Atomic and Molecular Oxygen as Chemical Precursors in the Oxidation of Ammonia by Copper

Abstract

The role of at. and mol. oxygen precursors in the overall catalytic cycle for ammonia dissocn. is analyzed using first-principle d. functional calcns. Adsorption energies for ammonia, mol. oxygen, NHx, NO, and various intermediates and adatoms were computed from geometry optimized calcns. on the model Cu(8,3) cluster of the Cu(111) surface. Reported values systematically underpredict exptl. adsorption energies by 30 kJ/mol due to the finite cluster size. Attractive and repulsive lateral interactions were important in accessing accurate adsorption energies. At. oxygen enhances N-H bond activation; however, it also acts to poison active surface sites and inhibit ammonia dissocn. kinetics. Transient mol. oxygen adsorbs weakly in both parallel (-17 kJ/mol) and perpendicular orientations (-10 kJ/mol) to the surface. Parallel adsorption appears to be a precursor for oxygen dissocn., whereas perpendicular adsorption is the precursor for ammonia dissocn. 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 mol. path in which hydrogen is abstracted sequentially is the most favored of all of the four paths studied. In light of the exptl. O2 dissocn. energy over Cu(111),transient O2 is more likely than hot at. oxygen as the dominant chem. precursor for ammonia dissocn. Subsequent dissocn. of the NHx fragments lead to N*. While enthalpic considerations favor recombinative desorption ofN2, at reaction conditions the MARI is at. oxygen thus making the recombinative desorption of NO more likely reaction path. [on SciFinder (R)]