Lewis acidity surface sites

E. Probing Bronsted and Lewis Acid Surface Sites by Adsorbed Molecules... 

A parallel set of changes occurs on thermal desorption of py. It is possible therefore to correlate the relative strengths of the Lewis acid surface sites, as determined from the thermal behavior of the py 8a vibrational modes, with the temperature-dependent changes observed in the -OH stretching region. The correspondences are shown in Table 7.2. 

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

Due to the formation of Ca/Al mixed oxide on the surface, the Ca -modified alumina has a completely different structure compared to the spinel one This leads to a different type of surface Lewis acid/basic sites, rendering the catalyst 30 times less active. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

Correlation of the Stretching Frequencies of Adsorbed CO with the Surface Electric Fields at Lewis Acid (Cr3+) Sites... 

We have seen that cations are exposed by the surface dehydration of rutile. These cus ions act as Lewis acid (acceptor) sites for the adsorption of pyridine, ammonia... 

The surface properties of amorphous silicas are largely influenced by the nature of the surface silanol (SiOH) groups (1-3). Lewis acid-base sites are absent unless the silica has been activated at very high temperatures, Bronsted acidity at the gas-solid interface is low or nonexistent, and the siloxane bridges are relatively unreactive toward most molecules. This chapter discusses some methods that employ chemical modification and H-D exchange to probe the nature of the surface hydroxyl groups on... 

In additional experiments it has been shown that iron is interacting with platinum, i.e., it is located in atomic closeness to Pt. In the bimetallic nanocluster, due to the high electropositivity of iron, there is an electron transfer from iron to platinum. The net result is the formation of electron deficient iron species at the Pt surface. The authors suggested that these electron-deficient or low-valent iron species on the Pt surface might act as Lewis acid adsorption sites. These sites, due... 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

Notably, catalysts with redox properties, such as molybdenum-, chromium-, and vanadia-based catalysts, show high activity in various oxidative dehydrogenation reactions of hydrocarbons [45 8]. Factors influencing the reaction also include acid-base bifunctionality, which plays an important role in CO2-mediated dehydrogenation reactions [49]. Both basic sites and Lewis-acid vacant sites are important for hydrocarbons activation [50]. In fact, an enhanced basicity results in an improved performance because of the rapid desorption of the electron-rich alkenes, whereas Lewis acid sites enhance the dehydrogenation process [51]. In addition, in the presence of CO2 as feed, surface basicity favors the adsorption and reactivity of the acid CO2 molecules [52] (see also previous chapters). 

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