One of the most successful fuel cell uses the reaction of hydrogen with oxygen to form water. The cell was used for providing electrical power in the Apollo space program. The water vapors produced during the reaction were condensed and added to the drinking water supply for the astronauts. In the cell, hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide solution. Which of the following can be used as catalysts in order to increase the rate of electrode reactions in this fuel cell?