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Pt γ-Al 2 O 3 catalysts treated in oxygen between 100 and 600 °C and in hydrogen at 500 °C were studied by uv-vis reflectance spectroscopy. The formation of different oxidized Pt surface species previously indicated by TPR studies [H. Lieske, G. Lietz, H. Spindler, and J. Volter, J. Catal. 81, 8 (1983)] was confirmed by characteristic uv-vis spectra. The results are used as the basis for a model describing the types of surface reactions and details of the platinum surface species formed in oxygen and in hydrogen, and for a model of the sintering in oxygen. The amount of soluble platinum was found to correspond with the amount of highly dispersed platinum. Hence, only surface platinum atoms are soluble.

The total oxidation of n-heptane in oxygen and in air on Pt Al 2 O 3 has been studied with respect to pretreatment and reaction conditions. Calcination of the samples between 500 and 900 °C does not decrease the activity, although the dispersion is dramatically diminished. Added chloride mainly deactivates dispersed samples. The catalysts are more active in air than in oxygen. Under certain conditions a pronounced oscillatory behavior of the oxidation is observed. The results can be explained by a dual-site model: active metallic Pt competes in the combustion with the less active oxidic surface complex [PtIV]s.

Pt γ-Al 2 O 3 catalysts with and without chloride were treated at different temperatures in oxygen or hydrogen and were studied by temperature-programmed reduction and by hydrogen adsorption. Two surface oxides, α- and β-[PtO2]s, and two chloride-containing surface complexes, [PtIV(OH)xCly]s and [PtIVOxCly]s, could be found and a comprehensive scheme of surface reactions is proposed. Redispersion of Pt in oxygen is possible only in the presence of chloride and is connected with the formation of [PtIVOxCly]s. A model for the redispersion is proposed.