Nanostructure of Pd/SiO2 supported catalysts

Pd/SiO2 supported catalysts were investigated using anomalous small-angle X-ray scattering (ASAXS), wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). Samples containing about 3 wt.% and 6 wt.% of metal were obtained by impregnation, calcined at 773 K and reduced at 623 K. Two populations of nanoparticles were found in both samples. In the 3 wt.% sample, one population consisted of very small nanoparticles (average 3 nm), and the other of larger ones (average 13 nm). A higher metal load seems to cause a broadening of the size distribution of the first population and a consequent increase in its average dimension (6 nm), rather than to influence the larger nanoparticles. Despite the relatively high metal content and temperatures, the population of small particles was predominant in both samples: 80% in the 3 wt.% sample and 57% in the 6 wt.% one. A large number of the smallest particles (<3 nm) was present in the 6 wt.% sample as well, and the total metal surface available was about the same in the two samples in spite of the different amounts of metal. Anomalous scattering has proven to be essential to quantitative analysis.