Dissociative chemisorption, metal

H] Asscher M, Haase G and Kosloff R 1990 Tunneling mechanism for the dissociative chemisorption of Nj on metal surfaces Vacuum 41 269... 

The molecular chemisorption of CO on various alkali-modified metal surfaces has been studied extensively in the literature. It is well established that alkali modification of the metal surface enhances both the strength of molecular chemisorption and the tendency towards dissociative chemisorption. This effect can be attributed to the strongly electropositive character of the alkali, which results in donation of electron density from the alkali to the metal and then to the adsorbed CO, via increased backdonation into the... 

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

The classical and traditional view is that for dissociative chemisorption of diatomic molecules to occur at metal surfaces, it is essential that two adjacent (vacant) sites are available ... 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

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. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

In our third example (52), dissociative chemisorption of Li2, B2, C2, 02, N2, F2, CO, NO and ethylene on (100)W and Ni surfaces was examined. The metal surfaces are represented by means of nine-atom clusters, arranged as in Fig. 35. Experimental geometry was used for the adsorbates. The standard EHT method was used, i.e. with charge-independent atomic ionization potentials. Charge transfer between adsorbate and surface was explored... 

From thermodynamic considerations it is evident that bulk nickel cannot be oxidized by CO2. However, it is not justified to conclude from this that dissociative chemisorption of CO2 will not occur. Consider, for example, the chemisorption of oxygen or hydrogen which on several metals takes place under conditions where bulk oxides or hydrides are not at all thermodynamically stable. Dissociative adsorption of CO2 has indeed been observed by Eischens and Pliskin (35). 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

Schematic representation of the cross section of the surface layer of a metal oxide. , Metal ions O, oxide ions. The metal ions in the surface layer (a) have a reduced coordination number. They thus behave as Lewis acids. In the presence of water the surface metal ions may first tend to coordinate H20 molecules (b). For most of the oxides dissociative chemisorption of water molecules (c) seems energetically favored.

The heat of chemisorption is, of course, the energy difference between the chemical bonds formed and those broken. One of the strongest bonds to be broken in dissociative chemisorption on metals is the N-N bond of N2. This chemisorption is known to be rate limiting in ammonia synthesis. Brill et al. reported in 1967 field emission results indicating that N2 adsorption on Fe is strongest on the (111) face." Then-suspicion that this might be the initial step in ammonia synthesis over Fe catalysts... 

FIGURE 9.2. Surface topography of (111) face of bcc metal with probable sites for dissociative chemisorption of N2 on bcc Fe. 

A feature common to both ir complex mechanisms is the nature of the second reagent in the exchange reaction [Eqs. (11), (12a), (12b)], namely heavy water or deuterium gas. Water is generally preferred in exchange reactions as it does not produce hydrogenated by-products. The important aspect concerning water and deuterium gas is the rapid exchange between these compounds on transition metal catalysts, which has been explained by dissociative chemisorption. 

An asterisk ( ) here represents an adsorption site. There are several observations which provide strong support for the association mechanism.266 First, Hf is observed from some metals such as Au, which is known not to show dissociative chemisorption of H2 at low temperatures. Second, for some metals such as Ir, even though chemisorption of H2 is dissociative, few Hj ions are observed. Third, when a H2-D2 mixed gas is used, atomic exchanges always occur regardless of whether hydro-... 

As a first example of the use of the d band model, consider the trends in dissociative chemisorption energies for atomic oxygen on a series of 4d transition metals (Figure 4.6). Both experiment and DFT calculations show that the bonding becomes... 

Dissociative chemisorption energies calculated by density functional theory for various molecules on a number of stepped transition metal surfaces. All values are given in eV per molecule. Positive and negative values signify endothermal and exothermal chemisorption reactions, respectively. 

Another example of the interaction of water with a relatively simple metal oxide surface is provided by the water vapor/a-Al203(0001) system (Figure 7.9(a)). Oxygen Is synchrotron radiation photoemission results indicate that significant dissociative chemisorption of water molecules does not occur below 1 torr p(H20) [149]. However, following exposure of the alumina (0001) surface to water vapor above this threshold p(H20) , a low kinetic energy feature in the Is spectrum grows quickly,... 

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