Oxygen Surface Chemistry Research Articles

Impact of nanoparticle exsolution on dry reforming of methane: Improving catalytic activity by reductive pre-treatment of perovskite-type catalysts

Nanoparticle exsolution is regarded as a promising alternative to classical catalyst synthesis routes. In this work, we compare the catalytic performance of nanoparticles formed by in-situ exsolution during dry reforming of methane with particles pre-formed by reductive pre-treatment. The experiments were conducted on three perovskite-type oxides. Using a combination of in-situ and operando spectroscopic investigations (x-ray diffraction, near ambient pressure x-ray photoelectron spectroscopy) and the correlation to the obtained catalytic results, we could highlight that pre-formed nanoparticles strongly enhance the activity compared to in-situ exsolution. Scanning electron microscope images recorded after catalytic tests revealed that nanoparticles formed during reductive pre-treatment are bigger on average than particles formed in-situ. Furthermore, B-site doping with Co or Ni significantly enhanced the catalytic activity. Importantly, the perovskite host lattice was stable in all experiments, thus providing the necessary enhanced oxygen surface chemistry which is the key to the coking resistance of the investigated materials. Additionally, we observe a temperature dependent change of mechanism leading to different product ratios.

Influence of carbon nanostructure and oxygen moieties on dopamine adsorption and charge transfer kinetics at glassy carbon surfaces

Abnormal levels of the neurotransmitter dopamine have been linked to a variety of neurochemical disorders including depression and Parkinson's disease. Dopamine concentrations are often quantified electrochemically using biosensors prepared from carbon electrode materials such as carbon paste or glassy carbon. The charge transfer kinetics of dopamine is highly sensitive to carbon surface termination, including the presence of certain oxygen functional groups and adsorption sites. However, the nature of the binding sites and the effects of surface oxidation on the voltammetry of dopamine are both poorly understood. In this work the electrochemical response of dopamine at glassy carbon model surfaces was investigated to understand the effects of altering both the carbon nanostructure and oxygen surface chemistry on dopamine charge transfer kinetics and adsorption. Glassy carbon electrodes with low oxygen content and a high degree of surface graphitisation were prepared via thermal annealing at 900 °C, whilst highly oxidised glassy carbon electrodes were obtained through electrochemical anodisation at 1.8 V vs Ag/AgCl. The carbon surface structure and composition in each case was studied via X-Ray Photoelectron Spectroscopy. Voltammetry in solutions of dopamine at acidic pH confirmed that both annealing and anodisation treatments result in carbon surfaces with rapid charge transfer kinetics. However, dopamine adsorption occurs only at the low-oxygen, highly-graphitized carbon surface. Density functional theory studies on graphene model surfaces reveal that this behaviour is due to non-covalent interactions between the π-system of dopamine and the basal sites in the annealed surface. Simulations also show that the introduction of oxygen moieties disrupt these interactions and inhibit dopamine adsorption, in agreement with experiments. The results clarify the role of oxygen moieties and basal plane sites in facilitating both the adsorption of and charge transfer to DA at carbon electrodes.

Molecular simulation aided nanoporous carbon design for highly efficient low-concentrated formaldehyde capture

Although recent experimental studies have demonstrated that doping of nanoporous carbons with nitrogen is an effective strategy for highly diluted formaldehyde capture, the impact of carbon surface chemistry and the pore size on formaldehyde capture at ∼ppm concentrations is still poorly understood and controversial. This work presents a combined theoretical and experimental study on dynamic formaldehyde adsorption on pure and oxidized nanocarbons. We find using Monte Carlo simulations and confirm experimentally that cooperative effects of pore size and oxygen surface chemistry have profound impacts on the breakthrough time of formaldehyde. Molecular modeling of formaldehyde adsorption on pure and oxidized model nanoporous carbons at ∼ppm pressures reveals that high adsorption of formaldehyde ppm concentrations in narrow ultramicropores <6 Å decorated with phenolic and carboxylic groups is correlated with long formaldehyde breakthrough times measured in the columns packed with specially prepared oxidized activated carbon fiber adsorbents with the pore size of ∼5 Å.

Role of graphene oxide surface chemistry on the improvement of the interlaminar mechanical properties of resin infusion processed epoxy‐carbon fiber composites

The present study deals with the influence of graphene oxide functional groups on their ability to reinforce an epoxy resin when forming carbon fiber composites. Composites were processed through the direct vacuum infusion of the doped resin into carbon fiber fabrics. We used graphene oxide nanosheets with two different chemical characters: as‐produced graphene oxide, with a high oxygen content and acid character, and a simple ammonia base‐washed graphene oxide, which to a great extent removes the oxidative debris or highly oxidized fulvic‐like entities, resulting in an average lower oxygen content and cleaner surface sheets than as‐produced graphene oxide. Base‐washed graphene oxide performed considerably better in both tensile and mode‐I interlaminar properties of carbon fiber composites. The fracture energy required for the onset of mode I interlaminar fracture toughness was enhanced 31% when using as‐produced graphene oxide and 60% when using base‐washed graphene oxide by adding 0.2 wt% only. More interestingly, base‐washed graphene oxide produces a higher delamination resistance along the entire range of crack growth. The effect of adding graphene oxide with a cleaner surface and lower oxygen surface chemistry allows direct chemical bonding matrix‐graphene when the resin is curing, promoting a better interface fiber‐resin and consequently, improving the reinforcement efficiency. POLYM. COMPOS., 39:E2116–E2124, 2018. © 2017 Society of Plastics Engineers

Surface chemistry of oxygen on aluminum--Performance of the density functionals: PBE, PBE0, M06, and M06-L.

We investigated the performance of the density functional theory (DFT) functionals PBE, PBE0, M06, and M06-L for describing the molecular and dissociative adsorption of O2 onto pure and doped Al(111) surfaces. Adsorption of O2 was studied at the perfect Al(111) surface and compared with the case where an additional Al atom was present as an adatom. Additionally, we studied how these functionals perform when different dopants are present at the Al(111) surface in two distinct geometries: as an adatom or as a substitutional atom replacing an Al atom. The performance of the different functionals is greatly affected by the surface geometry. The inclusion of Hartree-Fock exchange in the functional leads to slight differences in adsorption energies for molecular adsorption of O2 . These differences become very pronounced for dissociative adsorption, with the hybrids PBE0 and M06 predicting more exergonic adsorption than PBE and M06-L. Furthermore, PBE0 and M06 predicted trends in adsorption energies for defective and perfect surfaces which are in line with the experimental knowledge of the effects of surface defects in adsorption energies. The predictions of the non-hybrids PBE and M06-L point in the opposite direction. The analysis of the contributions of the van der Waals (vdW) forces to the adsorption energies reveals that the PBE and PBE0 functionals have similar difficulties in describing vdW interactions for molecular adsorption of O2 while the M06 functional can give a description of these forces with an accuracy which is at least similar to that of the correction of the D3 type.

Identifying the Active Oxygen Species in SnO2 Based Gas Sensing Materials: An Operando IR Spectrsocopy Study

This work demonstrates that it is possible to follow the surface chemistry of oxygen on SnO2 based gas sensing materials using operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The inherent difficulties, due to the intrinsic properties of the studied oxide and the limitations of the method, were overcome by comparing the results obtained for two different materials and by use of isotopically labeled gases together with the simultaneous measurement of the sensor signals. In spite of the differences in the surface composition and reactivity between the different materials, the experimental results show that the reactive oxygen species are similar in nature and the gas recognition takes place by the interplay of surface reduction and (re)oxidation.

Factors governing the adsorption of ethanol on spherical activated carbons

Ethanol adsorption on different activated carbons (mostly spherical ones) was investigated covering the relative pressure range from 0.001 to 1. Oxygen surface contents of the ACs were modified by oxidation (in HNO3 solution or air) and/or by thermal treatment in N2. To differentiate the concomitant effects of porosity and oxygen surface chemistry on ethanol adsorption, different sets of samples were used to analyze different relative pressure ranges (below 1000ppmv concentration and close to unity). To see the effect of oxygen surface chemistry, selected samples having similar porosity but different oxygen contents were studied in the low relative pressure range. At low ethanol concentration (225ppmv) adsorption is favored in oxidized samples, remarking the effect of the oxidizing treatment used (HNO3 is more effective than air) and the type of oxygen functionalities created (carboxyl and anhydride groups are more effective than phenolic, carbonyl and derivatives). To analyze the high relative pressure range, spherical and additional ACs were used. As the relative pressure of ethanol increases, the effect of oxygen-containing surface groups decreases and microporosity becomes the most important variable affecting the adsorption of ethanol.

Insight into the immobilization of ionic liquids on porous carbons

Very different carbon materials have been used as support in the preparation of supported ionic liquid phase samples (SILP). Some of them have been oxidized, either strongly (with ammonium persulfate solution) or weakly (with air at 300°C, 2h). The purpose is to establish which properties of the supports (e.g., porosity -volume and type-, surface area, oxygen surface chemistry and morphology) determine the IL adsorption capacity and the stability (immobilization) of the supported IL phase. The ionic liquid used in this work is 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6]). For each support, samples with different amounts of ionic liquid have been prepared. The maximum IL that can be loaded depends mainly on the total pore volume of the supports. For comparable pore volumes, the porosity type and the oxygen surface content have no influence on the IL loading. The supported IL fills most of the pores, leaving some blocked porosity. The stability of the supported IL phase (especially important for its subsequent use in catalysis) has been tested in water under general hydrogenation conditions (60°C and 10bar H2). In general, leaching is low but it increases with the amount of IL loaded and with the oxidation treatments of the supports.

Defects in surface chemistry--reductive coupling of benzaldehyde on rutile TiO₂(110).

The surface chemistry of oxygen and oxygenates on Rutile TiO2(110) is of great interest for various applications such as heterogeneous catalysis and photo catalysis. Though it is generally accepted that surface defects are active sites, the role of subsurface defects is under debate. We have therefore investigated the influence of the bulk defect density on the reductive coupling of benzaldehyde to stilbene as a model system. Using IRRAS we identify stilbene diolate as a reduction intermediate. The concentration of this intermediate is proportional to the bulk defect density, whereas adsorption of benzaldehyde at lower temperatures is not affected, which indicates a dominant role of Ti interstitials at temperatures above 400 K.

Effect of the porous texture and surface chemistry of activated carbons on the adsorption of a germanium complex from dilute aqueous solutions

The effect of the adsorption properties and surface chemistry of activated carbons on the adsorption of germanium (IV) from dilute aqueous solutions (50 ppm) was investigated. A large number of activated carbons were obtained by activating three precursors, one coal and two lignocellulosic chars, with alkaline hydroxides, NaOH and KOH, and different activating agent/precursor ratios (from 1/1 to 5/1). The characterization of their textural properties, assessed by N 2 and CO 2 isotherms, and their oxygen surface chemistry, assessed by temperature-programmed desorption experiments, confirm that a large variety of materials with different surface areas, pore size distributions and surface chemistries are available for this study. To increase the Ge adsorption, an organic compound (catechol) was used as a ligand in order to form a germanium complex. Such a complex works very well, allowing its good adsorption on the activated carbons. The results showed that to achieve the highest Ge adsorption, the activated carbon should show a high apparent surface area, a wide microporosity and a moderate oxygen surface content that may assure the wettability of the solution but that does not produce large repulsive interactions.

Competitive Etching and Oxidation of Vicinal Si(100) Surfaces

ABSTRACTExposure of a vicinal Si(100) surface to oxygen at around 550 C produces etching-mediated step recession. In addition, some oxide islands are formed which locally pin receding steps. We develop an atomistic lattice-gas model for this process which accounts for the interplay between oxygen surface chemistry (adsorption, diffusion, oxide formation, and etching via SiO desorption) and the silicon surface and step dynamics (anisotropic diffusion and aggregation of di-vacancies formed by etching, and ad-dimer attachment-detachment dynamics at steps incorporating anisotropic energetics). Kinetic Monte Carlo simulation of this model produces step morphologies retaining some qualitative but not quantitative features of their equilibrium structure (alternating rough SB steps and smooth SA steps), except for pinning which produces protruding “fingers”. These features are seen in Scanning Tunneling Microscopy studies.

Work function changes and surface chemistry of oxygen, hydrogen, and carbon on indium tin oxide

The surface chemistry of indium tin oxide (ITO) has been investigated with Auger electron spectroscopy (AES) and high resolution electron energy loss spectroscopy (HREELS). A vibrating Kelvin probe (KP) with a graphite reference was used to monitor the absolute work function ( Φ) of ITO as a function of chemical modification. The ITO was exposed in situ to molecular hydrogen (H 2), hot-filament-activated oxygen ( O 2 ∗ ), and hot-filament-activated deuterium ( D 2 ∗ ). The initial Φ of ITO was determined to be 5.2 eV, and surface chemical changes had strong effects on this value, as seen by KP. Exposure of clean ITO to O 2 ∗ increased Φ to ∼5.6 eV, but the increase was short-lived. The changes in Φ over time were correlated with the uptake of carbon impurities in ultra high vacuum (UHV), as monitored by AES. The HREELS of ITO revealed significant hydrocarbon impurities. Chemical reduction of ITO produced a metallic surface and dehydrogenated the adsorbed hydrocarbons. Both re-oxidation of metallic ITO and oxidation of clean ITO temporarily removed adventitious carbon from the surface, but oxidized ITO adsorbed an even larger quantity of carbon over time.

Photoelectron spectroscopic investigations and exoelectron emission of CVD diamond surfaces modified with oxygen and potassium

The surface chemistry of oxygen and alkali (K) treated CVD diamond surfaces was studied with X-ray induced photelectron spectroscopy (XPS) and UV-light induced photoelectron spectroscopy (UPS). The oxygen modification was achieved ex-situ using oxidizing acids (hydrochloric and nitric acid) or in-situ by evaporation of K from a SAES getter source. XPS analysis of the C 1s and O 1s spectra demonstrated that the acid treatment leads to a considerably uptake of oxygen (or OH groups). Owing to the presence of surface oxygen the C 1s peaks splits into two components with binding energies of 284.2 (diamond) and 287.3 eV (carbon bonded to oxygen). The adsorption of K was followed with XPS and UPS (HeI) spectroscopy. With XPS the evaluation of the K 2p states demonstrates that the amount of K which can be accommodated at 300 K depends on the oxygen precoverage; the maximum obtainable K coverage increases with the surface oxygen concentration. In the presence of K the C 1s state from the CO bonding shifts to lower binding energy, 286.4 eV. With UPS a broad unstructured emission probably from carbon 2p + 2s states is detected between 3.5–10 eV below E F from the untreated diamond surface. The emission between 0.0–2.5 eV was fairly low (band gap of diamond), it could be considerably reduced by the acid treatment of the sample. This is suggested to be owing to the removal of graphitic or amorphous carbon impurities. Owing to the presence of oxygen the broad emission peak shifted to higher binding energy (from 6.8 to about 8.0 eV). Low doses of K lead to a reduction of the 2p + 2s emission from carbon and oxygen, with a center of the reduction at 5.0 eV. For higher K coverages new K-induced states evolves. One state is located in the band gap of diamond near E F ; further peaks were found at 3.5, 7.0 and about 14 eV below E F . K covered diamond surfaces showed a strong exoelectron emission after oxygen exposure in vacuum.

Effect of low-intensity laser radiation during oxidation of the GaAs(110) surface

Considerable interest in the use of laser irradiation as a means of processing (annealing, cleaning, alloying, etc.) semiconductor materials, as well as the importance of the oxygen surface chemistry on III-V semiconductors, has led us to study the effect of low-intensity (?3 W/cm2) laser radiation on the oxidation behavior of (110) surfaces of GaAs cleaved in UHV (&amp;lt;10−10 Torr). The oxygen sticking probability in the submonolayer coverage range has been increased by a factor of 103 (from a probability of ∠10−9 to ∠10−6) by uniform irradiation of the GaAs surface with a continuous wave Ar+ laser (λ = 5145 Å) during the oxygen exposure. We find the data cannot be explained in terms of either heating of the surface or excitation of the oxygen by the laser radiation; it appears that the most likely explanation of the phenomena is an increase in the density of electrons and/or holes at the surface. A limiting step in the oxygen uptake process is the breakup of the oxygen molecule; this dissociation would be increased by increasing the number of electrons at or near the surface. In chemical terms, the reconstructed GaAs surface has no orbitals suitable for binding oxygen—such orbitals can be provided by changing the charge state of a surface atom via optical excitation.

The unified model for Schottky barrier formation and MOS interface states in 3–5 compounds

The object of the work reported here has been to develop an understanding on an atomic basis, of the interaction between semiconductor and metals or oxygen overlayers which determine the electronic characteristics of the interface, e.g. the Schottky barrier height of metal-semiconductor interface or the density and the energy position of oxide-semiconductor interface states. The principal experimental tool used has been photoemission excited by monochromatized synchrotron radiation (10 < hv < 300 eV). Extreme surface sensitivity is obtained by tuning the synchrotron radiation so that the minimum escape depth is obtained for the excited electrons of interest. In this way only the last two or three atomic layers of the solid is sampled. By changing hv, core levels or the valence bands can be studied. Using a metallic reference, the Fermi level position at the surface, E Fs, can be directly determined. GaAs, InP and GaSb have been studied. On a proper cleaved surface, there are no surface states in the semiconductor band gap; thus, no pinning of E Fs. Pinning of E Fs can then be monitored as metals or oxygen are added to the surface — starting from sub-monolayer quantities. Two striking results are obtained: (1) pinning position is independent of adatom for oxygen and a wide range of metals, and (2) the pinning is completed by much less than a monolayer of adatoms. These results can not rationally be explained by the pinning being due to the levels produced directly by the adatoms. Rather, they suggest strongly that the adatoms disturb the semiconductors surface forming defect levels. This is supported by appearance of the semiconductor atoms in the metal and by the disordering of the semiconductor surface by sub-monolayer quantities of oxygen. Since these basic experiments have been reported previously they are only reviewed briefly here. When metals or oxygen are added under very gentle conditions, the following levels are formed (all energies relative to the conduction band minimum). III–V Acceptor (missing atom) Donor (missing atom) GaAs 0.65 eV (As) 0.85 eV (Ga) InP 0.45 eV (P) 0.1 eV (In) GaSb 0.5 eV (Sb) below VBM (Ga) These results explain why Schottky barrier gates will provide useful FET's on n-GaAs but not n-InP. Likewise they predict that MOS or MIS gates will be practical for n-InP but not n or p GaAs. Studies of the oxygen surface chemistry find the As oxides to be unstable and P oxides to be stable — reinforcing the prediction. Recent work of others is reviewed and alternate identification of the missing atoms in the defects is discussed. Some of the new process possibilities opened up by this work are considered.

Basic studies of the oxygen surface chemistry of silver: Oxygen, dioxygen, oxide and superoxide on rubidium-dosed Ag(111)

The oxygen chemistry of the rubidium-dosed Ag(111) surface has been examined by LEED, Auger spectroscopy and thermal desorption spectroscopy. As with the other alkali metals, Rb enormously enhances the sticking probability of oxygen, but for the first time evidence is obtained for the formation of alkali-induced dioxygen surfaces species. Such species can arise in either of two ways: (i) by oxygen chemisorption in the presence of subsurface Rb on an otherwise clean surface (ii) by the formation of rubidium Superoxide as a surface compound at high Rb loadings. LEED shows the development of four distinct surface structures and the oxygen desorption spectra exhibit up to five distinct features. A consistent interpretation of these observations is presented in terms of the formation of chemisorbed atomic oxygen, dioxygen, normal rubidium oxide and rubidium Superoxide.

Basic studies of the oxygen surface chemistry of silver: Oxygen, dioxygen, oxide and superoxide on rubidium-dosed Ag(111)

Basic studies of the oxygen surface chemistry of silver: Oxygen, dioxygen, oxide and superoxide on rubidium-dosed Ag(111)