Adsorbed probe molecules

The most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. Sometimes infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on adsorption sites on a catalyst. We will first summarize the theory behind infrared absorption. 

Lercher, J.A., Gruendling, C. and Eder-Mirth, G. (1996) Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 353. 

Fig. 6 Normalized quadrupole coupling constants, Cq/Cq(0) as a function of proton affinity of adsorbed probe molecules. Cq(0) is the quadrupole coupling constant of the unloaded acidic zeolite (assumed to be loaded with N2 molecules, proton affinity of494 kJ/mol). Data are taken from DFT calculations of Ehresmann [234] and Koller [232], and NMR measurements of Jiao [233], and Marthala [235]...

K. C. Hartner, J. W. Carr, and J. M. Harris, Total internal reflection fluorescence for adsorbed probe molecule studies of liquid/solid interfacial environments, Appl. Spectrosc. 43, 81-86 (1989). 

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. 

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... 

Further characterization of BaAl O based on FTIR of adsorbed probe molecules (C02 and pyridine) and measurements of isoelectric point29 showed that the surface was basic. Because of the low bulk Ba content of this material such basic character can hardly be explained assuming a statistical exposure of the different crystallographic planes. This suggests that the Ba-containing mirror planes are preferentially exposed. 

In iron bearing clays the time law of the luminescence decay of Ru(byp)3 + is multiexponential (29). In the long time limit (>1 /is) the decay approaches an exponential law, but at shorter time ( 300 ns), a more complex decay has been observed. The decay mechanism has been explained by a simple localized model based on the assumption that on each particle the quenching process can be described as occurring in an ensemble of small independent subsystems. Each subsystem is composed of an excited probe system (with a very reduced mobility) and of the nearest lattice sites of the solid which may be occupied by the quencher ions. For the sake of simplicity, it has been assumed that the adsorbed probe molecules occupy the sites of a superlattice which matches the lattice containing randomly distributed quenchers (Fe, for instance)... 

Characterization of Chemical Nature of Specifically Adsorbed Probe Molecules... 

In the present chapter we will summarize results of two different evaluation procedures for the surface roughness of carbon blacks. In the mono-layer regime we refer to the scaling behavior of the estimated BET-surface area with the size of adsorbed probe molecules (yardstick method). On smooth flat surfaces the BET-area is independent of the adsorbed probe or applied yardstick, while on rough surfaces it decreases with increasing probe (yardstick) size due to the inability of the large molecules to explore smaller cavities. This is shown schematically in Fig. 5. 

The Lewis acidity of the sodium cation in the two positions at the rings with two or three A1 centers is different judged both by the Mulliken charge and the frequency shift of an adsorbed probe molecule carbon monoxide. The cation is more acidic in syn position, q(Na) = 0.61 e, and... 

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

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