Temperature-dependent dynamics of NH3-derived Cu species in the Cu-CHA SCR catalyst

Abstract

The Cu-exchanged CHA zeolite (Cu-CHA) is a promising catalyst for the NH₃-assisted selective catalytic reduction (NH₃-SCR) of harmful nitrogen oxides (NO_x, x = 1, 2), combining high hydrothermal stability with good performance in the 200–550 °C range. Despite many recent breakthroughs in the molecular-scale understanding of this catalyst, several open questions remain to ultimately unravel the NH₃-SCR mechanism across the operation-relevant temperature range. In this context, we apply in situ XAS and UV-vis–NIR spectroscopy to assess the nature and thermal stability of NH₃-derived Cu-species in a commercial Cu-CHA deNO_x catalyst. Both techniques evidence fast and complete ‘solvation’ by NH₃ of the framework-coordinated Cu^II and Cu^I ions formed upon thermal activation of the catalyst. Our results confirm that NH₃ desorption at T > 200 °C is accompanied by Cu^II → Cu^I reduction phenomena, while the compresence of pre-adsorbed NH₃ with gas-phase NO greatly enhances the reduction rate and efficiency. By applying state-of-the-art multivariate curve resolution (MCR) analysis, we elaborate these insights in a quantitative picture of Cu-speciation during NH₃ temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments. MCR analysis confirms recent theoretical predictions for the thermal stability of [Cu^I(NH₃)₂]⁺ species and allows us to experimentally identify the framework-coordinated O_fw–Cu^I–NH₃ intermediate formed upon desorption of a NH₃ ligand from [Cu^I(NH₃)₂]⁺.