ESR of adsorbed hydrogen

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

It is evident from the data presented that ultraviolet light has different and more specific effects on forming radicals in polyolefins than those previously found for ionizing radiation. Furthermore, hydrogen atoms have been observed in the ESR spectra from ultraviolet experiments (6, 24). The hydrogen radicals, however, are not believed to be located in the polymer matrix but to be formed in the glass or adsorbed on the glass surface of the sample cell (6). 

We were interested in the change in the oxidation state of Pd (II), incorporated in the zeolite, during heat treatment in oxygen or in vacuo. Hydrogen and carbon monoxide interactions were also studied. The experiments involved two techniques ESR, which provides direct identification of palladium in an ionic state, and IR spectroscopy, which gives information on the superficial structure of the exchanged zeolite and on the adsorbed species. 

Weakly adsorbed CO is characterized by a band around 2100 cm-1 and by bands below 2000 cm-1 (1935 and 1895 cm-1). All these bands are formed upon CO adsorption following hydrogen reduction of the Pd(II) ions. No intermediate species can be detected during this reduction, and the Pd+ ions observed by ESR are present in amounts too small to give strong bands. The reversible chemisorption must take place on the Pd°. 

Defects which contain or are derived from a surface impurity are an ever-present possibility in any but the most painstaking experiments. With carefully cleaned metals in ultrahigh vacuum, any defects may be presumed to arise from the metal itself, but with most samples, in the usual experimental systems the surface is surely eontaminated with residual water, atmospheric gases, or worse. Adsorbed free radicals produced from such contaminants can sometimes be seen by ESR, and groups containing hydrogen are often identifiable by infrared spectroscopy. With or without such markers, the participation of surface... 

Ammonia, n-butylamine, and pyridine are used extensively as gaseous bases for the determination of strength and amount of a solid acid. However, ammonia and a-butylamine in which hydrogen atoms are attached to the nitrogen atom have a tendency to dissociate (e.g., NH3 = NH2 +H ) and adsorb on both acidic and basic sites depending on the kinds of solids and the adsorption condition. In this regard, triethyl-amine, which is much more difficult to dissociate, is recommended for use as an adsorbate. Care must also be taken in the use of pyridine, because pyridine has recendy been found to adsorb on strong basic sites to form an anion radical of pyridine. It appears necessary to check the adsorbed states of basic molecules by IR or ESR spectroscopy. 

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