A DFT study on Cu-ZSM-5 as a catalyst for NO decomposition: Possible activity of a Cu(I) pair located at the T3 tetrahedral sites

The decomposition of NO catalysed by Cu+ pairs coordinated at the Al-substituted T3 sites of the 10-membered rings of Cu-ZSM-5 was studied by means of DFT/ONIOM calculations. Three mechanisms for NO decomposition were defined on the active site, all based on the formation and further decomposition of an hyponitrite anion, which gives the N2O molecule and a [Cu‒O‒Cu]2+ bridge, where both copper ions are oxidized. N2O can react with the oxidized active site according to two different spin-allowed mechanisms which give immediately the products, N2 + O2. Otherwise, N2O reacts with a new reduced site according to a spin-forbidden mechanism which gives N2 and a second [Cu‒O‒Cu]2+ bridge. When the latter mechanism occurs, reduced sites are restored by further reaction of [Cu‒O‒Cu]2+ with NO and desorption of NO2, which is finally decomposed on a second [Cu‒O‒Cu]2+ site, giving O2 + NO. Turnover frequencies (TOFs) calculated by means of the Energetic Span model showed that one of the spin-allowed mechanisms should be relatively fast, whereas the spin-forbidden one is expected to be extremely slow. Computational results were discussed with reference to experimental data available in the literature.