TRANSITION-METAL OXIDE CATALYZED CARBON-BLACK OXIDATION - A STUDY WITH O-18(2)

Physical mixtures of carbon black and several transition metal oxides (Cr2O3, Co3O4, Fe2O3, MoO3, V2O5, and K2MoO4) were treated batchwise i n O-18(2) at 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 ((CO)-O-16, (C O)-O-18, (CO2)-O-16, (COO)-O-16-O-18, and (CO2)-O-18) in experiments, in which O-18(2) and O-16(2) were fed alternately, showed that the amo unt of gas-phase oxygen incorporated in the products decreases in the series Cr2O3 > Co3O4 = Fe2O3 > MoO3 > V2O5 > K2MoO4. Based on this tre nd, a surface redox mechanism is proposed for Cr2O3, Co3O4, and Fe2O3 catalyzed carbon black oxidation in which only surface oxygen (lattice oxygen of the oxide located in the surface layer) participates. Consi dering the formation of carbon surface oxygen complexes, and the relat ively high fraction of O-18-labeled products obtained for Cr2O3/carbon black mixtures, a spill-over mechanism, involving adsorbed surface ox ygen, is suggested to run in parallel with the surface redox mechanism for Cr2O3. A ''classical'' redox mechanism, in which lattice oxygen f rom the hulk of the oxide is active, is proposed for MoO3, V2O5, and K 2MoO4. Significant carbothermic reduction of MoO3 and V2O5 was observe d in the absence of gas-phase oxygen. For K2MoO4 gas-phase oxygen is n eeded to keep the reaction going, although the highest amount of O-16- labeled products was obtained. K2MoO4 is not carbothermally reduced at the temperatures used. A ''push-pull'' mechanism seems to be operativ e 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. (C) 1998 Academic Press.