In-situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis

545 Pages·2007·10.29 MB·English

by  Shi-Gang Sun

Preface
Electrocatalysis is at the heart of many important high technological and in-
dustrial processes including: energy conversion (fuel cells, batteries, and super-
capacitors), green synthesis (electrosynthesis of both organic and inorganic
compounds), nanomaterials (nanostructuring and nanofabrication), biotech-
nology (biochips and sensors), micro-systems and electronics (MEMS and in-
terconnects in integrate circuits).
Over the last 20 years electrochemistry has undergone a shift in perception of
the electrode as a "Jellium", which simply acts as an electron sink or source, to
the dynamic, atomically discrete electrode, which participates fully in electrode
processes. This shift was predominantly enabled and certainly facilitated by the
development of in situ spectroscopic and imaging methodologies in electro-
chemical surface science. From many, those that have been used most frequently
are infrared spectroscopy (IR), ultra-violet/visible reflection spectroscopy (UV/
vis), Raman spectroscopy (Raman), mass spectroscopy (differential electro-
chemical mass spectroscopy (DEMS), the electrochemical quartz crystal micro-
balance (EQCM)), electron spin resonance (ESR) and nuclear magnetic
resonance (NMR). In addition, nonlinear optical spectroscopies (sum frequency
generation (SFG), second harmonic generation (SHG)) and x-ray spectroscopies
(EXAFS, XANES, and SXS) have also been employed. Furthermore, the com-
bination of in situ spectroscopic techniques with scanning probe methods such
as scanning tunneling microscopy (STM) and/or atomic force microscopy
(AFM) has provided novel, exciting, and crucial insights into the processes at
and near the electrode surface on the molecular and atomic levels.
This book comprises 51 chapters covering different in situ spectroscopic
techniques employed in studies of adsorption at the electrode and electro-
catalysis, such as infrared spectroscopy (Chapters 1-8), sum frequency gene-
ration (Chapter 9), Raman spectroscopy (Chapter 10), x-ray spectroscopy
(Chapters 11 and 12), electron spin resonance (Chapter 13), nuclear magnetic
resonance (Chapter 14), and scanning tunneling microscopy together with mass
spectroscopy (Chapter 15). The chapters of the book were contributed by
prominent scholars worldwide and are aimed at a wide range of professionals
interested in new applications of spectroscopy, electrocatalysis, electrochemis-
try, surface science, energy conversion, materials science, and bioscience. The
state-of-the-art presentations are based on each contributor's recent work on
mechanism and structure of the electrode/electrolyte interface.
The breadth of the present book makes it ideal for instructing newcomers
as well as for aiding established scientists and engineers in the field of
iv ecaferP
electrochemistry and electrocatalysis. Last but not least, the volume should
help to develop new ideas: basic and applied, and to develop new in situ surface
analytical methodologies to respond to challenges of electrochemistry and
electrocatalysis in the future.
Shi-Gang Sun
Paul Andrew Christensen
Andrzej Wieckowski
List of Contributors
H. Baltruschat, Institute of Physical and Theoretical Chemistry, University of
Bonn, Roemerstr. 164, D-53117 Bonn, Germany
A. Berne, Departamento de Qu{mica F{sica and Instituto de Electroqu{mica,
Universidad de Alicante, Apartado ,99 Alicante, E-03080, Spain
R. BuBar, Institute of Physical and Theoretical Chemistry, University of Bonn,
Roemerstr. 164, D-53117 Bonn, Germany
G.A. Camara, Departamento de Qu{mica, Universidade Federal de Mato Grosso
od Sul, P.O. Box 549, 79070-900, Campo Grande, MS, Brazil
K.-C. Chang, Materials Science Division, Argonne National Laboratory, 9700 .S
Cass Avenue Argonne, IL 60439, USA
P.A. Christensen, School of Chemical Engineerin9 and Advanced Materials,
Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne
NE1 7R ,U UK
.S Ernst, Institute of Physical and Theoretical Chemistry, University of Bonn,
Roemerstr. 164, D-53117 Bonn, Germany
J.M. Feliu, Departamento de Qu{mica Fisica and Instituto de Electroqu{mica,
Universidad de Alicante, Apartado ,99 Alicante, E-03080, Spain
F. Hahn, Laboratory of Catalysis ni Organic Chemistry LA CCO, Electrocataly-
sis Group, UMR 6503 CNRS-University of Poitiers, ,04 avenue du recteur
Pineau, 86022 Poitiers Cedex, France
A. Hamnett, The University of Strathclyde, McCance Building, John Anderson
Campus, Glasgow 1G 1XQ, UK
F. Hernandez, Institute of Physical and Theoretical Chemistry, University of
Bonn, Roemerstr. 164, D-53117 Bonn, Germany
J. Inukai, Department of Chemistry, University of Illinois, 600 South Mathews
Avenue, Urbana, IL, USA
T. Iwasita, Instituto de Qu{mica de Sdo Carlos, Universidade de Sdo Paulo, P.O.
Box 780, 13560-970 Sdo Carlos, SP, Brazil
J.-M. Jin, School of Chemical Engineerin9 and Advanced Materials, Bedson Build-
in9, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK
.V Komanicky, Materials Science Division, Argonne National Laboratory, 9700 .S
Cass Avenue Argonne, IL 60439, USA
vii
iiiv tsiL of srotubirtnoC
C. Korzeniewski, Department of Chemistry and Biochemistry, Texas Tech
,ytisrevinU Lubbock, TX ,1601-90497 USA
J.-M. L6ger, Laboratory of Catalysis ni Organic Chemistry LACCO, Electro-
catalysis Group, UMR 6503 CNRS-University of Poitiers, ,04 avenue ud
recteur Pineau, 86022 Poitiers Cedex, France
W.-F. Lin, School of Chemical Engineerin9 and Advanced Materials, Bedson
Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7R ,U
UK
J. Lu, Department of Chemistry, Wuhan University, Wuhan 430072, China
C.A. Lucas, Oliver Lodge Laboratory, Department of Physics, University oj
Liverpool, Liverpool, L69 7ZE, UK
N.M. Markovic, Materials Science Division, Argonne National Laboratory,
Argonne, IL 60439, USA
A. Menzel, Materials Science Division, Argonne National Laboratory, 9700 .S
Cass Avenue Argonne, IL 60439, USA
M. Osawa, Catalysis Research Center, Hokkaido University, Sapporo ,1200-100
Japan
.B Ren, State Key Laboratory of Physical Chemistry of Solid Surfaces and De-
partment of Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
A. Rodes, Departamento ed acim'tuQ F{sica and Instituto ed ,acim{uqortcelE
dadisrevinU ed Alicante, Apartado ,99 Alicante, E-03080, Spain
E.S. Smotkin, Department of Chemistry and Chemical Biology, Northeastern
,ytisrevinU Boston, MA 02115, USA
S.-G. Sun, State Key Laboratory of Physical Chemistry of Solid Surfaces,
Department of Chemistry, College of Chemistry and Chemical ,gnireenignE
Xiamen University, Xiamen 361005, China
A. Tadjeddine, UDIL-CNRS, Univ. Paris-Sud, BP ,43 91898 Orsay, France
Z.-Q. Tian, State Key Laboratory of Physical Chemistry of Solid Surfaces and
Department of Chemistry, College of Chemistry and Chemical ,gnireenignE
Xiamen University, Xiamen 361005, China
E.V. Timofeeva, Department of Chemistry, Kent State University, P.O. Box
5190, Kent, OH 44242-0001, USA
Y.V. Tolmachev, Department of Chemistry, Kent State University, P.O. Box
5190, Kent, OH 44242-0001, USA
Y.Y. Tong, Department of Chemistry, Georgetown University, 37th and 0
Streets, NW, Washington, DC 20057, USA
F. Vidal, Institut sed Nanosciences ed Paris, CNRS-UMR ,8857 Universitd Pierre
et Marie Curie-Paris ,6 Campus Boucicaut, 140, eur ed Lourmel, 75015 ,siraP
ecnarF
A. Wieckowski, Department of Chemistry, University of Illinois, 006 South
Mathews Avenue, Urbana, IL, USA
t&L of srotubirtnoC xi
D.-Y. Wu, State Key Laboratory for Physical Chemistry of Solid Surfaces and
Department of Chemistry, Colleye of Chemistry and Chemical ,gnireenignE
Xiamen University, Xiamen 361005, China
H. You, Materials Science Division, Argonne National Laboratory, 9700 .S Cass
Avenue Argonne, IL 60439, USA
Z.-Y. Zhou, State Key Laboratory of Physical Chemistry of Solid Surfaces,
Department of Chemistry, College of Chemistry and Chemical Enyineering,
Xiamen University, Xiamen 361005, China
L. Zhuang, Department of Chemistry, Wuhan University, Wuhan 430072, China
In-situSpectroscopicStudiesofAdsorptionattheElectrodeandElectrocatalysis
S.-G.Sun,P.A.ChristensenandA.Wieckowski(Editors)
r2007ElsevierB.V.Allrightsreserved
Chapter 1
In-situ FTIR Studies on the Acid–Base
Equilibria of Adsorbed Species on Well-Defined
Metal Electrode Surfaces
Antonio Berna´ , Antonio Rodes, Juan M. Feliu
Departamento de Quı´mica Fı´sica and Instituto de Electroquı´mica, Universidad de
Alicante, Apartado 99, Alicante, E-03080, Spain
1. Introduction
Infrared spectroscopy is one of the most useful techniques for the in-situ char-
acterization of interfaces at a molecular level [1–7]. Information obtained from
in-situ infrared spectra is related to the nature of adsorbed species, adsorbate
bondinggeometry,adsorbate–adsorbateinteractionsand,indirectly,thesurface
adsorption sites. The use of infrared spectroscopy coupled with electrochemical
systemsiscomplicatedbecauseofthestrongabsorptionofinfraredradiationby
the solvent [4–7]. Two different approaches have been applied in order to solve
this problem. The first one relies on the use of an internal reflection config-
uration [8,9]. Under these conditions, the penetration of the infrared beam into
the solution side of the interface is limited to a fraction of a micron. In the
so-called Kretschmann configuration [10], the infrared window is also used as
the substrate for the deposition of a thin metal film acting as the working
electrode [11–19]. Major limitations of the internal reflection experiments are
related to the stability of the thin film electrodes (thickness typically around
20nm) and to the control of the surface structure. On the other hand, adsorb-
ates on metallic thin film electrodes exhibit enhanced infrared absorption (the
so-called surface-enhanced infrared absorption or SEIRA effect [13]) similar to
that reported for molecules adsorbed on thin metallic films [20,21]. The high
intensity of infrared absorption bands, together with the existence of proper
1
2 A.Berna´etal.
conditions for mass transport and the low time constant for the spectroelectro-
chemical cells used in internal reflection experiments, allow the spectroscopic
study of the kinetics of electrode processes [15,17,18]. Moreover, lower
interference from infrared absorption from bulk water (related to the limited
penetration of the evanescent wave into the solution side of the interface)
offers a unique opportunity of gaining information about the metal–water and
adsorbate–water interactions [13,16,19].
Thesecondapproachforthein-situinfraredcharacterizationoftheelectrode/
solution interface is based on external reflection experiments. In this case,
the reflecting surface of a bulk electrode is pushed against an infrared window
withalowrefractiveindexinsuchawaythatthethicknessofthesolutionlayer
sampled by the radiation is reduced to a few microns (thin layer configuration)
[3,5–7]. High electric resistance and hindered mass transport conditions make
the thin layer configuration unsuitable for kinetic studies [7,13,19]. At the same
time, interference due to a still significant infrared absorption from bulk water
impedes the observation of reliable bands coming from interfacial water. How-
ever, external reflection experiments are still advantageous in many cases
because they allow the spectroscopic detection of the consumption and/or
formation of reactants, intermediates and reaction products (both adsorbates
and solution species trapped in the thin layer).
The external reflection configuration can be also used to study the structural
aspects of the adsorption processes since it can be used to probe the reflecting
surface of any bulk electrode material, including well-defined single crystal sur-
faces[3,5–7].Thepreparationandhandlingofthelatterhavebeenfacilitatedin
the last decades by the development of well-established preparation procedures
forplatinum[22–24],gold[25],rhodium[26]andpalladium[27,28]singlecrystal
electrodes. In the case of palladium, the occurrence of the hydrogen absorption
reaction limits the potential range where other electrochemical processes can be
studied. The deposition of epitaxial palladium monolayers on well-defined
platinumorgoldsingle crystalsurfacesoffers areliable alternativeto theuse of
bulk palladium single crystal electrodes [29,30].
Among the surface processes taking place at the metal/solution interface, the
specific adsorption of anions has attracted a lot of attention since it plays a key
role in the understanding of electrochemical reactions [31–33]. The specific
adsorption of anions coming from weak acids often involves an effect of ad-
sorption on the dissociation equilibria when compared to the behaviour ob-
served in solution. Examples include hydrogensulfate [5,34,35] (also named as
bisulfate for historical reasons), carbon dioxide [36,37], hydrogencarbonate (in
the following named as bicarbonate) [38] and carboxylic acids (such as acetic
[39], trifluoroacetic [40], oxalic [41,42], citric [43] and trimesic [44] acids) on
platinum and gold single crystal electrodes. In the cases of carbon dioxide
In-situ FTIR Studies onthe Acid–BaseEquilibria 3
and carboxylic acids, adsorption of the deprotonated species has been spec-
troscopically detected in solutions where the acid form prevails.
In this work, in-situ infrared spectroscopy has been used in an external
reflection configuration to extend the study of acid–base equilibria of the ad-
sorbed species coming from carbon dioxide and oxalic acid on well-defined
singlecrystalsurfacesof differentmetals.Asmentionedabove,carboxylicacids
are good candidates as probe molecules for studying the acid–base character of
the electrode/solution interface. Related adsorption processes are very easily
followed spectroscopically since their main characteristic bands,related to their
differentC–Ovibrations,lieinthemid-IRspectralregion,namelybetween2000
and 1000cm(cid:1)1.
Thischapterisorganizedasfollows.Asafirsttopic,theadsorptionbehaviour
of CO , which could be considered to be the simplest carboxylic acid, will be
2
studied on several electrode materials (platinum, gold, rhodium and epitaxial
palladiummonolayersdepositedonPt(111)electrodes).Thisstudyisrestrictedto
metalsurfaceswiththe(111)orientationinordertoavoidcomplicationscoming
fromthereductionofcarbondioxidetoadsorbedcarbonmonoxideat(110)and
(100) sites [36,45–51]. The effect of pH on the surface carbonate–bicarbonate
equilibrium at each electrode surface will be presented in Section 3.1. A similar
approachwillbefollowedinthestudyoftheadsorptionofoxalicacidonPt(111)
and Pt(100) electrode surfaces (Section 3.2). The role of surface site distribution
(namely, the step and terrace surface distributionin regular stepped surfaces) on
the surface acid–base equilibria of oxalic acid and its anions will beexplored for
the same material electrode, i.e. platinum electrodes (Section 3.3). Experimental
results reported in Section 3 will be discussed in more detail in Section 4.
2. Experimental
Platinum, gold and rhodium single crystal electrodes were prepared following
theproceduredescribedbyClavilier[22].Samplesemployedforelectrochemical
experiments were about 2mm in diameter whereas the diameter of the oriented
electrodes used for the in-situ infrared spectroscopy experiments were around
4mm. Prior to any experiment, the working electrode was annealed in a gas-
oxygen flame. Platinum and rhodium samples were cooled down in a reductive
atmosphere (H +Ar) and then transferred to the (spectro)electrochemical cell
2
under the protection of a droplet of water saturated with these gases [23]. Gold
electrodes were cooled down in air and the surface protected with a water
droplet in equilibrium with air [25].
Cyclic voltammetry and in-situ infrared experiments were performed in
perchloric acid solutions with different pH in the presence of carbon dioxide
4 A.Berna´etal.
(N48, L’Air Liquide) or oxalic acid (pro analysi from Merck). Perchloric acid
solutions were prepared from the concentrated acid (Suprapur from Merck)
and ultrapure water (Elga-Vivendi). Potassium perchlorate (pro analysi from
Merck)andsodiumhydroxide(SuprapurfromMerck)wereusedtoadjustionic
strengthand solution pH, respectively. Insome of the infrared experiments, the
working solutions were prepared in deuterium oxide (Merck, 99.95%), which
was used as received. Potentials were measured against a reversible hydrogen
electrode in the working solution (RHE). Prior to each experiment, the test
solutionwasdeaeratedbybubblingAr(N50,L’AirLiquide).Theelectrochemi-
caldepositionofpalladiumwasperformedinasecondcellcontaininga10(cid:1)5M
Pd2+ solution in 0.l M H SO [29]. Once a palladium coverage close to a
2 4
monolayerwasattained,theelectrodewasrinsedwithultrapurewaterandthen
transferred to the cell containing a palladium-free solution.
External reflection infrared experiments were carried out with a Fourier-
Transform Nicolet Magna 850 spectrometer equipped with a narrow-band
MCT detector. The spectroelectrochemical cell [5,7] was provided with a
CaF prismatic window bevelled at 601. Spectra were collected at different
2
electrode potentials either with p- or s-polarised light with a resolution of
8cm(cid:1)1. The spectra are represented as the ratio (cid:1)log(R /R ) where R and R
1 0 1 0
are the reflectance at the sample, E , and reference potentials, E , respectively.
1 0
The reference spectrum was collected at a potential between 0.10 and 0.30V, as
the electrode surface is free of adsorbed anions in this potential range. Inter-
ferograms were co-added at each potential with the electrode potential being
alternated from the reference to the sample potential every 100 interferograms.
The total number of interferograms collected at each potential (either 500 or
1000) was chosen to reach a convenient signal to noise ratio.
3. Results
3.1. Experimental results for M(111) electrodes in CO -saturated solutions
2
The interaction of carbon dioxide with platinum electrodes has been the object
of many studies in the past. Initial studies with polycrystalline electrodes
showed the formation of strongly adsorbed species in the hydrogen adsorption
region [52,53] which, in a generic way, were noted as ‘‘reduced CO ’’. The
2
structural aspects of this surface reaction have been reported by different au-
thors in various voltammetric [36,45–51] and in-situ infrared [36,45–49] studies
with platinum single crystal electrodes. As first reported for polycrystalline
electrodes [54,55], the spectroscopic results obtained for single crystal surfaces
showed that ‘‘reduced CO ’’ could be identified mainly as adsorbed carbon
2
monoxide. The extent of this surface reaction, which can be followed from the
In-situ FTIR Studies onthe Acid–BaseEquilibria 5
blockage of the hydrogen adsorption sites and the development of the typical
C–O stretching bands for adsorbed CO, was found to be higher for the (110)
sites and almost negligible for (111) terrace sites [46–48,50,51]. Similar conclu-
sions were reached from voltammetric experiments for the reduction of carbon
dioxide at palladium [56] and rhodium [57] electrode surfaces.
As indicated above, carbon dioxide is not reduced to CO at the Pt(111)
electrode surface at a significant rate [36,37,50,51]. Fig. 1A shows typical cyclic
voltammograms obtained for a well-ordered Pt(111) electrode surfaces in CO -
2
free(curvea)andCO -saturated(curveb)0.1Mperchloricacidsolution.Cyclic
2
voltammograms recorded for a palladium monolayer deposited at the Pt(111)
electrode, noted in the following as Pd/Pt(111), are shown in Fig. 1B for com-
parison. As previously discussed [36,37], the main effects of the presence of
carbon dioxide in the voltammetric behaviour of Pt(111) are the disappearance
of the sharp voltammetric peaks at 0.80V and the negative shift of the broad
adsorption states initially between 0.60 and 0.85V. This voltammetric behav-
iour is characteristic of the presence of specifically adsorbed anions that com-
pete with the adsorption of hydroxyl anions coming from surface water
dissociation. The relation between the adsorption states centred at ca. 0.65V
and the adsorption/desorption of anionic species was also established from
the charge density curves obtained by combining the voltammetric curves
Fig. 1. Cyclicvoltammograms of(A)Pt(111) and(B) Pd/Pt(111)electrodesin (a)0.1MHClO
4
(dashed-dotted line);(b)0.1MHClO +CO (sat.)(solidline) and(c)10(cid:1)3MHClO +0.099M
4 2 4
KClO +CO (sat.) (dashedline) solutions. Scanrate: 50mVs(cid:1)1.
4 2