Proposed mechanism for the catalyzed oxidation of CO by O3: Are NO and NO2 catalysts or intermediates? The steps below represent a proposed mechanism for the catalyzed oxidation of $\ce{CO}$ by $\ce{O3}$:
  
  $$\begin{align}
&amp;\text{Step 1: } &amp; \ce{NO2(g) + CO(g) &amp;-&gt; NO(g) + CO2(g)} \\
&amp;\text{Step 2: } &amp; \ce{NO(g) + O3(g) &amp;-&gt; NO2(g) + O2(g)} \\
\end{align}$$
  
  Which of the following statements is true based on this mechanism?
  
  
  The overall products are $\ce{NO2}$ and $\ce{O2}$.
  The overall products are $\ce{NO}$ and $\ce{CO2}$.
  The overall products are $\ce{NO}$ and $\ce{O2}$.
  Either $\ce{NO2}$ or $\ce{NO}$ can be viewed as a catalyst. (correct answer)
  Both $\ce{NO2}$ and $\ce{NO}$ are intermediates.
  


Based on the given proposed mechanism for the catalyzed oxidation of $\ce{CO}$ by $\ce{O3}$, the answer for above question suggests that either $\ce{NO}$ or $\ce{NO2}$ can be considered as a catalyst.

Thus, I'm very confused as to why the answer is 4. I would think it is 5) because $\ce{NO}$  is produced first and than consumed later on. I understand $\ce{NO2}$  does not have that same pattern, however I thought that it cannot be a catalyst because $\ce{O3}$  is already a catalyst.
My take on this problem is as follows. Suppose a flask contains an equimolar mixture of $\ce{CO}$ and $\ce{O_3}$. The net reaction between the two gases will be 

$$\ce{CO (g) + O3 (g) -&gt; CO2 (g) +  O2 (g)} \tag{1}$$

But it needs a catalyst. If $\ce{NO_2}$ gas is introduced, then the reaction proceeds via the two reactions given in the OP's question:

$$\ce{NO_2 (g) + CO (g) -&gt; CO2 (g) +  NO (g)} \tag{2}$$
and $$\ce{NO (g) + O3 (g) -&gt; NO2 (g) +  O2 (g)} \tag{3}$$

So the nitrogen oxides cycle around. On the other hand, if $\ce{NO}$ gas is introduced, then the reaction proceeds via 'flipped' equations:

$$\ce{NO (g) + O3 (g) -&gt; NO2 (g) +  O2 (g)} \tag{4}$$
and $$\ce{NO_2 (g) + CO (g) -&gt; CO2 (g) +  NO (g)} \tag{5}$$

and again the nitrogen oxides cycle around. So it does not really matter whether $\ce{NO_2}$ gas or $\ce{NO}$ gas is introduced first: the two nitrogen oxides are the overall catalytst and either could be considered to be an intermediate. Basically, this is a distinction without a difference.