ATOMIC AND MOLECULAR-OXYGEN AS CHEMICAL PRECURSORS IN THE OXIDATION OF AMMONIA BY COPPER

The role of atomic and molecular oxygen precursors in the overall cata lytic cycle for ammonia dissociation is analyzed using first-principle density functional calculations. Adsorption energies for ammonia, mol ecular oxygen, NHx, NO, and various intermediates and adatoms were com puted from geometry optimized calculations on the model Cu(8,3) cluste r of the Cu(111) surface. Reported values systematically underpredict experimental adsorption energies by 30 kJ/mol due to the finite cluste r size. Attractive and repulsive lateral interactions were important i n accessing accurate adsorption energies. Atomic oxygen enhances N-H b ond activation; however, it also acts to poison active surface sites a nd inhibit ammonia dissociation kinetics. Transient molecular oxygen a dsorbs weakly in both parallel (-17 kJ/mol) and perpendicular orientat ions (-10 kJ/mol) to the surface. Parallel adsorption appears to be a precursor for oxygen dissociation, whereas perpendicular adsorption is the precursor for ammonia dissociation. The mechanism in which hydrog en atoms are abstracted sequentially to form OOH intermediate [E* (ap parent) = 0 kJ/mol] is favored over that in which two hydrogens are si multaneously transferred to form water directly [E(apparent) = +67 kJ /mol]. The nonactivated transient molecular path in which hydrogen is abstracted sequentially is the most favored of all of the four paths s tudied. In light of the experimental O-2 dissociation energy over Cu(1 11), transient O-2 is more likely than ''hot'' atomic oxygen as the do minant chemical precursor for ammonia dissociation. Subsequent dissoci ation of the NHx fragments lead to N. While enthalpic considerations favor recombinative desorption of N-2, at reaction conditions the MARI is atomic oxygen thus making the recombinative desorption of NO more likely reaction path.