Dissociative Adsorption Of Oxygen Molecules Research Articles

Wolkenstein’s Model of Size Effects in CO Oxidation by Gold Nanoparticles

The Wolkenstein’s theory of catalysis and the d-band theory of formation chemical bonds between transition metal catalysts and adsorbates were used to develop the approach applied to the kinetics of CO oxidation by gold nanoparticles. In the model, within the framework of the mechanism of the reaction going through dissociative adsorption of oxygen molecules and reaction with gas-phase CO molecules, weak and strong chemisorption states of intermediates (O, CO2) were taken into account in the kinetic equations by introducing reversible electronic steps corresponding to electron transfers between the intermediates and the catalyst. As a result, we obtain the expression for the reaction rate, which exhibits a volcano-shape dependence upon the size of the gold nanoparticles at the conditions when the intermediates fractions are not small compared to the empty active sites of the catalyst. It is supposed that the approach can be also applied to the Langmuir-Hinshelwood mechanism.

Surface plasmon coupled chemiluminescence during adsorption of oxygen on magnesium surfaces.

The dissociative adsorption of oxygen molecules on magnesium surfaces represents a non-adiabatic reaction exhibiting exoelectron emission, chemicurrent generation, and weak chemiluminescence. Using thin film Mg/Ag/p-Si(111) Schottky diodes with 1 nm Mg on a 10-60 nm thick Ag layer as 2π-photodetectors, the chemiluminescence is internally detected with a much larger efficiency than external methods. The chemically induced photoyield shows a maximum for a Ag film thickness of 45 nm. The enhancement is explained by surface plasmon coupled chemiluminescence, i.e., surface plasmon polaritons are effectively excited in the Ag layer by the oxidation reaction and decay radiatively leading to the observed photocurrent. Model calculations of the maximum absorption in attenuated total reflection geometry support the interpretation. The study demonstrates the extreme sensitivity and the practical usage of internal detection schemes for investigating surface chemiluminescence.

Edges of FeO/Pt(111) Interface: A First-Principle Theoretical Study

An understanding of the reaction mechanisms of oxide/metal bicatalysts is important for their design to achieve better catalytic performance. Using the density functional theory calculations based on the GGA+U approach, the ferrous oxide (FeO) clusters on Pt(111) were systematically investigated as a model of oxide/metal bicatalyst since they showed high catalytic capacity on the preferential oxidation of carbon monoxide. Our calculations showed that the role of the coordinatively unsaturated ferrous (CUF) atoms at the edges of the FeO/Pt(111) interface was to help the dissociative adsorption of oxygen molecules. The oxygen atoms at the edges in the intermediate were more chemically active according to the analysis of their electronic properties. They can selectively attract the carbon monoxide molecules to oxide them. After the desorption of carbon dioxide molecules, the CUF atoms at the edges can be reproduced. The high efficiency and selectivity of FeO/Pt(111) bicatalysts were, therefore, explained usi...

DFT studies of oxidation routes for Pd9 clusters supported on γ-alumina

This research is focused on the analysis of adsorbed bare and oxidized Pd(9) nanoparticles supported on γ-alumina. From first-principle density functional theory calculations, several configurations, charge transfer and electronic density of states have been analyzed in order to determine feasible paths for the oxidation process. Studies of Pd/PdO nanoparticles prove that they are stable at γ-alumina supports. It is shown that the Pd(9) nanoparticle favors dissociative adsorption of oxygen molecules. The most energetically preferable sites for adsorption are close to the contact between the cluster and the support, where one oxygen atom interacts with a 5-coordinated aluminium atom, and the remaining oxygen is in contact with the closest palladium atom. After first dissociation, one oxygen atom creates a bridge between the palladium atom and the 5-coordinated aluminium atom and the second oxygen atom moves to the top of the Pd(9) cluster, making a bridge between two palladium atoms. Subsequent dissociations arise analogously, with the difference that oxygen atoms in the second layer of the palladium cluster occupy hollow sides of the cluster. Investigation of the charge distribution in each oxidation step reveals that charge transfer increases towards the Pd/PdO nanoclusters. The electronic density of states indicates that gradual oxygen molecule adsorption and dissociation shift the highest states of the Pd/PdO nanoparticles in different ways. The overall investigation is found to be beneficial for studying methane oxidation.

Oxygen electroreduction at catalysts PtM (M = Co, Ni, or Cr)

Bimetallic catalysts PtM (M = Co, Ni, or Cr) are synthesized. They exceed purely platinum commercial catalyst E-TEK (20 wt % Pt) in its mass activity (mA/mgPt) and specific activity (mA/cPt2) in the oxygen reduction reaction. According to XRD data, the high-temperature synthesis involving metal N4-complexes, chloroplatinic acid, and XC72 carbon black as precursors, yields alloys (or solid solutions) of the metals. The higher activity of the bimetallic catalyst PtCo/C is likely to be caused by the practically entire formation of solid solutions (Pt3Co and PtCo), unlike PtNi and PtCr where nickel and chromium exist also as oxides that decorate the electrode surface and partly block active centers. It is shown that the mechanism of the oxygen reduction reaction at the synthesized catalysts is similar to that of oxygen reduction at the purely platinum catalyst. The slow stage in the process is transfer of the 1st electron; at potentials more positive than 0.6 V the reaction mainly yields water. The higher electrocatalytic activity of the bimetallic systems is caused by the alloy formation, which leads to changes in the bond length between platinum atoms. The achieving of the optimal bond length, as a result of the alloy formation, provides appropriate conditions for dissociative adsorption of oxygen molecules; the surface coverage with oxygen-containing particles adsorbed from water (which block active centers for O2 adsorption) decreased. The increase in the activity may also be caused by the formation of the “core-shell” structures whose surface is enriched with platinum whose surface properties are changed under the ligand action of the core formed by the metal alloy

Real-time electron dynamics simulation of the adsorption of an oxygen molecule on Pt and Au clusters

The time correlation between the electron dynamics and the ion dynamics on the dissociative adsorption of oxygen molecules on a platinum surface and a gold surface modeled by a cluster has been investigated using the Ehrenfest TDDFT molecular dynamics method. It is found that the electron transfer from the surface to O 2 occurs more easily on the Pt(0 0 1) surface than on the Au(0 0 1) surface, and that the transfer of one electron from Pt to O 2 is completed when the O 2 reaches ∼1.6 Å from the Pt surface.

Degradation of Carbon Supports in PEFC Cathode Electrode Investigated by Electrochemical STM: Effects of Platinum and Oxygen on Enhanced Carbon Corrosion

The corrosion of highly oriented pyrolytic graphite (HOPG) model electrodes' surfaces with platinum nanoparticles and oxygen molecules in an aqueous solution was investigated using electrochemical scanning tunneling microscopy in order to understand the degradation of carbon supports found in polymer electrolyte fuel cells (PEFCs) cathode electrode. Structural changes of HOPG surfaces with platinum nanoparticles and oxygen due to the corrosion were observed directly at atomic levels under the real conditions at which PEFCs operate. The STM observations revealed that the graphite surface oxide formation due to corrosion of the HOPG surfaces with platinum nanoparticles occurred initially at step edge sites, i.e., at defect sites and then grew toward terraces. This indicates that platinum particles and oxygen on the HOPG surface accelerated the corrosion. Surface oxygen species formed through the partial oxidation of water and the dissociative adsorption of oxygen molecules likely enhance the oxidation rate of graphitic oxide species.

Impedance spectroscopy of perovskite air electrodes for SOFC prepared by laser ablation method

Perovskite thin film electrode for SOFC was prepared on a YSZ single crystal by laser ablation method. The deposited film appeared in pore-free and dense polycrystals oriented in a specific direction. The crystal orientation was controlled by the surface structure of the single crystal substrate. The electrochemical impedance spectroscopy was used to disintegrate the electrode reaction into the constituting basic several processes. For each process, the pO 2 and temperature dependence was discussed. In the surface reactions, the dissociative adsorption of oxygen molecules on La 0.8Sr 0.2CoO 3 (LSC) is rate limiting, while the charge transfer process becomes a limiting step on La 0.8Sr 0.2MnO 3 (LSM). The kinetics of these processes shows dependence on the kind of electrode material and the crystal plane of perovskite surface.

Oxygen diffusion through silver cathodes for solid oxide fuel cells

The oxygen reduction mechanism at the interface O 2,Ag/YSZ (yttria-stabilized zirconia) was investigated by impedance spectroscopy and current-overpotential recording. Oxygen partial pressure was varied between 10 −4 and 1 atm, and temperature between 600 and 850°C. Both dense and porous silver electrodes were studied. Bulk diffusion of oxygen atoms through the solid silver was identified as the rate-determining mechanism. A second, smaller contribution is ascribed to dissociative adsorption of oxygen molecules on the silver electrode. Good agreement with known data for the diffusion and solubility of oxygen in silver is obtained.

Low‐Temperature Operation of Solid Electrolyte Oxygen Sensors Using Perovskite‐Type Oxide Electrodes and Cathodic Reaction Kinetics

The lowest limit of operation temperature for the oxide solid electrolyte oxygen sensors was affected by the electrode materials. The sensors with silver and cobalt‐based perovskite‐type oxide electrodes exhibited theoretical EMF's at lower temperatures than those with Pt electrodes. In particular, the sensor with attained theoretical EMF even at 200°C. The electrode materials operative at low temperatures tend to have low cathodic over‐potential. From the kinetic analysis of the exchange current, the rate‐determining step is expected to be the dissociative adsorption of oxygen molecules at high temperatures and the charge transfer process at low temperatures.

Cathode and anode materials and the reaction kinetics for the solid oxide fuel cell

The I−V characteristics of a solid oxide fuel cell are strongly affected by the electrode materials when the cell is operated at low temperatures. Perovskite-type oxide, La-Sr-Co-O, La-Sr-Mn-O system, was effective as cathode of the fuel cell. The rate determining step is expected to be the dissociative adsorption of oxygen molecules at high temperatures, and the charge transfer process at low temperatures. The cell with an yttria stabilized zirconia electrolyte exhibited larger cathodic polarization than that with a ceria-based oxide, and large anodic resistance. Among metals, Ni anode exhibited a small overpotential.

Kinetics of oxygen absorption by .alpha.-zirconium

The rate of oxygen absorption by polycrystalline {alpha}-zirconium has been measured at oxygen pressures of 6.7 {times} 10{sup {minus}5} and 1.3 {times} 10{sup {minus}4} Pa over the temperature range 973-1098 K by using samples prepared under ultrahigh-vacuum conditions, and the mechanism for the early stage of absorption is discussed. The absorption rate is explained by using a model in which the absorption process comprises three successive steps: the dissociative adsorption of oxygen molecules on the zironium surface with an interaction between oxygen adatoms, the transfer of adatoms at the surface sites to the outermost bulk sites, and the diffusion of oxygen atoms into the bulk. The absorption rate is found to be limited by adsorption under the conditions studied. Kinetic parameters in the model have been evaluated. The activation energy for adsorption is (7.4 {plus minus} 0.6) {times} 10{sup {minus}20} J per O atom, and the energy of the interaction, which is attractive, is {minus}4 {times} 10{sup {minus}21} J per O-O pair. The adsorption site has a potential energy lower by (2.2 {plus minus} 0.1) {times}10{sup {minus}19} J per O atom than the site in the bulk.

Electrode reaction at Pt, O 2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model

This study aims to make clear the reaction kinetics at Pt, O 2(g)/zirconia electrodes in the oxygen partial pressure, P O 2 , of ∼ 10 −4 − 1 atm at ∼ 400–∼800°C. By a critical review on the preceding studies, problems were pointed out in the application of the Langmuir adsorption isotherm to the P O 2 dependence of electrode conductance, in the assumption of electric double layer at the electrode interface, and of the inconsistency between the recent reaction model of surface diffusion controlled kinetics and the absolute rate theory. It was shown that the charge transfer kinetics cannot be the rate determining step (RDS) of the electrode reaction. The possible RDS was concluded to be either (i) dissociative adsorption of oxygen molecules on the Pt surface or (ii) surface diffusion of O ad atoms on the Pt surface to the Pt/zirconia contact. The diffusion of O ad atoms on the Pt surface was considered to be proportional to θ(1−θ)(∂μ O/∂ x), where θ is the occupancy of O ad atoms on Pt and μ O is the oxygen chemical potential on the Pt surface. The rate equation, current-potential relationship, and the electrode conductivity, σ E, were calculated for the cases the RDS is (i) and (ii), respectively. By comparing the calculated σ E versus log P O 2 relationship with the reported ones, it was shown that the RDS is (i) for T ≲ 500°C and is (ii) for T ≳ 600°C. In the former case, σ E is essentially constant irrespective to P O 2 , and in the latter case, σ E maximum appears on the σ E versus log P O 2 relations.

Electrode reaction at Pt, O 2(g)/stabilized zirconia interfaces. Part II: Electrochemical measurements and analysis

In order to make clear the reaction kinetics at the Pt, O 2(g)/stabilized zirconia electrodes, measurements were made on the dc electrode conductivity, σ E, between 370 and 800°C and anodic and cathodic polarization at 700°C in the P O 2 range of 10 −4−1 atm. Above 600°C, the log σ E versus log P O 2 plot showed maximum. Below 500°C, σ E was essentially constant irrespective of P O 2 . At 700°C, when the P O 2 was high, the limiting current appeared for anodic polarization. In the lower P O 2 , the limiting current was observed at the cathodic polarization. The results were analyzed based on the reaction model proposed in Part I of the present paper. Above 600°C, the diffusion of adsorbed oxygen atoms on the Pt surface was confirmed to be the rate determining step. The activation energy of the surface diffusion of O ad atoms on Pt was found to be 41 ± 2.5 Kcal/mol, and the heat of adsorption of O ad on Pt was 53 ± 4 Kcal/mol. Below 500°C, the reaction rate was determined by the dissociative adsorption of oxygen molecules on the Pt surface near the Pt/zirconia boundary. The activation energy of this reaction was about 37 ± 4 Kcal/mol.

Absorption of gaseous oxygen by liquid cobalt, copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts’ method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600°C, and for copper was 1250°C. For initial oxygen pressures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm3 (STP) O2/cm2 of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm3 (STP) O2/cm2 under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and proportional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissociative adsorption of oxygen molecules at the gas/oxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative adsorption of oxygen molecules at the gas/oxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen absorption rate compared to that of liquid cobalt.

Absorption of gaseous oxygen by liquid iron

A constant volume technique was used to measure the rate of absorption of oxygen by liquid iron. The absorption process is found to proceed in two stages. When pure oxygen comes into contact with the surface of molten iron, an extremely rapid disappearance of oxygen from the gas phase accompanied by strong local super-heating of the melt at the gas/metal interface is observed. The rate of oxygen uptake during this stage is found to be dependent upon oxygen pressure and gas/metal interfacial area, but essentially independent of temperature, volume, and stirring conditions of the melt. The second phase commences after a few seconds with simultaneous cooling of the melt surface and the appearance at the gasmetal interface of a distinct third phase. The rate of absorption of oxygen during the second stage is markedly lower than the first stage. The presence of dissolved oxygen in the liquid iron has no influence on the absorption kinetics during either stage. A theoretical interpretation of the second stage is presented. A mechanism is proposed involving dissociative adsorption of oxygen molecules at the oxide/gas interface.