. This is reflected by a
drop in the Cu metal dispersion from 12.4% for Cu/ZnO to between
5.4% and 8.4% depending on La loading (Table 1). A similar influ-
ence was observed when 8 wt% Al2O3 was included in the Cu/
ZnO, with Cu dispersion decreasing from 12.4% to 3.9%.
not diluted with
a-Al2O3 in this series of experiments due to the
need for post-reaction characterisation. Further details regarding
the calculations for CO conversion are provided in Supporting
information (A).
3.2. XPS analysis
Changes to surface bonding with increasing La dopant concen-
tration were monitored by measuring the binding energies (BE)
of Cu 2p3, Zn 2p3 and La 3d5 core electrons. BEs and surface atomic
Cu/Zn and La/Cu ratios for La-doped Cu/ZnO catalysts are summa-
rised in Table 2. Binding energy of O1s was not considered due to
complications with the overlapping contribution of oxygen from
CuO, La2O3 and the ZnO support.
3. Results
3.1. Structural properties of the catalysts
The XRD spectra of nominal 37 wt% Cu-loaded ZnO-based nano-
particles synthesised by FSP (Fig. 1) showed the hexagonal struc-
ture of ZnO [27] was maintained over the range of La-doping
concentrations. The elevated Cu loading was also detectable by
XRD and gave a weak Bragg reflection at 2h = 38°, belonging to
the monoclinic CuO (1 1 1) phase [28]. There was no detectable
XRD reflection from La2O3 crystallites and no shift in Bragg posi-
tions of ZnO with increasing La loading. Evaluation of the ZnO crys-
tallite size from XRD using the Scherrer formula (Table 1) indicated
Cu addition reduced the crystal size from 20 nm to 17 nm. The CuO
(1 1 1) XRD reflection, indicated by (s) in Fig. 1, was too small for
accurate crystallite size evaluation, hence CuO crystallite size could
not be determined. Increasing La addition promoted further ZnO
crystal size reduction to 13 nm at 11 wt% La. The decrease in crys-
tallite size is reflected in the TEM images of neat, Cu-loaded and
Cu/La-loaded ZnO (Fig. 2a–d). The TEM micrographs also indicated
a shift in ZnO morphology moving from a rod-like shape for neat
ZnO (Fig. 2a) to spherical particles upon Cu incorporation
(Fig. 2b). Likewise, the decreasing crystal size influenced the spe-
cific surface areas of the neat and composite materials (Table 1)
with a corresponding increase in this characteristic with increasing
La loading. Although not depicted here, all the prepared samples
exhibited Type II N2 adsorption–desorption isotherms, suggesting
The BEs of Cu 2p3/2 and Zn 2p3/2 gave values of 933.9 eV and
1020.9 eV, respectively, corresponding to CuO and ZnO (Table 2).
These values differ slightly to those of Garbassi et al. [30] who re-
ported values of 933.4 eV and 1022.2 eV, respectively. This may
arise from the different synthesis routes during catalyst prepara-
tion as Garbassi and Petrini [30] used co-precipitation. The shift
in BEs when comparing the neat ZnO, CuO and the composite
material (Table 2) provides evidence of an interaction between
the Cu2+ and Zn2+ species. The presence of Cu2+ species can be read-
ily identified by a strong satellite peak at 944–945 eV [30,31] as
seen in Fig. 3. This satellite peak implies copper species on the cat-
alyst exist in the Cu2+ oxidation state following FSP synthesis [31].
The lack of a peak at 932 eV indicates negligible Cu0 or Cu+ is pres-
ent on the catalyst surface [32]. Inclusion of La into the system
shifted the Cu 2p3/2 peak maxima (Fig. 3) from 933.4 eV at low
La loadings (1 wt%) to 933.7 eV at higher La loadings (11 wt%)
and was accompanied by a decrease in La 3d5/2 peak maxima (Ta-
ble 2) from 835.4 to 835.1 eV demonstrating an interaction be-
tween CuO and La2O3. The simultaneous and opposing changes
to Cu 2p and La 3d BEs infer a decrease in electron density in the
Cu nucleus by increasing La dopant concentration. La2O3 inclusion
had no influence on the ZnO peak maxima.
The surface elemental ratios of Cu and Zn relative to La were
estimated from intensity ratios normalised by an atomic sensitivity
factor [33]. These ratios are listed in Table 2. It can be seen that the
Cu/Zn ratios are lower than bulk ratios obtained from ICP analysis,
indicating depletion in the Cu concentration on the particle sur-
face. On comparing the La/Zn and La/Cu ratios, an opposite effect
was observed, where the surface elemental ratio is higher than
the bulk ratio.
11.0% La
5.6% La
2.3% La
1.1% La
3.3. TPR analysis
0% La
La2O3
ZnO
The TPR profile of pure FSP-prepared CuO (Fig. 4) shows a
strong reduction peak at 290 °C which is assigned to the reduction
of CuO to metallic Cu as described by Eq. (5). The lack of any addi-
tional peaks agrees with XPS results indicating no Cu+ species are
present [34,35]. Peak shoulders appear for the composite material
between 170 °C and 200 °C and are more pronounced for the
11.0 wt% La-doped and F-COM catalysts. Although bulk ZnO reduc-
tion is thermodynamically feasible, temperatures as high as 650 °C
are required to reduce ZnO to metallic zinc [36] with no ZnO reduc-
tion peaks observed in this study (up to 400 °C). The Cu/ZnO com-
posite material possesses a lower reduction temperature (ꢃ210 °C)
than either of the neat materials, with this peak ascribed to CuO
CuO
Fig. 1. XRD spectra (Cu Ka, k = 1.542 Å) of FSP-made nominal 37 wt% Cu/ZnO doped
with varying La loadings. Pure CuO, ZnO and La2O3 are included for comparison. The
solid circles (ꢄ) represent characteristic peaks of ZnO hexagonal wurzite-phase. The
open circle (s) denotes peaks of monoclinic CuO (1 1 1) phase.

76
R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
Table 1
Physicochemical properties, apparent activation energy and turnover frequency of neat, La-doped and Al-doped Cu/ZnO catalysts.
Catalyst (nominal
weight ratio)
BET surface area
(m2 gꢀ1), SSA
Bulk composition in weight
ratioa (mole ratio) (%)
Crystallite
sizeb (nm)
Cu metal
dispersion (%)
Cu surface areac
(m2 gꢀ1), CSA
Ea
TOFd
(kJ molꢀ1
)
(sꢀ1
)
Cu
Zn
La
dCuO dZnO
CuO
ZnO
37% Cu/ZnO
35.8
42.8
67.5
100
0
0
100
62.7
(62.0)
60.5
(60.2)
59.4
(59.5)
55.9
(57.6)
51.9
(54.5)
54.9
(55.9)
0
0
0
25
–
–
–
20
17
1.4
0
12.4
7.6
0
27.4
–
–
–
–
37.3
(38.0)
38.4
(39.3)
38.3
(39.4)
37.5
(39.7)
37.1
(40.1)
37.0
42.9
43.1
33.6
31.6
32.6
60.9
3
0.008
0.012
0.018
0.009
0.004
0.063
Cu/ZnO/La (37:62:1)
Cu/ZnO/La (37:61:2)
Cu/ZnO/La (37:58:6)
Cu/ZnO/La (37:50:11)
72.0
72.8
75.2
80.4
80.3
1.1
(0.5)
2.3
(1.1)
5.6
(2.7)
11.0
(5.4)
8.1e
(5.3)
–
–
–
–
–
16
15
14
13
14
7.6
5.4
8.4
6.5
3.9
17.2
12.3
18.8
14.4
8.6
F-COM Cu/ZnO/Al2O3
(37:55:8)
(38.8)
a
b
c
Determined by ICP analysis.
Determined by XRD and Scherrer formula from averaging ZnO(1 0 0), ZnO(0 0 2) and ZnO(1 0 1) diffraction lines.
Determined by N2O decomposition and the calculation of the copper surface area, it is assumed that a reduced copper surface has a surface density of 1.46 ꢂ 1019 copper
atoms mꢀ2
.
d
TOF for the overall WGS reaction on Cu/ZnO-based catalysts at 230 °C, 1 atm total pressure, 7% CO, 8.5% CO2, 23% H2O, 37.5% H2 and 25% N2.
Weight ratio of Al2O3 component.
e
(a)
(b)
20nm
20nm
(c)
(d)
20nm
20nm
Fig. 2. TEM images of FSP-prepared (a) ZnO; (b) 37.3 wt% Cu/ZnO; (c) 5.6 wt% La-doped 37.5 wt% Cu/ZnO and (d) 11.0 wt% La-doped 37.1 wt% Cu/ZnO.
reduction and the temperature shift ascribed to interaction be-
tween the two oxides. The inclusion of La into the particles had
minimal effect on the CuO reduction temperature up to a 2.3 wt%
loading. Beyond this, La inclusion shifted the reduction tempera-
ture to higher values: 220 °C for 5.6 wt% and 230 °C for 11.0 wt%.
The TPR profile of F-COM is included for reference and shows a
reduction peak at ꢃ235 °C.
3.4. TPD analysis
The TPD spectra of water adsorbed on the reduced catalysts are
presented in Fig. 5. Water is strongly adsorbed on the neat FSP-ZnO
surface as indicated by its gradual desorption from ꢃ270 °C on-
wards, which accelerates at ꢃ350 °C and culminates in a peak max-
ima at 420 °C. Similar chemisorption of H2O on ZnO has been

R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
77
Table 2
Binding energies of core electrons and XPS atomic ratios of as-prepared FSP catalysts.
Catalyst (nominal weight ratio)
XPS binding energies (eV)
Surface elemental ratioa (%)
Cu 2p3/2
Zn 2p3/2
La 3d5/2
Cu/Zn
La/Zn
La/Cu
CuO
ZnO
37% Cu/ZnO
Cu/ZnO/La (37:62:1)
Cu/ZnO/La (37:61:2)
Cu/ZnO/La (37:58:6)
Cu/ZnO/La (37:50:11)
933.9
–
–
–
–
–
–
–
–
–
–
–
–
–
1020.9
1021.5
1021.5
1021.6
1021.5
1021.5
933.4
933.4
933.5
933.6
933.7
39.47 (61.29)
49.98 (65.28)
50.06 (66.22)
51.78 (68.92)
62.59 (73.58)
835.4
835.3
835.2
835.1
1.42 (0.83)
2.68 (1.85)
6.50 (4.69)
2.79 (1.27)
5.36 (2.79)
12.54 (6.80)
28.72 (13.47)
17.98 (9.91)
a
Values in parenthesis are elemental ratio determined by ICP analysis.
0%La
1.1%La
2.3%La
5.6%La
11.0%La
F-COM
FSP ZnO
CuO
11.0%La
5.6%La
2.3%La
1.1%La
0%La
925
930
935
940
945
950
200
250
300
350
400
450
500
550
600
Temperature (oC)
Binding energy (eV)
Fig. 3. Cu 2p3/2 XPS photoemission peak for FSP-prepared nominal 37% Cu/ZnO
catalysts with varying La loadings. Pure FSP-prepared CuO is included for
comparison.
Fig. 5. H2O-TPD profiles of FSP-prepared 37% Cu/ZnO catalysts with increasing La
dopant loadings. F-COM represents the Al-doped catalyst and is included for
comparison.
La loading is increased, the initial H2O desorption front becomes
steeper, the peak maxima increases and tapering off of the peak
is slower (incomplete by 600 °C). In the case of F-COM, Fig. 5 indi-
cates that while its desorption profile exhibits similarities to the
Cu/ZnO profile, the presence of Al2O3 in a Cu/ZnO system does
not facilitate H2O adsorption, as demonstrated by the lower H2O-
TPD maxima. Reduced FSP-CuO was not observed to adsorb/desorb
water (result not shown).
F-COM
11.0%La
5.6%La
2.3%La
1.1%La
0%La
The findings demonstrate, while H2O is strongly chemisorbed to
ZnO, the inclusion of CuO has a substantial influence on the ZnO
adsorption sites, shifting desorption to lower temperatures and
broadening the desorption peak. Increasing La addition boosts
H2O desorption, encouraging greater desorption at the lower tem-
perature and further broadening the adsorption site distribution.
The CO-TPD spectra on the reduced La/Cu/ZnO samples are
shown in Fig. 6. Weakly chemisorbed CO was evident as small
CO signals were detected at around 60–80 °C for all the samples
which agrees with literature [39,40]. A dominant peak was also ob-
served for all samples, exhibiting a maximum at between 300 °C
and 320 °C. As the La loading increased, the amount of desorbed
CO decreased and the peak maxima shifted to slightly higher tem-
peratures. The presence of Al2O3 in the structure, as shown by the
F-COM profile in Fig. 6 was detrimental to CO adsorption. CO-TPD
on neat FSP-ZnO and reduced neat FSP-CuO did not show evidence
of any CO adsorption (results not shown). Robert and Griffin stud-
ied a series of Cu/ZnO catalysts [39], prepared by different meth-
ods, using CO-TPD coupled with infrared (IR) spectroscopy. They
found CO did not chemisorb on ZnO in their Cu-containing cata-
CuO
50
100
150
200
250
300
350
400
Temperature (oC)
Fig. 4. H2-TPR profiles of FSP-prepared 37% Cu/ZnO catalysts at different La dopant
loading. Pure FSP-prepared CuO is included as a reference.
demonstrated by others [37,38]. Introducing Cu into the structure
distinctly alters the H2O-TPD profile with more rapid desorption
beginning at ꢃ270 °C, where it reaches a broader maxima between
370 and 400 °C, and then tapers off to around 600 °C. While La
addition does not alter the general shape of the Cu/ZnO H2O
desorption profile, it does introduce some modifications. As the

78
R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
Cu/ZnO catalyst loaded with 2.3 wt% La invoked a substantial de-
0%La
crease in Ea (10 kJ molꢀ1 difference), implying the forward WGS
reaction was promoted. At greater La loadings, despite the decrease
in CO conversion, the Ea remained relatively constant. No methane
was detected in this study suggesting high selectivity of the Cu/
ZnO catalysts. The stainless steel reactor did not contribute to
LT-WGS activity in this study (see Fig. S3 in Supporting informa-
tion (C)).
1.1%La
2.3%La
5.6%La
11.0%La
F-COM
3.6. Catalyst stability and post-reaction characteristics
The stability of three catalyst samples: (i) 37.3 wt% Cu/ZnO; (ii)
2.3 wt% La-doped-38.3 wt% Cu/ZnO; and (iii) F-COM during the
WGS reaction (at 300 °C) was assessed under 25 h of continuous
run-time. CO conversion with operating time is shown in Fig. 8.
Cu/ZnO conversion dropped by ꢃ11% over the first 5 h after which
it decreased a further 4% over the next 20 h. The presence of
2.3 wt% La markedly improved active site stability, although some
deactivation was evident with an observed ꢃ5% decrease in CO
0
100
200
300
400
500
600
Temperature (oC)
Fig. 6. CO-TPD profiles of FSP-prepared 37% Cu/ZnO catalysts at increasing La
dopant loadings. F-COM represents the Al-doped catalyst and is included for
comparison.
50
45
40
35
30
25
20
15
lysts but adsorbed only on the copper component [39]. To confirm
the carbonaceous species deposited on the catalyst surface during
FSP preparation did not influence CO-TPD results, the accompany-
ing CO2 signals are provided in Fig. S2 in the Supporting informa-
tion (B).
3.5. WGS performance of catalysts
The effect of La content on the activity of FSP-prepared La/Cu/
ZnO catalysts for the LT-WGS reaction is given in Fig. 7. While add-
ing 1.1 wt% La had negligible influence on activity, promotion with
2.3 wt% La increased CO conversion by ꢃ5% (to 44%) over the un-
doped Cu/ZnO catalyst. Further La addition was detrimental to
LT-WGS activity with CO conversion dropping to ꢃ50% (to 23%)
of the undoped material for 5.6 wt% La loading. Apparent activa-
tion energies (Ea) were determined (Table 1) from the rates of
CO consumption over the La/Cu/ZnO catalysts between 150 °C
and 300 °C. The turnover frequencies (TOF) were also evaluated
assuming the active sites were surface copper metal (Table 1).
37%Cu/ZnO
2.3%La
F-COM
10
5
0
0
5
10
15
20
25
Time on stream (h)
Fig. 8. CO conversion with time by 37.3% Cu/62.7% ZnO, 2.3% La/38.3% Cu/59.4%
ZnO and F-COM (37% Cu/54.9% ZnO/8.1% Al2O3) for the LT-WGS reaction. Reaction
conditions: temperature = 300 °C; flow rate = 100 mL minꢀ1 (7% CO, 8.5% CO2, 23%
H2O, 37.5% H2 and 25% N2); catalyst loading = 0.10 g; pressure = 1 atm.
50
45
40
35
30
25
20
15
10
5
0
0
2
4
6
8
10
12
La loading (wt %)
Fig. 7. Effect of lanthanum doping on CO conversion by 37 wt% Cu/ZnO (nominal)
for the LT-WGS reaction. Reaction conditions: operating temperature = 300 °C, total
flow = 100 mL minꢀ1 (7% CO, 8.5% CO2, 23% H2O, 37.5% H2 and 25% N2); catalyst
loading = 0.10 g; pressure = 1 atm.
Fig. 9. Specific surface area and Cu surface area changes for 37.3% Cu/62.7% ZnO,
2.3% La/38.3% Cu/59.4% ZnO and F-COM (37% Cu/54.9% ZnO/8.1% Al2O3) following
the LT-WGS reaction for 25 h. Reaction temperature = 300 °C.

R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
79
(a)
Before
After
Before
After
F-COM
F-COM
2.3%La
38.3%Cu/ZnO
2.3%La
38.3%Cu/ZnO
37.3%Cu/ZnO
37.3%Cu/ZnO
200
300
400
500
600
0
100
200
300
400
500
Temperature ( oC)
Temperature (oC)
Fig. 11. Comparison of TPO curves for selected catalysts before and after the 25 h
LT-WGS reaction at 300 °C.
(b)
Before
After
catalyst also lost H2O adsorption sites, however, to a lesser extent
than the undoped material. In the case of F-COM, the loss of H2O
adsorption sites was small. Similar behaviour was observed for
CO-TPD (Fig. 10b), with a substantial portion of the CO adsorption
sites lost during reaction for the undoped and La-doped Cu/ZnO
and a minor number lost for F-COM.
37.3%Cu/ZnO
2.3%La
38.3%Cu/ZnO
To assess whether a build-up of carbonaceous species contrib-
uted to catalyst deactivation, TPO experiments were conducted
on the fresh and used catalysts. Fig. 11 shows no accumulation of
carbonaceous species occurred on the catalyst surface during the
LT-WGS reaction for the neat and La-doped Cu/ZnO system. In fact,
it appears intrinsic carbonaceous species present on the fresh cat-
alyst were removed during the WGS reaction as the CO2 signal is
generally lower for the used material. A strong CO2 peak at
240 °C from the used-F-COM suggests a possible build-up of for-
mates or carbonates occurs on the catalyst surface over the 25 h.
Given there is no loss in activity for F-COM, it appears they behave
only as spectator species.
F-COM
0
100
200
300
400
500
600
700
Temperature (°C)
Fig. 10. Comparison of: (a) H2O-TPD profiles and (b) CO-TPD profiles; for selected
catalysts before and after 25 h LT-WGS reaction at 300 °C.
conversion over the 25-h period. Initial CO conversion by F-COM
was approximately 15% and 20% lower than the undoped and La-
doped catalysts, respectively; however, its activity was maintained
over the 25-h time frame.
4. Discussion
To aid in identifying the sources of catalyst deactivation/stabil-
ity, surface areas (Fig. 9) and TPD profiles (Fig. 10) of the post-reac-
tion materials were assessed. Fig. 9 indicates the Cu/ZnO catalyst
lost 33% and 18% of its initial specific surface area (SSA) and copper
surface area (CSA), respectively, during the reaction which may ac-
count for the activity loss incurred by this material. While La addi-
tion gave a mildly improved resistance to sintering (23% and 25%
loss of its initial SSA and CSA, respectively) compared with the un-
doped Cu/ZnO, the loss of these attributes remained substantial
but was not reflected by the catalyst activity. The presence of alu-
mina in the catalyst make-up gave a lower initial activity but im-
parted increased stability for both the catalyst structure and the
CSA. The 25-h reaction presented a small decrease in SSA (ꢃ8%),
and negligible change in CSA. Fig. 9 also lends some support to
the notion that Cu metal is not solely responsible for LT-WGS activ-
ity, as previously reported by Guo et al. [8]. Comparing the initial
CSA’s with initial LT-WGS activities indicates no correlation be-
tween the two.
4.1. Structural and surface features of Cu/ZnO-based catalysts
Transition of the rod-shaped FSP-ZnO crystallites into smaller
spherical ZnO crystals, as depicted in the TEM images (Fig. 2a
and b), has been observed by others [41]. Zhu et al. [41] reported
the morphology of nanostructured ZnO crystals could be altered
by varying the growth temperature and Cu to Zn ratio. Cu plays
an important role in confining the lateral growth of ZnO due to
competition between Zn and Cu for oxygen [41] which may be evi-
dent in the oxidative combustion process during FSP synthesis
[42]. Increasing La or Al-dopant loading effectively decreased the
Zn content in the catalyst bulk and had the effect of increasing
the Cu loading relative to Zn. This is expected to induce enhanced
competition for oxygen by the Cu during synthesis and may be the
source of the increasingly smaller ZnO crystallite sizes as demon-
strated by XRD (Table 1) and TEM (Fig. 2a–d).
The decrease in metallic Cu surface area with increasing La
loading may arise from the formation of a solid solution between
CuO and La2O3 crystals. Jacobs and Jayadevan derived a phase dia-
gram for the Cu–La–O system at 930 °C [43] and indicated a solu-
bility limit of 40 mol% CuO in La2O3 can be achieved without
altering the La2O3 structure.
Surface characterisation of the three post-reaction catalysts by
H2O-TPD and CO-TPD is shown in Fig. 10a and b, respectively.
Fig. 10a indicates the 37.3%Cu/ZnO lost almost half its H2O adsorp-
tion sites over the 25-h reaction period. The 2.3% La-loaded Cu/ZnO

80
R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
Intensification of the TPR profile shoulder (Fig. 4) as La loading
would remain constant over all the Cu/ZnO-based catalysts. This
finding suggests Cu species other than metallic Cu may be respon-
sible for the LT-WGS activity. Cu has been reported by Okamoto
et al. [48] to coexist in various oxidation states on the Cu/ZnO cat-
alyst surface. Using XPS on reduced Cu/ZnO, they revealed for high
copper content catalysts (>25 wt% CuO), the predominant Cu metal
species was well-dispersed metallic copper (Cu0) with Cu+ species
also present, originating from CuO dissolved in the ZnO lattice.
More recently, Harikumar and Rao [49] confirmed the existence
of Cu0 and Cu+ species on their reduced Cu/ZnO using Auger spec-
troscopy analysis. Additionally, surface Cu+ has a high affinity to
bind with CO molecules [49–53]. For instance, Didziulis et al.
[54] conducted extensive work on heats of adsorption of CO on dif-
ferent well-defined copper sites and on the ZnO surface. They asso-
ciated a high affinity of CO binding site with Cu+ centres on
ZnO(0 0 0 1) and ZnO(1 0 1 0) dimers [54].
H2O-TPD studies (Fig. 5) indicated the ZnO substrate was
responsible for adsorbing water, although Cu addition substan-
tially altered the H2O desorption profile of the support, shifting
the peak maxima to a lower temperature. La addition appeared
to generally increase the amount of H2O bound to the catalyst sur-
face compared with neat Cu/ZnO. Lanthanum oxide is known to be
highly hygroscopic [55], forming lanthanum hydroxide when ex-
posed to water vapour and then desorbing water under thermal
treatment (370 °C), to return to its oxide form (Eq. (6)) [56].
was increased in the Cu/ZnO system may be due to a reduction in
surface CuO that is kinetically faster and precedes bulk CuO reduc-
tion [35]. The shoulder is significant for the 11.0 wt% La-doped and
F-COM catalysts possibly due to their high specific surface areas
(80.4 and 80.3 m2/g, respectively). That is, the particles possess a
lower average size which in turn can affect the kinetics of surface
CuO reduction [35].
Pestryakov and Davydov [44] used CO adsorption with IR spec-
troscopy to show the surface basicity of La2O3 to be very high due
to the strong charge density on La2O3 oxygen ions. They were able
to observe an electron donor–acceptor interaction between Cux+
ions and La2O3 which indicated a loss of electron density in the
Cu2+ nucleus due to electron scavenging properties of the La2O3.
We observed from XPS a diminishing electron density in the Cu2+
nucleus manifested as a gradual shift in the Cu 2p3/2 spectra (Ta-
ble 2) to higher binding energies (Fig. 3), with increasing La load-
ing. This agrees well with Pestryakov and Davydov’s work [44].
The decreasing Cu2+ electron density implies the Cu2+ and O2ꢀ
bond is strengthened by La addition as the Cu2+ becomes more pos-
itively charged. This is reflected in the H2-TPR profiles with the
gradual shift in reduction peaks from 205 to 225 °C for 2.3–
11 wt% La/Cu/ZnO (Fig. 4) where the oxygen has become increas-
ingly harder to remove from CuO.
The lower Cu/Zn surface ratio from XPS analysis compared with
the bulk ratios acquired from ICP implies the CuO is not uniformly
distributed throughout the ZnO matrix. The homogeneous solubil-
ity limit of Cu in ZnO has been reported as 10 mol% [45] which is
considerably lower than the Cu loading used in this FSP catalyst.
This alludes to the idea that CuO may exist (in part) as inclusions
within the ZnO particles. Additionally, similar transition tempera-
tures from vapour to liquid phase at atmospheric pressure for CuO
at 2000 °C [46] and vapour to solid phase at 1900 °C for ZnO [47]
may induce component segregation during the FSP formation pro-
cess. In the case of La ratio with respect to Cu and Zn, the XPS sur-
face elemental ratio is consistently higher than the bulk ratio
(Table 2), suggesting an enrichment of La on the catalyst surface
due to nucleation of the La2O3 component after CuO and ZnO dur-
ing synthesis.
La2O3 þ 3H2O $ 2LaðOHÞ3
ð6Þ
Notably, increasing La content increased the amount of H2O des-
orbed between 280 °C and 400 °C. This temperature range overlays
the CO desorption temperature range and suggests La may increase
the amount of H2O adsorbed at the expense of CO, particularly at
loadings greater than 2.3 wt%.
Overall, the TPD results imply the ZnO substrate is primarily
responsible for adsorbing the H2O molecules, although the addition
of Cu to the system lowers the strength by which they are bound to
the surface. Cu on the other hand appears responsible for CO
adsorption, although this does not derive from its metallic state
but some other species, possibly Cu+. This site bifunctionality rules
in favour of the associative mechanism during LT-WGS. That is, fol-
lowing H2O and CO adsorption on the catalyst surface, carbona-
ceous intermediates are formed which then decompose into the
desired products, CO2 and H2 [11,57,58].
4.2. Catalytic behaviour, active sites and mechanism
The activity results (Fig. 7) demonstrated a decrease in CO con-
version for La loadings greater than 2.3 wt%. This coincided with
the shift to higher CuO reduction temperatures at La loadings
greater than 2.3 wt% during TPR. The shift may be explained by
the XPS findings which indicated increasing La addition decreased
Cu core electron density, making it more difficult to remove O from
the surface CuO. Coupled with this, CO-TPD results (Fig. 6) showed
La loadings of 5.6 wt% and above promoted a significant loss in CO
adsorption capacity of the catalyst. The loss in CO adsorption
capacity may derive from the increase in La loading on the surface
(with increasing La content) blocking surface active sites or elec-
tronic changes it may invoke on the adsorption sites. Given the
Cu component of the catalyst is reported to be responsible for CO
adsorption [39], it appears that increasing the La content beyond
2.3 wt% has a detrimental influence on both its electronic proper-
ties and capacity for adsorbing CO, in turn promoting the observed
loss in catalyst activity.
Table 1 demonstrated that La addition decreased Cu metal dis-
persion by between 6.5% and 12.4% over the neat Cu/ZnO. Relating
this finding to the activity results in Fig. 7 indicates no apparent
correlation between the two, implying this characteristic is not
responsible for LT-WGS catalyst activity. This notion is supported
by the changing TOF values provided in Table 1, whereby if the sur-
face Cu metal was responsible for the LT-WGS activity then the TOF
The inclusion of La into the catalyst promotes a mild improve-
ment in activity at 2.3 wt% after which higher loadings are detri-
mental to catalyst performance. The higher activity at 2.3 wt% is
reflected in the lower activation energy at this loading (compared
with 1.1 wt% La and neat Cu/ZnO), while further increases in La
loading appear to promote H2O adsorption at the expense of CO.
This may provide the source of the observed lower activities. Liu
and Rodriguez [59] concluded that dissociation of water is the
rate-limiting step in a Cu catalytic system for WGS reaction. Hence,
the presence of La increases the water adsorption and provides an
initial decrease in activation energy for the water dissociation.
4.3. Catalyst stability and deactivation
It is well documented that Cu/ZnO is susceptible to sintering
[6,7] during WGS operation, with a small amount of refractory
oxide such as Al2O3 or Cr2O3 typically added to stabilise the Cu/
ZnO structure, post-reaction catalyst characterisation showed
two effects were evident. First (with the exception of the Al-loaded
system), there were losses in catalyst surface area and Cu surface
area, the extent depending on the additive. Secondly, there was a
loss in CO and H2O adsorption capacity for the neat and La-loaded
catalysts but not for the Al-loaded system. The loss in substrate
and Cu surface areas for the neat Cu/ZnO may account for the ob-

R. Kam et al. / Journal of Catalysis 273 (2010) 73–81
81
[2] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic
Processes, John Wiley & Sons, Ltd., Hoboken, NJ, 2006.
[3] C. Rhodes, G.J. Hutchings, A.M. Ward, Catal. Today 23 (1995) 43–58.
[4] E. Bogel-Łukasik, R. Bogel-Łukasik, M.N.d. Ponte, Monatsh. Chem. 140 (2009)
1361–1369.
[5] D.S. Newsome, Catal. Rev: Sci. Eng. 21 (1980) 275–318.
[6] T. Shishido, M. Yamamoto, I. Atake, D. Li, Y. Tian, H. Morioka, M. Hondac, T.
Sano, K. Takehira, J. Mol. Catal. A: Chem. 253 (2006) 270–278.
[7] T. Shishido, M. Yamamoto, D. Li, Y. Tian, H. Morioka, M. Honda, T. Sano, K.
Takehira, Appl. Catal. A 303 (2006) 62–71.
served decrease in LT-WGS activity with time; however, this is not
the case for the La-loaded system with comparative losses in these
two characteristics not emulated by the activity decrease. This im-
plies the La acts to preserve the active sites despite the occurrence
of thermal sintering. Thermal sintering does appear to govern the
relative losses in CO and H2O adsorption capacity of the catalysts,
although the extent of loss in adsorption capacity is not closely cor-
related with the activity results. These findings also succinctly
demonstrate the capacity of Al for stabilising the Cu/ZnO structure,
albeit at the expense of activity.
[8] P.-J. Guo, L.-F. Chen, G.-B. Yu, Y. Zhu, M.-H. Qiao, H.-L. Xu, K.-N. Fan, Catal.
Commun. 10 (2009) 1252–1256.
[9] M.S. Spencer, Top. Catal. 8 (1999) 259–266.
[10] H. Schaper, E.B.M. Doesburg, L.L.v. Reije, Appl. Catal. 7 (1983) 211–220.
[11] T. Shido, Y. Iwasawa, J. Catal. 129 (1991) 343–355.
[12] D.C. Grenoble, M.M. Estadt, D.F. Ollis, J. Catal. 67 (1981) 90–102.
[13] N.A. Koryabkina, A.A. Phatak, W.F. Ruettinger, R.J. Farrauto, F.H. Ribeiro, J.
Catal. 217 (2003) 233–239.
[14] G.C. Chinchen, M.S. Spencer, K.C. Waught, D.A. Whan, J. Chem. Soc. Faraday
Trans. 183 (1987) 2193–2212.
[15] J.L.C. Fajin, M.N.D.S. Cordeiro, Francesc Illas, J.R.B. Gomes, J. Catal. 268 (2009)
131–141.
[16] R. Burch, R.J. Chappell, S.E. Golunski, Catal. Lett. 1 (1988) 439–444.
[17] Y. Okamoto, K. Fukino, T. Imanaka, S. Teranishi, Chem. Lett. (1984) 71–74.
[18] G. Ghiotti, F. Bocuzzi, Catal. Rev: Sci. Eng. 29 (1987) 151–182.
[19] T.v. Herwijnen, W.A.D. Jon, J. Catal. 63 (1980) 83–93.
[20] W.Y. Teoh, L. Mädler, D. Beydoun, S.E. Pratsinis, R. Amal, Chem. Eng. Sci. 60
(2005) 5852–5861.
TPO characterisation indicated deactivation did not result from
the build-up of carbonaceous species. F-COM was the only catalyst
to show carbonaceous species formed and remained on the surface
during reaction but there was no corresponding activity decrease.
This agrees with the findings by Guo et al. who reported a similar
build-up of carbonate species on their Cu/ZnO/Al2O3 catalyst; how-
ever, they suggested the presence of these species on the catalyst
surface, rather than sintering, coincided with deactivation [8].
5. Conclusions
[21] M.J.L. Gines, N. Amadeo, M. Laborde, C.R. Apesteguia, Appl. Catal. A 131 (1995)
283–296.
[22] J.R. Jensen, T. Johannessen, H. Livbjerg, Appl. Catal. A 266 (2004) 117–122.
[23] J.W. Evans, M.S. Wainwright, A.J. Bridgewater, D.J. Young, Appl. Catal. 7 (1983)
75–83.
[24] X. Liu, W. Ruettinger, X. Xu, R. Farrauto, Appl. Catal. B 56 (2005) 69–75.
[25] G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer, K.C. Waugh, Appl. Catal.
36 (1988) 1–65.
[26] J.R. Ladebeck, J.P. Wagner, Handbook of Fuel Cells – Fundamentals, Technology
and Applications, Part 2 3 (2003) 190–201.
La addition has been demonstrated to affect the activity and
stability of Cu/ZnO catalysts under realistic feed conditions for
the LT-WGS reaction. In particular, doping with 2.3 wt% La de-
creased the apparent activation energy of the Cu/ZnO system and
improved the WGS activity compared to undoped Cu/ZnO. Further
increase in La loading appears to promote H2O adsorption at the
expense of CO. This may provide the source of the observed lower
activities. Moreover, the La stabilised the active sites over an ex-
tended operating period with the rate of deactivation markedly de-
creased despite a substantial decrease in specific and metallic
copper surface area during reaction. These results implied metallic
copper sites were not responsible for the LT-WGS activity. XPS
characterisation revealed electron interaction between Cu and La
components which corroborates the increase in Cu2+ binding en-
ergy to O2ꢀ from increased concentration of La dopant. This is re-
flected by the increasing temperature required to reduce the CuO
to metallic copper. The CO and H2O-TPD studies imply the WGS
mechanism proceeds via an associative route in the La-doped Cu/
ZnO catalytic system.
[27] G. Shen, J.H. Cho, J.K. Yoo, G.-C. Yi, C.J. Lee, J. Phys. Chem. B 109 (2005) 5491–
5496.
[28] W. Wang, Y. Zhan, G. Wang, Chem. Commun. (2001) 727–728.
[29] S.E. Pratsinis, Prog. Energy Combust. Sci. 24 (1998) 197–219.
[30] F. Garbassi, G. Petrini, J. Catal. 90 (1984) 106–112.
[31] C.C. Chusuei, M.A. Brookshier, D.W. Goodman, Langmuir 15 (1999) 2806–2808.
[32] W.-L. Dai, Q. Sun, J.-F. Deng, D. Wu, Y.-H. Sun, Appl. Surf. Sci. 177 (2001) 172–
179.
[33] M. Daturi, C. Binet, J.-C. Lavalley, A. Galtayries, R. Sporkenb, PCCP 1 (1999)
5717–5724.
[34] F. Nishida, I. Atake, D. Li, T. Shishido, Y. Oumi, T. Sano, K. Takehira, Appl. Catal.
A 337 (2008) 48–57.
[35] M. Turco, G. Bagnasco, U. Costantino, F. Marmottini, T. Montanari, G. Ramis, G.
Busca, J. Catal. 228 (2004) 43–55.
[36] T. Imoto, Y. Harano, Y. Nishi, S. Masuda, Bull. Chem. Soc. Jpn. 37 (1964) 441–
444.
[37] C. Zwicker, K. Jacobi, Surf. Sci. 131 (1983) 179–194.
[38] R. Zhang, A. Ludviksson, C.T. Campbell, Surf. Sci. 289 (1993) 1–9.
[39] D.L. Roberts, G.L. Griffin, J. Catal. 110 (1988) 117–126.
[40] J. Strunk, R.N. d‘Alnoncourt, M. Bergmann, S. Litvinov, X. Xia, O. Hinrichsen, M.
Muhler, PCCP 8 (2006) 1556–1565.
In comparison with the Cu/ZnO/Al2O3 system, the La-doped Cu/
ZnO is superior in terms of WGS activity even though a loss in
activity is apparent over 25 h of WGS operation at 300 °C.
Acknowledgments
[41] Y. Zhu, C.-H. Sow, T. Yu, Q. Zhao, P. Li, Z. Shen, D. Yu, J.T.-L. Thong, Adv. Funct.
Mater. 16 (2006) 2415–2422.
[42] R. Strobel, A. Baiker, S.E. Pratsinis, Adv. Powder Technol. 17 (2006) 457–480.
[43] K.T. Jacob, K.P. Jayadevan, J. Mater. Sci. 37 (2002) 1611–1620.
[44] A.N. Pestryakov, A.A. Davydov, Appl. Surf. Sci. 103 (1996) 379–483.
[45] S.V. Ketchik, T.P. Minyukova, L.I. Kuznetsova, L.M. Plyasova, T.M. Yurieva, G.K.
Boreskov, React. Kinet. Catal. Lett. 19 (1982) 345–349.
[46] E. Mack, G.G. Osterhof, H.M. Kraner, JACS 45 (1922) 617–623.
[47] D.F. Anthrop, A.W. Searcy, J. Phys. Chem. 68 (1964) 2335–2342.
[48] Y. Okamoto, K. Fuklno, T. Imanaka, S. Teranishi, J. Phys. Chem. 87 (1983) 3747.
[49] K.R. Harikumar, C.N.R. Rao, Appl. Surf. Sci. 125 (1998) 245–249.
[50] E.I. Solomon, P.M. Jones, J.A. May, Chem. Rev. 93 (1993) 2623–2644.
[51] D.F. Cox, K.H. Schulz, Surf. Sci. 249 (1991) 138–148.
[52] G.E. Parris, K. Klier, J. Catal. 97 (1986) 374–384.
The authors acknowledge Dr. Bill Bing Gong and Katie Levick of
the University of New South Wales for their assistance in collecting
and interpreting XPS spectra and TEM images, respectively. The
author is grateful for Dr. Wey Yang Teoh for his assistance during
FSP synthesis.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2010.05.004.
[53] K.-D. Jung, O.-S. Joo, Bull. Korean Chem. Soc. 23 (2002) 1765–1768.
[54] S.V. Didziulis, K.D. Butcher, S.L. Cohen, E.I. Solomon, JACS 111 (1989) 7110–
7123.
[55] J. Kwon, M. Dai, M.D. Halls, E. Langereis, Y.J. Chabal, R.G. Gordon, J. Phys. Chem.
C 113 (2009) 654–660.
References
[56] A. Neumann, D. Walter, Thermochim. Acta 445 (2006) 200–204.
[57] T. Shido, Y. Iwasawa, J. Catal. 140 (1993) 575–584.
[58] Y. Amenomiya, G. Pleizier, J. Catal. 76 (1982) 345–353.
[59] P. Liu, J.A. Rodriguez, J. Phys. Chem. 1226 (2007) 164705.
[1] W. Vielstich, A. Lamm, H.A. Gasteiger, Catalyst development for water–gas
shift, in: J.R. Ladebeck, J.P. Wagner (Eds.), Handbook of Fuel Cells
–
Fundamentals, Technology and Applications, vol. 3, John Wiley
Chichester, UK, 2003, p. 190.
& Sons,