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Novel Spillover Interrelating Reversible Electrocatalysts for Oxygen and Hydrogen Electrode Reactions Jelena M. Jaksic,† Diamantoula Labou,†,‡ Georgos D. Papakonstantinou,† Angeliki Siokou,† and Milan M. Jaksic*,† ICEHT/FORTH, 26500 Patras, Greece, and Faculty of Agriculture, UniVersity of Belgrade, 11080 Belgrade, Serbia ReceiVed: April 29, 2010; ReVised Manuscript ReceiVed: September 22, 2010
New striking prospects both in low and medium temperature polymer electrolyte membrane fuel cell (PEMFC) and in water electrolysis (WE) have been opened by the interactive supported individual (Pt) or prevailing hyper-d-electronic nanostructured metal clusters (WPt3, NbPt3, HfPd3, ZrNi3), grafted upon and within high altervalent capacity hypo-d-oxides (WO3, NbO2, TaO2, TiO2) and their proper mixed valence compounds, to create a novel type of alternating polarity (alterpolar) interchangeable composite electrocatalysts for hydrogen and oxygen electrode reactions. Whereas in aqueous media Pt (Pt/C) features either chemisorbed catalytic surface properties of Pt-H or PtdO, missing any effusion of other interacting species, a new generation of composite SMSI (strong metal-support interaction) electrocatalysts in condensed wet state primarily characterizes interchangeable extremely fast reversible spillover of either H-adatoms or the primary oxides (Pt-OH, Au-OH) or the invertible bronze type behavior of these significant interactive electrocatalytic ingredients. Altervalent hypo-d-oxides impose spontaneous dissociative adsorption of water molecules and pronounced membrane spillover transferring properties instantaneously resulting with corresponding bronze type (Pt/HxWO3) under cathodic and/or its hydrated state (Pt/W(OH)6), responsible for Pt-OH effusion, under anodic polarization, this way establishing instantaneous reversibly revertible alterpolar bronze features (Pt/ H0.35WO3 S Pt/W(OH)6) and substantially advanced electrocatalytic properties of these composite interactive electrocatalysts. Such nanostructured type electrocatalysts, even of mixed hypo-d-oxide structure (Pt/H0.35WO3/ TiO2/C, Pt/HxNbO3/TiO2/C), have for the first time been synthesized by the sol-gel methods and shown rather high stability, electron conductivity and nonexchanged initial pure monobronze spillover and catalytic properties. Such a unique electrocatalytic system, as the striking target issue of the present paper, has been shown to be the superior substantiation of the revertible cell assembly for spontaneous reversible alterpolar interchanges between PEMFC and WE. The underpotential spillover double layer charging and discharging properties of the primary oxide (M-OH), interrelated with the interactive self-catalytic effect of dipoleoriented water molecules, has also been pointed out. Introduction The present study represents the theoretical sublimation and embodiment of so far stepwise achieved improvements in electrocatalysis for hydrogen and oxygen electrode reactions, based on individual and hypo-hyper-d-d-interelectronic combinations of intermetallic phases, interactive supported upon corresponding hypo-d-oxides, and leading to the reversible interrelating composite nanostructured electrocatalysts of bronze Pt/H0.35WO3 versus its hydrated Pt/W(OH)6 type.1-5 In the same context, such an invertible bronze type behavior enables us to substantiate the reversibly revertible alterpolar cell between PEMFC (polymer electrolyte membrane fuel cell) and water electrolysis (WE), as a target objective for energy conversion, primarily enabling hydrogen production and its immediate straightforward reversal fueling. Meanwhile, such a system necessarily implies the instantaneous reversible alterpolar properties of the same electrode from hydrogen evolution (HER) to cathodic oxygen reduction (ORR), and from oxygen evolution (OER) to anodic hydrogen oxidation (HOR), and vice versa. In such a constellation of longer stepwise development, the present * Author for correspondence,
[email protected]. † ICEHT/FORTH. ‡ University of Belgrade.
study emphasizes the need for a thorough review and combination of the main older sequential data and prehistory, to get the final version of a specific electrocatalytic theory and discuss it in the light of new insights. Composite Preview Structure of the Present Study. The first introductory part of the present paper reveals the correlation between intermetallic (hypo-hyper-d-d-interelectronic) bonding effects and electrocatalysis for hydrogen electrode reactions, the latter being further improved and reinforced by the same individual or prevailing hyper-d-electronic (elements with paired d-electrons) metal interaction with hypo-d- (elements with less and up to 5d-electrons) oxide supports. The next part displays the H-adatoms spillover features primarily upon the bronze type and other hypo-d-oxide supported catalysts (M/TiO2). The following discussion is devoted to the spontaneous adsorptive dissociation of water molecules upon hypo-d-oxide catalytic supports. The latter enables the substantial membrane properties of individual and mixed valence hypo-d-oxide compounds (Nb2O5,TiO2), further consequently resulting with the primary oxide (M-OH) dipolar spillover features. Finally, in the introductory theoretical part are employed the recent ab Intio DFT conclusions relating the high d-electronic density of states at the Fermi level with electrocatalytic activity, thence enabling us to select best interacting hypo-hyper-d-d-electronic sup-
10.1021/jp105491k 2010 American Chemical Society Published on Web 10/08/2010
Reversible Electrocatalysts ported electrocatalysts, as the synergistic active composites primarily for the ORR, and other hydrogen end oxygen electrode reactions. Experimental procedure reveals in detail the novel nanostructured sol-gel deposition of bronze type supported and homogeneously distributed electrocatalysts. Cyclic voltammetry has been employed to prove (i) the primary oxide spillover phenomena, (ii) to estimate the widespreading speed of the latter and (iii) from its dipolar nature, equivalent and reversible deposition and desorption (no reaction), confirm the double layer charging and discharging. In the same respect, formaldehyde and its rather fast reversible reaction with the primary oxide was additionally used in the next step to prove the broad potential range of generation and existence of the latter, and its spillover properties. Some controversies associated with the primary oxide catalytic effects and some preceding hints concerning its existence and spillover contributions were then consequently reviewed and pointed out. XPS (X-ray photoelectron spectroscopy) and DRIFT (diffusion reflectance infrared Fourier transform) scans have been further invoked to prove the a priori presence of the primary oxide at the interactive (SMSI) hypo-d-oxide supported electrocatalysts and its readiness for catalytic reaction. The XRD (X-ray diffraction) method contributed to prove the directional homogeneous interactive grafting distribution of Pt nanostructured clusters upon such catalytic supports. Finally, the relation between the primary oxide spillover and mechanism of the ORR, in the light of electrocatalytic improvements of the latter, were proved and illustrated. The first principle was also invoked to prove the primary oxide spillover and its universal relations with work function and polarization. Concluding remarks have been drawn therefrom. Hypo-Hyper-d-d-Interelectronic Bonding Effects in Electrocatalysis. The whole electrocatalytic theory1 relies on Friedel6 hypo-hyper-d-d-electronic correlations and Brewer7 intermetallic bonding model, which implies that the stronger the d-dbonding effectiveness, the more strengthened and exposed arise d-orbitals within the symmetric intermetallic phases, thereby the weaker are adsorptive strengths of intermediates (M-H, M-OH) in the rate determining steps (RDS), therefore the easier their cleavage and further, consequently, the higher the reaction rate and the overall catalytic activity.1-5 The same Brewer7 type d-d-intermetallic bonding model has in addition been the basis for the Tauster8 promotional strong metal-support interaction (SMSI) effect, with the far-reaching consequences in heterogeneous catalysis, electrocatalysis and systematically predetermined interactive grafting and thereby rather homogeneous even distribution of prevailing hyper-d-metallic catalysts on hypod-oxide supports.2-5 Meanwhile, the pronounced both cathodic and anodic interactive spillover (effusion) contributions within the SMSI have been of striking primary significance for the present theory and its embodiment in electrocatalysis of hydrogen and oxygen electrode reactions in low and medium temperature (L&MT) PEMFC and water electrolysis.2-5 Namely, whereas in aqueous media Pt (Pt/C) features either chemisorbed catalytic surface properties of Pt-H and/or PtdO, missing any effusion of other interacting species, a new generation of composite SMSI electrocatalysts in condensed wet state primarily characterizes extremely fast reversible spillover interplay of either H-adatoms or the primary oxides (Pt-OH, Au-OH) as the significant interactive electrocatalytic ingredients.4,5 Spillover Effect, Its Causes and Consequences. The first phenomenon of spillover in heterogeneous catalysis was noticed and defined by Boudart9,10 for the Pt/WO3 system, who pointed out that after dissociative adsorption on Pt surface, the rather
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18299 fast interactive effusion of H-adatoms over W(OH)6 becomes dramatically speeded up even at ambient conditions in the ultimate presence of condensed at least monolayered water.11,12 Such experimental evidence is of substantial significance for both the extremely fast spillover migration and the reversible substrate reduction. The latter finally leads to the corresponding form of electrocatalytically active bronze (Pt/H0.35WO3) for cathodic processes, in which nonstoichiometric incorporated hydrogen obeys the same free reactive function like adsorptive H-adatoms (Pt-H), and is the main source for the reaction. For the present study it is of the same significance to infer the dissociative hydrogen adsorption followed by the fast spillover and this way establishing the storage capacity of H-adatoms upon sol-gel developed surface of carbon current collecting supports (like E-tek Vulcan XC-72) of electrocatalysts, in particular facilitated by the metallic part of the catalyst13-16 and hypo-d-oxide support. Ab initio molecular orbital studies have shown that adsorption of H-adatoms is an exothermic process stable on the graphite basal plane14 and hydrogen spillover implies the transfer of electrons to acceptors within the support, this way modifying the chemical nature of the latter, activating previously inactive substrate, and inducing H-adatoms spillover sorption like mobile repulsive dipoles.13 In such a way oxygen functional groups usually facilitate H-adatoms spillover and their reductive interaction (rough value 1200 mol/(cm2/s), with linear velocities of 100 cm/s at edges of Pt crystallites, reducing down to a still rather high value of 5 cm/s midway between Pt particles), so that formation of Mobronze is an even faster process relative to tungsten.14 In addition, hypo-d-electronic transition metal ions usually feature several altervalent states giving rise even to interactive mixed valence compounds, such as, for example, TiO2/WO3, TiO2/NbO2, or TiO2/TaO2. Such an oxide network, in particular of polyvalent (high altervalent capacity) hypo-d-elements, when in hydrous state, substantially behaves as an ion exchange membrane.2,17-19 In fact, gels (aero and xerogels) are biphasic systems in which solvent molecules are trapped inside an oxide network, and such a material can be considered as a water-oxide membrane composite.17-19 Altervalent Membrane Ionic Transfer and Effusion. Such membrane properties are the consequence of a strong first principle thermodynamic confirmed evidence (density functional calculations, DFC),20 that water molecules undergo spontaneous dissociative adsorption on anatase and even rutile titania, and more so on the higher altervalent oxides21 of tungsten, tantalum, and/or niobium (Figure 2, ref 4). In addition, the first-principles molecular-dynamic simulations show the existence of a mechanism for thermodynamically favored spontaneous dissociation of water molecules even at low coverage of oxygen vacancies of the anatase (101) surface and, consequently to the Magneli phases (TinO(2n-1), in average Ti4O7), as substantially suboxide structure significant as both highly electronic conductive and interactive catalyst support.2-5,22 In fact, this is the status of reversible open circuit dissociative adsorption of water molecules at the equilibrium state. Meanwhile, in the presence of the nanosized metallic part of the catalyst, a directional electric field (or, electrode polarization) further disturbs such established equilibria and dynamically imposes further continuous forced dissociation of water molecules and, as a consequence, their membrane transport properties and the resulting spillover features.2-5 The main causes and consequences of substantial significance then are as follows: (i) Continuous undisturbed reversible anodic transferring membrane mechanism of the altervalent changes
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(W6+ S W5+ S W4+ S W3+ S W2+ S W1+, and similarly so for Ti4+, Ta4+, and Nb5+),17-19 as long as enough moisture supply and catalyst polarization is provided2-5
WO3 + 3H2O S W(OH)6
(1-a)
W(OH)6 + M ⇒ W(OH)5+ + M-OH + e-
(1-b) W(OH)5+ + 2H2O ⇒ W(OH)6 + H3O+ M + 2H2O S M-OH + H3O+ + e-
SCHEME 1: Model Presentation for the SMSI Effect, Resulting in the Spillover Transfer of H-Adatoms within the Bronze Nanostructure (Pt/H0.35WO3, part a), and/or the Primary Oxide (M-OH) Effusion, as a Dipole along the Hydrated Counterpart (Pt/W(OH)6, part b), Further Continuously Transferring Them upon the Metallic Part (Pt) of the Catalyst, Otherwise Both Originating from the Hypo-d-Oxide Continuously Fed by a Moisture Stream to Such as a Composite (M/TiO2,WO3) Electrocatalyst Structure and Maintaining Them in the Reversible Interchangeable Equilibrium (Pt/H0.35WO3 S Pt/W(OH)6)
(1-c)
(1)
Such an interchangeable mechanism imposes the anodic OH-transfer within such an ion exchange membrane, yielding with the consequent spillover of rather interactive primary oxide (M-OH) dipoles over metallic catalyst particles (Scheme 1).4,5 Alternatively, (ii) Cathodic spillover of H-adatoms exerts spontaneous reduction of hydrous tungsten oxide into corresponding bronze (Pt/H0.35WO3). These two spillover processes are highly fast and reversible,9-16 so that every change of catalyst polarity imposes instantaneous altering of W-bronze into its hydrated oxide (W(OH)6),4,5,9-13 and vice versa. Meanwhile, so far the problem was in unattainable nanostructured Pt-bronze, while catalytic activity exponentially increases with decreased Pt nanosize approaching maximum at monatomic dispersion.23 Such an embodiment dream has now been solved by the grafting implementation of Pt-acac (Pt-acetylacetonate) within colloidal particles of peroxopolytungstic acid.1-5 In fact, the spontaneous cathodic reduction of TiO2 (and ZrO2, HfO2), by spillover chemisorbed H-adatoms results in the corresponding suboxide state (TiOx, 2 > x > 1) “encapsulating” Pt nanoparticles.24 The latter leads at moderate high temperature (300 and down to 200 °C) both to the interphase transformation into stable intermetallic phase (TiPt3) (Figure 3 of ref 5)3 that strengthens the overall SMSI effect and finally to the highly stable and electron conductive nonstoichiometric Magneli phases themselves.4 Meanwhile, hypo-d-oxides of higher altervalent states (W,Nb,Mo,Ta) do not feature such encapsulation appearances and by H-adatoms spillover lead straightforwardly to the corresponding bronzes, while their properly produced oxides, like Magneli phases, provide rather high electronic conductivity. Such state of the art and broad experimental evidence enables the results from heterogeneous catalysis to be directly employed in electrocatalysis, with substantial requirements of perfect current collection and integration of every single nanoparticulate catalyst into the whole electrode system satisfied.2-5 The directional electric field (and/or polarization) then enables fast spontaneous reversible alterpolar changes otherwise missing in classical heterogeneous catalysis. Electrocatalysis and d-Electronic Density of States versus Fermi Level. The electrocatalytic reaction mechanism by ab initio DFT, for both HER and ORR, implies that a d-band centered near the Fermi level (EF) can lower the activation energy as the bonding orbital passes EF, the critical step for reduction processes taking place when the antibonding orbital passes the Fermi level of the metal from above and picks up electrons to become filled.25 In electro-oxidation reactions (HOR, OER), it is the bonding orbital that passes the Fermi level from below and gets emptied. Consequently, a good catalyst for these reactions should have a high density of d-states near the Fermi
level.26,27 Thus, the present concept consists of the proper hypohyper-d-d-interelectronic combinations of transition elements both being of rather high densities of d-states at the Fermi level (Figure 1), in which hypo-d-oxide components (W, Nb, Ta, Ti), besides the substantially high SMSI bonding effect, as typical oxophilic d-metals, in addition involve their pronounced membrane spillover properties for the primary oxide transferring and effusion. In such a state, our aim has been to keep the composite transition element ingredients with their initial high densities of d-states, in this way even to increase the latter by their SMSI interbonding effect, and at the same time to use the benefits of the primary oxide spillover for the overall reaction, in particular
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Figure 1. (a) Surface densities of d-band states of some selected hyper-d-electronic transition metals. The integral over the densities has been normalized to unity; the vertical line indicates the Fermi level. Indications and labels: dashed-dotted line, Pt(111); thick line, Au(111); thin line, Ag(111); dotted line, Cu(111); dashed line, Ni(111). (b) Surface densities of d-band states (bulk values differing from exact surface densities for less than 3%) of some selected hypo-d-electronic transition metals (W, Ta, Mo, Nb, color labeled) (calculated by Professor Wolfgang Schmickler, University of Ulm, Germany).
for the ORR. In this respect, Figure 1b reveals why Nb and Ta, by the position in the periodic table of elements, are electrocatalytically predestined for even higher activity than W, Mo, and Ti. The first embodiment and substantiation of the bronze type electrocatalyst ideas, at larger microscale particle size up to Pt wire surrounded by the former, belongs to Tseung et al.,28-30 who codeposited Pt/H0.33WO3 from an admixture of chloroplatinic and peroxopolytungstic acid. Apparently these authors missed the age of sol-gel nanostructured dispersion to approach an upper level of catalytic accomplishments. The achievements, nevertheless, were remarkable, very impressive, revealing, and instructive. However, the point is that these authors recognize H-adatom spillover even at anodic polarization, and although they were aware of the otherwise observed striking effect of primary oxide (OHads), the latter has been present in all of their schemes only indirectly. In other words, the effect of primary oxide was experimentally proved, but its spillover properties were not shown. Meanwhile, the definition and significance of (W, Nb, Ta, V, Mo) bronze originate from and are attributed to Glemser and Naumann.31 Methods Experiments for continuous generation and spillover of both Pt-OH and H-adatoms were performed in a simple membrane
fuel cell, 4.0 × 4.0 (cm × cm) in the projected electrode areas. Composite hypo-d-oxides supported nanostructured Pt electrode was sol-gel deposited from Pt-acac (2,4-pentandionate) precursor as 30.0 wt % Pt, 0.4 mg cm-2 of geometric electrode surface, 2-4 nm in average size, and 15.0 m2 g-1 Pt in actual surface area, UPD (underpotential deposited) H-adatoms, CO striping, and BET method confirmed. The hypo-d-oxide support was 5.0 mol % WO3 + 95.0 mol % anatase TiO2, both sol-gel produced as a submonolayered 20 wt % deposit. Peroxopolytungstic acid colloidal solution was prepared from elemental pure W, first dissolved in 30 wt % H2O2, the excesses of the latter then being removed by the catalytic reaction with Pt catalyst, then dissolved in pure ethanol and deprived of water by triple distillation, as an azetropic mixture. The corresponding submonolayer mixed hypo-d-oxide upon Vulcan XC-72 carbon (250 m2 g-1) with anatase titania was then obtained from Ti-isopropoxide by simple addition under proper stirring and hydrolysis, followed by supercritical drying in liquid CO2, to keep its sol-gel developed surface area (about 180 m2 g-1), further by calcinations at 300 °C. Pt-acac was then grafted with its homogeneous distribution by the spontaneous uniform widespread hypo-hyperd-d-bonding effect upon evenly exposed hypo-d-oxide composite and then decomposed by a hydrogen stream at 250 °C. Such a composite and nanostructured electrocatalyst was then deposited on DuPont’s Nafion-117 membrane using Nafion
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sludge, layer by layer, each followed by water evaporation, to get the desired composition. More experimental details can be found in referred papers of the same authors.2-5 Since nanostructured bronze type (W,Nb,Ta) electrocatalysts have been synthesized for the first time, their achievement deserves more descriptive details. These have been produced either as 20 wt % deposit of individual (WO3, Nb2O5, or TaO2) high altervalent capacity bronze type hypo-d-oxides, or each of them in 5-10 mol % admixture with 90-95 mol % anatase TiO2, to yield together the same (20 wt %) composite structure of hypo-d-oxides, both relative to 70 wt % Vulcan carbon and 10 wt % Pt. Whereas WO3, as above, resulted from colloidal peroxopolytungstic acid, Nb2O5, and TaO2 yield from proper hydrolysis of corresponding ethoxides again in anhydrous ethanol. For submonolayers of individual or composite hypod-oxides upon nanosized carbon, Pt-acac was added and hydrolyzed together with alkoxides, yielding an even distribution of grafted nanostructured catalyst metallic Pt clusters. Since these hypo-d-oxides feature a rather developed surface area (g170 m2 g-1), supercritical drying may be avoided. However, in the absence of Vulcan carbon, when composite hypo-d-oxides imply advanced electron conductivity and current collection, supercritical drying precedes Pt interactive grafting. Namely, such a procedure enables the support surface area to be more developed, and thereby as a priori prepared, provides the more homogeneous distribution growth of the exposed metallic part of electrocatalysts upon them, avoiding Pt deposition in mesopores and within the bulk of the support. The conductivity primarily depends on the composition and thermal treatment of mixed hypo-d-oxide supports (cf. refs 32-34). The photoemission experiments were carried out in an ultrahigh vacuum system (UHV) which consists of a fast entry specimen assembly, a sample preparation, and an analysis chamber.35 The base pressure in both chambers was 1 × 10-9 mbar. Unmonochromatized Mg KR line at 1253.6 eV and an analyzer pass energy of 97 eV, giving a full width at halfmaximum (fwhm) of 1.7 eV for the Au 4f7/2 peak, were used in all XPS measurements. The XPS core level spectra were analyzed using a fitting routine, which can decompose each spectrum into individual mixed Gaussian-Lorentzian peaks after a Shirley background subtraction. Regarding the measurement errors, for the XPS core level peaks we estimate that for a good signal-to-noise ratio, errors in peak positions are of about (0.05 eV. The binding energy (BE) scale was calibrated by assigning the main C 1s peak at 284.6 eV. The samples were in powder form and before the introduction into the ultrahigh vacuum they were pressed into pellets with thickness of 1 mm and diameter 1 cm. The phase analysis and characterization of the interactive mixed metal oxides/carbon supported Pt catalysts was carried out by X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer, Cu KR radiation. The diffractometer was equipped with a Lynx Eye solid state ultrafast detector. The spectra shown in the corresponding figure have been corrected for the background and for the Cu KR2 radiation contributions. Results and Discussion In such a constellation, an unusual and specific PEMFC electrode assembly has been created, in which a metallic part of the interactive supported electrocatalyst (Pt) is interconnected with two solid electrolytes: (a) Nafion-117 membrane for proton transfer after hydrogen dissociative adsorption from gas phase (Pt-H) and electrochemical desorptive charging in the subsequent step, and (b) the composite interactive hypo-d-oxide
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Figure 2. Cyclic voltammograms of mixed hypo-d-oxides supported nanostructured Pt electrode (Pt/TiO2,WO3), scanned in He stream, at negligible moisture content (curve 1) and at water vapor saturation (curve 2).
structure (Pt/WO3 or Pt/Nb2O5 and/or their mixed valence compounds with TiO2). The latter arises capable for simple hydration, originating from continuous adsorptive dissociation of moisture20-22 supply (eq 1-a)), followed by the hydroxide ionic migration transport under the anodic polarization field as a driving force (eq 1-b and 1-c), and resulting in the reversible (back and forth) primary oxide (dipole repulsion homogenized) spillover distribution all over the exposed anodic Pt surface (the overall eq 1), available for CO tolerance (above 600 ppm). Alternatively such a spillover mechanism yields Pt-bronze as the effect of fast H-adatoms reductive effusion at the cathode (Pt/H0.35WO3) during cathodic polarization.9-16 Potentiodynamic Evidence of the Primary Oxide Spillover. In this respect an intermolecular compatible hypo-d-oxide composite mixed valence architecture (5.0 mol % WO3, 95.0 mol % TiO2 ) 20 wt %), as the interactive catalytic submonolayer support of rather high altervalent capacity, capable of withstanding alkaline media, too, has been selected to investigate the primary oxide and H-adatoms spillover properties. In fact, the reversible spillover evidence achieved by potentiodynamic scans in such a typical membrane cell emerges as one of the most convincing dynamic confirmations of the M-OH effusion and its wide-spreading velocity (Figure 2). In this respect, cyclic voltammograms scanned upon Pt/WO3,TiO2/C electrocatalyst at low moisture content of He stream, insufficient for WO3 (or TiO2) hydration, (eq 1-a), repeatedly reveal similar potentiodynamic spectra characteristic for carbon-supported Pt itself, but with pronouncedly high double layer charging capacity, because of the accompanying parallel charging of Vulcan carbon particles beside the metal (QDL ) 1.07 C, assessed by the method displayed by Schmidt et al.36). Meanwhile, it is worth noting that the reversible peaks of both the Pt-OH growth and reduction partially take place even within the usually double layer (DL) charging range. In other words, such a situation reveals that the primary oxide dipoles contribute both to the UPD and OPD double layer charging capacity by the spillover effect, both by preceding and broadening the usual main reversible peak of Pt-OH, otherwise characteristic for plain Pt only within the defined potential range.37-43 In fact, even classical voltammograms show a straight electrolytic connection between the narrow DL charging range and the further following highly expanded in charge density reversible peaks of the primary oxide adsorption and desorption,37-43 and nothing else occurs along the potential axis after H-adatoms desorption. Namely, the primary oxide certainly participates in the double
Reversible Electrocatalysts layer charging on plain Pt after H-adatoms desorption, but in the absence of Pt-OH additional rich source and larger developed adsorptive electrode surface, such as is an individual or composite hypo-d-oxide support, it is not possible to see the more explicitly delineated spillover charge contribution. In fact, we should notice two distinct things: (a) the electrochemical reversible adsorptive deposition and corresponding desorption peaks of the primary oxide on plain metal electrodes at some a priori predestined fixed potential range,37-43 but by all means electrolytically connected with the DL charging range, and (b) the additional spillover facilitated and enriched generation of M-OH (Pt-OH) from the membrane type of richer and more pronounced effusion transfer of hydrated hypo-d-oxide supports, provided by continuous and enough water supply under the electric field and Pt polarization, and by a rather developed electrode surface area. The partial participation of moisture in the gas stream supply, along with available adsorptive surface of the electrode, define the extent of the charge capacity for both the primary oxide peak itself and double layer extension and growth, the phenomenon not marked on the RDE and other polished plain Pt type electrodes in aqueous media. In other words, if the DL charge capacity was not UPD self-connected with the main reversible peaks of M-OH adsorption and desorption, how would one evidence the CO oxidation enabled by the primary oxide for 600 mV further in the rather negative potential range and all along such an interval (see, for example, Figure 7 in ref 4)? In contrast to such fairly common occurrences, a continuous supply of saturated water vapor in the He stream at higher temperature (80 °C), imposing condensation and leading to the appearance of rather wet titania-tungstenia mixed altervalent oxide composite, as the interactive catalytic support, is accompanied by the unusual phenomenon of a dramatic expansion of two reversible pairs of peaks of both the primary oxide and H-adatoms deposition and desorption. The latter have both been of the enormously high spillover charge capacity and for Pt-OH (UPD and OPD) shifted toward both much more negative and far positive potential limits. In fact, the latter arises just as the effect of the primary oxide equivalent dipole charging and discharging of the double layer! Namely, just as stated above, nothing else takes place in between. Every cessation in the steam supply instantaneously imposes the sudden reversible shrinkage of both such rather exaggerated pair of peaks down to the same preceding initial typical potentiodynamic shape similar and close to the nanostructured Pt/C voltammogram spectra, with less pronounced extension in the double layer charging or discharging extents. And vice versa, the renewed saturate water vapor feeding immediately leads to their exactly repeated sharp former primary oxide peaks growth and the same former charge capacities.4,5 Such an appearance without exception behaves as a typical reversible transient phenomenon by its endless repetition, and never appears upon the plain Pt/C electrocatalyst. Even more so, the question is what happened with previously preceding distinctly delineated and separated typical double layer charging capacity range estimated within the nearly dry He supply (Figure 2, shaded area), since now the H-adatoms UPD desorption peaks directly merge with the prevailing broad reversible primary oxide spillover deposition and appear in dramatically expanded charge capacity (247 versus 47 mC cm-2, or in the ratio of about 5.3:1)? In fact, 0.4 mg of Pt cm-2 effectively corresponds to 200 cm2 of exposed Pt surface (BET assessed 50 m2 g-1 Pt), or to 42.0 mC cm-2 of charge capacity per projected geometric surface of electrode, in good agreement with the UPD H-adatoms determined desorption value under
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18303 dry conditions. Exactly the same relates to the H-adatoms adsorption and Pt-OH desorption peaks in the equivalence to their corresponding reversible counterparts. Does it mean that the rather fast spillover of H-adatoms over reversibly fast renewing tungsten bronze (Pt/H0.35WO3), including its own nonstoichiometric H-amount, along with adsorptive effusional deposition on, and subsequent desorptive removal from exposed carbon surface, as its auxiliary storage,13-16 can enable us to reduce the interactive supported Pt within such a composite electrocatalyst (Pt/WO3,TiO2/C), for an order of magnitude (≈75 µg cm-2), and at the same time still keep the initial polarization properties characteristic for dry catalyst? In fact, the striking electrocatalytic conclusion and corresponding contribution of the present study have resulted from the experimental evidence that such a catalytic effect, primarily characteristic for tungstenia supported Pt, equally yields and even exceeds its harmonic hypod-oxide combination with prevailing anatase titania content, otherwise compatible and already used in photovoltaics.44 Since two distinctly different cyclic voltammogram shapes appear only as the result of the difference in water vapor supply, all other parameters being the same, an unequivocal theoretical conclusion and the best heuristic experimental confirmation have been derived from the interactive character and specific interexchangeable double (H-adatoms and Pt-OH) spillover properties of an unique altervalent hypo-d-oxide structure under directional electrode features of polarized nanostructured Pt electrocatalyst. The same conclusion equally concerns the equivalent combinations of Nb2O5 and TaO2 (even MoO3) with anatase titania, which at some broader ratios, when properly thermally treated, feature rather high electron conductivity (above 300 S cm-1) and corresponding membrane transfer capability with equal dual spillover properties. Meanwhile, the electron conductivity is a bulk property, while the membrane transfer mostly takes place above the rather developed hydrated hypo-d-oxide surface. As a consequence, the whole phenomenon in pronouncedly more wet condensation conditions and further continuous water vapor supply arises much more facilitated in the reversible cathodic H-adatoms spillover yielding Pt--bronze and/or, vice versa, its anodic transference into hydrated state and sources (W(OH)6, Ti(OH)4, Nb(OH)5, Ta(OH)4) for Pt-OH effusion, both being the fast reversible reactions. Thus, the primary oxide dipole species undertake both the unusually pronounced double layer charging role and/or exaggerated UPD and OPD adsorption and corresponding desorption within much broader potential range. Consequently, the whole system behaves pronouncedly reversible and smoothly alterpolar both for the primary oxide and H-adatoms spillover and as the whole in the reversibly revertible alterpolar coexisting interrelation between tungsten bronze and its hydrated state (Pt/H0.35WO3 S Pt/W(OH)6). Such equilibrium of rather fast reversible reactions and instantaneous revertible alterpolar interrelations is of fundamental thermodynamic and practical meaning and significance, and consequently imposes new and unpredictable farreaching prospects in electrocatalysis. In other words, two coexisting and interfering reversible pairs of peaks fast altering between H-adatoms (adsorption or desorption) effusion and Pt-OH spillover (and/ or its backward removal) along the potential axis, with all interacting consequences imprinted upon cyclic voltammetry spectra, inherently testify by their exaggerated potentiodynamic features to the reversible interrelations between two coherent dual alterpolar electrode properties. The point and substance are that extremely fast H-adatoms spillover within hypo-d-oxide structure highly facilitates their effusion over carbon catalyst
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support. Such a convincing state of experimental evidence implies the equivalence in the endlessly reversibly repeatable alternative polarity and electrocatalytic properties of so composed L&M PEMFC in conjunction with equally catalytically advanced WE, in particular important for the impurity (frequent alterpolar relation) effect of the latter. In this respect, both peaks of Pt-OH adsorption and desorption are equal and of enormous charge capacity (QPt-OH(a) ) QPt-OH(c) ) 1.453 C), as the dc capacitance of rather developed electrode surfaces, and as they are highly reversible, they keep the same extents after simultaneous multiple and repeating number of cycles at any other time. Such a state of experimental facts implies the conclusion that the bipolar primary oxide structure (Pt-OH, Au-OH) establishes the reversible transient adsorption and charging capacity even on rather developed active carbon surface (C-OH), but by no means takes place in its oxidation. Such a statement has been confirmed by an online mass spectrometer, which identifies CO2 only within the oxygen evolving potential limits. The enriched potentiodynamic experimental evidence, in accordance with the theory of instantaneous reversibly revertible alterpolar properties of the bronze type electrocatalysts and extremely broad potential range of the primary oxide adsorption and desorption, unambiguously testify now that the Pt-OH (AuOH, M-OH) in all circumstances in aqueous media both UPD and OPD charges and discharges double layer (eq 1) and thence is available for reaction within broad potential limits. For example, formaldehyde oxidation starts exactly at its reversible potential value (0.32 V vs RHE), at the usual lower DL charging potential limits, even merges with the second UPD desorption peak of H-adatoms and extends as a rather exaggerated broad twin (double) peak all along the anodic scan until the beginning of OER (Figure 3a),45 similarly and correspondingly to Figure 2. A similar cyclic voltammogram, in accordance with the present theoretical model, and as the result of the Pt-OH spillover effect, has also been scanned by Tseung et al.46 for the anodic glucose oxidation on a Pt-bronze microelectrode (Figure 1, ref 46). In the same sense anodic CO oxidation on composite hypo-d-oxides supported Pt or Pt,Ru catalysts takes place even within the usual interval of H-adatoms desorption (Figure, ref 4), and can even be brought with the reversible bronze type Pt electrocatalyst under the conditions to be initiated just above the HER (nearly at 0.0 V vs RHE). In other words, Pt-OH arises available for reaction not only within its nominal reversible adsorption and desorption peak limits in regular mineral acid or base aqueous solutions but, depending on the reactant concentration, affinity, and its actual reaction rate, along a broad and extendable potential range. Controversies Associated with the Primary Oxide in Literature. The main ambiguities and controversies, meanwhile, come from the observed and recorded fact that anodic CO oxidation occurs all along a negative potential range from 0.1 V vs RHE28-30 down to and within the usual primary oxide adsorption peak, both in acidic and alkaline media (Kucernak and Offer47 and references there in). Namely, the problem now is how to explain the almost UPD appearance of the “hydroxyl adlayers” far away from its reversible peak along the potential axis of regular cyclic voltammograms, and being able to undertake oxidation of strongly adsorbed CO species equally at all such negative potential values and levels. Namely, instead of OHads, OH*, in the present paper we use the Conway37-42 term, to label it, Pt-OH, and this way pronounce its dipolar features,48 otherwise significant for uniform spillover distribution upon and allover an available surface. In such a respect, for
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Figure 3. (a, b) Cyclic voltammograms scanned on a polycrystalline Pt wire electrode in alkaline (0.1 M KOH, dashed lines) solution and in admixture of formaldehyde (0.01 M (a) and 0.1 M HCHO (b), full lines) at 200 mV · s-1 sweep rate between hydrogen and oxygen potential evolving limits. Labels: (a) reversible hydrogen adsorption peak; (b) irreversible Pt surface oxide (PtdO) desorption peak; (c and d) successive peaks of anodic aldehyde oxidation; (e) sudden sharp current jump and reverse peak of repeated HCHO oxidation; (g and f) reversible H-adatoms oxidation and desorption peaks.
example, Burke49 attributed the small middle peak noticed in the hydrogen desorption range at 200 mV on Pt under otherwise very clean conditions, to the formation of “premonolayer hydroxyl species” on noble metals, and in that way explained the broader potential ability limits of Pt to oxidize adsorbed COads. For the present study, meanwhile, it is of specific significance and of a general importance that Markovic and Ross50 have scanned the broad butterfly-like reversible peaks of greater charge capacities and extension along the potential axis in the primary oxide growth and desorption upon Pt(111) and Pt(100). Such experimental evidence clearly testifies to the ability of the hydroxyl species existence along the wider extended potential range and limits on the Pt surface. Since the strong adsorptive bonds of CO adlayers block and isolate the Pt surface, thereby retarding and even preventing anodic hydrogen oxidation, and complicating the overall issue, the spillover effect of the hydroxyl species even within the regular double layer potential range was more simply studied by the oxidation of
Reversible Electrocatalysts formaldehyde (Figure 3, panels a and b).45 Such a heterogeneous reaction is rather fast, mass-transfer limited, and since for more than an order of magnitude exceeds the interchanges of PtsOH into PtdO, while HCHO and water are mixable in all possible ratios, as long as diffusion exceeds the reaction rate, the generated hydroxyl species becomes instantaneously consumed (Figure 3b). The overall experimental evidence now testifies to the ability of primary oxide to undergo spillover (or effusion) above the clean Pt catalytic electrode surface, to impose double layer charging and discharging by its dipole structure and properties48 (eq 1), and in such a way behaves both with the UPD and OPD features. Thus, the Pt-OH, as a reversible species relative to the formaldehyde (glucose46 and other similar aldehyde type compounds45) oxidation reaction, is able to exert and impose the anodic and substantially (potentio)dynamic reaction within an unusually wide, both negative and positive potential range (Figure 3, panels a and b), the same and corresponding to the formerly marked limits in Figure 2 for the broader DL charging. The alternative might be the direct electron transferring for anodic oxidation of formaldehyde, but if so, such a peak would never extend along any longer potential range, and then Au by no means could exceed Pt in its catalytic activity.45 On the contrary, since both the monolayer primary oxide adsorptive growth and its coupled consecutive steps of formaldehyde oxidation are fast reversible reactions, the entire mutual twin’s peak of their two or bielectronic exchange interaction extends all along from UPD H-adatoms desorption almost to the oxygenevolving limits (Figure 3, panels a and b) and correspondingly follows Pt-OH generation along the potential axis (see also Figure 1, ref 46). Such a state of experimental facts testifies that as long as the diffusional flux of HCHO exceeds the Pt-OH generation rate, the anodic deposition of PtdO is effectively suppressed by their preceding instantaneous reaction. As a consequence, the surface oxide of Pt starts its dramatically postponed growth later along the potential axis, depending on HCHO concentration (or, its flux) and consequently appears quite close to the oxygen-evolving potential limits.2,45 Thence, the characteristic cathodic peak for the surface oxide (PtdO) desorption along the reversal negative potential sweep arises remarkably shortened in its charge capacity, and thereby the electrode surface becomes deprived much earlier of such a deposit along the potential axis. The overall result of such unusual and fascinating potentiodynamic occurrences then is that the well-pronounced renewing and repeating anodic formaldehyde oxidation peaks sharply grow again within the reverse cathodic scans and at hysteretic negative potential limiting values. Finally, as a corollary conclusion, the experimental evidence implies that the reaction of aldehyde oxidation cannot take place on the surface oxide (PtdO) covered electrode. Meanwhile, such a dramatically exchanged nominal potentiodynamic Pt (and Au,2,45 too) spectral image in the aldehyde presence, can now be considered as the reliable persuasive for the spillover effect and behavior of the primary oxide, including the reversible UPD and OPD double layer charging and discharging, (eq 1) in both reverse scan directions. The same relates to the potentiodynamic behavior of both Pt and Au in admixtures of other simple aldehydes, alcohols, and even monosaccharides in acidic and alkaline solutions, certainly including their specific structural and steric features and properties as impacts on the overall potentiodynamic spectra.45 Preceding and Supporting Hints for M-OH Effusion and Reactions. The creation of such a highly distinct Pt-OH spillover behavior within the composite Pt/tungstenia/titania/C or Pt/niobia/titania/C interactive supported electrocatalysts, primarily based on the continuous wetness feeding and spontaneous dissociative adsorption of water molecules20-22 (Figure 2,
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18305 ref 4), and further resulting in well-defined membrane migration transfer properties (eq 1), has clear clues in additional experimental facts: (i) XPS analysis of Pt/TiO2 nanostructured electrocatalysts, in particular as a function of temperature, the resulting hydrogen reduction, and the overall thermal effect, has distinctly shown an a priori presence of Pt-OH at the interface,51 ready for just considered spillover interactions and renewable transport (Figures 10 and 11, ref 52). (ii) Haruta53 has pointed out that advanced Au/TiO2 interactive supported heterogeneous catalysts feature their pronouncedly high activity for direct CO oxidation only when provided with the moisture in the gas stream above some critical content. (iii) The catalytic water-gas shift (WGS) reaction (H2O + CO w H2 + CO2), has 2 to even 3 orders of magnitude higher reaction rate on the same metal catalyst interactive (SMSI) supported on hypo-d- than upon sp-oxides,54,55 and by the electrochemical promotion (EPOC) has already been predestined for further catalytic advances and the efficient separation of final products.56 (iv) Ertl et al.57 have in addition shown and experimentally proved by HREEL (high resolution electron energy loss spectroscopy) spectra along with STM images that the Doebereiner catalytic reaction of hydrogen oxidation upon Pt surface, even at rather low temperatures (140 K), proceeds with remarkable amounts of the primary oxide (OHads), as the decisive and accumulated intermediate, and was shown to include the autocatalytic or self-catalytic step with adsorbed water molecules
Oads + H2Oads ⇒ 2OHads
(2)
In the light of the conclusive results illustrated within the present study, the reversible reaction (eq 1), along with the pronounced and decisive effect of polarized water dipoles in the double layer, certainly arises and represents the electrochemical and electrocatalytic equivalent to the Ertl mechanism (eq 2). In support of this, it might be of complementary significance to refer to Kucernak and Offer47 “the hydrogen bonded water-OHads network”, as the diffusional rate determining spillover factor of further reaction with CO. In addition, Iwasita and Xia58 inferred the importance of adsorbed water in electrocatalytic reactions and in particular the growing interaction between the 3a1 orbitals of water molecules with the d-band of catalytic metal substrate. In a similar sense the present paper points out the feeding (“pumping”) moisture effect for the continuous primary oxide spillover generation in electrocatalysis for PEMFC and membrane type WE, while Mukerjee et al.59 tackled the “water activation” as a decisive argument. (v) Cairns et al.60 ascribe the increased electrocatalytic effect of Pt interactive supported on titania and/or tungstenia to the formation of new active sites at the interface and involved the adlineation model to explain such catalytic appearances. Namely, the adlineation implies that the active sites in a two-phase catalyst are the linear boundaries between the phases. (vi) The anodic promotion of Pt catalyst (EPOC) or the NEMCA (non-Faradaic electrochemical modification of catalyst activity) effect for hydrogen oxidation in aqueous environment is impossible in acidic media and rather high in alkaline solutions,61,62 with primary oxide as the facilitating intermediate state, while various anions have practically neither positive nor negative influence.63 Namely, the entire anodic polarized Pt promotion for hydrogen (H-adatoms) oxidation takes place within the potential range, where there exists the surface oxide (PtdO) coverage of a strong adsorptive bonding, and discharge
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Figure 4. DRIFT spectra for H2O and M-OH: (a) Mo/TiO2 after reduction under H2 at 300 °C, He flow at RT; (b) As (a) for Pt-Mo/TiO2 and He flow at 350K; (c) As (b) under H2 flow. Since M-OH becomes depleted in hydrogen flow, the same correspondingly occurs with water.
of hydroxide ions to yield surface oxide (Pt-OH) then plays the decisive catalytic role. (vii) The preceding specific potentiodynamic investigations with the plain nanostructured polycrystalline and Magneli phases (TinO(2n-1), in average Ti4O7, Atraverda’s, Mansfield, UK, “Ebonex”) supported Pt have revealed (Figure 12, panels a and b, ref 4) the following substantially significant observation: Within the reversible range of low Tafel line slope, the ORR proceeds upon a partially oxide covered Pt surface, as at its open circuit potential, while the Pt surface deprived of its oxides, requires much higher polarization (120 mV/dec) for the same reaction to occur. In this respect the hypo-d-suboxide supported Pt electrocatalyst appears to possess significant advantages with its spillover of the primary oxide (Pt-OH). (viii) Watanabe64 was the first to recast cationic selective membranes with mixed hygroscopic oxides (primarily anatase TiO2) and show that the water uptake is significantly higher than that of the pristine Nafion. As a result of the water adsorption on the oxide surface, the back-diffusion of the cathode produced water has been enhanced and the water electro-osmotic drag from anode to cathode becomes reduced and balanced. In addition, Zr, Ce, Ti phosphates are also known as good inorganic ion exchangers, while heteropolyacides of W and Mo (we in particular propose Nb and Ta, in addition), feature remarkable proton (and at different arrangement even
hydroxyl) conductivity.65 Thus, the initial idea,64 in the light of the H-adatoms and hydroxyl group spillover properties considered in the present paper, has now a much broader and fundamentally more significant meaning, in particular concerning the substantial problems and consequently unique prospects and significance of alkaline selective membranes. (ix) Interactive hypo-d-oxide supported and nonsupported electrocatalysts (both Pt and RuPt) exhibited dramatically different activity for CO tolerance in LT PEMFC (Figure 7, ref 4) and provide new additional potentiodynamic evidence for the M-OH spillover effect.4,5 Namely, ever since Watanabe66 has shown that Ru even in a submonolayer core-shell deposit, or while alloying with Pt, shifts the primary oxide growth to a much more negative potential range and enables CO tolerance, the primary oxide spillover became of substantial significance for PEMFC.67 Similarly, the hypo-d-oxide supported Pt and Ru (Pt/TiO2/C, Ru/TiO2/C) in their behavior versus these two pure metals (Pt/ C, Ru/C) themselves, or even their alloys, RuPt/C and RuPt/ TiO2/C, are pairs of couples of distinctly different catalytic properties too, the interactive alternatives featuring an even more advanced primary oxide spillover effect.4,5 Hypo-d-oxides, primarily anatase titania, zirconia, and hafnia, and even more so tungstenia, niobia, and tantalia, facilitate spillover of M-OH, (Figure 7, ref 4). Such facts clearly point to the overall effect
Reversible Electrocatalysts SCHEME 2: Model Presentation of Electrochromic Individual WO3 Layers and Composite Combinations of Tungstenia and Titania with Corner and Age Sharing Crystal Units within the Consistent Mixed Valence Compounds, SEM and TEM Confirmed for Amorphous Overall Structure: (a and b) as Deposited Films, (c and d) Colored and (e and f) Bleached Hypo-d-oxidesa
a
Courtesy of Dr. Satoshi Hashimoto and Hideki Matsouka.68,69
and advantages of membrane type OH- transferring within TiO2, WO3, Nb2O5, TaO2, and their mixed networks of catalyst support, finally resulting in the primary oxide spillover, relative to the plain carbon (Pt/C). In other words, while Ru itself facilitates Pt-OH and Ru-OH spillover transfer in RuPt composite electrocatalyst,66,67 the supporting effusion effect of titania advances the whole same effect for more than 300 mV relative to RuPt/C catalyst. Anodic CO oxidation upon Ru,Pt/ TiO2/C starts even within the potential range of UPD desorption of H-adatoms and becomes much more pronounced in the charge capacity relative to Ru,Pt/C. This important result is one of the most significant confirmations of the present interactive and dynamic spillover catalytic model, as implemented in electrocatalysis. In such a context, Figure 4 illustrates the primary role of water (left peaks) versus M-OH (right peaks) by DRIFT relevance, as the a priori initial source of primary oxides. It should also be inferred that mixed anatase (and even rutile) titania, and in particular tungstenia, form intermolecular solid oxide solutions of a high altervalent capacity (Scheme 2), compatible both in amorphous and crystalline forms of the edge sharing TiO6 and the corner sharing WO6 octahedrons, with pronouncedly increased electrochromic features even at high contents of the former.68,69 In fact, highly charged W6+ cations, like Nb5+, additionally favor the reversible acidic dissociation of water molecules,20-22 and thereby such electrochromic layers exhibit well-defined ion exchange and electron conductive
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18307 properties.4,5,17-19 Thus, one of the fundamental contributions of the present paper is to show that prevailing anatase titania within composite catalytic support with tungstenia (and/or niobia, tantalia and even molybdenia), which is stable for alkaline media too, also behaves in a compatible way and regarding the (Pt/H0.35WO3 S Pt/W(OH)6) reversibility, features the same properties as pure tungsten bronze itself! XPS and Other Spectral Evidence for the Primary Oxide Appearance and Spillover. The link between the basic polycrystalline Pt (85% in (111) crystal structure), as a comparative model issue, and various individual or mixed valence hypo-doxide compounds of titania (TiO2) with WO3, Nb2O5, MoO3, TaO2, interactive supported nanostructured Pt electrocatalysts, imposed the imperative need for surface characterization of the latter, in particular regarding the primary oxide (Pt-OH) spillover and CO tolerance.3,4,51,52 In ex situ XP spectroscopy measurements, Figure 5 shows the corresponding Pt4f deconvoluted spectra of the Nb2O5, TiO2-Nb2O5, TiO2-WO3, and TiO2 itself, interactive supported Pt catalysts. The intensity ratio of the Pt 4f doublet components was 3:4 and their splitting energy was 3.3 eV.51 Three distinct components participate in the electrocatalytic surface, at three different binding energies, 71.1, 72.7, and 74.8 eV, and in this order were attributed to the metallic platinum itself, the primary (Pt-(OH)), and nonstoichiometric surface PtOx oxides,52 respectively (compare Figure 10, ref 52). The latter arises as the minor component among others and does not even appear at the Pt 4f spectrum of the Nb2O5-TiO2 supported Pt catalyst. Such a decisive and conclusive remark is of substantial and fundamental significance for the present study: The primary oxide is prevailing and a priori initially available, in particular in the condensed wet state of the electrocatalysts. In this respect, one should recall the selfcatalytic effect of water molecules on the primary oxide appearance and existence.57 The percentage contribution of each component to the total peak area is shown in Table 1, which has some deeper theoretical significance. Namely, the roots and expectancies for the primary oxide spillover are indicated there on an almost quantitative scale basis. The decisive and pronounced cooperative effect of anatase titania is clearly indicated by its individual rather high initial primary oxide surface percentage, while the Pt/Nb2O5,TiO2 features the most creative synergistic properties for the Pt-OH generation that further reflects on and definitely defines the overall electrocatalytic activity for the ORR. In the same context one should mention the shrinkage of the same XPS Pt-OH peak capacity by both the thermal and hydrogen reduction effect at higher (573 K) temperature (see Figure 4, ref 4), as the additional congenial experimental evidence for the preceding existence of primary oxide available for further instantaneous spillover distribution under electrode polarization. DRIFT imprints in Figure 4 in a similar and congenial manner reveal the same primary oxide thermal and reduction features. In Figure 6 the deconvoluted Ti 2p spectra of TiO2-Nb2O5 and TiO2 supported Pt catalysts are comparatively presented. The main doublet in both spectra, at binding energy (Ti 2p3/2) 458.8 eV is attributed to Ti4+ species.35,70 In the case of the TiO2-Nb2O5 mixed support, a second doublet is apparent at binding energy (Ti 2p3/2) 455 eV. This is attributed to the contribution of Ti atoms in the Ti3+ state. It should be inferred here that Ti atoms in the other mixed hypo-d-oxide support TiO2-WO3 are detected only in the Ti4+ state.32 In Figure 7 the deconvoluted Nb 3d XPS core level spectra of TiO2-Nb2O5 and Nb2O5 supported Pt catalysts are presented. In both cases only one doublet is apparent at binding energy (Nb 3d5/2) 207.1 eV, which is characteristic for the Nb5+ state. Namely, it is
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Jaksic et al. being much more likely at high temperatures.71 Quantitative analysis of the present results, using the Ti 2p and Nb 3d peak intensities (areas) corrected by the atomic sensitivity factors,72 shows that the appearance of each Ti3+, corresponds to one introduced Nb5+ ion. Thus, in the present case, and in agreement with the existing literature, it seems that the introduction of Nb oxide into the composite altervalent mixed compound network with anatase titania finally causes the partial reduction of Ti4+ to Ti3+ per every Nb5+ ion introduced, and perfectly suits the revealed relevant cooperative membrane transferring mechanism (eq 1).4,5 Now, one is faced with a rather interesting situation concerning Nb oxide structure. At relatively low temperatures of calcination, the latter arises crystallized as Nb2O5, wellconfirmed by the XPS analysis, while much more stable NbO2 appears above 900 °C. Such experimental evidence reveals the fifth OH- ion to be the most easily transferable within the overall spillover mechanism
Nb(OH)5 + Pt S Nb(OH)4+ + Pt-OH + e-
(3)
Meanwhile, XPS analysis has also revealed a further interrelating mechanism of similar exchanges with hydrated anatase titania
Nb(OH)4+ + Ti(OH)4 ⇒ Nb(OH)5 + Ti(OH)3+
(4)
or, when summed up
Ti(OH)4 + Pt S Ti(OH)3+ + Pt-OH + e-
Figure 5. Deconvoluted Pt 4f XPS core level spectra of Nb2O5, TiO2-Nb2O5, TiO2-WO3, and TiO2 interactive (SMSI) supported Pt electrocatalysts.
TABLE 1: Percentage Contribution of Each Pt 4f Component to the Total Peak Area % contribution of Pt 4f components catalyst support
Pt0 (71.1 eV)
Pt(OH) (72.7 eV)
PtOx (74.8 eV)
Nb2O5 Nb2O5-TiO2 WO3-TiO2 TiO2
67.2 65.3 61.9 56.8
23.7 34.7 26.7 31.9
9.1 11.4 11.3
found in the literature, and well-known that Nb5+, when inserted into a titanium dioxide network, usually causes an effect in the charge compensation.71 The addition of such a charge can be compensated either by creating one vacancy of Ti per four introduced Nb ions or by the reduction of Ti4+ to Ti3+ per each inserted Nb5+ ion. Both of these effects can occur, with the latter
(5)
the entire formalism clears up the equivalence between titania and niobia for the primary oxide spillover, as already concluded from Table 1. XRD Characterization of Hypo-d-Oxide Supported Electrocatalysts. The spectrum for the Pt/(Nb2O3-TiO2)/C catalyst shows a highly overlapped region between 35° and 50° (Figure 8). In this region one expects the reflections for the Pt(111) and -(200), TiO2 anatase (004) and (200), and the graphitic (100) crystallographic planes, as shown with the labeling straight lines drawn therein. Evidently, all the above-mentioned reflections are essentially overlapped. In this respect, it is not possible to estimate exactly the Pt and TiO2 particles size, while reflections attributable to Nb specimens are absent, due to their very small amount. Nb oxide reflections are also absent from the XRD spectrum of the Pt/Nb2O3/C catalyst, though the amount of niobia is now significantly higher as compared to the Pt/(Nb2O3-TiO2)/C sample. The only reflection visible in the Pt/Nb2O3/C sample is ascribed to the Pt(111) crystallographic plane. By fitting this particular peak with a Gaussian function, it is possible to estimate the average Pt particle size by applying the Scherrer equation. It was found to be approximately 2.2 nm, this way indicating the highly and uniformly dispersed nature of such a catalyst. From H2 chemisorption measurements, the average Pt particle size was estimated to be 3.7 nm (77 m2/g of Pt assuming spherical particles, while the electrochemically active surface area, UPD H-adatoms estimated by the corresponding potentiodynamic desorption peaks, is even lower, 57.5 m2/g of Pt). The significantly larger Pt particles estimated by the H2 chemisorption indicates the interaction between the Pt particles and the oxide support that hinders the H2 adsorptive properties of Pt crystallites, leading to particle size overestimation. On the other hand, the particle size determination by XRD is usually overestimated (the overlapping effect), and since the size
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Figure 6. Deconvoluted Ti 2p XPS core level spectra of TiO2-Nb2O5 (left) and TiO2 (right) supported Pt catalysts.
Figure 7. Deconvoluted Nb3d XPS core level spectra of TiO2-Nb2O5 (left) and TiO2 (right) supported Pt catalysts.
Figure 8. XRD spectra of the Pt supported on Nb2O3/C and (Nb2O3-TiO2)/C. The lines drawn in the figure show the positions of the respective reflections.
is close to the instrument limits, such a state of experimental evidence strongly suggests that Pt particles are rather highly and evenly dispersed and strongly bonded on the hypo-d-oxide catalytic support. The absence of any niobia reflections indicates that Nb oxides are either highly sub- to monolayer dispersed on the carbon particles surface (as essentially the surface species), or less probably in amorphous state, which has been shown not to be the case by decisive XPS analysis. In fact, the present electrocatalyst has been a priori planned and calculated by the mass and surface ratio between hypo-d-oxide and Vulcan carbon particles to be deposited as submonolayer (Nb2O5 having 170 m2 g-1, while carbon particles being in higher amount and of much larger surface area, 260 m2 g-1). Such a unique XRD experimental insight convincingly testifies to the appearance and existence of rather evenly high
Pt(111) bronze dispersion and distribution upon highly developed hypo-d-oxide support. Such a state relative to the standard Pt/C electrocatalyst, otherwise suffering from agglomeration, characterizes remarkably increased electrocatalytic activity, a much longer lasting catalyst because of the SMSI bonding effect, while the interactive structure enables Pt metal recovery, which is another high-quality achievement of the novel nanostructured bronze type electrocatalysts. Primary Oxide Spillover and Mechanism of the ORR. Finally, in the light of everything presented so far, concerning the ORR, its kinetics and electrocatalysis, it is unequivocally clear that under cathodic polarization, the desorption of primary oxide proceeds as a fast reversible and independent electrocatalytic reaction (eq 1),37-42 and consequently any accumulative adsorption coverage is impossible (ΘM-OH w 0). Thus, the primary oxide could not be the rate-determining species for the cathodic ORR,73 in particular since the former also participates in some fast steps of the entire mechanism, for example, (eq 6a-c)74
O2 + M + H3O + e- ⇒ M-OOH + H2O(RDS) (6-a) M-OOH + M ⇒ M-OH + M ) O
(6-b)
M-OH + M ) O + 3H3O+ + 3e- ⇒ 5H2O + 2M (6-c) Such a conclusion by strong analogy also comes from the dependence (∆Φ ) f(∆UWR)), displayed in the graphs of general fundamental catalytic promotion (EPOC) significance (Figure 8, ref 75). From such dependences, equally as from the highly
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Figure 9. Tafel plots for the cathodic ORR scanned on RDE in 0.5 M HClO4 solution at 25 °C for E-tek, Inc.: Pt/C (Vulcan XP-72, closed circles); Pt/Ebonex (Magneli phases, closed triangles); Pt/Nb2O5 (20 wt %)/C (70 wt %) (open triangles); and Pt/Nb2O5 (5 mol %),TiO2 (95 mol %) (open circles) (measured and plotted by Professor Nedeljko V. Krstajic, University of Belgrade).
reversible Pt-OH adsorption/desorption peaks themselves (Figure 1 and Figure 6 of ref 5), it is clear that the spillover chemisorption is possible only either at anodic or cathodic potentials, but not consistently in both of them. Namely, cathodic and anodic polarizations are alternatively either adsorptive or desorptive spillover causes and consequences. Unfortunately, the primary oxide (Pt-OH) almost spontaneously degrades after the monolayer coverage within the narrow potential range of regular acid/base voltammograms into the stable surface oxide (PtdO),37-43 which features pronounced irreversible properties within a broader potential interval. Under such experimental circumstances, similar to hydrogen peroxide (HO-OH) splitting, the ideal direct oxygen evolution (OER) from the primary oxide at its reversible potential
2M-OH f 1/2O2 + H2O + 2 M + 2e-
(7)
is prevented by the much faster, much more stable and highly polarizable surface oxide growth. In this respect, recent experimental evidence with hypo-d-oxide supported nanostructured Pt electrocatalysts have revealed the following:2-5 (i) As the effect of Pt-OH spillover, the ORR was doubled in its rate upon Pt/(Magneli phase) relative to unsupported Pt/C catalyst (Figure 10, ref 4), and for more than an order of magnitude more so upon Pt/WO3 and Pt/NbO2, or Pt/WO3,TiO2 and Pt/ NbO2,TiO2 composite electrocatalysts,76 under all other conditions being the same (Figure 9), the latter being the main confirmation and contribution of the displayed theory, as well as the main result of the present paper. (ii) The initial potential of the ORR (Figure 9) was shifted for more than 100 mV toward more positive values (at 1.05 V vs RHE, Figure 12, ref 4).76 (iii) All along the reversible part of the Tafel plot (60 mV/dec) for the ORR, the Pt surface has been found partially covered by oxides (Figure 12, ref 4), while at higher polarization (120 mV/dec range) the Pt surface becomes fully deprived of the latter. Thus, to approach the reversible oxygen electrode (ROE), both for the ORR and OER, and at the open circuit, one has to provide continuous primary oxide presence and supply within the entire potential range and its interference with the surface oxide (PtdO, AudO). In other words, the reversible behavior implies some interference between PtsOH and PtdO. Such a conclusion also comes from and is supported by the EPOC or
NEMCA promotion of catalytic hydrogen oxidation in aqueous media.61-63 In this respect, the present concept involves the higher altervalent capacities (W6+, Mo6+, Nb5+, Ta4+) hypo-doxide (SMSI) interactive catalytic supports, to use the advantage of some slow steps within their entire membrane transferring mechanism, affording M-OH still available even during cathodic polarization (compare Figure 8, ref 75). In the same context, it is necessary to infer that our bronze type interactive supported electrocatalysts are permanently under established equilibria (Pt/H0.35WO3 S Pt/W(OH)6), in particular in the state of continuous moisture supply (the Ertl57 condition of selfcatalytic effect). It might be necessarily useful to stress in the same context again that the electrochemical reaction equilibrium (eq 1) in the primary oxide generation, has the same equivalence, meaning, and significance as the Ertl57 autocatalytic mechanism (eq 2). Namely, in such a respect, these both clearly show the effect of (in particular the polarized) dipole water molecules for the primary oxide generation within the electrocatalyst double layer interface. In this respect, the present paper has brought us closer to and shown the way toward the reversible oxygen electrode, as the main goal in aqueous electrochemistry and electrocatalysis. Ever since Tseung et al.28-30 obtained remarkable electrocatalytic effects with macro- and microbronze type electrocatalysts, there have been lots of congenial and similar approaches and achievements in electrocatalysis worthy of mention. The first striking and promising issue of remarkable electrocatalytic effect for the ORR has been verified by Savadogo (Figure 11, ref 77), with developed Pt surface simply in the presence and relative to the absence of peroxopolytungstic acid. Adzic et al.78 recently used high-temperature baked NbO2 supported Pt and referred to the remarkable improvement of catalytic activity for the ORR, while other approaches70,79,80 are more or less similar to and rely on other related achievements with hypo-d-oxide supports.4,5,81 The common point is that all authors agree that the primary oxide is a rather decisive intermediate in the kinetics for the ORR, but while some consider Pt-OH for obstacle and rate-determining species,73,78,82,83 others show its striking electrocatalytic facilitating and speeding up contributions.4,5,79,81 In other words, while proceeding from the plain Pt (Pt/C), through the suboxide Magneli phases (Ti4O7 in average), and further to the hypo-d-oxide mixed valence composite supports of higher altervalent capacity, as XPS analysis shows (Table 1), the initial amount of the primary oxide (Pt-OH) increases, while correspondingly the electrocatalytic activity for the ORR76 (Figure 9) increases too and convincingly strengthens such an assertion. Meanwhile, for the time being of the pioneering achievements in actual electrochemical science is of primary significance to advance electrocatalytic activity, while novel in situ experimental methods on the individual “electrocatalytic activity spots” of subnanosize particle level,84-86 might soon answer and clearup all our questions concerning the RDS and overall mechanism for the ORR. First Principle and Spillover Correlations. On the basis of first principle thermodynamic analysis,75,87-89 it has been proven that polarization of electrocatalytic working (W), relative to the reference (R) electrode (∆UWR), imposes work function changes (∆Φ), and the latter consists of the primary oxide spillover (∆µM-OH) that reflects in the surface potential changes,2 e(∆χ), because of the imposed M-OH ordered package at the interface and, thereby, the resulting dipole repulsion and double layer charging capacity
∆UWR ) ∆Φ ) e∆χ ) ∆µM-OH
(5a)
Reversible Electrocatalysts consistent with the entire theory and experimental evidence displayed and discussed within the present study. Namely, Vayenas et al.75,87-89 in many significant papers have shown that the straight linear dependence between work function (∆Φ) and imposed surface potential changes (e∆χ), by dipole adsorption at the catalytic surface, straightforwardly reveal the spillover phenomena and their substantial significance for and contribution to the electrocatalytic activity. Concluding Achievements Nanostructured reversibly interchangeable (Pt/H0.35WO3 S Pt/W(OH)6) alterpolar bronze type, and its TiO2 modified (Pt/ Nb2O5,TiO2/C, Pt/WO3,TiO2/C) electrocatalyst issues for hydrogen and primarily oxygen electrode reactions (HER, HOR, ORR, OER), have for the first time been developed and produced by a proper sol-gel synthetic method. Since all these four electrode reactions are instantaneously reversibly interchangeable on the same electrocatalyst of doubled (bronze and hydrated state) properties and feature substantially pronounced individual reversible properties for each of them, several distinctly significant achievements resulted therein: (i) Electrocatalytic activity for the ORR has been advanced for more than an order of magnitude (Figure 9) relative to the standard nanostructured Pt/C electrocatalyst, enabling us to reduce Pt load for more than four times in L&MT PEMFC and still keep the same preceding activity. (ii) Such reversibly alterpolar and multifunctional electrocatalyst composite enables a still expensive, but functionally, technologically and catalytically ideal revertible cell system between PEMFC and WE and opens the ways toward cheaper ones. (iii) All proper combinations between higher altervalent capacity hypo-d-oxides (Nb2O5, WO3, TaO2) and anatase TiO2, properly thermal produced, feature high electron conductivity and enable us to avoid the nanosized carbon current collector and support in PEMFC and WE, which is decisive for anodic oxygen evolution. (iv) Since interactive (SMSI) d-d-bonded on suitable hypod-oxide supports, Pt nanoparticles are first properly grafted upon them, much more uniformly distributed and evenly dispersed at the low optimal nanostructure and sizes. The corresponding supported Pt nanoclusters are thereby prevented from agglomeration that implies longer lasting electrocatalytic activity, and even afford possibilities for Pt recovery, otherwise so far missing in LT PEMFC operation and uses. (v) XP spectra have been used to show an a priori preexistence of the primary oxide species, as predestined for spillover by the membrane transfer through the interactive (SMSI) bonded hypo-d-oxide supports, and being able and available for the double layer charging and discharging. XPS analysis has also revealed and confirmed the membrane mechanism in Pt-OH transference and spillover (confirmed surface Ti3+ versus Nb5+ in the ratio 1:1). In a similar respect XRD scans contributed to further supportive conclusions concerning the particle size and interionic interference between titania and niobia, or tungstenia. In addition, potentiodynamic spectra scanned under the wet and dry PEM has been employed to prove the spillover of primary oxide and its UPD and OPD double layer charging and discharging. Cyclic voltammetry in admixtures of formaldehyde are cited to reinforce such phenomenological statements, while Ertel57 auto- or self-catalytic effect of water molecules, particularly within the double layer structure, was added to complete the overall phenomenon of effusion.
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18311 (vi) The present and former experimental evidence4,5 show that within the reversible part of a Tafel plot, in particular, hypod-oxide supported Pt electrocatalysts are partially covered by its oxides and thereby polarization for the ORR starts shifted at remarkably more positive potential values and, consequently, increasingly acquiring and approaching the reversible oxygen electrode properties (cf. refs 90 and 91). The former attempts to approach such a reversible feature by continuous in situ physical removal of surface oxide have failed while the electrode was the subject of more pronounced polarization. Thus, one conclusion is that the present paper has shown the way toward the reversible oxygen electrode, as one of the main goals in aqueous electrochemistry, primarily and at least for the ORR. Acknowledgment. Thanks go to Professor Nedeljko V. Krstajic, University of Belgrade, for some decisive experiments carried out and providing what are so far the best existing Tafel plots in the ORR, being based on the Pt/Nb-bronze. The authors would like to acknowledge the significant and decisive help of Professor Slavko Mentus, University of Belgrade, with the preparation of the present study. References and Notes (1) Jaksic, M. M. J. Mol. Catal. 1986, 38, 161. (2) Neophytides, S. G.; Zafeiratos, S.; Jaksic, M. M. J. Electrochem. Soc. 2003, 150, E512. (3) Neophytides, S. G.; Murase, K.; Zafeiratos, S.; Papakonstantinou, G.; Paloukis, F. E.; Krstajic, N. V.; Jaksic, M. M. J. Phys. Chem. B 2006, 110, 3030. (4) Jaksic, J. M.; Krstajic, N. V.; Vracar, Lj. M.; Neophytides, S. G.; Labou, D.; Falaras, P.; Jaksic, M. M. Electrochim. Acta 2007, 53, 349. (5) Krstajic, N. V.; Vracar, Lj. M.; Radmilovic, V. R.; Neophytides, S. G.; Labou, D.; Jaksic, J. M.; Tunold, R.; Falaras, P.; Jaksic, M. M. Surf. Sci. 2007, 601, 1949. (6) Friedel, J.; Sayers, C. M. J. Phys. (Paris) 1977, 38, 697. (7) Brewer, L. Science 1968, 161, 115. (8) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. (9) Boudart, M.; Vannice, M. A.; Benson, J. E. Z. Phys. Chem. (Muenchen, Ger.) 1969, 64, 171. (10) Kohn, H. W.; Boudart, M. Science 1964, 145, 149. (11) Benson, J. E.; Kohn, H. W.; Boudart, M. J. Catal. 1966, 5, 307. (12) Vannice, M. A.; Boudart, M.; Fripiat, J. J. J. Catal. 1970, 17, 359. (13) Lueking, A. D.; Yang, R. R. Appl. Catal., A 2004, 265, 259. (14) Kim, J.-G.; Regalbuto, J. R. J. Catal. 1993, 139, 175. (15) Agarwal, R. K.; Noh, J. S.; Schwarz, J. A. Carbon 1987, 25, 219. (16) Takagi, H.; Hatori, H.; Yamada, Y. Carbon 2005, 43, 3037. (17) Livage, J.; Henry, M.; Sanchez, S. Prog. Solid State Chem. 1988, 18, 259. (18) Judeinstein, P.; Livage, J. J. Mater. Chem. 1991, 1, 621. (19) Livage, J.; Guzman, G. Solid State Ionics 1996, 84, 205. (20) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. Phys. ReV. Lett. 1998, 81, 2954. (21) Valdes, A.; Kroes, G. J. J. Chem. Phys. 2009, 130, 114701. (22) Tilocca, A.; Selloni, A. J. Chem. Phys. 2003, 119, 7445. (23) Mavrikakis, M.; Stoltze, P.; Norskov, J. K. Catal. Lett. 2000, 64, 101. (24) Haller, G. L.; Resasco, D. E. In AdVances in Catalysis, Eley, D. D., Pires, H., Weisz, P. B., Eds.; Academic Press: San Diego, CA, 1989; Vol. 36, p 173. (25) Santos, E.; Schmickler, W. Angew. Chem., Int. Ed. 2007, 46, 8262. (26) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238. (27) Christoffersen, E.; Liu, P.; Ruban, A.; Skriver, H. L.; Norskov, J. K. J. Catal. 2001, 199, 123. (28) Tseung, A. C. C.; Shen, P. K.; Chen, K. Y. J. Power Sources 1996, 61, 223. (29) Tseung, A. C. C.; Chen, K.-Y. Catal. Today 1997, 38, 439. (30) Shen, P. K.; Chen, K.-Y.; Tseung, A. C. C. J. Electroanal. Chem. 1995, 389, 223. (31) Glemser, O.; Naumann, K. Z. Anorg. Allg. Chem. 1951, 265, 288. (32) Bockhimi; Morales, A.; Novaro, O.; Lopez, T.; Sanchez, E.; Gomes, R. J. Mater. Res. 1995, 10, 2788. (33) Tanaka, K.; Kakimoto, K.; Ohsato, H. J. Cryst. Growth 2006, 294, 209. (34) Uekawa, N.; Kudo, T.; Mori, F.; Wu, Y. J.; Kakegenawa, K. J. Colloid Interface Sci. 2003, 264, 378.
18312
J. Phys. Chem. C, Vol. 114, No. 43, 2010
(35) Siokou, A.; Ntais, S. Surf. Sci. 2003, 540, 379. (36) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 2354. (37) Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A. J. Electroanal. Chem. 1973, 43, 9. (38) Angerstein-Kozlovska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051. (39) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429. (40) Angerstein-Kozlowska, H.; Conway, B. E.; Tellefsen, K.; Barnett, B. Electrochim. Acta 1989, 34, 1045. (41) Conway, B. E.; Barnett, G.; Angerstein-Kozlowska, H.; Tilak, B. V. J. Chem. Phys. 1990, 93, 8361. (42) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331. (43) Tian, F.; Jinnouchi, R.; Anderson, A. B. J. Phys. Chem. C 2009, 113, 17484. (44) Jose, R.; Thavasi, V.; Ramakrishna, S. J. Am. Ceram. Soc. 2009, 92, 289. (45) Ristic, N. M.; Kotorcevic, M.; Lacnjevac, C. M.; Jokic, A. M.; Jaksic, M. M. Electrochim. Acta 2000, 45, 2973. (46) Zhang, X.; Chan, K.-Y.; Tseung, A. C. C. J. Electroanal. Chem. 1995, 386, 241. (47) Kucernak, A. R.; Offer, G. J. Phys. Chem. Chem. Phys. 2008, 10, 3699. (48) Koper, M. T. M.; Van Santen, R. A. J. Electroanal. Chem. 1999, 472, 126. (49) Burke, L. D. Electrochim. Acta 1994, 39, 1841. (50) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117. (51) Zafeiratos, S.; Papakonstantinou, G.; Jaksic, M. M.; Neophytides, S. G. J. Catal. 2005, 232, 127. (52) Jaksic, J. M.; Vracar, Lj. M.; Neophytides, S. G.; Zafeiratos, S.; Papakonstantinou, G.; Krstajic, N. V.; Jaksic, M. M. Surf. Sci. 2005, 598, 156. (53) Date, M.; Haruta, M. J. Catal. 2001, 201, 221. (54) Panagiotopoulos, P.; Kondarides, D. I. Catal. Today 2006, 112, 49. (55) Panagiotopoulos, P.; Kondarides, D. I. Catal. Today 2007, 127, 319. (56) Kokkofitis, C.; Karagiannakis, G.; Stoukides, M. Catal. Today 2007, 127, 330. (57) Volkening, S.; Bedurftig, K.; Jacobi, K.; Wintterlin, J.; Ertl, G. Phys. ReV. Lett. 1999, 83, 2672. (58) Iwasita, T.; Xia, X. H. J. Electroanal. Chem. 1996, 411, 95. (59) Teliska, M.; Murthi, V. S.; Mukerjee, S.; Ramaker, D. E. J. Electrochem. Soc. 2005, 152, A2159. (60) Shim, J.; Lee, C.-R.; Lee, H.-K.; Lee, J.-S.; Cairns, E. J. J. Power Sources 2001, 102, 172. (61) Neophytides, S.; Tsiplakides, D.; Stonehart, P.; Jaksic, M. M.; Vayenas, C. Nature 1994, 370, 45. (62) Neophytides, S. G.; Tsiplakides, D.; Stonehart, P.; Jaksic, M. M.; Vayenas, C. G. J. Phys. Chem. 1996, 100, 14803. (63) Labou, D.; Neophytides, S. G. Top. Catal. 2007, 44, 451.
Jaksic et al. (64) Watanabe, M.; Uchida, H.; Seki, Y.; Emori, M.; Stonehart, P. J. Electrochem. Soc. 1996, 143, 3847. (65) Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. Chem. Mater. 2003, 15, 4896. (66) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (67) Davies, J. C.; Hayden, B. E.; Pegg, D. J.; Rendall, M. E. Surf. Sci. 2002, 496, 110. (68) Hashimoto, S.; Matsuoka, H. Surf. Interface Anal. 1992, 19, 464. (69) Hashimoto, S.; Matsuoka, H. J. Electrochem. Soc. 1991, 138, 2403. (70) Fuentes, R. E.; Garcia, B. L.; Weidner, J. W. ECS Trans. 2008, 12, 239. (71) Arbiol, J.; Cerda, J.; Dezanneau, G.; Cirera, A.; Peiro, F.; Cornet, A.; Morante, J. R. J. Appl. Phys. 2002, 92, 853. (72) Seah, M. P. In Practical Surface Analysis, 2nd; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1990; Vol. 1, p 201. (73) Anderson, A. Electroochim. Acta 2002, 47, 3759. (74) Ma, Y.; Balbuena, P. B. Chem. Phys. Lett. 2007, 440, 130. (75) Tsiplakides, D.; Vayenas, C. G. J. Electrochem. Soc. 2001, 148, E189. (76) Elezovic´, N. R.; Babic´, B. M.; Gajiæ-Krstajic´, Lj.; Radmilovic´, V.; Krstajic´, N. V.; Vrac´ar, Lj. M. J. Power Sources 2010, 195, 3961. (77) Jaksic, M. M. Electrochim. Acta 2000, 45, 4085. Savadogo, O. In New Materials in Electrochemical Systems; Savadogo, O., Roberge, P. R., Veziroglu, T. N., Eds.; Polytechnique de Montreal: Montreal, QC, 1995; p 544. (78) Sasaki, K.; Zhang, L.; Adzic, R. R. Phys. Chem. Chem. Phys. 2008, 10, 159. (79) Micoud, F.; Maillard, F.; Gourgaud, A.; Chatenet, M. Electrochem. Commun. 2009, 11, 651. (80) Fouda-Onana, F.; Savadogo, O. Electrochim. Acta 2009, 54, 1769. (81) Tominaka, S.; Momma, T.; Scrosati, B.; Osaka, T. J. Power Sources 2010, 195, 4065. (82) Wang, J. X.; Markovic, N. M.; Adzic, R. R. J. Phys. Chem. B 2004, 108, 4127. (83) Wang, J. X.; Zhang, J.; Adzic, R. R. J. Phys. Chem. A 2007, 111, 12702. (84) Schwamborn, S.; Stoica, L.; Chen, X.; Xia, W.; Kundu, S.; Muhler, M.; Schuhmann, W. Chem. Phys. Chem. 2010, 11, 74. (85) Nagalah, T. C.; Maljusch, A.; Chen, X.; Bron, M.; Schuhman, W. Chem. Phys. Chem. 2009, 10, 2711. (86) Chen, X.; Eckhard, K.; Zhou, M.; Bron, M.; Schuhmann, W. Anal. Chem. 2009, 81, 7597. (87) Metcalfe, I. S. J. Catal. 2001, 199, 247. (88) Metcalfe, I. S. J. Catal. 2001, 199, 259. (89) Tsiplakides, D.; Archonta, D.; Vayenas, C. G. Top. Catal. 2007, 44, 469. (90) Bockris, J. O’M.; Shamshul Huq, A. K. M. Proc. R. Soc. London, Ser. A 1956, 237, 277. (91) Hoare, J. P. J. Electrochem. Soc. 1979, 126, 1502.
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