(584ch) Controlling Metal Nanoparticle Size on Carbon Supports with Surface Tension

Strong Electrostatic Adsorption (SEA) has been used for numerous metals and common oxide and carbon catalyst supports, to synthesize ultrasmall supported metal nanoparticles. In this context, “ultrasmall” refers to sizes of 1 - 2 nm. It is believed that the lower limit of nanoparticle size is caused by the formation of nanodroplets of precursor solution as the powdered supports dried – while there is no large scale migration of the metal complexes as they are strongly attracted to the support surface, surface tension replaces electrostatic interaction as drying occurs and the small deposits of precipitated precursor dictate the ultimate, reduced particle size. To achieve yet smaller particle/cluster size, we now focus on decreasing the degree the precursor agglomeration during drying.

We hypothesize that metal nanoparticle size can be controlled by increasing the hydrophilicity of the carbon supports; with lower contact angles the volume of precursor nanodroplets is expected to decrease. To test this hypothesis, Ketjen Black 300 J (KB300J) and Norit SX Ultra (Norit) carbon supports, with PZCs 10 and 8 respectively, were oxidized using varied concentrations and exposure times of nitric acid to decrease the surface hydrophilicity. SEA was employed with Tetraammineplatinum(II) nitrate or Tetraamminepalladium(II) nitrate for lower PZC supports and Chloroplatinic acid or Tetrachloropalladium(II) for unoxidized supports. X-Ray Diffraction (XRD) and Scanning Transmission Electron Microscopy (STEM) were employed to characterize the nanoparticle size and atomic isolation of each dried and reduced catalyst. Platinum catalysts exhibit trends of decreasing particle size with higher degrees of oxidation excluding the most oxidized carbons which exhibited higher degrees of agglomeration. Dried catalysts exhibited trends of increased atomic isolation with as successive oxidation occurred with the most oxidized carbons exhibiting near entire atomic isolation. Catalysts for both carbons using Palladium will be reported.