Abstract
The acidic properties of MgO, Al 2O 3, and TiO 2 were studied using pyridine adsorption. Infrared spectroscopy and gravimetric adsorption measurements indicate no acid sites on MgO, while Lewis acid sites were observed on the surfaces of Al 2O 3 and TiO 2. Doping Fe onto MgO and A1 2O 3 was shown by Mössbauer spectroscopy to produce only highly coordinated Fe (e.g., sixfold coordination). A small amount of low coordination iron (e.g., fourfold coordination) was observed on TiO 2. Pyridine adsorption measurements showed that addition of Fe on MgO did not generate acidity, whereas iron produced a small number of sites on Al 2O 3 and iron addition caused a selective poisoning and strengthening of the acid sites on TiO 2. All acids in this series of single component and binary component oxides were Lewis acids. A model of Lewis acidity is proposed in that the existence of coordinatively unsaturated cations responsible for the acidic properties can be predicted using Pauling's electrostatic bond strength rules. This model is also shown to be valid for iron cations deposited on SiO 2.
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