Hydroxyl population oxides

In this section, we wiU consider the hydroxyl population on some materials of practical interest including oxides, zeolites, and porous phosphates, which have found application as catalysts and catalyst supports as well as metal organic frameworks. 

There are many investigations of the hydroxyl population on various aluminas because of the extraordinary importance of this oxide in catalysis. Noticeably, the hydroxyl spectra hardly depend on the polymorph form, although variations in the relative intensities of the OH bands are observed even with a single polymorph. Compared to oxides like MgO or Zr02,... 

In conclusion, partially dehydroxylated oxide surfaces exhibit a large inventory of surface OH groups and water molecules together with Lewis acidic and Lewis basic sites with coordinative unsaturation (structures II and III of Scheme 1). The hydroxyl population is the souree of protons that cause enhanced surface electrical conductivity and catalytic activity. It is significant that the increase in the conductivity value is paralleled by increases in either the amount of weakly bound protons or their mobility [48]. Almost all metal oxides are active in catalytic isomerization of alkenes, which is one of the least demanding reactions in terms of the requirements for the acid strength of active sites [34]. Studies on several oxide systems show that the activity is lost after extensive dehydration and is partially restored by... 

Silica-alumina is the most popular mixed oxide support, combining characteristic features of silica and alumina, including (i) high surface area, (ii) persistent OH population at high temperature and (iii) strong Lewis acidic sites. The predominant surface hydroxyl groups on silica-alumina are [=Si-OH], while [Als-OH] surface species have not been observed [79, 89, 90]. Note that the silica-alumina bulk is mainly composed of [=Si-0-Si=] along with [=Si-0-Als] moieties (Lewis... 

The causes of the increase in atmospheric methane are not clear. It is possible that the concentration of hydroxyl radical in the atmosphere is diminishing and consequently that the rate of oxidation of methane is decreasing, i.e., the residence time of methane in the atmosphere is becoming longer. Alternatively, with the growth in human population and the intensification of paddy rice production, the increase in methane may be due to larger populations of ruminants and to an increase in anaerobic... 

Inducible NOS (iNOS) Under oxidative stress NOS reacts with superoxide anion to form toxic hydroxyl radicals. Inducible NOS is found in glial cells of the CNS and is induced in response to injury. Levecque and Hague both reported association of iNOS with PD in European and Finnish populations, respectively (Levecque et al., 2003 Hague et al., 2004). Hancock confirmed the association in North Americans, but only in early-onset cases (one member of the family with age-at-onset <40 years) (Hancock et al., 2005). Levecque also found an interaction betw een smoking and iNOS, which Hancock et al. also recently reported in an even larger dataset. (Hancock et al., 2006). 

The existence of such species has been confirmed by IR spectroscopy for CO2 interactions with several metal oxide surfaces.(23-26) Finally, hydrogen can adsorb either heterolytically (H+ on an anion and H- on a cation) or reductively (forming two hydroxyl groups) on oxides (21). Depending upon relative surface populations, desorption is possible in each of these fashions. For heterolytic adsorption, pair-sites are again indicated. 

The effect of pH on 4-chlorophenol oxidation is shown in Figure 4. The results show that oxidation is favored under both acidic and basic conditions. This fact implies that different reaction mechanisms may be operative and that photocatalytic oxidation is affected by both H+ and OH- ions. At high pH, the number of hydroxyl ions on the TiO surface increases because of the abundance of OH- ions, thereby increasing the population of -OH radicals. Hickling and Hill (25) suggested that, at high pH, the adsorbed OH- group can be readily converted to OH radical upon irradiation. 

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