Pyridine, adsorption spectra

The samples were submitted to the sulfidation procedure described above, followed by 2 h of heating at 673 K, under vacuum (about 2x10 3 Pa). After cooling under vacuum, pyridine was adsorbed at room temperature for 30 minutes. The samples were then outgassed in three steps of 1 h the first one at room temperature and the others at 423 K and 523 K. Spectra were taken before pyridine adsorption and after each outgassing step, with a FTIR spectrometer Bruker IFS-88 (spectral resolution set at 1 cm ). Each spectrum represented the average of at least 50 scans. 

The major addic sites on H-MOR are Bronsted sites determined by pyridine adsorption studies above 80 % of addic sites are Br0nsted sites and the rest are Lewis add sites [4,5]. After adsorption of NH3, 0.3 kPa of EA are admitted on H-EDTA-MOR at 473 K (Figure 6F) adsorbed NH3 is easily replaced by EA to produce deformation bands of NH3+ (1597 cm-i, 1497 cm-t), CH2 (1460 cm-i). This spectrum is quite the same as the spectrum in Figure 6A. The results suggest that adsorption of EA is much stronger than that of NH3. When adsorbed EA is heated up to 573 K (Figure 6G-6H), the spectra are almost the same as the spectra in Figure 6B and 6C. 

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. 

Figure 4 Infra-red spectra of pyridine adsorbed on sample G5-Ce steamed at 550°C (spectrum a) and 650°C (b) and on a FCC commercial catalyst steamed at 775°C (c). Pyridine adsorption at 25°C and desorption at different temperatures.

In the two cases, the Ru(II) complexes are six-coordinated, however, the pyridine complex is soluble in water whereas [Ru(Dipy)2L2]Cl2 is insoluble because of a strong intermolecular interaction. The polymeric nature of the ligand manifests itself in a 10 run shift of the adsorption spectrum (in the UV-region) of the macromolecular complex due to lowering its resonance stabilization, differences... 

Figure 12.20 Typical adsorption spectrum for pyridine on a zeolite. Adsorption at I50°C and thermodcsorption by steps up to 350 C (L Lewis, B Bronsicd).

Figure 12.21 Typical adsorption spectrum for pyridine on an alumina. Adsorption at 25 C and thermodesorption by steps up to 350°C.

Figure 4. Spectra of species formed by pyridine adsorption on activated cloverite after evacuation at b) r.t., c) 423 K, d) 523 K, e) 623 K, f) 723 K. Comparison with the spectrum of activated cloverite (a).

The extraordinary enhancement of the Raman spectrum of pyridine adsorbed on silver has resulted in the investigation of pyridine as a candidate for possible enhancement on other metals. More recently, SERS has been used to probe the adsorption of molecules on metals that are of catalytic importance. For example, SERS spectra have been collected of pyridine adsorbed on metallic rhodium in the first demonstration of surface-enhancement from metallic rhodium. While the enhancement of pyridine on silver is thought to arise from an electrodynamic enhancement, the enhancement of the pyridine Raman spectrum by rhodium is thought to arise from a chemical effect since rhodium, unlike silver, is not a free electron metal. 

The acidity distribution was obtained by pyridine adsorption-desorption (Table 3). The presence in the IR spectrum of a band at 1455 cm was assigned to the pyridine coordinatively bonded to Lewis acid sites, while a band at 1545 cm was attributed to the pyridinium ion using the known values of the extinction coefficient of the two IR signals [12] it was possible to determine the Lewis and Bronsted acid sites densities. The number of acid sites observed in the used catalyst was about half of that present in... 

Characterization of catalysts The zeolite structure was checked by X-ray diffraction patterns recorded on a CGR Theta 60 instrument using Cu Ka, filtered radiation. The chemical composition of the catalysts was determined by atomic absorption analysis after dissolution of the sample (SCA-CNRS, Solaize, France). Micropore volumes were measured by N2 adsorption at 77 K using a Micromeritics ASAP 2000 apparatus and by adsorption of cyclohexane (at P/Po=0.15) using a microbalance apparatus SET ARAM SF 85. Incorporation of tetrahedral cobalt (II) in the framework of Co-Al-BEA and Co-B-BEA was confirmed by electronic spectroscopy [18] using a Perkin Elmer Lambda 14 UV-visible diffuse reflectance spectrophotometer. Acidity measurements were performed by Fourier transform infrared spectroscopy (FT-IR, Nicolet FTIR 320) after pyridine adsorption. Self-supported wafer of pure zeolite (20 mg/cm ) was outgassed at 673 K for 6 hours at a pressure of lO Pa. After cooling at 423 K, the zeolite was saturated with pyridine vapour (30 kPa) for 5 min, evacuated at this temperature for 30 min and the IR spectrum was recorded. 

The nature of the acidity was investigated in order to explain the catalytic activity of the calcined Co-substituted P-zeolite and the role played by the aluminic sites of this solid. A pyridine adsorption followed by IR spectroscopy measurements was performed on the calcined catalyst. It has been shown that adsorption of pyridine emphasized two distinct bands at 1548 cm and 1451 cm corresponding respectively to the adsorption on Brbnsted and Lewis sites [22], In the case of the calcined Co-substituted zeolite, only a weak band at 1548 cm appeared in the IR spectrum. Thus, we deduced that very few Bronsted sites were present in the catalyst. This could explain that the oxidation of cyclohexane into adipic acid in the presence of calcined Co-substituted aluminic P-zeolite was not inhibited. 

Solid-state ion exchange with CaCl2/H-MOR and MgC /H-MOR was also monitored by IR. Figure 7 demonstrates, as an example, the removal of acidic OH groups (spectrum 2a) in H-MOR due to replacement of the protons by Ca +. Subsequent pyridine adsorption gave rise to a sharp band at 1446 cm typical of pyridine coordinatively bonded to calcium cations and a smaller band at 1455 cm which is due to pyridine attached to true Lewis sites (spectrum 2b). Most likely, both types of pyridine coordination contributed to the small signal at 1610 cm. Note, however, that only a very weak pyridinium ion band at 1540 cm was observed. Another sample, which had been prepared via solid-state ion exchange (spectrum 2a), was contacted with small amounts... 

Adsorption of pyridine subsequent to spectrum c gave rise to a band at 1452 cm typical of pyridine coordinatively bonded to lanthanum cations only a tiny pyridinium ion band at 1542 cm was observed. When, however, generation of spectrum d (i.e. after HgO contact of the sample) was followed by pyridine adsorption, the band of acidic OH groups at 3630 cm was completely removed and a strong band at 1542 cm (pyridium ions) appeared. 

During the dehydration process some of the water molecules dissociate under formation of an increasing amount of Bronsted acid OH groups in the supercages and the sodalite units [8]. However, the amount of these acid sites is small, as has been proved by pyridine adsorption. From IR spectra bands at 1390, 1488, 1542 and 1632 ciiT should be expected [lO]. Figure 2 (lower spectrum) proves the almost total absence of these bands. On the other hand the strong bands at 1455, 1490 and 1605 cm indicate the presence of Lewis acid sites e.g. Cu cations [lO]. The responsible band at 1455 cm splits into a doublet indicating two different Lewis acid sites which probably are due to the free Cu(ll) cations and the true Lewis acid sites or residual Na" " ions. 

Pyridine adsorption shows a dramatic increase of Bronsted acid centers paralleled by the decrease of Lewis acid centers (see upper spectrum of Fig. 2). This is in agreement with the observation of the growing amount of reduced Cu particles accompanied by a loss of Lewis acid sites. From our experiments a correlation between cluster size and the water and hydrogen partial pressure must be assumed. 

The catalysts were characterized by X-ray diflfractometry (XRD) and infi-ared (IR) spectroscopy. Acidity of the catalysts was tested by pyridine adsorption monitored by IR spectroscopy. Self-supported wafers pressed fi-om zeolite powder (thickness 15 mg cm ) were placed in the sample holder and outgassed at 770 K in vacuum (final vacuimi was better than 10 Pa) for 2 horns followed by cooling to room temperature where the spectrum of the activated zeolites were registered. 1.33 kPa pyridine was adsorbed at 473 K for 1 hour followed by evacuation at the same temperature for 1 h. For calculating the concentration of acid sites extinction coefficients available in the literature were used [15]. 

Fig. 9.21 A series of SNIFTIR spectra recorded for pyridine adsorption at the Au(llO) surface using p-polarized light. The base potential , was set to -0.75 V (SCE) and the sample potential 2 varied. Its values are indicated at the corresponding spectrum. The solution was 0.1 M KCIO4 and 0.001 M pyridine in D2O. Taken with permission from Refs. [36] and [40].

Figure 2.38 Acid properties of AI-ITQ-33 zeolite measured by pyridine adsorption and subsequent (attempted) desorption at increasing temperatures. On the left is plotted the hydroxyl stretching region, where (a) is the IR spectrum after thermal treatment at 673 K under vacuum and (b) is the spectrum after adsorbing pyridine followed by desorption at 423 K. On the right is shown the C-C stretching region of the adsorbed pyridine after evacuation at (c) 423, (d) 523, and (e) 623 K. Reproduced from Ref. (462).

As a first example for illustrating the application of Raman spectroscopy in characteri2ing the orientation of surface species, we consider pyridine adsorption on an Ag surface [84], for several reasons. The first SERS experiment was carried out using pyridine as the adsorbed species. Secondly, pyridine has a large Raman cross section, relatively simple molecular structure, and a good assignment of bands appearing in its normal Raman spectrum and SER spectrum. Thirdly, pyridine is an excellent model molecule for surface coordination studies. Eourthly, interactions of the pyridine molecule with the metal surface involve both the it and lone-pair electrons. 

Spectrum c shows the result of pyridine adsorption then evacuation on the activated sample. C-H stretches of the adsorbed molecules give rise to the features at 3200-2900 cm , ... 

Fig. 13. IR spectra after degassing at 400 K and subsequent pyridine adsorption a Na-Y b the mixture BeCl2/Na-Y c sample of spectrum (b) heated at 725 K (after [73], 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... 

Figure 8. Vibrational spectra of pyridine. Upper curve EELS spectrum of PYR adsorbed at Pt(lll) (pH 3) lower curve mid-IR spectrum of liquid PYR (18). Experimental conditions adsorption at -0.1 V from 1 mM PYR in 10 mM KF (pH 3), followed by rinsing with 2 mM HF (pH 3) other conditions as in Figure 4.

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... 

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