(308e) Synthesis and Characterization of Dendrimer-Derived Ir/Γ-Al2O3 Catalysts

We report the synthesis of 1wt% Ir/g-Al₂O₃ catalyst using
a dendrimer-metal nanocomposite (DMN) precursor to control the resulting Ir
particle size.  Fourth generation hydroxyl-terminated polyamidoamine (PAMAM-OH)
dendrimer was combined with IrCl₃ in aqueous solution at room
temperature in a metal-dendrimer molar ratio of 40:1.  The complexation process
between Ir and the dendrimer amine and amide groups was followed using ultraviolet-visible
(UV-Vis) spectroscopy.  The results suggest that seven days are required to
reach equilibrium, after which the DMN solution saturated with H₂. 
The resulting DMN precursor is delivered to a g-Al₂O₃
support via standard wet impregnation.  The dendrimer is subsequently removed
by thermal decomposition in order to produce nanoparticles on the support. This
process was investigated using transmission Fourier transform infrared (FTIR)
spectroscopy to follow the decomposition of dendrimer into various fragments
and their subsequent desorption from the surface.  In addition, FTIR
measurements during CO adsorption were performed to identify the catalyst
activation treatment that exposed the greatest number of metal sites. Two different
optimal treatments were found:  350°C in
O₂ for 30 min/400°C in H₂
for 1 hr treatment or H₂ at 400°C
for 2 hrs.  In addition, this catalyst has been characterized using high
resolution transmission electron microscopy (HRTEM) to obtain particle size
distributions and extended X-ray absorption fine structure (EXAFS) spectroscopy
develop a better understanding of the dendrimer-Ir molecular interactions. 
These results will be compared with similar measurements from traditionally
prepared Ir catalysts.  The effectiveness of this novel approach for control of
metal particle size/dispersion will be discussed in light of these findings.