Structure of Chemisorbed

Structure, adsorbate-adsorbate interactions are reflected in the low-energy electron diflfraction structures observed at coverages between Fe H = 1 0.25 and 1 1 [378]. 

High-resolution electron-energy-loss spectroscopy spectra of c(2x2) H/Fe(l 1 0) [191] are interpreted from selection rules, isotope effect, and wave number as H in the short bridge position [191]. 

By secondary ion mass spectroscopy from H2 adsorbed on Fe the signal for H2 is stronger than for H [379]. This is taken as evidence that H2 does not dissociate [379]. However, this argument cannot be correct in view of the large difference in stability between H-atoms and H2-molecules in the gas phase. 

For the chemisorption of hydrogen on the catalyst, Emmett et al found a complex behavior [163, 380, 381]. Adsorption of H2 on the catalyst was detected at — 90°C and above + 100 °C [381]. Transients in the adsorption when the temperature is suddenly changed in the range 0-210 °C has also been observed by others [382]. Presumably this behavior is caused by two reactions where the low temperature reaction is a weakly exothermic equilibrium adsorption and the high temperature reaction is a reaction limited by a high activation energy. 

The initial enthalpy of chemisorption has been determined for single crystal surfaces of Fe. For H/Fe(l 00) — 86 kJ/mole was found by temperature programmed desorption [383] for H/Fe(l 10) — 109 kJ/mole [378] by temperature programmed desorption or — 24.2 kcal/mole [384] by He-scattering for H/Fe(l 11) —88 kJ/mole by temperature programmed desorption [378]. For D/Fe(l 10) — 24.2 kcal/mole has been determined from He-scattering [384] and for D/Fe(l 1 1) — 104 kJ/mole by calorimetry [385]. 

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Fig. VIII-10. (a) Intensity versus energy of scattered electron (inset shows LEED pattern) for a Rh(lll) surface covered with a monolayer of ethylidyne (CCH3), the structure of chemisorbed ethylene, (b) Auger electron spectrum, (c) High-resolution electron energy loss spectrum. [Reprinted with permission from G. A. Somoijai and B. E. Bent, Prog. Colloid Polym. ScL, 70, 38 (1985) (Ref. 6). Copyright 1985, Pergamon Press.]...

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-... 

Bare, A M., Binnig, G., Rohrer, H., Gerber, C., et al. Real-Space Obser/ation of 2 x 1 Structure of Chemisorbed Oxygen on NI(110) by Scanning Tunneling Microscopy/ Physical Review Letters, 52(15), 1304-1307 (April 9, 1984). Beebe, T.P., Jr., etaL Direct Observation of Native DNA Structures with the Scanning Tunneling Microscope, Science. 370 (January 20, 1989). 

V. Other Experimental Methods for Investigating the Structures of Chemisorbed Hydrocarbons on Metals... 

An exact interpretation of the basic reasons for the effect of the carrier on the structure of chemisorbed CO cannot yet be advanced. Qualitatively it appears that the alumina makes it easier for the platinum to provide electrons for bonding between the CO and the surface. This is based on a comparison of several possible electronic structures of the chemisorbed CO. The bond between the surface and the carbon of the linear CO could be either a double or a single bond,... 

The infrared spectra of chemisorbed molecules provide relatively clear and direct evidence concerning the structure of these molecules. Most of the problems to which the infrared techniques have been applied have been stimulated by an interest in heterogeneous catalysis. Since chemisorption is vital to catalysis and since the structure of chemisorbed molecules can be determined by infrared, it is reasonable to ask what has been learned about catalytic activity from these spectra. The number of cases where even a tenuous relationship between the spectra and activity is seen is not large. However, the infrared experiments were not designed specifically to seek such relationships. Despite this, interesting observations concerning catalytic activity have been made and will be described here to illustrate the type of reasoning involved rather than to claim well-defined relationships. 

Figure 3.4 Structures of chemisorbed oxygen on graphite. (Adapted from ref. 78.)...

The structure of chemisorbed CO2 species was determined by IR measurements of preadsorbed CO2. Figure 2 presents the IR spectra obtained on the catalysts after CO2 adsorption and sequential evacuation at 25 and 100 °C. The parent NaY zeolite exhibited infrared bands at 1710, 1690, 1645 and 1370 cm" . Adsorption of CO2 on Cs-exchanged zeolites revealed similar IR bands 1690, 1650 with shoulders at 1600, 1390 and 1330 cm". According to the literature [14,15], bidentate carbonates form on Lewds acid-Bronsted base pairs pair site, where... 

Solid-state NMR is proving to be a powerful technique for the study of reactions at surfaces. For example, NMR has been used in catalysis studies for determining the structure of chemisorbed molecules and for monitoring changes occurring in those stmctures as a function of temperature. 

While the structure of chemisorbed acetylene has not been definitely resolved, several studies determining the infrared spectra of acetylene chemisorbed on several metals have been carried out. 

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