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This paper is published as part of Faraday
Discussions volume 140:
Electrocatalysis - Theory and Experiment at the Interface
Preface
Preface
Andrea E. Russell, Faraday Discuss., 2009
DOI: 10.1039/b814058h
Introductory Lecture
General discussion
Faraday Discuss., 2009,
DOI: 10.1039/b814699n
Papers
Papers
Differential reactivity of Cu(111) and
Cu(100) during nitrate reduction in acid
electrolyte
Sang-Eun Bae and Andrew A. Gewirth,
Faraday Discuss., 2009
DOI: 10.1039/b803088j
The role of anions in surface
electrochemistry
D. V. Tripkovic, D. Strmcnik, D. van der Vliet,
V. Stamenkovic and N. M. Markovic, Faraday
Discuss., 2009
DOI: 10.1039/b803714k
Molecular structure at electrode/electrolyte
solution interfaces related to
electrocatalysis
Hidenori Noguchi, Tsubasa Okada and Kohei
Uosaki, Faraday Discuss., 2009
DOI: 10.1039/b803640c
From ultra-high vacuum to the
electrochemical interface: X-ray scattering
studies of model electrocatalysts
Christopher A. Lucas, Michael Cormack, Mark
E. Gallagher, Alexander Brownrigg, Paul
Thompson, Ben Fowler, Yvonne Gründer,
Jerome Roy, Vojislav Stamenković and Nenad
M. Marković, Faraday Discuss., 2009
DOI: 10.1039/b803523g
A comparative in situ195Pt electrochemicalNMR investigation of PtRu nanoparticles
supported on diverse carbon nanomaterials
Fatang Tan, Bingchen Du, Aaron L. Danberry,
In-Su Park, Yung-Eun Sung and YuYe Tong,
Faraday Discuss., 2009
DOI: 10.1039/b803073a
Electrocatalysis: theory and experiment at
the interface
Marc T. M. Koper, Faraday Discuss., 2009
DOI: 10.1039/b812859f
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Discussion
Surface dynamics at well-defined single
crystal microfacetted Pt(111) electrodes: in
situ optical studies
Iosif Fromondi and Daniel Scherson, Faraday
Discuss., 2009
DOI: 10.1039/b805040f
Spectroelectrochemical flow cell with
temperature control for investigation of
electrocatalytic systems with surfaceenhanced Raman spectroscopy
Bin Ren, Xiao-Bing Lian, Jian-Feng Li, PingPing Fang, Qun-Ping Lai and Zhong-Qun
Tian, Faraday Discuss., 2009
DOI: 10.1039/b803366h
Bridging the gap between nanoparticles
and single crystal surfaces
Payam Kaghazchi, Felice C. Simeone, Khaled
A. Soliman, Ludwig A. Kibler and Timo Jacob,
Faraday Discuss., 2009
DOI: 10.1039/b802919a
Mesoscopic mass transport effects in
electrocatalytic processes
Y. E. Seidel, A. Schneider, Z. Jusys, B.
Wickman, B. Kasemo and R. J. Behm,
Faraday Discuss., 2009
DOI: 10.1039/b806437g
Nanoparticle catalysts with high energy
surfaces and enhanced activity synthesized
by electrochemical method
Zhi-You Zhou, Na Tian, Zhi-Zhong Huang, DeJun Chen and Shi-Gang Sun, Faraday
Discuss., 2009
DOI: 10.1039/b803716g
Discussion
General discussion
Faraday Discuss., 2009,
DOI: 10.1039/b814700k
Papers
On the catalysis of the hydrogen oxidation
E. Santos, Kay Pötting and W. Schmickler,
Faraday Discuss., 2009
DOI: 10.1039/b802253d
Hydrogen evolution on nano-particulate
transition metal sulfides
Jacob Bonde, Poul G. Moses, Thomas F.
Jaramillo, Jens K. Nørskov and Ib
Chorkendorff, Faraday Discuss., 2009
DOI: 10.1039/b803857k
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Influence of water on elementary reaction
steps in electrocatalysis
Yoshihiro Gohda, Sebastian Schnur and Axel
Groß, Faraday Discuss., 2009
DOI: 10.1039/b802270d
Co-adsorbtion of Cu and Keggin type
polytungstates on polycrystalline Pt:
interplay of atomic and molecular UPD
Galina Tsirlina, Elena Mishina, Elena
Timofeeva, Nobuko Tanimura, Nataliya
Sherstyuk, Marina Borzenko, Seiichiro
Nakabayashi and Oleg Petrii, Faraday
Discuss., 2009
DOI: 10.1039/b802556h
Aqueous-based synthesis of ruthenium–
selenium catalyst for oxygen reduction
reaction
Cyril Delacôte, Arman Bonakdarpour, Christina
M. Johnston, Piotr Zelenay and Andrzej
Wieckowski, Faraday Discuss., 2009
DOI: 10.1039/b806377j
Size and composition distribution dynamics
of alloy nanoparticle electrocatalysts
probed by anomalous small angle X-ray
scattering (ASAXS)
Chengfei Yu, Shirlaine Koh, Jennifer E. Leisch,
Michael F. Toney and Peter Strasser, Faraday
Discuss., 2009
DOI: 10.1039/b801586d
Discussion
General discussion
Faraday Discuss., 2009,
DOI: 10.1039/b814701a
Papers
Efficient electrocatalytic oxygen reduction
by the blue copper oxidase, laccase,
directly attached to chemically modified
carbons
Christopher F. Blanford, Carina E. Foster,
Rachel S. Heath and Fraser A. Armstrong,
Faraday Discuss., 2009
DOI: 10.1039/b808939f
Steady state oxygen reduction and cyclic
voltammetry
View Article Online
Jan Rossmeisl, Gustav S. Karlberg, Thomas
Jaramillo and Jens K. Nørskov, Faraday
Discuss., 2009
DOI: 10.1039/b802129e
Intrinsic kinetic equation for oxygen
reduction reaction in acidic media: the
double Tafel slope and fuel cell applications
Jia X. Wang, Francisco A. Uribe, Thomas E.
Springer, Junliang Zhang and Radoslav R.
Adzic, Faraday Discuss., 2009
DOI: 10.1039/b802218f
A first principles comparison of the
mechanism and site requirements for the
electrocatalytic oxidation of methanol and
formic acid over Pt
Matthew Neurock, Michael Janik and Andrzej
Wieckowski, Faraday Discuss., 2009
DOI: 10.1039/b804591g
Surface structure effects on the
electrochemical oxidation of ethanol on
platinum single crystal electrodes
Flavio Colmati, Germano Tremiliosi-Filho,
Ernesto R. Gonzalez, Antonio Berná, Enrique
Herrero and Juan M. Feliu, Faraday Discuss.,
2009
DOI: 10.1039/b802160k
Electro-oxidation of ethanol and
acetaldehyde on platinum single-crystal
electrodes
Stanley C. S. Lai and Marc T. M. Koper,
Faraday Discuss., 2009
DOI: 10.1039/b803711f
Discussion
General discussion
Faraday Discuss., 2009,
DOI: 10.1039/b814702g
Concluding remarks
All dressed up, but where to go?
Concluding remarks for FD 140
David J. Schiffrin, Faraday Discuss., 2009
DOI: 10.1039/b816481a
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PAPER
www.rsc.org/faraday_d | Faraday Discussions
Introductory Lecture
Electrocatalysis: theory and experiment
at the interface
Marc T. M. Koper
Received 25th July 2008, Accepted 25th July 2008
First published as an Advance Article on the web 22nd August 2008
DOI: 10.1039/b812859f
Introduction
Browsing through the history of Faraday Discussions typically serves as a good
indicator of the development of concepts and challenges in physical chemistry.
The first Royal Society Discussion Meeting held under the name ‘‘Faraday Discussions’’, at the University of Manchester in 1947, was on the topic of ‘‘Electrode
Processes’’. Among the list of contributors were illustrious names such as Randles,
Levich, Frumkin, Bockris, Butler, Eley, and Heyrovsky†. This was the time that
electrochemical measuring techniques, such as electrochemical impedance spectroscopy, were developed, and electrochemists were still very much in the dark about the
molecular nature and theoretical description of electrode reactions, in particular
hydrogen evolution, which was a prominent discussion topic. One remarkable statement was that made by Eley,1 who seriously questioned whether it would ever be
possible to measure on a clean platinum surface in an electrolyte solution.
By 1968, at a Faraday Discussion on ‘‘Electrode Reactions of Organic
Compounds’’ at the University of Newcastle-upon-Tyne, some remarkable leaps
in development had taken place. Most prominently, there was by then a theory of
electron transfer reactions, introduced at the meeting by Marcus. The contributions
by Parsons and Conway were essentially concerned with the kinetic modeling of
electrocatalytic reactions, whereas Hush used semi-empirical quantum-chemical
calculations (!) to study bond breaking. Breiter had a contribution on methanol
oxidation at platinum, and there was an interesting paper by Brummer and Cahill
on the interaction between adsorbates, such as chemisorbed hydrogen and carbon
monoxide. The relation with some of the relevant issues still raised at this meeting
is remarkable. However, there was the nagging problem of specific adsorption, as
put forward by Parsons2: ‘‘The greatest uncertainty in the adsorption behaviour
arises from the lack of knowledge about the way the relative adsorption of different
species depends on the nature of the metal.’’ Thirsk, in his concluding remarks,
also mentioned the ‘‘resistance to theoretical attack’’ of adsorption as one of the
main problems of electrochemical science.
Five years later, at a meeting at Oxford University called ‘‘Intermediates in
Electrochemical Reactions’’, one of the founding fathers of electrochemical surface
science, Heinz Gerischer, was quite optimistic about the possibilities of new
Leiden Institute of Chemistry Leiden University, PO Box 9502, 2300 RA Leiden, The
Netherlands. E-mail: m.koper@chem.leidenuniv.nl
† At the meeting it was pointed out by Professor Roger Parsons, who was also present at the
Faraday Discussion 1 in 1947 as a graduate student of Professor Bockris, that the contributors
from the Soviet Union and Eastern Europe (Frumkin, Levich, Ershler, Heyrovsky) were not
able to be present at the meeting at Manchester. This severely limited discussion of their
important work and significantly delayed the impact of their contributions. This clearly
illustrates the importance of discussion in person which characterises the Faraday Discussions.
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surface-sensitive techniques to probe intermediates in electrode reactions.3 Indeed,
this was the meeting where the field witnessed the introduction of new non-electrochemical methods, such as reflectance spectroscopy (Kolb and MacIntyre) and
online mass spectrometry (Bruckenstein). However, only few participants of that
meeting would have foreseen the explosion of new techniques that was going to
transform interfacial electrochemistry in the next 10–20 years. In fact, Randles, in
his closing address, seemed rather reserved about the future of electrochemistry.4
He felt that, somehow, ‘‘chemistry has lost its glamour’’, though ‘‘electrochemistry
still has vitality and a potential for continuing usefulness’’. However, ‘‘we should
refrain from barren mathematising, we should use the techniques we have where
they are most appropriate and think actively about possible new ones’’.
The lesson to be learnt? Just browse through the next Faraday Discussion volume
94 on electrochemistry ‘‘The Liquid/Solid Interface at High Resolution’’, held in
Newcastle-upon-Tyne in 1992. There is no lack of glamour in the papers that
were contributed to that meeting! Scanning tunneling microscopy and other
scanning probe techniques had revolutionized the field of surface science and
electrochemists such as Kolb, Itaya, and Bard, amongst many others, were actively
studying the unprecedented potential of these new techniques. Also spectroscopic
techniques such as infrared spectroscopy, Raman spectroscopy and non-linear
methods, such as second harmonic generation, had become part of the modern
electrochemist’s toolbox. In the flood of STM papers, theory remained somewhat
underexposed (apart from one isolated theory paper by Nagy, Heinzinger and Spohr
on the modeling of water at platinum5).
The most recent Faraday Discussion meeting on electrochemistry was in 2002 in
Berlin, entitled ‘‘The Dynamic Electrode Surface’’. 2007 Nobel Laureate Gerhard
Ertl, Heinz Gerischer’s successor at the Fritz-Haber-Institute at Berlin, pointed
out in his Introductory Lecture the importance of looking at surface reactions at
different time- and length scales, and the different experimental and theoretical tools
needed for their proper description.6 Theory and computational chemistry, in particular quantum-chemical calculations based on density functional theory (DFT) and
(kinetic) Monte Carlo simulations of surface reactions, were beginning to play an
important role in the interpretation of experimental results, as exemplified in the
papers by Rikvold,7 Weaver,8 and myself.9 In the electrocatalysis talks, the role of
surface diffusion, especially Pt-bonded CO, was a matter of intense debate.
Since 2002, theory and computational chemistry have played an increasingly
important role in fundamental electrocatalysis work. Reactions such as hydrogen
oxidation and evolution, oxygen reduction, methanol oxidation and carbon
monoxide oxidation have all been studied using modern computational techniques,
such as first-principles DFT calculations and Monte Carlo simulations, which both
complement and give input for the more widespread kinetic modeling approaches.
The interaction with experiment has been become very fruitful, and it is this synergy
that provided the motivation for the present Faraday Discussion.
My aim in this paper is to illustrate the ideas and concepts on which I believe this
interaction or ‘‘interface’’ of theory and experiment should be based, and then to
illustrate this on two topics of my own current research focus. Since I am primarily
interested in moving electrocatalysis forward as a science, as opposed to ‘‘technology’’, I will only discuss the general fundamental challenges that we face, as
opposed to the specific challenges related to e.g. fuel cell catalysis and the development of better and more efficient low-temperature fuel cells (which is undoubtedly
one of the main drivers of the field).
Theoretical and computational electrochemistry
A famous statement made by Dirac in 192910 claims that ‘‘all of chemistry’’ follows
from the laws of quantum mechanics, and the main difficulty lies in the fact that
the equations are ‘‘too complex to be solved’’. However, in his statement Dirac did
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not foresee (or did not include) the invention of the computer, ultimately made
possible by the development of quantum mechanics itself. Much of modern theoretical chemistry is concerned with developing clever ways of using the computing power
of computers to solve the complex equations. The success of theoretical chemistry has
been immense, as illustrated by the award of the 1999 Nobel Prize to Pople and Kohn,
two founding fathers of modern computational quantum chemistry. However,
modern quantum chemistry still suffers—to some extent—from Dirac’s demon: the
number of atoms that can be included in a full-blown ‘‘first principles’’ computation,
especially when coupled to dynamics, is limited to at best a few hundred. The impressive work of Neurock and coworkers, as exemplified by their paper in this volume, is
a good example of what is currently achievable. The extrapolation to real sizes and
realistic time scales is by no means trivial, and involves approximation schemes,
the accuracy of which is often unclear and difficult to assess. The accuracy and/or
reliability of model calculations (and therefore of the inherent approximations)
may be evaluated on the basis on a comparison to experimental data, but this is
a practice which, at least in the author’s opinion, should be carried out with great
care, no matter how successful and gratifying such a comparison may often appear.
So what would be an ideal but still practical way to model an electrocatalytic
system? In other words: how to go from quantum mechanics to, say, a real voltammogram? First of all, it is imperative that the atomic structure of the electrode surface is
known accurately. Experimentally, this implies working with well-defined single crystals, and knowing how the exact structure of the single-crystalline or nanoparticulate
surface and possible defects influence reactivity. This aspect becomes less important if
one, for instance, compares the activity of a series of metals for a certain reaction, but
at the expense of losing quite a bit of detail. Next, one has to set up a hierarchy of
calculations that will finally lead to the prediction of the experimental outcome,
where typically one level of calculation delivers the input for the next level. For
instance, one may carry out a series of first-principles DFT calculations to estimate
interaction energies between adsorbates on an electrode surface, and rate constants
for reactions between adsorbates (based on transition state theory). These numbers
may be input for a lattice-gas kinetic Monte Carlo (KMC) simulation of an extended
surface, say 1000 1000 lattice points. The KMC simulation may directly give the
desired macroscopic variable, such as for instance the electric current, as well as
much more, such as the time-dependent distribution of adsorbates on the surface.
Essentially, the KMC simulation serves as a way to estimate the system’s partition
function, i.e. as a simulation method to sample the long-time statistics of the system
in a way that would be impossible by quantum chemistry alone. On the other hand,
one often approximates the statistics of the system by assuming a perfect mixing, also
known as the ‘‘mean-field approximation’’, which is the basis of most kinetic
modeling approaches and of, for instance, the well-known Frumkin isotherm.11 In
this case, a KMC simulation should in principle still be carried out in order to confirm
the validity of the mean-field approximation. Note, however, that the complexity of
the system often forces one to make many more (implicit) assumptions: the lattice-gas
approximation, the assumption of the additivity of interaction potentials (sometimes
three-particle interactions may be included but even this is an approximation),
entropic or other solvent effects are often neglected as first-principles free energy
calculations are extremely expensive, assumptions about the exact structure of the
surface and role of defects, assumptions about which reactions to include and which
not, assumptions about the reaction mechanism (sometimes reactions may be
included the rates of which are difficult to calculate and therefore they are estimated
or guessed), etc. Although there may be examples where many of these approximations may be (or may appear to be) reasonable for the particular system under consideration, it will still be necessary to compare carefully to experimental data as well as
to more accurate simulation data to finally assess the reasonableness of a model.
A key challenge in first-principles simulations of electrode systems is the introduction of the electrode potential. Since all existing quantum chemistry codes work with
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a fixed number of electrons in the simulation box, rather than with a fixed electrochemical potential of the electrons, applying an electrode potential to the simulation
of a half cell is usually approximated by adjusting the charge distribution within the
simulation cell. This can be achieved by applying an electric field to the cell,8,12,13
adding or removing electrons to the metal slab representing the electrode,14,15 or adding electron-drawing or withdrawing adsorbates to the metal surface.16 The first two
methods should also screen the electrostatic field or charge by an artificial background. The corresponding electrode potential is determined at the end of the fully
converged calculation, typically by referring the Fermi level of the metal electrons to
a field-free reference somewhere in the simulation cell, in combination with the
known relation between the vacuum scale of the metal work function and the normal
hydrogen electrode.14–16 Practically all current first-principles calculations including
the effect of the electrode potential are carried out in this fashion. The fundamental
limitation of these methods is that they are essentially coulostatic methods, and
cannot readily be applied to mapping out the reaction path of a charge transfer
reaction, as most electrode reactions are. During a charge transfer reaction under
coulostatic conditions, the electrode potential will change during the reaction. The
preferred way to circumvent this problem is to switch to a grand-canonical simulation method,17 in which the electrochemical potential of the electrons in the half cell
is held fixed rather than their number. The computational limitation of this method
is that it requires an additional iterative loop in the already very time-consuming
simulation, such that in practice the method is still hardly used.
An interesting approach that allows a rapid assessment of the influence of the
electrode potential was suggested by Nørskov et al.,18 and is based on the very simple
assumption that the only effect of the electrode potential is to change the energy of
the electrons. For the adsorption reaction of hydrogen:
H+ + e $ Hads
(1)
the reaction energy is written as:
DGreaction(E) ¼ DGads(Hads;E) DGads(H+ + e;E)
(2)
Nørskov et al. make the assumption that the first term on the right-hand side of eqn
(2) does not depend on potential E, whereas the second term is simply equal to e0E,
with the potential referred to the NHE (normal hydrogen electrode), as the energy of
[H+ + e] is 0 at the potential of the NHE by convention. In mathematical terms, the
first assumption implies that
dDGads(Hads)/dE ¼ 0
(3)
In so far as the electrode potential is proportional to the interfacial field F, this is
equivalent to stating that the adsorbate (in this case Hads) forms an apolar bond
with the surface, or, more accurately, that its static surface dipole moment is zero.
As a result, the quantity (1/e0)dDGreaction(E)/dE, which is known as the electrosorption valency,19 is equal to the (integer) number of electrons transferred in the adsorption reaction. Though this assumption seems reasonable for reaction (1), it remains
to be seen how valid it is in general, and it should certainly be tested for every adsorbate considered. Note that the relationship between the electrosorption valency and
the surface dipole moment is not a trivial one, and depends on the structure of the
double layer.20
Electrocatalytic oxidation of carbon monoxide
The oxidation of carbon monoxide is not only one of the favorite model reactions in
electrocatalysis, but it is also a hugely important reaction in the development of
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more efficient low-temperature fuel cells. We have studied the CO electro-oxidation
on various stepped single crystals of platinum and rhodium,21–24 and have demonstrated the importance of low-coordination step and defect sites in the oxidation
mechanism. However, there remain quite a few fundamental issues that I believe
are not yet fully understood. One of them is the importance of COads surface
mobility in the oxidative stripping of pre-adsorbed CO, an issue that was quite
intensely debated 6 years ago at the Faraday Discussion in Berlin. The other is
the nature of the oxygen-donating species. I will discuss these two issues in this
and in the next session.
From the DFT point-of-view, CO seems to be a somewhat problematic molecule,
as DFT has difficulty in correctly predicting the preferred adsorption site on
a Pt(111) surface. Whereas experimentally CO prefers atop coordination
on Pt(111) (in UHV at low CO coverage),25 most DFT calculations predict the threefold hollow site to be most stable.26 This is typically explained by the tendency of
DFT to overestimate bonding interactions, which are stronger for higher coordination. Olsen, Philipsen and Baerends27 have recently shown that using a localized basis
set and taking care of achieving full convergence of the DFT calculations, the atop
site is found to be the preferred site on Pt(111). Nevertheless, their binding energies
seem somewhat low compared to experiment. These observations clearly underpin
the statement by Feibelman et al.26 that ‘‘DFT calculations cannot be used as
black-box simulation tool.’’ Typically, qualitative or relative predictions are more
reliable than quantitative or absolute predictions, having error bars of ca. 0.1 and
0.3–0.5 eV, respectively. At any rate, these calculations suggest the qualitative conclusion that the corrugation potential for CO on Pt(111) should be rather flat, in agreement with the experimental observation that CO mobility on Pt(111) is high.
The oxidation of carbon monoxide, under electrochemical conditions, is believed
to follow the Langmuir–Hinshelwood-type mechanism originally suggested by
Gilman:28
H2O + * $ OHads + H+ + e
(4)
COads + OHads / COOHads / CO2 + 2* + H+ + e
(5)
The second step in this mechanism, the CO + OH combination reaction, is
believed to be rate-determining, primarily because the Tafel slope observed for
CO monolayer oxidation is ca. 70–80 mV dec1,29,30 close to the theoretical value
of 60 mV dec1 expected for an EC mechanism. DFT calculations indeed show
that there is sizeable barrier for the CO + OH reaction: Shubina et al.31 report ca.
0.6 eV on Pt(111) in the absence of water, whereas Janik and Neurock32 have
obtained values of 0.5–0.3 eV in the presence of water, depending on whether the
surface was charged or not. Note that these values apply to T ¼ 0 K, hence these
are not free activation energies.
By carrying out extensive chronoamperometry measurements on a series on stepped Pt surfaces in sulfuric acid, we have shown that the rate for CO monolayer stripping is proportional to the step density, and that there is no evidence for CO slowly
reaching the active step sites. This strongly suggests that OHads formation takes
place preferentially on the step sites, and the CO diffusion on the terrace is rapid,
in agreement with the flat corrugation potential predicted by DFT.
More recently, we have performed a similar series of experiments, but in alkaline
media.33 Alkaline media typically show higher catalytic activities than acidic media,
even if the potential scale is converted to the reversible hydrogen electrode to correct
for trivial pH effects.34,35 I believe that these observations are not well understood,
and cannot be explained simply by referring to the higher affinity of OH for step sites
in alkaline media, as this effect must have been accounted for by the RHE scale.
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Fig. 1 CO stripping (thick solid line) and the subsequent cyclic voltammogram (thin solid line)
for Pt(111), Pt(15 15 14), Pt(554), Pt(533), Pt(553) and Pt(110) in 0.1 M NaOH, sweep rate
20 mV s1, CO adsorbed at a potential of 0.1 V, no CO in solution during stripping. Reproduced with permission from ref. 33.
Fig. 1 shows the CO stripping voltammetry on a number Pt single crystals surfaces
in 0.1 M NaOH. The remarkable observation here is that the CO voltammetry
exhibits as many as 4 features. We can take the CO stripping voltammetry on
Pt(554) as an example. By comparing to Pt(111), Pt(15 15 14), and Pt(553), we
can conclude that the high-potential stripping peak between 0.72 and 0.80 V is
due to the oxidation of CO on (111) terraces. Inspection of the surfaces with (110)
and (100) step sites, we conclude that the stripping peak at around 0.6 V is CO oxidation at (110) sites, and that peak at ca. 0.70 V is due to CO oxidation at (100) sites.
Note the small feature at 0.7 V in the stripping curve for Pt(554) (and Pt(553) and
Pt(110) as well), which we ascribe to CO oxidation at a small amount of defects
of (100) orientation. Finally, a broad low-potential potential feature, which can start
at a potential as low as 0.35 V, is observed on all surfaces. This feature was also
observed by Spendelow and Wieckowski36 in their studies of CO adlayer oxidation
on lightly disordered Pt(111) in alkaline media. By combining voltammetry with
scanning tunneling microscopy, they suggested that this feature is due to CO oxidation on small monoatomically high islands on the Pt(111) surface, which present
low-coordination sites (essentially kink sites) that are particularly active for CO
oxidation. Following their assignment, we suggest that this low-potential feature
is CO oxidation on ‘‘kink’’-type sites, or defects in the steps. This leads to the
remarkable observation that a single voltammogram such as that shown for
Pt(554) reveals as many as 4 different active oxidation sites for CO: kink sites,
(110) step sites, (100) sites, and (111) terrace sites, in decreasing order of activity.
Such an observation is only possible if the mobility of CO on the surface is low,
as in the case of high CO mobility most if not all CO would oxidize at the most active
oxidation sites.
A simple way to probe the role of low CO mobility is to study the scan rate dependence of the CO stripping voltammetry, as shown in Fig. 2 for Pt(554). It is observed
that at high scan rates (500 mV s1) there is a significant amount of CO oxidizing
on the terraces. However, as the scan is lowered, the charge corresponding to CO
oxidizing at the terraces decreases until finally at 5 mV s1 it is almost negligible.
This clearly suggests that at low scan rates, CO has more time to diffuse to the
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Fig. 2 CO stripping (thick solid line) and the subsequent cyclic voltammogram (thin solid line)
for Pt(554) in 0.1 M NaOH at different sweep rates, Eads ¼ 0.1 V. Reproduced with permission
from ref. 33.
step sites and react there. When the amount of CO reacted in the steps or on the
terraces, as estimated from the corresponding stripping charges, is plotted as a function of the square root of the scan rate, linear relationships are observed. This ‘‘Cottrell-like’’ behavior is another strong indicator for the important role of slow
CO surface diffusion on Pt(111) terraces in alkaline media. The slow diffusion of
CO on the (111) terrace is also manifested in the chronoamperometric transients.
Kinetic Monte Carlo simulations of a model for CO oxidation on stepped surfaces37
show that in such a case, the transient is composed of two parts: an initial exponential current decay corresponding to a one-dimensional instantaneous nucleation and
growth along the step, followed by a peak corresponding to an instantaneous nucleation and growth onto the terrace (see Fig. 3). Experimental transients for CO oxidation on stepped Pt in alkaline media indeed display the same characteristics.38
The reason for the significantly reduced mobility of terrace-bound CO on stepped
Pt electrodes in alkaline media as compared to acidic media has not been fully clarified yet. In acidic media, as well as on Pt(111) in UHV, the high mobility of CO is
ascribed to the conclusion made above, namely that CO does not have a strong
preference for a specific adsorption site on Pt(111) (as confirmed by DFT, although
not in all its details) and therefore it should be able to move over the Pt(111) surface
almost barrierless. In alkaline media, the electrode potential is effectively more negative than in acidic media, however, even if the actual potential scale used is the
reversible hydrogen electrode (RHE). This implies that the Fermi energy of the Pt
in alkaline media (say pH ¼ 13) is about 0.7 eV higher than in acidic media at
pH ¼ 1. If the free energy of the adsorbate under consideration is not dependent
on pH, such as with CO, this may have a significant influence on the way it binds
to the Pt surface. From DFT calculations on both Pt(111) clusters and slabs,39–41
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Fig. 3 Results of Kinetic Monte Carlo simulations of CO stripping on a stepped surface in the
absence of diffusion of CO on the terrace. The stripping voltammetry (a) exhibits three features:
CO oxidation at steps (ca. 0.62 V), CO oxidation at terraces (ca. 0.72 V), and OH adsorption on
terraces (ca. 0.87 V). The snapshots correspond to a (554) surface, where light (dark) blue is
CO on steps (terraces), light (dark) grey is water on steps (terraces) and dark (light) red is
OH on steps (terraces). Figure b shows the chronoamperometry at 0.68 V, and clearly displays
exponential decay first (oxidation of CO at steps) and then a peak corresponding to CO oxidation on terraces. Reproduced with permission from ref. 37.
it has been found that CO binds more strongly to the surface at negative potentials,
especially to multifold coordination sites such as bridge sites and hollow sites. This
preference for multifold coordination at higher Fermi level or more negative potential can be explained qualitatively by the Blyholder model.42 At more negative potential the influence of back donation of metal electrons into the CO 2p* orbital
becomes more prominent, and since the interaction between the 2p* orbital and
the Pt d band is a bonding interaction, and therefore prefers to interact with as
many surface atoms as possible, a stronger preference for multifold coordination
may be expected. Recent FTIR experiments in our group43 on CO at Pt(111) in alkaline media confirm the difference in the electronic interaction with acidic media,
both through a significant change in C–O stretching frequency (which roughly corresponds to 0.7 V times the Stark tuning slope in cm1 V1) as well as a clearly
increased band intensity corresponding to bridge-bonded CO. However, a significantly more corrugated binding energy surface for CO on Pt(111) in alkaline media
18 | Faraday Discuss., 2008, 140, 11–24
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is not straightforwardly implied by these data. On the other hand, the FTIR data
seem to suggest that the product of CO oxidation in alkaline media, carbonate,
remains adsorbed on the surface until quite positive potentials. Strongly adsorbed
carbonate may have a negative effect on the CO surface mobility, similarly to
what we concluded for CO oxidation on rhodium single crystals in sulfuric acid,44
where strongly adsorbed sulfate severely hampers CO surface diffusion.
Formation of OHads on platinum
All our experiments, as well as those by various other authors, suggest that CO is
oxidized by OH that is absorbed in a step or defect on the Pt surface. It is therefore
all the more disconcerting that step-bonded OH has remained invisible in both
spectroscopic and voltammetric experiments. Its apparent voltammetric invisibility
is illustrated in Fig. 4, which compares the voltammetry of Pt(111) and a stepped
Pt surface, Pt(15 15 14), in 0.1 M NaOH. Before we will attempt to explain
the ‘‘anomalous’’ features of the stepped surface, let us discuss how the Pt(111)
voltammogram may be modeled using a combination of DFT and statistical
mechanics.
The Pt(111) voltammogram displays the well-known reversible features corresponding to H adsorption on the terrace (<0.35 VRHE) and OH adsorption on the
terrace (>0.6 VRHE). These regions are reasonably well predicted by DFT calculations. Employing eqn (2) above and the assumptions stipulated there, Rossmeisl
et al.45 have calculated from DFT the equilibrium potentials for the reactions:
H+ + e $ Hads
(6)
H2O $ OHads + H+ + e
(7)
and
to be 0.09 and 0.81 VRHE respectively. This implies that at T ¼ 0 K, water is stable at
Pt(111) between 0.09 and 0.81 VRHE, in reasonable agreement with the experiments
at room temperature. Rossmeisl et al. also calculated the field dependence of the
adsorption energy of H, O and OH to check if indeed eqn (3) is satisfied for these
adsorbates. All three adsorbates indeed show a variation of less than 0.1 eV within
a field range of 0.3 V Å1 to 0.3 V Å1, i.e. ca. 1 to 1 V (if the double layer thickness would be ca. 3 Å).
Fig. 4 Blank voltammetry of Pt(111) and Pt(15 15 14) in 0.1 M NaOH. For explanation, see
text.
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At room temperature, it is well known that the coverage q of H ‘‘upd’’ (underpotential deposition) adsorbates on Pt(111) follow a Frumkin isotherm to a good
approximation:
q
DGH ðEÞ
z3HH q
þ
¼ cH exp
exp
(8)
1q
RT
RT
where z is the surface coordination number of a surface atom (z ¼ 6 for Pt(111)), and
DGH(E)¼DGH,ads+ e0E
(9)
From Jerkiewicz’s temperature-dependent experiments46 and our Monte Carlo
simulations,47 values for the H upd adsorption energy DGH(E) and the nearestneighbor interaction energy 3HH may be determined, as summarized in the second
column of Table 1. Fig. 5 shows the ‘‘hydrogen upd region’’ predicted by this model,
compared to the exact Monte Carlo simulations. It is seen that the Monte Carlo
simulation show a bit more structure, due to the relatively strong interactions, ca.
0.047 eV per pair of neighboring H adsorbates. Nevertheless, the mean-field approximation or Frumkin isotherm is a reasonable approximation. In the third column of
Table 1, we give the values for the adsorption energy and the nearest-neighbor interaction as estimated from the DFT calculations by Karlberg et al.48 The nearestneighbor interaction energy is a bit smaller than the experimental estimate, where
we note that the DFT calculations were performed without water on the surface.
The Monte Carlo isotherm predicted by the DFT values is very close to the
mean-field prediction,48 as the lateral interaction is very weak.
The ‘‘OH adsorption region’’ on Pt(111) is more difficult to model. A simple
model suggested by Rossmeisl et al.49 elsewhere in this volume models the OH
adsorption on Pt(111) by a Langmuir isotherm with a maximum OH coverage of
1/3 ML. Using the DFT value for DGOH,ads mentioned above (0.81 eV), a reasonable
agreement with experiment is obtained although a few important details are not reproduced or explained. The final coverage of OH on Pt(111) is ca. 0.4 ML in acidic
media but in fact depends on pH, being slightly higher in alkaline media. The voltammogram (i.e. the derivative of the isotherm) displays a sharp peak in acidic media
(but not in alkaline media), which has been explained as an order–disorder phase
transition in the OH adlayer,47 or as caused by the adsorption of two different kinds
of OH.50
Fig. 4 compares the voltammetry of Pt(111) in 0.1 M NaOH with that of
a Pt(15 15 14) surface, which has 30-atom wide (111) terraces separated by steps
of (110) orientation. Whereas on the Pt(111) terrace H and OH adsorption lead to
two separate features, introduction of step sites leads to only one additional feature
in the voltammetry at ca. 0.25 V (note a small feature at ca. 0.4 V in Fig. 4 which
corresponds to step sites of (100) orientation). Furthermore, the feature is sharp
instead of broad, implying attractive lateral interactions, which is at least unexpected. The charge corresponding to this peak is ca. 1 electron per step atom in
acidic media,51 the reason why traditionally it has been attributed to hydrogen
adsorption on the step site. However, if that were true, where is the feature
Table 1 Hydrogen UPD adsorption energy and nearest-neighbor interaction energy as
estimated from experiment and/or fit from DFT calculations, on a Pt(111) electrodes, on the
(110) step site of a Pt[n(111)(110)] electrode, and on Pt(100) electrode
DGH,ads/eV
3HH/eV
References
exp./fit (111)
DFT (111)
exp./fit (110) step
exp./fit (100)
DFT (100)
0.21
0.047
46,47
0.16
0.019
48
0.025
0.20
53
0.45
0.014
56
0.27
0.0066
48
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Fig. 5 Monte Carlo simulation and mean-field ‘‘Frumkin’’ approximation of the hydrogen
region on Pt(111) using the parameters in the second column of Table 1.
corresponding to OH adsorption on the step site? Finally, whereas the features corresponding to H and OH adsorption on the (111) terrace show no significant pH
dependence on the RHE scale, the step-related feature is observed at a more positive
potential in alkaline media, shifting by ca. 10 mV pH1 on an RHE scale.52
Table 1, fourth column, gives the values for DGH,ads and 3HH estimated from the
peak corresponding to the (110) step site in perchloric acid solution,53 if it is assumed
that only H adsorbs on the step with a maximum step coverage of 1. Note that these
numbers would imply that adsorbed H not only experiences attractive interactions
on step sites, but also that H has a lower affinity to step sites than to terrace sites.
This is in disagreement with ultra-high vacuum results, where it has been found
that H has a higher affinity for step sites.54 By studying the co-adsorption of H
and water on a stepped Pt surface in UHV, we have recently shown that these results
can also not be explained by the interaction with water. In fact, a Pt surface covered
with H, be it on terraces or in steps, tends to be hydrophobic.55
A more general theory for sharp voltammetric peaks was formulated recently in
relation to a model for the voltammetry of Pt(100) in bromide containing solution.56
The adsorption of bromide on Pt(100) is accompanied by a sharp peak in which
adsorbed H is quickly replaced by adsorbed bromide (see Fig. 6). We have modeled
this voltammetric peak using a simple lateral interaction model that was solved using
Monte Carlo simulations. First, we estimated the adsorption energy and interaction
of upd H on Pt(100) by fitting the blank voltammetry of Pt(100) in perchloric acid
solution, the results of which are also given in Table 1. Note that, as expected,
H adsorbs more strongly on Pt(100) than on Pt(111), but that the interactions
between the adsorbed H are weaker than on Pt(111). These results are in good qualitative agreement with DFT calculations,48 as given in the last column of Table 1.
Introducing the potential dependent co-adsorption of Br into the model, a sharp
peak is obtained, giving a good fit of the experiment (dashed line in Fig. 5), if we
assume DGBr,ads ¼ 0.27 eV and 3HBr ¼ 0.055 for a pair of H and Br sitting on neighboring sites, and an infinite repulsion between two Br on neighboring sites. The
latter assumption leads to a maximum coverage of 0.5 ML of Br, in a c(2 2)
adlayer, in agreement with experiment.57 Because the interaction between H and
Br is stronger than between H, and the interaction between two adsorbed Br at
next-nearest neighbor sites is small, the interparticle repulsion exceeds the sum of
the intraparticle repulsion and the effective interaction for competing adsorbates
may be negative (i.e. attractive). For a mean-field based derivation of this condition,
we refer the reader to the original paper.56 This condition is typically satisfied if
a small and a large adsorbate compete for surface sites.
A similar explanation may be applied to the sharp peak observed in the ‘‘hydrogen
region’’ of stepped Pt surfaces. If we assume that the actual reaction corresponding
to that peak is:
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Fig. 6 Modeling hydrogen and bromine competitive adsorption from the voltammogram of
Pt(100) in HClO4 0.1 M + KBr 102 M (solid line, positive scan), by using Monte Carlo simulations using the interaction energies mentioned in the text (dashed line). The inset shows the
corresponding coverages of hydrogen (solid line) and bromine (dashed line) as a function of
the potential. Reproduced with permission from ref. 56.
Hads + xH2O $ xOHads + (1 + x)H+ + (1 + x)e
(10)
the sharp peak may be explained by the competition between H and OH. Also, reaction 10 would explain why only a single peak is observed, and not two. On the other
hand, the peak charge of 1 electron per step atom would be more difficult to explain
with this model, and reaction 10 would also not explain the anomalous pH dependence.
Since it is very difficult to see adsorbed OH on Pt in a spectroscopic experiment, we
have recently tried to adsorb OH in a step site of a stepped Pt(533) surface in UHV.
Our tactic was similar to a method employed previously for Pt(111): by pre-adsorbing atomic oxygen and subsequently dosing water, O will react with H2O to form
chemisorbed OH on the Pt(111) surface.58 In a temperature-programmed desorption
experiment, this manifests as a water desorption peak that appears at higher temperature than without pre-adsorbed O. This apparent hydrophilicity is due to the stabilization of water by the exothermic reaction of water with O to OH. A similar
experiment with atomic oxygen pre-adsorbed in the steps of a Pt(533) surface,
without O on the terraces, does not yield a clear apparent stabilization of a monolayer of water when subsequently dosed on the Pt(533)–Ostep surface.59 This would
suggest that the reaction to OH does not take place to a significant extent in the
step sites, presumably because the relative stability of atomic oxygen is higher in
the step than on the surface. This obviously raises the question what the product
of water dissociation is under electrochemical conditions. I believe that this is
a crucial question for which at this moment we do not have a consistent answer.
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Conclusions
The papers in this volume amply illustrate the glamour and vitality of modern electrocatalysis and interfacial electrochemistry, and the immense impact that modern
spectroscopic techniques and modern computational chemistry, and especially their
combination, are having on our understanding of the electrochemical interface at the
molecular level. Heinz Gerischer, in his Introductory Lecture to the Faraday Discussion 56 in 1973,3 quoted Julius Tafel from his 1904 paper in which he introduced the
famous Tafel equation:60 ‘‘The problem of electrode polarisation in electrolysis has
been studied scientifically for about one hundred years. It is, therefore, scarcely
possible to find some new aspects which previously have not been touched already,
either in experiment or speculations.’’ Now, another one hundred years later, I
would conclude almost the opposite: there are many aspects that we have not yet
explored and many observations that we still do not fully understand. However,
the potential of the modern tools that we have at our disposal to tackle these issues
is formidable and there is no question in my mind that this will lead to major
advances in both the understanding and the applications of electrochemistry.
These are indeed exciting times to be an electrochemist. Not only is the unprecedented power of modern experimental and computational tools an excellent enabler
for innovative fundamental research work, with the ever louder cry for alternative
and more sustainable energy sources and devices, electrochemistry has every reason
to put itself at the center of attention. Even prominent non-electrochemists admit
that our future energy technology will have electrochemistry as one of its cornerstones.
In a 2007 Science paper, Whitesides and Crabtree61 have identified long-term research
areas that should not be forgotten, and many of them are of a partial or even complete
electrochemical nature. In the largely personal translation of this electrochemist:
1. The oxygen electrode (both oxygen reduction and oxygen evolution),
2. (Electro-)catalysis by design (in essence the theme of this meeting),
3. Various aspects of photoelectrocatalysis,
4. (Electro-)chemistry of carbon dioxide,
5. (Electro-)chemistry of complex systems (‘‘emergent behavior’’, nonlinearity,
innovative (electro-)chemical engineering),
6. Efficiency of energy use,
7. (Electro-)chemistry of small molecules (H2O, CO, small inorganic nitrogen
compounds),
8. New (but sensible) ideas.
These long-term research areas provide us with plenty of challenges to explore the
potential of the interface between theory and experiment in electrocatalysis, and will
continue to be discussion topics at Faraday Discussions in the decades to come.
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