Adsorption Energy Of Atomic Oxygen Research Articles

Nodal PtNi nanowires with Pt skin and controllable Near-Surface composition for enhanced oxygen reduction electrocatalysis in fuel cells

The oxygen reduction reaction (ORR) takes place on the surface of catalyst, and the precise control of surface component at atomic scale has been an important factor to effectively improve the performance of catalysts. Herein, a highly active and stable catalyst is designed by constructing Pt skin and controllable near-surface atomic distribution of PtNi alloy nanowires (NWs). It is found that the d-band center of Pt-skin down-shifts with Pt ratio in the surface/near-surface of PtNi alloy decreasing, and correspondingly the adsorption energy of atomic oxygen (ΔEO) on PtNi alloys weakens, confirmed by experimental results and theoretical calculations. The optimum PtNi/C-NH3 NWs exhibit the enhanced mass activity (1.02 A mgPt−1) and specific activity (3.86 mA cm−2) for ORR, which are 5.7-fold and 11.4-fold higher than those of commercial Pt/C, respectively. Moreover, the H2/air fuel cell of PtNi/C-NH3 NWs exhibits a current density of 308 mA cm−2 at 0.8 V, which is superior to that of the 2020 target of the U.S. Department of Energy (DOE). Our study provides a new perspective and approach in promoting the electrocatalysis on alloy nanomaterials for ORR.

Initial oxidation and surface stability diagram of Ge(100) as a function of the temperature and oxygen partial pressure through ab initio thermodynamics

Density functional theory calculations in conjunction with thermodynamic modeling were performed to examine the oxygen adsorption on a Ge(100) c(4 × 2) surface and the subsequent initial oxidation. For several possible adsorption sites, the adsorption energy of atomic oxygen as well as the atomic configuration and electronic properties of the adsorbed structure were examined. Then the effects of the surface coverage of oxygen from 1/64 to 1/4 monolayers on the adsorption energy were considered. Through the surface Gibbs free energy as a function of the temperature (T) and oxygen partial pressure (), the (T, ) surface stability diagram was predicted for the O/Ge(100) c(4 × 2) surface. The theoretical prediction well reproduced previous experimental observations and provides an insight to control the initial oxidation process of Ge surface with tuned T

A First-Principles Study on the Oxygen Adsorption and Interface Characteristics with a-GeO2 of Ge[001] Nanowire

First-principles calculations were performed to study the atomic and the electronic structures of the oxygen adsorption on the sidewalls of Ge[001] nanowires. For several possible adsorption sites, the adsorption energy of atomic oxygen, as well as the atomic configuration and electronic properties of the adsorbed structure, were examined. The adsorption stability is not much affected by the nanowire diameter for diameters ranging from 0.8 nm to 2 nm while the electronic structures like band gap and gap states, vary with the nanowire diameter. The interface between the Ge nanowire and amorphous GeO2 (a-GeO2) was also investigated. The interface region is composed of Ge suboxides like those between Ge bulk slab and a-GeO2. The valence band offsets between Ge nanowire and a-GeO2 decreases considerably as the diameter of nanowire shrinks while the conduction band offsets show less dependence on the diameter of the nanowire.

Computational Study of Site-Specific Correlations among Oxygen Reduction Intermediates on Pd3Y (111)

In recent years, Pd3Y (111) alloy system has emerged as one of promising catalyst materials for oxygen reduction chemistry. This system is used in this work as a model system to study the site-specific adsorption energies of 4 ORR intermediates, O, OH, H2O, and OOH. Trends in energetics are estimated to assess the presence of scaling correlations among the adsorbates. Two types of site mappings are introduced and applied in correlating the energy of an adsorbed oxygen atom on hollow sites to its equivalent adsorption energies on top sites. Both types demonstrate relevant correlations to the other 3 intermediates, one correlates with OOH, and the other with H2O and OH. Discrepancies in energetic trends are explained in light of calculated charge densities and their implying effect on the observed correlations. OH and H2O strongly interact with the second layer of slab and the ground state adsorption of O, while OOH adsorption does not coincide with the second layer charge and correlates instead with its average effect that is reflected in the average adsorption energy of atomic oxygen. The results indicate how the energetics of one type of surface site may be affected by the energetics on another type. The underlying approach of site-mappings is expected to find high application in screening of catalyst materials using computational chemistry.

Descriptors controlling the catalytic activity of metallic surfaces toward water splitting

Periodic density functional theory (DFT) calculations have been used to unravel the existence of Brønsted–Evans–Polanyi (BEP) relationships for water dissociation on metallic surfaces which constitutes the rate determining step for the technologically important water gas shift reaction. In addition, it is predicted that nickel surfaces possessing low coordinated atoms could be effective for catalyzing water dissociation. Finally, it is shown that the adsorption energy of atomic oxygen on a given metallic surface provides an excellent descriptor of the activation energy for water dissociation on that surface, thus allowing the screening of large number of metallic and bimetallic systems in a simple way.

Structure Sensitivity for Forward and Reverse Water-Gas Shift Reactions on Copper Surfaces: A DFT Study

The DFT-GGA calculations have clearly reproduced the experimentally observed structure-sensitivity of forward and reverse water-gas shift reactions on Cu(111), Cu(100), and Cu(110) assuming the “redox mechanism”. The reason for the structure-sensitivity has also been explored by the transition-state structure analysis and the metal d-band center analysis. It is concluded that the difference in virtual adsorption energy of atomic oxygen or other strongly adsorbed species at the transition state is essential to account for the structure-sensitive reactions.

Ab Initio Study of Stability and Site-Specific Oxygen Adsorption Energies of Pt Nanoparticles

We have employed ab initio calculations based on density functional theory in order to study stability and oxygen adsorption energies of Pt nanoparticles. For particles with sizes up to 200 atoms and various geometric shapes, we have explored the dependence of cohesive energies on atomic coordination number and on lattice strain effects. A simple empirical relation, which is consistent with the well-known Gibbs−Thomson relation, represents the cohesive energy over the range of considered sizes and shapes. For hemispherical cuboctahedral particles with 37 and 92 atoms, we have generated contour plots of the adsorption energy of atomic oxygen on all nanofacets. These plots furnish the known trend of strongly enhanced oxygen adsorption energies in comparison to extended surfaces. We found that the interplay of geometric effects, involving the periodic arrangement of surface atoms and edge effects on nanofacets, causes the high site-selectivity of Pt−oxygen interaction energies, with the largest adsorption energies found at the edges. Particle relaxation upon oxygen adsorption exhibits a significant influence on adsorption energies. The presented results provide a map of the peculiar site-selectivity of adsorption at Pt nanoparticles, which should be accounted for in building detailed models of reaction mechanisms and reactivity.

Support effects and catalytic trends for water gas shift activity of transition metals

Water gas shift activity measurements for 12 transition metals (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Re, Ir, Pt, Au) supported on inert MgAl 2O 4 and Ce 0.75Zr 0.25O 2 are presented, to elucidate the influence of the active metal and the support. The activity is related to the adsorption energy of molecular CO and atomic oxygen on the metal; the latter is a good measure for the reactivity of the metal towards H 2O. Generally, the activity of the catalysts with the Ce 0.75Zr 0.25O 2 support is higher, compared to the corresponding MgAl 2O 4-supported catalysts. Exceptions are Cu and Au, which have a higher activity on the MgAl 2O 4 support and are both characterized by weak CO adsorption. For the MgAl 2O 4-supported catalysts a volcano-type relation between the activity and the adsorption energy of atomic oxygen on the metal is obtained. The maximum activity is found for metals with a binding energy of oxygen around −2.5 eV. No clear correlation exists with the adsorption energy of CO. In contrast, the activity for the Ce 0.75Zr 0.25O 2 support increases with increasing adsorption strength for CO, and based on a relatively low activity of Cu the activity does not seem to depend on the adsorption energy of oxygen. Such a change in activity-descriptor for the different supports can be rationalized by the possibility that water dissociation occurs on the redox-active Ce 0.75Zr 0.25O 2 support, whereas the MgAl 2O 4 support is inactive.

Detailed Mechanism for CO Oxidation on AuNi3(111) Extended Surface: A Density Functional Theory Study

The present density functional theory (DFT) calculations have clearly reproduced the experimental phenomena for low-temperature CO oxidation on the AuNi3(111) alloy surface. Our results indicate that the AuNi3(111) flat surface is also an effective candidate for the catalysis of CO oxidation due to its low activation energy (0.13 eV) compared to Au nanoparticles (0.40 eV), stepped Au(211) (0.46 eV), and supported Au catalysis (0.36∼0.40 eV). Our results also indicate that the catalytic reactivity of CO oxidation is strongly related to the adsorption energy of atomic oxygen, and a possible guide for the design of catalysts is that the metal should have modest adsorption ability for atomic oxygen, like Ni and Pd, rather than too weak, like Pt, or too strong, like Mo.

Theoretical Study of the Adsorption and Dissociation of Oxygen on Pt(111) in the Presence of Homogeneous Electric Fields

The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/A (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.

A theoretical study of dioxymethylene, proposed as intermediate in the oxidation of formaldehyde to formate over copper

The density functional theory-based hybrid method B3LYP was used to study the interaction of the dioxymethylene species with the copper (111) surface. This species has been proposed experimentally as one possible intermediate in the oxidation of methanol catalysed by metal surfaces. The H 2CO 2 species is very unstable, and this makes the experimental study too difficult. As far as we know, there are no direct theoretical or experimental studies of H 2CO 2 adsorption on metal surfaces in the literature. The experimental knowledge is limited to the IR frequencies obtained for adsorption on metal oxides. In this study, two different clusters and two different adsorption sites have been studied. A two-layer Cu 7 cluster was used to model the H 2CO 2 and [H 2CO 2] 2− adsorption on a small copper island, and a large three-layer Cu 30 cluster was used to model the H 2CO 2 adsorption on a copper (111) surface. These clusters were used to extract information concerning the energetics, geometry and IR frequencies for the dioxymethylene adsorption. When compared with a similar species, formate, dioxymethylene is stabilized more efficiently on the cross-bridge site than on the aligned-bridge site, which is the preferred orientation for formate. However, the oxygen-to-surface distances are similar, and the same is observed for the bonding type, which is mainly ionic. A bridge-bonded conformation is predicted for adsorption on the two sites considered. The comparison of the adsorption energy of the dioxymethylene species and the adsorption energy of atomic oxygen and free formaldehyde yields an interesting result: H 2CO 2(ads) is energetically more stable than adsorbed O (ads) and H 2CO (g). The IR frequencies are in good agreement with experimental data obtained for dioxymethylene adsorption on several oxide surfaces.