Metal oxide surfaces hydroxylation/dehydroxylation

From recent literature data, there are many cases in which the resulting surface products, after anchoring, are not in a zero-valent state but rather form a mixture of species so that the metal is both in zero-valent and partially oxidized states. Most studies on functionalized supports have been made on hydroxylated oxide surfaces. The metal particles produced on such supports are not of an unusually high degree of dispersion. The nature of the decomposition process on dehydroxylated supports has not been as thoroughly studied as on hydroxylated supports. Preliminary results, however, indicate that formation of highly dispersed zero-valent metal particles may be obtained (64). 

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... 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

In conclusion, we have found that AU/Y-AI2O3 catalysts can be deactivated both thermally and by CO oxidation. Successful regeneration of a thermally deactivated catalyst is accomplished by exposure to H2O, whereas a reaction-deactivated catalyst can be regenerated by exposure to H2 at room temperature. The latter can be regenerated also by the selective CO oxidation reaction, which is conducted in the presence of H2. The results can be explained with a reaction mechanism in which the CO oxidation reaction proceeds via the formation and decomposition of a surface formate and bicarbonate and an active site consisting of an ensemble of Au-hydroxyl group and metallic Au atoms. Deactivation is due to dehydroxylation of the Au-hydroxyl group or the formation of a rather inactive carbonate. 

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