DISSOCIATION OF AMMONIA ON A COPPER SURFACE AND THE EFFECT OF OXYGEN COADSORPTION - A QUANTUM-CHEMICAL STUDY

The adsorption and initial dissociation pathways of ammonia on an 11 atom model cluster of the copper (111) surface, and the effects of coadsorbed oxygen are analyzed through local density functional calculations, including non-local corrections to the final adsorption energies. The results demonstrate that the presence of oxygen increases the adsorption energy and promotes the dissociation of ammonia over copper. All examined dissociation reactions were endothermic. Dissociation in the absence of oxygen has the highest activation barrier of all steps analyzed and is the most endothermic with an overall energy change of + 176 kJ/mol. In presence of oxygen, however, the energy needed for the dissociation of ammonia is considerably lower with an overall energy of + 48 kJ/mol. The dissociation of ammonia in the presence of oxygen has the same overall reaction energy (+ 48 kJ/mol) regardless of whether the ammonia was initially adsorbed onefold or threefold. The activation energies, + 132 (ammonia initially onefold) and + 173 kJ/mol (ammonia initially threefold), demonstrate that the system favors the pathway where ammonia is initially onefold. It is concluded that ammonia dissociation proceeds via the intermediate formation of adsorbed hydroxyl.