Kinetics and Products of the Reaction of Graphene Nanoplatelets with Noble Metal Ions to Nanocomposites with Single Atoms and Clusters

Abstract

Metal-supported graphene nanocomposites with single atoms or small clusters are of interest for various catalytic processes, including applications in batteries, fuel cells, water electrolysis, and chemical synthesis. Typically, graphene oxide is reduced in the presence of metal salts to produce metal-graphene nanocomposites. However, graphene itself has reductive properties and can react with metal ions in higher oxidation states in a suitable solvent. While direct reactions (dip and coat or wet coating) with metal salts have been described several times, less is known about the kinetics. This study investigates the reaction of suspended graphene nanoplatelets (GNP) in aerated water with the chlorocomplexes of gold(III), iridium(IV), platinum(IV), and palladium(II) to form nanocomposites covered with single atoms and small clusters. The maximum metal loading ranges from 3.3 mass% for palladium to 44 mass% for gold, increasing with redox potential. At high redox potentials, such as those of Ir(IV) and Au(III), the reactions follow pseudo-first order kinetics. In contrast, at lower potentials, such as those of Pt(IV) and Pd(II), the reaction adhere to pseudo-second order. This data enable kinetically controlled metal coating of the GNP. In contrast to the use of a reducing agent, gold, platinum, and palladium are present on the GNP in different oxidation states, which can be specifically modified, as shown for platinum-coated GNP. Iridium(IV) has been deposited as anhydrous and hydrated iridium(IV) oxide. The nanocomposites have great potential as single-atom catalysts. The described process can be transferred to other transition metals and is sustainable because the reaction media can be recycled.