Reduction and cluster growth of palladium in zeolite Y containing transition metal ions. X-ray absorption studies

Abstract

845anchored to the air/solution interface. From this result is easily inferred a folding of the alkanediyl chain and its penetration into the inner micelle. The folding can also be examined by the following approach. The partial molar volume v of tetradecanesulfonate anion in the micellar state is estimated to be 254.4 cm3 mol-l 2o by applying the Stokes' radius of 1.84 A to the sodium ion. Then, the micellar surface area (S) made only of surfactant ions is given by where the micelles are assumed to be spherical due to relatively small aggregation numbers. On the other hand, the distance (6) between anionic head groups is related with the above surface area in the form s = (47)'/3(3fiV)*/3 (7)where the hexagonal packing is assumed as for the anionic groups. From the u and fi values obtained above, the distance d can be evaluated for each counterion. At the same time, the distance between two cationic charges in the counterions is also evaluated from the CPK model. These two distances are plotted against the carbon number of the alkanediyl group of the counterions ( Figure 6 ) . From this figure, the distance between the two cationic charges of the counterions becomes longer than that between anionic head groups of micellized surfactant ions above an eight CH2 groups separation of counterions, which also strongly indicates the folding of the alkanediyl chain of the counterions.The above two simplified calculations substantiate the assumption made from the cmc change in the previous paper1* that the divalent cationic charges whose separation is more than eight methylene groups become anchored to the micellar surface due to electrostatic attraction by anionic head groups of surfactant ions, while the alkanediyl chain folds and penetrates into a hydrophobic inner micelle, leading to a decrease in cmc. The effect of a second transition metal on the reducibility and agglomeration behavior of palladium in zeolite Y is studied by EXAFS spectroscopy. Special attention is given to the potential bonding interactions between both metal constituents that could result in "chemical anchoring" of the noble metal to the support via the unreduced cocation. Zeolite Y was coexchanged with Fe2+ or Co" and Pd2+ and with each ion alone. Dehydration in oxygen atmosphere and reduction with hydrogen was performed at different temperatures up to 623 K. The structure of the samples during various stages of pretreatment and reduction was studied with EXAFS on both absorption edges. Palladium is partially reduced to Pdo at room temperature and forms small metal clusters at higher temperatures. Iron is oxydized under oxygen atmosphere and is partially present in the form of iron oxide particles, which are redispersed under reducing conditions. A small enhancement of the reducibility of palladium is observed if iron is present in the zeolite, but neither a chemical anchoring nor an effect on the final agglomeration process of palladium is detected by the presence of iron cations. In contrast, coexchange with cobalt results in a substantially h...

EXAFS Characterization of Rh−Pt Metal Clusters Supported on NaY Zeolite

“…The detected Pt−Pt distance has a value of 2.76 Å (Table ), in good agreement with the Pt−Pt distance in pure Pt (2.78 Å). Besides the Pt−Pt contribution, two more contributions were identified due to the Si and O atoms contained in the framework of the zeolite, similar to results obtained with other metals in zeolite samples. − …”

Elucidating the mechanism of heterogeneous Wacker oxidation over Pd-Cu/zeolite Y by transient XAS

“…From EXAFS analysis, the average Pd-O/N coordination number decreased from 3.5 to 3.1 while the bond distance remained at 2.01 Å. A second Pd-Pd (metallic) coordination shell appeared at 2.76 Å with a coordination number of 1.5, suggesting the reduction of a portion of Pd(II) to metallic nanoparticles 33 , which could be linked to the loss of Wacker activity observed in Pd1Cu5-Y ( Supplementary Fig. 3).…”

Formation and Growth of Pd Nanoparticles Inside a Highly Cross-Linked Polystyrene Support: Role of the Reducing Agent

“…Therefore, it is of particular interest to look at the FT-IR spectra not only in the 1700–1300 cm –1 region, but also in the 2200–1700 cm –1 region, where carbonyl species give their contributions (Figure a). The IR spectrum at the beginning of the heating ramp (black) shows a broad absorption band at 2115 cm –1 , assigned to CO adsorbed on Pd­(II) species (likely, a few defect sites in palladium acetate particles). − This band decreases in intensity upon increasing temperature, without completely disappearing. Simultaneously, a complex series of IR absorption bands appear in two main regions: (i) 2000–1900 cm –1 , typical of metal carbonyls, and (ii) 1900–1700 cm –1 , characteristic of organic carbonyls.…”