Adsorbed hydrogen

An interesting point is that infrared absorptions that are symmetry-forbidden and hence that do not appear in the spectrum of the gaseous molecule may appear when that molecule is adsorbed. Thus Sheppard and Yates [74] found that normally forbidden bands could be detected in the case of methane and hydrogen adsorbed on glass this meant that there was a decrease in molecular symmetry. In the case of the methane, it appeared from the band shapes that some reduction in rotational degrees of freedom had occurred. Figure XVII-16 shows the IR spectrum for a physisorbed H2 system, and Refs. 69 and 75 give the IR spectra for adsorbed N2 (on Ni) and O2 (in a zeolite), respectively. 

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

Ethylene, C2H4, can adsorb in two modes the weaker Jt-bonded ethylene, in which the C=C double bond is above a single metal atom, or the stronger di-cr bonded ethylene in which the two C-atoms of the ethylene molecule bind to two metal atoms (Fig. 6.37). We consider the (111) metal surface. Hydrogen adsorbs dis-sociatively and is believed to reside in the threefold hollow sites of the metal. 

The fits indicate that the Langmuir-Hinshelwood model describes the measurements very well. The equilibrium constants point to a relatively strong adsorption of thiophene and, in particular, H2S, while adsorption of hydrogen is weak. Hence the term K may safely be ignored in Eq. (32). The order in H2 is 0.93, i.e. close to one, which is another indication that hydrogen adsorbs only weakly. 

Discuss the range of values that the orders in hydrogen, carbon monoxide and methanol may assume if hydrogen adsorbs more weakly than the other gases. 

Hydrogen adsorbs dissociatively on almost all metals. At 500 K the H atoms diffuse freely over the surface. Desorption occurs associatively by recombination of two H atoms, while desorption of atomic hydrogen can be ignored. 

Figure 6. Amount of hydrogen adsorbed and catalytic activity (at 323K) versus the degree of reduction of M0S2.

Szabo S, Nagy F. 1978. Investigations of bismuth adsorption via the ionization of hydrogen adsorbed on platinized platinum in hydrochloric acid solutions. J Electroanal Chem 87 261-265. 

Hydrogen binds to As sites at the surface when the GaAs electrode is electron rich when the GaAs electrode is electron poor, the hydrogen adsorbates are replaced by OH species at the As sites. Changes in potential were determined by interrupting the cyclic potential scans every 100 ms for a lmn period at various... 

Fig. 8. The adsorption of hydrogen on copper-nickel catalysts as a function of the copper content. The circles represent the total amount of hydrogen adsorbed at room temperature at 10-cm pressure. The triangles represent the amount of strongly adsorbed hydrogen, i.e., the amount not removed by a 10-min evacuation at room temperature following the completion of the initial adsorption isotherm. The amount of strongly adsorbed hydrogen is determined as the difference between the initial isotherm and a subsequent isotherm obtained after a 10-min evacuation (74).

Most measurements show that under ambient conditions, the amount of hydrogen adsorbed in SWNTs is small (<1 wt%), whereas under cryogenic conditions, hydrogen... 

A snapshot of hydrogen adsorbed in the idealized hexagonal bundle of the (10,10) CNTs at 3.8 MPa and 77 K. (Reproduced from Kowalczyk, P., Hoyst, R., Terronesb, M., and Terrones, H., Phys. Chem. Chem. Phys., 9, 1786-1792, 2007. With permission.)... 

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