Activation temperature surface hydroxyl

Several authors have proposed that CH4 combustion over PdO occurs via a redox mechanism [82-85]. Methane activation through assisted hydrogen extraction is generally regarded as the rate-determining step, although there is not a general consensus on the nature of the adsorption sites. Further, desorption of H2O by decomposition of surface hydroxyls has been reported to play a key role in reaction kinetics at temperatures below 450 °C [67, 86]. 

Like Cr/silica catalysts, the activity of Cr/AlP04 is improved with increasing calcining temperatures up to about 800°C. Again this probably reflects the condensation of surface hydroxyls, which are believed to interfere with polymerization. 

The photocatalytic activity of MgO, too, is considered to be caused by surface hydroxyl groups. This is shown in an investigation of the hydrogen-deuterium exchange at room temperature 78>. Monochromatic light was used in the range between 1800 and 4000 A and with a band width of 96 A. The reaction is treat-... 

The events are complicated by several factors. On the one hand, there is the loss of physically and strongly bound water in the monolayer with increasing activation temperature. Also, it is not established whether or not this combined water resides as hydroxyl groups. Finally, the response of Ge-O-Ge groups, if formed, to interaction with propanol (or to water) as a function of strain removal by heat treatment (cf. silica and alumina surfaces) is not at all understood. 

Because of the weakness of the Bronsted acidity of the surface hydroxyl groups, one can suppose that the synchronous mechanism is relatively efficient for these substitution reactions. The simplest reaction of this type is H/D isotopic heteroexchange with D-containing molecules. The heteroexchange with water is known to proceed rapidly at a temperature as low as 20°C. With molecular deuterium, such exchange occurs at 350-400°C with an activation energy of 15-20 kcal/mol (55). 

The effect of H2 dosage at room temperature has been summarized in Section III.A, and the analogous isothermal experiments performed at 20 K have been discussed in Section III.C. The room temperature experiment allowed previous authors to single out the presence of Families I and II of surface hydroxyl and hydride species (Fig. 9), and the experiments conducted at 20 K allowed Gribov et al. to observe the transient molecularly adsorbed H2 precursor of Family I species (Fig. 10) and, thus, to establish an energetic scale in the activation barrier of the heterolytic H2 dissociation (Scheme 2) between Families I and II. 

Adhesion is a complex phenomenon based on a number of different mechanisms. In contrast to simple mechanical adhesion, silanes enable a sealant to bond chemically to a substrate, resulting in more durable adhesion. The bond is less susceptible to the negative effects of moisture and temperature. The nature of the substrate surface also plays an important role in achieving good adhesion. The more chemically active sites (preferably hydroxyl groups) the substrate has, the... 

The surface of alumina is covered by five distinct types of surface hydroxyls in their coordination to the aluminum. The total hydroxyl groups of the y-alumn-ina is about 3 /imol/m. Upon heating to temperatures above 200°C, there is a consequent loss of these surface hydroxyls. Even though there is a loss of surface hydroxyls that may participate as weak Brpnsted sites, the activity of the alumina increases with increase of hydroxyl loss, because they are converted into Lewis acidic and basic sites which may act as stronger adsorption sites. The activity of the five types of hydroxyl sites on the alumina is dependent on the amount of water present on the surface. Furthermore, the highest surface activity would be obtained with lesser amounts of physically sorbed water. The presence of Na20, a common impurity of the y-alumina, is known to affect the pH of the y-alumnina to a more basic alumina [3],... 

A more delicate item may be the effect of alloy formation between noble metals and the metal components of the supports. In the case of Pt/Sn02, low-temperature (at 120 °C) reduction is required, which leads to both the formation of Pt-Sn alloys and the formation of surface hydroxyls at the perimeter [68]. On the other hand, in the case of Au/Ti02, vacuum evacuation or reduction dramatically suppresses the initial catalytic activity, which can be recovered gradually during CO oxidation in excess O2. The removal of oxygen species at the perimeter interface is deleterious to supported Au catalysts. 

Mo(CO)3 ads> which is 100% molecularly dispersed. Above 200° C remaining CO groups of Mo(CO)3 ads ar liberated, but detectable amounts of Mo(CO)2,ads Slid Mo(CO)ads do not accumulate. The likely dominant species after activation at 270° C is (a-0")2Mo . At higher temperatures, e.g 500° C, the zero-valent decarbonylated molybdenum is oxidized by the surface hydroxyl groups of alumina to an average oxidation number of about 5.6. Molybdenum hexacarbonyl on highly dehydroxylated alumina, le., pretreated at 950° C, behaves differently Mo(CO)3 a[Pg.114]

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