Preparation of carbon nanofiber supported platinum and ruthenium catalysts: Comparison of ion adsorption and homogeneous deposition precipitation

Carbon nanofiber (CNF) supported platinum catalysts have been prepared using Pt(NH3)(4)(NO3)(2) as a precursor by two different ion adsorption techniques, one at a constant pH and one in which the pH is gradually and homogeneously increased from 3 to 6 by hydrolysis of urea. The latter method resembles the procedure of homogeneous deposition precipitation (HDP). Characterization of the CNF support was performed by acid-base titration, thermogravimetric mass spectrometry, and X-ray photoelectron spectroscopy, and for the various platinum catalysts, transmission electron microscopy, H-2-chemi sorption, and X-ray fluorescence/inductively coupled plasma-atomic emission spectrometry were utilized. With both synthesis techniques from diluted precursor solutions homogeneously distributed, highly dispersed and thermally stable metal particles were obtained with an average particle size of 1-2 nm. With the HDP method for the Pt/CNF catalysts, a linear relationship between the number of acidic oxygen-containing groups on the surface of activated CNF and the metal loading has been found. For the highest loaded catalyst, a platinum/adsorption site ratio of 0.5 was established, corresponding to about 0.7 Pt(NH3)(4)(2+) molecules/nm(2). Furthermore, it has been established that with this procedure higher platinum loadings (similar to4 wt%) can be achieved than with the ion adsorption procedure (<2 wt%). The HDP method using RuNO(NO3)(3)(H2O)(2) also turned out to be suitable for the preparation of small (1-2 nm) uniform ruthenium particles on CNF with a high thermostability.