Chemisorbed species, monitoring

By monitoring the vibration spectra of chemisorbed species as a function of coverage, crystal surface, and temperature, the location and site symmetries of the adsorbed atoms (4-fold, 3-fold, bridge, on-top) can be monitored and variations in site occupancy can be determined. 

We have used pyrrole as an XPS probe molecule and monitored the line of this molecule chemisorbed on alkali exchanged X and Y zeolites [26,57]. We also studied this adsorbed species on the same samples by IR [58] and microcalorimetry [59]. A similar study was also performed using chloroform as the probe molecule in IR [60] and XPS [61]. 

Upon heating, the loaded HZSM5 catalyst in a gas stream containing both reactants, the coadsorption complex was observed up to temperatures of 453 K. The decrease in its concentration occurred in parallel with the appearance of the first reaction products (xylenes)(see Figure 3). Thus, we concluded that it is likely to be a possible precursor to the transition state in the methylation reaction. This is supported by the fact that under reaction conditions the rate of methylation of toluene was found to be directly proportional to the surface concentration of the activated methanol species [23,10]. We think that during the reaction only a small concentration of the bimolecular complex exists which can not be monitored by IR spectroscopy. Its abundance should, however, depend upon the concentration of chemisorbed methanol. 

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