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

The stability of the Pd, PdO and Pd 3 species at different crystallographic sites on the channels of the protonated and deprotonated MOR zeolite and their capacity to adsorb CO, NO and NO 2 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 NO 2 adsorption exhibits positive values for Δ Η ads and Δ G ads for almost 40% of the Pd/MOR structures considered. NO adsorbs preferably on Pd 3 /MOR systems than on those with one Pd atom. It was found that the stability of the NO-(OC)Pd/MOR and NO 2 –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. • From the TD-DFT results the Pd moieties supported in MOR are identifiable. • The most stable (Pd, OC-Pd, ON-Pd)-MOR systems have the Al atoms two T-sites apart. • NO adsorbs preferably on Pd 3 /MOR rather than on the (Pd, PdO)-(H-MOR, MOR) systems. • In presence of NO the PdO is converted into Pd. • The Pd formal oxidation state is +1 in two (ON)(OC)–Pd/MOR structures.