Bridging OH groups

Figure 4.9 Total and projected density of states forthe hydroxylated (top) and reduced (bottom) TiOjfl 1 0) surface, calculated using the B3LYP hybrid functional. The Ti3+ states are localized on (a) the Ti ion between the two bridging OH groups, Tilton (d) the Ti ion nearest to the oxygen vacancy, Tij (b), (c) on a five-...

BET results are reported in Table 19.1. A decrease in the BET surface areas was observed with addition of Pt and Na to zirconia however, the surface areas remained well above 100 m2/g. The activation of reduced defect sites over zirconia has been suggested49 to occur via either (1) the formation of an oxygen vacancy defect or (2) the formation of a type II bridging OH group. The latter is interpreted to be the result of the dissociative adsorption of H20 at the oxygen vacancy sites, or from the dissociation and spillover of H2 from the metal to the oxide surface. TPR experiments49 demonstrated that 2% Pt addition shifted reduction peaks for the zirconia surface to <200°C. 

Lamotte, Lavalley, and coworkers—IR characterization of Type II bridging OH groups and formate species. In the 1980s, Lamotte et al.503 506 also concluded that primarily formate species formed upon CO adsorption to H2-treated thoria. Comparison of the spectra of ceria with the previous studies on thoria gave them a... 

A possible pathway for this scrambling is presented in Scheme 5. First, protons from acidic bridging OH groups of the zeolite catalyst are transferred to the adsorbed octene molecules. Subsequently, carbenium ions are formed, and the =CH2 groups are transformed into CH3 groups. The further scrambling of the C label over the hydrocarbon chain can proceed via species incorporating protonated... 

Figure 20 shows the H MAS NMR spectra of zeolite HZSM-5 (ksi/wai — 21.5) before (a) and after (b) adsorption of acetone-tfg at room temperature. The ll MAS NMR signals at 4.0 and 1.8 ppm are attributed to bridging OH groups (SiOHAl) and silanol groups (SiOH), respectively (Fig. 20a). Upon adsorption of 0.33 mmol...

The conversion of methanol to hydrocarbons (MTHC) on acidic zeolites is of industrial interest for the production of gasoline or light olefins (see also Section X). Upon adsorption and conversion of methanol on calcined zeolites in the H-form, various adsorbate complexes are formed on the catalyst surface. Identification of these surface complexes significantly improves the understanding of the reaction mechanism. As demonstrated in Table 3, methanol, dimethyl ether (DME), and methoxy groups influence in a characteristic manner the quadrupole parameters of the framework Al atoms in the local structure of bridging OH groups. NMR spectroscopy of these framework atoms under reaction conditions, therefore, helps to identify the nature of surface complexes formed. 

As found for H-SAPO-34, the hydration of H-SAPO-37 is separated into two successive steps. At a water adsorption of more than 6mmol/g, only a hydration of Bronsted acidic bridging OH groups occurred, whereas upon further hydration, a coordination of water molecules to Al atoms was found (277). In contrast to the adsorption of water in H-SAPO-34, the adsorption of water at framework Al atoms... 

Hydration of the NH4-form of SAPO-34 and SAPO-37, that is, of materials that were ammoniated at the bridging OH groups, caused a coordination of water molecules exclusively to Al atoms in =P-O-A1= bridges. This process led to a hydrolysis of the framework (220). No hydrolysis of the silicoaluminophosphate framework occurred, provided that not only the bridging OH groups (SiOHAl), but also the aluminophosphate framework (=P-O-A1=) was covered by ammonia. The latter finding may explain the stabilizing effect of preloaded ammonia on silicoalumino-phosphates toward hydration and weak hydrothermal treatments as recently observed for H-SAPO-34 (227). 

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