Temperature-dependent dynamics of NH3-derived Cu species in the Cu-CHA SCR catalyst
The Cu-exchanged CHA zeolite (Cu-CHA) is a promising catalyst for the NH3-assisted selective catalytic reduction (NH3-SCR) of harmful nitrogen oxides (NOx, 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 deNOx catalyst. Both techniques evidence fast and complete ‘solvation’ by NH3 of the framework-coordinated CuII and CuI ions formed upon thermal activation of the catalyst. Our results confirm that NH3 desorption at T > 200 °C is accompanied by CuII → CuI 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 [CuI(NH3)2]+ species and allows us to experimentally identify the framework-coordinated Ofw–CuI–NH3 intermediate formed upon desorption of a NH3 ligand from [CuI(NH3)2]+.
- This article is part of the themed collection: Selective catalytic reduction of NOx