Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and the Formation of Stable Pd Clusters in Zeolites

Abstract

Spontaneous dispersion and clustering processes of Pd were measured by means of the energy-dispersive EXAFS method. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. In contrast to H-type zeolites, simple oxidation of the agglomerated Pd was observed on Na-ZSM-5. The structural change of Pd was followed in the atmosphere of hydrogen. The clustering processes of metal Pd depended on the kind of zeolite, and these were categorized into three groups. The first group, i.e., Na-ZSM-5 and H-beta, showed monotonic agglomeration of metal Pd by increasing the reduction temperature. The second group consisted of H-ZSM-5 and H-mordenite where the formation of Pd6 clusters was found. On these zeolites, the generation of Pd6 clusters was reversibly observed upon the repetition of reduction and oxidation treatments. The third group consisted of H-Y and USY zeolites where the formation of Pd13 clusters was observed. From these findings, it was concluded that the crystal structure and acid sites of zeolites had profound influences on the dynamic behavior and the genesis of Pd clusters with various structures.

X-Ray Photoelectron Spectroscopy Investigation of Pd-Beta Zeolite Catalysts with Different Acidities

“…In the case of Pd-H-ZSM the EXAFS studies showed that during calcinations the dispersion of PdO was dependent on the concentration of Brønsted acid sites [31]. The ratio of Pd/ PdO was also stated to depend on the concentration of the BAS in the zeolite [14][15][16][17][18][19][20]. During the reduction, however, the larger Pd particles ([3 nm) were formed and they migrated to the outer surface of the zeolite [31].…”

“…ions in Pd/Mordenite reduced at 150°C [7]. The ratio Pd/PdO was proposed to depend on the concentration of the BAS in the zeolite [14,20]. In the case of Pd/La 2 O 3, interactions between the metal particles and support have been detected [21].…”

“…Strongly interacting supports were observed usually to shift the cluster binding energies to lower levels due to the capability of Pd clusters to interact with more than one proton [4,12,13]. The change in metal particle size, which is proposed to depend on either the pore structure of zeolite due to the steric confinement or strong interaction between BAS and metallic Pd [12,14,15] can change the binding energy measured by XPS, especially for the particles less than 5 nm [11,16,17]. Furthermore, the change of the chemical environment of the metal changed the binding energy of Pd 3d 5/2 , when Pd electrode was polarized in KOH [10].…”

Formation of nanometer-sized Au particles on USY zeolites under hydrogen atmosphere

“…Zeolites are promising supports for metals because the presence of strongly acidic sites in these materials promotes high dispersion of metal particles. In fact, metal clusters of Pd and Pt with high dispersion were obtained on zeolites having Brønsted acid sites [14,15]. Ultrastable Y (USY) zeolites contain strongly acid sites [16] (ca.…”

Real time monitoring of the evolution of Ni2+ environment in faujasite upon rehydration by in situ dispersive-EXAFS

“…This value is significantly higher than the Ni-O distances (from 1.87 to 2.05 Å ) determined for dehydrated samples by EXAFS (Extended X-ray Absorption Fine Structure) [5][6][7], the number of nearest oxygen neighbours being furthermore lower (between 3 and 4). These discrepancies prompted us to analyse precisely the evolution upon rehydration of the coordination shell around Ni 2+ in a NiNaX sample and this was made possible owing to the recent development of dispersive-EXAFS for the in situ study of catalytic materials, allowing the recording of complete absorption spectra in very short times [8,9].…”