Pd^Ilocation and adsorbate interactions in PdH-SAPO-34 studied by EPR and electron spin–echo modulation spectroscopies

EPR and electron spin–echo modulation (ESEM) spectroscopies have been used to monitor the location of PdI and its interaction with water, methanol, ethanol, ethene, benzene, carbon monoxide and ammonia in sili-coaluminophosphate type 34 (SAPO-34) molecular sieve containing PdII by ion exchange. After activation at 600 °C, three different PdI 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 PdI ions in an activated sample to PdII ions complexed to O2–, indicating water decomposition. Adsorption of methanol and ethanol causes a change in the EPR spectrum which indicates some relocation of PdI to allow better coordination with one molecule of the alcohol. Exposure to ethene also changes the EPR spectrum, indicating interaction of PdI with it. ESEM shows that the PdI species coordinates to one ethene molecule. The adsorption of carbon monoxide results in a PdI complex with three molecules of carbon monoxide based on resolved 13C superhyperfine splittings. Upon adsorption of ammonia, one molecule of ammonia coordinates to PdI based on resolved nitrogen hyperfine coupling.