Platinum Metals Review - Volume 35, Issue 4, 1991

In many chemical reactions catalysed on platinum surfaces it is necessary that two reactants be adsorbed simultaneously. Often one reactant is so strongly adsorbed that it blocks the adsorption of the second; such a reaction is said to be self-poisoned. An example is the oxidation of carbon monoxide, where carbon monoxide forms a strongly adsorbed monolayer which effectively blocks the adsorption and decomposition of oxygen. Photoelectron microscopy shows, however, that oxygen can penetrate the carbon monoxide film at special defect sites, typically inclusions or microdust particles, on the platinum. From these special adsorption sites the oxygen rapidly reacts with neighbouring adsorbed carbon monoxide. Reaction fronts initiate at these sites and rapidly propagate across the surface. A second type of self-poisoning occurs in decomposition reactions for which vacant surface sites are necessary; for instance, the decomposition of nitric oxide in the presence of hydrogen. A monolayer film of nitric oxide poisons the reaction not by blocking the adsorption of hydrogen, but rather by preventing the dissociation of nitric oxide which requires a neighbouring unoccupied surface site. Empty sites are provided on impurity particles which weakly adsorb nitric oxide and initiate reaction fronts. Impurity sites also initiate reaction fronts when graphite is removed from platinum by oxidation. In order to avoid self-poisoning in catalytic reactions, these studies suggest that special adsorption sites should be introduced artificially to provide vacant sites by adsorbing only weakly the reactants causing self-poisoning.

When nuclear fuel is irradiated in a power reactor a wide range of chemical elements is created by the fission of uranium and plutonium. These fission products include palladium, rhodium and ruthenium, and could in principle constitute a valuable source of these three metals. Their separation front the fuel during reprocessing operations is, however, a complex mutter. Various processes have been proposed and evaluated, mainly on a laboratory scale. To date none of them has been established as applicable on a commercial scale, but investigations with this aim are continuing in several countries. Even a complete separation of the platinum group metals from other nuclides would yield a radioactive product, because of the presence of active isotopes of the platinum group metals. These would be expected to restrict the practical utilisation of platinum group metals created by nuclear fission, unless an isotope separation technique can be developed, or the metals are stored until the radioactivity has decayed.