Applications of EPR to a study of the hydrogenation of ethene and benzene over a supported Pd catalyst: detection of free radicals on a catalyst surface

Abstract

EPR has not been used extensively in the field of catalysis despite it being the most sensitive technique available for detecting free radicals and paramagnetic metal ions, which are intermediates or participants in many catalytic processes. In this paper, we describe its application to the detection of paramagnetic intermediates on the surface of a palladium catalyst during heterolytic hydrogenation reactions. EPR, in conjunction with spin trapping, is shown to provide a convenient, simple method for detecting hydrogen adatoms generated by the dissociative chemisorption of hydrogen on alumina-supported palladium catalysts (<0.04% Pd) at room temperature. By using D₂ we have also been able to demonstrate directly the occurrence of hydrogen/deuterium spillover onto the alumina surface by H/D atom transfer to surface hydroxy groups. Alkyl and aromatic free redical intermediates formed by reaction of the H˙ adatoms with alkenes and benzene have also been observed by EPR at a catalyst surface for the first time. The hydrogen atoms reacting with the spin traps, ethene and benzene, are not strongly chemisorbed hydrogen (Pd—H) but those weakly adsorbed H-adatoms in equilibrium with H₂(g).