Fourier transform IR study of NOx adsorption on a CuZSM-5 DeNOx catalyst

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO₂, NH₃, H₂O). NO₂ adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu²⁺NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N₂O₃ → N₂O₄ → NO₂ → NO₃. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO₂, N₂, O₂, and are only slightly affected by H₂O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NO_x into highly active surface nitrates.