So as to allow the conversion of the gases in the catalytic converter, in most of the cases the gases need to go through a ceramic substrate.
This ceramic substrate has a structure made of cells with a catalytic layer that contain elements such as Pt, Pd and Rh.
These elements allow the oxidation and reduction to enable the conversion of the adverse gases.
For these reactions to happen is necessary to get very high temperature within the catalytic converter. This is the main reason why nowadays they do place the catalytic converter near the engine.
Moreover, depending on the design, some tubes, flanges, and even lambda sensor can also be added to monitor the catalytic converter.