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I am a complete neophyte to Stirling engines, just beginning my
reading to become somewhat literate. I have read about various
operating gases: air, O2, N2, He. Even freon. CO2 has exceptional
compression and expansion properties depending on heat/cold. Has
research ever been done with CO2 in Stirling engine use?

thanks anyone for response.

i dont know im also keen to get the answer for the same. did u know the answer

The working gas has been studied in depth, including by real government labs. The answer is that the lightest gases of hydrogen and helium are the best.

Hydrogen, number one theory wise, has issues with exploding when being handled by people, and does strange things like leaks slightly right through metal. On future large scale commercial Stirlings, this is not a big issue, as it can be made and pumped back in.

Helium is best for small projects, as it is almost as good as hydrogen, but a lot safer.

For all practical purposes, though, you can just use good old air. There are bigger issues to resolve with Stirling engines, and the small margin gained by changing working fluid is not yet important.