Adsorption Of Oxygen-containing Species Research Articles

The effect of oxygen-containing species on corrosion behavior of Ta (1 1 0) surface: A DFT study with an experimental verification

Adsorption of oxygen-containing species on the surface of tantalum (Ta) electrode significantly affects its electrochemical corrosion behavior. Density-functional theory (DFT) is employed to investigate the adsorption energies, structural properties and electronic structures of atomic oxygen (O) and molecular water (H2O) on Ta (110) surface. The adsorption behavior of H2O at room temperature is also studied based on ab initio molecular dynamics (AIMD). We find the passivation of Ta metal is mainly attributed to the strong adsorption of oxygen atoms. Thermodynamic results show that bulk Ta2O5 is easily formed at room temperature, which is the fundamental reason for the spontaneous passivation of Ta (110) surface. The formation of an oxygen monolayer (1.00 ML) on Ta (110) surface dramatically increases the work function, making the equilibrium potential of Ta electrode move in the positive direction, thus slowing down the corrosion rate of Ta metal. However, the adsorption of H2O causes a negative work function change, which promotes its anodic dissolution. The electrochemical impedance spectra (EIS) of tantalum foil in three different NH4F-methanol electrolytes (pure, 0.01 M water and oxygen saturated) shows that the charge transfer resistance increases in the sequence RH2O < Rpure < Roxygen, which can be well explained by the results of DFT calculations.

Oxygen reduction at platinum electrodes: The interplay between surface and surroundings properties

Summary In this work, recent progress in the understanding of the mechanism of the oxygen reduction reaction at Pt surfaces is shortly reviewed. Specifically, the presence of a soluble and short-lived intermediate different to H2O2 in the ORR reaction path and the interrelated effect between the surface arrangement, adsorption of oxygen-containing species and water structure in the ORR reactivity in acid environments are discussed. Besides, the influence of the proton concentration on the ORR product distribution, the existence of a chemical step and the possible role of the soluble intermediate as a bifurcation point in the mechanism are also analyzed.

Origin of enhanced photocatalytic activity and photoconduction in high aspect ratio ZnO nanorods

Faceted ZnO nanorods with different aspect ratios were synthesized by a solvothermal method by tuning the reaction time. Increased reaction leads to the formation of high aspect ratio ZnO nanorods largely bound by the prism planes. The high aspect ratio rods showed significantly higher visible light photocatalytic activity when compared to the lower aspect ratio structures. It is proposed that the higher activity is due to better charge separation in the elongated 1D structure. In addition, the fraction of unsaturated Zn(2+) sites is higher on the {1010} facets, leading to better adsorption of oxygen-containing species. These species enhance the production of reactive radicals that are responsible for photodegradation. The photocurrent for these ZnO nanostructures under solar light was measured and a direct correlation between photocurrent and aspect ratio was observed. Since the underlying mechanisms for photodegradation and photocurrent generation are directly related to the efficiency of electron-hole creation and separation, this observation corroborates that the charge separation processes are indeed enhanced in the high aspect ratio structures. The efficiency of photoconduction (electron-hole pair separation) could be further improved by attaching Au nanoparticles on ZnO, which can act as a sink for the electrons. This heterostructure exhibits a high chemisorption of oxygen, which facilitates the production of highly reactive radicals contributing to the high photoreactivity. The suggested mechanisms are applicable to other n-type semiconductor nanostructures with important implications for applications relating to energy and the environment.

Computational and experimental study of the Volcano behavior of the oxygen reduction activity of PdM@PdPt/C (M = Pt, Ni, Co, Fe, and Cr) core–shell electrocatalysts

The activity of oxygen reduction electrocatalysts is governed by the Sabatier principle and follows a Volcano curve as a function of the oxygen-binding energy. Density functional theory calculations show that the oxygen-binding energy decreases in steps of about 10kJ/mol in a series of core–shell Pd3M@Pd3Pt (M=Ni, Co, Fe, Mn, and Cr) electrocatalysts, leading to a gradual, Volcano-like variation in the oxygen reduction activity. A series of carbon-supported PdM@PdPt (M=Ni, Co, Fe, and Cr) nanoparticles with similar particle sizes were prepared by an exchange reaction between PdM nanoparticles and an aqueous solution of PtCl42-. The variation in the surface electronic structure of the core–shell structures was evaluated by Pt 4f7/2 X-ray photo-electron spectroscopy and by CO-stripping voltammetry and agrees with the first principle calculations. At 0.85V, the PdM@PdPt/C core–shell electrocatalysts show a 6-fold variation in activity, following the Volcano trend predicted by the calculations. The Pt mass activity of the Volcano-optimal PdFe@PdPt/C catalyst is an order of magnitude higher than the activity of commercial 3.0-nm Pt/C catalysts. The core–shell catalysts also display a high methanol tolerance, which is important for use in direct methanol fuel cells. Calculated Pt–M segregation energies suggest that the Pd3M@Pd3Pt core–shell structures are stable, in particular in the presence of 1/4ML CO. Adsorption of oxygen-containing species may induce surface segregation of the 3d transition metal, except for the Volcano-optimal ORR catalyst, Pd3Fe@Pd3Pt.

Surface Properties of Nickel Electrodes in an Alkaline Electrolyte: An Ellipsometry Study

Adsorption of oxygen-containing species (OCS) on a polycrystalline nickel electrode in an alkaline electrolyte is studied by electrochemical and ellipsometric methods in the potential region from hydrogen evolution to oxygen evolution. It is concluded that OCS have stoichiometric composition Ni(OH)2 up to 1.35 V. The potential of the OCS formation onset, the oxide monolayer thickness, and the surface coverage by OCS in the interval 0 ≤ θ ≤ 1 are determined. The oxidation and reduction of a nickel oxide layer, repeatedly cycled at 1.35–1.5 V, suggests that this process is chemically reversible and the thickness of the oxide formed at 1.35 V is constant. It is shown that keeping the electrode under open-circuit conditions at 0.7–0.9 V for 10–20 h leads to the formation of nonelectroconducting oxide “NiOz.”

Electrocatalytic oxidation of preadsorbed monolayer of CO on polycrystalline Pt 60–Ru 40 electrocatalyst: nucleation and growth of oxygen-containing species

The kinetics and mechanism of nucleation and growth of oxygen-containing-species, during the electro-oxidation of preadsorbed saturated monolayer of CO on polycrystalline Pt 60–Ru 40 electrodeposited catalysts, were investigated in a CO-free perchloric acidic solution, using stripping voltammetry and potentiostatic pulse technique. The surface composition of investigated catalysts was determined using XPS. The plots of CO oxidation rate vs. time ( j– t curves) displayed responses quite typical for the processes controlled by the nucleation and growth phenomena. Therefore, the overall rate of the CO oxidation reaction can be expressed in terms of the rate of nucleation and growth of oxygen-containing species in adsorbed CO monolayer. With the increase in CO oxidation potential above +0.5 V vs. NHE, the change from a 2D-kinetically controlled nucleation to a 3D-diffusionally controlled nucleation mechanism was observed. Enhanced electrocatalytic activity of the investigated Pt–Ru surface toward CO electro-oxidation, compared to pure Pt surface, was discussed in terms of the propensity of Ru atoms nucleation sites toward the adsorption of oxygen-containing species and in respect to the observed difference in the intrinsic rate constants for CO oxidation on Pt vs. Ru surface atoms. Potentiostatic and voltammetric CO stripping experiments clearly showed the bifunctional character of our Pt–Ru catalyst, with enhanced synergistic properties.

X-ray photoelectron spectroscopy of [formula omitted] interfaces on self-assembled monolayers of 16-mercaptohexadecanoate

X-ray photoelectron spectroscopy (XPS) is used to identify the interactions at a metal/organic interface formed by evaporation of Cr overlayers on the methyl ester groups at the surface of a self-assembled monolayer (SAM) of 16-mercaptohexadecanoate [HS(CH 2) 15COOCH 3] on gold. The high reactivity of Cr with COOCH 3 groups promotes the growth of a relatively smooth Cr layer on top of the SAM. The CrCOOCH 3 interaction forms a dominant species for Cr/end group ratios less than 1, which is similar to that formed in the CrCOOH interaction on a SAM, but an additional species corresponding to a O 1s component at a higher binding energy is also formed that is not understood. For higher Cr coverages, a Cr(III) oxide is formed by dissociation of oxygen primarily from the SAM end groups. Adsorption of oxygen-containing species from the UHV ambient was found to have little effect on the results for less than 0.6 nm Cr coverage.

ChemInform Abstract: Palladium Electrode in Oxygen‐Saturated Aqueous Solutions. Potential Dependent Adsorption of Oxygen Containing Species and Their Effect on Oxygen Reduction.

AbstractThe effects of potential and pH on the adsorption of oxygen containing species at Pd electrodes and the influence of this adsorption on the kinetics of the oxygen reduction (OR) reaction, particularly in the low current‐density region with the Tafel slope of ‐60 mV/dec, are investigated.

Palladium Electrode in Oxygen‐Saturated Aqueous Solutions: Potential Dependent Adsorption of Oxygen Containing Species and Their Effect on Oxygen Reduction

Adsorption of oxygen species at palladium electrodes at various potentials, , was determined by a programmed potentiodynamic method in solutions of different pH. In all solutions examined, a linear relationship between and coverage,θ, with oxygen species was established in the range . An equation is obtained relating θ to and pH. This linear relationship extends from about vs. RHE to about 0.95V vs. RHE. This is the same potentials region in which the Tafel slope of −60 mV/dec has been reported for oxygen reduction at Pd electrodes. In view of the intermediated coverages, it is suggested that the first charge transfer under Temkin adsorption condition is the rate determining step for oxygen reduction. Fractional reaction orders with respect to H+, previously reported for the reduction in the Tafel region of −60 mV/dec, are now accounted for by the adsorption of oxygen species under Temkin conditions.