The role of oxygen during the catalytic oxidation of ammonia on Co 3 O 4 (1 0 0)

Abstract The adsorption selectivity and dehydrogenation energy barriers of NH 3 , NH 2 and NH on the (1 0 0) surface planes of Co 3 O 4 are determined by means of density functional methods. Stepwise hydrogen abstraction is effected by lattice O 3o associated with octahedrally coordinated surface Co atoms. The final H-abstraction, from NH, leads directly to the formation of gaseous product NO with the creation of a lattice oxygen vacancy. Reaction of this vacancy with gas-phase O 2 repairs the vacancy and creates surface-adsorbed O * which is also capable of abstracting H from NH 3 * , NH 2 * and NH * , the final step leading to directly again to NO formation. The mobile surface OH * formed from the O * -mediated abstraction steps is also capable of abstracting H from the NH x * species, leading ultimately to surface N * which then easily extracts a lattice O 3o to form NO and a new vacancy. The overall mechanism to form NO is a complex cycle of lattice- and surface-mediated abstractions. The hydrogen budget in the reaction shows corresponding complexity. Surface H * (formed when lattice O 3o abstracts H from NH x ) is stable and immobile but it can be abstracted by surface OH * to form water. OH * disproportionation reaction also forms water.