So what is...?

In automotive catalytic converters one generally finds Platinum/ Rhodium or Platinum/ Palladium compounds.
 

Pt		Platinum
		Platinum is a silvery-white metal that is expandable and has a melting point of 1772 degrees Celsius. Its boiling point is set at 3827 degrees Celsius. Thinly spread Platinum-constructs like the Platinum sponge serve as the catalysing basis, or the converting element, of organic compounds. The automobile sector constitutes the largest area for industrial use of Platinum. In 1995, 47.7 tons constituting 32% of overall Platinum use were used in the construction of automotive catalytic converters. This is the largest percentage ever, with continually rising figures.
Pd		Palladium
		Also referred to as "light Platinum" is silvery-white and is also expandable. It possesses excellent catalysing properties, and is able to absorb 9.5 times as much hydrogen correlative to its mass, when brought to a temperature of 1000 degrees Celsius. Palladium is used in combination with Platinum in the field of emissions-catalysation. During the production of Hydrogen peroxides industries widely use un-mounted Palladium-catalytic as well as mounted Palladium-catalytic converters, depending on individual production procedures.
Rh		Rhodium
		Rhodium also belongs to the metallurgical order of the Platinum-metals. The high catalysing properties of all metals included in the order of Platinum-metals defines their importance in the field of catalysing systems; most attention is given to the mounted catalysing systems, which one finds on cars.