Adsorbed Oxygen Atoms Research Articles

Quantification of the Contribution of Heterogeneous Surface Processes to Pollutant Abatement during Heterogeneous Catalytic Ozonation.

Heterogeneous surface processes such as adsorption and oxidation with surface-adsorbed reactive oxygen species (ROSad, e.g., adsorbed oxygen atom (*Oad) and hydroxyl radicals (•OHad)) have been suggested to play an important role in pollutant abatement during heterogeneous catalytic ozonation (HCO). However, to date, there is no reliable method to quantitatively evaluate the contribution of heterogeneous surface processes to pollutant abatement (fS) during HCO. In this study, we developed a method by combining probe compound-based experiments with kinetic modeling to distinguish heterogeneous surface processes from homogeneous bulk reactions with aqueous O3 and ROS (•OH and superoxide radicals (O2•-) in the abatement of various pollutants (e.g., atrazine, ibuprofen, tetrachloroethylene, and perfluorooctanoic acid) during HCO with reduced graphene oxide. The results show that the pollutants that have a low affinity for the rGO surface (e.g., ibuprofen and tetrachloroethylene) were essentially abated by homogeneous bulk reactions, while the contribution of heterogeneous surface processes was negligible (fS < 5%). In contrast, heterogeneous surface processes played an important or even dominant role in the abatement of pollutants that have a high surface affinity (e.g., fS = 32-82% for atrazine and perfluorooctanoic acid). This study is a critical first step in quantitatively evaluating the role of heterogeneous surface processes for pollutant abatement during HCO, which is crucial to understanding the mechanism of HCO and designing catalysts for effective pollutant abatement.

Structural heredity in the casting of eutectic cast iron

It has been shown that the effect of structural inheritance in casting eutectic cast irons can be explained from the standpoint of nanostructured crystallization of foundry alloys. Proposed is a mechanism of structural heredity when casting eutectic cast irons. This mechanism is determined by the stability of the crystallization centers of microcrystals of austenite, graphite, cementite. This stability has been shown to depend on the concentration of adsorbed oxygen atoms. The higher this concentration, the less stable the centers of crystallization of microcrystals of austenite, graphite, cementite in melts of eutectic cast irons, and vice versa. With an increase in overheating and (or) the holding time of melts, the concentration of adsorbed oxygen atoms in them increases. As a result, structural stability during remelting of eutectic cast irons is reduced and the effect of structural heredity is disturbed.

First-Principles Study of Adsorption of Atomic Oxygen on PdZn(111) Surface

First-Principles Study of Adsorption of Atomic Oxygen on PdZn(111) Surface

Adsorption of Oxygen and Nitrogen Atoms on the SiO2 Surface: Molecular Dynamics Calculation Based on the Quantum-Mechanical Potential

Adsorption of Oxygen and Nitrogen Atoms on the SiO2 Surface: Molecular Dynamics Calculation Based on the Quantum-Mechanical Potential

Enhanced precision lapping techniques for fused quartz optical elements: Synergistic integration of mechanical and mechanochemical processes

Fused silica is a material of great importance in the field of precision optical equipment due to its excellent optical properties, e.g. photolithography, laser fusion devices, etc. However, the material's inherent hardness and brittleness render it a challenging material to process. Therefore, an enhanced precision lapping technique was proposed, i.e., a synergistic integration of mechanical and mechanochemical lapping processes. Firstly, a synergistic lapping tool with an outer layer of porous diamond abrasive and an inner layer of CeO2 abrasive was fabricated by hot press forming method. The reduction of layer thickness due to abrasion and detachment of the porous diamond abrasive layer was analyzed, and the lapping mechanism of porous diamond on fused silica materials was revealed. The surface creation mechanism of the CeO2 abrasive layer on fused silica material was analyzed by molecular dynamics simulation and the results were validated by enhanced precision lapping experiment. The results demonstrated that the surface atoms of fused silica are removed in a ‘remote disordered’ manner by conventional diamond abrasive grains, whereas the surface atoms of fused silica are removed in a ‘proximally ordered’ manner by the fine porous diamond abrasive grains through the micro-multi-flute cutting. Consequently, the grinding of a porous diamond abrasive layer can significantly reduce the processing damage, with the surface roughness of the lapping Ra reaching 199 nm. The lapping of CeO2 induces the formation of an oxygen bridge on the surface of fused silica, which then produces bending vibration. This is followed by the adsorption and diffusion of oxygen atoms, which form the -Ce-O-Si intermediate with the oxygen atoms shared with the fused silica. This process results in the formation of CeSiO3 and CaSiO3 compounds. The final surface quality with a surface roughness of Ra 21 nm was obtained by mechanochemical lapping of the cerium oxide abrasive layer. The single mechanical-mechanochemical composite ultra-precision lapping process provides technical support for the efficient and low-damage processing of hard and brittle components.

Regulation of d-band center of TiO2 through fluoride doping for enhancing photocatalytic H2O2 production activity

Titanium dioxide (TiO2) has received extensive attention for photocatalytic hydrogen peroxide (H2O2) production, with the d-band center related to the adsorption performance, which affects the photocatalytic reaction process. Herein, an ingenious strategy to lower the antibonding-orbital occupancy in the Ti 3d orbital by fluoride ion (F−) doping is proposed, with density functional theory calculations predicting that F-doping into TiO2 induces a non-uniform charge distribution and enables an upshift of the d-band center in F/TiO2. This manipulation provides accessible active centers with favorable d-band energy levels, which can improve the charge-transfer behavior, strengthen the interaction between the adsorbed oxygen and the photocatalyst, and reduce the adsorption energy of oxygen, eventually promoting the photocatalytic H2O2 production rate. The experimental results further confirm that a lower antibonding-orbital occupancy can intensify the adsorption of atomic oxygen at the Ti sites. Electron paramagnetic resonance experiment reveals that the presence of F− ions in the lattice induces the formation of Ti3+ centers that localize the extra electron needed for charge compensation. Femtosecond transient absorption (fs-TA) spectroscopy suggests that photogenerated electrons are transferred from the conduction band of F/TiO2 to the Ti3+ surface states and surface F− ions, expediting the separation of electrons and holes. Consequently, with F− doping in TiO2, the photocatalytic H2O2 production yields improved from 277 to 467 μmol·g−1·h−1, with ethanol as a sacrificial reagent. This study provides a new strategy for regulating the d-band center to optimize the adsorption strength between the photocatalyst and oxygen atoms and achieve enhanced photocatalytic H2O2 production performance.

Unraveling the characteristics of adsorption and diffusion of oxygen atoms on the surface of Zr and ZrO2 crystals with point defects from molecular dynamic simulations

Unveiling the characteristics of adsorption and diffusion of oxygen atoms on the surface of zirconium alloys at microscale is significant during the initial stages of zirconium oxide layer formation in PWRs or LWRs. In this work, Adsorption and diffusion of oxygen atoms on the surface of Zr and ZrO2 crystals have been investigated using molecular dynamic simulations. A special focus of this work is on the diffusion coefficient calculation based on our developed unbiased approach. The results show that a weak effect of temperature on the adsorption rate of ZrO2. The greater influence of grain boundary orientation than the concentration of Frenkel defects on the adsorption rate of oxygen atoms. The number of oxygen atoms that diffuse deep into the studied samples after adsorption on the free surface increases with increasing temperature. Moreover, for crystals that have a certain concentration of Frenkel defects in the volume, the diffusion coefficients are greater than for defect-free crystals. The diffusion coefficient of adsorbed oxygen atoms is smaller for crystal system with grain boundary of Zr/Zr. This distinct feature may be solving the experimental puzzle of oxidation rate of zirconium alloys under neutron irradiation.

Enhancing LSCF-based air electrode durability: Insights into sulfur poisoning and air purification

The electrochemical performance of perovskite-type air electrodes in solid oxide cells is strongly affected by a trace amount of SO2 as the air impurity. The present work aims to study air purification efficiency in suppressing the degradation rate for the LSCF-GDC air electrode while operating under different SO2 concentrations. For this purpose, the electrochemical performance of symmetrical cells was evaluated using impedance spectra measured over 320 h and compared with intrinsic electrode degradation in synthetic air. Accordingly, the intrinsic degradation rate was measured at 0.05 mΩ cm2 h−1 while exposing the cell to synthetic air containing 10 and 100 ppb SO2, causing a severe increase to 0.17 and 0.30 mΩ cm2 h−1, respectively. Nevertheless, the calculated degradation rate under purified air containing 100 ppb SO2 was similar to the LSCF-GDC intrinsic degradation rate, representing the high practicality of filter material to adsorb SO2 species. Interpreting the impedance spectra using a distribution of relaxation times (DRT) and a complex nonlinear least squares fit (CNLS) revealed that the intrinsic degradation mechanisms were the limitation of dissociative surface adsorption of oxygen and charge transfer to the adsorbed oxygen. At the same time, inhibition of the O2 adsorption caused by the deposition of SrO, SO32−, and SrSO4 phases and increasing surface diffusion resistance for the adsorbed oxygen atoms were the rate-determining steps in SO2-containing synthetic air. Lastly, the effective suppression of degradation rate by gas purification indicates an extensive use of filter material to extend the lifespan of SOCs.

Coverage-Dependent Site-Specific Placement and Correlated Diffusion of Atomic Oxygen on Moiré-Patterned Graphene on Ru(0001).

Nano-periodic arrays of atomic oxygen are visualized on epitaxial graphene on Ru(0001) via STM following supersonic beam exposure to non-equilibrium fluxes of atomic oxygen. Self-organization of atomic oxygen on graphene is directed by the intrinsic moiré pattern of the ruthenium-graphene interface. Atom-resolved STM imaging reveals the richness of multiparticle interactions, leading to correlated atomic diffusion and placement. Pair-distribution functions demonstrate that repulsive oxygen-oxygen interactions play an increasingly important role in the site specificity and diffusivity of atomic oxygen on the moiré lattice with increasing coverage. Atomic visualization shows the number of oxygen atoms in a local region changes overall diffusion rates and promotes the correlated motion of oxygen atoms. Understanding the site specificity of oxygen adsorption and diffusive behavior of atomic oxygen on epitaxial graphene on Ru(0001) provides insight for both the synthesis and stability of moiré-templated two-dimensional materials which show promise as platforms for next-generation quantum materials and catalysts.

Adsorbed oxygen atoms for improving the oxidative dehydrogenation of ethane over B-site-doped layered perovskite La2Ti2O7

In this study, Nd, Ga, and Sm were used to partially replace Ti in layered perovskite La2Ti2O7 (A2B2O7 type) with the aim of developing a highly efficient low-temperature catalyst for the oxidative dehydrogenation of ethane (ODHE). Ethylene was successfully produced by the ODHE over the modified catalysts at temperatures of as low as 600 °C, and the Ga-doped catalyst exhibited the highest ethylene yield of 16.9 % at 625 °C with a high gas hourly space velocity of 35,273 h−1 and short reactant–catalyst contact time of 0.1 s. Thermal activation improved the ethylene yield to 33.6–39.4 % at 650–700 °C with a short contact time (<0.1 s) and productivity of approximately 10 gethylene·gcatalyst−1·h−1, which exceeds reported values. However, the pure catalytic activity could not be depicted at higher temperatures (>700 °C) because the reaction became more complex owing to thermally activated processes such as thermal dehydrogenation, thermal–oxidative dehydrogenation, and reforming reactions. The effects of B-site doping were explored through in situ characterization and density functional theory calculations, and a mechanism for the ODHE was proposed based on the results. Adsorbed oxygen atoms, which form by the dissociation of molecular oxygen at high temperatures, are responsible for the selective dehydrogenation of ethane to ethylene. Overall, the electronic modification of the B-site doped La2Ti2O7 catalysts improved the catalytic activity at 525–575 °C, and the experimental and calculation results confirmed that dissociatively adsorbed oxygen atoms were responsible for the improved ODHE.

Adsorption and diffusion behavior of oxygen atoms on Ti3SiC2 (0001) surface

Ti3SiC2 is the most typical 312 MAX phase, which has attracted more attentions in many fields. In this paper, the (0001) plane of Ti3SiC2 was built. The adsorption energy and diffusion path of oxygen atom at different sites were calculated by first principles. There are three different adsorption sites for oxygen atom on Ti3SiC2 (0001) surface. By calculating the adsorption energy of the three adsorption sites, it was found that the adsorption energy of the oxygen atom at the center site is the lowest. The diffusion path of the oxygen atom was simulated and showed that oxygen atom located at the center of Ti-Si tetrahedron after the diffusion into the bulk phase. The effects of solid solution elements on the diffusion activation energy of oxygen atoms were calculated. suggesting that the solid solution of V, Mo and W will greatly increase the energy barrier, thus preventing the inward diffusion of oxygen atoms.

Effect of Electrode Potential on the Surface Chemical Composition of Au-Pd Nanoparticles

Palladium (Pd) nanoparticles have been investigated for the hydrogen evolution reaction, the hydrogen oxidation reaction, the oxygen reduction reaction, and the methanol and ethanol oxidation reaction. The high methanol/ethanol tolerance of Pd is noteworthy.A powerful approach to further improving the catalytic properties of Pd is through alloying with suitable co-metals. In the case of the ethanol oxidation reaction in alkaline media, Pd alloyed with Au has demonstrated excellent activity exceeding even that of highly active Pt catalysts (1). A complete miscibility, small lattice mismatch, and contrasting properties of Pd and Au make Au-Pd a unique model system. Marked changes in the reactivity of Au-Pd catalysts have been reported when conditioning and operating conditions are altered, which should be related to physico-chemical changes in the nanoparticles. Indeed, dealloying/dissolution and segregation of metals at the surface of nanoparticles (NPs) takes place during conditioning and operation, which have pronounced effect on the activity and stability of NPs. The migration of metals from the bulk to the nanoparticles’ surface may take place during pre-catalytic conditioning and during operation, and can be triggered by the presence of oxidation and reducing agents (2), changes in temperature, and potential cycling (3). Thus, it is critical to investigate the effect of electrode potential on metals dissolution and segregation occurring in alloyed nanoparticles.In the present work, Au-Pd NPs prepared by pulsed laser ablation in liquids were used. Pulsed laser ablation is a simple method to synthesize alloy NPs with high purity out of different material systems and with different compositions. Typically, alloy-metal targets or pressed micro-powder mixture targets are ablated or fragmentated in liquid, respectively. Notably, the technique does not require any surfactant additives to achieve NPs stability in the electrolyte, thus allowing for NPs to remain free of associated contamination from the capping agents. We will show how one can fine-tuned the Au and Pd surface fraction of bimetallic Au-Pd nanoparticles using electrochemistry. The method involves electrochemical cycling in alkaline medium at a suitable upper vertex potential, resulting in a controllable diffusion of Pd atoms from the core to the shell. This preferential outward diffusion of Pd atoms occurs because of the electrochemical adsorption of oxygen atoms at the surface of NPs, which is driven by the application of a potential. This potential has to be positive enough for oxygen atoms to adsorb at the surface of the NPs, and therefore is dependent on the nature of the surface atoms. On AuPd NPs that are enriched with Au atoms, a potential of +1.5 V vs. RHE must be applied to form ca. 1 ML of adsorbed oxygen at the surface of gold and initiate the diffusion of Pd atoms from the core to the NP surface. This approach marks a new avenue in the development of compositionally-controlled electrocatalysts and can be tailored to suit different applications with a simple switch of voltage or a controlled pretreatment.

Enhancement of electronic and magnetic properties in Cr2O3 monolayer honeycomb-kagome by hydrogenation and oxygenation

This study investigates the structural, electronic, and magnetic properties of pristine, hydrogenated, and oxygenated Cr2O3 monolayer honeycomb-kagome (HK) surface using first-principles calculations based on density functional theory (DFT). Molecular dynamics and phonon dispersion calculations demonstrate the thermal and dynamic stability of these systems, implying their feasibility for laboratory synthesis. The results revealed that the adsorption of hydrogen and oxygen atoms on the Cr2O3 monolayer surface exhibited negative adsorption energies, indicating that these structures are energetically favorable. Specifically, the adsorption of oxygen atoms transforms the initial half-metallic nature of the monolayer into semiconducting behavior. Moreover, the introduction of hydrogen and oxygen atoms considerably enhances the Curie temperature of the surface. These findings offer a practical approach for enhancing the electronic and magnetic properties of Cr2O3 monolayer HK surface, thereby facilitating their prospective application in spintronics.

Oxygen adsorption in Ag(m)@X(n) (X=Pd, Pt) core-shell nanoalloys (m+n=55), structural and energetic properties: A DFT study

The interaction between surface atoms and adsorbates is a major element of the reactivity and stability of the catalyst. Previous experimental research has indicated potential associations between oxygen adsorption and Ag surfaces. The principal objective of this study is to identify the atomic oxygen adsorption effect on the structural families and energies of Pt13Ag42 and Pd13Ag42 core-shell nanoalloys using the Gupta and DFT approaches. Results showed that Pt and Pd atoms were located in the inner sites of the nanoalloys, and Ag atoms were found on the surface. The findings also indicated a positive relationship between the Ag–O bond number and energetic stability in the oxygen adsorption of the Pt13Ag42 and Pd13Ag42 nanoalloys. Our general results are that chiral Dh is the most suitable configuration for oxygen adsorption due to DFT relaxations, and the energy gains of similar configurations in both nanoalloys are almost equal.

Solvent-free synthesis of ZrO2 nanoparticles/nitrogen-rich two-dimensional carbon nanosheet composites as efficient catalysts for ORR

An innovative, high-performance, non-noble-metal-based oxygen reduction reaction (ORR) catalyst must be developed utilizing a new and cost-effective technique. ZrO2 has the merits of high stability, low toxicity, good catalytic activity, and low enthalpy for atomic oxygen adsorption. However, the poor conductivity restricts its application as an ORR catalyst. Here, we successfully prepared ZrO2 nanoparticles / nitrogen-rich two-dimensional carbon nanosheet composites as ORR catalysts using a straightforward, solvent-free, one-step pyrolysis approach. Due to the unique component and structure characteristics, ZrO2@N-C-900 demonstrates remarkable ORR performances in alkaline electrolytes, such as high half-wave potential (0.82 V vs. RHE), higher current density (JL = 6.4 mA cm−2), and outstanding long term stability (95 % for 36,000 s). Especially, when used in a Zn-air battery, ZrO2@N-C-900 exhibited a power density of 148 mW cm−2, exceeding that of 20 % Pt/C.

Synthesis and application of mixed conductive and oxygen-permeable materials based on BaFeO3-δ

Cobalt-free perovskite oxide BaFeo3-δ (BFO) exists in mixtures with various phase structures and exhibits poor oxygen ion conductivity, thereby limiting its application as a cathode material in solid oxide fuel cells (SOFC). In this study, the stabilization of a single cubic phase structure of BFO through doping with Gd at the A-site or Cu at the B-site and the lowest temperature required for the synthesis of such a structure are investigated. A large fraction of adsorbed oxygen atoms and a low valence state of Fe are found in BaFe0.9Cu0.1O3-δ, resulting in improved performance. Furthermore, the ideal sintering temperatures for Ba0.9Gd0.1FeO3-δ cathode and BaFe0.9Cu0.1O3-δ cathode are 900 °C and 850 °C, respectively. At 700 °C, the polarization resistance of the BaFe0.9Cu0.1O3-δ cathode is 0.439 Ω·cm2 and that of the Ba0.9Gd0.1FeO3-δ cathode is 0.545 Ω·cm2.

Self-assembled La0.6Sr0.4FeO3-δ-La1.2Sr0.8NiO4+δ composite cathode for protonic ceramic fuel cells

The oxygen reduction reaction at the cathode is an essential process for protonic ceramic fuel cells. Composite cathode materials are commonly used towards the multiple requirements including high surface oxygen activity as well as sufficient electronic and ionic conductivities. In this study, a cobalt-free composite cathode composed of a perovskite La0.6Sr0.4FeO3-δ phase and a Ruddlesden-Popper La1.2Sr0.8NiO4+δ phase is synthesized with a self-assembly technology. The cathode process is mainly controlled by (I) the reduction of adsorbed oxygen atom to O− on the surface and (II) the migration of O− from the surface into the lattice. The former benefits from the high electrical conductivity of La0.6Sr0.4FeO3-δ, and the latter is accelerated by La1.2Sr0.8NiO4+δ attributed to its superior oxygen activity. The one-pot synthesized composite cathode shows an enhanced synergistic effect due to the uniform distribution of the two phases at the nanoscale. The cathode shows the lowest polarization resistances of 0.055 and 0.095 Ω cm2 at 700 °C in oxygen and air, respectively. The results show that self-assembled La0.6Sr0.4FeO3-δ-La1.2Sr0.8NiO4+δ nanocomposite is a promising cathode material for protonic ceramic fuel cells.

Performance characterization and kinetic analysis of atmosphere-breathing electric propulsion intake device

An atmosphere-breathing electric propulsion (ABEP) system is designed to capture and compress the rarefied atmosphere through an intake device in very-low-Earth-orbit (VLEO). This study numerically characterizes the intake performance according to the design parameters, including the geometry of the tapered chamber, gas-surface interaction (GSI) models, surface temperature, aspect ratio, and sizing. The design of grid ducts is examined for different gaps, lengths, and yaw angles. The influence of intermolecular collisions is also investigated. The diffuse GSI model is chosen as a realistic model at VLEO due to the adsorption of atomic oxygen. The direct simulation Monte Carlo method is employed to calculate the capture efficiency, compression ratio, particle flow rate, and number density as the four indicators of intake performance. The flow conditions are obtained using the NRLMSISE-00 model, with a target altitude range of 180–220 km. In addition, a comprehensive design is proposed and the related propellant characteristics are investigated using kinetic analysis. Multi-modality is found in the particle velocity distribution, which implies non-equilibrium of the ABEP propellant.

Reactivity of diamanes against oxidation: A DFT study

Diamanes, ultrathin diamond films represent a very promising class of two-dimensional carbon materials that attracts attention due to broad potential applications starting from sensing and optics and finishing with mechanical applications. We investigate here the ability of diamanes with hydrogenated and fluorinated surfaces to confront oxidation by performing density functional calculations. Simulations for adsorption of atomic and molecular oxygen show greater oxidation resistance for fluorinated diamanes in comparison with hydrogenated ones making these 2D materials promising for various applications where intensive oxidation is present.

Investigation of time variations of redox reactions in SrRuO3 through interface-sensitive resistance measurements

We used an interface-sensitive resistance measurement technique to observe the time evolution of the early stages of redox reactions at the interface between the metallic oxide SrRuO3 and a silver epoxy electrode at around room temperature (∼280–320 K). On exposure to a reducing gas (CO or H2), the interface resistance gradually increased. The time variation functions of the resistance increases for CO and H2 were similar, although the magnitude was greater for H2 than for CO. After substitution of O2 for the reducing gas, the interface resistance decreased to almost the initial value, i.e., that before exposure to a reducing gas. The resistance variations can be well explained by the time variations of the oxygen deficiency at the SrRuO3 interface and subsurfaces as a result of reduction or oxidation. We regarded the reactions as pseudo-first-order reactions and evaluated the rate constants of the SrRuO3 redox reactions at a SrRuO3–Ag interface. For the reduction (resistance increase) process, a single exponential component was enough to fit the data, which suggests that the reduction proceeds successively from outside to inside the bulk. Adsorption of oxygen atoms in the oxidation (resistance decrease) process involved several rate constants, at least up to three exponential components, depending on the prior degradation by a reducing gas. The effective activation energy of each redox reaction was evaluated from an Arrhenius plot.