Hydrogen–Water Deuterium Exchange over Unsupported Group VIII Noble Metals

Abstract

Specific activities of unsupported powders of all six Group VIII noble metals have been determined for hydrogen – water deuterium isotope exchange. The metal surface areas, which are required to calculate the specific activities were measured by hydrogen chemisorption and were checked by electron microscopy. Specific activities were measured as a function of temperature in the range 353 to 573 K and also as a function of the partial pressure of hydrogen and water at suitable temperatures and over a tenfold range of partial pressures.The variation in the specific activities was Pd < Ir ≤ Ru < Rh < Os < Pt, and these specific activities varied over a range of about 1000. The observed orders with respect to hydrogen and water are shown to be consistent with a mechanism in which chemisorbed hydrogen atoms exchange with physically adsorbed water.From the orders and the apparent activation energies, the chemical activation energies (E0) were calculated. These varied randomly within the range 61 ± 6 kJ mol−1 for all the metals studied. Previously we showed that there was a correlation of E0 with the work function of the metal when metals were supported on a highly graphitized carbon black, and suggested that electron donation from the carbon to the metal was responsible for the correlation. This suggestion is supported by the present results which show that E0 is relatively constant for all six metals in the absence of a support.