Does anyone know why the automotive industry settled upon Palladium as the metal of choice to use in their catalytic converters?
IIRC, it’s cheaper than and does just as well, if not better than, platinum.
Actually, modern catalytic converters have two different catalysts that serve different purposes. One stage uses a rhodium-based catalyst to reduce NO (nitric oxide, one of the major contributors to smog) back to nitrogen. The other stage uses either palladium or platinum (or both) to oxidize carbon monoxide (a byproduct of incomplete combustion) to carbon dioxide and also to break down hydrocarbons (another major contributor to smog) into CO[sub]2[/sub]. These reactions actually occur in the same chamber, filled with a mixture of the two catalysts. Earlier catalytic converters contained only the palladium- or platinum-based oxidation step, possibly because the price of rhodium was once extremely high (but has declined sharply since 1990), and possibly because of the increasing focus on nitrogen oxides as a major source of smog.
I won’t get into why Pd and Pt are good choices for oxidation catalysts, but they are, and Rh is also a very good choice for a reduction catalyst. Other metals, such as copper or chromium, could be used in the oxidation step; presumably cobalt might serve instead of rhodium. However, the amounts of the valuable metals in a catalytic converter is not all that large, and they do their jobs very well. Once the engine has warmed up, the converter removes up to 80-90% of the major pollutants in the car’s exhaust. (Their effectiveness is strongly dependent on exhaust temperature.) This level of efficiency justifies the relatively high costs of the metals used.
I’m doing this from memory. It is my understanding that in the US, platinum is the usual choice for catalytic converters.
Palladium is more commonly used in the far east.