On the Mechanism of the H–D ExchangeReaction in Ethane over Platinum Catalysts

Abstract

The catalytic exchange of hydrogen atoms for deuteriums inethane was studied over a platinum foil between 540 and 640 K byusing a closed-loop microbatch reactor with mass spectrometrydetection. An activation energy of 27.1 kcal/mol and kineticorders of −0.55 and 1.00 with respect to deuterium and ethane,respectively, were obtained on the platinum foil, in agreementwith previous reports on other forms of platinum. The focus ofthis study was on the identification of surface intermediates byperforming a detailed analysis of the resulting products. Theexchange product distribution was in all cases U-shaped, withmaxima at the singly and fully deuterated ethane molecules,again the same as on supported platinum catalysts and onplatinum films and (111) single crystals. The uniqueness of thework reported here is that it describes the first completequantitative determination of the distribution of symmetric andasymmetric deuterium-substituted products. In particular,13C-NMR was used to determine that the yield forCH2D–CH2Dis more than twice that forCH3CHD2, a result that suggests thatadsorbed ethylene is one of the main intermediates in themechanism for complete exchange. A sequence of steps thatincludes the formation of ethyl, ethylene, and ethylidynesurface intermediates is discussed.