Intensity with adsorbed intermediates

The existence of several adsorbed states of an olefin on metal surfaces is shown by infrared spectroscopic studies [68]. This technique has the advantage that it yields direct information regarding the chemical identity of the various adsorbed species, although there are limitations to its use. One of the main limitations is that the presence of surface intermediates may not be revealed if the appropriate band intensities are too weak [69]. In this context, it has been suggested [70] that the C—H bands associated with carbon atoms which are multiply bonded to the surface are too weak to be observed. Pearce and Sheppard [71] have also proposed the operation of an optical selection rule, similar to that found with bulk metals [72], in determining the bands observed with adsorbed species on supported metal catalysts. In spite of these limitations, however, the infrared approach has contributed significantly to the understanding of the nature and reactivity of adsorbed hydrocarbons. 

It was natural to turn to 5-(2-formylfuryl) methyl ether (the methyl ether of 5-hydroxymethyl-2-furaldehyde) as a possible cause of these results. This compound has an absorption peak at 284 mp. It was found82 that this compound is difficult to purify, and treatment with adsorbents leaves a bright-yellow solution. A similar color is found with 2,5-diformyl-furan, in solutions of which, color develops spontaneously on exposure to light. The dye may be closely related to the ether-dimer of 5-(hydroxy-methyl)-2-furaldehyde, perhaps with one ring open. It may be significant that development of the intense, yellow color is not observed in solutions of 5-(hydroxymethyl)-2-furaldehyde, for its acyclic form is presumably the intermediate that decomposes to levulinic and formic acids. 

Directly. Under reaction conditions where adsorbed reactants, intermediates, and products display significant IR absorption band intensities, the transient intensities may be quantitatively monitored. Considerable detailed studies are required to correlate these intensities with surface concentrations. 

Heterogeneous photocatalysis involves photoactivation of the semiconductor catalyst, wherein illumination produces electron-hole pairs, which may migrate to the surface to become trapped reactants such as holes (h ) or electrons (e ). These may react with adsorbed reactant A directly or through some intermediates. The second step in Eq. (7.331) is considered to be light-activated, thus the rate constant of step (2) varies with light intensity in the following fashion i , where a is typically between 0.5 and... 

Upon UV light illumination, the photocatalytic reactions were initiated at the surface of the Pt/Ti02 catalyst, resulting in the formation of CO2, H2O, and intermediate species. Because of the overlap of the bands of CH3CH20Had (adsorbed ethanol) with the bands of the intermediate species, the intensity variations in the 1300-1750 cm region can be revealed through the difference spectra obtained by subtracting the spectrum at 0 min (i.e., before the reaction) from the subsequent... 

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