Oxide surface hydroxyl groups

This prolongs tire lifetime of the hole thus increasing the probability to oxidize surface hydroxyl groups or adsorbed water to yield a hydroxyl radical ... 

Strongly binding anions, such as molybdate, sorb on oxides by a ligand-exchange mechanism that involves the exchange of an oxide-surface hydroxyl group (SOH) for an aqueous anion (A ) (Stumm, Kummert, and Sigg, 1980 Sposito, 1984 Stumm, 1992). This reaction results in the formation of an inner sphere complex and is illustrated as follows ... 

The hydrogen-bonding interactions within the complexes W2CI4-(y-OR)2(OR)2(ROH)2 and V Cli y-OR)2(ORT)2(Rf0H)2 may provide the molecular analogues with which to model the structure and reactivities of transition metal oxide catalysts that possess surface hydroxyl groups. The thermal treatment which is often carried out in the pretreatment of metal oxides (leading to the loss of -OH... 

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

Another key contribution of the Schwarz group was the recognition of the dramatic influence of oxide surfaces on bulk solution pH. In a landmark 1989 paper, Noh and Schwarz [7] demonstrated the method of mass titration, in which successive additions of oxide cause stepwise shifts in solution pH. This procedure is illustrated in Figure 6.7 [7], As indicated in Figure 6.1, the protonation-deprotonation chemistry of the surface hydroxyl groups is coupled to the liquid-phase pH. In mass titration, as the mass (or more appropriately, the surface area) of oxide in solution increases, the solution pH is brought to the PZC of the oxide, at which point no driving force for proton transfer exists... 

Oxides, especially those of Si, Al and Fe, are abundant components of the earth s crust. Hence a large fraction of the solid phases in natural waters, sediments and soils contain such oxides or hydroxides. In the presence of water the surface of these oxides are generally covered with surface hydroxyl groups (Fig. 2.1). 

These functional groups contain the same donor atoms as found in functional groups of soluble ligands i.e. the surface hydroxyl group on a hydrous oxide has similar donor properties as the corresponding counterparts in dissolved solutes, such as hydroxides, carboxylates, e.g., (S-OH is a surface group)... 

Fig. b shows a schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ("s"), inner-sphere complexes ("a"), outer-sphere complexes ("P") and the diffuse ion swarm ("d"). (Modified from Sposito, 1984)... 

Similarly, the rate of oxygenation of Fe(II) bound to the surface hydroxyl groups of a hydrous oxide can be expressed in terms of the surface species. Thus,... 

In aqueous suspensions, metal oxides have amphoteric properties (4). One can describe the adsorption/desorption of H+ and counterion on surface hydroxyl groups (SOH) using the following mass action equations ... 

Photogenerated holes mineralise the organic pollutant. This initially involves the oxidation of surface hydroxyl groups (TiOH) to surface hydroxyl radicals (TiOH +), which then oxidise the organic pollutant and any reaction intermediates. 

Surface hydroxyl groups are to be expected on all metal oxides. Only a few of the many insoluble oxides have been studied in this respect so far. With advances in the techniques of determination of traces of impurity elements or groups by physical methods or by micro-analytical methods in combination with skillful preparative techniques, further progress in our knowledge of the surface chemistry should be expected. Interesting technological applications may be foreseen. 

---