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Chemically, the combustion of hydrocarbons and atmospheric air is a simple equation. What you put in on an atomic level is what comes back out.
However, air isn't the same in one place as it is in another. And fuel isn't always 100% the same either. Although C8H18 is the atomic formula, there are traces of sulfur and lots of other atoms in the stuff you pump into your tank (and don't get me started on additives!).
So the designer of a fuel and ignition system can't possibly get the ratio right every time. Stoichiometric is a guessing game when it leaves the paper.
Add needs for transition of load/speed, number of ignition events and combustion chamber physics, and it's even more of a challenge to combine every atom in a way that maximizes power and efficiency without causing nitrogen oxides, sulfur oxides, carbon monoxide, and other nasty byproducts.
So the catalytic converter catches the bits that slip through the cracks. A well-designed car could possibly (note that word!) pass an emissions test without one--if everything is perfect. But if not, the catalyst is a safety net.
As the car wears, injectors clog, spark plug gaps grow, a bad load of gas, a stuck thermostat or a bad oxygen sensor can make the catalyst do its job a lot more and you'd never pass without it.
A modern car will NOT pass an emissions test without a catalyst. Even if the rear O2 sensors are disabled so the catalyst warning light doesn't come on, modern vehicles use ignition advance and extra fuel injection to ensure the catalyst stays at the ideal temperature for breaking apart oxides to recombine into water and CO2.
More hydrocarbons in the catalytic converter = higher temps = higher catalyst efficiency (until it melts). So modern cars add that temperature and exhaust flow to ensure cleaner tailpipe gasses.
So it's not junk science, but I don't think it's the healthiest solution to emissions either. It works, and it's cheap compared to other systems that have been proposed.
Your carbon footprint is the same, unless you want to build a diesel-style filtrate system to hold the carbon in place. Atoms in = atoms out (+ energy released in the reaction).
...if you read this far, I'm sorry for the long boring post.
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