Preparation of Carbon Nanofiber Supported Platinum and Ruthenium Catalysts: Comparison of Ion Adsorption and Homogeneous Deposition Precipitation

Abstract

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, H2-chemisorption, 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)42+ molecules/nm2. Furthermore, it has been established that with this procedure higher platinum loadings (∼4 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.