Activation of Dioxygen on Isolated Transition Metal Ions in Nitrogen-Doped Carbon

The activation of O2 is an important process for a variety of reactions from the electrochemical reduction of O2 for fuel cells to thermochemical oxidative dehydrogenation reactions. Platinum particles supported on carbon are often reported to be highly active catalysts for these reactions, but their high costs limit their large-scale use. This high cost has prompted research into atomically efficient transition metal ions (M) doped into a conductive nitrogen-carbon support (N-C). These M-N-C catalysts have shown promise for comparable activity on the same magnitude as platinum electrodes for the electrochemical oxygen reduction reaction (ORR) and Pt nanoparticles for the thermochemical oxidative dehydrogenation of alcohols. Although the M-N-C catalysts facilitate O2 activation for both thermal and electrochemical reactions, the coordination environment surrounding the active site and the mechanism for O2 activation remain elusive. To avoid the complications of solvent and applied potential, CO oxidation and 2-propanol oxidative dehydrogenation were used as probe reactions in this dissertation.
A catalyst with isolated Co ions in nitrogen-doped carbon (Co-N-C) was synthesized and imaged by scanning transmission microscopy, confirming the presence of isolated Co ions. The Co-N-C catalyst converted CO at temperatures as low at 198 K, with a negative apparent activation energy at low temperatures, whereas low temperature catalytic activity was not observed for the other Co-containing materials (mixed Co oxide, Co on carbon black, and Co on silica). Comparison of the reaction orders measured over the Co-containing catalysts revealed nearly first-order behavior in both CO and O2¬ over the Co-N-C catalyst compared to zero order in CO and positive order in O2 for the other Co-containing catalysts. Isotope transient analysis indicated the surface coverage of reactive intermediates leading to CO2 on Co-N-C was low relative to CO and the turnover frequency was high (exceeding 0.3 s-1), even at 273 K.
Quantum chemical density functional theory (DFT) calculations indicated weak binding of both CO and O2 to the Co ion surrounded by 4 pyridinic N atoms. Molecular dynamic simulations with CO and O2 resulted in a plausible reaction path for CO oxidation whereby CO assisted the activation of O2 with the carbon support without involving a redox cycle on the Co ion. The reaction path was consistent with the experimental kinetic parameters for low temperature CO oxidation on the Co-N-C catalyst.
A quantum chemical DFT screening of other M-N-C catalysts for similar interactions between the transition metal and reactants CO and O2 suggested that the transition metals in the same column as Co are potentially good catalysts for low temperature O2 activation. Thus, a Rh-N-C catalyst was synthesized and tested for CO oxidation activity. Steady-state CO oxidation at low temperature over Rh-N-C had positive reaction orders in both CO and O2 as well as a nearly zero apparent activation energy, whereas supported Rh nanoparticles were not active at these low temperatures. The experimental observation of low temperature CO oxidation catalyzed by Rh-N-C validates the results from the DFT screening study.
The gas phase oxidative dehydrogenation of 2-propanol was also explored on Co-N-C. High initial activity for the Co-N-C catalyst at 473 K sharply deactivated to a steady-state rate that was similar to a metal free N-C catalyst. The similar steady-state rates of a Co-containing material and a Co-free material indicate that Co sites may not turn over at the steady state. The reaction was also tested in aqueous solvent and similar to the gas phase reaction, the Co-N-C catalyst had high initial activity but deactivated completely by 15 min. The observed performance of highly active species (associated with Co) and the less active N-C (Co-free) warrant further study.