I read a variety of replies to the question from Chris, many well thought out and some, like the one below, remind me that there are still some that think the world is flat :-).  A catalyst doesn't create any new compounds it just allows a chemical reaction to take place at a lower temperature than it otherwise could.  At any rate it is a good question, and catalysts are not being ignored because they are a bad idea.  It's just the difficulties involved.  First, as some suggest, they add weight and cost and as most anything added, potentially reduce reliability.  All normal catalysts are adversely affected by lead contamination and one tank of leaded fuel will probably make the catalyst ineffective.  But there are real benefits and they depend on the type of catalyst and the engine operation.  Any catalyst will work in the rich mode (reducing), lean mode (oxidizing) and at stoiciometric (both simultaneously), but some work better in one mode than another.  Stoiciometric is the mixture that has the chemically correct amount of fuel and air, just rich of peak EGT.  In the rich mode oxides of nitrogen (NOX) will be "reduced", creating nitrogen and oxygen, which then reacts with the hydrocarbons to create CO2 and water.  In the lean mode it will oxidize CO and HC, creating CO2 and water, but ignores NOX.  In the stioc mode it does both.  Catalysts are typically very effective, reducing the above pollutants by at least 90%.  Even in the lean mode and engine will emit significant amounts of HC - about 300ppm.  In the rich mode HC and CO emissions can be very high - at least 10 times what they are in the lean mode, but a catalyst cannot oxidize in the lean mode.  So the first step to take if you are concerned about emissions is to operate LOP.  Only one problem - operating lean of stoic will increase the production of NOX to several thousand ppm.  Peak EGT will produce maximum NOX.  Then why not use an O2 sensor and operate at stoic, like all modern cars?  That would work just fine, except for the potential lead contamination.  Unlike some posts would indicate, there is no such thing as "complete combustion" and an engine operating in any regime, whether in a car or aircraft will produce significant amounts of HC, CO and NOX.  The Feds have left us alone because we don't produce very much and the difficulty of improving things is large.  Incidentally, the crankcase breather probably emits half of the HC produced by the engine.  Feed it back to the intake and you've done a lot to reduce emissions.  Have I done it?  No, because even a simple thing like that creates a number of problems.

Gary Casey