Novel insights on the Pd speciation in Pd/SSZ-13 and on the role of H2O in the Pd reduction by CO

Pd speciation induced by the combined effect of CO and water on Pd/SSZ-13 samples prepared by both impregnation and ion exchange was examined by FT-IR spectroscopy of CO adsorbed at room temperature and at liquid nitrogen temperature on anhydrous and hydrated samples. Starting from the literature findings related to the CO reducing effect on Pd cations, the present work gives precise spectroscopic evidences on how water is necessary in this process not only for compensating with H+ the zeolite exchange sites set free by Pd reduction, but also for mobilizing isolated Pd2+/Pd+ cations and making possible the reduction reactions. The aggregation of some Pd+ sites, just formed by the reduction and mobilized by the hydration, gives rise to the formation of Pd2O particles. Also, Pd0(1 0 0) sites are observed with CO on hydrated sample, formed by the aggregation and reduction of isolated Pd cations. Moreover, Pd0(1 11) sites are formed on the surface of PdOx particles during CO outgassing. The observation of the combined effect of water and CO allowed to define assignments of IR bands related to carbonyls of Pd in different oxidation states and coordination degrees.