You need to spend a bit of time learning chemistry.
Just because there are two oxygen atoms bonded to a carbon atom (CO2) it does NOT mean that the oxygen is free to react as normal, diatomic oxygen (O2) would.
Both gasses are inert (for our purposes). Nitrogen tends to be drier from the bottle, but CO2 can be had that is just as dry as nitrogen.
We fridgys have started using CO2 again as a refrigerant for low temp systems. At -23F it had damned well better be dry. Don't believe me? Google R744.....
What I like about CO2 is that you get one heck of a lot more gas in the cylinder due to it being liquified. A little moisture doesn't scare me a bit as I know that I will be pulling a deep vacuum and installing new driers.
LOL...where was Uncle when this thread popped up.
I am going to share my experiences and what I learned from Uncle Clover.
We have a 65lb tank od Bone Dry co2. It is a CLass purity and is beter for sweeping systems because you get all liquid in the tank. We hace a co2 pump we use to fill 15lb aluminum tanks for field use.
I have done five large tonnage resi installs with 25 to 50 ft line sets with Bone DRy and had enough for a few more jobs. Nitro can not match that. Bone DRY co2 is processed for welding as a shield gas. The only advantage Nitro has (and it is a slight one) is the specific gravity is lower than co2.