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

Abstract

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 H₂ is found to have an autocatalytic character resulting in the rapid formation of large (N₁^Pt–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.