Adsorption of precious metal ions onto electrochemically oxidized carbon fibers

Abstract

Electrochemically oxidized carbon fibers (ECF) adsorbed a prodigious amount of Ag+ in contrast to oxygen plasma and nitric acid treated carbon fibers. The amount of adsorbed Ag+ reached 3700 μmol/g after 5652 C/g of electrochemical oxidation. This value approaches the 4050 μmol/g of Ag+ which adsorbed onto steam-activated Kenaf-based carbon (with a surface area of 1284 m2/g determined by N2/BET) under the same adsorption conditions. ECF oxidized to 9540 C/g adsorbed more than its own weight of Ag+ (12 608 μmol/g). These fibers exhibited a surface area of 115 m2/g (CO2–DR). Two different reactions occurred during Ag+ adsorption. These reactions were ion exchange adsorption between Ag+ and acidic functions (carboxyl) and redox adsorption between Ag+ and reducing functions such as catechol groups on these electrochemically oxidized fibers (ECF). The redox capability was expressed by the reaction electric potential (E) using the Nernst equation. High resolution XPS C 1s spectra of ECFs (level of oxidation 5300 C/g), before and after Ag+ adsorption, showed that the carbon atoms present in phenolic, alcohol or ether groups and those present in carbonyl or quinone groups increased after Ag+ adsorption. X-ray diffraction and X-ray photoelectron spectroscopy (XPS) Ag 3d spectra of the ECF showed that adsorbed Ag+ was reduced to Ag0 after both Ag+ adsorption and subsequent post-heat treatment of the fibers under N2 at 550°C for 30 min. Only about one-third as much Au3+ adsorption occurred versus the extent of electrochemical oxidation as was observed for Ag+. This ratio matches the requirement that three electrons are required to convert Au3+ to Au0 versus one to convert Ag+ to Ag. High resolution angle resolved XPS (ARXPS) Pd 3d and Pt 4f spectra show that there are two different Pd oxidation states and three different Pt oxidation states present after adsorption of Pd2+ and Pt2+ onto ECF. The peak areas as a function of take off angle showed that substantial amounts of Pd0 and Pt0 are present in addition to Pd2+ and Pt2+ and Pt4+ on the outermost surface regions of oxidized fibers.