If your car is producing particularly black or dark exhaust smoke from the muffler or exhaust system, it could be because of a substantial amount of carbon buildup in your car's engine or a bad catalytic converter.
http://www.carsdirect.com/car-repair/diagnosing-a-faulty-catalytic-converter
The inside of the catalytic converter is a honeycomb set of passageways or small ceramic beads coated with catalysts. A chemical reaction takes place to make the pollutants less harmful. There are many passages for the exhaust gases to flow, to allow for the maximum amount of surface area for the hot gases to pass.
http://auto.howstuffworks.com/question4821.htm
The catalytic converter is a device installed in the exhaust system to significantly reduce the emission levels of hydrocarbons (HC), carbon monoxide (CO), and in the case of most newer catalytic converters, oxides of nitrogen (NOx).
http://www.smogtips.com/cat.cfm
Poor Performance Issues. A faulty catalytic converter can also cause your car to perform less efficiently and produce far lower levels of power. If you experience an inability to drive at normal speeds even when giving the car a lot of gas, there's a good chance that a bad catalytic converter is the problem.
http://www.carsdirect.com/car-repair/diagnosing-a-faulty-catalytic-converter
Your car's catalytic converter is the first line of defense against pollution and global warming causing carbon-based emissions. A faulty catalytic converter can lead to a host of problems that can range from being only a minor annoyance to drastically affecting the performance of your vehicle.
http://www.carsdirect.com/car-repair/diagnosing-a-faulty-catalytic-converter
Location: The catalytic converter exterior is usually made of stainless steel. The CAT looks very similar to a muffler except it contains a catalyst substrate within and is normally located in the exhaust system before the muffler and after the exhaust manifold.
http://www.smogtips.com/cat.cfm
Use Your Nose. One of the surest ways to tell if your car is catalytic converter is bad is to simply use your nose. A faulty catalytic converter will often produce a horrible smell that smells like a combination of rotten eggs and sulfur.
http://www.carsdirect.com/car-repair/diagnosing-a-faulty-catalytic-converter
Your vehicle's Underhood Emissions label can provide you with information regarding the type and requirement of this component. If your vehicle requires a catalytic converter it must be present, installed and functioning properly. 95% of vehicles on the road in California are required to be equipped with a CAT. Operation: The CAT is a catalyst, it typically plays no part on how well your engine runs. The catalytic converter only begins working after exhaust has exited the engine's combustion chambers. When hot exhaust gases are forced through the catalytic converter, they contact the catalyst substrate.
http://www.smogtips.com/cat.cfm
Check the Exhaust System. Assessing the cause of a failing catalytic converter can sometimes be traced back to an error in the fuel system. A distinctive sign of this is a restriction in the performance of the exhaust system. It is indicative of the melting of catalyst materials inside.
http://www.doityourself.com/stry/6-symptoms-of-a-failing-catalytic-converter
Inside a Catalytic Converter. After the engine exhaust gases pass through the catalytic converter, the gases go through the muffler or mufflers, depending on the make of the automobile. Some vehicles use a pre-converter as well, to perform a similar function.
http://auto.howstuffworks.com/question4821.htm
what happens when catalytic converter is bad
A faulty catalytic converter will often produce a horrible smell that smells like a combination of rotten eggs and sulfur.