One of the main disadvantages of vehicles powered with diesel engines are the emissions of  nitrogen oxides and soot particles. In this context, the urea-selective catalytic reduction  (SCR) process is considered to be the most promising way to decrease  NOx emissions produced by the diesel heavy duty vehicles. When urea is used as reducing agent, it is hydrolised into ammonia and carbon dioxide. The ammonia so produced reacts with NO to form the non contaminating  byproducts water and nitrogen. The commercial catalyst based on vanadium (a toxic element) tends to form N₂O, a noxious greenhouse gas, at high reaction tempertaures. We have recently found the natural mordenites based catalysts and porous phosphate heterostructures (PPH), with copper or iron species as active phases, are execllent for the SCR -NO using ammonia or methane as reducing agent, and the very low formation of N₂O was observed. In this communciation, this catalytic reaction was study by means of X-ray Absorption Spectroscopy (XAS) using synchrotron radiation, where the chemical environment of Cu and Fe under real operation conditions is checked, since the oxidation state of both elements during the catalytic process is a subject of many controversies. Moreover,  the evolution of the chemical state duting the SCR of NO give us some answers to explain why N₂O is not formed with these catalysts, in comparison with some other systems. 

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