The role of sodium surface species on electrochemical promotion of catalysis in a Pt/YSZ system: The case of ethylene oxidation

Article abstract of DOI:10.1016/j.jcat.2013.03.015

The role of sodium addition as foreign (impurity) species on the electrochemical promotion of ethylene oxidation in a Pt/YSZ system was investigated. It was found that the presence of sodium surface species on the catalyst surface can significantly affect its catalytic and electrocatalytic properties, but there is no clear evidence at this stage that such species are necessary for the observation of EPOC. Under negative polarisation, low coverage sodium was found to have a pronounced effect on the electrochemical promotion of ethylene oxidation as an electronic promoter. The reaction changed behaviour from electrophilic at low sodium coverage (0.11%) and low to intermediate oxygen partial pressure (p_O2 ≤ 3.0 kPa) to electrophobic at high sodium coverage (65%) and under high oxygen partial pressures (p_O2 = 8.0 kPa). In between the two sets of conditions, the reaction showed volcano-type behaviour depending on the coverage of sodium and gas-phase oxygen partial pressure.

Full text of DOI:10.1016/j.jcat.2013.03.015

Journal of Catalysis 303 (2013) 100–109
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
The role of sodium surface species on electrochemical promotion of catalysis
in a Pt/YSZ system: The case of ethylene oxidation
Naimah Ibrahim a,b, Danai Poulidi a,1, Ian S. Metcalfe a,
⇑
a
School of Chemical Engineering and Advanced Materials, Newcastle University, Merz Court, Newcastle upon Tyne NE1 7RU, UK
School of Environmental Engineering, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The role of sodium addition as foreign (impurity) species on the electrochemical promotion of ethylene
oxidation in a Pt/YSZ system was investigated. It was found that the presence of sodium surface species
on the catalyst surface can significantly affect its catalytic and electrocatalytic properties, but there is no
clear evidence at this stage that such species are necessary for the observation of EPOC. Under negative
polarisation, low coverage sodium was found to have a pronounced effect on the electrochemical promo-
tion of ethylene oxidation as an electronic promoter. The reaction changed behaviour from electrophilic
at low sodium coverage (0.11%) and low to intermediate oxygen partial pressure (pO2 6 3.0 kPa) to elec-
trophobic at high sodium coverage (65%) and under high oxygen partial pressures (pO2 = 8.0 kPa). In
between the two sets of conditions, the reaction showed volcano-type behaviour depending on the cov-
erage of sodium and gas-phase oxygen partial pressure.
Received 9 January 2013
Revised 13 March 2013
Accepted 20 March 2013
Keywords:
EPOC
Surface impurities
Sodium promotion
Platinum
Ethylene oxidation
Ó 2013 Elsevier Inc. All rights reserved.
1
. Introduction
Catalyst promoters are very often used in heterogeneous
protonic [14,15], and alkali ion conductors [7,16–18]. The promot-
ing species (that originate from the support) depend on the choice
of catalyst support (i.e. oxygen ions for an oxygen ion conductor,
etc.).
catalysis as the means to improve catalyst performance by enhanc-
ing the activity or changing the selectivity of a catalyst. Promoters
can be supplied to the catalyst either during preparation (ex situ) or
in situ via some form of catalyst–support interaction [1]. Electronic
promoters (i.e. promoters that affect the catalyst performance by
changing the chemisorptive bond properties of the reactants and
reaction intermediates) have attracted a lot of attention in the past
three decades, especially in the area of Electrochemical Promotion
of Catalysis (EPOC). EPOC studies the effect that the in situ supply
of promoting species via the application of an external electrical
While extensive studies exist to predict and investigate the
behaviour of catalytic systems under controlled spillover (EPOC)
conditions, where the supply of promoting species is dictated by
the imposed electrical overpotential or current, very little work
has been carried out to investigate the effect of pre-existing species
(that can be found on the catalyst surface in the form of uncharac-
terised impurities) such as metal cations. Such species may have
promoting properties and are often (as in the case of alkali metals)
used as additives during catalyst preparation in order to enhance
the catalytic activity [19]. In other cases, pre-existing impurities
(such as silicon) may block the catalyst active sites, thus poisoning
the catalyst [20]. In fact, the question whether pre-existing impu-
rities on the catalyst surface may be responsible for the observed
rate modification in EPOC, via some interaction with the spillover
species (oxygen or other) has not been successfully addressed to
date.
In this work, we will investigate the role of sodium addition to a
platinum catalyst-film interfaced with an yttria-stabilised-zirconia
(YSZ) dense electrolyte membrane under catalytic (open-circuit)
and electropromoted (polarised) conditions. Ethylene oxidation
will be used as the probe reaction. Sodium is chosen in this study
due to the promoting properties it has been found to have in het-
erogeneous catalysis [21–25]. This study aims to study the com-
bined effect of ex situ (sodium) and in situ (electrochemically
(
(
or chemical) overpotential between a metallic catalyst/electrode
supported on an ionic conducting membrane) and a counter
electrode has on the catalyst performance [2,3]. It has been found
3,4] that the origin of EPOC lies in the backspillover (from the
support to the catalyst) of ionic promoting species that form
together with their mirror charge) an overall neutral double layer
[
(
on the catalyst surface, thus changing the work function of the cat-
alyst and modifying its activity and selectivity. EPOC has been
studied for a wide range of reactions [5–8], catalysts [9–12] and
different ionic conducting supports such as oxygen ion [12,13],
⇑
Corresponding author.
E-mail address: i.metcalfe@newcastle.ac.uk (I.S. Metcalfe).
Current address: School of Chemistry and Chemical Engineering, Queen’s
1
University Belfast, David Keir Bld., Stranmillis Rd., Belfast BT9 5AG, UK.
0
021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.03.015

N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
101
supplied oxygen ions) promoters in order to assess the possible
interactions between them and their combined effect on promo-
tion. To that effect, samples with sodium surface coverage from
alytic reaction in a number of ways, such as by causing a change in
the work function of the catalyst (a long-range effect) or by mod-
ifying the activity of catalyst sites in the vicinity of the sodium spe-
cies (short-range effect). Moreover, sodium species may directly
interact with the gas-phase reactants by creating different reaction
intermediates and pathways that can affect the catalytic reaction
rate. When studying the electropromoted behaviour of a catalyst
system, the addition of another type of promoter (oxygen ions in
the case studied here) leads to a complex system where possible
interaction not only between each promoting species and the reac-
tion components (i.e. catalyst, reactants and intermediates) but
also between the two types of promoters could influence the
behaviour of the reaction. A complex model would be required in
order to fully predict and explain the behaviour of such a system.
In this work, we will, in the first instance, test a very simple model
by making certain assumptions about the interactions between all
of the species. We will initially disregard any short-range effects
caused by sodium addition and also any interaction between the
sodium- and oxygen-promoting species. We would anticipate this
assumption to be valid at low promoter coverage (both sodium and
oxygen ions). Under these assumptions, we would expect equal
work function changes to cause equal rate modification regardless
of the origin of the work function change (i.e. sodium addition ex
situ or oxygen spillover in situ by electrical polarisation) and that
the total rate change observed as a result of the combined promo-
tion by sodium and oxygen species could be predicted by the total
catalyst work function change.
0
.11% to 65% have been used in this work. We must of course stress
that we should not (and in fact do not in this work) treat surface
species of coverage higher than 10% as impurities as such cover-
ages would be expected to have very pronounced effects even on
the nature of the catalyst. It is however, important for our work
to study a wide range of sodium surface coverage in order to obtain
a more complete picture of the role of sodium in the catalytic and
electropromoted behaviour of the catalyst.
In heterogeneous catalysis and surface science, alkali metals
such as sodium are known to affect the activity of a metal catalyst
and the use of alkali metals as catalyst promoters has been well-
documented [26–29]. For example, a study by Bertolini et al.
showed that the coverage of the alkali metal affects the adsorption
of gases (in the case of their study oxygen and carbon monoxide)
on the catalyst surface [28]. The study showed that at low cover-
age, the alkali metal was mostly in ionic form and could induce
large modification to the electronic properties of the neighbouring
platinum surface atoms, causing an increase in the adsorption of an
electron acceptor like CO on the catalyst surface. In contrast, at
high coverage, the alkali metal was mostly in metallic form and
had a great affinity for oxygen, thus enhancing oxygen adsorption.
The effect of sodium on the activity of a metal catalyst may also be
viewed as a change in the catalyst work function [26,30,31]. Some
of the electronic charge of the sodium atoms can be transferred to
the platinum metal because the ionisation energy of a sodium
atom (5.1 eV) is lower than the work function of platinum
Based on this model, we would expect that sodium addition and
negative polarisation (i.e. removal of oxygen ion species from the
catalyst surface) to have a qualitatively similar effect on the cata-
lytic reaction rate as they both lower the work function of the cat-
alyst. The effect of promotion would depend upon the behaviour of
the reaction, i.e. electrophilic, electrophobic, volcano or inverted
volcano. According to Vayenas’s reaction classification [8] for an
electrophilic reaction, the rate decreases as the work function of
the catalyst increases, while for an electrophobic reaction, the rate
increases for increasing work function i.e.:
(
5.9 eV) [30]. The charge forms dipoles at the surface resulting in
a lower platinum work function. We can see therefore that there
may be an analogous effect between the sodium addition- and
polarisation-induced work function changes with respect to the
observed rate modification on the catalyst system. In EPOC, sodium
has been successfully used as a promoter via the use of sodium ion
00
conducting supports such as b -Al
NASICON) [17,18,34].
Ethylene oxidation over platinum is a well-studied probe reac-
2 3 3 2
O [16,32,33] and Na Zr SiPO12
(
@
r=@
r=@
U
U
< 0
> 0
ð1Þ
tion for EPOC. In the case of ethylene oxidation, the promotional
behaviour of the system has been found to be dependent upon
the gas-phase composition and on the ratio between the partial
pressure of oxygen over partial pressure of ethylene [35].
@
ð2Þ
A reaction exhibiting volcano-type behaviour has a rate maximum
at UWR = 0 while a reaction with inverted volcano behaviour shows
a rate minimum at UWR = 0, where UWR is the potential between the
working and reference electrodes.
2
. Model of promotion
Next, we will propose a model that will help us evaluate the role
of sodium addition on the catalytic and electropromoted properties
of the Pt/YSZ system. As discussed earlier, sodium may affect a cat-
For an electrophilic reaction, we would see a rate increase
caused by sodium addition and negative polarisation and a rate
decrease caused by positive polarisation (i.e. supply of oxygen ions
Table 1
Estimation of sodium loading on platinum catalyst and percentage of sodium coverage. Sample A is used for kinetic studies and Samples B–J are used for XPS analysis.
Samplea
Volume NaOH (
l
L)
NaOH concentration (M)
Na (atom)
Na/Pt surface area (atom cm
ꢀ2
)
% Na coverage
A000
A104
A102
A502
Nominally ‘clean’ sample
1.00 ± 0.25
ꢀ
ꢀ
ꢀ
04
02
02
13
12
1 ꢁ 10
1 ꢁ 10
5 ꢁ 10
6.0 ꢁ 10
1.1 ꢁ 10
0.11
11
65
15
14
6.1 ꢁ 10
1.1 ꢁ 10
16
14
3.6 ꢁ 10
6.5 ꢁ 10
B000
C112
D108
E104
F103
G102
H502
J101
Nominally ‘clean’ sample
1.00 ± 0.25
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
12
08
04
03
02
02
01
05b
04
ꢀ9
1 ꢁ 10
1 ꢁ 10
1 ꢁ 10
1 ꢁ 10
1 ꢁ 10
5 ꢁ 10
1 ꢁ 10
6.0 ꢁ 10
6.0 ꢁ 10
6.0 ꢁ 10
6.0 ꢁ 10
6.0 ꢁ 10
3.0 ꢁ 10
6.0 ꢁ 10
1.1 ꢁ 10
1.1 ꢁ 10
09b
13b
14b
15b
16b
16b
08
ꢀ5
1.1 ꢁ 10
1.1 ꢁ 10
12
1.1 ꢁ 10
0.11
1.1
11
54
110
13
1.1 ꢁ 10
14
1.1 ꢁ 10
14
5.4 ꢁ 10
15
1.1 ꢁ 10
a
ꢀyy
Sample xyy; the most recent NaOH concentration = x ꢁ 10
M.
b
Not cumulative.

1
02
N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
to the catalyst) and vice versa for an electrophobic reaction; the
combined effect of sodium addition and positive polarisation
would be harder to predict as the total work function change could
be positive or negative depending on the sodium coverage and ap-
plied potential. For inverted volcano behaviour, we would expect
to observe a rate increase for any perturbation of the work function
of the catalyst (sodium addition or oxygen spillover) and the oppo-
site is expected for reactions with volcano-type behaviour.
tion, the discrete rate data points represent average values taken
from transient experiment; these averages were taken when the
rate was assumed to have reached steady state, and for the purpose
of our work, this meant that the rate variation was less than 5%.
When reporting transient rate data, no error bars are used in the
figures.
Two commonly used parameters to express the magnitude of
promotion [3], i.e. rate enhancement ratios,
ciencies,
addition to EPOC:
q
, and faradaic effi-
K
, will be employed in discussing the effect of sodium
3
. Experimental
q
¼ r=r
o
ð3Þ
An experimental rig was constructed using stainless steel
Swagelok compression fittings and Perfluoroalkoxy (PFA) tubing
as described in [36]. The flow of gas to the electrochemical reactor
was controlled by electronic mass flow controllers (MFCs) provided
K
¼
D
r=ði=2FÞ
ð4Þ
where r is the electropromoted catalytic rate, r
o
is the unpromoted
by Chell Hastings. The gases used here were 20% O
2 2 4
/He, 10% C H /
(
open-circuit) catalytic rate, Dr is the difference on the reaction rate
He and CP grade He (N5) provided by BOC with typical flowrates of
ꢀ1
between polarised and open-circuit conditions, i is the applied cur-
rent density and F is the Faraday constant. In addition, under open-
circuit conditions where the modification of the catalytic rate is
only caused by sodium (and not the applied overpotential), the rate
enhancement ratio, qNa, is defined as the ratio of the sodium-mod-
ified sample reaction rate (rNa) over the rate of the ‘clean’ sample
2
00 ml (STP) min . The flowrates of the gases were also measured
at the outlet using a Varian digital flowmeter (1000 series). The
single-chamber electrochemical reactor and three-electrode pellet
system used in the study have already been described in detail in
our previous work [36]. The solid electrolyte disc of 20 mm diam-
eter and 2 mm thickness was prepared by uniaxial pressing at
ꢀ2
(rclean), both measured under open-circuit conditions:
approximately 1 ton cm of 2.2 g of 8 mol% YSZ powder provided
by Pi-Kem Ltd., UK, and sintered at 1500 °C for 12 h. After shrink-
age during sintering, the resulting pellet had a diameter of
qNa ¼ rNa=rclean
ð5Þ
1
5 mm and a thickness of 1.5 mm. On one side of the disc a plati-
num catalyst film (Metalor) was deposited as the working elec-
trode, while on the other side, two gold films (Metalor A1118)
that served as the counter and reference electrodes were depos-
ited. The resulting working and counter electrodes were each cal-
Scanning electron microscopy (SEM), using a JEOL 5300’s scanning
electron microscope (SEM) fitted with energy-dispersive X-ray
(EDX) light element analysers, was used in order to obtain pre-
and post-operation images of the catalyst film and to investigate
the presence of elements on fresh and used sodium-modified sam-
ples. Note that the EDX measures the presence of elements to a
depth of several micrometers into the sample surface. Therefore,
the EDX results could not be used to quantify the elemental constit-
uents on the platinum surface, rather for simple identification and
distribution mapping of significant elements such as platinum and
sodium. SEM analysis of the ‘clean’ and sodium-modified samples
has been presented in our previous work [36].
culated to have
approximately 0.88 cm . The electrochemical measurements were
conducted using an Ivium Compactstat, while the gases were ana-
a geometric projected surface area, A, of
2
lysed using a BINOS CO
Sodium was deposited dropwise using 1
NaOH) solutions (Alfa Aesar) on the platinum surface using a fixed
2
analyser and a Varian gas chromatograph.
lL of sodium hydroxide
(
micropipette followed by drying in air at 400 °C for 1 h. Table 1
summarises the sodium loading (per platinum surface area) and
the percentage of nominal sodium coverage. Details of the sodium
deposition method and the calculation of sodium loading/coverage
are as reported in our previous work [37]. Unless otherwise stated,
the sodium loadings reported in Table 1 are cumulative. The sam-
ples are named as Sample xyy, where x and yy denotes the most re-
cent concentration of the NaOH solution deposited on the platinum
X-ray Photoelectron Spectroscopy (XPS) measurements were
also carried out to study the chemical state and composition of
the catalyst surface. The samples (B000–J101) used for the XPS
measurements were eight fresh platinum films made from Heraeus
platinum paste and supported on YSZ solid electrolyte, seven of
which were impregnated with variable concentration of NaOH
ꢀ
yy
ꢀ12
ꢀ1
surface (NaOH concentration = x ꢁ 10
M). Kinetic studies were
solutions (ranging from 10
to 10 M) following the same so-
carried out on Sample A (which was then subsequently added with
dium deposition method as for the samples used for kinetic exper-
iments. The XPS measurements were performed using a Thermo
ꢀ4
ꢀ2
NaOH solution from 10
to 5 ꢁ 10 M, resulting in Samples
A104–A502) under open and closed circuit conditions at 350 °C.
Ethylene partial pressure was kept constant at 0.5 kPa for all the
experiments reported here while the oxygen partial pressure was
varied between 0.5 and 8 kPa. The reaction rates reported here
K-Alpha electron spectrometer with a monochromated Al K
a
ꢀ19
X-ray source (1486.6 eV, 1 eV = 1.6302 ꢁ 10
J) with a spot size
of approximately 400
l
m on the sample surface. The instrument
has a hemi-spherical electron energy analyser; survey spectra were
taken at pass energy of 200 eV and narrowscan spectra around the
principal peaks of the elements present (i.e. C, O, Na, Pt, Zr and Y)
with pass energy of 50 eV. The narrowscan spectra were taken
from four points in a line for each of the eight samples. These
points on each line were about 2.5 mm apart, and shown in
Fig. 1. For each line scan, the first three points were on the plati-
num region, and the last point was on the YSZ region. The base
2
are expressed in terms of moles of CO produced per unit area per
unit time (where the area used for the rate calculations is the geo-
metric projected surface area of the catalyst/electrode) and are cal-
culated based on the outlet composition of the reactor. In the
figures where discrete data points for reaction rates are used, error
bars of 5% are included in the plots. This accounts for inaccuracies in
2
the measurement of the CO partial pressure and the total volumet-
ꢀ8
ric flow rate at the outlet of the reactor and possible fluctuations of
the electronic mass flow controllers used to deliver the flow of
gases to the reactor. We believe this to be a good estimate, based
on the manufacturer’s specification for the instruments used and
the fact that during repeat experiments under similar conditions,
the calculated reaction rates did not differ by more than 5%. In addi-
pressure in the analysis chamber was kept below 1 ꢁ 10 mbar
during data acquisition. The pass energy of the analyser was set
at 50 eV for the narrowscans, for which the resolution as measured
by the full width at half maximum (FWHM) of the Ag 3d5/2 peak
was 0.85 eV. The binding energies were referenced to Au using
the manufacturer’s automatic calibration software.

N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
103
coverage in Fig. 3. An increase in sodium coverage increases the
Na 1s intensity. It appears that sodium is non-uniformly distrib-
uted on each acquisition point (the highest scatter is observed
for the sample with the highest sodium coverage). The presence
of sodium on Point 4 could be due to the mobility of sodium and
sodium impurities pre-existing on the YSZ electrolyte; the manu-
facturer reports that the YSZ powder contains approximately
Pt
YSZ
P = Spectra
acquisition
point
P1 P2 P3 P4
.5mm
2
1
2
00 ppm Na O). As we can see from Fig. 3, the ‘clean’ sample
(
B000) has similar Na 1s peak intensity to the samples with cover-
age less than 1% (Samples C112–E104). This could possibly be due
to impurities pre-existing on the platinum surface and/or the de-
ionised water used to prepare the NaOH solutions (pre-existing
impurities will be more significant for solutions of low NaOH con-
centration and thus for samples of low coverage).
Spectra acquired in
a region approximately
d = 400 µm
Fig. 1. Schematic drawing of the four acquisition points on each sample during XPS
measurements.
4.2. Kinetic studies
4
. Results and discussion
Next, we will discuss the results obtained from the kinetic
experiments performed on ‘clean’ and sodium-modified samples
under open-circuit and polarised conditions.
4.1. XPS Analysis
The surface of a nominally ‘clean’ Pt/YSZ catalyst system (Sam-
4
.2.1. Open-circuit conditions
Fig. 4 shows the CO production rate of all samples under open-
circuit conditions, while Table 2 lists the rate enhancement ratios
caused by sodium addition, Na. As we can see from Fig. 4, the rate
of CO production is positive order with respect to oxygen partial
ple B000) and the Na-modified samples (C112–J101) were charac-
terised using XPS (these were all fresh samples and were not used
in kinetic experiments). Typical O 1s, Na 1s, C 1s and Pt 4f spectra
are shown in Fig. 2 (only those obtained from Point 3 in Fig. 1 are
shown). Similar spectra with respect to the peaks of O 1s, Na 1s,
C 1s and Pt 4f were obtained for all other points in the platinum
regions, except for some shifts in the binding energies, and changes
in the peak intensities probably due to non-homogeneity in
sodium distribution on the catalyst surface.
The nominally ‘clean’ sample in Fig. 2a exhibited two main O 1s
components at binding energies of 530.2 and 532.2 eV. The former
indicates the presence of lattice oxygen, this signal belongs to the
oxidic contribution of the YSZ [38], and the latter indicates the
existence of oxygen from other groups such as hydroxides, carbon-
ates and/or oxides on the surface of the platinum films [39–41].
Increasing the sodium coverage on the catalyst surface masks the
O 1s signal arising from the solid electrolyte (the first peak), while
at the same time increases the second peak, suggesting that this
peak may be associated with sodium. As can be seen from
Fig. 2b, a broad peak for Na 1s can be seen for the ‘clean’ sample,
as well as for the samples of low sodium coverage (below 1%). This
broad peak (which could represent more than one chemical states
of sodium on the catalyst platinum surface, such as sodium carbon-
ates, sodium oxides or hydroxides) becomes sharper and larger as
the sodium coverage increases. For Samples H502 and J101, the
binding energy of Na 1s peaks at 1071.5 eV indicating the presence
of sodium carbonates on the platinum surface [42]. This can be fur-
ther confirmed from the C 1s spectra (shown in Fig. 2c) that also
indicate the presence of carbonates (binding energy
2
q
2
pressure. This is true for all the samples (i.e. ‘clean’ and sodium-
modified). This has also been consistently reported in the literature
(
e.g. [9]). From Fig. 4, we can see that sodium addition in general
increases the open-circuit rate. This effect becomes more pro-
nounced at high sodium coverage and for low oxygen partial pres-
sure in the reaction mixture. Thus, the highest enhancement ratios
(q
Na ꢂ 4) are observed for 65% sodium coverage and oxygen partial
pressures up to 1.5 kPa (stoichiometric oxygen to ethylene ratio).
The results obtained under open-circuit conditions are, in the first
instance, in qualitative agreement with the proposed model (for an
electrophilic reaction), in the sense that, in general, sodium addi-
tion resulted in an increase in the reaction rate as predicted by
the model. Of course, whether this promotion is simply due to
the work function changes caused by sodium, or also caused by
additional interaction between the sodium species and the reac-
tants or the oxygen-promoting species, cannot be assessed at the
moment and will be discussed in more detail when the experi-
ments under polarisation are discussed.
4.2.2. Polarised conditions
Figs. 5 and 6 show the reaction rate transients during polarisa-
tion which lasts for 7 min (420 s). The reaction rate is monitored
for an additional 5 min prior to polarisation, and approximately
1
5 min after polarisation (in some cases the final open-circuit step
is longer as the return to the initial open-circuit rate can be slower
depending on the experimental conditions). In Fig. 5a and b, we can
see the transients for an oxygen partial pressure of 3.0 kPa (results
for lower partial pressures are not shown but showed similar
trends) while Fig. 6a and b show the behaviour under an oxygen
partial pressure of 8.0 kPa (high pO2). For both Figs. 5 and 6, figure a
corresponds to negative polarisation with an overpotential of ꢀ1 V
and figure b corresponds to positive polarisation at an overpotential
of 1 V. In addition, Table 3 shows the enhancement ratios and
faradaic efficiencies observed under all experimental conditions
studied here. As can be seen here, the reaction shows in general
electrophilic behaviour for sodium coverage less than 1% (Sample
A104) and oxygen partial pressure up to 3.0 kPa (i.e. fuel-rich to
mildly fuel-lean conditions) since application of a negative poten-
tial (Fig. 5a) leads to an increase in the reaction rate while positive
potential causes a rate decrease (Fig. 5b). This has also been
2
89.2 ± 0.2 eV) for Samples H502 and J101 [42]. The C 1s peak at
binding energies above 284 eV is normally assigned to graphitic
carbon species that may originate from impurities on the catalyst
surface due to sample preparation and handling. The binding en-
ergy of Pt 4f7/2 at 70.8 ± 0.3 eV in Fig. 2d indicates that the plati-
num catalyst on all samples is in metallic form [41]. This
suggests that sodium addition has not significantly modified the
chemical state of the platinum catalyst.2
The intensity of the Na 1s peaks for all samples at the four dif-
ferent acquisition points is plotted against the nominal% sodium
2
In Point 1 of some samples (spectra not shown), there is an indication of the
existence of platinum hydroxide species at 72.3 ± 0.3 eV [NIST X-ray Photoelectron
Spectroscopy Database 2000, National Institute of Standards and Technology (NIST),
http://srdata.nist.gov/xps/, Accessed on 12 November 2012].

1
04
N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
(a)
(b)
(c)
(d)
Fig. 2. XPS spectra of nominally ‘clean’ and Na-modified Pt/YSZ catalyst systems.
observed in our previous work [37]. The effect of sodium (at a low
coverage) becomes less pronounced at high oxygen partial pres-
sures (as seen in Fig. 6) where no significant rate modification is ob-
served; in fact polarisation appears to simply stabilise the
oscillating reaction rate for both the clean sample and the 0.11%
coverage (the stable rate is slightly lower than the oscillatory rate
observed prior to and after polarisation). However, as the sodium
coverage increases, the effect of polarisation at high oxygen partial
pressures becomes more pronounced. This indicates that under
these conditions, there may be an interaction between the sodium
surface species and the oxygen both from the gas phase and from
the oxygen ion supply (this interaction is expected at high sodium
coverage as indicated in our model discussion earlier on). As shown
in Table 3, the reaction shows volcano-type behaviour for oxygen
partial pressures of 8.0 kPa (at all sodium coverages) while the oxy-
gen partial pressure where volcano behaviour is first observed de-
creases with increasing sodium coverage (so for
a sodium
coverage of 65%, the reaction shows volcano-type behaviour even

N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
105
(
a)
Fig. 3. Relative intensity of sodium and platinum from XPS measurement versus
nominal% sodium coverage.
(
b)
Fig. 4. Open-circuit rates at varying oxygen partial pressures. Reactant partial
pressures, variable
ture = 350 °C; total flow rate = 200 ml min
pO2 = 0.5, 1.5, 3.0 or 8.0 kPa, pC2H4 = 0.5 kPa; tempera-
ꢀ
1
.
Fig. 5. Transient plots under intermediate oxygen partial pressure and polarised
conditions: (a) = +1 V. Top: current density, i versus time; bottom:
= ꢀ1 V; (b)
CO versus time. Reactant partial pressures, pO2 = 3.0 kPa, pC2H4 = 0.5 kPa; temper-
ature = 350 °C; Total flow rate = 200 ml min
Table 2
g
g
Rate enhancement ratios caused by sodium modification.
2
ꢀ
1
.
p
O2 (kPa) Rate enhancement ratio,
q
Na
A104
A102
A502
ꢀ
20 l
A cmꢀ2), this was not the case under positive polarisations
where, in general, higher sodium coverages resulted in higher
Sodium enhancement
0.5
1.5
3.0
8.0
0.58
1.04
0.90
1.06
1.29
1.90
1.90
0.87
3.96
3.81
1.82
1.27
current densities (current densities varied from 20 to
ꢀ2
1
50
l
A cm ).
A
similar behaviour was observed during
electrochemical measurements under non-reactive atmospheres
(
not shown here) where a limiting current density of approxi-
ꢀ
2
mately 20 lA cm had been observed for cathodic polarisation
at sub-stoichiometric oxygen to ethylene ratios, pO2 = 0.5 kPa). In
addition, for the case of a sodium coverage of 65% and an oxygen
partial pressure of 8.0 kPa, the reaction becomes electrophobic as
a rate enhancement is only observed under positive polarisation
lower than ꢀ0.5 V, while no limiting currents were observed for
anodic polarisation in the range of overpotentials used. This sug-
gests that sodium addition does not affect the rate of oxygen
reduction taking place at the three-phase boundary under cathodic
polarisation. In contrast, the supply of oxygen ions from the elec-
trolyte to the three-phase boundary and the catalyst surface is
facilitated by sodium addition (as indicated by the increased cur-
rents observed). This may be related to e.g. the creation of sodium
oxide or hydroxide species on the catalyst surface and may be
linked to the changing behaviour of the reaction at high sodium
(Fig. 6b).
With respect to the polarisation experiments, we can see from
Figs. 5 and 6 that the observed currents under negative and posi-
tive polarisation show a different behaviour as a function of the so-
dium coverage. While under negative polarisation the observed
current densities were very similar for all samples (around

1
06
N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
(
a)
2
Fig. 7. CO production rate under stoichiometric conditions as a function of work
function changes (work-function changes for sodium coverage of 0.11% and 11%
were calculated from work-function measurements performed in air; the work-
function change corresponding to 65% coverage is estimated by linear extrapolation
from the measured values). Reactant partial pressures, pO2 = 1.5 kPa, pC2H4 = 0.5 kPa;
ꢀ
1
temperature = 350 °C. Total flow rate = 200 ml min
.
5
. Discussion
The experimental data under varying sodium coverages and
oxygen partial pressures exhibit the following key features:
ꢃ Increasing sodium coverage up to 65% appears to promote the
open-circuit catalytic activity of the Pt/YSZ system for ethylene
oxidation; the sodium-induced rate enhancement is reduced by
an increase in the oxygen partial pressure
ꢃ The system exhibits electrophilic behaviour at low sodium cov-
erage and under fuel-rich conditions to mildly fuel-lean condi-
tions (pO2 = 0.5–3.0 kPa), however this promotional behaviour
appears to be changed to volcano and finally electrophobic by
increasing oxygen partial pressure and sodium coverage
(
b)
We will now test the work-function-dependent model pro-
posed earlier on with the experimental data obtained in this work.
We will initially consider the case of low oxygen partial pressures
where the reaction showed electrophilic behaviour; this is shown
in Fig. 7. In order to test our model, we must first estimate the work
function changes of the platinum film caused by sodium addition.
In our previous work [36], we had measured via the Kelvin Probe
technique the work functions of several platinum films of different
Fig. 6. Transient plots under high oxygen partial pressure and polarised conditions:
a) = +1 V. Top: Current density, i versus time; Bottom: CO rate
(
g
= ꢀ1 V; (b)
g
2
versus time. Reactant partial pressures, pO2 = 8.0 kPa, pC2H4 = 0.5 kPa; tempera-
ture = 350 °C. Total flow rate = 200 ml min 1.
ꢀ
coverage from electrophilic to volcano and electrophobic (for both
high oxygen partial pressure and sodium coverage).
Table 3
Rate enhancement ratios and faradaic efficiencies of all samples.
p
O2 (kPa)
Rate enhancement ratio,
q
Faradaic efficiency,
A000
K
A000
A104
A102
A502
A104
A102
A502
Negative polarisation
0.5
1.5
3.0
8.0
1.35
1.27
1.28
1.00
2.23
2.03
1.84
0.94
1.30
1.32
0.88
0.83
1.01
0.83
0.76
0.75
ꢀ138
ꢀ177
ꢀ330
ꢀ2.12
ꢀ505
ꢀ871
ꢀ1323
172
ꢀ299
ꢀ724
336
ꢀ55.2
322
473
666
1170
Positive polarisation
0.5
1.5
3.0
8.0
1.02
0.89
0.82
0.80
0.94
0.89
0.85
0.81
1.10
0.89
0.72
0.92
0.85
0.79
1.00
1.12
8.37
ꢀ65.7
ꢀ33.3
ꢀ157
ꢀ283
29.5
ꢀ108
ꢀ104
1.93
ꢀ148
ꢀ1142
ꢀ1015
ꢀ84.9
ꢀ169
ꢀ45.6
347

N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
107
(a)
(i)
(ii)
(iii)
(
b)
(
a)
(b)
(c)
Fig. 8. Schematic representation (a) and actual example (b) of the rate changes with positive and negative polarisations as a function of sodium coverage and oxygen partial
pressure.
3
sodium coverage (namely ‘clean’, 0.11% and 11%). From these val-
ues, we can calculate the work-function changes between the clean
and the sodium-modified samples. In order to calculate these
changes, we use the average work function of all the samples of
the same sodium coverage (and also calculate the corresponding
standard error) and find the difference between the average work
function of the clean samples and the average work function corre-
sponding to each sodium coverage (in addition, the corresponding
standard error of the difference is also calculated). Based on these
calculations, we obtained that the work function changes (with re-
spect to the work function of a ‘clean’ sample) for samples of 0.11%
and 11% sodium coverage were ꢀ100 ± 70 and ꢀ200 ± 80 meV,
respectively. We must, however, note that these measurements
were performed in air (at room temperature) and may as a result
deviate from work function changes caused by sodium addition
under operating conditions. The work function change caused by
a sodium coverage of 65% was extrapolated from the measured val-
ues assuming a linear dependence between work function [3] and
sodium coverage and is estimated to be ꢀ950 ± 100 meV (in this
case, the corresponding error was calculated by the errors of the
linear extrapolation); it is possible that an even larger error may
be associated with this estimation as work function changes may
not in fact have a linear dependence on sodium coverage under
the operating conditions. For the electrically induced work-func-
tion changes, we assume that
DU
¼ eDU
WR (we have not included
any error bars associated with electrically induced work function
changes). Fig. 7 shows the rate dependence on work function change
for an oxygen partial pressure of 1.5 kPa. The three sets of data in the
dotted boxes correspond to the nominally ‘clean’ sample (circle) and
ꢀ4
those modified with 10 M (0.11% Na coverage, triangle) and
ꢀ2
10 M (11% Na coverage, star) NaOH solutions (for open-circuit, po-
sitive and negative polarisation). As we can observe in the figure, in
general the reaction rate changes at low sodium coverage (0.11%)
follow the proposed work-function-dependent behaviour for an
electrophilic reaction, where negative polarisation generally en-
hances the reaction rate due to a decrease in the catalyst work func-
tion, while positive polarisation reduces the reaction rate as it
increases the catalyst work function. Sodium addition enhances
the rate increase caused by negative polarisation and reduces the
rate decrease for positive polarisation.
While we can comment on a qualitative agreement between the
proposed model and the experimental data, we also notice that a
3
In EPOC studies the change in the work function of a catalyst due to electrical
polarisation (
D
U
el) is linked to the applied potential between the working and the
el = ne WR, where n
reference electrode (UWR) or overpotential via the relation:
D
U
DU
depends on operating conditions and electrode morphology [I.S. Metcalfe, Journal of
Catalysis, 199 (2001) 247–258].

1
08
N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
deviation from the model exists; this suggests a possible interac-
tion between either the two promoting species or between the
promoters and the reactants or intermediates. In particular, we
can observe that while a sodium coverage of 0.11% does not signif-
icantly change the reaction rate, it induces a much higher electro-
promoted rate at negative polarisation than that of a ‘clean’ pellet
indicating an interaction between the oxygen and sodium promot-
ing species. The total rate increase for the sodium-modified elec-
tropromoted sample is not therefore equal to the addition of the
rate changes from sodium addition and polarisation alone. This
can be seen in Figs. 5a and 7. Moreover, as discussed earlier on
and is also shown in Fig. 7 at high sodium coverage, the reaction
changes behaviour to volcano-type and the rate data deviate signif-
icantly from the model. These observations suggest that the two
promoting species interact with each other, thus creating a more
complex response of the system than the one anticipated by the
proposed model.
Next, we will further discuss the role of sodium on the changing
behaviour of the reaction under EPOC conditions. As mentioned
earlier, the reaction behaviour changes with increasing oxygen par-
tial pressure and sodium coverage from electrophilic, to volcano
and finally to electrophobic. This is shown schematically in Fig. 8a
while Fig. 8b illustrates this shifting behaviour with experimental
data. Under polarisation, the electrophilic behaviour observed at
low sodium coverage and under low oxygen partial pressure can
simply be explained by stronger ethylene (electron donor reactant)
adsorption in comparison with oxygen (electron acceptor reactant)
in accordance with the more ionic character sodium exhibits at low
coverage. An increase in sodium coverage (and also gas-phase oxy-
gen) however may result in an increase in oxygen adsorption and
possibly also modify the competitive adsorption between oxygen
and ethylene, as sodium becomes more metallic [28]. In the inter-
mediate region of sodium coverage and oxygen partial pressure,
volcano behaviour should then be observed (where both reactants
are strongly adsorbed), while at high sodium coverage, when
eventually the oxygen becomes more strongly adsorbed than the
ethylene, the electrophobic behaviour can occur. This change in
the apparent behaviour of the reaction, which appears to be caused
primarily by the sodium addition, is in fact a manifestation of the
complexity of the interaction between all the species of this cata-
lytic system. The interaction between the two promoting species
could be used to explain some of the differences in experimental re-
sults presented by different research groups on nominally similar
catalyst systems where the raw materials are often obtained from
different sources. As an example, while most studies conducted
on the electrochemical promotion of ethylene oxidation on Pt/YSZ
show that the reaction exhibits electrophobic behaviour for a wide
range of conditions [42,43] in our work, the reaction showed clear
electrophilic behaviour for a nominally clean pellet; this may sim-
ply be due to the presence of different impurities on the raw mate-
rials used for the preparation of the different samples. This suggests
that while surface species may not be necessary for the occurrence
of EPOC, they can significantly affect the behaviour of the system.
tive effect between sodium addition and polarisation. However,
experimental results indicated that the two promoter species are
not completely independent of each other especially at high so-
dium coverage. With increasing sodium coverage, the reaction
behaviour changes from electrophilic to volcano-type (possible to
an increase in the chemisorptive bond strength of oxygen on the
platinum catalyst), while for high sodium coverage, where the so-
dium is known to adopt a more metallic character, interaction with
oxygen becomes more significant and the reaction shows electro-
phobic behaviour.
Acknowledgments
NI would like to thank the Ministry of Higher Education Malay-
sia for a scholarship. DP acknowledges the support of EPSRC
through Grants EP/G025649/1 and EP/G012865/1. X-ray photo-
electron spectra were obtained at the National EPSRC XPS User’s
Service (NEXUS) at Newcastle University, an EPSRC Mid-Range
Facility. The authors would like to thank Dr. Maria Elena Rivas
for useful discussions.
References
[1] S. Brosda, C.G. Vayenas, J. Wei, Appl. Catal. B: Environ. 68 (2006) 109–124.
[2] C.G. Vayenas, S. Bebelis, S. Neophytides, J. Phys. Chem. 92 (1988) 5083–5085.
[
3] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical
Activation of Catalysis: Promotion, Electrochemical Promotion, and Metal–
Support Interactions, Kluwer Academic/Plenum Publishers,, New York, 2001.
[4] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627.
[
[
5] M. Stoukides, C.G. Vayenas, J. Catal. 70 (1981) 137–146.
6] S. Souentie, L. Lizarraga, A. Kambolis, M. Alves-Fortunato, J.L. Valverde,
P. Vernoux, J. Catal. 283 (2011) 124–132.
[7] A. de Lucas-Consuegra, A. Caravaca, M.J. Martín de Vidales, F. Dorado,
S. Balomenou, D. Tsiplakides, P. Vernoux, J.L. Valverde, Catal. Commun. 11
(
2009) 247–251.
[
8] C.G. Vayenas, S. Brosda, C. Pliangos, J. Catal. 203 (2001) 329–350.
[9] C.G. Vayenas, B. Lee, J. Michaels, J. Catal. 66 (1980) 36–48.
[
[
10] C. Pliangos, I.V. Yentekakis, X.E. Verykios, C.G. Vayenas, J. Catal. 154 (1995)
124–136.
11] O.A. Mar’ina, V.A. Sobyanin, V.D. Belyaev, V.N. Parmon, Catal. Today 13 (1992)
567–570.
[
[
[
12] M. Konsolakis, I.V. Yentekakis, J. Hazard. Mater. 149 (2007) 619–624.
13] I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170–188.
14] D. Tsiplakides, S.G. Neophytides, O. Enea, M. Jaksic, C.G. Vayenas, J.
Electrochem. Soc. 144 (1997) 2072–2078.
[15] A. Thursfield, S. Brosda, C. Pliangos, T. Schober, C.G. Vayenas, Electrochim. Acta
8 (2003) 3779–3788.
4
[
[
16] C.G. Vayenas, S. Bebelis, M. Despotopoulou, J. Catal. 128 (1991) 415–435.
17] P. Vernoux, F. Gaillard, C. Lopez, E. Siebert, J. Catal. 217 (2003) 203–208.
[18] P.D. Petrolekas, S. Brosda, C.G. Vayenas, J. Electrochem. Soc. 145 (1998) 1469–
477.
1
[
19] G. Rothernberg, Catalysis: Concepts and Green Applications, Wiley-VCH,
Weinheim, 2008.
[20] E. Mutoro, B. Luerßen, S. Günther, J. Janek, Solid State Ionics 180 (2009) 1019–
033.
1
[
[
21] J. Gaube, H.F. Klein, Appl. Catal. A: Gen. 350 (2008) 126–132.
22] A.R. de la Osa, A. De Lucas, J.L. Valverde, A. Romero, I. Monteagudo, P. Coca,
P. Sánchez, Catal. Today 167 (2011) 96–106.
[
[
[
23] J. Haber, M. Nattich, T. Machej, Appl. Catal. B: Environ. 77 (2008) 278–283.
24] Y. Wu, M. Liu, Z. Ma, St. Xing, Catal. Today 175 (2011) 196–201.
25] Y. Zhai, D. Pierre, R. Si, W. Deng, P. Ferrin, A.U. Nilekar, G. Peng, J.A. Herron, D.C.
Bell, H. Saltsburg, M. Mavrikakis, M. Flytzani-Stephanopoulos, Science 329
(
2010) 1633–1636.
[
[
26] T. Aruga, Y. Murata, Prog. Surface Sci. 31 (1989) 61–130.
27] F. Arena, F. Frusteri, A. Parmaliana, Appl. Catal. A: Gen. 187 (1999) 127–140.
6
. Conclusions
[28] J.C. Bertolini, P. Delichère, J. Massardier, Surface Sci. 160 (1985) 531–541.
[
[
[
29] W.D. Mross, Catal. Rev. 25 (1983) 591–637.
30] W. Schröder, J. Hölzl, Solid State Commun. 24 (1977) 777–780.
31] J. H o} lzl, F.K. Schulte, Work function of metals, in: J. H o} lzl, F.K. Schulte,
The study presented here showed that the presence of foreign
surface species on a catalyst surface can significantly affect its cat-
alytic and electrocatalytic properties. We cannot however, based
on the evidence obtained from this work, assess the necessity of
such surface species for the occurrence of EPOC. In the case of so-
dium addition onto a platinum catalyst, the observed behavioural
changes have been partially explained by a model that considers
work function changes caused by each promoter species indepen-
dently. The model can, for low sodium coverage, predict an addi-
H. Wagner (Eds.), Solid Surface Physics/Contributions, Springer-Verlag, Berlin,
1979.
32] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.G. Lintz, Cataly. Today 11 (1992)
[
[
[
[
303–438.
33] I.V. Yentekakis, A. Palermo, N.C. Filkin, M.S. Tikhov, R.M. Lambert, J. Phys.
Chem. B 101 (1997) 3759–3768.
34] P. Vernoux, F. Gaillard, C. Lopez, E. Siebert, Solid State Ionics 175 (2004) 609–
613.
35] S. Bebelis, M. Makri, A. Buekenhoudt, J. Luyten, S. Brosda, P. Petrolekas,
C. Pliangos, C.G. Vayenas, Solid State Ionics 129 (2000) 33–46.

N. Ibrahim et al. / Journal of Catalysis 303 (2013) 100–109
109
[
[
[
[
36] N. Ibrahim, D. Poulidi, M.E. Rivas, I.D. Baikie, I.S. Metcalfe, Electrochim. Acta 76
2012) 112–119.
37] N. Ibrahim, M.R. Jalil, D. Poulidi, I.S. Metcalfe, Solid State Ionics 225 (2012)
86–389.
38] S.G. Neophytides, S. Zafeiratos, S. Kennou, Solid State Ionics 136–137 (2000)
01–806.
39] T. Grzybek, G.C. Maiti, D. Scholz, M. Baerns, Appl. Catal. A: Gen. 107 (1993)
15–124.
[40] S. Dahle, F. Voigts, W. Maus-Friedrichs, Thin Solid Films 520 (2012) 1842–
1846.
[41] NIST X-ray Photoelectron Spectroscopy Database 2000, National Institute of
Standards and Technology (NIST). <http://srdata.nist.gov/xps/> (12 November
2012).
(
3
8
[42] S. Bebelis, C.G. Vayenas, J. Catal. 118 (1989) 125–146.
[43] S. Bebelis, C.G. Vayenas, J. Catal. 138 (1992) 588–610.
1