Ni Metal Surface Research Articles

Analysis of KLL Auger spectra excited by X-rays from Ni and Cu metal surfaces

Ni and Cu KLL Auger spectra excited by X-rays from polycrystalline metal foils were measured with good energy resolution and intensity earlier. Auger spectra of 3d transition metals contain satellite peaks due to the atomic excitation processes. Because of the complexity of the measured spectral shape a complete explanation of the spectra was not given in the previous works. A new analysis of the measured spectra is presented here, with improved description of effects of inelastic electron scattering of the electrons in the solid sample and using complex peak shapes to model the satellite structure that follows each diagram line. The energy loss part of measured spectra due to the bulk plasmon excitations, surface plasmon excitations and intrinsic loss processes was removed using the Partial Intensity Analysis method based on energy loss distributions obtained from experimental reflection electron energy loss spectra of the same Cu and Ni metal foils.Relative Auger-transition energies derived from measured spectra of copper are in good agreement with previous experimental works and the results of cluster molecular orbital multielectron (DV-ME) calculations. The intensity ratio I(3P2/3P0) shows better agreement with the result of relativistic calculations than in previous works. In the case of nickel the relative Auger-transition energies are in good agreement with the previous results. According to the new evaluation four satellite peaks were identified on the low energy side of each diagram line in the Auger spectra of Ni.

Effect of ZrO2 on catalyst structure and catalytic methanation performance over Ni-based catalyst in slurry-bed reactor

A series of ZrO2-promoted Ni-based catalysts were prepared by different methods to study the effect of ZrO2 addition manners on catalytic methanation of CO in a slurry-bed reactor. These catalysts were characterized by N2 adsorption–desorption, XRD, H2-TPR, H2-chemisorption, XPS, TEM, and CHNS. The results show that the ZrO2-promoted catalyst prepared by impregnation–precipitation method exhibits higher Ni dispersion, metal surface area and catalytic performance than catalysts prepared by the coprecipitation and impregnation method. With the increase of ZrO2 molar percentage in ZrO2–Al2O3 support, the catalysts shows that the Ni dispersion and metal surface area first increases and then decreases. Catalyst with 20% ZrO2 exhibits the highest CO conversion and CH4 yield under the reaction conditions of 280 °C, 1.0 MPa, H2/CO = 3.0 and 3000 mL g−1 h−1 gas hourly space velocity. Moreover, 20% ZrO2 on catalyst surface performs superior catalytic stability, and prevents the carbon deposition and Ni aggregation.

Influence of Rare Earth (La, Pr, Nd, Gd, and Sm) Metals on the Methane Decomposition Activity of Ni–Al Catalysts

Rare earth (RE = La, Pr, Nd, Gd and Sm) metal-doped Ni-Al (Ni-RE-Al) hydrotalcite precursors were obtained by coprecipitation and calcined to form mixed oxide catalysts. The physicochemical characteristics of calcined and reduced Ni-RE-Al samples were determined by X-ray powder diffraction, Brunauer–Emmett–Teller surface area, H2 temperature-programmed reduction, O2 pulse chemisorption, UV-diffuse reflectance spectroscopy, electron spin resonance spectrometry, and Fourier transform infrared spectroscopy. The catalysts were evaluated for CH4 decomposition at 550 °C until their complete deactivation. The deactivated catalysts were examined by transmission electron, scanning electron, and Raman spectroscopy and elemental analysis. The Raman spectra indicated the presence of both ordered and disordered carbon in deactivated catalysts. A correlation is drawn between H2 production rates and the Ni metal surface area of catalysts. The addition of La to Ni-Al dramatically changed the Ni behavior, leading to highe...

Influence of La on reduction behaviour and Ni metal surface area of Ni–Al2O3 catalysts for COx free H2 by catalytic decomposition of methane

A series of Ni:Al:La mixed oxides derived from hydrotalcite precursors with varying La/Al mole ratio were examined for COx free H2 production by catalytic decomposition of CH4. The physico-chemical characteristics of Ni:Al:La were determined by XRD, UV-DRS, SEM, TEM, Raman spectroscopy, BET-surface area, TPR and O2 pulse chemisorption measurements. Addition of La to Ni–Al improved the Ni metal surface area and the reduction behaviour of NiO particles is dramatically changed. Particle size of Ni was similar to the size of carbon nano fibre in the deactivated catalyst. A direct correlation between Ni metal surface area and the CH4 decomposition activity was observed.

Hydrocarbon-fueled solid oxide fuel cells with surface-modified, hydroxylated Sn/Ni–Ce0.8Gd0.2O1.9heterogeneous catalyst anode

Ni-catalyst-based anode-supported solid oxide fuel cells (SOFCs) operating with H2 fuel are highly efficient, low-emission, electrochemical energy conversion devices. However, the economical production of hydrogen is a challenge, and there is immense interest to use hydrocarbon fuels directly with SOFC. Unfortunately, the solid-carbon generated continuously by hydrocarbon cracking accumulates on the Ni-metal surface and leads to rapid decline in performance. We show here that 1 atom% Sn (with respect to Ni) incorporated onto a surface-modified Ni–Ce0.8Gd0.2O1.9 (GDC) cermet anode with extended three-phase boundaries, where O2− ions, electrons, and fuels get together, acts as an excellent decoking agent. The hydrophilic Sn doped on the surface of Ni is hydrated and hydroxylated by reacting with the water vapour produced by the electrochemical oxidation of the fuel, and the hydroxyl groups aid the oxidation of neighbouring carbon and drive off as CO2. Moreover, the enlarged three-phase boundaries of the anode allow O2− ions to rapidly reach the deposited carbon as well as the hydrocarbon gas and electrochemically oxidize them. With this modified Sn/Ni–GDC catalyst anode, GDC electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3−δ cathode, more than 200 h of operation with a methane-fueled SOFC is demonstrated here.

Following Molecules through Reactive Networks: Surface Catalyzed Decomposition of Methanol on Pd(111), Pt(111), and Ni(111)

We present a model of the surface kinetics of the dehydrogenation reaction of methanol on the Pd(111), Pt(111), and Ni(111) metal surfaces. The mechanism consists of 10 reversible dehydrogenation reactions that lead to the final products of CO and H2. The rate coefficients for each step are calculated using ab initio transition state theory that employs a new approach to obtain the symmetry factors. The potential energies and frequencies of the reagents and transition states are computed using plane wave DFT with the PW91 exchange correlation functional. The mechanism is investigated for low coverages using a global sensitivity analysis that monitors the response of a target function of the kinetics to the value of the rate coefficients. On Pd(111) and Ni(111), the reaction COH → CO + H is found to be rate limiting, and overall rates are highly dependent upon the decomposition time of the COH intermediate. Reactions at branches in the reaction network are also particularly important in the kinetics. A sto...

A DF-vdW study of the CH4 adsorption on different Ni surfaces

A systematic density functional (DF) theory based study of methane (CH4) adsorption on the three lowest-index Miller Ni surfaces plus two stepped Ni surfaces is presented. A standard GGA type functional (PBE) has been used to compute the total energy and the van der Waals (vdW) contribution included to properly described the weak molecular interaction of CH4 with the Ni metal surfaces. The surfaces are represented by a periodic supercell approach and several sites and molecular orientations have been explored with one, two and three H atoms pointing towards the surface. Although all adsorption energy values are small, taking into account dispersion terms allows one to distinguish the effect of the surface structure on methane adsorption.

Effect of Partially Fluorinated Alkyl Chain on Friction and Surface Energy

Abstract Partially fluorinated carboxylic acid derivatives were synthesized and their macroscopic frictions and wetting properties were examined. The carboxylic acid adsorbs on a Co–15 wt % Ni metal surface with high orientation, but the ester has a weak interaction and its film is isotropic, which are suggested by the surface energy and the FTIR measurements of the film. The outermost part of the acid is occupied by fluorinated alkyl groups and the surface energy is 10 mJ m−2, which agrees with values for the fluorinated self-assembled monolayers. The friction coefficient does not depend on the fluorine content in the lubricant molecules, not only for the highly oriented layer, but also for the isotropic film. This seems to be caused by the anchoring effect. Macroscopic friction is largely dependent on the polar group. The alkyl chains in the monolayer of the semifluorinated materials contain gauche conformers, and the odd–even effect was insignificant in the film. The molecules of the odd CH2 units in the monolayer are more inclined on average and the friction and surface energy are slightly greater than the even ones.

Characteristics of La-modified Ni-Al2O3 and Ni-SiO2 catalysts for COx-free hydrogen production by catalytic decomposition of methane

Hydrotalcite precursors of La modified Ni-Al2O3 and Ni-SiO2 catalysts prepared by co-precipitation method and the catalytic activities were examined for the production of COx-free H2 by CH4 decomposition. Physico-chemical characteristics of fresh, reduced and used catalysts were evaluated by XRD, TPR and O2 pulse chemisorptions, TEM and BET-SA techniques. XRD studies showed phases due to hydrotalcite-like precursors in oven dried form produced dispersed NiO species upon calcination in static air above 450 °C. Raman spectra of deactivated samples revealed the presence of both ordered and disordered forms of carbon. Ni-La-Al2O3 catalyst with a mole ratio of Ni : La : Al = 2 : 0.1 : 0.9 exhibited tremendously high longevity with a hydrogen production rate of 1300 molH2 ·mol−1Ni. A direct relationship between Ni metal surface area and hydrogen yields was established.

Random phase approximation applied to solids, molecules, and graphene-metal interfaces: From van der Waals to covalent bonding

The random phase approximation (RPA) is attracting renewed interest as a universal and accurate method for first-principles total energy calculations. The RPA naturally accounts for long-range dispersive forces without compromising accuracy for short range interactions making the RPA superior to semi-local and hybrid functionals in systems dominated by weak van der Waals or mixed covalent-dispersive interactions. In this work we present plane wave-based RPA calculations for a broad collection of systems with bond types ranging from strong covalent to van der Waals. Our main result is the RPA potential energy surfaces of graphene on the Cu(111), Ni(111), Co(0001), Pd(111), Pt(111), Ag(111), Au(111), and Al(111) metal surfaces, which represent archetypical examples of metal-organic interfaces. Comparison with semi-local density approximations and a non-local van der Waals functional show that only the RPA captures both the weak covalent and dispersive forces which are equally important for these systems. We benchmark our implementation in the GPAW electronic structure code by calculating cohesive energies of graphite and a range of covalently bonded solids and molecules as well as the dissociation curves of H2 and H2+. These results show that RPA with orbitals from the local density approximation suffers from delocalization errors and systematically underestimates covalent bond energies yielding similar or lower accuracy than the Perdew-Burke-Ernzerhof (PBE) functional for molecules and solids, respectively.

Acceleration effect of thiourea on the oxidation reaction of hypophosphite ion on Ni surface

The acceleration effect of thiourea on an electroless Ni deposition reaction was analyzed through experimental and theoretical approaches. In the experimental approach, the entire deposition process was physically separated into the anodic and the cathodic reactions by preparing two cells connected by a salt bridge. The cells contained either a bath without metal (Ni) ions or a bath without a reducing agent (hypophosphite ions). The current flowing between the two separated baths increased when thiourea was added only to the anodic reaction bath, which indicated that the acceleration effect of thiourea was mainly on the anodic reaction of the hypophosphite ions. Based on this experimental result, in the theoretical calculation approach, a co-adsorption system consisting of thiourea, hypophosphite, and a Ni metal surface was analyzed using density functional theory. Among all the elementary steps of a hypophosphite reaction, thiourea primarily should promote the adsorption step owing to the following two factors. One was the molecular interaction between thiourea and the hypophosphite ion. The other was the restructuring of the electronic states of the Ni surface owing to the adsorption of thiourea, which enhanced the interaction between hypophosphite and its adsorption site on the Ni surface.

A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel-cell electrodes, generally comes from S-containing species such as hydrogen sulfide (H(2)S). The S-containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O(2) into gaseous SO(2). According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO(2) are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO(2) formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO(2) desorption at either room temperature or high temperatures.

The study of polycrystalline nickel metal oxidation by water vapour

The oxidation of polycrystalline nickel (Ni) metal surfaces with water (H 2O) vapour was studied at 25 °C and 300 °C using dose rates considerably higher than in previous studies. The reaction rate was followed to a point where the oxide thickness was several nanometers. This was done using X-ray photoelectron spectroscopy (XPS) and modeling of the spectral profiles using QUASES™. The surface reaction rate is much slower than that with oxygen gas (O 2) and it terminates within a few nanometers. The films formed were found to be composed of a defective nickel oxide (NiO) containing both divalent nickel (Ni 2+) and trivalent nickel (Ni 3+) as had been observed previously for reaction of Ni with O 2. The reason for the much slower reaction rate in the case of H 2O vapour appears to be linked to a slower place exchange with a surface hydroxyl (OH (ads)) intermediate. This adsorbate species appears only to be stabilised on metallic Ni and termination of oxide growth is believed to occur once all the available surface metal sites have been covered with oxide.

Normal spectral emittance of a nickel metal surface with rectangular microcavities

AbstractSpectral control of thermal radiation emitted from rectangular microcavities (0.5×0.5×0.5µm3) was investigated through emission experiments at high temperatures. The microcavities were fabricated on a mirror‐finished Ni metal surface. Through measurement of the normal spectral emittance, the maximum emittance was obtained around a wavelength of 0.87µm, which was very close to that (0.894µm) estimated from the cavity resonance theory. The emittance reached a maximum value of 0.95, and then decreased drastically with increasing wavelength, from the cut‐off wavelength. For a longer wavelength range from 1.7µm, it was equal to the emittance of the mirror‐finished surface. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20248

Development of Methane Decomposition Catalysts for COx Free Hydrogen

Now-a-days, catalytic decomposition of methane (CDM) into hydrogen and carbon is a promising technique for production of fuel cell grade hydrogen. The Ni based catalysts seems promising particularly for the production of COx free H2 by methane decomposition process. The CDM activity and longevity of the Ni based catalysts are mainly influenced by the amount of Ni and type of support material. In this paper the CDM activity results are correlated with NiO crystallite size, Ni metal surface area and acidity of the catalysts. In case of bimetallic catalysts addition of Cu to Ni catalysts lead to enhance the CDM activity at higher temperature thus resulting in the increased concentration of hydrogen in the outlet stream. Finally, some of the carbon-based catalysts are studied for methane decomposition activity at higher temperature. The surface changes over carbon catalysts with methane decomposition are studied using various characterization techniques.

Synthesis and Characterization of Au@Co and Au@Ni Core−Shell Nanoparticles and Their Applications in Surface-Enhanced Raman Spectroscopy

Au@Co and Au@Ni core−shell nanoparticles with controllable shell thicknesses were prepared by reduction of Co2+ and Ni2+ salts with hydrazine hydrate in ethanol over preformed Au seeds. Both cyclic voltammetric and surface-enhanced Raman studies using CO as the probe molecule confirmed the core−shell structure of the synthesized nanoparticles. The Au@Co and Au@Ni nanoparticles dispersed on a glassy carbon electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine. High-quality SERS spectra of CO absorbed on Co and Ni have been obtained through the high enhancement of the core−shell nanoparticles. The originally low surface enhancement of the Co and Ni can be substantially improved giving total enhancement factors up to 103-104. Such a SERS-active substrate can be used as an alternative substrate for investigating adsorption and reactions occurring on the Co and Ni metal surfaces.

Pure H2 Production by Decomposition of Methane Over Ni Supported on Hydroxyapatite Catalysts

The catalytic decomposition of CH4 for the production of pure H2 is carried out over Ni supported on hydroxyapatite [Ca5(PO4)3(OH)] catalysts at 650 °C and atmospheric pressure. CH4 decomposition activity is decreased with time on stream and finally deactivated completely. The physicochemical properties of the fresh catalysts are characterized by XRD, DTA/TG, TPR and SEM techniques along with CHNS analyses of the used samples. It is found that the 30 wt% Ni/HAp displayed higher H2 production rates over the other Ni loadings, which is correlated with Ni metal surface area measured by O2 pulse chemisorption.

The evaluation of the stability of Ni/MgO catalysts for the gasification of lignin in supercritical water

The lignin gasification in supercritical water with 20 wt.% Ni/MgO catalyst showed high catalytic performance, however, the deactivation of catalyst was observed when this catalyst was reused successively three times. The used catalyst was characterized by BET, XRD, TPR, CO chemical adsorption techniques, TPO and TEM observation, and the reasons of deactivation were discussed in detail. When the mixture of Ni/MgO catalyst and THF insolubles obtained in the previous run was used as a catalyst for the next run, the carbon yield of gas products considerably dropped due to the presence of char-like carbonaceous products and the formation of Mg(OH)2 phase. Moreover, the deactivation of catalyst was also observed even after used catalyst was regenerated by oxidation at 1023 K for 1 h and reduction at 1173 K for 1 h. The decrease in Ni metal surface area of catalyst treated by oxidation and reduction probably caused catalyst deactivation. The deactivation of catalyst during reuse test with Ni/MgO catalyst occurred regardless of regeneration treatments.

Structure and growth of oxides on polycrystalline nickel surfaces

AbstractThe oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O2) at 25 and 300 °C and pressures near 130 Pa, was studied using X‐ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni2+) and trivalent (Ni3+) species could be distinguished using Ni 2p spectra, while surface adsorbed O2 and atomic oxygen (O) species could be differentiated from bulk oxide (O2−) using O 1s spectra. Oxide thicknesses and distributions were determined using QUASES™, and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300 °C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25 °C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300 °C. Reaction of a Ni (100) single crystal under comparable conditions showed much slower reaction rates compared to polycrystalline specimens. The higher reaction rate of the polycrystalline materials is attributed to grain boundary transport of Ni cations. Oxide thickness was measured on a microscopic scale for polycrystalline Ni exposed to large doses of O2 at 25 and 300 °C. The thickness of oxide was not strongly localized on this scale. However, the QUASES™ analysis suggests that there is localized growth on a nanometric scale—the result of island formation. Copyright © 2007 John Wiley & Sons, Ltd.

Hydrogen production from the gasification of lignin with nickel catalysts in supercritical water

Hydrogen production from the gasification of lignin with Ni/MgO catalysts in supercritical water was conducted using stainless steel tube bomb reactor. Ni/MgO catalysts were prepared by impregnation method and were calcined at 773–1173 K in air for 8 h. The results of characterization for reduced Ni/MgO catalysts showed that Ni metal and NiO–MgO phase are formed after the reduction of calcined catalyst by H 2 . Furthermore, Ni metal surface area, which was calculated by CO chemical adsorption technique, decreased with increase in calcination temperatures. It was found that the carbon yield of gas products was increased with increase in Ni metal surface area except 10 wt% Ni/MgO (773 K) catalyst. Thus, it can be supposed that there is an optimal Ni particle size for the gasification of lignin in supercritical water. It should be noted that 10 wt% Ni/MgO (873 K) catalyst showed the best catalytic performance (carbon yield 30%) under reaction condition tested. It was concluded that Ni/MgO catalyst is a promising system for the gasification of lignin in supercritical water.