Adsorbates benzene

Calculate the rotational contribution to the entropy of adsorption of benzene on carbon at 35°C, assuming that the adsorbed benzene has one degree of rotational freedom. 

Cyclohexadiene and benzene form identical structures on Pt(l 1 1) at low pressures (Figures 7.23 and 7.24). 1,3-Cyclohexadiene dehydrogenates to form benzene on the surface, while benzene adsorbs molecularly. Figure 7.24b schematically shows the adsorbed benzene structure at low pressure. The STM images of the C6 cyclic hydrocarbons show three different adsorbed structures on Pt(l 1 1). Cyclohexene and cyclohexane partially dehydrogenate to form rc-allyl, 1,4-cyclohexadiene adsorbs in a boat configuration, and both 1,3-cylohexadiene and benzene adsorb as molecular benzene on the surface. 

Few studies have been made of benzene chemisorption by the volumetric method. Zettlemoyer et al. (8) have examined the adsorption of benzene vapor at 0°C on powders of nickel and of copper. First, the monolayer coverage of argon (vm) A, was measured. The argon was then removed by pumping and the amount of benzene required to form a monolayer, (vmi) Bz, was measured. Weakly adsorbed benzene was then removed by pumping, after which further benzene adsorption provided the value (vm2) Bz. Some results are reproduced in Table I. On the assumption that the same extent of surface is accessible both for argon and for benzene adsorption, it is clear that complete monolayers of benzene were not achieved, that some (Ni) or all (Cu) of the benzene was adsorbed reversibly. It was considered that only the irreversibly adsorbed benzene was chemisorbed, the remainder being physically adsorbed. Thus chemisorption of benzene on copper appeared not to occur. The heat of adsorption of benzene on nickel at zero... 

Field electron emission coupled with flash-filament studies have been employed by Condon and Hansen to study benzene chemisorption on tungsten (21). Evidence was obtained for the chemisorption of benzene by a single bond (probably of -character) to the surface. This form of asso-ciatively adsorbed benzene [(I), Scheme 1] appeared to exist in equilibrium with cr-adsorbed-CeHs (II) and adsorbed atomic hydrogen. 

Furthermore, ir-arene complexes of transition metals are seldom formed by the direct reaction of benzene with metal complexes. More usually, the syntheses require the formation of (often unstable) metal aryl complexes and these are then converted to ir-arene complexes. The analogous formation of w-adsorbed benzene at a metal surface via the initial formation of ff-adsorbcd phenyl, merits more consideration than it has yet been given. It is to be hoped that the recognition and study of structure-sensitive reactions will allow more exact definition of the sites responsible for catalytic activity at metal surfaces. The reactions of benzene, using suitably labeled materials, may prove to be useful probes for such studies. 

The SIMS experiments were performed by sputtering both adsorbed benzene and pyridine at various coverages from Ag(lll) surfaces. For... 

Farkas and Farkas suggested that the critical step in hydrogenation involved the simultaneous addition of two hydrogen atoms to an adsorbed benzene molecule, whereas exchange with deuterium required the prior dissociation, on the surface, of benzene to form a phenyl radical and a hydrogen atom. The phenyl radical then combined with a deuterium atom, which had been produced by the dissociation of a deuterium molecule, and the monodeuterobenzene was desorbed. 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

Su, B-L and Norberg, V. (1998) Migration of adsorbed benzene molecules from cations to 12R windows in the large cages of Cs(Na)-EMT zeolite upon coadsorption of NH3. Langmuir, 14, 2353-2360. 

Figure 1.8. Plan view of the structure of the Ni(lll) surface with adsorbed benzene, showing the local adsorption geometry at low ( 0.10) coverage, and the local and long-range ordering geometry in the slightly higher coverage (-Jl x /7)R19° phase. The H atom positions are schematic only and have not been determined experimentally.

Table II shows that at temperatures between 450 and 550 K all the close-packed surfaces investigated, and additionally Cu(100), Pd(100), and Fe(110), give similar spectra. These have latterly been attibuted to a surface species that we designate a(CCH). The spectrum has bands at ca. 3000 cm 1 (m), r CH 1360-1300 cm-1 (m, bd), vCC and 870-730 cm-1 (s), 5CH or yCH. We designate this species a(CCH) to distinguish it from others given the same formula but with notably different spectra that we discuss later. A particularly good example of an a(CCH) spectrum was observed for the species on Pd(lll) (24), whereby it was clearly distinguished from a spectrum from adsorbed benzene, which also has a strong absorption (at 730 cm-1) in the low-wavenumber region. Another example is provided by the species on Ni(lll) at 550 K (14).

At an early stage, Palazov and Shopov et al. (241, 242) established that cyclohexene, like cyclohexane, was retained as 77-adsorbed benzene after evacuation of a Ni/Si02 sample at room temperature. 

The adsorption behavior of benzene on dehydrated NaX and NaY zeolites has been investigated directly by H and 13C NMR measurements of the adsorbed benzene and indirectly by the combination of 129Xe NMR and isotherm measurements of the co-adsorbed xenon. Powdered zeolite samples of various Si/Al ratios and with varied adsorbate concentrations were investigated. Detailed macroscopic and microscopic adsorption phenomena of benzene in NaX and NaY zeolites, including the loading capacity, mobility, and sites of adsorption are presented in terms of measurements of NMR linewidths and chemical shifts. 

---