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 catalytic cycle for ammonia dissociation is analyzed using first-principle density functional calculations. Adsorption energies for ammonia, molecular oxygen, NH_x, NO, and various intermediates and adatoms were computed from geometry optimized calculations on the model Cu(8,3) cluster of the Cu(111) surface. Reported values systematically underpredict experimental adsorption energies by 30 kJ/mol due to the finite cluster size. Attractive and repulsive lateral interactions were important in accessing accurate adsorption energies. Atomic oxygen enhances N-H bond activation; however, it also acts to poison active surface sites and inhibit ammonia dissociation kinetics. Transient molecular oxygen adsorbs weakly in both parallel (-17 kJ/mol) and perpendicular orientations (-10kJ/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 hydrogen atoms are abstracted sequentially to form OOH∗ intermediate [E∗ (apparent) = 0 kJ/mol] is favored over that in which two hydrogens are simultaneously transferred to form water directly [E∗(apparent) = +67 kJ/mol]. The nonactivated transient molecular path in which hdyrogen is abstracted sequentially is the most favored of all of the four paths studied. In light of the experimental O₂ dissociation energy over Cu(111), transient O₂ is more likely than "hot" atomic oxygen as the dominant chemical precursor for ammonia dissociation. Subsequent dissociation of the NH, fragments lead to N∗. While enthalpic considerations favor recombinative desorption of N₂, at reaction conditions the MARI is atomic oxygen thus making the recombinative desorption of NO more likely reaction path.