Diffusion Of Subsurface Oxygen Research Articles

Is Subsurface Oxygen Necessary for the Electrochemical Reduction of CO2 on Copper?

It has recently been proposed that subsurface oxygen is crucial for the adsorption and subsequent electroreduction of CO2 on copper. Using density functional theory, we have studied the stability and diffusion of subsurface oxygen in single crystals of copper exposing (111) and (100) facets. Oxygen is at least 1.5 eV more stable on the surface than beneath it for both crystal orientations; interstitial sites are too small to accommodate oxygen. The rate of atomic oxygen diffusion from one layer below a Cu(111) surface to the surface is 5 × 103 s-1. Oxygen can survive longer in deeper layers, but it does not promote CO2 adsorption there. Diffusion of subsurface oxygen is easier to the less-dense Cu(100) surface, even from lower layers (rate ≈ 1 × 107 s-1). Once the applied voltage and dispersion forces are properly modeled, we find that subsurface oxygen is unnecessary for CO2 adsorption on copper.

Sand grain size composition influences subsurface oxygen diffusion and distribution of an endemic, psammophilic lizard

AbstractReptile species endemic to dune ecosystems worldwide possess morphological and behavioral adaptations for burying in sand. Specializations for burying and subsurface breathing among these animals are advantageous only where sand conditions permit. The patchy distributions of many psammophilic species are presumably due to the occurrence of suitable areas where attributes of the sand facilitate locomotion, burying, subsurface breathing and nesting. The endemic, dune‐dwelling Sceloporus arenicolus does not occur in areas where sand grain size composition is relatively fine, and the distribution of the species appears limited to areas with coarse‐grained sand. However, the exact mechanism by which this occurs is unknown. We hypothesized that subsurface breathing is inhibited in fine sand, and tested the prediction that fine sand restricts diffusion of oxygen. We compared oxygen diffusion rates in sand with grain size compositions matching sites where S. arenicolus was present (coarse sand) to diffusion rates in samples from sites where it was absent (fine sand). We found that samples with relatively coarse sand from sites where S. arenicolus was present had higher oxygen diffusion rates than samples with finer sand where S. arenicolus was absent. These results corroborated our prediction and support the hypothesis that subsurface breathing by S. arenicolus is constrained by fine sand. This is the first step in a line of research on the role of sand grain size composition in the life history of dune‐dwelling reptiles and more experiments can build on this study. Our results have broad implications for the conservation of all strictly psammophilic species by improving our understanding of the habitat elements necessary for their survival and persistence.

Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on adsorbate diffusion and dehydrogenation at vanadium oxide substrate

We discuss the details of important steps of the selective catalytic reduction (SCR) of NO(x) at model V2O5(010) substrate. First, diffusion processes at the substrate surface are considered where hydrogen and ammonium, NH4, are used as examples. Hydrogen diffusion, a prerequisite for water formation involving substrate oxygen, is described by diffusion paths between adjacent surface oxygen sites. Corresponding energy barriers are determined mainly by the flexibility and the amount of distortion of the oxygen atoms which participate in the O-H-O bridge formation at the transition state. Further, diffusion of sub-surface oxygen to fill surface oxygen vacancies of the V2O5(010) substrate has been considered and results in reactive surface sites which have not been discussed so far. NH4 diffusion at the V2O5(010) surface can be described as a combined tumbling and rotation process characterized by quite low diffusion barriers which make the adsorbate rather mobile. Finally, hydrogenation and dehydrogenation of different NH(x) species at the V2O5(010) substrate surface are studied where special emphasis is given to the influence of surface reduction simulated locally by oxygen vacancies. The results confirm experimental findings of the presence of both NH2 and NH4 species after ammonia adsorption at the V2O5(010) surface.

Energetics of oxygen embedment into unreconstructed and reconstructed Cu(1 0 0) surfaces: Density functional theory calculations

Atomic oxygen embedment into a Cu(1 0 0) surface is studied by density functional theory calculation and the nudged elastic band method. As the oxygen coverage increases on the unreconstructed surface from 0.25 monolayer (ML) to 0.75 ML, the energy barrier for oxygen embedment decreases and an energetically favorable sub-surface site is found at 0.75 ML coverage. At a fixed oxygen coverage of 0.5 ML, the oxygen embedment energetics vary with the surface morphology and the embedment is found to be more probable for reconstructed structures compared to the bare surface. On the missing-row reconstructed surface, we find that the energy barrier for atomic oxygen embedment is smaller through the missing-row compared to other paths, suggesting a mechanism for the formation of sub-surface oxygen structures that are consistent with a recent experiment. The energy barrier for sub-surface oxygen diffusion is predicted to be less than that for on-surface diffusion.

Subsurface miscibility of metal overlayers with V, Nb and Ta substrates

The high solubility and diffusivity of oxygen in Nb, Ta and V are responsible for the difficulty in the preparation of a clean, well-ordered (1 0 0) surfaces of these early transition metals. The deposition and subsequent annealing of a metal overlayer on Nb(1 0 0) are a convenient route for the preparation of flat surfaces with the Nb lattice constant and a metallic character. Such well-ordered, stable, inert and easily reproducible Nb(1 0 0)-like surfaces can be produced due to the suppression of oxygen surface segregation by a suitable layer blocking sub-surface oxygen diffusion. For example, a Nb(1 0 0)-like surface can be obtained by annealing thin Au or Pd films deposited on Nb(1 0 0), since this results in a Au–Nb or Pd–Nb alloy situated just below the surface which preserves the structure of the Nb-lattice and effectively suppresses the oxygen segregation toward the surface. In contrast, Ag and Cu layers do not show this property as these metals do not form a bulk alloy with Nb. The presence or absence of bulk alloying and its relation to surface oxygen contamination is a general phenomena observed for many metallic layers deposited on V, Nb and Ta substrates. A discussion of results reported in the literature is given for these adsorbate systems.

Mathematical modelling of the unsteady-state oxidation of nickel gauze catalysts

The oxidation of nickel gauze catalysts by O 2 was investigated by transient response methods. Qualitative description of the experimental transient responses have shown that the oxidation of metallic nickel proceeds in two stages. The first step is a fast dissociation of gaseous oxygen on the catalytic surface, followed by oxygen diffusion from the surface into the bulk. This slow second step explains the long tailing observed from the experimental transient responses. Furthermore, a rise of temperature was observed during the nickel oxidation. A kinetic model has been developed on the basis of the experimental results in order to provide quantitative information about this reaction. For nickel gauze oxidation, a non-isothermal model combining the subsurface oxygen diffusion with an exponential distribution for the activity of surface sites gave a good agreement with experimental transient data. As a result, the activation energies and the pre-exponential factors of the rate constants as well as the characteristic oxygen diffusion times were determined.

The Role of Subsurface Oxygen in the Catalytic CO-Oxidation Reaction

The CO/CO2 conversion performed over a Ru(0001) surface shows the decisive role of subsurface oxygen which explains the discrepancy between former UHV results and high-pressure experiments. At oxygen pressure higher than about 10—3 mbar the oxygen atoms start to penetrate the bulk. This subsurface oxygen has to be considered as a species which accompanies the high-pressure surface reactions where oxygen is involved. This new oxygen phase exhibits unique properties clearly different from adsorbed oxygen as well as from regular ruthenium oxides. The presence of subsurface oxygen raises the yield for CO-oxidation reaction by more than two orders of magnitude. Oxygen atoms forming the subsurface phase seem to be very mobile as can be deduced from the reaction kinetics which show that the CO2 formation is limited only by the thermal diffusion of subsurface oxygen towards the surface.

The Correlation of Subsurface Oxygen Diffusion with Variations of Silver Morphology in the Silver–Oxygen System

Silver undergoes pronounced morphological changes following high-temperature treatment in various gas atmospheres. SEM analysis shows that high-temperature treatment in oxygen leads to pronounced facetting of the silver surface.In situXRD shows a strong texturing of the polycrystalline bulk and an expansion of the unit cell resulting from the dissolution of oxygen. Two distinctly different forms of subsurface oxygen have been identified and the influence of morphological changes on the mechanism leading to their formation clarified. The first of these species is Oβ. It is bulk-dissolved oxygen which diffuses via an interstitial mechanism through low-resistance diffusion paths such as grain boundaries and open crystalline planes. Temperature programmed desorption spectroscopy analysis reveals that high-temperature oxygen pretreatment results in the formation of crystalline surfaces exhibiting different oxygen diffusion barriers. At elevated temperatures, pure thermal reordering dominates over oxygen-induced restructuring of the silver bulk and surface. The near-surface region is, therefore, comprised primarily of close-packed crystalline planes at elevated temperatures. The barrier for interstitialcy diffusion is overcome at temperatures in excess of 923 K, allowing bulk-dissolved, atomic oxygen to (Oβ) segregate into these low-indexed planes. This likely occurs via an interstitialcy diffusion mechanism where oxygen substitutes for silver atoms in the lattice. This oxygen species is referred to here as Oγ. Analysis of ISS depth profiling of silver foils treated under various conditions shows that the diffusion coefficient is a positive function of the oxygen concentration. This is a direct result of the oxygen-induced recrystallization of the silver resulting in a lowered barrier to diffusion.In situXRD shows that oxygen located in octahedral holes of silver preferentially diffuses in the [110] direction. This requires the anisotropic displacement of silver atoms. The resulting strain culminates in a slight expansion of the silver unit cell and the preferential growth of crystals in the shape of needles. STM imaging of the surface shows the macroscopic silver facets to be composed of smaller, columnar crystallites, thus confirming the validity of the model based on the XRD data.

Modelling the dynamics of the N 2O reduction on iron oxide

The reduction of N 2O to N 2 on iron oxide under transient conditions was studied in the temperature range of 270–340°C. Six different models were used to simulate the experimental data. The conventional second-order reaction between gas phase N 2O and surface oxygen vacancies failed to describe the dynamic experiments. Upon introducing linear or exponential distributions of the second-order surface reaction rate constant in function of the degree of reduction of the surface, satisfactory fittings to the experiment could not be achieved either. It was necessary to take into account subsurface oxygen diffusion to describe the low rates of reaction measured at low degrees of reduction of the catalyst. The combination of subsurface oxygen diffusion with a linear activity distribution for the surface reaction provided a good description of the transient N 2O reduction. Finally, the surface reaction rate constant as well as the diffusion coefficient of oxygen in iron oxide could be fitted with the Arrhenius law.

Modeling CO Oxidation on Silica-Supported Iron Oxide under Transient Conditions

The oxidation of CO on silica-supported hematite (Fe2O3) was studied by the step−response method in a tubular fixed-bed reactor, at temperatures ranging between 270 and 350 °C. The oxidation process appeared to proceed through two stages. Firstly, oxygen atoms adsorbed on the surface of hematite react with gas phase CO according to an Eley−Rideal mechanism. Once that adsorbed oxygen has been consumed to some extent, surface oxygen from the lattice of iron oxide is removed in a second stage involving CO adsorption and CO reactive desorption steps, thus generating surface oxygen vacancies. Further reduction of hematite proceeds through diffusion of subsurface oxygen into surface oxygen vacancies. On this basis, a kinetic model was developed, which quantitatively describes the transient behavior of the oxidation process. The activation energies and pre-exponential factors of the rate constants and characteristic subsurface oxygen diffusion time could be determined.

Interaction of Oxygen with Supported Ag–Au Alloy Catalysts

The effects of alloying silver with gold on the oxygen adsorption properties of Ag are studied over a set of 15% (Ag–Au)/α-Al2O3catalysts of variable alloy composition, using microgravimetric and temperature desorption techniques. Three adsorbed oxygen species are observed on the alloy surfaces at elevated temperatures, namely molecular, atomic and subsurface. It is found that geometric and electronic alterations induced by the presence of Au influence the adsorption characteristics differently for each species. When the Au content of the surface increases, the population of atomic oxygen decreases, its activation energy of adsorption increases, and the Ag–O bond weakens, while the enthalpy of adsorption decreases linearly from approximately 170 kJ/mol over pure Ag to about 70 kJ/mol over surfaces containing 24 at.% Au. Molecular oxygen adsorption on Ag is favored by the presence of Au, as indicated by the decrease of its activation energy of adsorption with increasing Au content. Finally, subsurface oxygen diffusion is strongly inhibited by the presence of gold, and the activation energy of this process increases from 100 kJ/mol for pure Ag to 400 kJ/mol for samples containing 24 surface at.% Au. These results are discussed evoking geometric and electronic alterations induced by the presence of Au on the catalytic surfaces.

Influence of surface oxygen vacancies on the catalytic activity of copper oxide Part 2. Oxidation of methane

The role of surface oxygen vacancies in the activity of a copper oxide on alumina catalyst for the oxidation of methane was investigated using temperature-programmed reduction and continuous flow experiments. It was established that the initial reactivity of the catalyst towards reduction with methane depends on the presence of oxygen vacancies. During reduction, the influence of the vacancies is diminished by fast diffusion of subsurface oxygen to the surface and by water formed during the oxidation. The water and the subsurface oxygen are responsible for blocking adsorption sites for methane. The catalytic activity thus does not vary markedly with the total number of oxygen vacancies.

Identification of molecular and subsurface oxygen on stepped Rh(711)

The adsorption of oxygen on Rh(711) was studied over a wide range of oxygen coverages using temperature programmed desorption, high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). Oxygen adsorption on Rh(711), a Rh(100) surface with four atom terraces and a (111) step, differs significantly from the behavior observed on the flat Rh(100) surface. Unlike Rh(100), the Rh(711) surface shows a strong O-O stretch at 1079 cm −1, considerable amounts of subsurface oxygen, and a LEED pattern which indicates a surface reconstruction. Rh(711) adsorbs oxygen dissociatively at 100 K and at low coverages, gives a single HREELS Rh-O mode which varies from about 410 to 435 cm −1 with increasing coverage, and a sharp c(2 × 2) LEED pattern. Desorption from this state grows in with the peak temperature shifting from 1250 to 1135 K characteristic of second-order desorption kinetics. We conclude that oxygen atoms adsorb initially on the terraces, rather than the steps, on this surface. At higher coverages, a new first-order or quasi-zero-order peak appears at 835 K at the same time that desorption from a molecular state is observed in two peaks at 125 and 150 K. On the fully saturated surface, the O-O stretch is observed at 1079 cm −1 which we assign to a superoxo molecular oxygen species. At high oxygen coverages, complicated LEED patterns are observed that suggest a reconstruction of the clean Rh(711) surface structure. O/Rh AES peak intensity ratios show that oxygen goes subsurface at these high coverages and HREELS shows two atomic oxygen stretches at 413 and 534 cm −1, assigned to oxygen in four- and three-fold sites, respectively. The results show that the four-atom wide Rh(100) terraces are different from flat Rh(100) single crystals with respect to: atomic oxygen ordered structures, facile subsurface oxygen diffusion, and HREELS detection of molecular oxygen.

Incongruent Sputtering in Nio(100) and CoO(100) with 2 keV Argon Ions

ABSTRACTAlthough NiO(100) sputters congruently at 300 K under 2 keV argon ion flux, sputter reduction is observed above a substrate threshold of 350 K. The reduction kinetics are described quantitatively by Fickian diffusion of subsurface oxygen coupled to differential sputtering at the sample surface. Two additional surface oxygen species are detected on sputter reduced NiO by XPS and these species have not been found on either stoichiometric or chemically reduced NiO(100).CoO(100) also shows sputter reduction, although only surfaces that have been pre-annealed to 973 K show significant reduction. Under these conditions, an epitaxial layer of Co3 O4 forms on the CoO substrate and this layer has a threshold temperature to sputter reduction of 550 K. Reduction kinetics for the epitaxy appear similar to those of NiO(100).

Initial stages of hydrogen reduction of NiO(100)

The reduction of single crystal NiO(100) under hydrogen has been followed by AES, XPS and LEED for the pressure range of 1.0 × 10 −7 to 1.3 × 10 −6 Torr and for substrate temperatures of 150–350°C. The kinetics of reduction are controlled both by the rate of removal of lattice oxide at the surface and by the diffusion of subsurface oxygen to the oxygen-depleted surface. The rate of oxygen removal is first-order in surface oxide concentration and in hydrogen pressure. An induction period precedes the reduction reaction and its length is postulated to be controlled by surface defect concentration. The stoichiometric and reduced lattice oxygen species appear to be chemically identical and give a single symmetric XPS peak at 529.4 eV. Nickel spectra indicate a shift in XPS binding energies from those expected of the oxide to those of nickel metal early in the reduction process, although LEED indicates the NiO(100) surface lattice to remain the stable structure for surface reduced to approximately 20% of the stoichiometric oxygen concentration. Ni(100) island formation is observed, with Ni 〈010〉 and 〈001〉 directions along the NiO 〈010〉 and 〈001〉, respectively, but only after the NiO surface is severely depleted in oxygen.