Ammonia adsorbed silica

Calculate the rotational contribution to the entropy of adsorption of ammonia on silica at -30°C, assuming (n) that the adsorbed ammonia retains one degree of rotational freedom and (b) that it retains none. In case (n) assume that the nitrogen is bonded to the surface. 

Since 1905, when Coblentz obtained the first IR spectrum, vibrational spectroscopy has become an important analytical research tool. This technique was then applied to the analysis of adsorbates on well-defined surfaces, subsequently moving towards heterogeneous reaction studies. Terenin and Kasparov (1940) made the first attempt to employ IR in adsorption studies using ammonia adsorbed on a silica aerogel containing dispersed iron. This led to a prediction by Eischens et al. from Beacon Laboratories in 1956 that the IR technique would prove to be extremely important in the study of adsorption and catalysis. For an excellent review article in IR spectroscopy, see Ryczkowski and references therein and for a more recent review with applications, see Topsoe. ... 

Figure 12.5 Irreversibly adsorbed and/or chemisorbed ammonia on silica gel as a Junction of the reaction temperature, with and without TCS modification.

For NH3 adsorption, experimental evidence has shown that the main interaction mechanism is H bonding of Si-OH to the N atom in NH3 [124], Ammonia gas is a widely used chemical in industry, and it has to be removed to less than one ppm, for instance, from the gaseous effluents of ammonia fertilizer plants, urea plants, and other sources [127], It is evident that silica is an excellent adsorbent of NH3 [124-126], Also, adsorption of ammonia on silica gel has received considerable attention recently, owing to its potential use in solar energy cooling cycles [128],... 

A germane attempt to rank adsorbate ligands in terms of their Cu d-d splitting when the cation (Cu ) is coordinated on a silica surface had earlier been made by Trouillet et ai, in a Cu/Si02 matrix. Trouillet and co-workers applied the rule of averaged environment to conclude that the order was ethylenediamine (en) > ammonia > water > silica (Si02). A series similar to this is obtained from the present study, but includes pyridine and acetone. Thus, from Table 2 and Figure 7, we can therefore conclude that it is possible to use IR spectroscopy to monitor ammonia in the environment. 

Fig. 150. Thin-layer chromatogram of phospholipids and other polar lipids of bovine milk [159 a]. 1 carbohydrate (lactose) and protein, 2 sphingomyelin, 3 phosphatidyl choline, 4 phosphatidyl serine, 5 phosphatidyl inositol, 6 phosphatidyl ethanolamine, 7 cerebroside dihexoside ( ), 8 cerebroside mono-hexoside ( ), P fatty acids, 10 neutral lipids. Adsorbent Silica gel HR. Solvents I, chloroform-methanol-water-28% aqu. ammonia (130 + 70+8 + 0.5) II, chloroform-acetone-methanol-acetic acid-water (50 + 20 + 10 + 10 + 5). Time 40 min in each direction. Indicator charring with chromic sulphuric acid solution...

FIGURE 14.1 Diagram of the general procedure for embedding nanoparticles in silica colloids in the first step, PVP is adsorbed on the colloidal particles (1). After transfer in an alcohol, for example, ethanol or butanol, the particles are adsorbed on amino-functionalized silica colloids (2). Finally, after addition of ammonia, a silica shell is grown on the nanoparticle-decorated colloid by consecutive additions of tetraethoxysilane (3). (From Graf, G. et al. 2006. Langmuir 22 5604-5610. With permission.)... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

Reforming catalysts are typically platinum on a silica-alumina support. The catalysts are deactivated by feedstock contaminants such as organic nitrogen, sulfur, ammonia, and H2S. For this reason, the reformer charge is hydrotreated to remove these components. Also, any trace metal contaminants will be adsorbed onto the hydrotreating catalyst. 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

Fig. 72. I5N CP/MAS NMR spectra at 20.3 MHz spectra of I5N-enriched pyridine (Py) adsorbed on silica-alumina in the presence of varying amounts (in grams) of n-butylamine (NBA). B, L, and HB denote Bronsted, Lewis, and hydrogen-bonding sites, respectively. Chemical shifts are given in ppm from liquid ammonia (371).

When H2S is chemisorbed on platinum or water adsorbed on silica, the S—H and 0—H stretching bands appear at positions close to those observed for H2S and H20 in the gaseous state. These results, together with those found for chemisorbed ammonia, indicate that the stretching vibrations of the single bonds to hydrogens are not markedly affected, by formation of coordinate bonds between a surface and the central atom of the molecule. 

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