Surface Ni Atoms Research Articles

Stability of metal nanoparticles formed during reduction of alumina supported nickel and cobalt catalysts

The formation of nickel and cobalt nanoparticles in hydrogenation catalysts and their stability against sintering during the reduction of the oxidic precursors were investigated. The morphology of the catalysts was manipulated by varying the reduction conditions. The catalysts were characterized using temperature programmed reduction (TPR), hydrogen chemisorption, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HREM). The transformation of the oxidic precursor into the active phase was monitored using quasi in situ HREM, which proved to be an excellent technique to visualize the formation of metal nanoparticles. For the nickel catalyst the reduction temperature plays a crucial role, whereas time is more critical for the cobalt catalyst. The sintering rate of cobalt is considerably lower than that of nickel during reduction. It is concluded that the activation energy for sintering is significantly higher for nickel than for cobalt. A model is proposed which depicts the structure of both types of catalysts in their oxidic and reduced state. TPR and XPS results indicate that the passivated catalysts contain approximately two oxygen atoms per surface Ni or Co atom.

Surface segregation of Ti atoms during NiTi alloy sputtering

The paper addresses NiTi alloy sputtering by 9 keV He and Ar ions and discusses the experiment performed by V.S. Chernysh et al. about 10 years ago. The binary collision simulation has been applied to extract the concentrations of surface Ni and Ti atoms from the experimental data. The results of simulations favor segregation of Ti for both He and Ar ion bombardment. The effect of non-symmetric surface collisions (Ti on Ni and Ni on Ti) was found to be negligible. A pronounced effect of the interatomic (target–target) potential is noted.

Structure‐sensitivity of CH3 dissociation on Ni(100)from first‐principles calculations

AbstractFirst‐principles calculations are performed to explore the CH3 dissociation on Ni(100). The CH2 species bind most strongly to the hollow site with two CH bonds pointing toward the neighboring surface Ni atoms. Through the calculations of local density of states (LDOSs) and partial charge density, the CHNi three‐center bond is regarded as the key factor determining the CH3 adsorption. The CH3 dissociation on Ni(100) is investigated along two different reaction pathways. After zero‐point energy (ZPE) correction considered, the energy barriers for the CH3 dehydrogenation are calculated to be 0.46 and 0.54 eV. Because these barriers are much lower than that for the CH3 dissociation on Ni(111), the CH3 dehydrogenation is found to be structure‐sensitive. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Giant perpendicular magnetic anisotropy of an Ir monolayer on a NiAl(001) surface

Using the state-of-the-art full potential linearized augmented plane-wave method, we have investigated the magnetic properties of Os and Ir monolayer (ML) film on NiAl(001) surface. It has been found that the one ML of Os and Ir film can have ferromagnetic ground state with magnetic moment of 0.35 and $0.64{\ensuremath{\mu}}_{B}$ on Ni terminated surface, whereas both films display no sign of magnetic state on Al terminated surface. In addition, the surface Ni atom has an induced magnetic moment of $0.26{\ensuremath{\mu}}_{B}$ in Ir/NiAl(001), while only $0.09{\ensuremath{\mu}}_{B}$ is observed in Os/NiAl(001). We attribute the existence of magnetism to the interaction between $5d$ of adlayer and $3d$ of surface Ni. Moreover, we have obtained that the Os/NiAl(001) and Ir/NiAl(001) films show a perpendicular magnetic anisotropy (PMA). Surprisingly, it appears that the Ir/NiAl(001) has a giant PMA energy of 7.18 meV.

Mixed platinum–gold electrocatalysts for borohydride oxidation prepared by the galvanic replacement of nickel deposits

Mixed platinum/gold coatings were prepared by partial galvanic replacement of nickel layers that had been electrodeposited onto glassy carbon electrode substrates. The process (termed “transmetalation”) involves the spontaneous replacement of surface Ni atoms by Pt and Au upon immersion of the former metal into equimolar mixed chloroplatinic and chlorolaurate acid solutions. The multimetallic deposits on glassy carbon substrates, PtAu(Ni)/GC, were characterised by SEM/EDS, XRD, AES and electrochemical techniques. Smooth films with a bulk composition enriched in Au and minimum quantities of Ni left were obtained. Pt and Au were found to be alloyed and both Pt and Au to co-exist with Ni down to the core of the layers. The PtAu(Ni) deposits displayed typical Pt and Au surface electrochemistry indicating the formation of a continuous noble metal shell; the estimated electroactive surface areas point again to an excess of Au on the surface. The activity of the PtAu(Ni)/GC electrodes towards the oxidation of borohydride was studied by voltammetry at a rotating disc electrode, RDE, and compared to that of bulk Pt and Au. As expected, the Pt–Au alloy catalyst showed behaviour intermediate to that of its pure components. However, in the limiting current potential range the alloy tended more to bulk Au behaviour (number of electrons was closer to eight) while at low overpotentials it resembled the catalytic activity of pure Pt.

Atomistic simulation of Ni–Pd alloy sputtering by 0.5–50 keV Ar ions

Sputtering of Ni5Pd and NiPd5 alloys by 0.5–50 keV Ar ions has been studied by means of binary-collision simulation. Special attention was given to the angular distributions of sputtered atoms at the steady-state conditions and to the relevant concentrations of Ni and Pd surface atoms, S Ni and S Pd, respectively. The simulation results were compared with the experimental data published recently. The best-fit values of S Ni and S Pd favor segregation of Pd in Ni5Pd and segregation of Ni in NiPd5. For a Ni5Pd alloy, it is expected that the ratio S Ni/S Pd will increase with energy, at least up to 10–20 keV.

O.272 Local hyperthermia in treatment of tumors of head and neck

The catalytic performance of Ni/TiO2 and Ni–Ir/TiO2 catalysts in the selective hydrogenation of cinnamaldehyde (CAL) has been investigated. The Ni–Ir/TiO2 presented higher activity than the Ni/TiO2. Here, we studied and gave some insights into the interaction between Ni and Ir species and the role of Ir using X-ray diffraction (XRD), transmission electron microscope (TEM), hydrogen temperature-programmed reduction (H2 TPR), hydrogen temperature-programmed desorption (H2 TPD), and X-ray photoelectron spectroscopy (XPS). The size of Ni particles on the Ni–Ir/TiO2 was 10.1 nm, smaller than that (12.7 nm) on the Ni/TiO2. The reducibility of Ni was improved by addition of a small amount of Ir, as confirmed with the H2 TPR analysis. The Ni–Ir/TiO2 could be reduced at a temperature of 352 °C, which is lower than the temperature for Ni/TiO2 (385 °C); moreover, a new reduction peak appeared at 240 °C due to the stronger interaction of Ni–Ir species, which was certified by XPS analysis. The H2 TPD results indicate that the hydrogen spillover effect may occur in Ni–Ir/TiO2. The electronic structure of the surface Ni atoms was modified upon addition of Ir, resulting in an enhanced activity of the Ni–Ir/TiO2 catalyst, about four times as high as that of the Ni/TiO2 catalyst.

Atomic structure and electronic properties of Ni 3Al(0 0 1) surface

We present the results of experimental and theoretical study of the structural and electronic properties of the (0 0 1) surface of an ordered, paramagnetic Ni 3Al alloy. Experimental part of this work is based on STM measurements which enabled us to obtain, for the first time, images of the Ni 3Al(0 0 1) surface with atomic resolution. Topographies of these images indicate the presence of a superstructure with the square unit cell and the lattice parameter equal to the distance between nearest surface Al or Ni atoms. We have observed that this order of the Ni 3Al(0 0 1) surface has a long-range character. This is remarkably different from the Ni 3Al(1 1 1) surface, where a (2 × 2) order (indicated by previous STM study) appears only within small surface domains. Our theoretical study of Ni 3Al(0 0 1) is based on ab initio calculations performed in the framework of density-functional theory and the use of a plane wave basis set. The obtained results indicate that changes of the atomic positions caused by the relaxation process are small and practically limited to the three topmost atomic layers. However, in the relaxed structure, surface Al atoms have higher vertical positions than surface Ni atoms, in accordance with experimental data provided by LEED measurements. We have also analyzed the differences between results of structural calculations performed independently using LDA and GGA descriptions of the exchange-correlation effects, and found that the GGA results are in a considerably better agreement with LEED data than the corresponding LDA results or the results of earlier theoretical studies.

C-C 5H 5 on a Ni(1 1 1) surface: Theoretical study of the adsorption, electronic structure and bonding

In the present work the ASED-MO method is applied to study the adsorption of cyclopentadienyl anion on a Ni(1 1 1) surface. The adsorption with the centre of the aromatic ring placed above the hollow position has been identified to be energetically the most favourable. The aromatic ring remains almost flat, the H atoms are tilted 17° away from the metal surface. We modelled the metal surface by a two-dimensional slab of finite thickness, with an overlayer of c-C 5H 5 −, one c-C 5H 5 − per nine surface Ni atoms. The c-C 5H 5 − molecule is attached to the surface with its five C atoms bonding mainly with three Ni atoms. The Ni Ni bond in the underlying surface and the C C bonds of c-C 5H 5 − are weakened upon adsorption. We found that the band of Ni 5 d z 2 orbitals plays an important role in the bonding between c-C 5H 5 − and the surface, as do the Ni 6s and 6p z bands.

Mixed layer formation of copper overlayers on Ni(1 1 0) surface

Copper overlayer formation on the Ni(1 1 0) surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Atom-resolved STM images showed that initially deposited Cu is replaced with surface Ni atoms forming atom-size depressions on the Ni(1 1 0) terraces and a Ni-rich quasi-one-dimensional island along the [ 1 1 ¯ 0 ] direction. Further Cu deposition yields a mosaic structure on the islands, indicating Cu/Ni mixed layer formation. From the quantitative measurement of the Cu/Ni ratio on the substrate and the islands, impinging Cu will be replaced with surface Ni whereas expelled Ni and directly impinging Cu to the island form the mixed island. The number of Cu atoms in the islands, however, more than the directly impinging Cu, indicate significant Cu/Ni replacement at the periphery of the island.

Photoelectrochemistry of Mesoporous NiO Electrodes in Iodide/Triiodide Electrolytes

Mesoporous NiO electrodes were used in photoelectrochemical cells using iodide/triiodide-based redox electrolytes. Cathodic photocurrents were found upon visible light excitation. These photocurrents were enhanced when coumarin 343 dye was adsorbed at the NiO surface or when a unanchored dye (coumarin 337) was added to the electrolyte. The proposed reaction for photocurrent generation is as follows: in the absence of dyes, excitation of triiodide leads to the generation of diiodide radicals, which subsequently accept electrons from the NiO electrode. Enhanced photocurrent generation in the presence of dyes is possibly due to energy transfer from excited dye molecules to triiodide. Charge transport studies of C343-sensitized NiO suggest that hole transport in mesoporous NiO is due to hopping of positive charge between Ni-surface atoms.

Interaction of carbon clusters with Ni(1 0 0): Application to the nucleation of carbon nanotubes

In order to understand the first stages of the nucleation of carbon nanotubes in catalytic processes, we present a tight-binding Monte Carlo study of the stability and cohesive mechanisms of different carbon structures deposited on nickel (1 0 0) surfaces. Depending on the geometry, we obtain contrasted results. On the one hand, the analysis of the local energy distributions of flat carbon sheets, demonstrate that dangling bonds remain unsaturated in spite of the presence of the metallic catalyst. Their adhesion results from the energy gain of the surface Ni atoms located below the carbon nanostructure. On the other hand, carbon caps are stabilized by the presence of carbon atoms occupying the hollow sites of the fcc nickel structure suggesting the saturation of the dangling bonds.

Modeling adsorption of CO2 on Ni(110) surface

The adsorption of CO2 on Ni(110) surface is simulated within the framework of density functional theory using ab-initio pseudopotentials. Beyond possible adsorbed geometries with C2v symmetry, suggested by analogy with the formate adsorption, a geometry with lower symmetry but higher adsorption energy is found and analyzed. Common features of the adsorbed geometries are non-negligible electronic charge transfers from the metal to the molecule ranging from 0.5 to 0.9 electrons in different cases and bending of the molecule with C atom closest to the surface. In the clean surface case we found an enhancement of the magnetic moments of surface and subsurface Ni atoms with respect to bulk, which is partially attenuated by CO2 chemisorption. Both local spin density (LSDA) and generalized gradient correction (GGA) approximations are used for the exchange-correlation functionals: at variance with GGA, LSDA functional is not reliable for energetics, although the results concerning geometric and some electronic properties are almost similar with the two functionals.

Finite temperature quantum distribution of hydrogen adsorbate on nickel (0 0 1) surface

Finite temperature quantum behavior of hydrogen and deuterium adsorbates on Ni(0 0 1) surface has been simulated using path-integral Monte Carlo technique. The adsorbate–surface interaction is described by the many-body alloy potential form, fitted to the adsorption parameters from DFT calculations. This allows consideration of substrate atom dynamics. Temperatures 100 K and 300 K have been considered and contribution of the thermal motion of Ni surface atoms is analyzed. At low temperatures the quantum delocalization of the adsorbate is considerable, and therefore, temperature dependence of distributions is weak. In this case, the isotope effect is larger. At higher temperatures, however, the thermal dynamics of the substrate dominates all studied phenomena and classical description may be sufficient. By using a semi-classical description of the hydrogen adsorbate temperature dependence of the distributions and energetics becomes strong at all temperatures, providing that quantum description is necessary for the correct picture of H/Ni(0 0 1) system.

Evidence for a mixed CoNiO layer at theCo∕NiO(001)interface from surface x-ray diffraction

The clean and cobalt-covered NiO(001) surface was studied by surface x-ray diffraction. The NiO(001) surface prepared by annealing in air at $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is characterized by a strong expansion of the top interlayer spacing ($\ensuremath{\approx}14%$ relative to the bulk NiO-spacing of $2.085\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$) and an outward relaxation $(0.28\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ of the oxygen atoms, which can be related to a high (27%) concentration of oxygen vacancy defects. In addition we find an oxygen-layer (most likely as part of the $\mathrm{O}{\mathrm{H}}^{\ensuremath{-}}$ anion) on top of the surface Ni atoms at a distance of $2.33\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Deposition of 0.8 monolayers of Co induces a complex surface restructuring involving the formation of double layer CoO-islands above an oxygen deficient interfacial oxide layer $([\mathrm{Ni}\mathrm{Co}]{\mathrm{O}}_{0.5})$ in registry with the underlying NiO single crystal. Our results support previous spectroscopic studies suggesting the presence of uncompensated interfacial spins responsible for the exchange bias.

Theoretical calculations of CH4 and H2 associative desorption from Ni(111): Could subsurface hydrogen play an important role?

The results of theoretical calculations of associative desorption of CH(4) and H(2) from the Ni(111) surface are presented. Both minimum-energy paths and classical dynamics trajectories were generated using density-functional theory to estimate the energy and atomic forces. In particular, the recombination of a subsurface H atom with adsorbed CH(3) (methyl) or H at the surface was studied. The calculations do not show any evidence for enhanced CH(4) formation as the H atom emerges from the subsurface site. In fact, there is no minimum-energy path for such a concerted process on the energy surface. Dynamical trajectories started at the transition state for the H-atom hop from subsurface to surface site also did not lead to direct formation of a methane molecule but rather led to the formation of a thermally excited H atom and CH(3) group bound to the surface. The formation (as well as rupture) of the H-H and C-H bonds only occurs on the exposed side of a surface Ni atom. The transition states are quite similar for the two molecules, except that in the case of the C-H bond, the underlying Ni atom rises out of the surface plane by 0.25 A. Classical dynamics trajectories started at the transition state for desorption of CH(4) show that 15% of the barrier energy, 0.8 eV, is taken up by Ni atom vibrations, while about 60% goes into translation and 20% into vibration of a desorbing CH(4) molecule. The most important vibrational modes, accounting for 90% of the vibrational energy, are the four high-frequency CH(4) stretches. By time reversibility of the classical trajectories, this means that translational energy is most effective for dissociative adsorption at low-energy characteristic of thermal excitations but energy in stretching modes is also important. Quantum-mechanical tunneling in CH(4) dissociative adsorption and associative desorption is estimated to be important below 200 K and is, therefore, not expected to play an important role under typical conditions. An unexpected mechanism for the rotation of the adsorbed methyl group was discovered and illustrated a strong three-center C-H-Ni contribution to the methyl-surface bonding.

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.

First-principles study on exchange force image of NiO(0 0 1) surface using a ferromagnetic Fe probe

Exchange force of a ferromagnetic Fe probe on antiferromagnetic NiO(0 0 1) surface has been investigated by means of a first-principles calculation. Calculated exchange force images show a clear spin image when the probe is located within 1 Å above the contact point. We can see antiferromagnetic pattern of the surface Ni atoms along the [1 1 0] direction, and asymmetric feature around surface O sites. The main contrast of Ni comes from the direct exchange interaction between the Fe probe and the surface Ni atom, while the asymmetric image possibly comes from the super exchange interaction between the Fe probe and the second layer Ni atom via the surface O. Such asymmetric feature is a key proof of the exchange force microscope image on observation.

Vibrational EELS and DFT study of propionic acid and pyruvic acid on Ni(100): Effects of keto group substitution on room-temperature adsorption and thermal chemistry

Abstract The room-temperature adsorption and thermally induced processes of propionic acid and pyruvic acid on Ni(1 0 0) have been investigated by electron energy loss spectroscopy (EELS). Computational vibrational analysis of the optimized bidentate structures for acid–Ni model complexes (involving the organic acid and a Ni atom) has been performed by using the two-layer ONIOM method with the Density Functional Theory and used to interpret the vibrational EELS data. Dehydrogenation of the hydroxyl group is found to result in bonding of the carboxylate group in the propionate and pyruvate adspecies to either a single Ni surface atom in a bidentate configuration or two neighbouring Ni atoms in a bridge configuration. Given the similarities in the total energies and related vibrational frequencies obtained by the calculations in the case of pyruvate adspecies, it is difficult to differentiate the alternate adsorption structure, in which the keto O and hydroxyl O atoms are bonded to a Ni atom in a five-member chelate ring configuration. Furthermore, temperature-dependent EELS studies show that both the propionate and pyruvate adspecies could decompose upon annealing to above 400 K and further dissociate to CO adspecies above 550 K and to C and/or O above 600 K.

Hydrogenation of carbon monoxide on Ni(1 1 1) investigated with surface X-ray diffraction at atmospheric pressure

Using a specially designed UHV chamber that can be pressurized to several atmospheres, we investigated the production of methane from a mixture of CO and H 2 close to atmospheric pressure using a Ni(1 1 1) surface as model catalyst. The reaction products were monitored with a gas analyzer and the surface structure with the surface X-ray diffraction technique. At temperature between 100 and 200 °C approximately, a Ni 3C epitaxial film develops on the surface and no methane production occurs. At temperatures above 400 °C, clear methane production was detected while no ordered carbide film was present on the surface. During the reaction, the Ni crystal truncation rods are almost identical to that of a freshly prepared surface under UHV, indicating that there is no significant rearrangement of the Ni surface atoms.