High Oxygen Coverage Research Articles

Roughening of Pt nanoparticles induced by surface-oxide formation

Using density functional theory (DFT) and thermodynamic considerations we studied the equilibrium shape of Pt nanoparticles (NPs) under electrochemical conditions. We found that at very high oxygen coverage, obtained at high electrode potentials, the experimentally-observed tetrahexahedral (THH) NPs consist of high-index (520) faces. Since high-index surfaces often show higher (electro-)chemical activity in comparison to their close-packed counterparts, the THH NPs can be promising candidates for various (electro-)catalytic applications.

Reaction mechanism for methanol oxidation on Au(1 1 1): A density functional theory study

The microscopic reaction mechanism for methanol oxidation on Au(111) surface has been thoroughly investigated by means of density functional theory (DFT) computations. The adsorption geometries and energies were obtained for all the adsorbates, including the reactants, the products, and various possible intermediates on the metal. According to different oxygen conditions, we propose two possible reaction pathways for methanol oxidation on Au(111): (1) HCHO esterification: the intermediate formaldehyde and methoxy couple to yield methyl formate at low oxygen coverage or without the presence of oxygen atoms; (2) HCHO oxidation: the formaldehyde is oxidized to form formate at high oxygen coverage, which further dissociates to give CO2. Our study emphasizes the critical role of oxygen coverage during the methanol oxidation reaction, and can perfectly explain the difference in product distributions observed in previous experiments.

Low Temperature CO Oxidation on Ruthenium Oxide Thin Films at Near-Atmospheric Pressures

Ruthenium model catalysts in the form of thin ruthenium oxide films grown on Ru(0001) were studied in the CO oxidation reaction at near-atmospheric pressures. The surfaces were prepared under vacuum conditions prior to the reactivity measurements carried out in a circulating flow reactor using gas chromatography. The films possessing oxygen in amounts equivalent to 1–4 monolayers (MLE) on Ru(0001) as determined by electron spectroscopy, exposed both the oxidic (RuO2(110)-like) and O/Ru(0001) surfaces. In addition, one-dimensional oxide structures were observed by scanning tunneling microscopy, which are tentatively assigned to the intermediate state for a crystalline ruthenium oxide thin film that covered the entire surface at higher oxygen coverages. At low temperatures studied (400–470 K), the reaction sets in only in the presence of oxidic structures, i.e. when the oxygen coverage, on average, exceeds 1 MLE. The reaction rate slightly increases with increasing the nominal film thickness up to 7 MLE, reflecting primarily the lateral growth of oxide phases. The disordered oxide films showed even higher reactivity. The results suggest that surface ordering and oxide film thickness are not critical for the superior catalytic activity of ruthenium oxides in this reaction.

Selective oxidation of propylamine on oxygen-covered Au(111): a DFT study

The reaction mechanism for selective oxidation of propylamine on oxygen-covered gold has been studied by the density functional theory (DFT) and generalized gradient approximation (GGA) with slab model. Our calculation results indicated that the adsorption energy of propylamine decreases with the increasing oxygen coverage, that is -0.38, -0.20 and -0.10 eV on clean, 2/9 monolayer (ML) and 2/3 monolayer (ML) oxygen, respectively. The adsorption energies of the intermediates also have the trend of the gradual lower. The present work also indicated that the final product distribution depends on the oxygen coverage: propylamine undergoes N-H bond and C-H bond cleavage to produce propionitrile and water at low-oxygen-coverage (θ(o) = 2/9 ML), and to yield propionitrile, propionaldehyde and water at high-oxygen-coverage (θ(o) = 2/3 ML). The energy barrier of the first step of propyamine oxidation (CH(3)CH(2)CH(2)NH(2) → CH(3)CH(2)CH(2)NH) is 0.16 eV (θ(o) = 2/9 ML) and 0.38 eV (θ(o) = 2/3 ML). On the second step, the barrier energy is 0.16 (θ(o) = 2/9 ML) and 0.25 (θ(o) = 2/3 ML) eV of CH(3)CH(2)CH(2)NH → CH(3)CH(2)CH(2)N, next both C-H breakage and the barrier energy is 0.20 eV (CH(3)CH(2)CH(2)N → CH(3)CH(2)CHN) and 0.25 eV (CH(3)CH(2)CHN → CH(3)CH(2)CN) on low oxygen coverage, and 0.15 eV (CH(3)CH(2)CH(2)N → CH(3)CH(2)CHN) and 0.26 eV(CH(3)CH(2)CHN → CH(3)CH(2)CN) on the high oxygen coverage. The additional reaction step of CH(3)CH(2)CHN → CH(3)CH(2)CHO occurs on the high oxygen coverage, and the associated barrier is 0.41 eV. The calculation results show that the oxidation of propylamine can occur at room temperature due to the lower energy barrier. Furthermore, it was found that the energy barrier for the possible reaction steps at the low oxygen coverage is generally smaller than that on high oxygen coverage, which agrees with the experimental results.

C-N coupling on transition metal surfaces: A density functional theory study

We have investigated the formation of C-N bonds from individual atoms and single hydrogenated moieties on a series of transition metals. These reactions play a role in HCN formation at high oxygen coverage, also known as Andrussow oxidation, and they are fundamental to understand the ability of other materials to form part of alloys where Pt is the major component. Dehydrogenations take place quite easily under these high oxygen conditions and thus, the C+N, HC+N, and N+CH recombinations to form HCN or its isomer CNH might represent the rate-limiting steps for the reaction. For all the metals in the present study we have found that the activation energy for the reactions between H(x)C and NH(y) (x,y = 0,1) involved in C-N formation follow a linear relationship with the adsorption energy of the N atom. This is due to the common nature of all these transition states, where N-containing fragments get activated from three-fold hollow sites to bridge positions. The slopes of the linear dependence, though, depend on the valence of the N fragment, i.e., smaller slopes are found for NH moieties with respect to N ones.

Structure of rhenium surfaces in an oxygen environment

Using density functional theory in conjunction with thermodynamic considerations, we studied different clean and oxygen-covered Re surfaces: low-index Re(0001), Re(10$\overline{1}$0), Re(10$\overline{1}$1), and Re(11$\overline{2}$1) as well as high-index Re(13$\overline{4}$2), all surfaces that were found to be relevant in oxygen-induced surface faceting of rhenium. We found that on more open surfaces oxygen adsorption begins at lower oxygen chemical potentials, which can be correlated to the stronger O--surface interaction. Furthermore, at high-oxygen coverages close-packed Re(0001) becomes significantly more stable than the other Re surfaces that have been investigated. In addition to the stability of surface structures, we also provide quantitative information on the geometries and binding energies, which are of relevance for understanding the properties and catalytic behavior of Re-based catalysts operating under oxygen-rich conditions.

Evolution of a Pt (111) surface at high oxygen coverage in acid medium

Density functional theory is used to study Pt (1 1 1) surface oxidation via analysis of buckling, reconstruction, and variation of atomic charges on the top Pt layer at 0.50 and 0.5625 monolayers of adsorbed oxygen. Hydronium ions enhance oxidation and Pt buckling through proton transfer to the adsorbed O, whereas the chloride species plays an additional oxidant role enhancing Pt detachment. At the fuel cell operation temperature of 80 °C, ab initio molecular dynamics results after 4 ps suggest a small influence of the temperature on surface oxidation and moderate reconstruction effects.

The internal energy of CO2 produced by the catalytic oxidation of CH3OH by O2 on polycrystalline platinum

The dynamics of steady state catalytic methanol oxidation on a polycrystalline Pt surface over a range of surface temperatures and reactant flow conditions were investigated by monitoring the kinetics with mass spectrometry and the internal state distribution of nascent CO(2) with tunable diode laser absorption spectroscopy. The results indicate that CO(2) formation proceeds via three distinct reaction pathways. The first produced CO(2), which is vibrationally excited relative to CO(2) in thermal equilibrium with the surface and shows preferential excitation in the asymmetric stretch. This pathway proceeds via the decomposition of CH(3)OH and the subsequent oxidation of nascent CO adsorbed to Pt in a weakly held precursor state. CO(2) production via this pathway is favored at high surface temperatures and high oxygen coverage. The second forms CO(2), which is vibrationally deactivated relative to CO(2) in thermal equilibrium with the surface and exhibits no preferential excitation among its three nondegenerate vibrational modes or the rotational energy. This pathway involves the decomposition of CH(3)OH and subsequent oxidation of nascent CO adsorbed to Pt in a more strongly held chemisorbed state. CO(2) production via this pathway is favored at low surface temperatures and low oxygen coverage. The third forms CO(2) with preferential excitation in the asymmetric stretch but with less overall vibrational excitation than CO(2) from the first pathway and more vibrational excitation than CO(2) from the second. This third pathway occurs via the complete dehydrogenation of CH(3)OH and subsequent oxidation of nascent CO adsorbed to Pt in a bridged state bound through both ends of the molecule. CO(2) production via this pathway is favored at intermediate surface temperatures and oxygen coverage, conditions which favor overall oxidation to form CO(2).

Methane combustion on Pd-based model catalysts: Structure sensitive or insensitive?

The C-H breaking of methane on the clean and the oxygen precovered palladium single crystal surfaces with the simplest orientations, namely, the dense (111), (100), the more open (110), and the stepped (111) surfaces, the corresponding O/Pd surfaces with different coverage of oxygen, as well as the palladium oxide PdO(100) and PdO(110) surfaces, has been studied with the density functional theory-generalized gradient approximation method using the repeated slab models. The adsorption energies under the most stable configuration of the possible species and the activation energy barriers of the reaction are obtained in the present work. Through systematic calculations for the C-H breaking of methane CH(4)-->CH(3)+H on these surfaces, it is found that such a reaction is structure sensitive on clean palladium and oxygen precovered palladium surfaces with lower oxygen coverage, but it is insensitive on oxygen precovered palladium surfaces with higher oxygen coverage and on palladium oxides. These results are in general agreement with the experimental observations.

Improved electrochemical properties of Ni/YSZ anodes infiltrated by proton conductor SZY in solid oxide fuel cells with dry methane fuel: Dependence on amount of SZY

Improved electrochemical properties of Ni/YSZ anodes for solid oxide fuel cells (SOFCs) were obtained when a proton conducting material, SrZr 0.95 Y 0.05 O 3 - α (SZY), was infiltrated to the anode. The synthesis of SZY in Ni/YSZ anodes involved first adding an SZY precursor solution by the infiltration method and then calcinating at 1300 °C to induce the chemical decomposition reaction of the SZY precursor. When the amount of infiltrated SZY was increased, the performance of SOFCs with Ni/YSZ–SZY increased, while the degradation of the anode decreased in dry methane fuel. Based on the performance, anode potential, and degradation ratio of the SOFCs, the optimal amount of infiltrated SZY was about 1/14 (SZY/NiO by wt). The improved electrochemical properties might be due to electrochemically produced H 2O and the high oxygen coverage near the triple phase boundary of SZY-infiltrated Ni/YSZ.

Surface segregation of Si and its effect on oxygen adsorption on aγ-TiAl(111) surface from first principles

We perform first-principles calculations based on the density-functional theory tostudy the surface segregation of Si and its effect on the oxygen adsorption on aγ-TiAl(111) surface for a range of oxygen coverage0<Θ≤1.0 monolayer (ML). The calculated results show that the alloying Siatoms prefer occupying surface Ti sites to the sites in the bulk ofγ-TiAl, which suggests the occurrence of Si surface segregation. When oxygen atoms adsorb on a pureγ-TiAl(111) surface, the most favorable sites are the adsorption sites with more Ti atoms as theirnearest neighbors in the surface layer at all the calculated coverages and the interactions betweenadsorbed oxygen atoms are repulsive. However, when oxygen atoms adsorb on an Si-alloyedγ-TiAl(111) surface, the interactions between the adsorbed oxygen atoms are attractive at oxygen coverage0<Θ≤1.0 ML. Meanwhile, the interactions between O and Al atoms become stronger whereas thosebetween O and Ti atoms become weaker relative to oxygen adsorbed on a pureγ-TiAl(111) surface. The atomic geometry and density of state are analyzed. Theresults show that the surface ripple of the top metal layer for oxygen on a pureγ-TiAl(111) surface is Ti upwards, while that for oxygen on an Si-alloyedγ-TiAl(111) surface is Al upwards at high oxygen coverage (Θ≥0.50 ML). This effect of Si is of benefit to the nucleation of alumina, which is attributed to Si surfacesegregation and an increase of the surface Al:Ti ratio. This can help to explain why alloying theγ-TiAl(111) surface with Si could favor the formation of theAl2O3 scale at the first stage and result in good oxidation resistance in experiments.

First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221)

The role of subsurface oxygen in Ag(111) and the step edge of vicinal Ag(221) on CO oxidation were studied by using density functional theory calculations. For high oxygen coverage, the formation of subsurface oxygen is not only energetically favorable but also kinetically likely. CO oxidation with on-surface atomic oxygen is facile with a barrier of about 0.16 eV, whereas the reaction with subsurface oxygen is hindered because of the significant barrier for diffusion of the subsurface oxygen to the surface from the subsurface region. It is found that the adsorption of molecules (CO, atomic O and O2) is stabilized in the presence of subsurface oxygen, and surface reactivity is enhanced. An energetically favorable reaction pathway for CO oxidation with O2 is identified, with a reaction barrier of 0.23 eV via a unique four-center Ο2···CΟ intermediate. A catalytic cycle for CO oxidation on Ag(111) in the presence of subsurface oxygen is proposed. On Ag(221) surfaces, our calculations show that both the adsor...

Formaldehyde interaction with adsorbed oxygen on open-circuit platinum electrodes in aqueous sulfuric acid solutions

The open circuit potential transients and cathodic potentiodynamic pulses were measured upon formaldehyde (methylene glycol) interaction with pre-adsorbed oxygen (Oads) on Pt/Pt and pc Pt electrodes in aqueous sulfuric acid solutions. The slowest interaction of CH2(OH)2 with Oads was observed in the high coverage range of the electrode surface (θO ∼ 0.2 0.8 to 1). The process rate in this range is determined by the direct reaction of Oads with CH2(OH)2 molecules from the bulk solution. In the middle surface oxygen coverage range (θO 0.2 to 0.8), CH2(OH)2 interaction with Oads takes place by the mechanism of “conjugated reactions”. The kinetic parameters of reactions for CH2(OH)2, HCOOH, and CH3 OH were compared. The rate of CH2(OH)2-Oads interaction on Pt electrodes in the high oxygen coverage range was found higher by an order of magnitude than that of HCOOH and by two orders of magnitude than in the case of CH3OH.

Mechanism of ammonia oxidation over PGM (Pt, Pd, Rh) wires by temporal analysis of products and density functional theory

The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary ( NH 3 + O 2 ) and secondary ( NH 3 + NO ) interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N 2O, and N 2 are equivalent on the three noble metals. NO is a primary reaction product, while N 2 and N 2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH 3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH 3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of O 2 / NH 3 = 10 , both N 2O and N 2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage.

Chemical composition and reactivity of water on clean and oxygen-covered Pd{1 1 1}

The chemical composition and dissociation behaviour of water adsorbed on clean and oxygen pre-covered Pd{1 1 1} was studied using high-resolution time-resolved and temperature-programmed X-ray photoelectron spectroscopy. We find that water remains intact at all temperatures up to desorption on the clean surface and at high oxygen coverage(0.69 ML) when a surface oxide is formed. The highest desorption peaks occur at 163 K from the clean surface and at 172 K from the surface oxide. At the intermediate coverage of 0.20 ML oxygen reacts with coadsorbed water at 155 K, to generate a mixed H 2O/OH layer exhibiting a ( 3 × 3 ) R 30 ° diffraction pattern, which is stable up to 177 K. The measured ratio between intact water and the hydroxyl species in this layer varies between 1.5 and 2 depending on temperature.

DFT study of reconstructed Cu(1 0 0) surface with high oxygen coverages

Adsorption energies and structures of a Cu(1 0 0) surface with an oxygen coverage from 0.5 ML to 1.5 ML have been studied using DFT methods. The calculations include both on-surface and mixed on- and sub-surface sites for oxygen. Also several energy barriers for oxygen transfer below the surface have been calculated. Calculations show that mixed structures are energetically more favourable than pure on-surface adsorption at coverages higher than 0.75 ML and barriers for oxygen atoms are low, around 0.5 eV. Experimental results show that during the oxidation process, disordered metal penetrating islands were not seen until after several structural changes, where the oxygen is expected to be on the surface. This is an opposite trend to what we observed in our calculations. An explanation for this discrepancy is that the dissociation of oxygen molecules is very slow on a reconstructed Cu(1 0 0) surface. However, after dissociation, oxygen atoms will diffuse easily below the reconstructed surface and our calculations are in agreement with the experimentally observed disordered islands.

Activated Adsorption of Ethylene on Atomic-Oxygen-Covered Ag(100) and Ag(210): Formation of an Oxametallacycle

The activated adsorption of ethylene on atomic-oxygen-covered Ag(100) surface and on an open-type step edge thereon was studied using density-functional-theory. On perfect Ag(100), such adsorption results in the formation of an oxametallacycle (OMC), with an activation energy slightly larger than 0.3 eV. We find that this activation energy is only weakly dependent on the coverage of on-surface oxygen (for ϑ ≤1/2 ML), whereas the OMC−surface interaction is substantially reduced at high oxygen coverage. Three types of OMCs have been identified on the (100) surface, which display similar stability, and the transformation between them is facile with activation energies below 0.1 eV. We find that the presence of subsurface oxygen reduces the activation energy for OMC formation and substantially increases the OMC−surface interaction. The reactivity of the step edge toward the OMC formation strongly depends on the local coverage of oxygen. Our calculations indicate that the relative stability of the OMC intermed...

Ab initiostudy of the oxidation on vicinal Si(001) surfaces: The step-selective oxidation

Initial oxygen adsorption structures and oxidation reaction on vicinal Si(001) surfaces with single-layer ${S}_{A}$ and ${S}_{B}$ steps were studied using ab initio total energy and electronic structure calculations. At the very early stage of the oxygen adsorption on Si(001), the calculations in this study show that oxygen atoms are preferentially incorporated into sites at the rebonded ${S}_{B}$ step edge. Various stable oxidation structures at the rebonded ${S}_{B}$ step were identified based on total energy calculations. Their stabilities are concerned with the structural attribute of the rebonded step-edge Si atoms (RSAs) with dangling bonds and weak rebonds. In addition, the proximity of the RSAs helps form the stable oxidation structures in high oxygen coverage regions that have in common a one-dimensional (1D) -Si-O- chain structure along the step edge as a basic 1D oxide building element. By forming this 1D chain structure, it is possible to effectively reduce the number of dangling bonds (DBs) at the step edge. This indicates that the reduction of the number of step-edge DBs plays a crucial role in the formation of oxidation complexes at the steps with local strain, as seen on flat semiconducting surfaces. For more detailed information, the electronic properties of the oxidation complexes were also calculated. The calculated site-projected density of states and scanning tunneling microscopy images of the oxidation structures exhibit characteristic features distinct from those of the clean vicinal Si surface. All these results clearly show that the ${S}_{B}$ step acts as a strong sink for the oxidation of Si as suggested in experiments.

Interaction of oxygen with TiN(001):N↔O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.

Hydrogen-assisted reduction of the Ni(110)(3×1)–O surface studied by metastable-induced electron spectroscopy

Hydrogen-assisted reduction reactions at oxidized Ni(1 1 0) surfaces have been studied by metastable-induced electron spectroscopy (MIES). We prepared an O/Ni(1 1 0) surface with the high oxygen-coverage (3 × 1) surface phase, “HC-(3 × 1)”. This surface has been known to show no reactions to molecular hydrogen gas at RT, which differs from other surface phases, low-coverage (3 × 1) and (2 × 1), that undergo prompt reduction reactions at RT. We exposed the HC-(3 × 1) surface to molecular hydrogen while elevating the substrate temperature. In the temperature range from 450 to 550 °C, MIES showed a decrease in the emission intensity of the O2p-induced peak in parallel with a recovery of the Ni3d-induced peak intensity. Evidence of reactions of the HC-(3 × 1) surface to the incident molecular hydrogen was obtained on the MIES spectrum only at temperatures above 500 °C, where a H1s-induced peak appeared at the same energy as the case of initial H adsorption on the clean Ni(1 1 0) surface. This result means that molecular hydrogen takes effect on the HC-(3 × 1) O/Ni(1 1 0) surface but at reduced oxygen-coverages. A surface-structure dependency of the hydrogen-assisted reduction reactions at the O/Ni(1 1 0) surface is discussed.