The CO adsorption on ordered Cu-Pd alloy surfaces and surface alloys has been studied using density functional theory (DFT) within the framework of the generalized gradient approximation (GGA). On the surface alloys, the CO adsorption energy at the top sites decreases with increasing concentration of the more reactive metal Pd. This surprising ligand effect is caused by the effective compressive strain induced by the larger size of the Pd atoms. On the other hand, at the most favorable adsorption sites the CO binding becomes stronger with increasing Pd concentration which is caused by an ensemble effect related to the availability of higher coordinated adsorption sites. At the surfaces of the bulk alloys, the trends in the adsorption energy as a function of the Pd concentration are less clear because of the strong Pd-Cu interaction and the absence of effective strain effects.

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We have studied the interaction of an oxygen molecule with Al clusters and Al(111) using both wave-functionbased quantum chemistry methods and density functional theory (DFT). These calculations were motivated by the fact that molecular beam experiments indicate that the adsorption of O 2 on Al(111) should be activated whereas periodic DFT calculations yield purely attractive adsorption paths for almost all impact configurations of O 2 on Al(111). On small Al 4 clusters, accurate wave-function-based quantum chemistry methods find a non-vanishing barrier in the O 2 adsorption. The DFT calculations for slabs and larger Al clusters confirm the important role of spin effects for the O 2 dissociation barrier on Al. The results indicate that exchange-correlation effects play a crucial role for the determination of the adsorption barrier in the O 2 /Al system but their determination is hampered by serious technical problems that are discussed in detail.

Any technologically important chemical reaction typically involves a number of different elementary reaction steps consisting of bond-breaking and bond-making processes. Usually, one assumes that such complex chemical reactions occur in a step-wise fashion where one single bond is made or broken at a time. Using first-principles calculations based on density functional theory we show that the barriers of rate-limiting steps for technologically relevant surface reactions are significantly reduced if concerted reaction mechanisms are taken into account.