Infrared spectroscopy, surface hydroxyl

A noteworthy feature of the photoacoustic spectra shown in Figure 2 Is the presence of water librations. These are frustrated rotations and have been observed for ice (24) by infrared spectroscopy, as well as for water adsorbed on Ft and Ag surfaces by electron energy loss spectroscopy (25-27). The three libration modes have been associated with the bands at 600, 538 and 468 cm" > this set of peaks occurs for water adsorbed on both the hydroxylated and methoxylated silica. 

Moffat and Neeleman (139) investigated the adsorption of ammonia on boron phosphate by means of infrared spectroscopy. Ammonia appear to dissociate on this solid. Although absorption bands arising from NH4+ and coordinated ammonia were obtained, the authors feel that the presence of NH + does not necessarily indicate that Br0nsted sites were initially present on the BP04 surface. Hydroxyl groups that might be formed when ammonia dissociates could react with dry ammonia to form NHi+. 

The structural and spectral complexity of clay minerals is sufficient to consider a single mineral as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and structural hydroxyls (25.) exchangeable and structural transition metal cations (2fL and this work), adsorbed species including atmospheric gases (22), organic materials (2) accessory minerals (2SL) and, possibly, trapped hole centers (0 -centers). Thus it is of interest to adapt NIRA to studies of mineral surface activity. We have done this by examination of a small set of highly homologous clays in which laboratory control of only one variable at a time could be accurately achieved and independently confirmed. 

For the physical methods, infrared spectroscopy is the most commonly applied technique for monitoring and controlling the surface hydroxylation of silica. Mid infrared spectroscopy is limited for quantitative applications, since it is very difficult to distinguish between adsorbed water and actual surface hydroxyl groups. 

Whereas infrared spectroscopy classifies the surface hydroxyls according to their bond strength (free and bridged silanols), 29 Si NMR spectroscopy distinguishes single (either isolated or vicinal) from double (geminal) silanols. This technique can thus be regarded upon as complementary to infrared spectroscopy. 

Alumina surfaces previously exposed to water vapor (or moist air) at temperatures above 100°C are terminated by a monolayer of hydroxyl groups. The presence of hydroxyl groups in the surface has been shown by deuterium exchange and infrared spectroscopy (113-116) and by chemical methods (19). 

Deuteration of the accessible hydroxyl groups is accomplished with saturated deuterium oxide vapor at room temperature. The extent of deuteration and therefore accessibility can be estimated gravimetrically by infrared spectroscopy. Accessibility rather than crystallinity is measured because deuteration of the hydroxyl groups on crystallite surfaces can also occur. 

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... 

Using diffuse reflectance Fourier transform infrared spectroscopy, Adhikari et al. studied the binding of oleic acid (88), triacylglycerol (89), and phosphatidylcholine (90) on silica gel at room temperature. Their interpretation was that oleic acid and triacylglycerols bind to silica surface hydroxyls via hydrogen bonding interactions between the carboxylate and ester carbonyls of these molecules, respectively. In contrast, phospholipids hydrogen bond to silica surface hydroxyls via the phosphate... 

Oare earth forms of zeolites X and Y type faujasites possess superior catalytic properties for various reactions such as alkylation, isomerization, and cracking (9, 12, 18). Structural studies involving x-ray diffraction and CO chemisorption have been made to locate the positions of the rare earth (11, 14, 16). Hydroxyl groups and their relationship to surface acidity have been studied by infrared spectroscopy, utilizing the adsorption of pyridine and other basic molecules (2, 6, 21, 22, 23). Since much of the previous research has involved measurements on mixed rare earth faujasites, a need existed for a more systematic study of the individual rare earth zeolites, in regard to both structural and catalytic properties. The present investigation deals with the Y, La, Ce, Pr, Sm,... 

The infrared spectroscopy shows the presence of surface SiOH groups [35,36]. Infrared spectra also reveal the surface B-OH groups but only after degasing of CPG at elevated temperatures under vacuum conditions [32-34,37], Geminal OH structures bonded with Si as well as B atoms can be detect among hydroxyl groups [38]. 

The activation of carbon dioxide was studied over a zirconium dioxide catalyst via infrared spectroscopy and 0-labeled reactants. The carbon dioxide adsorbed on the surface as either a carbonate or a bicarbonate species. The carbonate species formed as a result of CO2 Interaction with lattice oxygen. The bicarbonate species formed from CO2 interaction with a hydroxyl group. There was no direct interconversion between the carbonate and the bicarbonate. It is proposed that the bicarbonate can be converted to the formate via molecular CO. 

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