Adsorbate Interactions of Paramagnetic Palladium(I) Species in Pd(II) -Exchanged K-L Zeolite

Adsorbate interactions of paramagnetic palladium® species in Pd(II)-exchanged K-L channel type zeolite are studied by electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies. Three different preparation methods for PdK-L by hydrogen and oxygen treatments produced different Pd® ESR signals. Preparation A produced one Pd® species which is suggested to be located at the center of hexagonal prisms of the L zeolite framework. Preparation C produced an additional Pd® signal which is assigned to the center of double eight rings of the zeolite framework. Preparation B produced three Pd® signals including a new Pd® signal not observed in preparations A and C. ESR and ESEM after the adsorption of various adsorbates are almost the same for PdK-L preparations A-C. Equilibration with water results in a stable Pd® species coordinating to one water molecule. Adsorption of benzene produces two new Pd® species, each coordinating to one molecule of benzene based on ESEM analysis. Adsorption of methanol or ethanol results in a new Pd® species along with a broad ESR signal which is attributed to the formation of small palladium clusters. ESEM shows that the new species is due to the coordination of Pd® to one molecule of methanol or ethanol. Upon adsorption of ethylene, two new Pd® species occur that show slow time-dependent changes of their ESR intensities. ESEM analysis indicates that each of the Pd® species coordinates to one molecule of ethylene. ESR and ESEM results indicate that equilibration with ammonia forms a new Pd® species interacting with one molecule of ammonia at a relatively long distance. On the basis of its identical ESR spectrum to that for NH₂ adsorption, it is suggested that one NH₂ moiety of hydrazine weakly interacts with Pd® at a long distance as for NH₃.