GENESIS AND GROWTH OF PT PARTICLES ON MGO - CHARACTERIZATION BY X-RAY-ABSORPTION SPECTROSCOPY AND INFRARED-SPECTROSCOPY

Samples prepared by the reaction of [Pt(C3H5)(2)] With MgO powder were treated in H-2 at temperatures in the range 25-150 degrees C and char acterized by X-ray absorption spectroscopy and infrared spectroscopy. As the temperature of treatment in H-2 was increased, the infrared spe ctra indicated the disappearance of bands attributed to the allyl liga nds, and the sample became gray, the color characteristic of a support ed metal. Adsorption of CO was characterized by a single infrared peak in the nu(CO) region at 2070 cm(-1), consistent with that observed fo r CO chemisorbed on metallic Pt, The Fourier transforms of the EXAFS f unctions measured near the Pt L(III) absorption edge for each sample c onfirm the presence of metallic Pt, showing peaks at approximately 2.7 , 3.8, 4.7, and 5.6 Angstrom, consistent with the first, second, third , and fourth nearest neighbors in bulk fee Pt. The EXAFS results indic ate that the particle size increased from approximately 8 Angstrom aft er treatment in H-2 at room temperature, to about 9 Angstrom after a s imilar treatment at 50 degrees C, to about 10 Angstrom after treatment at 150 degrees C. The metal-support interface was characterized by tw o metal-support oxygen contributions, one at 2.18 Angstrom and one in the range 2.7-2.9 Angstrom. The total Pt-O coordination number decreas ed monotonically as the treatment temperature in H-2 increased. This r esult confirms that the fraction of Pt atoms in contact with the suppo rt decreased as the particle size increased.