ELECTRON-MICROSCOPY AND PT-195 NUCLEAR-MAGNETIC-RESONANCE OF PLATINUMPARTICLES IN A ZEOLITE-Y MATRIX

We present the first Pt-195 NMR spectra of platinum particles embedded in a zeolite, and compare them with earlier results on oxide-supporte d Pt. Slight variations in the details of the preparation can make a b ulk-like NMR signal appear, indicating the presence of particles large r than roughly 1.5 nm, that cannot fit into the zeolite supercage. TEM observations confirm that they are nevertheless inside the matrix. Al though the particles have nominally clean surfaces, a fraction of Pt a toms is not in a metallic environment: this may be caused by interacti on with the framework and its counterions, and/or by an intrinsic size effect. Hydrogen chemisorption seems to restore the metallic characte r at least partially. This may be related to the change of lattice con stant between clean and hydrogen-covered particles, known to occur fro m EXAFS studies.