Benzene chemisorption

Section II will summarize experimental work designed to determine the extent and manner of benzene chemisorption by the use of physical methods and the radioisotope carbon-14. In Section III, evidence obtained by use of deuterium as a tracer is examined. The relatively small amount of information concerning the mode of benzene chemisorption that is afforded by studies of its hydrogenation is presented in Section IV. 

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... 

Benzene chemisorption on platinum-alumina in the range 26°-470°C has been measured in a flow system by Pitkethly and Goble (7). A small dose of benzene was injected into a stream of inert carrier gas and transported to the reactor the effluent was then sampled repeatedly and analyzed by gas-liquid chromatography. Information concerning the adsorption and desorption of benzene was obtained from the shape of the subsequent benzene concentration versus time curves. Evidence was obtained for four types of adsorption of benzene ... 

Suhrmann and co-workers have examined the effects of benzene chemisorption upon a number of physical properties of evaporated metal films... 

Resistance changes upon benzene chemisorption have also been used by Gryaznov et al. (20) in an attempt to characterize adsorption sites at the surfaces of thin (< 10, 10-20, and 20-30 A) platinum films. 

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. 

The stepped nickel 9(lll)x(lll) and stepped-kinked Ni 7(lll)x(310) surfaces displayed a benzene coordination chemistry that was quantitatively and qualitatively identical with that of the Ni(lll) surface except that not all the benzene was displaced by trimethylphosphine indicating that either a small percentage ( 10%) of the benzene on these surfaces either was present in different environments or was dissociatively (9) chemisorbed see later discussion of stereochemistry. Benzene chemisorption behavior on Ni(110) was similar to that on Ni(lll) except that the thermal desorption maximum was lower, vl00°C, and that trimethylphosphine did not quantitatively displace benzene from the Ni(110)-C H surface. In these experiments, no H-D exchange was observed with CgHs + C D mixtures. 

This step differs from the rds proposed for gas phase hydrogenation of benzene, for which hydrogen adsorption is considered slow, inhibited by strongly adsorbed benzene (559). If a similar step were operative in the electroreduction, the Tafel slope would be twice the observed value. However, insensitivity of the electrocatalytic reaction to temperature (56) indicates strong benzene chemisorption, in agreement with catalytic results. 

G.D. Waddill and L.L. Kesmodel. Benzene Chemisorption on Palladium Surfaces. I. High Resolution Electron Energy Loss Vibrational Spectra and Structural Models. Phys. Rev. 5 31 4940 (1985). 

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... 

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