Ni Alloy Surface Research Articles

SURFACE COSEGREGATION OF CHROMIUM AND SULFUR IN SINGLE CRYSTALLINE AUSTENITICFe–Ni–CrALLOYS

In this work we have studied cosegregation of sulfur and chromium on well-defined austenitic Fe–Cr–Ni alloy surfaces. For this purpose we have investigated (100)-oriented surfaces of Fe–14.8%Cr–17.8%Ni(cS=9 wt-ppm) and Fe–18%Cr–14%Ni (cS=18 wt-ppm) single crystals using Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). Segregation experiments were carried out under UHV conditions at temperatures ranging from 500 to 900°C. Cosegregation of chromium and sulfur is very pronounced on the Fe–14.8%Cr–17.8%Ni(100) alloy surface, while on Fe–18%Cr–14%Ni(100) the surface concentration of cosegregated chromium is rather small. On both surfaces sulfur segregation is characterized by a c(2 × 2) LEED pattern. The results of the present study are compared with a previous study using ferritic Fe–15%Cr–S(100) alloy single crystals.

99/01959 Energetics study on syngas (CO + H 2) methanation reaction on Ni, Cu and Ni-Cu alloy surfaces by bond-order conservation model : Xia, W.-S. et al. Huaxue Xuebao, 1998, 56, (8), 773–779. (In Chinese)

Bond - order conservation - Morse potential method has been some modified for alloy systems. Comparison with Ni(100) and Cu(100), syngas (CO + H2) methanation activity on NiCu(100) has been studied. The results show, due to the introduction of Cu, methanation on NiCu has much lower activity than on Ni, which can be explained by the formation of deposited Csfrom CO dissociation on Ni surfaces. Also, we investigate the rate - determining - step of methanation reaction on those surfaces, and the influences of the deposited surface species at reaction beginning on the reactivity of CO methanation.

Effect of Cu powder as an additive material on the inner pressure of a sealed-type Ni–MH rechargeable battery using a Zr-based alloy as an anode

Extensive and systematic work on improving the surface catalytic activity of electrodes has been carried out to decrease the internal pressure of a sealed Ni–MH battery in which Zr 0.9Ti 0.1(Mn 0.7V 0.5Ni 1.2) 0.92 alloys are used in the anode. In order to improve the surface catalytic activity of a Zr–Ti–Mn–V–Ni alloy which are closely related to the inner pressure behavior in a sealed cell, the electrode was fabricated by mixing the alloy with Cu powder. By replacing 50% of carbon black with Cu powder the inner cell pressure rarely increased with cycles. This was due to the active gas recombination reaction. After measuring the surface area of the electrode and the surface catalytic activity, it was found that the surface catalytic activity for the oxygen recombination reaction was much more improved by the addition of Cu powder. This phenomenon is due to the thin Cu layer on the Zr–Ti–Mn–V–Ni alloy surface which is formed by alternate dissolution and precipitation reactions of mixed Cu powder during the charge–discharge cycle. This thin Cu layer has the role of preventing the alloy surface from oxidation and enhancing the possibility of the presence of metallic Ni, which is believed to have a high catalytic activity for the oxygen recombination reaction, on the MH surface. The inner pressure of the cell with the Zr–Ti–Mn–V–Ni alloy was lowered to a level equaling that of the commercial AB 5-type alloys by the addition of Cu powder.

The STRUCTURE OF THE c(2×2) Mn/Ni(001) SURFACE ALLOY BY PHOTOELECTRON DIFFRACTION

Surface alloys are two-dimensional metallic systems that can have structures that are unique to the surface, and have no counterpart in the bulk binary phase diagram. A very unusual structure was reported for the Mn–Ni system, based on a quantitative LEED structure determination, which showed that the Mn atoms were displaced out of the surface by a substantial amount. This displacement was attributed to a large magnetic moment on the Mn atoms. The structure of the Mn–Ni surface alloy was proposed to be based on a bulk termination model. Magnetic measurements on Mn–Ni, however, showed conclusively that the magnetic structure of these surface alloys is distinct from the bulk alloy analogs. For example, bulk MnNi is an antiferromagnet, whereas the surface alloy is ferromagnetic. This suggests that the proposed structure, based on bulk termination, is not complete. Photoelectron diffraction techniques were used to investigate this structure, using both a comparison to multiple scattering calculations and photoelectron holography.

Photoexcited processes at metal and alloy surfaces: electronic structure and adsorption site

Photoexcited processes of NO and CO at photon energies ranging from 2.3 to 6.4 eV are investigated on Pt(111), Ni(111) and Pt(111)Ge surface alloys by reflection-absorption infrared spectroscopy and resonance-enhanced multiphoton ionization. The branching between three competitive processes of desorption, recapture and dissociation upon laser irradiation is dramatically changed on the three surfaces. On Pt(111), NO is either photodesorbed or photodissociated depending on the coverage, while NO is exclusively photodissociated on Ni(111). UV-photon irradiation of NO on Pt(111)Ge, on the other hand, induces only desorption of NO. Desorption of CO bound at the on-top site of Pt(111) is induced by laser irradiation. The electronic mechanism for photodesorption and competitive branching is discussed in terms of the electronic structure of the substrate and the adsorbate.

A Theoretical Study of Butadiene Adsorption on the Pd–Ni Bimetallic System

The calculated electronic perturbation of the butadiene molecule is identical when it is adsorbed either on the Pd–Ni bimetallic system or on the pure Pd surface. However, the calculations provide evidence that the butadiene adsorption strength is much smaller in the former case. The orbital by orbital analysis reveals that the π-dz2interaction is reduced, whereas the π*-dyzis enhanced with respect to the pure Pd surface. Nevertheless, since the total electron transfers from the π orbitals to the π* orbitals within the butadiene molecule are similar, a two-electron analysis alone is unable to explain the large difference between the butadiene adsorption energy on the Pd–Ni surface alloy and that on pure Pd. This difference is partly related to anti-bonding interactions since the Pd–Ni bimetallic system exhibits a larger four-electron destabilization (roughly +25%). Evidence is also presented for a reduced electron exchange between the Pd–Pd adsorption site and its neighboring atoms in the case of the Pd–Ni bimetallic system with respect to the pure Pd and consequently a diminished coupling with the electron reservoir created by the extended surface. Hence, the adsorbing Pd clusters of a bimetallic system appear more isolated from the remaining part of the solid than they do in the pure Pd.

The Pd/Ni(110) Bimetallic System: Surface Characterisation by LEED, AES, XPS, and LEIS Techniques; New Insight on Catalytic Properties

Pd deposits on Ni(110) have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), low-energy ion scattering spectroscopy (LEIS), and X-ray photoelectron spectroscopy techniques and their catalytic properties tested by the butadiene hydrogenation reaction at room temperature. Their catalytic activity is greatly enhanced by appropriate annealings. Superstructure indicative of a long range ordering of Pd atoms on Ni is evidenced for low Pd coverages neither before nor after annealing. For higher coverages, a LEED superstructure appears after annealing. AES and LEIS studies reveal a sharp Pd depth profile and the existence of a Pd–Ni surface alloy even at room temperature. The Pd 3d core level binding energy is shifted upwards by at least 0.4 eV with respect to pure Pd surface atoms and remains unchanged in the course of annealings. Hence geometrical rearrangements have to be taken into consideration to explain the activity enhancement.

Hydrogen chemisorption on Pt/Ni (111) systems

Extensive self-consistent real-space recursion-method calculations were performed for the Pt overlayer or the Pt7 cluster on the Ni(111) surface and for hydrogen chemisorption on these systems. Correlations between the surface-atom local density of electronic-state properties before chemisorption, surface reactivity, and the initial-state contribution to the metal core-level shifts, respectively, is documented and discussed. The experimentally observed catalytic properties of PtNi alloy surfaces are also briefly considered. © 1996 John Wiley & Sons, Inc.

Structural and electronic properties of an ordered alloy surface: rippling and bonding in [formula omitted

By using the full potential linearized augmented plane wave method, the structural, electronic and magnetic properties of the c(2 × 2)- Sn Ni(001) surface alloy system have been determined. Interestingly, the presence of Sn atoms drastically reduces th of the density of states at E F and almost completely removes the magnetic moment of the topmost Ni atom. Thus the magnetic and chemical properties of the alloyed Ni(001) surface should be strongly altered. We found that the Sn atoms ripple by 0.36 Å over the Ni atoms in the surface plane, which agrees well with the measured value, 0.44±0.05 A ̊ .

Influence of alloyed Sn atoms on the chemisorption properties of Ni(111) as probed by RAIRS and TPD studies of CO adsorption

The chemisorption of CO on NiSn alloys formed on a Ni(111) surface has been studied with reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Formation of the (√3 × √3) R30°- Sn Ni(111) surface alloy strongly suppresses CO adsorption. Only 0.04 ML CO can be chemisorbed on this surface at 110 K, exclusively at atop sites. The binding energy of this adsorbed CO is reduced to only about 15 kcal/mol. Additionally, no significant effect of subsurface Sn on CO chemisorption on this alloy was observed. These results are in sharp contrast to our previous results for CO chemisorption on the (√3 × √3) R30°- Sn Pt(111) surface alloy, where the chemisorbed CO saturation coverage, adsorption site distribution and desorption temperature were quite similar to those properties of the Pt(111) surface. We ascribe the influence of alloyed Sn atoms on the CO chemisorption properties of this Sn Ni(111) surface alloy to be due to repulsive SnCO interactions at the NiCO distance required for chemisorption. The differences in the chemisorption properties of these PtSn and NiSn alloys are rationalized by considering the different sizes of the surface unit cells and the location of Sn with respect to the surface plane (the Sn buckling distance). These results have important implications for the discussion and determination of ensemble sizes at bimetallic surfaces since the surface chemistry depends strongly on the vertical position of the modifier.

Core hole screening mechanism of adsorbates

The concept of the cooperative core hole screening mechanism is discussed in detail by using a cluster configuration interaction (CI) approach. The XPS core hole spectral features of adsorbate/substrate systems such as CO Ni and N 2 Ni depend on the local metal s-d population of the substrate metal atom. The spectral features of the coadsorbed systems such as CO/H/Ni and of N 2 adsorbed on a Ni alloy surface are different from those of CO Ni and N 2 Ni , respectively. These spectral feature changes are explained by the cooperative core hole screening mechanism. The absence of the shake-up satellite in the XAS spectrum is also explained in the same manner.

Structure and properties of samarium overlayers and Sm/ Ni surface alloys on Ni(111)

Development of the Ni(111)/Sm interface as a function of rare earth loading and substrate temperature has been studied by LEED, XPS, UPS, Δφ and auxiliary measurements. At ambient temperature growth proceeds by the Stranski-Krastanov mechanism involving initial formation of a metastable low density, √3 bilayer of Sm adatoms Sm atoms in the bilayer are trivalent, divalent character appearing for thicker films; angle-resolved XP data sugges that the divalent Sm is surface-localised and therefore that this mixed valency is heterogeneous rather than homogeneous in nature. Thermal treatment results in the formation of a series of ordered surface alloys whose structures may be rationalised in terms of known Ni/Sm bulk alloys. The most important (most stable) of these surface alloys is based on the Ni 17Sm 2 structure and it appears to be capped by an overlayer of trivalent Sm.

An electron spectroscopic study of the adsorption of nitrogen on a Ni/TiO2 catalyst prepared in situ in the spectrometer: evidence for dissociative adsorption in the strong-metal-support-interaction (SMSI) state

Nitrogen is dissociatively adsorbed on an annealed Ni/TiO2 surface just as on a Ti–Ni alloy surface while it is molecularly adsorbed on a Ni/Al2O3 surface.

Surface composition analyses of Co–Ni alloys under Ar+ ion bombardment by AES and ISS: A possible standard sample for surface chemical analysis

AbstractThe change in surface composition of Co(50.4 at.%)–Ni alloy under Ar+ ion bombardment was investigated by Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). The surface concentrations were estimated by employing a spectrum synthesis method, since the Co and Ni signals in both AES and ISS spectra overlap each other. The results based on the quantifications by AES and ISS have indicated that the composition of the Co–Ni alloy surface under Ar+ ion bombardment is the same as the bulk composition within the accuracy in quantification, leading to the conclusion that ion bombardment does not cause surface composition change in the Co–Ni alloy. This supports the proposal that the Co–Ni alloy system is an appropriate reference material for surface chemical analysis.

Surface composition of CoNi alloys under Ar + ion bombardment

CoNi alloy surfaces under Ar ion bombardment were investigated by AES. The surface concentration was estimated by using direct spectrum synthesis of differential mode spectrum, together with the usual peak to peak estimation. No significant ion-induced surface segregation was observed, suggesting that this alloy system is favorable as a standard sample for AES analysis.

Studies on the effect of surface segregation on hydrocarbon conversion over CuNi alloys

The experimental results of Sinfelt, Carter and Yates on the activity of ethane hydrogenolysis and cyclohexane dehydrogenation reactions over the CuNi alloy surface have been interpreted in terms of an electronic theory of chemisorption-induced surface segregation and the bond order conservation (BOC) model. It is found that the adsorbates alter the segregation properties of the CuNi alloy which, in turn, affect the activities of these reactions.

A core-level binding-energy shift analysis of CO, H, and O adsorption on Cu–Ni surfaces

The equivalent core approximation (Z+1) is used with a Born–Haber cycle analysis to analyze the Ni 2p3/2 surface core-level binding-energy shifts which occur upon adsorption of CO, H, and O on Ni(100). The analysis gives values for the heats of desorption of these gases from Cu–Ni alloy surfaces. Perhaps more importantly, it also provides a quantitative determination of how chemisorption of CO, H, and O modify the heats of surface segregation for Cu–Ni surfaces.

The interaction of hydrogen with Ni-rich (111)Cu-Ni surfaces

The hydrogen chemisorption on Ni-rich (111)CuNi alloy surfaces with different surface compositions has been studied by means of thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and work function measurements during absorption and desorption. With respect to the interaction with hydrogen these alloy surfaces may, in a first approximation, be described as consisting of active Ni ensembles diluted by inert Cu. Desorption spectra observed after adsorption at 120 and 250 K show nearly the same overall features as those known from pure (111)Ni with two desorption states (β 1, and β 2) and second order desorption kinetics indicating dissociative adsorption. Increasing Cu surface content leads to a rapid decrease of the adsorbed amount of hydrogen, indicating a strong ensemble effect, and to a higher relative intensity of the low temperature desorption maximum (β 1) as compared to the high temperature one (β 2). This result shows that the hydrogen adsorption on the Ni ensembles is influenced by the surrounding Cu atoms. This may be due to the fact that Cu prevents lateral ordering of the adsorbed hydrogen, at least on smaller Ni islands. These findings are in keeping with those of our LEED investigations and work function measurements. As with pure Ni the work function increases during the development of the β 2-state and decreases if β 1 is filled. The initial work function increase rapidly becomes smaller for higher Cu surface content and even vanishes for higher concentrations. The (2 × 2) LEED adsorption structure associated to the β 2-state can only be observed at small Cu content. Studies of the adsorption kinetics indicate that Cu atoms may form precursor sites for the adsorption on Ni sites.

Interaction of carbon monoxide with clean and oxygen covered Cu(111)-Ni surface alloys

Cu(111)-Ni surface alloys were prepared by dissociation of nickel carbonyl on clean Cu(111). The adsorption of CO is studied with AES and ellipsometry. At pressures higher than 10 -2 Pa CO, migration of Ni to surface layers occurs, which makes it impossible to measure adsorption isotherms. The removal of preadsorbed oxygen on Cu(111)-Ni with CO is also studied with AES and ellipsometry. A reaction scheme is proposed to describe the two different types of reaction curves. At temperatures below 200°C, the overall apparent activation energy is 40 kJ/mol, at higher crystal temperatures this value drops to 10kJ/mol. CO is found to react with oxygen on the surface, which is replenished by incorporated (subsurface) oxygen.

Interaction of carbon monoxide with clean and oxygen covered Cu(111)-Ni surface alloys

Interaction of carbon monoxide with clean and oxygen covered Cu(111)-Ni surface alloys