On the mechanism of the H-D exchange reaction in ethane over platinum catalysts

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-loop microbatch reactor with mass spectrometry detection. An activation energy of 27.1 kcal/mol and kinetic orders of -0.55 and 1.00 with respect to deuterium and ethane, respectively, were obtained on the platinum foil, in agreement with previous reports on other forms of platinum. The focus of this study was on the identification of surface intermediates by performing a detailed analysis of the resulting products. The exchange product distribution was in all cases U-shaped, with maxima at the singly and fully deuterated ethane molecules, again the same as on supported platinum catalysts and on platinum films and (111) single crystals. The uniqueness of the work reported here is that it describes the first complete quantitative determination of the distribution of symmetric and asymmetric deuterium-substituted products. In particular, 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 complete exchange. A sequence of steps that includes the formation of ethyl, ethylene, and ethylidyne surface intermediates is discussed. (C) 1996 Academic Press, Inc.