Ethylidyne, chemisorption

Ethylidyne chemisorption also restructures the metal surface. This is shown for Rh(l 11) in Figure 2.27. The metal-metal distances expand for those rhodium atoms that bind to the carbon of the ethylidyne molecule located in the threefold site. This expansion forces the next nearest-neighbor rhodium atom more into the surface, which becomes corrugated as a result. The rhodium atom in the second layer directly underneath the ethylidyne binding site moves upward, closer to the organic molecule which is now bound more strongly to four instead of only three metal atoms. 

The answer to the first question is undoubtedly a positive one. The classical papers by Beeck et al., Rideal et al., and others have shown that ethylene disproportionates upon chemisorption into ethane and carbonaceous (adsorbed) residues (see 162). This disproportionation takes place at relatively low temperatures at room temperature and lower (see 162 for review). Moreover, the intensity analysis of LEED data has shown that upon chemisorption of ethylene, ethylidyne structures are formed. Similar structures are also formed by dissociative adsorption of higher olefins (181,182). There is thus no doubt with regard the first question. 

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

This is clearly shown in Figure 7. Although the chemisorption of ethylene on Pt(lll) has been studied by numerous techniques (93, 99, 100, 101, 102, 103), there is still debate over the precise geometry of the stable surface species. Proposed structures include ethylidyne ( C-CH3) (100, 101), ethylidene (>CH-CH3) (93, 99), and a vinyl species (>CH-CH2 ) (102, 103). 

In the case of chemisorption of acetylene on Pd(lll), high-resolution electron energy loss spectroscopy (HREELS) data suggest that ethylidyne coexists with vinylidene ( =C=CH2) at this surface (31). In their ultra-... 

These conditions must be satisfied in order to correctly apply the steady-state approximation to a reaction sequence. Consider the H-D exchange with ethylidyne (CCH3 from the chemisorption of ethylene) on a platinum surface. If the reaction proceeds in an excess of deuterium the backward reactions can be ignored. The concentrations of the adsorbed ethylidyne species have been monitored by a technique called secondary ion mass spectroscopy (SIMS). The concentrations of the various species are determined through mass spectroscopy since each of the species on the surface are different by one mass unit. Creighton et al. [Surf. Sci., 138 (1984) L137] monitored the concentration of the reactive intermediates for the first 300 s, and the data are consistent with what are expected from three consecutive reactions. The results are shown in Figure 4.2.2. 

Kesmodel LL, Dubois LH, Somoijai GA (1978) Dynamical DEED study of C H and C H, chemisorption on Pt(lll) evidence for the ethylidyne CH -C group. Chem Phys Lett 56 267... 

The ethylidyne species, CH3C, is formed by the dissociative chemisorption of ethene on metals. Although present on a metal during catalytic hydrogenation of ethene, ethylidyne is not an intermediate it is a spectator [78]. Chemisorbed ethylidyne has been characterised by vibrational spectroscopy and low energy electron diffraction by comparison with the model compound... 

Figure i 0-3 shows the bonding from the adsorption of ethylene on a platinum surface to form chemisorbed ethylidyne. Like physical adsorption, chemisorption is an exothermic process, but the heat.s of adsorption are generally of the same magnitude as the heat of a chemical reaction fi.e., 40 to 400 kj/mol). If a catalytic reaction involves chemisorption, it must be carried out within the temperature range where chemisorption of the reactants is appreciable. 

Figure 2.26. Bonding geometry of ethylidyne on the Rh(lll) and Pt(lll) crystal faces. Ethylidyne forms as a result of ethylene chemisorption.

The chemisorption of organic molecules on transition metals usually reduces their work function. According to the available experimental data, the chemisorption of ethylene in the form of ethylidyne reduces the work function by about —1.2 eV ( — 1.9 X 10 J), corresponding to the formation of a surface dipole of -1-0.9 Debye (3 X 10 C m). Benzene chemisorption reduces the work function by — 1.4 eV... 

Ethylidyne restructures the Rh(l 11) crystal face [30], sulfur restructures the Fe(l 10) face [7], and carbon restructures the Ni(lOO) face [6, 46]. The surface metal atoms move into new equilibrium p>ositions upon chemisorption in different ways, and there is evidence of restructuring even in the second substrate layer under the surface. Review the available data and point out the important electronic and structural parameters that influence the nature and magnitude of chemisorption-induced surface restructuring. 

Accepting the existence of active sites implies also that there are also inactive centres or perhaps overactive centres that are quickly inactivated by the destructive chemisorption that occurs so easily with hydrocarbons. The accidental or deliberate removal of such sites by autogenic toxins (i.e. carbonaceous deposits , ethylidyne etc.), or by other poisonous species such as sulfur compounds, allows the remaining active centres to exhibit reactions of a structure-insensitive type that could not take place while the overactive centres were in existence. This situation may arise not only through surface heterogeneity, but also on a plane surface by operation of the Principle of Maximum Occupancy (Section 4.2) by which the preferred first reaction is that which utilises the largest size of active... 

The molecular chemisorption of ethylene is observed below room temperature, but at higher temperatures the alkene can be cleaved with formation of ethylidyne complexes of the type ... 

Competitive reactions catalyzed by Pt involve hydrogenation-dehydrogenation, hydrogenolysis, isomerization and dehydrocyclization. Stable ethylidyne forms on Pt surfaces by chemisorption... 

Kessmodel LL, Dubois LH, Somorjai GA. LED analysis of acetylene and ethylene chemisorption on the Pt(lll) surface evidence for ethylidyne formation. J Chem Phys. 1979 70 2180. 

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