Particle development and characterisation in Pt(acac)(2) and Pt(acac)(2)/GeBu4 derived catalysts supported upon porous and mesoporous SiO2: effect of reductive environment, and support structure

In situ, time resolved energy dispersive EXAFS (EDE) has been used in conjunction with temperature programmed reduction/decomposition (TPR/TPD), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and diffuse reflectance infrared spectroscopy (DRIFTS) to probe the evolution and character of Pt and PtGe alloy particles, derived from Pt(acac)(2) and GeBu4 precursors, supported upon amorphous and mesoporous silicas. The reduction kinetics and final particle size distributions obtained are found to be functions of the reductive environment, the support architecture, and the presence of GeBu4. Reduction of the Pt-only systems in H-2 is found to have an autocatalytic character resulting in the rapid formation of large (N1Pt–Pt>8) particles at ca. 390 K. Reduction in the absence of hydrogen and/or in the presence of co-adsorbed GeBu4, results in a considerable retardation of particle growth and shows a dependence upon the support architecture. Both EDE and DRIFTS show that, in the case of PtGe alloy system elemental Pt particles form first (in the region of 400-500 K) and it is only at temperatures in excess of 500 K that significant alloy (Pt3Ge) formation is observed along with a concomitant reduction in average particle size. This same pattern of behaviour is also observed when Pt particles are pre-formed prior to the introduction of GeBu4 and subsequent reduction. These results are discussed in terms of the reductive processes at work in these systems, the support architecture, and the effects of retained carbonaceous materials on developing particles

Simultaneous determination of structural and kinetic parameters characterizing the interconversion of highly dispersed species: the interaction of NO with Rh-I(CO)(2)/gamma-Al2O3

Energy-dispersive EXAFS (EDE), combined with mass spectrometry and a flow microreactor system, has been used to investigate the reaction of an Al2O3-supported Rh-I(CO)(2) species with NO. This combined in situ approach uniquely permits a priori analysis of the structures of the species involved (on a time scale of ca. 2 s) and simultaneous determination of reaction mechanism and kinetic parameters. In the current case, it is found that the Al(O)Rh-I(CO)(2)Cl species reacts to form an intermediate Al(O)Rh(NO)(2)Cl Cl species (v approximate to 0.357 +/- 0.125 s(-1), E-act approximate to 11 +/- 1.25 kJ mol(-1)), which subsequently forms an (AlO)(2)Rh(NO)Cl- species and N2O(g) (v approximate to 2 +/- 0.5 x 10(4) s(-1), E-act approximate to 40 +/- 3.5 kJ mol(-1)) showing a bent (134 degrees) RhNO bond. This combination of rapid and complementary techniques should be applicable to a wide range of disciplines where quantitative structural and kinetic determinations are of importance