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In chemistry, a catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze. In the human body, enzymes are naturally occurring catalysts responsible for many essential biochemical reactions.

In the catalytic converter, there are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as the materials are extremely expensive. Some of the newest converters have even started to use gold mixed with the more traditional catalysts. Gold is cheaper than the other materials and could increase oxidation­, the chemical reaction that reduces pollutants, by up to 40 percent.

Most modern cars are equipped with three-way catalytic converters. This refers to the three regulated emissions it helps to reduce.

­The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2

honeycomb catalyst
Ceramic honeycomb catalyst structure.

The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O2 => 2CO2

There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.






Mufflers, Ask a Mechanic


Ask a RB Performance Mechanic � Mufflers and Exhaust


Q: My car is older. Lately, it seems to be getting noisier. How do I know if my muffler went bad?

A: Your car�s exhaust system is pretty complex. Excess noise could be from a number of sources in the exhaust system including cracks or holes in manifolds, gaskets, pipes or the muffler itself. To find out for sure where the noise is coming from, just stop by your local RB Performance shop. We can not only find it, we can fix it, too.

Q: How do mufflers actually work? Are mufflers made of special soundproof material?

A: Your exhaust system � and the muffler in particular � is very sophisticated, although you can�t really appreciate it just looking under your car. If you were to cut it open you would likely find what looks to be a bundle of metal tubes with holes in them. And while they might look simple, inside the muffler these tubes work together to force sound waves to hit one another in very precise ways, canceling the sound in the process. With all this hidden engineering, it�s no wonder mufflers can�t be a one-size-fits-all replacement part.

Q: Besides excess noise and rough idling, what are some other things I need to watch out for with my car�s exhaust system?

A: Excessive rust or broken rubber hangers can cause mufflers, pipes, and other parts of the exhaust system to hang extremely low or break, and that can be hazardous, especially to folks driving behind you. However, these aren�t always things that can be easily assessed by just peeking under your car. As a rule, we recommend having your exhaust system professionally inspected at least once a year.