Oxygen reduction reactions in the SOFC cathode of Ag/CeO2

The interactions between oxygen molecules and a silver surface or a CeO₂(111) supported atomic layer of silver are predicted using first-principles calculations based on spin polarized DFT with PAW method. The juncture between the CeO₂(111), the atomic layer of silver, and O₂ represents a triple-phase boundary (TPB) whereas the interface between silver surfaces and O₂ corresponds to a 2-phase boundary (2PB) in a solid oxide fuel cell (SOFC). Results suggest that the O₂ dissociation process on a monolayer of silver supported by CeO₂(111) surfaces (or TPB) with oxygen vacancies has lower reaction barrier than on silver surfaces (or 2PB), and the dissociated oxygen ions can quickly bond with subsurface Ce atom via a barrierless and highly exothermic reaction. The oxygen vacancies at TPB are found to be responsible for the lower energy barrier and high exothermicity because of the strong interaction between subsurface Ce and adspecies, implying that oxygen molecules prefer being reduced at TPB than on silver surfaces (2PB). The results suggest that, for a silver-based cathode in a SOFC, the adsorption and dissociation of oxygen occur rapidly and the most stable surface oxygen species would be the dissociated oxygen ion with − 0.78|e| Bader charges; the rate of oxygen reduction is most likely limited by subsequent processes such as diffusion or incorporation of the oxygen ions into the electrolyte.     

Electrode reaction at Pt,O2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model

This study aims to make clear the reaction kinetics at Pt, O₂(g)/zirconia electrodes in the oxygen partial pressure, P_O2, 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_O2 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_O2 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_O2, and in the latter case, σ_E maximum appears on the σ_E versus log P_O2 relations.     

Interpretation of kinetics of iron surface oxidation involving the real structure of single crystal samples

The kinetics of oxidation of iron surface has been studied by AES method. The effects of oxygen diffusion into the lattice defects have been considered in the discussion of the mechanism of the oxygen adsorption. The real sticking coefficient has been determined as a function of oxygen coverage (S=1?θ in the range of 0<θ<0.9). The oxidation of iron surface occurs in two steps. At the first step the dissociative oxygen adsorption occurs for the coverage 0<θ _O<1 and the rate of the oxygen molecule adsorption is limiting. At the second step, in the range of oxygen coverage 1<θ _O<2, the reconstruction of the iron surface occurs with the formation of free adsorption sites. At this step the sticking coefficient of oxygen is almost constant (S≈0.1).     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

A kinetic Monte Carlo/UBI-QEP model of O2 adsorption on fcc (1 1 1) metal surfaces

The previously developed kinetic Monte Carlo model of molecular oxygen adsorption on fcc (1 0 0) metal surfaces has been extended to fcc (1 1 1) surfaces. The model treats uniformly all elementary steps of the process—O₂ adsorption, dissociation, recombination, desorption, and atomic oxygen hopping—at various coverages and temperatures. The model employs the unity bond index—quadratic exponential potential (UBI-QEP) formalism to calculate coverage-dependent energetics (atomic and molecular binding energies and activation barriers of elementary steps) and a Metropolis-type algorithm including the Arrhenius-type reaction rates to calculate coverage- and temperature-dependent features, particularly the adsorbate distribution over the surface. Optimal values of non-energetic model parameters (the spatial constraint, a travel distance of “hot” atoms, attempt frequencies of elementary steps) have been chosen. Proper modifications of the fcc (1 0 0) model have been made to reflect structural differences in the fcc (1 1 1) surface, in particular the presence of two different hollow sites (fcc and hcp). Detailed simulations were performed for molecular oxygen adsorption on Ni(1 1 1). We found that at very low coverages, only O₂ adsorption and dissociation were effective, while O₂ desorption and O₂ and O diffusion practically did not occur. At a certain O + O₂ coverage, the O₂ dissociation becomes the fastest process with a rate one-two orders of magnitude higher than adsorption. Dissociation continuously slows down due to an increase in the activation energy of dissociation and due to the exhaustion of free sites. The binding energies of both molecular and atomic oxygen decrease with coverage, and this leads to greater mobility of atomic oxygen and more pronounced desorption of molecular oxygen. Saturation is observed when the number of adsorbed molecules becomes approximately equal to the number of desorbed molecules. Simulated coverage dependences of the sticking probability and of the atomic binding energy are in reasonable agreement with experimental data. From comparison with the results of the previous work, it appears that the binding energy profiles for Ni(1 1 1) and Ni(1 0 0) have similar shapes, although at any coverage the absolute values of the oxygen binding energy are higher for the (1 0 0) surface. For metals other than Ni, particularly Pt, the model projections were found to be too parameter-dependent and therefore less certain. In such cases further model developments are needed, and we briefly comment on this situation.     

Electronic decoupling of surface layers from bulk and its influence in oxidation catalysis: A molecular beam study

Interactions between oxygen and Pd-surfaces have important implications, especially towards oxidation reactions, and influence of subsurface oxygen to oxidation reactions is the focus of the present study. In our efforts to understand the above aspects, CO oxidation reactions have been carried out with mixed molecular beam (MB), consisting CO and O₂, on Pd(1 1 1) surfaces under a wide variety of conditions (T = 400-900 K, CO:O₂ = 7:1 to 1:10). A new aspect of the above reaction observed in the transient kinetics regime is the evidence for oxygen diffusion into Pd subsurface layers, and its significant influence towards CO oxidation at high temperatures (≥600 K). Interesting information derived from the above studies is the necessity to fill up the subsurface layers with oxygen atoms to a threshold coverage (θ_O-sub), above which the reactive CO adsorption occurs on the surface and simultaneous CO₂ production begins. There is also a significant time delay (Γ) observed between the onset of oxygen adsorption and CO adsorption (and CO₂ production). Above studies suggest an electronic decoupling of oxygen covered surface and subsurface layers, which is slightly oxidized, from the metallic bulk, which induces CO adsorption at high temperatures and simultaneous oxidation to CO₂.     

Oxidation of TiNi surface with hyperthermal oxygen molecular beams

We report results of our detailed studies on the initial oxidation process of TiNi with a 2 eV hyperthermal oxygen molecular beam (HOMB) and thermal O₂ in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy (XPS) measurements in conjunction with synchrotron radiation (SR). In the early stage of oxidation, the precursor mediated dissociative adsorption is the dominant reaction mechanism. In the oxide formation process at higher O coverage, HOMB has the advantage in the dissociation process of O₂ molecule and can grow TiO₂ layers with the underlying TiO_x-rich and/or Ni-rich layers. We succeeded in fabricating thick Ni-free TiO₂ layer, possibly blue colored rutile TiO₂, combining HOMB and surface annealing.     

Theoretical analysis of reactivity on Pt(1 1 1) and Pt-Pd(1 1 1) alloys

The reactivity of Pt-Pd alloy surfaces towards the oxygen reduction reaction is studied as a function of the alloy overall composition and surface atomic distribution and compared to that on pure Pt surfaces. The systems include Pd and Pt monolayers on various substrates and Pt₃Pd, PtPd and PtPd₃ surfaces of ordered alloys. Reactivity is evaluated on the basis of the adsorption strength of oxygenated compounds which are intermediate species of the four-electron oxygen reduction reaction, separating the effect of the first electron-proton transfer from that of the three last electron-proton transfer steps. None of the alloys are found to provide better sites than those of pure Pt both for O₂ dissociation and for the reduction of O and OH to water; with the skin surfaces being the closest to pure Pt. The results are discussed in relation to those found in 10-atom clusters of similar compositions and to experiments.     

Oxidation of ethylene on Pd(100); bonding of ethylene and scavenging of dehydrogenation fragments by surface oxygen

The adsorption and reaction of C₂H₄ on oxygen covered Pd(100) was studied with high resolution electron energy loss spectroscopy (EELS) and temperature programmed reaction spectroscopy (TPRS). The clean Pd(100) surface at 300 K was exposed to O₂ to produce atomic oxygen in the p(2×2) structure for coverages between 0.05 and 0.25. The EELS and TPRS measurements were conducted following saturation coverage of the oxygen covered surface by C₂H₄ at 80 K. Both the di-σ- and π-bonded forms of C₂H₄ were stable on the surface for θ_O less than 0.25. The π-bonded form desorbed without reaction between 100 and 300 K, but the di-σ-bonded form underwent dehydrogenation above 250 K. The C₂H₄ dehydrogenation products were reactive towards atomic oxygen and produced H₂, H₂O, CO, CO₂, and adsorbed C. Oxygen preadsorption inhibited C₂H₄ Oxidation by limiting the formation of di-σ-bonded C₂H₄, and the fully developed p(2×2)O overlayer, corresponding to θ_O = 0.25, was sufficient to block completely the reaction of ethylene. The extent of reaction decreased in a 2:1 ratio to the increase in oxygen coverage, and indicated that oxygen islands blocked C₂H₄ dissociation. Only the π-bonded form of C₂H₄ was stable on the surface for θ_O greater than 0.25; the saturation coverage of π-bonded C₂H₄ of 0.25 was the same as for clean Pd(100).     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Electron beam induced effects on gas adsorption utilizing auger electron spectroscopy: Co and O2 on Si: I. Adsorption studies

The effect of electron beam monitored gas adsorption on the clean Si surface is studied using Auger electron spectroscopy. It is shown that the beam affects the AES adsorption signal of CO and O₂ on Si by dissociating the adsorbed molecules on the surface and subsequently promoting diffusion of atomic oxygen into the bulk. A qualitative explanation of the adsorption data is presented and the initial sticking probability of O₂ on Si (111) surface is estimated to be S0 = 0.21.     

An oxygen reduction catalytic process through superoxo adsorption states on n-type doped h-BN: A first-principles study

Dioxygen adsorption and activation on metal-ligand systems are the key elements for biological oxidative metabolisms and also catalyst design for the oxygen reduction reaction (ORR). We show, through first-principles calculations, that similar dioxygen adducts can form on metal-free n-type doped hexagonal boron nitride (h-BN) nanostructures. The density of electron donors determines the charge state of dioxygen, either in superoxo and peroxo, which exactly correlates with the ‘end-on’ and ‘side-on’ configurations, respectively. Activated O₂ in the superoxo state shows a better catalytic performance possibly mediating the direct four-electron reduction. The formation of hydrogen peroxide (H₂O₂) is practically eliminated, and thus we suggest that a surface coated with the n-type doped h-BN can be the basis for an ORR catalyst with increased stability.     

One-step sonochemical syntheses of Ni@Pt core–shell nanoparticles with controlled shape and shell thickness for fuel cell electrocatalyst

We demonstrate a facile one-step method to synthesize Ni@Pt core–shell nanoparticles (NPs) with a control over the shape and the Pt-shell thickness of the NPs. By adjusting the relative reactivity of the Pt and Ni reagents in ultrasound-assisted polyol reactions, two Ni@Pt NP samples of the same composition (Ni/Pt = 1) and size (3–4 nm) but with different particle shape (octahedral vs. truncated octahedral) and different Pt-shell thicknesses (1–2 vs. 2–3 monolayer) are obtained. The control is achieved by using different Ni reagents, Ni(acac)₂ (acac = acetylacetonate) and Ni(hfac)₂ (hfac = hexafluoroacetylacetonate). A reaction mechanism that can explain all of the observations is proposed. The Ni@Pt NPs show up to threefold higher mass activity than pure Pt NPs in oxygen reduction reaction. Between the two Ni@Pt NP samples, the one composed of octahedral NPs with the thicker Pt-shell has higher activity than the other.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.     

Density functional theory-based analysis on O2 molecular interaction with the tri-s-triazine-based graphitic carbon nitride

The structural and electronic properties of O₂ molecular adsorption on the Tri-s-triazine-based graphitic carbon nitride (g-C₃N₄) surface was investigated through first principles calculation based on density functional theory (DFT). Here, we show that the O₂ molecule is merely physisorbed on the surface of g-C₃N₄ through the interaction of its lowest unoccupied molecular orbital (LUMO) with the orbitals of the 2-coordinated nitrogen atoms of the surface. Though physisorbed, a stronger molecular adsorption was found as compared with its adsorption on pure graphene sheets. We also found that the O₂ molecule gains very small amount of electron charges from the surface, which, together with a stronger adsorption energy, may attribute to a more effective oxygen reduction reaction (ORR) site as compared with pure graphene. These results would then be important for reactions with intermediate surface oxidation step in a carbon and nitrogen-based catalyst, and could lead to realization of an effective materials design for surface application, e.g. towards a more efficient catalyst for the ORR on the cathode side of the proton exchange membrane fuel cell (PEMFC).     

Adsorption and dissociation of O2 on CuCl(1 1 1) surface: A density functional theory study

The adsorption and dissociation of O₂ on CuCl(1 1 1) surface have been systematically studied by the density functional theory (DFT) slab calculations. Different kinds of possible modes of atomic O and molecular O₂ adsorbed on CuCl(1 1 1) surface and possible dissociation pathways are identified, and the optimized geometry, adsorption energy, vibrational frequency and Mulliken charge are obtained. The calculated results show that the favorable adsorption occurs at hollow site for O atom, and molecular O₂ lying flatly on the surface with one O atom binding with top Cu atom is the most stable adsorption configuration. The O-O stretching vibrational frequencies are significantly red-shifted, and the charges transferred from CuCl to oxygen. Upon O₂ adsorption, the oxygen species adsorbed on CuCl(1 1 1) surface mainly shows the characteristic of the superoxo (O₂⁻), which primarily contributes to improving the catalytic activity of CuCl, meanwhile, a small quantity of O₂ dissociation into atomic O also occur, which need to overcome very large activation barrier. Our results can provide some microscopic information for the catalytic mechanism of DMC synthesis over CuCl catalyst from oxidative carbonylation of methanol.     

An Auger spectroscopic study of the kinetic behavior of adsorbed oxygen on palladium

The adsorption of oxygen on polycrystalline palladium, the kinetics of the reaction of adsorbed oxygen with carbon monoxide and the amount of adsorbed oxygen present during the catalyzed reaction, $\text{CO +}\text{1}\text{2}\text{O}{}_2\text{→ CO}{}_2$, were studied by Auger electron spectroscopy. At temperatures below 783 K, the initial sticking probability is high (~0.8). Adsorbed oxygen and CO react with high probability and low activation energy to form carbon dioxide. The reaction is first order with respect to carbon monoxide pressure and zero order in oxygen coverage. Oxygen coverages measured during the CO-oxidation reaction decrease sharply around P_CO ? P_O2 and are very small when P_CO >P_O2. The reaction kinetics are discussed using a modified Eley-Rideal mechanism involving strongly adsorbed oxygen atoms and surface carbon monoxide in a short-lived state. The oxygen adsorption phenomena are correlated with the reaction kinetics.     

Oxygen adsorption properties of YBaCo4O7-type compounds

The oxygen adsorption and desorption of newly found compounds RBaCo₄O₇ (R = Y, Dy–Lu, In) were investigated by thermogravimetry (TG) in the temperature range from room temperature to 1100 °C. The influence of Co replaced by Zn and Fe on the oxygen diffusion properties of YBaCo₄O₇ was also studied. TG results showed clearly that all RBaCo₄O₇ compounds basically experience two oxygen adsorption and desorption processes in the temperature range 20∼1100 °C in oxygen flow. One happens at about 200∼450 °C and the another happens at about 660∼1050 °C. The differences between the resulting states by adsorbing oxygen at lower and high temperature were discussed based on the X-ray diffraction (XRD) patterns and TG data. We showed evidence that the oxygen adsorption at the lower temperature has a small activation energy, while the oxygen adsorption at the higher temperature has a large activation energy. The oxygen adsorbed at high temperature will destroy the RBaCo₄O₇ structure. Zn substituting in the YBaCo_4 − xZn_xO₇ influences the oxygen diffusion behavior prominently, the amount of oxygen adsorbed becomes increasingly weak with the increase of Zn content and disappears completely for the samples with x ≥ 2.0. However, replacement of Co by Fe has little effect on the oxygen absorption process.     

The adsorption of nitrogen dioxide on the Ag(110) surface and formation of a surface nitrate

The adsorption of NO₂ on the Ag(110) surface has been characterized by temperature programmed reaction spectroscopy and high resolution electron energy loss spectroscopy. At 95 K the NO₂ is dimerized to N₂O₄ in a multilayer and a distinct molecular layer, both of which desorb below 200 K. The first adsorption layer contains chemisorbed NO₂ and NO₃, the latter formed by a reaction between NO₂ and its decomposition products. Part of the NO₂ is molecularly chemisorbed via the oxygen atoms with a proposed symmetric bidentate geometry, desorbing at 270 K. Nitrogen dioxide also undergoes partial dissociation to nitrogen and oxygen adatoms. An NO₃ species is formed by the reaction of NO₂ with the oxygen adatoms produced from the partial dissociation of NO₂. The NO₃ is attached to the surface via one oxygen atom and has C_2v symmetry; it decomposes below 500 K. The geometry of both the chemisorbed NO₂ and NO₃ have analogues among inorganic metal complexes.     

Dissociations of O2 molecules on ultrathin Pb(111) films: first-principles plane wave calculations

Using first-principles calculations, we systematically study the dissociations of O₂ molecules on different ultrathin Pb(111) films. According to our previous work revealing the molecular adsorption precursor states for O₂, we further explore why there are two nearly degenerate adsorption states on Pb(111) ultrathin films, but no precursor adsorption states existing at all on Mg(0001) and Al(111) surfaces. The reason is concluded to be the different surface electronic structures. For the O₂ dissociation, we consider both the reaction channels from gas-like and molecularly adsorbed O₂ molecules. We find that the energy barrier for O₂ dissociation from the molecular adsorption precursor states is always smaller than that from O₂ gas. The most energetically favorable dissociation process is found to be the same on different Pb(111) films, and the energy barriers are found to be influenced by the quantum size effects of Pb(111) films.