Do all reactants have appear in the first mechanism of the rate law?

I have the following reaction, 
$$\ce{CO + NO2 -> CO2 +NO}$$
The "second" mechanism is given:
$$\ce{NO3 + CO -> NO2 + CO2}$$


So when searching for the first mechanism, I found this one to work:


$$\ce{NO2 + NO2 -> NO3 + NO}$$. 


Would this be considered a valid mechanism? Does it matter at all that one of the reactants does not show up in the first reaction but does so in the second reaction?




Your net equation is


$$\ce{CO + NO2 -> CO2 +NO}.$$


To explain the mechanism, the steps have to add up to the net equation. If you add up the two reactions that you suggested,


$$\ce{NO2 + NO2 -> NO3 + NO}$$
$$\ce{NO3 + CO -> NO2 + CO2}$$


and cancel species that appear on both sides, you get the net reaction. Which step you call first or second is not really important, but in the order I wrote them, you see how the $\ce{NO3}$ shown as a reactant in the second step is produced by the first reaction step.


As Zhe said in his comments, if there is a catalyst you would see species in the equations of the steps that don't even appear in the net reaction. So, for example, if your reaction were catalyzed by platinum, you would write


$$\ce{CO + NO2 ->[Pt] CO2 +NO}$$.


Then, nobody would be surprised if platinum appears in one of the steps of the proposed mechanism. In fact, you would expect it to play some part in the reaction.


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