New Method for Obtaining Palladium Particles on Y Zeolites. Characterization by TEM, 129Xe NMR, H2 Chemisorption, and EXAFS

Palladium particles on Y zeolites were prepared by a new method involving exchange of [Pd(en)]2+ (en = ethylenediamine) with zeolite counterions and autoreduction of Pd(II) by the ligand in inert gas. An in situ extended X-ray absorption fine structure (EXAFS) experiment at the Pd K edge on a [Pd(en)]2+/NaY sample shows reduction of Pd(II) to Pd(0) by the amine between 373 and 473 K. Pd/NaY and Pd/HY samples prepared by this method at 525 K were characterized by TEM, 129Xe NMR, H2 chemisorption, and EXAFS. These techniques were used to determine the local structure, size, and location of the Pd particles of freshly reduced samples (Pd/NaY, Pd/HY) and after heating in dioxygen at 573 K, followed by treatment with dihydrogen (Pd/NaYt, Pd/HYt). The TEM results show that the freshly reduced samples have particle sizes averaging 40 and 15 Å on NaY and HY zeolites, respectively. Treatment in dioxygen does not change particle sizes on HY zeolite but some large particles are seen on NaY zeolite. 129Xe NMR results show that in freshly reduced samples the particles are surrounded by organic residues of the ligand which can be removed by dioxygen treatment. 129Xe NMR and EXAFS results on samples without organic residues confirm that particles are smaller on Pd/HYt than on Pd/NaYt. Chemical anchoring of Pd clusters by zeolite H+ is proposed to explain the different results for the zeolites. While H2 chemisorption data on Pd/NaYt agree with other techniques, the amount of chemisorbed hydrogen on Pd/HYt disagrees completely with results of previous techniques, suggesting hydrogen suppression phenomena on the acid zeolite.