High Oxygen Coverage Research Articles

The Effect of Surface Oxygen Coverage on the Oxygen Evolution Reaction over a CoFeNiCr High-Entropy Alloy.

Developing cost-effective and highly active electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing sustainable energy applications. High-entropy alloys (HEAs) made from earth-abundant transition metals, thanks to their remarkable stability and electrocatalytic performance, provide a promising alternative to expensive electrocatalysts typically derived from noble metals. While pristine HEA surfaces have been theoretically investigated, and the effect of oxygen coverage on conventional metal electrocatalysts has been examined, the impact of surface oxygen coverage on the electrocatalytic performance of HEAs remains poorly understood. To bridge this gap, we employ density functional theory (DFT) calculations to reconstruct the free energy diagram of OER intermediates on CoFeNiCr HEA surfaces with varying oxygen coverages, evaluating their impact on the rate-limiting step and theoretical overpotential. Our findings reveal that increased oxygen coverage weakens the adsorption of HO* and O*, but not HOO*. As a result, the theoretical overpotential for the OER decreases with higher oxygen coverage, and the rate-limiting step shifts from the third oxidation step (HOO* formation) at low coverage to the first oxidation step (HO* formation) at higher coverage.

Heterogeneously Catalyzed Partial Oxidation of Styrene on a Silver Surface

AbstractThe epoxidation of olefins on Ag/O systems is a significant industrial‐scale process within heterogeneous catalysis. However, the details of the surface reaction remain controversial, and it has been highly challenging to reconcile the findings from cataltyic studies under reaction conditions with the highly detailed static studies under carefully controlled ultra‐high vacuum (UHV) conditions. In this study, we combine molecular beam surface scattering and ion imaging techniques to explore the partial oxidation of styrene. This experimental approach enhances the sensitivity to the extent that we can directly observe the partial oxidation product, styrene oxide, under UHV conditions. We note that partial oxidation exclusively occurs at high oxygen coverages, which we attribute to the reaction of styrene with electrophilic oxygen formed specifically at elevated coverages.

Characterization of adsorption sites on IrO2via temperature programmed O2 desorption simulations.

This study presents simulations of temperature-programmed desorption (TPD) profiles using desorption energy data from density functional theory (DFT) calculations. We apply this method to investigate the desorption of oxygen (O2) from IrO2(110) to gain insight into the kinetics of oxygen coupling and desorption, important elementary steps in the oxygen evolution reaction (OER). Initially, we confirm the thermodynamically stable adsorption site for oxygen in the pristine IrO2(110) as IrCUS, even with a high oxygen coverage. We successfully simulate TPD for O2 desorption, achieving good agreement with experimental TPD data for different initial oxygen exposures when including more than one adsorption site. We identify a new adsorption site, related to the formation of steps on IrO2(110)(IrCUS-step-0.5), that is essential for reproducing the experimental TPD. Our findings suggest that the observed TPD peaks are the result of different adsorption sites on the surface, rather than solely a lateral interactions effect. This work provides insight into the behavior of oxygen adsorption on IrO2, with implications for understanding surface reactivity and catalytic processes involving this material.

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.

Water and Hydroxyl Reactivity on Flat and Stepped Cobalt Surfaces

Hydroxyl adsorbates generally appear as transient species during water formation from adsorbed oxygen and hydrogen atoms on a metal surface, a reaction that is part of the catalytic cycle in various important surface-catalyzed reactions such as Fischer–Tropsch synthesis. In the present work, temperature-programmed desorption and in situ synchrotron XPS were used to study water adsorption and OH reactivity on a flat and a stepped cobalt single crystal surface. Water adsorbs intact on the flat Co(0001) surface and desorbs around 160 K. Electrons induce dissociation of water and produce OH species at low temperature. Hydroxyl species can also be formed by the reaction between Oad and H2O, but only for high initial oxygen coverage while low coverage Oad appears largely unreactive. Reactive hydrogen species (H atoms) produced by a hot tungsten filament hydrogenate adsorbed oxygen atoms at low temperature already and both OHad and H2O are formed. In all cases, hydroxyl adsorbates react around 190 K to form water via 2 OHad → H2O (g) + Oad associated with an activation barrier of 40–50 kJ mol–1. Water readily dissociates on the step sites exposed by vicinal Co(10–19). A part of the OHad species recombine to form water and oxygen between 200 and 300 K, while decomposition of OHad into Oad and Had dominates above 370 K. For catalysis, the high reactivity of step sites for water dissociation and the high stability of OHad at these sites implies that O removal from these sites may be difficult and may limit the overall rate of Fischer–Tropsch synthesis on cobalt catalysts.

Role of Fluxionality and Metastable Isomers in the ORR Activity: A Case Study

Understanding the dynamic reconstruction of active sites and the fluxional behavior of subnanoclusters has become the heart of operando modeling techniques. An accurate description of catalysis phenomena extends toward incorporating the activity of thermally accessible ensembles of metastable isomers in addition to the global minima (GM). Considering the Pt13 cluster as a model catalyst for gas-phase oxygen reduction reaction (ORR), we have studied the role of metastable isomers and unravelled their distinct catalyst dynamics leading to high fluxionality. The different adsorption energetics and ORR activity featured by the metastable isomers reveal the importance of addressing the fluxional behavior of subnanoclusters. A detailed mechanistic ORR investigation provided isomers possessing an activity comparable to GM. Furthermore, a statistical ensemble representation demonstrates the substantial role of metastable isomers within 0.4 eV relative energy difference from GM, with negligible activity enhancement of isomers with high energy at 300 K. Ab initio thermodynamics-based stability analysis represents different energetics associated with the Pt13 clusters at high oxygen coverage. This study highlights the importance of exploring the role of metastable isomers in determining the cumulative catalytic activity of subnanometer clusters.

High local oxygen coverage causes initial oxidation of UN(001) surface

Uranium mononitride (UN) is a promising nuclear fuel, however, during its storage and operation, the surface oxidation or the initial corrosion of UN always takes place. The limited understanding of atomistic corrosion mechanism hinders the improvement of UN corrosion resistance. Here, the interactions between adsorbates (O2 and H2O molecules and O adatoms) and the pristine UN(001) surface, the UN(001) surface with N vacancies (Nvac_UN(001)), and the UN(001) surface with N vacancies occupied by O atoms (O2N_UN(001)) are investigated by using density functional theory calculations. O2 molecule is found to spontaneously dissociate on these surfaces, while the H2O molecule is weakly adsorbed on the top site of U atom. Due to the attractive OO lateral interactions, high local O coverage (e.g., 4O coordinated to a U atom) is easily resulted, where significant relaxation of surface U atoms is observed. The maximum vertical displacement of U atoms is more than 1 Å at medium O coverage (e.g., 3O coordinated to a U atom). N vacancy and O-doping on the UN(001) surface do not change the vertical displacement of U atoms, but alter the adsorption energies of O atoms. The competition between surface U-O and subsurface U-N interactions leads to the formation of uranium cavities in the surface, preluding the dissolution as oxides of U atoms away from the surface. Our results provide new insights for the understanding of initial oxidative corrosion of UN surface.

Anion-catalyzed active dissolution model for the electrochemical adsorption of bisulfate, sulfate, and oxygen on gold in H2SO4 solution

The mechanism of the electrochemical adsorption of bisulfate, sulfate, and oxygen on polycrystalline gold in H2SO4 (0.5 M) was modeled using a mechanistic analysis of potentiodynamic polarization and electrochemical impedance spectroscopy data acquired at multiple dc potentials in the electric double-layer and pre-oxide formation region. Electrochemical quartz-crystal microgravimetry and optical ellipsometry provided evidence for the potential-dependent adsorption of solution species on the gold surface. Also, despite the high surface coverages of oxygen on the electrode surfaces, dissolution was observed over the entire the potential region studied. The anion-catalyzed active dissolution model accounts well for the experimentally observed current density and fractional surface coverages.

First-principles investigation of interaction between the atomic oxygen species and carbon nanostructures

• The following trends in the value of adsorption energies of triplet oxygen atoms on the carbon nanostructures were identified: diamond-like structures (sp 3 ) < graphene-like structures (sp 2 ) < fullerenes & nanotubes (sp 2 , pentagons, high curvature). • For the graphene flake and fullerene structures there are common precursors for carbon desorption in the form of CO/CO 2 molecules: (1) ether-ether-epoxy precursor and (2) epoxy-ether-epoxy. • Etching of sp 3 hybridized carbon structures can proceed faster than etching of sp 2 structures. • For graphene structure, unzipping process is more favorable than reduction process at lower temperatures. In this work we perform a systematic theoretical study of oxidation process of different carbon nanomaterials by atomic oxygen. In order to understand the atomistic mechanism of interaction between atomic oxygen and carbon nanostructures and to explain experimental results on their oxygen treatment, first-principles calculations were performed. Influence of spin state of the atomic oxygen on its interaction with different carbon nanostructures (finite size graphene flake, carbon nanotube, fullerene, diamondoid) was investigated. We found that the structure and stability of the adsorption complexes are determined by the oxygen spin state and depend on the carbon nanostructure morphology and hybridization. The atomistic mechanisms of chemical transformations driven by high local oxygen coverage, such as CO/CO 2 desorption and graphene unzipping, were investigated. Calculated barriers of CO 2 desorption allow to explain experimental data on thermal reduction of the oxidized carbon structures. In case of graphene, competition between channels resulting in CO 2 desorption and plane unzipping was observed. We found that for the temperatures ~400 K and lower unzipping process is favorable, since CO 2 desorption precursor formation proceeds through higher activation barrier (1.3 eV) and requires temperatures above 450 K.

Investigation of the oxygen coverage of propylene epoxidation on Ag(1 1 1) surfaces from DFT

The direct epoxidation of propylene on Ag(111) under different oxygen coverage conditions was systemically studied on the basis of density functional theory calculation and the general trends of oxygen coverage on the two parallel competitive reaction paths in propylene oxide (PO) formation: α-H stripping versus epoxidation and the generation of PO versus acetone (AC) were explored. Then, the PO formation selectivity was investigated. Results showed that increase in oxygen coverage enhanced the adsorption strength of adsorbed species, except the O* species. In the primary reaction of propylene epoxidation, high oxygen coverage facilitates the first α-H stripping process, and opposite trend was observed in epoxidation process. As for the secondary chemistry in propylene epoxidation, increase in oxygen coverage benefits PO and AC formation. The coverage of O* has considerable influences on the primary reaction but small influence on the secondary reaction. Although a high oxygen coverage benefits the first α-H stripping, the second α-H stripping has higher barrier than that in secondary chemistry, and thus the PO formation would be better than that of acrolein formation at higher oxygen coverage.

Strained Pt(221) Facet in a PtCo@Pt-Rich Catalyst Boosts Oxygen Reduction and Hydrogen Evolution Activity

Over the last years, the development of highly active and durable Pt-based electrocatalysts has been identified as the main target for a large-scale industrial application of fuel cells. In this work, we make a significant step ahead in this direction by preparing a high-performance electrocatalyst and suggesting new structure-activity design concepts which could shape the future of oxygen reduction reaction (ORR) catalyst design. For this, we present a new one-dimensional nanowire catalyst consisting of a L10 ordered intermetallic PtCo alloy core and compressively strained high-index facets in the Pt-rich shell. We find the nanoscale PtCo catalyst to provide an excellent turnover for the ORR and hydrogen evolution reaction (HER), which we explain from high-resolution transmission electron microscopy and density functional theory calculations to be due to the high ratio of Pt(221) facets. These facets include highly active ORR and HER sites surprisingly on the terraces which are activated by a combination of sub-surface Co-induced high Miller index-related strain and oxygen coverage on the step sites. The low dimensionality of the catalyst provides a cost-efficient use of Pt. In addition, the high catalytic activity and durability are found during both half-cell and proton exchange membrane fuel cell (PEMFC) operations for both ORR and HER. We believe the revealed design concepts for generating active sites on the Pt-based catalyst can open up a new pathway toward the development of high-performance cathode catalysts for PEMFCs and other catalytic systems.

Effect of lattice oxygen in nickel-based catalysts with different support on coke resistance of filamentous coke in CO2 reforming of tar

ABSTRACT For CO2 reforming of tar, deactivation of catalysts caused by filamentous coke deposition limits its application in solid waste treatment. The filamentous coke removal can be aided by the lattice oxygen of active mental sites, however, there is no information on the influence of lattice oxygen in catalyst support on filamentous coke oxidation. To investigate the activity and resistance to coke deposition of different catalysts, based on a comparison of the performance of Ni/ZrO2, Ni/SiO2, Ni/MgO, and Ni/attapulgite catalysts, the influence of lattice oxygen on filamentary coke deposition is investigated in this study. In CO2 catalytic reforming experiments, the Ni/ZrO2 catalysts showed higher coke conversions (10%–23%) than the Ni/SiO2, Ni/MgO, and Ni/attapulgite catalysts. In the meantime, the lattice oxygen coverage of the Ni/ZrO2 superficies reaches 55.59%, giving Ni/ZrO2 a better resistance to filamentary coking. The high lattice oxygen coverage also improves the oxidation of the coke superficies by lattice oxygen. However, there was still 31.66% filamentary coke aggregation on the Ni/ZrO2 catalyst superficies. In conclusion, inhibiting the formation of filamentous coke and promoting the oxidation of filamentous coke are key to improving the efficiency of CO2 reforming of anaerobic tar. The results of the research can be used as a reference for the improvement of catalysts.

Effects of high-temperature CeO2 calcination on the activity of Pt/CeO2 catalysts for oxidation of unburned hydrocarbon fuels

High temperature (800 °C) pre-calcination of CeO2 support decreases the surface defects and improves the mobility of surface lattice oxygen. As a result, the supported Pt clusters have higher oxygen coverage and superior HC oxidation activity.

Study on CO/CO2 formation mechanism of Zigzag model coke with high oxygen coverage based on DFT theory

The clean and efficient utilization of carbon resources is becoming more and more important, in energy, material, and chemical engineering field, but the mechanism of coke oxidation, especially that of the CO2/CO desorption is not fully studied yet. In this paper, density functional theory was used to study the oxidation mechanism of Zigzag char structure with high coverage of O2, which is related to an oxidation under lower temperature or high pressure. Based on the corresponding quantum chemistry calculation, it is shown that there are several possible pathways for the CO2 desorption process, which may need rearrange to form the structure containing O–C–O clusters. And successively, multiple intermediate reaction steps are required to complete the desorption of CO2. Other than in the literature that the COO–O–C functional group formed first, with then the C–O bond broken and CO2 desorbed respectively, a novel pathway with two C–O bonds broken simultaneously to generate CO2 was found. It results from a functional group of COO–char formed, and certain alternative pathways via C–C bonds breaking were also dealt with, as well as related CO desorptions. The reaction model built was validated by theoretical and experimental results from literature satisfactorily.

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, Oe and nucleophilic, On). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low Oe coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of Oe on Ag(111) revealed that Oe was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH2O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH2O selectivity and methanol conversion, where 93% CH2O selectivity can be achieved for an oxygen surface coverage of θO = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of ∼4 for θO = 0.26 ML with a suppression of CH2O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity.

Surface phase stability of PdAg core-shell nanoalloys in oxidizing atmospheres and its relevance to surface atomic charge

The early stage of oxidation on Pd6@Ag32 and Ag6@Pd32 core-shell nanoalloys are calculated by the surface phase stability diagrams using a first-principles atomistic thermodynamics method in oxidizing atmospheres. Ag38 nanoparticle with ΔμO = − 0.95 eV is more stable than Pd38 nanoparticle with ΔμO = − 1.3 eV. Unexpectedly, Pd6@Ag32 core-shell nanoalloy with ΔμO = − 0.9 eV exhibits better surface phase stability than Ag38 nanoparticle but Pd-segregated Pd6@Ag32 nanoalloys have lower stability than Ag38 nanoparticle. Meanwhile, Ag6@Pd32 core-shell nanoalloy with ΔμO = − 1.5 eV displays lower surface phase stability than Pd38 nanoparticle but Ag-segregated Ag6@Pd32 nanoalloys show better surface phase stability than Pd38 nanoparticle. Surface-segregated Pd6@Ag32 and Ag6@Pd32 core-shell nanoalloys with more surface Ag atoms tend to possess the higher surface phase stability. More interestingly, the order of surface phase stability follows the same trend of atomic charges, that is, the more negative charges corresponding to the lower surface phase stability. In addition, oxidation of PdAg nanoalloys can greatly change electronic structure and atomic charges and have diverse influences on catalytic properties. Unlike bulk PdAg alloys, the oxygen-induced Pd surface segregation in Pd6@Ag32 core-shell nanoalloy takes place under high oxygen coverage rather than vacuum and medium oxygen coverage. The high oxygen coverage upto 24 oxygen atoms can segregate Pd to outer shell in Pd6@Ag32 core-shell nanoalloy and restrain the segregation of Ag onto Ag6@Pd32 core-shell nanoalloy. Our results can provide useful information for designing PdAg-based catalyst materials with appropriate surface phase stability towards fuel cells.

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.

The effect of oxygen coverages on hydrogenation of Mg (0001) surface

The H2 dissociation and subsequent atomic H diffusion on the oxide Mg(0001) surface have been investigated using density functional theory calculations. Our calculation indicates that at low oxygen coverages the H2 dissociation is the rate-limiting step for surface hydrogenation. The increase of oxygen coverages from 0 to 1/4 monolayer results in decreased H2 dissociation energy barriers from 0.93 to 0.76 eV, indicating that partial oxidized Mg(0001) surface facilitates hydrogenation. This can attributed to the enhancement of H binding strength on the oxidized Mg surface. However, the atomic H diffusion energy barriers increase with increasing H binding strength on the surface. At high oxygen coverage (1 monolayer) the diffusion of H atom from surface to subsurface is the rate-limiting step for the hydrogenation. The H atom diffusion pathways are blocked by surrounding O2− anions, hence H atom needs to overcome a significantly high energy barrier of 2.38 eV to diffuse from 1 monolayer oxide surface to subsurface. This indicates that high oxygen coverages would impede the surface hydrogenation.

Superbound Excitons in 2D Phosphorene Oxides.

The optical excitations in layered phosphorene oxides are studied via ab initio calculation together with GW approximation for the self-energy and solving the Bethe-Salpeter equation (BSE) for the excitations. It is found that the electronic structure of phosphorene oxides closely depends on the oxygen concentration. For the high oxygen coverage structure P4O10, it shows a strong localized molecular-like electronic structure with exciton binding ( Eb) energy up to 3.0 eV, which is several times larger than the ordinary Eb value in various low-dimensional materials. This study may provide an alternative way to design functional layered materials with large exciton binding energies by controlling the oxidation level in phosphorene oxides.

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.