Species adsorbed

All heterogeneous catalysis relies on the existence of reactions between adsorbed molecules or atoms (the Langmuir-Hinshelwood mechanism) or between adsorbed molecules or atoms and free molecules (the Rideal mecharrism). 

The bulk of 13 C work done to date falls into one of the two categories, adsorbed species, or catalytic reactions. This section will be so sub-divided. 

work on adsorbed species has been comprehensively reviewed up to mid-1981 by Derbyshire,2 and to a lesser extent by Duncan and Dybowski.1 Work to mid-1982 has been covered by Hays.46 Substrates such as Si02, A1203, zeolites, graphite, carbon black, and certain metals have been dealt with. A great deal of information has been tabulated showing the means to distinguish between different acid sites, the mode of attachment of the adsorbed species, diffusion processes and rates, not to mention catalytic processes which occur at the surface. 

A recent interesting application of 13C n.m.r. has been in the investigation of the role of organic base in the preparation of ZSM-type catalysts. Tetra-propylammonium ions, for example, are very effective in the synthesis of crystalline ZSM-5. The 13C n.m.r. spectra of these occluded ions have been found to be sensitive to different environments within the zeolite,122-124 and hence can serve as useful probes in understanding the role of these template-like molecules in the synthesis mechanism and, as such, provide excellent complementary information to that obtained from Si and A1 

13 C relaxation studies on AW-dimethylaniline adsorbed on Si02 gel and octadecylsilanized Si02 gel provided information on mechanisms of molecular motion of these adsorbed species.130 

The combination of 29Si and 13C n.m.r. can be used as a means to identify chemically bound species on Si02,87 89,133 and an obvious area of application lies in the chemistry of Si02-supported catalysts. 

A Rh/Ce02 catalyst prepared from a chloride precursor was also studied [90]. After H2 adsorption a H NMR line appeared, upfield shifted from the main signal due to ceria-held species. It was ascribed to H atoms adsorbed on Rh particles, the shift arising from Fermi contact with the metal conduction electrons thus specific 

The experimental detection and quantitation of surface species on soil particles and other natural colloids is a difficult area of research because of sample heterogeneity, low surface concentrations and the need to investigate solid adsorbents in the presence of liquid water. Unambiguous information about the molecular structure and stability of adsorbed species can be obtained only with in situ surface spectroscopy (see also Chapter 3). Invasive spectroscopic methods that require sample desiccation or high-vacuum techniques (e.g. electron microscopy, X-ray 

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

To return to the theme of rearrangement, the effect of the presence of foreign atoms on the arrangement of a surface has been successfully demonstrated by Haas et alf They used various methods to produce different amounts of O and C on an Ir(lOO) surface  

Adsorbed Species.—Now that some features of the substrate have been examined we can turn our attention to adsorbed species. As Saijo et alf say in introducing 

In interpretation of LEED patterns there seems to be conflict between protagonists of 

There are still some obscurities and puzzles in the results from LEED. By way of striking a cautionary note attention is drawn to the very careful and critical study of adsorption of O on Fe(lOO) by Brundle. He found (2 x 1) and (1 X 1) LEED patterns when this surface was exposed to O2. One might have expected that these corresponded to 0 0.5 and 0- 1, but, as Brundle em- 

RU SECOND LAYER ATOM RU THIRD-LAYER ATOM 

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Langmuir adsorption isotherm A theoretical equation, derived from the kinetic theory of gases, which relates the amount of gas adsorbed at a plane solid surface to the pressure of gas in equilibrium with the surface. In the derivation it is assumed that the adsorption is restricted to a monolayer at the surface, which is considered to be energetically uniform. It is also assumed that there is no interaction between the adsorbed species. The equation shows that at a gas pressure, p, the fraction, 0, of the surface covered by the adsorbate is given by ... 

Although still used the Langmuir equation is only of limited value since in practice surfaces are energetic inhomogeneous and interactions between adsorbed species often occur. 

Derive the equation of state, that is, the relationship between t and a, of the adsorbed film for the case of a surface active electrolyte. Assume that the activity coefficient for the electrolyte is unity, that the solution is dilute enough so that surface tension is a linear function of the concentration of the electrolyte, and that the electrolyte itself (and not some hydrolyzed form) is the surface-adsorbed species. Do this for the case of a strong 1 1 electrolyte and a strong 1 3 electrolyte. 

It is thus seen that the dipole-induced dipole propagation gives an exponential rather than an inverse x cube dependence of U x) with x. As with the dispersion potential, the interaction depends on the polarizability, but unlike the dispersion case, it is only the polarizability of the adsorbed species that is involved. The application of Eq. VI-43 to physical adsoiption is considered in Section XVII-7D. For the moment, the treatment illustrates how a long-range interaction can arise as a propagation of short-range interactions. 

IRE Infrared emission [110] Infrared emission from a metal surface is affected in angular distribution by adsorbed species Orientation of adsorbed molecules... 

HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

ESD Electron-stimulated (impact) desorption [148, 149] An electron beam (100-200) eV) ejects ions from a surface Surface sites and adsorbed species... 

ESDIAD Electron-stimulated desorption ion angular distribution [150-152] A LEED-like pattern of ejected ions is observed Orientation of adsorbed species... 

LA Light absorption [192, 193] UV-visible adsorption by Nature of adsorbed species... 

NMR Nuclear magnetic resonance [223, 224] Chemical shift of splitting of nuclear spin states in a magnetic field H [225], C [226, 227], N [228], F [229], 2 Xe [230] Other Techniques Chemical state diffusion of adsorbed species... 

M. L. Hair, Vibrational Spectroscopies for Adsorbed Species, ACS Symposium Series No. 137, A. T. Bell and M. L. Hair, eds., American Chemical Society, Washington, DC, 1980. 

Various functional forms for / have been proposed either as a result of empirical observation or in terms of specific models. A particularly important example of the latter is that known as the Langmuir adsorption equation [2]. By analogy with the derivation for gas adsorption (see Section XVII-3), the Langmuir model assumes the surface to consist of adsorption sites, each having an area a. All adsorbed species interact only with a site and not with each other, and adsorption is thus limited to a monolayer. Related lattice models reduce to the Langmuir model under these assumptions [3,4]. In the case of adsorption from solution, however, it seems more plausible to consider an alternative phrasing of the model. Adsorption is still limited to a monolayer, but this layer is now regarded as an ideal two-dimensional solution of equal-size solute and solvent molecules of area a. Thus lateral interactions, absent in the site picture, cancel out in the ideal solution however, in the first version is a properly of the solid lattice, while in the second it is a properly of the adsorbed species. Both models attribute differences in adsorption behavior entirely to differences in adsorbate-solid interactions. Both present adsorption as a competition between solute and solvent. 

These authors doubt that such interactions can be estimated other than empirically without fairly accurate knowledge of the structure in the interfacial region. Sophisticated scattering, surface force, and force microscopy measurements are contributing to this knowledge however, a complete understanding is still a long way off. Even submonolayer amounts of adsorbed species can affect adhesion as found in metals and oxides [74]. 

These concluding chapters deal with various aspects of a very important type of situation, namely, that in which some adsorbate species is distributed between a solid phase and a gaseous one. From the phenomenological point of view, one observes, on mechanically separating the solid and gas phases, that there is a certain distribution of the adsorbate between them. This may be expressed, for example, as ria, the moles adsorbed per gram of solid versus the pressure P. The distribution, in general, is temperature dependent, so the complete empirical description would be in terms of an adsorption function ria = f(P, T). 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

The nature of reaction products and also the orientation of adsorbed species can be studied by atomic beam methods such as electron-stimulated desorption (ESD) [49,30], photon-stimulated desoiption (PDS) [51], and ESD ion angular distribution ESDIAD [51-54]. (Note Fig. VIII-13). There are molecular beam scattering experiments such... 

The work function across a phase boundary, discussed in Sections V-9B and VIII-2C, is strongly affected by the presence of adsorbed species. Conversely,... 

Such attractive forces are relatively weak in comparison to chemisorption energies, and it appears that in chemisorption, repulsion effects may be more important. These can be of two kinds. First, there may be a short-range repulsion affecting nearest-neighbor molecules only, as if the spacing between sites is uncomfortably small for the adsorbate species. A repulsion between the electron clouds of adjacent adsorbed molecules would then give rise to a short-range repulsion, usually represented by an exponential term of the type employed... 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Ref. 205). The two mechanisms may sometimes be distinguished on the basis of the expected rate law (see Section XVni-8) one or the other may be ruled out if unreasonable adsorption entropies are implied (see Ref. 206). Molecular beam studies, which can determine the residence time of an adsorbed species, have permitted an experimental decision as to which type of mechanism applies (Langmuir-Hinshelwood in the case of CO + O2 on Pt(lll)—note Problem XVIII-26) [207,208]. 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

If we consider the optical response of a molecular monolayer of increasing surface density, the fomi of equation B 1.5.43 is justified in the limit of relatively low density where local-field interactions between the adsorbed species may be neglected. It is difficult to produce any rule for the range of validity of this approximation, as it depends strongly on the system under study, as well as on the desired level of accuracy for the measurement. The relevant corrections, which may be viewed as analogous to the Clausius-Mossotti corrections in linear optics, have been the... 

The second issue concerns molecular specificity. For a simple measurement of SHG at an arbitrary laser frequency, one caimot expect to extract infomiation of the behaviour of a system with several possible adsorbed species. To make the technique appropriate for such cases, one needs to rely on spectroscopic infomiation. In the simplest iiiiplementation, one chooses a frequency for which the nonlinear response of tlie species of interest is large or dominant. As will... 

Little L H 1966 Infrared Spectra of Adsorbed Species (New York Academic)... 

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