SORPTION OF GASEOUS HYDROCARBONS AT FUEL CELL CATALYSTS OF THE PLATINUM METAL GROUP

The chemisorption of propane and n-pentane on Raney platinum in the presence of varying amounts of pre-adsorbed water was studied at 100C by a combined gravimetric and mass spectrometric technique. It was found that the amount of saturated hydrocarbon chemisorbed is independent of the amount of pre-adsorbed water. If water is present on the surface, a portion of it is displaced upon chemisorption of hydrocarbons. At high coverages of the surface with pre-adsorbed water the amount of displaced water is constant. The ratio of displaced water molecules to chemisorbed propane molecules is about 1 with propane and 0.6 with n-pentane. If oxygen is chemisorbed on the catalyst surface before water and/or hydrocarbon is admitted, propane chemisorption is strongly enhanced. At a coverage of 1 oxygen atom for every 2 platinum surface atoms, the subsequent propane chemisorption is increased by a factor of about 2.5. Moreover, if on top of the oxygen layer water is adsorbed, the ratio of displaced water molecules to chemisorbed propane molecules is increased to 2. A qualitative scheme of surface reactions which is consistent with the observations is presented and the implications for the hydrocarbon fuel electrode are discussed. (Author)