Abstract
Hydrogen-deuterium exchange in methanol over Al2,O3, platinum black and alumina supported platinum catalysts have been investigated by means of mass spectrometry. The hydrogen-deuterium exchange proceeds faster over Pt/Al2O3 than either platinum black or alumina, and the rate (as well as the rate of dimethyl ether formation) increases with platinum dispersion. From the reactions CH3OH + D2, CD3OH + H2 and CD3OH + D2 the different exchange rates of the CH3 and OH groups (R1, and R2, respectively) were determined. For all catalysts studied the exchange in the OH group was two or three orders of magnitude faster than CH3 group. Exchange in the methyl group needs higher activation energy: R1/R2 ratio increases with temperature. Catalyst pretreatment has an important effect: higher activation temperature results in a higher R1/R2 ratio. A methematical method is suggested for the evaluation of exchange rates of a molecule with two different types of hydrogen and a model is given for the interpretation of measured data: methanol is activated mainly on Al2,O3, forming surface methoxide groups on the Lewis acidic sites. This methoxide can react with deuterium atoms having been activated over platinum sites. With increasing dispersion the metal-support interface is enhanced which leads to higher rate of exchange.
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