The overall reaction provided is as follows:
NO₂ + CO → NO + CO₂
The reaction proceeds under the following two step mechanism:
Step 1: NO₂ + NO₂ → NO + NO₃ (slow)
Two molecules of nitrogen dioxide react to form NO and NO₃ which is the slow step in the mechanism. The NO₃ that is formed goes on to react with the carbon monoxide that leads to the formation of NO₂ and CO₂ and this is the fast step in the mechanism.
Step 2: NO₃ + CO → NO₂ + CO₂ (fast)
Now we combine both reactions to get the overall reaction:
NO₂ + NO₂ → NO + NO₃
NO₃ + CO → NO₂ + CO₂
NO₂ + CO → NO + CO₂
The overall reaction agrees with the reaction provided in the question.
