Combining computational and in situ spectroscopies joint with molecular modeling for determination of reaction intermediates of deNOx process-CuZSM-5 catalyst case study

Interaction of NO with CuIZSM-5 catalysts was spectroscopically investigated in static (IR and EPR) and flow (IR) regimes, complemented by DFT quantum chemical calculations. Particular attention was paid to the elucidation of the NN bond formation mechanism, which (along with OO bond making) is one of the key issues of a deNOx reaction. The active sites ({CuI}ZSM-5, {CuO}ZSM-5), the intermediates ({CuINO}ZSM-5}, {CuIN2O2}ZSM-5}), and the low-temperature spectators (up to 423 K) ({CuI(NO)2}ZSM-5}) appearing during this process were identified. Their assignment and molecular structure was ascertained by joint use of computational spectroscopy and DFT modeling. A new method of discrimination between the conformers of surface dinitrosyls based on the calculations of the relative IR intensities of the symmetric and antisymmetric vibrations was proposed, and the mechanistic importance of copper dinitrosyl conformation was discussed. The inner-sphere versus outer-sphere mechanistic dichotomy of the NN bond conception was rationalized in terms of the spin density repartition within the CuNO moiety of the mononitrosyl intermediate.