Surface spectroscopy distinguishing adsorption from

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

In addition to calorimetry, information to establish the mode of adsorption is often obtained spectroscopically. Changes in the optical properties of (groups on) the surface or the adsorptive may be monitored. As an Illustration of the former. Rochester studied adsorption from the gas euid liquid phase on rutile (TiOj) and used infrared spectroscopy to distinguish between attachment at different surface hydroxyls. As an example of an ESR study, McBride investigated the adsorption of fatty acids on amorphous alumina from methanol by labelling them with a spin probe. Relevant information could be... 

Furthermore, a closer look to experimental data and a tentative balance between CO consumption, CO2 formation and O2 uptake showed the irreversible loss of oxygen on Pt/A Os as well as the significant retention of carbon on ceria-alumina supports. In fact surface carbonated species were clearly identified by FT-IR spectroscopy (1200-1600 cm" ) upon CO adsorption [34-36]. An even more detailed study of Pt/A Oa and Pt/Ce02 systems [37] evidenced that the OSC of alumina systems mainly originates from the WGS reaction with alumina surface OH groups. Furthermore, in the case of ceria-supported catalysts, three contributions to the OSC could be distinguished the OSC related to the metal particle (M/MO), the OSC of ceria and the OSC originating from the metal-ceria contact. 

The singly bonded H atoms perpendicular to the surface can be distinguished from the H atoms more strongly bonded between two Pt centers by IR spectroscopy. In addition, molecular adsorption of hydrogen at a surface site also occius. An analogy to... 

Infrared spectroscopy has previously been shown to be a powerful tool for studying reactions at surface acid sites using a variety of surface-sorbed probe molecules (12, 13, 21,22). For example, the protonation of NH to when adsorbed at Bronsted sites can be clearly distinguished from NH retained at Lewis sites using IR (12). Similarly, the formation of pyridinium ions upon the adsorption of pyridine at Bronsted sites is spectrally distinct from pyridine sorbed at Lewis sites (12). We have taken advantage of IR spectroscopy to identify and quantify the degradation products, TriPB and DPBA, as pathway-specific probes of TPB reactivity at respective Bronsted and Lewis acid sites of phyllosilicates (13). 

In general, when results obtained from titration methods are compared with the catalytic activity, the correlation is not necessarily good. One of the reasons is that only a small fraction of the total acid sites measured by titration are active for a given catalytic reaction. The low temperature at which amine adsorption is measured with the use of color indicators favors adsorption on aU sites, including the weakest ones. Thermal desorption methods may discriminate between sites of different strength but are unable to distinguish between Lewis and Bronsted sites. When coupled with vibrational spectroscopy data, the TPD technique may, indeed, be adequate for analysis of surface acidity [136]. However, it is almost... 

It was shown in the previous chapters that the behavior of bound water is affected by many factors, which could be elucidated from comparison of the results obtained such methods as low temperature NMR spectroscopy, TSDC, DSC, adsorption, and quantum chemical modeling. The NMR, TSDC, and DSC are sensitive to transition of phase water-ice and can distinguish different phases in terms of quantity. These methods allow obtaining structural information on interfacial water (UWCSD), pore structure (PSD, surface area, and volume of pores in different ranges), and the thermodynamic characteristics of bound water or other liquids (such as changes in Gibbs free energy... 

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