Electron paramagnetic resonance and electron spin echo modulation spectroscopic studies on the location and adsorbate interactions of paramagnetic Pd ion species in PdII-exchanged K-L zeolite

Abstract

Dehydration and adsorbate (O₂, H₂O, CD₃OH, CH₃OD, CH₃CH₂OD, C₂D₄, CO) interactions of paramagnetic palladium ion species in Pd^II-exchanged K-L channel type zeolite are studied by electron paramagnetic resonance (EPR) and electron spin echo modulation (ESEM) spectroscopies. The EPR spectrum of an activated sample at ca. 400–500 °C shows the formation of one Pd^I species which has g_∥= 2.821 and g_⊥= 2.125 and which is suggested to be located at the centre of double eight rings of the L zeolite framework. Adsorption of oxygen on an activated sample oxidizes the reduced Pd^I to Pd^II interacting with a superoxide ion. Equilibration with methanol or ethanol results in a broad isotropic EPR signal which is attributed to the formation of small charged palladium clusters. ESEM shows that the Pd clusters coordinate to one molecule of methanol or ethanol. EPR and ESEM results indicate that equilibration with C₂D forms a new Pd^I species with g_∥= 2.661 and g_⊥= 2.125 coordinating to one molecule of ethene. Adsorption of carbon monoxide results in a complex containing two molecules of carbon monoxide based upon resolved ¹³C superhyperfine and analysis of ¹³C modulation.