Benzene, chemisorbed structures

The chemisorption of hydrocarbons, ethylene, cyclohexene, n-heptane, benzene and naphthalene at room temperature and above were studied on both the Au(l 11) and Au[6(l 11) x (100)] stepped surfaces (29). The difference in the adsorption characteristics of hydrocarbons on gold surfaces and on platinum surfaces is striking. The various light hydrocarbons studied (ethylene, cyclohexene, n-heptane, and benzene) chemisorb readily on the Pt(lll) surface. These molecules, on the other hand, do not adsorb on the Au(lll) surface under identical experimental conditions as far as can be judged by changes that occur in the Auger spectra. Naphthalene, which forms an ordered surface structure on the Pt(lll) face, forms a disordered layer on adsorption on the Au(l 11)surface. 

H. Jobic, J. Tomkinson, J.P. Candy, P. Fouilloux A. Renouprez (1980). Surf Sci., 95, 496. The structure of benzene chemisorbed on Raney nickel a neutron inelastic spectroscopy determination. 

In Figure 2.30 the bond-breaking sequences for ethylene and benzene chemisorbed on the Rh(l 11) surface are compared. At low temperatures the structures of the chemisorbed molecules are different. As the temperature is increased, benzene appears to break into three short-lived acetylene molecules, which become C2H spe-... 

Figure 4.7. Experimentally determined structures of benzene chemisorbed on various single crystal surfaces. (A) On Pt(lll) (disordered) " (B) on Pt(lll) (ordered with co-adsorbed CO) (C) on Rh(lll) (ordered with co-adsorbed CO) (D) on Ru(001) (E) on Ni(lll) (ordered with coadsorbed NO the same structure is found without NO) " (F) on Ni(l 10). In (C), (D) and (E) the molecules are adsorbed over a cph site.

Chemisorption of benzene at 297°C on Ni(110) occurred in a rather different manner. Several patterns, some streaked, were observed, and they followed the same sequence and showed the same behavior as those obtained when acetylene was chemisorbed on this surface (29). These structures have not been fully elucidated, but the streaked patterns suggest (i) that the mobility of adsorbed species along the "furrows of the (110) face is easier than their mobility across them, and (ii) that dissociation of the carbon skeleton of benzene and the formation of other structures occurs. 

Itaya s group presented images of benzene, naphthalene and anthracene on Cudll), and naphthalene and anthracene on Rh(l 11)/ Wandlowski and coworkers monitored adsorption of uracil on gold surfaces They reported imaging chemisorbed molecules as well as physisorbed molecules and determined their adlattice structures. They also made a correlation between the structure and lateral interaction forces of adsorbed molecules. They showed that application of sufficiently positive electrode potentials results in uracil deprotonation, leading to different surface structure and geometric orientation. 

Figurt 2.29. The urt uee iruciurc of hen/ene in a di iunJen J monnlayer un Pt(ll J). The chemisorbed benzene layer remains disordered in the absence of coadsorbed CO. Note the bending of the benzene molecule into a boal-IIke surface structure.

Clearly both A and B provide models for benzene binding at (1,1,1) metal surfaces, which may prove to be compelling analogues of chemisorbed arene states. The only structural precedent for B is the distantly related system in which one benzene of a unit bonds to a triangle of Ti atoms in the B-type man-... 

Although in the earlier sections we have omitted mention of the chemisorption of benzene and other aromatic molecules so as not to muddy the waters unnecessarily, there have been a number of structure determinations of chemisorbed benzene that are conveniently described at this point. On surfaces of trigonal symmetry (i.e. fee (111) and eph (001)) the molecule sometimes forms disordered overlayers, which can be forced into long-range order by co-adsorbing carbon... 

The structures and registries of chemisorbed benzene on Rh(lll) have been thoroughly scrutinized by surface-sensitive techniques such as LEED, EELS, and angle-resolved UV photoemission spectroscopy (ARUPS) in UHV [43]. These previous studies revealed various structures for benzene, including well-known structures such as c 2 i x 4)rect and (3 x 3), depending on whether CO was present unintentionally or intentionally in the UHV chambers. Although it has been repeatedly demonstrated that the adlayer structures of benzene on Rh and Pt were greatly affected by the presence of CO in the adlayer, the structure of the pure benzene adlayer has not yet been fully understood. Neuber et al. reported that a completely new structure with a /19 x yi9)i 23.4° symmetry appeared for the pure benzene adsorption on Rh(lll) under cleaner UHV conditions in the absence of CO, and the previously known structures of 0(2 3 x 4)rect and (3 x 3) were found to appear upon admission of CO [48]. 

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