Pt/SiO 2: VI. The effects of pretreatment on structures

Abstract

Pt catalysts with a wide range of percentage metal exposed have been examined by wide-angle and small-angle X-ray scattering, extended X-ray absorption fine structure (EXAFS), and X-ray fluorescence. The wide-angle studies were carried out in a special cell that permitted treatment in a gas environment at moderate temperatures. For high percentage metal exposed, the wide-angle patterns (as well as all EXAFS spectra) were obtained with X rays from the storage ring at Cornell University. After exposure to air, a catalyst with 63.5% Pt exposed consists of particles of crystalline Pt 3O 4 with a core of Pt, whereas for 81% exposed, the catalyst is nearly fully oxidized. There is no indication of any difference between the PtPt distance in these catalysts and that in bulk Pt, nor is there any difference for 40% exposed or less (for which the exposed catalyst is Pt metal), even after reductions in H 2. There is little contact area between the catalyst and the support. For the lowest percentage metal exposed (6.3%), reductions in hydrogen do not change the microstrains within particles, but there is reduction in particle size, which is greater the higher the reduction temperature. The activity of this catalyst decreases in the same manner. The mean-square amplitude of vibration increases with the temperature of H 2 reduction for several catalysts, as does the catalytic activity.