The 17O resonance from oxygen atoms directly bound to Cu II species in a low copper loaded Chabazite is reported. Water has been shown to play a strong effect on the reactivity of Cu species in zeolites promoting specific reaction pathways 29, 30, 31, promoting dynamic catalytic mechanisms at the cross road between homogeneous and heterogeneous catalysis 32, 33, 34. Here we show that the 17O isotopic labelling of the framework provides a unique source of information about the local binding environment of the active site enabling the rationalization of structure-property relationships in Cu-loaded zeolites under, for instance, different hydration conditions. 17O EPR has been used in different context to derive structural information on paramagnetic species 23, 24, 25, 26, 27, 28. However, this approach cannot be applied to investigate the Cu–O interaction due to the paramagnetic nature of Cu II ions. Indeed 17O solid state NMR has revealed invaluable to address important issues in this context, providing unique insights into crucial aspects related to the local structure of aluminosilicate zeolites 21, 22. The exploitation of hyperfine techniques for the investigation of the metal-oxygen bond requires therefore the isotopic enrichment of oxide solid systems that involves both cost and effort, which however can be very rewarding. The only magnetic isotope of oxygen is 17O ( I = 5/2) but its natural abundance (0.037%) is by far lower than the value necessary to detect a hyperfine structure. However, the most revealing and important piece of information that can be extracted from this technique, the detection of hyperfine couplings (a measure of the interaction between the electron spin and the nuclear spin of atoms in contact with the unpaired electron) for the coordinating oxygen atoms, is currently missing. One crucial aspect to understand and control the catalytic potential of such species is the detailed knowledge of their structure and of the intimate features of chemical bonding, which include covalency, ionicity, electron and spin delocalization.įor paramagnetic Cu II species ( S = 1/2) the sensitivity and selectivity of EPR techniques are in principle ideally suited to obtain detailed information on the topological distribution and the nature of the chemical bonding of Cu II species in zeolites and indeed EPR has been abundantly used to investigate such systems 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. These range from isolated single ion sites 9 to polynuclear species 10, all featuring unsaturated coordination and the possibility for adsorption of small molecules. The high activity of copper-containing zeolites in oxidation and reduction reactions is typically associated with the redox transformations of copper species. These make it an ideal model system to address fundamental questions of structure–performance relationships in the general context of metal-exchanged zeolite catalysis 1. the Si/Al ratio and the copper content on the catalytic performance are still under debate. However, the exact nature of the active sites and the impact of e.g. Cu-CHA catalysts have been widely investigated and it is agreed that isolated Cu II ions at ion-exchange positions are the active sites for the NH 3-SCR reaction 8. Among other systems, the Cu-Chabazite (Cu-CHA) redox catalyst is particularly attractive as it couples industrial and environmental relevance-a Cu-CHA catalyst for diesel engine exhaust was commercialized since 2008-and structural simplicity. Copper-exchanged zeolites have been in the focus of comprehensive studies for decades 1, 2 and are a subject of evergreen interest in catalysis due to applications ranging from NO x removal 3, 4 to the direct conversion of methane to methanol 5, 6, 7.
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