Reducibility of Cu-zeolites and stability of Cu+ monocarbonyl adducts: Qualitative and quantitative relationships from MCR-XAS and DFT

Abstract

A series of Cu-exchanged zeolites differing in framework topology and composition are studied through in situ X-ray Absorption Spectroscopy (XAS) to highlight differences in reducibility depending on the characteristics of the different samples. After reduction at high temperature in NH3, the samples are exposed to CO at 50 °C followed by a temperature-programmed desorption to study the different stability of carbonyl species forming on the Cu+ sites. Multivariate Curve Analysis applied to the XAS data allows extracting concentration profiles and pure spectral profiles on the species involved during several steps of the reaction. While all the samples are quantitatively reduced after treatment in NH3, the stability of the [Cu(CO)]+ sites shows a dependence on the topology of the materials. DFT calculations on possible sites occupied by Cu in the different frameworks are consistent with the stability trends observed experimentally and, together with isothermal volumetric CO adsorption data, confirm the particular instability of the Cu+-monocarbonyls formed in the CHA framework.