Formation and adsorbate interactions of paramagnetic Pd(I) species in Pd(II)-exchanged NaK- and H-clinoptilolite

The formation of monovalent palladium in PdNaK-clinoptilolite where Pd(II) is introduced into extraframework sites as [Pd(NH3)(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 el ectron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectr oscopies. Dehydration at 473 K produces one Pd(I) species in PdH-clinoptilo lite but no ESR signal in PdNaK-clinoptilolite. This indicates that the sta bility of Pd(I) between PdH-clinoptilolite and PdNaK-clinoptilolite is diff erent, probably due to the different locations and environments of Pd in th ese systems. Hydrogen reduction of Pd(II) in these two materials after acti vation reveals that Pd(II) ions in PdNaK-clinoptilolite occupy relatively a ccessible sites in comparison to those in PdH-clinoptilolite. The interacti ons of Pd(I) formed by thermal reduction of PdH-clinoptilolite with various adsorbates are also studied. The ESR studies coupled with ESEM measurement s show that Pd(I) in PdH-clinoptilolite interacts rapidly with molecules sm aller than methanol, such as hydrogen, water, ammonia, and carbon monoxide, and forms a stable complex with them. However, adsorption of benzene and p yridine 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 a re too big to enter.