Manganese deception on graphene and implications in catalysis

Abstract

Heteroatom-doped metal-free graphene has been widely studied as the catalyst for the oxygen reduction reaction (ORR). Depending on the preparation method and the dopants, the ORR activity varies ranging from a two-electron to a four-electron pathway. The different literature reports are difficult to correlate due to the large variances. However, due to the potential metal contamination, the origin of the ORR activity from “metal-free” graphene remains confusing and inconclusive. Here we decipher the ORR catalytic activities of diverse architectures on graphene derived from reduced graphene oxide. High angle annular dark field scanning transmission electron microscopy, X-ray absorption near edge structure, extended X-ray absorption fine structure, and trace elemental analysis methods are employed. The mechanistic origin of ORR activity is associated with the trace manganese content and reaches its highest performance at an onset potential of 0.94 V when manganese exists as a mononuclear-centered structure within defective graphene. This study exposes the deceptive role of trace metal in formerly thought to be metal-free graphene materials. It also provides insight into the design of better-performing catalyst for ORR by underscoring the coordination chemistry possible for future single-atom catalyst materials.

Toxicological response of the model fungus Saccharomyces cerevisiae to different concentrations of commercial graphene nanoplatelets

“…Although the described analyses gave us good insights into the morphology and composition of this particular lot of commercial GN (ref: 806625; lot: MKBW5736V), none of these analytical techniques allowed the identification and quantification of trace elements 32 . However, since the presence of trace metal impurities in graphene derivatives, either contained in the graphite precursor or transferred by reactants used in the nanomaterial preparation, has been previously described [33][34][35][36][37][38] , a trace element analysis of GN was done by inductively coupled plasma mass spectrometry (ICP-MS) to fully characterize its composition. All metallic elements identified and their concentrations are displayed in Table 1.…”

N‐Doped Defective Graphene from Biomass as Catalyst for CO2 Hydrogenation to Methane

“…Of note is the fact that none of the defective graphenes used has been prepared using any metal that could contaminate the defective graphenes and that chemical analysis of these samples has shown that the metal content of these defective graphenes are in the ppm level. This contrast with some samples of reduced graphene oxide whose preparation requires the use of an excess of oxidizing reagent, typically MnO 4 − , that is retained later in the material in some proportion . Also worth noting is the fact that the pyrolytic procedure at 900 °C for preparation of these doped defective graphene samples make them stable at temperatures in the range from 300 to 500 °C that will be used in the catalytic study.…”

3D Carbon Materials for Efficient Oxygen and Hydrogen Electrocatalysis

“…As sulfate anions are known to adsorb on Pt surfaces, perchloric acid is recommended for the ORR performance tests in acid, and it is also suggested for the porous carbons. Metal‐free catalysts can sometimes contain trace amounts of metal, perhaps from the synthesis steps that can be the sole cause of electrochemical activity, and so care should be taken to ensure there are no trace metals if claiming ORR catalysis in metal‐free materials . The electrolyte (KOH) can contain some metals (Fe) in ppm levels as well, perhaps leached from the catalysts used in its preparation steps, therefore, electrolytes should be screened for trace metals with inductively coupled mass spectroscopy (ICP‐MS) and purified where possible .…”