Structure surface

Surface Structure, - It has been long recognized that the surface of a catalyst is not homogeneous because it is composed of various crystal planes and 

Information on the heterogeneity of adsorption of H2 by Ni, Cu, and Cu-Ni alloys has been obtained by Takeuchi et The catalysts were 

Further evidence that the active sites, for the hydrogenation of ethylene on Ni, are probably the step-edges and terraces has come from field emission studies.  

Another important consideration in the characterization of catalysts is that the surface may undergo rearrangement upon exposure to a potential adsorbate. Bodys and Campbell have described an experiment in which radioactively labelled Kr was incorporated homogeneously into a Ni film. This was subsequently used for H2 adsorption studies. From the amount of [85-Kr] released on adsorption of H2 it was concluded that the initial stage of chemisorption (perhaps representing 5-10% coverage) caused considerable disturbance of the surface metal atoms and thus the release of incorporated [85-Kr]. 

The nature of the surface of the solid catalyst is ultimately vital to its performance and value. The number of catalytically active sites, their dispersion over the surface of the solid, their accessibility to substrate molecules and their activity or strength are fundamental properties that can be influenced by the nature of and any pretreatment of the support, the method of preparation of the 

Infrared spectroscopy [especially diffuse reflectance IR (DRIFTS)] 

Thermal analysis (including evolved gas analysis and temperature programmed desorption) 

Total surface area pore volume pore size distribution pore shape 

Identification of surface species measurement of surface polarity via dye adsorption 

The first diffraction pattern from a metal surface involved He scattering from W(211) 249,250 appeared when the incident beam was normal to the troughs in 

The patterns, particularly at non-grazing incidence, are very similar to those produced by scattering of light on echelette gratings, with most of the diffracted intensity concentrated in one or two beams, and Comsa et alf have made a 

Cantini, and R. Tatarek, J. Phys. F Metal Phys., 1976, 6, L237. 

Helium scattering has been reported also from a Cu(lOO) crystal which was roughened electrochemieally. It was assumed that the surface consisted of randomly oriented 100 terraces separated by monatomic steps and the observed variation in scattered intensity as a function of incidence angle was attributed to interference between beams scattered from various terraces. It was concluded that atomic steps separated by terraces 50 unit meshes wide could be detected readily. 

As with metals, diffraction experiments using covalent crystals have proved difficult also. In this case the open structure should produce pronounced corrugation but with Si(lll) broad scattering distributions, showing no significant structure, were observedit is probable that inelastic effects are important in these systems. Very recently, however, high-quality patterns have been published by Cardillo and Becker 

According to Eq. 1.1, the density or ordering ofthe boundary layer at the solid wall governs the oscillatory force between two approaching surfaces. Therefore any factors that influence this ordering will in turn influence the force profile. These factors include not only the properties of the intervening medium but also the chemical and physical properties ofthe surface itself, as discussed next. 

Section 5.3 considered NMR spectroscopic approaches to the bulk characterization of oxides and oxidation catalysts. Gatalytic activity is, however, intrinsically linked with the nature of the catalyst surface and hence a number of techniques have been developed in order to probe this. As discussed in Section 5.1, two of the most significant parameters impacting on catalyst activity are the acid-base characteristics of a surface and the redox properties of the material, and NMR techniques exist to probe both of these characteristics. One of the most common techniques to probe surface structure is GP-MAS NMR, in particular CP from hydrogen to the nucleus of interest-either the metal or the oxygen of the metal oxide. Historically, the source of surface H species has often been those naturally present on the catalyst surface, as chemisorbed hydroxyls or physisorbed water. As such, much of the work in this area involves the study of supports such as Si02. Applications of CP-MAS and other spectroscopic approaches to the study of oxide surfaces are outlined in the following sections. 

In addition to studies of sihca supports work has also been carried out focusing on aluminas. However, the appHcation of H — Al CP-MAS NMR is complicated as compared to H — Si CP, as this involves transfer between a half-integer nucleus ( H) and a quadmpolar nucleus ( Al). The issues surrounding this have, however, been discussed in depth by a number of workers [183, 185-190]. Despite these challenges, the application of H — Al CP-MAS NMR to catalytic systems has been successfully demonstrated. The first such study was carried out by Morris and Ellis in 1989 [186] for hydrated y-Al203. However, this was only partially successful as it failed to observe any tetrahedrally coordinated surface aluminum as result of processes which are unfavorable to CP, including fast spin-lattice relaxation [186]. 

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A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

I.P.P.D and its relatives have become standard procedures for the characterization of the structure of both clean surfaces and those having an adsorbed layer. Somoijai and co-workers have tabulated thousands of LEED structures [75], for example. If an adsorbate is present, the substrate surface structure may be altered, or reconstructed, as illustrated in Fig. VIII-9 for the case of H atoms on a Ni(llO) surface. Beginning with the (experimentally) hypothetical case of (100) Ar surfaces. Burton and Jura [76] estimated theoretically the free energy for a surface transition from a (1 x 1) to a C(2x 1) structure as given by... 

Above 81.5 K the C(2x 1) structure becomes the more stable. Two important points are, first, that a change from one surface structure to another can occur without any bulk phase change being required and, second, that the energy difference between dtemative surface structures may not be very large, and the free energy difference can be quite temperature-dependent. 

Fig. VIII-8. Surface structures (a) (1 x 1) structure on the (100) surface of a FCC crystal (from Ref. 76) (b) C(2 x 1) surface structure on the (100) surface of a FCC ciystal (from Ref. 76). In both cases the unit cell is indicated with heavy lines, and the atoms in the second layer with pluses. In (b) the shaded circles mark shifted atoms. (See also Ref. 77.)...

ADAM Angular-distribution Auger microscopy [85] Surface atoms silhouetted by Auger electrons from atoms in bulk Surface structure... 

AFM Atomic force microscopy [9, 47, 99] Force measured by cantilever deflection as probe scans the surface Surface structure... 

AM Acoustic microscopy [100] High-frequency acoustic waves are rastered across sample Surface and below-surface structure... 

FEM Field emission microscopy [62, 101, 102] Electrons are emitted from a tip in a high field Surface structure... 

FIM Field ion microscopy [63, 62, 103] He ions are formed in a high field at a metal tip Surface structure... 

NSOM Near-Held scanning optical microscopy [103a] Light from a sharp tip scatters off sample Surface structure to 3 nm... 

PTM Photon tunneling microscopy [12] An interface is probed with an evanescent wave produced by internal reflection of the illuminating light Surface structure... 

SFM Scanning force microscopy Another name for AFM Surface structure... 

STM Scanning tunneling microscopy [9, 19, 31] Tunneling current from probe scans a conducting surface Surface structure... 

SIAM Scanning interferometric apertureless microscopy [103b] Laser light is reflected off the substrate, and scattering between an AFM tip and sample is processed interferometrically Diffraction Surface structure... 

HEED High-energy electron diffraction [104] Diffraction of elastically back-scattered electrons (-20 keV, grazing incidence) Surface structure... 

LEED Low-energy electron diffraction [62, 75, 105] Elastic backscattering of electrons (10-200 eV) Surface structure... 

RHEED Reflection high-energy electron diffraction [78, 106] Similar to HEED Surface structure, composition... 

PED Photoelectron diffraction [107-109] x-rays (40-1500 eV) eject photoelectrons intensity measured as a function of energy and angle Surface structure... 

The importance of the solid-liquid interface in a host of applications has led to extensive study over the past 50 years. Certainly, the study of the solid-liquid interface is no easier than that of the solid-gas interface, and all the complexities noted in Section VIM are present. The surface structural and spectroscopic techniques presented in Chapter VIII are not generally applicable to liquids (note, however. Ref. 1). There is, perforce, some retreat to phenomenology, empirical rules, and semiempirical models. The central importance of the Young equation is evident even in its modification to treat surface heterogeneity or roughness. ... 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Some fascinating effects occur in the case of CO on Pt(lOO). As illustrated in Fig. XVI-8, the clean surface is reconstructed naturally into a quasi-hexag-onal pattern, but on adsorption of CO, this reconstruction is lifted to give the bulk termination structure of (110) planes [56]. As discussed in Section XVIII-9E very complicated changes in surface structure occur on the oxidation of CO... 

Fig. XVI-8. (a) The quasi-hexagonal surface structure of clean Pt(lOO) surface, (b) Adsorption of CO lifts this reconstruction to give the structure corresponding to the termination of (100) planes (from LEED studies). [Reprinted with permission from G. Ertl, Langmuir, 3, 4 (1987) (Ref. 56). Copyright 1987, American Chemical Society.]...

The plan of this chapter is as follows. We discuss chemisorption as a distinct topic, first from the molecular and then from the phenomenological points of view. Heterogeneous catalysis is then taken up, but now first from the phenomenological (and technologically important) viewpoint and then in terms of current knowledge about surface structures at the molecular level. Section XVIII-9F takes note of the current interest in photodriven surface processes. 

The technique of low-energy electron diffraction, LEED (Section VIII-2D), has provided a considerable amount of information about the manner in which a chemisorbed layer rearranges itself. Somotjai [13] has summarized LEED results for a number of systems. Some examples are collected in Fig. XVlII-1. Figure XVIII-la shows how N atoms are arranged on a Fe(KX)) surface [14] (relevant to ammonia synthesis) even H atoms may be located, as in Fig. XVIII-Ih [15]. Figure XVIII-Ic illustrates how the structure of the adsorbed layer, or adlayer, can vary wiA exposure [16].f There may be a series of structures, as with NO on Ru(lOTO) [17] and HCl on Cu(llO) [18]. Surface structures of... 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Much surface work is concerned with the local atomic structure associated with a single domain. Some surfaces are essentially bulk-temiinated, i.e. the atomic positions are basically unchanged from those of the bulk as if the atomic bonds in the crystal were simply cut. More coimnon, however, are deviations from the bulk atomic structure. These structural adjustments can be classified as either relaxations or reconstructions. To illustrate the various classifications of surface structures, figure A1.7.3(a ) shows a side-view of a bulk-temiinated surface, figure A1.7.3(b) shows an oscillatory relaxation and figure A1.7.3(c) shows a reconstructed surface. 

Figure Al.7.6. Schematic diagrams of the DAS model of the Si(l 11)-(7 x 7) surface structure. There are 12 adatoms per unit cell in the outennost layer, which each have one dangling bond perpendicular to the surface. The second layer, called the rest layer, also has six rest atoms per unit cell, each with a perpendicular dangling bond. The comer holes at the edges of the nnit cells also contain one atom with a dangling bond.

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]. 

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