Dissociation of ammonia on a copper surface and the effect of oxygen coadsorption: a quantum-chemical study

The adsorption and initial dissocn. pathways of NH3 on an 11 atom model cluster of the Cu(111) surface, and the effects of coadsorbed O were analyzed through local d. functional calcns., including non-local corrections to the final adsorption energies. The results demonstrate that the presence of O increases the adsorption energy and promotes the dissocn. of NH3 over Cu. All examd. dissocn. reactions were endothermic. Dissocn. in the absence of O 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 O, however, the energy needed for the dissocn. of NH3 is considerably lower with an overall energy of +48 kJ/mol. The dissocn. of NH3 in the presence of O has the same overall reaction energy (+48 kJ/mol) regardless of whether the NH3 was initially adsorbed 1-fold or 3-fold. The activation energies (+132 for NH3 initially 1-fold and +173 kJ/mol for NH3 initially 3-fold) demonstrate that the system favors the pathway where NH3 initially is 1-fold. The NH3 dissocn. proceeds via the intermediate formation of adsorbed OH