Chemisorption defined

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Flead and Silva used occupation numbers obtained from a periodic FIF density matrix for the substrate to define localized orbitals in the chemisorption region, which then defines a cluster subspace on which to carry out FIF calculations [181]. Contributions from the surroundings also only come from the bare slab, as in the Green s matrix approach. Increases in computational power and improvements in minimization teclmiques have made it easier to obtain the electronic properties of adsorbates by supercell slab teclmiques, leading to the Green s fiinction methods becommg less popular [182]. 

As discussed already in Chapter 2 the work function, , of a solid surface is one of the most important parameters dictating its chemisorptive and catalytic properties. The work function, (eV/atom) of a surface is the minimum energy which an electron must have to escape from the surface when the surface is electrically neutral. More precisely is defined as the energy to bring an electron from the Fermi level, EF, of the solid at a distance of a few pm outside of the surface under consideration so that image charge interactions are negligible. 

Evidence of dissociative chemisorption resulting in the final formation of atomic carbon and its incorporation in the metal lattice at 1000°K to form the carbide was reported. These methods when used in combination are informative, and the surfaces studied are clean and well defined. It is to be hoped that other metals will be so studied in the future. 

These and other data (10) show that hydrogen chemisorption is operationally of two types Type I chemisorption which is removed by evacuation for 15 min at room temperature, and type II chemisorption which is not removed by evacuation at room temperature even after several hours. The type I chemisorption appears to be independent of the amount of type II chemisorption (compare runs 3 and 5). Figure 2 show s an isotherm for type I adsorption, as defined. This is a typical curve for chemisorption and suggests that type I chemisorption occurs on sites corresponding to roughly 5% of the BET Vm value. (The designation type I and type II chemisorption was chosen in preference to fast and slow because not all of the type II chemisorption is slow. For example, the amount of adsorption in curve 1 of Fig. 1 is 0.154 cm3/gm after 2 min. We would estimate at least one-third of this adsorption is type II. Thus, some type II irreversible chemisorption is quite rapid.)... 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

The chemisorption of species occurs at specific sites on the electrode, for example on top of certain atoms, or in the bridge position between two atoms. Therefore, most adsorption studies are performed on well-defined surfaces, which means either on the surface of a liquid electrode or on a particular surface plane of a single crystal. Only fairly recently have electrochemists learned to prepare clean single crystal electrode surfaces, and much of the older work was done on mercury or on amalgams. 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

The work function plays an important role in catalysis. It determines how easily an electron may leave the metal to do something useful for the activation of reacting molecules. However, strictly speaking, the work function is a macroscopic property, whereas chemisorption and catalysis are locally determined phenomena. They need to be described in terms of short-range interactions between adsorbed molecules and one or more atoms at the surface. The point we want to make is that, particularly for heterogeneous surfaces, the concept of a macroscopic work function, which is the average over the entire surface, is not very useful. It is more meaningful to define the work function as a local quantity on a scale with atomic dimensions. 

Since the presence of chemisorption (V and AT) states gives rise to the formation of localized covalent bonds between the adatom and substrate, we are interested in how the occurrence of localized states is governed by the values of the parameters za, zs and r/, which define the adatom-substrate interaction. Localized states exist, if one or both of (1.39) and (1.44) have real roots //,fc, which, since cosh /y,fc > 1 and e k > 1, exist for a given ri in regions of the zazs-plane depicted by the two hyperbolas... 

As defined in Chap. 4, the chemisorption energy is the difference between the initial and final energies of the system. Although we could use the expression (4.85) for AE, it is more convenient to work with that of App. L, with a slight modification to account for double adsorption. Specifically, we have... 

Fig. 3 shows a topographic image of a Pt/y-A s catalyst. Contrast from particles is clearly separated from the substrate topography. On the other hand pores on the substrate are well defined. If the aperture includes some portion of the dark field spot then the resolution for small particles is improved. Fig. 4 shows an image of a 100% dispersed catalyst (as measur ed by chemisorption methods) in which particles of about 5 A can be seen. 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

Chemisorption involving covalent bonds as well as bound residue formation is also excluded, which is defined as any organic carbon remaining after exhaustive extraction that results from degradation of parent molecules. 

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