Atomically chemisorbed adsorbates

Figure A.9 A potential energy diagram of an atom chemisorbed on the model metal jellium shows the broadening of the adsorbate orbitals in the resonant level model.

Figure 2.14. Top Comparison of the XES (occupied states) and XAS (unoccupied states) spectra of atomic N adsorbed on Ni(100) and Cu(100) with separated p components. The intensity scaling between the XES and XAS spectra is arbitrary. Bottom Calculated density of states projected onto the px, and pz valence states of N chemisorbed onto a Cu(100) and Ni(100) surface in the c(2 x 2) structure. Reproduced from [32].

A potential curve of an endothermically chemisorbed atom or molecule represents an excited state with respect to the normal state of the physically adsorbed atom or molecule. When cesium atoms are adsorbed on salt layers or on cesium oxide, they are adsorbed as atoms and not, as they would be on metal surfaces, as ions. Ionization can be brought about by absorption of light 172) or by thermal excitation (173). 

In Sec. V,9 we spoke about the heat of adsorption of a cesium atom chemisorbed in the form of an ion on a tungsten surface. The value (Qa)i = 68.8 kcal./mole holds for the adsorption on a bare tungsten surface. It has been known for many years that the heat of adsorption decreases with an increase in the amount of cesium atoms adsorbed (316). In Fig. 29 the full-line curve shows the decrease according to the figures of Taylor and Langmuir (817). These authors could represent Qe (the heat of adsorption as a function of 8) by an empirical equation ... 

The chemisorption of chalcogen atoms on Ni surfaces has been investigated by many techniques (see Table VI). As a result of LEED and other studies the geometrical structure of chalcogens chemisorbed on nickel surfaces is well established. Chalcogen atoms generally adsorb in the site with maximum coordination on close packed nickel surfaces 4-fold coordinated on the (100) and... 

Titration is a technique that can be used to measure the number of surface metal atoms. The procedure involves first chemisorbing (chemical bonds formed between adsorbing species and surface atoms) molecules onto the metal atoms exposed to the reaction environment. Second, the chemisorbed species are reacted with a second component in order to recover and count the number of atoms chemisorbed. By knowing the stoichiometry of these two steps, the number of surface atoms can be calculated from the amount of the recovered chemisorbed atoms. The technique is illustrated for the problem at hand ... 

The influence of substrate structure is briefly as follows. The active entity (H in hydrogenation and dehydrogenation reactions) must be adsorbed, even in the surface layer, in sites which make crystallographic sense, as these proper sites will be those of lowest potential energy on the surface. For example, atoms chemisorbed on metals will be located in interstitial surface sites which fall on the lattice of interstitial sites for the crystal as a whole. Adsorption immediately above surface atoms is improbable, adsorption in surface interstices probable. Diffusion into the solid from the surface is then by a path similar to the subsequent stages in the bulk solid. The oxygen atoms adsorbed on oxides will occupy proper anion lattice sites and will either create or destroy defects. The defects created will be able to diffuse immediately beneath the surface and appear at the site where their properties are required. Molecular reactants of any size cannot usually penetrate the catalyst surface, but atoms or radicals may be separated from them. The molecule itself is chemisorbed with efficiency only if certain structural inter-relations are satisfied. Beeck and coworkers (62) have demonstrated this adequately for the adsorption of ethylene on various metals and on various crystal planes of the same metal. The adsorption of gases... 

Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

Chemisorption Chemical adsorption occurs when a chemical bond or a partial chemical bond is formed between the surface (adsorbent) and the adsorbate, leading to the specificity of the process. In gen- eral, but not always, the formation of a bond limits the coverage to at most one chemisorbed adsorbate for every surface atom. This limit of a single layer or monolayer is exploited later to derive a statement of conservation of sites. The upper limit need not have a one-to-one adsorbate to surface atom stoichiometry for example, saturation can occur after one-third of all sites are occupied. 

Fig. 27. 01 s core level spectra of molecular O2 and atomic O adsorbed on Pt(lll). Different phases exist at different temperatures Physisorbed oxygen at 25 K chemisorbed O2 at 90 K second chemisorbed O2 phase at 138 K atomic oxygen at > 150 K. All spectra excited with mono-... 

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