Surface compositions

Surface Composition. - Bimetallic catalyst systems have received much interest because variation in alloy composition offers a ready method of altering the metallic properties of the catalyst. For a range of Fe-Ni catalysts, Matsuyama et al have attempted to answer a question fundamental to such systems, namely, how does the surface composition compare with that of the bulk Powdered Fe-Ni catalysts were prepared from a solution of Fe(N03)2 and Ni(N03)2- The mixture also contained radioactive [63-Ni] which emits 3 radiation with an f max of 67 keV and a penetration of about 200 layers of heavy metal. It was possible to measure the amount of Ni which existed in the surface layers of the alloy, since 3-cmission from the underlayers of the metal was weakened by self-absorption. 

The radioactivity of the samples was measured with a 27t counter. A thin film of Mylar or Shealon was placed between the sample and the cathode of the 27t counter to filter out the weakened /3-rays. It was possible to calculate the number of layers emitting /3-rays from the equation 

Robertson and Webb ° have synthesized silica-supported Ru carbonyl catalysts which are active for the hydrogenation of but-l-ene. On heating, these catalysts undergo certain irreversible changes, accompanied by the loss of CO. Using [ CO]-labelled Ru3(CO)i2 on silica, the authors were able to show that decomposition occurred according to the following reaction sequence  

Approximate values of x andy were calculated to be 8 and 5, respectively. The subcarbonyl Ru3(CO)s was claimed to be the catalytically active species in the hydrogenation of but-l-ene. 

Catalytic events such as adsorption, breaking, and forming of bonds are obviously associated with the surface of the solid. Any information regarding the composition of the surface is therefore essential in providing a good understanding of the catalyst. Multiple approaches are generally used, especially chemisorption and spectroscopic methods. 

Chemisorption has been applied to numerous catalytic systems including the TMS catalysts, and valuable information on the active surface area or catalytic sites densities has been obtained. However, it is important to keep in mind that the interpretation of the results is subject to the assumption that the stoichiometry of the chemisorption is known. It is well known that on metal surfaces, dispersion is calculated using one hydrogen atom per metal atom. Consequently, dispersion higher than 100% is not unusual for highly dispersed catalysts with particle sizes below 15 A, reflecting the existence of metal atoms associated with more than one hydrogen. This assumption is also made for TMS catalysts. 

A good example for sulfides is provided in the chemisorption of oxygen on the edge surface of MoS2. If site densities are calculated using one oxygen atom per molybdenum on the edge, the densities are inexact, particu- 

A similar relationship can be observed with promoted M0S2. Each family of catalysts has its own linear correlation, which cannot be compared to each other directly because of the corrosivity problem. More recently, low-temperature oxygen chemisorption has been claimed to be more reliable, but it also lacks a well-determined stoichiometry (52). Oxygen chemisorption has also been applied to tungsten and rhenium sulfides, as well as promoted molybdenum and tungsten sulfides. In the isotropic class, it has been applied only to ruthenium sulfide, in which case it gives approximately the same result as a BET measurement due to the isotropic nature of this sulfide (41). 

STM image of ReS2 basal plane (57). Atomic spacing corresponds to actual ReS2 basal plane spacing. 

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A general prerequisite for the existence of a stable interface between two phases is that the free energy of formation of the interface be positive were it negative or zero, fluctuations would lead to complete dispersion of one phase in another. As implied, thermodynamics constitutes an important discipline within the general subject. It is one in which surface area joins the usual extensive quantities of mass and volume and in which surface tension and surface composition join the usual intensive quantities of pressure, temperature, and bulk composition. The thermodynamic functions of free energy, enthalpy and entropy can be defined for an interface as well as for a bulk portion of matter. Chapters II and ni are based on a rich history of thermodynamic studies of the liquid interface. The phase behavior of liquid films enters in Chapter IV, and the electrical potential and charge are added as thermodynamic variables in Chapter V. 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

Smith [113] studied the adsorption of n-pentane on mercury, determining both the surface tension change and the ellipsometric film thickness as a function of the equilibrium pentane pressure. F could then be calculated from the Gibbs equation in the form of Eq. ni-106, and from t. The agreement was excellent. Ellipsometry has also been used to determine the surface compositions of solutions [114,115], as well polymer adsorption at the solution-air interface [116]. 

The broken bond approach has been extended by Nason and co-workers (see Ref. 85) to calculate as a function of surface composition for alloys. The surface free energy follows on adding an entropy of mixing term, and the free energy is then minimized. 

Metals A and B form an alloy or solid solution. To take a hypothetical case, suppose that the structure is simple cubic, so that each interior atom has six nearest neighbors and each surface atom has five. A particular alloy has a bulk mole fraction XA = 0.50, the side of the unit cell is 4.0 A, and the energies of vaporization Ea and Eb are 30 and 35 kcal/mol for the respective pure metals. The A—A bond energy is aa and the B—B bond energy is bb assume that ab = j( aa + bb)- Calculate the surface energy as a function of surface composition. What should the surface composition be at 0 K In what direction should it change on heaf)pg, and why ... 

The various spectroscopic methods do have in common that they typically allow analysis of the surface composition. Some also allow an estimation of the chemical state of the system and even of the location of nearest neighbors. 

The principal use of Auger spectroscopy is in the determination of surface composition, although peak positions are secondarily sensitive to the valence state of the atom. See Refs. 2, 82, and 83 for reviews. 

These equations indicate that the energy of the scattered ions is sensitive to the mass of the scattering atom s in the surface. By scanning the energy of the scattered ions, one obtains a kind of mass spectrometric analysis of the surface composition. Figure VIII-12 shows an example of such a spectrum. Neutral, that is, molecular, as well as ion beams may be used, although for the former a velocity selector is now needed to define ,. ... 

SXES spectroscopy [111] ejects K electrons and the spectrum of the resulting x-rays is measured Spectroscopy of Emitted Electrons state of adsorbed molecules surface composition... 

AES ARABS Auger electron spectroscopy [77, 112-114, 117] Angle-resolved AES [85, 115] An incident high-energy electron ejects an inner electron from an atom an outer electron (e.g., L) falls into the vacancy and the released energy is given to an ejected Auger electron Surface composition... 

XPS X-ray photoelectron spectroscopy [131-137] Monoenergetic x-rays eject electrons from various atomic levels the electron energy spectrum is measured Surface composition, oxidation state... 

ISS Ion scattering spectroscopy [153, 154] Inelastic backscattering of ions (-1 keV ion beam) Surface composition... 

LEIS Low-energy ion scattering [155-157] A monoenergetic beam of rare-gas ions is scattered elastically by surface atoms Surface composition... 

SIMS Secondary-ion mass spectroscopy [106, 166-168] (L-SIMS liquids) [169, 170] Ionized surface atoms are ejected by impact of -1 keV ions and analyzed by mass spectroscopy Surface composition... 

APS Appearance potential spectroscopy (see AES) Intensity of emitted x-ray or Auger electrons is measured as a function of incident electron energy Surface composition... 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

With the exception of the scanning probe microscopies, most surface analysis teclmiques involve scattering of one type or another, as illustrated in figure A1.7.11. A particle is incident onto a surface, and its interaction with the surface either causes a change to the particles energy and/or trajectory, or the interaction induces the emission of a secondary particle(s). The particles that interact with the surface can be electrons, ions, photons or even heat. An analysis of the mass, energy and/or trajectory of the emitted particles, or the dependence of the emitted particle yield on a property of the incident particles, is used to infer infomiation about the surface. Although these probes are indirect, they do provide reliable infomiation about the surface composition and structure. 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

AES Auger electron spectroscopy After the ejection of an electron by absorption of a photon, an atom stays behind as an unstable Ion, which relaxes by filling the hole with an electron from a higher shell. The energy released by this transition Is taken up by another electron, the Auger electron, which leaves the sample with an element-specific kinetic energy. Surface composition, depth profiles... 

SIMS Secondary Ion mass spectroscopy A beam of low-energy Ions Impinges on a surface, penetrates the sample and loses energy In a series of Inelastic collisions with the target atoms leading to emission of secondary Ions. Surface composition, reaction mechanism, depth profiles... 

An example in which XPS is used for studying surface compositions and oxidation states is illustrated in... 

Most soHd catalysts used on a large scale are porous inorganic materials. A number of these and the reactions they catalyze are summarized ia Table 1 (10). Catalysis takes place as one or more of the reactants is chemisorbed, chemically adsorbed, on the surface and reacts there. The activity and selectivity of the catalyst depend strongly on the surface composition and stmcture. 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

The most useful application of ISS is in the detection and identification of sur-fece contamination, which is one of the major causes of product failures and problems in product development. The surface composition of a solid material is almost always different than its bulk. Therefore, surface chemistry is usually the study of unknown surfaces of solid materials. To better understand the concept of surface analysis, which is used very loosely among many scientists, we must first establish a definition for that term. This is particularly Important when considering ISS... 

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