Clean Ni Surface Research Articles

Carbon dioxide molecular interactions with hydrogenated Ni(111) surface: a DFT study

Within the density functional theory (DFT) study, we considered the contact of carbon dioxide (CO2) with the hydrogenated Ni(111) surface covered with adsorbed hydrogen. The hydrogenation of CO2 over the Ni(111) surface causes the formation of [HC(OH)2], [COOH], [HCOO], and [C(OH)2] reaction intermediates confirmed by the present DFT investigation. The formation of the [HC(OH)2] and [C(OH)2] intermediates is observed for the first time. When CO2 has approached the on-top Ni site on a partially hydrogen-covered Ni(111) surface, the [COOH] intermediate is successfully formed. Hydrogen adsorption is a reason for an increase in the CO2 adsorption energy compared to that of the CO2 adsorbed on the clean Ni(111) surface. Our results can help understand the CO2 adsorption on the nanoscale Ni metal. They will be useful in guiding experiments in developing better nanomaterials for efficient gas sensing.

Two-step model for reduction reaction of ultrathin nickel oxide by hydrogen

Nickel (Ni) is used as a catalyst for nitric oxide decomposition and ammonia production but it is easily oxidized and deactivated. Clarification of the reduction process of oxidized Ni is essential to promote more efficient use of Ni catalysts. In this study, the reduction processes of ultrathin oxide films formed on Ni(111) surfaces by thermal oxidation under vacuum and a hydrogen atmosphere were investigated by in situ time-resolved photoelectron spectroscopy. On the basis of these results, we propose a reaction model for the reduction of Ni oxide films. Our results show that the reduction of Ni oxide films on heating under vacuum does not yield a clean Ni(111) surface owing to formation of a residual stable suboxide structure on the Ni(111) surface. Conversely, in a hydrogen atmosphere of 1 × 10−5 Pa, the Ni oxide was completely reduced and a clean Ni(111) surface was obtained, even when heating below 300 °C. The reduction in a hydrogen atmosphere was best described by a two-step reaction model. The rate of the first step depends on the reduction temperature, and the rate of the second step depends on the H2 pressure. The rate-limiting process for the first step is surface precipitation of O atoms and that of the second step is dissociation of H2 molecules.

OH and O mediated interaction of CO2 with Ni(1 1 0) surface, and its implications on biomimetic CO2 hydration

Pre-coverage of a clean Ni(1 1 0) surface with OH and O plays an important role in activating CO2 molecules during a host of surface catalytic reactions involving CO2 and H2O. Appreciating this observation, the present work was designed to focus on providing detailed mechanistic insights into CO2 hydration reaction by Ni(1 1 0) surface, pre-covered by oxygenated surface species formed as a result of partial as well as complete dissociation of H2O. Density functional theory calculations were employed to investigate CO2 hydration with modified Ni(1 1 0) surfaces acting as biomimetic catalysts exhibiting carbonic anhydrase-like mechanism. Bader charge analysis was done to probe the charge distributions in different catalytic systems under study. Potential energy surfaces were developed for the formation of bicarbonate and carbonate. Our computational analysis conclusively demonstrates the feasibility of Ni(1 1 0) to act as biomimetic CO2 hydration catalyst, and the plausibility of carbonate formation to compete with biomimetic CO2 hydration.

Ni/NiO hybrid nanostructure supported on biomass carbon for visible-light photocatalytic hydrogen evolution

Well-dispersed Ni-based nanocrystals with Ni/NiO hybrid nanostructure are built on cellulose-derived carbon (Ni-C-500), which exhibits outstanding photocatalytic performance in the hydrogen evolution from water. Control experiments confirm that the optimal nickel content in Ni-C-500 is 16 wt% and the optimum calcination temperature is 500 °C. The hydrogen evolution rate on Ni-C-500 in the presence of Eosin Y and triethanolamine (TEOA) reaches 13.5 mmol/gcat/h in the first hour, which is even higher than that on commercial Pt/C catalyst (11.4 mmol/gcat/h). The carbon support facilitates the transfer of photogenerated electrons from photosensitizer to Ni-based nanocrystals, efficiently preventing the recombination of photogenerated electrons and holes during the photocatalytic procedure. Density functional theory (DFT) calculations further demonstrate that the NiO islands on Ni(111) surface facilitate the adsorption of water molecules because of the interaction between the oxygen atom of NiO island and the hydrogen atom of water. Furthermore, produced Had around NiO island of Ni(111) surface is more easily to form hydrogen on Ni/NiO hybrid nanostructure than on clean Ni(111) surface. Ni/NiO hybrid nanostructure supported on biomass carbon for visible-light photocatalytic hydrogen evolution

Interface promoted CO2 methanation: A theoretical study of Ni/La2O3

We have conducted theoretical study on Ni/La2O3 interface and clean Ni surface to promote the efficiency of CO2 methanation following experimental observation. A feasible interface between La2O3(011) and Ni(111) is constructed with a reasonable mismatch. The evolution of the main intermediates during CO2 methanation together with activation barriers and reaction energies for specific elementary steps are determined for both Ni and interface surfaces. We reveal that the La2O3 phase in the interface plays a part in binding carbon dioxide to form lanthanum carbonate, resulting in an estimated two-order faster reaction rate with respect to the pure Ni surface.

Study on hydrodeoxygenation mechanism of anisole over Ni (111) by first-principles calculation

Anisole, as an aromatic oxygenate lignin model compound, which can be selectively cleaved into benzene over Ni/C at high temperature. On the basic of experimental results, the hydrodeoxygenation (HDO) mechanism of anisole was investigated on clean Ni (111) surface by using computational techniques. Based on static energies, the microkinetic modeling on clean Ni (111) and H-covered Ni (111) were built in which the modeling of H-covered Ni (111) approximately simulated the real catalytic system under lower temperature and higher H2 pressure. As a result, the microkinetic analysis proved the phenol was the major product in the anisole decomposition on clean Ni (111) surface. However, the phenol was preferred to decompose rather than desorb from Ni surface because of the high desorption energy, suggesting that the benzene mainly originated from the secondary dissociation of phenol. Differently, the decreasing of energy barrier for anisole hydrogenation led to the formation of methylcyclohexane as main product on H-covered Ni (111) surface by microkinetic analysis. The differences in products distribution between Ni (111) and H-covered Ni (111) provided a mechanistic understanding for anisole conversion under different experimental conditions.

Growth and characterization of nickel oxide ultra-thin films

The oxidation of the Ni(111) surface under ultrahigh-vacuum conditions is studied experimentally with low-energy electron diffraction and high-resolution X-ray photoelectron spectroscopy. Exposure of the clean Ni(111) surface to molecular oxygen at room temperature followed by annealing at 400 K leads to the formation of two different structures (2×2) and (33×33)R30∘, prior to the formation of the NiO(111) monolayer. The O 1s core levels indicate that the obtained oxide is terminated by oxygen atoms while the valence band measurements clearly reveal the band gap of NiO. The energy difference between the Fermi level and the maximum of the valance band is extracted and is found to be 0.47 eV.

Effect of Near‐Surface Dopants on the Epitaxial Growth of h‐BN on Metal Surfaces

AbstractEpitaxial growth of ultrathin overlayers on solid substrate is critically dependent on the surface structure, and in this work near‐surface doping is identified as another important growth factor. It is shown that growth of hexagonal boron nitride (h‐BN) on Ni(111) through chemical vapor deposition or surface ammonization can be strongly modulated by near‐surface B doping. Epitaxial h‐BN islands form on clean Ni(111) surface, while both epitaxial and nonepitaxial h‐BN islands grow on Ni(111) containing near‐surface B atoms. Quantitative correlation of epitaxial growth and near‐surface doping is unambiguously demonstrated. In situ spatially resolved surface science measurements based on photoemission electron microscopy and low energy electron microscopy in combination with density function calculations reveal that near‐surface B atoms weaken the interaction between h‐BN overlayer and Ni surface, which favor the nonepitaxial and metastable h‐BN structures. The present work suggests that near‐surface doping acts as an effective route to influence epitaxial growth of two‐dimensional (2D) material overlayers on solids.

Molecular or dissociative adsorption of water on clean and oxygen pre-covered Ni(111) surfaces

Water adsorption and dissociation on clean and oxygen pre-covered Ni(111) surfaces have been computed systematically by using density functional theory and ab initio atomistic thermodynamics.

DFT and microkinetic investigation of methanol synthesis via CO2 hydrogenation on Ni(111)-based surfaces.

A DFT study of methanol production via CO2 hydrogenation reactions on clean Ni(111) and Ni(111)-M (M = Cu, Pd, Pt, or Rh) surfaces has been performed. The reaction network of this synthesis reaction has been determined using energy profiles. The competing reaction network between the formate-mediated route and the carboxyl-mediated route is also presented. Both routes are equally possible in mediating the overall synthesis reactions. A simple selectivity analysis based on the energy barrier shows that methanol synthesis is more preferred rather than formic acid (HCOOH) or carbon monoxide (CO) production. A mean-field kinetic analysis is also employed to determine the kinetic performance of all catalytic surfaces. The formate-mediated route is found to be energetically and kinetically more dominant than the carboxyl-mediated route. Cu, Pd, and Pt dopants are successful in increasing the kinetic performance of the clean Ni(111) surface in the formate route and Cu, Pt, and Rh dopants in the carboxyl route.

H2 Dissociation on H Precovered Ni(111) Surfaces: Coverage Dependence, Lattice Motion, and Arrangement Effects

Hydrogen molecule dissociation on metal surfaces is a prototype reaction to study the gas–surface interaction. The dissociation rate constants of H2 on H atom precovered Ni(111) surfaces are calculated using the quantum instanton method in full dimensionality. Four different arrangements of the preadsorbed H and the dissociated H2, in which the preadsorbed H is located at the nearest (H2/H1–Ni(111)), second-nearest (H2/H2–Ni(111)), third-nearest (H2/H3–Ni(111)), and fourth-nearest (H2/H4–Ni(111)) neighbor sites of the bridge site where the H2 is dissociated, are considered. Compared to the dissociation rates of H2 on a clean Ni(111) surface (H2/Ni(111)), the dissociation rates of H2/H1–Ni(111) are much smaller. For instance, the former is 5.22 times larger than the latter at 300 K. This is because there is a strong repulsive interaction between the preadsorbed H and H2, which hinders the dissociation of H2. The dissociation rates of the four arrangements increase in the order of H2/H1–Ni(111), H2/H2–Ni(11...

DFT study of the formate formation on Ni(111) surface doped by transition metals [Ni(111)-M; M=Cu, Pd, Pt, Rh

We report on a theoretical study of the formation of formate (HCOO) from the reaction of CO2 gas and a pre- adsorbed H atom (CO2 (g) + *H → *HCOO) on Ni(111) surface doped by transition-metals [Ni(111)-M; M= Cu, Pd, Pt, Rh] by means of density functional theory (DFT) calculations. This *HCOO formation reaction is one of the most important rate- limiting steps in the methanol synthesis process. We find that the presence of transition metal doping on the first-layer of Ni(111) surface could reduce the activation barrier of this reaction [up to ~38.4%, compared to clean Ni(111) surface].

Density functional theory study of SO2-adsorbed Ni(1 1 1) and hydroxylated NiO(1 1 1) surface

Spin polarized, DFT+U periodic calculation is used as an effective way to model the adsorption process of SO2 on hydroxylated NiO(111) surface. The adsorption of atomic O and O2 on the clean Ni(111) surface is calculated to investigate the forming process of passive film. The molecular and dissociated adsorptions of H2O on NiO(111) surface are evaluated to construct defect-free hydroxylated NiO(111) surface. The adsorption of SO2 and atomic O on clean Ni(111) surface is also investigated to compare with the adsorption capacity between passive film and substrate. With respect to the single adsorption process of SO2 on defect-free hydroxylated NiO(111) surface, the effects of O vacancy of surface and atomic O closed to the surface are investigated. The calculation results show that there is no chemical adsorption of SO2 on the defect-free hydroxylated NiO(111) surface with or without atomic O. Either single SO2 or SO2 with atomic O prefer adsorbing on the hydroxylated NiO(111) surface with O vacancies. The adsorption behavior is strengthened with the increase of percentage of surface O vacancy. The existence of atomic O leads to the production of SO3 on the hydroxylated NiO(111) surface and strengthens the adsorption capacity of SO2. Furthermore, the results also reveal the relationship between the charge transfer and the adsorption energy of SO2 and atomic O on the hydroxylated NiO(111) surface and clean Ni(111) surface. We inferred that broken passive film susceptibility to corrosion compare with substrate when surface O vacancies aggregate and its concentration large enough.

Effect of carbon on the Ni catalyzed methane cracking reaction: A DFT study

• Effect of carbon deposition on kinetic properties of methane dissociation is studied. • Existence of surface and subsurface C atoms destabilized CH x species adsorption. • CH x activation is hindered with the deposition of C on and in the Ni (1 1 1) surface. To understand the effects of carbon atoms on the Ni catalyzed methane cracking reactions, methane dissociation on clean, surface-carbon-covered, and subsurface-carbon-accumulated Ni(1 1 1) surfaces were investigated using density functional theory (DFT). The results show that the existence of surface and subsurface C atoms destabilized the adsorption of the surface hydrocarbon species when compared to the clean Ni(1 1 1) surface. The projected density state (PDOS) analysis shows that the deposition of C atoms on and into the Ni surface modified the electronic structure of the Ni surface, and thus reduced the catalytic activity of the bonded Ni atoms. Moreover, it was found that the presence carbon atoms increase the CH x ( x = 4–1) species activation barriers especially on the surface carbon covered (1/4 ML) Ni(1 1 1) surface, where CH x ( x = 4–1) species encounter highest energy barrier for dissociation due to the electronic deactivation induced by C Ni bonding and the strong repulsive carbon CH x interaction. The calculations also show that CH x dissociation barriers are not affected by its neighboring C atom at low surface carbon coverage (1/9 ML). This work can be used to estimate more realistic kinetic parameters for this system.

Analysis of sulfur-induced selectivity changes for anhydrous methanol dehydrogenation on Ni(100) surfaces

The absorption of sulfur on the Ni(100) surface has been reported to influence its product selectivity for methanol dehydrogenation. While dehydrogenation on the clean Ni(100) surface primarily produces a mixture of carbon monoxide and hydrogen, preadsorption of 0.25–0.33 monolayers of sulfur shifts the selectivity to mainly formaldehyde and hydrogen. Density functional theory calculations of clean and sulfur-modified Ni(100) surfaces demonstrate that sulfur destabilizes methanol dehydrogenation reaction intermediates in a manner consistent with this shift in selectivity. Microkinetic modeling of the reaction system further indicates that these changes alter the reaction selectivities from CO to CH2O production in the steady state. The reaction selectivities of metal alloy surfaces, which also have heterogeneous surface morphologies, are hypothesized to be influenced in a similar manner as these S/Ni(100) surfaces.

X-ray absorption from large molecules at metal surfaces: Theoretical and experimental results for Co-OEP on Ni(100)

Metal octaethylporphyrins (M-OEP), M-N(4)C(20)H(4)(C(2)H(5))(8), adsorbed at a metallic substrate are promising candidates to provide spin dependent electric transport. Despite these systems having been studied extensively by experiment, details of the adsorbate geometry and surface binding are still unclear. We have carried out density functional theory calculations for cobalt octaethyl porphyrin (Co-OEP) adsorbate at clean and oxygen-covered Ni(100) surfaces as well as for the free Co-OEP molecule where equilibrium structures were obtained by corresponding energy optimizations. These geometries were then used in calculations of Co-OEP carbon and nitrogen 1s core excitations yielding theoretical excitation spectra to be compared with corresponding K-edge x-ray absorption fine structure (NEXAFS) measurements. The experimental NEXAFS spectra near the carbon K-edge of Co-OEP bulk material show large intensity close to the ionization threshold and a triple-peak structure at lower energies, which can be reproduced by the calculations on free Co-OEP. The experimental nitrogen K-edge spectra of adsorbed Co-OEP layers exhibit always a double-peak structure below ionization threshold, independent of the layer thickness. The peaks are shifted slightly and their separation varies with adsorbate-substrate distance. This can be explained by hybridization of N 2p with corresponding 3d contributions of the Ni substrate in the excited final state orbitals as a result of adsorbate-substrate binding via N-Ni bond formation.

Structural stability of stepped nickel surfaces

The structural stability of stepped Ni(755) and Ni(771) surfaces in the temperature range 20–500°C and during adsorption of oxygen and different forms of carbon has been studied using scanning tunneling microscopy and low-energy electron diffraction. A phase transition from the structure with double steps at room temperature to the structure with single steps at a temperature above 350°C has been observed on the clean Ni(755) surface. This transition disappears after oxygen adsorption at a temperature above 350dgC. In this case, the structure on the Ni(755) surface with single steps is stabilized in contrast to Ni(771), which tends to faceting during oxygen adsorption. It has been shown that fullerenes C60 form an array of one-dimensional chains at the upper boundaries of steps of the substrate, when they are adsorbed on the Ni(755) surface.

First-principles study of the structural and magnetic properties of graphene on a Fe/Ni(1 1 1) surface

The structure and spin-resolved electronic states of a graphene-adsorbed Fe/Ni(1 1 1) surface are investigated and compared with a graphene/Ni(1 1 1) surface using first-principles calculations. Nine possible geometries are studied with Fe and C atoms at different sites with respect to the topmost Ni atoms. Geometries with one C atom located on top of an Fe atom (C1) and one at a hollow (fcc or hcp) site (C2) are the most energetically favourable. The electronic states of graphene are significantly modified by the interaction with the Fe/Ni(1 1 1) surface. The dominant π states of the C2 atom are drastically shifted towards the Fermi level and become highly positive-spin-polarized due to the corresponding spin-down states located above the Fermi level. The level shift is very small for the spin-up π states of the C1 atom but obvious for the spin-down states due to spin splitting induced by Fe atoms, resulting in a negative spin polarization at shallow levels and a positive one at deeper levels. The adsorption of graphene on Fe/Ni(1 1 1) is stronger than that on the clean Ni(1 1 1) surface.

CO-promoted formation of the alkali-oxygen bond on Ni(111)

High-resolution electron energy loss spectroscopy was used to study the coadsorption of alkali metals (Na, K) and oxygen on clean and CO-modified Ni(111) surfaces. We unambiguously show that on an alkali-precovered surface, the alkali-O bond was not formed upon O(2) exposure. On the contrary, the alkali-O bond was readily observed by exposing to O(2) the Ni(111) surface precovered with an alkali+CO phase. This enhanced oxidation rate of alkali metals in the presence of CO molecules was ascribed to the short-range CO-induced modification of the electronic charge of alkali-metal adatoms.

First Principles Investigation for H2 Dissociative Adsorption on Ni and Cr-Decorated Ni Surfaces - An Application to Alkaline Polymer Electrolyte Fuel Cell

In this research, density functional theory (DFT) calculations for H2 dissociative adsorption on Ni(111) and Cr-decorated Ni surfaces are performed to compare and understand the adsorption mechanism on both surfaces as an application to alkaline polymer electrolyte fuel cell (APEFC). The Cr-decorated Ni (CrML/Ni(111)) surface is constructed by placing a monolayer of Cr atoms on the topmost layer of a Ni (111) surface. The potential energy scans for H2 dissociative adsorption on both surfaces reveal that the activation barrier of Cr-decorated Ni surface is lower than that of the pristine Ni surface. For both surfaces, the H atoms are dissociated with a distance of 1.8 A; but H2 on Cr-decorated Ni surface has greater adsorption energy than for H2 on Ni(111) surface. The density of states (DOS) shows that the presence of Cr increased the unoccupied states in the region around the Fermi level as compared to clean Ni surface. The increase in unoccupied states is supported by the computed total charges of the atoms before and after the Cr monolayer is added on the Ni surface which also implies a strong metal-to-metal interaction between Ni and Cr atoms. The difference in the charges of the H atoms and the surface atoms before and after the adsorption on the surfaces shows an easier transfer of charges between H atoms and surface atoms of Cr-decorated Ni surface than the surface atoms of Ni surface. [DOI: 10.1380/ejssnt.2010.325]