Pyridine surface basicity

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Basic nitrogen species present on the surface of activated carbons or carbon fibers, like in the case of H2S, were found to enhance the sulfur dioxide uptake. Polyacrylonitrile (PAN)-based activated carbon fibers are examples of good adsorbents for SO2 removal [82, 92]. Although role of nitrogen present in the carbon matrix was not emphasized by Lee and coworkers [93] in their studies of SO2 adsorption on PAN-based activated carbon fibers, [93] Kawabuchi and coworkers noticed a significant increase in the sorption capacity when activated carbon fibers were modified with pyridine and basic nitrogen functionalities were introduced to the surface [93]. Pyridine provided basic functionality, which increased catalytic removal of SO,. . 

The probe molecules that are used to investigate surface acidity should be chosen accordingly to their ability to accept proton from the surface active site, or to donate electron pair to the solid surface. The molecules that fulfil these demands are, for example, ammonia, pyridine, or hydrocarbons. Similarly, the probe molecules that can be used to trace the basic site of solid catalysts must be able either to donate a proton or to accept electron(s). Importantly, many species (that even do not contain hydrogen in their formula, which is a demand according to Lowry-Brdnsted theory) can function as Lewis acid, accepting electron pair. Hence, the molecules that could be chosen to investigate surface basicity are, for example, dioxides of carbon or sulphur. 

SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

The pH-independence of Tdhpz at Pt indicates that the driving force for coordination of the nitrogen heteroatom to the Pt surface is much larger than that for protonation even in molar acid. This behavior is in contrast to that of pyridine, where protonation of the nitrogen heteroatom in molar acid hinders N-coordination to the surface (H). Such a difference in chemisorption characteristics may be related to the fact that the basicity of the nitrogen heteroatom in pyridine (pKb =8.8) is much greater than that of the nitrogens in pyridazine (pKb = 11.8) (23.). ... 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

Figure 9. A comparison between the catalytic performance through 2,6-xylenol yield, and relative acidity (solid diamonds) obtained from the area of v8a and vl9b bands from pyridine adsorbed on catalyst surface at lOOOC and the relative basicity (solid circles) obtained from the area of the bands between 1700 and 1250 cm-1 from C02 adsorbed on Cul-xCoxFe204 catalysts at 250C.

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

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