Site-Specific Molecular Spectroscopy Reveals Monomeric Cu Active Sites in SSZ-13 Zeolite for Methane Oxidation and NOx Reduction.

Copper-zeolite catalysts exhibit superior performance in selective catalytic reduction (SCR) of NOx and partial oxidation of methane (POM), but identification of their active sites remains elusive and is often controversial. Recent reports suggest that dimeric copper species are responsible for the active sites for these reactions. We demonstrate that infrared and UV-resonance Raman spectroscopy (using 244 nm laser) combined with UV-vis spectroscopy can reveal the anchoring and speciation of copper in SSZ-13 zeolite, particularly the dominant monomeric copper species that cannot be observed by Raman spectroscopy with commonly employed visible laser excitation (∼400-600 nm). Electronic structure modeling corroborates the spectroscopic features and reveals the vibrational coupling modes associated with the interfacial bonding between copper and the zeolite framework. The pseudo-square-planar monomeric Z2Cu species ("Z" represents the zeolite framework charge site) were found to be the active sites for both SCR and POM at ∼400°C, whereas monomeric ZCuOH species are responsible for SCR at lower temperatures.