Reactivity Of Metal Surfaces Research Articles

Book Review: Metal–Surface Reaction Energetics. Theory and Applications to Heterogeneous Catalysis, Chemisorption, and Surface Diffusion. Edited by E. Shustorovich.

Book Review: Metal–Surface Reaction Energetics. Theory and Applications to Heterogeneous Catalysis, Chemisorption, and Surface Diffusion. Edited by E. Shustorovich.

The structure and stability of metal surfaces modified by silane Langmuir-Blodgett films

Monomolecular LB films of n-octadecyltrichlorosilane (n-OTS) and chlorodimethyloctadecylsilane (CODS) have been produced on metal substrates like Fe and Ni in order to protect the reactive metal surface against corrosive species like H 2O, O 2 and SO 2. To characterize defects within the monomolecular LB films, they were studied by UHV methods such as scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM). In dry or humid air atmospheres it is possible to measure a work function topography of the LB film covered materials with a scanning Kelvin microprobe. In dry air the work function topography corresponds to the distribution of dipole potentials. Therefore it is possible to analyze intrinsic defects within the monomolecular LB film on the metal surface. In a humid atmosphere the work function is mainly given by the local corrosion potential of the metal substrate and therefore delamination processes of the monomolecular film can be studied as well as the stability of the LB film coated metal substrate.

The reactivity of metal surfaces

The relations between the microscopic description of the dynamics of chemical reactions at metal surfaces and our understanding of the activity of heterogeneous catalysts is discussed. It is shown that in some cases we start understanding some of the factors determining the catalytic activity of a given surface. Examples will be given, ranging from detailed reaction dynamics studies of simple adsorption reactions to a prediction of the trends in the ammonia synthesis rate along the transition metal series.

Chemically modified metal surfaces—a new class of composite materials

AbstractThis promising method of chemically modifying reactive metal surfaces with organic molecules prior to the attachment of polymers could have a significant impact on work aimed at the optimization of the performance of composite materials. Strong chemical bonds can be formed between a suitably prepared metal surface and certain monomers, which resist even an electrochemical polarization of the interface at large cathodic or anodic overpotentials. These chemical bonds can, however, only be achieved if a suitable structure and reactivity is generated, for example, by using electrochemical techniques. Therefore, further technological progress in this important area requires the cooperation of scientists from rather different scientific disciplines like electrochemistry, surface science and organic chemistry, in order to optimize the structure of the organic monomers and to provide the best possible bonds to the different metals in use, taking into account the rather different reactivity of technologically important media.

Energetics of metal-surface reactions: back-of-the-envelope theoretical modeling

The slow progress of quantum mechanical modeling of surface energetics makes it necessary to develop working alternatives. One of them is a phenomenological analytic approach based on bond-order conservation (BOC) and making use of Morse potentials (MP). This paper highlights the ‘back-of-the-envelope’ formalism of the BOC-MP model, and provides representative examples of calculations of the heats of chemisorption and the dissociation activation barriers for diatomic and some polyatomic adsorbates. The model findings are in good agreement with experiment. It appears that the BOC-MP model is well suited to treat the energetics of metal-surface reactions.

The effects of ultrasound on nickel and copper powders

The use of high-intensity ultrasound to enhance the reactivity of metal surfaces has become a routine synthetic technique. In spite of this, the origin of rate enhancements in both stoichiometric and catalytic reactions remains unexplored. To this end, we have examined the effects of ultrasound on metal powders in terms of their particle and surface morphology, their atomic composition, and their reactivity. Specifically, we have discovered that ultrasonic irradiation of Ni and Cu powders leads to dramatic changes in morphology: individual surfaces are smoothed and particles are consolidated into extended aggregates. Surface composition was probed by Auger electron spectroscopy depth profiles, which revealed that ultrasonic irradiation removed the surface oxide coating, but also produced (especially for Cu) a deposition of surface carbon. The effects on reactivity can be substantial: ultrasonic irradiation enhances the activity of Ni powder as a hydrogenation catalyst by > 10 5 and significantly increases the reactivity of Cu powder as a stoichiometric reagent. We beleive that these effects are due to interparticle collisons driven by the turbulent flow created by the ultrasonic field. It is likely that the origin of the enhanced chemical reactivity comes from the removal of the surface oxide passivating layer which these collisions induce.

Ultrasonic irradiation of copper powder

Ultrasonic irradiation enhances the reactivity of metal surfaces. Because of this, the sonochemical activation of various metals has become a routine synthetic technique. The mechanisms of heterogeneous sonochemistry, however, remain largely unexplored. In comparison, a detailed understanding of the mechanism of homogeneous sonochemistry has recently been developed. To more fully understand the origins of heterogeneous sonochemistry, we have examined the effects of ultrasonic irradiation on copper in terms of surface morphology, atomic composition, and reactivity. We report here that ultrasonic irradiation of Cu powder leads to substantial changes in all three areas. We believe that these effects are due to interparticle collisions driven by the shock waves created by the ultrasonic field: the origin of the enhanced chemical reactivity comes from the removal of the surface oxide passivating layer and surface damage that these collisions induce.

Fast atom bombardment (FAB) generation of neutral and charged reactive intermediates for rare gas matrix isolation spectroscopic studies. An ESR investigation of 111,113CdOH and 111,113Cd+ in neon and argon matrices at 4 K

An experimental arrangement is presented which allows the combination of neutral atom bombardment in the 3–10 keV range with the rare gas matrix isolation technique for generating and trapping reactive intermediates and cation radicals for electron spin resonance (ESR) and other spectroscopic investigations. The potential of the experimental procedure for studying metal surface reactions is discussed. Neon and argon matrix results are reported for the bombardment of several gaseous and metal samples. The 111,113CdOH radical has been characterized by ESR and compared with previous findings for CdCN, CdF, and CdH. Neon matrix measurements for CdH and Cd+ have been obtained with the FAB method which allow a comparison to be made with previous argon matrix findings. This represents the first magnetic data comparison of neon and argon matrix results for a metal hydride type radical (MH) and a cation radical. These comparisons are especially important for gauging the ‘‘gas-like’’ nature of matrix results for such highly reactive radicals.

A theoretical model of metal surface reactions

Metal surface reactions are modeled with a novel theoretical construct in which periodic trends can be scrutinized. The theoretical model is succinctly presented and a conspectus of periodic trends, based on the model, is explored. Periodic trends are discussed in the contexts of chemisorption bond energies, electron transfer between metal surface and adsorbate, stereochemical features of chemisorption states for closed-shell diatomic and linear X-CN or X-NC molecules, and hydrocarbon reactions. Hydrocarbon C-H bond-breaking processes are analyzed in terms of d-level occupancy, electron transfer, and stereochemistry of intermediates. Conceptually and computationally, the metal surface is characterized as a good electron donor: antibonding molecular orbitals of the adsorbate species appear to be significant contributors to the chemisorption bond and also play a decisive role in bond-breaking processes. No aspect of the model projections is inconsistent with the experimental data although the electronic characterization of some chemisorption states are counter to commonly held perceptions.

Resonant Charge Transfer in Atom–Metal Surface Reactions

In recent years there has been a rising interest in atom–metal surface interactions. Especially resonant charge exchange in atom–metal surface scattering has been the subject of many theoretical and experimental studies. Atoms which easily surface ionize like the alkalis when adsorbed at a metal surface form ionic bonds between adsorbate and substrate resulting in a surface dipole. A surface coverage of about half a monolayer then results in a dipole layer, by virtue of which the work function of the metal is decreased by a factor of three. Especially the pioneering work of Gadzuk resulted in a theoretical description and calculation of this effect from first principles. These theoretical considerations also offer the possibility of a quantitative prediction of surface ionization. Recently the possibility of surface ionization – positive as well as negative ionization – by scattering ions or neutrals from a metal surface has been explored very intensively. This large interest is mainly due to the possibilities of forming intense H− and possibly Li− ion beams by scattering the respective positive ions from low workfunction metal surfaces, like cesium covered tungsten. The intense negative ion beams after neutralization will be needed for heating plasmas in connection with fusion. This paper will be concerned with the theoretical and experimental progress in the field of surface ionization.

Theoretical description of molecule-metal interaction and surface reactions

Recent progress in the theoretical description of the interaction between molecules and metal surfaces and the reactivity of metal surfaces and of chemisorbed molecules is described. Electronic factors affecting associative and dissociative chemisorption, sticking, surface chemiluminescence and other surface reactions are illustrated with a few examples. In these dynamical processes several important features can be distinguished: the downward shift of molecular affinity levels upon approaching the surface and the fact that the active region usually lies outside the static equilibrium position studied by common spectroscopies. Further, the high reactivity of metals can in several cases be traced back to the fundamental and unique property of metals having a quasi-continous spectrum of electronic excitations. The qualitative concepts and considerations are based on results of self-consistent calculations on simple models that stress the extended nature of the conduction-electron states. In a few cases the implications have been tested by dynamical experiments.

Metal clusters in catalysis VIII Reduction of triple bonds

AbstractReduction of triple bonds in simple molecules remains a relatively difficult, yet very important, type of transformation. The scientific challenge is a catalytic demonstration of this reaction paradigm with molecules like N2, CO, RCN, RNC, RN2+, and RCCR. Presented in a structural and electronic rationale is a potential solution to the challenge with metal cluster catalysts. Relationships of the rationale to metal surface and metalloenzyme reactions are discussed. Basic structural forms considered in the context of bond order reduction and resultant ease of reduction (hydrogenation) are: magnified image.

A discussion on the energies involved in specific adsorption of ions

The change in specific adsorption of I− ions on the series of metals Au, Hg, Bi, Pb, Cd, and Ga is analyzed using data of specifically adsorbed charge and shift in potential of zero charge. Factors determining the change in adsorbability are discussed in the light of previous formulations. It is shown that the work connected with water desorption as an ion becomes adsorbed, usually neglected or underestimated in previous discussions, is very likely to be the main factor determining the change in adsorbability along the series of metals. A rough estimation of energies involved in water desorption suggests that metal—water surface bonds are probably weak on sp-metals so that they are unable to affect the reactivity of metal surfaces with respect to the gas phase as strong covalent surface bonds are involved, for instance in the hydrogen evolution reaction. Conversely, the strong effect of water desorption on the specific adsorption of ions may be an indication of ion—metal interactions to be substantially independent of the nature of the metal. This suggests that covalent contributions to the surface bond are apparently minor for metals more electropositive than Au.

A discussion on the energies involved in specific adsorption of ions

The change in specific adsorption of I − ions on the series of metals Au, Hg, Bi, Pb, Cd, and Ga is analyzed using data of specifically adsorbed charge and shift in potential of zero charge. Factors determining the change in adsorbability are discussed in the light of previous formulations. It is shown that the work connected with water desorption as an ion becomes adsorbed, usually neglected or underestimated in previous discussions, is very likely to be the main factor determining the change in adsorbability along the series of metals. A rough estimation of energies involved in water desorption suggests that metal—water surface bonds are probably weak on sp-metals so that they are unable to affect the reactivity of metal surfaces with respect to the gas phase as strong covalent surface bonds are involved, for instance in the hydrogen evolution reaction. Conversely, the strong effect of water desorption on the specific adsorption of ions may be an indication of ion—metal interactions to be substantially independent of the nature of the metal. This suggests that covalent contributions to the surface bond are apparently minor for metals more electropositive than Au.

Nato advanced study institute of electronic structure and reactivity of metal surfaces

Nato advanced study institute of electronic structure and reactivity of metal surfaces

Formula omitted] NATO advanced study institute on electronics structure and reactivity of metal surfaces

formula omitted] NATO advanced study institute on electronics structure and reactivity of metal surfaces

On the surface chemical reactions of metal and oxide XPS samples at 300–400° at a high vacuum produced by oil diffusion pumps

Metal and oxide surface reactions formed by heating in the spectrometer at 300–400° at a vacuum of ca. 10 −9 Torr (oil diffusion pumps) were studied. As a result of spectral observations before and after heating, the metals studied were classified into five groups. In the first group, oxide films on the metal surface are easily evaporated because of the high vapour pressure of oxide; in the second, the oxide films are easily reduced in the spectrometer; in the third, the oxide film formed on the metal is reduced but the bulk oxide is not easily reduced; in the fourth, very stable oxide films are formed and the bulk oxide is also stable; and finally in the fifth, the oxide film formed on the metal is apparently reduced, yet the bulk oxide is very stable.

Equilibrium hydrogen transfer between benzene and C 6 hydrocarbons over supported metal catalysts

A kinetic method is presented for the study of the hydrocarbon-surface interaction taking place during the catalytic conversion of hydrocarbons. The method involves the study of the rate of redistribution of an isotopic tracer between hydrocarbon molecules in thermodynamic equilibrium with the catalyst surface. A general discussion of the approach is given; its potentialities and advantages in studying the individual steps of hydrocarbon reactions are analyzed. These ideas are applied to the study of the rate of hydrogen transfer between cyclohexane and other C 6 hydrocarbons on the one side and benzene on the other over Pt, Pd, Ir, Rh, and Ru supported on Al 2O 3 and S jO 2. The investigation covered the effect on the reaction rate of the ratio of C 6 hydrocarbon to benzene and of metal particle size at 117 °C. With few exceptions the transfer rate increased upon increasing the ratio of cyclohexane to benzene and upon addition of molecular H 2 to the reactants. For the cyclohexane-benzene combination the relative activitv among the metals investigated was Pt > Pd > Ir > Ru > Rh. In the range of Pt crystallite size studied (12–2000 Å) the hydrogen surface coverage was influenced by the particle size, but the reaction rate constant per unit of metal surface area was little dependent upon crystallite size. Reactive surface efficiencies were in the range 10 −4 to 10 −7. Experiments on the hydrogen transfer between benzene and other C 6 hydrocarbons produced the following sequence of reaction efficiency: cyclohexene > cyclohexane > methyl-cyclopentane > n-hexane, 2,3-dimethylbutane. A surface equilibrium reaction and a rate-controlling step consistent with the experimental results are postulated. The nature of the reactive metal surface, particularly the role of the adsorbed hydrogen, is pointed out. The conclusions are analyzed in the framework of the present understanding of metal catalysis.

Volatile fluoro- and trifluoro-methyl diphosphorus compounds containing phosphorus in two valencies

Abstract : Reported attempts to prepare quinquevalent diphosphorus compounds from bis(trifluoromethyl)iodophosphine sulphide, (CF3)2P(S)I and iodothiophosphoryl difluoride, SPF2I, were unsuccessful. It was found that the reaction of the above iodophosphorus compounds with stoicheiometric quantities of metallic mercury at room temperature gives the desired diphosphorus disulphide compounds 2X2P(S)I + Hg yields X4P2S2 + HgI2(X = F,CF3). The reactions are slow and require vigorous agitation to renew the reactive metal surface. (Author)

770. The mechanism of contact between metal surfaces—the penetrating depth and the average clearance: T Tsukizoe and T H Isakado, J Basic Eng, 87 (3), 1965, 666–674

A commercial 300 keV high-resolution electron microscope has been modified for ultrahigh-vacuum operation by the provision of a novel specimen chamber, additional treatment- and cryochambers and extra pumping capacity. Measured vacuum levels after bakeout cycles are in the mid to low 10-9 Torr range. The side-entry-type specimen holder incorporates a flexible bellows connection to minimize vibrations and thermal drift problems. The interpretable resolution of the microscope of 0.20 nm has been retained after completion of the UHV conversion, and the microscope has subsequently been used in the profile imaging and reflection microscopy geometries to study a variety of semiconductor, oxide and metal surfaces and surface reactions. These include the 7X7 Si{111} and 1X1 GaP(110) surface reconstructions, and phase transitions between 2X1 and 3X1 reconstructions of the CdTe(100) surface. The effects of electron irradiation, and also annealing, on MgO and rutile surfaces have been documented.