The reaction of tetrakis(dimethylamino) ethylene (TDE) and oxygen in the presence of a hydroxyl source is a well known chemiluminescent reaction. We have previously reported that metal oxides could be used to enhance this reaction, and attributed this enhancement to the presence of hydroxyl groups. However, as the morphology of a metal oxide changes, the nature of hydroxyl groups changes. For example, small metal oxide crystallites have more corners and edges, which lead to coordinatively unsaturated sites. Hydroxyl groups at these sites will likely have different reactivities than hydroxyls on coordinatively saturated sites. We have studied TDE chemiluminescence in the presence of nanocrystalline MgO as a possible means to enhance TDE chemiluminescence, but also to investigate how differences in surface reactivity for different hydroxyl groups.

The type of hydroxyl groups present on nanocrystalline MgO was characterized using FTIR. Isolated hydroxyls at 3-coordinated (3C) and 4-coordinated (4C) Mg2+ cations were noted at 3740 and 3695 cm-1, respectively. The band intensity of the 4C hydroxyl groups and the hydrogen bonded hydroxyl groups, a broad band at 3660 to 3100 cm-1, decrease with the increase of treatment temperature for nanocrystalline MgO. As the treatment temperature was raised to 673K, the band of 4C hydroxyl groups disappeared completely. Meanwhile, the band intensity of 3C hydroxyl groups remained constant until 773K. Treating nanocrystalline MgO in steam led to a sharp and intense peak at 3695 cm-1, and a much smaller, broad peak due to hydrogen bonded hydroxyl groups. The number and types of hydroxyl groups for nanocrystalline MgO treated at different temperatures was further characterized by interaction with triethylaluminum (AlEt3).

Pretreated nanocrystalline MgO was tested for its ability to catalyze TDE chemiluminescence. The emission intensity and duration of TDE chemiluminescence was measured with a spectrometer. It was found that the enhancement of TDE chemiluminescence is independent from the total residual hydroxyl groups but strongly influenced by the hydroxyl groups adsorbed on lower coordination Mg2+ cations (especially 4C hydroxyl groups). The maximum emission intensity from TDE chemiluminescence catalyzed by thermally pretreated nanocrystalline MgO was gradually decreased with increasing pretreatment temperature. A 7.2 times enhancement in maximum emission intensity was observed when TDE chemiluminescence was catalyzed by steam-treated MgO (which had extensive 4C hyroxyl groups) versus being catalyzed by nanocrystalline MgO treated at 673K (a temperature at which no 4C hydroxyl groups were detected). Nanocrystalline MgO lost its ability to catalyzed catalyzing TDE chemiluminescence when the pretreatment temperature was over 673K. For catalysts treated under those conditions, TDE chemiluminescence in the presence of nanocrystalline MgO was nearly identical to TDE chemiluminescence without the presence at nanocrystalline MgO.

These results show that surface reactivity of hydroxyl groups significantly change based on the saturation of the sites upon which the hydroxyl groups were adsorbed. Furthermore, nanocrystalline metal oxides offer unique chemical reactivity because of the higher number of corners and edges that lead to isolated hydroxyl groups on coordinatively unsaturated sites. Finally, these results suggest that TDE chemiluminescence can be used as a probe of the types of hydroxyl groups present on metal oxides.