Introductory Lecture- ElectroCatalysis Teory and Experient at the Interface

8/8/2019 Introductory Lecture- ElectroCatalysis Teory and Experient at the Interface

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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., 2009DOI: 10.1039/b814058h

Introductory Lecture

Electrocatalysis: theory and experiment atthe interface Marc T. M. Koper, Faraday Discuss., 2009DOI: 10.1039/b812859f

Papers

The role of anions in surfaceelectrochemistry D. V. Tripkovic, D. Strmcnik, D. van der Vliet,V. Stamenkovic and N. M. Markovic, Faraday Discuss., 2009DOI: 10.1039/b803714k

From ultra-high vacuum to the

electrochemical interface: X-ray scatteringstudies of model electrocatalysts Christopher A. Lucas, Michael Cormack, MarkE. Gallagher, Alexander Brownrigg, PaulThompson, Ben Fowler, Yvonne Gründer,Jerome Roy, Vojislav Stamenković and NenadM. Marković, Faraday Discuss., 2009DOI: 10.1039/b803523g

Surface dynamics at well-defined singlecrystal microfacetted Pt(111) electrodes: in

situ optical studies Iosif Fromondi and Daniel Scherson, Faraday

Discuss., 2009DOI: 10.1039/b805040f

Bridging the gap between nanoparticlesand single crystal surfaces Payam Kaghazchi, Felice C. Simeone, KhaledA. Soliman, Ludwig A. Kibler and Timo Jacob,Faraday Discuss., 2009DOI: 10.1039/b802919a

Nanoparticle catalysts with high energysurfaces and enhanced activity synthesizedby electrochemical method Zhi-You Zhou, Na Tian, Zhi-Zhong Huang, De-Jun Chen and Shi-Gang Sun, Faraday Discuss., 2009DOI: 10.1039/b803716g

DiscussionGeneral discussion Faraday Discuss., 2009,DOI: 10.1039/b814699n

Papers

Differential reactivity of Cu(111) andCu(100) during nitrate reduction in acidelectrolyte Sang-Eun Bae and Andrew A. Gewirth,

Faraday Discuss., 2009DOI: 10.1039/b803088j

Molecular structure at electrode/electrolytesolution interfaces related toelectrocatalysis Hidenori Noguchi, Tsubasa Okada and KoheiUosaki, Faraday Discuss., 2009DOI: 10.1039/b803640c

A comparative in situ 195Pt electrochemical-NMR investigation of PtRu nanoparticlessupported on diverse carbon nanomaterials Fatang Tan, Bingchen Du, Aaron L. Danberry,In-Su Park, Yung-Eun Sung and YuYe Tong,Faraday Discuss., 2009DOI: 10.1039/b803073a

Spectroelectrochemical flow cell withtemperature control for investigation ofelectrocatalytic systems with surface-enhanced Raman spectroscopy Bin Ren, Xiao-Bing Lian, Jian-Feng Li, Ping-

Ping Fang, Qun-Ping Lai and Zhong-QunTian, Faraday Discuss., 2009DOI: 10.1039/b803366h

Mesoscopic mass transport effects inelectrocatalytic processes Y. E. Seidel, A. Schneider, Z. Jusys, B.Wickman, B. Kasemo and R. J. Behm,Faraday Discuss., 2009DOI: 10.1039/b806437g

Discussion

General discussion Faraday Discuss., 2009,DOI: 10.1039/b814700k

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Papers

On the catalysis of the hydrogen oxidation E. Santos, Kay Pötting and W. Schmickler,Faraday Discuss., 2009DOI: 10.1039/b802253d

Hydrogen evolution on nano-particulatetransition metal sulfides

Jacob Bonde, Poul G. Moses, Thomas F.Jaramillo, Jens K. Nørskov and IbChorkendorff, Faraday Discuss., 2009DOI: 10.1039/b803857k

Influence of water on elementary reactionsteps in electrocatalysis Yoshihiro Gohda, Sebastian Schnur and AxelGroß, Faraday Discuss., 2009DOI: 10.1039/b802270d

Co-adsorbtion of Cu and Keggin typepolytungstates on polycrystalline Pt:interplay of atomic and molecular UPD Galina Tsirlina, Elena Mishina, ElenaTimofeeva, Nobuko Tanimura, NataliyaSherstyuk, Marina Borzenko, SeiichiroNakabayashi and Oleg Petrii, Faraday Discuss., 2009DOI: 10.1039/b802556h

Aqueous-based synthesis of ruthenium–selenium catalyst for oxygen reduction

reaction Cyril Delacôte, Arman Bonakdarpour, ChristinaM. Johnston, Piotr Zelenay and AndrzejWieckowski, Faraday Discuss., 2009DOI: 10.1039/b806377j

Size and composition distribution dynamicsof alloy nanoparticle electrocatalystsprobed by anomalous small angle X-rayscattering (ASAXS) Chengfei Yu, Shirlaine Koh, Jennifer E. Leisch,

Michael F. Toney and Peter Strasser, Faraday Discuss., 2009DOI: 10.1039/b801586d

Discussion

General discussion Faraday Discuss., 2009,DOI: 10.1039/b814701a

Papers

Efficient electrocatalytic oxygen reductionby the blue copper oxidase, laccase,directly attached to chemically modifiedcarbons Christopher F. Blanford, Carina E. Foster,Rachel S. Heath and Fraser A. Armstrong,Faraday Discuss., 2009DOI: 10.1039/b808939f

Steady state oxygen reduction and cyclicvoltammetry Jan Rossmeisl, Gustav S. Karlberg, ThomasJaramillo and Jens K. Nørskov, Faraday Discuss., 2009DOI: 10.1039/b802129e

Intrinsic kinetic equation for oxygenreduction 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., 2009DOI: 10.1039/b802218f

A first principles comparison of themechanism and site requirements for theelectrocatalytic oxidation of methanol andformic acid over Pt Matthew Neurock, Michael Janik and Andrzej

Wieckowski, Faraday Discuss., 2009DOI: 10.1039/b804591g

Surface structure effects on theelectrochemical oxidation of ethanol onplatinum single crystal electrodes Flavio Colmati, Germano Tremiliosi-Filho,Ernesto R. Gonzalez, Antonio Berná, EnriqueHerrero and Juan M. Feliu, Faraday Discuss.,2009DOI: 10.1039/b802160k

Electro-oxidation of ethanol andacetaldehyde on platinum single-crystalelectrodes Stanley C. S. Lai and Marc T. M. Koper,Faraday Discuss., 2009DOI: 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., 2009DOI: 10.1039/b816481a

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Introductory LectureElectrocatalysis: theory and experimentat 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 goodindicator of the development of concepts and challenges in physical chemistry.

The first Royal Society Discussion Meeting held under the name ‘‘Faraday Discus-sions’’, at the University of Manchester in 1947, was on the topic of ‘‘ElectrodeProcesses’’. Among the list of contributors were illustrious names such as Randles,Levich, Frumkin, Bockris, Butler, Eley, and Heyrovsky†. This was the time thatelectrochemical measuring techniques, such as electrochemical impedance spectros-copy, were developed, and electrochemists were still very much in the dark about themolecular nature and theoretical description of electrode reactions, in particularhydrogen evolution, which was a prominent discussion topic. One remarkable state-ment was that made by Eley,1 who seriously questioned whether it would ever bepossible to measure on a clean platinum surface in an electrolyte solution.

By 1968, at a Faraday Discussion on ‘‘Electrode Reactions of OrganicCompounds’’ at the University of Newcastle-upon-Tyne, some remarkable leapsin development had taken place. Most prominently, there was by then a theory of electron transfer reactions, introduced at the meeting by Marcus. The contributionsby Parsons and Conway were essentially concerned with the kinetic modeling of electrocatalytic reactions, whereas Hush used semi-empirical quantum-chemicalcalculations (!) to study bond breaking. Breiter had a contribution on methanoloxidation at platinum, and there was an interesting paper by Brummer and Cahillon the interaction between adsorbates, such as chemisorbed hydrogen and carbonmonoxide. The relation with some of the relevant issues still raised at this meetingis remarkable. However, there was the nagging problem of specific adsorption, as

put forward by Parsons2: ‘‘The greatest uncertainty in the adsorption behaviourarises from the lack of knowledge about the way the relative adsorption of differentspecies depends on the nature of the metal.’’ Thirsk, in his concluding remarks,also mentioned the ‘‘resistance to theoretical attack’’ of adsorption as one of themain problems of electrochemical science.

Five years later, at a meeting at Oxford University called ‘‘Intermediates inElectrochemical Reactions’’, one of the founding fathers of electrochemical surfacescience, 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: [email protected]

† At the meeting it was pointed out by Professor Roger Parsons, who was also present at theFaraday Discussion 1 in 1947 as a graduate student of Professor Bockris, that the contributorsfrom the Soviet Union and Eastern Europe (Frumkin, Levich, Ershler, Heyrovsky) were notable to be present at the meeting at Manchester. This severely limited discussion of theirimportant work and significantly delayed the impact of their contributions. This clearlyillustrates 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-electro-chemical methods, such as reflectance spectroscopy (Kolb and MacIntyre) andonline mass spectrometry (Bruckenstein). However, only few participants of thatmeeting would have foreseen the explosion of new techniques that was going totransform interfacial electrochemistry in the next 10–20 years. In fact, Randles, inhis closing address, seemed rather reserved about the future of electrochemistry. 4

He felt that, somehow, ‘‘chemistry has lost its glamour’’, though ‘‘electrochemistrystill has vitality and a potential for continuing usefulness’’. However, ‘‘we shouldrefrain from barren mathematising, we should use the techniques we have wherethey are most appropriate and think actively about possible new ones’’.

The lesson to be learnt? Just browse through the next Faraday Discussion volume94 on electrochemistry ‘‘The Liquid/Solid Interface at High Resolution’’, held inNewcastle-upon-Tyne in 1992. There is no lack of glamour in the papers thatwere contributed to that meeting! Scanning tunneling microscopy and otherscanning probe techniques had revolutionized the field of surface science andelectrochemists such as Kolb, Itaya, and Bard, amongst many others, were activelystudying the unprecedented potential of these new techniques. Also spectroscopic

techniques such as infrared spectroscopy, Raman spectroscopy and non-linearmethods, such as second harmonic generation, had become part of the modernelectrochemist’s toolbox. In the flood of STM papers, theory remained somewhatunderexposed (apart from one isolated theory paper by Nagy, Heinzinger and Spohron the modeling of water at platinum5).

The most recent Faraday Discussion meeting on electrochemistry was in 2002 inBerlin, entitled ‘‘The Dynamic Electrode Surface’’. 2007 Nobel Laureate GerhardErtl, Heinz Gerischer’s successor at the Fritz-Haber-Institute at Berlin, pointedout in his Introductory Lecture the importance of looking at surface reactions atdifferent time- and length scales, and the different experimental and theoretical tools

needed for their proper description.

6

Theory and computational chemistry, in partic-ular quantum-chemical calculations based on density functional theory (DFT) and(kinetic) Monte Carlo simulations of surface reactions, were beginning to play animportant role in the interpretation of experimental results, as exemplified in thepapers 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 increasinglyimportant role in fundamental electrocatalysis work. Reactions such as hydrogenoxidation and evolution, oxygen reduction, methanol oxidation and carbonmonoxide 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 synergythat provided the motivation for the present Faraday Discussion.

My aim in this paper is to illustrate the ideas and concepts on which I believe thisinteraction or ‘‘interface’’ of theory and experiment should be based, and then toillustrate this on two topics of my own current research focus. Since I am primarilyinterested in moving electrocatalysis forward as a science, as opposed to ‘‘tech-nology’’, I will only discuss the general fundamental challenges that we face, asopposed to the specific challenges related to e.g. fuel cell catalysis and the develop-ment of better and more efficient low-temperature fuel cells (which is undoubtedlyone of the main drivers of the field).

Theoretical and computational electrochemistry

A famous statement made by Dirac in 192910 claims that ‘‘all of chemistry’’ followsfrom the laws of quantum mechanics, and the main difficulty lies in the fact thatthe 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 madepossible by the development of quantum mechanics itself. Much of modern theoret-ical chemistry is concerned with developing clever ways of using the computing powerof computers to solve the complex equations. The success of theoretical chemistry hasbeen 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 impres-sive work of Neurock and coworkers, as exemplified by their paper in this volume, isa good example of what is currently achievable. The extrapolation to real sizes andrealistic 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/orreliability of model calculations (and therefore of the inherent approximations)may be evaluated on the basis on a comparison to experimental data, but this isa practice which, at least in the author’s opinion, should be carried out with greatcare, 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 voltammo-gram? First of all, it is imperative that the atomic structure of the electrode surface isknown accurately. Experimentally, this implies working with well-defined single crys-tals, and knowing how the exact structure of the single-crystalline or nanoparticulatesurface 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, butat 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. Forinstance, one may carry out a series of first-principles DFT calculations to estimate

interaction energies between adsorbates on an electrode surface, and rate constantsfor reactions between adsorbates (based on transition state theory). These numbersmay be input for a lattice-gas kinetic Monte Carlo (KMC) simulation of an extendedsurface, say 1000 Â 1000 lattice points. The KMC simulation may directly give thedesired macroscopic variable, such as for instance the electric current, as well asmuch 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 partitionfunction, i.e. as a simulation method to sample the long-time statistics of the systemin 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, alsoknown as the ‘‘mean-field approximation’’, which is the basis of most kinetic

modeling approaches and of, for instance, the well-known Frumkin isotherm.11 Inthis case, a KMC simulation should in principle still be carried out in order to confirmthe 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-gasapproximation, the assumption of the additivity of interaction potentials (sometimesthree-particle interactions may be included but even this is an approximation),entropic or other solvent effects are often neglected as first-principles free energycalculations are extremely expensive, assumptions about the exact structure of thesurface and role of defects, assumptions about which reactions to include and whichnot, assumptions about the reaction mechanism (sometimes reactions may beincluded the rates of which are difficult to calculate and therefore they are estimatedor guessed), etc. Although there may be examples where many of these approxima-tions may be (or may appear to be) reasonable for the particular system under consid-eration, it will still be necessary to compare carefully to experimental data as well asto more accurate simulation data to finally assess the reasonableness of a model.

A key challenge in first-principles simulations of electrode systems is the introduc-

tion of the electrode potential. Since all existing quantum chemistry codes work with




a fixed number of electrons in the simulation box, rather than with a fixed electro-chemical potential of the electrons, applying an electrode potential to the simulationof a half cell is usually approximated by adjusting the charge distribution within thesimulation 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 add-ing electron-drawing or withdrawing adsorbates to the metal surface.16 The first twomethods should also screen the electrostatic field or charge by an artificial back-

ground. The corresponding electrode potential is determined at the end of the fullyconverged calculation, typically by referring the Fermi level of the metal electrons toa field-free reference somewhere in the simulation cell, in combination with theknown relation between the vacuum scale of the metal work function and the normalhydrogen electrode.14–16 Practically all current first-principles calculations includingthe effect of the electrode potential are carried out in this fashion. The fundamentallimitation of these methods is that they are essentially coulostatic methods, andcannot readily be applied to mapping out the reaction path of a charge transferreaction, as most electrode reactions are. During a charge transfer reaction undercoulostatic conditions, the electrode potential will change during the reaction. Thepreferred way to circumvent this problem is to switch to a grand-canonical simula-

tion method,17 in which the electrochemical potential of the electrons in the half cellis held fixed rather than their number. The computational limitation of this methodis that it requires an additional iterative loop in the already very time-consumingsimulation, such that in practice the method is still hardly used.

An interesting approach that allows a rapid assessment of the influence of theelectrode potential was suggested by Nørskov et al.,18 and is based on the very simpleassumption that the only effect of the electrode potential is to change the energy of the electrons. For the adsorption reaction of hydrogen:

H+ + eÀ$ Hads (1)

the reaction energy is written as:

DG reaction(E ) ¼ DG ads(Hads;E ) À DG ads(H+ + eÀ;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+ + eÀ] is 0 at the potential of the NHE by convention. In mathematical terms, thefirst assumption implies that

dDG ads(Hads)/dE ¼ 0 (3)

In so far as the electrode potential is proportional to the interfacial field F , this isequivalent to stating that the adsorbate (in this case Hads) forms an apolar bondwith the surface, or, more accurately, that its static surface dipole moment is zero.As a result, the quantity (1/e0)dDG reaction(E )/dE , which is known as the electrosorp-tion valency,19 is equal to the (integer) number of electrons transferred in the adsorp-tion reaction. Though this assumption seems reasonable for reaction (1), it remainsto be seen how valid it is in general, and it should certainly be tested for every adsor-bate 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 inelectrocatalysis, but it is also a hugely important reaction in the development of




more efficient low-temperature fuel cells. We have studied the CO electro-oxidationon various stepped single crystals of platinum and rhodium,21–24 and have demon-strated the importance of low-coordination step and defect sites in the oxidationmechanism. However, there remain quite a few fundamental issues that I believeare not yet fully understood. One of them is the importance of COads surfacemobility in the oxidative stripping of pre-adsorbed CO, an issue that was quiteintensely 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 thisand 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 ona Pt(111) surface. Whereas experimentally CO prefers atop coordinationon Pt(111) (in UHV at low CO coverage),25 most DFT calculations predict the three-fold 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 coordina-tion. Olsen, Philipsen and Baerends27 have recently shown that using a localized basisset and taking care of achieving full convergence of the DFT calculations, the atopsite is found to be the preferred site on Pt(111). Nevertheless, their binding energies

seem somewhat low compared to experiment. These observations clearly underpinthe statement by Feibelman et al.26 that ‘‘DFT calculations cannot be used asblack-box simulation tool.’’ Typically, qualitative or relative predictions are morereliable than quantitative or absolute predictions, having error bars of ca. 0.1 and0.3–0.5 eV, respectively. At any rate, these calculations suggest the qualitative conclu-sion that the corrugation potential for CO on Pt(111) should be rather flat, in agree-ment with the experimental observation that CO mobility on Pt(111) is high.

The oxidation of carbon monoxide, under electrochemical conditions, is believedto follow the Langmuir–Hinshelwood-type mechanism originally suggested byGilman:28

H2O + *$ OHads + H+ + eÀ (4)

COads + OHads/ COOHads/ CO2 + 2* + H+ + eÀ (5)

The second step in this mechanism, the CO + OH combination reaction, isbelieved to be rate-determining, primarily because the Tafel slope observed forCO monolayer oxidation is ca. 70–80 mV decÀ1,29,30 close to the theoretical valueof 60 mV decÀ1 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 Neurock 32 haveobtained values of 0.5–0.3 eV in the presence of water, depending on whether thesurface was charged or not. Note that these values apply to T ¼ 0 K, hence theseare not free activation energies.

By carrying out extensive chronoamperometry measurements on a series on step-ped Pt surfaces in sulfuric acid, we have shown that the rate for CO monolayer strip-ping is proportional to the step density, and that there is no evidence for CO slowlyreaching the active step sites. This strongly suggests that OHads formation takesplace 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 alkalinemedia.33 Alkaline media typically show higher catalytic activities than acidic media,even if the potential scale is converted to the reversible hydrogen electrode to correctfor 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 sitesin alkaline media, as this effect must have been accounted for by the RHE scale.




Fig. 1 shows the CO stripping voltammetry on a number Pt single crystals surfacesin 0.1 M NaOH. The remarkable observation here is that the CO voltammetryexhibits 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), wecan conclude that the high-potential stripping peak between 0.72 and 0.80 V isdue 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 oxida-tion 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) andPt(110) as well), which we ascribe to CO oxidation at a small amount of defectsof (100) orientation. Finally, a broad low-potential potential feature, which can startat a potential as low as 0.35 V, is observed on all surfaces. This feature was alsoobserved by Spendelow and Wieckowski36 in their studies of CO adlayer oxidationon lightly disordered Pt(111) in alkaline media. By combining voltammetry with

scanning tunneling microscopy, they suggested that this feature is due to CO oxida-tion on small monoatomically high islands on the Pt(111) surface, which presentlow-coordination sites (essentially kink sites) that are particularly active for COoxidation. Following their assignment, we suggest that this low-potential featureis CO oxidation on ‘‘kink’’-type sites, or defects in the steps. This leads to theremarkable observation that a single voltammogram such as that shown forPt(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 activeoxidation sites.

A simple way to probe the role of low CO mobility is to study the scan rate depen-dence of the CO stripping voltammetry, as shown in Fig. 2 for Pt(554). It is observedthat at high scan rates (500 mV sÀ1) there is a significant amount of CO oxidizingon the terraces. However, as the scan is lowered, the charge corresponding to COoxidizing at the terraces decreases until finally at 5 mV sÀ1 it is almost negligible.This clearly suggests that at low scan rates, CO has more time to diffuse to the

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 rate20 mV sÀ1, CO adsorbed at a potential of 0.1 V, no CO in solution during stripping. Repro-duced with permission from ref. 33.




step sites and react there. When the amount of CO reacted in the steps or on theterraces, as estimated from the corresponding stripping charges, is plotted as a func-tion of the square root of the scan rate, linear relationships are observed. This ‘‘Cot-trell-like’’ behavior is another strong indicator for the important role of slowCO 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 surfaces 37

show that in such a case, the transient is composed of two parts: an initial exponen-

tial current decay corresponding to a one-dimensional instantaneous nucleation andgrowth along the step, followed by a peak corresponding to an instantaneous nucle-ation and growth onto the terrace (see Fig. 3). Experimental transients for CO oxida-tion on stepped Pt in alkaline media indeed display the same characteristics. 38

The reason for the significantly reduced mobility of terrace-bound CO on steppedPt electrodes in alkaline media as compared to acidic media has not been fully clar-ified yet. In acidic media, as well as on Pt(111) in UHV, the high mobility of CO isascribed to the conclusion made above, namely that CO does not have a strongpreference for a specific adsorption site on Pt(111) (as confirmed by DFT, althoughnot in all its details) and therefore it should be able to move over the Pt(111) surfacealmost barrierless. In alkaline media, the electrode potential is effectively more nega-tive than in acidic media, however, even if the actual potential scale used is thereversible hydrogen electrode (RHE). This implies that the Fermi energy of the Ptin alkaline media (say pH ¼ 13) is about 0.7 eV higher than in acidic media atpH ¼ 1. If the free energy of the adsorbate under consideration is not dependenton pH, such as with CO, this may have a significant influence on the way it bindsto the Pt surface. From DFT calculations on both Pt(111) clusters and slabs,39–41

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, E ads¼ 0.1 V. Reproduced with permissionfrom ref. 33.




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. Thispreference for multifold coordination at higher Fermi level or more negative poten-tial can be explained qualitatively by the Blyholder model.42 At more negative poten-tial the influence of back donation of metal electrons into the CO 2p* orbitalbecomes more prominent, and since the interaction between the 2p* orbital andthe Pt d band is a bonding interaction, and therefore prefers to interact with asmany surface atoms as possible, a stronger preference for multifold coordinationmay be expected. Recent FTIR experiments in our group43 on CO at Pt(111) in alka-line media confirm the difference in the electronic interaction with acidic media,both through a significant change in C–O stretching frequency (which roughly corre-sponds to 0.7 V times the Stark tuning slope in cmÀ1 VÀ1) as well as a clearlyincreased band intensity corresponding to bridge-bonded CO. However, a signifi-cantly more corrugated binding energy surface for CO on Pt(111) in alkaline media

Fig. 3 Results of Kinetic Monte Carlo simulations of CO stripping on a stepped surface in theabsence 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 onterraces (ca. 0.87 V). The snapshots correspond to a (554) surface, where light (dark) blue isCO on steps (terraces), light (dark) grey is water on steps (terraces) and dark (light) red isOH on steps (terraces). Figure b shows the chronoamperometry at 0.68 V, and clearly displaysexponential decay first (oxidation of CO at steps) and then a peak corresponding to CO oxida-

tion on terraces. Reproduced with permission from ref. 37.




is not straightforwardly implied by these data. On the other hand, the FTIR dataseem to suggest that the product of CO oxidation in alkaline media, carbonate,remains adsorbed on the surface until quite positive potentials. Strongly adsorbedcarbonate may have a negative effect on the CO surface mobility, similarly towhat 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 platinumAll our experiments, as well as those by various other authors, suggest that CO isoxidized by OH that is absorbed in a step or defect on the Pt surface. It is thereforeall the more disconcerting that step-bonded OH has remained invisible in bothspectroscopic and voltammetric experiments. Its apparent voltammetric invisibilityis illustrated in Fig. 4, which compares the voltammetry of Pt(111) and a steppedPt surface, Pt(15 15 14), in 0.1 M NaOH. Before we will attempt to explainthe ‘‘anomalous’’ features of the stepped surface, let us discuss how the Pt(111)voltammogram may be modeled using a combination of DFT and statisticalmechanics.

The Pt(111) voltammogram displays the well-known reversible features corre-sponding to H adsorption on the terrace (<0.35 VRHE) and OH adsorption on theterrace (>0.6 VRHE). These regions are reasonably well predicted by DFT calcula-tions. Employing eqn (2) above and the assumptions stipulated there, Rossmeislet al.45 have calculated from DFT the equilibrium potentials for the reactions:

H+ + eÀ$ Hads (6)

and

H2O$ OHads + H+ + eÀ (7)

to be 0.09 and 0.81 VRHE respectively. This implies that at T ¼ 0 K, water is stable atPt(111) between 0.09 and 0.81 VRHE, in reasonable agreement with the experimentsat room temperature. Rossmeisl et al. also calculated the field dependence of theadsorption energy of H, O and OH to check if indeed eqn (3) is satisfied for theseadsorbates. All three adsorbates indeed show a variation of less than 0.1 eV withina field range of À0.3 V AÀ1 to 0.3 V AÀ1, i.e. ca. À1 to 1 V (if the double layer thick-ness would be ca. 3 A).

Fig. 4 Blank voltammetry of Pt(111) and Pt(15 15 14) in 0.1 M NaOH. For explanation, seetext.




At room temperature, it is well known that the coverage q of H ‘‘upd’’ (underpo-tential deposition) adsorbates on Pt(111) follow a Frumkin isotherm to a goodapproximation:

q

1 À q¼ cHþexp

DG HðE Þ

RT

exp

z3HHq

RT

(8)

where z is the surface coordination number of a surface atom (z ¼ 6 for Pt(111)), and

DG H(E )¼DG H,ads+ e0E (9)

From Jerkiewicz’s temperature-dependent experiments46 and our Monte Carlosimulations,47 values for the H upd adsorption energy DG H(E ) and the nearest-neighbor interaction energy 3HH may be determined, as summarized in the secondcolumn 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 Carlosimulation 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 approx-

imation 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 inter-action as estimated from the DFT calculations by Karlberg et al .48 The nearest-neighbor interaction energy is a bit smaller than the experimental estimate, wherewe 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 themean-field prediction,48 as the lateral interaction is very weak.

The ‘‘OH adsorption region’’ on Pt(111) is more difficult to model. A simplemodel suggested by Rossmeisl et al.49 elsewhere in this volume models the OHadsorption on Pt(111) by a Langmuir isotherm with a maximum OH coverage of 1/3 ML. Using the DFT value for DG OH,ads mentioned above (0.81 eV), a reasonable

agreement with experiment is obtained although a few important details are not re-produced or explained. The final coverage of OH on Pt(111) is ca. 0.4 ML in acidicmedia but in fact depends on pH, being slightly higher in alkaline media. The vol-tammogram (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 phasetransition in the OH adlayer,47 or as caused by the adsorption of two different kindsof 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 stepsof (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 featurein the voltammetry at ca. 0.25 V (note a small feature at ca. 0.4 V in Fig. 4 whichcorresponds to step sites of (100) orientation). Furthermore, the feature is sharpinstead of broad, implying attractive lateral interactions, which is at least unex-pected. The charge corresponding to this peak is ca. 1 electron per step atom inacidic media,51 the reason why traditionally it has been attributed to hydrogenadsorption 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)Â(110)] electrode, and on Pt(100) electrode

exp./fit (111) DFT (111) exp./fit (110) step exp./fit (100) DFT (100)

DG H,ads/eV À0.21 À0.16 À0.025 À0.45 À0.27

3HH/eV 0.047 0.019 À0.20 0.014 0.0066

References 46,47 48 53 56 48




corresponding to OH adsorption on the step site? Finally, whereas the features cor-responding 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 positivepotential in alkaline media, shifting by ca. 10 mV pHÀ1 on an RHE scale.52

Table 1, fourth column, gives the values for DG H,ads and 3HH estimated from thepeak corresponding to the (110) step site in perchloric acid solution,53 if it is assumedthat only H adsorbs on the step with a maximum step coverage of 1. Note that thesenumbers would imply that adsorbed H not only experiences attractive interactionson 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 foundthat H has a higher affinity for step sites.54 By studying the co-adsorption of Hand 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 coveredwith H, be it on terraces or in steps, tends to be hydrophobic.55

A more general theory for sharp voltammetric peaks was formulated recently inrelation 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 whichadsorbed H is quickly replaced by adsorbed bromide (see Fig. 6). We have modeledthis voltammetric peak using a simple lateral interaction model that was solved usingMonte Carlo simulations. First, we estimated the adsorption energy and interactionof upd H on Pt(100) by fitting the blank voltammetry of Pt(100) in perchloric acidsolution, 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 qual-itative 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 sharppeak is obtained, giving a good fit of the experiment (dashed line in Fig. 5), if weassumeDG Br,ads¼À0.27 eV and 3HBr¼ 0.055 for a pair of H and Br sitting on neigh-boring sites, and an infinite repulsion between two Br on neighboring sites. Thelatter assumption leads to a maximum coverage of 0.5 ML of Br, in a c(2 Â 2)adlayer, in agreement with experiment.57 Because the interaction between H andBr is stronger than between H, and the interaction between two adsorbed Br atnext-nearest neighbor sites is small, the interparticle repulsion exceeds the sum of the intraparticle repulsion and the effective interaction for competing adsorbatesmay 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 ‘‘hydrogenregion’’ of stepped Pt surfaces. If we assume that the actual reaction correspondingto that peak is:

Fig. 5 Monte Carlo simulation and mean-field ‘‘Frumkin’’ approximation of the hydrogenregion on Pt(111) using the parameters in the second column of Table 1.




Hads + xH2O$ xOHads + (1 + x)H+ + (1 + x)eÀ (10)

the sharp peak may be explained by the competition between H and OH. Also, reac-tion 10 would explain why only a single peak is observed, and not two. On the otherhand, the peak charge of 1 electron per step atom would be more difficult to explainwith this model, and reaction 10 would also not explain the anomalous pH depen-

dence.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-adsorb-ing atomic oxygen and subsequently dosing water, O will react with H2O to formchemisorbed OH on the Pt(111) surface.58 In a temperature-programmed desorptionexperiment, this manifests as a water desorption peak that appears at higher temper-ature than without pre-adsorbed O. This apparent hydrophilicity is due to the stabi-lization of water by the exothermic reaction of water with O to OH. A similarexperiment 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 mono-layer of water when subsequently dosed on the Pt(533)–Ostep surface.59 This wouldsuggest that the reaction to OH does not take place to a significant extent in thestep sites, presumably because the relative stability of atomic oxygen is higher inthe step than on the surface. This obviously raises the question what the productof water dissociation is under electrochemical conditions. I believe that this isa crucial question for which at this moment we do not have a consistent answer.

Fig. 6 Modeling hydrogen and bromine competitive adsorption from the voltammogram of Pt(100) in HClO4 0.1 M + KBr 10À2 M (solid line, positive scan), by using Monte Carlo simu-

lations using the interaction energies mentioned in the text (dashed line). The inset shows thecorresponding coverages of hydrogen (solid line) and bromine (dashed line) as a function of the potential. Reproduced with permission from ref. 56.




Conclusions

The papers in this volume amply illustrate the glamour and vitality of modern elec-trocatalysis and interfacial electrochemistry, and the immense impact that modernspectroscopic techniques and modern computational chemistry, and especially theircombination, are having on our understanding of the electrochemical interface at themolecular level. Heinz Gerischer, in his Introductory Lecture to the Faraday Discus-sion 56 in 1973,3 quoted Julius Tafel from his 1904 paper in which he introduced thefamous Tafel equation:60 ‘‘The problem of electrode polarisation in electrolysis hasbeen studied scientifically for about one hundred years. It is, therefore, scarcelypossible to find some new aspects which previously have not been touched already,either in experiment or speculations.’’ Now, another one hundred years later, Iwould conclude almost the opposite: there are many aspects that we have not yetexplored 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 issuesis formidable and there is no question in my mind that this will lead to majoradvances in both the understanding and the applications of electrochemistry.

These are indeed exciting times to be an electrochemist. Not only is the unprece-

dented power of modern experimental and computational tools an excellent enablerfor innovative fundamental research work, with the ever louder cry for alternativeand more sustainable energy sources and devices, electrochemistry has every reasonto put itself at the center of attention. Even prominent non-electrochemists admitthat ourfuture energy technology will have electrochemistry as one of its cornerstones.In a 2007 Science paper, Whitesides and Crabtree61 have identified long-term researchareas that should not be forgotten, and many of them are of a partial or even completeelectrochemical 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 willcontinue to be discussion topics at Faraday Discussions in the decades to come.

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Introductory Lecture- ElectroCatalysis Teory and Experient at the Interface

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