Adsorbate interactions of paramagnetic palladium species in PdII-exchanged H-rho zeolite characterized by EPR and electron spin echo modulation spectroscopies

Abstract

Interactions of paramagnetic palladium ions in H-rho zeolite with oxygen, water, methanol, ethanol, propanol, ammonia, carbon monoxide, hydrogen, ethene, benzene and pyridine have been studied by EPR and electron spin echo modulation (ESEM) spectroscopies. Pd^II was reduced to paramagnetic Pd^I by a thermal activation process. The EPR spectrum of an activated sample shows the formation of one Pd^I species, which is suggested to be located in octagonal prism sites of the rho zeolite framework. Pd^I interacts with water vapour or molecular oxygen, to form Pd^II–O₂^–, indicating the decomposition of water. Equilibration with methanol, ethanol or propanol results in a broad isotropic EPR signal which is attributed to the formation of small palladium clusters. ESEM shows that the Pd clusters coordinate to one molecule of methanol or ethanol. Interestingly, an activated sample with adsorbed ethanol produces an additional anisotropic signal, besides the broad isotropic signal, which is also coordinated to one molecule of ethanol. Propanol requires relatively much longer times for cluster formation, compared with methanol and ethanol. Adsorption of ammonia produces a Pd^I complex containing four molecules of ammonia, based upon resolved nitrogen superhyperfine coupling. Adsorption of carbon monoxide results in a Pd^I complex containing two molecules of carbon monoxide, based upon resolved ¹³C superhyperfine coupling. Upon adsorption of hydrogen, a new Pd^I species occurs, which disappears upon exposure to oxygen. EPR and ESEM results indicate that exposure to ethene leads to two new Pd^I species, each of which coordinates to one molecule of ethene.