The channels plugged on the outlet side are called inlet cells - where exhaust gases fall into the filter. In contrast, channels with plugged ends on the inlet side are called outlet cells, where the exhaust gases escape.

The exhaust gases go through such an obstacle must squeeze through the porous walls leaving larger particles trapped inside the plugged channels.

The main function of the catalytic coating is to provide a substrate for catalytic (noble) metals. In addition, the catalytic coating may physically separate and prevent undesirable reactions between components of the complex catalytic system.

Catalytic coating materials include inorganic non-noble metal oxides such as alumina, silicon oxide, cerium oxide, titanium dioxide, zirconium oxide, and zeolites. Some of them are used as catalyst carriers, others are added to the catalytic coating as promoters or stabilizers, while others have catalytic activity.

Good catalytic coating materials are characterized by high thermal stability. The catalytic coating is applied to a monolith with a water-based suspension.

The noble metal catalyst (s) may be present in the catalytic slurry or they are used in a second step called impregnation. During impregnation, the catalytically coated monolith is immersed in an aqueous solution containing catalytic precursors. The catalyst is dried and calcined to its final form. During calcination, the catalyst precursors decompose to form a final catalyst, usually a metal or a metal oxide. The most common catalysts are platinum group (PGM) metals, such as platinum (Pt), palladium (Pd) and rhodium (Rh) alone.

Ceramic (protective) mat
It is wrapped around a monolith. Provides thermal insulation, protection against mechanical shocks and vibrations of the vehicle.