Low Oxygen Coverage Research Articles

Nanotribological Properties of Oxidized Diamond/Silica Interfaces: Insights into the Atomistic Mechanisms of Wear and Friction by Ab Initio Molecular Dynamics Simulations.

Controlling friction and wear at silica-diamond interfaces is crucial for their relevant applications in tribology such as micro-electromechanical systems and atomic force microscopes. However, the tribological performance on diamond surfaces is highly affected by the working environment where atmospheric gases are present. In this work, we investigate the effects of adsorbed oxygen on the friction and wear of diamond surfaces sliding against silica by massive ab initio molecular dynamics simulations. Different surface orientations, O-coverages, and tribological conditions are considered. The results suggest that diamond surfaces with full oxygen passivation are very effective in preventing surface adhesion, and as a result present extremely low friction and wear. At low oxygen coverage, Si-O-C bond formation was observed as well as atomistic wear initiated from C-C bond breaking at extreme pressure. The analysis of electronic structures of the configurations resulting from key tribochemical reactions clarifies the mechanisms of friction reduction and atomistic wear. Overall, our accurate in silico experiments shed light on the influence of adsorbed oxygen on the tribological properties and wear mechanisms of diamond against silica.

Water Formation Kinetics on Co(0001) at Low and Near-Ambient Hydrogen Pressures in the Context of Fischer–Tropsch Synthesis

Understanding the kinetics of oxygen removal from catalytically active metal surfaces by hydrogen is important for several catalytic reactions such as Fischer–Tropsch synthesis, methanation of CO or CO2, and the reverse-water–gas-shift reaction. Motivated by FTS, a Co(0001) single crystal model catalyst was used to study the kinetics of oxygen removal through reaction with hydrogen. Kinetic studies in the 10–7–10–4 mbar H2 pressure regime show that water formation is first order in the surface hydrogen concentration while the order in oxygen concentration changes from one at low oxygen coverage to zero at high oxygen coverage. In situ XPS shows that the hydrogen surface concentration saturates around 10–1 mbar and on this basis the typical temperature of 450 K needed for water formation in this pressure regime can be considered as typical for high pressures as well. The absence of OH buildup during the reaction points to O + H as the rate-limiting step, with a barrier of ∼120 kJ mol–1. Such a high barrier shows that slow removal of adsorbed oxygen from the surface of reactive metal catalysts such as cobalt may be rate-limiting for the overall reaction.

Pt nanoparticles under oxidizing conditions - implications of particle size, adsorption sites and oxygen coverage on stability.

Platinum nanoparticles are efficient catalysts for different reactions, such as oxidation of carbon and nitrogen monoxides. Adsorption and interaction of oxygen with the nanoparticle surface, taking place under reaction conditions, determine not only the catalytic efficiency but also the stability of the nanoparticles against oxidation. In this study, platinum nanoparticles in oxygen environment are investigated by systematic screening of initial nanoparticle-oxygen configurations and employing density functional theory and a thermodynamics-based approach. The structures formed at low oxygen coverages are described by adsorption of atomic oxygen on the nanoparticles whereas at high coverages oxide-like species are formed. The relative stability of adsorption configurations at different oxygen coverages, including the phase of fully oxidized nanoparticles, is investigated by constructing p-T phase diagrams for the studied systems.

The dual role of the surface oxophilicity in the electro-oxidation of ethanol on nanostructured Pd/C in alkaline media

• Pd/C electrocatalysts are used for amperometric sensors and also for Fuel Cells. • Size-induced effects on Pd/C resulted in distinct tendency to interact with oxygen. • Electrocatalytic activity towards ethanol oxidation depends on the surface oxophilicity. • PdO acts both as a promoter and/or inhibitor towards ethanol oxidation. • The catalytic activity can be enhanced by 57% only by varying surface oxophilicity. The ability of a surface to interact with oxygen (oxophilicity) plays a very important role in several catalytic, electroanalytic and electrocatalytic reactions. To date, however, the role of the oxophilicity in the electrochemical oxidation of ethanol was never shown for pure nanostructured Pd electrocatalysts supported on carbon, a promising material to be used as a non-enzymatic amperometric sensors for ethanol and also as an electrocatalyst for the Direct Ethanol Fuel Cells. In this work, Pd nanoparticles were prepared by chemical reduction at room temperature in the presence of varying amounts of sodium citrate in order to produce particles with distinct sizes. Then, we used cyclic voltammetry in 0.1 mol L -1 KOH to obtain an oxophilicity trend among various nanostructured Pd/C samples. The oxidation of ethanol in alkaline media demonstrated that the surface oxophilicity has a dual role in the kinetics of the reaction: a promoter at low and medium oxygen coverages, and a spectator (inhibitor) species at high surface coverages. Thus, by a proper control of the surface oxophilicity the current density at a constant potential can be enhanced by 57%, highlighting the importance of that parameter in the electro-oxidation on ethanol on Pd-based materials. Our results have a great impact for the design of more active non-enzymatic ethanol sensors and also for the development of more active electrocatalysts for Direct Ethanol Fuel Cells.

The influence of temperature on the nature and stability of surface-oxides formed by oxidation of char

A coal char has been oxidized isothermally at temperatures comprised between 300 and 1073 K. The pre-oxidized chars have been subjected to Temperature Programmed Desorption (TPD) and to core-level high-resolution X-ray photoelectron spectroscopy (XPS) analysis using Synchrotron radiation to infer the nature of the carbon oxides that populate the surface and their evolution throughout thermochemical processing. For low oxygen coverages and mild oxidation temperatures the prevailing carbon-oxygen moieties are epoxy. Raising the oxidation temperature up to ~723K the edge carbon oxygen complexes (ether-hydroxyl and carbonyl-carboxyl) increase. The amounts of CO + CO2 desorbed during TPD also increase with temperature and duration of oxidation for relatively mild oxidative treatments (temperature below ~723K). Upon further increase of the oxidation temperature the amount of CO + CO2 decrease and the ratio of CO/CO2 increases remarkably. Altogether, results suggest the existence of a strong link between a remarkable shift of surface oxides from epoxy to ether/carbonyl and the desorption of CO and CO2. Moreover, the CO/CO2 ratio during desorption can be well correlated with the relative abundance and stability of epoxy moieties with respect to the “edge” oxides. Results are analyzed in the frame of a semi-lumped kinetic model of carbon oxidation with a focus on the role and nature of surface oxides as intermediates in carbon gasification reactions.

Methane dehydrogenation on Cu and Ni surfaces with low and moderate oxygen coverage

AbstractFirst‐principles density functional theory calculations are carried out to evaluate energy barriers and mechanisms for the dehydrogenation reactions of CH4 on clean and oxygen‐covered surfaces of Cu (111) and Ni (111) with low and moderate oxygen coverage. In the presence of oxygen, two possible pathways have been evaluated. The more likely pathway, which is further analyzed, is that CH4 loses an H to the surface O. Results from this pathway agree with previous findings showing that oxygen promotes CH4 dissociation on Cu (111) and hinders that on Ni (111). In addition, our results show lower energy barriers on Cu with higher oxygen coverages up to 0.38 monolayer. However, such an increase in oxygen coverage did not show any favorable effect for CH4 dissociation on Ni (111). The findings are analyzed through electronic factors revealed by charge analysis and density of states.

Ethylene Epoxidation on Ag(100), Ag(110), and Ag(111): A Joint Ab Initio and Kinetic Monte Carlo Study and Comparison with Experiments

Ethylene epoxidation is commercially one of the most important selective oxidation reactions. Despite extensive research into different catalytic materials, silver catalysts remain unrivaled in industrial applications, albeit with significant doping. Experiments have already shown that different silver facets exhibit different catalytic performance in terms of turnover frequencies as well as selectivity. In this work, we present extensive first-principles simulations and kinetic Monte Carlo (KMC) modeling of the reaction on the three pristine silver surfaces Ag(100), Ag(110), and Ag(111) and on the missing-row reconstructed Ag(110). To better understand the kinetics on different surfaces and veraciously describe the surface coverages, we explicitly take into account the lateral interactions between the adsorbates and their effect on the activation barriers. We show that the most stable Ag(111) surface maintains very low oxygen coverage while being the least active surface and only moderately selective. Ag...

Oxygen adsorption on spontaneously reconstructed Au(511)

The four-fold site on {100} facets, which are potential catalytic sites on high-curvature gold nanoparticles, is difficult to prepare on a gold single crystal due to surface reconstruction. Here, the Au(511) surface with a high density of step edges and a well-maintained {100} local structure was studied by scanning tunneling microscopy (STM), temperature programmed reaction spectroscopy (TPRS), and density functional theory (DFT) calculations. Annealing at 550 K and 870 K induces reconstruction on the Au(511) surface to a mixture of (311) and (711) micro-terraces. At room temperature and low oxygen coverage, oxygen adsorbs at both three- and four-fold sites on the surface, leading to a well-ordered zig-zag structure. Oxygen atoms cause significant orbital hybridization and a slight displacement of the gold atoms to which they bind. High oxygen coverages induce the formation of clusters, which are not active towards oxidation of isopropanol at room temperature and a pressure of 10−10 mbar. Oxygen adsorption saturates at 1 monolayer (ML), and two separate oxygen recombinative desorption peaks were observed at coverages above 0.11 ML. Our characterization of this surface and the O adsorption structure allows future studies of {100} micro-terraces for oxidation reactions, which contributes to studies on gold-based catalysts with an irregular shape.

Temperature Effect of Oxygen Reduction Reaction Activity on Pt-Pd/C Core-Shell Catalyst Investigated By X-Ray Absorption Spectroscopy

Introduction Environment and global warming issue is one of the main global issues around the world. The polymer electrolyte fuel cells (PEFCs) supposed to be one of the main devices used in the global renewable energy system and pure hydrogen energy society in the future. PEFC cathode catalyst requires an excess amount of platinum due to the degradation of oxygen reduction reaction (ORR) activity. High performance, high durability and low cost cathode catalyst is expected for the wide use of PEFC devices. Therefore, it is important to reduce the use of platinum in the catalyst and improve its catalytic activity as well as durability. Core-shell structure catalyst is one of the solutions to achieve a higher catalytic activity and less use of platinum. Compare with the common used platinum catalyst, Pd-core/Pt-shell catalyst is one of the promised catalyst to use less platinum, reduce the cost and possess high performance. However, the factor of controlling ORR activity in core-shell catalyst still has not well understood such as potential/temperature-dependent surface structures and oxidation states of platinum. Experimental In this research, we synthesized the 1 mono-layer Pt shell on carbon-supported palladium(Pd) core by copper under potential deposition (Cu-UPD) method, investigated its oxygen reduction reaction (ORR) activity at various temperatures and the oxygen coverage of platinum by rotating disk electrode (RDE) electrochemical measurements and operando X-ray absorption fine structure (XAFS, Fig. 1) respectively. Through the results of electrochemical measurements and XAFS, the relationship among ORR activity and oxygen coverage of the surface Pt shell at different temperature, the change of electronic structure and local structure of Pt atoms in Pt-Pd/C core-shell catalyst at different temperature in the same condition with electrochemical measurement (N2-saturated) and real working condition (O2-saturated) have been discussed. Results and discussion The result of electrochemical measurements shows Pt-Pd/C core-shell catalyst possess higher ORR activity and lower oxygen coverage than common Pt/C catalyst. In the high temperature (50~70°C), the ORR activity of Pt-Pd/C catalyst decrease with the increase of oxygen coverage and temperature, shows the decrease of active sites on the surface of Pt shell and the change of mechanism or structure of Pt-Pd/C catalyst. The result of XAFS shows the core-shell catalyst possess compressive strain and less Pt-Pt bond distances than common Pt/C catalyst in N2-saturated electrolyte, the expand of Pd core and Pt-Pt bond extend occurs at high temperature, explained the high ORR activity and less oxygen coverage of Pt-Pd/C core-shell catalyst in room temperature and the activity loss in the high temperature. In O2-saturated electrolyte, XAFS result shows Pt-O bond formation is dramatically increase and in the high temperature almost all the Pt shell oxidized to PtO2, Pt-Pd bond remarkable extends, shows the loss of ORR activity of the Pt-Pd/C catalyst in high temperature (60°C) in O2-saturated condition. Conclusion This research investigated ORR activity on Pd core-Pt shell with the change of temperature. The core-shell catalyst had a compressive strain and less Pt-Pt bond distances than common Pt/C catalyst in N2-saturated electrolyte, however, the expand of Pd core and Pt-Pt bond extend occurs at high temperature. Acknowledgments This research is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Selective Activation of Methyl C–H Bonds of Toluene by Oxygen on Metallic Gold

In accord with its gas phase acidity, toluene is activated by atomic oxygen adsorbed on Au(111) at the methyl C–H bonds. Titration of this species from the surface with HCOOD yields C6H5CHD2, indicating that much of the toluene has been doubly deprotonated at the methyl carbon. Even at low surface coverages of atomic oxygen the thusly-activated species yields only CO, CO2 and water as gas phase products when heated above room temperature, indicating the facile formation of an adsorbed benzoate, not simple oxygen insertion into the carbene to form benzyl benzyloxy. At higher O pre-coverages benzoic acid forms between 400 and 500 K.

First-Principles Investigation of Titanium Nanoparticle Oxidation

We perform first-principles calculations to investigate the initial stages of titanium nanoparticle oxidation. We determine the most stable structure of a 181-atom decahedral nanoparticle with various oxygen coverages ranging from a single atom to full oxidation of the surface. Linear Oad–Ti–Oad bonding configurations on the nanoparticle surface are found to be most stable for low oxygen coverage. The degree of lattice expansion is observed to gradually increase with increasing oxygen content up to 8.2% for full oxidation of the surface. To investigate likely mechanisms for subsequent subsurface oxidation, we calculate energy barriers for many inequivalent oxygen diffusion pathways. We find that the most favorable pathways involve penetration of oxygen into subsurface octahedral sites in the center of facets where the strain is largest. The results provide atomistic insight into the oxidation behavior of Ti nanoparticles and highlight the important role played by adsorption induced strain.

Adsorption of Carbon on Partially Oxidized Low-Index Cu Surfaces.

We use first-principles calculations to study the carbon adsorption on copper slabs of (100) and (111) surfaces predosed by oxygen atoms. Our results show that on both surfaces, an incoming carbon atom has the ability to replace and completely desorb a previously surface-adsorbed oxygen atom producing CO and CO2 molecules in the gas phase. By comparison, the (111) surface is better suited for oxygen desorption, and an incoming carbon atom can more easily bond to and desorb oxygen atoms even at low oxygen coverages. We examine this mechanism at two different temperatures for both surfaces at 0.5 ML oxygen coverage. An implication of this process is the experimentally proven cleaning effect of predosing copper surfaces with oxygen before graphene growth in the chemical vapor deposition process. Conversely, adsorption and diffusion of carbon atoms, both of which are necessary for the nucleation and growth of carbon nanotubes, may be hindered by the presence of the oxidized or partially oxidized surfaces.

Initial-Stage Oxidation of Ni3Al(100) and -(110) from Ab Initio Thermodynamics

The microscopic mechanisms of the initial-stage oxidation of the Ni3Al(100) and Ni3Al(110) surfaces are comparatively studied using ab initio calculations based on density-functional theory and thermodynamics considerations. The surface energies of the two surfaces as functions of aluminum and oxygen chemical potentials are constructed and show that the formation of any antisite defects is not favorable at the Ni3Al(100) surface, whereas Al antisite defects are favorable at the Ni3Al(110) surface. The surface phase diagrams of the Ni3Al(100) and -(110) surfaces with different antisite defects and at the various oxygen coverages are determined. These results show that oxygen adsorption enhances Al surface segregation at the initial stage of oxidation for both surfaces and that the Ni3Al(100) surface is thermodynamically more favored to oxidize completely at a lower oxygen coverage than the Ni3Al(110) surface.

Mechanistic insight into the facet-dependent selectivity of ethylene epoxidation on Ag nanocatalysts

Ethylene epoxidation reaction to selectively produce the ethylene oxide (EO) is a crucial heterogeneous catalytic chemical process in industry. However, the intrinsic catalytic selectivity depending on the facet of commonly used Ag catalysts is still far from understanding. In this work, we used the periodical density functional theory (DFT) calculations to gain an atomistic insight into the facet-dependent reaction selectivity of the ethylene epoxidation on Ag catalysts. We investigated the adsorption of atomic or molecular oxygen and oxametallacycle (OMC) intermediate, and further study the dissociation of molecular oxygen and the reaction mechanism of ethylene epoxidation on (111), (110), (100), and (211) surfaces of Ag catalysts. Our results indicate that the adsorption of the O atom is not facet-dependent under low oxygen coverage. Moreover, although the adsorption of molecular oxygen is facet-dependent, the dissociation of molecular oxygen would not have the essential influence on selectively producing EO in the ethylene epoxidation reaction. In addition, the catalytic selectivity of ethylene epoxidation reaction is facet-dependent, following the order of (110)>(100)>(111)>(211), which is consistent with the order of the adsorption strength of the OMC intermediate. Our results show that the stability of the OMC intermediate has an essential effect on the selectivity to EO in the ethylene epoxidation reaction. We have faith in that our results can provide useful information which would guide the intellectual design of the catalysts for ethylene epoxidation by controlling their morphologies.

Dissociative mechanism of oxygen reduction reaction (ORR) on Pd-Cu disordered binary alloy metal surfaces: A theoretical study

Density functional theory (DFT) in conjunction with virtual crystal approximation is used to study the oxygen reduction reaction (ORR) as a function of the cooper concentration in Pd–Cu disordered binary alloy metal surfaces and as a function of applied bias. The study is realized calculating the free energy diagrams of all intermediates. Reactivity is evaluated on the basis of an oxygen reduction reaction (ORR) dissociative mechanism of four steps, it involves the splitting of the O–O bond in O2 after its adsorption, followed by hydrogenation of atomic O to OH. Then, the OH species undergoes another hydrogenation to yield H2O. For each concentration of Cu, results for low oxygen coverage are presented at zero cell potential (U = 0), at the equilibrium potential (U = 1.23 V), and at the highest potential (U = 0.80 V) where all reaction are exothermic. The results indicate that at the ORR equilibrium potential of 1.23 V, the reactivity of all surfaces is shown to be limited by the rate of OH removal from the surface, while that at a cell potential of 0.80 V, the ORR reactivity of different surfaces is dictated by the hydrogenation of O∗ and strength of oxygen adsorption. The results are compared with other non-Pt alloys.

Calculations of oxide formation on low-index Cu surfaces.

Density-functional theory is used to evaluate the mechanism of copper surface oxidation. Reaction pathways of O2 dissociation on the surface and oxidation of the sub-surface are found on the Cu(100), Cu(110), and Cu(111) facets. At low oxygen coverage, all three surfaces dissociate O2 spontaneously. As oxygen accumulates on the surfaces, O2 dissociation becomes more difficult. A bottleneck to further oxidation occurs when the surfaces are saturated with oxygen. The barriers for O2 dissociation on the O-saturated Cu(100)-c(2×2)-0.5 monolayer (ML) and Cu(100) missing-row structures are 0.97 eV and 0.75 eV, respectively; significantly lower than those have been reported previously. Oxidation of Cu(110)-c(6×2), the most stable (110) surface oxide, has a barrier of 0.72 eV. As the reconstructions grow from step edges, clean Cu(110) surfaces can dissociatively adsorb oxygen until the surface Cu atoms are saturated. After slight rearrangements, these surface areas form a "1 ML" oxide structure which has not been reported in the literature. The barrier for further oxidation of this "1 ML" phase is only 0.31 eV. Finally the oxidized Cu(111) surface has a relatively low reaction energy barrier for O2 dissociation, even at high oxygen coverage, and allows for facile oxidation of the subsurface by fast O diffusion through the surface oxide. The kinetic mechanisms found provide a qualitative explanation of the observed oxidation of the low-index Cu surfaces.

Adsorption and Reactions of Water on Oxygen-Precovered Cu(110)

Using scanning tunneling microscopy we studied the adsorption and reactions of water on a Cu(110) surface partially covered with oxygen in the O(2 × 1) phase formed by Cu–O chains. The experiments were carried out from low (77 K) to higher temperatures to sequentially open reaction channels that require increased activation energy. At 77 K, water adsorbs at the edges and on top of the Cu–O chains. Heating to 155 K caused water molecules to react with the oxygen atoms in the chains to produce OH. These hydroxyl groups form elongated H-bonded structures mixed with excess unreacted water molecules. The Cu atoms freed in the reaction form small clusters at the location of the initial Cu–O chains. Heating to 180 K leads to a second reaction that desorbs all excess water (i.e., water not H-bonded with OH), leaving H2O–OH zigzag chains along [110] directions. At the low oxygen coverage studied here (<0.12 ML) the extent of the partial water dissociation reaction is stoichiometrically determined by the amount of ...

Chemisorption of oxygen and subsequent reactions on low index surfaces of β–Mo2C: Insights from first-principles thermodynamics and kinetics

Oxygen chemisorption on β–Mo2C surfaces, the subsequent CO/CO2 desorption and oxygen diffusion to the carbon vacancy have been investigated by density-functional theory. The most stable structures together with the energetics of oxygen stepwise adsorption, CO/CO2 desorption and oxygen diffusion to the carbon vacancy were identified. We examined the effect of oxygen coverage on the morphology of β–Mo2C by plotting the equilibrium crystal shape. Thermodynamic effect of temperature and reactant or product pressure on the CO/CO2 desorption were investigated. The CO/CO2 desorption is more favorable at the saturated oxygen coverage than the low oxygen coverage thermodynamically. The subsequent oxygen diffusion to the carbon vacancy after CO/CO2 desorption may happen depending on the surfaces and oxygen coverage.

First-principles calculations for initial oxidation processes of SiC surfaces: Effect of crystalline surface orientations

The initial oxidation processes on both the C-face and Si-face SiC surfaces are investigated by performing electronic-structure calculations within the density functional theory. We find the characteristic features of stable structures and adsorption–desorption behavior depending on the surface orientation of SiC. The desorption of C atoms as CO molecules on the C-face occasionally occurs in addition to the adsorption of O atoms even for the surfaces with low oxygen coverage, whereas the adsorption of O atoms preferentially occurs on the Si-face. These calculated results indicate that the incorporation of O atoms along with the desorption of C atoms is predominant even during the initial oxidation processes on the C-face of SiC. Moreover, on the C-face, three-coordinated O atoms can be formed at the topmost layer of the SiC surface, resulting in the formation of SiO2-like layers whose characteristics are different from those on the Si-face. These findings suggest that the differences in carbon-desorption behavior and resultant structural difference of SiO2 layers between the C-face and the Si-face are possible origins of the orientation dependence of oxidation rate observed in SiC oxidation.

Methanol-Selective Oxidation Pathways on Au Surfaces: A First-Principles Study

With density functional theory, all elementary steps of methanol (CH3OH) dehydrogenation and oxidation on atomic-oxygen-covered or OH-covered Au (111) surfaces are systematically studied. Our results suggest that on low oxygen coverage Au (111) surface the production of CH2O and CO start from α-H elimination and β-H elimination, respectively. The selective oxidation pathway is controlled by thermodynamics of the first step rather than kinetics. The overall energy barrier to produce CO is 0.39 eV corresponding to gas-phase methanol, which indicates that the reaction can proceed at low temperature. On high oxygen coverage Au (111) surface, the elimination of α-H and one β-H can take place simultaneously to form CH2O for the cooperative interaction of two nearby atomic oxygen. The missing observation of CH2O may come from the fact that the newly formed CH2O is ready to react with surface atomic oxygen and hydroxyl to form CH2OO(H) rather than desorption from the surface. The rate-limiting step of the oxidati...