when the atmospheric air is exposed to a high temperature approximately 3000 C, nitric oxide is formed with high temperature then when it is cooled down it reacts with oxygen and change into nitrogen dioxide.

The primary reaction occurs rapidly when nitric oxide reacts with the oxygen in air.  If you want to study the reaction in detail, e.g., kinetics, the main issues will be with your gas-handling system and to make sure the nitric oxide is free from oxygen before mixing it with air. Temperature control of the apparatus may not be so easy.  Are you planning to use commercial tanks of nitric oxide or to prepare it chemically?  Pure nitric oxide or mixtures with inert gases (e.g., argon)?  Would something like this (http://www.sierrainstruments.com/products/50series.html) help?

It might be possible to better address your questions if you could send some additional information about your reaction system and your experimental objectives.

Thanks John,

I'm interested in oxidizing NO from flue gas to NO2. Thus, the temp. is around 300 C.

In the lab I have 1% NO balanced in nitrogen. I have tried to oxidize it by bubbling the gas through H2O2 and on different solid catalysts based on active carbon and nothing...

I may have to think a bit more on this, but my first guess is that the NO2 is reacting with water to form nitrite/nitrate and then simply stays in solution as ions.  How are you (trying) to detect the NO2?

I hope to get nitrite/nitrate.

I measure it with gas analyzer (MRU Optima 7) in which I don't see any decrease in NO or formation of NO2.

Can you upload a schematic of the system?

Oxidation of nitric oxide

John, sorry for my drawing skills, but this is a sketch of my system. The reactor for liquid is a gas washing bottle (see picture). For solid catalyst, I am using a glass tube filled with the powder and glass wool at the ends to prevent the powder moving by the gas flow.

Michael, thank you very much for these papers. they are very informative.

You could run the NO gas over potassium permanganate.  This is used as one part of an NO/NO2 scrubber that you can purchase.  The permanganate is adsorbed onto molecular sieves.  The downside is the potential loss of the resulting NO2 due to adsorption to the sieves.