ON THE MECHANISM OF THE H-D EXCHANGE-REACTION IN ETHANE OVER PLATINUMCATALYSTS

The catalytic exchange of hydrogen atoms for deuteriums in ethane was studied over a platinum foil between 540 and 640 K by using a closed-l oop microbatch reactor with mass spectrometry detection. An activation energy of 27.1 kcal/mol and kinetic orders of -0.55 and 1.00 with res pect to deuterium and ethane, respectively, were obtained on the plati num foil, in agreement with previous reports on other forms of platinu m. The focus of this study was on the identification of surface interm ediates by performing a detailed analysis of the resulting products. T he exchange product distribution was in all cases U-shaped, with maxim a at the singly and fully deuterated ethane molecules, again the same as on supported platinum catalysts and on platinum films and (111) sin gle crystals. The uniqueness of the work reported here is that it desc ribes the first complete quantitative determination of the distributio n of symmetric and asymmetric deuterium-substituted products. In parti cular, C-13-NMR was used to determine that the yield for CH2D-CH2D is more than twice that for CH3CHD2, a result that suggests that adsorbed ethylene is one of the main intermediates in the mechanism for comple te exchange. A sequence of steps that includes the formation of ethyl, ethylene, and ethylidyne surface intermediates is discussed. (C) 1996 Academic Press, Inc.