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 e lectron microscopy (TEM) and extended X-ray absorption fine structure spect roscopy (EXAFS). Samples containing about 3 wt.% and 6 wt.% of metal were o btained by impregnation, calcined at 773 K and reduced at 623 K. Two popula tions of nanoparticles were found in both samples. In the 3 wt.% sample, on e 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 consequen t increase in its average dimension (6 nm), rather than to influence the la rger nanoparticles. Despite the relatively high metal content and temperatu res, 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 smal lest particles (<3 nm) was present in the 6 wt.% sample as well, and the to tal 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 es sential to quantitative analysis.