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Moderate

Heterogeneous Catalysis

APCHEM-5LJUPN

A common example of heterogeneous catalysis (also referred to as surface catalysis) takes place in the catalytic converter of an automobile. The converter consists basically of a sort of honeycomb structure of platinum and palladium.

The job of the converter is to convert oxides of nitrogen to elemental nitrogen and oxygen gases. The nitrogen is released into the atmosphere. Any carbon monoxide resulting from incomplete combustion of the fuel is converted to carbon dioxide by the oxygen gas.

The catalytic converter in an automobile has an extremely long (essentially indefinite) lifetime. That is, over the normal life of a car (even as much as 300,000 miles or more!) it tends to maintain its effectiveness.

Which of the following is the MOST likely explanation for their observed durability?

A

Platinum and palladium are very large atoms, and there are huge numbers of them in a converter. It takes an extremely long time for them to be completely consumed.

B

As they convert the potential pollutant compounds to "green" products, those products are released from the metal surfaces, leaving a fresh reactive surface.

C

Typically, for a process occurring in the presence of a surface catalyst, the order of reaction is found to be zero. Zero order means the rate of reaction is independent of concentration, so it does not matter how much of the catalyst is still effective.

D

There is so much surface area in the converter that there is always some space available for reaction to occur.