Surface adsorbed pyridine

A similar but very weak resonance reported at 30 ppm in 7-AI2O3 heated at 600°C has been attributed to 5-coordinated aluminium atoms on the surface of the oxide particles (Pecharroman et al. 1999). Cross-polarisation experiments between Al NMR and the protons from surface-adsorbed pyridine (Morris and Ellis 1989) or ammonia (Coster et al. 1994) have been used to investigate the surface state of cat-alytically active 7-AI2O3. No NMR evidence has been found of the hypothetical 3-coordinated Al theoretically predicted to occur at the alumina surface. Even assuming for this species a 8iso of 95-100 ppm, a Xq of 10 MHz and an t value of one, Coster et al. (1994) found no experimental evidence for its existence, prompting a recent explanation based on density functional calculations that the 3-coordinated surface Al atoms... 

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... 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Role of the organic feed XRD powder patterns and FT-IR spectra confirmed that pure MFI-type Ti-silicalite (TS-1) was obtained [18-23], with a surface area of 530 m g". FT-IR spectra of adsorbed pyridine (Fig. 39.2) showed the presence only of weak Bronsted and Lewis sites [24,25], as confirmed by the complete evacuation from the surface at 373 K. 

The first report of the SERS spectrum of a species adsorbed at the electrode/ electrolyte interface was by Fleischman et al (1974) and concerned pyridine on silver. The Raman spectrum of the adsorbed pyridine was only observed after repeated oxidation/reduction cycles of the silver electrode, which resulted in a roughened surface. Initially, it was thought that the 106-fold enhancement in emission intensity arose as a result of the substantially increased surface area of the Ag and thus depended simply on the amount of adsorbate. However, Jeanmarie and Van Duync (1977) and Albrecht and Creighton (1977), independently reported that only a single oxidation/reduction cycle was required to produce an intense Raman spectrum and calculations showed that the increase in surface area could not possibly be sufficient to give the observed enhancement. 

Nicotinic acid and related meta-carboxylic acids display the remarkable characteristic that coordination of the pendant carboxylic acid moieties to the Pt surface is controlled by electrode potential. Oxidative coordination of the carboxylate pendant occurs at positive electrode potentials, resulting in disappearance of the 0-H vibration and loss of surface acidity as judged by absence of reactivity towards KOH. Carboxylate in the 4-position of pyridine (as in INA) is virtually independent of electrode potential, whereas strong coordination of ortho-carboxylates to the Pt surface is present at most electrode potentials. Adsorbed pyridine carboxylic acids are stable in vacuum when returned to solution the adsorbed material displays the same chemical and electrochemical properties as prior to evacuation. 

The surface nethoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabilities were studied by Temperature-Programmed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed ammonia was... 

As shown in Table 1, the amount of irreversibly adsorbed pyridine dropped to one third its original value after modification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of Bpy peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. The results in Table 1 aslo show that the drop in acidity paralleled that... 

Subtractively normalized interfacial FTIR has been employed [242] to study the changes in the surface coordination of pyridine molecules on Au(lll). It has been deduced from the experiments that pyridine molecule is positioned upright at positive potentials and its plane rotates somewhat with respect to the electrode surface. In situ FTIR has also been used [243] to investigate adsorption of pyridine on Au(lll), Au(lOO), andAu(llO) electrodes. For the low-index electrodes, the behavior of band intensity located at 1309 cm and corresponding to the total adsorbed pyridine, agreed with the surface excess results obtained earlier from chronocoulometry. 

Spectra of adsorbed pyridine have been recorded for the MoCo-124 catalysts, for which the final calcination temperature after the cobalt impregnation has been varied. It turns out that the 400 and 500°C calcined samples and the 650 and 700°C calcined samples show very similar spectra. Therefore we show only the spectra of the 400°C (low calcined) and the 650°C (high calcined) samples. Figure 4 shows spectra after desorption at 150 and 250°C. Few Brdnsted acid sites are observed in the low calcined MoCo-124 samples. The reflection spectra (Figure 1) indicate for these low calcined samples the presence of cobalt on the catalyst surface, because no cobalt aluminate phase could be detected. The high calcined samples do show the presence of Brdnsted acid sites the presence of a cobalt aluminate phase is concluded from the reflection spectra (Figure 1) for these samples. 

It has been found by measuring spectra of adsorbed pyridine on various C0O-M0O3-AI2O3 systems that there is a clear interaction of the cobalt ions with the Mo03 Al203 surface layer. This conclusion is based upon the occurence of a second Lewis band in the spectrum of adsorbed pyridine, which has been observed in the whole region of the calcination temperatures applied. 

Although the basic principles behind this Intact ejection mechanism can be Illustrated with carbon monoxide, the extrapolation to large bloorganlc molecules Is not necessarily obvious. Calculations have been performed for a series of organic molecules adsorbed on a Ni(OOl) surface to understand the mechanisms of molecular ejection (8-12). The first molecules which have more than just a few atoms examined are benzene which it-bonds on a metal surface and pyridine which can either ir-bond or o-bond on a metal surface. Larger structures, whose sizes approach the diameter of bloorganlc molecules, are naphthalene, biphenyl and coronene whose adsorption structures are unknown. All the molecules except pyridine are assumed to ir-bond on the surface. 

The surface was actually a film of native aluminum oxide it did not adsorb pyridine but did adsorb chloroform showing the oxide to have no acid sites, but basic sites. Treatment of the aluminum oxide with aqueous carbonate solutions clearly enhanced the basicity, as evidenced by stronger adsorption of chloroform. By observing the temperature coefficient of adsorption isotherms with ellipsometry one can actually determine heats of adsorption on a square centimeter of flat surface. 

Pyridine is desorbed from the silica gel surface in a two-stage desorption process, figure 5.14. The two peaks appear at 333-363 K and 423-453 K, indicating that two different adsorption/desorption sites exist on the silica gel surface. Since pyridine is adsorbed on both types of hydroxyl groups, the two adsorption sites are the isolated and the vicinal hydroxyl groups. 

Shen et al. (219) reported on the photoluminescence and IR spectra of pyridine adsorbed on 7-AI2O3 to measure the surface acidities of alumina. They observed that neither the photoluminescence nor the IR spectra of adsorbed pyridine showed any evidence of Brpnsted acidity on y-AbOs pretreated at 673 K, but they showed four different weak OH bands when the alumina was treated at 873 K. 

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