CHARACTERIZATION BY EXAFS, NMR, AND OTHER TECHNIQUES OF PT NAY ZEOLITE AT INDUSTRIALLY RELEVANT LOW CONCENTRATION OF PLATINUM/

In-situ extended X-ray absorption fine structure (EXAFS), H-1 and Xe n uclear magnetic resonance (NMR), H-2 chemisorption, X-ray powder diffr action (XPD), and high-resolution electron microscopy (HREM) technique s were used to understand, as precisely as possible, carefully prepare d Pt/Y zeolite samples at industrially relevant low levels of Pt loadi ngs. These techniques were used to determine the local structure, size , and location of the metal particles for a series of 0.8 wt % Pt/NaY zeolite catalysts which have been reduced at 300, 500, and 650 degrees C. The EXAFS and NMR results show that the metal particles are smalle st, as expected, for the sample reduced at 300 degrees C, with an aver age size of 11.3 Angstrom and containing 30 atoms, assuming a spherica l shape. These particles are located primarily inside the supercages. For the samples reduced at 500 and 650 degrees C, the size and locatio n of the Pt particles are distinctly different from those for the 300 degrees C sample; i.e., the particles are larger and increasingly loca ted outside the supercages. The sizes of the Pt particles obtained fro m the first shell analysis for the samples reduced at 500 and 650 degr ees C are 23 and 45 Angstrom, respectively. The Pt particles for the s ample reduced at 650 Angstrom C are completely outside the zeolite cry stallite, while for the 500 degrees C sample 23% of the particles are outside. The XPD results show that the full-width-half-maximum (fwhm) of the zeolite X-ray diffraction peaks decreases as the reduction temp erature increases, indicating enhanced crystallinity of the framework and repair of the possible damage from the agglomeration and movement of the Pt particles, Comparison of EXAFS results of samples prepared w ith different techniques shows that the average metal particle size is 10 +/- 2 Angstrom for reduction temperatures up to 360 degrees C, and above this a rapid growth of metal particles is seen.