The impact of the zeolite local environment on the stability and spectroscopy features of the (CO, NO, NO2)–Pd-(H-MOR, MOR) systems

Abstract

The stability of the Pd, PdO and Pd3 species at different crystallographic sites on the channels of the protonated and deprotonated MOR zeolite and their capacity to adsorb CO, NO and NO2 were investigated as a function of the Al – Al distance using DFT. For the characterization of these species, we considered Hirshfeld charges, the UV–vis spectra, the vibration frequencies of the probe molecules adsorbed on the Pd-MOR systems, as well as a thermodynamic analysis of the adsorption processes. The DFT results show that the most stable Pd-MOR, OC-Pd-MOR and ON-Pd-MOR structures are those in which the Al atoms are two T-sites apart. From the TD-DFT calculations it was possible to distinguish the Pd species in the Pd-MOR systems. In general, the adsorption of CO and NO is a thermodynamic favored process. Conversely, the NO2 adsorption exhibits positive values for ΔΗads and ΔGads for almost 40% of the Pd/MOR structures considered. NO adsorbs preferably on Pd3/MOR systems than on those with one Pd atom. It was found that the stability of the NO-(OC)Pd/MOR and NO2–Pd/MOR systems is related to the oxidation state of the Pd atom. After analyzing one set of experimental CO and NO vibrational frequency results of Pd-MOR systems, it became clear that the determination of the Pd speciation must consider the particular experimental conditions under which the measurements have been performed.