Surface basidty

LaValley, J.C. (1996) Infrared spectro-metric smdies of the surface basidty of... 

Independent studies have shown that corona treatments increase both the surface acidity and basicity. Ty ncally, add indexes are increased from 8.7 to 18.9, while surface basidty parameters are increased by corona treatment fiom 24.5 to 27.4. A major increase in the acid/base parameters would not necessarily affed interactions with a neutral matrix sudi as PE on the other hand, acid/base interactions bdween filxes are practically doubled as a result of corona treatment... 

Finally, a more precise method for the characterization of the acidity and the basicity of the active carbon sites is needed. TPD of weak acids and bases may contribute. Basidty is probably more important, since we have shown that i) global surface basicity is more affected by nitric acid treatment than surface acidity, and ii) the amount of the different surface acidic sites is not directly related to the catalytic effects. 

Nevertheless, we have to be aware that large differences exist betwem different commercial silicas. Muller (13,14) measured the pH of suspensioiis of 5% silica in water. This is a measure of the surface acidity or basidty. The results are shown in Table 9.2. Subsequent measurement of the sodium contort of some of the silicas demonstrated t at least for some of the rilicas a high sodium content was responsible for the alkaline pH of the suspension. 

The basicity has been proposed to be one of the fundamental criteria for a good reactivity in oxidative methane coupling (16, 17). Two methods have recently been described (3, 5, 18) for basicity measurements. One is through benzoic add (Bronsted basidty), the other one through the adsorption and the desorption (TPD) of CO2 (Lewis basidty). The last one has been used by sending CO2 pulses on the surface at 20°C followed by TPD (see experimental). The amount of CO2 adsorbed at 20°C on the ASnOs (1) and (II) perovskites are given Table 4. 

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