Electro-oxidation of methanol on Pt-Rh alloys

Article abstract of DOI:10.1016/j.electacta.2006.12.016

Methanol adsorption and electro-oxidation on Pt-Rh alloys have been studied in aqueous 0.5 M H₂SO₄ for a broad range of alloy surface composition including the pure Pt and Rh metals. Adsorption results have been compared with equivalent data obtained for CO and CO₂ adsorption on these alloys. Current densities of continuous methanol oxidation on Pt, Rh and a Pt-Rh alloy with optimum surface molar fraction of Rh have been measured. Although on the pure Pt and Rh metals the methanol adsorption products exhibit similar energetic stability, as judged from the peak potential of electro-desorption, on the Pt-Rh alloys, there is a lowering of the stability. Similar behavior is observed for the CO and CO₂ adsorption products, however, the lowering for methanol is much less than for CO and CO₂. In the case of methanol, the maximum lowering is obtained for a surface molar fraction of Rh equal to ca. 0.65 and it is the same alloy surface composition that results in maximum lowering of the stability of the CO₂ adsorption products, but not of the CO adsorption products (optimal fraction of Rh equal ca. 0.10). Structural similarity of the methanol and the CO₂ adsorption products finds support in similar values of the electrons-per-site parameter obtained. Pt-Rh alloys show insufficient electrode potential improvement over Pt in continuous methanol electro-oxidation due to the susceptibility of Rh to strong poisoning by the methanol adsorption products, which switches off the bi-functional mechanism of methanol electro-oxidation on this alloy. The presence of Rh in the alloy with Pt additionally strongly lowers the methanol electro-oxidation turnover rate of the Pt component.

Full text of DOI:10.1016/j.electacta.2006.12.016

Electrochimica Acta 52 (2007) 5565–5573
Electro-oxidation of methanol on Pt-Rh alloys
a
a
a
b,∗
´
Wojciech Tokarz , Hanna Siwek , Piotr Piela , Andrzej Czerwinski
a Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland
b Faculty of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland
Received 8 September 2006; received in revised form 1 December 2006; accepted 2 December 2006
Available online 17 December 2006
Abstract
Methanol adsorption and electro-oxidation on Pt-Rh alloys have been studied in aqueous 0.5 M H2SO4 for a broad range of alloy surface
composition including the pure Pt and Rh metals. Adsorption results have been compared with equivalent data obtained for CO and CO2 adsorption
on these alloys. Current densities of continuous methanol oxidation on Pt, Rh and a Pt-Rh alloy with optimum surface molar fraction of Rh have
been measured.
Although on the pure Pt and Rh metals the methanol adsorption products exhibit similar energetic stability, as judged from the peak potential of
electro-desorption, on the Pt-Rh alloys, there is a lowering of the stability. Similar behavior is observed for the CO and CO2 adsorption products,
however, the lowering for methanol is much less than for CO and CO2. In the case of methanol, the maximum lowering is obtained for a surface
molar fraction of Rh equal to ca. 0.65 and it is the same alloy surface composition that results in maximum lowering of the stability of the CO2
adsorption products, but not of the CO adsorption products (optimal fraction of Rh equal ca. 0.10). Structural similarity of the methanol and the
CO2 adsorption products finds support in similar values of the electrons-per-site parameter obtained.
Pt-Rh alloys show insufficient electrode potential improvement over Pt in continuous methanol electro-oxidation due to the susceptibility of Rh
to strong poisoning by the methanol adsorption products, which switches off the bi-functional mechanism of methanol electro-oxidation on this
alloy. The presence of Rh in the alloy with Pt additionally strongly lowers the methanol electro-oxidation turnover rate of the Pt component.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Fuel cells; Menthanol; Pt-Rh alloy; Electro-oxidation
1. Introduction
blocks the surface for further methanol adsorption. Binary Pt-
based catalysts have been discovered that overcome this problem
to a large extent. A second metal is introduced that, firstly, has
a greater ability to form surface oxygen species compared to Pt
(bi-functional effect [4]), and secondly, has a lower d-band elec-
tron density than Pt and, thus, is capable of weakening the bond
between the strongly adsorbed intermediate and Pt by withdraw-
ing electron density from the bond (the ligand effect [5]). Among
all the second metals studied so far, Ru is the most successful in
methanol electro-oxidation.
A cyclic voltammetry comparison of Rh and Ru indicates
that Rh could potentially provide for a bi-functional effect, too,
because it is capable of water electro-decomposition with for-
mation of surface oxide species at a potential ca. 250 mV lower
than Pt is [6]. Also, Rh resists corrosion in the strongly acidic
environment of the DMFC just as well as Ru [7] making it a
good second-metal candidate from the stability standpoint. An
important difference between Ru and Rh, however, is that Rh,
similarly to Pt, adsorbs hydrogen under-potentially and is also
Electro-oxidation of methanol is an important research topic
because methanol is a good candidate fuel for low-temperature
fuel cells [1,2]. It has a high energy density at ambient tem-
perature and pressure and low-temperature electro-catalysts are
available, at which it relatively easily oxidizes directly to CO2,
whereby design of compact direct methanol fuel cell (DMFC)
systems is possible [3].
Methanol electro-catalysis in DMFCs currently relies on plat-
inum. The dehydrogenative adsorption of methanol on Pt starts
at a potential as low as 0.2 V versus SHE, however the final
product of this step is a strongly adsorbed intermediate, which
∗
Corresponding author. Tel.: +48 22 822 02 11x305/568 24 43;
fax: +48 22 568 23 90.
E-mail addresses: andrzej.czerwinski@ichp.pl, aczerw@chem.uw.edu.pl
(A. Czerwin´ski).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.12.016

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W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
capable of adsorbing methanol [8,9]. On the other hand, from
quantum chemical calculations of adsorption energies and acti-
vation energies relevant to the COads electrochemical removal
by water-generated OHads from the surface of 10-atom, Pt-based
binary alloy clusters in vacuo [10] it follows that, contrary to Ru,
addition of Rh to Pt should not increase significantly the CO
portion of the supporting electrolyte taking special care that no
oxygen be introduced into the cell. In the case of all the results
presented here the adsorption potential was +0.015, +0.008,
and +0.200 V versus SHE for CO, CO2, and methanol adsorp-
tion, respectively. Theseadsorptionpotentialsassuredmaximum
electrode coverage by the adsorption products.
`
tolerance of the cluster. This is surprising vis-a-vis the cyclic
All chemicals and gases used were of high-purity grade and
were used without additional purification. Prior to adsorption or
continuous methanol oxidation experiments, the electrodes were
cleaned by in situ electrochemical cycling to obtain well-defined
voltammograms certifying high system purity.
voltammetry data and the studies of carbon oxides adsorption
and electro-desorption from Pt-Rh alloys conducted in our group
[11,12], andthiscontrastwasoneoftheincentivesforthepresent
study.
Some enhancement of methanol electro-oxidation on a Pt-
Rh electrode relative to pure Pt electrode has been first reported
over 40 years ago by Binder et al. [13]. Since then, only
one literature report appeared that includes results of methanol
electro-oxidation on a Pt-Rh alloy. In this paper by Choi et al.
[14] nano-crystalline Pt-Rh fuel cell-type electrodes were tested
in the DMFC anode reaction. It was concluded that Pt-Rh (2:1)
gives considerable improvement over pure Pt in the fuel cell, but
the magnitude of this improvement was in disagreement with the
included cyclic voltammetry data. In a paper by Oliveira et al.
[15] a smooth Pt electrode with an under-layer of Rh (several
monolayers of Rh covered by several monolayers of Pt) was
claimed to give a considerable increase in the methanol electro-
oxidation current density relative to the same electrode without
the Rh under-layer. Given that the electrode with Rh and the
pure Pt electrode produced overlapping cyclic voltammograms
in the supporting electrolyte and that the cyclic voltammograms
in the presence of methanol differed only by a scaling factor (no
potential shifts), the improvement from addition of Rh is diffi-
cult to understand. Considering the ambiguity and the limited
amount of the literature data on the topic, it was again found
useful to study methanol electro-oxidation on Pt-Rh alloys in
detail.
Electrochemical techniques were performed with a Volta-
Lab PGZ 301 potentiostat/galvanostat. The EQCM was from
UELKO, Poland. Unless noted otherwise, in cyclic voltamme-
try experiments, a scan rate of 50 mV s−1, an anodic vertex
potential of +1.25 V versus SHE, and a cathodic vertex poten-
tial of +0.05 V versus SHE were used. The working electrode
potential was measured against a mercury/mercury sulfate or a
silver/silver chloride reference electrode, but all the potentials
are given versus SHE.
3. Results and discussion
3.1. Preparation and characterization of a Pt-Rh alloy with
a defined surface molar fraction of Rh
Rhodium is a metal less noble than platinum. Its thermody-
namic dissolution potential in acid is ca. 600 mV more negative
[7]. It is, therefore, possible to selectively remove Rh from the
Pt-Rh alloy by electro-oxidation. Also, by adjusting the condi-
tions of electro-deposition of the alloy, it is possible to obtain a
starting alloy with a Rh-rich surface. These two facts open up
a possibility to prepare Pt-Rh alloy electrodes with a controlled
surface molar fraction of Rh.
Fig. 1 presents a series of cyclic voltammograms (CVs) con-
secutively recorded on a freshly electrodeposited Pt-Rh alloy.
2. Experimental
All electrodes used in this study were limited volume
electrodes (LVEs) [16–18], which were electrodeposited poten-
tiostatically on gold (99.99 Au) from aqueous solutions
containing H2PtCl6, RhCl3, and HCl. The electro-deposition
potential was 0.180 V versus SHE in all cases. The deposited
layer thickness was a few micrometers and the surface rough-
ness factor was in the range 50–200. The gold support was either
a 0.5-mm diameter wire or, in the case of electrochemical quartz
crystal microbalance (EQCM) studies, a layer of gold affixed to
the quartz crystal as the working electrode.
All electrochemical studies were performed in deoxygenated
aqueous 0.5 M H2SO4 solutions at room temperature. The strip-
ping technique was used for studying the adsorption of methanol
and carbon oxides [19]. For CO and CO2 adsorption, a pressure
of 1 bar of the respective gas was maintained during the adsorp-
tion step, after which the gas was thoroughly flushed from the
cell with nitrogen or argon. In the case of methanol adsorption,
a concentration of 1 M methanol was used. After the adsorp-
tion step, the methanol was removed from the cell by replacing
the methanol-containing electrolyte with a fresh, deoxygenated
Fig. 1. Transformation of the cyclic voltammogram (CV) of the freshly electro-
deposited Pt-Rh alloy (short-dashed line) upon potential cycling. Solid, gray
lines and arrows indicate changes in characteristic CV features with increasing
number of cycles. CV of Pt (solid, black line) and Rh (long-dashed line) included
for reference.

W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
5567
The short-dashed, black-line CV characterizes the initial state
of the alloy. Firstly, one can see that the alloy is perfectly homo-
geneous because a single surface oxide reduction peak always
characterizes it. Secondly, by comparing this CV with the CVs
of pure Rh and pure Pt (Fig. 1, long-dashed and solid black
lines, respectively) one can see that, initially, the Pt-Rh alloy
electrode’s surface is indeed rich in Rh. As the alloy electrode’s
potential is cycled, the shape of the CVs gradually changes (gray
lines) with major tendencies indicated by arrows in the figure.
Noticeably, the electrode’s surface becomes more and more like
Pt. Thus, indeed, by potential cycling one obtains a perfectly
homogeneous Pt-Rh alloy with a defined surface molar fraction
of Rh.
The change of the alloy electrode’s composition was also
studied with the EQCM technique. The crystal frequency has
been monitored during a series of potential cycling experiments
similar to the one presented in Fig. 1 but every experiment in the
series had the anodic vertex potential higher than the preced-
ing experiment. Fig. 2 presents the changes of the frequency
measured at the anodic vertex potential over 10 consecutive
cycles. One can notice that up to the anodic vertex potential
of 0.7 V there was no change in the frequency and therefore no
change in the electrode’s mass, either. For higher vertex poten-
tials, a frequency drop/mass loss was recorded. Although the
potential of 0.8 V, at which the mass loss is first measured, is
somewhat higher than the thermodynamic dissolution potential
of Rh (0.312 V [7]), it is certainly not high enough for claim-
ing dissolution of Pt. Therefore, the mass loss can be treated as
due to the loss of Rh. The rate of the mass loss increased with
increasing anodic vertex potential, until a certain point. Then,
the dissolution process decelerated. The evolution of the elec-
trode presented in Fig. 2 roughly corresponds to the evolution
presented in Fig. 1, which allows for the conclusion that the
drop in the rate of mass loss is due to Rh depletion from the
electrode’s surface.
Fig. 3. Molar fraction of Rh on the surface of the Pt-Rh alloy electrode as a
function of the number of potential cycles, to which the freshly electro-deposited
electrode was subjected.
One can easily determine the surface composition of the Pt-
Rh alloy from the peak potential of the surface oxide reduction
peak in the CV. It has been shown that the peak potential varies
linearly with the composition [6,20,21]. Such a treatment of
data similar to the data from Fig. 1 but obtained in a cumula-
tive experiment, in which electrode potential cycling in the base
electrolytewasinterruptedregularlyformethanolstripping(data
not shown), resulted in a relationship between the surface molar
fraction of Rh and the number of potential cycles in the range
0–1.25 V, which is depicted in Fig. 3. It is an interesting obser-
vation that Rh is particularly prone to electro-dissolution from
the alloy electrode’s surface when its surface molar fraction is
between 0.3 and 0.7. The electro-dissolution process is signifi-
cantly slower both for low and high molar fractions of Rh. This
can be interpreted in terms of changes in the surface energy of
the alloy, which should be higher for the intermediate than for
the extreme surface compositions.
Thus, the above method provides a continuum of homo-
geneous Pt-Rh alloy electrodes with precisely known surface
composition, on which methanol, and other electrode processes,
can be studied.
3.2. CO, CO2, and methanol adsorption on Pt-Rh alloys
and electro-desorption of their adsorption products
In order to point out major similarities and differences
between the adsorption processes of these molecules, general
schemes of the processes are provided in Fig. 4. In the case
of CO adsorption on a metal surface with suitable affinity to
carbonaceous adsorbates, such as the alloys studied here, the
process is likely a simple chemical adsorption without charge
transfer over distances longer than the length of the metal-
carbon bond (Fig. 4a). It is known from spectroscopic studies
that the CO adsorption product is either a CO molecule bonded
linearly via the carbon atom on top of a single adsorption site
(metal atom), or the same molecule bonded to two adjacent
adsorption sites with the carbon atom forming a bridge between
them [22].
Fig. 2. EQCM frequency drop vs. anodic vertex potential of cycling for the
freshly electro-deposited Pt-Rh alloy. Frequency change measured at anodic
vertex potential over 10 consecutive cycles with each anodic vertex value.
The experiment was cumulative with anodic vertexes increasing in the order
indicated. Cathodic vertex potential was constant and equal 0 V vs. SHE.

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W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
Fig. 4. Generaladsorptionreactionschemefor(a)CO, (b)CO2, and(c)methanol
adsorption on the electrode surface.
CO2 adsorption is a much more complicated process because
it can only proceed with electron transfer from the electrode to
the CO2 molecule via adsorbed hydrogen (Fig. 4b) [23]:
Fig. 5. Charges analysis of a stripping cyclic voltammetry experiment. Thick,
solid line is the stripping cycle. Thick, dashed line is the cycle immediately
following the stripping cycle. Scan direction indicated by arrows.
(x − 2y + 4)H+ + (x − 2y + 4)e → (x − 2y + 4)H(ads)
CO2 + (x − 2y + 4)H(ads) → CHxOy(ads) + (2 − y)H2O
(1)
(2)
It is therefore a reductive adsorption requiring more than one
adsorption site to complete a single molecule adsorption act.
In the case of CO2 adsorption on platinum metals it is usu-
ally assumed that x = 0 and y = 1, meaning CO(ads) is the only
adsorption product, just as in the case of CO adsorption [24].
However, given the complexity of the process and considering
some previous radiometric results [25], one simple product is
unlikely.
Although, unlike CO2 adsorption, methanol adsorption does
not require a source of or sink for electrons such as the external
circuit, it is also a charge-transfer, this time oxidative adsorption
process, in which molecular hydrogen is evolved (Fig. 4c):
where n is the number of sites blocked by one molecule of the
adsorbate. “Blocking” in that sense is either forming a chemical
bond with the site or making the site unavailable for adsorption
due to space limitation and/or electronic interaction. Conse-
quently, when CO(ads) is the form of the adsorbate, for which
n = 1 or 2 is expected, an eps value between 1 and 2 should be
obtained. (In this case, eps − 1 should give the fraction of sites
binding CO(ads), for which CO is attached in the on-top position.)
Experimental (average) eps values outside this range prove the
adsorbate is, at least partially, something other than CO(ads)
.
Careful treatment of the stripping voltammetry data obtained
in this work allows for precise calculation of the eps parame-
ter. Fig. 5 presents the details of the analysis using an example
methanol stripping experiment performed with a Pt-Rh alloy
electrode. In order to calculate the average eps for a stripping
experiment, we must find the charge corresponding to oxida-
tive removal of the carbonaceous adsorbate and the number of
sites, which were blocked by the adsorbate prior to its oxidative
removal.
For calculating the stripping charge, we first notice that two
cathodic potential scans in Fig. 5, one recorded immediately
after the anodic stripping scan and the second taken from the
cycle following the stripping cycle, almost perfectly overlap.
This attests to the fact that the amount of surface oxide present on
theelectrodeattheendofthestrippingscanisexactlyequaltothe
amount of oxide present at the end of the anodic scan belonging
to the cycle following the stripping cycle. We can, therefore, find
the charge corresponding to oxide formation from this second
anodic scan (Qoxide in Fig. 5). By subtracting this charge from a
charge obtained from integration of the stripping scan up to the
CH3OH + (y − 1)H2O → CHxOy(ads) + (2y − x + 2)H+
+ (2y − x + 2)e
(3)
(4)
(2y − x + 2)H+ + (2y − x + 2)e → (y − x/2 + 1)H2
In the case of methanol adsorption on Pt it has been found by
spectroscopy that the deviation from x = 0 and y = 1, if any, is
very small [26–28]. However, such data for other metal surfaces,
particularly alloy surfaces, is missing. By comparing the CO2
and the methanol adsorption processes (Fig. 4b and c), one can
suspect a certain similarity in the products formed, because in
both cases adsorbate formation requires an ensemble of adsorp-
tion sites with the sites adjacent to the CHxOy(ads)-binding site
being involved in atomic hydrogen adsorption.
In the present work, the electrons-per-site (eps) parameter
is used to characterize the CO, CO2 and methanol adsorption
products. It is the number of electrons required to oxidatively
remove the adsorbate molecules from the adsorption sites, taken
for one adsorption site. If one assumes, which can be safely done
in the case of the studied molecules, that the oxidative removal
of the carbonaceous adsorbate leads to exclusive formation of
gaseous CO2, then, for CHxOy(ads), the eps is given by:
ꢀꢀ
anodic vertex (Q“CO +oxide in Fig. 5) the charge corresponding
solely to carbonaceous adsorbate removal is extracted.
The number of sites occupied by the adsorbate, expressed as
charge, is obtained in the following way. From the analysis of
Woods[29]itfollowsthatthetrueelectrodeareainthecaseofthe
Pt and Rh solid electrodes can be found by integrating the hydro-
gen current on the CV recorded at a slow scan rate down to the
x + 2 · (2 − y)
eps =
,
(5)
n

W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
5569
cathodic vertex potential corresponding to the cathodic current
minimum before hydrogen evolution (cf. Fig. 5). To calculate
a charge corresponding to all the sites, i.e. the whole electrode
area, the obtained hydrogen charge must be divided by a factor,
which originates from the fact that at the cathodic vertex poten-
tial, there is still a fixed fraction of sites not covered by H(ads)
.
Those factors have been determined to be 0.77 and 0.59 for Pt
and Rh, respectively. We propose that in the case of the Pt-Rh
alloy electrode, a corresponding weighted factor, which can be
obtained from the known surface molar fraction of Rh, is effec-
tive. Thus, by finding the hydrogen charge from the CV cycle
following the stripping cycle (QH,max in Fig. 5) and knowing the
surface composition of the alloy we can calculate the number of
coulombs corresponding to the entire true electrode area.
When the carbonaceous adsorbate is present on the elec-
trode’s surface, a number of adsorption sites is usually still
available for hydrogen adsorption, as evidenced by a significant
hydrogen electro-desorption charge. We can find this charge by
integrating the stripping scan in the potential region of hydrogen
electro-desorption. This charge (QH,min in Fig. 5) gives the num-
ber of sites occupied by H(ads) at the electrode potential equal
to the cathodic vertex potential. Just as in the case of absence
of the carbonaceous adsorbate, it is reasonable to assume that
also in the presence of it, besides the CHxOy(ads)-binding and
the H(ads)-binding sites, there are still free adsorption sites at the
electrode’s surface. These sites are inaccessible to H(ads) at the
cathodic vertex potential, just as the free sites in the absence
of the carbonaceous adsorbate. Although it has not been veri-
fied experimentally yet, we propose to assume that, both in the
absence and in the presence of the carbonaceous adsorbate, the
number of free sites constitutes the same fraction of the num-
ber of H(ads)-binding sites, with the proportion being described
by the correction factor discussed above. It is a safe assumption,
because when the electrode coverage by the carbonaceous adsor-
bate is small, it should be well fulfilled, and when the coverage
is high, i.e. QH,min and the fraction of free sites are small, the
possible error in estimating the number of free sites will have
a marginal influence on the calculated eps. Thus, the number
of sites occupied by the adsorbate, expressed as charge, is cal-
culated from the difference of QH,max and QH,min (Fig. 5), and
from the correction factor.
The charges discussed above were corrected for the double
layer charging in a way revealed in Fig. 5. It is assumed that
the double layer capacitance of the electrode is influenced only
by the carbonaceous adsorbate. In other words, in the case of
all the potential scans following the anodic stripping scan in
Fig. 5, the double layer charging current is assumed constant
for the entire potential range of the polarization and equal to the
smallest anodic current recorded in those scans. (Such a treat-
ment neglects the influence of H(ads) and surface oxide on the
electrode’s double layer capacitance, however it is considered a
good first approximation. In any case, the double layer-charging
corrections are small relative to the faradaic charges measured.)
When the carbonaceous adsorbate is present, the double layer
charging current is smaller and its value can be read off the
stripping scan as the current of the minimum between the H(ads)
electro-desorption and the stripping peak. For the potential range
Fig. 6. Stripping scans of the CO adsorption products from Pt, Rh, and Pt-Rh
(17% Rh). Anodic scan of Rh in supporting electrolyte included for reference.
Curves descriptions and corresponding electron-per-site (eps) values shown in
graph.
of the stripping peak, i.e. from the potential of the latter mini-
mum to the potential of the current minimum after the stripping
peak, a linear increase of the double layer charging current is
assumed.
Based on the above assumptions, the formula for the experi-
mental eps used in this work to analyze all the stripping data is
derived:
(Q‘‘CO’’+oxide−Qoxide) · [0.59 · xRh + 0.77 · (1 − xRh)]
eps=
,
(QH,max−QH,min
)
(6)
where xRh is the electrode’s surface molar fraction of Rh. The
same assumptions lead to calculation of the electrode’s coverage
by the carbonaceous adsorbate, θ, using the following formula:
QH,min
θ = 1 −
(7)
QH,max
Fig. 6 presents stripping CVs for CO adsorption on Pt, Rh,
and Pt-Rh (17% Rh). The first distinct feature of the data is
that the main electro-desorption peak recorded for the Pt-Rh
alloy is significantly shifted to negative potential compared to
both Pt and Rh. Apparently, the CO(ads) on the alloy electrode
is easier to oxidize. A hint on why it may be is found in the
calculated eps values. The eps value for the alloy electrode (1.74)
is significantly higher than the eps values for the pure metals,
Pt in particular (1.13). Assuming CO(ads) is the only adsorption
product in the case of CO adsorption, one can conclude that
on the Pt-Rh electrode the majority of CO is bonded in the on-
top position (74%), whereas on the pure metals the fraction of
bridge-bonded CO is more significant (e.g. 87% for Pt). Since
CO bonded on top is weakly bonded CO, this would explain the
shift of the stripping peak.
A second important feature of CO adsorption revealed by
Fig. 6 is the fact that the process is capable of suppressing surface
oxide formation. This has been shown in the figure using the
example of Rh. The CO(ads) electro-desorption current for Rh
grows starting at ca. 0.7 V, with the current recorded at potentials
lower than this value being practically zero. Inspection of the Rh
base-scan shows that without the carbonaceous adsorbate the

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W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
Fig. 8. Stripping scans of the methanol adsorption products from Pt, Rh, and Pt-
Rh (60% Rh). Anodic scan of Rh in supporting electrolyte included for reference.
Curves descriptions and corresponding electron-per-site (eps) values shown in
graph.
Fig. 7. Stripping scans of the CO2 adsorption products from Pt, Rh, and Pt-Rh
(62% Rh). Anodic scan of Rh in supporting electrolyte included for reference.
Curves descriptions and corresponding electron-per-site (eps) values shown in
graph.
surface oxide formation begins around 0.4 V, i.e. 300 mV more
negative. Similar, although weaker, suppression of surface oxide
formation is seen for the Pt-Rh (17% Rh) surface, but not for the
Pt surface (data not shown).
Electro-desorption of the CO2 adsorption products from the
surfaces of Pt, Rh and Pt-Rh (62% Rh) is shown in Fig. 7. In
general, the features of the stripping CVs for CO2 are similar to
those recorded for CO. There is a lowering of the stripping peak
potential on Pt-Rh relative to pure Pt and pure Rh. The blocking
of surface oxide formation by the CO2 adsorption products is
also present. For CO2, however, the eps values are generally
higher than for CO (with exception of the Pt-Rh alloy). In the
case of Rh and Pt-Rh, the stripping peaks are also twice as broad
for CO2 as for CO, suggesting there is a range of different CO2
adsorption product structures present on these surfaces.
For pure Rh and CO2 a particularly high eps value of 2.04
has been found. Comparison of the stripping and the base-scans
for Rh in the potential range of H(ads) electro-desorption (Fig. 7)
reveals a low coverage of the electrode by the carbonaceous
adsorbate and an accompanying high coverage by H(ads). In
our minds, these conditions favor both formation of CO(ads)
in the on-top configuration (eps = 2) as well as, perhaps, for-
mation of an adsorbate more reduced than CO(ads), e.g. x = 1,
y = 1, n = 1 (eps = 3, Eq. (5)). This could explain the high exper-
imental eps, although the value itself is still too close to the 1–2
range to warrant the existence of the more reduced carbonaceous
adsorbate.
Fig. 9. Stripping scans of the CO, CO2, and methanol adsorption products from
Pt-Rh (60% Rh). Curves descriptions and corresponding electron-per-site (eps)
values shown in graph.
for methanol (1.42) than for CO2 (2.04). The difference for Rh
can, however, be understood by considering that Rh has a low
H(ads) coverage at the CO2 adsorption potential, and that the
presence of H(ads) is critical for CO2 adsorption (vide Eqs. (1)
and (2), Fig. 4b, and the above discussion of CO2 on Rh). Since
H(ads) is not a substrate but an intermediate product of methanol
adsorption, coverage of Rh by the methanol adsorption products
can be high, which possibly leads to a lower eps.
All in all, however, we can conclude from stripping that the
CO2 adsorption products on the studied surfaces appear to be a
mixture of species more complicated than the products of CO
adsorption.
Fig. 8 presents stripping data for methanol adsorption prod-
ucts on the studied surfaces. Comparison of Figs. 7 and 8 reveals
a strong similarity between the corresponding stripping scans
of CO2 and methanol adsorption products. Comparison of the
experimental eps values confirms the similarity of the products,
with the exception of Rh, for which a lower eps has been found
Thus, the methanol adsorption products are also less stable
on the Pt-Rh alloy surface relative to the pure Pt and pure Rh sur-
faces. These products also block surface oxide formation on Rh
andPt-Rh(datanotshown). Moreover, theCO2 andthemethanol
adsorption products appear to be very similar in their electro-
desorption profiles (energy) and, perhaps, structure. This can be
understood upon some similarities in the general requirements
of CO2 and methanol adsorption (Fig. 4b and c).
Fig. 9 includes stripping scans recorded for CO, CO2, and
methanol adsorption on a Pt-Rh alloy with a 60%-Rh surface

W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
5571
composition. One can notice that in all the three cases the
voltammetric behavior of the adsorption products is different.
The stripping peak onsets, positions, and widths are all different.
Although these differences in the case of CO are reflected also
by the high eps value (1.89) that sets it apart from the cases of
CO2 and methanol, the same support for the stripping differ-
ences between CO2 and methanol is not found when their eps
values are compared (1.42 and 1.33, respectively). Clearly, the
experimental eps value itself does not fully describe the adsorp-
tion products. This is not surprising, because the same average
experimental eps value could, in principle, be obtained with
mixtures of adsorbate species having different basic eps num-
bers (Eq. (5)) and existing in different ratios on the electrode’s
surface. Such different mixtures producing the same experimen-
tal eps values (Eq. (6)) would exhibit different voltammetric
behavior.
The differences and similarities between the various adsorp-
tion products studied here are further evidenced by the influence
of the Pt-Rh alloy surface composition on the stripping peak
potential. This is presented in Fig. 10. Firstly, for all the adsorp-
tion products studied, there is the clear lowering of the stripping
peak potential in the case of the alloys compared with the pure
metals, however the maximum amount of the lowering is dif-
ferent in the case of each adsorbate-generating substance (130,
110, and 50 mV relative to pure Pt, for CO, CO2, and methanol,
respectively). Also, the shape of the relationship is different for
CO than for CO2 and methanol. In the case of CO, a small frac-
tion of Rh on the alloy’s surface is sufficient to strongly lower the
stripping peak potential relative to pure Pt, with further amounts
of Rh bringing virtually no change up to 60% Rh. The optimum
surface amount of Rh for electro-desorption of the CO adsorp-
tion product is around 10%. The relationships obtained for the
CO2 and the methanol adsorption products are both parabola-
like and the optimum alloy surface composition is the same in
both cases, and equal to ca. 65% Rh.
mustbeconcludedthat, forthePt-Rhalloys, theadsorptionprod-
ucts are different in the case of all the three substances studied.
The particularly weak influence of Rh on the stability of the
methanol adsorption products on Pt-Rh alloys is an interesting
observation, which suggests there are important mechanistic dif-
ferences between the superficially similar adsorption processes
of CO2 (Fig. 4b) and methanol (Fig. 4c) on these surfaces.
3.3. Continuous methanol electro-oxidation on Pt-Rh alloys
In the methanol adsorption experiments presented so far suf-
ficient time was given (30 min) for the slow but irreversible
dehydrogenative adsorption process to complete before strip-
ping. Thus, the stripping peak potentials only attest to the
thermodynamic stability of methanol adsorption products in
their final state and no information is extracted on the kinetics of
the adsorption or electro-desorption in a dynamic situation. In
order to fully assess the applicability of these electro-catalysts
to DMFC anodes, their dynamic interaction with methanol must
be probed. To do this, continuous methanol oxidation on Pt, Rh,
and Pt-Rh alloy (ca. 65% Rh) has been studied.
The recorded steady-state current densities as a function of
electrode potential are presented in Fig. 11a. (The current densi-
ties presented here have been recorded in stirred 1 M methanol
solution on the relatively smooth LVE-type electrodes. Such
conditions allow the measured current density to be envisioned
Although a certain similarity between the CO2 and the
methanol adsorption products is again evident from Fig. 10, it
Fig. 11. (a) Continuous methanol oxidation current density vs. electrode poten-
tial and (b) electrode coverage by methanol adsorption products vs. adsorption
potential for Pt (circles), Rh (squares), and Pt-Rh (ca. 65% Rh) (triangles). Cur-
rent density was measured with the wire-type working electrode immersed in a
stirred solution of 1 M methanol and 0.5 M H2SO4 after 30 min from potential
step to the target oxidation potential. Methanol adsorption time was 30 min at
each adsorption potential.
Fig. 10. Influence of the surface molar fraction of Rh of the Pt-Rh alloy on the
strippingpeakpotentialforCO(circles), CO2 (squares), andmethanol(triangles)
adsorption products.

5572
W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
as purely kinetic current density because the limiting methanol
current density under such conditions is estimated to be on the
order of 100 A cm−2.) It can be noticed in the figure that the
potential of the onset of current recorded on the Pt-Rh alloy
is ca. 50 and 20 mV more negative relative to pure Pt and pure
Rh, respectively. These values are in very good agreement with
the methanol stripping data presented in Fig. 10 (triangles),
confirming that the ability to electro-desorb the poisoning
methanol adsorption products is the key factor governing
the behavior of the electro-catalyst in continuous methanol
electro-oxidation. A direct confirmation of this relationship is
found in Fig. 11b, where the steady-state electrode coverage by
the methanol adsorption products is presented as a function of
the adsorption potential. It is seen that the rise of the steady-state
methanol oxidation current perfectly correlates with the fall
of the coverage in the case of all the three electrode surfaces
studied.
Fig. 11b data also allows verification whether the Pt and
Rh components of the homogeneous Pt-Rh alloy retain their
affinity to methanol measured on the pure metal electrodes.
This is done by verifying the fulfillment of the equation
xRh·θRh + (1 − xRh)·θPt = θPt-Rh. The assessment done for the
potential region of maximum θ leads to [left = 0.73] = [right =
0.72], meaning the affinity is retained.
A second observation from Fig. 11a is that the current densi-
tiesrecordedforpurePtareca. 90andca. 40timeshigherthanfor
pure Rh and for the Pt-Rh (ca. 65% Rh) alloy, respectively. Tak-
ing into account the alloy composition, it can be concluded that
the methanol electro-oxidation turnover rate of Pt in the alloy
is more than 10 times lower than the corresponding turnover
rate of Pt in pure Pt. This result points to severe kinetic limi-
tations introduced to the methanol electro-oxidation process on
the alloy electrodes by the presence of Rh.
Continuous methanol electro-oxidation on a metal surface
requires an initial sequence of dehydrogenation steps (dehy-
drogenative adsorption), described, in summary, by Eq. (3),
followed by removal of the strongly adsorbed intermediate,
CHxOy(ads), by surface oxygen species:
Fig. 12. Electrodecoveragebymethanoladsorptionproductsvs. adsorptiontime
for Pt (circles), Rh (squares), and Pt-Rh (ca. 67% Rh) (triangles). Adsorption
potential: 0.200 V vs. SHE.
ponent of the alloy accelerates methanol adsorption on the Rh
component of the alloy to the level characteristic of pure Pt.
Therefore, the adsorption step is likely not a bottleneck in con-
tinuous methanol electro-oxidation on the Pt-Rh alloy, and the
influence of Rh on the further steps of the process needs to be
considered.
In a “working” methanol electro-oxidation alloy, like Pt-Ru,
reactive surface oxygen species are available from Ru already
at a low electrode potential. As shown during the discussion of
methanol adsorption in the case of the Pt-Rh alloy, the methanol
adsorption products poison the surface of the Rh component
to such an extent that the surface oxygen species cannot form
on Rh. Thereby, the step described by Eq. (8) is certainly
inhibited, at least in the case of the Rh sites. Another possibility
is that the Rh surface oxygen species are non-reactive in the
recombination process (Eq. (9)), however no evidence for that
is presented here. In other words, the bi-functional mechanism
of methanol electro-oxidation on Pt-Rh appears to be switched
off.
As to the functioning of the Pt component of the alloy itself,
although the influence of Rh on the rate of OH(ads) generation
on Pt in the Pt-Rh alloy is unlikely, it must be considered. The
investigation of the kinetics of surface oxide formation on these
electrodes is ongoing in our laboratory. Before this data is avail-
able, it can only be hypothesized that the most likely mechanism
oftheinhibitiveinfluenceofRhonPtinthealloyisthroughslow-
ing down of the last step (Eq. (9)). It must be borne in mind that
in order for the combination reaction between the carbonaceous
adsorbate and the surface oxygen species to proceed fast, either
the adsorbate and the oxygen species must occupy neighboring
adsorption sites, or unhindered surface diffusion of at least one
of these species must be possible. It is suspected that surface
oxygen species on Pt are quite mobile [30]. Rh present in the
homogeneous alloy in a substantial amount (e.g. 65% in the case
shown in Fig. 11) certainly limits the number of neighboring Pt
sites, adversely influencing the activity of the Pt component of
the alloy. Moreover, Rh in these amounts could, in principle, also
H2O → OH(ads) + H+ + e
(8)
CHxOy(ads) + OH(ads)
→ CO2 + (y − 1)H2O + (x − 2y + 3)H+ + (x − 2y + 3)e
(9)
For the entire process to be fast, all the sub-processes need to
`
be fast. Vis-a-vis these considerations, the inhibitive influence
of Rh in the alloy can be on any of these sub-processes. The
first possibility, namely a supposed inhibition of the methanol
adsorption step (Eq. (3)) by the presence of Rh, has been veri-
fied in this work by studying the rate of methanol adsorption
on the various electrodes. This data is shown in Fig. 12. It
is evident from the figure that not only the Rh component
of the ca. 67%-Rh Pt-Rh alloy does not slow down methanol
adsorption on the Pt component of the alloy, but that the Pt com-

W. Tokarz et al. / Electrochimica Acta 52 (2007) 5565–5573
5573
interfere with the surface diffusion of OH(ads) bonded to Pt, thus,
Acknowledgements
introducing a transport-type limitation. To a certain extent, these
possibilities can be studied using Pt-Rh alloy surface composi-
tions with more Pt, and this is also a part of our further research
plan.
Statutory funding from the Department of Chemistry, War-
saw University and the Industrial Chemistry Research Institute,
Warsaw is greatly acknowledged.
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