Ammonia-Containing Species Formed in Cu-Chabazite As Per In Situ EPR, Solid-State NMR, and DFT Calculations

Nowadays, the most attractive technology for the elimination of nitric oxides from the exhaust gas of diesel vehicles is the selective catalytic reduction with ammonia (NH₃–SCR–NO_<i>x</i>) using Cu zeolite with the chabazite structure as the catalyst. Isolated copper species are the active sites, but the reaction intermediates and the overall reaction mechanism are still under debate. Here, we study the interaction of ammonia with zeolite Cu-SSZ-13 (CHA topology) with a uniform distribution of Cu²⁺ sites prepared in one pot and a conventional Cu-ZSM-5 (MFI topology) for comparison. In situ EPR and solid-state NMR spectroscopies combined with DFT calculations have allowed the identification of NH₄⁺, [Cu­(NH₃)₅]²⁺, [Cu­(O_f)₂(NH₃)₂]²⁺, [Cu­(O_f)₃NH₃]²⁺, [Cu­(NH₃)₂]⁺, and [CuO_f(NH₃)]⁺ (O_f being framework oxygen) under different conditions. The results demonstrate that ammonia is able to reduce Cu²⁺ to Cu⁺ and provide new information on the species formed in Cu-SSZ-13, which have important implications for the elucidation of the SCR reaction mechanism