Spectroscopy of Adsorbed Probe Molecules

Among the spectroscopic techniques, one of the most widely used to characterize the basic properties of alkaline earth metal oxides is infrared (IR) spectroscopy of adsorbed probe molecules (41,47-49) this is described below. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Species I were observed on MgO and CaO, and species II were observed on MgO, CaO, and SrO as well as on hydroxylated MgO (JJ). They were monitored by the NH2 absorption band in the region near 1550cm (55-57). Dissociative chemisorption has also been reported for pyridine on pairs in low co- 

It was found that proceeding in the series from MgO to CaO to SrO, the ease of formation of C5H4N ions increased, in agreement with the increasing basicity of the oxides (5S). 

The spectral behavior of CO bonded to metal atoms (metal carbonyls) has been used to characterize the surface of solids (61). For instance, it is known that metal carbonyl interacts with surface site of metal oxides and zeolites to form a Lewis-type adduct where a CO ligand of the metal carbonyl interacts (via the oxygen atom) with surface OH groups or with co-ordinatively unsaturated metal ions (surface Lewis acid sites) (62,63). On the other hand, thermal treatment of the metal carbonyl support adducts lead to loss of CO with formation of subcarbonyls, which are anchored to the support (64,65). Papile et al. (66) reported the characterization 

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In general, spectroscopic techniques and, in particular IR spectroscopy of adsorbed probe molecules such as the ones mentioned above, provide information about the nature of the basic sites on oxide surfaces. However, they do not give information about the number and strength distribution of the basic sites on a solid... 

Vibrational spectroscopy of adsorbed probe molecules is one of the most powerful tools to assess the acidic properties of catalysts. Acidity studies of dealumi-nated Y zeolites (main active component of FCC catalysts) or other zeolitic catalysts are reported using mostly Fourier Transform Infrared Spectroscopy (FTIR) with CO adsorption at 77 K or FTIR-pyridine/substituted pyridines adsorption at 425 K [22-26]. FTIR acidity studies of commercial FCC catalysts are even more scarce... 

The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 133... 

IR spectroscopy of adsorbed probe molecules is mostly performed with either the transmission/absorption technique or with the DRIFT technique. In the tiansmis-sion/absorption technique, self-supporting pressed disks ofthe pure oxide powders... 

NMR spectroscopy of adsorbed probe molecules can also be used to characterize... 

Vibrational spectroscopy of adsorbed probe molecules for investigation of supported catalysts - Estimation of the strength and concentration of various surface sites... 

Acid-base characterisation by IR spectroscopy of adsorbed probe molecules... 

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