Oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an oxygen-permeable membrane at high pressures

Oxygen permeation fluxes across the dense Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membrane disks were measured under an air/helium oxygen partial pressure gradient at high pressures (up to 10 atm) and various temperatures (973–1123 K). The fabricated BSCFO membrane exhibited good oxygen permeability with a high oxygen permeation flux of 2.01 ml min^− 1cm^− 2 (thickness: 1.37 mm) at 1123 K and 10 atm. Oxygen permeation results were analyzed theoretically using the surface exchange current model. The dependences of the oxygen permeation fluxes on the oxygen partial pressure gradient, suggested that the bulk oxygen ionic diffusion was the rate-limiting step for the overall oxygen permeation process across the BSCFO membrane. The ambipolar diffusion coefficients (D_a), the oxygen vacancy diffusion coefficients (D_v) and the oxygen ionic conductivities (σ_i) of the BSCFO material at different temperatures (973–1123 K) were calculated. It was found that BSCFO possessed high oxygen diffusion coefficients and ionic conductivities, which resulted in the good oxygen permeability of BSCFO. In addition, the BSCFO membrane exhibited good stability of oxygen permeation at 1123 K, while the deterioration of oxygen permeation stability was observed at 1098 K due to structural changes occurring at the surface of the BSCFO membrane disk as demonstrated by XRD.     

Adsorption and decomposition of formic acid on the NiO(111)-p(2 × 2) surface: TPD and steady state kinetics studies

The adsorption and decomposition of formic acid on NiO(111)-p(2 × 2) films grown on Ni(111) single crystal surface were studied by temperature-programmed desorption (TPD) spectroscopy. Exposure of formic acid at 163 K resulted in both molecular adsorption and dissociation to formate. The adsorbed formate underwent further dissociation to H₂, CO₂ and CO. H₂ and CO₂ desorbed at the same temperatures of 340, 390 and 520 K, while CO desorbed at 415 and 520 K. The desorption features varied with the formic acid exposure. Two reaction channels were identified for the decomposition of formate under equilibrium with gas-phase formic acid with a pressure of 2.5 × 10⁻⁴Pa, one preferentially producing H₂ and CO₂ with an activation energy of 22 ± 2 kJ mol⁻¹ and the other preferentially producing CO and H₂O with an activation energy of 16 ± 2 kJ mol⁻¹. The order of both reaction paths was 0.5 with respect to the pressure of formic acid.     

Initial stages of oxidation of binary alloys: The case of the stepped Pt3Ti(510) single crystal surface

The oxidation of the (510) oriented surface of the ordered fcc Pt₃Ti alloy was studied by low energy electron diffraction and other surface sensitive techniques. The clean Pt₃Ti(510) surface showed an ordered array of surface steps. Exposing the surface to oxygen at pressures in the range from 1×10⁻⁷ to 1×10⁻⁵ Torr at temperatures of 800 K or higher caused the formation of titanium oxide islands on the surface. This process caused the progressive disappearance of the ordered array of steps, which was replaced by large facets oriented along the (100) and (210) planes.     

A vibrational characterisation of the O/A1(111) system: a reassignment of HREELS data

In light of recent STM measurements of the O/A1(111) system, we reassign the dipole active modes observed at low coverage to resolve discrepancies between the interpretation of Strong, Firey, deWette and Erskine [Phys. Rev. B 26 (1982) 3483], invoking subsurface oxygen, and a variety of other studies which find no evidence for surface oxygen. The STM results, which show that very small island sizes are stabilized over an exposure range up to ∼ 200 L with a total coverage ≤ 0.2 ML, are incompatible with the assumption of long range periodicity required for lattice dynamical modeling. The consequence is that vibrational modes polarized parallel to the surface may become dipole active. Within an Al₃O cluster model appropriate to exposures ≤ 3 L where most oxygen atoms are isolated species in three-fold hollow sites, the strong feature at 584 cm⁻¹ (72 meV) is still attributed to top-layer oxygen motion perpecdicular to the surface (the symmetric Al₃O stretch) but the second intense feature at 480 cm⁻¹ (60 meV) is assigned to the umbrella mode involving predominantly Al motion parallel to the surface rather than the motion of two AlO layers moving perpendicular to the surface out of phase with each other. The lowest frequency mode near 224 cm⁻¹ (28 meV) derives from the frustrated translation of the cluster perpendicular to the surface. At higher exposures (> 10 L) where multiple oxygen islands begin to appear, totally symmetric combinations of the E-derived asymmetric Al₃O stretching motion polarised nominally parallel to the surface become dipole allowed and can be assigned to the loss at 850 cm⁻¹ (105 meV), which was previously attributed to subsurface oxygen.     

The initial oxidation of Cr(100)

Electron energy loss spectroscopy (EELS), Auger spectroscopy, and low-energy electron diffraction (LEED) have been used to study oxygen chemisorption on and the initial oxidation of Cr(100). With O₂ exposures up to 5 L, CrO vibrational frequencies between 495 and 545 cm⁻¹ are observed. A CrO stretching frequency at 635 cm⁻¹, probably due to rhombohedral Cr₂O₃, is observed to emerge strongly by ≈ 60 L. Based on a sequence of O₂ exposures at 300 K and on a second sequence with 625 and 1175 K anneals, a model of the initial oxidation of Cr(100) is presented. Subsurface oxygen in interstitial sites with Cr atoms maintaining bulk positions is proposed to act as a nucleus for subsequent oxide growth. According to this model, oxide growth at 300 K occurs primarily through domain expansion, while frequent creation of new domains occurs at 625 K. At elevated temperatures, competition between domain growth and diffusion into the bulk is observed.     

CO adsorption isotherms on Cu(100) at elevated pressures and temperatures using infrared reflection absorption spectroscopy

Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Cu(100) surface. Adsorption isotherms were determined at CO pressures from 10⁻⁶ to 10 Torr, and at temperatures from 115 to 340 K, and the isosteric heats of adsorption (δE_ads) evaluated as a function of CO coverage. For increasing CO coverages between 0-0.15 monolayers (ML), δE_ads decreases sharply from 16.7 to 12.7 kcal/mol. From 0.15 to 0.35 ML, δE_ads remains approximately 12.7 kcal/mol and exhibits little coverage dependence. These results are in excellent agreement with previously reported data for the CO/Cu(100) system acquired at much lower pressures (<10⁻⁴ Torr) and temperatures (<275 K). At substrate temperatures above 240 K and at pressures > 10⁻⁴, significant bathochromic shifts of the CO frequency to lower wavenumbers are observed.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Oxygen chemisorption,surface oxidation,and the oxidation of carbon monoxide on cobalt (0001)

Oxygen chemisorbs on clean Co(0001) at 300 K with an initial sticking probability of ~0.3. The chemisorbed overlayer (which is very reactive towards CO) readily undergoes conversion to cobalt oxide, even at room temperature. This transformation is accelerated at higher temperatures, and the oxygen uptake rate falls as CoO growth proceeds. At a certain point, however, the uptake rate rises sharply, and this behaviour is ascribed to nucleation and growth ofCo₃O₄. This interpretation is consistent with the available Δφ, Auger, LEED, and reactivity data. Thus Δφ changes sign as lattice penetration by the ad sorbed oxygen takes place, and this is accompanied by a shift and broadening of the O(KLL) Auger signal. LEED indicates the epitaxial growth firstly of CoO(111) and then, at higher oxygen exposures, of Co₃O₄(111). At 300 K CO rapidly reduces the Co₃O₄ surface back to CoO, and the oxidation/reduction behaviour by O₂/CO appears to be completely reversible. Steady-state measurements yield a value of 19 ± 7 kJ mol^?1 for the activation energy to CO₂ production from CO + O₂. Earlier photoelectron spectroscopic studies by other authors are considered in the light of these results.     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

A study of water vapour adsorption on the (110) surface of copper

The adsorption of water vapour on the (110)Cu face has been studied by AES and Δφ measurements in the 5 × 10^?9 to 3 × 10^?7 Torr range between 75 and 500°C. At lower temperatures, an initial physisorption of oriented water dipoles produces a fast initial Δφ decrease. Further adsorption causes no important changes of the Cu surface potential. At higher temperatures (above 100°C) a partial dissociation of the water molecules leads to a dissociative chemisorption producing a Δφ increase after the initial Δφ decrease due to water physisorption.     

Oxygen on Ni(110): Surface phases and related absolute coverages

The adsorption of oxygen on Ni(110) was investigated by nuclear reaction analysis (NRA), XPS, Δφ, temperature programmed reaction spectroscopy (TPRS) and LEED. At 423 K, (3 × 1), (2 ×1) and (3 ×1) phases are formed in sequence with increasing O₂ exposure. The coverage in the (2 ×1) phase was determined by NRA, the coverages in the other phases being determined via this calibration by XPS, TPRS and Δφ. Contrary to previous reports, the maximum in the intensity of half-order beams from the (2 × 1) phase is associated with a coverage of (5.6 ± 0.5) × 10¹⁴ O atoms cm⁻² or 0.49 ± 0.05 monolayers, and not 0.25 monolayers. The two (3 ×1) phases are associated with θ = 0.33 ± 0.03 and 0.64 ± 0.06 monolayers respectively. Oxygen adsorbed at 295 K is not at thermodynamic equilibrium. Annealing to T > 400 K causes significant decreases in Δφ and the formation of the (2 ×1) phase for θ > 0.3.     

Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe₃O₄-supported Pd model catalyst by molecular beam (MB) methods, time resolved IR reflection absorption spectroscopy (TR-IRAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The well-shaped Pd particles were prepared in situ by metal evaporation and growth under ultrahigh vacuum (UHV) conditions on a well-ordered Fe₃O₄ film on Pt(1 1 1).It is found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (∼10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface, stabilized by the iron-oxide support. Their formation and reduction is fully reversible. Upon decomposition, oxygen is released which migrates back onto the metallic part of the Pd surface. In consequence, the Pd interface oxide layer acts as an oxygen reservoir, the capacity of which by far exceeds the amount of chemisorbed oxygen on the metallic surface.Additionally, Pd surface oxides can also be formed at temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature. This effect is addressed to different onset temperatures for oxidation of the particle facets and sites. It is shown that the presence of Pd surface oxides sensitively modifies the adsorption and reaction properties of the model catalyst, i.e. by lowering the CO adsorption energy and CO oxidation probability. Still, a complete reduction of the Pd surface oxides can be obtained by extended CO exposure, fully reestablishing the metallic Pd surface.     

Reactivity of carbon dioxide at nickel (110)

Adsorption and reactivity of carbon dioxide at the clean and oxygen precovered Ni(110) surface has been studied by means of EELS and LEED. On the clean surface two different types of CO₂ molecules have been observed by EELS at 135 K, one being the undisturbed linear configuration. With increasing temperature the linear molecule changes into a different species which precedes dissociation at 220 K into CO and O. EELS and LEED data of the intermediate species support the assumption that it is a bent CO⁻₂ anion adsorbed in C_2v symmetry with twofold oxygen coordination to the surface. Oxygen preadsorption stabilizes the linear CO₂ molecule up to higher temperatures which does not convert into a bent species in this case. Instead, a reaction product of CO₂ and O is found and interpreted as a carbonate species.     

Electron paramagnetic resonance of Yb<Superscript>3+</Superscript> ions in a concentrated YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> compound with heavy fermions

The EPR signal from localized ytterbium ions was observed in an undoped YbRh₂Si₂ compound with heavy fermions in the temperature range from 1.5 to 25 K. The exponential contribution dominating the temperature dependence of EPR line width at temperatures above 15 K was shown to be caused by the random transitions from the ground to the first excited Stark sublevel of the Yb³⁺(4f¹³) ion with the activation energy Δ=115 K.     

Reduction of oxidized Fe(100) by hydrogen

The interaction of hydrogen with the oxide layer on Fe(100) has been studied with ellipsometry, AES and LEED. The oxide layer formed at room temperature on Fe(100) rearranges at elevated temperatures, resulting in a reconstructed oxide phase in deeper layers, plus a single monolayer of oxygen on top of the surface. This monolayer is unchanged upon heating. These surfaces are exposed to hydrogen pressures up to 2 × 10⁻² Torr at crystal temperatures between 473 and 643 K. The reduction proceeds via a mechanism of dissociative adsorption of hydrogen on an oxygen filled site. A continuous transport of oxygen from deeper layers to the surface region occurs on a time scale which is fast in comparison with the observed reaction rate. These oxygen containing reaction sites are related to the reconstructed oxide, since a single monolayer of oxygen on Fe(100) is inactive to hydrogen in the pressure range measured. The apparent activation energy for the reaction between the oxide overlayer on Fe(100) and hydrogen is 59 ± 4 kJ/mol at the initial oxygen coverage.     

Surface sensitive spectroscopic study of the interaction of oxygen with Al(111) - low temperature chemisorption and oxidation

The interaction of an atomically clean Al(111) surface with O₂ has been studied using a combination of electron energy loss spectroscopy (EELS) and Auger spectroscopy (AES). Oxygen dissociatively adsorbs and occupies both surface and subsurface binding sites under all exposure conditions in the temperature range 122–700 K. Surface sites are preferentially occupied at low exposures, while higher exposures increasingly favor population of subsurface sites. Studies of O₂ adsorption at temperatures as low as 131 K have shown that formation of Al₂O₃ occurs at high oxygen exposures. The Al₂O₃ produced exhibits a 54 eV Auger transition and a characteristic vibrational spectrum with loss features at 430, 645, and 880 cm⁻¹. Argon ion bombardment of thin monolayer level Al₂O₃ layers leads to preferential loss of Al₂O₃ and a reduction in the subsurface-to-surface oxygen ratio. Electron bombardment of similar, thin Al₂O₃ layers is ineffective in inducing desorption of surface species, whereas thick Al₂O₃ layers are strongly influenced by electron bombardment, as judged from AES behavior. Qualitative models for O₂ adsorption, oxidative annealing, and damage by ion and electron bombardment are given.     

The low temperature water formation reaction on Pt(111): A static SIMS and TDS study

The formation of water by the reaction of preadsorbed oxygen with hydrogen on a Pt(111) surface has been characterized, using secondary ion mass spectroscopy, below the desorption temperature of H₂O (180 K). The concentration of chemisorbed water was monitored during the reaction by following the SIMS H₃O⁺ signal. Reaction profiles were measured over a temperature range of 120 to 153 K, and an H₂ pressure range of 10^-9 to 10^-6 Torr. Under all conditions the reaction profiles were characterized by an induction time, a region of rapid reaction, and finally a steady decline in the rate. In the rapid region, an overall activation energy of 2.9 ± 0.3 kcalmol^-1 and a half-order H₂ pressure dependence were observed. At low initial oxygen concentrations the induction time increased and the maximum rate decreased. The reaction was slow in the absence of gas phase hydrogen, even when the surface coverage of hydrogen was relatively high. Water and hydrogen thermal desorption spectra, measured after stopping the reaction by removal of gas phase hydrogen, were complex functions of the H₂ exposure, exhibiting several peaks between 170 and 400 K. However, after an exposure large enough to drive the reaction to completion, only one H₂O peak at 173 K and one H₂ peak at 350 K were observed. The results indicate that only a fraction of the total H(a) on the surface was readily available for reaction during H₂ exposure at T ? 153 K. the remainder either recombined to form H₂ or reacted with O(a) during the thermal desorption ramp. There is good evidence for a surface rearrangement during the induction period. A model is proposed which involves the formation of water clusters that accelerate the rate.     

Ionic and electronic conductivities,stability and thermal expansion of La10 − x(Si,Al)6O26 ± δ solid electrolytes

Apatite-type La_10 − xSi_6 − yAl_yO_{27 − 3x/2 − y/2} (x = 0–0.33; y = 0.5–1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50–26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770–870 K. The average thermal expansion coefficients are (8.7–10.8) × 10^− 6 K^− 1 at 373–1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes.     

Negative molecular ion formation and work function changes: Experimental results for CO and CO2 scattering from nickel (+potassium) surfaces

The scattering of CO⁺ and CO⁺₂ at grazing incidence from Ni(111)+K and clean Ni(111), Ni(110) surfaces produces CO, CO₂ and dissociated species. The observation of negative species O⁻ and CO⁻₂ is strongly dependent on the K coverage or work function of the surface. The dissociation of CO⁺ (CO) is weakly changed by the presence of K, whereas in the CO⁺₂ (CO₂) case dissociation via CO⁻₂ → CO + O⁻ is strongly increasing with K coverage.     

Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.