Adsorption of pyridine

The discussion in the previous section suggests that adsorption of pyridine on the catalyst is a necessary prerequisite for the formation of 2,2 -bipyridine but as platinum catalysts, which are poisoned by... 

Figure 39.2. FT-IR spectra of TS-1 catalyst after adsorption of pyridine and following evaeuation at increasing temperature.

Pyridine. Pyridine and its methyl substituted derivatives (picolines and lutidines) were found to polymerize electrochemically and, under certain circumstances, catalytically. This behavior was not expected because usually pyridine undergoes electrophilic substitution and addition slowly, behaving like a deactivated benzene ring. The interaction of pyridine with a Ni(100) surface did not indicate any catalytic polymerization. Adsorption of pyridine below 200 K resulted in pyridine adsorbing with the ring parallel to the surface. The infrared spectrum of pyridine adsorbed at 200 K showed no evidence of either ring vibrations or CH stretches (Figure 5). Desorption of molecular pyridine occurred at 250 K, and above 300 K pyridine underwent a... 

White28 indicated that the adsorption of pyridine molecule can be used to determine the concentration of Bronsted and Lewis acid sites. When IR is used in conjunction with thermal desorption, an estimation of the acid strength distribution can be obtained. 

IR spectra of the samples with and without pyridine were recorded on an FT-IR spectrometer (PE 430) with a resolution of 1 cm. In order to measure the pyridine adsorption, the samples were pressed into thin pellets, and placed into a quartz cell with CaF2 windows. The sample pellets were evacuated at 400°C for 2 h (< 10 Torr). After cooling down to room temperature, the pellets were exposed to pyridine environment (10 Torr) at 25°C. IR spectra were recorded after adsorption of pyridine for 1 h and evacuation at 150, 250, 350, and 450°C for 1 h. 

The band centered at 3615 cm disappears by adsorption of pyridine and further outgassing at 150°C, thus confirming its assignment to stretching vibration of acidic OH groups. For samples with a different Si/Al ratio, a direct correlation is observed between the intensity of this band and that of protonated pyridine at 1540 cm (Table I). 

Figure 3 Infra-red spectra of pyridine on sample G5-Ce, steamed at 550°C. Adsorption of pyridine at room temperature, then evacuation at different temperatures.

Adsorption of pyridine on goethite and on hematite involved both coordination to the Lewis acid sites and hydrogen bonding to surface OH groups. Trimethylchlorosil-ane reacted with the surface OH groups on these oxides to form surface trimethyl siloxyl groups and HCI (Rochester and Topham, 1979c). 

The r, N curve for the adsorption of pyridine from an aqueous solution at a water-uncharged mercury interface. The surface tension measurements employed are those found by Gouy Ann. de Ghimie et de Physique, vill. ix. 130, 1906), whilst the pyridine activities are derived from the vapour pressure data of Zadwiski... 

Adsorption of pyridine and its derivatives on different metal electrodes has been studied very intensively [11], both at polycrystalline Au surface and different... 

On a pc-Au electrode in 1 M NaF vertically oriented pyridine molecules have been observed at 0.7 V (versus Ag/AgCl), applying in situ IR. In contrast, they have not been detected at this potential in electrochemical method [240]. Considering the fact that adsorption of pyridine on gold electrodes is a replacement reaction and taking into account the results obtained from quartz crystal microbalance experiments, the conclusion has been made that adsorption of one pyridine molecule is accompanied by the removal of 10-12 water molecules [241]. 

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. 

Applying in situ infrared spectroscopy and STM, Cai et al. [253] have studied adsorption of pyridine on Au(lll) electrodes from aqueous NaCl04 solutions. It has been found that pyridine molecule is flatly adsorbed on the surface at negative potentials. Its molecular plane rises up as the applied potential and surface concentration increase. Moreover, orientation of pyridine molecule changed with the applied STM potential. Ikezawa et al. [243] have used in situ FTIR spectroscopy to investigate adsorption of pyridine on Au(lll), Au(lOO),... 

In situ subtractively normalized interfacial FTIR spectroscopy has been employed [254] to study adsorption of pyridine on the Au(llO) electrode surface. The compound adsorbed via the nitrogen atom and the tilting angle decreased progressively with the increasing electrode potential, as it appeared from the IR data. 

Adsorption of pyridine on Au(llO) electrodes was also studied using reflection anisotropy spectroscopy [255]. Reflection anisotropy of pyridine/Au(110) system has been attributed to n-jT transitions, the band of which was shifted compared to their spectral position in the gas phase, due to the interaction of the lone electron pair orbitals at N atom with the gold surface. 

Figure 3. Total amount of residual adsorbed pyridine (full symbols) and fraction of Lewis sites with respect to the total number of acid sites (open symbols) as a function of the desorption temperature (after adsorption of pyridine at room temperature).

Catalysts were characterized by means of X-ray diffraction (Phillips diffractometer PW3710, with CuKa as radiation source), UV-Vis-DR spectroscopy (Perkin-Elmer Lambda 19) and chemical analysis. Measurements of surface acidity were carried out by recording transmission FT-IR spectra of samples pressed into self-supported disks, after adsorption of pyridine at room temperature, followed by stepwise desorption under dynamic vacuum at increasing temperature (Perkin-Elmer mod 1700 instrument). The procedure for chemical analysis is described in detail in ref. (13). 

Zhu, S., Bell, P.R.F. Greenfield, P.F. (1988) Adsorption of pyridine onto spent Rundle oil shale in dilute aqueous solution. Water Res., 22, 1331-1337 Zimmermann, F.K., Groschel-Stewart, U., Scheel, I. Resnick, M.A. (1985) Genetic change may be caused by interference with protein-protein interactions. Mutat Res., 150, 203-210... 

---