PdI location and adsorbate interactions in PdH-SAPO-34 studied by EPR and electron spin–echo modulation spectroscopies

Abstract

EPR and electron spin–echo modulation (ESEM) spectroscopies have been used to monitor the location of Pd^I and its interaction with water, methanol, ethanol, ethene, benzene, carbon monoxide and ammonia in sili-coaluminophosphate type 34 (SAPO-34) molecular sieve containing Pd^II by ion exchange. After activation at 600 °C, three different Pd^I species are observed: A₁(g_⊥= 2.177), A₂(g_⊥= 2.070) with a common g_‖= 2.92. These correspond to three different site locations in the framework. A₁ is assigned to the least accessible site III in the centre of a hexagonal prism, A₃ to site I displaced from a six-ring into the ellipsodial cage and A₂ to the most accessible site IV near an eight-ring window based on adsorption of oxygen and hydrogen and ³¹P modulations from the SAPO framework observed by ESEM. Oxygen and water oxidize Pd^I ions in an activated sample to Pd^II ions complexed to O₂^–, indicating water decomposition. Adsorption of methanol and ethanol causes a change in the EPR spectrum which indicates some relocation of Pd^I to allow better coordination with one molecule of the alcohol. Exposure to ethene also changes the EPR spectrum, indicating interaction of Pd^I with it. ESEM shows that the Pd^I species coordinates to one ethene molecule. The adsorption of carbon monoxide results in a Pd^I complex with three molecules of carbon monoxide based on resolved ¹³C superhyperfine splittings. Upon adsorption of ammonia, one molecule of ammonia coordinates to Pd^I based on resolved nitrogen hyperfine coupling.