Chemisorption studies infrared

Chemisorption Studies. - Infrared spectroscopy is usually the technique of choice in chemisorption studies on catalytic surfaces because of its rapid acquisition of data, good... 

Fourier Transform Infrared Spectroscopy remains a very important technique in all chemisorption studies. A summary of band assignments, specific for PC13 chemisorption, is presented in table 11.4.30,34,35... 

Nevertheless, C02 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively to their environments. Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of 13C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by C02 adsorption. Due to the very detailed information on the structure of sites on oxide surfaces that can be obtained by C02 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has to be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful. 

The chemisorption studies of Parris and Klier (43) using the Cu/ZnO catalyst have been mentioned earlier. Carbon monoxide was irreversibly bonded at room temperature to the surface of the binary catalysts that were also active in methanol synthesis however, this irreversible adsorbate could be desorbed as CO, which indicates that it was not a surface carbonate but rather a strongly bonded carbonyl-type CO. Infrared studies of this chemi-sorbate are lacking and it would be very desirable to determine the structure of this surface species. 

Various modem accessories (ATR crystals, acoustic detectors, infrared microscopes, polarization modulation technique) as well as hyphenation techniques have substantially expanded the field of application of infrared spectroscopy. Applications of IR spectroscopy to surface investigations (characterization of the surface, physi-sorption and chemisorption studies, catalytic properties) are reviewed in [7-9]. Applications of hyphenated techniques, in particular combinations with chromatography, are given in [8]. 

K. Stoppek-Langner, J. Goldwasser, M. HouaUa and D.M. Hercules, Infrared and carbon dioxide chemisorption study of Mo/ZrO, catalysts, Catal. Lett., 32(3—4), 263-271, 1995. 

Chemisorption state of carbon monoxide. The adsorption of carbon monoxide on the surface of solid catalyst has been studied extensively. As early as in 1950s, Esschens et al. have concluded that there are two states of the CO chemisorption by infrared spectrum One is linear structure, which is a single metal atom adsorptive site, that is, the carbon atom is adsorbed by a metal atom, with the wave number being at about 2,050 cm the other state is the bridge-like structure, which is double-metal atoms adsorptive site, that is, carbon atom is adsorbed by two adjacent metal atoms, with the wave number being at about 1,905 cm. ... 

Infrared was one of the first techniques applied to the analysis of molecules absorbed in a surface, and the first studies were conducted by Terenin and Kasparov in 1940, where ammonium was absorbed in the surface of a catalyst consisting of iron dispersed in silicone gel. Subsequently, the chemisorption study of CO in metals and metallic oxides was studied in the literature [7, 8, 32]. 

Dalla Betta and Boudart [74] conducted a very complete study on the preparation and characterization by Hj-chemisorption and infrared spectroscopy of Pt/Y zeolites ion-exchanged with various bivalent cations. Table 1 gives the H/Pt ratios measured on a 5 wL% Pt/CaY sample prepared by ion exchange with PtfNHjll cations and reduced under different conditions. 

Westermark, G., Kariis, H., Persson, 1. and Liedberg, B. (1999) An infrared study on the chemisorption of tertiary phosphines on coinage and platinum group metal surfaces. Colloids and Surfaces A - Physicochemical and Engineering Aspects, 150, 31-43. 

Infrared studies show that when water is adsorbed on the surface, the background intensity in the hydroxyl region increases new bands may appear but hydrogen-bonding effects make such conclusions uncertain. If such a catalyst is then exposed to hydrogen (or deuterium), no bands due to adsorbed hydrogen (or deuterium) are observed. Thus, adsorption of water apparently occurs on the active sites and blocks out type I chemisorption. 

Spectroscopic methods, in the infrared region, have been rapidly developed in scope and power since 1949. Excellent reviews of this topic have been given by Eischens and Pliskin 126) and, more recently, by Sheppard 127). In chemisorption, new species are formed and drastic changes take place between, say, the frequencies of a CO molecule in the gas phase and those of one adsorbed on platinum 128). Extensive work has been done in the physical adsorption field by Terenin and his co-workers (reviewed elsewhere, see 126,127). Most of this work has been concerned with changes which adsorption produces in the surface OH groups of porous glass. These groups may be considered part of the adsorbent spectral studies of the adsorbate as such have been less frequently made. 

Br0nsted acid sites depends on the structure of the amine. Chemisorption data for amorphous oxides (54) show that 2,6-dimethylpyridine (which contains methyl groups that hinder coordination of the nitrogen atom with Lewis acids) is a more selective reagent for the determination of Br0nsted acidity than an unhindered amine such as pyridine. Jacobs and Heylen (44) arrived at a similar conclusion on the basis of an infrared study of amines chemisorbed on Y zeolite. They also found that the poisoning effectiveness of 2,6-dimethylpyridine is much greater than that of pyridine for the catalytic titration of Y zeolite. 

Though chlorided alumina is not strongly acidic at low chloride levels, an infrared study by Tanaka and Ogasawara (114) demonstrates that chemisorption of HC1 on y-alumina does form Brpnsted acid sites as well as... 

The work described in the previous section was essentially concerned with the physical rather than chemical adsorption of some highly polarizable molecules on zeolites. With pyridine, such information can sometimes be obtained as easily using infrared spectroscopy. However, transmission IR spectroscopy cannot so easily be used to study chemisorption on oxides if it is essential to obtain low frequency spectral data (e.g., adsorbent-adsorbate stretching modes) because of the opacity of most oxides over much of the low frequency spectral region. Recent work has shown that the Raman technique can be extremely useful in this context (4). 

A large variety of problems related to the nature of the adsorption processes have been studied by infrared spectroscopy. The most extensive and productive application of this method has been in studies of chemisorption on supported-metal samples. Spectra of physically adsorbed molecules have provided important information on the interaction of these molecules with the surface of the adsorbent. Experimental developments have reached a state where it is evident that the infrared techniques are adaptable to practically all types of samples which are of interest to catalytic chemists. Not only are the infrared techniques applicable to studies of chemisorption and physical adsorption systems but they add depth and preciseness to the definitions of these terms. 

Recent work by Selwood (9), based on changes in the magnetization of nickel during chemisorption of ethylene, indicates that ethylene is associatively adsorbed on bare nickel. He suggests that the discrepancy between this result and the dissociative chemisorption indicated by the infrared experiments is due to factors such as the relative activity of the sample surfaces and temperature effects caused by the heat of chemisorption. Low-temperature infrared experiments in which ethylene is studied at —78° C. are expected to provide evidence on the importance of the above factors in determining the course of ethylene chemisorption. 

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