CuNi Alloy Surfaces Research Articles

Controlling Heterogeneous Catalysis of Water Dissociation Using Cu-Ni Bimetallic Alloy Surfaces: A Quantum Dynamics Study.

Copper-Nickel bimetallic alloys are emerging heterogeneous catalysts for water dissociation which is the rate-determining step of the industrially important water gas shift (WGS) reaction. Yet, the detailed quantum dynamics studies of water-surface scattering in the literature are limited to pure metal surfaces. We present here a three-dimensional wave packet dynamics study of water dissociation on Cu-Ni alloy surfaces, using a pseudodiatomic model of water on a London-Eyring-Polanyi-Sato (LEPS) potential energy surface in order to study the effect of initial vibration, rotation, and orientation of the water molecule on the reactivity. For all the chosen surfaces, reactivity increases significantly with vibrational excitation. In general, for lower vibrational states the reactivity increases with increasing rotational excitation but it decreases in higher vibrational states. Molecular orientation strongly affects reactivity by helping the molecule to align along the reaction path at higher vibrational states. For different alloys, the reaction probability follows the trend of barrier heights and the surfaces having all Ni atoms in the uppermost layer are much more reactive than the ones with Cu atoms. Hence the nature of the alloy surface and initial quantum state of the incoming molecule significantly influence the reactivity in surface catalyzed water dissociation.

How Low Nucleation Density of Graphene on CuNi Alloy is Achieved.

CuNi alloy foils are demonstrated to be one of the best substrates for synthesizing large area single‐crystalline graphene because a very fast growth rate and low nucleation density can be simultaneously achieved. The fast growth rate is understood to be due the abundance of carbon precursor supply, as a result of the high catalytic activity of Ni atoms. However, a theoretical understanding of the low nucleation density remains controversial because it is known that a high carbon precursor concentration on the surface normally leads to a high nucleation density. Here, the graphene nucleation on the CuNi alloy surfaces is systematically explored and it is revealed that: i) carbon atom dissolution into the CuNi alloy passivates the alloy surface, thereby drastically increasing the graphene nucleation barrier; ii) carbon atom diffusion on the CuNi alloy surface is greatly suppressed by the inhomogeneous atomic structure of the surface; and iii) a prominent increase in the rate of carbon diffusion into the bulk occurs when the Ni composition is higher than the percolation threshold. This study reveals the key mechanism for graphene nucleation on CuNi alloy surfaces and provides a guideline for the catalyst design for the synthesis of graphene and other 2D materials.

Reactive wetting of TiC-Ni cermet by Ag-Cu-Zn alloy during evaporation

In order to investigate the effect of the evaporation on wetting, reactive wetting of TiC-Ni cermet by molten Ag-Cu-Zn alloys with different Zn contents was studied at 810 °C using a sessile drop method in vacuum. The vapor recoil force induced by the strong evaporation of Zn in the Ag-Cu-25Zn alloy drove the spreading of the molten Ag-Cu-25Zn alloy. Furthermore, the sequential evaporation of Zn in the Ag-Cu-25Zn alloy caused the formation of a (Cu, Ni) solid-solution layer at the interface between the TiC-Ni cermet and the Ag-Cu-Zn alloy. Thus, the (Cu, Ni) solid-solution layer could serve as a metallization layer on the surface of the TiC-Ni cermet, which promoted the non-wetting to wetting transition of the TiC-Ni cermet by the Ag-Cu-25Zn alloy. The equilibrium contact angle of the Ag-Cu-25Zn alloy on the TiC-Ni cermet was 19.9°, approximately equal to that of a Ag-28Cu eutectic alloy on a Cu-Ni alloy surface. The final contact angle of the Ag-Cu-Zn liquid alloy on the TiC-Ni cermet first decreased and then increased as the Zn content in the liquid was increased from 0% to 30%. It reached a minimum value in the Ag-Cu-25Zn alloy.

Insight Into the Effect of CuNi(111) and FeNi(111) Surface Structure and Second Metal Composition on Surface Carbon Elimination by O or OH: A Comparison Study with Ni(111) Surface

A density functional theory (DFT) calculation has been carried out to systematically investigate the mechanism of surface carbon elimination by O and OH on both the alloy FeNi(111) and CuNi(111) surfaces, including the homogeneous and the segregated surfaces, respectively; meanwhile, the obtained results are compared with those on the pure Ni(111) surface in order to probe into the effects of CuNi(111) and FeNi(111) surface structure and second metal composition on the performance of surface carbon elimination. Our results show that compared to the pure Ni(111) surface, the introduction of Fe into Ni increases the adsorption of O, OH, and C species, while it weakens the adsorption of CO and COH; the incorporation of Cu into Ni decreases the adsorption ability of C, O, OH, CO, and COH species. The mechanism of surface carbon elimination by O and OH shows that OH species is more effective for carbon elimination than O species on Ni(111), CuNi(111) surface and the segregated FeNi(111) surface; meanwhile, CuNi(111) and FeNi(111) surface structure and second metal composition have obvious effect on the performance of carbon elimination. Compared to Ni(111), FeNi(111) surface is not favorable for carbon elimination, while CuNi(111) surface is beneficial for carbon elimination, in which the Cu enriched surface is much more favorable than the 1:1 Cu surface and the pure Ni(111), indicating that the segregated CuNi(111) surface with Cu enrichment significantly accelerates carbon elimination. Moreover, the good linear relationship exists between the average adsorption energy of C + O or C + OH and the activation barrier of the C + O(OH) reaction. As a result, once carbon is formed on the segregated CuNi alloy surface with Cu enrichment, carbon deposits can be timely eliminated, which can well explain the reported experimental facts that CuNi bimetallic catalysts with Cu surface enrichment display excellent carbon-resistance ability in CH4/CO2 reforming.

Synthesis of CuNi/C and CuNi/γ-Al2O3Catalysts for the Reverse Water Gas Shift Reaction

A new polyol synthesis method is described in which CuNi nanoparticles of different Cu/Ni atomic ratios were supported on both carbon and gamma-alumina and compared with Pt catalysts using the reverse water gas shift, RWGS, reaction. All catalysts were highly selective for CO formation. The concentration of CH4was less than the detection limit. Cu was the most abundant metal on the CuNi alloy surfaces, as determined by X-ray photoelectron spectroscopy, XPS, measurements. Only one CuNi alloy catalyst, Cu50Ni50/C, appeared to be as thermally stable as the Pt/C catalysts. After three temperature cycles, from 400 to 700°C, the CO yield at 700°C obtained using the Cu50Ni50/C catalyst was comparable to that obtained using a Pt/C catalyst.

Microstructure, mechanical, and corrosion properties of surface of CuNi alloy produced by punching and annealing treatment

The influence of annealing on the formation of nanocrystalline of CuNi alloy surface was investigated by evaluating the microstructure, mechanical properties, and corrosion behavior of as-processed condition (using severe plastic deformation by punching process) and its annealed condition. It was observed that the microstructure changed after annealing of punched sample using an atomic force microscope. Mechanical resistance and corrosion resistance were also characterized using nanoindentation test, electrochemical test, electron work function, and microtribometer test. It was found that the punched and subsequent annealed samples have increasing hardness, elastic behavior (η), and corrosion resistance. Therefore, annealing can lead to the final formation of nanocrystalline and corresponding stability of grain boundary, which are responsible for the increasing mechanical properties and corrosion resistance.

A surface analytical examination of passive layers on [formula omitted] alloys: Part I. Alkaline solution

The electrochemical formation of passive layers on Cu20Ni, Cu50Ni and Ni20Cu in various electrolytes has been examined with X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). In this paper the results of investigations in alkaline solution (1.0 N NaOH) are discussed. On the surface of CuNi alloys first an Ni(OH)2 layer is formed. Potential and time resolved measurements yield similar results for the structure of passive layers. At short times and low potentials the rapidly growing Ni(OH)2 leads to an accumulation of Cu at the metal surface which causes the formation of an oxide layer underneath, consisting mainly of CuO. At more positive potentials NiO enters the oxide sublayer. Cu2O appears to a minor extent at the metal/oxide interface during the initial stages of CuO formation and is no longer detected when CuO grows to a larger thickness at sufficiently positive potentials. In the potential range of the beginning of transpassive behaviour the top Ni(OH)2 layer is oxidized to NiOOH. Parallel to the NiOOH formation Cu(OH)2 is incorporated into the hydroxide overlayer. This transpassive oxidation causes a shift of the work function to larger values as determined by UPS and to the same shift of all XPS signals of oxide constituents to smaller binding energies similar to previous observations for pure Ni and FeNi alloys. The passive layer on CuNi alloys reflects the properties of both metal components.

Stoichiometry effects at Cu-Ni alloy surfaces during 5 keV Ar ion sputtering at room temperatures

The surface and subsurface compositional changes of Cu-Ni alloys with different stoichiometries were studied in situ using 5 keV Ar ion sputtering and X-ray photoelectron spectroscopy. The altered layer formed at room temperature exhibits enrichment of Cu-atoms at the surface due to segregation phenomena and diffusion. The size of the effect depends on the initial composition of the alloy. Additional information on the elemental depth distribution was obtained by varying the photoelectron take-off angle. Modeling of the preferential sputtering of Cu-atoms and of the depth distributions was also carried out. The simulations allowed to distinguish between the temperature and ballistic effects during the ion beam bombardment.

Sputtering and transformation of one phase Cu-Ni alloy surfaces under ion bombardment

Sputtering yields and changes of relief for Cu-Ni alloys with different concentration (2.5, 10, 20, 50, 80% of Ni) under 600 eV Ar+ ion bombardment in the temperature range 100–600°C were investigated. It has been proved that the sputtering rate of alloys decreases with an increase of the bombardment dose. Analysis of irradiated surfaces by the SEM method has shown that ion bombardment stimulates the growth of cone-like features on the surface. For high doses (5–6 × 1019ioncm2), the surface appeared to be covered by narrow disposed hills with a height of a few μm. The sputtering yield, density and sizes of the hills very weakly depend on the dose of bombardment.

Analysis of Corrosion Products Formed on Cu-Ni Alloys Immersed in Sea Water

Abstract A method was developed for separation and analysis of corrosion products formed on the surface of Cu-Ni alloys immersed in sea water polluted by sulphide ions. This method is based on the selective dissolution of oxidation compounds by suitable solvents dissolving the metal matrix only to a negligeable extent. The following solvents were used: 1) methanol to dissolve Na+, Cu2+, Ni2+ chlorides and sulphates; 2) glycine to dissolve bivalent metal compounds - Cu2+, Ni2+ oxides, sulphides, oxysulphates, oxychlorides and oxycarbonates; 3) ammonia solution to dissolve Cu+ compounds (i.e. Cu2O and CuCl); 4) potassium cyanide to dissolve CU+ sulphides. Reasonable agreement between chemical and X-Ray analysis results was observed only for copper compounds, since nickel and iron compounds could not tie observed by X-Ray diffraction. The results of Auger and chemical analyses better agree with each other, yet no Fe compounds could be detected. This is to be attributed to the non-homogeneous corrosion layer ...

The work function as a monitor for thermal desorption spectroscopy: CO desorption from Ni sites on a Ni-rich (110) Cu-Ni surface

A new method for the evaluation of thermal desorption spectra is described. Using this approach, based on the usual assumptions of transition-state theory, the coverage dependence of the Gibbs free activation energy G D (θ) of desorption reactions can be determined. As experimental input it suffices to measure any physical property which is proportional to θ during time-programmed desorption as a function of time and surface temperature. If the isosteric heat and the activation energy of adsorption are known, the activation entropy of desorption and the Arrhenius preexponential factor can be estimated from G D. Work-function data on CO desorption from Ni-rich (111) Cu-Ni alloy surface are used to discuss this method.

Evidence for Crystal-Face Specificity in Surface Segregation of CuNi Alloys

We present experimental evidence for crystal-face specificity in surface segregation at CuNi alloy surfaces using angular x-ray photoemission spectroscopy, secondary-ion mass spectroscopy, and "chemisorptive titration" techniques. More copper enrichment is found on the (100) surface than the (111) surface of a 17% Cu-83% Ni alloy, in agreement with predictions from current segregation theories.

Hydrogen desorption from Ni sites on (110) Cu-Ni surfaces

Hydrogen adsorption on Ni-rich (110) Cu-Ni alloy surfaces has been studied by means of thermal desorption spectroscopy. After adsorption near room temperature the hydrogen desorption spectra exhibit a coverage dependence similar to that known from pure (110)Ni. Besides a slightly composition dependent desorption energy the alloy surfaces behave like a (110)Ni surface diluted by practically inert Cu. These results are compared to those reported by Yu, Ling and Spicer.

Sputtering of CuNi alloys at elevated temperature

In situ AES measurements of the surface compositions of both a sputter-deposited layer and the sputtered surface of CuNi alloys at high temperatures reveal that Cu atoms are sputtered from the surface of Cu 50Ni 50 alloys much more than Ni atoms, and that the equilibrium state is no longer attained during sputtering at temperatures above 200°C. This results in a depletion layer of denuded zone of copper of significant depth in the samples, the depth depending on the sample temperature. Assuming that the sputtering process does not change for CuNi alloys at sample temperatures below 400 C from that for pure Cu and Ni samples, the present results suggest that the substantial enrichment of copper at the outermost atom layer occurs.

Sputtering of Cu-Ni alloys at elevated temperature

In situ AES measurements of the surface compositions of both a sputter-deposited layer and the sputtered surface of Cu-Ni alloys at high temperatures reveal that Cu atoms are sputtered from the surface of Cu 50-Ni 50 alloys much more than Ni atoms, and that the equilibrium state is no longer attained during sputtering at temperatures above 200°C. This results in a depletion layer of denuded zone of copper of significant depth in the samples, the depth depending on the sample temperature. Assuming that the sputtering process does not change for Cu-Ni alloys at sample temperatures below 400°C from that for pure Cu and Ni samples, the present results suggest that the substantial enrichment of copper at the outmost atom layer occurs.

Chemisorption of carbon monoxide on copper-nickel alloy surfaces

The adsorption of CO on Cu-Ni alloy surfaces has been studied at 300 and 120 K using LEED, AES, TDS, and work function measurement. The alloys have been prepared as thin (111) epitaxial films evaporated on mica, and as poly crystalline foils. At 300 K the alloy surfaces show an adsorption behavior similar to that of pure Ni: the work function increases to a saturation value which is higher for Ni-rich surfaces than for Cu-rich. The isosteric heat of adsorption (106 kJ/mole) is nearly as high as with pure Ni. At 120 K the alloys exhibit a more Cu-like adsorption behavior: the work function passes through a minimum which becomes deeper at higher Cu surface concentration. The isosteric heat of adsorption at low temperatures (50 kJ/mole) is nearly as low as for pure Cu. From TDS it can be shown, that the binding energy of the highest (Ni-like) adsorption states increases with increasing Ni surface concentration. At the (111) alloy surfaces no LEED superstructures due to CO adsorption could be observed.

SIMS, AES, work function and flash desorption measurements of the CO adsorption on NiCu alloy surfaces

The chemisorption of CO on Cu, Ni and CuNi alloy surfaces was examined by SIMS, work function measurements and desorption spectroscopy. Using a dynamic SIMS technique the M +, M + 2, MCO + and M 2CO + emission at different temperatures (100–400 K) was measured as a function of CO exposure. In agreement with the work function and desorption experiments an increase of M + and MCO + emission due to the CO adsorption on Cu was found only at low temperatures (100–190 K). On the Ni surface an increase of Ni +, NiCO + and Ni 2CO + was measured up to 400 K. The adsorption of CO on CuNi alloy surfaces — as derived from the work function measurements — can be described by the assumption of two different states of adsorbed carbon monoxide. They can be characterized by different binding energies and from sign and magnitude different work function changes. These states were interpreted as adsorption at Ni or Cu sites of the alloy surfaces, respectively. To a certain extent the SIMS results from the alloy surfaces are incompatible with the work function measurements and desorption spectroscopy and the SIMS studies on the pure metals. A Cu + emission with comparable intensity to the Ni + emission was found for alloys with bulk concentrations of 60 and 40 at% Cu at 300 K. The ratio Ni + Cu + was nearly independent of CO pressure and temperature. The measured ratios of Cu + 2 ( Cu + + Ni +) , Ni + 2 ( Cu + + Ni +) and CuNi + ( Cu + + Ni +) with values about 10 −2 can be explained the basis of a statistical arrangement of Cu and Ni atoms in the alloy surface. The intensities of the MCO + emissions are 10 2 times smaller than the corresponding values of the pure metals. No emission of M 2CO + was found on CuNi during CO adsorption.

On the selective catalytic behaviour of the Cu-NI alloy surface

The selective catalytic behaviour of the Cu x Ni 1− x alloy surface is studied. The concentration dependence of the catalytic activity with respect to chemical reactions involving cracking and non-cracking of C-C bonds is calculated for a simple model for C-C bonds. Surface segregation and local environment effects on chemisorption both calculated from a microscopic electronic theory using tight-binding approximation are taken into account.

Surface composition and surface cluster size distribution of Cu-Ni alloys via a monte carlo method

A Monte Carlo simulation of the Cu-Ni alloy surface was performed in order to determine surface composition and surface atom arrangement as a function of bulk composition, surface crystallographic orientation and temperature. The highly surface active nature of copper demonstrated in previous studies was confirmed, and surface nickel atom arrangement was shown to be a sensitive function of first-layer composition and of crystallographic orientation. Configurational energies were computed in a manner which accounted for the variation of atom-atom bond energies with coordination number. The effect of this method of computation on first-layer composition and composition profile was shown to be significant. Based upon the computed composition profile, an estimate of “surface” composition as it would be determined by Auger electron spectroscopy was made and was shown to be in reasonable agreement with published experimental results.

The effect of surface composition on the surface electronic structure of the CuNi alloys: A study by UPS

We report a.u.v. photoemission study of the surface electronic structures of the well annealed and argon sputtered surface of CuNi alloys. The UPS results indicate substantial Cu enrichment on the annealed alloy surface, in agreement with previous AES measurements. Furthermore, individual peaks in the surface electronic structure can be associated with the Ni or Cu component of the alloy, in accord with the CPA theory and the more recent ATA theory.