Chemisorption bond

Different types of chemisorption sites may be observed, each with a characteristic A value. Several adsorbed states appear to exist for CO chemisorbed on tungsten, as noted. These states of chemisorption probably have to do with different types of chemisorption bonding, maybe involving different types of surface sites. Much of the evidence has come initially from desorption studies, discussed immediately following. 

Note that the van der Waals forces tliat hold a physisorbed molecule to a surface exist for all atoms and molecules interacting with a surface. The physisorption energy is usually insignificant if the particle is attached to the surface by a much stronger chemisorption bond, as discussed below. Often, however, just before a molecule fonus a strong chemical bond to a surface, it exists in a physisorbed precursor state for a short period of time, as discussed below in section AL7.3.3. 

A very extreme version of surface corrugation has been found in the nonactivated dissociation reactions of Fl2 on W [, ], Pd and Rli systems. In these cases, the very strong chemisorption bond of the FI atoms gives rise to a very large energy release when the molecule dissociates. In consequence, at certain sites on the surface, the molecule accelerates rapidly downliill into the dissociation state. At the unfavourable sites, there... 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

Can one further enhance the performance of this classically promoted Rh catalyst by using electrochemical promotion The promoted Rh catalyst, is, after all, already deposited on YSZ and one can directly examine what additional effect may have the application of an external voltage UWR ( 1 V) and the concomitant supply (+1 V) or removal (-1 V) of O2 to or from the promoted Rh surface. The result is shown in Fig. 2.3 with the curves labeled electrochemical promotion of a promoted catalyst . It is clear that positive potentials, i.e. supply of O2 to the catalyst surface, further enhances its performance. The light-off temperature is further decreased and the selectivity is further enhanced. Why This we will see in subsequent chapters when we examine the effect of catalyst potential UWR on the chemisorptive bond strength of various adsorbates, such as NO, N, CO and O. But the fact is that positive potentials (+1V) can further significantly enhance the performance of an already promoted catalyst. So one can electrochemically promote an already classically promoted catalyst. 

The chemisorptive bond A-M is a chemical bond, thus chemisorption is reactant- and catalyst-specific. The enthalpy, AH, of chemisorption is typically of the order of -1 to -5 eV/atom (-23 to -115 kcal/mol, leV/molecule=23.06 kcal/mol). 

It therefore becomes important first to examine the chemisorption of promoters on clean catalyst surfaces and then to examine how the presence of promoters affects the chemisorptive bond of catalytic reactants. 

Adsorbates acting as promoters usually interact strongly with the catalyst surface. The chemisorptive bond of promoters is usually rather strong and this affects both the chemical (electronic) state of the surface and quite often... 

Increasing catalyst surface work function causes an increase in the heat of adsorption (thus chemisorptive bond strength) of electropositive (electron donor) adsorbates and a decrease in the heat of adsorption (thus chemisorptive bond strength) of electronegative (electron acceptor) adsorbates. 

The key of the promotional action is the effect of electropositive and electronegative promoters on the chemisorptive bond of the reactants, intermediates and, sometimes, products of catalytic reactions. Despite the polymorphic and frequently complex nature of this effect, there are two simple rules always obeyed which can guide us in the phenomenological survey which follows in this chapter. 

Similar is the effect of S coadsorption on the CO TPD spectra on Pt(lll) as shown in Figure 2.29. Sulfur coadsorption weakens significantly the chemisorptive bond of CO. 

It is also clear that in the present case oxygen is the electron acceptor (A) while CO is the electron donor (D). It has been already discussed that CO is an amphoteric adsorbent, i.e., its chemisorptive bond involves both electron donation and backdonation and that, in most cases, its electron acceptor character dominates. However, in presence of the coadsorbed strong electron acceptor O (see section 2.5.2.1) it always behaves as an electron donor. 

Rule 1 Electropositive adsorbates strengthen the chemisorptive bond of electron acceptor (electronegative) adsorbates and weaken the chemisorptive bond of electron donor (electropositive) adsorbates. 

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

This is an important result and shows that the dramatic decrease in catalytic activation energy, EA, upon increasing is due to the decrease in Ed and concomitant weakening of the Pt=0 chemisorptive bond upon increasing UWr and O. 

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function ethylene epoxidation and deep oxidation on Ag.5... 

The chemisorptive bond is a chemical bond. The nature of this bond can be covalent or can have a strong ionic character. The formation of the chemisorptive bond in general involves either donation of electrons from the adsorbate to the metal (donation) or donation of electrons from the metal to the adsorbate (backdonation).2 In the former case the adsorbate is termed electron donor, in the latter case it is termed electron acceptor.3 In many cases both donation and backdonation of electrons is involved in chemisorptive bond formation and the adsorbate behaves both as an electron acceptor and as an electron donor. A typical example is the chemisorption of CO on transition metals where, according to the model first described by Blyholder,4 the chemisorptive bond formation involves both donation of electrons from the 7t orbitals of CO to the metal and backdonation of electrons from the metal to the antibonding n orbitals of CO. 

FI. Increasing work function 0 (e.g. via addition of electronegative promoters) strengthens the chemisorptive bond of electron donor adsorbates (D) and weakens the chemisorptive bond of electron acceptor adsorbates (A). 

The double layer is described by its effective thickness, d, and by its field strength E (Fig. 6.15). The adsorbed moleculeJias a dipole moment P. It is well documented100 that the local field strength E can affect strongly not only the chemisorptive bond strength but also the preferred orientation of the adsorbate (Fig. 6.16). 

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