Basic surfaces

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

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. ... 

Lavalley, J.C. (1991) Use of probe molecules for the characterization of the surface basicity of divided metal oxides, Trends Phys. Chem., 2, 305. 

Lavalley, J.C. (1996) Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 377. 

Another important factor affecting carbon deposition is the catalyst surface basicity. In particular, it was demonstrated that carbon formation can be diminished or even suppressed when the metal is supported on a metal oxide carrier with a strong Lewis basicity [47]. This effect can be attributed to the fact that high Lewis basicity of the support enhances the C02 chemisorption on the catalyst surface resulting in the removal of carbon (by surface gasification reactions). According to Rostrup-Nielsen and Hansen [12], the amount of carbon deposited on the metal catalysts decreases in the following order ... 

Table IV. Surface basicity (0g), posTtional (kj 2) and geometric (kc t)isomerization constant of 1- and 2-Butene at 297 K and energy of activation for diffusion of 1-Butene adsorbed on SnSbO catalysts.

R = H or Si are generated. An increase in the surface basicity (molar ratio Na2C03 or K2C03 0s = 10-20 1) leads to anionic [Os(CO)3(OR)2UOR) " (R = H or Si= m > 1) species up to [Os(CO)3(OH)3]". The low reactivity of this latter species explains why low yields of carbonyl clusters are obtained when adding to the siUca surface a stronger base such as an alkali hydroxide instead of an alkali carbonate [14, 60]. 

Surface Basicity and Base Strength of Aikaiine Earth Metal Oxides... 

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [166]. The most common acidic probe molecules used are CO2 (p/fa = 6.37) and SO2 (p/fa = 1.89). Carboxylic acids such as acetic acid can also be used but dimmers can be formed, particularly at high coverage. Pyrrole may also be used, particularly at low adsorption temperature, but has sometimes shown some amphoteric character [103]. Hexafluoroisopropanol has also been used to characterize the surface basicity of some solids [145]. 

This is the case as shown in Table 1, runs 4,5, provided that basic sites are free from adsorbed H20 and C02 (compare runs 3 and 4, Table 1). Notably, once again the ratio hexan-3-one/hexen-3-ols appears unaffected by the catalyst precursor and/or pretreatment (runs 3,4, Table 1). These observations suggest that in the transfer hydrogenation of 4-hexen-3-one, the substrate is coordinated on a weak acid site while propan-2-ol must be coordinated on an adjacent surface basic site [7,24]. This is confirmed by the lack of reduction products observed over Mg(OH)2 and MgCl2 (runs 8,9, Table 1). 

With the aim of suppressing the concurrent condensation reactions a number of doped MgO catalysts was prepared and their catalytic activity investigated (Table 2). Surface basicity of these catalysts was measured by means of temperature programmed desorption of irreversibly adsorbed C02 (see methods section). Desorption peaks in the TPD experiments are considered to appear at higher temperatures as the basic sites on the surface... 

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