Electrochemical activation of an oblique angle deposited Cu catalyst film for H2 production

Article abstract of DOI:10.1039/c4cy01524j

A novel Cu catalyst film was prepared by oblique angle physical vapour deposition (OAD) on a K-βAl₂O₃ solid electrolyte (alkaline ionic conductor) for catalytic/electrocatalytic purposes. This technique allowed us to obtain a highly porous and electrically conductive Cu catalyst electrode which was tested in the partial oxidation of methanol (POM) reaction for H₂ production and its catalytic activity was in situ enhanced via electrochemical promotion of catalysis (EPOC). The electropromotional effect was reversible and reproducible, and allowed us to increase both hydrogen and methyl formate production rates by almost three times under optimal promotion conditions (320 °C, 2.2 × 10^-7 mol of K⁺ transferred). The observed promotional effect was attributed to a decrease in the Cu catalyst work function as a consequence of the controlled migration of electropositive K⁺ ions which favoured the chemisorption of electron acceptor molecules (O₂) at the expense of the electron donor ones (CH₃OH). Under the reaction conditions these ions formed some kinds of potassium surface compounds as demonstrated by SEM, EDX and XPS post-reaction characterization analyses. The obtained results demonstrate the interest of the used catalyst-electrode preparation technique for the electrochemical activation of non-noble metal catalyst films. This journal is

Full text of DOI:10.1039/c4cy01524j

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Electrochemical activation of an oblique angle
deposited Cu catalyst film for H2 production
Cite this: Catal. Sci. Technol., 2015,
5, 2203
Jesús González-Cobos,a Víctor J. Rico,b Agustín R. González-Elipe,b
a
a
*
José L. Valverde and Antonio de Lucas-Consuegra
A novel Cu catalyst film was prepared by oblique angle physical vapour deposition (OAD) on a K-βAl2O3
solid electrolyte (alkaline ionic conductor) for catalytic/electrocatalytic purposes. This technique allowed us
to obtain a highly porous and electrically conductive Cu catalyst electrode which was tested in the partial
oxidation of methanol (POM) reaction for H2 production and its catalytic activity was in situ enhanced via
electrochemical promotion of catalysis (EPOC). The electropromotional effect was reversible and
reproducible, and allowed us to increase both hydrogen and methyl formate production rates by almost
three times under optimal promotion conditions (320 °C, 2.2 × 10−7 mol of K+ transferred). The observed
promotional effect was attributed to a decrease in the Cu catalyst work function as a consequence of the
controlled migration of electropositive K+ ions which favoured the chemisorption of electron acceptor
molecules (O2) at the expense of the electron donor ones (CH3OH). Under the reaction conditions these ions
formed some kinds of potassium surface compounds as demonstrated by SEM, EDX and XPS post-reaction
characterization analyses. The obtained results demonstrate the interest of the used catalyst-electrode
preparation technique for the electrochemical activation of non-noble metal catalyst films.
Received 19th November 2014,
Accepted 9th January 2015
DOI: 10.1039/c4cy01524j
www.rsc.org/catalysis
are the design of scaled-up reactors and material cost minimi-
zation. With regard to the former, increasingly more compact
Introduction
Alkali ions have been widely employed as promoters of metal
catalysts to modify their activity and selectivity. In this sense,
and efficient reactor designs based, for example, on mono-
lithic configurations or hollow fibre membranes can be found
in the literature.4–8 Several EPOC studies have also dealt with
the improvement of catalytic materials and the development of
electrodes consisting of catalysts highly dispersed on gold,9
mixed ionic electronic conductors,10 carbonous materials11–13
and yttria-stabilized zirconia (YSZ),14–17 or catalysts based on
non-noble (lower cost) metals such as Ni,11,18–21 Fe4,22,23 and
Cu.7,24–27 A summary of these studies along with the studied
catalytic reaction in each case is reported in Table 1.
Important features of these metallic catalyst films such as
their porosity, surface area and metal dispersion strongly
depend on the preparation method. Different techniques
have been used in EPOC studies for the preparation of the
active catalyst film such as deposition and calcination of an
organometallic paste,4,17,20,22 impregnation of a metal precur-
sor solution,9,10,18 spray deposition in a carbon matrix12 and
other less conventional techniques which enable a better con-
trol of the microstructure of the film, including Physical
Vapour Deposition (PVD) methods such as sputtering,5–7,14,24,25
Cathodic Arc Deposition (CAD)13 and Pulsed Laser Deposition
(PLD).28 In the present work, we propose for the first time in
the literature a modification of the PVD technique called
oblique or glancing angle deposition (OAD or GLAD) for the
preparation of the film. By this method the substrate is placed
the electrochemical promotion of catalysis (EPOC) is
a
straightforward technique to study the effect of electronic pro-
moters on the reaction rate.1 The EPOC phenomenon is
based on the electrochemical pumping of promoter ions from
a solid electrolyte support to a catalytically active metal
film.1 Hence, while the conventional catalytic promotion is
carried out by adding a specific amount of a promoter during
the preparation step of the catalyst, in the case of the electro-
chemical promotion, the electrically induced back-spillover of
the promoter species enables the in situ control and enhance-
ment of the catalytic performance of the catalyst film during the
reaction step.2 This phenomenon has been applied with a large
variety of catalysts and solid electrolytes (cationic, anionic or
mixed conductors) in a wide variety of catalytic reactions of
industrial and environmental interest.3 Nowadays, some of the
main challenges of EPOC for further technological progress
a Department of Chemical Engineering, School of Chemical Sciences and
Technology, University of Castilla-La Mancha, Avda. Camilo José Cela 12,
13005 Ciudad Real, Spain. E-mail: Antonio.lconsuegra@uclm.es;
Fax: +34 926295437; Tel: +34 926295300
b Laboratory of Nanotechnology on Surface, Institute of Materials Science of
Sevilla (CSIC-Uni. Sevilla), Avda. Américo Vespucio 49, 41092 Sevilla, Spain
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Table 1 Most relevant EPOC studies in the search for scale-up reactors and material cost minimization
Efficient and compact reactor designs
Reactor configuration
Electrochemical catalyst Reaction under study
Fe/CaIn0.1Zr0.9O3-α NH3 synthesis
Ref.
4
Multi-pellet configuration
Monolithic electropromoted reactor (MEPR) Rh/YSZ/Pt
C2H4 oxidation and NO reduction by C2H4 in presence of O2 5 and 6
Cu–TiO2/YSZ
Pt/LSCF
CO2 hydrogenation
C2H4 oxidation
7
8
Hollow fibre membranes
Highly dispersed electrochemical catalysts
Non-noble metal catalysts
Electrochemical catalysta
Reaction under study
Ref. Electrochemical catalysta
Reaction under study Ref.
Pt–Au/YSZ
Pt–LSCF–GDC/GDC
Ni–CNF/YSZ
Ru–CNF/YSZ
Pt–C/K-βAl2O3
Pt–DLC/K-βAl2O3
C2H4 oxidation
C3H8 oxidation
CO2 hydrogenation
9
10
11
Ni/K-βAl2O3
Ni/CsHSO4
Water-gas shift
C2H4 hydrogenation
CH4 steam reforming 20
CO2 hydrogenation
CO2 hydrogenation
NH3 decomposition
NH3 synthesis
18
19
Ni/YSZ (solid oxide fuel cell)
Ni–CNF/YSZ
Ni/YSZ (tubular reactor)
Fe/K2YZrIJPO4)3
Fe/SZY
11
21
22
23
4
CO and C3H6 oxidation
CH3OH partial oxidation
CH3OH steam reforming
CH3OH partial oxidation
NO reduction by C3H6
12
13
Au–YSZ/K-βAl2O3
Pt/YSZ
Pd/YSZ (honeycomb monolith) CH4 oxidation
14
15
16
Fe/CaIn0.1Zr0.9O3-α (multi-pellet reactor) NH3 synthesis
Cu/Na-βAl2O3
NO reduction by CO
24 and 25
PtCu/Nafion (PEM fuel cell)
Cu–TiO2/YSZ (monolithic reactor)
PROX, water-gas shift 26
Pt–YSZ/Na-βAl2O3
CH4 steam reforming
17
CO2 hydrogenation
7
CH4 partial oxidation
CH4 autothermal reforming
Cu/K-βAl2O3 (tubular reactor)
CO2 hydrogenation
27
a
Unless otherwise stated, the employed reactor presented a single pellet configuration.
in an oblique or glancing angle configuration with respect to
the evaporated flux of the deposition material to enhance the
shadowing effects during the film growth.29 As a result, highly
porous films formed by tilted nanocolumns are obtained,
which are characterized by a high gas-exposed surface area and
a controlled microstructure depending on the vapour incident
angle, the rotation speed of the substrate, and the temperature
during the deposition, among other factors. The open porosity
of these films is very useful to prepare host materials for
devices with optical, photonic or magnetic applications30,31 or
for the development of gas and liquid sensors.29 They have also
been used for different energy related applications including
photoelectrochemical cells,29 solid-state hydrogen storage32
and electrocatalysis for oxygen reduction in polymer electrolyte
membrane fuel cells33 and 2-propanol oxidation in alkaline
direct alcohol fuel cells.34
In the present study, a novel Cu electrode prepared by
OAD with both suitable catalytic and electrical properties has
been applied in the electrochemical promotion of the partial
oxidation of methanol (POM) reaction. This exothermic reac-
tion is especially interesting for on board H2 production due
to the liquid nature of methanol, its high H/C ratio and its
availability from a wide variety of sources. The selection of a
Cu catalyst is justified by its lower cost in comparison with
noble metals such as Pt and Pd and its proven catalytic activity
for the POM reaction.35–39 For this selection we have also
taken into account previous studies showing the suitability of
Cu-based catalysts to study the fundamental aspects of the
EPOC phenomenon40 and its application in different catalytic
reactions such as NO reduction by CO,24,25 preferential oxidation
of CO26 and CO2 hydrogenation.7,27 However, to the best of
our knowledge, this is the first time that this non-noble
metal catalyst is electrochemically promoted in a H2 produc-
tion reaction.
Experimental
Electrochemical catalyst preparation
The electrochemical catalyst consisted of a continuous thin
Cu layer (geometric area of 2.01 cm2), which also behaved as
a working electrode (W), deposited on one of the sides of a
19 mm-diameter, 1 mm-thick K-βAl2O3 (Ionotec) pellet which
was used as a cationic solid electrolyte (i.e. as source of K+
promoter ions). In the first place, inert Au counter (C) and
reference (R) electrodes were prepared on the other side of
the electrolyte by applying thin coatings of gold paste (Fuel
Cell Materials 233001) followed by calcination at 800 °C for
2 h (heating ramp of 5 °C min−1). Then, the active Cu catalyst/
working electrode film was deposited by evaporation of a Cu
target (Goodfellow, 99.9999% purity) under vacuum condi-
tions by bombardment with a high kinetic energy (<5 keV)
and intensity (150 mA) electron beam. The vapour was con-
densed onto the surface of the substrate IJK-βAl2O3 pellet) at
room temperature in two steps by varying the zenithal angle,
α, formed between the direction perpendicular to the substrate
surface and the evaporation direction. Firstly, to provide a good
electrical conductivity to the catalyst/working electrode, a fairly
compact Cu layer was grown at α = 0° on the K-βAl2O3 pellet.
The microstructure of this compact film consisted of adjacent
straight nanocolumns perpendicular to the substrate (β = 0°)
and an approximate thickness of 0.8 μm. Subsequently, a
porous Cu layer with a tilted columnar microstructure was
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deposited in a similar way to that of the former but fixing the
zenithal evaporation angle at α = 80°. This top layer presented
a porosity of around 50% associated with the large void spaces
between separated nanocolumns tilted by an angle β ≈ 60°
with respect to the direction perpendicular to the substrate. It
was expected that the high porosity of this outer layer favours
the catalytic activity of the copper film. The total thickness of
the obtained Cu catalyst/working electrode was 1.6 μm approx-
imately, and the final metal loading was 1.08 mg Cu cm−2
(1.69 × 10−5 mol Cu cm−2). Thus, this novel preparation tech-
nique allowed the preparation of a suitable copper electrode com-
bining both good electrical and catalytic properties, as shown
below. Similar layers were also prepared at different evapora-
tion angles on silicon supports for characterization purposes.
More details about the experimental setup employed in the
thin catalyst film preparation can be found elsewhere.41,42
The resultant Cu/K-βAl2O3/Au electrochemical catalyst was
placed into a single pellet, continuous stirred tank reactor
(CSTR) where all of the electrodes were exposed to the same gas
atmosphere. Although the Cu film had a high electrical con-
ductivity after its preparation (surface electrical resistance
around 8 Ω), prior to the catalytic activity measurements it
was subjected to 25% H2 stream (Ar balance) while heating
to 280 °C (ramp of 5 °C min−1) to ensure its complete reduc-
tion. The three electrodes (working, counter and reference)
were connected to an Autolab PGSTAT320-N potentiostat–
galvanostat (Metrohm Autolab) using gold wires (Alfa Aesar,
99.95% purity). Hence the electrochemical promotion (EPOC)
experiments were carried out by varying the applied catalyst
potential measured between the working and reference
electrodes (VWR) according to the procedure generally used in
conventional three-electrode electrochemical cells.1
supply K+ from the K-βAl2O3 pellet to the Cu electrode. After
1 hour, the electrochemical catalyst was cooled down to room
temperature under the reaction atmosphere. The applied
potential was interrupted (open circuit conditions) at 100 °C.
The aim of this procedure was to “freeze out” the catalyst sur-
face conditions pertaining to its electropromoted state. Then
the electrochemical catalyst was transferred to different char-
acterization equipment. Initially, the used catalyst film was
characterized by XRD to examine the final crystalline
structure. SEM was also performed along with Energy-
Dispersive X-ray spectroscopy (EDX) elemental mapping anal-
ysis by means of a Bruker X-Flash Detector 4010 to identify
the surface segregation of different elements. X-ray photoelec-
tron spectra (XPS) were also recorded with a PHOIBOS-100
spectrometer with a Delay Line Detector (DLD) from SPECS,
which worked in a constant pass energy mode fixed at 30 eV.
Monochromatic Mg Kα radiation was used as the excitation
source and the binding energy (BE) scale of the spectra was
referenced to the C1s signal of graphitic carbon taken at 284.6 eV.
Catalytic activity measurements
The catalytic experiments were carried out in the experimen-
tal setup employed previously.14 The reaction gases (Praxair, Inc.)
were certified standards (99.999% purity) of air (as source of
O2 for the methanol partial oxidation reaction), H2 (reducing
agent) and Ar (carrier gas). The gas flow rates were controlled
by a set of mass flow meters (Bronkhorst EL-FLOW) while
methanol was fed by sparging Ar through a thermostated sat-
urator containing liquid CH3OH (Panreac, 99.8% purity). All
lines placed downstream from the saturator were heated
above 100 °C to prevent condensation. Methanol partial oxi-
dation (POM) experiments were carried out at atmospheric
pressure with an overall gas flow rate of 6 NL h−1, tempera-
tures from 280 °C to 360 °C and a feed composition of
CH3OH/O2 = 4.4%/0.3% (Ar balance). Reactant and product
gases were on-line analyzed by using a double channel gas
chromatograph (Bruker 450-GC) equipped with Hayesep
Q-Molsieve 13× consecutive columns and a CP-Wax 52 CB col-
umn, along with thermal conductivity and flame ionization
detectors, respectively. The detected reaction products were
H2, CO2, H2O and HCOOCH3 (methyl formate). The error in
the carbon atom balance did not exceed 5%, indicating no
consistent loss of material and no significant formation of
other oxygenated species.
Electrochemical catalyst characterization
The deposited porous Cu layer was firstly characterized by
X-ray diffraction (XRD) analysis using a Philips PW-1710
instrument with Cu Kα radiation (λ = 1.5404 Å). The surface
and in-depth microstructure of the Cu films deposited on sil-
icon substrates were examined by plan-view and cross-
sectional scanning electron microscopy (SEM) analysis using
a Hitachi S4800 field emission microscope operated at 2 keV.
Given the possibility that the Cu electrode could be oxidized
under the reaction conditions or damaged by the development
of cracks and thus lose its electrical conductivity, the in-plane
surface electrical resistance was continuously measured by
connecting the catalyst/working electrode to an additional
gold wire and a digital multimeter. Cyclic voltammetry was
also performed at a scan rate of 5 mV s−1 with the
potentiostat–galvanostat before the EPOC experiments. In
this way, a potential range between +1 and −1 V was bounded.
Moreover, the electrochemical catalyst was characterized
after the electrochemical promotion experiments. Previously,
the Cu catalyst film was exposed to the reaction mixture
IJCH3OH/O2 = 4.4%/0.3%) at 320 °C while applying a negative
potential, VWR = −0.5 V, in order to electrochemically
Results and discussion
Characterization of the Cu catalyst film as-prepared by OAD
Fig. 1 shows the X-ray diffractogram of an oblique angle
deposited copper film on a silicon substrate at α = 80°. This
diffractogram evidences that the as-deposited Cu catalyst film
exhibited a well-defined face-centered cubic (FCC) crystalline
structure (JCPDS, 89-2838) depicting the (111), (200), (220),
(311) and (222) diffraction peaks of copper at 2θ = 43.2°,
50.4°, 74.1°, 89.9° and 95.1°, respectively. A very intense
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inner layer of the copper electrode and provided a very good
electrical conductivity (6–8 Ω) as well as a high mechanical
stability to the electrocatalytic cell. Nevertheless, as observed
in the top view of Fig. 2b, this film presents some diffusion
surfaces at the interfacial boundaries of Cu columns which
may allow the migration of potassium ions therethrough to
the gas-exposed Cu catalyst surface. On the other hand,
Fig. 2c and d show the microstructure of the copper layer pre-
pared by OAD at α = 80°. A thickness of around 0.8 μm and a
microstructure consisting of tilted and separated nano-
columns with β = 60° can be deduced from the cross section
image. The open porosity of this layer, which in practice was
extended from the interface with a dense copper underlayer
up to the external surface of the catalyst, provided a free
access and intimate contact with the reaction gases. Although
not all columns presented exactly the same height, a quite
uniform nanocolumn cross-sectional diameter of around
120 nm and a column density of the order of 109 columns
per cm2 can be also deduced from these images. Similar
microstructures obtained by OAD have been reported for a
Fig. 1 XRD spectrum of the Cu film as-deposited by OAD on a silicon
substrate.
large variety of materials (see for example previous studies on
31,42
Co,44 Pt–Ni,33 TiO2 30,31,41 and Ta2O5
films).
Hence, very interestingly, the OAD technique allowed the
deposition of metallic films with a suitable porous micro-
structure for catalytic and electrocatalytic purposes. Herein,
an electrode film was developed consisting of a copper
compact-porous bilayer (which will be henceforth treated as a
single film) that was utilized as an electrochemical catalyst in
the POM reaction.
Si(400) peak was also found at 2θ = 69.1° (JCPDS, 35-1241)
which corresponded to the substrate.
The microstructure of the Cu layers with different porosi-
ties prepared by either normal or oblique evaporation (i.e.,
OAD) on silicon substrates was characterized by SEM.
Fig. 2a and b show the cross-section and plan-view images,
respectively, of the rather compact layer prepared at α = 0°
and reveal that it was formed by perpendicular, thin and
close-packed nanocolumns as expected for metal thin films
deposited under the conditions of zone I of the zone struc-
ture model.43 This first compact Cu layer constituted the
Electrochemical promotion of methanol partial oxidation
Influence of the applied potential and reaction mecha-
nism considerations. The Cu/K-βAl2O3/Au solid electrolyte
cell was tested in the methanol partial oxidation reaction
with a feed composition of CH3OH/O2 = 4.4%/0.3% at 320 °C
in order to study the phenomenon of electrochemical promo-
tion of catalysis (EPOC). A potential interval of 1 V was
selected based on cyclic voltammetry measurements. Fig. 3
shows the current (I) variation with the applied potential
(VWR) under such reaction conditions during the cyclic
voltammetry performed at a quite slow scan rate of 5 mV s−1
.
This voltammogram was found to be very reproducible from
the second cycle. A totally null oxidation current was not
obtained at +1 V. However, both cathodic (negative) and
anodic (positive) scans must be influenced by the charging
current associated with the Cu–KβAl2O3 interface, which is
directly proportional to its capacitance and the scan rate.45
The most important feature is the appearance of clear
cathodic and anodic peaks in the studied potential range,
which can be linked to the formation and decomposition of
promoter-derived surface compounds, respectively. The inte-
gration of both peaks from their respective baselines showed
to be quite similar, with a difference of near 10%. Moreover,
this similarity was even higher by considering the positive
and negative I vs. time curves obtained during the EPOC
Fig. 2 Cross-sectional and top view SEM images of the Cu films
as-prepared on a silicon substrate at a normal geometry (a and b,
respectively) and at an angle of deposition of 80° (c and d, respectively).
These layers constituted the inner and outer Cu layers in the electro-
chemical catalyst.
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Fig. 3 Current (I) variation vs. the applied potential (VWR) during the
cyclic voltammetry between +2 and −2 V (scan rate = 5 mV s−1).
Methanol partial oxidation conditions: CH3OH/O2 = 4.4%/0.3%, 320 °C.
experiments as will be observed below. This means that a
positive potential of 1 V led to the decomposition of practi-
cally all the promoter compounds previously formed on the
catalyst surface during the cathodic scan. One can find in the
literature a wide range of positive unpromoted potentials
employed in alkali-based electropromoted systems, from
+0.1 V24 or +0.7 V,40 up to +3 V12 or +4 V.27 In the present
study, a positive potential of 1 V was selected as the lowest
possible value to avoid the deterioration of the catalyst film
and/or the solid electrolyte, while providing a completely
reversible promotional effect.
Fig. 4 shows the dynamic response of the reaction rate of
the detected products, i.e., hydrogen, carbon dioxide and
methyl formate, as well as the current (I) vs. time curves
obtained upon the application of different catalyst potentials
(VWR) between +1 and −1 V at a temperature of 320 °C. Each
polarization step was performed for 1 hour to reach a steady-
state rate value. Similar to other EPOC studies based on K+-
conductor materials,12–14,18,22,46,47 the application of a posi-
tive potential (VWR = +1 V) at the beginning of the experiment
led to the removal of the potassium promoter from the cata-
lyst/working electrode that could have thermally migrated
from the solid electrolyte. In this way, VWR = +1 V was defined
as the reference state potential and its imposition before
each different polarization allowed the reaction to start from
the unpromoted state and to check the reversibility and
reproducibility of the promotional effect. It can be observed
in Fig. 4a that under unpromoted conditions the Cu catalyst
film already showed certain catalytic activity for the produc-
tion of hydrogen and carbon dioxide, as well as water
(not depicted), from the methanol partial oxidation reaction
(eqn (1)).
Fig. 4 Response of H2, CO2 and HCOOCH3 production rates (a) and
obtained electric current (I) (b) vs. time to step changes in the applied
catalyst potential (VWR). Methanol partial oxidation conditions:
CH3OH/O2 = 4.4%/0.3%, 320 °C.
1
(1)
CH3OH  O2  2H2  CO2
2
Carbon dioxide was the main product obtained in this
reference (unpromoted) state. No carbon monoxide was
detected but an appreciable amount of methyl formate
(HCOOCH3) was also produced. This latter compound is a
very interesting by-product with many industrial applications,
for example, as a precursor in the synthesis of formic acid,
formamide and dimethylformamide. The decrease in the
applied potential to VWR = +0.5 V caused only a slight
increase in H2 and CO2 production rates. This potential was
close to the open circuit value (V0WR), which oscillated in the
range of 0.3–0.5 throughout this entire EPOC study performed
at different reaction temperatures. Hence, this first decrease
in the applied potential did not constitute a clear negative
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overpotential or a significant transfer of potassium species.
However, under the application of VWR = 0 V, the reaction rate
of all products (H2, CO2 and HCOOCH3) sharply increased
(electrochemically promoted state), being H2 the main reac-
tion product. It should be noted that, in this case, the applied
potential was already lower than the open circuit potential
value leading to the observed negative current values
(Fig. 4b), i.e., under these conditions, K+ ions were electro-
chemically pumped from the K-βAl2O3 pellet to the Cu catalyst
film. An increase in the alkali concentration on the catalyst
surface upon decreasing the applied potential has been previ-
ously demonstrated, for instance, by Professor Lambert and
co-workers by means of in situ XP spectroscopy studies on
Cu,24,25,40 Pt48 and Rh.49 Hence, the observed modification of
the Cu catalytic activity with the applied potential can be
attributed to a decrease in the catalyst work function, which
was a consequence of the migration of electropositive ions
leading to the modification of the chemisorption properties
of the Cu surface during the catalytic reaction.40
According to the theory of the electrochemical promotion
of catalysis,1 under electropromoted conditions (VWR ≤ 0 V
in this work) the back-spillover of K+ ions from the solid
electrolyte onto the catalyst/working electrode would
strengthen the bond between copper and electron-acceptor
adsorbates, while it would weaken the bonding to the
electron-donor reactants. Under POM conditions, these mole-
cules should be identified as O2 and CH3OH, respectively.
Hence, the enhancement of the Cu catalytic activity experi-
mentally observed under negative polarization is in good
agreement with other POM studies37 where a positive order
of the reaction rate was found with respect to the oxygen par-
tial pressure (for low values). Likewise, an increase in the
methanol reaction rate with O2 concentration was found in
previous EPOC studies with different electrochemical
catalysts.52–54 Moreover, this kind of positive promotional
effect upon cathodic polarization (electrophilic EPOC behav-
iour) was also observed over Cu catalysts in other studies on
NO reduction by CO24,25 and, very recently, on CO2 hydroge-
nation under certain reaction conditions.27 In the present
study, the adsorbed oxygen would play an important role by
favouring the O–H bond rupture in eqn (3). Nevertheless, the
rate-limiting step in this kind of process is usually considered
the methoxy decomposition (eqn (4) and (9)).37,38 The electro-
philic EPOC effect could have also favoured the C–H bond
cleavage in the methanol and intermediate decomposition
reactions.53,54
It can also be observed in Fig. 4a that all the product reac-
tion rates (H2, CO2 and HCOOCH3) were restored to their ini-
tial values upon every positive polarization at VWR = +1 V.
This demonstrates the complete reversibility and reproduc-
ibility of the electropromoted effect, as well as the high
stability of the Cu electrode throughout the whole experiment.
Moreover, optimal promotional conditions were found at
VWR = 0 V. A further decrease in the applied potential down
to −0.5 and −1 V led again to an electropromotional effect. In
this case, a less pronounced increase in all the reaction rates
with respect to those found under unpromoted conditions
occurred, which can be attributed to the detrimental effect of
an excess of alkali promoter and the concomitant formation
of alkali-derived surface compounds that may partially block
the catalytically active sites. This is a common effect observed
both in EPOC13,14,18,22,46,47 and in conventional alkali chemical
promotion studies.55,56
Regarding the reaction mechanism, it is widely accepted
that the first reaction step in methanol oxidation processes is
the formation of methoxy species from methanol which is
favoured by the presence of surface oxygen through a disso-
ciative adsorption mechanism:36–38
O2 + 2* → 2O*
(2)
(3)
CH3OH + O* + * → CH3O* + OH*
where * denotes a free adsorption site on the catalyst surface.
Then, as proposed in an illustrative scheme by Sakong,38 sev-
eral consecutive reactions may take place leading to the pro-
duction of CO2 (eqn (4)–(6)), H2 (eqn (7)) and H2O (eqn (8)).
CH3O* + O* → H2CO* + OH*
H2CO* + O* → HCOO* + H*
HCOO* → CO2 + H*
(4)
(5)
(6)
(7)
(8)
2H* → 2* + H2
H* + OH* → H2O + 2*
On the other hand, the formation of methyl formate can
be explained by considering the large set of possible reaction
pathways and intermediate species reported elsewhere36,39,50,51
for methanol oxidation over Cu-based catalysts. For instance,
the following reaction mechanism could be proposed:51
Regarding the possible oxidation of copper during the
POM reaction, the in-plane surface electrical resistance was
continuously measured leading to values lower than 10 Ω
throughout the entire study. As it will be later discussed,
these low resistance values would indicate the catalyst/work-
ing electrode to mainly remain in its metallic state during
the reaction. It should be also mentioned that the methanol
conversion obtained in this work (not depicted) did not
exceed 5%, which was probably influenced by the characteris-
tics of the catalyst, in film form, and thus a still low geomet-
ric area. Nevertheless, it is important to stress that both cata-
lytic activity and selectivity of the Cu film in the POM
CH3O* + O* → HCOO* + H2 + *
CH3O* + HCOO* → HCOOCH3* + O*
HCOOCH3* → HCOOCH3 + *
(9)
(10)
(11)
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reaction can be in situ improved under polarization condi-
tions and tuned depending on the applied electric current or
potential via electrochemical promotion. This feature of the
EPOC phenomenon has been previously shown in other stud-
ies on POM over alkali-promoted noble metal catalysts.13,14
In the present study, the Cu-catalyzed H2 production was
enhanced up to three times at 320 °C, the H2 selectivity being
increased from 27.4% (unpromoted conditions, VWR = +1 V)
up to 39.1% (optimum electropromoted conditions, VWR
=
0 V). These values were calculated as 100 × moles of H2/(2 × moles
of reacted CH3OH). Moreover, from the integration of the differ-
ent current vs. time curves, the amount of the promoter
electrochemically transferred and the promoter to copper
ratio (θK+) can be estimated at each applied potential through
the following equations:
t
I
Promoter supply/mol K 
dt
(12)

0 nF
 0t
I
dt
(13)
nF
N
K
where n is +1 in this case, F is the Faraday constant (96
485 C mol−1), t is the polarization time and N is the total
amount of Cu deposited (3.4 × 10−5 mol of Cu). This parameter
was estimated in this way due to the difficulty of calculating
the Cu active sites in this electrocatalytic system. The potas-
sium to copper ratio varied from 1 × 10−3 (at VWR = +0.5 V) to
8.5 × 10−3 (at VWR = −1 V). Hence, the EPOC phenomenon
allows the determination of the optimum promoter to copper
ratio (θK+,opt) which leads to the maximum reaction rate. In this
study, this value is θK+,opt = 6.3 × 10−3, obtained at VWR = 0 V.
Influence of the reaction temperature
The influence of the reaction temperature on the copper cata-
lytic activity was studied at
a fixed feed composition
IJCH3OH/O2 = 4.4%/0.3%) and different applied potentials.
Fig. 5 shows the variation of the steady-state reaction rates of
the different products vs. the applied catalyst potential at
different reaction temperatures between 280 and 360 °C. In
order to quantify the magnitude of the electropromotional
effect, the maximum rate enhancement ratios (ρmax) for the
optimum applied potential are also included in the figure.
This parameter was calculated using the following equation:
Fig. 5 Steady-state H2 (a), CO2 (b) and HCOOCH3 (c) production rates
vs. the applied potential (VWR) as well as the corresponding maximum
production rate enhancement ratios (ρmax) at different reaction
temperatures (280–360 °C). Methanol partial oxidation conditions:
CH3OH/O2 = 4.4%/0.3%.
r
1
i 
(14)
r
i,0
result, at the three studied temperatures, all the product reac-
tion rates (H2, CO2, HCOOCH3) increased because of the
potassium promotional effect, evidencing an electrophilic
EPOC behavior (Δr > 0, ρ > 1, upon negative polarization).1
It should be noted that at every reaction temperature, the
same operational procedure was carried out as that in the
experiment shown previously (Fig. 4), i.e., a potential of +1 V
was applied at the end of every experiment and before each
where, for each compound, ri and ri,0 are the promoted (VWR
< +0.5 V) and unpromoted (VWR = +1 V) catalytic production
rates, respectively.
Starting from the reference unpromoted state (VWR = +1 V)
and according to the previous considerations, the decrease in
the applied potential caused the electrochemical pumping of
K+ ions from the solid electrolyte to the Cu catalyst film. As a
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different applied potential. In this way, both the reversibility
and the reproducibility of the electropromotional effect were
verified at all the reaction temperatures, as well as the stabil-
ity of the catalytic activity confirmed throughout the entire
study.
the EPOC experiments are shown in Fig. 6. It presents five
diffraction peaks corresponding to the typical face-centered
cubic structure (JCPDS, 89-2838) of metallic copper. An aver-
age Cu particle size (d) of 43 nm can be estimated from the
width of the main diffraction peak, Cu(111), by means of
Scherrer's formula. Peaks associated with the K-βAl2O3 pellet
(JCPDS, 21-618) can also be observed. No diffraction peaks
related to potassium compounds like carbonates or bicarbon-
ates were found. Neither diffraction peaks associated with
copper oxides like CuO nor Cu2O were detected. However,
the minority presence of these compounds cannot be ruled
out because some overlapping may take place with the peaks
of alumina, for instance, in the case of the CuO peak at
2θ = 35.4° (JCPDS, 80-1917).
Fig. 7 shows the SEM micrograph of a selected region of
the catalyst-electrode where highly grown surface crystals
were observed. Its corresponding elemental mapping and
spectra by EDX are also reported. The columnar structure
observed in Fig. 2 seems to disappear in this figure, although
both micrographs were not obtained under exactly the
same operation conditions. In the case of some morphology
changes, the most feasible reason would certainly be Cu
sintering that could take place during the experiment at high
temperature (360 °C).57 However, the catalyst porosity
seemed to be preserved. In fact, the reversibility and repro-
ducibility of both the catalyst activity and the promotional
effect were verified in the whole temperature range studied
(Fig. 4 and 5), also after cooling from 360 °C. As previously
highlighted, the catalyst showed the same unpromoted cata-
lytic activity after several working hours. In Fig. 7b1 and b2,
large concentrations of potassium (in blue) were found on
Under unpromoted conditions (VWR = +1 V), the Cu
catalytic activity increased with the reaction temperature,
in good agreement with other POM studies over conventional
39,50
Cu supported catalysts (e.g., Cu/ZnO,37 Cu/ZnO/Al2O3
and Cu/ZrO2 35). As an exception, a decrease in the methyl for-
mate production rate was found at the highest temperature
(360 °C), which can be attributed to its decomposition or
further oxidation to CO2 as reported in other studies with
Cu-based catalysts.39,50,51 Meanwhile, under optimum
electropromoted conditions (VWR = 0 V), an increase in
both the H2 and HCOOCH3 production rate enhancement
ratios (ρ) was found when increasing the temperature
from 280 to 320 °C, due to an increase in both the copper
catalytic activity and the solid electrolyte ionic conductivity at
fixed potential. The less pronounced EPOC effect found at
360 °C should be likely related to the higher unpromoted
reaction rate (r0). Moreover, in line with the experiment
in Fig. 4, a possible poisoning effect by potassium-derived
surface compounds can be observed under the application of
the lowest applied potentials (VWR = −0.5 and −1 V).
These POM results at different temperatures showed that
the catalytic performance of this novel Cu film prepared by
OAD can be enhanced in a controlled and reversible way via
alkaline electrochemical promotion. Under optimal reaction
conditions (320 °C, VWR = 0 V), the hydrogen and methyl for-
mate production rates increased by 2.63 and 2.70 times,
respectively. It must be stressed that this maximum enhance-
ment of the H2 production rate IJρH ,max = 2.63) was higher,
2
for instance, than that previously obtained in another EPOC
study (i.e., ρH ,max = 1.63) with an electrochemical catalyst
2
based on platinum nanoparticles (0.014 mg Pt cm−2) and sim-
ilar methanol partial oxidation conditions (320 °C and
O2/CH3OH = 0.08).13 Therefore, these results show that under
the explored reaction conditions a non-noble metal catalyst
(such as Cu) can be electrochemically activated in a similar
or even more pronounced way than a noble metal catalyst
such as Pt. This way, one can suggest that EPOC phenomena
could be used to increase the catalytic activity of a non-noble
metal up to the typically higher activity of a noble metal,
for example, in H2 production processes.
Post-reaction characterization of the electrochemically
promoted Cu catalyst
In the first place, it should be noted that the reaction rates
observed under the last two potentiostatic transitions at
320 °C (before the post-reaction characterization), i.e. +2 and
−0.5 V, were very similar to those obtained during the first
experiment performed at 320 °C (Fig. 4). This verifies the
good reproducibility of Cu catalytic activity even after heating
up to 360 °C. The XRD spectra of the used catalyst film after
Fig. 6 XRD diffractogram of the electrochemical catalyst after the
EPOC experiments (320 °C, CH3OH/O2 = 4.4%/0.3%, VWR = −0.5 V for
1 h).
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Fig. 7 Top view SEM image of a selected area of the electrochemical catalyst (a) after the EPOC experiments (320 °C, CH3OH/O2 = 4.4%/0.3%,
VWR = −0.5 V for 1 h), along with the corresponding elemental mapping (b1 and b2) of Cu (green), K (blue) and N (red) and the EDX spectra from
different regions (c1 and c2).
certain areas of the Cu surface (in green). This seems to indi-
cate that some potassium-derived surface compounds were
formed on the Cu surface during the EPOC experiments and
might block the Cu active sites causing the decrease in the
reaction rates observed in Fig. 4 and 5 under the application of
high negative potentials. Under our working conditions, the
most feasible K-derived surface compounds are potassium
oxides and/or carbonates formed by reaction with O2, H2O
and carbon-containing molecules in the reaction mixture.
These types of compounds were previously found over other
metal catalysts such as Rh,58 Pt47 and Ag,46 and may be
obtained, for instance, through the following electrocatalytic
reactions:
different areas of the micrograph. Fig. 7c2 shows that the K
and O signals were much higher in the spectra taken in the
agglomerates than those from the flat region (Fig. 7c1). More-
over, it is also worth noting that nitrogen (red points) was
found to be uniformly distributed on the film surface. This ele-
mental nitrogen can only proceed from the fed air stream, and
its presence on the catalyst surface will be confirmed and fur-
ther discussed by XPS analysis.
The chemical state and composition of the outer surface
layer of the Cu catalyst were studied by XPS. The Cu2p
photoemission (BE = 932.4 eV) and Cu LVV Auger (kinetic
energy 915.14 eV) spectra obtained (Fig. 8a) can be used to
determine the Auger parameter of copper (AP = BE photo-
emission peak + kinetic energy Auger peak) which presented
a value of 1847.54 eV, typical of Cu+ (ref. 59) (neither Cu0 nor
Cu2+). This finding would agree with previous results in the
sense that the Cu active state in the methanol partial oxida-
tion reaction could be considered Cu+.50 However, it should
be noted that XPS characterization is only sensitive to a sur-
face thickness of a few nanometers and that, therefore,
the Cu2O observed at the outermost catalyst layer could also
be attributed, at least in part, to the exposure of the sample
to the air atmosphere during its transfer from the reaction
cell to the characterization equipment. An oxidation limited
to the outermost surface layer agrees with the very low
1
2K  O2  2e  K2O
(15)
2
1
2K  O2  CO2  2e  K2CO3
(16)
2
Although the potassium distribution along the catalyst
thickness was not studied, it was important to demonstrate
that K+ ions were able to migrate through the catalyst film
and reach the outermost surface. The presence of potassium
compounds was also revealed by EDX analysis taken in
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Fig. 8 XPS spectra of the electrochemical catalyst after the EPOC experiments (320 °C, CH3OH/O2 = 4.4%/0.3%, VWR = −0.5 V for 1 h). Spectra
taken at different binding energy regions to show the presence of the different elements present on the surface of the sample (a). The
corresponding surface atomic composition expressed in percentages (b).
surface electrical resistance (below 10 ohms) and the XRD
analysis of the utilized catalyst that should remain mainly as
metallic copper under the studied reaction conditions. On
the other hand, the N1s peak observed at 398 eV demon-
strated the presence of some reduced nitrogen on the catalyst
surface, probably derived from a certain K-promoted interac-
tion between nitrogen from air and produced hydrogen. It is
well known that ammonia synthesis is typically promoted by
potassium.56 Furthermore, ammonia synthesis reaction has
been electrochemically promoted by H+ ions in previous stud-
ies with Fe catalysts under both high pressure4 and atmo-
spheric pressure.23 The C1s signal in the spectrum of the
used sample also shows the presence of surface carbon
mostly in the form of graphitic or hydrogenated carbon. This
indicates that a certain coking could have taken place at the
highest reaction temperatures. However, at least part of the
O, C and K amounts observed in this figure can be associated
with the promoter derived species as previously mentioned. The
aluminium also found in this XPS spectrum can be attributed
to some uncovered regions of the K-βAl2O3 electrolyte which
might have appeared due to small scratches caused by
the contact with the gold wires. A quantitative estimation of
the elemental concentration in the examined outer layer of
the utilized electrocatalyst is also included in Fig. 8b.
Conclusions
From the previous results and discussion, we can conclude
that the OAD technique is a straightforward method to pre-
pare a kind of novel Cu catalytic film with a tunable micro-
structure. Combining the physical vapour deposition at nor-
mal and oblique angles for the fabrication of a bilayer film
has yielded a catalytic layer with both a high surface electrical
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conductivity and a high porosity (around 50%), very suitable
for catalytic and electrocatalytic applications.
12 A. de Lucas-Consuegra, A. Princivalle, A. Caravaca,
F. Dorado, A. Marouf, C. Guizard, J. L. Valverde and
P. Vernoux, Appl. Catal., A, 2009, 365, 274–280.
13 A. de Lucas-Consuegra, J. González-Cobos, V. Carcelén,
C. Magén, J. L. Endrino and J. L. Valverde, J. Catal.,
2013, 307, 18–26.
14 J. González-Cobos, D. Horwat, J. Ghanbaja, J. L. Valverde
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17 A. De Lucas-Consuegra, A. Caravaca, P. J. Martínez,
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In this study we have also shown that a non-noble catalyst
can be electrochemically activated in the partial oxidation of
methanol. The catalytic activity of the Cu electrode was
enhanced by electrochemically supplying K+ ions in a con-
trolled, reversible and reproducible way. The observed pro-
motional effect can be explained according to the rules of
chemical/electrochemical promotion.
Under electrochemical promotion conditions, potassium-
derived compounds were formed on the Cu catalyst surface
as determined by post-reaction EDX and XPS characteriza-
tion. The formation of some adsorbed nitrogen functional
groups also seemed to take place under the promotional
effect of potassium due to the presence of N2 and H2 in the
reaction atmosphere.
18 A. De Lucas-Consuegra, A. Caravaca, J. González-Cobos,
J. L. Valverde and F. Dorado, Catal. Commun., 2011, 15, 6–9.
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Acknowledgements
We gratefully acknowledge the Spanish Ministry of Economy and
Competiveness (projects MAT2013-40852-R, MAT2013-42900-P
and CTQ2013-45030-R) for financial support to this work.
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