Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO2 nanoparticles

The Pd/TiO2 nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO2 were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O2 dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO2 particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO2 nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti–O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO2.