The Influence of the Substrate on Hydrogen–Water Deuterium Exchange over Carbon Supported Platinum

Abstract

Rate data for hydrogen–water deuterium isotope exchange are reported for a number of platinum on carbon catalysts whose carbon supports were graphitized to varying degrees before the metal was deposited on the support. The exchange over platinum black was studied for comparison. Over carbon supported catalysts, the rate was almost independent of hydrogen pressure and increased approximately linearly with water pressure. The rate controlling step seems to be the transfer of deuterium between chemisorbed hydrogen atoms and physisorbed HDO.Activation energies for the rate determining step decreased from 14 kcal/mol over unsupported platinum to as low as 2 kcal/mol over highly graphitized supports. Rates increased with heat treatment temperatures up to 2100 °C. These effects are ascribed to electron transfer from the carbon support to the metal, resulting in a weaker Pt—H bond, and faster exchange. However they may be partially the result of non-linear adsorption of water.