Chemisorption weak bond

We will now write reactions using a gas G, which is adsorbed on MG [SOU 70fj by means of a chemisorption weak bond (see section 6.4.3.1). The purpose of these reactions is again either to incorporate an element of the gas in the sohd MG or to... 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

This is easy to understand In the former case the backspillover species (O2 ) is also a reactant in the catalytic reaction. Thus as its coverage on the catalyst surface increases during a galvanostatic transient its rate of consumption with C2H4 also increases and at steady state its rate of consumption equals its rate of creation, I/2F. This means that the backspillover O2 species reacts with the fuel (e.g. C2H4) at a rate which is A times slower than the rate of reaction of more weakly bonded chemisorbed oxygen formed via gaseous chemisorption. 

Increasing catalyst potential and work function leads to a pronounced increase in total oxygen coverage (which approaches unity even at elevated temperatures) and causes the appearance of new chemisorption states. At least two such states are created on Pt/YSZ (Fig. 4.43) A strongly bonded one which, as discussed in Chapter 5, acts as a sacrificial promoter during catalytic oxidations, and a weakly bonded one which is highly reactive and causes the observed dramatic increase in catalytic rate. 

The surface area and the dimensions and volume of the pores can be determined in many ways. A convenient method is based on measurement of the capacity for adsorption. The experimental techniques do not differ from those used for chemisorption (see Section 3.6.3). The fundamental difference between physi.sorption and chemisorption is that in chemisorption chemical bonds are formed, and, as a consequence, the number of specific sites is measured, whereas in physisorption the bonds are weak so that non-chemical properties, in particular the surface area, are determined. 

Figure 4 depicts the different forms of chemisorption for a Na atom by means of the symbolic valence signs. In weak bonding the valence electron of the Na atom remains unpaired (see Fig. 2a), and in this sense the free valence of the Na atom may be considered unsaturated. This form of bond thus represents the radical form of chemisorption, which is symbolically depicted in Fig. 4a. Upon transition to strong n- or p-bonding a free electron or, respectively, a free hole of the lattice becomes involved in the bond the electron becomes localized and coupled to the valence electron of the Na atom (see Fig. 2b) or, respectively, the free hole recombines with the valence electron of the Na atom (see Fig. 2c).

The removal of an electron from an acceptor level or a hole from a donor level denotes, as we have seen, not the desorption of the chemisorbed particle but merely its transition from a state of strong to a state of weak bonding with the surface. The neglect of this weak form of chemisorption (i.e., electrically neutral form) which is characteristic of all papers on the boundary-layer theory of adsorption makes it quite impossible to depict the chemisorbed particle in terms of an energy level, i.e., to apply the energy band scheme depicted in Fig. 10 and used in these papers. ... 

Chuvylkin et al. (54) have used this approach to discuss EPR signals arising from weak R02 surface complexes in a number of systems where the g tensor does not fit the pattern expected [Eq. (6) and Fig. 3] from the ionic model. This is not discussed quantitatively, but they conclude that the appearance of covalently bonded oxygen is impossible without a favorable orientation of appropriate electronic orbitals. A similar covalent bonding approach has been considered theoretically for the chemisorption of oxygen on silicon surfaces (55). Examples of weakly bonded oxygen are given in Section IV,E. 

Chemisorption is vital in catalysis. Transition metals such as Fe, Pd, Pt, Ir, Ni, Co, Cr, Mn, Ti, Hf, Zr, V, Nb, Mo, W, Ru, and Os have the ability to chemisorb simple molecules such as 02, CO, H2, C02, C2H4, and N2 [24,25,27], However, if chemisorption is very strong, the catalytic sites are blocked. Therefore, it is necessary that an intermediate between weak chemisorption, when there is no reaction, and strong chemisorption, when the sites are blocked [24,25], is available. In this sense, the first d-block metals form especially stable surface bonds, while the noble metals form weak bonds. These properties are unfavorable to catalysis. Hence, the best metallic catalysts are in between these two groups [27],... 

Alkali-metals are frequently used in heterogeneous catalysis to modify adsorption of diatomic molecules over transition metals through the alteration of relative surface coverages and dissociation probabilities of these molecules.21 Alkali-metals are electropositive promoters for red-ox reactions they are electron donors due to the presence of a weakly bonded s electron, and thus they enhance the chemisorption of electron acceptor adsorbates and weaken chemisorption of electron donor adsorbates.22 The effect of alkali-metal promotion over transition metal surfaces was observed as the facilitation of dissociation of diatomic molecules, originating from alkali mediated electron enrichment of the metal phase and increased basic strength of the surface.23 The increased electron density on the transition metal results in enhanced back-donation of electrons from Pd-3d orbitals to the antibonding jr-molecular orbitals of adsorbed CO, and this effect has been observed as a downward shift in the IR spectra of CO adsorbed on Na-promoted Pd catalysts.24 Alkali-metal-promotion has previously been applied to a number of supported transition metal systems, and it was observed to facilitate the weakening of C-0 and N-0 bonds, upon the chemisorption of these diatomic molecules over alkali-metal promoted surfaces.25,26... 

The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to NEMCA conditions is also manifest clearly by temperature-programmed-desorption (TPD) [26] as shown in Fig. 10. The strongly bonded backspillover oxygen species (peak desorption temperature Tp=750-780 K) displaces the normal chemisorption state of atomic oxygen obtained via gas phase adsorption (Tp=740 K) to a significantly more weakly bonded state (Tp=680 K). The pronounced rate enhancement in NEMCA studies of catalytic... 

In general terms, physical adsorption, or physisorption, refers to weak bonding of molecules to surfaces through the interactions of induced or permanent dipoles and/or quadrupoles, whereas chemisorption describes adsorption where transfer of chemical charge between adsorbate and surface takes place. Physisorption is characteristically observed at low temperatures, is not an activated process and is completely reversible. Chemisorption, by contrast, involves the formation of bonds, persists to elevated temperatures and can lead to chemical changes. For the adsorption of molecules on microporous solids, important physisorption interactions include the uptake of simple non-polar molecules such as dinitrogen and dioxygen on cationic forms of zeolites whereas the adsorption of molecules onto acid sites is the most important type of chemisorption, because of its importance in catalysis. 

The three types of strong bonds between a solid and adsorbed molecules, radicals, or atoms are all relevant for surface chemistry. The two types of weak bonds between a solid surface and adsorbed molecules can also be observed. A reaction between a surface and a molecule that involves strong bonds is called chemisorption. Reactions under low driving forces are called physisorption. Chemisorption refers to strong bonding as between atoms and physisorption to weak bonding as between molecules. 

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