Transition Metal Oxide Catalyzed Carbon Black Oxidation: A Study with18O2

Physical mixtures of carbon black and several transition metal oxides (Cr2O3, Co3O4, Fe2O3, MoO3, V2O5, and K2MoO4) were treated batchwise in18O2at 625–675 K in a high-vacuum batch reactor. Three reaction mechanisms are proposed to be operative in the catalyzed oxidation of carbon black. The isotopic reaction product composition (C16O, C18O, C16O2, C16O18O, and C18O2) in experiments, in which18O2and16O2were fed alternately, showed that the amount of gas-phase oxygen incorporated in the products decreases in the series Cr2O3> Co3O4= Fe2O3> MoO3> V2O5> K2MoO4. Based on this trend, a surface redox mechanism is proposed for Cr2O3, Co3O4, and Fe2O3catalyzed carbon black oxidation in which only surface oxygen (lattice oxygen of the oxide located in the surface layer) participates. Considering the formation of carbon surface oxygen complexes, and the relatively high fraction of18O-labeled products obtained for Cr2O3/carbon black mixtures, a spill-over mechanism, involving adsorbed surface oxygen, is suggested to run in parallel with the surface redox mechanism for Cr2O3. A “classical” redox mechanism, in which lattice oxygen from the bulk of the oxide is active, is proposed for MoO3, V2O5, and K2MoO4. Significant carbothermic reduction of MoO3and V2O5was observed in the absence of gas-phase oxygen. For K2MoO4gas-phase oxygen is needed to keep the reaction going, although the highest amount of16O-labeled products was obtained. K2MoO4is not carbothermally reduced at the temperatures used. A “push–pull” mechanism seems to be operative for K2MoO4. Oxygen spill-over might also occur in the MoO3-, V2O5-, and K2MoO4-catalyzed carbon black oxidation, in view of the formation of carbon surface oxygen complexes, but this plays a minor role in the overall oxidation mechanism.

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