Electron spin resonance and electron spin echo modulation spectroscopic study of Pd(I) location and adsorbate interactions in PdH-SAPO-34 molecular sieve

Abstract

Abstract Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies have been used to monitor the location of Pd(I)and its interaction with water, methanol, ethanol, ethylene, benzene, carbon monoxide and ammonia in silicoaluminophosphate type 34 (SAPO-34) molecular sieve containing Pd(II) by ion-exchange. After activation at 600 °C, three different Pd(I) species are observed: A 1 (g ⊥ = 2.177), A 2 (g ⊥ = 2.136) and A 3 (g ⊥ = 2.070) with a common g ‖ = 2.92. These correspond to three different site locations in the framework. A 1 is assigned to the least accessible site HI in the center of a hexagonal prism, A 3 to site I displaced from a six-ring into the ellipsoidal cage and A 2 to the most accessible site IV near an eight-ring window based on adsorption of oxygen and hydrogen and 31 P 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 O 2− indicating water decomposition. Adsorption of methanol and ethanol causes a change in the ESR spectrum which indicates some relocation of Pd(I) to better coordinate with one molecule of the alcohol. Exposure to ethylene also changes the ESR spectrum indicating interaction of Pd(I) with ethylene. ESEM shows that the Pd(I) species coordinates to one ethylene. The adsorption of carbon monoxide results in a Pd(I) complex with three molecules of carbon monoxide based on resolved 13 C superhyperfine splittings. Upon adsorption of ammonia, one molecule of ammonia coordinates to Pd(I) based on resolved nitrogen hyperfine coupling. Upon adsorption of big molecules such as benzene, however, no change of ESR spectrum is observed, and no deuterium modulation was detedcted on ESEM spectrum, indicating no detectable interaction between Pd(I) and benzene.