Evacuation absorbents, adsorbents

The infrared spectra were recorded after equilibrating the reduced and evacuated solids with an excess of pyridine vapor and further evacuation at various temperatures. After evacuation at 423 K there is no more physically adsorbed pyridine. There is no characteristic band of pyridine adsorbed on Bronsted acid sites (no appearance of the 19b vibration at 1540-45 cm" ) [11,12]. The OH groups observed on the solids are thus non acidic. The existence of Lewis acid centers (coordinatively unsatured Al " or Zr ) is proven by the presence of the 19b vibration at 1440-50 cm" and of the 8a vibration at 1610-1620 cm". The absorbances of the 1440-50 cm" band show that the acidity difference between the Pd/Al203 and PdyZr02 solids is not significant. 

Any material which has been exposed to - or manufactured in - atmospheric air will have gas adsorbed on the surface, or dissolved or absorbed in the bulk. On evacuation, therefore, the walls of a vacuum system and the components and material being processed inside will tend to re-emit this gas. This phenomenon is generally termed outgassing. 

Acidities as expressed by the absorbances of the infrared bands are plotted in Figure 4. Data were recorded under 2 mm of pressure and after evacuation for 30 minutes to remove reversibly adsorbed species. The 3230 cm" band, like the 1538 cm" band, is caused by pyridinium ions and can be useful for estimating acidity when the latter band becomes too intense. Although some scatter exists, the Bronsted acidity tends to increase with ionic radius. Nearly 30-40% of this acidity is reversible. In the 1450 cm" region, 2 types of adsorbed pyridine can be distinguished, 1 species absorbing at 1452 cm" and the other at 1438 cm" The latter band is more intense and removed easily by evacuation. The remaining 1452 cm" band arises from Lewis acidity. Its intensity is the same for all the rare earth faujasites. 

IR spectroscopy was applied for the measurements of adsorption and desorption of pyridine. Prior to the pyridine adsorption, all samples were activated in vacuum (lO Pa) at 675K except when they were previously evacuated at 975K. Pyridine adsorbed on Bronsted acid sites, true Lewis acid sites and sodium cations was characterised by IR bands at -1550, -1455 and -1442 cm", respectively [12]. Table 2 shows the ratio of the absorbance at -1550 cm 1 (PyB) to that at -1455 cm 1 (] L) for various samples. These results indicate that all samples, even when evacuated at 975K possessed Bronsted acid sites. However, the Lewis acid sites dominated except with the samples treated at lower temperatures. One should stress that evacuation at 975K led to a higher reduction of Bronsted acid sites than heating at the same temperamre in air. The lower PyB/PyL ratio observed on niobium-modified NH4NaY... 

The adsorption of NO and CD3CN were studied by FT-IR (Nicolet Mx-IE). Heatable, high vacuum IR cuvette with samples in the form of self supporting pellets( 10 mg/cm2) were used. Before measurements, all samples were evacuated at 350°C overnight. In all the experiments the equilibrium amount of either NO (2 Torr) or CD3CN ( 1 Torr) was adsorbed at 25°C for 15 minutes and spectrum recorded. The normalized absorbance of the respective bands was measured after desorption at room temperature and 100 °C its variation was selected to... 

In view of the uncertainties inherent in Hammett indicator determinations of surface acidity by visual means, a study was made of the spectral behavior of dyes adsorbed on several silica-alumina catalysts and silica gel (62). The effects of catalyst water content, dye concentration, catalyst composition and pretreatment on the spectra of the adsorbed dyes were examined. The Hammett indicator dyes employed and their corresponding pKA values are summarized in Table II. Reference spectra were determined for the base-form of the dye in iso-octane or methylene chloride solution and for the acid-form in an aqueous sulfuric acid or ethanolic-hydrogen chloride solution. Dyes were adsorbed from isooctane solutions onto thin plates of optically transparent catalysts which were installed in evacuated cells of design similar to that shown in Fig. 4. The catalysts samples were routinely pretreated by calcination in oxygen at 500° to remove any organic contaminants, followed by evacuation at this temperature. To examine the effect of variable water content on the spectra the samples were rehydrated in an atmosphere of wateir vapor for 24 hr after pretreatment and subsequently evacuated at some lower temperature. Dye solutions were introduced through a side arm. These solutions were suitably dilute so that the absorbance due to dissolved dye was either below the limits of detection or, at... 

That the observed spectrum was the result of a chemical reaction between the hydrocarbon and the catalytically active centers of the silica-alumina surface (chemisorption), and not due to a general sur-fatochromic spectral shift, was demonstrated from the spectrum of this compound adsorbed on a nonacidic or very weakly acidic silica gel (29). The spectrum (Fig. 30, Curve B) of silica gel exposed to triphenylmethane vapor for 1000 hours at 100°C was identical to the spectrum (Curve A) of an alcoholic solution of this hydrocarbon. The close agreement between these spectra suggested that on silica gel the triphenylmethane was physisorbed. This was further evidenced by the marked loss of spectral intensity (Curve C) attendant to a four hour evacuation at 100°C. In contrast, on silica-alumina where the hydrocarbon was chemisorbed as the carbonium ion, no decrease in absorbance was noted even after 48 hr evacuation at 275°C. These data constituted the first direct demonstration of the formation of carbonium ions as a consequence of chemisorption of a tertiary hydrocarbon on the surface of a cracking catalyst by a reaction involving the rupture of an aliphatic C-H bond. The generality of this process of carbonium ion... 

CO probe FTIR measurements were performed using a Nicolet Magna-750 spectrometer. The samples (about 20 mg) were pressed into pellets and reduced in situ in a dedicated cell at 473 K in H2 flow for 2 h, followed by outgassing and cooling to ambient temperature. CO adsorption was performed at room temperature by injecting pulses in the cell until the catalysts saturation. Then, the samples were evacuated at room temperatme for 1 h. Difference spectra were obtained from the absorbances before and after adsorption of the probe molecule. The CO-adsorbed spectra were decomposed into different bands by using spectrum adjustment software on the basis of four peaks located at about 2095, 2024, 2068 and 1865 cm and corresponding respectively to the... 

Tin Oxide. - To our knowledge this system has been studied (with one exception) only by Thornton and Harrison at the University of Nottingham, who have produced seven papers on the subject. I.r. has been used to investigate the surface as a function of the evacuation temperature. Molecular H2O is largely removed at 320 K and fully removed at 473 K. H-bonded -OH groups are present, and the free -OH absorbs at 3640 cm . CO2 yields carbonates and bicarbonates. CO is not adsorbed as such, but forms carbonates by the partial reduction of Sn to Sn ". Adsorption of both NH3 and pyridine reveals Lewis basicity only. The adsorption of small organic molecules shows the oxidizing properties of tin oxide as indicated by CO. 

Once the loading phase is finished the raised pressure in the adsorber is reduced by pressure release or evacuation, which then decreases the absorbability of the adsorbate on the adsorbent. The released adsorbate may be stripped using a strip... 

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