Formation and Adsorbate Interactions of Paramagnetic Pd(I) Species in Pd(II)-Exchanged NaK- and H-Clinoptilolite

Abstract

The formation of monovalent palladium in PdNaK-clinoptilolite where Pd(II) is introduced into extraframework sites as [Pd(NH 3 ) 4 ] 2+ by liquid-state ion exchange at 298 K, is compared to that observed in PdH-clinoptilolite where Pd(II) is incorporated by solid-state ion exchange at 823 K, using electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies. Dehydration at 473 K produces one Pd(I) species in PdH-clinoptilolite but no ESR signal in PdNaK-clinoptilolite. This indicates that the stability of Pd(I) between PdH-clinoptilolite and PdNaK-clinoptilolite is different, probably due to the different locations and environments of Pd in these systems. Hydrogen reduction of Pd(II) in these two materials after activation reveals that Pd(II) ions in PdNaK-clinoptilolite occupy relatively accessible sites in comparison to those in PdH-clinoptilolite. The interactions of Pd(I) formed by thermal reduction of PdH-clinoptilolite with various adsorbates are also studied. The ESR studies coupled with ESEM measurements show that Pd(I) in PdH-clinoptilolite interacts rapidly with molecules smaller than methanol, such as hydrogen, water, ammonia, and carbon monoxide, and forms a stable complex with them. However, adsorption of benzene and pyridine on thermally reduced PdH-clinoptilolite produces no ESR signal due to a Pd(I)-benzene complex or a Pd(I)-pyridine complex, suggesting that Pd(I) is located at a site in eight-ring channels where benzene and pyridine are too big to enter.

Comparative ESR and Catalytic Studies of Ethylene Dimerization on Pd(II)-Exchanged Clinoptilolite, Mordenite, Ferrierite, and SUZ-4

“…The facts that no new ESR signal is observed after adsorption of these molecules on SUZ-4 and that Pd(I) species O increases with annealing time at room temperature indicate that these adsorbates reduce the remaining Pd(II) to Pd(I) in SUZ-4. A similar behavior has been observed in PdNaK-clinoptilolite 14 where Pd(II) located in a main 10-ring channel is reduced by methanol at room temperature. However, the stability of Pd(I) in clinoptilolite is different from that of Pd(I) in PdHK-SUZ-4, reflecting different locations of Pd(I) in these two materials.…”

“…This single isolated Pd(I) is reproduced by hydrogen reduction of PdH-clinoptilolite, PdH-mordenite, and PdHferrierite at high temperature showing a higher ESR intensity, compared to that of Pd(I) observed by thermal reduction. The Pd(I) species found in mordenite, ferrierite, and SUZ-4 are axially symmetric while Pd(I) in clinoptilolite 14 has rhombic symmetry. These four zeolites show no big difference in the reducibility of Pd(II) upon thermal reduction.…”

“…The accessibility of Pd(I), based on the interaction with various sizes of adsorbates, is different in these four zeolites. A previous study concerning the nature and adsorbate interactions of Pd(I) in clinoptilolite 14 suggested that Pd(I) is located at a site which is not accessible to benzene and pyridine, probably a site within an 8-ring channel parallel to the main 10-ring channel of clinoptilolite, while molecules smaller than pyridine easily interact with Pd(I). Adsorption of methanol and pyridine on thermally reduced mordenite and ferrierite at room temperature leads to the formation of new ESR species due to Pd(I) complexes with methanol and pyridine.…”

“…13 In the past, ESR and ESEM spectroscopies were employed to obtain the information about the location of Pd(I) and its local environment in PdH-clinoptilolite, which is necessary for application of this Pd-exchanged clinoptilolite in catalytic reactions. 14 Thermal reduction of PdH-clinoptilolite produced a single isolated Pd(I) (g 1 ) 2.927, g 2 ) 2.168, and g 3 ) 2.136) in the clinoptilolite structure where Pd(I) was located at a site which was not accessible to the molecules as large as pyridine but was readily coordinated with adsorbates such as ammonia, methanol, and ethylene. Adsorption of ethylene on thermally reduced Pd(I)H-clinoptilolite at room temperature led to the formation of three new ESR species due to the interaction of Pd(I) with ethylene.…”

Novel Preparation of Palladium Nanoclusters Using Metal Nitrates and Their Catalysis for Oxidative Acetoxylation of Toluene in the Presence of Molecular Oxygen

“…The evolution of CO 2 ascertains the presence of oxygen species on the Pd cluster surface. Since the reactivity of the surface oxygen species was high enough to oxidize a CO molecule even at 0 °C, the Pd sites responsible for the CO oxidation would be Pd 2 O species on the cluster surface . The ratio of the Pd 2 O species to the surface Pd atoms can be evaluated from the amounts of the formed CO 2 and the CO uptake, which is depicted in Figure (b).…”