Electrochemical promotion of environmentally important catalytic reactions

The performance of conventional heterogeneous metal catalysts may be enhanced by the addition of so-called promoter species that are used to modify the intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical methods provide an alternative, radically different and uniquely efficacious method of catalyst promotion. Substantial and reversible changes in catalyst perfomance can be induced by back-spillover ions pumped from a solid electrolyte to the surface of a catalytically active electrode: one hasin situ control of the working catalyst. Studies of the electrochemical promotion of NO reduction over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is pumped to the catalyst surface. We have examined the NO+CO reaction and the reaction of NO with propene. Both reactions are relevant to control of automotive and other emissions, and both exhibit strong electrochemical promotion. By simulating lean-burn engine conditions, we have also demonstrated that EP of a Pt catalyst very substantially enhances the ability of NO to oxidise propene in an oxygen-rich atmosphere. Reaction kinetic data obtained as a function of catalyst potential, temperature and gas composition indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is the critical reaction-initiating step. XP spectroscopy under the appropriate conditions of temperature and catalyst potential confirms that the mode of operation of the elctrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Electrochemical promotion of the NO reduction by C2H4 on Pt/YSZ and by CO on Pd/YSZ

The effect of electrochemical promotion was investigated for the catalytic reduction of nitric oxide with ethylene and carbon monoxide on polycrystalline Pt and Pd, respectively, deposited on yttria-stabilized zirconia (YSZ). It was found in both cases that applying negative potentials and thus lowering the catalyst work function results in a pronounced increase in the catalytic rate and in the selectivity to nitrogen. A 7-fold increase was observed for the NO+C₂H₄ reaction on Pt while a 2-fold increase was obtained for the NO+CO reaction on Pd. The induced changes in catalytic rates were found to be 7 to 50 times higher than the rates of ion transfer from the catalyst surface. In both reactions, the observed electrophilic behavior can be attributed to the strengthening of the chemisorptive NO bond and concomitant enhanced dissociation of NO as the catalyst potential and work function is decreased. Forced periodic oscillations of the applied current was investigated and resulted in a enhanced production of CO₂, but an intermediate selectivity towards N₂, as compared to constant current application. The effect of the cycling waveform, frequency and amplitude was studied and provided evidence that the synergy observed during the cycling experiment results from a favorable transient coverage of adsorbed species on the catalyst surface as the catalyst potential oscillates from negative to positive values. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Synthesis and characterization of polyhedral and quasi-sphere non-polyhedral Pt nanoparticles: effects of their various surface morphologies and sizes on electrocatalytic activity for fuel cell applications

In this article, polyhedral and non-polyhedral Pt nanoparticles were prepared by modified polyol method using AgNO₃ as a good structure-modifying agent. Their TEM and HRTEM images showed the particle size in the range of 8–16 nm for both the above cases. The structures and properties of the surfaces of Pt nanoparticles were investigated through cyclic voltammetry in dilute perchloric acid (HClO₄) electrolyte solution. A comparison of the electrocatalytic property in methanol electrooxidation was made. Here, the effects of polyhedral and non-polyhedral morphologies on their catalytic properties were studied. The results revealed that the special catalytic activity of quasi-sphere non-polyhedral Pt nanoparticles is higher than that of polyhedral Pt nanoparticles. In addition, Pt nanoparticles of un-sharp and quasi-sphere morphologies exhibit the tolerance to poisoning species better than that of Pt nanoparticles of sharp and polyhedral morphologies due to the various morphologies of the catalyst surfaces in the chronoamperometric plots. Therefore, these experimental evidences showed the morphology-dependent catalytic property according to the various morphologies and complexity of their catalyst surfaces.     

Binary-surfactant (Brij 35 + Tween 20) assisted preparation of highly dispersed Pt nanoparticles on carbon

Pt nanoparticles supported on Vulcan XC-72R, synthesized by a surfactant-stabilized colloidal method, exhibited excellent properties as anode catalyst for low-temperature fuel cell. The Pt/C catalyst prepared with binary-surfactant (Brij 35 + Tween 20) at 10 times CMC had an average particle size of 2.8 nm with quite a narrow distribution between 2 and 4 nm. Our preparation method resulted in complete reduction of Pt and full loading of Pt nanoparticles on the carbon. The home-made Pt/C catalyst showed higher EAS and better catalytic activity than a commercial Pt/C catalyst. The method used in this study provided an easy and reproducible procedure for the preparation of Pt nanoparticles supported on carbon.     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Catalytic soot oxidation in microscale experiments: Simulation of interactions between co-deposited graphitic nanoparticle agglomerates and platinum nanoparticles

Continuously regenerating catalytic soot traps are under development to reduce particulate emissions from diesel exhaust. A good understanding of the processes that take place during soot oxidation is needed to optimize diesel soot trap performance. To gain insight into these processes from the perspective of nanoparticle technology, the effects of catalyst particle size and the interparticle distance between soot and catalyst particles were measured. A model catalyst was prepared by depositing Pt nanoparticles on a SiO/SiO₂-coated transmission electron microscope (TEM) grid. A soot surrogate composed of graphitic nanoparticle agglomerates generated by laser ablation was deposited on the same surface. This system simulates, morphologically, catalytic soot traps used in practice. The reaction was carried out in a tubular flow reactor in which the gas phase simulated diesel exhaust gas, composed of a mixture of 10% O₂ and 1000 ppm NO with the remainder N₂. The progress of the carbon nanoparticle oxidation was monitored off-line by analysis of electron microscopy images of the agglomerates before and after reaction. This experimental method permitted the correlation of reaction rate with particle sizes and separation distances as well as catalyst surface area in the direct environs of the soot particles. The experimental results revealed no effect of Pt catalyst particle size in the range 7–31 nm on the rate of reaction. Also observed were a decrease in the rate of reaction with increasing distance between carbon agglomerates and catalyst particles and a linear dependence of the reaction rate on the fractional catalyst surface area coverage.     

Pt nanoparticles supported over Ce–Ti–O: the solvothermal and photochemical approaches for the preparation of catalytic materials

Ce–Ti–O supports with different Ce/Ti molar ratios were synthesized by the solvothermal method using hexadecyltrimethylammonium bromide. Pt nanoparticles were then supported by photochemical deposition. The shape, size, and structure of these materials were analyzed by high-resolution transmission electron microscopy. The single CeO₂ support was also prepared, consisting of agglomerated cubic particles ranging from ~3 to 8 nm. When titania was combined with ceria, a nanostructured architecture was produced, evidencing the strong influence of Ti in the support structure. Photodeposition of Pt nanoparticles is more efficient on Ce–Ti–O supports than in pristine CeO₂. Crystalline Pt nanoparticles (mainly of ~2 to 4 nm) were detected. The catalytic properties of the materials were tested in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. It was observed that Pt supported on Ce–Ti–O is more active and selective than Pt on CeO₂ or TiO₂ separately. The catalyst with 40 mol% Ce leads to total conversion of cinnamaldehyde in a few minutes; however, higher selectivity toward the desired product (cinnamyl alcohol) was obtained with higher amounts of Ce (50 mol%).     

Coupled catalytic oscillators: Beyond the mass-action law

We present Monte Carlo simulations of the reaction kinetics corresponding to two coupled catalytic oscillators in the case when oscillations result from the interplay between the reaction steps and adsorbate-induced surface restructuring. The model used is aimed to mimic oscillations on a single nm catalyst particle with two kinds of facets or on two catalyst particles on a support. Specifically, we treat the NO reduction by H(2) on a composite catalyst containing two catalytically active Pt(100) parts connected by an inactive link. The catalyst is represented by a rectangular fragment of a square lattice. The left- and right-hand parts of the lattice mimic Pt(100). With an appropriate choice of the model parameters, these sublattices play a role of catalytic oscillators. The central catalytically inactive sublattice is considered to be able only to adsorb NO reversibly and can be viewed as a Pt(111) facet or a support. The interplay of the reactions running on the catalytically active areas occurs via NO diffusion over the boundaries between the sublattices. Using this model, we show that the coupling of the catalytically active sublattices may synchronize nearly harmonic oscillations observed on these sublattices and also may result in the appearance of aperiodic partly synchronized oscillations. The spatio-temporal patterns corresponding to these regimes are nontrivial. In particular, the model predicts that, due to phase separation, the reaction may be accompanied by the formation of narrow NO-covered zones on the left and right sublattices near the boundaries between these sublattices and the central sublattice. Such patterns cannot be obtained by using the conventional mean-field reaction-diffusion equations based on the mass-action law. The experimental opportunities to observe the predicted phenomena are briefly discussed. (c) 2001 American Institute of Physics.     

Electrochemical vs. conventional promotion: A new tool to design effective, highly dispersed conventional catalysts

Pt-group metals exhibit strong Electrochemical Promotion (EP) by sodium during reactions related to emission control catalysis, such as NO reduction by hydrocarbons. Close similarities are found between electrochemically promoted catalysts and catalysts conventionally promoted and highly dispersed on large surface area supported materials. These similarities include (i) overall kinetic behaviour and (ii) the dependence of the activity and selectivity on Na loading. For example, using both methods of Na-promotion, the catalytic reduction of NO by propene exhibited rate enhancements by up to an order of magnitude accompanied by very pronounced increases of the system selectivity towards N₂. Among other things, our results serve to validate further the interpretation offered for the EP (or NEMCA) phenomenon. More importantly, they demonstrate that the insight obtained from EP studies can be used to design successfully effective catalyst formulations that were previously untried, thus opening up new areas for investigation in the frontiers between catalysis and electrochemistry. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y₂O₃-stabilized-ZrO₂ (YSZ), an O²⁻ conductor, TiO₂, a mixed conductor, and Nafion 117 solid polymer electrolyte (SPE), a H⁺ conductor and also on Pd films deposited on YSZ and β″-Al₂O₃ a Na⁺ conductor. Four catalytic systems were investigated, i.e. C₂H₆ oxidation on Pt/YSZ, C₂H₄ oxidation on Pd/YSZ and Pd/β″-Al₂O₃, C₂H₄ oxidation on Pt/TiO₂ and H₂ oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution. In all cases pronounced and reversible non-Faradaic electrochemical modification of catalytic rates was observed with catalytic rate enhancement up to 2000% and Faradaic efficiency values up to 5000. All reactions investigated exhibit a pronounced electrophobic behaviour which is due to the weakening of chemisorptive oxygen bond at high catalyst potentials. Ethane oxidation, however, also exhibits electrophilic behaviour at low potentials due to weakened binding of carbonaceous species on the surface. The general features of the phenomenon are similar for all four cases presented here showing that the NEMCA effect is a general, electrochemically induced, promoting catalytic phenomenon not depending on the reaction and the type of supporting electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Bimetallic alloy Pt/Ag nanoparticles with enhanced catalytic activity for formic acid oxidation

Here, we report the synthesis of Pt/Ag bimetallic alloy catalyst through combining the ion implantation and electrodeposition method. Ag nanoparticles are employed as the seeds for the growth of Pt nanoparticles. Pt/Ag alloy catalyst demonstrates much higher catalytic activity than pure Pt catalyst, which is about three times more active on the basis of equivalent Pt electrochemically active surface area than that of the pure Pt catalyst. The ion implantation of Ag efficiently enhances the catalytic activity of Pt catalyst for formic acid oxidation.     

An X-ray photoelectron spectroscopy investigation of a novel Pd–Pt colloid catalyst

A Bi-promoted charcoal-supported Pd–Pt oxidation catalyst prepared from colloidal NOct₄Cl-stabilized Pd–Pt nanoparticles was investigated by means of X-ray photoelectron spectroscopy (XPS). Pd 3d, Pt 4f, Bi 4f, C 1s and O 1s spectra of the colloid, the supported colloid catalyst and a conventional charcoal-supported Pd–Pt/Bi coimpregnation catalyst (Degussa, CEF 196 RA/W) were measured. Both catalysts were explored unused (as-prepared) and after deactivation in the heterogeneous catalytic oxidation of glucose to gluconic acid. The spectra are analyzed to elucidate the higher starting activity of the Pd–Pt/Bi/C colloid catalyst, especially the role of the promotor Bi and the mechanisms leading to catalyst deactivation. The higher starting activity of the colloid catalyst is explained by the presence of completely reduced Pd and Pt, threevalent Bi and a smaller particle size in contrast to the conventional catalyst which contains partly oxidized Pd and a non-unique chemical state of Bi. The deactivation of both catalysts is suggested to be due to metal dissolution, particle growth and chemical poisoning.     

Heterogeneous adsorption and catalytic oxidation of benzene, toluene and xylene over spent and chemically regenerated platinum catalyst supported on activated carbon

The heterogeneous adsorption and catalytic oxidation of benzene, toluene and o-xylene (BTX) over the spent platinum catalyst supported on activated carbon (Pt/AC) as well as the chemically treated spent catalysts were studied to understand their catalytic and adsorption activities. Sulfuric aqueous acid solution (0.1N, H₂SO₄) was used to regenerate the spent Pt/AC catalyst. The physico-chemical properties of the catalysts in the spent and chemically treated states were analyzed by using nitrogen adsorption-desorption isotherm and elemental analysis (EDX). The gravimetric adsorption and the light-off curve analysis were employed to study the BTX adsorption and oxidation on the spent catalyst and its modified Pt/AC catalysts. The experimental results indicate that the spent Pt/AC catalyst treated with the H₂SO₄ aqueous solution has a higher toluene adsorption and conversion ability than that of the spent Pt/AC catalyst. A further studies of H₂SO₄ treated Pt/AC catalyst on their catalytic and heterogeneous adsorption behaviours for BTX revealed that the activity of the H₂SO₄ treated Pt/AC catalyst follows the sequence of benzene > toluene > o-xylene. The adsorption equilibrium isotherms of BTX on the H₂SO₄ treated Pt/AC were measured at different temperatures ranging from 120 to 180 °C. To correlate the equilibrium data and evaluate their adsorption affinity for BTX, the two sites localized Langmuir (L2m) isotherm model was employed. The heterogeneous surface feature of the H₂SO₄ treated Pt/AC was described in detail with the information obtained from the results of isosteric enthalpy of adsorption and adsorption energy distributions. Furthermore, the activity of H₂SO₄ treated Pt/AC about BTX was found to be directly related to the Henry's constant, isosteric enthalpy of adsorption and adsorption energy distribution functions.     

First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions

First-principles-based kinetic Monte Carlo simulation was used to track the elementary surface transformations involved in the catalytic decomposition of NO over Pt(100) and Rh(100) surfaces under lean-burn operating conditions. Density functional theory (DFT) calculations were carried out to establish the structure and energetics for all reactants, intermediates and products over Pt(100) and Rh(100). Lateral interactions which arise from neighbouring adsorbates were calculated by examining changes in the binding energies as a function of coverage and different coadsorbed configurations. These data were fitted to a bond order conservation (BOC) model which is subsequently used to establish the effects of coverage within the simulation. The intrinsic activation barriers for all the elementary reaction steps in the proposed mechanism of NO reduction over Pt(100) were calculated by using DFT. These values are corrected for coverage effects by using the parametrized BOC model internally within the simulation. This enables a site-explicit kinetic Monte Carlo simulation that can follow the kinetics of NO decomposition over Pt(100) and Rh(100) in the presence of excess oxygen. The simulations are used here to model various experimental protocols including temperature programmed desorption as well as batch catalytic kinetics. The simulation results for the temperature programmed desorption and decomposition of NO over Pt(100) and Rh(100) under vacuum condition were found to be in very good agreement with experimental results. NO decomposition is strongly tied to the temporal number of sites that remain vacant. Experimental results show that Pt is active in the catalytic reaction of NO into N₂ and NO₂ under lean-burn conditions. The simulated reaction orders for NO and O₂ were found to be +0.9 and ?0.4 at 723?K, respectively. The simulation also indicates that there is no activity over Rh(100) since the surface becomes poisoned by oxygen.     

The relationship of the catalytic activity and the open-circuit potential of Pt interfaced with YSZ

This study deals with the relationship between open-circuit potential and catalytic activity of the system Pt/YSZ. Temperature-programmed desorption (TPD) of oxygen was carried out in order to investigate the link between the oxygen coverage and the potential. Then, catalytic activity in parallel with the potential value was measured between 200 °C and 500 °C for NO oxidation, C₃H₈ combustion, C₃H₆ combustion and the selective catalytic reduction (SCR) of NO by C₃H₆ in the presence of oxygen. It was found that potential measurement can give precious information on the competitive adsorption between oxygen and other reactants. Finally, it seems to be also a good way to anticipate the NEMCA behavior for reactions, which involve oxygen. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions

Experimental studies employing surface science methods indicate that kinetic oscillations, chaos, and pattern formation in heterogeneous catalytic reactions often result from the interplay of rapid chemical reaction steps and relatively slow complementary processes such as oxide formation or adsorbate-induced surface restructuring. In general, the latter processes should be analysed in terms of theory of phase transitions. Therefore, the conventional mean-field reaction–diffusion equations widely used to describe oscillations in homogeneous reactions are strictly speaking not applicable. Under such circumstances, application of the Monte Carlo method becomes almost inevitable. In this review, we discuss the advantages and limitations of employing this technique and show what can be achieved in this way. Attention is focused on Monte Carlo simulations of CO oxidation on (1 0 0) and (1 1 0) single-crystal Pt and polycrystal Pt, Pd and Ir surfaces and of NO reduction by CO and H₂ on Pt(1 0 0). CO oxidation on supported nanometre-sized catalyst particles and NO reduction on composite catalysts are also discussed. The results show that with current computer facilities the MC technique has become an effective tool for analysing temporal oscillations and pattern formation on the nanometre scale in catalytic reactions occurring on both single crystals and supported particles.     

Effect of combination of noble metals and metal oxide supports on catalytic reduction of NO by H2

We investigated the effects of combination of noble metals M (Rh, Pd, Ir, Pt) and metal oxide supports S (Al₂O₃, SiO₂, ZrO₂, CeO₂) on the NO + H₂ reaction using planar catalysts with M/S two layered thin films on Si substrate. In this study, NO reduction ability per metal atom were evaluated with a specially designed apparatus employing pulse valves for the injection of reactant molecules onto catalysts and a time-of-flight mass spectrometer to measure multiple transient products: NH₃, N₂ and N₂O simultaneously as well as with an atomic force microscopy to observe the surface area of metal particles. The catalytic performances of Rh and Ir catalysts were hardly affected by a choice of a metal oxide support, while Pd and Pt catalysts showed different catalytic activity and selectivity depending on the metal oxide supports. This assortment is consistent with ability to dissociate NO depending on metals without the effect of any support materials. There, the metals to the left of Rh and Ir on the periodic table favor dissociation of NO and those to the right of Pd and Pt tend to show molecular adsorption of NO. Therefore, the catalytic property of noble metals could be assorted into two groups, i.e. Rh and Ir group whose own property would mainly dominate the catalytic performance, and Pd and Pt group whose interaction with metal oxides supports would clearly contribute to the reaction of NO with H₂. NO reduction activity of Pd and Pt was found to be promoted above that of Rh and Ir, provided that Pd and Pt were supported by CeO₂ and ZrO₂.     

Growth of single-walled carbon nanotubes from size-selected catalytic metal particles

Single-walled carbon nanotubes (SWNTs) were synthesized using size-controlled catalyst nanoparticles created by the pulsed laser ablation method. Specifically, the alloy particles (Co/Mo or Co/Pt) were prepared by ablation of the target alloy materials in an inert gas atmosphere. Size selection was performed using a differential mobility analyzer (DMA). The obtained nanoparticles were deposited on a quartz substrate from which SWNTs were grown by the alcohol catalytic CVD (ACCVD) technique that was developed by the authors group. AFM and Raman scattering analysis revealed that SWNTs were successfully synthesized. It seems the Co/Mo alloy catalyst was more effective for the synthesis of SWNTs than the Co/Pt catalyst, though this is a preliminary result to be further investigated. PACS 36.40.-c; 61.46.+w; 65.80.+n; 78.30.Na; 81.07.de     

Synthesis of platinum and platinum–ruthenium-modified diamond nanoparticles

With the aim of developing dimensionally stable-supported catalysts for direct methanol fuel cell application, Pt and Pt–Ru catalyst nanoparticles were deposited onto undoped and boron-doped diamond nanoparticles (BDDNPs) through a chemical reduction route using sodium borohydride as a reducing agent. As-received commercial diamond nanoparticles (DNPs) were purified by refluxing in aqueous nitric acid solution. Prompt gamma neutron activation analysis and transmission electron microscopy (TEM) techniques were employed to characterize the as-received and purified DNPs. The purified diamond nanoparticulates, as well as the supported Pt and Pt–Ru catalyst systems, were subjected to various physicochemical characterizations, such as scanning electron microscopy, energy dispersive analysis, TEM, X-ray diffraction, inductively coupled plasma-mass spectrometry, X-ray photoelectron spectroscopy, and infrared spectroscopy. Physicochemical characterization showed that the sizes of Pt and Pt–Ru particles were only a few nanometers (2–5 nm), and they were homogeneously dispersed on the diamond surface (5–10 nm). The chemical reduction method offers a simple route to prepare the well-dispersed Pt and Pt–Ru catalyst nanoparticulates on undoped and BDDNPs for their possible employment as an advanced electrode material in direct methanol fuel cells.     

Thermal desorption study of Oxygen adsorption on Pt, Ag & Au films deposited on YSZ

The Thermal Desorption or Temperature Programmed Desorption (TPD) technique has been used for the study of oxygen adsorption on Pt, Ag and Au catalyst films deposited on YSZ. The catalyst film was deposited on the one side of the YSZ specimen while on the other side gold counter and reference electrodes were deposited, constructing a three-electrode electrochemical cell similar to those used in Electrochemical Promotion studies. Oxygen adsorption has been carried out either by exposing the samples to gaseous oxygen (gas phase adsorption) or by the application of a constant current between the catalyst/working electrode and the counter electrode (electrochemical adsorption) or by mixed gas phase and electrochemical adsorption. Oxygen adsorption was carried out at temperatures between 200 and 480 °C. After exposure to gaseous oxygen, normal adsorbed atomic oxygen species have been observed on Pt and Ag surfaces while there was no detectable amount of adsorbed oxygen on Au. Electrochemical O²⁻ pumping to Pt, Ag and Au catalyst films creates strongly bonded “backspillover” anionic oxygen, along with the more weakly bonded atomic oxygen. Electrochemical O²⁻ pumping to Pt, Ag and Au catalyst films in presence of preadsorbed oxygen creates strongly bonded “backspillover” anionic oxygen, with a concomitant pronounced lowering of the T_p of the more weakly bound preadsorbed atomic oxygen. The two oxygen species co-exist on the surface. The activation energy for oxygen desorption or, equivalently, the binding strength of adsorbed oxygen was found to decrease linearly with increasing catalyst potential, for all three metal electrodes. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland Sept. 13–19, 1997