Chemisorption theory of ammonia on copper

We present local-density-approximation calculations of ammonia adsorption on copper clusters of different sizes (6 to 18 atoms) modelling the (100) and (111) surface. Including for some of the copper atoms only one instead of eleven electrons explicitly in the calculation, did not always work satisfactorily. Comparison of adsorption energies for clusters of related geometry indicates a preference for onefold adsorption. This is due to the Pauli repulsion of the lone-pair orbital of ammonia with the copper 3d electrons. which is minimal for onefold adsorption. as well as an interaction with 4s electrons, which is most attractive in the onefold geometr

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 O

The oxidation of ammonia by copper

We present local spin density approximation calculations on the adsorption and dissociation of ammonia n a copper cluster which models the Cu(111) surface. Preadsorbed oxygen increases the adsorption energy of ammonia, and promotes the dissociation of ammonia. All examined dissociation reactions in the absence of oxygen are endothermic. Dissociation of ammonia to NH2 with oxygen is also found to be endothermic, but less than for systems without oxygen. Subsequent dissociation to NH is thermally neutral, whereas NH dissociation to nitrogen is exothermic. The most endothermic steps in the catalytic reaction cycle of the ammonia oxidation reaction appear to be the dissociation of ammonia and the desorption of wate